Magnetic coating compositions for magnetic recording materials

A magnetic coating composition for use in the production of a magnetic recording material is described. The composition is comprised of at least ferromagnetic metal powder and/or ferromagnetic alloy powder and nitrocellulose having an average degree of polymerization of from 10 to 55, or at least ferromagnetic metal powder and/or ferromagnetic alloy powder, nitrocellulose having an average degree of polymerization of from 10 to 55, and at least one polymeric binder. This magnetic coating composition has excellent dispersibility of the magnetic powder, and can provide a magnetic recording material which has excellent surface smoothness and orientation ratio, as well as excellent durability.

FIELD OF THE INVENTION 
The present invention relates to magnetic coating compositions for magnetic 
recording materials. More particularly, the invention relates to magnetic 
coating compositions capable of producing magnetic recording materials 
containing ferromagnetic metal powder and/or ferromagnetic alloy powder 
and having excellent dispersibility of the powder and durability. 
BACKGROUND OF THE INVENTION 
Magnetic coating compositions containing ferromagnetic metal powder and/or 
ferromagnetic alloy powder, which are to be used in the production of 
magnetic recording materials, are usually prepared by fully dispersing 
such magnetic materials and polymeric binders along with suitable organic 
solvents by means of a dispersing machine, e.g., a ball mill and a sand 
mill. If necessary, other materials may be added such as antistatic 
agents, lubricants, abrasives, hardeners, and so forth. The degree of 
dispersion of the magnetic coating composition is closely related to or 
responsible for the surface smoothness, orientation ratio, and so forth of 
the magnetic recording material produced therefrom. Further, it is the 
most important factor relating to the electromagnetic transforming 
characteristics of the magnetic recording material. Therefore, in order to 
produce magnetic coating compositions having much higher degree of 
dispersion, it is quite important to select suitable combinations of 
magnetic materials and polymeric binders. 
Furthermore, the durability of the magnetic recording material varies 
greatly with the combination of magnetic material and polymeric binders. 
Thus, also from a viewpoint of durability, it is very important to select 
suitable combinations of magnetic materials and polymeric binders. 
Heretofore, as such polymeric binders, vinyl chloride/vinyl acetate-based 
copolymers, polybutyral resins, phenoxy resins, thermoplastic polyurethane 
resins, acrylonitrile/butadiene-based copolymers, synthetic rubbers, etc. 
have been used alone or in combination with each other. 
When, however, these polymeric binders are used in the preparation of 
magnetic coating compositions containing ferromagnetic metal powder or 
ferromagnetic alloy powder, such metal type magnetic materials are 
insufficiently dispersed in the polymeric binders. This is because metal 
type magnetic materials have very poor dispersibility compared with other 
oxide type magnetic materials. Thus, magnetic recording materials produced 
from these metal type magnetic coating compositions have disadvantages in 
that the surface properties are poor and the orientation ratio is low. 
On the other hand, magnetic coating compositions using conventional oxide 
type magnetic materials and nitrocelluloses, e.g., HIG1/2 and RS1/2 are 
described in Japanese Patent Publication Nos. 48,003/77, 48,004/77, 
4,122/75, 48,126/74, 46,921/74 and 46,921/72, and Japanese Patent 
Application (OPI)Nos. 43,405/77, 44,904/76 and 67,605/75. These magnetic 
coating compositions are superior particularly in dispersion 
characteristics. However, when the metal type magnetic materials are used 
in place of such oxide type magnetic materials, the resulting magnetic 
coating compositions are seriously increased in viscosity or become gel, 
as reported in Japanese Patent Publication No. 22,063/72. The increase in 
viscosity is caused by the surface activity of the metal type magnetic 
materials being completely different from those of the conventional oxide 
type magnetic materials. Accordingly, it is not possible to obtain a 
product in a liquid paint form. 
