[0001] The invention relates to a process for preparing a sinterable polymeric powder comprising thermoplastic polymers such as poly(ethylene terephthalate), involving the step of mechanical agitation. The invention further relates to a sinterable polymeric powder obtainable by the process of the present invention and to the use of the sinterable polymeric powder for improving the tensile properties of an additive manufactured product.

[0002] For polymeric powders such as polyesters, used in sintering application, powder flow is often an issue which industry practitioners struggle to address. Poor powder flow can cause issues when such powder is used in applications such as Additive Manufacturing (AM) or 3D printing resulting in the formation of defective products with poor tensile properties. To address the issue of powder flow, polymeric powders are subjected to cryogenic grinding. However, such a process may result in the formation of powders having rock like shape and morphology and fail to produce powders with the desired flow property.

[0003] Other techniques that may be used to improve the flow property of a polymeric powder include techniques such as i) phase separation and precipitation, ii) super critical phase separation, iii) gas phase rounding of polymer powder in a downer reactor, iv) emulsification, v) fiber break-up and vi) emulsion polymerization. These processes have so far yielded limited success. For example, the precipitation process requires the use of hazardous organic solvents, and a subsequent step of removing such solvents which is both time and capital intensive. Further different polymers require different solvents and processing conditions, making such a process complex and inefficient.

[0004] As a further example, process such as gas phase rounding of polymer powder in a downer reactor involves the use of expensive reactor systems and external heat source. Typically in such a process the downer reactor is used to produce round particles by spraying aerosol of the polymer melt down a pipe with heating elements on the wall reactor. The aerosol subsequently solidifies as it travels down the pipe creating a round shape. The downer reactor are quite large in size and often suffers from slow production rate, rendering such a process to be both capital intensive and inefficient. [0005] EP4008742A1 relates to a composition comprising a powder based on at least one polyaryl ether ketone, said composition having at least a first endothermic peak and a second endothermic peak, the first endothermic peak having a peak temperature strictly greater than 280°C, and the second endothermic peak having a peak temperature equal to a value of 200°C to 280°C; the endothermic peaks are measured on a thermogram obtained by differential scanning calorimetry, according to the standard ISO 11357-3: 2018, on first heating, using a temperature ramp of 20°C/minute.

[0006] US20210129383A1 relates to a method for producing a powder comprising at least one polymer for use in a method for the additive manufacture of a three-dimensional object is described. The method includes the step of mechanically treating the powder in a mixer with at least one rotating mixing blade, wherein the powder is exposed to a temperature TB and TB is at least 30° C. and is below the melting point Tm of the polymer (determined according to DIN EN ISO 11357) if the polymer is a semi-crystalline polymer, or wherein TB is at least 30° C. and wherein TB is at most 50° C. above the glass transition temperature Tg of the polymer (determined according to DIN EN ISO 11357) if the polymer is a melt-amorphous polymer.

[0007] Although, such publications are promising, there is still scope to further improve the flow properties of thermoplastic polymers such as poly(ethylene terephthalate), especially when used for additive manufacturing application.

[0008] Therefore, it is an object of the present invention to provide a process for improving the flow property of a sinterable thermoplastic polymer powder such as poly(ethylene terephthalate). Another, objective of the present invention is to provide sinterable thermoplastic polymer powder that may be used to form additive manufactured products having improved tensile properties.

[0009] Accordingly, the one or more objectives of the present invention is achieved by a process for preparing a sinterable polymeric powder, comprising the steps of: a. introducing a powder raw material into a mixing apparatus, wherein the powder raw material comprises polymeric powder particles of at least one thermoplastic polymer; and b. subjecting the powder raw material to mechanical agitation for (i) a time period of > 0.1 minute, preferably > 0.5 minute; and (ii) at a temperature of > the glass transition temperature (Tg) of the thermoplastic polymer and < 10° C above the peak melting temperature (T _p , _m ), preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 10° C below the peak melting temperature (T _p , _m ), preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 15° C below the peak melting temperature (T _p , _m ), preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 20° C below the peak melting temperature (T _p , _m ), of the thermoplastic polymer, wherein T _g is determined in accordance with ISO 11357-2 (2013) and (T _p , _m ) is determined in accordance with ISO 11357-2 (2013), first heating run.

