Reference Work Entry

Handbook of Adhesion Technology

pp 1505-1526

Recycling and Environmental Aspects

  • Chiaki SatoAffiliated withPrecision and Intelligence Laboratory, Tokyo Institute of Technology Email author 

Abstract

Recycling and environmental aspects of adhesion technology are discussed in this chapter. Adhesively bonded adherends should be often separated before they can be recycled. For this purpose, dismantlable adhesives, which can be separated with stimulations, have been developed recently. There are lots of technical processes to realize such adhesives. For example, softening of adhesive, expansion force due to blowing agents or thermally expandable microcapsules, chemical degradation, electrochemical reaction, and taking advantage of interfacial phenomena can be applied to dismantlable adhesives. These adhesives have many applications such as temporary joints of work materials, shoe making, integrated circuits (IC) chip fabrication, electronics, housing construction, car fabrication, and so on. The most used trigger to start the separation of these adhesives is heating. Many kinds of methods including infrared, induction, and microwave heating are recently available. In addition, novel dismantlable adhesives, which have anisotropy in strength or are triggered by electric currents, have been presented.

Abstract

Recycling and environmental aspects of adhesion technology are discussed in this chapter. Adhesively bonded adherends should be often separated before they can be recycled. For this purpose, dismantlable adhesives, which can be separated with stimulations, have been developed recently. There are lots of technical processes to realize such adhesives. For example, softening of adhesive, expansion force due to blowing agents or thermally expandable microcapsules, chemical degradation, electrochemical reaction, and taking advantage of interfacial phenomena can be applied to dismantlable adhesives. These adhesives have many applications such as temporary joints of work materials, shoe making, integrated circuits (IC) chip fabrication, electronics, housing construction, car fabrication, and so on. The most used trigger to start the separation of these adhesives is heating. Many kinds of methods including infrared, induction, and microwave heating are recently available. In addition, novel dismantlable adhesives, which have anisotropy in strength or are triggered by electric currents, have been presented.

Introduction

Since adhesive bonding has many advantages such as low cost, light weight, easy application, etc., it is increasingly being used by modern industries. Adhesive bonding is more desirable than the other joining methods in terms of energy saving, so that its environmental load is small. However, the consumption of adhesives has been increasing drastically due to the industry globalization and the recent fast-growing of developing countries, and the total environmental load of adhesive bonding is also increasing. To meet environmental issues, the total environmental loads, although they are small, have to be reduced by all possible means.

Adhesive bonding has many aspects related to the environmental issue. One of them is energy consumption in some processes such as production, transportation, application, and curing of adhesives. Another one is recycling of adhesively bonded materials. Strictly speaking, there are other aspects such as recycling of adhesive itself and environmental contamination including vaporized organic chemical compounds (VOC). This chapter treats the recycling issue of adhesively bonded materials. The recycling of the adhesive itself is not dealt with because it is very difficult so far.

Adhesively bonded joints are not easy to separate if they are bonded with strong adhesives or the bonded area is large. Therefore, dismantlability of the joints is not required contrarily to the other joining methods such as mechanical fastening. The fact leads to scarce use of adhesion for substrates that have to be separated. However, a new type of adhesive called a “dismantlable adhesive” has been recently required to meet the demands to separate the adherend for recycling. For instance, a dismantlable design for a product is essential because the materials bonded together become bad wastes difficult to be recycled. However, if the materials can be separated, they become resources. The separation of parts is important even if they are bonded adhesively. Thus, dismantlable adhesives, which can be separated when and as we like, are increasingly required. An advantage of adhesive bonding is the ability to join dissimilar materials. However, it becomes a disadvantage if such materials cannot be recycled. Dismantlable adhesives are also appropriate to solve this problem.

Dismantlable adhesives, however, are thought difficult to realize because a good adhesive has been apprehended as strong and not being able to separate so far. The effort to realize a good adhesive has been concentrated into this point. The capability to separate when we like is a totally different concept from the past efforts.

As a matter of fact, dismantlable adhesives, if they are weak in strength, have been present since fairly early times. Hot-melt adhesives, for instance, can be separated by heating. However, strong adhesives, especially structural adhesives, present a high technical hurdle to be dismantlable. The trend has been changing for the last decade and many dismantlable adhesives have been proposed as a kind of novel functional adhesives. Although their applications are mainly temporary joints of works or product reworks so far, adherend recycling has become an issue recently. In addition, the adhesives are applied to make separation mechanisms for aircrafts, space crafts, or robots. This chapter deals with dismantlable adhesives, whose development has proceeded very fast recently, and shows the results of recent research and technical trends.

Impact of Adhesion Bonding in the Environmental Issues

Categories of Environmental Issues

The main environmental issues can be categorized in three following items:
  1. 1.

    Hazardous material emission

     
  2. 2.

