Method for producing clean thermoplastic particles

Abstract

A process for increasing purity of a low density polyethylene (LDPE) composition, comprising the steps of: a) providing a melted composition comprising LDPE having Mn of at least 5.0 kg/mol according to size exclusion chromatography, Mw of at least 50 kg/mol according to size exclusion chromatography, a density of 915 to 935 kg/m.sup.3 according to ISO1183 and a melt flow rate of 0.10 g/10 min to 80 g/10 min according to ISO1133:2011 measured at 190 C. and 2.16 kg, and b) providing particles from the melted composition by: b1) mixing a supercritical fluid in the melted composition to obtain a solution saturated with the supercritical fluid and b2) expanding the solution through an opening to obtain the particles.

Claims

1. A process for increasing purity of a low density polyethylene (LDPE) composition, comprising the steps of: a) providing a melted composition comprising LDPE having Mn of at least 5.0 kg/mol according to size exclusion chromatography, Mw of at least 50 kg/mol according to size exclusion chromatography, a density of 915 to 935 kg/m.sup.3 according to ISO1183 and a melt flow rate of 0.10 g/10 min to 80 g/10 min according to ISO1133:2011 measured at 190 C. and 2.16 kg, and b) providing particles from the melted composition by: b1) mixing a supercritical fluid in the melted composition to obtain a solution saturated with the supercritical fluid and b2) expanding the solution through an opening to obtain the particles.

2. The process according to claim 1, wherein the supercritical fluid is selected from the group consisting of CO.sub.2, NH.sub.3, H.sub.2O, N.sub.2O, CH.sub.4, ethane, propane, propylene, n-butane, i-butane, n-pentane, benzene, methanol, ethanol, isopropanol, isobutanol, chlorotrifluoromethane, monofluoromethane, toluene, pyridine, cyclohexane, cyclohexanol, o-xylene, dimethyl ether and SF.sub.6.

3. The process according to claim 1, wherein the supercritical fluid is SF.sub.6.

4. The process according to claim 1, wherein the LDPE has a melt flow rate as determined using ISO1133:2011 (190 C./2.16 kg) of from 0.10 to 70 g/10 min.

5. The process according to claim 1, wherein the LDPE particles obtained by step b) comprises essentially no amount of low molecular weight ethylene derived polymers with 31-59 carbons as determined by mass spectrometry direct inlet probe system.

6. The process according to claim 1, wherein the LDPE particles obtained by step b) comprise at most 1000 ppm of low molecular weight ethylene derived polymers with 10-32 carbons as determined by programmed temperature vaporisation-gas chromatography-mass spectrometry.

7. The process according to claim 1, wherein the mixing is performed using a mixing element, wherein the mixing element is selected from the group consisting of a static mixer, a stirrer and an extruder.

8. The process according to claim 1, wherein the melted composition provided in step a) comprises at least 95 wt % of the LDPE.

9. The process according to claim 1, wherein step a) involves the steps of: a1) polymerizing ethylene to obtain a composition comprising LDPE and ethylene, a2) removing ethylene from the composition of step a1) by a high pressure separator and a3) removing ethylene from the composition of step a2) by a low pressure separator to obtain the melted composition.

10. The process according to claim 1, wherein step a) involves the steps of providing a solid composition comprising the LDPE having Mn of at least 5.0 kg/mol according to size exclusion chromatography, Mw of at least 50 kg/mol according to size exclusion chromatography, a density of 915 to 935 kg/m.sup.3 according to ISO1183 and a melt flow rate of 0.10 g/10 min to 80 g/10 min according to ISO1133:2011 measured at 190 C. and 2.16 kg and melting the solid composition.

11. A process for making a master batch, comprising forming a masterbatch of the particles produced according to the process of claim 1, wherein the particles are not grinded.

12. The masterbatch according to claim 11.

13. A process for making a carpet backing, comprising forming a carpet backing from the particles produced according to the process of claim 1, wherein the particles are not grinded.

14. The carpet backing according to claim 13.

15. A process for increasing purity of a low density polyethylene (LDPE) composition, comprising the steps of: a) providing a melted composition comprising at least 97 wt % LDPE having Mn of at least 5.0 kg/mol according to size exclusion chromatography, Mw of at least 50 kg/mol according to size exclusion chromatography, a density of 915 to 935 kg/m.sup.3 according to ISO1183 and a melt flow rate of 0.10 g/10 min to 80 g/10 min according to ISO1133:2011 measured at 190 C. and 2.16 kg, by a1) polymerizing ethylene to obtain a composition comprising LDPE and ethylene, a2) removing ethylene from the composition of step a1) by a high pressure separator and a3) removing ethylene from the composition of step a2) by a low pressure separator to obtain the melted composition, and b) providing particles from the melted composition by: b1) mixing a supercritical fluid in the melted composition to obtain a solution saturated with the supercritical fluid and b2) expanding the solution through an opening to obtain the particles, wherein the LDPE particles obtained by step b) comprise at most 500 ppm of low molecular weight ethylene derived polymers with 10-32 carbons as determined by programmed temperature vaporisation-gas chromatography-mass spectrometry.

16. The process according to claim 15, wherein the LDPE particles obtained by step b) comprises essentially no amount of low molecular weight ethylene derived polymers with 31-59 carbons as determined by mass spectrometry direct inlet probe system.

17. The process according to claim 15, wherein the LDPE has a melt flow rate as determined using ISO1133:2011 (190 C./2.16 kg) of 0.10 to 50 g/10 min.

