JP3330273B2 - Heat-shrinkable polylactic acid-based film and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a biaxially oriented uniaxially heat-shrinkable polylactic acid-based film which decomposes in a natural environment, more specifically, poly (L-lactic acid), poly (D-lactic acid), poly (DL-lactic acid).
Lactic acid) or a mixture of these, which is excellent in impact resistance and substantially heat-shrinkable in one axis direction.

[0002]

2. Description of the Related Art Heat shrinkable films are widely used for shrink wrapping, shrink labels, cap seals and the like, utilizing the property of shrinking by heating. For these, a stretched film made of a vinyl chloride resin, a polystyrene resin, and more recently a polyester resin has been used. In particular, the film obtained by stretching in the uniaxial direction and setting the direction as the main shrinkage direction is used as a label film or a bundling film for various plastic containers and glass containers such as polyethylene terephthalate (PET) bottles. Is expanding.

[0003] However, these materials have a high calorific value and may damage the combustion furnace during the incineration process. Furthermore, a vinyl chloride resin film, which is still used in a large amount, cannot be burned due to its self-extinguishing property. In addition, plastic products, including materials that cannot be incinerated, are often landfilled, but due to their chemical and biological stability, they remain with little decomposition and shorten the life of landfills. I'm offended. Therefore, a material that has a low heat of combustion, decomposes in soil, and is safe is desired, and many studies have been made. One example is polylactic acid. Polylactic acid has a heat of combustion less than half that of polyethylene, and hydrolyzes spontaneously in soil and water, and then becomes harmless degradation products by microorganisms. At present, studies are being made to obtain containers such as films, sheets and bottles using polylactic acid.

[0004] Polylactic acid can also be obtained by uniaxial stretching.
An axially shrinkable film can be obtained, and can be used as the label or the binding film. However, polylactic acid has a disadvantage that it is brittle as it is, and although uniaxial stretching improves brittleness in the direction, particularly strength and elongation, it remains brittle in a direction perpendicular to the direction. On the other hand, a biaxially oriented film obtained by stretching biaxially has improved brittleness in both the longitudinal direction and the width direction and has increased strength, as shown in JP-A-5-508819, JP-A-6-23836 or JP-A-7-205278. Is disclosed. Also,
Japanese Patent Application Laid-Open No. 7-2027041 describes that a polylactic acid-based film having excellent thermal dimensional stability can be obtained biaxially by stretching polylactic acid having high crystallinity and then performing heat treatment. However, this does not make it possible to obtain a desired uniaxially shrinkable film.

[0005]

Accordingly, the present invention has improved brittleness by using a crystalline polylactic acid-based polymer and adjusting the preheating temperature, the stretching temperature, and the stretching ratio, that is, it has excellent impact resistance. An object of the present invention is to provide a biaxially oriented uniaxially contractible polylactic acid-based film.

[0006]

That is, the gist of the present invention is to provide poly (L-lactic acid), poly (D-lactic acid) and poly (DL-lactic acid).
-Lactic acid) or a mixture thereof, the tensile elongation at break in the longitudinal and width directions of the film is 30% or more, and the heat shrinkage at 100 ° C is 30% or more in either direction, and The biaxially oriented uniaxially heat-shrinkable polylactic acid-based film has a heat shrinkage in a direction perpendicular to the shrinkage direction of 10% or less.

Further, poly (L-lactic acid), poly (D-milk)
Acid), poly (DL-lactic acid) or a mixture thereof, in which the unstretched sheet is successively biaxially stretched. After the unstretched sheet is uniaxially stretched, the crystallization temperature Tc of the film after uniaxially stretching is obtained.
Preheating at the above temperature, and subsequently stretching the second axis in a direction perpendicular to the uniaxial stretching direction.
An object of the present invention is to provide a method for producing the above-mentioned biaxially oriented uniaxial heat-shrinkable polylactic acid-based film.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.
The polylactic acid in the present invention includes poly (L-lactic acid) whose structural unit is L-lactic acid, poly (D-lactic acid) whose structural unit is D-lactic acid, and a copolymer of L-lactic acid and D-lactic acid. There is a polymer, poly (DL-lactic acid), and also a mixture thereof.

