NL8201314A - PROCESS FOR MANUFACTURING PREPARATIONS FROM CROSS-TURNED POLYOLEFINS IN THE FORM OF OPEN-CELL FOAM. - Google Patents

Description

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Process for the production of products from cross-linked polyolefins in the form of open-cell foam.

The invention relates to a process for the production of products from cross-linked polyolefins in the form of open-cell foam.

Among the methods hitherto proposed for the production of cellular products of cross-linked polyolefins, especially polyethylene, the methods using decomposition type organic blowing agents are common. These methods, as described in Japanese Patent Publications 8840/1965, 10 18832/1967 and 22674/1968, generally involve the steps of first cross-linking polyethylene by an organic peroxide or by exposure to electron beams and then decomposing the blowing agent so as to provide a cellular structure to the cross-linked polyethylene. Furthermore, a method is known in which a foamable and cross-linkable material containing a polyolefin, a blowing agent and a cross-linking agent is heated in a closed mold under elevated pressure, after which the pressure applied to the molded material is released, resulting in foamed , cross-linked polyethylene, as well as the so-called "two-step method", as described in Japanese Patent Publication 29381/1970, wherein the foamable and cross-linkable polyolefin material is heated in the same manner as above to partially decompose the blowing agent and then further heated under atmospheric press to decompose residual blowing agent.

Since the decomposition of blowing agent and cross-linking agent in these latter two methods is effected by heating the material in the closed form under pressure, the cross-linking reaction of polyethylene takes place but the foaming is suppressed and the expansion of polyethylene only takes place after removal of the applied pressure. Therefore, the latter methods are the same as the former with respect to the principle that polyethylene is first cross-linked and then expanded.

The foamed products of cross-linked polyolefins obtained by the above methods have a closed cell structure. With these methods it will be difficult to obtain an open cell foamed product. This is because, unlike in the reactive foaming encountered in the production of polyurethane foam, the foaming of cross-linked polyolefin produces closed cells and the membranes surrounding these cells are so strong that they will not be broken even when a compressive force is applied, whereby such closed cells would be converted into open cells, and even if the membranes are forcefully broken in some way, the broken cell membranes will not be retained as they are. Due to the melt elasticity that polyolefins usually have, broken cell membranes cannot be retained as they are without regard to the specific type of blowing agent and the presence or absence of cross-linking reaction. The expansion of the expanding gas is accompanied by the phenomenon of contraction of cell membranes or occurrence of empty cavities. This phenomenon becomes more pronounced with an increasing degree of expansion of polyolefin foam.

Under the above conditions, the majority of the commercially available open cell foam products are polyurethane foam. Polyolefins, however, exhibit excellent weather resistance compared to the soft urethane resin characteristic of the resins capable of producing single cell foam products and also have very good chemical and water resistance. Hence, a long-awaited open-cell foamed article made from this resin.

To date, some methods directed to the production of open-cell foam products from polyolefins have been proposed, for example, the method of mixing polyolefins with a water-soluble powder, such as starch, and then leaching the water-soluble powder. from the mixture, and the sintering method wherein the polyolefin powder is sintered. However, with these methods hardly any cellular products of very low expansion ratio, on the order of about 2 to 3 times the original volume, are obtained.

Recently, methods have been proposed which effect the breaking of the closed cell membranes of a foamed cross-linked polyethylene through the action of compressive force. One of these methods is described in Japanese Patent Publication 10350/1974. This method comprises cooling the foamed article of a closed-cell thermoplastic resin to a temperature below the second order transition temperature (embrittlement temperature) of the thermoplastic resin and rolling the cooled foamed article to produce a cellular advance. produce an open-cell mixture. This method accomplishes the conversion of closed cells into open cells by sacrificing to some extent the strength of the thermoplastic resin itself. Another method is described in Japanese Patent Application Laid-Open No. 63172/1979. This method comprises producing a foamed article of polyethylene containing an inorganic filler and subjecting the formed article to compression so as to break the membranes of the closed cells and convert the cells into open cells. This method accomplishes the conversion of closed cells into open cells by adding a large amount of inorganic filler to the resin, sufficient to decrease the strength of the resin.

