Method for obtaining an at least partially foamed building panel

Abstract

The invention relates to a method for obtaining an at least partially foamed building panel, comprising the following steps of providing a mixture comprising polyethylene terephthalate having an intrinsic viscosity of at most 0.75 dL / g according to ASTM D4603-18 and at least one chain extender, feeding the mixture to an extruder, c) heating the mixture to a temperature of at least 240 °C and subjecting the mixture to shear forces to obtain a melt, injecting at least one foaming agent in the melt and extruding the melt to obtain an at least partly foamed core layer of a panel.

Description

[0001] Method for obtaining an at least partially foamed building panel

[0002] The invention relates to a method for obtaining an at least partially foamed panel, in particular a building panel. The invention also relates to such panel.

[0003] The global demand to repurpose plastic waste materials, specifically polyolefins such as post-consumer recycled polyethylene terephthalate (PCR PET), in an environmentally sound and cost-effective manner is increasing. As the lifespan of recycled plastic packaging including recycled PET is as short as virgin plastic packaging, and most plastic can be recycled only once or twice due to polymer degradation, recycled packaging plastics such as PET also get discarded and introduced in the waste stream relatively quickly after recycling. Recycling these plastics therefore often is seen to delay, rather than avoid, final disposal. It is therefore further desirable to physically recycle such waste materials into value- added products which may be utilized for longer periods of time, such as in the construction industry, as structural and / or decorative (building) panels, to avoid or delay re-introducing these into the waste stream. It is finally desirable to improve the physical properties of such structural and / or decorative (building) panels, to enable them to be used in load-bearing applications and exposed to the elements.

[0004] It is further desired to create value and economic incentive for the recycler, that is, make it more desirable for a recycler to create new products from plastic packaging and / or clothing waste, rather than discard and / or incinerate it.

[0005] Finally, it is desired to enhance the structural performance and durability of such panels. However, realizing a foamed composition presents significant challenges, as foaming of polymer-based core layers typically relies on the controlled expansion of gases within a polymer matrix to achieve reduced density while maintaining mechanical integrity. In core compositions with a high mineral-to- polymer ratio however, several difficulties may arise related to intrinsic viscosity, particularly in PET derived from post-consumer recycled sources, as this type of PET has a particularly low intrinsic viscosity due to thermal and hydrolytic degradation, which are further exacerbated by a high mineral content which further reduces IV and foaming capacity, leading to inconsistent cell formation and possible collapse upon processing. An excessive viscosity and lower melt flowability reduce extrusion efficiency, leading to die clogging and uneven material distribution. Poor dispersion may cause mineral agglomeration or weak interfacial adhesion between PET and mineral fillers, negatively impacting mechanical performance of the final product. The relatively high processing temperature of PET compared to other thermoplastics also imposes difficulty for controlled nucleation and, therefore, uniform expansion without risk of thermal degradation. Moreover, a relatively strong interfacial interaction between the mineral filler and polymer may hinder the foaming efficiency and may result in increased brittleness, thus compromising the impact resistance of the panel.

[0006] Hence, one goal of the present invention is to provide a polyethylene terephthalate- based product, in particular an at least partially recycled polyethylene terephthalate-based product, most particularly an at least partially foamed polyethylene terephthalate-based product comprising a high mineral content, that overcomes the problems with traditional materials and recycling outcomes. Furthermore, the invention also aims to provide a method to produce said polyethylene terephthalate-based product, in particular an at least partially recycled polyethylene terephthalate-based product, which may address or solve partially or entirely at least some of the aforementioned challenges.

[0007] In a first aspect, the present invention relates to a method for obtaining an at least partially foamed panel, in particular a building panel, comprising the following steps: a) providing a mixture comprising polyethylene terephthalate (PET) preferably having an intrinsic viscosity of at most 0.75 dL / g in particular according to ASTM D4603-18, and preferably at least one chain extender and at least one mineral filler, wherein the mineral filler is preferably selected from silicon dioxide, magnesium dihydroxide, calcium carbonate, magnesium carbonate, talc, chalk, montmorillonite, dolomite, or any combination thereof, b) feeding the mixture to an extruder, c) heating the mixture to a temperature of at least 200°C, in particular at least 240 °C and subjecting the mixture to shear forces to obtain a melt, d) providing or injecting at least one physical foaming agent in the melt, e) extruding the melt to obtain an at least partly foamed core layer of a panel. The method according to the present invention enables the production of an at least partially foamed polyethylene terephthalate panel, in particular an at least partially foamed polyethylene terephthalate building panel. Polyethylene terephthalate benefits of being extremely light, strong, and durable and requires less energy to manufacture than similar alternatives. The use of polyethylene is beneficial for the manufacturing of (building) panels as it can replace existing non- sustainable materials such as polyvinylchloride (PVC), oriented strand board (OSB) and / or high density fibreboard (HDF) which are currently the norm in the building industry. Due to the obtained panel, or at least the core layer thereof, being at least partially foamed, a relatively low-weight panel can be obtained which benefits of the material characteristics of polyethylene terephthalate. The method according to the present invention in particular provides for the provision of a multipurpose (building) panel in an environmentally sound and cost-effective manner. It was found that the structural performance and durability of the panels by increasing the mineral content of their polymeric composition is enhanced, which is beneficial in the current application. The method further benefits from the at least partially foamed PET core layer. The partial foaming of the PET in the core layer introduces voids within the polymer matrix thereof creating at least partially cellular structure therein, which significantly reduces the panel’s overall density. This reduction in density directly contributes to a lightweight product, which is particularly advantageous for applications requiring ease of handling, transportation, and installation, such as in construction and interior design industry. Unlike known panels, which typically rely on solid PET cores or incorporate heavy fillers to achieve strength, the panel of the present invention achieves comparable or even superior performance with a significantly lighter structure. It is highlighted, however, that despite the reduction in density, the at least partially foamed PET core layer, and the panel in whole, retains high mechanical integrity. Known panels often suffer from a trade-off between density and mechanical performance, wherein reducing density leads to substantial compromises in strength.

[0008] In addition, the structure of the partially foamed PET core layer maintains sufficient density, in particular in critical regions, to ensure excellent screw pullout strength. This may allow the panel to be used in applications where secure fastening is required, such as in modular constructions or the installation of heavy fixtures. Furthermore, the at least partially foamed core layer distributes stress efficiently across its structure, which helps to maintain high tensile and compressive strength. Consequently, an excellent strength-to-weight ratio is achieved making the panel of the present invention ideal for use in structural applications where maintaining strength while minimizing weight is critical. The at least partially foamed PET core layer also imparts high dimensional stability to the panel in whole under environmental fluctuations. This represents a significant improvement over known panels, which are often prone to dimensional instability due to inconsistent material properties or reliance on heterogeneous compositions. The cellular structure of the core layer contributes to mitigate the effects of thermal expansion and contraction, ensuring consistent panel dimensions and performance overtime. This property is particularly important in applications subjected to temperature variations, such as exterior building panels. moreover, the method of the present invention enables the use of recycled PET while also reducing, or even avoiding, chemical foaming agents thereby minimizing its environmental footprint. Known methods commonly utilize chemical foaming agents that release harmful byproducts during production, contributing to environmental degradation. The partial foaming reduces the quantity of raw PET material required for production, resulting more affordable panels without compromising performance. This is further amplified when using recycled PET, enhancing the product’s cost-effectiveness and sustainability. By contrast, many known methods rely heavily on virgin materials, making them less sustainable and cost-efficient. It is also emphasized that the voids within the foam in the core layer reduces the overall material consumption, which aligns with sustainable production practices. Additionally, known panels often lack the capability to tune properties effectively, limiting their versatility in application. Lastly, the partial foaming of the core layer allows manufacturers flexibility in production by making the properties of the panel tuneable depending on the degree of foaming and cell structure of the core layer. This customization enables manufacturers to optimize the panel’s performance for specific applications, ranging from load-bearing construction elements to decorative wall panels.

[0009] The polyethylene terephthalate as applied in the method according to the present invention typically has an intrinsic viscosity of at most 0.75 dL / g according to ASTM D4603-18, or an equivalent standard. It is also imaginable that the provided mixture has an intrinsic viscosity of at most 0.75 dUg according to ASTM D4603-18. A relatively low intrinsic viscosity may correspond to relatively short polymer chains, which is characteristic to recycled PET or rPET. Advantageously, at least part of the polyethylene terephthalate molecules are elongated by at least one chain extender. Hence, the mixture which is to be extruded may comprise at least one chain extender. The use of at least one chain extender will increase branching and / or entanglement within the polyethylene terephthalate polymer chains. The use of at least one chain extender may be understood to increase the intrinsic viscosity (IV) of the mixture. The increased IV in turn prevents the collapse of cells formed by the injection of a physical foaming agent in the melt. A high IV is representative for long polymer chains, such as is characteristic for virgin PET. These long polymer chains result in improved mechanical properties of the panel. A low IV is representative for short polymer chains. Short polymer chains are for example formed due to the degradation during PET use and / or recycling processes in combination with the initial IV of PET prior to recycling. In addition, a low IV could in part also be caused by a relatively narrow PET molecular weight distribution. A low IV is herein defined as an IV of 0.75 dUg or lower, typically ranging from 0.60 dL / g up to and including 0.75 dL / g. A high IV is herein defined as an IV higher than 0.75 dL / g, typically ranging between 0.75 dL / g and 0.85 dL / g.

[0010] The intrinsic viscosity (IV), [q], is an indicator of the viscosity of the composition and can be estimated with the following formula: [rj] = KMa

[0011] Herein is [q] the intrinsic viscosity; the viscosity attributed to the solute, rather than the viscosity of the solvent itself. M is the molecular weight in g / mol, K is a specific attribute specific to each polymer-solvent system, and a is a constant for a particular polymer. For PET specifically a is approximately 0.72. The intrinsic viscosity of PET influences its processing behaviour and mechanical properties and is typically linked to its molecular weight - a molecular weight of 15,000- 20,000g / mol corresponds to an intrinsic viscosity of 0.55-0.67 dL / g, 24,000- 36,000g / mol corresponds to 0.75-1.00dL / g. The intrinsic viscosity in particular depends on average molecular weight, the molecular weight distribution and the degree of polymer chain entanglement. A low intrinsic viscosity is linked to reduced mechanical properties in extrudates and challenges in foaming processes, whereas a high intrinsic viscosity tends to enhance the mechanical properties of extrudates and facilitates foaming. The method according to the present invention enables the upcycling of a low intrinsic viscosity PET composition, more specifically a recycled PET or rPET composition, to form an at least partially foamed board.

[0012] Step c) could also be a step of heating the mixture to a temperature to at least the melting temperature of the mixture, and in particular of the PET, and subjecting the mixture to shear forces to obtain a melt. The mixture is preferably heated to a temperature of at least 200°C, in particular at least 240 °C, more in particular at least 250 °C. It is also imaginable that the mixture is heated to a temperature in the range of 250-260°C.

[0013] The method according to the invention enables the use of recycled polyethylene terephthalate (rPET), in particular post-consumer recycled polyethylene terephthalate (PCR PET). Recycled polyethylene terephthalate typically has a relatively low intrinsic viscosity due to a shortening of molecular chains due to UV radiation as well as hydrolytic and thermal degradation, which may cause challenges in processing. Post-consumer recycled PET generally has a lower intrinsic viscosity compared to virgin PET, due to a low molecular weight and narrow molecular weight distribution. These difficulties in processing can be overcome by applying a method according to the present invention. The invention also enables for the provision of recycled polyethylene terephthalate (rPET), in particular post-consumer recycled polyethylene terephthalate (PCR PET) and virgin polyethylene terephthalate in the method and / or the panel.

