WO2015140374A1

WO2015140374A1 - Process to produce a highly expanded open cell cross-linked polyolefin foam and foam

Info

Publication number: WO2015140374A1
Application number: PCT/ES2015/070177
Authority: WO
Inventors: Miguel Ángel RODRÍGUEZ PÉREZ; Cristina SAIZ ARROYO; Josías TIRADO MEDIAVILLA
Original assignee: Cellmat Technologies, S.L.
Priority date: 2014-03-18
Filing date: 2015-03-12
Publication date: 2015-09-24
Also published as: ES2546218B1; ES2546218A1

Abstract

The invention describes a novel process for producing a highly expanded open cell cross-linked polyolefin foam comprising the following steps: (i) preparing one or more compositions forming highly expanded open cell cross-liked polyolefin foams, such as polyethylene, (ii) molding one or more solid pre-forms from the composition or compositions; (iii) obtaining a precursor of the final foam to be obtained, comprising a core (5) prepared from one or more solid pre-forms obtained in the previous step covered at least in part by a cross-linked polyolefin skin (6) with a specific melting temperature, viscosity and gas permeability; (iv) subject the precursor to pressure and temperature in a first mold (7), followed by the opening of the same and a first expansion; (v) subject to temperature the pre-expanded piece (9) resulting from the previous step in a second mold (10), followed by the opening of the same and a second expansion, and demolding the expanded open cell cross-linked polyolefin foam (11). The invention also describes the polyolefin foam obtainable by the process of the present invention and its use as acoustic absorber, as thermal insulator, or as oil absorber, as comfort element in seats and mattresses, as gasket or as component to dissipate mechanical vibrations and impacts.

Description

POLYOLEFINE FOAM MANUFACTURING PROCEDURE

OPENED CELL RETICULATES AND FOAMS OBTAINED

5 Field of the invention

The present invention relates to a new process for producing highly expanded, open cell crosslinked polyolefin foam with good temperature resistance by the compression molding method. The invention also relates to the foam obtained by means of this method and its uses in general.

Background of the invention

The current manufacturing process of highly expanded crosslinked polyolefins foams by compression molding was anticipated by the

15 companies Sanwa Chemical Co. and Furukawa Electric Co. (Japan) that marketed their products in the late 1960s. This method was disclosed in Japanese Patent No. 29381/1970, which consists primarily of heating under pressure, in a first phase, a crosslinkable and expandable polyolefin composition that then so

An additional 20 in a second phase is heated at atmospheric pressure to finish decomposing the foaming agent.

The polyolefin foam that is obtained by this method and similar ones has a closed cell cell structure. It is very difficult to obtain an open cell structure in this type of foam, the opposite of what happens with

25 polyurethane foams. For this reason the vast majority of commercial open-cell foams are made of polyurethane and most of the cross-linked polyolefin foams are closed-cell.

The conventional method of achieving an open cell structure in crosslinked polyolefin foams is by breaking the walls.

30 cell phones applying a compression and / or shear force to the foam, which was previously primarily closed cell. This methodology has different variants to achieve more or less breakable cells, but all of them have the disadvantage of adding stages to the manufacturing process and giving rise to foams with little temperature resistance. There are also direct methods that seek to break cell walls during foaming with the disadvantage that foams with high degrees of expansion and / or high open cell contents are not achieved.

5 Various patents seek to achieve an open cell structure in polyolefin foams by different procedures and for different purposes:

In the Japanese patent. No. 47-31695 the closed cell foam is cooled to a temperature below or near that of the glass transition and then compressed to break its cell walls.

Japanese Patent No. 54-63172 is characterized by adding an inorganic filler in large proportions to the formulation to produce a closed cell foam that can be subsequently broken by deformation.

Japanese patent No. 55-42100 describes a process of preparing a

15 open-cell polyethylene foam by pressure and heating of a mixture of polyethylene, foaming agent, crosslinking agent and a large amount of amorphous polypropylene.

In Japanese patent No. 59-23545 the cells are broken by compression of a closed cell foam at a temperature between 0 ° C and 40 ° C without being achieved

20 100% open cell.

US 4,424,181 and US 4,499,210 disclose a formulation suitable for directly producing open cell crosslinked polyolefins foams by reversing the decomposition temperature of the crosslinking agent and the foaming agent. That is presumably achieved by the

Presence in the formulation of silicone oil or its derivatives and trifunctional radicals that increase the speed of crosslinking.

US Patent 4,501, 71 1 supposes an improvement of the foregoing where the presence of the trifunctional reactive monomer is not necessary and the silicone oil is replaced by a silicone block copolymer.

US Patent 4,435,346 is characterized by the expansion of the material containing the foaming agent and the crosslinker at atmospheric pressure under conditions where the peak of the ratio between the degree of crosslinking and the percentage of decomposition of the foaming agent is less than twenty. This prepares the foam for the subsequent rupture of its cell walls when subjected to mechanical stress.

In US Patent 4,877,814 the possibility of adding a high melting powder resin to the formulation is considered. This results in a closed cell foam that is then fractured by compression.

European patent EP 0 452 527 A1 partially resolves the low temperature resistance by irradiation of the open cell foam already made with ionizing radiation.

European Patent EP 0 043 052 discloses a foamable formulation based on an ethylene ionomer resin that provides an open cell foam.

International patent application WO 98/21252 describes a method that includes a cell rupture stage after the expansion of a silane crosslinked polyolefin foam and additionally also with organic peroxides, increasing the percentage of open cell by more than 50% up to more than 80%. The method may include a puncture stage to further increase the percentage of open cell.

None of the above procedures allows to obtain crosslinked polyolefin foams that are 100% open cell, in which there is a high reduction in density, in which the breakage of the cells is not necessary after the end of the foaming process and that provide materials with temperature resistance equivalent to those of closed cell foams made from the same polyolefins.

