Reference Work Entry

Handbook of Adhesion Technology

pp 291-314

Composition of Adhesives

  • Hyun-Joong KimAffiliated withLaboratory of Adhesion & Bio-Composites, Program in Environmental Materials Science, Seoul National University Email author 
  • , Dong-Hyuk LimAffiliated withLaboratory of Adhesion & Bio-Composites, Program in Environmental Materials Science, Seoul National University
  • , Hyeon-Deuk HwangAffiliated withLaboratory of Adhesion & Bio-Composites, Program in Environmental Materials Science, Seoul National University
  • , Byoung-Ho LeeAffiliated withLaboratory of Adhesion & Bio-Composites, Program in Environmental Materials Science, Seoul National University

Abstract

Adhesives and sealants are generally developed and prepared for many applications such as packaging, construction, automobile, electronic, etc. An adhesive formulation will depend on the base materials and requirements of a particular application. Development managers or formulators have to have a public knowledge about the chemical composition and role of many components for reducing trials and errors. This chapter focuses on the definition and function of adhesive composition such as primary resins, solvents, fillers, plasticizers, reinforcements, and various additives.

Abstract

Adhesives and sealants are generally developed and prepared for many applications such as packaging, construction, automobile, electronic, etc. An adhesive formulation will depend on the base materials and requirements of a particular application. Development managers or formulators have to have a public knowledge about the chemical composition and role of many components for reducing trials and errors. This chapter focuses on the definition and function of adhesive composition such as primary resins, solvents, fillers, plasticizers, reinforcements, and various additives.

Introduction

An adhesive is a polymer mixture or polymerizable material in a liquid or semiliquid state that adheres substrates together (Petrie 2000). Adhesives may be composed of many components such as polymer, oligomer, filler, and additives from either natural or synthetic sources. It is very important to understand the components for adhesive formulation. The information of composition gives us an adhesive selection guide based on functional properties, curing mechanisms, and other relevant information supplied by the adhesive manufacturer.

An adhesive is a complex formulation of many components that have a unique function. The adhesive manufacturer or developer selects actual ingredients depending on the end-user requirement, the application, processing requirement, and the cost as shown in Fig. 13.1 . The various components of an adhesive formulation include the following: primary resins, solvents, fillers, plasticizers, reinforcements, thickeners and thixotropic agents, film formers, antioxidants, antifungal agents, emulsifiers, and wetting agents (Petrie 2000).
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Fig. 13.1

Flow chart of adhesive applications

The adhesive formulator gets a lot of information from raw material suppliers, books, papers, and patents. The number of possibilities for innovation seems to be endless. However, the formulation of an adhesive is fixed according to the formulator’s experience and education. Knowledge of how to incorporate ingredients together into a practical, workable formulation is also required.

Primary Resins

The primary resins of adhesives and sealants are the principal component that provides a lot of characteristics such as wettability, adhesion strength, thermal property, chemical resistance, and environmental resistance. The word “resin” means a hydrocarbon secretion of many plants, particularly coniferous trees. In the adhesive industry, the primary resin means a polymer that is the main chain in the adhesive molecular structure. The understanding of primary resins is essential for adhesive curing, application, reliability, and adhesion failure analysis. For example, a modified epoxy acrylate is one of the primary resins for the main sealants of the liquid crystal display (LCD) (Park et al. 2009). The main sealants are used to adhere the thin film transistor and the color filter. To produce sealants for LCD, the primary resins have to have absolutely high purity to prevent pollution of the liquid crystal. However, some primary resins have a large range of performance depending on molecular weight, molecular structure, additives ratio, etc. Table 13.1 gives a classification of applications and primary resins (Dostal 1990).
Table 13.1

Various adhesive classifications and primary resins

Main category

Subcategory

Primary resins

Characteristics

Anaerobic adhesive

Polyester, urethane, epoxy, silicone, acrylate

Elastic adhesive

Silicone, urethane, polysulfide

Conductive adhesive

Epoxy, acrylate, polyimide, silicone, EVA, phenol

Flame-retardant adhesive

Polybenzimidazole, polyquinoxazoline, polyphenylquinoxazoline, polyimide, bismaleimide, epoxy

Damping adhesive

Silicone, polyvinylalchol

Curing methods

Quick-drying-glue

Cyanoacrylate

UV-curable adhesive

Acrylate, polyene

EB curable adhesive

Acrylate

Visible light curable adhesive

Acrylate, polyene, polythiol

Hotmelt adhesive

SBC, polyamide, polyacrylate, EVA

Applications

Medical adhesive

Fibrin, gelatin, cyanoacrylate, polyurethane

Shoe adhesive

Polyurethane, phenol, SBR, rubber latex

Structural adhesive

Phenol, epoxy, nitrile-phenol, vinyl phenol, epoxy, urethane, acrylate

EVA ethylene-vinyl acetate, EB electron beam, SBC styrene block copolymers, SBR styrene-butadiene-rubber

Adhesives are classified by many methods such as dispensing method, application, and primary resin. The classification by primary resin is very useful to select a good adhesive for a given application, because the name of the primary resin involves the chemical structure of the back bone and the sketchy properties such as adhesion strength and heat resistance. In this chapter, the classification of primary resins by chemical composition consists in thermosets, thermoplastics, and elastomeric resins. A thermosetting resin is a prepolymer in a soft solid or viscous state that changes irreversibly into an insoluble polymer network by curing, which can be induced by the action of heat or radiation, or humidity. Thermosetting materials are generally stronger than thermoplastic materials due to 3-D network of bonds, and are also better suited to high-strength and high-temperature applications. Table 13.2 shows some typical thermosetting resins and their characteristics for adhesives and sealants (Dostal 1990). A thermoplastic resin is a polymer that can turn to a melting liquid when it is heated and returns to solid when it is cooled down. Table 13.3 shows some thermoplastic resins and their properties (Dostal 1990). The molecular structure of thermoplastic resins is linear or branched. Linear and branched polymers are often soluble in solvents such as chloroform, benzene, toluene, and tetrahydrofuran (THF). More detailed information for each primary resin is given in Chap.​ 14.
Table 13.2

