Method for producing thermoplastic polyurethane (PU) foams

Abstract

The invention relates to a foaming process of thermoplastic PU polymers comprising the following steps consisting of: Providing thermoplastic PU polymers which are swelled or solubilized in a solvent, the PU polymers having the structure R1-NH-COO- R2; filling an hermetic mould of the thermoplastic PU polymers swelled or solubilized in a solvent; Applying a controlled vacuum in the hermetic mould until a predetermined pressure is attained, under a predetermined temperature higher than the boiling point of the solvent at the used predetermined pressure and lower than the thermoplastic PU polymer Tg once dry, during a predetermined period of time for obtaining the PU foams exhibiting a low density.

Description

[0001]Method for producing thermoplastic polyurethane (PU) foams Description The invention relates to the field of polyurethanes (PU). More specifically the invention relates to a process for producing PU foams to provide them with specific targeted properties. Polyurethanes (PUs) are nowadays the most versatile and the sixth most widely industrially applied polymer family. Yet, increasing concerns related to isocyanate monomers toxicity have stimulated active research for alternatives synthesis and processing routes. PUs are employed in a wide range of applications, notably in the form of foams. Depending on their composition, PU foams can vary in structure from soft flexible foams used as cushioning materials to rigid foams used in insulating or structural materials. PU foams are produced by two concomitant reactions with the (multi- functional) isocyanates in presence of water. Firstly, the (multi-functional) isocyanates react with (multi-functional) alcohol to produce highly branched or crosslinked polymers, eventually creating a thermoset. Secondly, (multi-functional) isocyanates react with water to produce CO2(g)gas that drives the foam expansion. As a result, the PUs on which foams are based are highly branched or crosslinked, resulting in thermosetting materials which do not melt on heating; although some thermoplastic PUs are also produced for other types of applications and in significantly smaller quantities. Use of isocyanate-based monomers in PU synthesis raises severe health concerns. Regular isocyanates are actually synthesized using phosgene, a highly reactive and toxic gas. Isocyanates themselves are very toxic and powerful irritants to the mucous membranes of the eyes as well as to gastrointestinal and respiratory tracts. Direct skin contact can also cause significant inflammation. Isocyanates are known to cause chronical asthma issues and can also sensitize workers, making them subject to severe asthma attacks if they are exposed again. Moreover, PU synthesis most often requires the use of a catalyst, typically organotin compounds. The success of this catalyst is related to its high activity at low loading. However, it can be hardly removed from the final polymers. PU foams derived from (multi-functional) isocyanates are associated with environmental issues because isocyanates raw materials, in particular methylene diphenyl 4,4'-diisocyanate (MDI) and toluene diisocyanate (TDI), the most widely used isocyanates in the PU industry, and the corresponding aromatic diamines are classified as CMR (carcinogenic, mutagenic and reprotoxic substances). Therefore, there is a need to produce PUs in the form of foam. Thus, researchers in industry and in academia have put significant efforts in the past years to find alternatives to standard PUs, involving non-toxic reagents. Among PUs, NIPUs, non-isocyanate PUs, can be obtained from different polymerization path and processes. Different types of reaction may then be used to produce NIPUs, namely but not limited to, polyaddition of cyclic dicarbonates and diamines, polycondensation reactions or ring-opening polymerizations, but preferably polyaddition of cyclic dicarbonates and diamines. As for the path, one could list: step-growth polyaddition of cyclic decarbonates and diamines, step-growth polycondensation of linear activated decarbonates and diamines, step-growth polycondensation of linear activated carbamates and diols, ring-opening polymerization of cyclic carbamates to name only the most reported ones (cf. Figure 1). Production of PU foams are widely reported in the literature and they involve the use of conventional chemical or physical blowing agents which may generate toxic and flammable VOCs in the atmosphere, and are thus risky and complicated to handle at an industrial scale. Moreover, it is also reported that PU polymers used for the production of foams are exclusively chemically or physically crosslinked