contact adhesives . a fastest and easy way for bonding
DESCRIPTION
What is Contact Adhesive ? A bonding agent that is applied to two surfaces and allowed to dry before being pressed together or A liquid adhesive which dries to a film that is tack-free to other materials but not to itself. The adhesive is applied to both surfaces to be joined and dried at least partially. When pressed together at light to moderate pressure, a bond of high initial strength results.TRANSCRIPT
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Contact Adhesive
Let us understand what is meant by
Contact Adhesive and how is it different
from Pressure Sensitive Adhesive
What is Contact Adhesive ?
A bonding agent that is applied to two
surfaces and allowed to dry before being
pressed together or
A liquid adhesive which dries to a film
that is tack-free to other materials but
not to itself. The adhesive is applied to
both surfaces to be joined and dried at
least partially. When pressed together at
light to moderate pressure, a bond of
high initial strength results.
What is Pressure Sensitive Adhesive ?
PSA is adhesive which forms bond
when pressure is applied to marry the
adhesive with the adherent. No solvent,
water, or heat is needed to activate the
adhesive.
Pressure sensitive adhesives (PSA)
adhere to most surfaces with very slight
pressure. Pressure sensitive adhesives
form viscoelastic bonds that are
aggressively and permanently tacky at
room temperature. They
adhere with just finger or hand pressure
and do not require activation by water,
solvent or heat.
Now let us know more about Contact
Adhesives in this bulletin.
Contact adhesive is widely used
because of it’s easy application, fast
setting properties, and excellent
adhesion to many different substrates.
The contact adhesive is coated on both
surfaces to be joined. After the carrier/
solvent evaporates, the two surfaces are
brought together under slight pressure,
and the adhesive bonds to itself. A high
degree of handling strength occurs
immediately, and the joint strength
increases further with time.
Contact adhesives are available with
solvent or water based Technology. The
base polymer is generally poly
Chloroprene, but Polyurethane, Styrene
Butadiene Rubber, Natural rubber ,
Butyl Rubber and Acrylic polymers are
also used. Rubber and elastomers are
characterized by their high degree of
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flexibility and elasticity (high reversible
elongation).
Bond Strengths
Contact adhesives are available in a
variety of bond strengths
Specifications
Specification for Contact adhesives
include thermal properties, mechanical
properties, and electrical and optical
properties . However Service
temperature, thermal conductivity, and
coefficient of thermal expansion (CTE)
are key thermal properties. Tensile
strength and elongation are important
mechanical properties. Electrical and
optical properties for contact adhesives
include dielectric strength, dielectric
constant, index of refraction, and
transmission.
Process ability/ Coat ability
Temperature and humidity are two
important factors that affect the rate at
which the cement( Contact Adhesive )
solidifies. High temperatures cause
the adhesive to bond more quickly,
while low temperatures reduce the rate
at which the liquid evaporates, resulting
in slower drying. High humidity
causes contact cement to dry slowly,
while low humidity speeds up the
process. When precise placement is
important, slower bonding is valuable,
and allows plenty of time to position the
objects before the cement is fully
bonded. High temperatures and low
humidity are ideal when time is short.
Mechanism of Adhesion
For Contact adhesives is based on
intermolecular diffusion. Both surfaces
are coated for bonding, and the carrier/
solvent is allowed to evaporate
sufficiently so that the adhesive film
becomes dry (non-tacky) to the touch.
When the adhesive coated substrates are
brought together auto adhesion occurs.
Immediate diffusion of the polymers on
each surface occurs across the interface,
and the early bond strength is extremely
high. After a relatively short period of
time, the interface no longer exists and
the full cohesive strength of the polymer
prevails.
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For auto adhesion to take place several
important factors are involved.
The polymer/ elastomer must be
capable of diffusion. The degree of
diffusion is influenced by the nature of
the polymer and the choice of solvent .
Therefore, only certain polymers and
solvent combinations have auto adhesive
characteristics.
For maximum diffusion , the substrates
must be in intimate contact. This will
depend greatly on the elastic stress and
plastic flow components of the adhesive
composition. Thus, the coating
uniformity and rheology of the adhesive
composition is influential in achieving
good bond strength. Also, the effects of
pressure, time, and temperature on the
joint area is also very important in
achieving optimum diffusion.
Sufficient drying time must be allowed
before bonding to remove about 90%
of the solvent or water . Generally for
solvent-based contact adhesives the
drying time is about 30 minutes under
ordinary conditions of temperature and
humidity and much longer for water-
based adhesives. Forced drying with
heat can reduce this time significantly.
After the adhesive is coated and dried on
a substrate surface, there is a definite
time span, known as the open time,
during which diffusion may occur under
specific conditions of temperature and
pressure.
During this open time, the adhesive film
is changing in physical properties due to
continuing loss of solvent, phase change,
and crystallization of the base polymer.
