WO2013030318A1

WO2013030318A1 - Complexly shaped anisotropic foam manufacturing

Info

Publication number: WO2013030318A1
Application number: PCT/EP2012/066932
Authority: WO
Inventors: Kelly VANDEN BOSCHE; Ignace Verpoest; Jan IVENS; Miguel Angel RODRIGUEZ - PEREZ
Original assignee: Katholieke Universiteit Leuven, K.U.Leuven R&D; Universidad De Valladolid
Priority date: 2011-08-31
Filing date: 2012-08-31
Publication date: 2013-03-07
Also published as: GB201114958D0

Abstract

The present invention relates to a novel method of manufacturing of anisotropic cellular materials using a multi-step expansion process, matrix stabilization, and directed foam growth directions. This method is useful for customizing the cellular architecture of a foam such that specific desired macroscopic properties can be obtained, such as: density, the plateau strength in the growth direction, the ratio between stiffnesses in orthotropic directions (anisotropy), the shear stiffness, ratio between thermal conductivities in different directions, etc...

Description

Complexly shaped anisotropic foam manufacturing

Field of Invention

The present invention relates to a novel method of manufacturing of anisotropic cellular materials using a multi-step expansion process, matrix stabilization, and directed foam growth directions, as well as to materials thus obtained. This method is useful for customizing the cellular architecture of a foam such that specific desired macroscopic properties can be obtained, such as: density, the plateau strength in the growth direction, the ratio between stiffnesses in orthotropic directions (anisotropy), the shear stiffness, ratio between thermal conductivities in different directions, etc...

Background of the Invention

Idealized foam materials may be comprised of isotropic cells. However, in practice, most cellular materials do exhibit some a shape-anisotropy ratio, R, that is not equal to 1 . In other words, due to the manufacturing methods, slight anisotropy in the cellular geometry is common. In general terms, the cells are most of the time elongated in the direction of greatest expansion of the foam and /or directions in which the foamed material is stretched (for instance the cellular structure of foamed materials produced by extrusion is typically elongated in the extrusion direction). While the cells may even be temporarily extremely elongated, i.e. during very fast expansions in a single direction, the cells quickly return to a more equalized shape to reduce their total surface energy. In the absence of specific influences, therefore, shape-anisotropy ratios generally remains less than three (R<3), and with the vast majority of foams having an R value typically less than 1 .4.

If more pronounced anisotropy is needed to produce specific macroscopic properties (mechanical, thermal, etc .), then special influencing parameters must be designed into the production process. Some prior art describes foaming methods that produce cells with highly anisotropic cell geometries. The patents that describe these methods are:

(1 ) EP 0 41 1 437 B1 - describes a polyethersulphone processing method which result in cells with 5<R<12. For this method, the foam is processed in large blocks in a press under very high pressures (5-30bar). Thick blocks are produced using this method and it is not possible to produce directly (without using further cutting operations) complex parts. Cell size is oriented in one direction (the expansion direction of the foam. (2) GB 2 458 333 A - describes a 2-step foaming process that results in slightly anisotropic foam, whereby after a first expansion step the foamable polymer composition is expanding to foam an initial foam body having an isotropic cell structure. Here, negative pressures are applied to the foam during/after the second foaming step to avoid collapse of the foam due to gravity. Low anisotropy ratios between 1 and 1 .6 are achieved. Typically, only flat or semi-flat geometries can be made.

(3) DE 603 07 827 T2 and WO 2007/129886 A1 - describe the production of sandwich panels with an in-situ foamed thermoplastic core. Although it is stated that the core is considerably anisotropic, no description given about how this material is foamed, or its exact structure or properties.

(4) US 6 213 540 B1 - describes an anisotropic extruded thermoplastic material. The anisotropic material (example PP) exhibits almost twice the compressive strengths as the same material foamed to give an isotropic geometry. While shape anisotropy ratios are not defined in this patent, it can be estimated to roughly 1 .6-2 based on the compressive strength ratio between the anisotropic and isotropic materials (according to theoretical models - figure 6.22 Gibson and Ashby)

(5) US 6 342 171 B1 - describes the production of a carbon foam, which is made by carbonizing a polymeric foam at extremely high temperatures (900-1600 'Ό). In order for the cellular geometry to withstand these temperatures, the material is significantly stabilized using oxidative stabilization. This is to cause the material not to collapse. Encouraging any particular cellular geometry or orientation is achieved before stabilization.

Other prior art for instance also disclose methods using shape memory polymers, however these methods require a fast and high pressure drop (in 10 minutes) against collapsing of a foam. The latter method results in foams comprising a high anisotropy ratio, however with small cell dimensions in the μηι range. Unlike any of the prior art described above, it is desirable to develop and define a foam production method which (1 ) allows for a controlled design of the cellular geometry, (2) supports more significantly anisotropic structures, R > 3, (3) can produce foams in complex shapes, such as curved structures (foaming into final shape instead of shaping after foaming) and (4) can produce foams in a significant range of foam macroscopic densities. A need still exists for an improved method combining more or all of the advantages described above, e.g. an improved method for manufacturing complexly shaped anisotropic foams. Summary of the invention

It is an object of embodiments of the present invention to provide efficient means and methods for manufacturing anisotropic foams, as well as to provide anisotropic foams thus obtained. It is an advantage of embodiments of the present invention that efficient means and methods for manufacturing anisotropic foams in a significant range of foam macroscopic densities are provided, as well as foams thus obtained.

It is an advantages of at least some embodiments of the present invention to solve the shortcomings in the prior-art and to provide a foam production method which (1 ) allows for a controlled design of the cellular geometry, (2) supports more significantly anisotropic structures, R > 3, (3) can produce foams in complex shapes, such as curved structures (foaming into final shape instead of shaping after foaming) and (4) can produce foams in a significant range of foam macroscopic densities.

The above objective is accomplished by a method according to embodiments of the present invention.

In a first aspect, the present invention provides methods of producing a foam body having an anisotropic cell structure, the method comprising the steps of

- providing a stabilized prefoam (a), whereby providing a stabilized prefoam (a) may comprise or consist of performing a first blowing step applied on a compounded polymer to form a prefoam (a') and performing a stabilization step applied on said prefoam to form a stabilized prefoam (b');

- a blowing step applied on said stabilized prefoam to form a fully expanded foam material (b);

- a stabilization applied on said expanded foam to provide a stabilized foam body having anisotropic cell structure (c);

whereby a growth direction is imposed during at least the second blowing step.

In a preferred embodiment the stabilized prefoam is not fully hardened during the stabilization step (b') and thus the stabilized prefoam is not fully hardened when the secondary blowing step (b) is applied. The purpose of this stabilization step (b') is to suppress the nucleation of new cells during the secondary blowing step (b.). Therefore, the material should be well stabilized (lightly cross-linked material) but not too rigid (i.e. a fully cured thermoset material), and the temperatures and pressures should be applied such that the material is still pliable enough for expansion but not too soft/viscous that new bubbles can easily form. The exact parameters (i.e. cross-link densities, temperatures, pressures, etc.) will be dependent on the precise system (matrix, blowing agent, type of stabilization, etc ..) which is used, but can be easily determined by conducting experiments on nucleation and growth of cells at different processing parameters. When the situation is achieved that growth of existing cells proceeds preferentially to the nucleation of new cells (although it is not excluded that some new cells will be formed during the second blowing step), this requirement has been achieved. In other words, the stabilization step on the prefoam may be applied until the situation is achieved that growth of existing cells proceeds preferentially to the nucleation of new cells when the blowing step (b) is applied.

The stabilized prefoam may have an anisotropic cell structure. The stabilized prefoam may be transversely isotropic material.

In preferred embodiments the stabilization step (b') on the prefoam, to provide a stabilized prefoam, is applied until the situation is achieved that growth of existing cells proceeds preferentially to the nucleation of new cells. As an advantageous result, a foam body can be achieved with high anisotropy ratios as compared to the prior art, because a well stabilized prefoam is used as a starting point.

In preferred embodiments, cross-linking is used to stabilize a prefoam and to provide a stabilized prefoam, which advantageously results in a foam body with a high anisotropy ratio and preferably comprising cell dimensions in a mm range. In prior art, cross-linking is carried out to a degree sufficient to raise the melt strength of the polymer to withstand the pressure which causes melt fracture of the polymer to occur. The latter is typically related to instability of a polymer as it is extruded through a die and not to enhance anisotropic properties of a foam body.

The cell structure in the stabilized prefoam and foam body may be macroscopic on the scale of mm. The prefoam may comprise cell widths ranging from 0.1 mm to 2.5mm. The prefoam may comprise cell lengths ranging from 0.1 mm to 2.5mm. The foam body may comprise cell widths ranging from 0.1 mm to 2.5mm. The foam body may comprise cell lengths ranging from 0.2mm to 15mm. The stabilization of the prefoam (b') may prevent nucleation of the cells of the prefoam.

The blowing step (b) may be applied when the stabilized prefoam is in the situation that growth of existing cells proceeds preferentially to the nucleation of new cells.

By using cross-linking to stabilize a prefoam and foam body, the process according to embodiments of the invention advantageously is an entirely chemical process of blowing and stabilization.

In preferred embodiments, the stabilization step (b'), by for instance crosslinking, provides a prefoam that can be manipulated into complex shapes and which is dimensionally stable at temperatures necessary for subsequent blowing steps. As an advantageous result, the prefoamed preform, i.e. a semi-sphere, will retain its shape at high temperatures, whereas the unstabilized polymer could flow under the force of gravity. In embodiments of the invention, a compounded polymer comprises a polymer mixed with the preferred additives at a mixing temperature, whereby said mixing temperature is preferably higher than the melting temperature (T_m) or the glass transition temperature (T_g) for completely amorphous polymers. Preferably a mixing temperature is used which is lower than the decomposition temperature of a first blowing agent, when for instance using a chemical blowing agent, or lower than the activation temperature of a crosslinking agent for example if the first blowing is performed with a physical blowing agent.

