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Glycolysis recycling of rigid waste polyurethane foam from refrigerators ab

ab

ab

ab

ab

P. Zhu , Z.B. Cao , Y. Chen , X.J. Zhang , G.R. Qian , Y.L. Chu

ab

& M. Zhou

c

a

College of Environmental and Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, People's Republic of China b

Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling, College of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310018, People's Republic of China c

Semiconductor Manufacturing International (Shanghai) Corporation, 18 Zhangjiang Road, Shanghai 201203, People's Republic of China Published online: 27 May 2014.

To cite this article: P. Zhu, Z.B. Cao, Y. Chen, X.J. Zhang, G.R. Qian, Y.L. Chu & M. Zhou (2014) Glycolysis recycling of rigid waste polyurethane foam from refrigerators, Environmental Technology, 35:21, 2676-2684, DOI: 10.1080/09593330.2014.918180 To link to this article: http://dx.doi.org/10.1080/09593330.2014.918180

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Environmental Technology, 2014 Vol. 35, No. 21, 2676–2684, http://dx.doi.org/10.1080/09593330.2014.918180

Glycolysis recycling of rigid waste polyurethane foam from refrigerators P. Zhua,b∗ , Z.B. Caoa,b , Y. Chena,b , X.J. Zhanga,b , G.R. Qiana,b , Y.L. Chua,b and M. Zhouc a College

of Environmental and Chemical Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, People’s Republic of China; b Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling, College of Environmental Science and Engineering, Zhejiang Gongshang University, Hangzhou 310018, People’s Republic of China; c Semiconductor Manufacturing International (Shanghai) Corporation, 18 Zhangjiang Road, Shanghai 201203, People’s Republic of China

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(Received 23 July 2013; final version received 15 April 2014 ) Rapid growth of rigid waste polyurethane (WPUR) foam from refrigerators attracts the attention all over the world. In this study, glycolysis was chosen to treat WPUR from scrapped refrigerators collected in Shanghai, China. Glycolysis reagents and catalysts were selected. The results indicated that the glycolysis efficiency of ethylene glycol (EG) was higher than that of diethylene glycol, and the catalytic efficiency of alkali metal salts (NaOH) was more excellent than that of triethanolamine and organic salts of alkali metal (NaAc). When EG was 100%WPUR as a glycolysis reagent and NaOH was 1%WPUR as a catalyst at a constant temperature of 197.85◦ C for 2 h, the glycolysis product had the highest glycolysis conversion rate. In order to maximize the recycling of WPUR, regenerative Polyurethane was performed by adding 10% distilled mixed polyol, which conformed to the QB/T 26689-2011 requirements. Keywords: rigid waste polyurethane foam; scrapped refrigerators; glycolysis regents; glycolysis condition; regeneration

1. Introduction Polyurethane (PU) foam is widely used in refrigerators, end-of-life vehicles, and packing material.[1–3] However, its extensive use inevitably leads to the increasing amount of waste polyurethane (WPU) foam, especially the waste rigid polyurethane (WPUR) foam from scrapped refrigerators. According to statistics, the total of refrigerators’ WPUR in China is 25 million tons in 2004, but will increase to 100 million tons in 2015.[4,5] Thus, the disposal of WPU has become urgent. Landfill is the most commonly used technology by far in Shanghai, but limited to the refractory nature of WPU.[1,6] In contrast, chemical recycling has been an alternative approach, which realizes the re-conversion of WPU into raw materials for the regeneration of PU.[7] It includes hydrolysis, glycolysis, alcoholysis, fractionation, and amino alcoholysis.[8–10] Glycolysis is chosen with the following advantages: (1) the reaction temperature is low, (2) the glycolysis time is short, and (3) the glycolysis product can be used to produce new PU totally.[11] Its mechanism is simplified as transesterification reactions of the urethane bond with low-weight glycols as the following reaction.

