Ground silica and liquid radiation curable composition including same

Abstract

Vapor-phase or precipitated silica, which is ground in a first compression grinding step to a tap density of 200% to 1200% of the initial tap density of the vapor-phase or precipitated silica, and in a second expansion grinding step to a tap density of 10% to 50% of the tap density after the first compression grinding step.

Description

[0001] The present invention relates to ground silica, radiation curable compositions comprising ground silica, and their use in additive manufacturing applications.

[0002] Fumed silica has been considered as a rheology modifier in liquid radiation-curable formulations. Due to the thixotropic effect, the concentration of fumed silica in the formulation is usually low.

[0003] Precipitated silica has been used in tire rubber to reduce rolling resistance, as a filler in adhesives and sealants, as a reinforcing filler, and in silicones to increase tear resistance.

[0004] WO 2015189837 A1 describes a method for using hydrophilic fumed silica as a rheology modifier in photopolymers. However, the silica loading in the matrix is ​​limited because the viscosity increases with loading at 6-8 wt%.

[0005] US20050040562 A1 describes the use of 10-50 nm colloidal silica particles in photosensitive resins for 3D printing applications. The addition of silica increases the hardness of the printed product, but also increases brittleness and reduces elasticity.

[0006] US20070205528 A1 discloses a 3D printing resin that requires a viscosity of 200 mPa.s. This viscosity is achieved by adding colloidal silica nanoparticles to the resin. The addition of colloidal silica makes the resin easier to print, but the toughness of the printed part is significantly reduced.

[0007] US20050040562 A1 discloses a 3D printing resin with a viscosity below 500 mPa.s. Silicon dioxide colloidal nanoparticles are added to the resin to reduce the viscosity. This makes the resin easier to print, but the toughness of the printed part is also significantly reduced.

[0008] WO 2022 / 171406 discloses fumed silica that has been subjected to a heat treatment to reduce the silanol content while maintaining the BET surface area to achieve good dispersibility and thixotropic properties in paints, coatings and other liquid formulations.

[0009] US 6593393 B2 describes the use of fumed silica ground by ball mill compression for use in coating formulations. The compaction of the silica results in a larger particle size. This allows for easier dispersion in the coating formulation, but on the downside, the larger particles settle more quickly in the coating formulation. US 6593393 B2 proposes a second, optional grinding step, but does not mention the benefits of such an additional grinding step for use in coating formulations.

[0010] WO 2014098956 A1 describes the use of fumed silica surfaces functionalized with methacrylate groups as fillers in 3D printing resins to produce rigid dental polymers. The fumed silica reduces the porosity of the printed parts and increases the hardness of the polymer. This is achieved at the expense of elasticity, resulting in increased brittleness of the 3D printed material. As described in WO 2014098956 A1, the silica has a particle size of less than 60 nm, and the aggregate particle size is less than 200 nm.

[0011] It is an object of the present invention to provide ground silica and liquid radiation-curable compositions comprising said ground silica which at least reduce the disadvantages of the prior art and to provide liquid radiation-curable compositions which can produce articles having high toughness, good elasticity and reduced brittleness.

[0012] This object is achieved by grinding the fumed or precipitated silica in a first compression grinding step to an average tap density of 200% to 1200% of the initial tap density of the fumed or precipitated silica, and grinding it in a second expansion grinding step to an average tap density of 10% to 50% of the average tap density after the first compression grinding step.

[0013] The order in which the fumed or precipitated silica is milled is crucial to the present invention. The first milling step is a compression milling step, followed by a second milling step, which is an expansion milling step. The first compression milling step forms larger particles, while the second expansion step further reduces the particle size. Surprisingly, it has been discovered that the fumed or precipitated silica according to the present invention, when added to a liquid radiation-curable composition for 3D printing, can achieve unique properties in printed three-dimensional objects. The printed objects exhibit high tensile toughness and, at the same time, high elongation at break.

[0014] The average tap density is measured according to DIN / ISO 787 / 11.

[0015] Fumed silica is a spherical silica particle that is sintered at the neck to form longer chained fractal secondary structures. The milling method using fumed silica mentioned hereafter can control the particle structure to the submicron range of 100nm-500nm.