In some embodiments the temperature of the blowing step on the compounded polymer for forming a prefoam (a'), when for instance a chemical or physical blowing agent is used according to embodiments of the invention, is preferably higher than T_m or T_g for completely amorphous polymers, so that the polymer is preferably viscous and able to deform easily. In alternative embodiments, the blowing temperature of the blowing step for forming a prefoam is preferably smaller than or equal to the activation temperature of the crosslinking agent used. This blowing temperature is preferably only equal to the temperature of crosslinking in the case that the two processes are separated in time kinetically (i.e. in the case of DCP cross-linking, whereby the blowing is fast and the crosslinking is slow, though they occur at the same temperature).

In other embodiments comprising cross-linking as the stabilization step (b') to provide a stabilized prefoam, whereby said cross-linking is preferably chemical crosslinking, is performed at a temperature higher than T_m or T_g for completely amorphous polymers such that the polymer is advantageously viscous, and the polymer molecules have mobility. In alternative embodiments crosslinking is performed at a temperature higher or equal to the activation temperature of the blowing agent used for blowing to form a prefoam. The cross- linking temperature is preferably equal to the temperature of first blowing step in the case that the two processes are separated in time kinetically (i.e. in the case of DCP cross-linking, whereby the blowing occurs fast and the crosslinking is slow, though they occur at the same temperature). In yet other embodiments crosslinking is performed at a temperature lower than the decomposition temperature of a second blowing agent, for example when using a chemical blowing agent according to embodiments of the invention. Embodiments where a physical blowing agent is used, the pressure is preferably kept high enough that the gas released from a chemical blowing agent is kept in the solution during this step, then the temperature can exceed the temperature of the second blowing step.

In alternative embodiments, wereby a stabilization step is carried out, the stabilization step (b'), preferably radiation crosslinking, is preferably carried out over a wide range of temperatures. The range of temperatures can reach as low as room temperature, but preferably an elevated temperature as compared to the room temperature is used for more polymer molecule mobility. The crosslinking temperature is preferably performed at a temperature lower than the decomposition temperature of a second blowing agent, for example when using a chemical blowing agent according to embodiments of the invention. In embodiments where radiation crosslinking is used, the latter can lead to unintended temperature increases, and this is preferably controlled such that the material stays within allowable limits, i.e. below the decomposition temperature of the polymer or below the blowing temperature of an incorporated chemical blowing agent.

In other embodiments of stabilization (b'), silane crosslinking can also be applied, this process preferably has following steps:

(i) incorporation of the silane into the polymer, either by grafting of vinylsilane onto the polymer backbone or by copolymerization of vinylsilane with ethylene in the polymerization reactor, and

(ii) : cross-linking in the presence of water, generally catalyzed by tin compounds or other suitable catalysts. This second step can be controlled and made during or after the extrusion process.

Embodiments of the invention provide that a blowing step (a', b) be applied, whereby said blowing is achieved using preferably a chemical or physical blowing agent, is performed at a temperature higher than T_m or T_g for completely amorphous polymers such that the polymer is viscous and able to deform easily. After crosslinking these values, e.g. T_m and/or T_g, may be different from the original polymer. Preferably the second blowing is performed at a temperature lower than the decomposition temperature of the polymer used or lower than the decomposition temperature of any potential additional blowing agent used in additional steps, e.g. if using a chemical blowing agent. In embodiments where a blowing step is applied, e.g. a blowing step for forming a prefoam or a blowing step applied to the stabilised prefoam, whereby said blowing step is achieved by preferably applying a chemical blowing agent, a positive pressure between 0-20 Mpa is preferably applied, and more preferably a pressure between 0-5 Mpa. Preferably the pressure is applied to advantageously delay the time of nucleation after decomposition of the agent by keeping the evolved gas in solution with pressure. The preferred pressure depends on the critical solution pressure for the gas/polymer combination according to embodiments of the invention. In preferred embodiments, the rate of nucleation and growth is controlled by applying a specific depressurization rate and thus affecting the cell geometry/size. Depressurization rates from 0.01 MPa/s to 40 MPa/s are preferably used and more specifically preferably between 0.1 -10 MPa/s. In preferred embodimenst natural back pressure is built, by for instance limiting expansion, this advantageously results in better moulding capabilities, e.g. completely filling a mould, etc. In embodiments where a blowing is applied, e.g. blowing for forming a prefoam or blowing on the stabilised prefoam, whereby said blowing is achieved by preferably applying a physical blowing agent, a pressure between preferably 5-40 Mpa and more preferably preferably between 10-30 Mpa is applied, preferably with controlled depressurization rates from 0.01 MPa/s to 80 MPa/s, and more specifically preferably between 0.1 -10 MPa/s.

In preferred embodiments, mixing of the polymers, in order to preferably obtain a compounded polymer, is achieved within a few minutes for example on a tuned extruder. Mixing efficiency can for instance be tested by measuring distribution of additives and preferably adapted according to the achieved results.

In embodiments comprising a first blowing step for forming a prefoam (a'), whereby said first blowing step preferably is achieved by using a chemical blowing agent, the time of blowing preferably depends on the degradation temperature of the blowing agent, e.g. until this temperature has been reached. This is a relatively fast process resulting in a blowing time in the order of minutes. The blowing time is preferably long enough to reach desired expansion, but not so long such that the foam collapses. The time therefore preferably depends on the viscous properties of the polymer material, and the kinetics of the decomposition of the blowing agent, which are both of course, dependant on the temperature.

In other embodiments comprising a first blowing step for forming a prefoam (a'), whereby said first blowing step is preferably achieved by using a physical blowing agent, the blowing time can depend on the type of process (e.g. batch, continuous extrusion, etc). The major limiting time, is the time for dissolution of blowing agent into the polymer. This depends on the pressure and temperature of the system (which affect solubility and diffusion rates), and size of the polymer to be foamed. In embodiments of the invention, times can range from a few seconds for polymers above the melting temperature to several hours for polymers in the solid state.

Embodiments of the invention comprising cross-linking as part of the stabilization step (b'), whereby said cross-linking is preferably chemical cross-linking, the cross-linking step is preferably long enough to achieve desired cross-link density and thus stabilization of the polymer matrix. The latter is preferably dependant on the type and concentration of cross- linking agent and kinetics of cross-linking, thus the temperature and mobility of polymer molecules.

Alternative embodiments of the invention comprising cross-linking as part of the stabilization step (b'), whereby said cross-linking step is preferably achieved by radiation, the cross- linking step is preferably long enough to achieve desired crosslink density and thus stabilization of the polymer matrix. The latter is preferably dependant on the kinetics of cross-linking, thus the temperature and mobility of polymer molecules as well as the intensity of radiation. Cross-linking can continue to proceed long after application of radiation based on the kinetics of cross-linking.

In embodiments of the invention comprising a blowing step (b) on the stabilised prefoam, whereby said blowing step is achieved by preferably using a chemical blowing agent, the blowing step (b) may be a relatively fast process, resulting in a blowing time in the order of minutes, linked to the degradation temperature of the blowing agent e.g. once this temperature has been reached. The blowing step on the stabilised prefoam is preferably long enough to reach desired expansion, but not so long such that the foam collapses. The time therefore preferably depends on the viscous properties of the polymer material, and the kinetics of the decomposition of the blowing agent, which are both of course, dependant on the temperature.

Alternative embodiments of the invention comprising a blowing step on the stabilised prefoam(b), whereby said blowing step is achieved by preferably using a physical blowing agent, the time of the blowing step (b) preferably depends on type of process (batch, continuous extrusion, etc). The major limiting time, is the time for dissolution of blowing agent into the polymer. This depends on the pressure and temperature of the system (which affect solubility and diffusion rates), and size of the polymer to be foamed. Times can range from a few seconds for polymers above the melting temperature to several hours for polymers in the solid state.

In a preferred embodiment the cell structure in the stabilized prefoam and foam body is macroscopic on the scale of mm. Preferably, the prefoam comprises cell widths ranging from 0.1 mm to 2.5 mm and cell legnth cell lengths ranging from 0.1 mm to 2.5 mm. Preferably, the foam body comprises cell widths ranging from 0.1 mm to 2.5 mm and cell lengths ranging from 0.2 mm to 15 mm.

As, according to embodiments of the invention, the cell structure of the stabilized prefoam and foam body is macroscopic and on the scale of mm, the longer cell walls exhibit a lower bending resistance which advantageously results in a foam which has a low shear resistance perpendicular to the cell lengths. This lower shear resistance can lead for instance to lower rotational acceleration when e.g. used in a helmet application. The foam produced using a method according to embodiments of the present invention is beneficial in various applications, such as applications of vibration damping, or for increased thermal conductivity, whereby a foam produced according to embodiments of the invention will have lower insulating properties because of the larger cells in the mm range. In another preferred embodiment the blowing step (b) is applied after a crosslink density has been achieved such that the growth of existing cells proceeds preferentially to the nucleation of new cells.

In yet another preferred embodiment the material is supported and the cells are prevented from obtaining a spherical geometry during the guided growth of the foam in blowing steps (b), due to the stabilization of the prefoam. In other words, the material may be supported during the growth of the foam in blowing step (b), due to the stabilization of the prefoam. In a preferred embodiment of the present invention the stabilization step (b') results in a stabilized prefoam wherein new cells are not are not easily nucleated and therefore, the gas generated in the second blowing step is diffused to existing cells leading to an expansion of those cells.

In another preferred embodiment the growth direction imposed during the blowing step (b) is obtained by placing the stabilized prefoam into a mould or by placing internal structures such as inserts. In an alternative embodiment the growth direction is imposed by applying external pressure or vacuum.

