22 P o l y m e r s a n d the Technology o f Electrical Insulation J. H. LUPINSKI
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General Electric Company, Corporate Research and Development, Schenectady, NY 12301
General Description Historical Development Direct Conductor Insulations Other Insulating Resins Structural Materials Polymers for Electronic Applications
General Description The function of electrical insulation is to isolate conductors of different electrical potential from one another. This potential difference is commonly expressed in volts; the greater the voltage difference, the greater the stress on the electrical insulation. Because conductors carry the currents that make electrical equipment work, the tendency is to pack as many of them as possible in the available space. Consequently, electrical insulation is only allotted a small fraction of the total volume of a device, and thus high demands are put on small amounts of insulating material that is frequently present in rather vulnerable shapes, for example, thin coatings and films. The total spectrum of electrically insulating materials covers dielectric solids, dielectric liquids such as oils, silicone fluids, polychlorinated biphenyls (PCBs) and dielectric gases (SF , freons). This chapter will primarily be devoted to an overview of that part of the dielectric solids area in which polymers are the dominant ingredient, although inorganic materials may be present as fillers. After a few notes about historical developments, the material to be discussed in this chapter is organized as indicated in the table of contents. Emphasis will be placed on application aspects and polymer properties not l i k e l y mentioned in other chapters. (References to further general reading are given at the end of this chapter.) However, there will be occasional references to the basic chemistry of some polymers used in special circumstances. Applications in which the polymer system does not perform a major 6
0097 6156/ 85/ 0285 0515S06.00/0 © 1985 American Chemical Society
In Applied Polymer Science; Tess, Roy W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 10, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch022
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d i e l e c t r i c f u n c t i o n , for example, b a t t e r y cases, i n which polypropylene p r i m a r i l y provides a s t r u c t u r a l l y sound container for the a c i d and the b a t t e r y p l a t e s , f a l l o u t s i d e the scope of t h i s chapter. Numerous polymer compositions are a p p l i e d as e l e c t r i c a l i n s u l a t i o n . Some of them are only used i n s m a l l volumes for s p e c i a l a p p l i c a t i o n s . The materials used i n r e l a t i v e l y large volume i n 1980 are p o l y e t h y l e n e (high d e n s i t y + low d e n s i t y ) , 211,000 tons; p o l y v i n y l c h l o r i d e ) (PVC), 177,000 tons; and p o l y e s t e r s , 77,000 tons. Because the chemistry of s e v e r a l of the polymers used i n e l e c t r i c a l i n s u l a t i o n has already been described i n other chapters, i t w i l l not be repeated here. However, there i s one c h e m i c a l feature that i s quite c h a r a c t e r i s t i c for polymeric e l e c t r i c a l i n s u l a t i o n s , namely the c r o s s - l i n k e d form. E s s e n t i a l l y a l l of the magnet wire and cable i n s u l a t i o n s , the potting, encapsulating, and impregnating resins, are—in t h e i r f i n a l forms and shapes—cross-linked polymer systems. A few e x c e p t i o n s are found among p o l y m e r i c m a t e r i a l s such as some p o l y e s t e r s , p o l y p r o p y l e n e , and p o l y i m i d e f i l m s w h i c h a r e e s s e n t i a l l y t h e r m o p l a s t i c m a t e r i a l s and free of c r o s s - l i n k s . The reason why c r o s s - l i n k e d polymers are so common among i n s u l a t i n g materials i s that such polymers are i n s o l u b l e and i n f u s i b l e and p r o v i d e f o r e x c e l l e n t solvent resistance and good thermal c a p a b i l i t i e s , that i s , no melting and flowing during overheating of e l e c t r i c a l equipment. In general, the t o t a l amount of polymeric e l e c t r i c a l i n s u l a t i o n i s o n l y a minute f r a c t i o n of a l l the m a t e r i a l s that c o n s t i t u t e a transformer, a motor, or other rotating equipment. Yet i t i s often a f a i l u r e i n a few dimes worth of i n s u l a t i o n that causes a complex piece of equipment to f a i l . I t i s t h e r e f o r e e s s e n t i a l t h a t the q u a l i t y of the material i s s t r i c t l y c o n t r o l l e d , and i t i s e q u a l l y important that a q u a l i f i e d material be applied by r e l i a b l e , w e l l proven a p p l i c a t i o n methods. Good i n s u l a t i o n i s a key to e l e c t r i c a l equipment with long-term r e l i a b i l i t y i n spite of the fact that i t i s often functioning under adverse conditions such as exposure to o i l and s o l v e n t s (hermetic motors f o r r e f r i g e r a t o r s ) or to extreme c o n d i t i o n s of temperature and humidity (transformers i n outdoor i n s t a l l a t i o n , t r a c t i o n motors for l o c o m o t i v e s , and off-highway v e h i c l e s used i n mining operations). In addition to the general testing for mechanical, thermal, and c h e m i c a l p r o p e r t i e s w h i c h i s q u i t e common f o r p l a s t i c s i n n o n e l e c t r i c a l a p p l i c a t i o n s , the i n s u l a t i n g materials have to pass stringent e l e c t r i c a l tests that are designed to show imperfections and n o n u n i f o r m i t i e s that are not r e a d i l y detected by other t e s t methods. For example, i n an i n s u l a t e d conductor that has v a r i a t i o n s i n i t s 2 - m i l - t h i c k i n s u l a t i n g layer as shown schematically i n Figure 1, a test probe (not shown) at an e l e c t r i c a l p o t e n t i a l of 10 V with respect to the conductor at ground p o t e n t i a l w i l l r e a d i l y r e v e a l the p i n h o l e at A but i t w i l l not show the v a r i a t i o n s i n i n s u l a t i o n t h i c k n e s s at B, C, and D. However, a t e s t probe at 100 V w i l l a l s o break down the t h i n l a y e r at B and thus r e v e a l one of the t h i n spots. Common t e s t v o l t a g e s of 1500 to 3000 V can r e a d i l y f i n d i m p e r f e c t i o n s such as those shown at C and D. S p e c i f i c a t i o n s r e q u i r e that o n l y a very s m a l l number of i m p e r f e c t i o n s can be tolerated per unit length of conductor.
