Waterborne polyurethane (WPU) that uses water as the primary solvent, has emerged as an environment-friendly polymeric material compared to solvent-polyurethanes because of its low volatile organic compound (VOC) emission, nontoxic and nonflammable [[1], [2], [3]]. WPU has numerous advantages, such as high flexibility, excellent chemical and abrasion resistance, good applicability, and etc [4]. It has been widely used in coatings, adhesives, leathers, textile laminating, foams and other commercial products [1,5]. However, the main components (polyols, diisocyantes, hydrophilic chain-extenders) used for WPU synthesis are derived from petroleum feedstocks [6]. With the increased concerns over the environment sustainability, the excessive consumption of petroleum resources and the fluctuating price of crude oil [7,8]. The development of renewable resources for the production of WPU has become more and more urgent and important.
Recently, vegetable oils (VOs) and their derivatives have been paid much attention to the development of chemical products because of their sustainability, ready availability and relatively low cost. [9] VOs are triglycerides that contain numerous reactive sites, such as C
C double bonds, ester groups, hydroxyl groups and etc, which provides excellent platforms for the development of high-value added monomers and polyurethanes [10]. With the exception of lesquerella oils, castor oil (CO) is the only one used directly as the polyol for the preparation of polyurethane. Liang et al. have successfully synthesized castor oil-based cationic and anionic waterborne polyurethanes respectively [2,11]. The polyurethane dispersions demonstrate excellent storage stabilities at room temperature. The WPUs exhibit tunable mechanical properties, and more interesting, the cationic waterborne polyurethane was found to present the antibacterial activity against Vibrio parahaemolyticus. Otherwise, numerous strategies have been developed to the modification of VOs. For example, a novel, fully bio-based hydrophilic chain-extender has been synthesized from epoxidized soybean oil by epoxidation/ring-opening method [12]. And the tunable waterborne polyurethanes were successfully prepared from the chain-extender and different polyols. The WPUs with up to 74 % bio-based content exhibit excellent thermal and mechanical properties. Alagi et al. have transformed VOs to polyols through the efficient thiol-ene reaction, and the polyols were successfully incorporated into the polyurethanes. The polyurethanes exhibit excellent elastomeric properties and tensile strength [13]. Thus, vegetable oils and their derivatives have obvious advantages in the preparation of polyurethanes, and have great potential as major alternatives to petroleum-based polymers. Unfortunately, the thermal and mechanical properties of most vegetable oil-based waterborne polyurethanes are not able to match polyurethanes from petroleum monomers. The weak rigid structures of triglycerides-specific flexible fatty acid chain, and limited active reaction sites of VOs will result in the low cross-link density of vegetable oil-based WPU [14]. Moreover, the competition between food supply and biodiesel production from vegetable oils may arise in the future.
Glycerol, as a renewable clean energy, derived from nature resources or biodiesel industry. It has extensive applications in food industry, pharmaceuticals, creams, syrups, polyurethanes, etc [15,16] because of its low-price and widespread availability. According to the report, about 100 kg glycerol can be generated as the byproducts during the production of each tone of biodiesel, and almost 1 million tonnes of glycerol is prepared annually in Europe [15]. The price of the crude glycerol is appropriate ＄0.1/kg [17]. Therefore, transformation of low-value glycerol into value-added polymers is imperative. Recently, numerous polyurethanes have been developed from glycerol. For example, Hejna et al. prepared the rigid polyurethane-polyisocyanurate foams from crude glycerol and castor oil. The introduction of glycerol-based polyols decreased the thermal conductivity coefficient and enhance the mechanical properties of foams. Moreover, the compressive strength of the resulting foams was significantly improved compared to the reference samples [18]. Li et al. synthesized the polyurethane wood adhesives from crude glycerol. The wood adhesives exhibit excellent thermal stability, chemical resistance and bond strength compared with the petroleum glycerol-based PU adhesives [19]. However, few studies have been reported on the preparation of waterborne polyurethanes from glycerol. The value-added transformation of glycerol to bio-based waterborne polyurethanes has a significant potential to enhance the economics of bio-diesel resources.
In this study, the glycerol was chosen for the preparation of bio-based waterborne polyurethane. The use of glycerol polyol provides beneficial enhancement about its role within the process. The effect of glycerol functionalities and content on the thermal and mechanical properties of the castor oil-glycerol-based waterborne polyurethanes were investigated. The stability of the WPU dispersion was characterized by zeta-sizer, and the thermal and mechanical performances of the WPU films were characterized through DMA, TGA, DSC, Pencil hardness, Crosshatch adhesion, and tensile testing. In addition, the effect of glycerol content on the surface wettability of materials was investigated.
