Method for preparing a layered foamed polymer material

Abstract

The present invention provides a layered foamed polymeric material lacking discontinuities at the interfaces between the layers, and Providing a foamable polymer material; solubilizing the one or more blowing agents in the foamable polymer material under pressure and at a temperature above 20°C; and · Instantaneous pressure release process; The manufacturing method thereof includes: A manufacturing method wherein the solubilization step is carried out using a pressure profile of said one or more blowing agents that can be varied over time. [Selection diagram] [Figure 1B]

Description

Technical Field

[0001] The present invention relates to a method for preparing a layered foamed polymer material from both the perspective of cell morphology and density.

[0002] In particular, the above method uses an operation for solubilizing a physical blowing agent necessary for subsequent foaming and is characterized by conditions that can change over time. More specifically, the conditions that can change over time in the solubilization step generate a non-uniform profile of the concentration of the physical blowing agent in the polymer, which, upon foaming, generates a corresponding non-uniform morphology and density.

Background Art

[0003] Recently, there has been growing interest in "gradient" foamed materials with improved structural and functional properties, characterized by a uniform structure in terms of cell density and / or morphology.

[0004] This has been shown by recent scientific research both theoretical, numerical and experimental. Patent documents describe the use and advantages of such layered foamed structures or those having a gradient morphology and / or density. The publication of US Patent Application No. 2015125663 describes the use of different layers of a polymer foam assembled in a "gradient" pattern in energy absorption from impacts in a helmet.

[0005] Another field related to the cellular foams is the field of expansion for sintering (also known as steam-chest molding or bead foaming), which is very common in the production of sintered foam polystyrene products, but more recently has also been used for, among others, polypropylene, thermoplastic polyurethane and polylactic acid. In this field, in the production of products, pre-expanded beads (which are approximately spherical in shape and close to 1 millimeter in size) are used which are inserted into a mold and covered with steam or a hot gas to achieve the final foaming and sintering of the beads. The final foaming is mainly due to a blowing agent, such as pentane, still contained in the pre-expanded beads and released upon heating.

[0006] Indeed, the possibility of preventing the loss of blowing agent due to diffusion during the period between pre-expansion and final foaming is of great technical importance. The loss due to this diffusion phenomenon is a limitation in the problems of storage and method standardization of pre-expanded beads when products have to be produced using beads that have undergone different storage periods (seasoning).

[0007] Having pre-expanded beads that can utilize a radially layered morphology, for example a dense intermediate layer or an outer layer that acts as a barrier to prevent the loss of blowing agent in the inner layer, would solve the above problems.

[0008] Some commercially available products, such as golf balls or sports impact protection devices, are laminated products having one or more foam layers (see, for example, US Patent Application No. 2015 / 0283432 to Sullivan et al. and US Patent No. 4486901 to Donzis et al.).

[0009] However, techniques adjusted to create such a layered structure are probably artificial, far from industrialization and / or difficult.

[0010] For example, "Comparison of compressive properties of layered syntactic foams having gradient in microballoon volume fraction and wall thickness" by Nikhil Gupta et al., Mater. Sci. Eng A 427 (2006) pp. 331-342 describes a laboratory strategy involving the placement of hollow glass microspheres of different dimensions in a polymer.

[0011] In European Patent Applications Nos. 1452191 and 1878450, operations for creating a laminated structure by using composite materials are described. In particular, in European Patent Application No. 1452191, a mixture of two polymer materials is used with a concentration gradient that varies from a step mainly composed of a first material to a step mainly composed of a second component, and in European Patent Application No. 1878450, a composite pre-foam is made by using the precise placement of fillers and / or fibers in a mold together with a foaming polymer material. These operations require both the use of composite materials and the complex preparation and distribution of the components of these materials.

[0012] More frequently, as described in WO 2016102291, WO 2014041516, TW200918316A, and Erheng Wang et al., "The blast resistance of sandwich composites with stepwise graded cores", International Journal of Solids and Structures 46 (2009) 3492-3502, the coupling of several layers of foams with different morphologies and / or densities is described and used by means of bonding or fusing. Apart from the complexity of the coupling operation, it should be noted that these have discontinuities at the joints that are problematic from the perspective of performance and design, especially when carried out by means of bonding or heat fusion. Another approach was introduced by Zhou C et al., "Fabrication of functionally graded porous polymer via supercritical CO2 foaming", Composites: Part B 42 (2011) 318-325, and the use of "non-equilibrium profiles of blowing agent concentration" has been investigated. In this method, the material to be foamed is partially saturated with the blowing agent by a solubilization process carried out at a constant temperature and pressure for a time shorter than that required to achieve a uniform concentration of the blowing agent. In this case, the outermost part of the sample to be foamed (with respect to the free surface in contact with the blowing agent under pressure, i.e., the innermost part) contains a lower concentration of the blowing agent than the part adjacent to the said surface (more external part), and the concentration of the blowing agent easily reaches an equilibrium state in accordance with the external pressure of the blowing agent. As a result, the internal part of the sample has less or insufficient foaming, while the external part is fully foamed. The design of such a structure presupposes knowledge of the diffusion coefficient of the blowing agent in the polymer, which is available for a number of polymer / blowing agent systems.

[0013] The described method is somewhat limited because, despite the simplicity of manufacturing, it only allows for the configuration of a single-gradient foamed material, characterized by fewer foamed layers inside the sample and more foamed layers outside. The symmetry of the gradient clearly depends on the sample shape and the sample surface that can be approximated by the foaming agent.

Prior Art Documents

Patent Documents

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Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Patent Document 7

Patent Document 8

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Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0016] The method of the present invention aims to overcome the drawbacks of the methods known in the art.

[0017] In particular, the applicant has observed that by using the method of the present invention, a layered foam structure can be obtained in a simple and low-cost manner.

[0018] Furthermore, the applicant has observed that the layered foam structure obtained by the method of the present invention does not show discontinuity at the interface between the layers, but rather shows a gradual change in both the density and morphology of the bubbles present in the foam material. The applicant has also observed that the method of the present invention makes it possible to obtain a layered foam structure with a wide range of design choices, from both the perspective of the number of layers and their morphology and density.

[0019] The applicant has observed that these and other advantages can be obtained by means of a method for preparing a layered foam material that includes at least one non-equilibrium step in the mass transport of the blowing agent in the polymer, using at least one condition that can change over time in the solubilization step prior to foaming.

[0020] The applicant has observed that the use of one or more conditions that can change over time in the solubilization step of one or more blowing agents in the polymer generates a non-uniform profile of the concentration of the blowing agent in the same polymer, which, upon foaming, generates a corresponding non-uniform morphology and density.

[0021] The applicant believes that the penetration depth L in the polymer of any change in the boundary conditions in the mass transport characterized by a constant characteristic time T (such as the period of a sine wave) pen is given by Equation (1).