Typical thermosetting resins for adhesives and sealants

 

Advantages

Limitations

Epoxy

• High strength

• Good solvent resistance

• Good gap-filling capabilities

• Good heat resistance

• Wide range of formulations

• Relatively low cost

• Exothermic reaction

• Exact proportions needed for optimum properties

• Short pot life

Polyurethanes

• Various cure times

• Tough

• Excellent flexibility even at low temperature

• One- or two-component, room- or elevated-temperature cure

• Moderate cost

• Both uncured and cured are moisture sensitive

• Poor heat resistance

• May revert with heat and moisture

• Short pot life

• Special mixing and dispensing equipment required

Cyanoacry lates

• Rapid room-temperature cure

• One-component system

• High tensile strength

• Long pot life

• Good adhesion to metal

• Dispense easily from package

• High cost

• Poor durability on some surfaces

• Limited solvent resistance

• Limited elevated-temperature resistance

• Bonds skin

Modified acrylics

• Good flexibility

• Good peel and shear strengths

• No mixing required

• Will bond dirty(oily) surfaces

• Room-temperature cure

• Moderate cost

• Low hot-temperature strength

• Slower cure than with cyanoacrylates

• Toxic

• Flammable

• Odor

• Limited open time

• Dispensing equipment required

Phenolics

• Good heat resistance

• Good dimensional stability

• Inexpensive

• Modification of toughness by adding elastomeric resins

• Brittle

• Possibility of pollution due to formaldehyde as curing agent

Table 13.3

Typical thermoplastic resins for adhesives and sealants

 

Advantages

Limitations

Acrylate

• Good UV resistance

• Good hydrolysis resistance (better than rubber)

• Good solvent resistance

• Good temperature use range (−45∼260°C)

• Good shear strength

• Long service life

• Poor creep resistance

• Fair initial adhesion

• Moderate cost

Polyvinyl alcohol

• Water soluble resin

• Good wettability to porous substrate such as wood

• Quick set

• Heat seal available

• Good oil/grease resistance

• Odorless/tasteless

• Poor water resistance

• Poor heat resistance

• Poor creep resistance

Ethylene vinyl acetate

• Application to hotmelt

• Good wetting and adhesion

• Good flexibility

• Poor heat resistance

• Poor creep resistance

Polyamide

• Application to hotmelt

• Good heat resistance (better than ethylene vinyl acetate)

• Good resistance to dry cleaning fluids

• Expensive

• Low flexibility

• Poor resistance to impact

Styrene Block Copolymer

• Cheap

• Good heat resistance

• Good toughness

• Low water absorption

• Poor adhesion to nitrile, neoprene, natural rubber

Polyolefins

• Good wettability to various substrates due to low surface energy

• Excellent peel strength

• Various applications

• Heat seal available

• Low molecular weight constituents for commercial grade

• No tacky

Polysulfone

• High heat resistance

• Very tough

• High strength

• Excellent creep resistance

• Good chemical resistance to strong acid and alkali.

• Poor chemical resistance to polar organic solvent and aromatic hydrocarbons

• Low processability

Solvents

Solvents are liquids comprising one or more components that are volatile under the specified drying conditions and can dissolve film-forming agents purely and physically without chemical reactions (DIN EN 971-1 1996). Solvents can lower the viscosity of the formulation to make it easier to apply and to help liquefy the primary resin so that the other additives may be easily incorporated into the formulation (Petrie 2000).

Diluents are defined as ingredients used in conjunction with the true solvent to increase the bulk of another substance without causing precipitation (LeSota 1995). Diluents are mainly used to lower the viscosity and modify the processing conditions of adhesives and sealants. Diluents participate in the partial components of the final adhesives because they have a lower volatility than solvents.

Solvents are used to control the viscosity of the adhesives so that they can be applied more easily. They are also used to aid in formulating the adhesive by reducing the viscosity of the primary resin so that additions of other components and uniform mixing may be achieved more easily. When solvents are used in the adhesive formulation, the similarity of the solubility parameter between the solvent and the primary resin is very important.

The solubility parameter (δ-value) is divided as the following equation:
$$ {\delta_{\text{total}}}{ } = { }\sqrt {{\delta_{\text{D}}^2 + \delta_P^2 + \delta_H^2}} $$
(13.1)
δ D = parameter for nonpolar contribution; δ P = parameter for polar contribution; δ H = parameter for hydrogen bridges contribution.
The indices relate to the nonpolar dispersion forces, the dipole forces, and the interactions caused by hydrogen bridges. Table 13.4 lists the solubility parameters of various solvents (Goldschmidt and Streitberger 2007).
Table 13.4

Solubility parameters δ total of different solvents and their disperse contribution δ D, polar contribution δ P, and hydrogen bridging contribution δ H, (unit in [J/cm3]1/2) (Goldschmidt and Streitberger 2007)

 

