JPH0323097B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention can be used in the stretch packaging field.
This invention relates to vinylidene chloride film.

[Prior art]

When wrapping an article using a film, the article is wrapped in a state in which tension is applied to the film so that it is slightly stretched, and the article is tightly wrapped using the stretch recovery force generated in the film, which is called stretch packaging. Packaging methods are known.

This packaging method is mainly used at food counters such as supermarkets, where the contents of fresh foods, cooked foods, etc. are divided into containers such as trays and sold in large quantities in quantities that are easy for consumers to purchase and use economically. It is widely used for the exterior of packaging containers.

In addition, the film properties that can be used for this purpose are that it firstly stretches well under moderate stress, and then has so-called stretchability (stretching recovery ability), which allows it to instantly recover from its stretching when the stress is released. In addition, transparency is required so that the contents can be clearly seen.

Therefore, among single-layer films with these characteristics, polyvinyl chloride stretch films have a monopolistic market share, and some special olefin-based multilayer films [for example, the product name (Santech S) Film) Asahi Kasei five-layer film]
etc. are known to exist.

[Problem that the invention seeks to solve]

However, the single-layer vinyl chloride film, which is the stretch film on the market mentioned above, has poor gas barrier properties and poor water vapor barrier properties, which causes the content to sag (weight decreases due to water evaporation). On the other hand, stretch films with a multilayer structure can largely eliminate the sagging phenomenon, but the multilayer structure with a special composition of ingredients The increasing cost of stretching and the narrowing of the applicable range of stretchability have become unavoidable problems.

The present invention solves the above-mentioned problems of stretch films based on a completely new idea of whether it is possible to impart stretchability to a vinylidene chloride resin layer with high gas barrier properties.
The purpose is to provide a novel stretch film.

[Means for solving problems]

A single layer or a laminated film of 3 or less layers in which the vinylidene chloride/methyl acrylate copolymer resin layer containing 7 to 20% by weight of the methyl acrylate component accounts for 80% or more of the total thickness of the film, and the total thickness is 5 to 20% by weight. Micron stretched film is used for stretch packaging applications.

[Method for manufacturing the film of the present invention]

The contents of the present invention will be explained in detail below.

First, an example of the method for producing the film of the present invention will be introduced from the viewpoint of deepening understanding of the film used in the present invention. FIG. 1 is a conceptual diagram of an example of an apparatus that can employ special film-forming conditions as a stretching film-forming method for a vinylidene chloride resin film, which is convenient for obtaining the film of the present invention.

To introduce the manufacturing method in FIG. 1, the resin A supplied from the hopper 1 into the extruder 2 is heated, melt-mixed, and temperature-controlled, and is then transferred to the extruder 2 by the thrust of the screw 3. It is extruded into a cylindrical shape from the slit part of an annular die 4 attached to the tip of the cylindrical parison A'. This parison
A' is rapidly cooled from the inside and outside to a low temperature of 20℃ or less with refrigerants 5 and 6, reversed with pinch rolls 7 and 7', and heated to about 30℃ in the preheating zone between pinch roll groups 8 and 8' and 9 and 9'.
Warmed to ℃. The current process is not much different from the conventional vinylidene chloride resin film forming method. The problem is the subsequent inflation biaxial stretching with pinch roll groups 10, 10' and 11, 11', and the subsequent pinch roll groups 17, 17', 18, 18', 19, 1.
This is due to the combination of the shrinking process performed between 9' and 9'.

First, the biaxial stretching in the previous step is performed at a larger stretching ratio of 5 to 6 times in the vertical direction and 5 to 6 times in the horizontal direction, compared to the conventional vinylidene chloride-vinyl chloride copolymerization. Since the bubble shape is not sufficiently fixed due to resin crystallization in the stretching orientation, it is not easy to fold the inflation bubble as it is, as it is accompanied by a substantial puncture phenomenon.

The inventors of the present invention believe that the causes of this phenomenon are mainly due to the following two reasons, and have added countermeasures to the equipment conditions.

