JP2910091B2 - White polyester film and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a magnetic card, a label, a sticker, a home delivery slip,
Receiving paper for video printers, posters, maps, dust-free paper,
The present invention relates to a white polyester film used as a base material of a display plate, an electronic white board, a photographic paper, a copy paper, and the like, and a method for producing the same.

[Prior Art] Plastic films used in the above applications are required to be white and opaque. Conventional polyolefin resin with calcium carbonate added (JP-B 63-6431)
Although films based on polyolefin resins such as 0) are known, these films are susceptible to heat and have problems such as low mechanical strength. In order to improve these drawbacks, polyester films based on polyester and added with polypropylene (JP-A-63-168441).
Etc.) and those obtained by adding an incompatible thermoplastic resin to polyester (JP-A-49-134755, 62-11762).

However, although the conventional polyester film has fine bubbles inside and can reduce the specific gravity by lowering the apparent density, the voids increase due to the lower specific gravity, and as a result, the heat shrinkage increases. Problems arise. When the heat shrinkage rate is large, heat such as a drying process after print processing or the heat of a head to be printed may cause deterioration of the flatness of the polyester film, heat shrinkage of the film may cause printing misalignment, In this process, problems such as impairment of workability and aesthetics and formation of wrinkles are caused.

[Problems to be Solved by the Invention] Accordingly, an object of the present invention is to provide a white polyester film having a small apparent density and excellent thermal dimensional stability and flatness.

[Means for Solving the Problems] As a result of earnest studies, the present inventors have added polyalkylene glycol to polyester to which a specific polyolefin resin has been added, and further stretched under specific stretching conditions when performing biaxial stretching. Later, by performing heat treatment at a high temperature, the polyester film was further reduced in specific gravity, but the heat shrinkage was not increased, and it was found that a film excellent in dimensional stability and flatness could be obtained, and the present invention was completed. did.

That is, the present invention has at least a white polyester layer, and as a whole of the white polyester film, the apparent density is 0.5 g / cm ³ or more and less than 1.0 g / cm ³ , 120 ° C., 3
A white polyester film having a heat shrinkage in a longitudinal direction and a width direction at 0 minute of 1.0% or less.

Further, the present invention, after accelerating the crystallization of the melt obtained by mixing the polyester and the incompatible thermoplastic resin and polyalkylene glycol in the cooling process, after slightly elongating in the longitudinal direction near the glass transition temperature of the film, Further, there is provided a method for producing a white polyester film, wherein the film is stretched in the longitudinal direction in multiple stages, and then the film is transversely stretched while increasing the temperature in the width direction.

[Effect of the Invention] According to the present invention, a low-density polyester film having excellent thermal dimensional stability, flatness, and whiteness is provided. As a result, it is possible to prevent the flatness of the polyester film from being deteriorated due to heat such as drying and heating, and to obtain a low-cost film which is free from printing misalignment and has excellent workability.

[Specific description of the invention] The apparent density of the white polyester film of the present invention is:
0.5 g / cm ³ or more 1.0 g / cm less than ^3, preferably 0.6 g / cm ³ or more zero.
Less than 95 g / cm ^3, more preferably 0.7 g / cm ³ or more 0.9 g / cm ³
Is less than. When the apparent density is 1.0 g / cm ³ or more, not only the cushioning property and lightness of the film are lost, but also the cost is increased with respect to paper and the competitiveness as a substitute for paper is lost. On the other hand, if the apparent density is less than 0.5 g / cm ³ , the amount of microbubbles inside becomes too large, and sufficient mechanical strength cannot be obtained.

The heat shrinkage in the longitudinal direction and the width direction of the white polyester film of the present invention is not more than 1.0% at 120 ° C. for 30 minutes, preferably
0.9% or less. When the heat shrinkage is more than 1.0%, the heat shrinkage is lacking, the dimensional stability is deteriorated, and the flatness, printability and the like are reduced.

The white polyester film of the present invention contains countless fine bubbles in a layer made of polyester,
Light is scattered by these bubbles to make the color white and opaque.

