PUR Glue - Strongest Bookbinding Adhesive Available

Published in New England Printer & Publisher

by Frank Shear

Do adhesive bound books need to stay together? Of course they do. For the ultimate in raw strength and flexibility, don't trust anything less than PUR glue. PUR lasts longer than ordinary hot-melt and cold-emulsion adhesives, stands up to extreme temperatures better, looks great, and holds difficult coated stock with an iron clad grip. In short, PUR is the strongest bookbinding adhesive available on today's market.

PUR glues are considered by many bindery experts to be the most flexible and durable bookbinding adhesives on the market. They yield products that lie flatter and require less backbone preparation than other types of glue. PUR is clear, less of it is needed to create a solid bond and it yields stunningly beautiful products. And, since PUR cures to a semi-soft state, books bound with PUR glue lay flatter than those bound with traditional EVA (ethylene vinyl acetate) hot-melts.

Over time, hot-melt perfect bound books lose their strength and are subject to losing pages. This is OK if a book has a short expected life, but for many types of products, failing glue is a serious problem. Until recently, there wasn't a cost-effective solution to the aging glue problem, but in the late 1980s, PUR (polyurethane reactive) glue hit the bookbinding scene.

Why PUR?

PUR glues aren't subject to the normal limitations of hot-melt glues and don't melt or crack, even when exposed to the most difficult environmental conditions. Try holding a PUR bound book underwater. The pages will get soggy and lose their structural integrity, but the glue will hold just fine. Page pull tests conclusively demonstrate that PUR is at least twenty percent stronger than traditional hot-melts and simulated aging tests prove that PUR lasts far longer than paper.

PUR's bond is so strong that even previously Smythe sewn and notch-perfect bound books are now routinely bound with PUR glue. Today many museum-quality art books, coffee table books, and frequently used library reference materials are being bound with less costly PUR.

PUR glues are excellent for tackling tough adhesive binding jobs. If your paper grain isn't parallel to the spine, use PUR glue. If your project is on thick or heavily coated stock with a lot of clay fillers, use PUR. If your book has ink or coatings bleeding into the spine, use PUR. If your book will be exposed to extreme temperatures or moisture conditions, use PUR.

Choosing the right glue and binding method depends upon knowing the printed product's end use. Before beginning, consult with your finisher to determine the best glue and binding method for your project. Products benefiting from PUR glue include:

• Perfect bound books with 80# coated text or heavier. 
• Signatures with ink, varnish, UV or aqueous coating bleeding into the gutter (grind-off margin). 
• Books that are frequently opened or closed.
• Books that are used in extreme environmental conditions. (For example, maps and atlases placed on car dashboards routinely bake at temperatures exceeding 140ºF.) 
• Otabind and RepKover layflat adhesive binding.

PUR can be used to bind books that have failed during conventional perfect binding. For example, Seaboard Bindery once was asked to bind a catalog with 80# coated text. After asking a few questions, Seaboard Bindery discovered that the previous version of the project had "failed" because of exceptionally heavy use: End-users cracked spines and lost pages as they repeatedly opened books for photocopying. After Seaboard Bindery bound the next version with PUR, complaints disappeared.

Technical Aspects

Unlike hot-melt adhesives that cure within seconds of application, PUR glue requires a 24-hour curing period. During this time, PUR books must remain exposed to natural air because the bonding chemicals in PUR cure by drawing moisture out of the air. (Contrary to the behavior of regular hot-melt glue, PUR actually cures faster on humid days.) If uncured PUR bound books are shrink wrapped or stretch wrapped, there will be severe disappointment upon discovering stringy strands of glue clinging to poorly bound pages.

When sending a job out for PUR binding, let your bindery know what type of printing press was used. Heat-set web run jobs should have time to acclimatize to the surrounding atmosphere and absorb moisture before beginning the PUR binding process. As soon as these heat-set web jobs arrive on a bindery's loading dock, all stretch wrap should be removed as soon as possible so the acclimatizing process can begin.

PUR produces smoother, less wavy spines than traditional perfect binding because it runs at cooler operating temperatures and draws less moisture out of the paper. In general, very thin books (approaching 1/16") as well as those with cross grain signatures should be bound with PUR glue.

The following is a brief list of some PUR benefits:

• Tough Jobs. PUR formulations can successfully bind the most difficult jobs; even those with thickly coated stock running against the grain. 
• Great Flexibility. By itself, PUR has excellent layflat properties. However, the ultimate in layflat friendliness is achieved when PUR is combined with the patented Otabind layflat process. 
• Ink Solvent Resistance. PUR glues are virtually unaffected by the migration of printing ink vehicles into the spine. 
• Extreme Heat. PUR glues hold firm under any kind of atmospheric condition. Frequently, regular hot-melt glues fail at 120ºF. In many parts of the country, temperatures inside cars exceed 120ºF on hot summer days. 
• Extreme Cold. PUR glues will hold at temperatures as low as minus (-) 40ºF. Hot-melts are flexible only to (+) 40ºF, and then may crack.

Testing Strength and Flexibility

Most PUR formulations cure too slowly to be physically tested during production. Although PUR glue is clear, some PUR formulations now have an ultraviolet indicator built into the glue which allows operators to view these glues under a black light and easily determine the quality of glue application. Once cured, regular hot-melt bound pages should withstand 2.5 lbs. of pressure per linear inch before yielding from the spine. PUR glues routinely withstand 4 to 7 lbs. of pressure before being dislodged.

Another PUR test is called the "subway" test. Here, the PUR operator folds back the product cover-to-cover and looks for adequate connecting filaments of glue crossing the spine, commonly called "stringing." If you see consistent stringing from head to food, and if the entire spine is tacky to the touch, then you know you have good coverage. Your product will be strong!

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Should PUR glue be used on a standard short-life 96-page book, with 60# offset text, with no ink or coatings bleeding into the gutter? No, because the additional expense of PUR glue isn't worth it. However, for many other products requiring more durability and flexibility, be smart and choose the strongest bookbinding glue available: PUR!

Frank Shear is President of Seaboard Bindery, a service-oriented trade bindery located in Woburn, Massachusetts. His company specializes in offering high quality perfect, PUR and layflat adhesive binding, saddle stitching, Wire-O™, plastic coil, laminating and other bindery solutions. Call Frank at (781) 932-3908.