The current concerns for the environment and limited availability of petrochemical resources have resulted in the use of lignocellulosic biomass attracting considerable attention, as it is biodegradable, readily available, and renewable [1
]. Lignocellulosic biomass from forestry and agriculture could be an abundant feedstock for producing biofuels, such as fuel oil, and bio-based chemicals. Bio-polyols obtained using the thermochemical method, e.g., pyrolysis or liquefaction of lignocellulosic biomass, have promising properties for manufacturing formaldehyde resins [2
] and bio-based polyurethane foams [4
] with comparable attributes to their petroleum analogs.
Liquefaction of biomass is an effective method for processing biomass resources, which is classified into two categories based on the reaction conditions [6
]: (1) a process requiring a high reaction temperature and pressure-proof reactor; and (2) a process performed using reactive solvents, i.e., phenols, polyhydric alcohols, and carbonates, in the presence of strong acid at relatively low temperature and atmospheric pressure. The liquefaction conditions for woody biomass at temperatures below 200 °C have been optimized in the second category [7
]. These processes are based on solvolysis of lignocellulosic materials, where the liquefaction products can be used as raw materials for producing valuable polymer products.
The pore size and density of foams produced from bio-based feedstocks can be easily manipulated to meet the needs of various applications. Polyols (polyether and polyester) and isocyanate are the two main raw materials for producing polyurethane foam, and are currently obtained from petroleum-based products [10
]. The major drawback of petroleum-based products is that they are non-renewable and not biodegradable. Due to increasing concerns over dwindling fossil resources and the environmental impact of their use, obtaining polyols from renewable biomass is an attractive alternative for the polyurethane industry. Most biomasses consist of cellulose, hemicellulose, and lignin, which contain more than one hydroxyl group in the molecular chains, which can be used as an alternative to petroleum-based polyols for preparing polyurethane materials [11
]. Agricultural wastes, such as cornstalks and wheat straw, are eco-friendly and readily available feedstocks for sustainable production of energy and chemicals [12
]. We previously achieved liquefaction of woody biomass using mixed solvents of alcohol and water, without requiring a catalyst [13
]. The use of agricultural residues for producing high-value bio-products can help build a sustainable economy while having a significant positive impact on the environment. To date, there has been little research on liquefaction of cotton stalks for bio-polyols and the subsequent production of polyurethane foams.
Cotton stalks are rich in cellulose and have the potential to be used as a feedstock for the production of paper, industrial fuel, and composites. Currently, cotton stalks and other agricultural waste are generally burned, which wastes potential resources, pollutes the environment, and results in health problems for farmers. A large amount of energy and resources are being used for the current economic development in China. The increasing use of fossil fuels and burning of agricultural waste have exacerbated pollution problems, especially in urban areas. Therefore, it is meaningful to study methods for increasing the value of all biomass, while reducing environmental pollution. Previous studies have focused on the applications of cotton stalks as a renewable energy resource, which can be converted into bioethanol using available pretreatment methods to meet biofuel requirements. However, few studies have focused on cotton stalks as a feedstock for polyurethane materials. The aim of this study was to demonstrate a process for liquefying cotton stalk powder using polyethylene glycol (PEG) 400 and glycerin as liquefaction solvents.
The effects of the liquefaction conditions on the properties of the polyols and polyurethane foams produced using this feedstock were investigated. The optimal processing conditions for the liquefied product considering the quality of the polyurethane foams were studied as a function of the residue fraction. Bio-polyols with promising material properties were produced using liquefaction conditions of 150 °C, reaction time of 90 min, catalyst content of 3 wt.%, and 20 w/w% cotton stalk loading. Bio-polyols produced under preferential conditions showed hydroxyl numbers of 325–360 mg KOH/g, acid numbers below 5.00 mg KOH/g, and viscosities of 850–7950 mPa·s. Polyurethane foams produced under optimal conditions showed densities of 78–123 kg/m3 and compressive strength of 28–267 kPa. These results suggest that PEG–glycerin mixtures can be used as a liquefaction solvent to produce bio-polyols from agricultural waste biomass such as cotton stalk, which can subsequently be used to prepare polyurethane foams. The produced bio-polyols and polyurethane foams showed material properties comparable to their analogs from petroleum solvent-based liquefaction processes.