Process for the preparation of biaxially extended polyester film

A biaxially extended polyester film comprising light calcium carbonate particles, spherical silica particles and .gamma.- or .delta.-alumina particles or a mixture thereof treated with a silane coupling agent prepared by the inventive process possesses excellent surface and physical properties.

FIELD OF THE INVENTION 
The present invention relates to a process for the preparation of a 
biaxially extended polyester film having excellent surface and physical 
properties. 
BACKGROUND OF THE INVENTION 
Polyesters such as polyethylene terephthalate (PET) are known to possess 
good chemical stability, physical and mechanical strength, durability, 
heat resistance, chemical resistance, weather resistance and electrical 
insulation property; and, therefore, have been widely used in 
manufacturing various articles including medical devices, capacitors, 
packaging and labelling materials, photographic film and magnetic 
recording media. 
In general, it is well known that the physical properties such as the slip 
property and abrasion resistance of a polyester film greatly influences 
the processability, running property and quality of the film. Polyester 
films having embossed surface are often prepared so as to improve such 
properties of the film. The embossing can be typically formed by 
incorporating inert particles such as particles of calcium carbonate, 
silica and kaolin into the raw materials during the production of the 
film, or by forming inorganic particles in situ. 
However, such use of inorganic particles have the disadvantages that light 
calcium carbonate particles tend to wear out easily due to their low 
hardness, and when a film containing them is used as the base film of a 
video tape, it is easily scratched, e.g., by a guide roll during a 
magnetic layer coating process thereof and by a calendering roll during a 
calendering process. The scratching of the base film may cause the 
drop-out phenomena of the magnetic tape. 
As an attempt to solve such problems, Japanese Patent Laid- open 
Publication No. 214734/1990 discloses the use of light calcium carbonate 
particles and .alpha.-, .gamma.- or .delta.-alumina particles to enhance 
the scratch resistance of a polyester film. Further, Japanese Patent 
Laid-open Publication No. 151231/1992 describes the use of a mixture of 
light calcium carbonate particles and a colloidal silica to impart scratch 
and abrasion resistance to a polyester film. 
However, the use of alumina particles brings out the problems that a stable 
slurry containing a high alumina content is difficult to prepare and the 
life time of a filter used in a polymerization reactor becomes shortened, 
due to the thickening effect of the alumina particles. 
SUMMARY OF THE INVENTION 
It is, therefore, a primary object of the present invention to provide a 
biaxially extended polyester film having excellent surface and physical 
properties, by way of treating alumina particles with a silane coupling 
agent in a glycol to obtain a glycol slurry of treated alumina particles 
and directly using the slurry together with calcium carbonate particles 
and spherical silica particles in subsequent polycondensation reaction of 
a polyester monomer transesterified. 
In accordance with one aspect of the present invention, there is provided a 
biaxially extended polyester film comprising 0.01 to 4.0 wt %, based on 
the weight of the polyester, of light calcium carbonate particles having 
an average diameter ranging from 0.01 to 3 .mu.m, 0.01 to 4 wt % of 
spherical silica particles having an average diameter ranging from 0.1 to 
1.0 .mu.m and 0.01 to 4 wt % of .gamma.- or .delta.-alumina particles or a 
mixture thereof having an average diameter ranging from 0.005 to 3 .mu.m 
and a Mohs hardness of 6 or more, wherein said alumina particles are 
pretreated with 0.05 to 5 wt %, based on the amount of alumina particles 
employed, of a silane coupling agent of formula (I): 
EQU R.sup.1 -R.sup.2 -Si-(OR.sup.3).sub.3 (I) 
wherein, 
R.sup.1 is an organic functional group such as a methacryl 
##STR1## 
or an amino (--NR'.sub.2) group, each R' being independently hydrogen or 
an alkyl group; 
R.sup.2 is a C.sub.1-5 alkylene group; and 
R.sup.3 is a C.sub.1-3 alkyl group. 
In accordance with another aspect of the present invention, there is 
provided a process for preparing a biaxially extended polyester film which 
comprises transesterifying and polycondensing a monomer mixture for the 
preparation of a polyester resin, melt-extruding the polyester resin to 
form a sheet and biaxially extending the sheet to produce the polyester 
film, wherein a glycol slurry of .gamma.- or .delta.-alumina particles or 
a mixture thereof having an average diameter ranging from 0.005 to 3 .mu.m 
and a Mohs hardness of 6 or more which are pretreated with 0.05 to 5 wt % 
of the silane coupling agent of formula (I), light calcium carbonate 
particles having an average diameter ranging from 0.01 to 3 .mu.m and 
spherical silica particles having an average diameter ranging from 0.1 to 
1.0 .mu.m are introduced as slip agents to the product of the 
transesterification step at a temperature ranging from 170.degree. to 
193.degree. C.

