This invention relates to photo curable formulations that contain polymerizable brominated compounds for UV curing-based printing inks and three-dimensional (3D) printing applications .

Photopolymers are light sensitive polymeric materials, which changes their physical or chemical properties when exposed to a light source, mainly UV light. UV curing is a photo polymerization process that uses UV energy to harden the photopolymer resin. Upon absorption of the UV energy, a photoinitiator produces free radicals that initiate cross- linking between the photopolymer chains, and results in cure or solidification of the resin.

The growing market of UV curable resins is due to their advantages in terms of saving time (rapid curing) and energy, reducing waste (low V.O.C content) and their low temperature process. Based on these advantages, radiation-curing technology has enjoyed an outstanding growth in the field of coatings, printing inks, and adhesives. For decades, photo polymerization has been widely used in numerous commercial applications such as electronics, optics, fabrication of devices, sealants and surface modifications.

Recently, thanks to the combination of fast curing and the ability to focus the light source on minimum spot diameters, several methods and apparatuses have been developed for the production of three-dimensional objects by irradiation of photopolymers. The most efficient technology for UV curing based 3D printing is stereolithography (SLA) . SLA 3D printing technologies are based on bottom up fabrication by selective polymerization of monomers, by light irradiation. The fabrication of the object is mainly done by Digital Light Processing (DLP) , in which the ink is present in a bath and the light source is focused at various spots, or by inkjet printing in which each inkjet printed layer is exposed to UV radiation. The SLA 3D ink formulations are typically composed of monomers or oligomers in liquid form, with dissolved photo-initiators which are activated by the light source of the printer.

The number of materials manufactured by 3D printing has increased by a large extent for a variety of applications and the demand for printing objects with specific properties has increased .

Fabricated polymeric parts must meet certain physical and chemical characteristics in order to be suitable for the intended application. One major requirement of polymeric items in many applications is that they must be flame retarded. Non flammable articles are critical for many applications such as electronics, telecommunications, and transportation.

Polymers are typically flame retarded through the addition of various flame retarding compounds into the base polymer system. Flame retardants can be either (i) simple additives, i.e., they can be added and mixed into the base polymer by various techniques like melt-blending and extrusion; or (ii) reactive compounds, i.e., compounds having reactive functional groups that are thermo-polymerized into the polymeric matrix. Because of the high temperature processing, it is required that the flame retardant be thermally stable in order to avoid decomposition of the flame retardant. The flammability characteristics of polymers are quantifiable according to a method specified by Underwriter Laboratories standard UL 94, where an open flame is applied to the lowermost edge of a vertically mounted test specimen made of the tested polymer formulation. The specimens used in the UL 94 test method vary in thickness (typical thicknesses are ~3.2 mm, ~1.6 mm, ~0.8 mm and ~0.4 mm) . During the test, various features of the flammability of the test specimens are recorded. Then, according to the classification requirements, the tested polymer formulation is assigned either V-0, V-l or V-2 rating at the measured thickness of the test specimen. Polymer formulation assigned the V-0 rating is the less flammable .

It has been reported in WO 2005/054330 that flame retarded radiation curable compositions suitable for producing 30- objects able to pass UL 94 V-0 rating are difficult to prepare. A large variety of flame retardants were tested in WO 2005/054330, falling into different categories (bromine- containing flame retardants, phosphorous-containing flame retardants, nitrogen-containing flame retardants and inorganic flame retardants) . The flame retardants were added to radiation curable compositions which were used to prepare test specimens in a rapid prototyping machine. The conclusion reached was that a combination of flame retardants that belong to different classes is required to impart sufficient flame retardancy to test specimens prepared in a rapid prototyping machine. Amongst the bromine-containing flame retardants tested in WO 2005/054330 there was also a bromine-containing acrylate: 2 , 2 ' , 6, 6 ' -Tetrabromobisphenol A ethoxylate (1 EO/phenol) diacrylate which has the following formula:

(see Table 1 on page 21 of WO 2005/054330) . Examples C-8 and 12 of WO 2005/054330 illustrate that the compound depicted above (solid at room temperature) cannot be used solely and needs to be combined with two additional flame retardants, i.e., as part of a ternary mixture, at a total loading level of 42% by weight of the ternary mixture of flame retardants based on the total weight of the composition, to achieve a composition passing the UL 94 V-0/3.2 mm test.