∗ Corresponding

author. Email: [email protected]

© 2014 Taylor & Francis

Urethane groups in WPUR are ruptured and substituted by glycolysis regents, with recovered polyol and aromatic compounds as by-products.[12] The secondary reaction is also inevitable, leading to a variety of by-products, such as isocyanate, amine, and unsaturation, which bring in toxicity and poor quality of regenerated PU. Therefore, the choice of various glycolysis parameters has been a decisive aspect, especially on an industrial scale.

Environmental Technology Table 1.

The distinctions of WPUR foam and WPUF.

parameters Cellular structure

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Density/(kg/m3 ) Hydroxyl value/(mg KOH/g)

WPUR

WPUF

A few of cells is open 30–40 400–600

A lot of cells are open 20–25 40–60

Glycolysis is applied to the treatment of WPUR and waste flexible polyurethane foam (WPUF) usually, through owning several distinctions listed in Table 1.[13] Numerous researches have been published mainly about the influence of glycolysis reagents and catalysts on the glycolysis of WPUF. Ethylene glycol (EG) can achieve a high glycolysis efficiency in the absence of catalysts,[14] while promoted by tertiary amine.[15] Diethylene glycol (DEG) is an alternative glycolysis reagent combined with a series of effective catalysts including potassium octoate, potassium acetate, stannous octoate, and potassium hydroxide, which are widely used in WPUF from industrial samples and waste cars.[16–20] Diethanolamine (DEA) was also effective whether it was used alone or mixed with DEG.[21,22] Now the glycolysis of diphenylmethane diisocyanate (MDI) WPUF has been developed up to a pilot scale by the European Diisocyanate and Polyol Producers Association (ISOPA).[23] On the other hand, the researches about WPUR are only a small part. First, Mural et al. had examined the dissolution time of WPUR from waste refrigerators under various glycolysis conditions, which showed that more addition of dipropylene glycol and KOH, smaller size of WPUR pieces, and higher temperature (170–200 ◦ C) could shorten the dissolution time.[24] Meanwhile Wu et al. had used DEG and KAc as the glycolysis reagent and catalyst, respectively. The optimum condition was the concentration of DEG = 100% WPUR and KAc = 2% WPUR at 220 ◦ C for 2 h.[25] Combined with an extruder, DEA also had a high glycolysis efficiency without a catalyst at 175–200 ◦ C for 2 h.[26] However, it can be seen that these researches are still far from enough and lack systemic study about the glycolysis and regeneration of WPUR. The aim of this research was to build a more comprehensive and economically viable system about the optimum glycolysis and regenerative conditions for the recycle of the WPUR from waste refrigerators made in China. The roles of glycolysis reagents (EG and DEG) and catalysts (NaOH, NaAc and triethanolamine) were investigated, according to the mechanism of glycolysis. Through distillation and regeneration, physical parameters of the regenerative PU were compared with GB/T 26689-2011,[27] which contained appearance structure, microstructure, density, dimensional stability, compressive strength, and thermal conductivity.

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Table 2. The mass ratio of glycolysis reagent/WPUR and catalyst/WPUR in different steps.

Steps

Glycolysis reagent/WPUR Catalyst/WPUR (%WPUR) (%WPUR)

3.1.1. Comparison of 150 glycolysis reagent 3.1.2. Choice of catalyst 150 3.1.3. Optimal 80, 100, 150, 200, and 300 EG/WPUR 3.1.3. Optimal 100 NaOH/WPUR 3.2. Properties of 100 glycolysis product

