WO1994013466A1

WO1994013466A1 - Biaxially oriented penbb film for magnetic recording medium and magnetic recording media made therewith

Info

Publication number: WO1994013466A1
Application number: PCT/US1992/010688
Authority: WO
Inventors: Cynthia Bennett; E-Won Choe; John Anthony Flint; Bodo Kuhmann
Original assignee: Hoechst Aktiengesellschaft; Hoechst Celanese Corporation
Priority date: 1992-12-09
Filing date: 1992-12-09
Publication date: 1994-06-23
Also published as: EP0671991A1; EP0671991A4; JPH08504466A

Abstract

Disclosed herein is a biaxially oriented PENBB film having a surface modified to produce improved slip and/or improved adhesion to magnetic coatings. The surface modification includes particles, a continuous or discontinuous coating, or a combination thereof. In the PENBB of the invention the dicarboxylic acid derived repeating units include 25 mole percent or more of units derived from 4,4'-bibenzoic acid.

Description

BIAXIALLY ORIENTED PENBB FILM FOR MAGNETIC RECORDING MEDIUM AND MAGNETIC RECORDING MEDIA MADE THEREWITH

Background Of The Invention

1 . Field Of Invention

This invention pertains to a base film for magnetic recording media. More particularly, it relates to a base film suitable for high density magnetic recording media .

2. Description Of The Prior Art

There is an increasing tendency to develop magnetic recording media that may contain larger quantities of information recorded in a unit volume. For magnetic recording material that are tape based, this trend demands both higher recording density per unit surface area and that thinner tape substrates be available so that longer lengths of magnetic tape may be wound in the cartridge space available. Thinner substrates require a film with greater modulus (stiffness, i.e. tensile modulus) and strength to withstand the forces encountered within recording and replay equipment.

The most common base film used is polyethylene terephthalate (PET) polymer. In order to meet the needs for increased tensile properties films made from this polymer have been subjected to higher orientation stresses to produce "tensilized " films, wherein the tensile properties in the machine direction (lengthwise in the magnetic tape) are increased . Such processes produce increased machine direction strength but generally at the expense of the transverse αirection strength and modulus or higher shrinkage when the film or tape is subjected to elevated temperature or both. Reduced transverse direction moαuius means increased tendency of the tape to fold over lengthwise, which causes jamming in the tape player and renders the tape useless. Such shrinkage results in dimensional change that distorts the recorded signals leading to destruction of the information stored or to distorted video signals. The demands for high information storage density also requires that the recording surface (the magnetic susceptible layer) of the tape be extremely smooth and free from protrusions. On the other hand, the need for the film or tape to be handled or processed at high speeds through film and tape manufacturing processes, recording, and end-use equipment requires good slip characteristics in the film. Thus it is required that the polymer based film suitable for high density recording tape have both excellent smoothness and good slip characteristics.

Various means have been described to achieve the necessary balance of smoothness and handleability properties. Included in such means are the use of inert particles ("filler") within the polymer. Such particles may be inorganic or organic. They may be added separately during the polymer manufacturing step, or may be formed in situ during polymer manufacture such as by catalyst precipitation. Such particles must be of controlled particle size and distribution to avoid any large size particles sufficient to cause drop¬ outs (of stored information). Bimodal size distribution of particles has been shown to be preferred .

Another means to provide preferred surfaces for handleability of film and magnetic tape is to coat one or both sides of the film, preferably during the film manufacturing stage, with coatings designed to cause surface roughness sufficient to provide good slip without causing undue protrusions. In addition to providing good slip, such coatings may also be designed to provide improved adhesion of the magnetic susceptible layer to the sub¬ strate. The coatings may be specific to adhesion for magnetic oxide layers or for ultra thin metallic layers.

Summary Of The Invention

It is a primary object of the present invention to provide a magnetic tape base which has improved stiffness (tensile modulus) and high strength at room and elevated temperature which may be used to record information at high density. Another object of this invention is to provide a film base upon which ultra-thin magnetic recording layers may be deposited. Such recording layers would have excellent adhesion to the base, and be free from excessive surface roughness that would cause reduction or loss of signal. A further object of the invention is to provide a film base which exhibits good slip and machine processability.

