GB2098993A - Open-celled foamed olefin polymers (including polybutadiene) - Google Patents

Abstract

An open-celled foamed polymer of an olefine or butadiene having a high open-cell ratio and a high degree of expansion is produced by blending the polymer with a chemical blowing agent and a cross-linking agent and shaping the blend whilst maintaining its gel percent at zero, decomposing the cross-linking agent and blowing agent concurrently by heating the composition under atmospheric pressure in 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 and exerting a mechanical deformation to the resulting foam to rupture the cell membranes.

Description

SPECIFICATION Open-cell foamed polyolefins DESCRIPTION This invention relates to a method for the production of open-cell foamed polyolefins.

Methods using organic decomposition type blowing agents for the manufactuyre of cellular articles of cross-linked polyolefins, particularly of polyethylene are known. Such methods, as disclosed in Japanese Patent Publication Nos. 8840/1965, 18832/1967 and 22674/1968, generally comprise the steps of first cross-linking polyethylene by means of an organic peroxide or by exposure to electron beams and thereafter decomposing the blowing agent thereby imparting a cellular structure to the cross-linked polyethylene.In addition, there is known a method in which a foamable and cross-linkable composition containing a polyolefin, a blowing agent and a cross-linking agent is heated in a closed mould under increased pressure and thereafter the pressure applied to the composition in the mould is released resulting in the foamed cross-linked polyethylene, and also the "two-step" method, as disclosed in Japanese Patent Publication No.29381/1970, in which the foamable and cross-linkable polyolefin composition is heated in the same manner as above partially to decompose the blowing agent and thereafter further heated under atmospheric pressure to decompose the remaining blowing agent.In the latter two methods, since the decomposition of blowing agent and cross-linking agent is effected by heating the composition in a closed mould under pressure, the cross-linking reaction of polyethylene takes place but the foaming is suppressed, and the expansion of polyethylene occurs only after the release of applied pressure. Therefore, the latter methods are same as the former methods in principle that polyethylene is first cross-linked and thereafter expanded.

The foamed products of cross-linked polyolefins obtained by the above methods have a closed cell structure. By these methods, it would be difficult to obtain a foamed product having an open cell structure. This is because, unlike the reactive foaming such as is involved in the production of polyurethane foam, the foaming of cross-linked polyolefin according to the prior methods produces closed cells, and the membranes enclosing these cells are so tough that, even under application of compressive force, they will not be ruptured to transform such closed cells into open cells. Even if the membranes are forcibly ruptured somehow or other, the ruptured cell membranes will not be retained as they are. Owing to the melt elasticity possessed commoniy by polyolefins, such ruptured cell membranes cannot be retained.With the escape of expanding gas, there ensures the phenomenon of contraction of cell membranes or occurrence of empty cavities. This phenomenon becomes more conspicuous with the increasing expansion degree of polyolefin foam.

In these circumstances, most commercially available open-cell foamed articles are of polyurethane foam. However, polyolefins exhibit excellent weathering resistance as compared with the soft urethane resin typifying those resins which are capable of producing open-cell foamed articles and also have very good resistance to chemicals and to water. Thus an open-cell foamed polyolefin has long been awaited.

So far, a few methods aimed at the production of open-cell foamed articles of polyolefins have been proposed, for instance the method which comprises blending polyolefin with a water-soluble powder such as starch and thereafter dissolving out the water-soluble powder from the mixture, and the sintering method in which the polyolefin powder is sintered. By these methods, however, there are only obtained cellular products of very low expansion ratio of the order of about 2 to 3 times the original volume.

recently, there have been proposed methods which effect rupture of the membranes of closed cells of a foamed cross-linked polyethylene by the action of compressive force. One of these methods is disclosed in Japanese Patent Publication No. 1 0350/1974. This method comprises cooling the foamed article of a thermoplastic resin having closed cells to a temperature below the second-order transition temperature (brittle temperature) of the thermoplastic resin and roll pressing the cooled foamed article thereby producing a cellular article having open cells. This method accomplishes the transformation of closed cells to open cells by sacrificing the strength of the thermoplastic resin itself to some extent.

