CA1053339A - Capacitor having a hazy polypropylene film - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Hazy polypropylene film having a uniformly irregular or raised relief surface is provided for use as a die-lectric medium in dielectric liquid impregnated electrical capacitors. In the blown tube process for manufacturing the film, the polypropylene tube emanating from an extruder has a section of the tube controlled in temperature. The temperature control causes continuous and uniform spherulite formation having a crystalline structure on the outer sur-face of the tube. Subsequent inflation of the tube causes the desired hazy irregularity to occur on the outer surface.

Description

36--CA--3102 (A) ~5333~
~ ` HAZY POLYPROPYLENE FILM
.; . . :
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. BACKGROUND OF THE INVENTION
";,. ` .
i This invention relates to a synthetic resin ~
,.,.,~, . ;
s polypropylene film and more par-ticularly to a hazy polypro- ; -~$~, pylene film defined as a film having a uniformly irregular or --raised relief surface whose irregularities are in a pre-determined form or pattern and with a predetermined degree of relief . ;-.,.' ~! 10 Polypropylene film has found widespread use as the dielectric medium in dielectric liquid impregnated ;
. ~"; :, r;ll electrical capacitors. In the production of one such ~;
capacitor, very thin polypropylene strips are interleaved `;
with aluminum foil strips and the composite wound into a ,.. ~ ~ . .
tight roll f ~ . The roll is placed in a suitable housing and impregnated with a dielectric liquid impregnant.
Alternatively the aluminum foil strips may be replaced with `
metal coatings formed on appropriate dielectric strips.
Polypropylene film, as produced from the well known blown `
tube and draft and tentering processes, usually has very `~
smooth blocky surfaces which tend to cling very tightly to ~`-each other or to other adjacent surfaces. This creates a ;~
problem in attempting to impregnate the film when incorporated in a tightly wound capacitor roll. It is particularly difficult to cause the impregnant, such as an oil, to ~,1 completely permeate the capacitor roll particularly into ;;
the interfaces of polypropylene 5trips and polypropylene and foil strips.
For this reason many prior art attempts have been made to expedite lmpregnation of capacitor rolls by roughening the adjacent film strips, foils, etc. whether ;i by etching, embossing, abrading, or forming processes. x -r ! -- 1 ,~ . " .

; ' 9 36-CA-3102(A) ~ i ' The result of these processes have not been entirely satisfactory for a number of reasons. Particularly, many . of these roughening processes tend to affect the physical strength of the film as well as dielectric strength. Also .'j ~ ! , .
.~ forming processes which increase the overall thickness of -A'~ film, such as corrugation and embossing, increase the overall , thickness of the roll and therefore make the final roll .. ` excessively large. The blown tube process has been noted to ^. :
produce film having surface areas which are slightly .:~
roughened. However, the roughness was non uniform and spor-. adic, and not subject to predetermination in the film making process. I
OBJECTS OF THE INVENTION
Accordingly, it is an object of this invention to ~ provide an improved uniformly irregular surface or hazy ,;,...
~-~ polypropylene film.
....
It is another object of this invention to con- I .
tinuously provide, in the film manufacturing process, a uniformly irregular or haze surface on polypropylene Eilm which preserves film strength and integrity while improving impregnation characteristic.
It is a further object of this invention to provide : by means of the control parameters in a blown tube ilm -;~ manufacturing process a uniformly irregular surface or hazy ~j film whose irregularities are of increased concentration and , ;,;!:! . .
degree of raised relief.
SUMMARY OF THE INVENTION
. ...................................................................... ..
In one preferred form of this invention, the polypropylene tube or stalk emanating from an extruder in a film making process has a sectlon thereof predeterminedly ';rl ' temperature controlled to cause extensive continuous and ~.; uniform spherulite formation with predominately type III
!`' , s~ - 2 : . . -36-CA-3102(A3 ~333~ :
i cxystal structuxe on the outer surface of the stalk. Sub-sequent inflation of the stalk causes the desired irregularity ox haze to occur on the outer surface thereof.

