Magnetic recording medium

Abstract: A magnetic recording medium comprising a magnetic layer which comprises a magnetic powdery material, a first carbon black having a specific surface in terms of a BET value of from 80 m²/g to 900 m²/g and an oil absorption in terms of a DBP value of not less than 110 ml/100g, and a second carbon black having a specific surface in terms of a BET value of not less than 50 m²/g and an oil absorption in terms of a DBP value of less than 110 ml/100g.

Description: FIELD OF THE INVENTION The present invention relates to a magnetic recording medium such as a magnetic tape, a magnetic sheet, a magnetic disc or the like. BACKGROUND OF THE INVENTION Generally, a magnetic recording medium such as a magnetic tape is produced by coating and drying a magnetic coating liquid comprising a magnetic powdery material, binder resin, and the like on a support. In such a magnetic recording medium, particularly in video tape, carbon black is osed in combination with the above materials in order to satisfy simultaneously some of the properties such as the conductivity, coefficient of friction, surface nature (i.e., electromagnetic conversion property), wear resistance, and the like. As the technique of this kind there are known those conventional techniques disclosed in, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication ) Nos. 5426/1984, 1614/1984, 218039/1983 and 211321/1983, and Japanese Patent Examined Publication Nos. 20203/1978, 9041/1979 and 4968/1982. These techniques, however, were inadequate to provide a recording medium having an excellent conductivity and a low coefficient of friction, i.e., being excellent in the runnability as well as in the wear resistance, without roughening the surface, i.e., without deteriorating the electromagnetic conversion property. Namely, in the above-mentioned techniques, where carbon black whose particle sizes are not more than 40 nm were used, the conductivity is not always improved. For example, carbon black with a particle size of 23 nm, when used alone, must be incorporated into the magnetic layer in a quantity of not less than 15 parts by weight to 100 parts by weight of a magnetic powdery material to make the surface resistivity lower than 10 10 Ω/sq. However, if such a large amount of carbon is added, the electromagnetic conversion property is lowered. Also, in the carbon black whose particle sizes are not more than 40 nm, the friction on the magnetic layer's surface increases. If, in order to reduce the friction, a large amount of a lubricant is added, then there occur such phenomena as the deterioration of the electromagnetic conversion property, bleed-out phenomenon, etc. Where a large amount of carbon black whose particle sizes are not less than 50 nm is used, the electromagnetic conversion property is lowered. Also, the carbon black whose particle sizes are not less than 50 nm do not always improve the electromagnetic conversion property but rather generally lower the electromagnetic conversion property as in the case of carbon black having particle sizes of not more than 30 nm and a BET value of not less than 500 m 2 /g or carbon black having particle sizes of not less than 40 nm. The document JP-A 59-213 027 discloses a magnetic recording medium comprising two distinct carbon blacks of BET values between 200 m 2 /g and 500 m 2 /g and between 40 m 2 /g and 200 m 2 /g, respectively. As examples for the first of these carbon blacks, Vulcan XC 72® and Conductex 975® are mentioned. The document JP-A 61-24016 teaches to use a conductive carbon black having a BET value of less than 500 m 2 /g and a DBP value of 100 to 200 ml/100 g in combination with a graphitized carbon black. SUMMARY OF THE INVENTION It is an object of the present invention to provide a magnetic recording medium which is improved in the running durability as well as in the conductivity without deteriorating the electromagnetic conversion property. The above object of the present invention is accomplished by a magnetic recording medium comprising a magnetic layer containing a first carbon black whose specific surface is in terms of a BET value of from 80 m 2 /g to 900 m 2 /g and whose oil absorption is in terms of a DBP value from 110 ml/100g to 160 ml/100g, and a second carbon black whose specific surface is in terms of a BET value of not less than 50 m 2 /g, and whose oil absorption is in terms of a DBP value of less than 110 ml/100g. BRIEF DESCRIPTION OF THE DRAWINGS The drawings show examples of the present invention. Figure 1, Figure 2 and Figure 3 are partially enlarged sectional views of the respective