    Greenhouse gas emission

     
  3. 3.

    Use of finite resources

     

In this chapter, hazardous material emission is not treated (see Chap.​ 39). However, these items are not independent from each other and it is difficult to discuss them separately. Thus, the first item will be shortly referred in this chapter too.

Global warming is the most serious and important issue among environmental issues. There is a lot of theory or controversy on the effect brought by the problem and the time limit that we have for preventing vital results. However, it is certain that the final result is devastating to the global environment unless we can reduce, or sustain in the worst case, greenhouse gas emission. In this case, irreversible warming must occur within some decades. Although it is very rare for adhesives to emit greenhouse gases directly, the gases, mainly carbon dioxide, are possibly emitted during their production, application, curing, or disposal processes.

The third item, use of finite resources, may seem different from environmental issues. However, this should be included in environmental issues because the most important finite resource is fossil fuels such as petrol, and their combustion causes carbon dioxide. In addition, avoiding the use of finite resources without their recycling is indispensable to realize sustainable society. Form this viewpoint, this item is also discussed in this chapter.

Greenhouse Gas Emission

The main greenhouse gas emitted during the processes of adhesive production, use, and disposal is carbon dioxide. In the production process of adhesives, energy due to fossil fuel may be used and this causes carbon dioxide. Energy should be used in the curing process of adhesives too. Since the raw materials of adhesives are mainly made from petrol, carbon dioxide should also be produced by incineration in the disposal process. In addition, the other greenhouse gases such as hydrocarbons or organic solvents may be emitted to the atmosphere during the production or application process of adhesives. Such greenhouse gases have higher warming effects per unit than carbon dioxide. Therefore, even their total amount of emission is small, attention should be paid.

Use of Finite Resources

Some raw materials of adhesives are derived from petrol. Therefore, if petrol resources are depleted, the production of adhesive cannot be maintained. This depletion is an ultimate situation, but even the price rise of petrol can give serious and negative impact to adhesive industries. An alternative option is the use of other fossil carbon resources such as coal. Coal is more abundant than petrol in terms of reserve. However, its carbon dioxide emission per unit is larger than that of petrol. Anyway, true sustainable societies cannot be realized while finite fossil resources such as petrol or coal are widely and often used. Energy consumed in the production process of adhesive and its curing process may be derived from fossil fuels too.

Basic Strategies to Meet the Challenge for Environmental Issues

The basic strategies to solve the environmental problems can be categorized as follows:
  1. 1.

    Use of eco-conscious materials

     
  2. 2.

    Provide reworkability or dismantlability to adhesives

     
  3. 3.

    Recycling of adhesives itself

     
  4. 4.

    Optimization of the bonding process

     
  5. 5.

    Design optimization and lifetime cycle assessment of adhesively bonded joints

     

In this section, they are explained respectively.

Use of Eco-Conscious Materials

Selection of raw materials for adhesives is very important to reduce environmental loads. At first, the raw materials should be environment friendly. Synthetic polymers have relatively larger energy consumption in this production process than other materials such as fillers, including alumina powders, silica aerogels, calcium carbonate or talc consisting of minerals. Therefore, the composition of an adhesive is influential to the total environmental loads. In addition, if an adhesive is burned in the disposal process of the product bonded with it, carbon dioxide is emitted. As seen above, the presence of fillers can help to reduce the carbon dioxide emission because the portion of polymer in the adhesive decreases.

In order to reduce more effectively the environmental loads, use of recycled materials or carbon-neutral natural materials should be considered. Reclaimed rubbers are often used as the raw material of Pressure Sensitive Adhesive (PSA). Polystyrene foam used as shipping supplies can be solved with solvents and used as a raw material for adhesives. As such, recycled materials can be used to produce adhesives for the purposes of reducing environmental impacts and costs.

Use of carbon-neutral natural materials is another option. Since they absorb carbon dioxide from the air during their glowing, the total amount of carbon dioxide in the atmosphere is not changed even if these materials are burned. Historically, natural materials are mainly used as raw materials for adhesives. For instance, glue, lacquer, and casein have been widely used to make adhesives (Fay 2005). They are natural materials and carbon-neutral. The other example of natural material use is the case of PSA. To make PSA, natural rubbers for the main component and rosins for tackifiers, which are derived from plants, are often used. They are also carbon-neutral. Recently, other natural materials have been investigated for raw materials of adhesives. For example, polylactide resin can be applied to the main components of adhesives (Viljanmaa et al. 2002). Lignophenolic resins derived from lumber (Kadota et al. 2004) and chitosan resin from exoskeleton of crustacean are other examples (Yamada et al. 2000; Umemura et al. 2003). Soy beans oils can be epoxidized and used for adhesive main components (Ratna and Banthia 2000). The powder of lumbers such as cork is promising as fillers for adhesives because it can give ductility to the cured bulk of adhesive (Jos and Leite 2007).

