Recycled polymers for 3D printing

Abstract

A powder, called composite-based additive manufacturing (CBAM), is formed from waste products from various processes (injection molding, selective laser sintering) that are processed to have a particle size and distribution optimal for 3D printing technology. Alternative recycling processes include milling and sieving, emulsion extrusion, and liquid-liquid phase separation.

Description

[0001] Cross-references to related applications

[0002] This application claims priority to U.S. Provisional Patent Applications Nos. 63 / 133,666 (filed January 4, 2021) and 63 / 184,755 (filed May 5, 2021), which are incorporated herein by reference in their entirety. Technical Field

[0003] This invention belongs to the field of 3D printing processes and systems. More specifically, the improvements are for the material inputs and raw materials of systems for producing 3D printed parts. These improvements fall under the subfield of 3D printing pioneered by the Agents, which the Agents refer to as Composite-Based Additive Manufacturing (CBAM). Background Technology

[0004] 3D printing has many limitations. In particular, they often have significant material limitations. For example, selective laser sintering or similar methods such as high-speed sintering and processes based on it, like HP's multi-jet melting and Stratasys' selective absorption melting, primarily use materials called PA12 or PA11. This is because PA12 and PA11 have large thermal processing windows, which are unavailable for most thermoplastic polymers. Nevertheless, the sintering process requires polymers that have melted within a 10°C processing window. In the case of PA12 and PA11, these are expensive resins, and conventional processes for producing powders by precipitation are costly. This results in low availability and high cost. The polymer sintering process requires holding the polymer powder in the bed at high temperatures (just below the polymer's melting point) for extended periods. For polymers such as PA12 and PA11, this high-temperature heating can lead to polymer chain elongation and molecular weight increase. These phenomena can reduce the ability of the used material to be reused under standard sintering parameters. This, in turn, necessitates the complete disposal of waste powder or its mixing with the original material. This means the sintering process requires more than twice the amount of powder on average for this part, which is used in the production of the component, doubling the material cost. Because this powder must be discarded, this results in significant waste. A similar problem is more pronounced in stereolithography, where resins and photopolymers are more expensive than thermoplastic resins, but there is also substantial waste due to the limited activation life of two-component systems used by companies like Carbon. Stereolithography also requires expensive photoinitiators, and the thermosetting plastics produced by stereolithography are difficult to recycle.

[0005] Similarly, injection molding, blow molding, and thermoforming generate scrap from waste and dross, etc. For several reasons, including inclusions, impurities, and variations in melt flow and crystallinity, these primary processed materials do not perform as well as the virgin material. This means that in these production processes, the recycled material does not perform consistently in the manner necessary for large scale production. Increased utilization of recycled material would result in environmental improvements and lower cost of polymer raw materials. This would be beneficial to the introduction of methods using recycled polymers in 3D printing processes, which would be even more beneficial if the processing of the recycled material into powder for such use eliminated impurities as a byproduct of the processing. In addition to the issues mentioned above, post-consumer recycled materials, such as PET beverage bottles, are difficult to use in the recycled form due to contamination, colorants, and lack of uniformity in the material and its properties.

[0006] This impossible object CBAM process (Composite Based Additive Manufacturing) does not have these issues. CBAM is disclosed in a number of patent documents, including the following, each of which is hereby incorporated by reference in its entirety: 10,967,577, 10,046,552, and 9,827,754. It can use almost any thermoplastic powder. There is almost no waste since the excess powder is captured during the printing process and reapplied. In addition, there is no restricted thermal processing window since the heating is done in a flattener separate from the printing.

