I LLU M I NAT O R
March 2013
Making It Stick: Guide to Adhesives for Labeling Applications
By Ingrid Brase, Guest TLMI Illuminator Contributor and Market Segment Director, Pressure Sensitive
Adhesives, Henkel Corporation
Today’s labels serve many important functions: they provide information related to the product and are a
valuable marketing tool for brand managers. However, the label cannot convey this information if it does
not adhere well to the container or product. This article will attempt to provide some basic understanding
of adhesives, what makes the label stick.
Adhesives come in many forms and types. For labeling applications,
adhesives are available in two main forms: pressure sensitive and those
applied at point of adhesion (glue-applied). Glue applied adhesives were
the original products used most broadly. However, over time they have been
replaced by pressure sensitive adhesives. For the end user, the pressure
sensitive adhesive label has many advantage, key amongst them ease of
use. Instead of having to deal with rolls of labels, stocks of adhesive and
maintaining application equipment, now there are only rolls of labels to deal
with as pressure sensitive labels are delivered ready to apply.
Pressure sensitive adhesive labels comprise three main components: the
facestock, liner and adhesive. Facestocks, the ultimate label, vary by use
and include paper and film materials. Liners protect the adhesive on the
reverse side of the label and allow for easy dispensing. The final component,
adhesive, must stick to the surfaces where the label needs to adhere.
Adhesive performance is described by three key characteristics: adhesion, cohesion and tack. Tack is the
initial grab or attraction of the adhesive to a surface with no external pressure applied. Other terms used to
describe tack are “thumb appeal” or “quick stick”. This is an especially critical property in autodispensing
of the label where sometimes the only force is slight air pressure and immediate adhesion to the substrate
is critical. Adhesion is used to describe the bond strength between the adhesive and the substrate. Here
the development of the bond does involve some pressure application leading to the adhesive “wetting out”
on the surfaces that is, forming a continuous film between the label facestock and substrate. Adhesion
is measured by peel strength. Peel values are obtained by applying a force to remove the label from a
surface at a 180 or 90 degree angle. Cohesion is best described as the internal strength of the adhesive.
This is reported as shear strength or the ability of the adhesive to hold in place when weight is applied.
Shear is critical for prevention of edge lift, especially on very curved surfaces where the facestock wants
to return to its flat rigid shape. For labels all three properties are important; relative importance is very
dependent on the type of label.
The three adhesive properties can be used to divide adhesives into three very broad categories.
Permanent adhesives have higher adhesion and last for the life cycle of the substrate. They form a bond
that makes the label difficult to remove from the substrate. Removable adhesives will generally have
lower adhesion to substrates and remove cleanly with the facestock remaining intact even after aging.
Repositionable adhesives will build a stronger bond over time; they exhibit some “open time” where they
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I LLU M I NAT O R
March 2013
remove cleanly allowing for the label to be adjusted. Adhesives are specifically formulated to achieve the type of
permanence, repositionability or removability required in end use.
So what are the components used to create pressure sensitive adhesives? For label applications, two specific
polymer types are generally used; acrylics and rubber. Acrylics are available as water based emulsions at high
solids, usually 50+ %, or in solvent. Various acrylic esters and acrylic acid are polymerized to create the base
polymer. Rubbers, both synthetic and natural based, are usually applied as 100% solids hot melts. The majority
of rubber based hot melts use synthetic rubbers which are copolymers of styrene isoprene or styrene butadiene.
Hot melts are delivered by melting the solid material whereas emulsions and solvent based acrylics are applied
wet then dried. The polymers deliver the adhesion and cohesive strength of the adhesive. Performance is further
modified by the addition of other ingredients. Hot melts contain tackifiers which improve tack and adhesion,
process oils to aid coatability and antioxidants to provide stability. Acrylic emulsions are formulated with
surfactants and thickeners to improve coatability and wet out, as well as crosslinking agents to improve cohesive
strength. Solvent based acrylics utilize specific solvent blends to aid drying and wet out, as well as crosslinking
agents to build cohesive strength.
The selection of the correct adhesive chemistry to use is dependent upon available coating equipment as well as
end use performance targets. The table below summarizes the differences between the adhesive forms.
