WO2025191175A1

WO2025191175A1 - Additive manufacturing with pyrolyzed lignocellulosic filler and foaming agent

Info

Publication number: WO2025191175A1
Application number: PCT/EP2025/057125
Authority: WO
Inventors: Bart TAMBUYSER; Yannick AERTS
Original assignee: Carboganic
Current assignee: Carboganic
Priority date: 2024-03-15
Filing date: 2025-03-14
Publication date: 2025-09-18
Anticipated expiration: 2026-09-15
Also published as: BE1032472B1; BE1032472A1

Abstract

In a first aspect, the present invention relates to a masterbatch for forming a foamable additive manufacturing compound, comprising: i) a thermoplastic matrix, ii) a pyrolyzed lignocellulosic filler, and iii) a foaming agent.

Description

DESCRIPTION

ADDITIVE MANUFACTURING WITH PYROLYZED LIGNOCELLULOSIC FILLER AND FOAMING AGENT

Technical field of the invention

[001] The present invention relates to additive manufacturing materials and methods, and more in particular to masterbatches and foamable additive manufacturing compounds therefor comprising a pyrolyzed lignocellulosic filler dispersed in a thermoplastic matrix.

Background of the invention

[002] Additive manufacturing — also referred to as ‘3D printing’ — is the process of joining materials to make objects from 3D model data, usually layer upon layer and under computer control. It is particularly useful to make unique (or a small number of identical) purpose-tailored products, for example in rapid prototyping, patient-matched implants, etc. Notwithstanding, it is increasingly used in larger scale industrial production, with applications across various sectors including aerospace, automotive, healthcare, and consumer goods.

[003] One of the challenges in the field of additive manufacturing is the creation of large products, which can be time-consuming and resource-intensive, often requiring significant amounts of material and energy. Additionally, the weight of large printed products can be problematic, both in terms of the manufacturing process itself (e.g. sagging and/or collapsing of the printed material under its own weight) and the logistics of handling and transporting the finished product. To combat this, the use of foaming in additive manufacturing has previously been explored, which would allow theoretically allow to create more lightweight structures. Moreover, this could also improve the thermal insulation properties of the printed material. The foaming process is typically initiated by incorporating a chemical or physical foaming agent into the material, which, upon activation, causes the material to expand by forming voids therein.

[004] However, the use of foaming in additive manufacturing presents several challenges. First, the expansion of the material can lead to a reduction in the mechanical properties of the final product, such as its strength and hardness. This weakening causes sagging, collapsing and/or settling of the material into an undesired shape during and/or after the deposition process; problems which are already a challenge within additive manufacturing even without foaming, but increasingly so with foaming as the cellular structure of the foam as such can (partially) collapse, resulting in changes in volume of the printed material and thus more pronounced faults. With layer-by-layer printing, this issue is further exacerbated as the weight of the upper layers can deform the still-soft lower layers, leading to inaccuracies in the final shape and structural integrity of the printed product. Additionally, the foamed products may be more susceptible to breaking or collapsing under external forces due to their reduced density and strength.

[005] Furthermore, controlling the foaming process to achieve consistent, reproducible results is complex. Not only must the foaming be carefully balanced to ensure that the material expands appropriately without compromising the product's structural integrity or dimensional accuracy. Additionally, despite thorough mixing, the foaming is often found to occur unevenly through the material. [006] The integration of certain fillers into thermoplastic matrices could potentially help address some of the above issues. Forthis, (chopped) glass or carbon fibres have been considered; however, these could generally not achieve the desired results. CN110698806A and CN110698807A disclose a recycled regenerated plastic extruded to a wire for printing, comprising — among others — 60-70 parts of a recycled plastic (ABS, PP or PET), 5-20 parts of glass fibres, 1-15 parts of biochar and 0.1-5 parts of a foaming agent. Notwithstanding, the integration of fillers brings challenges in its own right due to issues with compatibility, dispersion and the potential for increased water absorption, which negatively affect the processing and performance of the material.

[007] Despite the advancements in the field, there thus remains a need for further innovation. In particular, there is a need for materials that can be used to form strong, lightweight and large objects through additive manufacturing, while simultaneously addressing the challenges outlined above.

Summary of the invention

[008] It is an object of the present invention to provide good materials for additive manufacturing. It is a further object of the present invention to provide methods and products associated therewith. This objective is accomplished by masterbatches, foamable additive manufacturing compounds, methods for forming such masterbatches and/or compounds, additive manufacturing methods and additive manufacturing products according to the present invention.

[009] Notably, in the prior art — including CN110698806A and CN110698807A, but also others — where a matrix is compounded with a foaming agent and filler, foaming is performed before additive manufacturing as part of forming the additive manufacturing compound as such (e.g. before extruding into a wire of filament). Accordingly, the additive manufacturing compound as such is foamed, not foamable. By contrast, the present inventors realized that this is not the best stage to perform foaming. Indeed, when foaming priorto additive manufacturing, e.g. due to the pressure and forces exerted on the additive manufacturing compound during these steps (especially if the additive manufacturing involves melting/softening of the additive manufacturing compound, as is often the case), the foamed additive manufacturing compound typically loses at least some — and typically substantially all — of its foaminess (i.e. the foam’s cellular structure is at least partially broken down) as it is processed by the additive manufacturing system and printed. Accordingly, the printed product is no longer substantially foamed and not particularly lightweight. Additionally, the components at the same time typically undergo some internal reorganization, which deteriorates the positive effect of the (lignocellulosic) fillers on the structural integrity of the printed product.

[010] Instead, by carefully controlling the temperature after addition of the foaming material, the additive manufacturing compound can be kept in an unfoamed — but foamable — state and only activated to foam during additive manufacturing (e.g. activated while orshortly before passing through the printing nozzle), so that the foaming occurs/continues as the additive manufacturing compound is being added to the printed product. In doing so, not only does the foamed material not have to undergo extensive processing prior to printing (thereby better maintaining its foaminess), the printing speed is effectively increased because the additive manufacturing compound increases in volume as it is being printed. In other words, the volume of additive manufacturing compound being printed is larger than the volume of additive manufacturing compound being supplied for printing (e.g. the volume leaving the printing nozzle). Accordingly, larger products can be printed more quickly. Moreover, a given size of product can be printed with less additive manufacturing material (compared to a unfoamed additive manufacturing material).

