Synthesis of Unsaturated Polyester Resins from PET Wastes: Effect of a Novel Co-catalytic System. on Glycolysis and Polyesterification Reactions

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1 Designed Monomers and Polymers ISSN: (Print) (Online) Journal homepage: Synthesis of Unsaturated Polyester Resins from PET Wastes: Effect of a Novel Co-catalytic System on Glycolysis and Polyesterification Reactions Sina Chaeichian, Saeed Pourmahdian & Faramarz Afshar Taromi To cite this article: Sina Chaeichian, Saeed Pourmahdian & Faramarz Afshar Taromi (2008) Synthesis of Unsaturated Polyester Resins from PET Wastes: Effect of a Novel Co-catalytic System on Glycolysis and Polyesterification Reactions, Designed Monomers and Polymers, 11:2, To link to this article: Published online: 02 Apr Submit your article to this journal Article views: 869 View related articles Citing articles: 3 View citing articles Full Terms & Conditions of access and use can be found at

2 Designed Monomers and Polymers 11 (2008) Synthesis of Unsaturated Polyester Resins from PET Wastes: Effect of a Novel Co-catalytic System on Glycolysis and Polyesterification Reactions Sina Chaeichian, Saeed Pourmahdian and Faramarz Afshar Taromi Polymer Engineering Department, Amir Kabir University of Technology, 424 Hafez Avenue, P.O. Box , Tehran, Iran Abstract Poly(ethylene terephthalate) (PET) waste materials were depolymerized by propylene glycol (PG) in presence of a novel catalytic system (zinc acetate with cyclohexylamine as a co-catalyst) and the effect of co-catalytic system on glycolysis reaction was compared with that of a commonly used catalytic compound (zinc acetate). The glycolyzed product was reacted with maleic anhydride (MA) and phthalic anhydride (PA) to prepare unsaturated polyester resin. The characteristics of the reaction products in each step were studied by 1 H-NMR, FT-IR, GPC and intrinsic viscosity (IV). It was found that the depolymerization of PET bottles in the presence of co-catalytic system proceeded faster compared with zinc acetate-catalyzed reaction. Moreover, in synthesis of unsaturated polyester resins, it was observed that the reaction rate of glycolyzed ingredients prepared by co-catalytic system with anhydrides was higher than that of glycolyzed product made by zinc acetate. It was concluded that cyclohexyleamine has a catalytic effect on unsaturated polyester resin synthesis. Koninklijke Brill NV, Leiden, 2008 Keywords Unsaturated polyester resin, cyclohexylamine, recycling, poly(ethylene terephthalate), glycolysis reaction, co-catalytic system 1. Introduction Unsaturated polyester (UP) resins are one of the most widely used reinforced plastics due to their low cost and versatility. In addition, UP resins can be tailor-made to meet specific requirements by proper choice of chemical building blocks. These materials are produced by reacting an unsaturated dibasic acid or a mixture thereof with a saturated dibasic acid with a polyhydric alcohol. The most commonly used diols are ethylene glycol (EG), propylene glycol (PG) and neopentyl glycol (NPG) whereas the diacids include orthophthalic anhydride, isophthalic acid, adipic acid, maleic anhydride and fumaric acid. Each of these materials can af- * To whom correspondence should be addressed. Tel.: (98-21) ; Fax: (98-21) ; chaeichian@yahoo.com Koninklijke Brill NV, Leiden, 2008 DOI: / X298080

