JP3207219B2 - Low-expanded particles of polyolefin resin and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to foaming agents such as CFC and HC.
A method for producing a polyolefin-based resin as a foaming agent by impregnating polyolefin-based resin particles in a gas phase with carbon dioxide that meets the FC regulations, and a method for producing a polyolefin-based resin for the first time, which was achieved for the first time by a conventional method, "bubble 1 The present invention relates to a low-magnification expanded particle of a polyolefin resin, which is improved in that the apparent film thickness per unit is thick within a certain range (4 to 26 μm) .

[0002]

2. Description of the Related Art Conventionally, as foamed particles to be subjected to in-mold molding, it is considered that foamed particles having a homogeneous structure "foamed particles having a closed cell structure and uniform cell diameter and particle diameter" are ideal. This is because foamed particles with a homogeneous structure are better when they are filled in a mold and heated to expand the foamed particles to form a closely fused molded body in which the spaces between the particles are filled. It is based on the fact that the filling state in the mold cavity is uniform, the expansion ability previously given to the foamed particles is greatly expressed, and the characteristics of the obtained molded body are essentially improved. It is considered.

[0003] For example, Japanese Patent Publication No. 61-11253 and Japanese Patent Publication No. 2-50944 disclose that foamed particles having a low expansion ratio are prepared in advance, and expanded in a stepwise manner to obtain expanded particles having a target expansion ratio. The method of obtaining is described. This technical concept is based on the fact that foamed particles having a uniform closed cell structure are required for re-expansion of foamed particles. Is a homogeneous closed cell structure and particle shape (particle size)
Are taught to be in the same state. However, Japanese Patent Publication No. 61-11253 and Japanese Patent Publication No. 2-5094
In JP-A No. 4 (KOKAI) No. 4, the technical content of a method for obtaining low-magnification expanded particles of a polyolefin resin having a large “apparent film thickness per cell” and a foaming method having a large “apparent film thickness per cell” are described. There is no disclosure of the specific effects exerted by the particles.

[0004] On the other hand, the use of carbon dioxide as a blowing agent for polyolefin resins has been described in, for example, Japanese Patent Publication No. 62-47 / 1987.
No. 77 and Japanese Patent Publication No. 62-61227, which are well known. In addition, foamed resin particles obtained by using carbon dioxide as a foaming agent for resin particles can be expanded into an in-mold molded article by adding an inorganic gas such as air to the expanded resin particles and imparting expandability thereto, followed by heat molding in a mold. For example, Japanese Patent Publication No. 63-419
No. 42, and the like. In these three inventions, the problem of depletion of the ozone layer is emphasized as a global environmental problem, and has been used as a foaming agent. For example, volatile organic foaming agents such as dichlorodifluoromethane and monochlorodifluoromethane have been restricted in use by CFC and HCFC regulations. Attention.

However, it is not easy to use carbon dioxide as a blowing agent for resin particles. The reason is that carbon dioxide is generally less soluble in a resin than a volatile organic foaming agent, and it is difficult to impregnate it directly into a polyolefin resin. That the impregnated carbon dioxide escapes too quickly from the resin,
This is probably because there is a problem that impregnated carbon dioxide itself becomes a cell nucleus and inhibits cell growth, and high-quality expanded particles that can be formed in a mold cannot be obtained. For this reason, in the above three invention techniques, all of the aqueous dispersion medium in which carbon dioxide is dissolved is contacted and impregnated with the resin particles, and `` the obtained expandable particles are dispersed with the dispersion medium under high temperature and high pressure. It is characterized by the fact that it employs a method of releasing and foaming under a low-pressure atmosphere.

[0006]

However, in the above three inventions using carbon dioxide, there are disadvantages in that foamed particles having a homogeneous structure "foamed particles having a closed cell structure and uniform cell diameter and particle diameter" cannot be obtained. Further, there is a drawback that the expanded particles have poor expandability, and as a result, there remains a problem that expanded particles suitable for in-mold molding cannot be obtained. This is because of the inherent difficulty of the foaming method itself, i.e., fluctuations in the foaming agent component and pressure in the container and in the dispersion medium caused by impregnation of the foaming agent and release of the foamable particles, resulting in the foamed particles being obtained. Unsolved problems that cause the bubble structure of the particles to vary, and ii) the bubble structure and particle shape of the particles tend to be irregular due to the temperature gradient in the particle group generated when the particles are released together with the dispersion medium. Remains. In addition, it is presumed that the difficulty of using carbon dioxide as a blowing agent to reduce the foaming has been added.

In view of the above-mentioned situation, the present inventors have conducted long-term research on using carbon dioxide as a foaming agent for resin particles. As a result, the "apparent film thickness per bubble" is smaller than that of a conventional product. It has succeeded in obtaining thick , improved low expansion ratio expanded particles. And surprisingly,
The foamed particles completed in the present invention are superior to any of the low-foamed foamed particles obtained using a conventional volatile organic foaming agent in the expansion performance and the maintainability of the closed cell ratio, and are used for in-mold molding. The present inventors have investigated the fact that the foamed particles are suitable for the present invention and have completed the present invention. Thus, an object of the present invention is to provide a CFC,
Using carbon dioxide to clear HCFC regulations,
It is to provide a production method capable of obtaining expanded polyolefin resin particles having excellent expansion performance and excellent maintainability of a closed cell rate, compared to any of the conventional expanded particles having a low expansion ratio,
As a result, an object of the present invention is to provide expanded polyolefin resin particles having a low expansion ratio suitable for in-mold molding.

