Polyester-polyurethane composition and use thereof

A composition containing CrO.sub.2 and a polyester-polyurethane wherein the chain extender contains a mixture of 1,4-butanediol and 1,6-hexanediol; and use thereof, especially in magnetic recording media.

TECHNICAL FIELD 
The present invention is concerned with polyester-polyurethane compositions 
and use thereof. The polyester-polyurethanes employed in the present 
invention are used as the binder for CrO.sub.2 and especially for 
producing flexible magnetic recording media. The present invention is 
particularly useful for magnetic recording media containing ferromagnetic 
chromium dioxide pigment. 
BACKGROUND ART 
Flexible magnetic recording media generally comprises magnetic pigment or 
particles, polymeric binder, lubricant, dispersant, and other minor 
additives. The majority of magnetic particles of practical importance are 
metal oxides. Interactions which exist between the magnetic particles and 
the binder can effect the frictional characteristics of the media, such as 
tape. 
Particle-binder interactions that are desirable from the standpoint of 
tape-media performance are those interactions which maintain separation of 
the individual particles, reinforce the mechanical properties of the 
binder, and hold the particles to the tape's flexible substrate in a 
cohesive coating. Undesirable interactions between the binder and the 
magnetic particles can lead to deterioration of magnetic performance or to 
deterioration of the coating's mechanical properties. 
Interaction between the binder and the particles is aggravated 
significantly by the fact that the majority of magnetic coatings contain 
magnetic-oxide particles in excess of 70% of the coating by weight and as 
much as 50% by volume. In order to achieve these high particle loadings, 
strong interactions between the particle and the polymeric binder are 
necessary. 
Polyester-polyurethanes (a type of thermoplastic elastomer) are widely used 
as binders for flexible magnetic recording media. These binders are 
rubbery materials which can be melted and cooled reversibly, without major 
changes occurring in their chemical or physical properties. Their unique 
properties, which are a direct result of the block-copolymer nature of 
these materials, make them significantly different from other elastomers, 
such as natural or synthetic rubber. 
These materials are composed of segments or blocks of chemically different 
units. The polyester portion, or soft segment, is composed of a repeating 
series of ester-linked units, and is itself a short-chain-length polymer. 
The polyester segments are formed by the reaction of a difunctional 
carboxylic acid with a difunctional alcohol, such that the ester is 
terminated substantially with alcohol or hydroxyl end groups. This 
polyester portion typically has a molecular weight of from 500 to 4000, 
corresponding to chains composed of from 4 or 5 ester units to as many as 
20. The effect of an increase in the length of the soft segment is 
generally an increase in the elasticity of the polyurethane. In general, 
it is the soft-segment portion of the polyester-polyurethane that 
determines the low temperature and the elastomeric properties of the 
binder. 
The other component in the polyester-polyurethane is the polyurethane or 
hard segment portion. This portion possesses a markedly different chemical 
and mechanical behavior from that exhibited by the polyester soft 
segments. In general, the hard segment is a hard, rigid polymer with a 
melting point near 200.degree. C. The hard segment is usually produced 
from a difunctional, aromatic diisocyanate, such as 4,4'-diphenylmethane 
diisocyanate (MDI) which is reacted with a difunctional alcohol such as 
1,4-butanediol. The hard segment usually has a very short chain length in 
the case of polyester-polyurethane elastomers used in solvent-based 
magnetic media coatings because the hard segment is not particularly 
soluble in the common solvents (examples are THF and MIBK) used in the 
manufacture of magnetic tape coatings. In addition, the size of the hard 
segment blocks has been found to increase hardness, modulus, and flow 
temperature at the expense of elasticity and toughness. For flexible 
magnetic recording tape, a balance of properties is sought so that the 
binder can be adapted to the requirements of magnetic recording. 
It has been found, in accordance with the suggestions in U.S. Pat. No. 
