SURFACE PROPERTIES MODIFICATION OF DIGITALLY MANUFACTURED ARTICLES VIA REACTIVE SILICON-CONTAINING PRECURSOR POLYMERS

Abstract

Methods for forming a coated digitally manufactured part include forming an article by a digital manufacturing method; coating a surface of the article with a reactive silicon-containing precursor polymer; and treating the polymer to form a silica-containing coating, thereby forming the coated digitally manufactured part. An article includes a digitally manufactured part having surface striations; and a coating encapsulating the digitally manufactured part and comprising silica. An article includes a digitally manufactured part (i) formed by selective lase sintering, (ii) comprising a surface defined by coalesced particles, and (iii) having a surface roughness R.sub.a of at least 0.1 microns; and a coating encapsulating the part and comprising silica. A composition comprising polysilazane is described.

Claims

1. A method for forming a coated digitally manufactured part, the method comprising: forming an article by a digital manufacturing method; coating a surface of the article with a reactive silicon-containing precursor polymer; and treating the polymer to form a silica-containing coating, thereby forming the coated digitally manufactured part.

2. The method of claim 1, wherein the digital manufacturing method comprises at least one of additive manufacturing, or laminated object manufacturing.

3. The method of claim 2, wherein additive manufacturing comprises at least one of vat polymerization, material extrusion, binder jetting, powder bed fusion, or material jetting.

4. The method of claim 2, wherein laminated object manufacturing comprises at least one of selective deposition lamination, wet lamination, dry lamination, wax lamination, or solventless lamination.

5. The method of claim 2, wherein forming the article further comprises subtractive manufacturing.

6. The method of claim 5, wherein subtractive manufacturing comprises at least one of CNC milling, laser engraving, or negative photoresist polymer lithography.

7. The method of claim 1, wherein the article comprises a material selected from the group consisting of metal, polymer, dielectric, ceramic, semiconductor, and combinations thereof.

8. The method of claim 1, wherein coating the surface comprises at least one of vapor deposition, solution deposition, or application of solid material.

9. The method of claim 1, wherein the reactive silicon-containing precursor polymer comprises at least one of a silazane-containing polymer or a siloxane-containing polymer.

10. The method of claim 1, wherein the reactive silicon-containing precursor polymer comprises a solid powder.

11. The method of claim 1, wherein the reactive silicon-containing precursor polymer comprises a liquid.

12. The method of claim 1, wherein the reactive silicon-containing precursor polymer comprises polysilazane dissolved in a solvent.

13. The method of claim 12, wherein the reactive silicon-containing precursor polymer comprises a second solvent.

14. The method of claim 13, wherein the second solvent comprises at least one of a reactive silicone and an unreactive silicone.

15. The method of claim 1, wherein the reactive silicon-containing precursor polymer comprises at least one of reactive or unreactive silicone.

16. The method of claim 1, wherein the reactive silicon-containing precursor polymer comprises a particulate additive.

17. The method of claim 16, wherein the particulate additive comprises at least one of silica-, ceramic-, or metal-oxide based particles.

18. The method of claim 16, wherein the particulate additive comprises silica spheres, chopped glass fibers, or combinations thereof.

19. The method of claim 1, wherein the reactive silicon-containing precursor polymer comprises a chemistry modifier.

20. The method of claim 19, wherein the chemistry modifier comprises functional silane.

21. The method of claim 1, wherein the reactive silicon-containing precursor polymer comprises a pigment.

22. The method of claim 21, wherein the pigment is selected from the group consisting of cobalt oxide, titanium dioxide, iron oxide, copper flake, and carbon black

23. The method of claim 1, wherein the reactive silicon-containing polymer comprises a rheology modifier.

24. The method of claim 1, wherein the coating has a thickness selected from a range of 1 nm to 500 microns.

25. The method of claim 24, wherein the coating has a thickness selected from a range of 250 nm to 75 microns.

26. The method of claim 25, wherein the coating has a thickness selected from a range of 1 micron to 50 microns.

27. The method of claim 1, wherein treating the polymer comprises exposing the polymer to an oxygen-containing humid environment for a predetermined time and temperature.

28. The method of claim 1, wherein the coating has a hardness of 4 GPa to 30 GPa.

29. The method of claim 1, wherein the coating has a surface roughness R.sub.a of 0.5-25 microns.

30. The method of claim 1, wherein the coating has a water wettability contact angle of 90-180°.

31. The method of claim 1, wherein the coating comprises silica.

32. An article comprising: a digitally manufactured part having surface striations; and a coating encapsulating the digitally manufactured part and comprising silica.

