Strength Properties at Elevated Temperature

The changes in tensile properties, as the temperature rises from 23°C to 210/240°C, follow a consistent pattern and decrease uniformly. It is specially observed that these PUs show no sharp melting points accompanied by rapid loss in dimensional stability. This is a characteristic usually shown by the traditional covalent crosslinked rubbers such as NR, CR, etc., and is of importance in engineering applications where it is essential to avoid failures of a catastrophic nature which may occur if a polymer possesses a sharp melting point. 

Retention of tensile strength at elevated temperature is seen to reach a maximum at 40% excess NCO with the EDO system and at 30% with the mixed EDO + CHDM system. With both systems, elongation at break values remain at consistently high levels throughout the temperature ranges up to 210 and 240 C, respectively. Also, the PUs remain dimensionally stable and elastic as demonstrated by their possession of measurable elastic modulus at the highest test temperatures used. 

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DDS can achieve a high HDT, with good retention of strength properties at elevated temperatures. Post cure times can be reduced by using a BF3MEA catalyst at the expense of a shortened pot life. When used without a catalyst, it is usually used at about 10% excess, c. m-phenylene diamine (MPD)... 

An alternative basket type has been developed and licensed which is constructed out of Metamic-HT, a metal matrix composite made by embedding nanoparticles of aluminum oxide and fine boron carbide powder on the grain boundaries of aluminum resulting in improved structural strength properties at elevated temperatures. This allows the basket to serve as both a structural component and a neutron absorber. 

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