[0016] Precipitated silica is used in tire rubber to reduce rolling resistance, as a reinforcing filler in adhesives and sealants, and in silicones to increase tear resistance. The primary structures typically have a diameter of 5 nm to 100 nm.

[0017] Colloidal silica is a spherical, discrete silica particle with a size less than 100 nm, mostly in the 20-60 nm range, and lacks a secondary fractal network structure. Due to its spherical shape and lack of porosity, colloidal silica is often chosen as a filler to reduce porosity. However, colloidal silica offers little enhancement to mechanical properties.

[0018] Discrete colloidal silica is used herein as a comparative example to distinguish the fractal network structure of interconnected fumed silica, which may confer beneficial properties in terms of improved tensile strength and stability within the polymer matrix.

[0019] Preferably, the particle size of the ground fumed or precipitated silica according to the invention is in the range of 100 nm to 500 nm.

[0020] Fine particles of colloidal silica are generally preferred in the art because the small particle size results in a lower viscosity of the 3D printing photopolymer composition. Surprisingly, superior properties were achieved with the fumed silica particles according to the present invention compared to fine particles of colloidal silica.

[0021] Surprisingly, larger particle sizes of fumed silica, preferably in the range of 100 nm to 500 nm, lead to better properties than colloidal silica in radiation curable compositions if the particles are subjected to a two-step milling process according to the invention.

[0022] Fumed or precipitated silicas processed in a compression milling and subsequent expansion milling step result in unexpected new properties when used in liquid radiation curable compositions.

[0023] The tap density and surface area show a return to the original, unground material. Surprisingly, new properties are obtained that have a beneficial effect on the mechanical properties of the cured photopolymer resin. The use of fumed or precipitated silica according to the invention makes it possible to maintain toughness without sacrificing elasticity and without producing a brittle material.

[0024] Preferably, the fumed or precipitated silica according to the present invention is characterized in that it is ground in a first compression grinding step to a tap density of 200% to 1000% of the initial tap density of the starting material, more preferably to a tap density of 200% to 800% of the initial tap density of the starting material, and most preferably to a tap density of 200% to 600% of the initial tap density of the starting material.

[0025] The thus ground fumed or precipitated silica is then ground in a second expansion grinding step to a tap density that is preferably 10% to 30% of the tap density after the first compression grinding step.

[0026] The first compression milling step is preferably carried out using a ball mill, hammer mill, or impact mill, more preferably a ball mill. The second expansion milling step is preferably carried out using an air jet mill or a pin mill. In the air jet mill, a jet compressed to a gauge pressure range of between 2 and 20 bar is used, with a material flow rate of between 2 and 500 kg / h.

[0027] The fumed or precipitated silica according to the present invention preferably has a tap density of 20-100 g / l, more preferably 20-80 g / l, most preferably 20-60 g / l.

[0028] Preferably, the fumed or precipitated silica according to the invention is surface-modified with silanes, organomodified silanes, siloxanes, organomodified siloxanes or mixtures thereof.

[0029] More preferably, the fumed or precipitated silica according to the present invention is surface-modified with an organomodified silane selected from the group consisting of (meth)acrylate silanes, epoxy silanes and hexamethyldisilane.

[0030] Most preferably, the ground silica according to the present invention is fumed silica.The fumed silica is preferably surface-modified with an organo-modified silane selected from the group consisting of (meth)acrylate silanes, epoxy silanes and hexamethyldisilane.

[0031] The use of fumed or precipitated silica according to the present invention allows for the application of larger particle sizes, ranging from 100 nm to 500 nm, resulting in superior performance for additive manufacturing. Prior art teaches the use of smaller particles, less than 60 nm ( US20050040562 A1 ), but this results in a decrease in the toughness of the printed parts. Surprisingly, the silica according to the present invention reduces the loss of toughness in printed parts, despite its larger particles.

[0032] Without wishing to be bound by theory, it is believed that the expansion milling step facilitates the dispersion of silica into the radiation curable resin with lower agglomerate counts, while the compression milling step reduces the viscosity of the resin without sacrificing the benefits of the expansion milling step.

