DE3231013A1 - Improved synthetic resin film with structured surface layer, process for the production thereof and electric capacitor produced therewith - Google Patents

Description

Dr. rer. not. Horst Schüler? °? ° '"nMuri / Μαΐηι, i". August 1982; |

Telex 04-16759 mopot d Postscheck-Account ι 282420-402 Fronkfurt / M. Bank account ι 225/0389 Deutsche Bank AG, Frankfurt / M.

8924-36-CA-3527

GENERAL ELECTRIC COMPANY

1, River Road
Schenectady, NY, USA

Improved synthetic resin film having a structured surface-\\ chenschicht, process for its preparation and produced electrical capacitor $ \

The invention relates to a synthetic resin film with a structured or roughened surface, in particular to a Hazy polypropylene film as a dielectric, mainly for use in electrical capacitors and processes for their manufacture.

The US-PS 4 255 381 of the same applicant discloses a bubble process for producing a film with struc-

(Rl

rougher or rough surface, known as Hazy ^v polypropylene film, in which hot polypropylene melt is extruded in tube form from a nozzle, the tube is cooled on a mandrel, reheated and inflated to form a biaxially stretched bubble. The bladder is then slit into strips of film, particularly as a dielectric material for electrical capacitors.

The use of prepared according to the method described Hazy film in an electrical capacitor is disclosed in commonly assigned U.S. Patent 4,228,481 claimed. The continued and increasing use of Hazy film as a dielectric for electrical capacitors

has to search for better methods of making improved Types of Hazy Film, and more importantly Hazy Film, with more predetermined and enhanced structural features such as the predictably uniform room factor or the roughness and economy of manufacture.

It has now been found that the above improvement can be achieved by a process using a polypropylene material exposed to some electron beam radiation and then biaxial the material into a thin film is stretched.

The invention can be better understood in conjunction with the following description and figures; illustrated by these

1 shows a simplified method and apparatus for electron irradiating a polypropylene strip;

FIG. 2 shows a simplified apparatus and method for stretching the polypropylene strip of FIG. 1;

Figure 3 is a photomicrograph of one form of polypropylene film strip with a structured surface of the method and the device of FIG. 2;

4 is a photomicrograph of another form of textured polypropylene strip Surface;

Fig. 5 is a capacitor winding employing the textured surface film of the present invention;

Fig. 6 is a power capacitor that modified

Windings of Figure 5 used;

Figure 7 is a schematic illustration of the tube blowing process for the production of polypropylene film.

Referring to Figure 1, there is shown a schematic illustration of an electron beam / film processing apparatus

10 disclosed. The apparatus 10 includes a feed roller

11 for flat or smooth synthetic resin film 12 and a take-up roller 13 for treated film. The film strip 12 can be viewed as an intermediate stage and can be a tubular stem, as it emerges from the pipe blowing apparatus as disclosed in the aforementioned US Pat. No. 4,255,381. The film strip 10 may also be the strip that comes out of the extruder of a stretch-and-tension film-making process, or a strip available by other well known methods of making polypropylene strips such as other extrusion and casting processes. The filmstrip 12 is taken from the roller 11 directly under an electron irradiation device 14, the suitable shielding devices 15, one that focuses the electron beam Structure 16 and an electron gun 17 comprises. The elec-

,, heat-resistant

electron beam source 18 is usually a (Crefractory) metal, like tungsten, which is electrically heated to drive out electrons. The electrons are let through from the source exit a protective slot in a curtain-like arrangement 19 and meet the film strip 12, which is in Moved longitudinally in one direction relative to the radiation device 14. U.S. Patents 3,702,412 and 3 780 308 are an example of this. However, the filmstrip 12 can be made to be in several Moved directions, e.g., both longitudinally and laterally, and the irradiation device 10 may be movable be made. This movement can provide the film with a structure that is in a predetermined order follows, for example, a predetermined geometric pattern

the movie. Furthermore, the irradiation process can be focused in this way or directed to create a structure on predetermined areas of the film, e.g., where a finished Film strips only have textured edges or hai edges. Both the type and the degree of exposure train you special degradation process across the »surface of the film with superficial penetration. During this degradation process, the perfection of a surface layer of crystals is unknown can be changed, and some networking can be done.

