HK1024205B - Thermo-shrinkable polyester film - Google Patents

Description

BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION:

The present invention generally relates to a thermo-shrinkable polyester film, and specifically, to a thermo-shrinkable polyester film suitable for a labeling film.

2. DESCRIPTION OF THE RELATED ART:

Conventionally, thermo-shrinkable films used as labels or coverings provided on bottles are mainly formed of poly(vinyl chloride), polystyrene, polyethylene and the like. Recently, thermo-shrinkable polyester films have been a focus due to the following problems with the above-mentioned materials. Poly(vinyl chloride) generates chlorine gas when burned for disposal. Polyethylene is difficult for use in printing characters and graphics on a film formed thereof. Moreover, when these materials are used for labels around polyethylene terephthalate (PET) bottles, the material of the labels needs to be distinguished from the material of the PET bottles in order to recycle the PET bottles.

Conventional thermo-shrinkable polyester films are not satisfactory as labeling films. The films shrink rapidly resulting in wrinkles, non-uniform shrinkage, and distortion. Further, the post-shrinkage films are easily ruptured by an external impact.

In order to solve these problems, Japanese Publication for Opposition JP-A-7-77757 discloses a method for significantly reducing the elongation at rupture in the direction perpendicular to a main shrinking direction of the film to improve the post-shrinkage state of the film.

Japanese Laid-Open Publication JP-A -58-64958 discloses a method for reducing the shrinking force of the film to improve the post-shrinkage state of the film.

However, the methods disclosed in these documents do not provide a satisfactory post-shrinkage state of the film when applied to produce labels for small PET bottles. Such bottles' labels are shrunk by passing the bottles through a shrinkage tunnel for a relatively short time.

Furthermore, demand for the recycling of bottles formed from thermoplastic polymers has increased greatly in order to deal with the environmental problems. Especially, the recycling of PET bottles is of great concern, and it is imperative to urgently establish a recycling system therefor. Labels provided around PET bottles generally include, for example, stretched labels formed of polyolefin and the like, thermo-shrinkable labels formed of polyester, polystyrene, vinyl chloride and the like, and tack labels formed of polypropylene and the like. The PET bottles are typically recycled as follows. The bottles are collected from consumers with the labels thereon and eventually brought to recycling companies or plants. The bottles are washed and then treated by primary pulverization to remove the labels. The substance obtained by the pulverization still includes a great amount of the label material. Accordingly, the substance is further treated by secondary pulverization and separation of the label material using specific gravity in liquids, de-watering and drying, separation of the label materials using wind, and pelletization. Thus, recycled pellets are obtained.

Recently, in order to improve the label removal efficiency, the labels are detached from the bottles after the bottles are washed but before pulverization and pelletization.

For performing such a process, labels having perforations are used to facilitate the detachment of the labels. The labels are detached from the bottles by tearing the labels along the perforations.

Films obtained by the methods disclosed in the above-mentioned documents are inferior in the ease of tearing along perforations after thermally shrunk and thus are not satisfactory for a shrinkable label film.

Conventional thermo-shrinkable polyester films which are shrinkable in one direction have further problems in that wrinkles, non-uniform shrinkage and insufficient shrinkage, occur when subjected to shrinkage to cover containers having complicated shapes such as, cosmetic containers. fragrance and toiletry containers, or liquid medication containers.

In order to solve such a problem, Japanese Laid-Open Publication JP-A-3-29763 discloses a thermo-shrinkable polyester film which is shrinkable in a direction perpendicular to the main shrinking direction. Japanese Laid-Open Publication JP-A-5-185510 discloses a method for restricting the drawing ratio in both the longitudinal and transverse directions of the film so that the shrinkage ratio in the direction perpendicular to the main shrinking direction is about 15%.

EP-A-0 271 928 and EP-A-0409288 disclose further examples of thermo-shrinkable polyester films for labeling purposes.

However, there is still a used for polyester films with improved shrinkage behaviour in the main shrinking direction and with improved anti-rupture properties in the direction perpendicular to the main shrinking direction.

