FI82477B - FOER FARING FOR FRAMSTAELLNING AV TVAERBUNDNA POLYOLEFINSKUMPRODUKTER. - Google Patents

Abstract

1. Process for preparing cross-linked polyolefin foamed articles with closed cells, in which a cross-linkable, expandable composition is heated under pressure for a preset period of time and subsequently heated again at atmospheric pressure for a preset period of time, characterised in that a composition containing in 100 parts by weight polyolefin with a melt ----------------------------------flow rate (MFR) ranging ----------------------------------from 0.1 to 5.0 g/10 min 0.2 to 1.0 parts by weight of a cross-linking agent, 10 to 30 parts by weight of a blowing agent and 0 to 0.6 parts by weight of an auxiliary foaming agent, is filled into a closed mould and heated therein for 20 to 70 min at temperatures ranging from 120 to 160 degrees C at a pressure of more than 10 bar and moulded into a shape corresponding to the mould, in which case a crosslinked, essentially unfoamed product with a gel content of approximately 10 to 70% and a degree of expansion ranging from 1 to 3 times the original volume is obtained, and subsequently this cross-linked, unfoamed product is foamed by heating at atmospheric pressure in a mould, which is not gas-tight, which corresponds to a desired shape, in which case a thick, tough foamed article with uniform, fine cells is obtained.

Description

1 82477

The present invention relates to a process for the preparation of rigid and thick crosslinked polyolefin foam products, the foam cells of which are smooth, finely divided and closed. For example, a method well known in the art has been used to prepare low density crosslinked polyolefin foam, which comprises crosslinking polyolefin by ionizing radiation and then heat crosslinking the crosslinked polyolefin, and a method comprising crosslinking the polyolefin chemically and then crosslinking the polyolefin foam. pressure or nor-15 paint air pressure. As an improvement over the latter two methods, there is provided a manufacturing process (e.g. GB Patent No. 1,328,525) which comprises, as a first step, heating a composition comprising a polyolefin, a crosslinking agent and a blowing agent under pressure for a period of time to partially decompose the crosslinking agent and blowing agent. and forming an intermediate foam and, in a second step, further heating the intermediate foam at normal atmospheric pressure for a predetermined time, whereby the crosslinking agent and the blowing agent which remain unchanged in the first step decompose. This method makes it possible to obtain a crosslinked polyolefin foam which has relatively uniform, finely divided and closed cells and a low density and a high thickness. However, in the first step, this method requires the use of a me-30 stable mold with a reduced vertical profile of the mold. If the metal mold used is not narrowed, as mentioned above, the removal of the intermediate foam cannot be carried out without difficulty. The vertical profile of the metal mold used in the first step is thus narrowed and the foam 35 is obtained by allowing the polymer to expand during the removal of the applied pressure, thereby leaving the metal mold. During this step, the polymer expands into a shape similar to the shape of a metal mold because there is no supporting part in the mold. When this intermediate foam is foamed directly by heating it in a mold having a rectangular cross-section which is not airtight at normal atmospheric pressure, rounding then occurs in the tapered part of the intermediate foam. To eliminate this phenomenon, the tapered portion must be removed by cutting the intermediate foam before the final flotation. Thus, the disadvantage of this method is that the efficiency of the material is poor and the machinability is poor.

In addition, when using a method in which the crosslinking and flotation of the polyolefin is performed in two steps, i. in the first flotation step using pressure 15 and in the second flotation step at normal atmospheric pressure, it is difficult to obtain high-tough cell membranes. This means that any cell membranes formed are pressurized under pressure and bond to each other, resulting in a foam with closed cells (namely, closed cell foam as disclosed in Japanese Patent Publication JP 56 (1981) -130).

The present invention relates to a process for producing a thick, crosslinked polyol-25 foam with a high expansion ratio, a high toughness which cannot be obtained by a conventional process and whose cells are smooth, finely divided and closed.

The crosslinked poly-30 olefin foam produced according to the invention has the above-mentioned excellent properties together with high material efficiency and satisfactory processability.