0 10 10 0.5, 1, 2, 5, 10, and 15 1

2. Materials and methods 2.1. Materials WPUR of waste refrigerators was provided by Senlan Environmental Protection (Shanghai) Co., Ltd (China) and the waste refrigerators were produced by Sony Corporation of China. A pulverizer (FW-200, The New Connaught Instruments Equipment Co. (China)) was used to smash the WPUR into pieces until passing through a 5 mm mesh. Polyether 455, fresh polyether polyol, and MDI were bought from Guodu Chemical (Kunshan) Co., Ltd (China). Other reagents were all of AR grade produced from Shanghai Sinopharm Chemical Reagent Co., Ltd (China). 2.2. Methods The experiments consisted of two parts as follows. 2.2.1. Glycolysis of WPUR Glycolysis was carried out in a three-necked round-bottom flask equipped with a stirrer, thermometer, and reflux condenser as shown in Figure S1. Glycolysis reagents and catalysts were added at a certain mass ratio given in Table 2 and heated to the boiling temperature of the glycolysis reagent added (EG, 197.85 ◦ C, DEG, 244.8 ◦ C), and then WPUR pieces were fed at a rate of 1.5–3 g/min. The measurement of glycolysis time was started at the moment of complete dosing of WPUR into the reactor. Finally, the glycolysis product was sampled within different glycolysis times (0, 1, 2, 3, 4, 5 h) and tested with the following methods. 2.2.2. Glycolysis conversion rate (X) The glycolysis conversion rate can be observed from the conversion of the –NCOO– functional group (at about 1720 cm−1 ) in the urethane.[25] Because the aromatic ring in PU is more stable than the –NCOO– in the glycolysis temperature range, the conversion of the –NCOO– can be estimated by the relative absorption intensity of the – NCOO– (at about 1720 cm−1 ) to that of the aromatic ring

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(at about 1450 cm−1 ), and calculated as follows: X (100%) = 1



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⎤ the relative absorption intensity of the ⎢ −NCOO − (at about 1720 cm−1 ) ⎥ ⎢ ⎥ ⎢ ⎥ to that of the aromatic ring ⎢ ⎥ −1 ⎣ ⎦ (at about 1450 cm ) in glycolysis product ⎞ . − ⎛ the similar relative absorption ⎝ intensity of the − NCOO ⎠ −to the aromatic ring in WPUR

The chemical structures of the glycolysis product were determined by Fourier transform infrared spectroscopy (FTIR) (Perkin Elmer 1725X, Rigaku, Japan) with a KBr pellet method. The sample preparation was as follows: 1 mg of liquid substance was grounded with 100 mg of KBr and pressed into a transparent sheet. Next, the sheet was analysed by FTIR. 2.2.3. Hydroxyl value The hydroxyl value of glycolysis product was determined by the titration method (ASTM (American Society for Testing and Materials) D2849 method B: pressure-bottle phthalation).[28] 2.2.4. Viscosity Glycolysis product was transferred to the beakers. The viscosity determination was carried out at 24 ◦ C and measured by a NDJ-1 viscometer (Shanghai Hengping Instruments Factory, China). 2.2.5. TG-DTA The thermal behaviour of WPUR pieces was examined by thermal analysis (TG-DTA; STA 449C; Netzsch-Geratebau GmbH, Germany). The samples (25.990 mg) were placed in a Pt-Rh crucible and heated from 25 ◦ C to 800 ◦ C at a rate of 10 ◦ C/min in a helium atmosphere. 2.2.6. Regeneration of PU First is distillation, as purification is necessary due to the excessive glycolysis reagent contained in the sampled glycolysis product (0, 1, 2, 3 h). It was performed at a constant temperature (240 ◦ C) of 2 h with the equipment shown in Figure S1, to obtain the excessive glycolysis reagent (distilled EG) and mixed polyol separately. According to the methods described above, hydroxyl value and viscosity of distilled EG and mixed polyol were also determined. The molecular weight of mixed polyol was assessed by gel permeation chromatography (GPC, LC-10AT VP, Shimadzu Corporation, Japan). The pillar type was 803C and the