and thus not likely to be recycled mechanically, which is known to be the less energy intensive recycling approach. For example, reference is made to EP 3199569A1, EP3760664A1, WO2021 / 067976A1, US2012 / 0329892A1 and WO2021 / 128822A1. Therefore, there is still a need to develop a PU foaming process with the following advantages, while not being limited to them, (i) control pore dimensions while allowing very low foam density, (ii) applicable to thermoplastics (i.e., not chemically or physically crosslinked) PU, (iii) no need for conventional chemical or physical blowing agent, (iv) ease of implementation at an industrial scale, and (v) improved sustainability (by avoiding the use of isocyanate monomers or blowing agents and promoting mechanically recyclable polymer solutions). By adapting the foaming conditions, the inventors selected thermoplastic PUs, including isocyanate based PUs and NIPUs adapted for this new foaming process. Requirements for said PUs are the following: - Linear PU chains (potentially chain extended) - Decent PU molecular weights and melt viscosity. - High Tg. - Solubility in common and volatile solvents, especially non toxic solvents, (i.e., easier to recycle). In parallel, the foaming process parameters are adapted to both the polymer and the selected solvent: - The temperature must be higher than the solvent boiling point at the selected pressure and lower than the polymer Tg, very preferably at ambient temperature. - The pressure can be left constant of varied during the process, but in nay case it is adapted to ensure a rather slow growing of the foam to better control pore size. - Time is mainly proportional to the foam growing, and depending on the amount of solvent collected and recycled. The invention concerns a foaming process of thermoplastic PUs comprising the following steps consisting of a) Providing thermoplastic PU polymers which are swelled or solubilized in a solvent, the PU polymers having the structure R1-NH-COO-R2, wherein R1= R2and are, independently, selected from the group consisting of one or more of the following entities selected from the group consisting of: unsubstituted or substituted linear or branched aliphatic, cycloaliphatic or aromatic moieties optionally containing hetero atoms in their main structure or as pendent group and oligomeric / (co-)polymeric species such as polyolefins, poly(alkylene oxides), poly(siloxanes), poly(dienes), poly(olefins), poly(amides); b) Filling an hermetic mold of the thermoplastic PU polymers swelled or solubilized in a solvent; c) Applying a controlled vacuum in the hermetic mold until a predetermined pressure is attained, under a predetermined temperature higher than the boiling point of the solvent at the used predetermined pressure and lower than the thermoplastic PU polymer Tgonce dry, during a predetermined period of time for obtaining the PU foams exhibiting a density of from 0.03 to 0.1. The inventors have then found that thermoplastic PU polymers which in turn are transformed into PU foams, are specifically selected according to the requirements mentioned above, with a three-step process and leading to PU foams, the whole presenting the following requirements: - The thermoplastic PU polymers of the process are selected to allow the preparation PU foams, thus without the use of undesirable and toxic chemical or physical blowing agents and using specific solvents, advantageously being non toxic. - The PU polymers are exhibiting (i) high Mnvalues, typically higher than 4 kDa, better higher than 50 kDa, or, in some examples, having values in the range of 4 - 50 kDa, more preferably of from 4 - 25 kDa, and (ii) high Tgvalues, typically higher than 30 °C, better higher than 50 °C, and in some embodiments about 80 °C, or, in some examples, having values in the range of 30 - 95 °C, said values being measured according to ASTM D3418-21 standard, and (iii) are not chemically or physically crosslinked. - PU foams are exhibiting low density in the range of 0.030 - 0.1 preferably presenting average pores diameter, the pores being interconnected, within the range of 5 - 500 µm, preferably 50 - 200 µm. The density may be measured by the knowledge of the volume of the foamed part and its weight. Advantageously, they are made recyclable mechanically, presenting mechanical resistance, for example performing a compressive test according to ASTM D1621-00 standard, heat stability, thermal conductivity in the range of 0.013 - 0.048 W / mK, and sound insulation properties, and having heat diffusivity values, typically in the range of 0.01 - 0.70 mm² / s, when using ISO 22007-2 standard. The thermal degradation of the polymer may advantageously be beyond 200 °C. According to step