Urethane cross-links, temperature effects

Figure 7. Elastically effective cross-link density versus bake temperature for an acrylic-urethane coating as a function of polyol to isocyanate ratio.

The low heat deflection temperature of PLA limits its use for several application fields, such as in packaging materials and electronic components. The introduction of rigid building blocks [63] or cross-links [64] is known, for instance, to increase the glass transition temperature and/or heat resistance of lactic acid based polymers. The effect of different amounts of comonomers in the prepolymers on the Tg and mechanical properties of poly(ester-urethane)s is demonstrated in Table 3.2. The heat resistance of poly(ester-ure-thane)s can be improved by the copolymerization of lactic acid with D,L-mandehc acid. This broadening of the operating temperature range is of clear practical importance. The incorporation of other comonomers that impede rotation and make polymer chains less mobile also causes an increase in Tg, even if the same comonomers can depress the rate of poly condensation [50]. 

Several vinyl ether functional poly(urethane) powder coatings have been formulated that show a sharp melt temperature at 40°C or 60°C and no cross-linking until 120°C. This wide gap between the melt and crosshnking temperature allows better flow and leveling prior to full cure. The crosshnking temperature is effectively lowered through the use of a latent cationic initiator that decomposes just prior to crosslinking (36). The lowered cure requirements of these powders allow one to use them on a wider variety of plastic components. 

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