HIGH CRYSTALLINE POLY(LACTIC ACID) FILAMENTS FOR MATERIAL-EXTRUSION BASED ADDITIVE MANUFACTURING

Abstract

Provided is a new and better solution to the problems associated with the premature softening of PLA filaments in the additive manufacturing of three dimensional articles. It is based upon the finding that poly (lactic acid) filaments with high crystallinity offer much better resistance to heat-induced softening. The crystalline poly (lactic acid) filament can accordingly be used in the additive manufacturing of three dimensional articles without encountering the problems associated with premature softening, such as poor quality and printer jamming. The crystalline poly (lactic acid) filaments can also be used in additive manufacturing of three dimensional articles without compromising the quality of the ultimate product, reducing printing speed, increasing cost, or leading to increased printer complexity. It more specifically discloses a filament for use in three-dimensional printing which is comprised of crystalized poly (lactic acid), wherein said filament has a diameter which is within the range of 1.65 mm to 1.85 mm.

Claims

1. A filament for use in three-dimensional printing, said filament being comprised of crystalized poly(lactic acid), wherein said filament has a diameter which is within the range of 1.65 mm to 1.85 mm or which is within the range of 2.75 mm to 3.15 mm.

2. The filament as specified in claim 1, wherein said filament has a diameter which is within the range of 1.70 mm to 1.80 mm.

3. (canceled)

4. The filament as specified in claim 1, wherein said filament has a diameter which is within the range of 2.80 mm to 3.05 mm.

5. A filament for use in three-dimensional printing, said filament being comprised of crystalized poly(lactic acid), wherein said crystalized poly(lactic acid) has a melting point which is within the range of about 145 C. to about 185 C.

6. The filament as specified in claim 1, wherein said crystalized poly(lactic acid) has a degree of crystallinity which is within the range of 5 percent to 40 percent.

7. The filament as specified in claim 5, wherein said crystalized poly(lactic acid) has a degree of crystallinity which is within the range of 10 percent to 30 percent.

8. The filament as specified in claim 1, wherein said crystalized poly(lactic acid) exhibits virtually no heat of crystallization.

9. The filament as specified in claim 1, wherein the poly(lactic acid) contains a nucleating agent.

10. The filament as specified in claim 1, wherein said filament is capable of passing through a ring gauge having an internal diameter which is 0.15 mm larger than the average diameter of the filament at a speed of 50 meters/minute without breaking.

11. In the process of manufacturing a three-dimensional article by additive manufacturing which includes extruding a filament of poly(lactic acid) into a desired geometric shape, the improvement which comprises said filament of poly(lactic acid) being comprised of crystalized poly(lactic acid).

12. The process as specified in claim 11, wherein the crystalized poly(lactic acid) has a degree of crystallinity which is within the range of 5 percent to 40 percent.

13. The process as specified in claim 12, wherein said crystalized poly(lactic acid) has a degree of crystallinity which is within the range of 10 percent to 30 percent.

14. The process as specified in claim 11, wherein said crystalized poly(lactic acid) exhibits virtually no heat of crystallization.

15. The process as specified in claim 11, wherein said crystalized poly(lactic acid) has a melting point which is within the range of about 145 C. to about 185 C.

16. The process as specified in claim 11, wherein said filament has a diameter which is within the range of 1.65 mm to 1.85 mm.

17. The process as specified in claim 16, wherein said filament has a diameter which is within the range of 1.70 mm to 1.80 mm.

18. The process as specified in claim 11, wherein said filament has a diameter which is within the range of 2.75 mm to 3.15 mm.

19. The process as specified in claim 18, wherein said filament has a diameter which is within the range of 2.80 mm to 3.05 mm.

20. The process as specified in claim 11, wherein said filament is as specified in claim 5.

21. A method for manufacturing crystalline poly(lactic acid) filaments which are particularly useful in the additive manufacturing of three dimensional articles, said method including the steps of: (1) extruding molten poly(lactic acid) into the form of an amorphous filament, (2) collecting the amorphous filament on a spool to make a spool of amorphous poly(lactic acid) filament, and (3) heating the spool of amorphous poly(lactic acid) filament to a temperature of at least the glass transition temperature of the poly(lactic acid) for a period of time which is sufficient to substantially crystallize the poly(lactic acid), and (4) allowing the spool of crystallized poly(lactic acid) filament to cool to ambient temperature.

