JPH0257577B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing polyethylene foams, particularly cross-linked polyethylene foams.

Conventionally, the method for producing cross-linked polyethylene foam has been to add a foaming agent that generates gas when heated to polyethylene, melt-knead it at a temperature lower than the decomposition temperature of the foaming agent, mold it, and then make the molded product ionizable. A method of crosslinking with radiation or a chemical crosslinking agent and then foaming by heating to a temperature higher than the decomposition temperature of the foaming agent is known.

In particular, polyethylene foams using low-density polyethylene produced by a high-pressure method are produced and sold in large quantities on a commercial scale. Also known are cross-linked foams made using high-density polyethylene with a density higher than 0.935 g/cm ³ produced by a medium-pressure method or a low-pressure method.

However, because high-density polyethylene foam has a high polymer melting point, it is extremely difficult to knead and mold it at a temperature lower than the decomposition temperature of the blowing agent, which tends to cause the blowing agent to decompose, making it difficult to obtain a foam with uniform cells. Therefore, despite its excellent mechanical properties and heat resistance, it is not produced and sold on a commercial scale.

On the other hand, foams made from low-density polyethylene produced by high-pressure methods are already produced in large quantities, but their performance is somewhat inadequate in terms of mechanical properties and heat resistance, and there are limits to their applications. It was hot.

The object of the present invention is to improve the performance of conventional foams made from low-density polyethylene, particularly in terms of vacuum formability and tensile elongation at break.

In order to achieve the above object, the present invention has the following composition: more than 50 parts but not more than 80 parts of low-density polyethylene (A) having a melting point of less than 115°C and a density of not more than 0.935 g/cm ³ and a carbon number of 4 as a copolymer component. Polyethylene copolymerized with ~20α-olefin, melting point 115℃~130
°C, density 0.940g/ ^cm3 or less, melt flow rate
0.1 to 50g/10min, less than 50 parts of polyethylene (B) with a molecular weight distribution of 6 or less The main component is 20 parts or more, and a blowing agent that generates gas when heated is added to this to a temperature lower than the decomposition temperature of the blowing agent. This method is characterized by a method for producing a polyethylene foam, in which after melt-kneading and molding, the molded product is crosslinked by a radiation crosslinking method, and then heated to a temperature higher than the decomposition temperature of the foaming agent to foam it.

However, in the present invention, the unit "parts" for the amount of mixture or amount added indicates "parts by weight."

The low-density polyethylene (A) of the present invention corresponds to that conventionally used as a raw material for cross-linked polyethylene foams that have been produced and sold in large quantities on a commercial scale. It is not possible to obtain a high-performance foam.

Another type of polyethylene (B) used in the present invention has a melting point of 115°C to 130°C, a density of 0.940 g/cm ³ or less, preferably 0.935 g/cm ³ or less, a melt flow rate of 0.1 to 50 g/10 min, and a molecular weight distribution. is 6 or less, and the copolymerization component is C _{4 to 20} , preferably C _{5 to 10} α.
-It is a copolymerized product of olefin.

The mixing ratio of low density polyethylene (A) and polyethylene (B) is more than 50 parts but less than 80 parts for the former and less than 50 parts for the latter.
Polyethylene (B) accounts for 20 parts or more, that is, 20% or more and less than 50% of the sum of both. Within this range, a polyethylene foam can be obtained that maintains the good melt-kneading properties of low-density polyethylene (A) and has high performance, particularly high tensile elongation at break.

The melting point referred to here is a differential scanning calorimeter (DSC).
After melting and recrystallization (second stage) measured by
This is the temperature at which the absorption peak of melting occurs. Density is A
Measured using the density gradient tube method specified in STM D1505.

If the amount of other resins such as high density polyethylene, polypropylene, ethylene vinyl acetate copolymer, polybutadiene, etc. is 30 parts or less per 100 parts of the total amount of low density polyethylene (A) and polyethylene (B), the effect of the present invention is not impaired. There is no problem even if they are mixed within a range.