SUMMARY OF THE INVENTION

According to one aspect of the invention, there is provided a thermo-shrinkable polyester film comprising:

• a polyester having a glass transition point of 58 to 68 °C and including at least one diol having from 3 to 6 carbon atoms,
• and wherein the film has a thermal shrinkage ratio in the main shrinking direction of 20 % or more after being treated in water of 70 °C for 5 seconds, 35 to 55 % after treated in water of 75 °C for 5 seconds, and 50 to 60 % after being treated in water of 80 °C for 5 seconds, anda probability of elongation at rupture of the film in the direction perpendicular to the main shrinking direction, as assessed by the tensile test in accordance with the method defined by JIS C2318, being 20 % or less, of 10 % or less after the film is shrunk by 5 % by treatment in water of 75 °C for 10 seconds.

In one example of the invention the thermoshrinkable polyester film has a thickness of dispersion of 6 % or less.

Furthermore, according to the present invention there is provided a method of producing the thermoshrinkable polyester film as above-described, said method comprising the steps of:

• drying and extruding the polyester at 200 to 300 °C, followed by rapid cooling to form an undrawn film;
• preheating the undrawn film at a low hot air speed such that the heat transfer coefficient is 0.0013 cal/cm2 • sec • °C or less;
• drawing the undrawn film using a tenter in either a transverse or longitudinal direction at a drawing ratio of 3X or more, at a temperature of less than Tg+15 °C, and at a heat transfer coefficient of 0.0009 to 0.0017 cal/cm2• sec • °C; and
• heating the drawn film at 70 to 100 °C.

In one example of the invention, the thermal shrinkage ratio in the main shrinking direction after being treated in water of 95°C for 10 seconds is 40% or more, and the thermal shrinkage ratio in the direction perpendicular to the main shrinking direction after being treated in water of 95°C for 10 seconds is 15 to 30%.

Thus, the invention described herein makes possible the advantages of providing a thermo-shrinkable polyester film which exhibits a satisfactory post-shrinkage state with substantially uniform shrinkage and no wrinkles or distortions in all possible uses, including labels for bottles, and is easily torn along perforations formed therein.

This and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

• Figure 1 is a perspective view of a thermo-shrinkable polyester film according to the present invention which is formed into a tube; and
• Figure 2 is a side view of a tube-shaped thermo-shrinkable polyester film according to the present invention, which is provided around a bottle, having two parallel lines of perforations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described by way of illustrative examples with reference to the accompanying drawings.

Polyester used for a thermo-shrinkable polyester film according to the present invention preferably contains at least one diol having from about 3 to about 6 carbon atoms (e.g., propanediol, butanediol, neopentyldiol, and hexanediol) and is adjusted to have a glass transition point (Tg) of 58 to 68°C.

Preferably, the polyester used in the present invention does not contain diols having about 8 or more carbon atoms (e.g., octanediol), polyhydric alcohols containing 3 or more hydroxyl groups (e.g., trimethylol propane, trimethylol ethane, glycerin, and diglycerin), or polycarboxylic acids (e.g., trimellitic acid, pyromellitic acid and anhydrides thereof). A thermo-shrinkable polyester film obtained using a polyester containing one of these diols, polyhydric alcohols and/or carboxylic acids is not preferable because the elongation at rupture of the post-shrinkage film in the direction perpendicular to the main shrinking direction tends to be undesirably low.

In the case where the polyester used in the present invention contains an aliphatic carboxylic acid (e.g., adipic acid, sebacic acid or decanedicarboxylic acid), such an acid is preferably contained in a content of 3 mol% or less. A thermo-shrinkable polyester film obtained using a polyester containing an aliphatic carboxylic acid at a content of 3 mol% or more is not preferable because the elongation at rupture of the post-shrinkage film in the direction perpendicular to the main shrinking direction tends to be undesirably lowered.

Preferable acids contained in the polyester used in the first embodiment include telephthalic acid, isophthalic acid, and naphthalenedicarboxylic acid. Preferable diols usable in addition to a diol having from about 3 to about 6 carbon atoms include ethylene glycol and 1,4-cyclohexane-dimethanol. It is preferable that diethylene glycol, triethylene glycol and polyethylene glycol are excluded. Especially, since diethylene glycol tends to exist as a by-product at the time of polymerization of polyester, the content of diethylene glycol is preferably restricted to less than 4 mol%.

In the case where two or more types of polyesters are mixed, the amount of acid and diol disclosed in this specification are the contents of the acid and the diol in the entire polyester whether or not ester interchange occurs after the mixture.

