MXPA96004964A

MXPA96004964A - Metallized poliolef sheet

Info

Publication number: MXPA96004964A
Application number: MXPA/A/1996/004964A
Authority: MX
Inventors: Kochem Karlheinz; Hatke Wilfried; Grosse Kreul Theo
Original assignee: Ticona Gmbh*
Priority date: 1995-10-20
Filing date: 1996-10-18
Publication date: 1997-06-01

Abstract

The present invention relates to a single or multi-layer polyolefin sheet, metallised on one or both sides, comprising at least one outer layer essentially of a cycloolefin polymer, which before metallization had not been subjected to any Procedure to increase the surface tension

Description

SHEET. METALLISED.J3E POLYOLEPHINE DESCRIPTIVE MEMORY The invention concerns metallized sheets, especially metallized on both sides, which are excellently suited as a dielectric in condensers. The sheets according to the invention are sheets of polyolefins, and in particular those made of polymers of cycloolefins, which, surprisingly, before metallization, must not undergo any process - in other cases usual for polyolefins - in order to increase surface tension or surface energy (such as corona treatment). For the use of polymeric films as a dielectric in capacitors, the dielectric loss factor d, the thermal stability, ie the stability of the mechanical (eg contraction) and electrical properties of the sheet at high temperature, is of great importance, as well as metallizability. The low dielectric loss factors are of particular interest in the case of applications of alternating current at high frequencies, since with low values of tan d the power of electrical losses is also low. A high power of electrical losses - and therefore a high value of tan d - means a heating, so that finally the thermal stability of the material of the sheet can be exceeded and the capacitor can be damaged or destroyed. An ideal dielectric for capacitors has correspondingly a low dielectric loss factor simultaneously with a high thermal stability. In relation to metallisability, it is known that polyester sheets are easier to metallize than polyolefin sheets, since these, before metallization, must be subjected to a surface treatment in order to achieve an adhesion of the metal on the metal. the sheet. The metallization of thin films for use in condensers is the subject of intense research efforts. Among the poly (ethylene terephthalate) [PET], currently used predominantly as a dielectric, and polypropylene [PP] there is a fundamental difference in this respect. Because of the polar structure of the polymer, the PET has a critical surface tension of approximately 43 mN / m, which is sufficient to guarantee adhesion to the metal, e.g. ex. aluminum. The critical surface tension of the polyolefin sheets is located, on the contrary, with values of 30 to 33 mN / m, in a margin that is not sufficient to guarantee an adherence to the metallic layer applied from the vapor phase. For this reason, the surfaces of the polyolefin sheets must be treated with different procedures, in order to increase the surface tension, and achieve wettability, tack and metallizability characteristics. The procedure that is used with the highest frequency is the treatment with an alternating current voltage at high frequency (10-60 kHz, 10-20 kV), this is the so-called corona treatment. In this case, the surface tension can be increased up to 50 mN / m. In the case of sheets of polyolefins, especially of biaxially oriented sheets based on polypropylene, surface tensions of 36 to 42 mN / m are usually adjusted by means of a corona discharge. The disadvantages of a corona treatment consist, however, in that p. ex. the surface tension is time-dependent and in the course of the treatment low molecular weight fragments of the polymer chain are formed, which can lead to a weakening of the bond between the polymeric surface and a metallic layer applied from the vapor phase . For economic reasons, it is desirable to form a capacitor based on a sheet metalized on both sides and another non-metallized sheet. As described in the patent document of the United States of America US-A-3,900,775, this is made possible for example by the use of a sheet of poly (ethylene terephthalate) metallized on both sides and a sheet of non-metallized polypropylene. It is a disadvantage of this structure, however, the value of tan d of poly (ethylene terephthalate), greatly increased compared to that of polypropylene. For reasons of the best dielectric loss factor, polyolefin sheets are preferred to polyester sheets in AC applications. The economical manufacture of polypropylene sheets coated from the vapor phase (metallized) on both sides, however, is essentially more difficult and is not currently carried out on an industrial scale. One problem is the corona treatment, which must be carried out on both sides of the sheet before metallization. This leads, due to the electrostatic charges applied in such a case, to a sticking (caking) of the sheet on the coil. By means of the adhesion forces that appear when unwinding, loads are generated in turn, which make subsequent subsequent coating with the metal from the vapor phase impossible. This problem can be overcome, according to the German patent document DE-A-28.02.769, by carrying out a discharge of the sheet before the uniform covering from the vapor phase. However, this also constitutes an additional procedural step and thus an additional source of errors, and is therefore uneconomical. There continues to be a need for a metallizable polyolefin sheet, preferably metallized on both sides, in which the disadvantages of the prior art and having a low dielectric loss factor and high thermal stability are avoided. The object of the present invention was also to provide a method for producing a metallised polyolefin sheet, preferably on both sides, which avoids the disadvantages of the prior art, especially the additional step of the process intended to increase the surface tension. Surprisingly, it was finally discovered that of the large number of polyolefins, cycloolefin polymers - contrary to all expectations - can be metallized without any treatment that increases surface tension. Correspondingly, the problem posed by the mission is solved by a polyolefin sheet of one or several layers, metallized on one or both sides, consisting of at least one of the outermost layers of the non-metallized polyolefin sheet, in essence, of a cycloolefin polymer, which before the metallization had not been subjected to any procedure to increase the surface tension. The expression "metallized" on one or both sides' "means that the sheet carries a metal layer on one or both of the surfaces. The expression "of one or several layers" means that the non-metallized sheet is either a monolamina, that is to say it consists only of one layer, or is constituted by multiple layers, and correspondingly it can be constituted by two, three, four, five or still more layers. It is essential according to the invention in this case that the monolimine, or at least one outermost layer of the multilayer film, consists essentially of a cycloolefin polymer. The term "essentially consists of a cycloolefin