[0033] Radiation-curable resins containing silica according to the present invention have lower viscosities than resins not containing silica according to the present invention. The addition of fumed or precipitated silica according to the present invention enables low sedimentation rates and increases in ultimate tensile strength and tensile modulus while maintaining the toughness of the printed radiation-cured three-dimensional articles.

[0034] Therefore, the present invention also relates to a liquid radiation curable composition comprising the fumed or precipitated silica according to the present invention in a concentration of 5 wt% to 50 wt% of the total liquid curable composition. More preferably, the liquid radiation curable composition comprises the fumed or precipitated silica according to the present invention in a concentration of 5 wt% to 25 wt%, most preferably 5 wt% to 15 wt%, of the total liquid curable composition.

[0035] In general, the unexpected effects of the ground fumed or precipitated silicas according to the invention can be achieved in all liquid curable compositions.

[0036] Preferably, the liquid curable composition comprises a reactive oligomer selected from the group consisting of: (meth)acrylates, ethoxylated (meth)acrylates, urethane (meth)acrylates, epoxies, epoxy (meth)acrylates, bisphenol ethoxylated (meth)acrylates, polyester (meth)acrylates, thiol-enes, siloxane (meth)acrylates, unsaturated polyesters, polyether (meth)acrylates, unsaturated polyureas, phenolic polymers, silicones, cyclic carbonate (meth)acrylates, hyperbranched acrylates, pure acrylates, silicone acrylates, acrylic (meth)acrylates, vinyl esters, polysiloxane (meth)acrylates, amide imides, poly(ethylene glycol), thiol-functionalized polysiloxanes, vinyl terminated poly(dimethylsiloxane), epoxy aliphatic (meth)acrylates, polycarbonate (meth)acrylates.

[0037] More preferably, the liquid curable composition comprises a reactive oligomer selected from epoxy resins, (meth)acrylates, ethoxylated (meth)acrylates, epoxy (meth)acrylates, bisphenol ethoxylated (meth)acrylates or urethane (meth)acrylates, polyester (meth)acrylates, bisphenol (meth)acrylates.

[0038] The liquid curable composition further comprises a reactive monomer selected from the group consisting of (meth)acrylates and epoxy resins or mixtures thereof.

[0039] The liquid radiation curable composition according to the present invention preferably comprises

[0040] Component a) 40 wt% to 80 wt% of one or more reactive oligomers selected from epoxy resins, (meth)acrylates, ethoxylated (meth)acrylates, epoxy (meth)acrylates, bisphenol ethoxylated (meth)acrylates or urethane (meth)acrylates, polyester (meth)acrylates, and bisphenol (meth)acrylates.

[0041] Component b) 20 wt% to 90 wt% of one or more reactive monomers selected from (meth)acrylates and epoxy resins;

[0042] Component c) 5 wt% to 50 wt% of fumed or precipitated silica, which is ground in a first compression grinding step to a tap density of 200% to 1200% of the initial tap density of the fumed or precipitated silica, and ground in a second expansion grinding step to a tap density of 10% to 50% of the tap density after the first compression grinding step.

[0043] Component d) 0 wt% to 15 wt% of a crosslinking agent reactive with components a) and / or b);

[0044] Component e) 0.01 wt% to 10 wt% of one or more photoinitiators;

[0045] Component f) 0 wt% to 20 wt% of one or more additives selected from pigments, fillers, dispersants, defoamers, antioxidants, light stabilizers, light absorbers or free radical inhibitors;

[0046] Provided that the sum of all components equals 100%.

[0047] As used herein, oligomer refers to an intermediate in a polymerization reaction involving two or more units.

[0048] The term "(meth)acrylate" refers to methacrylate, acrylate, or a mixture of the two.

[0049] Reactive means that components a) and b) can form multiple polymeric crosslinked networks with themselves, each other, or with component d) in the presence of free radicals, anions, nucleophiles, or combinations thereof.

