Highly loaded compositions

Abstract

A composition comprises a liquid and 40-60 wt.% fibres having an average diameter of ≤10 microns and an aspect ratio of 5-20, wherein the composition has a dynamic viscosity of ≤400 mPa.s at one or mo

Description

Compositions including solid fillers are critical in several industries where the physical properties of the final product, such as strength, stiffness, durability, mechanical or thermal stability, density, or conductivity, to name a few, are enhanced by the inclusion of fillers. The fillers may not only contribute to functional aspects of the compositions, but also to esthetical ones (e.g., increasing color density in inks). Additionally, as they can enhance certain (e.g, mechanical) properties of the compositions comprising them, they can reduce the amount of the other materials usually required to achieve a similar effect, reducing costs and / or weights. In some cases, such desirable effects of the fillers on the final products require a relatively high contents of fillers, such as constituting more than 20% by weight of the total composition. When the fillers serve to supplement base materials that are also in solid form before their joint mixing, difficulties in obtaining an homogeneous composite may stem from differences in the dimensions, the chemical substances, the shapes, and / or the surface characteristics of the respective components and any like factors that may impact their ability to homogeneously distribute themselves among the others upon mixing, or on the contrary their tendency to agglomerate in absence of even distribution, leading to segregation of some of the constituents. Uneven mixing of solid fillers with base materials can lead to significant issues in producing homogeneous composites, such as poor mechanical properties, inconsistent performance, or reduced durability. Mixing technologies, methods of treating the various constituents and specific additives (e.g., anti-caking agents and flow aids) have been developed to reduce or eliminate this problem of evenly combining materials of a same solid phase. However, when the base materials are in liquid form, incorporating amounts of solid fillers high enough to achieve a sought effect still presents several challenges. For illustration, the incorporation of a solid filler in a liquid may result in a viscosity so high as to detract from the workability of the composition. Moreover, such highly loaded liquid compositions often exhibit a tendency to phase separate, and a poor viscosity stability. The decreased workability of the composition may in turn deeply reduce the processing speed. This problem is particularly severe in processes expected to be completed in relatively short periods of time. For illustration, flowable compositions are often used in printing processes, and highly viscous ones may retard the processing speed to an extent annihilating the commercial relevance of the technology, or limiting it to high-end products, the cost of which could compensate for the length of the process. For example, in stereolithography highly viscous (filler loaded) resins can be so flow-resistant that they will not readily form a smooth layer of a photo curable composition to be thereafter solidified by suitable radiation. Staying with curable compositions as may be used in three-dimensional (3D) printing to illustrate the problems encountered by compositions highly loaded with fillers, such compositions may not only benefit from having a low enough initial viscosity, but for this viscosity to remain sufficiently stable over time. The tendency of highly loaded compositions to display increasing viscosity over time (e.g., due to rampant polymerization) results in less efficient, and more costly, (e.g., additive manufacturing) processes. Alternatively, or additionally, the instability of the highly loaded compositions may result in phase separation, whereby the inorganic filler is no longer homogeneously suspended in the surrounding curable liquid, usually resulting in a high-viscosity bottom portion supersaturated with the filler and a low-viscosity, largely unfilled top portion. This progressive settling of the filler cannot always be prevented by the inclusion of anti-sedimentation agents, to the extent their presence could be permitted or tolerated for the sake of the end-product and in the process leading to its manufacturing. While it is theoretically possible to rely on frequent and often vigorous recirculation of the highly loaded compositions to prevent or reduce such phase separation, this is a time- and energy-consuming process. SUMMARY With a view to mitigating at least some of the afore mentioned problems, there is provided in a first aspect of the present disclosure, a composition including a relatively high amount of fillers as hereinafter set forth in more detail and claimed in Claim 1 of the appended claims and claims depending therefrom. The composition may comprise fibers being present at a weight concentration by total weight of the composition between 40 wt.% and 60 wt.%, between 44 wt.% and 60 wt.%, or between 47 wt.% and 60 wt.%, and may therefore be referred to as a highly loaded composition. Furthermore, the highly loaded composition can be a curable composition. While the present highly loaded compositions were developed for the sake of curable compositions as applicable to a 3D printing process, and will be described mainly in this context, this is not their sole possible use and reference in the following to exemplary curable compositions should not be construed as limiting. In some embodiments, the (e.g., curable) composition comprises a (e.g., curable) liquid and fibers having an average diameter Dave not exceeding 10 pm and a unitless aspect ratio ASP between the average lengths Lave and diameters Dme of the fibers comprised between 5 and 20, the composition having a dynamic viscosity of 400 mPa s (one millipascal-second) or less, at at least one temperature between 40°C and 120°C, as measured using a Brookfield type viscometer. In some embodiments, the average diameter Dme of the fibers is between 150 nm and 10 pm, between 150 nm and 7 pm or between 150 nm and 5 pm, their average length Lave is between 1 pm and 50 pm, and the aspect ratio between the two dimensions (ASP= Lave / Dme) is between 5 and 15, between 7 and 15, or between 7 and 12. Notably, in presence of 0.1 wt.% of the fibers dimensioned as herein described, the liquid has a viscosity not exceeding 1.2-fold the viscosity of the liquid in absence of the fibers, as measured at at least one temperature between 40°C and 120°C using a Brookfield type viscometer. In some embodiments, and despite its relatively high loading in fibers, the composition has a dynamic viscosity of 300 mPa-s or less, 200 mPa-s or less, or 100 mPa-s or less, at at least one temperature between 40°C and 120°C. Advantageously, in some embodiments, when the liquid is curable, after spreading of the composition and full curing thereof, the cured composition has, in a direction of spreading, a linear coefficient of thermal expansion CLTE between 5xl0~6 K’1 and 30x1 (f6 K4, between 10xl0’6 K’1 and 30xl0’6 K'1, or between 15xl0’6 K’1 and 30xl0’6 K'1, as measured by thermomechanical analysis. The fibers loaded into the composition may include two populations, a first population of fibers having a first average diameter Di between 150 nm and 500 nm, between 150 nm and 400 nm, or between 150 nm and 350 nm; and a second population of fibers having a second average diameter D2 between 500 nm and 10 pm, between 500 nm and 8 pm, or between 500 nm and 6.5 pm. In some embodiments, the fibers can be characterized by a unitless ratio DR between the average diameters D2 and Dj (D2 / D1) of the two constituting populations being in the range of 3 to 70, in the range of 3 to 50 or in the range of 3 to 30. A dimension of the fibers can be determined by any suitable standard method, for instance by analysis of microscopy images, the values being typically averaged for at least 300 fibers. In some cases, the first population of fibers has a first average length Li between 1 pm and 30 pm, between 3 pm and 25 pm, or between 3 pm and 20 pm; and the second population of fibers has a second average length L2 between 5 pm and 50 pm, or between 5 pm and 40 pm. A unitless ratio LR between L2 andLi (L2 / L1) can be in the range of 1 to 40, or in the range of 1 to 30. When the highly loaded composition is a curable composition, the liquid is a curable liquid which can be selected from a group consisting of modified or unmodified epoxy compounds (including epoxy acrylates), acrylates (including mono-, di- and tri-acrylates); urethane acrylates; polyether acrylates; vinyl ethers; polycyclic aromatic hydrocarbons; and derivatives and / or combinations thereof. Suitable modified or unmodified epoxy compounds and epoxy acrylates can be selected from a group comprising bisphenol-A based epoxy acrylate; amine modified bisphenol-A based epoxy acrylate, bisphenol-A diglycidyl ether di-acrylate, epoxyacrylate oligomers, epoxycyclohexylmethyl epoxy-cyclohexane and carboxylate7-oxabicyclo[4.1.0]hept-3-ylmethyl-7-oxabicyclo[4.1.0]heptane-3-carboxylate; and derivatives and / or combinations thereof. Suitable modified or unmodified mono-, di- and tri-acrylates can be selected from a group comprising isobomyl acrylate, hexanediol diacrylate, tripropylene glycol diacrylate, pentaerythritol triacrylate, trimethylol-propane triacrylate, and ethoxylated trimethylolpropane triacrylate; and derivatives and / or combinations thereof. Suitable modified or unmodified urethane acrylates can be selected from a group comprising aliphatic urethane acrylate, aliphatic urethane diacrylate and aliphatic polyester urethane diacrylate; and derivatives and / or combinations thereof. Suitable modified or unmodified polyether acrylates can be selected from a group comprising amine modified polyether acrylate, amine modified polyether diacrylate, and propoxylated (2PO) neopentyl glycol diacrylate; and derivatives and / or combinations thereof. Suitable modified or unmodified vinyl ethers can be selected from a group comprising vinyl ether, diethylene-glycol divinyl ether, vinyloxyethoxy ethyl acrylate; and derivatives and / or combinations thereof. Suitable modified or unmodified polycyclic aromatic hydrocarbons can be selected from a group comprising benzo-cyclobutenes, benzocyclobutadienes, benzocyclooctatetraenes; and derivatives and / or combinations thereof. The fibers or the populations of fibers highly loaded in the present compositions can each be independently selected from fibers consisting of carbon, ceramics (including alumina, zirconia and glass), clay minerals (including halloysite), natural polymers (including cellulose, chitosan and lignin), synthetic polymers (including polyesters, polypropylenes, fluoropolymers, such as polytetrafluoroethylene (PTFE), and polyamides, such as aramid and nylon, and combinations thereof. When the one or more populations of fibers are fibers consisting of glass, the glass fibers can be selected from a group comprising E-glass, D-glass, and TEOS-based electro-spun glass. The (e.g., curable) composition highly loaded with the fibers as herein described, can in some embodiments further comprise beads having an average diameter not exceeding 5 pm and a unitless aspect ratio between their average longest and smallest dimensions comprised between 1 and 3, the beads optionally being present at a weight concentration by total weight of the composition of up to 10 wt.