Polymer powder and article made from the same

Abstract

A powder composition suitable for laser sintering for printing a three-dimensional object obtained by pre-heating a raw PEKK to evaporate a liquid solvent in the raw PEKK and the grinding the raw PEKK to form a PEKK powder, the powder composition having a first fraction comprising a polyetherketoneketone (PEKK) powder having a plurality of particles, the plurality of particles having a mean diameter D50 in the range of 30 ?m to 150 ?m. The plurality of particles of the first fraction are substantially non-spherical.

Claims

1. A powder composition suitable for laser sintering for printing a three-dimensional object obtained by pre-heating a raw PEKK to evaporate a liquid solvent in the raw PEKK and then grinding the raw PEKK to form a PEKK powder, the powder composition comprising: a first fraction comprising a polyetherketoneketone (PEKK) powder having a plurality of particles, the plurality of particles having a mean diameter D50 in the range of 60 ?m to 70 ?m; a second fraction comprising a plurality of carbon fiber; wherein the plurality of particles of the first fraction are irregularly shaped and substantially non-spherical due to the particle-on-particle contact impacts during a jet milling process, wherein the temperature of the raw PEKK during the pre-heating is greater than 200 C and is less than a temperature at which raw PEKK begins to melt; wherein after the pre-heating the PEKK powder of the first fraction has a purity greater than 99.50% as determined by the ASTM E1868 loss-on-drying test method; wherein the plurality of particles of the first fraction is sieved to reduce the particles less than 30 ?m in diameter and greater than 150 ?m in diameter so that the powder is configured for use in selective laser sintering.

2. The powder composition of claim 1, wherein the plurality of PEKK particles are angular.

3. The powder composition of claim 2, wherein the PEKK powder of the first fraction has a purity greater than 99.75% as determined by the ASTM E1868 loss-on-drying test method.

4. A powder composition suitable for laser sintering for printing a three-dimensional object obtained by pre-heating a raw PEKK to evaporate a liquid solvent in the raw PEKK and then grinding the raw PEKK to form a PEKK powder, the powder composition comprising: a first fraction comprising a polyetherketoneketone (PEKK) powder having a plurality of particles, the plurality of particles having a mean diameter D50 in the range of 60 ?m to 70 ?m; a second fraction comprising a plurality of carbon fiber; wherein the plurality of particles of the first fraction are irregularly shaped and substantially angular due to the particle-on-particle contact impacts during a jet milling process, wherein the PEKK powder of the first fraction has purity greater than 99.50% as determined by the ASTM E1868 loss-on-drying test method as a result of subjecting the raw PEKK to a heat treatment that is greater than 200 C. and is less than a temperature at which raw PEKK begins to melt.

5. The powder composition of claim 4, wherein the plurality of PEKK particles are angular.

6. The powder composition of claim 5, wherein the PEKK powder of the first fraction has a purity greater than 99.75% as determined by the ASTM E1868 loss-on-drying test method.

7. The powder composition of claim 6, wherein the plurality of particles of the first fraction is sieved to reduce the particles less than 30 ?m in diameter.

8. The powder composition of claim 4, wherein the second fraction comprises between 5% and 30% of the powder composition by weight.

9. The powder composition of claim 8, wherein the second fraction comprises 15% of the powder composition by weight.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates an SLS machine.

(2) FIG. 2 is an image showing a magnified view of raw PEKK flakes.

(3) FIG. 3 plots the temperature as a function of time during the heating and cooling steps of an embodiment of the present method.

(4) FIG. 4 plots the temperature as a function of time during the heating and cooling steps of another embodiment of the present method.

(5) FIG. 5 is a magnified view of a plurality of PEKK particles after the grinding step of the present method.

(6) FIG. 6 is a table showing data of a PEKK powder composition excluding carbon fiber.

(7) FIG. 7 is a table showing data of a PEKK powder composition including carbon fiber.

DETAILED DESCRIPTION

(8) An aspect of the present invention involves a method for preparing polymer powders for SLS or LS.

(9) One class of polymer powders is polyaryletherketones (PAEK) polymers. PAEKs are of interest in the SLS process because parts that have been manufactured from PAEK powder or PAEK granulates are characterized by a low flammability, a good biocompatibility, and a high resistance against hydrolysis and radiation. The thermal resistance at elevated temperatures as well as the chemical resistance distinguishes PAEK powders from ordinary plastic powders. A PAEK polymer powder may be a powder from the group consisting of polyetheretherketone (PEEK), polyetherketoneketone (PEKK), polyetherketone (PEK), polyetheretherketoneketone (PEEKK) or polyetherketoneetherketoneketone (PEKEKK).