When the filmstrip 12 has been suitably bombarded with electrons, it is placed in a simple film straightener and stretched (while still warm) in both the longitudinal and transverse directions to give it a permanent stretch. One such process is called biaxial orientation. A schematic representation of the stretching process is shown in FIG.

In Fig. 2, the stretching device 20 comprises a film strip 21 of one or more oppositely arranged Staples 22, 23 are gripped across the machine direction and one or more staples 24, 25 are gripped in the machine direction will. The clamps are moved in the direction of the arrows / in order to stretch the film strip 21 permanently.

During the stretching process, the irradiated surface layer stretches The film is different from the rest of the film due to the crystalline perfection and / or crosslinking that is on surface irregularities occur. These irregularities show up as protrusions from the planes of the original film and are usually arranged randomly across the surface to form a structured Surface similar to that disclosed in the aforementioned US Pat. No. 4,255,381. The film surface is described as an alternative with fibril-like elements in loosely circular

shaped and raised relief shape, which is about like
Craters and depressions appear in the film surface.

The analysis of the polypropylene film according to the invention with
Irradiated and structured surfaces by IR spectroscopy showed no distinguishable change in the chemical properties
Composition. According to scanning electron microscopy, the surface morphology shows a cloudy surface, similar to that
Hazy film as in connection with U.S. Patent 4,255,381 above
described. Films that have higher doses of radiation (20 megarads)
had been exposed, however, showed a degenerate one
elliptical. Fibril morphology, where the fibrils were discontinuous and discrete, closely spaced from one another
form holding islands. Representative examples of these
Concepts are illustrated in FIGS. 3 and 4. 'i

Fig. 3 is a photomicrograph showing a shape of a %

inventive film with a structured surface; '|

vividly. The magnification factor in Fig. 3 is 500 times at: |

at a 30 ° angle and clearly shows the fibrils that lie on the I.

Film give a pebbly effect, with different 'f

Projections and Depressions. The height (and depth) of the protrusions I.

depends on the degree of irradiation and the stretching process and |

parameters controlled. , i

Fig. 4 is a photomicrograph showing another form i: |

of a surface structured according to the invention? |

light. This form is caused by increased levels of loading "i *

radiation than in the case of Fig. 3 "In Fig. 4 50Of ache '\

Enlargement at a 30 ° angle creates a cluster or island effect '■ {

From before jumps from a film level. ^! |

The Method of Making a Polypropylene Film Strip ■ '·

with a structured surface according to the invention Practice is described in the following example. - '

example 1

In a typical practice according to the invention, probes of polypropylene cast from a blowpipe device subjected to an irradiation treatment as schematically shown in FIG Fig. 1 shown. The irradiation was carried out under a nitrogen blanket or atmosphere. The radiation dose varied between 5 and 20 megarads at 175 and 200 kV acceleration voltage. Lower doses did not deliver pronounced cloudiness. The / AnguBn £ material got through the Irradiation zone at about 15.24 m / min, about 25.4 cm / s. The extrudate was then placed on a stretching frame in a 6 ratio biaxially oriented at about 140 to 160 ° C. The following table illustrates the results obtained.

Tabel

Movie Mega
wheel Acceleration
tension
kV thickness Cloudiness
(%) by-
average Space factor
% C. 0 0 13.8 4.6 3.2 1 5 175 11.0 ⁷ ' ⁹ 5.8 2 10 175 11.9 16.9 11.0 3 20th 175 20.0 23.4 3.8 4th 10 + 10 175 12.0 21.9 7.3

In the example above, 10 + 10 megarads means two treatments of 10 megarads each. The above data show that irradiated and biaxially oriented films provide significant haze and space factors in the 7-10% category, what makes them excellent solid dielectric films for electrical capacitors. Increased irradiation can

Cause crosslinking in the polypropylene film, a result which is desirable in many capacitors because the crosslinked polypropylene is more resistant to swelling and solution is when it comes into contact with certain dielectric fluids. See U.S. Patent 2,948,666, U.S. Patent No. 2,948,666 Describing irradiation and subsequent heating as a crosslinking process.

