WO2019083876A1

WO2019083876A1 - Reduction of shrinkage or warping in objects produced by additive manufacturing

Info

Publication number: WO2019083876A1
Application number: PCT/US2018/056847
Authority: WO
Inventors: Bob E. FELLER; Leah Marie HEIST; Xinyu Gu
Original assignee: Carbon, Inc.
Priority date: 2017-10-26
Filing date: 2018-10-22
Publication date: 2019-05-02

Abstract

Provided herein is a polymerizable liquid useful for the production of a three-dimensional object by additive manufacturing, the polymerizable liquid comprising a mixture of: (a) at least one free-radically polymerizable constituent; (b) a free radical photoinitiator; and (c) optionally, but for some embodiments preferably, a free radical thermal initiator (for example, this constituent being optional in a method where a free radical thermal initiator is included in a wash liquid). Also provided is a method of making a three-dimensional object, comprising: (a) producing by stereolithography a three-dimensional object from a polymerizable liquid as described herein; (b) washing the object; and then (c) optionally, but in some embodiments preferably, further curing the object.

Description

REDUCTION OF SHRINKAGE OR WARPING IN OBJECTS PRODUCED BY ADDITIVE MANUFACTURING

Field of the Invention

The present invention concerns resins and methods for additive manufacturing, particularly for stereolithography techniques such as continuous liquid interface production.

Background of the Invention

A group of additive manufacturing techniques sometimes referred to as "stereolithography" create a three-dimensional object by the sequential polymerization of a light polymerizable resin. Such techniques may be "bottom-up" techniques, where light is projected into the resin onto the bottom of the growing object through a light transmissive window, or "top-down" techniques, where light is projected onto the resin on top of the growing object, which is then immersed downward into the pool of resin.

The recent introduction of a more rapid stereolithography technique known as continuous liquid interface production (CLIP), coupled with the introduction of "dual cure" resins for additive manufacturing, has expanded the usefulness of stereolithography from prototyping to manufacturing {see, e.g., US Patent Nos. 9,211,678; 9,205,601; and 9,216,546 to DeSimone et al.; and also in J. Tumbleston, D. Shirvanyants, N. Ermoshkin et al., Continuous liquid interface production of 3D Objects, Science 347, 1349-1352 (2015); see also Rolland et al, US Patent Nos. 9,676,963, 9,453,142 and 9,598,606).

Objects produced by such techniques can be succeptible to shrinkage and warping, such as when green objects produced from a dual cure resin undergo a second cure (such as by baking). Since accurate reproduction of objects is an important goal of additive manufacturing, new techniques to reduce shrinkage or warping are needed.

Summary of the Invention

A first aspect of the invention is a polymerizable liquid useful for the production of a three-dimensional object by additive manufacturing, the polymerizable liquid comprising a mixture of: (a) at least one free-radically polymerizable constituent; (b) a free radical photoinitiator; and (c) optionally, but for some embodiments preferably, a free radical thermal initiator (e.g., this constituent being optional in a method where a free radical thermal initiator is included in a wash liquid, as discussed below).

A second aspect of the invention is a method of making a three-dimensional object, comprising: (a) producing by stereolithography (e.g., by continuous liquid interface production) a three-dimensional object from a polymerizable liquid as described herein; (b) washing the object (e.g., in a wash liquid optionally containing a free radical thermal initiator); and then (c) optionally, but in some embodiments preferably, further curing the object (e.g., by heating).

In some embodiments, the heating step is carried out in an inert fluid atmosphere

(e.g., in an inert gas, such as nitrogen, argon, carbon dioxide, etc., or in inert liquid, such as an aqueous bath (e.g., distilled water, salt water, etc.) an organic liquid bath (e.g., mineral oil, inert fluorinated organic), etc.). In some embodiments, the fluid comprises an inert liquid, and the method further comprises the step of deoxygenating the inert liquid.

In some embodiments, the further curing or heating step is carried out at an elevated pressure.

The foregoing and other objects and aspects of the present invention are explained in greater detail in the drawings herein and the specification set forth below. The disclosures of all United States patent references cited herein are to be incorporated herein by reference.

Brief Description of the Drawings

Figure 1 shows a pair of test parts produced by additive manufacturing from a polyurethane dual cure resin, to which a thermal free radical initiator had been added. The part on the right was baked in ambient (air) atmosphere, while the part on the left was baked first in an inert (nitrogen) atmosphere (and then baked in air).

Detailed Description of Illustrative Embodiments

The present invention is now described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art. As used herein, the term "and/or" includes any and all possible combinations of one or more of the associated listed items, as well as the lack of combinations when interpreted in the alternative ("or").

"Free radical photoinitiator" as used herein includes type I free radical photoinitiators, such as phosphineoxide or hydroxyacetophenone (HAP), and/or type II free radical photoinitiators, such as a benzophenone photoinitiator (optionally but preferably in combination with a co-initiator (e.g., an alcohol or amine)). Particular examples include, but are not limited to, diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), diphenylphosphinyl(2,4,6-trimethylphenyl)methanone; benzophenone; substituted benzophenones; acetophenone; substituted acetophenones; benzoin; benzoin alkyl esters; xanthone; substituted xanthones; diethoxy-acetophenone; benzoin methyl ether; benzoin ethyl ether; benzoin isopropyl ether; diethoxyxanthone; chloro-thio-xanthone; N-methyl diethanol- amine-benzophenone; 2-hydroxy-2 -methyl- 1 -phenyl-propan- 1 -one; 2-benzyl-2-

(dimethylamino)-l-[4-(4-morpholinyl)phenyl]-l-butanone; and mixtures thereof. See, e.g., US Patent No. 9,090,765 to Henkel.

