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PUR and perfect binding

May 1, 2003 12:00 AM

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There are a number of binding options, including 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, according to Xavier Ardanaz, marketing manager for adhesives manufacturer National Adhesives Corp. (Bridgewater, NJ), which introduced the first generation of PUR to the U.S. bookbinding industry.

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 hotmelt 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.


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. Ardanaz says the newer PUR products grab and hold heavily coated papers together tenaciously. Page pulls reportedly average up to 40 percent to 60 percent better with PUR than with traditional adhesives.

The exec adds that 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.

Excellent lie-flat qualities

When applied at its recommended thickness — 0.01 inch — PUR is considerably more flexible than standard EVA hotmelts. This ability to lie flat when open allows for hands-free reading. According to National Adhesives, the world's largest software seller and a Texas-based computer manufacturer both chose PUR for their instruction manuals several years ago. The firms believed that the lie-flat characteristic was important to their customers.

PUR has been approved as an adhesive for Otabind-style binding.

Other benefits

PUR offers other benefits for the bookbinder:

Heat and cold resistance

Books bound by PUR will not fail at extreme temperatures — even over 200°F or under -40°F. These properties were especially important to a binder in Utah who sent books to both the Sahara Desert and to areas in Siberia. According to the binder, customers' adhesive-related complaints were virtually eliminated once the company switched from hotmelt to PUR.

Less wrinkling of backbone

Binding cross-grained paper with standard hotmelt adhesives causes considerable wrinkling in the book's gutter area. PUR's lower operating temperature, however, doesn't drive paper moisture away from the backbone. PUR is also somewhat malleable as it cures, allowing the paper fibers to return to their original orientation.

Less “chip-out”

Since PUR is applied at half the standard thickness of hotmelt, there is much less chance for the material to build up on trimmer knives and nick the cover material during trimming.

Square backbone

Because only a small amount of PUR needs to be used for binding, less adhesive is squeezed out when the cover station and side clinchers attach the cover and form the back.

Solvent resistance

According to bindery consultant and former RIT professor Werner Rebsamen, PUR is the only binding material that is totally resistant to solvents and oils. Samples of the cured PUR adhesive have been immersed in oils and solvents used in the printing industry, such as methyl ethyl ketone (MEK) and alcohol, and these chemicals have had no effect on the PUR film. On the other hand, some of these chemicals drastically soften, and even dissolve, standard hotmelt films.

Hardcover advantages

Binders that are gluing off book blocks for hardcover or edition-bound books can realize benefits beyond those of standard perfect binders:

Cost savings vs. smythe sewing

For many years, the industry standard to produce rounded, hardcover books was to use Smythe sewing and glue the sewn books with a flexible liquid adhesive prior to rounding. According to a study in Europe, binders can realize a cost savings of 30 percent to 40 percent by milling off the backbone of the folded signatures and then gluing them with PUR.

Ardanaz of National Adhesives notes that sewing is also more labor-intensive, and requires a greater capital investment.

(In fairness to sewing technology, however, Rebsamen notes that München, Germany-based FOGRA Graphic Technology Research Assn., which researches printing technology, handles approximately 400 adhesive-binding complaints each year. Many of the complaints reportedly involve PUR.)

Roundability and roundness retention

Many hardcover books are rounded and backed to provide both aesthetics and strength to the final product. A significantly greater roundness can be obtained by gluing off the book block with PUR, followed by inline rounding and backing. As the PUR cures with the book in the rounded state, the retention of the round will be maximized. Books that have been glued off with standard hotmelt, and subsequently rounded, have no such “memory.”

Application equipment

Special precautions must be taken with PUR to prevent premature cross-linking and to facilitate cleanup. Both open-pot and closed-extrusion systems have been developed to specifically deal with these needs.

Open-pot systems

The original North American open pot designed specifically for PUR was produced by web and sheetfed printer Commercial Printing Co. (Medford, OR) on a Müller Martini Star Plus binder. The Nordson Corp. (Westlake, OH), a manufacturer of glue premelters and nozzle extruders, then developed an innovative single wheel, which could be retrofitted on some existing binders. Within a few years, Kolbus began offering a specially designed open pot that would fit its binders. Soon after, Müller Martini began offering a PUR glue pot for certain models, such as the Corona, Acoro and Star binders.

Closed-extrusion system

The initial closed-extrusion system consisted of a premelter, a holding tank and recirculating hoses that fed the application head. This technology had limited success in the U.S., and a number of years were spent vainly attempting to improve this system.

Inatec GmbH (Langenfeld, Germany) then developed a novel system that involved extruding PUR onto the cover as it traveled from the binder feeder to the cover station, where the cover is attached to the book block. This worked well when the paper stock was easy to bind and the roughing station produced a smooth backbone. Penetration depended on the pressure of the cover station.

The newest technology, developed by Nordson, pumps the adhesive through a heated hose. The PUR is extruded onto the bottom of each book, which is held in place by a clamp (for more information, see “Almost perfect,” April 2002, p. 46). Since last year, there have been three systems installed in the U.S.; several more are operating in Europe. This system allows the glue nozzle to be turned on and off, and operators reportedly can control where the glue is applied. The innovation also keeps air away from the extruder at all times, which helps prevent excess waste of the PUR.