[0008]

The object of the present invention is to provide a foamed particle of the present invention, that is, a foamed particle of a closed cell structure made of a polyolefin resin and having a low expansion ratio, wherein the foamed structure of the foamed particle is: Foaming ratio is 1.5 to 3.8c
m ³ / g, bubbles are almost uniformly distributed in the direction of particle diameter, and “ apparent film thickness per bubble” is in a range of 4 to 26 μm. The low-expanded particles of the resin-based resin, and the production method of the present invention, that is, the polyolefin-based resin particles are held in a gas atmosphere in a high-pressure state lower than the critical pressure of carbon dioxide, and the resin content of the resin particles is 100%. The resin particles are impregnated with less than 5 parts by weight of carbon dioxide gas with respect to parts by weight to form expandable resin particles, which are then heated and expanded, and the expansion ratio is 1.5 to 3 It can be easily achieved by adopting a method for producing low-expanded polyolefin resin particles, which is characterized by making expanded particles of 0.8 cm ³ / g.

Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic enlarged cross-sectional view (× 150) of the expanded particles of the present invention (gas phase impregnation with carbon dioxide: impregnation amount 1.6 parts by weight, expansion ratio 2.6 cm ³ / g). FIG. 2 is an enlarged schematic (× 150) cross-sectional view of a foamed particle of a comparative product (having a liquid phase impregnated with carbon dioxide dissolved, impregnating amount cannot be measured, and an expansion ratio of 2.6 cm ³ / g). FIG. 3 is a schematic enlarged cross-sectional view (× 150 times) of a conventional expanded particle (having a liquid phase impregnation of monochlorodifluoromethane, an impregnation amount of 4.0 parts by weight, and an expansion ratio of 2.6 cm ³ / g).

First, in FIGS. 1 to 3, these are low-foamed particles (in this case, the expansion ratios are shown as uniform) using a polyolefin-based resin as a base resin. Generally, these particles are rich in closed cells (90% or more closed cells ), so that they are impregnated with an inorganic gas such as air at the time of practical use, heated and expanded to a target expansion ratio, and subjected to in-mold molding. It is known as a foamed particle having a usefulness. Looking at the characteristics of the foaming structure of each of the three types of foaming particles in comparison with FIGS. 1 to 3, the low-foaming particles of the present invention (FIG. 1) have relatively large air bubbles and relatively uniform diameters. In this state, they are almost evenly arranged in the resin (particles).

On the other hand, in the foamed particles of the comparative product (FIG. 2), a small number of cells having a large diameter and a large number of cells having an extremely small diameter are mixed and distributed in the resin (particles). It can be seen that the expansion ratio is maintained by the bubbles. Further, the foamed particles of the conventional product (FIG. 3) have a foam structure having a large number of bubbles, in which bubbles having a relatively small diameter are arranged almost uniformly in the resin (particles) in a state where the diameters vary. It turns out that it is.

In general, the expansion ratio is determined by the relationship between the bubble diameter and the number of bubbles occupying a certain volume. However, even in the case of particles having characteristics such as the expanded particles of the present invention, as particles having an expansion ratio having a certain range, when it is desired to express this as a whole, the average cell diameter, the number of cells per unit area, etc. When trying to express the features, the operation of averaging and expressing the variation in a range may inadvertently expand the actual expression content, and there is a possibility that the expression will include the foam particles of the conventional product . Therefore, the present inventors have devised a method of expressing a foamed structure that can be distinguished from conventional products and that can express the characteristics of the particles of the present invention.

That is, it is a constituent factor of the expanded particles of the present invention, and the expansion ratio is 1.5 to 3.8 cm ³ /
in the range of g, the bubbles are evenly distributed in the direction of the particle diameter, "apparent film thickness per one bubble" is expressed to be in the range of 4～26Myuemu. First,
Indicates a region of a degree of expansion that can be realized as the expanded particles of the present invention. That is, it means that the particles are in a very low foaming state limited to a narrow range. The reason for this limitation is that if the expansion ratio is less than 1.5, the resin amount in the whole particles becomes too large,
In the case of thermal expansion in which an inorganic gas such as air is impregnated, a gas impregnating operation becomes difficult, and a defect occurs in which the expanding ability to be exerted is reduced. Conversely, when the expansion ratio is 3.8 or more, the diameter of all the cells constituting the particles is too small or the distribution of the cell diameters is large, and the expanded particles defined in the present invention are not suitable. Can't embody. This is a unique phenomenon of the present invention in which carbon dioxide impregnated in the gas phase is used as a blowing agent. The expansion ratio viewed from the above-mentioned phenomenon tendency is desirably selected from the range of 2 to 3.2 cm ³ / g.