33. The article of claim 32, wherein the surface striations have an average height selected from a range of 250 nm to 100 microns.

34. The article of claim 32, wherein the coating has a thickness selected from a range of 1 nm to 500 microns.

35. The article of claim 34, wherein the coating has a thickness selected from a range of 250 nm micron to 75 microns.

36. The article of claim 35, wherein the coating has a thickness selected from a range of 1 micron to 50 microns.

37. The article of claim 32, wherein the coating has a hardness of 4 GPa-30 GPa.

38. The article of claim 32, wherein the coating has a surface roughness R.sub.a of 0.1 to 10 microns.

39. The article of claim 32, wherein the coating has a water wettability contact angle of 90°-180°.

40. An article comprising: a digitally manufactured part (i) formed by selective laser sintering, (ii) comprising a surface defined by coalesced particles, and (iii) having a surface roughness R.sub.a of at least 0.1 microns; and a coating encapsulating the digitally manufactured part and comprising silica.

41. A composition comprising: 1 wt % to 99 wt % polysilazane comprising perhydropolysilazane in di-n-butyl ether; and 1 wt % to 99 wt % a solvent comprising methyl ethyl ketone.

42. A composition comprising: 1 wt % to 99 wt % polysilazane comprising perhydropolysilazane in di-n-butyl ether; 1 wt % to 99 wt % solvent comprising methyl ethyl ketone; and 0.1 wt % to 15 wt % additive comprising bis(triethoxysilane) perfluoropolyether.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0040] FIG. 1 is a photograph of a part produced via DLP composite additive manufacturing (uncoated) and a photograph of a coated version of the same part, coated in accordance with an embodiment of the invention. The coated part has a glossier finish and more mechanical robustness.

[0041] FIG. 2 is a graph illustrating a typical reduction of the release force of an injected article from a 3D printed mold, coated in accordance with an embodiment of the invention, in comparison to an uncoated 3D printed mold.

[0042] FIG. 3 is a photograph of a coated injection mold tool insert, displaying a glossy sheen, manufactured in accordance with an embodiment of the invention.

[0043] FIG. 4 is a schematic diagram illustrating a structure of a digitally manufactured part before and after being coated in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

[0044] As used herein, digital manufacturing means translation of a digital design to a physical object by way of a printing process (layer-by-layer). Digital manufacturing includes 3D printing.

[0045] As used herein, reactive silicon-containing precursor polymer means a polymer or polymer solution that contains silicon atoms in the structure of the polymer and is adapted to react to form a new material.

[0046] As used herein, silane means any derivative of an inorganic compound with four substituents on silicon.

[0047] As used herein, siloxane means a molecule containing a silicon atom bonded to an oxygen atom bonded to another silicon atom.

[0048] As used herein, silazane means a polymer in which silicon and nitrogen atoms alternate to form the basic backbone.

[0049] Many metals, glass, ceramics or plastics with OH groups on the surface are easily wetted by polysilazanes. Reaction of Si—N with OH leads to the formation of Si—O-metal bonds generating good adhesion of the coating to the substrate. The free surface of the coating can react with humidity thereby creating a siloxane-like structure with excellent easy to clean properties.

##STR00001##

Polysilazane Adhesion on Polar Surface (Wikipedia—Polysilazane)

[0050] A family of coatings for 3D printed articles is described. These coatings may be applied to different materials, such as metal, polymer, dielectric, ceramic, and/or semiconductor, and may be used for various applications, such as for coating 3D printed injection molding tools and tool inserts, based on reactive silicon-containing precursor polymer, such as polysilazanes-containing polymer or siloxane-containing polymer, combined with a variety of optional additives to impart different characteristics to the printed part. The coating of 3D printed parts enhances the surface properties, such as lowering the surface roughness, altering the aesthetics, and enhancing the thermomechanical properties, especially for injection molding applications. Enhancements to the thermomechanical properties include, but are not limited to, increased strength, increased stiffness, mechanical fatigue resistance, reduced surface roughness (e.g. striations, defects), coefficient of thermal expansion (CTE) reduction, increased thermal fatigue resistance and others.