In another preferred embodiment of the present invention the stabilization step (b') results in a stabilized prefoam with a stabilized matrix which supports the normally transient shape of the growing cells during the blowing step (b) and discourages the energetic spring back to a more isotropic shape of the cells, therefore allowing anisotropic geometries that are generated during the second blowing step to be retained in the final foam body.

In yet another preferred embodiment of the present invention the stabilization step (b') results in a stabilized prefoam: (1 ) wherein new cells are not are not easily nucleated and therefore, the gas generated in the second blowing step is diffused to existing cells leading to an expansion of those cells, and (2) with a stabilized matrix which supports the normally transient shape of the growing cells during the blowing step (b) and discourages the energetic spring back to a more isotropic shape of the cells, therefore allowing anisotropic geometries that are generated during the second blowing step to be retained in the final foam body.

The foam body having anisotropic cell structure may be a stabilized foam body.

As a result of embodiments of the invention, the cell density in a prefoam according to embodiments of the invention advantageously can directly reflect the cell density in a foam according to embodiments of the invention. In addition, the sizes and density of the cells can be affected by the distribution of a first blowing agent in the material, by additives such as for instance nucleating agents, by methods to affect the diffusion of the gasses in the polymer (i.e. using an agent that produces a gas that is slower at diffusing, or by allowing less time for gas diffusion such as through rapid pressure release - will decrease the cell size and increase the cell density). Macroscopic cells in a prefoam according to embodiments of the invention can lead to bigger and longer cells in the final foam. In alternative embodiments one can optionally increase the ranges, for instance, one can decrease the cell size down to a few microns if needed, for instance for microcellular foams which can be used and applied for many applications.

The compounded polymer may comprise a polymer mixed, at an appropriate mixing temperature, with the preferred additives (e.g. blowing agents, crosslinking agents) needed to perform the blowing steps (a') and (b) and the stabilization steps (b') and (c). In another preferred embodiment a growth direction may be imposed during the blowing step of the compounded polymer by placing the compounded foam in a mould which restricts the growth in 1 - or 2-directions, or by placing internal structures such as inserts or by applying external pressure or vacuum.

The method may be adapted for producing an anisotropic foam body with a shape- anisotropy ratio R larger than 3.

During a mixing phase, according to embodiments of the invention, preferably no special pressures are applied, unless the mixing step is combined with a first blowing step (i.e. extrusion with a physical blowing agent) in these embodiments, the prefered pressure ranges for blowing should preferably be followed according to embodiments of the invention. During a blowing step, in one embodiment, there may be applied no external pressure.

The growth direction imposed during the blowing step (b) may be obtained by applying external pressure or vacuum.

In embodiments where a first blowing (a') is performed using a chemical blowing agent, a pressure between 0-20 MPa is preferably applied, and more specifically between 0-5 MPa. In preferred embodiments, pressure is applied for delaying the time of nucleation after decomposition of the agent by keeping the evolved gas in solution with the applied pressure. The pressure applied preferably depends on the critical solution pressure for the gas/polymer combination. In other preferred embodiments pressure is applied to control the rate of nucleation and growth by for instance applying a specific depressurization rate and thus affecting the cell geometry/size. In preferred embodiments depressurization rates from preferably 0.01 MPa/s to 40 MPa/s and more specifically between 0.1 -10 MPa/s are used. In emdobiment where natural back pressure is build op, by for instance limiting expansiop, advantageously better moulding capabilities, such as e.g. completely filling a mould, etc are achieved.

In embodiments where a first blowing (a') is performed using a gaseous physical blowing agent, a pressure between 5-40 Mpa is preferably applied, and more specifically between 10-30 Mpa, preferably with controlled depressurization rates from 0.01 MPa/s to 80 MPa/s and more specifically between 0.1 -10 MPa/s.

In embodiments where cross-linking is performed as part of the stabilization step (b'), whereby said stabilization step is achieved using chemical or radiation cross-linking, no particular pressure is needed during, unless if pressure is preferred for another reason, such as when in combination with a moulding step or to prevent nucleation of dissolved gasses. Crosslinking can be carried out above the decomposition temperature of the second blowing agent, and cell nucleation is preferably delayed by keeping the evolved gas in solution. Pressure thus needed preferably depends on the critical solution pressure for the gas/polymer combination.

In another preferred embodiment the stabilization (c) is done by means of cooling.

In another preferred embodiment, the stabilization (c) is done by means of cross-linking.

In yet another preferred embodiment the stabilized prefoam is cut before the blowing step

(b). The prefoam can be cut or shaped into a complex structure like e.g.. a 3-D curvature, and placed into a mould for foaming into the final shape. It can be molded into a complex shape having a three dimensional curvature.

The prefoam may be held in a desired shape during the stabilization step.

In yet another preferred embodiment the stabilized prefoam is produced (a') by foam injection moulding. The polymer is injected into and foamed within a mould of complex geometry such that no cutting or shaping is neccessary. This preform can be placed into a matching, yet thicker mould for the blowing step (b).

In a preferred embodiment the compounded foam comprises a thermoplastic base polymer which is stabilized by free-radical or radiation cross-linking or a base polymer which is a thermoplastic which is stabilized by a physical means such as cooling.

In another preferred embodiment the base polymer of the compounded foam is a thermoset which is stabilized by a B-stage system of partial curing.

In another preferred embodiment the blowing of (a') and (b) is achieved by either physical or chemical blowing agents, or a combination of the two.

The method may be adapted for manufacturing anisotropic foams with complex shapes having at least a three dimensional curvature.

At least some embodiments of the present invention are advantageous due to the following four functionalities:

(1 ) Foam can be produced with direct control over the anisotropy, shape, and size of the cellular structure.

(2) Shape-anisotropy ratios can easily exceed 3 (R>3) without the use of added pressure. Large anisotropies are possible without added pressures because the material is supported during the growth of the foam in secondary blowing steps, due to the stabilization of the prefoam.

(3) Because external pressure does not need to be applied to induce growth in any one direction, the foam can be produced in complex geometries (as opposed to a block or sheet) while maintaining control over the direction of cell growth, and hence anisotropy. e.g. curved shapes and materials with holes or inserts are possible.

(4) A large range of relative densities, e.g. 0.02 and 0.5, are also possible by varying the amount of blowing agent in the polymer and the processing conditions.

The present invention also relates to the use of a cross-linking agent for improving a shape- anisotropy ratio of an anisotropic foam. Such use may be for obtaining said shape- anisotropy ratio to greater than 3. Such use may be for stabilizing said anisotropic foam. The present invention also relates to a foam body having an anisotropic cell structure, the foam body being manufacture using a method as described above.

The present invention also relates to a foam body having an anisotropic cell structure, the foam body comprising a plurality of cells, whereby the majority of cells has a cell length in the range 1 mm to 15mm.

The present invention also relates to a system for producing a foam body having an anisotropic cell structure, the system comprising at least one blowing means and one stabilization means, the system furthermore being configured for performing a method as described above.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Definition of terms

When the terms "anisotropy" and "isotropy" are used in this description, they refer to the geometrical shape of foam cells which are characterized by one or two elongated or compressed principle cell dimensions (as e.g. illustrated in Figure 10). The terms anisotropy and isotropy can also be used to describe macroscopic mechanical (or other) properties of a material, however, that is not what is defined here, even though mechanical anisotropy is related to geometrical anisotropy of the cells.

The "shape-anisotropy ratio" or "R" is defined in Cellular Solids: Structure and Properties (1999) by Gibson and Ashbyl as the ratio between the two principle cell dimensions (L) in a 2D surface: R=L1/L2. A foam can be easily defined with three R values by defining directions 1 , 2 and 3 as orthotropic axes (relating to cell orientation), then by finding their relationships to each other, we can define R12, R13, and R23 which may all be different. For an idealized isotropic material, then these relationships are reduced: R12 = R13 = R23 = 1 . For a transversely isotropic material (which is a type of anisotropic material), i.e. a foam with a pronounced expansion in a single direction (direction 1 ), then these relationships are reduced: R23 = 1 and R12 = R13 > 1 . Figure 10 illustrates this concept,

"growth direction(s)" or "expansion direction(s)" are defined as the principle dimensions in which the material is permitted to expand during foaming. Free, unhindered expansion may result in a foam which expands evenly in 3-dimensions. By external forces, i.e. applied pressure, effects of gravity, restrictive moulds, vacuum, etc ., the foam may be restricted to grow in only 1 - or 2-dimensions. It is expected that the cells will have the most elongated cell lengths (L) in the direction(s) of greatest macroscopic expansion,

"blowing step" or "expansion step" or "foaming step" are all synonyms which refer to a processing step in which a blowing agent, either physical (i.e. dissolved pentane) or chemical (i.e. the chemical decomposition of azodicarbidimide), is activated by external factors (temperature, pressure, pressure drop, time, etc ..) to release a gas leading to the creation (nucleation) of new bubbles (cells) or the enlargement of existing cells in the material, and the expansion of the material macroscopically.

"stabilization" refers to a chemical (i.e. curing or cross-linking) or physical (i.e. cooling) process to rigidity and stabilize (i.e. the material behavior is altered such that it resists cell nucleation and/or further expansion at given process temperature) the foam matrix material, "matrix" refers to the solid material which is foamed. The modification of the composition of the matrix material affects both the foaming process and the final cellular structure and the properties of the cell walls and struts of the foam.

"Expansion ratio" is defined as the ratio between the density of the solid matrix and the density of the foamed product, i.e. measures the volume increase due to foaming.

"Relative density" is the density of the foam divided by the density of the solid matrix. This parameter is the inverse of the "expansion ratio"

A "mixing temperature" as used herein refers to a temperature needed to melt a polymer such that all of the additives can be compounded with it.