25 In view of the foregoing, there is therefore a need in the prior art to provide a new method of obtaining highly expanded open cell crosslinked polyolefin foam, which is simpler and more economical as well as reproducible, reliable and Scalable to industrial level.

30 Description of the Figures

Figure 1: Schematically represents the process of the present invention. The figure shows the following stages of the process: step (i) of preparing a forming composition (A); step (ii) of obtaining a preform (B); preexpansion stage (iv) (C); and stage (v) of second expansion (D), where the following numerical references refer to the following elements:

(1) components of the foam-forming composition that are introduced into an internal mixer (2) (it can also be an extruder); (3) homogeneous and molten mixture of the components; (4) calender; (5) laminated solid preform; (6) skin or preform coating; (7) press mold where the first expansion takes place; (8) closing and opening the press mold; (9) crosslinked and pre-expanded block; (10) press mold where the second expansion takes place; and (1 1) final foam of the invention.

Description of the invention

In a first aspect the invention relates to a process for the manufacture of a highly expanded open cell crosslinked polyolefin foam. The process is based on the fact that the inventors have surprisingly discovered that it is possible to obtain a highly expanded open cell crosslinked foam by working with a very low degree of crosslinking to produce the intercommunication of the foam cells and using a quantity of foaming agent greater than the usual one to compensate for the loss of gas that is produced by obtaining an open cell and to help cell opening, while using a skin with particular characteristics, defined below, which is used to coat the preform that it results in the open cell foam as detailed below. This skin makes it difficult for the gas generated in the first and second stage of expansion to escape, for the pre-expanded foam resulting from the first expansion to adhere to the mold, for the foam to collapse during the second expansion and for the foam to adhere to the mold during the second expansion. In the context of the present invention the term crosslinked refers to the presence of carbon-carbon covalent bonds between chains of linear or branched polymers of polyolefin type. This cross-linking is typically achieved by the use of organic peroxides or by irradiation of polyolefin with electrons or gamma radiation.

The term "highly expanded" in relation to a polyolefin foam is a conventional term used in the field of art. In this invention 5 highly expanded foam is understood as a foam with a degree of expansion (increase in volume versus starting solid) of between 6 and 55 times, which typically equals material densities between 15 and 150 kg / m3. io The term "open cell" in relation to polyolefin foam refers to the foam having an open cell content, equal to or greater than 90%, preferably equal to or greater than 95%, more preferably equal to or greater than 97 %, and even more preferably equal to or greater than 99%. In an even more preferred embodiment it is 100%. Open cell content characterizes

15 the degree of interconnection between the foam cells. In the present invention "open cell content" was determined by gas pycnometry following the procedure of ASTM D 2856-94 (1998). According to this regulation a material with 100% open cell has all the cells of the same connected to the outside of the material through holes in the

20 cell walls and / or absence of said walls. The foam of the present invention, as indicated below, is characterized by having an open cell content that reaches 100%.

The term "closed cell" refers in the present invention, in relation to a polyolefin foam, to an open cell content equal to or less than 20%, measured according to ASTM D 2856-94 (1998)

The process for the manufacture of a highly expanded open cell crosslinked polyolefin foam, hereinafter, the process of the invention, comprises the following steps:

(i) Prepare one or more highly expanded open cell crosslinked polyolefin foam forming compositions,

(ii) Mold one or more solid preforms from the one or more of the compositions prepared in step (i);

(iii) Obtain a foam precursor to be obtained which comprises a core prepared from one or more solid preforms obtained in the previous stage at least partially covered by a skin of a material

5 having a melting temperature equal to or lower than that used in the following stages (iv) and (v), an extensional viscosity greater than 10 ⁴ Pas measured at 150 ^Q C and a nitrogen permeability less than 4 sweeping measured at temperature ambient,

(iv) Subject the foam precursor to be obtained at pressure and temperature in i or a first mold, followed by opening it taking place a first expansion,

(v) Submit the pre-expanded shaped part resulting from the previous stage at temperature in a second mold, where a second expansion occurs,

15 (vi) Remove the expanded open-cell cross-linked polyolefin foam from this second mold.

The highly expanded open cell crosslinked polyolefin foam forming composition that is prepared in step (i) comprises at least 20 a polyolefin, at least one crosslinking agent, at least one foaming agent, and optionally one or more additives, and characterized by presenting:

(a) a crosslinking agent content X 'of between X / 10 and X / 1, 1 where X corresponds to the crosslinking agent content of the corresponding

25 equally expanded cross-linked cross-linked polyolefin foam forming composition, and

(b) a content of foaming agent Y 'of between 4Y and 1, 1 Y where Y corresponds to the content of foaming agent of the corresponding crosslinked polyolefin foam forming composition also

30 expanded closed cell.

X and Y refer to parts per 100 parts of polyolefin. The person skilled in the art knows how to obtain a foaming composition of Highly expanded closed cell crosslinked polyolefin, and determine the amount X of crosslinking agent and the amount Y of foaming agent, in the case of wishing to obtain a highly expanded crosslinked polyolefin foam of closed cell from a previously polyolefin

5 selected, depending on the chemical nature of the agents, the specific density desired in the final foam and the conditions of the expansion procedure. In other words, for each particular case, the expert can determine the contents of X and Y, so that cross-linking is achieved in a range that on the one hand allows the generation of stable cell walls during the expansion process, that is, walls Cell phones that offer a resistance that allows them to stretch without breaking in the expansion phase and a sufficient gas generation in the temperature range of the procedure to reach the desired relative density and degree of expansion.

fifteen

In a preferred embodiment the content of crosslinking agent X 'is between X / 7 and X / 1, 5, more preferably between X / 5 and X / 1, 75 and even more preferably between X / 4 and X / 1, 8 .

In another preferred embodiment, the content of foaming agent Y 'is between 3Y and 1, 2Y, more preferably between 2.5Y and 1.3Y, and even more preferably between 2.0Y and 1.4Y.