As the polymerization method, any known method such as a condensation polymerization method and a ring-opening polymerization method can be employed. For example, in the polycondensation method, polylactic acid having an arbitrary composition can be obtained by directly dehydrating polycondensation of L-lactic acid or D-lactic acid or a mixture thereof. In the ring-opening polymerization method, lactide, which is a cyclic dimer of lactic acid, can be used to obtain polylactic acid using a selected catalyst while using a polymerization regulator and the like as necessary. Lactide includes L-lactide, which is a dimer of L-lactic acid, D-lactide, which is a dimer of D-lactic acid, and DL-lactide comprising L-lactic acid and D-lactic acid. By mixing and polymerizing, polylactic acid having any composition and crystallinity can be obtained.

In order to increase the molecular weight, a small amount of a chain extender, for example, a diisocyanate compound, an epoxy compound or an acid anhydride can be used. The preferred range of the weight average molecular weight of the polymer is from 60,000 to 700,000. If it is below this range, practical physical properties are hardly exhibited, and if it is over this range, the melt viscosity is too high and molding processability is poor.

Next, the film forming conditions of the unstretched sheet will be described. The polylactic acid-based polymer is sufficiently dried to remove water, and then melted by an extruder. Since the melting temperature varies depending on the composition, it is preferable to appropriately select the melting temperature accordingly. In practice, a temperature range of 140 to 250 ° C. is usually chosen.

It is preferable that the polymer melt-formed into a sheet is rapidly cooled by contact with a rotating casting drum (cooling drum). The temperature of the casting drum is suitably 60 ° C. or less. Above this, the polymer sticks to the casting drum and cannot be pulled off. In addition, since crystallization is promoted and spherulites develop and cannot be stretched, it is preferable to set the temperature in the above-mentioned temperature range and rapidly cool to make them substantially amorphous.

Although the stretching method in the present invention is not particularly limited, it is advantageous to use a sequential biaxial stretching method in film production. In the sequential biaxial stretching method, a film stretched in a uniaxial direction is subsequently preheated to stretch a second axis in a direction orthogonal to the uniaxial stretching direction. At present, a sequential biaxial stretching apparatus that stretches in the longitudinal direction of the film (longitudinal stretching) and then stretches in the width direction (lateral stretching) is often used. After transverse stretching), stretching in the longitudinal direction (lateral stretching) may be performed.

Poly (L-lactic acid), poly (D-lactic acid), poly (L-lactic acid)
In the case of li (DL-lactic acid) or a mixture thereof, the stretching direction is appropriately selected in the range of 1.5 to 5 times each in the longitudinal direction and the width direction, and the stretching temperature is in the range of 50 to 90 ° C. The plane orientation degree ΔP of 0 × 10 ⁻³ or less is 3.0 ×
By increasing it to 10 ⁻³ or more, a tough biaxially oriented film can be obtained even with a small thickness.

The biaxially oriented film has an increased tensile strength as compared with a non-oriented film, and also has improved brittleness (impact resistance). One measure of improvement in impact resistance can be indicated by tensile elongation at break, where the elongation of a non-oriented film is several percent, whereas that of a biaxially oriented film is several tens to several hundreds in both the longitudinal and width directions. % Elongation can be obtained. That is, a film excellent in practicality can be obtained by biaxial stretching. However, the biaxially oriented uniaxially shrinkable film of the present invention cannot be obtained as it is.

An important point of the present invention is that the crystalline poly (L-
Lactic acid), poly (D-lactic acid), poly (DL-lactic acid) or
Is to select the preheating temperature, the stretching temperature, and the stretching ratio of the second axis appropriately in consideration of the stretching temperature of the first axis, the stretching ratio, and the Tc of the film after the uniaxial stretching.

The film of the present invention is obtained by uniaxially stretching an unstretched sheet and then subjecting the film to a crystallization temperature T after the uniaxial stretching.
It is obtained by preheating at a temperature equal to or higher than c and performing stretching in the second axis. The non-oriented (unstretched) polylactic acid sheet has a crystallization temperature Tc depending on the composition ratio of L-lactic acid units and D-lactic acid units.
Is 90 ° C. or higher. However, by performing uniaxial stretching, the molecular chains are oriented, and Tc tends to be lower than 90 ° C. Therefore, after preheating at a temperature equal to or higher than Tc of the uniaxially stretched film, by performing the second axial stretching, a part of the oriented chains in the film is crystallized, and the shrinkage in the first axial stretching direction can be suppressed. .

