Process for the preparation of biaxially oriented polyester film

A biaxially oriented polyester film with excellent levelling property is produced by the inventive process which comprises: (a) melt-extruding and extending biaxially a polyester resin to shape it into a film; (b) treating the film thermally under a condition that formulae (1), (2) and (3) are satisfied; and (c) relaxing the film at a temperature range of Tm-60.degree. C. to Tm-140.degree. C. during the step (b): EQU Tm-60.degree. C.<T1<Tm-10.degree. C. (1) EQU 50<T1-T2<100 (2) EQU 5<(T1-T2)/t<25 (3) wherein: PA1 Tm represents the melting point of the film (.degree.C.), PA1 T1 represents the initial temperature for the thermal treatment of the film (.degree.C.), PA1 T2 represents the terminal temperature for the thermal treatment of the film (.degree.C.), and PA1 t represents time for the thermal treatment (sec.).

FIELD OF THE INVENTION 
The present invention relates to a process for the preparation of a 
biaxially oriented polyester film having an excellent levelling property, 
by way of controlling thermal treatment conditions of the film. 
BACKGROUND OF THE INVENTION 
Polyesters, especially polyethylene terephthalates (PET), possess good 
mechanical properties, e.g., tensile strength, elasticity and impact 
strength, as well as good chemical stability, heat resistance and 
electrical insulation property; therefore, they have been widely used in 
various applications such as capacitors, packaging materials, graphics and 
magnetic recording media. 
For use in such applications, polyester films are generally subjected to 
successive biaxial extension and thermal treatment processes. The 
biaxially oriented film treated thermally by a conventional method has a 
significant disadvantage, in that the film has a poor levelling property 
due to residual extension and contraction stresses applied during the 
thermal treatment process. The poor levelling property of the film entails 
poor appearance, coatability and windability, which causes blurring of 
prints and also curling of the film when laminated. 
Therefore, numerous attempts have been made to ameliorate such problems. 
For example, Japanese Patent Publication No. 72-3196 discloses a process 
for treating the portion having poor levelling property using a separate 
thermal relaxation apparatus. However, this method has some disadvantages, 
e.g., the relaxation apparatus is not easily adapted to an existing 
system, and the difficulty in continuous monitoring and mending defective 
portions when it is installed in a continuous film production system. 
U.S. Pat. No. 4,293,508 teaches a method for minimizing the time needed for 
longitudinal extension of the film. This method still requires the use of 
complicate equipments that are difficult to harmonize with the existing 
film production line. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a process 
for conveniently preparing a biaxially oriented polyester film having an 
excellent levelling property, by way of controlling thermal treatment 
conditions of the film. 
In accordance with an aspect of the present invention, there is provided a 
process for preparing a biaxially oriented polyester film which comprises: 
(a) melt-extruding and extending biaxially a polyester resin to shape it 
into a film; (b) treating the film thermally under a condition that 
formulae (1), (2) and (3) are satisfied; and (c) relaxing the film at a 
temperature range of Tm-60.degree. C. to Tm-140.degree. C. during the step 
(b): 
EQU Tm-60.degree. C.&lt;T1&lt;Tm-10.degree. C. (1) 
EQU 50&lt;T1-T2&lt;100 (2) 
EQU 5&lt;(T1&lt;T2)/t&lt;25 (3) 
wherein: 
Tm represents the melting point of the film (.degree. C.), 
T1 represents the initial temperature for the thermal treatment of the film 
(.degree.C.), 
T2 represents the terminal temperature for the thermal treatment of the 
film (.degree.C.), and 
t represents time for the thermal treatment (sec.).

DETAILED DESCRIPTION OF THE INVENTION 
A biaxially oriented polyester film is typically prepared by melt-extruding 
and biaxially extending a polyester resin to shape it into a film, and 
treating the film thermally for heat-setting. 
In accordance with the present invention, a biaxially oriented polyester 
film having an excellent levelling property can be provided by way of 
controlling temperatures during the thermal treatment step such that they 
lie within the ranges specified by the present invention. 
A polyester resin suitable for use in the present invention may be prepared 
by polycondensation of an acid component, such as a dialkyl ester of an 
aromatic dicarboxylic acid with a diol component, such as an alkylene 
glycol. 