Another of the methods is disclosed in Japanese Patent Application laid open to public inspection No.

63172/1979. This method comprises producing a foamed article of polyethylene containing an inorganic filler and subjecting the formed article to compressive force thereby rupturing the membranes of closed cells and transforming the cells into open cells. This method attains the transformation of closed cells into open cells by adding to the resin a large amount of the inorganic filler, enough to lower the strength of the resin.

The former method, however, has a disadvantage that a very long time is required to cool the foamed article having extremely low thermal conductivity to a temperature below the brittle temperature (-1 000C) of the resin and the method, when desired to be carried through in a short period of time, is applicable only to foamed sheets of very small thickness.

The latter method also has a disadvantage that the method itself is hardly practicable and, if it is materialized by special technical efforts, the addition of the large amount of inorganic filler inevitably decreases the degree of expansion and increases the bulk density.

In any event, successful transformation of closed cells of a foamed cross-linked polyolefin to open cells on a commercial scale remains yet to be accomplished. This is because the resin used as the raw material of the foamed cross-linked polyolefin is so tough, by nature, that the membranes of closed cells in the foamed article will not be ruptured under application of compressive force and, even if the compressive force is great enough to rupture such membranes, the compressive force is transmitted only in the surface region of the foamed article. The compressive force transmitted to the deep portion of the foamed article is no longer great enough to rupture the membranes in that portion.

The invention provides a method for the production of open-cell foamed polyolefin, the method comprising blending a polyolefin with a chemical blowing agent and a cross-linking agent to obtain a foamable and cross-linkable composition, forming the foamable and cross-linkable composition into a desired shape whilst maintaining its gel percent at zero, heating the shaped composition at a suitable foaming temperature under atmospheric pressure in such conditions that the peak of the ratio of the degree of cross-iinking to the degree of decomposition of the blowing agent is not more than 20 to decompose the cross-linking agent and the blowing agent concurrently, thereby giving rise to a foamed product of cross-linked polyolefin having cells enclosed with very thin membranes capable of being easily ruptured by the action of mechanical force, and mechanically deforming the foamed product to rupture the cell membranes.

The method according to the invention utilizes the adjustment of the decomposition rate of the blowing agent relative to the rate of cross-linking reaction. When the foamable and cross-linkable composition is heated under atmospheric pressure, the cross-linking reaction and the decomposition of blowing agent take place, and curves can be obtained for the reaction rates of the cross-linking and the decomposition of the blowing agent. When the foamable and cross-linkable composition of which gel percent is maintained at zero is heated under atmospheric pressure and the ratio (y) of the degree of cross-linking to the degree of decomposition of the blowing agent against the heating time is plotted on logarithmic graph paper, there can be obtained the curve as shown in the accompanying drawing.Such a curve cannot be obtained when the foamable composition is cross-linked in advance as in the prior methods.

Degree of cross-linking Y= Degree of decomposition of blowing agent Degree of cross-linking: Gel percent of resin at a certain heating time Degree of decomposition of blowing agent: Ratio of the degree of expansion at the same heating time as above to the final degree of expansion of the foamed product obtained.

The term "gel percent" means the ratio of the weight of the sample after extraction to that before extraction, the extraction being carried out by refluxing with trichloroethylene as solvent for 24 hours in a soxhlet extractor using a glass filter of from 40 to 50 y. The gel percent is calculated by the following equation. The degree of cross-linking is proportional to the increase of gel percent.

W, - (1 - x) W, Gel percent = ----- x 100 A A CW T -o T -x)+0.7-x+ In the equation, WO = Weight of the sample before extraction, W1= Weight of the sample after extraction, T = Total parts by weight of the components, A = Parts by weight of the blowing agent, C = Parts by weight of the fillers, and X = Decomposition degree of the blowing agent.

Hence, A --(1 -- x)W, = Weight of the remaining blowing agent in the sample, T A 0.7--x =Weight of the residue of decomposed blowing agent in the sample, and T C -- W, = Weight of the fillers in the sample.