"Thermal free radical initiator" as used herein may be any suitable thermal free radical initiator, numerous examples of which are known. Particular examples include, but are not limited to, l ,l-bis(tert-amylperoxy)cyclohexane, tert-amyl peroxybenzoate, 4,4-azobis(4- cyano valeric acid), 2,2'-azobisisobutyronitrile (AIBN), benzoyl peroxide, 2,2-bis(fert- butylperoxy)butane, l,l-bis(tert-butylperoxy)cyclohexane, 2,5-bis(tert-butylperoxy)-2,5- dimethylhexane, 2,5-bis(teri-butylperoxy)2,5-dimethyl-3-hexyne, bis(l-(tert-butylperoxy)-l - methylethyl)benzene, 1,1 -bis (tert-butylperoxy)-3,3,5-trimethyl-cyclohexane, tert-butyl hydroperoxide, tert-butyl peracetate, tert-butyl peroxide, tert-butyl peroxybenzoate, tert- butylperoxy isopropyl carbonate, cumene hydroperoxide, cyclohexanone peroxide, dicumyl peroxide, lauroyl peroxide, 2,4-Pentanedione peroxide, peracetic acid, potassium persulfate, and combinations thereof. See, e.g., Aldrich, Applications: Free radical initiators (2016).

1. POLYMERIZABLE LIQUIDS (RESINS).

Numerous resins for use in additive manufacturing are known and can be used in carrying out the present invention. See, e.g., US Patent No. 9,205,601 to DeSimone et al.

In some embodiments, the resin is a dual cure resin. Such resins are described in, for example, Rolland et al., US Patent Nos. 9,676,963; 9,598,606; and 9,453,142, the disclosures of which are incorporated herein by reference. Resins may be in any suitable form, including "one pot" resins and "dual precursor" resins (where cross-reactive constituents are packaged separately and mixed together before use, and which may be identified as an "A" precursor resin and a "B" precursor resin).

Particular examples of suitable resins include, but are not limited to, Carbon, Inc. rigid polyurethane resin (RPU), flexible polyurethane resin (FPU), elastomeric polyurethane resin (EPU), cyanate ester resin (CE), epoxy resin (EPX), or urethane methacrylate resin (UMA), all available from Carbon, Inc. (Redwood City, California, USA).

Note that, in some embodiments employing "dual cure" polymerizable resins, the part, following manufacturing, may be contacted with a penetrant liquid, with the penetrant liquid carrying a further constituent of the dual cure system, such as a reactive monomer, into the part for participation in a subsequent cure. Such "partial" resins are intended to be included herein. See, e.g. , WO 2018/094131 (Carbon, Inc.), the disclosures of which are incorporated herein by reference.

In overview, polymerizable liquids for carrying out the present invention comprise a mixture of:

(a) at least one free-radically polymerizable constituent, such as: (i) a blocked or reactive blocked prepolymer, (ii) a blocked or reactive blocked polyisocyanate, (Hi) a blocked or reactive blocked polyisocyanate chain extender, and (iv) combinations of two or three of the foregoing (e.g., in a combined amount of from 5 to 90 percent by weight),

(b) optionally, but in some embodiments preferably, at least one additional chain extender (e.g., in an amount of from 1 or 5 to 30 percent by weight when present),

(c) a free radical photoinitiator (e.g., in an amount of from 0.1 to 4 percent by weight),

(d) a free radical thermal initiator (e.g., in an amount of from 0.1 to 4 percent by weight),

(e) optionally, but in some embodiments preferably, a polyol and/or a polyamine

(e.g., in an amount of from 5 to 90 percent by weight), and

(f) optionally, but in some embodiments preferably, a reactive diluent (e.g., included in an amount of from 1 to 40 percent by weight when present);

(g) optionally, but in some embodiments preferably, at least one non-reactive light absorbing pigment or dye (e.g., titanium dioxide, carbon black, and/or an organic ultraviolet light absorber) (e.g., in an amount of from 0.001 to 10 percent by weight, when present); and/or (h) optionally, but in some embodiments preferably, a filler (e.g., in an amount of from 1 to 50 percent by weight, when present).

In some embodiments, the at least one free-radically polymerizable constituent comprises a blocked or reactive blocked prepolymer (e.g., in an amount of from 5 to 70, 80, or 90 percent by weight). In some more particular embodiments, the reactive blocked prepolymer comprises a compound of the formula A-X-A, where X is a hydrocarbyl group and each A is an independently selected substituent of Formula X:

where R is a hydrocarbyl group, R' is O or NH, and Z is a blocking group (e.g., a group having a reactive epoxy, alkene, alkyne, or thiol terminal group).

In some embodiments, the at least one free-radically polymerizable constituent comprises a blocked or reactive blocked polyisocyanate (e.g., in an amount of from 5 to 70,

80, or 90 percent by weight). In some more particular embodiments, the reactive blocked polyisocyanate comprises a compound of the formula A'-X'-A', where X' is a hydrocarbyl group and each A' is an independentl selected substituent of Formula (Χ'):

where Z is a blocking group (e.g., a group having a reactive epoxy, alkene, alkyne, or thiol terminal group).

In some embodiments, the at least one free-radically polymerizable constituent comprises a blocked or reactive blocked diisocyanate chain extender (e.g., in an amount of from 5 to 70, 80, or 90 percent by weight). In some more particular embodiments, the reactive blocked polyisocyanate chain extender comprises a compound of the formula A"-X"- A", where X" is a hydrocarbyl group, and each A" is an independently selected substituent of Formula (X"): where R is a hydrocarbyl group, R' is O or NH, and Z is a blocking group (e.g., the blocking group having a reactive epoxy, alkene, alkene, or thiol terminal group).

In some embodiments, the at least one additional chain extender comprises at least one diol, diamine or dithiol chain extender.

In some embodiments, the reactive diluent comprises an acrylate, a methacrylate, a styrene, an acrylic acid, a vinylamide, a vinyl ether, a vinyl ester, polymers containing any one or more of the foregoing, and combinations of two or more of the foregoing.

