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Shoe Industry

  • José Miguel Martín-MartínezEmail author
Living reference work entry

Abstract

This chapter constitutes one of the very few reviews in the existing literature on shoe bonding, and it gives an updated overview of the upper-to-sole bonding by using adhesives. The surface preparation of rubber soles and the formulations of the solvent-borne and waterborne polyurethane and polychloroprene adhesives are described in more detail. The preparation of the adhesive joints and the specific adhesion tests in shoe bonding are also revised. Finally, the most recent developments dealing with shoe bonding are described.

Keywords

Shoe bonding Polyurethane adhesive Polychloroprene adhesive Waterborne adhesives Surface preparation Leather upper Rubber sole Mechanisms of adhesion Testing 

1 Introduction

The attachment of the upper to the sole by means of adhesives in shoe manufacturing is one of the most exigent bonding technologies. Several different types of materials are used in shoe manufacturing most of them varying in nature and composition from year to year because of fashion requirements. Furthermore, the geometrical shape and the design of the upper and the sole change often in male and female shoes, and several models and kinds of shoes can be manufactured (casual shoes, sport shoes, boots, safety shoes, sanitary and orthopedic shoes). For each shoe, bonding must be specifically designed, and this situation happens twice a year (spring and fall seasons in shoe industry). This is one of the reasons for the difficulty in establishing unified methodologies for shoe bonding as there is no time for shoe manufacturers to react once debonding appears, i.e., the upper and the sole separate during use (Fig. 1).
Fig. 1

Upper-to-sole detachment in a shoe

Another particular feature in shoe industry is the lack of quality control of the materials and the adhesives used for bonding. Shoe manufacturers have extensive knowledge on the technology of making shoes, but their knowledge on bonding comes mainly from the experience and the advice of the adhesive manufacturers. Certainly, the bonding of the shoe parts is more of an art than a technology, and the shoe manufacturers are always worried and even scared when they have to attach the upper to the sole in the new models of shoes in the season. In fact, it is a common practice in several famous shoe brands to sew the upper to the sole simply because they do not trust in the quality of the adhesive bonding!

The above given arguments justify the writing of a review on the bonding of shoe industry based mainly on the experience of the author in the field acquired during more than 25 years.

The bonding with adhesives has been introduced in the footwear industry as an alternative to sewing or the use of nails, staples, or tacks to bond several parts of the shoes, the most critical part being the upper-to-sole bonding. The introduction of adhesive technology in shoe manufacturing provided several advantages: (i) More flexible and homogeneous joints were obtained; (ii) the stresses during use were similarly distributed all along the joints; (iii) improved aesthetic properties were obtained, and new design and fashion shoes were possible; and (iv) automation was feasible. However, as a limitation, the bonding with adhesives in shoe industry needs a severe control of all steps involved in the formation of the joints to avoid adhesion problems, mainly the separation of the sole from the upper. Although several parts of a shoe are bonded with adhesives (mounting, heel covering, box toe bonding, sticking of the soft lining, lift attachment, shank and cushion bonding, etc.), this chapter will be devoted only to the upper-to-sole attachment because it is the most critical bonding operation in shoe manufacturing.

Depending on the type of shoe, different bonding performance is required. Due to fashion issues, the bonding of the casual and the free-time shoes is relatively low exigent as the requested durability is not too high (3 years maximum). However, the sport shoes are extremely exigent in bonding as low weight, high performance under impact and flexural stresses, high comfort, and high durability are mandatory requirements. Safety shoes are the most exigent in terms of bonding because they have to work in the presence of solvents, high or low temperature, high-impact resistance, and all these should last for a long time.

There are not many reviews in the existing literature dealing with the adhesive bonding in shoe industry. Some Spanish books (Diputación de Alicante 1974; Amat-Amer 1999) devoted to shoe industry include some basic technical chapters dealing with the use of the adhesives for bonding operation, but they have not been translated to English. In the 1960s and 1970s, the Prüf- und Forschungsinstitut Pirmasens (PFI) in Pirmasens (Germany) (Fisher and Meuser 1964; Fisher 1971) and the Shoe Allied Trade Research Association (SATRA) in Kettering (England) (Pettit and Carter 1973) published excellent reports dealing with several specific aspects of the adhesive joints in shoe industry. More extensive literature dealing with the solvent- and waterborne polyurethane adhesives used in shoe industry has been published (some to be cited are Pettit and Carter 1973; Schollenberger 1977; Dollhausen 1985; Kozakiewicz 1991; Sultan Nasar et al. 1998; Frisch 2002), but minor attention has been paid to polychloroprene adhesives (some to be cited are Whitehouse 1986; Kelly and McDonald 1963; Harrington 1990; Guggenberger 1990; Lyons and Christell 1997; Martín-Martínez 2002). On the other hand, the surface preparation procedures of the upper and sole materials for shoe bonding have been widely considered in the literature (some to be cited are Oldfield and Symes 1983; Extrand and Gent 1987; Hace et al. 1990; Fernández-García et al. 1991; Lawson et al. 1996; Cepeda-Jiménez et al. 2002).

Most of the books devoted to footwear consider only a chapter devoted to adhesives or surface preparation of materials before bond formation, but rarely both aspects are described in the existing literature, and the understanding of the complex nature of the bonding process in shoe manufacturing is almost absent. A previous chapter written by the author dealing with shoe bonding was published in 2005 (Adams 2005), and the present chapter is an update of that chapter in which the most recent scientific contributions have been addressed. Recently, a review dealing with the use of adhesives in the footwear industry has been published (Paiva et al. 2016). Obviously, the reader will find some similarities between the two chapters wroten by the author, but as a comprehensive updated view on shoe bonding is given, the reading of this chapter will certainly compensate for it.

This chapter is divided into four sections. The first section provides an overview of the bonding in the shoe industry. Conventional surface preparation of different sole and upper materials is described in section two, with special attention to the chlorination of rubber soles by using organic solvent solutions of trichloroisocyanuric acid. Both solvent-borne and waterborne polyurethane and polychloroprene adhesives are described in detail in section three with special reference to their formulations. Finally, the testing , the quality control, and the durability of the adhesive joints in shoe bonding are considered in section four.

2 Overview of the Shoe Bonding Process

Although each shoe may be constituted by materials of different nature with different shapes and the adhesives commonly used are polyurethane or polychloroprene adhesives, to produce an adequate bonding, the different operations/steps given in Fig. 2 must be followed.
Fig. 2

Schematic overview of the bonding process in shoe manufacturing (upper-to-sole attachment)

The first operation consists in the surface preparation of the materials to be bonded (upper and sole). Irrespective of the inherent difficulty of bonding these materials, solvent cleaning and roughening are generally carried out. For difficult-to-bond materials, more aggressive chemical surface treatments and/or primer application are mandatory. Once the materials are ready for bonding, the adhesive is prepared by adjusting its viscosity and by adding isocyanate hardener, mainly for improving the durability of the adhesive joints. The application of the adhesive is a critical step to reach good bonding as the method of application (brush, roller, etc.), and the amount of adhesive determine the success of the joint; the adhesive is usually applied to both materials to be bonded, and, therefore, contact adhesives are used (reactive polyurethane hot melts are an exception because of they are applied on the sole only). The next step in the joint formation consists in the evaporation of the most solvent of the adhesive on the surfaces to be bonded, and because of the absence of tack, reactivation (i.e., sudden heating with infrared or halogen lamps) of the adhesive layers is carried out followed by immediate joining of the melted adhesive films applied on the two materials to be bonded followed by the application of pressure for a given time. Finally, the joints are cooling down before placing the shoe in the box.

Interestingly, green adhesion is a critical issue for shoe manufacturers. It is a general practice to place the thumb nail at the shoe front and try to separate the adhesive joint. If the joint resists the press of the thumb nail, the joint is considered acceptable. If it is not the case, sewing is a common practice!

In terms of the mechanisms of adhesion (see chapter “Theories of Fundamental Adhesion”) involved in upper-to-sole bonding, mechanical interlocking is important. Considering that most of the upper materials are porous and the adhesives used in shoe bonding are applied as liquids, an acceptable penetration of the adhesive into the upper is expected, and then the mechanical adhesion is generally favored. However, because several synthetic sole materials are nonporous and have in general a relatively low surface energy, both mechanical (e.g., roughening) and chemical (e.g., halogenation) surface preparations are necessary, and enhanced chemical adhesion is required. Therefore, the main mechanisms of adhesion involved in shoe bonding are mechanical interlocking and chemical. Furthermore, in the bonding of some rubber soles, the weak boundary layers produced by surface contaminants and/or by migration of antiadherend moieties to the interface during joint formation must be removed before joint formation.

In the following sections, the materials and their surface preparation prior to be bonded, the adhesives used, and the manufacture and testing of the upper-to-sole joints will be separately discussed. Each section will consider an overview of the current state of the art in shoe industry, and future trends will be highlighted too.

3 Surface Preparation of Upper and Sole Materials for Bonding

Depending on fashion, each year different materials have been and are currently used in the manufacturing of shoes, including the rubber soles (vulcanized styrene–butadiene rubber (SBR), thermoplastic rubber, ethylene propylene diene monomer (M-class) rubber (EPDM)) and different polymeric materials (leather; polyurethanes; ethylene–vinyl acetate copolymers, EVA; polyvinyl chloride, PVC; polyethylene, Phylon). In order to produce adequate adhesive joints, the surface preparation of those materials is required (see part related to Surface Treatments). Surface preparation procedures for these materials must be quickly developed, and the validity of these treatments is generally too short because of fashion constrictions. Several procedures have been established to optimize the upper-to-sole bonding; the most of them are based in the use of organic solvents. Due to environmental and health issues, the solvents should be removed from the surface preparation procedure, and several environmental-friendly procedures for the surface preparation of different materials have been proposed.

Most of the upper and sole materials used in shoe industry cannot be directly joined by using the current adhesives (polyurethane and polychloroprene adhesives) due to their intrinsic low surface energy, the presence of contaminants and release agents, and antiadherend moieties on the surface. Therefore, the surface preparation of upper and sole must remove contaminants and weak boundary layers, and roughness and chemical functionalities able to produce adequate bond strength should be created.

A review on the formulation and characteristics of the main materials used in shoe manufacturing (except adhesives) was published by Guidetti et al. 1992. To the author’s best knowledge, no additional reviews on the subject have been published later.

In this section, a short review of the main materials used in shoe bonding as well as their current surface preparation procedures will be considered. In the last part of this section, the current and alternative surface treatments for upper and sole materials are described.

3.1 Upper Materials

Leather

Full-chrome , semi-chrome, and vegetable-tanned leathers are still the main source and most common upper material in shoe manufacturing. In general, the porous nature of the leather facilitates its bonding with adhesives in solution, mainly solvent-borne polychloroprene adhesives. To obtain good adhesion, the weak grain layer of the leather must be removed by using rotating wire brushes (roughening) to expose the corium (the part of the leather with higher cohesion of the collagen fibers) to the adhesive (SATRA 1963). In general, deeper roughening of upper leather produces a stronger bond than light roughening. Furthermore, two consecutive adhesive layers are generally applied on the roughened leather. First, a low-viscosity adhesive solution (incorrectly called primer) is applied to fill the pore entrances of the corium fibers and to facilitate the wettability of the leather surface. At least 5 min later (or alternatively when the adhesive layer becomes dried), a second high-viscosity adhesive solution of the same nature than the previous one is applied; diffusion of the polymer chains between the two adhesive layers is produced, and a homogeneous adhesive layer with high cohesion is created on the upper leather surface.

The main issues that make difficult the bonding of upper leather are the existence of finishing on the surface to be bonded and the presence of high grease content. Deep or slight scouring or roughening is sufficient to remove the finishing and the weak grain leather. However, in thin leathers (such as buffalo or calf leathers), mild surface scouring followed by application of a primer is necessary to avoid leather tearing; the primer must increase the cohesion of the collagen fibers, and also it has to be compatible with the adhesive. Several primers for thin leathers have been proposed (Ferrandiz-Gómez et al. 1994), but they generally require the application of moderate temperature for being effective. Furthermore, different low-viscosity reactive solvent-based polyurethane primers containing about 10 wt% free isocyanate groups have been developed for improving the adhesion of difficult-to-bond leather (Vélez-Pagés 2003).

Depending on the amount (more than 11 wt% is critical) and type of grease (mainly unsaturated fatty acids) in the leather, poor adhesive bond can be produced. The fat may diffuse through to the surface of the polyurethane adhesive film (possibly while the adhesive still wet, with the solvent acting as the vehicle) and interfere with its subsequent coalescence with the adhesive or the sole. When polychloroprene adhesive is used, the reaction of the fatty acids with some components in the adhesive formulation (zinc oxide, magnesium oxide) forms insoluble metal soaps (salts of fatty acids) at the interface that impart antiadherend properties. Polyurethane adhesives are more tolerant to greasy leather than polychloroprene adhesives. The roughening of the leather is always necessary, and the addition of 5 wt% isocyanate hardener to the adhesive just before application helps to reach good adhesive joints.

Solvent-borne polychloroprene adhesives are excellent for bonding leather because of their high wettability and permanent tack (heat activation is not needed). However, these adhesives have poor performance with most of the rubber soles and with several synthetic upper materials. In these cases, polyurethane adhesives provide better performance.

Synthetic Upper Materials

Several synthetic upper materials such as canvas, textiles, nylon, PVC on woven or nonwoven, and poromerics are used in shoe manufacturing. As for leather, most of these materials need roughening (to remove finishing) and/or solvent wiping, followed by application of two consecutive adhesive layers, to produce good adhesive joints.

