Reduction in VOC and FOG values of polypropylene grades by aeration

1. A process for reducing the volatile and semi-volatile organic compounds (VOC and FOG values) of a polypropylene composition to below 20 μg/g (VOC, VDA 278 October 2011) and below 250 μg/g (FOG, VDA 278 October 2011), the polypropylene composition including a polypropylene homopolymer and/or a polypropylene random copolymer,

the process comprising the steps of:

a) providing an aeration vessel having:

at least one inlet for aeration gas,

at least one outlet for exhaust gas,

an inlet for a raw polypropylene composition at the top of the aeration vessel,

an outlet for the polypropylene composition at the bottom of the aeration vessel;

wherein the polypropylene composition is present as a packed bed;

b) initiating a counter-current flow of the polypropylene composition and aeration gas, wherein the aeration gas is nitrogen or air or mixtures thereof;

c) by

feeding particles of the raw polypropylene composition showing a VOC value of greater than about 150 μg/g and an FOG value of greater than about 350 μg/g (VOC and FOG values according to VDA 278 October 2011), into said aeration vessel from the top, the polypropylene homopolymer and/or the polypropylene random copolymer having a melting point (Tm) of greater than 150° C., a melt flow rate MFR 2 (230° C.) of 4 to 160 g/10 min, and a xylene soluble fraction having a melt flow rate MFR 2 from 10 to 40 g/10 min prior to aeration, wherein the particles of the raw polypropylene composition are in pellet form and the pellets have a diameter D in the range of from 2.8 to 4.0 mm, wherein a polypropylene composition weight flow through the aeration vessel is from 70 to 170 kg/h referenced to an aeration vessel with a volume of 1.5 m 3 ;

feeding the aeration gas into said aeration vessel via the at least one inlet at the bottom, wherein a total volumetric airflow used is from 2 Nm 3 /kg to 4 Nm 3 /kg, the total volumetric airflow being the volume of gas flowing into the aeration vessel per hour referenced to the polypropylene composition weight flow;

withdrawing the exhaust gas via the outlet for exhaust gas;

withdrawing the aerated polypropylene composition via the outlet at the bottom of the aeration vessel;

d) maintaining said aeration gas flow for an aeration time of from 3 to 9 hours,

wherein, the temperature of the aeration gas is from 100° C. to 150° C., and wherein a Reynolds number (Re) of the gas flow is from 15 to 50, wherein the Reynolds number for the flow of aeration gas through the packed bed is defined by formula (I):

Re=(ρ· v s ·D )/μ  (I)

where:

ρ is the density of the aeration gas at the temperature used (kg/m 3 ),

μ is the kinematic viscosity of the aeration gas at the temperature used (kg/m s),

v s is the superficial velocity, defined as Q/A where Q is the volume flow rate of the aeration gas, (m 3 /s) and A is a cross sectional area (m 2 ) of the aeration vessel, and

D is a diameter (m) of the particles,

wherein the polypropylene composition is not agitated during aeration and the aeration is not a fluidized bed process.

2. The process according to claim 1 , wherein the aeration gas is air.

3. The process according to claim 1 , wherein the process is continuous.

4. The process according to claim 1 , wherein the pellets are pre-heated before being added to the aeration vessel.

5. The process according to claim 1 , wherein at least one component of the raw polypropylene composition originates from a solution polymerisation process.

6. The process according to claim 1 , wherein the polypropylene composition after aeration has a ratio of FOG/VOC of more than 5.0.

7. The process according to claim 1 , wherein the temperature of the aeration gas is from 100° C. to 140° C.

8. The process according to claim 1 , wherein the exhaust gas is subjected to a purification step and then recycled back into the inlet for the aeration gas.

9. The process according to claim 1 , wherein the exhaust gas passes a heat exchanger before being discharged into the atmosphere.

10. The process according to claim 1 , wherein the aeration vessel is cylindrical, or conical, or cylindrical with a cone shaped bottom portion.

11. The process according to claim 1 , wherein the polypropylene composition contains at least one slip agent.

12. A product obtainable by the process according to claim 1 .

13. The process according to claim 1 , wherein the polypropylene composition weight flow through the aeration vessel is from 70 to 100 kg/h referenced to an aeration vessel with a volume of 1.5 m 3 .