Details of the foregoing constituents are given in the references cited above, and in the additional description below.

C. Additional resin ingredients. The liquid resin or polymerizable material can have (among other things) solid particles suspended or dispersed therein. Any suitable solid particle can be used, depending upon the end product being fabricated. The particles can be metallic, organic/polymeric, inorganic, or composites or mixtures thereof. The particles can be nonconductive, semi-conductive, or conductive (including metallic and non-metallic or polymer conductors); and the particles can be magnetic, ferromagnetic, paramagnetic, or nonmagnetic. The particles can be of any suitable shape, including spherical, elliptical, cylindrical, etc. The particles can be of any suitable size (for example, ranging from 1 nm to 20 μιη average diameter).

The particles can comprise an active agent or detectable compound as described below, though these may also be provided dissolved or solubilized in the liquid resin as also discussed below. For example, magnetic or paramagnetic particles or nanoparticles can be employed.

The liquid resin can have additional ingredients solubilized therein, including pigments, dyes, diluents, active compounds or pharmaceutical compounds, detectable compounds (e.g., fluorescent, phosphorescent, radioactive), etc., again depending upon the particular purpose of the product being fabricated. Examples of such additional ingredients include, but are not limited to, proteins, peptides, nucleic acids (DNA, RNA) such as siRNA, sugars, small organic compounds (drugs and drug-like compounds), etc., including combinations thereof. Dyes/non-reactive light absorbers. In some embodiments, polymerizable liquids for carrying out the present invention include a non-reactive pigment or dye that absorbs light, particularly UV light. Suitable examples of such light absorbers include, but are not limited to: (i) titanium dioxide {e.g., included in an amount of from 0.05 or 0.1 to 1 or 5 percent by weight), (ii) carbon black (e.g., included in an amount of from 0.05 or 0.1 to 1 or 5 percent by weight), and/or (Hi) an organic ultraviolet light absorber such as a hydroxybenzophenone, hydroxyphenylbenzotriazole, oxanilide, benzophenone, thioxanthone, hydroxyphenyltriazine, and/or benzotriazole ultraviolet light absorber (e.g., Mayzo BLS® 1326) (e.g., included in an amount of 0.001 or 0.005 to 1, 2 or 4 percent by weight). Examples of suitable organic ultraviolet light absorbers include, but are not limited to, those described in US Patent Nos. 3,213,058, 6,916,867, 7,157,586, and 7,695,643, the disclosures of which are incorporated herein by reference.

Fillers. Any suitable filler may be used in connection with the present invention, depending on the properties desired in the part or object to be made. Thus, fillers may be solid or liquid, organic or inorganic, and may include reactive and non-reactive rubbers: siloxanes, acrylonitrile-butadiene rubbers; reactive and non-reactive thermoplastics (including but not limited to: poly(ether imides), maleimide-styrene terpolymers, polyarylates, polysulfones and polyethersulfones, etc.) inorganic fillers such as silicates (such as talc, clays, silica, mica), glass, carbon nanotubes, graphene, cellulose nanocrystals, etc., including combinations of all of the foregoing. Suitable fillers include tougheners, such as core-shell rubbers, as discussed below.

Tougheners. One or more polymeric and/or inorganic tougheners can be used as a filler in the present invention. See generally US Patent Application Publication No. 20150215430. The toughener may be uniformly distributed in the form of particles in the cured product. The particles could be less than 5 microns (μιη) in diameter. Such tougheners include, but are not limited to, those formed from elastomers, branched polymers, hyperbranched polymers, dendrimers, rubbery polymers, rubbery copolymers, block copolymers, core-shell particles, oxides or inorganic materials such as clay, polyhedral oligomeric silsesquioxanes (POSS), carbonaceous materials (e.g., carbon black, carbon nanotubes, carbon nanofibers, fullerenes), ceramics and silicon carbides, with or without surface modification or functionalization.

Core-shell rubbers. Core-shell rubbers are particulate materials (particles) having a rubbery core. Such materials are known and described in, for example, US Patent Application Publication No. 20150184039, as well as US Patent Application Publication No. 20150240113, and US Patent Nos. 6,861,475, 7,625,977, 7,642,316, 8,088,245, and elsewhere. In some embodiments, the core-shell rubber particles are nanoparticles (i.e., having an average particle size of less than 1000 nanometers (nm)). Generally, the average particle size of the core-shell rubber nanoparticles is less than 500 nm, e.g., less than 300 nm, less than 200 nm, less than 100 nm, or even less than 50 nm. Typically, such particles are spherical, so the particle size is the diameter; however, if the particles are not spherical, the particle size is defined as the longest dimension of the particle. Suitable core-shell rubbers include, but are not limited to, those sold by Kaneka Corporation under the designation Kaneka Kane Ace, including the Kaneka Kane Ace 15 and 120 series of products, including Kaneka Kane Ace MX 120, Kaneka Kane Ace MX 153, Kaneka Kane Ace MX 154, Kaneka Kane Ace MX 156, Kaneka Kane Ace MX170, Kaneka Kane Ace MX 257 and Kaneka Kane Ace MX 120 core-shell rubber dispersions, and mixtures thereof.

Organic diluents. In some embodiments, diluents for use in the present invention are preferably reactive organic diluents; that is, diluents that will degrade, isomerize, cross-react, or polymerize, with themselves or a light polymerizable component, during the additive manufacturing step. In general, the diluent(s) are included in an amount sufficient to reduce the viscosity of the polymerizable liquid or resin (e.g., to not more than 15,000, 10,000, 6,000, 5,000, 4,000, or 3,000 centipoise at 25 degrees Centigrade). Suitable examples of diluents include, but are not limited to, iV,N-dimethylacrylamide, N-vinyl-2-pyrrolidone, and N- vinyl formamide, or a mixture if two or more thereof. The diluent may be included in the polymerizable liquid in any suitable amount, typically from 1, 5 or 10 percent by weight, up to about 30 or 40 percent by weight, or more.