[0051] Embodiments of the described methods and articles may provide one or more of the following: [0052] The reactive silicon-containing precursor polymer deposits a silica or silica-like precursor layer onto digitally manufactured, e.g., 3D printed articles, and after reaction with oxygen-containing reactants, leaves a glossier, smoother and harder finish then may otherwise exist. This is applicable to digitally manufactured articles made of metals, polymers, dielectrics, ceramics, ceramic-filled polymers, semiconductors, and/or combinations thereof. [0053] When applied to injection molding tooling, these coating can appreciably aid the mold release process by (1) serving as a permanent or semi-permanent release agent, (2) reducing the surface energy and associated release force of an injected polymer from the mold, and (3) reducing the surface roughness from the printing process to further reduce the required release force. [0054] This coating makes the surface of the 3D printed part more durable. When applied to injection molding tooling this durability can result in a higher number of injection runs per mold before failure, which is a very significant result in the industry. [0055] This coating makes the surface of the 3D printed part smoother. When applied to injection molding tooling this smoothness can result in smoother injection molded parts generated from this tooling. Surface finish of this injection molding parts is critical to customers. [0056] This coating makes the surface of, e.g., SLA/DLP printed parts maintain high resolution and integrity at higher temperatures, minimizing CTE effects. When applied to injection molding tooling this high temperature stability can result in better maintaining the mold geometry at the higher temperatures experienced during the injection of molten polymer.

[0057] The coating of silica onto digitally printed parts may enhance the surface properties, including lowering the surface roughness, altering the aesthetics, and enhancing the thermomechanical properties, e.g., for injection molding applications.

[0058] Suitable coatings may be formed by reacting reactive silicon-containing precursor polymers with oxygen and water in the air to form coatings of silica; suitable silica precursor materials are, for example, polysilazanes, perhydropolysilazanes, polydimethyldisilazane, and organic precursors containing di- and/or trialkoxysilazane moieties.

[0059] The coatings described herein may be applied to 3D printed parts made of metal, polymer, dielectric, ceramic, semiconductor and/or combinations thereof. The reactive silicon-containing precursor-based coatings can improve the durability of 3D printed parts, especially at high temperature, by creating a covalently bonded and fully integrated glass-like layer that encompasses the printed part. Further, these coatings work to smooth the surface of a printed part which reduces surface-based stress concentrations during usage as well as alters aesthetics by making the part appear visually glossier. A glossy finish is often important with 3D printed parts for aesthetics with many 3D printing processes relying on solvent-based or thermal-based processes to try to smooth a printed part with time and energy consuming post-processing steps.

[0060] Referring to FIG. 1, such coating materials can achieve high adhesion to the surface of 3D printed parts. As illustrated, part 10 is formed by DLP composite additive manufacturing and is uncoated. Part (b) is a coated version of the same part, coated in accordance with an embodiment of the invention. Part 10′ is visibly smoother with a glossier finish, as well as with more mechanical robustness.

[0061] In some embodiments, these reactive silicon-containing precursor polymer-based coatings, e.g., silazane-based coatings, are useful for 3D printing injection molding tooling and tooling inserts. Specifically, silazane-coated 3D printed molds appreciably aid the mold release process by (1) serving as a permanent or semi-permanent release agent, (2) reducing the surface energy and associated release force of an injected polymer from the mold (shown in FIG. 2), and (3) reducing the surface roughness from the printing process to further reduce the required release force.

[0062] In particular, FIG. 2 illustrates how the release force of an injected article made by 3D printed mold decreases when the molding tooling has a polysilizane-based coating, in comparison to an uncoated molding tooling. To create the data in this figure, two identically printed mold inserts were tested for the strength required to demold the injected plastic part. One insert was coated with polysilazane and the other was left uncoated. The strength needed to demold the injected plastic part is much greater when using the uncoated insert instead of the coated insert, e.g., approximately 10 times greater.

[0063] Also, referring to FIG. 3, a coated injection mold tool insert 30 has a glossy sheen that is useful as a release agent.

[0064] Moreover, coating printed molds with the disclosed reactive silicon-containing precursor polymer-based coatings increases mold durability, which may result in a higher number of injection runs per mold before failure. Being able to increase the number of parts per mold increases the number of potential applications.

[0065] Further, 3D printed parts with coatings formed from disclosed reactive silicon-containing precursor polymers may be smoother when an overcoat is applied over the striations and other surface defects from the printing process. This smoothening also translates to injection molded parts generated from this tooling. Surface finish of this injection molding parts is critical to certain users. For example, users who want to injection mold aerodynamic parts are highly sensitive to surface roughness of the end part, as it greatly affects surface drag. Furthermore, polysilazanes-based coatings make the surface of SLA/DLP printed injection molding tooling more stable at higher temperatures. This stability can come in the form of higher modulus as well as reduction of surface stress concentrations during usage. This geometric and mechanical stability can result in better maintenance of the mold geometry during the injection of molten polymer.