In Applied Polymer Science; Tess, Roy W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on February 10, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch022
22.
LUPINSKI
Electrical Insulation: Polymers and Technology
F i g u r e 1. Schematic r e p r e s e n t a t i o n o f i m p e r f e c t i o n s uniformities occurring in e l e c t r i c a l insulation.
and
In Applied Polymer Science; Tess, Roy W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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Today's r o l e of polymers i n e l e c t r i c a l i n s u l a t i o n i s q u i t e extensive and complex. This has not always been the case, and i t i s i n t e r e s t i n g to c o n s i d e r how the present s t a t e of development was reached.
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H i s t o r i c a l Development P r i o r to the development of polymer insulated wire, the separation of c o n d u c t o r s was a c h i e v e d by w r a p p i n g the c o n d u c t o r w i t h n o n c o n d u c t i n g f i b r o u s m a t e r i a l s u c h as c o t t o n and s i l k . Subsequently, such wrapped i n s u l a t o r s were impregnated with resinous m a t e r i a l s of n a t u r a l o r i g i n . As our understanding of polymer chemistry advanced, s y n t h e t i c r e s i n s were a p p l i e d to the bare conductors without the need for fibrous wrappings to keep conductors separated. Presently, a l l magnet wire i s made by f i l m coating the bare wire with synthetic enamels. However, by no means has the system of wrapped wire, impregnated by r e s i n s , disappeared. T h i s s o - c a l l e d served w i r e i s made w i t h more modern m a t e r i a l s such as mica, g l a s s f i b e r s , and s p e c i a l l y t r e a t e d paper, and i t i s used i n h i g h - v o l t a g e equipment where a r e l a t i v e l y l a r g e s e p a r a t i o n between the conductors i s r e q u i r e d . Frequently the wrapping i s a p p l i e d over a l r e a d y i n s u l a t e d magnet w i r e , and the t o t a l c o n s t r u c t i o n i s subsequently impregnated with l i q u i d r e s i n t h a t i s cured to form a r i g i d s t r u c t u r e . These i n s u l a t i n g m a t e r i a l s must have e x c e l l e n t mechanical and thermal properties. Whereas the advancement of r e s i n materials science resulted i n a p p l i c a t i o n of i n s u l a t i o n to conductors by the enameling process ( f i l m c o a t i n g ) , the advancement of polymer p r o c e s s i n g t e c h n o l o g y allowed extrusion of molten resinous materials d i r e c t l y to the bare conductors. This technology i s p r i m a r i l y used to produce s i n g l e or multistrand PVC coated ( hook-up ) wire for a p p l i c a t i o n i n radios, TV s e t s , and other e l e c t r o n i c equipment. P o l y o l e f i n i n s u l a t e d multistrand cables are used for underground power d i s t r i b u t i o n . In a d d i t i o n , advanced polymer p r o c e s s i n g t e c h n o l o g y a l s o provided for s t r u c t u r a l materials such as sheets and f i l m s , tubing, laminates, and other composite materials. Table I gives a general o v e r v i e w of the types of a p p l i c a t i o n f o r the v a r i o u s polymer systems. In some a p p l i c a t i o n s such as i n underground or underwater cable, e l e c t r i c a l i n s u l a t i o n has to perform under r e l a t i v e l y constant temperatures; i n other a p p l i c a t i o n s the equipment i s subjected to wide ranging thermal c y c l e s , for example, d i s t r i b u t i o n transformers and d i r e c t c u r r e n t t r a c t i o n motors f o r l o c o m o t i v e s . Both are exposed to c o l d w i n t e r and hot summer weather. I n t e r n a l heating because of overload conditions enhances the effect of high ambient temperatures i n the summer. E l e c t r i c a l i n s u l a t i o n s are characterized by t h e i r c a p a b i l i t y to f u n c t i o n at c e r t a i n temperatures. A w i d e l y accepted thermal c l a s s i f i c a t i o n i s described as f o l l o w s : ff
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Class 90—Unimpregnated materials based on natural products such as c e l l u l o s e (e.g., c o t t o n and paper) or animal p r o t e i n ( s i l k ) for use below 90 °C.
In Applied Polymer Science; Tess, Roy W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
In Applied Polymer Science; Tess, Roy W., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985. Composites
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