The invention relates to a process for the preparation of crosslinked polyolefin foam products in which the cells are closed, whereby a crosslinkable, expandable

II

3,82477 The composition is heated under pressure for a predetermined time, after which the composition is reheated at atmospheric pressure for a predetermined time. The process is characterized in that the closed mold is filled with a composition containing 100 parts by weight of a polyolefin having a melt index (MFR) of 0.1-5.0 g / 10 min, 0.2-1.0 parts by weight of a crosslinking agent. , 10-30 parts by weight of blowing agent and 0-0.6 parts by weight of foaming aid, and the composition is heated for 20-70 minutes at 120-160 ° C at a pressure of more than 10 bar and molded into a mold corresponding to the crosslinked, substantially a non-foaming product with a gel content of about 10-70% and a degree of expansion of 1-3 times the original volume, after which the resulting crosslinked, non-foaming product 15 is foamed by heating at atmospheric pressure in a non-gas-tight mold corresponding to the desired shape to give a thick foamed product with uniform fine-grained cells. The process yields a tough and thick foam having smooth, fine-grained, closed cells surrounded by unbreakable membranes using a polyolefin having the above-mentioned specific MFR value by combining the polyolefin as a crosslinking agent, a blowing agent and a foaming aid with the addition of the above. 25 Ia thermal casting step using pressure and this casting using heat so as to obtain the above-mentioned, determined values of gel percent and expansion ratio and then carrying out the final flotation at normal atmospheric pressure.

The term "compressive force" (kp / cm2) as used in this brochure 30 refers to the value of the pressure formed in a metal mold placed in a hydraulic press, calculated by the following formula.

35 4 82477 (hydraulic gauge pressure with piston) x (piston cross-sectional area)

Compression force (kp / cm2) = --------------------------------- pin of the mixture filled with the mixture in contact with the hot plate 5 buck

The term "gel percentage" refers to a value determined by performing a 24-hour extraction on a given sample (polyolefin-10 nicotine composition) with trichlorethylene (Triclene) as solvent in a Soxhlet reflux extractor equipped with a 40-50 micron pore size glass filter and a glass filter. (in percent) after extraction to the weight of the sample before extraction.

The term "degree of expansion" means the ratio of the volume of the crosslinked, non-foamed resin composition formed (or the volume of the foam finally formed, indicating the expansion ratio of the final product) to the volume of the metal mold.

The quality of the polyolefin used according to the present invention is determined by the MFR (melt flow rate) value. If the MFR value of the polyolefin is less than 0.1, the resin is too rigid to stretch and thus too difficult to expand. Conversely, if the MFR value is greater than 5.0, gas leakage occurs in the resin during thermal foaming and thus no foam is formed. The amounts of crosslinking agent and other substances added vary slightly depending on the quality of the polyolefin used and the desired expansion ratio. In general, the amount of the crosslinking agent is in the range of 0.2 to 1.0 part by weight, the amount of the blowing agent is in the range of 10 to 30 parts by weight, and the amount of the blowing agent is in the range of 0 to 0.6 parts by weight, respectively, per 100 parts by weight of polyolefin. By using a polyolefin having a predetermined MFR of the above and by adding the above-mentioned additives in the above-mentioned specified proportions, a thick foam can be produced very easily, the cells of which are smooth, finely divided and closed surrounded by membranes which are not broken by compression. .

The pressure used during thermal casting must be greater than about 10 kp / cm2. Due to the uniformity and fine distribution of the individual cells of the foam and the suitability of the device for pressure operation, this pressure is preferably 10, in particular in the range from 20 to 200 kp / cm 2.

During the above-mentioned step of thermal casting, a very small amount of foaming agent is initially dispersed. The foam cores formed by the initial disintegration can be compressed to an extremely small size by performing heat casting using a pressure in the range of 10 to 200 kp / cm 2, preferably in the range of 20 to 200 kp / cm 2, preferably for 20 to 70 minutes. During the next thermal foaming step, the presence of these foam cores allows the foaming to proceed steadily and 20 even, finely divided cells are formed. If the pressure during thermal casting is less than 10 kp / cm2, the foam finally formed contains coarse, uneven cells.

The gel percentage and the expansion ratio of the crosslinked, non-foaming resin composition formed during the above-mentioned thermal casting vary depending on the type of resin used and the amounts of foaming agent, foaming aid and crosslinking agent added. It has been shown that, despite these variables, the resulting foam develops high toughness and contains uniformly fine, closed cells enclosed in refractory films if the aforementioned gel percentage is in the range of 10 to 70%, preferably between 20 and 65% and even more preferably between 25 and 60% and the expansion ratio is 1 to 3 times the original volume. If the gel percentage is greater than 70%, crosslinking proceeds excessively and the elongation of the resin decreases, making it impossible to produce a thick foam with a high expansion ratio. Conversely, if the gel percentage is less than 10%, the strength of the resin is so deficient that hardy, closed cells and cell membranes are prone to rupture. It is also difficult to obtain a tough foam containing uniform, finely divided closed cells when flotation at normal atmospheric pressure. However, when casting with heat under the above conditions, a tough foam containing uniform, finely divided, closed cells can be obtained, as intended in accordance with the present invention.

