ODA Polyimide in a Fused Silica Capillary

Article pubs.acs.org/IECR

Depolymerization of ODPA/ODA Polyimide in a Fused Silica Capillary Reactor and Batch Autoclave Reactor from 320 to 350 °C in Hot Compressed Water Fen Huang, Yuanyuan Huang, and Zhiyan Pan* Department of Environmental Engineering, Zhejiang University of Technology, Hangzhou 310032, People’s Republic of China S Supporting Information *

ABSTRACT: Depolymerization of polyimide synthesized from 4,4′-oxidiphthalic anhydride and 4,4′-diaminodiphenyl ether monomers (ODPA/ODA PI) in hot compressed water was carried out in a fused silica capillary reactor (FSCR) and batch autoclave reactor system. The phase behavior of ODPA/ODA PI in water during the heating, reaction, and cooling stages of the process in FSCR was observed with an image recording system from under the microscope. The effects of temperature (320−350 °C) and reaction time (30−90 min) on the depolymerization yield and products yields in batch autoclave reactor were investigated at a fixed H2O/PI ratio, and optimal conditions were established. Additionally, an understanding of the reaction pathway of ODPA/ODA PI depolymerization in hot compressed water was developed.

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supercritical water above 200 °C and at sufficiently high pressure) and its mixing behavior highlighted the opportunity to use the extraordinary properties for chemical reactions. In hot compressed water, Toray13 found that polyimide film synthesized from 1,2,4,5-benzenetetracarboxylic anhydride (PMDA) and 4,4′-diaminodiphenyl ether (ODA) monomers (PMDA/ODA PI) could be depolymerized back to the monomer units. The PMDA/ODA PI film investigated in the research was slurried, and nitrogen was employed as the gaseous medium. A conversion of 100% was obtained without catalysts and under reaction conditions of 300 °C and 30 MPa for 30 min in hot compressed water, providing 80% of the monomers. When the depolymerization of PMDA/ODA PI was carried out in methanol at 250 °C and 10 MPa for 30 min, a conversion of 100% was obtained, and the yield of monomers could reach 95%. The above research results showed that the depolymerization of polymer in hot compressed water systems was a feasible and environment friendly method for the recycling of monomers. However, nitrogen was needed to displace the air in the reactor and had to be supplied for an hour to allow for the desired pressure of 30 MPa to be reached. Furthermore, not much is known about changes in phase behavior during depolymerization reactions in hot compressed water, and these behaviors are important considerations for the practical recycling and handling of polymers such as polyimide. In the current research, changes in the phase behaviors of ODPA/ODA PI depolymerization reactions in hot compressed water were observed in a fused silica capillary reactor (FSCR) and recorded under a microscope at different temperatures. This new methodology was developed in our previous study.14−16 The reaction product in the vapor phase of the FSCR was analyzed directly by Raman spectroscopy.17 Further

INTRODUCTION Polyimides (PI) have been in mass production since 1961, and since then, hundreds of polyimides of different structures have been researched and progressed into industrial applications. Polyimide materials are lightweight, flexible, highly chemically resistant, and thermally stable up to 310 °C over extended periods of time and 450 °C over shorter periods of time. Polyimides are also resistant to weak acids but are not recommended for use in alkaline environments. In light of their properties, polyimides are wildly used in the electronics industry for flexible cables and as an insulating film on magnet wire.1,2 Disposal of waste PI has become a serious environmental and economic problem because of increased PI production. The three main methods of recycling for polymer wastes are chemical recycling, material recycling, and thermal recycling. Conversion of polymers to their constituent monomer by chemical recycling is the most desirable process of recycling.3 Some studies focused on the depolymerization of PI have reported the recycling of monomers from PI with alkaline, which is typically carried out with aqueous solutions of NaOH or KOH.4 Several papers indicated monomer yields of up to 80% using 10−30 wt % alkaline solution and reaction times over the range of 3−15 h.5−7 Pseudofirst-order kinetic data were obtained with kobs (the pseudofirst-order rate coefficients) in the range 0.1−0.9 min−1. On the basis of Stephans’ study,8 Jiang9 proposed a model of PI alkaline depolymerization, which predicted that PI would totally depolymerize to the corresponding monomers in a 10 wt % aqueous NaOH solution at 60 °C for 9 h. However, the main disadvantages for this type of recycling were long reaction times, the requirement for large amounts of alkaline, and the deteriorating properties of monomers. In recent years, the reactivity and depolymerization of organic compounds in water has received a great deal of attention.10,11 The studies by Kruse’s research group11,12 of the properties of hot compressed water (HCW, here subcritical and © 2012 American Chemical Society