Accelerators. In some embodiments, the liquid may include a deoxygenating compound as an accelerator of stereolithography (particularly CLIP). An example of a suitable such accelerator is triphenylphosphine.

2. PRODUCTION BY ADDITIVE MANUFACTURING.

Polymerizable liquids or resins as described herein may be used to make three- dimensional objects, in a "light" cure (typically by additive manufacturing) which in some embodiments generates a "green" intermediate object, followed in some embodiments by a second (typically heat) cure of that intermediate object. Techniques for additive manufacturing are known. Suitable techniques include bottom-up or top-down additive manufacturing, generally known as stereolithography. Such methods are known and described in, for example, U.S. Patent No. 5,236,637 to Hull, US Patent Nos. 5,391,072 and 5,529,473 to Lawton, U.S. Patent No. 7,438,846 to John, US Patent No. 7,892,474 to Shkolnik, U.S. Patent No. 8,110,135 to El-Siblani, U.S. Patent Application Publication No. 2013/0292862 to Joyce, and US Patent Application Publication No. 2013/0295212 to Chen et al. The disclosures of these patents and applications are incorporated by reference herein in their entirety.

In some embodiments, the intermediate object is formed by continuous liquid interface production (CLIP). CLIP is known and described in, for example, US Patent Nos. 9,21 1,678, 9,205,601, 9,216,546, and in J. Tumbleston, D. Shirvanyants, N. Ermoshkin et al, Continuous liquid interface production of 3D Objects, Science 347, 1349-1352 (2015). See also R. Janusziewcz et al., Layerless fabrication with continuous liquid interface production, Proc. Natl. Acad. Sci. USA 113, 11703-11708 (2016). In some embodiments, CLIP employs features of a bottom-up three-dimensional fabrication as described above, but the irradiating and/or advancing steps are carried out while also concurrently maintaining a stable or persistent liquid interface between the growing object and the build surface or window, such as by: (i) continuously maintaining a dead zone of polymerizable liquid in contact with said build surface, and (ii) continuously maintaining a gradient of polymerization zone (such as an active surface) between the dead zone and the solid polymer and in contact with each thereof, the gradient of polymerization zone comprising the first component in partially-cured form. In some embodiments of CLIP, the optically transparent member comprises a semipermeable member (e.g., a fluoropolymer), and the continuously maintaining a dead zone is carried out by feeding an inhibitor of polymerization through the optically transparent member, thereby creating a gradient of inhibitor in the dead zone and optionally in at least a portion of the gradient of polymerization zone. Other approaches for carrying out CLIP that can be used in the present invention and potentially obviate the need for a semipermeable "window" or window structure include utilizing a liquid interface comprising an immiscible liquid (see L. Robeson et al., WO 2015/164234), generating oxygen as an inhibitor by electrolysis (see I Craven et al., WO 2016/133759), and incorporating magnetically positionable particles to which the photoactivator is coupled into the polymerizable liquid (see J. Rolland, WO 2016/145182). After the intermediate three-dimensional object is formed, it is optionally washed, optionally dried (e.g., air dried) and/or rinsed (in any sequence). It is then further cured, preferably by heating (although further curing may in some embodiments be concurrent with the first cure, or may be by different mechanisms such as contacting to water, as described in US Patent No. 9,453, 142 to Rolland et al.).

3. WASHING.

Objects as described above can be washed in any suitable apparatus, preferably with a wash liquid as described herein.

Wash liquids that may be used to carry out the present invention include, but are not limited to, water, organic solvents, and combinations thereof (e.g., combined as co-solvents), optionally containing additional ingredients such as surfactants, chelants (ligands), enzymes, borax, dyes or colorants, fragrances, etc., including combinations thereof. The wash liquid may be in any suitable form, such as a solution, emulsion, dispersion, etc.

In some preferred embodiments, where the residual resin has a boiling point of at least

90 or 100 °C (e.g., up to 250 or 300 °C, or more), the wash liquid has a boiling point of at least 30 °C, but not more than 80 or 90 °C. Boiling points are given herein for a pressure of 1 bar or 1 atmosphere.

Examples of organic solvents that may be used as a wash liquid, or as a constituent of a wash liquid, include, but are not limited to, alcohol, ester, dibasic ester, ketone, acid, aromatic, hydrocarbon, ether, dipolar aprotic, halogenated, and base organic solvents, including combinations thereof. Solvents may be selected based, in part, on their environmental and health impact (see, e.g., GSK Solvent Selection Guide 2009).

Examples of alcohol organic solvents that may be used in the present invention include, but are not limited to, aliphatic and aromatic alcohols such as 2-ethyl hexanol, glycerol, cyclohexanol, ethylene glycol, propylene glycol, di-propylene glycol, 1,4-butanediol, isoamyl alcohol, 1,2-propanediol, 1,3 -propanediol, benzyl alcohol, 2-pentanol, 1-butanol, 2-butanol, methanol, ethanol, t-butanol, 2-propanol, 1-propanol, 2-methoxyethanol, tetrahydrofuryl alcohol, benzyl alcohol, etc., including combinations thereof. In some embodiments, a C1-C6 or C1-C4 aliphatic alcohol is preferred.

Examples of ester organic solvents that may be used to carry out the present invention include, but are not limited to, t-butyl acetate, n-octyl acetate, butyl acetate, ethylene carbonate, propylene carbonate, butylenes carbonate, glycerol carbonate, isopropyl acetate, ethyl lactate, propyl acetate, dimethyl carbonate, methyl lactate, ethyl acetate, ethyl propionate, methyl acetate, ethyl formate etc., including combinations thereof.