PVC uppers may cause lack of adhesion due to the migration of plasticizer into any material with which the PVC comes in contact. Light roughening is necessary, or alternatively methyl ethyl ketone (MEK) wiping can be applied followed by the application of polyurethane adhesive containing 5 wt% isocyanate hardener. Alternatively, the application of a primer based on nitrile rubber followed by application of two-component polychloroprene adhesive may perform adequately. PVC-coated fabrics may have acrylic or urethane coatings on top. Acrylic coatings are readily soluble by MEK wiping, whereas the urethane coatings need tetrahydrofuran wiping.

Polyurethane-coated fabrics can be bonded by using polyurethane adhesives. Mild scouring using a fine rotary wire brush must be used to completely remove the coating before applying the adhesive.

Poromerics (nonwoven fabric supporting a microporous urethane layer) also need to be roughened with extreme care. When nylon, polyester, or cellulose interlayers are present, the application of isocyanate-based primers promotes the adhesion.

Nylon fabric uppers show serious adhesion problems. The best bonding is obtained by mild roughening followed by applying 2 wt% isocyanate-based primer; the application of two-component polyurethane adhesive is also necessary.

3.2 Sole Materials

Several kinds of sole materials are currently used in shoe manufacturing, the rubber soles are the most common. Few soles are made of natural products, such as leather or cork. The leather soles are not difficult to bond because they are porous and easily wetted by adhesive solutions. A primer is required to produce good bond with solvent-borne polychloroprene adhesives. The use of polyurethane adhesives is also feasible but not so effective, and there is the need of the previous application of low-viscosity polyurethane primer to which 2.5 wt% isocyanate hardener must be added. On the other hand, the cork soles are widely used for sandals, and they are joined with polychloroprene adhesives. In some cases, a thin EVA midsole is attached to the cork to improve its abrasion resistance and comfortability by using two-component polychloroprene adhesive (Martin 1971).

Rubber Sole Materials

Rubber soles are by far the most common in shoe industry. Vulcanized or unvulcanized (thermoplastic) rubber soles are generally used. In general, the bonding is produced with one- or two-component polyurethane adhesives, and a surface treatment is always required to produce good adhesive joints.

Thermoplastic Rubber (TR) Soles

Styrene–butadiene–styrene (SBS) block copolymers are adequate raw materials to produce thermoplastic rubber (TR) soles. SBS block copolymers contain butadiene domains – soft and elastic – and styrene domains – hard and tough. Because the styrene domains act as virtual cross-linking in the SBS structure, the vulcanization is not necessary to provide dimensional stability. The TR soles generally contain polystyrene (to impart hardness), plasticizers, fillers, and antioxidants, and processing oils can also be added. The TR soles have low surface energy, so a surface modification is needed to reach proper adhesion to polyurethane adhesive. Special adhesives for the TR soles have been developed to avoid surface preparation, but they have poor creep resistance.

Due to their softness, the TR soles cannot be roughened. The TR soles tend to swell by application of adequate solvents, and the mechanical interlocking of the adhesive is favored; however, vigorous rubbing must be avoided to maintain the mechanical integrity of the sole. Chemical treatments are necessary to improve the adhesion of the TR to polyurethane adhesive, mainly cyclization (treatment with sulfuric acid) and halogenation.

The TR can be treated with concentrated sulfuric acid to yield a cyclized layer on the surface (Cepeda-Jiménez et al. 2001). This layer is quite brittle, and when it is flexed and stretched, microcracks are developed which help in the subsequent bonding by favoring the mechanical interlocking of the polyurethane adhesive with the TR. The thickness of the cyclized layer depends on the length of the treatment with sulfuric acid. On the other hand, the treatment with sulfuric acid mainly produces the creation of highly conjugated C–C bonds and the sulfonation of the butadiene units of the TR material. The immersion of TR for less than 1 min, the neutralization with ammonium hydroxide (it extracts the hydrogen from the sulfonic acid and leaves stabilized SO3 NH4 + ion pair), and the high concentration of the sulfuric acid (95 wt%) are essential to produce an adequate surface treatment. The treatment with H2SO4 increases the T-peel strength of surface-treated TR/polyurethane adhesive joints (Fig. 3). On the other hand, the styrene content in the TR determines the effectiveness of the treatment with sulfuric acid. The lower is the styrene content in the TR, the more noticeable are the modifications produced on the surface. Furthermore, the styrene content in the TR affects the extent but not the nature of the surface modifications produced by treatment with sulfuric acid.
Fig. 3

T-peel strength values (15 min and 72 h after joint formation) of sulfuric acid-treated TR rubber/polyurethane adhesive joints as a function of the immersion time (Cepeda-Jiménez et al. 2001)

Halogenation is the most common surface treatment for TR. The use of the chlorination surface treatment in improving the adhesion of several types and formulations of rubbers has been extensively demonstrated, and it is cheap and easy to apply. Furthermore, the chlorination surface treatment removes contaminants and antiadherend moieties from the rubber surface and imparts improved durability and aging resistance to the upper-to-sole joints, and the treated surfaces remain reactive with the polyurethane adhesive for at least 3 months after surface preparation. Immersion in aqueous chlorine or in sodium hypochlorite aqueous solution is a very effective surface preparation for TR soles although fast evolution of chlorine is produced. Therefore, the organic chlorine donors for the treatment of the TR have been proposed alternatively for allowing moderate evolution of chlorinated species during surface treatment. The most commonly used chlorinating agent is an organic solvent (ketone or ester) solution of trichloroisocyanuric acid (TCI) – 1,3,5-trichloro-1,3,5-triazin-2,4,6-trione – (Fig. 4) which is very cheap and common (it is generally used as chlorinating agent in swimming pools). The organic solvent in the TCI solutions determines the degree of rubber wetting, and furthermore, the actual chlorinating species are produced by reaction of the TCI with the organic solvent. The use of the TCI solutions is quite satisfactory for the TR surface preparation although it should be applied with a soft brush and left to dry during moderate time to avoid degradation of the TR surface by the solvent (Carter 1971).
Fig. 4

Chemical structures of DCI (sodium dichloroisocyanurate) and TCI (trichloroisocyanuric acid)

For avoiding the use of organic solvents in the chlorination agents, acidified sodium hypochlorite aqueous solutions containing 1-octyl-2-pyrrolidone as wetting agent have been successfully used for the treatment of the TR soles (Cepeda-Jiménez et al. 2003a). The treatment is restricted to about 1 μm depth surface of the TR, and the enhanced adhesion is due to the improved wettability and the creation of chlorine moieties on the rubber surface. Furthermore, aqueous solutions of sodium dichloroisocyanurate (DCI) (Fig. 4) have been used to increase the adhesion of the TR soles (Cepeda-Jiménez et al. 2002). The chemical structure of DCI is somewhat similar to that of TCI, and relatively concentrated DCI water solutions are necessary to obtain good peel strength values. On the other hand, successful surface chlorination treatment of the TR has been obtained by using aqueous N-chloro-p-toluenesulfonamide solutions (obtained by acidifying chloramine T solutions) (Cepeda-Jiménez et al. 2003a) – Fig. 5. The actual chlorinating species is SO2-C6H4-NCl-Cl. The surface treatment is carried out by immersion of the TR in the aqueous chlorinating solution at 20–80 °C for about 1 min. The effectiveness of this treatment was ascribed to the introduction of chlorine and oxygen moieties on the rubber surface.
Fig. 5

Chemical structure of chloramine T

Electrochemical treatments have also been successfully proposed to improve the adhesion of the TR (Brewis et al. 1997). The treatment consists in the immersion of the TR in an electrochemical cell of silver nitrate in 3.25 M nitric acid. After treatment, the rubbers have to be washed with nitric acid and then with bidistilled deionized water until neutral pH is reached.

Several environmental-friendly surface preparations of the TR sole materials by means of radiations have been more recently proposed. These treatments are clean (no chemical or reaction by-products are produced) and fast, and furthermore online bonding at shoe factory can be produced, so the future trend in the surface modification of the rubber soles will be likely directed to the industrial application of those treatments. Corona discharge, low-pressure radio-frequency (RF) gas plasma, and ultraviolet (UV) enriched in ozone – UV-ozone – treatments have been successfully used at laboratory scale to improve the adhesion of several TR sole materials.

Corona discharge has been used to improve the adhesion of the TR to polyurethane adhesives (Romero-Sánchez et al. 2003a). Although the treatment with corona discharge improved the wettability of the TR due to the formation of polar moieties on the surface, and surface cleaning and removal of contaminants (mainly demolding silicone moieties) were produced, the peel strength values moderately increased.

Low-pressure RF gas plasma has been shown effective to enhance the adhesion of the TR to different polyurethane adhesives (Ortiz-Magán et al. 2001). Different gases (oxygen, nitrogen, and oxygen–nitrogen mixtures) were used to generate the RF plasma. The treatment produced the partial removal of hydrocarbon moieties from the TR surface, and the formation of oxygen moieties and surface roughness was also produced. On the other hand, the treatment of SBS rubber with low-pressure RF plasma in CCl4 has also been proposed for improving its adhesion to polyurethane adhesive, and a drastic increase in the peel strength was observed after only a few seconds of treatment which was ascribed to the chemical bonding between the carbon–oxygen moiety species on the SBS surface and the isocyanate groups of the polyurethane adhesive (Tyczkowski et al. 2009). Water molecules strongly attached to the SBS surface were also found, and they reduced the adhesion by partial blocking of the functional groups against the action of isocyanates. On the other hand, because no correlation between the peel strength and the surface free energy was observed, the thermodynamic adhesion was unimportant, and the dominant role of the chemical adhesion was confirmed.

The surface treatment of the TR with UV–ozone produced with low-pressure mercury vapor lamp enriched in 175 nm wavelength has been shown to successfully increase its adhesion to polyurethane adhesive (Romero-Sánchez et al. 2003b). The UV–ozone treatment produced important surface modifications (improved wettability, roughness) incorporating oxygen and nitrogen moieties at the TR surface. The peel strength values increased after UV–ozone treatment of the TR, in a greater extent by increasing the treatment time (Fig. 6). More recently, several SBS rubbers containing different amounts of calcium carbonate and/or silica fillers were surface treated with UV–ozone to improve their adhesion to polyurethane adhesive (Romero-Sánchez and Martín-Martínez 2008). The nature and content of fillers determined the extent of surface modification and adhesion of SBS rubber treated with UV–ozone. The UV–ozone treatment improved the wettability of all rubber surfaces, and chemical (oxidation) and morphological modifications (roughness, ablation, surface melting) were produced. The increase of the UV–ozone treatment to 30 min led to surface cleaning due to ablation and/or melting of the most external rubber layers, and also incorporation of more oxidized moieties was produced. As a consequence of these surface modifications, the UV–ozone treatment increased the adhesive strength in all joints made with polyurethane adhesive, more noticeably in the joints made with the SBS rubbers containing silica filler. On the other hand, although the chemical modifications were produced earlier in unfilled rubber for short time of treatment with UV–ozone, they were more noticeable in silica-filled rubbers with low silica content for extended length of treatment. However, the oxidation process seemed to be inhibited for the SBS rubbers containing calcium carbonate filler.
Fig. 6

T-peel strength values of UV-  ozone treated TR/polyurethane adhesive joints as a function of the UV treatment time (Romero-Sánchez et al. 2003)

Vulcanized NR and SBR Soles

Although natural rubber (crepe) and nitrile rubber soles (common in the manufacturing of safety shoes due to its chemical inertness) are used, sulfur-vulcanized natural rubbers and styrene–butadiene rubbers (SBR) are the most common sole materials. The vulcanization system contains sulfur and activators (N-cyclohexyl-2-benzothiazole sulfenamide, dibenzothiazyl disulfide, hexamethylenetetramine, zinc oxide), and fillers (silica and/or carbon black, calcium carbonate) are added to control hardness and abrasion resistance. Antioxidants (zinc stearate, phenolic antioxidant) and antiozonant (microcrystalline paraffin wax) are also necessary to avoid degradation processes and early aging. In general, vulcanization is carried out in a metal mold placed in hot plate press at about 180 °C for allowing the reaction of the zinc oxide and the stearic acid to produce zinc stearate, which is combined with hexamethylenetetramine to form an unstable complex. After vulcanization, the contact with moisture or different solvents apparently causes the breakdown of this complex with the appearance of the zinc stearate that migrates to the vulcanized NR or SBR sole which acts as an antiadherend material to polyurethane adhesive (Pettit and Carter 1964). On the other hand, during cooling after vulcanization, the paraffin wax migrates to the rubber surface also producing adhesion problems.

The causes of poor adhesion in vulcanized NR and SBR soles are quite diverse. Over-vulcanized external rubber layer is too stiff to support peel strength stresses. The presence of silicone release agents may reduce the bond strength of the vulcanized SBR sole joints. The presence of antiadherend moieties on their surfaces, their low surface energy, and the migration of low-molecular moieties (antiozonant wax, processing oils) to the interface once the adhesive joint is produced are other causes for the poor adhesion in vulcanized NR and SBR. The paraffin wax and zinc stearate are concentrated in a layer covering the vulcanized rubber surface with a thickness of about 2.0 μm. The factors favoring or inhibiting the migration of antiozonant wax to the vulcanized NR and SBR surfaces have not been fully understood. The temperature is one key factor that determines the migration of the antiozonant paraffin wax to the rubber surface. The antiozonant migration with the temperature in vulcanized rubber has been recently studied (Torregrosa-Coque et al. 2012) by heating at 40–90 °C for 15 h in an oven. The heating of vulcanized SBR at different temperatures caused a partial removal of paraffin wax from the surface, and at a temperature close to the paraffin wax melting point, the crystals of paraffin wax on the vulcanized SBR surface were melted, causing a decrease in the thickness of the paraffin wax film on the surface. Furthermore, the migration of zinc stearate to the paraffin wax layer on the vulcanized SBR surface occurred by heating at 90 °C, and even after heating at 90 °C, a thin film of paraffin wax always remained on the vulcanized SBR surface.