[0022]

Number

[0023] Here, D is the diffusion rate of the blowing agent in the polymer.

[0024] When the change in the boundary conditions in mass transfer is instantaneous, the relationship itself shows the thickness affected by the change at each time T after the imposition of the change itself.

[0025] The applicant has observed that the change in the boundary conditions in mass transfer can be substantially affected by the following three variables. · Pressure change · Change in the composition of the expanding gas · Temperature change

[0026] The applicant has found that the pressure change can be appropriately adjusted by first using a management program that can cause periodic changes characterized by waveforms such as sine, triangle, rectangle, sawtooth, etc. for a certain period T.

[0027] The applicant has also found that the pressure change can be appropriately adjusted by using a management program that can operate non-periodic changes rapidly or slowly according to linear, parabolic, exponential, impulsive profiles, etc.

[0028] Similarly, the applicant has found that the change in the composition in the expanding gas can be appropriately adjusted by changing the partial pressures of two or more blowing agents (such as nitrogen and carbon dioxide) characterized by different diffusion coefficients D.

[0029] Finally, the applicant has observed that the temperature change, whose kinetics follow the kinetics of energy transport and is characterized by other properties of the system (thermal diffusivity), can be superimposed / combined in a calibrated manner on the kinetics of mass transfer that depends on pressure and the composition of the expanding gas to obtain different layers.

[0030] Therefore, the applicant has found that by appropriately designing the solubilization process by means of the periodic change in the pressure of one or more blowing agents and by using temperature changes, it is possible to obtain a foamed polymer having a multilayer structure in which each layer has a specific different morphology and density.

Means for Solving the Problems

[0031] Therefore, a first object of the present invention is represented by a method for preparing a layered foamed polymer material by means of using one or more blowing agents, the method comprising: · a step of providing a foamable polymer material; · a step of solubilizing the one or more blowing agents in the foamable polymer material under pressure and at a temperature above 20°C; and · a step of instantaneously releasing the pressure; comprising wherein the solubilization step is carried out using a pressure profile of the one or more blowing agents that can change over time.

[0032] A second object of the present invention is represented by a foamed polymer material having a multilayer structure obtained by the method according to the first object of the present invention.

[0033] A third object of the present invention is a foamed polymer material having a multilayer structure and a uniform composition, wherein the multilayer structure comprises at least two layers, each of the at least two layers has a specific different morphology and / or density, and a stepwise change is shown in the density and / or morphology at the interface between the at least two layers present in the foamed material, characterized by the foamed polymer material.

[0034] In particular, the foamed polymer material according to the third object of the present invention is characterized by not showing discontinuity in morphology and / or density at the interface between the at least two layers.

[0035] A fourth object of the present invention is a manufactured product made of a foamed polymer material having an overall or partially multi-layer structure and a uniform composition, wherein the multi-layer structure includes at least two layers, each of the at least two layers has a specific different morphology and / or density, and a step change is shown in the density and / or morphology at the interface between the at least two layers present in the foamed material. It is represented by a manufactured product characterized by this.

Brief Description of the Drawings

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[0066] <Detailed Description of the Invention> The expression "polymer material" refers to a polymer material including a thermoplastic or thermosetting homopolymer or copolymer, or a mixture thereof.

[0067] The expression "foamable polymer material" refers to a polymer material that can absorb a blowing agent under certain temperature and pressure, enable nucleation of bubbles upon release of the same pressure, and resist the straightening stresses during the growth of bubbles until solidification.

[0068] The expression "blowing agent" refers to a substance that can cause the expansion of a polymer material by forming bubbles inside the polymer material.

[0069] The expression "multilayer structure" refers to a structure including two or more layers, preferably three or more layers.

[0070] The expression "uniform composition" refers to a composition composed of a polymer material having a uniform and constant composition at all points.

[0071] The term "discontinuous" refers to a net distinct boundary between two adjacent layers typical of a composite material made by heat bonding means or by an adhesive of two layers having different structures made separately.

[0072] The term "density" refers to the ratio between the weight of a given volume of a layer of polymer material and this volume.

[0073] The term "morphology" refers to the shape, size and number per unit volume of the bubbles formed within the foamed polymer material.

[0074] The term "foamed polymer material" refers to a polymer material in which bubbles are formed by means of a blowing agent inside.

Embodiments for Carrying Out the Invention

[0075] The first object of the present invention is represented by a method for preparing a laminated foamed polymer material by means of using one or more blowing agents, the method comprising: · a step of providing a foamable polymer material; · a step of solubilizing the one or more blowing agents in the foamable polymer material under pressure and at a temperature exceeding 20°C; and · a step of instantaneously releasing the pressure; including wherein the solubilizing step is carried out using a pressure profile of the one or more blowing agents that can change over time.

[0076] According to the first object of the present invention, the pressure profile preferably changes in a periodic or non-periodic manner over time.

[0077] According to the first object of the present invention, the pressure profile preferably changes over time in a periodic manner with a waveform selected from the group consisting of sine, triangle, rectangle, sawtooth waveform type, or combinations thereof.

[0078] According to the first object of the present invention, the pressure profile preferably changes over time in a non-periodic manner according to linear, piecewise, curve, parabola, exponential, instantaneous profile, or combinations thereof.

[0079] According to the first object of the present invention, the pressure profile changes from a minimum pressure equal to atmospheric pressure to a maximum of 300 bar, more preferably from atmospheric pressure to 250 bar, and advantageously from atmospheric pressure to 200 bar.

[0080] According to a first object of the present invention, the pressure profile preferably includes a step having at least one pressure profile that increases over time and a step having at least one pressure profile that decreases over time.

[0081] According to a first object of the present invention, the pressure profile advantageously includes a step having at least one pressure profile that is constant over time.

[0082] According to a first object of the present invention, the solubilization step is carried out using a foaming agent or a mixture of two or more foaming agents, preferably a mixture of two foaming agents. Advantageously, the solubilization step can be carried out by changing the concentration of the foaming agent over time. In particular, the concentration of the foaming agent in the mixture can vary over time.

[0083] According to a first object of the present invention, the solubilization step is preferably carried out at a temperature including between 20 °C and 350 °C, more preferably between 30 °C and 250 °C, and advantageously between 50 °C and 200 °C.

[0084] According to a first object of the present invention, the one or more foaming agents are selected from the group consisting of inert gases, carbon dioxide, and substituted or unsubstituted aliphatic hydrocarbons having 3 to 8 carbon atoms (linear, branched, cyclic). Advantageously, the foaming agent is selected from the group consisting of nitrogen, carbon dioxide, n-butane, isobutane, n-pentane, and isopentane. Preferably, the substituted aliphatic hydrocarbons include halogenated hydrocarbons, in particular chlorofluorocarbons, hydrochlorofluorocarbons, and fluorocarbons, such as 1,1,1,2-tetrafluoroethane (Freon R-134a), 1,1-difluoroethane (Freon R-152a), difluoromethane (Freon R-32), pentafluoroethane (Freon R-125), and the like.

