JPH04363338A - Production of polyolefin-crosslinked foamed product - Google Patents

Abstract

PURPOSE:To produce a polyolefin-crosslinked foamed product reduced in the pin holes and having an excellent tensile strength by molding, crosslinking and foaming a composition obtained by kneading components in two kneading steps to homogeneously disperse a foaming agent in the composition. CONSTITUTION:(A) A1: 100 pts.wt. of ethylene-vinyl acetate copolymer and A2: 20-200 pts.wt. of a foaming agent, preferably a foaming agent having a decomposition temperature of 160-270 deg.C, such as azodicarbonamide, are kneaded at a temperature below the decomposition point of the component A2 to produce a foaming agent-containing flowable composition. (B) 50-95wt.% of polypropylene (its copolymer is also possible), B250wt.% of a linear low density copolymer of ethylene with a 4-8C alpha-olefin, and 5-40wt.% of the composition obtained in the process A are kneaded at a temperature below the decomposition point of the component A2 to produce a composition for crosslinked foamed products (the content of the component A2 is 4-20wt.% based on the total amount of the composition). (C) The composition obtained in the process B is molded, crosslinked and subsequently foamed to produce the objective polyolefin crosslinked foamed product.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention relates to a method for producing a polyolefin crosslinked foam, and in particular, a method for producing a polyolefin crosslinked foam, in which a blowing agent is uniformly dispersed, the number of pinholes is small, and the tensile strength at break is low. The present invention relates to a method for producing a polyolefin crosslinked foam with excellent tensile properties. [Prior Art and Problems to be Solved by the Invention] Polyolefin foams have excellent mechanical strength, flexibility, texture, heat resistance, chemical resistance, etc., and are used as interior materials for automobiles, insulation materials,
It is widely used in cushioning materials for sporting goods and food packaging, as well as other sound-deadening materials and exterior materials. [0003] Among these, polyethylene foam has excellent flexibility, texture, elongation, etc. due to its low melting point. On the other hand, polypropylene foam has excellent mechanical strength such as tensile strength and bending strength, as well as heat resistance, but due to its high melting point, molecular scission progresses considerably during foaming, resulting in large and bulky foam cells. It tends to be uniform. Moreover, the initial decomposition temperature of ordinary blowing agents is about 170 °C, which makes them not suitable for polypropylene resins, which have a high melting point.Furthermore, blowing agents with high decomposition temperatures generate only a small amount of gas, so they are not suitable for use with polypropylene resins. In order to obtain a high foaming ratio, a large amount of foaming agent is required, which poses the problem of being uneconomical. [0004] Therefore, attempts have been made to solve the drawbacks of both polypropylene and linear low-density polyethylene, or by creating compositions of polypropylene, linear low-density polyethylene, and ethylene-vinyl acetate copolymer. Such polyolefin crosslinked foams are produced by dry-blending a foaming agent, which is usually a powder, with a raw material resin, forming a raw sheet by extrusion molding, subjecting this raw sheet to radiation crosslinking, and heat treatment. However, the above method has the problem that it is difficult to perform sufficient kneading to avoid decomposition of the blowing agent, and it is difficult to uniformly disperse the blowing agent. If there are areas in the unfoamed molded article where the blowing agent is poorly dispersed, large bubbles, pinholes, etc. are likely to be present in the foam, and the mechanical properties of the foam are also likely to deteriorate. [0006] As a countermeasure to this problem, a so-called masterbatch, which is a mixture of the resin component and the foaming agent, is prepared in advance so that the foaming agent is in a significantly concentrated state.