. - DESCRIPTION OF THE D~WINGS ~: ~
:'. ' ;;'~ :.: ' This invention will be better und~rstood when taken in connection w;th the following description and drawings in which:
Fig. 1 is a schematic illustration of a blown tube ;

o~ bubble pr~cess employed for producing polypropylene film.
Fig. 2 is a photomicrogxaph of a non hazy film.

Fig~ 3 ~s a photomicrograph of a piece of `
~; polypropylene film i~llustrating surface irregularity of 20 haze.
Fig. 4 is a photomicrograph of a piece of polypro-' pylene film illustrating surface irregularity of 30~ haze.
; Fig. 5 is a photomicrograph of a piece of polypro-pylene film i1lus*rating surface irregularit~ of 40~ haze.
; Fig. 6 is an illustrat~on of a capacitor embodying `;
, /, ~: , .
,~ the hazy film of this in~ention.
F1g. 7 is an ~llustration of a capacitor roll embQdyIn~ the hazy ~ilm of this ~n~ention.
DESCRIPTIO~ OF PREFERRED EMBODIMENT5 , Referring now to the blown tube apparatus 10 of , `~
Flg. 1~ polypropylene resin in the form of pellets 11 are fed into a hopper 12 and then into an extruder 13 where they ;

are heated to form a very soft or molten mass of pol~v~propylene.
,. . .
This mass is extruded ~rom the extruder 13 through a die 14 n tubular form and comes into contact with a cooling mandrel z~

15 where it begins to crystallize in the form of a stalk 16.
From the cooling mandreI 15, the stalk 16 is pulled through -: a pair of rolls 17 which compress the stalk in sealing relationship to an air tube 18 which passes therebetween in a `' ,.

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.. . ... . . .. . ..

~ ~~ 36-CA-3102(A) 333~
,,. groove in the rolls. After passing the rolls 17 the cooled ~` and crystallized stalk 16 is reheated to its softening ~,' t,emperature by a suitable heating means 19, for example, ;~-. ..
'; radiant heaters, and then insufflated by introducing air ....
.;~' under pressure through air tube 18. Insufflation provides a~,,~ , .
~ controlled bubble or large tube 20 which stretches the poly-,'~,'. propylene stalk in both the horizontal and vertical direction by a factor of about 6 to provide biaxially oriented polypro- ~-~'. pylene film. The bubble 20 is then collapsed between a further pair of nip rolls (not shown) and hauled off to a ;.~
~,' .slitter where the bubble is slit into one or more strips which '; .
are then wound on a take-up winding roll. Typical structures , ~.
and processes for producing films from a blown tube process ~, are found in the U.S. Patents, 2,720,680-Gerow; 3,235,632--',;, Lemmer; and 3,223,764-Kahn.
il In the practice of the above described film making ;,:'3 process, it was found that from time to time certain surface ,. l ., .
~'. irregularities appear in the film. These irregularities ,~
differed markedly from batch to .batch of film and were also `,:
~", 20 not uniformly distributed over the surface or coextensive .. ~," .
~i with the surface of a film of large area. Typically these ~' :

,~.'.: irregular surfaces were non uniform both with respect to dis-,~ " . .tribution over the sur:Eace of the film and non uniform with ~, respect to the degree of the irregularity. In addition, the .~.,'.' pattern of irregularity formation was not the same over the surface of the film, and, the presence or absence of irregular- '~

~, ities was not readily predetermined and at times was considered accidental.
. , ,' The irregularities in the film created an opaque .

,, 30 kind of film which was believed in many cases to be less '" desirable than clear smooth film. It was then discovered that ` some of this roughened film had some quite deslrable impre- ~.
, .