After the open time has been exceeded,
the nature of the adhesive film is such
that diffusion or auto adhesion will not
occur and good adhesive properties will
not be achieved.
Characteristics of Contact Adhesive
The contact adhesive can hold a load for
substantially longer times than PSAs ,
because of very high the shear strength
and peel strength of contact adhesives
Therefore , contact adhesives are
generally considered as "semi-structural
adhesives" with properties falling
between those of conventional structural
adhesives (thermosetting epoxies,
urethanes, etc.) and non-structural
elastomeric pressure sensitive adhesives.
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Contact adhesives offer very high bond
strengths immediately after the joints are
made. The assembled components can
immediately undergo additional
finishing operations without clamping or
fixturing.
As a matter of fact , sometimes
difficulties are encountered in working
with contact adhesives due to their
immediate grab or "green strength"
which prevents any repositioning due to
misalignment.
Contact adhesives, have very good
toughness and elongation . As a result,
they withstand rapid and wide changes
in temperature, moisture, and
considerable vibration
The chemical and heat resistance of
contact adhesives are much similar to
structural and non-structural adhesives.
The hot strength of the assembled
component is generally good, depending
on the adhesive formulation. In many
cases the hot strength is sufficient to
permit post forming of thermoplastic
substrates without the adhesive
excessively softening or releasing due to
the temperature of the process.
The chemical and moisture resistance of
contact adhesives are generally
sufficient for using them in many
exterior and interior industrial
applications in the building and
construction, automotive, and footwear
industries.
The tensile shear strength, heat and
chemical resistance lie between
thermoplastic PSAs and structural
adhesives.
The Advantages and Disadvantages of
Contact Adhesive
Advantages
Immediate joint strength development
Easy application by hand and application
can be made on-site
Very good adhesion to many materials,
even non-porous
Flexible bonds due to elastomeric base
polymer
Good resistance from –40°C to +130°C
depending on formulation
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Good aging resistance indoors; aging
outdoors will depend on base polymer
and formulation
Dis advantages
Immediate set does not allow for
repositioning
Generally flammable solvents are
required, and some solvents are toxic
Waterborne systems have long drying
times and somewhat poorer performance
than solvent borne systems
Not a gap filler (substrates should be
smooth and parallel)
Critical drying and open times
depending on the formulation.
Application Methods
Contact adhesives are generally applied
by spraying, curtain coating, roller
coating, or brushing. Spraying is by far
the most important commercial
application method as it permits rapid
coverage of adhesive and minimizes the
drying time by breaking-up the adhesive
as it is applied. Roller and curtain
coating are used to apply adhesive to flat
sheets such as the cores and skins of
panels. Adhesives with relatively slow
drying solvents usually are used with
roller or curtain coating methods. The
slow drying adhesives are necessary to
prevent their drying out or building-up
in viscosity on the roller or curtain
coater.
Solvents are removed by prolonged air-
drying or by forced drying with heat.
Certain sprayable contact adhesives,
known as dry spraying adhesives, lose so
much solvent during spraying that the
adhesive film is practically ready for
bonding immediately after application.
This type of adhesive has been used
successfully when very fast assembly
times are required or when bonding a
substrate, such as polystyrene foam, that
may be susceptible to solvent. Another
way of achieving this effect is to use hot
spraying. In this method the adhesive
passes through heaters as it travels along
the fluid line to the spray gun. When the
adhesive leaves the spray gun, the
elevated temperature combined with the
large volume of air passing over the
adhesive tends to evaporate the solvent
rapidly.
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Types of Contact Adhesives
Natural rubber was the first polymer
used in contact adhesive formulations,
but it has been almost entirely displaced
by synthetic polymers. Today, the main
polymer used in contact adhesive
formulation is polychloroprene.
Polyurethanes, acrylic copolymers, and
styrene-butadiene copolymers are also
used in certain formulations.
Contact adhesives based on these
polymers are available in either solvent
solution or water dispersion. Although
the solvent-based adhesives generally
Polymer Advantages Disadvantages
Neoprene
Rapid strength
development
High shear
strength
Good
temperature and
chemical
resistance
Limited open time
Limited formulation
capability
SBR
Good bond to
LSE substrates
Properties
dependent on
styrene to
butadiene ratio
Less heat and
solvent resistance
than others
Requires tackifier
addition
Polyurethane
Resistance to
plasticizer
migration
Adhesion to
many substrates
Freedom in
formulation
Adhesive must be
in the amorphous
state when
substrates are mated
Low temperature
resistance and
chemical resistance
Acrylic
copolymers
Tack and
ultimate strength
is less than
others
Good bond to
LSE substrates
Good
environmental
resistance
Mainly latex
emulsions
Less moisture
resistance than
other contact
adhesives
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have greater strength and durability,
environmental and legislative pressures
have encouraged the development of
water-based systems. Water borne
contact adhesives often have problems
with slow evaporation rate, less tack,
and microbe buildup in the wet adhesive.
