Process for making skinless PVC

Abstract: Process for preparation of polyvinyl chloride (PVC) comprising the steps of: mass polymerizing vinyl chloride monomer in the presence of at least one oil soluble emulsifier and at least one initiator in a first reactor having an agitator stirring with high shear 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 at least one water soluble emulsifier; and polymerizing the mixture in the second reactor having an agitator stirring with shear lower than in the first reactor until a desired conversion is reached.

Description: The present invention relates to a process for preparation of polyvinyl chloride (PVC), especially to a process for preparation of skinless polyvinyl chloride. Various processes for the preparation of polyvinyl chloride are known in the art. US 3,522,227 discloses a process for mass polymerization of vinyl chloride monomer for increasing 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. Polymerization is done with no water present. US 3,687,919 discloses a process for polymerizing ethylenic monomers capable of polymerization in mass to form spherical particles with controlled size distribution wherein the polymerization 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. Polymerization is done with no water present. However, the mass polymerization processes of US 3,522,227 and US 3,687,919 are low productivity 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 firstly as an initial continuous phase consisting of monomer in the presence of a catalyst which is soluble in said monomer. The monomer is polymerized under agitation to provide a conversion of monomer to polymer up to about 5-20 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 invert the reaction wherein water becomes the continuous phase. The mixture is then polymerized to the desired conversion. 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 during 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. US 4,775,702 discloses an aqueous phase inversion process for producing skinless and crosslinked PVC resins comprising polymerizing vinyl chloride monomer together with at least one cross-linking agent wherein the process is first conducted with the monomer phase as the continuous phase and after from about 1 percent to about 10 percent conversion of monomer to polymer water is added to invert the process such that the aqueous phase is the continuous phase and the monomer phase is a discontinuous phase. 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 at least one oil soluble emulsifier and at least one initiator in a first reactor having high shear agitation with a fast stirring speed in a range of 100-1500 rpm until a desired conversion in the range of 1-15 percent is reached; transferring the content of the first reactor into at least one second reactor which has been charged with demineralized water and at least one water soluble emulsifier; and polymerizing the mixture in the second reactor having agitation with lower shear than the agitation in the first reactor until a desired conversion in the range of 60-100 percent is reached. According to the present invention, the stirring speed in the first reactor is preferably in the range of 750-1200 rpm, and the stirring speed in the second reactor is in the range of about 75-500 rpm, preferably 95-350 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 55-75°C, more preferaby 65°C, and the temperature in the second reactor is in the range of about 45-75°C, more preferably 50-70°C. According to the present invention, a process is provided wherein the conversion in the first reactor is in the range of about 1-15 percent, preferably 7-10 percent, and the conversion in the second reactor is in the range of about 60-100 percent, preferably 75-85 percent. Further, the volume of the first reactor may be in the range of 11 to 75 m 3 , preferably in the range of 25 to 75 m 3 , and the volume of the second reactor may be in the range of about 81 to 150 m 3 , preferably in the range of 50-150 m 3 . Preferably, one first reactor may be used for up to five second reactors, and the first reactor may be positioned above the second reactor and may be connected by solid pipe or blowdown line to the second reactors. The oil soluble emulsifier may be sorbitan monolaurate, sorbitan palmitate, sorbitan tristearate, sorbitan trioleate, preferably sorbitan palmitate or various sucrose ester mixtures of sucrose distearate, mono stearate, tristearate, dipalmitate, tetrastearate, pentastearate, hexastearate, heptastearate, or octastearate, preferrably mixtures of hexastearate, pentastearate and heptastearate with a hydrophilic/lipophylic balance value between 1-15 preferably 1-10. The oil soluble emulsifier be employed in an amount of 100 - 10,000 parts per million by weight, based on the weight of the vinyl chloride monomer. The oil soluble emulsifier may be further selected from sucrose esters. The initiator may be selected from the group comprising di-2-ethylhexyl peroxydicarbonate, 1,1,3,3-tetramethylbutly peroxyneodecanoate, tert-butyl peroxyneoheptanoate, tert-butyl