GB2042929A - Magnetic recording tape - Google Patents

Abstract

A magnetic recording tape comprising a polyethylene terephthalate substrate and a magnetic material-containing layer closely adhered onto the substrate, which tape exhibits a low dropout number increase and a small initial dropout number. The polyethylene terephthalate substrate satisfies the formula: 23.7 A-1.351H2-1231.97</=20 wherein A is the average particle size of the polyethylene terephthalate crystals, and H2 is the average number of projections present in a unit area of 1 mm2 on the surface of the substrate, which projections have a height of from 0.54 to 0.80 micron. The substrate having the surface projections is prepared by incorporating projection-forming materials into the reaction mixture for forming the polyethylene terephthalate or into the polyethylene terephthalate. As the projection- forming material, calcium carbonate is used in combination with calcium acetate and/or kaolin. The proportion of calcium carbonate is less than about 25 weight % based on the total weight of calcium carbonate plus calcium acetate and/or kaolin.

Description

SPECIFICATION Magnetic recording tape BACKGROUND OF THE INVENTION This invention relates to a magnetic recording tape exhibiting a reduced dropout number increase from that of the prior art.

Polyethylene terephthalate is popularly used in magnetic recording tapes, such as audio tapes and video tapes, as a substrate material on which a magnetic material layer is to be formed. Audio and video properties and other physical properties of magnetic recording tapes vary depending upon not only the magnetic material layer but also, the substrate.

SUMMARY OF THE INVENTION It now has been found that the physical properties of a magnetic recording tape prepared from a polyethylene terephthalate substrate, particularly, the initial dropout number of the tape observed when the initially recorded tape is reproduced, and the increase in the dropout number observed during the repeated recording-reproduction of the tape, are greatly influenced by the size of the polyethylene terephthalate crystals and the surface roughness of the polyethylene terephthalate substrate.

The main object of the present invention is to provide a magnetic recording tape having a polyethylene terephthaiate substrate, which tape, when compared to the prior art tapes, exhibits a reduced dropout number increase observed during the repeated recording-reproduction of the tape, and which tape further exhibits a relatively small initial dropout number observed when the initially recorded tape is reproduced.

In accordance with the present invention, there is provided a magnetic recording tape comprising a poZyethylene terephthalate substrate and a magnetic material-containing layer closely adhered onto the substrate, characterized in that said polyethylene terephthalate substrate satisfies the formula (I):: 23.7A~1 .351 H2~1231 .97 ~ 20 (I) wherein A is the average particle size of the polyethylene terephthalate crystals, determined as hereinafter mentioned, and H2 is the average number of projections present in a unit area of 1 mm2 on the surface of the substrate, which projections have a height falling within the range of from 0.54 to 0.80 micron;; said surface projections being formed either (1 ) by incorporating a mixture of finely divided kaolin and finely divided calcium carbonate into an ester-interchange reaction mixture or a polycondensation mixture in the course of manufacturing polyethylene terephthalate, or into the moiten polyethylene terephthalate in the step of forming the polyetnylene terephthalate substrate, the amount of the finely divided calcium carbonate being less than about 25% by weight based on the total weight of the finely divided calcium carbonate and the finely divided kaolin, or (2) by incorporating finely divided calcium carbonate or a mixture of finely divided kaolin and finely divided calcium carbonate into an ester-interchange reaction mixture or a polycondensation mixture in the course of manufacturing polyethylene terephthalate, or into the molten polyethylene terephthalate in the step of forming the polyethylene terephthalate substrate, and further incorporating calcium acetate into an ester-interchange reaction mixture of dialkyl terephthalate and ethylene glycol, followed by polycondensation of the interesterified product, thereby to precipitate particles of the calcium compounds represented by the following formula, during the polycondensation::

wherein n is an integer of from 0 to 3, the amount of the finely divided calcium carbonate being less than about 25% by weight based on the totai weight of the finely divided calcium carbonate, the calcium acetate and, if any, the finely divided kaolin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The polyethylene terephthalate (hereinafter referred to as "PET" for brevity) substrate of the magnetic recording tape of the invention is characterized as exhibiting the crystal particle size A and the surface roughness which satisfy the aforesaid formula (I).

