Technical Articles
Can Cure Times Be Reduced for PUR Adhesive Binding?
By: Kris Koch, President, Allied Bindery Company
Published in Printing Views
One of the key advancements to occur in the perfect binding world has been the use of PUR (polyurethane reactive) adhesives. PUR has been in use in the graphic arts industry for about a decade, and its advantages are numerous. It offers an extremely strong and durable binding, even in the most extreme usage environments and climates. It’s compatible with all types of stocks, including synthetics. PUR adhesive also offers the ability to form a strong bond on sheets containing even the heaviest ink and coating coverage. In short, it’s the best adhesive available for today’s demanding adhesive binding projects.
If there is one aspect to PUR adhesive that’s posed a challenge to this point, it’s the relatively slow curing rates compared with standard EVA hot melt binding adhesives. Most perfect binding providers that use PUR have found that bound books must be allowed at least 24 hours to fully cure. However, new glue formulas and binding techniques have narrowed that window by allowing books to be handled much sooner. That has allowed binders to improve turnaround times while still offering the same high quality product.
Adhesive Chemistry 101
Part of the reason why PUR is slower to cure than standard hot melt adhesives is the chemical makeup of the glue. Traditional EVA (ethyl vinyl acetate) perfect binding adhesives cure by dispersing moisture into the surrounding atmosphere. As the glue dries, it stiffens and cures to create a firm bond with the book block and cover. There are a wide range of EVA glue formulas, most of which are engineered to cure within minutes of application.
By contrast, PUR adhesives absorb moisture from both the surrounding atmosphere and the paper itself during the curing process. While this process may take longer, the strength of the bond that results is vastly superior. As with EVA adhesives, PUR is available in several formulas that vary in how quickly they cure. Some of the newer PUR formulas have chemical catalysts that allow books to be handled much sooner than the typical 24-hour window. These “catalyzed” PUR adhesives are engineered to promote sufficient grain strength as soon as possible to allow books to be handled even if they’re not fully cured.
The Quest for Better Turnaround Times
Of course, “sufficient” is a relative term. At Allied Bindery, we have been using PUR adhesives extensively for several years. That experience has allowed us to become adept at judging the point at which a wide range of book varieties can be handled. However, as production quantities and turnaround times have become shorter, the curing time of PUR adhesives has become more of a factor in the planning process. As a result, many customers have inquired as to exactly when in the curing process PUR-bound books can be handled for subsequent production – essentially asking for numbers to clarify what we mean when we say they have “sufficient” strength.
We were equally curious about the capabilities of the adhesive, so we recently conducted extensive testing with our PUR adhesive manufacturer, Henkel Technologies of Lewisville, Texas, to determine exactly how much page pull strength we are able to achieve at various points during curing. The tests were conducted at our facility using a book currently in production that represented a “typical” PUR project: a 190-page, ½” thick catalog printed on 50lb. coated offset paper.
As luck would have it, the tests were conducted under atmospheric conditions even Henkel representatives deemed a “worse case scenario”: an inside temperature of 67 degrees Fahrenheit and 21% relative humidity. The humidity was the biggest factor, as PUR draws moisture from the atmosphere to cure; the higher the relative humidity, the faster curing will take place. With 21% humidity, the adhesive was going to have a tough time finding enough moisture to cure quickly.
The testing was conducted on a typical Michigan winter day: 31 degrees Fahrenheit with light snow. That made for dry conditions not only in the plant, but also in the paper. The cold weather outside the plant and temperate, dry conditions within the plant meant the paper was parched, so to speak. Since PUR adhesive pulls moisture from the stock during curing, there was little moisture here for the adhesive to find.
Surprisingly Good Results
Henkel representatives pulled 36 books that morning for page pull testing throughout the day. After conducting the cure rate trial, Henkel determined that the books were 90% cured after seven hours under those worst-case conditions. That 90% is the point at which paper begins to tear during page pull tests, meaning the adhesive bond is stronger than the internal strength of the paper. For practical purposes, that means it’s more than strong enough for the books to be run through subsequent processes without shifting or shimmying.
Under high-heat, high-humidity conditions, the manufacturer stated that those books would have been 90% cured in four hours. However, it has been our experience that books can realistically be handled even sooner than that in most cases, something with which Henkel representatives concurred. After just three hours under our far-less-than-ideal conditions, the PUR-bound books achieved an average page pull strength of greater than 2.5, which is the GPO industry standard page pull strength for fully-cured EVA adhesive-bound books.
What does all of this mean? Simply put, PUR-bound books are able to achieve sufficient adhesion and page pull strength – by which we mean greater than that of a fully-cured perfect-bound book – in a fraction of the 24-hour window normally associated with PUR adhesive curing. That allows binders with PUR capabilities to produce books within even the most demanding turnaround schedules without sacrificing strength, durability, flexibility or quality.
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Allied Bindery Company is a full-service post press services company located in Madison Heights, Michigan. Allied Bindery Company specializes in a complete range of adhesive binding services, including PUR binding, perfect binding, notch binding and more. Additional capabilities include folding, trimming, cutting, collating, saddle stitching, shrink wrapping and more. Kris Koch can be reached at (800) 833-0151 or kris@alliedbindery.com.
