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Synthesis and characterization of waterborne polyurethane dispersion from glycolyzed products of waste polyethylene terephthalate used as soft and hard segment
Author’s Accepted Manuscript Synthesis and characterization of waterborne polyurethane dispersion from glycolyzed products of waste polyethylene terephthalate used as soft and hard segment Xing Zhou, Changqing Fang, Qian Yu, Rong Yang, Li Xie, Youliang Cheng, Yan Li www.elsevier.com/locate/ijadhadh
PII: DOI: Reference:
S0143-7496(16)30253-6 http://dx.doi.org/10.1016/j.ijadhadh.2016.12.010 JAAD1943
To appear in: International Journal of Adhesion and Adhesives Received date: 16 May 2014 Accepted date: 21 December 2016 Cite this article as: Xing Zhou, Changqing Fang, Qian Yu, Rong Yang, Li Xie, Youliang Cheng and Yan Li, Synthesis and characterization of waterborne polyurethane dispersion from glycolyzed products of waste polyethylene terephthalate used as soft and hard segment, International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2016.12.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and characterization of waterborne polyurethane dispersion from glycolyzed products of waste polyethylene terephthalate used as soft and hard segment Xing Zhou, Changqing Fang*, Qian Yu, Rong Yang, Li Xie, Youliang Cheng, Yan Li
Xi’an University of Technology, Xi’an 710048, China
Corresponding author. Tel.: +86 29 82312038, fax: +86 29 82312512.
[email protected] Abstract
Waste polyethylene terephthalate (PET) bottles were collected, cleaned and then depolymerized by glycolysis with neopentyl glycol (NPG) and dipropylene glycol (DPG), in the presence of N-butyl titanate catalyst. The product, named glycolyzed oligoesters, obtained through the depolymerization, were employed respectively in hard segment and soft segment in the synthesis of novel waterborne polyurethane dispersions (PUDs) via a simple and environmentally benign process. In addition, a polyurethane dispersion without glycolyzed oligoesters was synthesized as a comparison. The bulk structure of PET glycolyzed oligoesters and PUDs films were characterized by Fourier transform infrared spectroscopy (FT-IR), H-nuclear magnetic resonance (1H NMR) and Gel permeation chromatography (GPC). The results illustrated that glycolyzed oligoesters were successfully introduced into the hard and soft segment of the polyurethanes. Furthermore, differential scanning
1
calorimetry (DSC) and thermogravimetric analysis (TGA) were used to investigate the thermal properties of the PET glycolyzed oligoesters and PUDs films. The results showed that the thermal resistance of waterborne polyurethanes obtained with glycolyzed oligoesters increased due to lower degrees of phase separation. X-ray diffraction indicated that all synthesized polyurethanes exhibited reduced degrees of orientation.
Due to the balance between hard-/soft-segment of the waterborne
polyurethane dispersions, the formulations containing glycolyzed oligoesters within the hard segment sections of the polyurethanes provided the best performance.
Keywords: PET waste; Glycolyzed oligoesters; Waterborne polyurethanes; Soft segment; Hard segment
1. Introduction Due to good mechanical properties, dimensional stability, electrical insulation, optical clarity, non-toxic and odorless characteristics, polyethylene terephalate (PET) has become one of the most versatile commodity thermoplastics which has been widely used in an extensive range of applications, such as packaging, fibers, films, textiles and electrical insulating materials. With increasing consumer demand, a large amount of PET waste has been generated mainly from PET bottles, including the increasing use of carbonated beverage bottles, mineral water bottles, juice, milk, medicine, and other packaging containers. Since waste PET is chemically inert, natural degradation via air or micro-organisms is difficult. This has resulted in a certain degree of environmental contamination and waste of resources. Therefore, it is 2
necessary and significant to explore how to recycle PET packaging waste and turn it into a useful resource.
There are two main ways to recycle waste PET: one is a physical method, in which PET waste products are handled by simple physical treatments such as directly blending, mixing, melting, granulation. Such recycled materials can then be used as low-end products for various applications such as tensioned membranes, spinning and engineering plastics to achieve secondary recycling. The other approach is via chemical degradation methods (so-called chemical recycling method), whereby degradation of PET via chemical reagents and heat can result in relatively low molecular weight products, such as dimethyl terephthalate (DMT), terephthalic acid (TPA), ethylene glycol (EG), bis hydroxy ethyl terephthalate (BHET) and other chemical products. These separated and purified products can be reused as monomers to synthesize new chemical products to accomplish the recycling of resources [1].Chemical processes applied in PET recycling are divided into glycolysis, hydrolysis, acidolysis, methanolysis, aminolysis, etc. The most attractive processes that have been studied extensively are glycolysis and methanolysis [2-5]. The glycolysis of PET is a molecular degradation reaction involving monohydric alcohols, diols or polyols, in the presence of some catalysts such as metal acetate, solid acid and ionic liquid.
Polyurethane dispersions (PUD) are one of the most important industrial materials due to their versatile properties, such as toughness, mechanical flexibility,
3
chemical resistance, strong adhesion, excellent abrasion resistance, etc. [6-8]. They are widely used in the manufacture of car paints, flexible foams for bedding, rigid foams for insulation, and adhesives [9]. In recent years, owing to environmental concerns around volatile organic components (VOCs), waterborne polyurethane dispersions (PUDs) have become a major research and development area [10-12]. Polyurethanes (PU) are copolymers which consist of alternating soft and hard segments forming a unique microphase-separated structure. The soft segment is normally a low molecular weight polyether or polyester, which has a direct influence on the flexibility and low temperature performance of the PU, whilst the hard segment generally consists of a diisocyanate extended by low molecular weight diols or diamines, which contribute to mechanical properties, such as tensile strength and hardness [13-16]. Thus, the introduction of different groups into both the soft and hard segments of a polyurethane will result in different degrees of separation, allowing the existence of a wide range of properties.
In this study, recycled waste PET was employed as a source of both soft and hard segment material to prepare waterborne polyurethanes followed by characterization to determine differences in both structure and properties. Although some studies have focused on incorporating recycled waste PET as soft segment to prepare varieties of polyurethanes [17-20], but very little progress has been made in using recycled waste PET as the hard segment and analyzing the difference between these two kinds of waterborne polyurethanes.
4
2. Experimental 2.1 Material
Waste PET water bottles were collected and utilized. After removing caps and labels, the bottles were cut into chips (average size 5 mm × 5 mm) which were washed with deionized water to remove adhesive materials and then dried in a vacuum oven at 50 ºC and 0.05 MPa for 4 h. Neopentyl glycol and dipropylene glycol purchased from Sinopharm Chemical Reagent Co., Shanghai, China. N-Butyl titanate purchased from Kelong Chemical, Chengdu, China, was used as the transesterification catalyst for the depolymerization of PET. Isophorone diisocyanate (IPDI), poly(propylene glycol) (PPG, molecular weight (Mw) = 2000, dried under vacuum at 120 ºC for 2h before use) and dimethylolpropionic acid (DMPA) were purchased from Jingchun Chemical, Shanghai, China. 1, 4-butanediol (BDO), triethylamine (TEA, 99 wt% purity), and 1-methyl-2-pyrrolidone (NMP, 99 wt% purity) were purchased from Fuchen Chemical, Tianjin, China. Dibutyltin dilaurate (DBTDL was purchased from Qingxi Chemical, Shanghai, China. Acetone was used throughout the process and deionized water was used as the dispersing phase. All the purchased reagents were of analytical grade and used without further purification.
2.2 Glycolysis of PET
Small chips of waste PET were glycolyzed by neopentyl glycol (NPG), dipropylene glycol (DPG) (the molar ratio of PET repeating unit to the mixed glycol was 1:3, and the molar ratio of the mixed NPG and DPG was 1:1) and 0.5% (w/w) 5
n-butyl titanate catalyst in a four-necked flask (500 ml) equipped with a mechanical stirrer, thermometer and spiral condenser in an electric-heated thermostatic oil bath. The glycolysis reaction was carried out at 190 ºC for 1h and then the temperature was raised to 210 ºC under reflux in a nitrogen atmosphere for about 5 h till all the small chips disappeared. The obtained glycolyzed oligoesters were dried in a vacuum oven at 45 ºC and 0.05 MPa for 8h. The glycolyzed oligoesters were then available for the synthesis of waterborne polyurethane dispersions. The glycolysis process of waste PET with NPG and DPG was performed according to Scheme 1.
