JP2847197B2 - Mixed resin foam and method for producing the same - Google Patents

Description

The present invention relates to a mixed resin foam and a method for producing the same,
More specifically, the present invention relates to a low-density polyethylene resin foam mixed with low-molecular-weight polystyrene and a method for producing the same.

[Conventional technology and its problems]

Conventionally, regarding the production of polyolefin-based resin foam, there are many known methods of extruding a polyolefin-based resin by adding a chemical foaming agent and / or a physical foaming agent. It is very difficult to get. In particular, this tendency is remarkable when trying to obtain a foam from a low-density polyethylene resin, and when trying to obtain a thick foam with a low density, the foam causes shrinkage, resulting in a low-density foam. However, there arises a problem that the product cannot be obtained, or even if the product is obtained, the product value is significantly reduced.

In order to solve such a problem, a method is known in which a polyolefin-based resin and a Freon gas-based blowing agent are melt-kneaded and then transferred to a low-pressure zone to obtain a low-density polyolefin-based resin foam (for example, US Pat. No. 3067147). In this method, in order to obtain a low-density foam, 1,2-dichloro-1,1,2,2 is used as a blowing agent.
-It is necessary to use tetrafluoroethane or the like. However, this blowing agent is expensive and has the problem of chlorofluorocarbon gas pollution.

In the above-mentioned conventional method, the reason for using Freon gas as a blowing agent is that, besides being non-flammable, its gas permeation rate to polyethylene is lower than that of organic compound gases such as hydrocarbons and ethers. It is slow. That is, from the foam, the blowing agent gas escapes into the outside air, and air enters the foam, but in general, in the case of polyethylene, the permeation rate of the organic compound gas is faster than that of air. As a result, the pressure inside the foam gradually decreases, eventually causing deformation and shrinkage and a decrease in compressive strength. On the other hand, Freon gas is generally used as a blowing agent for polyethylene-based foams because it does not cause deformation and shrinkage of the foams because it generally has a near air permeation rate. However, since this fluorocarbon gas has the above-mentioned problems, development of a technique for producing a foam without using fluorocarbon gas has been demanded.

[Problem of the Invention]

The present invention solves the above-mentioned problems found in the technology for producing a low-density polyethylene-based resin foam, and does not use a freon-gas-based blowing agent having a high ozone depleting ability or reduces the amount of such a freon-gas-based blowing agent. Accordingly, it is an object of the present invention to provide a method and a foam for producing a polyethylene resin foam having a small shrinkage after molding.

[Means for solving the problem]

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, in the presence of an anti-shrinkage agent, focused on the fact that the gas permeation rate of isobutane to a low-density polyethylene resin is small, and isobutane is a main component. By using a foaming agent or a mixed foaming agent of isobutane and a volatile foaming agent having a low gas permeation rate to other low-density polyethylene resins, foaming of a low-density polyethylene resin with a very high expansion ratio and very small shrinkage It was found earlier that a body could be obtained (Japanese Patent Application No. 1-125898). However, even in this case, when the foam is very thick, such as about 100 mm, the air does not easily infiltrate in the center, so the pressure near the center is reduced and shrinkage occurs, and still satisfactory results are obtained. It was not something.

On the other hand, for polystyrene, the gas permeation rate of isobutane is slower than the permeation rate of air, and therefore, blending a polystyrene resin with a polyethylene resin is effective in suppressing the permeation rate of isobutane to a polyethylene resin. Seemed to be a means. Then, about 3 to 25 wt% of ordinary high molecular weight polystyrene, which does not lose the properties of the polyethylene resin, was mixed with the polyethylene resin and subjected to an extrusion test. When the foaming speed was too fast, the cells burst and the cells burst. Was not obtained.

Therefore, the present inventors have further studied diligently and found that a high-quality foam can be obtained by using a specific low-molecular-weight polystyrene, and have completed the present invention.

