JPS593139B2 - Manufacturing method of expandable polyethylene resin beads - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing expandable polyethylene resin beads.

A bead foaming method is known as a method 5 for producing molded foams of thermoplastic synthetic resin, and this method is widely used industrially for producing molded foams of polystyrene.

However, in the past, it has been difficult to apply this bead foaming method to foam molding of polyethylene, etc., for reasons such as the fact that a suitable solvent-based foaming agent for polyethylene 10 has not been found.

In recent years, various polyethylene bead molding methods using volatile solvent-based blowing agents have been proposed, but in all of these methods, the volatile solvent-based blowing agent is impregnated up to the center of the polyethylene particles. In addition, even if it were possible to impregnate the center, there are drawbacks such as the inability to preserve the beads due to the escape of the volatile solvent-based blowing agent, so it has not been put into practical use. It has not yet been reached. 20
There is also a method in which a pre-foam is made from a resin composition in which polyolefin such as polyethylene is mixed with a chemical blowing agent and an organic peroxide as a cross-linking agent, and then a molded foam is manufactured from this pre-foam. In recent years, various proposals have been made.

25 However, each of these methods has advantages and disadvantages, and no method has been found that is fully satisfactory.

The present inventors have conducted various studies on methods for obtaining expandable polyethylene resin beads that can be advantageously used in foam molding of molded objects. By extruding the resin composition into non-foamed strands, pre-foaming them, and then shredding them, they succeeded in obtaining excellent expandable polyethylene resin beads.

35 That is, the method for producing expandable polyethylene resin beads (hereinafter simply referred to as "expandable beads") of the present invention is to prepare a foamable resin composition obtained by kneading a chemical blowing agent and an organic peroxide with a polyethylene resin. This method is characterized by extrusion molding into a substantially non-foamed strand, then heating the strand to cause pre-foaming, and then cutting the obtained pre-foamed strand into pieces.

The polyethylene resin used in the present invention includes:
Polyethylene homopolymers such as low-density polyethylene (high-pressure polyethylene), high-density polyethylene (medium-pressure polyethylene or low-pressure polyethylene), copolymers of ethylene with small amounts of other monomers (e.g. propylene, butene, vinyl acetate, etc.) Examples include polymers.

In addition, in the present invention, a mixture of two or more of these polyethylene resins, a relatively small amount of other thermoplastic resins, rubbers, other organic substances,
Resin compositions containing 50% by weight or more of polyethylene resin, which are obtained by mixing inorganic fillers, inorganic pigments, etc.
It can be used as a polyethylene resin. As the chemical blowing agent added to the polyethylene resin in the present invention, a thermally decomposable organic blowing agent that is solid at room temperature and decomposes to generate gas at a temperature equal to or higher than the flow start temperature of the polyethylene resin is used. Such chemical blowing agents are already widely known, and examples include dinitrosopentamethylenetetramine, p-toluenesulfonyl semicarbazide, P,d-oxybisbenzenesulfonylhydrazide, and azodicarbonamide. Examples include nitroso-based, hydrazide-based, sulfonylcarbazide-based, and azo-based compounds. Two or more of these chemical blowing agents can also be used in combination. Azodicarbonamide is a particularly preferred chemical blowing agent from the viewpoint of ease of handling, adjustment of the amount of gas generated, and control of decomposition temperature. The amount of the chemical blowing agent added in the present invention varies depending on the expansion ratio of the final molded product obtained by foam molding the expandable beads, and the expansion ratio of the final molded product (the It refers to the volume ratio of the final molded product to the volume of the resin composition.

) is x times, it is preferable to add 0.7x parts by weight or more, preferably 0.9x parts by weight or more to 100 parts of the polyethylene resin used. The upper limit of its addition amount is 1.4x parts by weight, mainly for economic reasons. Since the expansion ratio of the final foam molded product produced from the expandable beads obtained by the present invention is usually 10 to 40 times, the amount of the chemical blowing agent added is 1/1 of the polyethylene resin.
00 parts by weight, it is usually about 10 to 40 parts by weight. The amount of such chemical blowing agent added in the present invention is significantly larger than the amount added in usual synthetic resin foam molding. This is because the diameter of the pre-foamed beads in the present invention is small, so the surface area per unit weight is large, and therefore a large amount of generated gas escapes, so a good foamed molded product can be obtained with a small amount of chemical blowing agent added. This is so that you will not be affected. To explain the above from a reverse perspective, for example, a resin composition in which 20 or 30 parts by weight of azodicarbonamide as a chemical blowing agent is added to 100 parts by weight of resin is prepared using a normal foaming method, for example, by blowing the foaming agent. If the added polyethylene resin is molded by compression molding or the like and foamed in one step without going through a pre-foaming process, 2.
When 0 parts by weight is added, the foaming ratio is about 3.5 times, or 3.
When 0 part by weight is added, it is possible to form a foam with a foaming ratio of 50 times or more.
In order to obtain a suitable molded product, when it is added in parts by weight, the expansion ratio of the final foamed molded product is at most 25 to 30 times, and when 30 parts by weight is added, the expansion ratio is at most 30 to 35 times. preferred. In the present invention, the organic peroxide added to the polyethylene resin acts as a crosslinking agent for the polyethylene resin.