In order to especially improve the post-shrinkage state of the inventive film, neopentyl glycol, which is a diol, is preferably used.

In order to improve the smoothness of the film, an inorganic or organic lubricant can be utilized. When necessary, additives such as, for example, a stabilizer, a colorant, an antioxidant, an anti-foaming agent, and an anti-electrostatic charge agent can also be utilized.

A polyester as described above is drawn at a temperature of Tg-5°C or more and less than Tg+15°C.

When the polyester is drawn at a temperature of less than Tg-5°C, the resultant film will not thermally shrink to a sufficient degree and the transparency of the resultant film is spoiled. When the polyester is drawn at a temperature of Tg+15°C or more, the elongation at rupture of the resultant film in the direction perpendicular to the main shrinking direction tends to be undesirably low.

The thermal shrinkage ratio in the main shrinking direction of the polyester film in water is 20% or more, preferably 25% or more, after being treated at 70°C for 5 seconds; 35 to 55%, preferably 40 to 55%, after being treated at 75°C for 5 seconds; and 50 to 60%, preferable 52 to 58%, after being treated at 80°C for 5 seconds. In the case where the thermal shrinkage ratio is less than 20% after being treated at 70°C for 5 seconds, less than 35% after being treated at 75°C for 5 seconds, or less than 50% after being treated at 80°C for 5 seconds, the end of the film is expanded like a flower leaf and tends to suffer from non-uniform shrinkage and wrinkles after the film is provided around the bottles as labels or the like and passed through the shrinking tunnel.

In the case where the thermal shrinkage ratio is more than 55% after being treated at 75°C for 5 seconds or more than 60% after being treated at 80°C for 5 seconds, the upper or lower ends of the film are extended obliquely or shift to a position above the prescribed position after the film is provided around the bottles as labels or the like and passed through the shrinking tunnel.

After the polyester film is shrunk by 5% by treatment in water of 75°C for 10 seconds, the probability of the elongation at rupture of the film in the direction perpendicular to the main shrinking direction being 20% or less is 10% or less, preferably 8% or less. When the probability is more than 10%, the resultant film provided around the bottle, as a label, is not easily torn along perforations formed therein; i.e., the film is torn in an unintended direction by the portion held in a hand. In order to allow satisfactory perforations to be formed in the film, the film preferably has a dispersion of thickness (average value) of 6% or less, preferably 3% or less.

. The thickness of the thermo-shrinkable polyester film is not limited to any numerical values. Preferably, the film is 1.0 to 200 µm thick, and more preferably, the film is 20 to 100 µm thick when used for labels.

Hereinafter, a method for producing a thermo-shrinkable polyester film will be specifically described.

A polyester is dried by a dryer such as, for example, a hopper dryer, a paddle dryer or a vacuum dryer and extruded in the form of a film at 200 to 300°C. The extrusion can be performed by any conventional method such as, for example, a T-die method or tubular method. After the extrusion, the film is rapidly cooled to obtain an undrawn film. The undrawn film is then drawn. Since the main shrinking direction is preferably the transverse direction in order to achieve the objectives of the present invention, an exemplary method for forming a film in the case where the main shrinking direction is the transverse direction will be described. In the case where the main shrinking direction is in the longitudinal direction, film formation can be performed in a similar manner except that the drawing direction is changed by 90 degrees.

Before the undrawn film is drawn in the transverse direction using a tenter, the undrawn film is preheated. In order to obtain a uniform thickness of the film, it is necessary to preheat the film to a prescribed temperature at such a low wind speed (i.e., speed of hot air) that the heat transfer coefficient is 0.0013 cal/cm² · sec · °C or less. Drawing in the transverse direction is performed at a drawing ratio of 3X or more, preferably 3.5X or more, at a temperature of less than Tg+15°C.

Then, the film is heat-treated at 70 to 100°C, thereby obtaining a thermo-shrinkable polyester film.

In order to restrict internal heat generation in the film which can accompany drawing and thereby reduce the non-uniformity of the temperature of the film in the transverse direction, the heat transfer coefficient during drawing is 0.0009 to 0.0017 cal/cm² · sec · C°, and preferably 0.0011 to 0.0017 cal/cm² · sec · °C.