[0014] The method requires the use of at least one physical foaming agent in order to enable foaming of the melt such that an at least partially foamed panel, or core layer, is obtained. Within the context of the present invention, a physical foaming agent is defined as a compound that does not form covalent bonds with any compounds present in the melt. Unlike chemical foaming agents, that typically undergo a reaction to transform to a gaseous compound, or to produce a gaseous compound, physical foaming agent merely undergo a phase transition at most. Physical foaming agents are typically inert. At least one physical foaming agent as applied in the method according the invention may be inert. Advantageously, physical foaming agents provide a solution for obtaining foamed PET, as no decomposition of physical foaming agents is required for their functioning. In addition, they allow for a higher foaming rate. In addition to the at least one physical foaming agent enabling the foaming of the core layer, the at least one (physical) foaming agent, and in particular the at least one supercritical (physical) foaming agent may in practise further function as a plasticizer enabling a better processability of the melt. The at least one physical foaming agent is introduced at a high pressure into the extrusion process. As a decrease in melt viscosity may occur after the introduction of supercritical foaming agent, therefore the temperature in the extruder after step d) may be reduced. It is conceivable that the temperature after step d) may be reduced to 230-240°C. It is conceivable that a modification in the screw / barrel design may offset the change in the pressure, where no temperature change is needed. In this particular case, said supercritical foaming agent may function at least partially as a plasticizing component, more specifically a transitory plasticizer.

[0015] The melt which is to be extruded typically has a saturation temperature (Tsat) and a saturation pressure (Psat). Supercritical CO2, N2or a combination thereof may be introduced as a physical foaming agent in the extruder at pressures greater than the saturation pressure, Psat, of the melt at the extrusion temperature. When the mixture passes through the die in step e), the pressure drops, reducing the solubility of the physical foaming agent, at which point the pressure, P, drops well below the Psat, resulting in phase separation and cell nucleation. During this so- called cell growth phase, the resulting non-critical CO2 and / or N2gas expands until the expansion force is equal to the opposing force provided by the melt's viscoelastic properties. During this step, cells may merge. As such, the melt must have a high melt strength to prevent the formed cells from collapsing during the stabilization process of cell formation. This melt strength is influenced by the IV but is also impacted by process parameters and could be directly and adversely impacted by non-polymeric content, such as but not limited to filler content.

[0016] The barrel may comprise at least one metering section, and in step d) the least one physical foaming agent may be injected in the melt at a location within the at least one metering section. Advantageously, at least one physical foaming agent can readily be injected in the metering section, as this section is already suited for adding compounds or materials to the extruder. In addition, adding the physical foaming agent to the melt in the metering section allows its homogenous mixing throughout the melt, and thus allow to obtain a homogenously foamed core layer.

[0017] The melt is preferably free of chemical foaming agents. The most commonly used chemical foaming agent for foaming building panels and flooring panels is azodicarbonamide (AC). Chemical foaming agents degrade and decompose into gases under the influence of heat. These chemical foaming agents however readily decompose at temperatures around 180 °C. PET has a melting temperature under atmospheric conditions that typically lies around 240 °C, depending among others on the number of crosslinks within PET, as well as depending on the molecular weight of PET. As such, the required temperature for processing PET is too high for typical chemical foaming agents. Moreover, these chemical foaming agents generally do not decompose completely, allowing for some residue to remain present in the foamed core layer, which may be a health hazard when used in a residence. In addition, production processes of chemical foaming agents are generally very polluting. Lastly, chemical foaming agents do not allow for relatively high foaming rates. However, in a possible embodiment, it is imaginable that a fraction, preferably lower than 1 wt.%, of chemical foaming agent is applied in addition to at least one physical foaming agent.

[0018] The method enables the provision of a product in particular a panel, such as but not limited to a floor panel, a building panel, a wall panel and / or a ceiling panel, or a core layer thereof. The panel can for example be a decorative panel. The method according to the present invention overcomes the difficulties which arise when foaming PET and in particular when foaming recycled PET. Therefore, the method enables the upcycling of PET. Non-limiting example for the use of a panel according to the invention and / or a panel obtained by the method according to the present invention are for building and decorative panelling, for example in high rise and high-risk buildings such as hospitals, schools and social venues. This is possible due to the good dimensional stability, indentation and impact resistance and fire proofness of the panel.

[0019] The extruder applied in the method according to the present invention typically comprises at least one extrusion die and at least one extrusion barrel, wherein in step d) the at least one physical foaming agent may be injected in the melt at a location within the barrel. In step e) the melt is extruded through at least one die. The injection of at least one physical foaming agent in the melt is advantageously performed in the extrusion barrel where at least one extrusion screw is provided for a homogenous mixing of the physical foaming agent and the melt. This results in homogenously distributed cells in the at least partly foamed core layer or panel.

[0020] The applied extruder can for example be a twin-screw extruder. The temperature of the at least one extrusion die is preferably adjustable. An increased die temperature during extrusion of the foamed core layer will typically result in increasing cell size and decreasing cell density. This could for example be due to the reduced melt viscosity at these elevated temperatures. A reduced die temperature during extrusion of a foamed core layer could results in reduced cell size and increased cell density, and an increase in crystallinity.

[0021] In a possible embodiment, at least one physical foaming agent may be selected from supercritical carbon dioxide (CO2), supercritical nitrogen (N2), or a combination thereof. It is also imaginable that at least one physical foaming agent comprises supercritical carbon dioxide and / or supercritical nitrogen. These compounds are abundant and available at relatively low cost. Advantageously, CO2, or carbon dioxide, is a greenhouse gas. Trapping this gas in a foamed PET panel further contributes to reducing the amount of CO2 gas in general. At least one physical foaming agent, such as CO2 or N2 can be introduced at a high pressure into the extruder, this process is referred to as supercritical foaming. Physical foaming agents, specifically CO2, provide several key advantages, as these have a relatively high solubility in polymers, specifically in PET, they are non-flammable, non-toxic, sustainable, cost effective, their production is non-polluting, and dissipate completely from the melt after production, as far as not being trapped inside cells formed by foaming. At least one physical foaming agent could for example be applied via at least one pump, for example a syringe pump. In a possible embodiment, the at least one physical foaming agent is applied such that an average migration velocity of at least part of at least one physical foaming agent through the melt is substantially equal to an average velocity of extrusion of the melt in the extrusion direction. It is also possible that the migration direction of at least part of at least one physical foaming agent through the melt is substantially opposite to the extrusion direction. In a preferred embodiment, the extruded foam comprises relatively small cells. Such relatively small cells may contribute to a relative dense structure and a better resistance to thermal expansion and contraction. Experimental data shows that relatively large cells may exhibit reduced stability after being exposed to temperature fluctuations. Furthermore, the extruded foam preferably has a normal cell size distribution, with an average cell size in a central region of the core being relatively large, and an average cell size in at least two outer region of the core, preferably regions adjacent to at least two surfaces, having a relatively small average cell size. Said cell size distribution may result in a good balance of stiffness, strength, and flexibility, which may contribute to better stability and flatness even after thermal changes. In addition, cell size can also be a factor impacting flatness and cupping wherein some panels that are produced by conventional means tend to bend up- or downward due to the upward movement of the supercritical gas injected in the material during extrusion and / or during the cell nucleation step, which may also cause undesired imbalance in cell size and / or density of the panel. To resolve this, it is conceivable that step e) is performed via vertical extrusion and / or vertically oriented extrusion, and / or comprises the step of reducing the die temperature on at least one surface of the extruded core layer during cell nucleation, thereby topically increasing the melt strength and reducing the ability of the foam to offgas through or move toward said surface, preventing deformation and / or cupping.

[0022] The at least partly foamed core layer or panel obtained in step e) may be a substantially homogenously foamed core layer. A variation in density of the substantially homogenously foamed core layer over a total length, height and / or width of the substantially homogenously foamed core layer may lie within 10% of an average density of the substantially homogenously foamed core layer, preferably within 8%, more preferably within 5%, most preferably within 3%. The introduction of the physical foaming agent in the melt allows for obtaining a very homogenous core layer. The core layer having a substantially homogenously foamed core layer is beneficial for the mechanical characteristics of the panel. It is also beneficial if the plurality of cells is substantially homogenously shaped and / or substantially homogeneously distributed over the core layer. The use of at least one physical foaming agent in a method according to the present invention enables the provisional of a substantially homogenously foamed core layer. The density variation typically corresponds to the relatively density of the at least one core layer. It is for example imaginable that the variation in density of the substantially homogenously foamed core layer over a total length, height and / or width of the homogenously foamed core layer lies within 30% of an average density of the homogenously foamed core layer, preferably within 25%, more preferably within 20%. In yet another possible embodiment, the at least partly foamed core layer or panel obtained in step e) may be a substantially heterogeneously foamed core layer. The method according to the present invention enables that a controlled foaming occurs wherefore the degree of homogeneity can be at least partially tuned. Hence, the method enables for the provision of a controlled and effective foaming process of PET.

[0023] In some embodiments, the foamed core layer may comprise a cell size and / or density gradient wherein the cell sizes may vary across the thickness of the core, with smaller cells on at least one surface, conceivably on at least two surfaces, preferably the top and bottom surface. Relatively larger cells may then be present in the inner region of the core layer. This variation in cell size distribution can be promoted by the cooling rate differences between the outer and inner regions during the extrusion process. During extrusion, the outer surfaces of the panel cool more rapidly due to direct contact with cooling surfaces, such as but not limited to the extrusion die, ambient air and / or cooling devices. This may lead to the formation of relatively smaller cells, as there is less time for the gas to expand and nucleate during solidification, while the core of the panel may experience slower cooling, partly because it is insulated by the outer layers, giving more time for gas expansion and nucleation, which potentially results in the formation of larger cells.

[0024] In a preferred embodiment, the provided melt comprises at least one chain extender. Preferably, the at least one chain extender (CE) comprises at least one multifunctional epoxy, multifunctional epoxide, bis-oxazoline, dianhydride, diisocyanate, tetrafunctional epoxy, tetrafunctional dianhydride, pyromellitic dianhydride (PMDA), tetraglycidyl diamino diphenyl methane (TGDDM), styrene- acrylic epoxide, styrene-acrylic oligomer epoxide, or any combination thereof. These chain extenders were found to work particularly well for increasing the IV of PET, in particular the IV of PCR PET. Chain extenders bind hydroxyl and / or carboxyl chain ends of PET oligomers with di- and polyfunctional reagents, thereby increasing their average molecular weight, enhancing chain entanglement and inducing the forming cross-bonds. In line therewith, at least one chain extender preferably comprises at least one functional reagent, wherein the at least one functional reagent preferably comprises a functional group selected from -OH, - COOH, or any combination thereof.

[0025] At least part of the polyethylene terephthalate may be post-consumer recycled polyethylene terephthalate having an intrinsic viscosity in the range of 0.50 - 0.75 dL / g, preferably in the range of 0.50 - 0.70 dL / g, more preferably in the range of 0.50 - 0.65 dUg. PCR PET has a relatively low cost. Therefore, increasing the IV of this PCR PET is advantageous, as the value of this PET is greatly enhanced. The subsequent foaming of the PET further enhances its value, as a high-quality core layer is obtained.

[0026] A weight percentage of polyethylene terephthalate in the mixture is at least 50wt%. It is possible that the weight percentage of polyethylene terephthalate ranges from 50.0 - 99.5 wt%, preferably from 55 - 90 wt.%, more preferably from 55 - 80 wt.%, based on the total weight of the mixture. It is also possible that the weight percentage of polyethylene terephthalate ranges from 50-70wt%. A relatively high percentage of PET ensures that the core layer can be properly foamed. Additives such as mineral fillers and / or flame retardants to the core layer in general reduce the ability of the core layer to foam and increase the rate at which cells collapse. In addition, the presence of contaminants may further reduce the formation of cells, in particular the formation of relatively small cells, which is one of the main reasons PET usually can only be recycled twice. Optionally, the polyethylene terephthalate is at least partially recycled PET, more preferably post-consumer recycled PET.

[0027] At least one chain extender may increase the average molecular weight and / or increase chain entanglement. It is also imaginable that at least one chain extender is configured to enable the formation of cross-bonds within the polymer chains. In a possible embodiment, the weight percentage of at least one chain extender in the mixture ranges from 0.5 - 5 wt.%, preferably from 0.5 - 3 wt.%, more preferably from 0.5 - 2 wt.%, most preferably from 0.5 - 1 wt.%, based on the total weight of the mixture. It was experimentally found that if at least one chain extender is applied in said, relatively small, concentrations a desired modification of the PET will occur.