a), the thermoplastic PU polymers are swelled or solubilized in a solvent, preferably low boiling point solvents and / or non toxic, volume thereof being advantageously just necessary for allowing said swelling or solubilization. Non limitative examples of solvents that may be used include alkanes, alkenes, chlorinated, amide, ethers, ketone and aromatic hydrocarbons, or mixture thereof, preferably but not limited to acetone, methyl ethyl ketone, 2,2,5,5-tetramethyloxolane (TMO), heptane methylene chloride, DMSO, 1,1,1-trichlormethane, diethyl ether, or mixture(s) thereof. R1and R2, independently, may advantageously be selected from the group consisting of linear or branched aliphatic moieties: C1-C4, C1-C6, C1-C8, C1-C12, cycloaliphatic moieties: C3-C6, aromatic moieties, such as substituted or non-substituted phenyl, and at least one alkylene group, preferably C2-C12. The heteroatom may be selected from the group consisting of N, S, O and P. The arrangement of the hermetic mold (step b)), otherwise named vacuum device, is not limited and may mainly depend on the quantity of swelled or solubilized thermoplastic PU polymers to be foamed. Any hermetic mold known in the art may be used with the proviso that a thermoplastic PU polymer foam is obtained. Typically, the thermoplastic PU polymers may be inserted in a closed and tight chamber or a mold wherein the vacuum is applied. For step c), the process requires to set the hermetic mold at a predetermined temperature higher than the boiling point of the solvent at the used pressure and lower than the polymer Tgonce dry, preferably adjusted at 5 ± 3 degree lower than the Tgof the respective polymers. In some embodiments, the predetermined temperature may be selected, in combination with the predetermined low pressure, for removing the solvent fast enough to make the polymer foam and solidify upon drying. Typical temperatures may be in the range of 50 - 120 °C. The pressure may be advantageously of 0.1 - 5 mbar for obtaining the desired PU foams. In some preferred embodiments, depending on the desired density of the final foam, a required amount of PUs (few grams) is first swelled in a minimum amount of a solvent (step b)) and is placed in a specially designed hermetic mold. The foaming procedure is completed within a predetermined period of time, preferably 1 min - 15 hours, while applying a dynamic vacuum of 0.5 - 1 mbar, optionally using an (oil or membrane) pump which may be integrated with recycling device such as a cold trap. The density of the formed foam is measured by dividing the weight of the finished foam by the mold volume. It is possible to produce thermoplastic PU foams, preferably being rigid, presenting densities preferably as low as 0.04 - 0.07 by this technique. According to a preferred embodiment, step c) may be performed as follows: - Incorporation of the thermoplastic PUs and an appropriate solvent in a closed mold, which shape is not limitative but arranged to perform the steps b) and c), at ambient pressure and controlled temperature, containing a solvent recycling system, - Reduction of the pressure to obtain PU foams, the solvent being recycled, - Drying of the obtained foams. The hermetic mold allows to obtain a desired predetermined low pressure as to perform the foaming of the PU polymer(s) (step c)). Typical desired pressures are 0.1 - 5 mbar. The duration or the predetermined period of time of the foaming step is depending on the nature of the thermoplastic PU polymers, the pressure drop and the temperature. Generally, the duration may be of from 1 min to 15 h, better of 4 min - 10 h, more preferably of starting range of 5 min to 4h - 8 h, when the predetermined temperature is within the range of 50 - 120 °C and the predetermined epressure to be attained is within 0.1 - 5 mbar. The hermetic mold used in steps b) and c) may include a solvent recycling device that ensures the recovery, eventually the purification, of the largest portion of solvent during step c), in order to reuse it, for example, in step b). After step c), the process may include a step d) of drying eventually the PU foams of high molecular weight. The process of the invention allows then the control of density of the PU foams. The thermoplastic PU polymers of step a) are defined as being, independently, thermoplastic isocyanate PUs and NIPUs presenting the chemical structure of step a), presenting the requirements set above. Thermoplastic isocyanate polyurethanes and processes for their production are well known. By way of example, isocyanate thermoplastic PUs can be produced via reaction of (a) isocyanates with (b) compounds reactive toward isocyanates and, if appropriate, (c) chain extenders, if appropriate