4,284,750, that thermoplastic polyurethane compositions having excellent 
mechanical and thermal properties, high hardness, and the capacity of 
binding or adhering to magnetic pigments can be formed by reacting (A) 
cyclohexanedimethanol and an acid selected from the group adipic acid, 
azelaic acid, and 1,12-dodecanedioic acid, including mixtures thereof; (B) 
a chain extender such as 1,4-butanediol; and (C) a diisocyanate such as 
MDI (methylene bis diphenyl diisocyanate, also known as 
4,4'-diphenylmethane diisocyanate). 
It has been found, however, that when magnetic chromium dioxide is employed 
as the ferromagnetic pigment, in place of iron oxide particles with 
binders disclosed in U.S. Pat. No. 4,284,750, certain problems are 
encountered. For instance, if one follows the teachings of U.S. Pat. No. 
4,284,750 when making chromium dioxide magnetic recording media, a 
substantial decay in the media's mechanical properties, such as the 
modulus (i.e., hardness, stiffness, load bearing capacity), occurs within 
the temperature range of 10.degree. C. to 50.degree. C. If one tries to 
improve the media by thermal annealing, only slight improvement results. 
In U.S. Ser. No. 567,291, certain problems concerning coating performance 
are overcome. In particular, it has been found, according to said 
application, that polyester-polyurethanes, of the general type defined in 
U.S. Pat. No. 4,284,750, become satisfactory for use with chromium dioxide 
particles when the polyurethane possesses increased hard segment content 
in the range 37% to 40% by weight and, preferably, 40% with the soft 
segment molecular weight being in the range of about 500 to 1500. 
However, it has been found that such binders are not entirely satisfactory 
from a processability standpoint. In particular, such binders tend to be 
brittle. Also, such binders have a tendency to lose cohesive integrity and 
adhesion to substrates such as polyethylene terephathalate substrates. 
Further, concerning various contributions recognized by the prior art to 
the structural and mechanical properties of polyurethanes made by the hard 
segment content, attention is directed to the following publications. 
R. J. Zdrahala, et al., "J. Elast. Plast.", Vol. 12, p. 184, 1980. 
S. L. Cooper and A. V. Tobolsky, "J. Appl. Poly. Sci.", Vol. 10, p. 1837, 
1966. 
K. C. Frischland and S. L. Reegen, Ed., "Advances in Urethane Sci. Tech.", 
Vol. 3, pp. 36-65, 1974. 
T. E. Lipatova, et al., "Poly. Sci. U.S.S.R.", Vol. 20, p. 2305, 1979. 
W. Nierzwicki and E. Szpilewicz, "J. Appl. Poly. Sci.", Vol. 23, p. 2147, 
1979. 
R. J. Zdrahala, et al., "J. Elast. Plast.", Vol. 12, p. 225, 1980. 
C. S. Schollenberger, "Advances in Chemistry Series 176", American Chemical 
Society, 1979. 
SUMMARY OF THE INVENTION 
The present invention makes it possible to overcome the above discussed 
problems with respect to brittleness, loss of cohesive integrity, and loss 
of adhesion without a concomitant loss of other desirable properties. In 
particular, the present invention is concerned with compositions 
comprising CrO.sub.2 in combination with certain polyester-polyurethanes. 
The polyester-polyurethanes are reaction products of a hydroxy-terminated 
polyester, a diol chain extender, and a diisocyanate. In accordance with 
the present invention, the chain extender must be a mixture of 
1,4-butanediol and 1,6-hexanediol in a weight ratio of about 35:65 to 
about 65:35 of 1,4-butanediol to 1,6-hexanediol in order to obtain the 
advantages of the present invention. In addition, the hydroxyl-terminated 
polyester is a reaction product of a difunctional alcohol having 
hydrolytic stability and a dicarboxylic acid. 
The polyester has a hydroxyl number of about 150-260. The resultant 
hydroxyl number of the polyester and chain extender is about 150-350. 
The relative proportions of the hydroxyl-terminated polyester, diol chain 
extender, and diisocyanate are selected to produce a thermoplastic 
polyester-polyurethane having a hard segment in the range of about 32% to 
about 60% by weight and a soft segment molecular weight (Mn) of about 415 
to about 2250. 