[011] Additionally, the inventors also identified further factors which positively contribute to the printing process and properties of the printed products. For example, pyrolyzed lignocellulosic filler — as compared to other fillers — were found to improve not only the properties of the printed products (e.g. structural integrity) but also contribute to good foaming. They do this in multiple ways: 1) their increased hydrophobicity (e.g. compared to untreated lignocellulosic filler) leads to a decreased moisture content in the foamable additive manufacturing compound (moisture results in increased printing faults and reduced reproducibility in thermoplastic additive manufacturing); 2) their porous nature allows them to function as a nucleation centre for foaming; 3) they are lightweight (i.e. have a low density) but provide good structural reinforcement, thereby helping to maintain the gas bubbles formed upon foaming (e.g. until solidification of the additive manufacturing compound) without sedimenting (which is a typical problem observed by the inventors with heavier fillers, such as glass or carbon fibre; which can explain their relatively poor performance; cf. supra), and 4) they act as a dispersion agent for the foaming agent, thereby helping to more uniformly and isotropically spread the foaming agent through the masterbatch and/or foamable additive manufacturing compound. Lignocellulosic fillers pyrolyzed at intermediate temperatures (e.g. 400 °C to 600 °C or 450 °C to 500 °C) particularly excel in this regard, by striking a good balance between between — on the one hand — improving their rheological effect, decreasing their moisture content and absorption, increasing their thermal stability and their ability to be uniformly and isotropically spread throughout the compound/printed product, and — on the other hand — not overreaching to the point of reducing again their hydrophobicity and increasing their hygroscopy.

[012] On top of the pyrolyzed lignocellulosic filler being lightweight — thereby providing good structural support with low sedimentation (cf. supra) — , also the foamed additive manufacturing compound (e.g. after being printed but while solidification is still ongoing) and the final printed product as such are particularly lightweight (thanks to the foaming and the lightweight filler). This creates a synergistic, compounding effect in that the pyrolyzed lignocellulosic filler provides improved structural support to the foamed additive manufacturing compound (e.g. to the gas bubbles, but also the printed product as a whole), while in turn the light weight of the foamed additive manufacturing compound as such requires less structural support. This results in particularly good printing properties (e.g. low sagging or collapse while printing) for the additive manufacturing compound.

[013] Moreover, also the thorough mixing of the thermoplastic matrix, pyrolyzed lignocellulosic filler, and foaming agent — for example through using a two-step process in which first a masterbatch is formulated and this masterbatch is subsequently diluted to a foamable additive manufacturing compound — was found to positively impact the uniformity and reproducibility of the printing process and printed product properties.

[014] In a first aspect, the present invention relates to a masterbatch for forming a foamable additive manufacturing compound, comprising: i) a thermoplastic matrix, ii) a pyrolyzed lignocellulosic filler, and iii) a foaming agent. [015] In a second aspect, the present invention relates to a foamable additive manufacturing compound, comprising: i) a thermoplastic matrix, ii) a pyrolyzed lignocellulosic filler, and iii) a foaming agent.

[016] In a third aspect, the present invention relates to a method for forming a masterbatch according to any embodiment of the first aspect, comprising: a) mixing a pyrolyzed lignocellulosic filler with a thermoplastic matrix, and b) — before, during or after step 15. a — mixing a foaming agent with the thermoplastic matrix.

[017] In a fourth aspect, the present invention relates to a method for forming a foamable additive manufacturing compound according to any embodiment of the second aspect, comprising: 1 a) compounding a masterbatch according to any embodiment of the first aspect with further thermoplastic matrix; or 2a) compounding a pyrolyzed lignocellulosic filler with a thermoplastic matrix, and 2b) — before, during or after step 2a — compounding a foaming agent with the thermoplastic matrix.

[018] In a fifth aspect, the present invention relates to a method for forming a product by additive manufacturing comprising: a) feeding a foamable additive manufacturing compound according to any embodiment of the second aspect into an additive manufacturing system, and b) additively manufacturing the product using the additive manufacturing system.

[019] In a sixth aspect, the present invention relates to a product obtainable by additive manufacturing of the foamable additive manufacturing compound according to any embodiment of the second aspect.

[020] It is an advantage of embodiments of the present invention that the method for forming a masterbatch or a foamable additive manufacturing compound may be performed such as to prevent activation of the foaming agent (e.g. by keeping the foaming agent below its foaming temperature), ensuring controlled activation and preventing premature foaming. It is a further advantage of embodiments of the present invention that foaming may be made to occur during additive manufacturing. It is yet a further advantage of embodiments of the present invention that the volume of additive manufacturing compound being printed is typically larger than the volume of additive manufacturing compound being supplied for printing, thereby improving the printing speed.

[021] It is an advantage of embodiments of the present invention that a thoroughly mixed foamable additive manufacturing compound can be obtained by first formulating a more concentrated masterbatch.

[022] It is an advantage that of embodiments of the present invention that the printed product can be a lightweight, but relatively strong, solid foam with uniformly distributed voids, providing good thermal insulation; reducing the weight of the object without significantly compromising its strength. It is a further advantage of embodiments of the present invention that a lightweight but strong printed product is often desirable both in terms of handling, shipping, etc., as well as in terms of specific applications (e.g. the printed products being lightweight is particularly useful in orthopaedic applications).

[023] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler may provide strength to the foamable additive manufacturing compound and the printed product, yielding an additive manufacturing compound with good printing properties and a strong printed product (particularly in view of it being lightweight).

[024] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler can impart a pronounced effect on the rheology of the compound. It is a further advantage of embodiments of the present invention that the compound can exhibit a Bingham effect, behaving as a rigid body at low stress and as a viscous fluid at high stress, which can be beneficial for the printing process and the structural integrity of the printed object. It is yet a further advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler can enhance the isotropy and uniformity of the printed product, which can be critical for the mechanical properties and aesthetic quality of the final object.

[025] It is an advantage of embodiments of the present invention that both the pyrolyzed lignocellulosic filler’s contribution to strength, rheology and lightweight improve the printing properties of the additive manufacturing compound (e.g. low sagging or collapse while printing), in turn improving the printability of the material an allowing flexibility in the shapes which can be printed (e.g. with high overhang angles and/or stacking relatively heavy layers).

[026] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler may be a porous material, which may act as a nucleating agent for foaming, and may act as a dispersion agent for the foaming agent; thereby facilitating homogeneous foaming and resulting in homogeneous properties of the product formed by the additive manufacturing.

[027] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler may have an increased brittleness (e.g. compared to compared to untreated pyrolyzed lignocellulosic filler), facilitating the production of a filler with a desired particle size distribution and enhancing the homogeneity, handleability, storage, transport, dosing and processing of the masterbatch and/or foamable additive manufacturing compound.

