Adhesives in various forms have been in use for centuries, and have
evolved over this time to fit many new applications. There are many types
of adhesives available today, each with unique characteristics. In general,
adhesives can be grouped into the following categories:
A) Water
Based - These are adhesives that use water as a carrier or
diluting medium, and set by allowing the water to evaporate or be absorbed
by the substrate. There are several types of water-based adhesives.
1)
Vegetable Glues - These are adhesives based on starch.
They are usually amber to brown in color, commonly know as dextrine
adhesives. These can also be made in a high viscosity, high tack version
called jelly gum. Relatively low cost adhesive commonly used in paper
bonding, packaging and labeling. Low moisture resistance. Bond lines
tend to be brittle.
2) Resin Cements - These are adhesives
based on an emulsion of EVA (Ethylene Vinyl Acetate) or PVA (Poly
Vinyl Acetate) polymers blended to an emulsion with water as a carrier.
Capable of bonding to wood, paper, some plastics and foams and many
other substrates. White in color. Bonds have higher degree of moisture
resistance than dextrine, but cost of resin cement is higher. Certain
resins may be blended with dextrines to form a hybrid product. Bond
lines have some flexibility, and are relatively clear when dry.
3) Animal/Protein Glues - The
two major types of adhesive in this category are hot animal glue (which
is made from processed animal parts) and casein glue (derived from
milk). Hot animal glue is amber to brown in color, and is applied
at approximately 140°F. It can be thinned with water. When first
applied it has very high tack, but dries to a non tacky film. Commonly
used in situations where the high tack will hold the parts together
while setting, but which will not be exposed to high temperatures
or high humidity. Casein glue is applied at room temperature, but
forms a bond with a high degree of moisture resistance. Commonly used
for labeling beer, champagne and some types of wine bottles. Casein
is light to tan in color.
4) Latex Cements - These adhesives
are a blend of latex or other elastomers in a water base emulsion.
In most cases they are applied to parts, allowed to dry, and form
a layer, which serves as a contact cement (two way cements). Some
types can also be applied to one surface and will form bonds as they
dry (one way cements). Can be formulated to remain tacky or become
dry to the touch (contact types). Generally white in color. Wide variety
of uses such as self-stick envelopes, fabric bonding, and leather
goods.
B) Thermal
Adhesives - Thermal adhesives are those adhesives that are
brought to a liquid state by heating, and are applied to the product
hot - either as liquid or as a high viscosity paste. The most common
types are hotmelt adhesives and waxes.
Hotmelt adhesives have seen tremendous development over the past thirty
years. These adhesives are blends of various polymers, but most are
based on a high percentage of EVA (ethylene vinyl acetate). To obtain
the desired characteristics, other polymers may be blended into the
mix, as well as waxes, oils, various types of rubber, and tackifying
resins.
Hotmelt adhesives can be used to bond many types of materials, and
are available in three general types categorized by how fast they set
up after application. The general categories are fast set, delayed set,
and pressure sensitive.
Fast setting hotmelts are types that form a bond very quickly as they
cool. These are used for situations where fast setting is important
such as the sealing of flaps on cartons or certain labeling applications.
Delayed set hotmelts are also known as adhesives having a "long
open time". These adhesives remain tacky for some period of time
after application, but eventually set to form a bondline that has very
little residual tack. They are useful in applications where the parts
must be positioned after application of the adhesive, or in situation
where the parts connot be assembled immediately after the adhesive is
applied. Shoe and leather goods assembly is an example of one area where
delayed set adhesive are used.
Pressure sensitive hotmelts remain tacky indefinitely after application.
This allows the adhesive to be applied to a part that may not be assembled
to a substrate for a long period of time. It also allows the bonding
of parts that are difficult to bond (example would be polyethylene foam).
In some cases pressure sensitive hotmelts are applied to a part and
put on silicone coated release paper. The release paper is peeled off
to expose the pressure sensitive layer, which is then bonded to the
substrate.
Pressure sensitive hotmelts are available in many degrees of tackiness,
from adhesives that form a temporary bond that can be easily broken
(fugitive bond adhesives) to very aggressive pressure sensitives that
will tear fiber from the substrates if removal is attempted.