3 188 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) fect final properties of the products. For instance, the general purpose (GP) resin is prepared by polycondensation of PG, maleic anhydride and phthalic anhydride. When phthalic anhydride is replaced by terephthalic acid certain properties such as heat distortion temperature (HDT) and chemical resistance are improved. Hence, UP resins which have terephthalate base in their structures are one of the most useful and versatile polyester resins; however, the direct use of TPA in synthesis of UPR is restricted because of its high melting point and processing difficulties arising from its sublimation during the course of the reaction. Thus, formulations involving terephthalic acid are more expensive, whereas UP resins prepared by glycolyzed poly(ethylene terephthalate) (PET) waste offer some economic advantages. Furthermore, synthesis of UPR from recycled PET can alleviate environment pollution. Therefore, the concept of preparing unsaturated polyester resin from the glycolyzed PET products is well developed [1 8]. The glycolysis of PET scraps with glycols in the presence of a suitable catalyst yields terephthalic oligomers by transesterification. Then, the oligoester diols may be reacted with maleic anhydride and/or other dibasic acids to form UPR. Due to the importance of the above-mentioned reactions, many studies have been focused on glycolysis of PET wastes. Different important factors such as reaction conditions (temperature, pressure, glycolysis time) [9 16], kind of glycol [17 24] and reactants ratio [25 27] were investigated. In addition, one of the most effective factors is catalyst because the reaction will be very sluggish if it is performed without a catalyst. The activation energy for the uncatalyzed glycolysis is about 32 kcal/mol, whereas a proper catalyzed process requires less than 20 kcal/mol [25]. Thus, various metal-containing catalysts, such as zinc acetate [26 30], lead-, cobalt- or manganese acetate [26, 29, 30] and titanium alkoxides [31] or other titanium compounds [5, 25] such as titanium (IV)-phosphate compound were used to facilitate the process. Zinc acetate is the most efficient catalyst in comparison with other metal-acetate compounds. Although the reaction proceeds faster in the presence of titanium alkoxides compared with the depolymerization catalyzed by metal acetate compounds, side-reactions proceed faster that yield products with an undesirable yellowish color. Therefore, titanium alkoxides are less used in glycolysis reactions of PET waste, whereas using zinc acetate in glycolysis reaction is more common in comparison with other transesterification catalysts. On the contrary, only a few investigators have examined co-catalytic systems in PET depolymerization and UPR synthesis even though they can have important roles in the reactions. Goje et al. [15] have referred to cyclohexylamine as a cocatalyst that increases the glycolysis rate, but the effect of cyclohexylamine was not examined entirely in glycolysis reaction of PET and it was not studied in UPR synthesis at all. The primary goal of this study was to investigate the role of a co-catalytic system in glycolysis reaction of PET and UPR synthesis. Hence, the effect of cyclohexylamine as the co-catalyst on the glycolysis raction in the presence of zinc acetate

4 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) (the major catalyst) was evaluated and an optimum amount of cyclohexylamine was found. Finally, UP resins were synthesized by reaction of glycolyzed products with anhydrides and the effect of the co-catalytic system on UPR synthesis was studied. 2. Experimental 2.1. Materials Post consumer PET beverage bottles produced by Damavand were crushed to small pieces after washing with acetone and methanol and removing adhesive and other materials. The intrinsic viscosity of PET in 60:40 (w/w) phenol/1,1,2,2- tetreachloroethane solution at 25 C was about dl/g, corresponding to a number-average molecular weight of g/mol. Propylene glycol and zinc acetate were obtained from Panreac and other materials were purchased from Merck All materials were used as received without further purification Glycolysis of PET Waste The PET waste was depolymerized with propylene glycol using 0.5% (w/w) zinc acetate, based on PET weight, as the major catalyst. The reaction was carried out at about 190 C under reflux and nitrogen blanket and the weight ratio of PET to PG was 1:1. The reactor was a four-necked, round-bottomed flask of 1-l capacity having a reflux condenser, thermometer and stirrer. Firstly, some experiments were carried out to evaluate the cyclohexylamine (CHA) effect. Then, the glycolysis of PET waste was carried out in six different reaction times, 3 to 8 h at 1-h intervals, with and without cyclohexylamine to investigate the progress of reactions in the presence of each kind of catalytic system. Glycolyzed products prepared by reaction in the presence of cyclohexylamine are coded as GPCi and without it are coded as GPZi. The products of glycolysis were analyzed to determine hydroxyl value and the amount of free glycol as follows Determination of Hydroxyl Value The hydroxyl values were determined by the conventional acetic anhydride/pyridine method [26] Removal of Free Glycol A weighed quantity of the glycolyzed products was extracted with water and filtrated. The filtrate containing water, free glycol and some soluble oligomers was concentrated by evaporation of water and then chilled to precipitate out of the watersoluble oligomers. The residue of filtration was dried in a vacuum oven Preparation of Unsaturated Polyesters The UP resin was prepared by reacting the glycolyzed product with maleic anhydride (MA) and phthalic anhydride (PA) to a fixed ratio of 1.2:1 for the hydroxyl to carboxyl groups. The hydroxyl number of the glycolyzed products before free glycol removal was used to determine the amount of anhydrides. The molar ratio