The blowing agents that generate gas when heated are those whose decomposition temperature is higher than the melting point of polyethylene (B), such as azodicarbonamide, hydrazodicarbonamide, barium azodicarboxylate salt, dinitrosopentamethylene, etc. Tetramine, nitroguanidine, P,P'-oxybisbenzenesulfonyl semicarbazide, and the like may be used alone or in combination, but are not limited to these.

The crosslinking method is carried out by a radiation crosslinking method.

In the case of radiation crosslinking, a method is used in which the formed sheet is irradiated with ionizing radiation such as electron beams, β rays, or γ rays, or a method in which an ultraviolet sensitizer such as benzophenone is mixed in advance and irradiated with ultraviolet rays. Ru.

When crosslinking is carried out by ionizing radiation, 0.1 to 10 parts of a crosslinking accelerator may be added to 100 parts of the total amount of low density polyethylene (A) and polyethylene (B),
In particular, the following polyfunctional compounds are suitable.

That is, divinylbenzene, diallylbenzene, divinylnaphthalene, divinylbiphenyl,
Divinylcarbazole, divinylpyridine and their nuclear substituted compounds and close analogs, polyacrylates and polymethacrylates of aromatic polyhydric alcohols such as ethylene glycol dimethacrylate, hydroxy nondimethacrylate, divinyl phthalate, diallyl phthalate, diallyl maleate, bis Polyvinyl esters of aliphatic and aromatic polycarboxylic acids such as acryloyloxyethyl terephthalate, polyallyl esters, polyacryloyloxyalkyl esters, polymethacryloyloxyalkyl esters, diethylene glycol divinyl ether, hydroxy nonvinyl ether, bisphenol A diallyl ether, etc. Polymers having unsaturated bonds such as polyvinyl ethers and polyallyl ethers of aliphatic and aromatic polyhydric alcohols, triallyl cyanurate, triallyl phosphate, trisacrylolyloxyethyl phosphate and polybutadiene are also applicable.

In addition, the polyethylene (B) used in the present invention exhibits even more excellent effects when the following polyethylenes are used.

That is, the melt flow rate (ASTM
D1238E is 0.1 to 50g/10min, density (ASTM
D1505) is preferably 0.915 to 0.935 g/cm ³ ,
A polyethylene (polyethylene ( B).

Even if an ethylene-propylene copolymer is used in place of the above-mentioned polyethylene (B), the effect of improving the tensile strength at break and tensile elongation at break of the foam is small.

The melt flow rate of polyethylene (B) is 0.1 to 50 g/10 min. If it is less than 0.1g/10min, the melt viscosity is high, and if it is more than 50g/10min, the melt viscosity is low, so there is a risk of impairing the moldability when preforming after mixing with the blowing agent and low density polyethylene (A). be. If the density is less than 0.915 g/cm ³ , the surface of the obtained foam may become sticky, and if it exceeds 0.935 g/cm ³ , the effect of improving the tensile elongation at break of the foam is small. Melting point is 115℃
If the temperature is less than 127°C, the effect of improving the tensile strength at break of the foam will be small, and if it exceeds 127°C, it will be necessary to increase the molding temperature when molding by mixing with a foaming agent and low density polyethylene (A). This is not preferred because there is a risk of foaming before crosslinking. When the molecular weight distribution (value of weight average molecular weight/number average molecular weight) exceeds 6, the effect of improving the tensile elongation at break of the foam is small.

Examples of the α-olefin having 4 to 20 carbon atoms, which is a component of the polyethylene (B) used in the present invention, include 1-butene, 1-pentene, 1-hexene,
3,3-dimethyl-1-butene, 4-methyl-1
-pentene, 4,4-dimethyl-1-pentene,
1-octene, 1-decene, 1-dodezene, 1-
One or more types selected from tetradecene, 1-octadecene, etc. can be mentioned. A small amount of propylene component may be contained as long as these α-olefins are used as constituent components, but
In that case, the content needs to be much lower than the α-olefin content. In order for the density of the entire polyethylene (B) to be within the above range, it is necessary for ethylene to be 88
It is sufficient that the content is between about 97% and 97% by weight.