That is, according to the present invention, there is provided a mixed resin foam comprising 97 to 75% by weight of a low-density polyethylene resin and 3 to 25% by weight of a polystyrene resin obtained by using isobutane or a foaming agent containing the same. A mixed resin foam is provided, wherein the polystyrene resin is a low molecular weight polystyrene resin having a weight average molecular weight in the range of 3 × 10 ⁴ to 15 × 10 ⁴ .

Further, according to the present invention, low-density polyethylene-based resin 97
A mixture resin of 3 to 25% by weight of a low molecular weight polystyrene resin having a weight average molecular weight in the range of 3 × 10 ⁴ to 15 × 10 ⁴ and isobutane or a foaming agent containing the same is prevented from shrinking. In the presence of the agent, melt kneading under pressure,
A method for producing a mixed resin foam, characterized in that the obtained foamable melt-kneaded product is transferred to a low-pressure zone and foamed.

The low-density polyethylene resin in the present invention, a polymer having a density in the normal temperature and pressure are mainly ethylene is 0.910~0.930g / cm ^3, other ethylene homopolymer, vinyl acetate, methacrylate And acrylate or propylene or other copolymers with a vinyl monomer copolymerizable with ethylene.

The low molecular weight polystyrene resin in the present invention has a weight average molecular weight measured by GPC of about 3 × 10 ⁴ to 15 × 10 ⁴ , preferably about 5 × 10 ⁴ to 10 × 10 ⁴ . When a high-molecular-weight polystyrene having a molecular weight larger than the above-mentioned molecular weight range is used, the above-mentioned polystyrene or low-density polyethylene exhibits a viscosity higher than the viscosity suitable for foaming during foaming of the mixed resin. It will be in a foaming state. On the other hand, with polystyrene having a molecular weight smaller than the above range, an effective foam shrinkage preventing effect cannot be obtained. In the present invention, it is particularly preferable to use a low-molecular-weight polystyrene having a melt viscosity at a foaming temperature (arbitrary temperature of 100 to 110 ° C.) substantially matching the melt viscosity of the polyethylene resin.

In terms of dimensional stability, the blending amount of the low-molecular-weight polystyrene resin is preferably as large as possible. However, in order to maintain the cushioning property, texture, and solvent resistance of the low-density polyethylene-based resin foam, 25 wt% or less. Preferably it is 5 to 15 wt%.
If it is less than 3% by weight, the effect of preventing shrinkage of the foam is not sufficient.

In the present invention, isobutane or one containing it is used as a foaming agent. Isobutane has a gas permeability coefficient for low-density polyethylene (measuring temperature 30 ° C) of 8.32.
× 10 ^-10 cc (STP) · cm / cm ² · sec · cmHg, which is easy to receive the effect of anti-shrinkage agent. Therefore, by using isobutane or a foam-containing agent as a foaming agent, shrinkage after molding is prevented, and a foam having low density and small shrinkage after molding can be easily obtained.

In the present invention, other low-boiling aliphatic hydrocarbons, for example, normal butane, pentane, isopentane, etc. have a large gas permeability coefficient to low-density polyethylene. Can not be used as. [Normal butane and normal butane / isobutane (7/3) mixture have a gas permeability coefficient of 27.5 and 24.5 cc (STP) · cm / cm ² · sec · cmHg and isobutane for low-density polyethylene at 30 ° C, respectively. It is big compared. However, in the present invention, it is possible to use an isobutane mixture containing a small amount of these low-boiling aliphatic hydrocarbons as a blowing agent. in this case,
The proportion of isobutane in the mixed blowing agent is at least 50 mol%, preferably at least 80 mol%.

In the present invention, the gas permeability coefficient (measuring temperature 30 ° C.) for low-density polyethylene is smaller than that of isobutane, that is, volatile foaming of 8.0 × 10 ⁻¹⁰ cc (STP) · cm / cm ² · sec · cmHg or less. A mixed blowing agent obtained by adding an agent to isobutane can also be used. By using this mixed foaming agent, a foam having a low density and a small shrinkage after molding can be obtained. In this case, the blowing agent having a small gas permeability coefficient for the low-density polyethylene also includes, for example, a fluorocarbon gas having a high ozone destruction ability, but in the case of the present invention, the use amount of such a fluorocarbon gas is reduced by using in combination with isobutane. As a result, the harmfulness of the chlorofluorocarbon gas can be suppressed. In this case, the mixing amount of the volatile foaming agent is preferably about 20 to 80 mol%.