Scheme 1. The glycolysis reaction of PET waste with NPG and DPG HO
CH2
CH2
O
O
C
C
O
CH3 O
CH2
CH2
n
OH
+
HO
CH2
C
CH2OH
CH3 CH3 HO
CH2
CH2
C
O
O
O
C
C
CH3 O
CH2
CH3
HO
CH2
CH2
O
CH
CH
O
O
C
C
O
CH2
CH
CH3 CH2
OH
O
CH2
CH2
n
OH
+
CH3 CH2
CH3 HO
CH2
CH3
CH3 HO
C
O
CH2
CH
O
OH
O
O
C
C
CH3 O
CH
CH3 CH2
O
CH2
CH
OH
2.3 Synthesis of waterborne polyurethane dispersions
Three groups of PUDs were synthesized to analyze the impact of glycolyzed oligoesters on final PU properties. The abbreviations of synthesized polyurethanes with waste PET as hard segment, soft segment and pure PUDs were PU1, PU2 and PU3, respectively. The differences among three samples were as follows: (1) in
6
synthesizing PU1, glycolyzed oligoesters of waste PET were introduced into the system as a low molecular weight chain extender, which forms the hard segments within the polyurethane; (2) in synthesizing PU2, glycolyzed oligoesters of waste PET and PPG were mixed with a molar ratio of 1:1 and utilized as oligomer polyols which form the soft segments of the polyurethane; (3) PU3 was synthesized without the waste PET glycolyzed oligoesters. Despite these differences, all the samples were synthesized according to the following process: Oligomer polyols and IPDI (molar ratio of isocyanate groups to hydroxyl groups, NCO/OH =4) were added to a four-necked flask (500 ml) equipped with a mechanical stirrer, thermometer and spiral condenser in an electrically-heated thermostatic water bath. The reaction was carried out at 80 ºC for 2.5 h, then DBTDL was added 30 minutes before the end of the first reaction, afterwards a pre-determined amount of DMPA dispersed in NMP at 60 ºC was added. The reaction was continued at 80 ºC for another 2 h. Subsequently, the resulting prepolymer was cooled to about 35 ºC, and then a low molecular weight chain extender with a small quantity of acetone and TEA dispersed in 120 g of deionized water were poured into the flask. Throughout the course of the experiment, a moderate amount of acetone was introduced into the system to reduce the viscosity. After the reaction, the residual acetone was removed in a vacuum oven at 50 ºC and 0.05 MPa for 1 h. The general recipe used for the preparation of polyurethane dispersions is listed in Table 1. The general reaction scheme for the synthesis of PUDs is illustrated in Scheme 2.
Scheme 2. Synthesis route of waterborne polyurethane dispersions 7
CH3 HO
～ OH +
CH3
+
CH2 NCO
OCN
HOCH2
C
CH2 OH
COOH CH3
H3C
NMP
O OCN
R
O
H
H
R
NCO
C
CH2
H
O CH2
CH2
C
O CH2
O
OCN H
COOH
O
O
R NCO～ OCN H
H
R NC
CH2
C
O CH2 -
COO NH Et 3
O
OCN +
H
O
O
CH2
4
O
H
H
R NC
O
CN
H
+
O
CH2
4
O
O
NCO
R
H
H
CH3
O
R NCO～ OCN
NCO
CH2
H
C
CH2
COOH
TEA
O
O
CN
O
H
CH3
O
R NCO～ OCN
H
H
R
H
H
Acetone
H
O
R NCO～ OCN
+
O
R NCO～ OCN
H
O
Water CH3
O
OCN
COOH BDO
CH3
Acetone
CH3
O
NCO～OCN
+
H
R
NCO H
CH2
C
CH2
COO-NH+ Et
3
Acetone Removal Waterborne Polyurethane CH3
R
CH2
=
H3 C
CH3
Table 1 Recipe for the preparation of polyurethane dispersions (weight in g).
Samples
Ingredient Oligomer polyols
IPDI
DPMA NMP Chain extender
TEA
PU1
50.059
22.231
4.562
7.577
7.471
3.445
PU2
50.103
17.165
3.622
6.017
1.624
2.735
PU3
50.032
22.198
3.938
6.539
2.593
2.974
2.4 Preparation of PUDs films
Films were prepared by casting aqueous dispersions onto Teflon surfaces, and allowing them to evaporate at room temperature for 3 days and then at 40 ºC in a vacuum drying oven for 12 h to allow complete removal of solvent. Finally, the coated films (still on Teflon) were stored in a desiccator to avoid moisture uptake. 8
2.5 Measurements and characterization
2.5.1 FT-IR measurement
To identify the structure of PUDs, the infrared spectra of the dried polyurethane films were obtained with a Fourier transform infra-red spectrometer (SHIMADIU FT-IR-8400S (CE)) and recorded by averaging 20 scans at a resolution of 16.0 cm-1. 2.5.2 1H NMR measurement 1
H NMR spectra were obtained on a Bruker-400MHz spectrometer, using
sodium 2, 2-dimethyl-2-silapentane-5-sulfonate (DSS) as an internal standard, with CDCl3 as solvent.
2.5.3 Gel permeation chromatography
The average molecular masses and molecular mass distributions of the glycolyzed products and obtained polyurethane dispersions were measured by gel permeation chromatography (GPC) (USA Waters, ALLIANCE) analysis with a DAWN EOS (λ=690.0 nm) and RI detector (Shodex RI-71). All samples were dissolved in tetrahydrofuran (THF) of HPLC grade and the flow rate of carrier solvent was 0.5 ml/min. Monodisperse polystyrene was used as the calibration reference.
2.5.4 TG analysis
Thermogravimetric (TG) experiments were carried out in a METTLER TOLEDO TGA/DSC 1 analyzer with Gos Controller GC10 STARe system. Film
9
samples ranging from 4 to 10 mg were placed in an alumina ceramic crucible and heated under argon (flow rate: 100 ml/min) from 30 to 700 ºC with a heating rate of 15 ºC/min.
2.5.5 DSC measurement
Differential scanning calorimetry (DSC) experiments were carried out in NETZSCH DSC 200 F3 Maia® instrument with a temperature range from - 100 to 120 ºC at a heating rate 10 ºC/min under an argon atmosphere (flow rate: 50 mL/min). To remove the thermal history from the PUDs, three consecutive runs were carried out: (1) Heating from room temperature to 120 ºC. (2) Cooling from 120 to -100 ºC (20 ºC/min). (3) Heating from -100 to 120 ºC (10 ºC/min).
2.5.6 XRD measurement
An X-ray diffractometer (XRD-7000, SHIMADZU LIMITED, Japan) was used to detect the crystallinity of the polyurethane films with monochromatic Cu Kα radiation (1.540598 nm). A scanning of 2θ angles between 10° and 60° under a scan speed of 8.0000 deg/min was carried out.
3. Results and discussion 3.1 FT-IR analysis
The bulk structure of glycolyzed oligoesters and synthesized waterborne polyurethane dispersions were estimated by FT-IR methods. Fig. 1 (a) shows the
10
FT-IR spectra of glycolyzed waste PET oligoesters obtained from glycolysis with a waste PET/glycol molar ratio of 1:3. The peak at 1720 cm-1 can be attributed to –C=O stretching which is a key band and confirms ester bond formation during the glycolysis of waste PET using glycol. This can be compared with the FT-IR spectra of currently used glycols which do not contain bands around 1750 cm–1 [20]. The band observed in the region of 3400 cm–1 can be attributed to free hydroxyl groups present in the glycolyzed waste PET oligoesters. The peaks at 1270 cm−1 and 1130 cm−1 can be ascribed to the asymmetric and symmetric bending vibrations of C–O–C from ester groups, respectively. Bands between 1500–1400 cm-1 originate from aromatic C–H while peaks appearing at 3000–2800 cm-1 relate to alkyl and C–H which originate from glycolyzed oligoesters formed during the glycolysis reaction. The peak at about 726 cm−1 can be ascribed to =CH of the benzene ring. These absorption peaks imply that the glycolyzed waste PET oligoesters are compounds having hydroxyl and ester groups.