examples. Figure 4 is a graph showing changes in the RF output according to the adding amount of carbon black. Figures 5, 6, 7 and 8 are graphs showing the respective characteristics of carbon black when its physical quantity is varied. DETAILED DESCRIPTION OF THE INVENTION It has now been found that, according to this invention, the aforementioned shortcomings of the conventional techniques can be adequately dissolved because the first carbon black and second carbon black having BET values and oil absorptions in their respective specific ranges are used in combination. That is, both first and second carbon blacks, since their BET values are as relatively large as not less than 80 m 2 /g and 50 m 2 /g, respectively, and less than 900 m 2 /g, tend to show high-density recording and high-electromagnetic conversion properties, and also, due to their oil absorptions, show not only their dispersibilities being maintained satisfactory but their friction coefficients reduced and also light-tightness improved. Furthermore, as regards the improvement on the conductivity, the first carbon black, since it shows an oil absorption of not less than 110 ml/100g, but not exceeding 160 ml/100g, easily takes a relatively structural construction to show a high conductivity and to lower friction. In addition, in the above, the BET value of the first carbon black should be preferably from 80 m 2 /g to 400 m 2 /g, and more preferably from 80 m 2 /g to 200 m 2 /g. The second carbon black, even if its adding amount is increased, does not deteriorate the surface nature nor lower the electromagnetic conversion property. In the second carbon black, the desirable range of its BET value is preferably 50m 2 /g to 400m 2 /g, more preferably 50m 2 /g to 200 m 2 /g. In this invention, in order to obtain the above effect, the adding amount of carbon black, as the total amount of both first and second carbon blacks, is preferable 3 to 20 parts by weight to 100 parts by weight of a magnetic powdery material, and more preferably from 3 to 15 parts by weight. There is a desirable range also in the proportion of both carbon blacks: The first carbon black should be used in an amount of preferably 0.1-10 parts by weight, more preferably 0.1-5 parts by weight, to 100 parts by weight of a magnetic powdery material, and the second carbon black should be in an amount of 3-15 parts by weight, more preferably 5-12 parts by weight). Useful examples of the first carbon black in this invention include Raven® 3200 manufactured by Columbian Carbon; Vulcan®-P, Vulcan®-9; and the like. Useful examples of the second carbon black include Raven® 1035, Raven® 1255, Raven® 1000 and Raven® 2000 manufactured by Columbian Carbon; Black Pearls®-L, Black Pearls® 1000, Black Pearls® 900 and Monarck®-800 by Cabot; CF-9® by Mitsubishi Chemical Industries; and the like. In addition, the terms 'specific surface' in above means the surface area per unit weight and is a physical quantity quite different from the average particle size; for example, there exist particles identical in the average particle size but differing in the specific surface; i.e., those having a large specific surface and those having a small specific surface. Measurement of the specific surface can be carried out by, e.g., a measuring method generally called the B.E.T. method, in which a magnetic powdery material is first deaerated being heated to around 250°C for 30 to 60 minutes to thereby remove adsorbates therefrom. and then is conducted into the measuring instrument, wherein its inside initial pressure of a nitrogen gas is settled to 4.9 Pa (0.5kg/m 2 ), whereby adsorption measurement is made with nitrogen at a liquid nitrogen temperature of -195°C. For more detail, reference can be made to J. Am. Chem. Soc., 60 309 (1938). As the measuring instrument for the specific surface (BET value), a powder measuring instrument called 'Quantasorb®' manufactured jointly by Yuasa Battery Co. and Yuasa Ionics Co. can be used. General explanations about the specific surface and measuring methods therefor are described in detail in the 'Measurement of Powder' (by J. M. Dallavalle and Clydeorr Jr.)