DETAILED DESCRIPTION OF THE INVENTION 
A polyester resin which may be used to prepare the polyester film of the 
present invention is produced by a process which comprises 
transesterifying and polycondensing a dialkyl ester of an aromatic 
dicarboxylic acid and an aliphatic glycol. The transesterification and 
polycondensation reactions may be carried out using batch or continuous 
processes, while a direct, one-step polymerization may also be used for 
the preparation of the polyester resin. 
Representatives dialkyl esters of aromatic dicarboxylic acids which may be 
used in preparing a polyester resin in accordance with the present 
invention include: dialkyl esters of terephthalic acid, isophthalic acid, 
naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, 
diphenoxyethanedicarboxylic acid, biphenyldicarboxylic acid, diphenyl 
ether dicarboxylic acid, anthracenedicarboxylic acid and 
.alpha.,.beta.-bis(2-chlorophenoxy)ethane-4,41-dicarboxylic acid, and 
others. Among them, dimethyl terephthalate are most preferred. 
Exemplary aliphatic glycols which may be used in the present invention 
include: ethylene glycol, trimethylene glycol, tetramethylene glycol, 
pentamethylene glycol, hexamethylene glycol and hexylene glycol, and 
others. Among them, ethylene glycol is most preferred. 
A preferred polyester for use in the present invention includes at least 80 
wt % of ethylene terephthalate repeating units, the remainder being 
copolymeric repeating units derived from other dicarboxylic acids or 
oxycarboxylic acids as well as other diols. Such dicarboxylic acids and 
oxycarboxylic acids include: isophthalic acid, 
p-.beta.-hydroxyethoxybenzoic acid, biphenyldicarboxylic acid, 
4,4'-dicarboxylbenzophenone, adipic acid, sebacic acid, sodium 
3,5-di(hydroxycarbonyl)benzene sulfonate, p-oxybenzoic acid and the like; 
and said other diols include: neopentyl glycol, diethylene glycol, 
cyclohexane dimethanol and the like. 
The transesterification catalyst which can be used in the present invention 
may be any one conventionally used in the art such as sodium, manganese, 
potassium, lithium, calcium, magnesium, barium, zinc, zirconium, cobalt, 
aluminum and cadmium compounds, and a mixture thereof. 
The polycondensation catalyst which can be used in the present invention 
may be any one conventionally used in the art such as titanium, germanium, 
tin, antimony, zinc, cobalt, aluminum, lead, manganese and calcium 
compounds, and a mixture thereof. 
The light calcium carbonate particles used as a slip agent in the present 
invention has an average diameter ranging from 0.01 to 3.0 .mu.m, 
preferably from 0.02 to 2.0 .mu.m; and may be used in an amount ranging 
from 0.01 to 4 wt %, preferably 0.05 to 2.0 wt %, based on the weight of 
the polyester. 
Spherical silica particles which are employed to increase running property 
of the polyester film has an average particle diameter ranging from 0.1 to 
1.0 .mu.m, preferably from 0.15 to 0.7 .mu.m; and, may be employed in the 
present invention in an amount from 0.01 to 4 wt %, preferably 0.05 to 2.0 
wt %, based on the weight of the polyester. 
Further, .gamma.- or .delta.-alumina particles or a mixture thereof to be 
used in the present invention has an average particle diameter ranging 
from 0.005 to 3 .mu.m, preferably from 0.01 to 1.5 pm, and a Mohs hardness 
of 6 or more; and may be employed in an amount ranging from 0.01 to 4 wt 
%, preferably from 0.05 to 2 wt % based on the weight of the polyester. 
The silane coupling agent, which is used for the treatment of the alumina 
in the present film, has a structure of formula (I): 
EQU R.sup.1 -R.sup.2 -Si-(OR.sup.3).sub.3 (I) 
wherein, 
R.sup.1 is an organic functional group such as a methacryl 
##STR2## 
or an amino (--NR'.sub.2) group, each R' being independently hydrogen or 
an alkyl group; 
R.sup.2 is a C.sub.1-5 alkylene group; and 
R.sup.3 is a C.sub.1-3 alkyl group. 
The silane coupling agent preferred in the present invention is the 
compound of the formula(I) wherein R.sup.2 is a propylene group and 
R.sup.3 is a methyl or ethyl group. 
