Positive temperature coefficient composition

The present invention is directed to a positive temperature coefficient composition comprising by weight based on total composition, 10-30% carbon black possessing a DBP absorption of about 125 cc/100 g carbon black or less; 10-40% chlorinated, maleic anhydride grafted, polypropylene resin; and organic medium capable of solubilizing the resin.

FIELD OF THE INVENTION 
This invention is directed to a positive temperature coefficient 
composition and in particular relates to such compositions which are 
suitable for automotive mirror heaters. 
BACKGROUND OF THE INVENTION 
It is well known in the art that the electrical properties of conductive 
polymers frequently depend upon, inter alia, their temperature; and that a 
very small proportion of conductive polymers exhibit what is known as PTC 
positive temperature coefficient behavior, i.e., a rapid increase in 
resistivity at a particular temperature or over a particular temperature 
range. The term "switching temperature" (Ts) is used to denote the 
temperature at which the rapid increase takes place. When the increase 
takes place over a temperature range (as is often the case) then Ts can 
conveniently be designated as the temperature at which extensions of the 
substantially straight portions of the plot of the log of the resistance 
against the temperature (above and below the range) cross. The resistance 
of PTC polymers continues to increase as the temperature rises above Ts 
until it reaches a maximum, called the Peak Resistance, at a temperature 
which is called the Peak Temperature; the resistance thereafter decreases 
more or less rapidly. 
Materials exhibiting PTC behavior are useful in a number of applications in 
which the size of the current passing through a circuit is controlled by 
the temperature of a PTC element forming part of that circuit. For 
practical purposes this means that the Ts of the material should lie 
between about -100.degree. C. and about 250.degree. C. and that the volume 
resistivity of the material at temperatures below Ts should be from about 
2.5 to about 10.sup.5 ohm cm. The lower limit on resistivity results from 
the requirement that, at temperatures above Ts the PTC element should be 
an insulator; if the resistivity of the element below Ts is less than 2.5 
ohm cm., then even after the increase in resistivity around and above Ts 
the resistivity will not be sufficiently high. The upper limit on 
resistivity results from the requirement that the PTC element should be a 
conductor at temperatures below Ts. The practical effect of these 
limitations on resistivity is to exclude from consideration conductive 
polymers having either very high or very low loadings of conductive 
filler. Another practical requirement for PTC materials is that the 
increase in resistance above Ts should be sufficiently high that the 
heater (or other device) is effectively converted from an electrical 
conductor to an electrical insulator by a relatively limited increase in 
temperature. A convenient expression of this requirement is that the 
material should have an R.sub.14 value of at least 2.0 or an R.sub.100 
value of at least 6, and preferably an R.sub.30 value of at least 4, where 
R.sub.14 is the ratio of the resistivities at the end and beginning of the 
14.degree. C. range showing the sharpest increase in resistivity; 
R.sub.100 is the ratio of resistivities at the end and beginning of the 
100.degree. C. range showing the sharpest increase in resistivity; and 
R.sub.30 is the ratio of the resistivities at the end and beginning of the 
30.degree. C. range showing the sharpest increase in resistivity. A 
further practical requirement for most PTC materials is that they should 
continue to exhibit useful PTC behavior, with Ts remaining substantially 
unchanged, when repeatedly subjected to thermal cycling which comprises 
heating the material from a temperature below Ts to a temperature above Ts 
but below the peak temperature, followed by cooling to a temperature below 
Ts. It is also preferred that the ratio of the peak resistance to the 
resistance at Ts should be at least 10:1. From the above one can see that 
property requirements are achieved by careful selection of fillers and 
polymer in order to obtain a useful PTC composition. The present invention 
will reduce material costs and extend battery life in consumer products 
such as automotive mirror heaters. 
PRIOR ART 
Conductive polymer compositions which exhibit PTC behavior and electrical 
devices comprising them are well known. Reference may be made, for example 
to U.S. Pat. Nos.: 5,206,482 Smuckler, 5,181,006 Shafe et al., 5,174,924 
Yamada et al., 5,093,036 Shafe et al., 4,935,156 van Konynenburg et al., 
4,818,439 Blackledge et al., 4,591,700 Sopory, 4,560,524 Smuckler, 
4,426,633 Taylor, 4,400,614 Sopory, 4,388,607 Toy et al., 4,237,441 van 
Konynenburg et al., 4,124,747 Murer et al., and J. Meyer in Polymer 
Engineering and Science, November 1973, No. 6, pages 462-468. 
