Synthesis and Characterization of Poly(ethylene terephthalate) from

May 19, 2015 - Novel biobased copolyesters based on 1,2-propanediol or 2,3-butanediol with the same ethylene skeletal structure as PETG. Ji-Hyun Kim ,...
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Synthesis and Characterization of Poly(ethylene terephthalate) from Biomass-Based Ethylene Glycol: Effects of Miscellaneous Diols Bin Xiao, Mingyuan Zheng,* Jifeng Pang, Yu Jiang, Hua Wang, Ruiyan Sun, Aiqin Wang, Xiaodong Wang, and Tao Zhang* State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023 Liaoning, China ABSTRACT: Using biomass-derived ethylene glycol (bio-EG) to synthesize poly(ethylene terephthalate) (PET) is of notable significance for alleviating the dependence on fossil energy resources. Bio-EG readily contains a small amount of miscellaneous diols, which derive from the side reactions in the catalytic conversion of biomass. To disclose the effects of miscellaneous diols on the synthesis and properties of PET, EG feedstock containing four 1,2-diols, i.e., 1,2-propylene glycol, 1,2-butanediol, 1,2pentanediol, and 1,2-hexanediol at 0−10% concentrations was used for the synthesis of PET. The molecular weights, intrinsic viscosities, and thermal and mechanical properties of obtained PET materials were measured. It was found that when the overall content of miscellaneous diols in EG was lower than 5%, the molecular weights and thermal properties of the prepared PET materials were very similar to that of PET synthesized from pure EG. The miscellaneous diols were less likely to be incorporated into PET resin because of the steric hindrance of the alkyl group in diols to the esterification and polycondensation reactions. Instead, they preferred to undergo dehydration reactions to form low-boiling-point aldehydes and hemiacetals, which could be removed from the reaction system during the reactions. Three bio-EG samples at purities of 99.9, 98.5, and 95.8 wt % were used for the bio-PET synthesis. Transparent and colorless bio-PET samples were obtained, demonstrating that the presence of miscellaneous diols does not have negative effects on the color quality of PET. The physical properties of bio-PET prepared with bio-EG at a purity of higher than 98 wt % were nearly the same as those of PET derived from pure EG. At a lower bio-EG purity of 95.8 wt %, the tensile strength of the obtained bio-PET sample was slightly decreased. The comprehensive results of property characterization show that bio-PET materials prepared with bio-EG at purity higher than 95 wt % could be used as widely as the conventional petro-PET resin without notable deterioration in their performance.

1. INTRODUCTION Because of the depletion of nonrenewable fossil energy resources and increasing concern about environmental issues, biomass utilization has been regarded as one of important ways to meet these challenges and realize the world development in sustainable ways.1−3 Through chemical or biological transformation of the sugar-based biomass, people can obtain not only liquid fuels but also a variety of oxygen-containing or hydroxyl-rich chemicals in particular for the synthesis of bulk quantities of polymers.4−7 For these purposes, biomass was converted to 2,5-furandicarboxylic acid for the synthesis of poly(ethylene 2,5-furandicarboxylate),6,8 succinic acid and 1,4butanediol for the synthesis of poly(butylene succinate),9 isosorbide for synthesis of polyester,10−12 lactic acid for the synthesis of poly(lactic acid),13 and ethylene glycol for the synthesis of poly(ethylene terephthalate) (PET).14−16 PET is one of the most highly produced man-made polymers with a capacity estimated to reach ca. 100 million tons in 2016.17 It is widely used in the manufacture of fibers, packaging articles, and films, which notably improve the quality of daily life.18−20 PET is synthesized from the monomers of ethylene glycol (EG) and purified terephthalic acid (PTA), both of which are dominantly obtained from petroleum currently.21 Replacing the petroleum-based PET with renewable-biomassderived monomers has attracted worldwide increasing attention. Synthesizing EG from biomass could possess a very high atom economy in terms of most of the hydroxyl groups in © XXXX American Chemical Society

