Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Extrusion molding
Reexamination Certificate
2000-03-20
2002-06-25
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Extrusion molding
C264S481000, C264S210700
Reexamination Certificate
active
06409958
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a method of producing a polyester film using the application of static electricity, and more particularly, relates to a method of producing a biaxially stretched polyester film with regulating casting conditions of a polyester film at a high speed so as not to induce surface defects caused by entrapping air microbubbles between a cooling drum and a molten resin sheet, and regulating longitudinal stretching conditions and lateral stretching conditions. The biaxially stretched polyester film is excellent in small thickness variation, resistance to cleavage and improvement in bowing phenomenon, and is particularly suitable for the support of a photographic film.
In a conventional method of producing a cast film by cooling rapidly to solidify a thermoplastic resin containing polyester reins, the application of static electricity is employed for adhering a sheet-shaped molten resin discharged from a die to a cooling drum (Japanese Patent KOKOKU 37-6142).
Besides, it is also disclosed in a reference “Kobunshi Jikken Gaku 5, Polycondensation and Polyaddition”, Kyoritsu Shuppan, 1980 that a metal catalyst is necessary for the synthesis of a polyester, such as polyethylene terephthalate.
Incidentally, when film forming speed of a cast film is raised in the above application of static electricity system, air entrapping problem occurs between a molten resin sheet and a rotating cooling drum exceeding a critical speed. As a result, indentations are formed as traces of air-entrapped on the surface of cooled sheet.
A considerable solving means for raising the critical speed, at which air entrapping occurs, is to increase the content of metal catalyst. That is, by increasing the content of metal catalyst, the specific resistance of the molten resin sheet is lowered, and thereby, adhesion of the sheet to a cooling drum is improved upon applying static electricity to inhibit entrapping of air. However, the increase of metal catalyst content induces problem of tone change caused by thermal degradation and the generation of metallic agglomerates in the molten resin.
Therefore, cast film forming is carried out by determining allowable surface conditions for the use of each sheet previously, preparing an optimum recipe of resin composition, and casting the sheet at a speed lower than the critical speed. Particularly, in the field of requiring severe allowable level of foreign matters, such as photographic film, there is a limitation of metal catalyst content by itself, and various proposals have been made for film forming with harmonizing film forming speed with tone change and the generation of metallic agglomerates.
For example, Japanese Patent KOKAI 2-9688 discloses a method of forming a uniform polyethylene terephthalate film by controlling the specific resistance of the polyester in a melted state to 500,000 to 500,000,000 &OHgr;·cm. However, according to this method, coloring occurred caused by thermal degradation in a small specific resistance region, i.e. when metal catalyst content is great, and air entrapping cannot be inhibited at a high speed in a large specific resistance region. Accordingly, a high speed film forming of a polyester film cannot be achieved for a photographic film.
Japanese Patent KOKAI 8-291216 discloses a method of inhibiting air entrapping by restricting the specific resistance of the polyester in a melted state to 1,000,000 to 10,000,000 &OHgr;·cm narrower than the above method. This method is preferable for high speed film forming, but is insufficient in tone change problem and metallic agglomerates problem for a photographic film.
Japanese Patent KOKAI 61-219622 discloses a method of forming a thermoplastic polymer sheet which comprises extruding the thermoplastic polymer in sheet on the surface of a rotating cooling drum on which a liquid membrane layer has been applied to solidify (called water application system), wherein locating the die in front of the vertical plane containing the rotating shaft of the cooling drum, and tilting the discharge angle of the molten thermoplastic polymer from the die against the vertical plane to shorten the traveling distance of the extruded sheet from the die to the surface of the cooling drum. Japanese Patent KOKAI 61-237619 discloses a modification of the above method by providing an adjusting plate for sheet edge portion on both side ends of the discharge port, and a molten resin sheet immediately after discharging travels in a state that both sides of the sheet are in contact with the adjusting plate. However, the above film forming methods according to the water application system are difficult of setting water application conditions so that traces of water drops do not remain on the surface of sheet, and a precision controller is necessary for the films requiring severed allowable level of surface defects, such as a support for a photographic film. Moreover, when the discharge angle of a die is tilted as disclosed in Japanese Patent KOKAI 61-219622, 61-237619, it is difficult to place a heat bolt for adjusting lip clearance, and a working space for rough adjustment by hand must be set higher than the cooling drum.
As mentioned heretofore, by the prior art, a polyester film without problems of tone change, metallic agglomerates and the like and useful for a photographic film support cannot be produced stably at a high speed without surface defects, such as air entrapping.
Incidentally, films useful for a support for a photographic film are required to be resistant to cleavage as well as small thickness variation. There is a prior art to decrease thickness variation by regulating longitudinal stretching conditions, such as regulating stretching distance or force, multistep longitudinal stretching, regulating film crystallinity immediately before longitudinal stretching. However, some polyesters, such as polyethylene-2,6-naphthalene dicarboxylate, do not have a one to one relationship between stress and elongation, that is, there are two or more elongation types against an stress. As a result, uneven elongation tends to occur in the width direction and the traveling direction during stretching a polyester film.
This problem can be avoided by stretching the film until the region of one to one relationship between stress and elongation. For example, in the case of polyethylene-2,6-naphthalene dicarboxylate, a stretched film without uneven thickness, i.e. having a small thickness variation, can be produced by stretching at a longitudinal draw ratio of about 5 times. However, the stretched film stretched by a longitudinal draw ratio of about 5 times is liable to be cloven, and accordingly, it is unsuitable for the support of a photographic film. When a film having resistance to cleavage required as the support for a photographic film is produced, the longitudinal draw ratio must be less than 4 times. The films stretched under conventional conditions in this region have an uneven thickness.
That is, Japanese Patent KOKAI 60-178025 discloses a means for restricting uneven Young's modulus in the width direction and uneven thickness by setting the longitudinal stretching distance 30 to 800 mm and the stretching force 0.2 to 10 kg/mm
2
. However, a polyethylene-2,6-naphthalene dicarboxylate film with a small uneven thickness cannot be obtained only by the means.
Japanese Patent KOKOKU 7-71821 discloses a means for obtaining a polyester being excellent in uniform thickness by stretching longitudinally in multiple step. However, films obtained by stretching a total longitudinal draw ratio of 4.0 times or more are insufficient in the resistance to cleavage required as the support for a photographic film, and breakage occurs during processing in the subsequent processes.
Japanese Patent KOKAI 6-210719 discloses a means for restricting uneven thickness by restricting crystallinity of a film immediately before longitudinal stretching to 0.5 to 20%, and a means for crystallization is a preliminary stretching, i.e. the longitudinal stretching is divided into two steps, and the firs
Hosono Yasuyuki
Kegasawa Tadahiro
Otoshi Masaaki
Ozaki Kazuo
Terai Fumitaka
Eashoo Mark
Fuji Photo Film Co. , Ltd.
Sughrue & Mion, PLLC
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