Examples of dibasic ester organic solvents include, but are not limited to, dimethyl esters of succinic acid, glutaric acid, adipic acid, etc., including combinations thereof.

Examples of ketone organic solvents that may be used to carry out the present invention include, but are not limited to, cyclohexanone, cyclopentanone, 2-pentanone, 3-pentanone, methylisobutyl ketone, acetone, methylethyl ketone, etc., including combinations thereof.

Examples of acid organic solvents that may be used to carry out the present invention include, but are not limited to, propionic acid, acetic anhydride, acetic acid, etc., including combinations thereof.

Examples of aromatic organic solvents that may be used to carry out the present invention include, but are not limited to, mesitylene, cumene, p-xylene, toluene, benzene, etc., including combinations thereof.

Examples of hydrocarbon (i.e., aliphatic) organic solvents that may be used to carry out the present invention include, but are not limited to, cis-decalin, ISOPAR™ G, isooctane, methyl cyclohexane, cyclohexane, heptane, pentane, methylcyclopentane, 2-methylpentane, hexane, petroleum spirit, etc., including combinations thereof.

Examples of ether organic solvents that may be used to carry out the present invention include, but are not limited to, di(ethylene glycol), ethoxybenzene, tri(ethylene glycol), sulfolane, DEG monobutyl ether, anisole, diphenyl ether, dibutyl ether, t-amyl methyl ether, t-butylmethyl ether, cyclopentyl methyl ether, t-butyl ethyl ether, 2-methyltetrahydrofuran, diethyl ether, bis(2-methoxyethyl) ether, dimethyl ether, 1,4-dioxane, tetrahydrofuran, 1,2-dimethoxyethane, diisopropyl ether, etc., including combinations thereof.

Examples of dipolar aprotic organic solvents that may be used to carry out the present invention include, but are not limited to, dimethylpropylene urea, dimethyl sulphoxide, formamide, dimethyl formamide, N-methylformamide, N-methyl pyrrolidone, propanenitrile, dimethyl acetamide, acetonitrile, etc., including combinations thereof.

Examples of halogenated organic solvents that may be used to carry out the present invention include, but are not limited to, 1,2-dichlorobenzene, 1,2,4-trichlorobenzene, chlorobenzene, trichloroacetonitrile, chloroacetic acid, trichloroacetic acid, perfluorotoluene, perfluorocyclohexane, carbon tetrachloride, dichloromethane, perfluorohexane, fluorobenzene, chloroform, perfluorocyclic ether, trifluoroacetic acid, trifluorotoluene, 1,2-dichloroethane, 2,2,2-trifluoroethanol, etc., including combinations thereof.

Examples of base organic solvents that may be used to carry out the present invention include, but are not limited to, Ν,Ν-dimethylaniline, triethylamine, pyridine, etc., including combinations thereof.

Examples of other organic solvents that may be used to carry out the present invention include, but are not limited to, nitromethane, carbon disulfide, etc., including combinations thereof.

Examples of surfactants include, but are not limited to, anionic surfactants (e.g., sulfates, sulfonates, carboxylates, and phosphate esters), cationic surfactants, zwitterionic surfactants, nonionic surfactants, etc., including combinations thereof. Common examples include, but are not limited to, sodium stearate, linear alkylbenzenesulfonates, lignin sulfonates, fatty alcohol ethoxylates, alkylphenol ethoxylates, etc., including combinations thereof. Numerous additional examples of suitable surfactants are known, some of which are described in US Patent Nos. 9,198,847, 9,175,248, 9,121,000, 9,120,997, 9,095,787,

9,068,152, 9,023,782, and 8,765,108.

Examples of chelants (chelating agents) include, but are not limited to, ethylenediamine tetraacetic acid, phosphates, nitrilotriacetic acid (NTA), citrates, silicates, and polymers of acrylic and maleic acid.

Examples of enzymes that may be included in the wash liquid include, but are not limited to, proteases, amylases, lipases, cellulases, etc., including mixtures thereof. See, e.g.,

US Patent Nos. 7,183,248 and 6,063,206.

In some embodiments, the wash liquid can be an aqueous solution of ethoxylated alcohol, sodium citrate, tetrasodium N,N-bis(carboxymethyl)-L-glutamate, sodium carbonate, citric acid, and isothiazolinone mixture. One particular example thereof is SIMPLE

GREEN® all purpose cleaner (Sunshine Makers Inc., Huntington Beach, California, USA), used per se or mixed with additional water.

In some embodiments, the wash liquid can be an aqueous solution comprised of of 2- butoxyethanol, sodium metasilicate, and sodium hydroxide. One particular example thereof is PURPLE POWER™ degreaser/cleaner (Aiken Chemical Co., Greenville, South Carolina,

USA), used per se or mixed with additional water. In some embodiments, the wash liquid can be ethyl lactate, alone or with a co-solvent. One particular example thereof is BIO-SOLV™ solvent replacement (Bio Brands LLC, Cinnaminson, New Jersey, USA), used per se or mixed with water.

In some embodiments, the wash liquid consists of a 50:50 (volume:volume) solution of water and an alcohol organic solvent such as isopropanol (2-propanol).

Examples of hydrofiuorocarbon solvents that may be used to carry out the present invention include, but are not limited to, 1,1, 1,2,3,4,4,5, 5, 5-decafluoropentane (Vertrel® XF, DuPont™ Chemours), 1,1,1,3,3-pentafluoropropane, 1,1,1,3,3-pentafluorobutane, etc.

Examples of hydrochlorofluorocarbon solvents that may be used to carry out the present invention include, but are not limited to, 3,3-dichloro-l,l,l,2,2-pentafluoropropane, l,3-dichloro-l,l,2,2,3-pentafluoropropane, 1,1-dichloro-l-fluoroethane, etc., including mixtures thereof.