Polymer component
Form
Hot Melt
Tack
Adhesion
to glass and metal
to plastics
Cohesive strength
UV stability
Aging
Temperature Resistance
Water/ Humidity Resistance
Solvent Resistance
Solvent
acrylic
50+ % solids
acrylic
40+ % solids
subject to biological
attack
flammable storage
and handling
drying required after
application
good - excellent
drying required after
application
good - excellent
Material Handling
Handling and storage
Application method
Die cutting
Emulsion
rubber
100% solids
heat to apply
fair - good
End Use performance
moderate - high
good -excellent
excellent
good - excellent
moderate
good - excellent
fair - good
fair - good
low - high
good - excellent
fair - excellent
good - excellent
poor - good
fair - good
poor - fair
good
poor - fair
good - excellent
good - excellent
good - excellent
fair - good
poor - good
good - excellent
good - excellent
good - excellent
excellent
poor - excellent
Adhesive manufacturers are constantly striving to improve shortcomings of the various adhesive types so
these properties should be used as general guidelines. For instance, in recent years clear UV-stable hot melt
adhesives have been offered as well as acrylic emulsions which are more water resistant.
The substrate that the adhesive needs to stick to plays a key role in both selection and coatweight. Uneven,
rough surfaces such as wood, masonry, fabrics, and cardboard, require higher coatweight or thickness of
adhesive to create a good bond for the label. The adhesive must be soft enough to flow into crevices but have
enough cohesive strength to hold the label in place. As previously mentioned, highly curved surfaces require
higher cohesive strength to hold the facestock in place and prevent wing-up of the label as the facestock tries
to revert to being flat. The cleanliness of the surface will also influence both adhesive type and formulation
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March 2013
components. Dust, oil and moisture all represent challenges to achieving a strong bond of the label to the
surface. Smooth, clean surfaces are relatively ease to bond with, thus both acrylic and hot melt adhesives will
work well.
The end use performance requirements are the ultimate driver for adhesive choice. These can be divided
into four categories: labelstock, application temperature, service temperature and environmental conditions
the label will endure in use. Labelstock selection from various types of paper to film will be the first challenge
for good adhesion. The adhesive must have enough affinity for the label stock to hold a strong bond. This is
especially critical for removable labels as the good adhesion to the face will allow the adhesive to remove
cleanly from the surface. Clear labelstock will require a clear adhesive for best visual effect. Acrylics and hot
melts can both be used for paper labels whereas some filmic stocks may require only acrylics be used.
Application temperature is the temperature at which the label is applied to the
surface. For instance, most meat packing rooms are kept at lower temperatures
so adhesives are specially formulated to maintain good tack and wet out at lower
temperatures. Service temperature is the temperature the label in use. A label may
be applied at room temperature and be required to maintain bond strength in both
hot and cold conditions. Acrylics generally have a broader service temperature range
than hot melts. Acrylics are the chemistry of choice for high temperature applications
where hot melts will begin to soften.
Finally, labels need to survive a broad range of environmental conditions in addition to temperature. These
can include humidity, solvents, and ultraviolet light. Solvent acrylics and hot melts will generally be more
humidity resistant than emulsion acrylics. Hot melts are subject to degradation by both solvent and UV light
although recently formulators have been able to improve the UV stability of hot melts allowing for use in clear
label applications. The lifecycle of the label is also a key parameter to consider with solvent acrylics having
longer lifespans than emulsions and hot melts being the most vulnerable to degradation over time.
In summary, there is a wide range of adhesives available to use in the manufacture of pressure sensitive
labels. Understanding the key end use requirements is a critical factor in making the correct choice. Dialog
with adhesive suppliers and extensive testing will ensure the correct choice for the job and allow the label to
perform as needed.
Ingrid Brase is a Market Segment Director for Henkel’s
Pressure Sensitive Adhesives business in North
America. Prior to this role at Henkel she had held
various marketing and business management positions
for the Pressure Sensitive Adhesives business of
National Starch and Chemical Company. She began her
career as a research chemist in the Starch Research
group at National Starch, advancing to increasing roles
of responsibility within the starch, specialty chemicals
and adhesives businesses. She holds a B.S. degree in
chemistry from SUNY/ Oneonta as well as an MBA from
Rider University. Ingrid is a member of AIMCAL board
of directors and also active in TLMI.
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