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Recovery of polyols from flexible polyurethane foam by split-phaseglycolysis: Study on the influence of reaction parameters

Carolina Molero, Antonio de Lucas, Juan F. Rodrguez*

Department of Chemical Engineering, University of Castilla-La Mancha, Avda. Camilo Jose Cela s/n, 13004 Ciudad Real, Spain

Received 11 October 2007; accepted 30 November 2007

Available online 8 December 2007

Abstract

The general purpose of polyurethane chemical recycling is to recover constituent polyol, a valuable raw material. Among suitable processes,

glycolysis, especially in two phases, allows better quality products. Potassium octoate a compound derived from cycloaliphatic carboxylic acids

shows suitable catalytic activity.

A detailed study of the main reaction parameters affecting the reaction and properties of the recovered polyol has been carried out. They

include carboxylate catalyst concentration, reaction temperature and mass ratio of treated foam to the glycolysis agent.

An increase in the reaction temperature and catalyst concentration enhances the degradation rate, however, it also affects the process

negatively by promotion of secondary reactions and contamination of the polyol phase. Related to the glycolysis agent amount, the minimum

quantity required to split the phases has been determined, as well as the optimal ratio.

2007 Elsevier Ltd. All rights reserved.

Keywords: Polyol; Polyurethane; Recovery; Glycolysis

1. Introduction

Polymeric compounds with several hydroxyl groups,

known as polyols, are essential components in the manufac-

ture of polyurethanes (PU). Polyether polyols generally have

a molecular range of 180e8000 g mol1 and are produced

by the polyaddition reaction of alkylene oxides like propylene

oxide (PO) and ethylene oxide (EO). By modifying molecular

weight and functionality, polyols provide a diverse range of

high performance properties in PU materials. Flexible PUfoams are the most important group among PU specialties,

reaching the 29% of the total production. They are widely

used in furniture, mattresses and automotive seats. Because

PUs are used in many everyday applications and industrial

uses, their wastes cause economical and environmental

problems. Such wastes comprise not only post-consumer

products but also scrap from slabstock manufacturing, which

can reach the 10% of the total foam production. An alternative

approach to landfilling, is chemical recycling to convert the

PU back into its starting raw materials, especially polyols.

Hydrolysis, treatment with esters of phosphoric acid, ami-

nolysis with low weight alkanolamines and glycolysis have

been described as suitable procedures to break down the PU

chain [1e6] by transesterification. Most of these processes

produce a liquid mixture of products containing the recovered

polyol and other hydroxyl active groups, which can be used

only in blending with raw materials. Nevertheless, betterquality products can be achieved from flexible PU foams using

a two-phase glycolysis, enabled by the higher molecular

weight of polyols used in this kind of PU. By means of an

excess of glycolysis agent, much larger than the stoichiometric

quantity, the reaction product splits in two phases, where the

upper layer is mainly formed by the recovered polyol from

the PU and the bottom layer by the excess of glycolysis agent

and reaction by-products[7]. In previous work[8,9], the two-

phase glycolysis of flexible PU foams treatment has been

investigated, in relationship with the glycolysis agent and* Corresponding author. Tel.:34 926 295300x3416; fax:34 926 295318.

E-mail address: juan.rromero@uclm.es(J.F. Rodrguez).

0141-3910/$ - see front matter 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2007.11.026

Available online at www.sciencedirect.com

Polymer Degradation and Stability 93 (2008) 353e361www.elsevier.com/locate/polydegstab

mailto:juan.rromero@uclm.eshttp://www.elsevier.com/locate/polydegstabhttp://www.elsevier.com/locate/polydegstabmailto:juan.rromero@uclm.es

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the catalysts. The primary aim of these studies was to establish

the influence of the glycolysing compound on the process and

the quality of the recovered polyol. The works reported the use

of alkaline carboxylates as novel catalysts for the transesteri-

fication of PUs, providing in the presence of diethylene glycol

(DEG), a recovered product suitable to be foamed again after

a slight purification[10].After the first approach to the novel polyether polyol recov-

ery process, a detailed study of the main reaction parameters

affecting the reaction and properties of the recovered polyol

has been carried out. It includes carboxylate catalyst concen-

tration, reaction temperature and mass ratio of treated foam

to the glycolysis agent.