Often a small amount of solvent, called a
coalescing aide, which is soluble in both
the elastomer and water is used to aid in
wetting as well as to improve the
coalescence of the latex particles.
Contact adhesives can be further
classified into curing and non-curing
types. The curing types have a cross-
linked molecular structure and provide
greater resistance to heat and chemicals.
However, they are generally supplied as
two component systems with a limited
pot life.
In addition to the base polymer and
carrier, contact adhesive formulations
may include tackifiers, stabilizers,
pigments and fillers, and crosslinking
agents.The main commercial types of
contact adhesives are described below
Polychloroprene
By far, the majority of contact adhesives
are formulated with polychloroprene
(Neoprene) elastomers. Rapid strength
development coupled with high ultimate
strength is typical of polychloroprene
contact adhesives. They also have auto
adhesion characteristics, high shear
strength, and resistance to oils and
chemicals.
Selection of the grade of
polychloroprene will be based on a
compromise between strength, strength
development, and open time
characteristics. The adhesive properties
directly affected by the type of
polychloroprene used are:
Initial tack, open time
Bond strength development
Hot bond strength (heat resistance)
Application properties and
Adhesive viscosity.
Polychloroprene elastomers prepared for
solvent-based adhesive formulation can
be dissolved in a wide variety of polar
and non-polar solvents. This provides a
range of solution viscosities and drying
times as well as specific adhesion to
many porous and nonporous substrates.
The choice of solvents used in the
manufacture of contact adhesive
depends not only on the solvent’s
dissolving power, but also on its
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evaporation rate. Solvent selection can
also influence the rate of crystallization.
Generally, the more efficient the solvent
and the lower the volatility, the longer
the open time for the adhesive.
Polychloroprene contact adhesives have
also been formulated from water
dispersed elastomers. Polychloroprene
lattices have not widely replaced
solvent-based contact cements as
originally thought. It takes a water borne
adhesive much longer to dry, and if
force dried the energy used is an added
cost. The auto adhesion properties of
water-based polychloroprene adhesives
provide lower initial strength and make
bonding at low pressures difficult. Also
the heat and moisture resistance of these
adhesives are generally lower than their
solvent-based counterparts because of
the presence of surface-active
ingredients. However, significant
development work has occurred over the
last several years to minimize these
problems.
Polychloroprene or chloroprene rubber
(CR), introduced by DuPont in 1931,
was the first synthetic rubber developed
that exhibited the elastomeric properties
of natural rubber. The new rubber was
given the trade name “Duprene” and
then renamed Neoprene. It remains one
of the most important specialty
elastomers with an annual consumption
of 300,000 tons worldwide.
The modern chloroprene process is
based on the conversion of butadiene
into the chloroprene monomer (2-
chlorobutadiene-1,3). With the aid of
free radical initiators, chloroprene in the
form of an aqueous emulsion is
converted into homo polymers or, in the
presence of co-monomers, into
copolymers.
In the production of solid-grade poly
chloroprene, the latex is coagulated,
drawn into a thin sheet, and then
chopped and dusted to form chips or
granules. Poly chloroprene is also
produced and sold in latex form, which
is used in waterborne adhesives.
Structure
Key roles in changing the molecular
structure of polychloroprene are played
by the polymerization conditions
(polymerization temperature, monomer
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conversion, polymerization process),
polymerization aids (co-monomers, type
and amount of molecular weight
modifier and emulsifier) and conditions
during finishing. The molecular weight
of polychloroprene is a function of the
conversion level and initiator content.
Another classification of commercially
available chloroprenes is the linearity of
the backbone. The polychloroprene
grades used in solvent-based adhesives
are highly linear and soluble. Lower-
molecular-weight chloroprenes can be
dissolved by simply stirring in solvent,
while higher-molecular-weight materials
often have to be masticated before easy
dissolution is possible. Latex-based
polychloroprenes have varying degrees
of gel structure and are marginally
solvent-soluble. Latex polychloroprenes
are often anionically stabilized with
rosin acid-based emulsifiers.
Applications
Some 60% of CR production is used in
the rubber industry for products such as
molded goods, cables, transmission belts
and conveyor belts. About one third of
CR production is used as a raw material
for adhesives (both solvent-based and
water-based). The balance of CR
production has different latex
applications such as dipped articles (e.g.,
gloves), molded foam and as a rubber
modifier for improved performance in
asphalt products.