22. The method as specified in claim 21, wherein the spool of amorphous poly(lactic acid) is heated to a temperature which is within the range of 55 C. to about 145 C. in step (3).

23. The method as specified in claim 21, wherein amorphous filament is drawn to a draw ration which is within the range of 1:3.5.

24. The method as specified in claim 21, wherein amorphous filament is drawn to a draw ration which is within the range of 1.1:1.75.

25. (canceled)

26. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is an illustration showing a typical printer head or extruder as is used in additive manufacturing printers.

[0015] FIG. 2 illustrates the premature softening of a filament in the filament barrel of a printer used in additive manufacturing.

[0016] FIG. 3 provides the DSC of as-extruded and heat-treated filaments which were made in accordance with the techniques of this invention.

DETAILED DESCRIPTION OF THE INVENTION

[0017] Poly(lactic acid), which is sometimes abbreviated as PLA, is a high molecular weight polyester which is synthesized by the polymerization of lactide monomer, which is a cyclic dimer of lactic acid, or 2-hydroxypropionic acid. Lactic acid is a chiral molecule with two enantiomeric forms, l-lactic acid d-lactic acid. Typically l-lactic acid and d-lactic acid are both present in PLA. The composition of l- and d-lactic acid is critical in determining the crystallization behavior of PLA, including the degree of crystallinity and crystallization kinetics. Most commercially available PLA has higher l-lactic acid content. When d-lactic acid content increases, the degree of crystallinity, melting temperature, crystallization rate all decrease. PLA will show very little tendency to crystallize when the content of d-lactic acid exceeds 15%. The PLA for the current invention is preferred to have an l-lactic acid content in the range of 85% to 100%. Examples of such PLA materials are 2500 HP, 4032D, 2003D, 4043D and 7001D from NatureWorks LLC.

[0018] Most PLA filaments used in FDM/FFF based 3D printing are produced by melt extrusion. In the melt extrusion process, fully dried PLA pellets, along with other ingredients, are fed into a polymer extruder (either single-screw or twin-screw) with a cylindrical die and continuously extruded. The extrudate is subsequently quenched/cooled and pulled by a puller to give the desired physical dimensions before being collected. The process can also include equipment such as melt or gear pumps (to ensure a stable output), laser micrometers (on-line measurement of the physical dimensions), etc.

[0019] Melt-extruded PLA often remains amorphous due to the slow crystallization rate of PLA. Those PLA filaments currently available for FDM/FFF based 3D printing are amorphous or have negligible crystallinity, resulting in a low softening temperature in the range of 55 C. to 65 C. (as dictated by the glass transition temperature). One way to induce crystallization in an extrusion process is to increase the orientation of the filament, which is generally done by using a large draw ratio and/or a cold drawing step in the extrusion process. However this was found to be undesirable for the 3D printing application as high orientation leads to too much relaxation of the polymer that can also cause the swelling issue as illustrated in FIG. 2.

[0020] This invention provides a novel method to manufacture PLA filaments with high degrees of crystallinity. The method of this invention involves the following key steps: (1) manufacturing the PLA filament using the melt-extrusion process; (2) spooling the filament on a spool; (3) heat-treat the spooled PLA filament at a temperature which is within the range of the glass transition temperature (Tg) of the PLA to about 80 C. above the Tg of the PLA) for an extended period of time which is sufficient to substantially crystallize the PLA in the filament.

[0021] As mentioned before, it is desired to have a relatively low draw ratio in the melt-extrusion process. Draw ratio, for simplicity, is defined as the ratio of the diameter of the cylindrical die used on the extruder to the final filament diameter. For example, if a 3 mm die is used to manufacture a filament of 1.75 mm diameter, the draw ratio is 3/1.75=1.714. Draw ratio affects both the heat-treat step (discussed later) and the 3D printing process. It was found that the draw ratio should be in the range of 1 to 3.5, and is preferred to be in the range of 1.1 to 1.75.

[0022] Before applying the heat-treat step, it is critical to have the filament spooled. Spooling allows the filament to be under slight tension which can help maintain the correct physical dimensions and prevent too much kinkiness when heat is applied.

