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Synthesis and characterization of polysulfone based on poly(ethylene terephthalate) waste.

INTRODUCTION

The growing interest of recycling of polyethylene terephthalate, (PET), is due to the widespread use of packaging made of this polymer mainly as beverage bottles. Beside the different methods of recycling polymer, chemical recycling is mainly important and applied in the case of post consumer polycondensates, which are very susceptible to solvolytic chain cleavage. In particular, solvolysis namely "controlled depolymerization by action of a reactive solvent" make it possible to return to the starting monomers or oligomers, by hydrolysis, glycolysis, or methanolysis. Also, to synthesize specialized products like polyurethane foams, unsaturated polyesters, polyamides, plasticizers, or alkyd resins, etc. [1],

Glycolysis of PET has been studied for many years. The most important glycols used are ethylene glycol (EG), diethylene glycol (DEG), and propylene glycol (PG) [2-4]. The products of depolymerization reaction of PET with EG are leading to bis (hydroxyethyl terephthalate), (BHET) and PET oligomers. The functionality of end groups of glycolysis products is very important; where, they can participate in new polymerization reactions, yielding graft copolymers or net works, such telechelic polymers called macromolecular monomers (macromonomers) [5].

4,4'-Dichlorodiphenyl sulfone (DCDPS) is used as a starting material in the polymerization of compounds such as Udel, Victrex, and Radel R. The polymerization occurs through a nucleophilic substitution reaction of DCDPS as difunctional nucleophiles with bisphenol A in dimethyl sulfoxide, DCDPS forms a material called Udel. Udel is a high performance amorphous sulfone polymer that can molded into a variety of different shapes. It is both rigid and temperature resistant, and has applications in many things e.g., plumbing pipes, printer cartridges, automobile fuses, ... DCDPS also reacts with bisphenol S to form Victrex. Like Udel, Victrex is a rigid and thermally resistant material with numerous applications [6], Polysulfones and their copolymers are characterized by their thermal stability, high glass transition temperatures, and their resistance to oxidation [7]. These kinds of polymers are used as filters in ultrafiltration of juices and beverages [8). Polysulfones are applied also, as novel membranes for affinity separation by surface modification of the electrospun nonwoven fiber mesh [9, 10). Furthermore, polysulfone can be utilized as membranes in refining drinking water via scavenging endocrine disrupting chemicals such as bisphenol A, which is used widely in the industries and causes pollution for rivers through factories drainage [11, 12], This category of polymers finds also its applications as ion beam selectivity in electronic applications [13]. Polysulfones were prepared via nucleophilic aromatic substitution of 4,4'-dichlorodiphenyl sulfone with bisphenates to produce high molecular weight homopolymers in a relatively short time [14], Polysulfone is extensively used in industry applications due to its low cost. In general polymer modifications are necessary to achieve desired features. Polysulfone have been widely used to overcome the problems associated with the brittleness of epoxy resins via end group functionalization or blending [15],

However, based on literatures published in this field, it could be stated that, no study has been led to synthesize a product of the glycolysed PET bis (hydroxyethyl terephthalate) with 4,4'-dichlorodiphenyl sulfone. On the other hand, the direct reaction of end group functions of oligomeric products of PET could lead to an interesting method to synthesis polysulfones.

The aim of this search is to evaluate the potentiality of hydroxytelechelic oligomers resulting from the glycolysis of PET waste with the prepared 4,4'-dichlorodiphenyl sulfone as a new method for valorization of PET wastes.

EXPERIMENTAL

Materials

Chloro-sulfonic acid, purity (as ClS[O.sub.2]OH) % by mass: Min. 99.0, free sulfuric acid (as [H.sub.2]S[O.sub.4]): Nil, free S[O.sub.3] (if any) as S[O.sub.3]% by mass: Max. 1.0, free hydrochloric acid as (HCl)% by mass: Nil. Chlorobenzene, vapor density 3.86 (vs. air), assay 99.8%. Dimethylformamide, Isopropanol, Potasium carbonate, Toluene, and all the previous ingredients were imported from

