Microwave-assisted aminolytic depolymerization of PET waste

European Polymer Journal 45 (2009) 2695–2700

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Microwave-assisted aminolytic depolymerization of PET waste N.D. Pingale, S.R. Shukla * Department of Fibers and Textile Processing Technology, Institute of Chemical Technology (Deemed University), Matunga, Mumbai 400 019, India

a r t i c l e

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Article history: Received 31 January 2009 Received in revised form 15 May 2009 Accepted 21 May 2009 Available online 2 June 2009

Keywords: PET waste Aminolysis Microwave irradiation Bis (2-hydroxyethyl) terephthalamide

a b s t r a c t Polyethylene terephthalate (PET) fibre waste and disposable soft drink bottle waste were subjected to depolymerization via aminolysis using excess of ethanolamine. The reaction was carried out under non conventional microwave energy in the presence of different simple chemicals as catalysts namely, sodium acetate, sodium bicarbonate and sodium sulphate. After repeated crystallization, pure bis (2-hydroxyethyl) terephthalamide (BHETA) was obtained with very high yields (nearly 90%). It was subjected to characterization by elemental analysis, melting point, FTIR, NMR and DSC. With the use of microwave energy, the process becomes economically viable since high yields of BHETA (>90%) at very low reaction time (4 min) could be obtained with common and cheap chemicals as catalysts. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Polyethylene terephthalate (PET) is a versatile polymer used in the manufacture of a variety of products differing widely in their physical characteristics and hence, the end uses. The use of huge volumes of PET is attributed to its excellent thermal and mechanical properties, crystal clear transparency and non-toxic nature [1]. Although PET does not create a direct hazard to the environment, its substantial contribution in solid waste generation and high resistance to natural degradative assimilation into the environment makes it a noxious material [2]. Two main sources of PET waste for recycling are the manufacturing waste and the post consumer waste [3]. Chemical recycling through depolymerization has the advantage of formation of materials, which can react to form the polymer itself or some other secondary valueadded products [4,5]. Hydrolysis, aminolysis, methanolysis and glycolysis are the possible routes to PET depolymerization [6–9]. Aminolysis reactions on polyesters have been reported way back in 60s with different approaches. The studies are mainly divided into two fields, namely study of PET mor-

* Corresponding author. Tel.: +91 22 2414 5616. E-mail address: [email protected] (S.R. Shukla). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.05.028

phology and the practical application of aminolysis. Popoola [10] studied the mechanism of weight loss and morphological changes occurring in PET on degradation with 40% aqueous methylamine. Awodi et al. [11] carried out characterization of PET remaining after depolymerization using 40% aqueous methylamine. Soni and Singh [12] degraded PET waste flakes with aqueous methylamine and ammonia at room temperature in the presence of cetyl ammonium bromide for various times up to 45 days to obtain N,N-dimethyl terephthalamide and terephthalamide, respectively as fine precipitates. Goje et al. [13] carried out PET aminolysis under reflux with hydrazine monohydrate in the presence of lead acetate as a catalyst and obtained terephthalohydrazide as a high value-added comonomeric product, used for preparation of a polyhydrazide comonomer. Spychaj et al. [14] conducted aminolysis of PET using some polyamines and triethanolamine and tested the products as epoxy resin hardeners and polyol components for rigid polyurethane foam synthesis. Microwave-assisted organic synthesis has revolutionized the chemical research [15,16]. Microwave irradiation as a heating technique offers many advantages over the conventional heating such as instantaneous and rapid heating with high specificity without contact with the material to be heated. It is, therefore, a popular technique for heating and drying materials and is utilized in many household and industrial applications [16,17].

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Nikje and Nazari [18] used microwave irradiation for glycolytic depolymerization of PET with alcohols. Hydrolytic depolymerization of PET in closed system under microwave irradiation 2 MPa pressure for 90–120 min led to the formation terephthalic acid, ethylene glycol and diethylene glycol as degradation products [19]. Similar studies were carried out by Zhang [20] using pure water at 220–230 °C at 2.0–2.5 MPa for 60–120 min. Krzan [21–23], during depolymerization with different glycols in the presence of zinc acetate catalyst and under microwave in a closed system, achieved complete PET solubilization in about 10 min. Our studies on glycolysis of PET bottle waste in presence of sodium carbonate, sodium bicarbonate and barium hydroxide catalysts under reflux in microwave irradiation has shown that the time of reaction reduced to 35 min as compared to 8 h under conventional heating system [24]. In the present study, we have attempted aminolytic depolymerization of PET waste (fibre and bottle) under reflux by using microwave irradiation for heating. The optimization of reaction parameters in terms of yield of bis (2-hydroxyethyl) terephthalamide (BHETA), which is the depolymerized product was carried out in presence of simple chemicals as catalysts, namely sodium bicarbonate, sodium acetate and sodium sulphate. 2. Experimental 2.1. Materials Poly (ethylene terephthalate) (PET) fibre waste was obtained from Reliance Industries, Mumbai. Discarded PET

