Various processes for the preparation of polyvinyl chloride are known in the art.

US 3,522,227 discloses a process for mass polymerization vinyl chloride monomer for increas- ing the density of the grains produced wherein the polymerization is carried out in a plurality of stages under similar conditions of pressure, temperature, and catalysis, comprising a prior stage proceeding to about 7-12 percent completion of polymerization accompanied by agitation of high turbulence, and a final stage proceeding in the absence of high turbulence to a selected end point of polymerization with relatively mild agitation peripherally applied to the polymer- containing mass.

US 3,687,919 discloses a process for polymerizing ethylenic monomers capable of polymeriza- tion in mass to form spherical particles with controlled size distribution wherein the polymeriza- tion mixture is agitated in a first reaction step with a high stirring speed and is than transferred to a second reactor provided with milder agitation.

However, the mass polymerization processes of US 3,522,227 and US 3,687,919 are low pro- ductivity processes and are therefore less economical than standard polymerization processes of vinyl chloride monomer which are based on a suspension technology.

US 3,706,722 discloses a process of polymerizing substantially water-insoluble polymerizable ethylenically unsaturated liquid monomer in aqueous dispersion. First, an initial continuous phase consisting of monomer in the presence of a catalyst which is soluble in said monomer is formed after which the monomer is polymerized under agitation to provide a conversion of monomer to polymer up to about 20 percent followed by addition of sufficient water to the monomer-polymer-mixture with agitation and in the presence of a water-soluble suspending agent to form a dispersion wherein water is the continuous phase. The mixture is then polymer- ized to convert essentially all of the remaining liquid monomer. The polymerization according to US 3,706,722 may be carried out in the presence of a surface active agent which is solubilized in said monomer and is carried out in only one reactor wherein the stirring speed of the agitator in that reactor is not changed. As is known by persons skilled in the art, the process of US 3,706,722 is a low productivity process and reactor fouling significantly occurs at the end of the polymerization. Therefore, the process of US 3,706,722 is less economical than a process for the preparation of polyvinyl chloride based on a standard suspension technology.

It is therefore an object of the present invention to provide a process for preparation of polyvinyl chloride which overcomes the drawbacks of the prior art, especially providing a process which produces skinless polyvinyl chloride with high productivity in an economical manner.

The object of the present invention is solved by a process for preparation of polyvinyl chloride (PVC) comprising the steps of : - mass polymerizing vinyl chloride monomer in the presence of an oil soluble emulsifier and at least one initiator in a first reactor having an agitator stirring with a first stirring speed until a desired conversion is reached; - transferring the content of the first reactor into at least one second reactor which has been charged with demineralized water and a water soluble emulsifier ; and - polymerizing the mixture in the second reactor having an agitator stirring with a second slower stirring speed than the agitator in the first reactor until a desired conversion is reached.

According to the present invention, the stirring speed in the first reactor is in the range of about 1000-1500 rpm, preferably 1200 rpm, and the stirring speed in the second reactor is in the range of about 200-500 rpm, preferably 350 rpm, and the second reactor is about 2-2,5 times bigger than the first reactor.

Preferably, the temperature in the first reactor is in the range of about 60-70°C, more preferably 65°C, and the temperature in the second reactor is in the range of about 50-60°C, more prefera- bly 58,5°C.

According to the present invention, a process is provided wherein the conversion in the first re- actor is in the range of about 1-15 percent, preferably 5 percent, and the conversion in the second reactor is in the range of about 60-100 percent, preferably 80 percent.

Further, the volume of the first reactor may be in the range of 1 1 to 75 m3, preferably in the range of about 25 to 75 m3, and the volume of the second reactor may be in the range of about 8 1 to 150 m3, preferably in the range of about 50-150 m3.

Preferably, the content of the first reactor may be transferred into up to five second reactors, and the first reactor may be positioned above the second reactor and may be connected by a solid pipe or blowdown line to the second reactor.

The oil soluble emulsifier may be Span 40 oil soluble emulsifier and may be employed in an amount of about 0,1 to 1,0 percent by weight, based on the weight of the vinyl chloride mono- mer.