%. In some cases, the beads can consist of glass, cellulose, ceramics, PTFE and combinations thereof, but need not be made of a same material as the fibers. Typically, fibers suitable for the present high loading have a specific surface area of at least 1.5 m2 / g and at most 20 m2 / g as measured by BET. In some cases, the fibers are made of materials having a specific gravity of at least 0.5 g / cm3 and at most 5 g / cm3, at least 0.5 g / cm3 and at most 2.5 g / cm3, or at least 1 g / cm3 and at most 2.5 g / cm3. Specific gravities of materials are known but can be independently measured by any standard method, and for example by a buoyancy method (based on Archimedes' principle that an object submerged in a fluid displaces an amount of fluid equal to its own volume, the specific gravity being calculated by weighing a sample of the material forming the fibers and measuring the apparent increase in volume of the liquid). In some embodiments, when the liquid is curable, after spreading of the composition and full curing thereof, the cured composition has, in a direction of spreading, a flexural strength between 200 MPa and 300 MPa, between 200 MPa and 400 MPa, or between 200 MPa and 500 MPa, as measured according to ASTM D6272. When the highly loaded composition is curable composition to be cured in a manufacturing process, the fibers can be selected to provide to an object cured from the composition at least one property selected from mechanical reinforcement, thermal insulation or conduction, electrical insulation or conduction, modulation of thermal expansion, and radiation blocking or adsorption. The composition may further comprise at least one additive the presence of all additives not exceeding 10 wt.% by total weight of the composition. Each additive can be adapted to the composition and be, for example, selected from a group consisting of curing initiators, curing catalysts, curing inhibitors, fire retardants, adhesion promoters, and viscosity modifiers, the additive being adapted to the curing method envisioned for the composition when including a curable liquid. In cases when the highly loaded composition is a curable composition, the liquid can be curable by application of energy selected from heat, eBeam, 1R and UV radiation. The highly loaded compositions according to the present teachings, in particular when curable compositions, can be for use in additive manufacturing. In a particular embodiment, the curable compositions can be used for manufacturing the insulating casing of a conductive pattern, such as may be found in PCBs. In a second aspect of the present disclosure, there is provided a process for additive manufacturing including in steps thereof the curing of a composition comprising a relatively high amount of fillers as hereinafter set forth in more detail. Features of different embodiments of the foregoing and additional benefits of the present disclosure are set out in the following detailed description taken in conjunction with the figures, and non-limiting examples. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described further, by way of example, with reference to the accompanying drawings, where like reference numerals or characters (or last digits thereof) indicate corresponding or like components. The description, together with the figures, makes apparent to a person having ordinary skill in the art how some embodiments of the disclosure may be practiced. The figures are for the purpose of illustrative discussion and no attempt is made to show structural details of an embodiment in more detail than is necessary for a fundamental understanding of the disclosure. For the sake of clarity and convenience of presentation, some objects depicted in the figures are not necessarily shown to scale. In the Figures: Fig, 1 is a schematic representation of a first 3D printing system; Fig, 2 is a schematic representation of a second 3D printing system; Figs. 3 to 7 show schematically the sequence of steps and implementing modules employed in forming a 3D article by using inter alia highly loaded compositions of the invention; Fig, 8 shows illustrations of the post-processing of an article after all its different layers have been deposited; Fig, 9 illustrates how devices can be arranged to form a 3D printing system able to form a stack of layers including regions of compacted first particles when the substrate and its transport mechanism are non-planar; Fig, 10 provides an exemplary alternative arrangement of devices, when the 3D printing system is intended for planar substrates and transport mechanisms, the printing system further illustrating how highly loaded compositions of the invention can be used; and Fig, 11 displays pictures captured by microscopy of particles that may serve as additives to curable liquids capable of serving as dielectric support material, panel A showing the rodshaped particles before being added to the curable liquid and panel B showing a cured version of the same. DETAILED DESCRIPTION The present compositions highly loaded with a filler were developed and tested in a demanding 3D printing process and will be described mainly in this context. However, this is not their sole use and applicability. Understandingly, they may also serve alternative printing processes, coating processes or any other manufacturing process in which highly loaded compositions can be beneficial, in particular when due to withstand pressure. Overview of Examplary 3D Printing Methods and Systems The 3D printing method of the present disclosure comprises forming a plurality of stacked layers, in which at least each intermediate layer is formed of contiguous regions of two or more different materials (e.g., particles having different compositions or properties and / or different adhesive compositions) that cover the whole of the area bounded only by the outer perimeter of each intermediate layer. In other words, the individual layers do not have any holes in them, though the perimeters of adjacent layers do not need to be the same. Consequently, during formation of each layer (or slice of a 3D object), there is a continuous underlying layer onto which material can be applied. Fig. 1 schematically shows an apparatus for carrying out a method of a first embodiment of 3D printing process. A substrate 10 is mounted on a transport mechanism shaped as a drum 12 that transports it sequentially through different processing stations. In a first station 14 a thin adhesive layer is selectively applied to regions onto which first particles are to be adhered, said regions (which need not be contiguous for any single type of material) being also termed first regions or foreground regions. The selective application may for example be carried out by jetting of an adhesive composition to the first regions. While the adhesive is tacky, the substrate passes through a second station 16 where first particles are made to adhere only to regions that have been coated with an adhesive. This process is then essentially repeated for second particles by passing the substrate through a third station 18 where a second adhesive coating is selectively applied to second regions (also called background regions or remaining regions) of the substrate 10 and then through a fourth station 20 where second particles are adhered only to regions coated with the second adhesive. If desired, the process can be repeated for third particles. Once all the different particles forming the various regions of a layer have been applied to the substrate 10, it is passed through a station 22 where the particles are compacted, and optionally levelled, to create a flat continuous surface onto which a new layer of particles can be applied. Such a compacting station may additionally be found after previous types of particles have been adhered to selectively applied regions of thin adhesive layer preceding the completion of a layer. In the setup illustrated in Fig. 1, such a situation would be depicted by the presence of two compacting stations, one downstream of station 16 and upstream of station 18, and the other as currently illustrated by station 22 downstream of station 20 and upstream of station 14. If the compaction achieved downstream of the last compacting station passed through during any given cycle is such that the flat continuous surface has an average roughness Ra of 3 pm or less (as could have been achieved by milling), separate levelling may not be necessary. Yet, in some embodiments, an even smoother surface (e.g., having an average roughness Ra of 2 pm or less, 1 pm or less, or 0.5 pm or less), may be required of the continuous intermediate layer, in which case a separate levelling station (not shown in present figure) may be further included downstream of station 22 and upstream of station 14 where a new cycle can be initiated. Depending on the desired degree of smoothness, the levelling station can include a milling device (for Ra of about 3.5 pm or less), a grinding device (for Ra of about 1.5 pm or less) or an abrading device (also known as a polishing device) (for Ra of about 0.5 pm or less). The topographic properties of a surface compacted and / or levelled by a compacting module or a levelling module can be monitored by any suitable method. For illustration, optical sensors suitable for distance measurements, light detection and ranging (LiDAR), 3D mapping, and laser scanning of the surface being assessed, or cameras capturing such information, may be used in connection with computerized analysis systems translating the sensors output into a sought measurement. Control systems may further be used to modify an operational parameter of the 3D printing system in response to the measured property (e.g., surface topography measured by laser triangulation). After completion of multiple cycles according to similar principles, there is formed a stack as shown at (A) in Fig. 8. Within this stack, the first particles, shown around the centreline, may be made of a metal or a ceramic and the second particles, surrounding the first particles, may be formed of a plastics material. To complete the production of an article made of the first particles, the stack may be consolidated as shown at (B) in Fig. 8, such as by sintering, to fuse the different layers of the first particles to one another and finally, as shown at (C) in Fig. 8, the second particles may be removed to leave behind only the desired article. It will be seen from this exemplary figure that the first particles are those serving to form the desired article and will therefore also be termed herein model particles, while the second particles serve only to allow the model particles to be deposited wherever necessary and are herein also termed support particles. The support particles can be made of material different from the model particles in order to permit their removal by a physical or chemical process. The manner of the removal will depend on the choice of support particles. For example, the support particles may dissolve in a solvent in which the model particles are not soluble, to permit chemical removal of the support particles, or they may have a lower melting point, allowing their removal by application of heat to which the model particles would be resistant. Alternatively, or additionally, the regions made of different materials may include different adhesives, the particles themselves optionally being made of a same material. For illustration, a first adhesive intended to adhere the first particles can be rendered tacky at a first temperature and a second adhesive intended to adhere the second particles can be rendered tacky at a second temperature. This alone may permit the joint application (at a same station and / or concomitantly) of the first adhesive and second adhesive, each in its respective intended region, the first adhesive being activatable at a first temperature (e.g., 80°C) sufficiently lower than the second temperature (e.g., 120°C) so that the second adhesive remains “inactive” when the first particles are applied to the first adhesive at its first temperature. The temperature may then be raised to reach the second temperature, thereby activating the second adhesive in regions uncoated by the first particles, the regions now rendered tacky being then able to adhere second particles. If the regions made of different materials differ from one another inter alia or only by the choice of the adhesives, then the adhesives can additionally be selected to facilitate or prevent removal of one type of particles’ coating over the other, the