in the presence of (d) catalysts and / or of (e) conventional auxiliaries and / or conventional additives, provided that said PUs are complying with the requirements set above. Reference is made for example to US2012 / 0329892. The thermoplastic NIPUs according to the invention may be synthesized according to known polymerization processes, for example and not limited to step-growth polyaddition of cyclic dicarbonates and diamines, step-growth polycondensation of linear activated dicarbonates and diamines, step-growth polycondensation of linear activated carbamates and diols and ring-opening polymerization of cyclic carbamates, provided that their structures and characteristic do comply with the requirements set above for PUs. Advantageously, the inventors have found two specific starting monomer types, respectively dicyclic carbonates and diamines, for the synthesis of thermoplastic NIPU polymers which in turn are transformed into NIPU foams, specifically adapted to the requirements mentioned above, with a three-step process using thus said monomers and leading to NIPU foams, the whole presenting the following requirements: - monomers of dicyclic carbonates and diamines are very advantageously selected to allow the preparation of thermoplastic NIPU polymers specifically tailored to obtain NIPU foams through the foaming process of the invention, thus without the use of undesirable and toxic chemical or physical blowing agents and using specific non toxic solvents. Both of these monomers, namely the dicyclic carbonate monomer (I), otherwise named cyclic biscarbonate, and the diamine monomer (II) are commercially available or very preferably obtained through known synthesis methods, as above defined. - Specific examples / demonstration of this approach has been produced. The step- growth polyaddition of cyclic dicarbonates and diamines was very preferably performed thanks to its short reaction times in a reactive extruder. Some rigid monomers leading to high Tg polymers were selected. More specifically, the synthesis of the thermoplastic NIPUs may be very preferably performed through the reaction of a dicyclic carbonate monomer (I) with a diamine monomer (II) wherein R is derived from a cyclic carbonate containing reagent and comprises one or more of the following entities selected from the group consisting of: linear or branched aliphatic, cycloaliphatic or aromatic moieties optionally containing hetero atoms in their main structure or as pendent group, oligomeric / (co-)polymeric species, such as poly(alkylene oxides), poly(siloxanes), poly(dienes), poly(olefins), poly(amides), and (co-)polymeric species in the form of cyclic carbonate terminated oligomers; R'’ is selected from the group consisting of linear or branched aliphatic moieties selected from the group consisting of C1-C8alkyl groups or any electron withdrawing group that has some activating effect on cyclic carbonates, R’ is derived from a amine containing reagent and comprises one or more of the following entities selected from the group consisting of: unsubstituted or substituted linear or branched aliphatic, cycloaliphatic or aromatic moieties or oligomeric / (co- )polymeric species such as polyolefins, poly(alkylene oxides), poly(siloxanes), poly(dienes), poly(olefins), poly(amides), or other (co-)polymeric species in the form of amine-terminated oligomers; at a temperature range of from 40 °C to 120 °C, during 1 min to 7 h, resulting in NIPU polymers. Preferred dicyclic carbonate monomers are five membered cyclic biscarbonates, noted 5-CC , selected from the group consisting of These could be obtained from respectively Diglycidyl ethers of bisphenol A (DGEBA), bisphenol AF (DGEBAF), and phenolphthalein (DGEPh) biscarbonates by for example reaction with gaseous CO2. Preferably, the diamine monomer (II), may be selected fr om the grou p co nsistin g of hexamethylenediamine (HMDA), 3,3′-diamino-N-methyld ipropylamine (DNDPA), p- xylylenediamine (p-XDA), Vestamin®IPD (VIPD, 3-aminomethyl-3,5,5- trimethylcyclohexylamine) and Vestamin®PACM (VPACM, 4,4′- Diaminodicyclohexylmethane), and PriamineTM1075 ). The thermoplastic NIPU polymers are exhibiting (i) hi Mnvalu ty hi than 4 kDa, better higher than 50 kDa, or, in some examples, having values in the range of 4 - 50 kDa, more preferably of from 4 - 25 kDa, and (ii) high Tgvalues, typically higher than 30 °C, better higher than 50 °C, and in some embodiments about 80 °C, or, in some examples, having values in the range of 30 - 95 °C, said values being measured according to ASTM D3418-21 standard, and (iii) are not chemically or physically crosslinked. Since both monomers are difunctional, the obtained NIPU polymers are linear (by opposition