Specialty hotmelts are also available which have characteristics tailored
to specific types of applications or temperature ranges. A few of these
are as follows:
a) Remoistenable hotmelts - This
is a type of fast-set hotmelt that is formulated with a component
of the blend being sensitive to moisture. If moistened, it forms a
tacky layer that can be used for envelope sealing or labels.
b) Polyamid hotmelts - These are
high temperature hotmelts with high performance characteristics. Generally
the application temperature is around 400° F, and structural strength
is higher than the more common types of hotmelt.
c) Reactive hotmelts - These are
hotmelts that are formulated with a chemistry similar to polyurethane
polymers. After application, an isocyanate component of the blend
reacts with moisture in the air or substrate to form a polyurethane
compound. Once cured, the material is no longer thermo plastic in
nature but has excellent flexibility, high bond strength, high moisture
and heat resistance, and resistance to most chemicals. The disadvantage
of reactive hotmelts is that they require specially designed application
equipment, since any adhesive that is exposed to atmospheric moisture
will react with it and form an inert polymer. Applications for the
reactive hotmelts are found in bookbinding, footwear construction,
and recreational vehicle assembly.
Waxes
are the oldest form of thermal adhesive, having been used for sealing
documents for centuries. In today's world they see use as laminating
adhesives for foils and films. Bonds are formed to the substrates when
hot, but the strength is sufficient to keep the materials bonded at
lower temperatures. One special form of adhesive wax is paste-up wax.
This is a blend of sticky waxes and tackifying agents that is used to
form a temporary bond that allows parts to be removed and repositioned
after bonding. It is used by newspapers and printers during page layout
(paste-up) process since it allows photos and columns of type to be
moved around as the page layout is being developed.
C) Two
part Adhesives
- These are adhesives that are made by mixing two or more components
that react chemically to form a chemically crosslinked adhesive. In
general, they are higher cost than other types of adhesives but also
provide very high strength bonds and outstanding performance characteristics.
The most common two part adhesives are epoxies, polyurethane's, acrylics,
and silicones.
Two part adhesives are able to cure in the absence of air or moisture,
and are often used to form structural bonds to metal, wood and plastic
components.
Epoxies consist of a base resin and a hardener. In most cases the
base resin is a high viscosity paste, and the hardener (catalyst)
a lower viscosity, but mixes can be formulated to differing viscosities
and mix ratios. Most types will set at room temperature, but some
require a heat cure to trigger the crosslinking reaction. Heat will
accelerate the cure rate of most epoxies, and will often help the
epoxy form better bonds and attain higher strength levels. Some types
of epoxies are available as single component pastes which are kept
cold to inhibit the reaction, but which will form bonds and crosslink
when exposed to heat.
Polyurethane adhesives are available as two part formulas or as
one-part components which are pre-mixed but mixed with a carrier material
such as solvent. Polyurethane's generally form bonds that are more
flexible than epoxies but are quite tough. Urethanes form strong bonds
to most materials, and can form strong bonds to rubber, plastics,
metal, wood, paper, ceramic, and fabrics. Most types are limited to
service temperatures below 250° F.
Polyurethane adhesive are available in a wide range of viscosities
and mix ratios. They must be very well mixed to obtain top quality
bonds. Some types contain isocanates or heavy metal catalysts than
can pose health risks to workers, and require extra handling precautions.
Acrylic adhesives are available either as two part adhesives or
as versions that are cured by exposure to ultraviolet light. Acrylic
adhesives produce bonds with excellent peel strengths as well as high
shear and impact strengths. They are generally more tolerant of dirty
or poorly prepared surfaces then other adhesives. Acrylics
Acrylics are available in a wide range of viscosities - from quite
watery to thick pastes. Most types (with the exception of the U.V.
cured types) are prepared by mixing the two components, but some types
are available that allow the one component to be applied to one substrate,
and the second component to the other. When the two substrates are
brought together, the reaction occurs to bond the parts. Acrylics
are generally limited to temperatures below 300° F.
Silicone adhesives are available as both one part and two part adhesives.
The one-part versions are known as RTV silicones (room temperature
vulcanizing) and cure by reacting with moisture in the atmosphere.