[028] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler can contribute to a reduced moisture content and uptake in the compound, which can be particularly advantageous in additive manufacturing processes where moisture can adversely affect the printing quality.

[029] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler can be compounded with a variety of thermoplastic matrices, offering flexibility in the choice of materials for different applications. It is a further advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler may have a higher thermal stability (e.g. as compared to untreated pyrolyzed lignocellulosic filler) and, thereby allowing to use thermoplastic polymers with higher melting points.

[030] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler may have a mean particle size that improves the ability to manufacture more uniform and isotropic products.

[031] It is an advantage of embodiments of the present invention that the foamable additive manufacturing compound can be provided in forms such as pellets, granules, flakes, grains, powder, filaments or wire, facilitating its use in different types of additive manufacturing systems. It is a further advantage of embodiments of the present invention that the foamable additive manufacturing compound may be a granular material, suitable for various additive manufacturing techniques such as fused granular fabrication or powder bed fusion, which can be particularly beneficial for the construction of large products.

[032] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler can act as a carbon sink, reducing the carbon footprint of the printed products.

[033] It is an advantage of embodiments of the present invention that the resulting compound can have an enhanced ‘touch and feel experience’ compared to traditional plastics, with the potential to imitate the effect of wood.

[034] It is an advantage of embodiments of the present invention that the compound can exhibit improved thermal features, such as lowered thermal conductivity, which can facilitate faster local heating during printing and reduce the cooling rate of the entire part, thereby avoiding excess residual stress within the polymer matrix.

[035] It is an advantage of embodiments of the present invention that the compound can have an elevated heat deflection temperature due to the presence of the pyrolyzed lignocellulosic filler, enabling higher loads of material during the printing process and potentially improving the performance of the printed object under thermal stress.

[036] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler can be thermostable, allowing for its use in compounds that are reusable for multiple printing cycles and recyclable for other applications.

[037] It is an advantage of embodiments of the present invention that the pyrolyzed lignocellulosic filler can be obtained from a variety of lignocellulosic biomass feedstocks, providing a sustainable source of material for the compound.

[038] It is an advantage of embodiments of the present invention that the foaming agent included in the compound can be selected from a range of physical or chemical foaming agents, providing versatility in the foaming process and the properties of the foamed product.

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

[040] Although there has been constant improvement, change and evolution of devices in this field, the present concepts are believed to represent substantial new and novel improvements, including departures from prior practices, resulting in the provision of more efficient, stable and reliable devices of this nature.

[041] The above and other characteristics, features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. This description is given for the sake of example only, without limiting the scope of the invention. The reference figures quoted below refer to the attached drawings. Brief description of the drawings

[042] FIG 1 is a flow chart for a preferred approach for forming a foamable additive manufacturing compound in accordance with embodiments of the present invention.

[043] FIG 2 to FIG 4 are schematic representations of side views of a product being formed by additive manufacturing in successive steps, in accordance with embodiments ofthe present invention. [044] FIG 5 is a photograph of a product being formed by additive manufacturing in accordance with embodiments of the present invention.

[045] FIG 6 is a photograph of a vertical cut through a printed product in accordance with embodiments of the present invention.

[046] FIG 7 is a photograph under 10 times magnification of a vertical cut through a printed product in accordance with embodiments of the present invention.

[047] In the different figures, the same reference signs refer to the same or analogous elements.

Description of illustrative embodiments

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

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

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

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

[052] Similarly, it is to be noticed that the term ‘coupled’ should not be interpreted as being restricted to direct connections only. The terms ‘coupled’ and ‘connected’, along with their derivatives, may be used. It should be understood that these terms are not intended as synonyms for each other. Thus, the scope of the expression ‘a device A coupled to a device B’ should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means. ‘Coupled’ may mean that two or more elements are either in direct physical or electrical contact, or that two or more elements are not in direct contact with each other but yet still co-operate or interact with each other.

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

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

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

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

[057] The following terms are provided solely to aid in the understanding of the invention.

[058] As used herein, and unless otherwise specified, the term ‘masterbatch’ refers to a concentrated mixture that is intended to be diluted by mixing with a base material (e.g. a thermoplastic matrix) to create a formulation for further use. Herein, the further use is generally additive manufacturing and the formulation is therefore referred to as a (foamable) additive manufacturing compound. Notwithstanding, other uses are also conceivable and the masterbatch can also be suitable for these; e.g. the masterbatch could be for forming a (foamable) injection moulding compound, to name but one example. As a concentrated mixture, a masterbatch typically contains a relatively high concentration of additives (e.g. pyrolyzed lignocellulosic filler and foaming agent) which are to impart specific properties to the final product. [059] As used herein, and unless otherwise specified, the term ‘thermoplastic matrix’ refers to a polymer material that becomes pliable or mouldable above a specific temperature and solidifies upon cooling. Within the present invention, this matrix is the base material in which other components (e.g. pyrolyzed lignocellulosic filler and foaming agent) are embedded/dispersed. Examples of specific embodiments of thermoplastic matrices include, but are not limited to, polylactic acid (PLA), polypropylene (PP), polystyrene (PS), polyamide (PA), polycaprolactone (PCL), polyvinyl chloride (PVC), and polyethylene terephthalate (PET).

[060] As used herein, and unless otherwise specified, ‘lignocellulose’ is a biomass material derivable from certain plants and/or trees, containing cellulose, hemicellulose and lignin. Note however that not every plant-based biomass/filler is necessarily lignocellulosic. Indeed, some plants contain little to no lignin, yielding instead a cellulosic biomass/filler. Likewise, for some fillers cellulose, hemicellulose and/or lignin are specifically separated from the biomass, so that the filler is also no longer lignocellulosic.

[061] As used herein, and unless otherwise specified, ‘pyrolyzed lignocellulosic filler’ refers to a material obtained by pyrolyzing lignocellulose. It is a carbon-rich product that is used herein as a filler in composite materials (e.g. a masterbatch or foamable additive manufacturing compound); typically to enhance their properties, such as mechanical strength and/or thermal stability. Examples of pyrolyzed lignocellulosic fillers include — but are not limited to — pyrolyzed wood, pyrolyzed bamboo and pyrolyzed flax. Note that in the prior art, ‘biochar’ or similar terms are sometimes used to refer to a pyrolyzed lignocellulosic filler, however these are broad umbrella terms which do not exclusively refer to pyrolyzed lignocellulosic filler. Indeed, ‘biochar’ is routinely used to cover carbonized biological biomass in general (i.e. not limited to pyrolysis and/or lignocellulose, but including also cellulosic and/or lignic materials). ‘Char’ in this context is thus not directly related to ‘charring’ as described below.

