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Bio-polyols synthesized from crude glycerol and applications on polyurethane wood adhesives
Industrial Crops & Products 108 (2017) 798–805
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Bio-polyols synthesized from crude glycerol and applications on polyurethane wood adhesives Shaoqing Cuia, Zhe Liua, Yebo Lia,b,
MARK
⁎
a Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Ohio Agricultural Research and Development Center, 1680 Madison Ave, Wooster, OH 44691-4096, USA b Quasar energy group, 8600 E. Pleasant Valley Rd, Independence, OH 44131, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Crude glycerol Bio-polyols Polyurethane adhesive Optimization Prediction model
Crude glycerol (CG), a byproduct of the biodiesel process, was converted through a one-pot thermo-chemical process to produce desirable bio-polyols (CG-polyols) which is suitable for polyurethane (PU) wood adhesives application in this study. The operating parameters (molar ratio of crude glycerol to fatty acid (RCG/FA), temperature, and reaction time) were optimized to obtain bio-based CG-polyols with preferred properties (hydroxyl number, acid number, and viscosity) for the production of PU-wood adhesives. The hydroxyl number of CGpolyols was greatly affected by the molar ratio RCG/FA and temperature, while the acid number was strongly dependent on temperature and time, and viscosity was associated with all three experimental factors. Reliable quantitative prediction models of hydroxyl number, acid number, and viscosity were then established, affording accurate prediction with the differences between predicted and measured data less than 2%. CG-polyols (hydroxyl number of 322 mg KOH/g, acid number of 1.7 mg KOH/g, and viscosity of 25 Pas) suitable for PU adhesives application was produced under the optimized reaction conditions (molar ratio RCG/FA of 1.5, temperature at 220 °C, and reaction time of 5 h). PU wood adhesives with maximum lap shear strength of 36.8 MPa were achieved from the above optimized bio-polyols through the reaction with isocyanate at a NCO/OH ratio (RNCO/OH) of 1.3. Properties of resulting PU wood adhesives presented good thermal stability, comparable lap shear strength and chemical resistance to petroleum based analogies.
1. Introduction Polyols are considered to be the most valuable compounds for pharmaceuticals, food science, and polymer chemistry (Tuck et al., 2014; Vanlede et al., 2015; Chattejee et al., 2014). In polymer chemistry, polyols are extensively used as raw chemicals to produce polyurethane (PU) products (foam, coatings, and elastomers) by reacting them with isocyanates (Moser et al., 2013; Hu et al., 2012; Ni et al., 2010). However, the current polyol industry heavily relies on petroleum, which exacerbates the depletion of a limited natural resource. Given the recent focus on global warming and sustainability, the development of bio-based polymers from renewable resources is of high interest (Desroches et al., 2012). Bio-polyols can be synthesized from vegetable oils via the functionalization of unsaturated sites along the fatty acid chains (Pillai et al., 2016; Alagi et al., 2016). Functionalization can be accomplished through various processing approaches, including epoxidation followed by ring opening (Petrovic et al., 2002), hydroformylation followed by
hydrogenation (Lysenko et al., 2004), ozonolysis followed by hydrogenantion (Petrovic et al., 2004), and a thiol-ene coupling route (Desroches et al., 2011). The resulting bio-polyols have been reported to exhibit properties suitable for PU product development, such as PU foams (Sonnenschein and Wendt, 2013). Nevertheless, vegetable oilbased bio-polyols consume large amounts of vegetable oil, thus competing with food supplies and a growing biodiesel industry. Some researchers have turned to the synthesis of bio-polyols derived from lignocellulosic biomass (Hu and Li, 2014a, 2014b; Chen and Lu, 2009; Hu and Li, 2012), which have shown acceptable performance for PU applications. The production of bio-polyols could be achieved via a liquefaction process that involves the addition of appropriate liquefaction solvents under a preferred reaction temperature and time with the presence of a catalyst (Hu and Li, 2012). But, the traditional liquefaction process also consumes a large amount of petroleum-derived solvents, which, consequently, lessens its renewability. Therefore, it is crucial to explore renewable sources of solvents and alternative approaches to obtain bio-polyols.
⁎ Corresponding author at: Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Ohio Agricultural Research and Development Center, 1680 Madison Ave, Wooster, OH 44691-4096, USA. E-mail address: [email protected] (Y. Li).
http://dx.doi.org/10.1016/j.indcrop.2017.07.043 Received 2 March 2017; Received in revised form 27 July 2017; Accepted 29 July 2017 0926-6690/ © 2017 Elsevier B.V. All rights reserved.
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purchased from Fisher Scientific (Pittsburgh, PA). Pyridine, ethyl alcohol, acetone, 98% concentrated H2SO4, and toluene were purchased from Pharmco-AAPER (Shelbyville, KY). All chemicals above were of reagent grade or higher purity, and they were used without further treatment.
Crude glycerol, the primary byproduct of the natural oil and animal fat transesterification process, is a promising renewable bio-feedstock with low cost, approximately $0.05 per pound (Kerr et al., 2007). Glycerol, an important chemical feedstock, is the predominant compound in crude glycerol; thus, extensive research has been conducted to convert crude glycerol to value-added chemicals through chemical and biological processes. Refined crude glycerol, produced as a result of the above processes, has been used in the applications of pharmaceutical, cosmetics, and PU industries (Clomburg and Gonzalez, 2013; Manosak et al., 2011). But the costly refining technologies for crude glycerol prevent its extended applications. Therefore, it is essential to investigate novel processes that can directly utilize unrefined crude glycerol. A few studies have reported the direct utilization of crude glycerol to produce crude glycerol based bio-polyols (CG-polyols) and PU products, but there have been no reports on synthesizing CG-polyols that are suitable for PU adhesives. Crude glycerol has been converted to biopolyols via a one-pot thermochemical conversion process which is a very simple and efficient reaction occurring in a flask to obtained desirable products without requiring further steps (Li et al., 2014; Luo et al., 2013). The obtained CG-polyols were used to produce PU foams that showed properties comparable to those of petroleum-based foams (Ugarte et al., 2017; Gaidukova et al., 2017). However, the properties of CG-polyols required for PU wood adhesive applications are greatly different from those for PU foams. Rigid PU foams are generally produced from polyols with a hydroxyl number around 400–600 mg KOH/ g and an acid number less than 5 mg KOH/g (Luo et al., 2013). But, the development of PU adhesives requires CG-polyols with a hydroxyl number between 250 and 350 mg KOH/g, an acid number less than 2 mg KOH/g, and a viscosity around ∼25–35 Pas (Kong et al., 2011). To the best of our knowledge, there have been no studies dedicated to investigating the synthesis of CG-polyols suitable for PU wood adhesives. With the purpose of synthesizing CG-polyols with preferred properties suitable for PU wood adhesives, the objectives of this study were to (1) investigate the effects of different operating parameters on the properties of CG-polyols; (2) establish models to predict the properties of CG-polyols and to optimize the operating parameters to produce desirable CG-polyols suitable for PU wood adhesives; (3) develop a suitable procedure to produce PU wood adhesives from CG-polyols; and (4) characterize the properties of CG-polyols obtained under the optimized experiment conditions and the properties of corresponding PU wood adhesives.
2.2. Synthesis of bio-polyols from crude glycerol The crude glycerol fraction was first distilled using a rotary evaporator at 60 °C under vacuum to remove volatiles (mainly methanol). Thermochemical conversion of crude glycerol to bio-polyols was carried out in a 250-mL three-necked flask equipped with a magnetic stirring and heating system (Thermo Electron Corp., Madison, WI), vacuum pump (DOA-P707-FB, Gast Manufacturing, Inc., Benton Harbor, MI), thermometer, and condenser. Designated amounts of distilled crude glycerol and fatty acids, at ratios R (CG/FA) of 1, 1.5 and 2, were added into the flask, and a catalyst (1% NaOH based on the total weight of reactant) was added subsequently. The reactions were conducted at different operating temperatures (180 °C ∼ 220 °C) with 15 inches of Hg vacuum and constant stirring (450 rpm) for 3 ∼ 6 h. The flask was then immediately removed from the heating panel and cooled to room temperature. Water and other volatiles generated during the thermal conversion process were collected by a glass Graham condenser. Crude glycerol-based bio-polyols, i.e. CG-polyols, were recovered from the flask. All CG-polyol production tests were conducted in triplicate. 2.3. Synthesis of PU adhesives The synthesis of PU adhesives was carried out at room temperature under nitrogen protection by mixing the synthesized CG-based polyols or PEG400 with polymeric methylene-4-4′-diphenyl isocyanate in the absence of chain extenders using dibutyltin dilaurate as a catalyst (0.1 wt% based on the total weight of polyols and polymeric methylene4-4′-diphenyl isocyanate). In this study, Dibutyltin dilaurate, which is a general-purpose catalyst, promoted the polymer forming and gelation reaction between the isocyanate and CG-based polyol. PU adhesives based on optimized CG-polyols under different molar ratios of NCO/OH (RNCO/OH) from 1.0 to 1.7 were prepared to investigate the effect of RNCO/OH on the properties of PU adhesives. The preparation of PU adhesives from PEG400, a petroleum-based glycerol, was similar to the procedure of PU adhesive from CG-polyols. The synthesis of PEG400based PU wood adhesives was carried out at a certain molar ratios of RNCO/OH of 1.3.
2. Methods and materials 2.4. Determination of hydroxyl number, acid number, and viscosity 2.1. Materials The hydroxyl number and acid number of obtained CG-polyols were measured following the standards of ASTM D4274-05D and D4662-08, respectively. The viscosity of obtained CG-polyols was measured using a digital Viscometer (Brookfield viscometer LVDV-II + PRO, Brookfield Ameter, Middleboro, MA) at a speed of 60 rpm, 35 °C.