peroxyneodecanoate, wherein di-2-ethylhexyl peroxydicarbonate is preferred. The initiator is employed in an amount of 100 - 1000 parts per million 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, methyl cellulose and methyl hydroxy propyl cellulose, wherein methyl hydroxy propyl cellulose is preferred. It is to be understood that the latter emulsifier groups are merely intended as an illustration and not in a limitative sense. The water-soluble emulsifiers may be employed in an amount of 500 - 5000 parts per million by weight, based on the weight of the vinyl chloride monomer. According to the present invention, the weight ratio of water to monomer in the second reactor is in the range of 1,0-1,5 to 1. Optionally, the second reactor may be additionally charged with vinyl chloride monomer and/or its comonomers. It is obvious that besides vinyl chloride at least one comonomer may be present in the first and/or second reactor. The comonomers copolymerizable with vinyl chloride, as referred to in this invention, encompass all comonomers which are known to be copolymerizable with vinyl chloride, such as vinyl bromide, vinylidene chloride and the like. Surprisingly, it was found that using the process according to the present invention polyvinyl chloride may be produced which is skinless and has good quality and which may be produced with high productivity. This is due to the specific variation of the suspension technology for preparing polyvinyl chloride, namely a mass pre-polymerization in a reactor with high shear agitation followed by a suspension polymerization in a second larger reactor with lower shear agitation than in the first reactor. Following the procedure of the present invention reactor fouling may be prevented. The process of the present invention is a process resulting in a clean reactor. 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 emulsifier 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 m 3 . 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 m 3 . 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. The vinyl chloride monomer is then added to the reactor 1. Afterwards, the agitator 4 is started to a stirring speed of about 750 to 1200 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, a second reactor 5 is charged with the recipe quantity of water and of the water-soluble emulsifier. The reactor 5 is 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 vinyl chloride monomer, and is vented to atmospheric pressure. Optionally further vinyl chloride monomer may be added to the reactor 5. Finally, the agitator 7 is started having a stirring speed in the range of 95 to 350 rpm, and the reactor 5 is heated to the desired temperature in the range of 50 to 70°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 conversion 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. The agitator is started at 1200 rpm and the mixture is heated to 65°C. Then 6,76 grams of di-2-ethylhexyl peroxydicarbonate are injected. 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 hydroxypropyl 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 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 1 has the characteristics as outlined in Table 1 below. Example Number Average Particle Size Microns Particle Size Dist. Apparent Bulk Density gm/cc Flow Time Seconds DOP Porosity cc/gm Heat Loss Percentage 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. The agitator is started at 1200 rpm and the mixture is heated to 65°C. Then 4,23 grams of di-2-ethylhexyl peroxydicarbonate are injected. 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 below. Example Number Average Particle Size Microns Particle Size Dist. Apparent Bulk Density gm/cc Flow Time Seconds DOP Porosity cc/gm Heat Loss Percentage 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. The agitator is started at 1200 rpm and the mixture is heated to 65°C. Then 5,07 grams of di-2-ethylhexyl peroxydicarbonate are injected. 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 below. Example Number Average Particle Size Microns Particle Size Dist. Apparent Bulk Density gm/cc Flow Time Seconds DOP Porosity cc/gm Heat Loss Percentage 3 264,5 56,27 0,494 22,38 0,337 0,205 Example 4 To a 4,35 liter reactor the following ingredients are added: 2,55 kg vinyl chloride monomer, 9,87 grams of S-170 sucrose ester oil soluble emulsifier. The agitator is started at 900 rpm and the mixture is heated to 65°C. Then 3,18 grams of di-2-ethylhexyl peroxydicarbonate are injected. After a 45 minute reaction time, the contents of the reactor are dropped to a 10,2 liter reactor that has been pre-charged with 4.95 kg of demineralized water and 274.55 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 330 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 4 has the characteristics as outlined in Table 4 below. Example Number Average Particle Size Microns Particle Size Dist. Apparent Bulk Density gm/cc Flow Time Seconds DOP Porosity cc/gm Heat Loss Percentage 4 237 70.145 0.545 22.475 0.249 0.2 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 invention in diverse forms thereof.

