Preparation and characterization of bio-polyol and bio-based flexible polyurethane foams from fast pyrolysis of wheat straw

Industrial Crops and Products 103 (2017) 64–72

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Preparation and characterization of bio-polyol and bio-based flexible polyurethane foams from fast pyrolysis of wheat straw Hongwei Li a,b , Nubla Mahmood b , Zhen Ma c , Mingqiang Zhu a,d , Junqi Wang a , Jilu Zheng a , Zhongshun Yuan b , Qin Wei a,∗ , Charles (Chunbao) Xu b,∗ a

College of Forestry, Northwest A&F University, Yangling, Shaanxi 712100, China Institute for Chemicals and Fuels from Alternative Resources (ICFAR), Western University, London, Ontario N6GA5B9, Canada c College of Food Engineering and Nutrition Science, Shaanxi Normal University, Xi’an, Shaanxi 710119, China d Beijing Key Laboratory of Lignocellulosic Chemistry, Beijing Forestry University, Beijing 100083, China b

a r t i c l e

i n f o

Article history: Received 13 April 2016 Received in revised form 23 January 2017 Accepted 25 March 2017 Keywords: Wheat straw Fast pyrolysis oil Bio-based flexible polyurethane foam Response surface methodology Resilience Thermal stability

a b s t r a c t Crude bio-oil from fast pyrolysis of wheat straw was extracted with ethyl acetate, and the extracted biooil was used as bio-polyol (hydroxyl number: 77.8 mg KOH/g) for the preparation of bio-based flexible polyurethane foams. GC–MS and FTIR analysis of the extracted bio-oil or bio-polyol evidenced that it has the presence of a larger amount of compounds containing multiple hydroxyl groups than the crude biooil. The foams were characterized for their physical, mechanical and thermal properties. Response surface methodology(RSM) was employed to optimize several independent variables (30–50 wt.% of bio-polyol, 10–30 wt.% of polymethylene polyphenylene isocyanate (PM200), 10–30 wt.% of polymeric diphenylmethane diisocyanate (PMDI), and 0.5–1.5 wt.% of cross linking agent, total amounts of isocyanates (PM200 and PMDI) was kept at 40 wt.%, all based on weight of total polyol) for their effects on the foam resilience. Under the optimized conditions, i.e., 30 wt.% bio-polyol, 17.9 wt.% PM200 and 22.1 wt.% PMDI, 0.5 wt.% cross linking agent, the resulted foam has resilience of 37.0% and is thermally stable up to 200 ◦ C. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Polyurethanes (PUs) are widely used for many engineering applications, such as insulation materials, automotive parts, bedding and furniture, electrical, flotation, packaging fields, and structural materials (Ge et al., 2000). The broad applications of PUs could be due to their diversified properties caused by different components and additives (Hakim et al., 2011). PU foam is realized through the reaction of isocyanates and polyols with other additives, blowing agents and catalysts (Xu et al., 2014). Currently, both polyols and isocyanates are mainly derived from petroleum resources. However, due to increasing concerns over the depletion of fossil fuels and their negative impacts on the environment and ecosystem, there is a growing interest in exploration for renewable feedstocks to replace petroleum derived polyols either partially or completely with bio-based polyols for the production of PU foams (Tanaka et al., 2008; Hu and Li, 2014). A number of vegetable oils have been used to prepare PU foams, such as soybean oil (Guo et al., 2000), palm oil (Tanaka et al., 2008),

∗ Corresponding authors. E-mail addresses: [email protected] (Q. Wei), [email protected] (C. Xu). http://dx.doi.org/10.1016/j.indcrop.2017.03.042 0926-6690/© 2017 Elsevier B.V. All rights reserved.

castor oil (Zhang et al., 2014), rape seed oil (Hu et al., 2002), linseed oil (Khoe et al., 1972) etc. However, some of these vegetable oils are produced on a very limited commercial scale. Bio-based polyols can also be produced by conversion of abundantly available biomass into bio-oils using chemical, thermochemical or biological methods. Chemical methods, mainly acid hydrolysis, needs special facilities; where biological method is a time consuming process. Bio-polyols obtained from the thermochemical method (e.g., pyrolysis or liquefaction) of lignocellulosic biomass have promising properties for their utilization in the preparation of PU foams, with comparable attributes to petroleum analogs (Hu et al., 2012; Yoshioka et al., 2013; Zhang et al., 2013). Commonly, the production of these polyols is realized by the liquefaction of lignocellulosic biomass in the presence of polyhydric alcohols as liquefying agents (Alma, 2007; Hakim et al., 2011; D’ Souza et al., 2014; Liu et al., 2014; Xu et al., 2014). Lignocellulosic biomass can be transformed into liquid oil via fast pyrolysis (Dahmen et al., 2012; Basile et al., 2014; Kohl et al., 2014), in the absence of oxygen (Henrich, 2007; Effendi et al., 2008). Bio-oils derived from agricultural and forestry residues/byproducts are more environmental friendly (Torri et al., 2016; Mahmood et al., 2013). Pyrolysis oil can be a renewable source of fuels and chemicals. Over hundreds of compounds have already

