PUR (Polyurethane Reactive) adhesives have been in bookbinding, primarily since 1989.  The first test in North America was run on the west coast using a homemade application system.  Since then, the use of PUR has increased dramatically.  In 1995, it was reported that there were 28 users of PUR, and by the end of 2010 this number has increased to 100+ users in North America.

Advantages of PUR over standard EVA hotmelts

PUR has been chosen over standard hotmelt adhesives for many reasons—with superior adhesion and layflat, being the most important ones.

  • Adhesion: PUR is unique in that it will bond to lacquer coatings, UV cured coatings, films such as mylar, as well as clay coated paper. 

  • Excellent Layflat: Films of PUR, when applied at the recommended thickness, are considerably more flexible than standard EVA hotmelts.  PUR is suggested to be applied at 0.25-0.3 mm because it is twice as strong as standard EVA hotmelts

While the adhesion and layflat qualities are very important in the use of PUR, there are many other qualities that provide benefits for the bookbinder.

  • Heat and Cold Resistance: Once the PUR adhesive has cross-linked, books bound by these adhesives will not fail at extreme temperatures over 1000C and below -400C.  These properties were especially important to a binder in the US who sent books to both the Sahara Desert region as well as areas in Siberia.  Since turning from hotmelt to PUR, this binder’s adhesive related complaints have been eliminated.

  • Less Wrinkling of Backbone: Binding cross-grained paper with standard hotmelt adhesives causes considerable wrinkling in the gutter area of the book.  The lower operating temperature of PUR does not drive the moisture in the paper away from the backbone, the way normal hotmelt does.  PUR is also somewhat malleable as it cures, allowing the paper fibres to return to their original orientation. 

  • Less “Chip-out”: Since PUR is applied at a low temperature and since it is applied at half the standard thickness of hotmelt, there is much less chance for the material to build up on the trimmer knives and nick the cover material during trimming. 

  • Square Backbone: The amount of PUR applied is considerably less than standard hotmelt, therefore less material is available to be squeezed out when the cover station and side nipping attach the cover and form the back.

  • Solvent Resistance: According to Werner Rebsamen’s article “Insidious Solvents” (WB Times, Aug-Sept 2005), PUR is the only binding material today that is totally resistant to solvents and oils.  Samples of the cured PUR adhesive have been immersed in oils and solvents, such as MEK and alcohol, that are used in the printing industry and these chemicals have had no effect on the PUR film.  Standard hotmelt films are softened drastically and even dissolved by some of these chemicals.

Then there are some additional benefits:

  • Cost savings vs. Smythe Sewing:   For many years the standard of the industry to produce rounded, hard cover books was to use Smythe sewing and glue the sewn books with a flexible liquid adhesive prior to rounding. A study in Europe has shown that savings of 30%-40% in cost can be realised by milling off the backbone of the folded signatures and then gluing them off with PUR.

  • Roundability and Round Retention: Many hard cover books are rounded and backed to provide both aesthetics as well as strength to the final book.  By gluing off the book block with PUR, followed by in-line rounding and backing, a significantly greater round can be obtained.  As the PUR cures with the book in the rounded state, the retention of the round will be maximised.  There will be no “memory” as is found when rounding books that have been glued off with standard hotmelts.

Application Equipment

Due to the nature of PUR, special precautions must be taken in order to prevent premature cross-linking and to facilitate clean up of the PUR.  Both open pot systems and closed extrusion systems have been developed since 1989 to specifically deal with these special needs.

  • Open Pot Systems: Initially, an open pot designed specifically for PUR was produced by commercial printing in Medford, Oregon.  It was on a Muller Martini Star Plus binder.  In order to take advantage of PUR, some of the binders on the west coast adopted this application technique. 

  • Closed Extrusion Systems: The initial closed extrusion system consisted of a premelter, a holding tank, and recirculation hoses feeding the application head.  This technology had limited success in the US.  A number of years were spent attempting to improve this system. The latest extrusion systems from Nordson Corporation and Robatech, Switzerland have made the application of PUR adhesives relatively simpler.

Concerns

One of the major concerns for the use of PUR is the price of the material.  While the price of PUR is about 3-4 times that of standard hotmelts, the application amount is generally half or less than the amount of hotmelt normally applied.  (Welbound has developed a price comparison calculator for PUR v/s Conventional EVA HMA. Please send a mail to This e-mail address is being protected from spambots. You need JavaScript enabled to view it , for a free copy). As one plant manager put it “a small increase in cost, more than justifies the use of PUR and I can sleep at night, knowing that the job will not be rejected.” 

Many users of PUR have found that switching back and forth from PUR to hotmelt is not justified because of this small difference in cost.  In addition, operators become much more familiar with the machine settings and are able to maintain the low application levels when PUR is used exclusively.

Another concern involves the emission of MDI, one of the ingredients used to make PUR.  The MDI is present in very small concentrations (parts per billion) and newer generations have cut the levels in half.

Then there is the length of time it takes the PUR to cure so the books can be trimmed and shipped.  PUR was first introduced into the United States bookbinding industry by the erstwhile National Adhesives (now a part of Henkel International) in 1989.  The first generation of PUR, while it could be trimmed inline, needed almost 24 hours before enough strength developed so the books could be shipped.  This delay in shipping was found to be completely unsatisfactory for most of the industry.

The second generation PUR was developed in 1994 that allowed inline trimming and sufficient strength within four hours of binding, if enough moisture was present in the paper and the relative humidity of the binding area was high enough.

The third generation PUR was developed in 1997.  This generation’s page pull development was achieved within an hour of production and only required the moisture of the paper to achieve final cure.  However, since the green strength was very high at an early time, especially in humid areas, it was found that the PUR was becoming very heavy in the glue pot. 

Finally, the fourth generation of PUR was developed.  This was a lower viscosity version of the third generation and maintained the high green strength, but extended the pot life.  This generation has replaced most of the third and second generation PUR, and is especially successful in the new extrusion systems and continues to be used in many systems today

The future of PUR

PUR began its usefulness in binding highly coated and  cross-grained paper and this market will continue to be the highest user.  More and more printers and publishers are aware of the benefits of PUR and this in turn is driving the demand.

Another emerging market for PUR usage is that of photo albums.  These books typically have very high quality paper stock and the purchasers require durability, flexibility, and strength only obtained by using PUR.

The next large increase for PUR will be in the hard cover binding area.  Because of the cost advantage of PUR binding over Smythe sewing, and the ability of the fourth generation of PUR to be rounded in line, many hard cover book manufacturers have been investigating PUR.

Another critical application is that of soft cover textbooks in countries that have hot summers. In order to prevent the books from falling apart, the publishers take the precaution of thread sewing or side stitching the book blocks, prior to pasting on a perfect binder. This bottleneck can be removed with the introduction off PUR.

Chuck Cline is a chemistry graduate who worked with the erstwhile National Adhesives for 30 years. He was part of the field trials of most of the 'path breaking inventions' in the adhesives industry including COOLBIND, ULTRACASE and PUR, in the North America. In 2001, he founded  'Binding Solutions LLC', an independent consulting firm.


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