(translated into Japanese 'Funtai no Sokutei' by Benda et al; published by Sangyo Tosho Publishing Co.), and also in the 'Kagaku Benran (Handbook of Chemistry)' (Applied Chemistry Sec., p.1170-1171, edited by the Japan Chemical Society, published on 30th April 1966 from Maruzen Co., Ltd.). (In the above'Kagaku Benran', the specific surface (Hihyomenseki) is merely called 'Hyomenseki' (surface area), but this is quite the same in meaning as the terms 'specific surface' in this specification.) As regards the foregoing 'oil absorption (DBP method)', DBP (Dibutyl phthalate) is added little by little to 100g of a pigment powder, and they are kneaded and in the meantime the state of the pigment is observed, and the ml value of the DBP obtained when finding a point at which the pigment is turned into the agglomerated state from the dispersed state is defined as the DBP oil absorption. The magnetic recording medium of this invention, as shown, for example, in Figure 1, has a magnetic layer 2 on its support 1, and also has a BC layer 3 on the reverse side of the support to the magnetic layer 1. This BC layer may or may not be provided. Examples of the magnetic powder, particularly ferromagnetic powder, to be used in magnetic layer 2 include magnetic powdery materials such as γ-Fe 2 O 3 , Co-containing γ-Fe 2 O 3 , Fe 3 O 4 , CO-containing Fe 3 O 4 , and the like. Of these, the preferred ones are the Co-containing iron oxides. Remarkable effects of such magnetic powdery materials can be obtained where the BET value thereof is not less than 35 m 2 /g, and more preferably not less than 40 m 2 /g. The magnetic layer 2 may contain a lubricant (such as silicone oil, graphite, molybdenum disulfide, tungsten disulfide, or one of fatty acid esters formed from monobasic fatty acids having 12 to 20 carbon atoms (such as stearic acid) and monohydric alcohols having 3 to 26 carbon atoms), an antistatic agent (such as graphite), and the like, in addition to the foregoing first and second carbon blacks. The magnetic layer may also contain a nonmagnetic abrasive particulate material. In that case, alumina (such as α-Al 2 O 3 (corundum), etc.), artificial corundum, fused alumina, silicon carbide, chromium oxide, diamond, artificial diamond, garnet, emery (principal components: corundum and magnetite), or the like may be used. The abrasive material content of the magnetic layer is desirable to be not more than 20 parts by weight to the magnetic powder, and the abrasive material's average particle size is preferably 0.5 µm, and more preferably not more than 0.4 µm. As the binder resin for the magnetic layer, at least a polyurethane may be used, which may be synthesized by the reaction of a polyol with a polyisocyanate. In addition to the polyurethane, if a phenoxy resin and/or a vinyl chloride-type copolymer also are to be contained, the binder resin, when applied to the magnetic layer, can improve the dispersibility of the magnetic powder to thereby increase the mechanical strength thereof. The use of the phenoxy resin and/or vinyl chloride-type copolymer alone hardens the layer to excess, but this can be prevented by incorporating the polyurethane, and further the incorporation of the polyurethane improves the adherence of the layer to the support or to the subbing layer. In the place of the above resins, cellulose-type resins, thermoplastic resins, thermosetting resins, reaction-type resins or electron radiation setting-type resins may also be used as the binder resin. The foregoing carbon black used in the magnetic layer 2 of this invention may be added also to the BC layer 3. The recording medium of Figure 1 may be one having a subbing layer (not shown) provided between magnetic layer 2 and support 1, or one having no subbing layer (the same shall apply hereinafter), or one having the support subjected to corona discharge treatment. As the material for support 1, a plastic such as polyethylene terephthalate or polypropylene; a metal such as Al, Zn, etc.; glass plates, BN, Silicon carbide; ceramics such as porcelain, earthenware, etc.; or the like, may be used. At the time of coating and forming the above magnetic layer or other layer, in advance adding a polyfunctional isocyanate as a cross-linking agent in a specified quantity to the coating liquid is desirable in respect of enabling to strengthening the magnetic layer. Useful examples of such the cross-linking agent include triphenylmethane triisocyanate, tris-(p-isocyanate-phenyl)-thiophosphite, polymethylenepolyphenyl isocyanate, and the like, in addition to the above-mentioned polyfunctional polyisocyanates. And also methylene diisocyanate-type and tolylene diisocyanate-type cross-linking agents are also useful. When the magnetic layer is hardened by electron beam radiation or the like, the addition of the isocyanate compound may be made or may be omitted. Figure 2 shows another example of the magnetic recording medium, wherein a OC layer 4 is provided on the magnetic layer 2 of the medium of Figure 1. This OC layer 4 is provided for the purpose of protecting the magnetic layer 2 