In step (i) of the process of the invention, one or more 25 open-cell cross-linked polyolefin foam forming compositions, hereinafter, forming composition depending on the type of foam, must be prepared. To this end, the person skilled in the art first determines the amounts of X and Y that should be used to obtain a highly expanded cross-linked cross-linked polyolefin foam, from a certain polyolefin, with a certain density and using certain conditions of the process of obtaining, and from said values of X and Y determines the necessary amounts (X 'and Y') that the corresponding polyolefin foam forming composition must present highly expanded open cell crosslinked from the same polyolefin, with the same density and using the same or similar conditions in the process of obtaining. The forming composition is prepared by contacting the components, and mixing them until a homogeneous and molten mixture is obtained. Mixing and melting can be done in conventional manner in any conventional device or apparatus such as an internal mixer or an extruder. The polyolefins that can be used in the present invention are for example low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), ethylene vinyl acetate copolymer (EVA) ), ethylene-methyl acrylate (EMA) copolymer, ethylene-butyl acrylate (EBA) copolymer, ethylene-propylene (EPM) copolymer, ethylene-propylene-diene (EPDM) copolymer, TPE (thermoplastic ethylene elastomers), etc. In a particular embodiment, two or more polyolefins are used to obtain a forming composition in any proportions. In another particular embodiment the polyolefin is LDPE. The crosslinking agent introduced to produce cross-linking of the polyolefin allows to produce a stable low density foam. As crosslinking agents, organic peroxides are typically used. Its selection in each particular embodiment is determined primarily by its decomposition temperature, which has to be significantly higher than the melting temperature of the polyolefin used. Examples of organic peroxides that can be used are, among others, 1,1-bis (terbutylperoxy) -3,5,5-trimethylcyclohexane; terbutylperoxy benzoate; 2,2-bis (terbutylperoxy) butane; dicumyl peroxide; diteramyl peroxide; diterbutyl peroxide; 1, 2- bis (terbutylperoxy-isopropyl) benzene; 2,5-dimethyl-2,5-bis (terbutyl-peroxy) hexane, and mixtures thereof. In a particular embodiment, dicumyl peroxide is used. Dicumyl peroxide (DCP) is typically used mixed with calcium carbonate or other inorganic materials to facilitate mixing and dispersion. Hereinafter and unless otherwise indicated, when the Description refers to DCP40 refers to a DCP mixed with calcium carbonate or other inorganic compound in a proportion 40% DCP / 60% calcium carbonate or inorganic compound.

5 The foaming agent is a chemical agent, which generates the expansion gas in stages (iv) and (v). Within the wide variety of products useful for practicing the invention, azodicarbonamide, azobisisobutyl nitrile, oxybis (benzenesulfonyl hydrazine), 5-phenyltetrazole, bicarbonate, citric acid or mixtures thereof can be used among others. In a particular embodiment, azodicarbonamide is used.

The forming composition may also contain conventional additives in usual amounts for this type of polyolefin foams. Examples of additives are the activators used to adjust the temperature of

15 decomposing the foaming agent and synchronizing the foaming reaction with that of crosslinking, such as transition metals (zinc, lead and cadmium), zinc oxide, polyols, urea, alcoholamines and some organic acids; process aids to improve processability and optionally others that confer special properties to the foam, such as

20 may be reinforcing charges, flame retardants, antistatic and electro-conductive agents, colorants, UV stabilizers, etc.

In general terms there are some practical restrictions on the use of these additives, so that they can only be incorporated in quantities

25, on the order of 40 parts per 100 parts of polymer in the forming composition preventing them from interfering with the elasticity of the cell wall necessary in the expansion or in the final properties of the foam. In a particular embodiment, the additive is aluminum or magnesium hydroxide which is used, for example, in 40% contents in order to generate foams.

30 halogen-free flame retardants.

In a particular embodiment, a composition of LDPE, DCP40 (ie dicumyl peroxide with 40% by weight of active agent) is prepared and azodicarbonamide to obtain a highly expanded open cell crosslinked foam with an average density of 25 kg / m3 under certain process conditions. For this, it is determined that for the corresponding case of wanting to obtain from the same materials of

5, under the same conditions, a highly expanded but closed cell crosslinked foam between 1, 1 and 2.2 parts of DCP40 is used per 100 parts of LDPE (equivalent to between 0.44 and 0.88 phr of peroxide of dicumyl in the mixture) and between 17 and 20 phr of azocarbonamide. Thus, the person skilled in the art can determine in order to obtain the composition io forming of step (i) that the amount of DCP40 crosslinking agent necessary must be reduced with respect to the previously mentioned amounts and be between 0.1 1 and 2.0 phr , particularly between 0.16 and 1.47, more particularly between 0.22 and 1.26, and even more particularly between 0.28 and 1.22. so that respectively the content of dicumyl peroxide in the

The mixture corresponds to a value between 0.044 and 0.8 phr, preferably between 0.06 and 0.59, more preferably between 0.09 and 0.50 and even more preferably between 0.1 1 and 0.49.

In the same way, the expert can determine that in order to obtain the forming composition 20 of step (i) the amount of azocarbonamide foaming agent must be increased with respect to the previously mentioned amounts and be between 18.7 and 80 phr, particularly between 20.4 and 60 phr, more particularly in the range between 22.1 and 50 phr, and even more particularly in the range between 23.8 phr and 40 phr.

25

In the case of other polyolefins, or mixtures of polyolefins, other final densities, other foaming agents and crosslinking agents, and certain process conditions, the person skilled in the art can determine the necessary amounts of foaming and crosslinking agents 30 X and Y for a closed cell foam, and to derive the corresponding amounts (X 'and Y') without need of inventive effort to obtain the open cell foam In step (ii) one or more solid preforms are molded from the one or more of the molten compositions prepared in step (i). In one particular embodiment, from each molten composition, one or more solid preforms are obtained. In another particular embodiment two or more molten compositions 5 can be used together to obtain the same solid preform. There is no limit on possible combinations of compositions to obtain all possible solid preform compositions in the present invention. io Molding can be done by any conventional molding technique, such as calendering, extrusion, or pelleting and compacting.