[0028] In a possible embodiment, the mixture which is to be extruded comprises at least one mineral filler. At least one mineral filler is preferably selected from silicon dioxide, magnesium dihydroxide, aluminium trihydroxide, calcium carbonate, magnesium carbonate, talc, chalk, montmorillonite, dolomite, or any combination thereof. The use of at least one filler could positively contribute to the obtain the desired characteristics of the material. At least one mineral filler, if applied, preferably has an average particle size of at least 200 mesh and preferably at least 300 mesh and / or at most 800 mesh, in particular according to ISO 565. At least one mineral filler, if applied, may have an average particle size ranging between 200 and 800 mesh and preferably between 300 and 400 mesh, in particular according to ISO 565. The use of such particle size is beneficial in order to ensure sufficient processability of the mixture / melt and to prevent that the mineral filler would negatively affect the foamed configuration of the panel or core layer. It is possible that at least 5 wt.%, preferably at least 10 wt.% of at least one mineral filler is applied. It is also imaginable that at least 20 wt.% or at least 30wt.% of mineral filler is applied within the mixture. In a further possible embodiment, the weight percentage of mineral filler ranges from 10 - 70 wt%, more preferably from 20-60 wt%, most preferably 30-50% by weight, based on total weight of the mixture. It is also conceivable that the weight percentage of mineral filler ranges from 0.5 - 50 wt.%, more preferably from 10 - 45 wt.%, most preferably between 20-45% by weight, based on total weight of the mixture.

[0029] It is also imaginable that the mixture comprises at least one nucleating agent. At least one nucleating agent, if applied, is preferably selected from talc, silicates, magnesium silicates, silica, chalk, montmorillonite (MMT), nanoclay, limestone, stearates, calcium stearate, zinc stearate, rubber particles, citric acid, sodium bicarbonate, sodium benzoate, or any combination thereof. The use of at least one nucleating agent may optimize dispersion of the cells within the foamed material. At least one nucleating agent will positively contribute to creating a substantially heterogenous melt composition, which will have a positive impact on the diffusion and dispersion of the (supercritical) foaming agent. The use of at least one nucleating agent may further provide improvement to the expansion rate and / or the nucleation process of the cells and / or improve the crystallization rate of PET. At least one nucleating agent preferably has an average particle size smaller than 500 mesh, preferably smaller than 800 mesh, in particular according to ISO 565. Preferably, at least one nucleating agent preferably has an average particle size ranging between 500 and 1500 mesh, preferably between 800 and 1200 mesh, more preferably between 900 and 1 100 mesh, in particular according to ISO 565. It is imaginable that the mixture comprises at least one mineral filler and at least one nucleating agent. Said least one mineral filler and at least one nucleating agent may comprise the same material or a similar functional group. However, at least the mesh size of the material will be defining whether the material will function as filler or as nucleating agent. A mineral filler material having a relatively large mesh size typically cannot function as nucleating agent for the mixture.

[0030] As indicated above, the desire to utilize and upscale (post-consumer) recycled PET has the difficulty to deal with a low intrinsic viscosity of the PET whilst a supercritical foaming process is to be applied. Adding an optional filler would typically lower the intrinsic viscosity even further, which also applies for the addition of at least one flame retardant. The use of at least one chain extender and / or at least one nucleating agent can overcome at least some or most of these challenges.

[0031] It is also imaginable that the mixture comprises at least one flame retardant. PET and in particular PET waste is on itself moderately flammable due to the presence of heteroatoms in the polymeric chain. Further, foamed polymers are typically more combustible with the same formulation than solids, due to an increased contact area between the polymer and the air, and the reduced volume concentration of any flame retardants present in the foam structure. Therefore, the use of at least one flame retardant could further improve the characteristics of the final product. If applied, at least one flame retardant is preferably an inorganic flame retardant, a phosphorous-based flame retardant, or a combination thereof. The use of at least one flame retardant may positively contribute to the fire-retardant properties of the panel. The at least one flame retardant could reduce the IV of the mixture. The mixture may comprise at least one flame retardant selected from zinc diethyl phosphinate, 2-chloro-5,5-dimethyl-1 ,3,2-dioxaphoshinane-2-oxide, halogenated flame retardants, zinc di-phosphinate, 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide, pentaerythritol phosphate alcohol, metal phosphorate, aluminium-alkylphosphinate, pentaerythritol spirobis(methylphosphonate), aluminium-diethylphosphinate, DOPO-O-PEPA, DEPzn, aluminium trihydroxides, magnesium dihydroxides, silicone-based compounds, montmorillonite or any combination thereof. At least one flame retardant could also act as nucleating agent.

[0032] In a further possible embodiment, the mixture comprises at least one metal stearate such as zinc stearate. Zinc stearate may be added as a facilitating agent and / or as flame retardant. It is for example imaginable that zinc stearate is applied in a ratio of 15-30:1 of flame retardant to zinc stearate, preferably 18-22:1 , more preferably around 20:1.

[0033] It is possible that at least one flame retardant is a halogenated flame retardant and / or a phosphorous-based flame retardant. In a beneficial embodiment, at least one flame retardant is or comprises pentaerythritol spirobis(methylphosphonate), a halogenated flame retardant and / or aluminium diethylphosphinate. The activation temperature of at least one flame retardant may be at least 250 °C, more preferably at least 275 °C, most preferably at least 300 °C. Said high activation temperature contributes to ensure that the flame retardants in the panel remain inactive under normal operating conditions or during lower-temperature processing. This stability prevents premature degradation of the core layer and ensures that the flame retardants are activated only when exposed to high temperatures, such as during a fire. By incorporating halogenated and / or phosphorus-based flame retardants with aforementioned activation temperature, the core layer retains its mechanical properties during normal use and processing. This ensures that the PET matrix remains strong and rigid, avoiding premature weakening or degradation caused by early activation of the flame retardants. As a result, the panel achieves enhanced fire resistance, thermal stability, and mechanical integrity, enabling it to perform effectively under fire conditions without compromising its structural properties during regular use.

[0034] In a beneficial embodiment of the method according to the present invention, the at least one chain extender is or comprises pyromellitic dianhydride (PMDA) and the mixture comprises at least one flame retardant and zinc stearate, wherein preferably a weight ratio between the at least flame retardant and the zinc stearate is between 10:1 and 30:1 , preferably between 15:1 and 25:1. It has been established that at aforementioned ratios, the PMDA effectively improves the molecular integrity of PET by repairing degraded chains, leading to increased molecular weight and enhanced thermal stability, thereby ensuring that the core layer retains its structure under high temperatures or prolonged use. The inventors have surprisingly found that at said ratio of the at least flame retardant and zinc stearate, a balanced composition where fireproofing properties are optimized without compromising the mechanical integrity or stability of the PET matrix is achieved.

[0035] In a further possible embodiment, at least one nucleating agent comprises montmorillonite, and at least one flame retardant is a phosphorus-based flame retardant, preferably aluminium-alkylphosphinate. It was surprisingly found that this embodiment is highly suitable for use as a building panel. The combination of montmorillonite and a phosphorus-based flame retardant significantly enhances the panel’s fire resistance. Montmorillonite promotes char formation and, along with the flame retardant, releases non-combustible gases during decomposition. This creates a protective barrier that slows flame spread and reduces heat release, greatly improving the panel's fireproof properties. Optionally, montmorillonite enhances the PET matrix by promoting a finer, more uniform crystalline structure, which improves the mechanical properties of the core layer and the panel as a whole. Montmorillonite may also reinforce the char layer formed during combustion, while the phosphorus-based flame retardant, such as aluminium-alkylphosphinate, boosts the thermal barrier properties of the char. This synergy particlu larlu contributes to the panel's thermal stability, with montmorillonite reinforcing the polymer matrix and aluminium-alkylphosphinate ensuring the core layer retains its integrity under high temperatures. Furthermore, phosphorus-based flame retardants, particularly aluminium-alkylphosphinate, produce less smoke and fewer toxic emissions compared to halogen-based alternatives, contributing to a safer environment during a fire. This dual action not only increases the flame retardant's effectiveness but also makes the panel safer and more durable for use in demanding applications. It is also imaginable that at least one flame retardant, if applied, comprises aluminium diethylphosphinate, montmorillonite and / or PSMP, and preferably wherein aluminium diethylphosphinate, montmorillonite and / or pentaerythritol spirobis(methylphosphonate) is present in a weight percentage from

[0036] 1 - 10 wt.%, preferably 2 - 8 wt.% based on total weight of the mixture.

[0037] It is imaginable that step c) includes heating the mixture to a temperature between 245 °C and 255 °C, and subsequently heating the mixture to a temperature between 265 °C and 275 °C, while the mixture is subjected to shear forces to obtain a melt. These temperatures are beneficial for the extrusion of PET. Possibly, after step d) the melt is cooled to a temperature ranging between 220 °C and 260 °C, preferably between 230 °C and 250 °C, more preferably between 235 °C and 245 °C. In this way, it can be prevented that the melt is cooled down too rapidly, which could result in that the orientation of the polymer molecules in the melt, which are randomly aligned, curved and entangled remain frozen and the solid has disordered structure. Relatively slow cooling will enable that some polymer chains take on a certain orderly configuration such that they align themselves in a substantially crystalline configuration.

[0038] It is also possible that after step e) of extruding the melt, a temperature of at least part of the at least partly foamed core layer is maintained within a crystallisation temperature range for at least a certain time period, wherein the crystallisation temperature range ranges between 20 °C, 15 °C or 10 °C above the glass transition temperature, Tg, of the polyethylene terephthalate present in the at least partly foamed core layer and 10 °C, 15 °C or 20 °C below the melting temperature, Tm, of the polyethylene terephthalate present in the at least partly foamed core layer. In yet a further possible embodiment crystallisation temperature range ranges from [(Tg+Tm) / 2] ± 20 °C. By controlling the temperature within the aforementioned crystallization temperature range, the PET matrix develops a more uniform and refined crystalline structure which consequently enhances the mechanical properties of the core layer. Operating within said range, between 10°C above the glass transition temperature (Tg) and 10 °C below the melting temperature (Tm) ensures that the PET matrix is neither too rigid nor too fluid during crystallization. This balance allows the PET to crystallize effectively without defects such as warping or cracking. Proper crystallization minimizes residual stress within the PET matrix which can occur if cooling happens too rapidly or at suboptimal temperatures thereby reducing the likelihood of deformation or failure over time. In an experimentally found embodiment, the crystallisation temperature ranges from 168 °C to 188 °C. It was also found that a relatively low crystallization temperature, preferably from 148 °C to 168 °C may promote the formation of relatively small crystallites, which are clustered segments where polymer crystals align, compared to those formed at higher temperatures. Relatively small crystallites could be particularly advantageous to the strength of the panel, in particular when the panel according to the invention is combined with an intricate interlocking mechanism or locking means.

[0039] The method may include that after step e) and prior to maintaining a temperature of at least part of the at least partly foamed core layer within the crystallisation temperature range, the at least partly foamed core layer is cooled to below the glass transition temperature Tgto form an at least partially amorphous panel. It is also possible that the temperature of the at least part of the at least partly foamed core layer is maintained within the crystallisation temperature range for at least 10 seconds, preferably at least 30 seconds, more preferably at least 1 minute, such as 1 - 3 minutes. This will have a possible effect on the polymer structure of the core layer. Maintaining the temperature of the at least partly foamed core layer within the crystallization range for 10 seconds to 3 minutes enhances the polymer structure by allowing the PET matrix to crystallize fully and uniformly. This promotes a well- ordered crystalline structure, improving mechanical properties like strength, rigidity, and impact resistance, essential for a durable, load-bearing panel. The extended duration also stabilizes the foam structure, preventing defects such as uneven density or warping, and minimizes residual stress, enhancing dimensional stability and thermal resistance. Overall, this process results in a high-performance panel with superior strength, stability, and durability.