Received: Revised: Accepted: Published: 7001

January 24, 2012 April 18, 2012 April 27, 2012 April 27, 2012 dx.doi.org/10.1021/ie300162m | Ind. Eng. Chem. Res. 2012, 51, 7001−7006

Industrial & Engineering Chemistry Research

Article

a heating collar, and heated to the desired reaction temperature. Temperature control to within ±1 °C was achieved with an XMT-type controller, and a K-type thermocouple was used to measure the temperature with the heating rate of 10 °C/min. Upon completion of the reaction, the reactor was removed from the heating collar and rapidly cooled in a water bath. Sample Analysis. The products of depolymerization from the batch autoclave reactor included a vapor product, solid products, and aqueous products. The vapor and solid products were identified by a Fourier-transform infrared (FT-IR) spectrometer (Thermo Nicolet, AVATAR-370, Waltham, MA) with a resolution of 1 cm−1. The aqueous phase was transferred to a volumetric flask and mixed with acetic ester to enable the contents of the flask to be extracted and qualitatively identified using an Agilent 6890 gas chromatograph equipped with a 5975C mass spectrometer (GC−MS) with a 30 m × 0.25 mm × 25 μm HP-5MS capillary column and quantified using an Agilent 6890 GC with flame ionization detection (FID). The temperature program consisted of a 3 min soak at 150 °C, followed by a constant-temperature ramp (30 °C/min) to 290 °C and a holding period of 6 min at the final temperature.

experiments to determine the effects of reaction temperature and time on the depolymerization of ODPA/ODA PI in hot compressed water were conducted in a batch autoclave reactor without displacing of the air in the reactor. On the basis of the experimental results, a mechanistic pathway for the depolymerization of ODPA/ODA PI in hot compressed water has also been proposed.

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EXPERIMENTAL SECTION Reagents and Equipment. Industrial grade polyimide (type: YS 20, cylinder φ 9 mm × 190 mm, molecular weight from 10 000 to 20 000 g/mol, Tg 265 °C, tensile strength(23 °C) 130 MPa) synthesized from 4,4′-oxidiphthalic anhydride (ODPA) and 4,4′-diaminodiphenyl ether (ODA) (ODPA/ ODA PI) was obtained from the Shanghai Research Institute of Synthetic Resins (Shanghai, China). The molecular structure of the polyimide is as shown in Scheme 1. For these experiments, Scheme 1. Chemical Molecular Structure of ODPA/ODA Polyimide

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RESULTS AND DISCUSSION Determination of Depolymerization Products. The vapor product was analyzed by Raman spectroscopy without sampling in FSCR and by a Fourier-transform infrared (FT-IR) spectrometer with sampling from the batch reactor after cooling. The results are shown in Figures 1 and 2. The standard CO2 and the vapor phase product were characterized by FT-IR analysis. As shown in Figure 2, the accordance of the characteristic peaks 669 cm−1 (C′O deformation vibration) and 2350 cm−1 (CO stretching) in FT-IR spectra for

cylinder polyimide was cut into φ 9 mm × 2 mm size pieces for depolymerization. Silica capillary tubing (665 μm o.d. and 300 μm i.d., with polyimide coating) was purchased from Polymicro Technologies LLC (Phoenix, AZ). The monomers, reagent grade 4,4′-oxydianiline, and 4,4′-oxybisbenzoic acid were purchased from Shandong Wanda Chemical Industries Co., Ltd. (Shandong, China) and Alfa Aesar (Tianjing, China), respectively. Reagent grade aniline, hydroquinone, and 4aminophenol were obtained from Shanghai chemical reagents plants (Shanghai, China). All reagents were used as received. To prepare the FSCR, a section of silica capillary of approximately 2−3 cm in length was cut, and one end of the tube was sealed with an oxyhydrogen flame. The ODPA/ODA PI and water were loaded into the capillary tube, and the open end of the tube was sealed with an oxyhydrogen flame to form a FSCR. Finally, the FSCR was loaded in the heating/cooling stage (Instec, INS0908051, U.S.) equipped with a digital temperature controller (Instec, Instec, STC200, U.S.). The phase change of the ODPA/ODA PI during heating was observed under a microscope and recorded onto a computer through a digital camera (JVC, TK-C1481, Japan). The experimental details are presented in the Supporting Information. Upon completion of the experiment, the depolymerization gas product was determined directly using Raman spectroscopy with a JY/Horiba LabRam HR Raman system. The system was equipped with 531.95 nm (frequency doubled Nd:YAG) laser excitation, a 10× Olympus objective with 0.25 numerical aperture, and a 1800 grooves/mm grating with a spectral resolution of 1 cm−1. Approximately 50 mW laser light was focused on the sample to acquire spectra from 100 to 4000 cm−1. The integration time was 100 s with three accumulations per spectrum. In addition, a batch autoclave reactor of volume 0.05 L was used to study the depolymerization of ODPA/ODA PI in hot compressed water. In a typical experiment, 1.2 g of ODPA/ ODA PI and 24 mL of deionized water were loaded into the batch autoclave reactor. The reactor was then sealed, placed in

