Positive temperature coefficient heater

Abstract: A self regulating heating device (12) for a mirror (10) includes a substrate (14) having an electrical buss system deposited on one surface including a plurality of interdigitated electrodes (32, 34, 36, 38) and two buss bars (16, 18). Stripes of positive temperature coefficient resistive material (40) are printed perpendicularly over the electrodes (32, 34, 36, 38) to form a plurality of heater areas (42, 44) and exposed substrate areas. A first adhesive layer (46) is deposited over the resistive stripes and exposed areas of the substrate (14) between the stripes. Preferably an electrical barrier layer (50) is secured to the adhesive layer (46) and another adhesive layer (52) on the other side of the barrier layer (50) is covered by a removable protective covering (54). The buss bars (16, 18) are tapered so that the power density at any point along their length is substantially equal to the average power density of the heater areas (42, 44).

Description: This invention relates to a heating device, especially a self regulating heating device. More particularly this invention relates to a heater using a positive temperature coefficient (PTC) resistive material which may be arranged for use in heating automotive-type outside rearview mirrors. Heating devices for glass plates including mirrors using positive temperature coefficient materials have been devised. Two such devices are disclosed in US-A-4,628,187 and US-A-4,631,391. These devices have certain disadvantages and shortcomings which the present invention overcomes. For example, the device described in US-A-4,631,391 uses individual spaced apart platelettes of PTC heater elements sandwiched between two heat conductive layers which do not provide uniform heating of the surface to be heated. In device described in US-A-4,628,187, an area principally at the periphery of the mirror occupied by the electrode material of the heating device is not heated resulting in a significant reduction in mirror heated area. Further, it should be noted that the electrode system in this device uses subsantial, wide, constant width silver buss bar conductor paths to carry the necessary current between the terminal connections and the electrode system. The wide conductors, not only result in significant "cold" areas of the mirror along the length of the conductors, but also requires significant quantities of the precious metal silver which significantly adds to the cost of the device. An electrical surface-heating device whereby the bus-bars are rendered heat-producing, and are tapered so that the wide end dissipates the same wattage as the narrow end, is disclosed in US-A-3 287 684. According to this invention a heating device comprises: a planar electrically insulative substrate; an electrical buss system on one surface of the substrate, including two buss bar and electrode patterns having a plurality of spaced apart parallel interdigitated electrodes, adjacent electrodes being connected to different ones of the buss bars each buss bar extending from one of a pair of terminal connection points to a free end; an electrically resistive layer of material having a positive temperature coefficient of resistance extending over the electrical buss system as a plurality of parallel spaced apart stripes transverse to the interdigitated electrodes to define a pluraltity of heater areas between adjacent electrodes; an adhesive layer deposited over the stripes of electrically resistive material and the substrate in the spaces between the stripes; means for achieving a predetermined similar power density at any location along each of the buss bars from their respective terminal connection point to the free end, the power density being substantially equal to an average power of the heater areas. The present invention provides a heating device that maximizes the surface area that is heated and also minimizes the use of conductor material by optimizing the size of the conductor paths. Preferably the width of the PTC material stripes varies in different areas of the substrate to achieve a desired power density and thus a desired differential heating effect. Typically the periphery suffers a greater heat loss and thus the width in this area is greater. Preferably the buss bars taper in width from their power terminals to their free ends. The taper of the buss bars reduces the quantity of conductive material required, thereby minimizing the quantity of conductive material which is usually precious silver material, and minimizes the overall cost to manufacture the heater. A particular example of a heater in accordance with this invention will now be described with reference to the accompanying drawings; in which:- Figure 1 is a plan of the heating device; Figure 2 is a transverse section taken along the line 2-2 shown in Figure 1; and, Figure 3 is a perspective view of a heating device attached to the back side of an automotive-type rearview mirror to be heated. Shown in Figure 3 is an automotive-type outside rearview mirror 10 having a heating device 12 according to the invention attached to a back side. The heating device 12 according to the present invention can be used in any other application where a self regulating heater is desirable. The embodiment disclosed herein however is specifically adapted for use in an automotive-type outside rearview mirror application which is subject to fogging, frosting, icing over and to being covered with snow making it desirable to have a device for overcoming such environmental effects. Further, this application is particularly suited for heating a device subject to changing ambient temperatures due to its ability to automatically control the temperature as a function