JPH0449573B2 - - Google Patents

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 in-mold foam molded products of polyolefin resin, the polyolefin resin is rarely 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 production of in-mold foam molded products of crosslinked polyolefin resins is not as easy as the production of in-mold foam molded products 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, or the closed cell ratio may decrease). (or even the shape of the pre-expanded particles may become 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, which reduces the yield of the molding process. tends to get worse. 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 article by heating to cause re-foaming and fusion between the expanded particles, at least two types of crosslinked polyolefin resin particles having different degrees of crosslinking are used. The purpose is to prepare the particles so as to satisfy the requirements described below, foam them separately, and then mix and mold the resulting pre-expanded particles of at least two types of crosslinked polyolefin resin. According to the manufacturing method of the present invention, by mixing and using pre-expanded particles of at least two types of crosslinked polyolefin resins with different degrees of crosslinking of the material resins, the range of conditions under which molding can be performed is significantly expanded, and as a result, The dimensional accuracy and physical properties of the molded 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 can be made 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, linear low density polyethylene, high density polyethylene, polypropylene, ethylene-propylene copolymer, ethylene- Examples include α-olefin copolymer, propylene-α-olefin copolymer, ethylene-butadiene copolymer, polybutene, and the like. The crosslinking treatment of the polyolefin resin may be carried out according to a conventional method. For example, particles having 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. Furthermore, in order to determine the degree of crosslinking of the group with the highest degree of crosslinking, the critical gel fraction Gc of the raw resin must be considered. However, the critical gel fraction Gc refers to crosslinked polyolefin resin particles that become difficult to foam as crosslinking progresses (that is, as the gel fraction increases), and when pre-foaming the particles, the temperature is raised significantly or a foaming agent is used. Among those that can be industrially foamed without using special conditions such as using a very large amount, this is the gel fraction with the highest degree of crosslinking, and this is the gel fraction for typical polyolefin resins. The values 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 3wt%) 50 Gel fraction correlated with degree of crosslinking Therefore, when expressing the degree of crosslinking, what is required in the present invention is
The method of dividing resin groups with different crosslinking treatments and the crosslinking treatment of each group is as follows (however, 1/2 of the above limit gel fraction Gc is Gb, and 1/1 of Gc
Let 00 be Ga. ). All groups of raw resins are crosslinked until their gel fractions are Ga or Gc. At least one group of raw material resins has a gel fraction of Ga.
The gel fraction of at least one other group of raw resins is crosslinked until the gel fraction reaches Gb or Gc. The total weight of the crosslinked polyolefin resin group with a gel fraction of Ga or more and less than Gb is W ₁ , and the gel fraction is
When the total weight of the Gb to Gc crosslinked polyolefin resin group is W ₂ , it is made such that 0.2≦W ₁ /W ₂ ≦6. Let the weighted average value of each gel fraction of the resin group with a gel fraction of Ga or more and less than Gb be G ₁ , and the weighted average value of each gel fraction of the resin group with a gel fraction of Gb or Gc.
When G ₂ is set, make sure that G ₂ −G ₁ ≧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 No. 56-1344, that is, the resin particles are made to absorb a volatile solvent-based blowing agent, and then heated together with a dispersion medium under pressure in a closed container, and then released into an atmosphere at a lower pressure than the inside of the container to cause foaming. There is a way. The expansion ratio to be determined depends on the type of foam molded product to be manufactured, and is usually between 3 and 3, although it is not particularly limited.
80 times is suitable, and particularly preferred is 5 to 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 step 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. This 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 cross-linked 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: Determined according to the following criteria ○ The range of vapor pressure that can mold a good product is 0.5Kg/cm ²
Exceeds △ Same pressure width exceeds 0.2Kg/cm ² 0.5Kg/cm ²
× Same as above Pressure width is 0.2Kg/cm ² or less Appearance: Judgment based on 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 (fusion ability between foamed particles; (controlling bending strength): Judgment based on the following criteria: ○ Does not break even when bent 90 degrees △ Partially cracks when bent 90 degrees × Breaks when bent 90 degrees Examples 1 to 6, Comparative Examples 1 to 8 Density 0.923 g/ cm ³ , melt index 1.5g/
After 10 minutes, low-density polyethylene particles with a critical gel fraction of 80% were divided into 6 groups A to F, and each group was treated with dicumyl peroxide (the amount used was within the range of 0.1 to 2.0% by weight based on the resin) for each group. (varying amounts) to obtain crosslinked low-density polyethylene particles with a gel fraction of 0.5 to 80%. Next, 100 parts of crosslinked resin particles (parts by weight,
(same below), 15 to 35 parts of dichlorodifluoromethane,
Each group of cross-linked polyethylene is separately prepared by charging 0.5 parts of finely divided aluminum oxide and 300 parts of water into an autoclave, heating it, and opening the outlet at the bottom of the autoclave at 100 to 140°C to release the contents into the atmosphere. Pre-expanded crosslinked polyethylene particles (Table 1) having an expansion ratio of about 20 times and different degrees of crosslinking were 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 above six groups of pre-expanded particles was individually
Or mix 2 groups at a time (mixing ratio 1:1), 300mm
Filled into a ×300mm ×30mm mold, 0.8~2Kg/cm ² G
It was molded by heating with water vapor. The results are shown in Table 2. In addition, in Comparative Example 7 and Comparative Example 8, the gel fraction was
This is an example of a combination of only Ga to Gb foam particles. Examples 7 to 12, Comparative Examples 9 to 13 Density 0.935 g/cm ³ , Melt index 4.5 g/
After 10 minutes, linear low-density polyethylene particles with a critical gel fraction of 60% were divided into 5 groups G to K, and the resin in each group was treated with dicumyl peroxide (the amount used was 0.1 to 0.1 to the resin).
(varied for each group within the range of 2% by weight) to obtain crosslinked linear low-density polyethylene particles with a gel fraction of 0.5 to 50%. Next, crosslinked resin particles 100
15 to 35 parts of dichlorodifluoromethane, 0.5 parts of finely divided aluminum oxide, and 300 parts of water are charged into an autoclave and heated, and at 110 to 145°C, the outlet at the bottom of the autoclave is opened and the contents are released into the atmosphere. By separately foaming each group of cross-linked polyethylene, pre-expanded cross-linked polyethylene particles with an expansion ratio of about 20 times and different degrees of cross-linking (Table 3)
I got it.