Crude glycerol samples with two separated fractions (crude fatty acid and glycerol fractions) were obtained from a biodiesel plant in Cincinnati, Ohio. The compositions of the two fractions are shown in Table 1. Imidazole, phthalic anhydride, phenolphthalein, standard hydrochloric acid (HCl) solutions (0.1 N), sodium hydroxide (NaOH) solutions (0.1 N and 10 N), Polyethylene glycerol with an average molecular weight of 400 (PEG 400), polymeric methylene-4-4′-diphenyl isocyanate, potassium hydroxide and Dibutyltin dilaurate were
2.5. Experimental design and data analysis Response surface methodology (RSM) with a central composite
Table 1 Compositions of crude glycerol sample. Crude samples Fatty acid fraction Glycerol fraction a b
Glycerol (wt%) 0.4 81.3
Methanol (wt%) a
BDL BDLa
Free fatty acid (wt%)
FAMEsb (wt%)
Soap (wt%)
Water (wt%)
Ash (wt%)
Others (wt%)
63.0 0.4
5.3 BDLa
0.2 0.3
0.6 10.5
0.5 3.0
30.0 4.5
: Below detection limit (BDL). : Fatty Acids Methyl Esters (FAMEs).
799
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Table 2 Central Composite Design and properties of the obtained CG-polyols (hydroxyl number, acid number and viscosity values). a
R(CG/FA) (X1)
b
Tem (C) (X2)
Time (Day) (X3)
Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 a b
1 −1 −1 1 −1 1 1 −1 0 0 0 0 1 −1 0 0 0 0
−1 1 −1 1 −1 1 −1 1 0 0 1 −1 0 0 0 0 0 0
−1 −1 1 1 −1 −1 1 1 1 0 0 0 0 0 0 0 0 0
Hydroxyl number (mg KOH/g)
Acid number(mg KOH/g)
Viscosity (Pas)
Measured
Prediction
Measured
Prediction
Measured
Prediction
238 226 256 249 210 234 365 343 332 327 352 305 418 385 403 418 424 396
232 222 251 241 222 222 360 352 355 337 360 322 422 416 424 428 422 416
2.67 1.65 3.33 3.30 1.11 1.06 2.75 2.72 4.36 6.05 2.20 2.20 2.75 2.75 4.86 3.12 2.63 1.95
2.54 2.01 2.95 2.79 1.11 0.99 2.54 2.41 3.95 5.79 2.11 2.36 2.54 2.41 4.55 3.18 2.11 1.69
20156 15557 13137 13737 15677 14377 24835 28874 23668 22555 21595 31593 26514 38312 25385 26633 27900 31334
19270 16270 13947 13947 15947 13947 26286 26286 25332 24332 23332 30332 27887 37887 28549 27549 28549 30549
molar ratio of crude glycerol to fatty acid. Temperature.
joined wood pieces were kept at room temperature and at humidity of 75 ± 5% for 7 days. Average value of three replicates for each sample was reported. Green strength is one of important indexes to evaluate adhesive properties. For green strength measurement, the prepared wood specimens were cured at room temperature and then directly subjected to lap shear tests at daily intervals up to 7 days. The chemical resistance of produced PU wood adhesives was tested under four conditions: cold water, boiling water, acid solution, and alkali solution. Wood specimens bonded with PU adhesives were immersed in cold water at 4 °C and hot water at 100 °C for 1 day, and in acid (hydrochloric acid, pH 2) and alkaline (sodium hydroxide, pH 10) solutions at 80 °C for 1 h. After that, the specimens were dried at room temperature for 7 days and then subjected to lab shear strength tests as described above.
rotatable design (CCRD) was applied in order to investigate the effects of three operating parameters (molar ratio of RCG/FA, temperature, and reaction time) on the physicochemical properties (hydroxyl number, acid number, and viscosity) of obtained CG-polyols. The levels of different experimental parameters defined as independent variables (RCG/ FA (x1), temperature (x2) and reaction time (x3)), measured data of target physicochemical features defined as dependent variables (hydroxyl number (y1), acid number (y2) and viscosity (y3)), and all experiment runs are listed in Table 2. The synthesis of PU adhesives based on obtained CG-polyols with desirable properties was carried out at different molar ratio of RCG/FA from 1.0 to 1.7. The optimized reaction conditions of the independent variables were selected from preliminary single factor tests. For comparison purposes, the data used for quantitative analysis in section 3.1 were transformed following the linear formula:
x ′ = (x i − x )/ x
(1)
3. Results and discussion
Where x ' is the normalized value, xiis the original value, and x is the mean value. To optimize experimental conditions for synthesizing appropriate CG-polyols for PU adhesives, regression models were developed to predict the performance of the CG-polyols based on the experimental conditions, and the optimized parameters were obtained with response surface methodology (RSM). RSM was carried out using Design-Expert Software Version 10.0 (State-Ease, Minneapolis, MN). The three-dimension (3D) response surface plot of variable response was grap hed with MATLAB (Math Works INC., Natick, MA).
3.1. Effects of operating parameters on the properties of CG-polyols As shown in radar plots of Fig. 1, with the value of RCG/FA increasing from 1.0 to 2.0, the hydroxyl number substantially increased from 0.06 (238 mg KOH/g) to 0.71 (424 mg KOH/g) and the viscosity value slightly increased from 0.23 (20.1 Pas) to 0.47 (27.9 Pas), but the acid number decreased from 0.43 (4.48 mg KOH/g) to 0.24 (2.75 mg KOH/ g). These results can be explained by increased amount of hydroxyl groups due to the increase of glycerin in the feedstock at higher RCG/FA which enhanced the esterification reactions between glycerol and fatty acids, leading to a decrease of acid number and increase of viscosity. The effect of temperature on the CG-polyols properties is shown in Fig. 1 (b). As the temperature increased from 180 °C to 200 °C, the acid number and hydroxyl number decreased by around 60% and 40%, respectively. This observed phenomenon is in agreement with previous reports (Hu and Li, 2014a, 2014b). As expected, the esterification between fatty acid and glycerol and the transesterification reaction between methyl ester of fatty acid (FAMEs) and glycerol were enhanced when the temperature increased, leading to decreases in acid and hydroxyl numbers and an increase in viscosity value. With temperatures higher than 200 °C, the decrease in acid and hydroxyl numbers slowed down, while the viscosity was continuously increased. Besides the enhanced esterification and transesterification reaction, higher temperatures might cause self-polycondensation of glycerol, which enlarged the
2.6. Characterization and properties test Fourier transform-infrared (FT-IR) spectra of produced CG-polyols and PU wood adhesives from CG-polyols and PEG 400 were obtained on a Spectrum Two IR spectrometer (PerkinElmer, Waltham, MA) with 32 scans at a resolution of 2 cm−1. Thermogravimetric analysis (TGA) was performed using a Q50 thermogravimeter (TA Instruments, New Castle, DE) by heating PU adhesive samples from 50 to 600 °C at a rate of 10 °C/ min under nitrogen atmosphere at a flow rate of 0.2 ∼ 0.5 m3/s. Lap shear strength of obtained PU wood adhesives was tested according to ASTM D 906 using a universal testing machine, Instron 3366 (Instron Corp., Norwood, MA). The synthesized PU adhesives was applied to the surfaces of two pieces of prepared wood strips at thickness of about 0.1 mm and an overlap joint area of 25 m × 30 m. Then, the 800
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Fig. 1. Radar plots of CG-polyols properties (hydroxyl number, acid number and viscosity) in the function of molar ratio RCG/FA (a), temperature (T) (b), and reaction time (t) (c).
the combined effects of molar ratio of RCG/FA and reaction time on hydroxyl number. It was obvious that the hydroxyl number increased quickly as the molar ratio RCG/FA increased from 1 to 3, while it decreased with increased reaction time. The maximum hydroxyl number of 445 mg KOH/g was reached with RCG/FA of 2.0 and reaction time of 6 h. Fig. 2 (b) shows the variance of acid number in a function of reaction time and temperature. The acid number was negatively affected when temperatures ranged between160 °C and 240 °C, while it was positively correlated to reaction time. It was predicted that the lowest acid number of 0.9 mg KOH/g could be obtained at a temperature of 215 °C after reaction for 6 h. Fig. 2 (c) is the response of viscosity in function of time and temperature. It is clear that the viscosity of the produced CG-polyols increased with increasing temperature and time. According to previous reports, adhesives with satisfactory bond strength (30.3 N/m2×105 ∼56.38 N/m2×105) and good chemical resistance can be developed from vegetable oil based polyols with a high hydroxyl number (300 ∼ 500 mg KOH/g), low acid number (0.5 ∼ 2.7 mg KOH/g), and suitable viscosity (12 ∼ 41 Pas at 25 °C) (Desai et al., 2003a; Desai et al., 2003b; Ang et al., 2014). CG-polyols with high viscosity are not encouraged for use in the synthesis of adhesive due to the practical difficulty of mixing with isocyanate, therefore, a reaction condition of lower temperature and shorter reaction time should be considered. However, higher temperature and longer reaction time are necessary to obtain low acid number, because acids existing in polyols can react with catalysts used in PU adhesives processes causing a decrease of catalytic efficiency. In addition, our previous researches suggested that CG-polyols with a hydroxyl number around 250 ∼ 350 mg KOH/g, acid number less than 2 mg KOH/g, and viscosity around 25–35 Pas were suitable for the production of PU adhesives with strong bond strength of 35 ∼ 40 MPa and stable thermal properties. Therefore, an optimized experimental condition with RCG/FA of 1.5, temperature of 220 °C, and reaction time of 5 h were strongly suggested.