8.0 × 10 gas permeability coefficient for low density polyethylene
Preferred specific examples of the volatile foaming agent of ^-10 cc (STP) · cm / cm ² · sec · cmHg or less include the following. The values in parentheses indicate the gas permeability coefficient for low-density polyethylene.

Chlordifluoromethane (5.23), 1,2-dichloro-1,1,2,2-tetrafluoroethane (2.
25), 1-chloro-1,1-difluoroethane (4.92) and the like.

Of these, chlorodifluoromethane and 1,2-dichloro-1,1,2,2-tetrafluoroethane are preferably used.

The proportion of the foaming agent used in the present invention is 2 to 15 parts by weight, preferably 5 to 10 parts by weight, based on 100 parts by weight of the low-density polyethylene resin, and is appropriately determined according to the density of the desired foam. Can be

In the present invention, a commonly used bubble nucleating agent may be used, if necessary. As the bubble nucleating agent, for example, an inorganic substance such as talc, or a chemical foaming agent such as decomposing at a melt-kneading temperature to generate a gas, or an acid-alkali of generating a carbon dioxide gas by reacting at a melt-kneading temperature. Combinations, for example, citric acid and sodium bicarbonate,
Such as alkaline salts of citric acid and sodium bicarbonate. By adding these cell nucleating agents, the size of the cells of the obtained foam can be arbitrarily adjusted, and the flexibility and feel of the foam can be improved.

Further, in the present invention, a shrinkage preventing agent is added in order to prevent shrinkage of the foam obtained by preventing the resin from passing through the blowing agent during foaming. As the shrinkage inhibitor, conventionally known ester-based, amine-based, or amide-based ones are used. Examples of the ester-based anti-shrinkage agent include polyoxyethylene monomyristate, polyoxypropylene monomyristate, polyoxyethylene monopalmitate, polyoxypropylene monopalmitate, polyoxyethylene monostearate, and polyoxypropylene monostearate. , Polyoxyalkylene higher fatty acid esters such as polyoxyethylene distearate, monolaurate glyceride, monomyristate glyceride, monopalmitate glyceride, monostearate glyceride, monoarachiate glyceride, dilaurate glyceride, dipalmitate glyceride, distearin Acid glyceride, 1-palmito-2-stearic acid glyceride, 1-stearo-2-myristate glyceride, tristearin Higher fatty acid glyceride such as glyceride and the like. Examples of the amine-based and amide-based anti-shrinkage agents include, for example, stearic acid amide,
Palmitic acid amide, stearic acid diethanolamide, stearic acid monoethanolamide, hydroxyethylamine, hydroxypropylamine and the like.

Usually, these anti-shrinkage agents are used by being previously mixed in a low-density polyethylene resin in a master batch. These anti-shrinkage agents have a gas permeability coefficient of 4.5 × 10 ⁻¹⁰ cc (ST 30 ° C.) for the low-density polyethylene of the blowing agent.
P) It is desirable to blend an amount of cm / cm ² · sec · cmHg or less into the low-density polyethylene. When showing the gas permeability coefficient of the foaming agent to the polyethylene resin blended with the anti-shrinkage agent, for example, 100 parts by weight of low-density polyethylene, when 1 part by weight of monostearic acid glyceride is a shrinkage inhibitor, Gas permeability coefficients of isobutane, chlorodifluoromethane, 1,2-dichloro-1,1,2,2-tetrafluoroethane and 1-chloro-1,1-difluoroethane for low-density polyethylene (measurement temperature 3
0 ° C) is 3.39 × 10 ^-10 , 3.83 × 10 ^-10 , 1.28 × 10 ^-10
And 2.75 × 10 ⁻¹⁰ cc (STP) · cm / cm ² · sec · cmHg.

In the present invention, various auxiliary components can be used, and inorganic fillers and the like can be used.

