JPH04213341A - Production of cross-linked olefin resin foam - Google Patents

Abstract

PURPOSE:To provide a cross-linked olefin resin foam causing no foam collapse on the surface and having a smooth and beautiful appearance and excellent physical properties. CONSTITUTION:A cross-linked olefin resin foam is prepd. by irradiating a sheet made from an olefin resin compsn. contg. an org. peroxide and a thermally decomposable foaming agent with a low-voltage electron beam of 50-400keV, pref. in an atmosphere of a low oxygen concn., to cross-link the surface of the sheet and then heating to carry out the cross-linking by the org. peroxide and the foaming by decomposition of the foaming agent.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to crosslinked olefin resin foams, and more particularly to a method for producing crosslinked olefin resin foams with a smooth and beautiful appearance with high productivity.

0002

[Prior Art] Olefin resin crosslinked foams are usually
It is produced by forming crosslinks in the resin to improve its viscoelastic flow properties and then foaming it. Methods for producing crosslinked olefin resin foams are broadly classified according to the crosslinking step that generates the above-mentioned crosslinked bonds. in particular,
(1) A method in which ionizing radiation is irradiated to generate radicals in olefin resin molecules to cause crosslinking. (2) Radicals generated by decomposition of organic peroxides extract hydrogen from the olefin resin to generate radicals and cause crosslinking. Chemical radical crosslinking method, (3) using a resin that has been grafted or copolymerized with a silicon compound containing an alkoxy group,
There is a chemical condensation crosslinking method in which a foamable resin composition is molded and then crosslinked by dehydration and condensation of alkoxy groups.

However, these methods have the following advantages as well as problems. (1) In the crosslinking method using ionizing radiation, ionizing radiation is irradiated before the start of foaming to impart a certain degree of crosslinking to the resin (generally 15 to 45% in terms of gel fraction).
The resin has the viscoelasticity necessary for forming cells during foaming, and the degree of crosslinking in the thickness direction can be made relatively uniform, resulting in a foam with uniform cells and a smooth and beautiful surface. be able to. However, if the degree of crosslinking is increased, the viscoelasticity of the resin becomes excessive, and the resin may not fully swell or cracks may occur on the surface, so the degree of crosslinking must be kept relatively low, resulting in poor heat resistance of the resulting foam. inferior in sex. Furthermore, since there is a limit to the ability of ionizing radiation to pass through the foamable resin composition sheet, a thick foam cannot be obtained by this method.

(2) Crosslinking with an organic peroxide is not only a crosslinking reaction before foaming, but also a crosslinking reaction that proceeds during or after foaming, so it is possible to achieve a relatively high degree of crosslinking, and the resultant foam It has excellent heat resistance and can also produce thick foams. However, since the surface layer of a foamable resin composition sheet is exposed to oxygen at high temperatures, oxygen molecules bond to the resin radicals generated by the hydrogen abstraction reaction caused by the decomposition radicals of organic peroxides, resulting in the loss of crosslinking reaction activity. , or cause oxidative decomposition of the resin. It also reduces the activity of decomposition radicals of organic peroxides. As a result, crosslinking of the surface layer of the foamable resin composition sheet hardly progresses, and during foaming, the cells in the surface layer portion are destroyed, resulting in a rough foam with an uneven surface.

(3) Since crosslinking using alkoxysilane is a crosslinking process based on a condensation reaction, there is almost no influence from oxygen and the crosslinking extends to the surface layer. Therefore, a foam with a smooth surface can be obtained, and thick products can also be produced. However, it requires a resin production process in which alkoxysilane is grafted or copolymerized, and furthermore, crosslinking requires long-term treatment and reaction in hot water or steam, making the process complicated and requiring special equipment. , disadvantageous in terms of productivity and cost.

[0006] In addition to the above-mentioned methods, as special methods, there are
In No. 39, a crosslinking reaction of an olefin resin composition sheet is carried out using ionizing radiation irradiation and an organic peroxide,
Methods have been proposed to improve heat resistance, moldability, appearance, etc.

