Biodegradable polyester additive manufacturing powder

Abstract

Powder compositions comprised of a biodegradable polymer such as a biodegradable polyester (e.g., polylactic acid) and an impact modifier (e.g., core shell rubber) subjected to further crystallization have shown increased toughness. Silicone acrylic core shell rubbers with or without a reactive surface are particularly suitable.

Description

BIODEGRADABLE POLYESTER ADDITIVE MANUFACTURING POWDERTECHNICAL FIELD

[0001] This disclosure relates to a method of forming a thermoplastic polyester powder useful for additive manufacturing.BACKGROUND

[0002] Powder-based methods of additive manufacturing include the following. Selective laser sintering (SLS) is a 3D-printing technique that uses a laser to fuse powder material in successive layers (see, for example, U.S. Pat. No. 5,597,589). High-speed sintering (HSS) and multi-jet fusion (MJF) 3D-printing employ multiple jets that similarly deposit successive layers of infrared-absorbing (IR-absorbing) ink onto powder material, followed by exposure to IR energy for selective melting of the powder layer. Electrophotographic 3D-printing employs a rotating photoconductor that builds the object layer-by-layer from the base.

[0003] Selective laser sintering (SLS), multi-jet fusion (MJF), and high-speed sintering (HSS) 3D-printing methods use the same type of free-floating, non-fixed powder bed where each successive layer of powder is metered out requiring a dry flow of the power to ensure good part integrity. They generally have the same material requirements for compatibility with the printing process since the additively built object will experience similar stresses, only with different heating mechanisms to obtain the melt phase. Typically, a free-body diagram of a 3D printed object can be used to determine the residual stresses expected in the printed object. This is necessary for successfully building the object. If the residual stress is too high or the powder does not flow adequately, the object will deform or have unacceptable defects.

[0004] The residual stresses have typically been minimized for these powder bed-based 3D printers by using crystalline or semicrystalline thermoplastic polymers having a sufficiently large window between its melting temperature and its recrystallization temperature. Unfortunately, this has limited the polymers that have successfully used to print large or complex parts using SLS and MJF methods (e.g., polyamides), thus limiting the use of theseadditive manufacturing methods. Likewise, the use of semi-crystalline polymers upon recrystallization after heating to make the additive manufactured article may limit the article’s properties.

[0005] There have been examples of induced crystallization of polymers such as polycarbonate as described in U.S. Pat. No. 3,214,407 and 4,853,462; U.S. Pat. Publ. No. 2021 / 0277180, and PCT Pub. No. WO 2017 / 033146. These all used volatile organic solvents and the separation of the powder from solvent (e.g., precipitation) to form powders having induced crystallization. Unfortunately, the use of these solvents may limit the use of such polymer articles, for example, those requiring food contact.

[0006] Generally, to realize the necessary particle size allowing the melting and fusion of the powders into adhered successive layers has required the cryogrinding and classification of the polymers. Cryogrinding unfortunately is an energy intensive and laborious process.Likewise, the grinding process tends, due to brittle fracture of the polymer masses result in powders having angular asperities as well as excessive fine particles may be formed that must be heated and reprocessed. Because of the particulate angularity arising from the milling and presence of fine particles the flow and distribution of the particles may result in defects in the manufactured articles. As a result, flow aids have been necessary in many instances to allow the dry metering of the powder with the required uniformity to realize defect free undeformed additive manufactured parts.

[0007] The use of biodegradable polyesters has been limited to fused filament deposition 3D printing methods with its limitations. Biodegradable polyesters are desirable for singular used items such as prototypes, prototype molds such as those used in thermoforming (e.g., mold to form dental retainers and the like) and items contacting food.

[0008] Accordingly, it would be desirable to provide biodegradable polyester powder for additive manufacturing.SUMMARY

[0009] Applicant has discovered that a biodegradable polyester (e.g., polylactic acid, “PLA”) composition having an impact modifier (e.g., core shell rubber) may have increased toughness when crystallized after being formed into a powder. Illustratively, the compositionis a powder comprised of PLA and an impact modifier comprised of a silicone acrylic core shell rubber.

[0010] An illustrative method comprises, compounding a biodegradable polymer (e.g., PLA) and an impact modifier to form a blend, milling the blend to form a milled powder, and crystallizing the biodegradable polymer. Crystallizing occurs post compounding and generally may occur before or after milling and may include crystallizing before or after forming an article from the milled powder, but desirably is performed before milling or on the milled powder and in the presence of the impact modifier. In an illustration, the impact modifier is a silicone acrylic core shell rubber. The crystallizing may improve the toughness by 50% or more.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is an optical micrograph of the powder composition of this invention.

