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PUR

Oct 1, 2004 12:00 AM


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The first reactive polyurethane (PUR) hot melts for perfect binding debuted at Drupa 1988. It was an undeniable adhesive achievement, an innovation that promised to push perfect binding beyond ordinary applications and into the dizzying heights of truly difficult jobs.

Today, PUR is gaining popularity for use with UV coated papers as well as those with high filler contents. It's also suitable for books subjected to high stress, such as computer manuals, and products exposed to extreme temperature fluctuations, including car manuals and road maps. (See Table 1.)

Nonetheless, PUR wasn't an overnight success. This article will briefly retrace the history of this binding option as well as the latest developments.

What is PUR?

PUR hot melts are mass-produced PUR prepolymers with reactive terminal groups, which when exposed to air humidity and/or paper humidity after application, undergo a cross-linking reaction. The prepolymers wet out and penetrate paper well, contributing to high adhesion and thus high page pulls.

After cross-linkage, PUR films show a high inherent strength (cohesion), resulting in extremely durable adhesive films. Not only are these films insensitive to printing ink oil and solvents, they also provide good aging resistance.

If you bind brochures with PUR hot melt, you can expect extreme cold and heat resistance. PUR bondings stick safely at temperatures to 120°C without quality loss, and remain elastic even at minus 40°C.

The glue-it-yourself approach

Despite these advantages, at the end of 1988, only two German bookbinders were using PUR hot melts for perfect binding. Performance wasn't an issue — PUR had a proven track record in many market segments. Equipment modifications — specifically reactive systems — were the real stumbling block.

Glue tanks, for example, must be coated so the extremely sticky adhesives can be removed when the tanks are cleaned. Also, to reduce the thermal stress on the adhesives, the operating temperature must be optimized.

(The previously mentioned PUR pioneers, the two German bookbinders, took a do-it-yourself approach and made their own application device adjustments.)

Drumming up support

Beyond application equipment issues, the need for new melting devices also impeded PUR's growth. Since PUR adhesives are moisture-curing systems, they must be protected against humidity during production, transport and application. That's why these systems are shipped in drums and emptied with “drum melters.” [Figs. 1 and 2]

Drum melters permanently seal out humidity while providing the convenience of PUR on-demand. Thermal stress on the system is reduced since only the actual amount required is melted.

Almost PUR-fect

Today more than 150 bookbinders in Germany and several hundred worldwide have switched from conventional to PUR hot melts. New bindery equipment, combined with the latest PUR hot melts, ensures processing that's practically trouble-free.

Users are no longer restricted to the classic roller application method for processing PUR hot melts. New developments include nozzle systems for applying the adhesives onto the bottom of each book [Fig. 3a]. With these nozzles, binders can reduce the application thickness to 0.3 mm for excellent lay-flat results.

By reducing glue film thickness (50 percent or less compared to standard ethylene vinyl acetate [EVA] hot melts), the mileage advantage for PUR hot melts almost compensates for its higher price.

For applications requiring a two-shot perfect binding process, you can get nozzle systems that apply adhesives onto the cover as it travels from the feeder to the station where the cover is attached to the bookblock [Fig. 3b].

Third-generation PUR hot melts

The first-generation reactive PUR hot melts of the late 1980s and early 1990s were subject to a rapid increase in viscosity during processing. These hot melts also had low green strength — that is, little initial strength. The chemical reaction leading to final strength — and thus excellent binding properties — took anywhere from three to six days, depending on the amount of humidity. Consequently, products couldn't be shipped until they had reached their expected performance level.

The second generation of PUR hot melts introduced in the late 1990s cured considerably faster. After a comparatively short period (less than three days), the final strength — and thus the usability — was guaranteed.

Generation three

Today's third-generation PUR product exhibits a fast chemical reaction. High-speed systems ensure that bound products can be used within six to 16 hours [Fig. 4].

These newly developed systems offer low processing viscosity and good initial strength as well as viscosity stability — a requirement for tank melters.

Although vendors have improved processing equipment and techniques dramatically, PUR hot melts require more attention to certain details than the conventional kind, particularly when it comes to maintenance. While users are restricted to cleaning products specifically engineered for compatibility with PUR's chemistry, these products are remarkably effective. They remove adhesive residues from dirty glue tanks, pumps, pipelines, tubes, application nozzles and all other application devices. Even cracked and charred adhesive residues can be removed.

Avoiding environmental issues

As with any new process, before using reactive PUR hot melts, you'll want to evaluate any environmental considerations.

Although processing today is low-risk when PUR hot melts are handled carefully, many U.S. and European operations face increasing pressure to reduce isocyanate emissions. “MicroEmission” PUR hot melts contain far less monomer isocyanate than traditional systems. These products contain less than 0.1 percent monomer isocyanate and thus are below the limit set by legislation that classifies PUR hot melt in the EU countries as a dangerous formulation. When melted, MicroEmission products reportedly release up to 90 percent less isocyanate vapor than conventional PUR hot melts.

Conclusion

Since the late 1980s, PUR has evolved from an intriguing innovation used by a few enterprising bookbinders to a state-of-art option offered by hundreds of postpress specialists around the world.

Reactive PUR hot melts fulfill all requirements for perfect binding, side gluing, back lining of thread- stitched or thread-sealed books. They offer excellent stability, fast curing and highest initial tack.


Contributing editor Dr. Hermann Onusseit joined Henkel (Dusseldorf, Germany) in 1984 as a lab manager for paper and packaging adhesives. He currently works in R&D. Contact him at hermann.onusseithenkel.com.

The author would like to thank Bill Leach from Henkel Adhesives for his valuable assistance.

What's behind PUR's popularity?

Perfect binding options include polyurethane reactive (PUR), ethylene vinyl acetate (EVA) hotmelt and polyvinyl acetate (PVA) cold emulsion adhesives, or Smythe sewing. PUR has only been in bookbinding since about 1989, and the first test in North America was run with a homemade application system. But many bindery managers have come to value it for premium books, annual reports, catalogs, directories and magazines.

Why PUR?

Since its introduction, PUR use has increased dramatically. In 1995, there were reportedly 28 PUR users; by the end of 2002, this number had increased to more than 50 in North America. PUR is chosen over standard hot melt adhesives for many reasons, the most important of which are its superior adhesion and the ability of a PUR-bound book to lie flat when open.

What can it do?

PUR is unique in that it will bond to lacquer and UV-cured coatings, films such as Mylar, as well as paper. Due to the polar nature of the polyurethane molecule, these adhesives also bond well to clay-coated papers.

The changes in paper weight, coating and inks used in today's printing applications reportedly pose little challenge for the latest PUR adhesives. Page pulls reportedly average up to 40 percent to 60 percent better with PUR than with traditional adhesives. And, while the migration of inks into a book's gutter can cause binding problems for traditional adhesives, PUR glues remain virtually unaffected by ink migration.
Source: “PUR and Perfect Binding,” AMERICAN PRINTER, May 2003. See www.americanprinter.com.