KR20040055101A - A Process for preparing Polyamide Film for Retort Pouch - Google Patents

Abstract

PURPOSE: Provided is a process for producing a polyamide film for a retort pouch excellent in a tensile strength, a heat resistance, and a shrinkage resistance, which can keep an excellent size-stability under the pressure and steam sterilizing condition. CONSTITUTION: The process for producing the polyamide film comprises the steps of: stretching a non-stretched polyamide film in the MD(machine direction) and TD(transverse direction) ratio of 2.8-3.5 times by a tubular method at 90-150deg.C, wherein the non-stretched polyamide film is obtained by using at least one or two polymer selected from the group consisting of nylon 4, nylon 6, nylon 7, nylon 8, and etc.; heat-treating the stretched film in a heat-fixing tower at 180-210deg.C before tentering; heat-treating the heat-fixed film in a tenter at 190-215deg.C.

Description

A process for preparing polyamide film for retort pouch

The present invention relates to a method for producing a polyamide film for a retort pouch. More specifically, ii) an unstretched film has a draw ratio in both directions of MD (machine direction) and TD (transverse direction) by a tubular film forming method. Stretching at 3.5 times; Ii) a method for producing a polyamide film for retort pouches (or pressurized steam sterilization) comprising the step of heat-treating the stretched film in a heat-setting tower before tenter and iii) heat-treating the heat-set film in tenter again. It is about.

Polyamide films are widely used as packaging materials for solid or liquid foods because of their excellent oxygen barrier properties, oil resistance, mechanical properties and thermal properties compared to other materials and excellent properties such as pinhole resistance at low temperatures. The polyamide film is used as a single layer film or as a laminate film laminated with a polyolefin-based film such as polystyrene and polypropylene, which is hygroscopic, in order to supplement the disadvantage of hygroscopicity. Recently, after fully-sealed foods are sealed in a retort pouch, the demand for a fully sterilized retort food is maintained by maintaining the taste and nutrition by autoclaving for a short time. The retort pouch is manufactured by laminating an outer layer of a thin film made of a polymer and an aluminum thin film. For example, CPP (Cast Polypropylene. Unstretched Polypropylene: inner surface) for high retort, which is the most severe sterilization condition, is manufactured. It has a four-layer structure consisting of / NY (nylon) / Al (aluminum) / PET (polyester: outer surface), in which case the polyamide film, which is an intermediate layer, increases gas barrier properties and sterilizes to prevent deterioration of food. It is inserted for the role of reinforcing pinhole resistance to prevent pinholes.

In general, polyamide films are excellent in heat resistance, but polyamide films for use in retort packaging materials that are subjected to pressurization and heat sterilization should have further improved heat resistance. In particular, high retort pouches are sterilized under pressure at a heating temperature of 130 to 140 ° C. for several minutes, so that they can withstand 1) very good pinhole resistance, 2) have excellent heat sealing properties and strong sealing area, 3 4) Hygienic and safe, and 4) Heat treatment time is shorter than cans, so the barrier property should be satisfied so as not to be damaged. The polyamide film that enters the intermediate layer of the retort pouch must have excellent barrier properties, mechanical properties such as tensile strength and moisture resistance, and must be stretched to give a high level of crystallinity. The shrinkage and anisotropy should be small in order to maintain the basic shape during processing and heat sealing and to prevent rupture or distortion of the envelope.

In particular, in order to prevent deformation such as shrinkage and curling even under autoclaved conditions, further improved heat resistance and shrinkage / strain resistance are required.

However, in the case of the stretched film, the shrinkage stress remaining due to the stretching and the relocation tendency above the Tg due to the fluidity of the polymer chains inevitably shrinks at a high temperature, especially under severe conditions such as pressurized steam sterilization used in the production of retort food. This will occur. Such shrinkage of the polymer film used in the retort pouch is a fatal defect because it causes deformation or breaking of the pouch. Therefore, the polyamide film used for the intermediate layer or the outer layer in the retort pouch should have excellent heat resistance and shrinkage resistance while maintaining excellent mechanical strength and the like.

In order to impart excellent heat resistance and shrinkage resistance, Korean Laid-Open Patent Publication No. 2002-0052481 aims to improve heat resistance and shrinkage resistance by removing shrinkage stress through a decrease in elongation of a film. By reducing the draw ratio of the amide film to 2.35 to 2.8 times and raising the temperature of the heat treatment heater roll to 160 to 200 ℃ to prepare a heat-resistant polyamide film with somewhat improved dimensional stability. However, in the above case, the polyamide copolymer is used to enhance the heat resistance and the drawing ratio is reduced, thereby improving the shrinkage resistance. However, the tensile strength of the final biaxially stretched polyamide film is significantly lower than that of the general polyamide film.

