Ind. Eng. Chem. Res. 2000, 39, 245-249
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APPLIED CHEMISTRY Chemical Recycling of Phenol Resin by Supercritical Methanol Jun-ichi Ozaki,* Subagijo Kastria Ingwang Djaja, and Asao Oya Department of Chemistry, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan
Thermosetting resin is one of the most difficult substances to be recycled, because it includes a highly cross-linked structure which gives the polymer chemical and thermal resistances. A preliminary study on the technique to recycle phenol resin was conducted in the present study, which was intended to be achieved by the use of a supercritical fluid. Supercritical methanol was employed because it has the following advantages over supercritical water: i.e., it allows milder reaction conditions and easier operation for the separation of the products from the solvent. The reaction of phenol resin in supercritical methanol was studied by varying the temperature in the range of 300-420 °C or changing the reaction time 30-150 min to understand the effects of the reaction temperature and of the reaction time. The conversion increased rapidly above 350 °C to give 94% at maximum at 420 °C, and the gas fraction also increased at the same time. When a longer reaction time was employed for the reaction at 400 °C, the conversion increased without giving additional gas product. From the point of view of liquefaction, the longer reaction at lower temperature would give better results. A gas chromatographic study revealed that the liquid product included phenol and its methylated derivatives. The carbon content of the solid product was higher than the initial phenol resin, showing that some carbonization took place during the reaction. Introduction The Japanese production of plastics in 1997 was reported to be 15.2 million tons, and the consumption of plastics was about 97 kg/yr per person.1 Artificially synthesized plastics are not always decomposed into soils spontaneously. There, we need to think about how to deal with the waste plastics. The waste plastics are currently disposed of by landfilling and incineration, and a very small amount of the plastics is subjected to be reused. Plastics are usually bulky and have low densities, so the dispositions by landfilling cause shortening of the lifetime of the lands.2 Combustion of plastics is usually said to generate high temperature that shortens the lifetime of furnaces. Further, combustion converts the plastics into carbon dioxide, water, and other compounds that sometimes are harmful such as dioxin. Landfilling and combustion should not be adequate ways of dealing with the waste plastics from this point of view. Materials should be recycled and reused. Three types of recycling for waste plastics are considered, that is, material recycling, chemical recycling, and thermal recycling.3 Material recycling is to collect and to reuse the waste plastics as raw materials without chemical treatments. There are two difficulties for undertaking this method. The waste plastics should be classified when they are collected to retrieve genuine regenerated plastics. Another is found in the degradation of properties during recycling. Chemical recycling is to decompose the waste plastics into their monomers * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +81-277-30-1352. Fax: +81277-30-1353.
or some useful chemicals by means of chemical reactions. Poly(ethylene terephthalate) (PET), which is vastly used as containers for soft drinks in the world, is known to be decomposed into monomers by hydrolysis,4,5 methanolysis,6,7 glycolysis,8-10 and reactions in supercritical water3 or methanol.11,12 Thermal recycling is to convert the waste plastic to fluid fuels.13 Because this method includes combustion of the product to obtain heat or power, this cannot be an ultimate recycling of plastics from the standpoint of effective uses of resources. The authors believe chemical recycling, which includes depolymerization and repolymerization, should be a promising technology for recycling of waste plastics. In considering chemical recycling, we have to be aware of the types of plastics, that is, thermoplastic resins and thermosetting resins. The production of thermosetting resins shares 12.6% of the total production of resins of both types according to 1997s statistics.1 Because these resins are used in electronic circuits and for matrix of composite materials, the consumption of these kinds of plastics is expected to grow. These materials are selected for these applications, because of their mechanical strength and thermal and chemical resistance, which are based on the highly cross-linked network structure in the polymer skeleton. These characteristics simultaneously prevent the recycling of thermosetting resins, because they do not melt or cannot be depolymerized easily. Even the highly pure waste materials, which occurred during manufacturing or molding of the polymers, are usually disposed of by landfilling without further utilization. The amount of utilization of thermosetting resins is expected to increase with the growth of electronic industries, because
10.1021/ie9904192 CCC: $19.00 © 2000 American Chemical Society Published on Web 01/13/2000
