JPH02289633A - Production of foamed polyolefin resin particle - Google Patents

Abstract

PURPOSE:To produce nonflammable foamed particles having low toxicity, high quality and high expansion ratio by impregnating monochlorodifluoromethane in polyolefin resin particles, expanding the particles and subjecting to secondary expansion and tertiary expansion under specific conditions. CONSTITUTION:The objective foamed particles are produced by (1) impregnating 2-8wt.% (based on the resin) of a foaming agent composed mainly of monochlorodifluoromethane into polyolefin resin particles and expanding the particles at a ratio of 2-5.5, (2) applying a pressure to said primarily expanded particles with a gas and expanding the particles at a ratio of 6-20 with hot steam (the expansion ratio between the primary expansion and the secondary expansion is <=8) to obtain secondarily expanded particles and (3) applying a pressure to the secondarily expanded particles with a gas and subjecting the particles to tertiary expansion with hot steam at an expansion ratio of 21-40 (the expansion ratio between the secondary expansion and the tertiary expansion is <=4).

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing foamed polyolefin resin particles, and more specifically, to a method for producing foamed polyolefin resin particles using a non-flammable blowing agent with low toxicity. Relating to a manufacturing method.

[Conventional technology]

A method of using a polyolefin resin as a base resin and crosslinking and foaming it to form crosslinked polyolefin resin foam particles;
Furthermore, a method of filling these expanded particles into a mold and heating them to form a molded body is already known (Japanese Patent Laid-Open No. 57-26435).
Publication No. 48-34391, Special Publication No. 51-
22951, Japanese Patent Publication No. 53-33996).
Currently, cross-linked polyolefin resin foam particles are mainly processed into molded bodies by the method described above and used as cushioning materials, heat insulating materials, packaging materials, soundproofing materials, etc., but recently, it has been difficult to taxidermy them as particles. It has also been developed for use as a filling material inside objects, pillows, cushions, etc.

By the way, the conventional crosslinked polyolefin resin foam particles as described above have been produced using fluorine-containing halogenated hydrocarbons such as trichloromonofluoromethane, dichlorodifluoromethane, and dichlorotetrafluoroethane as blowing agents. .

However, the fluorine-containing halogenated hydrocarbons listed above are now regulated as chemical substances that destroy the ozone layer in the atmosphere and cause environmental destruction.

In order to solve the above problems, CHzC1, CzHsChFt, CHz as unregulated halogenated hydrocarbons
C1z. Various methods using CHCI2F, CICI3, etc. are being studied.

However, most of these unregulated halogenated hydrocarbons are flammable and/or toxic, making them difficult to use in practice.

Monochlorodifluoromethane is nonflammable, has low toxicity, and has properties such as boiling point, vapor pressure, and solubility that are closest to dichloromethane, which is currently most commonly used.

However, even when attempting to produce foamed polyolefin resin particles using monochlorodifluoromethane using the conventional method, ■ air bubbles inside the particles are non-uniform, ■ the shape of the foamed particles is non-uniform, and ■ the particle size is not uniform. Phenomena such as large bulges, ■ too many open cell parts and no closed cell structure, and bubble punctures occur, resulting in poor fusion and poor appearance of the molded product.
There was a problem that dimensional shrinkage occurred.

On the other hand, as an inorganic gas, N. A method has been proposed in which polyolefin resin particles are impregnated with .

However, this method is not widely used due to economical and technical difficulties.

In addition, as an alternative to the above method using inorganic gas, CO
A method of impregnating it with 2 and It20 has also been proposed (Japanese Unexamined Patent Publication No. 142683/1983).

However, CO. which should contribute to foaming. Most of the CO is dissolved in water, and the amount of CO that is actually useful for foaming is small, which is uneconomical.In addition, the water in which CO is dissolved becomes acidic, shortening the life of the equipment, and furthermore, acid components may be present in the foam particles. The remainder poses a problem when used as a molded product for cushioning materials, etc.