We have now found a class of acrylate esters with aliphatically-bound bromine that are well suited for use in radiation (e.g., UV-light) curable ink compositions applied in 3D printing technologies. That is, the invention utilizes liquid bromine-containing acrylate esters having bromine atoms bonded to aliphatic (as opposed to aromatic) carbons. The preferred bromine-containing monomers to be used in this invention are represented by Formula A:

Formula A wherein X is an acrylic acid residue -C(0)-CH=CH ₂ or methacrylic acid residue -C (O) -CCH ₃ =CH ₂ . The individual compounds are therefore:

These compounds are chemically named:

3-bromo-2 , 2-bis (bromomethyl ) propyl ester of acrylic acid (i.e., 3-bromo-2 , 2-bis (bromomethyl ) -1-propyl acrylate or trinol acrylate (abbreviated herein "TA"); and

3-bromo-2 , 2-bis (bromomethyl ) propyl ester of methacrylic acid (i.e., 3-bromo-2 , 2-bis (bromomethyl ) -1-propyl methacrylate or trinol methacrylate (abbreviated herein "TMA") .

The aforementioned bromine-containing acrylate esters are liquids at room temperature; they have boiling points generally above 120°C, e.g., in the range from 130°C-150°C (1 mm Hg) , which is estimated to be >250°C at atmospheric pressure. Hitherto TA was shown to (i) undergo copolymerization with brominated aromatic monomers and multifunctional acrylates in the presence of a free radical initiator under heating to form lenses (WO 2007/007332); (ii) undergo emulsion copolymerization with acrylic monomers in water to form a latex (co-assigned PCT/ IL2017/ 050742 ; ºWO 2018/008018) and (iii) undergo copolymerization with various acrylates in response to UV light irradiation, to produce fire-resistant glass laminate for use in windows, in which the cured, bromine-containing composition constitutes an "interlayer" in the glass laminate (US 2015/0132584) .

The liquid bromine-containing acrylate esters depicted above are miscible with liquid resins used in 3D printing inks, forming clear homogeneous ink formulations, such that after the photo-polymerization takes place, the brominated acrylates become part of the formed polymers. That is, the bromine-containing acrylate has at least one acrylic group that can react with the other curable monomers and oligomers of the 3D resin ink formulation and is photo- polymerized into the polymeric matrix. The use of soluble flame retardants also prevent light scattering within the ink during the printing process, which may occur when solid, particulate flame retardants are used.

Accordingly, one aspect of the invention is a 3D printing resin formulation comprising:

A) at least one flame retardant, which is an aliphatic bromine-containing acrylate and/or methacrylate monomer that is liquid at room temperature;

B) non-halogenated UV light curable component, for example, monofunctional, difunctional and multifunctional acrylate or methacrylate, especially difunctional and multifunctional acrylate or methacrylate.

C) one or more photoinitiator ( s ) .

Regarding (A) , the concentration of the aliphatic bromine- containing acrylic monomer (s) is preferably above 20% by weight, more preferably above 30 wt%, e.g., from 30 to 40% by weight based on the total weight of the printable composition. As mentioned above, especially preferred are the compounds of Formulas A1 or A2 :

Formula A1 Formula A2

The brominated (meth) acrylate ester of Formulas A1 and A2 required as starting materials for this invention can be prepared by reacting aliphatic-bromine containing alcohol, e.g., tribromoneopentyl alcohol: CH2 Br

BrCH2— C— CH2— OH

CH2 Br with acrylic or methacrylic acid under conditions known in the art. The esterification reaction takes place in a solvent or a mixture of solvents, with the aid of a catalyst, generally in the presence of a polymerization inhibitor to prevent premature polymerization of the monomer. The crude liquid monomer is recovered using conventional techniques and can be purified. Further information including full preparative procedures can be found in US 3,165,502, US 3,480,600 and WO 2007/007332; and also WO 2011/045780 and US 2012/0203028, where the bromine-containing acrylate esters were purified by chromatography.