[062] As used herein, and unless otherwise specified, ‘pyrolysis’ is the process of thermally decomposing a material (e.g. a lignocellulosic biomass) in an inert atmosphere (e.g. under low- oxygen conditions, such as at most 10 mol%, at most 1 mol%, or at most 0.1 mol% of oxygen). Pyrolysis of lignocellulose increases the carbon-content of the material, so that it is a sub-form of the more general ‘carbonization’. Different types of pyrolysis may be distinguished, depending on the temperature range used. Herein, pyrolysis up to about 340 °C is referred to as ‘torrefaction’. Above 340 °C to about 1000 °C, one may speak of ‘charring’ (also referred to as ‘gasification’), which typically generates a gaseous product (e.g. syngas) and a char (e.g. a biochar). Although the char can be considered a by-product (e.g. a waste product) in some applications (e.g. in syngas production), it is in the context of the present invention the primary gasification product of interest. Both torrefaction and charring have become advanced processes that can be developed into larger, industrial volumes. In embodiments of the present invention, pyrolyzing the lignocellulosic biomass may be performed for a duration of from 10 min to 5 hours, preferably from 15 min to 4 hours, more preferably from 20 min to 3 hours, most preferably from 25 min to 2 hours, such as from 30 min to 45 min or 45 min to 1 hour.

[063] As used herein, and unless otherwise specified, the term ‘mean particle size’ refers to the (arithmetic) average size of the particles in a batch of material. This mean particle size can be determined by various methods; such as sieving (e.g. using an air jet sieve, such as an Air Jet Sieve E200LS), laser diffraction or microscopy. The mean particle size may be expressed as a number average or weight average, preferably a number average.

[064] As used herein, and unless otherwise specified, the term ‘foaming agent’ — also referred to as ‘blowing agent’ — is a substance that is capable of producing/expanding gas volume (gas bubbles) by chemical or physical means. When foaming occurs from within a polymer matrix, ideally a cellular structure is formed with pores/voids/spaces enveloped by the polymer matrix, which upon solidification results in a solid foam. Examples of chemical foaming agents include, but are not limited to, chemical agents that decompose (upon heating) to release gases (e.g. N2 or CCh), such as sodium bicarbonate, azodicarbonamide, sulfonyl hydrazide or sulfonyl semicarbazide. Examples of physical foaming agents include, but are not limited to, volatile liquids or gases, such as hydrocarbons (e.g. methane, butane or pentane) or (inert) gases (e.g. N2, CO2, Ar or H2). These vaporize and/or expand at the processing temperature to create gas bubbles.

[065] As used herein, and unless otherwise specified, the term ‘foaming temperature’ refers to the temperature at which the foaming agent activates and produces gas, causing foaming.

[066] As used herein, and unless otherwise specified, the term ‘envelope density’ — also known as ‘bulk density’ — refers to the overall density of a material, including the volume of its pores, voids and spaces (see also ASTM Standard D3766-24a). It is the mass of the material divided by its overall volume; i.e. the volume its envelope occupies. By contrast, the skeletal density refers to the mass of the material divided by its volume, excluding that of its pores, voids and spaces (i.e. by the volume of its ‘skeleton’). Envelope density and skeletal density are accessible for instance through gas pycnometry and/or medium (liquid or dry) displacement measurements. Envelope density in particular may be determined using wax immersion (e.g. cf. ASTM Standard C914-09), mercury displacement (e.g. cf. ASTM Standard C493-98) or displacement of a dry medium like DryF/o.

[067] As used herein, and unless otherwise specified, the term ‘foamable additive manufacturing compound’ refers to an additive manufacturing compound that is capable of foaming (e.g. upon activation, such as thermal activation) but is/has (substantially) not foamed. ‘Substantially not foamed’ refers to the material not having undergone significant foaming or expansion and retaining a predominantly solid, non-cellular structure.

[068] As used herein, and unless otherwise specified, the term ‘substantially not foamed’ refers to the state of the foamable additive manufacturing compound before the additive manufacturing process, indicating that the compound has not undergone significant foaming or expansion and retains a mostly solid, non-cellular structure. This term implies that any foaming present is minimal and does not affect the intended use of the compound in additive manufacturing.

[069] As used herein, and unless otherwise specified, the term ‘granular material’ refers to a material made up of discrete macro- or microscopic particles that are not bound together (such that they can typically flow freely when unconfined). Examples of granular materials include, but are not limited to, pellets, granules, flakes, grains or powder.

[070] As used herein, and unless otherwise specified, the term ‘compounding’ refers to the process of blending or mixing various materials (e.g. a thermoplastic matrix and one ore more additives) together to create a homogeneous mixture or compound. For example, using equipment such as extruders (single screw or twin screw, including co-rotating or counter-rotating), kneaders, mixers or blenders.

[071] As used herein, and unless otherwise specified, the term ‘additive manufacturing system’ refers to a machine or set of equipment used to create objects by additive manufacturing (e.g. sequentially layer by layer).

[072] As used herein, and unless otherwise specified, the term ‘fused granular fabrication’ refers to an additive manufacturing process where granular material is selectively fused together layer by layer to build a three-dimensional object. For the process may involve the use of heat or an adhesive to bond the granules together.

[073] As used herein, and unless otherwise specified, the term ‘fused granular fabrication’ (FGF) — also sometimes called ‘fused particle fabrication’ (FPF) — refers to an additive manufacturing process in which granular material is selectively fused layer by layer to build a three-dimensional printed product. For the foamable additive manufacturing compound of the present invention, this process may typically involve the application of heat to melt (or at least soften) the granules before fusing them together.

[074] As used herein, and unless otherwise specified, additive manufacturing product is considered ‘large’ if the product has at least one dimension of 1 m or larger, preferably 1 .5 or larger, more preferably 2 m or larger, yet more preferably 2.5 m or larger, most preferably 3 m or larger. For example, the manufactured product may have two — or even three — dimensions (e.g. perpendicular to one another) of 1 m or larger, preferably 1 .5 or larger, more preferably 2 m or larger, yet more preferably 2.5 m or larger, most preferably 3 m or larger.

[075] As used herein, and unless otherwise specified, the properties of a material may be those under standard conditions of normal temperature and pressure; i.e. 20 °C and 1 atm.

[076] In a first aspect, the present invention relates to a masterbatch for forming a foamable additive manufacturing compound, comprising: i) a thermoplastic matrix, ii) a pyrolyzed lignocellulosic filler, and iii) a foaming agent.

