FR2684332A1 - Process for manufacturing a film, based on an olefin polymer, and film thus obtained - Google Patents

Abstract

The invention relates to a process for manufacturing a film, based on an olefin polymer, having at least one rough face obtained by producing surface projections, characterised in that it comprises the operations consisting in coextruding, over a first layer (B) constituted by an olefin polymer, a second layer (A) constituted by the mixture of an olefin polymer (P) with a powder (D) having a chosen particle-size distribution, constituted by a solid material having a melting point (PF) above those of the other constituents of the first and second layers (A) and (B), and in subsequently drawing the film at a temperature below the said melting point (PF). Application to the production of dielectric films for capacitors.

Description

 The invention relates to the field of rough bi-stretched plastic films.

 The invention relates in particular to a method for obtaining a rough surface on a film essentially consisting of an olefinic polymer, advantageously polypropylene.

 The method according to the invention is applied in particular, but not exclusively, to the production of dielectric films for capacitors.

 Polypropylene films are increasingly used in industry for the manufacture of capacitors. In fact, polypropylene has excellent dielectric qualities and its low density allows the creation of compact consensors.

 A conventional technology for producing capacitors uses polypropylene films metallized on one of the faces. This configuration gives the capacitors interesting self-healing properties.

 It is also known to produce capacitors by a stack of conductive layers separated by a dielectric film.

 The dielectric film, metallized or not, is advantageously impregnated with an insulating oil to allow the capacitor to operate at high voltages.

 The impregnation of the film is typically done after it has been wound or assembled into sheets, by vacuum injection of a liquid or gaseous impregnator between the turns or the sheets.

 It is advisable to preserve a sufficient free space between the turns or the sheets so that the impregnator can progress on the surface of the films, which is obtained by using rough films.

 The roughness of the surface of the film used is also a very important parameter to control, because good control of the appearance of the surface will notably depend on the resistance of the metallization, or even the dielectric strength of the capacitor.

 It has for example been shown that a slight roughness makes it possible to improve the adhesion of metallic coatings while excessively pronounced surface irregularities lead to detachment of the metallization at the top of the roughness ridges.

 It has been proposed to obtain a rough surface by adding to the polypropylene nucleating agents which generate in the mass of the film different crystallization zones, also called phase B. By fusion of the ss phase of the polypropylene during the long stretching, obtains a film whose surface is studded with microscopic craters with a substantially elliptical outline. This type of surface appearance is described as elliptical roughness.

 However, during the impregnation with an insulating oil of a film having an elliptical roughness, the path of the impregnator on the surface of the film is hampered by the succession of ridges and hollows due to the craters.

 Furthermore, the known method for producing this type of roughness is difficult to control.

 Another method has also been proposed for obtaining a rough film which consists in coextruding onto homopolymer polypropylene a layer of a copolymer with a high ethylene content, then in stretching the film obtained, successively along the length and then across. The surface takes on a pleated appearance due to the entanglement of the microroughness thus formed and we speak of satin roughness. During the impregnation, the insulating oil must bypass the microroughness in its path which takes place perpendicular to the direction of the long stretching, therefore to the orientation thereof.

 For these two types of roughness obtained according to methods of the prior art, it can be seen that the dispersion of the dimension of the surface irregularities is significant. This dispersion is troublesome because it does not allow the impregnation to be properly controlled and, moreover, when the film is metallized on the rough face, this dispersion causes fluctuations in the thickness of dielectric between two metallized layers and a risk of breakdown in thinner regions.

 It is also advantageous to have, for applications relating to the lamination of objects or the production of filtration units, films of well defined roughness.

 The object of the invention is to remedy the drawbacks of rough films according to the prior art.

 The invention achieves this aim by a process for manufacturing a film, based on olefin polymer, having at least one rough face, the process being characterized in that it comprises the operations consisting in coextruding on a first layer constituted by an olefin polymer a second layer constituted by the mixture of an olefin polymer with a powder of selected particle size and constituted by a solid material with a melting point higher than those of the other constituents of the first and second layers, then in drawing the film at a chosen temperature lower than or equal to said melting point.

 According to an advantageous characteristic of the invention, the first layer consists of polypropylene homopolymer of isotacticity between 96 and 99.6, á.

 According to another advantageous characteristic of the invention, the mixture constituting the second layer comprises between 50 and 95% by weight of an olefin polymer. This olefin polymer is advantageously polypropylene homopolymer of isotacticity between 96 and 99.6, O or polypropylene random copolymer.

 We prefer a powder particle size between 1 and 5 µm.

 The powder advantageously comprises at least one of the following four polymers: polymethylpentene, polyester, polyamide PA6, polyamide PA6,6, or other polymers preferably compatible with polypropylene.

 The powder optionally comprises a non-polymeric mineral or organic substance.

 The invention also relates to a film based on olefin polymer having at least one rough surface, characterized in that it comprises at least a first layer constituted by an olefin polymer and a second rough layer constituted by a mixture of '' an olefin polymer with a powder of selected particle size, constituted by a solid material with a melting point higher than those of the other constituents of the first and second layers.

 Such a film is advantageously used to produce dielectric films for capacitors.