In other words, the first cause of the phenomenon when stretching the target resin.
One is that the tension in the part of the film where stretching has started and its thickness has become thinner is smaller than the tension in the part of the film at the starting point of stretching, so the stress of parison stretching is reduced to one point. Therefore, the longitudinal stretching ratio, which is given by the speed ratio of the roll group, increases the film's diameter in the direction of increasing the bubble diameter by the proportion that the stretching point is at a fixed location and does not push the longitudinal stretching. This contributes to thinning of the wall and creates the phenomenon of punctures. Another reason is that the film portion that has been stretched to a predetermined level has a strong elastic recovery force that causes it to contract and return to its pre-stretched state.
As a result, when the inflation bubble comes into contact with a deflator, etc. that tries to fold it, and resistance is generated there, it creates an uneven tension in the circumferential direction of the film, and that area becomes uneven in the form of a difference in wall thickness. It was considered that this would further promote uniformity and lead to the phenomenon of flat tires. Therefore, as a countermeasure, before inflating the parison, hot air is blown into the resin to warm it so that stretching begins with low stress, and cooling rings 16, 16', 16"..., and the film that has been thinned as the stretching progresses is cooled so that the film tension increases sequentially. Furthermore, the roll groups R1, R2, which constitute the deflator are cooled. R3, R4... are all drive type roll groups that can adjust and change the circumferential speed, and R4
The circumferential speed of is approximately 1, but the speed of the R1 side is increased by 13 to 16%, and the circumferential speed ratio in the process from R1 to R4 is assumed to be a deflator with a group of rolls that gradually decreases. The following film is forcibly rolled in and folded into pass rolls 12, 12', 12''...
After careful consideration to prevent rapid contraction,
Multi-stage heat treatment zones 17, 17', 18, 1
It is sent to a group of pinch rolls 8', 19, and 19'. In this shrinking process, gas is again injected into the cylindrical film that has been folded once, and the cylindrical film is heat-set in the inflated state.
The diameter of the bubble and the circumferential speed ratio of the pinch rolls are adjusted stepwise so that the contraction occurs by ~20% and about 10~25% in the lateral direction. 21 and 24 are devices for feeding heated gas, and 20 and 23 are heating devices. Furnace, 26 is a cooling device, 2
Reference numerals 8 and 28' denote winding shafts for the final film, and the film of the present invention is completed here. The vinylidene chloride resin film stretched in the above-mentioned manufacturing process exhibits characteristics that are slightly different from conventional commercially available vinylidene chloride resin films, due to the difference in the composition of its raw materials.

FIG. 2 shows the deformation stress (Kg/mm ² ) on the vertical axis and the deformation strain (%) on the horizontal axis, showing the relationship between the deformation stress applied to the film and the deformation strain generated in the film. In the figure, the solid line represents the stretched film of the present invention, and the broken line represents a conventionally known (commercially available) vinylidene chloride resin film (vinyl chloride content: 20%).

According to the results shown in Figure 2, commercially known films break at approximately 50% deformation strain (marked with X), whereas the stretched film of the present invention exhibits a large amount of strain depending on the deformation stress. When the deformation stress increases and the stress is released within the elastic limit of 2 kg/mm ² or less, the amount of strain recovers greatly, and the degree of recovery is more than 60% of the amount of strain (as indicated by the dotted line). part), that is, it has been shown to have stretching aptitude.

In the film of the present invention, the vinylidene chloride resin containing 7% by weight or more of methyl acrylate component has the above-mentioned stretchability at a practical level in combination with the above-mentioned film forming method. The higher the amount of this methyl acrylate component, the better the plasticizing fluidity of the resin during extrusion processing, but the extruded parison has more rubber elastic elements and the temperature range suitable for stretching film formation is narrower. It is difficult to increase the amount beyond 20% by weight because this will deteriorate the film forming properties and the gas barrier properties of the resulting film.

Ease of obtaining stretchability through extrusion film formation,
The amount of methyl acrylate component is desirably selected from the range of 9 to 15% by weight from the viewpoint of sufficiently satisfying both gas barrier properties of the obtained stretch film. Also, when adjusting the amount of methyl acrylate component at this time, it is better to mix high and low component amounts to reach the target value than if the target value was set from the beginning. Therefore, it tends to have superior extrusion film formability.

Utilizing this principle, the present invention has been achieved by reducing the content of liquid plasticizer, which has conventionally been necessary for plasticizing resin during extrusion processing, to a low level of 3% by weight or less. The film has a barrier value of 10 to 50 c.c./m ² in O ₂ TR (oxygen transmission rate).
Value at 24Hr.1 atm 20℃ 10μ, WVTR (water vapor transmission rate) 5 to 30g/ ^m2 , 24Hr, 38℃ 90%RH value
It can be made into 10μ.

