JPS59105036A - Production of extruded polyethylene resin foam - Google Patents

Abstract

PURPOSE:To obtain a high-expansion ratio foam free of volume shrinkage after foaming, by mixing polyethylene having a total methyl group number, a resin density, an m.p. and an MI, each of which falls within a specified range, with low- viscosity polystyrene and an organic volatile blowing agent, and extrusion- foaming the mixture. CONSTITUTION:100pts.wt. polyethylene having a total methyl group number of 5-20/1,000C, a resin density of 0.927-0.935, an m.p. of 110-120 deg.C, and an MI of 0.1-4 is mixed with 5-100pts.wt. polystyrene of an M.I. of 5-40. 100pts.wt. obtained resin mixture is further mixed with about 15-40pts.wt. organic volatile blowing agent (e.g., dichlorodifluoromethane), and the mixture is extrusion- foamed to obtain the purpose extruded polyethylene resin foam. In this way, a high-expansion ratio (about 30-60) foam substantially free of shrinkage can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a non-crosslinked extruded foam of polyethylene resin modified with polystyrene.

BACKGROUND ART It is conventionally known to uniformly heat and mix a polyolefin resin such as a polyethylene resin or a polypropylene resin with a foaming agent in an extruder, and to continuously extrude this mixture into the atmosphere to produce a foam.

However, when extrusion foaming is performed using a volatile foaming agent, the polyolefin resin gradually shrinks after foaming, resulting in a large volumetric shrinkage. For example, when low-density polyethylene is foamed using dichlorodifluoromethane as a foaming agent, the volume retention rate decreases to 50-60% on the first to third day after production, and it can be restored to 90% by curing at room temperature. about 30 to
It takes many days and has almost no value as a commodity.

To prevent shrinkage after foaming, improvements have been made such as selecting a specific blowing agent, blending polystyrene, or impregnating and polymerizing styrene to increase the rigidity of the base resin.

To select a blowing agent, there is a technique (Japanese Patent Publication No. 35-48415) that uses dichlorotetrafluoroethane, which has a gas permeation constant equivalent to that of air, for low-density polyethylene; however, dichlorotetrafluoroethane has a high unit price; Another drawback is that it is difficult to obtain a high magnification with a density of 25 f/10 or less.

Regarding polystyrene blends, for example,
No. 84662 describes mixing the two in the presence of a specific medium, and regarding impregnation polymerization, JP-A-52-121669 discloses polymerizing styrene contained in polyethylene resin.
Both disclose foaming each combined resin.

However, these techniques have poor physical properties, are complicated by requiring impregnation polymerization, and the foaming agent used has high gas permeability through the base material, so even if rigidity is imparted using polystyrene, the foam cannot be formed. There are problems such as not being able to sufficiently avoid the shrinkage of.

Regarding polystyrene blends, please refer to JP-A-57-10
There is a technique disclosed in No. 5428 that limits the MI (melt intex) ratio of polyethylene and polystyrene and the blend amount of polystyrene, etc., but even with these specifications alone, there are parts that cannot be implemented and foam molding is difficult. be. In other words, if a blowing agent such as trichloromonofluoromethane that has a solvent effect is selected for dispersion when polyethylene and polystyrene are mixed, although there is a problem with shrinkage of the foam after foaming, polystyrene is well dispersed in the polyethylene base material. However, it can exhibit foaming behavior that forms homogeneous cells. However, mechanical mixing using a blowing agent such as dichloro-difluoromethane, which has poor solvent effect, can only disperse polystyrene into a spherical or cylindrical seabird structure with a diameter of a few microns, and the foaming behavior that forms homogeneous cells cannot be achieved. I can't take it. This is because during the extrusion foaming process, from cell membrane formation to solidification, polystyrene dispersed in islands cannot follow the elongation of polyethylene, resulting in part of the cell membrane being damaged, resulting in open cells, or insufficient foaming. The blowing agent will dissipate over time, and in the end, a homogeneous foam with a high magnification cannot be obtained with such a mixture system.

The present inventors have discovered that when foaming a combination of polyethylene and styrene (c7) m BFI a, which have poor compatibility, using a blowing agent with poor solvent effect, such as dichloro-difluoromethane or propane, the difference in viscosity behavior occurs. It is necessary to prevent damage to the cell membrane, and for this purpose, we have considered it necessary to make the viscosities of each resin sufficiently close from cell membrane formation to solidification, and have conducted repeated studies.

In the combination of polyethylene and polystyrene, commonly used low density polyethylene (density 0.915~
0.925. MI 0.2-10) It is not suitable because it has a large viscosity difference with tofostyrene (there is a difference in temperature at which the foaming viscosity is indicated).

Normally, as a method for adjusting viscosity, changing the MI depending on the molecular weight is generally adopted, but for polyethylene, which is a crystalline polymer, changing the MI alone cannot significantly change the temperature at which the foaming viscosity is indicated. It is difficult to use this as an indicator to control the viscoelasticity to approach the viscoelastic range suitable for foaming polystyrene (see Figure 1).

Therefore, the present inventors took a hint from the fact that the melting point of high-density polyethylene is nearly 20°C higher than that of low-density polyethylene, and as a result of further intensive research on changes in viscosity behavior due to differences in molecular structure, they found that they could suppress branching of polyethylene and It has been found that the accompanying change in density brings about an increase in the melting point, greatly improving the temperature at which the foaming viscosity is appropriate, and making it possible to match the viscosity with the blended polystyrene.

