Linear Polyurethanes

Data have been published dealing with successful applications of HAS in stabilization of other polymers than PO elastomers, styrenic polymers, polyamides, polycarbonates, polyacetals, polyurethanes, linear polyesters, thermoplastic polyester elastomers, polyacrylates, epoxy resins, poly(phenylene oxide) or polysulfide [12]. In spite of their basicity, HAS may also be used for stabilization of PVC. This application includes less basic derivatives of piperidine and 1,4-dihydropyridine [12,13,145,146]. 

Figure 4 The dynamic storage modulus, of polyurethane (linear) and poly(ester-styrene-methyl methacrylate) (cross-linked) semi-II SIN S. The midrange compositions have very broad glass transitions, indicative of a microheterogeneous...

Tensile Strength and Elongation. The tensile strength of latex mbber foam has been shown to depend on the density of the foam (149,177) and on the tensile strength of the parent mbber (177,178). At low densities the tensile modulus approximates a linear relation with density but kicreases with a higher power of density at higher densities. Similar relations hold for polyurethane and other flexible foams (156,179,180). 

The physical properties of polyurethanes are derived from their molecular stmcture and deterrnined by the choice of building blocks as weU as the supramolecular stmctures caused by atomic interaction between chains. The abiHty to crystalline, the flexibiHty of the chains, and spacing of polar groups are of considerable importance, especially in linear thermoplastic materials. In rigid cross-linked systems, eg, polyurethane foams, other factors such as density determine the final properties. 

This process is based on the very high reactivity of the isocyanate group toward hydrogen present ia hydroxyl groups, amines, water, etc, so that the chain extension reaction can proceed to 90% yield or better. Thus when a linear polymer is formed by chain extension of a polyester or polyether of molecular weight 1000—3000, the final polyurethane may have a molecular weight of 100,000 or higher (see Urethane polymers). 

In addition to linear chain extension, excess diisocyanate leads to cross-linking iato a network because the diisocyanate groups can also react with the hydrogen atoms of the —NH— groups ia the chains. Furthermore, the weU-known polyurethane foam mbber can be made by a dding water to the mixture because the isocyanate groups react vigorously with water to Hberate carbon dioxide gas as follows ... 

As previously mentioned the initial research on polyurethanes was directed towards the preparation of fibre-forming polymers. Many poly hydroxy compounds and many di-isocyanates were used and the melting points of some of the more linear aliphatic polyurethanes produced cU"e given in Table 27.1. 

A wide range of polyurethane-type products has become available in recent years for coating applications. These include simple solutions of linear polyurethanes, two-pot alkyd-isocyanate and polyether-isocyanate systems and a variety of prepolymer and adduct systems. The coatings can vary considerably in hardness and flexibility and find use mainly because of their toughness, abrasion resistance and flexibility. Uses include metal finishes in chemical plant, wood finishes for boats and sports equipment, finishes for rubber goods and rain-erosion-resistant coatings for aircraft. One type of coating is potentially competitive with PVC leathercloth. Both alkyd-di-isocyanate and adduct-diisocyanate compositions may be coated on to fabrics from solutions of controlled viscosity and solids content. Such coated fabrics are soft, flexible and, unlike PVC leathercloth, free from plasticisers. 

The thermoplastic polyurethane (TPU) adhesives must, of necessity, contain low gel content because they must be processable in an extruder. Most adhesives are relatively linear, with a functionality of 2.0, although small amounts of branching may be introduced, usually at the expense of a lower melt flow. Good physical properties of TPU s are obtained when the thermoplastic urethanes have molecular weights of 100,000 or higher (see p. 56 in [63]). Most TPU adhesives are based on symmetrical polyesters with a fast crystallizing backbone or a backbone slightly modified to increase the open time. 

In these reactions, the monomers have two functional groups (whether one or two monomers are used), and a linear polymer results. With more than two functional groups present, crosslinking occurs and a thermosetting polymer results. Example of this type are polyurethanes and urea formaldehyde resins (Chapter 12). 

Polyurethane materials are extremely versatile in that it is possible to produce a large variety of structures which range in properties from linear and flexible to crosslinked and rigid. The crosslinked PURs are thermosets, which are insoluble and infusible and therefore cannot be reprocessed by extrusion without suffering extensive thermal degradation. At present, the main sources of recyclable waste are flexible PUR foams and automobile waste. Waste and scraps of these materials may consist of 15-25% by weight of total PUR foam production. 

Linear polyurethanes, 26 Linear step-growth polymerizations, 13 Lipase-catalyzed polyesterifications, 83 Lipases, 82, 84 catalytic site of, 84 Liquefied MDIs, 211, 226-227 Liquid carbon dioxide, 206 Liquid-castable systems, 201 Liquid crystal devices (LCDs), alignment coating for, 269-270 Liquid crystalline aromatic polyesters, 35 Liquid crystalline polyesters, 25, 26, 48-53... 

---