Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
2000-05-30
2001-11-13
Chen, Vivian (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S338000, C428S480000, C428S910000, C525S437000, C525S444000
Reexamination Certificate
active
06316096
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a biaxially oriented polyester film. More specifically, it relates to a biaxially oriented polyester film suitable for use in a thermosensitive stencil printing base sheet which has high printing sensitivity, is free from nonuniformity in the thickness and density of printed characters and figures and allows for clear stencil making and printing.
PRIOR ART
In recent years, much attention has been paid to thermosensitive stencil printing using a base sheet which is perforated by heating with a xenon flash lamp, thermal head or pulse radiation such as laser beams. The principle of this stencil making method is disclosed by JP-B 41-7623 (the term “JP-B” as used herein means an “examined Japanese patent publication”), and JP-A 55-103957 and JP-A 59-143679 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
A base sheet prepared by laminating together a film for a thermosensitive stencil printing base sheet and a porous base with an adhesive or by heat has been used as the base sheet used for thermosensitive stencil printing. A vinyl chloride film, vinylidene chloride copolymer film, polypropylene film and highly crystalline polyethylene terephthalate film have been used as the film for a thermosensitive stencil printing base sheet, and thin paper and polyester sheet have been used as the porous base.
However, they have the following disadvantages.
1) When a vinyl chloride or vinylidene chloride copolymer film is used as the film for a base sheet, printed characters are not clear.
2) When a polypropylene or polyethylene terephthalate film is used as the film for a base sheet, clear characters can be obtained but a clear image cannot be obtained by solid printing (a symbol or figure like a circle the inside of which is totally printed black and which has a large ink adhesion area, to be referred to as “solid printing” hereinafter).
3) A printed portion has nonuniformity in printing density.
4) A printed character is partially nonuniform in thickness.
5) Sensitivity is low and a light black printed character or the like cannot be obtained.
To eliminate the above disadvantages, JP-A 62-149496 proposes use of a film having small crystal fusion energy. However, this film involves such problems as blocking which occurs when a polymer chip is dried during the production process of the film and the adhesion of the edges of a longitudinally stretched film to the clip of a tenter type transverse stretching machine. Further, this film has a problem with printing quality that a polymer which is softened at the time of perforation is easily adhered to a thermal head and that a streak-like reversal marks are formed by the adhered polymer at the time of continuous stencil making. To solve these problems, JP-A 3-39294 proposes a film which shows two or more melting peaks in DSC temperature-elevation measurement and has already been put to practical use. However, there is a case where the sensitivity of this film as a thermosensitive stencil printing base sheet becomes insufficient for hardware for high-speed printing.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a biaxially oriented polyester film which is useful as a thermosensitive stencil printing base sheet.
It is another object of the present invention to provide a biaxially oriented polyester film suitable for use as a film for a thermosensitive stencil printing base sheet which enables clear character printing and solid printing, is free from nonuniformity in printing thickness, nonuniformity in printing density and wrinkles, and excellent in durability, and has high perforation sensitivity.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, the above objects and advantages of the present invention are attained by a biaxially oriented polyester film:
(A) which comprises a composition comprising at least two thermoplastic polyesters;
(B) which has at least two melting peaks which satisfy the following expressions (1) and (2) when measured by DSC at a temperature elevation rate of 2° C./min by superimposing a periodic temperature variation of ±1° C./min:
5° C.≦Tmp(max)−Tmp(min)≦20° C. (1)
200° C.≦½(Tmp(max)+Tmp(min))≦230° C. (2)
wherein Tmp(max) is the highest melting peak temperature (° C.) and Tmp(min) is the lowest melting peak temperature (° C.);
(C) which has on the film plane two crossing directions having a thermal shrinkage factor at 100° C. for 10 minutes of 16 to 25%; and
(D) which has a thickness of 0.2 to 7 &mgr;m.
The thermoplastic polyester resin used in the present invention is a polyester which shows plastic fluidity by heating, preferably a polyester which is essentially composed of a linear polymer chemically structurally. It may further contains a low molecular weight oligomer.
Typical examples of the thermoplastic polyester resin used in the present invention include polycarbonates such as polyethylene terephthalate, polybutylene terephthalate, polyethylene-2,6-naphthalene dicarboxylate, polycyclohexanedimethyl tere(iso)phthalate, polyethylene-&agr;,&bgr;-bis(2-chlorophenoxy)ethane-4,4-dicarboxylate and bisphenol A polycarbonate. In the present invention, homopolymers and copolymers of these polyesters are preferably used. The thermoplastic polyester resin in the present invention may contain a resin other than a polyester in limits that do not depart from the scope of the present invention.
That is, the film of the present invention may be not only a film which forms uniform composition microscopically, as exemplified by a homopolymer, alternating copolymer, random copolymer and compatible polymer blend but also a film which forms nonuniform composition microscopically, as exemplified by a block copolymer, graft copolymer and semi-compatible or non-compatible polymer blend if the macroscopical thermal properties (DSC behavior) and dynamic properties (dynamically viscoelastic behavior) of the film as a plane satisfy the above relationship (B). As for layer structure, the film may be single-layer or multi-layer (more than 2 layers). As for uniformity in the macroscopical thermal properties of the film as a plane, the thermal properties are desirably uniform in an area of at least 50 &mgr;m
2
or less, preferably 30 &mgr;m
2
or less, more preferably 10 &mgr;m
2
or less. When the thermal properties are uniform only in an area of 50 &mgr;m
2
or more, perforation properties differ by each dot of the thermal head, whereby printing density easily becomes nonuniform.
In the present invention, preferably, the composition comprising at least two thermoplastic polyesters comprises 30 to 70 wt % of a first aromatic polyester which comprises terephthalic acid as the main dicarboxylic acid component and ethylene glycol has the main glycol component, 30 to 70 wt % of a second aromatic polyester which comprises terephthalic acid as the main dicarboxylic acid component and tetramethylene glycol as the main glycol component, and 0 to 30 wt % of a third aromatic polyester which comprises terephthalic acid as the main dicarboxylic acid component and hexamethylene glycol as the main glycol component.
More preferably, the composition comprising at least two thermoplastic polyesters comprises 35 to 65 wt %, preferably 40 to 60 wt % of the first aromatic polyester, 35 to 65 wt %, preferably 40 to 60 wt % of the second aromatic polyester, and 0 to 15 wt %, preferably 0 to 10 wt % of the third aromatic polyester.
When the composition comprising at least two thermoplastic polyesters comprises three different aromatic polyesters, preferably, the first aromatic polyester is a poly(ethylene terephthalate/isophthalate)copolymer, the second aromatic polyester is polytetramethylene terephthalate, and the third aromatic polyester is a poly(hexamethylene terephthalate/isophthalate)copolymer.
In the present invention, the composition comprising at least two thermoplastic polyest
Murooka Hirofumi
Yoshida Tetsuo
Chen Vivian
Sughrue Mion Zinn Macpeak & Seas, PLLC
Teijin Limited
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