"The Wonders of PUR Glue"

Anyone in the binding industry knows that glue can be a pretty sticky issue sometimes particularly when it comes to perfect binding. In several past issues, I have mentioned the benefits of PUR (polyurethane reactive) glues. Because these adhesives have made such an incredible impact on the perfect binding industry, this article will explore them in more detail.

As a quick review, I'll briefly mention the three basic categories of glues used in perfect binding: EVA (ethylene vinyl acetate) hot-melts, PVA (polyvinyl acetate) cold emulsion adhesives, and PUR (polyurethane reactive) hot-melts.

EVAs or hot glues have been around for many years, are relatively inexpensive, cure quickly, and form a strong bond under most conditions. They do not hold up well in extreme temperatures, however, and tend to crack as a result. Hot-melts should be avoided when working with heavy-coated stocks or stocks over 70# in weight.

PVAs are applied cold. When they dry, their resins penetrate into the paper stock forming a solid bond. PVA adhesives cure to a semisoft state, providing more backbone flexibility than hot-melts, but they do not provide the same page strength as PUR glues.

When PUR glues came along in the early 1990's, they seemed the answer to many of the problems inherent to perfect bound books primarily flexibility, layflat qualities, and durability. Technically speaking, the polymers in PUR adhesives cross-link onto the stock via a chemical reaction with moisture thus facilitating adherence, increasing pull strength, providing greater resistance to hot and cold temperatures, and reducing waviness in wrong-grain projects.

The market for PUR glue, driven by products such as software manuals, reference books, and high-end catalogs has grown dramatically and now includes hard-cover books, catalogs, and annual reports. According to Chuck Cline, Technical Services Manager at National Starch and Chemical Company (one of the country's leading glue manufacturers), "PUR glue was first introduced in 1988 and has expanded over the past ten years to include over 36 users in the United States alone. Additional advances are being made with binders using PUR in Canada as well as in Latin America."

Check out the following advantages of PUR glue, as described by National Starch and Chemical:

Heat Resistance - Perfect bound materials hold up in temperature ranges all the way up to 200F. Conventional hot melts generally fail at 120F or less. (Printers should always ask their customers if the books will be shipped to an area with extreme climates and if any special consideration should be given to the end-use. For example, will the book be kept in a hot car for any length of time?)
Cold Resistance - PUR does not fracture at temperatures as low as minus (-) 45F. EVAs are generally flexible to plus (+) 25F while most rubber based materials remain pliable to minus (-) 10F.
Adhesion - Because of their chemistry and molecular weight, PURs can be formulated to adhere the most difficult stocks. This is particularly important in jobs with heavily coated stocks against the grain, traits common to annual reports, for example.
Flexibility - Books bound with PUR glue have excellent layflat qualities. The adhesive is very pliable, and less of it is required on the backbone. This is perhaps the most appealing aspect of PUR glue. Not only does it answer the problem of books that snap shut, or the "mousetrap" effect as it is called, there is far less unsightly glue seepage in the books. When PUR adhesives are used in combination with Otabind technology, the end result is the ultimate in layflat reader friendliness.
Low Application Temperature - For safety concerns, PURs must be applied at temperatures no higher than 250F. As a result, less fumes are generated less heat is put back into the book block, and less cooling time is needed to solidify the hot melt. (This saves precious time a rare commodity in our industry!)
Ink Solvent Resistance - PURs are virtually unaffected by the migration of printing ink vehicles. (PUR adhesives will not dissolve with exposure to things such as alcohol, toluene, and gasoline.)
Durability - All of the aforementioned benefits provide the printing industry with a ten-fold improvement in durability. Books destined for a long life or heavy use will retain their integrity. Durability is one of the premier reasons PUR glue has been such a success. In the past, perfect bound books were notorious for their lack of strength and durability over time. Now, even high-end catalogs that were previously sewn hold up great with PUR adhesives. Both page pull tests and flex tests measure substantially higher in books produced using PUR glue, with page pulls that average up to 40 to 60 percent better than with traditional adhesives.
There are many considerations when choosing an adhesive and, as with any product, PUR glue has its limitations. The type of adhesive is pricier than traditional glues making it prohibitive for certain jobs. As the product grows in popularity, however, and work continues to perfect it, glue manufacturers predict the cost will come down. Other downsides to PUR glue include the extra curing time necessary for acceptable page pulls because of the high moisture content necessary to activate PUR glues and the relatively low maximum temperatures below 285F necessary to avoid creating fumes that are irritants. There are newer formulations (e.g., "high green strength" PUR), however, that address these problems allowing effective page pulls after only 15 to 30 minutes of cure time. The rising demand for perfect bound books has placed greater demand on the postpress world to provide a better looking, longer lasting, more useful product (not much, right?). Determining whether PUR adhesives are right for a given job is contingent on several factors: the life expectancy of the book, the end-use, and the budget. My best advice is to be diligent about finding out these details before doing the job and consult with your binder they should be able to recommend the best adhesive for your job based on these factors. In other words, plan ahead, plan ahead, plan ahead!

I never thought glue would be such an issue in my life. That's a scary thought that perhaps I shouldn't think about too hard.