polymer" means that the monolimine, or at least one outermost layer of the multilayer film, consists of at least 90-100% by weight, preferably in at least 95-100% by weight, especially at least 98-99% by weight (based on the weight of the monolamina, or the outermost layer of the multilayer sheet) of a cycloolefin polymer. Optionally, the single-layer sheet, or the outermost layer, may additionally contain the additives that are usually employed in the manufacture of sheets. The expression "not submitted before metallization to any procedure to increase the surface tension" means that the sheet, after its usual manufacturing process, which usually includes extrusion, stretching and thermofixing, is not subjected to any additional treatment, which results in the increase of surface tension. For this concept, the usual procedures, such as crown or flame treatments, must be understood. It is essential according to the invention that the metallization can be carried out without the sheet being previously subjected to one such method. Cycloolefin polymers are materials that are distinguished by high hot shape stabilities, high modulus of elasticity, poor water absorption and good dielectric properties. The German Democratic Republic patent document DD-A-224,538 describes the manufacture of sheets based on copolymers of norbornene and ethylene by a sheet casting process. The manufacture of cycloolefin polymer films by melt extrusion is described in European Patent Documents EP-A-0,384.69, EP-A-0,610,814, EP-A-0,610,815 and EP-A-0,610. .816. The improvement of the mechanical properties of the sheets by monoaxial or biaxial stretching is also described in these documents. Documents DD-241.971 and DD-224.538 state that the sheets based on cycloolefins are distinguished by low dielectric loss factors (tan d). The values indicated for tan d, which reach up to 1.2-10 ~ 5, are located below the values found for the polymeric materials, which are used as dielectrics in capacitors according to the current state of the art. Only polystyrene has similarly low values. As further explained in document DD-241.971, the low values for tan d are of interest above all for applications of alternating current with high frequencies, since in these cases, due to the power of electrical losses in the sheet, it is possible to reach when warming up As a consequence of the combination of a high thermal stability (stability of mechanical and electrical properties at high temperatures) and a low value of tan d, cycloolefin polymers are optimally suitable as capacitor sheets, which can be used at high temperatures. temperatures and high frequencies. Polystyrene based sheets do not offer this advantage, since they begin to soften already at temperatures located around 90 ° C. The combination of the aforementioned properties makes the cycloolefin polymers suitable materials for use as a dielectric for alternating current applications with high frequencies. In addition, these materials present a good record of the electrical properties up to temperatures that are located slightly below the vitreous stages of the polymers. Therefore, cycloolefin polymers are especially suitable for use in capacitors, which are subjected to an alternating electric field at high frequencies and high temperatures. The cycloolefin polymers can be processed very well to give biaxially oriented sheets with good mechanical properties. The oriented films have modules in the range of 2.7 to 4.0 GPa, breaking strengths of 80 to 150 MPa and elongations at break in the range of 5 to 100%. The surface tension of these sheets is in the range of 30 to 31 mN / m and is therefore typical for polyolefin sheets. Also typical is the circumstance that the polar portion in the surface tension is very small. Therefore, it was expected that the sheets based on cycloolefin polymers - like other sheets based on polyolefins - could not be metallized without any prior treatment to achieve an increase in surface tension. Accordingly, it was all the more surprising that the cycloolefin polymer-based sheets could be metallized without any prior treatment with low surface tension. This behavior is also largely independent of the vitreous stage of the cycloolefin polymer. This invention is especially surprising, since in other researches it was discovered that, in order to be able to print the sheet, nevertheless a corona treatment is necessary. As with other polyolefin sheets, it increases the surface tension of the treated sheet. After corona treatment, values for surface tension are obtained that are typical for polyolefins. The cycloolefin polymers suitable for the invention are polymers containing from 0.1 to 100% by weight, preferably from 0.1 to 99% by weight, based on the total mass of the cycloolefin polymer, of polymerized units of at least a cyclic cycloolefin of formulas I, II, III, IV, V or VI, R wherein R1, R2, R3, R *, R5, Rs, R7 and R8 are the same or different and denote a hydrogen atom or a linear or branched hydrocarbon radical, a C6-C18 aryl radical, an alkylene-aryl radical C7-C20 or a cyclic C3-C20 alkyl radical or an acyclic C2-C20 alkyl radical, or two or more radicals R1 to R8 are cyclically bound, and the same radicals in the different formulas can have a different meaning, from 0 to 45% by weight, based on the total mass of the cycloolefin polymer, of polymerized units of at least one monocyclic olefin of the formula VII, where n. is a number from 2 to 10, from 0 to 99% by weight, based on the total mass of the cycloolefin polymer, of polymerized units of an acyclic olefin of the formula VIII, wherein R9, R10, R11, R12 are the same or different and mean a hydrogen atom or a C1-C10 hydrocarbon radical, for example an alkyl radical or a C6-C14 aryl radical. Also suitable are cycloolefin polymers which have been obtained by ring-opening polymerization of at least one of the monomers having the formulas I to VI, and subsequent hydrogenation of the products obtained. Preferably, the cycloolefin polymers contain polymerized units of at least one polycyclic olefin, especially of formula I or III, and of an acyclic olefin of formula VIII, preferably having from 2 to 20 C atoms, especially ethylene. Preferred are cycloolefin polymers, which contain polymerized units of polycyclic olefins with a basic norbornene structure, particularly preferably norbornene or tetracyclododecene. Also preferred are cycloolefin polymers, which contain polymerized units of acyclic olefins, such as o-olefins, particularly preferably ethylene. Copolymers of norbornene and ethylene as well as those of tetracyclododecene and ethylene are especially preferred. The proportion of polymerized units of acyclic olefins of the formula VIII is from 0 to 99% by weight, preferably from 5 to 80% by weight, particularly preferably from 10 to 60% by weight, based on the total weight of the polymer of cycloolefin. The cycloolefin polymers generally have glass temperatures between -20 ° C and 400 ° C, preferably between 50 ° C and 200 ° C. The viscosity index (in decalin, at 135 ° C, DIN 53728) is generally between 0.1 and 200 ml / g, preferably between 50 and 150 ml / g.