[0010] Preferably the peak melting temperature (T _p , _m ) of the thermoplastic polymer is < 300 °C, preferably < 260 °C, preferably < 250 °C, preferably < 240 °C, preferably < 225°C, where the (T _p ,m) is determined in accordance with ISO 11357-2 (2013), first heating run.

[0011] Preferably, the glass transition temperature of the thermoplastic polymer is > 70 °C, preferably > 75 °C, preferably > 80 °C and < 205°C.

[0012] Preferably the peak melting temperature (T _p , _m ) of the thermoplastic polymer is < 240 °C preferably < 225 °C and the glass transition temperature (T _g ) of the thermoplastic polymer is > 75 °C, preferably > 80 °C.

[0013] The sinterable polymeric powder obtained by the process of the present invention has improved flow properties and can be used in various industrial applications such as additive manufacturing. Accordingly, in an aspect of the invention, the invention relates to the use of the sinterable polymeric powder obtainable by the process of the present invention for improving the tensile properties of an additive manufactured product.

[0014] Preferably, wherein the mechanical agitation is sufficient to generate heat through frictional contact between the polymeric powder particles of the powder raw material. In other words, the mechanical agitation is carried out in a manner sufficient to generate frictional heat due to the collision between the polymeric powder particles. [0015] The mechanical agitation, for example generates heat at a temperature of > 70 °C and < 300 °C, preferably > 100 °C and < 250 °C, preferably > 150 °C and < 220 °C, preferably > 180 °C and < 210°C, preferably > 180 °C and < 205°C.

[0016] Preferably, the powder raw material is subjected to mechanical agitation for a time period of > 0.1 minute and < 20.0 minutes, preferably > 0.5 minute and < 15.0 minutes, preferably > 1.0 minute and < 10.0 minutes.

[0017] Preferably, the powder raw material is subjected to mechanical agitation at a temperature of > the glass transition temperature (T _g ) of the thermoplastic polymer and < 10° C above the peak melting temperature (T _p , _m ), of the thermoplastic polymer, preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 2° C above the peak melting temperature (T _p ,m), of the thermoplastic polymer, preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 2° C below the peak melting temperature (T _p , _m ), of the thermoplastic polymer, preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 10° C below the peak melting temperature (T _p , _m ) , of the thermoplastic polymer.

[0018] It is preferred that the powder raw material is subjected to mechanical agitation at a temperature of > the glass transition temperature (T _g ) of the thermoplastic polymer and < 10° C below the peak melting temperature (T _p , _m ), preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 15° C below the peak melting temperature (T _p , _m ), preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 20° C below the peak melting temperature (T _p , _m ),

[0019] At the glass transition temperature (T _g ) the amorphous regions of the thermoplastic polymer experience transition from a rigid state to a more flexible state. Glass transition temperature is a property of an amorphous polymeric material or particularly the property of the amorphous portion of a semi-crystalline thermoplastic polymer. When the ambient temperature is below T _g , the molecular chains of the amorphous portion of the semi-crystalline thermoplastic polymer are frozen in place and behave like solid glass. [0020] Peak melting point (T _p , _m ) also known as melt temperature is the critical temperature above which the crystalline regions in a semi-crystalline thermoplastic polymer are able to flow. Semicrystalline thermoplastic polymers such as polypropylene and polyesters, begin to soften above T _g . However, the thermoplastic polymers do not demonstrate fluid behavior until the Tp, _m range is achieved. In general, T _p , _m for a semi-crystalline polymer is higher than its T _g .