adhesives having different solubility / resistance towards a removing solvent. For instance, the first adhesive can be water-insoluble and the second adhesive water soluble, water being used to separate particles bound in the different regions upon completion of superposition of all compacted layers. It should be noted that removal of the support material is not essential as it may form part of the end-product. For example, in the manufacture of a circuit board, the support particles may form the insulating substrate while the model particles may form the desired electrical conductors. Fig- 2 differs from Fig. 1 in that instead of adhesives being applied to selective regions according to the particles to be coated thereby, an adhesive coating is applied to the entire substrate 10 and then selectively activated, such as by exposure to radiation or a suitable chemical agent. Thus, in the apparatus schematically depicted in Fig. 2 there is only one adhesive application station 30 where an adhesive is applied but is not tacky (or tacky enough) by the time it reaches a first activator station 32 where selected regions of the adhesive (e.g., first or foreground regions) are activated to remain tacky until desired particles are applied thereto. After first particle are applied to the activated regions of the adhesive by a station 34, the remaining regions of the adhesive (e.g., second or background regions) are activated in an adhesive activating station 36 to remain tacky till desired particles are applied thereto and are coated with second particles in station 38. As with the previous embodiment, the final station 40 serves to compact (and optionally level) all the particles constituting a layer to provide a continuous surface for the next deposition cycle and, similarly, such a compacting station may additionally be found downstream of station 34 and upstream of stations 36 being an adhesive activator or 38 being a dispenser of second particles. In both embodiments, the compaction (optionally achieving levelling) can be carried out by passing the coated substrate between rollers urged against one another with a desired force but movable relative to one another to allow for the height of the substrate to increase from cycle to cycle. Compaction can be performed at at least 25 MPa, at least 50 MPa, at least 75 MPa, or at least 100 MPa; the applied pressure typically not exceeding 500 MPa, and being of up to 400 MPa, or 300 MPa. Compaction can furthermore be performed at a temperature higher than room temperature, such as at 50°C or more, the temperature of compaction being selected in accordance with the thermal resistance of the materials being compacted. Typically, if heat is applied during compaction, it does not exceed 150°C. The embodiments of Figs. 1 and 2, which build an article onto a part-cylindrical substrate, are well suited to manufacture of flexible circuit boards, as the substrate can be bent after removal from the drum. To construct an article that is not necessarily flexible and that needs to have a flat base requires a different transport mechanism to move the substrate cyclically between different processing stations. It is possible for example, to replace the drum by an annular horizontal support surface that rotates about a vertical axis. As a further alternative, the substrate may be placed on a flat plate that is suspended from two continuous chains and each station may comprise features to retain the plate in a predetermined position during processing, to ensure correct alignment of the different regions. Though it has been assumed above that both the model and the support materials are made of particles, it would be possible for the support regions to be applied as a liquid that is then solidified (e.g, dried or cured if a curable liquid, such as a radiation curable resin that can be solidified by ultraviolet (UV), visible or IR light, electron beam (EB), or thermal energy). The highly loaded compositions of the present invention may serve this purpose. Liquid may be selectively applied to the support regions, such as by jetting, or one may coat with a liquid the entire substrate (including regions already coated with particles) and, following curing at a curing station, the solidified (e.g., cured) support materials may be removed from regions coated with model particles. In this case, a separate levelling device adapted to remove surplus support material will be required in addition to the compacting station adapted to compress the regions coated with model particles. The levelling device of the separate levelling station may, for example, employ an abrasive having a hardness greater than that of the cured support material but lower than that of the compacted model particles. While for the sake of efficient stacking of layers it may be desired to remove as little as possible from the regions of compacted model particles, this is not essential. It may, on the contrary, be desirable to expose the model particles compacted in an underlying layer to model particles to be freshly applied in a subsequent layer. This can be the case when model particles of adjacent layers are to be applied in overlapping regions, the exposure of the model particles of a previous layer facilitating later binding to model particles of a following layer in a consolidation process. When such exposure is required (e.g, for the additive manufacturing of a PCB), the levelling of the intermediate layer may be such as to achieve an average roughness Ra of 500 nm or less, 400 nm or less, 300 nm or less, or 200 nm or less. Figs. 3 to 7 illustrate schematically different apparatus for implementing different embodiments of a 3D printing method in which the present compositions can be used, wherein an arrow indicates the direction (X) of the process for completing the formation of a single layer of the stack. While illustrated with a transport mechanism adapted for a flat planar backing surface for stacked layers having a flat base, this should not be construed as limiting. Fig-3 shows the steps performed by the embodiment of Fig. 2 and uses the same reference numerals. A thin adhesive applicator 30 is followed by a selective adhesive activator 32. After passing through the model particle dispenser 34 while the selectively activated adhesive is still tacky, the substrate passes beneath a background adhesive activator 36 and a support particle dispenser 38 while the activated background adhesive is still tacky, before reaching the compacting station 40. While the second adhesive activator 36 can selectively activate (e.g., by selective irradiation or jetting of an activating chemical) the second regions of the substrate to be thereafter coated by second particles, this is not essential. For instance, if the thin layer of adhesive applied by applicator 30 over the entire surface of the substrate due to form a subsequent layer is heat activatable, then activation of the adhesive layer in the background regions not coated by model particles can be achieved by applying heat to the entire substrate, the model particles in the coated first regions blocking access to subsequently applied support particles. Fig-4 differs from Fig. 3 only in that it comprises two compaction stations, the model particles being compacted in a station 42 before activation of the background adhesive (at 36), deposition of support particles (at 38) on the exposed regions of the activated adhesive, and compaction of the support particles to the substrate in compacting station 40, where the model particles may be further compacted. In Fig. 5, which is based on the embodiment of Fig. 1 and uses the same reference numerals, the adhesive for the model particles is applied by a selective applicator 14A that is followed by an optional activator 14B that is not selective which can be used to increase, maintain or restore tackiness of the selective adhesive image. Likewise, the adhesive for the support particles is a selective applicator 18A that is followed by an optional activator 18B that is not selective. Otherwise, the process is the same as in Fig. 3. For illustration, selective applicators 14A and 18A may apply a thin layer of same or different heat activatable adhesives, which can optionally be non-selectively activated by heat at stations 14B and 18B, to the extent necessary to increase or renew their tackiness for adhering to the model or support particles respectively downstream dispensed. While in the present figure, the selective applicators of adhesives have been illustrated as being each downstream followed by a respective dispenser of particles 16 or 20, this arrangement should not be construed as limiting. For instance, if the adhesive compositions are distinct, e.g.. the first adhesive being activatable at a first temperature and the second adhesive being activatable at a second temperature greater than the first (and being inactive at the first temperature), then applicators 14A and 18A may be adjacent one another so as to form at once a layer with different regions of distinct adhesives. The first particles may then be applied by particle dispenser 16 at the first temperature (optionally with the assistance of heater 14B), this temperature rendering tacky only the regions of the first adhesive, and the second particles may be thereafter applied by particle dispenser 20 at the second temperature (optionally with the assistance of heater 18B), this higher temperature rendering tacky the regions of the second adhesive. As the regions of the first adhesive are by then already coated by the first particles, the second particles will only adhere to the second activated adhesive. In this case, the different adhesive compositions, even if jointly applied before entering a first dispenser of particles, may be viewed as serving the purpose of “differential” adhesion to a backing surface or underlying layer during the formation of the superposed layers. Alternatively, or additionally, the different adhesives may serve the purpose of “differential” removal of the different materials, if desired, following completion of superposition of the layers. For instance, the first adhesive can be insoluble in a “removing solvent” whereas the second adhesive would be soluble in the same solvent. Again, adhesive applicators 14A and 18A can be adjacent one another forming at once the different regions of distinct adhesives. Furthermore, in this case, the particles serving as model or support can be the same, since their removal may rely on the distinct adhesives binding them. Then, in such embodiment, a single particle dispenser 16 may suffice (optionally with the assistance of a device activating the layer comprising the distinct adhesives, e.g., heater 14B). As readily appreciated from these exemplary alternatives, an intermediate layer is said to be made of different materials in different regions of the layer, a) if the “building” material, whether particles or solidifiable liquids, are of different chemical and / or physical composition; b) if the “connecting” material, namely the adhesive compositions, similarly have different chemical and / or physical properties; or c) both. Fig. 6 illustrates an embodiment in which the support material is applied in liquid form. The first four processes in this embodiment can be the same as in Fig. 4 and have been allocated the same reference numerals. Following application and compaction of the model particles, in the station designated 60 a liquid resin is applied over the entire surface of the substrate. A squeegee 62 removes most of the liquid overlying the compacted model particles while leaving a film of the liquid resin over the remainder of the substrate. Following curing of the liquid resin in the station designated 64 the substrate passes through a levelling station 66 in which any cured resin overlying the compacted model particles is removed by milling, grinding, and / or abrasion, the material so eliminated being sucked away. If desired, an excess of model particles may be applied during the first four processes and