SPHERICAL POLYMERIC PARTICLE CONTAINING GRAPHENE NANOPLATELETS AS THREE DIMENSIONAL PRINTING PRECURSOR

Abstract

A processes and precursor are provided for use in selective laser sintering (SLS) that can create uniform packing densities that create good prints of 3D articles with a decrease in voids and incomplete infill. The resulting articles are electrically conductivity owing to a graphene coating thereby rendering such articles amenable to electroplating, or electrostatic coating processes. The process and precursor provide small diameter filled polymeric materials for 3D printing that are commercially viable to produce an article in a cost effective manner that has superior properties compared to conventional parts owing to reduced void volume and less residual inter-particle stress. The distribution of particles is spherical in shape and have a mean size polydispersity that varies by less than 5% in diameter. As a result of the control of polydispersity, the particles have the attribute of spontaneously forming closed packed arrangements common to crystals.

Claims

1. A powder material for use in selective laser sintering comprising: a plurality of particles having a size distribution, each of said particles having a polymeric matrix containing a loading of graphene nanoplatelets.

2. The powder material of claim 1 wherein said plurality of particles are spherical or tear drop have a mean size polydispersity that varies by less than 20% and a diameter of between 2 to 200 microns.

3. The powder material of claim 1 wherein said plurality of particles have a mean size polydispersity that varies by between 0.1 and 5% in diameter

4. The powder material of claim 1 wherein said polymeric matrix is formed of: polyamide, polycarbonate, polystyrene, polyethylenes, polypropylenes, polyetherketones, polyetheretherketones, poly aryl ether ketones, and block copolymers in which any of the aforementioned polymers alone or in combination constitute more than 50% of polymer repeat units.

5. The powder material of claim 1 wherein the loading of said graphene nanoplatelets range from 0.001 to 50% volume percent.

6. The powder material of claim 1 wherein the loading of said graphene nanoplatelets have a maximal linear extent in the three orthogonal X-Y-Z directions of between 3 and 50 nm, and secondary linear extent to at least 20 percent of the maximal linear extent.

7. The powder material of claim 1 further comprising a second type of particles that are sized to fit within interstitial spaces between contiguous particles of said plurality of particles.

8. A process of forming the powder material of claim 1 wherein said plurality of particles are produced by melt spraying.

9. A process of forming the powder material of claim 1 wherein said plurality of particles are produced by mechanical separation or classification.

10. A process of forming the powder material of claim 1 wherein said plurality of spherical particles are produced by microwave-atomized drying.

11. A process of forming an article comprising: exposing the powder material of claim 1 to selective laser sintering conditions; allowing sufficient time under the selective laser sintering conditions to induce fusion between two contiguous particles of said powder material to form a fused mass; repeating the exposing and allowing steps with positional variation to fuse additional particles to the fused mass to form the article.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The present invention is further detailed with respect to the following drawings that are intended to show certain aspects of the present of invention, but should not be construed as limit on the practice of the invention, wherein:

[0012] FIG. 1 is a scanning electron microscope (SEM) of a prior art asymmetric polymeric composite material (PA-12) currently used in selective laser sintered (SLS) polymeric 3D printing; and

[0013] FIG. 2 is a schematic of an inventive particle tailored for usage as a 3D printing precursor.

DETAILED DESCRIPTION

[0014] The present invention has utility as improved processes and precursor for use in selective laser sintering (SLS) that can create uniform packing densities that create good prints of 3D articles with a decrease in voids and incomplete infill. In some inventive embodiments, the resulting articles are electrically conductivity owing to the graphene coating thereby rendering such articles amenable to electroplating, or electrostatic coating processes. Embodiments of the invention provide small diameter filled polymeric materials for 3D printing that are commercially viable to produce an article in a cost effective manner that has superior properties compared to conventional parts owing to reduced void volume and less residual inter-particle stress. An inventive distribution of inventive particles is spherical in shape and have a mean size polydispersity that varies by less than 5% in diameter. In some inventive embodiments, the mean size polydispersity that varies by between 0.1 and 3% in diameter. In some inventive embodiments, the plurality of particles have a mean size polydispersity that varies by between 0.1 and 5% in diameter. As a result of the control of polydispersity, inventive particles have the attribute of spontaneously forming closed packed arrangements common to crystals.

[0015] It is to be understood that in instances where a range of values are provided herein, that the range is intended to encompass not only the end point values of the range, but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4.

[0016] Embodiments of the invention utilize sonication, melt spraying and microwave-atomized dried polymer to produce micron and sub-micron particles with a controlled spherical geometry that create an ideal printing system where the particle to particle distance is at a minimal threshold. Methods of formation of monodisperse polymer particles are known to the art. Kim, Shin-Hyun, et al. Microwave-assisted self-organization of colloidal particles in confining aqueous droplets. Journal of the American Chemical Society 128.33 (2006): 10897-10904; Wagner, Claudia S., Yan Lu, and Alexander Wittemann. Preparation of submicrometer-sized clusters from polymer spheres using ultrasonication. Langmuir 24.21 (2008): 12126-12128; and Kappler, P., et al. Size and morphology of particles generated by spraying polymer-melts with carbon dioxide. Sixth International Symposium on Supercritical Fluids: April 28-30 2003; Versailles. Institut National Polytechnique de Lorraine, 2003. In addition, control of the particle dispersity provides an added benefit to the process that allows for a better balance between melt-flow, conductive and convective processes that are occurring simultaneously during the print.

