Method of improving the slipperiness of polyester film

Abstract: The invention relates to a method for improving the slipperiness of a polyester film by adding spherical fine particles of inorganic oxide produced by hydrolyzing an organometallic compound in an alcoholic solution and having an average particle diameter of 0.05 to 5 micrometers and a relative particle size standard deviation of 1.0 to 1.5 to a polyester in an amount of 0.005 to 2% by weight based on the weight of the polyester.

Description: This invention relates to a method of improving the slipperiness of a polyester film by adding spherical fine particles of an inorganic oxide. Fine inorganic oxide particles have been extensively used heretofore as a polymer filler and a surface-treating additive. Improvement of the slipperiness of a polymeric film or fibers by including fine inorganic oxide particles is one example of their utility. Polyester films have been widely used in magnetic tapes, optical photography, capacitors and packaging materials. Various undesirable troubles presumably attributed to the poor slipperiness of polyester films may occur duing the process of polyester film production. A polyester film used to make a magnetic tape by coating or vacuum-depositing a magnetic layer on its surface requires especially good slipperiness. If the film slipperines is low, scratches and creases occur on the film surface during film production, coating or depositing of the magnetic layer, or film handling. This results in dropout, and the quality of the resulting magnetic tape is unsatisfactory. Furthermore, the magnetic tape product is required to have good slipperiness because it inevitably needs good travelling property. It has been the practice therefore to form a film from a polyester containing fine particles, thereby providing a roughened film surface and reducing its frictional resistance. On the other hand, as polyester films have been produced in small thicknesses and magnetic recording has been required to have a high density and high performance in recent years, the polyester films have been required to possess two apparently contradictory properties, slip-periness and surface flatness. As a compromise for these contradictory properties, it has been proposed to form fine and uniform raised and depressed parts on the film surface. Conventional methods proposed for this purpose include, for example, (1) a method which comprises precipitating a catalyst or a coloration inhibitor used in polyester synthesis partly or wholly during the polyester-forming reaction thereby to cause them to exist as fine particles in the polymer, and (2) a method which comprises externally adding fine inorganic particles at any desired stage of polyester synthesis. According to the method (1), the amount or particle diameter of the particles are difficult to control and the formation of coarse particles cannot be avoided because the particles are formed during polyester synthesis. In the method (2) commercially available materials such as silica, titanium oxide, silica-alumina, silica-magnesia, glass powder, barium sulfate, calcium carbonate, clay, mica, talc, calcium phosphate and magnesium phosphate which have an average particle diameter of 0.001 to 10 micrometers are used according to the end use of the films as the fine inorganic particles to be added (see, for example, Japanese Patent Publication No. 8216/1984 and Japanese Laid-Open Patent Publication No. 3645/1977). These fine inorganic particles used heretofore have a broad particle size distribution and are mostly irregularly-shaped owing to the methods of their production. For example, the fine particles of silica used are silica having an average primary particle diameter of 0.02 to 0.1 mirometer obtained by thermal decomposition of silicon halides, silica obtained by crushing agglomerated masses having a size of 1 to 5 micrometers obtained from sodium silicate by the wet method, and spherical particles of silica obtained by melting the crushed particles of natural silica. However, they have irregular particle shapes, or have a very broad particle size distribution even when they are nearly spherical, or contain agglomerated particles. According to the above methods, the slipperiness of the film surface is improved by formation of raised and depressed parts form on it. However, since the particles are non-uniform and the raised and depressed parts lack uniformity, there is a limit to the flattening of the film surface. It is an object of this invention to provide a method of improving the slipperiness of a polyester film which comprises forming uniform raised and depressed parts on the surface of a polyester film. According to this invention, there is provided a method for improving the slipperiness of a polyester film by adding spherical fine particles of inorganic oxide produced by hydrolyzing a compound of a metal selected from silicon, titanium, zirconium and aluminum which contains a hydrolyzable organic group and can form a hydrate when hydrolyzed in an alcoholic solution and having an average particle diameter of 0.05 to 5 micrometers and a relative particle size standard deviation of 1.0 to 1.5 to a polyester in an amount of 0.005 to 2% by weight based on the weight of the polyester. Spherical fine inorganic oxide particles which are applicable to this invention have an average particle diameter of 0.05 to 5 micrometers and relative particle size standard deviation of 1.0 to 1.5. The hydrolyzable metal compound as a raw material for fine inorganic oxide particles. denotes a compound of a metal selected from silicon, titanium, zirconium and aluminum which contain a hydrolyzable organic group and can form a hydrate when hydrolyzed. Alkoxides of the above metals are easily available commercially and inexpensive. Such metal alkoxides are represented by the general formula M(OR) m wherein M is a metallic