δ total

δ D

δ P

δ H

n-Hexene

14.9

14.7

0

0

Cyclohexane

16.8

16.8

0

0

Toluene

18.2

18.0

1.4

2.0

Xylene

18.0

17.8

1.0

3.1

Ethylbenzol

18.0

17.8

0.6

1.4

Styrene

19.0

18.6

1.0

4.1

n-Propnol

24.3

15.1

6.1

17.6

Isopropanol

23.5

15.3

6.1

17.2

n-Butanol

23.3

16.0

6.1

15.8

Isobutanol

21.9

15.3

5.7

15.8

2-Ethylhexanol

19.4

16.0

3.3

11.9

Cyclohexanol

23.3

17.4

4.1

13.5

Diacetonealcohol

18.8

15.8

8.2

10.8

Acetone

20.5

15.6

11.7

4.1

Methylethylketone

19.0

16.0

9.0

5.1

Methylisobutylketone

17.2

15.3

6.1

4.1

Cyclohexanone

19.8

17.8

7.0

7.0

Ethylacetate

18.6

15.1

5.3

9.2

Butylacetate

17.4

15.1

3.7

7.6

Butylcellosolve

18.2

16.0

6.3

12.1

Ethyldiglycol

19.6

16.2

7.6

12.3

Butyldiglycol

18.2

16.0

7.0

10.6

Water

47.8

14.3

16.3

42.6

Polystyrene

19.0

17.5

6.1

4.0

Polyvinylacetate

23.0

18.9

10.1

8.1

Polymethacrylacid methylester

22.0

18.7

10.1

8.5

Polyvinylchloride

22.4

19.1

9.1

7.1

Epoxy resin

23.4

17.3

11.2

11.2

The prediction of the miscibility between the polymer (primary resin) and solvent can be allowed from a comparison of the solubility parameters of the polymer (δ polymer) and solvent (δ solvent) because δ is a measure of the interaction forces between molecules of the material. Therefore, the difference of solubility parameter (δ polymerδ solvent) should be small for good miscibility.

Solvents can be classified into some categories by their chemical character into groups with common features: aliphatic, cycloaliphatic, and aromatic hydrocarbons, esters, ethers, alcohols, glycol ethers, and ketones. Table 13.5 shows the important physical performance indicators of solvents used for the coatings or adhesives such as the density, refractive index, boiling temperature or boiling ranges, and the vapor pressure or evaporation time. In addition, the flash point, ignition point, and explosion limits must also be needed for safety reasons (Goldschmidt and Streitberger 2007).
Table 13.5

Physical and safety-related data of important solvents (Goldschmidt and Streitberger 2007)

Type of solvents

Mol. Mass

Boiling point/region [°C]

Density at 20°C [g/cm2]

Refractive index at 20°C

Evaporation number

Vapor pressure at 20°C [hPa]

Flash point [°C]

Ignition temperature [°C]

Explosion limit [Vol%]

Aliphatic hydrocarbons and mixture

n-Hexane

86.2

65–70

0.675

1.372

1.4

190

−22

240

1.2/7.4

Solvesso 100

123

155–181

0.877

1.502

40–45

3

41

>450

0.8/7.0

Solvesso 150

135

178–209

0.889

1.515

120

1

62

>450

0.6/7.0

Solventnaphta

123

150–195

0.870

1.500

40–45

3

41

>450

0.8/7.0

Benzine 135/180

131

135–175

0.766

1.428

25–30

70

∼22

210

0.6/7.0

Cyclohexane

84.2

80.5–81.5

0.778

1.426

3.5

104

−17

260

1.2/8.3

Aromatic hydrocarbons

Xylene

106.2

137–142

0.874

1.498

17

90

25

562

1.0/7.6

Toluene

92.1

110–111

0.873

1.499

6.1

290

6

569

1.2/7.0

Styrene

104.2

145

0.907

1.547

16

60

31

490

1.0/6.3

Alcohols

Propanol

60.1

97.2

0.804

1.386

16

19

23

360

2.1/13.5

n-Butanol

74.1

117.7

0.811

1.399

33

6.6

34

360

1.4/11.3

Isobutanol

74.1

107.7

0.802

1.396

25

12

28

410

1.5/12

Ethylhexanol

130.2

183.5–185

0.833

1.432

600

0.5

76

250

1.1/12.7

Isotridecylalcohol

200.2

242–262

0.845

1.448

>2,000

<0.01

115

250

0.6/4.5

Glycol ether

Butylglycol

118.2

167–173

0.901

1.419

163

1

67

240

1.1/10.6

Propylglycol

104.2

150.5

0.911

1.414

75

2

51

235

1.3/15.8

Hexylglycol

146.2

208

0.887

1.429

ca. 1,200

0.08

91

220

1.2/8.4

Methyldiglycol

120.2

194.2

1.021

1.424

576

0.3

90

215

1.6/16.1

Butyldiglycol

162.2

224–234

0.956

1.431

>1,200

0.1

98

225

0.7/5.3

Methoxipropanol

90.1

122.8

0.934

1.403

25

 

38

270

1.7/11.5

Ethoxipropanol

104.1

132

0.896

 

33

<10

40

255

1.3/1.2

Esters

Butylacetate

116.2

123–127

0.880

1.394

11

13

25

400

1.2/7.5

Ethylethoxyproprionate

146.2

170

0.943

 

96

2

59

327

1.05/

Ethylacetate

88.1

76–78

0.900

1.372

2.9

97

−4

460

2.1/11.5

Isobutylacetate

116.2

114–118

0.871

1.390

8

18

19

400

1.6/10.5

Ethoxypropylacetate

146.2

158

0.941

1.405

 