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Aldrich chemicals-Germany. Ethylene glycol (EG) obtained from Scharlau S.A. was used for glycolysis. Magnesium acetate was supplied by Barcelona, Spain and used as catalyst. Postconsumer PET from soft water drink bottles, after removing caps and labels, was chopped into small pieces ([approximately equal to] 4 X 4 mm), cleaned thoroughly by washing with water and soap and then with distilled water. The clean PET wastes were dried at 80[degrees]C up to constant weight and then subjected to degradation. The melt flow index and the apparent viscosity of the PET waste were determined using Zwick Roell Melt Flow Indexer. The test was carried out according to ASTM D 1238. The total load was 2.16 kg and the operating temperature was 250[degrees]C. The result represents the average of three samples. The apparent viscosity was calculated from the following relation:

MFI = (14.13[rho]) [DELTA]P[D.sup.4]/I[[eta].sub.o] [16].

where MFI = Melt Bow index (2.28 gm/10 min.); [rho] = Density (1.41gm/[cm.sup.3]); D = Diameter of capillary (2.095 mm); L = length of capillary (8 mm); [DELTA]P = the pressure (62.692 kg/ [cm.sup.2]); [[eta].sub.o] = Apparent viscosity (3.0075 X [10.sup.3] cP).

Synthesis

Synthesis of 4,4'-Dichlorodiphenyl Sulfone (DCDPS). Fifty milliliter of chlorobenzene was charged into a round-bottomflask equipped with a mechanical shaker and connected to a dropping funnel filled with a stoichiometrical amount of chlorosulfonic acid. The dropping of the latter was running through 3 h. After completion of the reaction, it was left overnight. The produced crystals was then filtered and recrystallized from isopropanol and dried under vacuum. The reaction was described in Scheme 1.

Synthesis of Hydroxytelechelic PET Oligomers. The synthesis of hydroxytelechelic oligomers of PET was performed by glycolysis of PET with ethylene glycol at molar ratio [approximately equal to] 1:6.2 and 0.5 wt% Mn acetate, based on PET weight, was used as catalyst. The mixture was charged into a four-necked round bottom flask of 500 ml capacity, connected to a reflux condenser, thermometer, and mechanical stirrer. The reaction was carried out under reflux at 190-200[degrees]C for 6 h. At the end of a reaction time, distilled water was added in excess to the reaction mixture with continuous vigorous agitation, followed by filtration to separate the white precipitate from the reaction medium. The filtrate mainly contained unreacted ethylene glycol some water-soluble PET degradation products and the catalyst. The white precipitate (GP) was dried, and weighed. The glycolyzed products (GP) were analyzed for hydroxyl number (HN) according to a procedure described elsewhere [17], The reaction of this depolymerization process is described in Scheme 2.

Synthesis of BHET-PSU. BHET-PSU was synthesized using a 250 ml 2 necked round-bottom-flask fitted with a condenser, nitrogen inlet, Dean Stark trap and mechanical stirrer. BHET (6 g [approximately equal to] 7.5 mmol), DCDPS (3.5 g [approximately equal to] 6.3 mmol), and dried potassium carbonate (3.17 g [approximately equal to] 23 mmol) were added to 100 ml DMF and 20 ml toluene in the flask. The reaction mixture was heated under reflux at 180[degrees]C for 8 h (Scheme 2). Then the reaction was stopped and cooled to room temperature. The product was filtered to remove most of the solid part. After that the filtrate was poured in methanol to precipitate the polymer, separated by filtration, washed several times by methanol, dried in a vacuum oven at 60[degrees]C for about 12 h. The average number molecular weight of this part measured by GPC was 1787 g [mol.sup.-1] and was given the symbol (A). A is soluble in DMF, DMSO, and insoluble in chloroform or acetone.

The solid part of the reaction was dissolved in water in order to remove the potassium carbonate and other impurities. After that, the mixture was filtered and washed several times with distilled water and methanol and dried in vacuum oven at 60[degrees]C for about 12 h. The number average molecular weight of this part measured by GPC was 3162 g [mol.sup.-1] and was given the symbol (B). B is insoluble in chloroform, acetone, acetonitrile, tetrahydrofuran, and dimethyl formamide but partially soluble in dimethyl sulfoxide.

Instrumental Analysis

Fourier Transform Infrared (FTIR) Analysis. Infrared spectra were recorded using a JASCO FT/IR 6100E spectrometer (Tokyo, Japan).The samples were prepared by grinding the polymers together with KBr salt and then pressing as tablets.