The above-mentioned value of 1 to 3 times the initial volume for said expansion ratio represents the limits within which the foam cores formed by the initial dispersion of a very small amount of blowing agent during the above thermal casting are allowed to expand until the molded resin composition is removed from the metal mold. The expansion of the wax-20 composition is far from the concept of "foaming" and the molded product is thus referred to as a "crosslinked, non-foaming resin composition". This cast product can thus be easily removed from the metal mold. Thus, there is no need to use the aforementioned tapered metal mold in the method of the present invention, which has been a necessary requirement for conventional methods. The result is improved material efficiency and workability.

Due to the close interaction of the various conditions mentioned above, the present invention allows the production of a crosslinked polyolefin foam with cells that are smooth, finely divided and closed and surrounded by films that are tough and difficult to break.

The polyolefin used in the process 35 of the present invention is required to have the aforementioned predetermined MFR value of 7,82477. Polyethylene (having an MFR in the range of 0.1 to 5.0, preferably in the range of 0.5 to 4.0) and an ethylene / vinyl acetate copolymer (having an MFR of the same as polyethylene and having a vinyl acetate-5 droplet content in the range of 5 -20% by weight, preferably 5 to 15% by weight) are particularly preferred choices for the production of tough and thick foam with smooth, finely divided and closed cells. Poly-1,2-butadiene, ethylene / propylene copolymer, ethylene / butylene copolymer, copolymers of ethylene with up to 45% by weight of methyl, ethyl, propyl or butyl acrylate or methacrylate, chlorination products of these copolymers (of which chlorine content up to 60% by weight), mixtures of two or more polymers selected from the above-15 and mixtures of the above polymers with atactic or isotactic polypropylene are also useful. Naturally, all polymers are required to have an MFR in the range of 0.1 to 5.0.

Examples of crosslinking agents that can be used for the purposes of this invention include dicumyl peroxide, 1,1-ditertier-butyl peroxide-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-ditertier-butyl-peroxyhexane, 2, 5-dimethyl-2,5-diter tert-butyl peroxyhexine, α, α-diter tert-butyl peroxydisopropylbenzene, tertiary butyl peroxybenzoate. Among the above crosslinking agents, dicumyl peroxide is particularly preferred.

Examples of blowing agents that can be used in this invention include azotypic compounds such as azodicarbonamide and azodicarboxylate; nitro-type compounds such as dinitrosopentamethylenetetramine and trinitroso-trimethyltriamine; hydrazide-type compounds such as p, p'-oxybisbenzenesulfonylhydrazine-35 di; and sulfonyl semicarbazide type compounds such as p, p'-oxybisbenzenesulfonyl semicarbazide and toluenesulfonyl semicarbazide. Of the above crosslinking agents, azodicarbonamide is particularly preferred.

Examples of flotation aids that can be used in the present invention include compounds in which urea is a major component; metal oxides such as zinc oxide and lead oxide; and compounds in which the main component is salicylic acid, stearic acid, etc., for example, higher fatty acids and metal compounds of higher fatty acids.

In particular, the following describes a process for preparing a crosslinkable, foamable resin composition by the process of this invention, typically using dicumyl peroxide as a crosslinking agent, azo-dicarbonamide as a blowing agent, and zinc white, activated zinc white, or zinc stearate as a blowing aid.

When preparing a foam by expanding low density polyethylene, for example 30 times the initial volume, the resin composition is obtained by mixing together 100 parts (by weight; as usual thereafter) of low density polyethylene (MFR 0.1 to 5.0, preferably 0.5 - 4.0 g / 10 min, density 0.92-0.93 g / cm3), 14-17 parts azodicarbonamide, 0-25 0.4 parts activated zinc white or zinc white (or 0-0.6 parts zinc stearate) and 0.5 to 0.8 parts of dicumyl peroxide.