[0050] Crosslinkers are molecules that create bonds or short sequences of bonds that connect one polymer chain to another. These bonds can take the form of ionic bonds or, more commonly, covalent bonds. Common crosslinkers in photopolymers are difunctional, trifunctional, or tetrafunctional, such as dipropylene glycol diacrylate, 1,6-hexanediol diacrylate, trimethylolpropane triacrylate, trimethylolpropane ethoxylated triacrylate, and propoxylated pentaerythritol tetraacrylate.

[0051] A photoinitiator is a molecule that, when irradiated with light, absorbs photons and forms a reactive species that initiates a cascade reaction. Suitable photoinitiators include bis-acyl-phosphine oxides, diphenyl (2,4,6-trimethylbenzoyl) phosphine oxide or benzophenone.

[0052] The liquid radiation curable composition according to the present invention is particularly suitable for use in additive manufacturing processes, in particular VAT photopolymerization processes.Such additive manufacturing processes generally comprise repeated steps of depositing or layering and irradiating the composition to form a three-dimensional object.

[0053] Additive manufacturing (AM) technology has great potential for directly manufacturing end-use parts through a photopolymerization process in which a liquid resin material is cured layer by layer with the aid of radiation curing (e.g. UV) to produce a three-dimensional solid polymer object.

[0054] Therefore, the present invention also comprises the use of a liquid radiation curable composition according to the present invention in an additive manufacturing process comprising the repeated steps of depositing or layering and irradiating the composition to form a three-dimensional object.

[0055] The additive manufacturing process is preferably DLP-digital light processing, SLA-stereolithography, FFD-fused filament deposition or material jetting.

[0056] As described above, it has been unexpectedly discovered that liquid radiation curable compositions can yield printed three-dimensional objects with unique properties, having high tensile toughness while simultaneously having high elongation at break.

[0057] The present invention also includes three-dimensional objects formed by an additive manufacturing process using a liquid radiation curable composition containing fumed or precipitated silica according to the present invention. Example

[0058] Example 1

[0059] Fumed silica is synthesized by flame hydrolysis, in which silicon tetrachloride is reacted with hydrogen and oxygen. The resulting fumed silica has a density of 200 m³ / min, as measured according to ISO 9277. 2 The average tap density of the fumed silica is 55 g / L. The average tap density is measured according to DIN / ISO 787 / 11.

[0060] Will have 200m 2 This fumed silica having a surface area of ​​100 g / g and a tap density of about 50 g / L was mixed with 4 parts of water and 18 parts of 3-methacryloxypropyltrimethoxysilane, and the mixture was heat-treated at 140° C. under an inert gas. The obtained silica was designated as Type 1 and had the properties shown in Table 1. Due to the adhesion of particles caused by the moisture content, the surface area of ​​the particles decreased after the surface modification, and the exposed surface became smaller.

[0061] Example 2:

[0062] The fumed silica of Example 1, having an average tap density of 55 g / L, was ground using an air jet mill to a tap density of 16 g / L. Air jet milling is an expansion milling step. The milling of the fumed silica was performed in a single milling step. The fumed silica thus obtained was designated Type 2 and had the properties shown in Table 1.

[0063] Example 3:

[0064] The fumed silica of Example 1, having an average tap density of 55 g / l, was compacted to a tap density of 197 g / l using a continuously operated attritor. Milling in the attritor is a compression milling step. The milling of the fumed silica was performed in a single milling step. The silica thus obtained was designated Type 3 and had the properties shown in Table 1.

[0065] Example 4:

[0066] In a first compression milling step, the fumed silica of Example 1 having an average tap density of 55 g / l was compacted to a tap density of about 200 g / l using a continuously operated attritor mill. In a second step, the material from the first milling step was re-milled in an expansion milling step using an air jet mill to an average tap density of 42 g / L. The silica obtained was designated Type 4 and had the following properties:

[0067] The properties are shown in Table 1.

[0068]

[0069] Table 1: Properties of the fumed silica types of Examples 1 to 4

[0070] Surface area (m2) was measured according to ISO 9277. 2 / g).

[0071] The carbon content is determined according to ISO-3262-20.

[0072] Tap density measurement according to DIN / ISO 787 / 11

[0073] The oil absorption DOA (ml / 100 g) was measured according to ISO 19246.