The Nuclear Magnetic Resonance ([sup.1]H-NMR). [sup.1]H-NMR spectra were obtained from a JEOL ECA-50 NMR instrument (Akashima, Japan) at 500 MHz with tetramethylsilane as the internal reference. The spectra were obtained in dimethyl sulfoxide as solvent.

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Gel Permeation Chromatography (GPC). Molecular weight determination of the prepared products was done by gel permeation chromatography (GPC). This was carried out using Agilent 1100 series, Germany, a set of three high resolution PLgel 5 mm columns (100, 104, 105 [Angastrom]) on series. Tetrahydrofuran was used as an eluent at flow rate 1.0 ml [min.sup.-1]. The columns were calibrated by means of polystyrene as an internal standard and covering the molecular weight range of 1000-5,000,000 g/ mol.

Mass Spectra. The mass spectra of DCDPS (MS) was recorded using Thermo Scientific Trace GC Ultra, Coupled with ISQ Single Quadruple MS (USA).

Thermal Properties Evaluation. Thermogravimetric analysis (TGA) was achieved with a TGA7 thermogravimetric analyzer based on a DTG-60H detector (Narwhalk, CT). The analysis was performed in a platinum cell under a nitrogen atmosphere at a flow rate of 20 ml/min starting from room temperature up to 800[degrees]C at a heating rate of 10[degrees]C/min. Differential scanning calorimetric analysis (DSC) was realized with a Netzsch STA 409C/CD (Boston, USA). The test was carried out under a helium atmosphere at a flow rate of 50 ml/min. The sample was heated from room temperature up to 800[degrees]C at a heating rate of 10[degrees]C/min.

RESULTS AND DISCUSSION

Mass Spectra of DCDPS

There are several methods of synthesizing DCDPS but many of them give low yields, DCDPS was used as starting material in the proposed route for synthesis of polysulfones, polyether sulfones, and polyphenyl sulfones. The synthesis of DCDPS is illustrated in Scheme 1. Figure 1 showed the mass spectra of the white crystals (DCDPS) formed from the reaction of chlorobenzene and chlorosulfonic acid. It is clear that intense peaks up to at m/e 730 were obtained. The intense peak at m/e 286 was related to DCDPS [{Cl-ph-S[O.sub.2] -ph-Cl ([M.sub.n] = 287 g/mol)}. Low peaks at m/e 441 and 710 were also observed and may be related to {Cl-[(ph-S[O.sub.2]).sub.2] -ph-Cl ([M.sub.n] = 441 g/mol)) and (Cl-[(ph-S[O.sub.2]).sub.4] -ph-Cl) ([M.sub.n] = 710g/mol)]. These formulae masses could be represented as shown in Scheme 1.

Table 1 illustrates the number average chain length ([M.sub.n]) of DCDPS obtained by GPC analysis at retention time 32.593 min. The glycolysis of PET waste was carried out using ethylene glycol at molar ratio [approximately equal to] 1: 6.2 and 0.5 wt% [M.sub.n] acetate was used as catalyst, based on PET weight as previously state. The HN of the glycolysis products (GP) was determined and the data was used for the calculation of number average molecular weight (M.,) of GP product. (HN) and ([M.sub.n]) value of GP were 144 mg KOH/g and 780 g/mol, respectively. To confirm this prediction GPC analysis was carried out on glycolysis products nearly the same value was obtained at retention time 31.134 min, Table 1. The GP were used with DCDPS for the preparation of polysulfone and postulated scheme for this synthesis could be shown in Scheme 2.

Fourier Transform Infrared (FTIR)

FTIR spectrum of the BHET obtained from recycled soft drink bottles of PET was illustrated in Fig. 2. The band appeared around 3446 [cm.sup.-1] characterizes the stretching frequency of the hydroxyl group and the broadening of this band is attributed to hydrogen bonding between the terminal hydroxyl groups. The bands at region 2880-2958 [cm.sup.-1] stand for the CH stretching frequencies of CH, C[H.sub.2], aromatic and olefinic groups [18]. Furthermore, a strong band sited at 1717 [cm.sup.-1] due to the stretching frequency of the carbonyl group of acids and esters. The bands at 725 and 867 [cm.sup.-1] were due to the bending frequency of aromatic CH [18, 19].