The term i'crystal particle size A", used herein, refers to the crystal particle size that is calculated from the equation: A = (0.9 x A)/B.cosf wherein A is the particle size of a crystal in angstrom, B is the half height width in radian as determined as follows, 0 is Bragg angle and A is 2.2896 (angstroms).

The half height width is determined on the [ 1 00 ] #plane of a crystal specimen by using an X-ray diffraction apparatus (Model 4011~B2, supplied by RIGAKU ELECTRIC CO.). In this determination, a line drawn between two points corresponding to scattering strengths at 26 = 500 and 26 = 200, respectively, is employed as the base line.

The crystal particle size A is usually in the range of from approximately 45 to 75 angstroms. The crystal particle size A can be varied by suitably selecting the conditions under which the PET film is prepared. Particularly, the crystal particle size varies greatly depending upon the temperature at which the biaxially drawn PET film is heat-set. The higher the heat-setting temperature, the larger the crystal particle size. Generally, the intended crystal particle size can be obtained by suitably determining the heat-setting temperature in the range of from 1 800C to 2300 C.

The PET substrate has the surface roughness, defined by H2 above, satisfying the aforesaid formula (I). The "H2", used herein, is determined as follows. A film specimen is thinly metallized with aluminum and, then, the height and density of projections are measured. The projection height is measured by using a surface finish microscope (NIKON Model BFM, supplied by Nippon Kogaku K.K.) employing a visible green monochromic light. The projection heights obtained are classified into the following four groups.

H1: from 0.27 to 0.53 micron H2: from 0.54 to 0.80 micron H3: from 0.81 to 1.07 microns H4: from 0.08 to 1.34 microns The projections having height falling within the respective groups are counted in random samples having an area of 1.28 mm2 as follows. The microscope, of 240 magnification, is adjusted so that a photograph be obtained which has six to seven interference bands therein. The projections having heights falling within group H1 are single ring figures having lengths of at least 2 mm as measured along the direction parallel to the interference bands. The projections having heights falling within groups H2 and H3 are double ring figures and triple ring figures, respectively. The number of projections in the respective groups is expressed in terms of the number of projections present in the unit area of 1.0 mm2.

The PET substrate having the surface roughness, defined by H2, as mentioned above, may be prepared as follows.

(a) Dialkyl terephthalate and ethylene glycol are interesterified in the presence of calcium acetate as an ester-interchange catalyst, followed by polycondensation of the interesterified product under normal conditions, thereby to precipitate particles of the calcium compounds represented by the following formula, i.e., calcium terephthalate or a calcium salt of a PET oligomer, during the polycondensation.

wherein n is an integer of from 0 to 3.

Alternatively, finely divided particles of the aforesaid calcium compounds previously prepared may be incorporated in the polycondensation reaction mixture or the polycondensed product.

(b) Finely divided particles of inorganic substances, such as kaolin and calcium carbonate, are incorporated into the ester-interchange reaction mixture or the polycondensation reaction mixture, or into the molten PET in the step of forming the film.- The aforesaid method (a) and (b) may be employed alone or in combination. The height and density of projections present on the substrate surface can be varied by suitably selecting the amount and particle size of the particles to be formed or incorporated in the aforesaid methods (a) and (b).

Provided that the PET substrate has the surface roughness satisfying the aforesaid formula (I), the method whereby projections are formed on the surface of the PET substrate is not particularly limited.

That is, the increase of dropout number can be minimized not depending upon the particular method for forming the surface projections. However, when the surface projections are produced by a method wherein finely divided calcium carbonate particles are incorporated into the ester-interchange reaction mixture or the polycondensation reaction mixture or into the molten PET in the step of forming the film, the initial dropout number is undesirably large. Accordingly, it is preferable to use calcium acetate or kaolin, rather than to use calcium carbonate.

it now has been found that, even when finely divided calcium carbonate is used, the initial dropout number of the resulting tape can be reduced to the permissible extent, provided that finely divided calcium carbonate is used in combination with finely divided kaolin and/or calcium acetate and further that the amount of the finely divided calcium carbonate is less than about 25% by weight based on the total weight of the finely divided calcium carbonate and the finely divided kaolin and/or the calcium acetate.

