GB2042369A - Lubricating coatings for magnetic recording media - Google Patents

Abstract

The invention concerns lubricating coatings for magnetic recording media. Prior lubricating coatings for flexible recording media have not been reliable at high temperatures and high relative humidity. Prior lubricating coatings for rigid recording disks have not been resistant to removal when cleaned. These problems are solved by a lubricant which is a fluorinated telechelic polyether polymer having at least one polar group such as -CHNHCH2CH2OH, -COOCH3, -CONNCH2C6H5, and -COCF3. The telechelic polymer preferably has a Ferranti-Shirley viscosity of 20-2000 centipose and a number average molecular weight of at least 1000.

Description

SPECIFICATION Magnetic recording media lubricated with fluorinated telechelic polyether polymer The invention concerns the lubrication of magnetic recording media such as tapes, cards and disks.

It has been known since the advent of magnetic recording media that their recording surfaces should be lubricated to permit sliding contact with recording heads. The first lubricants were fatty acids and esters such as carnauba wax. These were either added to the dispersion of the magnetizable particles and binder resin or coated over the finished recording surface. The fatty acid and ester lubricants were considered to be satisfactory at ordinary ambient conditions but were not reliable at high temperatures and relative humidity, e.g., 400C and 80% relative humidity.

Of current flexible magnetic recording media, about 70 percent have fatty acid and ester lubrication and most of the others employ silicone lubrication. While U.S. Patents No.

3,490,946 and 3,778,308 suggest the use of fluorocarbon compounds as lubricants for magnetic recording media, they are little used as lubricants for flexible magnetic recording media. On the other hand, rigid magnetic recording disks are nearly always lubricated with fluorocarbon compounds such as highly fluorinated polyethers having a backbone comprising -CaaF2a-O- units wherein a is an integer from 1 to 4, for example,

where n is an integer providing a Ferranti-Shirley viscosity (hereinafter measured at 1640 sec-1) of about 75-1350 centiposes. Such polymers are available commercially from E. I. DuPont de Nemours 8 Co. as " Krytox" 143 fluorinated oils.A preferred member of this class has been Krytox" 143 AD which has a Ferranti-Shirley viscosity of 1322 centipoises and reportedly has a viscosity index (ASTM D2270) of 145 and a number average molecular weight of about 7000. When these fluorinated polumers are used to lubricate rigid disks, they tend to be removed ifthe disks are cleaned with isopropyl alcohol.

The present invention concerns a magnetic recording medium, the magnetizable layer of which has a lubricating coating that reliably provides effective lubrication at 40 C and 80% relative humidity. When used to lubricate rigid disks, the lubricating coating resists removal when wiped with a cloth containing isopropyl alcohol.

The lubricating coating of the invention comprises a fluorinated telechelic polyether polumer having a backbone comprising -C,F2a-O- units wherein a is an integer from 1 to 4, which backbone is terminated by at least one polar group such that jb2/MW is at least 19 x 10-4 Debye2-moles/g. "Telechelic" polymers have low molecular weight and known functional terminal groups.

The dipole moment 11 of the fluorinated telechelic polymer can be calculated from the Onsager Relation 2 9k T MW ( 47r de (ego+2) o S where k = Boltzmann's Constant T = Temperature in K MW = Molecular Weight No = Avogadro's Number d = Density 1s = Dielectric constant measured at low frequencies, Ico = Dielectric constant measured at high frequencies, where the approximation is made through Maxwell's Relations that = XD + (5~1 0%)D ~1 ~075 ?1D2 +(5#10%)#2#1.075 7702 where XD is the refractive index of the material at the sodium D line at 20 C.

The fluorinated telechelic polyether polymer preferably has a nur.ber average molecular weight of at least 1000 and a Ferranti-Shirley viscosity (measured at 1640 Sec- and 20 C.) of at least 20 centipoise. The number average molecular weight of fluorinated telechelic polyether polymers having more than one terminal polar group per polymer molecule is prefereably somewhat higher as compared to telechelic polymers having only one terminal polar group. For example, where there are two terminal polar groups per polymer molecule, the number average molecular weight preferably exceeds 1500.

Useful polar groups for the fluorinated telechelic polyethers include -CO2R wherein R is alkyl of 1 to 6 carbon atoms, aryl or alkaryl of 6 to 10 carbon atoms;

wherein each of R' and R" is hydrogen, alkyl of 1 to 6 carbon atoms, benzyl, or -R"'OH where R"' is an alkylene group of 2 to 6 carbon atoms; CbH2b H; -CbH2bNR'R";

C(OH)2CbF2b+ I wherein b is an integer of 1 to 4.