FTIR spectra of DCDPS Fig. 2 are characterized by the presence of CH stretching of phenyl at 3095 [cm.sup.-1]. The characteristic C-Cl stretching vibration band of DCDPS (monomer) showed at 1089 [cm.sup.-1] [20, 21]. In addition the stretching vibration in the para di-substituted benzene ring is usually assigned at [approximately equal to] 825 [cm.sup.-1]. The polycondensation of BHET and DCDPS in ratio 1:1 in toluene was recorded in Fig. 3 and the spectrum of the products A and B was characterized by a number of major bands that differ from the reactants (BHET and DCDPS). The intense absorption band characteristic of the C-S[O.sub.2]-C group to develop, particularly, the O=S=O asymmetric stretching at 1328 [cm.sup.-1], symmetric stretching at 1157 [cm.sup.-1] and the C-S-C asymmetric stretching at 764 [cm.sup.-1] [22], The characteristic band of C-Cl stretching vibration of DCDPS at 1089 [cm.sup.-1] decreased during the chemical reaction. This decrease is due to the depletion of the labile chlorine atom [20, 21]. The band at [approximately equal to]3423 [cm.sup.-1] was characteristic for the stretching frequency of -OH group, which is broad and intense especially in the product B due to the existence of hydrogen bonding. It is worth mentioning that the similarity of the FTIR spectra for the two products, namely A and B; confirms that the end products of the reaction of BHET and DCDPS is the same products but having different molecular weight. Table 2 illustrated the number average molecular weight ([M.sub.n]) of A and B obtained by GPC analysis with different polydispersity.

Nuclear Magnetic Resonance ([sup.1]H-NMR)

Further investigation for this reaction reactants and products was carried out using [sup.1]H-NMR and the signals for [sup.1]H-NMR were reproduced in Figs. 4 and 5. The most striking features in the all signals were:

For DCDPS, [sup.1]H-NMR spectrum, indicated the presence of two different aromatic protons in DCDPS at [delta] 7.8 ppm for [H.sub.a] and [delta] 7.4 ppm for [H.sub.b], Fig. 4. The [sup.1]H-NMR signals accorded very well with those reported in the Ref. [22].

For BHET the signal at [delta] 8.00 ppm indicates the presence of the four aromatic protons of the benzene ring in para position. Signals at [delta] 4.28 and 3.35 ppm are characteristic for the methylene protons of COOC[H.sub.2] and C[H.sub.2]-OH, respectively, Fig. 5.

For products A&B, the DCDPS and BHET reaction the hydroxyl end groups of the later are converted to polysulfone polymers with different molecular weights. The signals at [delta] 4.2-4.6 ppm indicates the presence of methylene protons COOC[H.sub.2] and C[H.sub.2]0 of the BHET. Signals appeared at [delta] 7.7 ppm were corresponding to the aromatic protons of BHET. Signals showed at [delta] 7.9 ppm related to the proton of [O.sub.2]S-Ar-H of the DCDPS. Signal appears at [delta] 7.8 might be due to the aromatic protons of [O.sub.2]S-ph-O, Fig. 5.

Differential Scanning Calorimetric (DSC)

Glass transition temperature, [T.sub.g], is one of the most important parameters for characterizing the polymeric material and is an indicator for its dimensional stability. [T.sub.g] depends on the heating rate of the experiment and the thermal history of the specimen as well as any molecular parameter affecting the chain mobility [23]. DSC thermogram recording was performed from room temperature to 800[degrees]C, at a heating rate 10[degrees]C/min for BHET, DCDPS, and they reached products (A and B). The thermogram of BHET illustrated two distinct endothermic peaks, starting from 108.93[degrees]C due to the melting point of BHET, which is in agreement with the known melting point of BHET reported in the Ref. [24] (Fig. 6). The second peak, centered about a maximum of 245.76[degrees]C is quite broad and can be related to the presence of an amount of high chain oligomers and this peak was not much lower than the PET melting temperature [25].

The DSC thermogram of DCDPS showed two distinct endothermic peaks starting from 140.85[degrees]C due to the melting of DCDPS, followed by an another endothermic peak at 284.85[degrees]C was attributed to the loss of sulfonic groups degradation of DCDPS [25, 26] (Fig. 6). For the products under investigation, Fig. 6 illustrated an endothermic peak at 142.89 and 147.61[degrees]C for A and B, respectively and which is due to the melting of A and B [26], For the product A an additional endothermic peak at 250.24[degrees]C that may be attributed to the decomposition of the product A. Endothermic peaks at 208.57, 468.47, and 621.47[degrees]C

could be detected for the product B. It should be noticed that the decomposition temperatures of product B were greater than that of product A (endothermic peak at 468.47 and 621.47[degrees]C for B as compared to 250.24[degrees]C for A product).

