ES2908524B2 - PROCEDURE FOR MANUFACTURING A POLYMERIC FOAM SHEET WITHOUT SOLID NON-FOAMED SKINS AND OBTAINED SHEET - Google Patents

Description

DESCRIPTION

MANUFACTURING PROCEDURE FOR A POLYMERIC FOAM SHEET

WITHOUT SOLID NON-FOAMED SKINS AND SHEET OBTAINED

field of invention

The present invention relates to the chemical sector, more preferably to the sector of the production of polymeric materials, even more preferably to the manufacture of sheets of polymeric foams without solid non-foamed skins or with a reduced content thereof. In the present invention, therefore, methods for the manufacture of cellular polymers without solid non-foamed skins are disclosed without there being any limitation in terms of the thickness of the sample, allowing even the foaming of thin polymeric sheets (with a thickness less than to 0.4mm). The process of the present invention is based on the use of a clean, inexpensive, efficient, and industrially scalable technique for foaming processes, such as: gas solution foaming, extrusion foaming, or injection foaming.

Background of the invention

Since the discovery of the first cellular and microcellular polymers in the 1980s (Eaves, D. Handbook of polymer foams; Eaves, D., Ed.; Rapra Technology Limited, 1993; Vol. 31; ISBN 1859573886), these materials They have aroused great interest due to their properties (lightweight, low cost, impact resistance, insulation). In recent years, much of the research on these materials has focused on achieving good control over the pore size and the homogeneity of the cell structure, with the aim of modifying, and generally improving, their physical properties (Notario , B., Pinto, J., Rodriguez-Perez, MA Nanoporous polymeric materials: A new class of materials with enhanced properties. Prog. Mater. Sci. 2016, 78-79, 93-139, doi:10.1016/j.pmatsci .2016.02.002; Miller, D.; Kumar, V. Microcellular and nanocellular solid-state polyetherimide (PEI) foams using sub-critical carbon dioxide II. Tensile and impact properties. Polymer (Guildf). 2011, 52, 2910-2919 , doi:10.1016/J.POLYMER.2011.04.049; Notario, B.; Pinto, J.; Rodríguez-Pérez, MA Towards a new generation of polymeric foams: PMMA nanocellular foams with enhanced physical properties. Polymer (Guildf). 2015, 63, 116-126, doi:10.1016/J.POLYMER.2015.03.003; Martin-de Leon, J.; Bernardo, V.; Rodríguez-Pérez, M. Low Density Nanocellular Polymers Based on PMMA Produced by Gas Dissolution Foaming: Fabrication and Cellular Structure Characterization. Polymers (Basel). 2016, 8, 265, doi:10.3390/polym8070265; Fujimoto, Y.; Ray, SS; Okamoto, M.; Ogami, A.; Yamada, K.; Ueda, K. Well-Controlled Biodegradable Nanocomposite Foams: From Microcellular to Nanocellular. Macromole. Rapid Commun.

2003, 24, 457-461, doi:10.1002/marc.200390068; Guo, H.; Nicolae, A.; Kumar, V. Solid-state poly(methyl methacrylate) (PMMA) nanofoams. Part II: Low-temperature solid-state process space using CO2 and the resulting morphologies. Polymer (Guildf).

2015, 70, 231-241, doi:10.1016/J.POLYMER.2015.06.031; Guo, H.; Kumar, V. Some thermodynamic and kinetic low-temperature properties of the PC-CO ₂ system and morphological characteristics of solid-state PC nanofoams produced with liquid CO ₂ . Polymer (Guildf). 2015, 56, 46-56, doi:10.1016/J.POLYMER.2014.09.061). Particularly, it has been observed that the reduction of the pore size entails an improvement in the mechanical properties and a decrease in the thermal conductivity. For this reason, in recent years nanocellular polymers and their production procedures have become a very interesting focus of research in the field of materials. In fact, the study and optimization of manufacturing processes for micro and nanocellular polymers has intensified, since it is known that the properties of a material are closely linked to the production method. Currently, there are various techniques for the manufacture of nanocellular polymers, however, not all processes offer the same characteristics or the same results. Some traditional methods such as phase separation or lithography use polluting organic solvents and/or are limited to the production of nanometer films. Due to these drawbacks, new techniques have been developed for the production of nanocellular polymers.

Some foaming processes, such as extrusion or injection foaming, and particularly the gas solution foaming procedure, have been identified as some of the most promising techniques for the production of microcellular and nanocellular polymers, obtaining great control over the pore size and on the homogeneity of the structure from the process parameters. These are clean techniques based on a physical gas dissolution procedure, commonly using CO2 as a foaming agent and without the use of dangerous and polluting organic solvents, as occurs in other techniques. However, these techniques have two clear drawbacks. On the one hand, the samples manufactured by these processes are characterized by presenting an outer zone of solid, non-foamed skin that surrounds the cellular structure. And, on the other hand, there is a limit regarding the thickness of the samples, below which it is not possible to produce foamed parts. Obviously, the fact that the materials present a solid skin intrinsic to the manufacturing method is an obstacle that limits certain properties and possible applications (for example, their use as filters, in catalysis, as sensors, as adhesives or for acoustic absorption). Therefore, there is a need to develop complementary techniques to these foaming processes that allow the reduction or elimination of solid skin.

Regarding gas solution foaming, it should be noted that for years this foaming procedure has been identified as one of the most interesting techniques for the manufacture of micro and nanocellular polymers. It is a purely physical procedure that consists of dissolving a gas, commonly CO2, inside the polymeric matrix. Precisely, CO2 is the most used gas in this process since the conditions of its supercritical fluid state are easily reached (7.3 MPa, 31 °C). Once these conditions are overcome, CO2 presents intermediate properties between a liquid and a gas. The most peculiar characteristic of supercritical fluids is that they have much lower viscosities than normal fluids, this allows them to behave like a gas when it comes to penetrating into the polymeric matrix and reaching higher solubilities. In addition, CO2 is a harmless gas that is easily removed from cell material without generating any residue. For all these reasons, the gas solution foaming procedure using CO2 is a technique widely used in the manufacture of cellular polymers. The procedure consists of 3 stages: saturation, desorption and foaming. In the first stage, CO2 is introduced into the sample at high pressure until the maximum gas solubility within the polymer is reached. In the desorption step, the pressure is abruptly reduced and the sample goes into a state of supersaturation. In this state, the separation between both phases and the appearance of nucleation take place (Costeux, S. CO ₂ -blown nanocellular foams. J. Appl. Polym. Sci. 2014, 41293, doi:10.1002/app.41293). At the same time, a process of diffusion of the dissolved gas in the matrix to the outside is taking place, where the concentration is reduced and a decreasing concentration gradient appears from the center. to the edges of the polymeric matrix. On the other hand, it is known that there is a threshold in the gas concentration below which the polymer fails to foam. Therefore, the decrease in the gas concentration in the periphery of the sample causes the appearance of the non-foamed solid skins in those areas and mentioned above. Finally, in the foaming stage, it is necessary to provide a certain amount of energy above a threshold so that the nuclei formed during the previous stage can grow and form the cells. This process can be carried out taking advantage of the temperature of the test itself during saturation (foaming in one stage) or introducing the sample in a thermal bath (for example, an oven or any other element that allows the temperature to be raised) at higher temperatures (foaming in one stage). two stages).

Obtaining cellular polymers without non-foamed skin and the possibility of foaming thin sheets can lead to the use as filters and membranes of porous materials obtained from improved foaming processes (Pinto, J.; Dumon, M.; Rodriguez -Perez, MA, Garcia, R., Dietz, C. Block copolymers selfassembly allows obtaining tunable micro or nanoporous membranes or depth filters based on PMMA, Fabrication method and nanostructures. J. Phys. Chem. C 2014, 118, 4656-4663 , doi:10.1021/jp409803u, J Pinto, D Morselli, V Bernardo, B Notario, D Fragouli, MA Rodriguez-Perez, A Athanassiou Nanoporous PMMA foams with templated pore size obtained by localized in situ synthesis of nanoparticles and CO2 foaming. Polymer (Guildf. 2017, 124, 176-185, doi:10.1016/J.POLYMER.2017.07.067). Undoubtedly, the possibility of foaming thin sheets, the connection between the cell structure and the external environment that would be obtained by eliminating the non-foamed solid skin, and the precise control that already exists over the cell structure depending on the process conditions, would allow obtaining a wide range of filtration membranes for different types and sizes of particles. In this line, Pinto, J. and colleagues (Pinto, J.; Dumon, M.; Rodriguez-Perez, MA; Garcia, R.; Dietz, C. Block copolymers self-assembly allows obtaining tunable micro or nanoporous membranes or depth filters based on PMMA; Fabrication method and nanostructures. J. Phys. Chem. C 2014, 118, 4656-4663, doi:10.1021/jp409803u.) were able to produce open-cell, polymer-based cellular structures with a very appropriate internal structure to act as filtration membranes for liquids or gases; however, the presence of a solid non-foamed skin on the outside of the samples limited its use in said applications.

On the other hand, the increase in porosity, surface area, and connectivity with the exterior of the cell structure achieved with the elimination of the non-foamed solid skin, would be a very interesting characteristic for the application of these materials in catalysis processes. (Germain, J.; Fréchet, J. M. J.; Svec, F. Nanoporous Polymers for Hydrogen Storage. Small 2009, 5, 1098-1111, doi:10.1002/smll.200801762), both using thin samples and larger pieces. In addition, these new advantages that these improvements would provide to these materials could also be beneficial in sectors where they are commonly used or their potential has been demonstrated, such as acoustic absorption (Notario, B.; Ballesteros, A.; Pinto, J. ; Rodríguez-Pérez, M. A. Nanoporous PMMA: A novel system with different acoustic properties. Mater. Lett.

