Low Cost Thermal Mastic for Polyurethane Foam Insulation
In refrigerators, freezers and other polyurethane foam insulated products a mastic is applied between evaporator tubing and metal walls to facilitate intimate camper refrigerator contact and heat transfer between the tubing and the foam. Typically the low cost thermal mastics employed in the industry contain a vehicle which is usually a high viscosity petroleum product and a filler such as powdered aluminum silicate clay and/or calcium carbonate, often with small amounts of wetting agent added to improve adhesion.
Object of the invention is to provide a thermal mastic of low cost that is compatible with polyurethane foam insulation. The present invention accomplishes this object by employing low viscosity chlorinated paraffins as a vehicle in combination with fillers and a wetting agent which are preferably selected from the group consisting of from about 25 to 40 weight percent, finely divided, organically bound, powdered or ground, non-metallic inorganic compounds. These compositions may have a kinematic viscosity between about 2600 centistokes at 24 C. and about 6000 centistokes at room temperature, a K-factor of about.65 B.t.u./in./hr./ft. and a coefficient of friction between about 1 to 10 lb./ft2. Moreover the thermal mastic has an adhesive property of about 8 to 15 lb./ft2.
Composition I This type of thermal mastic has excellent performance in the applications shown in FIG. 1. In addition to having a good adhesion to the foam insulation, the chlorinated paraffin containing paste also facilitated the expansion of the foam when it was injected into the cavity between the foam walls and allowed to cure. The cellular structure of the expanded foam was found to be unaffected by the paste containing the chlorinated paraffin.
The thermal mastic of this type has been tested in six standard chest freezer cabinets and the performance was satisfactory. The mastic was injected into the cavity between the foam wall and the evaporator coil of the refrigeration system before the cabinet was assembled into a finished product. The mastic provided excellent heat transfer between the refrigeration tube and the freezer wall. After several days of operation the mastic was cut away and the evaporator coil showed no evidence of degradation.