Regarding the non-quality polystyrene used here, the viscosity is represented by MI, but in the present invention, it is preferable to use polystyrene with a low viscosity, and the MI is 5 to 40.
The number is preferably in the range of 10 to 35, more preferably 10 to 35.

More specifically, the total methyl branching calculated from the formula (1) described later using IR spectrum is 5 per 1000 carbon atoms.
~20 pieces, preferably 5 to 10 pieces, and the resin density is 0
．． 927 to 0.985, preferably 0.930 to
0.935 te melting point is IIO~12°C, and M
I is 0.1-4, preferably 0. Only in the combination of polyethylene of J5-2 and polystyrene with MI of 5-40, preferably 10-40, the cell membrane solidifies without causing cell membrane rupture, and is 30-60 times more effective, more preferably 35
It was possible to obtain a highly foamed product with a closed cell ratio of at least 80% or more, usually 90% or more, which was expanded to ~50 times and hardly shrunk.

The target resin to be used can be one obtained by homogeneously tri-blending the granular or powdered respective component resins in a blender, or one obtained by melt-blending them once passed through a cross-wire extruder. In addition to the mixed resin specified in the present invention, other polyolefin resins and polystyrene resins may be blended as long as they do not make foam molding difficult.

The amount of blowing agent used to obtain the foam of the present invention can be freely selected within the range of 15 to 40 parts by weight based on 100 parts by weight of the mixed resin. In addition, there is no problem in using appropriate amounts of additives such as inorganic carbonates, silicates, higher fatty acid metal salts, substances generally used as nucleating agents, and lubricants 1 colorants for adjusting the cell diameter.

The method for evaluating characteristic values used in the present invention is shown below.

All-methyl-based granular or powdered polyethylene in O polyethylene is heated and pressed to 180° C. to form a film of about 60 μm, and after cooling, it is applied to an I4 spectrometer. The obtained IR spectrum is 1375
It is calculated using the following formula using the absorption of cm-1 and 1303 Cm-'.

×h1og female. 3-26)・・・・・・・・・(1)
However, α is the resin density of polyethylene (', /; 5J)l
; Sample thickness (resin density of C+ O polyethylene ASTM D-15054
Base ○Melting point of polyethylene resin AS'l'M J)-
2117 (M5000g, temperature 20σC) O Volume shrinkage of foam 2 minutes after extrusion, the diameter and length of the foam was measured by 1. The volume Vo is calculated by measuring with an accuracy of /120 mm. This foam was left in the air at room temperature, and the volumes Vl and VIO after 1 day and 10 days were measured and calculated using the same caliper,
Find the volumetric loading ratio using the following formula.

The resins used in Examples and Comparative Examples are shown in the table below.

Gradient Example 1 00 parts by weight of polyethylene Al and 15 parts by weight of polystyrene A were supplied to a tandem type extruder with a diameter of 40 mm to 50 mm, and after melt mixing, dichlorodifluoromethane was mixed through a blowing agent injection port provided in the middle of the extruder. resin 100
After press-fitting 30 parts per part by weight and melting and kneading, the temperature was 118°C.
The mixture was cooled to a temperature of 100.degree. C. and foamed by extrusion into the atmosphere through a tie with a circular orifice of 4 diameters.

The obtained foam has a cylindrical shape with a diameter of 25 mm and a density of 211 mm.
(1. It was a homogeneous foam with a cell diameter of 0.5 mm.

Example 2, Comparative Example 1 Polyethylene A, B, C, D, E, F, G, H, I,
Table 3 shows the results of extrusion and foaming in the same manner as in Example 1, except that polystyrene was changed to A, B, or C.

Example 3, Comparative Example 2 Polyethylene Al
Table 4 shows the results of extrusion foaming of 00 parts by weight in the same manner as in Example 1.

Example 4 Polystyrene A to 00 parts by weight of polyethylene AI
15 parts by weight were supplied to a tandem type extruder with a diameter of 55ff1g-gOmm, and after melt mixing, 30 parts of dichlorodifluoromethane was press-injected per 100 parts by weight of the mixed resin through a blowing agent injection port provided in the middle of the extruder, and after kneading, 115 parts by weight was added. The mixture was cooled to °C and extruded into the atmosphere through a rectangular die with a diameter of 2 mm x 8 Q l + 7 m through a molding die with a diameter of 25 mm x 140 ytnn.

The resulting foam was 25 ff1X 1-40 v'-,
Density 19.5F in a long rectangle of n! /1. Bubble diameter 0.5~
It was homogeneous between 1.0 and had excellent surface properties.

Table 5 shows the results of evaluation of the obtained foam based on JIS K-6767.

Table 5

[Brief explanation of the drawing]

FIG. 1 is a graph showing the temperature-viscosity relationship of two polyethylenes having different indexes (MI). The density of the sample polyethylene is 0922 for both curves (■) and (2), and the MI is 0.4 for (■) and 2.0 for (■).

Claims (3)

[Claims] (1) The total number of methyl groups is 5 to 20 per 1000 carbon atoms, the resin density is 0.927 to 0935, and the melting point is 110.
A polyethylene resin characterized by foaming a mixed resin of polyethylene with a temperature of ~120'C and an M■ of 0.1 to 4 and polystyrene with an M■ of 5 to 4° using an organic volatile foaming agent. Method for producing extruded foam. (2) The polystyrene to be mixed is polyethylene 1,0
The method for producing an extruded polyethylene resin foam according to claim 1, wherein the amount is 5 to 100 parts by weight based on 0 parts by weight. (3) The method for producing an extruded polyethylene resin foam according to claim 1, wherein the organic volatile blowing agent is dichlorodifluoromethane.