2016, 168, 76-79, doi:10.1016/J.MATLET.2016.01.037; Park, CP Cellular acoustic absorption polymer foam having improved thermal insulating performance 2001; Park, CP Perforated foams 1999) and thermal (Forest, C.; Chaumont, P.; Cassagnau, P.; Swoboda, B.; Sonntag, P. Polymer nano-foams for insulating applications prepared from CO2 foaming. Prog. Polym. Sci. 2015, 41, 122-145, doi:10.1016/J.PROGPOLYMSCI.2014.07.001). In the first case, the acoustic waves could be damped directly on the cellular structure more effectively than on the solid non-foamed skin, as has been shown in other open-celled cellular polymers (Zhang, C.; Li, J. ., Hu, Z., Zhu, F., Huang, Y. Correlation between the acoustbic and porous cell morphology of polyurethane foam: Effect of interconnected porosity. Mater. Des. 2012, 41, 319-325, doi:10.1016/J .MATDES.2012.04.031). In the second case, cellular polymers are currently used as thermal insulators due to their properties (low thermal conductivity) and their low cost. However, the use of these materials is limited to wall and roof cladding as they are not transparent. Therefore, it would be interesting to develop a material with adequate insulation and transparency properties to partially or totally replace the glass currently used in windows, since heat losses through them amount to around 45% ( Grynning, S., Gustavsen, A., Time, B., Jelle, BP Windows in the buildings of tomorrow: Energy losers or energy gainers. Energy Build. 2013, 61, 185 192, doi:10.1016/J.ENBUILD.2013.02 .029). This is a very significant amount of energy considering that they represent only a small area of the thermal envelope of an average building. Currently, there are lines of research in that address. Martin-de León, J. and colleagues (Martín-de León, J.; Bernardo, V.; Rodríguez-Pérez, M. Á. Key Production Parameters to Obtain Transparent Nanocellular PMMA. Macromol. Mater. Eng. 2017, 302, 3-7, doi:10.1002/mame.201700343) managed to produce transparent nanocellular polymers based on poly(methyl methacrylate) (PMMA) under extreme conditions of pressure (20 MPa) and temperature (-32 °C), through the foaming process. by gas dissolution. To obtain a high level of transparency in cellular polymers, it is necessary to reach low densities and reduce the cell size to tens of nanometers. The drawback of this study is that the appearance of the non-foamed solid skin formed during the manufacturing process negatively affects the thermal conductivity properties and therefore it is necessary to remove it by physical or chemical attack to obtain the transparent nanocellular material with high capacity. of thermal insulation. Therefore, avoiding the appearance of the non-foamed solid skin during the foaming process would help to decrease the density of the material, as well as the total thermal conductivity with respect to materials with a solid skin.

In addition, the high surface area of materials with surface porosity together with the small dimensions of the solid zones could also be used to generate dry adhesives (in English, Dry-Adhesives) with better performance than current ones. In this sense, there are previous works to achieve surface structures that simulate those of lizards (gecko) (A Davoudinejad, M M Ribo, D B Pedersen, A Islam and G Tosello, Direct fabrication of bio-inspired gecko-like geometries with vat polymerization additive manufacturing method Journal of Micromechanics and Microengineering, 28.8, 2018, Omar Tricinci, Eric V Eason, Carlo Filippeschi, Alessio Mondini, Barbara Mazzolai, Nicola M Pugno, Mark R Cutkosky, Francesco Greco and Virgilio Mattoli, Approximating gecko setae via direct laser lithography, Smart Materials and Structures 27, 7, 2018) and which are known to generate great adhesion due to their high surface area and the small dimensions of the elements that generate adhesion between materials.

To date, no great advances have been made in the production of cellular polymers avoiding the solid non-foamed skin. Despite this, there are some works that have pointed in this direction, such as, for example, the subsequent removal of the non-foamed solid skin by mechanical processes, either polishing the non-foamed solid skin (Martín-de León, J.; Bernardo, V.; Rodríguez-Pérez, M. Á. Key Production Parameters to Obtain Transparent Nanocellular PMMA. Macromol. Mater. Eng. 2017, 302, 3-7 , doi:10.1002/mame.201700343) or perforating the surface to establish a connection with the interior (Park, CP Cellular acoustic absorption polymer foam having improved thermal insulating performance 2001; Park, CP Perforated foams 1999; Nicolae, A.; Guo, HM Kumar, V. Method of calculating the fluid permeability of machined skin-covered porous sheets from experimental flow data. Int. Polym. Process. 2017, 32, 355-362, doi:10.3139/217.3352; Kolosowski, PA Method for providing accelerated release of a blowing agent from a plastic foam 1995; Lee, S.-T.; Brandolini, M. Partially perforated foam. 1999). The main disadvantage of these techniques is that they generate a series of stresses in the material, inducing the deterioration of the outer layers of the cellular structure. In addition, the use of these techniques in thin films is practically unfeasible, since severe and irreversible damage would be caused to the entire material. Another drawback would arise when producing these materials at an industrial level, since adding one more stage of this type to the manufacturing process would make it more complicated and expensive.

On the other hand, research has been done on techniques that avoid the formation of a solid non-foamed skin during the manufacturing process. First of all, it is necessary to know what causes the formation of the non-foamed solid skin. During the gas solution foaming process, the polymer is subjected to high pressure until it reaches maximum CO2 solubility. At the time of depressurization, a diffusion process begins where part of the gas manages to leave the polymeric matrix due to the pressure difference. As a result of this process, the outer areas of the polymer do not have a sufficient amount of gas to achieve foaming, and therefore the cellular structure is not formed in those areas. In addition, due to the same diffusion process, a pore size gradient appears between the non-foamed zone and the cell structure, decreasing homogeneity as a main consequence. As a solution to this problem, Siripurapu, S. and colleagues (Siripurapu, S.; Coughlan, JA; Spontak, RJ; Khan, SA Surface-constrained foaming of polymer thin films with supercritical carbon dioxide. Macromolecules 2004, 37, 9872-9879 , doi:10.1021/ma0484983) proposed a technique for foaming thin sheets by introducing them into a metallic mold that acts as a gas diffusing barrier. In this way, the diffusion process is limited on the faces with the greatest surface area, allowing only the entry and exit of the gas. through the edges of the sheet. This technique makes it possible to maintain a higher concentration of gas within the polymer during the foaming process, a concept closely related to the formation of nuclei that will later give rise to cells (Costeux, S. CO ₂ -blown nanocellular foams. J. Appl. Polym 2014, 41293, doi:10.1002/app.41293). However, the template also reduces the input diffusivity by limiting the diffusion process only through the edges of the sample, significantly increasing the saturation time that with this configuration is determined by the sample dimensions (length and width). Therefore, this technique is only valid for samples of a few millimeters, since in larger pieces the process would take considerably longer and would therefore be unfeasible. In addition, the pressure exerted by a rigid mold during the foaming process limits the expansion of the sample in that direction, thus preventing it from reaching low densities. Therefore, this technique is limited to the production of samples at the laboratory level and in no case would it be valid for manufacturing at the industrial level.

The above is not the only attempt to prevent the appearance of solid non-foamed skin in thin sheets. Morisaki, M. and colleagues (Morisaki, M.; Ito, T.; Hayvali, M.; Tabata, I.; Hisada, K.; Hori, T. Preparation of skinless polymer foam with supercritical carbon dioxide and its application to a photoinduced hydrogen evolution system. Polymer (Guildf). 2008, 49, 1611-1619, doi:10.1016/J.POLYMER.2008.01.049) use ethanol as a co-solvent in the foaming process. Using this technique, they manage to reduce the size of the solid skin in thin sheets and even the appearance of superficial porosity with the increase in ethanol. However, with this addition they obtain very heterogeneous cell structures with many defects. In addition, it must be taken into account that the incorporation of a co-solvent complicates the process. Additionally, it is not shown that the technique is valid for larger samples or slices, but only for thin slices.

Recently, it has been possible to produce porous thermoplastic polyurethane (TPU) polymeric membranes or sheets that can be used as filters (Ge, C.; Zhai, W.; Park, CB Preparation of Thermoplastic Polyurethane (TPU) Perforated Membrane via CO2 Foaming and Its Particle Separation Performance. 2019). The technique is based on the generation of pores on the polymer surface through heterogeneous nucleation promoted by the contact of another polymeric layer. In this In this case, they manage to manufacture a sheet of about 20 μm with a cellular monolayer, that is, they connect the two main surfaces of the sheet by means of cells that have a diameter similar to the thickness of the sheet. However, the idea of only generating superficial pores is only valid in extremely thin sheets where the thickness of the sheet is comparable to the pore size, since increasing said thickness would result in a sheet with superficial porosity on the outside generated by the heterogeneous nucleation but no cell structure inside. In addition, the polymeric layer that they use to promote heterogeneous nucleation only acts to generate pores on the surface, in no case as a gas diffusion barrier, since the material they use (polyimide, PI) is widely used in membranes for the separation of gases, which are characterized by good gas permeability properties (Lin, W.-H.; Chung, T.-S. Gas permeability, diffusivity, solubility, and aging characteristics of 6FDA-durene polyimide membranes. J. Memb. Sci. gas separation. J. Memb. Sci. 2001, 189, 231-239, doi:10.1016/S0376-7388(01)00415-X). Therefore, by increasing the thickness of the sheet, in this case, it is possible that no type of structure is formed inside the sheet due to the rapid diffusion of the gas and, therefore, the sheet does not present interconnections between the top and the bottom.

Therefore, in the state of the art there continues to be a need for processes for the manufacture of polymeric foam sheets avoiding or reducing the formation of non-foamed solid skins and allowing said processes to be useful for the manufacture of polymeric foam sheets of different sizes and/or thicknesses (from thin sheets with thicknesses less than 0.4 mm to samples of thick sheets with any thickness above said value of 0.4 mm, preferably with a thickness between 0.4 mm and 4 mm) and with a homogeneous porosity, including its surface. Consequently, the technical problem present in the state of the art would be to provide a process for the manufacture of polymeric foam sheets that makes it possible to obtain said foams directly without non-foamed solid skins (without the need for subsequent treatments to remove the solid skins), without limitation on the size, thickness and/or thickness of the polymeric foam and ensuring a homogeneous porosity in the sheets (including their surface).

Description of the invention

The inventors of the present invention, after extensive and exhaustive experiments, have discovered a process for manufacturing polymeric foam sheets based on the addition of a polymeric gas diffusion barrier, such as a polymer with low gas diffusivity, on top of the surfaces of the polymer sheet to be foamed. In this way, the diffusion of the gas is avoided or delayed, thus maintaining high levels of gas concentration within the matrix to be foamed. Incorporation of the polymeric gas diffusion barrier can be accomplished by various techniques. Preferably, as will be seen below, the polymer with low gas diffusivity is polyvinyl alcohol (PVOH), more preferably thermoplastic PVOH.

