Polyester film.

0001The present invention relates to a polyester film with improved dimensional stability, abrasion resistance, stretchability and improved sliding properties.

0002Biaxial or multiaxially oriented films made of polyester, e.g. polyethylene terephthalate, are used as a dielectric for capacitors because of their superior properties such as their tensile strength, their tensile strength, their modulus of elasticity, their transparency, their chemical and thermal resistance in the packaging sector and various technical fields such as the reprographic sector , as carrier foils for magnetic recording media such as used for video, audio and computer tapes, for magnetic cards and floppy disks and the like.

0003The polyester films must meet specific requirements for the various areas of application, which can be set via the raw material formulation or the process parameters during film production.

0004One of the prerequisites for proper handling of the films during production, processing and application is a sufficiently low coefficient of friction on the film surface. The rougher the film surface, the better the winding behavior and thus the yield in production.

0005In addition to a low coefficient of friction and good mechanical properties, polyester films for magnetic tapes must also have a uniform surface roughness as well as good abrasion resistance and dimensional stability.

0006Magnetic tape carrier foils are used to set a surface topography that meets these requirements<ul id="ul0001" list-style="none"><li>Incorporation of preferably inorganic pigments such as calcium carbonate, silicon dioxide, kaolin, barium sulfate, titanium dioxide and others,</li><li>- A controlled precipitation of precipitates from catalyst residues and monomers or oligomers of the polyesters used (production of so-called inner particles).</li></ul>

0007The use of inorganic pigments during the stretching process easily leads to the formation of voids at the points at which the matrix breaks away from the pigment grain. When further processing the film, such as when coating, ie application of the magnetic dispersion, parts of the polymer coating can then peel off in the area near the surface and, as a result of the abrasion caused thereby, the quality of the tapes in the coating and thus their electromagnetic properties deteriorate in an uncontrolled manner.

0008The more or less broad grain size distribution of inorganic pigments and the tendency to form agglomerates despite complex preparation processes also have a negative effect on the film quality.

0009The production of so-called inner particles (catalyst precipitate) of a defined size and amount is technically complex.

0010To improve the adhesion between the polymer and the incorporated particles that determine the surface topography of the film, DE-OS 30 19 073 describes instead of the inorganic particles micronized, crosslinked organic, hardened organic, globular or (see DE-OS 33 13 923) Incorporate cross-linked organic spherical particles with functional groups and narrow grain size distribution in the polyester. Films that contain cross-linked spherical particles with a narrow particle size distribution and functional groups for better integration in the matrix have a defined surface roughness and thus good electromagnetic properties. Since they still have good hatching and excellent transparency, they should be used as base films for packaging materials and technical applications such as photo films, magnetic tapes and others be particularly suitable, especially since their mechanical properties can be set via the process parameters of the manufacturing process.

0011In practice, however, it has been shown that the films have inadequate abrasion behavior and unsatisfactory dimensional stability and are therefore unsuitable for many applications.

0012It has also already been proposed to improve German patent application P 34 34 838, the dimensional stability and the abrasion resistance of polyester films by adding nucleating agents. However, such films do not yet have optimal sliding properties.

0013The present invention is therefore based on the object of providing a polyester film with a very uniform surface which has both good sliding properties and excellent abrasion resistance, improved dimensional stability and stretchability.

0014This object is achieved by a film of the type mentioned at the outset, the characteristic features of which can be seen in the fact that it contains 0.005 to 5.0% by weight of organic, in particular hardened or crosslinked, particles having a narrow particle size distribution in the range from 0.01 to 5. 0 _/um and a nucleating agent in an amount of 0.01 to 10 wt .-%, both data based on the weight of the film forming thermoplastic polymer.

0015In the context of the invention, thermoplastic polyester materials are polyester homopolymers and copolymers, mixtures of various polyesters and blends or blends of polyesters with other polymers.

0016The polyester can be produced both by the transesterification process, for example with the aid of the catalytic action of Zn, Ca, Mn, Li or Ge salts, and by the direct ester process.

0017Examples of polyesters are mono- or polycondensates made from terephthalic acid or isophthalic acid, 2,6-naphthalenedicarboxylic acid with glycols having 2 to 10 carbon atoms, such as polyethylene terephthalate, polytetramethylene terephthalate, polybutylene terephthalate, poly-1,4-cyclohexylene-dimethylenterepthalate, polyethylene-2,6- naphthalenedicarboxylate or polyethylene p-hydroxybenzoate.

0018The copolyesters can also contain adipic acid, sebacic acid, phthalic acid, isophthalic acid, 5-Na sulfoisophthalate, polyfunctional components such as trimelitic acid, among others, as building blocks.

0019The polyester mixtures can consist, for example, of polyethylene terephthalate and polybutylene terephthalate or polyethylene terephthalate and at least one alkali metal salt of a derivative of sulfonic acid, such as sulfosiopthalic acid.

