Removal of Oligomers from Poly(ethylene terephthalate) Resins by

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Removal of Oligomers from Poly(ethylene terephthalate) Resins by Hydrothermal Extraction Jun Inagaki,†,‡ Kensei Hirae,‡ Mitsuru Sasaki,*,‡ Motonobu Goto,§ and Katsuya Ito† †

Films Technology Center, Toyobo Co., Ltd., 10-24 Toyo-cho, Tsuruga, Fukui 914-0047, Japan Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan § Department of Chemical Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan ‡

ABSTRACT: In this study, hydrothermal extraction experiments were performed in a semibatch apparatus at varying temperatures and different extraction times to determine the optimum operating conditions for the removal of oligomers from poly(ethylene terephthalate) (PET) resins. High-molecular-weight PET in pellet form (Mw: 100 kDa) was used as the raw material. The results obtained suggest that hydrothermal treatment enables partial extraction of oligomers from PET resins. Using theoretical methods and the experimental data, we calculated the maximum reaction time at each temperature, considering the potential for application in plastic processing. A maximum extraction efficiency of 16% was achieved by hydrothermal extraction at 200 °C for 10 minutes. This novel technique is a promising tool for the development of a next-generation, lowoligomer, eco-friendly method in a short time.

1. INTRODUCTION Poly(ethylene terephthalate) (PET) is an important thermoplastic material. It has been widely used in the form of fibers, sheets, and films for packaging and other industrial applications because of its excellent characteristics, such as thermal stability, clarity, strength, and moldability. PET films are biaxially oriented semicrystalline films prepared by a process in which the amorphous cast is drawn in both machine and transverse directions. The biaxially oriented film is then heated and set to crystallize. The success of polymer films in general applications is attributed to the coupling of their basic polymer properties with the biaxial orientation and heat setting manufacturing processes, as described above. PET materials contain a small concentration of monomers and low-molecular-weight oligomers that are formed as byproducts during condensation polymerization of ethylene glycol and terephthalic acid. Two types of oligomers were present in PET: “linear” oligomers with a straight-chain structure and “cyclic” oligomers with a ring-shaped structure. It is known that the cyclic trimer is typically present in greater amounts than any other oligomer in PET. The copolymer of ethylene, diethylene glycol, and terephthalic acid are classified as linear oligomer. All these low-molecular-weight oligomers can migrate to the surface if the PET material is treated at high temperatures for tens of minutes.1 The presence of these oligomers on the surface can interfere with the manufacturing process. Therefore, it is important to prevent oligomeric migration to the surface of PET materials for the use of these materials in high-temperature processing. Several studies on the removal of oligomers from PET materials have been reported. The extraction of oligomers from PET resins using a liquid−solid Soxhlet extractor provided a high yield of oligomers but required an extremely long extraction time.2 Solid-state polymerization, which has been widely used in industrial processes, is an alternative method. © XXXX American Chemical Society

Although the concentration of oligomer in PET resin decreases, the molecular weight of PET increases by solid-state polymerization. The treated PET may result in low productivity and thermal degradation during the melting process owing to its high molecular weight.3 Furthermore, the efficiency of the oligomer removal is not sufficiently high. Supercritical fluid extraction is applied for extracting materials, such as essential oils and caffeine. For the extraction of PET oligomers, supercritical carbon dioxide extraction has been investigated as an analytical method.4−7 Sub- and supercritical fluids such as water and alcohol are also used in the chemical recycling of plastics. In the case of polyesters, the polymer is readily depolymerized into its monomers without catalysts in water or alcohol, which act as both reactants and solvents.8−11 On the other hand, in hydrothermal extraction, subcritical water is used as an extraction solvent. In the region near the critical point, water properties change rapidly with temperature and pressure. It offers a few advantages such as rapid hydrolysis and high solubility for polar components. These materials, such as sugars oligomers, have low solubility for water at room temperature. The hydrothermal method is used for these materials.12This work in this paper aims to remove soluble oligomers in water at hydrothermal conditions from PET. Therefore, in this study, hydrothermal extraction was conducted in a semibatch apparatus at varying temperatures and different extraction times to explore the optimum operating conditions for the removal of oligomers from PET. Received: November 27, 2012 Revised: May 11, 2013 Accepted: May 17, 2013