The present invention therefore solves the problems present in the state of the art and makes it possible to provide a process for manufacturing sheets of cellular polymeric foam without non-foamed solid skins, with high homogeneity in the cellular structure and with the appearance of high porosity. on the surface. As a consequence of the modification of the foaming process by adding the step indicated above (addition of a gas diffusion polymeric barrier), the polymeric foam sheets obtained according to the manufacturing process of the present invention have improved properties (absence of non-foamed solid skins , greater variety of dimensions, thicknesses and/or thicknesses and homogeneous porosity including their surfaces) so it is possible to expand their range of applications, as indicated above. The materials obtained through the manufacturing process of the present invention can be used in various applications such as filtration, catalysis, sensors, gas separation, adhesives, thermal insulation and acoustic absorption. In addition, the manufacturing process of the present invention makes it possible to manufacture transparent nanocellular thin films with a great capacity for thermal insulation, which undoubtedly represent a great revolution in sectors such as construction, the automotive industry or aeronautics.

In a first aspect, the present invention refers to a process for manufacturing a polymeric foam sheet that comprises the following steps: a) totally or partially covering the surface of a sheet comprising a polymer capable of being foamed, with a polymeric gas diffusion barrier; b) foaming the sheet from step a), and

c) removing the polymeric gas diffusion barrier from the covered surface of the foam sheet from step b).

In a final aspect, the present invention relates to a polymeric foam sheet obtained by the manufacturing process of the present invention.

As used herein "thin sheet" and its plural refer to a sheet with a thickness of less than 0.4 mm, more preferably the thickness is less than 0.4 mm and greater than or equal to 50 microns, even more preferably the thickness is less than 0.4 mm and greater than or equal to 100 microns, regardless of the manufacturing method.

As used herein "thick sheet" and its plural refer to a sheet with a thickness of 0.4mm or greater, more preferably 0.4mm to 4mm, regardless of manufacturing method.

In the present text, unless otherwise indicated or otherwise derived from the explanation given, when talking about thickness, it refers to thickness before the foaming step of the manufacturing process of the present invention (step b) of according to what will be explained later in this text. As is known in the state of the art, the thickness, after the foaming step, increases, since an expansion of the material occurs with said foaming.

Detailed description of the invention

As previously indicated, in a first aspect, the present invention refers to a process for manufacturing a sheet of polymeric foam that comprises the following steps:

a) totally or partially covering the surface of a sheet comprising a polymer capable of being foamed, with a polymeric gas diffusion barrier;

b) foaming the sheet from step a); and

c) removing the polymeric gas diffusion barrier from the covered surface of the foam sheet from step b).

The polymeric gas diffusion barrier prevents or at least delays the diffusion out of the sheet of the gas used in the foaming long enough to allow the formation and growth of the cells, thus allowing a correct and adequate foaming of the sheet, including the outermost areas of the central lamina. The incorporation of the gas diffusion polymeric barrier increases the gas concentration in the outer areas of the sheet, which produces an increase in nucleation and a reduction in the density of the central sheet without the need for additives, above all, but not exclusively, in thin sheets.

Preferably, the manufacturing process of the present invention is for manufacturing a microcellular or nanocellular, more preferably nanocellular, polymeric foam sheet.

In step a) of the manufacturing process of the present invention it is contemplated that the sheet comprises a single foamable polymer or a combination thereof, more preferably the sheet comprises a single foamable polymer.

In an embodiment in step a) of the manufacturing process of the present invention, the sheet comprising a polymer capable of being foamed is preferably a thin sheet, preferably with a thickness of less than 0.4 mm, more preferably the thickness is less than to 0.4 mm and greater than or equal to 50 microns, even more preferably the thickness is less than 0.4 mm and greater than or equal to 100 microns.

In another embodiment in step a) of the manufacturing process of the present invention, the sheet comprising a polymer capable of being foamed is preferably a thick sheet, more preferably with a thickness of 0.4 mm or greater, more preferably 0 .4mm to 4mm.

In step a) the polymer capable of being foamed can be any polymer known in the state of the art and that can be foamed, or combinations thereof. Preferably, in step a) of the manufacturing process of the present invention, the polymer capable of being foamed is selected from among amorphous and semi-crystalline polymers capable of being foamed in the gas solution foaming process; more preferably polymethyl methacrylate (PMMA), polycaprolactone (PCL), polystyrene (PS), polyetherimide (PEI), polycarbonate (PC), polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polyethylene terephthalate (PET) , thermoplastic polyurethane (TPU), polydimethylsiloxane (PDMS), natural rubber (NR), as well as copolymers, blends and composites derived therefrom, more preferably PMMA, PCL, PS, PC, as well as copolymers, blends and composites derived therefrom thereof. Even more preferably, in step a) of the manufacturing process of the present invention, the polymer capable of being foamed is PMMA. Therefore, in the most preferred embodiment, in the present manufacturing process in step a) the sheet comprises PMMA, more preferably, the sheet consists of PMMA.

In relation to the aforementioned PMMA, it preferably has a density between 1 and 2 g/cm3, more preferably it has a density of 1.19 g/cm3.

Also in relation to PMMA, preferably, said PMMA has a molecular weight between 50,000 and 150,000 g/mol, more preferably said PMMA has a molecular weight between 77,000 and 89,000 g/mol, still more preferably said PMMA has a molecular weight of 83,000 g/mol.

The polymeric gas diffusion barrier used in stage a) of the manufacturing process of the present invention has a very low or zero diffusivity to the gas or gases used for foaming in stage b). Preferably, the gas diffusion polymeric barrier used in step a) of the manufacturing process of the present invention has a lower diffusivity to the gas or gases used for foaming in step b) than the polymer capable of being foamed (included in section leaf).

Preferably, in stage b) CO2 is used for foaming. In this embodiment, the polymeric gas diffusion barrier is a polymeric diffusion barrier with very low or no CO2 diffusivity (preferably, a lower CO2 diffusivity than the foamable polymer (included in the sheet)).

In a preferred embodiment of the manufacturing process of the present invention, the gas diffusion polymeric barrier in stage a) comprises a polymer with very low or zero diffusivity to the gas or gases used for foaming in stage b), preferably a polymer with a very low or zero diffusivity of CO2, more preferably, the polymeric gas diffusion barrier in step a) comprises polyvinyl alcohol (hereinafter, PVOH, a polymer characterized by having very low CO2 diffusivities), polyethylene glycol (PEG), polyvinylpyrrolidone (PVP) or combinations thereof; more preferably, PVOH; more preferably, thermoplastic PVOH. In the most preferred embodiment, the polymeric gas diffusion barrier in step a) consists of PVOH, even more preferably the polymeric gas diffusion barrier in step a) consists of thermoplastic PVOH.

With respect to PVOH (preferably thermoplastic PVOH), it preferably has a molecular weight between 20,000 and 120,000 g/mol.

In a preferred embodiment, the thickness of the polymeric gas diffusion barrier (preferably PVOH, more preferably thermoplastic PVOH) is between 5 and 1,000 µm, more preferably between 20 and 300 µm, even more preferably between 50 and 200 µm.

In a preferred embodiment, in step a) of the manufacturing process of the present invention, the gas diffusion polymeric barrier completely covers the surface of the sheet that comprises a polymer capable of being foamed.

In another preferred embodiment, in step a) of the manufacturing process of the present invention, the polymeric gas barrier partially covers the surface of the sheet that comprises a polymer capable of being foamed. In this case, in step a) of the manufacturing process of the present invention, preferably the gas diffusion polymeric barrier totally or partially covers the surface of at least one of the two faces with the largest surface area of the sheet comprising a polymer capable of being foamed, more preferably, totally or partially covers the surface of the two faces with the largest surface area of the sheet comprising a polymer capable of being foamed, still more preferably, it completely covers the surface of the two faces with the largest surface area of the sheet.

Preferably, the polymeric gas diffusion barrier is at least one polymeric gas diffusion barrier film, more preferably two polymeric gas diffusion barrier films, as indicated above. Each of said two gas diffusion polymeric barrier films preferably fully or partially covers the surface of one of the two larger surface faces of the sheet, more preferably fully covering them. Additionally, each of said two gas diffusion polymeric barrier films preferably has a thickness of between 5 and 1,000 µm, more preferably between 20 and 300 µm, even more preferably between 50 and 200 µm.

It is contemplated that step a) of the manufacturing process of the present invention is carried out by any method known in the state of the art. Preferably, step a) is carried out by solvent evaporative deposition, spray impregnation, thermoforming, extrusion or co-extrusion; more preferably, thermoformed.

In a first preferred embodiment, in step a) of the manufacturing process of the present invention, the polymeric gas diffusion barrier is applied to the surface of the sheet by means of the solvent evaporative deposition method. In this case, a certain diluted amount of the gas diffusion polymeric barrier is deposited in a mold, applying a certain temperature to the whole, evaporation of the solvent is achieved and the formation of the corresponding film. Once dry, proceed in the same way to provide the polymer sheet capable of being foamed and another film of the gas diffusion polymeric barrier. The result is the superposition of the three compacted layers. Therefore, in this preferred embodiment, stage a) has the following sub-stages:

- ai) providing a mold;

- 82) depositing the desired quantity of the polymeric gas diffusion barrier diluted in a solvent in the mold;

- a3) evaporating the solvent to generate a film of the gas diffusion polymeric barrier, more preferably the evaporation is carried out at atmospheric pressure and at a temperature between 20 and 50 °C, more preferably the evaporation is carried out under pressure atmospheric and at a temperature of 25 °C;

- a4) depositing the desired quantity of the polymer capable of being foamed diluted in a solvent on top of the film of the gas diffusion polymeric barrier generated in step a3);

- a5) evaporating the solvent to generate a polymer sheet capable of being foamed, more preferably the evaporation is carried out at atmospheric pressure and at a temperature between 20 and 50 °C, more preferably the evaporation is carried out at atmospheric pressure and at a temperature of 25 °C;

- a6) depositing the desired quantity of the polymeric gas diffusion barrier diluted in a solvent, on top of the foamable polymer sheet generated in step a5);

- a?) evaporating the solvent from the gas diffusion polymeric barrier deposited in step a6) (more preferably the evaporation is carried out at atmospheric pressure and at a temperature between 20 and 50 °C, more preferably the evaporation is carried out at carried out at atmospheric pressure and a temperature of 25 °C), to thus generate a sheet of the polymer capable of being foamed whose surface is totally or partially covered with the polymeric gas diffusion barrier (preferably, the polymeric gas diffusion barrier covers totally or partially the surface of the two faces with the largest surface of the sheet, even more preferably, it totally covers the surface of the two faces with the largest surface of the sheet).

This first preferred embodiment is especially useful for sheets of the foamable polymer of small thicknesses, more preferably thin sheets (sheets less than 0.4 mm thick).