the biomass being preserved in the glycol product. Since 2008 and in the following years, a novel process for EG (direct conversion of lignocellulose to EG, DLEG) has been disclosed and developed.14−16 In the one-pot process with tungstic catalysts under hydrothermal conditions, the carbohydrates of cellulose, hemicellulose, and sugars were selectively transformed into EG with high yields of 60−76%.22−27 As for the other monomer, PTA, a number of researchers and companies have obtained its precursor, bio-p-xylene, by catalytic conversion of platform chemicals or raw biomass.28−33 Thus, all of these studies have presented an encouraging prospect for the total synthesis of green PET from renewable resources. On the other hand, it should be noted that, slightly different from the conventional petroleum-derived ethylene glycol (petro-EG), which has an ultrahigh purity, the biomass-derived ethylene glycol (bio-EG) product is prone to containing a small amount of other diols, such as 1,2-propene glycol (1,2-PG) and 1,2-butanediol (1,2-BDO), which are the byproducts in the catalytic conversion of biomass. Because these miscellaneous polyols have boiling points (less than 10 °C difference) very similar to that of EG, they are difficult and very energy consuming to be completely separated from the final EG Received: February 4, 2015 Revised: May 15, 2015 Accepted: May 19, 2015

A

DOI: 10.1021/acs.iecr.5b00487 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research product by a rectification process.34 For a new process, a rule of thumb is that only a significant benefit (in terms of potential market) will push its industrialization. To achieve successful commercialization of bio-EG production with good economic viability, it is highly necessary to learn the required purity of bio-EG for the synthesis of PET so that an optimum balance between the product purity and purification cost could be achieved. Thus, the effect of miscellaneous diols on the PET synthesis and the resulting resin properties need to be clarified. Up to now, however, little systematic study was reported on this aspect. PET polyester can be synthesized in two ways, i.e., direct esterification of PTA with EG and transesterification of dimethyl terephthalate with EG. The reaction conditions of these two methods are very similar in reaction temperatures and pressures.35,36 Since the late 1960s, direct esterification became the dominant route for the PET synthesis because of the better economic viability as a result of the development in PTA purification by recrystallization.36 Therefore, in the present study, the direct esterification method was investigated for the PET synthesis. The monomers of EG and PTA in the presence of four 1,2-diols at a variety of concentrations were used as feedstock. Besides 1,2-PG and 1,2-BDO, which are most readily contained in the bio-EG product, 1,2-pentanediol (1,2-PDO) and 1,2-hexanediol (1,2-HDO) were also investigated to probe the effect of the carbon chain length of the diols on the synthesis of PET. The properties of the obtained PET samples, including the molecular weights and thermal and mechanical properties, were characterized and compared. The correlation between the diol concentrations and polymer properties was discussed, and a suitable concentration range of miscellaneous diols in bio-EG was suggested for obtaining bio-PET materials with good properties similar to that of petroPET. Finally, homemade bio-EG samples were used as feedstocks for the synthesis of bio-PET, and the properties of bio-PET were compared to that of petro-PET. The results obtained in the present work would provide valuable references for the industrial production of bio-EG and bio-PET in the future.

