Biaxially oriented polyester film

Details Biaxially oriented polyester film Biaxially oriented polyester film

1999-08-09

2001-10-23

Chen, Vivian (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Physical dimension specified

C428S332000, C428S480000, C428S910000, C264S288400, C264S290200, C528S308000, C528S308100, C528S308600

Reexamination Certificate

active

06306496

ABSTRACT:

FIELD OF THE THE TECHNOLOGY
The present invention relates to a biaxially oriented olyester film. More specifically, it relates to a biaxially oriented polyester film which has good thickness nonuniformity in both longitudinal and transverse directions, uniform physical properties in a transverse direction and high strength in a longitudinal direction.
RELATED ART
A biaxially oriented polyester film having high strength in a longitudinal direction is generally a film having a large heat shrinkage factor in a longitudinal direction because it is stretched in a longitudinal direction at a high stretch ratio. Therefore, compared with a balanced (balanced type) film whose strengths in biaxial directions are nearly balanced and a transversely tensilyzed film whose strength in a transverse direction is made higher than strength in a longitudinal direction, the above film has larger shrinkage in a longitudinal direction in a transverse stretching zone and a heat-setting zone, and there is apt to occur such a phenomenon that the lengths in a longitudinal direction at a center portion and an edge portion in a transverse direction (width direction) differ from each other (this phenomenon may be referred to as “bowing” hereinafter). Owing to this bowing, the physical properties in a transverse direction of the film become nonuniform and there are caused large differences in heat shrinkage factor and mechanical properties between the center portion and the edge portion in a transverse direction, thereby bringing various troubles.
For example, in a magnetic recording medium, such troubles occur that a difference in skew caused by the contact with a magnetic head or by heat shrinkage factor due to differences in the physical properties of a film according to the location of a base film in a transverse direction of an entire film, or that the running of a film is curved by differences in the extension and contraction in a width direction of the film when a magnetic coating is introduced into a drying step after the coating has been made, thereby influencing productivity.
For obtaining uniform physical properties in a transverse direction of the film, the suppression of bowing is effective. As means of suppressing bowing, JP-A 57-57629 and JP-A 58-24418 propose a method for finely stretching a film in a heat-setting zone. However, this proposal is aimed to equalize the physical properties of a film in both longitudinal and transverse directions, and the stretch ratios in longitudinal and transverse directions are therefore made almost equal to each other. However, the proposal does not contain no mention of the equalization of physical properties in-a transverse direction in an area where the stretch ratio in a longitudinal direction is made 4.0 times or more as in a film of a type having increased strength in a longitudinal direction.
JP-A 9-57845 proposes a method of finely stretching a film in a transverse direction many times, but does not make any mention of stretching conditions for having a high stretch ratio in a longitudinal direction of 4.0 times or more.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a biaxially oriented polyester film which has good thickness nonuniformity in both longitudinal and transverse directions, uniform physical properties in a transverse direction and high strength in a longitudinal direction.
It is another object of the present invention to provide a process for producing the above biaxially oriented polyester film of the present invention.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, firstly, the above objects and advantages of the present invention are attained by a biaxially oriented polyester film which has (a) an F-5 value in a longitudinal direction of 13 kg/mm
2
or more, (b) a thickness nonuniformity in each of both longitudinal and transverse directions of 8% or less, and (c) a difference in a transverse direction of heat shrinkage factor at 105° C. in a longitudinal direction of 0.02 to 0.2% per 1 m in a transverse direction.
Secondly, the above objects and advantages of the present invention are attained by a process for producing a biaxially oriented polyester film comprising the steps of:
stretching a substantially amorphous unstretched polyester film to 4.0 to 6.0 times in a longitudinal direction;
stretching the so-obtained film to 3.0 times or more but less than 4.0 times in a transverse direction;
stretching the so-obtained film to 1.01 to 1.20 times in a transverse direction while heat-setting it; and
relaxing the so-obtained film by 0 to 20% in a transverse direction while cooling it.
DETAILED DESCRIPTION OF THE INVENTION
The biaxially oriented polyester film of the present invention is a longitudinally tensilyzed film having high strength in a longitudinal direction and a uniform distribution of physical properties in a transverse direction.
The above biaxially oriented polyester film has an F-5 value in a longitudinal direction of 13 kg/mm
2
or more, preferably 13 to 18 kg/mm
2
, more preferably 13.5 to 16.0 kg/mm
2
. This F-5 value can be provided by controlling the stretch ratio in a longitudinal direction of the film to 4.0 times or more, more specifically 4.3 times or more. To this end, stretching may be carried out in a single-stage longitudinal stretching or in a multi-stage longitudinal stretching with two or more stages.
In the biaxially oriented polyester film of the present invention, each of a thickness nonuniformity in a longitudinal direction and that in a transverse direction is 8% or less. When the thickness uniformity is more than 8%, the uniformity of the film cannot be accepted, which is disadvantageous.
The biaxially oriented polyester film of the present invention preferably has a thickness of 3 &mgr;m or more. The above thickness nonuniformity preferably has the following relationship with film thickness.
When the thickness of a film is 3 to 14 &mgr;m, the thickness nonuniformity satisfies the following expression:
−0.2t+3.5≦d≦−0.4t+9.2
wherein t is the thickness of the film (&mgr;m) and d is a thickness nonuniformity (%).
When the thickness of a film is more than 14 &mgr;m, the thickness nonuniformity (d) is 0.7 to 3.6%.
Further, in the biaxially oriented polyester film of the present invention, a difference in a transverse direction of heat shrinkage factor at 105° C. in a longitudinal direction is 0.02 to 0.2% per 1 m in a transverse direction.
The heat shrinkage factor in a longitudinal direction is preferably 3% or less, more preferably 1.5% or less at any location.
Further, in the above biaxially oriented polyester film, difference in a transverse direction of elongation at 1000° C. under a load of 1.000 g/mm
2
in a longitudinal direction (to be referred to as “TMA elongation” hereinafter) is preferably 0.01 to 0.1% per 1 m in a transverse direction.
The polyester to which the present invention is directed is a linear polyester comprising a dicarboxylic acid component and a glycol component.
Illustrative examples of the dicarboxylic acid component include aromatic dicarboxylic acids such as terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, diphenyldicarboxylic acid and diphenyl ether dicarboxylic acid. Out of these, terephthalic acid and 2,6-naphthalenedicarboxylic acid are preferred. Illustrative examples of the glycol component include ethylene glycol, propylene glycol, 1,4-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,4-cyclohexane dimethanol, diethylene glycol, neopentyl glycol and the like. Out of these, ethylene glycol and 1,4-butanediol are preferred, and ethylene glycol is particularly preferred.
Out of polyesters, polyethylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate and polybutylene terephthalate are preferred, and polyethylene terephthalate and polyethylene-2,6-naphthalene dicarboxylate are particularly preferred because they are excellent in mechanical properties and thermal properties.
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