In this first preferred embodiment, the foamable polymer sheet and the polymeric gas diffusion barrier are as explained above.

In this first embodiment, the solvent used for the gas diffusion polymeric barrier and the foamable polymer is any of those available in the state of the art that is compatible and does not unacceptably alter the characteristics of said gas diffusion polymeric barrier. gas diffusion or foamable polymer, respectively. More preferably, the solvent used with the polymeric gas diffusion barrier is distilled water and the solvent used with the foamable polymer is chloroform.

Alternatively, in a second preferred embodiment, in step a) of the manufacturing process of the present invention, it is carried out by means of the solvent evaporation deposition method, but the gas diffusion polymeric barrier is applied on the surface of a sheet of polymer capable of being foamed previously manufactured (preferably, by thermo-forming). In this case, the surface of one of the two faces with the largest surface of the sheet of polymer capable of being foamed is superimposed on a solution of the gas diffusion polymeric barrier. Upon evaporation of the solvent a film of the polymeric gas diffusion barrier is formed with the foamable polymer sheet adhered on top of it. By repeating the operation on the surface of the other face with a larger surface of the sheet, a sheet of the polymer capable of being foamed is obtained, totally or partially covered on its two faces with a larger surface by means of a gas diffusion polymeric barrier. Therefore, in this second preferred embodiment, step a) of the manufacturing process of the present invention has the following sub-steps:

- a1) providing a mold;

- a2) depositing the desired quantity of the gas diffusion polymeric barrier diluted in a solvent in the mold;

- a3) depositing a sheet of polymer capable of being foamed previously manufactured (preferably, by thermoforming), on one of its two faces with the largest surface, on the solution of the gas diffusion polymeric barrier from step a2);

- 84) evaporating the solvent to generate a film of the gas diffusion polymeric barrier that totally or partially covers the surface of one of the two faces with the largest surface area of the sheet, more preferably the evaporation is carried out at atmospheric pressure and at a temperature between 20 and 50 °C, more preferably the evaporation is carried out at atmospheric pressure and at a temperature of 25 °C;

- a5) depositing the desired quantity of the gas diffusion polymeric barrier diluted in a solvent on the surface of the other of the two faces with the largest surface area of the sheet;

- a6) evaporating the solvent from the gas diffusion polymeric barrier deposited in step a5) (more preferably the evaporation is carried out at atmospheric pressure and at a temperature between 20 and 50 °C, more preferably the evaporation is carried out at atmospheric pressure and at a temperature of 25 °C), to thus generate a sheet of polymer capable of being foamed whose surface is fully or partially covered with the polymeric gas diffusion barrier (preferably, the polymeric gas diffusion barrier completely covers or partially the surface of the two faces with the largest surface of the sheet, even more preferably, it completely covers the surface of the two faces with the largest surface of the sheet).

This second preferred embodiment is useful for sheets of the foamable polymer of any thickness, more preferably thick sheets (thickness 0.4 mm or greater).

In this second preferred embodiment, the foamable polymer sheet and the polymeric gas diffusion barrier are as explained above.

In this second embodiment, the solvent used for the polymeric gas diffusion barrier is any of those available in the state of the art that is compatible and does not unacceptably alter the characteristics of said polymeric gas diffusion barrier. More preferably, the solvent used with the polymeric gas diffusion barrier is distilled water.

In a third preferred embodiment, in step a) of the manufacturing process of the present invention, a certain amount of a solution of the gas diffusion polymeric barrier is deposited on the surface of one of the two faces with the largest surface of the sheet, once dried at a certain temperature, the sheet presents a thin film of the gas diffusion polymeric barrier adhered to the surface of one of the two faces with the largest surface area. Preferably, the procedure is repeated with the other side of the sheet with the largest surface area. Therefore, in this third preferred embodiment, step a) of the manufacturing process of the present invention has the following sub-steps:

- a1) providing a sheet of the polymer capable of being foamed previously manufactured (preferably, by thermoforming);

- a2) depositing the desired quantity of the gas diffusion polymeric barrier diluted in a solvent on the surface of one of the two faces with the largest surface area of the sheet;

- a3) evaporating the solvent to generate a film of the gas diffusion polymeric barrier that totally or partially covers the surface of one of the two faces with the largest surface area of the sheet, more preferably drying is carried out at atmospheric pressure and at a temperature between 20 and 50 °C, more preferably the drying is carried out at atmospheric pressure and at a temperature of 25 °C;

- a4) depositing the desired quantity of the gas diffusion polymeric barrier diluted in a solvent on the surface of the other of the two faces with the largest surface area of the sheet;

- a5) evaporating the solvent from the gas diffusion polymeric barrier deposited in step a4) (more preferably the evaporation is carried out at atmospheric pressure and at a temperature between 20 and 50 °C, more preferably the evaporation is carried out at atmospheric pressure and a temperature of 25 °C), to thus generate a sheet of polymer capable of being foamed whose surface is fully or partially covered with the polymeric gas diffusion barrier (preferably, the polymeric gas diffusion barrier completely covers or partially the surface of the two faces with the largest surface of the sheet, even more preferably, it completely covers the surface of the two faces with the largest surface of the sheet).

This third preferred embodiment is useful for sheets of the foamable polymer of any thickness. It is contemplated that in this embodiment the polymer sheet in this case is both a thin sheet (thickness less than 0.4mm) and a thick sheet (thickness 0.4mm or greater).

In this third preferred embodiment, the foamable polymer sheet and the polymeric gas diffusion barrier are as explained above.

In this third embodiment, the solvent used for the gas diffusion polymeric barrier is any of those available in the state of the art that is compatible and does not unacceptably alter the characteristics of said gas diffusion polymeric barrier. More preferably, the solvent used with the polymeric gas diffusion barrier is distilled water.

In a fourth preferred embodiment, step a) of the manufacturing process of the present invention is carried out by means of spray impregnation. In this case, the application of the polymeric gas diffusion barrier is carried out, preferably by impregnation by airbrush. After pulverizing the surface of one of the two faces with the largest surface of the sheet, it is dried. Preferably, the method (spraying and drying) is repeated with the surface of the other face with a greater surface area. In this case the thickness of the gas diffusion polymeric barrier is controlled by the concentration of the solution, the flow, the distance and the time of application on the sheet. Therefore, in this fourth preferred embodiment, step a) of the manufacturing process of the present invention has the following sub-steps:

- a1) providing a sheet of the polymer capable of being foamed previously manufactured (preferably, by thermoforming);

- a2) spraying the desired quantity of the polymeric gas diffusion barrier diluted in a solvent on the surface of one of the two faces with the largest surface area of the sheet (preferably, the spraying is carried out by impregnation with an airbrush);

- a3) evaporating the solvent to generate a film of the gas diffusion polymeric barrier, more preferably the drying is carried out under pressure atmospheric and at a temperature between 20 and 50 °C, more preferably the drying is carried out at atmospheric pressure and at a temperature of 25 °C; - a4) spraying the desired amount of the polymeric gas diffusion barrier diluted in a solvent on the surface of the other of the two faces with the largest surface area of the sheet (preferably, the spraying is carried out by impregnation with an airbrush);

- a5) evaporating the solvent from the polymeric gas diffusion barrier sprayed in step a4) (more preferably the evaporation is carried out at atmospheric pressure and at a temperature between 20 and 50 °C, more preferably the evaporation is carried out at atmospheric pressure and at a temperature of 25 °C), to generate a sheet of the polymer capable of being foamed whose surface is totally or partially covered with the polymeric gas diffusion barrier (preferably, the polymeric gas diffusion barrier covers totally or partially the two faces with the largest surface of the sheet, even more preferably, it completely covers the two faces with the largest surface of the sheet).

This fourth preferred embodiment is useful for sheets of the foamable polymer of any thickness. It is contemplated that in this embodiment the polymer sheet in this case is both a thin sheet (thickness less than 0.4mm) and a thick sheet (thickness 0.4mm or greater).

Additionally, this fourth preferred embodiment allows a chain sprayer with its subsequent drying, thus allowing continuous production, demonstrating great utility in scaling up to an industrial level.

In this fourth embodiment, in steps a2) and a4), spraying by airbrush impregnation was preferably carried out under the following conditions:

- Airbrush-sheet distance comprising polymer capable of being foamed:

between 10 and 30 cm, more preferably 20 cm.

- Concentration of polymeric gas diffusion barrier in the sprayed solution: between 2 and 20% by weight (mass/mass, hereinafter m/m), preferably between 5 and 15% by weight (m/m).

- Amount of solution deposited per surface unit (of the sheet comprising a polymer capable of being foamed): between 0.5 and 3 mL/cm2, more preferably between 1 and 2 mL/cm2, even more preferably 1.33 ml /cm2.

In this fourth preferred embodiment, the foamable polymer sheet and the polymeric gas diffusion barrier are as explained above.

In this fourth embodiment, the solvent used for the gas diffusion polymeric barrier is any of those available in the state of the art that is compatible and does not unacceptably alter the characteristics of said gas diffusion polymeric barrier. More preferably, the solvent used with the polymeric gas diffusion barrier is distilled water.

In a fifth preferred embodiment, step a) of the manufacturing process of the present invention is carried out by thermoforming. In this case, first of all, the films of the gas diffusion polymeric barrier and the foamable polymer sheet are previously produced separately. Next, the films and the sheet are adhered by applying pressure and temperature with, preferably a hydraulic press, to achieve compaction between the 3 components. During this procedure, the temperature and pressure parameters are adequately controlled in order to avoid the degradation of the materials and to achieve good adhesion between them. Preferably, the pressure is 0.98 MPa and the temperature is between 80 and 90 °C. Also, preferably, to avoid crushing and damaging the sheet, a mold with a specific thickness suitable for the sheet is used (preferably, a PMMA sheet).

Therefore, in this fifth preferred embodiment, step a) of the manufacturing process of the present invention has the following sub-steps:

- a1) provide a sheet of polymer capable of being foamed as well as at least one gas diffusion polymeric barrier film, all of them obtained by means of any technique available in the state of the art, preferably, by thermo-forming or by deposition by evaporation of the solvent, even more preferably by thermo-forming;

- a2) contacting the sheet of polymer capable of being foamed and the at least one gas diffusion polymeric barrier film, such that the at least one gas diffusion polymeric barrier film totally or partially covers the surface of sheet of foamable polymer; - a3) adhesion of the foamable polymer sheet and the at least one gas diffusion polymeric barrier film by pressure and temperature (preferably, the pressure is 0.98 MPa and the temperature is between 80 and 90 °C). Also, preferably, to avoid crushing and damaging the sheet, a mold with a specific thickness suitable for the sheet is used (preferably, PMMA sheet));

This fifth preferred embodiment is useful for sheets of the foamable polymer of any thickness. It is contemplated that in this embodiment the polymer sheet in this case is both a thin sheet (thickness less than 0.4mm) and a thick sheet (thickness 0.4mm or greater).

