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48

suggested that high particle velocities are required to form thin polymer splats with significant impact.3 The subject of “coalescence” (referring to the joining of smaller particles to form a continuous body) of traditional polymer powders has been reviewed by Mazur.4 Driving forces are intrinsic (surface tension) or extrinsic (applied temperature and pressure). Particle deformation, diffusion, and stress relaxation are the physical processes responsible for coalescence. Particle coalescence is stated to occur in two stages: neck growth and equilibration.4 Once the neck is formed, equilibration occurs by the interdiffusion of chains across the interface and the relaxation of mechanical stresses.5–7 A physical model describing the formation of combustion-sprayed PE-copolymer coatings has been developed by the author, based on classic particle-coalescence concepts, and will be discussed further in the

section “Case Study 1: EMAA Copolymer Coalescence.”8 Thermal-spraying of polymers is attracting new attention due to tightened environmental restrictions concerning the solvent content in industrial paints, increasing awareness of the annual costs of corrosion, and a new emphasis on life-cycle maintenance costs in the military. Thermal spraying is a viable option to industries facing environmental regulations, since it is a 100% solids process with zero volatile organic compounds (VOCs). Thermal-spraying of thermoplastic polymers has the following attributes in comparison with traditional painting processes:  it is a one-coat process (no need for a primer or sealer),  requires no cure time,  has an unlimited pot-life,  produces zero VOCs,  is repairable,  possesses superior mechanical properties, and  has the ability to produce highly loaded composite structures. Furthermore, thermal spraying is ideally suited to coating large structures on-site, such as architectural and infrastructure features that are not amenable to dip-andbake processes.9 Reports have also shown that polymers can be deposited in adverse weather conditions, such as high humidity and temperatures well below freezing, that would otherwise prevent the application of traditional paint systems.10

Materials Table I details the range of thermoplastics amenable to thermal spraying and

Table I: Commonly Sprayed Thermoplastic Resins and Their Associated Attributes in Coating Form. Thermoplastic Resin Polyethylene (PE) PE copolymers Ethylene methacrylic acid copolymer (EMAA) Ethylene acrylic acid copolymer (EAA) Ethylene vinyl acetate (EVA) Polyester Poly(ethylene terephthalate) (PET) Poly(butylene terephthalate) (PBT) Nylon (polyamide) Fluoropolymers Ethylene-tetrafluoroethylene copolymer (ETFE) Perfluoroalkoxy (PFA) Ketones Poly(ether ether ketone) (PEEK) Liquid-crystalline polymers (LCPs)

Property Conferred Barrier properties, toughness Adhesion, weathering resistance

Wear resistance, abrasion resistance

Wear resistance, abrasion resistance Chemical resistance, low friction

High temperature resistance High temperature, low gas and water p