The polyester used in the present invention may be any polyester that can form a film, for example,
Polyethylene terephthalate, polytetramethylene terephthalate, polyethylene-o-oxybenzoate,
Examples thereof include poly-1,4-cyclohexylene dimethylene terephthalate and polyethylene 2,6-naphthalenedicarboxylate. Of course, these polyesters may be homopolyesters or copolyesters. As the copolymerization component, diol components such as diethylene glycol, neopentyl glycol and polyalkylene glycol and adipic acid, sebacic acid,
Examples thereof include dicarboxylic acid components such as phthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, and 5-sodium sulfoisophthalic acid. Among them, polyethylene terephthalate has excellent water resistance, durability, chemical resistance, and the like when formed into a film, and is preferred as the polyester used in the present invention.

In the present invention, a polymer of an incompatible thermoplastic resin is added in order to make the polyester film contain air bubbles to give whiteness and lower the density. The incompatible thermoplastic resin used in the present invention preferably has a melting point of 200 ° C. or higher, and for example, a polyolefin resin is preferable. Preferred polyolefin resins are those which can form a film by mixing with polyester, and include, for example, polymethylpentene and 3-methylbutene-1-isotactic polystyrene. Also,
The copolymer is not necessarily limited to a homopolymer, and may be a copolymer thereof. Among them, polymethylpentene which is a polyolefin having a small critical surface tension is preferable. Particularly, polymethylpentene is preferable because it has good releasability from polyester and easily forms fine bubbles during stretching.

The addition amount of the incompatible thermoplastic resin is preferably 3
-30% by weight, more preferably 5-20% by weight. If the amount is less than 3% by weight, the amount of fine bubbles generated is small, and the apparent density of the white polyester film of the present invention cannot be easily obtained. On the other hand, if the amount exceeds 30% by weight, the stretchability of the film deteriorates, and the film-forming properties deteriorate.

Further, a polyalkylene glycol or a copolymer thereof can be added in order to finely disperse the incompatible thermoplastic resin particles and suppress the heat shrinkage. The specific gravity can be further reduced by adding a polyalkylene glycol or a copolymer thereof.

Polyalkylene glycol or a copolymer thereof used in the present invention include, but are not limited to, polyethylene glycol, polypropylene glycol, a 1: 1 copolymer of ethylene glycol and propylene glycol, methoxypolyethylene glycol, and the like. . In general, even when a surfactant is added, the dispersion state of the incompatible thermoplastic resin can be further reduced, but in this case, the adhesion between the polyolefin resin and the polyester is increased, and the generation of bubbles during stretching is inhibited. However, surprisingly, when polyalkylene glycol or a copolymer thereof is added, not only the dispersion state of the incompatible thermoplastic resin is refined, the generation of fine bubbles is promoted, but also the cleavage strength is improved and the heat shrinkage is improved. Improvements are also noted.

The addition amount of the polyalkylene glycol or the copolymer thereof in the present invention is preferably 0.5 to 5% by weight, more preferably 0.5 to 3% by weight. If the addition amount is less than 0.5% by weight, the effect of finely dispersing the incompatible thermoplastic resin resin is reduced, and the degree of improvement in cleavage strength and the degree of improvement in heat shrinkage are also reduced. If the amount exceeds 5% by weight, the thermal stability will be poor and the heat shrink resistance will be poor. Further, the white polyester layer becomes yellowish color, which is not preferable.

In particular, the polyalkylene glycol has a molecular weight in consideration of the stretchability of the film and the promotion of generation of fine bubbles.
Preferred are polyethylene glycols of 500 to 30,000, preferably 1,000 to 10,000. When the molecular weight is less than 500, the white polyester layer tends to have a yellowish color and the cleavage strength is often low. When the molecular weight exceeds 30,000, the cleavage strength often decreases.

Further, in consideration of the flatness, the ratio of the heat shrinkage in the longitudinal direction and the width direction of the white polyester film of the present invention, that is, the MD / TD ratio is preferably 5.0 or less, more preferably 4.5 or less. When the MD / TD ratio is larger than 5.0, the flatness is deteriorated, which may cause print misregistration and generation of folding lines.

In order to increase the whiteness and optical density of the white polyester film of the present invention, inorganic particles are added to the white polyester layer in an amount of 0.2.
It is preferable to add 05 to 25% by weight, preferably 0.5 to 15% by weight. Examples of the inorganic particles include talc, magnesium oxide, gypsum, barium sulfate, and magnesium carbonate. It is also preferable to add 0.001 to 0.5% by weight of a fluorescent whitening agent in order to increase the whiteness.