Specifically, in accordance with the present invention, there is provided a biaxially oriented film of PENBB (polyethylene naphthalate bibenzoate) which may contain particulate matter and/or coatings to provide slip, abrasion resistance, and/or adhesion.

Description Of Invention

PENBB as mentioned herein is a copolyester containing acid-derived units of which at least 25 mole-% are bibenzoate units.

(bibenzoate, BB)

The remainder of the acid-derived units are difunctional units such as 2,6-dicarboxy naphthalate, terephthalate, isophthalate, 5-sodium sulfoisopht- halate, adipate or poly-functional units such as trimellitic acid ester, pyromelli- tic acid ester, and p-hydroxy benzoate. The diol units of the copolyester may be chosen from ethylene glycol, diethylene glycol, propylene glycol, neopentyl glycol, 1 ,4-cyclohexane dimethanol, and p-xylene glycol.

The preferred copolyester would contain 4,4'-bibenzoate as acid derived units in the range 40 to 60 mole-% and 2,6-dinaphthoic acid in the range 60 to 40 mole-% . The diol component preferably comprises at least 80 mole-% ethylene glycol-derived units by weight, more preferably 95 to 100 mole-%.

U.S. Patent No. 3,008,934 discloses copolyesters containing as acid derived units 4,4'-bibenzoate and a host of other dicarboxylates including 2,6- naphthalic dicarboxylate. It also discloses oriented fibers and films prepared from these copolyesters, however, biaxially oriented PENBB films are not disclosed or envisioned . In particular, those films with improved stiffness (tensile modulus) and tensile strength in both MD and TD as well as thermostability, UV stability, hydrophobicity, dimensional stability and impermeability toward gases in comparison to PET film are not disclosed in U.S. Patent No. 3,008,934.

The copolyester is obtained by polycondensation of the corresponding diacid or lower dialkyl diester and the corresponding diol. Both components should normally be employed in roughly equimolar ratios. It may, however, be advantageous to employ one of the components — especially the diol — in excess, for instance in order to influence the reaction kinetics or to serve as a solvent. The polycondensation is carried out according to known processes used, e.g., in the production of polyethylene terephthalate (PET) . Usually about 1 00 mole-% of the dicarboxylic acid or dialkyldicarboxylate mixture are mixed with > 1 00 mole-% of the corresponding diol(s) . This mixture is then heated to about 200 ^° C, preferably in the presence of a transesterification catalyst, until sufficient lower alkyl alcohol or water has been removed from the mixture via distillation. This reaction yields an oligomer or a low molecular weight polyester, which is subsequently subjected to polycondensation, preferably in the presence of a stabilizer and/or catalyst. Useful stabilizers and catalysts can be polyphosphates, triorganyl phosphates, antimony trioxide or tetraalkoxy titanate(IV) or mixtures of triphenylphosphate and antimony trioxide. A preferred process for the production of such copolyesters is descriDed in U.S. Patent Application Serial No. 07/735,553 which is incorporated herein by reference. A further increase in molecular weight can be achieved by solid phase polycondensation at a temperature just below the melting point, under vacuum, or in a stream of dry air or inert gas.

In order to achieve the desired mechanical properties in the biaxially oriented PENBB film it is recommended that the IV value (inherent viscosity, as measured in a 1 : 1 weight-ratio mixture of pentafluorophenol and hexafluoroisopropanol at a concentration of 0.2 g/dl and a temperature of 25

^° C) of the PENBB polymer after extrusion be > 0.5 dl/g and preferably > 0.55 dl/g. To produce the biaxially oriented film, the polymer melt is extruded through a die onto a chill roll where it solidifies, is then biaxially oriented, heat set, optionally post treated, and then wound on a roll. The solidified film as extruded on the chill roll should be obtained in an essentially amorphous state. To achieve this, the melt film must be pinned to the chill roll by a known method such as electrostatic pinning or vacuum, air knife or the like.