In the above method, hydrogen abstraction radicals are generated in the olefin resin due to the irradiation energy of ionizing radiation, and decomposition radicals of the organic peroxide are also generated, and these radicals form bonds with each other. ,
Crosslinking efficiency is severely inhibited. In the conventional method, ionizing radiation is irradiated at a relatively high accelerating voltage, and if a large amount of organic peroxide is added, the above reaction may compete, resulting in the crosslinking required for foaming. On the other hand, if the amount of organic peroxide is small, it will decompose during irradiation, which will inhibit the progress of crosslinking during or after foaming, making it difficult to achieve high crosslinking. It is. Therefore, due to the competitive nature of the reaction and the compensating nature of the reaction activity,
The desired effect cannot be obtained, and it is extremely difficult to employ it in actual production.

Recently, after laminating a polyolefin resin composition sheet containing a blowing agent as a surface layer on the surface of an olefin resin sheet containing a blowing agent and a chemical crosslinking agent, the surface layer is irradiated with ionizing radiation. Then, a method of heating and foaming the whole has been proposed (Japanese Patent Application Laid-Open No. 1-286826).
issue). This method aims to combine the advantages of radiation crosslinking and chemical radical crosslinking, and is said to be able to produce foams with a smooth appearance. However, in this method, it is necessary to laminate sheets of different foamable resin compositions, and whether each sheet is molded in a separate process or laminated by coextrusion, the equipment becomes complex and heavy. In addition, since the lamination must be carried out at a relatively low temperature below the temperature at which the blowing agent and crosslinking agent decompose, the laminated interfaces are not necessarily integrated, which may cause separation between the layers or formation of air bubbles between the layers during foaming. There are cases where

[0009]

SUMMARY OF THE INVENTION It is an object of the present invention to overcome the problems of the prior art and to provide an excellent appearance and
An object of the present invention is to provide a method for producing a crosslinked olefin resin foam having good physical properties. That is, an object of the present invention is to provide a foam that is free from bubble breakage on the surface and has a smooth and beautiful appearance. Another object of the present invention is to provide a method for producing an olefin resin crosslinked foam that can adjust the degree of crosslinking of the foam from a relatively low degree of crosslinking to a relatively high degree of crosslinking, and that can meet various demands for quality. It's about doing. (For example, a foam with a low degree of crosslinking has good elongation during heat processing and molding, and a foam with a high degree of crosslinking has good heat resistance.)
Furthermore, it is an object of the present invention to provide a method for producing an olefin resin crosslinked foam that can also produce thick foams (thick products). Another object of the present invention is to provide a method for producing a crosslinked olefin resin foam that does not require complicated steps or equipment and has high productivity.

As a result of intensive research, the present inventors applied a low-voltage electron beam with a lower acceleration voltage to an olefin resin composition sheet containing an organic peroxide and a thermally decomposable blowing agent than in the prior art. It has been found that the above object can be achieved by irradiating and then heating to perform chemical crosslinking using an organic peroxide and decomposition foaming using a thermally decomposable foaming agent.

That is, when a low voltage electron beam of 50 to 400 keV is irradiated, the electron beam is transmitted only near the surface layer. Then, the organic peroxide in the surface layer is decomposed by irradiation with a low-voltage electron beam to eliminate its activity, and both sides of the sheet are irradiated with a relatively high dose to cause the necessary amount of crosslinking in the olefin resin ( Desirably, the foaming temperature is 1 to 10 Mrad), and the surface layer is made to have a degree of crosslinking (generally 10 to 40% in terms of gel fraction) to obtain the viscosity necessary for foaming, thereby preventing foam breakage during heating and foaming. Next, when heated and foamed, the inner layer, which accounts for most of the thickness of the foam, undergoes crosslinking due to the organic peroxide, which is unaffected by electron beam irradiation, resulting in a smooth and beautiful appearance and improved physical properties. Excellent foams can be obtained.

By the way, since the surface layer of a foamable resin composition sheet is exposed to oxygen when irradiated with an electron beam, the polyolefin radicals generated by the electron beam induce decomposition of the organic peroxide, and the resulting Since the peroxide radicals disappear by combining with oxygen in the air, the crosslinking of the polyolefin does not improve much, and as a result, sufficient surface smoothness may not be obtained. Therefore, by performing electron beam irradiation in an atmosphere with a low oxygen concentration, it is possible to stably obtain a foam with excellent appearance smoothness and good physical properties.

