METHOD FOR PREPARING POLYPROPYLENE PELLETS

Abstract

Method for continuously preparing polypropylene pellets having reduced low-molecular weight volatile organic compounds, the method comprising the steps ofa) preheating polypropylene pellets, b) feeding the preheated polypropylene pellets of step a) to a purge vessel and maintaining the pellets in said purge vessel while directing a flow of at least 10 Nm3 purge gas per hour per m3 of polypropylene pellets (Nm3/ m.sup.3 .sub.pp.hour) through the pellets, c) removing polypropylene pellets from said purge vessel, wherein the residence time of polypropylene pellets in said purge vessel is at least 24 hours and the polypropylene pellets are maintained at a temperature Tp of from 100 to 140 C., wherein in the preheating step a) the pellets are preheated to a temperature in the range of Tp20 C. and Tp+10 C.; wherein the polypropylene pellets have a temperature<40 C. before the preheating step a)

Claims

1. A method for continuously preparing polypropylene pellets having low FOG emission properties as determined in accordance with VDA 278, the method comprising the steps of a) preheating polypropylene pellets, b) feeding the preheated polypropylene pellets of step a) to a purge vessel via a feeding side and maintaining the pellets in said purge vessel while directing a flow of at least 10 Nm.sup.3 purge gas per hour per m.sup.3 of polypropylene pellets (Nm.sup.3/m.sup.3.sub.PP.hour) through said polypropylene pellets, c) removing polypropylene pellets from an extraction side from said purge vessel, wherein the residence time of polypropylene pellets in said purge vessel is at least 24 hours and the polypropylene pellets are maintained at a temperature of from 100 to 140 C., wherein in the preheating step a) the pellets are preheated to a temperature of not more than 20 C. lower than the temperature at which the pellets are maintained in the purge vessel and/or the pellets are preheated to a temperature of not more than 10 C. higher than the temperature at which the pellets are maintained in the purge vessel; wherein the polypropylene pellets have a temperature of <40 C. before the pellets are subjected to the preheating step a)

2. The method of claim 1 wherein the preheating comprises feeding polypropylene pellets to a preheating vessel.

3. The method of claim 1, wherein the residence time of polypropylene pellets in the purge vessel is at least 40 hours.

4. The method of claim 1, wherein the polypropylene pellets are maintained in the purge vessel at a temperature of from 105-135 C.

5. The method of claim 1, wherein the pressure in the preheating vessel and the purge vessel is in the range of from 900-1100 mbar.

6. The method of claim 1, wherein the polypropylene pellets obtained in step c) have an FOG value of at most 250 g/g as measured using VDA278 and/or wherein the polypropylene pellets before being preheated have a FOG value of more than 250 g/g as measured using VDA278.

7. The method of claim 1, wherein the purge vessel comprises a substantially cylindrical section of constant inner diameter D and a height L, such that L/D is at least 2.

8. The method of claim 1, wherein the polypropylene pellets have a bulk density of from 450-650 kg/m.sup.3.

9. The method of claim 1, wherein the polypropylene is a propylene homopolymer, a propylene random copolymer or a heterophasic propylene copolymer.

10. The method of claim 1, further comprising the steps of polymerising propylene and optionally one or more comonomers so as to form polypropylene powder, feeding said polypropylene powder and optional additives to an extruder, extruding the polypropylene and optional additives through a die so as to form one or more strands and cutting the obtained strand(s) into said polypropylene pellets.

11. The method of claim 10 wherein the cut pellets are consecutively fed to the preheating vessel while having a temperature in the range of from 40 to 80 C.

12. The method of claim 1, wherein in the preheating step a), the pellets are preheated to a temperature which is substantially the same as the temperature at which the pellets are maintained in the purge vessel.

13. The method of claim 1, further comprising the step of d) shaping the polypropylene pellets resulting from step c) into an article.

14. The method of claim 1, wherein the pellets are preheated to a temperature of from 60 to 135 C. in the preheating step a).

15. Polypropylene pellets obtained by the method of claim 1.

16. The polypropylene pellets of claim 15, wherein the polypropylene pellets have an FOG value of at most 250 g/g as measured using VDA278.