In this fifth preferred embodiment, preferably the at least one gas diffusion polymeric barrier film is two gas diffusion polymeric barrier films. In this case, in step a2) each film of the gas diffusion polymeric barrier is brought into contact with the surface of one of the two faces with the largest surface area of the sheet and in step a3) they are adhered to them, in such a way that in such a way that a sheet is obtained in which the surface of its two faces with the largest surface area is totally or partially covered with polymeric gas diffusion barrier, preferably totally covered.

In this fifth preferred embodiment, the foamable polymer sheet and the polymeric gas diffusion barrier are as explained above.

In a sixth preferred embodiment, step a) of the manufacturing process of the present invention is carried out by extrusion, more specifically, by extrusion of the polymer sheet capable of being foamed and the polymeric gas diffusion barrier and its subsequent compaction using the technique of molding by compression, so that a sheet of polymer capable of being foamed is generated whose surface is totally or partially covered with the polymeric gas diffusion barrier (preferably, the polymeric gas diffusion barrier covers totally or partially the two faces with the largest surface of the sheet, even more preferably, it completely covers the two faces with the largest surface of the sheet).

In this sixth preferred embodiment, the foamable polymer sheet and the polymeric gas diffusion barrier are as explained above.

In a seventh preferred embodiment, step a) of the manufacturing process of the present invention is carried out by co-extrusion of the sheet comprising a polymer capable of being foamed and the gas diffusion polymeric barrier. This embodiment makes it possible to directly obtain the sheet comprising the polymer capable of being foamed and the polymeric gas diffusion barrier joined together.

In this seventh preferred embodiment, the foamable polymer sheet and the polymeric gas diffusion barrier are as explained above.

In any of the embodiments explained above, it is contemplated that the process comprises additional steps and/or sub-steps to improve the adhesion between the sheet comprising at least one polymer capable of being foamed and the gas diffusion polymeric barrier. Examples of said additional steps and/or sub-steps would be: chemical etching of the sheet comprising at least one polymer capable of being foamed, for example, by immersing the sheet in a solution or using an airbrush, leading to the rupture of certain chemical bonds and the creation of attractive forces at the interface allowing better adhesion.

In the manufacturing process of the present invention, it is contemplated that stage b) of foaming is carried out by any of the foaming methods known in the state of the art. Preferably, in step b) the foaming is carried out by extrusion foaming, injection foaming or foaming. by gas solution, more preferably, in step b) the foaming is carried out by gas solution foaming.

On the other hand, in step b) of the manufacturing process of the present invention, foaming can be done using any gas known in the state of the art and useful for this purpose. Preferably, in stage b) of the manufacturing process of the present invention, CO ₂ is used to foam the sheet of stage a).

Therefore, in the most preferred embodiment, step b) of the manufacturing process of the present invention is carried out by gas solution foaming using CO2.

Foaming by gas solution, as previously indicated, comprises three sub-stages: saturation, desorption and foaming, which are as explained above.

Preferably, the saturation sub-step is carried out at a pressure between 10 and 70 MPa and at a temperature between -50 and 80 °C, more preferably at a pressure between 20 and 30 MPa and at a temperature between -30 and 60°C

Also preferably, in gas solution foaming, in the foaming sub-step, the temperature is between 20 and 200°C, more preferably between 60 and 100°C.

It is contemplated that the manufacturing process of the present invention comprises, after step b), a step c) of elimination of the polymeric gas diffusion barrier. Said stage of elimination of the gas diffusion polymeric barrier will depend on the characteristics of said barrier. In a preferred embodiment, in step c) the elimination of the polymeric gas diffusion barrier is carried out in a water bath (preferably distilled water), preferably with stirring (preferably between 300 and 600 rpm) or ultrasound, raising the temperature above room temperature (this facilitates the dissolution of the polymeric gas diffusion barrier (preferably PVOH) in water and therefore its removal from the foamed sheet). Obviously, the temperature must be such that it allows the elimination of the polymeric gas diffusion barrier without affecting the polymeric foam sheet (without it plasticizing or sheathing and/or avoiding the degradation of its cellular structure) or causing its degradation. Preferably, the temperature is increased to between 20 and 100°C, more preferably between 20 and 50°C.

It is contemplated that between stages a), b) and c) there may or may not be intermediate stages. Additionally, it is also contemplated that there may or may not be additional steps at the start or end of the manufacturing process of the present invention.

In an even more preferred embodiment, the manufacturing process of the present invention consists of steps a), b) and c), as explained above.

As is derived from what has been explained above and from the results included in the examples, the manufacturing process of the present invention makes it possible to solve the technical problems present in the state of the art and explained above. More specifically, the manufacturing process of the present invention makes it possible to obtain sheets of polymeric foam:

- Without solid non-foamed skins or greatly reducing their presence. Additionally, the sheets obtained present surface porosity, providing a correct or adequate connection between the internal cellular structure of the sheet and the external environment.

- Without limitation on the thickness or thickness of said sheets, being able to obtain correctly foamed sheets and with a thickness or thickness from 50 micrometers to several millimeters, preferably between 100 micrometers and 4 mm.

- Correctly foamed regardless of the thickness or thickness of the sheet and/or the cell or pore size of the foam: a homogeneous foam is achieved (there are no internal gradients in cell or pore size due to gas diffusivity) and since the gas diffusion polymeric barrier is not rigid, it allows a correct expansion of the polymer of the sheet and a correct formation of the cells.

Additionally, given the aforementioned characteristics for the polymeric foam sheets obtained with the manufacturing process of the present invention, it is important to note that said films of the present invention have a wide range of new applications (for example, filtration, catalysis, gas storage, adhesives, sensors and transparent thermal insulation for windows) and have improved performance in other applications in which are currently used sheets of polymeric foam (for example, thermal and acoustic insulation).

Regarding the manufacturing process of the present invention, which, as indicated above, allows solving the problems present in the state of the art, it is worth noting that:

- It is not expensive and it is respectful with the environment.

- It is easily scalable at an industrial level.

- It is versatile and simple since it can be used, adapted or incorporated into different forms of foaming (for example, extrusion, injection or gas dissolution), for the production of improved materials, such as cellular polymers without solid skins.

- Allows the elimination of the non-foamed solid skin during the foaming process in a preventive way (avoiding or greatly reducing its formation) without the need for chemical or physical attacks on already foamed samples.

- It makes it possible to foam thin sheets of polymeric foam without any limitation regarding the minimum thickness of the sheet.

- It allows fully foamed sheets to be obtained with high homogeneity in the cell structure while reducing the density without limiting the expansion of the sheets, allowing expansions not achievable by conventional foaming techniques.

- The sheets obtained through the manufacturing process of the present invention can be used in various applications such as filtration, catalysis, sensors, gas separation, adhesives, thermal insulation and acoustic absorption.

In a second aspect, the present invention relates to a polymeric foam sheet obtained by means of a manufacturing process of the present invention.

All the technical effects and uses mentioned above in the context of the manufacturing process of the present invention are directly applicable to the polymeric foam sheets of this second aspect of the invention.

Throughout the description and claims the word "comprises" and its variants are not intended to exclude other technical features. In addition, the word "comprises" includes the case "consists of".

In addition, the present invention covers all the possible combinations of particular and preferred embodiments explained above.

Description of the figures

For a better understanding, the present invention is described in more detail below with reference to the attached figures and the following non-limiting examples.

Figure 1 shows the different techniques or methods of incorporating the gas diffusion polymeric barrier (preferably PVOH, more preferably thermoplastic PVOH) on the surface of the sheet that comprises at least one polymer capable of being foamed (preferably PMMA ). That is, Figure 1 shows different ways of carrying out step a) of the manufacturing process of the present invention. Specifically, Figure 1A shows a variant of the deposition method by solvent evaporation, solution on solution; Figure 1B shows a variant of the sheet solution, solvent evaporative deposition method; Figure 1C shows another variant of the solvent evaporative deposition method, sheet solution; Figure 1D shows a variant of the spray impregnation method; and Figure 1E shows a variant of the thermoforming method.

More specifically, Figure 1A shows how a first sufficient quantity of the polymeric gas diffusion barrier (preferably PVOH, more preferably thermoplastic PVOH) diluted in a solvent (1) is deposited in a suitable mold (2). Next, a certain temperature is applied so that the solvent evaporates and thus a first film of diffusion polymeric barrier is formed. gases (4). Then, a sufficient quantity of the sheet comprising at least one polymer capable of being foamed (preferably PMMA) diluted in a solvent (21) is deposited on top of said first gas diffusion polymer barrier film (4). a certain temperature is applied so that the solvent evaporates and thus a sheet is formed that comprises at least one polymer capable of being foamed (3) attached or adhered to the first gas diffusion polymeric barrier film (4) that had previously been formed , to one of its faces with the largest surface. Next, a second sufficient quantity of the gas diffusion polymeric barrier diluted in a solvent (5 ) and a certain temperature is applied so that the solvent evaporates and thus forms a second gas diffusion polymeric barrier film (6) attached or adhered to the sheet that comprises at least one polymer capable of being foamed (3).

Figure 1B shows how a first sufficient quantity of the polymeric gas diffusion barrier (preferably PVOH, more preferably thermoplastic PVOH) diluted in a solvent (1) is deposited in a suitable mold (2). Next, the sheet comprising at least one polymer capable of being foamed (preferably PMMA) (3) (previously manufactured, by thermoforming or by the solvent evaporation method) is deposited using one of its two faces with the largest surface area and A certain temperature is applied so that the solvent evaporates and thus forms a first gas diffusion polymeric barrier film (4) attached or adhered to the sheet that comprises at least one polymer capable of being foamed (3). Next, a second sufficient amount of the polymeric gas diffusion barrier (preferably PVOH, more preferably thermoplastic PVOH) diluted in a solvent (5) is deposited in another suitable mold (7) and the solution is deposited on top of said solution. sheet comprising at least one polymer capable of being foamed (3) mentioned above, using its other face with the largest surface area (the one that does not have the first gas diffusion polymeric barrier film adhered to it (4)). Then, a certain temperature is applied so that the solvent evaporates and thus forms a second gas diffusion polymeric barrier film (6) attached or adhered to the sheet that comprises at least one polymer capable of being foamed (3).