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Table 1 Proximate and ultimate analysis of wheat straw. Proximate analysis (wt.%, d.b.)a , b d

Ultimate analysis (wt.%, d.a.f)a , c

VM

e

FC

Ash

C

H

O

N

S

Cl

Residue

71.31

19.79

8.90

43.20

5.00

39.41

0.60

0.10

0.28

11.41

a b c d e

On a dry basis. Determined by thermogravimetric analysis under N2 flow (20 mL/min) from 40 ◦ C to 900 ◦ C at 10 ◦ C/min. Ash free basis. VM: Volatile matters. FC: Fixed carbon.

Table 2 Physical properties of chemicals used. Ingredients

Functionality

Equivalent weight (g/mol)

−OH#a (mgKOH/g)

Comments

PMDI PM 200 Polyol HSH330N Diethanolamine (DEOA) Silicone oil A−1b A−33c T−9d Water

2.7 2.6 3.0 2.0 – – – – 2.0

135.0 135.5 5000 52.6 – – – – 9.0

– – 35.4 2133.1 – – – – 6233.3

NCO contents: 31.2% NCO contents: 30.5–32.0% Petroleum polyol Crosslinking agent Silicon surfactant Foaming catalyst Gelation catalyst Gelation catalyst Chemical blowing agent

a b c d

−OH#: Hydroxyl number. A-1: 70% solution of bis (2-dimethylaminoethyl) ether. A-33: triethylenediamine. T-9: stannous octoate.

been identified in bio-oils, though the amount of individual compound is low and isolation of single compounds is relatively difficult (Czernik and Bridgwater, 2004; Vamvuka, 2011). The aqueous phase isolated from the pyrolysis oil contains a variety of organic acids, mostly are formic, acetic acids which can react with lime to form calcium salts (Zhou et al., 1997; Pisupati and Bhalla, 2008). Due to the dwindling of fossil fuels resources and their associated environmental concerns, trends are moving towards bio-based chemicals/fuels/materials. Agricultural residue materials can be an ecofriendly and economic feedstock for bio-based chemicals/fuels/materials due to their unique chemical composition, low cost, abundant availability, and renewability (Sud et al., 2008; Wang and Chen, 2007). Moreover, it does not compete with food resources. Therefore, utilization of this low cost and abundantly available feedstock for the preparation of bio-products reduces the dependence on fossil resources and can produce sustainable products and materials. Therefore, the main objective of this study was to prepare bio-based flexible polyurethane (BFPU) foams using bio-polyol, extracted from fast pyrolysis oil of wheat straw. The bio-polyol was used to substitute petroleum-based polyols (≥30 wt.%) and the process parameters affecting the foaming process such as bio-polyol to petroleum-based polyol ratio, PM200 to PMDI ratio, amount of cross linking agent, were optimized using response surface methodology (RSM). The obtained BFPU foams were characterized in terms of their physical, mechanical and thermal properties and compared with the reference foams for their potential utilization as car cushion.

2. Materials and methods 2.1. Materials The bio-oil used in this study was fast pyrolysis oil of wheat straw (whose ultimate and approximate analysis is provided in Table 1) at 500 ◦ C with vapor residence time of 5 s, provided by the University of Science and Technology of China (Hefei, Anhui Province, China) and produced via fast pyrolysis of wheat straw at