Excellent adhesive properties and
formulating latitude at competitive
prices vs. other adhesives technologies
have contributed to the dominant
position of polychloroprene solvent
borne adhesives in many market
segments. Historically, polychloroprene
was used as a replacement for natural
rubber, which was in short supply during
World War II. However, synthetic
polychloroprene demonstrated clear
performance advantages over adhesives
formulated with natural rubber.
Polychloroprene contact adhesives are
used for bonding high-pressure
laminates, automotive trim, roofing-
membrane attachment, furniture, kitchen
cabinets, custom display cabinets,
interior and exterior panels, wall
partitions, shoe soles, and many other
applications where quick, high-strength,
permanent bonds are needed.
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The U.S. shoe industry was the first
major market penetrated by
polychloroprene contact adhesives.
Although this market has declined in the
United States during the past 15 years,
the shoe industry worldwide still is a
major consumer of polychloroprene
adhesives.
In the furniture and related industries, a
wide range of substrates must be
bonded, including wood; metal, such as
cold-rolled steel and aluminium; fabric;
paper; foam; plastic, such as polyvinyl
chloride, polystyrene, polyethylene,
polypropylene and acrylonitrile
butadiene styrene (ABS); fiberglass; and
materials used to construct high-pressure
laminates, for example, for countertops.
Essentially all furniture made from a
laminate of Formica on a less expensive
wood base is fabricated with contact
adhesives.
In the past, furniture manufacturing has
been dominated by one-part contact
adhesives dissolved or dispersed in
organic solvents, such as chlorinated
solvents and low-flash-point organic
solvents. Such one-part contact
adhesives are convenient to apply.
Recently, however, there has been a
desire to shift from organic, solvent-
based adhesive compositions to
aqueous-based or aqueous-dispersed
adhesive compositions for
environmental, health and safety
reasons.
Adhesive Properties
A significant performance advantage of
contact adhesives is auto-adhesion.
Polychloroprene contact adhesives
provide immediate green strength and
high ultimate bond strength for efficient
assembly. The substrates are coated with
adhesive, allowed to dry and then
combined under light to moderate
pressure. This results in an instant bond.
The mechanism is not a simple diffusion
process but diffusion with solubility.
Therefore, the residual solvent
concentration is an important factor.
Sufficient green strength is achieved so
that the assembled components can
undergo finishing operations without
additional clamping or fixturing.
The two main criteria that influence the
polymer selection when designing a
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formula for a contact adhesive are the
crystallization rate and viscosity.
Crystallization
Crystallization determines the rate of
initial strength development. Fast-
crystallizing polychloroprene grades
give rapid bond-strength development.
Crystallinity in a polymer inhibits the
diffusion process and hence reduces
auto-adhesion. A balance has to be
achieved between auto-adhesion
characteristics and ultimate mechanical
performance.
Unlike chemical crosslinking,
crystallization is reversible. The
crystallization of polychloroprene is
temperature-dependent, being very
pronounced at low temperatures, while
higher temperatures result in a gradual
loss of crystallinity. Adhesives that have
crystallized at room temperature de-
crystallize at elevated temperatures,
losing their strength. This can already be
noticed at 40 to 50 degrees C, and as the
temperature rises, the thermo plasticity
increases. If the bonds are
simultaneously subjected to a load, this
will eventually result in bond
destruction.
Polychloroprene adhesives that have
been de-crystallized at elevated
temperatures exhibit auto-adhesion. This
property is exploited in the production of
heat-activated adhesives.
Viscosity
Viscosity influences the inherent
strength of the adhesive film, the
solution viscosity and solids content.
The higher the polymer viscosity or
molecular weight, the higher the film
strength, the higher the adhesive
viscosity, or the lower the solids at a
given adhesive viscosity.
Polarity
The polarity of the polychloroprene
gives versatility. Polychloroprene
rubbers can be dissolved in a wide
variety of polar and non-polar organic
solvents to provide a range of solution
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viscosities and drying times, all with
good solution stability. Good specific
adhesion to a wide range of porous and
non-porous substrates can be achieved.
Open Time
Polychloroprene contact adhesives can
be formulated to provide a long open
time before the adhesive-coated
substrates are combined. This time span,
commonly referred to as “tack life” or
“open time,” is the period within which
a satisfactory bond can be formed under
contact pressure. Long open times
enable assembly operations to pre-coat
or stage the preparation of substrates
prior to bonding.
The shoe industry is a major consumer
of polychloroprene adhesives.
Solventborne Adhesive Formulations
Solvent-based polychloroprene contact
adhesives are formulated using a
polychloroprene rubber, a tackifying
resin (most commonly a tertiary-butyl
phenolic resin) and antioxidants/acid
scavengers, such as metal oxides.
Various grades of solid polychloroprene,
which generally differ in level of
crystallinity, are available for use in
solvent-based adhesives.