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these kinds of resins are commonly used for electric and electronic engineering. Hence, techniques for chemical recycling of thermosetting plastics should be important from the viewpoint of environmental conservation and of effective use of natural resources. Supercritical water has been applied to the chemical degradation of phenol resin14 and its model compounds.15,16 A similar approach to the coal liquefaction technique was also applied to novolak, resol, and epoxy resins, and liquid fuels were obtained by almost complete conversion.17 Possible merits of utilizing supercritical methanol over supercritical water are found in the following respects. First, the operation condition will be milder because the critical temperature and pressure of methanol are lower than those of water. This would widen the selection of materials for the reactors. Second, the separation of products from the solvent is much easier than the case using supercritical water, because the boiling point of methanol is lower than that of water. Additionally, alteration or modification of the product distribution would also be expected by changing the solvent. The present authors have been interested in controlling carbonization of phenol resin by modifying the polymer to obtain porous carbon fibers without an activation process.18-20 In the course of the study, we have found that the treatment with supercritical methanol gave liquid products from a phenol resin fiber. As far as we know, depolymerization of phenol resin in supercritical methanol has not been studied. This study was conducted to obtain preliminary information on the chemical recycling of phenol resin with supercritical methanol. The objectives of the study are (1) to know how the conversion of phenol resin is influenced by the reaction conditions and (2) to see what kinds of species are included in the liquid product. Experimental Section The used phenol resin fiber (KR-0204), of which the diameter was 10 µm, was supplied by Gun ei Chemical Co. Ltd. This was produced by spinning a novolak-type phenol-formaldehyde resin followed by curing to give a three-dimensionally cross-linked structure. A total of 200 mg of the fiber was charged in a stainless steel autoclave (reactor volume: 200 mL) with 70 mL of methanol. Air in the inner space of the autoclave was purged by nitrogen gas at 10 MPa. After returning the pressure to ambient pressure, the reaction was started by heating of the vessel in an electric furnace. The pressure inside the vessel was monitored by a pressure gauge attached to one of the ports of the autoclave. Because of the large heat capacity of the vessel, it normally took about 260 min to attain 400 °C and 300 min to attain 420 °C. The reaction temperature was controlled by a PID controller to maintain the temperature at the preset temperature (300, 350, 400, and 420 °C). During the reaction, the contents in the vessel was not agitated. The period kept at the preset reaction temperature was varied from 30 to 150 min by fixing the reaction temperature at 400 °C. Hereafter, this time is referred to as the nominal reaction time, because the time required for heating was long. After the vessel was cooled to room temperature, the products were taken out of the vessel. The liquid-solid mixture was filtered on a membrane filter made of poly(tetrafluoroethylene) (opening size: 1.0 µm). The solid product was rinsed with methanol several times and weighed after removal of the solvent by evacuation. The
Figure 1. Reaction temperature dependence of conversion based on the solid weights before and after the reaction in supercritical methanol. The reaction time was fixed at 60 min. The pressure generated during the reaction is also plotted as a function of the reaction temperature.
liquid product was retrieved from the filtrate by evaporating the solvents with a rotary evaporator, and the mass of the product was weighed. The gas product was not collected, and the yield of the product was calculated by subtracting the mass of the solid residue and of the liquid product from the initial mass of the resin. The conversion, X, was defined as follows:
X (%) ) 100(1 - Wf/Wi)
(1)
where Wi and Wf are the initial and final masses of the solid, respectively. The elemental composition of the solid product was measured by a conventional combustion technique. The liquid product was analyzed by GC-MS equipment (HP 6890 and 5973, Hewlett-Packard), in which a capillary column (HP-5MS) was installed. Helium gas was used as the carrier gas. For this measurement, the solid retrieved from the liquid product was dissolved in acetone to be injected to the gas chromatograph with a microsyringe. Results and Discussion Effect of Reaction Temperature. First, we studied how the reaction was influenced by changing the reaction temperature. During the reaction, the pressure inside the vessel increased with heating. Finally, it reached a steady value, when the temperature reached the preset reaction temperature. The pressure of each reaction temperature is plotted in Figure 1. To know the origin of the pressure, we separately measured the generated pressure of methanol during heating of the autoclave without phenol resin. There the pressures obtained were the same as the values shown in Figure 1. This indicated that the pressure generated during the reaction was not caused by the formation of gas products but by thermal expansion of methanol fluid. The reaction conditions employed in the present study exceeded the critical point of methanol (Tc ) 240 °C, Pc ) 8.09 MPa), as can be seen in Figure 1; hence, the reaction dealt with in this paper took place in supercritical methanols. The conversions of the reactions, of which temperature was varied, are also plotted in Figure 1. When the reaction temperature was 300 °C, a small conversion was obtained. A rapid increase in the conversion was seen above 350 °C, indicating that the reaction took place obviously in this temperature range. It should be noted that 98% of conversion was obtained, when the reaction was undergone at 420 °C.