Satisfactory performance using the Nordson technology requires that the binder either be new or reconditioned, so there is no clamp-height variation as the book block passes over the extrusion head, ensuring a consistent application. The cover station supplies the pressure to allow the PUR to penetrate into the backbone.

It should also be noted that PUR is not recommended for older perfect-binding equipment. Rebsamen observes that as the binders wear out, precise application becomes problematic.

Cost concerns

One of the major concerns about PUR is its cost. While the price of PUR is about three times that of standard hotmelts, the application amount is generally half. When taken in the context of the cost of the entire book, these numbers become relatively insignificant. On a typical 8½ × 11-inch book that is one inch thick, for example, standard hotmelt would cost 1.3 cents per book (assuming an application thickness of 0.025 mils at $1.35 per lb.). Using PUR on the same book would cost two cents. As one plant manager states, “such a small increase in cost — 70 cents per 100 books — more than justifies the use of PUR. I can sleep at night, knowing that the job will not be rejected.”

Many PUR users have found there's no cost justification to switching back and forth between PUR and hotmelt because of this small difference in cost. In addition, when PUR is used exclusively, operators become much more familiar with the machine settings and are able to maintain the low application levels.

There is also the cost of retrofitting bindery equipment to use PUR. Since PUR is supplied in 55-gal drums (or five-gal pails for small users), binders need a method of transferring the material into the open pot or to the extrusion head, the most common of which is use of a drum or pail unloader. These typically range from $25,000 to $50,000 just for the unloader. The application system is an additional cost, typically from $25,000 to $35,000.

Productivity issues

At one point, curing time also posed a concern. While first-generation PUR could be trimmed inline, it had to sit for almost 24 hours after binding so books would have enough strength to withstand the rigors of shipping.

Second-generation PUR, developed in 1994, allowed inline trimming and built up sufficient strength for shipping within four hours of binding, provided enough moisture was present in the paper and the binding area maintained a high level of humidity. This remained a problem in winter months, however, as the relative humidity dips below 20 percent in many Northern and Western plants.

Curing time only took one hour with third-generation PUR, and it only required the moisture of the paper. Books can be shrinkwrapped directly offline. This modification came with a negative property, however. Since the PUR achieved green strength relatively quickly, some customers found that the PUR would become very heavy in the glue pot. This was especially true in humid areas, and at plants that bound mostly thin books, where the PUR sat in the applicator pot for long periods of time.

Finally, the fourth generation of PUR was developed. This thinner version of the third generation maintained the desired high green strength but extended the pot life. This generation has replaced most of the second- and third-generation PUR, and is especially successful in the new extrusion systems.

Observes National Adhesives' Ardanaz, “These new adhesives let binders ship [product] on the same day they've bound the spines. Since the adhesives set fast, the productivity rate increases. And as that increases, operational flexibility improves too.”

Future of PUR

PUR began its usefulness in midsize trade binderies that produced a high percentage of trade journals and annual reports. Since many of these binderies continue to bind highly coated and cross-grained paper, this market will continue to be the highest user of PUR. Further, more publishers have been exposed to the benefits of PUR and are demanding this type of binding to ensure quality and longevity of their products. Even the smaller binderies recently have been asked to provide PUR binding, which has led to its expanded use in many areas of the country.

Hardcover binding will be the next area to witness large PUR adoption. Because of the cost advantage of PUR binding over Smythe sewing, and the ability of fourth-generation PUR to be rounded inline, many hardcover-book manufacturers have been investigating the adhesive. The use of PUR for hardcover binding will escalate as binders gain confidence that this technology can show cost savings, without sacrificing quality.

Selling customers on PUR

Even if printers and bookbinders understand the advantages of using polyurethane reactive (PUR) adhesive on projects, their customers may not be sold on using a more expensive adhesive. Trade bindery Steffens Bookbinders (Twinsburg, OH) has found a way to convince its customers that PUR is a better option: President Bill Turoczy tells customers to put a book bound with ethylene vinyl acetate (EVA) hotmelt and one bound with PUR in a freezer overnight, then open the books in the morning. “When these skeptics call me, they tell me they heard the EVA-bound books cracking, but the books bound with PUR opened just fine,” explains Turoczy.

The freezer experiment simply simulates what would happen if someone left a book in his or her car during the winter. To experiment for summer-weather durability, users can leave the book in the backseat during the day, when the temperature easily reaches up to 110°F. As the EVA adhesive starts to fail in the heat, pages loosen from the binding.

“The percentage cost increase to use a robust adhesive is so small that we recommend it unless the customer expresses extreme price sensitivity,” says Turoczy.

And some jobs really require the robust PUR adhesive: Steffens Bookbinders once handled a job involving a glossy, smooth-surface paper stock, ink extending into the gutter, diecuts, aqueous coating and high strength requirements. PUR was the only adhesive that could bind the pages, according to Turoczy.

Above all, however, Turoczy suggests that printers and binders test new and unknown paper stocks with various binding adhesives to ensure customer satisfaction and to minimize project headaches. At Steffens Bookbinders, testing is a part of daily operations, so that execs will have complete answers for any customer questions.

Source: National Adhesives Corp.