Process for the chemical recycling of polycarbonates

The invention relates to a process for the chemical recycling of polycarbonates by catalyzed reaction with diaryl carbonates to oligocarbonates which are crystallized, purified and then polycondensed back to polycarbonates, insoluble constituents optionally being removed before the crystallization step.

EXAMPLES 
Examples 1 to 4 
300 g (1.40 moles) of diphenyl carbonate were melted under nitrogen and, 
after the addition of 0.0026 g of KOH, the melt was heated to 200.degree. 
C. 1,016 g (4.0 moles) of bisphenol A polycarbonate with a molecular 
weight of around 28,000 were then added in portions while stirring. 
After everything had melted, the temperature was kept at 240.degree. C. for 
1 h, after which the melt was poured out, left to solidify and 
size-reduced. The material obtained was treated under nitrogen for 2 h at 
the boiling temperature with 2.5 to 3 times the quantity of solvent and 
then left to cool slowly while stirring. The crystal sludge formed from 
the oligomers was separated by filtration, compressed, washed with solvent 
and dried, the combined filtrates were concentrated by evaporation and the 
residue was distilled in a high vacuum, resulting in the formation of a 
crystalline distillate (essentially diphenyl carbonate) and a faintly 
colored glass-like residue. 
The results are set out in Table 1. 
TABLE 1 
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Distillate 
Carbonate* 
Melting 
Residue of 
diphenyl 
Residue 
Example 
Solvent 
oligomers 
range 
filtrate 
carbonate 
oligomers 
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1 Xylene 737 g 165-8* 
570 g 
57 g 510 g 
2 Cyclohexane 
990 g 172-9* 
321 9 
61 g 259 g 
3 Isopropanol 
988 g 135-9* 
325 g 
66 9 256 g 
4 Butyl acetate 
764 g 180-5* 
546 g 
63 g 483 g 
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*The average molecular weights Mw are between 1,800 and 2,200 
It can be seen from the Table 1 that, depending on the solvent and the 
procedure, approximately 700 to 1,000 g (50 to 76%) can be isolated from 
1,316 g of educt mixture in the form of a crystalline oligomer mixture 
suitable for the synthesis of polycarbonates. 
The diphenyl carbonate distilled off (approximately 20% of the quantity 
used) can be completely recycled into the process. More or less all (in 
the present case at least 90%) of the oligomer residue remaining after 
distillation can be reused, depending on the impurities. The remainder 
represents an outlet for secondary products from the polycarbonate damage 
and other components foreign to the product. 
A recyclate yield of more than 95% can be obtained in this way. 
Examples 5 to 8 
As described in Examples 1 to 4, a melt was prepared from 1,016 g (4 moles) 
of bisphenol A polycarbonate and 670 g (3.13 moles) of diphenyl carbonate 
and passed over a bed of granulated active carbon at 250.degree. C. and at 
a rate of 0.3 bed volume/h. 
The original viscosity fell from 1.171 to distinctly lower values. The 
results are set out in Table 2. 
The oligomer mixtures thus obtained can be crystallized and worked up as 
described in Examples 1 to 4. 
TABLE 2 
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Example Active carbon 
Viscosity 
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5 I 1.114 
6 II 1.118 
7 III 1.065 
8 IV 1.086 
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I Norit RS, a product of Norit 
II Norit R 2030, a product of Norit 
III Norit RB1, a product of Norit 
IV Sorbonorit 2, a product of Norit 
Examples 9 to 12 
As described in Examples 1 to 4, a melt was prepared from 1.016 g (4 moles) 
of bisphenol A polycarbonate and 1,700 g (7.9 moles) of diphenyl carbonate 
and passed as in Examples 5 to 8 over a bed of granulated active carbon at 
200.degree. C. and at a rate of 2 bed volumes/h. 
The results are set out Table 3. 
TABLE 3 
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(Initial viscosity 1.017) 
Example Active carbon* 
Viscosity 
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9 I 1.015 
10 II 1.017 
11 III 1.015 
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*See legend to Table 2