Rapid and efficient synthesis of isocyanate microcapsules via thiol-ene photopolymerization in Pickering emulsion and its application in self-healing coating

Abstract

We report an efficient and novel method for encapsulation of isophorone diisocyanate (IPDI) in polythioether microcapsules via thiol-ene click reaction based on Pickering emulsion, which has significantly improved and simplified the preparation process. The resultant microcapsules have spherical shapes with a layer of poly(glycidyl methacrylate) (PGMA) particles covering outside. The diameter ranges from 82.1 to 160.8 μm, which is controlled by the concentration of PGMA particles, and the typical shell thickness is about 9 μm. Moreover, the core content is up to 71% by adjusting the adding ratio of core/shell monomers. Remarkably, the microcapsules have excellently environmental stability that only 18% of the core content drops (from 63.2% to 51.8%) after immersion in water for seven days. These microcapsules could keep their integrity and uniformly disperse in acrylate coatings. In addition, they are proved to be a good self-healing candidate for coatings applied in wet environment.

Introduction

In the last decade, self-healing materials as important biomimetic and smart materials have received attractive attention due to their potential to endow materials with longer lifetime and less maintenance by repairing damages automatically without manual detection and intervention [1], [2], [3]. Microencapsulation has been one of the most efficient and widely used approaches in self-healing systems. In the first generation of self-healing microcapsules, White and coworkers prepared poly(urea-formaldehyde) (PUF) microcapsules that contained dicyclopentadiene (DCPD) as healing agent through in situ polymerization in an oil-in-water (O/W) emulsion [1]. Microcapsules together with Grubbs' catalyst were embedded in epoxy resin to form self-healing materials. When damage occurred, the microcapsules were forced to rupture and DCPD was released into the damaged area and polymerized by contact with the embedded catalyst, bonding the microcrack. Since then, self-healing materials based on microcapsules have been rapidly developed, including thermoset and thermoplastic materials [4], [5], [6], [7], coatings [8], [9], [10], fiber-reinforced polymer composites [11], and so on. According to special demands (type of matrix or service conditions), different healing agents were selected to be encapsulated, such as epoxy [7], [12], siloxanes [13], [14], [15], thiol/ene [16], thiol/isocyanate [17], maleimides [18] and glycidyl methacrylate [5].

As we know, water is an important factor to influence the corrosion process of steel structures. Therefore, the anticorrosion coatings should have the ability to prevent water from contacting with steel substrate through microcracks. Isocyanate groups are prone to react with water even the water vapor in humid environment, which can be developed to a one-part, catalyst-free self-healing system for materials that are exposed to moist or underwater environment. However, the high reactivity makes it difficult to produce isocyanate microcapsules and only a few researchers have succeeded. Previous work has been restricted to the encapsulation of solid or bulk forms of isocyanates [19], [20], [21]. Sottos and coworkers made the first attempt to encapsulate liquid isophorone diisocyanate (IPDI) via interfacial polymerization of toluene diisocyanate prepolymer and 1,4-butanediol [22]. The toxic chlorobenzene was used to dissolve the prepolymer, which would do harm to the environment in case of leakage. Later, Yang and coworkers prepared polyurethane (PU) microcapsules containing hexamethylene diisocyanate (HDI) using similar method and systematically investigated the effects of encapsulation parameters on the size and core content of microcapsules [9]. The core content was about 60%, but it dropped to zero after immersing microcapsules in water for 48 h. When exposed to the open air at room temperature for 1 month, the HDI content decreased from 60% to 45%. Hence, further optimization is required to solve the problem of high permeability of the microcapsules. Until now, there are only four other literatures that report the preparation and improvement of isocyanate microcapsules [23], [24], [25], [26]. Wang and coworkers made IPDI-loaded microcapsules and embedded oxygen plasma treated carbon nanotubes in poly(urea-formaldehyde) (PUF) to improve the micromechanical behavior of shells significantly [23]. Di Credico and coworkers studied the microencapsulation process of IPDI with different polymer shell (PU, PUF and double layered PU/PUF) to impart flexibility and tightness to the microcapsules [24]. Recently, Du Prez and coworkers reported a facile approach for encapsulation of the liquid hexamethylene diisocyanate isocyanurate trimer in polyurea microcapsules and functionalized the shell with different hydrophobic groups to increase the core content (up to 83%) and stability of microcapsules [25]. Besides, Wang and coworkers prepared IPDI-loaded microcapsules with multilayer shell based on lignin nanoparticles-stabilized O/W Pickering emulsion [26]. Nevertheless, the above work about preparation of isocyanate microcapsules usually requires heating, pH control or long reaction time. How to simplify the synthesis process, increase core content, improve the stability and produce microcapsules in large quantity still remain to be further investigated.

To ensure sufficient stability of emulsion and avoid demulsification, the polymerization was designed to proceed in Pickering emulsions which are stabilized by solid particles instead of low molar mass surfactants [27]. Because of the high energies attachment for solid particles held at the liquid–liquid interface, these particles were irreversibly adsorbed to form a flexible but robust colloidal layer at the interface [28], [29]. This remarkable stability of Pickering emulsion can avert coalescence of droplets, which makes it a versatile template for various structures, including core/shell microspheres [30], [31], multi-hollow microspheres [32], [33], [34] and microcapsules [26], [35]. For different types of Pickering emulsions, the critical factor is the wettability. It is reported that the hydrophilic particles easily result in O/W emulsions, while the hydrophobic particles might lead to water-in-oil (W/O) emulsions [36]. Therefore, numerous kinds of solid particles have been developed to stabilize the Pickering emulsions, such as polymer [29], [30], magnetic particles [37], silica [35], [38], TiO₂ [39] and clays [40]. In this paper, microcapsules are designed to make self-healing materials based on polymer (epoxy or acrylate). Hence, hydrolyzed PGMA particles with many hydroxyl groups were selected as stabilizer to facilitate the compatibility between polymer matrix and microcapsules.

To shorten the contact time of isocyanate and water and ensure sufficient core content of microcapsules, effective and efficient reactions need to be introduced. Thiol-click chemistry is well known for its high efficiency, high yield and insensitivity to oxygen or water [41]. Recently, more and more researchers have focused on the heterogenous polymerization via thiol-click reactions to prepare solid particles [42], [43], [44], porous microspheres [45] and capsules [46], [47]. Therefore, thiol-click chemistry was believed to be an excellent tool to produce microcapsules.

Herein, we present a facile route for fabrication of IPDI-loaded microcapsules via photoinitiated thiol-ene click chemistry in O/W Pickering emulsions at room temperature. Because of the efficiency of click reaction, reaction time was significantly shortened, and thus the side reaction of IPDI and water could be largely reduced. In this work, hydrolyzed poly(gylcidyl methacrylate) (PGMA) particles were selected as stabilizer of Pickering emulsion to form the O/W emulsion, in which the oil phase contained 1,3,5-tri-2-propenyl-1,3,5-triazine-2,4,6 (1H, 3H, 5H)-trione (TTT), IPDI and trimethylolpropane tris (3-mercaptopropionate) (TMMP), and the water phase was the aqueous suspension of hydrolyzed PGMA particles with the dissolved photoinitiator 2959. Under irradiation of UV light, TTT and TMMP were polymerized to produce a polythioether network, forming the shell of microcapsules. Remarkably, no toxic solvent is required to dissolve core or shell monomer and the resultant microcapsules had outstanding stability and considerable self-healing property.