molecular weight of CG-polyols and thereby increased the viscosity (Luo et al., 2013; ; Hu et al., 2012). Fig. 1 (c) shows the effect of reaction time on the physicochemical profile of CG-polyols. With the reaction time elapsed from 3 h to 6 h, acid number decreased substantially from 0.60 (6.05 mg KOH/g) to 0.09 (2.02 mg KOH/g); viscosity values increased from 0.18 (23.3 Pas) to 0.68 (27.8 Pas), while hydroxyl number slightly decreased. Polynomial regression model is a classical model widely used for process and parameter optimization (Yao, 2010). In the current study, a second order polynomial equation was employed as prediction model. Based on the relationships discussed above, the effects of operating parameters contributed differently to the hydroxyl number, acid number, and viscosity of CG-polyols. For example, the molar ratio RCG/ FA was found to substantially influence the hydroxyl number but marginally affect the acid number. Thus, the independent variable, molar ratio of RCG/FA, would be selected for developing prediction model of hydroxyl number, but it might be omitted when building model for acid number. Using the given structure of the regression model, all operating parameters were involved at first in order to fully determine the potential variables, after comparison, only variables at significant level (p < 0.001) were selected to develop prediction models. Following this criteria, the final prediction models of hydroxyl number, acid number, and viscosity prediction were established, as shown below. All three models had a satisfactory coefficient of determination (R2 > 0.95); and high significance level (p < 0.001), indicating these models could well explain the variations of CG-polyols properties under the changes of operating parameters.
y1 = −1138.90 + 441.87⋅x1 + 10.18⋅x2 + 0.36⋅x1⋅x2 − 113.89⋅x12 − 0.026⋅x x2 (2)
Y2 = 300.60 − 2.75⋅x2 − 0.94⋅x3 − 4416.67⋅x 2⋅x3 + 0.0064⋅x 22 + 0.14⋅x 32 (3)
Y3 = 75491.68 − 12517.84⋅x1 − 387.91⋅x2 − 348.09⋅x3 + 214.81⋅x1⋅x2 + 281.83⋅x1⋅x 3+4.73⋅x2⋅x3 − 6186.36⋅x12 − 0.17⋅x 22 + 17.23⋅x 32
3.2. Properties of CG-polyols produced at the optimized conditions
(4)
Where Y1,y2 and y3 are the hydroxyl number, acid number and viscosity of CG-polyols, and x1, x2 and x3 represents molar ratio R CG/FA, temperature and time, respectively. Values of CG-polyols properties (hydroxyl number, acid number and viscosity) predicted by aforementioned models are listed along with the measured values in Table 2. The differences between the measured and predicted values were averagely less than 2%, which again indicated that these prediction models were reliable and were capable of predicting values of hydroxyl number, acid number and viscosity accurately. The optimization of operating parameters to produce desirable CGpolyols properties for PU wood adhesives application was performed through response surface analysis, as shown in Fig. 2. Fig. 2(a) shows
CG-polyol with a hydroxyl number of 322 mg KOH/g, acid number of 1.7 mg KOH/g, and viscosity of around 25 Pas was obtained under the optimized reaction conditions (molar ratio RCG/FA of 1.5, temperature of 220 °C, and reaction time of 5), and it would be used for PU wood adhesives application. As shown in the Fourier transform infrared (FTIR) spectra (Fig. 3 (a)), a strong bond stretching band at 3445 cm −1 due to hydroxyl groups, a stretching band at 1745 cm−1 due to carbonyl groups of esters, and a stretching peak at 1100 cm −1 due to CeO bonds were observed in the spectra. The appearance of expected characteristic structures in this CG-polyol demonstrated the successful synthesis of bio-polyols from crude glycerol. Fig. 3(b) shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the CG-polyol. Two-stage weight loss was clearly observed with the 801
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Fig. 2. 3D response surfaces of the CG-polyols properties in the function of molar ratio of RCG/FA (a), temperature (b), and reaction time (c).
degradation starting at 180 °C and ending at 400 °C, presenting similar degradation trends with that of polyols derived from vegetable oil (Cayli and Kusefoglu, 2008). The first stage of weight loss occurred from approximately 180 °C to 300 °C, which is probably due to the evaporation of glycerin, C18-free fatty acid, and monoglyceride. The second small degradation occurred at the temperature range between 300 °C and 400 °C, which can be explained by the decomposition of diglycerides and triglycerides, shows a similar degradation curves with that of vegetable oil-based polyols (Cayli et al., 2008). 3.3. Effects of the molar ratio of NCO/OH on the properties of PU-wood adhesives The molar ratio of isocyanate functional group (NCO) to glycerol functional group (OH) (RNCO/OH) is one of the most important impact factors closely related to the properties of PU-wood adhesives (Desai et al., 2003a; Desai et al., 2003b). As shown in Fig. 4 (a), with the increase of RNCO/OH from 1.0 to 1.7, the lap shear strength of obtained PU wood adhesives produced from the optimized CG-polyols increased firstly from 1.0 to 1.3 and then gradually decreased till 1.7 with the maximum bonding strength up to 36.8 ± 3.5 MPa achieved at RNCO/ OH at 1.3. There are might be two reasons resulting in this peak around 1.3. First, with the increase of RNCO/OH, a higher crosslink density of PU adhesive was obtained, leading to an enhanced rigidity and increased bond strength. Besides, extra isocyanates resulting from increased RNCO/OH can react with some compounds of wood samples, and
Fig. 4. Effects of molar ratio of NCO to OH (RNCO/OH) on the lap shear strength (a) and curing time (b) of PU wood adhesives from CG-polyols.
therefore strengthen the adhesive bonding (Keyur et al., 2003). However, further increase of RNCO/OH beyond a critical ratio (1.3) likely caused more complex side reactions, for example, the reaction of isocyanate with water to form ureas and the reaction of ureas with isocyanate again to form biuret, which increases the stiffness of PU
Fig. 3. FTIR (a) and thermogravimetric analysis (TGA) (b) curves of CG-polyols.
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Fig. 5. FTIR spectra (a) and TGA curves (b) of PU wood adhesives from CG-polyols.
increasing over the first 7 days. More important, it is obvious that the averaged lap shear strength of PU wood adhesive from CG-polyols were substantially higher than that of PU adhesives from PEG400, suggesting that CG-polyols-based PU adhesives provided higher bonding strength than that of a petroleum-based analogs. The chemical resistance of these two PU adhesives (from CG-polyols and PEG400, respectively) was tested through the treatment of cold water, hot water, and acid and alkali solutions, as shown in Fig. 6 (b). PU adhesives from GC-polyols show superior resistance to cold water. But treatments of hot water, acid solution, and alkali solution cause negative effects on the adhesion strength. The possible reason is acid and alkali conditions cause stronger hydrolysis of PU wood adhesives than cold water. The sharp reduction in lap shear strength after alkali treatment was probably because of a great degradation of the wood surface due to penetration of the alkali solution, thereby destroying the contact surface between the wood and adhesive. Similar phenomenon was also observed in PU wood adhesives from PEG 400 but they showed much lower lap shear strength. The results were also in agreement with previous studies on chemical resistance of vegetable oil-based PU wood adhesives (Desai et al., 2003). Results from chemical resistance indicated that PU wood adhesives from CG-polyols have exhibited much better performance than PU-wood adhesives from PEG 400. The comparison of lap shear strength between the obtained PU wood adhesives from CG-polyols with commercial analogs was listed in Table 3. CGpolyols-based PU wood adhesives possessed comparable lap shear strength (36.8 ± 2.5 MPa) to that of commercial products (34.7 ∼ 35.5 MPa), indicating the PU wood adhesives from CG-polyols have promising potential on wood bonding applications. Curing time of the obtained CG-polyols-based PU wood adhesives were found to be around 5 ∼ 7 mins, much shorter than that of some commercial adhesives (around 10mins ∼ 45mins). Due to the fast curing time, the CGpolyols-based PU adhesives can be targeted for fast-curing adhesive applications. Further studies on prolonging the curing time could extend its potential applications.