[0085] According to a first object of the present invention, the polymer material is preferably selected from the group consisting of thermoplastic or thermosetting polymer materials.

[0086] Advantageously, the thermoplastic polymer material is selected from the group comprising polyolefins, polyurethanes, polyesters and polyamides.

[0087] Preferably, the thermosetting polymer material is selected from the group comprising polyurethanes, epoxy resins, melamine resins, polyphenols, and polyimides.

[0088] Preferably, the polymer material is a polymer and copolymer of styrene, ethylene, propylene, and other olefins such as polystyrene, polyethylene, and polypropylene. Optionally, the polymer material can include one or more comonomers. Comonomers can include, for example, alkylstyrene, divinylbenzene, acrylonitrile, diphenyl ether, α-methylstyrene, or combinations thereof. By way of example, the polymer material can include from about 0% to about 30% by weight, preferably from about 0.1% to about 15% by weight, more preferably from about 1% to about 10% by weight of comonomer.

[0089] Preferably, the polymer material can have a molecular weight Mw (measured by GPC) of from about 10,000 Daltons to about 500,000 Daltons, more preferably from about 150,000 Daltons to about 400,000 Daltons, even more preferably from about 200,000 Daltons to about 350,000 Daltons.

[0090] Advantageously, the polymer material has a sliding index, measured according to standard ASTM D 1238 at a temperature of 200 °C and a load of 10 kg, that includes between 1.0 g / 10 min and 20 g / 10 min.

[0091] Preferably, the polymer material used in the present invention has a uniform composition throughout its thickness, i.e., it has a uniform and constant composition at all points.

[0092] Advantageously, the method of the present invention enables the production of a multi-layer product having two or more layers while avoiding the use of composite materials comprising compositional gradients as described in European Patent Application No. 1452191 and European Patent Application No. 1878450.

[0093] Furthermore, the method of the present invention advantageously enables the creation of multiple gradients during the solubilization operation of the blowing agent (or mixture of blowing agents), and thus enables the production of multi-layer materials having three, four, five or more layers with different morphologies and / or densities.

[0094] Preferably, according to a first object of the present invention, the pressure is instantaneously released at a rate of 10 bar / second or more, more preferably 100 bar / second or more.

[0095] A second object of the present invention is represented by a foamed polymer material having a multi-layer structure obtained by the method according to the first object of the present invention.

[0096] Advantageously, according to a second object of the present invention, the foamed polymer material comprises at least two layers having different densities and / or morphologies without having a discontinuity in morphology and / or density at the interface between the at least two layers. Preferably, the foamed polymer material comprises at least two layers having different densities and / or morphologies and has a gradual change in density and / or morphology present in the foamed material.

[0097] According to a second object of the present invention, the foamed polymer material preferably consists of multi-layer pre-expanded beads or a multi-layer foamed sheet.

[0098] Advantageously, the multi-layer foamed sheet comprises at least one layer having a lower density and a finer morphology and at least one layer having a higher density and a coarser morphology.

[0099] Preferably, the above multi-layer foamed sheet includes at least one layer having a lower density and a coarser morphology and at least one layer having a higher density and a finer morphology.

[0100] Advantageously, the above multi-layer foamed sheet includes at least one layer having a lower density and a uniform morphology and at least one layer having a higher density. Advantageously, the above multi-layer foamed sheet includes at least one layer having a coarser morphology and a uniform density and at least one layer having a finer morphology.

[0101] Advantageously, the above multi-layer pre-expanded beads include at least one non-foamed layer and at least one pre-expanded layer, and the non-foamed layer is disposed at a radially outer position with respect to the pre-expanded layer.

[0102] Therefore, by the method of the present invention, for the first time, it becomes possible to obtain a foamed polymer material having a multi-layer structure lacking the discontinuities typical of materials obtained by bonding or fusing separately obtained layers together, thereby having a uniform composition, and thereby avoiding the use of composite materials.

[0103] Therefore, a third object of the present invention is a foamed polymer material having a multi-layer structure and a uniform composition, wherein the multi-layer structure includes at least two layers, each of the at least two layers having a specific different morphology and / or density, and showing a stepwise change in density and / or morphology at the interface between the at least two layers present in the foamed material.

[0104] In particular, the foamed polymer material according to the third object of the present invention is characterized in that it does not exhibit discontinuities in morphology and density at the interface between the at least two layers. Advantageously, according to the second and third objects of the present invention, the foamed polymer material comprises at least two layers having different densities and / or morphologies, and having a stepwise change in density and / or morphology present in the foamed material.

[0105] In particular, the multi-layer structure comprises at least three layers, and more specifically, three, four, five or more layers having specific different morphologies and / or densities.

[0106] A three-layer material is particularly preferred, and its embodiments are described in Examples 1, 4, 5, and 6, illustrated in FIGS. 2C, 5B, 6B, and 7B, and a five-layer material is particularly preferred, and its embodiments are described in Examples 2 and 10, illustrated in FIGS. 3B and 11B.

[0107] The polymer material according to the second and third objects of the present invention itself tends to be used for the creation of manufactured products having a multi-gradient and anisotropic morphology, i.e., a morphology determined by porosity.

[0108] The fourth object of the present invention is a manufactured product made of a foamed polymer material having an overall or partial multi-layer structure and a uniform composition, wherein the multi-layer structure comprises at least two layers, each of the at least two layers having a specific different morphology and / or density, and showing a stepwise change in density and / or morphology at the interface between the at least two layers present in the foamed material.

[0109] Preferably, according to the fourth object of the present invention, the multi-layer structure comprises a multi-gradient morphology.

[0110] Advantageously, according to the fourth object of the present invention, the multi-layer structure comprises an anisotropic morphology.

[0111] In particular, according to a fourth object of the present invention, the manufactured product is, for example, a protection system (shin guards, back braces, shoulder and elbow pads, knee pads, shells and pads, bulletproof vests), helmets and helms, orthopedic prostheses, dental prostheses, skin prostheses, scaffolds for tissue engineering, absorption and soundproof slabs and systems, heat insulation systems and slabs, soles and components for sports shoes, automotive panels, sports equipment, furnishings, packaging, filtration membranes and systems, sacrificial foams for ceramic materials and porous metals, foams for diffusers and ventilation devices, biomedical systems, pads and patches for controlled drug delivery, progressive mechanical response systems, progressive functional response systems, electromagnetic shielding systems, catalyst systems, foams for aerospace and aviation, foams for optoelectronics, floating systems, frames and chassis, and spectacle frames.

[0112] In particular, according to a fourth object of the present invention, the manufactured product is represented by, for example, an orthopedic prosthesis, i.e., a medical device capable of replicating the bone structure of a skeletal segment of the human or animal body with the same structural characteristics.

