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EC ratio on structure and properties of polyurethane foams prepared from untreated liquefied corn stover with PAPI
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 416–421
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cherd
Effects of CS/EC ratio on structure and properties of polyurethane foams prepared from untreated liquefied corn stover with PAPI Ti-peng Wang a,1 , Dong Li a,1 , Li-jun Wang b , Jun Yin a , Xiao Dong Chen a,c , Zhi-huai Mao a,∗ a
College of Engineering, China Agricultural University, 17 Qinghua Donglu, Beijing 100083, PR China College of Food Science and Nutritional Engineering, China Agricultural University, 17 Qinghua Donglu, Beijing 100083, PR China c Department of Chemical Engineering, Monash University, Clayton Campus, Victoria, Australia b
a r t i c l e
i n f o
Article history:
a b s t r a c t Corn stover (CS) was liquefied by using ethylene carbonate (EC) as liquefying solvent and
Received 18 July 2007
sulfuric acid as a catalyst at 170 ◦ C for 90 min. Polyurethane (PU) foams were prepared
Accepted 5 December 2007
from liquefied corn stover (LCS) and polymethylene polyphenylisocyanate (PAPI) by oneshot method at a [NCO]/[OH] ratio of 0.6 in the presence of blowing agent, surfactant and co-catalyst. The effects of CS/EC ratio (w/w) on the structural, mechanical and thermal prop-
Keywords:
erties were studied by means of Fourier Transform Infrared Spectroscopy (FT-IR), thermal
FT-IR
analysis (TGA/DSC) and universal tensile machine. FT-IR analysis indicated that urethane
Liquefaction
linkages were formed; free isocyanate groups existed in samples. All samples had a low
Polyurethanes
thermal stability and decomposition occurred in four successive stages. With the increase
Foams
in CS/EC ratio, tensile strength and Young s modulus first increased and then decreased, and
Thermal properties
elongation rate at break decreased. Properties of LCS-PU foams can be changed by varying the CS/EC ratio. © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Biomass resource such as agricultural residues are renewable natural polymers and easily obtainable. Only the amount of the world’s corn stover (CS) is estimated to be about 1.27 billion tons in 2004 (China Agriculture Yearbook, 2005). However, large quantities of agricultural residues are discarded, which not only increase the burden of environment but also cause that the valuable energy contained in agricultural residues are not involved in the energy cycle. In recent years, effective utilization of biomass resources has paid growing attention right from the starting to seeking a substitute for petroleum and environmental protection.
Liquefaction techniques can convert the solid lignocellulosic biomass into liquid products which contain some –OH groups and have potential values of substituting the polyester or polyether polyol to prepare PU foams (Bhunia et al., 1999), which can be friendly to the environment (Breslin, 1993). Some researches utilizing liquefied polyol products to prepare polyurethane have been reported (Montane et al., 1998; Yu et al., 2006; Kurimoto et al., 1992, 2000, 2001a,b; Lee et al., 2002; Yao et al., 1995, 1996; Wang et al., in press). Other studies utilizing the natural materials such as starch, soybean oil and cellulose to prepare or modify the properties and degradability of polyurethane have been carried out (Rivera-Armenta et al., 2004; Ciobanu et al., 2004; Huang and Zhang, 2002; Araujo et al., 2005; Lu et al., 2005; Ha and Broecker, 2002; Pechar et
∗ Corresponding author at: College of Engineering, China Agricultural University, P.O. Box 50, 17 Qinghua Donglu, Beijing 100083, PR China. Tel.: +86 10 62737351; fax: +86 10 62737351. E-mail address: [email protected] (Z.-h. Mao). 1 These authors contributed equally to this work. 0263-8762/$ – see front matter © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2007.12.002
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 416–421
al., 2006; Kwon et al., 2007). In fact, the characters of liquefied products from different liquefaction formulations are different (Liang, 2005) and the properties of PU foams depend on the structures of the polyols and isocyanate (Hepburn, 1982). However, in previous studies, the liquefied products from only one liquefaction formulation are used and investigated. The effects of liquefaction formulations on the properties of polyurethane have been reported very little (Kurimoto et al., 2000). The costs as well as mechanical properties are the main constraints that limit the biodegradable materials to be widely used (Lu et al., 2005). The increase in CS charge can be contributed to the decrease in the prices. This study was aimed at estimating the effects of CS/EC ratio (w/w) on structural, mechanical and thermal properties of LCS-PU foams at a [NCO]/[OH] ratio of 0.6. The effects of factors such as liquefaction time and the charge of catalyst will be further investigated. In this study, LCS-PU foam was prepared by oneshot method through co-polymerization of non pretreated LCS and PAPI in the presence of blowing agent, co-catalyst and surfactant. Structural, mechanical and thermal properties of LCS-PU foams were measured with FT-IR, universal tensile machine, thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC).