As has been clarified above, the film properties of the film of the present invention, such as gas barrier properties and stretchability, are obtained when the vinylidene chloride/methyl acrylate copolymer resin, which is a specific component, is stretched under certain stretching conditions. , and is comprised of a vinylidene chloride resin film.

Therefore, when the vinylidene chloride resin film is used in a single layer state, it is the simplest and most economical film, and its properties should be exhibited to the maximum extent.

However, in reality, a single layer that should be economical is not necessarily the best. This is because the types of properties that a single layer film can have do not necessarily satisfy the types of properties required for the intended use. For example, the surface of a vinylidene chloride resin film lacks the property of being sticky and self-adhesive compared to the surface of an ethylene vinyl acetate copolymer resin, and also lacks the sealing ability of making surfaces adhere to each other by heating. Therefore, it is possible to provide another resin layer on one or both surfaces of a vinylidene chloride resin film within a range of thickness that does not impair the stretchability of the film and utilize the surface characteristics of the provided resin. This is the scope of the technical idea of the present invention, which has succeeded in imparting stretchability to a vinylidene chloride resin film for the first time.
In the claims of this specification, the meaning described as a laminate film having three or less layers means the above-mentioned meaning.

The thickness of films used in the stretch packaging field generally ranges from 5 to 20 microns, most often from 8 to 15 microns. The vinylidene chloride resin film layer accounts for 80% of the total thickness.
% or more, preferably 85% or more. This limitation is the limit to which the stretchability of the vinylidene chloride resin can be fully utilized.

On the other hand, when stretching is carried out in a laminated state, it is desirable to use a resin type that easily imparts stretchability to the vinylidene chloride resin layer when laminated and stretched. From this point of view, the type of partner resin selected is preferably ethylene-vinyl acetate copolymer resin, polyurethane resin, polybutadiene, chlorinated polyolefin, ethylene-ethyl acrylate copolymer resin, etc. Among these, those with a melt index of 1 to 10 g/ Flexible ones in the 10 minute range are more advantageous.

[Effects of the present invention]

It is the first to impart stretchability to a vinylidene chloride resin film, and has significantly improved barrier properties compared to the currently used vinyl chloride film, contributing to suppressing loss of content and improving storage stability. Furthermore, compared to multilayer films with complex compositions and configurations, the same multilayer structure is simple and much more cost-effective, so it can be widely used as a barrier film suitable for stretch packaging.

Example-1 Vinylidene chloride-methyl acrylate copolymer resin containing 12% by weight of methyl acrylate component was
A diameter of 150 mm installed at the tip of an extruder with a single flight type screw of 90 mm L/D = 22.
The mixture was kneaded and melted through an annular die with a slit of 1.5 mm at a maximum temperature of 180°C and extruded into a cylindrical shape.

A tank filled with water whose temperature has been adjusted to 9°C is provided at the bottom of the die tip to maintain a distance of 6 cm between the bottom of the die and the water surface, and a refrigerant 5 is sealed in the cylindrical resin. , 7' to obtain a rapidly cooled parison with a diameter of 120 mm and a thickness of 240 μm.

Pinch roll 8, 8'-9,
The mixture was heated through a 9' room and heated by showering with water at 30°C. Then, this parison was sent out and passed continuously through pinch rolls 10, 10', and heated air at 40°C was blown into the hot air oven 13 through the hot air ring 14 to heat the resin to 40°C. cooling ring 16, 16',
Blow out cold air at a temperature of 15℃ from 16″ and 16, inject air into the parison, and adjust the longitudinal stretching ratio.
5.5, and the transverse stretching ratio was 5.5. At this time, the circumferential speed of the deflator roll R1 is the take-up pinch roll 1.
1,11', R2 was 1.09 times, R3 was 1.03 times, and R4 was 0.97 times. Next, the film taken out from the pinch rolls is passed through pinch rolls 17, 17' to 18, 18', 19, 19', and air is again injected into the cylindrical film between the pinch rolls to inflate it. and from 24
Blow out heated air at 40°C and 50°C to heat furnace 2.
The film was heat-treated by heating 0.23. Also, just before the pinch rolls 19, 19', cold air was blown out from the cooling device 26 to cool the film after the heat treatment. At this time, pinch rolls 18, 18', 19, 1
9' was gradually decreased from the circumferential speed of the pinch rolls 17, 17', and the amount of air injected into the film was adjusted to achieve the target shrinkage rate.
After shrinking by %, the edges are slit and the film is divided into two films and wound around the winding shafts 28, 28' to reduce the thickness.
A film of 11μ was obtained.