The present invention is also concerned with a magnetic recording media 
comprising a substrate coated with a composition comprising a 
ferromagnetic CrO.sub.2 pigment and a polyester-polyurethane, as defined 
hereinabove.

BEST AND VARIOUS MODES FOR CARRYING OUT THE INVENTION 
The polyester-polyurethane employed in accordance with the present 
invention, must be a reaction product of: 
a. a hydroxyl-terminated polyester which is a reaction of a difunctional 
alcohol having hydrolytic stability and a dicarboxylic acid or ester 
thereof or mixture of acid and ester wherein the polyester has a hydroxyl 
number of about 50-260; 
b. a chain extender containing a mixture of 1,4-butanediol and 
1,6-hexanediol in a weight ratio of about 35:65 to about 65:35 of the 
1,4-butanediol to the 1,6-hexanediol; 
c. the resultant hydroxyl number of said polyester and chain extender being 
about 150-350; 
d. a diisocyanate wherein the relative proportions of a, b, and d are 
selected to produce a polyester-polyurethane having a hard segment content 
in the range of about 32% to 60% by weight and a soft segment molecular 
weight (Mn) in the range of about 415-2250. 
The difunctional alcohol used to prepare the hydroxyl-terminated polyester 
must be hydrolytically stable and is preferably 1,4-cyclohexanedimethanol. 
However, other diols can be used, if desired. Examples of other diols are 
hydroquinone di (.beta.-hydroxyethyl) ether; and 2,2-dimethyl 
1,3-propanediol. 
The dicarboxylic acid or ester employed to prepare the hydroxyl-terminated 
polyester is generally a saturated aliphatic carboxylic acid, preferably a 
straight chain saturated aliphatic carboxylic acid having 6-12 carbon 
atoms, and most preferably, is adipic acid, azelaic acid, or 
1,2-dodecanedioic acid or ester thereof. Esters that can be used in 
preparing the polyester are generally alkyl esters such as the methyl 
esters including dimethyl 1,4-butanedicarboxylate; dimethyl 
1,7-heptanedicarboxylate; and dimethyl 1,10-decanedicarboxylate. 
Mixtures of dicarboxylic acids and/or esters can be employed when desired. 
The alcohol and acid are reacted in amounts and under conditions such that 
the hydroxyl-terminated polyester has a hydroxyl number of about 50-260 
and preferably about 150-180. The preparation of the hydroxyl-terminated 
polyester may be carried out in the presence of an esterification catalyst 
or combination of catalysts. Some catalysts in general use are derivatives 
of tin, such as, for example, stannous oxalate, stannous octoate, dibutyl 
tin oxide, dibutyl tin dilaurate, stannous chloride, and stannous 
fluoride. Combinations of tin catalysts and others may be employed as 
desired. 
The esterification reaction is carried out in an inert atmosphere, i.e., in 
a nitrogen atmosphere which aids in the prevention of degradation and 
excessive color in the polyester. 
The chain extender, in accordance with the present invention, is a mixture 
of 1,4-butanediol and 1,6-hexanediol. The weight ratio of the 
1,4-butanediol to the 1,6-hexanediol is about 35:65 to about 65:35, 
preferably about 40:60 to about 60:40, and most preferably, about 50:50. 
The chain extender can also, but not preferably, include minor amounts (up 
to about 10% by weight of the total chain extender) of other chain 
extenders such as ethylene glycol, propylene glycol, 1,3-butanediol, 
1,5-pentanediol, 1,4-cyclohexanedimethanol, and hydroquinone 
di(.beta.-hydroxyethyl) ether. 
The quantity of chain extender used is controlled by the hydroxyl number of 
the polyester which is in the range of from 50-260 and the hydroxyl number 
of the polyester chain extender blend which is in the range of 150-350. 
According to the present invention, the diisocyanate can be an aliphatic or 
aromatic diisocyanate and preferably is an aromatic diisocyanate. 
