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Q&A: Troubleshooting PUR Adhesive Problems

by Jen Clark

In the last 20 years, use of Polyurethane reactive binding adhesive, or PUR, has been gaining popularity in the binding industry, with particularly strong growth in the past 5-plus years.

Prior to PUR's inception, ethylene vinyl acetate (EVA) hot melt adhesives and cold adhesives were used, but not effectively, said Rick Pallante, industry specialist with the Nordson Corporation, Westlake, OH. A chemical reaction that occurs during the drying process makes PUR a popular alternative in perfect binding, paperback books and items published digitally. It creates a tight bond within paper fibers resulting in greater pull and flex strength. The reaction happens when the glue is exposed to moisture in the air.

Two features make PUR binding a good option for commercial printers: superior adhesion and excellent lie-flat qualities. Other benefits include resistance to heat and cold, less wrinkling of the backbone, less build up on trimmer knives resulting in fewer nicks to the cover material, solvent resistance and a book remains more flexible because it holds up to repeated usage better.

Question: What kinds of problems might a binding company using PUR encounter?

Answer: There are several, including costs, immediate pull test strength and misconceptions about health hazards.

a) Open wheel pots can be difficult to clean regularly and require serious maintenance on a regular basis. "The wheels and sometimes pots literally have to be sent off for cleaning, refurbishment and resurfacing," Pallante said. PURs also are more expensive than EVAs – sometimes two or three times more expensive per pound than an EVA. "It can be difficult for a binder to pass that extra cost on to a customer who isn't educated on PUR's benefits," he said. In addition, extended heating of PUR adhesives degrades the material and the bonding qualities.

"Nordson specializes in closed adhesive application systems – both tank and “bulk” (pail and drum) melters that feed our slot nozzle," Pallante said. The melters both minimize air exposure and use “melt-on-demand” technology whereby only the adhesive directly in contact with the melt platen or grid is heated as opposed to a melt tank that heats all the adhesive within it. Pressure-fed slot nozzles “feed” only the adhesive needed on the spine of a book and can somewhat mitigate the fact that PUR adhesives are more expensive than EVAs.

b) PURs do not provide sufficient immediate "pull test" strength. PURs provide exceptional bonding, but require up to 24 hours to reach full strength. The initial bond is strong enough to allow for trimming, packing and shipping. PURs also aren't widely known by binding customers.

"Many, if not most, binders are aware that a PUR bound book will fail an immediate pull test so it is simply a matter of education and reminding," Pallante said. With more experience using PURs, binder operators are learning to trust the adhesive to do its job without the need for immediate confirmation.

c) PURs have a "bad reputation." Early versions of hot melt polyurethanes were considered to be health hazards and required special handling,especially in regard to ventilation.

"PUR adhesives have gone through numerous changes through the years and are now less dangerous to be exposed to," he said.

Founded in 1954 and headquartered in Westlake, OH, Nordson has nearly six decades of experience with dispensing materials, and particularly adhesives. It has been proactive in understanding and promoting the use of PUR adhesives and the best, most efficient handling of them. For more information, visit www.nordson.com.