On the basis of the reached thermal analyses data it could be concluded that the prepared product B based on DCDPS and the glycolysis product of PET waste is characterized by acceptable high thermal stability [26].

Thermal Gravimetric Analysis (TGA)

TGA of the glycolysis product BHET is shown in Fig. 7. The first decomposition occurred in the range 150-291.58[degrees]C and rapid mass loss took place with 55.88%, which can explained that this was the most amount of component that had been decomposed. The second decomposition is occurred at range 310-422.51[degrees]C with mass loss 41.66% and it was considered as the thermal decomposition of PET produced by the dimer thermal polymerization during the thermogravimetric analysis [27, 28],

Figure 7 depictes TGA thermogram of DCDPS. The spectrum demonstrates only one endothermic peak and major mass loss ~96.77%. The decomposition is occurred at range 160-210[degrees]C. TGA profile of the products A and B presented in Fig. 7 and Table 3 indicates that the polymer B showed much higher thermal stability than A. The weight loss of the products A and B starts at 130[degrees]C. In addition three steps of degradation temperature were obtained for B and one for A as indicated in Fig. 7. The first stage of product B seemed to display at 190.17[degrees]C with mass loss 22.2%. The second one occurs at 389.9[degrees]C with mass loss 26.387%. The third stage arises at 530.9[degrees]C with mass loss 20.0%. The char yield of B (31.4%) at 800[degrees]C is higher compared to A (1.5%) at 700[degrees]C. The results suggest that high temperature performance and thermal stability of material B can be achieved by increasing the molecular weight of the product [29].

CONCLUSION

On the basis of the reached results, it could be concluded that-in addition to other oligomers-Bis (hydroxyethyl terephthalate), [BHET] is the predominately GP of PET wastes. The direct reaction of the obtained end group functions of the glycolyzed process of PET waste with 4,4'-dichlorodiphenyl sulfone could be considered as an interesting method to synthesis polysulfones. Two polysulfones (A and B) with different molecular weights, 1787 and 3162 g/mol. were obtained, respectively. On the basis of the thermal analyses data, the charring yield of product B was 31.4% at 800[degrees]C, which was higher than that of product A (1.5% at 700[degrees]C). The obtained results suggested that the acceptable high temperature performance and thermal stability of material can be achieved by increasing the molecular weight of the reaction products B. Chemical recycling of PET wastes offer a great variety of potential products and open a new field of applications.

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M.E. Tawfik, M.L. Tawfic

Department of Polymers and Pigments, National Research Center, Dokki, Egypt

Correspondence to: Magda Emil Tawfik; e-mail: magdaemileta@yahoo.com Contract grant sponsor: National Research Center.

Published online in Wiley Online Library (wileyonlinelibrary.com).

DOI 10.1002/pen.24005

TABLE 1. Molar masses of samples determined by GPC.

                    Retention
Samples   Solvent   time (min)   [M.sub.n]   [M.sub.w]   P.D

BHET        THF       31.13         807        1000      1.24
DCDPS                 32.59         500         586      1.17

TABLE 2. Molar masses of polysulphone products A and B determined by
GPC.

                    Retention
Samples   Solvent   time (min)   [M.sub.n]   [M.sub.w]   P.D

A          DMSO       29.13        1787        4931      2.76
B                     29.99        3162       11,523     3.64

TABLE 3. Thermal properties of polysulfone products A and B.

                                  TGA

                       1st peak                   2nd peak

           Temp. range      Mass      Temp. range      Mass
Samples   ([degrees]C)    loss (%)   ([degrees]C)    loss (%)

A         115.29-232.82    87.00     232.82-400.00    11.54
B         118.56-223.02    22.20     317.70-445.02    26.39

                                  TGA

                       3rd peak

           Temp. range      Mass     Total mass
Samples   ([degrees]C)    loss (%)   loss, (%)    Ash (%)

A              --            --        98.54       1.50
B         516.84-696.39    20.01       68.60       31.40
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Author:Tawfik, M.E.; Tawfic, M.L.
Publication:Polymer Engineering and Science
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Date:Jul 1, 2015
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