Replacement of Thermosets

To a lesser extent, the replacement of thermosets by PP-based compounds has also occurred in the area of thermoset polyester for large automotive parts such as grill opening panels however, the trend has not been so dominant as in the case of RIM. Grill opening panels, once made with thermoset polyester compounds, have switched to filled or glass fiber-reinforced PP. 

The automotive sector is expected to remain the fastest growing market for polyetherimide. There is still much scope for replacement of thermosets and other plastics by PEI in key automotive applications. Consumer products such as food service containers and microwavable ovenware, also offer good growth potential. 

Exceptional electrical properties and temperature resistance qualify the material for numerous electrical parts—connectors, coil bobbins, light sockets, terminal blocks, fuse holders, and motor parts, PBT provides weight reduction of final parts. PBT has replaced a number of thermoset materials and is particularly popular for appliance housings and fibers for paint brushes. 

Thc-changc-ovcr to automated processing of thermosetting materials calls for utilization of novel equipment, improved processing technology and replacement of powdered materials with up-to-date granulated materials. 

Thermoset polyurethanes are cross-linked polymers, which are produced by casting or reaction injection molding (RIM). For cast elastomers, TDI in combination with 3,3,-dichloro-4,4,-diphen5lmethanediamine (MOCA) are often used. In the RIM technology, aromatic diamine chain extenders, such as diethyltoluenediamine (DETDA), are used to produce poly(urethane ureas) (47), and replacement of the polyether polyols with amine-terminated polyols produces polyureas (48). The aromatic diamines are soluble in the polyol and provide fast reaction rates. In 1985, internal mold release agents based on zinc stearate compatibilized with primary amines were introduced to the RIM process to minimize mold preparation and scrap from parts tom at demold. Some physical properties of RIM systems are listed in Table 7. 

The raethylolated acrylamide type has been widely used in appliance finishes. These are cross-linked with melamine or epoxy modifiers and form hard, chemically resistant finishes. Detailed discussion of thermosetting acrylics can be found in References 2 and 8. Recently, topcoats based upon high-solids liquid polyesters have replaced thermosetting acrylics in response to environmental... 

The quality of moulded theimosets is highty dependent on a precisely repeatable cycle time. This is true also of thermoplastics, but is not so critical as in thermosets. The one phase of the (tycle not controlled is part removal from the mould this is at present done tty the machine operator. Whilst he does this, the safety gate protects him and the restart of the next (tycle is under his control and is delayed until he completes removal of the part and closes the safety gate. The human variability thus introduced (a change firom (tycle to cycle of onfy a few seconds) can affect product repeatability. This is one factor leading to the replacement of the human operator Ity robots for part removal. 

Unsaturated polyesters are the most versatile class of thermosetting polymers. They are macromolecules made up of an unsaturated component, (i.e., maleic anhydride or its trans isomer, fumaric acid, which provides the sites for further reaction) and a saturated dibasic acid or anhydride with dihydric alcohols or oxides (typically phthalic anhydride which can be replaced by aliphatic acid, like adipic acid, for improved flexibility). If blends of phthalic anydride (or isophthalic acid) and maleic anhydride/fumaric acid are used, ortho (or iso) resins (Figure 6.18) are obtained. On the other hand, if propoxylated or ethoxylated bisphenol A is used with fumaric acid, bisphenol A fumarates are obtained, if a blend of chlorendic... 

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