Reworkability or Dismantlability of Adhesives

The key word “3R” is often referred when environmental issues are discussed. This word means “reduce,” “reuse,” and “recycle.” In order to reuse or recycle adhesively bonded materials, joint separation is often required. To meet this demand, a new type of adhesive called “dismantlable adhesive,” which can be separated on demand, has been invented.

Defective products due to malfunctioned parts bonded adhesively or to inappropriate adhesion are very bad in terms of environment protection because they become a waste without any use. Therefore, dismantlable adhesive can be used to avoid the situation because bonded parts or inappropriate bonded areas can be separated in order to be repaired. This process is called “reworking” of defect products and involved in the processes of “reducing” wastes and “reusing” other nondefective parts in terms of 3R. This topic will be discussed in more detail in Sect. 58.4.

Recycling of the Adhesive Itself

Actual cases of adhesive recycling are still very rare because the amount of adhesive used in a product is much smaller than that of the adherend materials. In addition, adhesives consist usually of various kinds of materials. On the other hand, removing an adhesive layer from substrate is very difficult. Resolving obtained adhesive wastes by any solvent is also not easy because they are fully cured and cross-linked. However, the needs of sealant recycling are recently becoming important because the total amount of its use is huge. Thus, we should start to consider seriously the method to recycle sealant wastes.

Optimization of the Bonding Process

If the bonding process is not optimized, large environmental loads occur. For instance, although the surfaces of adherends often need pretreatment before bonding, there is room to reduce environmental loads. The use of oil accommodating adhesives is effective for the purpose because degrease and chemical treatments of adherends are not necessary. Otherwise, if solvents are used for degreasing, they are hazardous and their atmospheric release is undesirable. Collection systems of solvent vapor are useful for reducing the atmospheric release. The collected solvent vapor can be used again after condensation or thermally recycled, although carbon dioxide is emitted in this case.

It is also effective to reduce sub materials used in adhesion processes such as wiping cloths, gloves, masking tapes, removing agents, mixing cups, brushes, and static mixing nozzles. Since most of them are disposable, much waste is made unless the bonding process is optimized. In addition, release sheets of PSA become a waste too.

Energy consumption to cure adhesives is also a problem. Capabilities of ambient temperature curing or relatively low temperature curing are desirable to be installed for adhesives. Some acrylic adhesives can be cured quickly at room temperatures and they are strong and tough enough to be used for structural applications. A promising epoxy adhesive for structural use, which is ambient temperature curable and heat resistant due to nano-fillers, has been recently presented (Sprenger et al. 2004).

Design Optimization and Life Cycle Assessment of Adhesively Bonded Joints

Design optimization of adhesively bonded joints can contribute to environmental issues in a broad sense because it reduces the used materials and the weight of the joints. Precise knowledge of adhesives strength, modulus, and stress distribution in joints is helpful for designers, and it leads to rational design of the joints to minimize environmental loads. This point of view is likely to be missed, but very important. Adhesive making companies ought to provide positively the data to design joints appropriately. This is important not only for the optimal design, but also for preventing the duplication of basic tests that may be carried out by the adhesive maker and each user.

The other obligation of adhesive makers is disclosure of information for Life Cycle Assessment (LCA) of adhesively bonded joints. If these data are provided, the users of an adhesive can calculate the environmental impact associated with equivalent energy consumption or equivalent carbon dioxide emission. The later is called “carbon footprint” whose requests by users is recently increasing, and adhesives are not an exception. The users can evaluate the environmental impacts by LCA, and make rational decision to employ adhesive joining or not considering the total tradeoff of the whole life-cycle environmental loads of their products.

Types, Characteristics, and Applications of Dismantlable Adhesives

The main purpose of dismantlable adhesives is reworking of parts or recycling of materials in a product. Temporary joining of works also needs dismantlable adhesives. Reworking of parts is a process in which defective parts are removed and exchanged by normally functioning parts. If the parts are vitally important, defects of the parts have a huge influence on the product value. If the parts are joined with a dismantlable adhesive, they can be separated and exchanged easily, and that leads to an increased yield ratio of the product. Electronic devices have integrated circuit (IC) chips on their circuit boards bonded with epoxy adhesives called under fillers. Recently, reworkable underfillers, which are softened by heating and can be separated easily, are commercially available. Such reworkable adhesives are very promising because all devices are getting gradually complicated and the probability of error is increasing.

Dismantlable adhesives can be categorized as follows:
  1. 1.

    Thermoplastic adhesives, i.e., hot-melt adhesives

     
  2. 2.

    Adhesives including blowing agents or expansion agents

     
  3. 3.

    Adhesives including chemically active materials

     
  4. 4.