As the amount of water collected reached 95% of the theoretical value, the esterification reaction was stopped, and the second step polycondensation was then conducted at 260 °C. For the synthesis of PET for color measurement, a polyester stabilizer (triphenyl phosphate, 0.08 wt % of PTA) was added to the reactants before polycondensation. During the polycondensation reaction, the unreacted EG was removed under a slight vacuum (50 Pa) for 45 min; then the vacuum was quickly increased, and the reactants were stirred with a paddle agitator (50 Hz) for 50 min. After the polycondensation reaction was completed, the as-synthesized PET resin was unloaded at 240 °C and quenched in cold water with a pressure of nitrogen. 1 H NMR (TFA-d, 20 °C): δ 11.50 (s, 1H, −COOH), 8.32 (m, 4H, phenyl), 5.00 (m, 2H, −CH2−). 13C NMR (TFA-d, 20 °C): δ 170.98 (−CO−), 163.93 (CF3COO−), 132.23−135.77 (phenyl), 117.64 (CF3−), 66.31 (−CH2−). 2.3. Characterization. The molecular weights and intrinsic viscosities (IVs) of the PET samples were analyzed with a gel permeation chromatograph (Viscotek TDAmax), which was equipped with three detectors (refractive index, light scattering, and viscometry detectors), a Viscotek column (modified porous styrene−divinylbenzene copolymer, 300 mm length × 7.8 mm inside diameter), and a guard column (40 mm length × 7.8 mm inside diameter) from Polymer Laboratories. Chloroform was used as an eluent with a flow rate of 1.0 mL/min at 35 °C. The columns were calibrated with narrow polydispersity polystyrene standards (Polymer Laboratories). The differential scanning calorimetry (DSC) data of the PET samples were obtained with a TA Q1000 analyzer using a metallic aluminum vessel. The samples were heated in an argon atmosphere to 280 °C at a rate of 10 °C/min. The glass transition temperature (Tg) was obtained on the basis of analysis of the DSC data with TA Universal Analysis software. The melt temperature (Tm) was determined by the peak maximum temperature of the melting transition in the heating run. X is the crystal degree of the polyester, which was obtained from the melting enthalpy according to the following equation:

Χ=

2. EXPERIMENTAL SECTION 2.1. Materials. Purified terephthalic acid (PTA; 99%) was purchased from Shantou Longrui Chemical Company. Petroleum-derived ethylene glycol (petro-EG; 99.9%) was obtained from Acros Organics. Bio-EG was obtained from biomass conversion, as reported in our previous work and purified by rectification.14,24,26 Lithium acetate, 1,1,1,2,tetrachlorethane, phenol, and germanium dioxide were obtained from Sinopharm Chemical Reagent Co. Ltd. Deuterated trifluoroacetic acid (TFA-d; 99%) was purchased from Cambridge Isotope Laboratories Inc. 1,2-Propylene glycol (1,2-PG; 99%) was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. 1,2-Butanediol (1,2-BDO; 98%) was purchased from TCI, China. 1,2-Pentanediol (1,2-PDO; 98%) was purchased from Aladdin Chemistry Co. Ltd. 1,2Hexanediol (1,2-HDO; 97%) was purchased from Alfa Aesar. 2.2. PET Synthesis. EG and PTA at a molar ratio of 1.3 were introduced in a 1 L stainless-steel batch reactor (Polytex Company), which was equipped with a paddle agitator. Germanium dioxide (0.05 wt % of PTA) was added to the mixture before the reaction. The first step was esterification at 240 °C for 3−4 h under a nitrogen atmosphere, and the water produced from the reaction was removed by in situ distillation.

ΔHm ΔHm∞

(1)

wherein ΔHm is the melting enthalpy of PET (J/g), ΔH∞ m is the melting enthalpy of 100% crystalline PET, and its value is 125.6 J/g.37 NMR data of PET samples were obtained from AVANCE III 500 MHz (Bruker) using TFA-d as the solvent. Gas chromatography−mass spectrometry (GC−MS) analysis of the effluent in the esterification reaction was performed on Varian 450 GC and 320-MS equipped with a Varian CPWAX58 (FFAP) CB capillary column. The electron ionization mode was set at 70 eV with a mass range of m/z 30−450. The compound identification was obtained according to the NIST Mass Spectral Library (software version 2.0). Flexural and tensile tests of the PET samples were conducted on a universal testing machine (Sans Company model CMT4000) at 25 °C and at a speed of 2 mm/min according to GB/T9341-2008 and GB/T1040.1-2006. The specimen for the flexural test was 4 mm in thickness, 80 mm in length, and 10 mm in width. The specimen for the tensile strength test was dog-bone-shaped, which was made in a Hakke mini injection molding machine. Its gauge section had a thickness of 4 mm, a length of 80 mm, and a width of 3.2 mm. The impact test was B