2.27

54

325

1.0/9.8

Methoxypropylacetate

132.2

143–149

0.965

1.402

33

4.2

45

315

1.5/10.8

Pentylacetate

130.2

146

0.876

1.405

14

6

23

380

1.1/

Butylglycolacetate

160.2

190–198

0.945

1.415

250

0.4

75

280

1.7/8.4

Ketones

Methylisobutylketone

100.2

115.9

0.800

1.396

7

20

14

475

1.7/9.0

Acetone

58.1

56.2

0.792

1.359

2

245

−19

540

2.1/13

Cyclohexanone

98.2

155

0.945

1.451

40

3.5

44

455

1.1/7.9

Methylethylketone

72.1

79.6

0.808

1.379

2.6

96

−14

514

1.8/11.5

Isophorone

138.2

215

0.922

1.478

230

3

96

460

0.8/3.8

Others

Water

18

100

1.000

   

Propylencarbonate

102.2

242

1.208

 

>1,000

 

123

  
General solvent contents used for typical sealants are shown in Table 13.6 (Petrie 2000).
Table 13.6

General solvent contents for typical sealants (Petrie 2000)

Base resin

Solvent content,%

Polymer content,%

Acrylic

20–40

35–45

Latex emulsion

35–45

35–45

Polysulfide

0

30–45

Silicone

0

60–85

Urethane

0

30–45

The use of excessive solvent may cause a shrinkage problem when the sealant cures. Volume shrinkage will always be greater than the weight percent of solvent due to its much lower density than other components in the sealant. Toluene, xylene, petroleum spirits, water, and others are used as solvents in sealant formulations. In case of solvent mixtures, the balance of volatility between solvents is very important to avoid a trouble such as the sealant’s skin drying problem.

Fillers

Fillers are generally relatively non-adhesive materials added to the adhesive formulation to enhance mechanical strengths, thermal properties, adhesion performance, etc. Common fillers in conductive adhesives are listed in Table 13.7 . One of the purposes of using fillers is lowering the cost. The incorporation of fillers into the adhesive reduces the resin content and thus the product cost is also reduced. However, fillers can change adhesive properties. Filler selection and loading level are very important in formulation of adhesive because adhesive properties depend on filler type, size, shape, and volume contents. Common fillers are metal powder, kind of clay, dust, glass fiber, alumina, and so on. Sometimes fillers act as extenders or reinforcement materials.
Table 13.7

Filler for conductive adhesive in common adhesive formulations

Filler

Improvement in adhesive formulation

Aluminum

Electrical conductivity, thermal conductivity

Silver

Electrical conductivity

Carbon black

Electrical conductivity, color

Graphite

Electrical conductivity, lubricity

Copper

Electrical conductivity, machinability

Nickel

Electrical conductivity

Iron

Electrical conductivity, abrasion resistance

Chromium

Electrical conductivity

Molybdenum

Electrical conductivity

Tungsten

Electrical conductivity

Normally, adhesive can be conductive by adding conducting filler particles. The resin provides an interaction bond between substrates and conducting fillers as well. However, it is the conducting filler that provides the desired electrical interconnection path as depicted in Fig. 13.2 . To make conducting adhesives, the fillers must be in physical contact with each other. Thus, the electrical conductivity increases sharply when a percolation threshold level of well-dispersed conducting filler is accomplished. Figure 13.3 shows the resistivity versus conductive filler level (Petrie 2008).
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Fig. 13.2

Conductivity depends on contact between filler particles within the adhesive (Petrie 2008)

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Fig. 13.3

Conductive filler level versus resistivity in conducting adhesive formulation (Petrie 2008)

Alumina particles are one of the commonly used fillers for improving the thermal conductivity of adhesives in particular insulation adhesives. Aluminum and silver powders or flakes are used to improve the thermal and electrical conductivities for adhesives intended to be an electrical or thermal path. The filler volume content level is very important to get sufficient conductivity. However, excessive filler content might cause degradation in mechanical properties of the adhesives (Kahraman and Al-Harthi 2005; Kahraman et al. 2008).

One of the most effective techniques used to improve the electrical conductivity of polymers is the incorporation of conductive fillers in the polymer matrix. The most popular electrically conductive filler is silver due to its moderate cost and superior conductivity. Silver-coated inorganic particles and fibers are superior compared to carbon particles and fibers as components in epoxy-based adhesives regarding the electrical conductivity of the composite; because the electrical conductivity of silver is much higher than that of carbon. In addition to the high electrical conductivity of silver, the addition/application of silver-plated particles and fibers also leads to composites with high mechanical strength and modulus, low weight, and a high ratio of metal-plated fibers (Novák et al. 2004).

Among the various conductive particles, silver particle is probably the most common filler because of its excellent conductivity and chemical durability. Silver particles are easy to precipitate into a wide range of controllable sizes and shapes, so it can be used to change the percolation threshold of the adhesive formulation. Silver also exhibits high conductivity; however, the silver filler is expensive (Lin et al. 2009).

Graphite is usually used as an electro conductive filler due to its low cost and natural electrical conductivity. It also has a positive influence on the mechanical properties, as well as thermal and dimensional stability (Lin et al. 2009). Substitution of carbon black with renewable filler has been investigated in recent years. Carbon black has some advantages such as low cost, low density, high electrical conductivity, and, in particular, specific structures that enable the formation of conductive network (Wan et al. 2005; Jong 2007).

Copper has good electrical conductivity and high adhesion property in conductive adhesive systems. However, copper tends to form a nonconductive oxide surface layer (Zhao et al. 2007).