pillar temperature was 38 ◦ C at 1 ml/min (rate of mobile phase). The regeneration of PU contained three steps: (1) the mixed polyol and fresh polyether polyol were mixed at a series of mass ratios (mixed polyol/fresh polyether polyol = 0%, 10%, 15%, 18%, and 20%), (2) the mixture obtained from step 1 was stirred with MDI at a mass ratio of 1:1, and (3) the above product was foamed freely in paper cups. To obtain qualified products, physical indicators were detected based on the QB/T 26689-2011 standards. 2.2.7. Physical properties of regenerative PU The density, compressive strength, dimensional stability at 70 ◦ C of 24 h, and dimensional stability at −30 ◦ C of 24 h were tested according to GB/T 6343-2009, GB/T 8811-2008, GB/T 8813-2008, and GB/T 10295-2008.[29– 32] In addition, the compression strength was measured by an electronic universal testing machine (WDW-100A, Jinan Shijin Company, China) at 5 mm/min. Coefficient of thermal conductivity was measured by a YBF-3 thermal conductivity detector (Hangzhou Dahua Instrument Manufacturing Co., Ltd, China). 2.2.8. Apparent characteristics and cellular structure of regenerative PU The apparent images were took by digital camera and microstructure was observed through scanning electron microscope (SEM, JSM-6700F, JEOL Ltd, Japan) with an acceleration voltage of 15 kV. 3. Results and discussion 3.1. Glycolysis of WPUR 3.1.1. Comparison of glycolysis reagents As described that glycolysis is an intermolecular exchange in the urethane group, a suitable glycolysis reagent can make the urethane bonds break down and release the polyols.[12] Typical glycolysis reagents (EG and DEG) were selected and compared in Figure 1 and Figure S2. First, the characteristic absorption peaks of the –NCOO– at 1720 cm−1 are related to the extent of glycolysis.[25,33] In contrast, the absorption bands in 1455 cm−1 stands for the bending vibrations of methylene groups in the polyol chain. When EG was used as a glycolysis reagent, the peaks of the – NCOO– almost disappeared and 1455 cm−1 was clear after 5 h, while –NCOO– still existed obviously and 1455 cm−1 was invisible after 5 h with DEG. Combined with Figure 1, the glycolysis rate with EG was much higher than with DEG. As is known to all, DEG contains four carbon chains link to the end of the MDI segment, which is more than EG with two carbon chains. The more complex spatial structures of DEG result in the greater steric hindrance for its combination of the –NCOO– bonds, which makes the reaction slower and more –NCOO– to be left.[34] Additionally,

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(a) 1600 2100 1800

1400

1500

1300 Hydroxyl value

1200

1200

Viscosity

1100

Viscosity (mPa·S)

Hydroxyl value (mgKOH/g)

1500

900

1000 600 900

there were absorption bands in the spectral region of 3200– 3560 cm−1 due to stretching vibrations of OH groups in aromatic and fatty alcohol, in 2869–2970 cm−1 characteristic of stretching vibrations of CH bonds in aliphatic carbons, and in 2150 and 1950 cm−1 characteristic of cumulative double and triple bonds. These absorption bands came from the intermediate oligomer and final polyol.[16, p.221–228] It also has been mentioned that as the timing starts after the full dosing of WPUR, the ongoing glycolysis has been happening during the feeding process, leading to the high initial conversion at 0 h, which can be provided by the obvious absorption bands of OH groups (3200–3560 cm−1 ). On the other hand, glycolysis should be carried out below the degradation temperature of WPUR, in order to minimize pyrolysis and avoid by-production.[35] The thermal degradation of urethane group began around 245 ◦ C, then completed at 345 ◦ C as shown in Figure S3. The weight decrease at 472 ◦ C corresponded to ether bond thermolysis, according to the interval of degradation temperature 375– 500 ◦ C for several types of WPUR.[18] Taking into account that the boiling point of EG is 197.85 ◦ C (101 kPa) while that of DEG is 244.8 ◦ C (101 kPa), EG is chosen as the glycolysis reagent and 197.85 ◦ C the reaction temperature. 3.1.2. Choice of catalysts Catalysts play a key role in the urethane group transesterification process, with hydroxides, acetates, and amines as typical.[36] Here, triethanolamine, NaOH, and NaAc were chosen as representatives. When triethanolamine was used, the glycolysis product was of three phases containing two liquid layers and one solid phase of unreacted WPUR;

100

150

200

250

300

300

EG/WPUF (% WPUF)

(b) 1300

2700

1200

2400

2100

1100

1800

1000

Viscosity (mPa·S)