The quantity of diisocyanate employed is related to the equivalent weight 
of the hydroxy terminated polyester and chain extender blend. The 
relationship between the hydroxyl number and equivalent weight is given by 
the following expression: 
##EQU1## 
The ratio of the number of equivalents of the hydroxy terminated polyester 
and chain extender blend to the number of equivalents of diisocyanate is 
between 1-1.7, preferably in the range of from 1-1.05. 
The preferred diisocyanate is methylene bis diphenyl diisocyanate, also 
known as diphenyl methane-p,p'-diisocyanate, hereinafter referred to as 
MDI. Other operable diisocyanates may be the aliphatic diisocyanates such 
as tetramethylene diisocyanate, hexamethylene diisocyanate, and the like; 
the cycloaliphatic diisocyanates such as cyclohexyl diisocyanate, and the 
like; the aromatic diisocyanates such as the phenyl diisocyanates, the 
toluene diisocyanates, and the like; the dicycloaliphatic diisocyanates 
such as cyclohexyl methane diisocyanate, and the like; and the diaryl 
diisocyanates such as MDI, dichloro-diphenyl methane diisocyanate, 
dimethyl diphenyl methane diisocyanate, diphenyl dimethyl methane 
diisocyanate, dibensyl diisocyanate, diphenyl ether diisocyanate, and the 
like. 
The relative proportions of the hydroxyl-terminated polyester, chain 
extender, and diisocyanate must be such to yield a thermoplastic 
polyester-polyurethane having a hard segment content of about 32% to about 
60% by weight and a soft segment molecular weight (Mn) of about 415-2250 
and preferably about 600-900. 
Further, it is preferred that the molecular weight (Mw) of the polyurethane 
to be above 60,000. 
The polyurethanes employed in accordance with the present invention provide 
for a combination of high yield strength and high hardness along with high 
Young's Modulus. In addition, the composition of the present invention 
retains its desirable properties at elevated temperatures. For instance, 
the polyurethanes employed in the present invention generally exhibit 
hardness values significantly in excess of 100 Shore A Hardness and 
usually at least about 60 Shore D Hardness, at times up to about 90 Shore 
D Hardness, and preferably about 70 to about 75 Shore D Hardness. In 
addition, the polyurethanes employed pursuant to the present invention 
exhibit high yield strengths such as about 40 to about 70 MPa, along with 
high Young's Modulus such as about 1 to about 3 GPa as measured at room 
temperature. The polyurethane employed in the present invention also 
usually possess breaking elongation of about 200% to about 280% and more 
usually about 210% to about 216%. 
In accordance with the present invention, non-brittle compositions 
containing a high content of pigment can be obtained. The pigment is 
predominantly (e.g. at least 50% by weight of total pigment) ferromagnetic 
chromium dioxide. Other pigments can be admixed with the chromium dioxide. 
Examples of other pigments include ferromagnetic pigments such as magnetic 
iron oxide, magnetic cobalt-modified ion oxide, metallic iron, and 
magnetic ferrites. 
The most pronounced improvements achieved by the present invention are 
obtained when the pigment is entirely chromium dioxide. 
The chromium dioxide particles, per se, may be either stabilized or 
unstabilized from the effects of reductive degradation, as caused, for 
example, by the presence of water and certain easily oxidizable functional 
groups such as hydroxyl or amine. Stabilized particles are, however, 
preferred. 
The pigment is preferably employed in amounts of about 65% by weight to 
about 88% by weight, or up to the critical pigment volume, and most 
preferably about 78% to about 82% by weight. 
In addition, the compositions can include auxiliary constituents such as 
diluents, lubricants, and dispersants, when desired. 
An example of a suitable lubricant is tridecyl stearate which, when 
employed, is usually present in amounts of about 5 to 10% by weight of the 
composition. 
In addition, the composition can contain an organic polyisocyanate which, 
when present, is generally in amounts up to about 15% by weight, and 
preferably about 5% to about 10% by weight based upon the polyurethane. 
An example of a suitable polyisocyanate is poly (methylene bis diphenyl 
diisocyanate), also known as poly (methylene polyphenyl isocyanate), 
hereinafter referred to as pMDI. 