[077] By first formulating the thermoplastic matrix, pyrolyzed lignocellulosic filler and foaming agent into a more concentrated masterbatch (and later diluting it to a foamable additive manufacturing compound; cf. infra), the homogeneity and isotropical distribution of the pyrolyzed lignocellulosic filler and foaming agent can advantageously be improved, which in turn improves the uniformity and reproducibility of the printing process and printed product properties. In embodiments, the masterbatch may comprise from 50 wt% to 80 wt% of the pyrolyzed lignocellulosic filler, preferably 60 wt% to 70 wt%. In embodiments, the masterbatch may comprise from 1 wt% to 15 wt% of the foaming agent, preferably from 2 wt% to 10 wt%. The concentration of the pyrolyzed lignocellulosic filler and foaming agent is typically larger in the masterbatch than in the eventual foamable additive manufacturing compound (cf. infra). This can advantageously be beneficial to the manufacture and distribution of the masterbatch, because smaller volumes — compared to the foamable additive manufacturing compound as such — need to be processed and transported.

[078] In embodiments, any feature of any embodiment of the first aspect may independently be as correspondingly described for any embodiment of any of the other aspects. [079] In a second aspect, the present invention relates to a foamable additive manufacturing compound, comprising: i) a thermoplastic matrix, ii) a pyrolyzed lignocellulosic filler, and iii) a foaming agent.

[080] In embodiments, the foamable additive manufacturing compound may be for foaming during additive manufacturing, with the proviso that the foamable additive manufacturing compound is substantially not foamed prior to additive manufacturing. By delaying foaming of the additive manufacturing compound until printing, the cellular structure of the foam is advantageously maximally maintained and the printing speed is increased (cf. supra).

[081] In embodiments, the foamable additive manufacturing compound may comprise from 0.1 wt% to 65 wt% of the pyrolyzed lignocellulosic filler, preferably from 5 wt% to 50 wt%, more preferably from 10 wt% to 40 wt%. In other words, the pyrolyzed lignocellulosic filler may represent from 0.1 wt% to 65 wt%, etc. of the foamable additive manufacturing compound. In embodiments, the foamable additive manufacturing compound may comprise from 0.05 wt% to 5 wt% of the foaming agent. In other words, the foaming agent may represent from 0.05 wt% to 5 wt% of the foamable additive manufacturing. The concentration of the pyrolyzed lignocellulosic filler and foaming agent is typically lower in the foamable additive manufacturing compound than in a masterbatch (cf. supra).

[082] In embodiments, the pyrolyzed lignocellulosic filler may have an envelope density of from 0.1 g/cm³ to 0.7 g/cm³, preferably from 0.2 g/cm³ to 0.5 g/cm³, most preferably from 0.25 g/cm³ to 0.4 g/cm³. The pyrolyzed lignocellulosic filler may advantageously be lightweight, which helps to counter sedimentation, and thus improve uniform and isotropic spreading throughout the compound/printed product.

[083] In embodiments, the pyrolyzed lignocellulosic filler may have a mean particle size of from 1 pm to 5000 pm; preferably from 2 pm to 1000 pm, more preferably from 3 pm to 250 pm, yet more preferably from 5 pm to 100 pm; such as 5 pm to 10 pm or 5 pm to 15 pm. In embodiments, the pyrolyzed lignocellulosic filler may have a maximum particle size of from 1000 pm to 10000 pm, preferably from 2000 pm to 5000 pm. In embodiments, the pyrolyzed lignocellulosic filler may have a particle size distribution such that at least 90% — preferably at least 95% — of the particles have a particle size of from 1 pm to 5000 pm; preferably from 2 pm to 1000 pm, more preferably from 3 pm to 250 pm, yet more preferably from 5 pm to 100 pm; such as 5 pm to 10 pm or 5 pm to 15 pm. In embodiments, the pyrolyzed lignocellulosic filler may have a particle size distribution having a D10 of 1 pm or more; preferably 2 pm or more, more preferably 3 pm or more, yet more preferably 5 pm or more. In embodiments, the pyrolyzed lignocellulosic filler may have a particle size distribution having a D90 of 5000 pm or less; preferably 1000 pm or less, more preferably 250 pm or less, yet more preferably 100 pm or less. The particle size of the pyrolyzed lignocellulosic filler can advantageously be within a range that facilitates even distribution and effective reinforcement within the additive manufacturing compound and/or product.

[084] In embodiments, the pyrolyzed lignocellulosic filler may have been pyrolyzed at a temperature of from 220 °C to 1000 °C, preferably from 340 °C to 900 °C, more preferably from 400 °C to 600 °C, most preferably from 450 °C to 500 °C. Such lignocellulosic fillers, but especially those pyrolyzed at intermediate temperatures (e.g. 400 °C to 600 °C or 450 °C to 500 °C), advantageously strike a good balance between between — on the one hand — improving their rheological effect, decreasing their moisture content and absorption, increasing their thermal stability and their ability to be uniformly and isotropically spread throughout the compound/printed product, and — on the other hand — not overreaching to the point of reducing again their hydrophobicity and increasing their hygroscopy. Indeed, lignocellulosic fillers charred at higher temperatures typically show increased hygroscopic characteristics (tentatively attributed to their morphology, such as macro- and micropores therein).

[085] In embodiments, the foamable additive manufacturing compound may comprise a polymer, preferably polylactic acid, polypropylene, polystyrene, polyamide, polycaprolactone, polyvinyl chloride or polyethylene terephthalate. In some embodiments, the foamable additive manufacturing compound may comprise a polyester such as polylactic acid (PLA), polyethylene terephthalate (PET), polybutylene adipate terephthalate (PBAT), polybutylene succinate (PBS), polybutylene terephthalate (PBT), polycaprolactone (PCL), polytrimethylene terephthalate (PTT); especially polylactic acid or polyethylene terephtalate. The thermoplastic matrix can advantageously be selected in function of its suitability for the intended application of the final product, taking into various account factors such as (bio)degradability, strength, thermal properties, etc.

[086] In embodiments, the foamable additive manufacturing compound may comprise pyrolyzed wood, pyrolyzed bamboo or pyrolyzed flax; preferably pyrolyzed wood or pyrolyzed bamboo; most preferably pyrolyzed wood. The fillers can advantageously come from sustainable and renewable resources, which can reduce the environmental impact of the final product. Generally, biomass containing more lignin (e.g. wood and bamboo) may be preferred of those containing less (e.g. flax). [087] In embodiments, the foaming agent may be a physical or chemical foaming agent. In general, a wide variety of foaming agents may be used (cf. supra), e.g. depending on the specifics of the additive manufacturing process that is to be employed (e.g. compatibility with the thermoplastic matrix, lignocellulosic filler and desired properties for the printed product.