[0074] Particle size was determined as Z-average in ethanol using dynamic light scattering according to ISO 22412.

[0075] Unground surface-modified silica type 1 had a Z-average particle size of 575 nm (ISO 22412). Air jet-ground silica type 2 had a particle size of 325 nm. The compression milling step in Example 3 resulted in type 3 with a Z-average particle size of 666 nm. A particle size of 326 nm was achieved in type 4 by the compaction and expansion re-milling steps.

[0076] The tap density of the unground surface modified silica Type 1 material was increased from 55 g / l to 197 g / l in Type 3 before decreasing the tap density to 42 g / mL in Type 4 (42 g / mL in Type 4 is within a similar range to the tap density of the unground surface modified silica Type 1 at 55 g / mL).

[0077] While the tap density and surface area are similar in Forms 1 and 4, the particle sizes in ethanol are very different: the particle size of unmilled Form 1 is 575 nm, while the particle size of two-step milled Form 4 is 326 nm.

[0078] Quite surprisingly, these examples show that the use of silica ground in both the expansion and compression grinding steps can improve the printability and strength of photopolymer resins without sacrificing elasticity.

[0079] Example 5

[0080] Fumed silica was prepared according to Examples 6, 8, and 10 of WO 2022 / 171406. Fumed silica type composition 1 corresponds to Example 6, fumed silica composition 2 corresponds to Example 8, and fumed silica type composition 3 corresponds to Example 10. These fumed silica types have a tap density of 41-43 g / l and a viscosity of 186 m 2 / g-278m 2 / g of surface area.

[0081] According to Example 1 of US Pat. No. 6,593,393, a fumed silica composition 4 was produced by surface modification with hexamethyldisilazane and subsequent attritor milling and re-grinding with an air jet mill.

[0082] Fumed silica types Composition 1 Composition 2 Composition 3 Composition 4 Surface modification no no no yes vertical ball mill no no no yes Air jet grinding no no no yes <![CDATA[Surface area (m 2 / g)]]> 278 259 186 145 Carbon content [%] 2.7 Tap density [g / l] 41 41 43 188

[0083] Table 2: Properties of Fumed Silica Type Compositions 1 to 4

[0084] Surface area (m2) was measured according to ISO 9277. 2 / g).

[0085] Tap density measurement according to DIN / ISO 787 / 11

[0086] Example 6

[0087] A 3D printable radiation-curable composition containing 79 wt% isobornyl acrylate, 20 wt% bisphenol ethoxylated dimethacrylate, and 1 wt% bisacylphosphine oxide was prepared. Fumed silica type 1-4 was added to the resin at a concentration of 10 wt% and mixed in a high-speed mixer at 3000 rpm for 5 minutes.

[0088] Colloidal silica designated Type 5 was used to prepare a resin having 10 wt% of Type 5 in the same manner.

[0089] Colloidal silica is a dispersion of synthetic amorphous nano-sized silica particles in a reactive diluent and is commonly used as a filler in polymers. The colloidal silica used in Example 5 had an average particle size Z of 20 nm and a particle size of 140 nm. 2 / g surface area. Surface area (m2) was measured according to ISO 9277. 2 / g).

[0090] resin 0 1 2 3 4 5 Silica Type NA Type 1 Type 2 Type 3 Type 4 Type 5 Silica content [wt%] 0 10 10 10 10 10 Scraper fineness tester fineness [μm] 1 47 12 47 11 0 Viscosity [mPa.s] 20 835 700 52 107 21 Sedimentation rate [mm / year] NA 1 13 268 147 94

[0091] Table 3: Properties of resins including various types of silica particles

[0092] The particle fineness [μm] in the resin is checked according to DIN ISO 1524 using a scraper fineness gauge with a depth range of 50 μm.

[0093] The viscosity of the resin is determined in [mPa.s] according to ISO 3219.

[0094] The sedimentation rate in [mm / year] is measured according to ISO / TR 13097. A resin with a sedimentation rate of less than 200 mm / year is required to be proven to be stable.

[0095] Resin 1 containing silica type 1 had a very low sedimentation rate of 1 mm / year, but Resin 1 was not printable due to the high viscosity of 835 mPa.s.