Figure 1C shows how a first sufficient quantity of the gas diffusion polymeric barrier (preferably PVOH, more preferably thermoplastic PVOH) diluted in a solvent (1) is deposited on one of the faces with the largest surface of a sheet. comprising at least one polymer capable of being foamed (preferably PMMA) (3). Once dried at a certain temperature, a first film of the gas diffusion polymeric barrier (4) is formed, adhered to said sheet (3). As derived from Figure 1C, the procedure is repeated with the other side with the largest surface area of the sheet that comprises at least one polymer capable of being foamed (3) to generate a second film of the gas diffusion polymeric barrier (6 ).

In Figure 1D an airbrush (8) is used to spray a first sufficient quantity of the gas diffusion polymeric barrier (preferably PVOH, more preferably thermoplastic PVOH) diluted in a solvent (1) on one of the faces with the largest surface of a sheet comprising at least one polymer capable of being foamed (preferably PMMA) (3). Once dried at a certain temperature, a first film of the gas diffusion polymeric barrier (4) is formed, adhered to said sheet (3). As derived from Figure 1D, the procedure is repeated with the other side with the largest surface area of the sheet that comprises at least one polymer capable of being foamed (3) to generate a second film of the gas diffusion polymeric barrier (6 ).

Figure 1E shows how a first film of the polymeric gas diffusion barrier (preferably PVOH, more preferably thermoplastic PVOH) (4) and a second film of the polymeric gas diffusion barrier (preferably PVOH, more preferably thermoplastic) thermoplastic PVOH) (6) are each placed covering one of the two faces with the largest surface area of a sheet that comprises at least one polymer capable of being foamed (preferably PMMA) (3). Next, with a hydraulic press (9), said films (4 and 6) are pressed onto the sheet (3) so that they remain adhered or joined.

Figure 2 shows a scheme of the co-extrusion process to incorporate the gas diffusion polymeric barrier (preferably PVOH, more preferably thermoplastic PVOH) on the surface of the sheet that comprises at least one polymer capable of being foamed (preferably , PMMA). The process Co-extrusion is based on the combination of two separate extrusion processes. Extrusion processes allow working with materials above their melting point or glass transition point in order to mold them and obtain that material in the desired shape. In the co-extrusion process, each material is processed individually to the conditions that are required by the material itself, that is, the melting point or glass transition point must be exceeded depending on the material. The extrusion process consists of achieving a homogeneous mixture with a certain viscosity of the polymer by applying temperature and using a screw that rotates concentrically inside a tube at a constant speed. In addition, the extruder can have a double screw for improved mixing. Both parameters, temperature and spindle speed, are controlled in order to achieve a homogeneous mix of the material. Finally, the material leaves the extruder through a die that can be shaped into the desired shape, for example, in the form of a sheet or film. In the case of co-extrusion, shown in Figure 2, the two materials are processed in two different extruders (10 and 11), each with a screw (12 and 13), but the outlet nozzles (14, 15 and 16 ) of both materials are superimposed in such a way that the desired structure is generated continuously, without the need for additional manipulation: at the upper and lower ends, the first and second film of the gas diffusion polymeric barrier (4 and 6) and in the center, contained between said films (4 and 6) the sheet comprising at least one polymer capable of being foamed (preferably PMMA) (3). Thanks to this process, the 3 layers are overlapped one on top of the other, forming a sandwich. Using suitable process conditions for the two materials that make up the structure, good adhesion between them can be achieved.

Figure 3 shows a schematic of the foaming stage when the method used for foaming is gas solution foaming. Figure 3 shows the saturation substep (A), the desorption substep (B) and the foaming substep (C). In the saturation substage (A) it is observed how the sheet comprises at least one polymer capable of being foamed (preferably PMMA) (3) covered with gas diffusion polymeric barrier films (preferably PVOH, more preferably thermoplastic PVOH) (4 and 6) is introduced into a suitable container (17) together with the corresponding gas (preferably CO2) (18) at a suitable pressure and temperature to achieve maximum solubility of the gas (18) in the sheet that it comprises at least one polymer capable of being foamed (3).

In the desorption substage (B) the pressure is reduced and it is observed how the gas (18) is supersaturated inside the sheet that comprises at least one polymer capable of being foamed (3), consequently, the separation between both takes place. phases and the appearance of nucleation. At the same time, as explained above, in this substage a diffusion process of the gas (18) dissolved in the sheet that comprises at least one polymer capable of being foamed (3) towards the outside is taking place. In the foaming substage (C) shown in Figure 3, the sheet comprising at least one polymer capable of being foamed (3) covered with the gas diffusion polymeric barrier films (4 and 6) is introduced into a bath thermal (19) to increase the temperature and thus provide the necessary energy above a threshold so that the nuclei formed during the previous desorption substage (B) can grow and form the cells (20).

Figure 4 shows a micrograph of scanning electron microscopy in which the effect on foaming of using or not using PVOH as a polymeric gas diffusion barrier for the manufacture of a foamed PMMA sheet is compared. In this sense, the upper surface is the surface without PVOH and the lower surface is the surface with PVOH. In PMMA foaming, the conditions were: saturation pressure = 30 MPa, saturation temperature = 25 °C, foaming temperature = 60 °C.

Figure 5 shows scanning electron microscopy micrographs in which the effect on foaming of using or not using PVOH as a gas diffusion polymeric barrier for the manufacture of a foamed PMMA thin sheet is compared. Figure 5A shows a foamed PMMA sheet without PVOH; and Figure 5B shows a sheet of PMMA foamed with PVOH as a polymeric barrier for gas diffusion (in the image the PVOH is not seen because after the foaming of the PMMA, said PVOH has been removed). In PMMA foaming, the conditions were: saturation pressure = 30 MPa, saturation temperature = 25 °C, foaming temperature = 60 °C.

Figure 6 shows scanning electron micrographs comparing the surface of a foamed PMMA sheet without PVOH (Figure 6A); and a sheet of PMMA foamed with PVOH as a polymeric gas diffusion barrier (Figure 6B). In the foaming of PMMA, the conditions were: pressure of saturation = 30 MPa, saturation temperature = 60 °C. In this case the samples were foamed in a single stage (saturation and foaming were carried out at the same time in a single stage).

Figure 7 shows the thickness of the non-foamed solid skin of different sheets: sheets 1 to 7 of foamed PMMA with PVOH as a gas diffusion polymeric barrier (the details of sheets 1 to 7 are reflected in Table 3 of this document ); and sheets 9 to 11 of foamed PMMA without PVOH (details of sheets 9 to 11 appear in Table 3 of this document). The sheet number appears or is indicated on the abscissa axis; and the thickness of the non-foamed solid skin of the concrete sheet is indicated on the ordinate axis, as a percentage (percentage of thickness of the non-foamed solid skin in relation to the total thickness of the foamed concrete sheet). In this case, both thicknesses (solid skin and full sheet) are after foaming.

Figure 8 shows the relationship between the relative density and the thickness of the non-foamed solid skin in the sheets 1 to 11 indicated in Table 3 of the present document. The round ones refer to sheets of PMMA foamed with PVOH as a polymeric gas diffusion barrier; and the squares refer to sheets of foamed PMMA without PVOH. The abscissa refers to the thickness of the solid non-foamed skin of the concrete sheet, as a percentage (percentage of thickness of the solid non-foamed skin in relation to the total thickness of the concrete sheet). The ordinate axis shows the relative density of each of the sheets (dimensionless, coefficient between the density of the foamed sheet and the density of the unfoamed sheet). In this case, both thicknesses (solid skin and full sheet) are after foaming.

Figure 9 shows scanning electron micrographs of a PVOH-free foamed PMMA sheet (Figure 9A general view of the sheet; Figure 9B thickness of the non-foamed solid skin of the sheet; and Figure 9C sheet surface) and of a sheet of PMMA foamed with PVOH as a gas diffusion polymeric barrier produced by the thermoforming technique (Figure 9D general view of the sheet; Figure 9E thickness of the non-foamed solid skin of the sheet; and Figure 9F surface of the leaf). In PMMA foaming, the conditions were: saturation pressure = 30 MPa, saturation temperature = 25 °C, foaming temperature = 60 °C.

Figure 10 shows a scanning electron micrograph comparing the effect on foaming of using or not using PVOH as a polymeric gas diffusion barrier for the manufacture of a foamed PMMA sheet. In this sense, the upper surface is the PVOH foamed surface and the lower surface is the non-PVOH foamed surface. In PMMA foaming, the conditions were: saturation pressure = 70 MPa, saturation temperature = 25 °C, foaming temperature = 25 °C.

Figure 11 shows scanning electron microscopy micrographs of a foamed PMMA sheet on one of its larger surface areas without PVOH (Figure 11A general view of the sheet; Figure 11C detail of the surface) and, on the other, with PVOH (Figure 11B general view of the sheet; Figure 11D detail of the surface). In PMMA foaming, the conditions were: saturation pressure = 20 MPa, saturation temperature = -50 °C, foaming temperature = 60 °C.

EXAMPLES

Example 1: Gas solution foaming process as a method of manufacturing polymeric foams

The polymeric base chosen in this example and in which it is intended to reduce and eliminate the non-foamed solid skin, has been PMMA. The choice of this polymer is due to the fact that it is a widely known material used in the gas solution foaming process. The density of the PMMA used was around 1.19 g/cm3. The molecular weight was 83,000 g/mol. The solvent used for PMMA, where applicable, was chloroform.

On the other hand, as a polymeric gas diffusion barrier, a polymer with very low CO2 diffusivity such as PVOH was selected. Two types of thermoplastic PVOH were used in order to test the effectiveness of this material as a gas diffusing barrier. The molecular weight of the PVOH used ranged from 20,000-120,000 g/mol. The solvent used for the PVOH, when applicable, was distilled water.

Production of solid precursors

The PMMA sheets were produced using thermoforming techniques (for this, the PMMA sheet was kept for 10 minutes at 250 °C at atmospheric pressure; then 1 minute at 250 °C and a pressure of 2.18 MPa; and finally, 3 minutes at 25 °C and a pressure of 2.18 MPa) and by deposition by solvent evaporation (as explained earlier in this document, carrying out drying or evaporation at 25 °C and at atmospheric pressure). In the first case, sheets with a minimum thickness of 0.4 mm were obtained and in the second case sheets with a thickness between approximately 100 and 400 micrometers.