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dx.doi.org/10.1021/ie303265j | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL SECTION 2.1. Materials. An amorphous PET resin in pellet form with a molecular weight of 60 kDa was prepared by polycondensation of terephthalic acid and ethylene glycol using a conventional method as follows. When the esterification reaction vessel was heated to reach 200 °C, a slurry comprising 86.4 parts by mass of terephthalic acid and 64.4 parts by mass of ethylene glycol was charged, and while stirring, 0.017 parts by mass of antimony trioxide as the catalyst and 0.16 parts by mass of triethylamine were added. Next, heating was performed, and a pressurized esterification reaction was carried out under the condition of 0.34 MPa gauge pressure and 240 °C. The esterification reaction product obtained was transferred to a polymerization-condensation reaction vessel, and the temperature was gradually raised from 240 to 280 °C under reduced pressure. After completion of the polymerizationcondensation reaction, PET resin was crystallized by heating at 130 °C for 3 h. A PET resin with a higher molecular weight of 100 kDa and with 49% crystallinity was then prepared by solidstate polymerization of the crystallized PET. Two types of oligomers were present in the PET resins: “linear” oligomers with a straight-chain structure and “cyclic” oligomers with a ring-shaped structure. These structures were determined by HPLC analysis. The HPLC method is described in section 2.3. The different chemical structures of the linear and cyclic oligomers are shown in Figure 1. The copolymer of ethylene, diethylene glycol, and terephthalic acid is classified as a linear oligomer.

Figure 2. Schematic diagram of semibatch extraction system used in this study.

terminated. The extracted solution was collected in the extraction cell. 2.3. Analysis. The concentration of oligomers included in PET was determined by HPLC analysis. The HPLC method was described as follows. The precipitation method was used as preprocessing.13,14 The PET samples (approximately 100 mg) were precisely weighed and then dissolved in a hexafluoroisopropyl alcohol (HFIP)/chloroform mixture (3 mL, 2/3 v/ v, volume ratio). Chloroform (20 mL) was then added to the solution, and PET was precipitated with methanol (10 mL). The precipitated PET residue was filtered and dissolved in N,Ndimethylformamide (DMF, 10 mL). Thereafter, this solution was filtered by centrifugation and analyzed by HPLC (Hitachi Ltd., L-7000). The standard curve method was applied to absolute quantification. The standards of terephthalic acid, bishydroxyethylterephthalate (BHET), and cyclic trimer were used for the quantification of these compounds in oligomer solution. BHET was also used as standard for the quantification of the other linear oligomers. In the case of cyclic oligomers, cyclic trimer was used as standard. The typical analytical conditions for the HPLC analysis were as follows. Column: 1μ-Bondasper C18, 5 μm, 100 Ǻ , 3.9 × 150 mm (Waters Corporation); eluent A: 0.2% acetic acid/water (v/v); eluent B: acetonitrile; gradient B %: 10→100% (0−55 min); linear flow rate: 0.8 mL/min; detection wavelength (ultraviolet (UV)): 258 nm. The average molecular weight of the post-treated PET samples was estimated by a gel permeation chromatograph (GPC) equipped with a UV−Vis detector with a calibration curve derived from polystyrene standards of a known molecular weight. The PET samples were dissolved in a HFIP/chloroform mixture (2/3 v/v, volume ratio). Chloroform was then added to the solution. The post-treated PET samples (0.05% in mixed solvent) were obtained. A HFIP/chloroform mixture (2/98 v/ v, volume ratio) was used as the mobile phase with a flow rate of 0.6 mL/min at a column temperature of 40 °C. The carboxyl end groups were measured by neutralization titration as follows. The samples were dissolved in boiling benzyl alcohol, diluted with chloroform, and titrated with sodium hydroxide. Benzyl alcohol treated in the same manner was also titrated as a blank. The degree of crystallinity of PET samples was determined by densimetry. The density of PET samples was measured using a density gradient column prepared from calcium nitrate and distilled water at 30 °C.