adhesives and causes a decrease of adhesion strength (Moghadam et al., 2016; Kong et al., 2011). Fig. 4 (b) shows the effect of RNCO/OH on curing time. Unlike the effect on lap shear strength, the curing time of PU-wood adhesives was kept decreasing as the RNCO/OH increasing from 1.0 to 1.7. It is might because the residual glycerol in CG-polyol acting as a cross-linker to react with increasing isocyanates thereby resulting in a fast curing time. As shown in Fig. 5(a), a strong absorption band at 3340 cm −1 representing NeH group and an absorption band at around 1736 cm −1 indicating carbonyl groups of urethanes can be observed in the FTIR spectra of the PU wood adhesive), demonstrating the presence of urethane structures in the adhesive. The absorption band at 2274 cm−1 indicates the presence of excess NCO groups in the PU adhesive. The appearance of expected IR characteristic structures confirmed the successful synthesis of PU adhesive from CG-based polyols. As shown in Fig. 5(b), three stages of weight loss of the PU wood adhesive were observed in the TGA profile with degradation starting at 200 °C and ending at 550 °C. A similar degradation behavior of vegetable oil-based PU adhesives was also reported previously (Javni et al., 2000; Ni et al., 2010.) The first stage occurred from 200 °C to 300 °C, resulting from the dissociation of urethane linkages to isocyanates and alcohols and/or to amines and olefins with a loss of CO2 (Javni et al., 2000). The second and third stages occurring between 300 °C and 550 °C, probably due to the intensified decomposition of urethane bonds (Lee et al., 2007) and the scission of fatty acid chains in the polyol structures (Luo et al., 2013; Kong et al., 2011). 3.4. Comparison of properties of PU adhesives from CG-polyols to petroleum-based analogs PU adhesives derived from a petroleum based glycerol (PEG400) with an average hydroxyl number of 265 ∼ 305 mg KOH/g was introduce for the purpose to comprehensively evaluate the properties of obtained PU wood adhesives from the optimized CG-polyols. Fig. 6 shows changes of green strength and chemical resistance of aforementioned PU adhesives. Green strength is one of the important indexes to evaluate adhesive properties. It is closely related to the ability of an adhesive to hold the substrate before reaching its ultimate bond strength when completely cured. As shown in Fig. 6 (a), the lap shear strength of PU adhesives from CG-polyols increased markedly during the first 4 days, then slightly increased from day 5 to day 7. Approximate 85% of the maximum bonding strength of PU adhesives from CGpolyols was reached within the first 4 days. This indicated that the curing reaction occurred quickly during the first 4 days and then slowed down till a full curing occurring over 5 ∼ 6 days. For green strength variance of PEG400-based PU adhesive, the lap shear strength kept
4. Conclusions Results from qualitative analysis based on the radar plot and PCA classification, suggested that the hydroxyl number was mainly influenced by the molar ratio RCG/FA, reaction temperature and acid number were mainly influenced by reaction temperature and time, and viscosity was highly related to all three factors. The preferred conditions for developing desirable CG-polyols for PU wood adhesives were determined to be a molar ratio RCG/FA of 1.5, temperature of 220 °C, and reaction time of 5 h. All the established second-order polynomial models successfully predicted the response of dependent variables with 803
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Fig. 6. Green strength (plot (a)) and chemical resistance (b) of PU wood adhesives from CG-polyol and PU wood adhesives from PEG400. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Desai, S.D., Patel, J.V., Sinha, V.K., 2003b. Polyurethane adhesive system from biomaterial-based polyol for bonding wood. Int. J. Adhes. Adhes. 23, 393–399. Desroches, M., Caillol, S., Lapinte, V., Auvergne, R., Boutevin, B., 2011. Sythesis of biobased polyols by thiol-ene coupling from vegetable oils. Macromolecules 44, 2489–2500. Desroches, M., Escouvois, M., Auvergne, R., Caillol, S., Boutevin, B., 2012. From Vegetable Oil to polyurethanes: synthetic routes to polyols and main industrial products. Polym. Rev. 52, 38–79. Gaidukova, G., Ivdre, A., Fridrihsone, A., Verovkins, A., Cabulis, U., Gaidukovs, S., 2017. Polyurethane rigid foams obtained from polyols containing bio-based and recycled components and functional additives. Ind. Crop. Prod. 102, 133–143. Hu, S., Li, Y., 2012. Production and Characterization of biopolyols and polyurethane foams from crude glycerol based liquefaction of soybean straw. Bioresour. Technol. 103, 227–233. Hu, S., Li, Y., 2014a. Polyols and polyurethane foams from acid-catalyzed biomass liquefaction by crude glycerol: effects of crude glycerol impurities. J. Appl. Polym. Sci. 131, 40739–40748. Hu, S., Li, Y., 2014b. Polyols and polyurethane foams from base-catalyzed liquefaction of lignocellulosic biomass by crude glycerol: effects of crude glycerol impurities. Ind. Crop. Prod. 57, 188–194. Javni, I., Petrovic, Z.D., Guo, A., Fuller, Rachel, 2000. Thermal stability of polyurethane based vegetables oils. J. Appl. Polym. Sci. 77, 1723–1734. Kerr, B.J., Dozier, W.A., Bregendahl, K., 2007. Nutritional value of crude glycerin for nonruminants. In: Proceedings of the 23rd Annual Carolina Swine Nutrition Conference. 2007. Raleigh, NC. pp. 6–18. Keyur, P.S., Sujata, S.K., Natvar, K.P., Animesh, K.R., 2003. Castor Oil based polyurethane adhesives for wood-to-wood bonding. Int. J. Adhes. Adhes. 23, 269–275. Kong, X.H., Liu, G.G., Curtis, J.M., 2011. Characterization of canola oil based polyurethane wood adhesives. Int. J. Adhes. Adhes. 31, 559–564. Lee, C.S., Ooi, T.L., Chuan, C.H., 2007. Synthesis of palm oil-based diethanolamides. J. Am. Oil. Chem Soc. 84, 945–952. Li, C., Luo, X.L., Li, T., Tong, X.J., Li, Y., 2014. Polyurethane foams based on crude glycerol-derived bio-polyols: one-pot preparation of bio-polyols with branched fatty acid ester chains and its effects on foam formation and properties. Polymer 55, 6529–6538. Luo, X., Hu, S., Zhang, X., Li, Y., 2013. Thermochemical conversion of crude glycerol to bio-polyols for the production of polyurethane foams. Biorescource Technol. 139, 323–329. Lysenko, Z., Morrison, D. L., Babb, D. A., Bunning, D. L., Derstine, C. W., Gilchrist, J. H., Jouett, R. H., Kanel, J. S., Olson, K. D., Peng, W., Phillips, J. D., Roesch, B. M., Sanders, A. W., Schrock, A. K., Thomas, P. J., 2004. Aldehyde and alcohol compositions derived from seed oils. WO Patent 2004096744 A2. Manosak, R., Limpattayanate, S., Hunsom, M., 2011. Sequential-redining of crude glycerol derived from waste used-oil methyl ester plant via a combined process of chemical and adsorption. Fuel Process. Technol. 92, 92–99. Moghadam, P.N., Yarmohamadi, M., Hasanzadeh, R., Nuri, S., 2016. Preparation of Polyurethane Wood adhesives by polyols formulated with polyester polyols based on castor oil. Int. J. Adhes. Adhes. 68, 273–282. Moser, P., Comelio, M.L., Telis, V.R.N., 2013. Influence of the concentration of polyols on the rheological and spectral characteristics of guar gum. LWT-Food Sci. Technol. 53, 29–36. Ni, B., Yang, L., Wang, C., 2010. Synthesis and thermal properties of soybean oil-based waterborne polyurethane coatings. J. Therm. Anal. Calorim. 100, 239–246. Petrovic, Z. S., Javni, I., Guo, A., Zhang, W., 2002. Method of making natural oil based polyols and polyurethanes therefrom. US Patent No 6, 433, 121.
Table 3 Lap shear strength of PU wood adhesives from CG-polyol and three commercial PU wood adhesives. Sample
Lap shear strength (MPa)
Bostik’s Best adhesive J.E. Moser’s Wood Glue Sikaflex PU adhesive CG-polyol-based PU adhesive
35.5 30.2 34.7 36.8
± ± ± ±
2.7 3.6 2.1 2.5
R2 values of more than 0.98 (p < 0.05). The maximum lap shear strength of PU adhesives based on CG-polyols was achieved at molar ratio of RNCO/OH at 1.3. Properties of obtained CG-polyols showed expected chemical structures and good thermal stability. Performance of produced PU wood adhesives from CG-polyols exhibited good thermal properties, comparable bond strength and satisfactory chemical resistance to that of petroleum glycerol-based PU adhesives. Acknowledgements This project is supported by funding from USDA-NIFA Critical Agricultural Materials Program (No. 2012-38202-19288). The authors would like to thank Mrs. Mary Wicks (Department of Food, Agricultural and Biological Engineering, OSU) for reading through the manuscript and providing useful suggestions. References Alagi, P., Choi, Y.J., Seog, J., Hong, S.C., 2016. Efficient and quantitative chemical transformation of vegetable oils to polyols through a thiol-ene reaction for thermoplastic polyurethanes. Ind. Crop. Prod. 87, 78–88. Ang, K.P., Lee, C.S., Cheng, S.F., Chuah, C.H., 2014. Polyurethane wood adhesive from palm oil-based polyester polyol. J. Adhes. Sci. Technol. 28, 1020–1033. Cayli, G., Kusefoglu, S., 2008. Increased yields in biodiesel production from used cooking oils by a two-step process: comparison with one step process by using TGA. Fuel Process. Technol. 89, 118–122. Chattejee, A., Maity, B., Ahmed, S.A., Seth, D., 2014. Photophysics and rotational diffusion of hydrophilic molecule in polymer and polyols. J. Phys. Chem A 118, 12680–12691. Chen, F.G., Lu, Z.M., 2009. Liquefaction of wheat straw and preparation of rigid polyurethane foam from the liquefaction products. J. Appl. Polym. Sci. 111, 508–516. Clomburg, J.M., Gonzalez, R., 2013. Anaerobic fermentation of glycerol: a platform for renewable fuels and chemicals. Trends. Biotechnol. 31, 20–28. Desai, S.D., Anurag, L.E., Kumar, S.V., 2003a. Biomaterial based polyurethane adhesive for bonding rubber and wood joints. J. Polym. Res. 10, 275–281.
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Ugarte, L., Santamaria-Echart, A., Mastel, S., Autore, M., Hillenbrand, R., Corcuera, M.A., Eceiza, A., 2017. An alternative approach for the incorporation of cellulose nanocrystals in flexible polyurethane foams based on renewably sourced polyols. Ind. Crop. Prod. 95, 564–573. Vanlede, K., Kluijtmans, L.A.J., Monnens, L., Levtechenko, E., 2015. Urinary excretion of polyols and sugars in children with chronic kidney disease. Pediatr. Nephrol. 30, 1537–1540. Yao, F., 2010. Functional quadratic regression. Biometrika 97, 49–64.