In the present invention, the silane coupling agent may be added to an 
ethylene glycol slurry containing 20 to 60 wt % of alumina, in an amount 
ranging from 0.05 to 5.0 wt %, preferably from 0.1 to 5.0 wt % based on 
the weight of the alumina employed. The silane coupling agent may be 
preferably added to the ethylene glycol slurry, while stirring, at a 
temperature ranging from 30.degree. to 180.degree. C., preferably 
30.degree. to 120.degree. C., and the resulting mixture is preferably 
maintained for a period ranging from 40 to 80 minutes, more preferably 50 
to 70 minutes to treat alumina with the silane coupling agent. 
The ethylene glycol slurry of the treated alumina particles is preferably 
introduced, together with calcium carbonate particles and spherical silica 
particles, to the product of the transesterification step at a temperature 
ranging from 170.degree. to 193.degree. C. If the temperature is lower 
than 170.degree. C., the reaction proceeds slowly. Whereas if the 
temperature exceeds 193.degree. C., aggregated particles become larger, 
thereby shortening the life time of the filter used in the reaction. 
In addition to the slip agents described above, the polyester film of the 
present invention may also contain other common additives such as 
antioxidants, antistatic agents, heat stabilizers and dyes. Such additives 
may be added at any time during the preparation of the polyester, 
preferably during the transesterification step or immediately prior to the 
polycondensation step. 
The biaxially extended polyester film of the present invention may be 
prepared as follows. A polyester resin, having a molecular weight of about 
20,000 and containing the above-described alumina particles treated with 
silane coupling agent, spherical silica particles and calcium carbonate 
particles as well as other suitable additives, is melt-extruded into an 
amorphous cast sheet through a T-die, and the resulting sheet is 
subsequently quenched on a cold roll and then biaxially extended to 
produce a biaxially extended polyester film. At the biaxial extending 
step, the longitudinal and lateral extensions may be conducted at a 
temperature ranging from 60.degree. to 150.degree. C. in a draw ratio of 
2.5 to 6.0. 
The thickness of the biaxially extended polyester film may be controlled, 
depending on the final use of the film, typically in the range of 2 to 200 
.mu.m. 
The following Examples are intended to illustrate the present invention 
more specifically, without limiting the scope of the invention. 
In the Examples and Comparative Examples, the characteristics of the slip 
agents employed and the properties of the polyester films produced were 
evaluated in accordance with the following methods. 
1. Average Particle Diameter 
The average diameter of particles was measured as a volumetric average 
diameter in an ethylene glycol slurry by employing a centrifugation type 
granulometer (SA-CP2, Shimadzu in Japan). 
2. Slurry Viscosity 
The viscosity of an ethylene glycol slurry of slip agent particles was 
measured at a spindle speed of 60 rpm using B type viscometer (Brookfield 
in England) at room temperature. 
3. Molecular Weight 
The molecular weight of a polymer was measured by a molecular weight 
measuring apparatus (150C, Waters in U.S.A.) using m-cresol as the mobile 
phase at a flow rate of 1 ml/min. and at a column temperature of 
100.degree. C. 
4. Slurry Stability 
100 cc of an ethylene glycol slurry of slip agent particles was placed in a 
100 ml volumetric cylinder and allowed to stand while observing the status 
of the slurry. The stability of the slurry was evaluated on the basis of 
the following criteria: 
.COPYRGT. (excellent): when no phase separation occurs within 7 days 
.largecircle. (good): when phase separation occurs after 5 days 
.DELTA. (common): when phase separation occurs after 3 days 
X (poor): when phase separation occurs after 1 days 
5. Filter Life Time 
The number of batches which can be processed by a filter in a 
polymerization reactor for polymerizing a polyester resin was measured and 
the filter life time was evaluated on the basis of the following criteria: 
.COPYRGT.: When at least 50 batches are processed 
.largecircle.: When at least 30 batches are processed 
.DELTA.: When at least 10 batches are processed 
X: When at least 5 batches are processed 
6. Surface Smoothness 
The surface smoothness of a 30 mm.times.20 mm.times.15 .mu.m polyester film 
sample was determined with a contact type surface roughness gauge 
(SURFCORDER SE-30D, Kosaka Institute in Japan). 
Average Surface Roughness at Centerline(R.sub.a): Height of a line parallel 
to a mean line of a roughness curve where the areas of both sides of the 
line become equal. 
Highest Height at Centerline(R.sub.t): Distance from the highest point to 
the lowest point within the area measured. 
