JPH0257576B2 - - Google Patents

Description

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

Several methods are already known for producing cross-linked polyethylene foams, and production on a commercial scale is also being carried out. That is, a decomposable blowing agent is mixed with polyethylene, the sheet is formed by forming the sheet under temperature conditions where the blowing agent does not decompose, the sheet is irradiated with ionizing radiation (for example, an electron beam) to crosslink the polyethylene, and then A method of manufacturing a sheet-like foam by heating to a temperature above the decomposition temperature of the blowing agent, and instead of cross-linking polyethylene by irradiating it with ionizing radiation, a chemical cross-linking agent is mixed in advance with polyethylene. A method is known in which a sheet containing a crosslinking agent is made, the sheet is heated to decompose the chemical crosslinking agent, the polyethylene is crosslinked, and the sheet is heated and foamed almost simultaneously to produce a sheet-like polyethylene foam.

High-pressure low-density polyethylene is exclusively used in the production of these cross-linked polyethylene foams, which have the manufacturing advantage of excellent cross-linked foam moldability, as well as excellent weather resistance and flexibility. Taking advantage of its physical properties, it is used for cushioning materials, insulation materials,
It is widely used in various applications such as soundproofing materials, packaging materials, and building materials. However, this type of foam has poor thermal properties, tensile strength, tear strength, etc., so it may not be satisfactory for some uses, and there is a desire to improve it. On the other hand, high-density polyethylene type cross-linked foam is not easy to mold, and has problems in manufacturing technology such as difficulty in obtaining foam with satisfactory properties and physical properties such as lack of flexibility. Although it has the advantage of having excellent properties, it has not yet been produced on a large scale.

Furthermore, in addition to these crosslinked polyethylene foams, polypropylene crosslinked foams are also known.
Although it has excellent thermal and mechanical properties, it has the disadvantages of poor weather resistance, the need to add a large amount of stabilizer, and the difficulty of crosslinking with ionizing radiation or chemical crosslinking agents during production. .

Therefore, the present inventors have developed the weather resistance characteristic of foams made from conventional high-pressure low-density polyethylene.
As a result of intensive research in order to obtain a polyethylene crosslinked foam with improved mechanical properties such as thermal properties, tensile strength, and tear strength without impairing flexibility, we found that molding is possible by using a specific polyethylene. The present invention was accomplished by discovering that it is possible to easily produce a polyethylene crosslinked foam that has excellent thermal and mechanical properties, as well as good weather resistance, flexibility, and crosslinkability.

That is, the present invention uses metrofluorate: 0.1 to 50 g/10 min, density 0.910 to 0.940 g/cm ³
A decomposable blowing agent that generates gas when heated is added to polyethylene made of a copolymer of ethylene and α-olefin having a melting point of 110 to 130°C, and the melting point is lower than the decomposition temperature of the blowing agent. Production of a polyethylene foam characterized by melt-kneading at a low temperature, irradiating the molded product with radiation after molding, crosslinking without using a crosslinking agent, and then foaming by heating above the decomposition temperature of the foaming agent. The present invention provides a method.

The polyethylene used in the present invention (hereinafter abbreviated as polyethylene A) has a melt fluorate (ASTM D1238E) of 0.1 to 50 g/10 min,
Preferably 0.5-20g/10min, density (ASTM
D1505) is 0.910 to 0.940g/cm ³ , preferably
0.915 to 0.935 g/cm ³ and ethylene with a melting point of 110 to 130°C, preferably 115 to 127°C, and a carbon number of 4 to 20,
Preferably, it is a copolymer with an α-olefin having 5 to 20 carbon atoms, and more preferably an ethylene copolymer having a molecular weight distribution (value of weight average molecular weight/number average molecular weight) of 6 or less.

Even if an ethylene propylene copolymer or so-called high-pressure polyethylene is used instead of polyethylene A, both the moldability and toughness of crosslinked polyethylene cannot be improved.

In addition, polyethylene A has a low melt flow rate.
Items with a melt flow rate of less than 0.1g/10min have a high melt viscosity and are difficult to mold when a blowing agent is added, while items with a melt flow rate of over 50g/10min have a low melt viscosity and are molded with a blowing agent added. This is not preferable because the shape of the molded product becomes unstable in some cases. If the density is 0.910 g/cm ³ , the surface of the resulting foam will become sticky, and if it exceeds 0.940 g/cm ³ , the flexibility of the foam will be impaired and the tensile elongation will be low, so it is preferable. do not have. If the melting point is less than 110℃, the heat resistance of the foam will be insufficient, and if it exceeds 130℃, it will be necessary to add a foaming agent to raise the molding temperature, so it will be foamed before crosslinking. I don't like it because it's scary. Further, those having a molecular weight distribution (value of weight average molecular weight/number average molecular weight) of 6 or less are preferable because a foam having a higher tensile elongation can be obtained.