[0040] Possibly, at least part of the at least partly foamed core layer which is maintained at a temperature within the crystallisation temperature range is a part of the at least partly foamed core layer ranging from an outer surface of the at least partly foamed core layer inwardly to a center of the at least partly core layer by a distance of at most 25% of a total thickness of the at least partly foamed core layer, preferably by a distance of at most 15% of the total thickness of the at least partly foamed core layer. Limiting the crystallization process to a specific distance of up to 25%, preferably 15%, of the total thickness of the core layer strengthens the outer portion of the core layer, which is most exposed to mechanical stresses, impact, and wear, while leaving the inner portion less crystallized and lighter. This selective crystallization optimizes the panel’s overall weight, making it easier to handle and install without compromising surface durability. Additionally, maintaining this precise distance minimizes the risk of residual stresses, which can lead to defects such as warping or cracking during production or use. The inner core retains greater thermal insulation and flexibility, complementing the strength and rigidity of the crystallized surface. By focusing crystallization within this defined depth, the panel achieves a balance of mechanical robustness, thermal performance, and lightweight construction, making it efficient to produce and versatile for various applications.

[0041] It is possible that after step e) at least one outer surface of the at least partly foamed core layer is subjected to a temperature higher than the melting temperature, Tm, of the polyethylene terephthalate present in the at least partly foamed core layer, preferably higher than Tm- 10 °C, for at least 1 second, preferably at least 5 seconds, in particular in order to form at least one at least partially sealed outer surface. This so-called sealing step may seal at least part of the cells of at least one outer surface of the at least one core layer. It is conceivable that at least the top surface, preferably top and bottom surface of the at least partly foamed core layer is an at least partially sealed outer surface. This may be identified by a relatively low presence of cells, preferably lack of cells, present on said sealed outer surface.

[0042] At least one heating step applied in the method according to the present invention could also be a secondary crystallization step. By means of heating on at least one, preferably on at least two surfaces of the core layer, preferably on the top and / or bottom surfaces thereof, a crystallinity gradient may be provided on the at least partially foamed (semi-crystalline) PET core layer. It is for example imaginable that the at least one core layer is subjected to a secondary crystallization step, which is performed by subjecting the panel or core layer on at least one of its surfaces to a temperature gradient after step e). The applied temperature gradient will increase the crystallization rate at said surfaces and may promote the growth of crystallites and / or lamellae, which lamellae may form perpendicular to the temperature gradient. This perpendicular formation will result in the unexpected advantage of increased strength in the plane of the panel compared to a panel extruded according to conventional methods.

[0043] In one particularly advantageous embodiment, the at least one core layer could for example be subjected to a temperature gradient in a cooling section and / or by leading the at least one core layer over at least one, and preferably a pair of temperature controlled rollers. A core layer which has been quickly cooled down after extrusion, which is not commonly done in the art, may have a low crystallization rate by virtue of the cooling process and is therefore mostly amorphous in nature, in contrast with common panels, which are mostly crystalline in nature. A possible and particularly advantageous method according to the present invention provides for the provision of a panel, or core layer, comprising at least one substantially amorphous region and at least one substantially crystalline region. The at least one (secondary) crystallization steps is preferably performed at a temperature of (Tg+Tm) / 2. The temperature could also be 5 °C higher or lower. The temperature may also range from 148C-188C, preferably from 148C-168C, which may have an advantageous effect on the size of crystallites present in the panel, or core layer.

[0044] The method may further include the step of adhering at least one decorative top layer to the at least partly foamed core layer. At least one decorative top layer, if applied, may for example comprise at least one ply of cellulose-based layer and a cured resin, wherein the cellulose-based layer is preferably paper or kraft paper. Said ply of cellulose-based material may also be a veneer layer adhered to a top surface of the core layer. The veneer layer is preferably selected from the group consisting of wood veneer, cork veneer, bamboo veneer, and the like. Other decorative top layers that could possibly be applied for the present invention include a ceramic tile, a porcelain tile, a real stone veneer, a rubber veneer, a decorative plastic or vinyl, linoleum, and decorative thermoplastic film or foil. The top layer may possibly be further provided with a wear layer and optionally a coating. Examples of thermoplastics which could be used in such top layer are PP, PET, PVC and the like. It is also possible to provide on the top facing surface of the core an optional primer and print the desired visual effect in a direct printing or digital printing process. The decorative top layer can receive a further finishing with a thermosetting, EB- or UV-curing varnish or lacquer such as polyurethane, PUR, bio-PUR, an acrylic resin, non-isocyanate polyurethane, a melamine based resin, and / or combinations thereof. In other embodiments, the decorative top layer as used in a wall or floor panel can have increased visual appeal or aesthetics such as high gloss, textured design, matte bevel, or combinations thereof which can be achieved by having the said top layer comprising: (a) a digital printed, direct printed or transfer printed design with at least one layer of coating; (b) a polyethylene terephthalate or polyethylene terephthalate glycol (PETg) decor film and / or at least one polyethylene terephthalate wear layer; (c) electron beam (EB) cured foil, coating or paper; (d) excimer cured foil, coating or paper; (e) at least one layer of paper, foil, veneer, or ceramic; and / or combinations thereof. For a decorative floor panel, it is desirable that the decorative top layer comprises: (a) a digital printed or transfer printed design with at least one layer of coating such as a hotmelt resin, polyurethane (PUR) or acrylic coating; (b) cellulosic material such as an EB-cured paper, a resin impregnated paper, or a cellulosic material with a decorative print; (c) at least one layer of paper, foil, veneer, and / or ceramic; and / or combinations thereof. Moreover, the panel can be used as a decorative construction panel having a decorative top layer comprising at least one moisture barrier such as a PET or PETg layer and / or a sealer resin or sealing layer preferably comprising a bio-based resin, a bio-based PUR resin, an acrylic resin, a polyurethane resin, a melamine resin, and / or a non-isocyanate based polyurethane. In a further embodiment, the panel can also be a siding panel wherein the decorative top layer comprises (a) Econic tile; (b) at least one tile layer; (c) at least one metal layer such as an aluminium layer; (d) thermoplastic polyolefins (TPO) or resins formed via the combination of polypropylene (PP), uncrosslinked EPDM rubber, and polyethylene; (e) at least one PET or PETg layer; (f) at least one geopolymer layer; (g) at least one mineral layer; (h) at least one cementitious layer; and / or combinations thereof. It is, however, conceivable that the above examples are non-limiting and may apply to all decorative top layers as used for wall, floor, ceiling, construction, building, and / or siding panels. The thickness of said decorative top layer may range from 0.01 -2mm, preferably from 0.1 -1 mm, most preferably from 0.15-0.65mm.

[0045] The invention further relates to a panel, in particular a building panel, a floor panel, a wall panel and / or a ceiling panel, comprising at least one core layer in particular obtained or obtainable via a method according to the present invention, and at least one decorative top layer adhered, or attached, to the at least one core layer, wherein the at least one core layer comprises at least 50 wt.% polyethylene terephthalate based on total weight of the core layer, wherein the at least one core layer is at least partially foamed and comprises a plurality of cells, and preferably wherein an average diameter of the cells ranges between 0.01 - 0.50 mm. Within the context of the present invention, cells with an average diameter in the range of 0.01-0.20 mm classify was relatively small cells, and cells with an average diameter in the range of 0.30-0.50mm classify as relatively large cells. In particular, cells with an average diameter in the range of 0.01 -0.1 mm are relatively small cells, and cells with an average diameter in the range of 0.2-0.3mm are relatively large cells.

[0046] The panel according to the present invention can be made via a method according to the present invention. Any of the described embodiment for the method also apply for the panel according to the present invention. The panel benefits of the material characteristics of polyethylene terephthalate wherein the foamed configuration of the core layer results in the panel having a low density. The panel particularly benefits from its at least partially foamed PET core layer. The partial foaming of the PET in the core layer introduces voids within the polymer matrix thereof creating at least partially cellular structure therein, which significantly reduces the panel’s overall density. This reduction in density directly contributes to a lightweight product, which is particularly advantageous for applications requiring ease of handling, transportation, and installation, such as in construction and interior design industry. Unlike known panels, which typically rely on solid PET cores or incorporate heavy fillers to achieve strength, the panel of the present invention achieves comparable or even superior performance with a significantly lighter structure. It is highlighted, however, that despite the reduction in density, the at least partially foamed PET core layer, and the panel in whole, retains high mechanical integrity. Known panels often suffer from a trade-off between density and mechanical performance, wherein reducing density leads to substantial compromises in strength. The core layer, and thus the panel, benefits of a sufficient strength to weight ratio, good indentation and impact resistance and a good screw pullout strength, whilst being flexible enough to optionally sport a click or interlocking mechanism.

[0047] In addition, the structure of the partially foamed PET core layer maintains sufficient density, in particular in critical regions, to ensure excellent screw pullout strength. This may allow the panel to be used in applications where secure fastening is required, such as in modular constructions or the installation of heavy fixtures. Furthermore, the at least partially foamed core layer distributes stress efficiently across its structure, which helps to maintain high tensile and compressive strength. Consequently, an excellent strength-to-weight ratio is achieved making the panel of the present invention ideal for use in structural applications where maintaining strength while minimizing weight is critical. The at least partially foamed PET core layer also imparts high dimensional stability to the panel in whole under environmental fluctuations. This represents a significant improvement over known panels, which are often prone to dimensional instability due to inconsistent material properties or reliance on heterogeneous compositions. The cellular structure of the core layer contributes to mitigate the effects of thermal expansion and contraction, ensuring consistent panel dimensions and performance overtime. This property is particularly important in applications subjected to temperature variations, such as exterior building panels.

[0048] More importantly, the panel of the present invention enables the use of recycled PET while also reducing, or even avoiding, chemical foaming agents thereby minimizing its environmental footprint. Known panels commonly utilize chemical foaming agents that release harmful byproducts during production, contributing to environmental degradation. The partial foaming reduces the quantity of raw PET material required for production, resulting more affordable panels without compromising performance. This is further amplified when using recycled PET, enhancing the product’s cost-effectiveness and sustainability. By contrast, many known panels rely heavily on virgin materials, making them less sustainable and cost-efficient. It is also emphasized that the voids within the foam in the core layer reduces the overall material consumption, which aligns with sustainable production practices. Additionally, known panels often lack the capability to tune properties effectively, limiting their versatility in application. Lastly, the partial foaming of the core layer allows manufacturers flexibility in production by making the properties of the panel tuneable depending on the degree of foaming and cell structure of the core layer. This customization enables manufacturers to optimize the panel’s performance for specific applications, ranging from load-bearing construction elements to decorative wall panels. The core layer is in particular at least partially foamed and comprises a plurality of cells, wherein an average diameter of the cells ranges between 0.01 - 0.50 mm. The cells forming the foamed configuration are relatively small, which is beneficial for the characteristics of the panel. It is also imaginable that at least part of the cells has an average diameter of in the range of 0.01 - 0.30 mm, in particular in the range of 0.02 - 0.20 mm, more in particular in the range of 0.03 - 0.10 mm. Relatively small cells will result in a more stable product with a higher relative strength.

[0049] It is imaginable that at least one core layer comprises a top surface and a bottom surface, wherein at least part of the top surface and / or at least part of the bottom surface is substantially free from open cells. It is also imaginable that at least part of the top surface and / or at least part of the bottom surface is sealed. A sealed top and / or bottom surface of the core layer can positively contribute to the further processability of the panel.

[0050] In a possible embodiment, a variation in density of the core layer over a total length, height and / or width of the core layer lies within 10% of an average density of the core layer, preferably within 8%, more preferably within 5%, most preferably within 3%. A relatively low variation in density can positively contribute to the stability of the panel.