Films from the above examples were electrically tested as illustrated in the following table:

Table II

Film frequency dissipation factor dielectric constant

2.29 2.29 2.29

2.33 2.33 2.33

2.3
2.3
2.3

The irradiation method according to the invention is one in which the degree of irradiation and the focusing can be controlled are. The surface structured according to the invention can therefore also be used in two important ways that were previously difficult to control Areas are controlled, namely (1) the uniformity of the surface in a continuous manner and —_ —— _______-.

frequency Loss factor 1 0.07 10 0.06 100 0.03 1 0.07 10 0.07 100 0.07 1 0.05 10 0.06 100 0.16

(2) a predetermined uniform roughness. These Both factors are simply determined by the level of exposure and the speed of movement of the film through the

Irradiation zone controlled.

An important factor of the invention is that the treatment of the film is in the form of a solid particle treatment. It is precisely the bombardment of the film by electron particles in contrast to the light energy that causes the rapid and deep change in the surface layer of the crystals and networking. Particle bombardment can be broadly defined as atomic or sub-atomic particles whose equivalent wavelength is practically below that of UV light. Other particle bombardment processes include particles, like protons, neutrons etc. Solid particles work at extremely high energies and penetrate quickly and deeply into them Film surface and change the crystal structure more than the amorphous structure. Comparatively, it can be the shortest The wavelength of light, that of UV, must not be less than about 250 nm, while the equivalent wavelength of an electron beam at about 100 to 150 kV would be about 0.004 nm.

In a preferred form of the invention the film is heated well above 100 ⁰ C to near its melting point, wherein the three-mentioned steps are carried out, for example, for polypropylene to about 150 to 160 ° C, and then stretched sequentially. Exposure doses can vary from 5 to 20 megarads and above, and electron beam acceleration voltages can be from 100 to 200 kV and above. The rate of exposure to the film is in the range of about 12.7 to about 51.3 cm / s (25 to 100 feet / min), with about 50 feet more providing excellent results.

The product according to the invention with a structured surface is widely used in a dielectric Material in electrical capacitors as a means of facilitating the impregnation of the capacitor with a dielectric fluid. The strip can be metallized with a thin metal layer that is applied to each upper

323-013

surface of the strip can be applied. The invention _; (

contemporary film finds optimal use in electrical Kofi- fy

capacitors, where the film is the only dielectric. ; i |

For use as an electrical capacitor, there is ||

two important measurements for films with a structured surface ψ

area - (1) the turbidity and (!) the space factor. The turbidity; |

is determined by measuring § going through a film section

Light, i.e. generally perpendicular through the upper and un- |

over the surface instead of over the edges. In the 2nd

In practice, a Gardner laboratory opacimeter, one in the Han- '%

the apparatus available from Gardner Laboratory Corporation, ||

Bethesda, Md., Cat. No. HG1204. Also a |

Gardner digital photometer unit, Catalog No. PG55OO, can |

be used. The opacimeter directs light through the * i

Film and measures the intensity going into the film. Received % _t

Tene values are given in% haze of the film. Applied §

Test methods were standard methods used by the American Society for.!

Testing Materials, Tests ASTM-D1003, ASTM-D1044 and FTMS-406, l ';

Method 3022. t _t

The space factor is a more important measurement in dielectric;,; ■

Condenser films. During a turbidity measurement on the ground!?

position of the passage of light left a certain roughness specification? |

distant, it becomes too strong through internal coloring and 'j

Surface irregularities in the film, which affect the passage of light \\

rivers, influenced. Furthermore, the method of light transmission ||

gangs only provide an approximation for the space factor. If |

for example, a given film surface is only a few

ge, would have points protruding from the surface, would <* |

the method of measuring the light transmission indicates low turbidity. |

In fact, the film would have a high spatial factor. | If the given film has a larger number of small peaks
if the film were actually a low one, the light transmission method would give a high haze value

- 10 - ι

Space factor. A high turbidity value is related to a high space factor, e.g. only if predetermined Traversing conditions can be achieved. In the invention Polypropylene films with space factors below about 11%, with a typical range between about 7, are used in practice up to 11%. Higher exposure can result in an increased space factor.

The measurement of the space factor is done by a simplified direct calculation, since the space factor is an expression that the relationship of theoretical or fixed volume, for example a strip of polypropylene film, versus total volume, that the film strip occupies, denotes. For example, a film strip with a roughened surface have a total measured thickness that provides a given dimension from which a first calculation of the volume he follows. However, this volume includes the spaces, such as valleys between the projections or roughness peaks. A second calculation can be made on the actual volume of solid material in the strip. The difference between These two calculations, divided by the weight thickness, is the space factor, expressed in% available space in a roughened film over a theoretical fixed volume, e.g. a space factor of 10% indicates that 10% of the measured volume cannot be taken up by solid material. The weight average thickness is obtained by weighing a film sample of known length, width and density and calculating the thickness. The volume average thickness is obtained by measuring the film thickness with a micrometer. The difference between the two thicknesses, divided by the weight average thickness, is the space factor.