[0014] In the context of the above, the constituent features symbolize the features of the present invention. Specifically, the particles of the present invention are formed of a relatively small number of cell populations having a relatively large diameter, thereby ensuring an expansion ratio. That is, it means that the amount of the resin component located around each bubble is relatively large. The amount of this resin component is substantially
It shows the thickness . In the present invention, the expression of the relatively large amount of the resin component is referred to as “apparent film thickness per bubble”.
The characteristic is expressed as a range of “4 to 26 μm”. According to this coefficient, FIGS.
The expanded particles of No. 3 were 7.6 μm, 1.3 μm, and 2.8, respectively.
μm (average bubble diameters are 68 μm and 11 μm, respectively).
μm, 20 μm), which indicates that the bubble structure of the particles (FIG. 1) of the present invention is well represented and is classified as a known particle.

The requirement is to show the concept of the target expanded particle. That is, in the above coefficient expression, for example, there is a possibility that foamed particles having a structure as shown in FIGS. Expanded particles, such as 6 and 7 but is not clear whether existing, assuming that existing if the expanded beads of this type, the common sense, is considered to be unsuitable for mold molding. Therefore, in the present invention, by providing the requirements, foamed particles in which bubbles are unevenly distributed in the particle diameter direction as shown in FIGS.

FIGS. 4 and 5 are graphs showing typical effects of the expanded particles of the present invention. First, FIG. 4 shows the internal pressure [kg / cm ² · G] of the gas (air) applied to the foamed particles, and the expansion ratio [cm ³ /
g] is a graph showing the relationship. FIG. 5 is a graph showing the retention of closed cells in the expansion process confirmed in the experiment of FIG. In the figure, ○ indicates the case where the expanded particles of the present invention of FIG. 1 were used, ☆ indicates the case where the expanded particles of the comparative product of FIG. 2 were used, and □ indicates that the expanded particles of the conventional product of FIG. 3 were used. If you have
Is shown.

As is apparent from the results shown in FIGS. 4 and 5, the expanded particles of the present invention are superior to both the comparative product and the conventional product in both expansion performance and closed-cell retention, and have a good It can be seen that the foamed particles have a performance suitable for molding, particularly for in-mold molding with a high expansion ratio, which requires a repeated expansion operation. By the way, according to experiments by the inventors, the expansion efficiency of the imparted expansion capacity (internal gas pressure) is 73% or less for both the comparative product and the conventional product of the expanded particles, whereas the expansion efficiency of the conventional product is 73% or less. Shows a value of 85-87%. In addition, the maximum expansion ratio that can be made into highly expanded particles in a state where the closed cell ratio is maintained at 90% or more by one-stage expansion is about 12 times for the comparative product and about 10 times for the conventional product. It has been confirmed that the expanded particles of the present invention exhibit a value as high as about 18 times.

The above-mentioned phenomena and effects are caused by the fact that bubbles forming the expanded particles of the present invention have a large apparent film thickness, so that the expansion gas is prevented from escaping during the heat expansion process, and individual bubbles This is presumed to be due to the fact that the expansion of the bubble film along with the growth of the cells was easily performed. The above-mentioned expanded particles of the present invention have an average cell diameter of 40 to 110 μm (preferably 50 to 1 μm).
(00 μm), as the method of uniforming the bubble diameter, the presence of bubbles having a diameter outside the range of the apparent average bubble diameter ± 50% is 15% or less of the observable number of bubbles (strictly 10% or less). It is desirable. As described above, the completion of the foamed particles of the present invention having a uniform internal cell structure is uniform because the expansion ratio is also uniform. Therefore, if spherical resin particles having a uniform diameter are used as a raw material, the expanded particles having a uniform structure are obtained. "Expanded particles having a closed cell structure and uniform cell diameter and particle diameter" can be obtained, which means that the most suitable expanded particles for in-mold molding are completed, and their usefulness is shown.

Next, the method for producing expanded beads of the present invention will be described. The main point of the production method of the present invention is to impregnate the resin particles with carbon dioxide. A) Under a high-pressure atmosphere (specifically, 15 to 50 kg / cm ² · G) lower than the critical pressure of carbon dioxide, Contact impregnation of resin particles in a gaseous state (gas phase) with carbon; b) Impregnation amount is less than 5 parts by weight based on 100 parts by weight of resin content of resin particles. .

First, the necessity of the above-mentioned main point a) is, for example, to make the atmosphere above the critical pressure, to make carbon dioxide in a liquid (liquid phase) state, and to impregnate the resin particles with the pressure-resistant device. Not only does the cooling device become large-scale, but also the obtained foamed particles have a greater variation in cell diameter than that of FIG. Further, a method of contacting and impregnating a resin in a state in which carbon dioxide is dissolved in an aqueous dispersion medium, as represented by JP-B-62-44777, is performed by expanding and expanding as shown in FIG. Use would result in unsuitable foamed particles. On the other hand, in the impregnation of the present invention in an atmosphere of less than the critical pressure of carbon dioxide, the impregnation temperature can be performed at a low temperature, and the obtained expanded particles also have a homogeneous structure as shown in FIG. There are advantages.

