Page 301 - Handbook of Adhesives and Sealants

266 Chapter Seven

common unvulcanized elastomers such as the silicone, fluorocarbon,
chlorosulfonated polyethylene, and polyacrylate are more difficult to
bond. However, recently developed adhesive primers also improve the
bond of these elastomers to metal. Proprietary primers are available
from the manufacturers listed in Table 7.4. Suggestions for specific
primers to be used for particular elastomers and for certain applica-
tions can usually be obtained from the adhesive/primer manufacturer.
Surface treatment of the adherend before priming should, of course,
be according to good standards. Care must be taken so that the
primer/adhesive system is not wiped off the substrate during the flow
of the unvulcanized elastomer during molding. Equal care must be
exerted to not allow the elastomeric fluid to wash away mold release
from protected surfaces onto critical surfaces where adhesion will be
necessary.
Plastic inserts can similarly be bonded to unvulcanized rubber with
selected primers. The plastic part will need to be treated by one of the
processes discussed in the last chapter. The plastic must also be able
to withstand the rubber vulcanization temperatures. Primers are also
used for elastomer to fiber bonding. Applications include reinforced
hose, belts, and tires. Resorcinol formaldehyde resins are commonly
used as primers in these applications.

7.3 Adhesion Promoters
Adhesion promoters or coupling agents are a group of specialty bi-
functional compounds that can react chemically with both the sub-
strate and the adhesive. The adhesion promoter forms covalent bonds
across the interface that are both strong and durable. Adhesion pro-
moters can be applied directly to the substrate, similar to primers, or
they can be mixed with the adhesive itself. When mixed with the ad-
hesive, the coupling agent is capable of migrating to the interface and
reacting with the substrate surface as the adhesive cures. When ap-
plied directly to the substrate, adhesion promoters are applied in a
very thin coating that ideally is only one molecular layer thick.
Adhesion promoters usually consist of molecules with short organic
chains having different chemical composition on either end of the
chain. On one end is an organofunctional group that is particularly
compatible with the given adhesive material. At the other end of the
chain is an inorganic functionality that is especially compatible with
a given substrate. The adhesion promoter, therefore, acts as a chem-
ical bridge between the adhesive and the substrate.
Adhesion promoters were first used to treat glass fibers and other
fillers before they are incorporated into liquid resin to make composite
materials. In the fiber industry, adhesion promoters are also known