JPH08225674A - Flame-retardant olefinic resin composition for forming - Google Patents

Abstract

PURPOSE: To obtain a non-halogenic flame-retardant olefinic resin composition for foaming by incorporating a mixture of specific graphite and ammonium polyphosphate in two specific non-crosslinked olefinic resins for foaming. CONSTITUTION: This flame-retardant olefinic resin composition for foaming comprises 100 pts.wt. of a foaming olefinic resin, composed of (a) 80-99wt.% of a non-crosslinked olefinic resin having a weight average molecular weight M1 and 1-20wt.% of a non-crosslinked olefinic resin having a weight average molecular weight M2 , (which satisfies at lest either of the following two conditions (i) M1 =50,000 to 500,000, M2 <=7,000,000, M2 /M1 =5 to 100, and (ii) M1 =50,000 to 500,000, M2 <=7,000,000, M2 -M1 >=300,000) and 5 to 100 pts.wt. of a mixture of neutralized heat-expandable graphite and ammonium polyphosphate. The heat-expandable graphite is, for example, of a graphite interlaminar compound formed by treating a powder of natural scaly graphite or pyrolytic graphite with an inorganic acid and a strong oxidizing agent.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a flame-retardant olefin resin composition for foaming.

[0002]

2. Description of the Related Art Olefin resins such as polyethylene and polypropylene are inexpensive and have excellent physical and chemical properties. Foams obtained by expanding this resin at a relatively high ratio are heat insulating materials, cushioning materials, and soundproofing materials. It is preferably used for Such an olefin-based resin foam is generally obtained by allowing an olefin-based resin to contain a low boiling point organic solvent such as dichlorotetrafluoroethane as a foaming agent, a pyrolytic compound such as azodicarbonamide, or an inert gas such as carbon dioxide. It is obtained by melting the resin by heating and foaming the resin with the gas of the foaming agent.

However, an olefin resin is a crystalline resin, and when it is heated above its melting point, a rapid decrease in viscoelasticity occurs. However, since the temperature range showing viscoelasticity suitable for foaming is narrow, any foaming agent should be used. Even if it is used, it is difficult to obtain a stable olefin-based resin foam having uniform and fine bubbles and a high magnification (for example, 10 to 40 times) without strict temperature control. Therefore, in order to relatively easily obtain a foam having uniform and fine bubbles from an olefin resin and having a high magnification and stability, the resin is usually preliminarily crosslinked with an organic peroxide or ionizing radiation, A method of expanding the temperature range that exhibits viscoelasticity suitable for foaming is used.

However, when the olefin resin is crosslinked by the above-mentioned method, the production cost of the foam is increased correspondingly, and the resin of the obtained foam is crosslinked, so that the resin is completely crosslinked. It was difficult to heat and melt, and it was not possible to heat and melt a used olefin resin foam and use it as a recycled resin.

[0005] Further, with the expansion of applications of foams obtained by foaming an olefin resin which is originally flammable, performance as a flame retardant material is required, and flame retardation treatment is performed by various methods. As a method of making an olefin resin flame-retardant, a method of adding a halogen-containing compound is generally used. Halogen-containing compounds can certainly impart a high degree of flame retardancy, and have the advantage of relatively low deterioration of molding processability and mechanical strength of molded products, but generate a large amount of smoke during molding and burning. However, it was necessary to install special equipment to prevent corrosion of the equipment. Particularly in recent years, there is a strong demand for non-halogen flame retardancy, and in particular, it is necessary to impart a high degree of flame retardancy for use in building materials, etc. The reality is that it is difficult to obtain sex.

Under such circumstances, research on flame retardancy of resin by addition of hydrated metal oxide which does not generate a large amount of smoke when burning aluminum hydroxide, magnesium hydroxide, basic magnesium carbonate, etc. Has become popular. However, in order to impart sufficient flame retardancy to an olefin resin that is easily flammable only with these hydrated metal oxides, it is necessary to add a large amount of hydrated metal oxides, and as a result, the physical properties are It not only lowers but also adversely affects the foamability, and it is difficult to obtain a foam having a fine and uniform closed cell structure.

