Plastics surface preparation plasma treatment

An important newer use of fluorine is in the preparation of a polymer surface for adhesives (qv) or coatings (qv). In this apphcation the surfaces of a variety of polymers, eg, EPDM mbber, polyethylene—vinyl acetate foams, and mbber tine scrap, that are difficult or impossible to prepare by other methods are easily and quickly treated. Fluorine surface preparation, unlike wet-chemical surface treatment, does not generate large amounts of hazardous wastes and has been demonstrated to be much more effective than plasma or corona surface treatments. Figure 5 details the commercially available equipment for surface treating plastic components. Equipment to continuously treat fabrics, films, sheet foams, and other web materials is also available. 

Avseenko et al. (2001) immobilized antigens onto aluminum-coated Mylar films by electrospray (ES) deposition. Various surface modifications of the metallized films were studied to determine their abilities to enhance sensitivity. The plastic surfaces were firsf cleaned by plasma discharge treatment, followed by coating with proteins (BSA and casein) or polymers such as poly (methyl methacrylate) or oxidized dextran, or they were exposed to dichlorodimethyl silane to create hydrophobic surfaces. Protein antigen was prepared in 10-fold excess sucrose and sprayed onto the surfaces to form arrays with spot diameters between 7 and 15 pm containing 1 to 4 pg protein. 

As with metallic substrates, the effects of plastic surface treatments decrease with time. It is necessary to prime or bond soon after the surfaces are treated. Some surface treatments, such as plasma, have a long effective shelf life (days to weeks) between treatment and bonding. However, some treating processes, such as electrical discharge and flame treating, wiU become less effective the longer the time between surface preparation and bonding. 

Adhesives manufacturers are continually trying to develop adhesives to meet the needs of industry. One group of plastics that have been difficult to bond are polyolefins and related low-energy substrates (see Surface energy). They could not be bonded without elaborate surface preparation such as Flame treatment or Plasma pre-treatment, Corona discharge treatment or oxidative chemical methods. 

Epoxy and nitrile-phenolic adhesives have been used to bond these plastics after surface preparation. The surface can be etched with a sodirim sulfiiric-dichromate add solution at an elevated temperature. Flame treatment and corona discharge have also been used. However, plasma treatment has proven to be the optimum siuface process for these materi2ds. Shear strengths in excess of 3000 Ib/in have been reported on polyethylene treated for 10 min in an oxygen plasma and bonded with an epoxy adhesive. Polyolefin materials can also be thermally welded, but they cannot be solvent cemented. 

Treatment of certain polymeric surfaces with excited inert gases greatly improves the bond strength of adhesive joints prepared from these materials. With this technique, called plasma treatment, a low-pressure inert gas is activated by an electrode-less radio-frequency discharge or microwave excitation to produce metastable species which react with the polymeric surface. The type of plasma gas can be selected to initiate a wide assortment of chemical reactions. In the case of polyethylene, plasma treatment produces a strong, wettable, cross-linked skin. Commercial instruments are available that can treat polymeric materials in this manner. Table 7.10 presents bond strength of various plastic joints pretreated with activated gas and bonded with an epoxy adhesive. 

Almost any substrate that will be powder coated, whether it is metal, medium-density fiberboard (MDF), or some other material, requires some kind of surface freafmenf. Surface preparafion (prefreafmenf) of wood and more specifically, MDF preparation consist of sanding, removal of confami-nanfs, and board condifioning. MDF boards fhaf will be coafed by UV curable powders should have a moisture content of 49%. If is also a common practice fo preheaf MDF boards prior to application of fhe UV powder. Mefal surfaces are always cleaned by washing and usually freated by a phosphafe primer. Conductive plastics are washed and dried and then coated without any further treatment. Nonconductive plastics are made conductive by applying a primer coat or treated by plasma or flame. ... 

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