T In the accompanying drawing, the peak A of the curve indicates the ratio (y) of the degree of crosslinking to the degree of decomposition of the blowing agent at which the decompositon of blowing agent most lags behind the cross-linking of the resin compound. That is to say, at the heating time of this point A the distance between the cross-linking curve and the decomposition curve of the blowing agent is widest. The greater the peak value A of the ratio (y), the more the foaming is delayed with respect to the cross-linking. On the other hand, the smaller the peak value A of the ratio (y), the less the foaming is delayed with respect to the cross-linking, that is to say, the cross-linking reaction and the foaming phenomenon of the foamable and cross-linkable composition concurrently took place.

Surprisingly, it has now been discovered that there is a limit to the peak A in the ratio (y) for obtaining a foamed product having cell membranes capable of being easily ruptured by the action of mechanical force. The peak value A of the ratio (y) is influenced by the type of resins used and the amounts of the cross-linking agent and the blowing agent. However, in spite of these parameters, it has been found that if the peak ratio A is not more than 20, there can be obtained a foamed product having cell membranes suitable for manufacturing the open-ceil foamed article. The maximum value of 20 for the peak ratio A is critical, but it is preferable to control the reactions such that the peak ratio does not exceed 15.

Therefore, the expression "concurrent decomposition of cross-linking agent and blowing agent" as used herein means that the decomposition of cross-linking agent and blowing agent is effected in such conditions that the peak of the ratio (y) is not more than 20. It will be a good practice to subject various polyolefins to a preliminary foaming to determine the range of the amounts of cross-linking agent, blowing agent, foaming aid, if required, and their optimum foaming temperatures which satisfy said conditions. In the actual operations, one can select the amounts of each components within the range thus determined.

To describe the invention more specifically, a given polyolefin is mixed with a blowing agent, a cross-linking agent and, if required, a foaming aid, a filler and a pigment, and the resultant mixture is kneaded, suitably with a heated mixing roll. The composition obtained is placed in the mould having a desired cavity profile and, under pressure applied with a press, thermally shaped at a temperature of from 11 50C to 1 550C, preferably from 1 200C to 1 400C, and thereafter removed from the mould. In place of shaping at an elevated temperature and under pressure, the composition after kneading may be shaped by heating it in the mould to which the pressure is not applied or by directly passing it through an extruder or a calendering roll.However, since heating in the shaping step raises the foamable and cross-linkable composition to the thermally excited state and, as a result, contributes to the more smooth concurrent decomposition of the cross-linking agent and the blowing agent in the following foaming and cross-linking step, it is preferable to carry out the shaping of the composition under heating. For instance, when the shaping is carried out without heating and without applying pressure, the cells of the foamed product obtained in the following foaming and cross-linking step are coarse and non-uniform, which is somewhat undesirable. In this thermal shaping it is important that the foamable and cross-linkable composition should be shaped maintaining its gel percent at zero. The heating time and temperature should be selected accordingly.Therefore, the shaping temperature is required to be lower, preferably by more than 200 C, than the foaming temperature in the following foaming and crosslinking step. If the gel percent is not maintained at zero in the thermal shaping step, i.e. if cross-linking of the polyolefin occurs in the thermal shaping step, there will be obtained a final product having an open cell ratio of less than 50%. In addition, if the shaping is carried out at an elevated temperature and under pressure, the cell size of the obtained foamed product decreases with increased heating time.

Therefore, it is possible to vary the appearance and the tactile impression of the final foamed article by varying the heating time.

In the thermal shaping step, a very small amount of the blowing agent may be decomposed, and as a result the shaped composition may expand to about twice the original volume when removed from the mould. This very small degree of foaming prior to cross-linking is acceptable. It can be considered that the abovementioned difference in cell size is due to nuclei for cells being formed by this predecomposition of the blowing agent.