Further, as a method of imparting flame retardancy without using a halogen-containing compound, for example, JP-A-6-2547 is used.
Japanese Unexamined Patent Publication No. 6 discloses a resin composition in which a two-component mixed flame retardant of thermally expansive graphite and a phosphorus compound is used for a polyolefin resin, which resin composition is obtained by extrusion foaming. There is no description that it is applied to resin foam. Furthermore, since the heat-expandable graphite obtained by treating with an inorganic acid and a strong oxidizer is used as it is in the resin composition without being subjected to neutralization treatment, it inhibits the stability of the resin composition and causes kneading and molding. At times there was a risk of corroding the equipment.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned drawbacks, and an object thereof is to obtain a highly flame-retardant material by using a conventionally known foaming method without using any halogen-containing compound. It is an object of the present invention to provide a flame-retardant olefin resin composition for foaming, which allows a given resin foam to be obtained relatively easily and stably.

[0009]

The flame-retardant olefin resin composition for foaming of the present invention comprises a non-cross-linking olefin resin (a) and a non-cross-linking olefin resin (b) having different weight average molecular weights for foaming. It is composed of a mixture of olefin resin, neutralized thermally expandable graphite and ammonium polyphosphate.

The stretchable olefin resins (a) and (b) used in the present invention include polyethylene (low density, medium density, high density), polypropylene, ethylene-propylene copolymer, ethylene-vinyl acetate copolymer. Examples include a polymer, an ethylene-ethyl acrylate copolymer, an ethylene-propylene-diene copolymer, and the like.

As the non-crosslinked olefin resin, the weight average molecular weight of the non-crosslinked olefin resin (a) is M
_1, and when the weight average molecular weight of uncrosslinked olefin resin (b) was M _2, between the M ₁ and M _2, (b) M ₁ = 5 million in to 50 million in, M ₂ ≦ 700 million in, M ₂ / M
₁ = 5 to 100, or (b) M ₁ = 50,000 to 500,000, M ₂
Those satisfying one of the two conditions of ≦ 7 million and M ₂ −M ₁ ≧ 300,000 are used.

If the weight average molecular weight M ₁ of the non-crosslinked olefin resin (a) is less than 50,000, the viscosity at the time of melting becomes too low and the bubbles are broken during foaming, so that a good foam cannot be obtained. If it exceeds 500,000, the fluidity of the resin remarkably deteriorates and molding in an extruder becomes difficult, so the range is limited to 50,000 to 500,000. In particular, when polyethylene is used as the non-crosslinked olefin resin (a), M ₁ = 50,000 to 200,000 is preferable, and when polypropylene is used, M ₁ = 300,000 to
500,000 is preferable. Further, M ₂ / M ₁ = 6~80 are preferred.

When the weight average molecular weight M ₂ of the non-crosslinked olefin resin (b) becomes smaller than the condition (a) or (b), the resin composition mixed with the noncrosslinked olefin resin (a) is obtained. Has not seen much improvement in melt tension,
The resin cannot be expanded at a high magnification. Also,
When the weight average molecular weight M ₂ becomes large and deviates from the condition (a) or (b), the non-crosslinked olefin resin (a) is not uniformly dispersed, and the foaming ratio of the obtained foam is increased. Fluctuates and becomes unstable. On the other hand, if the weight average molecular weight M ₂ exceeds 7,000,000, the fluidity of the resin is remarkably reduced, and molding in an extruder becomes difficult. In particular, the range of M ₂ / M ₁ = 6~80 are preferred.

The weight average molecular weights M ₁ and M ₂ are
It is a value measured by gel permeation chromatography (GPC).

In the foaming olefin resin comprising the above-mentioned non-crosslinked olefin resin (a) and (b), when the proportion of the non-crosslinked olefin resin (b) is decreased, the melt tension of the resin composition is improved. No, the resin cannot be foamed at a high magnification, and when the resin composition is increased, the melt viscosity of the resin composition is greatly increased, and the tension of the bubble film during foaming becomes too large, so that the resin does not sufficiently foam. Limited to 20% by weight.

As the above-mentioned heat-expandable graphite, conventionally known substances can be used. For example, powders of natural scaly graphite, pyrolytic graphite, quiche graphite, etc. can be used.
Inorganic acids such as concentrated sulfuric acid, nitric acid, and selenate and treated with strong oxidants such as concentrated nitric acid, perchloric acid, perchlorate, permanganate, dichromate, and hydrogen peroxide to remove graphite intercalation compounds. It is a crystalline compound that is produced and maintains the layered structure of carbon.