Time is another key factor that determines the migration of the antiozonant paraffin wax to the rubber surface causing the formation of a weak boundary layer of nonrubber contaminants which is deleterious for adhesion of rubber to polyurethane adhesives (Torregrosa-Coque et al. 2011a). Figure 7 shows the ATR-IR spectra of polyurethane coating on MEK surface cleaned vulcanized SBR just after application and after 2 weeks. The increase of the time favored the migration of paraffin wax which is noticed by the appearance of the methylene bands in the ATR-IR spectra of the vulcanized SBR at 2848, 2915, and 2950 cm−1 on the polyurethane coating.
Fig. 7

ATR-IR spectra obtained with ZnSe prism of polyurethane coating on MEK surface cleaned vulcanized SBR for different time after polyurethane coating deposition. 3100–2700 cm−1 region (Torregrosa-Coque et al. 2011b)

One of the key steps in the manufacturing of rubber/adhesive joints is reactivation, i.e., sudden heating of the thin adhesive layers on the substrates to be joined under infrared (IR) radiation to 80–90 °C for a few seconds to allow diffusion of the polymeric chains under pressure. This reactivation may cause the migration of low-molecular-weight additives to the rubber surface causing a lack of adhesion. The influence of the reactivation temperature (40–90 °C) on the surface properties of sulfur-vulcanized styrene–butadiene rubber on the extent of the diffusion of the paraffin wax and zinc stearate to the rubber surface has been studied (Torregrosa-Coque et al. 2011a). The reactivation of the rubber at different temperatures produced changes in the morphology and thickness of the paraffin wax layer on the surface. By increasing the reactivation temperature, a partial removal of paraffin wax was produced, and the thickness of the paraffin wax film on the rubber surface was reduced. On the other hand, the increase in the reactivation temperature increased the surface area of the melted paraffin wax layer that prevented migration from the rubber bulk, but a critical reactivation temperature at 90–100 °C existed at which the migration of zinc stearate to the paraffin wax layer on the rubber surface was favored. Furthermore, it has been demonstrated that the paraffin wax migrated from the vulcanized rubber bulk to the polyurethane–rubber interface, and, then, it diffused across the polyurethane coating and migrated later to the polyurethane coating surface (Torregrosa-Coque et al. 2011b). On the other hand, the extent of migration of the paraffin wax depended on the time after polyurethane adhesive deposition, and the faster migration was produced 1 h after applying the polyurethane adhesive.

The application of surface treatments to NR and SBR soles should produce improved wettability; creation of polar moieties able to react with the polyurethane adhesive, cracks, and heterogeneities should be formed to facilitate the mechanical interlocking with the adhesive; and an efficient removal of antiadherend moieties (zinc stearate, paraffin wax, processing oils) has to be reached. Several types of surface preparation involving solvent wiping, mechanical and chemical treatments, and primers have been proposed to improve the adhesion of vulcanized NR and SBR soles. Chlorination with solutions of trichloroisocyanuric acid (TCI) in different organic solvents is by far the most common surface preparation for vulcanized NR and SBR.

Roughening of the SBR soles removes the over-vulcanized layer and surface contaminants (oils, molding release agents, antiadherend moieties, etc.) and creates roughness. After roughening, residues of the treatment must be removed by solvent wiping and/or the application of pressurized air. Although the removal of weak layers and the creation of surface heterogeneities may produce improved adhesion of the vulcanized NR and SBR soles, an additional chemical treatment is mandatory for achieving sufficient adhesive strength.

The soles can be chemically treated with concentrated sulfuric acid to improve their adhesion to polyurethane (Cepeda-Jiménez et al. 2000). This treatment is not restricted to the surface but also produces a bulk modification of the rubber because a decrease in the tensile strength and the elongation at break are caused. If the SBR contains paraffin wax, the treatment with sulfuric acid promotes its migration to the surface, and solvent wiping with petroleum ether is mandatory for obtaining good adhesion.

Several chlorinating agents have been used for SBR sole bonding. Acidified sodium hypochlorite aqueous solutions have been used successfully (Table 1), and the mechanism of chlorination has been established (Vukov 1984) that consists in the formation of H2ClO+ species which produces the evolution of chlorine; then, the chlorine reacts with the double C=C bond of the rubber producing chlorinated hydrocarbon moieties on the surface and some cross-linking.
Table 1

T-peel strength values (kN/m) of surface-chlorinated rubber/polyurethane adhesive /leather joints

Rubber

As-received

NaClO + HCl

SBR 1

1.8 (A)

5.6 (R)

SBR 2

0.2 (A)

14.0 (R)

SBR 3

0.9 (A)

7.8 (R)

A: Adhesion failure between the rubber and the adhesive

R: Cohesive rupture of the rubber

The most commonly used chlorinating agent in the chemical treatment of the SBR sole is ethyl acetate or MEK (butan-2-one) solution of trichloroisocyanuric acid (TCI). The organic solvent in the TCI solution determines the degree of rubber wetting, and the actual chlorinating species are α-chloro ketones in TCI/MEK solutions, whereas acid chlorides are the reactive moieties in TCI/ethyl acetate solutions (Pastor-Blas et al. 2000). The reaction of these chlorinating species with the double C=C bonds of the rubber produces C–Cl species, and the reaction is not restricted to the surface, but it is penetrating about 100 μm depth. On the other hand, the improved adhesion of the SBR soles treated with TCI solutions is due to the contribution of several mechanisms of adhesion : mechanical adhesion (cracks and pits are produced on the surface), chemical adhesion (chlorinated hydrocarbon, C=O moieties), removal of weak boundary layers (zinc stearate, paraffin wax), and thermodynamic adhesion (a decrease in contact angle is produced, i.e., improved wettability) (see chapter “Thermodynamics of Adhesion”). Furthermore, unreacted solid prismatic TCI crystals deposit during the application of the chlorination treatment on the SBR surface which can be dissolved in contact with the organic solvent in the polyurethane adhesive facilitating the creation of in situ reactive species.

The effectiveness of the chlorination treatment of the SBR soles with TCI solutions strongly depends on several experimental variables of the treatment (Martín-Martínez 2008). The most relevant ones are the application procedure of the TCI solution (brushing providing the best performance), the time between the application of the TCI solution and the polyurethane adhesive (a minimum of 10 min is necessary), the TCI concentration in the solution (less than 3 wt% TCI is recommended), and the roughening or solvent wiping before chlorination (more extensive treatment is produced).

The active chlorine content of the ethyl acetate solutions containing 3 wt% TCI determined the extent of the surface treatment of vulcanized SBR. The active chlorine concentration in the TCI solutions was not the only parameter determining the adhesion of vulcanized SBR, as the highest adhesive strength was not achieved by treating the rubber with the TCI solution with higher active chlorine content (García-Martín et al. 2010). Furthermore, the chlorine concentration in the TCI solutions was not stable in the course of time; the increase in time between TCI solutions preparation and SBR treatment allowed an increase in adhesive strength, the highest value corresponded to the joint produced with the vulcanized SBR treated with the TCI solution prepared for 60 days.

The influence of the temperature on the effectiveness of the chlorination treatment of vulcanized SBR with TCI solutions in organic solvents has been addressed. Vulcanized SBR was treated with 1 and 2 wt% TCI solutions in ethyl acetate and then thermally treated at 23 °C, 50 °C, or 65 °C for different time (Yin et al. 2012). Although this study was not intended for shoe industry, the results obtained revealed that the increase of the chlorination temperature up to 50 °C was very effective for SBR surface modification by TCI, leading to enhanced surface wettability, creation of chlorinated hydrocarbon moieties, and increased shear strength (from 2.0 to 9.7 MPa). However, later study (Yañez-Pacios et al. 2013) has shown that the application of temperature during chlorination of high antiozonant content vulcanized NR with TCI solutions in MEK did not increase the adhesion strength of the upper-to-sole joints. However, changes in wettability, surface chemistry, and roughness of the rubber were produced, all being irrelevant for improving adhesion likely due to the migration of antiozonants to the polyurethane adhesive-vulcanized NR interface with time after joint formation.

Due to organic solvent restrictions in the industry, aqueous chlorinating solutions have been tested as an alternative to TCI solutions in ethyl acetate or MEK. Aqueous solutions of sodium dichloroisocyanurate (DCI) have been recently demonstrated to increase the adhesion of vulcanized SBR soles (Cepeda-Jiménez et al. 2002). The surface treatment with aqueous DCI solutions creates C–Cl moieties, removes antiadherend moieties from the SBR surface, and creates roughness. The use of a low DCI concentration in water is sufficient to obtain good peel strength values in SBR/waterborne polyurethane joints. Furthermore, vulcanized NR has been treated with aqueous sodium hypochlorite solution for various chlorination times up to 30 min (Radabutra et al. 2009). The degree of chlorination increased with the chlorination time up to 10 min then leveled off, but the roughness and stiffness increased gradually with chlorination time. The maximum peel strength was found at 1 min of chlorination time and decreased afterward because of the increase in the surface stiffness that acted as weak boundary layer.

As for TR, successful treatments of vulcanized NR and SBR soles with aqueous N-chloro-p-toluenesulfonamide solutions and electrochemical treatments have been reported at laboratory scale. Similarly, the treatment of SBR sole with oxidant inorganic salts (acidified potassium dichromate, potassium permanganate, Fenton’s reactive) has been shown to be partially successful (Brewis and Dahm 2003).

Primers have also been used to improve the adhesion of SBR soles. Due to its high reactivity, isocyanate wipe has been proposed, but the most common treatment uses organic acid solutions. Solutions of lactic acid applied with stiff-bristled brush and with a scrubbing action are adequate to remove metal soap contamination (i.e., zinc stearate) on the surface of SBR sole and even replace roughening (Carter 1969). Solutions of different carboxylic acids (fumaric acid, maleic acid, acrylic acid, succinic acid, malonic acid) in ethanol have also been used as primers of SBR sole (Pastor-Sempere et al. 1995). The nature of the carboxylic acid determines the rate of diffusion into the polyurethane adhesive and the extent of rubber-adhesive interfacial interaction. Finally, mixtures of TCI and fumaric acid solutions have been tested in improving the adhesion of difficult-to-bond SBR sole, but it was demonstrated that the effectiveness of this treatment was mainly due to chlorination by TCI (Romero-Sánchez and Martín-Martínez 2003).

Treatment with supercritical fluids was also tested to remove zinc stearate and waxes from vulcanized SBR soles (Garelik-Rosen S, Torregrosa-Maciá R, Martín-Martínez JM (2003), unpublished results). The relative effectiveness of this treatment was ascribed to the dissolution of antiadherend moieties by penetration of the fluid into the rubber surface.

An alternative to water-based treatment is the use of low-pressure RF gas plasmas in the enhancement of the adhesion of vulcanized rubbers. The treatment in oxygen plasma for 1 min is enough to noticeably increase the adhesion of vulcanized SBR sole to polyurethane adhesive (Pastor-Blas et al. 1998). Later, three different configurations (direct, etching, and secondary downstream) of low-pressure RF oxygen plasmas for length of treatment between 1 and 10 min were used to modify the surface of vulcanized SBR (Torregrosa-Coque and Martín-Martínez 2011c). The direct oxygen plasma was the most aggressive treatment and the secondary downstream plasma the less one, and the oxygen plasma treatment caused surface oxidation and ablation on the SBR surface, as well as an increase of the temperature that also determined the extent of paraffin wax migration which was produced for at least 24 h after oxygen plasma treatment; as a consequence, poor adhesion to polyurethane adhesive was obtained, due to the creation of a weak boundary layer of paraffin wax at the rubber–polyurethane interface.

Vulcanized ethylene propylene diene polymethylene (EPDM) rubber surface was treated with low-pressure RF argon–oxygen plasma to improve adhesion with compounded natural rubber (NR) during co-vulcanization (Basaka et al. 2011). The plasma treatment changed both surface composition (formation of C–O and –C=O functional groups) and roughness of EPDM rubber, and consequently increased peel strength was obtained. In a different study, vulcanized NR containing intentionally noticeable excess of processing oils in its formulation was treated with argon–oxygen (Ar–O2) (2:1, vol/vol) low-pressure plasma for achieving a satisfactory level of adhesion to waterborne polyurethane adhesive (Cantos-Delegido and Martín-Martínez 2015). The effectiveness of the Ar–O2 low-pressure plasma treatment depended on both the configuration of the plasma chamber shelves and the treatment time, the direct configuration provided the most effective surface modification of the vulcanized NR. However, even important surface modifications were produced by Ar–O2 low-pressure plasma treatment, adhesion was not improved due to the creation of weak boundary layer at the polyurethane–rubber interface after joint formation. Heating at 80 °C for 12 h prior to Ar–O2 low-pressure plasma treatment enhanced the extent of the surface modifications, and improved adhesion was obtained for treatment times higher than 600 s.