bottles were procured from local market. The bottles after removing caps and labels were cleaned by boiling in a weak detergent solution followed by washing and drying. Subsequently they were cut into chips of approximately 1 cm2 size. 2.2. Chemicals All the chemicals including sodium acetate, sodium bicarbonate and sodium sulphate were of analytical reagent grade. 2.3. Aminolysis of polyester waste A 700 W Electrolux (17L) domestic microwave oven was used for the aminolysis reaction. It was modified to allow fitting of a condenser as described in the previous communication [24]. The PET waste was treated with ethanolamine under reflux using microwave oven at maximum power in the presence of sodium acetate, sodium bicarbonate and sodium sulphate as catalysts, for time period up to 7 min. The catalyst concentration was varied between 0.3% and 1% (w/w). At the end of the reaction, water was added in excess to the reaction mixture with vigorous agitation. The depolymerized product was obtained as a residue after filtration. The filtrate contained unreacted ethanolamine and the virtual monomer bis (2-hydroxyethyl) terephthalamide (BHETA). White crystals of BHETA were obtained by first concentrating the filtrate by boiling and then chilling it. White crystalline powder of BHETA was purified by repeated crystallization from water, dried in an oven at 80 °C and weighed for estimating the yield.

Table 1 Effect of time on % yield of BHETA. Time (min)

BHETA yield (%) PET fibre waste

3 4 5 6 7 8

PET bottle waste

Na acetate

Na sulphate

Na bicarbonate

Na acetate

Na sulphate

Na bicarbonate

88 90 90 90 90 –

84 94 93 93 94 –

81 87 91 91 91 –

74 85 86 88 90 90

75 85 88 88 88 88

75 78 82 86 89 89

Catalyst concentration 0.5% (w/w). PET:EA 1:6.

Table 2 Effect of catalyst concentration on % yield of BHETA. Catalyst conc.

BHETA yield (%) PET fibre waste

PET bottle waste

% (w/w)

Na acetate

Na sulphate

Na bicarbonate

Na acetate

Na sulphate

Na bicarbonate

0.2 0.3 0.5 0.7 1

89 90 90 90 90

91 94 94 94 93

89 90 90 91 91

– 88 89 91 90

– 88 88 89 90

– 88 89 89 89

Time was selected on the basis of optimum yield as indicated underlined in Table 1. PET:EA 1:6.

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BHETA yield (%) PET fibre waste

1:4 1:6 1:8 1:10

PET bottle waste

Na acetate

Na sulphate

Na bicarbonate

Na acetate

Na sulphate

Na bicarbonate

84 90 91 91

79 93 92 93

84 90 90 90

83 88 91 92

67 88 88 88

83 89 91 90

Time and catalyst concentration were selected on the basis of optimum yield as indicated underlined in Tables 1 and 2, respectively.

Table 4 Comparative reaction parameters and BHETA yield for sodium bicarbonate catalyst. PET fibre waste

BHETA yield (%) Time Catalyst conc. % (w/w) PET:EA

PET bottle waste

Conventional

Microwave

Conventional

Microwave

91 8h 1 1:6

90 5 min 0.3 1:6

83 8h 1 1:6

89 7 min 0.3 1:6

2.4. Characterization of BHETA

(DSC) (Shimadzu 60) at the heating rate of 10 °C/min from 25 °C to 200 °C in nitrogen atmosphere.

Melting point of the purified BHETA was determined in an open capillary. Elemental analysis was carried out by using Heraus Combustion Apparatus. For 1H NMR, the glycolyzed residue was dissolved in solvent CDCl3. Tetramethyl silane was used as an internal standard and the spectrograph was recorded on JEOL, FT-NMR (60 MHz). FTIR spectrum was recorded on Shimadzu FTIR Spectrophotometer (Model 8400S). The melting characteristic was determined by differential scanning calorimeter

3. Results and discussion Aminolysis of PET waste with excess ethanolamine (EA) under reflux by conventional heating in presence of the catalysts glacial acetic acid, sodium acetate and potassium sulphate has been reported earlier [25]. The results show that the aminolytic depolymerization led to the synthesis

Fig. 1. Mechanism of aminolytic depolymerization of PET.

