Abstract
Cassava bagasse (CB) was liquefied using polyethylene glycol to prepare polyol. The liquefied cassava bagasse (LCB) was reacted with polymeric diphenylmethane diisocyanate (PMDI) to produce novel bio-based polyurethane foams (PUF) via a one-pot process. The results showed that the density of the LCB-PUF was in the range of 0.027 to 0.039 g/cm3. When the LCB was used to instead of polyether polyol with a replacement ratio rising from 20% to 100%, the compressive strength of the LCB-PUF decreased from 0.171 to 0.057 MPa. The TGA results indicated that a higher replacement ratio (≥60%) of the LCB had a negative effect on the thermal stability of LCB-PUF, and the PUF with heterogeneous walls and irregular pore shapes was mainly caused by the high replacement ratio of the LCB. In addition, the mechanical properties of the LCB-PUF were improved through the adjustment of the [NCO]/[OH] ratio and the foam additive content.
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Preparation of Bio-based Polyurethane Foams from Liquefied Cassava Bagasse and Polymeric Diphenylmethane Diisocyanate
Zhengyi Tao,a,b Qiulin Yang,a,c,* Dan Huo,a,* Fangdong Zhang,a Yonglin Zhang,a and Zeen Chen a
Cassava bagasse (CB) was liquefied using polyethylene glycol to prepare polyol. The liquefied cassava bagasse (LCB) was reacted with polymeric diphenylmethane diisocyanate (PMDI) to produce novel bio-based polyurethane foams (PUF) via a one-pot process. The results showed that the density of the LCB-PUF was in the range of 0.027 to 0.039 g/cm3. When the LCB was used to instead of polyether polyol with a replacement ratio rising from 20% to 100%, the compressive strength of the LCB-PUF decreased from 0.171 to 0.057 MPa. The TGA results indicated that a higher replacement ratio (≥60%) of the LCB had a negative effect on the thermal stability of LCB-PUF, and the PUF with heterogeneous walls and irregular pore shapes was mainly caused by the high replacement ratio of the LCB. In addition, the mechanical properties of the LCB-PUF were improved through the adjustment of the [NCO]/[OH] ratio and the foam additive content.
Keywords: Cassava bagasse; Liquefaction; Polyurethane foam; Compressive strength
Contact information: a: Tianjin Key Laboratory of Pulp & Paper, Tianjin University of Science & Technology, Tianjin 300457, China; b: Shanying International Holding Co., Ltd., Maanshan, 243021, China; c: Jiangsu Province Biomass Energy and Materials Laboratory, Institute of Chemical Industry of Forest Products, Chinese Academy of Forestry, Nanjing, 210042, China;
* Corresponding authors: qiulinyang@tust.edu.cn; danhuo223@tust.edu.cn
INTRODUCTION
Along with the fast depletion of fossil oil and environmental issues associated with the extensive use of fuels, it is desirable to develop eco-friendly and economical resources instead of the traditional petrochemical-based products. Cassava bagasse (CB) is a type of low-cost and earth-abundant biomaterial. Apart from small quantities of protein, lipid, and ash, CB mainly consists of 40 to 64% un-extracted starch, and 15 to 51% fiber, and its composition mostly depends on the type of cassava and the process of starch extraction (Zhang et al. 2016).
According to Food and Agriculture Organization (FAO) statistics, the global output of CB is expected to be approximately 266 million tons by 2020, 7 to 10% of which is used for industrialization (Pandey et al. 2000). To date, CB research has been mainly related to the development of value-added products, such as a single cell protein, bio-energy, biochemical, enzyme, antibiotics, bioactive secondary metabolites, etc. (Pandey and Soccol 1998; Ekop et al. 2019). However, due to the complicated composition of fermentation broth and the high cost of separation, the utilization of the CB is currently restricted (Zhang et al. 2013). Regarding the composite material, some studies show that CB can be used to produce a CB-kraft paper composite, cellulose whiskers, reinforcing rubber, thermoplastic for food packaging, and biodegradable films, etc. (Pasquini et al. 2010; Carvalho et al. 2018).
Biomass liquefaction with polyhydric alcohols affords large molecules that can be subsequently polymerized to produce adhesives, plastics, and other polymers. For the liquefaction process of lignocellulosics, there are mainly two reactions occurring: decomposition and polycondensation reactions, where the composition of molecules extracted will be impacted by the choice of liquefying solvent and catalyst (Niu et al. 2011; Esteves et al. 2017). Compared to the other liquefied biomaterials such as pine wood, the liquefied CB being rich in starch is more suitable for producing novel bio-based polyurethane foam (PUF), which has prospective applications in packaging, construction, etc. (Lee and Lee 2018).
In the present work, the CB was liquefied into polyol, then it was further synthesized into an environmentally friendly bio-based composite. A convenient method that was conducted at low temperature and low pressure was designed and optimized to decrease the energy consumption and pollution during PUF production. Moreover, the CB was liquefied into the bio-based polyol and directly reacted with isocyanate to generate PUF, which could reduce production cost and the utilization of various types of polyurethane.