The shaped foamable and cross-linkable composition is then heated under atmospheric pressure thereby concurrently decomposing the blowing agent and cross-linking agent. In this step the shaped composition is preferably heated in an atmosphere of nitrogen or in a heating medium, for instance a metal bath containing Rose's metal, Wood's metal or the like, an oil bath, or a molten salt bath containing one or more salts such as sodium nitrate, potassium nitrate and potassium nitrite. The shaped composition is preferably placed in an openable mould or metal box which is not airtight and heated in the heating medium kept at a suitable foaming temperature. Otherwise, the openable mould or metal box which is not airtight may be provided with a heater on the surface of its metal plate or with a jacket through which a heating medium such as steam or heating oil is circulated.By the use of this openable mould, the foamable composition is indirectly heated by the heater or heating medium.

Besides', the shaped composition may be covered with a metal sheet or the like capable of moving up and down and heated in such a state. After the heating for a predetermined period, the composition is cooled to obtain a cooled and foamed product. The foaming temperature is selected within the range of from 1 450C to 21 00C, preferably from 1 600C to 1 900C, to suit the particular type of polyolefin actually used, and the heating time is suitably from 10 to 90 minutes, preferably from 1 5 to 40 minutes. Thus, there can be obtained a foamed article having closed cells, the membranes of which are easily ruptured by mechanical deformation. The degree of cross-linking is similar to that of the foamed product produced by the prior methods (up to about 95 gel percent).

The foaming and cross-linking step is, however, preferably carried out in two stages. In the twostage process, the conditions for foaming and cross-linking polyolefin are mild, and thus the decomposition of the cross-linking agent and blowing agent can be accomplished at a lower peak value A. Furthermore, heterogeneous heat conduction in the foamable and cross-linkable composition may be eliminated and the composition may be homogeneously heated. As a result, there will not arise phenomena such as surface cracking resulting from partial unevenness of foaming in the composition, the collapse and the escaping of gas. Furthermore, it is possible to increase the expansion ratio of the foamed article obtained up to about 70 times the original volume at will and the thickness up to about 1 50 mm.Therefore, this two-stage process is particularly suitable for producing thicker foamed articles or the foamed articles having higher expansion ratios more than 20 times the original volume.

More specifically, in the first stage the shaped composition is heated as above described, i.e. in an atmosphere of nitrogen or in a metal or molten salt bath, at a temperature of from 1 450C to 1 800C for a period of from 5 to 60 minutes, preferably from 10 to 45 minutes, and thereafter the intermediate product is removed from the heating medium. In the second stage the intermediate product is further heated as above described at a temperature of from 1 700C to 21 00C for a period of from 5 ot 50 minutes, preferably from 1 5 to 40 minutes, and subsequently cooled to give rise to a foamed article with low density. In the first stage, it is preferable to decompose from 5 to 70% of the blowing agent, and for the gel percent of the resin to reach from about 20 to about 80%.If the degree of decomposition of the blowing agent and the gel percent are very high, the advantages of the two-stage process will not be derived.

The foamed article obtained as above is mechanically deformed, suitably by being passed between two rolls rotated at an equal speed, with the result that the compression so applied will rupture the closed membranes of the foamed article and consequently convert the closed cell structure to the open cell structure.

The open-cell foamed polyolefin obtained by the method of the invention possesses outstanding properties favourably comparable with the properties of foamed polyurethane, and the open cell ratio thereof determined by the Remington Pasier Method (ASTM D 1 940--62T) is equal or nearly equal to 100%.

The polyolefins which are preferably used in this invention are low-density polyethylene, mediumdensity polyethylene, high-density polyethylene, poly-1,2-butadiene, ethylene propylene copolymer, ethylene-butene copolymer, ethylene-vinyl actate copolymer, copolymers of ethylene with up to 42% of methyl, ethyl, propyl or butyl acrylate or methacrylate, chlorinated products of the above homopolymers or copolymers having a chlorine content of up to 60% by weight, mixtures of two or more thereof, and mixtures of one or more thereof with isotactic or atactic polypropylene.

To suit the purpose of this invention, the cross-linking agent ought to decompose in polyolefin at a temperature at least higher than the flow point of polyolefin. Organic peroxides which decompose upon being heated to liberate free radicals capable of giving rise to intermolecular or intramolecular crosslinked bonds and, therefore, serve advantageously as radical generators meet this requirement.