However, the former method has the drawback that it takes a very long time to cool the foamed article with an extremely low thermal conductivity to a temperature below the embrittlement temperature (-100 ° C) of -100 ° C. the resin and the method, if it is to be carried out in a short time, can only be used for foaming sheets with a very small thickness.

The latter method also has the disadvantage that it is hardly practicable and, if it is carried out by special technical efforts, it is inevitable that the addition of the large amount of inorganic filler reduces the degree of expansion and increases the mass density.

In any event, successful closed cell conversion of a foamed cross-linked polyolefin into open cells on a commercial scale has not yet been accomplished. This is because the polyethylene resin, etc. used as the starting material of the foamed cross-linked natural polyolefin is so strong that the closed cell membranes in the foamed article will not be broken upon application of compressive force and even if the compressive force is sufficient large to break such membranes, the compressive force only penetrates into the surface area of the foamed article. The compression force transmitted to the deeper portion of the foamed article is not great enough to break the membranes in that portion. Therefore, the desired conversion of closed cells into open cells has not hitherto been achieved.

It is therefore an object of the invention to provide a method for easily producing an open-cell foamed article from cross-linked polyolefin solely by effecting mechanical deformation on the foamed article without any other special treatment or the addition of a filler is required.

Another object of the invention is to provide a method for the production of an open-cell foamed product of cross-linked polyolefin with a large thickness and a high degree of expansion.

8201314 - 5 -

Another object of the invention is to provide open-cell foamed crosslinked polyolefin articles which have very advantageous properties and a desired thickness with a high degree of expansion.

To achieve the above-described and other purposes of the present invention, there is provided a method for the production of an open-cell foamed article of cross-linked polyolefin, wherein the polyolefin is mixed with a decomposition type chemical blowing agent and a cross-linking agent to obtain a foamable and cross-linkable material, the material forms into a desired shape while maintaining its gel percentage at zero, the formed material heats at a suitable foaming temperature under atmospheric pressure under such conditions that the peak of the ratio of the degree of cross-linking until the degree of decomposition of the blowing agent is not more than 20 to decompose the cross-linking agent and the blowing agent simultaneously, resulting in a foamed product of cross-linked polyolefin with cells coated with very thin membranes that can be easily broken through the action of mechanical force and this foamed product mechanically deforms to break the cell membranes and convert the closed cells into open cells.

The drawing is a graph showing the change in the ratio of the degree of crosslinking to the degree of blowing agent decomposition as a function of the heating time at a suitable foaming temperature.

The method of manufacturing an open-cell crosslinked polyolefin foam product of the invention consists primarily in adjusting the blowing agent decomposition rate relative to the rate of the crosslinking reaction.

35 In the theory of the foaming of cross-linked polyolefin, the so-called "Pre-cross-linking / 8201314 Λ - 6 - subsequent foaming", as mentioned above, is generally accepted by those skilled in the art. In other words, it is believed that the expanding gas from the resin leaks upon expansion unless the viscosity of the resin is increased by its cross-linking, however, it has been found that the elongation of pre-cross-linked resin is low and thus it is hardly possible to produce a foamed product with cells that are coated through very thin membranes which are suitable for obtaining an open-celled foamed product It has now been found according to the present invention that it is possible to produce said foamed product by the cross-linking reaction and the foaming of the foamable and cross-linkable material simultaneously effecting the material 15 being held in the condition in which the gel percentage is zero.

The meaning of the term "simultaneous decomposition of cross-linking agent and blowing agent" will now be explained.

When the foamable and crosslinkable material is heated under atmospheric pressure, the crosslinking reaction and the blowing agent decomposition take place and the crosslinking curve and the blowing agent decomposition curve are obtained, respectively. When the foamable and crosslinkable material whose gel percentage is kept at zero under atmospheric pressure is heated and the ratio (y) of the degree of crosslinking to the degree of decomposition of the blowing agent is plotted against the heating time on logarithmic graph paper the curve as shown in the accompanying drawing can be obtained. However, when the foamable material previously cross-linked according to the prior art is heated under atmospheric pressure, a similar curve cannot be obtained.

Degree of cross-linking Degree of decomposition of the blowing agent 35 8201314

V.

