QUESTION: In the manufacturing world, the recycling of materials and the use of “green” materials are popular trends that are increasingly coming under legislative pressures. What about the recycling of adhesives and sustainability in the adhesives industry?
ANSWER: First of all, it is necessary to remind ourselves that many adhesive products have always been based on natural, sustainable materials, including starch-based adhesives and glues based on casein, animal, and fish byproducts. However, many adhesives will continue to be based on petroleum sources, and we do need to maximize the recycling of these adhesives. Industry challenges include the necessity for adhesives to be recyclable themselves and to avoid interfering with the recycling of the products to which they are bonded.
In paper and cardboard recycling, a lot of adhesives can now be sorted out easily, including pressure-sensitive adhesives (labels and tapes) and hot-melt adhesives used in packaging. New technologies in the bottle labeling industry—such as two-layer labels, which expand at different rates when heated—allow for the rapid removal of labels in hot bottle washing solutions.
Structural adhesives pose a difficult problem, since they are generally thermoset materials and are designed to be very strong with excellent heat and solvent resistance. In the automotive industry, epoxies and polyurethanes are now widely used for the structural bonding of metals and windshields. Until now, their effect on recycling has tended to be ignored because the very high temperatures used in re-melting the metals vaporizes the adhesives. However, as increasingly larger amounts of adhesives are used, we can expect some legislative pressures to emerge.
New technologies have been designed to simplify the de-bonding of assemblies. Microspheres, for example, release gas when subjected to high temperatures; this causes an adhesive to crack and the bond to fail. In the bonding of automotive plastics, some success has been achieved with thermoset acrylic adhesives, which are used in such small amounts that they do not change properties when the plastics are reground for recycling. In the future, we may need to research the use of more thermoplastics for structural bonding, which would allow easier recycling.