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Table 1. Results of DSC and GPC Analysis of PET Samples Synthesized from EG and PTA in the Presence of 1,2-PG at Different Contentsa entry

1,2-PG content/%

Tg/°C

Tc/°C

Tm/°C

X/%

Mn × 104

Mw × 104

Mw/Mn

IV/(dL/g)

1 2 3 4 5 6 7 8

0 0.1 0.2 0.5 1 2 5 10

73.4 73.0 73.5 73.4 73.5 73.6 72.3 71.8

133.4 131.2 133.8 132.3 135.0 135.4 132.2 151.9

243.8 243.3 243.7 243.9 243.3 242.4 240.3 234.7

24.7 23.0 23.3 24.2 24.3 24.7 21.5 17.9

3.57 4.21 4.16 3.35 3.47 3.56 3.46 2.62

4.64 5.36 5.30 4.53 4.62 4.44 4.70 3.45

1.29 1.27 1.28 1.35 1.33 1.25 1.36 1.31

0.62 0.66 0.68 0.59 0.62 0.59 0.68 0.47

The 1,2-PG content in EG is defined as the mole ratio of 1,2-PG to EG × 100%; Tg is the glass transition temperature, obtained on the basis of DSC data with TA Universal Analysis software; Tc is the cool-crystallization temperature, obtained from the DSC heating trace; Tm is the melting temperature; X is the crystal degree, obtained from the melting enthalpy in equaton 1; Mn is the number-average molecular weight, obtained with GPC; Mw is the weight-average molecular weight, obtained with GPC; Mw/Mn is the polydispersity of PET; IV is the intrinsic viscosity of PET, obtained with GPC. a

notably reduce the polymerization degree. However, at a higher content of 10% 1,2-PG, the molecule weight of PET was reduced remarkably. On the basis of the data listed in Table 1, it can be calculated that the molecule weight of PET was decreased by ca. 25% in this case. The melting temperature (Tm) of PET also qualitatively reflects the change of the molecular weights. A higher Tm value indicates a larger molecule. When the 1,2-PG content was lower than 5%, the Tm value of PET (240.3−243.9 °C) was very close to that of PET synthesized with pure EG (243.8 °C) and that (245.24 °C) reported in the literature.39 At 10% 1,2PG content, the Tm value remarkably decreased to 234.7 °C, suggesting that the molecular weight of PET decreased. This is consistent with the results of Mn, Mw, and IV discussed above. Thus, it can be concluded that when the EG feedstock contained 1,2-PG at a content of higher than 5%, the molecular weight of PET would decrease notably. This should be ascribed to the stereohindrance effect of the methyl group in 1,2-PG, which hinders the esterification reaction of the 2-OH group and the polycondensation reaction followed. Consequently, the length of the PET macromolecular chain was shortened. On the other hand, from the point of view of chemical reactivity, the secondary alcohol of 1,2-PG is also less active than the primary alcohol of EG for esterification and polycondensation. This would further result in the shortened length of PET molecules. In addition, it should be mentioned that the polydispersity coefficient (Mw/Mn) of the PET samples did not change with the 1,2-PG content in the range of 0−10%, suggesting that the degree of branching of PET was not affected by the addition of 1,2-PG to the EG feedstock. 3.2. Effects of C4−C6 Diols on the PET Properties. Because 1,2-BDO is another major miscellaneous diol that is likely to be present in the bio-EG product, we further investigated its effects on the PET synthesis. Moreover, in order to more clearly probe the effect of the carbon chain lengths of the miscellaneous diols on the polymer synthesis, 1,2-PDO and 1,2-HDO were also investigated. The results of DSC and GPC analysis of the as-synthesized PET samples are compared in Figures 1−3. As shown in Figure 1, regardless of the kind of miscellaneous diols present in EG, all of the Tm values of PET samples leveled off at ca. 243 °C when the contents of the three miscellaneous diols were below 2%. With the diol content increasing to 5%, the Tm values slightly decreased to 240 °C and then further dropped to 235 °C when the diol content increased to 10%.

conducted in a pendulum impact testing machine (Sans Company model ZBC1000), and the specimen size was the same as that for the flexural test. The color measurement of the PET samples was performed on a whiteness tester (Beijing Kangguang WSC-100).