Solvent selection and adhesive solids
will vary depending upon end use and
method of application. The selection of
the grade of polychloroprene and the
tackifier allows the adhesives formulator
to design products having specific
properties, such as long or short tack or
open time, controlled development of
ambient- and/or elevated-temperature
bond strength, and ultimate peel and
shear strength. The cured adhesives are
generally resistant to heat, oxidation,
water, solvents and other chemicals.
Solvent Selection
Solvent blends are normally used in
polychloroprene contact adhesives. Non-
flammable chlorinated solvents, once
popular, have been replaced due to
environmental restrictions. The choice of
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solvents or a solvent mixture used in the
manufacture of adhesives depends not
only on their dissolving action but also
on evaporation rate. Non-solvents are
often added to solvents that dissolve
polychloroprene without impairing the
dissolving action of the resulting
mixtures. The actual rate at which an
adhesive dries is also influenced by the
solvent retention of the formulation as
well as the absorption and permeability
of the substrates to be bonded.
PROPERTIES OF COMMONLY USED SOLVENTS FOR NEOPRENE RUBBER
SOLVENT MOL. WT SOLUBILITY RELATIVE FLASH
PARAMETER EVAPORATION RATE POINT
ACETONE 58.08 10.00 1160 15
CYCLO HEXANE 84.16 8.20 720 40
ETHYL ACETATE 88.10 9.10 615 45
HEPTANE 100.20 7.40 386 25
HEXANE 86.70 7.30 1000 -25
IPA 60.09 11.50 300 70
MEK 72.10 9.30 572 35
PENTANE 72.15 7.00 2860 < -50
TOLUENE 92.13 8.90 240 44
XYLENE 106.16 8.80 63 80
P. NAPHTHA * 7.60 275 48
Antioxidants and Acid Acceptors
Another common characteristic of
adhesive formulations based on
polychloroprene is that they require the
use of antioxidants and acid acceptors.
The adhesives are commonly modified
by the incorporation of magnesium
oxide and zinc oxide to inhibit
dehydrochlorination. An example of this
is the use of zinc oxide, which is often
used in a dispersed form in
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Polychloroprene-based formulations.
The zinc oxide serves to act as an acid
acceptor and as a crosslinking agent with
carboxyl functional groups to improve
heat resistance.
Magnesium oxide and zinc oxide are
both acid acceptors for trace amounts of
hydrogen chloride evolved during aging.
The presence of metal oxides improves
the storage stability of the adhesives and
the resistance to aging of the bonds.
Magnesium oxide will also prevent
scorching during mastication.
Magnesium oxide and zinc oxide are
generally used at about 4 parts per
hundred parts rubber (phr) of each.
Historically, the metal oxides were
milled into the polychloroprene to
ensure a fine dispersion that would stay
suspended in the adhesive. With the
direct dissolving process that is mainly
used today, they have to be dispersed
differently. Generally, these products
have to be used in a dispersed form,
which has the overall effect of reducing
the stability of the formulation and
increasing its cost. The use of metal
oxides with extremely small particle size
makes incorporation easier. Pre-reacting
the magnesium oxide with reactive
alkylphenolic resins can also improve
the incorporation into the adhesive.
Alternatively, the metal oxides can be
dispersed in a master batch (containing
the metal oxides at about the 10% to
20% level) that is milled into the
polychloroprene.
Resins
Resins are also used in polychloroprene
contact adhesives, the most common
being the alkyl phenolic resins. These
substances have little effect on the final
crystallizing of polychloroprene but
improve adhesion characteristics. They
are normally added at about 10-50 phr.
Tertiary-butyl phenolic resins improve
both the auto-adhesive and the oxidative
stability of the adhesive. The free
methylol groups on the resin react with
the magnesium oxide present to form a
high-melting-point resinate that also
improves the heat resistance. Adhesive
formulations containing these resins
have high initial strength, are rapid-
setting and have excellent final bond
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strength, but their tack range is limited
compared to other resins.
As alternatives to the alkyl phenolics,
terpene phenolic resins, rosin-modified
phenolic resins, rosin esters, and
hydrocarbon and coumarone resins can
be added to improve the tack and
adhesion. Coumarone resins and rosin
esters with a low melting point can
improve the auto-adhesive
characteristics but only by sacrificing
heat resistance and oxidative stability.
Less tack but greater strength is obtained
with terpene phenolic resins, rosin-
modified phenolic resins and rosin esters
with a relatively high melting point.
The aging properties of bonds produced
from polychloroprene adhesives are
influenced by the resins incorporated.
Resins sensitive to oxidation, such as
rosin esters and coumarone resins, cause
embrittlement or softening in adhesive
films within a relatively short period of
time. These resins must therefore only
be used in combination with suitable
antioxidants. It is likewise advisable to
use antioxidants when terpene phenolic
resins are employed. Alkyl phenolic
resins have no undesirable aging effect
on bonds obtained with polychloroprene
adhesives.
