Magnetic recording medium and process for producing the same
FIELD OF THE INVENTION 
     The present invention relates to a magnetic recording medium and a process for producing the same, and more particularly, to a magnetic recording medium having good electric characteristics and high wear resistance. 
     BACKGROUND OF THE INVENTION 
     Most conventional magnetic recording mediums are produced using a binder of a thermoplastic resin such as vinyl chloride acetate resin, vinyl chloride/vinylidene chloride resin, cellulose resin, acetal resin, urethane resin and acrylonitrile butadiene resin which are used either alone or in combination. However, one defect of such mediums is that the magnetic layer is unsatisfactorily wear-resistant and tends to foul the tape transport path. 
     Methods are known that use a thermosetting resin such as melamine resin or urea resin or blend the thermoplastic resins described above with a binder that forms crosslinkages by chemical reaction, such as an isocyanate compound or an epoxy compound. However, such crosslinkable binders have two important disadvantages: (1) the resin solution having magnetic particles dispersed therein does not have long keeping quality (i.e., has a short pot life) and the resulting magnetic coating composition, and hence the magnetic tape, tends not to have consistent physical properties; and (2) after drying the applied magnetic coating, it must be heated for curing, and this results in prolonged processing time. 
     To overcome these disadvantages, Japanese Pat. No. 12423/72 and Japanese Patent Application (OPI) Nos. 13639/72, 15104/72 and 77433/75 (the term &#34;OPI&#34; as used herein refers to a &#34;published unexamined Japanese patent application&#34;) propose methods for producing a magnetic recording medium which use a binder made of an acrylate ester oligomer and monomer and cure the dried magnetic coating by irradiation with electron beams. However, such methods are not capable of producing a magnetic recording medium having good electrical characteristics and mechanical properties. 
     There has been an increasing demand for providing magnetic recording media with better electrical characteristics by using a magnetic coating composition comprising a good dispersion of ferromagnetic particles. A poor dispersion of ferromagnetic particles can cause low output or noise. The conventional method of curing the magnetic coating by electron beams is not satisfactory since if a high concentration of magnetic particles are contained therein to achieve higher-density recording, the resulting magnetic coating composition has a short pot life and cannot provide a magnetic tape having electrical characteristics that withstand practical use. On the other hand, a thin base is required for achieving extended recording and to meet this requirement, the mechanical properties of the magnetic coating must be improved. Furthermore, with the use of multi-functional video cassette recorders, there has been a demand for video tape that withstands service under hostile conditions, but it has not yet been possible to prepare a magnetic recording medium having sufficiently satisfactory mechanical properties. 
     SUMMARY OF THE INVENTION 
     One object of the present invention is to provide a magnetic recording medium having good electrical characteristics and a process for producing such magnetic recording medium. 
     Another object is to provide a magnetic recording medium having a magnetic layer having good mechanical properties and a process for producing such magnetic recording medium. 
     Still another object is to provide a process for producing a magnetic recording medium that uses a magnetic coating composition having a long pot life and uniform physical properties. 
     A further object is to provide a magnetic recording medium having high wear resistance and a process for producing such magnetic recording medium. 
     A still further object is to provide a process for producing a magnetic recording medium that obviates the heating step for curing the applied magnetic coating composition. 
     To meet the above objects, the present inventors have made extensive studies on the conventional method of using a thermoplastic and thermosetting binders, methods of using a binder that forms crosslinkage by chemical reaction, and methods of using a binder that cures upon irradiation with electron beams, and have accomplished the invention described hereunder. 
     The objects of the present invention are achieved by a process which includes the step of applying to a base a composition comprising (1) a compound having an unsaturated bond polymerizable by electron beams, (2) a compound having one or more carbon-carbon unsaturated bonds, (3) ferromagnetic particles, and (4) an organic solvent, and polymerizing and curing the applied coating by irradiation with electron beams, said compound having one or more unsaturated bonds being such that at least 10% thereof is adsorbed on the ferromagnetic particles in a mixture of 10 parts by weight of a 1 wt% solution of the compound and 1 part by weight of the ferromagnetic particles. 
