Chinese Chemical Letters  2014, Vol.25 Issue (12):1550-1554   PDF    
Synthesis and characterization of azo dyestuff based on bis(2-hydroxyethyl) terephthalate derived from depolymerized waste poly(ethylene terephthalate) fibers
Meng-Juan Lia,b      , Yan-Hong Huangb, An-Qi Jub, Tian-Shi Yub, Ming-Qiao Gea,b     
aKey Laboratory of Eco-Textiles (Jiangnan University), Ministry of Education, Wuxi 214122, China;
bCollege of Textile & Clothing, Jiangnan University, Wuxi 214122, China
Abstract: This work aimed at effectively utilizing the chemically depolymerized waste poly(ethylene terephthalate) (PET) fibers into useful products for the textile industry. PET fibers were glycolytically degraded by excess ethylene glycol as depolymerizing agent and zinc acetate dihydrate as catalyst. The glycolysis product, bis(2-hydroxyethyl) terephthalate (BHET), was purified through repeated crystallization to get an average yield above 80%. Then, BHET was nitrated, reduced, and azotized to get diazonium salt. Finally, the produced diazonium salt was coupled with 1-(4-sulfophenyl)-3-methyl-5-pyrazolone to get azo dyestuff. The structures of BHET and azo dyestuff were identified by FTIR and 1H NMR spectra and elemental analysis. Nylon filaments dyed by the synthesized azo dyestuff with the dye bath pH from 4.14 to 5.88 showed bright yellow color. The performances of the dyestuff were described with dye uptake, color fastness, K/S, L*, a*, b*, and ΔE* values.
Key words: Waste PET fibers     Depolymerization     Bis(2-hydroxyethyl) terephthalate     Azo dyestuff    
1. Introduction

Poly(ethylene terephthalate) (PET) is a non-toxic synthetic polymer widely used in packaging,electronics,and textiles. Although PET has no direct contamination to the environment, it is non-biodegradable,requires huge space to landfill,and releases large amounts of noxious gas. Chemical recycling of postconsumer PET seems to be the best method to deal with this issue. Chemical recycling by means of depolymerization can transform PET waste into oligomers and produce some value added products [1, 2].

The chemical depolymerization of PET has been a most important and interesting method for recycling of PET waste in recent years [3, 4, 5, 6]. Many attempts have been carried out: Carta and Zhang et al. recycled PET waste by means of hydrolysis [7, 8]. Imran et al.converted post-consumer PET waste into bis (2-hydroxyethyl) terephthalate (BHET) by using excess ethylene glycol [9]. Pingale and Shamsiet al.depolymerized PET fiber waste and soft drink bottle wasteviaaminolysis using excess ethanolamine [10, 11]. Yanget al.recycled PET by means of methanolytic depolymerization [12]. Compared with the other above-mentioned methods,glycolysis of PET waste is a convenient method to recycle polymer because of the relatively mild reaction conditions and decent yield of oligomer.

Oligomers derived from depolymerized PET can be used as feedstock to produce valuable products: Roy and Li et al. transformed recycled PET into polyurethane foams [13, 14]. DuqueIngunza et al. synthesized unsaturated polyester resin from glycolyzed postconsumer PET wastes [15]. Attaet al. recycled PET waste into polyurethane for petroleum sorbent [16]. Shukla, Palekar,Choiet al.transformed PET waste into hydrophobic textile dyestuffs and disazo disperse dyes [17, 18, 19].

In this article,we provided a method to recycle PET fibers into azo disperse dyestuff. The waste PET fibers were depolymerized to BHET,nitrified,reduced,diazotized,and coupled with 1-(4-sulfophenyl)-3-methyl-5-pyrazolone to synthesize azo disperse dyestuff successively. The performances of the dyestuff were described with dye uptake,color fastness,K/S,L*,a*,b*,and DE* values of dyed nylon filaments. 2. Experimental 2.1. Materials

Waste PET fibers,nylon filaments,and fabrics were washed with water and acetone successively,and then dried at 808C for 8 h until the weight of the fibers remained constant. Ethylene glycol (EG),zinc acetate dihydrate,conc. nitric acid,conc. sulfuric acid, conc. hydrochloric acid,acetic acid,tin chloride dehydrate,1-(4-sulfophenyl)-3-methyl-5-pyrazolone,sodium nitrite,sodium acetate,sodium carbonate and carbamide,all of CP grade,were purchased from Sinopharm Chemical Reagent Co.,Ltd. (Shanghai, China). 2.2. Glycolysis of PET

Waste PET fibers were chemically depolymerized into bis-(2-hydroxyethyl) terephthalate (BHET) by utilizing ethylene glycol [14]. A 500 mL four-necked round-bottom glass flask equipped with a thermometer,a reflux condenser,a nitrogen catheter,and a magnetic stirrer was charged with 50.00 g PET,150.00 g EG,and 0.10 g zinc acetate dihydrate. The glycolysis reactions (Scheme 1) were processed at 198°C under nitrogen atmosphere for 4 h. The glycolysis products were purified through repeated crystallization to get an average yield of BHET above 80%.

