Ind. Eng. Chem. Res. 2007, 46, 7891-7895
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Hygroscopic Poly(ethylene terephthalate) by Nonaqueous Alkaline Glycolysis Hyung-Min Choi Department of Organic Materials and Fiber Engineering, Soongsil UniVersity, Seoul, Republic of Korea 156-743
Controlled glycolysis of poly(ethylene terephthalate) (PET) was carried out under nonaqueous alkali conditions to obtain an enhanced hygroscopicity. Effectiveness of an alkali catalyst was directly related to its alkalinity. Treatment conditions such as alkali concentration, temperature, and time were varied, and their effects were studied by evaluation of wetting time, contact angle, weight loss, strength and elongation, and dye sorption. The glycolyzed PET under these alkali conditions showed instant wetting in most cases at 80 °C or above but with some loss of strength. However, a careful selection of treatment conditions can minimize strength loss while maintaining exceptional hygroscopicity. Effect of glycolysis on conformational changes on PET was investigated by FTIR, DSC, and SEM-EDS analyses. 1. Introduction Poly(ethylene terephthalate) (PET) has been utilized as fibers and for other purposes in nonfibrous end uses. PET especially enjoys wide applications in the apparel area due to its high strength and dimensional stability. However, it has an inherent drawback; i.e., it has a very low hygroscopicity, resulting in low comfort and high propensity toward pilling and electrostatic charge.1-4 Several techniques have been examined to enhance the hygroscopicity of PET fibers. These methods include hydrolysis by strong acid or alkali,2,3 amine treatment,4 and treatment by various radiations (plasma).5,6 Among them, hydrolysis with alkali has been commercially applied, but it still gives some problems such as generation of undesired ionic groups. In addition, amine treatment of PET produces a yellowing effect and considerable strength loss4 and radiation treatment requires substantial initial capital investment to modify the fabric.5,6 Therefore, a new, better process has yet to be developed to make PET more hygroscopic. Decomposition of PET with nonaqueous glycolysis has been exclusively examined as a method for recycling of PET bottles to the formation of its monomers, terephthalic acid and diols.7-11 PET waste was depolymerized by ethylene glycol in the presence of different catalysts at high temperature (g150 °C) under atmospheric pressure7 or under elevated pressure.9 Conventional catalysts for PET synthesis have been mainly used, and zinc acetate is known to be the most effective catalyst among them.1,9 More recent research, however, revealed that alkali catalysts, such as sodium hydroxide, sodium carbonate, and sodium bicarbonate, were equally effective in the decomposition of PET with glycolysis.1,7 These alkali catalysts are preferred over heavy metal catalysts because of environmental concerns and economic reasons. Considering the use of glycolysis on recycling of PET, one might expect that controlled decomposition of PET at low temperature ( SBC, as shown in Figure 1. Retention of elongation also followed the same trend (not shown). Glycolysis with alkali catalyst considerably improved the hygroscopicity of the treated PET as indicated by instant wicking in most cases (Table 1). Hygroscopicity of PET fabrics glycolyzed with SBC was less than than those of the fabrics treated with SH and SC, but wicking time was still less than 10 s, indicating a substantial improvement in its hygroscopicity while maintaining strength retention equal to or greater than 90%. Therefore, an appropriate alkali catalyst could be selected depending on the end-use application based on required hygroscopicity and strength retention. It is interesting to note that, in the absence of alkali, the glycolysis at 80 °C showed no weight loss and did not enhance the hygroscopicity of the treated PET. The wicking time was rather longer than that of the untreated PET. This was probably due to an increase in the surface crystallinity of PET due to chemicrystallization occurring during glycolysis.13,14 This effect, however, disappeared on glycolysis at 100 °C; i.e., the fabric became more hygroscopic. It has been reported that density changes during the degradation process13 were attributed to a so-called “chemicrystallization” process. The idea was that chain scission only occurred in the amorphous regions.13 In the absence of alkali, chemicrystallization tended to occur, especially at 80 °C treatment. Glycolysis could induce two opposite effects on the crystallinity of PET: decrease due to hydrolysis by alkali or chain scission, and increase by chemicrystallization. Therefore, a resulting crystallinity of the glycolyzed PET will be dependent on temperature, catalyst concentration, and other glycolysis conditions. 