500 ◦ C with vapor residence time of 5 s. The pyrolysis oil obtained was a mixture of bio-oil, aqueous phase and other impurities, and was further purified before its utilization in the foam preparation. The water in the oil was removed at 50 ◦ C using rotary evaporator under −0.09 MPa vacuum (Chaala et al., 2004). The dewatered biooil was then extracted using equal volume of ethyl acetate to collect the water insoluble fractions, followed by removing the extraction solvent via rotary evaporating at 50 ◦ C under reduced pressure. The extracted bio-oil (water insoluble fraction) was termed as biopolyol. Other chemicals used in this work are all CAS reagent grade chemicals, purchased from Sigma-Aldrich and used without further purification, including polyethylene glycol HSH330N, polymeric diphenylmethane diisocyanate (PMDI), polymethylene polyphenylene isocyanate (PM200), surfactant (methyl silicone oil), catalysts including 70% solution of bis (2-dimethylaminoethyl) ether (A1), triethylenediamine (A-33) and stannous octoate (T-9) etc. The physical properties of all the chemicals used in the foam preparation are given in Table 2. 2.2. Preparation of flexible polyurethane foams All bio-based flexible foams were prepared using one shot method. The foaming system used in this study was comprised of two mixtures i.e., white mixture and black mixture. White mixture includes polyols, catalysts, stabilizing agent (surfactant), cross linking agent and blowing agent. Where, black mixture contains PM200 (NCO contents: 31.2 wt.%) and PMDI (NCO contents: 30.5–32.0 wt.%). According to our preliminary experiments, the bio-based foams prepared with the weight ratio of total polyols to isocyanates 2.5 showed good physical properties, therefore, for better control we prepared bio-based PU foams and the reference form under the same weight ratio of total polyols to isocyanates. As shown in Table 3, for all three bio-based PU foam formulas, the isocyanate indexes are in the narrow range of 0.66-0.68, not differing too much from that of the reference foam (0.72). Briefly, the foaming procedure employed comprises the following steps: (1) the desired amount of white mixture was weighed in a cup, followed by the premixing of the ingredients at a speed

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Table 3 Foam formulas. Weight (wt.%)

Ingredients

polyol HSH330N Bio-polyol Diethanolamine A−1a A−33b Stannous octoate water PM 200 PMDI Isocyanate index

Ref. Foam

Optimal foam

P40 foamc

P50 foamd

100 – 0.5 0.1 0.6 0.25 3.0 17.9 22.1 0.72

70 30 0.5 0.1 0.6 0.25 3.0 17.9 22.1 0.68

60 40 0.5 0.1 0.6 0.25 3.0 17.9 22.1 0.67

50 50 0.5 0.1 0.6 0.25 3.0 17.9 22.1 0.66

a

A-1: 70% solution of bis (2-dimethylaminoethyl) ether. A-33: triethylenediamine. c P40 foam: foam prepared with 40% bio-polyol according to the optimized formulation recipe. d P50 foam: foam prepared with 50% bio-polyol according to the optimized formulation recipe. b

of 3000 rpm for 180 s to obtain a homogeneous mixture, (2) a precalculated amount of black mixture was then vigorously stirred in a separate cup for 120 s at 3000 rpm and then, (3) the black mixture was added into the white mixture and mixture was stirred for 5–8 s at high speed and then the final mixture was transferred into an open mold. The dimensions of the mold used were 100 mm × 100 mm × 20 mm. All the foam samples were left in the fume hood for 72 h for curing prior to analysis. 2.3. Characterizations of bio-oil, bio-polyol and foams ASTM D 4274-99 method was used for determination of hydroxyl numbers of bio-oil and bio-polyol as follows: 2 g sample and 20 mL of acetylation reagent were heated for 2 h at 98 ◦ C (The acetylation reagent consisted of a mixture of 127 mL acetic anhydride and 1000 mL pyridine). This was followed by the rinsing of the flask with 20–30 mL of water, and the mixture was titrated with a 0.5 N sodium hydroxide solution to the equivalence point. The hydroxyl number in mg KOH/g of the sample was calculated by the following equation: Hydroxyl number(OH#) = [(B − A) × N × 56.1]/W

(1)

Where, B is the amount (ml) potassium hydroxide solution required for the titration of the blank free of bio-oil, and A is amount (ml) of potassium hydroxide solution required for the titration of the acetylated bio-oil sample, N is the normality of the potassium hydroxide solution, and W is the weight of sample (g). NCO/OH (isocyanate index) can be calculated according to the following equation: NCO/OH = (%NCO × 1000 × WNCO/42) 56.1 +



MAd × WAd/56.1 +





MOH × WOH/



MH2O × WH2O/56.1

(2)