Figure 1. Raman spectrogram of the vapor phase before reaction (A) and after reaction (B). The vapor product was determined directly using Raman spectroscopy without sampling at room temperature after the reaction in hot compressed water at 330 °C for 30 min in a FSCR. Approximately 50 mW laser light was focused on the sample to acquire spectra from 100 to 4000 cm−1 (shown in a) and the spectra from 1200 to 1500 cm−1 (shown in b). The integration time was 100 s with three accumulations per spectrum. The signals (lower and upper bands and hot bands) indicated CO2 was the only vapor product of ODPA/ODA PI depolymerization. 7002

dx.doi.org/10.1021/ie300162m | Ind. Eng. Chem. Res. 2012, 51, 7001−7006


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Table 1. Main Aqueous Products of the ODPA/ODA PI Depolymerization Detected by GC−MS Following Depolymerization Reaction at 330 °C for 30 min retention time/min

CAS

chemical names of products

contents/%

2.04 2.84 6.38 6.79 8.41 11.94

67-64-1 141-78-6 106-51-4 62-53-3 123-31-9 101-80-4

acetone (solvent) ethyl acetate (solvent) p-benzoquinone aniline hydroquinone ODA

0.34 97.30 0.1 0.88 1.03 0.12

water at a reaction temperature of 330 °C for 30 min contained p-benzoquinone (0.1%), aniline (0.88%), hydroquinone (1.03%), and 4,4′-ODA (0.12%) according to the GC chromatogram. The depolymerization yield of ODPA/ODA PI was defined as follows:

Figure 2. FT-IR spectrogram of standard CO2 sample (a) and the vapor phase product from the batch reactor at a reaction temperature of 330 °C for 30 min (b).

depolymerization yield of PI(%) weight of PI feed − weight of remaining solid = × 100% weight of PI feed

standard CO2 suggests the gas product is CO2. The results indicated that CO2 was the only gas formed from the ODPA/ ODA PI depolymerization in hot compressed water. The standard 4,4′-oxybisbenzoic acid and the solid product were characterized by FT-IR analysis. As shown in Figure 3, the

The yield of products is defined as follows: yield of products(%) weight of products recovered after reaction = × 100% theoretical content of products in PI feed

Phase Changes of ODPA/ODA PI in Hot Compressed Water in FSCR. Images of the phase changes occurring during the heating, reaction, and cooling stages of the depolymerization process are given in Figure 4. It is clear that during the heating process the ODPA/ODA PI swelled at 208.1 °C and softened at 300.1 °C. Three phases (i.e., ODPA/ODA PI in solid form, aqueous fluid, and vapor phase) were present in the FSCR below 300.1 °C. The ODPA/ODA PI melted at 318.6 °C and formed spherules at 329.8 °C, which coexisted with the aqueous and vapor phases. Figure 4b indicates the phase changes of ODPA/ODA PI in the condition of reaction time from 0 to 30 min at 330 °C. The liquid ODPA/ODA PI spherule shrank and depolymerized gradually, and a gas bubble appeared following 10 min at 330 °C. We noted that ODPA/ODA PI did not dissolve totally in hot compressed water, and three phases were present following 30 min at 330 °C. During the cooling process (as shown in Figure 4c), phase separation occurred at 298.2 °C, and increasing numbers of gas bubbles appeared. Needlelike particles began to appear at 149.3 °C, becoming bulk solid in appearance as the temperature decreased gradually to 45.3 °C. Gaseous CO2, which was produced by the depolymerization of ODPA/ODA PI, was detected in the vapor phase by Raman spectroscopy in the FSCR directly at 25 °C (as shown in Figure 1). Effect of Temperature and Time on the Depolymerization of ODPA/ODA PI. To investigate the effects of the reaction temperature and reaction time on the depolymerization of ODPA/ODA PI, experiments were carried out in the batch autoclave reactor at temperatures ranging from 320 to 350 °C under pressures of 11.5−17.5 MPa with reaction times of 30−90 min in hot compressed water. The experimental results for the depolymerization yield of ODPA/ODA PI are shown in Figure 5. As the reaction temperature and reaction

