Adhesive formulators are normally concerned with the strength of the bonds their products form, and how they perform during their lifetime. But with the focus on careful use of resources increasing, should we also be considering ways to make it easier to remove them when the products theyre used to assemble stop being useful?
Whatever materials a product is made of, including adhesives and polymers, their isolation, processing and assembly uses energy. Recycling a material from a product after its finished with can reduce this energy usage. Consequently, its important to cleanly remove adhesives before starting the recycling process.
Table 1: Approximate energy content of some |
For many pressure sensitive adhesives, removal from their substrates is straightforward. Thats especially true of microsphere pressure-sensitive adhesives, which contain particles larger than those found in conventional emulsion adhesives. These make contact between the adhesive and substrate surfaces discontinuous, enhancing removability.
Yet for more securely-attached materials, usually no thought is given to removal at the end of the products life in designing the structural adhesive that they are likely to use. At first glance it seems like hot-melt adhesives at least would allow products to be readily disassembled, given that their adhesion is reversible with temperature. However, it is difficult to heat just the adhesive, and once the substrates become hot they pose a risk to anybody handling them.
Some researchers have suggested that removability can be achieved in thermosetting adhesives by formulating them to contain materials that expand on heating.2 By setting the expansion temperature to be significantly above the cure temperature, these could deliver enough force to selectively separate substrates with a one-off post usage heating. Potential heat-expansible materials include microballoons and graphite, but each have their own problems. Microballoons can only accommodate low heat levels and provide weak adhesives. Likewise heat-expansible graphites particle size makes its use difficult, and it needs removal temperatures that are too high.
Including a thermally-activated oxidizing agent in an adhesive that can break it down is another option.3 As well as its oxidising effect, at a critical temperature limit the oxidising agent undergoes a highly exothermic reaction, raising the adhesive to a temperature high enough to drive its decomposition. The presence of the oxidising agent can raise handling problems for users however, and some oxidizing reagents can react with any amines in an adhesive to create bubbles. These bubbles prematurely decrease bond strength in polyurethane or epoxy adhesives that are typically cured with amines.
This year researchers sought to resolve this issue by pre-reacting amine groups with oxidizing agents. This makes oxidizing chemicals like perchlorates, chlorates, nitrates and nitrites less dangerous and reduces any foaming that would detract from their original adhesive strength. Nitrates, whose decomposed gas consists mainly of nitrogen, are preferable as oxidizing agents with regard to impact on the environment.
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