16.5 Thermoplastic Substrates

Unlike the thermosetting resins, the thermoplastic resins will soften on heating or on contact with solvents. They will then harden on cooling or on evaporation of the solvent from the material. This is a result of the non-crosslinked chemical structure of thermoplastic molecules. Tradenames of common thermoplastic materials can be found in Table 15.2. The following are important characteristics of thermoplastic resins that can affect their joining capability:

• Many thermoplastic compounds are alloyed and really consist of two or more resins.

• Additives and mold release agents are commonly employed in the formulation.

• Thermoplastics exhibit a relatively high degree of water absorption and mold shrinkage.

• Dimensional changes due to moisture migration, thermal expansion, etc. are generally greater than with other materials.

• The properties of the surface, such as surface energy and crystallinity, may be different from the bulk, (this is especially true for thermoplastics that are molded at very high temperatures).

• Many grades of the same material are available (e. g., high flow, high density).

Thermoplastic materials usually have a lower surface energy than thermosetting materials. Thus, physical or chemical modification of the surface is sometimes necessary to achieve acceptable bonding. This is especially true of the crystalline thermoplastics such as polyolefins, linear polyesters, and fluoropolymers. Methods used to increase the surface energy and improve wettability and adhesion include:

• Oxidation by chemical means or by flame treatment

• Roughening of the surface by electrical discharge

• Gas plasma treatment

As with metal substrates, the effect of plastic surface treatments decrease...