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DK4036_book.fm Page 1 Monday, April 25, 2005 12:18 PM

94
Pressure-Sensitive
Adhesives and
Adhesive Products

94.1 Introduction ......................................................................94-1
94.2 Adhesives............................................................................94-1
94.3 Adhesive Properties...........................................................94-2
Tack • Peel Adhesion • Shear Resistance • Other Tests • Dynamic
Mechanical Analysis
94.4 Products .............................................................................94-5
Tapes • Labels • Other Products
94.5 Processing ..........................................................................94-5
D. Satas* Coating • Drying • Slitting and Die-Cutting
Satas & Associates References .....................................................................................94-7

94.1 Introduction
Pressure-sensitive adhesives (PSAs) adhere to various surfaces just by application of a light pressure and
usually can be easily and cleanly separated. To meet these basic requirements, the adhesive must be
sufficiently soft in order to be tacky to touch and also have a sufficient cohesive strength to maintain its
integrity upon separation.*
The invention of rubber-based PSAs is attributed to Dr. Henry Day.1 Such an adhesive consisted of India
rubber, pine gum, and other ingredients. The first patent was issued to Shecut and Day in 1845.2 The early
uses were limited to medical applications, and the progress of further development was slow. The first
industrial applications of pressure-sensitive tapes appeared only in 1920 and labels in the late 1930s.
The technology of PSAs and adhesive products is covered extensively in the Handbook.3

94.2 Adhesives
The basis of a PSA is an elastomer made tacky by addition of tackifying resins. Natural rubber was the
first material used and still is among the most important elastomers for compounding of PSAs. The
general composition of such compounded adhesives is as follows.
Elastomer, 30 to 60%
Tackifier, 40 to 60%
Plasticizer, 0 to 30%

*Deceased.

94-1

© 2006 by Taylor & Francis Group, LLC


94-2 Coatings Technology Handbook, Third Edition

Filler, 0 to 30%
Antioxidants, 1 to 2%
Other elastomers used widely in addition to natural rubber are styrenic block copolymers, especially
those with isoprene midblock. Block copolymers made the basis for successful hot-melt adhesives. Styrene
butadiene rubber, polyisobutylene, and butyl rubber are also used.
These elastomers must be tackified with resins compatible with the elastomer forming a tacky material.
The resins may be synthetic of petroleum origin (based on C5 or C9 monomers) or they may be based
on wood rosin collected from coniferous trees (or from tall oil, a by-product of paper manufacturing).
Such rosins are chemically altered to improve their stability.
Another category of PSAs is copolymers of acrylic esters. These polymers are inherently tacky and do
not require compounding with resins or other additives, although they may be sometimes compounded
to enhance or change their properties. They are clear and have a good resistance to oxidation and
ultraviolet (UV) light. Acrylic copolymers quickly became the most important materials for pressure-
sensitive products.
Silicone PSAs are also available. They are expensive and their usage is small, but their usefulness at
high and low temperatures is unique and not approached by other polymers.
PSAs are available in several forms: as solutions in organic solvents, as aqueous emulsions, as hot-
melts, as 100% solids for calendaring or extrusion applications, and as 100% reactive materials for thermal
or irradiation curing after application. Solution adhesives are the easiest to apply and are the most versatile
in their properties. They are not desirable environmentally, because of solvent emission; but many
companies have invested in solvent recovery equipment and can effectively handle solution adhesives.
Hot-melt application has the advantages of environmental acceptability, high coating speeds, and less
expensive equipment. Hot-melt adhesives are based mainly on block copolymers. Aqueous adhesive
emulsions, along with hot-melts, are the fastest growing types. Acrylic adhesives are important as aqueous
emulsions, though SBR and natural rubber latexes are also used. The 100% solid adhesives are used in
calendaring processes and can be applied by extrusion. There is considerable current interest in 100%
reactive adhesives, especially cured by UV radiation, but their future is questionable.

94.3 Adhesive Properties
The main properties of PSAs are considered in the trade to be tack, peel adhesion, and shear resistance.
Sometimes these properties are thought to constitute the adhesive performance. If under the adhesive
performance we understand the adhesive product behavior in its application, then these test values,
although important, are not sufficient.

94.3.1 Tack
Tack is the capability of the adhesive to form a bond at low pressure during a short time period. It is
measured (most frequently) as the force required to separate such an instantaneous bond formed at low
pressure. A subjective measure of tack is experienced by placing a thumb over the adhesive surface and
quickly removing it. Tack is tested by several techniques, and it is basically defined by the testing method.
Thus, the probe tack test has been designed to approximate thumb tack, the loop tack and quick stick
techniques are similar to peel testing, and the rolling ball tack test employs a steel ball rolling on the
adhesive surface. While tack is the main property that makes the adhesive pressure sensitive, its numerical
determination depends on the testing technique used, and the tack value is not that important in
describing the adhesive performance. Figure 94.1 shows the loop tack measurement.