the levelling may remove some of the model particles as well as the overlying cured resin. While levelling is described in this figure as being performed on a cured resin, such a step may alternatively be performed when the materials forming an intermediate layer are all in the form of particles. Taking Fig. 6 for illustration, in this case station 60 would dispense support particles, squeegee 62 would ensure the support particles fill the regions of the substrate not previously coated by the model particles, station 64 would in turn ensure all particles are compacted before entering the levelling station 66. While Figs. 1 to 6 have schematically illustrated different methods and some alternative apparatuses for their implementation showing that all layers of the stack can be formed on a common transport mechanism (e.g., adapted to convey a flat planar or a curved backing surface and growing substrate), this should not be construed as limiting. For instance, some layers can be formed on a separate surface before being transferred to the transport mechanism onto an initially vacant surface or on layers previously formed thereon or transferred thereto from additional separate surfaces. Fig. 7 illustrates such an embodiment and differs from Fig. 6 in that there are two thin adhesive applicators. A first thin adhesive applicator 30a is configured to coat non-selectively the entire surface of the substrate transported by the flat transport mechanism 52 illustrated horizontally in the figure. A second thin adhesive applicator 30b is configured to non-selectively coat a separate surface, shown as drum 54, the downstream selective adhesive activator 32 and model particle dispenser 34 being now adjacent to and operative on drum 54 so as to form thereon a layer including regions selectively coated with model particles. These regions of model particles are transferred in mirror image from drum 54 to the thin adhesive applied on the upper surface conveyed by the flat transport mechanism 52 at the nip 56 therebetween. This relocation of the layer at the transfer nip 56 can take place if the thin adhesive applied by applicator 30a to form a flat adhesive is still tacky, and the model particles have a greater adhesion to the flat adhesive than to the adhesive to which they were initially adhered to on the surface of the drum. If this is not the case, an additional adhesive activator (not shown) can be included downstream of the thin adhesive applicator 30a and upstream of the nip 56 to ensure that the thin layer of adhesive be tacky enough to detach the regions of model particles from the drum to effect their transfer. While a compacting station 42 is shown downstream of the transfer nip 56, this may not be necessary if the pressure applied to effect transfer at the nip can additionally achieve a desired compaction of the model particles. The following processes can be as illustrated in Fig. 6, the support material being applied as a liquid resin in the station designated 60, most of the liquid overlying the compacted model particles being removed by squeegee 62 while leaving a film of the liquid resin over the remainder of the substrate which is solidified in the station designated 64. The substrate then passes through a levelling station 66. Alternatively (and not shown in Fig. 7), the support material can be applied in the form of particles, in which case the processes following compaction of the transferred model particles can be as illustrated for instance in Fig. 4. While Figs. 1 to 7 have for simplicity schematically illustrated different 3D printing methods and some exemplary printing systems for their implementation showing layers formed of two different types of material, this should not be construed as limiting. As can be appreciated, some steps of the method may be repeated for additional (e.g., a third, a fourth, and so on) material, the apparatus accordingly including additional corresponding devices adapted to the performance of such steps for the materials desired in addition to the first and second one. A set of devices adapted to the performance of any of the steps described herein, such as enabling the formation of regions of a specified material on the backing surface, can be referred to as a sub-system. Hence, if a third material is to be applied similarly to a second material, the apparatus may further comprise a sub-system identical to the one selected for the application of the second material. However, they need not be identical, as some devices of such two sub-systems can be redundant and omitted in one of the two sub-systems. Alternatively, the additional material can be applied by any other equipment, means or devices, suitable for said material and / or its intended purpose in the layer or stack thereof, the method being accordingly adapted to further comprise appropriate steps. For illustration, when manufacturing PCBs, the substrate may be capable of conveying a plurality of patterns of (e.g., first) conductive materials. The patterns formed on a common substrate can be the same or different. In such a case, a manufacturing process would ultimately include the division of the substrate into sections, each split region bearing the individual patterns constituting the sought chips. In the present method, separation between adjacent patterns formed on a common substrate can be facilitated or fully achieved by use of an additional “separating” material during the formation of the stack of layers. For instance, once layers of particles made of an electrically conductive material have been applied in repeated cycles and compacted to form the shape of the desired circuit of each future chip, a separating material can be applied to surround this pattern at any specified distance therefrom, the outline of the separating material from top view being same or different than the outer shape of the pattern surrounded thereby. For example, if the pattern from top view can be approximated to a rectangle, the separating material can be deposited to form a larger rectangle or a circle having a diameter larger than the longest dimension of the rectangular pattern each encompassing the regions to be separated from one another. If the patterns are the same or have a similar outer shape and dimensions (e.g., regardless of the topography of the inner circuit), the separating material is typically applied midway between two adjacent patterns outermost contours. The separating material can, depending on the shapes of the patterns, be shared by adjacent patterns in part of their perimeters. For example, if the patterns from top view can be approximated to rectangles, the separating material can be deposited at a distance of only one edge to separate adjacent rectangular patterns. If the patterns have different outer shapes (e.g., a square, a rectangle, a circle, a triangle, etc.), the separating material can be applied at any distance from an outermost contour of one pattern / shape as long as it does not intersect with an adjacent pattern or the separating material surrounding the second pattern / shape. The separating material can be applied by a movable dispensing injector, static nozzles of suitable jetting equipment or any like device capable of forming, around the patterns, “walls” or “fences” of sufficient height (ie., at least matching the height of the pattern). Then, a curable liquid can be applied as aforesaid to cover the plurality of patterns and their separating walls. Following curing, the cured outer surface can be levelled to at least expose the tips of the separating walls. The separating material can advantageously be made of a substance that can be removed with relative ease. The separating material needs be adapted to the formation of the walls (e.g., maintain a desired height), optionally compatible with the curing method of the curable material (e.g., both being UV curable at similar wavelengths) but can, for instance, be thereafter capable of being removed by a suitable solvent (e.g., by water, if water-soluble). In this case, a same, or different, separating material can be used to form a sacrificial layer prior to the formation of the patterns on the common substrate, so that the removal of the separating material(s) result in the physical separation (splitting) of the patterns individually “encapsulated” in the cured material (e.g., in a dielectric cured epoxy). As readily appreciated, while in preceding paragraphs an additional material (e.g., other than model or support as can be applied by the previously described methods or devices) further included in the present methods in reliance on additional dedicated dispensers was illustrated by a separating material due to be removed so as to physically detach patterns from neighboring ones formed on a same substrate, such method steps, or dedicated equipment, are not limited to these sole substances / purposes. For illustration, the additional material may be intended to remain where deposited, only serving to fence the patterns one from the functional and / or visual effect of another without necessarily physically detaching them. In such a case, the material can be termed a fencing material, but any other nomenclature can be used to similarly refer to a material suited for the named function of their presence which can also be purely ornamental. Regardless of the number of materials that may be used to generate any intermediate layer, the form in which they are applied (e.g., as particles or a jettable liquid) and of the way by which each such material is applied or transferred to form such a layer, the present method is believed to achieve such additive manufacturing in a relatively fast manner. The efficiency of such a process, its productivity over time, can be provided in volume of materials than can be formed by unit of time, for instance in cubic centimeter per hour (cm3 / h) or liter per hour (1 / h) under the operating conditions (e.g., speed of the transport mechanism) selected for the 3D printing system of relevance. In some cases, the present printing methods and / or apparatuses enable a 3D build speed of at least 5 cm3 / h, at least 10 cm3 / h, at least 25 cm3 / h, at least 50 cm3 / h, at least 100 cm3 / h, at least 200 cm3 / h, at least 300 cm3 / h, or at least 400 cm3 / h for manufacturing of superposed layers involving a liquid, such as can be required for the preparation of PCBs or similar electronic circuits. When the present printing methods and / or apparatuses rely on solid materials substantially devoid of a liquid carrier, the 3D build speed can be further increased and be of at least 0.5 1 / h, at least 1 1 / h, at least 2 1 / h, at least 4 1 / h, at least 6 1 / h, at least 8 1 / h, or at least 10 1 / h. Once the layers including regions of at least particles of first material being compacted have been superposed one on the other to form a 3D-object, or a part thereof, the stack may optionally be detached from the backing surface upon which the layers were prepared. The materials providing at least for the first particles (hence the first regions of each intermediate layer) can be functionalized (e.g., by consolidation, as in debinding processes, sintering or any like treatment adapted to render the object functional). Printing systems schematically illustrating examplary arrangements of the afore described modules are depicted in Figs. 9 and 10. Fig. 9 displays for clarity only a part of the devices as may be arranged in a printing system 1400 to form stack of layers as previously illustrated in Fig. 2, the transport mechanism of the substrate (hence, its backing surface) being a drum. In this figure the transport mechanism drum 12 rotates counterclockwise along a circular path, the first station being a non-selective adhesive applicator 1000, which comprises a dipping cylinder 110 rotating in a heated bath containing a melt adhesive, which transfers therefrom to a first rotatable cylinder 100 and therefrom as a thin layer of adhesive to the outer surface of drum 12. A metering device 130 is schematically illustrated