That is, the organic peroxide generates free radicals at a temperature equal to or higher than the flow initiation temperature of the polyethylene resin, and acts as a radical generator that causes crosslinking between resin molecular chains. The organic peroxide as a crosslinking agent used in the present invention has a decomposition temperature of 140 to 180°C, preferably 140°C in order to have a half-life of 1 minute.
Organic peroxides that have a temperature of ~160°C are preferred. For example, α-dicumyl peroxide (the decomposition temperature that can give a half-life of 1 minute is 171°C), 2,5-dimethyl-2,5-
Dibenzoyl peroxyhexane (temperature: 162°C),
t-Butylperoxyisopropyl carbonate (same temperature 158℃), 1,1-bis-t-butylperoxy-3,3,5-trimethylcyclohexane (same temperature 14℃)
8℃) and other organic peroxides. Two or more of these organic peroxides can be used in combination, and their combination is preferred for adjusting the decomposition temperature. As will be explained in detail later, the resin content of the expandable beads obtained by pre-foaming and cutting the strands in the present invention has a gel fraction of 20 to 20.
It is desirable that the gel fraction be cross-linked to 60%, and in order to do so, it is necessary to cross-link the gel fraction of the resin immediately before the start of expansion during pre-foaming of the strands to a value close to the above value. .

For this purpose, it is desirable to use an organic peroxide whose decomposition temperature falls within the above-mentioned range in order to obtain a half-life of 1 minute. The amount of organic peroxide used as a crosslinking agent is usually 0.00 parts by weight per 100 parts by weight of the polyethylene resin.
The amount is 5 to 3 parts by weight, preferably 1.5 to 2.5 parts by weight. If it is less than 0.5 parts by weight, the gel fraction will be low, causing large bubbles to be generated in the pre-foaming process, or shrinkage of the molded product when pre-foamed expandable beads are reheated in a mold and foam-molded. Easy to wake up.

On the other hand, if the amount exceeds 3 parts by weight, the foaming ratio of the final molded product will be low, or the beads will be prevented from fusing together, making it impossible to obtain an integral molded product. In addition, when the crosslinking reaction is carried out only by heating during pre-foaming, it is desirable to use two or more types of crosslinking agents in combination. It is desirable to use a mixture of two or more crosslinking agents in an amount of at least 0.3 parts by weight per 100 parts by weight of the polyethylene resin. In the present invention, it is desirable to further add a foaming aid to the polyethylene resin that acts to lower the decomposition temperature of the chemical foaming agent.

If the decomposition temperature of the chemical blowing agent is too high, it may be necessary to increase the steam pressure when heating the pre-foamed beads obtained by the production method of the present invention with steam in a mold to form the final foamed product. This is because there is a disadvantage in terms of equipment. Various foaming aids can be used for this purpose depending on the type of chemical foaming agent, and examples thereof include various organic compounds such as salicylic acid, phthalic acid, and stearic acid, urea, , metal oxides, and mixtures containing these as main components. In the present invention, a foamable resin composition obtained by mixing the polyethylene resin, a chemical blowing agent, and an organic peroxide described in detail above, and further mixing a foaming aid, etc. as necessary, is used. The material is first extruded into a substantially unfoamed strand.

The kneading is preferably carried out at a temperature of 100 to 110°C using a common kneader such as a roll or a kneader. Further, although a normal extruder can be used for extrusion molding, an extruder having a rapid compression type screw is preferably used. Furthermore, if the polyethylene resin is in powder form and can be easily and uniformly mixed with other additives, the mixture may be fed directly to the extruder and kneaded in the extruder. It may also be extruded.
Usually, the extrusion temperature is 100 to 12
It is extruded at about 0°C. The shape of the extruded strand can have a variety of arbitrary cross-sectional shapes, such as circular, oval, and polygonal cross-sections. Further, it is desirable that the average diameter of the extruded strand before pre-foaming is usually in the range of 1 to 3 Ttm. In the present invention, the substantially non-foamed strand thus obtained by extrusion molding is
Next, it is heated to a temperature higher than the decomposition temperature of the chemical foaming agent to pre-foam it.

