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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