JP4364413B2 - Water-based gas barrier coating agent, film coated with the same, and method for producing the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thermoplastic resin film excellent in gas barrier properties and a gas barrier coating agent.
[0002]
[Prior art]
Thermoplastic resin films such as polyamide and polyester are excellent in strength, transparency, moldability, and gas barrier properties, and thus are used in a wide range of applications as packaging materials. However, when used for applications that require long-term storage, such as retort-treated foods, higher gas barrier properties are required.
[0003]
In order to improve gas barrier properties, a film in which polyvinylidene chloride (PVDC) is laminated on the surface of these thermoplastic resin films has been widely used for food packaging, etc. PVDC uses organic substances such as acid gas during incineration. In recent years, there has been a growing interest in the environment, and there is a strong demand for a shift to other materials.
[0004]
On the other hand, polyvinyl alcohol (PVA) is an alternative material to PVDC. This material does not generate toxic gases and has a high gas barrier property in a low-humidity atmosphere, but as the humidity increases, the gas barrier property decreases rapidly and may not be used for packaging foods that contain moisture. Many.
[0005]
A film made of a copolymer of vinyl alcohol and ethylene (EVOH) is known as a film that improves the deterioration of gas barrier properties under high humidity of PVA, but the gas barrier properties under high humidity are maintained at a practical level. In order to do so, it is necessary to increase the ethylene content to some extent. When EVOH is used as a coating material, it must be dissolved using an organic solvent or a mixed solvent of water and an organic solvent, which is not desirable from the viewpoint of environmental problems, and requires a recovery step of the organic solvent. Therefore, there is a problem that the cost becomes high.
[0006]
As a method of coating a film with a liquid composition comprising a water-soluble polymer and developing a high gas barrier property even under high humidity, the film is coated with an aqueous solution comprising a partially neutralized product of PVA and polyacrylic acid or polymethacrylic acid. A method has been proposed in which both polymers are crosslinked by ester bonds by heat treatment (Japanese Patent Laid-Open No. 10-237180). In this method, in order to sufficiently advance the esterification reaction, heating at a high temperature for a long time is required, and there is a problem in productivity.
[0007]
On the other hand, it has been widely known that a polymer containing a maleic acid unit reacts with a hydroxyl group such as PVA or polysaccharide to make it water resistant. For example, JP-A-8-66991 discloses that a layer composed of 25-50% partially neutralized isobutylene-maleic acid copolymer and PVA has excellent water resistance. Japanese Patent Laid-Open No. 49-1649 proposes a method for making a PVA film water resistant by mixing an alkyl vinyl ether-maleic acid copolymer with PVA.
[0008]
[Problems to be solved by the invention]
In order to solve the above problems, the present inventors intend to provide a thermoplastic resin film that can maintain a high gas barrier property even under high humidity, and a method that can be produced industrially at a low cost. It is.
[0009]
[Means for Solving the Problems]
As a result of diligent research, the inventors of the present invention blended an alkaline compound with a coating solution composed of PVA and isobutylene-maleic anhydride, and adjusted the pH of the solution to a specific range, so that the PVA and isobutylene-maleic anhydride As a result, it was found that a film having excellent gas barrier properties under high humidity can be obtained by coating the surface of the thermoplastic resin film on the surface of the thermoplastic resin film and drying it.
That is, the gist of the present invention is as follows.
(1) A mixture of polyvinyl alcohol and isobutylene-maleic anhydride copolymer having a mass ratio of 97/3 to 20/80, and a mixture of an alkali compound having a boiling point of 200 ° C. or less and an alkaline earth metal hydroxide , A water-based gas barrier coating agent having a pH of 6 to 9.
( 2 ) A gas barrier film obtained by forming a film comprising the water-based gas barrier coating agent according to (1) above on at least one surface of a thermoplastic resin film.
( 3 ) A method for producing a gas barrier film, wherein the water-based gas barrier coating agent according to (1) is applied to a thermoplastic resin film and then heat-treated at a temperature of 150 ° C or higher.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0011]
Examples of the thermoplastic resin film to which the coating agent is applied in the present invention include polyamide resins such as nylon 6, nylon 66, and nylon 46, polyester resins such as polyethylene terephthalate, polyethylene naphthalate, polybutylene terephthalate, polybutylene naphthalate, and polypropylene. And a film made of a polyolefin resin such as polyethylene or a mixture thereof, or a laminate of these films. Polyethylene terephthalate and nylon 6 are particularly preferred. These may be unstretched films or stretched films.
