Layered self-regulating heating article

DETAILED DESCRIPTION OF THE INVENTION From the foregoing discussion it is apparent that by utilizing the teaching of the present invention thermoplastic polymer compositions having PTC characteristics can suitably be employed as a heating element, which element will approximate Type I characteristics notwithstanding that the PTC material per se would ordinarily manifest Type II, III, or IV characteristics.
In particular, virtually all of the polymeric PTC materials known to the prior art as hereinabove described may suitably be used as the PTC layer in a heating element constructed according to the teaching of the present invention.
Additionally, the novel PTC materials described in copending commonly assigned application of Horsma et al, now abandoned, Ser.
No.
510,035, filed Sept.
27, 1974 and in copending, concurrently filed, commonly assigned application of Horsma et al entitled "Positive Temperature Coefficient of Resistance Compositions", Ser.
No.
601,639 (now abandoned) are suitable.
Suitable conductive fillers for the polymeric PTC composition useful in the present invention in addition to particulate carbon include graphite, metal powders, conductive metal salts and oxides, and boron or phosphorous doped silicon or germanium.
Preferably for best operability when used in heating apparatus of the type disclosed and claimed herein, the PTC material exhibits an increase in resistance of at least a factor of six (6) for a temperature increase of 30.
degree.
C.
or less starting at T.
sub.
s.
As mentioned hereinabove, although prior art disclosures stress the practical advantages and importance of providing resistive compositions manifesting a type I resistance temperature characteristic, the number of such compositions available is relatively small notwithstanding the claims of the prior art.
Most of the hitherto disclosed compositions in fact possess type II and type III resistance characteristics.
Thus a method of enabling PTC material compositions having inherent type II or III resistance characteristics to manifest essentially type I behavior very greatly increases the number of compositions available for use in heating or other resistive devices