The biaxial orientation of the film is achieved by stretching the film at elevated temperature in the machine direction (MD) and transverse direction (TD) . This stretching can be either simultaneous or sequential . In the case of sequential stretching the first stretching step can be in either MD or TD, followed by stretching in the other direction. The orientation in MD can also be achieved in several steps, either one after another prior to stretching in TD, or before and after the TD stretching. Preferred temperatures for stretching lie between the glass transition temperature and about 30 ° C above the cold crystallization temperature of the PENBB copolymer composition in use (both temperatures can easily be measured on amorphous films by DSC). Suitable total stretch ratios in MD and TD lie between 1 to 2 and 1 to 10, preferably between 1 to 2.5 and 1 to 5. The product of the MD and TD total stretch ratios should be between 1 and 30 preferably between 5 and 20. Biaxial drawing is performed such that the birefringeance is < 0.2, preferably < 0.1 to ensure adequately isotropic properties. Birefringeance as mentioned herein is the absolute value of the difference between the maximum and minimum refractive indices in the plane of the film, as measured on common in¬ struments such as Abbe refractometer, optical bench or compensators. In order to optimize properties, such as shrinkage, relaxation steps can be included in the orientation and heat setting processes.

The heat setting takes place at a temperature between the cold crystallization temperature and the melt temperature of the copolymer composition. In some cases a surface treatment such as corona, plasma or flame treatment should be employed before winding the biaxially oriented film on a roll. Particles may be incorporated in the polymer during the polymerization process. They may be added as glycol slurries to the esterification (or transesterification) stage, using proper care to prevent agglomeration of the particulates. They may also result from precipitation of catalyst residues selected for their ability to properly influence the surface roughness. Further, they may be added later, for example, in the form of master batches added to the melt stream prior to extrusion. The preferred route is to add the properly dispersed and filtered slurries of the additives to the initial esterifica- tion/transesterification stage. It is further preferred that more than one type of particulate be added, each exhibiting a different mean particle size and distribution, and each of suitable physical shape, hardness and inertness, to provide a bimodal distribution of surface protrusions with an R_a of less than 0.1 μm. One such preferred formulation would include from 0.01 weight-% to 0.5 weight-% of inert particles having an average particle size ranging from about 1 0 to 1 000 nm and from 0.02 weight-% to about 0.1 weight-% of calcium carbonate having an average particle size of 0.5 to about 30 μm.

Coatings may be applied to one or both sides of the biaxially oriented PENBB film to provide the desired surface characteristics of slip, handleability, smoothness, and adhesion. It is preferred that these coatings be as thin as possible, consistent with achieving the desired properties. Thinner coatings are less expensive and also enhance the recycleability of the film. To achieve this, it is preferred that dilute dispersions or solutions of the coating formula¬ tions be applied during the film manufacturing steps before or between the orientation stages . Such in-line coatings may then be dried and/or cured in the transverse draw and/or annealing (heat setting) steps.

The typical coating would be applied from a water dispersion and at a solids concentration within the range of about 0.5 to 1 5 percent preferably about 3 to 10 percent. The preferred solids level is such as to yield a final dry coating thickness of 50 nm or less, preferably 5 to 30 nm.

The coatings are polymeric in nature and may be thermoplastic or crosslinkable. They may be single polymers or copolymers and may be used alone or with additives such as inert particle pigments, lubricants, dispersants, crosslinking agents, and the like. It is preferred that they be dispersed or dissolved in a carrier such as water, since such a carrier can be more safely employed in the film manufacturing process.

A typical copolyester coating with good adhesion characteristics to be applied in-line would contain the following units:

(a) about 60 to 75 mole-% of terephthalic acid;

(b) about 1 5 to 25 mole-% of at least one aliphatic dicarboxylic acid of the formula HOOC-(CH₂)_n-COOH wherein n ranges from 1 to 1 1 ;

(c) greater than 6 up to about 1 5 mole-% of at least one sulfomonomer containing an alkali metal sulfonate group attached to a dicarboxy aromatic nucleus; and

(d) stoichiometric quantities of 100 mole-% of at least one copolymerizable aliphatic or cycloaliphatic alkylene glycol having about 2 to 1 1 carbon atoms.