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-~- 36-CA-3102~A) ~
533~
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~na1tion chaxacte~istics. ~or example, when selected samples :
of maximum roughness of this film were used to wind exper-imental capacitor rolls, the film did not cling or stick to ; itself or adjacent surfaces. ~lso, the roughness of the film ; provi:ded a certain degree of separation of adjacent surfaces so that an impregnating liquid could more easily enter the space. Accordingly not only did enhanced and uniform roughness ~
"!,~,, become a desirable feature, but also its predictability, `
control, and uniformity as applied to a film making process ~ -`~' 10 become necessary criteria.
It has now been discovered that certain blown tube , .~.......................................................................... .
? ~xocess temperature controls may be employed to factually ' predetermine the presence or absence of irregularities on the i,':.; !
film surface. Even more importantly, it has been discovered that these ixregulaxities can be produced uniformly over the film surface in increased concentration and degree of relief.
Where irregular~ties are of a certaln kind, and the surface ~`
~;' pattern of irregularities is uniform over the total surface of the film, and where the degree of raised relief falls within prescribed limits ~nd is also uniform over the surface ~ ~ , . . .
~;~ of the film, the film is described as hazy film in accordance ~,s;",.',; with this inVention.
tl Measurement of surface roughness or irregularity may be given in terms of haze of haziness of film. Haze is determined by the measurement of light passing through a .
section o~ the film, i.e. generally perpendicularly through the top and bottom surfaces rather than edge~ise. In the practice of the present invention, a Gardner Laboratory Haze :-.1 :~,' ~` Meter, a commercially available device of Gardner Laboratory ,.............................. . . .
Corporation, Bethesda, Maryland, catalog #HG1204, was employed. Also utilized was a digital photometric unit, catalog #PG5500. The Haze Meter directs light through the ~;

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, --~ 36--CA--3102 (A) 5~339 ~`~ film and measures the intensity of light after passing through .
the film as compared to -th~ intensity entering the film.
Values obtained are given in terms of percent haziness of the film. Test methods employed were standard American Society for Testing Materials, Tests ASTM-D1003, AST~-D1044 and FTMS `
": ' 406, Method 3022. ~`
With a Haze Meter measurements are taken at small intervals across a film strip, for example, 15 measurements for a film strip about 1 meter in width. An average reading ~` 10 is taken for the 10 highest haze readings, and an average is taken for the lowest 5 readings. The hàze description given . .~.................................................................... . .
to the total film is a composite of the two averages, e.g.
40/20 haze. When a single haze number is given, the number, for example, 30~ haze, is the single number obtained from the Haze Meter.
Also, a Gardiner Gloss Meter was employed, catalog #GG9042 utilizing ASTM test D2457-70. This device measures reflected light from a film surface; also in terms of haze.

i Hazlness of the film may be controlled by effect-uating or controlling the temperature conditions of a predetermined area of the stalk, in the equipment of Fig. 1, ' and may be clearly demonstrated by heating or cooling a specific small area of the stalk by rapid spray cooling, for example, and noting the resultant haze differences in the bubble. Temperature control may be applied to the stalk 16, !, for example, in the form of a heater ring 21 in chamber 22 as illustrated in Fig. 1. A suitable temperature control fluid such as air may be circulated through one or more ; rings 21 to spray through suitable apertures therein and come into contact with stalk 16, and thereby elevate or reduce ` the temperature of the stalk 16 over a predetermined axial length thereof. Such heating or cooling may be obtained . ' ' ., ., ,. ..

:
- ~ 36-CA-3102(A) S~333~
, .. .
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from many kinds of apparatus known in the art which can elevate or reduce the temperature of an object moving there-through whether by direct or indirect contact, by liquids or gasses, or by radiation, as typical examples.
Temperatuxe control, however, must be exercised at -~
a predetermined position or area of stalk 16. Polypropylene extrudes from the dle 14 in a molten condition and approaches and moves coaxially about mandrel 15 to be progressively cooled and crystallized as stalk 16. In the cooling of stalk 16, a circumferential "frost" line or margin appears at a position of about 6 inches to 24 inches from the die 14. This line is a visible one and is the crystallization line denoting -~
the general area by which full crystallization of the polypro- ;
pylene stalk has occurred. The temperature control of this , .
invention, which is ordinarily heat addition to the stalk, `

;j must be applied after the die and before the frost line, and ~-, preferably at an intermediate section.