by a metering blade configured to control the thickness of the adhesive layer forming on the outer surface of cylinder 100. A pneumatic piston 124 which may engage (or disengage) the cylinder 100 transferring the this film adhesive to the substrate transporting drum 12 is also depicted. The thin layer of adhesive transferred to a substrate on drum 12 can then be selectively activated by device 500. Device 500 can be a laser emitting device such as detailed in the Applicant’s earlier patent applications WO 2016 / 189510, WO 2016 / 189511 and WO 2019 / 030694, capable of selectively heating and rendering tacky regions of a thermally activatable adhesive. The layer of adhesive including its selectively activated regions is then brought into contact with particles applied thereto by a particle applicator 250 being coated on its outer surface 252 by transfer roller 230 of dispenser 2000, following which the particles having adhered to the activated adhesive may be compacted by a compacting device 3000. The compacting device 3000 is schematically illustrated as comprising a compacting roller 310, a heater 312 and a pneumatic piston 300 for urging, when desired, the compacting roller into contact with the applied particles. For sake of clarity, this figure lacks stations 36, 38 and 40 as shown in Fig. 2 and stations 60, 62, 64 and 66 as shown in Fig. 6, station 60 serving to dispense second materials in liquid form, such as the present highly loaded compositions. Double arrows positioned next to pneumatic pistons 124, 254 and 300, indicate that the outermost cylinder of the devices including these pistons may be brought into contact with, or away from, the transporting drum and the layers being built thereon, the same controlled engagement / disengagement of the dispenser 2000 being illustrated by double arrows 256. In Fig. 9, adhesive applicating cylinder 100 of device 1000 and compacting roller 310 of compacting device 3000 are shown engaged with drum 12, whereas particle applicator cylinder 250 of particle dispensing device 2000 is shown disengaged from the transport drum 12. When the surface of the applicator cylinder 250, such as an elastomeric sleeve 252 having release properties, is urged into contact with the substrate at least selectively coated with an active adhesive so as to transfer particles thereto (e.g., by moving downward in Fig. 9), the dispenser 2000 can be disengaged from the applicator. This can be achieved by any mechanism (schematically represented by double arrows 256) able to displace the dispenser, and in particular the transfer cylinder 230 away from (or towards) the applicator. Once the applicator disengages from the substrate, the dispenser may reengage with the surface 252 so as to replenish the coat of particles in regions partially depleted by the preceding transfer. To facilitate the reformation of a homogenous coat of particles on the continuous blanket, it can be advantageous to set the last transfer cylinder coming into contact with the outer surface of the applicator to be in relative slippage therewith, e.g., by having a different linear speed such as at least 10% higher or lower, to favorize a rubbing effect on the particles. In Fig. 10, in contrast to Fig. 9, the transport mechanism of a printing system 1500 is planar. In this figure, in which the direction of printing is from left to right, a platform 600 that can be controlled to move back and forth on guiding rails 610 (and forming therewith a transport mechanism 630) is illustrated with a substrate 620 thereon. The substrate is passed by the transport mechanism 630 through a first station 700 at which an adhesive can be selectively applied to desired regions of the substrate. The station 700 may comprise an inkjet head capable of moving in any direction parallel to the plan of the substrate (e.g., typically in a scanning direction perpendicular to the printing direction), but may alternatively comprise a static print bar constituted from a plurality of inkjet heads, the nozzles (or arrays of nozzles) of the inkjet heads spanning the width of the substrate in a direction traverse to the printing direction. Inkjet heads are known and need not be detailed herein. They can be purchased from several reputable manufacturers, amongst them Eastman, Epson, Fujifilm Dimatix, Konica Minolta, Kyocera, Ricoh, Toshiba, Seiko and Xaar, each having models adapted to any desired viscosity and jetting method (including the controllable varying of droplet volume). Depending on the inkjet head, the adhesive liquid to be jetted thereby and the particles to be adhered thereto, lines may be formed to have a width of 50 pm or more, the minimal spacing between adjacent lines being typically of at least 50 pm. Following the formation of the desired image of adhesive, the substrate is transported to a second station at which particles of a first material can be applied to the adhesive. The device capable of transferring the particles to the adhesive can be the particle applicator 2000 previously detailed. In the present illustration, cylinder 230 which acted as a last transfer cylinder preceding the applicator cylinder 250 in Fig. 9 can now directly serve to transfer the particles to the adhesive and de facto serve as applicator cylinder. The mechanism 256 which previously served to disengage the dispenser from an external applicator cylinder 250 whilst it was urged into contact with the substrate, may now serve to urge the applicator cylinder 230 into contact with the substrate, so that layers of particles may transfer from its outer surface to the adhesive image formed at the first station. The substrate partially coated with the adhesive and the particles thereon can proceed to a third station 710 at which excess liquid from the adhesive can be removed. Device 710 can be a drier, heating the substrate by convection or radiation {e.g., IR). While a single device 710 is illustrated in this figure downstream of applicator 2000, a second one may be positioned upstream of the applicator of particles if the adhesive selectively applied by the inkjet head(s) 700 is too liquid / insufficiently tacky to retain the particles upon passing beneath the applicator of dispenser 2000. Following its drying, by one or more driers 710, the substrate and the particles adhered thereto arrive at the compacting nip formed by urging the compacting device 3000 against the platform 600. The layer including the compacted stack of particles can then be sintered by any suitable sintering device 720, the sintering being performed either for each cycle, for every few cycles, or upon completion of all cycles of additive manufacturing. In the latter case, the sintering device 720 need not be included in the printing system 1500 and can be an off-line device. If needed, following compaction and optional sintering, the substrate can be cleaned by a cleaning device 730, configured to remove particles having parasitically transferred to and / or sintered on unintended regions of the substrate. An additional, or alternative, cleaning device 730 (not shown in the figure) can be disposed upstream of the compacting device 3000 and downstream of the optional dryer 710. The cleaning device 730 can comprise a rotating brush and suctions means to remove the surplus material. In the apparatus exemplified in this figure, the second material is applied in liquid form by a suitable applicator 740 which may further include a squeegee 742 to ensure that the curable liquid uniformly coats the surface of the substrate. While in the present illustration, the squeegee 742 would wipe the liquid in direction opposite the printing direction as the substrate proceeds to a following station, alternatively it (or an additional squeegee) can be movable and configured to wipe the liquid in a direction transverse to the printing direction. The substrate then proceeds to a curing station 750, the exact curing device being selected in accordance with the liquid material to be cured (e.g., the device including a UV source when the liquid is UV-curable). Finally, the upper surface of the substrate can be milled, ground or abraded by a levelling device 760, following which the levelled surface can be inspected by an inspecting device 770 before being sent back to the first station for a new cycle and a new layer. While not apparent in the figure, an encoder is provided in the transport mechanism, allowing the substrate to be correctly positioned with respect to the inkjet head(s) for the adhesive of a subsequent cycle / layer to be properly registered with respect to the adhesive selectively applied in a previous cycle / layer. Alternatively, or additionally, proper registration between cycles (or between stations, if selective) can be achieved by optical sensors suitably disposed in the apparatus to monitor signals (e.g., fiducial marks) indicative of positioning of the substrate with respect to any device due to interact therewith. As new layers are formed, the distance between the outermost layer being formed and the stations and devices interacting therewith is modified. A desired gap (or lack thereof) can be maintained by moving the substrate 620 on the platform 600 up or down in a Z-direction, to a Z-level as desired, alternatively, or additionally, the devices (e.g., 2000, 3000, 730, 740-742, and 760) can be displaced up or down to achieve the desired distance. For instance, while the compacting device 3000 and the levelling device 760 are positioned to contact the uppermost surface of the substrate or layers formed thereon when the transport mechanism is moving in the printing direction, these same devices could be lifted to avoid contact in the reverse direction when the substrate is brought back to home position in order to start a new cycle. While these systems are intended for 3D printing, using them for a unique printing cycle can alternavely achieve 2D printing or coating depending on the devices actually present and / or operating in the arranged systems. Highly loaded compositions according to the present teachings may be used to form, in dependence upon the fillers and base materials selected for their preparation and their relative proportions, temporary sacrificial layers or separating materials, or permanent fencing or encapsulating materials. These compositions, the use of which was schematically illustrated in devices or modules 60 and 740 in the exemplary processes described above, will now be considered in more detail in the following. Highly Loaded Curable Compositions When the support material can remain in the end-product, it may be provided in the form of a liquid which can undergo hardening. Materials may relatively solidify by building more complex structures starting from simpler ones (which can be liquid at the time they are applied to the substrate). A classic example is provided by pre-polymers which may form larger (e.g., thermosetting) polymers by ongoing cross-linking, a polymerization process also known as curing. Curing of curable materials can be induced by chemical additives (also called curing agents) or by physical means (e.g., light, heat, radiation). Chemically curable materials can be epoxy resins, their typical curing agents including polyamines, polyamides, anhydrides, imidazoles and phenols; polyester resins, their typical curing agents including styrene monomers, MEKP (Methyl Ethyl Ketone Peroxide), and cobalt naphthenate: polyurethane resins, their typical curing agents including isocyanates and polyols; acrylic resins, their typical curing agents including free radical initiators such as peroxides or azo compounds; silicone resins, their typical curing agents including silanes and organometallic compounds; phenolic resins, their typical curing agents including formaldehyde-based resins or other phenolic compounds; vinyl ester resins, their typical curing agents including peroxides and amines; cyanate ester resins their typical curing agents including anhydrides; and diallyl phthalate (DAP) resins, their typical curing agents including peroxides. Other specialty