In the case where the wind speed during the preheating causes the heat transfer coefficient to be more than 0.0013 cal/cm² · sec · °C; or the wind speed during drawing causes the heat transfer coefficient to be less than 0.0009 cal/cm² · sec · °C, then the thickness of the film tends to be non-uniform. Such a film is not preferable because when multiple color printing is desired because the patterns of different colors are not generally aligned.

When three-color printing is performed to evaluate the post-shrinkage state of the film, a film having a thickness dispersion of 6% or less allows for easy overlapping of patterns of different colors, whereas a film having a thickness dispersion of more than 6% does not tend to provide such satisfactory results.

The film can be drawn slightly in the longitudinal direction in addition to one-axis drawing in the transverse direction performed using the tenter. Such two-axis drawing can be performed either by sequential drawing or parallel drawing. When necessary, re-drawing can be performed in the longitudinal or transverse direction.

Hereinafter, the present invention will be described in more detail by way of examples.

<Evaluation>

First, the films are evaluated on the following points.

(1) Thermal shrinkage ratio

A film is cut into square pieces of about 10 cm × 10 cm. The square pieces are put in water of a prescribed temperature ± 0.5°C with no load for a prescribed period of time to be thermally shrunk. The resultant film pieces are measured in the longitudinal and transverse directions, and the thermal shrinkage, ratio is obtained by expression (1). Among the longitudinal direction and the transverse direction, the direction having a greater thermal shrinkage ratio is set as a main shrinking direction. $\text{thermal shrinkage ratio =}\text{((pre-shrinkage length - post-shrinkage length)/pre-}\text{shrinkage length) × 100(\%)}$

(2) Post-shrinkage state

Five-hundred milliliter round bottles (height: 20.6 cm; diameter of central part: 6.5 cm: produced by Kabushiki Kaisha Yoshino Rogyosho: used for GOGO-NO KOCHA (product name; Kirin Beverage Company) provided with films therearound are passed through using a steam tunnel (SH-1500-L; Fuji Astec Inc.) with a passage time of 2.5 seconds and a temperature of 80°C. The number of the samples used is 20.

Before the test, the films around the bottles are subject to three-color printing with green, gold and white ink (Toyo Ink Mfg. Co., Ltd.). The post-shrinkage state is visually evaluated by the following criteria.

• ○ : No wrinkles are observed; the films are not shifted to a position above the prescribed position; no insufficient shrinkage is observed.
• X : The films are shifted to a position above the prescribed position; or insufficient shrinkage is observed.

(3) Anti-rupture property

The films used in item (2) contains printing thereon are put around cans for canned coffee and put in water of 75°C for 10 seconds to be shrunk by 5%. The samples obtained are subjected to a tensile test using a tensile test apparatus (UTM-4L: Tokyo Seiki Co., Ltd.) in accordance with the method defined by JIS C2318.

The evaluation is conducted by the number of samples in which the elongation at rupture in the direction perpendicular to the main shrinking direction is 20% or less. The number of samples used is 50. The values in Table 1 represent the ratio of the number of samples having such elongation at rupture with respect to the total number of sample of 50.

(4) Tg (glass transition point)

The temperature of approximately 10 mg of undrawn film is raised from -40°C to 120°C at a rate of 20°C/min. using a differential scanning calorimeter (DSC220; Seiko Densh1 Kogyo Kabushiki Kaisha) to obtain heat absorption curves . A tangent is drawn before and after the transition point of each heat absorption curve, and the intersection of the tangents is set as the glass transition point Tg.

(5) Ease of tearing along perforations

As shown in Figure 1, the films are made into tubes (width: 106 mm; length: 54 mm) by the impulse seal method. Two parallel lines of perforations are formed in the length direction of each tube. The lines of perforations are 10 mm apart from each other. The films are put around PET bottles and shrunk by the method described in item (2) above. The number of samples are 50.

The films are torn along the perforations from the top (near the cap) toward the bottom of the bottles. The number of the samples in which the film is torn in an unintended direction is counted. The ratio of such samples against the total number of samples is set as the defective llnear tearing ratio(%). In Figure 2, L1 represents the length of each perforation, and L2 represents the distance between the perforations. In the example shown in Figure 2. the ratio of L1 to L2 is about 2.5 to about 3.0.