     According to the present invention, a magnetic recording layer is formed by using a binder of a compound that contains in the molecular chain one or more carbon-carbon unsaturated bonds curable by electron beams and which has great adsorbability on ferromagnetic particles plus a compound curable by electron beams, and upon irradiation with electron beams, there occurs a surprising increase in the dispersibility of the ferromagnetic particles so as to form a magnetic coating composition having long storage quality, thus producing a magnetic tape having greatly improved electrical characteristics and wear resistance. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Suitable examples of the compound having one or more carbon-carbon unsaturated bonds and having great adsorbability onto the ferromagnetic particles are aliphatic acids, amides, phosphoric acid, sulfuric acid, phosphorous acid, and sulfurous acid. Specific examples include unsaturated monoaliphatic acids such as acrylic acid, crotonic acid, 3-butenoic acid, 2-pentenoic acid, 3-octenoic acid, 2-decenoic acid and 10-undecenoic acid; unsaturated polybasic acids such as maleic acid, fumaric acid, itaconic acid, 2-butene-1,4-dicarboxylic acid and muconic acid; unsaturated aliphatic acid amides such as acrylamide, crotonamide, 2-penteneamide and maleinamide; and monoalkyl unsaturated esters of sulfuric acid and phosphoric acid such as β-acryloyloxyethyl phosphate and vinylsulfone. 
     According to the finding of the present inventors, a magnetic coating composition comprising a highly uniform dispersion of ferromagnetic particles can be produced by adding such compounds at least 10% of which is adsorbed on ferromagnetic particles in a mixture of 10 parts by weight of a 10 wt% solution of these compounds and 1 part by weight of the ferromagnetic particles. The desired dispersion is not obtained if only less than 10% of these compounds can be adsorbed on the ferromagnetic particles. 
     The compound having an unsaturated bond polymerizable by electron beams is a compound having one or more carbon-carbon unsaturated bonds. Examples are compounds containing an acryloyl group, acrylamido group, allyl group, vinyl ether group or vinyl thioether group, and unsaturated polyesters. Specific examples include methyl acrylate and homologous acrylic acid alkyl esters; styrene and homologous α-methylstyrene and β-chlorostyrene; acrylonitrile, vinyl acetate and vinyl propionate. Two or more unsaturated bonds may be present in the molecule. Examples of such compound are found in Kankosei Jushi Data-Shu (A List of Data on Photosensitive Resins), published by Sogo Kagaku Kenkyusho, December 1968, pp. 235-236. Particularly preferred compounds are unsaturated esters of polyols such as ethylene diacrylate, diethylene glycol diacrylate, glycerol triacrylate, ethylene diacrylate and pentaerythritol tetraacrylate, and glycidyl acrylate having an epoxy ring. A compound having one unsaturated bond in the molecule may be used in combination with a compound having two or more unsaturated bonds. 
     These compounds are oligomeric compounds as described below. Particularly preferred compounds are those which have an acrylate group at terminal ends of the straight chain or the side chain, and such compounds are described in A. Vrancken, Fatipec Congress, Vol. 11, p. 19 (1972). An example is represented below: ##STR1## The polyester unit of the above formula may be replaced by a polyurethane unit, epoxy resin unit, polyether unit and polycarbonate unit, or a mixture thereof. These compounds generally have a molecular weight of from 1,000 to 20,000, but are not limited thereto. A mixture of the above named monomers and polymers may be used as the compound polymerizable by electron beams. 
     In addition to the above binder components, if desired, thermoplastic resins such as vinyl chloride/vinyl acetate copolymer, cellulosic resins, acetal resins, vinyl chloride/vinylidene chloride resins, urethane resins and acrylonitrile butadiene resins may be added alone or in admixture to the magnetic coating composition of the present invention in order to achieve the desired mechanical strength of the resulting magnetic layer. 
     The compound adsorbable to the ferromagnetic particles is used in an amount of from 0.1 to 10 wt%, preferably from 0.2 to 5 wt%, and most preferably 0.2 to 3 wt%, based on the weight of the ferromagnetic particles. If the amount of the compound is too small, the dispersibility of the ferromagnetic particles is decreased, and if the amount is too large, more energy is required for the subsequent polymerization. The total of the binder components in the magnetic coating composition (the organic components of the coating composition excluding the organic solvent) is preferably from 7 to 0.1 part, and more preferably from 0.5 to 0.2 part by weight per part by weight of the ferromagnetic particles. 
     Suitable ferromagnetic particles include finely divided ferromagnetic iron oxide, chromium dioxide, and metal alloys. The ferromagnetic iron oxide and chromium dioxide particles have a length to breadth ratio of from about 2/1 to 20/1, and preferably at least 5/1, and an average length of from about 0.2 to 2.9 μm. The ferromagnetic alloy particles are such that at least 75 wt% thereof is metal and at least 80 wt% of the metal is a ferromagnetic metal (i.e., Fe, Co, Ni, Fe-Co, Fe-Ni, Co-Ni, or Fe-Co-Ni) and have a major axis of not more than about 1.0 μm. 
     Suitable organic solvents include ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone; esters such as methyl acetate, ethyl acetate, butyl acetate, ethyl lactate and glycol acetate monoethyl ether; ethers and glycol ethers such as glycol dimethyl ether, glycol monoethyl ether and dioxane; aromatic hydrocarbons such as benzene, toluene and xylene; and chlorinated hydrocarbons such as methylene chloride, ethylene chloride, carbon tetrachloride, chloroform, ethylene chlorohydrin and dichlorobenzene. 
     After coating a magnetic composition, the magnetic coating may be smoothed whereby electric properties can be markedly improved. For smoothing the surface of the magnetic coating, calendering can be performed with two mirror finishing rolls or with a mirror finishing roll and an elastic roll. The mirror finishing roll may be a metal roll, and the elastic roll may be a cotton roll or synthetic resin (e.g., nylon or polyurethane) roll. 