Examples of hydrofluoroether solvents that may be used to carry out the present invention include, but are not limited to, methyl nonafluorobutyl ether (HFE-7100), methyl nonafluoroisobutyl ether (HFE-7100), ethyl nonafluorobutyl ether (HFE-7200), ethyl nonafluoroisobutyl ether (HFE-7200), l,l,2,2-tetrafluoroethyl-2,2,2-trifluoroethyl ether, etc., including mixtures thereof. Commercially available examples of this solvent include 3M™Novec™ 7100 and 3M™Novec™ 7200 (3M™) engineered fluid (3M, St. Paul, Minnesota, USA).

Examples of volatile methylsiloxane solvents that may be used to carry out the present invention include, but are not limited to, hexamethyldisiloxane (OS- 10, Dow Corning), octamethyltrisiloxane (OS-20, Dow Corning), decamethyltetrasiloxane (OS-30, Dow Corning), etc., including mixtures thereof.

Other siloxane solvents (e.g., NAVSOLVE™ solvent) that may be used to carry out the present invention include but are not limited to those set forth in US Patent No. 7,897,558.

In some embodiments, the wash liquid comprises an azeotropic mixture comprising, consisting of, or consisting essentially of a first organic solvent (e.g., a hydrofiuorocarbon solvent, a hydrochlorofluorocarbon solvent, a hydrofluoroether solvent, a methylsiloxane solvent, or a combination thereof; e.g., in an amount of from 80 or 85 to 99 percent by weight) and a second organic solvent (e.g., a C1-C4 or C6 alcohol such as methanol, ethanol, isopropanol, fert-butanol, etc.; e.g., in an amount of from 1 to 15 or 20 percent by weight). Additional ingredients such as surfactants or chelants may optionally be included. In some embodiments, the azeotropic wash liquid may provide superior cleaning properties, and/or enhanced recyclability, of the wash liquid. Additional examples of suitable azeotropic wash liquids include, but are not limited to, those set forth in US Patent Nos. 6,008,179, 6,426,327, 6,753,304, 6,288,018, 6,646,020, 6,699,829, 5,824,634, 5,196,137, 6,689,734, and 5,773,403, the disclosures of which are incorporated by reference herein in their entirety.

When the wash liquid includes ingredients that are not desired for carrying into the further curing step, in some embodiments the initial wash with the wash liquid can be followed with a further rinsing step with a rinse liquid, such as water (e.g., distilled and/or deionized water), or a mixture of water and an alcohol such as isopropanol.

4. FURTHER CURING.

After washing, the object is in some embodiments further cured, preferably by heating or baking.

Heating may be active heating (e.g., in an oven, such as an electric, gas, solar oven or microwave oven, heated bath, or combination thereof), or passive heating (e.g., at ambient (room) temperature). Active heating will generally be more rapid than passive heating and in some embodiments is preferred, but passive heating— such as simply maintaining the intermediate at ambient temperature for a sufficient time to effect further cure— is in some embodiments preferred.

In some embodiments, the heating step is carried out at at least a first (oven) temperature and a second (oven) temperature, with the first temperature greater than ambient temperature, the second temperature greater than the first temperature, and the second temperature less than 300 °C (e.g., with ramped or step-wise increases between ambient temperature and the first temperature, and/or between the first temperature and the second temperature).

For example, the intermediate may be heated in a stepwise manner at a first temperature of about 70°C to about 150°C, and then at a second temperature of about 150°C to 200 or 250 °C, with the duration of each heating depending on the size, shape, and/or thickness of the intermediate. In another embodiment, the intermediate may be cured by a ramped heating schedule, with the temperature ramped from ambient temperature through a temperature of 70 to 150 °C, and up to a final (oven) temperature of 250 or 300 °C, at a change in heating rate of 0.5°C per minute, to 5 °C per minute. (See, e.g., US Patent No. 4,785,075). In some embodiments, the heating step is carried out in an inert gas atmosphere. Inert atmosphere ovens are known, and generally employ an atmosphere enriched in nitrogen, argon, or carbon dioxide in the oven chamber. Suitable examples include but are not limited to those available from Grieve Corporation, 500 Hart Road Round Lake, Illinois 60073-2898 USA, Davron Technologies, 4563 Pinnacle Lane, Chattanooga, TN 37415 USA, Despatch Thermal Processing Technology, 8860 207th Street, Minneapolis, MN 55044 USA, and others.

In other embodiments, the heating step is carried out in an inert liquid bath. Suitable inert liquids may be aqueous liquids (i.e., pure water, salt solutions, etc.), organic liquids (e.g., mineral oil, fluorinated, perfluorinated, and polysiloxane organic compounds such as perfluorohexane, perfluoro(2-butyl-tetrahydrofurane), perfluorotripentylamine, etc. (commercially available as PERFLUORTNERT® inert liquids from 3M Company), and mixtures thereof. These inert liquids can be deoxygenated, if necessary, such as by bubbling an inert gas such as nitrogen through the liquid, by boiling the inert liquid, by mixing oxygen- scavenging agents with the inert liquid medium (or contacting them to one another), etc., including combinations thereof (see, e.g., US Patent No. 5,506,007).

In some embodiments, the further curing or heating step (whether carried out in a liquid or gas fluid) is carried out at an elevated pressure (e.g., elevated sufficiently to reduce volatilization or out-gassing of residual monomers, prepolymers, chain extenders, and/or reactive diluents, etc.). Suitable pressure ranges are from 10 or 15 psi to 70 or 100 psi, or more.