[0007] In CBAM, a computer model divides the part to be printed into slices of cross sections. Using a printing technique (e.g., inkjet printing), a liquid is printed on a porous plate in a shape corresponding to one of the object cross sections. The porous plate is typically carbon fiber, but can also include fiberglass or other suitable substrate. Likewise, the printing can occur at the end of a roll (or web) with cutting occurring at a downstream stage. The printed sheet is saturated with powder (typically thermoplastic powder) such that the powder adheres only to the printed areas and not to the non-printed areas. Various methods (e.g., vacuum, vibration, air knives) are employed to remove the non-adhered powder from the sheet. The sheet is then moved to a stacking station where it is placed on top of a previous sheet (if present) that has undergone similar processing to the immediately adjacent object cross section. The stacking machine uses tapered alignment pins to keep the sheets aligned, which fit into the holes printed on such sheets at the upstream printing stage. The process is repeated as many times as needed for the cross sections to produce an assembly block of multiple base sheets, each stacked on top of another in the exact order needed to represent all cross sections of the 3D object. To cause the powder on the printed areas to melt and coalesce around the fibers, the assembly block undergoes subsequent processing in the form of compression and heating. The assembly block resulting after the pressing and heating is then subjected to abrasion to remove the base material, e.g., the brittle carbon fiber areas that were not printed. The melted / fused areas remain with such abrasion and thus emerge from the process with the intended shape of the final 3D printed part as defined by the computer model. Advantageously, the use of carbon fiber and thermoplastic powder in this manner results in a part that is very durable, very well suited for high tolerances needed in industrial applications - thus, it is a "composite-based" 3D printed part. The '552 patent describes various aspects of the aforementioned system, as well as embodiments of subsystems that implement each stage (e.g., material supply, flexographic printing, powdering, depowdering, stacking, etc.). For base material that is not brittle, e.g., PET, a chemical removal process as described in the referenced patent can be used.

[0008] One of the advantages of the impossible object CBAM process is the ability to use high performance thermoplastic materials, such as PEEK, PEKK, PAEK, PPS, and PEI. PEEK, for example, is a very expensive material, costing about $65 per pound. Conventional PEEK is difficult to convert into a flexible powder because the milling process produces multi-faceted particles visible in FIGS. 1A and IB (prior art). FIG. 1A illustrates a raw PEEK block (which is not an issue for recycled PEEK as discussed herein), and FIG. IB (same as FIG. 1A, but at 500x magnification) illustrates the varying particle size. These powders have a large size distribution and a large amount of small particles; the overall average particle size D50 = 30 microns.

[0009] One of the challenges of the CBAM process is to get a sufficient amount of polymer (thermoplastic powder) on the sheet. Because the mass of the particles is calculated as the cube of the particle diameter, larger particles significantly increase the mass of the deposited polymer. Higher deposition mass results in stronger parts and requires less compression, which means that the thickness of the layers is greater, which in turn speeds up the process.

[0010] In the case of most powders, a flow agent such as Aerosil 200 (produced by Wacker Chemie AG, Essen, Germany) is added to the powder to improve flow properties. For example, in recycled PEEK, the angle of repose of the unprocessed powder is > 40°, after Aerosil 200 the angle of repose is < 35° (as measured by ASTM C1444). This makes the powder flow better in the CBAM process.

[0011] In the case of PEEK as an example, there is a large amount of PEEK supplied that is discarded in many processes, such as in injection molding. Discarded PEEK material typically comes from industries like pharmaceuticals or aerospace that only use virgin material for regulatory reasons. For several reasons, many molders do not regrind and use discarded parts, gates, and runners. Generally, in injection molding, only a small portion of the regrind material can be used to make new parts. SUMMARY

[0012] In CBAM, it has been discovered that PEEK injection molding scrap material can be regrind and reused in high proportions or completely. In addition and unexpectedly, this regrind injection molding scrap, when pulverized, results in a better particle size distribution and larger particles than virgin regrind material purchased at the manufacturer, such as at Solvay. The discarded injection molding material (such as parts, gates, and runners) is ground and screened for the appropriate particle size distribution and further used in the CBAM process.

[0013] In addition, it has been discovered that polyolefins, PET, or other polymers that are waste products of other non-CBAM technologies can be reprocessed and recycled using an emulsification technique. The emulsification process includes heating and mixing the thermoplastic material in a mutually insoluble liquid, then agitating or whipping at high speed to form droplets in the mixture, then cooling to solidify the thermoplastic material, and thereafter recovering the powder from the mixture. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1A (prior art) shows a virgin PEEK block at optical (low) resolution.