[Problem B to be Solved by the Invention] The present invention is a method for foaming monochlorodifluoromethane, which is nonflammable and has low toxicity, but has conventionally been thought to be unable to produce highly foamed polyolefin resin foam particles of good quality. The object of the present invention is to propose a method for obtaining highly foamed foam particles of good quality by using the foam particles as a foaming agent.

[Means to solve the problem]

According to the present invention, monochlorodifluoromethane is used as a blowing agent in polyolefin resin particles, and 2 to
8% by weight impregnation to obtain expandable resin particles, expand the expandable resin particles to an expansion ratio of 2 to 5.5 times to obtain primary expanded particles, and apply gas pressure to the primary expanded particles. After that, the secondary foamed particles are foamed with heated steam at 6 to 20 times and the expansion ratio between the primary foaming and the secondary foaming (hereinafter referred to as the secondary foaming ratio) is 8 times or less. The secondary foamed particles are then subjected to gas pressure, and then heated with steam to a foaming ratio of 21 to 40 times, and the foaming ratio between the secondary foaming and the tertiary foaming (hereinafter referred to as the tertiary foaming ratio). ) proposes a method for producing expanded polyolefin resin particles characterized by obtaining tertiary expanded particles by foaming to a size of 4 times or less.

Polyolefin resins used as base resins in the present invention include homopolymers of olefins such as polyethylene and polypropylene, copolymers of different olefins such as copolymers of ethylene and propylene, and copolymers of ethylene and vinyl acetate. It is a copolymer of an olefin containing 50 mol% or more of an olefin component, such as a copolymer of ethylene and methyl methacrylate, and other monomers.
When this material is crosslinked, it may be crosslinked by electron beam irradiation or addition of an organic radical generator. In this case, crosslinking is preferably carried out until a high crosslinked state with a gel fraction of 30% or more is achieved.

To prepare particles of this crosslinked polyolefin resin,
For example, this is carried out by melt-extruding an olefin resin to form a strand, crosslinking it by irradiating it with an electron beam, and then cutting the strand and granulating it.

The size of the particles at this time is not particularly limited, but is usually 0.
Selected within the range of 3 to 3III1.

In the present invention, the base resin particles prepared in this manner are impregnated with monochlorodifluoromethane as a blowing agent in an amount of 2 to 8% by weight based on the weight of the resin, and the expandable resin particles are foamed. Foam to a magnification of 2 to 5.5 times
Secondary foamed particles are obtained, and then gas pressure is applied, and then heated steam is used to foam the foam to a ratio of 6 to 20 times and a secondary expansion ratio of 8 times or less to obtain secondary foamed particles. It is characterized in that tertiary expanded particles are obtained by foaming in the same manner to a ratio of 21 to 40 times, and at a tertiary expansion ratio of 4 times or less.

The above-mentioned primary foaming can also be carried out by impregnating the base material particles obtained as described above with a blowing agent mainly consisting of monochlorodifluoromethane, and then heating and foaming them until a desired expansion ratio is achieved. According to the method disclosed in JP-A No. 47-2643, olefin resin particles are simultaneously impregnated with a crosslinking agent and a blowing agent mainly composed of monochlorodifluoromethane in an aqueous liquid, and the mixture is heated to effect crosslinking and foaming at the same time. It's okay. In addition, cross-linked olefin resin particles are heated and impregnated with a blowing agent mainly composed of monochlorodifluoromethane in an aqueous liquid in a pressurized system, and after reaching a nearly equilibrium impregnation state,
Primary foamed particles can also be obtained by pressurizing the inside of the system with an inert gas such as air, nitrogen, carbon dioxide, helium, etc., and releasing resin particles out of the system at the same time as other compositions to cause foaming. .

The amount of monochlorodifluoromethane impregnated into the polyolefin resin particles is selected in the range of 2 to 8% by weight based on the weight of the resin. The monochlorodifluoromethane-based blowing agent used to contain the blowing agent usually contains other physical blowing agents, such as halogenated hydrocarbons such as ethylene chloride and ethyl chloride, in an amount of 10% by weight or less, preferably 5% by weight or less. They can be used by mixing in different amounts. When considering the working environment, it is desirable that the monochrome difluoromethane be pure or in a state close to pure.