When preparing a foam by expanding an ethylene / vinyl acetate copolymer 30 times the original volume, a resin composition is obtained by mixing 100 parts of an ethylene / vinyl acetate copolymer (MFR value 0.1 to 5.0, preferably 0.5 to 4). 0 g / 10 min, vinyl acetate content 5-20%, preferably 5-15%), 14-17 parts azodicarbonamide, 0-0.4 35 parts activated zinc white or zinc white (or 9 82477 0-0.6 parts zinc stearate) and 0 .5 to 0.8 parts of dicumyl peroxide.

When preparing a foam by expanding the polyethylene 40 times the original volume, a resin composition is obtained by mixing 100 parts of low density polyethylene (MFR value 0.1 to 5.0, preferably 0.5 to 4.0 g / 10 min and density 0). , 92 -0.93 g / cm3), 18-22 parts of azodicarbonamide, 0-0.3 parts of activated zinc white or zinc white (or 0 -10 0.6 parts of zinc stearate) and 0.5-1.0 parts of dicumyl peroxide .

Further, when preparing the foam by expanding the ethylene / vinyl acetate copolymer to 40 times the original volume, a resin composition is obtained by mixing 100 parts of the ethylene / vinyl acetate copolymer (MFR number 0.1 to 5.0, preferably 0.5 to 4.0). g / 10 min, vinyl acetate content 5-20%, preferably 5-15%), 18-22 parts of azodicarbonamide, 0-0.3 parts of activated zinc white or zinc white (or 20 0-0.6 parts of zinc stearate) and 0, 5 to 1.0 parts of dicumyl peroxide.

The term "MFR value" as used herein means the quantity (expressed in g / 10 min) determined by the method of JIS K-6760-1971.

If the mixture formed as described above is heated in the casting step at a temperature in the range of 120 to 160 ° C for 20 to 70 minutes using a pressure in the range of 20 to 200 kp / cm 2, a crosslinked, non-foaming resin composition having a gel percentage of 10 - 70% and an expansion ratio of 1 to 3 times the original volume. The crosslinked, non-foamed resin composition molded as described above is then foamed by heating in a mold of the desired shape in a non-airtight mold at normal atmospheric pressure to produce a tough, crosslinked polyethylene foam having smooth, finely divided and closed fractured cells. .

The present invention is described in detail below. First, a crosslinkable, foamable resin-5 pig composition obtained by mixing together a polyolefin, a blowing agent, a crosslinking agent and, if desired, a foaming aid in the above proportions is compressed into the cavity of the sealable mold and heat-heated under pressure at the above conditions. The crosslinked, non-foaming resin composition obtained by casting is removed from the mold. This resin composition is placed in a mold of the desired shape which is not airtight, such as a mold having a rectangular cross-section, and is heated for a predetermined time, for example, by holding in a metal bath using Rose or Wood metal; oi-bath was; in a salt bath using a molten mass of one or more salts selected from the group consisting of sodium nitrate, potassium nitrite and potassium nitrate; or under a stream of nitrogen. Alternatively, the above-mentioned resin composition is placed in a mold having a rectangular cross-section, the outer wall of which is provided with a tube for a heating medium (such as water vapor) or covered with a movable iron plate, etc., and then heated for a predetermined time. The heated resin composition is then cooled to obtain a foam. The heating temperature is in the range of 145 to 210 ° C, preferably 160 to 190 ° C, and the heating time is in the range of 10 to 90 minutes, preferably 15 to 40 minutes, as described above. The result is a highly flexible, tough and thick foam whose cells are smooth, finely divided and closed surrounded by unbreakable films.

In addition, the present invention allows heating 35 during the aforementioned thermal flotation to be performed at a normal atmospheric pressure of 11,82477 in two separate steps, allowing the crosslinking conditions for flotation to be reasonable. The object of the present invention to carry out the flotation in two steps is based on uniformly balancing the use of heat in a foamable, crosslinkable resin composition, i. to eliminate heat unevenness in the thickness direction of the above composition. As a result, surface cracking in the case of partially uneven foaming, rounding and gas leakage can be prevented, the expansion ratio of the foam can be increased to about 70 times the original volume, and the thickness of the foam can be adjusted to about 150 millimeters. This two-stage thermal foaming method thus proves to be particularly effective for obtaining a foam with a high final thickness and a foam with a high expansion ratio, such as more than 20 times the original volume. In particular, the above-mentioned two-stage thermal foaming is performed by heating a crosslinked, non-foaming resin composition molded according to the above procedure to 145-180 ° C in the first step of thermal foaming as described above for 5-60 minutes, preferably 10-45 minutes, then removing the formed foam and then in the second step, this intermediate foam is placed in a mold of the desired shape which is not airtight and is heated to a temperature in the range of 170-210 ° C as described above for 5-50 minutes, preferably 15-40 minutes, and thus the heated foam is cooled with a low density foam. to obtain. The first step of thermal foaming described above is preferably carried out in such a way that 5 to 70% of the foaming agent can be dispersed. If the amount of scattering of the blowing agent is too large, the above-mentioned effect is no longer obtained by dividing the thermal foaming into two stages.

