Technical Articles
Glue Choices In The Bindery: Take The Right Tack
Published in New England Printer & Publisher
by Frank Shear
Every
bookbinding project is different. Your bindery professional must be
able to choose the right glue for every book. Let there be no
mistake: The many factors involved make this no easy task. Properly
adhering pages to a spine or crash sheet requires first selecting the
right family of glue (usually EVA or PUR) and second, the right glue
within that family.
There are
literally hundreds of bookbinding glues available on the market today and
each has a different combination of adhesive strength, tack, drying speed
and other important properties. The endless combinations of paper
substrates, coatings and end uses demand a scientific approach and an
artisan's touch. Keeping the myriad of important factors straight
is just the beginning.
Glue Families
Ethylene
vinyl acetate (EVA) hot melt glues are commonly used in traditional
perfect binding. Hot melt EVAs are made from a combination of solid
thermoplastic polymers, tackifying resins and waxes that function with the
right viscosity when properly heated in a glue pot. Although
different EVA glues demonstrate different properties, EVA is the right
type of glue for most adhesive binding projects of easy to medium
manufacturing difficulty. EVAs demonstrate strong adhesive
properties and "set" quickly. Modern EVA formulations are able
to bind a wider range of paper stock than yesteryear and no longer are
subject to as much chemical breakdown during the aging process.
After application, most EVA glues rigidly bind pages together, much like a
clamp. EVAs are subject to cold crack when stored in very cold
warehouses. However, some newer formulations have good flexibility
properties and are appropriate for layflat (Otabind) adhesive binding jobs
on uncoated stock.
Polyurethane
reactive hot melt (PUR) glues consist of solid polymer moisture curing
elements and are appropriate for difficult to "impossible" bookbinding
projects. EVA hot melts don't come close to matching PUR's
exceptional performance. Extremely durable PUR is the right glue for
frequently used books printed on heavy weight coated stock with inks and
other coatings bleeding into the spine gutter. PUR cures when
exposed to moisture in the air and is highly resistant to both re-melting
and cold cracking. Not surprisingly, PUR bound books are able to
withstand the most severe temperature and moisture conditions.
Moreover, PUR glues yield products that lie flatter and require less
backbone preparation than any other ultra-durable glue. On the
downside, PUR costs a bit more and requires an extended curing time,
usually about 24 hours. PUR technology has gained widespread
acceptance in New England's graphic arts community and has significantly
reduced the need for costly, slow and labor-intensive Smythe-sewn
signatures.
A third class of
bookbinding glue - cold applied polyvinyl acetate (PVA) - is not used
in general soft cover bookbinding because of long curing times and lack of
adaptability of most machinery to the constant cleaning necessary from job
to job. PVAs can be difficult to apply and can yield inconsistent
results when applied improperly. PVA glues (as well as animal glues)
do lay flat and are used mostly on telephone books and extremely porous
stocks.
The last class
of glues worthy of mention is the pressure-sensitive glue family.
Although pressure-sensitive glues aren't used for binding pages to
spines, they are useful in adhering hinge scores to the first and last
pages of perfect and layflat adhesive bound book blocks. They
accommodate long "open" time needs of the Otabind layflat adhesive
binding process. Although some companies prefer hot melt EVA side
glues, many prefer pressure-sensitive glues because of their reliability
and ease of use.
Page
Pull Strength Testing
In
general, binderies err on the side of strength, not flexibility, when
making glue decisions. Since strength is so important, good
binderies test all jobs as they are being run. The industry standard
for page pull strength is 2.5 pounds per linear inch, which means that a
page in a book with an 11" binding edge must withstand at least 27œ
pounds of applied pressure before pulling loose from the spine. The
best binderies have more stringent page pull standards and routinely
reject books if testing results fall below 3 pounds per linear inch.