Another typical coating with improved adhesion for thin film metal deposits, useful in high density recording, would consist of a water dispersible copolyester consisting essentially of the condensation product of the following components, or their polyester forming equivalents:

(a) about 65 to 95 mole-% of isophthalic acid;

(b) about 0 to 30 mole-% of at least one aliphatic dicarboxylic acid of the formula HOOC(CH₂)_nCOOH where n ranges from 1 to 1 1 ;

(c) about 5 to about 1 5 mole-% of at least one sulfomonomer containing an alkali metal sulfonate group attached to a dicarboxylic aromatic nucleus; and

(d) stoichiometric quantities of about 100 mole-% of at least one copoly- merizable aliphatic or cycloaliphatic alkylene glycol having about from 2 to 1 1 carbon atoms.

Other coating formulations may contain one or more of the following: acrylic esters, methacrylic esters, silicones, amino-silanes, metallo-organic long chain alkyl salts. Dual coatings of soluble intermediates may be applied consecutively to subsequently interact to form insoluble particulate matter on the surface. One such embodiment of this concept is described in U.S. Patent No. 4,880, 695. Inert particles may be added to the coating dispersion (solution) as an alternate means to provide a controlled surface roughness. Selected embodiments of the present invention are illustrated below in the Examples. However, the scope of the invention is not limited to the embodiments illustrated or described herein.

The following test methods were used to determine the characteristics of the biaxially oriented films:

Measurement methods

Amorphous samples are required for determination of T_a and T_cc by means of DSC. On first heating at a heating rate of 20 K/min a step- wise increase in the heat capacity occurs at T_g. Above T_g exothermic cold crystallization takes place at T_cc (exothermic peak temperature). Densities were determined in agreement with ASTM D1 505-68 by dipping samples in density gradient columns. Either mixtures of CC1₄/heptane or aqueous ZnCI₂ solutions were used to prepare the density gradient columns.

The mechanical properties were determined via a tensile test on film strips 100 mm x 1 5 mm in size. The modulus of elasticity was determined applying tension at a rate of 10 %/min between 0.4 and 0.6 percent elongation. The tear strength and elongation at break were measured applying tension at a rate of 100 %/min.

EXAMPLE 1 289 parts by weight of dimethyl 2,6-naphthalene dicarboxylate, 322 parts by weight of dimethyl 4,4'-bibenzoate, 368 parts by weight of ethylene glycol and 0.7 parts of manganese acetate tetrahydrate are initially introduced into a conventional polycondensation reactor provided with a blanketing gas line (N₂), pressure equalization, a thermometer, a condenser, a vacuum connection and a stirrer. The mixture is heated at 220 ° C for 2.5 hours, during which time methanol is distilled off. 0.675 parts by weight of triphenyl phosphate and 0.2259 parts of antimony trioxide are then added as polycondensation catalysts and the mixture is heated to 270 ^° C, with stirring. Vacuum is applied and the temperature is raised to 285 ° C and maintained for 2.5 hours. A portion of the copolyester thus obtained is ejected from the reactor in the form of melt under nitrogen pressure and solidified as a thin, clear, amorphous film. A glass transition temperature (T_g) of 1 23 ^° C and a cold crystallization temperature (T_cc) of 1 32 ^° C is determined for the film by means of DSC. The residual melt is granulated . The granules are white, opaque and crystalline. An IV value of 0.56 dl/g is determined for the granules (measured at a concentration of 0.1 g/ml in pentafluorophenol/hexafluoroisopropanol (weight ratio 1 : 1 ) at 25 ^° C) .

The granules are further condensed for 20 hours at 240 ^° C under vacuum in the solid phase. After this treatment the IV value is 1 .1 dl/g. As expected, no T_g or T_cc is discernable in the DSC recording for the crystalline granules condensed in the solid phase; the melting point (T_m) is 281 ^° C.