The initiator may be selected from the group comprising di-2-ethylhexyl peroxydicarbonate, isopropyl peroxypercarbonate, lauroyl peroxide, azobisisobutyronitrile and the like, wherein di- 2-ethylhexyl peroxydicarbonate is preferred.

The initiator is employed in an amount of about 0,05 to 0,5 percent by weight, based on the weight of the vinyl chloride monomer.

The water-soluble emulsifier may be selected from the group comprising alkyl or hydroxy alkyl cellulose ethers, such as methyl cellulose and methyl hydroxy propyl cellulose, wherein methyl hydroxy propyl cellulose is preferred.

The water-soluble emulsifies may be employed in an amount of about 1 to 10 percent by weight, based on the weight of the vinyl chloride monomer.

According to the present invention, the ratio of water to monomer in the second reactor is in the range of about 1,0-1,5 to 1.

Optionally, the second reactor may additionally charged with vinyl chloride monomer.

Finally, according to a specific embodiment of the present invention, at least one comonomer, such as vinyl bromide, vinylidene chloride and the like, may be present in the first and/or the second reactor.

Surprisingly, it was found that using the process according to the present invention polyvinyl chloride may be produced which is skinless and which may be produced with high productivity.

This is due to the specific variation of the suspension technology for preparing polyvinyl chlo- ride, namely a pre-polymerization in a reactor with a high stirring speed of the agitator followed by a suspension polymerization in a second larger reactor with a stirring speed which is much slower than the stirring speed of the agitator in the first reactor. Following the procedure of the present invention reactor fouling may be prevented.

Further advantages and features of the present invention will be explained in the following in more detail under consideration of the drawing which consists of only one figure which shows a diagrammatic view of the reactor system of the present invention.

According to the figure a first reactor 1 is provided with a supply line 2 for an oil soluble emulsi- fier and an initiator, and with a second supply line 3 for supplying vinyl chloride monomer. The first reactor 1 has, for example, a volume of 25 to 75 m3. Within the reactor 1 an agitator 4 is located. According to the figure, the reactor 1 is connected to five second reactors 5 by line 6.

The line 6 may be a solid pipe or a blowdown line. Preferably, the second reactors 5 are located below the first reactor 1 and have a volume of about 50 to 150 m3. Within the second reactors 5 there is each provided an agitator 7. Further, each reactor 5 is provided with a supply line 8 for supplying the reactor 5 with demineralized water and water-soluble emulsifier. Optionally, a further supply line (not shown) may be provided for the second reactor 5 for providing further vinyl chloride monomer or comonomer to that reactor 5.

The process according to the present invention is carried out as follows: To the reactor 1 a recipe quantity of the oil soluble emulsifier is added. Then the reactor 1 is closed and full vacuum is drawn, the reactor 1 is then purged which nitrogen, and full vacuum is drawn again. After that the reactor 1 is purged with vinyl chloride monomer and is vented to atmospheric pressure. Afterwards, the agitator 4 is started to a stirring speed of about 1000 to 1500 rpm and the reactor 1 is heated to the desired temperature which is in the range of about 60 to 70°C.

At the same time, the second reactors 5 are charged with the recipe quantity of water and of the water-soluble emulsifier. The reactors 5 are closed and full vacuum is drawn, whereupon the reactor 5 is purged with nitrogen and full vacuum is drawn again. After that, the reactor 5 is purged again with nitrogen, or optionally with vinyl chloride monomer, and is vented to atmos- pheric pressure. Optionally further vinyl chloride monomer may be added to the reactor 5. Fi- nally, the agitator 7 is started having a stirring speed in the range of 200 to 500 rpm, and the re- actor 5 is heated to the desired temperature in the range of 50 to 60°C.

As soon as both reactors 1 and 5 have reached their desired temperatures, the initiator is injected into the first reactor 1 and the polymerization in reactor 1 is carried out until the desired conver- sion is reached. After that, the content of the first reactor 1 is transferred into the second reactor 5 and the valves (not shown) between the two reactors 1,5 are closed and the reactor 1 is vented.