7. Abrasion Resistance 
The abrasion resistance was determined by running a 1/2 inch-wide film 
sample against a guide pin of a running tester TBT-300D(Yokohama System 
Institute in Japan) at a running speed of 3.3 cm/sec, and observing 
visually or with a microscope, the degree of white powder formation on the 
surface of the guide pin. 
The abrasion resistance was evaluated on the basis of the following 
criteria: 
.COPYRGT.:No white powder was formed on the surface of the guide pin. 
.largecircle.: Up to 20% of the surface of the guide pin was covered by a 
white powder. 
.DELTA.: Up to 50% of the surface of the guide pin was covered by a white 
powder. 
X: The whole surface of the guide pin was covered by a white powder. 
8. Scratch Resistance 
The scratch resistance was determined by running a 1/2 inch-wide film 
sample over a running length of 90 m against a guide pin of a running 
tester with a surface roughness of 0.2S, at a contact degree (.THETA.) of 
180.degree. and a contact speed of 3.3 cm/minute, and at a temperature of 
25.degree. C. and a relative humidity of 60%, while controlling the inlet 
tension of the tester to 30 g. After two such runs, the film surface was 
examined with a microscope for damages caused by the guide pin. 
The scratch resistance was evaluated on the basis of the following 
criteria: 
.COPYRGT.: 2 or less scratch lines were formed on the surface of the film. 
.largecircle.: 3 to 4 scratch lines were formed on the surface of the film. 
.DELTA.: 5 to 6 scratch lines were formed on the surface of the film. 
x : 7 or more scratch lines were formed on the surface of the film. 
9. Running Property 
The running property was measured by running a 1/2 inch wide tape at 
20.degree. C. and at a relative humidity of 60% using a tape running 
tester (TBT-300D, Yokohama System Institute in Japan), and then 
calculating the initial running friction coefficient .mu.k by using the 
following equation: 
EQU .mu.k=0.733log(T.sub.out /T.sub.in) 
wherein: 
T.sub.in is a tension of the tape at the inlet of the tester; and 
T.sub.out is a tension of the tape at the outlet of the tester. 
&lt;Running property at a high speed&gt; 
The running property of the film at a high speed was measured by rotating 
the guide pin either in the running or in the reverse direction, setting 
the winding angle of the film at 180.degree. and then measuring the 
running friction coefficient at a speed of 50 cm/sec and at a tension of 
300 g. 
The running property at a high speed was classified on the basis of the 
following criteria: 
.COPYRGT.: .mu.k.ltoreq.0.10: excellent 
.largecircle.: 0.10&lt;.mu.k&lt;0.20 : good 
.DELTA.: .mu.k=0.20 :common 
X : .mu.k&gt;0.20 :poor 
Example 1 
An ethylene glycol slurry containing 20 wt % of .gamma.-alumina was 
prepared, and then the average particle size of .gamma.-alumina was 
measured. The slurry was then heated to 60 OC and thereto was added 
H.sub.2 N(CH.sub.2).sub.3 Si(OCH.sub.2 CH.sub.3).sub.3 as a coupling 
agent, in an amount of 0.3 wt % based on the amount of .gamma.-alumina 
employed, to treat the surface of the .gamma.-alumina for 55 minutes. The 
viscosity of the slurry thus obtained was measured. 
Dimethyl terephthalate and ethylene glycol were mixed at a molar ratio of 
1:2 and the resulting mixture was transesterified in the presence of zinc 
acetate. Introduced to 100 parts by weight of the resulting product at a 
reactor temperature of 180.degree. C. were; 0.15 part by weight of the 
treated alumina particles in the form of ethylene glycol slurry obtained 
above, 0.30 part by weight of light calcium carbonate particles having an 
average diameter of 0.42 .mu.m and 0.25 part by weight of spherical silica 
particles having an average diameter of 0.28 .mu.m. Then, the resulting 
mixture was subjected to a polycondensation reaction in the presence of 
antimony trioxide to obtain a polyester resin having a molecular weight of 
approximately 20,000. 
The polyester resin thus obtained was dried and melt-extruded to form a 
cast sheet. The sheet was stretched in a draw ratio of 3.0 in both 
longitudinal and lateral directions at 90.degree. C. to provide a 
biaxially extended polyester film having a thickness of 50 .mu.m. The 
properties of the film were measured and the results are shown in Table I. 
Examples 2 and 3 and Comparative Examples 1 to 9 
The procedure of Example 1 was repeated except that the component, the 
particle diameter and the amount of the additives were varied as shown in 
Table I. 