curing agents can be used with the afore-said chemically curable resins. Resins that can be cured by using peroxides, azo compounds, or like initiators are often polymerizing through a free radical mechanism, and may therefore alternatively be referred to as free radical curable resins. Physically curable materials can be resins that harden through different mechanisms when exposed to specific types of energy, the resins being for instance heat curable, UV-curable, photo-curable, and e-beam curable. UV-curable resins include acrylate and methacrylate-based oligomers and photo-initiators that are acrylate or methacrylate derivatives; photo-curable resins include cationic epoxy resins and vinyl ether resins, their polymerization upon exposure to light being initiated by photoinitiators, such as diaryliodonium salts or sulfonium salts; e-beam curable resins can be epoxy resins and acrylic or methacrylic resins. The curable liquid can be adapted to the model particles, and compatible therewith, and can additionally be selected to suitably adhere to an underlying surface and preferably rapidly solidify when sufficiently exposed to the relevant curing agent and / or curing energy. Fundamentally, a material or a chemical composition is compatible with another composition or with a method if it does not prevent its activity or performance, or if it does not reduce it to an extent that would significantly affect the intended purpose. Such compatibility may be from a chemical standpoint, for instance, materials having respective moieties or active groups that may desirably interact with one another. From a physical standpoint, the compatibility of materials may relate, for instance, to materials intended to form contiguous regions having a similar coefficient of thermal expansion. From the standpoint of an intended use, the materials would be compatible if, for instance, not impairing the consolidation of the layers or their functionalization, such as permitting proper sintering, conductivity, RF or dielectric properties, as may be desired. For illustration, if the particles are to be sintered at an elevated temperature, then the curable liquids and their cured counterparts should be selected to be heat resistant at least up to the temperature of sintering. Likewise, if additional parts are to be attached to a stack of layers prepared as herein taught with cured regions set to remain in the final product, the curable liquids and their cured counterparts should be selected to be adapted to their future attachment (e.g, being heat resistant at least up to the temperature of soldering of electronic components to a PCB to form an integrated circuit). The curable liquids may further contain any desirable additive as can be selected according to an optional further function of the cured liquid, in addition to supporting other materials (e.g., compacted particles in the exemplified printing process), the additional role being sought during the manufacturing process and / or thereafter. Additives to curable liquids can, depending on their nature, either be consumed during the curing process or be stable therethrough, so as to remain present in the cured material. Additives can: serve for the formulation of the curable liquids (e.g., rheology modifiers, surfactants, etc. ); be facilitators of the curing process (e.g., initiators, catalysts, cross-linkers, etc.)', be protective agents to the model particles by blocking permeation of deleterious environmental factors (e.g., antioxidants, dessicants, etc.)-, or contribute to any sought chemical and / or physical property of the curable liquid and / or of the cured material resulting therefrom (e.g, thermal and / or electrical conductivity, or lack thereof, mechanical, optical, magnetic properties, flame retardation, etc.). The additives can be soluble in the curable liquids or dispersible therein, in which case they are often referred to as fillers. Mention is made of the delicate balance between a desire to use curable compositions with a concentration of a suitable filler high enough to achieve a sought goal, while maintaining the viscosity sufficiently low to retain workability (e.g., ease of application) and a brief enough processing speed. Considering prospective self-levelling to illustrate the latter, highly viscous compositions would be relatively flow-resistant and may significantly delay the natural formation of a smooth layer as compared to a more liquid (e.g., curable) composition, the lower viscosity of which shortening the time it takes to equilibrate as a smooth and even surface. The viscosity of liquid compositions can be adapted to their method of application, for instance, while extrusion may tolerate a paste like substance, jetting would typically require a relatively low viscosity, being typically achieved by the inclusion of a solvent. The nature of the solvent used, if necessary, for the preparation of a highly loaded composition may depend on the nature of the material dispersed therein. Its presence should be only sufficient to achieve a viscosity workable by the method of application (e.g., not exceeding 300 mPa.s if rapid spreading is desired). As the presence of a solvent may delay the process by the time necessary for its elimination, it should preferably be volatile under the process (e.g., temperature) conditions. To name a few, the solvent can be water, or short chain alcohols (e.g., methanol, ethanol, isopropyl alcohol, etc.) which tend to evaporate more rapidly, or combinations thereof. By way of non-limiting examples, mechanical properties of a cured material can be modulated by the addition of reinforcement fillers (such as glass fibers or silica particles). Reinforcement fillers (or any other solid additive) can be added to the curable liquids in any desired form (e.g., globular, rod-like, platelet-like, elongated fibers, etc.), such additives typically having at least one dimension (e.g., a rod having a diameter) in the nanometer range and being often referred to as nanofillers The dimensions of solid fillers, if any present in the curable liquids, can be selected in accordance with the method of application of the liquid during the manufacturing of the stacked layers. Vice versa, the method of application of the curable liquids can be selected in accordance with their composition. Understanding^, cured materials due to withstand compaction at elevated pressure (e.g., up to 500 Mpa), and optionally elevated temperature (e.g., up to 150°C), such step being repeatedly perform in a 3D printing process, may require a relatively high presence of fillers to provide suitable resistance. For illustration, silica can be added in the form of nanorods to a curable epoxy resin to serve for the manufacturing of a PCB. Particles of silica may contribute, among others, to the coefficient of thermal expansion (CTE, as can be measured as a change in ncrease in length, area, or volume in response to temperature) of the cured resin, the modulation of which to match or be close to the CTE of the model material being often desired. Moreover, silica may increase the mechanical strength (e.g., compressive strength, tensile strength or flexural strength) of the cured resin and desirably affect its dielectric properties. The silica can be purchased as chopped or unchopped fibers (e.g., such as Unifrax E-Glass® E-08-F) or prepared as nanofibers according to known methods, such as disclosed by Tepekiran et al. in Aerosol Science And Technology, Vol. 53, Issue No. 8, pp 921-932 (2019), the nanofilaments being subsequently manually ground using a mortar and pestle to yield nanorods or short fibers having on average a diameter between about 100 nm and 1,000 nm and a length between about 0.5 pm and 10-15 pm. Such silica nanorods were added to a UV-curable epoxy resin, Optimax® 8046-LV from Novachem Corporation at a significantly high weight percentage of 50 wt.%, poured on a rigid epoxy support having a thickness of 100 pm, the curable epoxy having a wet thickness of 30 pm, and the silica-filled epoxy was cured for 2 seconds under a UV-lamp having a wavelength of 395 nm (manufactured by Zhongshan Enbo Optoelectronics). A micrograph of such nanorods (milled fibers) as captured by FIB-SEM microscopy are shown in Fig. 11A before their embedment in the resin and in Fig. 11B in the cured resin. The dielectric constant or relative permittivity (Dk) of the silica filled epoxy was measured by parallel plate capacitor method (such as described in ASTM DI50) on a sample similarly prepared but with a cured epoxy thickness of 2 mm. A 2 mm thick pellet having a diameter of 13 mm was extracted from the cured silica filled coating, placed between conductive plates to yield a dielectric constant of 4.7 as measured at a frequency of 100 kHz and a temperature of 25°C. For comparison, a reference sample made of an FR4 sheet purchased from Vector Electronics was tested in the same way and found to have a highly similar dielectric constant of 4.6, demonstrating that the present methods, which enable a filler loading high enough to desirably modulate the properties of the cured solid, provide for the manufacturing of structures having dielectric constants comparable to commercialized product. In general, conventional FR4-boards typically have a dielectric constant between 3.5 and 5.5, or between 3.8 to 4.8, depending on the fiber glass weave style, the thickness of the board and of conductive copper, its resin content, temperature, moisture and frequency (typically set between 1 MHz to 10 GHz). In a separate experiment, the same Unifrax E-Glass® E-08-F glass fibers were milled using a mortar-mill into short fibers having an average length of 15 pm and incorporated at 50 wt.% into an alternative UV-curable epoxy resin, commercialized as EPV 9411 by Polymer G. The composition had a dynamic viscosity of 267 mPa s at 50°C, at which the fiber-supplemented curable liquid was poured, smeared and levelled into thin layers having a thickness of about 30 pm. Each layer was cured for 2 seconds under UV-light with wavelength of 395nm. This process was repeated to create specimens with a total thickness of about 2 mm. One specimen was punched into a pellet having a diameter of 13 mm to assess its dielectric constant as previously described, which was found to be of 4.7. Another fiber-supplemented specimen was tested for flexural strength in the direction of smearing (according to ASTM D6272) and displayed a strength of 212 MPa. For comparison, a reference specimen prepared by casting to have similar dimensions, but no glass fibers, displayed a flexural strength of 132 MPa. The coefficient of linear thermal expansion (CLTE) of the specimen with the glass fibers and its reference was measured by thermomechanical analysis performed from 25°C to 200°C. The measurements performed in the direction of smearing of the curable liquid resulted in a CLTE of the fiber-supplemented specimen of 26xl0'6 K'1, while in the perpendicular direction the CLTE of the specimen was found to be 88xl0'6 K’1. Without wishing to be bound by any particular theory, the distinct CLTEs observed in the different directions of the specimen are believed to result from the relative alignment of the fibers according to the smearing involved in its formation. For comparison, the CLTE for the reference sample lacking any filler was found to be 120x1 O'6 K’1. Therefore, it is seen that high loading of glass fibers into a curable liquid provides for a cured material having superior flexural strength and reduced CTLE, as compared to the same material cured in absence of any filler. Advantageously, the dielectric constants are suitable for the preparation of 3D printed articles such as PCBs. The relatively high loading is enabled by the dimensions of the fibers following their milling. If one were to try to incorporate even as little as 0.1 wt.% of the fibers as supplied into a liquid, this would result in an amorphous mass and not in a liquid of measurable viscosity. By the present method, wherein the fibers are relatively shorter than in their supplied form, adding 0.1 wt.