The dropout number increase of the magnetic recording tape has a good correlation with the value of (23.7A-1 .351 H2-1 231.97). When this value exceeds 20, the dropout number increases to a practically impermissible extent. This value of the PET substrate can be generally decreased to approximately~100. That is, this value may be varied, generally in the range of from approximately -100 to 20, more preferably from approximately -100 to 10.

The magnetic material layer formed on the PET substrate may be conventional. That is, the magnetic material layer can be comprised of approximately 60 to 80% by weight of a finely divided magnetic material, such as maghemite (y-Fe203), CrO2, Fe-Co alloy, magnetite (Fe304) and Co-doped or -adsorbed #-Fe203, and approximately 20 to 40% by weight of a binder resin. The binder resin includes, for example, a thermoplastic resin, such as a vinyl chloride/vinyl acetate copolymer, a styrene/butadiene copolymer, nitrocellulose, cellulose acetate or cellulose butyrate, or a thermosetting resin, such as a polyurethane resin, an epoxy resin or a melamine resin. Additives, such as a plasticizer, a dispersing agent, a lubricant and a colorant, may be incorporated in the magnetic material layer.

The formation of the magnetic material layer on the substrate may be carried out in a conventional manner, wherein a magnetic material, a binder resin and other additives are slurried in a suitable solvent, and then, the slurry is coated on the substrate, followed by drying and heat-treating.

The invention will be further illustrated by the following example, wherein percents and parts are by weight unless otherwise specified.

EXAMPLE 1 100 parts of dimethyl terephthalate and 70 parts of ethylene glycol were heated in the presence of calcium acetate and 0.035 part of manganese acetate to effect an ester exchange reaction. To the reaction product, there was added 0.04 part of antimony trioxide, 0.15 part of lithium acetate (dissolved in ethylene glycol), kaolin and calcium carbonate (dispersed in ethylene glycol). The amounts of calcium acetate, kaolin and calcium carbonate were varied in the ranges of from 0 to 0.20 part, from 0 to 0.06 part and from 0 to 0.06 part, respectively. The average particle size of the calcium carbonate was varied in the range of from 0.01 to 3 microns. Thereafter, 0.13 part of trimethyl phosphate was added to the mixture. Then, the resulting mixture was heated to effect polycondensation.The polyethylene terephthalate, so prepared, had an intrinsic viscosity of 0.620.

The PET was melt-extruded in a conventional manner and then, the extrudate was drawn, first in the longitudinal direction and then in the transverse direction. The drawing ratio was varied in the range of from 2 to 5 times the original length both in the longitudinal and transverse directions. Each drawn film was heat-set at various temperatures of from 1 800C to 2300 C. The heat-set film had a thickness ol about 15 microns.

Crystal particle size and surface roughness of the PET films, so prepared, are shown in Table I, below.

Each PET film was surface-treated by corona discharge at 1 0 watt.min/m2 and, then, coated with a magnetic material-containing a coating slurry by a doctor blade coating procedure. The coat thickness was about 4 microns, in terms of dry coat thickness. The coating slurry was composed of 70 parts of Co-adsorbed y-Fe203, 26 parts of a binder resin (13 parts of a urethane rubber, 9.0 parts of nitrocellulose and 4.0 parts of polyvinyl chloride) and 260 parts of methyl ethyl ketone. The coating slurry further contained 4 parts of an isocyanate hardener ("Desmodule L", supplied by Bayer A.G.). The coated film was air-dried and, then, heat-treated at a temperature of 600C for 24 hours.

The dropout number increase of the magnetic recording tape, so prepared, was determined as follows.

