Cast elastomers Hard segment content

Both series of polyurethanes were prepared using a prepolymer technique in which reactants were mixed at 70 °C/1 hour, cast into molds at 105 °C/2 hours, and cured at 80 °C/14 hours. The BD/MDI hard segment contents ranged from 0% (transparent, colorless homopolyurethanes) to 30% w/w (opaque, white copolyurethanes). All elastomers were characterized using DSC, dynamic mechanical, and tensile stress-strain measurements. 

The Durometer hardness was measured after the cast elastomers were aged for at least 7 days at 23 °C and 50% relative humidity. The details are given in Table 8.3. From the results, it is evident that as the crystalline hard segment content of the elastomer is decreased, the elastomer hardness also decreases. By analysing the soft and hard segments and their relation to hardness, it appears that the amorphous hard segment behaves like the polyether soft segment. 

Since the cast urethane elastomer properties are greatly influenced by the soft and hard segment contents and their phase separation behaviour, they are theoretically calculated from the molar ratios of the prepolymer and chain extenders and are reported in Table 8.12. Table 8.12 shows the hard segment content of the elastomer is 45.5% for HER-HP and is reduced to 38.2% for TG-275. This reduction in the hard segment content is due to the formation of 8.8% amorphous hard segment from TG-275 chain extender. 

On the other hand, a gradual increase in tensile strength and compression set values is observed for cast elastomers containing more amorphous hard segment contents. 

Cast Elastomers and Their Hard and Soft Segment Contents... 

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