Such is present in Mondur MRS which is commercially available. 
Other operable polyisocyanates may be either aliphatic polyisocyanates such 
as poly (isophorone isocyanate) or aromatic polyisocyanate such as poly 
(phenyl isocyanate), poly (toluene isocyanate), and the like. 
The amount of the polyisocyanate when used as discussed above is such that 
the resulting cured coating is essentially free of NCO (isocyanate) groups 
in the final form as used in preparing magnetic media. 
The compositions of the present invention are preferably used to prepare 
magnetic recording media and most preferably flexible magnetic recording 
media such as magnetic tape. 
For instance, coating compositions can be prepared by preparing a slurry of 
the magnetic particle, a dispersant, and a solvent; then preparing a 
binder solution of the polyester-polyurethane, a lubricant, and a solvent; 
and adding the binder solution to the slurry to form the coating. The 
coating can then be coated onto a flexible substrate, e.g., polyethylene 
terephthalate, to form magnetic recording tape. 
The apparent glass transition temperature (T.sub.g) of coatings of the 
present invention are preferably above normal room temperatures and most 
preferably about 40.degree. C. to 50.degree. C. This contributes to the 
retention of the physical properties of the compositions at elevated 
temperatures. 
The following non-limiting examples are presented to further illustrate the 
present invention: 
EXAMPLE 1 
Part A--Preparation of Hydroxyl Terminated Polyester 
Into a reaction vessel equipped with a sealed stirrer, nitrogen inlet and 
two connected reflux condensers, about 10.32 moles of molten (80.degree. 
C.-90.degree.C.) 1,4 cyclohexanedimethanol are added under constant 
stirring. To the stirred mass about 5.6 moles of adipic acid and about 2.4 
moles of azelaic acid are added together with a catalyst combination of 
about 1.0 grams of stannous oxalate and about 4.0 grams of phthalimide. 
Purging with nitrogen was begun. Steam is fed to the first condenser and 
cold water to the second one while raising the temperature of the mixture 
to about 225.degree. C. while continuously removing water. The reaction is 
allowed to proceed at this temperature for about four (4) hours, after 
which time the batch is allowed to cool to about 165.degree. C. and the 
nitrogen supply is discontinued. A vacuum of less than about 5.0 mm 
mercury is drawn and the reaction is allowed to continue for about five 
(5) hours at about 165.degree. C. while removing the remaining water and 
low molecular weight polyester. At the end of this time, the batch is 
cooled to about 100.degree. C. The resultant hydroxyl terminated polyester 
has an acid number of about 0.5 and a hydroxyl number of about 100. 
Part B--Preparation of Polyurethane Resin 
The polyester of Procedure A is blended with the chain extender which is a 
blend of 1,4-Butanediol/1,6-Hexanediol 1:1 by weight. Thus, into about 
1343.68 grams of polyester about 256.32 grams of chain extender are added 
to yield a polyester chain extender blend having a hydroxyl number of 260. 
To this entire polyol blend there are added about 1.0 grams of 
triethylenediamine and about 917.4 grams of MDI. The ingredients are 
thoroughly mixed and then cured in an oven at about 125.degree. C. for 
about one (1) hour. The resulting polyurethane polymer has a viscosity of 
about 1780 cps when measured as a 20% solution in tetrahydrofuran. 
##EQU2## 
EXAMPLE 2 
The polyester prepared in accordance with Part A of Example 1 is blended 
with the chain extender which is a blend of 1,4-Butanediol/1,6-Hexanediol 
1:1. 
Thus, into about 1298.24 grams of polyester about 301.76 grams of the chain 
extender are added to yield about 1600.0 grams of a polyester-chain 
extender blend having a hydroxyl number of about 290. To this entire 
polyol blend there are added about 1.0 grams of triethylenediamine 
catalyst and about 1025.8 grams of MDI to give a resulting polyurethane 
polymer having a viscosity of about 1500 cps when measured as a 20% 
solution in tetrahydrofuran and a % hard segment content of about 50.55 by 
weight. 
