13th International ConferenceResearch and Development in Mechanical Industry

RaDMI 201312 - 15. September 2013, Kopaonik, Serbia

THE KINETIC STUDY OF PET GLYCOLYSIS REACTION

Milica Rančić1, Jelena D. Rusmirović1, Svetlana D. Pešić1, Dušan M. Janković2, Enis S. Džunuzović3, Pavle M. Spasojević3, Aleksanddar D. Marinković3

1 Faculty of Forestry Science, University of Belgrade, Kneza Višeslava 1, 11030 Belgrade, SERBIA2 Military Academy, Pavla Jurišića - Šturma 33, 11000, Belgrade, SERBIA

3 Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11120 Belgrade, SERBIA

Summary: Polyethylene terephthalate (poly(ethylene terephthalate), PET, is a thermoplastic polyester resin and is used in synthetic fibers. Due to the presence of the ester functional group, PET polymer can react with different reagents and in that process, macromolecular polymer chains split into shorter oligomer chains. Following reagents can be used: water (hydrolysis process), alcohols (alcoholysis), two hydroxyl alcohols (glycolysis) and different amines (aminolysis). Herein, we report the investigation of kinetics of the glycolysis reaction, i.e. chemical depolymerization of the PET polymer, the reaction that takes part in the PET recycling proccess. The glycolysis reaction was performed with different diols, such as DEG, DG, DPG, Gly, TMP, TEG in order to study the kinetics of the PET glycolysis reaction and the influence of the type of the glycol reagent and catalyst.

Keywords: poly(ethylene terephthalate) (PET), recycling of PET

1. INTRODUCTION

Polyethylene terephthalate (poly(ethylene terephthalate), PET, is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination with glass fiber (CAS 25038-59-9). Structural part of polyethylene terephthalate is presented in Fig.1.

Figure 1. Structural formula of polyethylene terephthalate

Depending on its processing and thermal history, polyethylene terephthalate may exist both as an amorphous (transparent) and as a semi-crystalline polymer. The semicrystalline material might appear transparent (particle size < 500 nm) or opaque and white (particle size up to a few microns) depending on its crystal structure and particle size. Its monomer (bis-β-hydroxyterephthalate) can be synthesized by the esterification reaction between terephthalic acid and ethylene glycol with water as a byproduct, or by transesterification reaction between ethylene glycol and dimethyl terephthalate with methanol as a byproduct. Polymerization is through a polycondensation reaction of the monomers (done immediately after esterification/transesterification) with water as the byproduct [1].

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The majority of the world's PET production is for synthetic fibers (in excess of 60%), with bottle production accounting for around 30% of global demand. In the context of textile applications, PET is referred to by its common name, "polyester," whereas the acronym "PET" is generally used in relation to packaging. Polyester makes up about 18% of world polymer production and is the third-most-produced polymer; polyethylene (PE) and polypropylene (PP) are first and second, respectively. PET consists of polymerized units of the monomer ethylene terephthalate, with repeating C10H8O4 units. PET is commonly recycled, and has the number "1" as its recycling symbol. Due to the presence of the ester functional group, PET polymer can react with different reagents and in that process, macromolecular polymer chains split into shorter oligomer chains. Following reagents can be used: water (hydrolysis process), alcohols (alcoholysis), two hydroxyl alcohols (glycolysis) and different amines (aminolysis) [2-7]. Herein, we report the investigation of kinetics of the glycolysis reaction. This paper presents the study of the kinetics of the PET glycolysis reaction with different glycols in order to study the influence of the catalyst and structure of the glycol reagent.

2. TECHNICAL REQUIREMENTS

2.1. Materials

All chemicals were purchased from Fluka, Sigma-Aldrich and Zorka-pharma and used as received and Eurocat 9555 catalyst was purchased from Cores System d.o.o., Belgrade. 2.2. Instrumental analysis

Gas chromatographic analysis was performed on Varian 3400 apparatus which is equipped with flame ionization detector and a coloumn filled with OV-101with lenght of 2 m and diameter of 0.3175 cm (1/8’’). Analysis conditions: -Injector temperature: 250 C;-Detector temperature: 270 C;-Coloumn temperature: 80 oC (1 min) → 10 oC/min → 200 oC (15 min)-Carrying gas: nititrogen (purity 99,99%) - flow 1 cm3/min.-Airflow: 250 cm3/min (purity 99,99%);-Hydrogen flow: 25 cm3/min (purity 99,99%).The UV absorption spectra were measured with a Shimadzu 1700 UV/Vis spectrophotometer.

2.3. Method of catalytic PET depolimerization

The appropriate amounts of PET, glycol and catalyst (Eurocat 9555) were placed in a four-necked flask (250 ml or 500 ml) equiped with reflux condenzator, mechanical stirrer, termometer and system for N2

introduction. The glycolysis reaction was performed during 5 h at 205-220 ºC. Different glycols were used: diethylene glycol (DEG), propylene glycol (PG), dipropylene glycol (DPG), triethylene glycol (TEG), trimethylolpropane (TMP) and glycerol (Gly), with different glycol/PET molar proportion. When reaction ended, the glycolysis product was poured out, cooled down and, successively, purificated by dissolving in appropriate amount of dichloromethane, washed out twice with destilled water and dried with anhydrous sodium sulphate. After vacuum filtration, dichloromethane was removed by destilation and product was additionally dried on vacuume to remove water completely, as well as residual ethylene glycol.