The present invention has been completed based on these findings.

[0014]

[Means for Solving the Problems] According to the present invention, an olefin resin composition sheet containing an organic peroxide and a thermally decomposable blowing agent is irradiated with a low voltage electron beam of 50 to 400 keV. A method for producing a crosslinked olefin resin foam is provided, which comprises crosslinking the surface layer and then heating to perform chemical crosslinking using an organic peroxide and decomposition foaming using a thermally decomposable foaming agent. The electron beam irradiation in the present invention is preferably performed in a low oxygen concentration atmosphere such as an inert gas. The present invention will be explained in detail below.

(Crosslinking/Foaming Method) In the present invention, a foamable olefin resin composition sheet containing an organic peroxide and a thermally decomposable blowing agent is first exposed to a voltage of 50 to 400 keV, preferably 100 to 300 keV.
A low voltage electron beam of eV is irradiated. By irradiating the sheet with a low-voltage electron beam, the organic peroxide present in the surface layer is decomposed and the activity is erased, and a relatively high dose is applied to the sheet to cause the required amount of crosslinking to the olefin resin in the surface layer. Irradiate both sides. The dose depends on the type of olefin resin used, but is usually 1 to 10 Mrad, preferably 2 to 6 Mrad.
It is ad.

[0016] Electron beam irradiation is preferably carried out in an atmosphere with a low oxygen concentration, particularly when the oxygen concentration is 1000 ppm.
Below, it is more preferable to set it as 400 ppm or less. To achieve low oxygen concentrations, an inert gas is usually introduced into the electronic irradiation zone. Examples of the inert gas include nitrogen, helium, and argon, but nitrogen gas is preferred from the viewpoint of cost. Polyolefin radicals generated by low-voltage electron beam irradiation induce decomposition of organic peroxides present in the surface layer to generate peroxide radicals, but by creating an atmosphere with a low oxygen concentration, these radicals can be combined with oxygen. It is presumed that the bonding is suppressed or prevented and hydrogen is newly extracted from the polyolefin resin, thereby generating olefin radicals and promoting the crosslinking reaction.

[0017] In this way, the surface layer of the sheet has a degree of crosslinking (10 to 40% in terms of gel fraction) that provides the viscosity necessary for foaming, thereby preventing foam breakage during heating and foaming. Next, heating is performed to decompose the pyrolytic foaming agent and cause foaming. At this time, most of the organic peroxide is in the inner layer of the sheet and is not decomposed by the low-voltage electron beam irradiation, so that a sufficient crosslinking reaction proceeds. As a result of the above, it is possible to eliminate bubble breakage on the surface of the foam and obtain a smooth and beautiful foam. In addition, the organic peroxide in the inner layer of the sheet is not affected by low-voltage electron beams, so by changing the amount and type of organic peroxide,
The average degree of crosslinking of the foam can be adjusted.

[0018] In the above method, the electron beam needs to pass only through a limited surface layer and does not need to penetrate into the inner layer, so it is possible to produce thick products. Furthermore, since there is no need to laminate sheets of different compositions, there is no clear interface between the laminated layers or the degree of crosslinking before foaming, and there is no peeling between layers or generation of large bubbles during foaming. Moreover, by applying electron beam irradiation crosslinking to prevent bubble breakage on the surface layer, the foamable resin composition sheet exhibits shape retention properties against heat, and the organic peroxide is partially decomposed and pre-crosslinked before foaming. Even without any steps, it can be directly introduced into a foaming oven and crosslinked with an organic peroxide at the same time as foaming, increasing productivity.

In the method of the present invention, the acceleration voltage during electron beam irradiation is limited to 50 to 400 keV.
If it is less than 400 keV, the transmission thickness of the electron beam will be too thin and it will not be possible to sufficiently prevent foam breakage in the surface layer.On the other hand, if it exceeds 400 keV, the electron beam will be transmitted to relatively inner layers of the resin composition sheet. However, the decomposition of the organic peroxide and the competition of the above reactions impede the increase in the degree of crosslinking in the inner layer. Desirably 100-3
00 keV.

