Mechanismus of effective control of automotive coatings adhesion to polymers and composites

 
Polymers and lightweight composites rapidly replace metals in high-tech engineering applications in automotive, aerospace, shipbuilding, civil and other engineering applications. In the automotive industry, the weight reduction and associated fuel economy are the key drivers in increasing contents of polymeric materials in car bodies. The key polymers of interest are polyolefins, predominantly polypropylene, which finds wide use in the manufacture of bumper bars, interior body trim panels, instrument panel and other components. High-strength thermoplastic polymers and composites based on nylon, polyacetal and polycarbonate also find increasingly broad use. The aircraft manufacturers, in turn, are vitally interested in maximising the contents of fiber reinforced thermoplastic composites in the manufacture and assembly of large primary structures such as tail, wing and fuselage offering substantial weight reduction, up to 50%, in comparison with aluminium components. The key candidate thermoplastic polymers considered in such applications are polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherimide (PEI), polyphenylene sulfide (PPS) and other high-tech engineering plastics. Similar trend is observed regarding the use of fibrereinforced composite materials in body panels manufacture. It is demonstrated that significant enhancement of adhesion between coatings and polymers is achieved through the use of specific connector molecules chemically attached (grafted) to the molecular backbone of a polymeric substrate. The strength of the bond between coating and substrate, and consequently overall performance of coated product is controlled by physico-chemical structure, properties and spatial architecture of interphase, an intermediate zone between the substrate and the coating. Such interphase comprises an array of "connector chains" which, at one end, are chemically grafted to the molecular backbone the substrate whilst the unbonded "free end" on application of chemically crosslinking coating becomes chemically bonded to it. Alternatively, on contact with non-crosslinking coatings connector molecules enhance adhesion through interpenetration into the bulk coating. Due to chemical inertness, hydrophobicity, surface contamination and migrating functional and processing additives, most polymers require surface modification to ascertain high strength of coating-substrate adhesion. The key objective is creation of permanent bond which becomes unbreakable. Consequently, the coating or substrate become the weakest element of the coated structure throughout entire service life under the influence of all static, dynamic and environmental loads and impacts. Theoretical and engineering aspects of the problem, including criteria for attaining optimised bond properties are comprehensively discussed in this paper. The effectiveness of coating adhesion enhancement is shown to depend on surface density of grafted molecules and the length of individual molecules. Examples of a relatively simple and industry-feasible technology for surface grafting connector molecules for enhancing adhesion of automotive TPO's are also discussed in detail.