The α-olefin having 4 to 20 carbon atoms, which is a constituent component of polyethylene A used in the present invention, is as follows:
For example, 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 -Dodesen,
One or more types selected from 1-tetradecene, 1-octadecene, etc. can be mentioned. Note that a small amount of propylene component may be contained as long as these α-olefins are included, but in that case, the content needs to be considerably smaller than the α-olefin content. In order for the density of the entire polyethylene A to be within the above range, α-
Although it varies depending on the type of olefin, it is sufficient that the ethylene content is about 88 to 97% by weight.

The polyethylene A having the above properties used in the present invention is:
It is 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 slurry polymerization, gas phase polymerization, and high temperature solution polymerization.

The melting point of the polymer was measured using a differential scanning calorimeter (DSC).
20℃/min after melting the sample at 200℃ for 5 minutes using
This is the peak temperature when the endothermic curve is measured at a heating rate of 10°C/min after crystallization is performed by cooling to room temperature at a rate of 10°C/min after keeping at room temperature for 1 hour. The polyethylene A used in the present invention may have only one endothermic peak or may have multiple detected endothermic peaks, but
In the latter case, the highest peak temperature is taken as the melting point.

Molecular weight distribution (weight average molecular weight/number average molecular weight) was measured using a gel permeation chromatograph {measuring device: manufactured by 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}, the molecular weight distribution curve was determined, and the weight average molecular weight (hereinafter abbreviated as w) was determined using the universal calibration method using polystyrene as the standard.
It was determined by calculating the number average molecular weight (hereinafter abbreviated as n).

The polyethylene A used in the present invention may be other polyolefins, such as high-pressure low density polyethylene, high density polyethylene,
Polypropylene or the like may be mixed up to about 40%.

Naturally, it is also possible to add other thermoplastic resins to the present polyethylene A, and optional additives such as flame retardants, fillers, stabilizers, antistatic agents, etc. can also be added.

The decomposable blowing agent used in the present invention may be any agent as long as it has a decomposition temperature higher than the melting temperature of polyethylene A used in the present invention, but is preferably azodicarbonamide, especially if its main decomposition point is A temperature of 196℃ or higher is desirable. The decomposition point here refers to the Japanese Industrial Standard (JIS K-8004).
Using a melting point measuring device specified in
℃ at a rate of 2℃ per minute, and then at a rate of 1℃ per minute. This is the temperature at which the yellow color of the sample is completely decolored. In addition, hydrazodicarbonamide, azodicarboxylic acid barium salt, dinitrosopentamethylenetetramine, nitroguanidine, p,p'-oxybisbenzenesulfonyl semicarbazide, etc., which have a decomposition point equivalent to or at a high temperature as azodicarbonamide, may be used singly or in combination, especially It can also be used by mixing with azodicarbonamide.

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

In the case of radiation crosslinking, a method is employed in which the formed sheet is irradiated with ionizing radiation such as electron beams, β rays, and γ rays.

When crosslinking is carried out by ionizing radiation, 0.1 to 10 parts by weight of a crosslinking accelerator may be added to 100 parts by weight of polyethylene A, and the following polyfunctional compounds are particularly suitable. Divinylbenzene, diallylbenzene, divinylnaphthalene, divinylbiphenyl, divinylcarbazole, divinylpyridine and their nuclear substituted compounds and close analogs, polyacrylates and polyols of aromatic polyhydric alcohols such as ethylene glycol dimethacrylate and hydroquinone dimethacrylate. Polyvinyl esters of aliphatic and aromatic polycarboxylic acids such as methacrylate, divinyl phthalate, diallyl phthalate, diallyl maleate, bisacryloyloxyethyl terephthalate, polyallyl esters, polyacryloyloxyalkyl esters, polymethacryloyloxyalkyl esters, diethylene glycol di Polyvinyl ethers and polyallyl ethers of aliphatic and aromatic polyhydric alcohols such as vinyl ether, hydroquinone divinyl ether, bisphenyl A diallyl ether, triallyl cyanurate, triallyl phosphate, trisacryloxyethyl phosphate and polybutadiene. Polymers having unsaturated bonds can also be applied.

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

First, polyethylene A, a blowing agent, and if necessary, a crosslinking accelerator and other additives are mixed uniformly in a mixer, and the mixture is fed to an extruder and melt-kneaded so that the blowing agent does not decompose. Shape, preferably sheet form. Next, the molded article is irradiated with ionizing radiation to crosslink the polyethylene A. 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.

The sheet is kneaded and molded at a temperature lower than the decomposition temperature of the blowing agent, which is an essential requirement for producing a uniform sheet and a uniform foam. When the foaming agent is partially decomposed during sheet forming, the gas generated is retained in the sheet, resulting in a sheet with fine cavities. When such a sheet is heated and foamed, the cells in the foam become coarse and have a good quality. You can't get anything.