Fig. 1 (b) shows FT-IR spectra of polyurethane films of sample PU1, PU2 and PU3. It can be seen in Fig. 1 (b) that the FT-IR spectra of the PUDs are quite similar. The most relevant IR bands correspond to N-H stretching at 3332 cm-1, C-N stretching and δ N-H stretching at 1542 cm-1, C-H stretching from 2865 to 2972 cm-1 and C=O stretching from 1640 to 1712 cm-1. The FT-IR spectra show the characteristic C-O and N-CO-O stretching (at about 1242 and 1018 cm−1) and C-O-C stretching (at about 1103, 955 cm−1). According to these characteristic peaks the formation of urethane groups (-NHCOO-) can be confirmed. As depicted in Fig. 1 (b), 11
the peak at 3332 cm−1 can be assigned to NH groups H-bonded with urethane carbonyl groups, indicating that most of the NH groups form H-bonding with carbonyl oxygen. Moreover, the carbonyl stretching band regions in urethane and urea, the peaks of which should be at 1743 cm−1 and 1700 cm−1, display peaks at 1712 cm−1 and 1640 cm−1. The variation of the band locations can be attributed to the existence of H-bonding. The existence of H-bonding in three samples results in a significant phase separation between soft- and hard-segments. Meanwhile, a peak at about 726 cm−1 can be attributed to =CH of the benzene ring can be found only in PU1 and PU2, with a lower intensity compared to that of glycolyzed products according to Fig. 1 (a), which apparently shows that glycolyzed oligoesters have participated in the reaction and been successfully immitted into the main chain of the polyurethanes. Furthermore, it is observed that PU3 has a stronger vibration frequency of N-CO-O originating from urethane (at 1242 cm−1) than PU1, and a stronger vibration frequency of CH2 originating from polyol (at 1458 cm−1) than PU2, which may due to the introduction of glycolyzed products into the hard and soft segment, respectively.
12
110 100
Glycolyzed oligoesters
Relative transmittance (%)
90 80 70 60 50 40 30 20 10 0
3400
1468
3000-2800
726
1720
1270 1130 -10 4000 3600 3200 2800 2400 2000 1600 1200 800 Wavenumber/cm
400
-1
(a)
200
PU1 PU2 PU3
Relative transmittance (%)
150 3332
2972-2865
1712
2972-2865
1712
2972-2865
1712
726 1542
100
726
3332 50
1542
3332 0 4000
3500
3000
2500
1542
2000
Wavenumber/cm
1500
1000
500
-1
(b) Fig. 1. FT-IR spectra of the samples (a) Glycolyzed PET oligoesters; (b) The obtained PUDs 3.2 1H NMR analysis
13
The 1H NMR spectrum of the glycolyzed product is shown in Fig. 2. It can be seen that the signal at δ = 8.107 ppm ascribes to the presence of aromatic entities in the structure. Signals at δ = 4.21 and 4.30 ppm signify C-CH2-O-CO linked to CH2 present at α- position which justifies the presence of a substituted neopentyl group [19]. The signal of chemical shift at δ = 4.01 can be assigned to the presence of ethylene units in the structure. The signal at δ = 1.02 ppm represents C-CH2 attached as a β-substitution to the CH3 group. The signal observed at δ = 3.49 ppm can be attributed to a CH2-OH group attached as α-substitution. These results indicate the occurrence of a glycolysis reaction. The 1H NMR spectra of the polyurethane samples are shown in Fig. 3. The signal at 7.28 ppm ascribes to CDCl3 which was used as solvent during measurement. The peak at 3.57 ppm can be assigned to the methylene protons of (-CH2CH2-O-) groups in the BDO blocks. Peaks at 1.06, 3.07, 3.42 ppm can be assigned to methyl, methyne and methylene protons in PPG blocks, respectively. It can be seen that a weak peak appears at 7.02 ppm which can be assigned to the protons of -OCONH-. Peaks at 1.71 ppm and 1.15 ppm can be attributed to methylene protons of macroglycol and the methyl protons of DMPA, respectively. It is clear from Fig 3 that all the samples have a similar structure, confirming the formation of PUDs. However, a signal at 8.107 ppm, which can be ascribed to the presence of an aromatic entity, can be discovered only in PU1 at 8.136 ppm and PU2 at 8.127 ppm, demonstrating that glycolyzed products have been involved in the synthesis of the polyurethanes, which is in accordance with the FTIR results. Moreover, this deviation of signal of chemical shift 14
between PU1 and PU2 indicates that glycolyzed waste PET oligoesters have been introduced into different segments of the polyurethanes. When introduced into the hard segment it has a stronger influence on the chemical shift of the aromatic entity than that in the soft segment. In PU3 the signal at 8.107 ppm, corresponding to an aromatic entity, cannot be found, which illustrates that this is the characteristic peak of glycolyzed oligoesters.
Fig. 2. 1H NMR spectrum of glycolyzed oligoesters
Fig. 3. 1H NMR spectrum of the obtained PUDs 3.3 GPC analysis
15
The molecular weight and molecular weight distribution of glycolyzed waste PET oligoesters and polyurethane dispersions were confirmed by GPC analysis. Fig. 4 shows the GPC chromatogram of the glycolyzed products, where monodisperse characteristics are observed. According to Patel et al [17] and Cakic et al. [18], the number average molecular masses (Mn) of waste PET used for soft drink bottles are generally in the range of 2.5–3.0×104 g/mol. The number average molecular mass (Mn), the weight average molecular mass (Mw) and polydispersity index of the glycolyzed oligoesters obtained by GPC are listed in Table 2. The molecular weight Mn and Mw were observed to be 1499 and 1800 g/mol respectively with a polydispersity index of 1.2. These values show the reduction in molecular mass of the glycolyzed products to be nearly 6% of the waste PET molecular mass used for the reaction, which indicates that the glycolysis reaction is almost complete. Notably, during the course of the reaction, glycolyzed products are supposed to contain the PET depolymerized fraction, glycolyzed oligoesters and glycols (NPG, DPG and EG formed during the glycolysis reaction), which arises from the response of ester linkages in amorphous region/crystalline to the glycol [19]. The ester linkages in the amorphous region of PET are easily attacked by glycol, but this behavior is restricted when it comes to the crystalline region of PET where it is very difficult to replace EG linkages by NPG and DPG during transesterification. This limits the average molecular masses of glycolyzed oligoesters resulting in smaller values. Thus, the by-products in the glycolysis process cannot be avoided, of which bulk structure, purity and properties will be further investigated in our future research.
16
Fig. 5 shows the GPC chromatograms of the obtained PUDs. The Mn, Mw and polydispersity index of the polyurethanes obtained by GPC are summarized in Table 2. All the samples present monodisperse character and narrow molecular weight distributions. The chromatograms of the polyurethane dispersion show two clear separate peaks with low value of polydispersity. Meanwhile, it can be seen from Table 2 that the molecular weight and polydispersity index of the polyurethane dispersions decreased by using waste PET. These results imply that the introduction of glycolyzed oligoesters increases the possibility of chain formation within the polyurethane dispersions having varying lengths. This may arise from two kinds of glycolyzed oligoesters which are formed during the glycolysis reaction. When glycolyzed oligoesters are used as hard and soft segment, they exhibit different reactivity kinetics, thus, a reduction in molecular weight is expected [21]. According to the data in Table 2, PU2 has the smallest molecular weight and polydispersity index which could be due to the use of the glycolyzed oligoesters in the formation of the soft segment, following mixing with the PPG.
17
1200 Glycolyzed oligoesters 1000
MV
800
600
400
200 19
20
21
22
23
24
MINS
Fig. 4. GPC chromatogram of glycolyzed oligoesters
1600 1400 1200
MV
1000
PU1
800
PU2
600 PU3
400 12
13
14
15
16
17
18
MINS
Fig. 5. GPC chromatograms the obtained PUDs Table 2 Average molecular masses and polydispersity index of glycolyzed PET oligoesters and polyurethane dispersions.
Samples
Mw(g/mol)
18
Mn(g/mol)
Mw/Mn
Glycolyzed oligoesters
1800
1499
1.20
PU1
23740
17100
1.39
PU2
20990
15290
1.37
PU3
28290
20010
1.41
3.4 TG analysis
The thermal stability of these thermoplastic elastomers is generally not high, especially above their softening temperatures, and the degradation mechanism is very complex due to the variety of products formed in the process [22]. Usually, at a low heating rate, the degradation process results in differential weight loss (DTG) curves with several peaks, which indicates the complexity of the degradation. The peaks correspond to the temperatures at maximum rate of weight loss in the corresponding step. The thermal resistance of glycolyzed waste PET oligoesters and the PUDs (showed in Fig. 6) were analyzed by TG. Differential weight loss (DTG) curves for PU1 are presented in Fig. 7. As depicted in Fig. 6, glycolyzed waste PET oligoesters show two main degradation stages. According to Wang Hui et al. [23], the initial stage of degradation which occurred at 116 ºC may arise from the degradation of bis(hydroxyethyl) terephthalate (BHET) monomer formed during the glycolysis reaction. The later stage happening at 341ºC may ascribe to the degradation of glycolyzed oligoesters. 19
As to the PUDs, the residual water in PUDs is removed at 150-180 ºC before the decompositions. With increasing temperature, all the samples display three main degradation stages. Table 3 shows the decomposition temperature and weight loss percentage for each degradation stage. Due to the utilization of diol and diamine, two kinds of hard segments (urethane and urea) form in the PUDs. It is verified that the urethanes have lower thermal resistance than that of the urea [24, 25]. Therefore, the first decomposition process occurring at 220-250 ºC (shown in table 3) should correspond to the urethane hard-segments, and the second at around 300 ºC to those of urea. The third degradation stage produced at 357-371 ºC attributes to the decomposition of the soft-segment [26].