SUMMARY OF THE INVENTION 
The present inventors have carried out extensive investigations to produce 
magnetic coating compositions for use in the production of metal type 
magnetic recording materials (i.e., containing ferromagnetic metal powder 
or ferromagnetic alloy powder). As a result of these investigation the 
inventors have found that the use of nitrocellulose having a low degree of 
polymerization as one of polymeric binders produces magnetic coating 
compositions capable of producing the metal type magnetic recording 
materials having greatly superior dispersibility and durability. 
An object of the present invention is to produce magnetic coating 
compositions capable of producing magnetic recording materials containing 
ferromagnetic metal powder or ferromagnetic alloy powder and having 
excellent dispersibility of the powder and durability. 
The present invention, in one embodiment, relates to magnetic coating 
compositions for use in the production of magnetic recording materials, 
comprising at least ferromagnetic metal powder and/or ferromagnetic alloy 
powder, and nitrocellulose having an average degree of polymerization of 
from 10 to 55. 
In another embodiment, the present invention relates to magnetic coating 
compositions for use in the production of magnetic recording materials, 
comprising at least ferromagnetic metal powder and/or ferromagnetic alloy 
powder, nitrocellulose having an average degree of polymerization of from 
10 to 55, and at least one polymeric binder. 
DETAILED DESCRIPTION OF THE INVENTION 
The term "ferromagnetic metal powder or ferromagnetic alloy powder" is used 
herein to mean needle-like fine powders of iron, alloys composed mainly of 
iron or nickel. The alloys comprised mainly of iron or nickel used in the 
present invention contain at least 50% by weight, preferably 70% by weight 
or more, of iron or nickel. These needle-like fine powders can be produced 
by various techniques, typical ones of which are shown below: 
(1) Organic acid salts of ferromagnetic metal are decomposed by heating and 
then, reduced with reducing gases; 
(2) Needle-like oxyhydroxides with or without other metals incorporated 
thereinto, or needle-like oxides produced from the needle-like 
oxyhydroxides are reduced; 
(3) Ferromagnetic metals are vaporized in low pressure inert gases; 
(4) Metal carbonyl compounds are thermally decomposed; 
(5) Ferromagnetic powder is electrically deposited by the use of a mercury 
cathode, and then separated from the mercury; 
(6) A solution of ferromagnetic metal salts are reduced by adding a 
reducing agent thereto. 
The ferromagnetic metal powder or ferromagnetic alloy powder as used herein 
is not limited to specific ones. Examples of the powders include iron, an 
iron-cobalt alloy, an iron-nickel alloy, an iron-cobalt-nickel alloy, and 
a nickel-cobalt-phosphorus alloy. The powders have preferably an axis 
ratio of from 5/1 to 15/1 and a long axis of from about 0.2 to about 0.8 
.mu.m, can be used. 
The nitrocellulose as used herein is nitrocellulose having an average 
degree of polymerization of from 10 to 55, which is produced by nitration 
of a cellulose material, e.g., cotton linter and wood pulp, in known 
procedures to form nitrocellulose, and then, heating the thus-formed 
nitrocellulose along with water in an autoclave. Considering durability, 
it is preferred to use nitrocellulose having an average degree of 
polymerization of from 31 to 55. The heating time and temperature in the 
autoclave are determined depending on the degree of polymerization of the 
crude nitrocellulose used. Nitrocellulose having an average degree of 
polymerization of less than 10 has disadvantages in that the dispersion 
stability of the resulting composition is poor and the ferromagnetic metal 
powder or ferromagnetic alloy powder precipitates in the composition, and 
the resulting magnetic recording material has poor durability. On the 
other hand, when the average degree of polymerization is more than 55, the 
resulting coating composition has only poor dispersibility and can provide 
only a magnetic recording material which is quite insufficient with 
respect to its surface smoothness and orientation ratio. 
The amount of the nitrocellulose used is from 1 to 30% by weight, 
preferably from 5 to 20% by weight, based on the weight of the 
ferromagnetic metal powder and/or ferromagnetic alloy powder. When the 
amount is less than 1% by weight, the resulting composition has only poor 
dispersibility, and it can therefore provide only a magnetic recording 
material which is not satisfactory with respect to its surface properties 
and orientation ratio. On the other hand, when the amount is larger than 
30% by weight, although the dispersibility of the resulting composition 
are good, the density of a magnetic recording material produced from the 
composition is excessively decreased, resulting in a reduction in 
recording density. 