The above patents require a crystalline or semi-crystalline polymer, not an 
amorphous polymer like the polymer of the present invention. The 
crystalline character is taught in the art to be important for the 
self-regulating aspects of the PTC compositions. That is, the crystalline 
melt temperature affects the switching temperature and the temperature 
range in which the PTC properties are exhibited. 
Further reference is made to U.S. Pat. Nos.: 4,857,880 Au et al., 4,775,778 
van Konynenburg et al., 4,727,417 Au et al., 4,658,121 Horsma et al., 
4,560,498 Horsma et al., 4,534,889 van Konynenburg et al., and GB 
1,604,735 Raychem Corporation. 
This group of patents require a cross-linked polymer, not an uncrosslinked 
polymer like the polymer of the present invention. The group teaches that 
cross-linking is necessary to increase the stability of the polymer in the 
critical "hot zone", i.e., the temperature range in which the PTC behavior 
is exhibited. 
U.S. Pat. No. 5,198,639 to Smuckler and U.S. Pat. No. 4,774,024to Deep et 
al. disclose a composition containing "a polymer matrix" and "a polymeric 
component", respectively. In addition to the polymer component and a 
conductive filler, both patents require additional materials which are not 
solvents and which remain in the PTC composition. Smuckler requires, in 
the final PTC composition, a polymer-miscible monomeric crystallizable 
organic compound having a characteristic crystalline melt temperature 
below about 150.degree. F., the compound being selected from the group 
consisting of saturated hydrocarbons, organic acids, and alcohols. The 
final PTC composition that results after drying of the proposed 
formulation does not contain the monomeric organic compound disclosed in 
the present invention or any equivalent crystallinity. Deep requires the 
additional components of an arc-controlling agent and a lubricant or 
coupling agent comprising an organo-silicon compound, a stearate or a 
titanate. Neither of these components is found in the present invention's 
composition. 
U.S. Pat. No. 4,755,553 to Kishimura et al. discloses a primer composition 
with 1-100 parts by weight chlorinated carboxyl groups containing 
.alpha.-olefin polymer and 100 parts by weight solvent. In addition, 
carbon black is used as an organic pigment in the amount of about 0.01 to 
about 10% by weight. The different types of carbon are never 
distinguished. There is no teaching in Kishimura of using the disclosed 
invention as a composition for PTC application. In fact, as evidenced by 
Applicants experiments, it is shown that even the preferred black at a 
loading of less than 10% exhibits extremely poor if not zero PTC effect. 
SUMMARY OF THE INVENTION 
The present invention is directed to a positive temperature coefficient 
composition comprising, by weight, based on total composition, 10-30% 
carbon black possessing a DBP absorption of about 125 cc/100 g carbon 
black or less; 10-40% chlorinated, maleic anhydride grafted, polypropylene 
resin; and organic medium capable of solubilizing the resin. 
The present invention is further directed to a sheet comprising a cast 
layer of the novel positive temperature coefficient composition from above 
which has been heated to remove volatile organic solvent. 
The present invention is still further directed to a self-regulating heated 
mirror assembly which comprises a reflective mirror, the novel composition 
of the present invention, and spaced electrodes connected to a source of 
electrical power to pass current between electrodes.

DETAILED DESCRIPTION OF THE INVENTION 
I. Conductive Phase 
The composition contains electrical conductive fillers such as carbon 
black, graphite and the like in a filler to binder weight ratio of about 
50/100 to 300/100 or 10-30 wt. % and a preferred range of 15-20 wt. % 
based on total composition to provide an electrically conductive film. The 
preferred particulate filler is carbon black. The preferred blacks for 
many devices of the present invention, especially self-limiting heaters, 
are blacks having a low structure. Low structure carbon blacks consist of 
small primary aggregates allowing close packaging; high structure carbon 
blacks generally are more conductive and import higher viscosity in 
solution. A common test used to quantify low structure is the absorption 
of dibutyl phthalate (DBP) oil, measured in cc's of oil absorbed per 100 
grams of carbon black. Therefore, carbon blacks possess a DBP absorption 
of about 125 cc/100 g carbon black or less. Carbon blacks preferred are 
those like Cabot Monarch 120 which has a DBP of 72. A 25 micron thick film 
of the composition in its dried state has an electrical resistance of 
about 1-50 kohms and preferably 5-20 kohms. The type of black selected 
will influence the resistivity/temperature characteristics of the 
composition. Other types of carbon blacks for use in this invention are 
furnace and acetylene blacks but the less conductive thermal and channel 
process blacks can also be used. Conductive fillers such as silver may 
also be utilized. 