When the amount of monochlorodifluoromethane that is impregnated exceeds 8% by weight, which has a high dissipation property from resin particles, the difference in the concentration of the blowing agent between the center and the surface layer of the resin particles becomes large in a short period of time after impregnation until foaming, resulting in foaming. The internal bubbles of the particles vary considerably, and if the amount of impregnation is less than 2% by weight, the foaming ability becomes insufficient and it becomes difficult to obtain primary foamed particles with an expansion ratio of 2 to 5.5 times. The amount of monochlorodifluoromethane impregnated is 8
If the non-uniformity of the internal bubbles in the primary foamed particles exceeds the weight percentage, the bubble film will partially break during heat molding, reducing the expansion ability, and as a result, the fusion state of the molded product will be insufficient. .

In addition, in order to obtain a high magnification of more than 5.5 times per foamable resin particle by one foaming, it is necessary to foam at a high temperature and under unreasonable conditions, resulting in variations in the particle size of the resulting foamed particles. growing. If expanded particles with large particle size variations are used for in-mold molding, the mechanical properties and cushioning performance of the resulting molded product will deteriorate, and the surface of the molded product will be mixed with large and small particles, which is undesirable in terms of appearance. "do not have. On the other hand, if the expansion ratio of the primary foaming is less than 2 times, the expansion force of the expandable resin particles will be unnaturally suppressed, and it will be easily influenced by foaming conditions such as temperature unevenness of heated steam. , the variation in particle size of the resulting expanded particles increases. Next, in the present invention, the granular foam thus prepared is further subjected to secondary foaming and tertiary foaming, but it is necessary to perform a foaming ability imparting treatment prior to each foaming.

The foaming ability imparting treatment before the secondary and tertiary foaming can be carried out by impregnating monochlorodifluoromethane as in the case of the primary foaming, but generally the temperature is 40 to 8 oC and 2 to 2 oC.
It is preferable to infiltrate a foaming gas, such as an inert gas such as air, nitrogen, carbon dioxide, or helium, into the granular foam under conditions maintained at 0 kg/cd (gauge pressure). At this time, if the pressure does not reach 2 kg/Cli (gauge pressure), it takes more time than necessary to infiltrate the foaming gas into the foamed particles and complete the foaming ability imparting process, resulting in a decrease in productivity. If the pressure exceeds 20 kg/c+Il (gauge pressure), the equipment becomes too large, leading to problems such as increased equipment costs.

The secondary foaming is performed at a foaming ratio of 6 to 20 times, and at a secondary foaming ratio of 8 times or less. If the primary foamed particles are expanded to a high magnification of more than 20 times in a single foaming process, the thermal and mechanical loads applied to the cell membranes that make up the foamed particles will increase, eventually causing damage to the cell membranes and causing the foaming to deteriorate. The closed cell structure of the particles is damaged, and the mechanical performance and cushioning performance of the molded article are significantly reduced.

Furthermore, the expansion force of the expanded particles decreases, resulting in insufficient fusion of the molded article. If the expansion ratio in the secondary foaming is less than 6 times, it will be difficult to obtain a foam having the expansion ratio within the desired range in the tertiary foaming.

The tertiary foaming is performed at a foaming ratio of 21 to 40 times, and at a ratio of 3x foaming ratio or less. If the secondary foamed particles are made to have a high magnification of more than 40 times, the skin and/or cell membrane that constitutes the foamed particles will become extremely thin, and as a result, they will not be able to withstand the internal pressure of the cells, causing them to partially expand abnormally. Eventually, a phenomenon of tearing (hereinafter referred to as a puncture phenomenon) occurs.

Furthermore, even when the second expansion ratio and the third expansion ratio exceed 8 times and 4 times, respectively, the expansion of the foamed particles becomes uneven due to rapid expansion, and the skin of the foamed particles becomes uneven. and/or a puncture phenomenon occurs as a result of the bubble membrane becoming extremely thin in some areas. The molded product obtained from the expanded particles in which this puncture phenomenon has occurred is of poor quality and has significantly reduced mechanical performance and cushioning performance. In addition, marks of broken bubbles remain on the surface of the molded product, which is unfavorable in terms of appearance.