The thermal stability of bonds can be
considerably improved by adding a
polyisocyanate crosslinker to the
adhesive. The crosslinking of the
adhesive reduces its thermoplasticity.
The addition of polyisocyanate
crosslinker not only raises the thermal
stability but also improves the cohesive
strength and in many cases significantly
increases the adhesion to certain
substrates while improving solvent
resistance. Polyisocyanates must be
added to the adhesive solution
immediately before use. Resins reacting
with polyisocyanate, such as phenolic
resins, should be avoided. The pot life of
an adhesive to after-addition of a
polyisocyanate crosslinker is usually
several hours before the reaction
proceeds to the point where gelling
occurs. The pot life depends largely on
the composition of the adhesive solution.
Good heat resistance is important in
footwear for tropical countries, in
flooring materials in the vicinity of
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radiators or dark flooring materials that
are exposed directly to sunlight, and in
laminated kitchen counters (hot pots and
pans, vicinity of stoves). Heat resistance
is also important in many automotive-
trim applications that must maintain
bond integrity through heat-age cycles
(often in excess of 80 degrees C).
Polychloroprene adhesives are used to
attach roofing membranes.
Waterborne Adhesive Formulations
Historically, water-based systems were
sensitive to operating conditions. As a
result, their bonding properties were
inconsistent. The stability of
polychloroprene latices in combination
with other commonly used ingredients
necessary in adhesive compositions has
been a general problem. In recent years,
latex compositions with improved
stability have become available. They
are generally based on polychloroprene-
meth acrylic acid copolymers stabilized
with partially hydrolyzed polyvinyl
alcohol.
Many factors have limited the use of
waterborne adhesive formulations. They
have a major disadvantage, namely the
slow evaporation of the water. It takes a
waterborne adhesive much longer to dry
at ambient conditions than a solvent-
based adhesive. Attempts to provide
them in one-part form have achieved
only gradual industry acceptance since
they have had longer dry times than
conventional, solvent-based adhesives,
as well as a relatively slow rate of bond-
strength development. Quick drying of a
latex-based system can only be achieved
by an investment in drying equipment.
The contact bond properties of
polychloroprene latices are not as good
as solvent-based polychloroprene
adhesives in several respects. The
difference in auto-adhesion properties
gives lower initial bond strength and
makes bonding at low pressures
difficult. The contact ability of the
adhesive films is often insufficient after
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drying. Waterborne systems lack the
crystallization-inhibiting and tackifying
effect of a solvent remaining in the film.
Heat activation prior to bonding is often
necessary.
Adhesive Strength
To overcome such limitations, two-part
aqueous-dispersed adhesive systems
have been developed that demonstrate
high adhesive strength within seconds of
spraying. The adhesive composition is
one part of the two-part system with an
external coagulant as the second part in a
predetermined ratio. Coagulants such as
citric acid, lactic acid, acetic acid or zinc
sulfate are typically used.
The two parts are combined in a spray-
mixing process that involves the addition
of the coagulant at the spray head. Both
parts are conveyed separately into a
spray gun and become mixed in the
spray. Since the components only come
into contact with each other in the spray,
there is no need to be concerned about
pot life.
Heat Resistance
The heat resistance of adhesives based
on polychloroprene latices is generally
lower than solvent-based adhesives
because of the presence of surface-active
ingredients used to emulsify the latex,
tackifiers and other compounding
ingredients. Also the high heat resistance
achieved with solventborne CR contact
adhesives through the addition of an
alkylphenol-magnesium chelate complex
is not possible in water-based systems.
New grades of homopolymer
polychloroprene latex have recently
been introduced with increased chain
crosslinking that are relatively low in
crystallinity and exhibit improved heat
resistance. When compounded with a
terpene phenolic tackifier, they give a
high softening point comparable to that
of solventborne polychloroprene
adhesives.
Peel Strength
Many water-based adhesives formulated
from polychloroprene latices do not have
good initial peel strength. Adhesives
based on polychloroprene latex with
increased chain crosslinking blended
with low-crystallinity polychloroprene
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latex containing increased chain
branching have demonstrated both
improved heat resistance and excellent
peel strength.
Improving Properties by Crosslinking
Increasing the heat resistance of
waterborne systems to the level
achievable with two-component,
crosslinked, solventborne CR adhesives
is another problem recently addressed.
Aromatic polyisocyanates and
hexamethylene diisocyanate (HDI)
trimers are unsuitable for crosslinking
alkaline polychloroprene latices because
they cause coagulation of the latex. With
the availability of dispersions of
isophorone diisocyanate (IPDI) trimer, a
monomeric cycloaliphatic diisocyanate,
a sufficiently long pot life without
coagulation has been demonstrated in a
waterborne system. In combination with
polychloroprene latex with a
significantly higher amount of hydroxyl
groups in the polymer chains,
crosslinked adhesives can be made. The
use of such isocyanates allows a
crosslinking reaction to take place at
room temperature within a relatively
short time. Bond strength and heat-
resistance properties comparable to
isocyanate-crosslinked solventborne
systems can be achieved.