On the other hand, the PVOH sheets were produced in the form of films by thermoforming (for this, the PVOH film was kept for 10 minutes at 200 °C without pressure, then 1 minute at 200 °C and 2.18 MPa ; and finally 3 minutes at 25 °C and 2.18 MPa) and deposition by evaporation of the solvent (as explained previously in this document, carrying out drying or evaporation at 25 °C and at atmospheric pressure). A process of optimization of the thickness of the PVOH layers was carried out since the placement of a diffusion barrier that was too thick caused a significant increase in the saturation time of the PMMA sample. Therefore, it was necessary to find the balance that allowed PVOH to act as a diffusion barrier without reaching too high saturation times.

The incorporation of the PVOH on the surface of the PMMA was carried out by means of several techniques (deposition by solvent evaporation, thermoforming and impregnation by spraying; see Figure 1) with the aim of testing the versatility of the effect of the PVOH acting as a barrier of gas diffusion.

The incorporation of the PVOH onto the surface of the PMMA by means of solvent evaporation techniques was as previously explained in this document, the steps being drying or evaporation at 25 °C and atmospheric pressure.

The incorporation of the PVOH onto the surface of the PMMA by means of thermoforming was carried out as previously indicated in this document, using the following conditions: 6 minutes at 120 °C without pressure; then 1 minute at 120°C and 0.98MPa; and finally 3 minutes at 25 °C and 0.98 MPa,

The incorporation of the PVOH by spray impregnation was carried out as indicated hereinabove. More specifically, with the following characteristics:

- Airbrush-sheet distance comprising at least one polymer capable of being foamed: 20 cm.

- Concentration of polymeric gas diffusion barrier of the solution between 2 and 20% by weight (m/m), preferably between 5 and 15% by weight (m/m).

- Amount of solution deposited per surface unit of the sheet that comprises at least one polymer capable of being foamed: 3 mL on a 1.5x1.5 cm surface, that is, 1.33 mL/cm2.

Production of cellular polymers

The production of the cellular materials was carried out by means of the gas solution foaming process, as already indicated above. It must be taken into account that this process allows great control over the cell structure by modifying the parameters involved. Therefore, tests were carried out varying some of them with the aim of testing the versatility of the diffusion barrier in a wide range of situations (see Table 1). The foaming of the samples was carried out in one or two stages, using in the second case a thermal oil bath (instead of the water bath normally used), to prevent the dissolution of the PVOH during the foaming stage.

Regarding the thickness of PVOH used to act as a polymeric gas diffusion barrier, an optimal range of application was also determined (which allowed a compromise between achieving a good barrier effect against the diffusion of the gas outlet without increasing the time too much). of saturation of the matrix to foam). This optimum range in the thickness of the PVOH was set at between 5 and 1000 µm (see Table 1).

After foaming, the PVOH was removed (both in the present example and in all examples included herein). To do this, the samples were placed in distilled water at a temperature between 20 and 50 °C, paying attention to the thermal characteristics of the polymer to be foamed (PMMA) so as not to degrade the foam, and with stirring between 300 and 600 rpm ( revolutions per minute).

In order to contrast the effect of PVOH as a polymeric gas diffusion barrier, the analysis of the cell structure obtained (polymeric foam sheet) was carried out using the scanning electron microscopy technique. To do this, micrographs of both the section and the surface of the foamed sheets were taken in order to measure the thickness of the solid non-foamed skin and to confirm the interconnection of the interior cellular or pore structure with the exterior.

Table 1. Parameters of the foaming process by gas solution.

Effect of PVOH as a polymeric gas diffusion barrier

First of all, it is important to understand the qualitative effect offered by PVOH acting as a polymeric gas diffusion barrier. Gas diffusion in a flat thin film occurs mostly through the two faces with the largest surface area. Therefore, it can be considered that the diffusion process is limited to these two faces when the thickness of the sample is much smaller compared to its superficial dimensions, as is the case of the present invention (that is, as is the case of sheets). In order to observe the effect caused by PVOH as a polymeric gas diffusion barrier, a sheet was manufactured by covering it with PVOH (through solvent evaporative deposition, drying or evaporating at 25 °C and atmospheric pressure; the thickness of the film being of 50 μm PVOH) only on one of the main faces of the sheet in order to analyze the differences between both faces and understand the diffusion mechanisms that occur after the incorporation of PVOH. in the figure 4, a scanning electron microscopy micrograph of a sample where PVOH has been added to one of its faces (lower surface) can be seen. As can be seen in said Figure 4, the lower area of the polymeric matrix is fully foamed due to the effect of the PVOH, which significantly reduces the gas diffusivity through that surface and makes it possible to maintain an optimal gas concentration for foaming. However, the area where the diffusion barrier has not been placed (upper area) has a broad solid skin without foam. As previously mentioned, these differences are due to the diffusion process that occurs in the moments after depressurization, where part of the gas easily escapes through the surface without PVOH, radically reducing the concentration of CO ₂ in the area. and preventing foaming in that area. As a consequence, a solid non-foamed skin was formed on the entire upper part of the sample, which represents approximately 25% of the total thickness (after foaming) of the sample (Figure 4).

Also, it was possible to appreciate that, due to the same diffusion process, the cell structure presented a gradient in pore size as a consequence of the difference in gas concentration in each zone, resulting in a heterogeneous cell structure.

Therefore, it can be deduced that, by making full use of PVOH as a polymeric gas diffusion barrier, that is, placed on both sides with a greater surface area, the concentration gradient will disappear and the homogeneity of the entire sample will increase at the same time. while solid non-foamed skins from the edges are reduced and/or removed.

Example 2. Foaming of thin sheets

In the present example, the cellular structure of a thin sheet of PMMA without PVOH and another with the addition of PVOH on its surfaces or main faces is compared (Figure 5). Both thin sheets used in this experiment had an initial thickness (before foaming) of 100 microns.

In this example, PVOH has been added to the thin PMMA sheet using the thermoforming technique (according to what was explained in example 1) and the Foaming conditions were 30 MPa saturated pressure, 25 °C saturation temperature, and 60 °C foaming temperature.

On the one hand, it was observed that the thin sheet of PMMA not coated with PVOH barely foamed, only some porosity was intuited in the central area. On the other hand, the thin sheet to which the PVOH was incorporated as a polymeric gas diffusion barrier presented a very homogeneous cellular structure and foamed practically in its entirety (see Figure 5). Table 2 offers a comparison between the expansion experienced by the sheet, the relative density and the percentage of non-foamed solid skin with respect to the total thickness of the foam, being Esolid and Efoam the thicknesses of the precursor sheet and the foamed sheet, respectively. . The results show that the sum of the solid skins of the sheet with PVOH barely represented 7% of the total thickness of the sheet. While in the PMMA sheet without PVOH, more than 60% of the thickness failed to form any cellular structure (ie, failed to foam) (Table 2). In addition, it was observed that there is a significant difference in terms of expansion between both sheets. Additionally, after the incorporation of a gas diffusion polymeric barrier such as PVOH in thin sheets, a homogeneous cellular structure was achieved, reducing the thickness of the non-foamed solid skin and allowing a correct expansion of the sheet. Both results, the increase in expansion and the reduction of solid non-foamed skins, had a direct impact on a third parameter, which is the relative density (ratio between the density of the foamed sheet and that of the precursor sheet). As is logical, the reduction and elimination of the non-foamed solid skins caused an increase in the volume of the foamed zone and, therefore, a decrease in the total density was achieved. As can be seen, the relative density of a thin sheet of PMMA (without PVOH) is reduced from 0.899 to a value of 0.280 for the PMMA sheet with the PVOH coating of the present invention. Undoubtedly, it represents a very significant decrease in relative density, which is a very important parameter in the manufacture of cellular materials, since it is closely linked to the reduction in weight and therefore cost, and with the functionality of the material in the various applications raised throughout this report.

Table 2. Comparison of the expansion, the thickness of the non-foamed solid skin and the relative density between a thin sheet of PMMA without a PVOH coating and another of PMMA with a PVOH coating.

In addition to achieving the foaming of thin sheets of polymeric foam, one of the secondary objectives of the present invention would also be the interconnection between the cellular structure and the external environment. A clear evidence of this fact is the appearance of porosity on the surface of the sample where the PVOH layer was adhered. Figure 6 shows two images of the PMMA surface of both sheets. In the case of the foamed PMMA sheet without the PVOH coating, a perfectly smooth surface characteristic of solid skins that have failed to foam was observed (Figure 6A). While the PMMA sheet with the PVOH coating presented a surface full of porosity (Figure 6B). In fact, the dimensions of the surface pores in the PMMA sheet with the PVOH coating had similarity with the inner cell structure. This result undoubtedly demonstrates that the present invention makes it possible to manufacture totally porous polymeric foams, including thin sheets.

Given this evidence, it can be said that the incorporation of a PVOH gas diffusion polymer barrier proved to be an effective technique to achieve complete foaming of polymer sheets and avoid the formation of non-foamed solid skins, maintaining homogeneous structures, interconnecting the structure. cell with the external environment and without limiting the expansion as occurs in other procedures of the state of the art. It is important to indicate that the results obtained were independent of the PVOH incorporation technique used. However, the best results were obtained using the thermoforming technique (Figure 1E), which in turn has the advantage of being more easily scalable at an industrial level.

Example 3. Reduction and elimination of the non-foamed solid skin in thicker samples (thick sheets)

In accordance with the results obtained previously after the incorporation of PVOH in polymeric thin sheets, in this example the process of the present invention was tested also on thicker sheets. To do this, PMMA sheets with a PVOH coating were produced by increasing the thickness of the PMMA sheet, using different PVOH incorporation techniques. Subsequently, it was subjected to the gas dissolution foaming procedure together with PMMA sheets of the same thickness but without the PVOH coating, as a comparison.

Figure 7 shows the results of the skin thickness of different samples, both in the form of thin slices (6, 7 and 11) and thicker slices (1-5 and 8-10). Sheets 1-7 corresponded to PMMA with a PVOH coating where the PVOH was incorporated using the different techniques previously explained, while sheets 8-11 corresponded to PMMA without a PVOH coating. The results in Figure 7 correspond to sheets foamed under the same conditions: saturation pressure = 30 MPa, saturation temperature = 25 °C, foaming temperature = 60 °C. In addition, Table 3 offers the characteristics of each sheet as a complement to the results collected in Figure 7.