Although the foaming agent is mixed with a resin component to achieve a desired expansion ratio, the dispersion of the foaming agent is not always good, resulting in large bubbles, pinholes, etc., and an accompanying decrease in the mechanical properties of the foam. These issues have not been resolved. [0007] Accordingly, an object of the present invention is to provide a method for producing a crosslinked polyolefin foam in which a blowing agent is uniformly dispersed, the number of pinholes is small, and the tensile strength is excellent. [Means for Solving the Problems] In view of the above problems, as a result of intensive research, the present inventors have developed a resin composition that uses polypropylene, linear low density polyethylene, and ethylene-vinyl acetate copolymer as resin components. In the resin composition for polyolefin crosslinked foam, first, the ethylene-vinyl acetate copolymer, which is the component with the lowest melting point, and a blowing agent are kneaded to form a blowing agent-containing flowable composition, and then polypropylene and a linear low-flow composition are kneaded. A polyolefin crosslinked foam obtained by kneading density polyethylene and the foaming agent-containing flowable composition to obtain a composition for a crosslinked foam, radiation crosslinking of a molded product obtained from this composition, and then heating and foaming. They discovered that the foaming agent is uniformly dispersed, has a small number of pinholes, and has excellent tensile strength, and came up with the present invention. That is, the method for producing a polyolefin crosslinked foam of the present invention includes (A) 100 parts by weight of ethylene-vinyl acetate copolymer and 20 to 200 parts by weight of a blowing agent at a temperature below the decomposition temperature of the blowing agent. Producing a blowing agent-containing flowable composition by kneading at temperature, (B) (a)
50 to 95% by weight of polypropylene, (b) 50% by weight or less of linear low density polyethylene, and (c) 5 to 40% by weight of the foaming agent-containing flowable composition at a temperature below the decomposition temperature of the foaming agent. A composition for a crosslinked polyolefin foam is produced by kneading, and (C) the composition is crosslinked after being molded, and then foamed. The present invention will be explained in detail below. First, various ingredients that serve as raw materials will be explained. In the present invention, the ethylene-vinyl acetate copolymer (EVA) is an ethylene copolymer having a vinyl acetate content of 6 to 20% by weight, preferably 12 to 20% by weight. Such ethylene-
Melt index of vinyl acetate copolymer (EVA)
(MI, 190 °C, 2.16 kg load) is 1 to 2
0 g/10 minutes is preferred. Also, its density is 0.920
It is about 0.940 g/cm3. Furthermore, in the present invention, polypropylene is a crystalline polymer polymerized with propylene monomer as a main component, and includes not only homopolymers but also block copolymers or random copolymers of propylene and other α-olefins such as ethylene. . In the case of copolymers, propylene-ethylene random copolymers are preferred. In this case, the ethylene content is preferably 5% by weight or less, more preferably 0.3 to 4.0% by weight. Such polypropylene usually has a melt flow rate (MFR) of 1 to 50 g/10 minutes, preferably 3 to 30 g/10 minutes.
, JISK7210, load 2.16kg, 230℃)
has. Further, a propylene random copolymer (hereinafter referred to as PPDM) containing a non-conjugated diene comonomer may also be used. As explained above, the term "polypropylene" used herein is not limited to homopolymers of propylene, but should be understood to include each of the above-mentioned copolymers. Note that two or more types of the above copolymers may be used. In the present invention, the linear low density polyethylene is a linear copolymer of ethylene and an α-olefin having 4 to 8 carbon atoms. The above α-olefin is 4
-Methylpentene-1, 1-butene, 1-hexene and the like. Further, the ethylene content in the linear low density polyethylene is 90 mol% or more, preferably 90 mol% or more.
It is 5 mol% or more. Such linear low density polyethylene usually has a density of 0.910 to 0.940 g/cm3, preferably 0.910 to 0.930 g/cm3, and a density of 0.7 to 60 g/10 min, preferably 3 to 2
0g/10 min melt index (MI, JISK
7210, load 2.16 kg, 190 °C). In the present invention, the blowing agent is a compound that is liquid or solid at room temperature, but decomposes or vaporizes when heated above the melting point of polypropylene.