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Figure 2. Variation of product distribution for the reactions with various reaction temperatures. The composition of gas was obtained as the difference. The reaction time was fixed at 60 min.
As mentioned in the Experimental Section, it took a rather long period to attain 420 °C, i.e., 40 min for only a 20 °C increase; we have to be aware of the effect of the elongated reaction time, particularly in the temperature range 400-420 °C, where the reaction occurred significantly. The product distributions corresponding to the reaction discussed above are presented in Figure 2. Here, the gas composition was obtained by subtracting the masses of the solid and the liquid products from the initial mass of the phenol resin. So, if the solvents methanolswas incorporated in the reaction to influence the mass of the liquid products, then one should notice that the fraction of the gas products thus calculated would give a minimum composition. When the reaction temperature increased from 300 to 400 °C, the increase in the conversion mainly led to the formation of the liquid product. A further increase in the reaction temperature by 20 °C gave an increase in the conversion as mentioned above. The composition of the reaction products showed the following features: the gas fraction increased at the expense of the solid fraction; however, the liquid fraction was kept at the same value as that of the 400 °C reaction. There are two possibilities for the increase in the gas fraction. One is the direct conversion from the solid residue to the gas product, and the other is a gas formation by some cracking reaction of the liquid products. We believe that the use of an apparatus that allows rapid heating and gas analysis would give some clue for the gas formation reaction for this case. Effect of Nominal Reaction Time. The change in the conversion against the nominal reaction time is presented in Figure 3 along with the pressure during the reaction. The pressure did not vary in this case. The conversion was found to increase with the nominal reaction time and to have no relation to the pressure. It can be seen that 94% of conversion could be attained even at 400 °C, if a longer reaction time of as much as 150 min was employed. One might point out that the reaction would be completed during heating, because of the long heating period to attain the reaction temperature (normally 260 min to attain 400 °C). However, the conversion obtained for a 30 min reaction was less than that for 60 min, which neglected the pointed-out possibility. Figure 4 shows the product distribution of the reactions with different reaction times. When the reaction was continued for 30 min, the liquid composition was 67% and the gas composition was ca. 4%. With an increase in the time to 60 min, the liquid yield became ca. 80% and the gas composition did not show a significant variation. The liquid production was en-
Figure 3. Reaction time dependence of conversion based on the solid weights before and after the reaction in supercritical methanol. The reaction temperature was fixed at 400 °C. The pressure generated during the reaction is also plotted as a function of the reaction temperature.
Figure 4. Variation of product distribution for the reactions with various reaction times. The composition of gas was obtained as the difference. The reaction temperature was fixed at 400 °C.
hanced to 88% after the reaction for 150 min. As noted above, this reaction gave the highest conversion of 94%, which was coincidentally the same conversion as the reaction at 420 °C for 60 min. It is interesting to compare both reactions. First, in case one considers that the longer heating time to reach 420 °C from 400 °C would give the same effect as heating at a lower temperature for a longer period, we shall discuss the reaction pressures of both reactions. They showed definitely different pressures of 27 MPa for the 400 °C reaction and 31 MPa for the 420 °C reaction, from which we can say that the two runs had been performed at different conditions, and hence the difference discussed below should never be the effect of heating time but of the temperature. Now, let us compare the gas formation of the two reaction runs. The case of the reaction at 400 °C for 150 min gave a gas composition of ca. 7%; on the other hand, the reaction at 420 °C for 60 min gave a higher gas composition of 26%. In the present project, the component that we need to obtain is liquid, so the longer reaction at lower temperatures would be favorable than the shorter reaction at higher temperatures. Compound Distribution of Liquid Products. The gas chromatographic technique is applicable only for the compounds with lower molecular weight or lower boiling point; hence, if some oligomers of phenol resin are included in the product, we cannot notice them. The color of the liquid product was usually brown to orange, which suggested the presence of higher molecular weight compounds in the product. Here, we conducted a study to obtain preliminary information on what kinds of products were formed by this reaction. Figure 5 shows a typical chromatogram of the liquid product. As can
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Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 Table 1. Elemental Composition of the Solid Products after the Reactions Varying the Nominal Reaction Time sample
temperature (°C)
phenol resin solid residue #1 solid residue #2 solid residue #3
400 400 400
nominal time (min) 30 60 150
composition (%) C
H
O
74.9 84.2 84.9 89.9
5.5 6.0 5.5 3.9
19.6 9.8 9.6 6.2
Conclusions
Figure 5. Typical example of a GC-MS chromatogram. The presented data were obtained with the liquid product after the reaction at 400 °C for 150 min.