    Adhesives to which an electrochemical reaction on interfaces is applied

     

Thermoplastic Adhesives and Hot-Melt Adhesives

Thermoplastic adhesives, i.e., hot-melt adhesives can be separated. They are softened by heating. For instance, solid wax has been used for temporary joints of work materials. The work materials are joined and fixed on the bed of a machine tool with the melt solid wax, and separated by heating. The wax is usually used for many materials such as glass, ceramics, and silicones which are nonmagnetic and cannot be fixed with a magnetic chuck. The amount of the wax used is increasing, although it is a relatively old material. In electronics applications, several types of dismantlable adhesives are used for making IC chips. At first, silicone ingots are fixed with an adhesive and cut into wafers with a dicing saw. The adhesive can be softened by heating. The residue of the adhesive on the wafers can be washed off using a particular solvent or an alkali solution. Next, the wafer is fixed with another type of adhesive which can be separated mechanically using a scraper. The residue of adhesive on the wafer’s surface can also be washed off with an alkali solution.

Hot-melt adhesives began to be used for housing applications in order to recycle the materials. Figure 58.1 shows a bonding method of wall boards or ceiling boards on beams using a hot-melt tape and an induction heating machine. The method is called the “Allover Method” and has been utilized already by Japanese construction companies (Sekine et al. 2009). A key point is that the tape has a conductor layer made of aluminum alloy in it. To bond the boards on the beam, the tape is inserted between them. The process is easy because the tape has a weak tackiness. After that, the layer can be heated using the induction heating machine, and the board and beam are joined together. They can also be separated by heating because it is totally reversible.
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Fig. 58.1

Principle of the Allover method using induction heating (IH) and hotmelt adhesives (Sekine et al. 2009)

Combining thermosetting plastics and thermoplastics, reworkable adhesives having high strength, stability, and good utility, can be realized. Epoxy adhesives, which are thermosetting resins, are used often for electronics packaging. The introduction of a thermoplastic property to the epoxy adhesive leads to a novel function where the adhesive can be cured and melted by heating. This is called “thermally molten epoxy resin” and has become an important object for research recently (Nishida and Hirayama 2006). The resin is used as an under-fill adhesive to bond IC chips on circuit boards. If the IC chips are out of order, they can be removed and replaced by other chips. Such chips are usually bonded with an adhesive and solder, and both of them can be soften by heating. Therefore, the chips can be separated from the boards easily. The process is called “IC chip rework” (Fig. 58.2 ) and it is not frequent but occasional.
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Fig. 58.2

Reworkable underfillers used for integrated circuit (IC) chip bonding

Adhesives Including Blowing Agents or Expansion Agents

Mixture of blowing agents or expansion agents into an adhesive is frequently used as a method to provide dismantlability. For instance, thermally expandable microcapsules (TEMs) are used often, as shown in Fig. 58.3 . The TEMs expand by heating and an internal stress occurs in the matrix resin of the adhesive. The stress causes an interfacial separation between the adhesive and the adherend. The type of the matrix resin is important because the internal stress depends on its modulus and strength. Thermoplastic resins are suitable for the adhesive. They can deform much at high temperatures. The expanding force of TEMs induces large deformation in the matrix resin and that leads to interfacial fracture between the adhesive and the adherend (Ishikawa et al. 2005). In addition, the separated surface of the adhesive becomes rough because of the TEMs’ expansion, and the roughness becomes a barrier for the adhesive to stick again to the adherend. Therefore, the separated interface cannot be joined again even after the temperature decreases. In the case of a thermoplastic adhesive, the resin is softened over its glass transition temperature, and a similar phenomenon happens also in the case of thermosetting adhesives. Epoxy resins, for example, can be applied as a matrix of a dismantlable system, and they can be used for a wide variety of applications because of the high strength and heat resistance.
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Fig. 58.3

Dismantlable adhesive including thermally expandable microcapsules (TEMs)

As an example of application of this technique, a commercially available tape including TEMs can be shown, which has been used for temporary joints of IC chips die-cutting. The tape has double sided coats of a pressure sensitive adhesive (PSA). The bonding strength and modulus of the PSA are low, so that the tape can expand much with a small amount of TEM inclusion, and the expansion leads to an interfacial separation between the IC chips and the tape. Similar methods are applied to other applications such as adhesives or sealants for housing. A vinyl emulsion adhesive including TEMs is used to join fiber-reinforced plastics (FRP) or decorative steel sheets to plaster boards for fabrication of unit-bath structures in Fig. 58.4 . A silicone sealant including TEMs is used for sealing of kitchen sinks, etc., as shown in Fig. 58.5 . In both the cases above, the joint can be dismantled by heating (Ishikawa et al. 2004).
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Fig. 58.4