The optical density of the white polyester film of the present invention is 0.
It is 7 or more and 1.6 or less, preferably 0.8 or more and 1.6 or less. If the optical density is less than 0.7, the concealability of the film is small, so that the back side can be seen favorably. On the other hand, if the optical density exceeds 1.6, a large amount of fine bubbles must be contained, and the strength of the film becomes weak, which is not preferable.

The whiteness of the white polyester film of the present invention is preferably 80% or more and 110% or less, more preferably 8% or less.
5% or more and 105% or less. When the whiteness is less than 80%, the film becomes yellowish and impairs the high-grade appearance of printing. On the other hand, if the whiteness exceeds 110%, a bluish tint is imparted, which also impairs a sense of quality.

The white polyester film of the present invention may be configured as a single-layer film composed of only the white polyester layer, or may be configured as a composite film of such a white polyester layer and another film. There may be. In the case of a composite film, the white polyester layer according to the present invention is formed by laminating a polyester layer A to which an incompatible thermoplastic resin or a polymer thereof is not added on at least one surface of the white polyester layer as a composite film. Is preferred. When this polyester layer A is laminated on a white polyester layer to which polyalkylene glycol or a copolymer thereof has not been added,
Although the cleavage strength is low, when laminated on a white polyester layer to which polyalkylene glycol or a copolymer thereof is added, the cleavage strength is remarkably improved due to interaction with the polyalkylene glycol or the copolymer thereof. Further, when polyalkylene glycol or a copolymer thereof is added to the polyester layer A, the cleavage strength is further improved. Also,
Laminating the polyester layer A has an effect of preventing the flatness from being deteriorated. Further, in the case of a single-layer film, although the adhesiveness of the surface is reduced by the presence of the incompatible thermoplastic resin on the surface of the white polyester layer, it is possible to prevent the adhesiveness from being reduced by providing the polyester layer A on the surface. it can.

The thickness of the polyester layer A is preferably 10 μm or less. If it exceeds 10 μm, separation at the interface between the laminated polyester layer A and the fine bubble-containing layer tends to occur.

In the present invention, by adding inorganic particles to the laminated polyester layer A, the smoothness of the film surface is improved, and by lowering the gloss, a polyester film closer to paper can be obtained. Examples of the inorganic particles include calcium carbonate, silicon dioxide, titanium dioxide, barium sulfate and the like. The addition amount of the inorganic particles is preferably 0.05 to 25% by weight. If the addition amount is less than 0.05% by weight, the slip of the film becomes worse as compared with paper, and the productivity in post-processing such as printing is not good. If the addition amount exceeds 25% by weight, the surface of the film becomes weak and troubles such as paper dust easily occur during post-processing.

Next, a method for producing the polyester film of the present invention will be described.

After thoroughly vacuum drying the raw material obtained by adding the polyalkylene glycol during the polymerization reaction of the polyester raw material and the polyalkylene glycol or a copolymer thereof or during the polymerization reaction of the polyester or at the completion of the polymerization,
The chips of the incompatible thermoplastic resin are mixed, supplied to an extruder heated to 270 to 300 ° C., and formed into a sheet from a T-die. At this time, the polyalkylene glycol or its copolymer is preferably added at the time of polyester polymerization or at the completion of polymerization to form a master chip, but the three components may be kneaded in advance with a pelletizer or the like. In the case of laminating the polyester layer A, a separately dried polyester chip can be supplied to another extruder and laminated with a white polyester layer in a T-die to form a sheet.
Further, at this time, a master chip of inorganic particles may be mixed with a polyester chip and dried under vacuum.

The sheet-like film obtained as described above is cooled and solidified on a cooling drum at a temperature of 60 to 90 ° C., which is maintained at a higher temperature than the routine casting, to form an unstretched film. At this time, the temperature of the cooling drum is
By setting the temperature to a high temperature of 40 to 60 ° C. to 60 to 90 ° C., crystallization required for the production method of the present invention can be promoted. The obtained unstretched film is finely stretched in the longitudinal direction at a temperature not higher than the glass transition temperature of the film, and then guided to a group of rolls heated to 70 to 120 ° C., which is higher than the glass transition temperature.
Longitudinal stretching is performed in multiple stages or more, and further cooled by a group of rolls at 20 to 30 ° C. to obtain a uniaxially stretched film. The fine stretching ratio is preferably about 1.1 to 2 times.

