CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims the benefit of U.S. Provisional Patent Application No. 62/785,772, filed on December 28, 2018, the entirety of which is incorporated herein by reference for all purposes.

FIELD OF THE INVENTION

[0002] The disclosed subject matter relates to polyolefin powders and three dimensional (3D) printed parts made therefrom.

BACKGROUND

[0003] Selective laser sintering (SLS) is an additive manufacturing (AM) technique that uses a laser to sinter powdered material together to create a solid structure. Typically, powdered materials for SLS have a small particle size distribution (PSD) in order to obtain a printed part with high bulk density and a low level of intrinsic porosity. However, using powdered material with a small particle size in industrial processes can lead to powder transport, compaction, fluidization, vessel discharge, filter clogging, drag of the particles by transition points (reactor top), extruder feeding, etc. Accordingly, an opportunity to develop alternative powders remains.

SUMMARY

[0004] In various embodiments, a polyolefin powder is provided. The polyolefin powder includes a polyolefin polymer. The polyolefin may be selected from polypropylene,

polyethylene, propylene copolymers, or ethylene copolymers, including, for example, ethylene vinyl acetate, and combinations thereof. In some embodiments, the polyolefin may have bimodal, multimodal, or broad molecular weight distribution. In some embodiments, the polyolefin powder is not milled. In some embodiments, particles in the polyolefin powder can be a spherical, semi-spherical, potato-type, and/or an irregular shape. The shape can be defined by an aspect ratio between the maximum diameter to the minimal diameter. In some

embodiments, the aspect ratio can be from 1 to 2. In some embodiments, the polyolefin powder has porous particles. In some embodiments, the porous particles have a volume of pores in the range of about 0.001 to about 0.100 cm ³ /g. In some embodiments, the polyolefin powder can have a particle size distribution (PSD) of from about 5 microns to about 1000 microns.

[0005] In various embodiments, a three dimensional (3D) part made from a polyolefin powder is provided. The part can be produced by selective laser sintering or selective laser melting using a polyolefin polymer. In some embodiments, the polyolefin polymer can be polyethylene, polypropylene, or a mixture thereof. In some embodiments, the part can be characterized as having low porosity, good particle diffusion, high bulk density, and/or good mechanical properties.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The features and advantages of the disclosed subject matter will be more fully disclosed in, or rendered obvious by the following detailed description of the preferred embodiments, which are to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:

[0007] FIGS. 1A-1C are microscopic images of powders. FIG. 1A shows spherical particles; FIG. IB shows“potato-type” particles; and FIG. 1C shows particles made by cryogenic grinding.

[0008] FIGS. 2A-2D are micrographic images of polyolefin powder particles, in accordance with some embodiments described herein.

[0009] FIGS. 3 A and 3B are micrographic images of a part printed with polypropylene particles.

[0010] FIGS. 4A and 4B are micrographic images of parts printed with polyolefin powder particles, in accordance with some embodiments described herein.

[0011] FIGS. 5A-5D are perspective images of parts printed with polyolefin powder particles, in accordance with some embodiments described herein.

DETAILED DESCRIPTION

[0012] This description of the exemplary embodiments is intended to be read in connection with the accompanying drawings, which are to be considered part of the entire written description. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosed subject matter, the preferred methods and materials are described. Generally, nomenclatures utilized in connection with, and techniques of chemistry and material science are those well-known and commonly used in the art. Certain experimental techniques, not specifically defined, are generally performed according to conventional methods well known in the art and as described in various general and more specific references that are cited and discussed throughout the present specification.

[0013] In various embodiments, a polyolefin powder is provided. The polyolefin may be selected from polypropylene, polyethylene, propylene copolymers, or ethylene copolymers, including, for example, ethylene vinyl acetate, and combinations thereof. The polyolefins may include homopolymers, random copolymers, block copolymers, and heterophasic polymer compositions such as impact copolymers.

