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TOLEDO (SPAIN)KINETIC STUDY OF THE GLYCOLYSIS OF FLEXIBLE POLYURETHANE WASTESUSING STANNOUS OCTOATE AS CATALYST
D. Simón, M.T. García, A. de Lucas, R. Mazarro and J.F. Rodríguez
Department of Chemical Engineering, University of Castilla-La Mancha
Faculty of Chemistry, Avda. Camilo José Cela s/n 13071. Ciudad Real, Spain.
e-mail: juan.rromero@uclm.es

Abstract
Polyurethanes represent one of the most important groups of plastics, and so the increasing quantity of wastes makes
their recycling an urgent task. Among the processes suitable, glycolysis, and specially glycolysis in two phases is the onethat allows better quality products. It has been demonstrated that stannous octoate is the catalyst that provides thefastest decomposition rate and allows to obtain a polyol with similar characteristic to fresh polyol. In this work a study ofthe main reaction parameters has been carried out. The optimal catalyst concentration and the ratio of glycolysis agent(DEG) to the treated PU foam have been determined.

Keywords: Polyol; Polyurethane; Recovery; Glycolysis; Octoate

1. Introduction
Polyurethanes (PU) have been used in diverse areas and are
one of the most important groups of plastics because of theirversatility. Flexible PU foams are the most important group
among PU specialties. They are widely used in furniture,
mattresses and automotive seats. As a direct consequence of
their commercial success an increasing quantity of wastes isdisposed by landfilling. By this reason its recycling is now anurgent task. Two-phase glycolysis has been demonstrated tobe the method to obtain the best quality products. The
glycolysis of PU´s is a chemical recycling process based on theexchange reaction of polyols and glycols in the urethane bond.
The process allows the breaking down of the PU matrix [1] andthe release of the polyol. One the most decisive aspects is thecatalyst choice. This reaction has been traditionally catalyzed
by alkanolamines, titanium compounds, or acetates. Later theemployment of catalysts based on potassium and calcium
octoates opened up a new way to catalyze the glycolysis ofpolyurethanes. Recently stannous octoate have showed proper
catalytic activity with an enhanced quality of the recoveredpolyols and faster decomposition rates. In this work the optimal
stannous octoate concentration and the ratio of glycolysis agent(DEG) to the treated PU foam have been determined.

2. Materials and Methods
Industrial samples of flexible PU foam based on polyether
polyol [poly(propylene oxide-block-ethylene oxide) Mw=3500,
functionality with respect to OH groups:3, PD=1,06] and toluene
diisocyanate (TDI) were scrapped with an arbitrary diameter
ranging from 5 to 25 mm. These foams had been prepared in
the presence of a cell regulator (surfactant), crosslinking agent,catalyst, colouring agent, mineral loads and water as a foaming
agent. The scrap foam was reacted in several mass ratios, with
diethylene glycol (DEG)(PS from Panreac,Spain) as glycolysis
agent. The ratios of glycolysis agent to PU foam ranged from
0.9 to 1.5 by weight. Stannous octoate was used as catalyst(potassium 2-ethylhexanoate 46,4 % by weight in decyl alcohol(isomers mixture), from NUSA, Spain).

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The glycolysis reactions were carried out in a jacketed 1 L flaskequipped with stirrer and refluxing condenser under nitrogen
atmosphere to avoid oxidation. The glycolysis agent was placedin the flask and when the temperature raised the desired level,
the required quantity of scrap foam was added during an hourby means of a continuous feeder, according to its dissolution.
The feeding rate selected was 5 g min-1. The zero time for thereaction was taken when all the foam was fed (aprox during 1h). Temperature was maintained constant during the feeding
and the reaction. Experiments were carried out in thetemperature interval 179-189ºC.
The reaction mixture was periodically sampled and analyzed.
Gel permeation chromatography (GPC) was used to determine
the molecular weight distribution as well as the concentration ofpolyol in the products. Measurements were performed using
Tetrahydrofurane (THF) as eluent and poly(ethylene glycol)
standards.

3. Results and Discussion
As reported in a previous work, the transesterification ofurethanes with the hydroxyl groups of glycols proceeds very
slowly in the absence of a catalyst [2]. This fact has beenmarked in the literature [3] for this kind of interchange reactions
in condensation polymers. For this reason the choice of catalyst
for the process is an important factor affecting the properties of
the recovered products and the time taken to reach thecomplete degradation of the PU chain. In literature the catalysts
used in PU glycolysis include Lewis acids and bases such asamines, hydroxides, and alkoxides [4]. Recently, theperformance evaluation of new catalysts based on alkalineearth octoates has been carried out. In a previous reportstannous octoate had been demonstrated to be the catalyst
with the fastest decomposition rate [5]. In addition, this catalyst
provides a polyol with a purity greater than those obtainedusing other catalysts previously employed for this kind ofreaction. The use of stannous octoate as transesterification
catalyst for the PU glycolysis has provided a novel way toenhance the polyol recovery. In this work some of the main
reaction parameters have been studied.