The average degree of polymerization as used herein is measured as follows: 
A necessary amount (as determined so that .eta.sp is from 0.1 to 0.2) of 
nitrocellulose is accurately weighed in a 200-ml Elmenmeyer flask with a 
stopper, and 50 ml of a special grade acetone (defined by JIS-K3034) 
maintained at 20.+-.0.1.degree. C. was added thereto. After allowed to 
stand for about 16 hours, the resulting solution was gently shaken for 5 
minutes to obtain a sample solution. The flask was soaked in water 
maintained at 20.+-.0.1.degree. C., and allowed to stand for at least 20 
minutes. A 5-ml portion is sampled, placed in an Ostwald viscometer, 
allowed to stand at 20.+-.0.1.degree. C. for at least 20 minutes, and 
thereafter, the speed at which the sample solution flows down from Point 
A(upper line) to Point B(lower line) in a fine tube of the viscometer is 
measured. Separately, the time required for acetone to flow down from 
Point A to Point B is measured in the same manner as used in measuring the 
speed of the sample solution. On basis of the thus-obtained values, the 
degree of polymerization is calculated by the following equations (1), 
(2), and (3): 
##EQU1## 
where .eta.sp: specific viscosity 
t: time required for sample solution to fall down (sec.) 
t.sub.0 : time required for acetone to fall down (sec.) 
##EQU2## 
where [.eta.]: intrinsic viscosity 
C: concentration of sample (g/l) 
K': 0.315 
##EQU3## 
where DP: average degree of polymerization 
Km: 11.times.10.sup.-4 
Any polymeric binders conventionally used in the preparation of magnetic 
coating compositions for magnetic recording materials can be used with the 
nitrocellulose in the invention. Examples include vinyl chloride/vinyl 
acetate-based copolymers, vinyl chloride/vinylidene chloride-based 
copolymers, vinyl chloride/acrylonitrile-based copolymers, acrylic acid 
ester/acrylonitrile-based copolymers, acrylic acid ester/vinylidene 
chloride-based copolymers, acrylic acid ester/styrene-based copolymers, 
methacrylic acid ester/acrylonitrile-based copolymers, methacrylic acid 
ester/vinylidene chloride-based copolymers, methacrylic acid 
ester/styrene-based copolymers, polyurethane elastomers, polyvinyl 
fluoride, vinylidene chloride/acrylonitrile-based copolymers, 
butadiene/acrylonitrile-based copolymers, polyamide resins, polyvinyl 
butyral, cellulose acetate butyrate, cellulose acetate, cellulose acetate 
isobutyrate, cellulose acetate propionate, ethyl cellulose, polyester 
resins, styrene/butadiene-based copolymers, various synthetic rubber-based 
thermoplastic resins (e.g., polybutadiene, polychloroprene, and 
polyisoprene), modified natural rubbers (e.g., chlorinated rubber and 
cyclized rubber), and modified polyolefins (e.g., chlorinated polyethylene 
and chlorinated polypropylene). These polymers can be used alone or in 
combination with each other. 
The polymeric binder should be added in an amount of not more than 30 times 
(by weight) based on the weight of the nitrocellulose. Preferably the 
polymeric binder is added in an amount from equal to to 10 times the 
amount of the nitrocellulose. When the amount of the polymeric binder 
added exceeds 30 times (by weight) of the nitrocellulose, the 
dispersibility of the ferromagnetic metal powder or ferromagnetic alloy 
powder decreases, providing a magnetic recording material which is 
insufficient with respect to its surface properties and compounding 
properties. 
These polymeric binders may be added after dispersing the nitrocellulose 
and the magnetic material in an organic solvent as a medium, or may be 
added along with the nitrocellulose in dispersing the magnetic material. 
The use of these polymeric binders in combination with the abovedescribed 
nitrocellulose improves, in particular, the durability of a magnetic 
recording material produced from the resulting magnetic coating 
composition. 