[0012] Figure 2 is a particle size distribution of an illustration of a powder composition of this invention.

[0013] Figure 3 is a particle size distribution of an illustration of a powder composition of this invention.

[0014] Figure 4 is a graph of the toughness of additive manufactured articles formed using the powder composition of this invention.

[0015] Figure 5 is a graph of the toughness of additive manufactured articles formed using the powder composition of this invention.

[0016] Figure 6 is a differential scanning calorimetry (DSC) plot of an uncrystallized compounded polylactic acid.

[0017] Figure 7 is a DSC plot of a crystallized compounded polylactic acid.DETAILED DESCRIPTION

[0018] While the disclosure has been described in connection with certain embodiments, it is to be understood that the disclosure is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims, which scope is to be accorded the broadestinterpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.

[0019] The biodegradable polymer may be any known in the art such as biodegradable polyesters such as PLA. The PLA may be any form of PLA such as those formed using L- lactide, D-lactide, or combination thereof. Desirably, the amount of L-lactide is at least 50%, 60%, 70%, 80% or 90% to 98% or 100% (100% may include trace amounts of D-lactide) by weight of the monomer used to make the PLA. Other polyesters such as those known in the art that readily form crystalline or semi-crystalline polymers may also be used.

[0020] The PLA or polyester may have any Mw to realize the milled powder. Typically, the PLA has a weight average molecular weight (Mw) of about 10 kDa to 500 kDa. The melt flow rate of the PLA or polyester may be any useful for an additive manufacturing powder. Generally, the MFRs may be from 25, 50, 60 or 70 to 90, 100, 125 or 150 grams (210°C / 10 min, 2.16kg). Examples of suitable PLAs are available under the tradename INGEO Biopolymer 625F, 6252D, and 3260HP from Nature Works LLC and LUMINY L105 from Total Corbion PLA.

[0021] The impact modifier may any useful for toughening polymers such as core shell rubbers. Core shell rubber is comprised of particles having a core of elastomeric material and a shell of a protective material. Typically, the core is comprised of an elastomer having a low Tg to realize the toughening of the toughened thermoplastic polymer such as about 0 °C or less, about -25 °C or less, or about -40 °C or less. Exemplary core materials include polymers of siloxanes, silicones, ethylene, propylene, butadiene, acrylates, methacrylates and the like. The impact modifier may be present in any amount useful to realize the desired characteristics. Generally, the amount impact modifier is from 0.1%, 0.5%, 2% or 5% to 40%, 30%, 20%, 15%, or 10% by weight of the biodegradable polymer and impact modifier.

[0022] The shell is a relatively rigid polymer and may contain reactive groups that react with the polyester (reactive shell or reactive surface), but in some illustrations there may be essentially no reactive groups such as less than 1% to none of the groups at the surface are reactive (unreactive shell or unreactive surface). Exemplary reactive groups on the surface of the shell of the core shell rubber may include glycidyl, maleic anhydride, and the like. The shell may further comprise polymer chains derived from one or more monomers that form rigid polymer chains. Any monomers which form rigid polymer chains may be utilized. Themonomers may polymerize by free radical polymerization. The monomers may be capable of polymerizing in emulsion polymerization processes. Exemplary classes monomers arc alkyl (meth) acrylates, styrenics, acrylonitriles, and the like. Exemplary alkyl (meth)acrylates include alkyl acrylates, such as methyl acrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, 2-ethylhexyl acrylate, and alkyl methacrylates, such as hexyl methacrylate, 2- ethylhexyl methacrylate, n-lauryl methacrylate, n-butyl acrylate may be preferred. The shell may be prepared from alkyl (meth)acrylates, crosslinkers and graft-active monomer units. Multifunctional compounds may be used as crosslinkers. Examples include ethylene glycol dimethacrylate, propylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate and 1,4- butylene glycol dimethacrylate. The following compounds individually or in mixtures may be used for inserting graft-active sites: allyl methacrylate, triallyl cyan-urate, triallyl isocyanurate, allyl methacrylate. Allyl methacrylate may also act as cross-linker. These compounds may be used in amounts of about 0.1 to about 20 percent, based on the weight of core shell rubber. The preferred graft shell includes one or more (meth)acrylic acid (Ci-Cs)- alkyl esters, especially methyl methacrylate copolymerized with glycidyl(meth)acrylate.