Other liquid brominated acrylate esters to be considered for use in this invention have the following formula:

wherein X is selected from an acrylic acid residue -C(0)-CH=CH ₂ and methacrylic acid residue -C (0) -CCH ₃ =CH ₂ (one X may be hydrogen but generally both X moieties contain the (meth) acrylate functionality) . These esters (named herein didinol diacrylate or methacrylate) are obtainable from the corresponding di-alcohol:

by esterification with acrylic or methacrylic acid. However, experimental work conducted in support of this invention indicates that bromine-containing monofunctional acrylates, namely, trinol acrylate, generally perform better than bromine-containing multifunctional acrylates, achieving UL 94 3.2/V-O rating with lower loading levels.

It should be noted that the bromine incorporated into the UV light curable 3D printing resin formulation is bound to aliphatic carbons. That is, the compositions of the invention are generally devoid of aromatically-bound bromine acrylates. The compositions of the invention are also preferably devoid of additive (non-reactive) flame retardants, e.g., halogenated flame retardants. Trinol acrylate can be used as a sole flame retarding agent in the 3D printing resin composition, to achieve, for example, V-0 rating for 3.2 mm thick 3D printed specimen in the UL 94 test.

Regarding (B) , the concentration of the (non-halogenated) UV light curable components of the composition is generally not less than 40% by weight, e.g., not less than 45% by weight, for example, from 50 to 70% by weight. Experimental work conducted in support of this invention indicates that the brominated acrylate esters are soluble in a variety of commercially available stereolithography resins. The UV light curable resin preferably comprises one or more polyfunctional ethylenically unsaturated components which are multifunctional (meth) acrylates , e.g., monomers and oligomers having from 2 to 6 (meth) acrylate functional groups, e.g., at least one monomer or oligomer having from 3 to 6, e.g., from 4 to 6 (meth) acrylate functional groups (the examples below refer to acrylate monomers and oligomers for the purpose of illustration; corresponding methacrylates are also intended) : Tricyclodecane dimethanol diacrylate, commercially available from Sartomer as SR833S);

- pentaerythritol derivatives [for example, compounds represented by the linear formula (H2C=CHCC>2CH2 ) _n C (CH2OH) _4-n wherein n is 2, 3 or 4, e.g., pentaerythritol tetraacrylate; alkoxylated pentaerythritol derivatives represented by the linear formula (H ₂ C=CHC (O) - [O-CRH-CH2] _P -0-CH ₂ ) _n C (CH ₂ OH) _4-n wherein R is CH3 or H, n is 2, 3 or 4 and p is independently the number of alkoxylated units in each chain; for example alkoxylated pentaerythritol tetraacrylate (commercially available from Sartomer as SR 494); dipentaerythritol hexa (meth) acrylate, dipentaerythritol penta (meth) acrylate, or dipentaerythritol tetra (meth) acrylate) ;

- glycol derivatives, such as:

Including polyethylene glycol di (meth) acrylate:

[H ₂ C=C (CH ₃₎ CO ₂ CH ₂ ] ₃ CC ₂ H5 and ethoxylated and propoxylated derivatives thereof; both-terminal (meth) acrylic acid adduct of bisphenol A diglycidyl ether, ethoxylated bisphenol A di (meth) acrylate, propoxylated bisphenol A di (meth) acrylate, ethoxylated hydrogenated bisphenol A di (meth) acrylate, propoxylated- modified hydrogenated bisphenol A di (meth) acrylate, and ethoxylated bisphenol F di (meth) acrylate .

Further monomers classified according to their functionality include :

Monoacrylates: Carpolactone (meth) acrylate, Diethylene glycol butyl ether (meth) acrylate, Tetrahydrofurfuryl (meth) acrylate, isophoryl (meth) acrylate, Ethoxylated ( 4 ) phenol (meth) acrylate, Ethoxylated ( 4 ) nonyl phenol (meth) acrylate .

