This article talks about adhesives, looks at various polymer chemistries and curing options available, and provides information.
Environmentally friendly products
One thing many customers want is sustainable materials. There are several approaches the adhesives industry is using to move in this direction. In applications where performance requirements do not dictate the use of solvent-based adhesives, other materials allow manufacturers to avoid the emission of volatile organic compounds (VOCs) present in solvents such as toluene and xylene. Water-based acrylics, UV-curable adhesives, and hot melts all fall into the category of solvent-free options.
Further development of water-based and UV-curable PSAs so they can better compete with the performance of solvent-based products is a step toward more environmentally friendly solutions. This includes improvements in properties such as tack and shear as well as developing wider processing windows. Research efforts include working to balance the desire for faster cure times with the need to avoid build up of adhesive on rollers during the coating process.
Another aspect of sustainability is recyclability of paper products. Paper labels (Figure 3) and stamps used by the U.S. Post Office (Figure 4) are coated with recycling-compatible adhesives (RCAs), water-based adhesives that dissolve in water during the recycling process. PSAs that are not recycling-compatible break into small particles that can damage paper-making equipment.
Recycling of polyethylene terephthalate (PET) plastic products such as beverage bottles and clam shell packaging is another important challenge, with demand for recycled PET outpacing supply. Labels or stickers on these products complicate the recycling process since adhesive residue is generally left behind. The Association of Postconsumer Plastic Recyclers (plasticsrecycling.org) states, “It is harmful to recycling for a label and adhesive to be the source of technical interference with the recycling process.” They have created a detailed test protocol for evaluating the recyclability of labels and adhesives. Several water-based acrylic adhesives are listed as meeting this protocol.
Still, either heat or alkaline cleaners are required to remove adhesive residue from PET. A possible solution is adhesives that adhere extremely well to the label material but not as strongly to the substrate, such that the label would remain attached to the bottle or package long enough to serve its purpose to the consumer but could be easily removed in order for the plastic to be recycled. The technology to produce such PSAs is still developing.
A different approach to environmentally friendly adhesives is to move away from polymers that use petrochemicals as raw materials and replace them with plant-based materials. A variety of patents exist in this field. A 2012 patent from the Kansas State University Research Foundation, for example, proposes repositionable PSAs made from plant oil triglycerides. Plant-based materials have also made their way into actual products, such as rosin esters in hot melt PSAs. There is, however, a cost trade-off that makes it difficult for renewable products to compete in the marketplace.
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