3. RESULTS AND DISCUSSION

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3.1. PET glycolysis kinetics in the presense Eurocat 9555 catalyst

The chemism of the PET depolimerisation reactions, which could cotribute the overall kinetics of the investigated system, is presented on Figure 2. As it can be seen from the reaction pattern presented on the Figure 2, reaction system is very complex, so some assumptions should be made [8-15].

T: Structural unit of terephtalic acid -O2CC6H4CO2-E: Structural unit of ethylene glycol -CH2CH2-D: Structural unit of glycolysis reagent

Figure 2. The PET glycolisys reaction

Starting from the PET and glycolysis reagents, diols (HODOH), following changes can be expecting: 1. Glycolysis of the PET chains gives two kinds of oligomers with hydroxyl groups at their terminals, one

with DOH group and the other with EOH group (reaction 1; k1).2. Futher glycolysis of the PET chains, by glycolysis reagents or oligomers with terminal hydroxyl

groups, gives shorter chains. During the first reaction, HODOH is consumed and diol structural unit is embeded into chains. The extraction of EG occurs in reactions 6 (k12) and 4 (k7).

3. The formation of statistic copolymer structure is the consequence of the large number of equilibrium reactions.

At the beginning, the mixture consists of two phases: solid (PET) and liquid (HODOH). If the chemical structure (molar mass and composition) of the polyester is soluble, they transit to the solution what has two main consequences for the liquid phase: Increasing of the UV absorbance because of the presence of the terephthalic acid structural units Decreasing of the HODOH concentration as the consequence of the reaction with polyestersTherefore, important data can be obtained by analyzing of liquid phase: The UV absorbance show the portion of dissolved polyesters Free glycols, HODOH and MEG, can be determined by gas chromatography

3.2. The PET glycolysis with different glycols

The PET glycolysis with DEG, DPG, Gly and PG was performed at 220 oC without catalyst and at 190 oC with Eurocat 9555 catalyst. Using of the large glycol amount is necessary for the adequate dissolution of the PET flakes. Molar ratio, glycol/PET=4/1. Samples were analyzed by gas chromatography to follow the portion of free glycols formed. While EG is the reaction product, and its amount range from zero, characteristic marginal value determined by equilibrium, the change of the amount of glycolysis reagent (HODOH) is less pronounced. At the beginning of the reaction, PET is insoluble, so glycolysis reagent presents 100%. Afterwards, its amount decreases to the marginal value, because of pseudo-dissolving of polyesters in macroscopic homogenous solution and the incorporation of glycol structure units in polyestar chains by chemical reaction. Four typical glycolysis curves are presented on Figure 3.

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Figure 3. Typical curves of the decreasing of the glycol portion during the glycolysisreaction

Curve (a): Fast decreasing of glycol portion at the beginning corresponds to the complete dissolving of polyester without chemical reaction, while futher slow decreasing corresponds to the chemical reaction. This corresponds to non-catalyzed glycolysis reaction with DPG.

Curve (b): Very small initial glycol portion decreasing corresponds to weak polyester dissolving. It can be noticed that reactivity and solubility are very weak (e.g. Gly).

Curve (c): Fast decreasing of the glycol portion, where chemical reaction and polyester dissolving occur at the same time (e.g. DEG at 220 oC without catalyst).

Curve (d): Similar decreasing as for curve (b) (e.g. PG).

3.3. Non-catalyzed glycolysis at 220 oC

It was derived that increasing the EG portion is much faster with DEG, considering that for the other glycols, it takes more than 5 hours until the stabilization of the reaction and more than 20 hours till the end of the reaction. Depending on the glycol used, there is also difference in the initial time of the reaction:

DEG - 15 min. DPG - 4 h i 30 min. Gly - 90 min. PG - 90 min.

This results imply that non-catalyzed PET glycolysis with DPG, Gly and PG is slow reaction because of weak DPG chemical reactivity, while in Gly and PG, PET has low solubility, as well as polyester oligomers. Order of reactivity in non-catalyzed glycolysis at 220 oC is:

DEG >> PG >Gly>> DPG

3.4. Catalyzed glycolysis at 190 oC

The EG formation is faster during glycolysis with DEG than with DPG (Figure 4). The portion of EG reaches equilibrium value after one hour during glycolysis with DEG, while the time required for achieving the EG eqiulibrium value during glycolysis with DPG is 2 h i 30 min. It can be also noticed, that initial reaction time with DPG is 15 min. For the reactions with Gly and PG, reaction times are much longer- for Gly 2 h and for PG 90 min. After that time, the EG portion continiously increases unless it reaches 5 % from marginal value 18 hours after the beginning of the reaction.

