Ind. Eng. Chem. Res. 1999, 38, 1391-1395
1391
MATERIALS AND INTERFACES Decomposition of Prepolymers and Molding Materials of Phenol Resin in Subcritical and Supercritical Water under an Ar Atmosphere Yu-ichi Suzuki,† Hideyuki Tagaya,*,† Tetsuo Asou,‡ Jun-ichi Kadokawa,† and Koji Chiba† Department of Materials Science and Engineering, Faculty of Engineering, Yamagata University, Jonan, Yonezawa, Yamagata 992-8510, Japan, and Sumitomo Bakelite Co. Ltd., Takayanagi, Fujieda, Shizuoka 426-0041, Japan
Seven prepolymers of phenol resin were decomposed into their monomers such as phenol, cresols, and p-isopropylphenol by reactions at 523-703 K under an Ar atmosphere in subcritical and supercritical water. The total yield of identified products depended on the kind of prepolymers, and the maximum yield reached 78% in the reaction at 703 K for 0.5 h. The decomposition reactions were accelerated by the addition of Na2CO3, and the yields of identified monomers reached more than 90%. Two kinds of molding materials of phenol resin whose content of phenol resin was less than 50% were also decomposed mainly into phenol and cresols by the reaction in supercritical water. Introduction The chemical recycling of waste polymers has been gaining greater attention in recent years as a means of obtaining valuable products from waste plastics.1,2 Thermal cracking of thermoplastic resin is a well-known technique, and fluidized-bed pyrolysis technology has been under development.3,4 However, a chemical recycling process for thermosetting resin wastes such as phenol resin waste has not yet been developed. We have already reported that prepolymers of phenol resin were decomposed in hydrogen donor solvent, tetralin, at 673-713 K.5 However, the yields of phenol were only 5.3-13.7%. It is wellknown that the methylene bond connecting aryl groups is not easy to cleave.6,7 Phenol resin is known as compounds having high thermal stability because aromatic units are connected by methylene bonds as shown in Figure 1. Recently we have communicated that seven prepolymers and model compounds of phenol resin were decomposed into their monomers by reaction in supercritical water.8 It is well-known that water under supercritical conditions is much less polar and can homogenize substantial amounts of nonpolar organic compounds.9-11 Furthermore, it is pointed out that the supercritical water is emerging as a medium12 which could provide the optimum conditions for a variety of chemical reactions of substituted benzenes such as anisoles13 and guaiacol.14,15 It is also well-reported that supercritical water could provide the destruction condi* To whom correspondence should be addressed. Tel: +81-238-26-3116. Fax: +81-238-26-3413. E-mail: tc021@ dip.yz.yamagata-u.ac.jp. † Yamagata University. ‡ Sumitomo Bakelite Co. Ltd.
Figure 1. Structure of phenol resin.
tions of polymeric compounds such as cellulose,16 thermoplastic resin,17,18 rubber,19 and hazardous wastes.20,21 However, many decomposition reactions of organic compounds in supercritical water were carried out under the presence of oxygen gas and categorized to the reaction of supercritical water oxidation (SCWO).22,23 It was reported that neat pyrolysis and reaction in supercritical water of diphenylmethane without oxygen gas at 678 K led to no observable products.24 In this study we have carried out a decomposition reaction of prepolymers and molding materials of phenol resin to confirm the cleavage of the methylene bond in subcritical and supercritical water. Experimental Section Seven prepolymers of phenol resin were kindly provided by Dainippon Ink & Chemicals Co., Ltd. We have
10.1021/ie9805842 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/04/1999
1392 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 Table 1. Characteristics of Prepolymers A-G of Phenol Resin prepolymer
softening point (K)
mean MW
C (%)
H (%)
H/C
A B C D E F G
360 403 385 373 393 389 373
435 748 576 605 923 425 247
79.2 79.0 79.1 79.1 77.4 80.1 79.9
7.1 7.0 7.0 6.0 5.9 6.9 7.0
1.1 1.1 1.1 0.9 0.9 1.0 1.1