[0113] In particular, the orthopedic prosthesis according to a fourth object of the present invention includes orthopedic internal prostheses for the lower limbs (feet, ankles, knees, femurs, lumbar regions), upper limbs (hands, wrists, elbows, humeri, shoulders), and spine.

[0114] Here, the present invention is described with reference to the materials and methods described for interpretation, but is not limited to the purposes in the following experimental section.

Examples

[0115] <Experimental section> For the preparation of the foam samples, the batch foaming system shown in Figure 1 was used, and some of its details are shown below. Figure 1 shows a photograph (Figure 1A) and a diagram (Figure 1B) of the discontinuous foaming apparatus used in the present invention.

[0116] The reactor is cylindrical, thermally regulated and pressurized, and has a capacity of 0.3 L (HiP, BC-1 type). The reactor was modified to enable the measurement and control of relevant process parameters.

[0117] For temperature control, an electric heater (11) was used as the heating element, and a heat exchanger (12) with an oil bath was used as the cooling element.

[0118] The heater (11) and the heat exchanger (12) are controlled by a PID thermoregulator (Ascone X 1 model), which reads the temperature inside the reactor using a Pt100 probe (4).

[0119] A Schaevitz pressure transducer, model P943 (3), was used to measure the pressure during the saturation process and record the pressure pattern during the release of the blowing agent. Valve (1) was connected to the expansion gas supply source, while valve (2) was connected to the vacuum pump.

[0120] The pressure relief system consists of a HiP ball discharge valve, model 15-71 NFB (5), a HiP electromechanical actuator, model 15-72 NFB TSR8 (6), and a solenoid valve (7) connected to a pipe (8) for compressed air and a cable (9) for the solenoid valve actuation signal (7). With this system, the opening of the valve could be reproduced. The pressure pattern P(t) over time during pressure relief was recorded using a DAQ PCI6036E data acquisition system, National Instruments, Austin, Texas, USA.

[0121] The pressure program was managed by a Teledyne ISCO 500 D volumetric pump (Lincoln, Nebraska, United States of America). The pump could be controlled by a computer via the serial interface of the pump control device to perform any pressure program. Furthermore, the controller could handle up to four pumps for different fluids.

[0122] The creation of variable conditions in the solubilization process can occur by changing the solubilization pressure of the blowing agent, a periodic pattern (e.g., triangular or sine wave), or an aperiodic pattern (linear or curved profile), as described in the following examples.

[0123] <Example 1 - Invention> In this example, the polymer used is polystyrene (PS) with code N2380, supplied by Versalis SpA (Mantua, Italy), having an average molecular weight, density, and melt flow index equal to 300 kDa, 1.05 g / cm 3 and 2.0 g / 10 min at 200 °C and 10 kg.

[0124] The samples consist of PS cylindrical disks with a diameter of 25 mm and a thickness of 2.2 mm. Their sides (Ω in Figure 2A) are protected with a metal barrier film (which does not allow the blowing agent to pass through completely), and the surfaces exposed to contact with the blowing agent are only the two upper and lower bottoms (Σ and Σ' in Figure 2A). Thus, mass transfer occurs in the axial direction of the cylinder and can be studied as a one-dimensional problem. The use of the barrier film in this case, and in those described in Examples 2, 3, 4, 5, and 6, when the samples to be foamed are cylindrical, is determined only by the need to make the one-dimensional treatment of material transfer more rigorous. It is not considered essential to obtain a multilayer structure.

[0125] The samples were placed at room temperature in the batch foaming system shown in Figure 1 and described previously. Then, the reactor was closed and brought to a temperature of 100 °C.

[0126] Next, using the pressure profile described in Table 1, the system was subjected to a solubilization process of the expanding CO2 gas.

[0127] [Table 1]

[0128] As shown in Table 1, the pressure profile includes the following three stages: · In Step 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 150 bar with a linear ramp of 0.2 minutes (12 seconds). · In Step 2, the pressure of the expanding CO2 gas was increased from 150 bar to 200 bar with a linear ramp of 2.6 minutes (156 seconds). · In Step 3, the pressure of the expanding CO2 gas was decreased from 200 bar to 100 bar with a linear ramp of 5.2 minutes (312 seconds).

[0129] At the end of Step 3, the pressure was instantaneously released (at a maximum rate of 1000 bar / s) for foaming.

[0130] Immediately before the pressure was released, at the end of Step 3, using the data on the CO2 diffusion rate in PS at 100 °C reported by Sato et al. in "Solubility and Diffusion Coefficient of Carbon Dioxide in Poly(vinyl acetate) and Polystyrene", The Journal of Supercritical Fluids 19 (2001) 187 - 198, the concentration profile calculated using Comsol Multiphysics 10.0 simulation software and the above geometric configuration resulted in the results shown in Figure 2B.

[0131] In particular, three different foaming regions were recognized and identified in the foaming cylindrical disk: two near the bottom surface with a high concentration of foaming agent and a central region with a low concentration of foaming agent. From this concentration profile, different foaming abilities of different regions were found upon pressure release.

[0132] The regions with high concentrations of blowing agent foamed with a morphology and density that depend on the foaming conditions (here, specifically a temperature of 100 °C, a concentration of about 6 wt%, and a pressure release rate of 1000 bar / s), while the central region lacking blowing agent did not foam even under the same operating conditions as the high-concentration regions.

[0133] In particular, Figure 2C shows a scanning electron microscope image of a cross-section of the produced foam, which has an apparent three-layer structure. Two regions near the bottom foamed with a density of about 0.1 g / cm 3 and a minimum pore size of about 20 μm, while the central region did not foam. By using a step change in the concentration profile, a distinct separation was not created between the three regions. Instead, a step change in both density and morphology was detected. In particular, the decrease in the concentration of blowing agent towards the center of the disk resulted in an increase in the bubble size (from about 20 μm to about 300 μm) and an increase in density from 0.1 to 1 g / cm 3 Using specific solubilization programs, it is possible to design different numbers, thicknesses, morphologies, and densities of layers, as described in the following examples.

[0134] <Example 2 - Invention> For the composition, geometry, and arrangement of the barrier film, a PS sample similar to that described in Example 1 was placed at room temperature in the reactor of the batch foaming apparatus shown in Figure 1 and previously described. The reactor was then closed and brought to a temperature of 100 °C.

[0135] Next, using the pressure profile described in Table 2, a solubilization step of CO2 gas expanding into the system was performed.

[0136]

Table 2

[0137] As shown in Table 2, the pressure profile includes the following four stages: · In Process 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 130 bar with a linear ramp over 0.2 minutes (12 seconds); · In Process 2, the pressure was maintained at 130 bar for 120 minutes; · In Process 3, the pressure of the expanding CO2 gas was decreased from 130 bar to 80 bar with a linear ramp over 9.6 minutes (576 seconds); · In Process 4, the pressure of the expanding CO2 gas was increased from 80 bar to 130 bar with a linear ramp over 15.6 minutes (936 seconds).