Wall panels for a unit-bath room consisting of plaster boards and steel plates bonded with a dismantlable adhesive of vinyl emulsion including TEMs (Ishikawa et al. 2004)

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Fig. 58.5

Bonded and dismantled parts of a kitchen sink bonded with a dismantlable adhesive of silicone resin including TEMs (Ishikawa et al. 2004)

The technique has been applied to strong adhesives too. A project called “ECODISM,” which is the abbreviation of “Ecological and economical development of innovative strategy and process for clean maintenance and dismantling further recycling,” was carried out in Europe aiming at the bonding of car structures using dismantlable adhesives or primers (Papon et al. 2008; Alcorta et al. 2004; Bain and Manfre 2000). A urethane adhesive including TEMs and blowing agents (pTSH) was applied to joints between a windscreen and the body structure, as shown in Fig. 58.6 . A rear hatch having polymer-alloy panels and a glass was also fabricated with the dismantlable adhesive. Infrared (IR) lamps were used to heat the structure in order to separate it, and the structure could be dismantled within 150 s.
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Fig. 58.6

Application of a dismantlable adhesive with thermally expandable microcapsules and blowing agents in the car industry, the ECODISM project (Papon et al. 2008)

Epoxy adhesives are also targets for separation using TEMs, although these adhesives are very strong. For instance, a type of TEMs, shown in Fig. 58.7 , is mixed into an epoxy resin to be used as a strong dismantlable adhesive (Nishiyama et al. 2003). The microcapsules have a structure with a core and a shell. The shell consists of polyvinylidene, and the core is filled by isobutene. The average diameter of the TEMs is about 20 μm. The TEMs can expand due to temperature increases because the shell becomes soft and the inner pressure of the TEMs increases. The expansion starting temperature of the TEMs depends on the material characteristics of the shell and the filling hydrocarbon. There are a lot of commercially available grades having different temperatures of expansion beginning and maximum expansion volumes. The TEMs, shown in Fig. 58.7 , have an expansion starting temperature of 80°C and a maximum expansion volume of 70 times. Therefore, the temperature of dismantlement of an adhesive is selected as a function of the TEMs. Generally speaking, a grade of TEMs having a high expansion starting temperature has a smaller expansion volume than that of low temperature expansion TEMs. Therefore, an increase of the dismantlable temperature of an adhesive may induce a decrease of dismantlability because the expansion volume is vitally important to separate the bonded joints.
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Fig. 58.7

Structure of thermally expandable microcapsules (TEMs) (Nishiyama et al. 2003)

As mentioned before, since the stresses caused by the TEMs expansion is the main driving force for joint separation, the expansion characteristics should be known to improve the performance of a dismantlable adhesive. Several experiments have been carried out to investigate the expansion performance of a type of TEM (Nishiyama and Sato 2005). For the purpose, Pressure-Volume-Temperature tests (PVT tests) were conducted using an experimental set up called “PVT measurement equipments.” In these tests, TEMs are contained in a flexible vessel under hydrostatic pressure with silicone oil under different temperature conditions. The hydrostatic pressure is caused using a pump and the temperature increases using a heater. The volume change of the TEMs was measured using a differential transformer. Therefore, the volume change of TEMs under different conditions of pressure and temperature could be measured and the P-V-T relation could be determined by the tests. For instance, the TEM mixed in the epoxy resin can expand 8 times in volume at 100°C. The pressure, P, caused by the TEM for a volume expansion of 800% and T = 100°C could be determined as 1–2 MPa. The result is strange because the pressure is much lower than the strength of the epoxy resin. In other words, the pressure is too small to deform the epoxy matrix resin enough for dismantlement. However, the resin becomes soft enough to deform at the temperature of the TEMs expansion. Thus, the combination of the resin softening and the TEM expansion is the key for the adhesive’s large deformation and internal stress occurrence. In order to provide dismantlability to an adhesive by mixing TEMs, not only the expansion force of the TEMs is important, but also the low strength and modulus of the matrix resin at high temperatures. A resin, whose Tg is close to the volume expansion temperature of the TEM, whose modulus and strength decreases drastically around the Tg, and which is soft enough not to suppress the expansion of the TEM in its rubber plateau, has to be selected as the matrix for a dismantlable adhesive.

