Transesterification process for the production of polycarbonates having a low content of volatile compounds
View Patent ↗A multi stage process for the production of polycarbonates having low monomer content is disclosed. The process entails, in sequence, (i) transesterifying in the melt at least one dihydroxyaryl compound with at least one diaryl carbonate in the presence of at least one onium catalyst to produce a polycondensate that contains monomers, (ii) introducing into the polycondensate an inhibitor in an hypostoichiometric amount in relation to the catalyst and (iii) reducing in a finishing reactor the content of monomers in said polycondensate. The polycarbonate thus produced is characterized in that it contains less than 350 ppm of diphenyl carbonate, less than 40 ppm of 2,2-bis-(4-hydroxyphenyl)propane and less than 150 ppm of phenol.
1. A multi stage process for the production of polycarbonates having low monomer content comprising in sequence
(i) transesterifying in the melt at least one dihydroxyaryl compound with at least one diaryl carbonate in the presence of at least one onium catalyst to produce a polycondensate that contains monomers,
(ii) introducing into the polycondensate an inhibitor in an hypostoichiometric amount in relation to the catalyst said inhibitor selected from the group consisting of Lewis or Bronsted acid and an ester of a strong acid, and
(iii) reducing in a finishing reactor the content of monomers in said polycondensate
the process performed without alkali or alkaline-earth catalysts carried out in more than one reactor.
2. The process according to claim 1 , wherein the polycarbonate has a weight average molecular weight of at most 30,000 g/mole.
3. The process of claim 1 wherein the palycarbonate is characterized in that its content of OH end groups is between 50 and 750 ppm.
4. The process according to claim 3 characterized in that the OH end group content is between 70 and 500 ppm.
5. The process of claim 1 wherein the inhibitor has a pKa value not exceeding 5.
6. The process according to claim 1 wherein the inhibitor is a member selected from the group consisting of ortho-phosphoric acid and acid ester of ortho-phosphoic acid.
7. The process according to claim 1 wherein the amount of inhibitor is <8 ppm.
8. The process according to claims 1 wherein the reducing the content of monomers in said polycondensate is carried out at temperatures not exceeding 310° C.
9. The process according to claim 1 wherein the residence time in the finishing reactor is 15 to 150 min.
10. The polycarbonate obtained by the process according to claim 1 .
11. The polycarbonate of claim 10 wherein the polycarbonate contains less than 350 ppm of diphenyl carbonate, less than 40 ppm of 2,2-bis-(4-hydroxyphenyl) propane and less than 150 ppm of phenol.