[0017] It is appreciated that the particle morphology is readily controlled as to a variety of properties including at one of mean size, shape, size distribution, surface texture, and chemical composition through conventional techniques. It is also appreciated that several stocks of particles can be mixed and used in SLS processes.

[0018] While spherical particles have certain advantages as to spontaneous space filling, other particle shapes are operative herein; the other particle shapes illustratively include ovoid, ellipsoidal, cylindrical, tetrahedral, trigonal pyramidal, cuboidal, and polygonal shaped having from 9 to 36 facets. In some inventive embodiments, the institial voids between closed packed spherical particles are filled by a second type of particle is provided that is sized to fill the interstitial void alone or in combination with other individual second type of particles. As a result, shrinkage of an article with full densification is reduced. It is further appreciated that the second type of particle can vary relative to the particles as to graphene content, including no loading; size, composition; shape; surface texture; or a combination thereof.

[0019] While a lack of size distribution, also referred to herein synonymously as monodisperse, afford certain advantages, particle size distributions are readily approximated by theoretical curves illustratively are Gaussian, Poisson, binomial, or a combination thereof. The mean size for such distributions is readily shifted.

[0020] Surface textures are readily varied from smooth to include a texture. A texture is readily imparted by etching through chemical, mechanical, or plasma techniques conventional to the art. Mechanical etching is readily accomplished by tumbling with abrasives or embossing.

[0021] The present invention incorporates graphene into the powdered sphere polymer precursor material. The inclusion of graphene provides an additional benefit during the printing process, as for most SLS systems in that during the printing stage/the entire bed of powdered material is heated to allow for better melt flow and to minimize the energy gap that the laser must overcome during the print process. Previously, the use of heating is often in vein as most polymers are poor at thermal conduction, so heating the print stage is not relevant after a few layers. However, the use of graphene in embodiments of the invention has demonstrated the ability to increase the thermal conductivity of polymers through conductive and resistive methods, and allows for a better range of heating at the bed and in the print area.

[0022] Graphene is a 1 to 10 atom-thick layer of sp.sup.2 hybridized carbon atoms in a honeycomb-like, 2-dimensional sheet. Graphene is known to have excellent mechanical strength and flexibility, thermal and electric conductivities, and much higher optical cross section to laser light absorption relative to the polymer matrix forming the spherical particle. Graphene nanoplatelets have a maximal linear extent in the three orthogonal X-Y-Z directions of between 3 and 50 nm, and secondary linear extent to at least 20 percent of the maximal linear extent. In addition, the graphene has an aspect ratio between about 25 and 25,000 between the maximum linear extent and the minimum linear extent, synonymously referred to herein as thickness.

[0023] A schematic of an inventive particle is shown in cross section in FIG. 2 generally at 10. The particle 10 is composed of a polymer matrix 12 in which graphene nanoplatelets 14 are dispersed. The graphene nanoplatelets are not shown to scale for visual clarity and depicted to convey a distribution of orientations within the matrix 12. Polymers suitable to form a matrix 12 of a particle 10 illustratively include polyamide, polycarbonate, polystyrene, polyethylenes, polypropylenes, polyetherketones, polyetheretherketones, poly aryl ether ketones, and block copolymers in which any of the aforementioned polymers alone or in combination constitute more than 50% of the number of polymer repeat units of the resulting copolymer.

[0024] The diameter of a particle 10 is between 2 and 200 microns. The particle 10 in combination with other such particles has a polydispersity as detailed above. Loading of graphene nanoplatelets 12 in a matrix 14 range from 0.001 to 50 volume percent. In still other inventive embodiments, the graphene nanoplatelets 12 range from 0.001 to 50 volume percent of the particle 10.

[0025] Incorporation of the graphene into an inventive particle occurs, for example, by starting with polymerizable monomer, such as styrene is added to a polymerization inhibitor (antioxidant) and then mixed to obtain a monomer mixture. To the monomer mixture, 2% by weight of graphene nanoplatelets are added with mixing. The resulting mixture is used to form substantially monodisperse spheres according to a procedure of Wagner et al. above. There resulting particles are filtered and essentially monodisperse with a homogeneous loading of graphene therein. The resulting particles provide superior articles produced by SLS, as compared to using like size polystyrene lacking graphene content.

[0026] While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the described embodiments in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient roadmap for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes may be made in the function and arrangement of elements without departing from the scope as set forth in the appended claims and the legal equivalents thereof.