Polymer composition for selective sintering

Abstract

A polymer composition for the production of shaped objects via selective sintering includes ≥70.0 wt % of poly(ethylene terephthalate), wherein ≥25.0 wt % and ≤90.0 wt % of the poly(ethylene terephthalate has resulted from a selective sintering process as unsintered material. The polymer composition is a powder having a D.sub.10 of ≥10 and ≤40 μm, a D.sub.50 of ≥75 and ≤100 μm, and a D.sub.90 of ≥160 and ≤200 μm. The polymer composition allows for the production of an article having a continuous use temperature of ≥100° C., and results in a low change of molecular weight during exposure to selective sintering powder processing temperatures. Further, the polymer composition allows for a significant reduction of the waste material generated during selective sintering as the unsintered material does not have to be disposed of as waste but may be used again.

Claims

1. A polymer composition for the production of shaped objects via selective sintering wherein the polymer composition comprises ≥70.0 wt % of a thermoplastic polyester, wherein ≥25.0 wt % and ≤90.0 wt % of the thermoplastic polyester has been subjected to a thermal treatment, and the thermoplastic polyester that has been subjected to a thermal treatment is a thermoplastic polyester that has resulted from a selective sintering process as unsintered material, wherein the thermoplastic polyester is poly(ethylene terephthalate) having an intrinsic viscosity of ≥0.80 dl/g and ≤2.5 dl/g determined in accordance with ASTM D2857-95 (2007), a degree of crystallinity of ≥15.0%, and a heat of fusion of ≥50 J/g, and wherein the polymer composition is a powder having a D.sub.10 of ≥10 and ≤40 μm, a D.sub.50 of ≥75 and ≤100 μm, and a D.sub.90 of ≥160 and ≤200 μm.

2. The polymer composition according to claim 1, wherein the thermal treatment involves exposing the poly(ethylene terephthalate) to a temperature of more than 100° C. above the glass transition temperature T.sub.g and less than 10° C. below the peak melting temperature T.sub.p,m, wherein T.sub.g is determined in accordance with ISO 11357-2 (2013) and T.sub.p,m is determined in accordance with ISO 11357-3 (2011), first heating run.

3. The polymer composition according to claim 1, wherein the thermal treatment is performed for ≥1 hour.

4. The polymer composition according to claim 1 wherein the thermal treatment involves exposing the poly(ethylene terephthalate) to a temperature of ≥170° C. and ≤230° C.

5. The polymer composition according to claim 1, wherein ≥75.0 wt % of the poly(ethylene terephthalate) has been subjected to the thermal treatment.

6. The polymer composition according to claim 1, wherein the poly(ethylene terephthalate) is a poly(ethylene terephthalate) homopolymer.

7. The polymer composition according to claim 1, wherein the poly(ethylene terephthalate) has an intrinsic viscosity of ≥1.00 dl/g and ≤1.50 dl/g determined in accordance with ASTM D2857-95 (2007).

8. The polymer composition according to claim 1, wherein the polymer composition comprises ≥90.0 wt % of the poly(ethylene terephthalate) with regard to the total weight of the polymer composition.

9. The polymer composition according to claim 1, wherein the polymer composition further comprises ≥0.01 wt % and ≤5.00 wt % with regard to the total weight of the polymer composition of a flow agent selected from silica, alumina, phosphate, borate, titania, talc, mica, kaolin, attapulgite, calcium silicate, magnesium silicate or a combination thereof.

10. A process for the production of shaped objects using a polymer composition of claim 1, wherein the process comprises the steps of: (a) providing a quantity of a powder comprising the polymer composition of claim 1; (b) irradiating a portion of the polymer composition with a radiation source, such that the particles in that portion of the polymer composition absorb sufficient heat to reach a temperature above the peak melting temperature T.sub.p,m determined via differential scanning calorimetry, first heating run in accordance with ISO 11357-1 (2009); (c) terminating the exposure of the portion of the polymer composition to the irradiation source so that the temperature of the particles of the polymer composition decreases to below T.sub.p,m; and (d) removing the portion of the polymer composition that has not been subjected to irradiation by the energy source; wherein steps (a) through (d) are executed in this sequence.

11. A shaped object produced from the composition of claim 1 by selective sintering process, wherein the shaped object has a porosity of ≤5.0%.

12. The polymer composition of claim 1, wherein ≥50.0 wt % and ≤90.0 wt % of the poly(ethylene terephthalate) has been subjected to a thermal treatment.

13. The polymer composition of claim 1, wherein ≥50.0 wt % and ≤80.0 wt % of the poly(ethylene terephthalate) has been subjected to a thermal treatment.

Description

(1) A poly(ethylene terephthalate) (PET) homopolymer powder was used having the following properties: an intrinsic viscosity (IV) of 1.12 dl/g as determined in accordance with ASTM D2857-95; particle size distribution as determined in accordance with ISO 9276-2 (2014): D.sub.10=39 μm; D.sub.50=94 μm; D.sub.90=188 μm; and mean particle volume size=107 μm; weight average molecular weight (M.sub.w) of 117.1 kg/mol and number average molecular weight (M.sub.n) of 44.8 kg/mol, as determined in accordance with ISO 16014-1 (2012), and a polydispersity index M.sub.w/M.sub.n of 2.62

(2) The PET was a virgin PET, which is to be understood that the PET has not been subjected to any thermal treatment (such as in a selective sintering machine) after the production of the PET.