3. RESULTS AND DISCUSSION 3.1. Effects of 1,2-PG on the PET Properties. For EG production from the catalytic conversion of cellulosic biomass, 1,2-PG is the main byproduct.14−16 Therefore, its effects on the PET resin properties were investigated first. The PET samples were prepared with PTA and EG containing 1,2-PG at different concentrations. Table 1 presents the results of DSC and gel permeation chromatography (GPC) analysis. First of all, it could be noticed that the glass transition temperatures (Tg) of the as-synthesized PET samples were well kept at 73 °C with the 1,2-PG presence in the EG feedstock. Even at 10% content of 1,2-PG (mole ratio of 1,2-PG to EG), Tg was merely decreased by 1.6 °C compared with the PET sample synthesized with pure EG. This suggests that 1,2-PG does not significantly affect the Tg value of PET, which is one of the most important properties for material application. As for the cool-crystallization temperatures of the PET samples, they remained in a range of 131.2−135.5 °C when the 1,2-PG content was lower than 5% in the EG feedstock. These values were very similar to that of PET obtained from pure EG (entry 1). However, further increasing the 1,2-PG content to 10% in the EG feedstock notably increased the Tc value of PET up to 151.9 °C. It is reported that a higher Tc value of a PET sample indicates that the resin is getting more difficult to crystallize.38 That is to say that the crystal degree of PET was decreased to some extent when 10% 1,2-PG was contained in the EG feedstock for polymerization. This is well consistent with the data of the crystal degree (X) listed in Table 1. This should be attributed to the asymmetry of the molecular structure of 1,2-PG, which disturbed the regularity of the linear macromolecule of PET and consequently decreased the crystallinity of the PET resin. The changes in the lengths of the PET macromolecular chains as a function of the 1,2-PG content in the EG feedstock were identified by analysis of the molecular weights (Mn or Mw) and intrinsic viscosities (IV) of the PET samples. As shown in Table 1, the Mn, Mw, and IV values were basically unchanged when the 1,2-PG content increased from 0 to 2% and even up to 5%, indicating that 1,2-PG at such concentrations did not C

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3). Although there are some fluctuations in the profiles in Figure 2, it can still be clearly seen that when the diol contents were lower than 5%, the molecular weights of PET were basically kept in a narrow range similar to that of PET prepared with pure EG. However, at 10% content of miscellaneous diols, the Mw value of PET dropped by ca. 20% and reached 3.7 × 104. This is also very similar to that observed in the case of 1,2PG as discussed above. The profiles of the intrinsic viscosity in Figure 3 show that the values of IV of PET notably decreased at the 10% content of miscellaneous diols, further demonstrating that the molecular weights of PET decreased. Figure 4 shows the effects of the chain length of diols on the crystallinity and number-averaged molecular weights of PET Figure 1. Effects of the diol content in EG on the melt temperature (Tm) of PET. The diol content is the molar ratio of diol to EG in the feedstock.

Figure 4. Effects of the carbon chain length of 1,2-diols on the number-average molecular weight (Mn) and crystallinity (X) of PET. PET was synthesized with EG containing 10% miscellaneous diols. Figure 2. Effects of the diol content in EG on the weight-average molecular weight. The diol content is the mole ratio of diol to EG in the feedstock.

