JPH0363123A - Mixed resin low density foam and manufacture thereof - Google Patents

Abstract

PURPOSE:To make it possible to obtain foam having good modulus of elasticity by a method wherein balloon is formed by extruding mixture, which is prepared by blending high density polyethylene, low density polyethylene and polystyrene- based resin, the melt flow rate of which lies within the specified range, at specified blending ratio, together with low boiling point foaming agent through a circular die. CONSTITUTION:Mixed resin consists of 20-60wt.% of high density polyethylene, 20-40wt.% of low density polyethylene and 20-40wt.% of polystyrene-based resin. When foaming agent mainly made of butane is used, the enough amount to be mixed necessary for obtaining the foam, the density of which is less than 0.05g/cm<3>, is normally 10-20 pts.wt. to 100 pts.wt. of the mixed resin. Concretely, the mixed resin and the foaming agent are melted and kneaded in the pressurizing cylinder of an extruder so as to be extruded from the circular die of the extruder into open air in the form of balloon in order to obtain sheet-like foam by being passed between rolls for contact bonding the inner surface of the balloon. Thus, the foam having favorable bending rigidity, which depends upon the suitable thickness and bending modulus, is realized.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a mixed resin low-density foam made of a polyolefin resin and a polystyrene resin and a method for producing the same, and in particular to a cushioning packaging material for easily damaged fruits, etc. The present invention relates to a foam useful as a foam and a method for producing the same.

[Prior art and its problems]

BACKGROUND ART Conventionally, low-density polyethylene foam produced by extruding a foam together with a foaming agent from a circular die has been widely used as a cushioning packaging material for fruits and the like that are easily damaged. However, this type of low-density polyethylene foam has low bending rigidity and poor shape retention.

It has the disadvantage of being difficult to handle during packaging work. The reason for this is that due to the manufacturing process, it is only possible to manufacture products with a maximum thickness of about 6 m, and the flexural modulus is low.

A common method for obtaining a thick sheet is to form a balloon from a foam extruded from a circular die together with a foaming agent, and then pass the foam between rolls and press the inner surface of the balloon. Since low-density polyethylene foam has a slow crystallization speed, the above method allows the inner surface of the balloon to be crimped and a thick sheet can be obtained, but uneven thickness and corrugation are likely to occur, and a thick sheet with a uniform thickness cannot be obtained. I couldn't get it.

Therefore, in order to create a cushioning packaging material with excellent cushioning properties and bending rigidity, we used secondary processing to bond low-density foams together or to materials with high bending rigidity such as cardboard or foamed polystyrene sheets and low-density polyethylene foam. Packaging material is used.

However, since such packaging materials are manufactured through secondary processing, they have the drawback of high cost.

Foamed products made of high-density polyethylene alone are also known, but good results cannot be obtained unless fluorocarbon gas is used as a foaming agent, which poses the problem of ozone layer depletion due to fluorocarbon gas.

In addition, the polystyrene resin and high density polyethylene should be used in a ratio such that the total amount thereof is 10 to 60 parts by weight with respect to 100 parts by weight of the low density polyethylene, and the polystyrene resin should be a combination of the polystyrene resin and the high density polyethylene. It is also known to melt and knead together with a foaming agent under pressure at a ratio of 6 to 40 weight 2 based on the body weight, and then transfer the mixture to a low pressure zone to form a foam (Japanese Patent Publication No. 51-41149
Publication No.).

Since the mixed resin foam obtained by this method contains high-density polyethylene and polystyrene resin, it exhibits a better flexural modulus than low-density polyethylene foam.However, even with the above composition, the thickness is still uneven. This is because thick sheets with uniform thickness cannot be obtained due to the occurrence of corrugations, etc.

A cushioning packaging material with sufficient bending rigidity cannot be obtained.

[Problem of invention]

The present invention overcomes the drawbacks seen in the conventional cushioning packaging materials and provides a foam suitable as an inexpensive cushioning packaging material that has good bending rigidity due to suitable thickness and flexural modulus, and a method for producing the same. The task is to do so.

[Means to solve the problem]

As a result of extensive research to solve the above problems, the present inventors have discovered that high density polyethylene, low density polyethylene, and polystyrene resin having a specific melt flow rate (hereinafter abbreviated as MFR) are combined in a specific ratio. When using a mixed resin blended with , it is melt-kneaded together with a low boiling point blowing agent under pressure, then formed into a balloon using a circular die, transferred to a low-pressure zone, and passed between rolls and crimped on the inner surface of the balloon. The present inventors have discovered that a foam suitable for use as a cushioning packaging material can be obtained, having a thickness (6 mm or more) and a suitable flexural modulus, and has completed the present invention.

That is, according to the present invention, high density polyethylene 20 to 60
A mixed resin foam consisting of 20 to 60% by weight of low-density polyethylene zO and 20 to 60% by weight of low-density polyethylene zO, and the foam is formed by internally pressing a balloon-shaped foam to 0.05 g/cm3. There is provided a mixed resin low density foam characterized in that it has a density of less than or equal to.