[0012]
As a method for producing a film, a thermoplastic resin is heated and melted by an extruder and extruded from a T die, and is cooled and solidified by a cooling roll or the like to obtain an unstretched film, or is extruded from a circular die and is cooled by water or air. Solidify to obtain an unstretched film. In the case of producing a stretched film, a method in which an unstretched film is once wound or continuously stretched by a simultaneous biaxial stretching method or a sequential biaxial stretching method is preferable. From the viewpoint of performance such as mechanical properties and thickness uniformity of the film, a method in which a flat film forming method using a T die and a tenter stretching method are combined is preferable.
[0013]
The mass ratio of PVA and isobutylene-maleic anhydride copolymer in the present invention needs to be in the range of 97/3 to 20/80, preferably 90/10 to 40/60. When the mass ratio of PVA exceeds 97 or less than 20, the gas barrier property of the film particularly in a high humidity atmosphere is deteriorated.
[0014]
The PVA used in the present invention can be obtained by a known method such as complete or partial saponification of a vinyl ester polymer. Examples of the vinyl ester include vinyl formate, vinyl acetate, vinyl propionate, vinyl pivalate, vinyl versatate, etc. Among them, vinyl acetate is industrially most preferable.
[0015]
Other vinyl compounds may be copolymerized with the vinyl ester as long as the effects of the present invention are not impaired. Other vinyl monomers include unsaturated monocarboxylic acids such as crotonic acid, acrylic acid, and methacrylic acid and their esters, salts, anhydrides, amides, nitriles, and unsaturated acids such as maleic acid, itaconic acid, and fumaric acid. Examples thereof include dicarboxylic acids and salts thereof, α-olefins having 2 to 30 carbon atoms, alkyl vinyl ethers, vinyl pyrrolidones and the like.
[0016]
In this invention, in order to provide gas barrier property to the film surface, it is preferable that the vinyl alcohol unit in PVA is 40 mol% or more. When the vinyl alcohol unit is less than 40 mol%, the ester bond reaction rate with the isobutylene-maleic anhydride copolymer is lowered, and the gas barrier property is impaired. Further, when a large amount of a hydrophobic copolymer component is contained, the water solubility of the resin is impaired.
[0017]
As the saponification method, a known alkali saponification method or acid saponification method can be used, and among them, a method of alcoholysis using an alkali hydroxide in methanol is preferable. The saponification degree is preferably 80% or more, more preferably 97% or more, and particularly preferably 99% or more. If the degree of saponification is too low, the barrier performance is lowered, and at the same time, the water solubility of the polymer is impaired.
[0018]
Next, the isobutylene-maleic anhydride copolymer used in the present invention is obtained by polymerizing maleic anhydride and isobutylene by a known method such as solution radical polymerization.
[0019]
The isobutylene-maleic anhydride copolymer may be copolymerized with a small amount of another vinyl compound within a range not impairing the effects of the present invention. Other vinyl compounds include, for example, acrylic esters such as methyl acrylate, ethyl acrylate, butyl acrylate, and methyl methacrylate, vinyl esters such as vinyl formate and vinyl acetate, styrene, p-styrene sulfonic acid, Examples thereof include compounds having a reactive group that reacts with olefins having 2 to 30 carbon atoms such as ethylene, propylene, and isobutylene, and hydroxyl groups of PVA.
[0020]
The maleic acid unit in the isobutylene-maleic anhydride copolymer in the present invention preferably contains 10 mol% or more. When the maleic acid unit is less than 10 mol%, the formation of a crosslinked structure by reaction with the vinyl alcohol unit in PVA is insufficient and the gas barrier property is lowered.
[0021]
The maleic acid unit in the isobutylene-maleic anhydride copolymer used in the present invention tends to be a maleic anhydride structure in which the adjacent carboxyl group is dehydrated and cyclized in a dry state, and is opened when wet or in an aqueous solution. It becomes a maleic acid structure.
[0022]
The water-based gas barrier coating agent (hereinafter referred to as “coating agent”) in the present invention is suitable for an aqueous solution of PVA and isobutylene-maleic anhydride copolymer with respect to the carboxyl group in the isobutylene-maleic anhydride copolymer. It is necessary to adjust the pH of the liquid to a range of 6 to 9 by adding a certain proportion of alkali compound.
