Photo courtesy of Henkel Group/Nordson Deutschland GmbH
Table 1. Common Bookbinding Industry Definitions
Every day, we come into contact with one of the most
demanding adhesive applications: books. Adhesives are used in the manufacture
of books principally for the binding, but they are also used to make hardcover
book covers. Bookbinding demands sophisticated adhesive solutions for a very
Certain bookbinding applications require a
specific combination of adhesive strength, tack, drying speed and other
properties. The number of possible substrates, coatings and end uses are
seemingly limitless, considering that the final products include magazines,
catalogs, brochures, and more. Bookbinding adhesives must be able to be applied
in low-cost, high-speed processes, as well as be able to survive long-term
storage conditions and repetitive stress.
Hundreds of bookbinding formulations are
available on the market today, due to the significant demands placed on the
adhesives. However, modern bookbinding adhesives come from only a few families:
emulsion, hot-melt and reactive hot-melt. This article provides an introduction
to the processes and adhesives that are used in this industry. Conventional
formulations as well as innovative developments will be discussed. Important
definitions used in the bookbinding industry are included in Table 1.
Bookbinding Adhesive Processes
Two major commercial bookbinding techniques are used today:
traditional and perfect binding (or adhesive binding). Many variations exist
for each of these techniques as well. Traditional binding is generally used on
hardcover, or hardbound, books. However, perfect binding (developed originally
for paperback books) is being used widely as more durable adhesives are being
developed. In reality, perfect binding is one of many forms of adhesive
In perfect binding, adhesive alone is used
to attach the pages of a book together and attach the book’s cover. If only one
adhesive application is used to construct the book, it is called a “one-shot
process.” When two adhesive applications are required, it is a “two-shot
process.” In the latter process, the first application of adhesive binds the
bases of the book together. This adhesive is sometimes referred to as “primer”
glue. The second application of adhesive is used to attach the book’s cover to
the book spine. This adhesive is referred to as the “cover” glue.
The one-shot perfect binding process involves
the following steps.
- The signatures (a group of 16 or
32 double pages that are folded in the center) are gathered in proper
- The spine, or “backbone,” of the
book receives a “grind” or “roughening” so that adhesive can penetrate into the
- A coating of adhesive is applied
to permanently bind the leaves together with the cover.
Roughening is an important step in this
process. Roughening occurs by stacking and securing the signatures and then
using a milling station to remove a few millimeters of paper. Milling also cuts
shallow notches into the spine of the text block. The adhesive is then
carefully applied to the roughened signature group at a predetermined coating
weight. Lastly, the block gets placed onto a waiting cover that is wrapped and
clamped at the spine until the adhesive hardens.
Paper selection is also crucial to the
success of perfect binding. Modern paper stocks have more fillers and less
grain and paper fiber, which means they need exceptional spine roughening.
Aqueous coated stocks can also sometimes affect adhesion.
Perfect binding is growing in popularity
across all types of books (paperbacks, magazines and journals, as well as
traditional thick and thin hardbound books). It has become a well-mechanized
process with specialty equipment. However, many in the industry still believe
that perfect binding is an art, primarily because of the many factors
contributing to success.
To be competitive, the multiple steps
required in binding a book must occur inexpensively and at a very high speed.
As a result, hot-melt adhesives have achieved a strong competitive position
relative to adhesive emulsions, especially for long-run production jobs. Hot
melts provide the bookmaker with the capability of running at least 30% faster.
The operations of assembling the book sections, trimming the back, applying a
hot-melt adhesive, fastening the cover, and trimming the remaining sides of the
book generally must be completed in a time span of fewer than 10 seconds.
Table 2. Bookbinding Adhesives
Types of Bookbinding Adhesives
At one time, starch and animal glues were the dominant
bookbinding adhesives. These have gradually been replaced by hot-melt and
emulsion adhesives. However, lower-quality bindings (e.g., telephone books,
mail-order catalogs, etc.) may still be bound with these earlier adhesive
products. Table 2 provides a list of adhesive types commonly used in various
bookmaking operations. Adhesives currently used in bookbinding fall into three
categories: water-based emulsions, hot melts, and reactive hot melts (or “warm
Modern-day bookbinding adhesives are often
based on ethylene copolymers such as vinyl acetate ethylene (VAE) emulsions or
ethylene vinyl acetate (EVA) hot melts. In both types, ethylene polymer
segments in the molecule provides flexibility, while pendant ester (acetate)
groups yield an amorphous and chemically functional polymer, which is needed
for good adhesion. In reactive hot-melt adhesives, the base polymer is
generally polyurethane; from this backbone, the adhesive realizes strength and
Table 3. Starting Formulation for a Flexibilized PVAc Bookbinding Adhesive
Emulsion-Based Bookbinding Adhesives
Water-based emulsion adhesives are most often based on
polyvinyl acetate (PVAc) homopolymers or vinyl acetate ethylene (VAE)
copolymers. VAE adhesives are generally preferred, due to their inherent
flexibility without external plasticizers. These water-based adhesives are most
often used in the traditional binding processes, and are also used as a primer
adhesive in two-shot perfect binding, especially when the cover glue is a
With emulsion adhesives, the water carrier
provides excellent penetration into the paper. The penetration provides the
solid bond necessary to hold the book together. This generally results in
superior flex and pull-strength characteristics, and makes cold-emulsion
adhesives the most durable method of adhesive binding. As a result, emulsion
adhesives are used in traditional binding, even though they provide a slower
process than hot melts.
