5-7 OCTOBER

ISFR

2011

117

TOLEDO (SPAIN)CHEMICAL DEPOLYMERIZATION OF PET COMPLEX WASTES: HYDROLYSIS vs.
GLYCOLYSIS
M. Arieta-Araunabeña1 , S. Arnaiz 1, A. Asueta 1, I. Robertson1, L. Martínez 2, A. Aguado 2
1GAIKER Technology Center, Parque Tecnológico, 202, 48170 Zamudio (Spain)
2CARTIF Centro Tecnológico, Environmental Division, Parque Tecnológico de Boecillo 205, 47151 Boecillo (Spain)

e-mail1: arieta@gaiker.ese-mail2: lidmar@cartif.esAbstract
The huge increase in the generation of post-consumer plastic wastes has produced a growing interest about eco-efficient
strategies and technologies for their appropriate management and recycling. In response to this PROQUIPOL Project isfocused on developing, optimizing and adapting feedstock recycling technologies as an alternative of management for thetreatment of complex plastic wastes. Among the different plastic wastes studied, PROQUIPOL Project is working on
providing a suitable treatment to the highly colored and complex multilayered post-consumer waste fractions of PET bychemical depolimerization methods. Glycolysis and alkali hydrolysis processes have been studied with the aim of promoting
the transformation of PET into bis (2-hydroxyethyl) terephthalate (BHET) monomer and terephthalic acid (TPA) respectively.
In both cases operational conditions such as temperature, reaction time, catalyst to PET rate and solvent to PET rate havebeen considered to optimize product yield, achieving values nearly to 90% and monomer purities over 95% in bothprocesses. This paper presents results obtained for each treatment as well as a simplified comparison of technical, economicand environmental issues.

Keywords: depolimerization, alkali hydrolysis, glycolysis, PET waste

1. Introduction
Needs of content preservation have been responsible forthe development of barrier materials and marketingstrategies favored the introduction of high colored materials
associated to brands and quality perception. Currently, one
emerging challenge associated to PET recycling, is toprovide a suitable treatment to the highly colored andcomplex multilayered post-consumer waste fractions ofPET. These fractions are not appropriate for mechanicalrecycling because derived products present limitations
either in color, transparency or intrinsic viscosity, restricting
their applications and value. In accordance with this,PROQUIPOL project is assessing the glycolysis(degradation of PET with ethylene glycol by a catalytic
reaction to promote the formation of BHET [1]) and alkalinehydrolysis (degradation of PET by an aqueous solution ofNaOH to promote the formation of TPA [2]) as alternativesof management for these non-mechanically recoverable
fractions.

2. Materials and Methods
2.1. Materials
During this research work virgin PET supplied by
NOVAPET and samples of highly colored and complex
multilayered post-consumer PET wastes provided by
ECOEMBES, the Spanish integrated management system
for the light packaging waste, have been used. The caustic
soda used in hydrolysis runs has been purchased fromPANREAC. Meanwhile, glycolysis runs have employed EGinteriores libro.indd 117

purchased from JABER and Zn(Ac)2•2H2O as catalyst (zincacetate dihydrate 99.5%) from SCHARLAU.
2.2. Glycolysis process
Glycolysis experiments have been carried out in a 2 L
stainless steel reactor. It has an electrical resistance to heatand control the temperature in the reactor, and a stirrer toensure good mixing properties. The system can operate ata pressure of 6 bar. In a first step, a factorial design ofexperiments has been carried out with virgin PET toestablish the better conditions for the process. Then,optimum experiment has been repeated with postconsumer PET. Factors under study in the factorial designof experiments have been process temperature (195, 208and 220°C), reaction time (150, 180 and 210 min), EG/PET
ratio (2, 4 and 6 w/w) and ZnAc/PET (0.2, 0.6 and 1.0%w/w). After glycolysis reaction, boiling water has been
added in excess to the reaction mixture under vigorous
agitation. Then, the mixture has been filtrated obtaining asolid subproduct above the filter and a liquid phase. This
recovered liquid has been refrigerated to ensure thecrystallization of the BHET. A second filtration has been
performed to separate BHET crystals from EG. BHET has
been dried at 65°C until constant weight.
2.3. Alkaline hydrolysis process
Hydrolysis experiments have been carried out in a 1.8 L
batch operated stainless steel reactor with a maximum
operating pressure and temperature of 250 bar and 500°C,respectively. Reaction system included automated control,real time parameter monitoring, gas feeding lines and26/9/11 13:13:08

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6th International Symposium on Feedstock Recycling of Polymeric Materials

3. Results and Discussion
3.1. Glycolysis process results
As a result of the factorial design of experiments (DOE), theinfluences of temperature, reaction time, EG:PET andZnAc:PET weight ratios have been achieved. Considering
optimal operational conditions (195°C, 150 min, 6:1 and 1.0%w/w of catalyst), runs with post-consumer PET andrecycled EG have been carried out. The results prove theeffect of PET nature and reutilization of solvent over BHET
yield, decreasing from 90%w to nearly 82%w in both cases(see Figure 1).