[0138] At the end of Process 4, the pressure was instantaneously released (at a maximum rate of 1000 bar / s) for foaming.

[0139] Just before the pressure was released, at the end of Process 4, using the data on the CO2 diffusion rate in PS at 100 °C reported by Sato et al. in "Solubility and Diffusion Coefficient of Carbon Dioxide in Poly(vinyl acetate) and Polystyrene", The Journal of Supercritical Fluids 19 (2001) 187 - 198, the concentration profiles and the above-mentioned geometric configuration calculated using Comsol Multiphysics 10.0 simulation software resulted in the results shown in Figure 3A.

[0140] In particular, five different foaming regions were recognized and identified in the foaming cylindrical disk: two near the bottom with a high concentration of foaming agent, two in the intermediate region with a low concentration, and a central region with a high concentration of foaming agent again.

[0141] From this concentration profile, different foaming abilities in different regions were found upon pressure release.

[0142] Regions with a high concentration of blowing agent (two near the surface and one in the center) foam with a morphology and density that depend on the foaming conditions (here, specifically a temperature of 100 °C, a concentration of about 5 wt%, and a pressure release rate of 1000 bar / s) (shown as the "high-foaming region" in Figure 3B), while the two intermediate regions with a lower concentration of blowing agent (about 3% concentration) had a lower degree of foaming (shown as the "low-foaming region" in Figure 3B).

[0143] In particular, Figures 3B - 3D show some images of cross-sections of foams generated at different magnifications, having a distinct five-layer structure, taken with a scanning electron microscope. Two regions near the bottom and the central region foamed with a density of about 0.1 g / cm 3 and a pore size of about 20 μm. The two intermediate regions were less foamed with a density of 0.3 g / cm 3 and a pore size of about 80 μm.

[0144] Also, in this example, by using the observed stepwise change in the concentration profile, a distinct separation between different regions was not created. Instead, a stepwise change in both density and morphology was detected. Images 3C and 3D show some magnifications of the transition zone, where the change in the size of the bubbles can be noted.

[0145] The method of the present invention, unlike the method described in "Fabrication of functionally graded porous polymer via supercritical CO2 foaming" by Shu Xi et al., Composites: Part B 42 (2011) pages 318 - 325, allows for a wide degree of design freedom.

[0146] Furthermore, in the foamed samples, as seen in Figures 2C and 3B, it was observed that, unlike products laminated by coupling present in products obtained by methods known in the art, discontinuities in the morphology were not formed.

[0147] <Example 3 - Comparison> Regarding the composition, geometry, and arrangement of the barrier film, a PS sample similar to that described in Example 1 was placed at room temperature in the reactor of the batch foaming apparatus shown in Figure 1 and previously described. The reactor was then closed and brought to a temperature of 100 °C.

[0148] Subsequently, using the pressure profile described in Table 3, a solubilization step of CO2 gas expanding into the system was performed.

[0149]

Table 3

[0150] As shown in Table 3, the pressure profile includes the following two steps: · In Step 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 100 bar with a linear ramp over 0.2 minutes (12 seconds); · In Step 2, the pressure of the expanding CO2 gas was maintained at 100 bar for 180 minutes to allow complete solubilization of the foaming agent.

[0151] At the end of Step 2, the pressure was instantaneously released (at a maximum rate of 1000 bar / s) for foaming.

[0152] Immediately before the pressure was released, the concentration profile at the end of Step 2 was constant and equal to approximately 6 wt% of CO2.

[0153] From this concentration profile, upon release of the pressure, there was uniform foaming from both the perspective of density (about 0.1 g / cm 3 ) and morphology (average bubble size 50 μm).

[0154] This type of uniform structure is typical of the state of the art.

[0155] A graph showing the CO2 concentration value at the end of stage 2 (just before foaming), expressed as a percentage of the equilibrium concentration at maximum pressure, as a function of the spatial abscissa "cylindrical axis" x, is shown in FIG. 4A. A scanning electron microscope image of the obtained foam is shown in FIG. 4B.

[0156] <Example 4 - Comparison> Regarding the composition, geometric shape and arrangement of the barrier film, a PS sample similar to that described in Example 1 was placed in the reactor of the batch foaming apparatus shown in FIG. 1 and previously described at room temperature. Then, the reactor was closed and brought to a temperature of 100 °C.

[0157] Next, using the pressure profile described in Table 4, the system was subjected to a solubilization process of expanding CO2 gas.

[0158]

Table 4

[0159] As shown in Table 4, the pressure profile includes the following two steps: · In step 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 100 bar with a linear slope over 0.2 minutes (12 seconds); · In step 2, the pressure was maintained at 100 bar for 20 minutes;

[0160] At the end of step 2, the pressure was instantaneously released (at a maximum rate of 1000 bar / s) for foaming.

[0161] Just before the pressure is released, at the end of step 2, the concentration profile is described by the error function (erf) widely described in scientific and technical literature.

[0162] In particular, as described in Shu Xi et al., "Fabrication of functionally graded porous polymer via supercritical CO2 foaming", Composites: Part B 42 (2011) pp. 318 - 325, the concentration of the blowing agent within the cylindrical disk changes stepwise from the outside to the inside. In the outer layer, the gas concentration near the outer surface is 6% by weight, while in the inner layer, it is zero because the dissolution time (in this case 20 minutes) is much lower than the time required for complete dissolution.

[0163] A graph showing the CO2 concentration values at the end of stage 2 (just before foaming), expressed as a percentage of the equilibrium concentration at maximum pressure, as a function of the spatial coordinate "cylindrical axis" x, is shown in FIG. 5A. A scanning electron microscope image of the resulting foam is shown in FIG. 5B.

[0164] In this foam, two different foamed regions near the bottom surface that foamed with a density of approximately 0.1 g / cm 3 and a fine morphology, and a non - foamed central region, were recognized and distinguishable. In particular, the foamed regions have a non - uniform morphology, and the number of bubbles per unit volume decreases stepwise from the outer surface towards the inner surface (corresponding to an increase in the size of the bubbles), indicating a lower gas concentration from the outside to the inside. This type of structure is known to those skilled in the art (Shu Xi et al., "Fabrication of functionally graded porous polymer via supercritical CO2 foaming", Composites: Part B 42 (2011) pp. 318 - 325) and results from the "error function" solution to the problem of mass transfer of the blowing agent at a constant external pressure. Therefore, there is no possibility of changing the structure, for example, by changing the thickness of the foamed / non - foamed layer and / or the width of the gradient region. Moreover, it is impossible to have a non - monotonic pattern in the thickness concentration unless it is defined by reasons of symmetry with respect to the outer surface exposed to the atmosphere of the blowing agent.