Diacrylates: Tricyclodecane dimethanol di (meth) acrylate, Dioxane glycol di (meth) acrylate, Dipro[ylene glycol di (meth) acrylate (DPGDA) , Tripropylene glycol di (meth) acrylate, polyethylene glycol di (meth) acrylate, Ethoxylated bisphenol A di (meth) acrylate, propoxylated ( 2 ) neopentyl glycol di (meth) acrylate . Triacrylates: propoxylated glycerol tri (meth) acrylate, Tris ( 2-hydroxylethyl ) isocyanurate tri (meth) acrylate.

Higher acrylates: Dipentaerythritol penta/hexa (meth) acrylate, Alkoxylated pentaerythritol tetraacrylate, Di ( trimethylol ) prpane tetraacrylate.

Other examples of commercially available acrylates monomers and oligomers for UV curable formulations can be found in Hutchinson. Raw materials for UV curable inks. In: S. Magdassi, editor. The chemistry of inkjet inks. Singapore; World Scientific. (2010) .

Regarding (C) , the concentration of the photoinitiator in the 3D printing resin formulation is at least 0.1 % by weight, for example, from 0.25 to 5.0% by weight, e.g., from 0.3 to 1.0% by weight. As mentioned above, the photoinitiator absorbs light and generates free radicals that initiate the polymerization reaction. A detailed survey of many useful photoinitiators can be found in the paper authored by Gruber H. F., Photoinitiators for free-radical polymerization. Prog. Polym. Sci. 17, 953-1044 (1992) . Photoinitiators for free radical UV-curing are generally Type I photoinitiators selected from the classes of acylphopsphineoxides and acylphosphonates , benzoin derivatives, benzyl derivatives, dialkoxyacetophenones , -hydroxyalkylphenones , - aminoalkylphenones , O-acyl-a-oximinoketones , organic peroxides, halogenated ketones, organic sulfur compounds and phenylglyxiyklates . Acylphopsphineoxides display high efficiency in many photocuring processes on account of their high thermal stability and ability to rapidly initiate the free radical polymerization. As far as 3D printing applications are concerned, 2 , 4 , 6-trimethylbenzoyl- diphenylphosphine oxide (TPO) : commercially available in the market from various manufacturers, may be considered as the photoinitiator of choice; it has been shown to be operable in commercially available, low-cost, light-emitting diode-based 3D printers using digital light processing.

The 3D printing resin formulations are prepared by combining components A) , B) and C) with stirring at room temperature or under heating, e.g., up to 100°C, until homogeneous clear resin formulation is obtained. Secondary additives such as pigments and dyes for fabricating colored objects, stabilizers (heat, light, antioxidants e.g., UV stabilizer such 2 , 5-Bis ( 5-tert-butyl-benzoxazol-2-yl ) thiophene ) and flame retardant synergists like antimony trioxide can also be added, generally up to 10 % by weight.

The formulations can be used for photo-polymerization based printing technologies (3D printing, 2D) . In particular, the formulated inks described above may be used for fabricating flame-retarded 3D objects by stereolithography (SLA), digital light process (DLP) , inkjet techniques and extrusion based printing, to produce objects meeting high UL-94 standard rating .

Accordingly, another aspect of the invention is a method of creating 3D-printed objects in a layer-by-layer manner, comprising consecutively solidifying thin layers of a UV light curable resin composition comprising one or more liquid bromine-containing acrylates (e.g., the formulations described above) with ultraviolet light, optionally followed by post curing.

The UV light curable 3D printing resin formulation of the invention is especially suitable for creation of 3D objects by means of SLA systems. In their most general form, such systems, which are commercially available in the market, e.g., from Formlabs, are equipped with a resin tank, a UV light source and a build platform. The viscosity of the 3D printable resin composition of the invention intended for use in such SLA systems may be from 70 to 3000 centipoises (cp) , e.g., from 70 to 2000 cp, more specifically from 70 to 1000 cp . For example, some suitable formulations may have viscosity in the range from 70 to 150 cp; other suitable formulation may have viscosity in the range from 300 to 900 cp (measured at 25°C by Brookfield viscometer) .

