JPS6129975B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a crosslinked ethylene resin foam.

Conventionally, many methods are known for producing crosslinked foams of ethylene resin. For example, a method in which ethylene resin is mixed with a chemical foaming agent, molded, then irradiated with ionizing radiation to crosslink and then heated and foamed, or a method in which a chemical foaming agent and a photosensitizer are mixed in polyethylene resin, irradiated with ultraviolet rays, and crosslinked. Alternatively, the ethylene resin is mixed with a chemical blowing agent and an organic peroxide having a decomposition temperature lower than that of the blowing agent, and the mixture is heated to first decompose the organic peroxide and cause crosslinking. Next, there is a method of foaming by decomposing a chemical foaming agent.

However, in the method of irradiating ionizing radiation,
The equipment is large and its application is limited, or its handling requires advanced technology. moreover,
There is a problem with the permeability of ionizing radiation to the resin, and there are drawbacks such as the inability to obtain a thick foam. In addition, in the method of irradiating ultraviolet rays, the penetration of ultraviolet rays into the resin decreases in proportion to the amount of chemical foaming agent blended, requiring the use of a large amount of photosensitizer, and making it difficult to obtain thick foam. There are no drawbacks. On the other hand, in the method using an organic peroxide, it is necessary to crosslink the resin prior to foaming, so the molded product may be melted and deformed during heat crosslinking, resulting in a deformed foam, or If the organic peroxide is decomposed at a lower temperature in order to suppress thermal deformation, there are disadvantages such as the time required for crosslinking, which reduces productivity, or the use of a large amount of organic peroxide.

Therefore, Japanese Patent Publication No. 45-28503, Japanese Patent Publication No. 47-11908, etc. have been proposed as methods to improve such drawbacks.

However, the method disclosed in Japanese Patent Publication No. 45-28503 involves mixing polyolefin with a chemical blowing agent and an organic peroxide, molding the mixture, and then irradiating the molded product with ionizing radiation to first create a moderate degree of crosslinking in the molded product. , then
This is a method of heating to decompose a chemical blowing agent and an organic peroxide to form a foam. By combining the method of irradiating ionizing radiation with the method of using an organic peroxide, the drawbacks of each can be overcome. Although the part is overcome,
There are still major drawbacks in the method of irradiating ionizing radiation, such as the equipment and the high level of skill required for handling. The method disclosed in Japanese Patent Publication No. 47-11908 involves molding a thermoplastic resin mixed with a chemical blowing agent and an organic peroxide, and then strongly heating only the surface layer of the molded product for a short period of time. In this method, a crosslinked shell is formed in the layer, and then the entire molded product is heated to a temperature higher than the decomposition temperature of the organic peroxide and the chemical foaming agent to effect crosslinking and foaming. However, the method of crosslinking the surface layer of a molded article is based on thermal decomposition of an organic peroxide, and since heating is performed in two stages, temperature control is troublesome. In addition, the chemical blowing agent used must have a temperature higher than the decomposition temperature of the organic peroxide, which limits the selection of blowing agents, and even with ignited heating, there is an upper limit to the heating temperature. However, crosslinking of the surface layer only by heating has the disadvantage that it requires a crosslinking time of several minutes.

As a result of intensive research aimed at improving these drawbacks, the present invention has been developed by irradiating ultraviolet rays onto a molded article of ethylene resin containing a chemical blowing agent, an organic peroxide, a photosensitizing compound, and a polyfunctional monomer. It was discovered that the surface layer portion of the molded article could be easily crosslinked, and this was further investigated to arrive at the present invention.

That is, the present invention blends an ethylene resin with a chemical blowing agent and an organic peroxide having a decomposition temperature higher than the melting point of the resin, further adds a photosensitizing compound and a polyfunctional monomer, and mixes the mixture uniformly. The molded body is then irradiated with ultraviolet rays with a wavelength of 2600 Å or more in a molten state to crosslink the surface layer of the molded body, and then heated under normal pressure to remove the organic peroxide and chemical blowing agent. This is a method for simply and efficiently producing a thick crosslinked ethylene resin foam that does not undergo thermal deformation and is made by decomposing, crosslinking, and foaming.