[0023] The heat-treat step is the step that imparts crystallinity to the PLA filament. Annealing the filament at a temperature which is above the Tg of the poly(lactic acid) for an extended period of time allows the PLA to slowly crystallize. The temperature range suitable for this step is from the Tg of the PLA to 80 C. above the Tg of the PLA, and preferred to be in the range of 10 C. above the Tg of the PLA to 40 C. above the Tg of the PLA. For instance if the PLA has a Tg of about 60 C., the preferred heat-treat temperature will be in the range of about 70 C. to 100 C. The choice of temperature should be high enough to allow enough polymer chain mobility for crystallization to occur, but not so high as to induce significant sticking or even melting of the filament. For PLA it was found that a temperature which is within the range of 70 C. to 100 C. is a good general temperature range. The required time for the heat treatment depends on the temperature, and is recommended to be no less than 1 hour, and preferred to be 2 hours or more. For example, the optimum heat treatment profile for a PLA filament produced using 4043D from NatureWorks LLC was found to be 90 C. for 2 hours.

[0024] Nucleating agents may be used to expedite the heat-treat step, by increasing the rate of nucleation for the crystallization process. Examples of nucleating agents are: talc, silica, graphite, clay, inorganic salts, organic metal salts, inorganic pigments (such as titanium dioxide or carbon black), metal oxides, amides, and esters. Such nucleating agents can accordingly be included in the PLA at a level which is within the range of about 0.1 weight percent to about 2 weight percent. In cases where nucleating agents are included they are typically present at a level with is within the range of 0.5 weight percent to 1 weight percent.

[0025] It is critical to maintain the physical dimensions unchanged or of little change before and after the heat-treat step. In addition to the temperature, the draw ratio and spooling are both important. The draw ratio should not be too large, as a large draw ratio was found to cause too much kinkiness, change in diameter, and also sticking of the filament. The draw ratio should be in the range of 1 to 3.5, and is preferred to be in the range of 1.1 to 1.75. Spooling gives the filament slight tension, without which the filament will become kinky and the dimensions can change significantly. Since most 3D printing filaments are supplied in spools, it is also convenient to heat-treat the filament in spooled form.

[0026] The PLA filament disclosed in this invention, unlike any other PLA filaments made using conventional processes, exhibits high degrees of crystallinity. The degree of crystallinity can be characterized using differential scanning calorimetry (DSC). In a typical DSC experiment, a small (several mg) sample of the filament is heated at a constant heating rate, from ambient or sub-ambient temperature to a high temperature that is higher than the Tm of the filament. The heat flow data is collected and plotted against temperature. The degree of crystallinity can be calculated as:

[00001]$\left(100.Math..Math.\%\right)=\frac{.Math..Math.H_m-.Math..Math.H_c}{.Math..Math.H_f}100.Math.\%$

where H.sub.m, H.sub.c and H.sub.f are the heat of melting, heat of cold crystallization, and heat of fusion, respectively. H.sub.m and H.sub.c can be determined by integrating the endothermic melting peak and the exothermic cold crystallization peak, respectively, on the DSC curve. H.sub.f is taken from literature as 146 kJ/mol (Polymer Data Handbook, Oxford University Press, Inc., 1999). The key features of the PLA filament manufactured using the disclosed method are:

[0027] (1) The filament is fully crystallized and exhibit no or minimal cold crystallization (H.sub.c0) (whereas conventional, amorphous PLA will exhibit cold crystallization);

[0028] (2) The filament exhibits a degree of crystallinity in the range of 5-40%, more typically in the range of 10-30%.

[0029] The filament can be manufactured into almost any diameter. However the most commonly used diameters for 3D printing are about 1.75 mm and 3 mm. It is important for the diameter to have a small variation, as large variations in diameter can lead to poor printing quality and feeding problems. It is preferred for the filament to have a variation of less than 0.1 mm.

[0030] The filament useful herein is a solid, crystalized PLA filament at room temperature and/or at the temperature it is loaded into the printer and/or the printing head. This contrasts with other printers which employ, for example, liquid printing solutions. Without intending to be limited by theory, it is believed that the solid PLA filaments herein are able to be applied via, for example, direct mechanical pressure of the filament via the rollers 2 in FIG. 1 which then feed the filament to the heater block 4. The PLA is then melted by the heater block 4 and extruded out of the nozzle 5 to form the printed object 6. The PLA extruded from the nozzle typically cools down immediately upon extrusion from the nozzle 5 so as to then solidify into the printed object 6 formed from crystalized PLA. Accordingly, it is believed that due to the high Tg and crystallinity of the present filaments, the printed object 6 (formed from crystalized PLA) will more quickly reach the desired hardness. Furthermore, it is believed that the use of such a filament will reduce premature softening of the filament in the barrel of the printer and swollen polymers in the barrel, so as to avoid increased viscosity in the barrel. This in turn is believed to produce more consistent feeding of the PLA though the nozzle and to significantly reduce jamming of the printer/extruder. Without intending to be limited by theory, it is also believed that the present invention allows maintained and/or improved printing quality as the filament diameter may more closely match the internal diameter of the filament barrel. This also reduces the need for an active or passive cooling element on the filament barrel, thereby reducing printer complexity.