Figure 1. Structural formulas of PET oligomers.

2.2. Experimental Apparatus and Procedure. Figure 2 shows a schematic diagram of the extraction system used for the semibatch hydrothermal treatment. The system consists of a high-pressure liquid-chromatography (HPLC) pump (JASCO, Intelligent HPLC pump PU-2080 Plus), a stainless steel extraction cell with an internal volume of 25 mL, a preheating coil, a back-pressure regulator, and an extraction oven. The extraction cell was completely filled with the highmolecular-weight PET resin (Mw: 100 kDa). Deoxygenated water that was previously degassed for 30 min in an ultrasonic bath was supplied to the system at a constant flow rate of 2 mL/min using the HPLC pump. The water was heated to a desired temperature (150, 175, or 200 °C) via the preheating coil inside the extraction oven and then loaded into the extraction cell. The extraction pressure was controlled at 5 MPa by adjusting the back-pressure regulator. After 5−80 min, the supply of water was discontinued, and the extraction was B



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3. RESULTS AND DISCUSSION 3.1. Effect of Temperature and Time. This study primarily aimed to determine the potential of hydrothermal treatment as a practical method for removing oligomers from PET. Hence, the extractions were performed for 40 min at 150, 175, and 200 °C. The experimental results are listed in Table 1. Table 1. Effect of Hydrothermal Extraction Temperature after 40 min for PET Residues reaction temperature (°C)

Mw (-)

total amount of oligomers (ppm)

control 150 175 200

100,000 88,200 72,500 32,500

5,400 5,200 4,800 6,200

Figure 4. Time course of concentration of carboxyl groups Δ[COOH] at various temperatures.

case of kinetics as carboxyl groups is the report of Zimmerman.15 The reaction rate was expressed as shown in eq 1.16 1 kt = 10380(1 − Xc) + C ini ⎛ 10380(1 − Xc)(C ini + Δ[COOH]) ⎞ × ln⎜ ⎟ ⎝ C ini{10380(1 − Xc) − Δ[COOH]} ⎠ (1)

The weight-average molecular weight values ranged from 32,500 to 88,200 and decreased by hydrothermal extraction. These results indicate that the weight-average molecular weight decreased owing to PET hydrolysis, and hydrolytic degradation was promoted at higher temperatures. Table 1 also lists the total concentration of oligomers included in the PET residues after hydrothermal extraction for 40 min at 150 and 175 °C. The total concentration of oligomers can be decreased by hydrothermal treatment at these specific temperatures. The results at 200 °C were in contrast to our presumption. Linear oligomers other than the cyclic oligomers and monomers are defined as linear impurities in this study. The linear impurities markedly increased at 200 °C owing to extreme hydrolysis of PET. 3.2. Determination of the Maximum Reaction Time. An additional experiment for reaction time was performed to determine the optimum operating conditions. The average molecular weight of the PET residues estimated by GPC analysis is shown in Figure 3. These results indicate that

Cini are initial carboxyl end groups [eq/106 g], Δ[COOH] represents increased number of carboxyl end groups, Xc is the crystallinity [%], and t is the elapsed time [h]. The increase in the carboxyl end group concentration is presumably negligible as compared to the total ester bond concentration [10,380 eq/106 g] in this study. Therefore, eq 1 can be modified to eq 2. kt =

⎛ 1 Δ[COOH] ⎞ ln⎜1 + ⎟ 10380(1 − Xc) ⎝ C ini ⎠

(2)

The calculated relationship between the concentration of carboxyl end groups and the reaction time determined according to eq 2 is indicated in Figure 4 by the solid line. From this figure, it can be seen that the calculated values for the increase in the concentration of the carboxyl end groups generally correspond well with the measured results. The relationship between the carboxyl end group concentration and the weight-average molecular weight in our experimental results is shown in Figure 5. The weight-average molecular weight can be approximated by modeling the PET hydrolysis in accordance with the following expression (eq 3): M w = −2207 × [COOH] + 98320