10080

TPA yield, (%)

manual sampling for liquids, vapours and gases. In all runs,
40 g of PET have been charged to the reaction system.
Firstly, a set of reactions with virgin PET have been carriedout to assess the influences of temperature, particle sizeand PET:NaOH ratio. Finished the reaction, the mixture has
been filtered to remove non-reacted PET. After filtration,HCl has been added to promote the formation andprecipitation of TPA. The solution has been filtered again
where the solid phase, composed of TPA, has beencleaned with water to remove possible solid elements anddried at 80°C during 2 hours for later analysis.
2.4. Analysis of products
A FTIR spectrophotometer and a DSC thermal analysis
have been used to identify chemical structure and todetermine purity of reaction products, BHET and TPA.

virgin PET

6040200
Highly colored

MultilayeredFigure 2. TPA yield in PET hydrolysis process.

4. Conclusions
A technical comparison between the two chemical recycling
techniques shows that both achieved similar yields andpurities to respective products (TPA and BHET). Regarding
economic and environmental aspects, energy consumption
in the glycolysis process can be slightly higher due toboiling water addition and EG recovery, although raw
material cost is highly reduced. On the other hand, alkalinehydrolysis implies the use of HCl with the subsequently costand environmental issues in the waste management.
GLYCOLYSIS PROCESS FLOWCHARTMIXER

REACTORPRECIPITATORDISTILLATION

Cooled water
Hot waterPETWater

TC

Water

FIC

FilterTC

Filter

BHET
H2O

TC

EG+ H2O

Solid subproducts +Non-reacted PET

Solvent

EG100EGALKALINE HYDROLYSIS PROCESS FLOWCHART
REACTORBHET yield, (%)

MixtureECOEMBES post-consumer PET

PRECIPITATOR

80
PET
Water

FilterFilterTPA

virgin PET

60

TC

Non-reacted
PET

40

EG+ H2O
NaOH

HCl

20Figure 3. Industrial flowcharts of studied processes.

0Virgin PET with recycled EGECOEMBES post-consumer
PET

Figure 1. BHET yield in PET glycolysis process.
FTIR and DSC analysis of obtained BHET have been
compared with the derived from commercial BHET. Results
verify that chemical structures match up with thecommercial one and that purity reaches values near to 96%
in all cases.
3.2. Alkaline hydrolysis process results
After a set of reactions with virgin PET, the influences oftemperature, PET: NaOH molar ratio and particle size havebeen assessed to select later the appropriate operationconditions. According to results obtained for the alkalinehydrolysis of post-consumer highly colored and complex
multilayered PET in optimum conditions (150°C, 1-0.5 mm,2.4:1), PET nature doesn’t show a remarkable influence on
TPA yield, achieving in all cases yields similar or higher
than 90% (see Figure 2).
Characterization of alkaline hydrolysis products has been
analogous to that made in glycolysis. FTIR and DSC results
confirm that chemical structures are identical to commercial
one and purity values are over 95% in all cases.

interiores libro.indd 118

Finally, despite all, it can be concluded that economic
feasibility of both processes will be established bysocioeconomic factors such as PET consumption, the mostconsumed PET, virgin and recycled PET price or evenmechanical recycling situation.

References[1] C.Y. Kao, W.H. Cheng. Investigation of catalytic
glycolysis of polyethyelene terephthalate by differential
scanning calorimetry. Thermochimica Acta 292 (1997) 95[2] D. Paszun and T. Spychaj. Chemical Recycling ofPoly(ethylene terephtalate). Ind. Eng. Chem. Res. 36(1997) 1373-1383.

Acknowledgments
The authors wish to acknowledge the confidence andeconomic support of the Spanish Ministry of Education andScience to this work by financing the PROQUIPOL Project“Reference PID-560620-2009-4” and the supply of actual
post-consumer waste samples of PET by ECOEMBES.

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