0020Examples of polymers which can be incorporated or mixed into the polyester are polyolefin homo- or copolymers such as polyethylene, polypropylene, poly-4-methylpentene, ethylene-vinyl acetate copolymers, which in turn can be saponified, ionomers, polyamides, polycarbonates, Polytetrafluoroethylene, polysulfones and others

0021The hardened particles contained in the film can consist, for example, of melamiri-formaldehyde resin, benzoguanamine-formaldehyde resin, phenol-formaldehyde resin or an epoxy resin.

0022The crosslinked organic particles can be used as building blocks, inter alia unsaturated nonionic monomers such as esters of acrylic and methacrylic acid such as methyl methacrylate and butyl acrylate, the esters of unsaturated dicarboxylic acids such as maleic acid dialkyl esters, unsaturated vinyl compounds such as styrene, unsaturated nitriles such as acrylonitrile, functional monomers such as unsaturated carboxylic acids, hydroxyl-containing monomers such as hydroxyethyl group methacrylate monomers such as hydroxyethyl methacrylate acrylate Contain sulfonic acids among others.

0023Examples of crosslinking components are diallyl phthalate, divinylbenzene and others

0024Components which carry functional groups and are polymerized into the particles for this purpose so that they can preferably form covalent bonds between the polyester matrix and crosslinked particles in polyester synthesis are hydroxyethylene methacrylate, acrylic acid and methacrylic acid.

0025The degree of hardening and crosslinking of the particles can be varied within wide limits by the composition, in particular the hardening and crosslinking component.

0026It is essential that the organic particles, which may be hardened or crosslinked, are insoluble and not meltable during polymer synthesis and are also retained when the polymer is melted, in particular when producing shaped articles such as films or the regeneration of waste.

0027The particles can be produced by means of the known methods, for example by emulsion or suspension polymerization. They can be incorporated into the polymer matrix during polymer production in the form of aqueous or glycolic dispersions or via a concentrate. The incorporation during the polymer synthesis leads to a particularly good integration into the matrix and good distribution in the polymer itself.

0028The polymeric, preferably crosslinked or hardened, solid particles have a diameter in the range from 0.01 to 5 _/µm, preferably from 0.02 to 3.0 _/um, with a narrow grain size distribution. The quotient of the weight average of the particle diameter (D) and the number average of the particle diameter (D_n) is preferably &lt;1.1. To determine D_w and D_n see UE Woods, JS Dodge, IM Krieger, P. Pierce, Journal of Paint Technology, Vol. 40, No. 527, p. 545 (1968).

0029Depending on requirements, only particles of uniform size or mixtures of narrowly distributed particles can be used.

0030The film contains particles described above in an amount of 0.005 to 5% by weight, preferably 0.02 to 3% by weight, based on the weight of the film.

0031As nucleating agents, e.g. Alkali or alkaline earth metal salts of ester waxes or partially saponified ester waxes, for example montanic acid, ionic copolymers from ethylene and alkali metal salts of methacrylic acid, alkali metal salts of phenolsulfonic acids, alkali metal salts or alkaline earth metal salts of benzoates or stearates, sorbitol derivatives or inorganic nucleating agents.

0032Suitable inorganic nucleating agents are, for example, alkaline earth metal carbonates and oxides such as titanium dioxide and aluminum oxide, talc and silicates, boron nitride and others

0033The montanic acid is a mixture of acids consisting mainly of aliphatic monocarboxylic acids with a chain length between 26 and 23 carbon atoms. Suitable montan wax salts mainly contain metals from the 1st to 3rd main groups of the periodic table, preferably Li, Na, K, Be, Mg, Ca and Al. Na montanate is preferably used. Salts prepared by partially reacting the montanic acid with 0.1 to 1 equivalent of alkali metal hydroxide or oxide, preferably with 0.25 to 0.9 equivalent of sodium hydroxide solution, are used as partially neutralized montan wax salts.

0034Suitable montan wax ester salts are obtained by partial esterification of the montanic acid with up to 0.90 equivalent, preferably 0.5 to 0.8 equivalent, of dihydric alcohols with 2 to 4 carbons in the alkylene group and subsequent neutralization with oxides or hydroxides of the metals mentioned.

0035Particularly suitable diols are, for example, ethylene glycol, 1,2- or 1,3-propanediol and 1,3- or 1,4-butanediol.

0036Nucleating agents based on montanic acid and their use in foils are described, for example, in Japanese Laid-Open Patent Application 81 / 139.551 (see also Chem. Str. 96: 53440 g). This publication describes the transparency and the coefficient of friction of films produced with the additives mentioned.

0037However, it has surprisingly been found that films can be produced from polyester materials with added hardened or crosslinked organic particles and nucleating agents, which have particularly good abrasion resistance and improved dimensional stability and stretchability with excellent transparency. The additions of nucleating agents, which are preferably in the range from 0.1 to 5.0% by weight, based on the weight of the polyester, can be added to the polyester material during or after production. In practice, with regard to optimum dimensional stability and abrasion resistance, it has proven advantageous either to add the nucleating agent in the form of a masterbatch or to apply it to the dried granules by mixing.

0038In addition to the nucleating agents, the films of the present invention may also contain agents for optimizing the slip and sliding properties, such as, for example, inert inorganic particles, which may also be in colloidal form, catalyst residue particles and others, as well as other customary additives, such as antioxidants, antistatic agents, thermostabilizers, dyes and others.