Where, WNCO is the weight of isocyanate (g), MOH is the hydroxyl number of polyol (bio-polyol and Polyol HSH330N), WOH is the weight of polyol (g), MAd is the hydroxyl number of additives (crosslinking agent), WAd is the weight of additives (g), MH2O is the hydroxyl number of water, WH2O is the weight of water (g), 1000 is the unit conversion of mol to mMol, 42 is the molecular weight of NCO, 56.1 is the molecular weight of KOH. The filtered fresh sample of bio-oil and bio-polyol was dissolved in acetone to make a homogeneous solution for gas chromatography-mass spectrometer (GC–MS) analysis. GC–MS analysis was conducted with ThermoFinnigan GC–MS (ThermoFinnigan, California, US). The carrier gas (He) was sup-

plied at a constant flow rate of 1.0 mL/min. Separation was performed using a ThermoFisher TR-1MS capillary column (30 m × 0.5 mm × 0.25 ␮m). Injection port temperature was 250 ◦ C. The column temperature was maintained at 50 ◦ C for 2 min, and then programmed to increase to 250 ◦ C at a rate of 10 ◦ C/min, held for 10 min. Split injection was conducted at a split rate of 25:1. Mass spectroscopic conditions were as follows: electron impact source, electron energy: 70 eV, ion source temperature of 200 ◦ C, mass scanning range: 35–650 amu/s. Compounds were identified by the comparison of retention time and mass spectra using library data of mass spectra (NIST). The relative contents of compounds were measured by their peak areas. The dried samples of crude bio-oil and bio-polyol were analyzed by BRUKER Fourier transform infrared (FTIR) spectroscopy (Vetex70, Karlsruhe, Germany), for their functionality changes in the range of 400–4000 cm−1 (resolution: 4 cm−1 , scan: 16). The viscosity of the bio-oil and bio-polyol were measured using a NDJ-5S rotary viscometer (Shandong, China) at 25 ◦ C. The apparent density of the foam samples was measured according to ASTM D3574-11 test A with the sample size of 45.0 cm3 . The mechanical properties of the synthesized foams were measured at ambient conditions using the Texture Analyzer (TA. XT. plus) (Stable Micro Systems Ltd, Godalming, United Kingdom) according to GB/T 18941-2003 (resilience) and ASTM D3574-11 (tensile strength and tear strength). To analyze the mechanical properties of foam using Texture analyzer three different modes were used depending on the type of analysis. The tensile strength and tear strengths were measured in Mode two and three, respectively. The resilience was tested in Mode one. Mode one was set according to the following conditions: sample size of ∼50 mm × 50 mm × 18 mm; compression and resilience, probe P/36R, pre-test speed 1.00 mm/s, test speed 1.00 mm/s, post-test speed 1.00 mm/s, time interval 5 s, strain 50%, trigger force 5.0 g. The mode two was set as follows: sample size of 100 mm × 35 mm × 15 mm; extension and tensile strength, probe A/TG, pre-test speed 1.00 mm/s, test speed 1.00 mm/s, post-test speed 10.00 mm/s, distance 50 mm, trigger force 5.0 g. Mode three for measuring tear strength was set as follows: sample size of 50 mm × 40 mm × 18 mm; shear force and tear strength, probe A/CKB, pre-test speed 1.00 mm/s, test speed 1.00 mm/s, post-test speed 10.00 mm/s, strain 100%, trigger force 5.0 g. AJSM-6360LV scanning electron microscope (SEM) (JEOL, Chibaken, Japan) (15 kV accelerated voltage) was used to determine the morphology of the foam prepared with the optimized formulation recipe. Platinum-plated specimens were prepared prior to SEM inspection. FTIR analysis of the bio-based flexible foam obtained at the optimal formulation was also realized (resolution: 4 cm−1 , scan: 16, range, 4000–600 cm−1 ). The thermal stability of foam sample was determined by Shimadzu thermogravimetric analysis (TGA) using TA-60WS+ DTG-60A mode simultaneous thermal analyzer, under a nitrogen flow (50 mL/min) from room temperature to 800 ◦ C at a flow rate of 10 ◦ Cmin−1 . About 1–2 mg of the foam sample was used in each analysis. 2.4. Statistical experimental design Preliminary studies have been performed using single factor experimental design to screen for suitable levels of each factor. In single factor analysis seven factors i.e., bio-polyol to petroleum polyol weight ratio (3:7–7:3, w/w), polymethylene polyphenylene isocyanate (PM 200) to polymeric diphenylmethane diisocyanate (PMDI) weight ratio (0:4–4:0, w/w), 70% solution of bis (2-dimethylaminoethyl) ether (A-1) to triethylenediamine (A-33) ratio (A-1/A-33 0.1 wt.%/0.3–0.7 wt.%), amount of surfactant (0.8–1.2 wt.%), amount of cross linking agent diethanolamine (DEOA: 0.0–2.0 wt.%), amount of stannous