(10) More specifically, the method if useful for preparing PEKK polymers powders for use in LS. PEKKs are well-known in the art and can be prepared using any suitable polymerization technique, including the methods described in the following patents, each of which is incorporated herein by reference in its entirety for all purposes: U.S. Pat. Nos. 3,065,205; 3,441,538; 3,442,857; 3,516,966; 4,704,448; 4,816,556; and 6,177,518. PEKK polymers differ from the general class of PAEK polymers in that they often include, as repeating units, two different isomeric forms of ketone-ketone. These repeating units can be represented by the following Formulas I and II:
-A-C(?O)BC(?O)I
-A-C(?O)-D-C(?O)II
where A is a p,p-Ph-O-Ph-group, Ph is a phenylene radical, B is p-phenylene, and D is m-phenylene. The Formula I:Formula II isomer ratio, commonly referred to as the T:I ratio, in the PEKK is selected so as to vary the total crystallinity of the polymer. The T:I ratio is commonly varied from 50:50 to 90:10, and in some embodiments 60:40 to 80:20. A higher T:I ratio such as, 80:20, provides a higher degree of crystallinity as compared to a lower T:I ratio, such as 60:40.

(11) The crystal structure, polymorphism and morphology of homopolymers of PEKK have been studied and have been reported in, for example, Cheng, Z. D. et al, Polymorphism and crystal structure identification in poly(aryl ether ketone ketone)s, Macromol. Chem. Phys. 197, 185-213 (1996), the disclosure of which is hereby incorporated by reference in its entirety. This article studied PEKK homopolymers having all para-phenylene linkages [PEKK(T)], one meta-phenylene linkage [PEKK(I)] or alternating T and I isomers [PEKK(T/I)]. PEKK(T) and PEKK(T/I) show crystalline polymorphism depending upon the crystallization conditions and methods.

(12) In PEKK(T), two crystalline forms, forms I and II, are observed. Form I can be produced when samples are crystallized from melting at low supercoolings, while Form II is typically found via solvent-induced crystallization or by cold-crystallization from the glassy state at relatively high supercooling. PEKK(I) possesses only one crystal unit cell which belongs to the same category as the Form I structure in PEKK(T). The c-axis dimension of the unit cell has been determined as three phenylenes having a zig-zag conformation, with the meta-phenylene lying on the backbone plane. PEKK(T/I) shows crystalline forms I and II (as in the case of PEKK(T)) and also shows, under certain conditions, a form III.

(13) Suitable PEKKs are available from several commercial sources under various brand names. For example, polyetherketoneketones are sold under the brand name OXPEKK? polymers by Oxford Performance Materials, South Windsor, Connecticut, including OXPEKK?-C, OXPEKK?-CE, OXPEKK?-D and OXPEKK?-SP, OXPEKK?-N, OXPEKK?-ESD polymers. Polyetherketoneketone polymers are also manufactured and supplied by Arkema. In addition to using polymers with a specific T:I ratio, mixtures of polyetherketoneketones may be employed.

(14) Although the present disclosure is provided in reference to PAEK polymers, and more specifically PEKK polymers, the present disclosure is not so limited. A person of ordinary skill in the art and familiar with this disclosure would understand that aspects of the present invention could be employed with other types of polymers useful in LS and with other materials useful in LS, including, but not limited to, metals and ceramics.

(15) In reference to FIG. 1, an example of an LS system 10 is illustrated that includes a first chamber 20 having an actuatable piston 24 disposed therein. A bed 22 is disposed at an end of the piston 24. It should be understood that the term bed refers to the physical structure supported on the piston or the uppermost layer of powder disposed thereon. The temperature of the bed 22 can be variably controlled via a controller 60 in communication with heating elements (not shown) in and or around the bed 22. A second chamber 30 is adjacent to the first chamber 20. The second chamber 30 includes a table surface 32 disposed on an end of a piston 34 disposed therein. A powder 36 for use in the LS system 10 is stored in the second chamber 30 prior to the sintering step.

