Patent Application
20080074232
Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of this invention, with reference to the accompanying drawing, in which:
The PTC element of this invention, which is particularly useful in the manufacture of a PTC circuit protection device having a resistivity at 20° C. of less than 10 ohm-cm, has a PTC polymer composition comprising: 40-70 vol % of a non-cross-linked polyvinylidene fluoride (PVDF) or a cross-linked polyvinylidene fluoride formed by cross-linking the non-cross-linked polyvinylidene fluoride by irradiation to a dosage less than 8 Mrads; and 30-60 vol % of a particulate conductive filler. The non-cross-linked polyvinylidene fluoride has a melting flow rate (MFR) of less than 120 g/10 min.
Preferably, the cross-linked polyvinylidene fluoride is formed by cross-linking the non-cross-linked polyvinylidene fluoride by irradiating it to a dosage less than 4 Mrads.
Preferably, the non-cross-linked polyvinylidene fluoride has a melting flow rate ranging from 0.5-30 grams per 10 minutes, and a melting point ranging from 140 to 180° C.
The particulate conductive filler is preferably made from a conductive material selected from the group consisting of metal particles and particulate carbon black.
The merits of the PTC polymer composition of this invention will become apparent with reference to the following Examples.
PTC elements of Examples 1-29 and Comparative Examples 1-41 were prepared in a conventional manner that involved compounding, compress molding, laminating with nickel plated copper foils, and compressing steps, but differ from each other in the PTC polymer composition and in the irradiating dosage for the polyvinylidene fluoride of the polymer composition. The PTC polymer compositions of the Examples 1-29 and the Comparative Examples 1-41 are represented by corresponding formulation numbers shown in Table 1.
The PTC elements of Examples 1-29 and Comparative Examples 1-41 were tested in their electrical resistance stability. The tests included Trip Endurance Test, in which a current of 7.8 A was continuously applied to each test specimen at a voltage of 16V for 300 hours, and Cycle Test, in which a current of 7.8 A was applied to each test specimen at a voltage of 16V and at an interval of 1 minute on/1 minute off for 7200 cycles. The results of the electrical resistance stability test for Examples 1-29 and Comparative Examples 1-41 are shown in Table 2. As best shown in
Accordingly, the applicants have discovered that PTC polymer compositions containing non-cross-linked PVDF or low dosage irradiated cross-linked PVDF, which have an MFR of less than 120 g/10 min, exhibit an excellent electrical resistance stability.
While the present invention has been described in connection with what are considered the most practical and preferred embodiments, it is understood that this invention is not limited to the disclosed embodiments but is intended to cover various arrangements included within the spirit and scope of the broadest interpretation and equivalent arrangements.
