Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Patent
1993-08-05
1994-03-22
Carrillo, Ana L.
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
525432, 525436, C08L 7700
Patent
active
052965589
DESCRIPTION:
BRIEF SUMMARY
This invention relates to polymeric compositions that include aromatic polymers and to electrical wires or other articles that employ such compositions.
Insulation materials, for example those that are used for electrical wires, need to exhibit a number of desirable physical properties. For example they should have moderate to good electrical resistivity depending on whether they are used as primary jackets or primary insulation, and they should exhibit good mechanical properties for example good scrape abrasion resistance and cut through resistance. In addition, for many applications for example in the case of airframe wire, the insulation should exhibit good arcing and tracking resistance.
Tracking is a phenomenon associated with the formation of permanent and progressive conducting paths on the surface of the material by the combined effects of an electrical field and external surface pollution. Once commenced, the carbonaceous conducting deposits often extend quickly in dendritic fashion to give a characteristic "tree" pattern until failure occurs across the surface. Electrical tracking can occur when a damaged energised bundle of wires become wet e.g. from electrolytes or condensation. This tracking may lead to flashover and arcing that causes additional wires in the bundle to become damaged. A catastrophic cascade failure can result from a fault to a single wire if adjacent wires that are at a different electrical potential are also susceptible to tracking or if the bundle is in contact with a grounded structure. Tracking can occur at low voltages e.g. 100V a.c. or less but becomes less likely as the voltage is reduced.
A related phenomenon, to which a number of polymers are also highly susceptible, is that of breakdown due to arcing. In this case a potential difference between two conductors, or between a conductor in which the insulation has been mechanically damaged, and ground, can result in the formation of an arc between the conductors or between the conductor and ground. The high temperature of the arc causes the polymer to degrade extremely rapidly and form an electrically conductive carbonaceous deposit which can extend rapidly, as with wet tracking, and lead to catastrophic failure in which many or all of the wires in a bundle are destroyed. Arcing can occur at very low voltages, for example 24V d.c. or lower, and since, unlike tracking, no electrolyte or moisture is involved, it is a particularly hazardous phenomenon. Arcs may also be struck by drawing apart two conductors between which a current is passing as described for example by J. M. Somerville "The Electric Arc", Methuen 1959.
A number of highly aromatic polymers have been proposed in recent years for use as electrical insulation, these polymers exhibiting good mechanical properties especially at high temperatures, which is at least partly due to their relatively high glass transition temperatures, e.g. of at least 100.degree. C. However we have observed that such polymers are particularly susceptible to the tracking and arcing phenomena described above. In order to overcome, or at least reduce, the propensity of the polymers to track and to arc, we have proposed in International patent application No. WO 89/00756 blending the aromatic polymer with an aliphatic polymer. Although significant improvements in tracking and arcing performance have been obtained, this has been at some cost to the mechanical properties of the aromatic polymer especially at high temperatures. Furthermore, when the aliphatic polymer is replaced with a polymer, e.g. a polyamide, having both aromatic and aliphatic moieties and a relatively high glass transition temperature in order to improve the high temperature performance of the composition, instead of the mechanical properties of the blend being improved, we have found that certain properties are dramatically reduced, at least when the composition is extruded onto a wire, due to incompatibility of the polymers. This incompatibility is manifested in poor mechanical properties such as low ultimate elongatio
REFERENCES:
patent: 4612353 (1986-09-01), Andrews et al.
patent: 4657987 (1987-04-01), Rock et al.
Day Stephen
Hood Kathleen A.
Penneck Richard J.
Burkard Herbert G.
Carrillo Ana L.
Novack Sheri M.
Raychem Limited
Richardson Timothy H. P.
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