[0051] It is possible that the at least one core layer comprises at least one mineral filler. At least one mineral filler, if applied, is preferably selected from the group of: silicon dioxide, magnesium dihydroxide, calcium carbonate, magnesium carbonate, talc, chalk, montmorillonite, dolomite, or any combination thereof. It was found that said mineral filler material could positively contribute to the characteristics of the panel when applied in combination with polyethylene terephthalate. The at least one mineral filler preferably has a relatively large average particle size. It is for example imaginable that the at least one mineral filler has an average particle size ranging between 200 and 800 mesh, preferably between 300 and 400 mesh, in particular according to ISO 565. At least one mineral filler, if applied, may have an average particle size below 800 mesh and preferably below 600 mesh, in particular according to ISO 565. It is possible that at least 5 wt.%, preferably at least 10 wt.% of at least one mineral filler is applied. It is also imaginable that at least 20 wt.% or at least 30 wt.% of mineral filler is applied within the mixture. In a further possible embodiment, the weight percentage of mineral filler ranges from 10 - 70 wt.%, more preferably from 20 - 60 wt.%, most preferably between 30-50% by weight, based on total weight of the mixture. It has been experimentally found that mineral filler with particle sizes ranging from 200 to 800 mesh, preferably between 300 and 400 mesh, and incorporated in aforementioned proportions, 10-70wt % and preferably 20-60wt% or most preferably between 30-50wt%, achieves a balance between processability, product durability, and foaming efficiency, making it possible to produce lightweight, high-strength panels. Mineral fillers at said particle sized incorporated in the mixture at said amounts are found to be optimized for uniform dispersion within the mixture, effectively minimizing the risk of aggregation. Such uniformity not only enhances the mechanical stability of the resulting panel but also ensures that the fillers do not interfere with the foaming process or compromise the configuration of the foamed core layer. The particle sizes of the mineral filler or the mineral filler content significantly contributes to the stabilization of foam cells, preventing their premature collapse and ensuring that cell size and distribution remain consistent throughout the core layer. This stabilization is vital for achieving a uniform and mechanically robust structure. The foam cells’ uniformity is important in producing a panel with enhanced structural integrity, thermal resistance, and dimensional stability, which are essential attributes for their application in construction and other demanding environments.

[0052] Moreover, the optimized particle size and proportion of the filler ensure that the foaming process is not adversely affected, maintaining the integrity of the lightweight foamed structure. By stabilizing the foam cells and supporting consistent expansion during foaming, these fillers at the specified particle sizes and amount help produce panels with improved mechanical properties, such as flexural strength, impact resistance, indentation resistance, and reduced shrinkage or cupping. This ensures that the panels perform reliably even under varying environmental conditions.

[0053] The total volume of the plurality of cells divided by a total volume of the core layer preferably ranges from 0.34 to 0.93, such as from 0.34 to 0.50, from 0.50 to 0.70, or from 0.70 to 0.93. Hence it is imaginable that the void fraction of the at least one core layer is at least 30%, preferably at least 50%, more preferably at least 75%. The void fraction can for example range from 34 to 93%. A higher expansion rate typically corresponds to a higher void fraction. It is imaginable that the at least one core layer has an expansion rate between 1 -15, for example 1.5-2 or 3-6 or 7-10. According to the embodiments of this invention, a microfoamed panel comprising 30-40% mineral content such as calcium carbonate (CaCO3) and 60-70% thermoplastic such as PET has a low expansion ratio between 1 -1 .6 and a void fraction between 30-40% wherein the density of the mineral content is between 2000-2500 kg / m3 and the density of the thermoplastic is between 1200-1500 kg / m3. The resulting expansion ratio is relatively smaller than a foamed PET panel comprising mineral contents of less than 5%, which may have an expansion ratio of 12-15 and a void fraction between 90-95%. Any combination or range in between is within the conceivable bounds of the present invention, for example a panel comprising 5-30% mineral content and 70-95% PET forming a density of 650- 950kg / m3, further comprising at least one sealed surface, may be particularly suited to form the core of a flooring panel or structural building panel.

[0054] It is possible that the at least one core layer comprises at least one nucleating agent. If applied, the at least one nucleating agent preferably has an average particle size ranging between 500 and 1500 mesh, preferably between 800 and 1200 mesh, more preferably between 900 and 1 100 mesh, in particular according to ISO 565. The at least one nucleating agent preferably has a particle size which is smaller than 800 mesh, preferably smaller than 1000 mesh, in particular when determined according to ISO 565.

[0055] In order to improve the panel characteristics, and in particular the fire proofness of the panel, the core layer may comprise at least one flame retardant. The at least one core layer preferably comprises at least one flame retardant selected from zinc diethyl phosphinate, 2-chloro-5,5-dimethyl-1 ,3,2-dioxaphoshinane-2-oxide, halogenated flame retardants, zinc di-phosphinate, 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide, pentaerythritol phosphate alcohol, metal phosphonate, aluminium-alkylphosphinate, pentaerythritol spirobis(methylphosphonate), aluminium-diethylphosphinate, DOPO-O-PEPA, DEPzn, aluminium trihydroxides, magnesium dihydroxides, silicone-based compounds, montmorillonite or any combination thereof. The inventors have found that the inclusion of the aforementioned flame retardants effectively interrupts combustion processes thereby reducing flammability of the core layer and the panel in whole. It is deemed that these flame retardants in combination with the mineral fillers in the core layer forms a protective layer during heat exposure, insulating the core layer and delaying structural breakdown. It is deemed that this combination minimizes smoke and toxic gas emissions thereby ensuring safer usage in residential and commercial spaces. Most importantly, the aforementioned flame retardants are deemed to be effectively compatible with PET, maintaining its mechanical properties such as strength and rigidity while enhancing its thermal stability.

[0056] In a beneficial embodiment, the panel comprises at least one nucleating agent which comprises montmorillonite, and at least one flame retardant which is a phosphorus-based flame retardant, preferably aluminium-alkylphosphinate. It was experimentally found that this embodiment benefits of the montmorillonite functioning as a facilitating agent and a nucleating agent.

[0057] In a further possible embodiment, the at least one flame retardant comprises aluminium-alkylphosphinate present in 6 - 8 wt.% based on total weight of the core layer, and wherein the at least one nucleating agent comprises montmorillonite and / or polytetrafluoroethylene (PTFE) present in 0.5 - 2 wt.% based on total weight of the core layer, and optionally wherein the core layer comprises a chain extender comprising an epoxy-based styrene-acrylic oligomer present in 1 wt.% based on total weight of the core layer.

[0058] Optionally, the at least one flame retardant, if applied, comprises a halogenated flame retardant (HFR), aluminium diethylphosphinate, pentaerythritol spirobis(methylphosphonate) (PSMP), or any combination thereof. The at least one flame retardant is preferably present in 1 - 10 wt.%, preferably 2 - 8 wt.%, based on total weight of the core layer. It has been established that said flame retardants are highly effective at reducing the flammability of the core layer and the panel in whole by effectively interrupting the combustion process, promoting char formation, and releasing flame-inhibiting radicals, thereby providing robust fireproofing properties even at low concentrations of 1-10wt%. At this concentration, the mechanical properties of the core layer are not significantly compromised. In yet a further possible embodiment, the at least one nucleating agent comprises zinc stearate, and the core layer comprises a chain extender comprising pyromellitic dianhydride (PMDA). The addition of nucleating agents like zinc stearate enhances the crystallization of PET during the production process, thereby achieving improved mechanical properties such as strength, rigidity and dimensional stability. It imparts a more uniform crystalline structure in the core layer thereby contributing to the durability and resistance to deformation under load of the core layer and the panel in whole.

[0059] The average density of the at least one core layer ranges preferably between 100 - 1200 kg / m3. It is for example imaginable that the average density of the at least one core layer ranges between 100 - 250 kg / m3in particular for non-structural ceiling or wall panels, 200 - 600 kg / m3for example for structural wall panels, or 700 - 1200 kg / m3for example for load-bearing panels or flooring. It is also imaginable that the density of at least one core layer is in the range of 100 - 350 kg / m3. The obtainable relatively low densities are achieved by the foamed configuration of the PET core layer. The density could be at least partially tuned by applying at least one filler material, for example a mineral filler. In one particular embodiment where foaming is less desired or mineral content is of key importance, such as for countertops, it is imaginable that the average density of the at least one core ranges between 1200- 1800kg / m3.

[0060] It is possible that the panel, and in particular the core layer, comprises at least one antioxidant preferably selected from primary oxidants, phenol-based oxidants, sterically hindered phenols, 2,4-di-t-butylphenol, Irganox 1010, Irganox 1076, Irganox 1098, Irganox 1330, Irganox 245, Irganox 1135, secondary oxidants, antioxidant 168, Irgafos 126, Irganox PS 800, Irganox PS 802, or any combination thereof. It has been found that the addition of said at least one antioxidant protects the core layer during high-temperature extrusion or foaming thereby preventing breakdown and ensuring consistent quality of the panel produced. It is also established that the presence of said at least one antioxidant mitigates discoloration and aesthetic degradation caused by oxidative reactions, preserving the panel’s appearance over time. By combining primary and secondary oxidants such as Irganox 1010 and Irgafos 168, the core layer received comprehensive stabilization, addressing both free radical formation and hydroperoxide decomposition. As a result, the resulting core layer and panel in whole, is made more resilient to harsh environmental conditions such as UV exposure and temperature fluctuations.

[0061] The panel, and in particular the at least one core layer preferably has an average percent crystallinity of at least 1%, in particular determined according to ASTM- F2778. It is preferred that the average percent crystallinity of the at least one core layer ranges from 15 to 60%, more preferably from 20 to 50%, most preferably from 30 to 45%, in particular determined according to ASTM-F2778. A relatively high crystallinity is beneficial for the dimension stability of the panel. The panel of the present invention comprising a core layer having an average percent crystallinity of at least 1%, preferably ranging from 15% to 60%, and more preferably from 30% to 45%, imparts high crystallinity to the core layer which enhances dimensional stability, ensuring the panel retains its shape and size under mechanical or thermal stress. It also increases the panel’s mechanical strength and rigidity, enabling it to withstand greater loads and resist deformation. Additionally, a more crystalline core layer improves thermal resistance by raising the melting temperature and reducing the likelihood of thermal distortion, allowing the panel to perform effectively in high- temperature environments. Higher crystallinity reduces water absorption, making the panel resistant to warping, swelling, or degradation in humid or wet conditions. It also improves wear resistance, contributing to better durability against scratches and abrasion, particularly in high-traffic or industrial settings. By controlling crystallinity within the specified range, the panel achieves an optimal balance between toughness and flexibility, preventing brittleness while maintaining structural integrity. Furthermore, a highly crystalline core enhances long-term durability by resisting environmental stressors such as UV exposure, oxidation, and chemicals, extending the panel’s lifespan. This precise crystallinity range also stabilizes the material during manufacturing processes like extrusion or foaming, ensuring consistent quality and performance.

[0062] It is possible that a part of the core layer ranging from an outer surface of the core layer inwardly to a center of the core layer by a distance of at most 25% of a total thickness of the core layer, preferably by a distance of at most 15% of the total thickness of the core layer has an average percent crystallinity from 15 to 60%, more preferably from 20 to 50%, most preferably from 30 to 45%, and / or wherein the percent crystallinity of the core layer other than the part of the core layer ranges from 10 to 30%, more preferably from 15 to 25%, most preferably around 20%, in particular according to ASTM-F2778. The method according to the present invention enables for the provision of a panel or core layer which benefits of such crystallinity. It was further experimentally found that the improved crystallinity resulted in a reduction of the shrinking from 1% shrinking to 0.1% shrinking and a reduction in cupping of 2mm to 0.1 mm of the core layer when tested to ISO 23999. Hence, in a preferred embodiment the core layer has a shrinking rate of at most 0.1% and / or a cupping rate of at most 0.1 mm, in particular when tested to ISO 23999.