The present invention is explained in greater detail in the following non-limiting examples. EXAMPLE 1

Shrinkage Changes based on Baking Conditions

To investigate causes of shrinkage of a dual cure polyurethane additive manufacturing resin during the second "bake" cure of a green object produced from that resin, RPU70 resin (available from Carbon, Inc., Redwood City, California, USA) was supplemented with 0.5 percent by weight of l,l-bis(fert-amylperoxy)cyclohexane (specifically, LUPEROX® 531M80 free radical thermal initiator, Arkema, available from Millipore Sigma). The objects shown in Figure 1 were produced together on a Carbon Inc. Ml additive manufacturing apparatus and washed together, in accordance with conventional procedures. The two objects weighed the same after washing and before baking. The object on the left was baked first in an inert nitrogen atmosphere, then baked a second time in air to remove residual unpolymerized chain extender (isobornyl methacrylate; IBOMA), with both bake steps otherwise for conventional times and temperatures. The object on the right was baked in air in accordance with conventional procedures. Note the object on the left lost less volume and mass, even though it was baked longer overall.

These results indicate that ambient oxygen is inhibiting free radical polymerization in the part during the bake step, resulting in mass loss due to volatilization of unreacted IBOMA chain extender during the bake step. It appears that free radicals are lost after the initial production and wash steps, and that the added thermal initiator re-initiates free-radical polymerization during the bake step, thereby incorporating the IBOMA into the polymer network of the object.

EXAMPLE 2

Differential Mass Loss Causes Warping

Three test parts in the shape of a planar grid were additively produced in the same shape, then washed, and then baked, all in like manner as those in Example 1 above, but with variations in the production of each part as described below.

A first part was produced from RPU70 resin without added thermal initiator, and baked in air. Substantial warping was seen.

A second part was produced from RPU70 resin with 0.5 percent by weight thermal initiator added, and baked in air. Slightly less warping was seen as compared to the first part.

A third part was produced from RPU70 resin with 0.5 percent by weight thermal initiator added, and then baked in an inert nitrogen atmosphere. Considerably less warping was seen as compared to the first and second parts.

These results indicate that the differential mass loss resulting from volatilized chain extender results in warping, and that warping can be reduced by inclusion of a thermal free radical initiator, and further reduced by baking in an inert atmosphere. EXAMPLE 3

Influence of Time between Wash and Bake on Warping

Four test parts in the shape of a planar grid were made at the same time and in the same shape, on a Carbon Inc. Ml additive manufacturing resin, with Carbon Inc. RPU70 resin supplemented with 0.5% LUPEROX® thermal free radical initiator, and washed at the same time in accordance with conventional procedures. The parts were then baked for the same times and temperatures in accordance with conventional procedures, but differing as follows:

The first test part was baked in air. Conventional shrinkage resulting in warping was observed.

The second test part was baked in an inert nitrogen atmosphere immediately after being washed. The part appeared to bulge slightly, but was still superior to the first test part.

The third test part was allowed to rest in ambient air for several hours after wash, and then baked in an inert nitrogen atmosphere. This part exhibited little if any warping and was significantly improved as compared to the first test part.

The fourth test part was allowed to rest in ambient air over night, and then baked in an inert nitrogen atmosphere. This part exhibited slight warping, but was still superior to the first test part.

The foregoing is illustrative of the present invention, and is not to be construed as limiting thereof. The invention is defined by the following claims, with equivalents of the claims to be included therein.

Claims

We claim:

1. A polymerizable liquid useful for the production of a three-dimensional object by additive manufacturing, said polymerizable liquid comprising a mixture of:

(a) at least one free-radically polymerizable constituent;

(b) a free radical photoinitiator; and

(c) a free radical thermal initiator.

2. The liquid of claim 1, useful for the production of an object comprised of polyurethane, polyurea, or a copolymer thereof, said polymerizable liquid comprising a mixture of:

(a) said at least one free-radically polymerizable constituent selected from the group consisting of: (i) a blocked or reactive blocked prepolymer, (ii) a blocked or reactive blocked polyisocyanate, (Hi) a blocked or reactive blocked polyisocyanate chain extender, and (iv) combinations of two or three of the foregoing (e.g., in a combined amount of from 5 to 90 percent by weight),

(b) optionally at least one additional chain extender (e.g., in an amount of from 1 or 5 to 30 percent by weight when present),

(c) the free radical photoinitiator (e.g., in an amount of from 0.1 to 4 percent by weight),

(d) the free radical thermal initiator (e.g., in an amount of from 0.1 to 4 percent by weight),

(e) optionally a polyol and/or a polyamine (e.g., in an amount of from^5 to 90 percent by weight), and

(f) optionally a reactive diluent (e.g., included in an amount of from 1 to 40 percent by weight when present).

3. The liquid of any preceding claim, wherein said at least one free-radically polymerizable constituent comprises a blocked or reactive blocked prepolymer (e.g., in an amount of from 5 to 70, 80, or 90 percent by weight).

4. The liquid of claim 3, wherein said blocked or reactive blocked prepolymer comprises a compound of the formula A-X-A, where X is a hydrocarbyl group and each A is an independently selected substituent of Formula X:

where R is a hydrocarbyl group, R is O or NH, and Z is a blocking group (e.g., a group having a reactive epoxy, alkene, alkyne, or thiol terminal group).

5. The liquid of any preceding claim, wherein said at least one free-radically polymerizable constituent comprises a blocked or reactive blocked polyisocyanate (e.g., in an amount of from 5 to 70, 80, or 90 percent by weight).

6. The liquid of claim 5, said blocked or reactive blocked polyisocyanate comprising a compound of the formula A'-X'-A', where X' is a hydrocarbyl group and each A' is an independently selected substituent of Formula (Χ'):

7. The liquid of any preceding claim, wherein said at least one free-radically polymerizable constituent comprises a blocked or reactive blocked diisocyanate chain extender (e.g., in an amount of from 5 to 70, 80, or 90 percent by weight).

