JPS5968341A - Production of expanded molding - Google Patents

Abstract

PURPOSE:To produce easily an expanded molding having excellent dimensional accuracy and physical properties with good yields by increasing the freedom in selecting molding conditions, by mixing pre-expanded particles of at least two kinds of crosslinked polyolefin resins having different degrees of crosslinking and molding the mixture. CONSTITUTION:Each of polyolefin resins having a particle size of 0.5-5omega is heated in the presence of a crosslinking agent to crosslink it to the desired degree of crosslinking. A mixture of at least two kinds of the pre-expanded particles having different degrees of crosslinking is charged in a mold and heated to re-expand it and to fuse the particles to one another, whereby the desired expanded molding can be obtd. As said crosslinked polyolefin resin, for example, the one having a gel fraction of Ga-Gb and the one having a gel fraction of Gb-Gc which is at least 10 larger than that of the former are used in a weight ratio of 0.2-6, wherein Gc is gel fraction of an uncrosslinked starting resin, Gb=0.5Gc and Ga=0.01Gc.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a crosslinked polyolefin resin in-mold foam molded article.

When producing an in-mold foam molded article of polyolefin resin, it is rare that the polyolefin resin is foamed and molded as is, but the granular raw material resin is first crosslinked to make it easy to foam, and then pre-foamed. The pre-expanded particles are then typically filled into a mold and heated to form them. However, the manufacture of in-mold foam molded articles of crosslinked polyolefin resins is not as easy as the manufacture of in-mold foam molded articles of other resins, such as polystyrene. First, when crosslinking polyolefin resin particles, crosslinking conditions such as crosslinking agent, crosslinking temperature, and crosslinking time must be strictly controlled depending on the desired degree of crosslinking and the characteristics of the raw resin. If the degree of crosslinking becomes uneven due to insufficient process control in the crosslinking process, various problems will occur in the next pre-foaming process (for example, the expansion ratio may vary greatly between pre-foamed particles, the closed cell ratio may decrease, etc.) (or even the shape of the pre-expanded particles becomes non-uniform). However, on the other hand, pre-expanded particles of cross-linked polyolefin resin with a uniform degree of cross-linking have a very narrow suitable temperature range when molded in a mold, so even if you pay close attention to the molding conditions, the quality of the molded product tends to vary. The yield of the molding process tends to be poor.

The present invention overcomes the above-mentioned conflicting issues when producing a foam molded article of crosslinked polyolefin resin via pre-expanded particles, and easily produces a high quality crosslinked polyolefin resin foam molded article. The purpose is to provide a manufacturing method with high yield.

The present invention, which has succeeded in achieving the above object, is characterized by crosslinking a polyolefin resin, then foaming it in the form of particles, and filling the resulting crosslinked polyolefin resin pre-expanded particles into a mold. , when producing a crosslinked polyolefin resin in-mold foam molded product by heating to cause re-foaming and fusion between foamed particles,
The method involves preparing at least two types of crosslinked polyolefin resin particles having different degrees of crosslinking, foaming them separately, and mixing and molding the obtained at least two types of crosslinked polyolefin resin pre-expanded particles.

However, by heating and using pre-expanded particles of at least two types of cross-linked polyolefin resins with different degrees of cross-linking of the material resins, the range of conditions under which molding can be performed is significantly expanded. The dimensional accuracy and physical properties of the product are improved, and the yield in the molding process is also improved. Therefore, at the stage of crosslinking the raw material resin, the treatment conditions are sufficiently strict to obtain a resin with a uniform degree of crosslinking, and this can be used to smoothly carry out the preliminary foaming process.

Specific examples of polyolefin resins that can be made into foam molded products by the production method of the present invention include low density polyethylene, direct low density polyethylene, high density polyethylene, polypropylene, ethylene-propylene copolymer, ethylene-α-olefin Examples include copolymers, propylene-a-olefin copolymers, ethylene-butadiene copolymers, and polybutenes.

The crosslinking treatment of the polyolefin resin may be carried out according to a conventional method. For example, particles with a particle size of about 0.5 to 5 mm are heated in the presence of a crosslinking agent such as dicumyl peroxide to achieve a desired degree of crosslinking. However, in the present invention, the raw resin particles are divided into at least two groups, and each group is treated separately to be crosslinked to different degrees of crosslinking. The average degree of crosslinking of the entire resin mixed in the form of foam particles during the molding stage is determined by the physical properties such as cushioning properties required of the foam molded product, and the type of resin used.
In addition, in order to determine the degree of crosslinking of the phase having the highest degree of crosslinking, the critical gel fraction Gc of the raw resin must be considered. However, the critical gel fraction Gc is defined as
To industrially foam crosslinked polyolefin resin particles that are difficult to foam without having to use special conditions such as raising the temperature significantly or using a very large amount of foaming agent when pre-foaming them. The gel fraction of the one with the highest degree of crosslinking among those that are possible, and the values for typical polyolefin resins are as follows.