[0015] Figure IB (prior art) shows a close-up of virgin PEEK at 500x magnification and shows the associated non-uniformity of the powder particles.

[0016] Figure 2Recovered PEEK powder according to the grinding embodiment of the application is shown with a D50 between 80-100 microns.

[0017] Figure 3 A design drawing of an exemplary structure (solder tray) made according to the CBAM process using recycled PEEK according to the grinding embodiment of the application is shown.

[0018] Figure 4 A flow of recycled PEEK according to the grinding embodiment of the application is shown.

[0019] Figure 5 (Related art) A Russell ultrasonic Finnex device for high precision sieving is shown.

[0020] Figure 6 Recovered PEEK powder according to the grinding embodiment of the application is shown after addition of a flow agent.

[0021] Figure 7 A flow of recycled polymer according to the emulsification embodiment of the application is shown. DETAILED DESCRIPTION

[0022] Better particle size and distribution (D50 between 80 and 100 microns) results in better (strength and density) parts in CBAM. Figure 2 The output of the grinding embodiment described herein is depicted, maintaining this size and distribution. Since virgin thermoplastic materials have been universally discarded to date, and grinding is part of the raw material cost, this new use of high performance PEEK is less expensive in raw material cost than lower performance materials like PA12.

[0023] The recycled PEEK material used in CBAM results in a tensile strength of 140 MPa and a tensile modulus of 14 GPa, which are nearly identical to the performance of conventional PEEK powder. In addition, the use of this recycled powder reduces the carbon footprint of the end part. The use of this recycled powder can be extended to nearly any thermoplastic resin. These include, for example, PET from beverage bottles, polyolefins such as polyethylene and polypropylene, polystyrene, polycarbonate, and thermoplastic elastomers.

[0024] An exemplary end part made in the CBAM process from recycled PEEK is a solder tray. Figure 3 Two sides of a solder tray made from recycled PEEK powder according to the teachings herein are shown.

[0025] In CBAM, changes or variations in the melting point or melt viscosity (melt index) do not significantly affect the final part. Changes in polymer melt flow and viscosity generally have less impact in the CBAM process than in sintering-based processes. This is because the CBAM final processing step involves heating all the materials above the melting point and then compressing the assembly block to a preset thickness, which forces the molten thermoplastic material to flow and strengthen. The processes and results described herein show that recycled PEEK (and others) not only processes improved powder properties (e.g., greater size and better distribution for the base sheet covered in the powder stage), but also does so with typically discarded products from more traditional manufacturing processes. Positive environmental impacts inevitably arise, such as lower carbon footprint, opening up applications for injection molding waste and post-consumer recycled materials, such as PET from beverage bottles, or polyethylene or polypropylene from packaging, for example, which are not recycled for most of their products.

[0026] The method of using recycled materials is not limited to high-performance materials such as PEEK, but can be extended to many different materials, such as PET. PET material is used in other products, beverage containers, which have a large demand for recycling. This form of recycled material is about half the cost of the original material. Impurities and / or colorants that can be present in the recycled material are not a major problem for CBAM. This also applies to most thermoplastics, including polyolefins such as polyethylene or polypropylene, and engineering plastics such as PA12, as well as PEEK, PAEK, PEKK, PPS, PEI, etc. in many other thermoplastics. PA12 or PA11 powders from sintering processes can be recycled, extruded again, then converted to powder, for example, using emulsification or other processes, and reused in the CBAM process.

[0027] In addition, substrates made from recycled materials, such as non-woven PET supplied by Freudenberg Performance Materials (Weinheim, Germany), can be used and parts can be made using CBAM. This can be used with powder materials, such as EVA, and chemically remove excess material.