i2 82477 In the present invention, various additives (fillers) including carbon black, metal oxides such as zinc oxide, titanium dioxide, calcium oxide, magnesium oxide and silica, carbonates such as magnesium carbonate and calcium carbonate, non-fibrous materials such as wood pulp, various dyes, pigments and fluorine there is no appreciable detrimental effect on crosslinking, can be added to the starting material mixture to improve the physical properties and reduce the cost of the foam to be made.

Further, according to the method of the present invention, the diameter of the cells formed in the foam decreases in proportion to the increase in heating time during thermal foaming, as shown in Table 1 below. Thus, the appearance, feel, etc., of the foam produced finally can be intentionally changed by adjusting this heating time.

The crosslinked poly-20 olefin foam obtained according to the present invention has a wide range of applications in various applications including thermal insulation, noise shields, butt shields, packing pads, mattress interiors and packaging materials.

The invention is presented below by means of working examples.

25 Example 1

The composition containing 100 parts of low density polyethylene (MFR value 1.0 and density 0.920; the product manufactured by Mitsubishi Petro-Chemical Co., Ltd. is sold under the tradename "Yukalon YF-30"), 17 parts of azodicarbonamide-30 dia ( the product manufactured by Eiwa Chemical Industry Co. Ltd, sold under the tradename "Vinyhol AC ^ ^ SOS"), 0.1 part of activated zinc white and 0.6 part of dicumyl peroxide was kneaded with a mixing roller at 110 ° C. In a metal mold (750 x 360 x 28 mm) kept in a press heated to 150 ° C, the obtained

II

i3 82477 mixture under pressure (100 kp / cm2) for 40 minutes to cast a crosslinked non-foaming composition (750 x 360 x 28 mm). The gel percentage of this crosslinked, non-foaming composition was found to be 42% and the expansion ratio 1.5 to the original volume. The crosslinked, non-foamed composition obtained was then placed in a non-airtight metal mold (2080 x 1060 x 95 mm) with an outer wall provided with a tube for the heating medium, heated with steam at 1070 ° C for 80 minutes, cooled and removed. made of metal mold. This gave a foam which had a very high mechanical strength and whose cells were smooth, finely divided and closed.

The foam cells 15 thus prepared had a content of closed cells of 82%, a uniform thickness of 100 mm, a cross-sectional shape and dimensions substantially the same as the metal mold, and a density of 0.030 g / cm 3. When the foam was compressed five times in an amount of 50% by means of a compression machine, the cell walls remained unbroken and the concentration of closed cells unchanged. The recovery rate of the foam was 92% after standing for one hour.

In this case, the efficiency of the material was high and the machinability was satisfactory, because, unlike usual, the metal mold used did not need to be reduced and there was no tapered part in the intermediate foam that should have been removed by cutting.

Example 2

The crosslinked, non-foaming composition formed under the same conditions as in Example 1 was heated in a metal bath at 170 ° C for 35 minutes to effect 40% dispersion of the blowing agent and to form a first stage intermediate foam. This first stage foam was removed from the metal bath, then placed in a non-airtight metal mold (2080 x 1060 x 35 95 mm) and heated in a metal 184824 bath at 185 ° C for 40 minutes to completely disintegrate the remaining blowing agent, cooled and removed. from the mold. This gave a foam having the same physical properties as the foam of Example 5.