However, care must be taken because if the preheating temperature and the stretching temperature in the second axis are too high, the film may be broken before stretching at a desired magnification is completed.

Tc is determined by differential scanning calorimetry (DSC) of a film sample, and is calculated by a temperature rise rate of 10
It is determined from an exothermic peak generated at the time of crystallization that occurs during the temperature rise process when the temperature is raised at a rate of ° C./min.

Tc mainly depends on the crystallinity of the polymer itself, and a polymer having high crystallinity takes a low temperature. Incidentally, in the homopolymer of either L-lactic acid or D-lactic acid without a copolymer, Tc depends on the molecular weight.
１１０110 ° C. The melting point Tm is 180 ° C. or higher. Lactic acid units corresponding to each of these L-lactic acid homopolymers or D-lactic acid homopolymers are included,
Alternatively, when other aliphatic monomer components are copolymerized, the crystallinity decreases, Tc increases, and Tm decreases. An amorphous polymer that does not crystallize where Tc and Tm are about to overlap. Therefore, the film of the present invention cannot be obtained with such a polymer.

Further, by performing a heat treatment in an atmosphere at a temperature higher than the crystallization temperature after the biaxial stretching, the shrinkage rate can be further suppressed in both the longitudinal direction and the width direction of the film. In the present invention, heat treatment after biaxial stretching is not always necessary, but can be an effective means in controlling the shrinkage.

[0022]

The present invention is not limited by the following examples. The measurement values shown in the examples were measured under the following conditions. Also,
In this embodiment, the vertical direction indicates the longitudinal direction of the film, and the horizontal direction indicates the width direction of the film. (1) Tensile strength and tensile elongation at break Tensile strength and tensile elongation at break were measured for each of the length and width of the film sample using a Tensilon II type machine manufactured by Toyo Seiki Co., Ltd. in accordance with JIS-K7127. The pulling speed is 100 mm / min.

(2) Tc Crystal based on heat generation from a thermogram when 10 mg of a film sample was heated at a rate of 10 ° C./min based on JIS-K7122 using DSC-7 manufactured by PerkinElmer. The activation peak Tc was determined.

(3) Heat shrinkage rate A film sample was placed 100 mm × 100 m along the length and width.
m, immersed in warm water of 100 ° C for 5 minutes,
The length and width were measured, and the ratio of {(dimension before shrinkage) − (dimension after shrinkage)} to (dimension before shrinkage) was expressed as%.

(4) Impact Resistance The impact resistance was measured using a high-speed impact tester HTM-1 (Hydroshot) manufactured by Shimadzu Corporation. 100mm x 1
The film cut to 00 mm was fixed with a clamp, a drop was applied to the center of the film to give an impact, and the energy up to breakage was read. The measurement temperature is 23 ° C., and the falling speed of the drop is 3 m / sec.

(Example 1) The ratio of the structural unit composed of L-lactic acid to the structural unit composed of D-lactic acid was approximately 99: 1, the glass transition point was 57 ° C, the crystallization temperature was 102 ° C, and the melting point was 178.
° C, poly L-lactic acid having a weight average molecular weight of 130,000
Extruded from a T-die at 200 ° C with a single-screw extruder, quenched with a casting roll to obtain an unstretched sheet of about 300 µm, and then set to a film temperature of 75 ° C with an infrared heater, and set to a longitudinal (longitudinal) direction. 2.6) direction
Roll stretching was performed twice. Next, the film after longitudinal stretching is preheated in a tenter at 75 ° C., and then stretched 2.4 times in the transverse (width) direction with respect to the film width before longitudinal stretching in a stretching zone set at 75 ° C. to a thickness of 45 μm. Was obtained.
The heat treatment zone after the biaxial stretching was set at 60 ° C., and passed without substantial heat treatment. The passage time in each of the preheating, stretching and heat treatment zones was about 20 seconds. The Tc of the film after longitudinal stretching was 72 ° C. when measured by DSC. Table 1 shows the performance of the obtained biaxially oriented film.

(Example 2) In the same manner as in Example 1, the film was roll-stretched 2.2 times in the longitudinal (longitudinal) direction at 70 ° C.
After preheating at 80 ° C. with a tenter, the film was stretched 2.7 times the film width before longitudinal stretching in the transverse (width) direction in a stretching zone set at 76 ° C. The temperature of the heat treatment zone after the biaxial stretching was set at 65 ° C. to obtain films shown in Table 1. The passing time in each zone was about 30 seconds. The Tc of the film after longitudinal stretching was 76 ° C.