In the present invention, the heat-expandable graphite obtained by the above acid treatment is the heat-expandable graphite further neutralized with ammonia, an aliphatic lower amine, an alkali metal compound, an alkaline earth metal compound or the like. used. Examples of the aliphatic lower amines include monomethylamine, dimethylamine, trimethylamine, ethylamine, propylamine, butylamine, and the like, and examples of the alkali metal compound and alkaline earth metal compound include potassium,
Examples thereof include hydroxides, oxides, carbonates, sulfates and organic acid salts of sodium, calcium, barium, magnesium and the like.

When the above-mentioned thermally expandable graphite is used without being neutralized, the stability of the resin composition may be impaired and the molding machine and the like may be corroded during kneading and molding, which is not preferable.

When the particle size of the above-mentioned thermally expandable graphite becomes finer, the expansion degree of the thermally expandable graphite becomes smaller and the flame retardancy decreases,
When it is large, the effect of imparting flame retardancy is obtained, but when it is mixed with a polyolefin resin and kneaded, the dispersibility is poor, and the physical properties of the resulting molded article deteriorate, so 20-200 mesh is preferred.

Examples of the ammonium polyphosphates used in the present invention include ammonium polyphosphate and melamine-modified ammonium polyphosphate, which may be used alone or in combination of two or more kinds. The ammonium polyphosphates have a 2% weight loss temperature of 180 to 250 determined by thermogravimetric analysis from the viewpoint of improving flame retardancy.
The temperature at which the 5% weight loss temperature is in the range of 270 to 310 ° C is preferable.

The above 2% or 5% weight loss temperature is measured by using a thermogravimetric analyzer (abbreviated as TGA) at a heating rate of 10 ° C. /
Min, about 1 under the condition of 200 ml / min air flow
The temperature at which the weight of a sample decreases by 2% or 5% when the temperature of a 0 mg sample is raised to 30 to 900 ° C.

The neutralizing heat-expandable graphite and ammonium polyphosphates are mixed in a weight ratio of 9: 1 to 1: 1.
1: 9 is preferable. If the mixing ratio is out of the above range, there is almost no difference in the flame retardant performance from the case where the thermally expandable graphite and the ammonium polyphosphates are used alone.

In the resin composition of the present invention, if the amount of the mixture of the neutralized heat-expandable graphite and ammonium polyphosphate used is small, sufficient flame retardancy cannot be obtained, and if it is large, the foaming property is increased. Of the olefin resin for foaming, the amount is limited to 5 to 100 parts by weight, preferably 8 parts.
To 80 parts by weight, and more preferably 10 to 60 parts by weight.

A hydrated metal compound may be added to the above resin composition as a flame retardant aid within a range that does not impair foaming properties. As the hydrated metal oxide, aluminum hydroxide, magnesium hydroxide, basic magnesium carbonate,
Examples thereof include dawsonite, and these may be used alone or in combination of two or more.

The amount of the hydrated metal oxide used varies depending on the amount of the mixture of the neutralized heat-expandable graphite and ammonium polyphosphate, but if the amount is large, the foaming characteristics are impaired. It is preferably 100 parts by weight or less with respect to 100 parts by weight of the resin.

If necessary, the resin composition of the present invention may further contain a bubble nucleus adjusting agent, an antioxidant, an ultraviolet absorber, an antistatic agent, a lubricant, a coloring agent and the like. Examples of the bubble nucleus adjusting agent include talc, silica, calcium carbonate, calcium stearate, calcium silicate, and baking soda, and examples of the antioxidant include a phenol type, an organic thioic acid type, an organic phosphoric acid type, an organic amine. System, organic tin system, and the like.

In order to produce a non-crosslinked flame-retardant olefin resin foam from the resin composition of the present invention, known foaming methods such as extrusion foaming, pressure foaming and normal pressure foaming are adopted. The foaming agent used in these foaming methods is appropriately selected from known materials such as a low boiling organic solvent, an inert gas, and a thermal decomposition type foaming agent. Further, the expansion ratio of the foam is 10 to 40.
About twice is preferable.