Ordinarily, in a blown tube film making process the ;,l mandrel cooling temperatures, the stalk temperatures, and ., .,. ~ : ~ .
~ 20 the frost line are fairly established conditions and their ~
,~ ~. . .
` ranges are quite limited. For example, the temperature of the ; -polypropylene emanating from the die 14 is at about 455F.
~; with the frost line temperature being from about 200F. Under these conditions the crystallization characteristics of the stalk are also fairly established and the type of crystal . . .:. .
formation is somewhat constant.
The hazy film of this invention is caused by the controlled formation of a skin effect or specific crystal ~; layer in the stalk. The skin effect in the stalk may be described as a layer at the outer surface of the stalk which comprises an area of markedly increased density of spherulites : . .;
of type III polypropylene crystals. It is the presence of , ,j , _ 7 _ ~
,.,:; :-'. . .

~ ~ 36-CA-3102(A) : ~S~3~9 this skin effect in the stalk which, when the stalk is , biaxially oriented in the bubble, transforms the outer surface into the hazy surface of this invention. One explanation relating to the type III crystals to haziness is that the type III crystal is a transient crystal in the film making process. Tests have shown that the type III polypropylene .
- crystal melts during the reheating process at l9 (Fig.l) just prior to bubble stretching. Reheat temperatures are ,:.
- between about 285 Fo and 310 F. The type III polypropylene ,r crystal in the stalk has a density of about 0.8 and upon passing through the reheater l9 transform tG crystal I and II
types which have densities on the order of 0.9. Consequently the stretching of the bubble in combination with the density change of the crystals results in a discontinuity or crater-like effect which gives an indication of haze.
The skin effect which gives rise to hazy film can be caused to form by effectuating a certain temperature ^: :
differential between the polypropylene stalk at the mandrel surface and at an opposite point at the outer surface of the stalk. This temperature differential is also combined with the fact that the stallc is under tension as it is being pulled by the nip rolls 17. Therefore additional cooling or heating of the stalk changes the tension and stress therein. An increase of temperature results in the generation of shearing stresses in an on going process and provides a greater degree of nucleation in the outer surface of the stalk with more spherulites with type III crystal structure. This increased density of crystals in the outer surface define the skin effect. An important factor in hazy film is the formation of the skin effect on one surface of a stalk as the stalk is being processed to a final film form by means of a specific temperature control. The skin effect is a physical one and is 36-CA-3102(A) clearly discernible in a photomicrograph of a stalk section.
` ~he skin effect is caused to occur in one practice , . , ; of this invention by elevating the temperature of the outer :,.
s surface of the stalk by means of heating means 22 well before ~,, the frost line, very rapidly, without adversely disturbing " , .
the frost line, and without disrupting the essentially balanced heat flow conditions at the mandrel and in the stalk which : ,~
provide for yood stalk formation. Under these conditions it ?i s is believed that a favorable temperature gradient or environ-ment in the stalk is established which is favorable to sub-sequent skin effect nucleation. Furthermore the sudden addition of heat to a specific portion of the outer surface s of the stalk, or the sudden prevention of heat transfer there-from permits the existing tension to stretch that portion of the stalk and this stretching or shearing also favors `
nucleation~ The application of heat and the consequent shear `! stresses at a defined location before and adjacent to the frost line contributes to the production of the hazy film of this ~ invention. The amount of heat input on the outer surface ,~ 20 temperature of the stalk is best determined by empirical relationship and visual determinations, since the skin effect li and the resultant haze are clearly physical states which are.. ~ . .
easily measured.
Temperature control of this invention may be exercised without actual separate heat addition. For example, l a temperature distribution or gradient from the inside 'i surface to the outside surface of the stalk and a higher transient temperature at the outside are the characteristics which favor the described skin formation. Best results are obtained when the distribution curve is steep and the mark- `
edly higher temperature is at the extreme outer surface.
~ For example, in one practice of this in~ention, following .,~ `