(16) During operation of the LS system 10, a spreader 40 translates across a top surface of the first chamber 20, evenly distributing a layer of powder 36 across either the top surface of the bed 22, or the material previously disposed on the bed 22. The LS system 10 preheats the powder material 36 disposed on the bed 22 to a temperature proximate to a melting point of the powder. Typically, a layer of powder is spread to have a thickness of 125 ?m, however the thickness of the layer of powder can be increased or decreased depending on the specific LS process and within the limits of the LS system.

(17) A laser 50 and a scanning device 54 are disposed above the bed 22. The laser 50 transmits a beam 52 to the scanner 54, which then distributes a laser beam 56 across the layer of powder 36 disposed on the bed 22 in accordance with a build data. The laser selectively fuses powdered material (e.g., PEKK powder prepared according to the presently disclosed method) by scanning cross-sections generated from a three-dimensional digital description of the part on the surface of the bed having a layer of the powdered material disposed thereon. The laser 50 and the scanner 54 are in communication with the controller 60. After a cross-section is scanned, the bed 22 is lowered by one layer thickness (illustrated by the downward arrow), a new layer of powdered material is disposed on the bed 22 via the spreader 40, and the bed 22 is rescanned by the laser. This process is repeated until a build 28 is completed. During this process, the piston 34 in the second chamber is incrementally raised (illustrated by the upward arrow) to ensure that there is a sufficient supply of powder 36.

(18) The steps of the present method for preparing a PAEK powder (e.g., a PEKK powder) for use in SLS will now be discussed.

(19) First, raw PEKK is provided. The raw PEKK is commercially available from companies such as Arkema, Inc. of King of Prussia, Pa., and Cytec Industries Inc. of Woodland Park, N.J. The raw PEKK is typically swilled from a chemical reactor and then washed. The raw PEKK is a non-powder material. That is, the raw PEKK is not in the form of a powder. As shown in FIG. 2, the raw PEKK 200 is in the form of irregularly-shaped particles 202 (e.g., particles that are vaguely round, elongated, flat, etc.) and has an appearance similar to that of Rice Krispies? cereal. The irregularly-shaped particles of the raw PEKK have grain sizes that are orders of magnitude larger than 150 ?m, for example. The remainder of the raw PEKK can be in the form of a gel or gel-like form caused by an amount of liquid solvent (e.g., orthodichlorobenzene (ODCB)) other solvents and volatiles remaining from the process of producing the raw PEKK.

(20) After the step of providing the raw PEKK, a heat treatment step is performed that involves placing the raw PEKK into a shallow pan and heating both within a convection oven. The temperature is ramped up to 200? C. over a one-hour period. The temperature is held at 200? C. for several hours (e.g., 5 or 6 hours). The temperature is ramped up a second time to 225? C. The temperature is held at 225? C. for a minimum of one hour and for preferably between one and four hours. The temperature is then ramped up a third time to 250? C. The temperature is held at 250? C. for a minimum of one hour and for preferably between one and four hours. The temperature of the raw PEKK does not exceed 250? C. during the heat treating step, and thus remains below the melting temperature of the raw PEKK.

(21) The heat treatment step evaporates any remaining liquid solvent and other impurities, and causes at least substantially all of the raw PEKK to be in the form of irregularly-shaped particles. The heat treatment step also causes some coalescence of the irregularly-shaped particles. However, the bulk density of the raw PEKK remains low after the heat treatment step.

(22) The duration and temperature of each heating period during the heat treatment step can be selected based on the expected amount of impurities in the raw PEKK. The total duration of the heat treatment step is directly correlated to the amount (or expected amount) of impurities (e.g., liquid solvent) in the raw PEKK.

(23) Referring to FIG. 3, in one embodiment in which the raw PEKK was determined to be approximately 99.75% pure via the ASTM E1868 loss-on-drying testing method, the heat treatment step includes a first heating period 310 having a duration of one (1) hour, during which the temperature of the raw PEKK is ramped up from room temperature to approximately 200? C.; (ii) a second heating period 312 having a duration of seven (7) hours, during which the temperature of the raw PEKK is held at a temperature of approximately 200? C.; (iii) a third heating period 314 having a duration of 0.5 hours, during which the temperature of the raw PEKK is ramped up from approximately 200? C. to approximately 225? C.; (ii) a fourth heating period 316 having a duration of 1.5 hours, during which the temperature of the raw PEKK is held at a temperature of approximately 225? C.; (i) a fifth heating period 318 having a duration of 0.5 hours, during which the temperature of the raw PEKK is ramped up from approximately 225? C. to approximately 250? C.; (ii) a sixth heating period 320 having a duration of two (2) hours, during which the temperature of the raw PEKK is held at a temperature of approximately 250? C.