Claims: Process for preparation of polyvinyl chloride (PVC) comprising the steps of: mass polymerizing vinyl chloride monomer in the presence of at least one oil soluble emulsifier and at least one initiator in a first reactor having an agitator stirring with high shear in a range of 100-1500 rpm until a desired conversion in the range of 1-15 percent is reached; transferring the content of the first reactor into at least one second reactor which has been charged with demineralized water and at least one water soluble emulsifier; and polymerizing the mixture in the second reactor having an agitator stirring with shear lower than in the first reactor until a desired conversion in the range of 60-100 percent is reached. Process according to claim 1, wherein the stirring speed in the first reactor is in the range of 750-1200 rpm, and the stirring speed in the second reactor is in the range of about 75-500 rpm, preferably 95-350 rpm. Process according to claim 1 or 2, wherein the second reactor is 2-2,5 times bigger than the first reactor. Process according to any of the preceding claims, wherein the temperature in the first reactor is in the range of 55-75°C, preferably 65°C, and the temperature in the second reactor is in the range of 45-75°C, preferably 50-70°C. Process according to any of the preceding claims, wherein the conversion in the first reactor is in the range of 7-10percent, and the conversion in the second reactor is in the range of 75-85 percent. Process according to any of the preceding claims, wherein the volume of the first reactor is in the range of 1 liter to 75 m 3 , preferably in the range of 25 to 75 m 3 , and the volume of the second reactor is in the range of 8 liters to 150 m 3 , preferably in the range of 50-150 m 3 . Process according to any of the preceding claims, wherein one first reactor is used with up to five second reactors. Process according to any of the preceding claims, wherein the first reactor is positioned above the second reactor and is connected by solid pipe or blowdown line to the second reactor. Process according to any of the preceding claims, wherein the oil soluble emulsifier is selected from sorbitan or sucrose esters or mixtures thereof. Process according to claim 9, wherein the oil soluble emulsifier is employed in an amount of 100 to 10,000 parts per million by weight, based on the weight of the vinyl chloride monomer. Process according to any of the preceding claims, wherein the initiator is selected from the group comprising di-2-ethylhexyl peroxydicarbonate, 1,1,3,3-tetramethylbutly peroxyneodecanoate, tert-butyl peroxyneoheptanoate, tert-butyl peroxyneodecanoate, wherein di-2-ethylhexyl peroxydicarbonate is preferred. Process according to claim 11, wherein the initiator is employed in an amount of about 100 - 1000 parts per million by weight, based on the weight of the vinyl chloride monomer. Process according to any of the preceding claims, wherein the water-soluble emulsifier is selected from the group comprising alkyl or hydroxy alkyl cellulose ethers, methyl cellulose and methyl hydroxy propyl cellulose, methyl hydroxy propyl cellulose being preferred. Process according to claim 13, wherein the water soluble emulsifier is employed in an amount of 500-5000 parts per million by weight, based on the weight of the vinyl chloride monomer. Process according to any of the preceding claims, wherein the weight ratio of water to monomer in the second reactor is in the range of 1,0-1,5 to 1. Process according to any of the preceding claims, wherein the second reactor is additionally charged with vinyl chloride monomer.

Inventors: AL-KHODAIRI FAHAD, HEALY BRUCE A, ZERFA MOHAMMED, AL-KHODAIRI, FAHAD, HEALY, BRUCE A., ZERFA, MOHAMMED
Applicants: SAUDI BASIC IND CORP, SAUDI BASIC INDUSTRIES CORPORATION
Application year: 2001
Main classification: C08F2
IPC codes: C08F2, C08F14