The preparation of the cycloolefin polymers is carried out by a heterogeneous or homogeneous catalysis with organic metal compounds. Catalyst systems, based on mixed catalysts based on titanium or vanadium compounds, are described in DD 109,224, DD 237,070 and EP-A-0,156,464. EP-A-0,283,164, EP-A-0,407,870, EP-A-0,485,893 and EP-A-0,503,422 describe the preparation of cycloolefin polymers with catalysts based on soluble complexes of metallocenes. The methods of preparing cycloolefin polymers described in these documents are expressly mentioned herein. The cycloolefin polymers used according to the invention can contain the usual additives in the case of the manufacture of sheets, such as fine inert particles, which improve the slip and roll behavior. Such particles, which may be contained in an amount of 0 to 1%, are for example: SiO2, Al203, silicates with an SiO2 content of at least 30% by weight, amorphous and crystalline clay minerals, aluminosilicates, oxides of Mg, Zn, Zr and Ti, sulfates of Ca, Mg and Ba, phosphates of Li, Na and Ca (including the onohi-drógeno-salts and the dihydrogen-salts), benzoates of Li, Na and K, terephthalates of Ca , Ba, Zn and Mn, titanates of Mg, Ca, Ba, Zn, Cd, Pb, Sr, Mn, Fe, Co and Ni, chromates of Ba and Pb, carbon (eg carbon black or graphite), glass (glass powder and glass spheres), carbonates of Ca and Mg, fluorspar, sulphides of Zn and Mo, organic polymeric substances such as poly (tetrafluoroethylene) - polyethylene, talcum, lithium fluoride and the salts of Ca, Ba, Zn and Mn of organic acids. The sheet may also contain appropriate additives such as p. ex. stabilizers, neutralizing agents, lubricants and slip agents or antioxidants. In principle, additives that are used for polyolefins such as polyethylene or polypropylene, are also suitable for cycloolefin polymer sheets. As UV stabilizers, for example, absorbers, such as hydroxyphenyl-benzotriazoles, hydroxy-benzophenones, formamidine or benzylidene camphor, suffocation agents such as cinnamic acid esters or nickel chelates, radical scavengers such as sterically hindered phenols, can be used. peroxides decomposing agents such as complexes with nickel or zinc of sulfur-containing compounds or photostabilizers of the HALS type, as well as their mixtures. Lubricating or sliding agents which may be used are, for example: fatty acids as well as their esters, amides and salts, silicones or waxes such as PP or PE waxes. As antioxidants, for example, radical scavengers such as substituted phenols and aromatic amines and / or peroxide decomposition agents such as phosphites, phosphates and thio compounds can be added.