A resin formulation of the invention can be used in a "right- side up" SLA printer: the formulation is added to fill the resin tank. A mobile build platform that is submerged in the tank is able to slide down in an increment corresponding to the thickness of an individual layer to be built. A UV light source mounted above the tank cures the resin formulation at the surface. Hence the process is repeated by allowing the build platform to move down to build layers on top of the others to create the 3D object. The resin formulation of the invention can also be used in an "Upside-Down" (Inverted) SLA printer, where the mobile build platform is suspended above the resin tank. The UV light source is located under the tank and cures the resin through the transparent bottom of the tank. That is, a fresh resin layer to be cured repeatedly flows into the space between the transparent bottom of the tank and the build platform (more precisely, the last completed layer supported on the build platform) ; the height of this space corresponds to the thickness of an individual layer to be cured.

Regarding the post curing, the 3D object is removed from the build platform of the 3D printer, optionally washed to remove residual monomers which did not undergo reaction, and then exposed to light irradiation and/or heat, for example, in a post-curing chamber (the light irradiation is not necessarily UV light) . Post-curing may be mandatory for certain resin formulations while in many other cases it is not required, though recommended in order to improve mechanical strength and reduce tackiness to facilitate painting, etc.

Accordingly, another aspect of the invention is a method of creating a flame-retarded 3D object in a layer-by-layer manner, comprising consecutively solidifying thin layers of the UV light curable liquid resin composition described above with ultraviolet light, wherein the layer to be cured is the free surface of said liquid resin filling a resin tank in which a mobile build platform is submerged, wherein said build platform moves down incrementally to enable the fresh free surface to be cured onto the built-up 3D object.

Yet another aspect of the invention is a method of creating a flame-retarded 3D object in a layer-by-layer manner, comprising consecutively solidifying thin layers of the UV light curable liquid resin composition described above with ultraviolet light, wherein the layer to be cured is constrained in the space between a transparent bottom of a resin tank and a mobile build suspended above said tank, wherein said build platform moves up incrementally to enable fresh free resin to flow into said space and be cured to join the built-up 3D object.

In addition to the methods described above in relation to SLA 3D printers, the formulations of the invention can also be used in inkjet printing, in which each inkjet printed layer is exposed to UV radiation. Brominated acrylates-based photo formulations for 3D inkjet printing of the invention can meet the requirements of viscosity and surface tension for good jet-ability (e.g., with the aid of added wetting agent) . The viscosity of the printing formulation of the invention intended for inkjet printing is below 50 cp, e.g., below 40 cp, for example, from 20 to 40 cp, e.g., from 20 to 30cp (measured at 40°C using Brookfield viscometer) . The inkjet printing of the formulations of the inventions usually require heating of the print-head. The viscosity is adjusted to meet the requirements of the print-head used and the jetting temperature.

Another aspect of the invention is the use of one or more liquid bromine-containing acrylates, e.g., a compound of Formula A:

Formula A

as defined above, as flame retardant incorporated into a 3D-printed object.

A 3D-printed object made of one or more cured polymers (produced, for example, by SLA or inkjet printing) having a structural unit of the formula: incorporated into the polymer chains, characterized in that R comprises bromine atoms that are bound to aliphatic carbons, forms another aspect of the invention. Specifically, the bromine-containing structural unit incorporated into the polymer chains corresponds to the monomer of Formula A (e.g., trinol acrylate; namely, R is -CH2-C (CfhBr) 3) . The bromine content of the 3D-printed object is not less than 10%, e.g., not less than 12%, for example, from 12 to 26%, e.g., 15 to

26% (by weight based on the total weight of the composition) . 3D-printed objects of the invention are generally devoid of aromatically-bound bromine. 3D-printed objects of the invention are also preferably devoid of additive (non reactive) flame retardants, e.g., halogenated flame retardants .

It should be noted that solid bromine-containing acrylate monomers (e.g., with aliphatically-bound bromine) which are soluble in the resin formulations can also be used, in lieu of, or in addition to, the liquid bromine-containing acrylates described above.