As described above, it is possible to obtain a granular foam with a high expansion ratio and no variation in volume, density, etc. This material shows little change in volume and density, and therefore, when used as a raw material for mass production, it is possible to supply a constant amount by controlling the supply amount based on the weight standard.
The granular foam obtained by the method of the present invention has the advantage that the quality of the molded product will not vary because there is no risk of classification occurring in the storage tank or transport pipe.
Alternatively, a homogeneous foam molded article having a shape faithful to the mold can be obtained by performing the foaming ability imparting treatment again and then heat-forming in a mold according to a conventional method.

The foaming ability imparting treatment at this time can be carried out by permeating the foaming gas into the granular foam under high temperature and pressure.
This can also be done by compressing it to 0%. If the volume after compression is larger than 90%, in other words, if the volume reduction rate is smaller than 10%, the foaming ability during molding will be insufficient, resulting in a molded product with many wrinkles; When the volume after compression is less than 50%,
In other words, if the volume reduction rate exceeds 50%, the fusion between the particles inside the molded product will not proceed sufficiently during molding, making it impossible to provide a high-quality molded product. This compression treatment involves, for example, filling a granular foam before molding into a closed container or a mold,
It is advantageous to apply pressure to this, start hot molding while the pressure is maintained, and complete the molding while gradually releasing the pressure. It is preferable to apply pressure during this compression so that the pressure is applied almost uniformly to the entire granular foam. Therefore, in this respect as well, direct pressurization with pressurized gas is more convenient than mechanical compression. be.

9 This foaming ability imparting treatment by compression gives good results when using the above-mentioned granular foam obtained by the method of the present invention, and can be applied to granular foams with a lot of variation obtained by the conventional method. Then, it becomes impossible to mold. This is considered to be because even if the particles are compressed under the same conditions, the compressed state will actually differ depending on the density of each particle, causing variation in the foaming ability of each particle.

In addition, in-mold molding is usually 0. 3 ~2. 0 kg
/ co! (gauge pressure) using saturated steam, 20 to 1
This can be done by heating for 00 seconds.

In this way, by in-mold molding using the granular foam obtained by the method of the present invention, it is possible to produce an excellent foamed molded product with consistent quality on an industrial scale.

〔Example〕

Next, the present invention will be explained in more detail with reference to Examples. In each example, the terms "dispersion of bubbles inside the foamed particles, distortion of the shape of the foamed particles, variation of the foamed particles, closed cell ratio, puncture phenomenon, fusion, appearance, and performance" are used in the following meanings.

<Evaluation Method> 20 samples of Burtsky foamed particles of '' were visually observed at 30x magnification with respect to the cross section of the sample cut at the center cross section.

◎: The size of the bubbles is uniform within the particle cross section (see Figure 1 (A)) O: The bubbles in the surface layer are slightly larger than the center (see Figure 1 (B)) ×: The bubbles in the upper surface layer are extremely large compared to the center (
(See Figure 1 (C)) Samples of 50 particles, 10 particles each, were collected from five arbitrary locations, and each particle was measured using a magnifying glass with a scale to measure the maximum from the cross section of the sample cut at its center. Diameter (D
1) and the minimum diameter (D2) were measured, and the representative diameter (D+z) of each particle was determined from the arithmetic average value of the maximum diameter (D+) and the minimum diameter (D2).

Next, add the representative diameters of all 50 particles and divide that value by the total number of particles (50) to obtain the average diameter (n)υ Average value judgment scale for the variation of expanded particles ◎: R of all 50 particles is R≦10 Big and pi≦50:50
R of all particles is 10, R≦20, and 5<-R-≦10×: Those that do not fall under the above conditions.The rate of insanity is measured by the air pycnometer method (AST D-2856), and as follows: asked.

◎: Closed cell ratio is 95% or more. ○: Closed cell ratio is 90% or more and less than 95%. ×: Closed cell ratio is less than 90%. Observed and determined as follows.

◎: No banking phenomenon of the foamed particle skin or bubble film was observed.

