Polymer films, particularly polyester films, are subjected to heat treatment to remove residual strain arising from their manufacturing process. This heat treatment is called an annealing treatment.
In carrying out such an annealing treatment, there has been adopted a method of storing a film which has been rolled up (a film roll) for predetermined time in a thermostatic chamber which is kept the temperature therein at a high temperature, or a method of slowly passing a film roll through a continuous oven. When a film roll is heated ununiformly in such an annealing process, various problems arise such that the film roll suffers starring, winding wrinkles or winding ruggedness. In addition, a prolonged period is required for effecting an annealing treatment.
To solve the above described problems, the following techniques have been proposed.
(1) JP-B-05-19899 (the term "JP-B" means an "examined Japanese patent publication") discloses a heat treating method where a film placed in an oven for heat treatment is heated up to a heat treatment temperature while it is continuously wound up into a roll as spacers are put along both side edges of the film, and then the spacer-interposed roll is stored at the heat treatment temperature for a predetermined time.
(2) JP-A-59-207346 (the term "JP-A" means an "unexamined published Japanese patent application") discloses an apparatus for winding a web equipped with an air chamber arranged at a predetermined interval with a web so that the web is subjected to hot-air blowing from air chambers to thereby control the web temperature.
However, any of the above-cited proposals (1) and (2) have no description about a core temperature. Therefore, these proposals is disadvantageous that a wound film shrinks because of the temperature deference with a core, which results in surface defects (such as wrinkles, ruggedness, scratches and the like). In addition, these proposals have a problem in controlling the winding tightness of a film roll.
Furthermore, to avoid the above described starring, winding wrinkles or winding ruggedness, and to prevent from generating elliptical distortion and wrinkles, JP-A-63-31962 discloses a method of storing a plastic film that has been wound onto a reel into a roll with a space rate of from 0.6 to 0.95 while rotating the reel centering around the reel axis at a speed of from 5 to 1,000 revolutions per hour.
The method as described in JP-A-63-31962 is successful in preventing the generation of the elliptical distortion and wrinkles upon long-term storage. However, it has problems that temperature ununiformity is locally present in a thermostatic chamber used therein, and therefore, the effects on the reduction of annealing time and on the enhancement of annealing efficiency are inadequate.
Furthermore, conventional methods cannot avoid ununiformity in heating upon annealing treatment which results in temperature ununiformity of a film roll between the core side part and the periphery side part, and ununiform loading.
In the conventional methods, the entire film roll of a polymer film web is cooled to room temperature (about 25.degree. C.) after an annealing treatment, and thereafter the film roll is unwound.
An edge-thickening processing called "knurling", in which both edge parts of a polymer film web is deformed to be rugged, has so far been employed to prevent telescoping in the film roll. This knurling processing plays an important role in the above described annealing treatment as well, and it is not to be dispensed with for ensuring uniform heating and loading applied to a polymer film web.
Typical conventional techniques relating to edge-thickening processing, i.e., knurling, include the followings:
(3) JP-B-50-36459 discloses a method of winding a synthetic resin comprising subjecting a part of the film surface in the width direction (side zones on the film surface) to a treatment for enhancing antislip properties (specifically, a corona discharge treatment) followed by winding.
(4) JP-B-57-36129 discloses an apparatus for an edge thickening apparatus, with which edge parts of a molded thermoplastic sheet material are mechanically deformed to be rugged. Specifically, the apparatus comprises means for mechanical deforming which comprises a pair of rollers having a rugged surface and means for bringing them into pressure contact, in which at least one of the rollers has such surface roughness that striped gaps are arranged in the width direction on the curved surface of the roller so as to left traces of the gaps on the parts to thicken the sheet material.
(5) JP-A-U-01-109045 (the term "JP-A-U" means an "unexamined published Japanese utility model application") discloses an apparatus for winding a plastic film equipped with a heating device which enables hot-air to blow against both side edges of the film, to thereby locally form rough portions on the film surface.
In the conventional techniques, however, measures were not sufficiently taken to the problem that a film roll as a whole lacks uniformity in heated and loaded conditions. Furthermore, although the disclosed techniques relating to the knurling processing are effective measures to telescoping, there is no description with respect to measures for avoiding ununiformity on the film roll between the core side part and the surface side part occurring in an annealing treatment. In addition, these techniques do not provide sufficient effect for achieving suitable unwinding of a polymer film web from a roll that has been annealed. As a result, many defects occur in the film roll during and after the annealing treatment. Specifically, sharp swelling, cut-end-mark and imprints of a core texture are generated in the core part and ruggedness are generated in the periphery part during or after the annealing treatment. Furthermore, telescoping, starring wrinkles and wrinkles around the core are generated in an unwinding operation after the annealing treatment.