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Degree of cross-linking: gel percentage of resin at a given heating time.

Rate of blowing agent decomposition: ratio of the degree of expansion at the same heating time as above to the final degree of expansion of the resulting foamed product.

# *

Herein, the term "gel percentage" means the ratio of the weight of the sample after extraction to 10 wt. Weight before extraction, the extraction being carried out under reflux of trichlorethylene used as a solvent for 24 hours by means of a soxhlet extraction- device using a glass filter from 40 to µm. The gel percentage is calculated using the following equation. The degree of cross-linking is proportional to the increase in the gel percentage.

W3 - {| (1 - x) + | } WQ

Gel percentage = - x 100

20 WQ - {| (1 - x) + 0.7 | x + | } WQ

in which:

Wed *. weight of the sample for extraction,

Wj: weight of the sample after extraction, 25 T: Total number of parts by weight of the components, A: Number of parts by weight of blowing agent, C: Number of parts by weight of fillers, x: Degree of decomposition of the blowing agent, Λ γ (1-χ) ¥ ο: Weight of the residual blowing agent in the sample,

a

0.7 γχ Wq: Weight of the decomposed blowing agent residue in the sample and c - W: Weight of the fillers in the sample.

Γ o

In the figure, the peak A of the curve indicates the ratio (y) of the degree of crosslinking to the degree of decomposition of the blowing agent with the decomposition of 8201314 V ·.

- the blowing agent is the most behind on the crosslinking of the resin compound. That is, at the time associated with this point A, the distance between the cross-linking curve and the decomposition curve of the blowing agent is greatest. The greater the value of this ratio (y) at this peak, the greater the foaming retardation relative to the crosslinking. On the other hand, a smaller value of the said peak ratio shows that the delay of the frothing relative to the cross-linking is small, i.e. the cross-linking reaction 10 and the frothing phenomenon of the foamable and cross-linkable material occurred simultaneously.

Surprisingly, it has now been found that there is a peak limit in said ratio (y) in order to obtain a foamed product with cell membranes that are easily breakable by the action of mechanical force. The peak value of this ratio (y) is influenced by the type of resin used and by the amounts of the cross-linking agent or the blowing agent. Regardless of these parameters, however, it has been found that when said peak ratio is no more than 20, a foamed product can be obtained with cell membranes suitable for the manufacture of the open-cell foamed article. The said peak ratio value is critical, but it is preferable to have this peak ratio in the range of no more than 15.25 because in the case of specific resin types, a high degree of rigidity is required with respect to the reaction conditions units, etc., at a value close to 20.

Thus, the term "simultaneous crosslinking agent and blowing agent decomposition" as used herein means that the decomposition of the crosslinking agent and the blowing agent is effected under conditions such that the peak of the ratio (y) is not more than 20.

It is recommended to subject various polyolefins to a prior foaming to determine the range of amounts of cross-linking agent, blowing agent, foaming aid, if necessary, and the optimum foaming temperatures meeting the aforementioned conditions. In actual embodiments, the amounts of each of the components can be selected within the range so defined.