from being damaged, so that it needs to have a sufficient slidability. Upon this, the polyurethane resin (preferably phenoxy resin and/or vinyl chloride-type copolymer in combination) that is used in the above magnetic layer 2 is to be used as the binder resin of the OC layer 4. The surface roughness of OC layer 4, particularly in connection with the chroma S/N, should be set as Ra ≦ 0.01 µm, and Rmax ≦ 0.13 µm. In this instance, the surface roughness of the support 1 is desirable to be as smooth as Ra ≦ 0.01 µm and Rmax ≦ 0.13 µm. Figure 3 shows a magnetic recording medium that is constructed as a magnetic disc, wherein the support 1 has on each of both sides thereof the magnetic layer 2 and OC layer 4 similar to the above, and the OC layer 4 is allowed to contain a binder resin comprised principally of the foregoing polyurethane resin. EXAMPLES The present invention will be illustrated in detail by the following examples. The term 'part(s)' used hereinafter represents 'part(s) by weight'. Each of the magnetic powder-dispersed coating liquids obtained by dispersing by means of a ball mill the constituents given in the respective compositions in Examples 1 through 10 and Comparative Examples 1 through 4, after the addition of a polyisocyanate thereto, was coated on one side of a 15 µm-thick polyethylene terephthalate base by means of an extrusion coater, then dried, and then the coated product was subjected to calender-surface treatment. After that, it was slitted into 1.27 cm (1/2-inch)-wide tape rolls. The average thickness of the magnetic layer at this time was 4.0 µm. Each of the obtained tape samples was measured with respect to the following items. Consequently, the results as shown in Table 2 were obtained. RF output: A VTR deck for RF output measurement was used to measure the RF output at 4MHz. The output (in dB) obtained after 100-time-repeated video tape reproducing operations is shown in a relative value to that of the Reference Example (0dB) in Table 2. Lumi S/N: A noise meter (925 D/1) manufactured by Shibasoku K.K. was used to meaure the lumi S/N. The obtained values each is shown in terms of the difference from that of Example 1 as reference tape (0dB). The measurement was made with the high-pass filter being set to 4.2 MHz and the low-pass filter being set to 10 kHz. The VTR used was a JVC HR-D120. Chroma S/N: The measurement was made in similar manner to the RF output. Coefficient of Friction: The tape tensions at both the inlet and the outlet of the heat cylinder were measured to find µ = 1 π ln T 2 (outlet-side tension) T 1 (inlet-side tension). Surface resistivity: Measurement was made by using a surface potentiometer. Drop-out: Number of scratch defects that appeared on the reproduced image. Runnability test: Tape samples were continuously run through a video deck over a period of 200 hours at a temperature of 40°C with a relative humidity of 80% to measure degrees of the deterioration of the RF output, skew, and shedding of each sample. The runnability tests are shown in Table 2. The terms CF-9, Vulcan, Raven , Black-Pearls, XC-72, Sterling, Conductex, HS-500, Monarck, Asahi Carbon, Ketjen Black, Neospectra and Asahi Thermal, used in the following description, are trade-marks. From the results it is understood that Examples 1 through 10, particularly Examples 1 through 8, in which the two types of carbon black are used in combination according to this invention, are excellent in the following points as compared to the comparative examples. (1) High-level electromagnetic conversion properties can be obtained. (2) Surface resistivities of not more than 1x10 10 Ω/sq can be obtained. Consequently, dust hardly attaches to the surface, and drop-out defects are reduced. (3) The coefficient of friction on the surface of the magnetic layer is lowered. As a result, a recording medium excellent in the running durability can be obtained. (4) The use of the above types of carbon black improves the dispersing stability of a magnetic coating liquid to hardly form agglomerates, thus enabling to reduce the formation of drop-out defects. Also, aside from the above samples, different samples, wherein the total amount of the types of carbon black in the above Example 1 or Comparative Example 2 was varied, were prepared, and they were measured with respect to their RF output values, and the relation between the varied amounts of the carbon black and the RF values was found, and the results are shown in Figure 4 with the RF output value at the time when the amount of the carbon black is zero being regarded as 0dB. As is apparent from Figure 4, the