In a particular embodiment, a preform consisting of a calendering sheet is obtained. The sheets can be obtained with thicknesses

15 variables In another particular embodiment, a preform is obtained by extrusion.

The preform obtained by extrusion according to the present invention may in principle have any dimensions, shape, etc., such as circular, spheroidal, tubular, square, polygonal, block etc. Typically, the preform is in the form of a three-dimensional block with a thickness therefore superior to the sheet

20 obtained by calendering. The preform obtained by extrusion or calendering can be used directly in the next step (iii) as a core to coat it with a skin and obtain a precursor.

In the particular case of a sheet obtained by calendering, it is possible to obtain a core for the precursor in the next stage from several (same or different) sheets or overlapping (same or different) sheet fragments, one on top of the other, forming an assembly, as illustrated in Figure 1. The number of overlapping sheets and their thickness will determine the thickness of the precursor obtained. In a particular embodiment the core 30 comprises a single sheet. The sheets obtained by calendering can have different thicknesses, in general, between 0.5 and 5 mm.

In another particular embodiment the core comprises 2 or more sheets, typically 4, 5, 6, 7, 8 for example. In a particular embodiment the Overlapping sheets gives rise to a block-shaped core. In a more particular embodiment, the block has lateral dimensions (XY: x and y axes) much greater than its thickness, (Z: z dimension) which in an even more particular case is between 5 and 60 mm.

The present invention contemplates the option of preparing two or more solid preform forming compositions from which solid preforms of different composition and properties are obtained. These preforms can be combined to form different cores that generate foams according to the present invention with differentiated parts. Thus, in a particular embodiment, two or more solid preforms are obtained in the form of sheets. These sheets or parts thereof can in principle be superimposed in any way by generating a multitude of different cores from which foams according to the invention can be manufactured in a very versatile way with compositions, properties, non-uniform structures throughout

15 of thickness Z and lateral dimensions XY.

Illustratively, sheets can be prepared from LDPE and EVA sheets, and a core with both types of sheets can be prepared, resulting in a foam according to the invention having different properties, depending on how the sheets have been placed, by example with different properties

20 on each surface of the resulting block. It is also possible, for example, to prepare sheets composed of portions of sheets of different composition, and achieve foams according to the invention with different compositions, properties, and structures in different areas thereof.

Starting from the core, step (iii) comprises preparing a precursor of the open cell foam to be obtained comprising said core coated at least in part by a skin comprising a material having a melting temperature lower than that used in the following stages (iv) and (v), an extensional viscosity greater than 10 ⁴ Pas that typically corresponds

30 with the viscosity of a low density polyethylene crosslinked with dicumyl peroxides (using a DCP40 ratio of 1.9 phr) and said viscosity measured at 150 ^Q C, and a nitrogen permeability of less than 4 sweeps corresponding to the nitrogen permeability of a low polyethylene Density measured at room temperature. In a preferred embodiment the extensional viscosity of the skin is greater than 5 10 ⁴ Pas and more preferably greater than 10 ⁵ Pas. In another preferred embodiment the nitrogen permeability is less than 3 sweep and more preferably less than 2 sweep. With respect to permeability, it is desired to point out that the permeability (P) or permeability coefficient (P) of a given gas through a polymeric membrane is defined by the equation P = qt / AAp where q is the mass flow of gas through the membrane of area A and thickness t under a partial pressure gradient across the membrane Δρ. The gas permeability coefficient of polymeric membranes can be expressed in several units, although the most commonly used is the Sweeper (H. Alter, J. Polymer Science 57, 926, 1962) whose definition is

and where STP means standard temperature and pressure. In units of the International System: 1 Sweep = 3,348 x 10 ^"19 kmol m / (m ² s Pa)

In the first place as regards the properties of the skin, these make the skin in the expansions of the process of the invention can melt, and also can be stretched without surfing significant breaks. The material that forms the skin also has a viscosity such that neither it nor the constituent material of the molten core can escape from the molds used for expansion, and prevents or reduces the escape of gas generated during stages (iv) and (v ). In addition, the skin allows for its properties that when the molds used in the process of the invention are metallic, the demolding of the pre-expanded shaped piece resulting from step (iv) and the expanded open cell crosslinked polyolefin foam resulting from step (v) is easy without the material adhering significantly to the mold.

The term "skin" refers in the context of the present invention to a solid preform that is used to at least partially coat the precusror of the highly expanded open cell polyolefin foam. This, generally in sheet form, can have different thicknesses depending on, for example, the method of production but in general the typical thickness

5 is usually between 0.1 and 2 mm. The skin can be easily obtained in a conventional manner by the person skilled in the art. In this sense, the person skilled in the art can select, for example, one or more polyolefins from those mentioned above and prepare a homogeneous and molten mixture including, one or more crosslinking agents, optionally one or more foaming agents, and optionally one or more additives in suitable proportions of the conventional compounds defined above for the forming composition. From it, the skin can be obtained by molding by any conventional technique such as calendering, extrusion, etc. also described above for solid preforms. The skin can be a

Extruded sheet as defined above for the solid preform or being constituted by one or more sheets as also described above for the solid preform.

The skin can therefore be obtained by molding using chemical crosslinking, with the aforementioned crosslinking agents in conventional proportions 20 to generate closed-cell polyolefin foams with the necessary viscosity properties, etc. In a particular embodiment of the process of the invention. The skin can be obtained by co-extrusion together with the solid preform of step (ii).

Alternatively, the skin can also be prepared by physical crosslinking (irradiation with electrons or gamma radiation) from a polyolefin mixture without crosslinking agent.