[0063] In another possible embodiment, the panel, and in particular at least one core layer, could comprise a substantially amorphous region and at least one substantially crystalline region. It is for example imaginable that the at least one core layer comprises at least one substantially amorphous central region and at least one substantially crystalline outer region. The panel according to the present invention comprising a core layer having at least one substantially amorphous region and at least one substantially crystalline region provides flexibility, impact resistance, and toughness, allowing the panel to absorb energy effectively and resist cracking under while also ensuring the panel retains its shape and withstands mechanical loads. This dual-phase structure enables the panel of the present invention to achieve an optimal balance between flexibility and strength, making it suitable for applications requiring both durability and adaptability. As such, the MOE of the panel according to the invention is, depending on its foaming rate and application, between 800-2000Mpa for non-structural ceiling or wall panels, between 1800- 3000Mpa for structural wall panels, or between 3000-7000Mpa for decorative floor panels or load-bearing structural panels. Additionally, the crystalline regions of the core layer enhance thermal stability and resistance to deformation at elevated temperatures, while the amorphous regions help to distribute stress evenly, reducing the likelihood of localized failure. This combination also supports efficient manufacturing, as the amorphous regions improve processability during forming or extrusion, while the crystalline regions ensure structural integrity in the final product. It is for example possible that at a substantially amorphous central region is enclosed by at least two substantially crystalline outer regions. It is also possible that a substantially or relatively amorphous central region is adjacent to at least one relatively crystalline outer region. Said at least one crystalline outer region may benefit from lamellae oriented in a plane of the panel or core, relatively small crystallites, and / or relatively small cells.

[0064] In one embodiment, a panel may comprise a core layer, comprising 90-99.5% PET and 0.5-10% of Antioxidant 168. In another embodiment, a panel may comprise a core layer comprising 80-90% of PET, 6-11 % of Silicon dioxide, 3.5-7.5% of magnesium dihydroxide, and 1 -2% of antioxidant 168. In a fire resistant embodiment, such a panel may comprise a core layer of 60-80% of PET, 6.5-10% of calcium carbonate, 6.5-10% of silicon dioxide, 4.5-7% of magnesium carbonate, 3-5% of phosphorus-based flame retardants, 1-2% of antioxidant 1076. In another embodiment, a panel may comprise a core layer comprising 50-60% recycled PET, 20-30% calcium carbonate, 10-20% magnesium carbonate, 0.4-0.8% antioxidant 168. Any of the above formulations may comprise post consumer recycled PET and at least one chain extender which may not be detectable in a fully reacted finished composition.

[0065] The panel according to the present invention may comprise at least one backing layer comprising at least one ply of resin impregnated paper, wherein at least one ply of paper is impregnated with a resin composition comprising at least one thermosetting resin and / or at least one polymer adhesive. It is conceivable that the backing layer substantially equals the top layer. Hence, any of the described top layer could also be applied as backing layer attached to a bottom core surface of the core layer. The backing layer, if applied, can be directly attached to the bottom core surface of the core layer. The backing layer can for example be a balancing layer. The bottom core surface of the core layer can have equal characteristics as the upper core surface. In a preferred embodiment is the weight of the backing layer is higher than the weight of the top layer.

[0066] It is conceivable that the panel and in particular the core layer comprises at least one reinforcing layer. In a possible embodiment, the core layer comprises multiple core layers wherein optionally two adjacent core layers enclose a reinforcing layer. The presence of at least one reinforcing layer may further enhance the impact resistance of the core layer, and thus the panel. At least one reinforcing layer may for example be present in the form of a reinforcing mat, a membrane and / or a mesh. At least one reinforcing layer may for example comprise fiber glass, polypropylene, jute, cotton and / or polyethylene terephthalate.

[0067] Depending on the intended purpose of the panel, the panel, and in particular the core layer thereof may comprise coupling means. In a possible embodiment, the panel comprises two pairs of opposite side edges, wherein at least one pair of opposite side edges, and preferably each pair of opposite side edges, is provided with complementary coupling parts. The core layer of the panel according to the present invention may comprise at least one pair of opposing (side) edges, said pair of opposing (side) edges comprising complementary coupling parts configured for mutual coupling of adjacent panels. The coupling parts may form part of the core layer. The coupling parts of the panel may for example be interlocking coupling parts, which are preferably configured for providing both horizontal and vertical locking. Interlocking coupling parts are coupling parts that require elastic deformation, a click or a movement in multiple directions to couple or decouple the parts with or from each other. Any suitable interlocking coupling parts as known in the art could be applied. A non-limiting example is an embodiment wherein a first edge of said first pair of opposing edges comprises a first coupling part, and wherein a second edge of said first pair of opposing edges comprises a complementary second coupling part, said coupling parts allowing a plurality of panels to be mutually coupled; wherein the first coupling part comprises a sideward tongue extending in a direction substantially parallel to a plane defined by the panel, and wherein the second coupling part comprises a groove configured for accommodating at least a part of the sideward tongue of another panel, said groove being defined by an upper lip and a lower lip. It is conceivable the complementary coupling parts require a downward scissoring motion when engaging, or are locked together by means of a horizontal movement. It is further conceivable that the interconnecting coupling parts comprise a tongue and a groove wherein the tongue is provided on one side edge of one pair of opposing side edges, and the groove is provided on the other side edge, or an adjacent side relative to that of the tongue, of the same pair of opposing side edges.

[0068] Further possible embodiments according to the present invention will hereinafter be presented on the basis on the following non-limitative clauses. Clauses

[0069] 1 . Method for obtaining an at least partially foamed building panel, comprising the following steps: a) providing a mixture comprising polyethylene terephthalate having an intrinsic viscosity of at most 0.75 dL / g according to ASTM D4603-18, at least one chain extender, and optionally at least one mineral filler b) feeding the mixture to an extruder, c) heating the mixture to a temperature of at least 240 °C and subjecting the mixture to shear forces to obtain a melt, d) injecting at least one physical foaming agent in the melt, and e) extruding the melt to obtain an at least partly foamed core layer of a panel.

[0070] 2. Method according to clause 1 , wherein the extruder comprises an extrusion die and an extrusion barrel, wherein in step d) the at least one physical foaming agent is injected in the melt at a location within the barrel, and wherein in step e) the melt is extruded through the die.

[0071] 3. Method according to clause 1 or clause 2, wherein at least one physical foaming agent is selected from supercritical carbon dioxide, supercritical nitrogen, or a combination thereof.

[0072] 4. Method according to clause 2 or clause 3, wherein the barrel comprises a metering section, and wherein in step d) the least one physical foaming agent is injected in the melt at a location within the metering section.

[0073] 5. Method according to any one of clauses 1 - 4, wherein the at least partly foamed core layer obtained in step e) is a substantially homogenously foamed core layer, wherein a variation in density of the substantially homogenously foamed core layer over a total length, height and / or width of the substantially homogenously foamed core layer lies within 10% of an average density of the substantially homogenously foamed core layer, preferably within 8%, more preferably within 5%, most preferably within 3%. 6. Method according to any one of clauses 1 - 5, wherein the at least one chain extender comprises at least one multifunctional epoxy, multifunctional epoxide, bis-oxazoline, dianhydride, diisocyanate, tetrafunctional epoxy, tetrafunctional dianhydride, pyromellitic dianhydride, tetraglycidyl diamino diphenyl methane, styrene-acrylic epoxide, styrene-acrylic oligomer epoxide, or any combination thereof.

[0074] 7. Method according to any one of clauses 1 - 6, wherein the at least one chain extender comprises at least one functional reagent, wherein the at least one functional reagent preferably comprises a functional group selected from -OH, - COOH, or any combination thereof.

[0075] 8. Method according to any one of clauses 1 - 7, wherein at least part of the polyethylene terephthalate is a post-consumer recycled polyethylene terephthalate having an intrinsic viscosity in the range of 0.50 - 0.75 dUg, preferably in the range of 0.50 - 0.70 dL / g, more preferably in the range of 0.50 - 0.65 dL / g.

[0076] 9. Method according to any one of clauses 1 - 8, wherein a weight percentage of polyethylene terephthalate in the mixture ranges from 50.0 - 99.5 wt%, preferably from 55 - 90 wt.%, more preferably from 55 - 80 wt.%, based on the total weight of the mixture.

[0077] 10. Method according to any one of clauses 1 - 9, wherein a weight percentage of the at least one chain extender in the mixture ranges from 0.5 - 5 wt.%, preferably from 0.5 - 3 wt.%, more preferably from 0.5 - 2 wt.%, most preferably from 0.5 - 1 wt.%, based on the total weight of the mixture.

[0078] 11. Method according to any one of clauses 1 - 10, wherein the at least one mineral filler is selected from silicon dioxide, magnesium dihydroxide, calcium carbonate, magnesium carbonate, talc, chalk, montmorillonite, dolomite, or any combination thereof.

[0079] 12. Method according to clause 11 , wherein the mineral filler has an average particle size ranging between 200 and 800 mesh, preferably between 300 and 400 mesh, according to ISO 565. 13. Method according to clause 11 or 12, wherein a weight percentage of the mineral filler ranges from 10 - 70 wt.%, more preferably from 20 - 60 wt.%, most preferably between 30 - 50 wt.%, based on total weight of the mixture.

[0080] 14. Method according to any one of clauses 1 - 13, wherein the mixture comprises at least one nucleating agent, preferably selected from talc, silicates, magnesium silicates, silica, chalk, montmorillonite, nanoclay, limestone, stearates, calcium stearate, zinc stearate, rubber particles, citric acid, sodium bicarbonate, sodium benzoate, or any combination thereof.

[0081] 15. Method according to clause 14, wherein the at least one nucleating agent has an average particle size ranging between 500 and 1500 mesh, preferably between 800 and 1200 mesh, more preferably between 900 and 1100 mesh, according to ISO 565.

[0082] 16. Method according to any one of clauses 1 - 15, wherein the mixture comprises at least one flame retardant, preferably wherein the at least one flame retardant is an inorganic flame retardant, a phosphorous-based flame retardant, or a combination thereof.

[0083] 17. Method according to any one of clauses 1 - 16, wherein the mixture comprises at least one flame retardant selected from zinc diethyl phosphinate, 2- chloro-5,5-dimethyl-1 ,3,2-dioxaphoshinane-2-oxide, halogenated flame retardants, zinc di-phosphinate, 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, pentaerythritol phosphate alcohol, metal phosphonate, aluminium-alkylphosphinate, pentaerythritol spirobis(methylphosphonate), aluminium-diethylphosphinate, DOPO-O-PEPA, DEPzn, aluminium trihydroxides, magnesium dihydroxides, silicone-based compounds, montmorillonite or any combination thereof.

[0084] 18. Method according to clause 16, wherein the at least one flame retardant is pentaerythritol spirobis(methylphosphonate), a halogenated flame retardant, or aluminium diethylphosphinate. 19. Method according to any one of clauses 1 - 18, wherein the mixture comprises zinc stearate.

[0085] 20. Method according to any of clauses 16 - 18 and clause 19, wherein the at least one chain extender is pyromellitic dianhydride (PMDA), and wherein a weight ratio between the at least flame retardant and the zinc stearate is between 10:1 and 30:1 , preferably between 15:1 and 25:1.

[0086] 21 . Method according to clause 14 - 20, wherein the at least one nucleating agent comprises montmorillonite, and wherein at least one flame retardant is a phosphorus-based flame retardant, preferably aluminium-alkylphosphinate.

[0087] 22. Method according to any one of clauses 1 - 21 , wherein step c) of heating the mixture comprises heating the mixture to a temperature between 245 °C and 255 °C, and subsequently heating the mixture to a temperature between 265 °C and 275 °C, while the mixture is subjected to shear forces to obtain a melt.

[0088] 23. Method according to any one of clauses 1 - 22, wherein after step d) the melt is cooled to a temperature ranging between 220 °C and 260 °C, preferably between 230 °C and 250 °C, more preferably between 235 °C and 245 °C.

[0089] 24. Method according to any one of clauses 16 - 23, wherein an activation temperature of at least one flame retardant is at least 250 °C, more preferably at least 275 °C, most preferably at least 300 °C.

[0090] 25. Method according to any of clauses 16 - 24, wherein the at least one flame retardant comprises aluminium diethylphosphinate, montmorillonite and / or PSMP, and preferably wherein aluminium diethylphosphinate, montmorillonite and / or pentaerythritol spirobis(methylphosphonate) is present in a weight percentage from 1 - 10 wt.%, preferably 2 - 8 wt.% based on total weight of the mixture.