[0028] Example 1: Recycled PEEK from grinding

[0029] Regarding Figure 4The following process is representative of the steps required to prepare a batch of recycled PEEK material for the CBAM system. At step 10, injection molded scrap consisting primarily of PEEK is collected. At step 20, an operator grinds the scrap into particles, most of which contain the preferred size and distribution mentioned above: D50 above 30 microns, more preferably above 70 microns, and most preferably between 80-100 microns. At step 30, the operator can further screen the particles to further control the size and distribution to be within the desired range. At step 40, the operator can further add a flow aid. At step 50, the operator can second screen the particles (now containing the flow aid) to produce a result 60 - a desired final powder containing the desired particle size and distribution, and the flow aid.

[0030] In further detail regarding steps 10, 20, and 30, the PEEK resin used is obtained from an existing commercial injection molding operation. The sprue, runners, and other scrap material are collected and run through a grinding system to produce a fine powder. These particles are then processed through a Hosokawa Micro Air Jet Sieve tester equipped with a 140 mesh screen to obtain an initial powder batch.

[0031] In further detail regarding step 40, this material is treated with Aerosil 200 silica flow aid at a concentration of 1%. A high speed variable mixer is used to mix these flow aid particles with the PEEK powder to obtain a static angle between 28° and 34° as measured using ASTM C1444.

[0032] In further detail regarding step 50, the PEEK plus Aerosil mixture is second screened using a Russell Finnex Sieve Shaker Station equipped with a 140 mesh screen and ultrasonic sieve head (as shown in Figure 5 ).

[0033] In further detail regarding step 60, this second screened material (as shown in Figure 6 ) is then added to the CBAM system for production, for example, to the powderization subsystem.

[0034] The general reason that recycled PEEK from injection molding has been largely rejected by industry as a primary source of raw material in its raw form must deal with the polymer changes that occur in the material once heated to processing temperatures. It is believed that when this material, now in the form of scrap, is subjected to relatively high temperature heating and melting, and then cooling, the polymer chains are lengthened as compared to the original raw material. The injection molding process requires that the material have a consistent molecular weight and consistent melt flow and viscosity. This is available in the original powdered resin, but cannot be guaranteed in the regrind material. Conversely, as stated previously, the CBAM process accommodates polymers with longer chains and polymers with a low uniformity of chain length distribution.

[0035] Similarly, similar changes in viscosity and melt flow also occur in many pre-heated materials used in traditional sintering processes. This is why sintering scrap is often discarded as well. The pre-heated material exhibits inconsistent viscosity and melt flow that is unpredictable and prevents widespread reuse. Again, the CBAM process can accommodate these changes or variations in viscosity and melt flow while producing satisfactory workpieces.

[0036] Example 2: Recycled polymer from an emulsification process

[0037] Separate from the grinding implementation, the emulsification implementation similarly deploys recycled polymers with satisfactory end workpiece results. In this implementation, scrap polymer, which can be PEEK, olefin, PA12, etc., or any other polymer mentioned in this patent document, is melted and mixed with a mutually immiscible liquid. The heated mixture is then stirred or agitated at high speed to form droplets in the mixture. When the term "emulsification" is used, the resulting heated mixture can or can not precisely meet most technical definitions of emulsification, so long as it is a combined material that has the polymer, now melted, suspended or contained within it.

[0038] Upon cooling the two-phase liquid mixture, the polymer solidifies to form a powder that is then removed. This operation can take one of several forms. A continuous process such as emulsification can be used. One benefit of the emulsification process is that impurities and contaminants in the recycled material can be removed at the same time that the powder is produced. This means that post-consumer recycled polyethylene, polypropylene, or any color of PET can be used in this process. Furthermore, since the melt flow of the polymer is less important to CBAM, changes or variations in melt flow and molecular weight have only a small or non-existent impact on the process or the completed workpiece. Similarly, unlike many other processes where clear (more expensive) raw materials are considered to be more desirable material stock, both clear and colored raw materials are generally equally acceptable for CBAM workpieces.