In addition, if necessary, antistatic agents, lubricants, abrasives, 
dispersing agents, hardening agents, and so forth can be added to the 
magnetic coating compositions of the invention. Usually, carbon black is 
used as an anti-static agent; high molecular weight alcohol esters such as 
butyl myristate, and silicone oils are used as lubricants; chromium 
dioxide and alumina are used as abrasives; and lecithin and surface active 
agents are used as dispersing agents. Examples of useful hardening agents 
include a tolylene diisocyanate/trimethylolpropane adduct, which is most 
preferred. Other useful hardening agents include a hexamethylene 
diisocyanate/trimethylolpropane adduct, a hexamethylene diisocyanate/water 
adduct, an isophorone diisocyanate/trimethylolpropane adduct, and their 
isocyanurated products. 
The above-described additives are preferably added (with the total weight 
of the ferromagnetic metal powder and/or ferromagnetic alloy powder as 
100) in an amount of from 2 to 10 for the antistatic agent, from 1 to 5 
for the lubricant, from 1 to 5 for the abrasive, and from 0.5 to 2.5 for 
the dispersing agent. If the additives are included in lesser amounts than 
the above-defined ranges, the effect of each additive is insufficient, 
whereas in larger amounts than the ranges the durability of the resulting 
coating film is poor. 
The amount of the hardening agent added is determined depending on the 
types and amounts of the nitrocellulose and polymeric binder used. 
Preferably the hardening agent is added so that the NCO content of the 
hardening agent is from 0.2 to 1.0 mole per mole of the total OH content 
of the nitrocellulose and the polymeric binder(s) used in combination 
therewith. When the NCO content of the hardening agent is less than 0.2 
mole, the crosslink density is not sufficiently high, leading to the 
production of a magnetic recording material having poor durability. On the 
other hand, when it is more than 1.0 mole, the unreacted hardening agent 
remains in the resulting magnetic coating film, causing problems such as a 
reduction in the running properties of the ultimate magnetic recording 
material (e.g., a magnetic tape) and the contamination of a head. 
The use of the above-described additives makes the invention more useful. 
Typical methods of preparing the magnetic coating compositions of the 
invention will hereinafter be explained. 
(1) Ferromagnetic metal powder and/or ferromagnetic alloy powder and the 
nitrocellulose are mixed and dispersed for from 1 to 72 hours in the 
presence of a suitable organic solvent (for example, acetic acid esters 
such as methyl acetate, ethyl acetate and butyl acetate, ketones such as 
acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK) and 
cyclohexanone, tetrahydrofuran, N,N'-dimethylformamide, toluene, xylene, 
cyclohexane, and n-hexane) by means of a dispersing machine such as a ball 
mill, a sand mill and an attritor. Further, a polymeric binder, an 
antistatic agent, a lubricant, an abrasive, a hardening agent, etc. are 
added, and the resulting mixture is further mixed for from 1 to 10 hours 
to prepare the desired magnetic coating composition. 
(2) Ferromagnetic metal powder and/or ferromagnetic alloy powder, the 
nitrocellulose, a polymeric binder, an antistatic agent, a lubricant, an 
abrasive, etc. are mixed and dispersed for from 1 to 72 hours in the 
presence of a suitable organic solvent by means of a dispersing machine 
such as a ball mill, a sand mill and an attritor. Then, a hardening agent 
is added, and the resulting mixture is further mixed for from 1 to 10 
hours to prepare the desired coating composition. 
(3) Ferromagnetic metal powder and/or ferromagnetic alloy powder and the 
nitrocellulose are preliminarily kneaded in the presence of a suitable 
organic solvent by means of a kneading machine such as a kneader and a 
disper and, thereafter, the resulting mixture is processed in the same 
manner as in the preparation of conventional magnetic coating 
compositions, to prepare the desired coating composition. 