Pillai, P.K.S., Li, S., Bouzidi, L., Narine, S.S., 2016. Metathesized palm oil & novel polyol derivatives: structure, chemical composition and physical properties. Ind. Crops Prod. 84, 205–233. Sonnenschein, M.F., Wendt, B.L., 2013. Design and formulation of soybean oil derived flexible polyurethane foams and their underlying polymer structure/property relationship. Polymer 54, 2511–2520. Tuck, C.J., Muir, J.G., Barrett, J.S., Gibson, P.R., 2014. Fermentable Oligosaccharides, disaccharides, monosaccharides and polyols: role in irritable bowel syndrome. Expert Rev. Gastroenterol. Hepatol. 8, 819–834.
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Industrial Crops & Products journal homepage: www.elsevier.com/locate/indcrop
Bio-polyols synthesized from crude glycerol and applications on polyurethane wood adhesives Shaoqing Cuia, Zhe Liua, Yebo Lia,b,
MARK
⁎
a Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Ohio Agricultural Research and Development Center, 1680 Madison Ave, Wooster, OH 44691-4096, USA b Quasar energy group, 8600 E. Pleasant Valley Rd, Independence, OH 44131, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: Crude glycerol Bio-polyols Polyurethane adhesive Optimization Prediction model
Crude glycerol (CG), a byproduct of the biodiesel process, was converted through a one-pot thermo-chemical process to produce desirable bio-polyols (CG-polyols) which is suitable for polyurethane (PU) wood adhesives application in this study. The operating parameters (molar ratio of crude glycerol to fatty acid (RCG/FA), temperature, and reaction time) were optimized to obtain bio-based CG-polyols with preferred properties (hydroxyl number, acid number, and viscosity) for the production of PU-wood adhesives. The hydroxyl number of CGpolyols was greatly affected by the molar ratio RCG/FA and temperature, while the acid number was strongly dependent on temperature and time, and viscosity was associated with all three experimental factors. Reliable quantitative prediction models of hydroxyl number, acid number, and viscosity were then established, affording accurate prediction with the differences between predicted and measured data less than 2%. CG-polyols (hydroxyl number of 322 mg KOH/g, acid number of 1.7 mg KOH/g, and viscosity of 25 Pas) suitable for PU adhesives application was produced under the optimized reaction conditions (molar ratio RCG/FA of 1.5, temperature at 220 °C, and reaction time of 5 h). PU wood adhesives with maximum lap shear strength of 36.8 MPa were achieved from the above optimized bio-polyols through the reaction with isocyanate at a NCO/OH ratio (RNCO/OH) of 1.3. Properties of resulting PU wood adhesives presented good thermal stability, comparable lap shear strength and chemical resistance to petroleum based analogies.
1. Introduction Polyols are considered to be the most valuable compounds for pharmaceuticals, food science, and polymer chemistry (Tuck et al., 2014; Vanlede et al., 2015; Chattejee et al., 2014). In polymer chemistry, polyols are extensively used as raw chemicals to produce polyurethane (PU) products (foam, coatings, and elastomers) by reacting them with isocyanates (Moser et al., 2013; Hu et al., 2012; Ni et al., 2010). However, the current polyol industry heavily relies on petroleum, which exacerbates the depletion of a limited natural resource. Given the recent focus on global warming and sustainability, the development of bio-based polymers from renewable resources is of high interest (Desroches et al., 2012). Bio-polyols can be synthesized from vegetable oils via the functionalization of unsaturated sites along the fatty acid chains (Pillai et al., 2016; Alagi et al., 2016). Functionalization can be accomplished through various processing approaches, including epoxidation followed by ring opening (Petrovic et al., 2002), hydroformylation followed by
hydrogenation (Lysenko et al., 2004), ozonolysis followed by hydrogenantion (Petrovic et al., 2004), and a thiol-ene coupling route (Desroches et al., 2011). The resulting bio-polyols have been reported to exhibit properties suitable for PU product development, such as PU foams (Sonnenschein and Wendt, 2013). Nevertheless, vegetable oilbased bio-polyols consume large amounts of vegetable oil, thus competing with food supplies and a growing biodiesel industry. Some researchers have turned to the synthesis of bio-polyols derived from lignocellulosic biomass (Hu and Li, 2014a, 2014b; Chen and Lu, 2009; Hu and Li, 2012), which have shown acceptable performance for PU applications. The production of bio-polyols could be achieved via a liquefaction process that involves the addition of appropriate liquefaction solvents under a preferred reaction temperature and time with the presence of a catalyst (Hu and Li, 2012). But, the traditional liquefaction process also consumes a large amount of petroleum-derived solvents, which, consequently, lessens its renewability. Therefore, it is crucial to explore renewable sources of solvents and alternative approaches to obtain bio-polyols.
⁎ Corresponding author at: Department of Food, Agricultural, and Biological Engineering, The Ohio State University/Ohio Agricultural Research and Development Center, 1680 Madison Ave, Wooster, OH 44691-4096, USA. E-mail address: [email protected] (Y. Li).
http://dx.doi.org/10.1016/j.indcrop.2017.07.043 Received 2 March 2017; Received in revised form 27 July 2017; Accepted 29 July 2017 0926-6690/ © 2017 Elsevier B.V. All rights reserved.
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purchased from Fisher Scientific (Pittsburgh, PA). Pyridine, ethyl alcohol, acetone, 98% concentrated H2SO4, and toluene were purchased from Pharmco-AAPER (Shelbyville, KY). All chemicals above were of reagent grade or higher purity, and they were used without further treatment.
Crude glycerol, the primary byproduct of the natural oil and animal fat transesterification process, is a promising renewable bio-feedstock with low cost, approximately $0.05 per pound (Kerr et al., 2007). Glycerol, an important chemical feedstock, is the predominant compound in crude glycerol; thus, extensive research has been conducted to convert crude glycerol to value-added chemicals through chemical and biological processes. Refined crude glycerol, produced as a result of the above processes, has been used in the applications of pharmaceutical, cosmetics, and PU industries (Clomburg and Gonzalez, 2013; Manosak et al., 2011). But the costly refining technologies for crude glycerol prevent its extended applications. Therefore, it is essential to investigate novel processes that can directly utilize unrefined crude glycerol. A few studies have reported the direct utilization of crude glycerol to produce crude glycerol based bio-polyols (CG-polyols) and PU products, but there have been no reports on synthesizing CG-polyols that are suitable for PU adhesives. Crude glycerol has been converted to biopolyols via a one-pot thermochemical conversion process which is a very simple and efficient reaction occurring in a flask to obtained desirable products without requiring further steps (Li et al., 2014; Luo et al., 2013). The obtained CG-polyols were used to produce PU foams that showed properties comparable to those of petroleum-based foams (Ugarte et al., 2017; Gaidukova et al., 2017). However, the properties of CG-polyols required for PU wood adhesive applications are greatly different from those for PU foams. Rigid PU foams are generally produced from polyols with a hydroxyl number around 400–600 mg KOH/ g and an acid number less than 5 mg KOH/g (Luo et al., 2013). But, the development of PU adhesives requires CG-polyols with a hydroxyl number between 250 and 350 mg KOH/g, an acid number less than 2 mg KOH/g, and a viscosity around ∼25–35 Pas (Kong et al., 2011). To the best of our knowledge, there have been no studies dedicated to investigating the synthesis of CG-polyols suitable for PU wood adhesives. With the purpose of synthesizing CG-polyols with preferred properties suitable for PU wood adhesives, the objectives of this study were to (1) investigate the effects of different operating parameters on the properties of CG-polyols; (2) establish models to predict the properties of CG-polyols and to optimize the operating parameters to produce desirable CG-polyols suitable for PU wood adhesives; (3) develop a suitable procedure to produce PU wood adhesives from CG-polyols; and (4) characterize the properties of CG-polyols obtained under the optimized experiment conditions and the properties of corresponding PU wood adhesives.
2.2. Synthesis of bio-polyols from crude glycerol The crude glycerol fraction was first distilled using a rotary evaporator at 60 °C under vacuum to remove volatiles (mainly methanol). Thermochemical conversion of crude glycerol to bio-polyols was carried out in a 250-mL three-necked flask equipped with a magnetic stirring and heating system (Thermo Electron Corp., Madison, WI), vacuum pump (DOA-P707-FB, Gast Manufacturing, Inc., Benton Harbor, MI), thermometer, and condenser. Designated amounts of distilled crude glycerol and fatty acids, at ratios R (CG/FA) of 1, 1.5 and 2, were added into the flask, and a catalyst (1% NaOH based on the total weight of reactant) was added subsequently. The reactions were conducted at different operating temperatures (180 °C ∼ 220 °C) with 15 inches of Hg vacuum and constant stirring (450 rpm) for 3 ∼ 6 h. The flask was then immediately removed from the heating panel and cooled to room temperature. Water and other volatiles generated during the thermal conversion process were collected by a glass Graham condenser. Crude glycerol-based bio-polyols, i.e. CG-polyols, were recovered from the flask. All CG-polyol production tests were conducted in triplicate. 2.3. Synthesis of PU adhesives The synthesis of PU adhesives was carried out at room temperature under nitrogen protection by mixing the synthesized CG-based polyols or PEG400 with polymeric methylene-4-4′-diphenyl isocyanate in the absence of chain extenders using dibutyltin dilaurate as a catalyst (0.1 wt% based on the total weight of polyols and polymeric methylene4-4′-diphenyl isocyanate). In this study, Dibutyltin dilaurate, which is a general-purpose catalyst, promoted the polymer forming and gelation reaction between the isocyanate and CG-based polyol. PU adhesives based on optimized CG-polyols under different molar ratios of NCO/OH (RNCO/OH) from 1.0 to 1.7 were prepared to investigate the effect of RNCO/OH on the properties of PU adhesives. The preparation of PU adhesives from PEG400, a petroleum-based glycerol, was similar to the procedure of PU adhesive from CG-polyols. The synthesis of PEG400based PU wood adhesives was carried out at a certain molar ratios of RNCO/OH of 1.3.