Here, the term "substantially unfoamed strand" includes not only strands that are not foamed at all, but also strands that have a foaming ratio of 2 times or less. This is because the expansion ratio is smaller than that of the expandable polyethylene beads obtained by the method of the present invention and the molded foam obtained from the beads. For heating and foaming the strand, it is preferable to pass the continuously extruded strand through a heating zone to continuously pre-foam it, but discontinuous heating and foaming may also be used. For heating the strands, any heating means can be used, such as an infrared heater, a heating bath with hot air, steam, a salt bath (for example, one consisting of 50% potassium nitrate, 7 (:fl) sodium nitrate and sodium nitrite), etc. can. The heating temperature varies depending on the type of chemical blowing agent, whether or not a blowing aid is added, etc., but from the viewpoint of ease of controlling the amount of blowing agent decomposition, the shape of the cells in the foamed strand, and their appearance, etc. The temperature is preferably in the range of 150 to 200°C, and the heating time is preferably about 20 seconds to 5 minutes. The first important thing in the pre-foaming of the present invention is the degree of foaming.

The degree of pre-foaming is usually preferably such that the pre-foaming ratio falls within the range of theoretical foaming ratio x (0.2 to 0.4). Here, the theoretical expansion ratio is the expansion ratio when the entire amount of the chemical blowing agent added to the foamable resin composition is decomposed and the entire amount of gas generated thereby contributes to the foaming of the resin composition. That's true. If the pre-expansion ratio exceeds the above range, the foamed molded product tends to shrink when the obtained expandable beads are filled into a mold and foamed in the mold, which is not preferable. In addition, if the pre-expansion ratio does not reach the above range, the voids in the mold will become too large when the resulting expandable beads are filled into the mold, making it difficult to produce foam molded products with complex shapes. It becomes difficult to obtain a product that matches the shape of the mold. The second important thing in the pre-foaming of the present invention is the gel fraction of the polyethylene resin in the resulting expandable beads.

Here, the gel fraction of the polyethylene resin is the percentage of insoluble matter when the resin is immersed in xylene maintained at the boiling point temperature for 20 hours. In the present invention, the gel fraction of the resin in the expandable beads is 20 to 60%.
It is desirable to carry out the crosslinking reaction so that If the gel fraction is less than 20 (F6), the molded product tends to shrink during in-mold foam molding of expandable beads, and extreme shrinkage occurs especially when heated with saturated steam. If the fraction exceeds 60%, the beads will not be sufficiently fused together during in-mold molding of the expandable beads, making it impossible to obtain an integral molded product or a molded product with high strength.The expandable beads of the present invention In order to make the gel fraction 20 to 60%, there are two methods: crosslinking by heating temperature of the extruder when forming into strands, crosslinking by preheating the strands before prefoaming, and crosslinking by preheating the strands before prefoaming. There is a method of crosslinking at the same time as pre-foaming depending on the heating temperature.

If too much crosslinking is allowed to proceed when forming into a strand, it may solidify in the extruder and the extrusion may not be carried out well. Therefore, if crosslinking is performed at the same time during extrusion, the gel content is usually rate below 40%, preferably 30%
% or less, and when crosslinking more than this, it is desirable to use the other two methods in combination. Next, when crosslinking the strands by preheating before prefoaming, the temperature is below the decomposition temperature of the blowing agent, e.g.
This can be done by heating the strands at a temperature of 140° C. for several minutes or more.