5 190 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) for MA to PA was 1:1. The reactor was a round-bottom flask equipped with a condenser, thermometer and stirrer. Reactants were heated from room temperature to 170 C in 1.5 h and the reaction was carried out at 170 C under reflux and nitrogen blanket. The polyester formed by GPCi are coded as UPRCi and prepared by GPZi as UPRZi. The acid value was determined by titrating the solution of the weighed quantity of resin in acetone, with about 0.1 M standard alcoholic KOH solution using tymol blue as indicator [5] Characterization Gel-Permeation Chromatography The molecular weights of the glycolyzed products and UPR were measured by gel-permeation chromatography (GPC; Model Agilent 1100, calibrated with polystyrene standards). Three successive columns (PLgel 10 µ, 102, 103, 104 Agilent, mm) were applied. All samples were dissolved in tetrahydrofuran (THF) of HPLC grade at the constant concentration of 0.1 wt%. After filtration of samples, 200 µl of each sample was injected into the columns. The flow rate of carrier solvent was set on 1.00 ml/min H-NMR 1 H-NMR spectra were recorded on a Bruker AVANCE-300 MHz spectrometer in d 6 -acetone solution FT-IR Spectroscopy A FT-IR spectrometer (Model Bruker-Eqinox 55) was used to identify the chemical structure of UP resins Intrinsic Viscosity The intrinsic viscosity of UP resins was measured in tetrahydrofuran (THF) at 25 C to evaluate the progress of polyesterification reactions. 3. Results and Discussion 3.1. The Effect of CHA on Glycolysis Since it was claimed by Goje et al. [15] that cyclohexylamine (CHA) has a considerable effect on increasing the rate of PET conversion, the role of CHA was investigated in glycolysis reaction. In the first step, CHA was solely used as the main catalyst, 0.5 wt% based on PET weight, in glycolysis reaction to investigate its catalytic effect on the reaction. The mixture in the flask was heterogeneous after 4 h which shows that CHA dose not have any independent catalytic role in the reaction. Some experiments were carried out for 3 h to study the influence of CHA as a co-catalyst on the glycolysis reaction. In all experiments, the weight ratio of zinc acetate to PET was 0.5 wt% and the weight ratio of CHA to PET was 0, 0.07, 0.14, 0.2, 0.27 and 0.34 wt%. The relationship between the hydroxyl value of glycolyzed

6 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) Figure 1. Effect of cyclohexylamine on the hydroxyl value of glycolyzed products. Figure 2. Hydroxyl value of glycolysis products as a function of glycolysis time in the presence of zinc acetate with and without cyclohexylamine. products and the amount of co-catalyst is presented in Fig. 1. Although an ascending depolymerization rate with CHA amount was expected, an optimum amount was observed. Results revealed that the depolymerization rate in 0.2 wt% of CHA is maximal and by adding more CHA, the reaction progress decreased. To evaluate the effect of the catalytic system (zinc acetate and CHA) on the glycolysis reaction, experiments were carried out in six different reaction times (3 8 h at 1-h intervals). The progress of depolymerization of PET in the presence of zinc acetate with and without cyclohexylamine is shown in Fig. 2. As expected, in the presence of CHA the depolymerization of PET proceeded faster. Moreover, glycolysis reactions with CHA proceeded to a greater extent, leading to oligoester diols having lower average molecular weight in comparison with reactions without CHA. The results confirm that cyclohexylamine has a profound co-catalytic effect on glycolysis reaction Analysis of Oligoesters Molecular weights of four samples of glycolyzed products were determined by endgroup analysis [32] and GPC, and the results are summarized in Table 1. GPC chromatograms obtained for the products are shown in Fig. 3.

7 192 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) Table 1. Molecular weight characterization by end group and GPC analysis Oligoester Catalyst Reaction M n (g/mol) a M n (g/mol) b M w (g/mol) b M w /M n time (h) GPZ1 ZnAc GPC1 ZnAc, CHA GPZ2 ZnAc GPC2 ZnAc, CHA a From V(OH) data. b From GPC data. Figure 3. GPC chromatograms of glycolyzed products: (a) GPC1, (b) GPC2, (c) GPZ1, (d) GPZ2. The results of end-group analysis and GPC reveal that GPCi has a lower molecular weight than that of GPZi. These results are in agreement with the hypothesis that cyclohexylamine has a co-catalytic effect on the glycolysis reaction in the presence of zinc acetate. The polydispersity index of the oligomers for GPCi products has a lower value than that of GPZi products. GPC chromatograms show that the peak weight average molecular weight of the glycolyzed product decreases during the course of reaction, yet the variation is not considerable. There are three main peaks in each glycolyzed product which suggest that the products maybe composed of three main kinds of components. The 4th peaks (in early time of reaction) in glycolyzed products obtained after 5 h are

8 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) weak in a way that they disappeared during reaction progress and the long-chain oligoesters decompose into oligomers having lower molecular weights. Furthermore, results obtained from end-group analysis and GPC suggest that the produced oligomers mainly have 1 3 monomeric units in which dimers predominately constitute them. Different structures of oligomers are proposed in Scheme 1 and their theoretical characteristics are shown in Table 2. Theoretically, the second structure (M2) constitutes a little proportion of glycolyzed products. The existence of these M2 structure oligomers was also confirmed by 1 H-NMR spectroscopy. It seems that the number of oligomers having Scheme 1. Proposed structures of glycolyzed products.