Nickel is a good nominee for conductive filler in conducting adhesives because of the numerous benefits it possesses. Nickel shows chemical stability and oxidizes relatively slowly compared to copper. But, nickel has disadvantages compared to silver: nickel has a higher electrical resistance than silver (about 25% of silver). However, it is less expensive than silver filler (Goh et al. 2006).

There are many conducting fillers for adhesive formulation. Apart from the ones mentioned above, iron, chromium, molybdenum, tungsten, and other metal particles can be used as fillers for conducting adhesives.

Many other inorganic fillers are used for adding a function to adhesive. Table 13.8 shows the additional function of fillers. Aluminum oxide (flame retardant), lead (radiation shielding), mica or clay (electrical resistance), and silica and silicon carbide (abrasion resistance) are used to add additional function to the adhesive formulation.
Table 13.8

Filler for other function in common adhesive formulations

Filler

Improvement in adhesive formulation

Aluminum oxide

Flame retardant

Lead

Radiation shielding

Mica

Electrical resistance

Phenolic microspheres

Decrease density

Silica sand

Abrasion

Silicon carbide

Abrasion resistance

Titanium dioxide

Color

Zinc

Corrosion resistance

Generally, fillers in sealant formulation are used as additives to boost the viscosity of the sealant and get better gap-filling properties and to reduce the material cost of the sealants. In sealant formulation, fillers cannot affect reinforcement and improved strength. However, they can affect other properties such as water resistance and hardness, etc. The most common filler used in sealant is calcium carbonate, because of its many advantages such as abundant resource, low cost, and stability. Other fillers are clays, silica, titanium dioxide, carbon black, and iron oxide. Table 13.9 lists commonly used fillers in sealant formulations and functions in sealant systems.
Table 13.9

Fillers commonly used in sealants

Filler

Improvement in sealant formulation

Calcium carbonate

Adjustment of mechanical properties, thixotropic agent

Silica

Common for silicon resin

Clay

Mechanical modifier

Titanium dioxide

Colorant (for white)

Carbon black

Colorant (for black)

Iron oxides

Colorant

Glass fiber

Thixotropic agent

Fillers represent the largest part in terms of weight for many sealants. Calcium carbonate is the most widely used filler for sealant formulations. As a filler for sealants, calcium carbonate acts as an inert extender to reduce formulation cost, modifier for mechanical properties, and a rheological modifier. Normally, properties of fillers such as particle size, shape, and surface properties are very important factors for sealant properties. Figure 13.4 shows the typical effect of particle size on the viscosity in common adhesives (Chew 2003). Filler volume contents can vary significantly with being dependent on the primary resin and the formulation.
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Fig. 13.4

The viscosity versus particle size in common adhesives (Chew 2003)

Plasticizers

Plasticizers are substances of low or negligible volatility that lower the softening range and increase workability, flexibility, or extensibility of a polymer (ASTM D907-08b 2008). The main purpose of plasticizers is to modify the property of adhesives and sealants. The addition of plasticizers causes the improvement of flow and flexibility, but the reduction of the elastic modulus, stiffness, hardness, and the glass transition temperature (T g). As a result, the processibility and extrudability of adhesives and sealants can be improved by addition of plasticizers (Tracton 2007; Stoye and Freitag 1998).

Plasticizers also affect the adhesion property. Addition of plasticizer to the adhesive will always lower the cohesive strength, generally reduce the peel adhesion, and will have a variable effect on the tack depending on the type of plasticizer used (Satas 1999). Plasticizers must be compatible with other adhesive ingredients because of their inherent chemical characteristics and nonreactivity with other components (Petrie 2000).

Plasticizers and solvents are governed by the same laws of solubility and have the ability to increase the free volume of the polymer. However, solvents mostly serve as viscosity modifiers and plasticizers are used to modify the properties of the adhesives or sealants, such as softening and lowering the T g (Petrie 2000).

Plasticizers have polar and nonpolar components in the molecular structure. The polar components interact with the polar groups of the primary resins. Otherwise, the nonpolar components prevent the intermolecular interaction between plasticizers and the molecules of primary resins by steric hindrance. As a result, the mobility of adhesives or sealants can be promoted. Due to the same reason, T g is shifted to a lower temperature with an increase of the plasticizer content as shown in Fig. 13.5 (Goldschmidt and Streitberger 2007).
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Fig. 13.5

Lowering the glass transition temperature (T g) of a polymer by a plasticizer (Goldschmidt and Streitberger 2007)

When a compatible oil is added to an elastomer such as natural rubber, it acts as a plasticizer. The storage modulus (G′) of a natural rubber/aliphatic oil adhesive decreased at all frequencies with the increasing amount of plasticizing oil as shown in Fig. 13.6 (Satas 1999).
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Fig. 13.6

Effect of plasticizing oil on the storage modulus (G′) of natural rubber/aliphatic oil as function of frequency at 25°C (Satas 1999)

A variety of plasticizers can be used in adhesives and sealants as to their primary resin type. Paraffinic oils, phthalate esters, and polybutenes are typical plasticizers (Dostal 1990). Plasticizers for natural rubber adhesives, such as mineral oil or lanolin, are used to reduce the cost of the adhesive mass, and have a depressing effect on the peel adhesion (Satas 1999). Phthalates, chlorinated hydrocarbons, and aliphatic hydrocarbons are commonly used as plasticizers in urethane sealants (Dostal 1990). Most of sealants, except for silicones, contain plasticizers in their formulations. Silicone sealants can be plasticized only by low molecular weight silicone oils (Petrie 2000).