In addition, when crosslinking is performed at the same time as the heating during pre-foaming, fine bubbles will not be generated unless crosslinking has progressed to some extent before the strands start foaming, so the selection of the type and combination of crosslinking agents is important. Furthermore, it is necessary to accurately control the amounts added. It is preferable that the gel fraction has increased to at least 10% before the strands start foaming. In short, the same gel fraction is 20 to 60
%, the degree of crosslinking can be adjusted by selecting the type of organic peroxide used as a crosslinking agent, selecting a combination of crosslinking agents, selecting the amount of crosslinking agent added, and also by selecting the heating conditions during pre-foaming or the like. This can be done by selecting previous preheating conditions. For example, for 100 parts by weight of polyethylene, 1
, 1.8 parts by weight of 1-t-butylperoxy-3,3,5-trimethylcyclohexane, 30 parts by weight of azodicarbonamide
In the case of a strand molded from a resin composition kneaded with 4 parts by weight of zinc oxide, 4 parts by weight of zinc oxide, and 4 parts by weight of zinc stearate, when heated to 165 to 175°C for about 1 to 1.5 minutes, substantially The gel fraction of polyethylene can already be several tens of percent before the initial expansion begins.
It is easy to increase the gel fraction to 20 to 60% only by heating during pre-foaming. Thus, the pre-foamed strand is usually foamed 5 to 30 times, preferably 10 to 25 times, and gelled to a gel fraction of about 20 to 60 (fl).

The pre-foamed strands contain a foaming agent in an additive amount of 20 to 20%.
It is desirable that about 30 (fl) remain undecomposed for later in-mold foam molding. In the present invention, the pre-foamed strand thus obtained is then shredded using, for example, an ordinary cutter to form expandable beads.

In this case, the degree of shredding is extremely important as it has a significant impact on the performance of the resulting expandable beads. That is, since the pre-foamed strand has substantially no cell structure on its side surfaces, the expandable beads obtained by shredding the pre-foamed strand have only the cut surface (hereinafter referred to as the "cell structure cut surface"). It has a bubble structure. Therefore, depending on the degree of shredding of the pre-foamed strands, the resulting beads have a proportion of the cell structure cut surface to their total surface (
Hereinafter, this will be referred to as the "ratio of cell structure cut surface." ) changes, and the proportion of this cell structure cut surface has a great relationship with the performance of the beads. In the present invention, the pre-foamed strand is preferably shredded so that the proportion of the cell structure cut surface of the expandable beads is 60% or less, preferably 3501) or less.

If the proportion of the cell structure cut surface of the expandable beads is too large,
When producing in-mold foam molded products, shrinkage becomes large, which is undesirable. Usually, the shrinkage in in-mold foam molding of expandable beads is related to the degree of expansion of the expandable beads, for example, when the pre-expansion ratio is 5 times or less (theoretical expansion ratio XO.l
25), even if the cell structure cut surface is close to 100%, if it is molded with a relatively simple shape or with a final expansion ratio of about 10 times or less,
In-mold foam molding is possible. However, in general, especially when using beads with a large pre-expansion ratio, or when molding objects with a complex shape or a large final expansion ratio, if the proportion of the cut surface of the cell structure is too large, The shrinkage during in-mold foam molding increases, making it impossible to obtain the desired foam molded product. The reason why shrinkage occurs during in-mold molding when the proportion of the cell structure cut surface is large is that if the cell structure cut surface is large, the decomposition gas of the blowing agent escapes quickly during in-mold foaming, and foaming expansion does not proceed sufficiently. Also, because the foaming agent in the center of the strand is more decomposed during pre-foaming than the foaming agent in the periphery, beads with a high proportion of cut surfaces of cell structures do not have sufficient foaming power. It depends.

In order to produce an in-mold foamed product using the polyethylene foamable beads obtained by the manufacturing method of the present invention, the foamable beads can be filled into a predetermined mold and heated. The beads are foamed and expanded again to obtain a foam molded article having the shape of the mold core.

In this case, the amount of foamable beads filled into the mold is determined by the foaming ratio of the final foamed molded product, which is "expansion ratio/theoretical expansion ratio".
If the value is adjusted to 0.3 to 0.6, a good molded product can be obtained. However, since the expandable beads of the present invention generally have a large expansion ratio, it is usually desirable to fill the mold with beads. Thereby, a homogeneous foam molded product with accurate shape and dimensions can be easily obtained. On the other hand, when molding a foam molded article with a relatively simple shape, a good molded article can be easily obtained without necessarily filling the mold to the full. The mold used for foam molding using the pre-expanded expandable beads obtained by the manufacturing method of the present invention must have a structure that can close the expandable beads but cannot seal gas or liquid. is necessary.

That is, a mold with gaps or small holes is used.
Any heating means can be used to heat the mold, such as hot air, steam, a hot bath using a salt bath, or the like. The mold heating temperature varies depending on the heating means used, the size and shape of the mold, etc., but when using steam, the steam pressure is usually 3.
kg/Cd or more (temperature: about 135 to 140°C), and when using a heating bath, about 165 to 175°C is preferably used. Further, the heating time is appropriately determined depending on the size of the mold. The polyethylene foam beads obtained by the production method of the present invention can thus be advantageously used for in-mold foam molding.

