Page 268 - Handbook of Adhesives and Sealants

236 Chapter Six

the adhesive is a polar surface having higher surface energy. Such
‘‘engineered’’ surface treatments of polypropylene and polyethylene
claim to give a six fold increase in peel strength when bonded with an
epoxy adhesive. 32 Evidence indicates that covalent bond formation oc-
curs between the fluorinated surface and the amine component of the
epoxy hardener. It is claimed that nearly all plastic surfaces can be
effectively improved by fluorine treatment. 33,34
Another approach to active surface treatment is to graft polar mon-
omers onto surfaces of low energy polymers to alter the surface by
making them more polar. Very thin coatings can be applied by plasma
technology. Significant improvements in bond strength have been
made with polyolefin substrates and epoxy adhesives. 35–39 However, on
a commercial scale these procedures would require considerable cap-
ital investment and the use of hazardous chemicals. The use of con-
ventionally applied surface primers and adhesion promoters to treat
polymeric surfaces has proved to be a more desirable alternative on
the production line. These primers are discussed in Chapter 7.
Adhesive abrasion is a process developed for polymeric substrates
40
such as the fluorocarbons. It is primarily used in the microelectronics
industry. With this process, the plastic surface is abraded while im-
mersed in a liquid adhesive. The abraded substrates are then imme-
diately mated and allowed to cure. Abrasion in the presence of the
liquid adhesive produces free radicals that react directly with the ad-
hesive before they can be scavenged by atmospheric oxygen.
Although bond strength can be improved significantly by these
chemical methods, most are time consuming batch processes, often
taking hours to treat, wash, and dry parts. There is also the problem
of disposing of hazardous waste from these processes and the opera-
tor’s safety. Thus, the application has been mostly for small volume,
high value parts.
There are many industries performing fast, high volume assembling
or sealing operations on substrates with low surface energy. Many of
the high volume assembly applications selectively use thermoplastic
substrate materials so that thermal welding or solvent cementing as-
sembly processes can be used which do not require active surface
treatment. These processes are described in Chapter 15.

6.5.2 Active physical surface treatments
for polymeric materials
Because of the main disadvantages of chemical treatments (hazardous
nature and a slow, batch type process), a number of other active sur-
face treatments have been developed for polymeric materials. These
processes utilize the reactivity of the polymeric surface to gain change