The first reactor 1 is purged with nitrogen and may then be opened for inspection and cleaning.

The polymerization now proceeds in the second reactor 5, and when the pressure in the reactor 5 drops to a specified amount, a shortstop for stopping the polymerization is injected. Unreacted monomer is then vented from the reactor 5 and the content of the reactor 5 is stripped at 90°C for 10 minutes. Finally the reactor 5 is cooled down and its content may be discharged.

In the following the process of the present invention is further illustrated by way of examples.

Example 1 To a 4,35 liter reactor the following ingredients are added: 3,38 kg vinyl chloride monomer, 10,14 grams of Span 80 oil soluble emulsifier, and 6,76 grams of di-2-ethylhexyl peroxydicarbonate. The agitator is started at 1200 rpm and the mixture is heated to 65°C.

After a 20 minutes reaction time, the contents of the reactor are dropped to a 10,2 liter reactor that has been pre-charged with 5,07 kg of demineralized water and 105,6 grams of hydroxypro- pyl methyl cellulose. The pre-charged materials have been heated to 58,5°C with an stirring speed of 350 rpm. This reaction mixture is allowed to react for a total reaction time of 315 min- utes. The unreacted vinyl chloride monomer in the reactor is then vented, and the remaining ma- terial is dewatered and dried. The polyvinyl chloride obtained in Example 1 has the characteris- tics as outlined in Table 1 below.

Table 1 Example Average Particle Apparent Flow DOP Heat Number Particle Size Bulk Time Porosity Loss Size Dist. Density Seconds cc/gm Percentage Microns gm/cc 1 211, 5 36, 835 0, 451 21, 71 0, 349 0, 2 Example 2 To a 4,35 liter reactor the following ingredients are added: 3,38 kg vinyl chloride monomer, 16,90 grams of Span 40 oil soluble emulsifier, and 4,23 grams of di-2-ethylhexyl peroxydicarbonate. The agitator is started at 1200 rpm and the mixture is heated to 65°C.

After a 17 minute reaction time, the contents of the reactor are dropped to a 10,2 liter reactor that has been pre-charged with 5,07 kg of demineralized water and 140,83 grams of hydroxypropyl methyl cellulose. The pre-charged materials have been heated to 58,5°C with a stirring speed of 300 rpm. This reaction mixture is allowed to react for a total reaction time of 380 minutes. The unreacted vinyl chloride monomer in the reactor is then vented, and the remaining material is dewatered and dried.

The polyvinyl chloride obtained in Example 2 has the characteristics as outlined in Table 2 be- low.

Table 2 Example Average Particle Apparent Flow DOP Heat Number Particle Size Bulk Time Porosity Loss Size Dist. Density Seconds cc/gm Percentage Micronsgm/cc 2 301 24, 45 0, 465 20, 49 0, 409 0, 105 Example 3 To a 4,35 liter reactor the following ingredients are added: 3,38 kg vinyl chloride monomer, 13,52 grams of Span 40 oil soluble emulsifier, and 5,07 grams of di-2-ethylhexyl peroxydicarbonate. The agitator is started at 1200 rpm and the mixture is heated to 56°C.

After a 20 minute reaction time, the contents of the reactor are dropped to a 10,2 liter reactor that has been pre-charged with 5,07 kg of demineralized water and 140,83 grams of hydroxypropyl methyl cellulose. The pre-charged materials have been heated to 58.5°C with an stirring speed of 300 rpm. This reaction mixture is allowed to react for a total reaction time of 420 minutes. The unreacted vinyl chloride monomer in the reactor is then vented, and the remaining material is dewatered and dried.

The polyvinyl chloride obtained in Example 3 has the characteristics as outlined in Table 3 be- low.

Table 3 Example Average Particle Apparent Flow DOP Heat Number Particle Size Bulk Time Porosity Loss Size Dist. Density Seconds cc/gm Percentage Microns gm/cc 3 264, 5 56, 27 0, 494 22, 38 0, 337 0, 205 The features disclosed in the foregoing description, in the claims and/or in the accompanying drawing may, both separately and in any combination thereof be material for realizing the inven- tion in diverse forms thereof.