Examples

Materials

The materials used for preparing ink compositions (either for SLA printers or inkjet printing) are tabulated in Table 1 (FR is the abbreviation of flame retardant) :

Table 1

Flammability test

The flammability test was carried out according to the Underwriters-Laboratories standard UL 94, applying the vertical burn on specimens of 3.2 mm thickness.

Mechanical properties

The izod impact (un-notched) test was carried out according to ASTM D256-81 using Instron Ceast 9050 pendulum impact system. Tensile properties were determined according to ASTM D638 using Zwick 1435 materials testing machine (type 2 dumbbells were used, with a speed test of 5 mm/min) .

Thermal properties

Heat distortion temperature (abbreviated HDT; this is the temperature at which a polymer sample deforms under a specific load) was measured according to ASTM D-648 with load of 1820 MPa and heating rate of 120°C/hour.

Examples 1-9

Flame retarded photo-curable ink formulations

for SLA 3D-printers

Ink formulations were prepared by mixing various UV curable resins employed for 3D printing together with trinol acrylate (ICL-IP, Israel), TPO as photoinitiator and in some cases also 2 , 5-Bis ( 5-tert-butyl-benzoxazol-2-yl ) thiophene (Sigma) as a stabilizer. After stirring for 15 minutes in a hot water bath, the mixture was poured into the resin bath of the DLP 3D printer FormLabs 1+ (FormLabs, USA) . The printing was done at Clear 02 mode in resolution of 0.2 mm (Examples 1 and 2), Tough 02 mode in resolution of 0.1 mm (Examples 3, 4, 7 and 8), Grey 02 mode in resolution of 0.1 mm (Examples 5 and 6) and Clear 02 mode in resolution of 0.1 mm (Example 9) . The object was then washed with iso propyl alcohol (IPA) to remove uncured monomer residue and post-cured in an Asiga UV lamp for 3 min on each side of the object. The 3D printed objects prepared were conventional test specimens applicable for UL94 flammability test (127 mm in length, 12.7 mm in width and 3.2 mm in thickness) and mechanical tests.

The compositions of the printed specimens and the results of the flammability and mechanical tests are set out in Table 2.

Table 2

(the viscosity of the formulation of Example 1 was ~100cp, measured at 25°C by Brookfield viscometer; other formulations display higher viscosities, up to ~900cp) . The results indicate that the compositions pass the UL 94 V-0/3.2mm test with the aid of a reasonable loading level of trinol acrylate (generally not more than 40% by weight) acting as a sole active ingredient. Addition of a second bromine-containing flame retardant having two acrylate functional groups (Didinol diacrylate in Example 2) is possible, but it cannot compensate effectively for a reduction in the amount of trinol acrylate.

Example 10

Inkjet printing

An ink formulation was prepared by mixing 39.0 wt% SR833S (Sartomer, USA), 19.5 wt% SR494 (Sartomer, USA), 39.0 wt% Trinol acrylate (ICL-IP, Israel), 1 wt% Isopropyl-9H- thioxanthen-9-one (Aldrich, Germany) as a photo initiator, 1 wt% Ethyl 4 (dimethylamino) benzoate as a Photochemical co initiator and 0.5 wt% Byk 333 as a wetting agent. After mixing for 15 minutes in a hot water bath, the formulation was filtered through Rezist 30/1.0 PTFE syringe filter.

Printing experiments were performed with an Omnijet 100 printer (Omnijet, Korea), equipped with 10 pL cartridge (DMC- 11610) . The printhead contained 16 nozzles, and the diameter of each nozzle was 21 pm. The printing height was set to 1 mm, and the jetting frequency was set to 1 kHz, using dot spacing of 42 pm (equivalent to 600 dpi) . The printhead temperature was set to 40°C and the printing was performed on polyester sheet (the viscosity of the formulation was 30cp, measured at 40°C by Brookfield viscometer) . The printed pattern was cured during printing by a 395 nm UV flashlight (Ultraflame, Japan) that was assembled to the print-head.