Figure 1. Influence of glycolysis reagents and catalysts on the conversion rate (X ) of glycolysis product ( EG = 150%WPUR, T = 197.5 ◦ C; • DEG = 150%WPUR, T = 244.8 ◦ C;  EG = 150%WPUR, Triethanolamine = 10%WPUR, T = 197.5 ◦ C;  EG = 150%WPUR, NaAc = 10%WPUR, T = 197.5 ◦ C;  EG = 150%WPUR, NaOH = 10%WPUR, T = 197.5 ◦ C). * The timing started when WPUR was dosing totally; *‘Before treatment’ was the moment that WPUR was added into the reactor at first.

Hydroxyl value (mgKOH/g)

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50

1500 900

Hydroxyl value Viscosity

800

0

2

4 6 8 10 12 NaOH/WPUR(% WPUR)

14

1200 16

Figure 2. Influence of the mass ratio of EG/WPUR and NaOH/WPUR on hydroxyl value and viscosity of glycolysis product ((a) EG/WPUR, (b) NaOH/WPUR) (() hydroxyl value, (•) viscosity) ((a) EG-80, 100%, 150%, 200%, 300%WPUR, NaOH–10%WPUR, glycolysis time = 5 h, T = 197.5 ◦ C) ((b) EG = 100%WPUR, NaOH = 0.5%, 1%, 2%, 5%, 10%, 15%WPUR, glycolysis time = 5 h, T = 197.5 ◦ C).

the –NCOO– peaks still existed after 5 h as shown in Figure S4(a). However, when NaOH and NaAc were added separately, single-phase homogeneous glycolysis product was obtained and its –NCOO– peaks disappeared after 1 h (Figure S4(b)), which indicates the complete degradation of WPUR. The existence of 1455 cm−1 was derived from the production of polyol. Comparison with a series of conversion rates (X ) in Figure 1 indicated that NaOH and NaAc had similarly high catalytic efficiency on glycolysis, while triethanolamine had little, which caused the residual of WPUR in the solid phase. The decreasing stage of the conversion rate for triethanolamine may be due to its catalytic mechanism. The complex formed between triethanolamine, EG, and –NCOO– bonds causes a delay in the time required for the –NCOO– bonds’ decomposition, limited by its huge volume and strong steric hindrance. Because EG has a high hydroxyl value (1807.6 mg KOH/g) as the major factor to the value of the reaction

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(a)

(b)

Figure 3. Infrared analysis of distilled EG and mixed polyol within different times: (a) distilled EG; (b) mixed polyol (EG = 100%WPUR, NaOH = 1%WPUR, glycolysis time = 0.5,1,2,3 h, T = 197.5 ◦ C).

system, NaOH revealed higher catalytic efficiency which had less EG left and lower hydroxyl value (Figure S5).[37] During the glycolysis process, PU fragments were produced continually which contributed to the increase of hydroxyl value. When EG and WPUR reacted completely nearly after 2 h, the side reaction gradually enhanced, which can convert the hydroxyl group into other groups.[37] It resulted in the decrease of hydroxyl value after 2 h. The inferred mechanism should be explained as follows: NaAc as organic base salts can form coordination complex with –NCOO– bonds and EG, while NaOH as alkali metal salts form alkoxide through nucleophilic action. The latter has much less steric hindrance and stronger interaction, which results in a higher catalytic efficiency.[7] 3.1.3. Optimal conditions of glycolysis The mass ratio of glycolysis reagents, catalysts, and WPUR has a decisive influence on full reaction time, the quantity of

Table 3. Mass mean molecular weights (Mw ) of mixed polyol within different times. Glycolysis time (h) Mw

Polyether 455

0.5

1.0

2.0

3.0

5.0

524

872

769

521

525

529

polyol and economic feasibility.[25] Therefore, the effect of different mass ratios of EG/WPUR and NaOH/WPUR was compared in Figure 2 with T = 197.5 ◦ C and tsample = 5 h. The upward trend of the hydroxyl value in Figure 2(a) was derived mainly from the increase in the relatively surplus addition of EG. On the other hand, the viscosity value decreased with the EG/WPUR increases shown in Figure 2(a). Because EG had a lower viscosity value of 22.1 mPas than that of PU fragments of 4100 mPas (the

