Polyurethane Hot Melt Vs EVA Glue I
When it comes to perfect binding, there are really only two options as far as adhesive choices are concerned.
Up until the mid-1990s, the long-standing traditional method for perfect binding used a hot melt adhesive called Ethylene Vinyl Acetate (EVA). EVA glues are applied with a glue pot utilizing a wheel applicator. The properties of the glue itself allow for it to be re-melted many times, thus providing little waste during production. They also have quick set points requiring a small cure time before trimming. While they are acceptable for most offset papers, issues can arise when more difficult stocks are chosen.
Developed for perfect binding in 1989, polyurethane (PUR) hot melt adhesive has become the overwhelming preference within the binding industry. As with EVA, PUR adhesive requires the backbone of the book to be prepared in much the same way, utilizing milling knives, notching heads, and roughing and/or equalizing heads to create fiber. It is this fiber that the PUR molecularly bonds to during the curing process. While the first generations of this glue required a long set up time (often over 24 hours), the technology has improved greatly in the past decade. Current formulations of polyurethane hot melts attain 80% of their green strength in just a few hours.
Polyurethane hot melt bonds to the book through a chemical reaction. The properties designed within the adhesive draws moisture out of the air, technically making polyurethane hot melt adhesive a moisture-curing product. Therefore, the application method is critical.
Recently, hot melt adhesive manufacturers commonly take advantage of a state-of-the-art extrusion head system that applies the glue directly to the spine of the book with a slot applicator that sprays the adhesive evenly. This computer controlled system allows for precise application of the thickness, and as opposed to a polyurethane hot melt open method, it keeps the glue from curing in the pot. This also limits any waste and virtually eliminates any cleanup.
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