to chemically or physically crosslinked) and thus thermoplastics, which makes them easy to recycle mechanically (i.e., reprocessable by grinding and melt- processing). Accordingly, the synthesis conditions: two specific monomers, the temperature and, in some embodiments, an inert atmosphere, provide the NIPU polymers exhibiting the specified Mnand Tg, that are not chemically or physically crosslinked, and soluble or swellable in non-toxic solvents. The molar ratio of compound (I) and compound (II) was kept at 1:1. The temperature range for the reaction, 40 °C to 120 °C, is selected to avoid formation of by products, to improve yield, molecular weight, and to eliminate branching / crosslinking. The preferred temperature may be within 60 - 100 °C, and duration may be starting from 5 min to 7 h, which are the most preferable ranges for optimally avoid the drawbacks previously cited. Advantageously, NIPUs synthesis may be carried out using known melt reaction devices, such as a classical batch reactor or extrusion device or an extruder or a micro compounder or other melt processing equipment such as an internal mixer or a roll mill, known to the one skilled in the art which allows the polymerization to be done, i.e. the polymerization may be run by reactive extrusion or according to different embodiments by other known polymerization, as previously mentioned, using for example a twin-screw extruder. Step a) is performed under air or, preferably, inert atmosphere may be maintained during the step (a), the applied gas may be nitrogen or argon. As a non-limitative example, thermoplastic NIPUs may be obtained as follows: NIPUs may be synthesized by reactive extrusion using a lab-scale reaction device, such as a micro compounder equipped with conical co-rotating intermeshing twin- screws. Before charging any monomers, the barrel temperature is set at about 103 - 110 °C to attain a melt temperature of 100 °C during the process. The heating is done until a thermal equilibrium is reached, while a small flow of argon, typically few bubbles / min, is maintained through the entire process of reaction to maintain an inert atmosphere inside the micro compounder. Following the primary steps, monomers (cyclic biscarbonate and diamine) are fed together, upon need, with the solvent in the preheated micro compounder. With respect to the overall monomers content, a solvent is added, such as DMSO. The reaction time is of from 1 min to 270 min to synthesize thermoplastic NIPUs of different molecular weight. The rotational speed of the screw is kept constant for example at 100 - 130 rpm for all the synthesis. After completion of synthesis (potentially by reactive extrusion) polymers may be purified by precipitation in a non-solvent, such as ethanol, ethyl acetate, or water. Optional steps for obtaining thermoplastic NIPUs of said example may be omitted in some other examples of implementation of said NIPUs synthesis. The invention also relates to thermoplastic PU foams that may be obtainable by the process of the invention, exhibiting a density in the range of 0.030 - 0.1, and at least one characteristic selected from the group consisting of an average pores diameter within the range of 5 - 500 µm, preferably of 50 - 200 µm, Mnvalues higher than 4 kDa, better higher than 50 kDa, or in some examples having values in the range of 4 - 50 kDa, more preferably 4 - 25 kDa, Tgvalues higher than 30 °C, better higher than 50 °C, and in some embodiments about 80 °C, or, in some examples, having values in the range of 30 - 95 °C, said values being measured according to ASTM D3418-21 standard and heat diffusivity values in the range of 0.01 - 0.70 mm² / s, when using ISO 22007-2 standard. Examples The following non limitative examples and experimental implementations of the process according to some embodiments of the invention are given here below with appended figures. Figure 1 shows the synthesis of PUs with four possible routes, according to an embodiment of the invention. Figure 2 is an example of the implementation of step c) for the preparation of PU foams according to an embodiment. Figure 3 shows the morphological results of a series of thermoplastic NIPUs foams based on p-xylylenediamine and different cyclic biscarbonate monomers, according to an embodiment. Synthesis of NIPUs by reactive extrusion Different thermoplastic NIPUs are synthesized by reactive extrusion according to known polymerization conditions. Following the primary steps, monomers (cyclic biscarbonate and diamine in 1:1 molar ratio) were fed together with the solvent in a preheated micro compounder.30 wt % (with respect to the overall monomer content) of DMSO was added. After completion of syntheses, polymers may