As it could be observed, the incorporation of PVOH caused a significant decrease in non-foamed solid skins regardless of the thickness of the PMMA sheet and the technique of implantation of the PVOH coating. In the comparison between the thicker PMMA sheets (1 to 5 and 8 to 10) (thick sheets), the thickness of the solid skins in the PVOH-coated foamed PMMA sheets (1 to 5) was reduced to more than half with respect to foamed PMMA sheets without PVOH coating (8-10). While after the incorporation of PVOH in thin sheets of PMMA, it went from obtaining a slight porosity in the central zone to reaching almost complete foaming of the sheet and radically reducing the amount of non-foamed solid skins (see sheets 6-7 in front of 11).

Table 3. Samples from Figure 7, including details of the technique used for the application of the PVOH (when applicable), the thickness of the sample and the solid skin thickness result obtained

Samples 1-5: PVOH-coated foamed PMMA sheet (200 μm PVOH film thickness); Sample 6: PVOH-coated foamed PMMA thin sheet (50 μm PVOH film thickness); Sample 7: PVOH-coated foamed PMMA thin sheet (20 μm PVOH film thickness); Samples 8-10: Foamed PMMA sheet without PVOH coating; Sample 11: Thin sheet of foamed PMMA without PVOH coating

* Thickness of the original PMMA sheet (in μm) before foaming and ** Non-foamed solid skin thickness (%): percentage of non-foamed solid skin thickness in relation to the total thickness of the sheet after foaming.

The reduction of solid non-foamed skins into cellular polymers entails a series of advantages, one of which is the increase in volume of the foamed area, which consequently leads to a reduction in the total density of the polymer. cell phone. Figure 8 shows the relationship between the relative density and the thickness of the solid non-foamed skins formed. As expected, the reduction and disappearance of the non-foamed solid skins leads to a decrease in the total relative density of the sample. Therefore, the present invention can be used to achieve low densities when there is a limitation regarding the polymeric material or the foaming process.

Example 4. Analysis of different conditions of the process of the present invention

Finally, an evaluation of all the samples produced in examples 1 to 3 was carried out to determine which method of applying the PVOH and which conditions were the most favorable and appropriate. In principle, it was verified that the best results in terms of the reduction and elimination of solid skins were obtained in the samples manufactured using the thermoforming technique (Figure 1E), see results summarized in Table 4). Even so, the effect could be seen in all the PVOH incorporation techniques analyzed (see results shown in Table 3). The differences between all the methods may be due to the adhesion obtained between both polymers in each of the techniques; however, it is possible to achieve acceptable results in any case.

Table 4. Samples from Figure 9, including details of the technique used for the application of the PVOH (when applicable), the thickness of the sample and the solid skin thickness result obtained.

Sample 5. PVOH-coated foamed PMMA sheet (200 μm PVOH film thickness); Sample 9. Foamed PMMA sheet without PVOH coating * Original PMMA sheet thickness (μm): before foaming and ** Non-foamed solid skin thickness (%): Percentage of non-foamed solid skin thickness in relation to the total thickness of the sheet after foaming

Figure 9 shows a comparison between a foamed PMMA sheet using the conventional technique (that is, without a PVOH coating) and a foamed PMMA sheet using the manufacturing process of the present invention, that is, using a PVOH coating. (by thermoforming - see Figure 1E). In the general view micrographs (Figure 9A and Figure 9D) it can be seen that the cell structure was more homogeneous with the use of the PVOH coating.

In addition, complete foaming of the PVOH sheet was achieved with the method of the present invention (Figure 9E) while the PMMA sample without PVOH coating (Figure 9B) exhibited unfoamed solid skins of significant thickness. Lastly, the use of the PVOH coating as a polymeric gas diffusion barrier allowed foaming the surfaces of the sheet, achieving pores that interconnected the interior cellular structure with the exterior environment (Figure 9F). This fact, however, did not occur without the use of the process of the present invention, that is, on the foamed PMMA sheet without a PVOH coating (Figure 9C). It is important to emphasize that these results were obtained in a similar way for all the PVOH incorporation techniques and in a wide range of the parameters used (saturation parameters, foaming parameters, thickness of both materials).

Other conditions in the foaming process tested were:

1- Saturation pressure = 70 MPa, saturation temperature = 25 °C, foaming temperature = 25 °C.

The results obtained for these conditions are summarized in Table 5 and Figure 10. Figure 10 shows a thick sheet of PMMA in which a gas diffusion polymer barrier has been placed on the surface or upper face, while that the surface or underside remained uncoated. Subsequently, this sample was foamed according to the gas solution foaming process at the conditions indicated above. A clear difference was observed between both edges of the sample, again proving or demonstrating the effect of the polymeric gas diffusion barrier reducing and/or preventing the appearance of solid skins.

Table 5. Sample of Figure 10, including details of the technique used for the application of PVOH (when applicable), the thickness of the sample and the solid skin thickness result obtained.

2- Saturation pressure = 20 MPa, saturation temperature = -50 °C, foaming temperature = 60 °C.

The results obtained for these conditions are summarized in Table 6 and Figure 11. Figure 11 shows two faces (Figure 11A and Figure 11B) and their respective surfaces (Figure 11C and Figure 11D) of a thick sheet of PMMA, where one of them has been covered with the gas diffusion polymeric barrier on one of its faces with the largest surface area, specifically the one that corresponds to Figure 11B and Figure 11D; and not covered with the gas diffusion polymeric barrier on the other side with a larger surface area, Figure 11A and Figure 11C.

Table 6 shows the results corresponding to the skin thickness of both sides of the sample. The side where the gas diffusion polymer barrier has not been placed has a solid skin (Figure 11A and Figure 11C), while the side where the gas diffusion polymer barrier has been placed has almost completely foamed, even achieving generation of pores on the surface (Figure 11B and Figure 11D), which does not occur without the incorporation of the barrier.

Table 6 . Samples from Figure 11, including details of the technique used for the application of the PVOH (when applicable), the thickness of the sample and the solid skin thickness result obtained.

)

* Non-foamed solid skin thickness (%): percentage of non-foamed solid skin thickness in relation to the total thickness of the sheet)

Table 7 shows a comparison between the range of parameters where the procedure of the present invention works (first line) and the preferred range of parameters where the best results have been obtained (second line).

Table 7. Parameters of the process where the process of the present invention works (above) and parameters of the process of the present invention where the best results have been obtained (below).

In addition, as a complementary analysis, the use of the technique or procedure of the present invention in other polymeric bases has also been tested. It has been verified, in the same way as for PMMA, that the incorporation of a diffusion barrier on other polymeric bases has been effective and qualitative results similar to those of PMMA have been obtained. In other words, it has been possible to reduce the solid skin on the edges where the diffusion barrier has been placed (see results included in Table 8).

Table 8. Different polymeric bases used to obtain foamed sheets, including details of the technique used for the application of PVOH (when applicable), the thickness of the sample and the solid skin thickness result obtained.

* Initial thickness (μm): before foaming

** Non-foamed solid skin thickness (%): Percentage of non-foamed solid skin thickness in relation to the total thickness of the sheet after foaming

Claims (23)

1. Process for manufacturing a polymeric foam sheet comprising the following stages: a) fully or partially covering at least one surface of a sheet comprising a polymer capable of being foamed, with a polymeric gas diffusion barrier; b) foaming the sheet from step a), and c) removing the polymeric gas diffusion barrier from the covered surface of the foam sheet from step b). 2. Manufacturing method according to claim 1, characterized in that the polymer capable of being foamed is polymethyl methacrylate (PMMA), polycaprolactone (PCL), polystyrene (PS), polycarbonate (PC) as well as copolymers, mixtures and composite materials derived therefrom. . Method according to claim 2, characterized in that the polymer capable of being foamed is PMMA. 4. Manufacturing process according to claim 3, characterized in that the PMMA has a density between 1 and 2 g/cm3. Production process according to claim 3 or 4, characterized in that the PMMA has a molecular weight of between 50,000 and 150,000 g/mol. 6. Manufacturing method according to any of claims 1 to 5, characterized in that the gas diffusion polymeric barrier comprises polyvinyl alcohol (PVOH). 7. Manufacturing method according to claim 6, characterized in that the PVOH is thermoplastic PVOH 8. Production process according to claim 6 or 7, characterized in that the PVOH has a molecular weight between 20,000 and 120,000 g/mol. 9. Manufacturing method according to any of 1 to 8, characterized in that the thickness of the gas diffusion polymeric barrier is between 5 and 1000 μm. 10. Manufacturing method according to any of claims 1 to 9, characterized in that the thickness of the gas diffusion polymeric barrier is between 20 and 300 μm. 11. Manufacturing method according to any of claims 1 to 10, characterized in that in step a) the polymeric gas diffusion barrier totally or partially covers the surface of the two faces with the largest surface area of the sheet that comprises at least one polymer. capable of being foamed. 12. Manufacturing method according to any of claims 1 to 11, characterized in that in step a) the gas diffusion polymeric barrier completely covers the surface of the two faces with the largest surface area of the sheet that comprises at least one polymer capable of be foamed 13. Manufacturing method according to any of claims 1 to 12, characterized in that step a) is carried out by deposition by evaporation of the solvent, impregnation by spraying, thermoforming, extrusion or co-extrusion. 14. Manufacturing method according to any of claims 1 to 13, characterized in that step a) is carried out by thermoforming. Manufacturing process according to any of claims 1 to 14, characterized in that step b) is carried out by extrusion foaming, injection foaming or gas solution foaming. 16. Manufacturing method according to any of claims 1 to 15, characterized in that in stage b) CO2 is used for foaming. 17. Manufacturing process according to any of claims 1 to 16, characterized in that step b) is carried out by gas solution foaming. 18. Manufacturing process according to claim 17, characterized in that the foaming by gas solution comprises three sub-stages: saturation, desorption and foaming. 19. Manufacturing method according to claim 18, characterized in that the saturation substep is carried out at a pressure between 10 and 70 MPa and at a temperature between -50 and 80 °C. 20. Manufacturing method according to claim 18 or 19, characterized in that the saturation sub-step is carried out at a pressure between 20 and 30 MPa and at a temperature between -30 and 60 °C. 21. Manufacturing process according to any of claims 18 to 20, characterized in that in the foaming sub-stage the temperature is between 20 and 200 °C. 22. Manufacturing process according to any of claims 18 to 21, characterized in that in the foaming substage the temperature is between 60 and 100°C. 23. Polymeric foam sheet obtained by a manufacturing process according to any of claims 1 to 22.