In general, when isobutylene-maleic anhydride copolymer is dissolved in water to form an aqueous solution, it is known that the dissolution time can be shortened by adding an appropriate amount of an alkali compound. For example, when added to an equimolar amount with respect to the carboxyl group in the isobutylene-maleic anhydride copolymer, the liquid can be easily adjusted, but there is a problem that the transparency of the coating film is lowered when the film is applied and dried. Moreover, the gas barrier property of the film obtained is also low. On the other hand, if the blending amount of the alkali compound is too small, not only will it take a long time to dissolve the isobutylene-maleic anhydride copolymer, but the stability of the solution will decrease, and precipitation and turbidity will occur during coating. It is not preferable. This is presumably because the compatibility between PVA and isobutylene-maleic anhydride copolymer and the stability of the aqueous solution change depending on the amount of the alkali compound added.
According to the study by the present inventors, the most preferable result is obtained when the pH of the liquid is adjusted to the range of 6 to 9, and the film obtained by applying the coating agent thus adjusted to the film and subjecting it to a drying heat treatment Has high transparency of the coating film and excellent gas barrier properties under high humidity.
The alkali compound is not particularly limited as long as it can neutralize the carboxyl group in the isobutylene-maleic anhydride copolymer. Examples of the alkali compound include alkali metal and alkaline earth metal hydroxides, ammonium hydroxide, and organic ammonium hydroxide compounds. Can be mentioned.
[0023]
Of these, alkali compounds having a boiling point of 200 ° C. or lower are preferable, such as ammonium hydroxide, primary amines such as methylamine, ethylamine, propylamine, and butylamine, and secondary amines such as dimethylamine, diethylamine, and methylethylamine. And tertiary amines such as trimethylamine and triethylamine.
[0024]
Most of the coating agent containing a low boiling point alkali compound is volatilized in the process of coating, drying and heat-treating the film, so that the carboxyl group activity of the isobutylene-maleic anhydride copolymer is increased and the crosslinking reaction is increased. It is considered that the gas barrier property is promoted and excellent as a result.
[0025]
Moreover, it discovered that the further outstanding gas-barrier property was obtained by mixing and mix | blending the alkaline-earth metal used as a bivalent metal ion as an alkali compound. This is because the carboxyl group of isobutylene-maleic anhydride copolymer is cross-linked by alkaline earth metal ions, and in addition to the cross-linking formation of PVA and isobutylene-maleic anhydride copolymer, a synergistic effect is expressed. Conceivable.
[0026]
As a method for adjusting the coating agent, a known method may be used using a melting pot equipped with a stirrer. For example, a method in which PVA and isobutylene-maleic anhydride copolymer are separately made into an aqueous solution and mixed before use is preferable. At this time, the alkali compound is preferably mixed with an aqueous solution of isobutylene-maleic anhydride copolymer. When a low boiling point compound is used as the alkali compound, it is preferably carried out in a closed system so that the alkali compound does not evaporate and change its composition when dissolved by heating.
What is important in the present invention is to adjust the pH of the finally mixed coating agent to a range of 6 to 9, which is the kind of alkali compound and the mixing of PVA and isobutylene-maleic anhydride copolymer. Since it also changes depending on the ratio, the blending amount must be selected appropriately.
[0027]
In the present invention, the gas barrier properties of the resulting film can be further improved by adding a small amount of a water-swellable layered inorganic compound such as vermiculite, montmorillonite or hectorite to the coating agent.
[0028]
The film of the present invention is prepared by preparing a mixed solution of PVA, isobutylene-maleic anhydride copolymer and an alkali compound, coating this on the surface of the film, drying by heating, and further heat-treating at a temperature of 150 ° C. or higher. Obtained by. A small amount of an organic solvent such as alcohol can be added to the coating agent for the purpose of increasing the solubility, shortening the drying process, improving the solution stability, and the like.
In order to improve the gas barrier property of the film of the present invention, it is necessary that a crosslinking reaction by an ester bond occurs between PVA and isobutylene-maleic anhydride copolymer. A catalyst such as can also be added. In this case, when ammonium hydroxide or the like is used as the alkali compound, most of the alkali compound is evaporated by drying and heating after coating, and the catalyst is activated.