Two mechanical models of the interfacial debonding of dismantlable adhesives are proposed in Fig. 58.8 . One of them is the local crack generation model, in which the expansion of a TEM causes a small interfacial crack nearby the contact point between the TEM and the adherend. The crack expands to the neighbor cracks, and the cracks join and lead to the total fracture of the joint. The other model is the global stress generation model, in which the expansion of TEMs induces volume increase of the adhesive layer, leading to joint fracture due to the global residual stress concentrating around the tip of the adhesive layer or interfacial cracks. An experimental verification of the models has been carried out using a glass joint bonded by an adhesive, including TEMs. In this test, optical and microscopic observation was conducted through the glass adherend. No gas generation was observed. Small circumferential cracks around each TEM were observed as explained by the local crack generation model. However, global cracks occurred at the edges of the joint and propagated inside the joint. This implies that the global stress generation model is also valid. Thus, the results support both models, which happen simultaneously, although the contribution ratio of each effect is still unknown.
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Fig. 58.8

Dismantling experiments to observe the interfacial debonding caused by TEMs in an epoxy adhesive; debonded regions in the joint can be seen as the dark parts in the images at the bottom of the figure, and they are expanding with respect to temperature

Adhesives Including Chemically Active Materials

Mixture of chemically active materials is an effective method to dismantle adhesives. For example, expandable graphite and aluminum hydroxide can be used for the purpose. Expandable graphite is a flaky carbon having a layer structure of planar crystals. Chemicals such as acids are intercalated between the crystal layers. The acid is vaporized by heating, and water included in the acid is also vaporized. Therefore, the vaporized acid and the vapor can expand the layer structure as an accordion being pulled, increasing the volume drastically, as shown in Fig. 58.9 . Since the vaporized acid and vapor are hot, they can attack the resin. Aluminum hydroxide consists in a fine powder that is transformed into aluminum oxide and vapor at high temperature. The hot vapor can not only expand the resin, but also attack the resin in the same manner as the expandable graphite does. Water becomes superheated vapor over 100°C in the atmospheric pressure. The superheated vapor can cause chemical decomposition of the resin. An application of the technique is double-side PSA tapes that include aluminum hydroxide and which used for the joining heat sinks to large-size plasma display panels. They have to be dismantled in their recycling process because they are dissimilar materials. The joint can be separated applying a weak force because the PSA tape can be degraded by heating and the bonding strength decreases very much. Other types of chemicals, oxidizing agents such as ammonium perchlorate, can be used to attack the resin too.
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Fig. 58.9

Expandable graphite

Adhesives to Which an Electrochemical Reaction on Interfaces Is Applied

A novel adhesive: Electrelease (EIC Laboratories, MA, USA), which can be separated electrically, is already commercially available (Welsh et al. 2003). The interface between the adhesive and the anodic adherend becomes so weak by applying a DC current that the joint can be broken by hand, as shown in Fig. 58.10 . The adhesive was developed to bond sensors on the wing of aircrafts, and has been tested to be applied to a separation mechanism for space applications. The application of this technique weakens the joint, but not completely. Therefore, some external force has to be applied to separate the joint even after the electric voltage is imposed. For instance, if a weak load is applied continuously using bias springs to the joint, applying only an electric voltage causes the spontaneous separation of the joint. As just described, a one-time separation mechanism triggered electrically can be realized easily using this adhesive as shown, in Fig. 58.11 (Shiote et al. 2009). The reason why the adhesive can be weakened by electric currents is not completely understood. It is supposed that an electric current transmitted to the adhesive layer causes electrochemical reactions on the surface of the anodic adherend, and this leads to erosion of the metal-oxide layer of the adherend. A similar mechanism of delamination induced by electric currents is known as “cathodic delamination,” in which alkali materials precipitate at the interface and attack the resin (Horner and Boerio 1990). These delamination mechanisms are quite novel and very promising in terms of expanding the variety of dismantlable adhesives.
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Fig. 58.10

An electrically dismantlable adhesive (Electrelease®, EIC Laboratories) where PEG and PDMS denote polyethyleneglycol and polydimethylsiloxane, respectively (Shiote et al. 2009)

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Fig. 58.11

One-time separation mechanism using an electrically dismantlable adhesive (Shiote et al. 2009)

Miscellaneous Methods

PSAs are promising materials to realize dismantlable joints because separation capabilities are built in already. Recently, strong PSAs have been commercially available, and they have enough strength to be applied for semi-structural uses. However, it becomes a problem in terms of dismantlability if the strength is too high. To avoid this problem, new types of PSAs, which are strong enough but can be separated easily, have been developed.

Command Tab® (3M, MN, U.S.A.) and PowerStrip® (TESA, Hamburg, Germany) are examples. Tackiness of PSAs depends on the modulus. If the modulus is out of a particular range known as Chang’s “viscoelastic window,” the PSA loses the tackiness or the bonding strength (Chang 1991). For instance, the PSA of PowerStrip comprises block-copolymer molecular chains having soft and hard segments, and the modulus depends on the deformation (Krawinkel 2003). If the PSA is pulled enough, the modulus increases and the tackiness decreases. Therefore, the PowerStrip tape can be removed easily, as shown in Fig. 58.12 .
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Fig. 58.12

High strength strippable PSA tape (PowerStrip®, TESA)

This theory of the viscoelastic window can be applied to other materials. Mixture of cross-linkers is another way by which PSAs can be hardened by heating or UV irradiation and the tackiness decreased. Recently, many kinds of tapes that can be separated by this method are used for dicing of IC chips.