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:;: ~;
~ 36-CA-3102~A) ,: ~
: ~53;~9 the illustration of Fig. 1, the polypropylene resin was isotactic polypropylene commercially available from Dart Industries and extruded at the die ordinarily at about 450F.
,...
: .;. . .
, , However, for this example the temperature was reduced to ."""j.".
~"~ about 435F. The mandrel/film relationship was sliding ; ....................................................................... .
,~ friction of the film surface directly adjacent the metal ,rl ~.
mandrel. Stalk diameter was about 6 inches diameter, the wall thickness was about 17 mils and axial progress along the mandrel was about 35 feet per minute. A more uniform stretch- ~
. :
ing of the stalk was taking place and the resultant shearing , ~ stresses favored nucleation. At this point mandrel cooling ` was progressively reduced to effectuate an elevated temper-ature in the stalk without adversely affecting the frost line.
Cooling may be slightly reduced along the mandrel or at a more specific location with a resulting higher temperature in the outer surface of the stalk. At this point, heat :i '~i' radiation to the environment may also be controlled by suit-..... .
able shrouds or heat reflectors in the control zone 22. By this means a skin effect is also formed on the stalk and a ~` 20 hazy film is produced in the bubble. Haze measurements made .:.:. ~
~, on film produced in the above example showed a range of from ,: , 20 percent to 40 percent depending on the control temperature of the stalk at station 22.
."
i Polypropylene produced by practice of this i ~i; invention has a high degree of haziness, and the haziness is 'j,, uniquely uniform and similar over the entire film surface.

The specific haze structure is illustrated and compared in , Figs. 2 through 6.

Referring now to Fig. 2 there is illustrated a .:., .
~ 30 photomicrograph of a piece of polypropylene film made with a ;,?' non haze or prior bubble process. This film is 0.70 mil.

~ thick and shown at 75X magnification. What is seen are a ':' ~: -- 1 0 ~ . . . .

.i. ~
-~ 36-CA-3102~A) 5333~ -., .
. ~
few straight s~ratch lines and several small circumferential ~
lines which may be the ridges of slight cavities or depress- ;
. 1 , ~` ions.
'~ Referring now to Fig. 3 there is illustrated a , photomicrograph of a piece of 0.70 mil. thick polypropylene ' film of about 20 percent haze in accordance with the measuring practices of this invention as described and also -~
. .
s"' ! at a magnification factor of 75X. The circum~erential rims defining craters or depressions are now clearly visible and proliferate. The raised ridges or fibroids may be elevated . .

2-3 microns above the ordinary gage thickness of the film , and they are uniform and continuous. Uniformity and ~-l continuity means that a continuous run of film, i.e. several ' yards at least, or a bobbin supply of film has the ,' configuratlon and density of Fig. 3 coextensively over one ;" surface. -i.. . . ...
,;~ Referring now to Fig. 4 there is illustrated a ~ ;
piece of polypropylene film of 0.70 mil. in thickness magnified 75X whose haze reading is about 30% in accordance with the light measuring practices of this invention. In ,`
this illustration there are as described a great number of ,' fibroids or craters intertwined and overlapping, and co-~" extensive and uniform over a continuous run of film. The illustration gives an appearance of a pressed matte of very thin loose fibers. The structure is very conducive to impreg- J`' ' .. ~ ,~ .
nation, is non sticky and represents a good example of the ,,l hazy film of this invention.
Referring now to Fig. S there is illustrated a piece of polypropylene film of 0.70 mll. thickness magnified at a factor of 75X and whose haze measuremen~ by the practice 1'.' ' .~
of this invention is greater than about 40%. Fig. 5 illustrates myriads of crater-like depressions defined by , . .
,.. I , .

~,.