[0039] As a non-limiting example of the foregoing discussion of the present invention, the extrusion sub-step can be performed using a prior art process that converts PBT pellets into a powder (R.G. Kleinjnen, M Schmid, K. Wegener: Production and Processing of a Spherical Polybutylene Terephthlate Powder for Laser Sintering, Applied Science, 2019, Vol. 9, pp. 1308 ff.). In the previously published discussion, 6 kg of PBT acrylic fiber (TORAYCON) 1200M pellets (Toray, Tokyo, Japan) were mixed with 9 kg of PEG polyethylene glycol (Polyglykol) 3500S flakes (Clariant, Muttenz, Switzerland) in a laboratory scale single screw extruder (Brabender Extrusiograph, Duesseldorf, Germany) with a barrel temperature profile of 230°C to 250°C. The extrudate was discharged through a slit die and cooled to room temperature; during this cooling step, the molten PBT domains (T m = 225°C) solidified in the molten PEG matrix (T m

[0040] In 2.5 kg batches of the blend, the polyethylene glycol (PEO) solid matrix phase was dissolved with water, the blends were agitated in a concrete mixer, and after the PBT pellets that were insoluble in water had settled, the blends were rinsed twice more. The resulting wet PBT was then dried. Microscopy showed that 75% of the PBT pellets were nearly spherical (aspect ratio A = 1.22), while the remaining 25% of the PBT pellets retained the fiber shape created by the shear field of the extruder. The spherical PBT pellets were classified by sedimentation to remove the smallest particles (< 10 microns), then dried and sieved to 150 microns. The particle size distribution had D10 = 9 microns, D50 = 35 microns, and D90 = 110 microns.

[0041] The classified, spherical PBT pellets were mixed with 0.05% Aerosil R812 flow aid (Evonik, Essen, Germany) and used to print tensile test bars on a Superstation 2000 laser sintering machine (DTM, Austin, Texas, USA). To the detriment of the intended use disclosed in the prior art, it was discovered that the powder obtained by the emulsification process crystallized at higher temperatures for unknown reasons. While this change adversely affected the SLS parts, it unexpectedly provided a satisfactory raw material for the CBAM for the reasons described above.

[0042] ​A second liquid mixing process can also be advantageously utilized. If the waste / abandoned thermoplastic material is combined with a solvent (as opposed to a non-solvent), powder particles can be formed by a process of liquid-liquid phase separation (LLPS). Briefly, the polymer-solvent system is heated to form a homogeneous single phase - a solution. For suitable selection of polymer molecular weight and concentration, the single solution phase will separate upon cooling into regions of polymer-rich liquid dispersed in a polymer-lean matrix. Interfacial energy will cause the dispersed regions to become spherical. Further cooling of the two-phase mixture will result in crystallization in the polymer-rich droplets, fixing their shape, and allowing recovery of the powder by simple filtration. In contrast to the emulsion method, LLPS does not use high temperatures (greater than the drying T m ) or intense mechanical shear fields to form the particles.

[0043] A prior art source provides a descriptive example of the LLPS process to prepare a powder now considered suitable for CBAM 3D printing (M. A. Dechet et al: Production of polyamide 11 microparticles for Additive Manufacturing by liquid-liquid phase separation and precipitation. Chemical Engineering Science, Vol. 197 (2019), pp. 11-25). 20 g of PA11 (Rislan BMNO, Arkema) were combined with 80 g of 99.5% ethanol, denatured with 0.1% MEK, in a DBA-3 autoclave (Berghof), capable of magnetic stirring at 200 bar and 100 rpm. The temperature was raised to 190°C and held for 15 minutes before cooling at 0.5-3°C / min. When the temperature reached 50°C, the reactor was opened and the PA11 powder particles were recovered by filtration on a Buchner funnel with Whatman #1 filter paper. The molecular weight, as measured by gel permeation chromatography, was reduced by about 20% with this process.