○: 10 or less punctures on the skin or bubble membrane of the foamed particles ×: 1 or more punctures on the skin or bubble membrane of the foamed particles Severely-1 A flat plate with dimensions of 300 mm x 300 m x 50 openings for the length, width, and height, respectively. A cut with a depth of 20 mm was made in the height direction of the molded test piece, and the remaining part was torn by bending, and the number of torn and cracked particles was expressed as a percentage of the total number of expanded particles present in the cross section of the torn part, and was determined as follows. .

◎: 90% or more ○: 90% or less and less than 80%
Wrinkles, unevenness between foamed particles, and overall deformation and distortion on the surface of a flat plate-shaped test piece of I1×50 mII1 were observed with the naked eye and determined as follows.

◎: Very good condition 0: Good condition The compressive stress value when 25% strain is generated is determined (JIS-20234 method).

○: The molded product obtained by the method according to the example has the same compressive stress value as the molded product obtained by the method according to the example when compared with the molded product at the same magnification ×: The molded product obtained by the method according to the example when compared with the molded product at the same magnification The compressive stress value is inferior to that of the L value. Good: The quality performance is considered to be extremely good. The quality performance is considered to be good. The quality performance is considered to be good. The quality performance is considered to be good. The example is an experiment to demonstrate that there is an appropriate range for the impregnation position of the blowing agent that is impregnated into particles. Therefore, the expansion ratio for the primary foam particles was set to the same value.

The compositions shown in Table 1 were placed in an autoclave, and while stirring in the autoclave, the temperature was raised from room temperature to 210°C at a rate of 2°C/min to impregnate the composition until it reached an almost equilibrium impregnated state. After reaching the temperature, one end of the autoclave was opened while pressurizing the inside of the autoclave with compressed air, and the resin particles were released into the atmosphere together with other compositions to form expanded particles shown in Table 1.

(Left below) According to the results in Table 1, the appropriate content of the blowing agent is 8g/1
00g resin particles or less, preferably 6-2g/100g
It is shown that they are resin particles.

Experimental Examples 5 to 8 These experimental examples demonstrate that there is an appropriate range for the magnification of primary expanded particles.

For this purpose, the amount of foaming agent impregnated into the resin particles was made to be the same.

Low-density polyethylene (manufactured by Asahi Kasei Corporation, Suntec LD)
, density 0.930g/cd, gel fraction 57%) 1000
g, water 4000 g, monochlorodifluoromethane 23
0 g and 30 g of magnesium carbonate were placed in an autoclave, and heated to a predetermined temperature at a rate of 2°C/min while stirring, impregnating the foaming agent into the resin particles and bringing them into a state of almost equilibrium impregnation. After reaching the temperature, one end of the autoclave was opened while the inside of the autoclave was pressurized with compressed air, and the resin particles were released into the atmosphere together with other compositions to form expanded particles shown in Table 2.

According to the results in Table 2, the appropriate magnification of the primary expanded particles is 2.
It has been shown to be in the range of ~5.5 times.

Experimental Examples 9 to 14 These experimental examples demonstrate that there is an appropriate range for the magnification of secondary expanded particles. For this purpose, the magnification of the primary expanded beads was adjusted to 3 times and 5.5 times, and then the magnification of the secondary expanded beads was changed.

The primary expanded particles obtained in Experimental Example 6.7 are placed in a pressurized container, and then the inside of the pressurized container is pressurized with compressed air to add a predetermined amount of air into the bubbles of the expanded particles to expand them. The particles were then transferred to a foaming machine and heated and foamed with steam at a pressure of 0.6 kg/c+fl (gauge pressure) to obtain secondary foamed particles shown in Table 3. The closed cell ratio of these secondary expanded particles was evaluated.

The secondary foamed particles listed in Table 3 obtained in Experimental Examples 9, 10 and 11 were placed in a pressurized container, and then the pressurized container was pressurized with compressed air to inject a predetermined amount into the bubbles of the expanded particles. After adding air to form expandable particles, the particles are transferred to a foaming machine and the pressure is reduced to 0. 6
The mixture was heated and foamed with steam at kg/cd (gauge pressure) to obtain tertiary expanded particles shown in Table 4.