(4) Ferromagnetic metal powder and/or ferromagnetic alloy powder, the 
nitrocellulose and a polymeric binder are dispersed in the presence of an 
organic solvent by means of a dispersing machine such as a ball mill, and 
then, an antistatic agent, a lubricant, a hardening agent, etc. are added 
and mixed to prepare the desired coating composition. 
Of these methods, Method (3) is preferred, in which ferromagnetic metal 
powder and/or ferromagnetic metal powder and the nitrocellulose are 
preliminarily kneaded in the presence of an organic solvent, and then, a 
polymeric binder, a lubricant, a hardening agent, etc. are added and 
dispersed to prepare the composition of theinvention.

The following examples are given to illustrate the invention in greater 
detail. 
Preparation of Nitrocellulose 
Nitrocellulose HE 2000 (produced by Asahi Kasei Kogyo Kabushiki Kaisha: 
average degree of polymerization: 900; ethyl alcohol content: 30%) was 
added to 10 volumes of water, fully stirred, and then, dehydrated by a 
centrifugal separator. This procedure was repeated five times, and then, 5 
volumes of water was added. The resulting mixture was heated at 
140.degree. C. for 100 minutes, 120 minutes, 160 minutes, 200 minutes, 240 
minutes and 350 minutes, respectively, to prepare six kinds of 
nitrocelluloses. The average degrees of polymerization of these 
nitrocelluloses were measured with the results shown in Table 1. 
These nitrocelluloses are designated as Nitrocelluloses A to F as described 
in Table 1. 
TABLE 1 
______________________________________ 
Treating Time in 
Average Degree 
Autoclave (min.) 
of Polymerization 
Nitrocellulose 
______________________________________ 
100 52 A 
120 45 B 
160 32 C 
200 22 D 
240 13 E 
350 8 F 
______________________________________ 
EXAMPLE 1 
______________________________________ 
Amount 
(parts by weight) 
______________________________________ 
Ferromagnetic metal powder 
110 
(produced by Toda Kogyo Co., 
Ltd.; trade name: KM-1000) 
Nitrocellulose A (average 
10 
degree of polymerization: 
52) 
Thermoplastic polyurethane 
20 
resin (produced by B. F. 
Goodrich Corp.; trade Name: 
Estane 5703) 
MEK 100 
MIBK 100 
______________________________________ 
These ingredients were dispersed in a stainless steel ball mill for 72 
hours. Then, 1 part by weight of carbon black as an antistatic agent, 0.3 
part by weight of butyl myristate as a lubricant, and 3 parts by weight of 
Colonate L (trade name; produced by Nippon Polyurethane Co., Ltd.) as a 
hardening agent were added, and the resulting mixture was further mixed 
for 1 hour to prepare a magnetic coating composition. The magnetic coating 
composition was evaluated by the methods as described hereinafter. The 
results are shown in Table 2. 
Surface Smoothness 
A magnetic coating composition was coated on a polished glass plate having 
a smooth surface in a wet thickness of 254 .mu.m by means of a Baker 
applicator. The thus-formed coating film was then preliminarily dried at 
room temperature for 30 minutes and then hardened by heating at 80.degree. 
C. for 24 hours. Thereafter, the 60.degree./60.degree. specular glossiness 
of the coating film was measured by the use of a digital varied-angle 
glossmeter (manufactured by Suga Shikenki Co., Ltd.). The results are 
shown in Table 2, in which the values are shown with the glossiness of the 
standard black plate as 91.5. Higher surface glossiness shows higher 
surface smoothness. 
Orientation Ratio 
A magnetic coating composition was coated on a 15 .mu.m thick polyester 
film in a dry thickness of about 5 .mu.m and, before drying, was passed 
through a repulsive magnetic field to orientate the magnetic material 
contained in the magnetic coating film. Then, the coating film was 
hardened by heating at 80.degree. C. for 24 hours, and cut to a 1 inch 
square sample piece. With this sample piece, the squareness ratios in the 
direction of orientation, and in a direction perpendicular to the 
orientation direction were measured by a B-H curve tracer (produced by 
Riken Denshi Co., Ltd.), and the orientation ratio was determined by the 
following equation: 
EQU Orientation ratio=(Squareness ratio in orientation direction)/(Squareness 
ratio in perpendicular direction) 
Dispersion Stability 
A magnetic coating composition was placed in a precipitating tube made of 
glass having a diameter of 10 mm and a length of 300 mm and was allowed to 
stand in an atmosphere of 20.+-.0.1.degree. C. and 65.+-.3% RH for 48 
hours, and the condition of the composition was observed with the naked 
eye. The criteria for the determination were as follows: 
A: Precipitation and separation of ferromagnetic metal powder and/or 
ferromagnetic alloy powder were not observed. 
