In recent decades, tremendous efforts have been devoted to developing water-based materials as alternatives to solvent-based products, which is driven by increasing concern on release of volatile organic compounds and growing demands on green production (Mucci et al., 2023, Zafar et al., 2019). Water-based materials have found applications in various fields such as coating (Huang et al., 2023), ink (Zhao et al., 2023), adhesive (Kong et al., 2023), daily necessities (Zhang et al., 2020b), etc. However, one of the challenges faced by waterborne polymers is their inherent hydrophilicity, resulting in poor resistance to water and weathering, as well as insufficient compatibility with many of nonpolar materials (Wu et al., 2013). As a consequence, such materials often require frequent maintenance and have short lifespans, leading to increased production cost and substantial waste of resources.
To address this issue, utilization of crosslinking agents to improve the durability and longevity of water-based materials have been widely applied (Yang et al., 2023). Among different crosslinking agents, the extremely active isocyanate has been considered as one of the most efficient options since its unsaturated double bond (N
C=O, -NCO) can rapidly react with substances containing active hydrogen, including water, alcohol, and acid (Grostad and Pedersen, 2010). Indeed, it has been demonstrated that isocyanate can improve the properties of water-based materials by reducing the amount of hydrophilic groups (e.g., hydroxide groups) and generating crosslinked networks within polymer chains. To this end, isocyanate can be used as additives to prepare two-component adhesives, for instance, isocyanate/waterborne polyurethane coatings (Wang et al., 2022) or isocyanate/poly(vinyl acetate) (PVAc) emulsion adhesives (Zhang et al., 2020a). However, traditional method for mixing isocyanate into waterborne polymer upon process faces difficulties in achieving maximal benefits due to its hydrophobic properties (Grostad and Pedersen, 2010). More importantly, the extreme incompatibility between isocyanates and the materials dispersed in water leads to uneven mixing locally in the system (Guo et al., 2017), causing uncontrollable crosslinking reaction, rapid escape of CO2 bubbles (Bi et al., 2023), concentrated local stress (Chen et al., 2023), and short period of on-site operation (Grostad and Pedersen, 2010). Therefore, although isocyanates are capable of improving performances of waterborne materials, the overall strength of products can be counterbalanced by such detrimental effects, which are undesirable for realistic applications.
In order to efficiently use -NCO groups in an operational condition, alternative approaches on improving compatibility of isocyanate and waterborne materials are necessary. One potential solution is the use of compatibilizers or coupling agents that can bridge hydrophobic isocyanate and hydrophilic polymer (Grostad and Pedersen, 2010). For instance, addition of surfactants, e.g., sodium dodecyl sulfate (SDS), into the polymeric diphenylmethane diisocyanate (pMDI)/PVAc system can emulsify the pMDI, resulting in an improvement of the bonding performance (Zhang et al., 2020a). However, the presence of residual SDS in the film shortens the durability of the adhesion joint, thereby gradually weakening the bonded species. To address this issue, inserting hydrophilic groups, such as ether bonds (Zhang and Jiang, 2018), carboxyl groups (Guo et al., 2017, Zhang and Jiang, 2018), or sulfonic acid groups (Peng et al., 2019), into the molecular chain of isocyanate is proposed as an effective approach to enable self-emulsification of isocyanate, which can in situ improve the compatibility with waterborne polymer. Blocked isocyanates have also been reported as a means to achieve hydrophilicity and low reactivity simultaneously, leading to uniform and stable isocyanate dispersion (Li et al., 2013, Saeed and Shabir, 2013). Another approach is to transform active isocyanate into microcapsules (Ma et al., 2019, Lubis et al., 2020), which can protect the -NCO groups and reduce the consumption rate, prolonging the pot life of isocyanate/waterborne polymer mixture. However, additional pressure is typically necessary to release the encapsulated -NCO groups during application, and the microcapsules may harden over time, making it difficult to be broken (Sun et al., 2018). To overcome these challenges, further researches are therefore required to identify alternative methods on improving compatibility and efficient utilization of -NCO groups.
Structural design of the isocyanate chain emerges as a promising way to modify its inherent chemical properties. Accordingly, herein we propose a new structural model (Hw-Ho-NCO) that is consisted of hydrophilic part, hydrophobic segment, and -NCO groups, which can protect isocyanates (-NCO groups) in the water phase. The hydrophilic group is part of the hydrophobic segment, which can resemble as a surfactant-like structure. It is important to note that the -NCO group needs to be combined with the hydrophobic groups on the structure. To implement this model, isocyanates are modified by hydrophobic molecular chains containing hydrophilic groups. In recent years, there has been increasing interest in bio-derived renewable materials due to depletion of fossil resources and environmental concerns (Hai et al., 2021). Vegetable oils, such as castor oil extracted from Ricinus communis seeds, have shown great potential for this purpose (Paraskar et al., 2021). Castor oil, with its natural hydrophobic triglyceride segments and hydroxyl groups, has been widely used to modify isocyanates (Zhang et al., 2021), which can improve the water resistance, toughness, and biocompatibility of the resulting products, including waterborne polyurethane (Song et al., 2023), foam (Martins et al., 2021), and composites (Gama et al., 2019). The hydrophilic groups in the model structure can be introduced by modifying castor oil with maleic anhydride, wherein the carboxyl groups are successfully implanted into the molecular chains of castor oil (Chen et al., 2021). Additionally, the liquid polyphenyl polymethylene polyisocyanate (PAPI) is chosen as the raw isocyanate since its hydrophobic properties originated from plenty of aromatic groups are representative for exploration of the stability of isocyanate emulsions. Moreover, the high molecular and unpurified feature of PAPI pose the advantages of low toxicity and low cost (Hejna et al., 2020).
This research focuses on the potential of using Hw-Ho-NCO structural model to develop a stable isocyanate emulsion. Meanwhile, low amount of surfactant (SDS) is added to assist stabilization of the emulsions. Another objective of this study is to utilize the isocyanate emulsion as crosslinking agents to formulate waterborne composite adhesives (ethylene vinyl acetate copolymer resin, EVA-102, is used as the main component). The addition of isocyanate emulsion into EVA-102 enables better bonding strength, exceeding the Chinese national standard. Particularly, the pot life of the composite adhesive is extended to 8 h, which is meaningful for production of environmentally friendly and high-performance adhesives while maintaining suitable operability. Overall, this research shows an example that structural design is efficient to modify compatibility of isocyanate with waterborne polymers, contributing to develop waterborne composite adhesives.
