Segments, soft glass transition

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

Urethane structural adhesives have a morphology that is inverse to the toughened epoxy just described. The urethanes have a rubber continuous phase, with glass transition temperatures of approximately —50°C. This phase is referred to as the .soft segment . Often, a discontinuous plastic phase forms within the soft segment, and that plastic phase may even be partially crystalline. This is referred to as the hard segment . A representation of the morphology is shown in Fig. 3 [34]. 

Ethylene-butylene, glass transition and melting temperature for soft/hard segments, 7 649t Ethylene-butylene-isobutylene,... 

It is anticipated that the possibility of producing elastomeric lignin-containing PU s strongly depends on whether crosslink density can be held on a low level or not. It is also necessary to include soft segments, so that the glass transition temperature (Ta) of the PU falls below room temperature. 

Soft segments One of the two phases which make up polyurethane. The soft segment is composed of long-chain polyether or polyester polyols. The soft segment controls many of the polyurethane properties such as tensile and tear strength, hydrolysis and chemical resistance, glass transition temperature, and flexibility. 

Internal plasticizers are synthesized by copolymerization of suitable monomers. Polymeric non-extractable plasticizers, mostly copolymers having substantially lower glass transition temperatures due to the presence of plasticizing ( soft ) segments such as poly(ethylene-co-vinyl acetate) with approximately 45 % vinylacetate content, ethylene-vinyl acetate-carbon monooxide terpolymer, or chlorinated PE, are available for rather special applications in medicinal articles (Meier, 1990). In this case, the performance of the internally plasticized polymers is the principal advantage. However, copolymerization may account for worse mechanical properties. A combination with external plasticizers may provide an optimal balance of properties. For example, food contact products made from poly(vinylidene chloride) should have at most a citrate or sebacate ester based plasticizers content of 5 % and at most 10 % polymeric plasticizers. 

Table I. Soft-Segment, Glass-Transition-Temperature Dependence on Hard-Segment Content for PTMO/MDI/BD(ET) - and PTMA/MDI/BD (ES) -Segmented Copolymers...

Figure 14. Plots of the glass transition temperature of the soft segment phase, Tffs, Young s modulus E, and SAXS intensity, l(cps), vs. post-annealing time for a commercial polyester polyurethane (MDI/BD based), R53 (Hooker Chemical Company) (fixed s = 0.042 A 1 for SAXS data) (97).

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