Strongly acidic polar groups such as carboxylic or sulfonic acid groups are less desirable because they are potentially corrosive. This may be minimized by conversion to salts. Preferably the pKa of the telechelic polyether polymer is at least 1.0.

Preferred fluorinated telechelic polymers have the backbone - [ CF2CF20 ] m- [ CF20 ] n-, where m is an integer from about 6 to 32 and n is an integer from about 12 to 52, as disclosed in U.S.

Patents 3,810,874 and 4,085,1 37, or the backbone - [ -CF(CF3)CF20- ] -0 or -(-CF2CF(CF3)O-J-0, where n is an integer from about 6 to 20, as disclosed in U. S. Patents 3,250,808 and 3,699,145.

Such fluorinated telechelic polyethers are readily produced to have number average molecular weights within the preferred range of 1000-5000 and Ferranti-Shirley viscosities (measured at 1640-' sec) within the preferred range of 20-2000 centipose. The fluorinated telechelic polyether polymer may also contain -CF2-CF2- units which increase the separation between the ether oxygens and tend to make it necessary to avoid number average molecular weights toward the high end of the preferred range of 1000-5000 mentioned above. Otherwise, the viscosity of the fluorinated telechelic polyether polyerm might be too high to provide good lubrication.

Superior lubrication of flexible magnetic recording media has been attained by applying the fluorinated telechelic polyether polymer with a rotogravure coater from solutions or emulsions as dilute as 0.1 % by weight, thus producing what is believed to be a coating weight of about 10 mg/m2. Such a coating is approximately of monomolecular thickness. The actual coating weights are so small as to be difficult to measure, but superior lubrication has been achieved at coating weights which are believed to be about 10 to 150 mg/m2. It is currently preferred to apply the fluorinated telechelic polymer from a 0.4-0.8% solution or emulsion to provide coating weights of about 25-60 mg/m2. This should provide reasonable assurance of good lubricating properties under adverse environmental conditions for a number of years.

The fluorinated telechelic polyether polymers may be applied to rigid magnetic recording disks from dilute solutions or dispersions by wiping or by any other technique such as spraying or dipping. After drying, the surface should be buffed with a dry, soft tissue to distribute the lubricant and remove any excess. The buffing changes the appearance from a slight haze to a shine. If a 5-,ul drop of n-decane after 10 seconds has a diameter of about 3 mm, this indicates that a continuous coating has been applied. In the absence of any lubricant, the diameter of the drop may be about 10-12 mm.

Because it is not known how much of the lubricant has been removed by the buffing and because the coatings are thin, it is difficult to determine their thickness. Since a monomolecular layer of a fluorinated telechelic polyether polymer is believed to be on the order of 40 Angstroms in thickness, it is assumed that the coating should be at least that thickness. Electron scattering measurements suggest that thicknesses of 75 to 250 Angstroms should provide prefered results. It is believed that an unduly thick coating of a fluorinated telechelic polyether polymer on a rigid recording disk can cause undue drag.

It may be desirable to blend the fluorinated telechelic polyether polymer with other lubricants such as fatty acids and esters for such purposes as to provide reliable stop-motion to video tape recorders at high temperatures and high relative humidity. The fluorinated telechelic polyether polymer should comprise at least 10% by weight of the blended lubricants. Preferably the coating weight of the blended lubricants is from 25 to 150 mg/m2.

The backing member of each of the magnetic recording media of Examples 1-8 was a flexible biaxially-oriented polyethylene terephthalate film having a thickness of approximately 15 micrometers.

Example 1 A preferred fluorinated telechelic polyether polymer of the invention is

where m and n are each integers providing an average molecular weight of about 2000, and has a Ferranti-Shirley viscosity of 1440 cps. 0.3g of this telechelic polyether was emulsified in 60g of isopropyl alcohol, and the emulsion was applied with a rotogravure coating over the recording layer of a flexible magnetizable recording tape. The recording layer comprised fine acicular magnetizable iron oxide particles in a binder comprising crosslinked polyvinyl chloride copolymer and polyester polyurethane. The recording layer was specifically designed for helicalscan video recording except that it contained no lubricant.