Any material can be used as long as it does not substantially interfere with molding such as sheet formation or crosslinking reaction, but if the decomposition temperature is 160~
A temperature range of 270°C is preferred. Specific examples thereof include azodicarbonamide, azodicarboxylic acid metal salt, dinitrosopentamethylenetetramine, hydrazodicarbonamide, p-toluenesulfonyl semicarbazide, s-trihydrazinotriazine, and the like. Next, a method for producing the crosslinked polyolefin foam of the present invention using the above-mentioned components will be explained. First, a flowable composition containing a blowing agent is produced. The above blowing agent-containing fluid composition is produced by adding a blowing agent to ethylene-vinyl acetate copolymer (EVA) and kneading the mixture. The amount of blowing agent added is 20 to 2 parts by weight per 100 parts by weight of ethylene-vinyl acetate copolymer (EVA).
00 parts by weight, preferably 50 to 100 parts by weight. If the content of the blowing agent is less than 20 parts by weight, foaming will be insufficient, while if it exceeds 200 parts by weight, it will be difficult to mix uniformly. The content of the blowing agent in the ethylene-vinyl acetate copolymer depends on the desired expansion ratio and the resin component.
(polypropylene + linear low-density polyethylene + ethylene-vinyl acetate copolymer) may be set as appropriate depending on the total amount, but usually 1 to 100 parts by weight of the entire composition (resin component + blowing agent). 30 parts by weight, preferably 4
It is used in a range of 20 parts by weight, and the mixing amount can be changed as desired depending on the type and expansion ratio. Next, the blowing agent-containing flowable composition thus obtained, polypropylene, and linear low density polyethylene are kneaded. At this time, the melt-kneading temperature must be lower than the decomposition temperature of the blowing agent. The preferred melt-kneading temperature is 150-170°C. [0020] In the above kneading, the blending ratios of the various components are (a) polypropylene in an amount of 50 to 95% by weight, preferably 60 to 85% by weight, and (b) linear low density polyethylene in an amount of 50% by weight or less, preferably 2 to 40% by weight, and (c) the above-mentioned foaming agent-containing flowable composition is 5 to 40% by weight.
40% by weight, preferably 10-30% by weight. In the present invention, when kneading the fluid composition or resin component, a peroxide may be added in addition to the above components to carry out a crosslinking reaction. However, when the crosslinking reaction is carried out using ionizing radiation, it is not necessary to use an organic peroxide. When using organic peroxides,
It is preferable that the decomposition temperature is about 150°C or higher, and a specific example is methyl ethyl ketone peroxide (182°C or higher).
℃), t-butylperoxyisopropyl carbonate (153℃), dicumyl peroxide (171℃)
, cumene hydroperoxide (255°C) , 2,5
-dimethyl-2,5-di(t-butylperoxy)
Hexane (179°C), 2,5-dimethyl-2,5-
Di(t-butylperoxy) hexyne-3 (193℃
), di-t-butyl peroxyphthalate (159°C)
and so on. These organic peroxides are resin components 10
It is used in a proportion of 0.01 to 1.0 parts by weight, preferably 0.05 to 0.5 parts by weight. Further, in order to carry out the crosslinking reaction smoothly and efficiently, it is preferable to add a crosslinking aid. Examples of crosslinking aids that can be used in the present invention include trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, pentaerythritol pentaacrylate,
Examples include diallyl phthalate and diallyl maleate. The amount of the crosslinking aid added is 0.5 to 5.0 parts by weight, preferably 1.0 parts by weight, based on 100 parts by weight of the resin component.