The thickness of the foamable olefin resin sheet is usually 0.5 mm or more, preferably 1 mm or more. The thickness of the surface layer that is crosslinked by irradiation with a low-voltage electron beam is usually 0.1 to 0.5 mm, preferably 0.1 to 0.3 m.
It is about m. In sheets with a thickness of less than 0.5 mm, the electron beam affects the organic peroxide throughout the inner layer,
Difficulty achieving goals.

[0021] In the method of the present invention, when a non-aromatic organic peroxide having no benzene ring in its molecular structure is used, the benzene ring does not absorb the energy of the electron beam, making the above effects even more effective. can do.

[0022] Furthermore, the progress of crosslinking during heating and foaming can be stopped,
In order to prevent partial rupture of the foam layer, stabilize the cells, and further increase the closed cell ratio, the decomposition half-life must be 1.
an organic peroxide having a temperature of 140°C to 180°C in minutes;
It is effective to use a small amount, preferably 0.4 parts by weight or less, of a low-temperature decomposition type organic peroxide having a temperature of 120 to 140°C.

(Olefin Resin) As the olefin resin used in the present invention, for example,
Low density polyethylene, medium to high density polyethylene, linear low density polyethylene, polypropylene, butene-propylene copolymer, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer, vinyl chloride-ethylene copolymer, ethylene -acrylate copolymers, etc., and these can be used alone or in combination.

(Organic Peroxide) Specific examples of organic peroxides that can be used in the present invention will be shown together with the temperature at which the decomposition half-life is 1 minute. 2,4-dichlorobenzoyl peroxide (121°C), benzoyl peroxide (130°C), 1,1-di(t-butylperoxy)-3,3,5-trimethylcyclohexane (148°C), n-butyl-4, 4-di(t-butylperoxy)valerate (166°C), dicumyl peroxide (171°C), α,α′-bis(t-butylperoxyisopropyl)benzene (175°C), 2,5-dimethyl-2, 5-di(t-butylperoxy)hexyne-3 (179°C), t-butylperoxycumene (17
6℃) etc.

These organic peroxides can be used alone or in combination of two or more. Among these, non-aromatic 1,1-di(t-butylperoxy)-3,3,5-
Trimethylcyclohexane, n-butyl-4,4-di(
t-butylperoxy)valerate, 2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, and the like are more effective because their benzene rings absorb less electron beams. The amount of organic peroxide added is usually 0.4 to 2.0 parts by weight, preferably 0.6 to 1.5 parts by weight, per 100 parts by weight of the olefin resin.

(Thermal decomposable blowing agent) Examples of the blowing agent include azodicarbonamide, oxybisbenzenesulfonyl hydrazide, etc.
A pyrolytic blowing agent that generates air bubbles is used. The number of parts added may be changed depending on the desired degree of foaming, but it is generally 2.5 to 25 parts by weight per 100 parts by weight of the olefin resin.

(Other additives) The olefin resin composition used in the present invention may optionally contain foaming aids such as fatty acid metal salts and zinc white that improve the decomposition properties of the blowing agent, and trimethylolpropane trimethacrylate. Various additives such as cross-linking aids such as and divinylbenzene, antioxidants, colorants, flame retardants, and fillers can be added.

[0028]

[Examples] The present invention will be specifically explained below with reference to Examples and Comparative Examples. <Method for measuring physical properties> Degree of crosslinking: The degree of crosslinking is determined by cutting a small amount of the foam, immersing it in xylene at 120°C for 24 hours, removing the undissolved residue, and then vacuum drying at 80°C for 8 hours. and the ratio to the initial foam weight was calculated and shown.

Appearance: The surface layer of the obtained foam was visually observed and evaluated on the following five scales. ◎: Excellent smoothness and beauty. 〇: Excellent smoothness and beauty. Δ: Good smoothness and beauty. ×: Poor smoothness and rough surface.