94.3.2 Peel Adhesion
Peel adhesion is measured by applying the tape to a standard stainless steel panel and measuring the
force required to remove the tape by peeling at 180 or 90 degrees. The peel test gives important data in



Pressure-Sensitive Adhesives and Adhesive Products 94-3

FIGURE 94.1 Loop tack test.

A B

C
D

FIGURE 94.2 Schematic diagram of various peel tests: (A) 180-degree peel; (B) 90-degree peel; (C) drum peel (tape
unwind); (D) T-peel.

evaluating the adhesive. However, the peel values depend on the test conditions and on the stiffness of
the tape backing. We are measuring not only the force required to break the adhesive bond but also the
energy expended to deform the backing and the adhesive itself. The test results, therefore, are comparable
only to the adhesive applied to the same backing. Figure 94.2 shows the 180° peel test arrangement, and
Figure 94.3 shows the peel test results for tapes with various backings and various backing thicknesses.
In the case of paper label stock, the failure is paper failure, except in the case of removable adhesives.
In label stock, however, the release of silicone-coated liner from the adhesive is important, and the release
is tested similarly to the peel test.

94.3.3 Shear Resistance
Shear resistance is the only test of the main three tests that measures directly the cohesive strength of the
adhesive, and it correlates well with its viscosity. The shear resistance can be measured by recording the




5
A – Mylar Polyester Film
B – Aluminum Foil 1145-H19
4 C – Aluminum Foil 1145-0
Peel Force – Lbs. D – Urethane Rubber Film

B
3
C

2

A
D
1

2 4 6 8 10 12 14 16 18 20 22
Backing Thickness - In × 10−3

FIGURE 94.3 The dependence of peel force on the backing thickness in a 180-degree peel test. [Reprinted from
Adhesive Age, 9(8) (1966).]

FIGURE 94.4 Shear test.

time to failure (cohesive) under some static load. Shear creep is measured by the distance the adhesive
has moved in a given time under a static load. Shear resistance shows directly the capability of the adhesive
to carry the load, and it is an important factor in predicting the adhesive’s behavior under the load.
Figure 94.4 shows the shear test setup.

94.3.4 Other Tests
The main information expected from testing is the determination of whether the adhesive performs well
in the intended application. The above tests generally do not give such information, and therefore a large
number of speciﬁc application tests are used. Special tests are available or are improvised for various
applications, such as hospital and ﬁrst aid tapes, electrical tapes, packaging uses, masking and surface
protection tapes, and other uses.

94.3.5 Dynamic Mechanical Analysis
PSAs are viscoelastic materials, i.e., their properties depend upon the rate of force application. Therefore,
it is of interest to investigate the mechanical adhesive properties (such as moduli) at various shear rates.
Such data also appear to shed considerable light on the adhesive performance, which mainly depends
on the bulk adhesive properties. This test is covered in a separate chapter.




94.4 Products
PSA-coated products are subdivided into three main groups: tapes, label stock, and other products. Tapes
are narrow strips of material with adhesive applied on one side and wound in a roll. Generally no release
liner is needed; the adhesive unwinds from the outer side of the backing, which may be coated with a
special release coating.
Label stock is printable material with the adhesive coating on one side, which is protected by easily
removable silicone-coated liner.

94.4.1 Tapes
Tapes can be subdivided according to the backing material used: film tapes, fabric, paper, foil, foam, etc.
Special tape products are double-coated tapes, which have adhesive on both sides of a supporting sheet
(film, paper, nonwoven fabric, foam), and transfer tapes, which consist of an unsupported adhesive film.
Double-coated tapes and transfer tapes are protected by a silicone-coated release liner. These tapes are
heavily used for product assembly applications and mounting and holding uses.
Tapes are also subdivided according to their application. Packaging tapes are the largest single application.
This field is dominated by OPP film tape, but other film, paper, and glass fiber reinforced tapes are also
used. Medical tapes and related products constitute an important product area. Office tapes are dominated
by matte acetate film tape, although cellophane film tapes are still used. Electrical tapes include many
products starting with black vinyl electrician’s tape and ending with high-temperature fiber/film tapes with
silicone adhesive. The automotive industry consumes many different tapes: electrical harness wraps, paint
masking tapes, product (including foam) assembly tapes, and tapes for many other applications. The
construction industry requires paint masking tapes, foam tapes for gasketing, tapes for insulation of heating
and air conditioning ducts, and many other uses. The appliance industry uses wood grain and other surface
decorating tapes, tapes for attaching name plates, foam gasketing tapes, etc. Tapes are used in all industries
for many different applications. In most cases, PSAs are used for easier and more convenient application.