[0023] The core may have grafting sites on its outer surface to facilitate bonding of the shell to the core. The core is a particle having a sufficient size to positively impact the impact properties and the environmental stress crack resistance of the composition of the invention. The particles size may be a median particle size (D50 value) of about 0.05 microns or greater or about 0.1 microns or greater. The particles size may be a median particle size (D50 value) of about 5.0 microns or less, about 2.0 microns of less or about. 1.0 micron or less. The weight ratio of the core to the shell may be any as typically used in the art such as from about 1:99 or greater, about 2:98 or greater or about 3:97 or greater. The weight ratio of the core to the shell may be about 95:5 or less, about 90:10 or less or about 80:20 or less. The core shell rubber may include a silicone acrylic core shell rubber that may or may not have an epoxymodified shell such as those available from Mitsubishi Chemical under the tradename METAB LEN.

[0024] The powder may have further components such as those commonly used in thermoplastic polymers. Exemplary additives include ignition resistant additives, stabilizers, colorants, antioxidants, antistats, silicone oils, flow enhancers, mold releases, etc. Compounds which stabilize the PLA against degradation caused by, but not limited to heat,light, and oxygen, or a mixture thereof may be used. Fillers and reinforcements may also be present. Exemplary fillers include those that may be biodegradable or environmentally benign including, for example, organic fillers from ground organic matter (e.g., coffee, nut shells, rice husks, bamboo and the like), oxides (CaO and TiCh), talc, clay, wollastonite, mica, glass or a mixture thereof.

[0025] If used, such additives and / or fillers may be present in the compound compositions in an amount about 0.01 percent by weight or greater, about 0.1 percent by weight or greater, about 1 percent by weight or greater, about 2 percent by weight or greater, or about 3 percent by weight or greater based on the weight of the compositions. The additives and / or fillers may be present in an amount of about 40 percent by weight or less, about 30 percent by weight or less, about 20 percent by weight or less, about 15 percent by weight or less, about 10 percent by weight or less, about 5 percent by weight or less based on the weight of the composition (i.e., biodegradable polymer, impact modifier and any additives / fillers). The additives may be present in amounts up to 5 weight percent while fillers may be present in amounts up to 40 weight percent based on the weight of the compositions.

[0026] The compounding may be performed by any suitable method such as those known in the art, with polymer extrusion being applicable. Illustratively, compounding may be any suitable method for blending organic polymers under shear at temperatures sufficient to realize the desired blend PLA and impact modifier. The agitation may be any suitable, but typically requires high shear which may be provided by known apparatus such as extruders (e.g., twin screw), banbury mixers, colloid mills, homogenizers, ultrasonic agitation and combinations thereof. In particular it may be desirable to provide high shear by a twin screw extruder. The agitation may be provided the entire time for forming the composition, such as provided for by extrusion of the composition into pellets and the like. Pellets may be any shape or morphology and generally pellets may be any having an equivalent size diameter of at least 1 mm, 2 mm or 5 mm to about 5 cm, 2 cm or 1 cm.

[0027] The temperature may be any in which sufficient flow of the PLA occurs for time sufficient to realize the compounded composition. Desirably, the temperature is within 30°C or 20 °C of the melt temperature of the PLA and for a short time to reduce any decomposition or reduction of the molecular weight of the PLA. Typically, the temperatureis from about 150 °C to about 200 °C and the time may be from 1 minute, 5 minutes, 10 minutes or 15 minutes to 4 hours, 3 hours, 1 hour or 0.5 hour.

[0028] The milling may be performed as described in WO2022 / 245720, incorporated herein by reference. Illustratively, the milling typically comprises cooling of a thermoplastic polymer below glass transition temperature to its brittleness temperature. Brittleness temperature is as defined in ASTM D746 or ISO 974. The temperature may be further below the brittleness temperature. The cooling may be by any suitable such as those known in the art of cryomilling. Illustratively, the cooling may be refrigeration or exposure to dry ice or liquid nitrogen. The temperature of cryomilling may be any suitable temperature depending on the particular thermoplastic polymer and its glass transition temperature and brittleness temperature. Typical temperatures may be any temperature below or equal to 40 °C, 20 °C, 0 °C, -25 °C, -50 °C to about -75 °C, -100 °C, -150 °C, or -190 °C.