Atmospheric plasma torch treatment of vulcanized NR with different excess of paraffin wax and processing oils was carried out for improving adhesion to waterborne polyurethane adhesive (Kotrade et al. 2011). Similar chemical modifications (new C=O and C–N functional groups) and roughness were produced by plasma torch treatment in all rubbers, and the best results were obtained at a rubber surface nozzle of the plasma torch distance of 5 mm and a platform speed of 1 m/min. However, although the rubber surfaces were effectively modified by plasma torch, the adhesion to waterborne polyurethane adhesive was not improved. In a different study (Moreno-Couramjou et al. 2009), vulcanized NR was treated by atmospheric dielectric barrier discharges to improve its adhesion to silicone adhesive. The atmospheric plasma treatment with the use of allyl alcohol improved noticeably the adhesion by a factor of 10 due to the preferential formation of C–O bonds.

More recently the treatment with UV radiation combined with ozone (UV–ozone) in improving the adhesion of difficult-to-bond vulcanized NR (it contained an excess of processing oils) to waterborne polyurethane adhesive in footwear has been carried out (Moyano and Martín-Martínez 2014). Both the length of the treatment and the distance between the UV radiation source to the rubber surface were varied, and the effects of the treatment on the surface chemistry, wettability and surface energy, and topography of the rubber were analyzed. The treatment of the vulcanized rubber with UV–ozone removed hydrocarbon moieties and zinc stearate from the surface, surface oxidation (C–O, C=O, and COO- groups formation) occurred, and improved wettability and increased surface energy (mainly due to the polar component) were obtained. The UV–ozone surface treatment also caused heating of the surface and increased the adhesion of the vulcanized rubber to waterborne polyurethane adhesive; the highest adhesion was obtained in the joints made with UV–ozone-treated rubber for 3 and 6 min at a UV radiation source-rubber surface distance of 5 cm.

Polymeric Soles

Ethylene–vinyl acetate (EVA) block copolymer soles are common in sport and casual shoes due to their low density and easy coloring. EVA soles are difficult to bond with the polyurethane and polychloroprene adhesives commonly used in shoe industry, mainly due to their low surface energy. The higher the vinyl acetate content, the less difficult to bond the EVA sole is. The lightweight microcellular EVA soles can usually be adequately bonded after scouring using polyurethane and polychloroprene adhesives. The injection-molded EVA soles are more difficult to bond although roughening followed by application of two-component polychloroprene adhesive can give moderate bond strength (Martínez-García et al. 2003a). Corona discharge has been shown to be useful to improve the peel strength of injection-molded EVA soles containing 12 and 20 wt% vinyl acetate/polychloroprene adhesive joints. The improvement is ascribed to the creation of C=O and R-COO moieties. Treatment with sulfuric acid also provided improved adhesion of injection-molded EVAs with vinyl acetate contents between 9 and 20 wt%, because this treatment produces sulfonation and creation of oxygen moieties on EVA surface (Martínez-García et al. 2003b). Oxygen and other gases low-pressure plasmas are very effective in improving the peel strength of joints produced with EVA soles with different vinyl acetate contents and two-component polyurethane adhesive (Cepeda-Jiménez et al. 2003b). The treatment is more effective with non-oxidizing plasmas, giving rise to a high roughness. Finally, the treatment with UV–ozone has been developed to enhance the bond strength between EVA sole and polyurethane adhesive (Landete-Ruiz and Martín-Martínez 2005). The surface modifications produced by UV–ozone treatment of two ethylene–vinyl acetate (EVA) copolymers containing 12 (EVA12) and 20 wt% (EVA20) vinyl acetate have been studied (Landete-Ruiz and Martín-Martínez 2015). The treatment with UV–ozone improved the wettability of both EVAs due to the creation of new carbon–oxygen moieties, and the extent of these modifications increased with increasing the time of treatment; the modifications produced in EVA20 were produced for shorter lengths of treatment. The UV–ozone treatment also created roughness and heterogeneities, and whereas roughness formation prevailed on the UV–ozone-treated EVA12, important ablation was dominant on the treated EVA20. T-peel strength values in joints made with polychloroprene adhesive increased. Short length of UV–ozone treatment (1 min) produced higher T-peel strength in joints made with EVA20, whereas higher T-peel strength values in joints made with EVA12 were obtained after treatment for 5–7.5 min in which a cohesive failure into a weak boundary layer on the treated EVA surface was found. Finally, the aging resistance of the treated EVA/polychloroprene adhesive joints was good, and the surface modifications on the UV–ozone-treated EVAs lasted for 24 h after treatment at least.

Ethylene–vinyl acetate (EVA) copolymers intended for sport sole manufacturing may contain noticeable amounts of low-density polyethylene (LDPE) – Phylon-type soles – for improving abrasion resistance and decreasing cost; however, the EVA–PE blend had low polarity and showed poor adhesion. Good results have been obtained by applying extensive organic solvent wiping, followed by application of an UV-activated primer and by using two-component polyurethane adhesive. More recently, an effective environmental-friendly and fast surface treatment based on UV–ozone has been used to increase the wettability, polarity, roughness, and adhesion of EVA–PE material to leather with waterborne polyurethane adhesive (Jofre-Reche and Martín-Martínez 2013). The more extended length of treatment and the shorter UV source–substrate distance increased the wettability and created new carbonyl groups mainly, and the amounts of hydroxyl and carboxylic groups were increased. The UV–ozone treatment produced ablation and etching of the EVA–PE material surface, mainly in the vinyl acetate of the EVA; this topography was also caused by heating during UV–ozone treatment. For low length of UV treatment or high UV source–material distance, the modifications of the EVA–PE material were mainly produced in the ethylene causing the selective removal of vinyl acetate, whereas more aggressive conditions produced strong oxidation in the EVA–PE material. Finally, adhesive strength was noticeably increased in the UV–ozone-treated EVA–PE/polyurethane adhesive joints, and a cohesive failure in the leather was obtained.

Only polyurethane adhesives should be used to bond PVC soles . The adhesion problems of PVC derive from the presence of plasticizers and stabilizers (stearate type) able to migrate to the surface impeding the contact with the adhesive (creation of weak boundary layers). A solvent wiping on the PVC surface is usually effective in improving adhesion, and solvents such as MEK are adequate. On the other hand, treatment with 10 wt% aqueous solutions of sodium hydroxide has been successfully applied to increase the adhesion performance of plasticized PVC soles (Abbott et al. 2003).

Polyurethane soles usually contain silicone mold release agents which prevent adhesion. To remove them, mild roughening following by solvent wiping is necessary. The application of an isocyanate primer is very useful before polyurethane adhesive is applied. Solvent-free treatments have been proposed for polyurethane soles, mainly cryoblasting (Abbott et al. 2003). Cryoblasting consists in the bombardment of the PVC soles with particles of solid carbon dioxide at pressures about 0.4 MN/m2. The impact of the solid particles successfully removed mold release agents from the polyurethane surface improving their adhesion to polyurethane adhesive.

3.3 Adhesives in Upper-to-Sole Bonding

Contact adhesives based in one- and two-component polychloroprene (neoprene) and mainly polyurethane adhesives are the most commonly used in shoe industry to bond upper to sole (see chapters “Adhesive Families” and “Sealant Families”). These adhesives are bonded by autoadhesion which implies the application of the adhesive to both surfaces to be joined; diffusion of polymer chains must be achieved across the interface between the two adhesive films on the substrates to be joined to produce intimate adhesion at molecular level. To achieve an optimum diffusion of the polymer chains, both high wettability and adequate viscosity and rheology of the adhesive should be achieved.

In the past, polychloroprene adhesives were more extensively used in upper-to-sole bonding, but nowadays polyurethane adhesives are preferred. Polychloroprene adhesives have better tack and improved wettability than polyurethane adhesives, but the polychloroprene adhesives are not compatible with the surface-treated rubber soles and cannot be used to joint PVC soles. Therefore, polyurethane adhesives show higher versatility on broader range of substrates and also have lower oxidative degradation in time. However, they require always the surface preparation of the materials to be bonded.

3.4 Polyurethane Adhesives

In 1960s, the solvent-borne polyurethane adhesives replaced the polychloroprene adhesives in the bonding of PVC soles for sport shoes. Today, the use of solvent-borne polyurethane adhesives is in decline due to environmental regulations and safety issues. Shoe industry was initially reluctant to the use of waterborne polyurethanes due to higher cost and reduced green strength. However, the use of waterborne polyurethanes is more common nowadays because their solid content is high and small amount of adhesive is needed to produce a similar joint than with solvent-borne adhesives. On the other hand, during the last decade, solvent-free polyurethane adhesives have been tested in shoe bonding. Although these adhesives have been widely used and effectively tested in the automotive industry, the advent in the shoe manufacturing is not as successful, likely because different machinery and bonding concepts need to be implanted. Shoe industry is, in general, quite reluctant to change bonding technologies, particularly when it implies a cost increase!

In this section, the solvent-borne, the waterborne, and the solvent-free polyurethane adhesives will be considered separately.

Solvent-borne Polyurethane Adhesives

Elastomeric thermoplastic polyurethanes are the main components in solvent solution adhesives for shoe industry. These polyurethanes are generally prepared in the form of pellets or chips by reacting an isocyanate (such as MDI – 4,4′-diphenylmethane diisocyanate), a long-chain diol (polyester or ε-caprolactone type), and a chain extender (glycol) to produce a linear polymer with negligible chain branching and relatively low-molecular-weight (M w = 200.000–350.000 Da). The isocyanate to hydroxyl equivalent ratio (NCO/OH ratio) is usually kept near 1, thus producing a polymer with terminal hydroxyl groups which are able to form hydrogen bonds with the urethane moieties.

From a polymer physics point of view, the configuration of the elastomeric polyurethanes corresponds to a segmented structure (block copolymer of (AB)n type) consisting of soft and hard segments (Fig. 8). Typically the soft segments are composed of a rubbery polymer (mainly the polyol), the glass transition temperature (Tg) which is located well below ambient temperature. The hard segments are generally produced by the reaction of the isocyanate and the short chain glycol (chain extender) and have a rigid and crystalline structure. The nonpolar low-melting soft segments are incompatible with the polar high-melting hard segments. As a result, phase separation (segregation) occurs in the polymer network.
Fig. 8

Scheme of the segmented structure of thermoplastic polyurethane

Typical elastomeric polyurethanes used as adhesives in upper-to-sole bonding have a relatively low content of hard segments, and their properties are mainly determined by the soft segments. Therefore, these polyurethanes will be elastic in the range of temperature between the glass transition temperature (generally located between −30 and −40 °C) and the softening temperature of the elastomeric domains (50–80 °C). The low melting point permits the elastomeric polyurethane to be softened at relatively low temperature, with sufficient thermoplasticity (i.e., loss of cohesion at moderate temperature) and surface tack to ensure correct bond.

Solvent-borne polyurethane adhesives are generally prepared by dissolving the solid elastomeric polyurethane pellets or chips in a solvent mixture. Because polyurethanes have a linear molecular structure, the solid polymer does not need mastication prior to dissolving. The solubility of the elastomeric polyurethanes in ketone solvents is mainly governed by the degree of crystallinity in the polyester soft segments. The crystallinity can be varied by selecting the reactants and by controlling the molecular weight of the polyester. Generally, the elastomeric polyurethanes with low crystallization rates have long open times but poor peel strength and heat resistance. Those polyurethanes with high rates of crystallization have short open times but high peel strength and improved heat resistance. For this reason, mixtures of low and high crystallization rate polyurethanes are used to balance the open time and adhesion of the solvent-borne adhesives.

The solvents determine the viscosity and solubility of the elastomeric polyurethane, its storage stability, its wetting properties, and its evaporation rate when applied on a substrate. The most common solvents are aromatic (toluene, xylene), tetrahydrofuran, dioxane, cyclohexanone, esters (ethyl acetate, butyl acetate), and ketones (acetone, methyl ethyl ketone). Commonly, two or more solvents are employed, a low-boiling and a high-boiling solvent. The low-boiling solvent assures rapid flash-off of the majority of the solvent after applying the adhesive solution on the substrate. The higher-boiling solvent helps to control the crystallization kinetics of the soft segments, thereby helping to extent the open time of the adhesive. In fact, once crystallization of the soft segments occurs, the open time expires. Other function of the high-boiling solvent is to keep the viscosity of the adhesive low (this is the reason for being called diluents), allowing the adhesive solution to wet the substrate by facilitating the polyurethane to penetrate the substrate surface, thus improving the mechanical interlocking. On the other hand, the addition of toluene avoids gel formation of the adhesive solutions during storage.

The formulation of the solvent-borne polyurethane adhesives may include several additives such as tack and heat resistance modifiers, plasticizers, fillers, tackifiers, antihydrolysis agents, and cross-linkers. Carboxylic acid as an adhesion promoter can also be added. A typical formulation of a solvent-borne polyurethane adhesive contains less than 20 wt% solid content only.

The spotting tack and/or the heat resistance of the elastomeric polyurethane adhesives may be extended by adding resins (alkyl phenolic, epoxide, terpene phenolic, coumarone) or polymers (low-crystallizing polyurethane, acrylic, nitrile rubber, chlorinated rubber, acetyl cellulose) (Penczek and Nachtkamp 1987) having low miscibility with the polyurethane. To improve adhesion together with heat resistance, reactive alkyl phenolic resins, chlorinated rubber, or other chlorine-containing polymers can be added.

The main additives used in the formulation of solvent-borne polyurethane adhesives are described in more detail below.

Tackifiers. One of the limitations of the elastomeric polyurethane adhesives is the lack of high immediate (green) adhesion to rubbers and polymers. One way to increase the green strength of the elastomeric polyurethanes is the addition of tackifiers mainly rosins or hydrocarbon resins (Arán-Aís et al. 2002). Addition of rosin as internal tackifier (i.e., incorporated during the synthesis of the polyurethane) favors the miscibility between the tackifier and the polyurethane. The hard segment content of the elastomeric polyurethanes containing rosin is higher than in regular polyurethanes because two kinds of hard segments are produced, urethane and urea–amide (this is produced by reaction of the isocyanate groups with the carboxylic acid of the rosin). Addition of rosin increases the molecular weight of the polyurethane and retards its kinetic of crystallization, and the immediate adhesion to vulcanized SBR sole is also increased.