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Table 4 The 33 full factorial and Box-Behnken design for experiments along with actual responses. Foam ID

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Variables in coded units A

B

C

−1 −1 −1 0 +1 0 −1 +1 0 −1 0 +1 0 +1 +1 −1 0

+1 −1 −1 0 −1 0 +1 −1 0 +1 −1 −1 +1 −1 0 +1 +1

+1 −1 +1 0 −1 0 −1 +1 0 +1 0 +1 0 0 +1 0 −1

A

B

C

Bio-polyol (wt.%)

DEOA (wt.%)

PM 200 (wt.%)

30 30 30 40 50 40 30 50 40 30 40 50 40 50 50 30 40

1.5 0.5 0.5 1.0 0.5 1.0 1.5 0.5 1.0 1.5 0.5 0.5 1.5 0.5 1.0 1.5 1.5

30 10 30 20 10 20 10 30 20 30 20 30 20 20 30 20 10

Y (%)

30.5 35.5 34.0 27.2 26.6 28.9 31.0 21.6 26.3 32.9 24.4 23.7 29.8 26.2 27.0 31.9 24.4

Fig. 1. FTIR analysis of crude bio-oil and bio-polyol extracted using ethyl acetate.

octoate (T-9: 0.1–0.3 wt.%) and amount of blowing agent (water: 2.0–4.0 wt.%), all based on weight of total polyols, were analyzed. Then, based on the results of flexibility of foam and its structural uniformity the factors that showed the least influence on the final properties of the foam were removed from the analysis. Then, after the selection of most significant parameters, statistically designed experiments were constructed using response surface methodology (RSM) with Box-Behnken design (BBD) to investigate the effects of three independent variables (bio-polyol, DEOA, PM 200) on the foam properties which subsequently lead to an optimized flexible foam formulation recipe. All the levels of the three factors used are shown in the Supplementary Table S1, where the levels of bio-polyol, DEOA and PM 200, coded as A, B and C, are in the range of 30–50 wt.%, 0.5–1.5 wt.% and 10–30 wt.%, respectively. The levels of other factors were kept the same during the optimization experiment, i.e., A-1 0.1 wt.%, A-33 0.6 wt.%, T-9 0.25 wt.% and water 3.0 wt.%, where, A-1 represents 70% solution of bis (2dimethylaminoethyl) ether, A-33 is for triethylenediamine and, T-9 represents stannous octoate.

A total of 17 experiments were performed. Experimental levels of both coded/actual values of independent variables were presented in Supplementary Table S1. The total numbers of experiments along with their different parameter levels are provided in Table 4. The quadratic equation obtained from Box-Behnken model for predicting the optimal conditions can be expressed in the form of the following equation: Y = ˇo +

 i

ˇixi +

 i

ˇiix2ii +



ˇijxixj + ␧

(3)

i
where ˇ0, ˇi, ˇij are regression coefficients for the intercept, linear and interactions among factors, respectively; Y is the predicted response; xi and xj are independent variables in coded units and ␧ is the error term (Ghosh and Thakur, 2014). Table 4 shows all the three experimental variables along with their actual values. The data were subjected to analysis of variance (ANOVA) and the coefficient of regression (R2 ) was calculated to find out the best fit of the model. Two dimensional plots were also obtained to visualize the individual and interactive effects of the factors on the response surfaces. Optimization using the desirability function (numerical

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Table 5 Physical properties of obtained bio-oils. Physical Properties

Bio-oil

Bio-polyol

Hydroxyl number (mgKOH/g) Acid number (mgKOH/g) Viscosity (c.P.) NCc (%) pH

45.60 1.44 NDa 21.4 2.58

77.76 0.96 87 52.3 UMb

a ND: Not determined. The viscosity of bio-oil is too low to be determined accurately. b UM: Unmeaning. The pH value of bio-polyol is not determined. c NC: Nonvolatile content.