Further, according to the present invention, high density polyethylene, low density polyethylene, and polystyrene resin having an MFR of 5 g/10 minutes or more are mixed in a proportion of 20 to 60 weight S, 20 to 40 weight 2, and 20 to 40 weight 2, respectively. After melting and kneading under pressure with a foaming agent at a boiling point, a balloon is formed from a circular die, transferred to a low pressure zone, and the inner surface of the balloon is pressed between rolls to obtain an ungrooved foam with a density of 0.05 g/ad. A method for producing a mixed resin low density foam is provided.

The high-density polyethylene in the present invention is a polymer mainly composed of ethylene and has a density of 0.941 to 0.970 g/cm3 at room temperature and normal pressure, and is vinyl acetate.

Also included are methacrylates, acrylates, and copolymers with propylene and other vinyl monomers that can be copolymerized with ethylene. The high-density polyethylene used in the present invention is usually preferably one having an MFR of 0.03 to 10 g/lo min.

The low-density polyethylene in the present invention is a polymer mainly composed of ethylene and having a density of 0.910 to 0.930 g/cj at normal temperature and normal pressure, and includes various vinyl-based polymers that can be copolymerized with ethylene as described above. Copolymers with monomers are also included. The low-density polyethylene used in the present invention preferably has a normal MFR of 0.1-Log/10 minutes.

The polystyrene resin in the present invention refers to a polymer mainly composed of styrene, including styrene homopolymers and copolymers with other vinyl monomers that can be copolymerized with styrene, and generally impact-resistant polystyrene resins. Also included in the scope of the polystyrene resin of the present invention are copolymers or mixtures with rubber-based polymers mainly composed of polystyrene, or copolymers with diene monomers.

Note that in the present invention, when an impact-resistant polystyrene resin is used as the polystyrene resin, the flexibility and elasticity of the resulting foam can be improved. ta f
, in the present invention, as the polystyrene resin, M
A material with an FR of 5 g/10 minutes or more is used. MFR
If less than 5 g/10 minutes is used, it will be difficult to press the inner surface of the balloon-shaped foam.

In addition, MFR referred to in this specification is JIS K721
Condition 8 is used for polystyrene resin, and Condition 4 is used for polyethylene resin.

As the blowing agent used in the present invention, aliphatic hydrocarbons, halogenated hydrocarbons, or chlorofluorocarbon gas may be used alone or in combination. Specific examples of aliphatic hydrocarbons include propane, butane, isobutane, pentane, isopentane, etc., and examples of halogenated hydrocarbons include chlorine or bromine substituted products of these aliphatic hydrocarbons.

In addition, as chlorofluorocarbon gas, chlordifluoromethane,
Trifluoromethane, 1,2,2,2-tetrafluoroethane, l-chloro-1,1-difluoroethane, 1.
1-difluoroethane, 1-chloro-1,2,2,2-
Tetrafluoroethane, trichlorofluoromethane, dichlorodifluoromethane, 1,2-dichloro-1,2,
Examples include 2,2-tetrafluoroethane. When using the above blowing agent, it is desirable to select a blowing agent whose main component has a boiling point range of 120 to 20°C.

Further, mixing a small amount (40% by weight or less) of fluorocarbon gas with an aliphatic hydrocarbon or halogenated hydrocarbon as a foaming agent is an effective method from the viewpoint of obtaining a low-density foam.

The proportion of high density polyethylene in the mixed resin in the present invention is 20 to 60% by weight2, preferably 25 to 405 to 40% by weight.
The proportion of polyethylene in parts by weight is 20 to 40 parts by weight2, preferably 25 to 40 parts by weight2, and the proportion of polystyrene resin is 20 to 400 to 40 parts by weight, or 25 to 40 parts by weight. If the proportion of high-density polyethylene is less than the above range, mixing with low-density polyethylene and polystyrene resin will be poor, and if it exceeds the above range, the resulting foam will show significant shrinkage and will have poor rebound. Elasticity also becomes impaired.

If the proportion of low-density polyethylene is less than the above range, the recovery properties of the resulting foam after compression will be impaired; on the other hand, if it exceeds the above range, a surging phenomenon will occur, making it difficult for the resin and blowing agent to interact properly. It will not mix and it will be difficult to obtain a low density foam. If the proportion of the polystyrene resin is less than the above range, the shrinkage of the foam will increase and it will be difficult to press the inner surface of the balloon-shaped foam. On the other hand, if the amount exceeds the above range, the compatibility with the polyethylene resin during melt mixing will be poor, and the resulting foam will have too high rigidity and become brittle, and will also have poor recovery properties after compression.

The proportion of the blowing agent used in the present invention is 1:1 for the mixed resin.
00 parts by weight, 1 to 40 parts by weight, preferably 10 to 40 parts by weight
Butane (normal:iso=7:
When using a material containing 3) as the main component, the density is 0.0.
The amount necessary to obtain a foam with a weight of less than 5 g/cm3 may be generally 10 to 20 parts by weight per 100 parts by weight of the mixed resin.