Brittle adhesives have high shear and low peel strength. It is possible to take advantage of this aspect to make a kind of anisotropic adhesive. For example, a temporary joint adhesive, Lock n′ Pop® (Illinois Tool Works (ITW), WA, U.S.A.) is commercially available. The adhesive is strong enough to fix cardboard boxes due to the high shear strength during their transportation, but is easy to separate by applying vertical forces because of its low peel strength, as shown in Fig. 58.13 .
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Fig. 58.13

Anisotropic strength adhesive for a temporary joint of cardboard boxes (Lock n’ Pop®, Illinois Tool Works (ITW))

Recent Advances

The use of adhesives including TEMs has been expanding in many applications. For example, an adhesive has been introduced for shoe fabrication to bond uppers and soles (mid soles and outer soles) as shown in Fig. 58.14 (Mori and Harano 2009). The adhesive includes TEMs as expansion agents and diethylene glycol or ethylene glycol as heat-generation agents. Since diethylene glycol is a high-dielectric material, the temperature can be risen by microwave irradiation heating. The temperature rise can induce the expansion of TEMs and finally leads to the separation of the uppers and soles. Microwave irradiation has advantages such that low energy loss, and the necessary equipments have become inexpensive recently. For instance, even a home microwave oven can be used to dismantle the shoes. Requirements for adhesives used in sport shoes are many and severe. Bond strength, flexibility, and heat resistance are indispensable. To meet these requirements, a formulation, which includes an urethane resin, 5 wt% of TEMs, and over 20 wt% of high-dielectric materials, is typically used. At the moment, the adhesive is only applied to expensive shoes for top athletes because they want to reuse the uppers when the soles have to be replaced. However, the adhesive is ready to be used in ordinary shoe markets in terms of performance and price. Shoe industries have huge markets in which adhesives have to be used essentially because dissimilar materials are bonded. Since the total mass of production is enormous and has been increasing, the use of recyclable joining techniques is of significance, especially in the future.
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Fig. 58.14

A dismantlable adhesive application for shoe industry (Mori and Harano 2009)

The use of TEMs is convenient to provide adhesives with dismantlability, but it is not versatile because of some disadvantages. One of them is the small expansion forces produced by TEMs, by which a weak adhesive can be separated but strong ones such as rubber modified ductile epoxy adhesives cannot be separated because they have high strength and ductility suppressing interfacial fracture even at a higher temperature than its Tg. To overcome this disadvantage, there are two alternative ways: changing the expansion agents or modifying the matrix resin. As alternative expansion agents, expandable graphite and aluminum hydroxide are promising, as mentioned above. They are not only chemical attack agents, but also expansion agents having larger expansion capabilities than TEMs.

Modification of matrix resins is also important, but this is the next step and it has not been investigated often so far. A strategy is to modify the resin to be softer than the previous ones at high temperature so that the resin can deform due to the TEMs’ expansion. A problem is that the heat resistance of the resin might be damaged if the resin’s Tg becomes too low. Thus, a good compromise of heat resistance and dismantlability must be pursued.

Kishi et al. has tried to develop a heat resistant dismantlable adhesive for joining composite materials and metals, and modified the composition of an epoxy adhesive accordingly (Kishi et al. 2006). This research was conducted as part of the national project “R&D of Carbon Fiber-Reinforced Composite Materials to Reduce Automobile Weight” supported by New Energy and Industrial Technology Development Organization (NEDO) in Japan. As mentioned above, the resin should be soft at the temperature of the TEMs’ expansion. A problem is that typical heat resistant resins have moduli and strengths enough to suppress the TEMs’ expansion even at high temperatures, over their Tg. In other words, such heat resistant resins are not soft enough for TEMs to expand even at high temperature. Therefore, a kind of resin which has a high Tg and enough low modulus and strength above the Tg is necessary for heat resistant dismantlable adhesives. In addition, steep change of the modulus and strength of the resin around the Tg is also important to increase the maximum allowable temperature of the adhesive, as shown in Fig. 58.15 .
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Fig. 58.15

An idealistic variation of modulus for dismantlable adhesive’s matrix resin

Kishi et al. aimed a target for the adhesive as:
  1. 1.

    The maximum temperature for use is 80°C. Under this temperature, the resin has a modulus and a strength high enough to bear the applied loads.

     
  2. 2.

    Over 100°C, the resin has a steep decrease of the modulus and strength.

     
  3. 3.

    In the temperature range over 150°C, in which the resin is in rubbery condition, the resin should have a modulus and strength low enough for TEMs expansion.