In a particular embodiment of the precursor, the skin and the core to be coated may comprise the same polyolefin. In a more particular embodiment, the skin and the core comprise LDPE, and the mixture from which the skin is generated comprises a crosslinking agent and optionally foaming agent content that generates a closed cell polyolefin crosslinked foam. In an even more particular embodiment the skin is obtained from a composition containing LDPE, and a DCP content sufficiently high to generate a high cross-linking such as between 1, 1 and 2.2 phr of DCP at 40% active substance per 100 parts of LDPE resin (phr). The incorporation of foaming agent into the composition is optional.

In another particular embodiment, the skin may be formed by a polyolefin different from that used to generate the core. In the case, for example, of preparing highly expanded open cell LDPE foams, the skin may comprise a polyolefin selected from EVA, EBA, PP, HDPE, LLDPE, EPDM, TPEs, and mixtures thereof.

The coating of the core with the skin can be carried out by different methods. The skin can cover all or part of the generated core surface.

Thus, in a particular embodiment, the skin is positioned so that it covers the entire surface of the precursor. In another particular embodiment it is positioned so that it covers only a part of the surface thereof. For example in the case of a block, the skin can be placed on the upper and lower faces in the XY planes thereof. In a preferred embodiment the core is a three-dimensional block of XYZ dimensions. The skin in a particular embodiment covers the entire surface of the core (including the upper and lower faces located in the XY period, as well as the lateral faces located in the XZ and YZ planes) In another particular embodiment the skin covers only the two upper and lower faces of the core without covering the side faces located in the XZ and YZ planes. In yet another particular embodiment, the skin only covers one of the faces (the upper or lower) of the core without covering the lateral faces located in the XZ and YZ planes.

In order to cover the core with the skin, it is not necessary that the skin, nor the core, be preheated, since it is not necessary that there be a union between the skin and the solid of the core, which occurs in the later stages by application of temperature.

In a particular embodiment mentioned above, the precursor can be prepared directly by co-extrusion where in the outer layers of the material manufactured by co-extrusion the skins would be placed and in the center the core would be located.

Stage (iv): Subject the precursor to pressure and temperature in a first mold, followed by opening it and a first expansion.

This stage and the next stage are described in reference to Figure 1, for better understanding.

The precursor prepared in the previous stage which is formed by a core and a skin (5 and 6) is introduced into the mold (7) of a press where a first expansion is to be carried out. In a particular embodiment it is a hot plate press that transmits heat uniformly to the mold where the precursor material is housed. Once the mold (8) is closed and after a cycle that combines pressure and temperature together, the polymer melts and the decomposition of the crosslinking agent and foaming agent occurs.

The decomposition of the crosslinking agent causes the polymer to crosslink or crosslink, increasing the viscosity of the mixture. The viscosity is therefore controlled by the concentration of the crosslinking agent, looking for a viscosity high enough for the foam to be stable and low enough for the cell walls to fracture during the expansion process.

The decomposition of the foaming agent causes a sufficient amount of gas to be generated that will allow the desired expansion to be achieved. Due to the effect of the applied pressure, as long as it is higher than the pressure of the gas generated by the foaming agent when it decomposes, the gas is dissolved inside the polymer without producing any type of foaming while the pressure continues exercising. The pressure can be kept constant throughout the stage or it can be varied as long as it is higher than the gas pressure generated by the decomposition of the foaming agent.

5 After a period of time the press (8) opens, producing a rapid expansion of the material. In a particular embodiment, the previous application of mold release solutions on the surface of the molds facilitates extraction. The result of this stage is to obtain a pre-expanded and cross-linked shaped piece, (9).

i o It should be noted that at this stage a very precise control of the process variables is required to obtain the necessary curing of the polyolefin material and an adequate expansion ratio. As a general rule, the time under which the mold remains under conditions of pressure and temperature should be the minimum necessary for the decomposition of the

15 all of the crosslinking agent and a part of the foaming agent to achieve an expansion ratio of less than 10.

The temperature can be applied by heating the upper and lower plates of the press, by means of external heating side jackets, by infrared heaters, by electrical resistors inserted in the mold body, by oil circuits thermostating into the mold body or by any other process.

The temperature must be such that the polymer matrix melts and the decomposition of the crosslinking agent and part of the foaming agent. The heating process can be carried out by means of different steps of temperature if, for example, first it is desired to produce the fusion of the polymer and subsequently the decomposition of the crosslinking agent and the foaming agent.

The recommended pressure so that expansion does not occur when the mold remains closed is 15 MPa (150 bar) or higher.

After the processing cycle has elapsed, the pressure is released and the generated gas is no longer dissolved causing the precursor to expand. A pre-expanded shaped piece is thus obtained. Stage (v): Submit the pre-expanded shaped part resulting from the previous stage at temperature in a second mold, to generate a second expansion followed by opening it. At this stage the expansion of the material at atmospheric pressure is completed. For this, the pre-expanded shaped part extracted from the previous stage is introduced into the cavity of a hot mold (10), whose dimensions are those that must finally adopt the highly expanded crosslinked polyolefin foam, of open cell to be obtained according to the invention.

The temperature can be applied in the same way as to the mold of the previous stage and will be higher than the decomposition of the foaming agent. When the foam has completed the mold, it begins a cooling cycle that ends when the material reaches a temperature that allows its demoulding without major deformations. Typically these temperatures are below 50 ^QC .

The cooling can take place in different ways, by means of air circulation, submerging the mold in a water bath or other liquid for rapid cooling, introducing circuits through which the entry or exit of water or oil can occur in the body of the mold, etc.

After the cooling cycle, the mold is opened and the block (1 1) is removed. As in the previous stage, the previous application of mold release solutions on the surface of the molds facilitates the extraction of the block.