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6th International Symposium on Feedstock Recycling of Polymeric Materials

3.1. Catalyst concentration

3.3. Influence of temperature

Degradations reactions of the PU foams were carried out in thesame conditions, just varying the amount of catalyst added. InFig.1 the evolution of polyol concentration in the upper phasewith the reaction time is depicted as a function of the catalystamount used.

The beginning of thermal degradation of PU compounds andtheir derivatives is around 120-250 ºC. This range is strongly
influenced by the physical characteristic of the PU. With the aimof minimizing depolymerisation reactions the maximum
glycolysis temperature for the study was set at 189ºC that wasdetermined to be the optimal temperature in a previous report[7]. Fig.3. represents the evolution of polyol concentration in theupper phase with reaction time.

Fig.1. Evolution of polyol content in the upper phase during theglycolysis reaction of PU foams for different catalyst concentrations, inthe presence of DEG as glycolyzing agent. TR=189ºC; Wglycol ag./WPU=1.5

As can be seen, there is an amount of catalyst (6 g) from which
to up the reaction rate remain more or less constant. That is,
the addition of more catalyst does not improve the reaction rate,approaching zero order behavior [6]. For this reason the use of6 g (catalyst concentration in the glycolysis agent=1.3 %)of
stannous octoate would be the best choice.

3.2. Mass ratio of glycolysis agent
In a two-phase glycolysis of PU, the amount of glycol added ismuch larger than the stoichiometric quantity required toproduce the transesterification. The excess is used not only todisplace the equilibrium to the glycol substitution, but also topromote the phase splitting which allows the obtaining of apolyol-rich phase. However, a too large excess of glycol would
imply huge volume equipment requirements and largeramounts of bottom phase that has to be recycled by distillation.
In order to determine the optimal amount of glycol a series ofdegradation experiments were carried out. In these reactions all
the parameters were kept constant just varying the mass ratioof glycolysis agent to PU foam. Fig.2. shows polyol
concentration history in the upper phase.

Fig.2. Evolution of polyol content in the upper phase during theglycolysis reaction of PU foams for different ratios of glycolysis agent(DEG) to the treated foam. TR=189ºC; Catalyst concentration in theglycolysis agent=1.3%.

It can be observed that a mass ratio of glycolysis agent to PU
foam of 0.9:1 (500 g of PU foam) provides a low polyol content
in the upper phase. Mass ratios 1.125:1 (400 g of PU foam) and1.5:1 (300 g PU foam) let to obtain a similar polyol. The only
difference is the reaction time. Reaction time with the mass
ratio 1.125:1 is only ten minutes more than ratio 1.5:1. Thismeans that glycolysis reactions must be carried out at a mass
ratio glycolysis agent to PU foam of 1.125:1.

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Fig.3. Evolution of polyol content in the upper phase during theglycolysis reaction of PU foams at different temperatures. Catalyst
concentration in the glycolysis agent=1.3%; Wglycol ag./WPU=1.5.

Reactions carried out at 179ºC and 184ºC provides a slow
recovery process, as demonstrated by the low percentage ofpolyol in the upper phase. It has been demonstrated that aslight decrease in the temperature affects strongly todecomposition rate. Take into account this fact, the optimal
reaction temperature would be 189ºC.

4. Conclusions
Stannous octoate shows a proper performance as catalyst in
the glycolysis of PU wastes. As it had been reported in aprevious report, an increase in the reaction temperature andcatalyst concentration enhances the degradation rate. It has
been proved that the optimal conditions to carry out theglycolysis reaction are the following ones: Catalyst
concentration in the glycolysis agent =1.3 %; Mass ratio ofglycolysis agent to PU foam=1.125:1; Reaction
temperature=189ºC.

References[1] Modesti, M. Advances in Urethane Science and Technology
vol. 13, Tecnomic Publising CO, Lancaster, 1996, pp. 237-288.
[2] Molero. C, de Lucas. A, Rodríguez. JF. Recovery of polyols
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with new catalysts. Polym Degrad Stab 2006; 91:894-901.
[3] Kricheldorf HR, Denchev Z. Interchange reactions incondensation polymers and their analysis by NMR
spectroscopy. In: Fakirov S, editor. Transreactions incondensation polymers. Weinheim: Wiley-VCH;1999. p. 1-78.
[4] Bauer G (1991) Alkoholyse-Chemisches Recyclingverfahren
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[6] Stier U, Gähr F, Opperman W. Kinetics of transesterificationof dimethyl 2,6-naphthalenedicarboxylate with 1,3-propanediol.
J Appl Polym Sci 2001;80:2039-46.
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