(6) Thickness dispersion

The thickness of samples each having a longitudinal size of 5 cm and a transverse size of 50 cm is measured using a contact thickness meter (KG60/A; Anritsu Corporation). The thickness dispersion is calculated for each sample by expression (2). Furthermore, the average of distribution is evaluated by the following criteria. The number of samples is 50. $\text{Thickness dispersion=}\text{((maximum thickness-minimum thickness)/average thickness)}\text{× 100(\%)}$

• ○: average value of 6% or less
• Δ: average value of more than 6% but less than 10%
• X: average value of 10% or more

(7) Printability

The films are processed with three-color printing with green, gold and white with Ink Shrink EX (Toyo Ink Mfg. Co., Ltd.) in a euperimposing manner, using a PAS-type multiple-color gravure printing apparatus (Kabushiki Kaisha Higashitani Tekkosho). The printing speed is about 100 m/min. and the drying temperature is about 50°C. The number of samples is 10.

The offsetting length among the colors is measured by JIS Class 1 measure and the printability is evaluated by the following criteria.

• ○ : offsetting length of less than 1 mm
• Δ : offsetting length of 1 mm or more but less than 3 mm
• X : offsetting length of 3 mm or more

The following examples are performed using the following polyesters. "IV" refers to intrinsic viscosity.

• Polyester A: polyethylene terephthalate (IV: 0.75)
• Polyester B: ethylene glycol (70 mol%), neopentyl glycol (30 mol%) and terephthalic acid (100 mol%) (IV: 0.72)
• Polyester C: polybuthylene terephthalate (IV: 1.20)
• Polyester D: terephthalic acid (65 mol%), adipic acid (10 mol%), isophthalic acid (25 mol%), and butanediol (100 mol%) (IV: 0.72)

<Examples> (Example 1)

A polyester was obtained by mixing 25 wt.% of polyester A, 55 wt.% of polyester B, and 20 wt.% of polyester C, and then the mixture was extruded at 280°C and rapidly cooled, thereby obtaining an undrawn film. The glass transition point of the undrawn film was 66°C.

The undrawn film was preheated up to 85°C under the conditions of a heat transfer coefficient of 0.0008 cal/cm² · sec · °C and then drawn in the transverse direction at 74°C at a ratio of 4X by a tenter. Then, the film was heated at 80°C for 10 seconds. Thus, a thermo-shrinkable polyester film having a thickness of about 50 µm was obtained.

(Example 2)

A polyester was obtained by mixing 25 wt.% of polyester A, 50 wt.% of polyester B, and 25 wt.% of polyester C, and then the mixture was extruded at 280°C and rapidly cooled, thereby obtaining an undrawn film. The glass transition point of the undrawn film was 63°C.

The undrawn film was preheated up to 82°C under the conditions of a heat transfer coefficient of 0.0008 cal/cm²·sec·°C and then drawn in the transverse direction at 72°C at a ratio of 4X by a tenter. Then, the film was heated at 79°C for 10 seconds. Thus, a thermo-shrinkable polyester film having a thickness of 50 µm was obtained.

(Example 3)

A polyester was obtained by mixing 25 wt.% of polyester A, 45 wt.% of polyester B, and 30 wt.% of polyester C, and then the mixture was extruded at 280°C and rapidly cooled, thereby obtaining an undrawn film. The glass transition point of the undrawn film was 61°C.

The undrawn film was preheated up to 80°C under the conditions of a heat transfer coefficient of 0.0008 cal/cm² · sec ·°C and then drawn in the transverse direction at 69°C at a ratio of 4X by a tenter. Then, the film was heated at 77°C for 10 seconds. Thus, a thermo-shrinkable polyester film having a thickness of 50 µm was obtained.

(Example 4)

A polyester was obtained by mixing 40 wt . % of polyester A, 50 wt.% of polyester B, and 10 wt.% of polyester D, and then the mixture was extruded from a T die at 280°C and rapidly cooled by a chill roll, thereby obtaining an undrawn film. The glass transition point of the undrawn film was 67°C.

The undrawn film was drawn in the same manner as described in Example 1. Thus, a thermo-shrinkable polyester film having a thickness of 50 µm was obtained.

(Comparative example 1)

A thermo-shrinkable polyester film having a thickness of 50 µm was obtained in the same manner as described in Example 1 except that the drawing temperature was 83°C.