94.4.2 Labels
PSAs are increasingly used for labels. Although they are more expensive than labels secured by waterborne
or hot-melt adhesives, because a release liner is required, PSA label application equipment is less expen-
sive, it is easier and cleaner to run, and the adhesive is quite suitable for plastic surfaces, such as
polyethylene containers. PSAs are used for impervious foil labels; they are easier to use for intricate label
design, and they are used for transparent film invisible labels.
The label industry consists of two separate groups. Large manufacturing companies produce label
stock: unprinted adhesive-coated large rolls or sheets. These are sold to label manufacturers who print
and die-cut the labels. While there are only few label stock manufacturers, there are many label printers,
often servicing a limited geographical area.

94.4.3 Other Products
PSAs are used for many different products other than tapes and labels. The medical area is a large one
for a variety of dressings, transdermal drug delivery systems, and other products and devices that use
pressure-sensitive adhesives to secure these devices to the human body. Easily removable stickers (Post-
its) have been developed for office uses. There are many other office uses that employ PSAs. The list of
other products is difficult to exhaust, and this area is growing fastest and carries the highest markup.

94.5 Processing
PSAs are applied in thin layers on various substrates. Most of the coating is carried out roll-to-roll, i.e.,
the substrate is supplied as a wound roll, it is unwound, the adhesive is applied to the substrate’s surface,




Metering Roll

Dams

Applicator Roll

Backing Roll

FIGURE 94.5 Top nip fed three-roll reverse roll coater.

the adhesive is dried or otherwise hardened, and then the substrate is rewound into a large roll. Further
processing then starts with the material in a large coated roll.

94.5.1 Coating
The adhesives are applied by specialized coating equipment. They must be liquid, or sufficiently fluid,
to form a thin layer in the coating head. Coating equipment is discussed in several books.4,5 Several
techniques are used for PSA coating. The reverse roll coater (see Figure 94.5) is the most versatile and
frequently used machine for solvent-borne and aqueous adhesives. Knife-over-roll coaters are used for
heavier solvent-borne adhesive coaters. The slot orifice coater is the main technique for the application
of hot-melt adhesives, and this technique is also useful for the coating of aqueous emulsion adhesives.
Calendering is used for some specialized adhesive products. The compounded solid adhesive is squeezed
and formed into a thin sheet between heavy calender rolls. Calendering was the main coating technique
early in the development of PSA products.

94.5.2 Drying
If the adhesive is applied as solution or emulsion, it requires drying before it can be wound up. Forced
air convection dryers dominate this application, with less frequently used infrared heating. The web is
carried through the oven supported on idlers, sometimes on an apron, or sometimes supported by an
air cushion in floatation ovens. More detailed information on web drying can be found in several books.4,5
No drying is needed in hot-melt coating: cooling is sufficient to solidify the adhesive. UV irradiation is
also used as means of solidifying reactive adhesive coatings by the initiation of a polymerization reaction
in the reactive oligomer and monomer mixture.

94.5.3 Slitting and Die-Cutting
Large rolls produced on the drying equipment are slit to narrow tapes or to wide material in case of label
stock to be printed and die-cut. Several slitting techniques are used: shear, score, and razor blade. See
Reference 3 for more detailed information on the slitting of PSA products. The label stock is die cut by
cutting through the label material and leaving the backup release liner uncut. Rotary die cutters are
usually employed. The matrix between the labels is peeled off and the labels are then dispensed as needed.




References
1. C. W. Bemmels, “Pressure-sensitive tapes and labels,” in Handbook of Adhesives. I. Skeist, Ed. New
York: Van Nostrand Reinhold, 1977.
2. W. H. Shecut and H. H. Day, U.S. Patent 3,965 (1845).
3. D. Satas, Ed., Handbook of Pressure Sensitive Adhesive Technology, 3d ed. Warwick, RI: Satas and
Associates, 1999.
4. D. Satas, Ed., Web Processing and Converting Technology and Equipment. New York: Van Nostrand
Reinhold, 1984.
5. E. D. Cohen and E. B. Gutoff, Eds., Modern Coating and Drying Technology. New York: VCH
Publishers, 1992.


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