The polyethylene (B) having the above-mentioned properties used in the present invention can be obtained by polymerizing ethylene and α-olefin in a ratio that provides the required density by a so-called medium-low pressure method using a transition metal catalyst. In this case, a molecular weight regulator such as hydrogen may be used to obtain the desired melt flow rate. Polymerization can be carried out by various methods such as thriller polymerization, gas phase polymerization, and high temperature solution polymerization.

The melting point of the polyethylene (B) was determined using a differential scanning calorimeter (DSC), after melting a sample amount at 200℃ for 5 minutes.
This is the peak temperature when the endothermic curve is measured at a heating rate of 10°C/min after cooling to room temperature for crystallization at a rate of 10°C/min and keeping at room temperature for 1 minute. The polyethylene (B) used in the present invention may have only one endothermic peak or may have a plurality of endothermic peaks; in the latter case, the highest peak temperature is taken as the melting point.

In addition, the average molecular weight (weight average molecular weight/number average molecular weight) is measured using a Kölper permeation chromatograph [measuring device: Waters Associates (USA)].
Model 150C-LC/GPC, column: Toyo Soda Co., Ltd.
GMH-6 (10 ₃ - 10 ₇ Å mix gel), solvent:
o-dichlorobenzene, measurement temperature: 135°C], and the weight average molecular weight (hereinafter abbreviated as _w ) and number average molecular weight (hereinafter abbreviated as _o ) using the universal calibration method using polyethylene as the standard. It was obtained by calculating.

The method for manufacturing the foam of the present invention will be explained.

First, low-density polyethylene (A), polyethylene
(B), the blowing agent and, if necessary, a crosslinking accelerator and other additives are mixed uniformly in a mixer, the mixture is fed to an extruder, and the mixture is melt-kneaded without decomposition of the blowing agent to form a desired shape. Preferably, it is formed into a sheet.
Next, the molded article is irradiated with ionizing radiation. Crosslink polyethylene. Next, the foamable molded product is heated in a foaming machine to a temperature higher than the decomposition temperature of the foaming agent to cause foaming, and then taken out from the foaming machine and cooled.

As mentioned above, the present invention is based on low density polyethylene.
Since the method for producing polyethylene foam was to mix (A) and polyethylene (B) and crosslink and foam the mixture, the produced foam exhibits the following excellent effects.

(1) The foam has extremely excellent tensile elongation at break over a wide range of expansion ratios. Therefore, it exhibits superior room temperature processability and impact resistance properties compared to conventional polyethylene foams.

(2) When a vertical cup-shaped mold with a diameter D and a depth H is heated and straight-formed using a vacuum forming machine under optimal heating conditions, the limit at which the foam can be stretched into a cup shape without tearing is reached. When H/D is determined, one that satisfies the following equation is obtained. However, the D of the cup is constant at 50mm.

H/D>6d+0.30,0.02<d≦0.06 H/D>4d+0.42,0.06<d<0.15 where d: apparent density (g/cm ³ )0.02<d
<0.15 For example, according to the above formula, the H/D of a 40 times foamed product (density 0.025 g/cm ³ ) is 0.45 or more, and the H/D of a 30 times foamed product (density 0.033 g/cm ³ ) is 0.5 or more. , H/D of 10 times foamed product (density 0.1g/cm ³ ) is
0.82 or more, such performance was achieved for the first time by the present invention, and exhibits excellent vacuum formability.

Hereinafter, one embodiment of the present invention will be described based on examples.