(Comparative example 2)

A thermo-shrinkable polyester film having a thickness of 50 µm was obtained in the same manner as described in Example 1 except that the drawing temperature was 60°C. The films were entirely blushed at the exit of the tenter.

(Comparative example 3)

A polyester was obtained by mixing 25 wt. % of polyester A, 65 wt.% of polyester B, and 10 wt.% of polyester C, and then the mixture was extruded from a T die at 280°C and rapidly cooled, thereby obtaining an undrawn film. The glass transition point of the undrawn film was 69°C

The undrawn film was preheated up to 90°C under the conditions of a heat transfer coefficient of 0.0008 cal/cm²·sec·°C and then drawn in the transverse direction at 75°C at a ratio of 4X by a tenter. Then, the film was heated at 80°C for 10 seconds. Thus, a thermo-shrinkable polyester film having a thickness of 50 µm was obtained.

. (Comparative example 4)

A thermo-shrinkable polyester film having a thickness of 50 µm was obtained in the same manner as described in Example 1 except that the post-drawing heating temperature was 75°C.

(Comparative example 5)

A thermo-shrinkable polyester film having a thickness of 50 µm was obtained in the same manner as described in Example 1 except that the post-drawing heating temperature was 83°C.

(Comparative example 6)

A thermo-shrinkable polyester film having a thickness of 50 µm was obtained in the same manner as described in Example 1 except that the heat transfer coefficient was 0.0017 cal/cm² · sec · °C.

Table 1 shows the evaluation results of the films obtained in Examples 1 through 4 and Comparative examples 1 through 6. As seen from Table 1, the films obtained in Examples 1 through 4 exhibit satisfactory values in all the evaluation items including the pre-shrinkage state, ease of tearing along perforations, and thickness dispersion. As can be appreciated, the thermo-shrinkable polyester film in the first embodiment of the present invention is of satisfactory quality, highly practical and especially suitable for use as films for labels.

In contrast, the thermo-shrinkable polyester film in Comparative example 1 is inferior in ease of tearing along perforations due to the inferior anti-rupture property. The thermo-shrinkable polyester film in each of Comparative examples 3 and 5 suffer from wrinkles and insufficient shrinkage, resulting in an inferior post-shrinkage state. The thermo-shrinkable polyester film in Comparative example 4 shifts to a position different from a prescribed position when put around the bottle, resulting in an inferior post-shrinkage state. The thermo-shrinkable polyester film in Comparative example 6 is inferior in the thickness dispersion. As can be appreciated, the thermo-shrinkable polyester films in the Comparative examples 1 through 6 are of low quality and are not practical for use as label films.

As described above, a thermo-shrinkable polyester film according to the present invention exhibits a satisfactory post-shrinkage state with substantially uniform shrinkage and no wrinkles or distortions in all possible uses including labels for bottles, and is easily torn along perforations formed therein.

Claims (3)

1. A thermo-shrinkable polyester film comprising:

   a polyester having a glass transition point of 58 to 68 °C and including at least one diol having from 3 to 6 carbon atoms,

   and wherein the film has

   - a thermal shrinkage ratio in the main shrinking direction of 20 % or more after being treated in water of 70 °C for 5 seconds, 35 to 55 % after treated in water of 75 °C for 5 seconds, and 50 to 60 % after being treated in water of 80 °C for 5 seconds, and

   - a probability of elongation at rupture of the film in the direction perpendicular to the main shrinking direction, as assessed by the tensile test in accordance with the method defined by JIS C2318, being 20 % or less, of 10 % or less after the film is shrunk by 5 % by treatment in water of 75 °C for 10 seconds.
2. A thermo-shrinkable polyester film according to claim 1, the film having a thickness of dispersion of 6 % or less.
3. A method of producing the thermo-shrinkable polyester film of claim 1 or 2 comprising the steps of:

   - drying and extruding the polyester at 200 to 300 °C, followed by rapid cooling to form an undrawn film;

   - preheating the undrawn film at a low hot air speed such that the heat transfer coefficient is 0.0013 cal/cm² • sec • °C or less;

   - drawing the undrawn film using a tenter in either a transverse or longitudinal direction at a drawing ratio of 3X or more, at a temperature of less than Tg+15 °C, and at a heat transfer coefficient of 0.0009 to 0.0017 cal/cm² • sec • °C; and

   - heating the drawn film at 70 to 100 °C.