Improving Properties With Acrylics
Many of the commercially available
water-based contact adhesives are based
on polychloroprene latex and
acrylic/vinyl acrylic type emulsions.
Blends of polychloroprenes with soft
vinyl acrylics produce desirable
synergistic effects. The acrylate
dispersion provides some initial tack and
also a destabilizing effect that
accelerates coagulation.
Status of Waterborne Systems
Recent developments in polychloroprene
homopolymer and copolymer latices
with varying molecular structure have
demonstrated improved performance
approaching that of solventborne
systems.
Two-part waterborne adhesives utilizing
the spray-mixing system have improved
the application and efficacy of
waterborne adhesives. However, they
are not entirely satisfactory. The co-
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spraying equipment is expensive, the
equipment requires maintenance and the
ratio of the two parts (the coagulant and
the adhesive composition) must be
monitored during application. Thus,
there is a need for a true, one-part,
aqueous-based, fast-setting contact
adhesive.
Polyurethane
Urethane polymers for solution contact
adhesive are generally prepared from
bulk polymerization of difunctional
hydroxyl compounds with difunctional
isocyanates. These polymers are usually
supplied in bead or granular form and
can be dissolved in common ketonic
solvents. Like polychloroprene,
polyurethane resins are available with a
wide range of crystallization, solution
viscosity, and cohesive properties.
Polyurethane contact adhesives can be
applied similarly to polychloroprene
contact adhesives.
Polyurethane adhesives can also be
applied as a heat sealable contact
adhesive. The adhesive is exposed to
temperatures (70 deg -90ged C) above
the crystalline melting point before
joining pressure is applied. While the
adhesive is in the amorphous state, the
substrates are mated and the adhesive on
each substrate diffuses into the other.
Strength develops as the adhesive
recrystallizes. This type of adhesive and
processing are commonly used in the
footwear industry.
Polyurethane adhesives are often chosen
over polychloroprene adhesives when
greater resistance to plasticizer
migration is required or difficult to bond
polymeric substrates are involved. One
of the largest applications is in footwear
where high bond strengths to PVC
compositions are required.
Acrylic Copolymers
Contact adhesives have been developed
based on acrylic emulsions. These
emulsions provide high bond strength to
various surfaces, good environmental
resistance, high strength, and relatively
fast setting times without having to use a
solvent system with resulting
environmental and safety restrictions.
Acrylic emulsions are prepared by
conventional emulsion polymerization
processes. A blend of several monomers
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is used to achieve the desired copolymer
characteristics such as glass transition
temperature, flexibility, ultimate
strength, and autohesion. They are often
the same acrylic emulsions that are used
in pressure sensitive adhesives or sealant
formulations.
Styrene Butadiene Rubber (SBR)
SBR elastomer is a random copolymer
of styrene and butadiene formed through
emulsion polymerization. As with
polychloroprene, SBR can be coagulated
and re dissolved in a solvent or used as a
latex. The application and end-use
properties are very much dependent on
the ratio of the styrene to butadiene
along the molecular chain. Styrene
butadiene polymer systems have
relatively poor auto adhesive
characteristics and require compounding
with a tackifying resin to provide a
contact adhesive. High strength is
achieved by strong intermolecular forces
and entanglements. However, SBS
polymers are susceptible to oxidative
degradation, especially when stressed.
An antioxidant, such as a hindered
phenol, is necessary for bond durability.
Because of their good solubility in many
different solvents, SBR contact
adhesives have been used in applications
where the substrate may be sensitive to a
certain type of solvent, such as the
bonding of polystyrene foam. In this
application SBR contact adhesives are
used where the solvents are either short
chain linear hydrocarbons or alcohols.
Contact Adhesive Formulations
FORMULATION NO: 1
ADHESIVE BASED ON GRAFT
POLYMERS OF
NEOPRENE AND METHYL
METHACRYLATE
Material Quantity
1 NEOPRENE AD- G 20.000
2 METHYL METHACRYLATE 20.000
3 TOLUNE 80.000
4 MEK 20.000
Sub Total 140.000
SLOWLY HEAT THIS SOLUTION IN
A CLOSED REACTOR
BY INDIRECT HEAT / OR ON A
WATER BATH , TO 60 deg C
ADD
BENZOYL PEROXIDE TO INITIATE
POLYMERISATION as follows.