[0165] <Example 5 - Invention> Regarding the composition, geometry, and arrangement of the barrier film, a PS sample similar to that described in Example 1 was placed in the reactor of the batch foaming apparatus shown in Figure 1 and previously described at room temperature. Subsequently, the reactor was closed and brought to a temperature of 100 °C.

[0166] Subsequently, using the pressure profile described in Table 5, the system was subjected to a solubilization process of expanding CO2 gas.

[0167]

Table 5

[0168] As shown in Table 5, the pressure profile includes the following four stages: · In Step 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 200 bar with a linear ramp over 0.2 minutes (12 seconds); · In Step 2, the pressure of the expanding CO2 gas was maintained at 200 bar for 9.6 minutes; · In Step 3, the pressure of the expanding CO2 gas was decreased from 200 bar to 100 bar over 0.2 minutes (12 seconds); · In Step 4, the pressure of the expanding CO2 gas was maintained at 100 bar for 5 minutes;

[0169] At the end of Step 4, the pressure was instantaneously released (at a maximum rate of 1000 bar / s) for foaming.

[0170] Immediately before the pressure was released, at the end of Step 4, using the data on the CO2 diffusion rate in PS at 100 °C reported by Sato et al. in "Solubility and Diffusion Coefficient of Carbon Dioxide in Poly(vinyl acetate) and Polystyrene", The Journal of Supercritical Fluids 19 (2001) 187 - 198, the concentration profile calculated using Comsol Multiphysics 10.0 simulation software and the above-described geometry resulted in the results shown in Figure 6A.

[0171] In particular, three different foaming regions were recognized and identified in the foaming cylindrical disk: two near the bottom surface with a high concentration of foaming agent and a central region with a low concentration of foaming agent. Different from the concentration profile shown in FIG. 5A, in this case, the profile is initially flat in the high-concentration region and then decreases towards the inside. On the other hand, in the case of FIG. 5A, a region with a constant gas concentration was not obtained.

[0172] From this concentration profile, different foaming abilities of different regions were found upon pressure release.

[0173] The region with a high concentration of foaming agent foamed with a morphology and density that depend on the foaming conditions, while the central region lacking the foaming agent did not foam even under the same operating conditions as the high-concentration region.

[0174] In particular, FIG. 6B shows a scanning electron microscope image of the cross-section of the produced foam having an apparent three-layer structure. The two regions near the bottom surface foamed with a density of about 0.1 g / cm 3 and a minimum pore size of about 20 μm, while the central region did not foam. However, different from the case described in FIG. 5B, in this case, as a result of the outermost region having a certain concentration, the foaming morphology is more uniform, and only the inner part has a reduced number of bubbles (with an increase in the average size) as a result of the lower gas concentration.

[0175] <Example 6 - Invention> Regarding the composition, geometric shape, and arrangement of the barrier film, a PS sample similar to that described in Example 1 was placed in the reactor of the batch foaming apparatus shown in FIG. 1 and previously described at room temperature. Then, the reactor was closed and brought to a temperature of 100°C.

[0176] Next, using the pressure profile described in Table 6, a solubilization process of CO2 and 134a (1,1,1,2 - tetrafluoroethane) gases expanding into the system was performed.

[0177]

Table 6

[0178] As shown in Table 6, the pressure profile includes the following four stages: · In Step 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 100 bar with a linear slope over 0.2 minutes (12 seconds); · In Step 2, the pressure of the expanding CO2 gas was maintained at 100 bar for 9.6 minutes; · In Step 3, the pressure of the expanding CO2 gas was decreased from 100 bar to 0 bar in 0.2 minutes (12 seconds); simultaneously, in the same step, while the external pressure was equilibrated, a second gas 134a with a larger molecular weight was used to maintain the total external pressure at 100 bar; · In Step 4, the gas pressure of 134a was maintained at 100 bar for 2 minutes;

[0179] At the end of Step 4, the pressure was instantaneously (at a maximum rate of 1000 bar / s) released for foaming.

[0180] Just before the pressure was released, at the end of Step 4, using the data on the CO2 diffusion rate in PS at 100 °C reported by Sato et al. in "Solubility and Diffusion Coefficient of Carbon Dioxide in Poly(vinyl acetate) and Polystyrene", The Journal of Supercritical Fluids 19 (2001) 187 - 198, the concentration profile calculated using Comsol Multiphysics 10.0 simulation software and the above geometric configuration resulted in the results shown in Figure 7A. For the purpose of using this calculation, the diffusion coefficient of gas 134a was set much higher than that of CO2, and for all practical effects, the absorption of 134a in the polymer was ignored.

[0181] In particular, three different foaming regions were recognized and identified in the foaming cylindrical disk: two near the bottom surface with a high concentration of foaming agent and a central region with a low concentration (near 0) of foaming agent. Different from the concentration profile shown in FIG. 5A, in this case, the CO2 concentration is low near the surface.

[0182] From this concentration profile, different foaming abilities in different regions were found upon pressure release.

[0183] The regions with a high concentration of foaming agent foamed with a morphology and density that depend on the foaming conditions, while the central region lacking the foaming agent did not foam.

[0184] In particular, FIG. 7B shows a scanning electron microscope image of the cross-section of the produced foam with an obvious three-layer structure. The two regions near the bottom surface foamed, while the central region did not foam. However, different from the case described in FIG. 5B, in this case, the foaming morphology can be described by two gradients where the density of the bubbles moves from the outside to the inside, first increasing and then decreasing, corresponding to the pressure profile shown in FIG. 7A.

[0185] <Example 7 - Comparison> In this example, the polymer used is polystyrene (PS) with code N2380 supplied by Borealis (Mantua, Italy), having an average molecular weight, density, and melt flow index equal to 300 kDa, 1.05 g / cm 3 and 2.0 g / 10 min at 200 °C and 10 kg.

[0186] The samples consist of PS spheres with a diameter of 1 mm schematically shown in FIG. 8. These objects are used to sinter so-called pre-expanded beads in a system that uses steam to create the final product downstream of the pre-expanded bead sintering method. In this system, mass transfer occurs radially and can be studied as a one-dimensional problem.

[0187] A sample containing no barrier layer was placed in the batch foaming system shown in Figure 1 and described previously at room temperature. Then, the reactor was closed and brought to a temperature of 100 °C.

[0188] Subsequently, the system was subjected to a solubilization process of expanding CO2 gas using the pressure profile described in Table 7.

[0189]

Table 7

[0190] As shown in Table 7, the pressure profile includes the following two steps: · In Step 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 100 bar with a linear ramp over 0.2 minutes (12 seconds); · In Step 2, the pressure of the expanding CO2 gas was maintained at 100 bar for 10 minutes to allow complete solubilization of the foaming agent.

[0191] At the end of Step 2, the pressure was instantaneously released (at a maximum rate of 1000 bar / s) for foaming.