(24) Referring to FIG. 4, in another embodiment in which the raw PEKK was determined to be approximately 99.5% pure the ASTM E1868 loss-on-drying testing method, the heat treatment step includes: (i) a first heating period 410 having a duration of 0.5 hours, during which the temperature of the raw PEKK is ramped up from room temperature to approximately 200? C.; (ii) a second heating period 412 having a duration of four (4) hours, during which the temperature of the raw PEKK is held at a temperature of approximately 200? C.; (iii) a third heating period 414 having a duration of 0.5 hours, during which the temperature of the raw PEKK is ramped up from approximately 200? C. to approximately 225? C.; (ii) a fourth heating period 416 having a duration of 4 hours, during which the temperature of the raw PEKK is held at a temperature of approximately 225? C.; (i) a fifth heating period 418 having a duration of 0.5 hours, during which the temperature of the raw PEKK is ramped up from approximately 225? C. to approximately 250? C.; (ii) a sixth heating period 420 having a duration of five (5) hours, during which the temperature of the raw PEKK is held at a temperature of approximately 250? C.

(25) After the heat treating step, a cooling step is performed that involves powering-off the convection oven heater and allowing the raw PEKK to cool naturally. In some embodiments, such as those shown in FIGS. 3 and 4, the cooling step involves a cooling period 22 during which the temperature of the oven heater is decreased from approximately 250? C. to 75? C. over the course of 0.5 hours before the raw PEKK is allowed to naturally cool.

(26) After the cooling step, a grinding step is performed that involves grinding the raw PEKK to form what will hereinafter be referred to as the PEKK powder. The grinding step can be performed using known grinding techniques performed by companies such as Aveka, Inc. of Woodbury, Minn. Upon completion of the grinding step, the particles of the PEKK powder are significantly smaller (i.e., several degrees of magnitude smaller) than the particles of the raw PEKK. The particles of the PEKK powder are more consistent and regular in shape as compared to the particles of the raw PEKK; however, the particles of the PEKK powder are still irregularly-shaped in comparison to the spherical-shaped particles of prior art PEKK powders that are pre-heated as described above. In reference to FIG. 3, a magnified image of irregularly shaped particles 500 produced via the above process is shown.

(27) After the grinding step, an optional processing step is performed that involves sieving or otherwise processing the PEKK powder to eliminate particles having sizes outside a range appropriate for SLS (e.g., 30-150 ?m). In some embodiments, the LS composition is formed from PEKK powder and carbon fiber. It is known in the art to reduce or eliminate particles having a diameter below a cut-off point, for example 20 ?m, as it has been found that particles in this range may hinder use of the powder in the LS process or may degrade the mechanical properties of parts built via LS therefrom. For example, International Patent Application WO2014100320 discloses such a method for preparing powders for use in selective laser sintering.

(28) The above described milling process and powder preparation process is the subject of co-pending U.S. patent application Ser. No. 15/872,598 filed on Jan. 16, 2018 by Hexcel Corporation and titled Polymer Powder and Method of Preparing the Same. The disclosure of that reference is hereby incorporated by reference. The particles resulting from the milling process, are non-spherical and substantially angular. This is a result of the jet milling process that performs particle comminution via particle-on-particle impact. The inventors have discovered that the substantial non-spherical PEKK particles perform better in the LS process. For example, the non-spherical particles are more easily distributed on the bed table for the LS process and the non-spherical particles result in substantially stronger parts, particularly in the out of plane axis. The increased performance of non-spherical particles is due in part to the increased ability for larger and smaller particles to pack together enhancing the strength of the laser fusion.

(29) Another aspect of the invention is a PEKK powder manufactured according to the above-described method.

(30) In some embodiments, the method can be performed using an SLS machine made by EOS GmbH Electro Optical Systems of Krailling, Germany, such as the EOSINT P 800. In such embodiments, the PEKK powder is placed into an overhead gravity feed hopper and then forced to cascade down (i.e., flow down) several stepped shelves that cause the PEKK powder to be in a form that is not at all packed or tamped down. The PEKK powder is then fed through a recoater that causes the PEKK powder to be provided in a flat, level layer. There is no intentional heating prior to the PEKK powder reaching the workspace. The components of the SLS machine that feed and dose the PEKK powder before it reaches the workspace (e.g., the hopper, the recoater, etc.) can get warm, but this is unintentional and there is no monitoring of such heating.