EXAMPLE 3 
Part A 
Part A of Example 1 is repeated, except that the quantity of 
1,4-cyclohexanedimethanol used is about 11.52 moles to yield a polyester 
having an acid number of about 0.3 and a hydroxyl number of about 140. 
Part B 
The polyester prepared in accordance with Part A of this Example is blended 
with the chain extender (1:1 by weight 1,4-Butanediol/1,6-Hexanediol). 
Thus, into about 1374.72 grams of polyester about 225.28 grams of chain 
extender are added to yield a polyol blend having a hydroxyl number of 
about 275. To this entire polyol blend there are added about 1.0 grams of 
triethylenediamine and about 970.0 grams of MDI to give a resulting 
polyurethane polymer having a viscosity of about 3350 cps when measured as 
a 20% solution in tetrahydrofuran and a hard segment content of about 
46.5% by weight. 
EXAMPLE 4 
The polyester prepared in accordance with Part A of Example 3 is blended 
with the chain extender (1:1 by weight blend of 
1,4-Butanediol/1,6-Hexanediol. 
Thus, about 1416.48 grams of polyester and about 183.52 grams of chain 
extender are added to yield a polyol having a hydroxyl number of about 
250. To this entire polyol blend there are added 1.5 grams of 
triethylenediamine and about 885.8 grams of MDI to give a resulting 
polyurethane polymer having a viscosity of about 1900 cps when measured as 
a 20% solution in tetrahydrofuran and a hard segment content of about 43% 
by weight. 
EXAMPLE 5 
The polyester prepared in accordance with Part A of Example 3 is blended 
with the chain extender (1:1 by weight of 1,4-Butanediol/1,6-Hexanediol). 
Thus, into about 1332.96 grams of polyester (hydroxyl number about 140), 
about 267.04 grams of the chain extender blend are added to yield a polyol 
having a hydroxyl number of about 300. To this entire polyol blend there 
are added about 1.5 grams of triethylenediamine and about 1069.5 grams of 
MDI to give a resulting polyurethane polymer having a viscosity of about 
3000 cps when measured as a 20% solution in tetrahydrofuran and a hard 
segment content of about 50.0% by weight. 
EXAMPLE 6 
A slurry of magnetic chromium dioxide is prepared by mixing about 275.0 
pounds of chromium dioxide, about 8.3 pounds soybean lecithin, about 254.1 
pounds of tetrahydrofuran, and about 84.7 pounds of methylisobutyl ketone 
in a high-speed premix tank. This premix is then passed through a series 
of high-speed sandmills, or equivalent dispersion equipment, to produce a 
milled slurry of about 45-46% solids by weight. 
A binder solution containing about 13.4% by weight of solids is prepared by 
mixing about 46.0 pounds of the polyester-polyurethane obtained in 
accordance with the procedure of Example 1, about 5.4 pounds of a fatty 
acid lubricant (trydecyl stearate), about 248.5 pounds of tetrahydrofuran, 
and about 82.8 pounds of methylisobutyl ketone. 
This binder solution is then combined with an equal volume of the milled 
chromium dioxide slurry in a high-speed sandmill to give a final ink of 
about 32.7% solids and about 83.3% pigment by weight. Prior to coating of 
the ink, about 3.1 pounds of a functional isocyanate hardener Mondur MRS 
(NCO content .about.2.2-2.4) is mixed into the ink. The ink is then 
filtered prior to coating. 
The final ink is then applied to a moving web of flexible substrate of 
polyethylene terephthalate by means of hydrodynamic extrusion as described 
in U.S. Pat. No. 4,345,543 (disclosure of which is incorporated herein by 
reference). The coating thus applied is dried and calendered to obtain a 
coating of 140.mu. inches thickness and smooth surface (0.7.mu. inch RMS 
roughness) is obtained. It is processed into half-inch tape suitable for 
high-density digital recording. 
Adhesion and abrasion resistance are excellent. Magnetic performance is 
good, giving high signal amplitude and data reliability. The apparent 
glass transition temperature (T.sub.g) is about 23.degree. C. 
