Field of the Invention
The present invention relates to a temperature self-controlling heating composition having a positive temperature coefficient (referred to as PTC hereinafter), which can be used for a domestic heater such as a floor heater, a wall heater and the like.
Up to the present time, a temperature self-controlling heating composition which has been practiced is produced by the radiation crosslinking of a molded article of mixture of crystalline resins such as low density polyethylene and carbon black.
The electrical resistance of heating composition produced from a simple mixture of a crystalline resin and carbon black tends to sharply increase near the softening temperature (T1) of the crystalline resin and to decrease at a temperature higher than the melting point (T2), as shown by a solid line in FIG. 1. Accordingly, if the heating composition is heated by an outside heat source and the temperature of the composition rises higher than the melting point T2, the resistance of the composition becomes reduced and the temperature abnormally rises to a point which could possibly cause ignition. Further, there is a serious problem in that the resistance of the heating composition becomes gradually increased, and finally loses its heating ability, if an electrical potential is continuously or intermittently applied to the heating composition, even at ordinary temperatures.
The following is thought to be a cause of the above phenomenon. Although an electrical conductive path is formed in the case in which carbon black is homogeneously dispersed into a crystalline resin just after both are mixed, the carbon black, at a temperature higher than the melting point (T2) of the crystalline resin, begins a Brownian movement in the melted crystalline resin, and the Brownian movement increases as the temperature becomes higher, so that the opportunity for contact of adjacent carbon black increases. As the result of the above, the resistance reduces at a temperature higher than the melting point (T2) of the crystalline resin On the other hand, the reason for the increase of the resistance in the latter case, is considered to be that the electrical conductive path is interrupted by partial agglomeration (deterioration of dispersion) of the carbon black which will be induced by continuous or intermittent application of an electrical pressure.
Such agglomeration of carbon black will be caused by lower heat resistance of a crystalline resin, which is the dispersion medium for the carbon black. A heat saturated temperature of a temperature self-controlling heater is set up at a temperature lower than the melting point of the crystalline resin by about 20.degree.-30.degree. C., the reason being that the PCT property is dependent on the change of the specific volume of the crystalline resin in a melted state, and such a selection of the temperature that will be suitable. The heat saturated temperature, however, is a macrotemperature of the whole temperature self-controlling heating composition, and the microtemperature in the crystalline resin forming the electrical conductive path will rise higher than or near the melting point on some occasion. The crystalline resin will be sharply reduced in viscosity at a temperature higher than the melting point and become a liquid. The carbon black cannot be retained in the melted resin so as to partially agglomerate, and portions consisting of only the crystalline resin inherently insulative are formed within the electrical conductive path to make the heating composition highly resistive. As apparent from the above reasons, it had been considered difficult to stably retain carbon black dispersed in a crystalline resin alone. Therefore, a conventionally practiced heating composition is produced by the radiation crosslinking of a molded article made from a mixture of carbon black and a crystalline resin. As the crystalline resin subjected to the radiation crosslinking is improved in the heat resistance by the formation of a three-dimensional structure from the crystalline resin having a two-dimensional structure (prevention of the rapid change in physical properties near the melting point, especially decreases the viscosity), the agglomeration of the carbon black can be prevented. The relation of resistance (ordinate) and temperature (abscissa) of such an embodiment is shown in FIG. 1, in which the broken line indicates the resistance/temperature curve.
The temperature self-controlling heating composition containing such a crosslinked resin is too expensive because the cost of equipment for the radiation crosslinking is expensive, and is lacking in flexibility.