2.2.
Experiment
2.1.
Materials and methods
Corn stover with moisture content of 8–10% from a local farm in Beijing suburb of China was milled and only the fractions with particle size of 20–80 meshes were used for the liquefaction experiments. The components of corn stover were analyzed by ANKOM220 cellulose analysis (ANKOM company, USA). The results were soluble materials, 33.16%; cellulose, 34.15%; hemicellulose, 23.86%; lignin, 6.61%; acid insoluble materials, 1.86%. Sulfuric acid (97%, v/v) was used as the catalyst. Ethylene carbonate (99.90%) was used as the solvent in the liquefaction process. PM-200 as the PAPI was obtained from Yantai Wanhua Polyurethane Co., Ltd. (Shandong, China) and the NCO group content was 30.03%. Water and silicone were used as the blowing agent and surfactant, respectively. Triethylamine and dibutyltine dilaurate were used as the co-catalysts. All chemicals used were of reagent grade and were obtained from commercial sources. The chemical structures of cellulose, PAPI and ethylene carbonate were as follows:
Preparation of LCS
Oven-dried corn stover flour 40–80 g, ethylene carbonate 200 g and sulfuric acid 7.4 g were placed in a three-neck flask (1000 ml) equipped with a reflux condenser, a thermometer, and a motor-driven stirrer and refluxed at 170◦ C for 90 min with continuous stirring. Then, the flask was immersed into cold water to quench the reaction and the LCS as the biopolyols was collected for later analysis and use. The acid number of LCS was gained by titratation method with a 0.5 M sodium hydroxide solution, and the hydroxyl number was measured by referring to Kurimoto et al. (2000). The moisture contents of LCS were determined by the Karl Fischer method using a CBS-1A model moisture content meter, Beijing Chaosheng company (Beijing, China). Insoluble residues (unliquefied corn stover) ratio (IRR) of LCS was measured by the following method. A mixture of 2 g LCS was diluted using 50 ml dioxane–water (4:1, v/v) and the insoluble residues were filtered using a Buckner funnel and filter paper (the weight was marked). The residue rinsed thoroughly with the dioxane and 10 blank filter papers were heated for 24 h at 105 ◦ C. The average water content in blank filter paper was calculated. The insoluble residues ratio is given as follows: IRR =
2.
417
w2 − w3 w1
(1)
where w2 and w3 are the dry weight of the filter paper with the insoluble residues and the filter paper, respectively; w1 is the weight of the LCS, 2 g. The characteristics of LCS are listed in Table 1.
2.3.
Preparation of LCS-PU foams
The foams were prepared by the one-shot method. A mixture of 15 g LCS, 0.5 g water, 0.15 g silicone, and 0.3 g co-catalysts (triethylamine:dibutyltine dilaurate = 1:1, w/w) was mixed in a 150 ml polypropylene cup at 1000–1200 rpm for 1 min followed by the addition of 9.60–11.36 g PAPI ([NCO]/[OH] ratio, 0.6) and agitated at 1400–1600 rpm until cream time (about 6–12 s) at room temperature for co-polymerization. The polymerized mixture was poured and daubed onto smooth glass to uniform thin layer of PU foam. The obtained foams were cured for 7 d at room temperature, and then were conditioned for 16 h at 23 ◦ C, 50% RH (relative humidity) according to ISO 11841983 Standard. Every experiment was replicated three times. The [NCO]/[OH] ratio is given as follows:
418
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 416–421
Table 1 – The characteristics of LCS with different CS/EC ratio Sample names LCS1 LCS2 LCS3 LCS4 LCS5
CS/EC ratio (w/w)
Acid number (mg KOH/g)
0.20 0.25 0.30 0.35 0.40
19.18 20.61 17.21 9.32 12.64
where MPAPI is the content of the isocyanate group in PAPI (7.15 mmol/g), Mpolyol the content of the hydroxyl group in LCS (hydroxyl number/56.1, the unit is mmol/g), and WPAPI , Wpolyol and Wwater are the weights (g) of PAPI, LCS and water, respectively.
Infrared spectroscopy
Fourier transform infrared spectroscopy (FT-IR) (NICOLET, USA) spectra of LCS-PU foams were carried out in a NICOLET 560 by attenuated total reflectance (ATR).
2.5.
Thermo gravimetric analysis (TGA)
Thermo gravimetric analysis (TGA) (TA, USA) was carried out in DSC-TGA Q600 TA instrument. The samples of 4–8 mg were heated from 40 ◦ C to 700 ◦ C at the rate of 20 ◦ C/min under air flow of 100 ml/min.
2.6.
