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242   Chapter Six


            6.13. Certain polymeric surfaces, such as the polyolefins, become cross-
            linked during plasma treatment. The surface skin of polyethylene, for
            example, will become crosslinked so that if the part were placed on a
            hot plate of sufficient heat, the interior would turn to a molten liquid
            while the crosslinked outer skin held a solid shape. Other polymers
            are affected in different ways. Plasma treated polymers usually form
            adhesive bonds that are 2 to 4 times the strength of untreated poly-
            mers. Table 6.14 presents bond strength of various plastic substrates
            that were pretreated with gas plasma and bonded with an epoxy or
            urethane adhesive.

            6.5.2.4  Other physical surface treatments for polymers. Other surface
            treatments have been reported to enhance adhesion of low energy
            polymers. Bond strengths of some materials can be improved if the
            mating surfaces are etched or otherwise textured prior to joining. Ion
            beam etching and excimer laser radiation are two ways of doing this.
              Ion beam etching has been used on stainless steel, graphite, and
            fluorocarbon surfaces. 44  The resulting surface is composed of needles
            or spires that allow improved mechanical bonding. Bonds are stronger
            both in tension and in shear than those made on chemically etched
            plastics. An additional benefit of ion beam treatment is that its effect
            does not diminish with time as in the case of chemical etching.
              Excimer laser surface treatment has been used for preparing poly-
            ester sheet molding compounds (SMC) for adhesive bonding in the
            automotive industry. The excimer laser preparation of SMC surfaces
            occurs through the following stages: ablation of surface contaminates,
            selective ablation of calcium carbonate filler from the SMC, and re-
            moval of polyester resin from the SMC. 45,46
              UV irradiation has also been applied as a prebond surface treatment
            to a variety of plastics. 47  Basically, this process involves applying a 5%
            solution of benzophenone (a light sensitizer) to the surface of the part.
            The part is then briefly exposed to an ultraviolet light source. This
            causes chemical changes in the polymer surface, including an increase
            in wettability and a certain amount of crosslinking that reportedly
            strengthens the surface and improves the physical interaction neces-
            sary for adhesion. It is likely that this process promotes controlled
            degradation of the surface of the polymer, offering a better surface on
            which to bond. The process was originally developed for application to
            EPDM rubber, but it may also be applicable for polypropylene, poly-
            ethylene, and other thermoplastics.
              Primers are widely recognized for preparing metals for adhesive
            bonding. It has also been reported that certain polymeric surfaces,
            notably polyolefins, can be modified by applications of primer solutions
            such as chlorinated polyethylene. The probable mechanism by which
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