In particular, since the beads have a high pre-expansion ratio, they are suitable for easily forming molded foams of arbitrary shapes that are accurate and homogeneous in shape and size. In addition to foam molding using the above-mentioned mold, the beads obtained by the manufacturing method of the present invention can be heated in their bead shape to form a spherical foam, which can be used as a buffer filler or as a liquid surface. It can also be used as a heat insulating material. Example 1 30 parts by weight of azodicarbonamide (trade name Vinyhole AC+3A, manufactured by Eiwa Kasei Kogyo Co., Ltd.) was added to 100 parts by weight of low-density polyethylene (manufactured by Mitsubishi Chemical Industries, Ltd., trade name Novatec L-F25O) [note that this azodicarbonamide When 0.5 g of zinc oxide was added to 1 g of amide and suspended in liquid paraffin, the temperature was gradually raised from 2°C to 300°C to generate gas. (converted volume) of gas was generated.

], 1 part by weight of zinc oxide, 1 part by weight of zinc stearate, 1
, 1-bis-t-butylperoxy-3,3,5-trimethylcyclohexane (manufactured by NOF Corporation, trade name: Perhexa 3M) was added, and the mixture was thoroughly kneaded with a roll. The roll surface temperature during this kneading was 110
It was kept at ℃. The obtained kneaded material was brought to a die temperature of 11
Extrusion molding is performed using an extruder heated to 0° C. into a strand having a diameter of 2 to 4 mm.

The gel fraction of polyethylene in the obtained strand was 00
1), and there was no foaming at all. When the resulting non-foamed strand was immersed in a salt bath heated to 170°C and heated, substantial expansion began 30 seconds after the start of heating, and the strand had an expansion ratio of 20 times (
It was foamed to a preliminary expansion ratio/theoretical expansion ratio=0.29).

The resulting pre-foamed strand has a diameter of approximately 1011L.
The gel fraction of m1 was 55 (fl). In addition, the gel fraction of the strand which was taken out from the heating bath immediately before the substantial expansion started after heating of the strand and the heating was stopped had a gel fraction of 40 (fl). The pre-foamed strand thus obtained is cut into lengths of approximately 10 mm to obtain pre-foamed expandable beads.

The proportion of the foam structure cut surface of the obtained expandable beads was 33%.
It was hot. When a mold with pores is filled with these foam beads and immersed in a salt bath heated to 165°C for 5 minutes, a foam molded product conforming to the internal shape of the mold is obtained. The foaming ratio of the obtained foam molded product was 30 times (molded product foaming ratio/theoretical foaming ratio = 0.43
).

Example 2 To 100 parts by weight of the same low density polyethylene as in Example 1, 20 parts by weight of azodicarbonamide and 4 parts by weight of zinc oxide were added.
Parts by weight, 4 parts by weight of zinc stearate, 1,1-bis-t
1.8 parts by weight of -butylperoxy-3,3,5-trimethylcyclohexane is added and thoroughly kneaded using a roll having a surface temperature of 105°C.

The resulting mixture is extruded and shaped using an extruder heated to a die temperature of 11'C to obtain a strand with a diameter of 311.

The gel fraction of polyethylene in the obtained strand is 0%
There was no foaming at all. This non-foamed strand was immersed in a salt bath heated to 165°C and heated for 2 minutes, and the foaming ratio was 12 times (preliminary foaming ratio/theoretical foaming ratio 20x).
．． 23), a pre-foamed strand with a gel fraction of 55% and a diameter of about 711 mm.

Note that the gel fraction of the strand, which was taken out from the heating bath immediately before the start of substantial expansion after heating of the strand was started and the heating was stopped, was 40%. The resulting pre-foamed strands are cut into lengths of approximately 7 mm to form expandable beads.

The proportion of the foam structure cut surface of these beads was 33%.
When the obtained expandable beads were completely filled into a mold with pores and immersed in a salt bath heated to 165°C for 5 minutes and heated, the expansion ratio was 20 times (expansion ratio/theoretical expansion ratio - 0). A molded article conforming to the internal shape of the mold of .39) was obtained.