Differential scanning calorimetry (DSC)
Differential scanning calorimetry (DSC) (TA, USA) was carried out in Q10 TA instrument in two scans. First, 4–8 mg sample were placed in hermetic pans and were scanned up to 50 ◦ C to eliminate the effect of thermal history, thereafter cooled to −80 ◦ C followed by up to 50 ◦ C. All scans were done at the rate of 10 ◦ C/min under dry nitrogen flow of 50 ml/min. The glass transition temperature (Tg ) was determined in the second scan.
2.7.
Moisture content (%)
137.34 108.63 113.59 127.59 108.93
MPAPI × WPAPI [NCO]/[OH] ratio = . MPolyol × WPolyol + WWater × 2/18 × 1000 (2)
2.4.
Hydroxyl number (mg KOH/g)
Mechanical properties of foams
3.
Results and discussion
3.1.
Infrared spectroscopy
1.481 1.790 2.350 2.600 3.040
3 × o´ × 2 . 3 − 1
3.2.
TGA
Fig. 2 shows the typical thermo gravimetric curves of LCSPU2 and LCS-PU4 foams. The thermograms of samples were similar and decomposition mainly occurred in four successive stages. The detailed results of all samples are listed in Table 2. From Table 2, LCS-PU foam had a low thermal stability and with the increase of CS/EC ratio, the initial decomposition
(3)
where is the tensile strength and = L/L0 , L0 and L being the initial and final length of the samples, respectively.
1.98 2.14 4.69 6.56 11.62
The typical FT-IR spectrum of LCS-PU2 foam is shown in Fig. 1. The appearance of absorption peaks at 1720 cm−1 (C O), 1597 cm−1 (ˇN–H) and 1213 cm−1 (C–O) had confirmed the formation of urethane linkages as expected. The stretching vibrations of N–H bonds at 3311 cm−1 can overlap with hydroxyl bonds which belong to the water or liquefied corn stover. Furthermore, peak at 2238 cm−1 indicated the presence of isocyanate. In theory, when [NCO]/[OH] ratio is less than 1.0, the isocyanate will react completely with hydroxyl groups, no remain. In fact, a thin layer of isocyanate was observed in the surface of the samples in experiment. Moreover, the degradation peak temperature of isocyanates is at around 345 ◦ C; in the following themogravimetric analysis, the peak temperature of the third stage is about 345 ◦ C, which also indicated the presence of isocyanates. This could be explained in the sense that some hydroxyl groups did not react with PAPI in copolymerization process due to low reactivity, which resulted in the residues. However, obvious typical absorption peak of allophanates (1640 cm−1 ) was probably not observed in the spectrum, which can be the reason that the peak was weak and/or overlapped with other peaks. Other strong absorption peaks were attributed to carbonyl at 1800 and 1772 cm−1 , asymmetrical stretching vibration of –CH2 at 2927 cm−1 and aromatic at 1506 cm−1 .
The tensile strength ( b ) and elongation rate at break (εb ) of the foams were measured with a universal tensile machine (INSTRON-4411, INSTRON, England) at a crosshead speed of 300 mm/min at 23 ◦ C, 50% RH and Young s modulus was calculated. The grip distance was set at 86 mm. The dumbbell specimens of 120 mm × 10 mm (length × width) according to the ISO 1184-1983 standard were cut directly from the conditioned samples using a dumb-bell shape knife. The thicknesses of the foams were measured with a digital vernier caliper. All samples were measured five times and averages were obtained. Young s modulus (YM) was calculated by the following equation (Ferry, 1970): YM =
IRR (%)
Fig. 1 – FT-IR spectrum of LCS-PU2 sample.
419
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 416–421
Fig. 2 – Thermo gravimetric curves of LCS-PU2 and LCS-PU4 samples.
temperature defined as the point where weight loss beyond 0.5% increased trends. This indicates that crosslink density of foams increased with the increase in CS/EC ratio. From Table 1, with the increase in CS/EC ratio, IRR increased, which can be one of the reasons of the increase in crosslink density because the insoluble residues can act as the crosslinking agent in copolymerization (Rivera-Armenta et al., 2004). In liquefaction, because of sulfuric acid (97%, v/v) as a catalyst, it was possible that the surface cellulose in the insoluble residues was changed into cellulose sulphate. Moreover, due to liquefaction, some active OH groups were possibly exposed. The two factors could induce the insoluble residues to act as a cross-linking agent in co-polymerization. An initial thermal decomposition of all samples was due to the release of volatile components. The hard and soft segment influenced each other and the former could act as antioxidants in decomposition (Wang and Hsieh, 1997). So the first stage was mainly the degradation of soft segment, and the second stage was due to the decomposition of the hard segments. The third stage (peak temperature at around 345 ◦ C) could result in the degradation of isocyanate which did not react with polyol or water (Kurimoto et al., 2001a,b). Moreover, FT-IR had also indicated the presence of isocyanates in LCS-PU foams. The fourth stage is due to the burning of the char produced in the first three stages. Furthermore, some peaks existed between 400 ◦ C and 495 ◦ C of all samples, which could be the decomposition of insoluble residues.