[088] In preferred embodiments, the foamable additive manufacturing compound may be suitable for additive manufacturing comprising a fused granular fabrication (FGF). In embodiments, the foamable additive manufacturing compound may thus be a granular material, such as pellets, granules, flakes, grains or powder. For example, the granular material may be for feeding to an additive manufacturing system for fused granular fabrication (cf. infra). This embodiment offers the advantage of providing a compound in a form that is readily processable by additive manufacturing equipment, in particular FGF equipment.

[089] In other embodiments, the foamable additive manufacturing compound may be suitable for additive manufacturing comprising a fused filament fabrication (FFF). In these embodiments, the foamable additive manufacturing compound may typically be formed as a filament or wire.

[090] In embodiments, the foamable additive manufacturing compound may further comprise one or more additives; such as a maleic anhydride, chalk, talc, porous filler, activated carbon, perlite or porous high-melting thermoplastic powder. The addition of maleic anhydride (e.g. 3-5 wt%) in particular may be useful to improve the binding/adhesion between the pyrolyzed lignocellulosic filler lignocellulosic filler and the thermoplastic matrix. In preferred embodiments, no heavy fillers — in particular not heavy fibres (e.g. glass fibre or carbon fibre) — are included in the foamable additive manufacturing material. Heavy fillers have a tendency to sediment, which negatively affects the foaming of the foamable additive manufacturing compound (e.g. destabilizing the foam, leading to sagging and/or collapse), especially in the case of fibres which are intended to provide a structurally reinforcing effect instead (but achieve the opposite).

[091] In embodiments, any feature of any embodiment of the second aspect may independently be as correspondingly described for any embodiment of any of the other aspects.

[092] In a third aspect, the present invention relates to a method for forming a masterbatch according to any embodiment of the first aspect, comprising: a) mixing a pyrolyzed lignocellulosic filler with a thermoplastic matrix, and b) — before, during or after step 15. a — mixing a foaming agent with the thermoplastic matrix.

[093] In embodiments, the foaming agent may be left inactivated during step b. For example, the foaming agent may have a foaming temperature and the foaming agent may be kept agent below said foaming temperature (e.g. 20 °C to 50 °C below said foaming temperature) during step b. Preferably, the foaming may be kept inactive after step b, up until (or shortly prior to) printing the additive manufacturing compound (cf. supra).

[094] In embodiments, the method may comprise an additional step c — before, during or after step b — of: compounding one or more additives (cf. supra) with the thermoplastic matrix.

[095] In preferred embodiments, the mixing in step a and/or b may be high-shear mixing (e.g. using a high-shear mixer). Mixing such as high-shear mixing is particularly advantageous for adding the pyrolyzed lignocellulosic filler to the thermoplastic matrix, as the light weight of the pyrolyzed lignocellulosic filler — similar to dust/fine powder — makes it challenging in practice to add by traditional compounding (e.g. using an extruder).

[096] In embodiments, any feature of any embodiment of the third aspect may independently be as correspondingly described for any embodiment of any of the other aspects.

[097] In a fourth aspect, the present invention relates to a method for forming a foamable additive manufacturing compound according to any embodiment of the second aspect, comprising: 1 a) compounding a masterbatch according to any embodiment of the first aspect with further thermoplastic matrix; or 2a) compounding a pyrolyzed lignocellulosic filler with a thermoplastic matrix, and 2b) — before, during or after step 2a — compounding a foaming agent with the thermoplastic matrix. In embodiments, the former approach (i.e. via a masterbatch) may be preferred as it facilitates forming a foamable additive manufacturing compound in which the pyrolyzed lignocellulosic filler and foaming agent — and other additives, if present — are better (e.g. more uniformly and isotropically) spread throughout the compound.

[098] In embodiments, the foaming agent may be left inactivated during step during step 1 a and/or 2b. For example, the foaming agent may have a foaming temperature and the foaming agent may be kept agent below said foaming temperature (e.g. 20 °C to 50 °C below said foaming temperature) during step during step 1 a and/or 2b. Preferably, the foaming may be kept inactive after step during step 1 a and/or 2b, up until (or shortly prior to) printing the additive manufacturing compound (cf. supra). [099] In embodiments, the further thermoplastic matrix in step 1 a may be the same or a different thermoplastic matrix as the one selected for the masterbatch. In embodiments, the masterbatch may in step 1 a be diluted by a factor of 1 .2 to 5, preferably 1 .5 to 3; such as about 2.

[100] In embodiments, the method may comprise an additional step 1 b — before, during or after step 1 a — , or an additional step 2c — before, during or after step 2a — of: compounding one or more additives (cf. supra) with the thermoplastic matrix. In the context of step 1 b (i.e. wherein a masterbatch with thermoplastic matrix, pyrolyzed lignocellulosic filler and foaming agent has already been formed), the additives could optionally also include further foaming agent and/or further pyrolyzed lignocellulosic filler.

[101] In preferred embodiments, the compounding in step 1 a, 2a and/or 2b may be by using an extruder.

[102] In embodiments, any feature of any embodiment of the fourth aspect may independently be as correspondingly described for any embodiment of any of the other aspects.

[103] In a fifth aspect, the present invention relates to a method for forming a product by additive manufacturing comprising: a) feeding a foamable additive manufacturing compound according to any embodiment of the second aspect into an additive manufacturing system, and b) additively manufacturing the product using the additive manufacturing system.

[104] In embodiments, the method may further comprise foaming the foamable additive manufacturing compound. For example, foaming may be activated (e.g. thermally) during step b.

[105] In embodiments, the additive manufacturing may comprise a fused granular fabrication or fused filament fabrication, preferably fused granular fabrication.

[106] In some embodiments, feeding the foamable additive manufacturing compound into the additive manufacturing system may comprise feeding the foamable additive manufacturing compound from a compounder coupled to the additive manufacturing system. Accordingly, the foamable additive manufacturing compound may advantageously be formed (from scratch or from a masterbatch, cf. supra) and directly fed into the additive manufacturing system.

[107] In embodiments, step b may comprise heating the foamable additive manufacturing compound to melt (or at least soften) said foamable additive manufacturing compound. Typically, the additive manufacturing comprises heating the foamable additive manufacturing compound above its melting point, although in some alternative case — depending on properties of the foamable additive manufacturing compound, intended application of the product, and the specific technique used for the additive manufacturing — the foamable additive manufacturing compound may be heated only to above its glass transition temperature (but below its melt temperature). In some embodiments, said heating may simultaneously activate the foaming agent.