[0192] Immediately before the pressure was released, the concentration profile at the end of Step 2 was constant and equal to approximately 6 wt% of CO2. The gas concentration profile is shown in Figure 8A.

[0193] From this concentration profile, upon release of the pressure, there was uniform foaming from both the perspective of density (about 0.1 g / cm 3 ) and morphology (average bubble size 50 μm).

[0194] This type of uniform structure is typical of the state of the art.

[0195] A scanning electron microscope image of the obtained foam is shown in Figure 8B.

[0196] <Example 8 - Invention> Regarding the composition, geometry, and absence of the barrier film, a PS sample similar to that described in Example 7 was placed in the reactor of the batch foaming apparatus shown in Figure 1 and previously described at room temperature. Subsequently, the reactor was closed and brought to a temperature of 100 °C.

[0197] Subsequently, using the pressure profile described in Table 8, a solubilization step of CO2 and N2 gases expanding into the system was performed.

[0198]

Table 8

[0199] As shown in Table 8, the pressure profile includes the following four stages: · In Step 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 100 bar with a linear ramp over 0.2 minutes (12 seconds); · In Step 2, the pressure of the expanding CO2 gas was maintained at 100 bar for 10 minutes to allow complete solubilization of the blowing agent; · In Step 3, CO2 was exchanged with N2 · In Step 4, the pressure of the expanding N2 gas was maintained at 100 bar for 0.5 minutes.

[0200] At the end of Step 4, the pressure was instantaneously released (at a maximum rate of 1000 bar / s) for foaming.

[0201] The concentration profiles of the two gases at the end of Step 2, immediately before the pressure is released, are shown in Figure 9A.

[0202] From this concentration profile, there was bimodal foaming from both the density and morphology perspectives with the release of pressure.

[0203] The scanning electron microscope image of the obtained foam is shown in Fig. 9B. In particular, in the outer layer, since N2 was used as the foaming agent and foaming was carried out, the morphology is very fine. In the central region, it should be noted that the bubble density is lower because foaming was carried out with CO2. In the literature, it has been widely proven that N2 has a greater nucleation ability than CO2, even at low concentrations (at the same pressure), which determines the low density in the case of the use of CO2.

[0204] <Example 9 - Invention> Regarding the composition, geometry and absence of the barrier film, a sample of PS similar to that described in Example 7 was placed in the reactor of the batch foaming apparatus shown in Fig. 1 at room temperature. Then, the reactor was closed and brought to a temperature of 100 °C.

[0205] Next, using the pressure profile described in Table 9, a solubilization process of CO2 and 134a (1,1,1,2 - tetrafluoroethane) gases expanding into the system was carried out.

[0206]

Table 9

[0207] At the end of Step 4, the pressure was instantaneously released (at a maximum rate of 1000 bar / s) for foaming.

[0208] The concentration profiles of the two gases at the end of Step 2, immediately before the pressure is released, are shown in Fig. 10A.

[0209] From this concentration profile, when the pressure was released, foaming was only seen in the central layer.

[0210] A scanning electron microscope image of the resulting foam is shown in Figure 10B. In particular, note the presence of an outer non-foamed layer of approximately 50 μm and then how the foamed inner core is present. This morphology cannot be obtained without varying the saturation conditions.

[0211] <Example 10 - Invention> Regarding the composition, geometry, and arrangement of the barrier film, a sample of PS similar to that described in Example 1 was coated on one of the two bottom surfaces (surface Σ in Figure 2A) with a 100-micron-thick polymer film of poly(vinyl alcohol), PVA. This polymer has a carbon dioxide diffusion rate approximately two orders of magnitude lower than that of the polystyrene and can be effectively used to obtain an asymmetric concentration profile with respect to the plane parallel to the bottom surface of the centerline of the cylindrical sample. This sample was placed in the reactor of the batch foaming system shown in Figure 1 at room temperature. The reactor was then closed and brought to a temperature of 100 °C.

[0212] Next, using the pressure profile described in Table 10, the system was subjected to a solubilization step of expanding CO2 gas.

[0213] [Table 10]

[0214] As shown in Table 10, the pressure profile includes the following four stages: · In step 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 130 bar with a linear ramp over 0.2 minutes (12 seconds); · In step 2, the pressure of the expanding CO2 gas was maintained at 130 bar for 10 hours (600 minutes) to allow complete solubilization of the blowing agent; · In step 3, the pressure of the expanding CO2 gas was decreased to 80 bar with a linear ramp over 10 minutes; · In step 4, the pressure of the expanding CO2 gas was increased to 130 bar with a linear ramp over 10 minutes.

[0215] At the end of step 4, the pressure was instantaneously (at a maximum rate of 1000 bar / s) released for foaming.

[0216] Just prior to the pressure release, the concentration profiles of the two gases at the end of step 2 are shown in Figure 11A.

[0217] From this concentration profile, different foaming abilities in different regions were found upon pressure release.

[0218] Regions with a high concentration of foaming agent foamed more, with a morphology and density that depend on the foaming conditions, while regions with a low concentration of foaming agent had less foaming.

[0219] In particular, Figure 11B shows a scanning electron microscope image of the cross-section of the produced foam, which has a clearly asymmetric four-layer structure with respect to the center of the sample (shown as the "center line of the sample" in the image of Figure 11). To guide the eye, a figure showing the qualitative trend of the pore size in different regions of the SEM micrograph is inserted. The vertical dotted line indicates the contour of the different regions. C ... (the part in parentheses about being shown as the "center line of the sample" in the image of Figure 11 is already in the original text and is not translated separately here as it seems to be a reference note within the description).

[0220] <Example 11 - Invention> Samples of polycaprolactone, PCL, a biocompatible polyester used in the field of tissue engineering, were introduced into a porous metal container having the shape of the upper end of the femur shown in Figure 12A so as to confine and mold the foaming material. The container was then placed at room temperature inside the reactor of the batch foaming apparatus shown in Figure 1 and described previously. The reactor was then closed and brought to a temperature of 80 °C for 5 minutes and then to 60 °C.

[0221] Then, using the pressure profile described in Table 11, a solubilization step of expanding CO2 and N2 gases into the system was performed.

[0222]

Table 11

[0223] As shown in Table 8, the pressure profile includes the following four stages: · In Step 1, the pressure of the expanding CO2 gas was increased from atmospheric pressure to 100 bar with a linear gradient over 0.2 minutes (12 seconds). · In Step 2, the pressure of the expanding CO2 gas was maintained at 100 bar for 10 minutes to enable complete solubilization of the blowing agent. · In Step 3, CO2 was exchanged with N2 · In Step 4, the pressure of the expanding N2 gas was maintained at 100 bar for 3 minutes.

[0224] During Step 4, the temperature was set to 40 °C, and at the end of Step 4, the pressure was instantaneously released (at a maximum rate of 1000 bar / s) for foaming.