3.3.
Fig. 3 – DSC scans of LCS-PU foams.
Fig. 4 – The effect of CS/EC ratios on the tensile strength of LCS-PU foams.
range of the latter was larger than the former. It can be attributed to the difference of crosslink density. But Tg of LCSPU3, LCS-PU4 and LCS-PU5 were not observed, even in the conditions of the heating rate or when the weight of sample increased. These can be due to their large crosslink density which restricted the movement of molecule.
3.4.
Mechanical properties
DSC
The effects of CS/EC ratio on the DSC curves of LCS-PU foams are shown in Fig. 3. Distinct Tg of LCS-PU1 and LCS-PU2 were observed in the range of scanned temperature and the Tg
The relationships between CS/EC ratios and b, εb or Young s modulus are shown in Figs. 4–6. With an increase in the CS/EC ratio from 0.2 to 0.4, b first increased and reached its maximum (1.508 MPa) at a CS/EC ratio of 0.25, and then decreased to 0.497 MPa. At the same time, εb largely decreased from 63.06%
Table 2 – The decomposition temperature and weight loss of samples at every stage Sample names
LCS-PU1 LCS-PU2 LCS-PU3 LCS-PU4 LCS-PU5
LCS
LCS1 LCS2 LCS3 LCS4 LCS5
Stage 1
Stage 2
Stage 3
Stage 4
Ts
WL
Ts
WL
Ts
WL
Ts
WL
51.37 56.43 58.48 55.23 68.22
16.85 21.70 14.51 19.80 15.73
209.79 226.83 214.25 223.98 242.37
18.74 12.56 19.32 17.32 17.04
316.75 301.88 328.91 326.74 327.82
11.75 12.67 6.81 5.96 5.75
493.34 489.94 486.84 493.33 492.24
41.35 41.31 44.24 41.15 43.23
*Ts is the starting decomposition temperature (◦ C); WL is the weight loss of every stage (%).
420
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 416–421
Fig. 5 – The effect of CS/EC ratio on elongation rate at the break of LCS-PU foams.
Thermo gravimetric analysis (TGA) showed that LCS-PU foams had a low thermal stability, and decomposition of all samples mainly occurred in four successive stages. With the increase in CS/EC ratio, the initial decomposition temperature increased. Differential scanning calorimetry (DSC) analysis showed that LCS-PU1 and LCS-PU2 have obvious glass transition temperatures (Tg ) but the Tg of others was not observed. It can be attributed to the too large crosslink density which resulted in the disappear of Tg . Analysis of mechanical properties showed that the CS/EC ratio had evident effects on LCS-PU foams prepared from liquefied corn stover with PAPI. The b reached its maximum (1.508 MPa) at a CS/EC ratio of 0.25, Young s modulus reached its maximum (27.977 MPa) at a CS/EC ratio of 0.30, and εb decreased with an increase in CS/EC ratio. Foam cellular structure relative to crosslink density could have more important effects on the mechanical properties of LCS-PU foams. In conclusion, properties of LCS-PU foams can be changed by varying the CS/EC ratio.
Acknowledgements Research support was provided by the National Nature Science Foundation of China (No. 30471374), the Funding System for Scientific Research Projects of Doctor Subject of Chinese Advanced University (No. 20060019041, 20050019029), and the Key Project of Chinese Ministry of Education (No. 105014)
references
Fig. 6 – The effect of CS/EC ratio on Young’s modulus of LCS-PU foams.
to 2.38%, and Young s modulus increased from 3.642 MPa to 27.933 MPa and then slightly decreased to 21.409 MPa. Young s modulus reached its maximum at a CS/EC ratio of 0.30. From Table 1, with an increase in the CS/EC ratio, the residual water increased, which resulted in the increase in urea linkages formed through reaction of water and PAPI. So the hardness of the foams increased, b and Young s modulus increased and εb decreased. Simultaneously, the content of CO2 produced through the reaction between water and PAPI increased and resulted in the growing thinness of cell walls and the growing thickness of the PU foams. The main reason that b largely decreased from 1.508 MPa to 0.497 MPa at the CS/EC ratio between 0.25 and 0.40. Furthermore, it was observed in the co-polymerization process that with the increase in CS/EC ratio, the copolymerization reaction accelerated and the cream time decreased. This can result in the differences of components and/or kinds of –OH in the LCS from different CS/EC ratios. The components and kinds of –OH will be further investigated.
4.
Conclusions
PU foams were prepared from untreated liquefied corn stover at a [NCO]/[OH] ratio of 0.6. FT-IR analysis showed that urethane linkages were formed; there were residual isocyanates in PU foams.