element, m is an integer corresponding to the valency of the element, and R is an alkyl group, preferably a lower alkyl group having up to 8 carbon atoms. Specific examples are tetramethyl silicate, tetraethyl silicate, tetraisopropyl silicate, tetrabutyl silicate, tetramethyl titanate, tetraethyl titanate, tetraisopropyl titanate, tetrabutyl titanate, tetramethyl zirconate, tetraethyl zirconate, tetraisopropyl zirconate, tetrabutyl zirconate, tetra(2-ethylhexyl) titanate, trimethyl aluminate, triethyl aluminate, triisopropyl aluminate and tributyl aluminate. They may also be those having a plurality of alkyl groups such as dimethyldiethyl silicate and diethyldibutyl titanate. Other preferred metal compounds are derivatives of these alkoxides, for example low condensation products obtained by partially hydrolyzing these alkoxides, compounds of the general formula M(OR) m in which the alkoxy groups (OR) are partly replaced by a group capable of forming a chelate compound such as a carboxyl or beta-dicarbonyl group, and low condensation products obtained by partially hydrolyzing these alkoxide substituted compounds. Other metal compounds include, for example, acylate compounds of titanium, zirconium or aluminum such as zirconium acetate, zirconium oxalate, zirconium lactate, titanium lactate and aluminum lactate; and chelate compounds between titanium, zirconium or aluminum and a glycol, a beta-diketone or an amino alcohol, such as titanium acetylacetonate, zirconium acetylacetonate, titanium octyl glycolate, titanium triethanol aluminate and aluminum acetyl acetonate. The fine oxide particles may also be fine particles of composites formed between an oxide of silicon, titanium, zirconium and/or aluminum obtained by hydrolyzing compounds of silicon, titanium, zirconium and/or aluminum in the presence of an metal compound or inorganic salt of sodium, potassium, rubidium, cesium, magnesium, calcium, strontium, barium, boron, gallium, indium or tin, and an oxide of another metal. The proportion of the oxide of silicon, titanium, zirconium and/or aluminum in the fine oxide particles is not particularly limited, but is preferably at least 70%. In the first step of mixing the metal compound with an alcoholic solution to hydrolyze it and form a suspension of fine particles of the hydrate in the alcoholic solution, the method of mixing may be arbitrary. For example, they may be mixed at a time or portionwise. At this time, there is no particular restriction on the final concentration of the metal compound in the solution. Preferably, it is not more than 2 moles/liter because agglomeration of the fine hydrate particles does not appreciably occur in such final concentrations. The alcohol in the alcoholic solution is not particularly restricted. For example, methanol, ethanol, isopropanol, butanol, isoamyl alcohol, ethylene glycol, propylene glycol, etc. may be used singly or in combination. A minor amount of an organic solvent such as dioxane, diethyl ether, ethyl acetate, benzene, toluene and hexane may be included in the solution. The alcoholic solution is required to contain water necessary for hydrolysis. The content of water should be controlled to a favorable range since it affects the shape or particle size of the particles. Water may be supplied as a moisture in a gaseous phase. The hydrolysis is carried out, for example, by adding the above metal compound or its alcohol solution to the above alcohol solution and stirring the mixture at 0 to 100°C, preferably 0 to 50°C, for 10 minutes to 100 hours. At this time, in order to control the rate of hydrolysis, a catalytic component, for example a cation such as NH 4 + or Na + or an anion such as SO 4 2- or H 2 PO 4 - may be added. The presence or absence of such a catalyst component and its amount differ depending upon the starting materials, and are properly selected by considering its effect on the shape and size of the particles. By hydrolyzing the metal compound in the alcoholic solution under suitable conditions as stated above, a monodispersed suspension of fine particles of the hydrate can be obtained. Furthermore, by selecting the concentration of the starting material, the reaction temperature, the concentration of water, the type of the alcohol, the type of the solvent and the type and concentration of the catalyst within preferred ranges, the fine hydrate particles can be obtained as uniform spherical particles having an average particle diameter within the range of 0.05 to 5 micrometers and a relative particle size standard deviation of 1 to 1.5, prefarably 1 to 1.3. The fine hydrate particles sometimes have bonded thereto part of the organic groups derived from the starting materials, or contain the catalyst component adsorbed thereon. Depending on the starting material, the fine hydrate particles may partly be crystallized. But most of it is amorphous. To prevent formation of agglomerated particles, it is effective to control the activity of the surface of the fine particles by adding a coupling agent to the suspension of the particles in the alcoholic solution and subjecting the surface of the fine hydrate particles to a coupling treatment. Any coupling agent may be used for this purpose which contains at least one non-hydrolyzable organic group and at least one hydrolyzable group in the molecule. Silane-, titanate- and aluminum-type coupling agents which are readily available are preferably used. Examples of the silane coupling agents include alkoxysilanes having at least one (substituted) alkyl group, such as methyltrimethoxysilane, phenyltrimethoxysilane, benzyltriethoxysilane, methyltriisopropoxysilane, 3-chloropropyltrimethoxysilane, dimethoxydimethylsilane, diethoxymethylphenylsilane, ethoxytrimethylsilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethylaminopropyl)-trimethoxysilane, 3-glycidoxypropyltrimethoxysilane, allyltriethoxysilane and vinyltriethoxysilane; chlorosilanes