3.2. Contact Angle Measurement. Increase of the hygroscopicity of the glycolyzed PET fabrics was also observed during contact angle measurement. Contact angles of water drops of untreated control and glycolyzed PET under no alkali are 126.7° and 127.2°, respectively. These two samples clearly showed spherical water drops during the measurement. On the other hand, contact angles for the PET fabrics glycolyzed with SH were not able to be measured because of too fast wetting. The same phenomena were shown in most cases for the PET fabrics glycolyzed with the other two catalysts (not shown). 3.3. Methylene Blue Analysis. Methylene Blue (MB), a cationic dye, was employed to investigate the presence of -COOH groups on glycolyzed PET. Glycolysis of PET is an equilibrium reaction, with the reverse reaction being the wellknown polycondensation as shown below:10
-COC6H4COOC2H4O- + HOC2H4OH T -COC6H4COOC2H4OH + HOC2H4O- (4) Theoretically, no COOH group is expected to occur during glycolysis. However, in reality, color strength (K/S) values of the glycolyzed, MB-stained PET were greater than that of untreated control (Table 1). This suggested the presence of some COOH groups within the glycolyzed PET. Without alkaline catalyst, the K/S value was higher than that of the untreated, but was still lower than other K/S values of PET with SH. These results suggested the formation of a small amount of carboxylic group during the glycolysis. A simultaneous hydrolysis reaction occurring by residual water in the presence of alkali was a probable cause of carboxylic groups since a complete elimina-
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Figure 6. SEM micrographs of PET fabrics. Table 2. Effect of Catalyst Concentration on Peak Height of PET Fabric Treated by Glycolysis at 80 °C for 2 h height of FTIR absorption peaks (×10-2 cm-1) catalyst untreated no catalyst SH
SC
catalyst concn (%)
3431
2967
1713
1504
1340
1245
1096
1017
973
872
847
795
A973/A795 ratio
0.0 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0
0.041 0.102 0.084 0.096 0.063 0.104 0.103 0.105 0.102 0.109
0.048 0.294 0.336 0.290 0.278 0.307 0.343 0.347 0.349 0.344
1.685 11.52 10.81 9.544 9.330 11.22 12.61 12.00 11.64 11.93
0.033 0.770 0.935 0.614 0.610 0.752 0.771 0.764 0.739 0.767
0.186 2.334 2.382 1.977 1.919 2.314 2.456 2.424 2.317 2.487
1.169 10.63 9.428 8.456 8.427 10.04 11.21 10.74 10.38 10.82
0.712 7.863 6.985 6.305 6.282 7.408 8.331 7.943 7.874 8.014
0.219 3.545 3.651 2.880 2.876 3.459 3.628 3.577 3.558 3.831
0.048 0.910 0.848 0.751 0.728 0.941 0.938 0.930 0.905 0.980
0.059 2.645 1.933 2.166 2.077 2.556 2.930 2.647 2.721 2.581
0.028 0.621 0.671 0.495 0.473 0.570 0.613 0.578 0.589 0.661
0.012 0.697 0.737 0.655 0.578 0.749 0.685 0.691 0.684 0.713
4.02 1.31 1.15 1.15 1.26 1.26 1.37 1.34 1.32 1.37
Table 3. Weight Percent Carbon and Oxygen in PET Glycolyzed in the Presence of 0.5% SH glycolysis conditions untreated control EG 100%/2 h
temp (°C)
wt % C
wt % O
80 100 120 140
69.29 69.63 68.84 69.23 68.24
30.71 30.37 31.16 30.77 31.76
tion of water from the EG system was difficult. Therefore, K/S values with strong alkali were greater than those with less strong alkali such as SB and SBC. Nevertheless, the level of free COOH groups was rather small and it was not detected by FTIR spectroscopy. Undesired hydrolysis reaction could be completely eliminated by employing a distilled EG since the boiling point of EG (197.3 °C) is much greater than that of water. Low level of ionic group after the glycolysis treatment was an additional advantage of this process since other hygroscopic treatments such as amine and alkaline hydrolysis reactions produced considerable amounts of ionic groups that could alter the dyeing characteristics of regular PET. This fact was further confirmed by the small difference in ∆E values (all the samples were less than 1) between the glycolyzed PET and untreated control dyed with C.I. Disperse Blue 60. 3.4. Effect of Treatment Temperature. PET fabrics were exposed under controlled decomposition by EG in the presence of 0.5% SH and SC for 1 and 2 h at appropriate temperatures. As shown in Figure 2, losses in weight and strength were generally similar for the two catalyst systems, but differ in the level of losses. Again both losses were more severe with SH, regardless of the treatment time, due to the higher alkalinity of SH. For glycolysis at 80 °C, retention of the peak load of the treated PET decreased to around 70%, with the exception of SC for 1 h, while additional weight loss was quite small compared to that at 60 °C. This tended to reveal that significant