of the ambient temperature. That is, at elevated ambient temperatures, no heating is required, whereas at low ambient temperatures, such as below freezing, higher temperatures are desirable. Figures 1 and 2 show a preferred construction of the heating device 12. As shown in Figure 2, the heating device comprises an electrically insulating substrate 14 of for example MYLAR of about 0.007 inches (0.18mm) thickness Deposited on one side of the substrate 14 is an electrical buss system, shown best in the plan view in Figure 1. The buss system comprises a layer of printable, electrically conductive material preferably comprising an electrically conductive silver polymer such as the commercially available silver polymer 725 manufactured by Hunt Chemical. The conductive buzz system layer is preferably deposited on the substrate in a thickness within the range of about 8 to 10 microns. The buss system further includes two buss bars 16, 18 each electrically connected to and extending from one of two terminals 20, 22 which each comprise an eyelet 24 secured in a hole 25 in contact with a respective one of the buss bars and a contact terminal member 26 adapted to connect to an external power supply. Each buss bar 16, 18 extends along substantially opposite portions of the peripheral edge of the substrate terminating in free ends 28,30. Each buss bar is also tapered in decreasing area from its respective terminal connection toward its free end in a manner and for the purpose described herein below. Extending perpendicularly from each buss bar 16,18 are a plurality of conductor paths, such as paths 32, 34, 36, 38, defining a plurality of spaced apart, parallel, interdigitated electrodes. That is, adjacent electrodes connect to opposite buss bars and extend in opposite parallel directions terminating spaced from the other buss bar. Screen printed over the buss system is a layer of positive temperature coefficient electrically resistive material 40. The PTC material 40 is a screen printable PTC electrically conductive ink having a composition adjusted to have a desired electrical characteristic for the particular application. For example, for automotive outside rearview mirror applications, a preferred screen printable PTC material has been found to comprise an eythlene vinyl acetate co-polymer resin, such as Dupont 265 which comprises 28 percent vinyl acetate monomer and 72 percent eythlene monomer modified to have a sheet resistivity of 15,000 ohms per square. To achieve this electrical characteristic, this eythlene vinyl acetate co-polymer resin is first dissolved in an aromatic hydrocarbon solvent such as naptha, xylene or toluene at 80 degrees C and let down to where 20 percent of the total weight of the solution is solids. Carbon black such as CABOT VULCAN PF is then added and mixed to bring the total solid content to 50 percent by weight. This material is then passed through a three roll dispersing mill having a 0.1 to 1 mil (2-25 micron) nip clearance to further disperse and crush the solids. The material is then further let down with a 20% solids resin and solvent solution until the desired sheet resistivity is achieved. As noted, the PTC material is screen printed over the buss system and substrate in parallel spaced apart stripes perpendicular to the electrode pattern, as shown in Figure 1, and preferably in a thickness of about 2.5 - 5 microns so as to form a plurality of individual heating areas, such as 42, 44 on the substrate. When a voltage is applied across the terminals and thus across the electrode array, depending upon the ambient temperature and electrical characteristics of the PTC material, current will flow through the PTC material between the electrodes causing the individual heating areas to heat. As is known, the current flow and heating effect of the PTC material depends on its temperature which will change as the ambient temperature changes and, at a predetermined temperature of the PTC material, the resistivity of the material increases causing the material to no longer conduct current, whereby the heating areas no longer generate heat. Accordingly, it can be seen that the heating device is self regulating in accordance with the surrounding ambient temperature. It should be noted that the heating effect at any location on a heater is a function of the power density at that location which can be changed by changing the width of the PTC material stripe at that location. Accordingly, it is possible to increase or decrease the heating effect at any given area of the substrate in accordance with the specific thermodynamics of the application. For example, in automotive outside rearview mirror applications, heat loss from the mirror is greatest at the perimeter. Accordingly, the width of the PTC stripes can be increased, even to the point where adjoining stripes connect together as shown in Figure 1, so as to increase the power density and heating affect at those areas. Similarly, the width of the PTC stripes can be decreased, for example at the center of the mirror where heat loss is the least. The buss system includes a novel buss bar configuration. The current carrying requirements of each buss bar decreases with increasing distance from the power terminals. That is, the portion of each buss bar at, for example, location A in Figure 1 must carry all of the current requirements for all of the heating areas on the substrate, whereas at location B in Figure 1 the buss bar only needs to carry the current requirements for the last electrode pair in the system. Accordingly, if the buss bar size is maintained constant at, for example, a size sufficient to carry the maximum current requirement at location A, there will be little, if any resistance heating of the buss