"Radiation cross-linking" as used herein can be achieved for instance by gamma radiation, electron beam, x-ray and is preferably achieved without chemical additives, but with a directed dose of radiation. The degree of cross-linking is directly related to dose of radiation and polymer reactivity. Brief Description of Figures

Figure 1 : Schematic flowchart of one embodiment of the production method whereby a growth direction is imposed during the second blowing step by placing the stabilized prefoam into a mould, (a) compounded polymer matrix, (b) pre-foam, (c) stabilized pre-foam, (d) prefoam in mould, (e) foam after second blowing step; (1 ) pre-foaming step, (2) stabilization, (3 placing stabilized pre-foam in mould, (4) second blowing step, (5) final stabilization (not pictured).

Figure 2: Drawings of example mould or insert configurations, as can be used in embodiments of the present invention, (a) external mould restricting expansion to 1 - dimension (resulting in ellipsoidal anisotropic cells; R>1 ); (b) external mould restricting expansion to 2-dimensions (resulting disk shaped anisotropic cells; R<1 ); (c) external mould and inserts used. 1 -dimension expansion (R>1 ).

Figure 3: Example - Compounded matrix polyethylene containing cross-linking agent and chemical blowing agents, as can be used in embodiments of the present invention.

Figure 4: Example - Intermediate foam after 1 ) first blowing step(100% expansion), and 2) stabilization step (cross-linking), illustrating a result of a step in an embodiment of the present invention.

Figure 5: Example - Final foam after second blowing step, illustrating a result of a step in an embodiment of the present invention.

Figure 6: Prefoaming moulds as can be used in embodiments of the present invention

(a) Prefoaming mould used in Example 1

(b) Prefoaming mould used in Example 2

Figure 7: Schematic diagram illustrating the growth of existing cells during the second blowing stage, illustrating features of embodiments of the present invention.

Figure 8: The effect of different cross-linking treatments on geometry of the final foam, illustrating features of embodiments of the present invention.

Figure 9: Flowcharts showing optional processing steps, as can be performed in embodiments according to the present invention.

(a) Flowchart A: Basic steps of one embodiment of the invention wherein a growth direction is imposed during the second blowing step by using a mould with desired geometry (limited growth directions).

(b) Flowchart B: Including optional steps of cutting the prefoam and manipulating into a secondary mould for secondary foaming, i.e. extrusion prefoaming.

(c) Flowchart C: Including option steps in which both the prefoam and the second foaming step occur in shaped moulds, i.e. injection moulding prefoaming. Figure 10: Illustration of shape-anisotropy ratio, as can be obtained using embodiments of the present invention.

Figures 1 1 (a)-(b) illustrate a prefoam according to embodiments of the invention.

Figures 12 (a)-(b) illustrate a foam body according to embodiments of the invention.

The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

Any reference signs in the claims shall not be construed as limiting the scope.

In the different drawings, the same reference signs refer to the same or analogous elements.

Detailed Description of Figures

Figure 1 : Schematic flowchart of one embodiment of the production method

Shows the basic production method pictorially whereby a growth direction is imposed during the second blowing step by placing the stabilized prefoam into a mould. It can be seen that the cells are nucleated in the prefoaming step and expanded in the final blowing step.

(a) compounded polymer matrix; (b) pre-foam; (c) stabilized pre-foam; (d) prefoam in mould; (e) foam after final blowing

(1 ) pre-foaming; (2) stabilization; (3) placing pre-foam in mould; (4) final blowing (5) final stabilization (not pictured)

Figure 2: Drawings of example mould or insert configurations

(a) external mould restricting expansion to 1 -dimension (resulting in ellipsoidal anisotropic cells; R>1 )

(b) external mould restricting expansion to 2-dimensions (resulting disk shaped anisotropic cells; R<1 )

(c) external mould and inserts used. 1 -dimension expansion (R>1 )

Figure 3: Example - Compounded matrix polyethylene containing cross-linking agent and chemical blowing agents. Illustration of the compounded material before any expansion takes place.

Figure 4: Example - Intermediate foam after 1 ) first blowing step(1 .5 x expansion), and 2) stabilization step (cross-linking). Illustration of the stabilized prefoam. Note the small cells which are not significantly anisotropic (when compared with figure 5).

Figure 5: Example - Final foam after second blowing step. Illustration of the stabilized final foam. Note that the cells are significantly bigger (indicating no significant additional bubble nucleation) and significantly anisotropic (R>3, indicating the material was supported during foam growth, and the cells could not relax to a spherical shape.) Figure 6: Prefoaminq moulds

(a) Prefoaming mould used in Example 1 : This mould is used for the second (final) blowing step, thus in order to have a prefoam that fits into that mould, it was also conveniently used for prefoaming. Expansion was allowed in one direction, but it wasn't constrained, so the surfaces of the prefoam are not perfect in all cases. Forces due to gravity, viscosity, crosslinking shrinkage and thermal expansion can pull the prefoam away from the mould surfaces for non-ideal heat conduction. The secondary blowing step was carried out directly in the same mould without a need to cool down and change mould components. Components: 1 ) flat mould lid; 2) cross section of cylindrical mould; 3) flat mould bottom; 4) prefoam.

(b) Prefoaming mould used in Example 2: This mould was designed to have a lid with a long insert to constrain the prefoam at 5% less volume than the natural expansion. This reduced volume applied the pressure preferred to have a perfectly fitting prefoam, which improved crosslinking and foaming uniformity. The secondary foaming step was carried out in the same mould as (a). Components: 1 ) insert mould lid; 2) cross section of cylindrical mould; 3) flat mould bottom; 4) prefoam.

Figure 7: Schematic diagram illustrating the growth of existing cells during the second blowing stage.

(a) Illustration of the stabilized prefoam. Note the small cells (1 -28) which are not significantly anisotropic.

Illustration of the stabilized final foam. Note that the cells (1 -28) are significantly bigger than in the stabilized prefoam (a) (indicating no significant additional bubble nucleation) and significantly anisotropic (R>3, indicating the material was supported during foam growth, and the cells could not relax to a spherical shape).

Numbered cells (1 -28): represent cells that were present in the prefoam that were expanded during the second blowing step. It is clear that this is the preferential expansion mechanism in the second blowing step.

A stabilized prefoam provided according to some embodiments of the invention comprises cell widths which can range preferably from 0.1 mm to 2.5 mm, more specifically preferably from 0.2 mm to 1 .5 mm as illustrated in Figs. 1 1 (a)-(b), with the majority of cells falling in the 0.8-1 .2mm range. Cell lengths of the prefoam preferably range preferably from 0.1 mm to 2.5 mm, and more specifically preferably from 0.4 mm to 2 mm with the majority of the cells falling in the 1 -1 .5 mm range. This results in preferred aspect ratios, R (L:W ratio), for a prefoam according to embodiments of the invention ranging range from 1 - 3 with the majority of the cells having an aspect ratio between 1 .2-1 .7. In some embodiments the cell density of a prefoam is around approximately 850 cells/cm³, but can range from 50- 2000cells/cm³ depending on processing conditions.

Lettered cells (A-D) in stabilized final foam (b): represent cells that were not present (or were extremely small) in the prefoam, but that may have expanded during the blowing step (small percentage of total number of cells e.g. preferably <20%).

Remaining small dots in stabilized final foam (b): very small cells that originated during the second blowing step (nucleation) were not significantly expanded during the blowing step. There may be a large number of these small cells, but they are significantly smaller than the major part of cells (1 -28) that were expanded (e.g. preferably <80% of the size of the fully expanded cells.) It is common to find these small cells at the warm mould surfaces, where the energy available for nucleation is higher, but they can also be present in matrix-rich cell walls and struts.

In the foam body produced using a method according to embodiments of the invention, cell widths preferably range from 0.1 to 2.5 mm, more specifically preferably from 0.4 mm to 2 mm, with the majority of cells falling in the 1 -1 .5mm range as illustrated in Figs. 12 (a)-(b). Cell lengths range preferably from 0.2 to 15 mm, more specifically preferably from 0.9mm to 10 mm. The aspect ratios, R (L:W ratio), range from 2-10. While the macroscopic cells have stayed more or less the same in number, one can see micro-cells in the foam have been generated within the cell walls and struts. Foams produced by using preferred embodiments of the present invention comprise macroscopic cells which are oriented parallel to the growth direction.

It is demonstrated that the foam growth in the second blowing step is dominated by the preferential growth of existing cells which were nucleated during the first blowing step. It is also shown that there may be the nucleation of some limited new cells, especially in the area near the mould walls. However, these new cells are not the dominating expansion process. It is recognized that the expansion is dominated by the expansion of existing cells if there are roughly the same number of large cells after the second blowing step as there are nucleated cells for the same formulation/process after the first blowing step (preferably +/- 30%, more preferably +/- 20%). There may be a great number of very small, newly nucleated cells appearing in the second blowing step, but these are significantly smaller (e.g. at least 80% smaller) than the expanded cells.

The 2D cell density (top view of the prefoam and foam as illustrated respectively in Figs. 1 1 a and 12a) of a prefoam according to embodiments of the invention is around 120 cells per 100mm² cross sectional area. This parameter is the same when comparing the prefoam to the final foam. Advantageously, since cross sectional dimensions of the foam did not change in this direction (the foam is preferably grown only in the out-of-the-page direction, and constricted to the same cross sectional area), the top-view of the prefoam and the foam enables one to notice that significant new cells are not being formed and thus the nucleation of these cells are advantageously prevented. In fact, the macroscopic cell density of the final foam according to embodiments of the invention is slightly lower (-1 10 cells per 100mm²) than that of the prefoam (due to e.g. some cells merging because of walls stretching and breaking). It is clear, however that with the evolution of more gas (through decomposition of second blowing agent) it is preferential to expand the existing cells than to nucleate and grow new cells. Indeed, if we look at the side view of both the prefoam and foam as illustrated in Fig. 1 1 b and 12b respectively, we reach the same conclusion, there is about 80- 90 cells/100mm² of the prefoam, and about 80-90 cells/200mm² of the foam (but the foam has been expanded by a factor of 2.