8. The liquid of claim 7, said reactive blocked polyisocyanate chain extender comprising a compound of the formula A"-X"-A", where X" is a hydrocarbyl group, and each A" is an independently selected substituent of Formula (X"):

where R is a hydrocarbyl group, R' is O or NH, and Z is a blocking group (e.g., the blocking group having a reactive epoxy, alkene, alkene, or thiol terminal group).

9. The liquid of any preceding claim, wherein said at least one additional chain extender comprises at least one diol, diamine or dithiol chain extender.

10. The liquid of any preceding claim, wherein said reactive diluent comprises an acrylate, a methacrylate, a styrene, an acrylic acid, a vinylamide, a vinyl ether, a vinyl ester, polymers containing any one or more of the foregoing, and combinations of two or more of the foregoing.

11. The liquid of any preceding claim, further comprising:

(g) at least one non-reactive light absorbing pigment or dye (e.g., titanium dioxide, carbon black, and/or an organic ultraviolet light absorber) (e.g., in an amount of from 0.001 to 10 percent by weight, when present); and/or

(h) a filler (e.g., in an amount of from 1 to 50 percent by weight, when present).

12. The liquid of any preceding claim, wherein said free radical photoinitiator comprises a type I free radical photoinitiator, and/or a type II free radical photoinitiator.

13. The liquid of any preceding claim, wherein said free radical photoinitiator is selected from group consisting of: diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO), diphenylphosphinyl(2,4,6-trimethylphenyl)methanone; benzophenone; substituted benzophenones; acetophenone; substituted acetophenones; benzoin; benzoin alkyl esters; xanthone; substituted xanthones; diethoxy-acetophenone; benzoin methyl ether; benzoin ethyl ether; benzoin isopropyl ether; diethoxyxanthone; chloro-thio-xanthone; N-methyl diethanol- amine-benzophenone; 2-hydroxy-2-methyl- 1 -phenyl-propan- 1 -one; 2 -benzyl -2- (dimethylamino)-l-[4-(4-morpholinyl)phenyl]-l-butanone; and mixtures thereof.

14. The liquid of any preceding claim, wherein said thermal free radical initiator is selected from the group consisting of l ,l-bis(tert-amylperoxy)cyclohexane, tert-amyl peroxybenzoate, 4,4-azobis(4-cyanovaleric acid), 2,2'-azobisisobutyronitrile (AIBN), benzoyl peroxide, 2,2-bis(ter/-butylperoxy)butane, l,l-bis(tert-butylperoxy)cyclohexane, 2,5- bis(tert-butylperoxy)-2,5-dimethylhexane, 2,5-bis(tert-butylperoxy)2,5-dimethyl-3-hexyne, bis( 1 -(tert-butylperoxy)- 1 -methylethyl)benzene, 1 , 1 -bis (tert-butylperoxy)-3 ,3 ,5 -trimethyl- cyclohexane, tert-butyl hydroperoxide, tert-butyl peracetate, tert-butyl peroxide, tert-butyl peroxybenzoate, tert-butylperoxy isopropyl carbonate, cumene hydroperoxide, cyclohexanone peroxide, dicumyl peroxide, lauroyl peroxide, 2,4-Pentanedione peroxide, peracetic acid, potassium persulfate, and combinations thereof.

15. The liquid of any preceding claim, further comprising an accelerator (e.g., a deoxygenating agent).

16. The liquid of claim 15, wherein said accelerator comprises triphenylphosphine.

17. A method of making a three-dimensional object, comprising:

(a) producing by stereolithography (e.g., by continuous liquid interface production) a three-dimensional object from a polymerizable liquid of any preceding claim;

(b) washing said object; and then

(c) optionally, further curing said object.

18. The method of claim 17, wherein said further curing step is included and is carried out by heating.

19. The method of claim 18, wherein said heating step is carried out in an inert fluid atmosphere (e.g., in an inert gas, such as nitrogen, argon, carbon dioxide, etc., or in inert liquid, such as an aqueous bath (e.g., distilled water; salt water, etc.) an organic liquid bath (e.g., mineral oil, inert fluorinated organic), etc.).

20. The method of claim 19, wherein said fluid comprises an inert liquid, and said method further comprises the step of deoxygenating said inert liquid.

21. The method of any one of claims 17-20, wherein said further curing or heating step is carried out at an elevated pressure (e.g., elevated sufficiently to reduce volatization or out-gassing of residual monomers, prepolymers, chain extenders, and/or reactive diluents, etc.) (e.g., 10 or 15 psi to 70 or 100 psi).

22. The method of any one of claims 17-21, wherein said washing step is carried out with a wash liquid comprising an organic solvent.

23. The method of claim 22, wherein said wash liquid further comprises a thermal free-radical initiator (e.g., selected from l ,l-bis(tert-amylperoxy)cyclohexane, tert-a yl peroxybenzoate, 4,4-azobis(4-cyanovaleric acid), 2,2'-azobisisobutyronitrile (AIBN), benzoyl peroxide, 2,2-bis(ter/-butylperoxy)butane, l,l-bis(/ert-butylperoxy)cyclohexane, 2,5- bis(tert-butylperoxy)-2,5-dimethylhexane, 2,5-bis(tert-butylperoxy)2,5-dimethyl-3-hexyne, bis( 1 -(tert-butylperoxy)- 1 -methylethyl)benzene, 1 , 1 -bis (ter/-butylperoxy)-3 ,3 ,5 -trimethyl- cyclohexane, tert-butyl hydroperoxide, tert-butyl peracetate, tert-butyl peroxide, tert-butyl peroxybenzoate, tert-butylperoxy isopropyl carbonate, cumene hydroperoxide, cyclohexanone peroxide, dicumyl peroxide, lauroyl peroxide, 2,4-Pentanedione peroxide, peracetic acid, potassium persulfate, and combinations thereof) (e.g., included in said wash liquid an amount of from 0.1 to 5 percent by weight).