2. Experimental

2.1. Materials and methods

Industrial samples of flexible PU foam based on polyether

polyol [poly(propylene oxideeblockeethylene oxide) Mww3500, functionality with respect to OH groups: 3] and

toluene diisocyanate (TDI) were scrapped with an arbitrary

diameter ranging from 5 to 25 mm. These foams had been pre-

pared in the presence of a cell regulator (surfactant), crosslink-

ing agent, catalyst, colouring agent, mineral loads and water as

a foaming agent. The scrap foam was reacted in several mass

ratios, with diethylene glycol as glycolysis agent (DEG) (PS,

from Panreac, Spain). The ratios of glycolysis agent to PU

foam ranged from 0.75 to 2 by weight. Potassium octoate

was used as catalyst (potassium 2-ethylhexanoate 46.4% by

weight in decyl alcohol (isomers mixture), from NUSA,

Spain).The glycolysis reactions were carried out in a jacketed 1 L

flask equipped with stirrer and refluxing condenser under

nitrogen atmosphere to avoid oxidation. The glycolysis agent

was placed in the flask and when the temperature raised the

desired level, the required quantity of scrap foam was added

during an hour by means of a continuous feeder, according

to its dissolution. The zero time for the reaction was taken

when all the foam was fed. Temperature was maintained

constant during the feeding and the reaction. Experiments

were carried out in the temperature interval 175e195 C.

2.2. Characterization

At given time intervals aliquots were sampled, cooled and

centrifuged to ensure the total separation of phases. They

were dissolved in tetrahydrofuran (THF from Panreac, Spain)

at a concentration of 1.5 mg mL1 and then filtered (pore size

0.45 mm). Gel permeation chromatography (GPC) was used to

determine the molecular weight distribution (MWD) as well as

concentration of polyol in the products. Measurements were

performed with a Shimadzu chromatograph (Kyoto, Japan)

equipped with two columns (Styragel HR2 and Styragel

HR0.5) using THF as eluent at 40 C (flow: 1 mL min1)

and a refractive index detector. Poly(ethylene glycol) stan-

dards (from Waters, USA) were used for MWD calibration

and mixtures of the industrial starting polyether polyol and

DEG were used as concentration standards. The glycolysis

products were separated and their properties analyzed.

Hydroxyl number and acidity were determined by standard

titration methods (ASTMD-4274-88 and ASTMD-4662-93,

respectively). Amine values in products were determined by

a titration method based on ASTMD-2073-92, and the solventwas changed for a mixture of 1:1 tolueneeethanol. Water

content was determined by the Karl-Fisher method and the

viscosity was measured by a Brookfield LVTDV-II rotational

viscometer. All chemicals used in these analyses were of the

quality required in the standards. Chemical structures of gly-

colysate products were studied by Fourier Transform Infrared

Spectroscopy using a Perkin Elmer 16PCFT-IR spectrometer;

droplet samples were impregnated on KBr discs.

3. Results and discussion

3.1. Catalyst concentration

As reported in a previous work, the transesterification of

urethanes with the hydroxyl groups of glycols proceeds very

slowly in the absence of a catalyst [9]. This fact has been re-

marked in the literature[11]for this kind of interchange reac-

tions in condensation polymers. Catalysts can reduce the time

required to complete the polyol recovery process, however, the

amount used should be studied, at least due to economic

reasons. The use of potassium octoate as transesterification

catalyst for the PU glycolysis has provided a novel way to

enhance the polyol recovery, and the following step previously

to its implantation comprises a study in deep of main reactionparameters.