The practical impregnation temperature in this case is a certain temperature in the range of 5 to 20 ° C. in the case of gas phase impregnation of carbon dioxide alone, and 20 to 30 ° C. in the case of mixed gas phase impregnation with an impregnation aid. It is desirable to maintain each of them at a certain temperature in the range of in order to obtain a homogeneous cell structure. In other words, the role of the impregnation aid is to increase the economics of temperature control by bringing the impregnation temperature close to room temperature. Here, the impregnation aid is a volatile organic solvent that produces a plasticizing effect on the base resin. Among them, methyl chloride, methylene chloride, ethyl chloride,
Halogenated hydrocarbons such as ethylene chloride are desirable, and since methylene chloride is particularly nonflammable and has a relatively low boiling point of 40 ° C., it is easy to carry out post-treatment such as volatilization and removal from the impregnated particles, especially desirable.

Next, the necessity of the main point b) is that the resin particles
If the impregnation amount exceeds 5 parts by weight with respect to 100 parts by weight of the resin component, there is a phenomenon that the bubbles of the obtained expanded particles are miniaturized. This phenomenon does not disappear even if the foaming conditions are adjusted. This is probably because the carbon dioxide gas itself acts as a nucleating agent during foaming. However, if the impregnation amount is too small close to zero, foaming is naturally inhibited. According to the knowledge of the inventors, the amount of resin particles impregnated with respect to 100 parts by weight of resin is determined by the type of resin, and preferably 1.1 to 1.8 parts by weight of polyethylene resin and 2 parts by weight of polypropylene resin. The selection is made in the range of 0.5 to 4.5 parts by weight in accordance with the target expansion ratio.

The expandable resin particles satisfying the above-mentioned main points a) and b) and impregnated with carbon dioxide are heated to a suitable foaming temperature of the base resin and foamed. Foaming in this case is true foaming in which carbon dioxide as a foaming agent forms and grows bubbles in the resin particles, and the bubbles increase the volume of the particles and increase the expansion ratio. Therefore, as the adjustment conditions, it is important to mainly set and control the heating (foaming) temperature “base resin melting point− (1 to 5 ° C.)”, the rate of temperature rise, and the heating time. However, these differ depending on the type of foaming (method) apparatus to be employed, the size of the scale, and the like, so that it is difficult to express them numerically.

Therefore, in short, c) "the expansion ratio is 1.
The above-mentioned foaming conditions are set in accordance with the employed apparatus, with the index of “to make foamed particles of 5 to 3.8 cm ³ / g”.
Under foaming conditions in which the expansion ratio is less than 1.5, the obtained particles lack the volume of the bubble region occupying the resin component, and become particles which are difficult to expand and use later. On the other hand, under the expansion conditions in which the expansion ratio exceeds 3.8, the obtained particles have a reduced bubble size, or a phenomenon in which microbubbles and maximal bubbles are mixed occurs, which makes it difficult to expand and use later. Particles. Looking at the above phenomenon trends,
It is desirable to control the expansion ratio in a narrow range of 2 to 3.2 cm ³ / g in order to obtain high-quality expanded particles.

Further, it should be noted on the foaming conditions that the "heating rate", which indicates the speed at which the whole of the foamed particles in the foaming apparatus rises to the foaming temperature, is increased. This necessity is that the rate of carbon dioxide escaping from the resin is 2 to 4 times higher than that of a conventionally used fluorocarbon volatile organic foaming agent.
It is twice as fast. Therefore, increasing the “heating rate” is for effectively utilizing the impregnated carbon dioxide as a blowing agent. One of the measures to determine the suitability of the above-mentioned “heating rate” is the thickness dimension of the non-foamed skin formed on the surface of the obtained foamed particles. It is considered that the skin portion on the surface of the particle was generated as a result of the foaming agent (carbon dioxide) existing on the surface of the resin particles being preferentially escaping, and the thickness of the skin portion was less than 70 μm, preferably 60 μm. Is to keep it below. If the thickness dimension exceeds 70 μm, the expansion ratio of the particles does not reach the target value, and even if the expansion ratio reaches the target value, the particles are difficult to expand and use later. But,
Conversely, using the speed of the carbon dioxide escape rate, to create foamed particles having a structure in which the entire surface of the foamed particles is completely covered with a non-foamed skin portion having a thickness of 20 to 60 μm, These particles are extremely excellent in expansion performance, and high-quality expanded particles (expanded molded articles) having an expansion ratio of 80 to 100 cm ³ / g can be easily obtained by expansion and expansion in two to four stages.

According to the above-mentioned production method of the present invention, the expanded particles of the present invention having a high “closed cell film thickness” and a high closed cell structure, which are rich in expansion performance and closed cell retention, are obtained. , Can be created naturally. Generally, the expansion of expanded particles is performed by expanding the bubbles due to the thermal expansion of the gas in the individual cells constituting the particles.At this time, the expansion ability exerted depends on the tensile strength of the bubble film and the closed cells. It is greatly influenced by the high rate. In-mold molding using foamed particles based on the same principle uses the expanded strength of the foam film of the foamed particles adopted and the retention of the closed cell rate itself, resulting in an increase in the expandability that is exhibited, and foaming with a high closed cell rate A compact is formed in a state in which the particles are firmly heat-sealed with each other, thereby guaranteeing the quality characteristics of the final product. From such a viewpoint, it can be said that the expanded particles of the present invention are very useful and extremely excellent. In addition, this production method is achieved by using carbon dioxide, which meets the CFC and HCFC regulations, as a blowing agent, and its technical significance is extremely high.