optimization) was applied to optimize the level of bio-polyol, DEOA and PM 200 for maximum responses. Validation test was performed by duplicate runs under the optimized conditions obtained from numerical optimization to validate the adequacy of the model. The averaged value of the experiments obtained during the validation test was compared with the predicted value by the model to determine the accuracy and suitability of the model. 2.5. Preparation of the reference and bio-based PU foams The reference PU foam, and three bio-based PU foams (according to the foam recipe determined from the RSM), including the optimal foam and P40 and P50 foams (containing 40% and 50% bio-polyol, respectively) were prepared in accordance to the foam formulas as listed in Table 3. 3. Results and discussion 3.1. GC–MS analysis of bio-polyol The bio-polyol was characterized by GC–MS for the identification of its chemical composition. The percentage of each compound illustrated in the Supplementary Table S2 only represents the relative content of the compound over all the compounds that can pass through the GC column. There were also some unknown constituents identified in the bio-polyol. Furthermore, compounds having large molecular weights or high boiling points could not be detected under the used GC–MS conditions. The major compounds in the bio-polyol (ethyl acetate soluble fraction of the pyrolysis oil) were phenol and their derivatives (49.09%), which can be effective bio-polyols due to the presence of hydroxyl groups in their structure to react with isocyanates. The GC–MS results for the biopolyol thus evidence that its composition is suitable for application in the preparation of polyurethane due to the presence of multiple hydroxyl groups in their structure (Ge et al., 2000). 3.2. Physical properties of the crude bio-oil and bio-polyol The physical properties of the crude bio-oil and bio-polyol are shown in Table 5. Due to the presence of a high content of water, the viscosity of crude bio-oil was relatively low and it could not be measured accurately, while the bio-polyol appeared as a viscous liquid at room temperature, with viscosity of 87 cps at 25 ◦ C. The hydroxyl value of the crude bio-oil and bio-polyol were determined as 45.60 mg KOH/g and 77.76 mg KOH/g, respectively, although the hydroxyl number of the bio-polyol was slightly lower than that of ca. 85 mg KOH/g of hyper-branched polyols from soybean oil successfully used for producing polyurethane foams (Zlatanic et al., 2014). The acid value of the bio-polyol was determined as 0.96 mg KOH/g, compared with 1.44 mg KOH/g of crude bio-oil, suggesting rejection of acidic compounds to the water soluble fraction by solvent extraction. The hydroxyl number is a vital property of a polyol for PU preparation. Although, the hydroxyl number of

the bio-polyol obtained after extraction from the fast pyrolysis oil derived from wheat straw is slightly lower than those of the polyols obtained via biomass liquefaction (D’ Souza et al., 2014; Xu et al., 2014), it can still be a promising substitute to petroleum derived polyols for PU preparation. FTIR spectrum of the bio-polyol is similar to that of the crude bio-oil, which indicates that the extraction operation had almost minimal effects on the functional structure of the oil (Fig. 1). The stretching vibration at 3600–3200 cm−1 is associated with OH groups. The deformation vibrations at 1450–1370 cm−1 are related to C H chain formation, which is the reflection of the carbon skeleton of the bio-oil components. The peaks at 1200–1100 cm−1 are related to C O groups from alcohols, phenols and esters (Du et al., 2011) in the bio-oil and bio-polyol. 3.3. Optimization via RSM The 17 designed experiments were carried out in accordance with levels of bio-polyol, DEOD and PM 200 as given in Supplementary Table S1, at a fixed amount of A-33 0.6 wt.%, surfactant 1.0 wt.%, water 3.0 wt.% and T-9 0.25 wt.%, respectively. Table 4 shows the actual responses (resilience, Y) in the 17 designed experiments. Analysis of variance (ANOVA) was performed using Design Expert software 8.0.6 version to explain the effects of the input variables (A, B and C) on the response variable (Y) at a predefined confidence interval (CI) in terms of their linear, 2-factor interactions (2fi) and quadratic contributions. Supplementary Table S3 shows that linear contributions to all responses have significant effects than the interaction and quadratic contributions over the studied range of the factors in our experiments. The adequacy of the model was also analyzed by ANOVA, shown in Supplementary Table S4. The insignificant terms (AC, B2 and C2 ) were removed from the model and modified model and results are shown in Table 6. The obtained value of P (<0.001) for the model of resilience implies that the modified model was statistically more significant than original model for analyzing the resulted data. From Table 6, it was observed that the value of coefficient of variance (CV) of the modified model (5.65%) was less than that (5.92%) of original model (Supplementary Table S4), indicating that the modified model was more desirable and adequate to describe the response. In addition, the results show that the adequate precision (AP) value of both models are higher than the desirable value of 4.0, which implies that the predicted model can be used to navigate the space defined by Box-Behnken Design (BBD). As shown in Table 6, the dosages of bio-polyol and DEOA are highly significant (P < 0.001), and the dosage of PM 200 is also significant (P < 0.05) for foam resilience. PM 200 can easily react with water to generate an unstable carbamic acid, an intermediate that then spontaneously decomposes yielding carbon dioxide causing expansion of the foam (Kaushiva, 1999). The interaction between bio-polyol and DEOA was also significant. In consequence, the modified model Eq. (4) are shown below was selected. Resilience(%) = 118.32 - 3.64A - 22.78B - 0.28C + 0.49 AB + 0.25 BC + 0.036 A2