B: Separation was observed. 
Durability 
A magnetic coating composition was coated on a 15 .mu.m thick polyester 
film in a dry thickness of about 5 .mu.m and, before drying, was passed 
through a repulsive magnetic field to orientate the magnetic material in 
the magnetic coating composition. The resulting film was preliminarily 
dried at room temperature for 24 hours, subjected to calendering, hardened 
by heating at 60.degree. C. for 48 hours, and cut to a 1-inch width to 
produce a test tape. This test tape was mounted on a commercially 
available video tape recorder and the still mode reproduction was 
performed for 60 minutes. At the end of the time, the condition of the 
tape surface was examined with the naked eye and under an optical 
microscope. The criteria for the determination were as follows: 
A: No changes were observed. 
B: Some scratches were observed. 
C: The magnetic layer was completely removed. 
EXAMPLE 2 
A magnetic coating composition was prepared in the same manner as in 
Example 1 except that Nitrocellulose B (average degree of polymerization: 
45) was used in place of Nitrocellulose A, and its performance was 
evaluated. The results are shown in Table 2. 
EXAMPLE 3 
A magnetic coating composition was prepared in the same manner as in 
Example 1 except that Nitrocellulose C (average degree of polymerization: 
32) was used in place of Nitrocellulose A, and its performance was 
evaluated. The results are shown in Table 2. 
EXAMPLE 4 
A magnetic coating composition was prepared in the same manner as in 
Example 1 except that Nitrocellulose D (average degree of polymerization: 
22) was used in place of Nitrocellulose A, and its performance was 
evaluated. The results are shown in Table 2. 
EXAMPLE 5 
A magnetic coating composition was prepared in the same manner as in 
Example 1 except that Nitrocellulose E (average degree of polymerization: 
13) was used in place of Nitrocellulose A, and its performance was 
evaluated. The results are shown in Table 2. 
COMATIVE EXAMPLE 1 
A magnetic coating composition was prepared in the same manner as in 
Example 1 except that Nitrocellulose HIG1/2 (average degree of 
polymerization: 90; produced by Asahi Kasei Kogyo Kabushiki Kaisha) was 
used in place of Nitrocellulose A, and its performance was evaluated. The 
results are shown in Table 2. 
COMATIVE EXAMPLE 2 
A magnetic coating composition was prepared in the same manner as in 
Example 1 except that Nitrocellulose HIG1/4 (average degree of 
polymerization: 65; produced by Asahi Kasei Kogyo Kabushiki Kaisha) was 
used in place of Nitrocellulose A, and its performance was evaluated. The 
results are shown in Table 2. 
COMATIVE EXAMPLE 3 
A magnetic coating composition was prepared in the same manner as in 
Example 1 except that Nitrocellulose F (average degree of polymerization: 
8) was used in place of Nitrocellulose A, and its performance was 
evaluated. The results are shown in Table 2. 
COMATIVE EXAMPLE 4 
A magnetic coating composition was prepared in the same manner as in 
Example 1 except that a vinyl chloride/vinyl acetate copolymer (VAGH, 
produced by Union Carbide Corp.) was used in place of Nitrocellulose A, 
and its performance was evaluated. The results are shown in Table 2. 