The process of the invention optionally comprises an additional step for removing from the expanded crosslinked open cell polyolefin foam obtained in step (vi), the remains of material corresponding to the skin used in the process. The removal of the material can be done, for example, by machining or cutting using the same means used today for cutting, and slicing the closed cell polyolefin foams. The result of the process of the invention is a highly expanded crosslinked polyolefin foam (the ratio can reach up to 45 or what is the same, a density equal to or greater than 20 kg / m ³ , with a cellular structure of up to 100% open with high tortuosity and low permeability to 5 nitrogen, a good resistance to temperature (up to 100 ° C) and all this in a simple and direct way without requiring additional stages, such as cell rupture after obtaining the foam, which It constitutes a fundamental advantage of the process of the invention io In a further aspect the invention relates to a highly expanded open cell crosslinked polyolefin foam obtainable by the process of the invention, hereinafter foam of the invention.

This foam has been characterized by various methods making it clear that it generally presents good thermal resistance. In a particular embodiment the polyolefin is low density polyethylene, and has a thermal resistance of at least 90 ^Q C, preferably at least 100 ^Q C.

The foam of the invention has an open cellular structure, with at least an open cell content equal to or greater than 90% and preferably equal to or greater than 95%, more preferably equal to or greater than 97%, and even more preferably equal to or greater than 99%, and still more preferably 100%

The average size of the foam cells of the invention can be controlled through various process parameters and their values are generally in the range between 50 microns and 3,000 microns.

The foam of the invention is characterized by its very high resistance to air flow. So while for example the airflow resistance of open-cell polyurethane foams is of the order of 10,000 rayls / m, that of the foams of this invention is equal to or greater than 50,000 rayls / m and preferably equal to or greater than 150000 rayls / m. The foam also has very high tortuosity. In this sense, while the typical tortuosity of open-cell polyurethane foams is in the order of 2, that of the foams of the invention is equal to or greater than 5 5, preferably equal to or greater than 10. Tortuosity of an open cell cellular material is defined in the present invention as the distance that a gas molecule must travel to traverse the thickness of the material traveling through the pores of the structure divided by the geometric thickness of the material.

i o Finally, the foam also has high sound absorption; Thus, the normalized sound absorption coefficient is equal to or greater than 0.3 and preferably equal to or greater than 0.5

In addition, the foam has a mechanical behavior in compression that is very dependent on the speed of deformation. So at low speeds of

15 deformation of 10 ^"2 s ^{" 1} the material behaves like a flexible polyurethane foam in the post-collapse zone of the strain stress curve. That is, after the elastic zone and for deformations between 5 and 60% it is given at a practically constant effort value. However, when deformation rates are high (typically equal to or greater than 10 ^" V ¹ ) the

The material behaves like a closed cell crosslinked polyolefin foam in which the stress in the post-collapse zone (deformation between 5 and 60%) increases with the deformation produced. Therefore the material behaves as a flexible polyurethane foam at low deformation rates and as a closed-cell polyoelfin foam at high

25 deformation speeds. This fact is related to the high tortuosity of the structure that makes the gas at high deformation rates not have time to leave the structure and therefore contributes to the effort when compressed inside the foam.

The foams of this invention have a coefficient of thermal expansion

30 similar to that of the polymer from which they were manufactured and which is generally significantly inferior to that of the closed-cell foam made from the same polymer. The average cell size of the foams was determined using the intersection method, ASTM D3576-04 (2010) in which said size is determined by counting the number of cells intercepted by a grid drawn on foam micrographs. The micrographs were obtained by scanning electron microscopy in a JSM 820 from Jeol.

Tortuosity was determined following the methodology explained in the following references: Laurikis, W. In Low Density Cellular Plastics: Physical Basis of Behavior; Hilyard, N. C; Cunningham, A., Eds .; Chapman & Hall: London, 1994; Chapter 10 or M.A. Rodriguez-Perez, M. Álvarez-Lainez, J.A. de Saja, Microstructure and physical properties of open-cell polyolefin foams, Journal of Applied Polymer Science, 1 14, 1 176-1 186, 2009.

Resistance to air flow is an important magnitude for open cell cellular materials; It measures the resistance offered by the cellular structure of the material to the passage of an air flow. In this document this magnitude was determined using the procedure described in ISO 9053: 1991.

The acoustic absorption of a specific material refers to the amount of energy that the material is able to dissipate when an acoustic wave hits it. The acoustic absorption coefficient was determined following the methodology explained in ISO 10534-2. The result of this experiment are acoustic absorption coefficient curves as a function of frequency. In order to be able to characterize the material by a single numerical value of the acoustic absorption coefficient, the parameter normalized absorption coefficient was defined as the arithmetic average of the values of the absorption coefficient in the measured range. This parameter was used for example in the publication M.A. Rodriguez-Perez, M. Álvarez-Lainez, J.A. de Saja, Microstructure and physical properties of open-cell polyolefin foams, Journal of Applied Polymer Science, 1 14, 1 176-1 186, 2009.

The mechanical behavior of the compression materials and the thermal expansion coefficient were determined as described in the MA document Rodriguez-Perez, M. Álvarez-Lainez, JA de Saja, Microstructure and physical properties of open-cell polyolefin foams, Journal of Applied Polymer Science, 1 14,1 176-1 186, 2009. The temperature resistance of materials It was determined using the following procedure. Samples of the material under study of dimensions 5x5x5 cm ³ are conditioned under controlled conditions of humidity and temperature for 24 hours (typically 21 ± 2 ^Q C and 50% ± 10% relative humidity) and their geometric dimensions are measured. They are then placed in an oven at a certain temperature T _res for a period of 24 hours. Once removed from the oven they are reconditioned in the same conditions in which the initial dimensions were measured and the dimensions are measured after passing through the oven. The change in dimensions in each direction of space (x, yyz) is determined as the percentage of difference between the initial and final dimensions using the initial one as a reference. The thermal resistance of the material is determined as that temperature T _res for which the dimensional change in some of the three directions of space is 5%. It is said that the material can be used below that temperature without the foam increasing its density significantly.

