Elastomers from polymer polyols

In the manufacture of highly resident flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. Graft polyols are dispersions of free-radical-polymerized mixtures of acrylonitrile and styrene partially grafted to a polyol. Polymer polyols are available from BASF, Dow, and Union Carbide. In situ polyaddition reaction of isocyanates with amines in a polyol substrate produces PHD (polyhamstoff dispersion) polyols, which are marketed by Bayer (21). In addition, blending of polyether polyols with diethanolamine, followed by reaction with TDI, also affords a urethane/urea dispersion. The polymer or PHD-type polyols increase the load bearing properties and stiffness of flexible foams. Interreactive dispersion polyols are also used in RIM appHcations where elastomers of high modulus, low thermal coefficient of expansion, and improved paintabiUty are needed. 

For polyurethane production, Donnelly [109] has carried out the synthesis of copolyurethanes based on mixtures of commercial poly(THF diol)s with glucose. Complex products resulted, which can be represented by mono- or bis(glucoside) structures. From a variety of polyol blends, solid polyurethanes were prepared which ranged from linear, soluble, weak elastomers to polymers of higher transition temperature and stiffness, low solubility, and low extension under tensile load [110]. 

With amine initiators the so-called self-catalyzed polyols are obtained, which are used in the formulation of rigid spray foam systems. The rigidity or stiffness of a foam is increased by aromatic initiators, such as Mannich bases derived from phenol, phenolic resins, toluenediamine, or methylenedianiline (MDA). In the manufacture of highly resilient flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. 

Polyurethane adhesives are inherently quite flexible without added tougheners. Even the crosslinked polymers behave like thermoplastic elastomers with two-distinct phases the hard phase contributed by the isocyanate and the soft phase from the polyol. 

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. 

Polymers prepared from untipped PPG polyols have some inherent deficiencies. Table 1 shows data on injection molded TPU elastomers based on PTMG and PPG polyols. 

Reducing the monol content also improves the dynamic properties of cast elastomers. Figure 9.3 shows the DMTA response for the two polymers described above. The polymer derived from the ultra-low monol polyol has a flatter rubbery plateau region. The higher modulus in the rubbery plateau is consistent with the polymer s higher tensile modulus. The substantial reduction in tan delta (8) across the entire temperature range should also be noted. Lower tan delta translates into improved performance in dynamic applications due to lower heat buildup and improved rebound as noted above. 

Polyurethanes are very versatile polymers. They are used as flexible and rigid foams, elastomers, and coatings. Polyurethanes are available as both thermosets and thermoplastics. In addition, their hardnesses span the range from rigid material to elastomer. Thermoplastic polyurethanes will be the focus of this section. The term polyurethane is used to cover materials formed from the reaction of isocyanates and polyols. The general reaction for a polyurethane produced through the reaction of a diisocyanate with a diol is shown in Fig. 2.35. 

The chain extender structure strongly influences the PUs mechanical performance. Modifying the ratio between the polyol and chain extender, PUs may result in a change from a hard, brittle material to a rubbery elastomer, as a result of the variation of the HS concentration (defined as the ratio of the mass of the non-polyol components to the total mass of the polymer) [2-4]. 

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