EXAMPLE 6 
______________________________________ 
Amount 
(parts by weight) 
______________________________________ 
KM-1000 110 
Nitrocellulose C 
20 
Estane 5703 10 
MEK 100 
MIBK 100 
______________________________________ 
These ingredients were mixed and dispersed in a stainless steel ball mill 
for 72 hours. Using the thus-formed dispersion, a magnetic coating 
composition was prepared in the same manner as in Example 1, and its 
performance was evaluated. The results are shown in Table 2. 
EXAMPLE 7 
______________________________________ 
Amount 
(parts by weight) 
______________________________________ 
KM-1000 110 
Nitrocellulose C 
10 
MEK 50 
MIBK 100 
______________________________________ 
These ingredients were mixed and dispersed in a stainless steel ball mill 
for 72 hours. Then, 30 parts by weight of Estane 5703, 50 parts by weight 
of MEK, 1 part by weight of carbon black, 0.3 part by weight of butyl 
myristate, and 3 parts by weight of Colonate L were added, and the 
resulting mixture was further mixed to prepare a magnetic coating 
composition. The performance of the magnetic coating composition was 
evaluated in the same manner as in Example 1. The results are shown in 
Table 2. 
EXAMPLE 8 
______________________________________ 
Amount 
(parts by weight) 
______________________________________ 
KM-1000 110 
Nitrocellulose C 
3 
MEK 50 
MIBK 50 
______________________________________ 
These ingredients were mixed and preliminarily kneaded in a kneader 
(.SIGMA. blade type) for 4 hours. Then, the following ingredients were 
added thereto. 
______________________________________ 
Amount 
(parts by weight) 
______________________________________ 
Nitrocellulose C 
7 
Estane 5703 20 
MEK 50 
MIBK 50 
______________________________________ 
The resulting mixture was kneaded in a stainless steel ball mill for 68 
hours. Using the thus-kneaded mixture, a magnetic coating composition was 
produced in the same manner as in Example 1, and its performance was 
evaluated. The results are shown in Table 2. 
EXAMPLE 9 
A magnetic coating composition was prepared in the same manner as in 
Example 6 except that the amounts of the Nitrocellulose C and Estane 5703 
added were changed from 20 parts by weight to 27 parts by weight and from 
10 parts by weight to 3 parts by weight, respectively. The performance of 
the magnetic coating composition was evaluated. The results are shown in 
Table 2. 
EXAMPLE 10 
A magnetic coating composition was prepared in the same manner as in 
Example 6 except that the amounts of the Nitrocellulose C and Estane 5703 
added were changed from 20 parts by weight to 6 parts by weight and from 
10 parts by weight to 24 parts by weight, respectively. The performance of 
the magnetic coating composition was evaluated. The results are shown in 
Table 2. 
TABLE 2 
______________________________________ 
Surface 
Orienta- Disper- Total 
Glossi- 
tion sion Dura- Evalu- 
ness Ratio Stability 
bility 
ation* 
______________________________________ 
Example 
No. 
1 45 1.45 A A B 
2 55 1.65 A A A 
3 60 1.70 A A A 
4 66 1.75 A B B 
5 60 1.75 A B B 
6 65 1.80 A A A 
7 63 1.75 A A A 
8 68 1.80 A A A 
9 68 1.80 A B B 
10 47 1.62 A A B 
Comparative 
Example 
1 15 1.15 B C C 
2 27 1.27 B B C 
3 55 1.60 B C C 
4 22 1.20 B A C 
______________________________________ 
*Note [Total Evaluation 
A -- Excellent 
B -- Good 
C -- Not Good 
It can be seen from Table 2 that the magnetic coating compositions of the 
invention are much superior with respect to their dispersibility as 
compared with conventional compositions prepared using synthetic resins 
such as a vinyl chloride/vinyl acetate copolymer (Comparative Example 4), 
and nitrocellulose having a degree of polymerization outside the range of 
10 to 55 (Comparative Examples 1-3). The composition of the invention can 
provide magnetic recording materials which have excellent surface 
smoothness and orientation ratio, and further, excellent durability. Thus, 
the magnetic coating compositions of the invention are very useful in the 
production of video tapes and audio tapes. 
While the invention has been described in detail and with reference to 
specific embodiment 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.