A model system (phenyl isocyanate and acetic acid) has been studied to understand the reaction between the isocyanate group and the rosin (Irusta-Maritxalar ML, Fernández-Berridi MJ (2002), personal communication). The expected by-products of the reaction of the isocyanate and the carboxylic acid moieties are given in Eq. 1:
$$ {\displaystyle \begin{array}{l}\mathrm{R}-\mathrm{NCO}+{\mathrm{R}}^{\prime }-\mathrm{COOH}\to \left[\mathrm{R}-\mathrm{NHCO}\mathrm{OCO}-{\mathrm{R}}^{\prime}\right]\left(\mathbf{I}\right)\to \mathrm{R}\hfill \\ {}-\mathrm{NHCO}-{\mathrm{R}}^{\prime}\left(\mathbf{I}\mathbf{V}\right)+{\mathrm{CO}}_2\downarrow \kern0.55em \uparrow \mathrm{Heatingorseveraldays}\kern0.14em \mathrm{at}\kern0.14em \mathrm{roomtemperature}\phantom{\rule{0ex}{1em}}\mathrm{R}-\mathrm{NHCO}\mathrm{NH}\hfill \\ {}-\mathrm{R}\left(\mathbf{I}\mathbf{I}\right)+{\mathrm{R}}^{\prime }-\mathrm{COOCO}-{\mathrm{R}}^{\prime}\left(\mathbf{I}\mathbf{I}\mathbf{I}\right)\hfill \end{array}} $$
(1)

An unstable intermediate carbo-anhydride is formed (compound I), which under heating decomposes to produce urea (compound II) and anhydride (compound III) derivatives. These two compounds react by heating or by standing several days at room temperate to produce an amide compound (compound IV).

Fillers. Fillers should be highly dispersible in solution and should not settle down during the storage period of the adhesive. Whiting, talc, barite, calcium carbonate, attapulgite, and quartz flour have been suggested as fillers to lower the cost of the solvent-borne polyurethane adhesive, improve joint filling, and reduce the loss of adhesive during setting. In general, the fillers are incorporated during the preparation of the adhesive solutions by adding a small amount of solvent under high stirring speed for a short time, followed by addition of the other ingredients of the adhesive and all solvent under lower stirring rate and longer time (Maciá-Agulló et al. 1992).

Pyrogenic (fumed) silicas are the most common fillers of the polyurethane adhesives in shoe industry. When bonding highly porous materials (leather, textiles), fumed silicas are added to prevent undesirable penetration of the adhesive. The addition of the fumed silica adjusts the viscosity and rheology (thixotropy, pseudoplasticity) for application of the polyurethane adhesive solutions. The rheological properties of the polyurethanes containing fumed silica are due to the creation of hydrogen bonds between the silanol groups on the fumed silica particles and the urethane and carbonyl groups of the polymer, which increase the mechanical and rheological properties of the adhesives (Jaúregui-Beloqui et al. 1999; Vega-Baudrit et al. 2009; Bahattab et al. 2011, 2012; Donate-Robles et al. 2014). Consequently, the cohesion properties of the polyurethane are increased, and improved initial peel strength is obtained, in a greater extent by increasing the amount of fumed silica (Bahattab et al. 2012). Several parameters of the fumed silica determine its performance as rheological additive of solvent-borne polyurethane adhesives. Low particle size and degree of agglomeration, specific surface areas between 200 and 300 m2/g, and the hydrophilic nature of fumed silicas increase the rheological, mechanical, and adhesion properties of the solvent-borne polyurethane adhesives (Pérez-Limiñana et al. 2003; Bahattab et al. 2011).

Calcium carbonate fillers have also been added to solvent-borne polyurethane adhesives for improving their mechanical properties and initial adhesion (Donate-Robles and Martín-Martínez 2011a, b). The addition of precipitated calcium carbonate (PCC) filler produced a moderate increase in the rheological and viscoelastic properties of the polyurethane due to the poor dispersion of filler and the weak interactions between the PCC nanoparticles and the polymer chains. Furthermore, the first glass transition temperature of the polyurethane decreased by adding PCC filler and the crystallinity of the soft segments as well, because of the disruption of the degree of phase separation in the polyurethane. The initial adhesive strength in PVC/polyurethane adhesive/PVC joints increased noticeably by adding PCC filler; the greater the amount of filler, the greater the initial adhesive strength, and the highest final adhesive strength (72 h after joint formation) was obtained in the joint produced with the solvent-borne polyurethane adhesive containing 10 wt% PCC. The thermoplastic polyurethane–calcium carbonate interactions have been studied by using flow microcalorimetry and diffuse reflectance Fourier transform infrared spectroscopy (Donate-Robles et al. 2014). Three calcium carbonates (coated and uncoated precipitated calcium carbonate and natural ultramicronized uncoated calcium carbonate) were added to solvent-borne polyurethane adhesives, and stronger polyurethane-filler interaction was shown in the uncoated precipitated calcium carbonate due to the fact that more of the surface was available for interaction. In fact, the polyurethane strongly adsorbed onto untreated calcium carbonate via the ester groups, and the stearate treatment of calcium carbonate greatly reduced the strength of interaction with the polyurethane due to blockage of the surface adsorption sites.

Nanometric carbon black (CB) filler has been added to solvent-borne polyurethane adhesives for improving adhesion. The CB and the polyurethane pellets were adequately dispersed in methyl ethyl ketone matrix, and an increase in the number and size of the carbon black aggregates in the polyurethane matrix was obtained by increasing the carbon black loading (Alvarez-García and Martín-Martínez 2015). The addition of the carbon black improved the rheological and viscoelastic properties the adhesive, and the addition of higher amounts of CB changed the viscoelastic behavior of the polyurethane which became mainly elastic. On the other hand, the addition of CB loadings up to 12 wt% increased the thermal stability of the polyurethanes and increased their elongation at break without noticeable reduction in tensile strength. However, the polyurethane adsorbs less carbon black than fumed silica and calcium carbonate due to less accessible surface groups due to the presence of relatively important amount of micropores (Donate-Robles et al. 2014).

Carboxylic acids . It has been shown that improved adhesion can be obtained in joints produced with solvent-borne polyurethane adhesives containing small amount of different carboxylic acids. These carboxylic acids can be aliphatic or aromatic, can be unsaturated or saturated, may contain one or more carboxylic acid functionalities, and have hydroxyl, carbonyl, or halogen groups. Fumaric and maleic acids are the most common carboxylic acids added to the solvent-borne polyurethane adhesives for the joining of vulcanized SBR sole.

The effectiveness of the carboxylic acid as adhesion promoter is governed by its compatibility with the polyurethane and the solubility in the organic solvent selected to dissolve the polyurethane. It has been established (Pastor-Sempere et al. 1995) that the mechanism by which adhesion of SBR is increased is due to the carboxylic acid migration to the polyurethane adhesive surface once the adhesive joint is formed, removing zinc stearate and paraffin wax from the rubber surface. On the other hand, the carboxylic acid reacts with the polyurethane in the presence of a tiny amount of water which results in chain cleavage and disruption of polyurethane crystallinity (Eq. 2).

The addition of carboxylic acid decreases the molecular weight, tensile strength, and glass transition temperature of the polyurethane more markedly upon increasing the time after the adhesive is prepared and more noticeably for high amounts of carboxylic acid. As a consequence, lower viscosity of the polyurethane adhesive solutions and smaller peel strength values are obtained. The increased amount of the carboxylic acid in the solvent-borne polyurethane adhesive enhances its bond strength to SBR until a maximum value is reached. On the other hand, the nature of the polyurethane determines the degree of effectiveness of the carboxylic acid on adhesion of vulcanized rubber.

Cross-linkers. Polyisocyanates with functionality greater than two (such as p, p”, p”’ – triisocyanate triphenylmethane, thiophosphoric acid tris(p-isocyanatophenyl) ester) can be added to improve specific adhesion and heat resistance of the polyurethane adhesives (especially those with slower crystallization rate). Most of them have 20–30 wt% solids and 5.4–7 wt% free NCO, and the most common solvent is ethyl acetate. About 5–10 wt% polyisocyanate is generally added in the adhesive solution just before application on the substrate, and it acts as cross-linking agent. Since all isocyanates react with the residual hydroxyl groups on the polyurethane, in solution they yield adhesive systems with a limited pot life (1–2 h to several days). The addition of the polyisocyanate improves the spotting tack and the heat activation of the freshly dried adhesive films, but after several days they lose their capacity to be reactivated.

In finished bonds, the elastomeric polyurethanes show a relatively low cross-link density and then are exposed to outstanding hydrolytic degradation under high humidity and temperature that can lead to a partial or complete loss of strength during shoe use. The hydrolytic degradation of the polyurethane can be inhibited by adding 2–4 wt% carbodiimide to the adhesive solution because the carbodiimide forms acyl ureas by reaction of the carboxyl groups arising from the hydrolytic degradation of the polyurethane (Dollhausen 1988).

Waterborne Polyurethane Adhesives

Organic solvents are, in general, volatile, flammable, and toxic, in some degree. Further, organic solvent may react with other airborne contaminants contributing to smog formation and workplace exposure. Although solvent recovery systems and afterburners can be effectively attached to ventilation equipments, many shoe factories are switching to the use of waterborne adhesives.

Waterborne polyurethane adhesives are an environmental-friendly alternative to the solvent-borne polyurethane adhesives. Those are the more logical choice to replace the solvent-borne adhesives because they can be processed on the same machines, their performance in many applications is just as good as that of solvent systems, and they can be used economically despite their higher raw material and processing costs. However, the use of the waterborne polyurethane adhesives needs some minor changes in the current existing technology, essentially the additional heat required to remove water before joint formation. Furthermore, the waterborne polyurethane adhesives show additional limitations: (i) lack of tackiness at room temperature (heat activation is needed), (ii) poor wettability of some substrates (particularly greased leather), and (iii) polyurethane dispersions are thermodynamically unstable, and therefore they show relatively poor storage stability (dispersion collapses in the presence of metallic contaminants, at low temperature (generally below 5 °C), or by applying high stresses).

The polyurethane dispersions are constituted by urethane polymer particles wholly dispersed in water. The dispersion of the polymeric particles is feasible because of the presence of ionic hydrophilic groups chemically bonded to the main polymer chains which are oriented to the surface of the particles (Fig. 9). These dispersions are based on crystalline, hydrophobic polyester polyols (such as hexamethylene polyadipate), and aliphatic isocyanates (such as H12MDI – methylene bis(cyclohexyl isocyanate) – or IPDI, isophorone diisocyanate).
Fig. 9

Scheme of the structure of waterborne polyurethane adhesive

There are several routes to produce polyurethane dispersions, the two most common are the so-called acetone and prepolymer methods. In the acetone method , the polyol, the anionic internal emulsifiers (such as dimethylolpropionic acid or diethylamine sodium sulfonate), and the isocyanate are reacted to obtain a prepolymer with hydrophilic groups (García-Pacios et al. 2010). The prepolymer is dissolved in acetone for decreasing viscosity, and the ionic groups are neutralized by addition of an amine (trimethylamine is generally used to neutralize the carboxylic acid moieties). The chain extension of the prepolymer is carried out by addition of or diol; both hydrazine and ethylene diamine are commonly used as chain extenders because of their faster reaction with isocyanates than with water. Then, water is added for obtaining the polyurethane micelles by phase inversion, and the acetone is removed by distillation at reduced pressure. In the prepolymer method , two basic steps are needed in order to produce a waterborne polyurethane dispersion: a prepolymer step and a chain extension step. The prepolymer obtained by reacting the polyol and the aliphatic isocyanate is modified through an internal emulsifier containing hydrophilic groups, thereby eliminating the use of external emulsifiers. The prepolymer containing the internal emulsifier is reacted with a tertiary amine (such as triethylamine) to increase the length of the polymer chain and increase the molecular weight of the polyurethane. The amine reacts with the pendant carboxylic acid groups, forming a salt that under adequate stirring allows the dispersion of the prepolymer in water. The chain extension step takes place in the water phase. The waterborne polyurethane dispersions obtained by the acetone and the prepolymer methods contain high-molecular-weight polyurethane chains, and they have polyurethane and polyurea linkages.

The method for preparing the waterborne polyurethane dispersions determines their properties, and it has been scarcely studied. In general, the acetone method produces more homogeneous anionic dispersions and well-controlled polyurethane structure. It has been shown that the method of synthesis determined the mean particle size, the crystallinity, and the viscosity of the waterborne polyurethane dispersions (Barni and Levi 2003). In later study it has been shown that the acetone method produced dispersions with narrower particle size distributions and higher crystallinity than the prepolymer method, and the adhesive strength was similar in the joints made with waterborne polyurethane dispersions prepared with both methods (Pérez-Limiñana et al. 2006).

The waterborne polyurethane dispersions generally have a pH between 6 and 9 and higher solid content (35–50 wt%) and lower viscosities (about 100 mPa.s) than the solvent-borne polyurethane adhesives. They are high-molecular-weight linear polyester ureas or urethanes constituted by small rounded spherical particles of about 0.1–0.2 μm diameter.