bar along its length. This is particularity true at increasing distances from the power terminals toward location B. That is, the buss bar at greater distances from the terminals becomes increasingly oversized and will remain "cold" and there will be no electrical resistance heating effect in the area covered by the buss bars. The invention however, decreasingly tapers the buss bars from the power terminals to their free ends such that the power density at any location along the length of the buss bar is substantially equal to the average power density of all of the heating areas on the substrate. In this manner, the electrical resistance created by the sized buss bar, will create a heating effect substantially the same as that created by the heating areas. It should be noted that one skilled in the art knowing the electrical characteristic of the PTC material, conductive silver and voltage available at the power terminals can readily calculate the average power density of the heater areas and thus the buss bar size at all locations required to achieve the average power density at all locations along its length. Accordingly, the entire substrate from the center out to the periphery, including those areas beneath the buss bars, will be heated with substantially no cold spots. It can be appreciated therefore that substantially the entire surface area of the mirror will be heated. Another advantage of the tapered buss bar is that the quantity of silver required is minimized with the corresponding cost savings. Referring to Figure 2, a layer of acrylic pressure sensitive adhesive 46 is deposited over the PTC material. Because the PTC material is deposited in stripes, the adhesive is able to flow down to and adhere to the exposed substrate areas 48 in the spaces between adjacent stripes of PTC material. The adhesive adheres significantly better to the MYLAR substrate than to the PTC material and the integrity of the bond is significantly increased. A second insulating barrier layer 50 of MYLAR of about 0.001 inch 25 micron in thickness is secured by the adhesive layer 46 and functions to environmentally seal the conductor and PTC material and to electrically insulate the conductors from possible shorting or arcing to the member on which it is mounted. For example, without the barrier layer 50, the conductors could come into contact with or arc to a silver backing on the mirror. Another adhesive layer 52 is deposited on the barrier layer and a removable protective covering 54, such as paper, is retained to the adhesive layer 52. To count the heater on a mirror, the protective covering 54 is peeled off, the device is secured to the back of the mirror by the adhesive 52 and the power source is connected across the terminals 20, 22.

Claims: A heating device comprising: a planar electrically insulative substrate (14); an electrical buss system on one surface of the substrate, including two buss bar (16, 18) and electrode patterns (34, 36; 32, 38) having a plurality of spaced apart parallel interdigitated electrodes, adjacent electrodes (32, 34 and 36, 38) being connected to different ones of the buss bars (16, 18) each buss bar (16, 18) extending from one of a pair of terminal connection points (20, 22) to a free end; an electrically resistive layer of material (40) having a positive temperature coefficient of resistance extending over the electrical buss system as a plurality of parallel spaced apart stripes transverse to the interdigitated electrodes (32, 34, 36, 38) to define a plurality of heater areas (42, 44) between adjacent electrodes (32, 34); an adhesive layer (46) deposited over the stripes of electrically resistive material (40) and the substrate (14) in the spaces between the stripes; means for achieving a predetermined similar power density at any location along each of the buss bars (16, 18) from their respective terminal connection point (20, 22) to the free end, the power density being substantially equal to an average power density of the heater areas (42, 44). A heating device according to claim 1, wherein the means for achieving the predetermined power density includes each buss bar (16, 18) being decreasingly tapered from its respective terminal connection point (20, 22) to its free end. A heating device according to claim 1 or 2, wherein the buss bars (16, 18) extend generally along opposite edge portions of the substrate (14). A heating device according to any one of the preceding claims, which also includes an electrically insulative barrier layer (50) adhered to the adhesive layer (46). A heating device according to claim 4, which includes another adhesive layer (52) on the other side of the barrier layer (50) from the one adhesive layer (46). A heating device according to claim 5, further including a removable protective layer (54) on the side of other adhesive layer (52) remote from the barrier layer (50). A heating device according to any one of the preceding claims, wherein the stripes (28, 30) of resistive material (40) at least along a portion adjacent the periphery of the substrate (14) have widths greater than the widths of the other stripes of resistive material (40). A heating device according to any one of the preceding claims, wherein the electrically insulating substrate (14) has a predetermined shape conforming to that of a member to be heated. A heating device according to any one of claims 1 to 5 or claims 7 or 8 when dependent upon any one of claims 1 to 5, when adhered to the back face of a mirror (10) by the one or other adhesive film (46, 52).

Inventors: WATTS LESLIE MARK, WATTS, LESLIE MARK
Applicants: ILLINOIS TOOL WORKS, ILLINOIS TOOL WORKS INC.
Application year: 1989
Main classification: H05B3
IPC codes: H05B3, B60S1, B60R1, H01C7