Reinforcements

Reinforcements are used to enhance mainly mechanical properties. Many reinforcements are now available, and some are designed for a particular primary resin. There are various reinforcements for adhesive formulation. Generally, reinforcements act as the polymer resins in an adhesive system and reinforcing the internal bonding strength of the adhesive. Figure 13.7 depicts a schematic diagram of an adhesive with reinforcements. Reinforcements act like crosslink agents. It means that the reinforcement reduces the strain in a shear test and can enhance the mechanical strength of the adhesive. The main reinforcements in adhesive formulations are listed in Table 13.10 .
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Fig. 13.7

Schematic diagram of adhesive at shear strength test: (a) without reinforcement and (b) with reinforcement

Table 13.10

Common reinforcement agents

Reinforcement

Main function

Clay

Mechanical modifier

CNT

Mechanical modifier, electrical conductivity

Graphite

Mechanical modifier

Carbon black

Mechanical modifier, colorant

Recently, reinforcement agents in adhesive formulations have received attention due to very high reinforcement ability, simple preparation, and cost. One of the agents is inorganic clay which acts as a modifier of the mechanical and barrier properties. However, clay has some disadvantages for application as a reinforcement agent because of aggregation in the polymer resin. Therefore, many researchers have modified the surface of the clay. Figure 13.8 shows the clay loading contents versus lap shear strength. It can be seen that the modified clay gives an improved lap shear strength in relation to that of the pure clay (Maji et al. 2009; Osman et al. 2003).
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Fig. 13.8

Lap shear strength versus clay contents in polyurethane adhesive between aluminum and aluminum with PU adhesives (Maji et al. 2009)

Carbon nanotube (CNT) has excellent mechanical properties. CNT is very similar to graphite. A single-walled carbon nanotube (SWCNT) can be seen as a rolled sheet of graphite, while a multi-walled carbon nanotube (MWCNT) can be viewed as layers of many graphite sheets. Moreover, the stiffness is very high: SWCNT’s Young’s modulus is about 1 TPa and the shear modulus to be 0.45 TPa. Many researchers have used CNT to enhance the mechanical properties of adhesives. Figure 13.9 depicts the shear strength of an epoxy adhesive with different MWCNT contents (Hsiao et al. 2003).
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Fig. 13.9

The average shear strength of epoxy adhesive with different MWCNT weight fraction (Hsiao et al. 2003)

Sometimes, the reinforcement is a carrier in the adhesive composition such as in tapes and films. A carrier is usually a thin fiber fabric, cloth, or paper material. In pressure sensitive adhesives, the carrier is a backing substance for the adhesive being applied. Usually, the backing materials are used for functional or decorative reasons. Glass, nylon fabric, polyester, and paper are common carriers for adhesives. In this case, the carrier acts as a “reinforcement” to ease the use of adhesive. Figure 13.10 shows a schematic diagram of the carrier for an adhesive tape or film. A carrier is very useful to b-staged adhesive systems because of their semi-cured formulation. The carrier can act as support for adhesive.
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Fig. 13.10

Carrier for adhesive

Other Additives

Additives are functional chemicals added to adhesives to ease the process and improve some properties. Many additives, like antioxidants, thermal stabilizers, UV stabilizers, polymer processing aids, anti-blocking additives, slip additives, antifogging agents, antistatic additives, flame retardants, and colorants, have been used for centuries. Recently, additives are used as special functional materials to improve important properties of final products; hence additives application technology is essential for adhesive development.

Antioxidant is any substance that delays or inhibits oxidation during the adhesive manufacturing process, storage, and application. Oxidation is a chemical reaction that transfers electrons from a substance to an oxidizing agent. As a result, reaction produces free radicals (Zweifel 2004). Oxidation during compounding or processing can cause problems such as loss of strength, breakdown, or discoloration. Oxidation can also occur in the final product causing discoloration, scratching, and loss of strength, flexibility, stiffness, or gloss. Antioxidants have some ability to terminate radical chain reactions by removing free radical intermediates or inhibiting other oxidation reactions. Antioxidants can be classified by the chemical structure such as phenol type, aromatic amine type, thioester type, phosphate type, etc.

For example, hotmelt adhesives are 100% solid thermoplastic compounds that are solids at room temperature, but they are changed to liquid when heated to the melting temperature. When applied, hotmelts bond and cool rapidly. To make EVA based hotmelt adhesives, mixing process at high temperature is essential. To reduce the thermal degradation of hotmelt adhesives, 1–2 parts of a phenolic antioxidant by weight were used as a thermal stabilizer (Park et al. 2003; Park et al. 2006).

A UV stabilizer is a substance to prevent discoloration, surface crack, and decreasing of the mechanical properties by UV radiation that has a high energy wave in the range of 290–400 nm. In particular, UV stabilizers are essential additives for transparent plastics for outdoor application. UV stabilizers can be classified by chemical reaction, such as UV absorption, quenchers, and HALS (hindered amine light stabilizer). UV absorbers absorb the harmful UV radiation and dissipate it so that it does not lead to photosensitization. Typical UV absorbers are hydroxyl benzophenone, benzotriazoles, and modified acrylate. Quenchers are light stabilizers that are able to take over the energy absorbed by the chromophores present in plastic material (Bellus 1971). HALS is a radical scavenger that represents the most important development in light stabilization for many polymers. HALS acts by scavenging the radical intermediates formed in the photo-oxidation process. HALS’ high efficiency and longevity are due to a cyclic process wherein the HALS is regenerated rather than consumed during the stabilization process. HALS also protects polymers from thermal degradation and can be used as a thermal stabilizer.

