Polyurethane coating swelling

FIGURE A3.39 Swelling behaviour of a polyurethane coating resin (Viacryl/Desmodur). Contours are shown at 100% (—) and 200% (—) swelling in solvent. 

Cyclic oligomeric phosphonates with the varying degrees of structural complexity (Structure 5.4) are also available in the market.25 They are widely used as flame-retardant finishes for polyester fabrics. After the phosphonate is applied from an aqueous solution, the fabric is heated to swell and soften the fibers, thus allowing the phosphonate to be absorbed and strongly held. It is also a useful retardant in polyester resins, polyurethanes, polycarbonates, polyamide-6, and in textile back coatings. A bicyclic pentaerythritol phosphate has been more recently introduced into the market for use in thermosets as well as for polyolefins (preferably, in combination with melamine or ammonium polyphosphate)... 

Recently, new polymer-based bioactive coils have been investigated. Polymer coated platinum coils have been developed to swell on contact with blood. The hybrid-hydrogel coil can swell up to nine times its original volume, consequently maximizing coil volume while reducing the number of coils needed and surgical time [53]. Polyurethane coils have been shown to induce occlusion faster than metal coils in animal studies [54]. Furthermore, an SMP coil system has been developed from polyurethanes and tested in vitro [55]. 

As the substrate sheet, a thin plastic film or tightly packed nonwoven cloth, such as polyester spanbond nonwoven cloth of approximately 0.15 mm, is used. A synthetic rubber, such as styrene-butadiene rubber or thermoplastic polyurethane elastomer, is used as a binder. In addition, a surfactant for hydrophilicity, an antioxidant for prevention of thermodeformation, and a silica-type inorganic filler for prevention of tackiness are used. For the superabsorbent polymer particles, various synthetic polymers, for example, polyacrylate and polyvinyl-type superabsorbent polymers, can be used. For this apphcation, the particle sizes are an important parameter, because they polymer is required to be within the coating layer, and as the absorption rate is no retarded, quickly protrude from the layer when swelling. 

The most commonly used group of plastics in this class are polyurethanes which are resistant to most solvents, with the exception of n-methyl pyrollidone which swells the substrate but improves adhesion. RIM (reaction injection moulded) polyurethane finds use in the automotive sector for bumpers, spoilers and external trims. Polyurethanes are affected by heat because of their cellular structure, often craters or pimples are formed in the coating as a result of the substrate s porosity. Hence components are often manually prepared with primer filler, pore filler and several de-nib stages prior to the application of a topcoat. In recent years this high labour requirement has resulted in the replacement of polyurethane by alternative plastics. 

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