Chemical recycling of polycarbonate to raw materials by thermal decomposition with calcium hydroxide/steam

Article abstract of DOI:10.1246/cl.2005.282

We investigated the influence of the decomposition conditions (temperature: 500, 700, 900°C, steam concentration: 0, 35, 86 vol %, Ca(OH) ₂/PC molar ratio: 0 or 5) on polycarbonate (PC). Without steam and Ca(OH)₂ the liquefaction yield was 57 wt % (low and high boiling products 14, 43 wt %, respectively), the gasification yield 12 wt %, and the carbonization yield 31 wt % at 500°C. At a steam concentration of 86 vol %, 84 wt % liquids (low and high boiling products 17, 67 wt %, respectively), 1 wt % gases and 15 wt % residues were observed. The amount of low boiling products increased from 17 to 31 wt %, and the amount of high boiling products decreased from 67 to 49 wt % when Ca(OH)₂ was added at a steam concentration 86 vol %. At this time, the yield of phenol in the low boiling fraction increased from 15 to 74 mol % (20 wt % of all products). Copyright

Full text of DOI:10.1246/cl.2005.282

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82
Chemistry Letters Vol.34, No.3 (2005)
Chemical Recycling of Polycarbonate to Raw Materials by Thermal Decomposition
with Calcium Hydroxide/Steam
ꢀ
y
Toshiaki Yoshioka, Katsuya Sugawara, Tadaaki Mizoguchi, and Akitsugu Okuwaki
Environmental Conservation Research Institute, Tohoku University, Aramaki Aza Aoba 6-6-07, Aoba-ku, Sendai 980-8579
y
Graduate School of Environment Studies, Tohoku University, Aramaki Aza Aoba 6-6-07, Aoba-ku, Sendai 980-8579
(Received October 7, 2004; CL-041182)
We investigated the influence of the decomposition condi-
ꢁ
100
80
tions (temperature: 500, 700, 900 C, steam concentration: 0,
3
(
5
5, 86 vol %, Ca(OH)2/PC molar ratio: 0 or 5) on polycarbonate
PC). Without steam and Ca(OH)2 the liquefaction yield was
7 wt % (low and high boiling products 14, 43 wt %, respective-
gas
ly), the gasification yield 12 wt %, and the carbonization yield
ꢁ
3
1 wt % at 500 C. At a steam concentration of 86 vol %,
4 wt % liquids (low and high boiling products 17, 67 wt %, re-
60
8
low boiling
point liquid
spectively), 1 wt % gases and 15 wt % residues were observed.
The amount of low boiling products increased from 17 to
40
high boiling
point liquid
31 wt %, and the amount of high boiling products decreased from
67 to 49 wt % when Ca(OH)2 was added at a steam concentration
86 vol %. At this time, the yield of phenol in the low boiling frac-
2
0
solid
tion increased from 15 to 74 mol % (20 wt % of all products).
0
Ca(OH) /PC
0
5
0
0
5
0
0
0
0
0
0
2
Various methods for plastic recycling technology are now
developed. One of the proposed technologies is the conversion
to oil, performed by three large plants in Japan. Polyolefines,
such as polyethylene, polypropylene, and polystyrene, can be
converted into oil via thermal decomposition.
steam
conc.(%)
0
35 86
0
35 86
700
0 35 86
900
Temperature (°C) 500
Figure 1. Influence of the decomposition temperature, steam
concentration, and Ca(OH)2 on the yields of products.
PC is a plastic used in various applications, and in order to
use valuable resources effectively, it is necessary to recycle
them. Today physical recycling of PC is carried out. In chemical
recycling of PC, solvolysis is well known.1 In this process the
heat transfer between organic solvent and PC is very excellent.
Further it is possible to supply hydrogen required for the radical
stabilization during the degradation of the PC. It suppresses the
ꢁ
with filter plate at temperatures between 500, 700, 900 C with
a helium flow-rate of 50 mL/min or with steam/helium (steam
concentration: 0, 35, 86 vol %). In case of steam addition, water
–3
ꢁ
was vaporized at 450 C in a second reactor, and led into the de-
composition reactor. Liquid products were collected in cooling
traps cooled with iced water and liquid nitrogen. Gases were
gathered in a gas pack. Quantitative analysis was conducted us-
ing GC–MS, GC–FID, and GC–TCD.
4
generation of residual substance. Hu et al. reported that
96 mol % of bisphenol A (BPA) was obtained using a MeOH/
toluene system. 95 mol % of BPA was obtained by aminolysis.
1
2
However in order to extend the object of recycling to composite
materials containing PC, development of a new technology for
getting monomers and fuels is desired. Yoshioka et al. reported
that the addition of Ca(OH)2 to PET affects a high selectivity of
benzene without producing sublimation substances during the
The influence of the decomposition temperature, steam con-
centration, and Ca(OH)2 on the yields of gases, liquids, and sol-
ids is shown in Figure 1. Liquid is classified into the low boiling
fraction (<280 C) and the high boiling fraction (>280 C). The
yield of liquids was decreased with increasing temperature. On
the other hand, the yield of gases was increased. The yields of
liquids increased with increasing the steam concentration at
ꢁ
ꢁ
5
thermal decomposition. In order to apply this result also to