It is observed that the first decomposition temperature of PU3 is lowest, while that of PU1 and PU2 is almost the same, indicating that the thermal resistance of PU3 is lower than PU1 and PU2. PU1 and PU2 showed higher thermal stability due to the introduced glycolyzed products containing benzene rings and ester groups causing the crosslinking of the polyurethane backbone [27]. Hereby, glycolyzed waste PET oligoesters used as hard-/soft-segment in PUDs could increase the thermal resistance of the PUDs significantly. There is an interesting phenomenon that the second and third decomposition temperatures of PU2 are lower than PU1 and PU3, while that of PU1 and PU3 are of little difference, as are the weight loss values relating to these two decompositions. The differences arise from the amount of soft-segment of glycolyzed waste PET oligoesters in PU2. The weight loss values of PU2 show that the amount of hard-segment is lowest while that of soft-segment is highest. Under 20
normal circumstances, the degradation temperature of the soft-segment should be higher than that of the other two samples, but it turns out the third stage decomposition temperature of PU2 is the lowest, indicating that the soft-segment must contain a certain amount of hard-segment. Thereby the hard-segment reduces the degradation temperature of the soft-segment in PU2, which indicates that there is a lower degree of microphase separation. Furthermore, this phenomenon implies that when glycolyzed oligoesters of waste PET are employed as the soft-segment in PUDs, it can facilitate the dissolution of the soft-segment into the hard-segment.
Comparing the content of hard-/soft-segments in PU1 and PU3, it can be seen that when glycolyzed oligoesters of waste PET were used as the hard-segment, the hard- and soft-segment domain can achieve a more balanced stage (in PU1, hard-segment of 47.21% and soft-segment of 47.22%, while in pure PUDs, hard-segment of 43.98% and soft-segment of 50.77%). Thus, it can be concluded that the thermal resistance of waterborne polyurethane can be increased effectively when glycolyzed waste PET oligoesters were introduced into the synthesis process. Moreover, when glycolyzed oligoesters were used as the hard-segment, the hard-segment region and soft-segment region in PUDs can achieve greater balance, which may result in waterborne polyurethanes with better performance.
21
100
Weight loss (wt%)
80 60 40
Glycolyzed oligoesters PU1 PU2 PU3
20 0 0
100
200
300
400
500
600
Temperature (ºC)
Fig. 6. TG curves of glycolyzed PET oligoesters and the obtained PUDs
0.8 100
Weight loss (wt%)
0.0
60 40 20
-0.8
PU1 (Weight loss) PU1 (Deriv. weight change)
Deriv. weight change (wt%/ºC)
80
0 100
200
300
400
500
Temperature (ºC)
Fig. 7. TG and DTG curves of PU1 Table 3 Main decomposition of the PUDs
Second Samples
First decomposition
Third decomposition decomposition
22
Weight loss T1 (ºC)
(wt%)
T2 (ºC)
Weight
T3
Weight loss
loss (wt%)
(ºC)
(wt%)
PU1
243
7.16%
305
40.05%
371
47.22%
PU2
241
3.78%
294
33.14%
357
57.37%
PU3
224
5.93%
310
38.05%
371
50.77%
T1, T2 and T3 are the onset temperatures for the first, second and third decomposition 3.5 DSC analysis
Further information about thermal properties of glycolyzed oligoesters and the obtained PUDs could be received from the analysis of DSC (depicted in Fig. 8, third heating run). Glycolyzed oligoesters show only one glass transition temperature in the range from – 68.6 to – 47.5 ºC, indicating the amorphous and low temperature resistance of oligoesters. All the PUDs show a single glass transition temperature (Tg) in the range from – 66.3 to – 36.4 ºC due to the soft segment. It can be seen that the Tg values of the soft-segment in the three PUDs samples are substantially higher than that of pure PPG (Mn = 2000, Tg = – 69 ºC) [26]. The increase in Tg demonstrates that the hard-segment was dispersed in the soft-segment microdomains. In addition, a small exotherm peak at around – 29.2 ºC in PU3 and the higher Tg values found for both PU1 and PU2 are attributed to the higher degree of crosslinking and entanglements and enhanced thermal stability when glycolyzed oligoesters were introduced, which reduces the orderliness of the molecular chain structure and the 23
ability to crystallize. Furthermore, the Tg of PU2 is slightly higher than that of PU1 and PU3, which may be attributed to the lower phase separation caused by the –C=O contained within the soft segment. The Tm values of the soft-segment of all samples were not detected in the curves, indicating that the crystallization kinetics was very slow, due to the weak phase separation between the soft- and hard-segments. According to a previous study, the Tg of the hard segment is expected at about 80 ºC, however detection using DSC is not always possible due to the low sensitivity of DSC analysis for detecting second order transitions [28]. The Tg value of all the samples are reported in Table 4.
0.4 Glycolyzed oligoesters PU1 PU2 PU3
Heat Flow
EXO
Tg 0.0 Tg -0.4
Tg
Tg
-0.8
-1.2 -90
-60
-30
0
30
60
90
120
Temperature (ºC)
Fig. 8. DSC curves of glycolyzed PET oligoesters and the obtained PUDs Table 4 Glass transition temperature of glycolyzed oligoesters and PUDs samples
Sample Glycolyzed oligoesters
PU1
24
PU2
PU3
Tg (ºC)
– 68.6 ~ – 47.5
– 60.1 ~ – 46.7 – 50.3 ~ – 36.4 – 66.3 ~ – 52.1
3.6 XRD analysis
The crystallinity of glycolyzed oligoesters and the polyurethanes was detected by X-ray diffraction. As depicted in Fig. 9 (a), an absence of crystalline peaks proved that these compounds are totally amorphous, corresponding to the DSC results. The reagents used in the synthesis of glycolyzed oligoesters (neopentyl glycol and dipropylene glycol) seem to decrease or avoid the crystallization of polyesters for their chemical structures differ from the repeating PET units [29]. The X-ray diffractograms of the PUDs (Fig. 9 (b)) showed two main diffraction peaks at 2θ values of 18° (main diffraction peak) and 42° suggesting that the samples have a reduced degree of orientation structure, which should be characteristic of the soft segment in the segmented polyurethanes [30], this phenomenon is consistent with the results from the DSC analysis.
160 Glycolyzed oligoesters
140 120
Intensity (a. u)
100 80 60 40 20 0 -20 0
20
40
60 2(°)
25
80
100
(a)
1200 1000
Intensity (a. u)
800 600 400
PU3 PU2
200
PU1 0 0
10
20
30
40
50
60
70
80
90
2(°)
(b) Fig. 9. X-ray diffractogram of the samples (a) Glycolyzed PET oligoesters ; (b) The obtained PUDs
4. Conclusions Waste PET bottles were depolymerized by a mixture of neopentyl glycol (NPG) and dipropylene glycol (DPG) with a molar ratio of 1:3. The main purpose of this study was to investigate the difference between a variety of properties of PUDs prepared via recycled PET containing both soft segment and hard segments. Through FTIR and 1H NMR, it can be certified that glycolyzed oligoesters were introduced into the hard and soft segments of PUDs successfully. According to GPC experiments, it was observed that the average molecular weight of PUDs, obtained using glycolyzed oligoesters, decreased when compared to pure PUDs. According to TG, DSC and XRD analysis, it was found that introducing glycolyzed oligoesters into
26
different parts of the PUDs, the thermal properties were distinct, which may due to the different degrees of phase separation observed. The thermal resistance and Tg values of the PUDs increased when glycolyzed oligoesters were utilized. In addition, when glycolyzed oligoesters were employed as hard-segments, PUDs with better performance may be obtained because of the balance between hard-/soft-segment. It is supposed that glycolyzed products obtained from recycled PET would be applicable for use as both soft and hard segments during the synthesis of waterborne polyurethane dispersions.
Acknowledgements The authors acknowledge the financial support provided by Programs for New Century Excellent Talents in University of Ministry of Education of China (Grant No.: NCET-12-1045), Shaanxi Programs for Science and Technology Development (Fund No.2010K01-096), Key Program for Innovation Team in Shaanxi Province, Ph.D. Innovation fund projects of Xi’an University of Technology (Fund No. 310-252071501), and Program for Innovation Team in Xi’an University of Technology (Grant No. 108-25605T401).