glycolysis actually started around 80 °C by transesterification of EG, which was indicated by a substantial loss of strength. Both losses in weight and strength generally increased with increase in the treatment temperature, but with SH they leveled off at 120 °C. In most cases, the glycolyzed PET fabrics showed instant wetting regardless of the catalyst with exceptions at some low temperatures. The wetting times of water drops on the glycolyzed PET fabrics were 40 s for the SH catalysis and 240 s for the SC catalysis at 60 °C/1 h, 12 s for the SH catalysis and 120 s for the SC catalysis at 60 °C/2 h, and 4 s for the SH catalysis and 12 s for the SC catalysis at 80 °C/1 h. The treatment at 80 °C and 2 h with both SH and SC showed instant wetting in both cases. Nevertheless, even at 60 °C glycolysis the hygroscopicity of the treated PET was enhanced significantly, resulting in much faster wetting compared to that of untreated control fabric. Therefore, a careful selection of glycolysis conditions such as type of catalyst and treatment time and temperature could tailor the hygroscopicity and strength of PET suitable for appropriate end-use application. 3.5. FTIR Analysis. As shown in Table 2, glycolysis treatment substantially increased heights of the FTIR absorption peaks of the PET fabrics regardless of treatment conditions. Increase in height was generally greater with the absorption peaks for phenyl hydrogen vibrations such as 1504, 1017, and 872 cm-1 than the peaks for carbonyl (1713 cm-1) and hydroxyl groups (3431 cm-1). The ratio of the peaks at 973 and 794 cm-1 has been used to determine the effect of drawing on the crystallinity.15,16 The band at 973 cm-1 was due to the trans isomer of EG linkage, while the band at 795 cm-1 was the internal thickness band for normalizing the thickness of different substrates.15,16 The results indicated that this ratio was much lower in the glycolyzed PET than in the untreated control. The same effect was also observed at different glycolysis temperatures (not shown). The decrease in the ratio was probably due to combination of the
Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7895
crystallinity changes and disruption of chain extension in the amorphous regions with decreasing orientation.13,14 Transformation of the trans configuration of EG to the gauche configuration because of glycolysis could also be considered. In addition, the ratios were always lower in the PET fabrics glycolyzed with SH than those with SC at the same concentration of catalyst, substantiating the severe effect of SH. These morphological changes therefore occurred in PET by glycolysis contributed strength loss of the fabrics. 3.6. Thermal Analyses. DSC analysis was carried out to explore the effect of glycolysis on the thermal characteristics of PET. The melting endotherm peaks of PET were relatively sharp and were placed between 254 and 258 °C (not shown). There was no appropriate difference in Tm1 and Tm-max of PET treated with glycolysis at various temperatures as shown in Figure 3. On the other hand, in the second heating both Tm2 and Tm-max2 changes were small, except for the PET glycolyzed at 120 °C. This suggested a significant conformational change of PET by glycolysis at high temperatures such as 120 °C. Based on the heat of fusion of single-crystal PET (∆Hf ) 122 J/g), the percent crystallinity of EG-treated PET was calculated and is illustrated in Figure 5. The percent crystallinity of untreated PET control was 52.7%. Unlike Tm and Tm-max of the melting endotherms, a three-step trend was shown in the percent crystallinity of both the first and second heating curves of PETs treated with EG at different temperatures: decrease of percent crystallinity with EG treatment at 60 °C, then increase to a certain point at intermediate temperature (80 or 100 °C), and then decrease at higher treatment temperature. This trend was quite similar to the previous percent crystallinity data measures by the density of PET treated with amine such as 20% methylamine.17 The first stage was due to an attack in the amorphous regions with little change in the sample weight and crystallinity, and in the second stage, chain scission led to a rapid fall in sample weight and a rise in degree of crystallinity.2,17 The third stage, which showed a leveling off or slight decrease in crystallinity, was due to a gradual decrease in the reaction rate attributed to a slower attack on both amorphous