[0021] Alternatively, the peak melting point and the glass transition temperature may be determined by by means of DSC measurements according to the standard DIN EN ISO 11357. The measurements were carried out on a DSC device of the type “Mettler Toledo DSC823e” with automatic sample changer. The evaluations were carried out using the “STARe Software” software, version 9.30. Nitrogen 5.0, i.e. nitrogen with a purity of 99.999 percent by volume, was used as purge gas. Using DSC, a sufficiently wide temperature range was examined for each material (for the PEKK material examined, for example, the range from 0° C. to 360° C.). The heating and cooling rate was 20° C./min. The melting point was determined in the first heating run. The melting point corresponds to the peak temperature.

[0022] It is particularly preferred that the mechanical agitation of the powder raw material is subjected to mechanical agitation at a temperature of > the glass transition temperature (T _g ) of the thermoplastic polymer and < 2° C below the peak melting temperature (T _p , _m ) of the thermoplastic polymer, preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 10° C below the peak melting temperature (T _p , _m ), of the thermoplastic polymer.

[0023] At temperature above T _g but below T _p , _m , a skilled person would recognize the phase as a “rubbery region,” where the polymeric powder particles comprising the thermoplastic polymer is sufficiently soft and accordingly the powder shape and morphology may be suitably modified under conditions of mechanical agitation.

[0024] The thermoplastic polymer may be at least one polymer selected from the group consisting of a polyester, polyamide, polypropylene, polyethylene, polycarbonate (PC), polyetherimide (PEI), polyethyleneimine, styrene-acrylonitrile resin (SAN), polyether ether ketone (PEEK), poly(phenylene oxide) (PPO), polyether ketone ketone (PEKK), polysulfone sulfonate (PSS), polystyrene (PS) and copolymers thereof. Preferably the thermoplastic polymer is a polyester or polypropylene.

[0025] Preferably, the thermoplastic polymer is a polyester. Preferably, the thermoplastic polymer is a polyester selected from the group consisting of poly(ethylene terephthalate), poly(ethylene naphthalate), poly(ethylene furanoate), poly(trimethylene terephthalate), poly(ethylene succinate), and poly(hydroxyl butyrate). Preferably the thermoplastic polymer is poly(ethylene terephthalate).

[0026] Alternatively, the thermoplastic polymer is a polypropylene. The polypropylene may be selected from a group consisting of a polypropylene homopolymer, preferably iso-tactic polypropylene (iPP), a propylene-ethylene random copolymer (random PP).

[0027] The powder raw material may be subjected to mechanical agitation using a mixing blade. The mixing blade may be part of the mixing apparatus. Non-limiting examples of a mixing apparatus includes a blender, a Henschel mixer and any equivalent apparatus.

[0028] The mixing blade may be operated at a suitable rotational speed in order to generate the requisite frictional contact between the polymeric powder particles of the thermoplastic polymer. For example, the mixing blade may be operated at a rotation per minute (RPM) of > 200 and < 2000, preferably > 400 and < 1000.

[0029] The thermoplastic polymer may have an intrinsic viscosity of > 0.50 dl/g and < 2.5 dl/g, preferably an intrinsic viscosity of > 0.80 dl/g and < 2.0 dl/g as determined in accordance with ASTM D2857-95 (2007). The intrinsic viscosity may be so selected so as to aid the flow of the polymeric powder.

[0030] In an aspect of the invention, the powder raw material may be subjected to cryogenic milling prior to introducing the powder raw material into the mixing apparatus. The process of cryogenic milling may involve the steps of contacting the powder raw material with liquid nitrogen such that the powder raw material is cooled to a temperature below the glass transition temperature (T _g ) or the peak melting temperature (T _p , _m ) of the thermoplastic polymer. Thereafter, the powder raw material is grinded and optionally sieved prior to introducing the powder raw material into the mixing apparatus.