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PII: DOI: Reference:
S0143-7496(16)30253-6 http://dx.doi.org/10.1016/j.ijadhadh.2016.12.010 JAAD1943
To appear in: International Journal of Adhesion and Adhesives Received date: 16 May 2014 Accepted date: 21 December 2016 Cite this article as: Xing Zhou, Changqing Fang, Qian Yu, Rong Yang, Li Xie, Youliang Cheng and Yan Li, Synthesis and characterization of waterborne polyurethane dispersion from glycolyzed products of waste polyethylene terephthalate used as soft and hard segment, International Journal of Adhesion and Adhesives, http://dx.doi.org/10.1016/j.ijadhadh.2016.12.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis and characterization of waterborne polyurethane dispersion from glycolyzed products of waste polyethylene terephthalate used as soft and hard segment Xing Zhou, Changqing Fang*, Qian Yu, Rong Yang, Li Xie, Youliang Cheng, Yan Li
Xi’an University of Technology, Xi’an 710048, China
Corresponding author. Tel.: +86 29 82312038, fax: +86 29 82312512.
[email protected] Abstract
Waste polyethylene terephthalate (PET) bottles were collected, cleaned and then depolymerized by glycolysis with neopentyl glycol (NPG) and dipropylene glycol (DPG), in the presence of N-butyl titanate catalyst. The product, named glycolyzed oligoesters, obtained through the depolymerization, were employed respectively in hard segment and soft segment in the synthesis of novel waterborne polyurethane dispersions (PUDs) via a simple and environmentally benign process. In addition, a polyurethane dispersion without glycolyzed oligoesters was synthesized as a comparison. The bulk structure of PET glycolyzed oligoesters and PUDs films were characterized by Fourier transform infrared spectroscopy (FT-IR), H-nuclear magnetic resonance (1H NMR) and Gel permeation chromatography (GPC). The results illustrated that glycolyzed oligoesters were successfully introduced into the hard and soft segment of the polyurethanes. Furthermore, differential scanning
1
calorimetry (DSC) and thermogravimetric analysis (TGA) were used to investigate the thermal properties of the PET glycolyzed oligoesters and PUDs films. The results showed that the thermal resistance of waterborne polyurethanes obtained with glycolyzed oligoesters increased due to lower degrees of phase separation. X-ray diffraction indicated that all synthesized polyurethanes exhibited reduced degrees of orientation.
Due to the balance between hard-/soft-segment of the waterborne
polyurethane dispersions, the formulations containing glycolyzed oligoesters within the hard segment sections of the polyurethanes provided the best performance.
Keywords: PET waste; Glycolyzed oligoesters; Waterborne polyurethanes; Soft segment; Hard segment
1. Introduction Due to good mechanical properties, dimensional stability, electrical insulation, optical clarity, non-toxic and odorless characteristics, polyethylene terephalate (PET) has become one of the most versatile commodity thermoplastics which has been widely used in an extensive range of applications, such as packaging, fibers, films, textiles and electrical insulating materials. With increasing consumer demand, a large amount of PET waste has been generated mainly from PET bottles, including the increasing use of carbonated beverage bottles, mineral water bottles, juice, milk, medicine, and other packaging containers. Since waste PET is chemically inert, natural degradation via air or micro-organisms is difficult. This has resulted in a certain degree of environmental contamination and waste of resources. Therefore, it is 2
necessary and significant to explore how to recycle PET packaging waste and turn it into a useful resource.
There are two main ways to recycle waste PET: one is a physical method, in which PET waste products are handled by simple physical treatments such as directly blending, mixing, melting, granulation. Such recycled materials can then be used as low-end products for various applications such as tensioned membranes, spinning and engineering plastics to achieve secondary recycling. The other approach is via chemical degradation methods (so-called chemical recycling method), whereby degradation of PET via chemical reagents and heat can result in relatively low molecular weight products, such as dimethyl terephthalate (DMT), terephthalic acid (TPA), ethylene glycol (EG), bis hydroxy ethyl terephthalate (BHET) and other chemical products. These separated and purified products can be reused as monomers to synthesize new chemical products to accomplish the recycling of resources [1].Chemical processes applied in PET recycling are divided into glycolysis, hydrolysis, acidolysis, methanolysis, aminolysis, etc. The most attractive processes that have been studied extensively are glycolysis and methanolysis [2-5]. The glycolysis of PET is a molecular degradation reaction involving monohydric alcohols, diols or polyols, in the presence of some catalysts such as metal acetate, solid acid and ionic liquid.
Polyurethane dispersions (PUD) are one of the most important industrial materials due to their versatile properties, such as toughness, mechanical flexibility,
3
chemical resistance, strong adhesion, excellent abrasion resistance, etc. [6-8]. They are widely used in the manufacture of car paints, flexible foams for bedding, rigid foams for insulation, and adhesives [9]. In recent years, owing to environmental concerns around volatile organic components (VOCs), waterborne polyurethane dispersions (PUDs) have become a major research and development area [10-12]. Polyurethanes (PU) are copolymers which consist of alternating soft and hard segments forming a unique microphase-separated structure. The soft segment is normally a low molecular weight polyether or polyester, which has a direct influence on the flexibility and low temperature performance of the PU, whilst the hard segment generally consists of a diisocyanate extended by low molecular weight diols or diamines, which contribute to mechanical properties, such as tensile strength and hardness [13-16]. Thus, the introduction of different groups into both the soft and hard segments of a polyurethane will result in different degrees of separation, allowing the existence of a wide range of properties.
In this study, recycled waste PET was employed as a source of both soft and hard segment material to prepare waterborne polyurethanes followed by characterization to determine differences in both structure and properties. Although some studies have focused on incorporating recycled waste PET as soft segment to prepare varieties of polyurethanes [17-20], but very little progress has been made in using recycled waste PET as the hard segment and analyzing the difference between these two kinds of waterborne polyurethanes.
4
2. Experimental 2.1 Material
Waste PET water bottles were collected and utilized. After removing caps and labels, the bottles were cut into chips (average size 5 mm × 5 mm) which were washed with deionized water to remove adhesive materials and then dried in a vacuum oven at 50 ºC and 0.05 MPa for 4 h. Neopentyl glycol and dipropylene glycol purchased from Sinopharm Chemical Reagent Co., Shanghai, China. N-Butyl titanate purchased from Kelong Chemical, Chengdu, China, was used as the transesterification catalyst for the depolymerization of PET. Isophorone diisocyanate (IPDI), poly(propylene glycol) (PPG, molecular weight (Mw) = 2000, dried under vacuum at 120 ºC for 2h before use) and dimethylolpropionic acid (DMPA) were purchased from Jingchun Chemical, Shanghai, China. 1, 4-butanediol (BDO), triethylamine (TEA, 99 wt% purity), and 1-methyl-2-pyrrolidone (NMP, 99 wt% purity) were purchased from Fuchen Chemical, Tianjin, China. Dibutyltin dilaurate (DBTDL was purchased from Qingxi Chemical, Shanghai, China. Acetone was used throughout the process and deionized water was used as the dispersing phase. All the purchased reagents were of analytical grade and used without further purification.
2.2 Glycolysis of PET
Small chips of waste PET were glycolyzed by neopentyl glycol (NPG), dipropylene glycol (DPG) (the molar ratio of PET repeating unit to the mixed glycol was 1:3, and the molar ratio of the mixed NPG and DPG was 1:1) and 0.5% (w/w) 5
n-butyl titanate catalyst in a four-necked flask (500 ml) equipped with a mechanical stirrer, thermometer and spiral condenser in an electric-heated thermostatic oil bath. The glycolysis reaction was carried out at 190 ºC for 1h and then the temperature was raised to 210 ºC under reflux in a nitrogen atmosphere for about 5 h till all the small chips disappeared. The obtained glycolyzed oligoesters were dried in a vacuum oven at 45 ºC and 0.05 MPa for 8h. The glycolyzed oligoesters were then available for the synthesis of waterborne polyurethane dispersions. The glycolysis process of waste PET with NPG and DPG was performed according to Scheme 1.