[0066] The coating can also provide a hydrophilic or fluorous surface chemistry that is anti-stick to most relevant injection molding plastics. These interactions can be further enhanced with additional surface chemistry to the polysilazane coating including, but not limited to, silane chemistries. These silane chemistries can be incorporated into the polysilazane coating process or can be applied in a post-processing step.

[0067] In addition, once the polysilazanes-based coating is applied to the printed article, additional release agents can be used in tandem with the surface coating. For example, silicone-based release sprays can be additionally applied to a coated 3D printed mold to enhance the mold's performance by decreasing the release force and increasing the number of injection cycles before mold failure.

Details of Coating Process

[0068] In an embodiment, a 3D printed article may be coated as follows. An article, e.g., an injection mold tool, is formed by digital manufacturing, e.g. by additive manufacturing and/or laminated object manufacturing. Examples of additive manufacturing include vat polymerization (e.g., stereolithography), material extrusion (e.g., fused filament fabrication (FFF), binder jetting, powder bed fusion (e.g., selective laser sintering (SLS), and material jetting. Another example of digital manufacturing is laminated object manufacturing, e.g., selective deposition lamination, wet lamination, dry lamination, was lamination, and solventless lamination.

[0069] In some instances, subtractive manufacturing may be combined with additive manufacturing or laminated object manufacturing. Subtractive manufacturing includes, e.g., computer numerical control (CNC) milling, laser engraving, and negative photoresist polymer lithography.

[0070] The defining characteristics of articles formed by digital manufacturing include layer-by-layer deposition, or particle binding and sintering. These articles are clearly identifiable due to striations on the surface of the part, a coarse texture due to particle size, or unique internal features (lattices or channels, for example) that are only manufacturable via additive manufacturing. Surface striations may have an average height ranging from, e.g., 250 nm to 100 microns.

[0071] Any residue from formation of the article, i.e., the 3D printed injection mold tool, may be cleaned off with, e.g., organic solvent, soap, or an adequate mechanical removal method (such as wiping with a cloth or through ultrasonication). A surface of the article may then be coated with a reactive silicon-containing precursor polymer by, e.g., vapor deposition, solution deposition, and/or application of solid material, in accordance with embodiments of the invention.

[0072] The reactive silicon-containing precursor polymer may include, e.g., a silazane-containing polymer and/or a siloxane-containing polymer. It may include reactive and/or unreactive silicone. It may include a solid powder or a liquid. In some embodiments, it may include polysilazane dissolved in one or two solvents. For example, the first solvent may be 2-butanone and the second solvent may include a reactive silicone and/or an unreactive silicone.

[0073] The reactive silicon-containing precursor polymer may include a particulate additive, such as one or more of silica-, ceramic-, or metal-oxide bead particles. Examples of suitable additives include silica spheres, chopped glass, and fumed silica. Particulate additives generally serve two purposes. One purpose is to modify the theology of the coating to optimize for a particular coating process. Another purpose is to impart a particular texture to the tool, that one may not otherwise accomplish via printing conditions, to produce a specific texture on the final injection molded part.

[0074] The reactive silicon-containing precursor polymer may include a chemistry modifier, such as a functional silane, e.g., Fluorolink S10. A chemical modifier may be added for one or more several purposes. One purpose is to express a unique chemistry on the surface for modifying adhesion or Hansen-solubility parameter (wettability). Another purpose is to modify the bulk properties of the coating, especially crack resistance through thermal cycling. An example of this is a reactive siloxane polymer. A third purpose is to attribute a specific chemical signal, such as a non-reactive fluorophore.

[0075] The reactive silicon-containing precursor polymer may include a pigment, e.g., cobalt oxide, titanium dioxide, iron oxide, copper flake, and/or carbon black, thereby enabling determination of the color of the finished part.

[0076] The reactive silicon-containing precursor polymer may include a rheology modifier, e.g. fumed silica, to tailor the viscosity and rheology of the coating solution. Modification of the coating rheology enables optimization of coating thicknesses under different coating methods.