The adhesive characteristics of the waterborne polyurethanes are mainly defined by the melting point and the crystallization kinetics of the polymer backbone. It is highly desirable to activate the adhesive at room temperature, but most of the waterborne polyurethane adhesives have melting point above 55 °C and need reactivation. The crystallization kinetics defines the open time of the adhesive. On the other hand, unlike solvent-borne adhesives, the viscosity of waterborne polyurethane adhesives is not dependent on the molar mass of the polymer but on the solids content, average particle size of the dispersion, and the existence of additives in the formulation.

The adhesion properties of the waterborne polyurethane adhesives are greatly determined by the polymer, and the formulations are generally simple including only small amounts of a thickener and an emulsifier. When formulating the waterborne polyurethane adhesives, the application conditions (heat activation, contact bond time, pressure) and substrate type must be taken into account. Because the properties of the waterborne polyurethane adhesives are mainly determined by the structure of the polyurethane, several studies analyzing the influence of different raw materials and the experimental variables of the synthesis procedure have been published. It has been concluded that the content of ionic groups, the hard-to-soft segment ratio, the nature and molecular weight of the polyol, the nature of the chain extender, the degree of neutralization of the prepolymer, and the nature of the counterion determined the properties of the waterborne polyurethane dispersions (Kim and Lee 1996; Jang et al. 2002).

Several studies have considered the influence of the hard-to-soft segments or NCO/OH ratio on the properties of the waterborne polyurethane dispersions. Most studies have shown that the increase in the NCO/OH ratio increased the mean particle size and enhanced the viscoelastic properties of the polyurethane dispersions (Pérez-Limiñana et al. 2007; García-Pacios et al. 2010; Vicent and Natarajan 2014). The variation of the viscosity of the waterborne polyurethane dispersion with the NCO/OH ratio is controversial. García-Pacios et al. (2011a) found that the increase in the NCO/OH ratio in waterborne polyurethane dispersions prepared with polycarbonate diol decreased the mean particle size and increased the Brookfield viscosity of the dispersions; however, the opposite trend was described previously (Madbouly et al. 2005). On the other hand, the increase in the NCO/OH ratio increases the urea and urethane hard-segment content in the polyurethane, and, therefore, the mechanical properties (i.e., Young’s modulus and tensile strength) and the glass transition temperature increased by increasing the NCO/OH ratio of the waterborne polyurethane dispersion. On the other hand, the NCO/OH ratio affected the T-peel strength and single-lap shear adhesive strength of the waterborne polyurethane adhesives, and a maximum value is obtained for an NCO/OH ratio of 1.5 (García-Pacios et al. 2011a).

The influence of the nature, the structure, and the amount of the diisocyanate on the properties of waterborne polyurethane adhesives has been extensively studied. The cycloaliphatic diisocyanates (methylene bis(cyclohexyl isocyanate) – H12MDI – isophorone diisocyanate, IPDI) are the most commonly used because of the controlled reactivity during prepolymer formation. It has been shown that the waterborne polyurethane dispersions prepared with IPDI had lower mean particle size but lower mechanical properties than the ones prepared with H12MDI (Kim et al. 1994). Aromatic diisocyanates such as MDI and TDI have been used for preparing waterborne polyurethane dispersions, and they showed higher thermal and mechanical properties than the ones prepared with cycloaliphatic diisocyanates (Yang et al. 1999). Mixtures of aliphatic and aromatic diisocyanates have been used for preparing waterborne polyurethane dispersions, and the mechanical properties and adhesive strength were higher in the polyurethane made with MDI and lower with the one prepared with H12MDI (Rahman et al. 2014). However, after accelerated aging in salt water, the mechanical strength and adhesive strength were better in the polyurethanes prepared with mixture of H12MDI and MDI diisocyanate.

The nature (diol, diamine), chain length, and amount of the chain extender determined the properties of the waterborne polyurethane dispersions. The diol chain extender produced polyurethanes with high degree of phase separation, higher crystallinity, and good adhesive strength; this later is higher by increasing the chain length of the diol (Orgilés-Calpena et al. 2016a). On the other hand, the diamine chain extenders produced polyurea–urethanes that show lower crystallinity, lower degree of phase separation, and lower peel strength than the ones prepared with diol chain extenders (Rahman 2013).

With respect to other raw materials, the polyol determines in greater extent the properties of the waterborne polyurethane adhesives. The increase in the molecular weight of the polyol (i.e., soft segments) produces dispersions with lower mean particle size and increased toughness and elongation at break (Mumtaz et al. 2013), but both the hardness and tensile strength decrease (Wang et al. 2015). On the other hand, the peel strength decreases by increasing the molecular weight of the polyol (García-Pacios et al. 2013a). The nature of the polyol (polyester, polyether, polycarbonate diol, polycaprolactone) determines the degree of phase separation in the polyurethane and affects noticeably the properties of the waterborne polyurethane dispersions. Because of the existence of carbonate groups, the use of polycarbonate diol produces lower degree of phase separation and increased shear adhesion with respect to the use of polyester and polyether (García-Pacios et al. 2013b). More recently the polyols derived from natural oils have been used for the synthesis of waterborne polyurethane dispersions (Yang et al. 2014; Hu et al. 2015). The waterborne polyurethane dispersions prepared with natural dimer fatty acid-based polyester polyols exhibited excellent water resistance, outstanding hydrolytic resistance and superior thermal stability than the ones prepared with nonnatural-derived polyester polyols (Liu et al. 2011).

The properties of the waterborne dispersions are also affected by the nature and amount of the ionic groups . The increase in the content of dimethyl propionic acid (DMPA) decreases the mean particle size and increases the viscosity of the waterborne polyurethane dispersions (Huang and Chen 2007). Increased DMPA amounts result in the higher hard-segment contents and the increase in the mechanical properties of the polyurethanes (Cakić et al. 2013). Furthermore, the increase of the DMPA content decreased the adhesion of the polyurethane dispersions (Rahman and Kim 2006).

The influence of the solids content on the properties of the polyurethane dispersions has been studied, and contradictory findings with respect to their influence on the mean particle size have been found. It has been stated that the increase of the solids content caused an increase in the mean particle size that was related to the decrease in the viscosity of the polyurethane dispersions (Lee and Kim 2009). Another study concluded that the mean particle size of the polyurethane dispersion decreased by increasing its solids content, and an increase in the viscosity was found, and this was related to the decreased particle–particle interactions and the higher degree of phase separation (García-Pacios et al. 2011b). On the other hand, the increase in the solids content does not affect the peel strength and decreases the single-lap shear strength of the waterborne polyurethane dispersions (García-Pacios et al. 2011b). A recent study analyzed the structure and properties of high solids content waterborne polyurethane dispersions synthesized by combining ionic and nonionic monomers (Lijie et al. 2015). The adequate combination of the molecular weight of the polyether and the amount of DMPA improved the stability of the dispersion and increased the solids content of the polyurethane dispersion up to 52 wt%. Additionally, the glass transition temperature of the soft segments and the crystallinity of the polyurethane decreased, although the mechanical properties were improved.

Thickeners (polyvinyl alcohol, polyurethanes, polyacrylates, cellulose derivatives) are added to increase the low viscosity of the waterborne polyurethane adhesives and to avoid excessive penetration in porous substrates (Orgilés-Calpena et al. 2009a). The mechanism of thickening of the waterborne polyurethane adhesives by adding urethane-based thickener has been analyzed by confocal laser microscopy (Fig. 10). The addition of the thickener produces an entanglement in the polyurethane adhesive (seen as denser and whiter images in Fig. 10) which can be ascribed to the existence of interactions between the urethane-based thickener and the polyurethane. Several interactions take part in the thickening mechanism of the polyurethane adhesive dispersions by adding urethane-based thickener: (i) interactions between the terminal hydrophobic groups of the thickener with the polyurethane micelles, (ii) interactions by ionic adsorption of the polyester chains of the thickener with the ionic groups on the surface of the polyurethane particles, and (iii) interactions by hydrogen bonds between the urethane groups of the polyurethane dispersion and the urethane-based thickener (Orgilés-Calpena et al. 2009a, b). Figure 11 shows a schematic diagram of the thickening mechanism of waterborne polyurethane adhesive containing urethane-based thickener in which the formation of 3D micelle network is seen. On the other hand, the hard-to-soft segment ratios and the content of ionic groups determine the properties of the waterborne polyurethane adhesives. Thus, the addition of 5 wt% urethane-based thickener markedly increases the viscosity of the waterborne polyurethane adhesive more noticeably by decreasing the hard-to-soft segments ratio and by increasing the ionic group contents (Orgilés-Calpena et al. 2009a). On the other hand, as the hard-segment content of the thickened polyurethane adhesive decreases, the kinetics of crystallization is favored as a result of stronger polyurethane–thickener interactions.
Fig. 10

Micrographs of confocal laser microscopy corresponding to the waterborne polyurethane adhesives without (14D6) and with (14D6/5e) 5 wt% urethane-based thickener

Fig. 11

Thickening mechanism of waterborne polyurethane adhesive containing urethane-based thickener through the formation of 3D micelle network (Orgilés-Calpena et al. 2009b)

Emulsifiers (surface active agents) assist to stabilize the pH of the waterborne polyurethane dispersion and to decrease its surface tension to obtain improved wettability. The emulsifiers are always oriented on the interface between the polymer particles and the aqueous phase. Small amounts must be added to avoid loss in bonding characteristics.

On the other hand, to achieve particular performance, dispersions of several polymers (vinyl acetate, acrylic ester) and resins (hydrocarbon resin, rosin esters) can be added to waterborne polyurethane adhesives. Special attention must be paid to the compatibility with the polyurethane to avoid dispersion collapse. In general, fillers are not added to waterborne polyurethane adhesives although dispersions of silicas can be added for adjusting the viscosity and the rheology.

To produce adequate bond, the waterborne adhesive solution is applied to the two substrates to be joined, and the water is evaporated at room temperature for about 30 min or by heating with hot air or infrared lamps at 50–60 °C for a few minutes. When the water comes out, the polyurethane viscosity rises, and a continuous film is formed on the substrates. Then, the dry films are heat activated, and the melting of the crystalline soft segment is produced imparting tackiness to the films. The heat activation process of the waterborne polyurethanes is affected very strongly by the substrates to be bonded and by the process parameters such as temperature, pressure, and contact bond time. Increasing the adhesive film temperature, raising the pressure, and extending the time of contact have all similar effect, i.e., an increase of the actual contact area is produced, and better bond is obtained. After heat activation, the decrystallized polyurethane film is an amorphous viscoelastic melt with good flow properties. The decrease in viscosity after heat reactivation allows the adhesive film to wet the substrate, and the joining to the adhesive film on a second substrate under pressure is produced. Once the adhesive joint is formed, the bulk viscosity and the modulus of the adhesive increase, initially by cooling and afterward by the recrystallization of the polyurethane.

If an isocyanate is added as cross-linker of the waterborne polyurethane adhesive, the bond formation occurs much slower than with only the polyurethane, but a further increase in viscosity with time is obtained. The cross-linking reaction does not initially bring any increase in the peel adhesive strength at room temperature but increases the heat resistance of the bond. The most common cross-linkers are polyisocyanates (adequately modified with surface active agents to become emulsifiable in water), although azyridines, polycarbodiimides, and epoxies can also be used. The cross-linker is normally added to the dispersion and stirred in before application on the substrates. Once the cross-linker is added, the drying off of the water is important because the reaction with water reduces its effectiveness. Depending on the formulation, the adhesives can be applied over a period of 4–10 h without affecting the adhesion properties. The isocyanate cross-linker generally takes several days before the cross-linking reaction is completed. Unlike solvent-borne adhesives, the pot life of the adhesive dispersions is not associated with an increase in viscosity.

In general, the durability of the polyurethane adhesives is acceptable although they are water sensitive, particularly the polyether-based polyurethane. Protection against water degradation can be reduced by adding carbodiimides and/or polyisocyanates during curing.

The bond strength of the waterborne polyurethane adhesives is particularly dependent on the substrate. Good results are obtained with surface-chlorinated vulcanized rubbers and PVC, but adhesion to TR may be poor or variable (Abbott 1992). Adhesion to leather is sometimes insufficient, and roughening must be always carried out; adhesion to greasy or waterproof leathers and unroughened polyurethane coatings may also be difficult.

Solvent-free Polyurethane Adhesives

Currently, the waterborne adhesives are being introduced into the shoe industry. Their performance is quite similar to that of the solvent-borne adhesives, so it can be estimated that for several years, they will be used in shoe industry. However, the future seems to be directed through the use of moisture-curing holt-melt urethane and thermoplastic urethane adhesives as they are 100% solids systems and evaporation of solvents is not necessary. Although the hot-melt urethanes could replace waterborne adhesives, this could take longer to occur because of the vastly different equipment requirements and the change in the bonding concept by the shoe manufacturers.

Some literature has been published dealing with moisture-reactive hot-melt polyurethane adhesives (Frisch 2002). Most moisture-curing hot-melt adhesives are obtained by reacting a crystallizable polyol such as poly(hexamethylene adipate) and monomeric MDI (NCO/OH ratios = 1.5–2.2). A catalyst such as dimorpholinediethyl ether is also necessary. The polyester polyol plays a key role because the open time, the viscosity, and the glass transition temperature can be adequately tailored. A mixture of hydroxyl-terminated polyesters having different characteristics allows control of the adhesion and hardening time of the adhesive. Recently, reactive polyurethane hot-melt adhesives containing polycarbonate polyols derived from CO2 and 4,4′-diphenylmethane diisocyanate (MDI) have been developed from improving the T-peel strength of leather/polyurethane adhesive/vulcanized SBR joints, and they show good green and final strength before and after high temperature/humidity aging and hydrolysis tests (Orgilés-Calpena et al. 2016a, b).