Description

EXAMPLES

(1) A high pressure/high temperature apparatus for batch micronisation, electrically heated, able to operate from 200 barg up to 300 barg and from 180 C. up to 300 C. (temperature control as accurate as 1 C.) was filled with LDPE, assembled, purged and pre-pressurized with gas until a pressure of approximately 15 barg was reached. The system was then heated up to 120 C. with injection of additional gas reaching 50 barg. Subsequently the temperature and the pressure were adjusted up to pre-expansion conditions as summarized in Table 1 by adding gas until the system reached equilibrium. An expansion to atmospheric pressure was performed by opening the high pressure valve at the bottom of the equipment, with simultaneous feed of fresh gas preheated to operating temperature at operating pressure to the system. Conditions and gases used for the experiments are shown in Table 1 below. In all cases solidified micronized particles were obtained.

(2) TABLE-US-00002 TABLE 1 Pre-expansion Pre-expansion pressure temperature Example Polymer Gas (bar) ( C.) 1 LDPE1 SF6 300 250 2 LDPE2 SF6 295 251 3 LDPE2 CO2 305 256

(3) The levels of impurities in the particles obtained were determined by DIP-MS, as well as the level of impurities in the pellets of LDPE1 and LDPE2.

(4) The particles obtained by examples 1-3 as well as pellets of LDPE1 and LDPE2 were subjected to DIP-MS for the determination of the presence of low molecular weight ethylene derived polymers.

(5) In DIP-MS, solid samples are introduced into a quartz cup located on the tip of a probe, which enters the vacuum chamber through an inlet. The tip of the probe is directly introduced into the ionization chamber, close to the ionization source. In the presence of light volatile material the heat of the filament (supplying the electrons which ionize the molecules) under vacuum conditions is enough to vaporize the components and the detection of the signal begins immediately. Higher-boiling components need more heat to vaporize. Therefore, the temperature at which the vaporization occurs gives an indication of which hydrocarbons are present in the sample. When vaporization starts at a certain temperature, it can be understood that the sample contains hydrocarbons with number of carbons corresponding to said temperature and hydrocarbons with more number of carbons. Hence, a lower starting temperature for the vaporization means that there are hydrocarbons with lower carbon numbers in the sample.

(6) The tip of the probe is heated in a temperature-programmed mode to detect the different components of the sample with a procedure similar to fractional distillation. The complete setup is designed in such a way that a rapid ionization before thermolytic degradation is guaranteed and the heating rate is set to avoid too rapid vaporization of the sample and saturation of the signal.

(7) Results are summarized below.

(8) LDPE2; intensity increase starting at 155 C. which coincides with the release of C.sub.34

(9) LDPE1; intensity increase starting at 180 C. which coincides with the release of C.sub.41

(10) Example 1 (LDPE1 treated by SF6); intensity increase starting at 380 C. which coincides with the release of >C.sub.70

(11) Example 2 (LDPE2 treated by SF6); intensity increase starting at 340 C. which coincides with the release of >C.sub.70

(12) Example 3 (LDPE2 treated by CO2); intensity increase starting at 270 C. which coincides with the release of C.sub.60

(13) The results show that the particles of Examples 1-3 do not contain detectable amount of low molecular weight ethylene derived polymers with 31-59 carbons, in comparison with the reference materials which contain C34+ polymers (LDPE2 or C41+ polymers (LDPE1).

(14) PTV-GC-MS

(15) The particles obtained by examples 1 and 3 as well as pellets of LDPE1 and LDPE2 were subjected to PTV-GC-MS for the determination of the presence of low molecular weight ethylene derived polymers.

(16) For LDPE1 and LDPE2, 5 gram of the samples was extracted with 200 mL n-hexane using 16 hours boiling under reflux. The extracts were concentrated by evaporating the solvent to 10 ml.

(17) For Example 1 (LDPE1 treated by SF6) and Example 3 (LDPE2 treated by CO2), 0.2 gram of the samples was extracted with 200 mL n-hexane using 16 hours boiling under reflux. The extracts were concentrated by evaporating the solvent to 1 ml.

(18) The extracts were injected to the equipment without further treatment. Calculations were performed against an external standard of naphthalene.

(19) Following PTV-GC-MS equipment was used:

(20) GC Agilent 6890N

(21) Detector Agilent 5973 Mass detector

(22) Autosampler Agilent G2614

(23) Software ChemStation G1701 DA version D.00.01.27

(24) Column Agilent HP5MS 60M*0.250 mm, 1.0 m film

(25) Injection 50 C., 20 l

(26) Temperature program Initial 70 C., hold for 0.5 min, ramp 10 C./min until 300 C., hold for 20 min.

(27) Detection 6 min, 30-500 AMU

(28) TABLE-US-00003 total amount of C10-C32 (ppm) LDPE2 >560 LDPE1 >540 Ex 1 210 Ex 3 >480

(29) Amounts of some types of hydrocarbons in these samples were found to be as follows (in ppm):

(30) TABLE-US-00004 C23 C24 C32 Dodecane Tridecane alkene cycloalkane cycloalkane LDPE2 >15 >30 4.3 12 0.4 LDPE1 >15 >30 3.3 10 0.4 Ex 1 2.3 8.0 0.6 1.4 <0.1 Ex 3 10 >25 <0.1 2.8 <0.1

(31) The amount of the low molecular weight ethylene derived polymers with C10-C32 carbons is decreased by the PTGG treatment. The decrease by the use of SF6 is especially large.