To describe the present invention more specifically, a given polyolefin is mixed with a blowing agent, a cross-linking agent and optionally a foaming aid, a filler and a pigment, and the resulting mixture is kneaded with a heated mixing roll or the like. Thereafter, the obtained material is placed in the mold with a desired hollowing profile and pressure is applied with a press, thermoformed at a temperature within the range of from 115 to 155 ° C, preferably from 120 to 140 ° C, and then from removed the shape. Instead of said molding at an elevated temperature and under pressure, the material after kneading can be formed by heating it in a non-pressurized mold or by passing it directly through an extruder or through a calender roll to lead. However, since heating in this molding step brings the foamable and cross-linkable material to the thermally excited state and thus contributes to the more uniform simultaneous decomposition of the cross-linking agent and the blowing agent in the subsequent foaming and cross-linking step, the molding is preferred of the material under heating. For example, when the molding is performed without heating and without applying pressure, the cells of the foamed product obtained in the subsequent foaming and cross-linking step are coarse and uneven, which is somewhat undesirable. In this thermal molding, it is significant that the foamable and crosslinkable material should be formed while maintaining its gel percentage at zero, namely at the predetermined heating time and temperature, which will not crosslink polyolefin. Therefore, the molding temperature should be lower, preferably more than 20 ° C lower, than the foaming temperature in the subsequent foaming and cross-linking step. If the cross-linking of polyolefin takes place in this thermal shaping step, as shown in the comparative examples described below, a final product with an open cell ratio of less than 50% would be obtained, which cannot be an open cell foam product called. Furthermore, if said molding is carried out at an elevated temperature and under pressure, as in Examples XI to XV as described below, the cell size of the resulting foamy product becomes finer as the heating time increases. It is therefore possible to delicately vary the appearance and tactile impression of the final foamed article by varying the heating time. Incidentally, a very small amount of blowing agent can be pre-decomposed in this thermal heating step, and as a result, the molded material can expand to a degree approximately twice the original voltime when removed from the mold. However, this phenomenon does not fall within the concept of frothing and is acceptable for the present invention. It can be considered that the above-mentioned difference in cell size is due to the fact that the nuclei for cells can be formed by this blowing agent decomposition.

The foamable and cross-linkable material, formed as above, is then heated under atmospheric pressure to simultaneously decompose the blowing agent and the cross-linking agent. The meaning of the term "simultaneous decomposition of blowing agent and cross-linking agent" and the conditions thereof have already been explained above.

In this foaming and cross-linking step, the formed material is heated in an atmosphere of nitrogen or in a heating medium, for example, in a metal bath containing Rose metal, Wood metal or the like, an oil bath, a molten salt bath containing one or more salts such as sodium nitrate, potassium nitrate, potassium nitrite or the like. The formed material is preferably placed in an openable mold or metal box which is not airtight and heated in the said heating medium at a suitable temperature of change. It is also possible to provide the openable mold or metal box which is not airtight with a heating device on the surface of the metal plate thereof or with a jacket through which a heating medium such as steam, heating oil, etc. is circulated . By using this openable mold, the foamable material is indirectly heated by the heating device or heating medium. Furthermore, the formed material can be covered with a metal plate, etc., which is movable up and down and heated in such a state. After heating for a predetermined period of time, the material is cooled to obtain a cooled and foamed product.

The foaming temperature was selected within the range of from 145 to 210 ° C, preferably from 160 to 190 ° C, appropriate to the specific type of polyolefin used, and the heating time is within the period of from 10 to 90 minutes, preferably from 15 to 40 minutes. Thus, a foamed product can be obtained with closed cells, the membranes of which can be easily broken by exerting a mechanical deformation, at a degree of cross-linking comparable to that of the foamed product obtained by the methods of the prior art of the technique, (gel percentage up to about 95%).

According to the invention, the heating in the foaming and cross-linking step can be performed in two steps. In this two-step process, the conditions for foaming and cross-linking the polyolefin are mild, and therefore the decomposition of the cross-linking agent and the blowing agent can be accomplished more simultaneously in two steps. This two-step mode allows the heterogeneous heat conduction towards the thickness of the foamable and cross-linkable material to be eliminated and the material to be homogeneously heated. Accordingly, no phenomena such as surface cracks resulting from partial unevenness of material shift, collapse and gas escapement occur. Furthermore, it is possible to increase the expansion ratio of the foamed article obtained at will up to about 70 times the original volume and to a thickness of up to about 150 mm. Therefore, this two-step process is particularly suitable for producing thicker foam products or foam products with higher expansion ratios, more than 20 times the original volume.

To describe this two-step foaming and cross-linking process more specifically, in the first step, the foamable and cross-linkable material formed as mentioned above is heated in the same manner as described above, ie in a nitrogen atmosphere or in a metal bath, a molten salt bath, etc., at a temperature of 145 to 180 ° C for a period of 5 to 60 minutes, preferably 10 to 45 minutes, and then the intermediate is removed from the heating medium. In the second step, the intermediate is further heated in the same manner as mentioned above at a temperature of 170 to 210 ° C for a period of 5 to 50 minutes, preferably 15 to 40 minutes, then cooled to provide a foamed low density product.