Evaluation of Flow property and rounding of polymeric powder particles

[0031] The sinterable polymeric powder obtained by mechanical agitation of the powder raw material has improved flow property. A suitable metric for evaluating the improved flow property of the polymeric powder particles, is by determining the flowability ratio at a specific consolidation stress. A flowability ratio of 4.0 and above is indicative of a suitable flow property of the polymeric powder particles while a flowability ratio of less than 4.0 is indicative that the polymeric powder particles are cohesive and have resistance to flow.

[0032] Accordingly, in an aspect of the present invention, the sinterable polymeric powder has or is selected to have a flowability ratio of > 4.0, preferably > 4.0 and < 10.0, preferably > 5.0 and < 9.0, preferably > 5.0 and < 8.0, at a consolidation stress of 1000-3000 Pa and at a temperature between 50°C -70°C wherein the flowability ratio is defined as a ratio of the major consolidation stress (o 1) of the sinterable polymeric powder to the unconfined yield strength (o _c ) of the sinterable polymeric powder, and measured using the Schulze Ring Shear Tester in accordance with ASTM D 6773-02, and wherein the major consolidation stress (o 1) and the unconfined yield strength (o _c ) is determined using Mohr stress circles.

[0033] The improvement in the flow property of the sinterable polymeric powder is particularly evident when compared with the flowability ratio of the powder raw material. For example, the powder raw material has or selected to have a flowability ratio < 4.0 at a consolidation stress of 1000-3000 Pa and at a temperature between 50°C -70°C, wherein the flowability ratio is defined as a ratio of the major consolidation stress (o 1) of the powder raw material to the unconfined yield strength (o _c ) of the powder raw material, and measured using the Schulze Ring Shear Tester in accordance with ASTM D 6773-02, wherein the major consolidation stress (o 1) and the unconfined yield strength (o _c ) is determined using Mohr stress circles.

[0034] The flow test may be carried out by using a ring shear tester (Dr. Dietmar Schulze Schuttgutmesstechnik, Germany). The tester and the measuring device may be calibrated according to procedures described in ASTM D6773 - 02. The powder raw material may be subsequently subjected to specific consolidation stress. The unconfined yield strength and the major consolidation stress are derived from the two Mohr stress circles according to the procedure described by Jenike et al in the published article (Storage and flow of solids. Bulletin No. 123; Vol. 53, No. 26, November 1964, 1976.).

[0035] The sinterable polymeric powder has a suitable shape and morphology required for the polymer particles to be sinterable and be used for additive manufacturing application. For example, the sinterable polymeric powder may have an average particle diameter of > 90.0 pm and < 120.0 pm, preferably > 95.0 pm and < 110.0 pm as determined in accordance with ISO 9276-2 (2014).

[0036] The inventors further believe that the improved flow property of the sinterable polymeric powder may be attributed to the rounding of the polymer particles obtained as a result of the process of the present invention.

[0037] The extent of rounding of the polymeric powder particles may be evaluated by determining the number averaged mean circularity of the polymeric powder particles before and after the process involving mechanical agitation. A number averaged mean circularity value of 1.0, is indicative of a perfectly rounded polymeric powder particle and a value of 0.0 indicates an elongated powder particle. The number averaged mean circularity is measured by using the formula: Circularity = 4TT*(Area/Perimeter ^A2 ) using light microscopy. The sinterable polymeric powder may have a number averaged mean circularity of > 0.65 and <1.0, preferably > 0.75 and < 0.99, preferably > 0.8 and < 0.99, preferably > 0.85 and < 0.95, preferably > 0.85 and < 0.90.