[Table] Table 3 Symbol Gel fraction (%) G 0.5 H 6.5 I 18.3 J 32.2 K 50.0 Each of the above five groups of pre-expanded particles was individually
Alternatively, two groups were mixed (mixing ratio 1:1 unless otherwise noted), filled into a 300 mm x 300 mm x 30 mm mold, and molded by heating with steam at 0.8 to 2 Kg/cm ² G. The results are shown in Table 4.

【table】

Claims (1)

[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 the gap between the expanded particles. In producing a crosslinked polyolefin resin in-mold foam molded article by molding by causing fusion, prepare at least two types of crosslinked polyolefin resin particles that satisfy the following requirements and foam them separately, A method for producing a foamed molded product characterized by mixing and molding the obtained pre-expanded particles of at least two types of crosslinked polyolefin resin: where the critical gel fraction of the raw material resin is Gc, and 1/2 of Gc
When Gb is Gb and Ga is 1/100 of Gc, the gel fraction of all crosslinked polyolefin resins is
The gel fraction of at least one crosslinked polyolefin resin is in the range of Ga to Gc, and the gel fraction of at least one other crosslinked polyolefin resin is between Gb and Gc.
The total weight of the crosslinked polyolefin resin group with a gel fraction of Ga or more and less than Gb is W ₁ , and the gel fraction is
When the total weight of the Gb to Gc crosslinked polyolefin resin group is W ₂ , 0.2≦W ₁ /W ₂ ≦6,
In addition, the weighted average value of each gel fraction of the crosslinked polyolefin resin group with a gel fraction of Ga or more and less than Gb is _G1 , and the weighted average value of each gel fraction of the crosslinked polyolefin resin group with a gel fraction of Gb or Gc. When G _{2 is G 2} , G ₂ −G ₁ ≧10.