be purified by precipitation in a non-solvent, as mentioned in Table 2, which also gives the different monomers used. Molecular characterizations of synthesized thermoplastic NIPU polymers All the NIPU polymers are characterized by nuclear magnetic resonance (NMR) spectroscopy and Size exclusion chromatography (SEC), according to classical experimental implementations known in the field of the invention Thermal characterizations of synthesized thermoplastic NIPU polymers These are characterized by Differential scanning calorimetry (DSC) under nitrogen flow, and Thermogravimetric analysis (TGA) under air flow, according to classical experimental implementations known in the field of the invention. Rheological, dynamic mechano-thermal, and mechanical characterizations of thermoplastic NIPU polymers Prior to testing, the discs are conditioned for 24 hours under static vacuum and preserved inside a desiccator to avoid any water intake by the samples. Rheological measurements are recorded using an Anton Paar MCR 302 rheometer equipped with a CTD 450 temperature control unit. Experiments are carried out on 25 mm diameter parallel plate geometry in shear mode. The rheological measurements are recorded at a fixed temperature of 125 °C at a controlled strain of 0.1% while performing a frequency sweep in the range of 0.1 to 100 rad / s. A three min isotherm is performed before the start of the experiment to equilibrate the temperature. The gap between the plates is maintained at 1 ± 0.1 mm during all the experiments. Tensile samples measurements of dumbbell geometry are done as specified by ISO 37-4 and samples are submitted to the dynamic mechanical analysis (DMA) according to classical experimental implementations known in the field of the invention. Preparation of PU foams (example of a generic implementation) Thermoplastic PUs are foamed using vacuum compression molding in temperature. Depending on the desired density of the final foam, a required amount (1 - 2 g) of thermoplastic PU was swelled in a minimum amount of acetone (~ 1 g of polymer in ~ 1.5 mL of acetone) and was placed in a specially designed PTFE mold (30 mL) equipped with a perforated lid (3 - 5 holes of 2 mm diameter). The set temperature for the foaming procedure was adjusted at 5 ± 3 degrees lower than the Tgof the respective polymers. The foaming procedure was completed within 4 - 6 hours while applying a dynamic vacuum of 0.5 - 1 mbar using an Edwards oil-pump (integrated with liquid nitrogen cooled solvent trap). Density of the formed foam was measured by dividing the weight of the finished foam by the mold volume (30 mL). It is possible to produce rigid PU foams of densities as low as 0.04 - 0.07 by this technique. Characterization of PU foams Micro-computed X-ray Tomography (µ-CT) testing was performed to visualize the internal structure of the foams with a micron scale resolution, according to classical implementation conditions known in the art: Thermal conductivity of NIPU foams were measured at room temperature (25 ± 3 °C) according to the ISO 22007-2 standard. Mechanical performance of the produced foams was measured following ASTM D 1621-00 standard protocols. Example 1 An example of a schematic implementation of the foaming process is shown in Figure 2. Accordingly, PUs from step a) and an appropriate solvent are incorporated in a hermetic mold, which shape is not limitative but arranged to perform the steps b) and c), at ambient pressure and controlled temperature, containing a solvent recycling system. Further, the pressure in the closed mold is lowered to be of from 0.1 to 5 mbar to obtain NIPU foams, the solvent being recycled and a drying step is performed of the obtained foams. Example 2 Synthesis of NIPUs Thermoplastic NIPUs are synthesized by reacting cyclic biscarbonates and diamines by reactive extrusion. The amines were selected to obtain a broad range molecular flexibility for different NIPUs. This way, it is possible to design a full spectrum of NIPUs with distinctly different Tgvalues. After completion of each reaction, polymers were purified and subjected to different characterization techniques. Number average molecular weight (Mn) together with polydispersity index (Ð), yield, onset temperature of thermal degradation (Tonset), glass transition temperature (Tg), and modulus at 30 °C for all the NIPUs samples were gathered from individual experiments and are shown in Table 2. Table 2. Properties of a set of thermoplastic NIPUs (noted PHU_1 - PHU_8) synthesized by reactive extrusion based on different selected diamines and cyclic biscarbonates. S.N. Cyclic Amine MnaÐaYield TonsetTgcMod at 30 carbonate (g / mol) (%)b(°C) (°C) °Cc(MPa) PHU_1 Priamine 17200 2.1 65 290 