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.
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
Hazardous material emission
Greenhouse gas emission
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
Use of eco-conscious materials
Provide reworkability or dismantlability to adhesives
Recycling of adhesives itself
Optimization of the bonding process
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.
Thermoplastic adhesives, i.e., hot-melt adhesives
Adhesives including blowing agents or expansion agents
Adhesives including chemically active materials
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.
Adhesives Including Blowing Agents or Expansion Agents
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.
Adhesives Including Chemically Active Materials
Adhesives to Which an Electrochemical Reaction on Interfaces Is Applied
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.
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.
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.
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.
Over 100°C, the resin has a steep decrease of the modulus and strength.
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.
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.
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.