[0029] The milled powder may then be further crystallized by heating to a temperature above the glass transition temperature (Tg) of the biodegradable polyester to below its melting point (Tm) for a sufficient time to impart further crystallinity. Typically, the time is at least about 30 minutes to 24 hours with 1 to 5 hours generally being useful. The atmosphere may be any useful such as air, dry air, nitrogen, or an inert gas.

[0030] Illustratively, the powder generally has a particle size and size distribution that is useful for making additive manufactured articles and typically have an average or median particle size (Dso), by volume, from about 1 micrometer (pm), 10 pm, 20 pm, 30 pm or 40 pm to 150 pm, 125 pm, 110 pm or 100 pm. Likewise, to enable consistent heating and fusion of the powder, the powder desirably has a D90 of at most 300 pm, 200 pm or 150 pm. To aid in flowability the thermoplastic polymer powder desirably has a Dio of at least 0.1 pm, 0.5 pm or 1 pm by volume. D90 means the particle size (equivalent spherical diameter) in the particle size distribution, where 90% by volume of the particles are less than or equal to that size; similarly, D50 means the particle size (equivalent spherical diameter) in the particle size distribution, where at least 50% by volume of the particles are less than that size, and Dio means the particle size (equivalent spherical diameter) in the particle size distribution, where at least 10% by volume of the particles are less than that size. The particle size may be determined by any suitable method such as those known in the art including, for example, laser diffraction or image analysis of micrographs of a sufficient number of particles (-100 to-200 particles). A representative laser diffractometer is one produced by Microtrac such as the Microtrac S3500.Illustrations

[0031] Illustration 1. A powder comprising a biodegradable polyester and an impact modifier comprised of a core shell rubber.

[0032] Illustration 2. The powder of illustration 1, wherein the core shell rubber is a silicone acrylic core shell rubber having an unreactive shell.

[0033] Illustration 3. The powder of illustration 2, wherein the biodegradable polyester has been further crystallized.

[0034] Illustration 4. The powder of any one of the preceding illustrations wherein the biodegradable polyester is a polylactic acid.

[0035] Illustration 5. A method of forming a powder comprising (i) compounding a biodegradable polymer and an impact modifier to form a blend, (ii) milling the blend to form a milled powder, and (iii) crystallizing the biodegradable polymer.

[0036] Illustration 6. The method of illustration 5, wherein the crystallizing comprises heating the biodegradable polymer above its glass transition temperature and below its melting point.

[0037] Illustration 7. The method illustration 6, wherein the crystallizing is for a time of 1 hour to 5 hours.

[0038] Illustration 8. An article comprised of the powder of any one of illustrations 1 to 4 fused together.

[0039] Illustration 9. An article comprising the powder made by the method of any one of illustrations 5 to 7 fused together.

[0040] Illustration 10. The article of either illustration 8 or 9, wherein the article is an additive manufactured article.

[0041] Illustration 11. A method for forming a biodegradable polyester powder comprising compounding a biodegradable polymer to form pellets and milling the pellets to form a powder having a D50, by volume, from 10 pm, to 100 pm, a D90 of at most 200 pm and a D10 of at least 1 pm by volume.

[0042] Illustration 12. The method of illustration 11 further comprising crystallizing the biodegradable polymer after compounding.

[0043] Illustration 13. The method of illustration 12, wherein an impact modifier is present when crystallizing.

[0044] Illustration 14. A polylactic acid powder having a single melt peak.

[0045] Illustration 15. The poly lactic acid powder of illustration 14, wherein the polylactic acid powder has a D50, by volume, from 10 pm, to 100 pm, a D90 of at most 200 pm and a D10 of at least 1 pm by volume.

[0046] Illustration 16. The polylactic acid powder of illustration claim 14 or 15, wherein the polylactic acid powder is further comprised of an impact modifier.

[0047] Illustration 17. The polylactic acid powder of illustration 16, wherein the impact modifier is a core shell rubber.

[0048] Illustration 18. The polylactic acid powder of illustration 17, wherein the core shell rubber has an unreactive shell.