As the above-mentioned extrusion foaming, for example, the resin composition of the present invention is charged from the hopper of the extruder and melt-kneaded in the extruder, while dichlorotetrafluoro is introduced from a foaming agent injection hole provided in the middle of the extruder. There may be mentioned a method in which a low boiling point organic solvent such as ethylene or an inert gas such as carbon dioxide gas is pressed in and extruded from a mold having a predetermined shape to foam. Also, a mixture of the resin composition of the present invention and a pyrolytic foaming agent such as azodicarbonamide may be charged from the hopper of an extruder, melt-kneaded in the extruder, and extruded from a mold having a predetermined shape to foam. Good.

As the above-mentioned foaming under pressure, the resin composition of the present invention is melted and kneaded by an extruder or a roll, and this is molded into a sheet by pressing or the like, and the sheet is put in a pressure resistant container such as an autoclave and heated. A method may be mentioned in which a low-boiling-point organic solvent such as dichlorotetrafluoroethylene or an inert gas such as carbon dioxide is pressed into this, and then the pressure is released to foam.

As the above-mentioned normal pressure foaming, a mixture of the resin composition of the present invention and a pyrolytic foaming agent such as azodicarbonamide is kneaded with an extruder or roll at a temperature at which the foaming agent does not decompose, and this is pressed. And the like, and the foamable sheet is placed in a heating hot air oven or a heating bath and heated under normal pressure to foam.

[0031]

Next, embodiments of the present invention will be described. (Examples 1 and 2) Predetermined amount of non-crosslinked olefin resin having weight average molecular weight M ₁ shown in Table 1 and weight average molecular weight M ₂
A non-crosslinked olefin resin, a neutralized thermally expandable graphite, ammonium polyphosphate and an antioxidant were dry blended to prepare a flame retardant olefin resin composition for foaming. This resin composition was prepared by changing the cylinder temperature from the hopper side to the tip end at 140 ° C, 165 ° C, 140 ° C,
It was supplied into the extruder from a hopper of a vent type extruder (bore diameter 65 mmφ, L / D = 35) set to 140 ° C., respectively. While sufficiently melting and kneading the resin composition in the extruder, carbon dioxide gas as a foaming agent was continuously injected at a pressure of 90 kg / cm ² from a vent hole of the extruder, and the temperature was set to 113 ° C. while sufficiently kneading. 15kg / from mold
A foam was obtained by extruding into a rod shape at an extrusion rate of hr (extrusion foaming method).

(Example 3) Predetermined amounts of the non-crosslinked olefin resin having a weight average molecular weight M ₁ shown in Table 1, a noncrosslinked olefin resin having a weight average molecular weight M ₂ , and a neutralized thermally expandable graphite A flame-retardant olefin resin composition for foaming was prepared by dry blending ammonium polyphosphate and an antioxidant. This resin composition was melt-kneaded using a roll at 140 ° C. and then molded into a sheet having a thickness of 2 mm using a press. Then, this sheet was put into an autoclave and heated to 110 ° C., and carbon dioxide gas as a foaming agent was injected into the resin at a pressure of 90 kg / cm ² to obtain a resin composition 100.
After sufficiently dissolving 5 parts by weight of carbon dioxide with respect to parts by weight, the pressure in the autoclave was released to obtain a foam (pressurized foaming method).

Example 4 First, polypropylene having a weight average molecular weight of 460,000 [PP (2)] and a weight average molecular weight of 23 were used.
An equivalent amount of 0,000 of high-density polyethylene [HDPE (1)] was uniformly dissolved in hot xylene, vacuum dried at 80 ° C., and the obtained mixture was ground. 20 parts by weight of this pulverized product was added to polypropylene having a weight average molecular weight of 460,000 [PP
(2)] After adding 80 parts by weight, a predetermined amount shown in Table 1 of dry-blended neutralized thermally expandable graphite, ammonium polyphosphate and an antioxidant are added to the flame-retardant olefin for foaming. A resin composition was prepared. A vent type extruder (bore diameter 65 mmφ, whose cylinder temperature was set to 170 ° C., 220 ° C., 220 ° C., and 195 ° C., respectively, from the hopper side to the tip thereof was used.
It was fed into the extruder from a hopper of L / D = 35). While sufficiently kneading the resin composition in the extruder, carbon dioxide gas as a foaming agent was continuously injected at a pressure of 90 kg / cm ² from the vent hole of the extruder, and the gold was set at 160 ° C. while sufficiently kneading. A foam was obtained by extruding from a mold in a rod shape at an extrusion rate of 15 kg / hr (extrusion foaming method).