Scheme 1. The glycolysis reaction of PET waste with NPG and DPG HO
CH2
CH2
O
O
C
C
O
CH3 O
CH2
CH2
n
OH
+
HO
CH2
C
CH2OH
CH3 CH3 HO
CH2
CH2
C
O
O
O
C
C
CH3 O
CH2
CH3
HO
CH2
CH2
O
CH
CH
O
O
C
C
O
CH2
CH
CH3 CH2
OH
O
CH2
CH2
n
OH
+
CH3 CH2
CH3 HO
CH2
CH3
CH3 HO
C
O
CH2
CH
O
OH
O
O
C
C
CH3 O
CH
CH3 CH2
O
CH2
CH
OH
2.3 Synthesis of waterborne polyurethane dispersions
Three groups of PUDs were synthesized to analyze the impact of glycolyzed oligoesters on final PU properties. The abbreviations of synthesized polyurethanes with waste PET as hard segment, soft segment and pure PUDs were PU1, PU2 and PU3, respectively. The differences among three samples were as follows: (1) in
6
synthesizing PU1, glycolyzed oligoesters of waste PET were introduced into the system as a low molecular weight chain extender, which forms the hard segments within the polyurethane; (2) in synthesizing PU2, glycolyzed oligoesters of waste PET and PPG were mixed with a molar ratio of 1:1 and utilized as oligomer polyols which form the soft segments of the polyurethane; (3) PU3 was synthesized without the waste PET glycolyzed oligoesters. Despite these differences, all the samples were synthesized according to the following process: Oligomer polyols and IPDI (molar ratio of isocyanate groups to hydroxyl groups, NCO/OH =4) were added to a four-necked flask (500 ml) equipped with a mechanical stirrer, thermometer and spiral condenser in an electrically-heated thermostatic water bath. The reaction was carried out at 80 ºC for 2.5 h, then DBTDL was added 30 minutes before the end of the first reaction, afterwards a pre-determined amount of DMPA dispersed in NMP at 60 ºC was added. The reaction was continued at 80 ºC for another 2 h. Subsequently, the resulting prepolymer was cooled to about 35 ºC, and then a low molecular weight chain extender with a small quantity of acetone and TEA dispersed in 120 g of deionized water were poured into the flask. Throughout the course of the experiment, a moderate amount of acetone was introduced into the system to reduce the viscosity. After the reaction, the residual acetone was removed in a vacuum oven at 50 ºC and 0.05 MPa for 1 h. The general recipe used for the preparation of polyurethane dispersions is listed in Table 1. The general reaction scheme for the synthesis of PUDs is illustrated in Scheme 2.
Scheme 2. Synthesis route of waterborne polyurethane dispersions 7
CH3 HO
～ OH +
CH3
+
CH2 NCO
OCN
HOCH2
C
CH2 OH
COOH CH3
H3C
NMP
O OCN
R
O
H
H
R
NCO
C
CH2
H
O CH2
CH2
C
O CH2
O
OCN H
COOH
O
O
R NCO～ OCN H
H
R NC
CH2
C
O CH2 -
COO NH Et 3
O
OCN +
H
O
O
CH2
4
O
H
H
R NC
O
CN
H
+
O
CH2
4
O
O
NCO
R
H
H
CH3
O
R NCO～ OCN
NCO
CH2
H
C
CH2
COOH
TEA
O
O
CN
O
H
CH3
O
R NCO～ OCN
H
H
R
H
H
Acetone
H
O
R NCO～ OCN
+
O
R NCO～ OCN
H
O
Water CH3
O
OCN
COOH BDO
CH3
Acetone
CH3
O
NCO～OCN
+
H
R
NCO H
CH2
C
CH2
COO-NH+ Et
3
Acetone Removal Waterborne Polyurethane CH3
R
CH2
=
H3 C
CH3
Table 1 Recipe for the preparation of polyurethane dispersions (weight in g).
Samples
Ingredient Oligomer polyols
IPDI
DPMA NMP Chain extender
TEA
PU1
50.059
22.231
4.562
7.577
7.471
3.445
PU2
50.103
17.165
3.622
6.017
1.624
2.735
PU3
50.032
22.198
3.938
6.539
2.593
2.974
2.4 Preparation of PUDs films
Films were prepared by casting aqueous dispersions onto Teflon surfaces, and allowing them to evaporate at room temperature for 3 days and then at 40 ºC in a vacuum drying oven for 12 h to allow complete removal of solvent. Finally, the coated films (still on Teflon) were stored in a desiccator to avoid moisture uptake. 8
2.5 Measurements and characterization
2.5.1 FT-IR measurement
To identify the structure of PUDs, the infrared spectra of the dried polyurethane films were obtained with a Fourier transform infra-red spectrometer (SHIMADIU FT-IR-8400S (CE)) and recorded by averaging 20 scans at a resolution of 16.0 cm-1. 2.5.2 1H NMR measurement 1
H NMR spectra were obtained on a Bruker-400MHz spectrometer, using
sodium 2, 2-dimethyl-2-silapentane-5-sulfonate (DSS) as an internal standard, with CDCl3 as solvent.
2.5.3 Gel permeation chromatography
The average molecular masses and molecular mass distributions of the glycolyzed products and obtained polyurethane dispersions were measured by gel permeation chromatography (GPC) (USA Waters, ALLIANCE) analysis with a DAWN EOS (λ=690.0 nm) and RI detector (Shodex RI-71). All samples were dissolved in tetrahydrofuran (THF) of HPLC grade and the flow rate of carrier solvent was 0.5 ml/min. Monodisperse polystyrene was used as the calibration reference.
2.5.4 TG analysis
Thermogravimetric (TG) experiments were carried out in a METTLER TOLEDO TGA/DSC 1 analyzer with Gos Controller GC10 STARe system. Film
9
samples ranging from 4 to 10 mg were placed in an alumina ceramic crucible and heated under argon (flow rate: 100 ml/min) from 30 to 700 ºC with a heating rate of 15 ºC/min.
2.5.5 DSC measurement
Differential scanning calorimetry (DSC) experiments were carried out in NETZSCH DSC 200 F3 Maia® instrument with a temperature range from - 100 to 120 ºC at a heating rate 10 ºC/min under an argon atmosphere (flow rate: 50 mL/min). To remove the thermal history from the PUDs, three consecutive runs were carried out: (1) Heating from room temperature to 120 ºC. (2) Cooling from 120 to -100 ºC (20 ºC/min). (3) Heating from -100 to 120 ºC (10 ºC/min).
2.5.6 XRD measurement
An X-ray diffractometer (XRD-7000, SHIMADZU LIMITED, Japan) was used to detect the crystallinity of the polyurethane films with monochromatic Cu Kα radiation (1.540598 nm). A scanning of 2θ angles between 10° and 60° under a scan speed of 8.0000 deg/min was carried out.
3. Results and discussion 3.1 FT-IR analysis
The bulk structure of glycolyzed oligoesters and synthesized waterborne polyurethane dispersions were estimated by FT-IR methods. Fig. 1 (a) shows the
10
FT-IR spectra of glycolyzed waste PET oligoesters obtained from glycolysis with a waste PET/glycol molar ratio of 1:3. The peak at 1720 cm-1 can be attributed to –C=O stretching which is a key band and confirms ester bond formation during the glycolysis of waste PET using glycol. This can be compared with the FT-IR spectra of currently used glycols which do not contain bands around 1750 cm–1 [20]. The band observed in the region of 3400 cm–1 can be attributed to free hydroxyl groups present in the glycolyzed waste PET oligoesters. The peaks at 1270 cm−1 and 1130 cm−1 can be ascribed to the asymmetric and symmetric bending vibrations of C–O–C from ester groups, respectively. Bands between 1500–1400 cm-1 originate from aromatic C–H while peaks appearing at 3000–2800 cm-1 relate to alkyl and C–H which originate from glycolyzed oligoesters formed during the glycolysis reaction. The peak at about 726 cm−1 can be ascribed to =CH of the benzene ring. These absorption peaks imply that the glycolyzed waste PET oligoesters are compounds having hydroxyl and ester groups.