,:; ,- , .- . . , . .. ~

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; -- 36-CA-3102(A) ~ ,.,.~., ~
5333~ :
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; fibrous-like raised rims in overlapping and intertwined .i~.; ~
:j,.~.,.
; relationship. The appearance is somewhat similar to crackle glass. Fig. 5 represents an excellent form of haze of this ,, invention.
Hazy film of this invention is believed to be , ~; related to the skin effect above described, and the stress -~;- nucleation effect. The enhanced haze is so controlled by ~ these factors that it is now caused to appear as a complete ;~ or coextensive pattern over a substantial supply of tilm, i.e. a bobbin supply from which many capacitor rolls can be ~;~ wound.
Hazy films produced in accordance with the teach-ings of the present invention were incorporated in an electrical capacitor structure as illustrated in Fig. 6.
Referring now to Fig. 6 there is disclosed a capacitor 25 ~;. embodying the hazy film of this invention. Capacitor 25 .,. ~, :
typically follows the design and process set out in U.S. `~
~`~ Patents RE-27,824 - Cox; 3,754,173-Eustance; and ~- 3,724,043 - Eustance.
In Fig. 6 capacitor 25 may be denoted as a high , /,. . .
voltage AC power capacitor and more particularly a power ; factor correction capacitor. Capacitor 25 includes an appropriate casing member 25 with a cover 27 sealed thereto.
Capacitor 25 is provided with insulating bushing members 28 and 29 which insulate terminals 30 and 31 from cover 27.
Terminals 30 and 31 provide an electrical connection (not shown) to tap straps 32 and 33 of the roll sections 34 in . .;
the casing 26. Roll sections 34 are more fully described with respect to Fig. 7.
Referring now to Fig. 7, there is illustrated a , .~ .common example of a convolute or roll section 34 for use in `~ an impregnated type capacitor either of the high voltage AC
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~ 36-CA-3102(A) 533.39 .... .
type above described or in smaller motor run type of capacitor. A roll section 34 usually comprises alternate strips, 35 and 36, of a metal foil, and polypropylene film dielectric, 37 and 38. In a preferred practice of this invention, polypropylene strips 37 and 38 are usually less than 0.001 inch thickness and foil strips 35 and 36 are of aluminum. Placed adjacent the foil strips at appropriate '~ locations in the roll are tap straps 32 and 33 which are utilized as electrical connections for the electrode foil strips, and are suitable connected to terminals 30 and 31. --.
The noted strips are fairly tightly wound in roll form and -then flattened to the illustrated configuration.
, Impregnation of the capacitor 25 usually takes place by means of introducing an impregnant into one or more ` small holes in cover 27 which thexeafter are solder sealed.
i'1 During impregnation the capacitor 25 is generally submerged in a liquid impregnant which fills casing 26 and impregnates ;l the roll sections 34 therein. Certain pre-evacuation cycles, high temperatures and other process steps are also usually ' 20 employed.
One of the major processing disadvantages present l in impregnated capacitors, particularly roll section ;; capacitors, is the difficulty of obtaining essentially ,1 :.
complete impregnation of the roll. In order to provide, for example, a high voltage (above about 600 volts), AC power factor correction capacltor, essentially complete impregnation ~` should include a filling of all significant air spaces or ,! voids between the electrodes, whether these voids and interstices are between adjacent dielectric strips, between ! 30 dielectric strips and electrode strips, or within the , dielectric materials. `
In a wound roll section such as illustrated in .
.

' , : . . i ... .. . . . .

~ 53~39 36-CA-3102(A) :
Figs. 6 and 7, the impregnant must proceed axially through the roll to reach the inner most portions. If the dielectric material is porous, or will otherwise absorb and transmit or pass the impregnant under the conditions of impregnation, then the impregnant not only may pass rapidly axially through the roll, but a~so may pass transversely through adjacent dielectric strips. The hazy film of this invention~
whether adjacent another hazy surface or a foil surface or a film surface, will provide by means of its irregular surface, ' ~?
certain openings, apertures, and passageways which facilitate the entry and passage of the impregnant deep into the ~oll.
, In an exemplary practice of this invention capacitors were made up as described with respect to Fig. 7 with strips of hazy polypropylene and other identical ~;i capacitors were made up with smooth polypropylene dielectric strips. Results are as follows:
TABLE 1 t No._Failed/Tested 5500 Hrs.
;~ Life Test V-AC/ C Smooth Film Hazy Film Capacitors for the following test utilized strips i!,. of 50 gauge polypropylene and were 2 Uf 540V-AC units:

.,,,j .
! Life TestSmooth Film Hazy Film ~! -Total Failed/Tested at 17/70 2/25 - ~0-Hrs.
The hazy film of this invention may have an electrically conductive coating on either its hazy surface -~
or its opposite surface. Metallic coatings, such as aluminum, do not adhere as strongly as desirable on a smooth polypropylene surface. The kind of irregularity and the :

; ~ 36-CA-3102(A) ~ S~ ~39 pattexn of the irregular suxface of the hazy film of this invention provide better contact between the metal layer and the film and an improved bond. When the metal coating is on - the smooth side of the film an improved composite is produced r i.e. an electrode, a dielectric film and a hazy surface.
. . . .
Such met~llized strips have an important application in capacitors and elimina~e the need for a separate electrode foil.
.. .
;~' The hazy film of thls invention provides the major ~; advankage of facilitating impregnation. Because the haziness is uniform over the film the impregnation cycle can be .1~.~... . .
-~ ~ pxogrammed to ~nclude this benefit. Because the haziness /~ is of a considerable magnitude, liquid penetration is ~acilitated bet~een the hazy surface of a polypropylene strlp and ad~acent surfaces, whether of hazy polypropylene .... .
~; or other ~aterials.
..... .
The invention lS appllcable to crystalllzable thermoplastic materials generally whose manufacturing vil charactexistics approximate those of the isotactic ,`
polypxopylene of this invention. Included among these materials are forms of polypropylene such as syndiotactic pol~pxopylene and the copolymers and homopolymers of polypropylene as well as blends or of polypropylene with other synthetic resins. Also included are the other crystallizable polyole~ns.

The hazy film of this invention may be gainfully employed as electrical insulation materials and products ,~l particularly when winding and unwinding procedures are ~ involved, or impregnation or drying cycles are required.
.,.j ::
The hazy surface not only facilitates winding and reeling ~-because of its non stickiness characteristics, but also ~
.;, ~i , . .
provides passageways for removal of vapors in a drying cycle.

~;`' The hazy film of this invention is also particularly ~. ;. ~: .

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~ 36-CA-3102(A) ~.:
333~
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adaptable for printing thereon. One prior problem with :~.
smooth films is that commercial ink stamping would not bond well and was easily erased. The rough surface of hazy film provides better surface lack characteristics for inks and prints.
While this invention has been disclosed with respect to particular embodiments thereof, numerous modifications may be made by those skilled in the art without departing from its true spirit and scope. Therefore, it is intended that the appended claims cover all such modifications and variations which come within the true spirit and scope of the present invention.

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Claims (5)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. An electrical capacitor comprising:
(a) a casing having electrical terminal means thereon, (b) a capacitor roll in said casing, said roll consisting of two hazy film dielectric strips interleaved in alternating fashion with two metal electrodes, each of said dielectric strips having a smooth surface and an opposite hazy surface which is a uniformly irregular raised relief sur-face, (c) electrical connection means between said electrodes and said terminal means, and (d) a dielectric liquid impregnant in said casing and essentially completely impregnating said roll.

2. The capacitor of claim 1, wherein said two hazy film dielectric strips comprise two thin continuous poly-propylene strips each having a uniform coextensive haze on the hazy surface thereof, said haze having a haze measurement of from 20 percent to 40 percent.

3. The capacitor of claim 1, wherein said electrodes comprise aluminum foils.

4. The capacitor of claim 2, wherein said two polypropylene strips each have a metallic coating on the smooth surface thereof, said two metal electrodes being the two metallic coatings.

5. The capacitor of claim 4, wherein said metallic coating comprises aluminum.