(Example 5) Instead of carbon dioxide gas as a foaming agent, dichlorotetrafluoroethane (in the table, "Freon") was used.
Was displayed at a pressure of 50 kg / cm ² and was extruded in the same manner as in Example 4 to obtain a foam (extrusion foaming method).

(Embodiments 6 and 7) With a predetermined amount shown in Table 2,
Flame-retardant for foaming by dry-blending non-crosslinking olefin resin with weight average molecular weight M ₁ , non-crosslinking olefin resin with weight average molecular weight M ₂ , neutralized thermally expandable graphite, ammonium polyphosphate and antioxidant After preparing the water-soluble olefin resin composition, azodicarbonamide (in the table, "ADC
(Indicated as "A") 10 parts by weight were added and uniformly mixed. The mixture was heated at 135 ° C, 160 ° C, 135 ° C, 135 ° C from the hopper side to the tip.
Vent type extruder (caliber 6
It was supplied into the extruder from a hopper of 5 mmφ and L / D = 35). While sufficiently kneading the resin composition in the extruder,
A foaming sheet having a thickness of 2 mm was obtained by extruding into a sheet shape at an extrusion rate of 15 kg / hr from a mold set to 115 ° C. Next, this foamable sheet was introduced into a heating and foaming furnace at 240 ° C. for thermal foaming to obtain a foam (normal pressure foaming method).

(Example 8) Predetermined amounts of non-crosslinked olefin resin having a weight average molecular weight M ₁ shown in Table 2, noncrosslinked olefin resin having a weight average molecular weight M ₂ and neutralized thermally expandable graphite A flame-retardant olefin resin composition for foaming was prepared by dry blending ammonium polyphosphate and an antioxidant. Using this resin composition, a rod-shaped foam was obtained using carbon dioxide as a foaming agent in the same manner as in Example 4 (extrusion foaming method).

(Comparative Examples 1 to 4) In a predetermined amount shown in Table 3,
Flame-retardant for foaming by dry-blending non-crosslinking olefin resin with weight average molecular weight M ₁ , non-crosslinking olefin resin with weight average molecular weight M ₂ , neutralized thermally expandable graphite, ammonium polyphosphate and antioxidant A olefin resin composition was prepared. Using this resin composition, a rod-shaped foam was obtained using carbon dioxide as a foaming agent in the same manner as in Example 1 (extrusion foaming method). In Comparative Examples 2 and 4, the load of the extruder was increased and extrusion molding was impossible, and no foam was obtained.

(Comparative Examples 5 and 6) In the predetermined amount shown in Table 4,
Flame-retardant for foaming by dry-blending non-crosslinking olefin resin with weight average molecular weight M ₁ , non-crosslinking olefin resin with weight average molecular weight M ₂ , neutralized thermally expandable graphite, ammonium polyphosphate and antioxidant A olefin resin composition was prepared. Using this resin composition, a foam was obtained in the same manner as in Example 3 (pressure foaming method).

(Comparative Examples 7 and 8) With a predetermined amount shown in Table 4,
Flame-retardant for foaming by dry-blending non-crosslinking olefin resin with weight average molecular weight M ₁ , non-crosslinking olefin resin with weight average molecular weight M ₂ , neutralized thermally expandable graphite, ammonium polyphosphate and antioxidant A olefin resin composition was prepared. Using this resin composition, a foam was obtained in the same manner as in Example 5 (extrusion foaming method). In Comparative Example 8, the load on the extruder increased and extrusion molding was impossible in 15 minutes. Therefore, the foam obtained before that was used for performance evaluation.

(Comparative Examples 9 to 11) In the predetermined amount shown in Table 5,
Flame-retardant for foaming by dry-blending non-crosslinking olefin resin with weight average molecular weight M ₁ , non-crosslinking olefin resin with weight average molecular weight M ₂ , neutralized thermally expandable graphite, ammonium polyphosphate and antioxidant A olefin resin composition was prepared. Using this resin composition, a foam was obtained in the same manner as in Example 6 (normal pressure foaming method).

(Comparative Example 12) Predetermined amounts of non-crosslinked olefin resin having a weight average molecular weight M ₁ shown in Table 5, noncrosslinked olefin resin having a weight average molecular weight M ₂ and neutralized thermally expandable graphite A flame-retardant olefin resin composition for foaming was prepared by dry blending ammonium polyphosphate and an antioxidant. Using this resin composition, a foam was obtained in the same manner as in Example 1 (extrusion foaming method).