The moisture-curing hot-melt adhesives must be stored in nitrogen-sealed containers, and moisture must be fully excluded. These adhesives are crystalline solids at room temperature, and for application, heating is necessary (about 125 °C) to become low-viscosity liquids, and they are only applied on one of the substrates to be joined; the second substrate is immediately joined with minimal pressure. Simultaneously to the application of the adhesive, water can be sprayed onto the adhesives/materials in the amount necessary for facilitating the curing process. Although after cooling down the crystallinity is reached, about 24 h is necessary to develop the strength of a structural adhesive. Initial strength can be improved by adding polyester, thermoplastics, or polymerized acrylates. Addition of ketone–formaldehyde and terpene phenolic resins also increases adhesive performance. Limitations of these adhesives derive from the special equipments necessary for application and from the substrate nature (not all substrates can be bonded). However, some pre-reacted hot-melt adhesive types are liquids and may be applied by hand, by spraying, or even better by nozzles.

When the polyurethane hot-melt adhesive comes into contact with moisture, the irreversible curing of the adhesive is initiated. First, the water adds to the isocyanate group producing unstable carbamic acid, which in turn is transformed into a primary amine group. These amine groups generate linear urea bridges by reaction with additional isocyanate; a three-dimensional network is produced by cross-linking with more isocyanate groups, providing the structural bonding in moisture-cured hot-melt polyurethane adhesives.

The thermoplastic polyurethane adhesives are another future alternative for bonding in shoe industry. These adhesives have good holding strength after crystallization, but their cost is higher than the majority of the adhesives used in shoe industry. Most thermoplastic polyurethane adhesives are based in fast crystallization polyesters. They are prepared using a NCO/OH ratio near 2, and they have linear structure. They consist in two components (polyol and isocyanate) which have to be mixed at about 60 °C in the presence of a catalyst such as dibutyltin dilaurate; after reaction, the resulting polymer is pelletized and packed in nitrogen- and moisture-free atmosphere containers. Application temperatures are higher than 170 °C, and to produce bonding the same procedure as for moisture-cured hot-melt urethane is used.

3.5 Polychloroprene Adhesives

Although polychloroprene adhesives (more often called neoprene adhesives because of the trade name given by Du Pont to the first commercial product) were extensively used in the past in the sole-to-upper bonding, nowadays they have been practically substituted by polyurethane adhesives. Therefore, there is no sense in providing an extensive description of the polychloroprene adhesives in this chapter which the prime aim is to provide an actual overview. A detailed description of the polychloroprene adhesives for shoe bonding can be found in chapter 14 of the book entitled “Adhesive bonding. Science, technology and applications” (Adams 2005). Therefore, in this chapter, the main features of these adhesives will be only considered, and the reader can refer to that chapter for more detailed information.

Solvent-borne polychloroprene adhesives are less used in shoe industry because of the presence of aromatic organic solvents in their formulation which are currently unacceptable. However, the most recent developed waterborne polychloroprene adhesives can compete with waterborne polyurethane adhesives because of their good performance in the upper-to-sole bonding. Furthermore, they are extremely useful as nonpermanent adhesives in several mounting operations (previously to the sole-to-upper attachment) involving porous substrates in the shoe manufacturing.

Solvent-Borne Polychloroprene Adhesives

Polychloroprene adhesives are contact adhesives (see chapter “Adhesive Families”). The diffusion process in polychloroprene rubber adhesives is mainly affected by the solvent mixture of the adhesive (which determines the degree of uncoiling of rubber chains) and by the ingredients in the formulation (mainly the amount and nature of tackifier). The chemical nature and molecular weight of the polychloroprene greatly determine its adhesive properties. The polychloroprene adhesives show high peel strength; high green strength and noticeable resistance to moisture, chemicals, and oils; excellent aging properties; and excellent temperature resistance. These characteristics are important in shoe industry.

Polychloroprene elastomers are produced by free radical emulsion polymerization of 2-chloro-1,3-butadiene monomer. The emulsion polymerization of chloroprene involves the dispersing of monomer droplets in an aqueous phase by means of suitable surface active agents, generally at a pH of 10–12. Polymerization is initiated by addition of free radical catalyst at 20–50 °C. To obtain a high conversion in the polymerization reaction and processable polymer, the addition of sulfur, thiuram disulfide, or mercaptans is necessary (Whitehouse 1986).

The crystallinity in the polychloroprene is essential in sole-to-upper bonding in shoe industry. Crystallinity is produced by noticeable trans-1,4 addition (more than 90%) during polymerization. As a result of crystallization, the cohesive strength of the polychloroprene is much greater than that of the amorphous polymer. Crystallization is reversible under temperature or dynamic stresses. Thus, for a temperature higher than 50 °C, uncured polychloroprene adhesives lose their crystallinity, and upon cooling the film recrystallizes, and cohesive strength is regained. The increase in the crystallinity improves the modulus, hardness, and cohesive strength of the polychloroprene adhesives but decreases their flexibility.

A typical composition of solvent-borne polychloroprene adhesive for upper-to-sole attachment is given in Table 2.
Table 2

Typical composition of a solvent-borne polychloroprene adhesive. phr means parts per hundred parts of rubber

Polychloroprene

100 phr

Tackifying resin

30 phr

Magnesium oxide

4 phr

Zinc oxide

5 phr

Water

1 phr

Antioxidant

2 phr

Solvent mixture

500 phr

The role of the different components in solvent-borne polychloroprene adhesive is described below in more detail.

The sulfur-modified polychloroprenes with little or no branching are the most commonly used in shoe adhesives as they are soluble in aromatic solvents, and for difficult-to-bond substrates, methacrylic graft polymers show better performance. The polymer type influences several properties of the solvent-borne polychloroprene adhesives, mainly the molecular weight and the rate of crystallization. The increase in the molecular weight of the polychloroprene imparts higher solution viscosity, adhesive strength, and heat resistance. On the other hand, the increase in the crystallization rate of the polychloroprene improves the rate of development of the bond strength and the ultimate strength at high temperature, but a reduction in open time (tack) is obtained. However, the tack can be varied by changing the pressure during joint formation and by an adequate selection of the solvent; the lower the volatility of the solvent, the longer is the open time of the solvent-borne polychloroprene adhesive.

Metal oxides provide several functions in solvent-borne polychloroprene adhesives. The main function of the metal oxides in the polychloroprene adhesive formulations is acting as acid acceptor. Upon aging, small amounts of hydrochloric acid are released which may cause discoloration and substrate degradation. Magnesium oxide (4 phr) and zinc oxide (5 phr) act synergically in the stabilization of the solvent-borne polychloroprene adhesives against dehydrochlorination. On the other hand, the magnesium oxide retards scorch during mill processing of the polychloroprene adhesives and also reacts in solution with the t-butyl phenolic resin to produce an infusible resinate (a small amount of water is also necessary) which provides improved heat resistance. Furthermore, the zinc oxide produces a room temperature cure of the solvent-borne polychloroprene adhesives, giving increased strength and improved aging resistance.

Addition of resins to solvent-borne polychloroprene adhesives serves to improve its specific adhesion, increases tack retention, and increases hot cohesive strength. Para-tertiary butyl phenolic resins are the most common, and amounts between 35 and 50 phr are generally added. In general, the tack decreases by increasing the phenolic resin content in the polychloroprene adhesive, and bond strength reaches a maximum at about 40 phr, decreasing for higher amounts of phenolic resin.

Solutions of polychloroprene adhesives containing metal oxides and t-butyl phenolic resin may show phasing (e.g., clear upper layer and flocculated lower layer of metal oxides) on standing upon days or months. To recover the full utility of the metal oxides, agitation before use is sufficient. Phasing is ascribed to the wide molecular weight distribution of the t-butyl phenolic resins.

Terpene phenolic resins can also be added to solvent-borne polychloroprene adhesives to increase open tack time and to provide a softer glue line than the t-butyl phenolic resins. To provide adequate hot-bond strength, these resins are used in combination with a polyisocyanate-curing agent.

An antioxidant (about 2 phr) should be added to the polychloroprene adhesives to avoid oxidative degradation and acid tendering of the substrates. Derivatives of diphenylamine (octylated diphenylamine, styrenated diphenylamine) provide good performance, but staining is produced. To avoid staining, hindered phenols or bisphenols can be added.

Solvent affects the adhesive viscosity, the bond strength development, the open time, and the ultimate strength of the polychloroprene adhesives. Blends of three solvents (aromatic, aliphatic, oxygenate – e.g., ketones, esters) are generally added. Typical solvent blends for polychloroprene adhesives are given in Table 3. The open tack time of the solvent-borne polychloroprene adhesives partially depends on the evaporation rate of the solvent blend. If a solvent evaporates slowly, the solvent-borne polychloroprene adhesive will retain tack longer, whereas if the solvent evaporates quickly, the cohesive strength will develop more rapidly.
Table 3

Influence of the solvent on the viscosity and the tack of polychloroprene adhesives. Formulation: 100 phr polychloroprene elastomer; 4 phr MgO; 5 phr ZnO; 2 phr hindered phenolic antioxidant; 500 phr solvent mixture (Whitehouse 1986)

Solvent

Mixing ratio

Solution viscosity 20 °C (mPa.s)

Tack time (min)

Toluene

4460

35

Toluene/n-hexane/MEK

35/5/15

3500

25

n-Hexane/MEK

55/45

1520

15

Cyclohexane/acetone

80/20

3700

18

MEK/acetone

75/25

2100

15

Toluene/n-hexane/ethyl acetate

34/33/33

3600

30

Toluene/MEK/acetone

34/33/33

3400

22

n-Hexane/acetone

50/50

1140

7

Dichloromethane

4670

30

1,1,1-Trichloroethane

4600

38

Isocyanates can be added to solvent-borne polychloroprene adhesive solutions as two-part adhesive systems. The reaction of the isocyanate with polychloroprene that leads to improved heat resistance property has not been fully explained as there are no active hydrogen atoms in the polychloroprene to allow reaction with isocyanate group. The two-part adhesive system is less effective with rubber substrates containing high styrene content and with butadiene–styrene block (thermoplastic rubber) copolymers. To improve the specific adhesion to those materials, addition of poly-alpha-methylstyrene resin to solvent-borne polychloroprene adhesives is quite effective (Tanno and Shibuya 1967).

Waterborne Polychloroprene Adhesives

In recent years, the use of solvent-borne polychloroprene adhesives has been seriously restricted, and waterborne adhesives have been developed. Polychloroprene latex differs from its solid elastomer counterpart (used in solvent-borne polychloroprene adhesive) in that it is a gel polymer (e.g., insoluble in organic solvents). Latex systems derive their bond strength characteristics from the gel structure rather than crystallinity as in solvent solution systems. Higher gel content leads to the same properties than polymers with higher crystallinity (Lyons and Christell 1997). Polymers with higher gel content exhibit higher cohesive strength, modulus, and heat resistance, but tack, open time, and elongation at break are reduced.

The initial formulations of the polychloroprene latex adhesives contain essentially the same components than the solvent-borne adhesives, except that water-based ingredients have to be used, surfactant, thickener and antifoaming agent must be added, and the compounding is particularly exigent. A typical composition of initial waterborne polychloroprene adhesive is given in Table 4.
Table 4

Typical composition of the initial waterborne polychloroprene adhesive formulation

Polychloroprene dispersion

100 phr

Surfactant

As required

Antifoam

As required

Tackifying resin

50 phr

Thickener

As required

Zinc oxide

5 phr

Antioxidant

2 phr

In the last years, more simple and efficient formulations of waterborne polychloroprene adhesives for shoe industry have been developed (Kueker et al. 2016). The typical formulation of the current waterborne polychloroprene adhesives is given in Table 5 and consists in polychloroprene, acrylic or silica sol dispersion, zinc oxide, surfactant, antifoaming agent, and antioxidant. The preparation of these waterborne polychloroprene adhesives starts with the addition of the antioxidant to the polychloroprene dispersion, followed by slow addition of zinc oxide and the final incorporation of the acrylic or silica sol dispersion.
Table 5

Typical composition of the current waterborne polychloroprene adhesive formulation

Polychloroprene dispersion

100 phr

Surfactant

As required

Antifoam

As required

Silica sol dispersion

10 phr

Zinc oxide

1 phr

Antioxidant

2 phr

The final adhesive strengths obtained with the current waterborne polychloroprene adhesives are due to the cross-linking between the polychloroprene and the silica or acrylic micelles, and similar adhesive strengths to the ones obtained with solvent-borne adhesives are obtained, although the open time is generally longer. The use of silica sol dispersions with low mean particle size (i.e., about 5 nm) improves more the initial adhesive strength or wet bonding to leather than the use of silica sol dispersions with higher mean particle size, and the increase in the amount of silica causes softening and a decrease in the mechanical properties of the polychloroprene. Finally, the heat resistance of the polychloroprene adhesives can be noticeably increased by using hydroxylated polychloroprene dispersion due to the hydrogen bond formation with the silica sol particles; in fact when this hydroxylated polychloroprene dispersion is used, there is no need of adding polyisocyanate cross-linker for increasing the aging resistance, and this is even better than by using solvent-borne polychloroprene adhesives (Kueker et al. 2016).