     Preferably, the magnetic layer is calendered with a train of 30 rolls at a nip pressure of from about 25 to 400 kg/cm, preferably from 50 to 100 kg/cm, at a temperature of from about 10° to 150° C., more preferably from 10° to 70° C., and at a rate of from 5 to 200 m/min. Higher temperatures and nip pressure result in magnetic layer shedding or base deformation. If the calendering speed is less than about 5 m/min., the desired smoothing effect is not achieved, and if it is more than about 200 m/min., the smoothing operation becomes difficult. 
     In the process of the present invention, removal of organic solvent, smoothing of magnetic layer and irradiation with electron beams can be effected in any order. Thus, magnetic layer is smoothed by mirror finishing rolls before or after the organic solvent is removed from the magnetic layer. The removal of organic solvent can be effected before and/or after irradiation with electron beams. The organic solvent may be completely or partially removed. For instance, the magnetic recording medium is left to stand or dried under ordinary conditions to remove all or part of the organic solvent, and then the medium is smoothed. 
     The magnetic coating composition of the present invention may further contain suitable additives such as lubricant, polishing agent, corrosion inhibitor, or antistat (i.e., antistatic agent). Examples of useful lubricants are a saturated or unsaturated higher aliphatic acid, aliphatic acid ester, higher aliphatic acid amide, higher alcohol, silicone oil, mineral oil, edible oil, and fluorine-containing compound. These lubricants may be added at the time that the magnetic coating composition is prepared. Alternatively, they can be spread or sprayed onto the magnetic layer after drying, smoothing or curing (by electron beams) thereof either directly or in the form of a solution in an organic solvent. 
     The coating composition can be prepared by charging a mixer with the magnetic particles and all other ingredients simultaneously or sequentially. Alternatively, dispersant may be added simultaneously with the magnetic powder, then the compound that can be polymerized and cured by electron beams, and then the thermoplastic resin may be added. 
     Various mixers may be used to achieve thorough mixing of the magnetic coating composition. Examples are a two-roll mill, ball mill, sand grinder, disperser, high-speed impeller disperser and high-speed mixer homogenizer. 
     A magnetic coating is formed from the magnetic coating composition on a base by doctor coating, blade coating, air knife coating, squeeze coating, reverse roll coating and gravure coating. The magnetic coating composition is applied to the base to form a coating having a dry thickness of from about 0.5 to 15 μm. The dry thickness of the magnetic coating is determined by the use, shape and other specifications of the magnetic recording medium. 
     The magnetic coating thus formed on the base is dried after the magnetic particles in the coating are optionally oriented in an a.c. or d.c. magnetic field having a magnetizing force of from about 500 to 3,000 Oe. 
     The oriented magnetic layer is generally dried at between about 50° and 120° C., and preferably between 70° and 100° C., more preferably between 80° and 90° C., for a duration of from about 30 seconds to 10 minutes, preferably from 1 to 5 minutes, with air supplied at a rate of 1 to 5 kl/m 2 , preferably 2 to 3 kl/m 2 . Before the drying, the surface of the magnetic coating may be smoothed by a magnet smoother, smoothening coil, smoothening blade or smoothening blanket. 
     Suitable bases include polyesters such as polyethylene terephthalate and polyethylene-2,6-naphthalate; polyolefins such as polyethylene and polypropylene; cellulose derivatives such as cellulose triacetate; polycarbonate, polyimide and polyamideimide; non-magnetic metals such as aluminum, copper, tin, zinc and non-magnetic alloys including the same; paper, and apper coated or laminated with polyolefins. The non-magnetic base may be in the form of a film, tape, sheet, disc, card, drum or other suitable shapes, and a suitable material can be selected from among the various possible materials depending on the desired form and properties. The back surface (opposite the side having the magnetic coating) of the base may be provided with a back coating for various purposes, e.g., prevention of static buildup, transfer, and wow flutter, as well as improvement of the strength of the magnetic recording medium and the matting of the back surface. 
     Suitable electron beam accelerators include a Van de Graaff scanning accelerator, double scanning accelerator and curtain beam accelerator. A curtain beam accelerator is preferred because of its low price and high power output. The acceleration voltage is generally from 100 to 1,000 kilovolts, and preferably from 150 to 300 kilovolts. The absorbed dose is generally from 0.5 to 20 megarads, preferably from 2 to 10 megarads. An acceleration voltage lower than 100 kilovolts causes insufficient energy transmission, and a voltage higher than 1,000 kilovolts reduces the energy necessary for polymerization. If the absorbed dose is less than 0.5 megarad, the curing reaction is not sufficient to provide a magnetic medium having high strength, and if the dose is greater than 20 megarads, the efficiency of energy used for curing is reduced, or the web being irradiated generates heat, and in particular, the plastic base deforms. 
     According to the present invention, a magnetic recording medium having a smooth magnetic layer and having surprisingly good wear resistance and electrical characteristics can be produced. 
    