These are used most often as caulking and gasketing materials. Two
part silicones offer higher performance, and can be used for bonding
metal glass and ceramic components. These types of silicone adhesives
find use in the electronics industry.
The primary advantage of silicone adhesives and sealants are their
temperature resistance. Silicones can be formulated to withstand temperatures
as high as 500° F, but provide flexible bond lines or sealing
throughout their service range.
D) Moisture
Cure Adhesives - Moisture cure
adhesives are formulated to react with the moisture in the air or in
the substrates to form a cured polymer layer with high strength. They
are actually two component adhesives with one component being moisture.
The two best-known types are silicone and polyurethane. The silicones
are known as RTV silicones (room temperature vulcanizing), and are most
commonly used as caulking compounds, gasket compounds, and sealants.
Polyurethane moisture cure adhesives are available in liquid form.
In most cases the urethane monomer is dissolved in a solvent carrier,
and reaction with moisture occurs as the solvent evaporates. Some
types of water-borne urethanes are also available, but the newest
types of moisture cure urethanes are made in the form of hotmelt adhesives.
These are called reactive hotmelts, and exhibit a dual property. They
are applied like regular hotmelts, but after application begin to
crosslink with moisture to form a tough adhesive layer with high resistance
to heat, moisture, and impact.
E) Ultraviolet
Cure Adhesives - These are adhesives which contain monomers
that will crosslink upon exposure to ultraviolet light to form a polymer.
The crosslinking (or cure) can happen in less than a second at proper
energy levels, so these adhesive can be used in high speed situations.
Acrylic adhesives lend themselves to U.V. curing quite well, but U.V.
cure versions of silicones, urethanes/acrylic blends and cyanoacrylates
are also used.
Ultraviolet cures adhesives can form high strength bond lines on
materials which will pass the U.V. light. The primary advantage of
U.V. cure adhesives is the fast cure speed.
F) Cyanoacrylate
Adhesives - These are fast setting one component adhesives
that are popularly known as "crazy glue". Cyanoacrylates are
solvent free and react with the moisture on the surfaces of the substrate
materials to form a rigid plastic adhesive layer that has high strength
characteristics. The cured adhesive is very high in tensile and shear
strength, but low in peel strength.
Cyanoacrylates are expensive compared to other adhesives, but only
a very small amount is needed to cover the area to be bonded, since
this material works best when spread into a very thin bond line. The
material is available in a range of viscosities from water thin to
thickened versions that are in the form of thixotropic pastes or gels.
G) Anaerobic
Adhesives -
These adhesives cure to a solid polymer in the absence of oxygen. They
are commonly used as thread - locking compounds and retaining compounds
for metal parts such as bearings and shafts. Anaerobic adhesives remain
liquid as long as they are exposed to the atmosphere, but cure rapidly
once confined. They are packaged in special containers that can "breathe"
to prevent the materials from setting up in the containers. They are
easy to use and are available in a range of viscosities and bond strengths.
Some versions are available for making structural bonds between substrates
that need to be laminated.
H) Film
Adhesives - These are adhesives
that are made in the form of sheets. In most cased they are carried
on release paper, but some types are carried on release paper, but some
types are heat activated and do not require release paper. Film adhesives
are made from water base, solvent base, or hotmelt adhesives which are
cast into a thin film leaving only the adhesive. They find use in situations
where the release paper can be left in place and peel off prior to application
to the substrate. They are popular for mounting of plastic components
such as warning stickers, die cut parts such as letters and numbers,
and a multitude of other parts. This form of adhesive also finds use
for cold laminating of paper, plastics and films. The heat reactivated
versions find use in fabric bonding and industrial applications where
heat can be applied to the substrates to melt the adhesive.
Some types of film adhesives are cast onto a supporting material
such as a scrim cloth or nonwoven fabric. These prevent stretching
of the adhesive in use and simplify handling. Double back carpet tape
is made in this manner. Film adhesives tend to be expensive relative
to other adhesives because the cost of the release paper carrier must
be included in the price of the adhesive. For many applications, the
release paper stays with the product until it is applied, so the cost
premium is justified. Film adhesives are also useful in cases where
liquid adhesives might distort the substrates to be bonded. This is
the case with some types of thin papers, films and foils, especially
in low volume applications where ease of handling is of primary importance.