Table 13.11 shows an example of formulation of the UV-curable coatings, which is composed of isocyanate and acrylated urethane oligomers (Lee and Kim 2006). In addition, Fig. 13.11 shows chemical structures of light stabilizers. UV-cured film with both Tinuvin 384-2 and Tinuvin 292 showed the slight change of optical and mechanical properties. In the case of most UV-cured films, using a UV stabilizer is helpful to discoloration, gloss, and hardness properties after weathering.
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Fig. 13.11

Chemical structures of light stabilizers

Table 13.11

Formulation of UV-curable coatings

Components

Compositions (wt.%)

Sample #1

Sample #2

Sample #3

Sample #4

Sample #5

Sample #6

Sample #7

Oligomers

Ebecryl 210

23.5

57

55.2

55.2

55.2

45

Ebecryl 270

57

23.5

Monomer

Miramer M200

38

38

38

36.8

36.8

36.8

30

Photoinitiator

Micure CP-4

5

5

5

3

3

3

3

Irgacure 819

 

2

2

2

2

Light stabilizer

Tinuvin 384-2

 

3

 

2

Tinuvin 292

 

3

1

Pigment

Dupont R706

 

20

Total (wt.%)

 

100

100

100

100

100

100

100

Ebecryl 210 (aromatic urethane diacrylate)

Ebecryl 270 (aliphatic urethane diacrylate)

Miramer M200 (1,6-hexanediol diacrylate)

Micure CP-4 (1-hydroxy-cyclohexyl-phenyl-ketone)

Irgacure 819 (bis(2,4,6-trimethylbenzoyl)-phenylphosphine oxide)

Tinuvin 384-2(95% benzenepropanoicacid, 3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)-4-hydroxy-,C7e9-branched and linear alkyl esters, 5% 1-methoxy2-propyl acetate (UV absorber type))

Tinuvin 292 (mixture of bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-sebacate and 1-(methyl)-8-(1,2,2,6,6-pentamethyl-4-piperidinyl)-sebacate (hindered amine light stabilizer type))

Dupont R706 (titanium dioxide)

Flame retardants are materials that inhibit or resist the spread of fire. Flame retardants can remove thermal energy from the substrate by functioning as a heat sink or by participating in char formation as heat barrier. The additives can also provide flame retardancy by conduction, evaporation, or mass dilution or by participating in chemical reactions (Zweifel 2004).

Adhesives can be separated into two general classes: thermoplastics and thermosets. Thermoplastics can be formulated with halogen-containing and non-halogen-containing additives. Thermosets are commonly treated by adding flame retardants that chemically react with a resin precursor. Table 13.12 lists flame retardants that are typically used.
Table 13.12

Common flame retardants used within thermoset plastics

Resin

Flame retardant

Flame retardant level

Epoxy

Tetrabromobisphenol-A

18 wt.% Br

Unsaturated polyester

Tetrabromophthalic anhydride

Chlorendic acid/anhydride

10∼22 wt.% Br

15∼29 wt.% Cl

Polyurethane

Tetrabromophthalate diols

Pentabromodiphenyloxide

Dibromoneopentylglycol

15∼28%

6∼18%

5∼15%

Tackifiers are used in formulating rubber based on adhesives to improve the tack property. Tackifiers are low molecular weight compounds with high T g. There are two classes of tackifiers: the rosin derivatives and the hydrocarbon resins. The rosin derivatives include the rosins, modified rosins, and rosin ester. The hydrocarbon resins consist of low molecular weight polymers derived from petroleum, coal, and plants. The miscibility between tackifiers and the adhesives is important to choose a tackifier. The viscoelastic property of adhesives can be modified by blending of a miscible tackifier.

A curing agent or hardener is a substance added to an adhesive to promote the curing reaction. These affect curing reaction by chemically combining with the base resin and becoming part of the final polymer molecule. They are specifically chosen to react with a certain resin (ASTM D907-08b 2008). Curing agents will have an important effect on the curing features and on the fundamental properties on the adhesive system. A polyamide resin that is used in room-temperature curing epoxy adhesive system is an example of a curing agent (Petrie 2000). Polyamide curing agents are used in most “general-purpose” epoxy adhesives. They offer a room-temperature cure and bond well to many substrates that include elastomers, glass, and plastics. The polyamide-cured epoxy also provides a comparatively flexible adhesive with moisture resistance, fair peel strength, and thermal cycling properties. In general, mixing ratio is not critical. Within limits, the greater the quantity of polyamide in an epoxy formulation, the greater the flexibility, impact strength, and peel strength. However, T g is decreased as are the shear strength and temperature resistance. There are some polyamides with changing viscosity. The reaction with conventional di-glycidyl ether of bisphenol A (DGEPA) epoxy resins yields a comparatively low degree of exotherm. In Table 13.13 , the distinct features of curing agents used with epoxy resins in adhesive formulations are briefly stated (Petrie 2000).
Table 13.13

Characteristics of curing agents used with epoxy resins in adhesive formulations

Curing agent

Physical form

Amount requireda

Cure temp (°C)

Pot life at 24°Cb

Complete cure conditions

Max use temp

Triethylenetetramine

Liquid

11–13

21–135

30 min

7 days (24°C)

71

Diethylentriamine

Liquid

10–12

21–93

30 min

7 days (24°C)

71

Diethylaminopropylamine

Liquid

6–8

28–149

5 h

30 min (24°C)

85

Metaphenylenediamine

Solid

12–14

65–204

8 h

1 h (85°C)

149

2 h (163°C)

Methylene dianiline

Solid

26–30

65–204

8 h

1 h (85°C)