Figure 3. FT-IR spectrogram of a standard 4,4′-oxybisbenzoic acid sample (a) and sample of the solid product from the ODPA/ODA PI depolymerization in hot compressed water at a reaction temperature of 330 °C for 30 min (b).

accordance of the characteristic peaks of the benzene ring at 3066.1, 1689.1 (CO symmetrical stretching), 1596.1 (CC stretching), 1250.5 (C−O−C), and 770 (CO bending) in FT-IR spectra for 4,4′-oxybisbenzoic acid suggests the solid product is 4,4′-oxybisbenzoic acid. The main product identified in the liquid from the ODPA/ ODA PI depolymerization was the ODA monomer, as shown in Table 1. The GC chromatogram and MS plot of liquid are shown in Figures S1 and S2 in the Supporting Information. Amounts of p-benzoquinone, aniline, and hydroquinone were also detected in the ODPA/ODA PI depolymerization in hot compressed water. As shown in the Table 1, the liquid products of the ODPA/ODA PI depolymerization in hot compressed 7003



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Figure 4. Photomicrographs of ODPA/ODA PI in hot compressed water during the heating process (a), reaction at 330 °C at different time points (b), and the cooling process (c). Note that in a FSCR above 318.6 °C shown in (a), ODPA/ODA PI was starting to melt and formed spherules at 329.8 °C, as shown in (b). Solid product began to separate out when lowering the temperature to 45.3 °C, as shown in (c).

Figure 5. Effects of reaction time on ODPA/ODA PI depolymerization yield at different temperatures in hot compressed water.

Figure 6. Effects of time on yield of OBBA at different temperatures in hot compressed water.

time increased, the depolymerization yield of ODPA/ODA PI gradually increased. For a reaction of 30 min, the depolymerization yield of ODPA/ODA PI increased from 8.19% at 320 °C to 43.40% at 340 °C. Considering the phase changes of ODPA/ODA PI in Figure 4, water and solid ODPA/ODA PI cannot come in complete contact with each other at the lower temperature of 320 °C, which leads to a lower yield in the depolymerization reaction. The depolymerization yield of ODPA/ODA PI increased rapidly when the reaction temperature was 330 °C, especially for reaction times between 45 and 75 min. Furthermore, the depolymerization yield was greater than 99% when the reaction temperature was 330 °C for 75 min. When the reaction temperature was 350 °C, the depolymerization yield of ODPA/ODA PI exceeded 99% at 30 min. High temperature and prolonged reaction times were beneficial for the swelling of ODPA/ODA PI’s in hot compressed water. Effect of Temperature and Time on the Yield of OBBA. The yields of the ODPA/ODA PI products, OBBA and ODA, were determined from the solid and liquid products of depolymerization, respectively. First, the effect of temperature and time on the yield of the OBBA is presented in Figure 6.

The data show that changes of temperature had a significant influence on the yield of OBBA at different times. The yield of OBBA increased rapidly when the temperature was increased from 320 to 340 °C for a reaction time in the range of 30−60 min. It is apparent from Figure 6 that the maximum yield of OBBA obtained with 330 °C 75 min was higher than the maximum yield obtained at 340 °C 60 min. The results indicate that the generation rate of OBBA is greater than the decomposition rate during the low temperature. With the increasing temperature over extended periods, the decomposition rate of OBBA became greater than the generation rate. Following 30 min at 350 °C, the yield of OBBA was 81.04%, although the yield decreased when the residence time exceeded 30 min. The results demonstrated that the yield of OBBA obtained with a reaction temperature of 350 °C and reaction time of 30 min was significantly greater than the yield obtained following 90 min at the same temperature. Effect of Temperature and Time on the Yield of ODA. In addition, the effects of temperature and time on the yield of the ODA monomers of ODPA/ODA PI were analyzed. Figure 7 shows that for a reaction temperature of 320−330 °C, the ODA yield increased over time, and the maximum yield of 7004



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reaction pathway of ODPA/ODA PI depolymerization in hot compressed water under the experimental conditions is shown in Scheme 2. Scheme 2. Pathway of ODPA/ODA PI Depolymerization in Hot Compressed Water

Figure 7. Effects of time on the yield of the ODA monomer at different temperatures in hot compressed water.

