Application of rigid polyurethane integral-skin foams to thermal insulating unit cases for car air conditioners

Integral-skin foams of rigid polyurethane are sandwich structures consisting of a core layer of closed cells enclosed in rigid surface layers on both sides. We examined the layer composition of integral-skin foam with the objective of maximum flexural strength, and then studied possibilities of reconciling the strength and thermal insulating properties in housings for evaporators in car air conditioners; i.e., unit cases. This examination showed that the most practical density range (250 ≦ ρ_pall ≦ 500 kg/m³) provides vibratile resistance and thermal insulating properties. In actual car-running tests, a maximum 0.1 MPa stress was generated on unit cases with overall densities of 350 kg/m³, We found this to be 0.4% of the flexural strength of an integral-skin foam and 2% of the fatigue strength. In the forcible vibratile test, a stress of 0.5 to 1.0 MPa was generated at the resonance point of a unit case with 250 to 500 kg/m³ overall density. We found that these values are 2 to 5% of integral-skin foam's flexural strength and 10 to 25% of its fatigue strength. These values are of the same level as the conventional unit case made of polypropylene blended with talc. An integral-skin foam with an overall density of 250 kg/m³, nearly equal to half the weight of polypropylene, has the same level of resistance to vibration.