 Such a film is also used to make filter membranes.

 In this case, the powder used advantageously comprises an active principle improving filtration.

 A rough film according to the invention is also advantageously applied to the production of films for the lamination of objects.

Other characteristics and advantages of the invention will appear on reading the non-limiting description of a process and of the film obtained by the process according to the invention, applied to the production of a dielectric film for capacitors, and in view of the attached drawing in which: - Figure 1 shows in top view the appearance of the surface of a film
conventional with an elliptical roughness, - Figure 2 shows in top view the appearance of the surface of a film
conventional with a satin roughness, - Figure 3 is a schematic section, along the thickness, of a film
obtained by the method according to the invention and before stretching, - Figure 4 is a schematic section, along the thickness, of a film
obtained by the process according to the invention after drawing, - Figure 5 is a top view of a film obtained by the process according to
the invention to illustrate the path of the impregnant, - Figure 6 schematically illustrates the free space left in a volume
elementary on the surface of a conventional film having roughness
satin, - Figure 7 schematically illustrates the free space left in a volume
elementary on the surface of a film according to the invention, - Figure 8 is a schematic sectional view along the thickness of a film
rough to ellipses according to the prior art.

- Figure 9 is a sectional view along the thickness of a rough film to
pins according to the invention,
FIG. 1 shows the appearance of a rough surface known as an ellipse. It will be noted on examining this figure that the surface is studded with a large number of craters 10, the crest 15 of which defines a substantially elliptical closed contour.

 The path of the impregnator on such a rough surface is a succession of ridges and hollows which impede its progress.

 Figure 2 illustrates the appearance of a rough satin surface. We discern in this figure a tangle of microroughness 20, oriented in the direction of long stretching, giving the surface a pleated appearance. The direction of travel of the impregnator is substantially perpendicular to the direction of orientation of the microroughness 20 and this type of roughness is a brake on impregnation during the manufacture of the capacitors.

 FIG. 3 represents, before drawing, in diagrammatic section along the thickness, a film capable of being obtained by the method according to the invention.

 The film referenced as a whole by the index 100 was advantageously obtained by coextrusion on a first support layer B advantageously constituted by homopolymer polypropylene, a second layer A constituted by the mixture of an olefin polymer P with a powder D of selected particle size, consisting of a polymer or mixture of polymers with a melting point PF greater than those of homopolymer polypropylene and of olefin polymer P, then by drawing the film at a temperature below PF, typically at a cross-draw rate between 4 and 8 and at a long draw rate between 3 and 7.

 The first support layer B is advantageously constituted by polypropylene homopolymer with a drisotacticity of between 96 and 99.6%.

 According to an advantageous alternative embodiment of the present invention, the layer A consists of a mixture of 50 to 95% by weight of a polypropylene homopolymer with a very high rate of isotacticity, said rate advantageously being between 96 and 99.6 %, with a polymethylpentene powder D whose melting temperature 220 "C is higher than that of homopolymer polypropylene which is 1 70 C.

 Different particle sizes of powders can be used, but powders with an average grain size of between 1 and 5 μm are preferred.

 During the hot stretching of the film 100, care will be taken to remain below the melting temperature of the powder D, so that the grains of the powder D are not melted during the stretching.

 One can conceive the use of a powder D constituted by a mineral substance but one will prefer a powder D constituted by a polymer or mixture of polymers, whose chemical structure is similar to that of the other constituents of layers A and B.

 Not exhaustively, we will mention polyester with a melting point equal to PF = 230 "C, polyamides PA6 (PF = 255" C) or PA6.6 (PF = 218 "C).

 Referring to Figure 4 which shows in schematic section along the thickness the film 100 after stretching, it is noted that the grains of the powder D form spikes on the surface of the film.

 The number of peaks thus obtained per unit of PC surface area is easily deduced for those skilled in the art from the concentration of powder in the initial mixture A.

On the other hand, the average of the maximum hollow-peak deviations or average roughness depth used to evaluate it, conventionally designated by the symbol R z in standard DIN 4768, is obtained here directly by the diameter of the grains constituting powder . This highlights a very important characteristic of the invention, namely the possibility that the method according to the invention gives of easily independently controlling the parameters PC and R z
Figure 8 shows a rough film with ellipses in section along the thickness, corresponding to a given space factor and R z.

FIG. 9 shows in section a rough film with spikes obtained by the method according to the invention with the same space factor and the same R z
Those skilled in the art will readily understand that the films according to the invention make it possible to replace in their applications the films of the prior art while retaining, if necessary, their characteristics of space factor and Rz, while exhibiting an arithmetic roughness. Different ra and a volume offered to the improved impregnator.

 The path of the impregnator on the surface of a rough film obtained by the process according to the invention is illustrated in FIG. 5.

 It is easy to guess on examining this figure that the impregnator progresses more rapidly on the surface of this film than in the case of elliptical roughness or satin roughness according to the prior art. It follows that for an equivalent quality of impregnation of the capacitor, the space factor used in the manufacture of the latter can be reduced.