2. Methods and materials 2.4. Determination of hydroxyl number, acid number, and viscosity 2.1. Materials The hydroxyl number and acid number of obtained CG-polyols were measured following the standards of ASTM D4274-05D and D4662-08, respectively. The viscosity of obtained CG-polyols was measured using a digital Viscometer (Brookfield viscometer LVDV-II + PRO, Brookfield Ameter, Middleboro, MA) at a speed of 60 rpm, 35 °C.
Crude glycerol samples with two separated fractions (crude fatty acid and glycerol fractions) were obtained from a biodiesel plant in Cincinnati, Ohio. The compositions of the two fractions are shown in Table 1. Imidazole, phthalic anhydride, phenolphthalein, standard hydrochloric acid (HCl) solutions (0.1 N), sodium hydroxide (NaOH) solutions (0.1 N and 10 N), Polyethylene glycerol with an average molecular weight of 400 (PEG 400), polymeric methylene-4-4′-diphenyl isocyanate, potassium hydroxide and Dibutyltin dilaurate were
2.5. Experimental design and data analysis Response surface methodology (RSM) with a central composite
Table 1 Compositions of crude glycerol sample. Crude samples Fatty acid fraction Glycerol fraction a b
Glycerol (wt%) 0.4 81.3
Methanol (wt%) a
BDL BDLa
Free fatty acid (wt%)
FAMEsb (wt%)
Soap (wt%)
Water (wt%)
Ash (wt%)
Others (wt%)
63.0 0.4
5.3 BDLa
0.2 0.3
0.6 10.5
0.5 3.0
30.0 4.5
: Below detection limit (BDL). : Fatty Acids Methyl Esters (FAMEs).
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Table 2 Central Composite Design and properties of the obtained CG-polyols (hydroxyl number, acid number and viscosity values). a
R(CG/FA) (X1)
b
Tem (C) (X2)
Time (Day) (X3)
Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 a b
1 −1 −1 1 −1 1 1 −1 0 0 0 0 1 −1 0 0 0 0
−1 1 −1 1 −1 1 −1 1 0 0 1 −1 0 0 0 0 0 0
−1 −1 1 1 −1 −1 1 1 1 0 0 0 0 0 0 0 0 0
Hydroxyl number (mg KOH/g)
Acid number(mg KOH/g)
Viscosity (Pas)
Measured
Prediction
Measured
Prediction
Measured
Prediction
238 226 256 249 210 234 365 343 332 327 352 305 418 385 403 418 424 396
232 222 251 241 222 222 360 352 355 337 360 322 422 416 424 428 422 416
2.67 1.65 3.33 3.30 1.11 1.06 2.75 2.72 4.36 6.05 2.20 2.20 2.75 2.75 4.86 3.12 2.63 1.95
2.54 2.01 2.95 2.79 1.11 0.99 2.54 2.41 3.95 5.79 2.11 2.36 2.54 2.41 4.55 3.18 2.11 1.69
20156 15557 13137 13737 15677 14377 24835 28874 23668 22555 21595 31593 26514 38312 25385 26633 27900 31334
19270 16270 13947 13947 15947 13947 26286 26286 25332 24332 23332 30332 27887 37887 28549 27549 28549 30549
molar ratio of crude glycerol to fatty acid. Temperature.
joined wood pieces were kept at room temperature and at humidity of 75 ± 5% for 7 days. Average value of three replicates for each sample was reported. Green strength is one of important indexes to evaluate adhesive properties. For green strength measurement, the prepared wood specimens were cured at room temperature and then directly subjected to lap shear tests at daily intervals up to 7 days. The chemical resistance of produced PU wood adhesives was tested under four conditions: cold water, boiling water, acid solution, and alkali solution. Wood specimens bonded with PU adhesives were immersed in cold water at 4 °C and hot water at 100 °C for 1 day, and in acid (hydrochloric acid, pH 2) and alkaline (sodium hydroxide, pH 10) solutions at 80 °C for 1 h. After that, the specimens were dried at room temperature for 7 days and then subjected to lab shear strength tests as described above.
rotatable design (CCRD) was applied in order to investigate the effects of three operating parameters (molar ratio of RCG/FA, temperature, and reaction time) on the physicochemical properties (hydroxyl number, acid number, and viscosity) of obtained CG-polyols. The levels of different experimental parameters defined as independent variables (RCG/ FA (x1), temperature (x2) and reaction time (x3)), measured data of target physicochemical features defined as dependent variables (hydroxyl number (y1), acid number (y2) and viscosity (y3)), and all experiment runs are listed in Table 2. The synthesis of PU adhesives based on obtained CG-polyols with desirable properties was carried out at different molar ratio of RCG/FA from 1.0 to 1.7. The optimized reaction conditions of the independent variables were selected from preliminary single factor tests. For comparison purposes, the data used for quantitative analysis in section 3.1 were transformed following the linear formula:
x ′ = (x i − x )/ x
(1)
3. Results and discussion
Where x ' is the normalized value, xiis the original value, and x is the mean value. To optimize experimental conditions for synthesizing appropriate CG-polyols for PU adhesives, regression models were developed to predict the performance of the CG-polyols based on the experimental conditions, and the optimized parameters were obtained with response surface methodology (RSM). RSM was carried out using Design-Expert Software Version 10.0 (State-Ease, Minneapolis, MN). The three-dimension (3D) response surface plot of variable response was grap hed with MATLAB (Math Works INC., Natick, MA).
3.1. Effects of operating parameters on the properties of CG-polyols As shown in radar plots of Fig. 1, with the value of RCG/FA increasing from 1.0 to 2.0, the hydroxyl number substantially increased from 0.06 (238 mg KOH/g) to 0.71 (424 mg KOH/g) and the viscosity value slightly increased from 0.23 (20.1 Pas) to 0.47 (27.9 Pas), but the acid number decreased from 0.43 (4.48 mg KOH/g) to 0.24 (2.75 mg KOH/ g). These results can be explained by increased amount of hydroxyl groups due to the increase of glycerin in the feedstock at higher RCG/FA which enhanced the esterification reactions between glycerol and fatty acids, leading to a decrease of acid number and increase of viscosity. The effect of temperature on the CG-polyols properties is shown in Fig. 1 (b). As the temperature increased from 180 °C to 200 °C, the acid number and hydroxyl number decreased by around 60% and 40%, respectively. This observed phenomenon is in agreement with previous reports (Hu and Li, 2014a, 2014b). As expected, the esterification between fatty acid and glycerol and the transesterification reaction between methyl ester of fatty acid (FAMEs) and glycerol were enhanced when the temperature increased, leading to decreases in acid and hydroxyl numbers and an increase in viscosity value. With temperatures higher than 200 °C, the decrease in acid and hydroxyl numbers slowed down, while the viscosity was continuously increased. Besides the enhanced esterification and transesterification reaction, higher temperatures might cause self-polycondensation of glycerol, which enlarged the
2.6. Characterization and properties test Fourier transform-infrared (FT-IR) spectra of produced CG-polyols and PU wood adhesives from CG-polyols and PEG 400 were obtained on a Spectrum Two IR spectrometer (PerkinElmer, Waltham, MA) with 32 scans at a resolution of 2 cm−1. Thermogravimetric analysis (TGA) was performed using a Q50 thermogravimeter (TA Instruments, New Castle, DE) by heating PU adhesive samples from 50 to 600 °C at a rate of 10 °C/ min under nitrogen atmosphere at a flow rate of 0.2 ∼ 0.5 m3/s. Lap shear strength of obtained PU wood adhesives was tested according to ASTM D 906 using a universal testing machine, Instron 3366 (Instron Corp., Norwood, MA). The synthesized PU adhesives was applied to the surfaces of two pieces of prepared wood strips at thickness of about 0.1 mm and an overlap joint area of 25 m × 30 m. Then, the 800
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Fig. 1. Radar plots of CG-polyols properties (hydroxyl number, acid number and viscosity) in the function of molar ratio RCG/FA (a), temperature (T) (b), and reaction time (t) (c).
the combined effects of molar ratio of RCG/FA and reaction time on hydroxyl number. It was obvious that the hydroxyl number increased quickly as the molar ratio RCG/FA increased from 1 to 3, while it decreased with increased reaction time. The maximum hydroxyl number of 445 mg KOH/g was reached with RCG/FA of 2.0 and reaction time of 6 h. Fig. 2 (b) shows the variance of acid number in a function of reaction time and temperature. The acid number was negatively affected when temperatures ranged between160 °C and 240 °C, while it was positively correlated to reaction time. It was predicted that the lowest acid number of 0.9 mg KOH/g could be obtained at a temperature of 215 °C after reaction for 6 h. Fig. 2 (c) is the response of viscosity in function of time and temperature. It is clear that the viscosity of the produced CG-polyols increased with increasing temperature and time. According to previous reports, adhesives with satisfactory bond strength (30.3 N/m2×105 ∼56.38 N/m2×105) and good chemical resistance can be developed from vegetable oil based polyols with a high hydroxyl number (300 ∼ 500 mg KOH/g), low acid number (0.5 ∼ 2.7 mg KOH/g), and suitable viscosity (12 ∼ 41 Pas at 25 °C) (Desai et al., 2003a; Desai et al., 2003b; Ang et al., 2014). CG-polyols with high viscosity are not encouraged for use in the synthesis of adhesive due to the practical difficulty of mixing with isocyanate, therefore, a reaction condition of lower temperature and shorter reaction time should be considered. However, higher temperature and longer reaction time are necessary to obtain low acid number, because acids existing in polyols can react with catalysts used in PU adhesives processes causing a decrease of catalytic efficiency. In addition, our previous researches suggested that CG-polyols with a hydroxyl number around 250 ∼ 350 mg KOH/g, acid number less than 2 mg KOH/g, and viscosity around 25–35 Pas were suitable for the production of PU adhesives with strong bond strength of 35 ∼ 40 MPa and stable thermal properties. Therefore, an optimized experimental condition with RCG/FA of 1.5, temperature of 220 °C, and reaction time of 5 h were strongly suggested.