Example 3

A composition containing 100 parts of low density polyethylene (MFR 0.5 and density 0.920; manufactured by Mitsubishi Petro-Chemical Co. Ltd., sold under the tradename "Yukalon HE-60"), 23 parts of azodicarbonamide (manufactured by manufactured by Eiwa Chemical Industry Co., Ltd, sold under the tradename "Vinyhol AC ^ 50S"), 0.2 parts of zinc stearate and 0.6 parts of dicumyl peroxide were kneaded with a mixing roller at 110 ° C. The mixture was placed in 15 metal molds (720 x 33 x 25 mm) held in a press at 150 ° C and heated under pressure (100 kp / cm 2) for 40 minutes to cast a crosslinked, non-foaming composition (720 x 330 x 25 mm). . The gel percentage of this crosslinked, non-foaming composition was found to be 45% and the expansion ratio 1.8 times the original volume. The crosslinked, non-foaming molded composition was then subjected to thermal foaming under the same conditions as in Example 1. This gave a foam having very high mechanical strength and uniform, finely divided and closed cells.

The foam thus prepared had a closed cell content of 84%, a uniform thickness of 95 mm, a cross-sectional shape and dimensions substantially the same as the metal mold and a density of 0.023 g / cm 3. When this foam was compressed five times to 50% relative to the press machine, the cell walls remained unbroken and the concentration of closed cells remained unchanged. The recovery ratio was 93% after standing for one hour.

35 In the present case, the efficiency of the material was high and the workability was satisfactory, since, unlike the usual method, the metal mold used did not need to be narrowed and there was no tapered part in the intermediate foam which had to be removed by cutting.

5 Example 4

The crosslinked, non-foaming composition obtained under the same conditions as in Example 3 was subjected to thermal foaming under the same conditions as in Example 1. This gave a foam having the same physical properties as the foam obtained according to Example 3.

Example 5

The composition containing 100 parts of ethylene / vinyl acetate copolymer (vinyl acetate content 14%, 15 MFR 1.5 and density 0.397; the product manufactured by Toyo Soda Manufacturing Co., Ltd. is sold under the tradename "Ultra UE-630" ), 17 parts of azodicarbonamide (manufactured by Eiwa Chemical Industry Co., Ltd, sold under the tradename "Vinyhol AC ^ = / 50S"), 0.1 part of activated zinc protein and 0.8 part of dicumyl peroxide were kneaded on a roller at 75 ° C. The mixture was heated in a metal mold (750 x 360 x 28 mm) maintained in a press heated to 145 ° C under pressure (100 kp / cm 2) for 40 minutes to obtain a crosslinked, non-foaming composition (sheet). The gel percentage of this crosslinked, non-foaming composition was found to be 35% and the expansion ratio 2.0 times the original volume. The crosslinked, non-foaming sheet thus obtained was treated by the procedure of Example 2 except that the dimensions of the metal mold used in the second step of flotation were 2080 x 1060 x 95 mm. This gave a foam with very high mechanical strength and containing smooth, finely divided, closed cells.

The closed cells i6 82477 of the foam thus prepared had a content of 80%, a uniform thickness of 100 mm, a cross-sectional shape and dimensions substantially the same as the metal mold, and a density of 0.03 g / cm 3. When the foam was compressed five times at 50% relative to the press machine, the so-5 cell walls remained unbroken and the concentration of closed cells unchanged. The recovery value was 95% after one hour of standing.

In this case, the efficiency of the material was high and the workability was satisfactory because the metal-10 mold used, unlike the conventional method, did not need to be tapered and the intermediate foam did not have cut-outs.

Example 6

The crosslinked, non-foaming composition obtained under the same conditions as the foam of Example 5 was subjected to thermal foaming under the conditions of Example 1. This gave a foam having the same physical properties as the foam of Example 5.

20 Comparative Example 1

A composition containing 100 parts of low density polyethylene (MFR 7.0 and density 0.927; the product manufactured by Mitsubishi Petro-Chemical Co., Ltd., sold under the tradename "Yukalon YM-61"), 17 parts of azodicarbonamine-25 dia, 0.1 part of activated zinc white and 0.8 part of dicumyl peroxide were kneaded with a mixing roller at 110 ° C. Then, following the procedure of Example 1, a foam was obtained. The expansion ratio of the crosslinked, non-foamed sheet was found to be 1.4 times the volume of the original 30 and the gel percentage to be 40%.