[0031] The filament should be reasonably straight in order to feed properly into the printing head. As straightness or kinkiness is difficult to define, here we use a practical testing method to verify the straightness. The method involves passing the filament through a ring gauge with an internal diameter of d.sub.F+0.15 mm (d.sub.F being the average filament diameter) and a thickness of 8.5 mm at a speed of about 50 meters/minute. If the filament has large kinks, it will not be able to pass the ring gauge. These tests can be used as a quality assurance step for the filament with it being important for the filament to be capable of passing through the ring gauge at a speed of 50 meters per minute without breaking.

[0032] In addition to PLA, the filament can contain other ingredients, such as, but not limited to: colorants, pigments, fillers, fibers, plasticizers, nucleating agents, heat/UV stabilizers, process aids, impact modifiers, and other additives.

[0033] This invention is illustrated by the following examples that are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, parts and percentages are given by weight.

Example 1

[0034] PLA (grade 4043D from NatureWorks LLC) in the form of pellets was extruded using a 30 mm single-screw extruder equipped with a gear pump and a cylindrical die with a diameter of 2.25 mm to manufacture filament with a targeted diameter of 1.75 mm (draw ratio=1.286). The processing parameters are shown in Table 1. The extrudate was subsequently water-cooled, stretched by a puller to a final diameter of about 1.75 mm (continuously monitored by a dual-axis laser micrometer) and collected as a continuous filament on a large spool. The collected filament, without any post-processing, is designated as the as-extruded filament.

TABLE-US-00001 TABLE 1 2 5 (com- (gear 1 pres- 3 pump 6 Gear (feed sion (metering 4 en- (gear 7 pump zone) zone) zone) (flange) trance) pump) (die) (rpm) 170 C. 200 210 210 205 205 210 15

[0035] The filament on the large spool was then transferred to smaller spools. Each smaller spool contains about 750 grams of the as-extruded filament. The smaller spools loaded with as-extruded filaments were placed in a convection oven at 90 C. for 4 hours and then cooled in air to room temperature. The filament was designated as heat-treated filament.

[0036] The as-extruded filament and heat-treated filament were passed through a dual-axis laser micrometer to measure the diameter profile. The results are shown in Table 2.

TABLE-US-00002 TABLE 2 As extruded Heat-treated Average diameter (mm) 1.754 1.756 Standard deviation (mm) 0.016 0.017 Maximum diameter (mm) 1.799 1.793 Minimum diameter (mm) 1.729 1.730

[0037] As Table 2 suggests there was very little change in physical dimensions before and after the heat-treat step. The heat-treated filament has almost identical appearance to the as-extruded filament, based on visual inspection. Both filaments can pass a ring gauge with an internal diameter of 1.90 mm and a thickness of 8.5 mm at a speed of about 50 m/min.

Example 2

[0038] The as-extruded and heat-treated filaments in Example 1 were tested on a DSC instrument (Q2000, TA Instruments). The samples were heated from 20 C. to 200 C. at a heating rate of 40 C./min. The results are shown in FIG. 3. The dashed and solid curves are from the as-extruded filament and the heat-treated filament, respectively. Both samples show a glass transition at about 65 C. The as-extruded filament displays a peak at Tg due to physical aging. The main difference between the two filaments is in the melting behavior. The as-extruded filament shows both cold-crystallization and subsequent melting (see the inset in FIG. 3 for a magnified view of area in the dashed box). The degree of crystallinity is very low, i.e. <0.5%. Therefore the material remains almost completely amorphous. In contrast, the heat-treated filament shows no cold crystallization, indicating that the material had fully crystallized. The degree of crystallinity of the heat-treated filament is (calculated from the melting peak) about 15%.

[0039] Variations in the present invention are possible in light of the description of it provided herein. While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention. It is, therefore, to be understood that changes can be made in the particular embodiments described which will be within the full intended scope of the invention as defined by the following appended claims.