% of the fiber to a curable liquid of interest does not significantly affect the dynamic viscosity, such that the viscosity of the supplemented liquid is no more than 1.2-fold the viscosity of the liquid without any filler. This was confirmed by incorporating 0.1 wt.% of the milled glass fibers (E-Glass® E-08-F) into Optimax® 8046-LV or EPV 9411 and measuring the dynamic viscosity (with or without the fibers) at a temperature at which the liquid is sufficiently flowable for application. At 50°C the dynamic viscosity of Optimax® 8046-LV was of about 71 mPa.s, this viscosity being unaffected by the addition of the glass fibers. At the same temperature, the dynamic viscosity of EPV 9411 was of about 62 mPa.s, the viscosity only mildly increasing to 65 mPa.s in presence of the glass fibers. Similarly suitable cured materials were prepared by supplementing with 50 wt.% glass fibers prepared as previously discussed the following curable liquids EPV 3408 CO, EPV 938, EPV 8100 (of Polymer G); SunTronic® U6415 JET 9525 (of SunChem); Prelect UVD110 (of Agfa); which were tested as single material; and Ebecryl® 83, Ebecryl® 160, Ebecryl® 270, Ebecryl® 600, Ebecryl® 605, Ebecryl® 605 / 40, Ebecryl® 641, Ebecryl® 3203, Ebecryl® 3600, Ebecryl® 3703, Ebecryl® 3708, Ebecryl® LEO 10551, and Ebecryl® LED 03 (of Allnex); which were tested in various combinations one with another. The suitability of all curable compositions as currently supplemented with 50 wt.% of milled fibers of E-glass, which was preliminarily established in laboratory settings with manual application of the liquid as detailed for EPV 9411 and Optimax® 8046-LV, was confirmed in the automated printing system described with reference to Fig. 10. This second series of experiments was performed with curable compositions which have been stored since their preparations for periods of up to three months at 4°C. Notably, and despite the very high loading of milled fibers, the curable compositions retained flowable viscosities at their temperature of application and displayed no deleterious separations between the filler and the liquid medium. This suggests a stability of the present curable compositions which is unexpected of liquids highly loaded in solid materials. An additional or alternative additive that can be added to a curable liquid may be a coloring agent selected in accordance with a color conventionally associated with the products due to include the cured liquid. Thus, the cured material can be transparent or opaque, and clear or colored. The coloring agent, if present, which could provide for any desirable shade, should not affect the performance of the material once cured. Colors can be selected according to the shades of the compacted particles upon which the curable liquid is to be cast, so as to achieve contrast, brightness or any other visual effect in the finished product including the layers stacked according to the present teachings. Taking for illustration the manufacture of printed circuit boards, the model material of the particles being compacted could be electrically conductive (optionally following a treatment step, such as sintering) and the support material which may be applied in liquid form could be electrically insulating (e.g., epoxies, including glass or ceramic filled epoxies, ceramics, polyimide, polytetrafluoroethylene (PTFE) and combinations thereof, so as to separate and isolate the shapes of conductors formed on the PCB substrate. In this case, the different colors that can be added to the curable liquid may be used to distinguish the solder masks, facilitate the identification of the PCBs and ease their assemblies. While green has been predominantly used in solder masks of PCBs, colors other than green (e.g., red, blue, white, black, yellow, and purple) have recently been introduced in the preparation of solder masks to serve as PCB color codes. Sacrificial Compositions When separation of the object formed by the present 3D printing methods is desired, application of a sacrificial layer on the backing surface can precede the cyclical formation of layers including the compacted particles. This step, however, is not essential when separation can be achieved mechanically without adverse effect to the base of the 3D printed object being disconnected from the backing surface upon which it was produced. The sacrificial layer can be formed by a pre-treating module configured to dispense a sacrificial liquid so as to form a coat of uniform thickness on the backing (e.g., metallic) platform, the suitable module further comprising a drier to eliminate the liquid (e.g., a solvent in which suitable materials are dispersed or dissolved), so that the sacrificial layer formed thereby is substantially dry and flat when entering a first cycle of particle deposition. Alternatively, the sacrificial layer can be prepared separately according to the dimensions of the platform and / or of the object to be additively manufactured thereon. The layer can then be solidified (e.g., dried, cured or cooled, if made of a thermoplastic polymer) so as to form a flat sacrificial support (e.g., having a thickness of 500 pm or less, 250 pm or less, 100 pm or less, or 50 pm or less) to be placed on the backing surface at the beginning of the manufacturing process. Such ready-to-use sacrificial supports can be dispensed individually at the commencement of the process by suitable dispensers. In case the sacrificial layer is supplied in form of a separately prepared sacrificial support, it may be advantageous to include in the platform means to retain the sacrificial support in position. For illustration, a sacrificial support can be secured on a porous backing surface by way of suction applied from the rear side of the support. As the base surface of a 3D printed object to be formed thereon may reproduce the topography of the outermost surface of the sacrificial layer or support, in cases where the base surface is to meet specific requirements, it can be advantageous for the sacrificial surface to enable or facilitate obtaining such properties. For illustration, if the base surface is to be flat and relatively smooth (e.g, as may be required of PCBs), it can be beneficial to use a sacrificial layer or support that is similarly flat and smooth. Thus, a module configured to form a solid sacrificial layer or dispense a sacrificial support (if not previously prepared with desirable surface properties) may further include levelling members selected to provide a sought smoothness. Alternatively, the solid sacrificial layer or support dispensed by such a module can be transported to a levelling module of the printing system before being subjected to a first cycle of application of particles thereon. As already explained, to enable later separation, the sacrificial layer or support should preferably be easily soluble in a solvent (e.g., water) chemically inert with respect to the object being separated. But, as other materials previously detailed, the sacrificial layer (whether formed inline from a liquid or secured as a sacrificial support prepared offline) should furthermore be made of substances compatible with the process (e.g., pressure and temperature) and the particles or curable liquids to be subsequently applied thereon. It is reminded that similar principles apply to compositions that can be used to separate regions through slices of the 3D printed object. Thus, compositions intended to transiently isolate areas that may be later detached one from the other to form discrete articles out of the printed object, can be identical to sacrificial compositions as herein described. The sacrificial layer must ensure that the layers building-up thereon are adhered sufficiently to the backing surface to sustain compaction (and levelling, when applied) at the temperatures at which the 3D printing is performed. For illustration, if regions of the object are to include a cured liquid having a hardness in the Shore C or even Shore D range, then the sacrificial layer or support should preferably have a comparable or higher hardness at the printing temperature of compaction, to mechanically resist to this step so as to avoid deformation of the layers by local or total collapse of the sacrificial layer. Materials suitable for the preparation of the sacrificial liquid or sacrificial support may depend on the printing conditions being considered and materials envisioned. For example, one may use water-soluble polyvinyl alcohol (PVA) of appropriate grades for this purpose. Materials suitable for the preparation of sacrificial compositions can be polymers, such a polyvinyl pyrrolidones (PVP), polyvinyl alcohols (PVA), polyvinyl butyrals (PVB), polyamides (PA), polyacryl amide (PAM), poly(N-isoprolylacrylamide)s (PNIPAM), poly(acrylic acid)s (PAA), polymethacrylates (PMMA), polypropylenes (PP), polyethylenes (PE), poly(ethylene oxide)s (PEO), polyethylene terephthalates (PET), polyethylene glycols (PEG), sulfonated polyesters and poly(2-ethyl-2-oxazoline) (PEOX). They can also be selected from sugars, oligosaccharides and polysaccharides, such as glucose, maltose, cellulose, hydroxyethyl cellulose, cellulose acetate, amylose, chitosan, carnauba wax, agar agar, acrawax, and starch; or be combinations of the foregoing materials. However, since the sacrificial layer or support is required to withstand repeated compaction while substantially retaining its original dimensions, the materials that may be used alone for the sake of an adhesive composition or a particle binder may now need to be supplemented with fillers to augment their mechanical resilience. Fillers can be ceramics (e.g., alumina, silica, zirconia, zirconium(IV) silicate, etc.), minerals (e.g., calcium carbonate, clays, sodium chloride, talc, etc.), polysaccharides (e.g., cellulose, chitosan, lignin, starch, etc.), or any other material known as suitable for this mechanical reinforcing purpose. The fillers can be provided in any shape adapted to their function (e.g., beads or fibers). Notably, their presence in a sacrificial composition can be relatively significant, being for illustration of 30 wt.% or more, 50 wt.% or more, or even 70 wt.% or more. Sacrificial compositions may, as other substances already described, contain additives such as plasticizers, dispersants, rheology modifiers, stabilizers, lubricants, pH modifiers and like conventional additives, which need not be further detailed. If the flowability of the sacrificial composition is insufficient at their temperature of inline application or off-line preparation, a (e.g., volatile) solvent can be added to facilitate their spreading, the solvent being later eliminated by evaporation prior to the application of particles and compaction thereof. The following sacrificial compositions were prepared both in-line in a printing apparatus as illustrated in Fig. 9 and off-line for feeding to a printing apparatus as illustrated in Fig. 10. All were found suitable for the ongoing stacking and compacting of layers thereupon, and for their later separation from the base of the additively manufactured object, even following sintering at temperatures ranging from 200°C to 1500°C, depending on the particles used in the subsequently applied layers. 5 The polyvinyl alcohols (PVA) tested included Poval™ 3-86, Poval™ L-508, Elvanol™ 85-82, Elvanol™ 71-30, Mowiflex™ C-17, and Mowiflex™ H-15, and the polyvinyl butyral (PVB) materials tested included Mowital™ B60H and Mowital™ B16H, all available from Kuraray. Additional binding materials tested included polyvinyl pyrrolidone (PVP) as commercialized by BASF as Luvitec® K90 and rosin gum commercialized as Foral™ AX-E. 10 The fillers tested in combination with the above listed binding materials (and their relative concentration in the sacrificial compositions) included calcium carbonate (50 wt.%), cellulose (33 wt.%), chitosan (80 wt.%, 90 wt.%), corn starch (25 wt.%, 33 wt.%, 50 wt.%, 66 wt.%), lignin (3 wt.%), silica (50 wt.%), sodium chloride (43 wt.%, 73 wt.%), and zirconium silicate (91 wt.%). 15