Figure 8: The effect of different cross-linking treatments on geometry of the final foam.

(a) cross-linking of the stabilized prefoam is not sufficient;

(b) too much cross-linking in the stabilized prefoam;

(c) the final foamed material has the desired structure, indicating optimum cross-linking treatment (time, temperature, cross-linking agent concentration) during the first stabilization step.

This figure shows the results of experiments on different cross-linking treatments. When the cross-linking is not sufficient (a), extra cells are nucleated, and the result is an isotropic foam with many different sized cells. This foam was also relatively unstable, and cells break easily due to the instability of the matrix polymer. In another experiment (b), it was observed that when there was too much cross-linking, the matrix polymer was too stiff and did not expand fully to desired expansion ratios. In (c), when the final foamed material had the desired structure, it was determined that the cross- linking treatment (time, temperature, cross-linking agent concentration), was optimum. Figure 9: Flowcharts showing optional processing steps.

(a) Flowchart A: Basic steps of one embodiment of the invention wherein a growth direction is imposed during the second blowing step by using a mould with desired geometry (limited growth directions)..

(b) Flowchart B: Including optional steps of cutting the prefoam and manipulating into a secondary mould for secondary foaming. i.e. "extrusion prefoaming" Note: This is only ONE possible production method including optional steps submitted as an example. As long as all of the basic steps are present, any or all optional steps can be used to fit the particular material, processing limitations, and final goal. (c) Flowchart C: Including option steps in which both the prefoam and the second foaming step occur in shaped moulds, i.e. "injection moulding prefoaming" Note: This is only ONE possible production method including optional steps submitted as an example. As long as all of the basic steps are present, any or all optional steps can be used to fit the particular material, processing limitations, and final goal.

Figure 10: Illustration of shape-anisotropy ratio

A foam can be easily defined with three R values by defining directions 1, 2 and 3 as orthotropic axes (relating to cell orientation), then by finding their relationships to each other, we can define R ₂, Ri₃, and R₂₃ which may all be different. For an idealized isotropic material, then these relationships are reduced: Ri₂ = Ri₃ = R₂₃ = 1 . For a transversely isotropic material, i.e. a foam with a pronounced expansion in a single direction (direction 1 ), then these relationships are reduced: R₂₃ = 1 and Ri₂ = R₁₃ > 1 .

Figure 1 1 : CT image of prefoam resulting from the production method of Example 2

(a) CT slice of prefoam looking down at the top of the material (the subsequent expansion will be in the direction of the observer).

(b) CT slice of prefoam looking at the side of the material (the subsequent expansion will be in the direction of the top of the page).

Figure 12:CT image of foam resulting from the production method of Example 2

(a) CT slice of foam looking down at the top of the material (the final expansion has taken place in the direction of the observer).

(b) CT slice of foam lo looking at the side of the material (the final expansion has taken place in the direction of the top of the page).

Detailed Description of the Invention

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

Furthermore, the terms first, second and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequence, either temporally, spatially, in ranking or in any other manner. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner, as would be apparent to one of ordinary skill in the art from this disclosure, in one or more embodiments.

Similarly it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the claims following the detailed description are hereby expressly incorporated into this detailed description, with each claim standing on its own as a separate embodiment of this invention.

Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are meant to be within the scope of the invention, and form different embodiments, as would be understood by those in the art. For example, in the following claims, any of the claimed embodiments can be used in any combination.

In the description provided herein, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known methods, structures and techniques have not been shown in detail in order not to obscure an understanding of this description.

It is an object of the present invention to solve at least some of the shortcomings in the prior- art and to provide a foam production method which (1 ) allows for a controlled design of the cellular geometry, (2) supports more significantly anisotropic structures, R > 3, (3) can produce foams in complex shapes, such as curved structures (foaming into final shape instead of shaping after foaming) and/or (4) can produce foams in a significant range of foam macroscopic densities, advantageously to combine all four of these properties.

In a first aspect, the present invention relates to a method of producing a foam body having an anisotropic cell structure. Although embodiments of the present invention are especially suitable for manufacturing of complexly shaped anisotropic foams, embodiments of the present invention are not limited to producing complexly shaped foams but also suitable for forming less complexly shaped foams. The method comprises providing a stabilized prefoam, applying a blowing step on said stabilized prefoam to form a fully expanded foam material and applying a stabilization on said expanded foam to provide a foam body having anisotropic cell structure, whereby a growth direction is imposed during the blowing step. According to embodiments of the present invention, providing a stabilized prefoam may comprise obtaining as an input previously made stabilized prefoam or it may comprise manufacturing the stabilized prefoam, e.g. by applying a previous blowing step on a compounded polymer to form the prefoam and applying a stabilization step on said prefoam. In embodiments of the present invention, the method of producing anisotropic foams comprising: at least two blowing steps, in which at least one is separately activated from the others in time; a stabilization step of the matrix material occurring before the final blowing step; the imposition of a growth direction of the foam during at least the second blowing step; and final stabilization of the foam. The result is a foam with controllable macroscopic density and shape, and which is characterized by a pronounced anisotropic structure which can be tuned in size, orientation, and shape-anisotropy ratio to achieve desired properties.

Blowing agents, e.g. physical blowing or foaming agents, that can be used in embodiments of the invention are illustrated in Table 1 . Table 1 :

Blowing agents, e.g. chemical blowing agents, that can be used in embodiments of the invention are illustrated in Table 2. Table 2:

In Table 2 the decomposition temperature is the temperature in which the chemical degrades into other components, some of which are the gasses used for blowing. In addition, gas yield at STP is the volume of gas (x 10-3 m³) that is produced with full decomposition per kg of the blowing agent at Standard Temperature and Pressure (0°C and 100kPa).

According to at least some embodiments, a method of foaming is provided such that the expansion occurs in two or more discrete blowing steps, in which at least one is separately activated from the other blowing steps in time. At some time after the first and before the last blowing step, the matrix is stabilized either chemically (i.e. cross-linking) or physical (i.e viscosity increases by thermal methods). This stabilization suppresses the nucleation of new cells and favors the growth of existing cells, giving control over the size and number of the final cells. The stabilization also supports the material for foaming in complex geometries and it also helps to preserve the anisotropic cellular geometry that is developed during foam expansion. The use of one or more foaming moulds with specific geometries or alternatively inserted materials (guiding walls i.e. honeycomb structures) are used strategically to direct the growth direction of the foam, and thus the geometry of the resulting cellular structure. A wide range of materials can be used for this process. The major requirement for the polymer system is that the matrix is able to be stabilized, but not fully hardened during the stabilization step (i.e. thermoplastics such as ethylene copolymers, polypropylene, polyvinyl chloride, polyvinyl difluoride, etc. . ., that can be lightly cross-linked or increase their viscosity through thermal cooling, or alternatively thermoset materials such as polyurethane, epoxy, silicone, etc. . ., that can be B-staged, meaning a molecular network can be formed without complete curing until the end of the process). With that in mind, the general steps of an exemplary foaming method can be as indicated below, illustrating standard and optional steps of a method according to an embodiment of the present invention.

(1 ) Preparatory steps: Depending on the type of material, the starting matrix material additional preparation steps may optionally be performed: thermoset resin is mixed, thermoplastic material is compounded with chemical additives including at least one chemical blowing agent (i.e. azodicarbidimide, sodium bicarbonate, etc. . . ) and/or saturated with physical blowing agent(i.e. nitrogen, pentane, etc. . . ).

(2) Pre-foaming step: According to some embodiments, when no previously made stabilized pre-foam is obtained as input material, the starting material, optionally prepared as described above, may be pre-foamed. This may be in a mould or outside of a mould. It may be a batch or a continuous process. Expansion ratios for this step can vary from 1 .1 to 10. It may be separate or connected to step (1 ) i.e. extrusion foaming to compound and then immediately pre-foam the material in a foaming die. In this step, the unstabilized material should have a relatively low viscosity so bubbles are able to nucleate quite easily. Cell nucleation can be made easier or more controllable by using heterogeneous nucleation agents such as calcium carbonate, talc, mica, clays, etc...

(3) Optional steps that may be carried out on the pre-stabilized prefoam:

a. extra foaming steps: there may be one or more prefoaming steps that may be activated by different blowing agents and/or processing parameters. The number of cells can be controlled during these steps by the use of pressures and pressure release rates, heterogeneous nucleation, etc...

b. forming: The pre-stabilized foam can be formed into complex shapes by the means of injection moulding, thermoforming, cutting, etc.

c. placement in a mould: The prefoam may be placed in a mould before or after stabilization, depending on how stable the material is for manipulation.

(4) Stabilization step and thus providing a stabilized prefoam: If the stabilized prefoam is not obtained as input material, after the pre-foaming step, a stabilization step is performed. The material is chemically (i.e. free-radical or radiation cross-linking for thermoplastics or B-staging for thermosets) or physically stabilized (i.e. through cooling to increase the viscosity of the system). In this way, subsequent expansions are supported. This achieves two goals: (1 ) New cells are not as easily nucleated - therefore, the gas generated in the subsequent foaming step(s) is diffused to existing cells which are expanded. (2) The stabilized matrix supports the normally transient shape of the growing cells, and discourages the energetic spring back to a more isotropic shape. This allows for anisotropic geometries that are generated to be retained in the final foam. Alternative crosslinking agents that can be used in embodiments of the present invention are summarized in Table 3.