Resin Limit gel fraction (%) Low density polyethylene 80 Linear low density polyethylene 60 High density polyethylene
50 polypropylene
50 ethylene-propylene random copolymer
(Ethylene component: 3 wt%) 50 When the degree of crosslinking is expressed by the gel fraction, which has a correlation with the degree of crosslinking,
In the present invention, it is desirable to divide the resin groups that undergo different crosslinking treatments and to perform the crosslinking treatment for each group as follows (however, 1/2 of the above limit gel fraction Gc is Gb, and Gc 1/100 of is Ga.)

(1) All groups of raw resins have a gel fraction of G
It is crosslinked from a to Gc.

(2) At least one part of the raw resin is crosslinked until the gel fraction becomes Ga or more and less than Gb, and at least one other group of raw resins is crosslinked until the gel fraction becomes Gb or Gc.

(3) When W1 is the total weight of the crosslinked polyolefin resin group with a gel fraction of Ga or more and less than Gb, and W2 is the total weight of the crosslinked polyolefin resin group with a gel fraction of Gb or Gc, 0.2≦ Make sure that W1/W2≦6.

(4) G1 is the weighted average value of each gel fraction of the resin group with a gel fraction of Ga or more and less than Gb, and the gel fraction is between Gb and Gc.
When the weighted average value of each gel fraction of the resin group is G2,
Make sure that G2-G1≧10.

Each group of resin particles having a different degree of crosslinking is foamed separately for each group under suitable foaming conditions. The foaming method is arbitrary, but one example is the method according to the invention of Japanese Patent Publication No. 56-1344, in which a volatile solvent-based foaming agent is absorbed into resin particles, and then heated together with a dispersion medium under pressure in a closed container. There is a method of foaming by then releasing the material into an atmosphere with a lower pressure than the inside of the container. The expansion ratio to be determined depends on the type of foamed molded product to be manufactured, and is not particularly limited, but usually 3 to 80 times is appropriate, and particularly preferred is 5 times. ~60 times. Although it is desirable that the foaming ratios be as uniform as possible between raw resins having different degrees of crosslinking, strict consistency is not required.

The pre-expanded crosslinked polyolefin resin particles obtained after the foaming treatment are stored and aged in a pressurized gas such as nitrogen or air according to a conventional method until the next mixing molding, if necessary.

The process of molding the pre-expanded particles in a mold is carried out using a mixture of two or more types of cross-linked polyolefin resin pre-expanded particles having different degrees of cross-linking as described above. It can be done in exactly the same way as internal molding. That is, no special mold, type of heating medium, temperature, etc. are required. Rather, as mentioned earlier, in the present invention, the moldable temperature range is wide based on the use of a mixture of two or more types of crosslinked polyolefin resin pre-expanded particles. It is much easier than in the case of forming pre-expanded particles.

The present invention as described above facilitates the production of all kinds of foam molded products, such as packaging materials, cushioning materials, insulation materials, flotation materials, containers for food and other articles, made of crosslinked polyolefin resin, and improves product quality and production. The implementation effect is remarkable as it also makes it possible to improve performance.

The present invention will be described below with reference to Examples and Comparative Examples.

The method for measuring the gel fraction and other characteristic values is as follows.

Gel fraction: After treating a sample with hot xylene for 6 hours using a Soxhlet extractor, the undissolved content is weighed and calculated using the following formula.

Gel fraction (%) = undissolved weight x 100/sample weight Formability: Judgment based on the following criteria ○ The range of vapor pressure that can mold a good product is 0.4 Kg/cm2
△ Same as above Pressure width is 0.2Kg/cm2 or more 0.4K
Less than g/cm2 × Same as above Pressure width is less than 0.2 kg/cm2 Appearance: Judging according to the following criteria ○ Surface is smooth △ Unevenness or distortion is observed.

× Significant unevenness and distortion.