The particular structure of the foams of the present invention provides them with properties similar to flexible polyurethane foams, such as excellent resilience, very high acoustic absorption (better than that of low frequency polyurethane foams), excellent capacity as shock absorber and as thermal insulator, good impact behavior. In addition, the polyolefin character of the material makes it act as a specific oil absorbent (interesting for applications to separate oil from water) and provides very good chemical and weather resistance, as well as a lower environmental impact than that of flexible polyurethane foam.

In short, the open cell structure gives the material very different properties to those of closed cell foams, and all this however from a same material, that is, from a polyolefin. The latter has important relevance in the separation of materials in the recycling of components using materials that are formed by an open cell foam with another closed cell foam. Thus today these structures are constituted by an open cell polyurethane foam and a closed cell polyolefin foam; by using the open-cell polyolefin foam of this invention combined with a polyolefin closed-yield foam, a material of a unique chemical composition and therefore more easily reusable is obtained, which finds application in the recycling of components at the end of Its useful life.

In this sense, in a further aspect the invention relates to the various applications or the use of the foam of the present invention derived from its advantageous properties.

In a particular embodiment, the invention thus relates to the use of the foam of the invention as an acoustic absorbent, or as a thermal insulator, or as an oil absorber.

The foam of the invention is used in another particular embodiment as a material for the manufacture of seats, mattresses, etc., to provide comfort to them.

In another particular embodiment the foam of the present invention is used to obtain seals, since the material is normally hydrophobic and has a coefficient of thermal expansion equal to that of the starting polymer.

Furthermore, as the presence of the open cellular structure confers interesting characteristics in the dissipation of mechanical vibrations, in another particular embodiment the foam of the invention is used to manufacture elements that contribute to the mechanical dissipation of vibrations in automobiles, machinery, etc.

Finally, the material is also interesting for absorbing energy in impacts and therefore it is used for the preparation, for example, of packaging or sports protections. Examples Example 1: manufacturing process of an open cell crosslinked polyethylene foam.

A first composition was prepared by mixing the following components in a co-rotating twin screw extruder in the following proportions: low density polyethylene (Repsol PE003) with a flow rate of 2 g / 10 min measured at 190 ^Q C with 2, 16 kg 100 phr average particle size azodicarbonamide (D ₅₀ ) of 10 microns

30 phr 40% dicumyl peroxide in calcium carbonate (DCP40), 0.9 phr zinc oxide and 0.025 phr stearic acid 0.5 phr

A second formulation was also prepared with the same ingredients in the proportions of 100 parts, 19 phr, 1, 8 phr, 0.025 phr and 0.5 phr, respectively.

Subsequently the mixture of the first composition was pelleted, and 47 g of it was introduced into each of the four cavities present in a stainless steel mold and a cycle of 4 minutes was applied at 120 ° C and then others 4 minutes of cooling. The mold cavities had dimensions of 155 x 75 x 4 mm ³ . The process was repeated obtaining eight preforms with the dimensions of the mold cavities, of which 6 and a half were used as indicated below. At the same time, the mixture of the second formulation was calendered to obtain a sheet-shaped skin 1 mm thick.

A 290 g precursor was then made by placing six and a half preforms of the first composition (5) in a stack and a 1 mm sheet at the top and another at the bottom of the second formulation (6). Subsequently, the precursor was introduced into the mold of the phase 1 press (7) previously heated to 147 ° C. The mold was made of aluminum with dimensions of 155x75x25 mm ³ . The press (8) was closed with a force of 20 Tm and a 55 minute cycle was applied maintaining the closing force and the temperature.

After the end of the phase 1 cycle, the press (8) was opened releasing the pressure, which caused an expansion of the precursor. From the resulting pre-foam (9) the possible burrs were removed and without allowing time to cool it was introduced into the phase 2 mold (10). This mold had a lid with lockable closure, was constructed of aluminum and the interior dimensions were: 410x205x103 mm ³ . The mold was then closed and placed in an oven at 165 ° C for 100 minutes. After the phase 2 cycle, the mold was removed from the stove and allowed to cool in the air until it reached room temperature.

Finally, the phase 2 mold was opened and demoulded to obtain a foam block (1 1) of density 28 kg / m ³ , 100% open cell, an average cell size of 400 microns, a tortuousness of 18, a resistivity at the air flow of 149,000 rayls / m and a maximum operating temperature of 100 ° C.

The invention is not limited to the specific embodiments that have been described but also covers, for example, the variants that can be made by the average person skilled in the art within what follows from the claims.

Claims

one . - Procedure for obtaining a highly expanded open cell crosslinked polyolefin foam comprising the steps of:

5 (i) Prepare one or more highly expanded open cell crosslinked polyolefin foam forming compositions,

(ii) Mold one or more solid preforms from the one or compositions prepared in step (i);

(iii) Obtain a foam precursor to be obtained which comprises a core io prepared from one or more solid preforms obtained in the previous stage at least partly covered by a skin of a material having a melting temperature lower than the used in the following stages (iv) and (v), an extensional viscosity greater than 10 ⁴ Pas measured at 150 ^Q C and a nitrogen permeability less than 4 sweep 15 measured at room temperature,

(iv) Subject the precursor to pressure and temperature in a first mold, followed by opening it, which results in a first expansion,

(v) Submit the pre-expanded shaped part resulting from the previous stage 20 at temperature in a second mold, which results in a second expansion, followed by opening thereof and a second expansion, and

(vi) Remove the expanded open cell crosslinked polyolefin foam from this second mold.