[Examples 1 to 5, Comparative Examples 1 to 2] Low density polyethylene resin (MI 2.5, density 0.9
22) To 100 parts by weight, add 15 parts by weight of azodicarbonamide as a foaming agent, 1.5 parts by weight of zinc white as a foaming aid, and an organic peroxide in the proportions shown in Table 1,
After uniformly mixing with a Henschel mixer, the mixture was put into a 65 mmφ single screw extruder (L/D=26) and discharged into a sheet from a T-die to form a sheet with a thickness of 3 mm and a width of 450 mm. The temperature condition of the extruder is 130℃ from the compound input side.
The temperature of the mold was 125°C, and the extrusion rate was 30 kg/hour.

Thereafter, both sides of the sheet were irradiated with a self-shielded low-voltage compact electron beam irradiator at the voltage and dose shown in Table 1, and then placed in a heating oven at about 220°C to increase the thickness. A foam sheet with a size of about 7 mm, a width of about 800 mm, and a foaming ratio of about 30 times was obtained. The electron beam irradiation was performed in air. The results are shown in Table 1. For comparison, the case where the surface of the sheet was not irradiated and crosslinked only with organic peroxide,
The same table shows the case where both sides were irradiated with a high voltage (800 keV) electron beam.

[0032]

[Table 1] *The entire foam is broken and a satisfactory foam cannot be obtained.

003
3] [Examples 6 to 11, Comparative Example 3] Low density polyethylene resin (MI 2.5, density 0.9
22) For 100 parts by weight, 15 parts by weight of azodicarbonamide as a blowing agent, 1.5 parts by weight of zinc white as a foaming aid, and an organic peroxide (dicumyl peroxide) in the proportions shown in Table 2. After the mixture was added and mixed uniformly using a Henschel mixer, it was put into a 65 mmφ single-screw extruder (L/D=26) and discharged into a sheet from a T-die to form a sheet with a thickness of 2 mm and a width of 450 mm. The temperature conditions of the extruder are:
The temperatures were 130°C, 135°C, and 140°C from the compound input side, the temperature of the mold was 125°C, and the extrusion rate was 30 kg/hour.

Thereafter, the sheet was irradiated with a self-shielded low-voltage compact electron beam irradiator under a nitrogen atmosphere (oxygen concentration 200%).
ppm), a voltage of 200 KeV, and the dose shown in Table 2 on both sides of the sheet, and then placed in a heating oven at about 230°C to form a foam sheet with a thickness of about 7 mm, a width of about 800 mm, and a foaming ratio of about 30 times. Created. The results are shown in Table 2.

Table 2 also shows the cases in which the sheet surface was irradiated with an electron beam in air and, for comparison, the surface of the sheet was not irradiated and crosslinked only with the organic peroxide.

[0036]

[Table 2]

[0037]

According to the present invention, a smooth and beautiful foamed product can be obtained by crosslinking the sheet surface before foaming to prevent foaming.

Furthermore, according to the method of the present invention, only the surface layer of the sheet is affected by the electron beam, and the organic peroxide in the inner layer is not affected, so it is possible to increase the concentration of organic peroxide in the inner layer. It is possible to increase the degree of crosslinking by using this method, and it can also be used in applications that require heat resistance. Furthermore, since it is not necessary for the electron beam to pass through the inner layer, it is possible to produce thick products.

Furthermore, the sheet whose surface layer has been irradiated with a low-voltage electron beam can be directly crosslinked with an organic peroxide and foamed with a pyrolyzable foaming agent in a foaming oven, resulting in excellent productivity.

Claims (2)

[Claims] Claim 1: An olefin resin composition sheet containing an organic peroxide and a thermally decomposable blowing agent contains 50 to 40%
An olefin resin crosslinked foam characterized in that the surface layer is crosslinked by irradiation with a low voltage electron beam of 0 keV, and then heated to perform chemical crosslinking with an organic peroxide and decomposition foaming with a thermally decomposable foaming agent. manufacturing method. 2. The manufacturing method according to claim 1, wherein the electron beam irradiation is performed in a low concentration oxygen atmosphere.