The closed cell content of the foam was 80%, the uniform thickness was 95 mm, the cross-sectional shape and dimensions were essentially the same as those of the metal mold, and the density was 0.030 g / cm 3. If the foam thus obtained was compressed five times by 50 per cent with the press machine it i7 82477, the cell membranes ruptured and the concentration of closed cells decreased to 20 per cent. The recovery ratio was 60% after standing for one hour. The mechanical strength of this foam was very poor.

5 Comparative Example 2

A composition containing 100 parts of ethylene / vinyl acetate copolymer (vinyl acetate content 6%, MFR value 6 and density 0.923; the product manufactured by Mitsubishi Petro-Chemical Co., Ltd. is sold under the trade name 10 "Yukalon EVA-10K") and the same ingredients in the same proportions as in Example 5 were treated in exactly the same manner as in Example 5 to form a foam. The expansion ratio of the crosslinked, non-foamed sheet was found to be 2.0 compared to the original volume and the gel to be 15% 33.

The foam thus prepared had a closed cell content of 76%, a uniform thickness of 95 mm, a cross-sectional shape and dimensions substantially the same as the metal mold, and a density of 0.030 g / cm 3. If the foam was compressed 20 times in a 50% press machine, the cell walls ruptured and the concentration of closed cells decreased to 15%. The recovery ratio was 70% after standing for one hour. The mechanical strength of the foam was very poor.

25 Examples 7-9

Foams were prepared following the procedure of Example 1 except that the heating temperature during casting was 135 ° C, 145 ° C and 155 ° C, respectively.

The relationship between the heating temperature used during casting and the diameter of the cells in the finally obtained foam is shown in Table 1.

35 ie 82477

table 1

Example 789

Heating temperature (eC) during casting 135 145 155 5

Cell diameter (mm)

Comparative Example 3

The same composition as in Example 1 was treated under the same conditions as in Example 1 except that the heating temperature of the press was 115 ° C, to produce a foam. The expansion ratio of the crosslinked, non-foamed sheet was found to be 1.0 times the original volume and the gel percentage to 0.

The foam cell thus prepared had a closed cell content of 78%, a uniform thickness of 100 mm, a cross-sectional shape and dimensions substantially the same as the metal mold and a density of 0.030 g / cm 3. When the foam was compressed five times in a 50% amount in a press-20 machine, the cell walls ruptured and the concentration of closed cells decreased to 44%. The recovery value was 68% after standing for one hour.

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Claims (8)

A process for preparing cross-linked closed cell polyolefin foam products wherein a crosslinkable expandable composition is heated under pressure for a predetermined time, whereafter the composition is heated under atmospheric pressure for a predetermined time, characterized in that: a closed mold is filled with a composition containing 100 parts by weight of polyolefin having a melt index (MFR) of 0.1-5.0 g / 10 min, 0.2-1.0 parts by weight of crosslinking agent, 10-30 parts by weight of expansion agent and 0-0.6 parts of white foam auxiliary, and the composition is heated for 20-70 minutes at a temperature of 120-160 ° C at a pressure above 10 bar and molded into a mold corresponding to the mold to give a cross-linked, substantially non-bonding. foamed product having a gel content of about 10-70% and a 1-3 fold expansion ratio relative to the original volume, after which the obtained cross-linked, non-foamed product is foamed by heating under atmospheric pressure. in a form which is not gas-tight and which corresponds to the desired shape, whereby a thick, foamy product with evenly spaced cells is obtained. 2. A process according to claim 1, characterized in that the foaming is carried out by heating at a temperature of 145-210 ° C under atmospheric pressure. Process according to claim 1, characterized in that the foaming under atmospheric pressure by heating is carried out in two stages, wherein in the first stage the crosslinked, non-foamed product is heated until 5-70% of the expansion agent is decomposed and in the second stage the product heated until the remaining portion of the expansion agent is decomposed at a temperature higher than in the first step. 4. A process according to claim 3, characterized in that the heating in the first stage 22 is performed at a temperature of 145-180 ° C and in the second stage the heating is carried out at a temperature of 170-210 °. C. 5. A method according to claim 1, characterized in that the casting of the crosslinkable expandable composition is carried out at a pressure of 10-200 bar. 6. A process according to claim 1, characterized in that the cross-linkable, expandable composition is molded in a rectangular cross-section mold. 7. A method according to claim 6, characterized in that the outer side of the mold with a rectangular cross section is provided with channels for heating medium. 15 8. A process according to claim 1, characterized in that the crosslinkable, expandable composition further contains filler, pigment, dye and fluorescent substance.