Recently, the magnetic conditioning for 3 h of aqueous-based polychloroprene contact adhesive has been proposed for increasing its adhesion (Souza et al. 2016). To promote adhesion, a nanometric zinc oxide, carbon dioxide as catalyst, hydroxylamine, and a magnetic conditioning process before the application of the adhesive were carried out. The magnetic conditioning was performed in magnetic cell comprising of containers for circulation of the adhesive, two submerged centrifugal hoses for transfer of the adhesive, a set of magnets iron–boron–neodymium with magnetic field of 2120 gauss, gaussmeter, and agitators. Noticeable increase in shear adhesion was obtained.

The polychloroprene latex determines the initial tack and open time, the bond strength and the hot-bond strength, the application properties, and the adhesives viscosity. Because most of the latexes have low viscosities, most of the waterborne polychloroprene rubber adhesives are sprayable. Thickeners such as fumed silicas can be added to some formulations for increasing viscosity and thixotropy.

Grafting of methyl methacrylate (MMA) onto polychloroprene rubber latex (CRL) has been carried out by emulsion polymerization using a redox initiator, and the surface diffusion-controlled process model was proposed to describe the grafting mechanism (Zhang et al. 2012). The graft copolymer was blended with tackifier and filler for waterborne contact adhesive applications, and the 180° peel strength to canvas to other decoration materials showed similar performance than solvent-based contact adhesive and superior to the one obtained with polychloroprene and polymethyl methacrylate.

Based on the synergistic effect of polychloroprene latex and styrene–acrylate emulsion, waterborne contact adhesive containing 40 wt% styrene–acrylate emulsion and 1.25 wt% boric acid has been developed (Zhang et al. 2009). The blend had a good shelf stability, its set time was 5 min, its tensile strength was reasonable, and it had very high elongation at break. The performance of the waterborne contact adhesives was found to be comparable to the solvent-based contact adhesives because they offered good wetting ability to substrate, high initial tack was obtained, it possessed excellent crystallization ability, and it enhanced the cohesive strength of the adhesive.

Although noncompounded polychloroprene latex has good mechanical and storage stability, surfactants are added commonly for stabilization. They function by strengthening the interfacial film by maintaining or increasing the degree of solvation or by increasing the charge density on the latex particle. More precisely, surfactants are added to improve storage stability, substrate wetting, and attain improved freeze resistance. However, incorporation of surfactants has an adverse effect on cohesive properties and should be kept to a minimum. Water resistance and tack may also be affected. Excessive stabilization of the adhesive mixture may negatively affect coagulation (which is desirable in the wet bonding process). Anionic emulsifiers (alkali salts of long-chain fatty acids and alkyl/aryl sulfonic acids) or nonionic emulsifiers (condensation products of long-chain alcohols, phenols, or fatty acids with ethylene oxide) can be used.

Zinc oxide is the most effective metal oxide and plays three main functions: (i) promote cure; (ii) improve aging, heat, and weathering resistance; and (iii) acid acceptor. In general, 2–5 phr zinc oxide is added in latex formulations.

Resins influence the adhesion, open time, tack, and heat resistance of the waterborne polychloroprene adhesives. In some formulations, 30–60 phr is added, and attention should be paid to the pH and compatibility with the surfactant. The glass transition temperature, the softening point, the polarity, and the compatibility of the resin with the polymer determine the adhesive properties. Thus, the hot-bond performance is generally proportional to the softening point of the resin. t-Butyl phenolic resins cannot be used in waterborne polychloroprene adhesives because of colloidal incompatibility. Instead, terpene phenolic resins can be added to polychloroprene latex without great reduction in hot strength as the resin content is increased; however, the contact ability is reduced, and an adhesion failure is obtained, even at the 50 phr level. Furthermore, the terpene phenolic resins have relatively poor tack but impart good resistance to elevated temperatures to the polychloroprene latexes and require either heat activation or pressure to achieve adequate bond strength. Rosin ester resin emulsions are also effective in latex adhesives as they extend the tack life of the polychloroprene latexes, but they do not have the reinforcing characteristics of the terpene phenolic or alkyl phenolic resins. Hence, the cohesive strength and heat resistance are sacrificed to obtain surface tack.

New nitrogen-containing compounds have been proposed as highly efficient adhesion promoters for adhesive compositions based on polychloroprene (Keibal et al. 2011). On the other hand, similar antioxidants than for solvent-borne polychloroprene adhesives can be used in the formulation of waterborne polychloroprene adhesives.

Addition of thickeners increases the viscosity of the polychloroprene latex adhesives. Amounts up to 1 wt% of polyacrylates, methyl cellulose, alginates, and polyurethane thickeners can be used. Particular attention should be paid to fluctuations in pH when thickener is added in the formulations. For low pH (7–10) formulations, fumed silica or some silicates can be used.

Curing agents have little effect on the performance of latexes with the highest gel content, but they are sometimes used with low-gel polymers to improve hot-bond strength while maintaining good contactability. Suitable curing agents are thiocarbanilide either alone or in combination with diphenylguanidine, zinc dibutyldithiocarbamate, and hexamethylenetetramine. Two-part systems have been developed using more active materials such as aqueous suspension isocyanates and hexamethoxy melamine. These agents produce a cross-linking reaction at room temperature and give fast bond development but exhibit a finite pot life. The most-common use of the curing agents is with carboxylic latices. Isocyanates and melamines can be used, but zinc oxide is the most common curing agent. Zinc oxide cross-links carboxylated latices and improves bond strength by ionomer formation (Cuervo and Maldonado 1984). Carboxylated polychloroprene reacts slowly with zinc oxide in dispersed form, causing a gradual increase in adhesive gel content that can lead to restricted adhesive shelf life. Resin acid sites compete with the polymer acid sites for ZnII; the more resin acid sites, the more stable the adhesive is.

4 Testing of Adhesive Joints in Shoe Bonding

In order to produce an optimum adhesive bonding of upper to sole, apart from the adequate surface preparation of the sole and upper materials and the adequate choice of the adhesive, the procedure to produce the joint should be carefully controlled and optimized. To achieve adequate adhesion of upper to sole, the following issues should be considered.
  1. 1.

    Selection of the upper and sole materials. In general, formulators of upper and sole materials do not consider that they have to be bonded, but they pay more attention to match the hardness and mechanical properties and aesthetic of the materials. Furthermore, depending on fashion, different difficult-to-bond materials are used to produce shoes which make problematic to standardize bonding. In general, the formulation of the shoe materials must elude the presence of additives able to decrease the adhesion to polyurethane or polychloroprene adhesives (plasticizers, excessive amounts of processing oils, inadequate selection of antiozonants, and antioxidants), and especially the use of too greased leather upper must be avoided. On the other hand, the formulations must be repetitive, and first-class raw materials must be used. As a general rule, the use of adequately formulated uppers and soles avoid more than 90% of the bonding problems in shoe industry.

     
  2. 2.

    Selection of the adhesive. The adhesive must be selected considering the performance required for each joint. Adhesive must properly wet the upper and sole surfaces, and in this aspect, solvent-borne adhesives are excellent. One of the problems in the use of waterborne adhesives is their poor wettability, although an adequate formulation may solve quite satisfactorily this limitation. In the selection of an adhesive, the following aspects should be particularly considered: nature and formulation of the materials to be bonded; stresses produced during shoe use; environment of use (solvents, acid, or alkali media), only for safety shoes; adhesive application restrictions (viscosity, rheological properties); specific requirements of the adhesive (pot life, working scheme in the shoe factory): and safety regulations.

     
  3. 3.

    Design of the joint. Fashion dictates the shape and geometry of the shoes. Sometimes, the shoes become difficult to bond because of high heels or heterogeneous shapes of soles. In general, the shoe designers do not pay attention to the bonding.

     
  4. 4.
    Adequate bonding operation. Several cases of poor adhesion in shoe bonding arise from a deficient operation. As a summary, the following aspects must be properly obeyed to assure an adequate upper-to-sole bonding:
    • Surface treatments of upper and sole. The way to produce the surface preparation and the instruments used are critical. Furthermore, proper surface preparation for each upper and sole must be selected.

    • Adhesive application. A thin film of adhesive (about 100 μm thick) is applied to each of the two substrates to be bonded, and the solvent is removed by natural or forced evaporation. A heavier coat of adhesive is more likely to result in a cohesive failure in the substrate. The application procedure (brush, doctor knife, spray gum, roller, coater) and the amount of adhesive must be carefully controlled. The choice of the adhesive application devices depends mainly on the type and size of the materials to be bonded as well as on the rheological properties of the adhesive. Furthermore, the viscosity of the adhesive must be controlled, and the operation times (evaporation rate of organic solvents or water, open time, shelf life) must be strictly obeyed.

    • Adhesive film drying. Controlled drying of the shoe bottom cements is preferable to natural drying. Removed excess water or organic solvent from an adhesive film is not entirely governed by the process of evaporation but also by the speed of absorption into the substrate. Porous substrates (such as leather) absorb water or organic solvents without detrimental effect on bond strength, whereas nonporous substrates need the complete solvent removal to produce adequate adhesion. Furthermore, a force drying may produce skin formation on the surface of the coating (especially if heavier adhesive coating is applied) leading to poor coalescence of the adhesive. A slight trace of organic solvent in the cement film on the upper at sole attaching is beneficial in giving complete coalescence. When necessary, although very fast drying can be achieved by radiant heat, the use of IR radiation and hot air is a more convenient method, and both provide more uniform heating.

    • Bond formation. The components with the dry adhesive film are placed for 10–30 s in a “flash heater” – hot air or IR radiation can also be used – where a radiant heat source raises suddenly the temperature of the adhesive film above the crystalline melting of the polyurethane (heat activation or reactivation process). Recommended reactivation temperature for polyurethane adhesives ranges between 45 and 85 °C, depending on the substrates and the adhesive characteristics. An insufficient reactivation temperature causes non-coalescence of the adhesive, and an excessive reactivation temperature decreases creep because the substrate is too hot and the adhesive is still soft when sole and upper are attached (SATRA 1963). In some particular cases, the reactivation temperatures of the adhesives higher than 100 °C are recommended to produce good performance. While still in their amorphous state, the adhesive films are brought together under pressure (typically 35–200 kPa) for 10–30 s (stuck-on process). The time interval after the heat activation process during which the films adhere is known as the “spotting tack.” Both the applied pressure and the pressing time of the upper to the sole must be adequately selected for each joint.

    • Crystallization or curing of the adhesive. After bond formation, the adhesive joint needs time to gain sufficient cohesion. Time can vary, although in general a minimum of 24 h is required for crystallization, being 72 h the optimum. For waterborne adhesives, the curing time may be longer.

     
T-peel (see chapter “Fracture Tests”) and creep (see chapter “Creep Load Conditions”) tests are the most commonly used to establish the adhesion performance in shoe bonding. The peel test serves to determine the bonding properties of upper to sole in shoe industry. Peeling rate, material thickness, and size of test samples must be optimized. Standard T-peel tests require two rectangular test pieces of 150 mm long, 30 mm width, and 3 mm thick that are stuck together to cover it other to a length of at least 50 mm (Fig. 12). Standard peel rate is 100 mm/min. The joints are stored for 72 h in standard atmosphere (23 °C and 50% relative humidity) before carrying out the separation tests. Both the peel resistance and the way in which the separation occurs help to assess the bond. A minimum of five replicates, preferably ten, for each joint must be tested and averaged. Bond strength should be expressed as kN/m, although very often N/mm is used in the shoe industry. The loci of failure of the joints are generally expressed using the following capital letters:
  • A: Adhesion failure (detachment of the adhesive film from one of the materials)

  • C: Cohesive failure in the adhesive (separation within the adhesive film without detachment from the material)

  • N: Non-coalescence failure (failure of the two adhesive films without detachment from the material)

  • S: Surface cohesive failure of the material (breakdown of a substrate of low structural strength at its surface)

  • M: Cohesive failure of one of the substrates

Fig. 12

T-peel test sample

Under the influence of force and when heated, the adhesive layers of footwear material bonds suffer plastic flow. The creep test at constant temperature serves to assess the behavior of shoe material bonds when heated under the influence of a constant peeling force over a fixed time. Weight between 0.5 and 2.5 kg can be fixed on the lower holders outside a heating oven where the test pieces can be heated at 50–70 °C. The unbounded ends of five test pieces are bent apart carefully and inserted in the holders. Temperature is generally raised to 60 °C after a warm-up period of 1 h. The test pieces are loaded with the chosen weight constantly for 10 min. Then, the heating oven is opened, and the separations of the bonds are marked.

Moisture and temperature are the main agents able to degrade the shoe joints. Therefore, adequate aging tests have been developed to assure adequate performance. In general, addition of polyisocyanate hardener increases the durability and retards aging in most of the upper-to-sole adhesive joints. Several tests have been proposed to establish the aging resistance of upper-to-sole bonds. The most common aging tests involve the immersion in hot water, the exposition of the joints to 50 °C and 95% relative humidity for 1 week, freeze-thaw cycles, and/or UV light. Generally, after aging T-peel tests are carried out to determine durability.

5 Conclusions

This review considered the current state of the art of the main issues involved in the bonding process of upper to sole in footwear manufacturing. The main innovations in the surface treatments of soles of different materials and the waterborne adhesives were addressed.

Notes

Acknowledgment

The chapter was not possible without the help of several people who work in my laboratory. The financial support from the Spanish Research Agency (CICYT, MCYT, MICINN), the Generalitat Valencia (Consellería de Educación, Cultura y Deporte, Consellería de Industria), and the University of Alicante is greatly appreciated. Last but not least, my deep recognition and gratitude to Antonia Armentia-Agüero (Toñi) for her love and for her help in several figure designs.

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© Springer International Publishing AG 2017

Authors and Affiliations

  1. 1.Adhesion and Adhesives LaboratoryUniversity of AlicanteAlicanteSpain

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