[0225] Just before the pressure was released, the concentration profiles of the two gases at the end of Step 4 were similar to those shown in Figure 9A.

[0226] From this concentration profile, there was double foaming from both the density and morphology perspectives with the release of pressure.

[0227] Scanning electron microscope images of the obtained foams are shown in Figures 12B and 12C. Figure 12B shows the longitudinal cross-section, and Figure 12C shows the transverse cross-section. The similarity to the bone structure of the femur shown in Figure 12D is remarkable. In particular, the straight dotted line in Figure 12D indicates the region of the actual femur structure that is very similar to that shown in Figure 12B of the foam sample, while the circular line in Figure 12D indicates the region of the actual femur structure that is very similar to that shown in Figure 12C of the foam sample. Note the orientation and the shape of the elongated pores (during foaming, the pores follow the polymer flow lines) in Figure 12C similar to the actual femur during mold filling.

[0228] Therefore, by using the method of the present invention, artificial bone prostheses can be created easily and at low cost. Generally, by appropriately designing the mold, the level of polymer filling, the solubilization process, and the hydrodynamics of polymer mold filling during foaming, it is possible to create a multi-gradient porous structure with a desired cell morphology, density, and orientation, and to optimize the structural and functional anisotropic properties of the foamed material.

Claims

1. A method for preparing a laminated foamed polymer material by means of using one or more blowing agents, the method comprising: - a step of providing a foamable polymer material; - a solubilization step of solubilizing the one or more blowing agents in the foamable polymer material under pressure and at a temperature above 20 °C; and - a foaming step of foaming the one or more blowing agents by instantaneously releasing the pressure; wherein the solubilization step is carried out using a pressure profile of the one or more blowing agents that can change over time, and immediately before the pressure is released, a non-uniform concentration profile of the one or more blowing agents is formed inside the foamable polymer material, and the foaming step is characterized in that a density and / or morphology corresponding to the non-uniform concentration profile of the one or more blowing agents is formed inside the laminated foamed polymer material.

2. The method according to claim 1, wherein the pressure profile changes over time in a periodic or non-periodic manner.

3. The method according to claim 2, wherein the pressure profile changes over time in a periodic manner with a waveform selected from the group consisting of sine, triangle, rectangle, or sawtooth, or a combination thereof.

4. The method according to claim 2, wherein the pressure profile changes over time in a non-periodic manner according to linear, piecewise, curved, parabolic, exponential, instantaneous profiles, or a combination thereof.

5. The method according to any one of claims 1 to 4, wherein the pressure profile changes from a minimum pressure equal to atmospheric pressure to a maximum of 300 bar.

6. The method according to any one of claims 1 to 5, characterized in that it includes a step having at least one pressure profile that increases over time and a step having at least one pressure profile that decreases over time.

7. The method according to claim 6, characterized in that it includes a step having at least one pressure profile that is constant over time.

8. The method according to claim 1, characterized in that one blowing agent is used.

9. The method according to claim 1, characterized in that a mixture of two or more blowing agents is used.

10. The method according to claim 9, wherein the concentration of the blowing agent in the mixture changes over time.

11. The method according to claim 1, characterized in that the one or more blowing agents are selected from the group consisting of inert gases, carbon dioxide, and substituted or unsubstituted aliphatic hydrocarbons having 3 to 8 carbon atoms (linear, branched, cyclic).

12. The method according to claim 11, characterized in that the one or more blowing agents are selected from the group consisting of nitrogen, carbon dioxide, n-butane, iso-butane, n-pentane, iso-pentane, 1,1,1,2-tetrafluoroethane (Freon R-134a), 1,1-difluoroethane (Freon R-152a), difluoromethane (Freon R-32), and pentafluoroethane.

13. The method according to claim 1, characterized in that the polymer material is selected from the group consisting of thermoplastic and thermosetting polymer materials.

14. The method according to claim 13, characterized in that the thermoplastic polymer material is selected from the group consisting of polyolefins, polyurethanes, polyesters, and polyamides.

15. The method according to claim 13, characterized in that the thermosetting polymer material is selected from the group consisting of polyurethanes, epoxy resins, melamine resins, polyphenols, and polyimides.

16. A foamed polymer material having a multilayer structure obtained by the method defined in claims 1 to 15.

17. The foamed polymer material according to claim 16, characterized in that it comprises at least two layers having different densities and / or morphologies with a gradual change in density and / or morphology at the interface between the at least two layers.

18. The foamed polymer material according to claim 16 or 17, characterized in that the foamed polymer material consists of multilayer pre-expanded beads or a multilayer foamed sheet.

19. The foamed polymer material according to claim 18, characterized in that the multilayer pre-expanded beads comprise at least one non-foamed layer and at least one pre-expanded layer, and the non-foamed layer is arranged at a radially outer position with respect to the pre-expanded layer.

20. The multi-layered foamed sheet includes at least a first layer and a second layer, and the second layer has (i) a higher density and a coarser morphology, or (ii) a higher density and a finer morphology, or (iii) a uniform morphology and a higher density, or (iv) a uniform density and a finer morphology, compared to the first layer. The foamed polymer material according to claim 18, characterized in that.

21. A product made of the foamed polymer material according to claim 16, which has an overall or partial multi-layer structure and a uniform composition, the multi-layer structure includes at least two layers, each of the at least two layers has a specific different morphology and / or density, and shows a step change in density and / or morphology at the interface between the at least two layers present in the foamed material. A product characterized by that.

22. The manufactured product according to claim 21, wherein the multi-layer structure includes a multi-gradient morphology.

23. The manufactured product according to claim 21, wherein the multi-layer structure includes an anisotropic morphology.

24. The manufactured product is selected from the group consisting of a protection system (shin guard, back brace, shoulder and elbow pads, knee pads, shell and pads, bulletproof vest), helmet, orthopedic prosthesis, dental prosthesis, skin prosthesis, tissue engineering scaffold, absorption and soundproof slab and system, heat insulation system and slab, sole and components for sports shoes, automotive panel, sports equipment, equipment, packaging, filtration membrane and system, sacrificial foam for ceramic materials and porous metals, foam for diffusers and ventilation devices, biomedical system, pads and patches for controlled drug delivery, progressive mechanical response system, progressive functional response system, electromagnetic shielding system, catalyst system, foam for aviation and aerospace, foam for optoelectronics, floating system, frame and chassis and spectacle frame. The manufactured product according to claim 21.

25. The manufactured product according to claim 21, wherein the manufactured product is selected from the group consisting of orthopedic prostheses.

26. The manufactured product according to claim 25, wherein the orthopedic prosthesis is selected from the group consisting of orthopedic internal prostheses for the lower extremities (foot, ankle, knee, femur, lumbar spine), upper extremities (hand, wrist, elbow, humerus, shoulder), and spine.

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