Next, the uniaxially stretched film is guided to the tenter while gripping both ends of the film with clips, and is laterally stretched while being heated in the width direction perpendicular to the longitudinal direction in an atmosphere gradually heated to 80 to 220 ° C., and biaxially stretched film is stretched. Can be obtained. The stretching ratio is
It is preferable that the longitudinal stretching ratio <the transverse stretching ratio ≦ 5.0 times. When the transverse stretching ratio is larger than the longitudinal stretching ratio, the heat shrinkage in the width direction perpendicular to the longitudinal direction is deteriorated, but the MD / TD ratio is balanced and the flatness is improved.

The biaxially stretched film obtained as described above can have flatness and dimensional stability by performing heat setting at a high temperature of 200 to 240 ° C. in a tenter. When the heat treatment temperature is lower than 200 ° C., the heat shrinkage becomes too large, and the flatness is deteriorated in a portion to which heat is applied in a step such as post-processing, which may impair a beautiful feeling or cause generation of folding wrinkles. In addition, the relaxation rate when performing heat setting is 0 to 20%.
Is preferred from the viewpoint of dimensional stability. The heat-set film is gradually and uniformly cooled, and gradually cooled to room temperature to take up the polyester film of the present invention.

Hereinafter, methods for measuring the physical properties of the polyester film of the present invention will be described.

(1) Apparent density Measured using a carbon tetrachloride-n-heptane-based density gradient tube. More specifically, the value after standing in a thermostat at 25 ° C. for 24 hours is used.

(2) Heat shrinkage The film is cut into 10mm wide and 300mm long in the longitudinal or width direction, marked at 200mm intervals and fixed tension (5
g) After fixing below, measure the original length a (mm) of the marking interval. Next, a load is applied using a 3 g
Treat for 30 minutes while rotating in a hot air oven at ℃, and measure the marking interval b (mm) in the same manner as the original length measurement. The heat shrinkage is determined by the following equation, and the average value of five pieces is used.

Heat shrinkage (%) = (ab) / a Also, from the obtained heat shrinkage in the longitudinal and width directions, MD
Find the / TD ratio.

MD Heat shrinkage in the longitudinal direction TD Heat shrinkage in the width direction (3) Cleavage strength A polyurethane-based primer is applied to the film to form a 2 µm thick coating, and an unstretched polypropylene film (60 µm thick, Toray) at 50 ° C (Manufactured by Gosei Co., Ltd.) and an adhesive, aged at 40 ° C. for 48 hours, cut to 15 mm in width, peeled off one end, attached to Tensilon, and kept at a right angle to the part where the laminated part was peeled off. The average value of the strength of the five film tapes when the unstretched film is peeled off is defined as the cleavage strength. However, this does not exclude the case where the film was not broken and was peeled off with the adhesive layer. At this time, the peeling speed is 300 mm / min.

(4) Flatness The film is cut in the longitudinal direction to a width of 200 mm and a length of 350 mm, and both ends of the film are fixed in a frame of 320 mm in the longitudinal direction and 250 mm in width. Leave to observe the film surface. The flatness is evaluated as follows.

：: No wrinkles, sagging, etc. are observed on the film surface. △: 3 or less sagging in the longitudinal direction on the film surface ×: 4 or more sagging in the longitudinal direction on the film surface (5) Whiteness JIS-L-1015 When the reflectance at wavelengths of 450 nm and 550 nm is B% and G%, respectively, whiteness (%) is expressed by 4B-3G.

(6) Optical Density The films are stacked so as to have a thickness of about 150 μm, and the transmission density is measured using an optical densitometer (TR927, manufactured by Macbeth). Plot film thickness and optical density,
The optical density corresponding to a thickness of 0 μm is determined.

(7) Stretchability ：: A state in which a uniform film having no break in longitudinal stretching or tear in transverse stretching during film formation and without stretching unevenness can be collected.

Δ: The film was longitudinally torn during film formation, but could be easily repaired and a uniform film free from stretching unevenness could be collected.