measured their molecular weights as 247-923 by GPC chromatograms; however, their exact compositions were unknown. Two molding materials A and B of phenol resin were prepared by Sumitomo Bakelite Co. Ltd. The content of the phenol resin in molding materials was less than 50%. Prepolymers of phenol resin melted at the reaction temperature. However, the reactivity of the lump of molding materials was less than that of powder. Therefore, molding materials were reacted after pulverizing. A 10 mL tubing bomb reactor was used as the reactor in which the reaction temperature was attained in about 2 min.24 The typical reaction was carried out for 0.1 g of prepolymer or molding materials with 1.0 mL of water after flushing the reactor with argon. Calculation by using van der Waals equation of state suggested that supercritical conditions were attained in the reaction at 703 K. After the reaction, products were extracted by ether and measured by GC/MS and GC. Watersoluble compounds were measured by HPLC. Results and Discussion Reaction of Prepolymers of Phenol Resin. The exact compositions of seven prepolymers of phenol resin were not known; however, the molecular weights of seven prepolymers were measured as 247-923, as shown in Table 1. They have widely differing softening points and carbon and hydrogen contents. Seven prepolymers were treated in subcritical and supercritical water under an Ar atmosphere. By the thermal treatments of prepolymers B and E in subcritical water at 623 K for 1 h, peaks of GPC chromatograms shifted to longer retention time, indicating the increment of low molecular weight compounds, as shown in Figure 2. It was clarified that prepolymers were decomposed even in subcritical water under an Ar atmosphere. Reaction products were extracted by ether, and compounds 1-6 were identified by GC/MS as the main decomposition products, as shown in Figure 3. Production of these monomer compounds indicated the occurring of a cleavage reaction of the methylene bond in subcritical and supercritical water. No carbonization reaction was confirmed in these reaction conditions. Phenol and cresol were not produced by the reactions at 523 K for 1 h, as shown in Table 2. However, production of phenol and cresol was confirmed by the reactions for 10 h even at 523 K. Furthermore, in the reactions of prepolymers A-C at 673 K, production of cresol was confirmed by the reaction for 1 h although no production was confirmed in the reaction for 0.25 h. These results indicated that high molecular weight compounds in prepolymers were gradually decomposed into low molecular weight compounds and a long reaction time was required to obtain monomer compounds. The yields of phenol and cresol depended on the kinds of prepolymers and reaction conditions. In the reactions
Figure 2. GPC chromatograms of prepolymers before (s) and after (- - -) thermal treatment at 623 K for 1 h of (a) prepolymer B and (b) prepolymer E.
of prepolymers F and G at 523-673 K, the production of phenol was not confirmed. A decomposition reaction of phenol at 653-713 K was reported in which the reactions were carried out in the presence of oxygen gas.25 In the absence of oxygen, thermal decomposition of phenol at 733 K was reported.26 However, in this study we confirmed that phenol was stable in the reaction at 673 K in the absence of oxygen gas. It indicated that if phenol was produced, the extent of secondary decomposition reaction of phenol was small. These facts suggested that prepolymers F and G were cresol resins. The yields of identified products in the reactions at 523-673 K were not large; therefore, reactions were carried out in supercritical water at 703 K for 0.5 h. The main decomposition products from prepolymers A-C were compounds 1 and 5, indicating that prepolymers A-C were prepolymers of isopropylphenol resins. Also the main decomposition products from prepolymers F and G were compounds 2-4, indicating that they were prepolymers of cresol resins as described above. Similarly, prepolymers D and E were considered as prepolymers of phenol resin, because the production of phenol was large compared to yields of other products. As already reported,5 phenol yields in the reactions of prepolymer at 713 K with hydrogen donor compounds were 5.3-13.7%. In the reactions with supercritical water at 703 K, high phenol yields from 10.5 to 30.7% were obtained for prepolymers of isopropylphenol and phenol resins, as shown in Table 3.8 Especially, in the case of prepolymer C, the total yield of compounds 1-6 reached 77.9%. These results indicated that prepolymers of phenol resin easily decomposed into their monomers not depending on the compositions of prepolymers. Hydrolysis of condensation polymers such as polyesters and polyamide has been reported. For example,
Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 1393
Figure 3. Products in the reaction of prepolymers of phenol resin. Table 2. Decomposition Reaction of Prepolymers A-G of Phenol Resin in Subcritical and Supercritical Water total yield of phenol and cresol (yield of cresol) (wt %) 523 K prepolymer 1 h 10 h A B C D E F G
0.0 0.0 0.0 0.0
623 K
573 K for 1 h
0.25 h
6.3 1.1 3.3 5.9 0.2 3.1 0.1 2.0 2.2 (2.2)
673 K 1h
0.25 h
1h
8.2 11.9 7.6 16.5 (2.4) 9.3 6.0 4.7 11.3 (3.2) 6.5 9.9 5.6 13.0 (2.5) 1.2 1.4 2.8 (0.9) 4.3 (2.0) 1.7 3.4 2.3 (0.9) 8.6 (2.3) 2.0 (2.0) 2.7 (2.7) 1.0 (1.0) 3.9 (3.9) 2.2 (2.2) 4.7 (4.7) 2.0 (2.0) 5.2 (5.2)
Table 3. Decomposition Reaction of Prepolymers A-G of Phenol Resin in Supercritical Water at 703 K for 0.5 h yield of product (wt %) prepolymer
additive
1
A
none Na2CO3 none Na2CO3 Na2CO3a none Na2CO3 none Na2CO3 none Na2CO3 none Na2CO3 none Na2CO3
30.7 48.9 18.0 27.4 57.4 26.6 58.0 20.0 29.0 10.5 24.5 0.6 1.2 1.6 1.6
B C D E F G a
2
3
4
1.3 0.1 0.2 8.9 5.0 0.9 0.4 0.2 0.1 1.2 1.0 0.2 5.9 1.8 0.3 12.0 5.0 1.3 4.9 2.4 0.7 7.0 11.6 0.7 9.9 11.0 1.5 3.8 5.9 0.8 6.9 8.4 1.3 8.4 3.3 5.4 29.7 7.7 11.6 23.7 6.7 10.8 38.7 8.3 12.2
5
6
total
19.2 25.8 11.5 14.5 19.4 29.3 24.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
3.3 2.5 0.0 0.0 0.0 3.7 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
54.8 92.0 30.2 44.3 84.8 77.9 90.4 39.3 51.4 21.0 41.1 17.7 50.2 42.8 60.8
703 K, 1 h.
poly(ethylene terephthalate) (PET) was depolymerized completely to monomer at 538 K.18 However, the reactivity of aromatic compounds containing methylene bridges was fairly low, even the reaction temperature at 678 K.24 This study indicates that water is an excellent solvent for the decomposition reaction of methylene bridges contained in prepolymers of phenol resin. Effect of Na2CO3 Addition on the Decomposition Reaction. In the reactions at 703 K, the presence of carbonization product was confirmed. It indicated the
presence of condensation reactions. If the radical intermediates were produced in the reactions with supercritical water and production to the condensation reaction, hydrogen donor compound prevent the condensation reaction. Therefore, a hydrogen donor compound, tetrahydronaphthalene, was added to the reaction with supercritical water. However, no effect of the addition of tetrahydronaphthalene on the monomer yields was observed. The importance of the ionic reaction on the decomposition was expected in the reaction with supercritical water. Therefore, acid or base compounds were added. Acid addition was not effective; however, the addition of Na2CO3 was effective on the decomposition reactions of prepolymers. When the decomposition reactions of prepolymers were carried out by adding 0.1 wt % Na2CO3 for prepolymer, the total yields of compounds 1-6 fairly increased, as shown in Table 3.8 In the case of prepolymers of isopropylphenol resin, prepolymers A and C, phenol yields increased from 30.7% and 26.6% to 48.9% and 58.0%, respectively, and the total yields reached more than 90%. In the reaction of prepolymer B whose molecular weight was larger than those of prepolymers A and C, elongation of the reaction time was effective. The large effect of Na2CO3 addition on the decomposition reaction was attained by the addition of a catalytic amount of Na2CO3. It was reported that the decomposition of various compounds in supercritical water was initiated by nucleophilic attack of water on a saturated carbon atom.24 However, they also reported that diarylalkanes such as diphenylmethane were not attacked by water. Recently hydrogen supply reaction27 and oxygen supply reaction of water28 were confirmed by 13C NMR and the reaction product of a model compounds study of phenol resin, respectively. The presence of oxygen supply reaction suggested the participation of OH- ion to the reaction. The mechanism of the positive effect of Na2CO3 addition on the decomposition reaction is not clear; however, it was expected that the concentration of OH- increased by the addition of Na2CO3. Reaction of Molding Materials of Phenol Resin. We confirmed the decomposition reaction of molding materials of phenol resin in supercritical water at 703 K into their monomers. In the decomposition reaction of prepolymers of phenol resin, compounds 1-3 were