The polyolefin resin referred to in the present invention is:
A general term for polyolefin-based resins that can be foamed in a cross-linked state or in a non-cross-linked state, such as high-, medium-, low-, and ultra-low-density polyethylene resins, ethylene-vinyl acetate copolymer resins, polypropylene resins, and ethylene. -A single resin such as a propylene copolymer resin or a mixed resin of two or more thereof. Above all, the range of density is 0.92
It is desirable to use a polyethylene resin of ^{5 to} 0.940 g / cm ³ or an ethylene-propylene random copolymer resin having an ethylene component of 1 to 30% by weight, since the maximum effect is exhibited.

[0028]

[Evaluation method] The evaluation method used in the present invention is shown below. Foaming ratio: The volume (Vcm ³ ) of a known foamed particle having a weight (Wg) is measured by a submersion method, and the value obtained by dividing the volume by the weight is expressed as the foaming ratio (c).
m ³ / g).

Closed cell ratio; about 24 cm ³ of expanded particles having a known expansion ratio (cm ³ / g)
The true volume of the air is 9
Measured using Model 30 and the closed cell rate (%) from the following formula (I )
(S) is calculated.

(Equation 1) V _x : the volume (cm ³ ) of the true expanded particles measured by the above apparatus. V _a: the volume of the foamed particles [expansion ratio × Weight] (cm ^3). W: Weight of expanded particles (g). ρ: density of base resin of expanded particles (g / cm ³ ).

The internal pressure of the gas (air) applied to the expanded particles. The foamed particles imparted with expanding ability by pressurizing and heating device atmospheric pressure (0 kg / cm ² · G) taken out under, and measuring the weight of the foamed particles of about 150 cm ³ when passed 6 minutes, the following equation (2 ) ) To the internal pressure (kg / cm) of the gas (air)
² · G) Calculate (P) .

(Equation 2) W ₁ : The weight (g) of the foamed particles after the swelling ability has been imparted and taken out under atmospheric pressure (0 kg / cm ² · G) for 6 minutes. W ₀ : Weight (g) of the foamed particles when the internal pressure of the foamed particles is atmospheric pressure (0 kg / cm ² · G). V: Volume (cm ³ ) of the foam particles when the pressure inside the cells of the foam particles is atmospheric pressure (0 kg / cm ² · G). M: molecular weight of pressurized gas (air) (28.98 g / mol). R: gas constant (0.082 atm · L / mol · ° K). T: Converted value temperature (293 ° K).

The “apparent film thickness per cell”, “average cell diameter”, and “apparent average cell diameter” inside the expanded beads. Using an image processing device, an enlarged image of the bubbles in the cross section of the expanded particles is analyzed and counted based on the size and number of the bubbles, and the “apparent film thickness per bubble (μm)” and the “average bubble diameter (μm)” Is calculated. Next, the procedure will be described. i) Creation of an enlarged bubble image of the cross section of the expanded particles. A substantially central portion of the foamed particles is cut with a sharp blade, and the entire cut surface of the prespecified microscopic sample is enlarged and photographed using a scanning electron microscope. Magnification 15 from this film
A photographic image having a size of 0x and a size of 340 mm x 240 mm is created, a trace paper is placed on the photographic image, and the outline of the bubble is traced to create an enlarged bubble image of the cross section of the foamed particles.

Ii) Analysis of enlarged bubble image In the above enlarged bubble image, about 1 cm in the vicinity of the expanded particle core portion is used.
A bubble group located at a size of 60 mm × 160 mm square is converted to an image processing device (color image processor SPCCA-I).
I, a trade name; manufactured by Nippon Avionics Co., Ltd.), and a grayscale image process and a binary image process are performed.
m ² ) ”,“ number of bubbles (pieces) ”,“ area of each bubble (μ
m ² ) and its distribution ”and“ total bubble area (μm ² ) ”are calculated. From this calculation, “apparent film thickness per bubble (μm)”, “average bubble diameter (μm)”, and “apparent average bubble diameter (μm)” are calculated.

<Apparent film thickness per bubble (μm)
Assuming that the film has a uniform amount of the base resin occupied by one bubble in the average bubble diameter and completely covers the film, the “apparent film thickness per bubble (μm)” from the following equation (3) (T ) Calculate the value .

(Equation 3) A ₀ ; Image processing area (μm ² ) A _g ; Total bubble area (μm ² ) N; Number of bubbles (pieces) L; Average bubble diameter (μm)

<Average Cell Diameter (μm)> The cell diameter measured on the cross section obtained by cutting the expanded particles does not completely represent the group of cells constituting the particles. That is, assuming that the bubbles are spherical, some of the bubbles visible along the cut surface are cut at the maximum diameter (= diameter) part of the bubbles and some others are smaller than the maximum diameter, Alternatively, the cut bubbles are stochastically arranged in a portion that looks much smaller. Therefore, the bubble diameter of the present invention is expressed by the following equation (4)
Coefficient to the average value of the bubble area measured by more evaluation section (6
/ Π) is the “average bubble diameter (μm)”
(L) .

