Page 299 - Handbook of Adhesives and Sealants

264 Chapter Seven

primers used with cyanoacrylate adhesives produce adhesive bonds
with polypropylene and low density polyethylene that are sufficiently
strong to exceed the bulk shear strength of the substrate. They are
also sufficiently durable to withstand immersion in boiling water for
long periods of time. 14
It appears that one of the main reasons for improved adhesion by
primers with cyanoacrylate adhesives is that the solvents in the pri-
mer wet-out and swell the polyolefin. This facilitates interpenetration
of the low viscosity cyanoacrylate resin.
A similar effect seems to work with free radical cured acrylic ad-
hesive systems. In this system, the primer consists of a solution of a
copper salt, and the adhesive is based on methyl methacrylate mon-
omer. The bonds formed on low density polyethylene result in sub-
strate failure, and an interphase of mixed adhesive and polyethylene
is formed up to 1.5 mm thick. 15
Chlorinated polyolefins are used for priming low energy surfaces
such as polyolefins. These primers are based on either chlorinated
polyethylene or polypropylene and are usually used as a solvent based
solution. The application of chlorinated polyolefin as a primer on the
surface of polypropylene provides a significant improvement in the
adhesion of a latex topcoat to the polypropylene. 16,17 A primer based
on a solution of chlorinated polypropylene has been used to adhere
paint to polypropylene automobile bumpers with some interdiffusion
of primer into the part. Chlorine atoms in the outer surface of the
primer are believed to increase polarity and enhance paint adhesion
1
in this case. Solvent-free, water borne chlorinated polyolefin primers
based on emulsions and dispersions of chlorinated polyolefin have also
been developed. 18,19 They provide increase in bond strength and water
resistance for polypropylene and other thermoplastic polyolefin joints.

7.2.4 Primers for unvulcanized elastomers
Adhesion primers have also found significant application in treating
the surface of inserts that are to be embedded in elastomeric parts.
There are many instances when unvulcanized rubber is required to
bond to itself or to another substrate such as a metal insert. Primers
are generally used in these applications to provide a strong bond be-
tween the elastomer and the adherend. An example is the manufac-
ture of roller wheels. Here the metal insert (bearing assembly) needs
to be well bonded to the surrounding elastomer that serves as the
elastomeric wheel.
The primer is coated on the part and then air or heat dried. In the
case of the roller wheel, the metal hub must be surface treated, primed
and dried, and then carefully placed in the mold. The molten elasto-