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.The maximum bonding was found to occur when the surfacewas populated with -OH or -CHO, but not with ester linkages.This was ex-plained by the specific chemical speciation required for substrate reaction withisocyanate groups in the polyurethane coating (14).This study points out thatcovalent interactions should be expected to need chemical tuning to ensure thatthe proper surface species are provided to match the reactive group in the coating.3.8 Adhesion to Low-Energy Polyolefin SurfacesThe importance of low-cost material solutions is the primary reason to overcomethe difficulty of coating polyolefin surfaces.In addition to the use of adhesionpromoters and of the flame and plasma surface treatment previously discussed,two additional potential alternatives that are specific to polyolefins are discussedin the following text.3.8.1 Surface Functionalization Through Bulk ModificationThe addition of polar groups to the polyolefin backbone has been a widelyexplored technique for improved adhesion.The success of this approach relieson the ability to place a sufficient concentration of polar groups on the surface ofthe formed part.This surface concentration correlates to the bulk concentrationcombined with the mobility of the polar-modified polymer in the melt.Berg-breiter et al.demonstrated that blending block co-oligomers of PP and tert-butyl acrylate into unmodified PP produced a surface that provided surface acidfunctionality after hydrolysis (12).Improved wetting by the addition of the polarmaleic anhydride group directly onto PE was found to depend on the polarcontent of the material, processing conditions, and the extent to which the anhy-dride group was hydrolyzed after substrate formation.These materials were alsofound to offer better adhesion to a polar substrate than the same surface treatedwith oxygen plasma (15).Hailat combined a maleated PP and polyetheraminethat reactively coupled in the melt to produce a part having excellent adhesionto polyurethanes (16).Similarly, maleated polyolefins have been reacted withpolyamides and their copolymers to reactively produce a polar polyolefin in themelt that has improved adhesion (17,18).Although these chemistries have beendemonstrated at laboratory to pilot scale, none are currently practiced commer-cially.Issues that remain include the cost of the new polymers in comparisonto use of adhesion promoters, and the fact that such treatments reduce the heat-distortion temperature of the polymer below that dictated by paint oven tempera-tures for typical one-component coatings (19).Compared to one-component sys-tems, two-component systems cure at temperatures that do not compromise thephysical properties of the substrate, but do cost significantly more. 34 Babinec and Cornell4 ELECTROSTATIC PAINTINGPlastics play an increasingly important role in automotive exteriors due to theirlight weight, which enables compliance with fuel efficiency standards, and highlevel of design and processing flexibility.As discussed previously, consumersdemand that automotive components have a high aesthetic value.For exteriorvehicle body panels, alternatives to the painting process, such as molded-incoloring and in-mold laminates, have not yet produced a final appearance thatis uniformly acceptable to both customers and the manufacturing industry.Thedesire for a Class A finish thus keeps traditional painting techniques as the mostwidely practiced option for surface finishes.Coating the parts with high-qualitypaints is thus an absolute requirement for commercial success.This finishingstep is very expensive, with coating costs frequently equal to that of the unfin-ished part.Efficient coating is therefore critical for continued commercial viabil-ity in this application.A common figure of merit for the painting process is transfer efficiency(TE), which is the weight of paint delivered to the part divided by the totalweight that left the paint gun per part.Although each painting technique coversa range of efficiencies because of differences among painting shops, there arefundamental efficiency differences between the different processes.Table 12shows that electrostatic painting with automatic air atomization of the paint hasthe potential to be the most efficient, and thus the lowest cost, coating technique
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