[0038] The process of using light microscopy may involve using few milligrams of the polymeric powder particles, which may be applied on a microscope slide and subsequently spread by friction with the help of another microscope slide to reduce particle agglomeration. Images may be taken using an Axio Imager M1m light microscope (Carl Zeiss Microscopy GmbH, Germany) in transmitted light mode with a 10x objective. A motorized microscopy stage may be calibrated to enable scanning of the complete slide thereby generating an array of images, which may be digitally analyzed using Imaged software for determining the particle size and shape factor distributions (circularity) of the polymeric powder particles. [0039] In aspect of the invention, the invention relates to process for preparing a sinterable polymeric powder, comprising the steps of: a) introducing a powder raw material into a mixing apparatus, wherein the powder raw material comprises polymeric powder particles of at least one thermoplastic polymer; and b) subjecting the powder raw material to mechanical agitation for (i) a time period of > 0.1 minute, preferably > 0.5 minute; and (ii) at a temperature of > the glass transition temperature (Tg) of the thermoplastic polymer and < 10 °C above the peak melting temperature (T _p , _m ), of the thermoplastic polymer, preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 2° C below the peak melting temperature (T _p , _m ), preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 10° C below the peak melting temperature (T _p ,m), preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 15° C below the peak melting temperature (T _p , _m ), preferably > the glass transition temperature (T _g ) of the thermoplastic polymer and < 20° C below the peak melting temperature (T _p , _m ), wherein T _g is determined in accordance with ISO 11357-2 (2013) and T _p , _m is determined in accordance with ISO 11357-2 (2013), first heating run; wherein the mechanical agitation is sufficient to generate heat through frictional contact between the particles of the powder raw material, wherein the sinterable polymeric powder has or selected to have a flowability ratio of > 4.0, preferably > 4.0 and < 10.0, at a consolidation stress of 1000- 3000 Pa and at a temperature between 50°C -70°C wherein the flowability ratio is defined as a ratio of the major consolidation stress (o 1) of the sinterable polymeric powder to the unconfined yield strength (o _c ) of the sinterable polymeric powder, and measured using the Schulze Ring Shear Tester in accordance with ASTM D 6773-02, and wherein the major consolidation stress (o 1) and the unconfined yield strength (o _c ) is determined using Mohr stress circles. Preferably, the thermoplastic polymer is poly(ethylene terephthalate).

[0040] Preferably the process for preparing a sinterable polymeric powder, comprising the steps of: a) introducing a powder raw material into a mixing apparatus, wherein the powder raw material comprises polymeric powder particles of at least one thermoplastic polymer; and b) subjecting the powder raw material to mechanical agitation for (i) a time period of > 0.1 minute, preferably > 0.5 minute; and (ii) at a temperature of > the glass transition temperature (Tg) of the thermoplastic polymer and < 20° C below the peak melting temperature (T _p , _m ), wherein Tg is determined in accordance with ISO 11357-2 (2013) and T _p , _m is determined in accordance with ISO 11357-2 (2013), first heating run; wherein the mechanical agitation is sufficient to generate heat through frictional contact between the particles of the powder raw material, wherein the sinterable polymeric powder has or selected to have a flowability ratio of > 4.0 and < 10.0, at a consolidation stress of 1000-3000 Pa and at a temperature between 50°C -70°C wherein the flowability ratio is defined as a ratio of the major consolidation stress (o 1) of the sinterable polymeric powder to the unconfined yield strength (o _c ) of the sinterable polymeric powder, and measured using the Schulze Ring Shear Tester in accordance with ASTM D 6773-02, and wherein the major consolidation stress (o 1) and the unconfined yield strength (o _c ) is determined using Mohr stress circles, wherein the thermoplastic polymer is poly(ethylene terephthalate).

Sinterable polymeric powder

[0041] In an aspect of the invention, the invention relates to a sinterable polymeric powder obtainable by the process according to the present invention. Preferably, the sinterable polymeric powder has or is selected to have a flowability ratio of > 4.0, preferably > 4.0 and < 10.0, preferably > 5.0 and < 9.0, preferably > 5.0 and < 8.0, at a consolidation stress of 1000-3000 Pa and at a temperature between 50°C -70°C wherein the flowability ratio is defined as a ratio of the major consolidation stress (o 1) of the sinterable polymeric powder to the unconfined yield strength (o _c ) of the sinterable polymeric powder, and measured using the Schulze Ring Shear Tester in accordance with ASTM D 6773-02, and wherein the major consolidation stress (o 1) and the unconfined yield strength (o _c ) is determined using Mohr stress circles. Preferably, the thermoplastic polymer is poly(ethylene terephthalate).