0 to 3.0 parts by weight. [0023] In addition, various other compounding agents, specifically rubber substances, other resins, pigments, various fillers, flame retardant substances, and antioxidants are added for the purpose of improving the properties of the polyolefin crosslinked foam. Agents and the like can be mixed as appropriate. In addition,
These various compounding agents can be compounded into either the fluid composition or the resin component. Next, a method for producing a polyolefin crosslinked foam using a polyolefin crosslinked foam composition comprising the above-mentioned components will be explained. First, a kneaded product of the blowing agent-containing flowable composition having the above composition is extruded from a die such as an extruder. In this case, the kneading temperature is set at 150 to 170°C in order to prevent decomposition of the blowing agent. Next, a predetermined amount of the foaming agent-containing fluid composition is blended into polypropylene + linear low density polyethylene, and kneading is performed again. At this time, the temperature of the die is controlled at 150 to 170°C. Extrusion is performed using a T-die or the like to obtain a sheet-like molded product. [0025] The thus obtained molded body is crosslinked, and the crosslinked structure can be formed by using an organic peroxide as described above, or by using α rays, β rays (electron beam), γ
Conventionally used methods such as irradiation with ionizing radiation such as radiation, heating, etc. can be employed. Among these methods, a method by irradiation with ionizing radiation is preferred, and crosslinking by electron beam irradiation is particularly preferred. The amount of radiation irradiated varies depending on the type and amount of the crosslinking aid, the degree of crosslinking, etc., but is generally 0.1 to 50 Mrad, preferably 1 to 30 Mrad. [0026] The thus crosslinked polyolefin crosslinked foam resin composition is foamed at a temperature higher than the melting point of the resin component, preferably 200°C or higher, more preferably 200°C or higher.
This can be done by heating to a temperature of 30 to 280°C. The heating time required for foaming is usually 0.5 to 5 minutes. [Operation] In the present invention, first, a foaming agent-containing fluid composition of an ethylene-vinyl acetate copolymer and a foaming agent is produced, and then this foaming agent-containing fluid composition, polypropylene, A foam is produced by forming a composition with linear low-density polyethylene, radiation crosslinking the molded product obtained from this composition, and then heating and foaming it. The foam obtained by the method of the present invention has a small number of pinholes and has excellent tensile strength. The reason why such an effect is obtained is that in a resin system consisting of polypropylene and ethylene-vinyl acetate copolymer (EVA), which is characterized by linear low-density polyethylene, it has the lowest melting point. By adding a blowing agent to ethylene-vinyl acetate copolymer (EVA), which has good dispersibility in both low-density polyethylene and creating a foaming agent-containing fluid composition, the dispersibility of the blowing agent can be dramatically improved. This is thought to be to improve performance. EXAMPLES The present invention will be explained in more detail with reference to the following specific examples. The following resins were used as raw material resins. [1] Ethylene-vinyl acetate copolymer/EVA [melt index (MI, 190 °C, 2.16 kg load) 2.5 g/10 min, density 0.940 g/c
m3, vinyl acetate content 18% by weight] [2] Polypropylene propylene-ethylene random copolymer/RPP [melt flow rate (MFR, 230 °C, 2.16 kg load) 14 g/10 min, ethylene content 0.4 weight %] [3] Linear low density polyethylene/LLDPE [Melt index (MI, 190
°C, 2.16 kg load) 6 g/10 min, density 0.9
15 g/cm3] Examples 1 and 2 and Comparative Examples 1 and 2 Ethylene-vinyl acetate copolymer (EVA) 1
00 parts by weight, blowing agent (azodicarbonamide: A
50 parts by weight or 100 parts by weight of DCA) were kneaded using a twin-screw extruder at 120 to 140°C for 3 minutes to obtain blowing agent-containing flowable compositions (MB-1 and MB-
2) was manufactured. [0031] The foaming agent-containing flowable composition (MB-1 or MB-2) thus obtained, polypropylene (RPP), and linear low density polyethylene (LLDPE)
) in the proportions shown in Table 1, and further 3 parts by weight of a crosslinking aid (trimethylolpropane trimethacrylate and neopentyl glycol) and an antioxidant (Irganox 1010) per 100 parts by weight of each of the above components.