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available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/cherd
Effects of CS/EC ratio on structure and properties of polyurethane foams prepared from untreated liquefied corn stover with PAPI Ti-peng Wang a,1 , Dong Li a,1 , Li-jun Wang b , Jun Yin a , Xiao Dong Chen a,c , Zhi-huai Mao a,∗ a
College of Engineering, China Agricultural University, 17 Qinghua Donglu, Beijing 100083, PR China College of Food Science and Nutritional Engineering, China Agricultural University, 17 Qinghua Donglu, Beijing 100083, PR China c Department of Chemical Engineering, Monash University, Clayton Campus, Victoria, Australia b
a r t i c l e
i n f o
Article history:
a b s t r a c t Corn stover (CS) was liquefied by using ethylene carbonate (EC) as liquefying solvent and
Received 18 July 2007
sulfuric acid as a catalyst at 170 ◦ C for 90 min. Polyurethane (PU) foams were prepared
Accepted 5 December 2007
from liquefied corn stover (LCS) and polymethylene polyphenylisocyanate (PAPI) by oneshot method at a [NCO]/[OH] ratio of 0.6 in the presence of blowing agent, surfactant and co-catalyst. The effects of CS/EC ratio (w/w) on the structural, mechanical and thermal prop-
Keywords:
erties were studied by means of Fourier Transform Infrared Spectroscopy (FT-IR), thermal
FT-IR
analysis (TGA/DSC) and universal tensile machine. FT-IR analysis indicated that urethane
Liquefaction
linkages were formed; free isocyanate groups existed in samples. All samples had a low
Polyurethanes
thermal stability and decomposition occurred in four successive stages. With the increase
Foams
in CS/EC ratio, tensile strength and Young s modulus first increased and then decreased, and
Thermal properties
elongation rate at break decreased. Properties of LCS-PU foams can be changed by varying the CS/EC ratio. © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1.
Introduction
Biomass resource such as agricultural residues are renewable natural polymers and easily obtainable. Only the amount of the world’s corn stover (CS) is estimated to be about 1.27 billion tons in 2004 (China Agriculture Yearbook, 2005). However, large quantities of agricultural residues are discarded, which not only increase the burden of environment but also cause that the valuable energy contained in agricultural residues are not involved in the energy cycle. In recent years, effective utilization of biomass resources has paid growing attention right from the starting to seeking a substitute for petroleum and environmental protection.
Liquefaction techniques can convert the solid lignocellulosic biomass into liquid products which contain some –OH groups and have potential values of substituting the polyester or polyether polyol to prepare PU foams (Bhunia et al., 1999), which can be friendly to the environment (Breslin, 1993). Some researches utilizing liquefied polyol products to prepare polyurethane have been reported (Montane et al., 1998; Yu et al., 2006; Kurimoto et al., 1992, 2000, 2001a,b; Lee et al., 2002; Yao et al., 1995, 1996; Wang et al., in press). Other studies utilizing the natural materials such as starch, soybean oil and cellulose to prepare or modify the properties and degradability of polyurethane have been carried out (Rivera-Armenta et al., 2004; Ciobanu et al., 2004; Huang and Zhang, 2002; Araujo et al., 2005; Lu et al., 2005; Ha and Broecker, 2002; Pechar et
∗ Corresponding author at: College of Engineering, China Agricultural University, P.O. Box 50, 17 Qinghua Donglu, Beijing 100083, PR China. Tel.: +86 10 62737351; fax: +86 10 62737351. E-mail address: [email protected] (Z.-h. Mao). 1 These authors contributed equally to this work. 0263-8762/$ – see front matter © 2007 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.cherd.2007.12.002
c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 416–421
al., 2006; Kwon et al., 2007). In fact, the characters of liquefied products from different liquefaction formulations are different (Liang, 2005) and the properties of PU foams depend on the structures of the polyols and isocyanate (Hepburn, 1982). However, in previous studies, the liquefied products from only one liquefaction formulation are used and investigated. The effects of liquefaction formulations on the properties of polyurethane have been reported very little (Kurimoto et al., 2000). The costs as well as mechanical properties are the main constraints that limit the biodegradable materials to be widely used (Lu et al., 2005). The increase in CS charge can be contributed to the decrease in the prices. This study was aimed at estimating the effects of CS/EC ratio (w/w) on structural, mechanical and thermal properties of LCS-PU foams at a [NCO]/[OH] ratio of 0.6. The effects of factors such as liquefaction time and the charge of catalyst will be further investigated. In this study, LCS-PU foam was prepared by oneshot method through co-polymerization of non pretreated LCS and PAPI in the presence of blowing agent, co-catalyst and surfactant. Structural, mechanical and thermal properties of LCS-PU foams were measured with FT-IR, universal tensile machine, thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC).
2.2.
Experiment
2.1.