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Table 5 Elemental analysis of BHETA (formula, C12H16N2O4; molecular weight, 252; melting point, 221 °C). Element (%)

Observed

Calculated

C H N

57.10 6.25 11.05

57.14 6.34 11.11

of pure virtual monomer bis (2-hydroxyethyl) terephthalamide (BHETA) with yields as high as 91%. The selection of these catalysts was based on the fact that they have proved efficient in the glycolytic depolymerization of PET without any loss in the yield of the pure virtual monomer bis (2-hydroxyethyl) terephthalate (BHET) [8]. Moreover, they offer additional advantage of being simple chemicals that are non-toxic as against the heavy metal catalyst, lead acetate. The present work reports the results on aminolytic depolymerization of PET waste in excess of EA under reflux in a microwave heated setup in the presence of economical and ecofriendly chemicals namely sodium acetate, sodium sulphate and sodium bicarbonate as catalyst. Sodium acetate has been shown to give maximum yield of BHETA from aminolysis of PET fibre waste in EA [25], whereas sodium sulphate has produced maximum yield of BHET from glycolysis of PET bottle waste in ethylene glycol [8]. The results on optimization of the reaction parameters, viz., time of reaction (Table 1), catalyst concentration (Table 2) and PET:EA ratio (Table 3) under microwave irradiation indicates the yield of BHETA. From Table 1, it is clear that in all the cases the optimum reaction time was as low as only 4–5 min, which

gave nearly 90% yield of BHETA. In comparison, as much as 8 h were required for the conversion of PET into BHETA in the earlier studies conducted by using conventional heating source. This was attributed to the fact that microwave irradiation provides the momentum to overcome the barrier to reach the higher state and complete the reaction more quickly than the conventional source of heat energy [26]. Also, the BHETA yield was observed to be either similar or better under microwave irradiation, even with the extremely low time of reaction. As far as the effects of catalyst concentration (Table 2) and PET:EA ratio (Table 3) on the BHETA yield are concerned, it was observed that there was no difference in the optimized values for these two parameters in comparison to our earlier experiments with conventionally heated depolymerization. The comparative values of parameters for aminolytic depolymerization of PET in excess of EA in the presence of sodium bicarbonate catalyst are shown in Table 4. It can be clearly seen that the depolymerization reaction under microwave irradiation proves economical from the point of view of catalyst concentration and reduced time of reaction, which also leads to energy conservation. A slight difference in the values of BHETA yield, observed by using fibre and bottle waste, was attributed to the differences in the molecular weight and its distribution in the fibre grade PET and the bottle grade PET. The molecular weight is lower and the molecular weight distribution is narrower for the former than that for the latter. This imparts higher viscosity to the bottle grade PET, necessary for the blow-molding process, which is assisted through polymer modification using certain co-monomers and chain terminating agents. This polymer composition causes little decrease in the BHETA yield [27,28].

Fig. 2. FTIR spectra of BHETA.

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Fig. 3. NMR spectra of BHETA.

The mechanism of depolymerization through aminolysis is explained in Fig. 1. Ethanolamine has two nucleophilic centres. The amine group, being more electronegative than hydroxyl group of EA, attacks the ester linkage of PET easily to form BHETA. After repeated crystallization, purified BHETA was characterized by elemental analysis, and melting point, as indicated in Table 5. The characterization confirmed that the purified product of PET depolymerization is BHETA. The FTIR spectrograph (Fig. 2) of the purified monomer clearly shows the peaks at 1049 and 3282 cm 1, indicating the presence of primary alcohol. The peaks for secondary amide stretching are observed at 1315, 1550 and 3358 cm 1. The 1H NMR (Fig. 3) gave peak at d 8.59 corre-

sponding to –NHCO groups, d 3.25–3.65 corresponding to aliphatic CH2–CH2 proton, d 7.98 corresponding to aromatic ring protons and d 4.70 corresponding to –OH groups. The DSC scan (Fig. 4) also show reasonably sharp endothermic peak at 222 °C in agreement with the known melting point [29], whereas an endothermic peak at around 60 °C represents the phase change of BHETA.

4. Conclusion The aminolytic depolymerization of PET fibre and bottle waste under reflux in the presence of microwave radiations offers an economical technology to produce the virtual

Fig. 4. DSC of BHETA.

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monomer BHETA. The conservation in time and energy of reaction coupled with substitution of heavy metal catalysts by simple non-toxic and cheap chemicals are the added advantages. Further, BHETA was obtained in yields > 90% and possess reactive end groups, which indicate the potential of carrying out further reactions to obtain various valueadded products for use in different fields.

[7] [8] [9] [10] [11] [12] [13] [14]

Acknowledgement Authors are grateful to Council of Scientific and Industrial Research, New Delhi, Government of India, for funding this project. References [1] Karayannidis GP, Achilias DS. Macromol Mater Eng 2007;292:128–46. [2] Scheirs J. Recycling of PET. In: Polymer recycling: science, technology and applications. Wiley series in polymer science. Chichester, UK: John Wiley & Sons; 1998. [3] Nikles DE, Farahat MS. Macromol Mater Eng 2005;290(1):13–30. [4] Vaidya UR, Nadkarni VM. J Appl Polym Sci 1987;34(1):235–45. [5] Shukla SR, Harad AM, Jawale LS. Waste Manage 2008;28(1):51–6. [6] Lorenzetti C, Maaresi P, Berti C, Barbiroli G. J Polym Environ 2006;14(1):89–100.

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