0039The film according to the invention is produced by the extrusion process, the polyester material mixed with the nucleating agent being melted, extruded into a pre-film and quenched on a cooling roll. This film is then lengthways and / or crosswise at temperatures between the glass point of the polymer and 160^*C and a stretching ratio, preferably in the range from 2.0 to 6.0, and then heat-set at temperatures between 150 and 240 ° C. The number and sequence of the longitudinal and transverse stretching stages is not fixed, but depends on the requirements. The individual stretching processes "lengthwise" and "crosswise" can be carried out in one or more stages. Simultaneous longitudinal and transverse stretching (simultaneous stretching) is also possible.

0040The films resulting from the stretching process can only have strength in one direction (monoaxially stretched), have balanced mechanical properties in both directions (balanced films) or have special strengths in the transverse and / or longitudinal direction (tensilized or supertensilized films).

0041The films can also have two or more layers, wherein differently nucleated or formulated or differently nucleated or formulated and non-nucleated or formulated polyester materials can be combined to form multilayer films by coextrusion or lamination.

0042Due to the layer formation, the advantages of the different nucleating agents and additives can be used specifically for the intended use of the film.

0043The structure of the coextruded films can be both symmetrical and asymmetrical.

0044When carrying out the processes described above, it is particularly surprising that no increased crystallite formation is observed when the polyester melt is cooled on the cooling roll by the nucleating agent. The density of the pre-film is less than 1.34 g / cm³. Even when the film is stretched, no negative influences from the nucleating agent and the germs present for the formation of crystallites are observed. These germs only come into play during the heat fixation process step, whereby the tendency to shrink the film is greatly reduced.

0045Within the individual stretching stages there is a certain relationship between density, orientation and crystallite size or number. This ratio can be regulated by nucleating agents added to the film according to the invention.

0046Surprisingly, the mechanical properties of the film do not deteriorate.

0047In addition to the improved abrasion resistance, dimensional stability and stretchability of the film, further advantages of the present invention are that when the polyester granules are dried at higher temperatures, the addition of the nucleating agents results in a lower tendency to stick to the polyester particles. This increases the throughput in this operation. Thus, higher production speeds with the same film quality and thus an improvement in economy can be achieved with the present invention.

0048The surface properties and surface roughness can be influenced by additional coating of aer foils with solutions or dispersions which, inter alia, contain crosslinkable, curable or already hardened or crosslinked substances or particles.

0049Examples include copolyesters, polyurethanes, thermosetting acrylic acid derivatives, polysiloxanes, styrene-butadiene rubbers or substances which - as described above - can be incorporated into the polymer.

0050To improve the properties of the applied layer, the dispersions or solutions can contain organic additives such as Na montanate, fatty acid esters, silane or siloxane coupling reagents and inorganic additives such as colloidal SiQ₂-, Ti0₂-Particles included.

0051The coatings can be applied in-line, ie between the drawing stages or after orientation.

0052The invention is described in more detail by the following examples, which are not restrictive. Here, a raw material is used which contains finely divided, crosslinked acrylate-based polymer particles which have been produced by emulsion polymerization and have a narrow particle size distribution. They were incorporated into the raw material via dispersions under defined conditions during the production of the polyester. The abrasion behavior of the foils was determined on a measuring device in which a 12.5 mm wide foil strip with a constant web tension (1 N) was first pulled out of a cassette over a rubber cleaning roller, then over a fixed deflection pin, and then over two serving as measuring rollers Rubber rollers and then rolled up. The loading of the rubber rollers with abraded material was assessed on a scale from 1 to 5, ie from "very good" to "poor".

Example 1 (M)

00530.4% by weight, based on the weight of the polyester, of sodium montanate was admixed with a polyethylene terephthalate raw material of 1000 ppm of crosslinked organic particles of methyl methacrylate, butyl acrylate and hydroxymethyl acrylate produced by emulsion polymerization. The mixture was melted, formed into a film in a slot die and quenched into an amorphous film on a highly polished cooling roll. The pre-film was then gradually biaxially stretched, with an area stretch ratio of

0054A = 13 was reached. The biaxially oriented, approximately 14.4 μm thick film was then heat-set at 195 ° C.

Comparative Example 1 (V)

0055As in Example 1, a stepwise biaxially oriented film was made without the addition of sodium montanate.

0056Results:<tables id="tabl0001" num="0001"><img file="EP0188198A2_D0001.tif" /></tables>

Example 2

0057In accordance with Example 1, a film was formed which was placed on a highly polished roller with a surface temperature of 54 ° C. The amorphous pre-film was then stretched in stages at 117 ° C. and an area stretch ratio of λ = 4.7 along and at 95 ° C. with a cross stretch ratio of λ = 3.6 and heat-set at 195 ° C.

Comparative example (V)

0058As in Example 2, a stepwise biaxially oriented film was produced without the addition of sodium montanate.

Results

0059<tables id="tabl0002" num="0002"><img file="EP0188198A2_D0002.tif" /></tables>