GB2055865A - Mass polymerisation of vinyl chloride - Google Patents

Abstract

Blue coloration in vinyl chloride polymers produced by mass polymerisation in the presence of iron is reduced or eliminated by polymerising in the presence of a dissolved phosphorus acid, or salt or ester of such an acid.

Description

SPECIFICATION Polymerisation of vinyl chloride The present invention relates to a process for the mass polymerisation of vinyl chloride.

The material which is produced by the polymerisation of vinyl chloride in the presence of an initiator is known as PVC resin. In order that it can be used to make articles by for example moulding or extrusion it is necessary to mix various additives with the PVC resin to give what is sometimes known as a PVC compound which is a material which can be processed to make plastic articles. Well known examples of such additives are heat stabilisers, process aids, and lubricants.

It is disclosed in French patent specification 2 355 034 that heat stabilisers, lubricants and process aids may be incorporated into polyvinyl chloride produced by the mass polymerisation process by adding these materials to the mass polymerisation process itself. The specification states that any heat stabiliser may be used and lists organic phosphites as among stabilisers which can be used in the process.

A problem which is sometimes found with polyvinyl chloride produced by the mass polymerization process (hereinafter referred to as mass PVC) is the production of a blue colour on fusion under pressure into a sheet or other moulding. This colouration is quite different from the normal blackening due to thermal degradation which takes place on extended heating of PVC resin or mouldings formed from PVC resin. Polymers in which this blue colouration is observed usually exhibit poorer thermal stability, as judged by more rapid blackening or heating, in unstabilised and conventionally heat stabilised compounds. In addition the colour of the virgin polymer powder or the initial compound is usually more yellow than is observed with polymers which do not give the blue colouration on fusion. The formation of this blue colour may take place in some batches of mass PVC but not in others.

We have now found how to prevent or reduce this blue colouration, and the associated powder and compound colour problems, which has been a matter of considerable concern for the manufacture of the mass PVC.

According to the present invention the process for the mass polymerisation of vinyl chloride is characterised in that the polymerisation reaction mixture contains at least 4ppm of iron and in that polymerisation is carried out in the presence of a dissolved phosphorus acid or salt or ester of such an acid.

The mass polymerisation of vinyl chloride is well known in the art. Mass polymerisation may be carried out in a single stage or in two stages. It is particularly preferred to carry out a two stage mass polymerisation. In a preferred two stage polymerisation process polymerisation is carried out to a conversion of about 3% to 15%, preferably 7 to 12% by weight of monomer, with high speed agitation.

This first stage of the polymerisation is then followed by a second stage of polymerisation using low speed polymerisation as described in GB 1 047 489.

The process of the present invention may be applied to a vinyl chloride monomer to give a homopolymer. Alternatively the process may be applied to a mixture of vinyl chloride with up to 50% by weight of copolymerisable monomers, or preferably not more than 30% by weight of copolymerisable monomer.

The iron which is believed to be responsible for the production of blue colouration may be introduced into the polymerisation mixture in various ways. Iron contamination may result from corrosion of the reactor, which may be increased by use of acidic substances added to reduce fouling.

Certain techniques of removing monomer from the polymer in the reactor may lead to the introduction of iron. The process of the present invention is applied to polymerisation mixtures containing at least 4 ppm of iron. The level of iron contamination may be as high as 40 ppm. The process may for example be applied to polymerisation mixtures containing from 6 to 30 ppm of iron.

If a material is being added to have a stabilising effect on the polymer finally produced it will generally be desirable to add it towards the middle or end of the polymerisation process so as to avoid any risk of interfering with the very important initial polymerisation step which determines the particle morphology. We believe, however, that the presence of iron adversely affects the rate at which polymerisation takes place and we prefer to introduce the additive of the present invention at the beginning of the polymerisation step. Thus in the two-stage polymerisation process referred to above we prefer to introduce the additive at the beginning of the first step.