Accordingly, in the technical field, a polyamide film for retort pouches having excellent tensile strength and improved heat resistance and shrinkage resistance, no shrinkage in strength even after autoclaving, no shrinkage of the pouch (curling), and no shrinkage and excellent dimensional stability. There has been a need for a method that can produce.

MEANS TO SOLVE THE PROBLEM The present inventors earnestly researched and produced the polyamide film for retort pouches excellent in tensile strength, heat resistance, and shrinkage property by achieving the said objective, and extending | stretching and extending | stretching in the same ratio as a general polyamide film using the tubular film forming method. After heat-treatment in the tenter before and after the heat treatment in the tenter and heat-treat it in the tenter, it maintains high tensile strength and greatly improves heat resistance and shrinkage resistance to maintain excellent dimensional stability under severe conditions such as pressure steam sterilization. It has been found that polyamide films can be produced and the present invention has been reached.

As a result, the present invention is excellent in tensile strength, heat resistance and shrinkage resistance is improved, there is no decrease in strength even after pressure sterilization and shrinkage is suppressed without bending phenomenon (curling) of the pouch to produce a polyamide film for retort pouches excellent in dimensional stability And a polyamide film accordingly.

One aspect of the present invention for achieving the above object is the step of stretching the unstretched polyamide film by the tubular film forming method in the bidirectional draw ratio of MD (machine direction) and TD (transverse direction) of 2.8 to 3.5 times ; Ii) heat-treating the stretched film in a heat-setting tower before tenter; and iii) heat-treating the heat-set film in tenter again.

Another aspect of the present invention relates to a polyamide film having excellent tensile strength, heat resistance and shrinkage resistance produced according to the above production method.

Hereinafter, the present invention will be described in detail step by step.

Iv) Bidirectional stretching of the film by the tubular film forming method:

The unstretched polyamide film is stretched by a tubular film forming method with a draw ratio in both directions of MD (machine direction) and TD (transverse direction) of 2.8 to 3.5 times.

The unstretched polyamide film used in the present invention is nylon 4, nylon 6, nylon 7, nylon 8, nylon 11, nylon 12, nylon 6.6, nylon 6.9, nylon 6.10, nylon 6.11, nylon 6.12, nylon 6T, nylon 6 /6.6, nylon 6/12, nylon 6 / 6T, nylon 6T

( terephthalic acid) / 6I (isophthalic acid), and one or more polymers selected from the group consisting of nylon MXD6. Preferably, nylon 6 is used, and when introducing an aromatic nylon such as MXD6, further heat resistance can be achieved.

The viscosity of the polymer used in the present invention is in the range of 2.8 to 3.6, preferably 3.0 to 3.4, as measured by a 1% by weight polyamide resin solution of 95% sulfuric acid at 25 ° C. for stable bubble formation. It is preferable to prepare a polyamide chip for final film production by adjusting the content of unreacted lactam to 1% or less and preferably 0.5% or less through an extraction process after chip polymerization.

The polyamide chip thus prepared is introduced into an extruder heated to 240 ° C. to 270 ° C., melt kneaded, and quenched with a cooling air or cooling water to melt the polyamide melt discharged through a circular die to prepare an amorphous unstretched fabric.

After passing through the extruder and melting, a biaxially stretched film can be formed by the tubular method.

In the tubular method, nylon melt discharged from a circular die is generally quenched in a water tank to form an unoriented, amorphous unstretched fabric (first bubble), and transverse stretching (TD) by air pressure during the stretching tower is performed. It is subjected to longitudinal stretching (MD) by the new line speed (second bubble).

Since this invention uses the above-described tubular method, it has excellent physical property uniformity over the whole width, and can suppress anisotropy and bowing property to the maximum.

In more detail, in the tubular film forming process according to the present invention, the unstretched fabric discharged through the circular die is stretched at a draw ratio of 2.8 to 3.5 times and preferably 3.0 to 3.3 times for both MD and TD in the drawing tower. In general, when the shrinkage stress is reduced to improve the heat resistance, the draw ratio is decreased. However, when the draw ratio is low, the molecular orientation is poor, resulting in a decrease in tensile strength and gas barrier properties. In the present invention, by adopting the same draw ratio as the general polyamide film, it prevents the deterioration of strength and gas barrier property, and instead achieves the two objectives of maintaining strength and eliminating shrinkage stress by adding a separate heat treatment step and performing double heat treatment. can do. The stretching is carried out in a stretching section set at a temperature of 90 ° C to 150 ° C after preheating at around 60 ° C, and MD stretching by the speed difference between the nip rolls and TD stretching by the air pressure of the bubbles are simultaneously performed.