molecular weight of CG-polyols and thereby increased the viscosity (Luo et al., 2013; ; Hu et al., 2012). Fig. 1 (c) shows the effect of reaction time on the physicochemical profile of CG-polyols. With the reaction time elapsed from 3 h to 6 h, acid number decreased substantially from 0.60 (6.05 mg KOH/g) to 0.09 (2.02 mg KOH/g); viscosity values increased from 0.18 (23.3 Pas) to 0.68 (27.8 Pas), while hydroxyl number slightly decreased. Polynomial regression model is a classical model widely used for process and parameter optimization (Yao, 2010). In the current study, a second order polynomial equation was employed as prediction model. Based on the relationships discussed above, the effects of operating parameters contributed differently to the hydroxyl number, acid number, and viscosity of CG-polyols. For example, the molar ratio RCG/ FA was found to substantially influence the hydroxyl number but marginally affect the acid number. Thus, the independent variable, molar ratio of RCG/FA, would be selected for developing prediction model of hydroxyl number, but it might be omitted when building model for acid number. Using the given structure of the regression model, all operating parameters were involved at first in order to fully determine the potential variables, after comparison, only variables at significant level (p < 0.001) were selected to develop prediction models. Following this criteria, the final prediction models of hydroxyl number, acid number, and viscosity prediction were established, as shown below. All three models had a satisfactory coefficient of determination (R2 > 0.95); and high significance level (p < 0.001), indicating these models could well explain the variations of CG-polyols properties under the changes of operating parameters.
y1 = −1138.90 + 441.87⋅x1 + 10.18⋅x2 + 0.36⋅x1⋅x2 − 113.89⋅x12 − 0.026⋅x x2 (2)
Y2 = 300.60 − 2.75⋅x2 − 0.94⋅x3 − 4416.67⋅x 2⋅x3 + 0.0064⋅x 22 + 0.14⋅x 32 (3)
Y3 = 75491.68 − 12517.84⋅x1 − 387.91⋅x2 − 348.09⋅x3 + 214.81⋅x1⋅x2 + 281.83⋅x1⋅x 3+4.73⋅x2⋅x3 − 6186.36⋅x12 − 0.17⋅x 22 + 17.23⋅x 32
3.2. Properties of CG-polyols produced at the optimized conditions
(4)
Where Y1,y2 and y3 are the hydroxyl number, acid number and viscosity of CG-polyols, and x1, x2 and x3 represents molar ratio R CG/FA, temperature and time, respectively. Values of CG-polyols properties (hydroxyl number, acid number and viscosity) predicted by aforementioned models are listed along with the measured values in Table 2. The differences between the measured and predicted values were averagely less than 2%, which again indicated that these prediction models were reliable and were capable of predicting values of hydroxyl number, acid number and viscosity accurately. The optimization of operating parameters to produce desirable CGpolyols properties for PU wood adhesives application was performed through response surface analysis, as shown in Fig. 2. Fig. 2(a) shows
CG-polyol with a hydroxyl number of 322 mg KOH/g, acid number of 1.7 mg KOH/g, and viscosity of around 25 Pas was obtained under the optimized reaction conditions (molar ratio RCG/FA of 1.5, temperature of 220 °C, and reaction time of 5), and it would be used for PU wood adhesives application. As shown in the Fourier transform infrared (FTIR) spectra (Fig. 3 (a)), a strong bond stretching band at 3445 cm −1 due to hydroxyl groups, a stretching band at 1745 cm−1 due to carbonyl groups of esters, and a stretching peak at 1100 cm −1 due to CeO bonds were observed in the spectra. The appearance of expected characteristic structures in this CG-polyol demonstrated the successful synthesis of bio-polyols from crude glycerol. Fig. 3(b) shows the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of the CG-polyol. Two-stage weight loss was clearly observed with the 801
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Fig. 2. 3D response surfaces of the CG-polyols properties in the function of molar ratio of RCG/FA (a), temperature (b), and reaction time (c).
degradation starting at 180 °C and ending at 400 °C, presenting similar degradation trends with that of polyols derived from vegetable oil (Cayli and Kusefoglu, 2008). The first stage of weight loss occurred from approximately 180 °C to 300 °C, which is probably due to the evaporation of glycerin, C18-free fatty acid, and monoglyceride. The second small degradation occurred at the temperature range between 300 °C and 400 °C, which can be explained by the decomposition of diglycerides and triglycerides, shows a similar degradation curves with that of vegetable oil-based polyols (Cayli et al., 2008). 3.3. Effects of the molar ratio of NCO/OH on the properties of PU-wood adhesives The molar ratio of isocyanate functional group (NCO) to glycerol functional group (OH) (RNCO/OH) is one of the most important impact factors closely related to the properties of PU-wood adhesives (Desai et al., 2003a; Desai et al., 2003b). As shown in Fig. 4 (a), with the increase of RNCO/OH from 1.0 to 1.7, the lap shear strength of obtained PU wood adhesives produced from the optimized CG-polyols increased firstly from 1.0 to 1.3 and then gradually decreased till 1.7 with the maximum bonding strength up to 36.8 ± 3.5 MPa achieved at RNCO/ OH at 1.3. There are might be two reasons resulting in this peak around 1.3. First, with the increase of RNCO/OH, a higher crosslink density of PU adhesive was obtained, leading to an enhanced rigidity and increased bond strength. Besides, extra isocyanates resulting from increased RNCO/OH can react with some compounds of wood samples, and
Fig. 4. Effects of molar ratio of NCO to OH (RNCO/OH) on the lap shear strength (a) and curing time (b) of PU wood adhesives from CG-polyols.
therefore strengthen the adhesive bonding (Keyur et al., 2003). However, further increase of RNCO/OH beyond a critical ratio (1.3) likely caused more complex side reactions, for example, the reaction of isocyanate with water to form ureas and the reaction of ureas with isocyanate again to form biuret, which increases the stiffness of PU
Fig. 3. FTIR (a) and thermogravimetric analysis (TGA) (b) curves of CG-polyols.
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Fig. 5. FTIR spectra (a) and TGA curves (b) of PU wood adhesives from CG-polyols.
increasing over the first 7 days. More important, it is obvious that the averaged lap shear strength of PU wood adhesive from CG-polyols were substantially higher than that of PU adhesives from PEG400, suggesting that CG-polyols-based PU adhesives provided higher bonding strength than that of a petroleum-based analogs. The chemical resistance of these two PU adhesives (from CG-polyols and PEG400, respectively) was tested through the treatment of cold water, hot water, and acid and alkali solutions, as shown in Fig. 6 (b). PU adhesives from GC-polyols show superior resistance to cold water. But treatments of hot water, acid solution, and alkali solution cause negative effects on the adhesion strength. The possible reason is acid and alkali conditions cause stronger hydrolysis of PU wood adhesives than cold water. The sharp reduction in lap shear strength after alkali treatment was probably because of a great degradation of the wood surface due to penetration of the alkali solution, thereby destroying the contact surface between the wood and adhesive. Similar phenomenon was also observed in PU wood adhesives from PEG 400 but they showed much lower lap shear strength. The results were also in agreement with previous studies on chemical resistance of vegetable oil-based PU wood adhesives (Desai et al., 2003). Results from chemical resistance indicated that PU wood adhesives from CG-polyols have exhibited much better performance than PU-wood adhesives from PEG 400. The comparison of lap shear strength between the obtained PU wood adhesives from CG-polyols with commercial analogs was listed in Table 3. CGpolyols-based PU wood adhesives possessed comparable lap shear strength (36.8 ± 2.5 MPa) to that of commercial products (34.7 ∼ 35.5 MPa), indicating the PU wood adhesives from CG-polyols have promising potential on wood bonding applications. Curing time of the obtained CG-polyols-based PU wood adhesives were found to be around 5 ∼ 7 mins, much shorter than that of some commercial adhesives (around 10mins ∼ 45mins). Due to the fast curing time, the CGpolyols-based PU adhesives can be targeted for fast-curing adhesive applications. Further studies on prolonging the curing time could extend its potential applications.