Fig. 1 (b) shows FT-IR spectra of polyurethane films of sample PU1, PU2 and PU3. It can be seen in Fig. 1 (b) that the FT-IR spectra of the PUDs are quite similar. The most relevant IR bands correspond to N-H stretching at 3332 cm-1, C-N stretching and δ N-H stretching at 1542 cm-1, C-H stretching from 2865 to 2972 cm-1 and C=O stretching from 1640 to 1712 cm-1. The FT-IR spectra show the characteristic C-O and N-CO-O stretching (at about 1242 and 1018 cm−1) and C-O-C stretching (at about 1103, 955 cm−1). According to these characteristic peaks the formation of urethane groups (-NHCOO-) can be confirmed. As depicted in Fig. 1 (b), 11
the peak at 3332 cm−1 can be assigned to NH groups H-bonded with urethane carbonyl groups, indicating that most of the NH groups form H-bonding with carbonyl oxygen. Moreover, the carbonyl stretching band regions in urethane and urea, the peaks of which should be at 1743 cm−1 and 1700 cm−1, display peaks at 1712 cm−1 and 1640 cm−1. The variation of the band locations can be attributed to the existence of H-bonding. The existence of H-bonding in three samples results in a significant phase separation between soft- and hard-segments. Meanwhile, a peak at about 726 cm−1 can be attributed to =CH of the benzene ring can be found only in PU1 and PU2, with a lower intensity compared to that of glycolyzed products according to Fig. 1 (a), which apparently shows that glycolyzed oligoesters have participated in the reaction and been successfully immitted into the main chain of the polyurethanes. Furthermore, it is observed that PU3 has a stronger vibration frequency of N-CO-O originating from urethane (at 1242 cm−1) than PU1, and a stronger vibration frequency of CH2 originating from polyol (at 1458 cm−1) than PU2, which may due to the introduction of glycolyzed products into the hard and soft segment, respectively.
12
110 100
Glycolyzed oligoesters
Relative transmittance (%)
90 80 70 60 50 40 30 20 10 0
3400
1468
3000-2800
726
1720
1270 1130 -10 4000 3600 3200 2800 2400 2000 1600 1200 800 Wavenumber/cm
400
-1
(a)
200
PU1 PU2 PU3
Relative transmittance (%)
150 3332
2972-2865
1712
2972-2865
1712
2972-2865
1712
726 1542
100
726
3332 50
1542
3332 0 4000
3500
3000
2500
1542
2000
Wavenumber/cm
1500
1000
500
-1
(b) Fig. 1. FT-IR spectra of the samples (a) Glycolyzed PET oligoesters; (b) The obtained PUDs 3.2 1H NMR analysis
13
The 1H NMR spectrum of the glycolyzed product is shown in Fig. 2. It can be seen that the signal at δ = 8.107 ppm ascribes to the presence of aromatic entities in the structure. Signals at δ = 4.21 and 4.30 ppm signify C-CH2-O-CO linked to CH2 present at α- position which justifies the presence of a substituted neopentyl group [19]. The signal of chemical shift at δ = 4.01 can be assigned to the presence of ethylene units in the structure. The signal at δ = 1.02 ppm represents C-CH2 attached as a β-substitution to the CH3 group. The signal observed at δ = 3.49 ppm can be attributed to a CH2-OH group attached as α-substitution. These results indicate the occurrence of a glycolysis reaction. The 1H NMR spectra of the polyurethane samples are shown in Fig. 3. The signal at 7.28 ppm ascribes to CDCl3 which was used as solvent during measurement. The peak at 3.57 ppm can be assigned to the methylene protons of (-CH2CH2-O-) groups in the BDO blocks. Peaks at 1.06, 3.07, 3.42 ppm can be assigned to methyl, methyne and methylene protons in PPG blocks, respectively. It can be seen that a weak peak appears at 7.02 ppm which can be assigned to the protons of -OCONH-. Peaks at 1.71 ppm and 1.15 ppm can be attributed to methylene protons of macroglycol and the methyl protons of DMPA, respectively. It is clear from Fig 3 that all the samples have a similar structure, confirming the formation of PUDs. However, a signal at 8.107 ppm, which can be ascribed to the presence of an aromatic entity, can be discovered only in PU1 at 8.136 ppm and PU2 at 8.127 ppm, demonstrating that glycolyzed products have been involved in the synthesis of the polyurethanes, which is in accordance with the FTIR results. Moreover, this deviation of signal of chemical shift 14
between PU1 and PU2 indicates that glycolyzed waste PET oligoesters have been introduced into different segments of the polyurethanes. When introduced into the hard segment it has a stronger influence on the chemical shift of the aromatic entity than that in the soft segment. In PU3 the signal at 8.107 ppm, corresponding to an aromatic entity, cannot be found, which illustrates that this is the characteristic peak of glycolyzed oligoesters.
Fig. 2. 1H NMR spectrum of glycolyzed oligoesters
Fig. 3. 1H NMR spectrum of the obtained PUDs 3.3 GPC analysis
15
The molecular weight and molecular weight distribution of glycolyzed waste PET oligoesters and polyurethane dispersions were confirmed by GPC analysis. Fig. 4 shows the GPC chromatogram of the glycolyzed products, where monodisperse characteristics are observed. According to Patel et al [17] and Cakic et al. [18], the number average molecular masses (Mn) of waste PET used for soft drink bottles are generally in the range of 2.5–3.0×104 g/mol. The number average molecular mass (Mn), the weight average molecular mass (Mw) and polydispersity index of the glycolyzed oligoesters obtained by GPC are listed in Table 2. The molecular weight Mn and Mw were observed to be 1499 and 1800 g/mol respectively with a polydispersity index of 1.2. These values show the reduction in molecular mass of the glycolyzed products to be nearly 6% of the waste PET molecular mass used for the reaction, which indicates that the glycolysis reaction is almost complete. Notably, during the course of the reaction, glycolyzed products are supposed to contain the PET depolymerized fraction, glycolyzed oligoesters and glycols (NPG, DPG and EG formed during the glycolysis reaction), which arises from the response of ester linkages in amorphous region/crystalline to the glycol [19]. The ester linkages in the amorphous region of PET are easily attacked by glycol, but this behavior is restricted when it comes to the crystalline region of PET where it is very difficult to replace EG linkages by NPG and DPG during transesterification. This limits the average molecular masses of glycolyzed oligoesters resulting in smaller values. Thus, the by-products in the glycolysis process cannot be avoided, of which bulk structure, purity and properties will be further investigated in our future research.
16
Fig. 5 shows the GPC chromatograms of the obtained PUDs. The Mn, Mw and polydispersity index of the polyurethanes obtained by GPC are summarized in Table 2. All the samples present monodisperse character and narrow molecular weight distributions. The chromatograms of the polyurethane dispersion show two clear separate peaks with low value of polydispersity. Meanwhile, it can be seen from Table 2 that the molecular weight and polydispersity index of the polyurethane dispersions decreased by using waste PET. These results imply that the introduction of glycolyzed oligoesters increases the possibility of chain formation within the polyurethane dispersions having varying lengths. This may arise from two kinds of glycolyzed oligoesters which are formed during the glycolysis reaction. When glycolyzed oligoesters are used as hard and soft segment, they exhibit different reactivity kinetics, thus, a reduction in molecular weight is expected [21]. According to the data in Table 2, PU2 has the smallest molecular weight and polydispersity index which could be due to the use of the glycolyzed oligoesters in the formation of the soft segment, following mixing with the PPG.
17
1200 Glycolyzed oligoesters 1000
MV
800
600
400
200 19
20
21
22
23
24
MINS
Fig. 4. GPC chromatogram of glycolyzed oligoesters
1600 1400 1200
MV
1000
PU1
800
PU2
600 PU3
400 12
13
14
15
16
17
18
MINS
Fig. 5. GPC chromatograms the obtained PUDs Table 2 Average molecular masses and polydispersity index of glycolyzed PET oligoesters and polyurethane dispersions.
Samples
Mw(g/mol)
18
Mn(g/mol)
Mw/Mn
Glycolyzed oligoesters
1800
1499
1.20
PU1
23740
17100
1.39
PU2
20990
15290
1.37
PU3
28290
20010
1.41
3.4 TG analysis
The thermal stability of these thermoplastic elastomers is generally not high, especially above their softening temperatures, and the degradation mechanism is very complex due to the variety of products formed in the process [22]. Usually, at a low heating rate, the degradation process results in differential weight loss (DTG) curves with several peaks, which indicates the complexity of the degradation. The peaks correspond to the temperatures at maximum rate of weight loss in the corresponding step. The thermal resistance of glycolyzed waste PET oligoesters and the PUDs (showed in Fig. 6) were analyzed by TG. Differential weight loss (DTG) curves for PU1 are presented in Fig. 7. As depicted in Fig. 6, glycolyzed waste PET oligoesters show two main degradation stages. According to Wang Hui et al. [23], the initial stage of degradation which occurred at 116 ºC may arise from the degradation of bis(hydroxyethyl) terephthalate (BHET) monomer formed during the glycolysis reaction. The later stage happening at 341ºC may ascribe to the degradation of glycolyzed oligoesters. 19
As to the PUDs, the residual water in PUDs is removed at 150-180 ºC before the decompositions. With increasing temperature, all the samples display three main degradation stages. Table 3 shows the decomposition temperature and weight loss percentage for each degradation stage. Due to the utilization of diol and diamine, two kinds of hard segments (urethane and urea) form in the PUDs. It is verified that the urethanes have lower thermal resistance than that of the urea [24, 25]. Therefore, the first decomposition process occurring at 220-250 ºC (shown in table 3) should correspond to the urethane hard-segments, and the second at around 300 ºC to those of urea. The third degradation stage produced at 357-371 ºC attributes to the decomposition of the soft-segment [26].