(3) The PET powder material was divided into 5 sample portions, which were each subjected to a thermal treatment according to table I below. The thermal treatment simulates the level exposure to heat, both as conducted heat and as radiation heating from e.g. infrared lamps) to which material that is not sintered during a selective sintering process is exposed.

(4) TABLE-US-00001 TABLE I Thermal treatment conditions and molecular weights for PET powders Sample A B C D E Thermal treatment time None 24 h 48 h 72 h 96 h M.sub.w (kg/mol) 117.1 113.9 118.8 125.5 130.0 M.sub.n (kg/mol) 44.8 44.2 46.3 47.3 49.1 M.sub.w/M.sub.n 2.62 2.58 2.56 2.65 2.65

(5) The thermal treatment was performed using an oven is which the powder samples were placed under vacuum at a temperature of 210° C. for a time period at indicated above in table I. The M.sub.w and M.sub.n of the samples were each determined after the thermal treatment.

(6) Sample A can be understood to be a comparative example. It can be observed that the molecular structure as defined by M.sub.w, M.sub.n and the M.sub.w/M.sub.n ratio changes but does so only to a very limited extent. The numerical differences are within the variation of the GPC measurement method (±12%), and hence one can conclude that there is no statistically significant difference. Surprisingly, and unlike other polymer powders for SLS, the thermally treated material of samples B through E are still of such quality that they can be utilized as sinterable material in a selective sintering process, even without addition of virgin PET powder.

(7) The thermally treated samples B-E, which may also be referred to as the aged powder samples, and comparative sample A, were subjected to differential scanning calorimetry (DSC) in accordance with ISO 11357-1 (2009). A first melting curve and a first cooling curve were recorded at a heating and cooling rate of 10° C./min, in nitrogen atmosphere. From the DSC curves, the extrapolated melt onset temperature of first heating (T.sub.ei,m) in ° C. the peak melt temperature at first heating (T.sub.p,m) in ° C., the heat of fusion in J/g, the extrapolated crystallisation onset temperature of first heating (T.sub.ei,c) in ° C., the peak crystallisation temperature (T.sub.p,c) in ° C., the heat of crystallisation in J/g, and the sintering window in ° C. were determined. The sintering window was calculated as T.sub.ei,m−T.sub.ei,c. Results are presented in table II.

(8) TABLE-US-00002 TABLE II DSC results Sample A B C D E T.sub.ei, m 238 238 239 238 239 T.sub.p, m 242 242 242 242 242 Heat of fusion 57.0 59.7 60.5 61.2 61.4 T.sub.ei, c 186 186 185 185 184 T.sub.p, c 175 176 175 173 173 Heat of crystallisation 28.5 28.8 26.8 28.5 27.1 Sintering window 52 52 54 53 55

(9) From the results in table II, it can further be concluded that the suitability of the samples B-E for use in selective sintering processes is not significantly affected by the thermal treatment. For example, the sintering window even increases, indicating that the temperature range to which the material is exposed during the sintering process even becomes less critical.

(10) To further determine the properties of the sample materials and their suitability for utilisation in selective sintering processes resulting in objects having desired properties, a quantity of sample A and a quantity of sample E, i.e. the material that has undergone the severest thermal treatment, were each subjected to selective laser sintering to produce test bars of 2 cm width, 5 cm length and 3 mm thickness.

(11) The selective laser sintering (SLS) was performed using an SLS machine comprising a CO.sub.2 laser source. To each of the powder sample A and E, a quantity of 0.05 wt % Aerosil flow promoter was added. The materials were pre-dried prior to processing via SLS. The SLS process was conducted in an atmosphere having an oxygen content of ≤1.0 wt %. The SLS process conditions are presented in table III.

(12) TABLE-US-00003 TABLE III SLS process conditions. Powder bed temperature (° C.) 228 Piston temperature (° C.) 180 Cylinder temperature (° C.) 175 Feed temperature (° C.) 160 Laser power (W) 30 Scan speed (m/s) 5 Hatch distance (μm) 100 Layer thickness (μm) 100

(13) The produced bars were subjected to determination of density and the colour in the form of the yellowness index of the bars. Density in kg/m.sup.3 was determined in accordance with ASTM D792 (2013) according to method A; yellowness index (YI) was determined in accordance with ASTM E313 (2010). Results are presented in table IV.

(14) TABLE-US-00004 TABLE IV Results of testing of SLS bars Test bar 1 2 3 Material used 100 wt % of Powder blend of 80 100 wt % of for printing sample A wt % sample C, 20 sample E wt % sample A Density 1.362 1.369 1.366 YI 0.23 −1.21

(15) These results present the density of the printed bars to be relatively unaffected by the use of either a virgin material or a material comprising thermally treated material.

(16) With regard to the yellowness index, the results show that the YI of the bar produced using the thermally treated material is even better than that of the bar produced using virgin material, indicating that use of such thermally treated material leads to a desirable colour of the produced object.