149

2 h (163°C)

Boron trifluoride monoethylamine

Solid

1–4

135–204

6 months

3 h (163°C)

163

Methyl nadic anhydride

Liquid

80–100

121–38

5 days

3 h (160°C)

163

Triethylamine

Liquid

11–13

21–135

30 min

7 days (24°C)

82

Polyamides

Amine value 80–90

Semisolid

30–70

21–149

5 h

5 days (24°C)

c

Amine value 210–230

Liquid

30–70

21–149

5 h

5 days (24°C)

c

Amine value 290–320

Liquid

30–70

21–149

5 h

5 days (24°C)

c

aPer 100 parts by weight; for an epoxy resin with an epoxide equivalent of 180–190

bFive hundred gram per batch; with a bisphenol A-epichlorohydrin derived epoxy resin with an epoxide equivalent of 180–190

cHighly dependent on concentration

Catalysts are substances that markedly speed up the cure of an adhesive when added in a minor quantity compared to the amounts of the primary reactants (ASTM D907-08b 2008). Solidification and crosslink of the primary resins are caused by catalysts. The commonly used catalysts are acids, bases, salts, sulfur compounds, and peroxides. To influence curing, only small quantities are needed (Petrie 2000). Table 13.14 shows the lists of some catalysts and the reactions they catalyze (Hare 1994). The curing of the adhesives or sealants by transformation into a hardened state is fulfilled by chemical methods, for instance, oxidation, vulcanization, polymerization, or by physical action, such as evaporation of the solvents (Cognard 2005).
Table 13.14

Catalysts and reaction catalyzed

Catalyst

Reaction catalyzed

Base

Tris (dimethylaminomethyl phenol) (and its tri-2 ethylhexanoic acid salt)

Epoxy/polyamide, epoxy/polyamine, epoxy/novolac, epoxy/polysulfide, epoxy/epoxy

Dimetylaminomethyl phenol

Epoxy/polyamide or polyamine, epoxy/novolac, epoxy/polysulfide, epoxy/epoxy

Diazabicydoudecene and its ethyl hexanoic salt

Epoxy/novolac, epoxy/anhydrides

Nonyl phenol, phenol

Epoxy/polyamide, epoxy/polyamine

Benzyl dimethylamine

Epoxy/polyamide

1-Propylimidazle, 2-methylimidazole

2-Ehtyl-4-methyl imidazole and derivatives

Epoxy/epoxy, epoxy/dicyandiamide, epoxy/anhydride

Triethylene diamine

Epoxy/epoxy, epoxy/novolac, epoxy/amine, epoxy/acrylic, epoxy/polyester, isocyanate/hydroxyl (polyesters and acrylics)

Quaternary bases (benzyltrimethylammonium chloride)

Epoxy/dicyandiamide, epoxy/anhydride, epoxy/phenol

Acid

Paratoluene sulfonic acid (and its morpholine salt)

Epoxy/amino, epoxy/phenolic resole, alkyd/amino

Phosphoric acid

Epoxy/amino, epoxy/phenolic resole, alkyd/amino

Butyl phosphoric acid

Epoxy/amino, epoxy/phenolic resole, alkyd/amino

Acid ethyl phosphate

Epoxy/amino, epoxy/phenolic resole, alkyd/amino

Boron trifluoride monoethylamine

Epoxy/epoxy, epoxy/anhydride

Salicylic acid

Epoxy/amine (particularly cycloaliphatic and aromatic amines)

Metallics

Dibutyl tin dilaurate

Isocyanate/hydroxyl

Cobalt naphthenate, octoate systems, etc.

Isocyanate (aromatic)/hydroxyl, oxidizing, vinyl ester, acrylated epoxies and acrylated urethanes, silicone/silicone, polyester/polyester, cyanate ester trimerization, cyanate ester/epoxy

Tin naphthenate, octoate, etc.

Isocyanate/hydroxyl

Zinc naphthenate, octoate systems, etc.

Isocyanate (aromatic)/hydroxyl, oxidizing, cyanate ester trimerization, silicone/silicone, cyanate ester/epoxy

Manganese naphthenate, octoate, etc.

Isocyanate/hydroxyl, oxidizing system, cyanate ester trimerization, silicone/silicone, cyanate ester/epoxy

Ziroconium naphthenate, octoate, etc.

Isocyanate/hydroxyl, oxidizing system

Acetylacetonates of aluminum, chromium, iron, cobalt, copper acetylacetonates of zinc, copper, iron

Polyester, vinyl ester and acrylate polymerization and crosslinking, epoxy/epoxy vinyl ester (catalyzed with metallics) urethane reactions

Acetylacetonates of zinc

Trimerization of nitriles, epoxy/cyanate ester

Acetylacetonates of aluminum, copper, vanadium

Esterification reactions

Peroxides (as initiators)

Methyl ethyl ketone peroxide

Polyester/polyester, vinyl ester, acrylated epoxies and urethanes (catalyzed with metallics)

Cumene peroxide

acetylacetonates of zinc, copper, iron

Polyester/polyester acrylated epoxies and urethanes, vinyl ester (catalyzed with metallics)

Conclusions

There are various commercial adhesives in the market area and various requirements for a specific application: controlling flow, extending temperature range, improving toughness, lowering the coefficient of thermal expansion, reducing shrinkage, increasing tack, modifying electrical and thermal conductivity, etc. To meet specific requirements, the selection of a primary resin and its minor components is very important. Understanding the chemical composition of an adhesive or sealant is very useful to R&D centers, suppliers, and customers of the adhesives.

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