Examples of such organic peroxides include dicumyl peroxide, 1,1 -di-t-butyl-peroxy-3,3,5-trimethyl cyclohexane, 2,5-dimethyl-2,5-di-t-butyl peroxyhexane, 2,5-dimethyl-2,5-di-t-butyl peroxyhexene, a,a- di-t-butyl peroxydiisopropyl benzene, tertiary-butyl peroxy ketone and t-butyl peroxy benzoate. The organic peroxide which best suits the particular type of polyolefin used should be selected.

The blowing agents suitable for use in the invention are chemical blowing agent having decomposition temperatures higher than the melting point of the polyolefin. Examples of such chemical blowing agents include azo type compounds such as azodicarbonamide and barium azodicarboxylate; nitroso type compounds such as dinitrosopentamethylene tetramine and trinitrosotrimethyl triamine; hydrazide type compounds such as p,p'-oxybis (benzene sulphonyl hydrazide); and sulphonyl semicarbazide type compounds such as p,p'-oxybis (benzene sulphonyl semicarbazide) and toluenesulphonyl semicarbazide.

Besides the particular type of polyolefin used and the foaming temperature selected, the amounts of the cross-linking agent and the blowing agent are the significant factor exerting influence on the ratio (y) of the degree of cross-linking to the degree of decomposition of the blowing agent. If the amount of cross-linking agent is too much or the amount of blowing agent is very little, the peak valve A of the ratio (y) will easily exceed 20 and, as a natural consequence, it is hardly possible to produce an opencell 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 20.

Besides the above factors, it is possible to control the peak of ratio (y) by adding a foaming aid.

Thus, in the present invention, a foaming aid may be used depending on the particular type of blowing agent to be used. Examples of such aids include compounds having urea as a principal component; metal ozides such as zinc oxide and lead oxide; compounds having salicyclic acid, stearic acid, etc. as a principal component, i.e. higher fatty acids, metal compounds of higher fatty acids, etc.

To improve the properties of the open-cell foamed polyolefin and to reduce cost of its preparation, there may, if desired, be blended with the polyolefin, the chemical blowing agent and the cross-linking agent such compounding additives or fillers as do not harm the cross-linkage of polyolefins. Examples are carbon black; silicon dioxide; metal oxides such as zinc oxide, titanium oxide, calcium oxide and magnesium oxide; carbonates such as magnesium carbonate and calcium carbonate; fibrous filler materials such as pulp; various dyes, pigments, and fluorescent materials; and commonly used rubber compounding ingredients.

The foamed articles can be obtained by the method of the invention without impairing the advantageous properties of polyolefins and with high open-cell ratios falling within the range of from 97 to 1 00%. The method of the invention has further advantages of easy operation, short working time and high productivity.

The open-cell foamed polyolefins obtained by the method of the invention can be suitably used for cushioning media, filters, heat insulating materials, coaters, etc. Particularly, when the foamed polyolefins are used in clothes, noise abating materials and heat insulating materials so far produced by using soft polyurethane foams, they exhibit outstanding resistance to weathering and chemicals and high flame retardance and, therefore, warrant safe use.

The invention is illustrated by the following Examples.

EXAMPLE 1 A composition consisting of ethylene-vinyl acetate copolymer (proprietary product of Mitsui Polychemical Co., Ltd., marketed under trade name of "Everflex P-1 403", VAC 14% by weight), 17 parts by weight per hundred parts by weight of resin (phr) of azodicarbonamide (proprietary producty of Eiwa Chemical Industry Co., Ltd., marketed under trade name of "Vinyhol AC#50S"), 0.83 phr of dicumyl peroxide and 0.5 phr of zinc oxide was kneaded in a mixing roll at 850C. The resultant blend was charged in a mould (150 x 150 x 7 mm) within a press kept at 1 260C and heated under increased pressure for 30 minutes to form a foamable and cross-linkable sheet. The gel percent of this sheet was zero.The sheet obtained was then heated for 40 minutes in a metal bath kept at 1 700C to obtain an intermediary foamed product in which 30.5% of the blowing agent was decomposed. Thereafter, the intermediary foamed product was further heated in a metal bath kept at 1 900C for 20 minutes to obtain a foamed product in which the remaining blowing agent was completely decomposed. The peak value A of the ratio (y) in the foaming and cross-linking step was 10.4. After the cooling, the foamed product was passed between two rolls separated by a space of 3 mm and rotated at an equal speed to rupture the cell membranes. The foamed article obtained had a thickness of 23.0 mm, a bulk density of 0.03 g/cm3 and an open cell ratio of 100%.