The puncture phenomenon of these tertiary expanded particles was evaluated.

According to the results in Table 4 and Table 3, it is shown that the appropriate range of magnification for secondary expanded particles is 6 to 20 times. Experimental Examples 15 to 1
9 This experimental example is an experiment to demonstrate that there is an appropriate range of magnification for tertiary foaming. For this purpose, the magnification of the secondary expanded particles was adjusted to 6 times, 16 times, and 20 times, and then the magnification of the tertiary expanded particles was changed. According to the results in Table 4, 3
It has been shown that the appropriate range of magnification for the sub-expanded particles is 21 to 40 times.

Examples and Comparative Examples This example shows the fact that a foamed molded article made of expanded particles obtained by the manufacturing method of the present invention using a blowing agent mainly consisting of monochlorodifluoromethane for polyolefin resin particles shows excellent quality and performance. It is an attempt to prove that. To describe the experimental method below, each foamed particle obtained by the method corresponding to each experiment number shown in Table 5 was processed using a polyethylene foamed particle automatic molding machine (manufactured by Kasahara Kogyo ■, model PIONY-75-
MEF) was used to create a plate-shaped molded body (
Length x width x height dimensions are 300+nm x 300mm
x 300mm) was molded and evaluated. The evaluation results are shown in Table 6. The optimum molding conditions were selected for each molding condition. The objectives of each experiment will be explained below in order of experiment number.

In the Examples and Comparative Examples shown in Table 6 below, for example, Comparative Example 1 was obtained by repeatedly foaming resin particles with an impregnated amount of more than 8 parts, which was outside the requirements of the present invention, and then by the above-mentioned method. The molded product was obtained and evaluated, and it was found that the fusion state was poor and the performance was significantly inferior.

In addition, in Comparative Example 10, the magnification of each of the primary to tertiary expanded particles is outside the requirements of the present invention based on the amount of impregnated resin particles, and the resulting molded product has poor fusion and appearance. , all performance items were completely inferior.

Similarly, in Comparative Examples 2, 3.4, and 5, the magnification of the primary expanded particles was, and in Comparative Example 6, the magnification of the secondary expanded particles was
In Comparative Example 7, the magnification of the tertiary expanded particles was
In Comparative Example 9, the second expansion ratio and the third expansion ratio in Comparative Example 9 are outside the requirements of the present invention. One or more of them were defective. Compared to these Comparative Examples, the molded bodies made of expanded particles obtained by the method of the Examples that satisfy the constituent requirements of the present invention all had good fusion, appearance, and performance, and the effects of the present invention were recognized. .

〔Effect of the invention〕

According to the present invention, monochlorodifluoromethane, which is nonflammable and has low toxicity, but which has conventionally been thought to be unable to produce high quality polyolefin resin foam particles, is used as a blowing agent. Foamed polyolefin resin foam particles can be obtained.

[Brief explanation of the drawing]

FIG. 1(A), FIG. 1(B), and FIG. 1(C) are enlarged cross-sectional views of foamed particles showing a guideline for evaluating the dispersion of air bubbles within the foamed particles. Patent applicant: Asahi Kasei Industries, Ltd.

Claims (1)

[Claims] Polyolefin resin particles are impregnated with a blowing agent mainly consisting of monochlorodifluoromethane in an amount of 2 to 8% by weight based on the resin weight to obtain expandable resin particles, and the expandable resin particles are expanded to an expansion ratio of 2 to 5.5 times. After obtaining the primary foamed particles and applying gas pressure to the primary foamed particles, foaming is performed with heated steam to an expansion ratio of 6 to 20 times, and an expansion ratio of 8 times or less between the primary expansion and the secondary expansion. to obtain secondary expanded particles, and then
After applying gas pressure to the secondary foamed particles, they are expanded with heating steam to a temperature of 21 to 40 times, and the expansion ratio between the secondary foaming and the tertiary foaming is 4 times or less to obtain tertiary foamed particles. A method for producing expanded polyolefin resin particles characterized by