(Comparative Example 1) Extruded in the same manner as in Example 1,
A 0 μm unstretched film was obtained. Table 1 shows the evaluation of this film.

(Comparative Example 2) Extruded in the same manner as in Example 1,
A 50 μm non-stretched sheet was obtained, and a uniaxially stretched film was obtained by transverse stretching with a tenter without longitudinal stretching. The set temperatures of the preheating zone and the stretching zone are both 70 ° C., the stretching ratio is 2.5 times, and the temperature of the heat treatment zone after the stretching is 60 °.
° C. The transit time in each zone was about 60 seconds.
The evaluation of the obtained film is shown in Table 1.

Comparative Example 3 In Example 2, the temperature of the preheating zone in the transverse stretching and the temperature of the stretching zone were both 70 ° C.
A biaxially stretched film shown in Table 1 was obtained in the same manner except that

[0031]

[Table 1]

In both Example 1 and Example 2, the film was preheated at a temperature higher than Tc of the film after longitudinal stretching, and the film was transversely stretched. The resulting film has tensile strength in both length and width,
It is a biaxially oriented film having improved elongation and excellent impact resistance. In addition, it can be seen that the film shrinks substantially only in one axial direction, with little or no longitudinal shrinkage.

On the other hand, it can be seen that the unstretched film shown in Comparative Example 1 has no heat shrinkage and has poor practical strength.
Further, Example 2 shows a film that has been uniaxially stretched, and has excellent uniaxial shrinkage, but has poor tensile strength and elongation in the unstretched longitudinal direction, and is inferior in impact resistance. I understand. Further, in Comparative Example 3 in which biaxial stretching was performed, the tensile strength and elongation were improved in both the longitudinal and transverse directions, and the impact resistance was excellent. However, the film was preheated at a temperature lower than Tc of the film after longitudinal stretching, and the transverse stretching was performed. Is performed, the contractility in the vertical direction is not suppressed, and biaxial contractility is shown.

[0034]

According to the present invention, a uniaxial heat-shrinkable film having practically sufficient impact resistance can be obtained from a degradable polylactic acid-based polymer.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-185510 (JP, A) JP-A-5-212790 (JP, A) JP-A-6-23836 (JP, A) JP-A-6-23836 114934 (JP, A) JP-A-6-240037 (JP, A) JP-A-7-68639 (JP, A) JP-A-7-205278 (JP, A) JP-A-7-207041 (JP, A) JP-A-7-275653 (JP, A) JP-A-7-292134 (JP, A) JP-A-8-198955 (JP, A) JP-A-8-230036 (JP, A) JP-A-8-300481 JP-A-9-3177 (JP, A) JP-A-9-57849 (JP, A) JP-A-9-95605 (JP, A) JP-A-9-151310 (JP, A) Table Hei 4-504731 (JP, A) Special table Hei 5-507109 (JP, A) Special table Hei 5-508819 (JP, A) Special table Hei 6-50 4799 (JP, A) European Patent Application Publication 683207 (EP, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) B29C 61/00-61/10 B29C 55/00-55/30 C08J 5/18 C08G 63/00-63/91 CA (STN) REGISTRY (STN) WPI / L (QUESTEL)

Claims (2)

(57) [Claims] 1. Poly (L-lactic acid), poly (D-lactic acid),
Consisting of poly (DL-lactic acid) or a mixture thereof ,
The tensile elongation at break in the longitudinal direction and the width direction of the film are both 30% or more, and the heat shrinkage at 100 ° C. is 30% or more in any direction and the heat shrinkage in the direction perpendicular to the shrinkage direction. There biaxially oriented uniaxially heat-shrinkable polylactic acid film is 10% or less. 2. Poly (L-lactic acid), poly (D-lactic acid),
In sequentially biaxially stretching an unstretched sheet of poly (DL-lactic acid) or a mixture thereof, the unstretched sheet is pre-heated at a temperature equal to or higher than the crystallization temperature Tc of the film after uniaxial stretching, 2. The method for producing a biaxially oriented uniaxially heat-shrinkable polylactic acid-based film according to claim 1, wherein a second axis of stretching is performed in a direction perpendicular to the uniaxially stretching direction.