[0042] Advantageously, the process of the present invention, results in a sinterable polymeric powder having improved flow property as determined by the flow ratio. Preferably, the sinterable polymeric powder obtainable by the process according to the present invention has at least 70% preferably at least 72%, preferably at least 75%, preferably at least 75% and < 95%, improved flow ratio compared to the powder raw material. [0043] Preferably, the sinterable polymeric powder has a number averaged mean circularity of > 0.65 and <1.0, preferably > 0.75 and < 0.99, preferably > 0.8 and < 0.99, preferably > 0.85 and < 0.95, preferably > 0.85 and < 0.90.

Article

[0044] In an aspect of the invention, the invention relates to an article prepared from the sinterable polymeric powder of the present invention. The article may be an additive manufactured article, an automotive component, a healthcare article, a battery pack component. Preferably, the article is an automotive component. Preferably, automotive component include automobile interior component.

[0045] In an aspect of the invention, the invention relates to a process for preparing the article in accordance with the present invention, comprising the steps of: a) providing the sinterable polymeric powder in accordance with the present invention; b) irradiating a portion of the sinterable polymeric powder with a radiation source; c) terminating the exposure of the portion of the sinterable polymeric powder to the radiation; and d) removing the portion of the polymer composition that has not been subjected to irradiation and obtaining the article.

[0046] Non-limiting examples of radiation source may be a laser beam source, an ion beam source, an infra-red beam source or any other suitable radiation source. The polymeric powder particles of the sinterable polymeric powder is exposed to the radiation source such that the particles absorb sufficient heat to reach a temperature above the glass transition temperature (T _g ) or the peak melting temperature (T _p , _m ) of the thermoplastic polymer constituting the polymeric powder particles.

[0047] The present invention will now be further elucidated based on the following non-limiting examples.

EXAMPLES [0048] Materials: For the purposes of this example the following materials were used:

Table 1 [0049] Process and conditions of mechanical agitation: The following test protocols and standard were followed: Cryogenically milled powder raw material was introduced in a Henschel mixer and was subjected to mechanical agitation under conditions as provided in Table 2 below. The powder raw material was used as control (CE). The inventive samples (IE1-IE3) were the sinterable polymeric powder obtained by the process as described in this disclosure.

Table 2

[0050] Process determining the flowability ratio: The samples of sinterable polymeric powder (IE1-IE3) that were obtained and the control sample (CE) were evaluated for their flow property. For evaluating the flow property a ring shear tester (Dr. Dietmar Schulze Schuttgutmesstechnik, Germany) was used to measure the flow behavior of the powder. The powder samples were filled in the tester and the measuring device was prepared according to procedures indicated in ASTM D6773 - 02. The powder samples were subjected to consolidation stresses of 2000 Pa at 65 °C. The unconfined yield strength and the major consolidation stress were derived from the two Mohr stress circles according to the procedure described by Jenike et al in the published article (Storage and flow of solids. Bulletin No. 123; Vol. 53, No. 26, November 1964, 1976.). The flowability ratio was evaluated for each of the samples as the ratio of the major consolidation stress (o 1) of the polymeric powder to the unconfined yield strength (o _c ) of the polymeric powder and the result is provided in the table below:

Table 3

[0051] Results and Conclusion: From the results provided under Table 3, it is observed that the inventive sample IE1-IE3 had a significant improvement in the flowability ratio as compared to the sample CE which was not subjected to mechanical agitation as prescribed in the present invention. For example, the sample IE3 has nearly 83% higher flowability ratio than that of the sample CE (powder raw material) while sample IE1 has nearly 76% higher flowability ratio than that of the sample CE (powder raw material).

[0052] This result clearly indicates that the process as prescribed in the present invention improves the flow property of polymeric powder particles using the process prescribed in the invention.