[0096] Resin 2 containing silica type 2 also has a low sedimentation rate, but the viscosity is too high, so this resin is not suitable for 3D printing.

[0097] Resin 3 containing silica type 3 had a high sedimentation rate of 268 mm / year, which made the resin unstable for commercial use.

[0098] Resin 4, which contains silica type 4 ground using a two-step milling process combining compression milling and expansion milling, exhibited a sedimentation rate of 147 mm / year, meaning the resin is stable and suitable for commercial use. Resin 4 also exhibited a very low viscosity of 107 mPa.s, making it suitable for 3D printing even at room temperature.

[0099] Resin 5 containing 10 wt% of silica type 5 (colloidal silica) is also stable, with a sedimentation rate of 94 mm / year and a very low viscosity of 21 mPa.s, which allows for use in 3D printing applications.

[0100] The blade fineness is an indicator of the number of particle agglomerates. The dispersion quality of the photopolymer resin is checked according to ISO 1524 by means of a blade fineness meter with a gauge of 50 μm.

[0101] The blade gage used in 3D printable photopolymer resins must have a gage fineness of less than 15 μm. A gage fineness greater than 15 μm can result in visible silica agglomerates in 3D printed parts, which means that the visual and mechanical properties of the 3D printed parts are impaired.

[0102] The expansion milling step reduced the sample's sedimentation rate. Resin 4, which contained Type 4 silica that underwent an expansion milling step followed by a compression milling step, had a much lower annual sedimentation rate in the accelerated aging test than Resin 3, which contained Type 3 silica that was only compression milled. The sedimentation rate of Resin 4 was comparable to that of Resin 5, which contained colloidal silica, the most commonly used silica.

[0103] Resins 1 and 2 have the lowest sedimentation rates, but due to the high viscosity of silica in the resins at 835 mPa.s and 700 mPa.s, respectively, these resins are not suitable for 3D printing applications.

[0104] Example 7

[0105] 3D printable radiation curable compositions were prepared in the same manner as described in Example 6. Fumed silica type compositions 1 to 4 were added to the resin at a concentration of 10 wt % and mixed in a high-speed mixer at 3000 rpm for 5 min.

[0106] resin 6 7 8 9 Silica Type Composition 1 Composition 2 Composition 3 Composition 4 Silica content [wt%] 10 10 10 10 Scraper fineness tester fineness [μm] 27 50 50 Viscosity [mPa.s] 14366 12162 8666 44 Sedimentation rate [mm / year] NA NA NA 257

[0107] Table 4: Properties of Resins Containing Silica Particles Compositions 1 to 4

[0108] Resin 6 containing silica composition 1 was not printable due to the high viscosity of 14366 mPa·s. Furthermore, due to the high viscosity, the sedimentation rate could not be determined.

[0109] Resin 7 containing silica composition 1 was not printable due to the high viscosity of 12162 mPa·s. Furthermore, due to the high viscosity, the sedimentation rate could not be determined.

[0110] Resin 8 containing silica composition 1 was not printable due to the high viscosity of 8666 mPa·s. Furthermore, due to the high viscosity, the sedimentation rate could not be determined.

[0111] Resin 9 containing silica composition 4 had a viscosity suitable for printing, but a high sedimentation rate of 257 mm / year made the resin insufficiently stable for commercial use.

[0112] Example 8

[0113] For Example 6, the same 3D printable liquid radiation curable composition as shown in Example 5 included 79 wt% isobornyl acrylate, 20 wt% bisphenol ethoxylated dimethacrylate, and 1 wt% bisacylphosphine oxide.

[0114] Fumed silica types 1-4 were added to the resin at a concentration of 10 wt% and mixed in a high speed mixer at 3000 rpm for 5 min. Colloidal silica designated as type 5 was used to prepare a resin having 10 wt% of type 5 in the same manner.

[0115] Resins 0 to 5 were heated in a masked stereolithography 3D printer at 1.0 mW / cm -2 and 30 s exposure time for printing for each 100 μm deposition layer.

[0116] Printed samples were prepared according to ASTM D638 Type V bars.