[0044] The dry powder was treated with 0.5 wt% hydrophobic fumed silica (Aerosil, Evonik). By electron microscopy, the particles appeared as irregular isometric particulate aggregates; by light scattering particle size distribution, the volume average was D503= 150 microns. If the continuous cooling was interrupted by isothermal holding at 120°C or 130°C for 30 minutes, the D503= 90 microns or D503= 50 microns, respectively. The SLS processing window for the LLPS powder was 15°C-19°C, depending on the cooling conditions. Tensile specimens were made on a DTM Sinterstation 2000, with a bed temperature of 170°C and a power of 0.6 J / mm2 The parts were said to have good appearance, but no tensile data were reported.

[0045] The LLPS process is a batch process that requires agitation at high pressure. The particle size can be controlled by the cooling rate. LLPS is easier to remove impurities and additives than the emulsion process.

[0046] Additionally, waste from high speed sintering or the multi-jet fusion process from HP, for example, PA 12 that would otherwise need to be discarded, can be recycled and reused in the present emulsion embodiments, as the changes in molecular weight and crystallinity do not pose an obstacle to CBAM. The same is true for PET recycled nonwoven substrates, and cellulosic or other natural fibers, and water soluble paper, and olefins of thermoplastic polymers such as PET and polylactic acid, extractable from corn.

[0047] With respect to Figure 7 The following process is representative of the steps required to prepare a batch of recycled polymer material from a CBAM system according to the emulsion embodiments. In step 110, the operator obtains a resin from waste from another process or product, such as PET bottles. This waste can also be a collection of PA 12 that was discarded after a laser sintering process. In step 120, the operator converts the waste into flakes (if it is not already in a semi-pulverized or fully pulverized state). This can include the use of industrial grinders, such as those used in the grinding embodiments. In step 130, the operator in turn performs the emulsion process to convert the flakes into a powder. As mentioned previously, this process will use agitation or stirring during the heating in the immiscible liquids. Finally, in step 140, the resulting powder is placed in the CBAM printing process in order to make a finished workpiece.

[0048] Just as injection molded parts are structurally distinct from welded or carved parts having the same overall shape, one of skill in the art will appreciate that a 3D printed CBAM part constructed using any of the methods described above will be structurally distinct from a 3D printed CBAM part made with virgin polymer. At a microscopic level, the use of the original polymer material from which the powder was made according to any of the descriptions above will have different characteristics and properties based on, for example, increased differences in polymer chain length, crystalline structure, etc. in the finished part.

[0049] While the foregoing specification and examples provide a description of the present application, various modifications can be made to the application without departing from the spirit and scope thereof. It is to be understood that the foregoing embodiments are provided merely as examples and are not to be construed as limiting the scope of the application. Various other embodiments are within the scope of the claims.

Claims

1. A process for making 3D objects from recycled materials comprising: a. converting a polymer material that has been used in an industrial process or consumer product into a recycled powder; b. embedding the recycled powder into a powderization stage of a 3D printing process that deposits the powder onto a substrate plate to make a cross-sectional layer corresponding to a planar layer of a 3D object; the converting step comprising: first, processing the polymer material into an altered polymer that contains smaller particles than the originally contained polymer material; second, agitating or stirring the altered polymer in a mutually immiscible liquid to obtain sufficient shear to produce a further altered polymer suspended in the liquid; third, mixing the further altered polymer with a matrix to form a polymer-matrix mixture, wherein the matrix has a lower melting point than the further altered polymer; fourth, placing the polymer-matrix mixture in an extruder that is heated above the melting point temperature of both the altered polymer and the matrix; fifth, extruding the polymer-matrix mixture above the melting point temperature of both the altered polymer and the matrix to form an extrudate; sixth, cooling the extrudate below the melting point temperature of both the altered polymer and the matrix; seventh, dissolving the matrix with a liquid in a mixing device to obtain a processed polymer; and eighth, drying the processed polymer.

2. The process of claim 1, wherein, the processing step produces an altered polymer in the form of particles, flakes, spheres, microparticles.

3. A 3D object made according to the process of claim 1.

4. The 3D object of claim 3, wherein, the 3D object comprises a solder tray.

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