Scheme 1. Glycolysis of PET with ethylene glycol.
2.3. Synthesis of azo dyestuffs

BHET was nitrated and reduced in the general procedure (Scheme 2) [20]. A 500 mL three-necked round-bottom glass flask equipped with a magnetic stirrer,thermometer,and reflux condenser was charged with 10.00 g BHET. A mixture of 15 mL conc. sulfuric acid and 3 mL conc. nitric acid was added into the glass flask. The reaction was continued at 60°C for 1 h and at 80°C for 10 h successively. After the reaction,the mixture was refrigerated with ice water until the mixture reached room temperature. The precipitate was filtered with distilled water and ethanol 3 times successively. Then,the filtrate was dissolved in hot water to be recrystallized in the refrigerator. The formed crystal, nitro-bis(2-hydroxyethyl) terephthalate (NBHET),was dried in the oven at 60°C.

Scheme 2. Nitration and reduction of BHET.

A three-necked round-bottomed glass flask fitted with a stirrer, a reflux condenser,and a thermometer was charged with 26.65 g tin chloride dehydrate and 100 mL fuming hydrochloric acid. After the tin chloride dehydrate dissolved completely,10.00 g of NBHET was added gradually into the glass flask. The reaction continued at 80°C for 3 h. After the reaction,the precipitate was filtered with distilled water and dried in an oven at 60°C for 2 h,then purified by recrystallization from ethanol and dried in an oven at 60°C for 2 h to form amino-bis(2-hydroxyethyl) terephthalate (ABHET). Five gram ABHET was added into 4.5 mL hydrochloric acid at 8°C with magnetic stirring. Then,6.5 mL sodium nitrite aqueous solution (20% by weight ratio) was added slowly into the mixture and reacted at 5°C for 3 h to get diazonium salt (Scheme 3). The excess sodium nitrite was removed by carbamide.

Scheme 3. Diazo reaction ABHET.

The resulting diazonium salt solution was used immediately in the coupling reaction with 1-(4-sulfophenyl)-3-methyl-5-pyrazolone (Scheme 4). 1-(4-Sulfophenyl)-3-methyl-5-pyrazolone (5 g) was added into 50 mL distilled water at 5°C,and the solution of the pH was maintained at 8.0 by sodium carbonate. Diazonium salt (6.23 g) was dissolved into the mixture,and the pH of the solution was maintained at 7.0 by sodium carbonate for 2 h. The bright yellow colored dyestuff was filtered,washed with distilled water, and dried at 60°C for 2 h.

Scheme 4. Coupling reaction of diazonium salt with 1-(4-sulfophenyl)-3-methyl-5-pyrazolone.
2.4. Dyeing of nylon filaments

The performance of the synthesized dyestuff was evaluated by dyeing nylon filaments and fabrics. Dyestuff was dispersed in distilled water at 3% (o.w.f.) with dispersing agent. The pH of the dye bath was varied from 4.14 to 5.88 by acetic acid-sodium acetate buffer solution. Nylon filaments and fabrics were immersed in the dye bath at 95°C for 40 min. After dyeing,nylon filaments and fabrics were washed by distilled water and dried in an oven at 70°C. 3. Results and discussion 3.1. Characterization of BHET and synthesized dyestuff

Fourier-transform infrared (FTIR) spectra of BHET and the synthesized dyestuff were obtained on a Nicolet Nexus-470 IR Spectrometers (USA) with KBr as a reference material. The scanning range was 650-4000 cm-1 and the resolution was 1 cm-1 . The FTIR spectra of BHET,1-(4-sulfophenyl)-3-methyl-5-pyrazolone and the synthesized dyestuff are shown in Fig. 1. Three absorption peaks at 1510,1432 and 1286 cm-1 were due to benzene ring groups. The absorption bands due to -OH group shown at 3640-3230 cm-1 in Fig. 1(a) and (c) demonstrated the complete depolymerization of PET to BHET. The absorptions at 1188 and 1130 cm-1 shown in Fig. 1(b) and (c) were due to O==S==O.

Fig. 1. FTIR of BHET (a),1-(4-sulfophenyl)-3-methyl-5-pyrazolone (b) and azo dyestuff (c).

Elemental analyses of BHET and the synthesized dyestuff were carried out using a vario EL III elemental analyser (Germany). The formula of BHET is C12H14O6(mol. wt. 254.22),with its calculated result as: C,56.70%; H,5.55%; and with its observed result as C, 56.80%; H,6.02%. The formula of synthesized dyestuff is C22H22O10N4S1(mol. wt. 534.47),with its calculated result as: C, 49.44%; H,4.15%; N,10.48%; S,6.00% and with its observed result as C,49.42%; H,4.06% N,10.29%; S,5.75%.