EXPERIMENTAL
Materials
The fresh CB was obtained from a cassava starch mill (Jiahua Starch Factory, Wuming, China), shattered into powder using a Wiley mill (60-mesh; Retsch, North Rhine-Westohalia, Germany), dried at 54 °C for 24 h, and then stored in an airtight container. Polyethylene glycol (PEG#400), glycerin, polysorbate (Tween-80), and H2SO4 (Tianjin Kemiou Chemical Reagent Co., Ltd., Tianjin, China) were of reagent grade and were directly used as liquefaction reagents. The 4110 (polyether polyol, hydroxyl number 430 mg KOH/g) and PM-200 (polymeric diphenylmethane diisocyanate (PMDI), -NCO content 30.2%) were purchased from Maijia Chemical Co., Ltd. (Guangzhou, China) and Yantai Wanhua Polyurethane Co., Ltd. (Yantai, China), respectively. The foam stabilizer SD-60 (Polyether siloxane), the foaming catalyst T-12 (DBTDL, dibutyltin dilaurate), and triethylene diamine were obtained from Siltech New Materials Co., Ltd. (Suzhou, China), Air Products & Chemicals (China) Inc. (Shenzhen, China), and Tianjin Guangfu Science & Technology Co., Ltd. (Tianjin, China), respectively. The HCFC-141b (Shanghai 3F New Chemical Materials Co., Ltd., Shanghai, China) and water were used as the blowing agents.
Preparation of the LCB
A three-necked flask equipped with an atmospheric condenser, a thermometer, and a motor-driven stirrer was used for liquefaction. A total of 10 to 20 g of the CB and 50 to 80 g of the liquefaction reagents were placed into the three-necked flask, where the liquefaction reagents included (in that order) 35 to 48 g PEG#400, 8 to 18 g glycerin, 0.1 to 0.3 g Tween-80, and 0.5 to 2.0 g sulfuric acid. The flask was heated to 130 to 190 °C and kept at that temperature for 30 to 180 min, with a stirring speed of 200 rpm. After reaction, the flask was immersed in cold water to terminate the reaction, and then the reacted mixture was neutralized with sodium hydroxide (48 wt%) using an automatic titrator (AT-510; KEM (Kyoto Electronics Manufacturing Co., Ltd.), Kyoto, Japan) to obtain the liquefied cassava bagasse (LCB).
Methods
Characterization of the LCB
The moisture of polyol had a notable effect on the foaming process and morphology of the PUF. The Karl Fisher method equipped with a moisture content meter (EBU-610-KF; KEM, Kyoto, Japan) was used to determine the moisture content of the LCB samples. The detection of the residual CB was clearly described in a previous study (Yao et al. 1996). Approximately 5 g of the LCB was dissolved with 100 mL of dioxane/water solution (4/1, v/v) in a conical flask (250 mL). The insoluble residue was filtered using a solvent filter and an organic microporous membrane (0.45 μm). Then, the insoluble residue was dried to constant weight, and its content in the LCB mixture was calculated as follows,
(1)
where m1 is the initial dry weight (g) of the CB in the LCB mixture and m2 and m3 are the dry weights (g) of the membrane with and without insoluble residue, respectively.
For the hydroxyl number of the LCB, 1 g of the LCB was esterified with 25 mL of phthalate agent (150 g phthalic anhydride, 24.2 g imidazole, and 1000 g dioxane) at 110 °C for 20 min, then added to 75 mL of dioxane/water solution (2/1, v/v). The mixture was titrated with sodium hydroxide (1 mol/L) to the equivalence point using an automatic potentiometric titrator (AT-510; KEM, Kyoto, Japan) (Kurimoto et al. 2000). The hydroxyl number (mg KOH/g) of the LCB was calculated using Eq. 2,
Hydroxyl number = [(B – A)C × 56.1] / W (2)
where A is the volume (mL) of the sodium hydroxide solution required for titration of the LCB, B is the volume (mL) of blank solution, C is the normality (mg KOH/mL) of the sodium hydroxide, and W is the weight (g) of the LCB.
Preparation of LCB-PUF
The LCB mixtures and PMDI were reacted to prepare the foams by a one-pot process. A mixture consisted of 4110, LCB, SD-60, T-12, triethylene diamine, and traces of water was mixed uniformly in a paper cup at 500 rpm for 2 min, where the triethylene diamine was used as a collaborated foaming additive. Then the HCFC-141b and PMDI were added into the mixture and stirred (1500 rpm) at room temperature for copolymerization until the mixture formed a cream-like substance. The isocyanate index was calculated using Eq. 3,
(3)
where MPMDI is the content (7.43 mmol/g) of isocyanate group in PMDI, MLCB is the content (mmol/g) of the hydroxyl group in the LCB (hydroxyl number/56.1), 1000/9 (mmol/g) is the content of hydroxyl group in the water, WPMDI, WLCB, and Wwater are the weights (g) of PMDI, LCB, and water, respectively (Wang et al. 2008).