The manufacture of the cycloolefin sheets is carried out in a conventional manner known to a person skilled in the art, for example by casting films from a solution, extruding from a melt with wide-slit nozzles and subsequent monoaxial stretching. or biaxial, extrusion from a melt with annular nozzles and subsequent stretching by an air stream (sheet blown). Extrusion is preferred through wide-slit nozzles with sub-orientation biaxial orientation and thermofixation. In this case, the polymer is heated and melted within an extruder and extruded through a wide-slit nozzle onto a cooling roll; usually, the preliminary film thus obtained is removed after the cooling roller and then biaxially stretched, that is to say most of the time firstly in the machine direction and then in the transverse direction. This biaxial orientation is usually followed by a thermofixing, after which the sheet is rolled up. In this process, the sheet can be extruded either in the form of a monolmine or also in the form of a multilayer sheet, then comprising at least one outermost layer, essentially, of cycloolefin polymers, as described above. . The other layers of the multilayer film may also consist, for example, of cycloolefin polymers - optionally other than that used in the cover layer - but also of other polymers, especially polyolefins such as polyethylene or polypropylene. In this way sheets can be manufactured with a thickness spectrum of 2 to 50 μm, preferably 3 to 30 μm. The sheet thus manufactured can then be provided with a metal layer, without prior measures to increase the surface energy, that is, for example without prior corona treatment. Suitable metals are, for example, aluminum, zinc, mixtures of zinc and aluminum, or silver. Preferably, aluminum and zinc are used, as well as mixtures and / or alloys thereof. The metallization is carried out in a conventional manner, which is customary for a person skilled in the art, for example by treating the sheet in the vapor phase and in vacuum. The advantage according to the invention is to be seen in the fact that the polymer sheets of cycloolefins can be metallized not only on one side, but also on both sides from now on. From the metallized sheets based on cycloolefin polymers, capacitors can be manufactured according to conventional methods. Next, the invention is explained in more detail with the aid of Examples.

