Manufacturing Methods for PSAs
There are four popular chemistries resulting in a
pressure-sensitive adhesive laminate. One chemistry is achieved through the use
of a solvent. With this method, the adhesive components (usually rubber or
acrylic) are dissolved in a solvent and then coated onto a web. The solvent is
then evaporated using heat and air flow, leaving a dry yet sticky adhesive
(pressure-sensitive) that is wound in a roll.
Adhesives made with solvents often provide high performance, but can be
expensive and require extra care because of processing restrictions. Special
equipment is needed to properly dispose of the solvent, and there must be
vigilant awareness of safety issues when dealing with solvents.
Another way to manufacture a PSA is to create an emulsion (water-based). Here,
the acrylic polymer and other additives are dispersed, not dissolved, in water
and coated onto a web. Again, heat and air flow are used to evaporate the water
before winding into a roll. Today’s water-based emulsions can often provide the
same high level of performance as solvent-based acrylics and are generally a
good choice when lower cost and safety are important, since water-based
emulsions don’t require the extra steps that are needed with solvents.
The third method for creating a pressure-sensitive adhesive is developing a
hot-melt. Hot melts are typically a mixture of a polymer (such as thermoplastic
rubber), tackifying resins and a hydrocarbon diluent (wax or oil plasticizer),
which is heated until flowable. The hot-melt adhesive is then coated on a web
and cooled before winding into a roll.
Hot-melt adhesives typically exhibit excellent adhesion to substrates and
facestocks, have very good coatability, and are less expensive than most
solvent-based adhesives. Drawbacks generally include processing and safety
challenges, as well as difficulty performing under high temperatures. The major
advantage of a hot-melt system is its lack of solvents and water — meaning it
doesn't have to be dried in an oven. Hot melts are frequently referred to as
100%-solids (no water or solvent) technology.
Finally, PSAs can be created through the use of a UV-curable adhesive. These
adhesives usually require no mixing and cure in seconds when exposed to a UV
light source. These environmentally friendly adhesives are reactive compounds
that contain no solvents or other volatile substances. Because UV adhesives
cure so quickly and at temperatures much lower than thermal heating, they can
be used successfully on several heat-sensitive substrates such as plastic
films, paper, and synthetic fibers. The quick curing minimizes shrinkage,
providing the dimensional accuracy that is often called for in applications
such as electronics manufacturing. UV-curable adhesives also offer durability,
strength, insulation characteristics, and resistance to chemicals, moisture,
and temperature. Like hot melts, UV adhesives do not require an oven.
A decision on which method of adhesive manufacturing technology should be used
for a particular application is determined by the performance characteristics
needed.
Common Adhesive Types and Their Properties
In addition to considering the method in which the adhesive
is created (solvent-based, water-based, hot melt, or UV-cured), it is important
to understand the key adhesive component choices and how they will affect the
PSA’s performance in the application. The three main adhesive chemistry options
are rubber/resin, acrylic and silicone.
For many years, rubber/resin formulations have been available that combine
natural or synthetic rubber with tackifying resins, oils, antioxidants, or
other ingredients as needed. They are typically either solvent-based or used in
hot melts, and the aggressive nature of rubber adhesives makes excellent
bonding easy to achieve.
These adhesives are known for their high level of adhesion to a variety of
substrates and facestocks, as well as their high coatability. Rubber-based
adhesives provide high tack and peel, and make excellent general-purpose
adhesives, because of their ability to adhere well to several non-polar,
low-energy surfaces, including polyethylene and polypropylene. In general,
rubber-based adhesives are often a more economical choice compared to most
acrylics.
Disadvantages of rubber-based adhesives include limited effectiveness when
exposed to certain chemicals, UV rays, or high temperatures (over 150°F/66°C).
In addition, they are more susceptible to oxidation and may darken, lose their
tack, and become brittle if overexposed. Also, rubber/resin adhesives may turn
soft and gummy if plasticizers, used in most polyvinyl chloride films (PVC),
migrate into the adhesive.
The other type of adhesive chemistry commonly used in PSAs is acrylic. Acrylic
adhesives can either be solvent- or water-based, and are formulated by reacting
monomers with the desired properties, which are then typically crosslinked to
form the type of polymer needed. Monomers are the building blocks of polymers
and are considered to be either “soft” or “hard.” The combination of hard and
soft monomers can be adjusted based on the level of adhesive (polymer)
performance needed.
Crosslinking polymers improves the cohesive strength (internal strength of the
adhesive, or the ability to stick to itself) of the acrylic formulation and can
be accomplished in one of three ways: the use of thermal heat energy; by
self-curing when a solvent is removed during the drying process; or through UV
or EB non-thermal radiation.
In general, acrylics are relatively firm and durable, but are not as aggressive
as rubber-based adhesives. An acrylic adhesive can be modified to alter its
adhesion properties based on the selection of monomers used, the level of
polymerization created, or by adding tackifying resins or other chemicals.
The advantages of acrylic adhesives are typically the opposite of rubber
adhesives. Acrylics provide excellent temperature, UV and oxidation resistance;
therefore, they are often the preferred choice for outdoor applications. They
resist the negative plasticizing action that can occur with rubber and hold up
well to chemical exposure. Acrylic-based adhesives are very durable and have
clarity and color stability. Acrylics bond well to polar surfaces like metal,
glass, polyesters and polycarbonates, and have high peel, tack, and shear.
Disadvantages of acrylics usually include poor adhesion to low-energy surfaces,
such as polyethylene and polypropylene, as well as lower overall adhesion
compared to rubber unless the adhesive is highly engineered. In general, acrylics
are often more expensive than rubber-based adhesives.
Silicone adhesives are another chemical option for PSAs but are less common
than rubber or acrylic. These specialized adhesives consist of silicone
polymers that provide adhesion to silicon and other hard-to-adhere-to
materials. Silicone-based adhesives can maintain adhesion over a range of
temperatures; however, beyond their ability to adhere to difficult surfaces,
their overall adhesive strength is low. Silicone adhesives are also more expensive.