(Equation 4)

<Apparent average cell diameter (μm)> For each cell diameter inside the expanded beads, there is no correction method, a circle-equivalent diameter is obtained from the cell area of the measured value, and this numerical value is used as the cell diameter. Therefore, as the “apparent average bubble diameter” (L ₀ ) of these bubble diameters, a value calculated from the following equation (5) is used.

(Equation 5)

The appearance quality of the molded article is to evaluate the phenomenon that the molding dimension at the center is insufficient with respect to the molding dimension at the corner of the molded article. A ruler is applied horizontally, and the maximum dimension of a gap (sink) generated between the molded body and the surface of the molded body is measured.

The following three items are evaluated from the viewpoint that the molded article is a foamed particle having elastic cushioning performance. -1 Compressive strength Indicates the stress when the compact is compressed at a constant speed.
The stress under 5% strain is defined as the compressive strength, and is defined by JIS Z- 0235.
The evaluation is made according to the test method. -2 Compression set This indicates the rate of recovery after a given amount of strain is applied to the molded body over a long period of time, and is evaluated by the test method of JIS K-6767. -3 Repeated compression set This indicates the recovery ratio after a given amount of strain is repeatedly applied to the molded body, and is evaluated by the test method of JIS K-6767.

[0038]

EXAMPLES Hereinafter, the contents of the present invention will be described in detail with reference to examples, but these do not limit the scope of the present invention. (Example 1) Low-density polyethylene (Suntech LD, trade name; manufactured by Asahi Kasei Corporation), density 0.930 g / cm ³ , MI = 2.
(4 g / 10 min, melting point: 117 ° C.) was impregnated with dicumyl peroxide as a crosslinking agent in an aqueous suspension.
Heat decomposition for 5 minutes, gel fraction 60% (boiling toluene x
8 hours) to obtain crosslinked polyethylene resin particles having an average particle size of 1.2 mm. The crosslinked resin particles are accommodated in a pressure-resistant container, and the water vapor in the container is replaced with dry air. Then, carbon dioxide (gas) is injected as a foaming agent, and the pressure is 32 kg / cm.
^The resin particles were impregnated with carbon dioxide over 3 hours under the conditions of ² · G and a temperature of 11 ° C. The amount of carbon dioxide impregnated was determined by taking the resin particles out of the container into the atmosphere and measuring one minute later .
It was 1.6 parts by weight.

Next, the expandable resin particles are accommodated in a foaming apparatus (a degassing and heating system), and the temperature in the tank is raised from 100 ° C. to a foaming temperature of 114.2 ° C. over 20 seconds. While maintaining the temperature, the mixture was foamed by steam heating for 10 seconds to obtain crosslinked polyethylene foam particles. The expansion ratio of the expanded particles was 2.6 cm ³ / g. The internal cell structure of the obtained expanded particles was evaluated by the evaluation method described in the text, and FIG. 1 is a schematic diagram of a local sketch in which the cross section of the expanded particle was enlarged (× 150 times). According to FIG. 1, it can be seen that in the cell structure of the expanded particles of the present invention, cells having a relatively large diameter are formed of a relatively small number of cells in the resin.

Comparative Example 1 As a comparative example, the method of impregnating and foaming the blowing agent in the blowing agent and the crosslinked polyethylene resin particles of the example was changed as follows, and the same expansion ratio (2.6 cm) as in the example was used. ³ / g), and the same conditions as in the example were used.
As can be seen from FIGS. 2 and 3, the foam structure of the obtained expanded particles shows that the comparative product and the conventional product have a structure with a large number of bubbles in which bubbles having a non-uniform bubble diameter and a small diameter are mixed. . (i) Experiment No. 1 (Comparative product) 100 parts by weight of crosslinked resin particles, 16 parts by weight of carbon dioxide (solid) as a foaming agent, 450 parts by weight of water, and 3.0 parts by weight of basic magnesium carbonate as a dispersant were placed in a pressure-resistant container and stirred. After raising the temperature to 128 ° C. and holding the resin for 45 minutes to impregnate the foaming agent into the resin, the inside of the container was cooled to 118 ° C., and then the container was pressurized with nitrogen gas at 33 kg / cm ² · G. Open one end of the container, release foam into the atmosphere,
Expanded particles having an expansion ratio of 2.6 cm ³ / g were obtained. The impregnation amount of carbon dioxide could not be measured due to the method of impregnation and foaming of the foaming agent. The cell structure inside the obtained expanded particles was evaluated in the same manner as in Example 1, and is shown in FIG. (ii) Experiment No. 2 (Conventional product) Using monochlorodifluoromethane liquid as a foaming agent,
This resin and the crosslinked resin particles are placed in a pressure vessel, and the temperature is increased while stirring, and impregnated with the volatile organic foaming agent at 45 ° C. for 90 minutes (impregnation amount of 100 parts by weight of resin of the resin particles is 4. 0 parts by weight), and then foaming was performed in the same manner as in Example 1.
Expanded particles having an expansion ratio of 2.6 cm ³ / g were obtained. The cell structure inside the expanded particles was evaluated in the same manner as in Example 1, and is shown in FIG.

