Polyurethanes general

In most cases the Ta values are related directly to the lignin content of the network. Soft segment-free networks show more scatter than those containing alkylether components, and this may be attributed to the crosslink density of the materials. The method of soft segment incorporation, simple addition or covalently bonded, has a minimal impact on Ts. A comparison of the HDI and aromatic isocyanate-crosslinked networks, at a given lignin content, shows that the HDI-crosslinked polyurethanes generally have a lower Tg than the aromatic isocyanate-crosslinked networks. 

MDI-diol-cured polyurethanes generally can receive FDA approval for use, provided no mercury catalysts are used. Polyurethanes made from most amine-cured TDI material will not meet the requirements for FDA approval. Trimetyleneglycol di-p-aminobenzote (Versalink 740M) of the diamine... 

Biostability increases strongly by changing to polyether polyol based polyurethanes. Generally the polyethers are relatively non-toxic but nonbiodegradable products. 

Polyether polyurethanes are more resistant to hydrolysis than polyester-based urethanes, whereas the polyester polyurethanes have better fuel and oil resistance. Polyether polyurethanes generally have lower glass transition tanperatures than polyester polyurethanes. 

Abrasion and impact resistance of acrylic coatings can be improved with the addition of up to 30% of a water borne polyurethane. The addition of a polyurethane dispersion as a physical blend improves the inherent film strength and film formation of the aciylic latices. Polyurethanes generally have better film forming properties because of their lower MFFT at identical surface hardness. 

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