Materials and methods
Corn stover with moisture content of 8–10% from a local farm in Beijing suburb of China was milled and only the fractions with particle size of 20–80 meshes were used for the liquefaction experiments. The components of corn stover were analyzed by ANKOM220 cellulose analysis (ANKOM company, USA). The results were soluble materials, 33.16%; cellulose, 34.15%; hemicellulose, 23.86%; lignin, 6.61%; acid insoluble materials, 1.86%. Sulfuric acid (97%, v/v) was used as the catalyst. Ethylene carbonate (99.90%) was used as the solvent in the liquefaction process. PM-200 as the PAPI was obtained from Yantai Wanhua Polyurethane Co., Ltd. (Shandong, China) and the NCO group content was 30.03%. Water and silicone were used as the blowing agent and surfactant, respectively. Triethylamine and dibutyltine dilaurate were used as the co-catalysts. All chemicals used were of reagent grade and were obtained from commercial sources. The chemical structures of cellulose, PAPI and ethylene carbonate were as follows:
Preparation of LCS
Oven-dried corn stover flour 40–80 g, ethylene carbonate 200 g and sulfuric acid 7.4 g were placed in a three-neck flask (1000 ml) equipped with a reflux condenser, a thermometer, and a motor-driven stirrer and refluxed at 170◦ C for 90 min with continuous stirring. Then, the flask was immersed into cold water to quench the reaction and the LCS as the biopolyols was collected for later analysis and use. The acid number of LCS was gained by titratation method with a 0.5 M sodium hydroxide solution, and the hydroxyl number was measured by referring to Kurimoto et al. (2000). The moisture contents of LCS were determined by the Karl Fischer method using a CBS-1A model moisture content meter, Beijing Chaosheng company (Beijing, China). Insoluble residues (unliquefied corn stover) ratio (IRR) of LCS was measured by the following method. A mixture of 2 g LCS was diluted using 50 ml dioxane–water (4:1, v/v) and the insoluble residues were filtered using a Buckner funnel and filter paper (the weight was marked). The residue rinsed thoroughly with the dioxane and 10 blank filter papers were heated for 24 h at 105 ◦ C. The average water content in blank filter paper was calculated. The insoluble residues ratio is given as follows: IRR =
2.
417
w2 − w3 w1
(1)
where w2 and w3 are the dry weight of the filter paper with the insoluble residues and the filter paper, respectively; w1 is the weight of the LCS, 2 g. The characteristics of LCS are listed in Table 1.
2.3.
Preparation of LCS-PU foams
The foams were prepared by the one-shot method. A mixture of 15 g LCS, 0.5 g water, 0.15 g silicone, and 0.3 g co-catalysts (triethylamine:dibutyltine dilaurate = 1:1, w/w) was mixed in a 150 ml polypropylene cup at 1000–1200 rpm for 1 min followed by the addition of 9.60–11.36 g PAPI ([NCO]/[OH] ratio, 0.6) and agitated at 1400–1600 rpm until cream time (about 6–12 s) at room temperature for co-polymerization. The polymerized mixture was poured and daubed onto smooth glass to uniform thin layer of PU foam. The obtained foams were cured for 7 d at room temperature, and then were conditioned for 16 h at 23 ◦ C, 50% RH (relative humidity) according to ISO 11841983 Standard. Every experiment was replicated three times. The [NCO]/[OH] ratio is given as follows:
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Table 1 – The characteristics of LCS with different CS/EC ratio Sample names LCS1 LCS2 LCS3 LCS4 LCS5
CS/EC ratio (w/w)
Acid number (mg KOH/g)
0.20 0.25 0.30 0.35 0.40
19.18 20.61 17.21 9.32 12.64
where MPAPI is the content of the isocyanate group in PAPI (7.15 mmol/g), Mpolyol the content of the hydroxyl group in LCS (hydroxyl number/56.1, the unit is mmol/g), and WPAPI , Wpolyol and Wwater are the weights (g) of PAPI, LCS and water, respectively.
Infrared spectroscopy
Fourier transform infrared spectroscopy (FT-IR) (NICOLET, USA) spectra of LCS-PU foams were carried out in a NICOLET 560 by attenuated total reflectance (ATR).
2.5.
Thermo gravimetric analysis (TGA)
Thermo gravimetric analysis (TGA) (TA, USA) was carried out in DSC-TGA Q600 TA instrument. The samples of 4–8 mg were heated from 40 ◦ C to 700 ◦ C at the rate of 20 ◦ C/min under air flow of 100 ml/min.
2.6.
Differential scanning calorimetry (DSC)
Differential scanning calorimetry (DSC) (TA, USA) was carried out in Q10 TA instrument in two scans. First, 4–8 mg sample were placed in hermetic pans and were scanned up to 50 ◦ C to eliminate the effect of thermal history, thereafter cooled to −80 ◦ C followed by up to 50 ◦ C. All scans were done at the rate of 10 ◦ C/min under dry nitrogen flow of 50 ml/min. The glass transition temperature (Tg ) was determined in the second scan.