Firstly, degradation reactions of the PU foams were carried

out in the same conditions, just varying the amount of catalyst

added. InFig. 1the evolution of urethane oligomer content in

the polyol phase with reaction time is depicted as a function of

0 50 100 150 200 250 300 350

0

5

10

15

20

25

30

%b

yweight

reaction time (min)

0 %

1.1%

2.2 %

3.3 %

5.6 %

7.8 %

Fig. 1. Evolution of oligomer content in the upper phase during the glycolysis

reaction of PU foams for different catalyst concentrations, in the presence of

DEG as glycolyzing agent. TRa 190 C; Wglycol ag./WPU 1.5.

354 C. Molero et al. / Polymer Degradation and Stability 93 (2008) 353e361

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the catalyst amount used. The exchange reaction with DEG,

which occurs randomly according to Casassa theory of

statistical degradation[12], leads to a drastic decrease of PU

molecular weight. The oligourethanes so produced and their

subsequent disappearance give information about the degrada-

tion in that when complete disappearance has been achieved, it

is assumed that all the PU has been degraded. As observed,a small amount of catalyst induces a strong enhancement of

the degradation, which fully agrees with the common experi-

ence with other condensation polymers like polycarbonate

(PC)[13]. The successive increases in the catalyst concentra-

tion also keep decreasing the reaction time for the complete

degradation, apparently without restriction.

As a result of the PU degradation, the recovered polyether

polyol is released to the reaction medium. As shown in Fig. 2,

polyol release runs parallel to oligomer disappearance. At

given reaction times up to the end point, the more the catalyst

used the more the polyol present in the upper phase due to

faster degradation. Nevertheless, once the plateau value is

achieved, the polyol purity decreases with the catalystconcentration. In order to gain a better understanding of

the catalyst concentration influence on the process, in

Fig. 3the times required to reach the complete PU degradation

and the polyol concentrations obtained in the upper phase for

each concentration studied are presented.

As regards the degradation time, it must be noticed that at

low percentages a slight increase in the catalyst concentration

affects the reaction rate more markedly, as shown by the slope

of the graph of reaction rate versus the catalyst concentration.

The common way to discuss a reaction development is related

to kinetic calculations, but in the literature there are no kinetic

models for PU glycolysis. However, several analogies can befound with the transesterification processes (glycolysis) to

prepare or degrade common polyesters like PET (poly(ethyl-

ene terephthalate)).These products have gained increasing

commercial importance and therefore there is more literature

dealing with kinetic approaches to describe the influence oftheir transesterification catalysts. Stier et al. [14] have

proposed that the kinetic constant of transesterification of di-

methyl 2,6-naphthalenedicarboxylate (2,6-DMN) with monop-

ropylene glycol (MPG) and metal carboxylates as catalysts can

be expressed as

kk0 k0cat 1

where the constant k0 takes the uncatalysed reaction into ac-

count and the second term takes care of the catalyzed reaction

assuming that the reaction is of first order with respect to the

catalyst concentration. Nevertheless, several authors [14e16]

have observed a nonlinear correlation between reactivity andcatalyst concentration. The reaction order is about zero for

a large concentration range and first order for very low catalyst

concentration.

Experimental results show that glycolysis of PET is analo-

gous to glycolysis of PUs and the kinetic theory used for PET

can also be applied to PU. Firstly, the process can be

conducted in the absence of a catalyst, related to the kinetic

constant k0. Secondly, Fig. 3 shows that at low catalyst

concentration, there is a strong dependence with the improve-

ment in the reaction time, but at high concentrations the slope

of the dependence curve decreases. It is expected that at higher

catalyst concentrations the improvement in the reaction rate

would not be noticeable, approaching zero order behaviour.

On the other hand there is a conflicting effect on the process

because when the catalyst concentration is increased the final

concentration of polyol in the upper phase is reduced. This

concentration tends to a minimum steady value at the highest

catalyst concentrations. The GPC chromatograms of upper

phase samples obtained after the complete glycolysis of PU

(Fig. 4) at high and low catalyst concentrations show the

component profile obtained.

In both cases, the main peak corresponds to the recovered

polyol, followed by a group of peaks due to low molecular

weight reaction by-products and that corresponding to DEG.