Claims

1. A composition comprising a liquid and fibers having an average diameter Dave not exceeding 10 gm and a unitless aspect ratioASPbetween the average lengths Lave and diameters Dme of the fibers comprised between 5 and 20, the fibers being present at a weight concentration by total weight of the composition between 40 wt.% and 60 wt.%, between 44 wt.% and 60 wt.%, or between 47 wt.% and 60 wt.%, the composition having a dynamic viscosity of 400 mPas (one millipascal-second) or less, at at least one temperature between 40°C and 120°C, as measured using a Brookfield type viscometer, the liquid optionally being a curable liquid and the composition a curable composition.

2. The composition as claimed in claim 1, wherein the average diameter Dave of the fibers is between 150 nm and 10 pm, between 150 nm and 7 pm or between 150 nm and 5 pm, the average length Lave is between 1 pm and 50 pm, and the aspect ratio (ASP Lave I Dave) is between 5 and 15, between 7 and 15, or between 7 and 12.

3. The composition as claimed in claim 1 or claim 2, wherein in presence of 0.1 wt.% of the fibers, the liquid has a viscosity not exceeding 1.2-fold the viscosity of the liquid in absence of the fibers, as measured at at least one temperature between 40°C and 120°C using a Brookfield type viscometer.

4. The composition as claimed in any one of claim 1 to claim 3, the composition having a dynamic viscosity of 300 mPas or less, 200 mPa s or less, or 100 mPa s or less, at at least one temperature between 40°C and 120°C, as measured using a Brookfield type viscometer.

5. The composition as claimed in any one of claim 1 to claim 4, wherein the liquid is curable and after spreading of the curable composition and full curing thereof, the cured composition has, in a direction of spreading, a linear coefficient of thermal expansion CLTE between 5 and 30 xlO'6 K’1, between 10 and 30 xlO'6 K’1, or between 15 and 30 xlO'6 K’1, as measured by thermomechanical analysis.

6. The composition as claimed in any one of claim 1 to claim 5, wherein the fibers include two populations, a first population of fibers having a first average diameter Di between 150 nm and 500 nm, between 150 nm and 400 nm, or between 150 nm and 350 nm; and a second population of fibers having a second average diameter D2 between 500 nm and 10 pm, between 500 nm and 8 pm, or between 500 nm and 6.5 pm; a unitless ratio Dr between the average diameters D2 and Di (D2ID1) being in the range of 3 to 70, in the range of 3 to 50 or in therange of 3 to 30; a size of the fibers being determined by analysis of microscopy images and averaged for at least 300 fibers.

7. The composition as claimed in claim 6, wherein the first population of fibers have a first average length Li between 1 pm and 30 pm, between 3 pm and 25 pm, or between 3 pm and 20 pm; and the second population of fibers have a second average length L? between 5 pm and 50 pm, or between 5 pm and 40 pm; a unitless ratio Lr between L2 and Li (L2IL1) being in the range of 1 to 40, or in the range of 1 to 30.

8. The composition as claimed in any one of claim 1 to claim 7, wherein the liquid is curable and selected from a group consisting of modified or unmodified epoxy compounds (including epoxy acrylates), acrylates (including mono-, di- and tri-acrylates); urethane acrylates; polyether acrylates; vinyl ethers; polycyclic aromatic hydrocarbons; and combinations thereof.

9. The composition as claimed in claim 8 being curable, wherein the modified or unmodified:a) epoxy compounds and epoxy acrylates are selected from a group comprising bisphenol-A based epoxy acrylate; amine modified bisphenol-A based epoxy acrylate, bisphenol-A diglycidyl ether di-acrylate, epoxyacrylate oligomers, epoxycyclohexylmethyl epoxycyclohexane and carboxylate7-oxabicyclo[4.1.0]hept-3-ylmethyl-7-oxabicyclo[4.1.0]heptane-3-carb oxy late;b) mono-, di- and tri-acrylates are selected from a group comprising isobomyl acrylate, hexanediol diacrylate, tripropylene glycol diacrylate, pentaerythritol triacrylate, trimethylolpropane triacrylate, and ethoxylated trimethylolpropane triacrylate;c) urethane acrylates are selected from a group comprising aliphatic urethane acrylate, aliphatic urethane diacrylate and aliphatic polyester urethane diacrylate;d) polyether acrylates are selected from a group comprising amine modified polyether acrylate, amine modified polyether diacrylate, and propoxylated (2PO) neopentyl glycol diacrylate;e) vinyl ethers are selected from a group comprising vinyl ether, diethylene-glycol divinyl ether, vinyloxyethoxy ethyl acrylate; andf) polycyclic aromatic hydrocarbons are selected from a group comprising benzocyclobutenes, benzocyclobutadienes, benzocyclooctatetraenes and derivatives thereof.

10. The composition as claimed in any one of claim 1 to claim 9, wherein the fibers or the populations of fibers are each independently selected from fibers consisting of carbon, ceramics (including alumina, zirconia and glass), clay minerals (including halloysite), natural polymers (including cellulose, chitosan and lignin), synthetic polymers (including polyesters, polypropylenes, fluoropolymers, such as polytetrafluoroethylene (PTFE), and polyamides, such as aramid and nylon, and combinations thereof.

11. The composition as claimed in any one of claim 1 to claim 10, wherein the one or more populations of fibers are fibers consisting of glass, the glass fibers being selected from a group comprising E-glass, D-glass, and TEOS-based electro-spun glass.

12. The composition as claimed in any one of claim 1 to claim 11, further comprising beads having an average diameter not exceeding 5 pm and a unitless aspect ratio between the average longest and smallest dimensions of the beads comprised between 1 and 3, the beads optionally being present at a weight concentration by total weight of the composition of up to 10 wt.%.

13. The composition as claimed in claim 12, wherein the beads consist of glass, cellulose, ceramics, PTFE and combinations thereof.

14. The composition as claimed in any one of claim 1 to claim 13, wherein the fibers have a specific surface area of at least 1.5 m2 / g and at most 20 m2 / g as measured by BET.

15. The composition as claimed in any one of claim 1 to claim 14, wherein the fibers are made of a material having a specific gravity of at least 0.5 g / cm3 and at most 5 g / cm3, at least 0.5 g / cm3 and at most 2.5 g / cm3, or at least 1 g / cm3 and at most 2.5 g / cm3.

16. The composition as claimed in any one of claim 1 to claim 15, wherein the liquid is curable and after spreading of the curable composition and full curing thereof, the cured composition has, in a direction of spreading, a flexural strength between 200 MPa and 300 MPa, between 200 MPa and 400 MPa, or between 200 MPa and 500 MPa, as measured according to ASTM D6272.

17. The composition as claimed in any one of claim 1 to claim 16, wherein the liquid is curable and the fibers are selected to provide to an object cured from the curable composition at least one property selected from mechanical reinforcement, thermal insulation or conduction, electrical insulation or conduction, modulation of thermal expansion, and radiation blocking or adsorption.

18. The composition as claimed in any one of claim 1 to claim 17, further comprising at least one additive the presence of all additives not exceeding 10 wt.% by total weight of the composition, each additive being selected from a group consisting of curing initiators, curing catalysts, curing inhibitors, fire retardants, adhesion promoters, and viscosity modifiers, the additive being adapted to the curing method of the liquid when curable.

19. The composition as claimed in any one of claim 1 to claim 18, wherein the liquid is curable by application of energy selected from heat, eBeam, IR and UV radiation.

20. The composition as claimed in any one of claim 1 to claim 21, for use in additive manufacturing.

21. The composition as claimed in claim 20, wherein the composition a) is a curable composition remaining in cured form in a manufactured object, optionally serving as fencing material or as an insulating casing for a conductive pattern, or b) a temporary composition, optionally serving as removable support, separating material, fencing material, sacrificial layer or support.

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