[0029]
The thickness of the layer composed of PVA and isobutylene-maleic anhydride copolymer in the present invention is preferably 0.1 μm or more in order to sufficiently enhance the gas barrier property of the film.
The polymer concentration of the coating agent is appropriately changed depending on the viscosity and reactivity of the liquid and the specifications of the apparatus to be used, but is preferably in the range of 5 to 50% by mass of the entire solution. In a dilute solution, it becomes difficult to coat a layer having a sufficient thickness to exhibit gas barrier properties, and the subsequent drying process tends to take a long time. On the other hand, if the concentration of the solution is too high, problems may arise in the mixing operation, storage stability, and the like.
[0030]
The method for coating the film with the coating agent of the present invention is not particularly limited, and usual methods such as gravure roll coating, reverse roll coating, wire bar coating, and die coating can be used. The coating may be performed before the film is stretched or may be performed on the stretched film.
In order to perform coating prior to stretching, the film is first coated on an unstretched film and dried, and then supplied to a tenter-type stretching machine to simultaneously stretch the film in the running direction and the width direction (simultaneous biaxial stretching), or heat treatment, Alternatively, the film may be stretched in the running direction of the film using a multistage hot roll or the like, coated, dried, and then stretched in the width direction (sequential biaxial stretching) by a tenter type stretching machine. It is also possible to combine running direction stretching and simultaneous biaxial stretching with a tenter.
A method of coating prior to stretching, followed by stretching and heat treatment is a preferable method because a high temperature during stretching and heat treatment can be used for the crosslinking reaction.
[0031]
In the present invention, in order to cause a cross-linking reaction between PVA and isobutylene-maleic anhydride copolymer formed on one side or both sides of the film, it is necessary to perform heat treatment in an atmosphere at a temperature of 150 ° C. or higher, preferably 180 ° C. or higher. It is.
By setting the heat treatment temperature to 150 ° C. or higher, the crosslinking reaction can sufficiently proceed and a film having sufficient gas barrier properties can be obtained.
[0032]
The heat treatment may be performed in a hot air oven, or by a known method such as a method of bringing a film into contact with the surface of a hot roller or the like.
[0033]
[Action]
Although the action of the alkali compound in the coating agent is not clear, it is partially neutralized with a suitable amount of alkali, so that the solubility of isobutylene-maleic anhydride is improved and is more uniform with PVA in an aqueous solution. This is considered to be because the tightness of both polymers is maintained even after mixing and drying. Moreover, when there is too much quantity of an alkali compound, not only the compatibility of PVA and isobutylene-maleic anhydride will fall, but in the crosslinking reaction between both polymers, it reacts with a hydroxyl group, and also as a catalyst of the reaction It is considered that the activity of the acting carboxyl group is lowered and the crosslinking reaction is less likely to occur.
[0034]
【Example】
Next, the present invention will be specifically described with reference to examples.
[0035]
The gas barrier property was measured by measuring oxygen permeability in an atmosphere of 20 ° C. and 85% relative humidity with an oxygen barrier measuring device manufactured by Mocon.
[0036]
Reference example 1
As PVA, polyvinyl alcohol UF040G (degree of saponification 99.6%, average degree of polymerization 500) manufactured by Unitika Chemical Co. was dissolved in pure water to obtain a 10% by mass aqueous solution.
As isobutylene-maleic anhydride, Isoban 04 manufactured by Kuraray Co., Ltd. was dissolved in an aqueous solution containing 60 mol% ammonium hydroxide with respect to the carboxyl group to obtain a 10 mass% solution. Next, both aqueous solutions were mixed so that the mass ratio of PVA and isobutylene-maleic anhydride was 70/30, and stirred at room temperature to prepare a coating agent. The pH of the coating agent was 7.4.
This coating agent was coated on a biaxially stretched PET film (Embret PET12 manufactured by Unitika Co., Ltd., thickness 12 μm) with a Mayer bar so that the coating thickness after drying was about 2 μm, and 2 in a hot air dryer at 100 ° C. After drying for a minute, it was heat-treated in a hot air oven at 200 ° C. for 10 seconds.
The obtained film had an oxygen permeability of 101 ml / m ² · day · MPa.
[0037]
Comparative Example 1
The same operation as in Reference Example 1 was performed using only the PVA aqueous solution as the coating agent. The obtained film had an oxygen permeability of 450 ml / m ² · day · MPa and was insufficient in gas barrier properties for use in food packaging films.