ODA was only 6.43% at 60 min. Heating for longer periods resulted in a decrease in the ODA yield. The yield decreased gradually with increasing reaction time for temperatures in the range of 340−350 °C. It can be seen from Figure 7 that the highest yield of ODA in the aqueous phase was only 9.85% (340 °C, 30 min), which was much lower than the theoretical value. The experimental results for ODA yield reveal that the ODA yield decreases with increasing temperature and time, to a certain extent (beyond 60 min at 320−330 °C). This trend was attributed to the poor stability of ODA with the increasing temperature over extended periods of time. In the liquid phase of these reactions, p-benzoquinone, aniline, and hydroquinone were also detected. We supposed that ODA was decomposing and being broken into other substances, although this would need to be further investigated in the future.

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CONCLUSIONS The depolymerization of ODPA/ODA PI was examined in hot compressed water in a FSCR and batch autoclave reactor. Observation of changes in the phase behavior of ODPA/ODA PI in hot compressed water in the FSCR during the heating, reaction, and cooling stages of the process revealed that the ODPA/ODA PI swelled at 208.1 °C, intenerated at 330 °C, and did not dissolve completely in water following 30 min at 330 °C. The only gas product detected in the vapor phase by Raman spectroscopy of the FSCR directly was carbon dioxide. In addition, the same product was also detected in the vapor phase from the batch autoclave reactor by FT-IR. The main monomers of PI depolymerization were ODA and OBBA, which were analyzed both quantitatively and qualitatively by GC−MS and FT-IR, respectively, from the batch autoclave reactor. The optimum reaction conditions of 350 °C for 30 min afforded an ODPA/ODA PI depolymerization yield in excess of 99%. Furthermore, the yields of the OBBA and ODA monomer units were 81.04% and 8.18%, respectively. A detailed reaction mechanism of ODPA/ODA PI depolymerization in hot compressed water was developed by the results from the FSCR and batch autoclave reactor. The results have broadened the understanding of the effects of temperature and time on ODPA/ODA PI depolymerization in hot compressed water for chemical recycling of polymer wastes.

REACTION PATHWAY Following analysis of the phase changes of ODPA/ODA PI by microscopy during heating in hot compressed water inside a FSCR and the products of ODPA/ODA PI in hot compressed water in the batch autoclave reactor, we believe that the intermolecular forces of the ODPA/ODA PI chains decreased as the distances between molecules increased with increasing temperature. The ionic product of water (Kw) increased approximately 3-fold near the critical point, and comparatively adequate amounts of H+ and OH− were in aqueous solution. The imide rings of the ODPA/ODA PI chains were easily hydrolyzed by OH− ions forming polymeric acids. Subsequent hydrolysis of the imide bonds resulted in the formation of the ODA and OBBA and the breaking of the macromolecular chains. Carbon dioxide was the only gas product of the reaction, which was detected by both Raman spectroscopy directly in the FSCR and FT-IR with sampling from the batch autoclave reactor. According to the data, the actual yield of ODA is lower than the theoretically possible value after the reaction. Cleavage of ether bond due to the high temperatures and pressures is possible, and the overall stability of the system would also be reduced by the presence of an amino group in the para-position of benzene. On the basis of the industrial manufacture of hydroquinone by the oxidation of aniline, it can be suggested that p-benzoquinone was generated from aniline by oxidation from the O2−, which was generated by ODA decomposing into aniline. The p-benzoquinone can then combine with H+ to form hydroquinone in hot compressed water.17,18 A suggested

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ASSOCIATED CONTENT

S Supporting Information *

Materials and experimental method. This material is available free of charge via the Internet at http://pubs.acs.org. 7005



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Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: (+86) 571-88320061. Fax: (+86) 571-88320061. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support for this research was provided by the Natural Science Foundation of China (20677052 and 21077092). REFERENCES

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ODA Polyimide in a Fused Silica Capillary

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