Compositions – Electrically conductive or emissive compositions – Elemental carbon containing
Reexamination Certificate
2000-06-06
2001-03-06
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Elemental carbon containing
C252S512000, C252S513000, C252S514000, C338S0220SD
Reexamination Certificate
active
06197220
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to polymeric positive temperature coefficient (PTC) compositions and electrical PTC devices. In the invention relates to polymeric PTC compositions containing fibrillated fibers which exhibit improved over voltage capabilities and an enhanced PTC effect.
Electrical devices comprising conductive polymeric compositions that exhibit a PTC effect are well known in electronic industries and have many applications, including their use as constant temperature heaters, thermal sensors, low power circuit protectors and over current regulators for appliances and live voltage applications, by way of non-limiting example. A typical conductive polymeric PTC composition comprises a matrix of a crystalline or semi-crystalline thermoplastic resin (e.g., polyethylene) or an amorphous thermoset resin (e.g., epoxy resin) containing a dispersion of a conductive filler, such as carbon black, graphite chopped fibers, nickel particles or silver flakes. Some compositions additionally contain flame retardants, stabilizers, antioxidants, antiozonants, accelerators, pigments, foaming agents, crosslinking agents, dispersing agents and inert fillers.
At a low temperature (e.g. room temperature), the polymeric PTC composition has a contiguous structure that provides a conducting path for an electrical current, presenting low resistivity. However, when a PTC device comprising the composition is heated or an over current causes the device to self-heat to a transition temperature, a less ordered polymer structure resulting from a large thermal expansion presents a high resistivity. In electrical PTC devices, for example, this Wgh resistivity limits the load current, leading to circuit shut off. In the context of this invention T
s
is used to denote the “switching” temperature at which the “PTC effect” (a rapid increase in resistivity) takes place. The sharpness of the resistivity change as plotted on a resistance versus temperature curve is denoted as “squareness”, i.e., the more vertical the curve at the T
s
, the smaller is the temperature range over which the resistivity changes from the low to the maximum values. When the device is cooled to the low temperature value, the resistivity will theoretically return to its previous value. However, in practice, the low-temperature resistivity of the polymeric PTC composition may progressively increase as the number of low-high-low temperature cycles increases, an electrical instability effect known as “ratcheting”. Crosslinking of a conductive polymer by chemicals or irradiation, or the addition of inert fillers or organic additives may be employed to improve electrical stability.
In the preparation of the conductive PTC polymeric compositions, the processing temperature often exceeds the melting point of the polymer by 20° C. or more, with the result that the polymers may undergo some decomposition or oxidation during the forming process. In addition, some devices exhibit thermal instability at high temperatures and/or high voltages that may result in aging of the polymer. Thus, inert fillers and/or antioxidants, etc. may be employed to provide thermal stability.
Among the known inert fillers employed in PTC polymeric compositions are polymeric powders such as polytetrafluoroethylene (e.g., Teflon™ powder), polyethylene and other plastic powders, fumed silica, calcium carbonate, magnesium carbonate, aluminum hydroxide, kaolin, talc, chopped glass or continuous glass, fiberglass and fibers such as Kelvar™ polyaramide fiber (available from DuPont) among others. According to U.S. Pat. No. 4,833,305 by Machino et al., the fibers employed preferably have an aspect ratio of approximately 100 to 3500, a diameter of at least approximately 0.05 microns and a length of at least approximately 20 microns.
Polymeric PTC materials have found a variety of applications, such as self-regulating heaters and self-resettable sensors to protect equipment from damage caused by over-temperature or over-current surge. For circuit protection, the polymeric PTC devices are normally required to have the ability to self-reset, to have a low resistivity at 25° C. (10 &OHgr;cm or less), and to have a moderately high PTC effect (10
3
or higher) in order to withstand a direct current (DC) voltage of 16 to 20 volts. Polyolefins, particularly polyethylene (PE)-based conductive materials, have been widely explored and employed in these low DC voltage applications.
Polymeric PTC sensor devices that are capable of operating at much higher voltages, such as the 110 to 130 alternating current voltages (VAC) (“Line” voltages) present in AC electrical lines, in which the effective AC current may have peaks equivalent to 156 to 184 DC volts have recently been developed by Therm-O-Disc, Inc. Such polymeric PTC devices have been found to be particularly useful as self-resettable sensors to protect AC motors from damage caused by over-temperature or over-current surge. For example, and without limitation, such high voltage capacity polymeric PTC devices would be useful to protect the motors of household appliances, such as dishwashers, washers, refrigerators and the like.
In view of the foregoing, there is a need for the development of polymeric PTC compositions and devices comprising them that exhibit a high PTC effect, have a low initial resistivity, that exhibit substantial electrical and thermal stability, and that are capable of use over a broad voltage range, i.e., from about 6 volts to about 300 volts.
SUMMARY OF THE INVENTION
The invention provides polymeric PTC compositions and electrical PTC devices having increased voltage capabilities while maintaining a low RT resistance. In particular, the polymeric compositions also demonstrate a high PTC effect (the resistivity at the T
s
is at least 10
4
to 10
5
times the resistivity at 25° C.) and a low initial resistivity at 25° C. (preferably 10 &OHgr;cm or less, more preferably 5 m&OHgr; or less). The electrical PTC devices comprising these polymeric PTC compositions preferably have a resistance at 25° C. of 500 m&OHgr; or less (preferably about 5 m&OHgr; to about 500 m&OHgr;, more preferably about 7.5 m&OHgr; to about 200 m&OHgr;, typically about 10 m&OHgr; to about 100 m&OHgr;) with a desirable design geometry, and can withstand a voltage of 110 to 130 VAC or greater without failure for at least 4 hours, preferably up to 24 hours or more, after reaching the T
s
.
The polymeric PTC compositions of the invention, demonstrating the above characteristics, comprise an organic polymer, a particulate conductive filler, an inert filler including fibrillated fibers and, optionally, an additive selected from the group consisting of inorganic stabilizers, flame retardants, antioxidants, antiozonants, accelerators, pigments, foaming agents, crosslinking agents and dispersing agents. The compositions may or may not be crosslinked to improve electrical stability before or after their use in the electrical PTC devices of the invention. Preferably, the polymer component of the composition has a melting point (T
m
) of 100° C. to 200° C. and the PTC composition exhibits a thermal expansion coefficient of 4.0×10
−4
to 2.0×10
−3
cm/cm° C. at a temperature in the range of T
m
to T
m
minus 10° C.
The electrical PTC devices of the invention have, for example, the high voltage capability to protect equipment operating on Line current voltages from over-heating and/or over-current surges. The devices are particularly useful as self-resetting sensors for AC motors, such as those of household appliances, such as dishwashers, washers, refrigerators and the like. Additionally, PTC compositions for use in low voltage devices such as batteries, actuators, disk drives, test equipment and automotive applications are also described below.
REFERENCES:
patent: 4833305 (1989-05-01), Mashimo et al.
patent: 5250226 (1993-10-01), Oswal et al.
patent: 5382384 (1995-01-01), Baigrie et al.
patent: 5643502 (1997-07-01), Nahass et al.
patent: 5651922 (1997-07-01), Nahass et al.
patent: 5837164 (1998
Blok Edward J
Khadkikar Prasad
Rumler Joseph V.
Scoular Mark R.
West Jeffrey A.
Harnes Dickey & Pierce P.L.C.
Kopec Mark
Therm-O-Disc Corporation
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