After mixing at 500 rpm for 2 minutes using a Henschel mixer, the mixture was fed to an extruder (50 mmφ, L/D = 28, with a T-die) and extruded at an extrusion temperature of 170°C. , thickness 1.0
A sheet of mm was prepared. Next, apply 750kV, 5Mr to this sheet.
Crosslinking was carried out by irradiating with an electron beam at a dose of ad. Note that the gel fraction of the crosslinked sheet was about 40% by weight. Thereafter, it was placed in an air oven at 250°C for 2 minutes to decompose the foaming agent and cause foaming. For each crosslinked foam sheet thus obtained, the expansion ratio, tensile strength at break, tensile elongation at break, and number of pinholes were measured. The results are shown in Table 2 along with the blowing agent content. For comparison, a blowing agent was added to a composition consisting of polypropylene and linear low-density polyethylene and the same resin composition as in Example 1, and the mixture was mixed and melt-kneaded at once. The foaming ratio,
Tensile strength at break, tensile elongation at break, and number of pinholes were measured. The results are shown in Table 2 along with the blowing agent content. [0035] No.
1 Table Composition (parts by weight) Example 1 Example 2
Comparative example 1 Comparative example 2 RPP
70 70
70 70 LL
DPE 12 3
30 10
EVA - -
-20
MB-1-27
− −
MB-2 18
− − −
Content of foaming agent in raw sheet (1)
9
9 9 9 [
(1) The content is based on 100 parts by weight of the raw sheet, and the unit is parts by weight. Table 2 Characteristics
Example 1 Example 2 Comparative example 1 Comparative example 2 Expansion ratio (1)
25 25 25
25 Tensile breaking strength (2)
10.3 9.7
9.3 8.8 Tensile elongation at break (3) 450 550
325 350
Number of pinholes (4) 1
0 4 5
(1) The unit is double. (2) Value measured in accordance with JIS K6767 (unit: kg/cm 2 ). (3) Value measured in accordance with JIS K6767 (unit: %). (4) The number of pinholes per 1 m2 of foam sheet was counted (unit: number). As is clear from Table 2, the polyolefin crosslinked foams of Examples 1 and 2 produced by the method of the present invention had the following properties:
The values of tensile strength at break and tensile elongation at break were larger than those of the foams of Comparative Examples 1 and 2, and the number of pinholes was also significantly reduced. This is believed to be because the blowing agent is very well dispersed by the method of the present invention. Effects of the Invention As detailed above, in the present invention,
First, a blowing agent-containing flowable composition of an ethylene-vinyl acetate copolymer and a blowing agent is produced, and then a composition of this blowing agent-containing flowable composition, polypropylene, and linear low-density polyethylene is prepared. The molded product obtained from the composition is produced by radiation crosslinking and then heating and foaming, so the resulting foam has a small number of pinholes and is excellent in tensile strength. The polyolefin crosslinked foam obtained by the method of the present invention is suitable for use in automobile interior materials, sporting goods, cushioning materials for food packaging, heat insulating materials, sound deadening materials, and other exterior materials. be.

Claims (2)

[Claims] 1. (A) A blowing agent-containing fluid composition is prepared by kneading 100 parts by weight of an ethylene-vinyl acetate copolymer and 20 to 200 parts by weight of a blowing agent at a temperature below the decomposition temperature of the blowing agent. (B) (a)
50 to 95% by weight of polypropylene, (b) 50% by weight or less of linear low density polyethylene, and (c) 5 to 40% by weight of the foaming agent-containing flowable composition at a temperature below the decomposition temperature of the foaming agent. A method for producing a crosslinked polyolefin foam, characterized in that a composition for a crosslinked polyolefin foam is produced by kneading, (C) the composition is molded, crosslinked, and then foamed. 2. The method for producing a crosslinked polyolefin foam according to claim 1, wherein the content of the blowing agent is 4 to 20% by weight, with the entire composition for a crosslinked polyolefin foam being 100% by weight. A method for producing a characteristic polyolefin crosslinked foam.