Example 1 Preparation of a norbornene and ethylene polymer COC-A A 1.5 dm3 reactor was charged with 1 liter of a gasoline fraction (90-110 ° C boiling range) and 20 ml of a toluene solution of methylaluminoxane ( 10.1% by weight of a methylaluminoxane with a molar mass of 1300 g / mol according to cryoscopic determination) and stirred at 70 ° C for approximately 30 min, in order to eliminate any impurities present. After the solution was discharged, the reactor was charged with 480 cm 3 of an 85 weight percent solution of norbornene in toluene. By applying ethylene under pressure at several times (6 bar G (gauge)) the solution was saturated with ethylene and then 10 cm3 of the toluene solution of methylaluminoxane was poured into the reactor and stirred for 5 min at 70 ° C. . After having previously activated for 15 minutes, a solution of 5.43 mg of isopropylene- (1-cyclopentadienyl) - (1-indenyl) -zirconium dichloride in 10 cm3 of the toluene solution of methylaluminoxane was added. While stirring (at 750 revolutions per minute [rpm]), it was polymerized for 30 min at 70 ° C, the ethylene pressure being maintained by metering at 6 bar G. The homogenous solution of the reaction was discharged into a container and mixed with approximately 1 ml of water. Then, the solution was mixed with a filtering aid and filtered through a Buchner funnel under pressure. This solution is poured rapidly into 5 dm3 of acetone, stirred for 10 min and filtered through a Buchner funnel under pressure. The solid material obtained was washed with acetone. The filtered polymer was again dried for 15 hours at 80 ° C and at a pressure of 0.2 bar. 89.1 g of a colorless polymer were obtained. For the determination of the viscosity index, 0.1 g of the polymer was dissolved in 100 ml of decalin. The solution was measured in a capillary viscometer at 135 ° C. The viscosity index was 56.5 dl / g. The vitreous temperatures were determined with a DSC calorimeter from Perkin-Elmer. The respective vitreous temperature was determined with a heating rate of 20 ° C / min from the second heating curve, and was 175 ° C. The norbornene content was determined as 58% by moles by nuclear magnetic resonance (NMR) -13C spectroscopy. The molecular weight of the polymer was determined by gel permeation chromatography at 135 ° C. Patterns of polyethylene were used as standards. For the polymer, the following values were found: Mn (number average of the molar mass): 21,500 M "(weighted average of the molar mass): 45,000 Mw / Mn: 2.1 Preparation of a norbornene and ethylene polymer (COC-B) The polymerization was carried out as previously described for COC-A. However, isopropylene-bis (1-indenyl) -zirconium dichloride was used as the metallocene-based catalyst and the polymerization was carried out at a pressure of 20 bar G. A statistical copolymer was prepared, having a content in norbornene of 40% by moles, determined by 13 C-NMR, a vitreous stage of 75 ° C (measured by DSC) and a viscosity index of 120 ml / g (in decalin, at 135 ° C, 0.1 g / dl).

Preparation of a norbornene and ethylene polymer (COC-C) The polymerization was carried out as described above for COC-A. However, isopropylene-bis (1-indenyl) -zirconium dichloride was used as the metallocene-based catalyst and the polymerization was carried out at a pressure of 10 bar G. A statistical copolymer was prepared, which possessed a norbornene content of 53% by moles, determined by 13 C-NMR, a vitreous stage of 140 ° C (measured by DSC) and a viscosity index of 60 ml / g (in decalin, at 135 ° C, 0.1 g / dl).

Fabrication of a sheet (from COC-B) The COC-B was extruded at a temperature of 200 ° C to give a sheet with a thickness of 400 μm and a width of 250 mm. From this sheet, pieces of 200-200 mm2 were cut out and stretched simultaneously at 100 ° C longitudinally and transversally by a factor of 3.0 in a facility for sheet stretching (Karo III of the entity Brückner, Siegsdorf). The sheet thus obtained had the following properties: Thickness: 45 μm Module-E: 2, 9 GPa Breaking strength: 80 MPa Elongation at break: 20% Permeability to water vapor (23 ° C, relative humidity 85%): 0.6 g-40 μm / m2-d Surface tension: 30 mN Manufacture of a sheet (from COC-B) The COC-C was extruded at a temperature of 240 ° C to give a sheet with a thickness of 300 μm and a width of 250 mm. From this sheet, pieces of 200-200 mm2 were cut out and stretched simultaneously at 155 ° C longitudinally and transversally by a factor of 3.0 in a plant for the stretching of sheets (Karo III of the Brückner entity, Siegsdorf). The sheet thus obtained had the following properties: Thickness: 35 μm Module-E: 3.2 GPa Breaking strength: 90 MPa Elongation at break: 50% Permeability to water vapor (23 ° C, relative humidity 85%) : 1 g-40 μm / m2 • d Surface tension: 29 mN / m Metalization of the sheet The sheets obtained from the COCs B and C were cut into pieces with the DIN-A4 size and covered with aluminum from the vapor phase on one side and on both sides without any other surface treatment (metallization conditions) : pressure 10 ~ 5 mbar, time 11 min). The layer thickness of the aluminum applied from the vapor phase was approximately 40 nm. The adhesion of the aluminum layer on the sheets was tested using the ASTM D 3359 standard, but without cutting in the reticle, by gluing and quickly starting an adhesive strip (Tesafilm TP 104). In the cases of both sheets, no aluminum could be detached from both sides treated in the vapor phase. The permeability to water vapor (23 ° C, relative humidity 85 ° C) of the sheets treated in the vapor phase (manufactured from COC-B) was then, in the state treated in vapor phase on one side, of 0.5 g-40 μm / m2-d, and, in the state treated in vapor phase on both sides, 0.4 g-40 μm / m2-d.