[0014] In some embodiments, the polyolefin powder can be produced by a process comprising a catalyst. The catalyst can be or include any suitable material. In some

embodiments, for example, the catalyst is a heterogeneous supported catalyst. In some embodiments, for example, the catalyst is a homogeneous catalyst. In some embodiments, a co catalyst is used. In some embodiments, a carrier is used. In some embodiments, for example, the supported catalyst has a spherical, semi-spherical, potato-type, or irregular shape. In some embodiments, the catalyst and/or co-catalyst is a metal. In some embodiments, for example, the metal is titanium, aluminum, zirconium, hafnium, or any other suitable metal. In some embodiments, a Ziegler-Natta or Ziegler-Natta-type catalyst is used. The support can be any suitable material. In some embodiments, the support is a metal. In some embodiments, for example, the support includes magnesium or another suitable metal. In some embodiments, the carrier has a specifically defined size and shape. In some embodiments, for example, the carrier has a spherical, semi-spherical, potato-type, or irregular shape. In some embodiments, additional components are used. The additional components can be organic or inorganic materials. In some embodiments, for example, an additional component is used to modify the polymerization reaction.

[0015] In some embodiments, the polyolefin may have bimodal, multimodal, or broad molecular weight distribution. The molecular weight distribution can affect the sintering or melting processes, and ultimately the mechanical properties of a printed part. Lower molecular weight polymers may present good flowability and melt viscosity that would favor the coalescence of particles in selective laser sintering (SLS) or selective laser melting (SLM) processes. On the other hand, higher molecular weight polymers would favor the mechanical properties of the printed part; however, the poor diffusion of the high molecular weight chains could affect the coalescence of the particles resulting in fragile parts. In some embodiments of the present application, it was observed that the combination of different molecular weights could favor the coalescence of particles resulting in parts with better mechanical properties. Without being bound to a particular theory, the low molecular weight chains could act as a lubricant for the high molecular weight chains, and improve the coalescence of particles. A high molecular weight could yield one or more advantageous mechanical properties.

[0016] In some embodiments, the polyolefin powder is not milled.

[0017] In some embodiments, particles in the polyolefin powder can have any suitable shape. In some embodiments, for example, a particle in the polyolefin powder can have a spherical, semi-spherical, potato-type, or irregular shape. The polyolefin powder may comprise particles having the same shape or different shapes. In some embodiments, a substantial amount of the particles in the polyolefin powder have the same shape. The shape can be defined by an aspect ratio between the maximum diameter and the minimal diameter. Under that definition, aspect ratio = Dmax/Dmin, an aspect ratio of 1 represents a spherical particle. In some

embodiments, for example, the aspect ratio for the particles is from about 1 to about 2, from about 1 to about 1.8, from about 1 to about 1.5, from about 1 to about 1.4, from about 1 to about 1.3, from about 1 to about 1.2, or from about 1 to about 1.1.

[0018] In some embodiments, the polyolefin powder has porous particles. In some embodiments, the porous particles have a volume of pores in the range of about 0.001 to about 0.100 cmVg, from 0.002 to about 0.050 cmVg, from about 0.004 to about 0.010 cmVg. Without being bound to a particular theory, the porous particles may facilitate the absorption of energy, which would favor melting, diffusion, and coalescence of the particles.

[0019] In some embodiments, the polyolefin powder can have a particle size distribution (PSD) of from about 5 microns to about 1000 microns, from about 50 to about 500 microns, from about 75 to about 400 microns, from about 90 to about 350 microns, from about 90 to about 250 microns, from about 100 to about 230 microns, from about 100 to about 200 microns, from about 105 to about 195 microns, from about 100 to about 170 microns, from about 100 to about 150 microns, or from about 100 to about 130 microns. In some embodiments, the PSD is from about 3 microns to about 600 microns, or from about 15 microns to about 450 microns, or from about 30 microns to about 300 microns.

[0020] The polyolefin powder of the present application may be used to produce three dimensional (3D) parts by additive manufacturing techniques such as selective laser sintering or selective laser melting. The 3D printed part can have nearly any structure and/or configuration. Generally, properties of a printed part will vary depending on the particular polymer powder used to produce it and the conditions employed during the selective laser sintering or selective laser melting process. The 3D printed part can be characterized by its level of porosity, surface area, diffusion among particles, bulk density, and/or mechanical properties. The morphology and shape of a polyolefin powder will affect the properties of the 3D printed part. In some embodiments, parts produced with a polyolefin powder of the present application can be characterized as having low porosity, good particle diffusion, high bulk density, and/or good mechanical properties.

[0021] In some embodiments, particle size and particle shape can be adjusted to improve the compatibility or workability of the polyolefin powder. Without being bound to a particular theory, particle size and shape will generally effect the available contact points on the particle for sintering during the selective laser sintering process.