example for this invention is conspicuously smaller in the fall of the RF output than the comparative example. Subsequently, as given in Table 3, various further carbon blacks were used to prepare samples in Example 12 and Comparative Examples 5 through 15, and the obtained mediums were measured with respect to their characteristics with relation to the physical quantities of the carbon blacks. The results are shown in Figure 5 through Figure 8. The RF values in Figures 5 through 7 are shown in relative values to the RF output value of Example 1 regarded as 0dB. Figure 5: Shows the relation between the BET value and the RF output of the second carbon black when the first carbon black (Vulcan-P) was fixed. (According to data of Examples 1, 2 and 3 and Comparative Examples 12 and 13.) Figure 6: Shows the relation between the DBP oil absorption and the RF output of the second carbon black when the first carbon black (Vulcan-P) was fixed. (According to data of Examples 1, 2, 3 and 5 and Comparative Examples 10 and 11.) Figure 7: Shows the relation between the BET value and the RF output of the first carbon black when the second carbon black (Raven-1035) was fixed. (According to data of Examples 2 and 11 and Comparative Examples 5, 6, 7 and 8.) Figure 8: Shows the relation between the DBP oil absorption and the coefficient of kinetic friction of the first carbon black when the second carbon black (Raven-1035) was fixed. (According to data of Example 2 and Comparative Examples 5, 6, 9, 14 and 15.) As has been described above, in the first carbon black, a high RF output can be obtained only where the BET value thereof is from 80m 2 /g to 900m 2 /g, and the RF output can be improved when the oil absorption is made from 110 ml/100g to 160 ml/100 g to remarkably lower the coefficient of friction, and further, in the second carbon black, the RF output can be improved where the BET value thereof is not less than 50 m 2 /g and further improved when the oil absorption thereof is made less than 110 ml/100g. Thus, it is understood that the superiority of restricting these characteristics to the specified ranges in this invention is remarkable.

Claims: A magnetic recording medium comprising a magnetic layer which comprises a magnetic powdery material, a first carbon black having a specific surface in terms of a BET value of from 80 m 2 /g to 900 m 2 /g and an oil absorption in terms of a DBP value from 110 ml/100g, to 160 ml/100g, and a second carbon black having a specific surface in terms of a BET value of not less than 50 m 2 /g and an oil absorption in terms of a DBP value of less than 110 ml/100g. The magnetic recording medium of claim 1, wherein said specific surface of said first carbon black is in terms of a BET value of from 80 m 2 /g to 400 m 2 /g. The magnetic recording medium of claim 2, wherein said specific surface of said first carbon black is in terms of a BET value of from 80 m 2 /g to 200 m 2 /g. The magnetic recording medium of claim 1, wherein said specific surface of said second carbon black is in terms of a BET value of from 50 m 2 /g to 400 m 2 /g. The magnetic recording medium of claim 4, wherein said specific surface of said second carbon black is in terms of a BET value of from 50 m 2 /g to 200 m 2 /g. The magnetic recording medium of claim 1, wherein said first carbon black and said second carbon black are added in a total amount thereof of 3 to 20 parts by weight to 100 parts by weight of said magnetic powdery material. The magnetic recording medium of claim 6, wherein said total amount of said first carbon black and said second carbon black is 3 to 15 parts by weight to 100 parts by weight of said magnetic powdery material. The magnetic recording medium of claim 1, wherein said first carbon black is added in a quantity of 0.1 to 10 parts by weight to 100 parts by weight of said magnetic powdery material. The magnetic recording medium of claim 8, wherein said adding quantity of said first carbon black is 0.1 to 5 parts by weight to 100 parts by weight of said magnetic powdery material. The magnetic recording medium of claim 1, wherein said second carbon black is added in a quantity of 3 to 15 parts by weight to 100 parts by weight of said magnetic powdery material. The magnetic recording medium of claim 10, wherein said adding quantity of said second carbon black is 5 to 12 parts by weight to 100 parts by weight of said magnetic powdery material.

Inventors: MIYAKE TORU, NAKANO YASUSHI, MIYAKE, TORU, NAKANO, YASUSHI
Applicants: KONISHIROKU PHOTO IND, KONICA CORPORATION
Application year: 1987
Main classification: G11B5
IPC codes: C09D5, G11B5