In said first step, it is preferable to decompose 5 to 70% of the blowing agent, achieving a gel percentage of the resin material from about 20 to about 80%. If the blowing agent decomposition rate and gel percentage are very high, the above-mentioned advantages of this two-step process will not be achieved.

The foamed article, obtained as above, is compressed by passing between two rollers rotating at the same speed, with the result that the pressure thus applied will break the closed cell membranes of the foamed article and consequently closed cell structure will convert into an open cell structure.

8201314 - 13 -

The open-cell foamed articles obtained by the process of the invention have excellent properties that compare favorably with the properties of a polyurethane foamed article, and its open-cell ratio, determined in the same manner according to Remington Pariser Method (ASTM D 1940-62T) equal to or almost equal to 100%.

The preferred polyolefins used in this invention are low density polyethylene, intermediate density polyethylene, high density polyethylene, poly-1,2-butadiene, ethylene-propylene copolymer, ethylene-butene copolymer, ethylene vinyl acetate copolymer, copolymers of ethylene with at most 45% methyl, ethyl, propyl or butyl acrylate or methacrylate, chlorinated products of the above homo- or copolymers of which the chlorine content is at most 60% by weight blends of two or more of the above polymers and blends of the above polymers with isotactic or atactic polypropylene.

To meet the purpose of the invention, the cross-linking agent in the polyolefin should decompose at a temperature at least higher than the inflection point of the polyolefin. Organic peroxides which decompose upon heating to release free radicals capable of producing intermolecular or intramolecular cross-linked bonds and thus advantageously function as radical generators meet this requirement. Examples of such organic peroxides include, but are not limited to: dicumyl peroxide, 1,1-ditertiary-butyl-peroxy-3,3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-ditertiary-butyl-peroxyhexane, 2,5-dimethyl-2,5-ditertiary-butyl-peroxy-hexene, α, α-ditertiary-butyl-peroxydiisopropyl-benzene, tertiary-butylperoxy-ketone, tertiary-butylperoxybenzoate, etc. The organic peroxide that is best suits the specific type of polyolefin used should be selected.

The blowing agents useful in the present invention are chemical blowing agents whose decomposition temperature is higher than the melting point of the polyolefin. Examples of such chemical blowing agents include, but are not limited to: azotype compounds, such as azodicarbonamide and barium azodicarboxylate; nitrosotype compounds, such as dinitrosopentamethylene-tetramine and trinitrosotrimethyl-triamine; hydrazide-type compounds such as p, p'-oxybis (benzene sulfonyl hydrazide); sulfonyl semicarbazide type compounds such as p, p'-oxybis (benzenesulfonyl semicarbazide) and toluene sulfonyl semicarbazide, etc.

In addition to the specific type of polyolefin used and the foaming temperature selected, the amounts of the cross-linking agent and the blowing agent are significant factors affecting the ratio (y) of the degree of cross-linking to the degree of blowing agent decomposition . If the amount of cross-linking agent is too large or the amount of blowing agent is very low, the peak of said ratio (y) will easily exceed 20 and, as a natural consequence, it is hardly possible to produce an open-cell foamed article. Therefore, the amounts of the cross-linking agent and the blowing agent should be selected within the range in which the peak of said ratio (y) will not exceed 25.

In addition to the above factors, it is possible to control the peak of the ratio (y) by adding a foaming aid (see Example I and Comparative Example A, described below). Thus, in the present invention, a foaming aid can be used depending on the specific type of blowing agent to be used. Examples of such aids include, but are not limited to: compounds with urea as the major component; metal oxides, such as zinc oxide and lead oxide; compounds such as salicylic acid, stearic acid, etc. as the main component, i.e. higher fatty acids, metal compounds 8201314-15 of higher fatty acids, etc.

According to the invention, in order to achieve improvement of the properties of the prepared material and reduction of costs, if desired, compounding additives or fillers can be added to the material which do not have too detrimental effects on the cross-linking of the polyolefin, for example carbon black; metal oxides such as zinc oxide, titanium oxide, calcium oxide, magnesium oxide and silicon dioxide; carbonates, such as magnesium carbonate and calcium carbonate; fibrous fillers, such as pulp; various dyes; pigments; fluorescent materials and rubber compounding ingredients commonly used.