[0077] In an embodiment, the coating may be formed on the article by solution deposition. Here, the article's surface may be coated by, e.g., fully submerging the article into the coating solution described below. The article remains submerged for a suitable period of time, long enough to allow the ingress of solvent and polymer into the surface of the printed part on the sub-micron-scale e.g., 10-20 seconds, removed from the solution, and may then be drip dried with the aid of an air gun to rid the article of any drips or pools before drying completely. The reactive silicon-containing precursor polymer may then be treated to form a silica-containing coating, thereby forming a coated digitally manufactured part. For example, treating the polymer may include exposing the polymer to an oxygen-containing humid environment for a predetermined time and temperature. For example, the polymer may be dried at room temperature and cured at an elevated temperature, e.g., at 60-100° C. in a >25% humidity atmosphere for 2 hours.

[0078] Alternative coating methods may be used. For example, a spray coating may be applied to coat the digitally manufactured part. The spray coating may be best achieved with an airless spray device, and applied to achieve a thickness that is, for example, 1 nm to 500 microns, e.g., 250 nm to 75 microns or 1 micron to 50 microns.

[0079] The part may be subjected to high rotary forces, such as a high revolutions per minute (RPM) rotation, to remove excess coating solution, prior to drying.

[0080] Once a digitally manufactured part undergoes this process, its surface has a clear and glossy appearance, which increases hardness and decreases surface texture. The resulting coating is between a nanometer, e.g., tens of nanometers thick, up to 500 microns, e.g., 50 or 75 microns thick, and is clearly identifiable under a scanning electron microscope. Additional analysis by X-ray photoelectron spectroscopy (XPS), for example, allows for a composition analysis typically reveals a thin coating of SiO.sub.2 on the surface of the part, which is of a different composition than the body of the part.

[0081] A hardness of the coating may range from, e.g., 4 GPa to 30 GPa.

[0082] The coating may have a surface roughness R.sub.a of 0.5 microns to 25 microns, i.e., significantly less than striations typically present in digitally manufactured articles, which generally range from 35 microns-150 microns.

[0083] The coating may have a water wettability contact angle selected from a range of 0° to 180°. This range includes all possibilities, from poor adhesion to most materials (excellent for molding application and anti-fouling properties) to excellent adhesion to some materials (important for end use parts that may need to be glued to other components).

[0084] The coating may include silica. Silica can impart coating solution rheology modification, an engineered surface texture in the final cure coating, and/or impart a higher degree of scratch-resistance to the final coating.

[0085] Referring to FIG. 4, a digitally manufactured article 40, as printed, may include fibers 42 of, e.g., glass, ceramic, carbon, metal, silica, metal oxide, etc. disposed in a resin 44 (e.g., acrylate/methacrylate). The fibers extend beyond the surface of the resin and are exposed. These exposed fibers define striations 46 and may help anchor a silica coating through covalent bonding. After the digitally manufactured article 40 is coated to form a coated digitally manufactured part 40′ in accordance with embodiments of the invention, the coating 50 is anchored to the article and the striations are reduced in magnitude.

Preparation of Coating Solution

[0086] An exemplary coating solution is prepared by combining a perhydro-polysilazane solution (such as Durazane 2850 from Merck EMD which comes as a ˜22% solids solution in di-n-butyl ether) with a compatible solvent such as 2-butanone or methyl ethyl ketone to achieve a desired thickness of coating. The composition may include 1 to 99 wt % perhydro-polysilazane solution, and 1 to 99 wt % solvent; a suitable ratio of polysilazane to solvent may be about 1:1 by weight. In an embodiment, no dilution is made.

[0087] Additives to affect surface sheen, mechanical properties, wear properties, aesthetics and surface chemistry can be added at this time and mixed used a mechanical mixer. Examples of such additives are discussed above and may include, e.g., spherical silica with diameters in the range of 1-75 microns from Evonik to create texture, Fluorolink S10 from Solvay to develop a perfluorinated surface, and reactive silicone oils such as HMS-11 from Gelest to reduce the risk of cracking or brittle failure.

[0088] Another exemplary composition may include 1 wt % to 99 wt %, e.g., 2 wt % to 75 wt % polysilazane such as Durazane 2850; 0 wt % to 99 wt %, e.g., 1 wt % to 99 wt % solvent of methyl ethyl ketone; and 1 wt % to 15 wt %, e.g., 0.1 wt % to 10 wt % of an additive such as bis(triethoxysilane) perfluoropolyether (available as Fluorolink S10). A suitable ratio of polysilazane to solvent to additive may be about 6:5:1 by weight.

[0089] While the present invention has been described herein in detail in relation to one or more preferred embodiments, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for the purpose of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended to be construed to limit the present invention or otherwise exclude any such other embodiments, adaptations, variations, modifications or equivalent arrangements; the present invention being limited only by the claims appended hereto and the equivalents thereof.