[0091] 26. Method according to any one of clauses 1 - 25, wherein after step e) of extruding the melt, a temperature of at least part of the at least partly foamed core layer is maintained within a crystallisation temperature range, wherein the crystallisation temperature range ranges between 10 °C above the glass transition temperature, Tg, of the polyethylene terephthalate present in the at least partly foamed core layer and 10 °C below the melting temperature, Tm, of the polyethylene terephthalate present in the at least partly foamed core layer, preferably wherein the crystallisation temperature range ranges from [(Tg+Tm) / 2] + 20 °C to [(Tg+Tm) / 2] - 20 °C, most wherein the crystallisation temperature range ranges from 168 °C to 188 °C.

[0092] 27. Method according to clause 26, wherein after step e) and prior to maintaining a temperature of at least part of the at least partly foamed core layer within the crystallisation temperature range, the at least partly foamed core layer is cooled to below the glass transition temperature Tg.

[0093] 28. Method according to clause 26 or clause 27, wherein a temperature of the at least part of the at least partly foamed core layer is maintained within the crystallisation temperature range for at least 10 seconds, preferably at least 30 seconds, more preferably at least 1 minute, such as 1 - 3 minutes.

[0094] 29. Method according to clause 21 or clause 22, wherein the at least part of the at least partly foamed core layer which is maintained at a temperature within the crystallisation temperature range is a part of the at least partly foamed core layer ranging from an outer surface of the at least partly foamed core layer inwardly to a center of the at least partly core layer by a distance of at most 25% of a total thickness of the at least partly foamed core layer, preferably by a distance of at most 15% of the total thickness of the at least partly foamed core layer.

[0095] 30. Method according to any one of clauses 1 - 29, wherein after step e) at least one outer surface of the at least partly foamed core layer is subjected to a temperature higher than the melting temperature, Tm, of the polyethylene terephthalate present in the at least partly foamed core layer, preferably higher than Tm- 10 °C, for at least 1 second, preferably at least 5 seconds, to form at least one at least partially sealed outer surface.

[0096] 31 . Method according to any one of clauses 1 - 30, wherein a decorative top layer is adhered to the at least partly foamed core layer. 32. Building panel, in particular a decorative building panel, in particular suitable for assembly into a floor, wall or ceiling covering, comprising: at least one core layer, in particular obtained or obtainable via a method according to any one of clauses 1 - 31 ; and at least one decorative top layer adhered to the at least one core layer, wherein the at least one core layer comprises at least 50-70wt.% polyethylene terephthalate, preferably at least partially recycled polyethylene terephthalate, and optionally at least 30-50wt% of at least one mineral filler based on total weight of the core layer, wherein the at least one core layer is at least partially foamed and comprises a plurality of cells, and wherein an average diameter of the cells ranges between 0.01 - 0.50 mm.

[0097] 33. Panel according to clause 32, wherein an average diameter of the cells ranges from 0.01 - 0.30 mm, preferably from 0.02 - 0.20 mm, more preferably from 0.03 - 0.10 mm.

[0098] 34. Panel according to clause 32 or clause 33, wherein the at least one core layer comprises a top surface and a bottom surface, wherein at least one of the top surface and bottom surface is substantially free from open cells and / or is sealed.

[0099] 35. Panel according to any one of clauses 32 - 34, wherein a variation in density of the core layer over a total length, height and / or width of the core layer lies within 10% of an average density of the core layer, preferably within 8%, more preferably within 5%, most preferably within 3%.

[0100] 36. Panel according to any one of clauses 32 - 35, wherein the at least one core layer comprises at least one mineral filler, preferably selected from silicon dioxide, magnesium dihydroxide, calcium carbonate, magnesium carbonate, talc, chalk, montmorillonite, dolomite, or any combination thereof.

[0101] 37. Panel according to clause 36, wherein the mineral filler has an average particle size ranging between 200 and 800 mesh, preferably between 300 and 400 mesh, according to ISO 565. 38. Panel according to any one of clauses 32 - 37, wherein a total volume of the plurality of cells divided by a total volume of the core layer ranges from 0.34 to 0.93%. such as from 0.34 to 0.50, from 0.50 to 0.70, or from 0.70 to 0.93.

[0102] 39. Panel according to any one of clauses 32 - 38, wherein the at least one core layer comprises at least one nucleating agent, preferably wherein the at least one nucleating agent has an average particle size ranging between 500 and 1500 mesh, preferably between 800 and 1200 mesh, more preferably between 900 and 1100 mesh, according to ISO 565.

[0103] 40. Panel according to any one of clauses 32 - 39, wherein the at least one core layer comprises at least one flame retardant selected from zinc diethyl phosphinate, 2-chloro-5,5-dimethyl-1 ,3,2-dioxaphoshinane-2-oxide, halogenated flame retardants, zinc di-phosphinate, 9,10-dihydro-9-oxa-10- phosphaphenanthrene-10-oxide, pentaerythritol phosphate alcohol, metal phosphonate, aluminium-alkylphosphinate, pentaerythritol spirobis(methylphosphonate), aluminium-diethylphosphinate, DOPO-O-PEPA, DEPzn, aluminium trihydroxides, magnesium dihydroxides, silicone-based compounds, montmorillonite or any combination thereof.

[0104] 41 . Panel according to any of clauses 32 - 40, comprising at least one nucleating agent and at least one flame retardant, wherein the at least one nucleating agent comprises montmorillonite, and wherein the at least one flame retardant is a phosphorus-based flame retardant, preferably aluminium- alkylphosphinate.

[0105] 42. Panel according to any of clauses 32 - 41 , comprising at least one nucleating agent and at least one flame retardant, wherein the at least one flame retardant comprises aluminium-alkylphosphinate present in 6 - 8 wt.% based on total weight of the core layer, and wherein the at least one nucleating agent comprises montmorillonite and / or polytetrafluoroethylene (PTFE) present in 0.5 - 2 wt.% based on total weight of the core layer, and optionally wherein the core layer comprises a chain extender comprising an epoxy-based styrene-acrylic oligomer present in 1 wt.% based on total weight of the core layer. 43. Panel according to any of clauses 32 - 40, comprising at least one nucleating agent and at least one flame retardant, wherein the at least one flame retardant comprises a halogenated flame retardant (HFR), aluminium diethylphosphinate, pentaerythritol spirobis(methylphosphonate) (PSMP), or any combination thereof, wherein the at least one flame retardant is present in 1 - 10 wt.%, preferably 2 - 8 wt.%, based on total weight of the core layer, wherein the at least one nucleating agent comprises zinc stearate, and wherein the core layer comprises a chain extender comprising pyromellitic dianhydride (PMDA).

[0106] 44. Panel according to any one of clauses 32 - 43, wherein an average density of the at least one core layer ranges between 100 - 1200 kg / m3, such as between 100 - 250 kg / m3, 200 - 600 kg / m3, or 700 - 1200 kg / m3.

[0107] 45. Panel according to any one of clauses 32 - 44, herein the at least one core layer comprises at least one antioxidant selected from primary oxidants, phenol- based oxidants, sterically hindered phenols, 2,4-di-t-butylphenol, Irganox 1010, Irganox 1076, Irganox 1098, Irganox 1330, Irganox 245, Irganox 1135, secondary oxidants, antioxidant 168, Irgafos 126, Irganox PS 800, Irganox PS 802, or any combination thereof.

[0108] 46. Panel according to any one of clauses 32 - 45, wherein the panel and in a particular the at least one core layer has an average percent crystallinity of at least 1%, determined according to ASTM-F2778.

[0109] 47. Panel according clause 46, wherein the average percent crystallinity ranges from 15 to 60%, more preferably from 20 to 50%, most preferably from 30 to 45%, determined according to ASTM-F2778.

[0110] 48. Panel according to clause 46, wherein a part of the core layer ranging from an outer surface of the core layer inwardly to a center of the core layer by a distance of at most 25% of a total thickness of the core layer, preferably by a distance of at most 15% of the total thickness of the core layer has an average percent crystallinity from 15 to 60%, more preferably from 20 to 50%, most preferably from 30 to 45%, and / or wherein the percent crystallinity of the core layer other than the part of the core layer ranges from 10 to 30%, more preferably from 15 to 25%, most preferably around 20%, determined according to ASTM-F2778.

[0111] 49. Panel according to any of clauses 32 - 47, wherein the at least one core layer comprises at least one substantially amorphous region and at least one substantially crystalline region.

[0112] 50. Panel according to clause 49, wherein at least one substantially amorphous central region is enclosed by at least two substantially crystalline outer regions.

[0113] 51 . Panel according to any of clauses 32-50, wherein the panel has a shrinking rate of at most 0.1% and / or a cupping rate of at most 0.1 mm.

[0114] 52. Panel according to any of clauses 32-51 , wherein the panel comprises interlocking coupling means provided on at least two opposite edges

[0115] It will be clear that the invention is not limited to the exemplary embodiments which are described here, but that countless variants are possible within the framework of the attached claims and / or abovementioned clauses, which will be obvious to the person skilled in the art. In this case, it is conceivable for different inventive concepts and / or technical measures of the above-described variant embodiments to be completely or partly combined without departing from the inventive idea described in the attached claims.

[0116] The verb 'comprise' and its conjugations as used in this patent document are understood to mean not only 'comprise', but to also include the expressions 'contain', 'substantially contain', 'formed by' and conjugations thereof.

Claims

Claims1 . Method for obtaining an at least partially foamed building panel, comprising the following steps: a) providing a mixture comprising polyethylene terephthalate having an intrinsic viscosity of at most 0.75 dL / g according to ASTM D4603-18, at least one chain extender, and at least one mineral filler b) feeding the mixture to an extruder, c) heating the mixture to a temperature of at least 240 °C and subjecting the mixture to shear forces to obtain a melt, d) injecting at least one physical foaming agent in the melt, and e) extruding the melt to obtain an at least partly foamed core layer of a panel.

2. Method according to claim 1 , wherein the extruder comprises an extrusion die and an extrusion barrel, wherein in step d) the at least one physical foaming agent is injected in the melt at a location within the barrel, and wherein in step e) the melt is extruded through the die.

3. Method according to claim 1 or claim 2, wherein at least one physical foaming agent is selected from supercritical carbon dioxide, supercritical nitrogen, or a combination thereof.

4. Method according to claim 2 or claim 3, wherein the barrel comprises a metering section, and wherein in step d) the least one physical foaming agent is injected in the melt at a location within the metering section.

5. Method according to any one of claims 1 - 4, wherein the at least partly foamed core layer obtained in step e) is a substantially homogenously foamed core layer, wherein a variation in density of the substantially homogenously foamed core layer over a total length, height and / or width of the substantially homogenously foamed core layer lies within 10% of an average density of the substantially homogenously foamed core layer, preferably within 8%, more preferably within 5%, most preferably within 3%.

6. Method according to any one of claims 1 - 5, wherein the at least one chain extender comprises at least one multifunctional epoxy, multifunctional epoxide, bis- oxazoline, dianhydride, diisocyanate, tetrafunctional epoxy, tetrafunctional dianhydride, pyromellitic dianhydride, tetraglycidyl diamino diphenyl methane, styrene-acrylic epoxide, styrene-acrylic oligomer epoxide, or any combination thereof.

7. Method according to any one of claims 1 - 6, wherein the at least one chain extender comprises at least one functional reagent, wherein the at least one functional reagent preferably comprises a functional group selected from -OH, - COOH, or any combination thereof.

8. Method according to any one of claims 1 - 7, wherein at least part of the polyethylene terephthalate is a post-consumer recycled polyethylene terephthalate having an intrinsic viscosity in the range of 0.50 - 0.75 dUg, preferably in the range of 0.50 - 0.70 dL / g, more preferably in the range of 0.50 - 0.65 dL / g.

9. Method according to any one of claims 1 - 8, wherein a weight percentage of polyethylene terephthalate in the mixture ranges from 50.0 - 99.5 wt%, preferably from 55 - 90 wt.%, more preferably from 55 - 80 wt.%, based on the total weight of the mixture.

10. Method according to any one of claims 1 - 9, wherein a weight percentage of the at least one chain extender in the mixture ranges from 0.5 - 5 wt.%, preferably from 0.5 - 3 wt.%, more preferably from 0.5 - 2 wt.%, most preferably from 0.5 - 1 wt.%, based on the total weight of the mixture.