1394 Ind. Eng. Chem. Res., Vol. 38, No. 4, 1999 Table 4. Decomposition Reaction of Molding Materials A of Phenol Resin in Supercritical Water at 703 K for 15-90 min
Table 5. Decomposition Reaction of Molding Materials B of Phenol Resin in Supercritical Water at 703 K for 15-90 min
yield of product (wt %)
yield of product (wt %)
time (min)
additive (wt %)
1
2
3
7
total
time (min)
additive (wt %)
1
2
3
7
8
total
15 15 15 15 30 30 30 30 60 60 60 60 90 90 90 90
none Na2CO3 (0.1) Na2CO3 (0.3) Na2CO3 (0.5) none Na2CO3 (0.1) Na2CO3 (0.3) Na2CO3 (0.5) none Na2CO3 (0.1) Na2CO3 (0.3) Na2CO3 (0.5) none Na2CO3 (0.1) Na2CO3 (0.3) Na2CO3 (0.5)
1.0 4.3 6.1 6.7 2.5 4.7 7.0 7.9 3.6 8.8 7.4 7.2 3.0 5.4 7.1 7.4
1.0 4.0 4.5 4.8 0.9 3.4 6.5 5.3 5.1 8.5 7.7 7.3 4.3 6.1 7.9 7.6
1.3 3.1 5.4 6.2 1.9 2.2 6.8 5.9 3.9 8.2 7.5 7.3 4.4 6.7 7.2 7.7
0.5 1.2 1.6 1.7 0.8 0.4 2.5 1.6 2.5 3.0 2.8 2.6 2.1 2.8 2.8 2.7
3.8 12.6 17.6 19.4 6.1 10.7 22.8 20.7 15.1 28.5 25.4 24.4 13.8 21.0 25.0 25.4
15 15 30 30 60 60 90 90
none Na2CO3 (0.1) none Na2CO3 (0.1) none Na2CO3 (0.1) none Na2CO3 (0.1)
1.1 2.4 0.9 3.2 2.2 3.4 2.1 2.9
1.7 8.0 0.1 3.8 3.6 4.7 3.3 3.2
1.1 5.6 0.3 4.3 3.5 4.7 3.5 4.0
0.3 0.3 0.0 0.4 0.6 0.6 0.5 0.3
0.6 1.4 0.1 2.2 2.1 2.5 2.1 1.9
4.8 17.7 1.4 13.9 12.0 15.9 11.5 12.3
Table 5. The addition of Na2CO3 was effective especially in the reactions for 15 and 30 min. Conclusions We have confirmed that not only prepolymers of phenol resin but also molding materials of phenol resin were decomposed into their monomers by the reaction in subcritical and supercritical water under an Ar atmosphere. The decomposition reaction of thermoplastic resin in subcritical and supercritical water was known; therefore, a chemical recycling process in supercritical water can be applied to the mixture of various plastics including thermoplastic and thermosetting resins. Acknowledgment The authors acknowledge the experimental assistance of N. Tohji and O. Itabashi. We are grateful for the support of this work by Research Institute of Innovative Technology for the Earth (RITE).
Figure 4. Effects of Na2CO3 addition on the decomposition reaction of molding material A at 703 K. Na2CO3: none (4), 0.1% (O), 0.3% (y), 0.5% (b).
obtained as the main products. In molding materials, aromatic units are connected by methylene bonds threedimensionally and complexly, as shown in Figure 1. Therefore, not only production of compounds 1-3 but also production of compound 7 was confirmed. In the reactions of molding material A of phenol resin, total yields of compounds 1-3 and 7 reached 15.1% by the reaction for 60 min, as shown in Table 4. The content of phenol resin in molding material A was less than 50%, and the others were a bridging agent and a nonresin organic compound. Therefore, the actual yield of resin decomposition was calculated as more than 30%. These results indicated that methylene bonds in molding materials were also cleaved in supercritical water effectively. Na2CO3 addition was also effective on the decomposition reaction of molding material A. By the addition of catalytic amounts of Na2CO3, the yields of compounds 1-7 reached near 30%. The values corresponded to actual decomposition yields of phenol resin more than 50%. For the reactions for 15 and 30 min, phenol yield increased with an increase in the amounts of Na2CO3 addition, as shown in Figure 4. For the reactions for 60 and 90 min, the effects of the amounts of Na2CO3 on the decomposition yields were small. Molding material B of phenol resin was also decomposed in supercritical water, and the production of compounds 1-3, 7, and 8 was confirmed, as shown in
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Received for review September 14, 1998 Revised manuscript received January 11, 1999 Accepted January 13, 1999 IE9805842