As an example of a specific device for carrying out the present invention, a foam molding device having a structure in which an accumulator is connected to an extruder can be shown. In order to produce a foam using this apparatus, a low-density polyethylene resin, a foaming agent, and a shrinkage-preventing agent are melt-kneaded in a pressurized cylinder of an extruder, and the kneaded product is once under pressurized conditions. It is extruded into an accumulator, and then extruded to atmospheric pressure by a piston through a die orifice and passed between rotating belts. In this way, a plate-shaped foam molded article having a thickness of 30 mm or more, preferably 40 to 150 mm can be obtained. Further, in the present invention, a thin foam having a thickness of 0.5 to 10 mm can be efficiently obtained by extrusion foaming with a usual screw type extruder.

〔The invention's effect〕

The foam of the present invention has an extremely low density (density of 0.020 to 0.050).
g / cm ³ ) and having a thickness of 100 mm or more, very low shrinkage (5% or less), and is flexible and excellent in rebound resilience.

The foam of the present invention is advantageously used as a cushioning material or cushioning material for packaging or transporting home electric appliances, glass, pottery, etc. by utilizing such properties.

〔Example〕

 Next, the present invention will be described in more detail by way of examples.

Examples 1 to 6, Comparative Examples 1 to 4 100 parts by weight of a low-density polyethylene (hereinafter referred to as LDPE) having a melt index value of 2.0 g / 10 min and a density of 0.923 g / cm ³ and a shrinkage inhibitor (glyceride monostearate) and Polystyrene was blended in the proportions shown in Table 1, and this blend was blended with Table-
The various foaming agents shown in 1 were melt-kneaded at a pressure of 170 kg / cm ² G in an extruder having a discharge rate of 10 kg / hr, and then extruded into an accumulator. Next, from the accumulator (pressure 40 kg / cm ² G), the melt-kneaded material was heated at a temperature (foaming temperature) shown in Table 1 at 100 kg / hr (Example 1) or 620 k / h.
At a discharge speed of g / hr (Examples 2 to 6 and Comparative Examples 1 to 3), the film was discharged under atmospheric pressure through a die orifice provided in an accumulator, and the thickness was about 110 mm (Example 1) or about 70 m.
m (Examples 2 to 6 and Comparative Example 1) were obtained.

With respect to the foam obtained in this way, the density at room temperature after 7 days was measured, and the shrinkage of the foam was calculated by the following equation. Table 1 shows the results.

A: Volume of foam immediately after discharge B: Volume of foam after 3 months In addition, the code | symbol shown about the foaming agent in Table 1 means the following contents.

HCFC22: Chlordifluoromethane HCFC142b: 1-Chloro-1,1-difluoroethane The molecular weights shown for polystyrene in Table 1 are liquid chromatograph LC-3A manufactured by Shimadzu (column: Shima Gel HSG, mobile phase: THF2mol) / min).

Example 7 In Example 1, the average molecular weight of polystyrene was 3.
The experiment was performed in the same manner except that 3 × 10 ⁴ or 14.5 × 10 ⁴ was used. In these cases, good foams were obtained as in the case of Example 1.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C08J 9/00-9/42 B29C 47/00-47/96

Claims (2)

(57) [Claims] 1. A low-density polyethylene resin obtained by using isobutane or a foaming agent containing the same, 97 to 75% by weight.
And a polystyrene resin comprising 3 to 25% by weight of a polystyrene resin, wherein the polystyrene resin is a low molecular weight polystyrene resin having a weight average molecular weight in the range of 3 × 10 ⁴ to 15 × 10 ^4. Mixed resin foam. 2. A mixed resin comprising 97 to 75% by weight of a low-density polyethylene resin and 3 to 25% by weight of a low-molecular-weight polystyrene resin having a weight-average molecular weight in the range of 3 × 10 ⁴ to 15 × 10 ⁴ , and isobutane Or a foaming agent containing the same, melt-kneading under pressure in the presence of an anti-shrinkage agent, and transferring the resulting foamable melt-kneaded product to a low-pressure zone for foaming; Method.