The behavior of the deformation stress and deformation strain of the obtained film is shown in Figure 2. The breaking stress is 3.1Kg/mm ² , the elongation at break is 350%, and the elastic recovery rate when the deformation stress is 2Kg/mm ² is as follows. 70%, indicating that the film had excellent stretchability.

To measure the elastic recovery rate, measure the film with a width of 1 cm and a length of 10 cm.
Cut it out, Tensilon (manufactured by Toyo Baldwin)
UTM--100 type) gripping length 5cm, pulling speed
It is pulled at 1000mm/min and when the deformation stress reaches 2Kg/ ^mm2 , it is returned at the same speed and the amount of return until the stress becomes 0Kg/ ^mm2 is the ratio to the elongation when the deformation stress is 2Kg/ ^mm2 . It's what I asked for.

Example-2 Vinylidene chloride/methyl acrylate copolymer resin containing 12% by weight of methyl acrylate and ethylene/vinyl acetate copolymer resin [Product name: Ultrasen UE634 manufactured by Toyo Soda, vinyl acetate component 26%, melt index 4g/10 ], and the former is 90mm in diameter.
The one with a screw of L/D=22, the latter has a diameter
An extruder with 40mmL/D=24 screws,
The cylinder parts are kneaded and melted at maximum temperatures of 180°C and 190°C, respectively, and then merged in a circular two-layer, three-layer die attached to the tip of the extruder and extruded into a cylinder. - A parison made of methyl acrylate copolymer resin/ethylene vinyl acetate copolymer resin and having a thickness ratio of 5%/90%/15%, a diameter of 120 mm, and a total thickness of 280 μm was cooled in the same manner as in Example 1. For stretching, heated air at 60℃ was blown, the parison temperature was heated to 60℃, and the longitudinal stretch ratio was 5.7 and the transverse stretch ratio was 5.7.
The process was the same as in Example 1 except that the biaxial stretching was carried out, and the heat treatment was carried out at 60°C and
A film having a thickness of 12 μm was obtained by shrinking the film by 10% in the vertical direction and 20% in the horizontal direction in the same manner as in Example 1 except that heated air at 70° C. was blown out. The behavior of the deformation stress and deformation strain of the obtained film is that the stress at break is 3.3Kg/mm ² and the elongation at break is 330.
%, and the elastic recovery rate when the deformation stress was 2 Kg/mm ² was 65%. Even when vinylidene chloride/methyl acrylate copolymer resin is laminated with other resins, the vinylidene chloride/methyl acrylate copolymer resin layer accounts for a high proportion, so it has a high elastic recovery rate and elongation similar to a single layer film. It was a film with excellent stretchability.

[Brief explanation of drawings]

FIG. 1 shows an example of a convenient process diagram for making the film of the present invention, and FIG. 2 shows a characteristic diagram of the film of the present invention. A... Raw resin, A'... Parison, 1... Hopper, 2... Extruder, 3... Screw, 4...
...Die, 5...Refrigerant, 6...Refrigerant, 7,7'...
Parison take-up pinch roll, 8,8'...pinch roll, 9,9'...pinch roll, 10,1
0'...Parison delivery pinch roll, 11,
11'...Stretched film take-off pinch roll, 1
2, 12', 12'', 12...Stretched film pass roll, 13...Parison heating furnace, 14...Hot air ring, 15...Hot air, 16, 16', 16'', 1
6... Cold air ring, 17, 17', 18, 1
8', 19, 19'... Heat treatment device pinch roll,
20, 23... Heating furnace, 21, 24... Hot air ring, 22, 25... Hot air, 26... Cooling device, 2
7... Cold air, 28, 28'... Winding shaft, R1-R
4... Drive deflator.

Claims (1)

[Claims] 1 A single-layer film or a laminated film of 3 or less layers in which vinylidene chloride-methyl acrylate copolymer resin containing 7 to 20% by weight of methyl acrylate component accounts for 80% or more of the total thickness of the film, and the total thickness is 5 to 20% by weight. A stretch packaging film characterized by being a micron stretched film.