Unlike in the conventional art, where one succeeds in obtaining the required reduction in the strength of the resin, for example, by cooling the closed-cell foamed product to a temperature below the rosin temperature of the resin or by adding large amounts of inorganic fillers to the material to be foamed, the method according to the invention uses the setting of the blowing agent decomposition rate relative to the resin crosslinking rate. Accordingly, the present invention makes it possible to easily obtain open-cell foamed products of cross-linked polyolefins without compromising the beneficial properties of polyolefins. Furthermore, the final foamed articles can be obtained according to the invention with high open cell ratios, falling within the range of 97 to 100%, and of great thickness. The method of the invention has further advantages of easy practicability, short operating time and high productivity.

The open-cell foamed products of cross-linked polyolefins obtained by the process according to the invention can be used. are suitably used for cushioning media, filters, heat insulating materials, coating materials, etc. In particular, when the said foamed articles are used in garments, 8201314-16 - soundproofing materials and heat insulating materials hitherto were produced using soft polyurethane housings, they exhibit excellent weathering and chemical resistance and high flame retardancy and thus ensure safe use.

To illustrate the invention more specifically, the following examples are presented which are intended to be purely illustrative and not to be construed as limiting the invention.

Example I

A material consisting of ethylene-vinyl acetate copolymer (a product of Mitsui Polychemical Co., Ltd., marketed under the name "Everflex P-1403", VAC 14 wt%), 17 parts by weight per 100 wt. parts resin (phr) 15 azodicarbonamide (a product of Eiwa Chemical Industry Co.,

Ltd., marketed under the name "Vinyhol AC? M50S"), 0.83 phr dicumyl peroxide and 0.5 phr zinc oxide were kneaded in a mixing roll at 85 ° C. The resulting mixture was molded (150 x 150 x 7 mm) within a press, held at 126 ° C and heated under elevated pressure for 30 minutes to form a foamable and cross-linkable sheet. The gel percentage of this sheet was zero. The resulting sheet was then heated in a metal bath held at 170 ° C for 40 minutes to obtain a foamed intermediate in which 30.5% of the blowing agent had decomposed. Then, the foamed intermediate was further heated in a metal bath at 190 ° C for 20 minutes to obtain a foamed product in which the residual blowing agent had completely decomposed. The peak value of the ratio (y) in said foaming-30 and cross-linking step was 10.4. After cooling, the foamed product was passed between two rollers spaced 3 mm apart and rotating at equal speed to break the cell membranes. The foamed article obtained had a thickness of 23.0 mm, a mass density of 0.03 g / cm and an open cell ratio of 100%.

Here, the open cell ratio was measured in a manner similar to the Remington Pariser Method (ASTM D 1940-62T) and determined using the following calculation formula:

<Vs - V - <Δν - V

5 Open cell ratio ----- x 100 V - V_

s R

= —-X 100 V -

s R

1 o where V: Volume of the sample s v: Volume of the resin matrix (= sample weight W / R s density of the resin), and ^ V: Increase in volume.

15

Examples II to IV

The procedure of Example I was repeated using varying amounts of zinc oxide and dicumyl peroxide as shown in Table A. In each of the examples, a 100% open cell foamed article was obtained.

The blowing agent decomposition rate in the foamed intermediate was 51.7% in Example II, 69.0 X in Example III and II X in Example IV, and the peak value of the ratio (y) was 5.0 in Example II, 1.27 in Example III and 4.0 in Example IV.

Comparative Example A

The procedure of Example I was repeated without using zinc oxide. The peak value of the ratio (y) in the foaming and cross-linking step was 20.5. The foamed product obtained had an open cell ratio of 55.3%, demonstrating that the rupture of the cell membranes was only partially accomplished.

Example V

A foamed article was produced from a material consisting of ethylene-vinyl acetate-8201314-J8 copolymer (a product of Mitsubishi Petrochemical Co., Ltd. marketed under the name "Yukalon EVA-41H", VAC 16 wt. .%), 17 phr azodicarbonamide and 0.53 phr dicumyl peroxide in the same manner and under the same conditions as in Example I. The open cell ratio of the foamed product obtained was 100%.