[108] Preferably, the pyrolyzed lignocellulosic filler may be homogeneously dispersed throughout the foamable additive manufacturing compound. The pyrolyzed lignocellulosic filler can advantageously act as a nucleating agent, at which the formation or deposition of gas bubbles (for foaming) may be nucleated. The pyrolyzed lignocellulosic filler acting as nucleating agent may prevent the formation of a supersaturated foamable additive manufacturing compound, which could result in inhomogeneous foaming and a resulting product that has inhomogeneous mechanical characteristics. As the pyrolyzed lignocellulosic filler is herein preferably homogeneously dispersed throughout the foamable additive manufacturing compound — and moreover can act as a dispersing agent for the foaming agent — , said nucleation, and hence said foaming, occur homogeneously throughout the foamable additive manufacturing compound as well. This typically results in homogeneous properties of the product formed by the additive manufacturing. Advantageously, no additional nucleating agent — apart from the pyrolyzed lignocellulosic filler — needs to be added to the foamable additive manufacturing compound, while nevertheless good, homogeneous foaming can be achieved. Notwithstanding, in some embodiments, in addition to the pyrolyzed lignocellulosic filler, a further nucleating agent may be added.

[109] In embodiments, any feature of any embodiment of the fifth aspect may independently be as correspondingly described for any embodiment of any of the other aspects.

[1 10] In a sixth aspect, the present invention relates to a product obtainable by additive manufacturing of the foamable additive manufacturing compound according to any embodiment of the second aspect.

[1 11] The product may typically be a solid foam; i.e., a porous solid matrix material (with the pyrolyzed lignocellulosic fibre disperse therein) having voids (pores) spread through it. Said voids may be preferably uniformly present throughout the product.

[1 12] In embodiments, the product may have an envelope density of from 0.015 g/cm³ to 1 .0 g/cm³, preferably from 0.020 g/cm³ to 0.9 g/cm³, most preferably from 0.025 g/cm³ to 0.8 g/cm³. In embodiments, the envelope density of the product may be at least 90% or less of the density of the thermoplastic matrix as such, preferably 80% or less, more preferably 70% or less, most preferably 60% or less; such as 50% or less, or 40% or less. In preferred embodiments, the product may be expanded with respect to the foamable additive manufacturing material by a factor of 4 or more, preferably 10 or more, more preferably 20 or more, most preferably 40 or more. This generally corresponds to the envelope density of the product being 25% or less of the density of the thermoplastic matrix as such, preferably 10% or less, more preferably 5% or less, most preferably 2.5% or less. The product may advantageously be lightweight (i.e. have a relatively low density). In this regard, one could refer to an expansion factor below 4 a yielding a high density material, 4 to 10 as yielding a medium density material, 10 to 40 as yielding a low density material and above 40 as yielding an ultra-low density material.

[1 13] In embodiments, the product may have a mean (i.e. arithmetic average) cell (or ‘pore’/’void’) size of 300 pm or less, preferably 10 pm or less, more preferably 1 pm or less. Foams having a mean cell size above 300 pm may be referred to as conventional foams, from 300 pm to 10 pm as fine- celled foams, from 10 pm to 1 pm as microcellular foams, and 1 pm and below as nanofoams. The cell size typically determines the cell density, which is the number of cells per volume (e.g. per cm³). Conventional foams typically have fewer than 1 million cells per cm³, fine-celled foams in the order of 10⁶ to 10⁹ per cm³, microcellular foams have more than 10¹° cells per cm³.

[1 14] Aside from the mean cell size, also the cell size dispersity (i.e. the variability of the cell size) can be an important characteristic of the cell size distribution. Uniform cell sizes (low dispersity) typically advantageously result in a foam with homogeneous properties (e.g. mechanical properties), whereas a mix of large and small bubbles (high dispersity) result in heterogeneous properties. Different measures may be used to express the cell size dispersity, of which one is the coefficient of variation (CV). The latter is a normalized measure of dispersion, calculated as the ratio of the standard deviation (a) to the mean cell size (p) (CV = o / p). In embodiments, the product may for instance have cell size dispersion with a CV of 1 or below. The cell size, cell size distribution, mean cells size and cell size dispersity of a product can be accessed taking a 3D scan using scanning equipment or microscopy images on sections through the product, followed by analysis (e.g. using software) of the scan or images.

[1 15] In embodiments, any feature of any embodiment of the sixth aspect may independently be as correspondingly described for any embodiment of any of the other aspects.

[1 16] The invention will now be described by a detailed description of several embodiments of the invention. It is clear that other embodiments of the invention can be configured according to the knowledge of the person skilled in the art without departing from the true technical teaching of the invention, the invention being limited only by the terms of the appended claims.

Example 1 : Forming an additive manufacturing material

[1 17] Reference is made to FIG 1 , which is a flow chart representing a preferred method — via a masterbatch — for forming a foamable additive manufacturing compound in accordance with embodiments of the present invention. In this method, first, pyrolyzed lignocellulosic filler is provided by pyrolyzing lignocellulosic biomass (101). For example, the lignocellulosic biomass may be pyrolyzed at a temperature of 450 °C to 500 °C.

[1 18] The method continues with mixing the pyrolyzed lignocellulosic filler with a thermoplastic matrix (102) and mixing a foaming agent with the matrix material (103), to yield a masterbatch (104). These steps may be performed one afterthe other — in any order (e.g. following the path indicated by the full arrows or to striped arrows) — or may be performed both at the same time.

[1 19] Finally, the masterbatch (104) is diluted by compounding it with a further thermoplastic filler (105), to yield the foamable additive manufacturing material (106)

Example 2: Forming a product with an additive manufacturing material

[120] Reference is made to FIG 2, schematically depicting a building platform (1) and an additive manufacturing system (2) (e.g. a fused granular fabrication system) for forming a product by additive manufacturing onto said platform (1). A foamable additive manufacturing compound (4) in accordance with embodiments of the present invention, comprising a thermoplastic matrix, a pyrolyzed lignocellulosic filler and a foaming agent, is fed into the additive manufacturing system (2).

[121] The additive manufacturing system (2) comprises a nozzle (20) for depositing/printing the foamable additive manufacturing compound (4) onto the platform (1). The additive manufacturing system (2) typically comprises an extruder or similar system for softening (e.g. melting) — typically by heating — the foamable additive manufacturing compound (4) prior to printing.

[122] At the same time, the foaming agent is activated, so that foaming occurs as the additive manufacturing compound (4) is being printed. The foaming results in expansion of the additive manufacturing compound (4), so that volume of material being printed is larger than the volume of material exiting the nozzle. This is also visible in FIG 5, which is a photograph of a product being formed layer-by-layer by additive manufacturing, showing i.a. a clear difference in the diameter of the additive manufacturing compound exiting the nozzle compared to that of the layer being formed.