Figure 6. Estimated structure of the phenol resin used in the present study. This was given by Gun ei Chemical Co. Ltd.
be seen, the liquid product was composed of mainly six phenol substitutes, that is, phenol, o- and p-methylphenols, 2,6- and 2,4-dimethylphenols, and 2,4,6-trimethylphenol. The chemical structure of the resin, which was given by the supplier estimated from the elemental analysis and the solid-state NMR, is given in Figure 6. According to this, the phenolic nuclei typically have the structures of which rings are substituted by two to three methylene groups on average. If the action of methanol is merely to break the methylene bridges, then the main product should be cresols or xylenols. However, as can be seen in Figure 6, the production of trimethylphenol is obvious. This suggests a possibility of methylation of phenolic nuclei by the action of supercritical methanol. In fact, reactions of phenol with methanol under hightemperature and high-pressure conditions have been known to give methylation.21 The main product of the reaction varied with the condition; for example, o-cresol was produced at 300-360 °C and 4-7 MPa and 2,6xylenol at more severe conditions. Indeed, this reaction is employed in the U.S. and German industries to produce phenol derivatives, such as o-cresol and 2,6xylenol. To conclude the occurrence of methylation in the reaction, we need to obtain the absolute abundance of the phenols. Solid Products. The color of the solid was changed from the original color of the resin, gold, to black after the reaction. The elemental composition of the solid product is presented in Table 1. The reaction in supercritical methanol increased the carbon content and reduced the oxygen content. The results indicated that carbonization took place during the reaction. This observation is very interesting from the standpoint of development of new carbonization techniques. Properties of the solid residue will also be inspected.
In this study, we conducted a preliminary study on the liquefaction of phenol resin by supercritical methanol. The resin is one of the most difficult substances for recycling, because it has a highly cross-linked network structure. The obtained conclusions are as follows. (1) Supercritical methanol could liquefy phenol resin, and the reaction was obvious above 400 °C to give a conversion higher than 80%. (2) Both reaction conditions of a longer time at a temperature and a higher reaction temperature for a shorter time resulted in increasing the conversion. However, the former condition turned out to be favorable for obtaining a higher yield of liquid products. (3) The liquid product was found to include phenols. (4) The analysis of the solid products revealed a concentration of carbon atoms during the reactions. Acknowledgment The authors express their thankfulness to Gun ei Chemical Industry Co. Ltd. for providing phenol resin fibers used in the present study. We also thank Associate Professor Tadafumi Adschiri, Tohoku University, and Professor Takao Ikariya, Tokyo Institute of Technology, for their useful discussions. Literature Cited (1) Arai, Y., Ed. Toukei de miru Purasuchikku Sangyou no Ichinen. Purasuchikkusu 1998, 49, 18. (2) Voss, D. Plastics Recycling: New Bottles for Old. Chem. Eng. Prog. 1989, Oct, 67. (3) Sato, O.; Saito, N. Decomposition of Plastics with Supercritical Fluids. Nippon Enerugi Gakkaishi 1997, 76, 861. (4) Campanelli, J. R.; Kamal, M. R.; Cooper, D. G. A Kinetic Study of the Hydrolytic Degradation of Polyethylene Terephthalate at High Temperatures. J. Appl. Polym. Sci. 1993, 48, 443. (5) Yoshioka, T.; Sato, T.; Okuwaki, A. Hydrolysis of Waste PET by Sulfuric Acid at 150 °C for a Chemical Recycling. J. Appl. Polym. Sci. 1994, 52, 1353. (6) Vereinigte Glanzstoff-Fabriken, A.