EXAMPLE 7 
Example 6 is repeated, except that the polyester-polyurethane in accordance 
with the procedure of Example 2 is employed as the binder to provide a 
final ink containing about 33.2% solids. 
The ink thus prepared is coated onto a polyester substrate to give a 
coating of 181.mu. inches thickness and an average surface roughness of 
0.6.mu. inches RMS. Adhesion is somewhat lower than obtained with Example 
6, but the abrasion is slightly improved. Magnetic performance is good, 
giving high reliability and signal amplitude. The T.sub.g is about 
28.degree. C. 
EXAMPLE 8 
Example 6 is repeated, except that the polyester-polyurethane in accordance 
with Example 3 is employed as the binder to provide a final ink containing 
about 33.9% solids. 
The ink thus prepared is coated onto a polyester substrate to give a 
coating of 150.mu. inches thickness and 0.6.mu. inches RMS surface 
roughness. The adhesion of the coating is somewhat better than that 
achieved with the coating prepared in Example 7. Abrasion resistance 
appears to be improved as well. Magnetic performance is excellent, giving 
very high amplitude and data reliability. The media is also found to 
exhibit good frictional behavior (FIG. 1). The mechanical performance of 
the coating as measured as a free film with a dynamic mechanical analyzer 
(FIG. 2) is also found to be good. The apparent T.sub.g of the coating is 
observed at about 50.degree. C. This contributes to good control of the 
surface deformation of the coating for maintaining stable frictional 
performance. 
EXAMPLE 9 
Example 6 is repeated, except that the slurry of chromium dioxide contains 
about 46.6% solids and that the polyester-polyurethane prepared in 
accordance with the procedure of Example 4 is employed as the binder to 
provide a final ink containing about 33.1% solids. 
The ink thus prepared is coated onto a polyester substrate to give a 
coating of 152.mu. inches thickness and 0.5.mu. inches RMS. The coating 
adhesion is improved significantly as compared to Examples 7 and 8. 
Abrasion resistance is comparable to that measured for Example 8. Magnetic 
performance is excellent, giving very high amplitude and data reliability. 
The media is also found to exhibit somewhat better frictional behavior 
(FIG. 1) as compared to Examples 6 and 7, but slightly inferior to that 
achieved with the polyester-polyurethane used in Example 8. The apparent 
T.sub.g of the coating prepared using this polyester-polyurethane (Example 
4) is observed to occur at about 46.degree. C. 
EXAMPLE 10 
The procedure of Example 6 is repeated, except that the 
polyester-polyurethane prepared in accordance with the procedure of 
Example 5 is employed to provide a final ink containing about 32.5% 
solids. 
The ink thus prepared is coated onto a polyester substrate to give a 
coating of about 156.mu. inches thick with an average surface roughness of 
0.6.mu. inches RMS. The adhesion of the coating to the substrate is 
slightly less than that obtained with Example 9, while the abrasion 
resistance is slightly improved. Magnetic performance is found to be 
excellent, giving high amplitude and data reliability. The media is found 
to give the best frictional properties of all the resin iterations (FIG. 
1). The improved frictional performance is noted, despite the relatively 
lower modulus and T.sub.g observed for this coating as compared to 
Examples 8 and 9 (FIG. 2). 
FIG. 3 presents the observed swelling behavior of these resins (without 
CrO.sub.2 pigment) in methyl isobutyl ketone (MIBK), a non-solvent. It is 
noted that as the hard-segment content is increased, the swelling of the 
resin in MIBK decreases. Since MIBK is used as a diluent solvent in the 
preparation of the coated media, the swelling plays an important part in 
the characteristic composition of the tape surface and thus impacts the 
abrasion and frictional performance of the tape. This presumably arises 
from the control of the surface porosity and permeability of the coating 
which, in part, governs lubricant film integrity and stability. 
As a result, a balance between bulk mechanical properties and the control 
of the surface composition of the tape appears to be a crucial portion of 
the practice of this invention and methods thus described for making and 
characterizing superior performance magnetic media using chromium dioxide 
pigments.