Comparative Example A biaxially oriented, non-corona treated sheet, based on a very isotactic polypropylene (Trespaphan PM A 10, manufacturer Hoechst, thickness 10 μm) was cut into a model of DIN A4 size analogous to Example 1 and was covered with aluminum from the vapor phase on one side without any other surface treatment (the experimental conditions were analogous to those of Example 1). The surface tension on the face to be metallized was 31 mN / m before metallization. The thickness of the aluminum layer applied from the vapor phase was approximately 40 nm. The adhesion of the aluminum layer was tested as in Example 1. The aluminum could be detached from the surface of the sheet completely together with the adhesive tape.

Claims

NOVELTY OF THE INVENTION CLAIMS 1.- Single or multi-layer polyolefin sheet, metallised on one or both sides, consisting of at least one outer layer essentially of a cycloolefin polymer, which before metallization had not been subjected to no procedure to increase surface tension.
2. Metallized sheet based on a cycloolefin polymer according to claim 1, characterized in that the cycloolefin polymer is a polymer, containing 0.1 to 100% by weight, based on the total mass of the cycloolefin polymer, polymerized units of at least one cyclic cycloolefin of formulas I, II, III, IV, V or VI, wherein R1, R2, R3, R4, R5, R6, R7 and R8 are the same or different and mean a hydrogen atom or a C1-C30 hydrocarbon radical, or two or more radicals R1 to R8 are attached cyclic, with the same radicals in the different formulas having a different meaning, from 0 to 45% by weight, based on the total mass of the cycloolefin polymer, of polymerized units of at least one monocyclic olefin of the formula VII, HC = CH \ / (Vil) (CH2) ft where n is a number from 2 to 10, from 0 to 99% by weight, based on the total mass of the cycloolefin polymer, of polymerized units of an acyclic olefin of the formula VIII , wherein R9, R10 # R11, R12 are the same or different and mean a hydrogen atom or a hydrocarbon radical C? _ -C10.
3. - Metallized sheet based on a cycloolefin polymer according to claim 1 or 2, characterized in that the cycloolefin polymer is a copolymer of norbornene and ethylene.
4. Metallized sheet based on a cycloolefin polymer according to one or more of claims 1 to 3, the metallized sheet being characterized in that it has been metallised with aluminum, zinc, silver or mixtures or alloys based on two or more of these metals
5. Metallized sheet based on a cycloolefin polymer according to one or more of claims 1 to 4, the sheet being characterized in that it is a monollamine.
6. Metallized sheet based on a cycloolefin polymer according to one or more of claims 1 to 4, the sheet being characterized in that it is a sheet of 2, 3 or more layers.
7. Metallized sheet based on a cycloolefin polymer according to one or more of claims 1 to 6, the sheet being characterized in that it is metallized on both sides.
8. Metallized sheet based on a cycloolefin polymer according to one or more of claims 1 to 7, the sheet being characterized in that it has a thickness of 2 to 50 μm, preferably 3 to 30 μm.
9. Metallized sheet based on a cycloolefin polymer according to one or more of claims 1 to 8, the sheet being characterized in that it has been biaxially stretched.
10. Use of a metallized sheet based on a cycloolefin polymer according to one or more of claims 1 to 9 for the manufacture of capacitors.
11. Condenser, which contains a metallized sheet based on a cycloolefin polymer, according to one or more of claims 1 to 9.
12.- Use of a cycloolefin polymer for the manufacture of a metallized sheet, not having been subjected to the sheet before metallization to any procedure to increase the surface tension.