A major feature of the present invention is that a simple crosslinking method is used before heating, crosslinking and foaming an ethylene resin molded product containing a chemical blowing agent, an organic peroxide, a photosensitizing compound, and a polyfunctional monomer. The method involves first crosslinking the surface layer of the molded article using a method of irradiating ultraviolet rays.

That is, the crosslinking rate is not sufficient with the conventional method of crosslinking ethylene resin mixed with a photosensitizing compound by irradiating it with ultraviolet rays;
However, since crosslinking occurs only on the surface, a method of irradiating ultraviolet rays at room temperature or below the melting point of the ethylene resin is generally used. Moreover, when a chemical blowing agent is added to the ethylene resin, a problem arises in the transparency of ultraviolet rays.

On the other hand, according to the method of the present invention, in which an ethylene resin mixed with an organic peroxide, a photosensitizing compound, and a polyfunctional monomer is irradiated with ultraviolet rays, the crosslinking rate is significantly improved. Since it utilizes a long wavelength range of 2600 Å or more, which has excellent transparency to resin, crosslinking does not end with only the surface of the molded product, but also a moderately thick surface layer from the surface. As a result, for example, simply by irradiating a molded product in a molten state immediately after extrusion molding with ultraviolet rays, crosslinking occurs on the surface layer of the molded product, suppressing thermal deformation. It can be sent to the process. In the subsequent heating step, the molded product is crosslinked to the inside of the molded product without undergoing thermal deformation, and the advantage is that a good ethylene resin crosslinked foam can be continuously obtained by decomposing the chemical blowing agent. be.

The ethylene resin used in this invention is a copolymer mainly composed of polyethylene and ethylene. Specific examples include low-density polyethylene, medium- and high-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, ethylene-ethyl acrylate copolymer ionic copolymer (ionomer), and ethylene. -propylene copolymers, etc., or mixtures thereof. Furthermore, a mixture of the above-mentioned resin as a main component with other polymers such as polybutadiene, polyisoprene, and other rubber components can also be used.

The chemical blowing agent used in this invention has a decomposition temperature higher than the melting point of the ethylene resin used, and preferably higher than the decomposition temperature of the organic peroxide. Specifically, N・N′−
Dinitrosopentamethylenetetramine, trinitrosotrimethylenetetramine, P.P'-oxybisbenzenesulfonyl hydrazide, P-toluenesulfonyl semicarbazide, azodicarbonamide, barium azodicarboxylate, etc., and the decomposition residue may cause unpleasant odor and pollution. Azodicarbonamide, which has low properties and is inexpensive, is preferred. The chemical foaming agent is used in an appropriate amount depending on the foaming ratio, ranging from 1 to 20% by weight based on the ethylene resin. and,
In order to adjust the decomposition temperature of the chemical blowing agent, cadmium stearate, zinc stearate, zinc oxide, etc. can also be blended.

The organic peroxide used in this invention is
A material having a decomposition temperature equal to or higher than the melting point of the ethylene resin used and which effectively acts on the crosslinking of the ethylene resin is used. Preferred organic peroxides include 2,5-dimethyl 2,5-di(tertiarybutylperoxy)hexane, 2,
5-Dimethyl 2,5-di(tert-butylperoxy)hexyne-3,1,3-bis(tert-butylperoxyisopropyl)benzene, dicumyl peroxide, 1,1-bis(tert-butylperoxy) ) 3,3,5-trimethylcyclohexane, etc., but 1,3-bis(tertiarybutylperoxyisopropyl)benzene), which does not leave an unpleasant odor in the decomposition residue of the organic peroxide, is preferred.

The photosensitizing compound used in this invention is one that efficiently crosslinks ethylene resin by the action of ultraviolet light when used in combination with a polyfunctional monomer, such as benzophenone, anthraquinone, benzyl, β-methylanthraquinone, P −
Chlorobenzophenone, acetonephenone, etc. are carbonyl compounds that do not contain hydrogen donating groups in their molecules.