such as trimethylchlorosilane and diethyldichlorosilane; and acetoxysilanes such as acetoxytriethylsilane, diacetoxydiphenylsilane and triacetoxyvinylsilane. Examples of the titanate coupling agent include isopropyltriisostearoyl titanate and bis-(dioctylpyrophosphate) oxyacetate titanate. Examples of the aluminum coupling agent are acetoalkoxyaluminum diisopropylate. When an alkoxide or its derivative is used as the metal compound, it is especially preferable to use a coupling agent containing an alkoxy group as the hydrolyzable group because its reactivity is high. The amount of the coupling agent added is 0.1 to 10% by weight, preferably 0.5 to 5% by weight, based on the weight of the fine particles calculated as oxide. If its amount is lower than the lower limit, the effect of the coupling agent is small. Amounts above the upper limit are effective but not economical. The temperature for the coupling treatment is 0 to 100°C, preferably the temperature employed in the first step. The amount of the fine oxide particles added is 0.005 to 2% by weight based on the weight of the polyester. If it is less than 0.005% by weight, the effect of these particles on slipperiness is insufficient. If it is larger than 2% by weight, the physical properties such as tensile strength of the finally obtained film are undesirably deteriorated. The polyester film having improved slipperiness produced in accordance with this invention has excellent abrasion resistance and lends itself to excellent coating or vacuum-deposition of a magnetic layer in the production of magnetic tapes. Hence, it is especially suitable for production of magnetic tapes having excellent electromagnetic converting characteristics. The following examples illustrate the present invention in greater detail. The shape, average diameter, relative particle size standard deviation, specific surface area and crystallinity, can be measured or be determined by the following methods. Shape of the particles The shape of the particles was determined by observation under an electron microscope (magnification 50,000X). Average particle diameter and the relative particle size standard deviation The particle diameters of 100 particles arbitrarily selected from an electron micrograph (50,000X) of the fine particles were actually measured, and the average particle diameter and the relative particle size standard deviation were calcualted from the following equations. Relative particle size standard deviation = X + σ n-1 X In the above equation, Xi is the diameter of the Crystallinity of the fine particles An aliquot of the suspensions of the fine oxide particles or the fine hydrate particles was dried in vacuum at 50°C to remove completely volatile components such as the alcoholic solvent and glycol not bonded to the fine particles and to prepare a powder sample of the fine particles. The crystallinity of the powder sample was determined by X-ray diffractometry. Specific surface area The specific surface area of the powder sample obtained as above was measured by the BET method. PRODUCTION OF A SUSPENSION OF FINE HYDRATE PARTICLES IN AN ALCOHOLIC SOLUTION EXAMPLE 1-(1) Methanol (16 liters) and 1.5 kg of a 28% aqueous solution of ammonia were fed into a 30 liter glass reactor equipped with a stirrer, a dropping port and a thermometer, and mixed. The mixture was adjusted to 20°±0.5°C, and with stirring, a solution of 1.0 kg of tetramethyl silicate in 2 liters of methanol was added dropwise from the dropping port over 1 hour. After the addition, the mixture was continuously stirred for 2 hours to produce a suepension of fine silica hydrate particles (1a 1 ) in an alcoholic solution. The concentrations of the starting materials based on the total weight of the final solution were 0.32 mole/liter for tetramethyl silicate, 2.90 moles/liter for water, and 1.19 moles/liter for ammonia. The reaction conditions and the results of analysis of the fine particles are shown in Table 1. EXAMPLES 1-(2) - 1-(7) Example 1-(1) was repeated except that the type of the metal compound, the type of the alcohol, the concentrations of the starting materials based on the total weight of the final solution and the reaction temperature were changed as shown in Table 1. There were produced suspensions of fine silica hydrate particles (2a 1 ) to (3a 1 ), a suspension of fine titania hydrate paritcles (4a 1 ), a suspension of fine zirconia hydrate particles (5a 1 ), a suspension of fine alumina hydrate particles (6a 1 ) and a suspension of fine slilica-alumina compound hydrate particles (7a 1 ). The reaction conditions and the results of analysis are shown in Table 1.

Claims: A method for improving the slipperiness of a polyester film by adding spherical fine particles of inorganic oxide produced by hydrolyzing a compound of a metal selected from silicon, titanium, zirconium and aluminum which contains a hydrolyzable organic group and can form a hydrate when hydrolyzed in an alcoholic solution and having an average particle diameter of 0.05 to 5 micrometers and a relative particle size standard deviation of 1.0 to 1.5 to a polyester in an amount of 0.005 to 2% by weight based on the weight of the polyester. The method for improving the slipperiness of a polyester film according to claim 1, wherein the compound is a metal alkoxide or its derivative. The method for improving the slipperiness of a polyester film according to claim 1, wherein the compound is composed mainly of a compound of silicon, titanium and/or zirconium and the inorganic oxide is composed mainly of silica, titania, zirconia or a compound oxide thereof.

Inventors: KAMO MIDORI, NAKAHARA SABURO, TAKEDA MITSUO, YONEDA TADAHIRO, KAMO, MIDORI, NAKAHARA, SABURO, TAKEDA, MITSUO, YONEDA, TADAHIRO
Applicants: NIPPON CATALYTIC CHEM IND, NIPPON SHOKUBAI CO., LTD.
Application year: 1987
Main classification: C08J5
IPC codes: G11B5, C08K3, C01B13