9 194 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) Table 2. Theoretical amounts of hydroxyl value and molecular weight of M1 andm2structures Structure Oligomer Hydroxyl value (mg KOH/g) M n (g/mol) M1 Monomer Dimer Trimer M2 Monomer Dimer Trimer Figure 4. 1 H-NMR spectrum of GPC2. M2 structure reduces during the progress of the reaction and its approaching to the equilibrium state Characterization by 1 H-NMR Spectroscopy The glycolyzed products were analyzed by 1 H-NMR spectroscopy. The 1 H-NMR spectrum of GPC2 is shown in Fig. 4 and the probable assignments in Table 3. 1 H-NMR spectroscopy reveals that the number of methyle protons is much greater than that of methylene protons. Therefore, the possibility of the first structure (M1) is much higher than that of second structure (M2). In other words, the number of oligomers or monomers with terminal hydroxyl groups that are related to ethylene glycol bases is much lower.

10 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) Table 3. Assignments of characteristic peaks in the 1 H-NMR spectrum of GPC2 Group δ (ppm) Integration I Aromatic proton Polyesterification The glycolyzed products were polyesterified with maleic anhydride (MA) and phthalic anhydride (PA). For UPRs prepared from both types of glycolyzed products, the variation of acid value as a function of reaction time is shown in Fig. 5. It is evident that the polycondensation of GPCi with anhydrides is faster than that of GPZi. Also, the greater variation of acid value in the polyesterification of GPCi is an indication of more acid consumption and faster reaction progress of UPRCi. Intrinsic viscosity of prepared polyester samples was measured to investigate the reaction progress more precisely. The intrinsic viscosity (IV) of polyester samples during the course of reaction is shown in Fig. 6. An increasing trend of IV was observed during the course of reaction. Moreover, it is evident that the increasing rate of IV related to UPRCi is greater than that related to UPRZi. In other words, the esterification reaction of GPC2 proceeds faster and it suggests that cyclohexylamine probably has a catalytic effect on polycondensation reactions Gel-Permeation Chromatography Two polyester samples (UPRZ1 and UPRC1) were characterized by GPC to precisely investigate the average molecular weight and its distribution for polyester resins prepared from two different types of glycolyzed products. GPC chromatograms and their numeric results of polyester resins are shown in Fig. 7 and Table 4, respectively.

11 196 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) Figure 5. Acid value of UPRs prepared by polyesterification reaction of GPZ2 (yielding UPRZi) and GPC2 (yielding UPRCi) as a function of time. Figure 6. Intrinsic viscosity of UPRs prepared by polyesterification reaction of GPZ2 (yielding UPRZi) and GPC2 (yielding UPRCi) as a function of reaction time. Figure 7. GPC chromatograms of UP resins: (a) UPRC1, (b) UPRZ1.

12 S. Chaeichian et al. / Designed Monomers and Polymers 11 (2008) Table 4. Characterization of UPRZ1 and UPRC1 UPR Glycolyzed Time of Acid value IV M n M w M w /M n product esterification (mg KOH/g) (dl/g) (g/mol) a (g/mol) a reaction (h) UPRZ1 GPZ UPRC1 GPC a From GPC data. The greater average molecular weight and broader molecular weight distribution of UPRC1 point out the higher reaction rate; this observation proves the catalytic effect of cyclohexylamine on polyesterification reactions FT-IR Spectroscopy FT-IR spectroscopy was used to analyze functional group especially double bonds (C=C) in prepared polyester resins. The FT-IR spectrum of UPRC1 and UPRZ1 are shown in Fig. 8. According to Chen and Chen [20] the bands at 982 cm 1 and 1646 cm 1 are assigned to the double bonds of the polyester chains which proves the unsaturated structure of UPR samples. Bands in the range of cm 1 are related to terminal hydroxyl and the peaks imply that the hydroxyl content of UPRC1 is lower; it shows the higher consumption of the hydroxyl group in the corresponding esterification reaction and more progress of the reaction. 4. Conclusion The glycolysis of recycled PET for the production of unsaturated polyester resin was investigated. Indeed, the effect of an aromatic compound (cyclohexylamine) as a catalyst was studied in both steps. The obtained results showed that cyclohexylamine has no independent catalytic role in the glycolysis of PET with PG. On the other hand, it has a co-catalytic role in the presence of zinc acetate, as main catalyst, and it can improve the rate of glycolysis. Moreover, it was found that there is an optimum concentration of cyclohexylamine as a co-catalyst (0.2 wt%, based on PET weight). Analysis results suggested that the produced oligomers mainly have 1 3 monomeric units in which the dimers are dominant. After degradation, the oligoester diols were mixed with maleic anhydride and phthalic anhydride to form alkyd resins. The obtained results implied that the synthesis of unsaturated polyester resin from glycolyzed product by co-catalytic system proceeds faster which means that cyclohexylamine has a profound catalytic effect on esterification reactions.

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