Dimensional accuracy: Judgment based on the following criteria ○ Dimensional change rate with respect to the mold is less than 3% △ Dimensional change rate as above is 3% or more and less than 5% × Same as above change rate is 5% or more Fusibility (Fusability of foamed particles with each other; (controls bending strength): Judgment based on the following criteria ○ Does not break even when bent 90 degrees △ Partially cracks when bent 90 degrees 923g/cm3, melt index 1.5
g/10 minutes, low-density polyethylene particles with a limit gel fraction of 80% are divided into 6 groups A to F, and the resin of each group is dicumyl peroxide (the amount used is 0.1 to 2.0% by weight based on the resin). (varied for each group within the range of), and the gel fraction was
．． 5-80% crosslinked low density polyethylene particles. Next, 100 parts of crosslinked resin particles (parts by weight, same hereinafter), 15 to 35 parts of dichlorodifluoromethane, 0.5 parts of finely divided aluminum oxide, and 300 parts of water were charged into an autoclave and heated, and the autoclave was heated at 100 to 140°C. Each group of cross-linked polyethylene is separately foamed by opening the outlet at the bottom and discharging the contents into a flame, thereby producing pre-foamed cross-linked polyethylene particles (with an expansion ratio of approximately 20 times and different degrees of cross-linking). Table 1) was obtained.

Table 1 Symbol Gel fraction (%) A 0.6 B 5.6 C 17.0 D 38.5 E 53.0 F 76.7 Each of the six groups of pre-expanded particles above was used alone or in combination with two
Mix each group (mixing ratio 1:1), 300mm x 300
Filled into a mm x 30mm mold, 0.8-2Kg/cm
It was molded by heating with steam from step 2. The results are shown in Table 2.

Examples 9 to 14, Comparative Examples 7 to 11 Density 0.935 g/cm3, Melt index 4.5
g/10 minutes, linear low-density polyethylene particles with a limit gel fraction of 60% are divided into 5 groups from G to K, and each group's resin is mixed with dicumyl peroxide (the amount used is 0.1 to 2% by weight of the resin). %
(varied within the range of 0.5 to 10%) to obtain crosslinked linear low-density polyethylene particles with a gel fraction of 0.5 to 50%. Next, 100 parts of crosslinked resin particles, 15 to 35 parts of dichlorofluoromethane, and 0.0 parts of finely divided aluminum oxide.
5 parts and 300 parts of water were placed in an autoclave and heated.
By opening the outlet at the bottom of the autoclave at 110-145°C and releasing the contents into the flame, each group of cross-linked polyethylene is foamed separately, with a foaming ratio of about 20 times and different degrees of cross-linking. Crosslinked polyethylene pre-expanded particles (Table 3) were obtained.

Table 2 Note: The numbers in the "Moldability" column are the moldable vapor pressure [
Kg/cm2C].

Table 3 Symbol Gel fraction (%) G 0.5 H 6.5 I 18.3 J 32.2 K 50.0 The above five groups of pre-expanded particles were prepared individually or by mixing two groups. (Mixing ratio 1:1 unless otherwise noted)
, fill a 300mm x 300mm x 30mm mold,
It was molded by heating with steam of 0.3 to 2 Kg/cm2. The results are shown in Table 4.

Table 4 Note: The numbers in the "Moldability" column indicate the moldable vapor pressure [
Kg/cm2G].

*1 Mixing ratio 1:3 *2 Mixing ratio 5:1

Claims (2)

[Claims] (1) After crosslinking the polyolefin resin, it is foamed in the form of particles, and the obtained crosslinked polyolefin resin pre-expanded particles are filled into a mold and heated to cause re-foaming and fusion between the expanded particles. In producing crosslinked polyolefin resin in-mold foam moldability by molding by A method for producing a foam molded article, which comprises mixing and molding pre-expanded particles of a crosslinked polyolefin resin. (2) The manufacturing method according to claim 1, in which at least two types of crosslinked polyolefin resin particles having different degrees of crosslinking are prepared so as to satisfy the following requirements: where the critical gel fraction of the raw resin is Gc. , 1/2 of Cc is Gb, and G
When 1/100 of c is Ga, the gel fraction of all crosslinked polyolefin resins is within the range of Ga to Gc, and the gel fraction of at least one type of crosslinked polyolefin resin is Ga or more and less than Gb. , at least one other
The gel fraction of each type of crosslinked polyolefin resin is Gb to Gc, and the total weight of the crosslinked polyolefin resin group with a gel fraction of Ga or more and less than Gb is W1, and the gel fraction is Gb.
The total weight of the crosslinked polyolefin resin group from Gc to W
2, 0.2≦W1/W2≦6, and the weighted average value of each gel fraction of the crosslinked polyolefin resin group whose gel fraction is Ga or more and less than Gb is G1, and the gel fraction is Gb or Gc. When G2 is the weighted average value of each gel fraction of the crosslinked polyolefin resin group, G2-G1≧10.