    
     The present invention is now described in greater detail by reference to the following examples and comparative examples which are provided here for illustrative purposes only, and are not intended to limit its scope. In the examples and comparative examples, all parts are by weight. 
     EXAMPLE 1 
     A composition having the formulation indicated below was agitated in a ball mill for 50 hours. 
     
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                      parts                                               
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γ-Fe.sub.2 O.sub.3                                                  
                        100                                               
Nitrocellulose (RS 1/2H of Daicel Ltd.)                                   
                        10                                                
Urethane resin (condensate of adipic                                      
                        10                                                
acid, butanediol and tolylene                                             
diisocyanate)                                                             
Acrylic acid            0.5                                               
Ester acrylate oligomer (Aronix M 6100                                    
                        2                                                 
of Toagosei Chemical Co., Ltd.)                                           
Hexamethylene diacrylate                                                  
                        2                                                 
Methyl ethyl ketone     250                                               
Stearic acid            1                                                 
Butyl stearate          1                                                 
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     The resulting coating composition was applied to a polyethylene terephthalate base (20μ thick) by a doctor blade to form magnetic layer in a dry thickness of 8μ. The magnetic particles were oriented with a cobalt magnet, and the web was dried at 100° C. for one minute to eliminate the solvent. The web was passed through a 5-roll calender mill (cotton rolls and mirror finishing rolls) to smooth the magnetic layer (roll temperature: 40° C., nip pressure: 100 kg/cm). The web was then irradiated with electron beams to provide an absorbed dose of 5 megarads (acceleration voltage: 200 kilovolts, beam current: 10 milliamperes). The resulting tape was referred to as Sample No. 1 
     COMPARATIVE EXAMPLE 1 
     A magnetic tape was prepared as in Example 1 except that the paint composition did not contain acrylic acid. The tape was referred to as Sample No. 2. Sample Nos. 3 to 7 were prepared as in Example 1 except that the acrylic acid was replaced by the compounds indicated below. 
     COMPARATIVE EXAMPLE 2 (SAMPLE NO. 3) 
     