II. Polymer 
Characteristics of the polymer layer is that the polymer be substantially 
non-crystalline and non-crosslinked in nature. As used herein, the term 
"non-crystalline" refers to polymers having no more than about 0% 
crystallinity as determined by X-ray diffraction. About 10-40 wt. % 
polymer based on total composition is present in the instant invention. 
The preferred polymer of this invention is HYON.RTM. CP 826 
manufactured by E.I. du Pont de Nemours and Company, Wilmington, Del., but 
any chlorinated, maleic anhydride grafted, polypropylene resin may be 
used. In addition to the polymer being added to form the initial 
composition, 2-20 wt % additional HYON.RTM. medium (HYON.RTM. 
dissolved in a solvent) may be added to the composition to bring the 
resistivity value up to a level which will satisfy the needs of the heated 
mirror design. For example, if 4 ohms is the desired starting resistance 
of a mirror circuit and the dimensions are 5 inches by 15 inches, then 
only a certain resistivity value of the PTC carbon will satisfy these 
requirements. Balanced with that, is the desire to have a certain level of 
PTC activity, i.e., how quickly it will "shut off" or self-thermostat. The 
higher the resistivity, the higher the TCR. Thus, the more potent the PTC 
effect. The preferred ratio of the HYON.RTM. to solvent in the 
HYON.RTM. medium is 20/80 but the HYON.RTM. component may be in the 
range of 10-40. 
III. Organic Medium 
The inorganic particles are mixed with an essentially inert liquid medium 
(vehicle) by mechanical mixing. This mixture is then subjected to a three 
roll mill to assure proper dispersion of the particles to form a 
paste-like composition having suitable consistency and rheology for screen 
printing. The latter is printed as a "thick film" on conventional 
dielectric substrates in the conventional manner. 
Any organic, inert liquid may be used as the solvent for the vehicle so 
long as the polymer is fully solubilized. Solubilize herein is defined as 
the extent to which a substance mixes with a liquid to produce a 
homogeneous system or solution. Various organic liquids, with or without 
thickening and/or stabilizing agents and/or other common additives, may be 
used as the vehicle. Exemplary of organic liquids which can be used are 
dibutyl carbitol, for example, or beta-terpineol. 
EXAMPLES 
Compositions, Temperature Coefficient of Resistance (TCR) values, and 
Resistivity for the Examples hereinbelow are summarized in Table 1. 
Example 1 
20.0 grams of HYON.RTM. 826 resin (chlorinated, maleic anhyride grafted, 
polypropylene resin available from E.I. du Pont de Nemours and Company) 
was dissolved in 80.0 grams of a 50/50 (wt.) mixture of Dibutyl 
Carbitol/Beta-Terpineol. The mixture was heated at approximately 
80.degree. C. for 3 hours with a light yellow homogenous solution 
resulting. The solution was cooled for approximately 1 hour. At this time, 
20.0 grams of MONARCH 120 carbon powder (available from Cabot Corporation) 
was added to 80.0 grams of the above HYON.RTM. solution and mixed for 
30 minutes. This mixture was subjected to one cycle on the three roll-mill 
at a pressure of 200 PSI. Ten grams of the above resistive paste was used 
for all subsequent work. 
The resulting thick film resistive ink was applied to a 5 mil thick 
polyester substrate (MYLAR.RTM. available from E.I. du Pont de Nemours and 
Company) by the screen printing process. After printing a highly 
conductive polymer thick film conductor suitable for use on polyester 
substrates such as 5025, it was cured in an oven at 130.degree. C. for 5 
minutes. Subsequently, the resistive paste was printed over the edges of 
the silver ink and cured at 130.degree. C. for 5 minutes. Test parts were 
printed to measure the resistance/resistivity of the carbon paste at 
25.degree. C. and 125.degree. C. Initial resistivity values (25.degree. 
C.) were 0.95 Kohm/sq (Acceptable Kohm/sq. are within the range of approx. 