It is observed that the first decomposition temperature of PU3 is lowest, while that of PU1 and PU2 is almost the same, indicating that the thermal resistance of PU3 is lower than PU1 and PU2. PU1 and PU2 showed higher thermal stability due to the introduced glycolyzed products containing benzene rings and ester groups causing the crosslinking of the polyurethane backbone [27]. Hereby, glycolyzed waste PET oligoesters used as hard-/soft-segment in PUDs could increase the thermal resistance of the PUDs significantly. There is an interesting phenomenon that the second and third decomposition temperatures of PU2 are lower than PU1 and PU3, while that of PU1 and PU3 are of little difference, as are the weight loss values relating to these two decompositions. The differences arise from the amount of soft-segment of glycolyzed waste PET oligoesters in PU2. The weight loss values of PU2 show that the amount of hard-segment is lowest while that of soft-segment is highest. Under 20
normal circumstances, the degradation temperature of the soft-segment should be higher than that of the other two samples, but it turns out the third stage decomposition temperature of PU2 is the lowest, indicating that the soft-segment must contain a certain amount of hard-segment. Thereby the hard-segment reduces the degradation temperature of the soft-segment in PU2, which indicates that there is a lower degree of microphase separation. Furthermore, this phenomenon implies that when glycolyzed oligoesters of waste PET are employed as the soft-segment in PUDs, it can facilitate the dissolution of the soft-segment into the hard-segment.
Comparing the content of hard-/soft-segments in PU1 and PU3, it can be seen that when glycolyzed oligoesters of waste PET were used as the hard-segment, the hard- and soft-segment domain can achieve a more balanced stage (in PU1, hard-segment of 47.21% and soft-segment of 47.22%, while in pure PUDs, hard-segment of 43.98% and soft-segment of 50.77%). Thus, it can be concluded that the thermal resistance of waterborne polyurethane can be increased effectively when glycolyzed waste PET oligoesters were introduced into the synthesis process. Moreover, when glycolyzed oligoesters were used as the hard-segment, the hard-segment region and soft-segment region in PUDs can achieve greater balance, which may result in waterborne polyurethanes with better performance.
21
100
Weight loss (wt%)
80 60 40
Glycolyzed oligoesters PU1 PU2 PU3
20 0 0
100
200
300
400
500
600
Temperature (ºC)
Fig. 6. TG curves of glycolyzed PET oligoesters and the obtained PUDs
0.8 100
Weight loss (wt%)
0.0
60 40 20
-0.8
PU1 (Weight loss) PU1 (Deriv. weight change)
Deriv. weight change (wt%/ºC)
80
0 100
200
300
400
500
Temperature (ºC)
Fig. 7. TG and DTG curves of PU1 Table 3 Main decomposition of the PUDs
Second Samples
First decomposition
Third decomposition decomposition
22
Weight loss T1 (ºC)
(wt%)
T2 (ºC)
Weight
T3
Weight loss
loss (wt%)
(ºC)
(wt%)
PU1
243
7.16%
305
40.05%
371
47.22%
PU2
241
3.78%
294
33.14%
357
57.37%
PU3
224
5.93%
310
38.05%
371
50.77%
T1, T2 and T3 are the onset temperatures for the first, second and third decomposition 3.5 DSC analysis
Further information about thermal properties of glycolyzed oligoesters and the obtained PUDs could be received from the analysis of DSC (depicted in Fig. 8, third heating run). Glycolyzed oligoesters show only one glass transition temperature in the range from – 68.6 to – 47.5 ºC, indicating the amorphous and low temperature resistance of oligoesters. All the PUDs show a single glass transition temperature (Tg) in the range from – 66.3 to – 36.4 ºC due to the soft segment. It can be seen that the Tg values of the soft-segment in the three PUDs samples are substantially higher than that of pure PPG (Mn = 2000, Tg = – 69 ºC) [26]. The increase in Tg demonstrates that the hard-segment was dispersed in the soft-segment microdomains. In addition, a small exotherm peak at around – 29.2 ºC in PU3 and the higher Tg values found for both PU1 and PU2 are attributed to the higher degree of crosslinking and entanglements and enhanced thermal stability when glycolyzed oligoesters were introduced, which reduces the orderliness of the molecular chain structure and the 23
ability to crystallize. Furthermore, the Tg of PU2 is slightly higher than that of PU1 and PU3, which may be attributed to the lower phase separation caused by the –C=O contained within the soft segment. The Tm values of the soft-segment of all samples were not detected in the curves, indicating that the crystallization kinetics was very slow, due to the weak phase separation between the soft- and hard-segments. According to a previous study, the Tg of the hard segment is expected at about 80 ºC, however detection using DSC is not always possible due to the low sensitivity of DSC analysis for detecting second order transitions [28]. The Tg value of all the samples are reported in Table 4.
0.4 Glycolyzed oligoesters PU1 PU2 PU3
Heat Flow
EXO
Tg 0.0 Tg -0.4
Tg
Tg
-0.8
-1.2 -90
-60
-30
0
30
60
90
120
Temperature (ºC)
Fig. 8. DSC curves of glycolyzed PET oligoesters and the obtained PUDs Table 4 Glass transition temperature of glycolyzed oligoesters and PUDs samples
Sample Glycolyzed oligoesters
PU1
24
PU2
PU3
Tg (ºC)
– 68.6 ~ – 47.5
– 60.1 ~ – 46.7 – 50.3 ~ – 36.4 – 66.3 ~ – 52.1
3.6 XRD analysis
The crystallinity of glycolyzed oligoesters and the polyurethanes was detected by X-ray diffraction. As depicted in Fig. 9 (a), an absence of crystalline peaks proved that these compounds are totally amorphous, corresponding to the DSC results. The reagents used in the synthesis of glycolyzed oligoesters (neopentyl glycol and dipropylene glycol) seem to decrease or avoid the crystallization of polyesters for their chemical structures differ from the repeating PET units [29]. The X-ray diffractograms of the PUDs (Fig. 9 (b)) showed two main diffraction peaks at 2θ values of 18° (main diffraction peak) and 42° suggesting that the samples have a reduced degree of orientation structure, which should be characteristic of the soft segment in the segmented polyurethanes [30], this phenomenon is consistent with the results from the DSC analysis.
160 Glycolyzed oligoesters
140 120
Intensity (a. u)
100 80 60 40 20 0 -20 0
20
40
60 2(°)
25
80
100
(a)
1200 1000
Intensity (a. u)
800 600 400
PU3 PU2
200
PU1 0 0
10
20
30
40
50
60
70
80
90
2(°)
(b) Fig. 9. X-ray diffractogram of the samples (a) Glycolyzed PET oligoesters ; (b) The obtained PUDs
4. Conclusions Waste PET bottles were depolymerized by a mixture of neopentyl glycol (NPG) and dipropylene glycol (DPG) with a molar ratio of 1:3. The main purpose of this study was to investigate the difference between a variety of properties of PUDs prepared via recycled PET containing both soft segment and hard segments. Through FTIR and 1H NMR, it can be certified that glycolyzed oligoesters were introduced into the hard and soft segments of PUDs successfully. According to GPC experiments, it was observed that the average molecular weight of PUDs, obtained using glycolyzed oligoesters, decreased when compared to pure PUDs. According to TG, DSC and XRD analysis, it was found that introducing glycolyzed oligoesters into
26
different parts of the PUDs, the thermal properties were distinct, which may due to the different degrees of phase separation observed. The thermal resistance and Tg values of the PUDs increased when glycolyzed oligoesters were utilized. In addition, when glycolyzed oligoesters were employed as hard-segments, PUDs with better performance may be obtained because of the balance between hard-/soft-segment. It is supposed that glycolyzed products obtained from recycled PET would be applicable for use as both soft and hard segments during the synthesis of waterborne polyurethane dispersions.
Acknowledgements The authors acknowledge the financial support provided by Programs for New Century Excellent Talents in University of Ministry of Education of China (Grant No.: NCET-12-1045), Shaanxi Programs for Science and Technology Development (Fund No.2010K01-096), Key Program for Innovation Team in Shaanxi Province, Ph.D. Innovation fund projects of Xi’an University of Technology (Fund No. 310-252071501), and Program for Innovation Team in Xi’an University of Technology (Grant No. 108-25605T401).
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