Table 3:

(5) Optional steps on the stabilized prefoam (before final foaming):

a. extra foaming steps: extra foaming steps may also be carried out after stabilization, through any combination of blowing agents or varying processing parameters. After stabilization, expansion of cells should be energetically preferred over the nucleation of new cells.

b. cutting: the prefoam may need to be shaped in a secondary process of cutting, e.g. if the prefoam was made in a continuous way (i.e. extrusion), flat sheets may need to be cut into a preform shape to be inserted into a shaped mould. If the prefoam was made in a batch way (i.e. injection moulding or free foaming in a mould), it may need to be trimmed to remove injection ports, etc.

c. forming: the prefoam can be formed into a shape by folding, thermoforming, or otherwise clamping into the final foaming mould.

d. placement in a mould: the stabilized preform is placed into a different mould for final forming. The mould may be simply to restrict the growth in some direction, but may also be complex, curved, shaped, containing inserts or holes.

(6) Final foaming step: In the final foaming step, additional gas is evolved to achieve a secondary expansion between 1 .1 and 35, and more preferably between 1 .1 and 15 The stabilization achieved in step 4, together with specific process conditions chosen in this step work together so that nucleation of new bubbles is hindered and newly evolved gas energetically favors diffusing to established cells to expand them. The exact parameters that are preferred to achieve this (i.e. cross-link densities, temperatures, pressures, etc) will be dependent on the precise system (matrix, blowing agent, type of stabilization, etc ..) which is used, but can be easily determined by conducting experiments on nucleation and growth of cells at different processing parameters. When the situation is achieved that growth of existing cells proceeds preferentially to the nucleation of new cells, this requirement has been achieved. In general terms, the material is preferably well stabilized (lightly cross-linked material) but not too rigid (i.e. a fully cured thermoset material), and the temperatures and pressures applied such that the material is still pliable enough for expansion but not too soft/viscous that new bubbles can easily form. An example of how this is achieved is given in Example 1 below.

The relationship between the expansion ratio in this step and the number and expansion ratios of previously nucleated steps will determine the cell size of the final foam material. The size and geometry of the foaming moulds in this step will determine the cellular anisotropy: shape-anisotropy ratio and orientation.

(7) Final stabilization step: In order to handle the final product (for demoulding and end-use) without distorting it, the material needs to be fully stabilized either by chemical of physical means. This can be, for instance, by fully curing a resin, or by cooling the material down so that it is a solid material, and no longer having regions which are molten.

In a second aspect, the present invention relates to the use of a cross-linking agent for obtaining a predetermined shape - anisotropy ratio of an anisotropic foam. In some embodiments, such a ratio may be larger than 3. The cross-linking agent may be used for stabilizing the anisotropic foam. Features and advantages of methods as described in the first aspect may be equally applicable to the second aspect.

In a third aspect, the present invention relates to a foam body having an anisotropic cell structure, whereby the foam body is manufactured using a method as described in the first aspect. It is an advantage of embodiments of the present invention that a foam body can be produced in complex shapes and sizes. In this section sizes of the foam obtained by performing embodiments of the invention are discussed, whereby the sizes are based on sheets having an area and a thickness. As the size of the foam body depends on the size of the prefoam, first the dimensions of the prefoam will be discussed. Preferably a minimum area can be achieved in the range of 1 cm² (e.g. from a batch process), whereas a maximum area in the range of >1 m² is preferably achieved, e.g. 2m wide and infinite meters long for a continuous process such as for instance extrusion. The thickness of the prefoam preferably ranges from 1 -300mm and more specifically from 1 - 40mm, whereby the maximum thickness being limited by heat transfer for subsequent steps. In embodiments of the invention expansion of the prefoam is limited to the thickness direction. Depending on the application, other directions than the thickness direction can be used as well. Embodiments whereby foaming is limited to the thickness direction, the minimum and maximum area of the foam body preferably remain the same, e.g. from 1 cm² to 1 m². The foam thickness can be dependent on the expansion ratio of the foaming step. So starting from 1 -40 mm (with expansion ratios of 1 .1 -15 as indicated above), thickness ranges of preferably 1 .1 mm - 600 mm can be realized.

In embodiments where cutting, e.g. cutouts, are used to complexly shape the foam body, preferably cutouts or holes in the material are achieved by using a mould design with internal features, limited only by the ability to demould the foam body (i.e. an undercut geometry). As an advantageous result, cutouts (like for instance bicycle helmet vents) will help to guide the foaming and give better anisotropy. If it is not desirable to mould the foam directly with cutouts, any number of standard milling or cutting methods can be used to create holes or to shape the part, e.g. saw or waterjet cutting.

Embodiments of the invention enable curved foam bodies comprising of curvatures, whereby the curvature of the foamed part preferably depends on how the prefoam is formed. In embodiments where a prefoam is formed in injection moulding, a minimum radius of 1 mm can be achieved. In embodiments where a prefoam is formed using thermoforming, a minimum radius of 3 mm can be achieved. The maximum radius in both cases is flat, or infinity.

However, even though the prefoam can be formed with such small radii, it does not mean that this is appropriate for the foam body. The curvature of the final foam body preferably depends on the expansion ratio and the properties of the final foam body or part. The smaller the radius, the greater the deviation from the intended foam density will be within the area of the curvature, and in fact the greater the misalignment of the cell geometry. A rough approximation of reduction in foam density can be done as follows:

- a single curvature: density in region of curvature = intended density ^* (R/R+t)

- double curvature: density in region of curvature = intended density ^* (R²/[R+t]²)

where R is the radius of curvature, and t is the thickness of the foamed part, and assuming that the foam is able to fill the area of the curvature.

So one can see that the minimum radius according to embodiments of the invention are determined by the density that is needed in the area of curvature, and the thickness of the foamed material (i.e. for very low expansion ratios, very small radii can be easily achieved). For instance, for the purposes of bicycle helmets, and most typical foam applications, one would not expect curvatures to have a radius significantly smaller than 50mm, however this is by no means a limit, the limit are linked to the application type.

In a fourth aspect, the present invention relates to a system for producing a foam body having an anisotropic cell structure. The system may comprise an obtaining means for obtaining stabilized prefoam, a blowing means for applying a blowing step on a stabilized prefoam obtained in the obtaining means to form a fully expanded foam material and an stabilization means for applying a stabilization step on the expanded foam to provide a foam body having an anisotropic cell structure. In particular embodiments, the obtaining means may comprise a blowing means for performing a blowing step on a compounded polymer to form a prefoam and a stabilization means on the obtained prefoam. In one embodiment, the system thus may comprise at least a blowing means and a stabilization means as well as a controller for performing a method as described in the first aspect. In another embodiment, the system, which may be a production line, comprises an input means for obtaining a compounded polymer, a first blowing means for applying a blowing step to the compounded polymer for form a prefoam, a stabilization means for applying a stabilization step on the prefoam, a blowing means for blowing the stabilized prefoam to form a fully expanded foam material and a further stabilization means for applying a stabilization step to the expanded foam material. The system may comprise a controller for controlling the different system elements so that e.g. a method according to the first aspect can be performed. The latter may for example be performed in an automatic and/or semi-automatic way.

By way of illustration, embodiments of the present invention not being limited thereto, further examples of a method and system according to embodiments of the present invention are described and experimental results are discussed below.

In the section below guidelines are provided for advantageous processing parameters, embodiments of the present invention not being limited to such processing parameters. Temperature

a. Mixing temperature (temperature needed to melt the polymer enough such that all of the additives can be compounded with it.

·> melting temperature T_m (or glass transition temperatureT_g for completely amorphous polymers)

·< decomposition temperature T_dec0mposition of the first blowing agent (if using a chemical blowing agent), or activation temperature Taxation of the crosslinking agent (if the first blowing is done with a physical blowing agent).

b. First blowing temperature (for chemical and/or physical blowing agent)

• >melting temperature T_m (or glass transition temperature T_g for completely amorphous polymers) so that the polymer is viscous and able to deform easily

·< activation temperature Taxation of crosslinking agent. The temperature may only be equal to the temperature of crosslinking in the case that the two processes are separated in time kinetically (i.e. in the case of DCP cross-linking, the blowing is fast and the crosslinking is slow, though they occur at the same temperature).

c. Crosslinking, in case of chemical crosslinking

•>melting temperature T_m (or glass transition temperature T_g for completely amorphous polymers) so that the polymer is viscous, and polymer molecules have mobility.

·≥ activation temperature Taxation of first blowing agent. The temperature may only be equal to the temperature of first blowing in the case that the two processes are separated in time kinetically (i.e. in the case of DCP cross-linking, the blowing is fast and the crosslinking is slow, though they occur at the same temperature).

·< the decomposition temperature T_dec0mposition of the second blowing agent (if using a chemical blowing agent)

•If a physical blowing agent is used, or the pressure is kept high enough that the gas released from a chemical blowing agent is kept in solution during this step, then the temperature can exceed the temperature of the second blowing step.

d. Crosslinking, in case of radiation crosslinking

•Radiation crosslinking can be carried out over a wide range of temperatures. It can even be done at room temperature, but may be advantageous to have elevated temperature for more polymer molecule mobility.

·< the decomposition temperature T_{decomposition} of the second blowing agent (if using a chemical blowing agent)

•It is also important to be aware that radiation crosslinking can lead to unintended temperature increases, and this should also be controlled such that the material stays within allowable limits.

e. Second Blowing (for chemical or physical blowing agent)

•>the melting temperature T_m (or glass transition temperature T_g for completely amorphous polymers) so that the polymer is viscous and able to deform easily. Note that this value may have changed after crosslinking.