Using a commercially available VHS tape video tape recorder, a three stair step signal was recorded on a tape specimen at the optimum recording current and, then, reproduced. During the reproduction, the number of dropouts in output from the video head amplifier, which dropouts occurred in 1 5 minutes, was counted by using a dropout counter (made by NJS). The term "dropout" herein used refers to output reduction of at least 18 dB, continuing for a period of at least 20 micro-seconds. The determination of the number of dropouts was carried out at several locations on the tape specimen. The averate number of dropouts was referred to as "V,". The aforesaid recording-reproducing operation was repeated three times at the same locations on the tape specimen.The average number of dropouts as determined in the fourth (i.e. last) recording-reproducing operation was referred to as "V4". The dropout number increase is the difference between V4 and V,, expressed by the equation: Drop out number increase = V4-V1 The dropout number increase data, so obtained, are shown in Table I, below.

TABLE I Projection- Part icle P roj ect i on Value of Dropout Run forming size number formula number No. additive*l (angstroms) (H,) Y *2 increase 1 B 62.8 153.9 48.5 76 2 B 69.0 30.8 361.7 766.9 3 B 54.6 35.9 13.6 19.6 4 B 61.9 156.4 23.8 0 5 B 64.1 33.3 242.2 0 6 B 63.8 50.0 212.5 95.8 7 A 65.2 66.7 223.2 29.8 8 A 71.8 296.2 69.5 75.7 9 A 69.0 78.2 297.7 301.5 10 A 71.8 50.0 402.1 394.5 11 A 72.6 51.3 419.3 240.9 12 A 61.3 59.0 141.1 134.4 13 D (38 /62) 55.8 60.3 9.0 5 14 D (38 /62) 53.8 67.9 -48.6 9.9 *1 A: Calcium acetate B: Kaolin D:Calcium acetate plus calcium carbonate The numerals within the parenthesis refer to the proportion of calcium acetate ( /0) /calcium carbonate (%) *2 Value of formula Y: Y = 23.7A - 1.35#1H2 - 1231.97 The initial dropout number of the magnetic recording tape was also determined, which number was observed when the magnetic recording tape having recorded thereon a three stair step signal was reproduced. The results are shown in Table II, below.

TABLE II Projection- Particle Projection Run forming size number Dropout No. additive *1 (angstroms) (H2) number 15 B 63.5 41 154 16 B 52.8 40.8 72 17 C 60.4 45.1 280 14 D(38/62) 53.8 67.9 955.

18 C 61.3 58.6 884 19 A 60.7 62.5 118 20 A 60.7 62.5 136 21 A 65.8 226.7 108 22 A 65.8 226.7 159 23 A 46.5 35.2 65 *1 A: Calcium acetate B: Kaolin C: Calcium carbonate D: Calcium acetate plus calcium carbonate The numerals within the parenthesis refer to the proportion of calcium acetate (%) /calcium carbonate ( /0) It will be seen from Table II, above, that the initial dropout number obtained when calcium acetate or kaolin is used as a projection-forming additive (Run Nos. 1 5, 1 6, 19 through 23) is smaller than that obtained when calcium carbonate is used (Run Nows.14,17 and 18).

The initial dropout number was also determined for the tapes which were prepared by using a finely divided calcium carbonate additive in combination with a finely divided kaolin additive and/or calcium acetate (interesterification reaction catalyst). The results are shown in Table lil, below.