 We recall that we classically designate by space factor, the ratio 100 x (em - ep) / ep, where em denotes the micrometric thickness and where ep is the average thickness calculated by weighing a chosen surface of the film of known density.

 A comparison carried out in the following description between a film with a satin roughness and a rough film according to the invention shows that the latter film advantageously makes it possible to produce a capacitor resistant to higher electric fields, when a metallized electrode is deposited on the rough face.

 To simplify, we will consider that the rough parts are volumes associated with an elementary mesh. The dotted line in FIG. 6 shows a mesh 200 representative of the satin roughness according to the prior art. A microroughness 20 is modeled in this figure using a parallelepiped volume, of height R z, and the length L of which defines the length of the mesh. The width of the mesh I corresponds to the distance between two microroughness 20, increased by the width x of one of them.

On a typical film of the prior art, we measure on average
x = 6 pm, = = 20 um,
L = 20 pm,
R z = 3 pm, and the free space EL corresponds to the volume of the mesh thus defined, subtracted from the volume of the microroughness, which gives for the film considered, a free space of 840 pm3.

 Figure 7 refers to a mesh cut on the surface of a rough film according to the invention.

 For an equal mesh surface, it is easy to understand, on a comparative examination of FIGS. 6 and 7, that the same free volume EL can be obtained with a pin of lesser height Rz than the microroughness 20.

 Thus, with a spike whose base is a square of 4 μm on a side, and considering a mesh surface identical to that of the previous example, it is sufficient that the spike is 2.18 μm in height.

 The rough film according to the invention therefore advantageously makes it possible to lower the height of the roughness ridges while leaving an equivalent impregnation volume available.

 On the other hand, we have shown that the impregnator, the path of which is little impeded by the surface dots, spreads better on the surface of the film. For an equivalent quality of impregnation of the rough films, it is therefore possible to be satisfied with a lower free volume if a rough film with pins is used.

 For example, the prior art typically uses a space factor of 15.6.

 The use of a rough film according to the invention makes it possible to lower this factor to 12% and one can thus further reduce the height R of the pins.

 Obtaining, by the process according to the invention, films of lower R z is particularly advantageous since it makes it possible to reduce the variations in thickness of the dielectric, therefore to improve the dielectric rigidity of the capacitor which can then operate under a higher voltage, when a metallized electrode is deposited on the rough face.

 The invention also makes it possible to obtain films for filter membranes applied to the filtration of gases or liquids. It is then advantageous for the powder to contain an active principle which improves filtration, for example activated carbon.

 The invention also makes it possible to produce films for the lamination of objects which are particularly attractive to the consumer thanks to the good control of the surface appearance allowed by the method according to the invention.

Claims (18)

 1. Method for manufacturing a film based on olefin polymer, having at least one rough face, characterized in that it comprises the operations consisting in coextruding on a first layer (B) constituted by an olefin polymer , a second layer (A) constituted by the mixture of an olefin polymer (P) with a powder (D) of selected particle size, constituted by a solid material with a melting point (PF) greater than that of the other constituents of the first and second layers (A) and (B), and then proceed to stretch the film at a temperature less than or equal to said melting point (PF).  2. Method according to claim 1, characterized in that the first layer (B) comprises polypropylene homopolymer of isotacticity between 96 and 99.6  3. Method according to claim 1 or 2, characterized in that the mixture constituting the second layer (A) comprises between 50% and 95, ó by weight of an olefin polymer.  4. Method according to one of claims 1 to 3, characterized in that the dtolefin polymer (P) of the second layer (A) comprises polypropylene homopolymer of isotacticity between 96% and 99.6 96.  5. Method according to claims 1 to 3, characterized in that the olefin polymer (P) of the second layer (A) comprises polypropylene random copolymer.  6. Method according to one of claims 1 to 5, characterized in that the powder (D) has a particle size between 1 and 5 pm.  7. Method according to one of claims 1 to 6, characterized in that the powder (D) comprises at least one polymer.  8. Method according to one of claims 1 to 7, characterized in that the powder (D) comprises polymethylpentene.  9. Method according to one of claims 1 to 8, characterized in that the powder (D) comprises polyester.  10. Method according to one of claims 1 to 9, characterized in that the powder (D) comprises polyamide PA6.  11. Method according to one of claims 1 to 10, characterized in that the powder (D) comprises polyamide PA6,6.  12. Method according to one of claims 1 to 11, characterized in that the powder (D) comprises a mineral substance.  13. Method according to one of claims 1 to 12, characterized in that the powder (D) comprises an organic non-polymeric substance.  14. Film based on olefin polymer having at least one rough face, characterized in that it comprises at least a first layer (B) constituted by an olefin polymer and a second rough layer (A) constituted by a mixture of an olefin polymer (P) with a powder (D) of selected particle size, constituted by a solid material with a melting point higher than those of the other constituents of the first and second layers.  15. Film according to claim 14, characterized in that it is applied to the production of a dielectric film for capacitors.  16. Film according to claim 14, characterized in that it is used to produce filter membranes.  17. Film according to claim 16, characterized in that the powder (D) comprises an active principle improving filtration.  18. Film according to claim 14, characterized in that it is applied to the lamination of objects.