·< the decomposition temperature T_dec0mposition of the polymer

·< the decomposition temperature T_dec0mposition of any potential additional step blowing agent

(if using a chemical blowing agent)

Pressure

a. Mixing

•No special pressures are needed here, unless the mixing is combined with the first blowing step (i.e. extrusion with physical blowing agent) in which case, the pressure requirements for the blowing should be followed.

b. First Blowing (chemical blowing agent)

•0-20 MPa (more commonly 0-5 MPa)

•Pressure is only needed for:

-Delaying the time of nucleation after decomposition of agent by keeping the evolved gas in solution with pressure. Pressure needed depends on critical solution pressure for the gas/polymer combination.

-Controlling the rate of nucleation and growth (by applying a specific depressurization rate) - thus affecting the cell geometry/size. Depressurization rates from 0.01 MPa/s to 40 MPa/s (more commonly 0.1 -10 MPa/s)

-Building natural back pressure (by limiting expansion) for better moulding capabilities (completely filling a mould, etc).

c. First Blowing (physical blowing agent)

•5-40 MPa (more commonly 10-30 MPa) with controlled depressurization rates from 0.01 MPa/s to 80 MPa s (more commonly 0.1 -10 MPa/s) d. Crosslinking (chemical or radiation)

•No particular pressure is needed during this step, unless: -In the previous step, backpressure needed for moulding was applied, and is still needed -Crosslinking is being carried out above T_dec0mposition of the second blowing agent, and cell nucleation is preferably delayed by keeping the evolved gas in solution. Pressure thus needed depends on critical solution pressure for the gas/polymer combination. e. Second Blowing (chemical blowing agent)

•0-20 MPa (more commonly 0-5 MPa)

•Pressure is only needed for:

f. Second Blowing (physical blowing agent)

•5-40 MPa (more commonly 10-30 MPa) with controlled depressurization rates from

0.01 MPa/s to 80 MPa s (more commonly 0.1 -10 MPa/s)

Time

a. Mixing

•Usually achieved within a few minutes on a tuned extruder. Mixing efficiency should be tested by measuring distribution of additives

b. First Blowing (chemical blowing agent)

•Once the degradation temperature of the blowing agent has been reached, this is a relatively fast process, on the order of minutes. Is preferably long enough to reach desired expansion, but not so long that the foam collapses. The time therefore depends on the viscous properties of the polymer material, and the kinetics of the decomposition of the blowing agent, which are both of course, dependant on the temperature.

c. First Blowing (physical blowing agent)

• Time depends on type of process (batch, continuous extrusion, etc). The major limiting time, is the time for dissolution of blowing agent into the polymer. This depends on the pressure and temperature of the system (which affect solubility and diffusion rates), and size of the polymer to be foamed. Times can range from a few seconds for polymers above the melting temperature to several hours for polymers in the solid state. d. Crosslinking (chemical)

• Long enough to achieve desired crosslink density/stabilization of matrix. Dependant on the type and concentration of crosslinking agent and kinetics of crosslinking, thus the temperature and mobility of polymer molecules.

e. Crosslinking (radiation)

•Long enough to achieve desired crosslink density/stabilization of matrix. Dependant on kinetics of crosslinking, thus the temperature and mobility of polymer molecules as well as the dose of radiation. Crosslinking can continue to proceed long after application of radiation based on the kinetics of crosslinking.

f. Second Blowing (chemical blowing agent)

g. Second Blowing (physical blowing agent)

•Time depends on type of process (batch, continuous extrusion, etc). The major limiting time, is the time for dissolution of blowing agent into the polymer. This depends on the pressure and temperature of the system (which affect solubility and diffusion rates), and size of the polymer to be foamed. Times can range from a few seconds for polymers above the melting temperature to several hours for polymers in the solid state.

Example 1

The example shown in this description and the attached figures is that of a polyolefin (HDPE:

70-90 %wt) that is compounded with a chemical cross-linking agent (dicumylperoxide -

DCP: 1 -3 %wt ) and a chemical blowing agent (azodicarbidimide - AZDC 10-20 %wt). Other typical compounding additives are also present, such as a lubricant and antioxidant. The material is compounded in an extrusion process and foamed in the absence of applied pressures.

Steps for example 1 :

(1 ) The material is compounded in an extrusion process, carefully not exceeding any of the temperatures for degradation of DCP or AZDC. (see figure 3)

(2) The first foaming step for this material was carried out in the 120-140 degree Celsius temperature range. This temperature range activates the DCP to decompose into acetophenone, methane, and 2-phenyl isopropanol, the former is a free-radical responsible for the cross-linking of HDPE, and the latter two are gasses in this temperature range. It is clear then, that the decomposition of DCP leads to the evolution of gas. In HDPE, the diffusion of these gases through the material and nucleation of bubbles results in an expansion of the material as cells form. The degradation of the DCP and the evolution of its gaseous products, thus constitutes a first blowing step. These initial cells are (approximately) isotropic. This blowing step occurs almost immediately after the material reaches the temperature for this step, (see figure 4) . This expansion step leads to an expansion of 2 (i.e. an increase of volume by about 100% and a decrease in density of about 50%). This prefoaming step, in this particular example, was carried out in the same mould as the second foaming step (see figure 6a). This was only to produce a stabilized prefoam that fits perfectly into the shape of the foaming mould. This means that the first foaming step was also constrained to one direction of expansion, but because there is not yet sufficient matrix stabilization, the final cell shape of the prefoam is isotropic. Since the mould plays no significant role on the cellular geometry of the prefoam, any mould geometry that results in the preferred final macrostructural shape is appropriate here, and indeed it is even possible to prefoam the material without a mould and cut it into the appropriate shape for secondary or final foaming.

(3) During and after the prefoaming step, the final product of DCP degradation, acetophenone, in a slower process, begins the process of cross-linking (stabilizing) the material. The material is therefore held in this temperature range for some time for allowing adequate cross-linking and stabilization of the material well past end of the pre-foaming step. Adequate cross-linking time was determined by a series of tests that investigated the structure of the resulting foam when the parameters affecting crosslinking (time, temperature, and concentration of DCP) were varied during the cross-linking step. Figure 8 clearly shows this effect. Shown in this figure is the result of blowing after only 5 minutes of crosslinking at 133^QC (which is above the activation temperature of DCP and below the activation temperature of AZDC, the blowing agent). The cells are isotropic, and many newly nucleated cells were observed. In addition, this foam was highly unstable at higher temperatures, due to the weakness of the matrix - proving that the cross-linking was indeed not adequate. These experiments were carried out until an optimum procedure for cross-linking was found. Too little crosslinking resulted in an isotropic, unstable foam, with many cells being nucleated in the second blowing step. Too much cross- linking resulted in a foam that was too stiff to achieve desired expansion ratios.

(4) The fourth step, and final blowing step for this material, is the decomposition of AZDC within the temperature range 170-220 degrees Celsius. This blowing step is complete after a short time (e.g. 2-10 minutes where the absolute time is depending on temperature, which affects the rate of decomposition), and is carried out in a mould with a specific geometry, allowing expansion in only one direction - up. (For this experiment and setup, the AZDC degrades in two minutes at 220 degrees Celsius, or in 10 minutes at 190 degrees Celsius. It is however to be noted that these values are dependent on the heat conductivity of the mould, the amount of material, etc .). The cell structure is macroscopic - on the scale of mm, and the shape-anisotropy ratio, R > 3. (see figure 5). The final expansion ratio here of the material is around 7-12, corresponding to final relative densities in the range of 0.143 - 0.083.

(5) Final stabilization is achieved by quenching the material (in the mould) in a cold water bath, prior to demoulding.

The final material consists of HDPE foam with a low density and a high shape-anisotropy ratio R>3 (see figure 5). The large cells were achieved due to the hindrance of cell nucleation (expansion of existing cells was favored over new cell nucleation), due to the stabilization step 3 increasing the viscosity of the polymer material at the process temperature. The anisotropy was produced due to the expansion of the material in the second blowing step (step 4 above) in a mould which restricted growth to only one direction. Additionally, the stabilization (step 3 above) helped support the foam so that the cells, once elongated, did not revert back to a more spherical shape. The stiffness and the plateau strength of this material are higher in the direction consistent with the length of the cells.

Example 2

In a second example, the process was slightly improved by designing a special mould for the prefoaming step (see figure 6b). In this experiment, the same material formulations and processing parameters (time, temperature) were used as in example 1 ; however, a specialized prefoaming mould was used (in steps 2 and 3 above). This prefoaming mould (shown in figure 6.b) was designed to have the same shape and diameter as the final foaming mould (the same as Example 1 ). The difference for the mould shown in figure 6.b was that the thickness of this mould was reduced (compared to the mould shown in figure 6.a) such that the total volume in the mould is equal to 5% less than the final volume of the prefoam if it was allowed to expand naturally. This means that the prefoam completely fills the cavity and develops some back pressure leading to a very good contact between the material and the mould, on all sides. This improved contact improved thermal conductivity to the material, and therefore resulted in a much more homogenous prefoam. The crosslinking (during step 3) and final blowing homogeneity resulting from the blowing during step 4) was observed with a markedly more uniform cellular structure and decreased macroscopic geometrical abnormalities. It was also much less sensitive to changes in process parameters and heating method (oven, heating mantle, etc). In short, by using this mould during step 2 and 3, better foams can be obtained, under a larger range of process parameters.

Example 3

In a third example, LDPE or EVA or TPEs or blends of these materials can be used as polymer material, that is compounded with a chemical cross-linking agent Dicumyl peroxide (DCP) and a first blowing agent Dicumyl peroxide (gas production as by-product of crosslinking). The material is compounded and mixed at a mixing temperature of 100 to 120^QC. The first foaming step for this material was carried out in the 135 to 145 ^QC range. This temperature activates the DCP and foaming happens immediately with decomposition of DCP as crosslinking is a slower kinetic process. The second foaming step is carried out in the 155-180^QC range, whereby foaming is performed at atmospheric pressure, with and a second blowing agent OBSH.