Example 2 and Comparative Example 2 FIG. 1 (expanded particles of the present invention) obtained in Example 1 and Comparative Example 1
Fig. 2 (Expanded particles of comparative product), Fig. 3 (Expanded particles of conventional product)
Expanded particles having an expansion ratio of 2.6 cm ³ / g exhibiting the cell structure of the above are accommodated in a pressurizing / heating device, and the expansion pressures indicate the respective internal pressures of the expanded particles of 3, 4, 5 kg / cm ² · G. Noh was given. The conditions for the treatment for imparting the high expansion ability are as follows: high pressure air is used at a temperature of 80 ° C., the pressure is increased over 1 hour, and the pressure is 10 to 1
The pressure was adjusted so as to show the above internal pressure within a range of 5 kg / cm ² · G for 5 to 10 hours. Next, the expandable foamed particles are accommodated in a foaming apparatus, and the temperature is raised from the bath temperature of 100 ° C. to the foaming temperature of 114.5 ° C. over 15 seconds. did.

With respect to the obtained expanded particles, the expansion ratio and the closed cell ratio were evaluated by the evaluation methods described in the text, and the results are shown in graphs in FIGS. That is, FIG. 4 is a graph showing the relationship between the internal pressure of the gas (air) applied to the expanded particles and the expansion ratio that can be reached by heating and expanding the particles.
FIG. 5 is a graph showing the relationship between the expansion ratio and the closed cell ratio of the expanded particles obtained by heat expansion. 4 and 5
The circles in FIG. 1 are the lines when the expanded particles of the present invention of FIG. 1 are used, the circles are the lines when the expanded particles of the comparative product in FIG. 2 are used, and the squares are the expanded particles of the conventional product in FIG. Is a line in the case of using. It can be seen that the expanded particles of the present invention can highly increase the expansion ratio during repeated expansion (expansion) while maintaining a high closed cell ratio.

(Example 3 / Comparative Example 3) The shredded product of the low-density polyethylene used in Example 1 / Comparative Example 1 was adjusted so that the apparent film thickness per cell becomes the value shown in Table 1. And zinc stearate as a nucleating agent
For extruded shredded products with the amount added in the range of 0.3% by weight,
And pressure of 32 kg / c under carbon dioxide (gas) impregnation condition
m ² · G to 20 kg / cm ² · G, temperature 11 ° C to 13
° C (the carbon dioxide impregnation amount is 10 % of the resin content of the resin particles).
The exception that each was the) 1.2 parts by weight with respect to 0 parts by weight, and the primary foamed particle expansion ratio 2.0 cm ³ / g in the same manner as in Example 1, Comparative Example 1. For the expanded particles, the closed cell ratio, the apparent film thickness per cell, and the average cell diameter were evaluated by the evaluation method described in the text. The results are shown in Table 1.

Each of the primary foamed particles is housed in a pressurized (high-pressure air) / warming apparatus, pressurized at a temperature of 80 ° C. for 1 hour, and held at a pressure of 15 kg / cm ² · G for 8 hours. After increasing the gas (air) internal pressure of the foamed particles, the foamed particles are housed in a foaming apparatus, heated from a bath temperature of 100 ° C. to a foaming temperature in 15 seconds, and further steam-foamed for 8 seconds while maintaining the temperature. Then, secondary expanded particles having an expansion ratio of 8 cm ³ / g were once obtained. The foaming temperature was adjusted in the range of 112.5 ° C to 114.5 ° C so that the foamed particles had the above-mentioned expansion ratio. Next, after aging each of the secondary expanded particles at room temperature for 2 days, they are again stored in a pressurized (high-pressure air) / warming device, and
The pressure is raised to 10 kg / cm ² · G over 4 hours at a temperature of ° C., and the state is maintained for 1 hour to give an expanding ability.
Table 1 shows the results of evaluating the gas (air) internal pressure of the expanded particles by the evaluation method described in the text. The secondary expanded particles having this gas (air) internal pressure are housed in a foaming apparatus,
Steam heating foaming was performed in the same manner as in Comparative Example 2 to obtain tertiary foamed particles. The expansion ratio and closed cell ratio of the obtained tertiary expanded particles were evaluated by the evaluation methods described in the text, and the results are shown in Table 1.

[0045]

[Table 1] According to the results shown in Table 1, it can be seen that the expanded particles of the present invention are excellent in the level of application of gas (air) internal pressure, the high ultimate magnification during repeated expansion (expansion), the maintainability of the closed cell ratio, and the like.