adhesives and causes a decrease of adhesion strength (Moghadam et al., 2016; Kong et al., 2011). Fig. 4 (b) shows the effect of RNCO/OH on curing time. Unlike the effect on lap shear strength, the curing time of PU-wood adhesives was kept decreasing as the RNCO/OH increasing from 1.0 to 1.7. It is might because the residual glycerol in CG-polyol acting as a cross-linker to react with increasing isocyanates thereby resulting in a fast curing time. As shown in Fig. 5(a), a strong absorption band at 3340 cm −1 representing NeH group and an absorption band at around 1736 cm −1 indicating carbonyl groups of urethanes can be observed in the FTIR spectra of the PU wood adhesive), demonstrating the presence of urethane structures in the adhesive. The absorption band at 2274 cm−1 indicates the presence of excess NCO groups in the PU adhesive. The appearance of expected IR characteristic structures confirmed the successful synthesis of PU adhesive from CG-based polyols. As shown in Fig. 5(b), three stages of weight loss of the PU wood adhesive were observed in the TGA profile with degradation starting at 200 °C and ending at 550 °C. A similar degradation behavior of vegetable oil-based PU adhesives was also reported previously (Javni et al., 2000; Ni et al., 2010.) The first stage occurred from 200 °C to 300 °C, resulting from the dissociation of urethane linkages to isocyanates and alcohols and/or to amines and olefins with a loss of CO2 (Javni et al., 2000). The second and third stages occurring between 300 °C and 550 °C, probably due to the intensified decomposition of urethane bonds (Lee et al., 2007) and the scission of fatty acid chains in the polyol structures (Luo et al., 2013; Kong et al., 2011). 3.4. Comparison of properties of PU adhesives from CG-polyols to petroleum-based analogs PU adhesives derived from a petroleum based glycerol (PEG400) with an average hydroxyl number of 265 ∼ 305 mg KOH/g was introduce for the purpose to comprehensively evaluate the properties of obtained PU wood adhesives from the optimized CG-polyols. Fig. 6 shows changes of green strength and chemical resistance of aforementioned PU adhesives. Green strength is one of the important indexes to evaluate adhesive properties. It is closely related to the ability of an adhesive to hold the substrate before reaching its ultimate bond strength when completely cured. As shown in Fig. 6 (a), the lap shear strength of PU adhesives from CG-polyols increased markedly during the first 4 days, then slightly increased from day 5 to day 7. Approximate 85% of the maximum bonding strength of PU adhesives from CGpolyols was reached within the first 4 days. This indicated that the curing reaction occurred quickly during the first 4 days and then slowed down till a full curing occurring over 5 ∼ 6 days. For green strength variance of PEG400-based PU adhesive, the lap shear strength kept
4. Conclusions Results from qualitative analysis based on the radar plot and PCA classification, suggested that the hydroxyl number was mainly influenced by the molar ratio RCG/FA, reaction temperature and acid number were mainly influenced by reaction temperature and time, and viscosity was highly related to all three factors. The preferred conditions for developing desirable CG-polyols for PU wood adhesives were determined to be a molar ratio RCG/FA of 1.5, temperature of 220 °C, and reaction time of 5 h. All the established second-order polynomial models successfully predicted the response of dependent variables with 803
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Fig. 6. Green strength (plot (a)) and chemical resistance (b) of PU wood adhesives from CG-polyol and PU wood adhesives from PEG400. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) Desai, S.D., Patel, J.V., Sinha, V.K., 2003b. Polyurethane adhesive system from biomaterial-based polyol for bonding wood. Int. J. Adhes. Adhes. 23, 393–399. Desroches, M., Caillol, S., Lapinte, V., Auvergne, R., Boutevin, B., 2011. Sythesis of biobased polyols by thiol-ene coupling from vegetable oils. Macromolecules 44, 2489–2500. Desroches, M., Escouvois, M., Auvergne, R., Caillol, S., Boutevin, B., 2012. From Vegetable Oil to polyurethanes: synthetic routes to polyols and main industrial products. Polym. Rev. 52, 38–79. Gaidukova, G., Ivdre, A., Fridrihsone, A., Verovkins, A., Cabulis, U., Gaidukovs, S., 2017. Polyurethane rigid foams obtained from polyols containing bio-based and recycled components and functional additives. Ind. Crop. Prod. 102, 133–143. Hu, S., Li, Y., 2012. Production and Characterization of biopolyols and polyurethane foams from crude glycerol based liquefaction of soybean straw. Bioresour. Technol. 103, 227–233. Hu, S., Li, Y., 2014a. Polyols and polyurethane foams from acid-catalyzed biomass liquefaction by crude glycerol: effects of crude glycerol impurities. J. Appl. Polym. Sci. 131, 40739–40748. Hu, S., Li, Y., 2014b. Polyols and polyurethane foams from base-catalyzed liquefaction of lignocellulosic biomass by crude glycerol: effects of crude glycerol impurities. Ind. Crop. Prod. 57, 188–194. Javni, I., Petrovic, Z.D., Guo, A., Fuller, Rachel, 2000. Thermal stability of polyurethane based vegetables oils. J. Appl. Polym. Sci. 77, 1723–1734. Kerr, B.J., Dozier, W.A., Bregendahl, K., 2007. Nutritional value of crude glycerin for nonruminants. In: Proceedings of the 23rd Annual Carolina Swine Nutrition Conference. 2007. Raleigh, NC. pp. 6–18. Keyur, P.S., Sujata, S.K., Natvar, K.P., Animesh, K.R., 2003. Castor Oil based polyurethane adhesives for wood-to-wood bonding. Int. J. Adhes. Adhes. 23, 269–275. Kong, X.H., Liu, G.G., Curtis, J.M., 2011. Characterization of canola oil based polyurethane wood adhesives. Int. J. Adhes. Adhes. 31, 559–564. Lee, C.S., Ooi, T.L., Chuan, C.H., 2007. Synthesis of palm oil-based diethanolamides. J. Am. Oil. Chem Soc. 84, 945–952. Li, C., Luo, X.L., Li, T., Tong, X.J., Li, Y., 2014. Polyurethane foams based on crude glycerol-derived bio-polyols: one-pot preparation of bio-polyols with branched fatty acid ester chains and its effects on foam formation and properties. Polymer 55, 6529–6538. Luo, X., Hu, S., Zhang, X., Li, Y., 2013. Thermochemical conversion of crude glycerol to bio-polyols for the production of polyurethane foams. Biorescource Technol. 139, 323–329. Lysenko, Z., Morrison, D. L., Babb, D. A., Bunning, D. L., Derstine, C. W., Gilchrist, J. H., Jouett, R. H., Kanel, J. S., Olson, K. D., Peng, W., Phillips, J. D., Roesch, B. M., Sanders, A. W., Schrock, A. K., Thomas, P. J., 2004. Aldehyde and alcohol compositions derived from seed oils. WO Patent 2004096744 A2. Manosak, R., Limpattayanate, S., Hunsom, M., 2011. Sequential-redining of crude glycerol derived from waste used-oil methyl ester plant via a combined process of chemical and adsorption. Fuel Process. Technol. 92, 92–99. Moghadam, P.N., Yarmohamadi, M., Hasanzadeh, R., Nuri, S., 2016. Preparation of Polyurethane Wood adhesives by polyols formulated with polyester polyols based on castor oil. Int. J. Adhes. Adhes. 68, 273–282. Moser, P., Comelio, M.L., Telis, V.R.N., 2013. Influence of the concentration of polyols on the rheological and spectral characteristics of guar gum. LWT-Food Sci. Technol. 53, 29–36. Ni, B., Yang, L., Wang, C., 2010. Synthesis and thermal properties of soybean oil-based waterborne polyurethane coatings. J. Therm. Anal. Calorim. 100, 239–246. Petrovic, Z. S., Javni, I., Guo, A., Zhang, W., 2002. Method of making natural oil based polyols and polyurethanes therefrom. US Patent No 6, 433, 121.
Table 3 Lap shear strength of PU wood adhesives from CG-polyol and three commercial PU wood adhesives. Sample
Lap shear strength (MPa)
Bostik’s Best adhesive J.E. Moser’s Wood Glue Sikaflex PU adhesive CG-polyol-based PU adhesive
35.5 30.2 34.7 36.8
± ± ± ±
2.7 3.6 2.1 2.5
R2 values of more than 0.98 (p < 0.05). The maximum lap shear strength of PU adhesives based on CG-polyols was achieved at molar ratio of RNCO/OH at 1.3. Properties of obtained CG-polyols showed expected chemical structures and good thermal stability. Performance of produced PU wood adhesives from CG-polyols exhibited good thermal properties, comparable bond strength and satisfactory chemical resistance to that of petroleum glycerol-based PU adhesives. Acknowledgements This project is supported by funding from USDA-NIFA Critical Agricultural Materials Program (No. 2012-38202-19288). The authors would like to thank Mrs. Mary Wicks (Department of Food, Agricultural and Biological Engineering, OSU) for reading through the manuscript and providing useful suggestions. References Alagi, P., Choi, Y.J., Seog, J., Hong, S.C., 2016. Efficient and quantitative chemical transformation of vegetable oils to polyols through a thiol-ene reaction for thermoplastic polyurethanes. Ind. Crop. Prod. 87, 78–88. Ang, K.P., Lee, C.S., Cheng, S.F., Chuah, C.H., 2014. Polyurethane wood adhesive from palm oil-based polyester polyol. J. Adhes. Sci. Technol. 28, 1020–1033. Cayli, G., Kusefoglu, S., 2008. Increased yields in biodiesel production from used cooking oils by a two-step process: comparison with one step process by using TGA. Fuel Process. Technol. 89, 118–122. Chattejee, A., Maity, B., Ahmed, S.A., Seth, D., 2014. Photophysics and rotational diffusion of hydrophilic molecule in polymer and polyols. J. Phys. Chem A 118, 12680–12691. Chen, F.G., Lu, Z.M., 2009. Liquefaction of wheat straw and preparation of rigid polyurethane foam from the liquefaction products. J. Appl. Polym. Sci. 111, 508–516. Clomburg, J.M., Gonzalez, R., 2013. Anaerobic fermentation of glycerol: a platform for renewable fuels and chemicals. Trends. Biotechnol. 31, 20–28. Desai, S.D., Anurag, L.E., Kumar, S.V., 2003a. Biomaterial based polyurethane adhesive for bonding rubber and wood joints. J. Polym. Res. 10, 275–281.
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