Typical capacitors using the film of the present invention are illustrated in FIGS. In Fig. 5 is a capacitor winding 26 in partially unwound

♦ · -t ·

- 11 -

Shape shown. The winding 26 comprises a pair of AJpstand metal foil strip electrodes holding one another 27 and 28 and intervening polypropylene film strips 29 and 30. Further polypropylene film and foil strips complete the wrap with pairs of polypropylene film strips are present between metal foil electrode strips across the winding. Pick-up strips 31 and 32 are adjacent to the electrode strips in the winding 26 used to serve as electrical connections for the electrodes. As a composite capacitor are one or a plurality of capacitor windings 26 inserted into a suitable housing which is filled with dielectric liquid is, and the liquid can penetrate the winding, around the space between the turns of the winding as well to fill in the polypropylene material itself.

Such a capacitor, which uses multiple windings and is referred to as a power capacitor, is illustrated in FIG. 6. In FIG. 6, the capacitor 33 comprises a series of windings 26 which may be arranged in upper and lower rows in a double-packed configuration, all of which are immersed in the liquid 34. The housing 35 can extend over 65.0 cm in height and the coils 26 can be about 25.4 to 30.5 cm in length. The windings 26 comprise strips 31 and 32 are electrically DIAE -the one another and to terminals 36 and 37 verburi. ⁴ Where a single packed configuration (group of turns 26) is used, the turns are referred to as wide turns and can be about two feet or more in length. The strips 31 and 32 can be eliminated by using an exposed foil construction with such electrode foil protruding from a respective end of the winding and then the foil turns being soldered together at each end. Then a connection is made to the terminals 36 and 37. Because of the density of the turns of the windings 26 and the fact that the metal foil strips

- 12 -

If the electrodes essentially form an impermeable side barrier, the liquid must penetrate through the winding ends 38 and 38 (FIG. 5). Here, due to the increased temperatures of the "vacuum drying of the winding" as well as during the impregnation process, the overlapping polypropylene film strip edges at the ends of the winding tend to terminate with one another and with the electrode strips and swell tightly with adjacent strips when they absorb liquid, which prevents the liquid from penetrating In accordance with the invention, the use of the textured surface polypropylene film strips provides a controlled, permeable spacer device and pathways for liquid to penetrate the foils 26 from the winding ends in a unique and economical manner , such as aluminum deposited thereon, in order to act in the well-known metallized capacitor structures, whether in the described winding form or the form arranged one above the other, where the layers are flat and thick ht are stacked next to each other.

In practicing the invention, the method and apparatus of FIG. 1 may suitably be used with a well known polypropylene film manufacturing process be combined. For example, in Fig. 7 there is schematically illustrated a blowpipe device 40 as in Fig U.S. Patent 4,255,381. Referring to the blowpipe apparatus 40 of FIG. 1, polypropylene resin is molded of pellets 41 fed to a hopper 4 2 and then to an extruder 43 where it becomes very soft or melted Polypropylene mass is heated. This mass is extruded from the extruder 43 through a nozzle 44 in the form of a pipe or hose and comes into contact with the cooling core 45, on which it crystallizes in the form of a stem 46. From the cooling Core 45, the stem 46 is pulled over a pair of rollers 47, which press it sealingly against an air tube 48 which is in a

- 13 -

Recess in the rollers passes between them. To
after passing the rollers 47, the cooled and crystallized / ^ 46 is again through to the softening point
a suitable heating device 49, e.g. radiant heater,
heated and then by introducing compressed air through the
Air tube 48 inflated. This inflation results in a controlled bladder or large tube 50 which the Polyptopy lenextrudat both horizontally and vertically around a
Stretching a factor of about 6 to a biaxially oriented polypropylene film. The bladder 50 is then framed by roller 51
between a pair of nip rollers 52 to collapse ■ §)

brought and conveyed to a slitter 53, where the ^

compressed bladder into one or more strips cje- §j

which is then wound on take-up rollers 54, FIG

will. Typical structures and methods of manufacture -i;

of films using a blown tube process can be found in '!