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Hydroxyethyl acrylate   1 part                                            
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     EXAMPLE 2 (SAMPLE NO. 4) 
     
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CH.sub.2 ═CHCO.sub.2 CH.sub.2 CH.sub.2 OPO.sub.3 H (product           
                           1 part                                         
Kyoeisha Oil &amp; Grease Chemical                                            
Industry Co., Ltd.)                                                       
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     EXAMPLE 3 (SAMPLE NO. 5) 
     
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       Acrylamide     1 part                                              
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     EXAMPLE 4 (SAMPLE NO. 6) 
     
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       Maleic acid    0.5 part                                            
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     EXAMPLE 5 (SAMPLE NO. 7) 
     
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10-Undecenoic acid      1 part                                            
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     The compounds indicated in Table 1 were subjected to an adsorption test, the result of which is indicated in the same table. In the test, methyl ethyl ketone was used as a solvent. 
     
                       TABLE 1                                                     
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                          Adsorption                                      
Compound                  (%)                                             
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Acrylic acid              35                                              
β-Hydroxyethyl acrylate                                              
                          5                                               
CH.sub.2 ═CHCO.sub.2 CH.sub.2 CH.sub.2 OPO.sub.3 H                    
                          25                                              
Acrylamide                30                                              
Maleic acid               30                                              
10-Undecenoic acid [CH.sub.2 ═CH(CH.sub.2).sub.8 CO.sub.2 H]          
                          12                                              
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     The samples of Examples 1 to 5 and Comparative Examples 1 to 2 were subjected to dynamic friction coefficient test after 100 tape passes on a video tape recorder. Still life and squareness ratio (residual flux density/maximum flux density) measurements were also made. The results are indicated in Table 2 below. 
     
                       TABLE 2                                                     
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       Dynamic Friction                                                   
Sample Coefficient Change         Squareness                              
No.    after 100 Passes                                                   
                       Still Life Ratio                                   
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1      0.28            More than  0.81                                    
                       60 min.                                            
2      Measurement     10 min     0.62                                    
       impossible due to                                                  
       shed magnetic                                                      
       layer                                                              
3      0.43            15 min     0.63                                    
4      0.29            More than  0.81                                    
                       60 min.                                            
5      0.29            More than  0.79                                    
                       60 min.                                            
6      0.30            More than  0.82                                    
                       60 min.                                            
7      0.29            More than  0.76                                    
                       60 min.                                            
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     While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.