After the telechelic polymer coating was dried in an oven at about 50 C to a dried coating weight of about 37.5 mg/m2, the coating tape was slit to one-half-inch (1.27-cm) widths and then loaded into videocassettes of the "VHS" and "Betamax" formats. Two videocassettes of the Betamax format and one of the VHS format were tested on videorecorders at 40 C and 80% relative humidity through 500 cycles, at which point the tests were terminated. None of the tapes had failed. Current industry standards require 200 passes without failure. Both of the Betamax cassettes were further tested to 1000 cycles, again with no failures. One Betamax cassette was tested to 1500 cycles, and its tape did not fail.

Examples 1A and 1B Two Betamax videocassette tapes were prepared which were identical to those of Example 1 except having twice and half the coating weight, respectively. Each also passed 500 cycles at 40 C and 80% relative humidity without failure.

Example 1C The same fluorinated telechelic polymer was coated over the recording layer of each of several endless-loop, 8-track audio recording tapes, the recording layers of which comprised acicular iron oxide particles in nonmagnetizable binder. At least 54old improvement was noted in resistance to wear at ordinary room temperature and 50% relative humidity as compared to typical endless-loop, 8-track audio recording tapes now on the market.

Examples 1 D and 1 E Two Betamax videocassette tapes were prepared which were identical to those of Example 1 except that one lubricating coating contained 2.5 parts by weight of butyl myristate and the other, 2 parts by weight of myristic acid per part of the telechelic polymer. When tested at 40 C and 80% relative humidity, the first tape passed 200 cycles and the second passed 500 cycles without failure, whereupon testing was discontinued.

The following telechelic fluorinated polyether polymers were tested as lubricants for the same magnetic recording tape as in Example 1 except that the test results are here reported only for Betamax videocassettes.

viscosity Example No.ave. (Fer-Sh) No. MW Cps. 0 0 I II 2 CH3OC-CF2OiCF2CF2O)m+CF2O) n-CF2-COCH3 3500 54 3 HCCH2-CF2O+CF2CF2o) m+CF2O) ##CF2#CH20H 2000 24 4 ditto 4000 194 CF O I # 5 CF3-o+CF2-CF-o) nCFZ-C-CF# 2600 125 f F3 CF3 6 F(CF-CF2-O)n-CF-COOH 3200 1130 CF3 C1F3 7 F(CF-CF2-O)n-CF-CO2CH3 3200 345 CF CF O 1 3 1 3 II :F20),-#F--C-NH( 3300 20H33001785

The fluorinated polymers of Example 1-8 had the following values (eS being measured at 100 Hz, except Example 1 to 10,000 Hz and Example 8 at 1000 Hz): y2/MW Example m n d e0 eoo x104 1 8 14 1.772 23.0 1.876 809 2 16 28 1.750 2.86 1.805 53.7 3 8 14 1.793 2.80 1.809 49.4 4 16 28 1.814 2.55 1.805 37.6 5 -- 15 1.874 2.22 1.814 20.6 6 -- 18 1.80 2.36 1.825 27.6 7 -- 18 1.848 2.79 1.822 46.6 8 -- 18 1.90 3.68 1.843 80.2 Each test of a tape of Examples 2-8 was continued until failure or 500 cycles without failure at 400C and the indicated relative humidity.Each test was discontinued immediately after any failure such as jamming of the tape, visual scoring of its magnetizable layer, accumulations of visible residue on the video heads, a 6 dB loss in video signal, or sticking of the tape to the video capstan, audio stack or to any other surface. Results are summarized below:: Test Results Polymer Relative No. of Example Concentration Vehicle Humidity Passes 2 1% IPA 85% 500 + 0.5% do do 452 3 1% do 80% 500 + (2 tapes) 0.25% "Freon" do 500 + (Registered Trade Mark) 4 1% IPA 85% 119 do do do 30 do do do 500 + 5 do "Freon" 80% 500 + (2 tapes) (Registered Trade Mark) do IPA ' do do (2 tapes) 0.5% do do 71 6 do do do 500+ 7 1% do do do 0.5% do do do 8 1% do do 500 + do do do 40 0.5% do do 5 do do do 481 tested at 20 C 500 + indicates no failure at 500 cycles IPA = isopropyl alcohol Freon = CClF2CCl2F sold as "Freon" 113 ("Freon" is a Registered Trade Mark) Example 9 0.3g of the telechelic polymer of Example was dissolved in 19.7 g of CClF2Cl2F. While a 35cm rigid magnetic recording disk was being rotated at 3600 rpm, a soft tissue saturated with the solution was wiped across its recording surface several times. Then after two minutes during which the solvent volatilized, the surface was buffed with a dry soft tissue to distribute the lubricant and remove any excess. The recording surface of the rigid disk comprised fine acicular magnetizable iron oxide particles in a thermoset epoxy resin binder.