TABLE Ill Projection Number Projection- Proportion Particle Dropout Run forming of CaCO, size (H,) (H,) (H, ) (H,) hnxl03 number No. additive'l (O/o) '2 (angstroms) ^3 '3 *3 "3 *4 per meter 24 D 62 48.8 132.8 32.6 7.80 7.8 -10.7 530 25 D 32 50.0 112.7 32.5 3.40 0 - 9.0 640 26 E 20 53.3 61.2 39.1 1.25 0 - 9.7 120 27 E 30 53.3 42.2 25.8 3.90 0.8 - 5.1 360 28 D 10 38.4 40.0 51.8 0 0 7.9 80 29 D 10 40.5 34.2 37.1 1.00 0 3.0 100 30 D 10 51.3 29.5 38.2 1.70 0 4.3 180 31 D 10 49.7 37.1 73.2 3.90 0 6.5 120 32 D 10 49.7 43.0 33.9 3.90 0 - 1.5 160 33 D 10 43.8 41.7 37.3 2.60 0 3.7 140 34 D 10 43.3 49.5 38.2 1.70 0 3.1 140 *1 D: Calcium acetate plus calcium carbonate E:Kaolin plus calcium carbonate *2 Proportion of calcium carbonate in % by weight based on the total weight of calcium carbonate plus koalin or calcium acetate *3 Projection height H1: from 0.27 to 0.53 micron H2: from 0.54 to 0.80 micron H3: from 0.81 to 1.07 microns H4: from 1.08 to 1.34 microns *4 Birefringence (An) was determined as follows.

Using a polarization microscope (Model POH supplied by Nippon Kogaku K.K.), a retardation R (nm) is measured, wherein white light projected from the light source is transmitted through a green filter [546.1 (nm) ] and a compensator. Birefringence (An1) is calculated from the retardation R, so measured, according to the equation: An, = R/d wherein d is thickness of film specimen in ,um. The birefringence (An1), so calculated is corrected according to the equation: Corrected birefringence An = 1,000 An1/(0.009X3 - 0.11 3X2 + 2.1 14X + 100.229) wherein X is the deviated angle in degree of the molecular orientation direction from the longitudinal direction.

The corrected birefringence An is expressed in a plus number when na - > 0, and in a minus number when n~nss na > 0 where na and na are corrected refractive indexes as determined on the longitudinal direction of the film specimen and on the transverse direction thereof, respectively.

It will be seen from Table Ill, above, that, when the proportion of calcium carbonate is large (Run Nos. 24, 25 and 27), the dropout number of the tape is undesirably large. In contrast, when the proportion of calcium carbonate is small (Run Nos. 26 and 28 through 34), the dropout number is small.

Claims (3)

1. A magnetic recording tape comprising a polyethylene terephthalate substrate and a magnetic material-containing layer closely adhered onto the substrate, characterized in that said polyethylene terephthalate substrate satisfies the formula (I): 23.7A-1 .351 H2-1231 .97 < 20 ~ 20 (1) wherein A is the average particle size of the polyethylene terephthalate crystals, determined as hereinafter mentioned, and H2 is the average number of projections present in a unit area of 1 mm2 on the surface of the substrate, which projections have a height falling within the range of from 0.54 to 0.80 micron;; said surface projections being formed either (i) by incorporating both of finely divided kaolin and finely divided calcium carbonate into an ester-interchange reaction mixture or a polycondensation mixture in the course of manufacturing polyethylene terephthalate, or into the molten polyethylene terephthalate in the step of forming the polyethylene terephthalate substrate, the amount of the finely divided calcium carbonate being less than about 25% by weight based on the total weight of the finely divided calcium carbonate and the finely divided kaolin, or (ii) by incorporating finely divided calcium carbonate or a mixture of finely divided kaolin and finely divided calcium carbonate into an ester-interchange reaction mixture or a polycondensation mixture in the course of manufacturing polyethylene terephthalate, or into the molten polyethylene terephthalate in the step of forming the polyethylene terephthalate substrate, and further incorporating calcium acetate into an ester-interchange reaction mixture of dialkyl terephthalate and ethylene glycol, followed by polycondensation of the interesterified product, thereby to precipitate particles of the calcium compounds represented by the following formula, during the polycondensation:

wherein n is an integer of from 0 to 3, the mount of the finely divided calcium carbonate being less than about 25% by weight based on the total weight of the finely divided calcium carbonate, the calcium acetate and, if any, the finely divided kaolin.

2. A megnetic recording tape according to claim 1, wherein said polyethylene terephthalate substrate satisifes the formula: 23.7A- 1.351H2- 1231.97 < 10 wherein A and H2 are the same as defined in claim 1.

3. A magnetic recording tape substantially as herein described with reference to the Examples.