[0117] The mechanical properties of the printed samples are shown in Table 5 below.

[0118] resin 0 1 2 3 4 5 Silica Type NA Type 1 Type 2 Type 3 Type 4 Type 5 Silica content [wt%] none UTS[MPa] 26 31 32 33 30 29 Elongation at break [%] 133 79 91 38 94 62 Tensile modulus [MPa] 1379 1788 1781 1861 1789 1548 Toughness [MPa] 25 19 23 8.9 21 12

[0119] Table 5: Mechanical properties of printed bars using resins 0 to 5

[0120] The ultimate tensile strength (UTS) in [MPa] was determined according to ASTM D638.

[0121] Elongation at break in [%] measured according to ASTM D638

[0122] The tensile modulus in [MPa] was determined according to ASTM D638.

[0123] The toughness in [MPa] was measured according to ASTM D638.

[0124] Resin 0 did not contain any silica particles. In the absence of silica in Resin 0, the UTS of the printed V-bar was found to be 26 MPa. The elongation at break according to ASTM D638 was 133%, the tensile modulus was 1379 MPa, and the toughness was found to be 25 MPa.

[0125] For Resin 1, which contains 10 wt% fumed silica type 1, mechanical properties could only be determined after each printed layer of resin had been pushed to the center of the photopolymer VAT. Due to the resin's high viscosity, it did not flow. Resin 1 is not suitable for use as a 3D printing resin at room temperature. The mechanical properties in Table 5 are for reference only.

[0126] For Resin 2, which contains 10 wt% fumed silica type 2 treated with an expansion milling step, the situation is similar to that for Resin 1. Mechanical properties can only be measured after each layer of resin is pushed into the center of the photopolymer VAT, as Resin 2 also does not flow due to its high viscosity. Resin 2 is also unsuitable for use as a 3D printing resin. For both Resins 1 and 2, the printer must be paused to manually push the material into the printer. The mechanical properties in Table 5 are for reference only.

[0127] Resin 3 contains 10 wt% fumed silica type 3 treated with a compression milling step. The resin's viscosity makes it suitable for 3D printing. However, the sedimentation rate is too high for use as a commercial resin. The mechanical properties of the printed bars showed a UTS of 33 MPa, which is within a similar range to the other resins. The elongation at break was reduced to 38%, compared to 91% for Resin 2. The tensile modulus of the printed bars was measured to be 1861 MPa, and the toughness was 8.9 MPa. The toughness is significantly lower than that of the printed bars from Resin 0 or Resin 4.

[0128] Resin 4 contained 10 wt% fumed silica type 4, processed in the expansion and compression milling steps. Printing V-bars using Resin 4 restored the elongation at break from 38% for printed Resin 3 to 94%.

[0129] Comparing the properties of the printed bars of Resin 5, which contains 10 wt% colloidal silica type 5, with Resin 4 shows that Resin 5 has a slightly lower UTS at 29 MPa, a significantly lower tensile modulus at 1548 MPa, and a significantly lower elongation at break at 62%, relative to 94% for Resin 4. Toughness is reduced by 42% and is found to be 12 MPa.

[0130] Colloidal silica was used because it has been used in applications to improve tensile modulus (US20050040562A1). As can be seen in Example 6, Resin 4, which contains Type 4 silica, has a higher tensile modulus performance of 1789 MPa, which is higher than the 1548 MPa of Resin 5 (colloidal silica in Resin 5). The ultimate tensile strength (UTS), measured using V-bars according to ASTM D638, increased from 26 MPa in Resin 0 to 30 MPa using Resin 4. The elongation at break decreased but remained higher than Resins 1, 2, 3, and 5, especially compared to Resin 3, where the elongation at break deteriorated to 38%.

[0131] Resin 4 containing fumed silica type 4 according to the present invention can significantly improve the ultimate tensile strength UTS while maintaining the toughness of the printed material to a large extent.

[0132] Quite surprisingly, these examples show that the use of silica ground in both the compression and expansion grinding steps can improve the printability and strength of photopolymer resins without sacrificing elasticity.