U.S. Patents 2,720,680 and 3,235,632. J. J

Extrudate £ j

When the / 46 is led upwards on the cooling core 45 £ i

it moves past at a speed of about .. ·

7.62 to 15.24 m / min (approximately 25 to 50 feet / min) and is driven by ' one or more irradiation devices 14 with electrical. j

NEN irradiated as described in connection with FIG. ■ ".;

As the irradiated extrudate continues to rise, this %

heated to a temperature above about 125 ° C and then to the '·

Bladder mold 50 inflated as shown. The stretching process ν the stem shape to the bubble shape leads to a biaxial orientation

the polypropylene film in the bladder. The biaxial v '

Orientation is triggered by the fact that the ■

Pinch rollers 51 at the upper end of the bladder move them longitudinally;> ·

stretching direction while inflating the bladder? in since- i?

lighter direction. In the final state is a waste]> step of the bubble in the ratio of about 6: stretched 6, ie

6 times in the longitudinal direction and 6 times in the lateral direction. -U

The same modes of operation of irradiation are applied to the drawing "· ',

- 14 -:.

and tensioning systems, e.g., as illustrated in U.S. Patent 2,412,182. applicable, being a thick, flat strip or plate of a synthetic resin from a rectangular Nozzle exits. The strip is gripped by a number of clamps that hold it both lengthways and crossways stretch. The device and the method according to Fig. 1 are for the treatment of the nodding strip before stretching, Process adapted.

The film according to the invention can be used for other applications, e.g. for packaging purposes, association purposes and markings or markings, where printing or writing on a film surface is desired.

While the invention has been disclosed with respect to particular embodiments, numerous modifications are possible from FIG A person skilled in the art can be made without deviating from the inventive concept. Therefore, the claims are intended to include all such modifications and variations that are within the scope of the inventive concept.

Claims (19)

Expectations λ Method for producing a synthetic resin film with ^ - ^ structured surface, characterized that a) the film is bombarded with particles of high energy and b) the film is then stretched to achieve a rough structure on the bombarded surface of the film. 2. Method of making a synthetic resin film with rough structured surface, characterized in that a) the film is bombarded with particles of high energy, b) the film is subjected to an elevated temperature above about 100 ° C and c) the film is then stretched to achieve a rough structure on the bombarded surface. 3. Method of making a synthetic resin film with rough structured surface, characterized by the fact that & a) the film to an electron beam bombardment in Be ¹ ^ range from about 5 to about 20 megarads and about 100 to 200 kV at an exposure rate of between about 12.7 to _51r 3 cm / s (about 25 to 100 ft / min) exposed ¹ , b) the film at an elevated temperature above about 100 ° d subject and c) the film is then stretched to give the bombarded surface a rough structure. 4. The method according to claim 1, characterized in that it is biaxially stretched. 5. The method according to claim 4, characterized in that a polypropylene resin film is stretched. 6. The method according to claim 5, characterized in that both surfaces of the film are structured. 7. The method according to claim 5, characterized in that the electron beam is generated by a hot metal source will. 8. The method according to claim 5, characterized in that dab as the source an electrically heated / heat resistant metal , ^. , (refractory) is used. 9. The method according to claim 5, characterized in that the film is moved relative to the electron beam. 10. The method according to claim 7, characterized in that the movement takes place in several directions. • I 11. The method according to claim 7, characterized in that the movement in several directions in the horizontal i Level takes place. H 12. The method according to claim 5, characterized in that the movement is in one direction. 13. The method according to claim 3, characterized in that a film comprising polypropylene is used. (14. Synthetic resin film with structured surface layer obtained by / combination of electron beam treatment and biaxial stretching with statistical distribution of protrusions on the structured surface. 15. A synthetic resin film according to claim 14, containing polypropylene. 16. The synthetic resin film of claim 14, the space factor of which is over about 5%. 17. A synthetic resin film according to claim 14, both surfaces of which are structured. 18. A synthetic resin film according to claim 14, the polypropylene of which is crosslinked. 19. Electrical capacitor with a composite of alternating Layers of polypropylene film and aluminum electrodes, the film of which has a space factor of about 5%, which is through particle bombardment of the film and then stretching into a predetermined geometric pattern of Structuring has been preserved. '