Examples of phosphorus acids which may be used in the present invention are ortho phosphoric acid, polyphosphoric acid, meta phosphoric acid, phosphorous acid, hypophosphorous acid, phenyl phosphinic acid, and phenyl phosphonic acid. Among salts which may be used are sodium pyrophosphate, and sodium hexametaphosphate. Examples of esters which may be used are diethyl phosphite, triethyl phosphite, trinonyl phenyl phosphite, tri-2,4-tertbutyl phenyl phosphite, and distearyl pentaerythritol diphosphite.

It is particularly preferred to use phosphite esters in particular aliphatic esters. The esters preferably contain alkyl groups, and in addition preferably contain alkylene groups bridging oxygen atoms on the same phosphite group. Alternatively the alkylene group may bridge two oxygen atoms on different phosphite groups. Such compounds correspond to those formed by the esterification of a dihydroxy alcohol. It is particularly preferred if the alkylene group bridges two phosphite groups and also bridges two oxygen atoms in each phosphite group, the remaining oxygen of each phosphite group being linked to an alkyl group. This corresponds to a compound derived from a tetrahydroxy alcohol.

The preferred compounds are 3,9-di-alkoxy-2,4,8,10-tetraoxa-3,9-phospho spiro (6,6) hendecanes.

The spiro structure above corresponds to that of a diphosphite derived from pentaeriythritol. A particularly preferred compound of this class is 3,9-di(octadecoxy)-2,4,8,10-tetraoxa-3,9-phosphospiro (6,6) hendecane otherwise known as distearyl pentaerythritol diphosphite.

The alkyl and alkylene groups preferably each contain 2 to 20 carbon atoms. The alkyl groups more preferably each contain 10 to 20 carbon atoms.

Examples of suitable phosphites are disclosed in US 2 961 454 and US 3 205250.

Another preferred class of phosphites are aryl phosphites derived from alkyl phenols having a hindered phenolic group, for example, having a bulky group such as tertiary butyl in the two and/or six position on the phenolic group. The use of such materials may be particularly useful when the resulting polymer is subjected to steam stripping to remove monomer, as hydrolysis will give rise to a phenolic antioxidant and phosphorus acid.

The quantity of phosphorus acid, salt or ester present in the polymerisation reaction mixture is preferably in the range 5 ppm to 8000 ppm, more preferably 48 ppm to 500 ppm based on monomer.

It is believed that it is necessary for the phosphorus-containing additive of the present invention to be uniformly dispersed throughout the polymerisation mixture. Where the additive, e.g. ortho phosphorous acid or phosphorous acid is not in itself soluble in vinyl chloride it may be necessary to dissolve the additive in a solvent which is miscible with vinyl chloride, for example methanol.

The invention will now be described by reference to the following Examples.

Test A This is a comparative test not according to the invention. Vinyl chloride, 2-ethyl hexyl perdicarbonate (in the form of 40% solution) were introduced into a stainless steel reactor provided with a helical wall scraping vertical agitator. (The agitator does not in fact make contact with the wall. The clearance is, however, only a few millimetres). Polymerisation was carried out at 570C for five hours.

The polymer was then recovered and the colour of the polymer powder noted. The polymer was formed into a sheet without the addition of any further stabiliser and the colour of the sheet observed. A sample of the polymer was then mixed with a tin stabiliser and a plaque formed which was subjected to the ISO thermal stability test No. R305 and 1 800C. The time to formation of a black plaque was measured. The quantities of material used and the results obtained are shown in Table 1.

Comparative Test B This is a comparative test not according to the invention, and shows the harmful effect of iron on conversion, colour production and thermal stability.

The example is carried out as in comparative test A except that 10 ppm of iron (as FeCL3 in methanol) were added to the polymerisation mixture before polymerisation began. The results are shown in Table 1.

