Heat mat with thermostatic control

Abstract

A heating pad with a thermostatic control circuit coupled to a resistance heating element. The resistance heating element and an acrylic based polymer adhesive are glued between polyester film layers. The film layers and heating element are contained with a plastic sleeve. The thickness and properties of the polyester film and adhesive forms a fuse. The adhesive delaminates the polyester film at excessive temperatures causing the resistance heating element to sever thereby creating an open circuit that halts operation of the heating pad. The thermostatic control circuit includes a hysteresis circuit that compares analog signals across two thin film resistors to provide a control signal to a power controller to selectively vary the power output to the heating element. The thermostatic control circuit is an integrated circuit board disposed within the fused plastic sleeve.

What is claimed is: 
     
         1 . A heating pad comprising:
 a resistance heating element coupled to a thermostatic control circuit, wherein said resistance heating element is sealed within multiple layers comprising in order:   a plastic sleeve layer;   a layer of thermoplastic film;   a middle layer containing said resistance heating element comprising high resistance metal wire and an adhesive;   an additional layer of thermoplastic film; and   an additional plastic sleeve layer, wherein said plastic sleeve layers are larger than said thermoplastic film layers and said middle layer, and wherein said plastic sleeve layers are fused to each other at their edges beyond the periphery of said film layers.   
     
     
         2 . The heating pad of  claim 1 , wherein said plastic sleeve layers are made from polyvinyl chloride (PCV) and are fused to each other to form a waterproof heating pad. 
     
     
         3 . The heating pad of  claim 1 , wherein said thermoplastic film layers are made from polyester. 
     
     
         4 . The heating pad of  claim 3 , wherein said polyester film layers have a density between about 1.2 and 1.6 g/cm 3 , elongation between about 160 to 240% and at break values between about 100 and 140. 
     
     
         5 . The heating pad of  claim 3 , wherein said thermoplastic film layers have a light transmission of about 93%. 
     
     
         6 . The heating pad of  claim 3 , wherein said thermoplastic film comprises a polyethylene terephthalate (PET) film between about 1.8 and 2.2 mils thick. 
     
     
         7 . The heating pad of  claim 6 , wherein said PET film has a thermal heat value in the range of about 1.2 to 1.6% MD measured by the SKC method, and a thermal shrinkage value between about 0.3 and 0.7 TD measured at 150 degrees C.×30 minutes. 
     
     
         8 . The heating pad of  claim 1 , wherein said adhesive is an acrylic based adhesive. 
     
     
         9 . The heating pad of  claim 8 , wherein said adhesive comprises a solvent-evaporated cured acrylic based crosslinked polymer adhesive. 
     
     
         10 . The heating pad of  claim 9 , wherein said thermoplastic film layers are made from polyester. 
     
     
         11 . The heating pad of  claim 10 , wherein said cured adhesive and said polyester film layers comprise a fuse, wherein said adhesive delaminates the polyester film layers at temperatures above about 300° F. causing the resistance heating element to sever thereby creating an open circuit that halts operation of the heating pad. 
     
     
         12 . The heating pad of  claim 9 , wherein said cured adhesive has a loop tack of about 4.4 lbs/in, and a 180° peel adhesion of about 4.3 lbs/in utilizing a 15 minute dwell. 
     
     
         13 . The heating pad of  claim 9 , wherein said cured adhesive has a shear adhesion of 24+ hours utilizing ½ in×½ in×500 grams test conditions and a plasticity of about 2.4 mm. 
     
     
         14 . The heating pad of  claim 1 , wherein said metal wire is a metal alloy wire and includes a material selected from the group consisting of copper, nickel, stainless steel and combinations thereof. 
     
     
         15 . The heating pad of  claim 1 , wherein said thermostatic control circuit is connected between a mains electrical source and said resistance heating element. 
     
     
         16 . The heating pad of  claim 15 , wherein said thermostatic control circuit is embedded within an overmolded power plug. 
     
     
         17 . The heating pad of  claim 2 , wherein said thermostatic control circuit is sealed within the fused plastic sleeve layers. 
     
     
         18 . A method for manufacturing a heating pad comprising the steps of:
 providing two thermoplastic films and two outer protective sleeve layers which are larger than said thermoplastic films;   coating one of said thermoplastic films with adhesive;   placing a resistance heating wire in the adhesive; and   fusing said two outer protective sleeve layers together at their edges beyond the periphery of said thermoplastic films to sandwich the two thermoplastic films, the resistance heating wire and the adhesive therebetween.   
     
     
         19 . The method of  claim 18 , wherein said coating step comprises coating one of said thermoplastic films with an acrylic polymer dissolved in a solvent. 
     
     
         20 . The method of  claim 18 , wherein said fusing step seals a thermostatic control circuit within the outer protective sleeve layers.