[123] The pyrolyzed lignocellulosic filler — which acts as a reinforcing, dispersing and nucleating agent — facilitates homogeneous foaming and thus homogeneous expansion throughout the foamable additive manufacturing compound (4). Next, the printed additive manufacturing compound (4) cools down and solidifies, forming a solid foam. The nozzle (20) moves over the platform (1) to form a first layer (31) of the printed product.

[124] We now refer to FIG 3. After completion of the first layer (31), the additive manufacturing system (2) may proceed with the formation of a second layer (32) on top of the first layer (31), by moving the nozzle (20) over the first layer (31) and depositing the foaming additive manufacturing compound (4) onto the first layer (31). Due to the presence of the pyrolyzed lignocellulosic filler, the additive manufacturing compound (4) is both lightweight and strong. Therefore, the second layer (32) exerts only limited force on the underlying first layer (31), and, at the same time, the first layer (31) is, notwithstanding its low density and high porosity, sufficiently strong so that it does not bend through nor collapse.

[125] We now refer to FIG 4. A plurality of layers may be formed in this way to form the desired printed product (3). As may be observed in FIG 5 and the vertical cross section in FIG 6, the plurality of thus formed layers have a uniform shape and thickness, as a result of the additive manufacturing compound — thanks to multiple beneficial roles of the pyrolyzed lignocellulosic filler (cf. supra) — having a low density, being structurally reinforced and foaming uniformly. Accordingly, the resulting product is advantageously lightweight, yet strong, and provides good thermal and sound insulation due to foamed cellular structure of the product. The latter is visualized in the magnification in FIG 7, showing some larger pockets (which may be at least partially attributed to the act of cutting through the material) but overall a nice cellular structure with small voids that are uniform in both size and distribution.

[126] It is to be understood that although preferred embodiments, specific constructions, configurations and materials have been discussed herein in order to illustrate the present invention. It will be apparent to those skilled in the art that various changes or modifications in form and detail may be made without departing from the scope of the invention as defined in the appended claims.

Claims

1. A masterbatch (104) for forming a foamable additive manufacturing compound (106, 4), comprising: i) a thermoplastic matrix, ii) a pyrolyzed lignocellulosic filler, and iii) a foaming agent.

2. The masterbatch (104) according to claim 1 , comprising from 50 wt% to 80 wt% of the pyrolyzed lignocellulosic filler, preferably 60 wt% to 70 wt%.

3. The masterbatch (104) according to any of the previous claims, comprising from 1 wt% to 15 wt% of the foaming agent, preferably from 2 wt% to 10 wt%.

4. A foamable additive manufacturing compound (106, 4), comprising: i) a thermoplastic matrix, ii) a pyrolyzed lignocellulosic filler, and iii) a foaming agent.

5. The foamable additive manufacturing compound (106, 4) according to claim 4, for foaming during additive manufacturing, with the proviso that the foamable additive manufacturing compound (106, 4) is substantially not foamed prior to additive manufacturing.

6. The foamable additive manufacturing compound (106, 4) according to claim 4 to 5, comprising from 0.1 wt% to 65 wt% of the pyrolyzed lignocellulosic filler, preferably from 5 wt% to 50 wt%, more preferably from 10 wt% to 40 wt%.

7. The foamable additive manufacturing compound (106, 4) according to claim 4 to 6, comprising from 0.05 wt% to 5 wt% of the foaming agent.

8. The foamable additive manufacturing compound (106, 4) according to claim 4 to 7, wherein the pyrolyzed lignocellulosic filler has an envelope density of from 0.1 g/cm³ to 0.7 g/cm³, preferably from 0.2 g/cm³ to 0.5 g/cm³, most preferably from 0.25 g/cm³ to 0.4 g/cm³.

9. The foamable additive manufacturing compound (106, 4) according to claim 4 to 8, wherein the pyrolyzed lignocellulosic filler has a mean particle size of from 1 pm to 5000 pm.

10. The foamable additive manufacturing compound (106, 4) according to claim 4 to 9, wherein the pyrolyzed lignocellulosic filler is pyrolyzed at a temperature of from 220 °C to 1000 °C, preferably from 340 °C to 900 °C, more preferably from 400 °C to 600 °C, most preferably from 450 °C to 500 °C.

11 . The foamable additive manufacturing compound (106, 4) according to claim 4 to 10, wherein the thermoplastic matrix comprises a polymer, preferably polylactic acid, polypropylene, polystyrene, polyamide, polycaprolactone, polyvinyl chloride or polyethylene terephthalate.

12. The foamable additive manufacturing compound (106, 4) according to claim 4 to 11 , wherein the pyrolyzed lignocellulosic filler comprises pyrolyzed flax, pyrolyzed bamboo or pyrolyzed wood.

13. The foamable additive manufacturing compound (106, 4) according to claim 4 to 12, wherein the foaming agent comprises a physical or chemical foaming agent.

14. The foamable additive manufacturing compound (106, 4) according to claim 4 to 13, being a granular material.

15. A method for forming a masterbatch (104) as defined in any of claims 1 to 3, comprising: a) mixing a pyrolyzed lignocellulosic filler with a thermoplastic matrix, and b) — before, during or after step a — mixing a foaming agent with the thermoplastic matrix.

16. A method for forming a foamable additive manufacturing compound (106, 4) as defined in any of claims 4 to 14, comprising:

1 a) compounding a masterbatch (104) as defined in any of claims 1 to 3 with further thermoplastic matrix; or

2a) compounding a pyrolyzed lignocellulosic filler with a thermoplastic matrix, and

2b) — before, during or after step 2a — compounding a foaming agent with the thermoplastic matrix.

17. The method according to claim 15 or 16, wherein the foaming agent has a foaming temperature and wherein the foaming agent is kept below said foaming temperature during step b, 1a and/or 2b.

18. A method for forming a product by additive manufacturing comprising: a) feeding a foamable additive manufacturing compound (106, 4) as defined in any of claims 4 to 14 into an additive manufacturing system (2), and b) additively manufacturing the product using the additive manufacturing system (2).

19. The method according to claim 18, wherein step b further comprises foaming the foamable additive manufacturing compound (106, 4).

20. The method according to claim 18 or 19, wherein said additive manufacturing comprises a fused granular fabrication.

21. A product (3) obtainable by additive manufacturing of the foamable additive manufacturing compound (106, 4) as defined in any of claims 4 to 14.