With the disk spinning at 3600 rpm, a tissue soaked in isopropyl alcohol was pressed under full fingertip pressure against the disk for about 30 seconds while moving the tissue across the recording surface several times. The fluorinated telechelic polyether polymer was resistant to such cleaning as indicated by surface tension tests using n-decane.

The lubricated disk passed each of the Post-Cleaning Slide Test and the Post-Cleaning Sliding Friction Test as described below: Post-Cleaning Slide Test After cleaning with isopropyl alcohol as indicated above, the disk is rotated at 120 rpm on a data module spindle, a speed at which an IBM 3348-type recording head remains in contact with the recording surface. After 24 hours at ordinary room temperature, the disk and head are then examined with the naked eye, and failure is indicated if either there is a burnish mark on the recording surface or the head has accumulated a visible residue.

Post-Cleaning Sliding Friction Test During the Post-Cleaning Slide Test, the friction between the head and the recording surface is monitored periodically, and failure is indicated if more than 4.5 grams of frictional force is applied against the head, either continuously or intermittently at any time during the 24-hour period.

If in this test a value of more than 4.5 grams is observed initially, this may indicate that the coating of lubricant is too heavy, in which event the coating thickness may be reduced to an acceptable level by buffing with a dry soft tissue.

Example 10 The telechelic polymers of Examples 2, 5 and 7 and the following telechelic polymers were likewise applied to the recording surfaces of rigid magnetic recording disks as in Example 9, and each of these lubricated disks passed each of the Post-Cleaning Slide Test and Post-Cleaning Sliding Friction Test:

HOOC-C20+CF2CF20)m+CF20)n-CF2-COOH wherein m and n are each integers providing a number average molecular weight within the range of 1,000 to 5,000.

Example 1 1 When Example 1 was repeated except that the magnetic recording disk had a binder-free recording surface of U.S. Patent No. 3,973,072 (Anderson), both the Post-Cleaning Slide Test and the Post-Cleaning Sliding Friction Test were passed. Such a binder-free recording surface can be applied by electrodeposition, chemical reduction, electroless plating, vapor plating, evaporation or sputtering.

Claims (8)

1. Magnetic recording medium, the magnetizable layer of which has a lubricating coating comprising a fluorinated polyether polymer having a backbone comprising -C0F2,-0- units wherein a is an integer from 1 to 4, characterized by the feature that the backbone is terminated by at least one polar group, the y2/MW of the polymer being at least 19 X 10-4 Debye2moles/g.

2. Magnetic recording medium as defined in claim 1 further characterized by the feature that said at least one polar end group is selected from -CO2R wherein R is alkyl of 1 to 6 carbon atoms, aryl or alkaryl of 6 to 10 carbon atoms, or

wherein each of R' and R" is hydrogen, alkyl of 1 to 6 carbon atoms, benzyl, or -R"' OH where R"' is an alkylene group of 2 to 6 carbon atoms: -QH2#0H; #CbH2bN R1R1,;

or -C(OH2CbF2b+l wherein b is an integer of 1 to 4.

3. Magnetic recording medium as defined in claim 2 further characterized by the feature that said fluorinated polyether polymer is selected from

and m and n are each integers.

4. Magnetic recording medium as defined in any preceding claim further characterized by the feature that the Ferranti-Shirley viscosity of said fluorinated polymer is at least 20 centipoise.

5. Magnetic recording medium as defined in any preceding claim further characterized by the feature that the Ferranti-Shirley viscosity of said fluorinated polymer is at most 2000 centipoise.

6. Magnetic recording medium as defined in any preceding claim further characterized by the feature that said telechelic polymer has a number average molecular weight of at least 1000.

7. Magnetic recording medium as defined any preceding claim further characterized by the feature that up to 90% by weight of said lubricating coating comprises a fatty acid or ester and the coating weight is 25-150 mg/m2.

8. Magnetic recording medium substantially as described in any of the examples herein.