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Figure 4. Changes of the EG portion (left) and UV absorbance (right) during the PET glycolysis reaction at 190 oC with catalyst

From the figure 4. it can be seen that the UV absorbance increases much faster and it is stabilized faster with DEG (50 min) and DPG (75 min). As said, Gly and PG exhibit different behavior compared to DEG and DPG because they need much more time for the stabilization of the UV absorbance Similar results are obtained in the literature [16]. On the basis of the obtained results, it can be concluded that reactivity order for catalyzed glycolysis at 190 oC is following: DEG >DPG>> PG >Gly

3.5. The PET glycolysis kinetics with DEG

Reactions were carried out in four-necked reactor from 500 ml, equipped with condensator, termometer, mechanical stirrer, aparatures for the introduction of N2 and heater. 212g (2 mol) of DEG and 0,48g (1,41 mmol) or 0 g of Eurocat 9555 catalyst were added in the reactor and heated to the corresponding temperature (190-220 oC). 196 g (0,5 mol) of PET were heated to the same temperature before added in the reactor (beginning of the reaction) The mixture was stirring at 350 rpm. The sampling of the liquid phase was carried out in the equal time intervals and the samples were successively analyzed. The reaction kinetics was followed analyzing the liquid phase: Free glycols were analyzed by means of gas chromatography- decreasing of DEG and increasing of EG. Terephthalic acid concentration was followed by the UV spectroscopy after dissolving in THF (50 mg

of liquid phase in 10 ml THF).GC experiments: 300 mg of sample were measured and added to the solution wich contains acetic acid anhydride and pyridine in the ratio 50:50 v/v and 70 mg 1,4-butanediol (internal standard). The soltion was left for one hour at the room temperature before it is injected into the cromatograph at 100-165 oC. The UV spectra were recorded on Shimadzu 1700 spectrophotometer from the solutions which contain 50 mg of the samples in 10 ml THF as solvent and absorbance at 293 nm.

3.6. The influence of temperature for non-catalysed reactions

From previous results, it can be concluded that DEG showed as the most reactive glycol for the PET glycolysis. From that reason, further investigation (influence of the temperature and catalyst) of the glycolysis reaction was performed with DEG. Non-catalyzed PET glycolysis with DEG was investigated at different temperatures between 190 oC and 220 oC. At temperatures of 190 oC and 200 oC, the mixture was tranformed to heterogenious white liquid phase in which the particles of polyester were dispersed. At 210 oC, the liquid phase has white colour at the beginning, becoming more and more clear as the reaction proceeds unless it becomes tranparent after 270 min. At 220 oC, the mixture becomes clear much faster. GC results show decreasing of DEG and increasing EG in the mixture. (The UV absorbance change during the PET glycolysis reaction at 190, 200, 210 and 220 oC) and other data are presented in Table 1.

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Table 1. Influence of temperature on the PET glycolyse reaction with DEG

Catalyst T (oC)Approximate time of the end

of the reaction (min)t1/2 (min)1

Formation of EG UV absorbance Formation of EG UV absorbanceEurocat 9555 190 40 38 10 9,5

Without catalyst

190 >350 >300 200 170200 350 >300 120 60210 260 120 100 30220 140 25 50 13

1 Time for achieving the half of the terminal value

On the basis of obtained results, it can be concluded:1. The reactivity is much higher at 220 oC compared to lower temperatures2. The reactivity is much lower at 190 oC and 200 oC at which the liquid phase remains heterogenously white even after 5 hours of the reaction, and if the heating is continued, immediately becomes clear when temperature reaches 210 oC3. At lower temperatures, in the solid phase, the reactions are slower compared to the same reactions in the liquid phase.

3.7. The influence of the catalyst

The influence of Eurocat 9555 catalyst for the PET glycolysis reaction with DEG was investigated at 190 oC (beyond 190 oC, the raction is too fast with the catalyst). All aspects are faster with the catalyst and, also, increasing the portion of free glycols (DEG and EG) (Figure 5) and the UV absorbance have the parallel course until stabilization after 40 min.

Figure 6. The increasing the EG portion during the PET glycolysis reaction at 190 oC with and without catalyst

Therefore, in the liquid phase, the catalyst has the greater influence on the chemical reactivity, while the kinetic controll of the reaction in the solid phase is more efficient.

4. CONCLUSION

The main goal of this study is investigations of the kinetics of PET waste glycolysis by solvolytic reagents of different chemical structures (DPG, Gly, DPG and PGP) with and without catalyst eurocat 9555. Glycolysis of PET is a complex system of reactions, since it involves reactions in two phases (solid and liquid), followed with multiple alcoholysis reactions. The kinetic results presented here are obtained from

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chemical analysis of the liquid phase. The rate of PET glycolysis depends on the chemical composition of the solvolytic system (catalyst or not). Also it could be observed that effect of the Eurocat 9555 catalyst on the chemical reactivity is far more intense for DPG than for other used glycols. In the conclusion, the order of reactivity for both uncatalyzed and catalyzed glycolysis at 220 and 190oC, respectively, is DEG > DPG > PG > Gly.

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