After the degradation, no oligomers are detected; their peaks

0 50 100 150 200 250 300 350

0

10

20

30

40

50

60

70

80

90

reaction time (min)

%b

yweigh

t

0 %

1.1 %

2.2 %

3.3 %

5.6 %

7.7 %

Fig. 2. Evolution of polyol content in the upper phase during the glycolysis

reaction of PU foams for different catalyst concentrations, in the presence

of DEG as glycolyzing agent. TRa 190 C;Wglycol ag./WPU 1.5.

0

10 2 3 4 5 6 7 8 9

50

100

150

200

250

300

catalyst concentration (%)

reaction

time(min)

74

76

78

80

82

84

%polyolb

yweight

Fig. 3. Dependence of polyol concentration in the upper phase and time to

reach the complete polyol recovery as a function of the catalyst concentration.

TRa 190 C; Wglycol ag./WPU 1.5.

355C. Molero et al. / Polymer Degradation and Stability 93 (2008) 353e361

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should appear at retention times lower than the polyol. Al-

though the polyol recovery in the upper phase is quantitative,

a small amount of the bottom phase pollutes the upper phase,

which reveals itself as the low weight compounds group in

the GPC chromatograms. When the catalyst concentration is

increased, not only is the amount of carboxylate salt increased

in the reaction mixture but also the catalyst solvent which is

preferably dissolved in the polyol phase. Moreover the in-

crease of the catalyst concentration enhances the mutual phase

solubility and favours the recovered polyols becoming more

polluted. The properties of the polyol phases obtained, which

are reported in Table 1, are in agreement with the previous

statements: hydroxyl number, amine value and potassium

content, related to solubilisation of DEG and solvent alcohol,

reaction by-products and the catalyst salt, respectively, also

increase with the amount of catalyst employed and so does

the deviation from commercial conformities. Values of proper-

ties out of commercial conformity, especially amine value and

alkaline cations content can negatively affect the further

foaming of the recovered polyol in a new flexible PU [17]

and should be reduced to a minimum. An easy, low-cost wash-

ing process allows a good decrease in hydroxyl number but not

a strong one in those undesirable properties [10]. This factimplies that although the use of a carboxylate catalyst is

strongly recommended due to their improvement in the degra-

dation rate, the concentration used must be reduced in order to

obtain the highest quality of recovered polyol. Probably the

use of 2.2% of catalyst would be the best choice.

3.2. Influence of temperature

PU foams contain different structures, mainly correspond-

ing to the monomers (polyether polyol) and the groups derived

from the starting isocyanates, like urethane, urea, allophanate

and others. The beginning of thermal degradation of PUcompounds and their derivatives is around 120e250 C. This

range, which is strongly influenced by the physical character-

istics of the PU, namely, internal crosslinking, hydrogen bonds

and the inner crystalline structure[18], is quite similar to that

described for glycolysis, 180e220 C[19].

Among the groups which form a PU, biuret and allophanate

are the less stable, because their complete thermal degradation

can be reached at 170e180 C [20,21]. Decomposition of

these thermolabile structures yields the precursor groups: ure-

thane, urea, hydroxyl, isocyanate and amine. Urethane and

urea bonds, followed by isocyanurate structures are the follow-

ing in the stability range. It can be necessary to reach 270 C

to achieve their decomposition. Specifically, for urethane

groups related to primary and secondary alcohols of a flexible

polyether polyol ofMw3000 g mol1, Ravey and Pearce[20]

observed that degradation started at 200 C, whereas Lefebvre

et al. [22] observed that the degradation of urethane groups

started at 250 C for a similar PU. When a rigid polyether pol-

yol is linked to the urethane group, the beginning of the deg-

radation decreases to 230 C[23]. The urethane group thermal

degradation occurs randomly in the polymer structure and has

been described according the following mechanisms[20,24]:

Depolymerisation. Related to the dissociation of the group

in the precursors, polyol and isocyanate. This reaction also

retention time (min)

intensity(a.u.)

recoveredpolyol

DEG

5.6 %

1.1 %

Mw

460

Mw

3

00

10 12 14 16 18

Mw

180

Mw