17. The method of claim 14, wherein the pellets are preheated to a temperature of 80 to 125 C.

18. The method of claim 1, wherein in the preheating step a) the pellets are preheated to a temperature of not more than 10 C. lower than the temperature at which the pellets are maintained in the purge vessel and/or the pellets are preheated to a temperature of not more than 5 C. higher than the temperature at which the pellets are maintained in the purge vessel.

Description

[0054] The invention will now be further illustrated by the following non-limiting examples.

[0055] Test Method

[0056] FOG values were determined in accordance with VDA 278 standard, version October 2011. The measurement of FOG is well known to the skilled person and essentially consists in placing a sample in a desorption tube which is connected on one side with a carrier gas inlet and on the other side to GC-MS analytical equipment.

[0057] FOG according to VDA 278 is the sum of all organic compounds of low volatility, which have an elution time greater than or equal to n-tetradecane. FOG is calculated as tetradecane equivalent (TE). FOG according to VDA 278 represents organic compounds in the boiling point range of n-alkanes C.sub.14 to C.sub.32. VDA standards are issued by Verband der Automobilindustrie. The VDA standards used herein are available from Dokumentation Kraftfahrwesen (DKF); Ulrichstrasse 14, D-74321 Bietigheim-issingen, Germany or can be downloaded from their website (www.dkf-ev.de).

[0058] Experiments

[0059] Four venting experiments were carried out on a pilot scale venting silo comprising a conical outlet section and a substantially cylindrical purging section having an internal diameter of about 0.85 m and having a maximum capacity of about 1.7 m.sup.3. The experiments were conducted using a filling level in the purge vessel of about 720 kg of pellets having a bulk density of about 534 kg/m.sup.3. The experiments were carried out by continuous feeding and withdrawing of polypropylene pellets with a throughput of about 10 kg/hr. Accordingly the residence time was 72 hour in each experiment. The target temperature of the pellets in the silo and the flow rates of the purge gas was as shown in Table 1 below. The normal cubic metre per hour (Nm.sup.3/hr) means the cubic meter of purge gas at room temperature (23 C.) and a pressure of 1 atmosphere (i.e. ambient pressure).

[0060] The polymer pellets used in the Experiments were SABIC PP612 MK10, a heterophasic polypropylene having a melt flow rate of about 33 g/10 min (ISO 1133, 2.16 kg, 230 C.). All pellets were at room temperature (20 C.). In Exp_1, Exp_2 and Exp_3, the pellets were preheating. In Exp_4, the pellets were not preheated before they were purged.

[0061] The FOG of this material when corning directly from the plant (i.e. without being vented) is about 700 g/g.

[0062] An inlet for purge gas was located at the lower side of the cylindrical outlet section so that the purge gas moves countercurrent with the flow of pellets through the silo. The purge gas was dried air.

TABLE-US-00001 TABLE 1 Target Preheat (to FOG temperature target acc. to [ C.] Flow rate temperature VDA (of the purge [Nm.sup.3/hr .Math. of the purge 278 vessel) [Nm.sup.3/hr] m.sup.3.sub.pp] vessel) [g/g] Exp_1 105 40 31 Yes 265 Exp_2 125 40 31 Yes 200 Exp_3 125 15 12 Yes 310 Exp_4 125 15 12 No 450 (the target temperature was the temperature measured with a sensor positioned in the outlet section of the purge vessel)

[0063] The experimental results show that venting or purging can reduce the FOG to acceptable levels. What is further noticeable is that the flow rate of the purge gas is a relevant parameter for controlling the FOG. Finally it is clear from the results that pre-heating the pellets before they enter into the purge vessel has a significant impact on the FOG value. Without willing to be bound to it the present inventors believe that cold pellets may act as a cold trap onto which high(er) molecular weight oligomers released from pellets below the cold pellets (i.e. the hot pellets) condenses. By preheating the pellets this condensation is prevented or at least significantly reduced. Apart from that the pre-heating reduces the risk of forming agglomerates, as a result of the release of (re)crystallisation heat, to a minimum.