Emulsion adhesives cure to a semi-soft
state, which allows the spine to be more flexible, unlike most hot-melt
adhesives. Under extreme cold temperatures, emulsion adhesives do not crack.
However, they do have relatively weak page-pull strength; that is, pages can be
pulled out fairly easily.
PVAc emulsions have been used as
bookbinding adhesives; without any plasticizer, however, they will become hard
and brittle after time. Adding a plasticizer will soften the PVAc polymer and
eliminate embrittlement. A typical PVAc adhesive formulation for bookbinding is
provided in Table 3.
Externally plasticized homopolymers, such
as PVAc, will generally provide considerably stronger bonds than internally
plasticized copolymer. This is because the externally plasticized emulsion will
harden as plasticizer migrates into the environment or paper stock during
aging. This results in about a 100%-increase in pull-test values. Although this
increase in strength becomes stabilized with time, the loss of plasticizer can
be detrimental to other properties, such as flexibility and cold-temperature
Emulsion bookbinding adhesives can be
difficult to apply, due to their low solids content and foaming characteristics.
They can also yield inconsistent results when applied improperly. However,
emulsion adhesives provide books that can lay flat. They are used mostly on
telephone books and extremely porous stocks, in addition to most traditionally
Table 4. Starting Formulation for an EVA Hot-Melt Adhesive
Hot-Melt Bookbinding Adhesives
The term “perfect binding” was created in 1911 to describe
hot-melt adhesive application in the bookbinding process. At that time, hot
melts attempted to overcome the disadvantages of emulsion adhesives while
retaining the desirable characteristics of pull strength and flexibility. The
driving force toward hot-melt adhesives was their high speed, minimal waste and
relatively low cost.
Hot-melt adhesives have advanced so much
that the term “perfect binding” is now used to describe all adhesive binding,
not just the hot-melt segment. Hot-melt bookbinding adhesives are most often
based on ethylene vinyl acetate (EVA) copolymers. However, styrene block
copolymers (SBCs) and polyurethanes are also gaining a strong stature in the
In hot-melt bookbinding adhesives,
flexibility is highly valued; many conventional hot-melt polymers are
relatively rigid. Low-melt viscosity is also a requirement, since this is
generally positively correlated with page pull strength. As a result,
high-molecular-weight hot-melt polymers are generally used as a base polymer in
adhesive formulations along with various plasticizers, waxes, antioxidants, and
Adhesive-application temperature is
critical in this application because it directly affects how long the hot melt
remains “open” during the manufacturing process. If the hot melt is not hot
enough, open time will be too short and the book covers will not adhere
properly. If it’s too hot, the adhesive’s viscosity will be too thin, creating
excessive seepage and other problems.
Waxes are often used to reduce adhesive
viscosity and blocking characteristics. Soft, high-melting microcrystalline
waxes improve flexibility. Hard, high-melting microwaxes and synthetic waxes
improve elevated temperature properties. Migrating plasticizers can be used to
temporarily reduce an adhesive’s viscosity to an acceptable range. After aging,
the plasticizer migrates into the paper and a tougher-than-normal adhesive
remains as the binding.
Ethylene Vinyl Acetate
Hot-melt EVAs are made from a combination of solid
thermoplastic copolymers, tackifying resins and waxes that function with the
right viscosity when applied in bookbinding operations. EVA adhesives
demonstrate strong adhesive properties but, most importantly, they set quickly,
due to their hot-melt character.
After application, EVA adhesives rigidly
bind pages together. Modern EVA formulations have page-pull strength and flex
characteristics equal to emulsion adhesives. Modern EVA formulations are able
to bind a wider range of paper stock than older formulations, and are no longer
subject to as much chemical breakdown during the aging process. EVAs are
versatile enough to be used on both traditional and adhesive bindings. For
example, a “roundable” hot-melt adhesive can be used to create a rounded spine
in a traditional binding.