(3)

Figure 3. Relationship between the weight average molecular weight of PET residue and extraction time at 150, 175, and 200 °C.

degradation was promoted at higher temperatures. Similar results were also found with the number molecular weight. The results of crystallinity of the PET residue ranged from 45% to 49%. Figure 4 presents the data for the carboxyl end group concentration as a function of the reaction time at 150, 175, and 200 °C. It is evident that an increase in the time and temperature caused an increase in the carboxyl end group concentration. In addition, as their concentration increased, the reaction velocity of the hydrolysis increased. Therefore, the carboxyl groups catalyzed the PET hydrolysis. An important

Figure 5. Plot of the weight average molecular weight of PET residue vs concentration of carboxyl groups Δ[COOH]. C



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The minimum value for the weight-average molecular weight for film processing is assumed to be 60 kDa in our calculation. Using eq 3 and the experimental data, the minimum carboxyl end group concentration for practical use was determined to be 17.36 eq/ton. The maximum reaction time was calculated by substituting the minimum carboxyl end group value into eq 2. The results of this calculation are shown in Table 2. However, it is difficult to extend the reaction time at higher temperatures because the PET hydrolysis leads to a decrease in the molecular weight. Table 2. Determination of the Maximum Reaction Time reaction temperature (°C)

k (g/eq/h)

maximum time (h)

150 175 200

81.3 179 438

1.25 0.60 0.25

3-3. HPLC Analysis of the PET Residue. The results of the total concentration of oligomers included in the PET residues as a function of the reaction time at 150, 175, and 200 °C are presented in Figure 6.

Figure 7. Time course of total concentration of (a) linear oligomers and (b) cyclic oligomers in PET residue after hydrothermal extraction at different temperatures.

4. CONCLUSION The results obtained in this study suggest that hydrothermal treatment enables partial extraction of oligomers from the PET resin, and 16% of maximum extraction efficiency was achieved by hydrothermal extraction at 200 °C for 10 minutes. The decrease of weight-average molecular weight and the increase of linear oligomers were caused by the hydrolysis of PET. The degradation was affected by time and temperature. The extraction behavior is strongly related to the chemical structure of the PET oligomers and their solubility in high-temperature water. In conclusion, the total concentration of oligomers reaches a minimum owing to the hydrothermal treatment when a balance between the PET hydrolysis and extraction of the oligomers is achieved. This novel technique is a promising tool for the development of a next-generation, low-oligomer, ecofriendly method in a short amount of time.

Figure 6. Time course of total concentration of oligomers in PET residue after hydrothermal extraction at different temperatures.

With a short processing time, from the figure, it can be seen that hydrothermal extraction effectively removes the oligomers from PET. However, after a certain time, the concentration of oligomers increased with processing time. For example, it shows a drastic increase at 200 °C. The total concentration of oligomers after hydrothermal extraction for 40 min was greater than that in the initial PET sample. The results of the different types of oligomers can be seen in Figure 7. The upper image shows the data for the linear oligomers with a straight-chain structure, and the lower image shows the data for the cyclic oligomers with a ring-shaped structure. The concentration of cyclic oligomers decreased, while the concentration of linear oligomers increased after hydrothermal extraction. The linear oligomers presumably increased owing to the hydrolysis of PET, as described above. These results suggest that the total concentration of oligomers reaches a minimum owing to the hydrothermal treatment when a balance between the PET hydrolysis and extraction of the oligomers is achieved. However, the minimum point was distinct at 150 and 175 °C. The reactions at these temperatures follow the same tendency as the reaction at 200 °C, but the minimum point may shift to a longer reaction time because the rates of PET hydrolysis at 150 and 175 °C are lower than that at 200 °C.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +81-96-3423666. Fax: +81-96-342-3665. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Kumamoto University Global COE program “Initiative Center for Pulsed Power Engineering”. D



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ABBREVIATIONS PET = poly(ethylene terephthalate) REFERENCES

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Removal of Oligomers from Poly(ethylene terephthalate) Resins by

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