The degree of crosslinking suitable for the method of the present invention is 10 in terms of gel fraction.
~60%, preferably 15-50%. If the degree of crosslinking is too low, the bubbles during foaming will not be retained and will tend to aggregate, forming a large foam, and will also make it difficult to increase the foaming ratio due to gas escaping to the outside of the system.
If the degree of crosslinking is too high, the expansion of the foam will be hindered, the expansion ratio will be difficult to increase, and the extensibility during heating will decrease.
Thermoformability decreases. The gel fraction here is the sample
This is the weight percent of the insoluble portion when 0.2 g is immersed in 50 ml of tetralin at 135°C for 3 hours.

In addition, sheet foaming is carried out continuously under a normal pressure atmosphere by supplying a crosslinked foamable sheet to a heating foaming machine; There are two methods: one in which the foam is heated and foamed, one in which it is suspended from the top and heated with an infrared heater and hot air to foam, and one in which it is suspended above a liquid heat medium and heated from above with an infrared heater or hot air to foam. It is possible even if it is twisted.

As described above, the present invention uses a specific polyethylene A and uses a method for producing a foam by crosslinking and foaming it, so that the produced foam has the following excellent effects.

(1) The cross-linked foam of the present invention has a melting point measured by differential scanning calorimetry (DSC) (measured by melting by heating, cooling and crystallization, and then reheating). The endothermic peak of 120~
The temperature is 130℃, which is higher than the 110℃ of conventional cross-linked foam, and its heat resistance is improved.

(2) Since the crosslinked foam of the present invention has an extremely large tensile elongation at break (hereinafter abbreviated as l), it exhibits superior room temperature processability and impact resistance compared to conventional crosslinked foams.

Example 1 Melt fluorate 12.0g/10min, density 0.921
g/cm ³ , melting point 122°C, 100 parts by weight of polyethylene A whose copolymerization component is 1-hexene, decomposition point 201°C
13 parts by weight of azodicarbonamide and 5 parts by weight of crosslinking accelerator divinylbenzene were mixed and kneaded in a 90 mmφ single screw extruder to form a sheet with a thickness of 2.5 mm.
This sheet was irradiated with an electron beam of 4 Mrad to create a foam sheet, which was continuously fed onto a heated liquid bath at 230°C and heated with an infrared heater from above to cause foaming.

The resulting foam has a thickness of 5 mm, a density of 0.035, and a l
(MD) was 350%, and l(TD) was 280%.

Comparative Example 1 In Example 1, instead of polyethylene A, melt flow rate was 3.7 g/10 min, density: 0.923
A foam was made under the same ^conditions using 100 parts by weight of low-density polyethylene with a melting point of 109°C.

The resulting foam had a thickness of 5 mm, a density of 0.036, and a l
(MD) was 200%, and l (TD) was 150%.

Example 2 Melt fluorate 8.0g/10min, density 0.921
g/cm ³ , melting point 120°C, 80 parts by weight of polyethylene A using 1-octene as a copolymerization component, melt flow rate 3.7 g/10 min, density 0.923 g/cm ³ , melting point 109
20 parts by weight of low density polyethylene, decomposition point of 198 °C
Mix 9 parts by weight of azodicarbonamide and 4 parts by weight of ethylene glycol dimethacrylate at 90°C.
The mixture was kneaded using a mmφ single-screw extruder and formed into a sheet with a thickness of 2.4 mm. This sheet was irradiated with an electron beam of 6 Mrad and foamed under the same conditions as in Example 1.

The resulting foam had a thickness of 4 mm, a density of 0.065, and a l
(MD) was 340%, and l (TD) was 300%.

Comparative example 2 Melt flow rate 15g/10min, density 0.922
g/cm ³ , melting point 125°C, 100 parts by weight of polyethylene A using 1-hexene as a copolymer component, decomposition point 202°C
10 parts by weight of azodicarbonamide, dicumylper
0.8 parts by weight of oxide and 6 parts by weight of divinylbenzene were mixed and kneaded in a 90 mmφ extruder to form a sheet with a thickness of 4 mm. The obtained sheet was placed on a wire mesh in an infrared heating oven at 190℃ for about 2 minutes.
It was heated at 230°C for about 2 hours. The obtained foam has a thickness of 5 mm, a density of 0.052, and both l (MD) and l (TD).
It was 280%.

Comparative Example 3 In Comparative Example 2, a foam was produced by the method of Comparative Example 2 using the low density polyethylene used in Higaki Example 1 as the raw material and without adding divinylbenzene. The resulting foam has a density
0.049, l(MD) and l(TD) values were 170%.

Claims (1)

[Claims] 1 Melt fluorate: 0.1 to 50g/10min,
Density: 0.910 to 0.940g/ ^cm3 and melting point: 110
or 130℃ ethylene and α with 4 to 20 carbon atoms
- A blowing agent that generates gas when heated is added to polyethylene made of a copolymer with olefin, and the mixture is melted and kneaded at a temperature lower than the decomposition temperature of the blowing agent. After molding, the molded product is irradiated with radiation and cross-linked. A method for producing a polyethylene foam, which comprises crosslinking without using a foaming agent, and then foaming by heating to a temperature higher than the decomposition temperature of a foaming agent.