The polyfunctional monomer used in this invention has the effect of increasing the crosslinking efficiency in crosslinking by heat and ultraviolet rays when used in combination with an organic peroxide or a photosensitizing compound, and a typical example is polyethylene. Acrylic esters of glycol diacrylate and triacrylate, methacrylic esters of trimethylolpropane trimethacrylate, tetramethylolmethanetetramethacrylate, and allyl esters such as triallyl trimellitate, triallyl cyanurate, and triallylisocyanurate. Among these, triallyl cyanurate or triallyl isocyanurate is highly effective when used in combination with an organic peroxide or a photosensitizing compound. The appropriate amount of these organic peroxides, photosensitizing compounds, and polyfunctional monomers to be added is 0.05 to 1.5% by weight, respectively, based on the ethylene resin, and is preferably 0.05 to 1.5% by weight from the viewpoint of economy and efficiency. 0.2～
It is 1 part by weight.

Ethylene resin, chemical blowing agent, organic peroxide,
adding a photosensitizing compound and a polyfunctional monomer;
As a method for uniformly mixing, a commonly used kneading method using an extruder or the like is used, and the method is not particularly limited. Molded products such as tapes, plate-like sheets, rods, or molded products with irregular cross sections can be easily obtained using known extrusion equipment, and their thickness is particularly limited since only the surface layer of the molded product is irradiated with ultraviolet rays. There is no.

The ultraviolet light used in the present invention has a wavelength of
Ultraviolet rays having a wavelength of 2,200 Å or more, preferably 2,600 Å or more, have excellent transmittance to ethylene-based resins.
Irradiation sources that emit such ultraviolet rays include low pressure,
High-pressure, ultra-high-pressure mercury lamps, xenon lamps, etc. are used.

There are no particular restrictions on the conditions for this ultraviolet irradiation, but in the case of continuous heating following crosslinking of the surface layer portion, it is preferable and efficient to set the irradiation temperature to the melting point of the ethylene resin or higher. The ultraviolet rays are irradiated evenly from the periphery of the molded object.

Foaming in the present invention is carried out either by first cross-linking the surface layer of the molded product by irradiating it with ultraviolet rays to suppress thermal deformation, and then heating the inside of the molded product to appropriately cross-link it, or by performing the foaming process by cross-linking the inside of the molded product. Decomposition of the blowing agent occurs in parallel, and after it has expanded sufficiently, it can be heated subsequently to increase internal crosslinking. If the internal crosslinking is increased after foaming, a crosslinked ethylene resin foam with high heat resistance can be obtained.

The degree of crosslinking of the surface layer of the molded product by ultraviolet irradiation is preferably in the range of 20 to 60%, so that it can expand into a similar shape without causing thermal deformation during subsequent crosslinking and heating during foaming. In such a case, a foam with a smooth surface and uniform fineness can be efficiently obtained. If the degree of crosslinking in the surface layer of the molded product is less than 20%, sufficient heat resistance cannot be obtained and thermal deformation is likely to occur. A sufficiently expanded foam may not be obtained during crosslinking or foaming, or cracks may occur on the surface of the foam.
A good foam cannot be obtained.

In this invention, the heating method for decomposing the organic peroxide and the chemical blowing agent may be steam heating, heater heating, silicone oil bath, infrared heating, or the like.

The degree of crosslinking in this invention is defined by placing the sample in a 200-mesh wire mesh basket, performing reflux extraction with boiling toluene for 7 hours, drying, measuring the toluene insoluble content, and determining the ratio to the sample content before extraction. be done. In addition, the melting point of ethylene resin refers to the temperature when the crystalline portion of ethylene resin is in a molten state. Specifically, the melting curve is measured using a differential running calorimeter at a heating rate of 8°C/min. This refers to the peak temperature when

The present invention will be described below with reference to Examples, but is not limited to these Examples.