Characterization of the LCB-PUF
The compressive strength of the PUF was measured using a universal stress tester (11-3; Lorentzen & Wettre, Stockholm, Sweden) at 23 °C and 50% relative humidity, with a constant crosshead speed of 5 mm/min. Prior to detecting, the foams were cut into pieces (50 × 50 × 50 mm3), and the compressive strength was determined based on the stress, which was 12% lower compared to that in the yield point (the elastic limit or the point at which a material can no longer deform elastically). The samples were coated with a thin layer of gold (JEC-3000FC; JEOL Ltd., Tokyo, Japan; 20 mA for 60 s), and the cell structure was observed using scanning electron microscopy (SEM) at 10 kV (JSM-IT300; JEOL Ltd., Tokyo, Japan). Thermogravimetric analysis (TGA) was performed using a TA Instruments (SDT-600; New Castle, PA, USA) thermogravimetric analyzer, where the samples were heated from 50 to 900 °C, at a heating rate of 20 °C/min and a nitrogen flux of 40 mL/min.
RESULTS AND DISCUSSION
Liquefaction of the CB
The fresh CB and shattered CB were dried and used for SEM detection, as shown in Fig. 1. Three types of solid particles were present in the CB, including a spherical starch granule, a fiber, and a starch-fiber floc, where the starch occupied a large proportion of the CB. After mechanical pretreatment, the shattered CB demonstrated a more uniform particle size distribution (Fig. 1b). In the mechanical pretreatment, the fiber was cut into fragments, and its particle size was similar to the starch. Moreover, the starch granules were released from the fiber-starch floc. Thus, the mechanical pretreatment promoted the reagent to penetrate into the raw material more readily, which accelerated the liquefaction of the CB and improved the liquefaction efficiency. Hence, a mechanical pretreatment is important for the liquefaction of the CB.
The changes in the residual CB and the hydroxyl numbers of the liquefied CB mixtures are shown in Fig. 2. The CB was efficiently liquefied by the polyol when the sulfuric acid was added as a catalyst. When a moderate temperature of 130 °C was used, more than 70% of the CB was liquefied within 30 min. Compared to the lignocelluloses, such as cellulose, wood powder, and lignin, the CB had a higher liquefaction rate and liquefaction yield in the same polyol system, even under a mild condition (Zhang et al. 2014). The liquefaction rate of the starch was higher than that of the lignocellulose, and the components of lignocellulose including cellulose, hemicellulose and lignin were also had different liquefaction rates (Yao et al. 1996; Zhang et al. 2014). Thus, the high liquefaction yield was mainly related to the high content of starch in the CB, as well as the size improvement (specific surface area) of the mechanical pretreatment. The reduce trend of the unliquefied residue decreased with the decreasing starch content, and the residual CB was difficult to be further liquefied when the liquefaction time was over 30 min. Thus, the liquefaction of the CB mainly contained two processes, starch liquefaction and lignocellulose liquefaction, where the lignocellulose liquefaction required more time, even under a high temperature condition. In addition, further degradation and oxidation occurred in the liquefied CB mixture, in accordance with the changes of the hydroxyl group content (Yamada and Ono 2001). At the beginning, the hydroxyl content of the mixture increased along with the liquefaction process, until it reached the maximum value, then it rapidly decreased to the end of the process, even less than the initial value. This could have been attributed to the solvation effect between PEG#400 and glycerol (Yao et al. 1996). Furthermore, the utilization of PEG#400, glycerol, and Tween-80 decreased the re-polymerization (gelation) reaction, based on the content changes of the residue in the LCB mixture.
Fig. 1. SEM images of the CB (a) and shattered CB (b)
Fig. 2. Effects of temperature on the liquefaction of the CB
PEG#400/glycerol/ H2SO4/Tween-80 = 42/14/1.7/0.16; Liquefaction reagent/CB = 2.9/1
Morphological Characteristics of the LCB-PUF
The liquefied CB polyol was directly used to prepare polyurethane foam via reacting with PM-200 (PMDI) in a one-pot process. The components of the reaction medium for LCB-PUF preparation are shown in Table 1 (based on the LCB polyol mixture), and the appearance of the LCB-PUF is shown in Figs. 3 and 4.
In Fig. 3, when the 4110 was substituted by the LCB with a replacement ratio of 80%, the bio-based PUF was still successfully prepared, which had a regular shape and smooth surface. The color of the LCB mixture was continuously changed during the liquefaction process, from gray to burgundy, brown, and finally to black. When the addition of H2SO4 was increased, or the liquefaction temperature increased, the color of the LCB mixture darkened, correspondingly, the color of the LCB-PUF was also changed. For the self-foaming process, the density of the LCB-PUF was approximately 0.02 to 0.03 g/cm3, and when water was used as a blowing agent, the density increased to 0.062 g/cm3.