The open cell ratio was measured in a similar manner to Remington Pariser method (ASTM D 1940--62T) and determined by the following calculation formula.

(V5- VR) - (AV VR) Open cell ratio = x 100 Vs - RR Vs - #V = ----- x 100 V5VR (V5: Volume of sample, VR: Volume of resin matrix (= weight of sample/density of resin), and ,V: Increase in volume).

EXAMPLES 2 to 4 The procedure of Example 1 was repeated varying the amounts of zinc oxide and dicumyl peroxide as shown in Table 1. In each Example a foamed article having an open cell ratio of 100% was obtained.

The degree of decomposition of the blowing agent in the intermediary foamed product was respectively 51.7% in Example 2, 69.0% in Example 3 and 11% in Example 4, and the peak value A of the ratio (y) was 5.0 in Example 2, 1.27 in Example 3 and 4.0 in Example 4.

EXAMPLE 5 A foamed article was produced from a composition consisting of ethylene-vinyl acetate copolymer (proprietary product of Mitsubishi Petrochemical Co., Ltd. marketed under tradename of "Yukalon EVA--41 H", VAC 16% by weight), 17 phr of azodicarbonamide and 0.53 phr of dicumyl peroxide in the same manner and under the same conditions as in Example 1. The open cell ratio of the foamed article obtained was 100%.

EXAMPLE 6 A composition consisting of the same resin as used in Example 5, 17 phr of azodicarbonamide, 0.08 phr of zinc oxide and 0.73 phr of dicumyl peroxide was kneaded in the same manner as in Example 1. The resultant blend was charged in a mould (140 x 140 x 28 mm) within a press kept at 1 260C and heated under increased pressure for 30 minutes to form a foamable block. The foamable block obtained was then heated in a metal bath kept at 1 700C for 40 minutes to obtain an intermediary foamed product in which 27% of blowing agent was decomposed. Thereafter, the intermediary foamed product was placed in an openable mould (370 x 370 x 110 mm) which was not air-tight, and heated in a metal bath kept at 1 900C for 30 minutes to decompose the remaining blowing agent completely.After the cooling the foamed product was removed from the mould.

The foamed product was passed between two rolls separated by a space of 10 mm and rotated at an equal speed to rupture the cell membranes. A foamed article having a thickness of 100 mm, a bulk density of 0.03 g/cm3 and an open cell ratio of 100% was obtained.

EXAMPLE 7 A foamable sheet was obtained from a composition consisting of the same resin as used in Example 1, 17 phr of azodicarbonamide, 0.2 phr of zinc oxide and 0.63 phr of 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 1 900C for 1 5 minutes completely to decompose the blowing agent and crosslinking agent in a single step, resulting in a foamed product. After the cooling, the foamed product was converted to an open-cell foamed article having an open-cell ratio of 100% by passing between two rolls rotated at an equal speed in the same manner as in Example 1.

EXAMPLE 8 An open-cell foamed article was produced from a composition consisting of the same resin as used in Example 5, 35 phr of azodicarbonamide and 0.53 phr of dicumyl peroxide under the same conditions as in Example 1. The foamed article produced had an open-cell ratio of 100%, a thickness of 30 mm and a bulk density of 0.019 g/cm3.