Example VI

A material consisting of the same resin as used in Example V, 17 phr azodicarbonamide, 0.08 phr zinc oxide and 0.73 ph dicumyl peroxide was kneaded on

the same manner as in Example 1. The resulting mixture was molded (140 x 140 x 28 mm) within a press, held at 126 ° C and heated under elevated pressure for 30 minutes to form a foamable block. The foamable block thus obtained was then heated in a metal bath held at 170 ° C for 40 minutes to obtain a foamed intermediate in which 27% of the blowing agent had decomposed. Thereafter, the foamed intermediate was placed in an openable mold (370 x 370 x 20 110 mm) which is not airtight and heated in a 190 ° C

held metal bath for 30 minutes to completely decompose the residual blowing agent. After cooling, the foamed product was removed from the mold.

The foamed product was fed between two rollers spaced 10 mm apart and rotating at the same speed to break the cell membranes. A thick foamed open cell article, with a thickness of 100 mm, a mass density of 0.03 g / cm and an open cell ratio of 100%, was obtained.

Example VII

A foamable sheet was obtained from a material consisting of the same resin as used in Example I, 17 phr azodicarbonamide, 0.2 phr zinc oxide and 0.63 phr dicumyl peroxide in the same manner and under the same conditions as in Example 1. The foamable sheet obtained was heated in a metal bath kept at 190 ° C for 8201314-19-15 minutes to completely decompose the blowing agent and the crosslinking agent in a single step, resulting in a foamed product. After cooling, the foamed product was converted into an open cell foamed article having an open cell ratio of 100% by passing it between two rollers rotating at the same speed in the same manner as in Example I.

Example VIII

An open-cell foamed article was produced from a material consisting of the same resin as used in Example V, 35 phr azodicarbonamide, 0.53 phr dicumyl peroxide under the same conditions as in Example 1. The highly expanded open-cell foamed article had an open cell ratio of 100%, a thickness of 30 mm and a mass density of 0.019 g / cm.

Example IX

A material consisting of low density polyethylene (a product of Mitsubishi Petrochemical

Co., Ltd. marketed under the name "Yukalon LK-30", 3 density 0.918 g / cm, MFR 40), 17 phr azodicarbonamide and 0.2 phr zinc oxide were intimately mixed in a mixing roll at 100 ° C. The resulting mixture was molded (150 x 150 x 7 mm) in a press at 136 ° C and heated under elevated pressure for 30 minutes to form a foamable sheet.

This sheet was cross-linked and expanded under the same conditions as in Example I and then fed between two rollers rotating at the same speed as in Example I. An open cell foamed article with an open cell ratio of 100% and a thickness of 23 mm was thus obtained.

Example X.

An open cell foamed article was produced from a material consisting of low density polyethylene (a product of Mitsubishi Petrochemical 35 Co., LTD., Marketed under the name "Yukalon HE-30", density 0.92 g / cm , MFR 0.5), 17 phr azodicarbonamide and 8201314 - 20 -

0.13 phr dicumyl peroxide under the same conditions as in Example IX. The open cell foamed article had an open cell ratio of 100% and a thickness of 23 mm. Examples XI to XV

The procedure of Example I was repeated using the same material as in Example I and a varying heating time in a press of 1, 5, 10, 20 and 30 minutes. Each of the foamed articles had an open cell ratio of 100% and the thickness of the molded article and its appearance was unchanged regardless of the heating time. However, the cell size decreased with increasing heating time.

Comparative Examples B and C

The foamed articles were produced from the same material as used in Example V, under the same conditions as in Example V, except that the heating temperature in the press was changed to 145 ° C (Comparative Example B) and 151 ° C (Comparative Example C ). After the compression step, the foamed products according to test were found to have open cell ratios of 47.2 and 45.2%, respectively, showing that the cell membranes were only broken and that the foamed products were not open-celled. -foamed product will function. Under the conditions, the gel percentages of the foamed molded sheets were press 20.0% at 145 ° C and 31.2% at 151 ° C, respectively.

