contact adhesive ,formulation ,types of contact adhesives ,application , faq's ,resins , polychloroprene , chloroprene rubber, additives, solvents for adhesive, its mechanism, advantages & disadventages

1. ADHESIVE
 An adhesive is any substance applied to the surfaces of materials
that binds them together and resists separation.
 The development of modern adhesives began in 1690 with
the founding of the first commercial glue plant in Holland.
 Adhesives are formulated and synthesized.
 Low- to medium-performance products are based on natural
substances such as starch, dextrin, natural rubber or protein or
synthetic polymers such as polyvinyl acetate, polyvinyl alcohol, polyesters, acrylics, neoprene,
butyl rubber, phenolic.
 High-performance products are based on polymers such as epoxy, polysulfide, polyurethane,
cyanoacrylate and silicone which have enhanced properties including bond strength,
elongation capacity, durability or environmental resistance.

2. 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.
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.
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.

3. Types:
Contact adhesives are available with
 solvent based Technology
 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.
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.
Effect of Temperature and Humidity:
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 rat 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

4. 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.
 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.
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.
• Adhesives with relatively slow drying solvents usually are used with roller or curtain
coating methods.

5. Advantages:
 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 and non-structural elastomeric pressure
sensitive adhesives
 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
 Good aging resistance indoors; aging outdoors will depend on base polymer and formulation
Disadvantages:
 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.

6. 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 have greater strength and
durability.
• 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. In addition to the base
polymer and carrier, contact adhesive
formulations may include tackifiers,
stabilizers, pigments and fillers, and
crosslinking agents

7. Polychloroprene:
Polychloroprene or chloroprene rubber (CR), introduced by DuPont in 1931, was the first
synthetic rubber developed that exhibited the elastomeric properties of natural rubber. An
annual consumption of chloroprene rubber 300,000 tons world wide. The modern chloroprene
process is based on the conversion of butadiene into the chloroprene monomer (2-
chlorobutadiene-1,3).
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. 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. The choice of solvents used in the
manufacture of contact adhesive depends not only on the solvent’s dissolving power, but also
on its evaporation rate. Solvent selection can also influence the rate of crystallization
POLYCHLOROPRENE-BASED
FORMULATIONS:

8. Metal Oxides:
The zinc oxide and Magnesium oxide serves to act as an acid acceptor and as a crosslinking
agent with carboxyl functional groups to improve heat resistance and the storage stability.
Magnesium oxide will also prevent scorching during mastication.
Magnesium oxide and zinc oxide are generally used at about 4 parts per hundred parts rubber
of each.
Resins:
Resins are also used in polychloroprene contact adhesives, the most common being the alkyl
phenolic resins. 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. 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. Less tack but greater strength is
obtained with terpene phenolic resins.
Solvent Selection:
Solvent blends are normally used in polychloroprene contact adhesives. Nonflammable
chlorinated solvents, once popular, have been replaced due to environmental restrictions. The
choice of 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.

9. 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 Membrane attachments, and a wide variety of additional applications where quick, high
strength, permanent bonds are required

10.  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 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.
 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.
 Does humidity affect the drying time of contact adhesives
When humidity is low, drying is accelerated. When it is high, drying is slower.
per Kg.
FAQ’S

11.  What does viscosity mean
Viscosity is a measure of the thickness of the adhesive: the higher the viscosity, the thicker the
adhesive.
 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.
 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.
 Apply the adhesive by the hot spray method.
 What are the prime causes of bond failure
 Coating too thin (not enough glue)
 Bonding too soon
 Not enough pressure
 Exposure to high heat
FAQ’S