samples that were synthesized with EG containing 10% miscellaneous diols. Although the carbon number of diols was doubly increased from C3 to C6, the changes in the crystallinity and molecular weight were negligible. This indicates that, as homologues, the 1,2-diols tested herein have similar inhibitive effects on esterification and polycondensation. Taken altogether, 1,2-BDO, 1,2-PDO, and 1,2-HDO showed effects very similar to that of 1,2-PG upon PET synthesis. The methyl, ethyl, propyl, and butyl groups in such diols hindered the esterification and polycondensation reactions in similar ways. 3.3. Four Diols Coexisting in EG for PET Synthesis. When different miscellaneous diols were concurrently contained in the EG feedstock, their reactivities in the esterification and polycondensation reactions could be compared. PET samples were synthesized with EG feedstock containing four diols of 1,2-PG, 1,2-BDO, 1,2-PDO, and 1,2-HDO at a mole ratio of 1:1:1:1. The mole ratios of the overall diols to EG varied in a range of 1−10%. As shown in Table 2, similar to 1,2PG, the four diols had negligible effects on the Tm and Tc values of PET when the total content of diols was below 2%. Only when the overall diol content was increased to 5% or higher did the molecular weight of the PET materials start to decrease notably. The synthesis of PET included two-step reactions, i.e., esterification and the polycondensation reactions followed. More detailed reactive behavior of each miscellaneous diol in the two-step reactions was studied by analyzing the effluents during the PET synthesis. From Table 3, one can notice that the ratios of diols/EG in the effluents of esterification and the subsequent polycondensation reaction are much lower than

Figure 3. Effects of the diol content in EG on the intrinsic velocity of PET. The diol content is the mole ratio of diol to EG in the feedstock.

These results were consistent with that observed in the presence of 1,2-PG. Moreover, the difference in the carbon chain length of miscellaneous C4−C6 diols did not generate discernible different effects on Tm. The decreased Tm value at 10% diol content indicates that the molecular weights of PET materials dropped with the presence of excess C4−C6 diols in the EG feedstock. The Mw and IV values quantitatively showed the changes in the molecular weights of the PET samples as a function of the miscellaneous diol content in the EG feedstock (Figures 2 and D

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Industrial & Engineering Chemistry Research Table 2. Results of DSC and GPC Analysis of PET Samples Synthesized from EG and PTA in the Presence of Four Diols at Different Contents entry

overall diol content/%a

Tc/°C

1 2 3 4 5

0 1 2 5 10

133.4 148.2 137.4 152.7 157.7

Tm/°C

Mn × 104

Mw × 104

Mw/ Mn

IV/ (dL/ g)

243.7 242.0 242.6 238.2 233.1

3.57 4.20 4.04 3.36 2.50

4.64 6.62 5.32 4.82 4.11

1.29 1.57 1.31 1.43 1.64

0.62 0.67 0.69 0.67 0.56

a

The overall diol content contained four diols of 1,2-PG, 1,2-BDO, 1,2-PDO, and 1,2-HDO at a mole ratio of 1:1:1:1; the overall mole ratios of four diols to EG are listed in the second column.

Figure 5. Spectra of GC−MS analysis of the effluent in the esterification reaction with feedstock containing EG and 1,2-PG at a mole ratio of 1:1.

Table 3. Results of GC Analysis of the Effluents in the Esterification and Polycondensation Reactions with the EG Feedstock Containing Four Diolsa