Representatives of the dialkyl ester of aromatic dicarboxylic acid include: 
dialkyl esters of terephthalic acid, isophthalic acid, naphthalene 
dicarboxylic acid, cyclohexane dicarboxylic acid, diphenoxyethane 
dicarboxylic acid, diphenyl dicarboxylic acid, diphenylether dicarboxylic 
acid, anthracene dicarboxylic acid and 
.alpha.,.beta.-bis(2-chlorophenoxy)ethane-4,4'-dicarboxylic acid; and, 
among these, dimethyl terephthalate is most preferred. 
Exemplary alkylene glycol are ethylene glycol, trimethylene glycol, 
tetramethylene glycol, pentamethylene glycol, hexamethylene glycol and 
hexylene glycol. Among these, ethylene glycol is most preferred. 
The polyester resin of the present invention may comprise additives, e.g., 
antioxidant, UV absorbent, thermal stabilizer, crystallization 
accelerator, colorant, nucleating agent, lubricant, anti-blocking agent 
and the like, in amounts which would not deteriorate the effectiveness of 
the present invention. 
Further, in order to impart good winding and optical properties to the 
film, the polyester resin may comprise additional inert inorganic or 
organic particles which are insoluble in the polyester resin. These inert 
particles may be employed in amounts which would not affect the 
effectiveness of the present invention. 
Representatives of the inorganic particles include: oxides or other 
inorganic salts of a metal selected from Group II, III or IV of Periodic 
Table of Element, e.g., synthetic or natural calcium carbonate, wet or dry 
silica, calcium phosphate, magnesium carbonate, talc, alumina, sodium 
fluoride, titanium oxide, mica, aluminum hydroxide, calcium terephthalate 
and the like. 
Organic particles suitable for use in the present invention include: 
particles of a crosslinked polymer, elastomer or fluorine-containing 
polymer; and, among these, particles of a crosslinked polystyrene and 
fluorine-containing polymer are most preferred. 
The inert particles may be added to the polyester resin at any time before 
terminating the polycondensation of raw materials, and they are preferably 
added immediately after the transesterification step, or at the beginning 
of the polycondensation step. 
The biaxially oriented polyester film of the present invention may be 
obtained as follows: a polyester resin prepared by a conventional method 
is melt-extruded into an amorphous cast sheet by the T-die method well 
known in the art. The sheet is passed through rollers, which are 
maintained at a temperature ranging from T.sub.g +10.degree. to T.sub.g 
+50.degree. C. (T.sub.g : glass transition temperature of the polyester 
resin) and are rotated at different speeds, to extend the resin in the 
longitudinal (film proceeding) direction in a draw ratio of 2.0 to 5.0, 
and then, cooled to room temperature, thus providing a monoaxially 
oriented film. The monoaxially oriented film is transported to a tenter, 
where the film is extended in both longitudinal and traverse directions at 
a temperature ranging from T.sub.g '+10 to T.sub.g '+50.degree. C. 
(T.sub.g ': glass transition temperature of the monoaxially oriented 
polyester film) in a draw ratio of 3.0 to 5.0, thus providing a biaxially 
oriented film. 
The film so obtained is then subjected to a thermal treatment process, 
thereby to impart dimension stability to the film and to control its 
crystal structure. 
In accordance with the present invention, the thermal treatment of the film 
is carried out under a condition that formulae (1), (2) and (3) are 
satisfied, wherein the film was relaxed, by partially releasing the 
tensional stress applied to the film, at a temperature range of 
Tm-60.degree. C. to Tm-140.degree. C.: 
EQU Tm-60.degree. C.&lt;T1&lt;Tm-10.degree. C. (1) 
EQU 50&lt;T1-T2&lt;100 (2) 
EQU 5&lt;(T1&lt;T2)/t&lt;25 (3) 
wherein: 
Tm represents the melting point of the film (.degree.C.), 
T1 represents the initial temperature for the thermal treatment of the film 
(.degree.C.), 
T2 represents the terminal temperature for the thermal treatment of the 
film (.degree.C.), and 
t represents time for the thermal treatment (sec.). 