In the present invention, a commonly used cell nucleating agent may be used as necessary. Examples of this cell nucleating agent include an inorganic substance such as talc, or a cell that is decomposed at the temperature inside the extruder. A chemical blowing agent that generates gas, or an acid-alkali combination that reacts at the temperature to generate carbon dioxide, such as citric acid and sodium bicarbonate, or an alkali salt of citric acid and sodium bicarbonate. By adding these cell-planting agents, the size of the cells in the obtained foam can be arbitrarily set to 1111, and the flexibility, feel, etc. of the foam can also be improved. Furthermore, in the present invention, an anti-shrinkage agent may be added to the mixed resin, if necessary, in order to prevent the foaming agent from permeating the resin during foaming and suppress shrinkage of the foam. Examples of such substances include 1, for example, polyoxyethylene monomyristate, polyoxypropylene monomyristate, polyoxyethylene monopalmitate,
Polyoxypropylene monopalmitate, polyoxyethylene monostearate, polyoxypropylene monostearate, polyoxyethylene distearate, monolauric acid glyceride, monomyristic acid glyceride, monopalmitic acid glyceride, monostearic acid glyceride, monoarachic acid glyceride , dilauric acid glyceride, cybalmitic acid glyceride, distearic acid glyceride, 1-palmito-2-stearic acid glyceride, 1-stearo-2-myristic acid glyceride, and tristearic acid glyceride. Other foam nucleating agents, inorganic fillers, etc. can also be used.

The foam of the present invention can be manufactured using a conventional extrusion foam molding machine. That is, the mixed resin and foaming agent are melted and kneaded in a pressurized cylinder of an extruder, and this mixture is extruded into the atmosphere through a circular die of the extruder to form a balloon, and then passed between rolls and crimped on the inner surface of the balloon. In this way, a plate-shaped foam can be obtained.

The foam of the present invention has a density of less than 0.05 g/-, and preferred foams have a density of 0.015 to 0.0
When expressed in zero expansion ratio having a density of 25 g/cm 3 , the foam of the present invention exhibits an expansion ratio of at least 18 times or more.

〔Effect of the invention〕

The foam of the present invention has a suitable thickness and flexural modulus, and is therefore preferable as an inexpensive cushioning packaging material with good flexural rigidity.

By taking advantage of these characteristics, the foam of the present invention can be advantageously used for packaging agricultural products, home appliances, glass, etc., or as cushioning materials, cushioning materials, etc. for transportation.

〔Example〕

Next, the present invention will be explained in more detail with reference to Examples.

Examples 1-3. Comparative Examples 1 to 6 High-density polyethylene (HOPE) with a density of 0.958 g/aj and MFRo of 3, low-density polyethylene (HOPE) with a density of 0.923 g/cm3 and AFR of 0.26i (LDP and density of 1.05 g/aJ, Table Polystyrene resins (ps) with MFR values shown in Table 1 were used in the proportions shown in Table 1, and 1.0 parts by weight of monostearic acid glyceride was added as an anti-shrinkage agent to about 100 parts by weight of these resins. , the blowing agent butane (normal:iso=7:
3) was added at the rate shown in Table 1, and the discharge amount was 18 kg/h.
After melt-kneading at a pressure of 200 kg/aJG in an extruder of
A balloon was formed by extrusion at the temperature shown in (foaming temperature) under atmospheric pressure, and the inner surface of the balloon was pressed between rolls to obtain a plate-shaped foam.

Seven days later, the density and thickness of the foam thus obtained at room temperature were measured, as well as the flexural modulus and compressive residual strain. The results are shown in Table-1.

Note that the bending elastic modulus and compressive residual strain were measured according to the following method.

(1) Flexural modulus: Based on JIS K7203, test speed: 10 m/min, sample size: 150 x 25 mm, span: 100 m
, with a jig radius of 5 m.

(2) After compressing the 50% compression residual strain sample (size 50X50+w+s) to 5 parts of the total thickness (A) (compression speed 10 wxa/min),
The compression was immediately removed at the same speed as the compression, and the thickness was calculated from the thickness (B) at which the stress became zero and the original thickness (A).

-8 50% compressive residual strain (%) = It can be seen that it has recovery properties after compression.

Claims (2)

[Claims] (1) 20-60% by weight of high-density polyethylene, 20-40% by weight of low-density polyethylene and polystyrene resin 2
A mixed resin foam consisting of 0 to 40% by weight, and the foam is formed by pressing a balloon-shaped foam on the inner surface.
．． A mixed resin low-density foam having a density of less than 0.05 g/cm^3. (2) High-density polyethylene, low-density polyethylene, and polystyrene resin with a melt flow rate of 5 g/10 minutes or more, 20 to 60% by weight, 20 to 40% by weight, and 2% by weight, respectively.
After melting and kneading under pressure with a low boiling point blowing agent at a ratio of 0 to 40% by weight, a balloon is formed using a circular die, transferred to a low pressure zone, and the inner surface of the balloon is pressed between rolls to obtain a material with a density of 0.05 g. 1. A method for producing a mixed resin low-density foam, characterized in that a foam with a density of less than /cm^3 is obtained.