Example 4 and Comparative Example 4 FIG. 1 (expanded particles of the present invention) obtained in Example 1 and Comparative Example 1
Fig. 2 (Expanded particles of comparative product), Fig. 3 (Expanded particles of conventional product)
The expansion ratio of the secondary expanded particles of Example 3 and Comparative Example 3 was changed from 8 cm ³ / g to 9 cm ³ / g, and the expansion capacity was increased by using expanded particles having an expansion ratio of 2.6 cm ³ / g showing the cell structure of The steam heating foaming temperature of the applied secondary foamed particles of 114.5 ° C. is set to 1
By adjusting in the range of 10.5 to 114.5 ° C,
Tertiary expanded particles having an expansion ratio of 32 cm ³ / g were prepared in the same manner as in Example 3 and Comparative Example 3, except for the respective changes. After leaving the foamed particles at normal temperature and normal pressure for 2 days, they are housed in a closed container, and 63% of the original bulk volume (compressibility 37
%) And pressurized and compressed while maintaining the state in an in-mold forming die having a water vapor hole (inner dimensions 312 mm x 312 mm x
26 mmt) and heated with steam at a pressure of 1.3 kg / cm ² · G to expand and fuse the foamed particles to each other, then cooled and taken out from the molding die. Next, this molded body was aged in a hot air circulating constant temperature bath at 75 ° C. for 8 hours.
It was left at 3 ° C. for 3 days to obtain an in-mold molded product having an expansion ratio of 30 cm ³ / g. The appearance quality and properties of the molded articles were evaluated for each molded article by the above evaluation method, and the results are shown in Table 2.

[0047]

[Table 2] According to the results shown in Table 2, it can be seen that the expanded particles of the present invention also have excellent performance as an in-mold molded product.

Example 5 Non-crosslinked ethylene-propylene random copolymer resin [manufactured by Union Polymer Co., FM821, density 0.90 g / cm
³ , MFR7g / 10min (230 ° C, 2.16kg),
Ethylene content of 2.7% by weight] was used, and the carbon dioxide (gas) impregnation time of Example 1 and Comparative Example 1 was changed from 3 hours to 6 hours and the foaming temperature from 114.2 ° C to 130 ° C. Except for the modification, foamed particles having an expansion ratio of 2.6 cm ³ / g were produced in the same manner as in Example 1 and Comparative Example 1. The carbon dioxide impregnation amount was 3.5 per 100 parts by weight of the resin particles.
Parts by weight. For the expanded particles, the closed cell rate, apparent film thickness per cell,
As a result of evaluating the average cell diameter, 98%, 8.3 μm, 70%
Each value of μm was shown, and the bubble structure of the cross section of the expanded bead was observed in the same manner as in Example 1 and Comparative Example 1. As a result, bubbles having a relatively large diameter were uniformly distributed as in FIG. Was. Next, the foamed particles were accommodated in a pressurized (high-pressure air) / warming device, and the gas (air) internal pressure of the foamed particles of Example 2 and Comparative Example 2 was set to only 4 kg / cm ² · G, and the foamed particles were foamed. Example 2 except that the temperature was changed from 114.5 ° C. to 128 ° C.
-Steam heating foaming was performed in the same manner as in Comparative Example 2. For the obtained expanded particles, the expansion ratio according to the above evaluation method,
When the closed cell rate was evaluated, 13.5 cm ³ / g, 9
7%.

[0049]

As has been described in detail above, the expanded particles of the present invention, having the above-mentioned structure, have extremely excellent expansion performance, and can be repeatedly expanded (expanded) even if they are repeatedly expanded (expanded).
Good foamed particles having a high expansion ratio can be easily obtained while maintaining a high closed cell rate. If the expanded particles are used for in-mold molding, it is possible to provide an in-mold molded article having excellent appearance quality and molded article properties. In addition, since the production method of the present invention uses carbon dioxide as a blowing agent to clear CFC and HCFC regulations, it does not destroy the ozone layer, is nonflammable, has low toxicity, and is easy to handle. There are various advantages such as low cost and low cost. Therefore,
The present invention is outstanding for the industry.

[Brief description of the drawings]

FIG. 1 is a schematic view of a local area in which a cross section of a foamed particle of the present invention is enlarged (× 150).

FIG. 2 is a schematic diagram of a local part in which a cross-section of a foamed particle of a comparative product is enlarged (× 150 times).

FIG. 3 is a schematic diagram of a local part in which a cross section of a foamed particle of a conventional product is enlarged (× 150 times).

FIG. 4 is a graph showing the relationship between the applied internal pressure and the expansion ratio of the expanded beads of the present invention.

FIG. 5 is a graph showing the relationship between the expansion ratio and the closed cell ratio for the expanded particles of the present invention.

FIG. 6 is a conceptual schematic diagram of a cross-sectional periphery of a foamed particle excluded from foamed particles defined in the present invention.

FIG. 7 is a conceptual schematic view of a center portion of a cross section of a foamed particle excluded from the foamed particles defined in the present invention.

Claims (2)

(57) [Claims] 1. A foamed particle of a closed cell structure made of a polyolefin resin and having a low expansion ratio, wherein the expanded structure of the expanded particles has an expansion ratio in the range of 1.5 to 3.8 cm ³ / g. Are uniformly distributed in the direction of the particle diameter, and the “ apparent film thickness per bubble” is 4 to 26 μm.
m, a low-expanded particle of a polyolefin resin. 2. The polyolefin-based resin particles are held in a gas atmosphere in a high-pressure state lower than the critical pressure of carbon dioxide, and less than 5 parts by weight of carbon dioxide gas is added to 100 parts by weight of resin of the resin particles. A polyolefin-based resin, wherein the resin particles are impregnated to form expandable resin particles, which are then heated and expanded to form expanded particles having an expansion ratio of 1.5 to 3.8 cm ³ / g. A method for producing low-expanded resin particles.