PC, we performed thermal decomposition of PC by the addition
of Ca(OH)2 and considered the influence of the reaction temper-
ature, steam concentration and Ca(OH)2 addition on the decom-
position of PC.
ꢁ
ꢁ
500 and 700 C. The yield of liquids was 84 wt % at 500 C
ꢁ
and 71 wt % at 700 C in steam concentration 86 vol %. When
ꢁ
Ca(OH)2 was added to PC at 500 C, the yield of the low boiling
0
.60 g of PC pellets (ꢀ2 ꢂ 3 mm) or a mixture of PC pellets
fraction increased irrespective of the existence of steam, and
reached 17 wt % at a steam concentration of 86 vol % and the
high boiling fraction was 67 wt % in case of the molar ratio
(Ca(OH)2/PC) of 0. On the other hands, in case of the molar ra-
tio (Ca(OH)2/PC) of 5, the yield of the low boiling fraction was
31 wt %, and the yield of the high boiling fraction was 49 wt %,
respectively.
and 0.87 g of Ca(OH)2 powder (molar ratio (Ca(OH)2/unit struc-
ture of PC):5) was used as a sample. After checking for reaching
predetermined temperature and having become a stationary
state, the sample was slowly dropped into a reactor made of
quartz for 20 min. The moment of having supplied a sample
to a reactor was considered as the reaction start. The sample
was decomposed on quartz wool of the middle of the reactor
5
Figure 2 shows the composition of low boiling fraction at
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.3 (2005)
283
100
CH3
CO2
phenol
C O
O
C
O
CH3
OH
OH
OH
OH
CH3
PC
CO2
8
0
0
0
0
H O or
CH =CH
2
2
Ca(OH)
2
CH3
C H
2 5
p-methylphenol
p-ethylphenol
HO
C
OH
6
4
2
CH3
BPA
Ca(OH)2
CH
p-vinylphenol
tetramethylbenzene
3
HO
C
O
CH3
bisphenol A
others
CH2
HO
phenol
C
OH
Ca(OH) /PC
0
5
0
0
5
2
CH3
IPP
+
steam
conc. (%)
0
35
86
H O or
2
Ca(OH)2
ꢁ
Figure 2. Composition of low boiling fractions at 500 C.
CH3
CH3
OH
CH3
CH3
phenol
ꢁ
5
00 C. Main products of phenols were phenol, p-methylphenol,
p-ethylphenol, and p-vinylphenol, tetramethylbenzene, and
BPA. The total molar yield of main phenols in the low boiling
fraction was almost constant at about 80 mol %, when steam con-
centration was changed from 0 to 86 vol %. The yield of p-meth-
ylphenol, however, fell from 40 to 20 mol %, and the yield of
BPA increased from 8 to 36 mol % with increasing steam con-
centration. This result shows that the hydrolysis of the carbonate
group is preferred rather than cleavage of C–C bonds in a steam
atmosphere. Others consist of benzene, p-isopropylphenol,
biphenyl, 2-vinyl-2,3-dihydronaphtofuran, 4-(dimethylphenyl-
methyl)phenol, 5-tert-butylbiphenyl-2-ol, 1,2-bis(4-hydroxy-
phenyl)ethane, and 1,3-dimetyl-2(2,4,6-trimetylphenyl)-1,3-cy-
1 wt % when Ca(OH)2 was added. CO2 the most part of gases
was fixed as CaCO3, because the investigation of the solid reac-
tor residue by XRD showed a mixture of Ca(OH)2 and CaCO3.
We suppose the reaction mechanism of PC decomposition
with steam and/or Ca(OH)2. When NaOH is used as an additive
for decomposition of PC, Na2CO3, and BPA are generated (de-
ꢁ
1
composition with organic solvent, 60 C). Therefore, the same
degradation mechanism occurs using Ca(OH)2 as an additive.
6
It is well known that PC is decomposed into alkylphenols. PC
hydrolyzed into BPA preferentially in existing steam or
Ca(OH)2. This indicates that high selectivity of BPA by the ad-
dition of steam. Since Ca(OH)2 is a strong base, the hydroxy
group acts as an anion and cleavage of BPA proceeds. After that
ꢁ
clopentadiene. The product composition at 700 C was almost
ꢁ
7
the same as at 500 C. However, the yields of gases such as
H2, CO, CO2, and CH4 of a low molecular weight increased to
p-isopropenykphenol (IPP) and phenol are generated. IPP is
mainly decomposed into phenol and tetramethlbenzene.
ꢁ
3
0–48 wt % at 900 C, and the yields of liquids decreased to less
than 30 wt %. Carbonization has taken place since there is much
solid. It will be for the radical reaction by heat to occur more
quickly than hydrolysis. In addition, decomposition was not ob-
served at 400 C, but only melting of PC irrespective of the ex-
istence of steam. Remained PC was 97.3 wt % with steam con-
centration 0 vol % and 99.8 wt % with steam concentration
References
1
2
L.-C. Hu, A. Oku, and E. Yamada, Polymer, 39, 3841 (1998).
S. Hata, H. Goto, E. Yamada, and A. Oku, Polymer, 43, 2109
(2002).
ꢁ
3
K. Tsujita, N. Kawai, T. Takahashi, Y. Satou, T. Kamo, and
Y. Kodera, ‘‘FSRJ 2000,’’ (2000), p 29.
8
6 vol %.
4
5
Y. Sato, Kagaku to Kogyo, 54, 1347 (2001).
T. Yoshioka, E. Kitagawa, T. Mizoguchi, and A. Okuwaki,
Chem. Lett., 33, 282 (2004).
S. E. Hunter and P. E. Savage, J. Org. Chem., in press.
S. E. Hunter, C. A. Felczak, and P. E. Savage, Green Chem.,
6, 222 (2004).
The total molar yield of main phenols increased to 90 mol %
with decreasing the others yield by the addition of Ca(OH)2.
Moreover, the addition of Ca(OH)2 led to a high selectivity for
phenol (70 mol %) irrespective of steam concentration. In any
steam concentration, the yields of gases became less than
6
7
Published on the web (Advance View) January 29, 2005; DOI 10.1246/cl.2005.282