[0133] The photopolymer resin containing silica according to the present invention reduces the viscosity of the resin and can allow for a low sedimentation rate and an increase in UTS and tensile modulus while maintaining toughness.

[0134] When used in photopolymer resins, fumed silica treated in a compression milling and subsequent expansion milling step results in unexpected new properties. The tap density and surface area show a return to the original, unmilled material. Surprisingly, new properties are obtained that have a beneficial effect on the mechanical properties of the cured photopolymer resin.

[0135] Example 9

[0136] In a mask stereolithography 3D printer, the -2 Resin 9 was printed with an exposure time of 30 s for every 100 μm deposition layer. The printed samples were prepared according to ASTM D638 type V bars.

[0137] The mechanical properties of the printed samples of Resin 9 containing Silica Composition 4 are shown in Table 6 below.

[0138] Mechanical properties were determined as described in Example 8.

[0139] resin 9 Silica Type Composition 4 Silica content [wt%] 10 UTS[MPa] 24 Elongation at break [%] 27 Tensile modulus [MPa] 1429 Toughness [MPa] 3.6

[0140] Table 6: Mechanical properties of printed strips using resin 9

[0141] Besides the high settling velocity which already renders Resin 9 unsuitable for commercial use, the mechanical properties of the printed strips show low elongation at break and very low toughness.

Claims

1. Fumed or precipitated silica, which is ground in a first compression grinding step to a tap density of 200% to 1200% of the initial tap density of the fumed or precipitated silica, and ground in a second expansion grinding step to a tap density of 10% to 50% of the tap density after the first compression grinding step.

2. The fumed or precipitated silica according to claim 1, characterized in that The fumed or precipitated silica is ground in a first compression grinding step to a tap density of 200% to 800% of the initial tap density of the starting material, and in a second expansion grinding step the fumed or precipitated silica is ground to a tap density of 10% to 30% of the tap density after the first compression grinding step.

3. The fumed or precipitated silica according to claim 1 or 2, characterized in that The first compression milling step is performed in a ball mill, a hammer mill or an impact mill, and the second expansion milling step is performed in an air jet mill or a pin mill.

4. Fumed or precipitated silica according to any one of claims 1 to 3, characterized in that The tap density of the ground silica is 20-100 g / l.

5. Fumed or precipitated silica according to any one of claims 1 to 4, characterized in that The fumed or precipitated silica is surface-modified with silane, organo-modified silane, siloxane, organo-modified siloxane, or a mixture thereof.

6. The fumed or precipitated silica according to claim 5, characterized in that The fumed or precipitated silica is surface-modified with an organo-modified silane selected from the group consisting of (meth)acrylate silanes, epoxy silanes, and hexamethyldisilane.

7. A liquid radiation curable composition comprising the fumed or precipitated silica according to any one of claims 1 to 6 in a concentration of 5 wt% to 50 wt% of the total liquid curable composition.

8. The liquid radiation curable composition according to claim 7, comprising Component a) 40 wt% to 80 wt% of one or more reactive oligomers selected from epoxy resins, (meth)acrylates, ethoxylated (meth)acrylates, epoxy (meth)acrylates, bisphenol ethoxylated (meth)acrylates or urethane (meth)acrylates, polyester (meth)acrylates, and bisphenol (meth)acrylates; Component b) 20 wt% to 90 wt% of one or more reactive monomers selected from (meth)acrylates and epoxy resins; Component c) 5 wt% to 50 wt% of fumed or precipitated silica according to any one of claims 1 to 6; Component d) 0 wt% to 15 wt% of a crosslinking agent reactive with components a) and / or b); Component e) 0.01 wt% to 10 wt% of one or more photoinitiators; Component f) 0 wt% to 20 wt% of one or more additives selected from pigments, fillers, dispersants, defoamers, antioxidants, light stabilizers, light absorbers or free radical inhibitors; Provided that the sum of all components equals 100%.

9. Use of a liquid radiation curable composition according to claim 7 or 8 in an additive manufacturing process comprising repeated steps of depositing or layering and irradiating the composition to form a three-dimensional object.

10. A three-dimensional object formed by an additive manufacturing process using the liquid radiation curable composition according to claim 7 or 8.

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