Heat-resistant polyurethanes from

Heat-Resistant Polyurethanes from Solvofysis Lignin 387... 

Hirose, S., Yano, S., Hatakeyama, T., and Hatakeyama, H., 1989, Heat-resistant Polyurethanes from Solvolysis Lignin. ACS Symp. Ser. 397 382-389. 

Different materials can be used such as nylon, polyester (TS), and epoxy, but TS polyurethane (PUR) is predominantly used. Almost no other plastic has the range of properties of PUR. Modulus of elasticity range in bending is 200 to 1,400 MPa (29,000-203,000 psi) and heat resistance from 90 to over 200°C (122-392°F). The higher values are for chopped glass-fiber-reinforced RIM (RRIM). 

Molds can be made from either metallic or nonmetallic materials. Metallic and nonmetallic molds are heated to the same temperature, which can be up to 120°C. Metallic molds are much more heat conductive than molds made from polyurethane or epoxy. When handling the metal molds, clean, heat-resistant gloves must be worn to prevent burning of the skin. The weight of these molds must be taken into account when handling them. Suitable lifting equipment such as a crane, block and tackle, or a forklift must be used. 

The unit cost of polyurethanes varies greatly from the lowest raw material cost of the PPG/TDI based systems to a heat-resistant, optically clear aliphatic grade. The actual lowest volume cost is the cost of a system that will fully meet the needs of a specified item. The use of a grade that is too good for the application wastes money. Conversely, a grade that is not suited also costs money. 

Some typical elastomers are natural rubber, which is gathered from trees, SBR rubber, which is used a lot in motorcar tires, neoprene, as in wet suits and oil seals, EPDM, a general purpose rubber, butyl, a heat-resistant rubber with the ability to keep the air in car tires, nitrile for oil seals, silicones for heat resistance, fluoro-elastomers for chemical resistance, and last but not least, polyurethanes, which cover a number of the above fields. Table 1.1 shows some of the advantages of castable polyurethanes over conventional rubbers. 

Polyurethanes are also used to make paints, available as water-thinnable paints and powder coatings. Items painted with PU paints benefit not only from a lacquer film, but also from an improved chemical resistance. Polyurethane paints are scratch, abrasion, and impact resistant, and they adhere well to a variety of surfaces. Polyurethane adhesives are similarly good choices as they resist heat, water, solvents, grease, and oils. ... 

The synthesis of dimeric fatty acids is based on the reaction between a fatty acid with one double bond (oleic acid) and a fatty acid with two double bonds (linoleic acid) or three double bonds (linolenic acid), at higher temperatures in the presence of solid acidic catalysts (for example montmorillonite acidic treated clays). Dimerised fatty acids (C36) and trimerised fatty acids (C54) are formed. The dimer acid is separated from the trimeric acid by high vacuum distillation. By using fatty dimeric acids and dimeric alcohols in the synthesis of polyesters and of polyester polyurethanes, products are obtained with an exceptional resistance to hydrolysis, noncrystalline polymers with a very flexible structure and an excellent resistance to heat and oxygen (Chapter 12.5). Utilisation of hydrophobic dicarboxylic acids, such as sebacic acid and azelaic acid in polyesterification reactions leads to hydrolysis resistant polyurethanes. 

Commercial ABS/TPU blends contain thermoplastic polyurethane elastomer (TPU) as the main blend component. The blends were first introduced in 1990 by Dow Chemical Co., under the trade name Prevail . These blends characteristically exhibited low modulus (340-1,000 MPa) and high impact strength at low temperatures, e.g., notched Izod values of 370-1,500 J/m at —29 °C. The TPU component of the blend imparts high toughness and also allows paintability without a primer. ABS component imparts heat resistance (for paint ovens) and good tensile strength in the blend. The blend was projected to be useful for automotive soft bumper fascias, but faced stiff competition from TPOs. Typical properties of ABS/TPU blends are shown in Table 19.9. 

More recently, polyoxyalkylene polythioalkylene polyols have been claimed to be useful for improving the heat resistance in polyurethanes. Studies with model compounds have also shown that the structure of the glycol chain extender may also influence the thermal stability. Ethylene glycol-based materials were found to be less stable than compounds prepared from butanediol. 

Styrene-maleic anhydride copolymers (SMA) Excellent heat resistance compared to typical polystyrenes Can be reinforced with glass fiber to enhance heat resistance, and with rubber to prevent plastic from becoming too brittle. Are used in IP in BMW 3 and 5 series and in Fiat Coupe, covered with polyurethane foam. [63]... 

Phenolic foams are thermoset, partially open cell foams with good dimensional stability and heat resistance . Like polyurethanes they are foamable in situ, at densities from 16-320 kg/m. Phenolic foam properties are relatively unaffected by heat and moisture, although this is dependent upon the density. Chemical and solvent resistance is very good. Phenolic foams have been used as insulants and also in packaging applications. 

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