Proton nuclear magnetic resonance ( 1H NMR) spectra of BHET and synthesized dyestuff were recorded on a Bruker DRX-400 spectrometer (Germany) at 400 MHz in deuterated dimethylsulfoxide (DMSO-d6),and the chemical shiftsdwere measured using tetramethyl silane as an internal reference. Fig. 2 shows the structures and 1H NMR spectra of BHET and the synthesized dyestuff. The 1H NMR values are showed in Table 1.

Fig. 2. 1H NMR of BHET (a) and azo dyestuff (b).

Table 1
1H NMR values of BHET and azo dyestuff.
3.2. Variation of dyeing performances with pH

Absorbance of dyestuff in water was determined by an UVmini-1240 spectrophotometer (Shimadzu,Japan). A sample of UVspectra of dyestuff dispersed aqueous solution is shown in the inset of Fig. 3. The maximum absorption wavelength (lmax) of the dyestuff in water was a bout 395 nm. The absorbance-concentration curve was determined by the UV-spectra of dye bath in different concentrations and is shown in Fig. 3. The relation between concentration of dyestuff and absorbance could be expressed by:

whereCis the concentration of dyestuff (mM),Ais the absorbance. Dye uptake is an important performance of dyestuff,which could be expressed by:
where C0 and C1 are the concentrations of dye bath before and after dyeing (mmol/L). Then absorbance of dye bath before and after dyeing (A0 and A1) in different pH was measured.C0 and C1 were calculated from A0 and A1 by Eq. (1). The dye uptake of various pH dye bath was calculated by Eq. (2) and shown in Table 2. The dye uptake decreased from 76.39 to 5.81 while the pH of dye bath increased from 4.14 to 5.88. This result should be due to two reasons: the (NH3+-Ny-COO-) group of nylon fibers changed into (NH3+ -Ny-COOH) and positively charged in an acid solution. The dyestuff molecules and nylon fibers combined with Van der Waals’force and hydrogen bonding; nylon fibers hydrolyzed in an acid solution and formed lots of imino groups. Both of these reasons could facilitate adsorption and fixation of dyestuff on nylon fibers.
Fig. 3. Dependence of dyestuff concentration on absorbance. The inset demonstrates UV-spectra of dyestuff dispersed aqueous solution.

Table 2
Variation of dye uptake with pH of dye bath.

Color fastness characteristics of dyed nylon fabrics to washing and light were measured according to AATCC test method 61 and AATCC test method 16E [21],using an SW-12A wash fastness tester (Wuxi,China) and an ATLAS-150S light fastness tester,respectively. The washing fastness of dyed nylon fabrics was rated 4-5 on a scale of 5,where 5 is excellent. The light fastness of dyed nylon fabrics was found to be moderate (rating 4-5 on a scale of 8,where 8 is excellent) (Table 3).

Table 3
Color fastness of dyed nylon fabrics.

Relative color values (K/S,L*,a*,b*,DE*) of each dyed sample were measured by a COLOR-Eye-7000A spectrophotometer (GretagMacbeth,USA). The values of color coordinates L*,a* and b* represent lightness/darkness,red/green and yellow/blue tones of color,respectively [22, 23]. The data of color values (K/S) representatives of the color depth of the dyed fibers are given in Table 4. The positiveb* values indicated the yellowish tone of the dyestuff,whereas higher L* values confirm the color brilliancy. The value of DE* represents levelness of dyed samples which could be expressed by:

where DX* equals the difference between the maximum value and the minimum value (X=L,a,b). The lower its DE*,the better its levelness. The relative color values corresponded to the maximum absorption wavelength of the dyestuff (Table 4) and the bright yellow color of synthesized dyestuff and dyed nylon filaments (Fig. 4).
Table 4
Variation of dyestuff maximum absorption wavelength (lmax) and relative color values (K/S,L*,a*,b*,DE*) with pH of dye bath.

Fig. 4. Photographs of dye bath and dyed nylon filaments with various pH: (A) 4.14, (B) 4.39,(C) 4.69,(D) 5.26,(E) 5.60 and (F) 5.88.
4. Conclusion

By utilizing ethylene glycol as the depolymerizing agent,waste PET fibers were glycolytically depolymerized into bis(2-hydroxyethyl) terephthalate (BHET). Then,BHET was nitrified,reduced, diazotized and coupled with 1-(4-sulfophenyl)-3-methyl-5-pyrazolone to synthesize azo disperse dyestuff successively. The maximum absorption wavelength of the synthesized dyestuff was 394-396 nm,which corresponded to its bright yellow color. Nylon filaments and fabrics were dyed by the synthesized dyestuff with the pH from 4.14 to 5.88. Lower pH dye bath facilitated the dye uptake. The dyed nylon fabrics showed good washing fastness and moderate light fastness characteristics.


This work was financially supported by the National High-tech R&D Program of China (863 Program,No. 2012AA030313),the Open Project Program of Key Laboratory of Eco-Textiles (Jiangnan University),Ministry of Education,China (No. KLET1115),the Fundamental Research Funds for the Central Universities (No. JUSRP11201),and the Cooperative Innovation Fund-Prospective Project of Jiangsu Province,China (No. BY2012060).

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