EXAMPLE 9 A composition consisting of low-density polyethylene (proprietary product of Mitsubishi Petrochemical Co., Ltd. marketed under the tradename of "Yukalon LK-30", density: 0.918 g/cm3, MFR 40), 1 7 phr of azodicarbonamide and 0.2 phr of zinc oxide was intimately kneaded in a mixing roll at 1 000C. The resultant blend was placed in a mould (150 x 150 x 7 mm) within a press kept at 1 360C and heated under increased pressure for 30 minutes to form a foamable sheet. This sheet was cross-linked and expanded under the same conditions as in Example 1, and thereafter passed between two rolls rotated at an equal speed in the same manner as in Example 1. A foamed article having an open-cell ratio of 100% and a thickness of 23 mm was thus obtained.

EXAMPLE 10 An open-cell foamed article was produced from a composition consisting of low-density polyethylene (proprietary product of Mitsubishi Petrochemical Co., Ltd. marketed under the tradename of "Yukalon HE-30", density: 0.92 g/cm3, MFR 0.5), 17 phr of azodicarbonamide and 0.13 phr of dicumyl peroxide under the same conditions as in Example 9. The open-cell foamed article had an opencell ratio of 100% and a thickness of 23 mm.

EXAMPLES 11 to 15 The procedure of Example 1 was repeated using the same composition as in Example 1 and varying the heating time within the press to 1, 5, 10, 20 and 30 minutes. Each foamed article had an open-cell ratio of 100%, and the thickness of the shaped article and its appearance was unchanged regardless of the heating time. However, the cell size was reduced with the increase of heating time, as set out in Table 3.

Table 1 COMPOSITION AND SHAPING CONDITIONS Shaping Composition (phr) Conditions under Gel % Foaming Cross- pressure of Blowing aid linking shaped Example agent (zinc agent Temp. Time sheet No. Resin - (ADCA) oxide) (DCP) (OC) (min.) (%) 1 EVA '?-1403" 17 0.08 0.83 126 30 0 2 P-1403 17 0.20 0.83 126 30 0 3 P-1403 17 2.50 0.83 126 30 0 4 P-1403 17 0 0.33 -126 30 0 5 EVA "EVA-41H" 17 0 0.53 126 30 0 6 EVA-41H 17 0.08 0.73 126 30 0 7 EVA "P-1403" 17 0.2 0.63 126 30 0 8 EVA "EVA-41H" 35 0 0.53 126 30 0 9 LDPE "LK-30" 35 0.2 0.53 136 30 0 10 LDPE "HE-30" 35 0 0.13 136 30 0 11 EVA "P-1403" 35 0.08 0.83 126 1 0 12 P-1403 35 0.08 0.83 126 5 0 13 P-1403 35 0.08 0.83 126 10 0 14 P-1403 35 0.08 0.83 126 20 0 15 P-1403 35 0.08 0.83 126 30 0 Table 2 FOAMING CONDITIONS AND PROPERTIES OF FINAL PRODUCT Conditions Conditions of two step foaming Bulk of one step Thickness density foaming first step second step of final of final open cell open-cell Open-cell Example Temp. Time Temp. Time Temp.Time foam foam ratio No. ( C) (min.) ( C) (min.) ( C) (min.) (mm) (g/cm3) (%) 1 - - 170 40 190 20 23 0.030 100 2 - - 170 40 190 20 23 0.030 100 3 - - 170 40 190 20 23 0.030 100 4 - - 170 40 190 20 23 0.030 100 5 - - 170 40 190 20 23 0.030 100 6 - - 170 40 190 30 100 0.030 100 7 190 15 - - - - 23 0.030 100 8 - 170 40 190 20 30 0.019 100 9 - - 170 40 190 20 23 0.030 100 10 - - 170 40 190 20 23 0.030 100 11 - - 170 40 190 20 23 0.030 100 12 - - 170 40 190 20 23 0.030 100 13 - - 170 40 190 20 23 0.030 100 14 - - 170 40 190 20 23 0.030 100 15 - - 170 40 190 20 23 0.030 100 Table 3 Heating time in Example shaping step Cell size No. (min.) (mm) 11 1 1.5 12 5 0.9 13 10 0.8 14 20 0.6 15 30 0.6