The following performance evaluations were performed using the foams obtained in the above Examples and Comparative Examples, and the results are shown in Tables 1-5. (A) Foaming ratio It was calculated from the specific gravity ratio of the resin composition and the foam measured by a specific gravity meter. (B) Flame retardancy test The flame retardancy was evaluated by measuring the oxygen index according to JIS D1201.
Less than was judged as x. (C) Cell Structure and Appearance The cell structure and appearance of the foam were visually observed, and those having good cell structure and appearance were evaluated as ◯, and those having at least one of cell structure and appearance as poor were evaluated as x.

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

[Table 4]

[0047]

[Table 5]

In Tables 1 to 5, the following components were used. (Non-crosslinked olefin resin having a weight average molecular weight of M ₁ ) LDPE (1): low density polyethylene (“LF440HB” manufactured by Mitsubishi Chemical Co., Ltd.), M ₁ = 710,000 · PP (1): polypropylene (Mitsui Petrochemical (“F601” manufactured by Mfg. Co., Ltd.), M ₁ = 180,000 PP (2): Polypropylene (“E” manufactured by Mitsubishi Chemical
C8 "), M ₁ = 460,000 (non-crosslinked olefin resin having a weight average molecular weight of M ₂ ) -LDPE (2): low density polyethylene (" ZC-30B "manufactured by Mitsubishi Chemical Corporation), M ₂ = 140,000-HDPE (1): High-density polyethylene (“HIZEX Million 240M” manufactured by Mitsui Petrochemical Co., Ltd.), M ₂ = 2.3 million HDPE (2): High-density polyethylene (“HIZEX Million 320M” manufactured by Mitsui Petrochemical Co., Ltd.), M ₂ = 2.8 million PP (3): polypropylene ("E" manufactured by Mitsubishi Chemical Corporation
C9 ") M ₂ = 50.9 man-neutralized thermally expandable graphite: manufactured by Nippon Kasei Chemical Co., Ltd." CA-
60S ", average particle size 60 mesh, pH = 9.50-Ammonium polyphosphate:" Sumisafe P "manufactured by Sumitomo Chemical Co., Ltd. 2% weight loss temperature by TGA = 214 ° C, 5% weight loss temperature by TGA = 294 ° C As TGA Seiko Denshi's "Thermogravimetric analyzer (model: TG / DTA320) was used.-Antioxidant: ADEKA ARGUS 'Mark 328"

The pH of the neutralized heat-expandable graphite
Is the thermally expansive graphite 1 in 100 cm ³ of pure water at 25 ° C.
g was added, the mixture was stirred with a stirrer for 5 minutes, and then measured with a pH meter.

[0050]

The composition of the flame-retardant olefin resin composition for foaming of the present invention is as described above, and by foaming by a conventionally known foaming method, a non-crosslinked structure having uniform and fine bubbles at high magnification. The flame-retardant olefin resin foam can be obtained relatively easily and stably. In addition, the obtained resin foam exhibits excellent flame retardancy and does not generate a halogen-based gas during combustion, so that it can be used in a wide range of applications where flame retardancy is required, including the field of building materials. Further, since the obtained resin foam is a non-crosslinked product, it can be used as a recycled resin by completely heating and melting a used foam.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Qi Shirato 2-2 Kamitobaue Tonkocho, Minami-ku, Kyoto City Sekisui Chemical Co., Ltd. -2 Sekisui Chemical Co., Ltd.

Claims (1)

[Claims] 1. (a) 80 to 99% by weight of a non-crosslinked olefin resin having a weight average molecular weight of M ₁ and (b) a weight average molecular weight of M
₂ to 1 to 20% by weight of non-crosslinked olefin resin,
Between the weight average molecular weights M ₁ and M ₂ , (a) M ₁ = 50,000 to 500,000, M ₂ ≦ 7 million, M ₂ / M
₁ = 5 to 100, or (b) M ₁ = 50,000 to 500,000, M ₂
100 parts by weight of an olefin resin for foaming, and neutralized heat-expandable graphite and ammonium polyphosphates, which satisfy at least one of the two conditions of ≦ 7 million and M ₂ −M ₁ ≧ 300,000. A flame-retardant olefin resin composition for foaming, which comprises 5 to 100 parts by weight of a mixture.