Isocyanurate Crosslinking as a Means of Producing Thermally Stable Polyurethane Elastomers

Abstract

Castable polyurethane solid elastomers (PUS) based on the poly-caprolactone ( CAPA 225) and the cycloaliphatic diisocyanate trans 1,4–cyclohexane diisocyanate (CHDI) were synthesised to form two series of PUS: in one series the initially prepared prepolymer was chain extended with 1,4–butane diol (BDO) whilst in the second series a mixed chain extender system was used. The first PU elastomer series synthesised had the following composition CAPA 225/CHDI/BDO and covered the molar ratio range 1/2/1 to 1/3/1; the second series CAPA 225/CHDI/BDO + CHDM covered the molar range 1/3/2 to 1/4·5/2. In each series a controlled excess of diisocyanate was allowed to remain in the initially formed PUS: within a particular series each polymer differed from its immediate analogue by 5% excess NCO and the series covered the range 0–50% of excess NCO in a series of 5% steps. It was discovered that elastomers containing a molar excess of iso-cyanate which had been cast and initially cured at 120°C for 16 h were apparently free from reactive NCO groups and did not emit hazardous fumes when subjected to post-curing or rearrangement by heating to 130°C. When, after conditioning for I week, these PUS are post-cured at a higher temperature (16 h at 130°C) then a second crosslinking system is formed in situ in the PU which considerably enhances its thermal deformation and heat resistance properties. The conventional PU system possessing typically 0–0·2% of free NCO has a maximum service temperature of approximately 120°C; this is raised to 210–240°C by the use of this excess isocyanate technique. It is considered that the second in situ crosslinking system consists of isocyanurate type crosslinks. Tensile properties at elevated temperatures and dynamic mechanical thermal analysis (DMTA) were used to measure thermal stability and strength retention. An important feature of these CHDI based PUS is the remarkable independence of storage modulus to temperature changes. For example, over the temperature range 23–180°C for the BDO series and 23–200°C for the BDO + CHDM series no change is observed. Storage modulus E is defined as dynamic storage modulus with units Nm-2 measured at 10% tensile strain.