When stretched under the above conditions, the orientation crystallization of the film is promoted, and not only the removal of shrinkage stress but also an additional increase in crystallinity can be achieved in the heat treatment described later.

Ii) heat setting step in the heat setting tower:

The film stretched through the stretching tower is subjected to a first heat treatment in a (third bubble) bubble state in a heat setting tower installed before the tenter.

The first heat treated film as described above is subjected to the second heat treatment again in the tenter which will be described later. The use of such a triple bubble system with enhanced heat treatment not only minimizes the difference in physical properties for each part by enhancing the thermal stability and uniformity of physical properties of the film, but also has the advantages of greatly improving dimensional stability and bowing properties.

The primary heat treatment is to remove the shrinkage stress generated by the forced stretching, and in the present invention, in addition to the conventional tenter single heat treatment, the heat treatment is performed uniformly and effectively by the double heat treatment by performing a uniform heat treatment in a bubble state in the heat-setting column. The shrinkage stress can be eliminated.

The temperature of the heat-setting column is 180 ° C to 210 ° C, preferably 190 ° C to 200 ° C, and the heating method adopts dual heating using an infrared heater and hot air in that it heats the entire film uniformly. desirable. Unlike the tenter heat treatment, the bubble state heat treatment in the heat-setting column has an additional advantage that can effectively suppress problems of inequality and bowing and anisotropy, which are inevitably entailed during the tenter process, because there is no distinction between the edge and the center. have.

Iii) heat treatment in tenter:

The first polythermally bubbled polyamide film is heat-treated again in a tenter. More specifically, both ends of the bubble-shaped polyamide film are slitted and separated into two films, and then heat-treated in a tenter step to be wound in a mill roll winding roll.

At this time, the temperature of the tenter section is 190 ℃ to 215 ℃, preferably 200 ℃ to 210 ℃ for effective removal of residual shrinkage stress, by reducing the width of the stretched film

(Small treatment) further removed the shrinkage stress. In the narrow treatment, the relaxation rate obtained by the following formula is adjusted to 2% to 5%:

Relaxation rate = {film initial width (before passing tenter) -film final width (after passing tenter)} / film initial width (before passing tenter) × 100.

When the heat treatment temperature is lower than 190 ° C, the removal of residual stress is not sufficient, and when the heat treatment temperature is higher than 215 ° C, process problems such as film breakage occur due to excessive heating conditions. In addition, when the relaxation rate is lower than 2%, it is difficult to effectively remove the shrinkage stress, and when the relaxation rate is higher than 5%, process problems such as film sagging and fusion are not preferable.

After the heat treatment in the tenter, the final polyamide film is cut into square (20 cm × 20 cm) shapes and immersed in 100 ° C. hot water, so that the shrinkage measured in the film advancing direction (MD) and the width direction (TD) is 0.5% to 2%. . When the shrinkage rate exceeds 2%, the shrinkage or deformation of the film may be caused during the pressurized steam retort sterilization process, and when the shrinkage rate is less than 0.5%, the heat treatment conditions may be excessively severe, and the problem of bowing in the film may be exacerbated.

By the above method according to the present invention, it is possible to produce a polyamide film for a retort pouch which minimizes dimensional deformation due to heat shrinkage and suppresses a decrease in mechanical properties. The polyamide film produced can be used as a film constituting the retort pouch, according to a conventional method.

EXAMPLE

Hereinafter, the structure and effect of the present invention will be described in more detail with specific examples and comparative examples, but these examples are only intended to more clearly understand the present invention, and are not intended to limit the scope of the present invention.

The properties of the films obtained in Examples and Comparative Examples were evaluated by measuring the heat shrinkage and tensile strength of each obtained polyamide film by orientation.