Example 3 Ethylene monovinyl acetate copolymer (manufactured by Mitsubishi Yuka Co., Ltd.,
100 parts by weight of Yucalon HE6O (trade name), 20 parts by weight of azodicarbonamide (manufactured by Eiwa Kasei Co., Ltd., trade name Vinyhole AC◆3A), 4 parts by weight of zinc oxide, 1,1-
A mixture containing 1.8 parts by weight of bis-t-butylperoxy-3,3,5-trimethylcyclohexane (manufactured by NOF Corporation, trade name: Perhexa 3M) is thoroughly kneaded with a roll.

The roll surface temperature during this kneading was maintained at 110°C. The obtained kneaded product is extruded into a strand having a diameter of 2 to 4 using an extruder heated to 120° C. at a die temperature.

The gel fraction of ethylene vinyl acetate in the obtained strand was 8%, and the foam was 1.3 times larger. After cooling the obtained substantially non-foamed strand to room temperature,
When heated to 170° C. and immersed in a salt bath for 1 minute and 30 seconds, the product was foamed to an expansion ratio of 10 times (preformed product expansion ratio/theoretical expansion ratio = 2.0).

The pre-foamed strands obtained had a diameter of approximately 2 ml and a gel fraction of 45%. Note that the gel fraction of the strand, which was taken out from the heating bath immediately before the start of substantial expansion after heating of the strand was started and the heating was stopped, was 40%. The pre-foamed strand thus obtained is cut to a length of about 61 ml to form expandable beads.

The proportion of the foam structure cut surface of the obtained pre-foamed beads was 33%. When these pre-foamed beads were completely filled into a mold with pores and immersed in a salt bath heated to a temperature of 170°C for 5 minutes, a foamed molded product conforming to the internal shape of the mold was obtained. The foaming ratio of the obtained foam molded product was 20 times (molded product foaming ratio/theoretical foaming ratio 20.39).

Example 4 100 parts by weight of low-density polyethylene (manufactured by Mitsubishi Chemical Industries, Ltd., trade name Novatec L-F25O), 30 parts by weight of azodicarbonamide (manufactured by Eiwa Kasei Industries, Ltd., trade name Vinyhole AC4p3), and 20 parts by weight of zinc oxide. parts, zinc stearate 2.0 parts by weight, α dicumyl peroxide (manufactured by Nippon Oil & Fats Co., Ltd., trade name Percmil D) 1.75 parts
Parts by weight, 1,1-bis-t-butylperoxy-3,3
, 1.0 parts by weight of 5-trimethylcyclohexane (manufactured by NOF Corporation, trade name Perhexa 3M) was kneaded under the same conditions as in Example 1, and then kneaded in an extruder (dice temperature 105°C).
It was extruded into a 1 mm ψ strand shape.

The gel fraction of polyethylene in the obtained strand was 0%
There was no foaming at all. When the obtained non-foamed strand was immersed in a salt bath heated to 170° C. and heated for 2 minutes, the strand was foamed to a foaming ratio of 20 times (preliminary foaming ratio/theoretical foaming ratio = 0.29).

The obtained pre-foamed strand has a diameter of approximately 3 m1L1
Its gel fraction was 60%. In addition, just before the strand begins to expand substantially after the start of heating (approximately 4 hours after the start of heating)
The gel fraction of the strand that was removed from the heating bath at 5 seconds) and the heating stopped was 40%. The length of the pre-foamed strand thus obtained is approximately 3W! Cut into ml pieces to make expandable beads.

The foam structure cut surface ratio of the obtained expandable beads was 330
It was t:). These expandable beads were filled to the fullest into a mold with pores, and placed in a 101-degree autoclave at a vapor pressure of 3.
When heated for about 5 minutes with steam at kg/Cd (saturation temperature 133 degrees Celsius), a foam molded product that conforms to the internal shape of the mold can be obtained, and the foaming ratio of the obtained foam molded product is 2.
It was 7 times (molded product expansion ratio/theoretical expansion ratio 0.39).

Experimental Example When the expandable beads produced in Example 4 were completely filled into the same mold as in Example 4 and immersed in a salt bath heated to 175°C for 7 minutes, the inside of the mold It is possible to obtain a foamed molded product with the exact shape, and the foaming ratio of the obtained foamed molded product is 30 times (molded product foaming ratio/theoretical foaming ratio = 0)
．． 43).

Claims (1)

[Claims] 1. A foamable resin composition prepared by kneading a chemical foaming agent and an organic peroxide with a polyethylene resin is extruded into a substantially non-foamed strand, and then the strand is heated to pre-foam, A method for producing expandable polyethylene resin beads, which comprises then cutting the obtained pre-foamed strands into pieces.