Process of making biaxially oriented polyester films

Details Process of making biaxially oriented polyester films Process of making biaxially oriented polyester films

2000-10-27

2003-05-13

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

Reshaping running or indefinite-length work

C264S210700, C264S216000, C264S235800

Reexamination Certificate

active

06562274

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to biaxially oriented polyester films and their production methods. More specifically, it relates to biaxially oriented polyester films that are high in rigidity in all directions within the film plane, high in dimensional stability, and resistant to deformation under load, and serves as a base film for high density magnetic recording media that shows particularly high travelling durability when used as data-recording tape and improved preservability in tape's service environment, and it also relates to their production methods.
2. Description of the Prior Art
Recent magnetic recording tapes have become thinner and higher in recording density to permit the production of smaller products with longer recording time, and there are increased demands for tape products with smaller elongational deformation under tensile stress and longer preservability in tape's service environment. Under such conditions surrounding the development of magnetic recording tape products, there are increased demands for improved base film materials that are higher in strength as well as form stability and dimensional stability in the tape's service environment.
To provide base films that meet the above requirements, aramid materials have been used conventionally as they are high in strength and dimensional stability. Though they are high in price and disadvantageous in terms of cost, manufacturers have had to use them as there are no alternatives. On the other hand, in conventional methods for producing a high-strength biaxially oriented polyester film, film is once stretched in two directions, i.e., machine and transverse directions, and further stretched in the machine direction to ensure high strength in the machine direction (for example, JP-B-SHO 42-9270, JP-B-SHO 43-3040, JP-A-SHO 46-1119 and JP-A-SHO 46-1120). For additional increase in strength in the transverse direction, “longitudinal and transverse re-stretching processes” in which film is first re-stretched in the machine direction and then re-stretched in the transverse direction have been proposed (for example, such films are proposed in JP-A-SHO 50-133276 and JP-A-SHO 55-22915). High-strength polyester films produced by these conventional methods have such disadvantages as: 1) tape breaks during use, 2) insufficient rigidity in the transverse direction causes edge damage, 3) elongational deformation under stress or dimensional deformation due to environmental conditions result in a shift of recording tracks that cause errors when records are read out, and 4) insufficient strength brings about difficulty in thickness reduction and makes it impossible to achieve required magnetic conversion performance. Thus, many problems still remain to be solved to allow large-capacity, high-density magnetic recording tape to be produced from these films.
Further, another stretching method has been proposed in which preliminary stretching is performed prior to the above-mentioned stretching-orientation process. U.S. Pat. No. 5,409,657, for example, proposes a process in which film is subjected to preliminary stretching at a draw ratio of 1.2 to 3 times in the machine direction at temperatures of (polyester's glass transition temperature Tg+40)° C. to (crystallization temperature Tc−20)° C. followed by stretching in the transverse direction and the machine direction, and shows films that are strengthened only in the longitudinal direction. Further, JP-A-HEI 9-300455 also proposes (a) a process in which film is subjected to preliminary stretching at a draw ratio of 1.5 to 2.5 times in the transverse direction at temperatures of 100° C.-120° C., followed by stretching in the transverse direction and the machine direction, and (b) a process in which a preliminary stretching is performed at a draw ratio of 1.1 to 2.2 times in the machine direction at temperatures of 100° C.-120° C. in addition to the above-described process (a), and shows films that are strengthened only in the transverse direction. Furthermore, JP-A-SHO 58-145421 proposes a process in which film is stretched in the two directions simultaneously at a temperature of 115° C. or higher, followed by simultaneous biaxial stretching, with the aim of producing thin films and increasing the production speed, and shows films with small Young's modulus. Films produced by these technologies, however, are not high in rigidity in all directions and cannot solve the problems associated with applying the material to the production of high-density recording tape.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide biaxially oriented polyester films that are high in rigidity in all directions in the film plane, high in dimensional stability, and resistant to deformation under load, and serves as base film for high density magnetic recording media that show particularly high travelling durability when used as data-recording tapes and improved preservability in tape's service environment, and provide their production methods.
The present inventors have carried out studies to solve these problems, and achieved the invention after finding that film having a certain structure and certain physical properties after biaxial stretching and thermal treatment can serve to produce polyester magnetic recording tapes with reduced edge damage, increased travelling durability and improved preservability.
A biaxially oriented polyester film according to the present invention is 7.0 GPa or more in at least either the Young's modulus in the machine direction (YmMD) or in the transverse direction (YmTD), and in the range of 55° or more and 85° or less in the circumferential half-width of the diffraction line from the crystal plane in the direction of the polyester's backbone chain that is determined through crystal orientation analysis by wide angle X-ray diffractometry performed while rotating the polyester film around its normal.
Films as proposed by the present invention have such favorable embodiments as described below:
(a) The crystal size in the polyester's backbone chain direction is 45 Å or more and 90 Å or less.
(b) The sum of the Young's modulus in the machine direction (YmMD) and that in the transverse direction (YmTD) is 13 GPa or more and 25 GPa or less, and the Young's modulus in an diagonal direction (45° or 135°) is 6 GPa or more and 10 GPa or less.
(c) The creep compliance after being left for 30 minutes under the conditions of a temperature of 50° C. and a load of 28 MPa is 0.11 GPa
−1
or more and 0.35 GPa
−1
or less.
(d) The propagating tear strength of the film, converted to 5 &mgr;m thickness, in the transverse direction is 0.7 g or more and 1.8 g or less.
(e) The polyester is polyethylene terephthalate.
(f) At least either the ratio R
1
(=I
MD
/I
ND
) of the peak intensity in the machine direction (I
MD
) to that in the normal direction (I
ND
) at 1615 cm
−1
measured by laser Raman scattering or the ratio R
2
(=I
TD
/I
ND
) of the peak intensity in the transverse direction (I
TD
) to that in the normal direction (I
ND
) is 6 or more.
(g) The refractive index in the normal direction (n
ZD
) is 1.470 or more or 1.485 or less, and the planar orientation index (f
n
) is 0.175 or more and 0.195 or less.
(h) The density of the film is 1.385 or more and 1.400 or less.
(i) The heat shrinkage starting temperature of the film is 70° C. or more, and the heat shrinkage at the temperature of 80° C. is 0.5% or less.
Such biaxially oriented polyester films according to the present invention as described above serve favorably as base films for high density magnetic recording media, electrostatic capacitors, and thermal transfer ribbons.
Desirable polyester film production methods according to the present invention include, but not limited to, production method (I) and production method (II) described below.
Production method (I) is a biaxially oriented polyester film production method wherein substant

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