Example 1 80 parts of low density polyethylene (A) with a melting point of 109°C, a density of 0.923 g/cm ³ and a melt flow rate of 3.7 g/10 min,
Melting point 122℃, density 0.922g/cm ³ , melt flow rate 2.5g/10min 20 parts of polyethylene (B) copolymerized with 9% hexene, 12 parts of azodicarbonamide as a blowing agent, ethylene glycol dimethacrylate as a crosslinking accelerator. 3 parts at 160℃ using a 40mmφ extruder
The mixture was kneaded and formed into a sheet with a thickness of 2.5 mm. This sheet absorbs an electron beam with an accelerating voltage of 750kV.
The sheet was irradiated to 6 Mrad, immersed in a silicone bath at 220° C., and heated until the foaming agent was almost completely decomposed to cause foaming.

The resulting foam was removed from the silicone bath, washed with acetone and water, and then dried. The obtained foam had an apparent density of 0.035 g/cm ³ , a tensile elongation at break in the length direction of 250%, a tensile elongation at break in the width direction of 220%, and an H/D of 0.65 in the vacuum forming test. .

Comparative Example 1 In Example 1, polyethylene (B) was not used,
A foam was produced under the same conditions using 100 parts of low density polyethylene (A).

The obtained foam had an apparent density of 0.034 g/cm ³ ,
Tensile elongation at break in length direction: 190%, tensile elongation at break in width direction: 150%, H/D0.46 by vacuum forming test
It was nothing more than a simple thing.

Example 2 20% of low density polyethylene (A) with a melting point of 110°C, a density of 0.926g/cm ³ and a melt flow rate of 21g/10min
, melting point 109℃, density 0.923g/cm ³ , melt flow rate 3.7g/10min low density polyethylene (A')
40 parts, melting point 125℃, density 0.929g/cm ³ , melt flow rate 2.3g/10min, 40 parts polyethylene (B) copolymerized with 6% hexene, 17 parts azodicarbonamide blowing agent, divinyl crosslinking accelerator benzene 3
The sample was extruded in the same manner as in Example 1, irradiated with an electron beam at an absorbed dose of 4 Mrad, and foamed in the same manner as in Example 1.

The obtained foam had an apparent density of 0.026 g/cm ³ ,
The tensile elongation at break in the length direction is 250%, the tensile elongation at break in the width direction is 210%, and the H/D according to the vacuum forming test is
It was 0.05.

Comparative Example 2 In Example 2, polyethylene (B) was not used,
A foam was produced under the same conditions as in Example 2 using 33 parts of (A) and 67 parts of (A').

The obtained low-density polyethylene foam has an apparent density of 0.025 g/cm ³ and a tensile elongation at break in the longitudinal direction of 140.
%, the tensile elongation at break in the width direction was 110%, and the H/D in the vacuum forming test was only 0.42.

Comparative Example 3 In Example 3, polyethylene (B) was not used,
Example 3 using only 100 parts of low density polyethylene (A)
A foam was made in a similar manner.

The obtained foam had an apparent density of 0.083 g/cm ³ , a tensile elongation at break of 180%, and an H/
It was only D0.60.

Claims (1)

[Claims] 1 Polyethylene copolymerized with more than 50 parts but not more than 80 parts of low-density polyethylene (A) with a melting point of less than 115°C and a density of 0.935 g/cm ³ or less, and an α-olefin having 4 to 20 carbon atoms as a copolymerization component, which has a melting point 115℃～130
°C, density 0.940g/ ^cm3 or less, melt flow rate
0.1 to 50g/10min, less than 50 parts of polyethylene (B) with a molecular weight distribution of 6 or less The main component is 20 parts or more, and a blowing agent that generates gas when heated is added to this to a temperature lower than the decomposition temperature of the blowing agent. 1. A method for producing a polyethylene foam, which comprises melt-kneading and molding the molded product, crosslinking the molded product using a radiation crosslinking method, and then foaming the product by heating it to a temperature higher than the decomposition temperature of a foaming agent.