Example 4

In a forth example, HDPE is used as polymer material, that is compounded with a chemical cross-linking agent Di-tert-butyl peroxide and a first blowing agent Sodium Bicarbonate. The material is compounded and mixed at a mixing temperature 130-145^QC. The first foaming step for this material was carrier out in the 120 to 150 °C range. The cross-linking step is performed in the 170-180 °C temperature range. A second foaming step occurs at at 190- 210^QC, whereby foaming is performed at atmospheric pressure, whereby Azodicarbonamide is used as second blowing agent.

Example 5

In a fifth example, HDPE is used as polymer material, which is compounded with a crosslinking agent 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane and a first blowing agent which can be Nitrogen, carbon dioxide, a hydrocarbon (butane, pentane, ect), etc. The material is compounded and mixed at a mixing temperature of 130-145 °C. In addition the material is compounded in an extrusion process and foamed at an extrusion temperature between 150-160 °C. The extruding is performed under an applied pressure, after injection port of nitrogen of between 20-30 Mpa. The first foaming takes place at the die exit, and a second foaming is performed at 210-235^QC using Touluenesulfonylsemicarbazide as second blowing agent. The cross-linking step is preferably performed between 175-185^QC.

Example 6

In a sixth example, HDPE is used as polymer material, which is compounded by crosslinking using radiation, e.g. an electron beam. The cross-linking step is preferably performed at 25- 100 °C, which is obtained by passing through an electron beam. A first blowing agent, Sodium Bicarbonate, is then used. The first foaming step is performed at 120-150°C. The material is compounded and mixed at a mixing temperature of 130-145 'Ό. A second foaming is performed at a temperature between 190-210^QC using Azodicarbonamide under atmospheric pressure (ATM).

Example 7

In a seventh example, HDPE is used as polymer material, which is compounded with a crosslinking agent Di-tert-butyl peroxide and a first blowing agent which can be Sodium Bicarbonate. The first foaming step is performed at 120-150 °C. The material is compounded and mixed at a mixing temperature of 130-145 'Ό. The cross-linking step is preferably performed at a temperature between 170-180^QC. A second foaming is performed at a temperature between 190-210^QC using Azodicarbonamide under a pressure between 2 - 10 MPa during decomposition of the second blowing agent, followed by controlled release of pressure (0.1 - 1 MPa per second).

Example 8

In an eighth example, HDPE is used as polymer material, which is compounded with a crosslinking agent Dicumyl peroxide (DCP) and a first blowing agent which can be Dicumyl peroxide, whuch is the gas production as by-product of crosslinking. The first foaming step and crosslinking step is performed at a temperature between 135 to 145^QC whereby foaming happens immediately with decomposition of DCP, crosslinking is a slower kinetic process. The mixing temperature is between 125 to 130^QC. The second foaming is performed using a second blowing agent, namely C0₂ or N₂. The second foaming step is performed at a temperature between 160-180^QC and under a pressure between 20-30 MPa for several minutes in order to dissolve the blowing agent, C0₂ into the prefoam obtained after a first foaming step. The rate of depressurization is controlled (1 -10 MPa per second) to enable foaming.

All of the examples given mainly relate to thermoplastic materials; however a person skilled in the art would be able to also give other examples for instance by incorporating thermosetting materials. A thermosetting material can be a thermosetting plastic, also known as a thermoset, which is polymer material that irreversibly cures. The cure may be done through heat (e.g. generally above 200 'Ό, but for example also well under 100 'Ό with epoxy materials in a first curing step), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing. Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives. Once fully cured, a thermoset resin cannot be reheated and melted back to a liquid form. Advantageously, thermoset materials are generally stronger than thermoplastic materials due to their three dimensional network of bonds (cross-linking), and are also better suited to high -temperature applications.

It is to understood that the lists of chemicals given in the tables are not exhaustive, and it should be stated that other similar chemicals could be used for blowing and crosslinking. It is to be understood that this invention is not limited to the particular features of the means and/or the process steps of the methods described as such means and methods may vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It is also to be understood that plural forms include singular and/or plural referents unless the context clearly dictates otherwise. It is moreover to be understood that, in case parameter ranges are given which are delimited by numeric values, the ranges are deemed to include these limitation values. The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the present invention, and it is not intended to detail all those obvious modifications and variations of it which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the present invention, which is defined by the following claims. The claims are intended to cover the claimed components and steps in any sequence which is effective to meet the objectives there intended, unless the context specifically indicates the contrary.

Claims

1 . A method of producing a foam body having an anisotropic cell structure comprising the steps of:

(a) providing a stabilized prefoam;

(b) applying a blowing step on said stabilized prefoam to form a fully expanded foam material;

(c) applying a stabilization on said expanded foam to provide a foam body having anisotropic cell structure;

whereby a growth direction is imposed during the blowing step .

2. The method of claim 1 wherein said stabilized prefoam has an anisotropic cell structure, more specifically said stabilized prefoam is a transversely isotropic material.

3. The method of any of claims 1 or 2, wherein providing a stabilized prefoam comprises the steps of:

(a') applying a previous blowing step on a compounded polymer to form a prefoam and; (b') applying a stabilization step on said prefoam.

4. The method of claim 3, wherein the stabilized prefoam is not fully hardened during the stabilization step.

5. The method of any of claims 3 to 4, wherein the stabilization step on the prefoam is applied until the situation is achieved that growth of existing cells proceeds preferentially to the nucleation of new cells when the blowing step (b) is applied.

6. The method of any of the previous claims, wherein the cell structure in the stabilized prefoam and foam body is macroscopic on the scale of mm.

7. The method of claim 6, wherein the prefoam comprises cell widths ranging from 0.1 mm to 2.5 mm.

8. The method of any of claim 6 or 7, wherein the prefoam comprises cell lengths ranging from 0.1 mm to 2.5mm.

9. The method of any of claim 6 to 8, wherein the foam body comprises cell widths ranging from 0.1 mm to 2.5mm.

10. The method of any of claim 6 to 9, wherein the foam body comprises cell lengths ranging from 0.2mm to 15mm.

1 1 . The method of any of the previous claims in as far as dependent on claim 3, wherein stabilization of the prefoam (b') prevents nucleation of the cells of the prefoam.

12. The method of any of the previous claims wherein the blowing step (b) is applied when the stabilized prefoam is in the situation that growth of existing cells proceeds preferentially to the nucleation of new cells.

13. The method of any of the previous claims wherein the material is supported during the growth of the foam in blowing steps (b), due to the stabilization of the prefoam.

14. The method of any of the previous claims wherein providing a stabilized prefoam (a) comprises providing a stabilized prefoam wherein new cells are not are not easily nucleated and therefore, the gas generated in the second blowing step is diffused to existing cells leading to an expansion of those cells.

15. The method of any of the previous claims, wherein the providing a stabilized prefoam (a) comprises providing a stabilized prefoam with a stabilized matrix which supports the normally transient shape of the growing cells during the blowing step (b) and discourages the energetic spring back to a more isotropic shape of the cells, therefore allowing anisotropic geometries that are generated during the second blowing step to be retained in the final foam body.

16. The method of any of the previous claims wherein said foam body having anisotropic cell structure is a stabilized foam body.

17. The method of any previous claims wherein no external pressure is applied during a blowing step.

18. The method of any of the previous claims for producing a anisotropic foam body with a shape-anisotropy ratio R > 3.

19. The method of any of claims 3 to 18, wherein the compounded polymer comprises a polymer and the preferred additives needed for performing the blowing steps (a') and (b) and the stabilization steps (b') and (c).

The method of any of claims 3 to 19, wherein a growth direction is imposed during the previous blowing step on a compounded polymer to form a prefoam.

20. The method of claim 20, wherein a growth direction is imposed by placing the compounded polymer in a mould or by placing internal structures such as inserts or by applying external pressure or vacuum.

21 . The method of any of the previous claims, wherein the growth direction imposed during the blowing step (b) is obtained by placing the provided stabilized prefoam into a mould or by placing internal structures such as inserts.

22. The method of any of the previous claims, wherein the growth direction imposed during the blowing step (b) is obtained by applying external pressure or vacuum.

23. The method of any of the previous claims, wherein the final stabilization (c) is done by means of cooling.

24. The method of any of the previous claims, wherein the stabilized prefoam is cut and/or molded before the second blowing step.

25. The method of any of the previous claims, wherein the prefoam is cut and/or molded into a complex shape having a 3 dimensional curvature.

26. The method of any of the previous claims, wherein the prefoam is held in a desired shape during the stabilization step.

27. The method of any of claims 3 to 26, wherein the compounded foam comprises a thermoplastic base polymer which is stabilized by free-radical or radiation cross-linking.

28. The method of any of claims 3 to 27, wherein the compounded foam comprises a base polymer which is a thermoplastic which is stabilized by a physical means such as cooling.

29. The method according to claim 28, wherein the base polymer of the compounded foam is a thermoset which is stabilized by a B-stage system of partial curing.

30. The method according to any of the previous claims, wherein the method is adapted for manufacturing anisotropic foams with complex shapes having at least a three dimensional curvature.

31 . Use of a cross-linking agent for improving a shape-anisotropy ratio of an anisotropic foam.

32. Use of a cross-linking agent according to claim 31 for obtaining said shape-anisotropy ratio to greater than 3.

33. Use of a cross-linking agent according to claim 31 or claim 32, for stabilizing said anisotropic foam.

34. A foam body having an anisotropic cell structure, the foam body being manufacture using a method according to any of claims 1 to 30.

35. A foam body having an anisotropic cell structure, the foam body comprising a plurality of cells, whereby the majority of cells has a cell length in the range 1 mm to 15mm.

36. A system for producing a foam body having an anisotropic cell structure, the system comprising at least one blowing means and one stabilization means, the system furthermore being configured for performing a method according to any of claims 1 to 30.