Claims (17)

1. A method for the production of open-cell foamed polyolefin, the method comprising blending a polyolefin with a chemical blowing agent and a cross-linking agent to obtain a foamable and crosslinkable composition, forming the foamable and cross-linkable composition into a desired shape whilst maintaining its gel percent at zero, heating the shaped composition at a suitable foaming temperature under atmospheric pressure in 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 concurrently, thereby giving rise to a foamed product of cross-linked polyolefin having cells enclosed with very thin membranes capable of being easily ruptured by the action of mechanical force, and mechanically deforming the foamed product to rupture the cell membranes.

2. A method according to claim 1 in which the foaming temperature is from 145 to 21 00C.

3. A method according to claim 1 in which the concurrent decomposition by heating of the crosslinking agent and blowing agent in the shaped composition is effected by a two stage process, a primary heating in which from 5 to 70% by'weight of the blowing agent originally present in the composition is decomposed and a secondary heating in which the undecomposed blowing agent and cross-linking agent remaining in the primary foamed product is decomposed at a higher temperature than that in the primary heating.

4. A method according to claim 3 in which the primary heating is effected at a temperature of from 1 45 to 1 800C and the secondary heating is effected at a temperature of from 170 to 21 OOC.

5. A method according to any preceding claim in which the decomposition of the cross-linking agent and blowing agent in the shaped composition is effected by heating the composition in a metal bath, oil bath or molten salt bath or in an atmosphere of nitrogen gas.

6. A method according to any preceding claim in which the shaped composition is placed in an openable mould which is not airtight and provided with a heater or a jacket through which a heating medium is circulated, and the decomposition of the cross-linking agent and blowing agent in the shaped composition is effected by the indirect heating with the heater or the heating medium.

7. A method according to any preceding claim in which the shaping of the foamable and crosslinkable composition is carried out at an elevated temperature.

8. A method according to claim 7 in which the shaping temperature is from 11 5 to 1 550C and is lower than the foaming temperature.

9. A method according to any preceding claim in which the shaping of the foamable and crosslinkable composition is carried out under increased pressure.

10. A method according to any preceding claim in which the shaping of the foamable and crosslinkable composition is effected by use of an extruder or a calendering roll.

11. A method according to any preceding claim in which the mechanical deformation is effected by means of compression exerted with two rolls rotated at an equal speed.

12. A method according to any preceding claim in which a foaming aid is blended with the polyolefin, the chemical blowing agent and the cross-linking agent to obtain the foamable and crosslinkable composition.

13. A method according to any preceding claim in which a compounding agent or filler is blended with the polyolefin, the chemical blowing agent and the cross-linking agent to obtain the foamable and cross-linkable composition.

14. A method according to claim 13 in which the compounding agent or filler is a metal oxide, carbonate, fibrous filler material, dye, pigment, fluorescent material or rubber compounding ingredient or a mixture of two or more thereof.

1 5. A method according to any preceding claim in which the polyolefin is high-density polyethylene, medium-density polyethylene, low-density polyethylene, poly-1,2-butadiene, ethylenepropylene copolymer, ethylene-butene copolymer, ethylene-vinyl acetate copolymer, a copolymer of ethylene with up to 45% of methyl, ethyl, propyl or butyl acrylate or methacryiate, a chlorinated product of any of the above homopolymers or copolymers having a chlorine content of up to 60% by weight, a mixture of two or more thereof or a mixture of one or more thereof with atactic or isotactic polypropylene.

1 6. A method according to any preceding claim in which the cross-linking agent is an organic peroxide having a decomposition temperature higher than the flow temperature of the polyolefin.

17. A method according to any preceding claim in which the blowing agent is an azo type compound, nitroso type compound, hydrazide type compound or sulphonyl semi-carbazide type compound possessing a decomposition temperature exceeding the melting temperature of the polyolefin.

1 8. A method for the production of an open-cell foamed polyolefin, the method being substantially as described herein with reference to any of the Examples.