[0022] In some embodiments, a process for printing a 3D part comprises the steps of (a) disposing a layer of a polyolefin powder on a target surface; (b) directing an energy beam over a selected area of the polyolefin powder layer, in which the powder is sintered or melted; and (c) repeating said steps (a) and (b) to form the 3D part.

[0023] In some embodiments, a 3D part with lower porosity is obtained. In some embodiments, a 3D part with good diffusion among particles is obtained. In some embodiments, a 3D part with high bulk density, or good mechanical properties is obtained. In some

embodiments, for example, the 3D printed part is characterized by a bulk density of at least about 0.30 g/cm ³ , of at least about 0.50 g/cm ³ , of at least about 0.80 g/cm ³ , or of at least about 1.0 g/cm ³ . In some embodiments, the ratio of the bulk density of the 3D printed part to the bulk density of the polyolefin ranges from about 0.5 to 1, about 0.65 to 1, or about 0.85 to 1.

[0024] In some embodiments, the part demonstrates mechanical properties that are about 30% higher than that for a part prepared by injection molding. In some embodiments, the part demonstrates mechanical properties that are about 50% higher than that for a part prepared by injection molding. In some embodiments, the part demonstrates mechanical properties that are about 80% higher than that for a part prepared by injection molding. In some embodiments, the part demonstrates mechanical properties that are about 90% higher than that for a part prepared by injection molding.

EXAMPLES

[0025] The performance of a polyolefin powder according to some embodiments described herein was compared to the performance of a commercial-grade polypropylene powder in a selective laser sintering process. A comparison of 3D printed parts made from a polyolefin powder according to some embodiments described herein and printed parts made from a commercial-grade polypropylene powder was done.

Preparation of polyolefin samples and printed parts

[0026] Inventive Example: a polyethylene powder was produced using a Ziegler-Natta- type catalyst in a Hostalen Technology process. The polyethylene powder was a high density polyethylene (HDPE) with a density of 0.953 g/cm ³ (measured according ASTM D792), a MFI (melt flow index) of 0.33 g/10 min (measured at 190°C at 5kg according to ASTM D1238) with a bimodal molecular weight distribution. The volume of pores and surface area for the particles were determined to be 0.004 to 0.010 cm ³ /g and 0.5 m ² /g, respectively. No milling process or post-reactor treatment was conducted on the particles. As can be seen from the Figures 2A-2D, the HDPE contains a broad particle size distribution in which the particle sizes vary from about 30 pm to about 200 pm. The figures show the presence of porous particles, and particles having spherical, semi-spherical, irregular, or a potato-type shapes.

[0027] Comparative Example: a commercially available micronized polypropylene having a narrow particle size distribution in which the particle sizes vary from about 30 pm to about 50 pm.

[0028] The polyolefin powders from the inventive and comparative examples were used in a selective laser sintering process to produce 3D printed parts. After printing, a red dye was added to the parts to highlight the morphology of their surfaces.

Results

[0029] Figures 3 A and 3B show micrographies of a part printed with the commercially available micronized polypropylene according to the comparative example. The figures show sintering among the particles; however, the particles have maintained most of their spherical morphology, which indicates poor coalescence between the particles. In addition, the parts have high porosity. This effect may be due to sintering in a solid or semisolid state. It would be expected that the presence of pores in the printed part may affect its mechanical properties.

[0030] Figures 4A-4B show micrographies of a part printed with the HDPE powder according to the inventive example. As can be seen in the figures, the part has a smooth surface and low porosity. The volume of pores and the surface area for the printed parts were determined to be 0.000024 cm ³ /g and 0.05 m ² /g, respectively. Compared to the particles in the HDPE powder, a reduction in surface area of 10 times and a reduction in porosity of about 300 times were observed for the printed part. These results indicate good coalescence between the particles, and further that the sintering occurred in a liquid state with melting and diffusion of the polymer particles. It would be expected that the lower porosity of the printed part favors good mechanical properties.

[0031] Figures 5A-5D show parts produced with the HDPE powder according to the inventive example. As can be seen from the figures, the parts have unique structures and complex configurations. The parts demonstrated excellent mechanical properties. Thus, the inventive example parts demonstrate that polyolefin particles obtained directly from industrial reactors, without milling or any other modification or treatment, can be utilized in a selective layer sintering process to prepare improved parts.

[0032] The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of the disclosed subject matter. Modifications and adaptations to these

embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of the disclosed subject matter.

[0033] Although the subject matter has been described in terms of exemplary

embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments, which may be made by those skilled in the art.