1 Kohm/sq. to 60 Kohm/sq.) while TCR values at 125.degree. C. were 22500 
ppm/C. Typical TCR values for carbon inks that do not exhibit PTC effect 
are HTCR's below 6000, although a marginal PTC effect is seen as 6000 is 
approached. Commercially acceptable HTCR's are about 20,000 or greater. A 
value of 22500 indicates significant increase in resistance at the higher 
temperature as compared with the resistance at 25.degree. C. 
Example 2 
The same conditions were used as per Example 1. To 10 grams of ink of 
Example 1, 1.0 grams of HYON.RTM.-based medium was added, wherein the 
HYON.RTM. to solvent is in a ratio of 20/80. The mixture was mixed for 
10 minutes, and tested per the above. Initial resistivity values for this 
example were 2.1 K ohm/sq while the TCR values at 125.degree. C. 
(reference temperature=25.degree. C.) were 42800. 
Example 3 
The same conditions were used as per Example 1. Here, 3.0 grams of the 
HYON.RTM.-based medium was added to the paste from Example 1, wherein 
the HYON.RTM. to solvent is in a ratio of 20/80. The mixture was mixed 
for 10 minutes, and tested per the above. Initial resistivity values were 
8.1 K ohm/sq while the TCR values at 125.degree. C. (reference 
temperature=25.degree. C.) were 68900. 
Example 4 
20.0 grams of a Polyester resin (Goodyear Vitel-200) was dissolved in 80.0 
grams of DBE-9 solvent (available from E.I. du Pont de Nemours and 
Company). The mixture was stirred/heated to 80.degree. C. for several 
hours at which time a homogeneous solution results. 20.0 grams of MONARCH 
120 Carbon (available from Cabot Corporation) was then added to 80.0 grams 
of the Polyester-based solution and then processed as per Example 1. 
Resistivity values of parts made with this paste were 0.53 K ohms/sq. TCR 
values at 125.degree. C. (reference temperature=25.degree. C.) were 5317, 
indicating no PTC effect. 
Example 5 
The same conditions were used as per Example 1. Here, Sanyo 822S 
chlorinated polypropylene (sold through Philip Brothers Chemical Co., 74 
Mt. Paran Road, Atlanta, Ga. 30327) was used instead of the HYON.RTM. 
826 resin. Initial resistivity values were 1.37 K while HTCR values were 
15190. Clearly PTC activity exists. 
Example 6 
The same conditions were used as per Example 1. Eastman Chemical CP-343-1 
resin (Eastman Chemicals, Kingsport, Tenn.) was used instead of the 
HYON.RTM. 826. Initial resistivity values were 1.67 K while HTCR values 
were 22690. Again, PTC activity clearly exists. 
A summary of the Examples 1-8 is presented in Table 1 hereinbelow. 
Example 7 
The same conditions were used as per Example 1. Eastman Chemical CP-343-1 
resin was used. Initial resistivity values were 7750 K while HTCR values 
were 9585, indicating slight PTC effect. 
Example 8 
The same conditions were used as per Example 1. Eastman Chemical CP-343-1 
resin was used. Initial resistivity values were 0 K and HTCR was also 0, 
indicating no PTC effect. 
TABLE 1 
__________________________________________________________________________ 
Example 1 
Example 2 
Example 3 
Example 4 
Example 5 
Example 6 
Example 
Example 
__________________________________________________________________________ 
8 
HYON .RTM. 
16% 14.5% 12.3% 
Sanyo 822S 16% 
Eastman 16% 20.93% 21.85% 
Chemical 
Dibutyl 64% 58.2% 49.2% 64% 64% 64% 
Carbitol/Beta 
Terpineol 
Dibutyl Carbitol/ 70.07% 73.15% 
Terpineol 
Polyester resin 16% 
MONARCH 120 
20% 18.2% 15.4% 20% 20% 20% 9% 5% 
carbon powder 
HYON .RTM. 
-- 9.1% 23.1% -- 
based medium 
(Addition) 
Resistivity 
0.95 Kohm/sq 
2.1 Kohm/sq 
8.1 Kohm/sq 
.53 Kohm/sq 
1.37 Kohm/sq 
1.67 Kohm/sq 
7750 Kohm/sq 
0 Kohm/sq 
@ 25.degree. C. 
TCR @ 125.degree. C. 
22500 ppm/C 
42800 ppm/C 
78900 ppm/C 
5317 ppm/C 
15190 ppm/C 
22690 ppm/C 
9585 ppm/c 
0 
__________________________________________________________________________ 
ppm/c