×: Frequent tearing in horizontal stretching or breaking in vertical stretching,
Even if stretching is possible, uneven stretching occurs.

[Examples] The present invention will be described more specifically with reference to examples, but examples of the present invention are not limited thereto.

Examples 1 and 2, Comparative Examples 1 to 5 A 6 wt% polyethylene glycol master chip was mixed with a polyethylene terephthalate (intrinsic viscosity: 0.65) chip so as to have the ratio shown in Table 1, and vacuum-dried at 180 ° C. for 3 hours. After that, chips of polymethylpentene were mixed so as to have the ratio shown in Table 1, and the mixture was supplied to an extruder heated to 280 ° C. The film extruded into a sheet from the T-die was cooled and solidified by a cooling drum heated to a surface temperature of 75 ° C. The resulting unstretched film was guided to a group of rolls heated to 70 ° C, stretched 1.8 times in the longitudinal direction first, and then 90 ° C
After longitudinal stretching at the magnification shown in Table 1, the film was cooled with a roll group at 25 ° C. Next, the film stretched in the longitudinal direction was guided to a tenter while holding both ends of the film with clips, and was horizontally stretched in the widthwise direction at a magnification shown in Table 1 in a temperature-raised atmosphere heated to 90 to 220 ° C. The obtained biaxially stretched film was heat-set at 220 ° C. in a tenter, and then gradually cooled to obtain a polyester film having a thickness of 50 μm.

The physical properties of the obtained polyester film are as shown in Table 1. As can be seen from Table 1, the apparent density is 0.5 g / c.
is less than m ³ becomes weak cleavage strength. Further, when the addition amount of polyethylene glycol is 5% by weight or less,
Although the heat shrink resistance and the flatness are maintained, if it exceeds 5% by weight, on the contrary, the whiteness deteriorates and the whiteness becomes poor. Regarding the stretching ratio, when the longitudinal stretching ratio and the transverse stretching ratio are the same, the heat shrinkage in the longitudinal direction exceeds 1.0% and becomes poor.

Examples 3 to 6 A polyethylene terephthalate (intrinsic viscosity: 0.65) chip was mixed with a polyethylene glycol 6% by weight master chip so that polyethylene glycol was 1.0% by weight, and 10% by weight of polymethylpentene was further mixed.
After vacuum drying at 3 ° C. for 3 hours, a 50 μm thick film was obtained in the same manner as in the above example. However, the stretching of the film was performed at the magnifications shown in Table 2.

The physical properties of the obtained film are as shown in Table 2.
It can be seen that the heat shrinkage in the width direction deteriorates as the transverse stretch ratio is increased as compared with the longitudinal stretch ratio, but the MD / TD ratio is balanced and the flatness is improved.

──────────────────────────────────────────────────の Continuation of the front page (51) Int.Cl. ⁶ identification symbol FI // B29K 67:00 (58) Field surveyed (Int.Cl. ⁶ , DB name) C08J 9/00-9/14 C08J 5 / 18 B32B 1/00-35/00 B29C 55/00-55/30

Claims (5)

(57) [Claims] Claims: 1. A white polyester film having at least a white polyester layer having an apparent density of 0.5 g / cm ³ or more and less than 1.0 g / cm ³ at 120 ° C. for 30 minutes in the longitudinal direction and the width direction. A white polyester film having a heat shrinkage of 1.0% or less. 2. The ratio of heat shrinkage in the longitudinal direction and the width direction MD / TD
3. The white polyester film according to claim 1, wherein is not more than 5.0. 3. An optical density of from 0.7 to 1.6 and a whiteness of 80%.
3 or more and 110% or less.
The white polyester film according to 1. 4. The white polyester film according to claim 1, further comprising a polyester layer A laminated on at least one side of the white polyester layer. 5. A method in which a melt obtained by mixing a polyester with an incompatible thermoplastic resin and a polyalkylene glycol promotes crystallization in a cooling process, and then slightly elongates in a longitudinal direction around a glass transition temperature of the film. The method for producing a white polyester film according to any one of claims 1 to 3, wherein the film is stretched in the longitudinal direction in multiple stages, and then the film is transversely stretched while increasing the temperature in the width direction.