25

2. A method according to claim 1 wherein the highly expanded open cell crosslinked polyolefin foam forming composition comprises at least one polyolefin, at least one crosslinking agent, at least one foaming agent, and optionally one or more additives, and is characterizes

30 because it presents:

(a) a crosslinking agent content X 'of between X / 10 and X / 1, 1 where X corresponds to the crosslinking agent content of the corresponding crosslinked polyolefin foam forming composition likewise expanded closed cell, and

(b) a content of foaming agent Y 'of between 4Y and 1, 1 Y where Y corresponds to the content of foaming agent of the corresponding closed-cell cross-linked polyolefin foam forming composition.

3. The method according to claim 2 wherein the content of crosslinking agent is between X / 7 and X / 1, 5, preferably between X / 5 and X / 1, 75 and more preferably between X / 4 and X / 1 , 8.

4. Method according to any one of claims 2 or 3, wherein the content of foaming agent is between 3Y and 1, 2Y, preferably between 2.5Y and 1, 3Y, and more preferably between 2.0Y and 1, 4Y.

5. The method according to any one of claims 1 to 4, wherein the polyolefin is low density polyethylene (LDPE), linear low density polyethylene (LLDPE), high density polyethylene (HDPE), polypropylene (PP), copolymer ethylene-vinyl acetate (EVA), ethylene-methyl acrylate (EMA) copolymer, ethylene-butyl acrylate (EBA) copolymer, ethylene-propylene (EPM) copolymer, ethylene-propylene-diene (EPDM) copolymer, TPE (thermoplastic ethylene elastomers) or mixtures thereof.

6. The method according to claim 5, wherein the polyolefin is LDPE.

7. A method according to any one of claims 1 to 6, wherein the crosslinking agent is selected from the group consisting of 1,1-bis (terbutylperoxy) -3,5,5-trimethylcyclohexane; terbutylperoxy benzoate; 2,2-bis (terbutylperoxy) butane; dicumyl peroxide; diteramyl peroxide; diterbutyl peroxide; 1,2-bis (terbutylperoxy-isopropyl) benzene; 2,5-dimethyl-2,5-bis (terbutyl-peroxy) hexane and mixtures thereof.

8. The method according to claim 7, wherein the crosslinking agent is dicumyl peroxide.

9. The method according to any one of claims 1 to 8, wherein the foaming agent is selected from the group consisting of azodicarbonamide, azobisisobutyl nitrile, oxybis (benzenesulfonyl hydrazine), 5-phenyltetrazole, bicarbonate, citric acid and mixtures thereof.

10. The process according to claim 9, wherein the foaming agent is azodicarbonamide.

eleven . Process according to any one of claims 1 to 10, wherein the forming composition comprises LDPE, dicumyl peroxide and azodicarbonamide.

12. The method according to any one of claims 1 to 1 1, wherein the forming composition comprises LDPE, between 0.1 1 and 2.0 phr, preferably between 0.16 and 1.47, more preferably between 0.22 and 1, 26, and even more preferably between 0.28 and 1.22 of dicumyl peroxide with 40% by weight of active agent and between 18.7 and 80 phr, preferably between 20.4 and 60 phr, more preferably in the range between 22.1 and 50 phr, and even more preferably in the range between 23.8 phr and 40 phr of azodicarbonamide, and the highly expanded open cell crosslinked foam obtained has an average density of 25 kg / m3. .

13. A method according to any one of the preceding claims wherein a solid preform is obtained by a molding technique selected from calendering, extrusion, pelleting and compaction.

14. A method according to the preceding claim wherein the solid preform comprises at least one sheet obtained by calendering, two or more sheets obtained by superimposed calendering or a piece obtained by extrusion.

15. A method according to the preceding claim wherein the solid preform is a three-dimensional block with a thickness between 16 and 60 mm.

16. A method according to any one of the preceding claims wherein the skin used in step (iii) is a sheet with a thickness between 0.1 and 2 mm.

5

17. Method according to any one of the preceding claims wherein the skin and the core comprise the same polyolefin.

18. Method according to any one of the preceding claims in which the skin and the core comprise a different polyolefin.

19. The method according to any one of the preceding claims wherein the skin is obtained from a composition containing LDPE, a content of DCP40 between 1.4 to 2.2 phr of DCP at 40% active substance by

15 per 100 parts of LDPE resin (phr), and optionally a foaming agent.

20. A method according to any one of the preceding claims wherein the skin covers a block-shaped precursor such that (a) the skin is disposed on all faces of the block, or (b) on the lower and upper face

20 or (c) only on one of the faces, the upper or the lower.

twenty-one . Process according to any one of claims 1 to 12, wherein the precursor and the skin are prepared by co-extrusion.

22. A highly expanded open cell cross-linked polyolefin foam obtainable by the process of the invention

23. Polyolefin foam according to claim 22, wherein the polyolefin is low density polyethylene, and has a thermal resistance of at least 30 90 ^Q C, preferably at least 100 ^Q C.

24. Foam according to claim 22 or 23, which has an open cellular structure, with at least an open cell content equal to or greater than 90% and preferably equal to or greater than 95%, more preferably equal to or greater than 97%, and even more preferably equal to or greater than 99% or equal to 100%.

25. Foam according to any one of claims 22 to 24, having an average cell size in the range of 50 microns to 3,000 microns.

26. Foam according to any one of claims 22 to 25, which has an air flow resistance equal to or greater than 50,000 rayls / m, preferably equal to or greater than 150000 rayls / m.

27. Foam according to any one of claims 22 to 26, which has a tortuosity equal to or greater than 5, preferably equal to or greater than 10.

28. Foam according to any one of claims 22 to 27, having a sound absorption coefficient normalizing equal to or greater than 0.3 and preferably equal to or greater than 0.5

29. Use according to any one of claims 22 to 28 as an acoustic absorber, as a thermal insulator, or as an oil absorbent, as an element in the manufacture of seats and mattresses, as a gasket, as an element to dissipate mechanical vibrations and impacts, and as sports packaging or protections.

30. Use of the foam according to any one of claims 22 to 27 combined with another closed-cell polyolefin foam for the recycling of components at the end of their useful life.