(4)

where A = bio-polyol, B = DEOA and C = PM 200. 3.4. Response surface plots Response surface plots were generated (Fig. 2) based on the model Eq. (4) to visualize the interaction effects of independent factors on resilience of PU foams. The two plots illustrate the relative effects of AB and BC by keeping the third factor constant. Resilience was found to increase with increasing amount of DEOA, but for biopolyol, it decreased up to an medium dosage after which there was

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Table 6 Modified analysis of mean square. Source

Sum of squares

dfa

Mean square

F value

P-value

Prob > F

Model A B C AB BC A2 Residual Lack of fit Pure error Corresponding total C.V.b % APc R2

216.9 38.51 32.49 15.01 22.25 13.56 28.53 26.10 19.75 8.35 244.19 5.65 10.656 0.8894

6 1 1 1 1 1 1 10 6 4 16

36.02 38.51 32.49 15.01 22.25 13.56 28.53 2.81 3.29 2.09

12.82 13.70 11.56 5.34 7.92 4.83 10.15

0.0003 0.0041 0.0068 0.0434 0.0184 0.0527 0.0097

** ** ** * *

1.58

0.3433

Not significant

**

A: Bio-polyol. B: DEOA. C: PM 200. a df: degree of freedom. b C.V.: coefficient of variance. c AP: adequate precision.

a slight increase (Fig. 2A). However, increasing the amount of PM 200 along with the DEOA had a positive influence on the resilience of PUFs (Fig. 2B), though it declined at a high dosage of PM 200.

3.5. Validation of the model In order to further validate and confirm the suitability of the foam formula, the validation experiments were performed with the tested variables at its optimized level for the maximum resilience. The optimized conditions were obtained at bio-polyol 30 wt.%, DEOA 0.5 wt.% and PM 200 17.90 wt.%. Under these conditions, 36.7% resilience was predicted by the model. The experiment was performed at the determined optimal conditions. The result was found to be close to the predicted one with the resilience of 37.0%, very close to the predicted value (36.7%), verifying the adequacy and importance of foaming process optimization via statistical experimental design.

3.6. Mechanical properties

Fig. 2. 3-D surface plot showing the interaction of (A) bio-polyol and DEOA; (B) DEOA and PM 200 on resilience of synthesized foams.

Mechanical properties of the reference foam and the foam obtained at the optimal formulation recipe (Table 3) were measured and are shown in Table 7. Resilience of the foam obtained at the optimal formulation recipe was 37.0%, as mentioned in the previous section, which meet the requirement of Chinese standard (GB/T 10802-2006) for flexible foams, indicating that the synthesized foam with 30 wt.% bio-polyol could be used for car-cushion, though the resilience is slightly lower than that (40%) of the BPU foam from bark and starch (Ge et al., 2000). The tensile strength of the prepared 30 wt.% BPU foam is 80.76 kPa, slightly lower than the industrial requirement (82 kPa) for tensile strength (Zlatanic et al., 2014). Table 7 also shows the mechanical properties of BPU foams prepared with various amounts of bio-polyol, according to the optimal formulation recipe. It can be seen from the table that both the resilience and tensile strength values for P40 and P50 foams (containing 40% and 50% bio-polyol, respectively) are lower than that of the foam produced at the optimized condition (30% bio-polyol), while the tear strength values of the P40 and P50 foams are greater than that of the foam prepared at the optimized conditions.

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Table 7 Mechanical properties of BPU foams with various amounts of bio-polyol. Foam samples

Resilience (%)

Tensile strength (kPa)

Tear strength (N/m)

Apparent density (kg/m3 )

Reference foam Optimal foam P40 foama P50 foamb

50.0 ± 2.4 37.0 ± 1.5 27.2 ± 1.7 26.2 ± 3.6

85.0 ± 10.5 80.8 ± 13.2 41.4 ± 8.2 31.2 ± 5.3

130.0 ± 9.8 119.1 ± 12.1 136.3 ± 7.0 154.8 ± 6.1

96.1 ± 1.0 105.1 ± 1.0 107.5 ± 2.0 111.7 ± 1.5

a b

P40 foam: foam prepared with 40% bio-polyol according to the optimized formulation recipe. P50 foam: foam prepared with 50% bio-polyol according to the optimized formulation recipe.

Fig. 3. SEM morphologies of reference (a) and optimal (b) foams and macro photograph of reference (c) and optimal (d) foams.