The results of the measurements for the films thus obtained are shown in 
Table I. 
TABLE I 
__________________________________________________________________________ 
Additives Properties of film 
Slip Agent Used Surface 
Aver. Par. 
Slu. 
Slu. 
Molecular 
Smoothness 
Comp. Dia. Amount 
Vis.* 
Stab. 
Weight 
R.sub.a 
R.sub.t 
A.R. 
S.R. 
F. R. 
-- .mu.m 
wt % cps -- Mn .mu.m 
.mu.m 
-- -- -- -- 
__________________________________________________________________________ 
Ex. 
1 C+ S+ .gamma. 
0.42/0.28/ 
0.30/0.25/ 
75.sup.@ 
.circleincircle. 
18,900 
0.021 
0.325 
.circleincircle. 
.circleincircle. 
.circleincircle..sup.# 
.circleincircle. 
0.20 0.15 
2 C+ S+ .delta. 
0.53/0.50/ 
0.20/0.20/ 
67.sup.@@ 
.circleincircle. 
18,800 
0.022 
0.345 
.circleincircle. 
.circleincircle. 
.sup. .circleincircle..sup.## 
.circleincircle. 
00.20 
0.20 
3 C+ S+ .gamma. 
0.60/0.70/ 
0.15/0.15/ 
65.sup.@@@ 
.circleincircle. 
19,100 
0.024 
0.373 
.circleincircle. 
.circleincircle. 
.circleincircle..sup.# 
.circleincircle. 
0.35 0.25 
Com. 
1 C 0.40 0.30 -- -- 17,700 
0.021 
0.364 
X X -- X 
Ex. 
2 C 0.54 0.20 -- -- 17,800 
0.023 
0.100 
X X -- X 
3 C 0.65 0.15 -- -- 17,900 
0.028 
0.475 
X X -- X 
4 .gamma. 
0.10 0.25 1500 
X 18,600 
0.018 
0.288 
X .DELTA. 
X.sup.### 
.DELTA. 
5 .gamma. 
0.20 0.20 1500 
X 18,300 
0.023 
0.356 
X .DELTA. 
X.sup.### 
.DELTA. 
6 .delta. 
0.30 0.15 510 
X 19,500 
0.025 
0.380 
X .DELTA. 
.sup. X.sup.#### 
.largecircle. 
7 C+ .delta. 
0.40/0.10 
0.30/0.25 
510 
X 18,400 
0.027 
0.432 
X .DELTA. 
X.sup.### 
.DELTA. 
8 C+ S+ .gamma. 
0.53/0.50/ 
0.20/0.20/ 
1500 
X 19,500 
0.031 
0.553 
X .largecircle. 
X.sup.### 
.largecircle. 
0.20 0.20 
9 C+ S+ .delta. 
0.60/0.70/ 
0.15/0.15/ 
510 
X 19,300 
0.029 
0.456 
X .largecircle. 
.sup. X.sup.#### 
.largecircle. 
0.35 0.20 
__________________________________________________________________________ 
Footnote 
C: Light calcium carbonate, .gamma.: alumina, .delta.: alumina, S: 
spherical silica 
*The alumina content of the slurry was 20 wt %. 
R.sub.a : Average surface roughness at centerline, R.sub.t : Highest 
height at centerline 
A.R.: Abrasion resistance, S.R.: Scratch resistance, R.: Running property 
F.: Filter life time, @: H.sub.2 N(CH.sub.2).sub.3 Si(OC.sub.2 
H.sub.5).sub.3 (0.3 wt %), @@: CH.sub.2C(CH.sub.3)COO(CH.sub.2).sub.3 
Si(OCH.sub.3).sub.3 (0.4 wt %) 
@@@: CH.sub.2 CHCH.sub.2 O(CH.sub.2).sub.3 Si(OCH.sub.3).sub.3 (0.6 wt %) 
#: Alumina slurry was introduced at 170.degree. C., ##: Alumina slurry wa 
introduced at 193.degree. C. 
###: Alumina slurry was introduced at 145.degree. C., ####: Alumina slurr 
was introduced at 250.degree. C. 
As can be seen from the results in Table I, the films comprising alumina 
particles treated with a silane coupling agent, light calcium carbonate 
particles and spherical silica particles, prepared in accordance with the 
present invention, exhibit excellent surface and physical properties, and 
therefore, are useful in manufacturing various articles. 
While the invention has been described with respect to the above specific 
embodiments, it should be recognized that various modifications and 
changes may be within the scope of the invention as defined by the claims 
the follow.