EVAs are subject to cold crack when stored
in very cold warehouses. However, some newer formulations, with appropriate
plasticizers and modifiers, have good flexibility and are even appropriate for
lay-flat adhesive binding jobs. Guides to formulating EVA hot-melt adhesives
have been published.1
A typical starting-point
formulation for an EVA hot-melt adhesive for bookbinding is shown in Table 4.
Styrene Block Copolymers
Table 5. Hot-Melt SBS Assembly Adhesives for Bookbinding
SBC hot-melt adhesives exhibit excellent bond strength to
many different types of paper stock, including coated paper. Their network
molecular structure allows them to exhibit a degree of physical crosslinking
that provides high- and low-temperature resistance, creep resistance, and
excellent flexibility. They also provide a very fast set due to their
SBC-based hot-melt adhesives often require
the use of water-based primer to improve adhesion to the book spine. In this
way, SBC-based hot-melt adhesives are often used in making
rounded-spine-edition books. SBC-based adhesives have a good combination of low-cost,
adhesion, physical properties, and processing characteristics.
One of the better SBCs for bookbinding has
been found to be a radial block copolymer having a styrene content of about 35%
by weight, tackifying resins, and at least one wax diluent. This waxy diluent
is usually either a paraffin wax or a microcrystalline wax. A
styrene-butadiene-styrene (SBS) hot-melt adhesive formulation for bookbinding
is shown in Table 5.
Component Reactive Polyurethane Hot Melts
Reactive polyurethane hot-melt adhesives are relatively new
to bookbinding. Originally developed in the 1980s for use on both furniture and
automobiles, they are one of the strongest-binding adhesives available.
Hot-melt polyurethane adhesives are solid prepolymers with a lower melting
point than conventional hot-melt adhesives. The prepolymers are produced from
solid polyols and isocyanate, and have a low number of free isocyanate groups.
Several articles that include formulation details on reactive polyurethane
hot-melt adhesives are available.2
PUR hot melts are applied similarly to
conventional hot melts but at lower temperatures (i.e., 120-150° rather than
170-180°). The adhesive sets quickly when it cools from the melt form to a
solid, and at that point has reasonable strength. However, the adhesive will
continue to develop strength due to a chemical reaction that occurs between the
polyurethane and environmental moisture. Once cure is complete (generally in
24-48 hours), the bond is far superior to ordinary hot melts - by as much as
twice the strength. Due to the crosslinking provided by the moisture reaction,
PUR hot melts are also highly resistant to remelting, cold cracking and
The largest disadvantage of PUR hot melts,
as might be expected, is that books should not be tested for strength until 24
hours after adhesive application. Also, the application equipment and adhesive
formulations, relative to EVA products, are expensive.
To increase production speed, specially
accelerated PUR hot melts have been developed. Generally, organometallic
catalysts are used to accelerate cure, and complete cure can be realized within
a few hours. Because of this, the bonded book pages can be tested after just a
Polyurethane works well with all paper
stocks, including synthetics and UV-coated stock. PUR hot-melt adhesives are
more flexible than EVA adhesives, making them a popular choice for lay-flat
adhesive bonding applications. Because of their low application temperature,
PUR adhesives do not cause warpage of the substrate, and they can be applied in
a uniform, thin coating. This helps prevent wavy spines and other problems that
can be associated with higher-viscosity hot melts.
Table 6. Important Performance Properties of Books
Specifications and Requirements
Important performance properties of books include page-pull,
lay-flat, page-flex, cold-crack, easy open and spine flexibility
characteristics (see Table 6).
For optimum performance, the adhesive must
have excellent flexibility, toughness (without stiffness) and fatigue
resistance. These properties must be maintained at an elevated temperature
(60°) over long periods of time. In the bookbinding industry, the adhesive is
generally used as a non-pressure-sensitive assembly adhesive. Tack is usually
of less concern in the bookbinding industry than in other industries; in fact,
tack is often not wanted, and a non-blocking agent will be included in the
In general, binderies will err on the side
of strength, not flexibility, when making adhesive 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 lbs per liner inch.
Although bookbinding adhesives today are far superior to
those used several decades ago, the bookbinding industry continues to demand
increased performance from adhesive manufacturers. In particular, improvements
are required in drying characteristics, bond strength, flexibility, shape
retention, and versatility of both aqueous emulsion and hot-melt bookbinding