11. Method according to any one of claims 1 - 10, wherein the at least one mineral filler is selected from silicon dioxide, magnesium dihydroxide, calcium carbonate, magnesium carbonate, talc, chalk, montmorillonite, dolomite, or any combination thereof.

12. Method according to claim 11 , wherein the mineral filler has an average particle size ranging between 200 and 800 mesh, preferably between 300 and 400 mesh, according to ISO 565.

13. Method according to claim 11 or 12, wherein a weight percentage of the mineral filler ranges from 10 - 70 wt.%, more preferably from 20 - 60 wt.%, most preferably between 30 - 50 wt.%, based on total weight of the mixture.

14. Method according to any one of claims 1 - 13, wherein the mixture comprises at least one nucleating agent, preferably selected from talc, silicates, magnesium silicates, silica, chalk, montmorillonite, nanoclay, limestone, stearates, calcium stearate, zinc stearate, rubber particles, citric acid, sodium bicarbonate, sodium benzoate, or any combination thereof.

15. Method according to claim 14, wherein the at least one nucleating agent has an average particle size ranging between 500 and 1500 mesh, preferably between 800 and 1200 mesh, more preferably between 900 and 1100 mesh, according to ISO 565.

16. Method according to any one of claims 1 - 15, wherein the mixture comprises at least one flame retardant, preferably wherein the at least one flame retardant is an inorganic flame retardant, a phosphorous-based flame retardant, or a combination thereof.

17. Method according to any one of claims 1 - 16, wherein the mixture comprises at least one flame retardant selected from zinc diethyl phosphinate, 2- chloro-5,5-dimethyl-1 ,3,2-dioxaphoshinane-2-oxide, halogenated flame retardants, zinc di-phosphinate, 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, pentaerythritol phosphate alcohol, metal phosphonate, aluminium-alkylphosphinate, pentaerythritol spirobis(methylphosphonate), aluminium-diethylphosphinate, DOPO-O-PEPA, DEPzn, aluminium trihydroxides, magnesium dihydroxides, silicone-based compounds, montmorillonite or any combination thereof.

18. Method according to claim 16, wherein the at least one flame retardant is pentaerythritol spirobis(methylphosphonate), a halogenated flame retardant, or aluminium diethylphosphinate.

19. Method according to any one of claims 1 — 18, wherein the mixture comprises zinc stearate.

20. Method according to any of claims 16 - 18 and claim 19, wherein the at least one chain extender is pyromellitic dianhydride (PMDA), and wherein a weight ratio between the at least flame retardant and the zinc stearate is between 10:1 and 30:1 , preferably between 15:1 and 25:1.

21. Method according to claim 14 - 20, wherein the at least one nucleating agent comprises montmorillonite, and wherein at least one flame retardant is a phosphorus-based flame retardant, preferably aluminium-alkylphosphinate.

22. Method according to any one of claims 1 - 21 , wherein step c) of heating the mixture comprises heating the mixture to a temperature between 245 °C and 255 °C, and subsequently heating the mixture to a temperature between 265 °C and 275 °C, while the mixture is subjected to shear forces to obtain a melt.

23. Method according to any one of claims 1 - 22, wherein after step d) the melt is cooled to a temperature ranging between 220 °C and 260 °C, preferably between 230 °C and 250 °C, more preferably between 235 °C and 245 °C.

24. Method according to any one of claims 16 - 23, wherein an activation temperature of at least one flame retardant is at least 250 °C, more preferably at least 275 °C, most preferably at least 300 °C.

25. Method according to any of claims 16 - 24, wherein the at least one flame retardant comprises aluminium diethylphosphinate, montmorillonite and / or PSMP, and preferably wherein aluminium diethylphosphinate, montmorillonite and / or pentaerythritol spirobis(methylphosphonate) is present in a weight percentage from 1 - 10 wt.%, preferably 2 - 8 wt.% based on total weight of the mixture.

26. Method according to any one of claims 1 - 25, wherein after step e) of extruding the melt, a temperature of at least part of the at least partly foamed core layer is maintained within a crystallisation temperature range, wherein the crystallisation temperature range ranges between 10 °C above the glass transitiontemperature, Tg, of the polyethylene terephthalate present in the at least partly foamed core layer and 10 °C below the melting temperature, Tm, of the polyethylene terephthalate present in the at least partly foamed core layer, preferably wherein the crystallisation temperature range ranges from [(Tg+Tm) / 2] + 20 °C to [(Tg+Tm) / 2] - 20 °C, most wherein the crystallisation temperature range ranges from 168 °C to 188 °C.

27. Method according to claim 26, wherein after step e) and prior to maintaining a temperature of at least part of the at least partly foamed core layer within the crystallisation temperature range, the at least partly foamed core layer is cooled to below the glass transition temperature Tg.

28. Method according to claim 26 or claim 27, wherein a temperature of the at least part of the at least partly foamed core layer is maintained within the crystallisation temperature range for at least 10 seconds, preferably at least 30 seconds, more preferably at least 1 minute, such as 1 - 3 minutes.

29. Method according to claim 21 or claim 22, wherein the at least part of the at least partly foamed core layer which is maintained at a temperature within the crystallisation temperature range is a part of the at least partly foamed core layer ranging from an outer surface of the at least partly foamed core layer inwardly to a center of the at least partly core layer by a distance of at most 25% of a total thickness of the at least partly foamed core layer, preferably by a distance of at most 15% of the total thickness of the at least partly foamed core layer.

30. Method according to any one of claims 1 - 29, wherein after step e) at least one outer surface of the at least partly foamed core layer is subjected to a temperature higher than the melting temperature, Tm, of the polyethylene terephthalate present in the at least partly foamed core layer, preferably higher than Tm- 10 °C, for at least 1 second, preferably at least 5 seconds, to form at least one at least partially sealed outer surface.31 . Method according to any one of claims 1 - 30, wherein a decorative top layer is adhered to the at least partly foamed core layer.

32. Decorative building panel suitable for assembly into a floor, wall or ceiling covering, comprising: at least one core layer, in particular obtained or obtainable via a method according to any one of claims 1 - 31 ; and at least one decorative top layer adhered to the at least one core layer, wherein the at least one core layer comprises at least 50-70wt.% polyethylene terephthalate, preferably at least partially recycled polyethylene terephthalate, and at least 30-50wt% of at least one mineral filler based on total weight of the core layer, wherein the at least one core layer is at least partially foamed and comprises a plurality of cells, and wherein an average diameter of the cells ranges between 0.01 - 0.50 mm.

33. Panel according to claim 32, wherein an average diameter of the cells ranges from 0.01 - 0.30 mm, preferably from 0.02 - 0.20 mm, more preferably from 0.03 - 0.10 mm.

34. Panel according to claim 32 or claim 33, wherein the at least one core layer comprises a top surface and a bottom surface, wherein at least one of the top surface and bottom surface is substantially free from open cells and / or is sealed.

35. Panel according to any one of claims 32 - 34, wherein a variation in density of the core layer over a total length, height and / or width of the core layer lies within 10% of an average density of the core layer, preferably within 8%, more preferably within 5%, most preferably within 3%.

36. Panel according to any one of claims 32 - 35, wherein the at least one core layer comprises at least one mineral filler, preferably selected from silicon dioxide, magnesium dihydroxide, calcium carbonate, magnesium carbonate, talc, chalk, montmorillonite, dolomite, or any combination thereof.

37. Panel according to claim 36, wherein the mineral filler has an average particle size ranging between 200 and 800 mesh, preferably between 300 and 400 mesh, according to ISO 565.

38. Panel according to any one of claims 32 - 37, wherein a total volume of the plurality of cells divided by a total volume of the core layer ranges from 0.34 to 0.93%. such as from 0.34 to 0.50, from 0.50 to 0.70, or from 0.70 to 0.93.

39. Panel according to any one of claims 32 - 38, wherein the at least one core layer comprises at least one nucleating agent, preferably wherein the at least one nucleating agent has an average particle size ranging between 500 and 1500 mesh, preferably between 800 and 1200 mesh, more preferably between 900 and 1100 mesh, according to ISO 565.

40. Panel according to any one of claims 32 - 39, wherein the at least one core layer comprises at least one flame retardant selected from zinc diethyl phosphinate, 2-chloro-5,5-dimethyl-1 ,3,2-dioxaphoshinane-2-oxide, halogenated flame retardants, zinc di-phosphinate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10- oxide, pentaerythritol phosphate alcohol, metal phosphonate, aluminium- alkylphosphinate, pentaerythritol spirobis(methylphosphonate), aluminium- diethylphosphinate, DOPO-O-PEPA, DEPzn, aluminium trihydroxides, magnesium dihydroxides, silicone-based compounds, montmorillonite or any combination thereof.41 . Panel according to any of claims 32 - 40, comprising at least one nucleating agent and at least one flame retardant, wherein the at least one nucleating agent comprises montmorillonite, and wherein the at least one flame retardant is a phosphorus-based flame retardant, preferably aluminium-alkylphosphinate.

42. Panel according to any of claims 32 - 41 , comprising at least one nucleating agent and at least one flame retardant, wherein the at least one flame retardant comprises aluminium-alkylphosphinate present in 6 - 8 wt.% based on total weight of the core layer, and wherein the at least one nucleating agent comprises montmorillonite and / or polytetrafluoroethylene (PTFE) present in 0.5 - 2 wt.% based on total weight of the core layer, and optionally wherein the core layer comprises a chain extender comprising an epoxy-based styrene-acrylic oligomer present in 1 wt.% based on total weight of the core layer.

43. Panel according to any of claims 32 - 40, comprising at least one nucleating agent and at least one flame retardant, wherein the at least one flame retardant comprises a halogenated flame retardant (HFR), aluminium diethylphosphinate, pentaerythritol spirobis(methylphosphonate) (PSMP), or any combination thereof, wherein the at least one flame retardant is present in 1 - 10 wt.%, preferably 2 - 8 wt.%, based on total weight of the core layer, wherein the at least one nucleating agent comprises zinc stearate, and wherein the core layer comprises a chain extender comprising pyromellitic dianhydride (PMDA).

44. Panel according to any one of claims 32 - 43, wherein an average density of the at least one core layer ranges between 100 - 1200 kg / m3, such as between 100 - 250 kg / m3, 200 - 600 kg / m3, or 700 - 1200 kg / m3.

45. Panel according to any one of claims 32 - 44, herein the at least one core layer comprises at least one antioxidant selected from primary oxidants, phenol- based oxidants, sterically hindered phenols, 2,4-di-t-butylphenol, Irganox 1010, Irganox 1076, Irganox 1098, Irganox 1330, Irganox 245, Irganox 1135, secondary oxidants, antioxidant 168, Irgafos 126, Irganox PS 800, Irganox PS 802, or any combination thereof.

46. Panel according to any one of claims 32 - 45, wherein the panel and in a particular the at least one core layer has an average percent crystallinity of at least 1%, determined according to ASTM-F2778.

47. Panel according claim 46, wherein the average percent crystallinity ranges from 15 to 60%, more preferably from 20 to 50%, most preferably from 30 to 45%, determined according to ASTM-F2778.

48. Panel according to claim 46, wherein a part of the core layer ranging from an outer surface of the core layer inwardly to a center of the core layer by a distance of at most 25% of a total thickness of the core layer, preferably by a distance of at most 15% of the total thickness of the core layer has an average percent crystallinity from 15 to 60%, more preferably from 20 to 50%, most preferably from 30 to 45%, and / or wherein the percent crystallinity of the core layerother than the part of the core layer ranges from 10 to 30%, more preferably from 15 to 25%, most preferably around 20%, determined according to ASTM-F2778.

49. Panel according to any of claims 32 - 47, wherein the at least one core layer comprises at least one substantially amorphous region and at least one substantially crystalline region.

50. Panel according to claim 49, wherein at least one substantially amorphous central region is enclosed by at least two substantially crystalline outer regions.51 . Panel according to any of claims 32-50, wherein the panel has a shrinking rate of at most 0.1% and / or a cupping rate of at most 0.1 mm.

52. Panel according to any of claims 32-51 , wherein the panel comprises interlocking coupling means provided on at least two opposite edges.