35 645 5CC- PHU_2 DNDPA 21800 3.2 80 195 58 1 600 DGEBA PHU_3 HMDA 19000 2.3 64 200 52 1 380 PHU_4 VPACM 5 200 2.0 47 215 --d--dPHU_5 p-XDA 14500 2.1 80 230 72 1 385 PHU_6 VIPD 4 700 2.0 72 205 --d--d5CC- PHU_7 p-XDA 13200 1.9 65 220 88 1 870 DGEBAF 5CC- PHU_8 p-XDA 10000 3.0 80 200 92 2 900 DGEPh The values are associated toaSEC analysis;bTGA analysis;cLoss modulus (E") peak obtained by DMA analysis.dThe samples were too brittle for DMA studies Example 3: Fabrication and characterization of NIPU foams Following all the characterization of neat polymers, polymers based on p-XDA were foamed. The selection of the thermoplastic NIPU polymers to be foamed was mainly based on their high Tgand molecular weights. For the selected polymers the foamability was found to be linked with the values of complex viscosities. For example, it is possible to foam PHU_5 (5CC-DGEBA+p-XDA) with a greater ease compared other synthesized NIPUs. It is possible to produce thermoplastic foams with density as low as ~ 0.035 while using PHU_5 as the starting material. However, the lowest density of foams that could be achieved from PHU_7 and PHU_8 was ~ 0.06 and 0.07 respectively. For comparison, all the foams were fabricated to a density of ~ 0.07 and their micro-structure, mechanical properties and thermal insulation were characterized. To estimate pore size distribution and cell structure, NIPU foams were subjected to µ- CT analysis, according to classical experimental implementations. The experimental results of pore size distribution as obtained from µ-CT analysis are presented in Figure 3 (a-c). A representative picture of each foam along with a representative cross-sectional view of each of them, as obtained from µ-CT studies, are inserted in the Figure 3. From the analysis, majority of the cells (> 85 %) were found to be interconnected irrespective of the chemical composition of the associated polymer. The pore volume fraction as estimated by µ-CT analysis was found to be similar (70 – 85 %) for all the different types of foams, while the matrix volume fraction accounts for the rest. The average pore diameter and skewness of the distribution of the pores were estimated from the raw data and shown in Figure 3. From the analysis, the fabricated foams were found to be similar in terms of average pore size and its distribution. The average pore size of the foams was found to be in the range of 125 - 150 µm, while the skewness varied from 35 to 50. With a similar pore size distribution, all these foams were expected to have similar mechanical strength and similar heat insulation properties. Following the morphological studies, the NIPU foams were subjected to quasi-static mechanical experiments and the results are shown in Table 3. As expected, the NIPU foams with similar pore distribution behaves similarly under compression. The stress at 15 % deformation was found to vary within 230 to 300 kPa, while the samples experienced a permanent deformation of 5 – 7 %. The figures associated to density (0.06 - 0.07 vs.0.04 for the traditional rigid PU foams) and compressive strength for the fabricated foam samples (230 - 300 kPa at 15 % deformation vs.200 kPa at 10 % deformation) are close to those of traditional rigid PU foams, which are the market leaders for packaging and thermal insulation applications. Acknowledging the potential end use, the foams were subjected to thermal insulation studies, and the results are shown in Table 3. Thermal conductivity of the NIPU foams was found to be in the range of 0.043 - 0.047 W / mK, while thermal diffusivity varied within the range of 0.5 - 0.6 mm2 / s. The values are quite similar to the thermal conductivity of commercial crosslinked PU foams (0.048 W / mK) and expanded thermoplastic polystyrene (0.032 W / mK). Considering the resemblance of mechanical and thermal insulation properties of the newly fabricated thermoplastic NIPU foams in comparison to the traditional rigid thermoplastic PU foams, NIPU foams can outdistance the traditional crosslinked PU foams especially when the toxicity of the monomers are concerned. Table 3. Mechanical and heat insulation properties of a series of NIPU foams based on p-XDA and different cyclic biscarbonate monomers. Mechanical properties Heat insulation Density Stress Foams Monomers Permanent (g / cc) at 15 % Conductivit Diffusivity deformation compression y (W / mK) (mm2 / s) (%) (kPa) 5CC- -2a4.70 × 10 F.PHU_5 DGEBA / p- 0.06 ± 0.01 300 ± 90 5.0 ± 0.4 - 0.49 ± 0.04 ± 0.2 × 102XDA 5CC- 4.30 × 10-2F.PHU_7 DGEBAF / 0.06 ± 0.01 250 ± 25 7.2 ± 2.5 -20.67 ± 0.13 ± 0.2 × 10 p-XDA 5CC- 4.70 × 10-2F.PHU_8 DGEPh / 0.07 ± 0.01 230 ± 25 4.7 ± 0.6 -20.57 ± 0.0.6 ± 0.1 × 10 p-XDA a F.PHU_5 stands for foam of PHU_5

