Cast polyurethanes

The largest segment of the CASE family of polyurethanes are elastomers. Cast polyurethane elastomers reached a new dimension when high pressure impingement mixing led to reaction injection molding (RIM). This technology is used widely in the automotive industry, and reinforced versions (RRIM) and stmctural molded parts (SRIM) have been added in more recent years. 

The thermoplastic rubbers have properties similar to those of the cast polyurethane rubbers but, because of the absence of covalent cross-links, have rather higher values for compression set, a common problem with thermoplastic rubbers. Their main uses are for seals, bushes, convoluted bellows and bearings. 

The polishing pad, as another consumable material, also has a dominating effect in the CMP process, which is usually made of a matrix of cast polyurethane foam with filler material to control hardness of polyurethane impregnated felts. The pad carries the slurry on top of it, executes the polishing action, and transmits the normal and shear forces for polishing, thereby playing a very cnacial role in process optimization [44 6]. 

Wheels and tires are one of the major uses of cast polyurethane elastomers. We commonly see these on fork lift trucks and shopping carts, where their excellent abrasion resistance, resistance to oil, and good elasticity are valued. In industrial settings we find polyurethane covers on rollers used for paper, steel, and textile conveyor systems. In such applications, their excellent cut and abrasion resistance help prolong their useful life. If used in hot and humid conditions, polyether-based polyurethanes are preferred. 

Figure 4.2. Flowsheet of production of cast polyurethane elastomer articles from a prepolymer by the continuous method 1 - vessel to store prepolymer 2 - vessel to prepare prepolymer 3 - reactor for curing agent 4 -transfer pump 5 - metering pump 6 - mixing device 7- mold.

Cast polyurethanes cured with water would need the bulk of the carbon dioxide to be flashed off and the remainder held in place by compression molding. 

Before using a new mold, the dimensions should be checked. The coefficient of expansion of the mold material must be balanced against shrinkage of the cast polyurethane (normally 1.0 to 1.5%). It is normally much easier to remove material from a mold or master piece than to add to it. This should be taken into account when designing the mold. 

It is most important to realize that at this time the full properties of the material are not yet developed, and the molded polyurethane has to be returned to the curing oven for the period recommended by the system supplier. The curing time varies depending on the chemistry of the system. The maximum properties of the cast polyurethanes are developed after a further 1 to 2 weeks at ambient temperature. 

Along with the simple open-mold casting method, there are several other methods of casting polyurethanes. They include ... 

A balance has to be reached between various competing factors when casting polyurethanes. These competing factors are ... 

Polyurethanes have changes in their dimensions during curing, subsequent heating, and cooling. The shrinkage of hot-cast polyurethanes is a function of the temperature of the reaction. For a reaction temperature of 50°C, the linear shrinkage is approximately 1%. At 90°C it is close to 1.5%, and at 130°C it is 2.5%. 

Li, J. and I.M. Hutchings. "Resistance of Cast Polyurethane Elastomers to Solid Particle Erosion." Wear 135 (1990) 293-303. 

Polyurethanes find applications in a wide range of industries in competition with other plastics, ceramics, and metals. Cast polyurethanes are tough, abrasion resistant, have load bearing properties, and are chemical resistant. That makes them the first-choice materials in certain situations. The ability to be easily prototyped and have short production runs is a major positive factor for polyurethanes. 

The fabrication of polyurethanes into complex shapes is much easier than with metals. Large casting of up to 500 to 1,000 kg can be made from relatively simple molds and buckets, with minimal labor. The energy input into the production of a cast polyurethane part is low compared to the melting of a metal alloy. The actual molding costs are also lower. 

Latex rubber can be poured like polyurethanes, but they generally are only poured in thin sheets. Even thicker sheets such as Linatex require a very long cure time. Pouring is the method of choice for cast polyurethanes. Normally, rubbers require a compression molding press or injection molding machine to produce parts. This has high capital and mold costs. 

When a thin layer of polyurethane is cast over a metal reinforcing, the thickness of the polyurethane must be even and sufficient to prevent moisture from getting to the reinforcing and causing corrosion. A potential cause of problems is if an air bubble is trapped between the reinforcing and the cast polyurethane. 

Because of the radiation resistance of polyurethanes, they find use in the nuclear industry. Polyurethanes find an important use in short-run production where parts can be made accurately until more costly molds can be made, without the need for potentially expensive change. Polyurethanes can be used to make molds for casting polyurethane parts. A coat of mold release is required for easy part removal. These molds can be used for short-to-medium production runs. Polyurethane molds can be made from a variety of different materials. Only very low-density wood and high-moisture-containing materials such as plaster of Paris prove to be too difficult to prepare a mold. 

In general, polishing pads are composed of either a matrix of cast polyurethane foam with filler material to control hardness or polyurediane impregnated felts. Polyurethane is utilized because urethane chemistry allows the pad characteristics to be tailored to meet specific mechanical properties needs. Jairath et al. describe the manufacture of pad material as follows ... 

Formrez , [Witco] Adipate polyesters or polyether triok used for coatings, chain extenders, cast polyurethane elastomers, microcellular urethanes, potting compds., enctqrsulating agents, print-iiig tolls, coatings, adhesives. 

Primax. [Air Prods.] Modified UHMW-PE us in cast polyurethane formulations to impart increased abrasion resist, reduced coeff. of friction, improved tear str., reduced part dens. 

MBOCA is a man-made chemical and has not been found in nature (lARC 1974). It is produced commercially by reacting formaldehyde with 0-chloraniline (HSDB 1991 lARC 1974). Pure MBOCA is a colorless crystalline solid (Smith and Woodward 1983). The technical grade of MBOCA that is available in the United States comes mainly from Japan in the form of tan/yellow fused prills or pastilles. The diamine purity is 99.8%, typically with 0.2% free o-chloroaniline (monomer). Isomers are produced as side reactions such as trimers and tetrameres-diamines with three-and four-ring structures joined by methylene groups. Isomers constitute up to 8-10% of MBOCA. The dimer makes up to 90-92% of the MBOCA produced today for coatings and cast polyurethanes. There is no commercial use for pure dimer MBOCA other than for laboratory work. 

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