2.7.
Moisture content (%)
137.34 108.63 113.59 127.59 108.93
MPAPI × WPAPI [NCO]/[OH] ratio = . MPolyol × WPolyol + WWater × 2/18 × 1000 (2)
2.4.
Hydroxyl number (mg KOH/g)
Mechanical properties of foams
3.
Results and discussion
3.1.
Infrared spectroscopy
1.481 1.790 2.350 2.600 3.040
3 × o´ × 2 . 3 − 1
3.2.
TGA
Fig. 2 shows the typical thermo gravimetric curves of LCSPU2 and LCS-PU4 foams. The thermograms of samples were similar and decomposition mainly occurred in four successive stages. The detailed results of all samples are listed in Table 2. From Table 2, LCS-PU foam had a low thermal stability and with the increase of CS/EC ratio, the initial decomposition
(3)
where is the tensile strength and = L/L0 , L0 and L being the initial and final length of the samples, respectively.
1.98 2.14 4.69 6.56 11.62
The typical FT-IR spectrum of LCS-PU2 foam is shown in Fig. 1. The appearance of absorption peaks at 1720 cm−1 (C O), 1597 cm−1 (ˇN–H) and 1213 cm−1 (C–O) had confirmed the formation of urethane linkages as expected. The stretching vibrations of N–H bonds at 3311 cm−1 can overlap with hydroxyl bonds which belong to the water or liquefied corn stover. Furthermore, peak at 2238 cm−1 indicated the presence of isocyanate. In theory, when [NCO]/[OH] ratio is less than 1.0, the isocyanate will react completely with hydroxyl groups, no remain. In fact, a thin layer of isocyanate was observed in the surface of the samples in experiment. Moreover, the degradation peak temperature of isocyanates is at around 345 ◦ C; in the following themogravimetric analysis, the peak temperature of the third stage is about 345 ◦ C, which also indicated the presence of isocyanates. This could be explained in the sense that some hydroxyl groups did not react with PAPI in copolymerization process due to low reactivity, which resulted in the residues. However, obvious typical absorption peak of allophanates (1640 cm−1 ) was probably not observed in the spectrum, which can be the reason that the peak was weak and/or overlapped with other peaks. Other strong absorption peaks were attributed to carbonyl at 1800 and 1772 cm−1 , asymmetrical stretching vibration of –CH2 at 2927 cm−1 and aromatic at 1506 cm−1 .
The tensile strength ( b ) and elongation rate at break (εb ) of the foams were measured with a universal tensile machine (INSTRON-4411, INSTRON, England) at a crosshead speed of 300 mm/min at 23 ◦ C, 50% RH and Young s modulus was calculated. The grip distance was set at 86 mm. The dumbbell specimens of 120 mm × 10 mm (length × width) according to the ISO 1184-1983 standard were cut directly from the conditioned samples using a dumb-bell shape knife. The thicknesses of the foams were measured with a digital vernier caliper. All samples were measured five times and averages were obtained. Young s modulus (YM) was calculated by the following equation (Ferry, 1970): YM =
IRR (%)
Fig. 1 – FT-IR spectrum of LCS-PU2 sample.
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c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 416–421
Fig. 2 – Thermo gravimetric curves of LCS-PU2 and LCS-PU4 samples.
temperature defined as the point where weight loss beyond 0.5% increased trends. This indicates that crosslink density of foams increased with the increase in CS/EC ratio. From Table 1, with the increase in CS/EC ratio, IRR increased, which can be one of the reasons of the increase in crosslink density because the insoluble residues can act as the crosslinking agent in copolymerization (Rivera-Armenta et al., 2004). In liquefaction, because of sulfuric acid (97%, v/v) as a catalyst, it was possible that the surface cellulose in the insoluble residues was changed into cellulose sulphate. Moreover, due to liquefaction, some active OH groups were possibly exposed. The two factors could induce the insoluble residues to act as a cross-linking agent in co-polymerization. An initial thermal decomposition of all samples was due to the release of volatile components. The hard and soft segment influenced each other and the former could act as antioxidants in decomposition (Wang and Hsieh, 1997). So the first stage was mainly the degradation of soft segment, and the second stage was due to the decomposition of the hard segments. The third stage (peak temperature at around 345 ◦ C) could result in the degradation of isocyanate which did not react with polyol or water (Kurimoto et al., 2001a,b). Moreover, FT-IR had also indicated the presence of isocyanates in LCS-PU foams. The fourth stage is due to the burning of the char produced in the first three stages. Furthermore, some peaks existed between 400 ◦ C and 495 ◦ C of all samples, which could be the decomposition of insoluble residues.