Claims

Claims 1. A foaming process of thermoplastic PU polymers comprising the following steps consisting of a) Providing thermoplastic PU polymers which are swelled or solubilized in a solvent, the PU polymers having the structure R1-NH-COO-R2, wherein R1= R2and are selected from the group consisting of one or more of the following entities selected from the group consisting of: unsubstituted or substituted linear or branched aliphatic, cycloaliphatic or aromatic moieties optionally containing hetero atoms in their main structure or as pendent group and oligomeric / (co-)polymeric species such as polyolefins, poly(alkylene oxides), poly(siloxanes), poly(dienes), poly(olefins), poly(amides); b) Filling an hermetic mould of the thermoplastic PU polymers swelled or solubilized in a solvent; c) Applying a controlled vacuum in the hermetic mould until a predetermined pressure is attained, under a predetermined temperature higher than the boiling point of the solvent at the used predetermined pressure and lower than the thermoplastic PU polymer Tgonce dry, during a predetermined period of time for obtaining the PU foams exhibiting a density of from 0.03 to 0.

1.

2. The foaming process according to claim 1, wherein R1and R2, independently, are selected from the group consisting of linear or branched aliphatic moieties: C1-C4, C1-C6, C1-C8, C1-C12, cycloaliphatic moieties: C3-C6, aromatic moieties, such as substituted or non-substituted phenyl, and at least one alkylene group, preferably C2-C12. The heteroatom may be selected from the group consisting of N, S, O and P.

3. The foaming process according to any of claims 1 to 2, wherein the hermetic mold used in steps b) and c) includes a solvent recycling device.

4. The foaming process according to any of claims 1 to 3, wherein, after step c), the process may include a step d) of drying the PU foams of high molecular weight.

5. The foaming process according to any of claims 1 to 4, wherein, in step c), the predetermined temperatures are in the range of 50 - 120 °C and / or the pressure to be attained is of 0.1 - 5 mbar.

6. The foaming process according to any of claims 1 to 5, wherein the thermoplastic PU polymers are defined as being, independently, thermoplastic isocyanate PUs and thermoplastic NIPUs presenting the chemical structure of step a).

7. The foaming process according to claim 6, wherein a synthesis of the thermoplastic NIPUs is performed throughout the reaction of a dicyclic carbonate monomer (I) with a diamine monomer (II)(I)(II)h reinR is derived from a cyclic carbonate containing reagent and comprises one or more of the following entities selected from the group consisting of: linear or branched aliphatic, cycloaliphatic or aromatic moieties optionally containing hetero atoms in their main structure or as pendent group, oligomeric / (co- )polymeric species, such as poly(alkylene oxides), poly(siloxanes), poly(dienes), poly(olefins), poly(amides), and (co-)polymeric species in the form of cyclic carbonate terminated oligomers; R'’ is selected from the group consisting of linear or branched aliphatic moieties selected from the group consisting of C1-C8alkyl groups or any electron withdrawing group that has some activating effect on cyclic carbonates, R’ is derived from a amine containing reagent and comprises one or more of the following entities selected from the group consisting of: unsubstituted or substituted linear or branched aliphatic, cycloaliphatic or aromatic moieties or oligomeric / (co-)polymeric species such as polyolefins, poly(alkylene oxides), poly(siloxanes), poly(dienes), poly(olefins), poly(amides), or other (co- )polymeric species in the form of amine-terminated oligomers; at a temperature range of from 40 °C to 120 °C, during 1 min to 7 h, resulting in thermoplastic NIPU polymers.

8. The foaming process according to claim 7, wherein dicyclic carbonate monomers (I) are five membered cyclic biscarbonates, noted 5-CC, 5CC- DGEBA, 5CC-DGEBAF, 5CC-DGEPh obtained from respectively Diglycidyl ethers of bisphenol A (DGEBA), bisphenol AF (DGEBAF), and phenolphthalein (DGEPh) biscarbonates.

9. The foaming process according to any of claims 7 and 8, wherein the diamine monomer (II), is selected from the group consisting of hexamethylenediamine (HMDA), 3,3′-diamino-N-methyldipropylamine (DNDPA), and p- xylylenediamine (p-XDA), Vestamin®IPD (VIPD, 3-aminomethyl-3,5,5-trimethylcyclohexylamine) and Vestamin®PACM (VPACM, 4,4′-Diaminodicyclohexylmethane), and PriamineTM1075).10.The foaming process according to any of claims 1 to where of step a) is alkanes, alkenes, chlorinated, amide, ethers, ketone and aromatic hydrocarbons, or mixture thereof, preferably acetone, methyl ethyl ketone, 2,2,5,5-tetramethyloxolane (TMO), heptane, methylene chloride, DMSO, 1,1,1- trichlormethane or diethyl ether, or mixture(s) thereof. 11.The foaming process according to any of claims 1 to 10, wherein the predetermined period of time in step c) is of from 1 min to 15 h, better of 4 min to 10 h, more preferably of starting range of 5 min to 4h - 8 h, when the predetermined temperature is within the range of 50 - 120 °C and the predetermined pressure to be attained is within 0.1 - 5 mbar. 12.The foaming process according to any of claims 1 to 11, wherein the use of chemical or physical blowing agents is excluded. 13.A thermoplastic PU foam that is obtainable by the process of any of claims 1 - 12, exhibiting a density in the range of 0.030 - 0.1, a thermal conductivity in the range of 0.013-0.048 W / mK, and an average pores diameter within the range of 50 - 200 µm.

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