The PENBB granules having a melting point of 281 ^° C are melted in a single screw extruder at temperatures of 280 to 320 ^° C and extruded through a sheet die onto a cooling roll temperature controlled at 30 ^° C. A 1 20 μm thick pre-film is obtained which is clear and transparent. Its density is 1 .31 g/cm³ and the IV-Value is 0.78 dl/g. The mechanical properties are:

Longitudinal Transverse Modulus of Elasticity: 2.1 GPa 2.1 GPa Tear Strength: 74 MPa 74MPa

The PENBB pre-film is simultaneously biaxially oriented at 1 35 ° C to 4 by 4 times its original length and width using a simultaneous film stretching device. The density is 1 .31 g/cm³. The film thickness is 8 μm and the film is clear and transparent. The mechanical properties are:

Longitudinal Transverse Modulus of Elasticity: 5.8 GPa 5.1 GPa

Tear Strength: 1 54 MPa 1 98 MPa

biaxially oriented PENBB film is clamped in a tenter frame and heat treated at

260 ^° C for 1 0 minutes. The biaxially oriented film remains transparent (turbidity 3.7 %) and its density is 1 .33 g/cm³. The mechanical properties are: PENBB TYPICAL PET Longitudinal Transverse

Modulus of Elasticity: 8.9 GPa 8.1 GPa 4.0 - 4.5 Tear Strength: 204 MPa 256 MPa 1 50 - 200

EXAMPLE 2 A PENBB pre-film is formed as in Example 1 . The PENBB pre-film is sequentially biaxially oriented (first transversely then longitudinally: 3.5 x 3.5) at 1 25 ° C on a film stretching device. An 8 μm thick, clear biaxially oriented film is obtained. The biaxially oriented film is clamped in a tenter frame and heat treated at 260 ° C for 10 minutes. Its density is 1 .343 g/cm³. The mechanical properties are:

Longitudinal Transverse Modulus of Elasticity: 9.2 GPa 8.0 GPa Tear Strength: 237 MPa 182 MPa

EXAMPLE 3 A film is made according to the procedure of Example 1 , except that at the stage where the pre-film is formed, to the PENBB granules is added a master batch of PENBB containing 3 percent silica (Syloblock 44 from Grace

GmbH of Worms, Germany) of average particle size 1 .5 μm and distribution.

This master batch is prepared beforehand by adding the silica to the PENBB in a twin screw extruder. The master batch is added at a level to provide a concentration of about 2000 ppm in the film. This film can easily be wound and unwound and has a remarkably reduced tendency to block compared to film of Example 1 (which lacks silica particles) .

Claims

THAT WHICH IS CLAIMED IS:

1 . A biaxially oriented film comprising a copolyester containing recurring main acid derived units of formula (I)

wherein unit (I) comprises more than 25 mole percent of the total acid derived units of said copolyester, and main diol derived units of formula (II)

-0-R-O- (ID with R denoting an C_T-Ce-alkylene group , wherein the film has at least one surface modified to improve slip and/or improve adhesion to magnetic coatings.

2. A biaxially oriented film according to claim 1 , wherein an other main acid derived unit is a unit according to the formula (III)

A biaxially oriented film according to Claim 1 or 2, wherein R is

-CH₂-CH₂-

4. A biaxially oriented film according to any one or more of Claims 1 to 3, wherein said modified surface is achieved by including particles into the biaxially oriented film, the particles being selected from the group consisting of inorganic additives, organic additives, polymer catalyst precipitates and any combination of these. 5. A biaxially oriented film according to Claim 4, wherein said particles include a first set of particles having a first mean particle diameter and a second set of particles having a second mean particle diameter, wherein said first set of particles comprises a different composition than said second set of particles and said first mean diameter is different than said second mean diameter.

6. A biaxially oriented film according to any one or more of Claims 1 to 5, wherein said modified surface is achieved by a coating having a thickness of 50 nm or less.

7. A biaxially oriented film according to Claim 6, wherein said coating is discontinuous.

8. A biaxially oriented film according to Claim 6 or 7, wherein said coating comprises particles chosen from inorganic additives, organic additives, catalyst precipitates, and combinations thereof.

9. A biaxially oriented film according to Claim 6,7 or 8, wherein said coating is applied prior to or during orientation of said film.

10. A biaxially oriented film according to any one or more of Claims 1 to 9, wherein the film has a birefringeance of < 0.2 and the IV-value of the copolyester is > 0.5 dl/g.

1 1 . A magnetic recording medium comprising a biaxially oriented film according to Claim 1 and a ferromagnetic thin film formed on at least one surface of said biaxially oriented film.