3.3.
Fig. 3 – DSC scans of LCS-PU foams.
Fig. 4 – The effect of CS/EC ratios on the tensile strength of LCS-PU foams.
range of the latter was larger than the former. It can be attributed to the difference of crosslink density. But Tg of LCSPU3, LCS-PU4 and LCS-PU5 were not observed, even in the conditions of the heating rate or when the weight of sample increased. These can be due to their large crosslink density which restricted the movement of molecule.
3.4.
Mechanical properties
DSC
The effects of CS/EC ratio on the DSC curves of LCS-PU foams are shown in Fig. 3. Distinct Tg of LCS-PU1 and LCS-PU2 were observed in the range of scanned temperature and the Tg
The relationships between CS/EC ratios and b, εb or Young s modulus are shown in Figs. 4–6. With an increase in the CS/EC ratio from 0.2 to 0.4, b first increased and reached its maximum (1.508 MPa) at a CS/EC ratio of 0.25, and then decreased to 0.497 MPa. At the same time, εb largely decreased from 63.06%
Table 2 – The decomposition temperature and weight loss of samples at every stage Sample names
LCS-PU1 LCS-PU2 LCS-PU3 LCS-PU4 LCS-PU5
LCS
LCS1 LCS2 LCS3 LCS4 LCS5
Stage 1
Stage 2
Stage 3
Stage 4
Ts
WL
Ts
WL
Ts
WL
Ts
WL
51.37 56.43 58.48 55.23 68.22
16.85 21.70 14.51 19.80 15.73
209.79 226.83 214.25 223.98 242.37
18.74 12.56 19.32 17.32 17.04
316.75 301.88 328.91 326.74 327.82
11.75 12.67 6.81 5.96 5.75
493.34 489.94 486.84 493.33 492.24
41.35 41.31 44.24 41.15 43.23
*Ts is the starting decomposition temperature (◦ C); WL is the weight loss of every stage (%).
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c h e m i c a l e n g i n e e r i n g r e s e a r c h a n d d e s i g n 8 6 ( 2 0 0 8 ) 416–421
Fig. 5 – The effect of CS/EC ratio on elongation rate at the break of LCS-PU foams.
Thermo gravimetric analysis (TGA) showed that LCS-PU foams had a low thermal stability, and decomposition of all samples mainly occurred in four successive stages. With the increase in CS/EC ratio, the initial decomposition temperature increased. Differential scanning calorimetry (DSC) analysis showed that LCS-PU1 and LCS-PU2 have obvious glass transition temperatures (Tg ) but the Tg of others was not observed. It can be attributed to the too large crosslink density which resulted in the disappear of Tg . Analysis of mechanical properties showed that the CS/EC ratio had evident effects on LCS-PU foams prepared from liquefied corn stover with PAPI. The b reached its maximum (1.508 MPa) at a CS/EC ratio of 0.25, Young s modulus reached its maximum (27.977 MPa) at a CS/EC ratio of 0.30, and εb decreased with an increase in CS/EC ratio. Foam cellular structure relative to crosslink density could have more important effects on the mechanical properties of LCS-PU foams. In conclusion, properties of LCS-PU foams can be changed by varying the CS/EC ratio.
Acknowledgements Research support was provided by the National Nature Science Foundation of China (No. 30471374), the Funding System for Scientific Research Projects of Doctor Subject of Chinese Advanced University (No. 20060019041, 20050019029), and the Key Project of Chinese Ministry of Education (No. 105014)
references
Fig. 6 – The effect of CS/EC ratio on Young’s modulus of LCS-PU foams.
to 2.38%, and Young s modulus increased from 3.642 MPa to 27.933 MPa and then slightly decreased to 21.409 MPa. Young s modulus reached its maximum at a CS/EC ratio of 0.30. From Table 1, with an increase in the CS/EC ratio, the residual water increased, which resulted in the increase in urea linkages formed through reaction of water and PAPI. So the hardness of the foams increased, b and Young s modulus increased and εb decreased. Simultaneously, the content of CO2 produced through the reaction between water and PAPI increased and resulted in the growing thinness of cell walls and the growing thickness of the PU foams. The main reason that b largely decreased from 1.508 MPa to 0.497 MPa at the CS/EC ratio between 0.25 and 0.40. Furthermore, it was observed in the co-polymerization process that with the increase in CS/EC ratio, the copolymerization reaction accelerated and the cream time decreased. This can result in the differences of components and/or kinds of –OH in the LCS from different CS/EC ratios. The components and kinds of –OH will be further investigated.
4.
Conclusions
PU foams were prepared from untreated liquefied corn stover at a [NCO]/[OH] ratio of 0.6. FT-IR analysis showed that urethane linkages were formed; there were residual isocyanates in PU foams.
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