Examples 1 to 3

The polyamide 6 chip for film having a relative viscosity of 3.4 polymerized from caprolactam was introduced into an extruder heated at 260 ° C., melted and extruded through a circular die, and then cooled and solidified in a water bath to prepare an unstretched fabric. The unstretched fabric thus prepared was preheated at 60 ° C. and stretched three times in the TD direction by air pressure in a bubble state in a drawing tower set at 120 ° C., and three times in the MD direction by a nip roll speed difference. Subsequently, according to the heat treatment setting conditions as shown in Table 1, the first heat treatment was performed in the heat setting tower, and the film was separated into two sheets, followed by the second final heat treatment in the tenter. Thus formed biaxially stretched polyamide film was wound in a mill roll, and the tensile strength and thermal shrinkage of the final film were measured to evaluate the strength and dimensional stability, and the results are shown in Table 2.

Here, tensile strength was measured according to ASTM D-882 at 23 ° C. and 60% relative humidity, and thermal shrinkage was measured after shrinking a 20 cm × 20 cm square-shaped film in 100 ° C. boiling water for 30 minutes.

Comparative Examples 1 and 2

The polyamide 6 chip for film having a relative viscosity of 3.4 polymerized from caprolactam was introduced into an extruder heated at 260 ° C., melted and extruded through a circular die, and then cooled and solidified in a water bath to prepare an unstretched fabric. The unstretched fabric prepared as described above was preheated at 60 ° C. and stretched three times in the MD direction by TD stretching by air pressure in a bubble state and a nip roll speed difference in a stretching tower set at 120 ° C. Then, as shown in Table 1, the heat treatment process was performed only in the heat setting tower while changing the heat treatment setting conditions (Comparative Example 1), or the heat treatment was performed only in the tenter without the heat setting tower step (Comparative Example 2).

Production condition Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Preheating temperature (℃) 60 60 60 60 60 Drawing temperature (℃) 120 120 120 120 120 Draw ratio 3 × 3 3 × 3 3 × 3 3 × 3 3 × 3 Heat-fixing tower temperature (℃) 200 200 200 200 - Tenter temperature (℃) 205 207 210 - 210 Relaxation rate (%) 3.5 3.5 3.5 3.5

Production condition Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Tensile Strength (Kgf / cm ² ) MD 2,400 2,400 2,500 2,400 2,500 TD 2,300 2,400 2,500 2,300 2,400 % Elongation 130 135 130 110 120 Modulus (Kgf / cm ² ) MD 14,000 14,000 13,500 14,000 14,000 TD 14,000 13,500 13,500 13,500 14,000 Shrinkage (%) MD 2.0 1.5 1.0 5.0 4.7 TD 1.7 1.0 0.5 5.0 4.2 Boeing Province Good Good Good Great Bad

As can be seen in Table 2, in Examples 1 to 3, the heat shrinkage was reduced to 1/10 or less while maintaining the strength of the polyamide film produced by the conventional one-step heat treatment, thereby showing excellent dimensional stability. On the other hand, in the case of Comparative Example 1, the bowing property is excellent, but the heat shrinkage rate is excessive. Also, in the case of Comparative Example 2, the strength is excellent, but the heat shrinkage rate is excessive and the bowing property is poor. It is difficult to apply as a film for retort sterilization.

According to the present invention, a heat-resistant polyamide film having excellent dimensional stability can be obtained by suppressing shrinkage without deterioration in strength even after autoclaving and without warping (curling) of the pouch. In particular, through the heat treatment of the bubble state through the heat-setting column, it is possible to effectively suppress the difference between the physical properties of the edge and the center, the problems of bowing and anisotropy, and to obtain a film having uniform properties for each site.

Claims (5)

Iii) stretching the unstretched polyamide film by a TULA film forming method at a draw ratio of 2.8 to 3.5 times in both directions in the machine direction (MD) and the transverse direction (TD); Ii) heat-treating the stretched film in a heat-setting tower before tenter; and iii) heat-treating the heat-fixed film in a tenter again. According to claim 1, wherein the non-stretched polyamide film of step iii) nylon 4, nylon 6, nylon 7, nylon 8, nylon 11, nylon 12, nylon 6.6, nylon 6.9, nylon 6.10, nylon 6.11, nylon 6.22, Method for producing polyamide film, characterized in that obtained by using one or more polymers selected from the group consisting of nylon 6T, nylon 6 / 6.6, nylon 6/12, nylon 6 / 6T, nylon 6T / 6I, and nylon MXD6 . The heat treatment of the heat-setting column in step ii) is performed at a temperature of 180 ° C to 210 ° C, and in step iii). The heat treatment of the tenter phase is carried out at a temperature of 190 to 215 ℃ polyamide film production method. The polyamide film production method according to claim 1 or 2, wherein the tenter phase heat treatment in step (iii) is performed at a relaxation rate of 2% to 5%. A polyamide film produced by the process according to claim 1.