products of dehydration of 1,2-PG and the reactions between the resulting propyl aldehyde and EG and 1,2-PG. Evidently, intramolecular dehydration reactions of 1,2-PG took place, forming allyl alcohol and propyl aldehyde under the acidic reaction environment. It was reported that dehydration of 1,2PG followed two paths, i.e., a main route of pinacol rearrangement to form propyl aldehyde and a minor route of E1 elimination to form enols by the catalysis of acids.40,41 From the chemical point of view, the methyl group in 1,2-PG is a better electron donor than proton to stabilize the carbocation, which is the important intermediate in the pinacol rearrangement and E1 elimination reactions. Therefore, 1,2-PG has a higher activity for dehydration compared with EG. This should be the other main reason that only a small portion of 1,2-PG was incorporated into the final PET resin besides the steric hindrance of the alkyl groups to diol esterification and polycondensation. Other diols larger than 1,2-PG also possess alkyl groups and should have chemical properties similar to those of 1,2-PG, which readily undergo dehydration reaction to form enol, aldehyde, and hemiacetals. Moreover, the activity of dehydration would increase with the carbon length of the diols because of the enhanced electron donor capacity of the alkyl groups. This would lead to their lower contents in the final PET products. In addition, it should be mentioned that when the content of miscellaneous diols reached 10% and 50% in EG, the PET resin had difficulty in forming. This may be ascribed to two aspects. On the one hand, the EG concentration in the feedstock for the PET synthesis was significantly decreased. On the other hand, the stereohindrance effect of the methyl group in 1,2-PG hindered the esterification reaction of the 2-OH group and the polycondensation reaction. 3.4. Comparison of the Properties between Bio-PET and Petro-PET. On the basis of the study of the PET synthesis with petro-EG containing miscellaneous diols at different contents, we further tested the bio-EG feedstock for the bioPET synthesis. Three bio-EG feedstocks containing EG at concentrations of 95.8, 98.2, and 99.9 wt % were used, respectively. They were obtained from the catalytic conversion of biomass according to the work reported previously14,24,26 and the rectification followed. The detailed compositions of the bio-EG feedstocks are listed in Table 4. The thermal properties and molecular weights of synthesized bio-PET were compared with those of petro-PET in Table 5. Similar to the results of PET obtained from petro-EG

content/% entry

1,2-PG/EG

1,2-BDO/EG

1,2-PDO/EG

1,2-HDO/EG

A1 A2 A5 A10 B1 B2 B5 B10

0.04 0.24 0.39 0.88 0.07 0.15 0.31 0.71

0.03 0.20 0.34 0.74 0.05 0.14 0.28 0.62

0.03 0.26 0.29 0.64 0.05 0.16 0.24 0.55

0 0.06 0.13 0.28 0 0.08 0.20 0.46

a

A1−A10 represent the esterification process; B1−B10 represent the polycondensation process. The numbers beside A and B represent the mole ratio of overall diols to EG in the feedstock. Diols/EG represents the mole ratio of diols to EG in the effluents. The mole ratio among the four diols in the feedstock was 1:1:1:1.

those in the feedstock. This indicates that all of the miscellaneous diols tested in the present work are more reactive than EG to be converted. Moreover, the ratio of diols/ EG decreased with an increase of the carbon chain length of the diols. It seems that the larger diols had a higher reactive activity. Because the primary alcohol has a higher activity than the secondary alcohol in the esterification and polycondensation reactions, the higher reactive activity of 1,2-diols observed herein is possibly related to the intramolecular dehydration reaction, which was further manifested in the following experiment. To get more quantitative results, we further conducted a conditional experiment in which a mixture of EG and 1,2-PG at a mole ratio of 1:1 was used as the feedstock for the synthesis of PET. The obtained PET sample was characterized with 1H NMR to quantify the amount of 1,2-PG incorporated in the PET resin. The mole ratio of 1,2-PG to EG monomers in the PET sample was found to be as low as 0.17:1, demonstrating that most 1,2-PG did not participate in the polymer formation. However, as mentioned above, 1,2-PG was more reactive than EG during the PET synthesis process. The most plausible explanation might be that 1,2-PG was converted into lowboiling-point byproducts and removed from the reaction system during the two-step reactions. GC−MS analysis (Figure 5) of the effluent of the esterification reaction provided evidence for this supposition. It was found that there are remarkable amounts of allyl alcohol and hemiacetals contained in the effluent. They are the E

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Table 4. Compositions of Bio-EG and Petro-EG

Table 6. Mechanical Properties of PET Samples Prepared with Various EG Feedstocks

EG feedstock

1,2-PG/%

1,2-BDO/%

1,2-PDO/%

EG/%

petro-EG (99.9%) bio-EG (99.9%) bio-EG (98.2%) bio-EG (95.8%)