3.7. Sem The reference foam and the BPU foam prepared with the optimized formulation recipe were analyzed for their morphological characteristics using SEM (Fig. 3). The cells of the control/reference foam showed a honeycomb structure with uniform and smooth surface texture (Fig. 3a). In addition, it was found that the cells under the surface of the BPU foam maintain a high percentage of close area (Fig. 3b), which is detrimental to elasticity of flexible PUF. Cell membranes of BPU foam were much thicker than that of the reference foam and many of them were left intact after crushing. It can be seen that the BPU foam (Fig. 3d) has dark color and slightly coarse surface, in comparison with white color and smooth surface for the reference foam (Fig. 3c).

3.8. FTIR analysis of foams FTIR spectra of the reference foam and BPU foam prepared at the optimal conditions showed similar spectra, suggesting that the synthesized BPU foam has a similar structure as the reference foam (Fig. 4). The FTIR spectra indicates that the bio-polyol has chemical groups such as hydroxyl, C H, and C O groups, originating from methyl sugar derivatives, and phenolic derivatives. The band of −OH stretching vibration at 3600–3400 cm−1 is associated with OH groups of non-bonded polyol, the crystal water, or OH groups within the polymer structure. The intensities of transmission bands at 2972–2953 cm−1 and deformation vibration at 1450–1370 cm−1 are related to CH2 and CH chain formation, which is the main

carbon structure of polyurethane foam. Transmittance at wave number 1730 cm−1 is assigned to the ester carbonyl absorbance (Ugarte et al., 2014). It was observed that with the introduction of bio-polyol in the foam formulation, the intensity of ester carbonyl absorbance peak decreased due to the incorporation of ether groups in the formulation instead of ester groups. The intensities of transmission bands at 1500–1230 cm−1 can be related to C N and C N H chain formation, which is a reflection of the reaction between polyol and isocyanate (including tertiary amine catalysts).

3.9. Thermal stability of polyurethane foams The reference foam and the BPU foam obtained at optimum conditions were analyzed for their thermal stability using thermogravimetric analysis (TGA) (Fig. 5). As shown in Fig. 5, the weight loss below 150 ◦ C is considered to be due to the evaporation of water (Mahmood et al., 2015). All the foams were thermally stable up to 200 ◦ C, but after that the first stage of thermal degradation started. It can be seen that a peak of weight loss of the BPU foam was observed at the temperature of approx. 214 ◦ C, confirming the valid assumption that the large proportion of the bio-based polyols in PU foams may form new networking of macro particles. Those bio-polyols contain low molecular weight organics, leading to high volatility at around 210 ◦ C. The main decomposition of the foams takes place between 200 and 520 ◦ C in nitrogen atmosphere through a complex weight loss process with several mechanisms, such as scission, chain-end scission, repeated initiation recombination, vaporization and vapor phase reactions (Filip et al., 2011).

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Fig. 4. FTIR spectra of reference and optimal foam.

Fig. 5. Thermogravimetry (TG) cures and DTG cures of reference and optimal foams.

At around 240 ◦ C thermal decomposition of PU foam starts through decomposition of a urethane linkage leading to the formation of amines, carbon dioxide (Zhao et al., 2011). The second decomposition stage starts around 510 ◦ C to release components like aliphatic ethers, aldehydes and carbon monoxide evolving from the decomposition of polyols (Rogulska et al., 2015). At above 600 ◦ C, the weight loss of both foams is negligible. As illustrated in Fig. 5, the TGA/DTG results demonstrate that there is not much difference between BPU and the reference PU foams with respect to thermal stability.

4. Conclusions Optimum foaming conditions for producing bio-based polyurethane (BPU) flexible foams using bio-polyol derived

from pyrolysis oil of wheat straw using response surface methodology (RSM). The optimal foaming recipe was determined to be: 30 wt.% bio-polyol, 0.5 wt.% diethanolamine and 17.9 wt.% PM200 based on weight of total polyol, with which the BPU foam obtained has resilience of 37.0%. Tensile strength of the BPU foam obtained at optimal condition was 80.0 kPa, although slightly lower than that of the reference foam strength (85.0 kPa). Although the resilience and tensile strength values for the bio-based PU foams (BPU foams containing bio-polyol at 30–50% substitution ratio) are lower than those of the reference PU foam, while the tear strength values of all BPU foams are greater than that of the reference foam. In accordance to the Chinese standard (GB/T 10802-2006) for flexible foams, the prepared BPU foams have good potential in various applications such as for car-cushion use.

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