and crystalline regions.2,17 3.7. SEM Analysis. Energy dispersive spectroscopy (EDS) of SEM indicated that difference in weight percent of C and O shown in Table 3 was not substantial as expected since transesterification practically involved interchange of the same molecules. SEM micrographs showed that topological features of PET fibers did not change much during regular glycolysis treatment even at high temperatures such as 120 and 140 °C. 4. Conclusions Controlled glycolysis of PET was carried out under nonaqueous alkali conditions to obtain an enhanced hygroscopicity. This treatment considerably enhanced the hygroscopicity of the treated PET as indicated by instant wicking and contact angle measurement. The effectiveness of three alkali catalysts was directly related to their alkali strength, i.e., SH > SC > SBC. Both losses of weight and strength increased at higher treatment temperatures, but with SH they leveled off at 120 °C. At 2 h of treatment, the treated PET showed instantaneous wetting in most cases. The morphological changes that substantiated strength loss of the fabrics were confirmed by FTIR analyses, and the effect was greater with SH catalyst. DSC analysis indicated that melting peaks of the glycolyzed PET were relatively consistent
except at high temperature (120 °C), which showed a significant conformational change. Percent crystallinity of the glycolyzed PET revealed a three-step trend, and this behavior was quite similar to the previous experimental results. SEM micrographs showed that topographical features of PET fibers did not change much during regular glycolysis even at high temperatures such as 120 and 140 °C. Therefore, the glycolysis treatment of PET fabric at 80 °C for 1 h could be an optimal condition to obtain the balance between high hygroscopicity and desired physical properties. Acknowledgment The work was supported by the Soongsil University Research Fund. Literature Cited (1) Shukla, S. R.; Kulkarni, K. S. Depolymerization of Poly(ethylene terephthalate) Waste. J. Applied Polym. Sci. 2002, 85, 1765. (2) Ellison, M. S; Fisher, L. D.; Alger, K. W.; Zeronian, S. H. Physical Properties of Polyester Fiber Degraded by Aminolysis and by Alkaline Hydrolysis. J. Appl. Polym. Sci. 1982, 27, 247. (3) Chen, W.; McCarthy, T. J. Chemical Modification of Poly(ethylene terephthalate). Macromolecules 1998, 31, 3548. (4) Fukatsu, K. Mechanical Properties of Poly(ethylene terephthalate) Fibers Imparted Hydrophilicity with Aminolysis. J. Appl. Polym. Sci. 1992, 45, 2037. (5) Liu, C.; Zhu, Z.; Jin, Y.; Sun, Y.; Hou, M.; Wang, Z.; Chen, X.; Zhang, C.; Liu, J.; Li, B.; Wang, Y. Chemical Modifications in Polyethylene Terephthalate Films Induced by 35 MeV/u Ar ions. Nucl. Instrum. Methods, B 2000, 166-167, 641. (6) Liu, C.; Zhu, Z.; Jin, Y.; Sun, Y.; Hou, M.; Wang, Z.; Wang, Y.; Zhang, C.; Chen, X.; Liu, J.; Li, B. Study of Effects in Polyethylene Terephthalate Films Induced by High Energy Ar Ion Irradiation. Nucl. Instrum. Methods, B 2000, 169, 78. (7) Oku, A.; Hu, L. C.; Yamada, E. Alkali Decomposition of Poly(ethylene terephthalate) with Sodium Hydroxide in Nonaqueous Ethylene Glycol. J. Appl. Polym. Sci. 1997, 63, 595. (8) Guclu, G.; Kasgoz, A.; Ozbudak, S.; Ozgumus, S.; Orbay, M. Glycolysis of Poly(ethylene terephthalate) Wastes in Xylene. J. Appl. Polym. Sci. 1998, 69, 2311. (9) Chen, J. Y.; Ou, C. H.; 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. Polym. Sci. 1994, 54, 1731. (11) Kao, C. Y.; Cheng, W. H.; Wan, B. Z. Investigation of Catalytic Glycolysis of Polyethylene Terephthalate by Differential Scanning Calorimetry. Thermochim. Acta 1997, 292, 95. (12) Harnett, P. R.; Mehta, P. N. A Survery of Wicking. Text. Res. J. 1984, 54, 471. (13) Sammon, C.; Yarwood, J.; Everall, N. An FT-IR Study of the Effect of Hydrolytic Degradation on the Structure of Thin PET Films. Polym. Degrad. Stab. 2000, 67, 149. (14) Sammon, C.; Yarwood, J.; Everall, N. An FTIR-ATR Study of Liquid Diffusion Processes in PET films: Comparison of Water with Simple Alcohols. Polymer 2000, 41, 2521. (15) Mehta, R. E.; Bell, J. P. Chain-folding Measurements in Annealed Poly(ethylene terephthalate). J. Polym. Sci.: Polym. Phys. Ed. 1973, 11, 1773. (16) Mocherla, K. K.; Bell, J. P. Morphology of Uniaxially Oriented Poly(ethylene terephthalate). J. Polym. Sci.: Polym. Phys. Ed. 1973, 11, 1779. (17) Farrow, G.; Ravens, D. A. S.; Ward, L. M. The Degradation of Polyethylene Terephthalate by Methylamine-A Study by Infra-red and X-ray Methods. Polymer 1962, 3, 17.
ReceiVed for reView April 26, 2007 ReVised manuscript receiVed September 16, 2007 Accepted September 18, 2007 IE0705960