0 0 0.63 1.81

0 0 0.95 2.14

0 0 0.22 0.25

99.9 99.9 98.2 95.8

PET samples

tensile strength/ MPa

notched Izod impact strength/ MPa

flexural strength/ MPa

flexural modulus/ MPa

52.2

7.8

45.9

586

49.6

5.8

44.6

556

52.3

6.5

46.2

578

46.6

6.6

42.3

532

petro-PET (99.9%) bio-PET (99.9%) bio-PET (98.2%) bio-PET (95.8%)

containing the various miscellaneous diols discussed above, there were negligible differences in the thermal properties and molecule weights of bio-PET when the purity of bio-EG was higher than 98 wt %. Even for the bio-PET sample prepared with 95.8 wt % bio-EG, its glass transition temperature was just decreased by 2.6 °C. The mechanical properties of various PET samples, including the tensile strength, impact strength, and flexural strength, are shown in Table 6. For the bio-PET samples prepared with 99.9 and 98.2 wt % bio-EG, their mechanical properties were very close to those of petro-PET. Decreasing the bio-EG purity to 95.8 wt % led to a slight decrease in the tensile strength and flexural strength, but other properties were well retained within a range of experimental error. This means that PET was slightly changed to crisp when much miscellaneous diol was contained in the EG feedstock. Besides the thermal and mechanical properties, the color of bio-PET is also one of most concerned properties for resin applications, such as fiber and bottle manufacturing.42 Table 7 shows the color (CIELAB color space) of various PET samples. One can find that all of the bio-PET and petro-PET samples had very similar color values. They were transparent and colorless. The b* values of all of the PET samples were lower than 4.5, which was very close to that of a commercial highquality PET sample (b* value of −1 to +1). The a* values of the bio-PET samples were slightly lower, which might be related to some contamination of bio-EG during the rectification process. Considering that the bio-EG and PET samples were prepared on a small laboratory scale in this study, the color values of PET obtained herein have been rather good. If PET samples are prepared in a large-scale industrial production, the color quality could be further improved because of less exposure to contaminant and oxidation during the synthesis process.43,44 In conclusion, the presence of lower than 5 wt % miscellaneous diols in bio-EG did not show negative effects on the thermal and physical properties and color of the bioPET materials. Such bio-PET materials prepared with 95−98 wt % purity of bio-EG should be suitable for use as widely as the conventional petro-PET resin without deterioration in the performance.

Table 7. Color Values of PET Samples Prepared from Various EG Feedstocksa PET samples

L*

a*

b*

petro-PET (99.9%) bio-PET (99.9%) bio-PET (98.2%) bio-PET (95.8%)

69.90 64.32 63.10 65.90

0.66 −2.28 −7.05 −7.71

3.93 2.78 4.36 4.45

a

The value of L* is related to the transparency of PET between black and white (100); the value of a* is related to the color of red (100) and green (−100); the value of b* is related to the color of yellow (100) and blue (−100).

PET synthesis, a EG feedstock containing miscellaneous diols was tested at different concentrations. The diols involved 1,2PG, 1,2-BDO, 1,2-PDO, and 1,2-HDO, which are readily contained in the bio-EG product and very energy consuming to be separated. It was found that when the content of the diols was lower than 5%, the properties of the PET resin, including the molecular weight, intrinsic viscosity, and thermal properties, were very similar to those of the PET resin synthesized with pure EG. The diols are less likely to be incorporated into the PET resin because of hindrance of the alkyl group in the diols to the esterification and polycondensation reactions. Instead, they are more apt to undergo dehydration reactions to form aldehydes, enol, and hemiacetals, which have lower boiling points and can be largely removed from the reaction system. When the content of miscellaneous diols was higher than 10%, the PET resin became more difficult to synthesize. Transparent and colorless bio-PET samples were synthesized with 99.9, 98.2, and 95.8 wt % bio-EG. The presence of miscellaneous diols with content lower than 2 wt % did not show negative effects on the physical properties and the color of the bio-PET materials. Even with the 95.8 wt % bio-EG sample, merely the tensile strength of the derived bio-PET decreased slightly. The comprehensive physical and thermal properties of the bio-PET materials synthesized from the bioEG-containing diols (