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Environmental Technology (a)

(b)

(c)

(d)

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Figure 4. Properties of regenerative PU with different additions of mixed polyol: (a) density, (b) compressive strength, (c) dimensional stability at 70 ◦ C of 24 h, and (d) dimensional stability at −30 ◦ C of 24 h.

average value was obtained from the glycolysis product in this study), the heap of unreacted EG resulted in a decrease in the viscosity value when the mass ratio was above 100% EG/WPUR. However, when the mass ratio ranged from 80% to 100% EG/WPUR, the number of PU fragments increased, leading to the rise in the viscosity value. It means glycolysis reacts fully at 100% EG/WPUR. Figure 2(b) shows that the hydroxyl and viscosity value both have an upward trend first, then drop and the maximum points appear at NaOH /WPUR = 1%WPUR. As mentioned above, PU fragments are the major factor to the two values if glycolysis is sufficient.[25,33] When it was less than 0.5% WPUR, the content of NaOH was not enough, which resulted in lower catalytic efficiency, little production of PU fragments, and low hydroxyl value and viscosity. Then with the addition of NaOH reaching 1%WPUR, the two values both increased because of the rise in the catalytic efficiency. However, the large reduction followed can be explained as that the excessive alkali metal ions have a negative influence on glycolysis.[38]

In consequence, the optimal mass ratios of EG/WPUR and NaOH/PUR are 100%WPUR and 1%WPUR, respectively. 3.2. Properties of glycolysis product The glycolysis product contains mixed polyol, excessive EG, and by-products. Though the former two can be used for the regeneration of PU and glycolysis, respectively, it is a pity that the latter has a negative effect on the regeneration, which may act as a catalyst to destroy the balance during the foaming process.[7,39] To maximize the recycle, distillation was chosen to achieve purification, and performance analysis of distilled EG and mixed polyol was carried out. First, the FTIR peaks of distilled EG with glycolysis time from 0.5 to 3 h (Figure 3(a)) were basically the same as these of pure EG. However, the hydroxyl values of distilled EG were between 1740 and 1777 mg KOH/g as shown in Figure S6, which were lower than those of pure EG (1807.6 mg KOH/g). The reason was that the distilled EG still contained a small amount of NaOH and water, in

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Figure 5.

SEM of regenerative PU with different mass ratios (mixed polyol/fresh polyether polyol, %).

spite of few adverse impacts on glycolysis. Therefore, the distilled EG could be reused for secondary glycolysis. From Figure S6, the hydroxyl values of mixed polyol after distillation were between 847 and 875 mg KOH/g, which was higher than that of the industrial polyether polyol (polyether 455, 430–470 mg KOH/g). It can be explained that the mixed polyol was a mixture which contained various types of polyol with a wide range of hydroxyl value. Changes in the foam formulation or an extraction can reduce the high value without decreasing the quality of regenerative PU. The shoulder peak at the –O– peaks (1096 cm−1 ) from 1 to 3 h in Figure 3(b) proved

the existence of modified isocyanate MDI polyol, which was produced by side reactions.[37] Besides, the peaks of –NCOO– at 0 h showed the presence of unreacted WPUR, but then disappeared with the reaction completion. In addition, the mass mean molecular weights (Mw ) of the mixed polyol decreased as the reaction time proceeded (Table 3), because of the gradual breaking and shortening of the PU fragments. Then it reached a balance after 2 h for the glycolysis had went to equilibrium, which was just close to that of industrial polyether polyol (polyether 455, Mw = 524).[12,15] In order for the WPUR to react completely, the optimal glycolysis time is determined as 2 h.