-G. Conversion of Poly(ethylene terephthalate) into dimethyl terephthalate. Brit. Patent 755,071, 1956. (7) Vereinigte Glanzstoff-Fabriken, A.-G. Dimethl terephthalate. Brit. Patent 787, 554, 1957. (8) Oku, A.; Hu, L.-C.; Yamada, E. Alkali Decomposition of Poly(ethylene terephthalate) with Sodium Hydroxide in Nonaqueous Ethylene Glycol: A Study on Recycling of Terephthalic Acid and Ethylene Glycol. J. Appl. Polym. Sci. 1997, 63, 595. (9) Chen, J. Y.; Ou, C. F.; Hu, Y. C.; Lin, C. C. Depolymerization of Poly(ethylene terephthalate) Resin Under Pressure. J. Appl. Polym. Sci. 1991, 42, 1501. (10) Campanelli, J. R.; Kamal, M. R.; Cooper, D. G. Kinetics of Glycolysis of Poly(ethylene terephthalate) Melts. J. Appl. Polym. Sci. 1994, 54, 1731. (11) Adschiri, T.; Sato, O.; Machida, K.; Sato, N.; Arai, K. Recovery of Terephthalic Acid by Decomposition of PET in Supercritical Water. Kagaku Kogaku Ronbunshu 1997, 23, 505. (12) Sako, T.; Sugeta, T.; Otake, K.; Nakazawa, N.; Sato, M.; Namiki, K.; Tsugumi, M. Depolymerization of Polyethylene Tereph-
Ind. Eng. Chem. Res., Vol. 39, No. 2, 2000 249 thalate to Monomers with Supercritical Methanol. J. Chem. Eng. Jpn. 1997, 30, 347. (13) Uemichi, Y.; Hattori, M.; Itoh, T.; Nakamura, J.; Sugioka, M. Deactivation Behavior of Zeolite and Silica-Alumina Catalysts in the Degradation of Polyehthylene. Ind. Eng. Chem. Res. 1998, 37, 867. (14) Goto, J.; Otori, T.; Adschiri, T.; Arai, K. Chourinkai Suichuu deno Netsukoukasei Jushi no Bunkai. Nettowakuporima Kouentouronnkai Kouenyoushishuu 1996, 29. (15) Tagaya, H.; Suzuki, Y.; Kadokawa, J.; Karasu, M.; Chiba, K. Decomposition of Model Compounds of Phenol Resin Waste with Supercritical Water. Chem. Lett. 1997, 49. (16) Suzuki, U.; Tagaya, H.; Asou, T.; Kadokawa, J.; Chiba, K. Decomposition of Prepolymers and Molding Materials of Phenol Resin in Subcritical and Supercritical Water under an Ar Atmosphere. Ind. Eng. Chem. Res. 1999, 38, 1391. (17) Sato, Y.; Kodera, Y.; Kamo, T. Effect of Solvents on the Liquid-Phase Cracking of Thermosetting Resins. Energy Fuels 1999, 13, 364. (18) Ozaki, J.; Ohizumi, W.; Endo, N.; Oya, A.; Yoshida, S.; Iizuka, T.; Roman-Martinez, M. C.; Linares-Solano, A. Preparation
of Platinum Loaded Carbon Fiber by using Polymer Blend. Carbon 1997, 10-11, 1676. (19) Ozaki, J.; Endo, N.; Ohizumi, W.; Igarashi, K.; Nakahara, M.; Oya, A.; Yoshida, S.; Iizuka, T. Novel Preparation Method for Production of Mesoporous Carbon Fiber from Polymer Blend. Carbon. Carbon 1997, 35, 1031. (20) Imamura, R.; Matsui, K.; Takeda, S.; Ozaki, J.; Oya, A. A New Role for Phosphorus in Graphitization of Phenolic Resin. Carbon 1999, 37, 261. (21) Mukaiyama, T., Translator. Weissermel, K.; Arpe, H.-J. In Kogyo Yuuki Kagaku (Original: Industrielle organishe chemie Bedeute Vor- und Zwischenprodukte; Tokyo Kagaku Dojin: Tokyo, 1996; p 345.
Received for review June 14, 1999 Revised manuscript received October 26, 1999 Accepted November 6, 1999 IE9904192