[0038]
Reference Examples 2 to 5 , Example 1 and Comparative Example 2
A gas barrier PET film was obtained in the same manner as in Reference Example 1 except that the mixing ratio of PVA and isobutylene-maleic anhydride, the type and amount of the alkali compound were changed as shown in Table 1.
The oxygen permeability of the obtained film is shown in Table 1.
[0039]
Example 2
Using a biaxially stretched nylon 6 film having a thickness of 15 μm, a gas barrier nylon film was obtained in the same manner as in Example 1 .
The oxygen permeability of the obtained film is shown in Table 1.
[0040]
Example 3
As PVA, polyvinyl alcohol UF040G (degree of saponification 99.6%, average degree of polymerization 500) manufactured by Unitika Chemical Co. was dissolved in pure water to obtain a 20% by mass aqueous solution.
As isobutylene-maleic anhydride, Isoban 04 manufactured by Kuraray Co., Ltd. was dissolved in an aqueous solution containing 60 mol% ammonium hydroxide and 3% calcium hydroxide with respect to the carboxyl group to obtain a 20 mass% solution. Next, both aqueous solutions were mixed so that the mass ratio of PVA and isobutylene-maleic anhydride was 70/30, and stirred at room temperature to prepare a coating solution.
Next, the PET resin was extruded into a sheet at a cylinder temperature of 260 ° C. and a T die temperature of 280 ° C. using an extruder (75 mm diameter, L / D 45 slow compression type single screw) equipped with a T die, The film was brought into close contact with a cooling roll adjusted to a surface temperature of 10 ° C. and quenched to obtain an unstretched film having a thickness of 120 μm. Subsequently, the unstretched film was guided to a gravure roll coater, coated so that the coating thickness after drying was 20 μm, and dried in a hot air dryer at 80 ° C. for 30 seconds. Next, the film was supplied to a tenter simultaneous biaxial stretching machine, preheated at a temperature of 100 ° C. for 2 seconds, and then stretched at 95 ° C. at a magnification of 3 times in the machine direction and 3.5 times in the transverse direction.
Further, a heat treatment was performed at 200 ° C. for 15 seconds with a transverse relaxation rate of 5%, and after cooling to room temperature, the stretched film was wound up.
The oxygen permeability of the obtained film is shown in Table 1.
[0041]
Example 4
Nylon 6 resin was extruded into a sheet form at a cylinder temperature of 260 ° C and a T-die temperature of 270 ° C using an extruder equipped with a T-die (slow compression type single screw with a diameter of 75 mm and L / D of 45), and the surface temperature The film was brought into close contact with a cooling roll adjusted to 10 ° C. and rapidly cooled to obtain an unstretched film having a thickness of 150 μm. Subsequently, the unstretched film was guided to a gravure roll type coater, coated with the same coating liquid as prepared in Example 3 so that the coating thickness after drying was 20 μm, and dried in a hot air dryer at 80 ° C. for 30 seconds. did. Next, the film was supplied to a tenter simultaneous biaxial stretching machine, preheated at a temperature of 100 ° C. for 2 seconds, and then stretched at 170 ° C. at a magnification of 3 times in the machine direction and 3.5 times in the transverse direction. Next, heat treatment was performed at 200 ° C. for 15 seconds with a transverse relaxation rate of 5%, and after cooling to room temperature, the stretched film was wound up.
The oxygen permeability of the obtained film is shown in Table 1.
[0042]
[Table 1]
[0043]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, the thermoplastic resin film which has a high gas barrier property also under high humidity, and can be manufactured cheaply industrially is provided.

Claims (3)

A mixture of polyvinyl alcohol and isobutylene-maleic anhydride copolymer having a mass ratio of 97/3 to 20/80, and a mixture of an alkali compound having a boiling point of 200 ° C. or less and an alkaline earth metal hydroxide, and having a pH of 6 A water-based gas barrier coating agent that is ˜9. A gas barrier film obtained by forming a coating comprising the aqueous gas barrier coating agent according to claim 1 on at least one surface of a thermoplastic resin film. A method for producing a gas barrier film, comprising: applying the water-based gas barrier coating agent according to claim 1 to a thermoplastic resin film, and then performing a heat treatment at a temperature of 150 ° C or higher.