     
For the adhesive, they proposed a formulation, shown in Fig. 58.16 , including diglycidylether of bisphenol A (DGEBA) 828, DGEBA 1001, and glycidylphthalimide (GPI) as the base resins cured with hardeners dicyandiamide (DICY) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) that can increase the Tg of the resin. An important point of the formulation is the inclusion of GPI that has high polarity and can form strong bonds between the main molecular chains. The bonds work as if the cross-linking density of the resin increases at low temperatures. However, the bonds are dissociated at high temperatures, decreasing the substantial cross-linking density. The low modulus at high temperatures and the steep decrease of the modulus around the Tg are due to the destruction of the bonds. Thus, the resin has high modulus and strength in low temperature ranges, high Tg, steep decrease of the modulus, and low modulus over Tg, which are suitable characteristics for heat resistant dismantlable adhesives. For instance, the resin has a modulus higher than 3 GPa in the glass condition range around the ambient temperature. However, the modulus decreases to 2 MPa over 150°C.
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Fig. 58.16

Components of the heat resistant dismantlable adhesive (Kishi et al. 2006)

The resin was combined with an expandable graphite and was used to bond metal parts to carbon-fiber-reinforced plastics (CFRP) plate. The joint could be separated within 5 min at 250°C, as shown in Fig. 58.17 . The adhesive includes 10 wt% of the expandable graphite and it begins to expand around 200°C, which is much higher than the Tg of the resin. The margin between the expansion temperature of the expandable graphite and the Tg of the resin was decided considering the chemical stability of the expandable graphite at high temperatures. The temperature of 250°C in the dismantlable test was decided as a compromise between time and temperature of separation.
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Fig. 58.17

Specimen of metal parts bonded on a carbon reinforced plastic (CFRP) plate with a heat resistant dismantlable adhesive

Future Seeds

One of the promising seeds to realize novel types of dismantlable adhesives is the use of degradable polymers. Polyperoxide-polymers, for instance, can be applied for that purpose because the resin can be cured and re-liquefied reversibly by heating (Matsumoto and Taketani 2006). Adhesives using the resin can be separated and bonded many times. Another kind of polymer having weak points such as Azo groups in the molecular chains is also promising for heat-degradable dismantlable adhesives. A type of rubber such as thermoreversible hydrogen bond cross-linked (THC) rubber, shown in Fig. 58.18 , can be re-liquefied by heating, has been proposed recently (Chino et al. 2002; Chino and Ashiura 2001; Chino 2006). Such rubbers are also applicable for dismantlable adhesives.
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Fig. 58.18

Schematic illustration of thermoreversible hydrogen bond cross-linked (THC) rubber principle

Since almost all recent dismantlable adhesives need heating to be separated, heating methods are also important to improve the efficiency of the dismantling process. The necessity of heating will remain even in the future. Heating the whole structure including the adhesively bonded joints is not efficient. Local heating of the joints is more convenient. Induction heating and microwave heating methods are appropriate in terms of efficiency. A new method of microwave heating using nanoferrite particles, shown in Fig. 58.19 , has been proposed and it can be applied for the heating process of joints bonded with dismantlable adhesives (Sauer et al. 2004).
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Fig. 58.19

Microwave heating using nanoferrite as dipole conductors (Sauer et al. 2004)

Other stimulations except heating should be investigated in the future. Magnetism, radiation (neutron beams, gamma ray), hydrostatic pressure, compressive stress, UV irradiation, and high voltage discharge are candidates for the stimulation. Dry adhesion, which has been attracting attention recently, is another possibility to provide new ideas for joint dismantlement (Lee et al. 2007). Further investigation and research on interfacial phenomena must be conducted to obtain further knowledge because the ultimate dismantlable adhesive must be an adhesive with which its interface can be separated easily and completely with a small amount of energy used for the separation.

Conclusion

To promote the use of adhesives, eco-friendly attitudes of both adhesive makers and users are indispensable. Reducing carbon dioxide and energy consumption is the main topics of environmental issues. Recently, alternative options, such as the use of carbon-neutral or recycled materials for making adhesives, can be taken to meet the demands derived from the environmental issues. The optimization of bonding process and joint design is also very important and effective to reduce energy consumption and wastes. Recently, dismantlable adhesives have been available for joining the materials that have to be separated on demand. These adhesives can be applied to adherend recycling or product reworking.

There are many technical seeds for dismantlable adhesives already. Since dismantlable adhesive technology has still some minor problems such as low heat resistance or low durability, they should be solved soon by researchers or industries, and then the use of the adhesives will expand to many applications. Dismantlable adhesive technology has contradictory aspects: good bonding and easy separation. High bonding strength tends to induce difficult separation. Therefore, it is also important to clarify the minimum bond strength necessary for each application instead of pursuing high performance in strength.

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© Springer-Verlag Berlin Heidelberg 2011
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