[0049] Illustration 19. The polylactic acid powder of illustration 17, wherein the core shell rubber is a silicone acrylic core shell rubber.Examples

[0050] Natureworks 6252D PLA is compounded with the impact modifiers as shown in Table 1 to form compounded pellets. LOTRYL® 29 MA 03 is a copolymer of ethylene - methyl acrylate available from SK Functional Polymer. ELVALOY PTW is an ethylene terpolymer available from The Dow Chemical Company. LOTADER AX8900 is a random ethylene-methyl acrylate-glycidyl methacrylate terpolymer available from SK Functional Polymer. METABLEN S2200 is a silicone acrylic core shell rubber having a reactive modified epoxy surface available from Mitsubushi Chemical. METABLEN S2030 is a silicone acrylic core shell rubber without a reactive surface available from Mitsubishi Chemical.

[0051] The compounded pellets are milled as described in WO2022 / 245720 to form milled powder (Figure 1). A representative particle distribution is shown Figures 2 and 3 after screening the milled powder with a 106 micrometer screen. Complex parts are printed on a Farsoon 252P 3D printer, with the complex parts including dental molds useful forthermoforming dental retainers showing good detail and withstanding post article formation sand or microblasting.

[0052] To test the effect of the crystallization and impact modifiers, notched Izod impact strength (ASTM D256) bars are produced by injection molding at about 400 °F using Milacron ROBOSHOT SllOiB. For compounded pellets not further crystallized (Uncrystallized) by heating to 130 °C for two hours the toughness results are shown in Figure 4. Those compounded pellets crystallized by heating to 130 °C for two hours (Crystallized) are shown in Figure 5. From the results it is readily apparent that the further crystallizing of the biodegradable polymer powder substantially increases the toughness of an additive manufactured article made with the crystallized milled powder regardless of the impact modifier used. It has also been discovered that silicone acrylic core shell rubbers and particularly those that are unreactive (e.g., unreactive shell) give surprisingly good results with or without further crystallization. The further crystallizing may result in a further increase in toughness of at least 20%, 30%, 40% or 50% to 500% or 300%. The compounded PLA that has not undergone crystallizing displays a complex melting behavior with a large tail and appears to display two melt peaks indicative of a second crystalline phase (Figure 6). The further crystallized compounded PLA pellets display a sharper onset melting behavior (Figure 7) without a secondary peak as well as an enthalpy more than 10% greater than melt enthalpy of the compounded pellets that have not undergone crystallizing. That is the further crystallized compounded pellets display a single melt peak (i.e., in the absence of a tail and secondary peak indicative of a second crystalline phase.Tabic 1.

Claims

What is claimed is:

1. A powder comprising a biodegradable polyester and an impact modifier comprised of a core shell rubber.

2. The powder of claim 1, wherein the core shell rubber is a silicone acrylic core shell rubber having an unreactive shell.

3. The powder of claim 2, wherein the biodegradable polyester has been further crystallized.

4. The powder of any one of the preceding claims wherein the biodegradable polyester is a polylactic acid.

5. A method of forming a powder comprising (i) compounding a biodegradable polymer and an impact modifier to form a blend, (ii) milling the blend to form a milled powder, and (iii) crystallizing the biodegradable polymer.

6. The method of claim 5, wherein the crystallizing comprises heating the biodegradable polymer above its glass transition temperature and below its melting point.

7. The method claim 6, wherein the crystallizing is for a time of 1 hour to 5 hours.

8. An article comprised of the powder of any one of claims 1 to 4 fused together.

9. An article comprising the powder made by the method of any one of claims 5 to 7 fused together.

10. The article of either claim 8 or 9, wherein the article is an additive manufactured article.

11. A method for forming a biodegradable polyester powder comprising compounding a biodegradable polymer to form pellets and milling the pellets to form a powder having aD50, by volume, from 10 pm, to 100 pm, a D90 of at most 200 pm and a Dio of at least 1 pm by volume.

12. The method of claim 11 further comprising crystallizing the biodegradable polymer after compounding.

13. The method of claim 12, wherein an impact modifier is present when crystallizing.

14. A polylactic acid powder having a single melt peak.

15. The polylactic acid powder of claim 14, wherein the polylactic acid powder has a D50, by volume, from 10 pm, to 100 pm, a D90 of at most 200 pm and a Dio of at least 1 pm by volume.

16. The polylactic acid powder of either claim 14 or 15, wherein the polylactic acid powder is further comprised of an impact modifier.

17. The polylactic acid powder of claim 16, wherein the impact modifier is a core shell rubber.

18. The poly lactic acid powder of claim 17, wherein the core shell rubber has an unreactive shell.

19. The polylactic acid powder of claim 17, wherein the core shell rubber is a silicone acrylic core shell rubber.

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