Books
bound with EVA glue are easy to test because they fully cure within
seconds of being manufactured. Testing EVA glue strength is as
simple as taking a newly bound book to a page pull testing machine,
strapping it in and getting a page pull strength reading. This
entire testing procedure takes no longer than a minute and should be done
at regular intervals throughout all production runs. If your bindery
isn't consistently testing your glue bound books, demand that they do.
Testing
PUR books during production runs is more difficult because of their
required 24-hour curing period. Since this delay is completely
impractical during the course of normal business, traditional page pull
testing doesn't work. Instead, alternative testing methods have
been developed. A good way to gauge future PUR glue strength is to
slit the spine of a freshly bound book and bend it backwards,
cover-to-cover, exposing filaments of glue "strings." As long as
these strings are evenly distributed throughout the length and width of
the spine, and the overall glue thickness falls somewhere between 10 and
15 millimeters with no exposed bare areas, the book will perform as
intended. Once fully cured, PUR bound books routinely achieve
extraordinary page pull test results ranging between 4 and 7 pounds per
linear inch.
Manufacturing
& Other Considerations
Chemical
reactions. Paper manufacturers use different combinations of
coatings, clay fillers, pulp and bonding agents to make their papers what
they are: Different. Inks, varnishes and other offline paper
coatings have their own chemical properties and react differently to
different glues. In addition, end user habits and user environments
are all different. All of these factors can and do affect glue
performance. Literally thousands of chemical combinations must be
properly managed to achieve intended bookbinding results. Industry
horror stories abound. Even books that pass page pull strength tests
one day may fail the same test a few days later if solvents migrate
through substrates and negatively react with the glue. To anticipate
all that can happen, there simply is no substitute for decades of adhesive
binding experience.
Temperature.
Glue application temperature is critically important because it directly
affects how long glue remains "open" during the manufacturing process.
("Open" refers to the amount of time from when spine glue is applied
to when the cover is attached.) If the glue pot isn't hot enough,
open time will be too short and the covers will fail to adhere properly.
If it's too hot, the glue's viscosity will be too thin creating
excessive seepage and a host of other problems. Either way,
incorrect glue pot temperatures harm page pull strength, product
appearance or both.
Operator
attentiveness. Machine operators must be on the lookout for a
wide range of potential glue problems. Air bubbles and contaminants
can interfere with glue application, especially in the roller and glue
scraper areas. Operators need to constantly monitor the quality of
their work and keep glue pots and scrapers free from excessive dirt and
debris. Glue levels must always be high enough.
Outsourcing
communication. When outsourcing adhesive binding services, give
your bindery complete information about your job and let their
professionals recommend the right glue for your project. Provide
detailed information regarding book size, job components, book block
thickness, stocks used, any and all paper coatings and expected end use.
Problems happen during the quoting stage when too little information is
provided for what is initially called a "perfect binding" job.
What happens if you win this apparently routine job, print sheets and ship
them to your bindery? What if the text pages have varnish or inks
bleeding into the spine gutter? Unfortunately, this routine job is
suddenly anything but. Imagine your disappointment when you receive
the phone call from your astute bindery representative advising you that
the varnish will significantly hinder EVA performance. At this point
you won't have enough money in the budget to cover the additional PUR
costs
and you will be in a pickle. The bottom line is this: When
in doubt, provide more information to your bindery than you think
necessary.
* * *
Choosing the right glue for adhesive binding jobs isn't always easy. Printers and their sales forces should be armed with information about various glues and adhesive binding methods because it will help them win more business. During the project planning stage, communicate as much information as possible about your projects' end use. Let adhesive binding professionals help you plan your job. In doing so, you will eliminate unpleasant surprises and delight your customers. Isn't this the reason we're in business?
Frank Shear
is President of Seaboard Bindery, a service-oriented trade bindery located
in Woburn, Massachusetts. His company specializes in offering high
quality perfect binding, PUR and layflat adhesive binding, saddle
stitching, Wire-O, plastic spiral, and other bindery solutions. Call
Frank at (781) 932-3908 or e-mail him at frank@seaboardbindery.com.