(31) In reference to tables shown in FIGS. 6 and 7, a comparison of parts made from a first composition consisting essentially of PEKK powder and a second composition consisting essentially of PEKK powder and carbon fiber prepared in accordance with the presently disclosed method is shown. The powders were sold under the names OXPEKK?-N and OXPEKK?-ESD, wherein N includes PEKK powder and ESD includes PEKK powder blended with carbon fiber. In both cases, the LS cycle affects the physical and chemical properties of the powder, and the tensile properties of parts made from the powder improve with each successive generation. The comparison confirms that parts made with the carbon fibers in accordance with the presently disclosed invention are unequivocally stronger than parts made from pure PEKK powder. In both cases, the raw PEKK flake was treated in accordance with the proposed heat treatment steps and grinding steps.

(32) In reference to FIG. 6, Table 1 600 shows the properties of PEKK powder sold under the brand name OXPEKK?-N from Oxford Performance Materials, Inc. The left column, Cake Level, identifies the number of LS cycles that an OXPEKK?-N powder has been exposed to. Virgin refers to a powder that has not been exposed to LS process, while Cake A has been exposed to 1 LS process, Cake B2 LS process, etc. Each cake level was subjected to an LS build process that manufactured test rods in the x-plane. The tensile properties were determined pursuant to ASTM D638. The test rods were manufactured in a EOS P 800 sintering machine. The bed temperature was determined in accordance with the method set forth in International Patent Application WO2015031758 for a Method for Analytically Determining SLS Bed Temperature.

(33) In reference to FIG. 7, Table 2 700 shows the properties of ESD PEKK powder sold under the brand name OXPEKK?-ESD from Oxford Performance Materials, Inc. The powder was manufactured in accordance with the above described method. Specifically, consisted essentially of PEKK powder and carbon fiber in a ratio of 85/15 by weight. The D50 of the powder was between 60 ?m and 70 ?m and the L50 of the powder was between 70 ?m and 80 ?m. The particles were substantially non-spherical and the carbon fiber was mixed with the PEKK powder via high intensity mixing to partially embed the fiber into the particles.

(34) The left column, Cake Level, identifies the number of LS cycles that an OXPEKK?-ESD powder has been exposed to. Virgin refers to a powder that has not been exposed to LS process, while Cake A has been exposed to 1 LS process, Cake B-2 LS processes. Each ESD cake level was subjected to an LS build process that manufactured test rods in the x-plane. The tensile properties were determined pursuant to ASTM D638.

(35) The present method offers significant advantages over prior art methods for preparing a PEKK powder. By heat treating raw PEKK and then performing the grinding step to form PEKK powder, as opposed to pre-heating PEKK powder as is typically done in the prior art, the present method achieves a PEKK powder with particles that are irregularly-shaped in comparison to the spherical-shaped particles of prior art PEKK powders. Contrary to conventional wisdom, the irregularly-shaped PEKK powder particles yield enhanced consistency of performance and improved performance over the prior art spherical-shaped particles. The irregularly-shaped PEKK powder particles pack together more rigidly, providing greater bed stability and allowing for better shape accuracy in general. Also, the irregularly-shaped PEKK powder particles are believed to provide more resistance to machine transients that would otherwise cause distortions in the manufactured object. It is also believed that the shapes (e.g., size, shape distributions) of voids created by irregularly-shaped PEKK powder particles versus those of spherical-shaped PEKK powder particles offer a different mode of consolidation, yielding enhanced consistency of performance and possibly improved performance.

(36) The present disclosure describes aspects of the invention with reference to the exemplary embodiments illustrated in the drawings; however, aspects of the invention are not limited to the exemplary embodiments illustrated in the drawings. It will be apparent to those of ordinary skill in the art that aspects of the invention include many more embodiments. Accordingly, aspects of the invention are not to be restricted in light of the exemplary embodiments illustrated in the drawings. It will also be apparent to those of ordinary skill in the art that variations and modifications can be made without departing from the true scope of the present disclosure. For example, in some instances, one or more features disclosed in connection with one embodiment can be used alone or in combination with one or more features of one or more other embodiments.