Example 1 Low density polyethylene powder (density 0.921g/cm ³ ,
Mix 100 parts by weight of azodicarbonamide, 0.4 parts by weight of 1,3-bis(tertiarybutylperoxyisopropyl)benzene, 0.4 parts by weight of benzophenone, and 0.6 parts by weight of triallyl cyanurate to 100 parts by weight of MI3.0). Supply it to a 30mmφ extruder
Melt and knead at 130℃, form a sheet with a thickness of 2 mm and a width of 40 mm without decomposing the chemical blowing agent and organic peroxide. A high-pressure mercury lamp manufactured by Toshiba Electric Co., Ltd. (model H-400P) is used.
Eight tubes were installed, and the temperature inside the device was adjusted to 100-130°C. ] at a speed of 1 to 2 m/min, and irradiated with ultraviolet rays for 30 and 15 seconds from both sides of the sheet, respectively. Subsequently, it was introduced into a heating furnace using a far-infrared heater with a radiation wavelength of 0.75 to 100μ, and heated to cause continuous foaming. The obtained foamed sheet had an apparent density of 0.12.08 g/cm ³ , was free from thermal deformation, had a smooth sheet surface, and was a good foam.
Immediately after UV irradiation and before foaming, 0.5 mm of the surface layer of the sheet was scraped off and the degree of crosslinking was measured, and it was found to be 48.23%, and the degree of crosslinking inside the sheet was 10% and 0%. Furthermore, the degree of crosslinking of the foam sheet was 65%.

In addition, for comparison, when a foam was manufactured under the above conditions without irradiation with ultraviolet rays, the resulting foam sheet had large thermal deformation and a rough surface with many traces of collapsed air bubbles on the sheet surface. It was hot. The apparent density was 0.25 g/cm ³ and the cell diameter was large, and a good foam sheet could not be obtained.

Example 2 100 parts by weight of low density polyethylene powder, 10 parts by weight of azodicarbonamide and dicumyl peroxide
0.8 parts by weight, 0.4 parts by weight of benzophenone, and 0.6 parts by weight of triallyl cyanurate were mixed, formed into a sheet with a thickness of 5 mm, irradiated with ultraviolet rays for about 20 seconds under the same conditions as in Example 1, and then cross-linked by burning. A foam was produced. The resulting foamed sheet had a thickness of about 13 mm, an apparent density of 0.05 g/cm ³ , no thermal deformation, and a smooth and good surface. Immediately after UV irradiation, the degree of crosslinking in the surface layer of the sheet before foaming was measured and found to be 28%.

In addition, for comparison, foamed sheets were manufactured under the above conditions by heating and irradiating with ultraviolet rays for a case in which dicumyl peroxide and triallyl cyanurate were not mixed, and a case in which benzophenone and triallyl cyanurate were not mixed. did. In either case, the crosslinking rate is slow, the degree of crosslinking in the surface layer of the sheet is 0%, the thermal deformation is large, and if dicumyl peroxalide and triallyl cyanurate are not mixed, a foamed sheet cannot be obtained. When benzophenone and triallyl cyanurate were not mixed, the apparent density was 0.23 g/cm ³ and the surface was rough, and a good foamed sheet could not be obtained.

Example 3 100 parts by weight of ethylene/vinyl acetate copolymer (vinyl acetate content 15 wt%, MI 0.3), 10 parts by weight of azodicarbonamide, 0.4 parts by weight of 1,3-bis(tertiarybutylperoxyisopropyl)benzene, 0.4 parts by weight of benzophenone and 0.6 parts by weight of triallylisocyanurate were mixed and a crosslinked foam sheet was continuously produced under the same conditions as in Example 1. The obtained foamed sheet had an apparent density of 0.05 g/cm ³ , had a smooth surface, was free from thermal deformation, and had good rubber elasticity.

As detailed above, according to the method of the present invention, the surface layer of a molded product can be easily crosslinked by a simple method of irradiating ultraviolet rays, and thick crosslinked foam can be efficiently produced. can.

Claims (1)

[Claims] 1. A chemical blowing agent having a decomposition temperature higher than the melting point of the resin and an organic peroxide are blended into an ethylene resin,
Furthermore, a photosensitizing compound and a polyfunctional monomer are added, mixed uniformly and molded, and the molded product is then irradiated with ultraviolet rays with a wavelength of 2600 Å or more while the molded product is in a molten state to remove the surface layer of the molded product. A method for producing a crosslinked ethylene resin foam, which comprises crosslinking and then heating under normal pressure to decompose an organic peroxide and a chemical blowing agent to cause crosslinking and foaming. 2. The method according to claim 1, wherein the chemical blowing agent is azodicarbonamide.