The operating conditions maintained in the examples and in the comparative examples, and the results obtained are shown in Tables A and B below. The relationship between the cell size of the final foamed product and the heating time in the shaping step in the examples XI to XV are shown in Table C.

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Table C

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Claims (17)

1. A process for the production of open-cell foamed, cross-linked polyolefin products, characterized in that a polyolefin is mixed with a chemical blowing agent and a cross-linking agent to obtain a foamable and cross-linkable material, the foamable and cross-linkable material. material in a desired shape while maintaining its gel percentage to zero, heats the molded material at a suitable foaming temperature under atmospheric pressure under such conditions that the peak of the ratio of the degree of cross-linking to the degree of decomposition of the blowing agent is not more than 20 to decompose the cross-linking agent and the blowing agent simultaneously, to form a foamed product of cross-linked polyolefins with cells surrounded by very thin membranes which can be easily broken by the action of mechanical force , and said foamed product mechanically deforms to break the membranes of the cells. A method according to claim 1, characterized in that the decomposition of the cross-linking agent and the blowing agent in the shaped material is effected more simultaneously by two-step heating; wherein in the first heating 5 to 70% by weight of the blowing agent originally present in the material is decomposed and in the second heating the undecomposed blowing agent and cross-linking agent are decomposed remaining in the primary foamed product at a temperature higher than that during the first heating. Method according to claim 1 or 2, 35, characterized in that the decomposition of the cross-linking agent and the blowing agent in the formed material is effected 8201314 *. By heating the material in a bath selected from the group consisting of metal bath, oil bath and molten salt bath, or in an atmosphere of nitrogen gas. Method according to claim 1, 5, characterized in that the shaped material is placed in an openable form which is not airtight and is provided with a heating device or a jacket through which a heating medium is circulated, and the decomposition of the cross-linking agent and the blowing agent in the formed material 10 is effected by indirect heating with the heating device or the heating medium. A method according to claim 1, characterized in that the foaming temperature of the molded material falls within the range of 145 to 210 ° C. A method according to claim 2, characterized in that the foaming temperature of the molded material falls within the range from 145 to 180 ° C in the first heating and within the range from 170 to 210 ° C in the second heating. Method according to claim 1, characterized in that the shaping of the foamable and crosslinkable material is carried out at an elevated temperature. Method according to claim 1, characterized in that the shaping of the foamable and crosslinkable material is carried out under elevated pressure at an elevated temperature. 9. A method according to claim 7 or 8, characterized in that the molding temperature falls within the range of 115 to 155 ° C and is lower than the foaming temperature. 10. A method according to claim 1, characterized in that the shaping of the foamable and cross-linkable material is effected by using an extruder or a calender roll. Method according to claim 1, 8201314 - 26 - characterized in that the mechanical deformation is effected by means of compression applied with two rollers rotating at the same speed. Method according to claim 1, 5, characterized in that the foamable and cross-linkable material contains a foaming aid. 13. A method according to claim 1, characterized in that the foamable and cross-linkable material contains a compounding agent or filler, such as metal oxides, carbonates, fibrous fillers, dyes, pigments, fluorescent materials and rubber compounding ingredients. Process according to claim 1, characterized in that the polyolefin is selected from the group 15 of high-density polyethylene, medium-density polyethylene, low-density polyethylene, poly-1,2-butadiene, ethylene-propylene copolymer , ethylene-butene copolymer, ethylene-vinyl acetate copolymer, ethylene copolymers with up to 45% methyl, ethyl, propyl or butyl acrylates or 20-methaerylates, chlorinated products of the above homo- or copolymers with chlorine contents up up to 60% by weight, blends of two or more of the above polymers and blends of the above polymers with atactic or isotactic polypropylene. A method according to claim 1, characterized in that the cross-linking agent is an organic peroxide with a decomposition temperature higher than the flow temperature of the polyolefin. 16. A method according to claim 1, characterized in that the blowing agent is selected from the group of compounds of the azo type, compounds of the nitroso type, compounds of the hydrazide type and compounds of the sulfonyl semicarbazide. type, with decomposition temperatures above the melting temperature of the poly-35 olefin. 17. Methods and articles essentially as described in the description and / or the examples. 8201314