Plasticising thermosets

Sanding is carried out at this stage and, after clean-up, the final colour or top-coat is applied. There is some variation in the resin chemistry used. Alkyds crosslinked with melamine-formaldehyde are widely used for non-metallic pigmentation. Metallics are usually based on acrylics for better durability. The acrylic may be thermoset with melamine-formaldehyde or a thermoplastic lacquer (plasticised copolymer of methyl methacrylate). A thickness of about 50ftm is applied and stoved for 20 min at 130°C (lacquers receive a bake-sand-bake process for a smoother appearance). 

Thermosetting resin produced by the reaction of phenol and formaldehyde in the presence of either an acid or an alkaline catalyst. In rubber compounding these resins are used as plasticisers and reinforcing materials. Phenyl-a-Naphthylamine... 

The mechanical strength and operating temperature of plastics are low compared with that of metals. The mechanical strength, and other properties, can be modified by the addition of fillers and plasticisers. When reinforced with glass or carbon fibres thermosetting plastics can have a strength equivalent to mild steel, and are used for pressure vessels and pressure piping. Unlike metals, plastics are flammable. Plastics can... 

Exchange between product and pack can occur in both directions, e.g. certain labelling materials such as heat sensitive and self-adhesive labels when in contact with plastic materials. Both the plastic and the adhesives may contain plasticisers or migratory constituents. Most cellulosics use phthalate, sebacate, phosphate-type plasticisers (e.g. methyl phthalate (DMP) may be used in cellulose acetate). Plasticisers may also be found in poly vinyl chloride/acetate copolymers, polyvinyl acetate and polyvinyl alcohol formulations, polymethyl methacrylate, nylon and certain thermosetting resins. 

Often non reactive plasticisers, such as saturated polyester adipates or epoxidised soya bean oils, are added in small quantities to thermosetting acrylics to improve the flexibility or drawing properties of the coating. 

Extrusion is used for the produetion of intermediate products such as profiles, boards and sheets. Generally screw extruders are used. For the processing of thermosets, piston extruders are also suitable. The material is kneaded thoroughly and plasticised by friction and additional cylinder heating. It is fed through an opened mould (die) with a diameter corresponding to the desired profile (Figure 1.12). 

The more eeonomical process is the injection moulding of thermosets on screw injection moulding machines. The material is plasticised with a special screw. The optimum preparation of the melt means a plasticising unit with a corresponding feed unit and screw for material specific dosage. 

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