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Environmental Technology 3.3. Regeneration of PU The balance between the reactions that generate gas which is responsible for some cell features and the gelling reactions that contribute to the formation of the three-dimensional network is the key point for the regeneration.[40] Therefore, when three steps of regeneration were finished, the tests of the regenerative PU were put out. First, the apparent characteristics of the regenerative PU were shown in Figure S7. With the addition of mixed polyol increasing from 0% to 20%, the quality showed a downward trend. It was caused by the straight-chain structure in the modified isocyanate MDI polyol from the mixed polyol, which damaged the uniformity of threedimensional network and the growth of the hole-shaped structure.[25] Meanwhile, the physical properties of regenerative PU were compared with the standard values from GB/T 26689-2011. First, when the addition of mixed polyol was 0–15%, the density met the standard value (≤ 38 kg/m3 ) from Figure 4(a). Then in Figure 4(b), the compression strength dropped fast with the addition increasing. According to GB/T 26689-2011 (when relative deformation = 10%, compressive strength should ≥130 kPa), the percentage of mixed polyol should be 0–10%. Thirdly, the dimensional stability is another important indicator of PU used as insulation materials. When measured at 70 ◦ C of 24 h in Figure 4(c), the change rates of length, width, and height conformed to the standard (≤1.5%). Therefore, the regenerative PU could be used as a heat preservation material near 70 ◦ C. On the other hand, all the change rates tested at −30 ◦ C of 24 h in Figure 4(d) exceeded the standard (≤1%), indicating that if the regenerative PU is used as a heat preservation material below −30 ◦ C, modification might be necessary. Finally, the thermal conductivity had an upward trend with the addition increasing gradually. Referring to the standard (≤0.022 W/(m·K)), the added amount would be controlled below 10%. From the microscopic structure, PU is constituted by the space of three-dimensional skeleton, which supports the entire foam and enhances the supporting force of the foam.[17] In Figure 5(a) and 5(b), the cellular structure was almost spherical, numerous, and evenly distributed when the mixed polyol was added from 0% to 10%. However, when exceeded 15%, the cells became distorted and less uniform as in Figure 5(c) and 5(d). When the content of mixed polyol increased, more cells would start to form at the same time leading to the lack of space available for their growth, thus limiting the cell size distribution. Besides, the straight-chain structure in mixed polyol can also inhibit the expansion of regenerative PU and the growth of the cells. With the addition of mixed polyol above 10%, the cellular structure becomes nonuniform, causing the reduction of physical properties.[41] It is consistent with the physical parameters above.

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Table 4. Thermal conductivity of regenerative PU with different additions of mixed polyol. Mixing ratio Thermal conductivity (W/ (m·K))

Unmixed 10% 15% 18% 0.017

0.021 0.032 0.039

In order that the parameters of regenerative PU meet the GB/T 26689-2011, the addition of mixed polyol is determined to 10% (Table 4).

4. Conclusion Glycolysis had been carried out to treat the WPUR from waste refrigerators. Firsts, EG was chosen as the glycolysis reagent since it had a less complex spatial structure and smaller steric hindrance and lower boiling point than DEG. Compared with triethanolamine and NaAc, NaOH had higher catalytic efficiency, which could form alkoxide through nucleophilic action with –NCOO– and EG. Then the optimal conditions were EG/WPUR = 100% and NaOH/WPUR = 1% at 197.85◦ C of 2 h. According to the properties of the distilled glcolysis products, excessive EG could be reused and the mixed polyol could be recycled for the regeneration of PU. Based on the GB/T 266892011, the regenerative PU could meet the industrialized requirements with the addition below 10% mixed polyol. A large-scale application of this research and the specific mechanism of glcolysis and regeneration will be the subject of future papers. Acknowledgements The authors are grateful for support of the key personnel in the Shanghai Municipality (S30109), the Innovation Program of Shanghai Municipal Education Commission (14YZ002), and the Opening Project of Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling (SWTR-2012–05).

Supplemental data Supplemental data for this article can be accessed at 10.1080/09593330.2014.918180.

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Glycolysis recycling of rigid waste polyurethane foam from refrigerators.

Rapid growth of rigid waste polyurethane (WPUR) foam from refrigerators attracts the attention all over the world. In this study, glycolysis was chose...
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