When the initial temperature of the thermal treatment step, T1, is equal to 
or below Tm-60.degree. C., the dimension stability of the film 
deteriorates; when it is equal to or higher than Tm-10.degree. C., it 
becomes difficult to adjust the levelling property of the film. 
Further, when the difference between the initial and terminal temperatures 
of the thermal treatment, T1-T2, is equal to or less than 50.degree. C., 
the levelling property of the film is not easily controlled and, 
therefore, becomes poor. When the difference is equal to or greater than 
100.degree. C., temperature control in the tenter is difficult. 
Even if the formulae (1) and (2) are satisfied in accordance with the above 
requirements, the levelling property and productivity of the film is 
lowered if formula (3) is not met. 
In the present invention, relaxation of the film is carried out at a 
temperature range of Tm-60.degree. C. to Tm-140.degree. C. during the 
thermal treatment process. Any violation of this temperature requirement 
may make it difficult to control the levelling property and dimension 
stability of the film. 
In preparing the polyester film in accordance with the present invention, 
conditions for melt-extrusion, casting, and longitudinal and traverse 
direction extension may be suitably determined by persons skilled in the 
art. 
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 levelling property of the 
polyester film was evaluated in accordance with the following method: A 
thermally treated 1 m (length).times.1 m (width) film was spread over a 
black, plane plate and pressed for 30 minutes in order to remove air 
between the film and the plate. The film was then observed visually, and 
the percentage of uneven surface area to the total surface area of the 
film was determined. The levelling property of the film was classified as 
follows: 
A: When the percentage is less than 5% 
B: When the percentage is between 5% and 10% 
C: When the percentage is between 10% and 30% 
D: When the percentage is between 30% and 50% 
E: When the percentage is greater than 50% 
EXAMPLE 1 
Polyethylene terephthalate (PET) chips were melt-extruded through a T-die 
and quenched over a cooling drum to give a PET sheet. The sheet was 
extended in longitudinal direction by rollers at 95.degree. C. in a draw 
ratio of 3.5, to give a monoaxially oriented film, which was transferred 
to a tenter and extended at 120.degree. C. in traverse direction in a draw 
ratio of 3.5, yielding give a biaxially oriented PET film. The film was 
then treated thermally in a temperature section ranging from 220.degree. 
C. to 140.degree. C. for 10 seconds. During the thermal treatment, the 
film was relaxed by releasing the tensional stress applied to film at a 
relaxation rate of 1 to 5% when the temperature was within the range of 
190.degree. C. to 140.degree. C. 
The levelling property of the PET film thus prepared was evaluated in 
accordance with the above-mentioned method, and the results are shown in 
Table I. 
COMATIVE EXAMPLES 1 to 6 
The procedure of Example 1 above was repeated, except that the conditions 
of the thermal treatment and relaxation steps were varied as shown in 
Table I. 
The levelling properties of the PEN films thus obtained were measured, and 
the results are shown in Table I. 
TABLE I 
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Relax- Level- 
T1-T2 ation ling 
T1 T2 T1-T2 t /t(.degree.C./ 
Temp. Prop- 
(.degree.C.) 
(.degree.C.) 
(.degree.C.) 
(sec) 
sec) (.degree.C.) 
erty 
______________________________________ 
Ex. 1 220 140 80 10 8.0 190-140 
A 
Com. 200 160 40 10 4.0 190-140 
D 
Ex. 1 
Com. 230 80 150 10 15.0 190-140 
C 
Ex. 2 
Com. 230 130 100 3 33.3 190-140 
C 
Ex. 3 
Com. 230 130 100 30 3.3 190-140 
C 
Ex. 4 
Com. 220 140 80 10 8.0 200-180 
D 
Ex. 5 
Com. 220 140 80 10 8.0 160-80 D 
Ex. 6 
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As can be seen from Table I, the biaxially oriented polyester film prepared 
in Example 1 in accordance with the inventive method has an excellent 
levelling property, which presents a sharp contrast to the inferior 
levelling properties observed for the films prepared according to 
procedures outside the scope of the present invention. 
While the invention has been described with respect to the above specific 
embodiments, it should be recognized that various modifications and 
changes may be made within the scope of the invention as defined by the 
claims that follow.