EXAMPLE 1 This was carried out as comparative test B except that 400 ppm of distearyl pentaerythritol disphosphite were added before polymerisation began. The results are shown in the Table. This shows that the presence of the phosphite ester gave a marked improvement in conversion in comparison with results from Test B and overcame the problems of colouration which would otherwise have resulted from the presence of iron.

It is believed that the mechanism by which the phosphorus contianing additive of the present invention acts is by formation of a complex with iron. It is believed that the blue colour of unstabilised mass PVC sheet is due to a complex between iron and polyenes formed during PVC degradation. A similar blue/green colour was observed on mixing solutions of ferric chloride and beta-carotene (which contains a polyene sequence believed to be similar to those present in PVC). The suitability of a compound for use in the present invention was tested by adding the compound to a beta-carotene/iron complex. The following compound were found to remove the blue/green colouration: ortho phosphoric acid, phosphorous acid, hypophosphorous acid, polyphosphoric acid, phenyl-phosphinic acid, phenylphosphonic acid, metaphosphoric acid, sodium pyrophosphate, sodium hexametaphosphate, trim ethyl phosphate, triethyl phosphate, diethyl phosphite, triethyl phosphite, trinonylphenyl phosphite, tri-2,4-tertbutyl phenyl phosphite, distearyl pentaerythritol diphosphite, Sandoz PEPQ - phosphonite and dimethyl methyl phosphonate.

TABLE 1

A 1 B Monomer charge (kg) v - 20 2-ethyl hexyl perdicarbonate wt. of 40% solution (9) 35 Polymerisation temperature C < -~ 57 Agitation. Helical wall scraping 160 rpm for 25 minutes Agitation. Vertical axis 50 rpm thereafter Reaction time hrs. < . 5 Iron (as FeCI3 in methanol) ppm - 10 10 Weston 618 (Distearyl pentaerythritol diphosphite) ppm - 400 Conversion % 65 53 40 Colour of unstabilised sheet clear clear blue Colour of polymer powder pink white deep pink yellow Initial Plaque colour in ISO thermal stability test R305* paie pink pale straw deep yellow Time to black plaque in ISO thermal stability test R305* 110 mins 110 mins 90 mins Time to a deep straw plaque in a mill stability test for a Bated stabilised formulation 45 mins 40 mins 20 mins *Tested as tin stabilised compound at 1800C.

Claims (13)

1. The process for the mass polymerisation of vinyl chloride characterised in that the polymerisation reaction mixture contains at least 4 ppm of iron and in that polymerisation is carried out in the presence of a dissolved phosphorus acid or salt or ester of such an acid.

2. The process according to Claim 1 wherein the polymerisation reaction mixture contains 6 to 30 ppm of iron.

3. The process according to any one of the preceding claims wherein the polymerisation is carried out in the presence of a phosphite ester.

4. The process according to Claim 3 wherein the phosphite ester is an aliphatic phosphite.

5. The process according to Claim 4 wherein the phosphite ester contains an alkyl group.

6. The process according to either one of Claims 4 or 5 wherein the phosphite ester contains an alkylene group bridging oxygen atoms on the same phosphite group.

7. The process according to any one of Claims 4 to 6 wherein the phosphite ester contains an alkylene group bridging oxygen atoms on two different phosphite groups.

8. The process according to any one of Claims 4 to 7 wherein an alkyl and alkylene group where present, each contain from 2 to 20 carbon atoms.

9. The process according to any one of the preceding claims wherein the phosphite is a 3,9dialkoxy-2,4,8,1 (-tetraoxa-3,9-phosphospiro (6,6) hendeca ne.

10. The process according to Claim 3 wherein the phosphite ester is an aryl phosphite derived from alkyl phenols having a hindered phenol group.

11. The process according to any one of the preceding claims wherein the phosphorus acid or salt or ester is added at the beginning of the polymerisation step.

12. The process according to any one of the preceding claims wherein the quantity of phosphorus acid, salt, or ester present during polymerisation is in the range 5 to 8.000 ppm based on monomer.

13. The process substantially as hereinbefore described with reference to the examples.