5 BENZOYL PEROXIDE 1.000
50% DISP
CONTINUE HEATING TO 80 deg C
ADD
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A/O WINGSTAY L TO TERMINATE
THE REACTION
6 A/O WNGSTAY L 2.000
Total 143.000
FORMULATION NO: 2
NEOPRENE CONTACT
ADHESIVE
Material Quantity
1 NEOPRENE - AD 20.000
2 MgO 1.600
3 ZnO 1.000
4 A/O 0.400
23.000
Mix the above on a Rubber Mixing
Mill
Mixed Stock as above 23.000
5 t- BUTYL PHENOLIC RESIN 9.000
6 WATER 0.200
add Solvent as follows
7 TOLUENE + HEXANE+ ACETONE 120.000
Add Solvent in the Ratio of 2 Parts/ 4
Parts / 4 Parts
Total 152.200
FORMULATION NO: 3
Bayprene 320 10.000
Bayprene 233 10.000
MgO 1.400
ZnO 0.600
Heat Reactive Alkyl Phenol
Resin 20.000
Sub Total 42.000
Solvents
Ethyl Acetate 7.000
White Spirit Boiling Point 65/95
deg C 14.000
CycloHexane 28.000
MEK 20.000
Sub Total 69.000
Total 111.000
Alternative for Neoprene Rubber
SBC 's is a alternative to substute
Chloroprene Rubber in Solvent borne
Contact Adhesives.
Advantages of SBC over Chloroprene
Narrow mol weight distribution than CR
No pre mastication required
Lower viscosity
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Excellent solubility in a wide Range of
organic solvents
Thermoplastic nature
Excellent cohesive performance
Excellent Tack
Contact Adhesive Applications
There are five principal markets for
contact adhesives. These are footwear,
automotive, do-it-yourself (DIY),
building and construction, and furniture.
Contact adhesives are generally used in
applications where the type of substrate
is likely to vary (decorative laminates to
wood for countertops, shoe
compositions, etc.) and where fast
setting speeds and high ultimate strength
are required.
Contact bonding is preferred when
bonding two nonporous surfaces since
most of the solvent or carrier is removed
from the adhesive film during the open
time period. Any number of materials
can be bonded with contact adhesives.
Contact adhesives can be used for
furniture, kitchen cabinets, custom
display cabinets, interior and exterior
automotive trim, roofing
membraneattachments, and a wide
variety of additional applications where
quick, high strength, permanent bonds
are required.
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Now Just to Sum up
What is a contact cement
Contact cement is an adhesive, which,
when coated on two mating surfaces and
allowed to dry, provides an instant
permanent bond when brought into firm
contact.
How is contact cement applied
By brush, roller coater, notched trowel,
air spray (hot or cold), airless spray (hot
or cold ), curtain coater.
How much cement should be applied
Enough to coat the surface uniformly
and thick enough that, when completely
solvent-free, a tacky, some what resilient
film is left.
How soon should bonding be done
This may be done as soon as solvent is
completely gone. If you can press a
finger to the surface and pull away
cleanly and with no sign of “legginess”
the surface is dry. If there is any doubt at
all, give it a few extra minutes. If any
puddling has occurred, be sure the
puddles are dry. After dryness has been
reached, you have an hour to make the
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bond. This is called “open time”.
How is drying accomplished Drying is accomplished by ordinary air
exposure, use of fans, passage under
infra-red bulbs, or passage through a
heated tunnel. It all depends on the
production speed desired.
Does temperature affect the drying
time of contact adhesives
Drying takes place more slowly at lower
temperatures and is faster at higher
temperatures.
Does humidity affect the drying time
of contact adhesives When humidity is low, drying is
accelerated. When it is high, drying is
slower.
What does viscosity mean
Viscosity is a measure of the thickness
of the adhesive: the higher the viscosity,
the thicker the adhesive. Spray adhesives
have low viscosity to allow proper
atomization by the spray gun. Brush
adhesives have a higher viscosity to
provide easy spreading and prevent
soaking-in porous core materials.
What is the solids percentage of a
contact adhesive The solid portion of the adhesive is dry
adhesive content in it. The solvent is
only a carrier to provide an easy method
of application. Therefore, adhesives with
a higher solids percentage generally
contain more usable adhesive per Kg.
What is “blush” “Blush” is moisture condensing on
freshly applied adhesive due to the rapid
solvent evaporation cooling the air
below the dew point.
Can any precautions be taken to
prevent “blush”
Two courses of action are possible:
Install a drying heat tunnel and
use it for all panels immediately
after the adhesive is applied. This
will also decrease the drying time
and increase productivity.
Apply the adhesive by the hot
spray method. Using this method,
the adhesive is applied at about
135 degrees F and the cooling
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effect of solvent evaporation is
not sufficient to cause “blush.”
What are the prime causes of bond
failure
Coating too thin (not enough glue)
Bonding too soon
Not enough pressure
Exposure to high heat