Preparation of living and highly stable blended polyurethane emulsions for self-healing films with enhancive toughness and recyclability

Highlights

• The mixed emulsion of maleimide- and furan-modified waterborne polyurethane latexes is highly stable.

• Cross-linked films show robust mechanical property.

• Exhibiting excellent self-healing performance regarding cracks and physical properties.

• Showing ideal recyclability of the thermosets relying on the reversible DA chemistry.

Abstract

Widely applied waterborne polyurethane (WPUs) are expected to be functionalized with excellent mechanical property, ideal self-healing performance and desired recyclability, but it is a great challenge to integrate these attractive properties into a one-pot WPU platform. Herein, a novel double ingredients waterborne polyurethane (2K-WPU-DA-x), found on the furan modified WPU-F and maleimide decorated WPU-M dispersions for Diels-Alder (DA) cross-linking reactions, was designed and projected which exhibited interesting enhanced-mechanical, self-healable and recyclable properties. The stability and appearance of these novel waterborne polyurethane emulsions were well studied. In the following, the reversible nature of direct DA/retroDA reactions within the 2K-WPU-DA-x was extensively analyzed via FT-IR, UV–vis, and DSC measurements. On account of the DA bonds resulted a cross-linked structure of 2K-WPU-DA-1/1, the stress and strain at the break increased by more than 20 MPa and 250% respectively compared with linear structural WPU-F. At the same time, relying on the thermal-responsive dissociation/reassociation of DA covalent bonds, 2K-WPU-DA-x films shown outstanding self-healing ability, with a self-healing efficiency beyond 95%. Moreover, owing to the reversibility of 2K-WPU-DA-x, the networks exhibited readily recyclable and reshaping properties, showing a desired thermoplasticity.

Introduction

Versatile and environmentally friendly waterborne polyurethane (WPUs) are broadly applied in the coatings [[1], [2], [3], [4]], adhesives [5,6], textile finishing, inks [[7], [8], [9]] and other industries, owing to its extremely comprehensive balance and flexibly adjustable performance. However, the typically one-pot WPUs display poor mechanical and water-resistance performance caused by its lower cross-linking density [[10], [11], [12]]. Herein, two-component waterborne polyurethane (2K-WPUs), relying on hydroxy-functional resin dispersion and hydrophilic poly-isocyanate, have drawn much attention, which is an effective way to strengthen the WPUs’ physical performance because the polyaddition between –OH and –NCO enlarges the cross-linking density [[13], [14], [15], [16]]. Although the typical 2K-WPUs exhibit outstanding performance, the ideal mixture is not easy to achieve, and the mixture of –OH and –NCO based 2K-WPU will be networked within several hours, which seriously limits the usage [16,17]. Consequently, we intend to prepare a novel 2K-WPU system, depending on furan (WPU–F) and maleimide (WPU-M) components for reversible Diels-Alder (DA) chemistry. The typical [4 + 2] cycloaddition Diels-Alder reactions are taken place among furan and maleimide derivatives at a mild temperature, which result in cyclohexene adducts [[18], [19], [20], [21]]. Interestingly, the DA adducts reverse to the relative furan and maleimide groups with a higher temperature stimulus [22]. When the furan and maleimide are employed into the WPU chains, they prefer to stay inside the WPU particles, not the outside, which quite differs from the hydrophilic ionic constituent since its well hydrophobicity. As a result, the mixed dispersions of WPU-F and WPU-M can be maintained for a long time without DA cross-linking reactions, which might cause precipitation or catastrophic failure.

What's more, WPU films are easily damaged by the abrasion, fatigue, impact, etc. because WPU products are frequently exposed to the rough conditions over a long period [23,24]. In order to extend the service time of WPUs and enhance their durability, it is of great significance to impart self-healing ability to WPUs [25,26]. Among the diverse self-healing mechanisms, the reversible direct DA/retro DA chemistry has been extensively researched since it is simple, repeatable, effective, and minor of side reaction [20,27]. On the basis of the special direct DA/retro DA reactions, a variety of robust, multifunctional, and promising self-healing polymeric materials were prepared. Based on the dissociation/association of DA bonds between furan and maleimide derivatives, Gu's group has developed a self-healing and carbon nanotubes modified polyurethane composite [28], Turkenburg et al. manufactured self-repairing epoxy [29], and Jung and his co-workers prepared self-healable polymethacrylate [30]. In spite of various polymers were modified with reversible DA covalent bonds to achieve excellent self-healing capacity, rare reports concerned about environmentally friendly WPUs. Recently, Aizpurua et al. reported dynamic WPU materials based on DA chemistry, and the reversibility and kinetics of the characteristic DA reaction were well explored. Therefore, it is necessary to explore WPUs with appealing self-healing property [31]. Our previous work has prepared a one-pot self-healing WPU system functionalized by the Diels-Alder (DA) bonds which were introduced into the backbone of the WPU chains. Although the extraordinary self-repairing property was achieved, the mechanical properties were sacrificed seriously because of the incorporation of DA bonds [32]. Accordingly, we are planning to tackle this contradiction of the excellent mechanical stress and satisfying self-healing efficiency via designing a novel double component 2K-WPU-DA-x which is composed of furan modified ingredient and maleimide decorated component. Via regulating the ratio of WPU-M emulsion over WPU-F emulsion of the mixture, the 2K-WPU-DA-x films are expected to exhibit controllable and robust mechanical property.

At the same time, in order to achieve superior mechanical property, ideal solvent and deformation resistance of polymeric materials, an efficient method is the covalent cross-linking to form stiff networks, while those thermosets could not be reshaped or reprocessed like thermoplastics once being processed [[33], [34], [35]]. Because most of the thermosets cannot be recycled, it causes a terrible wastage of the natural non-renewable fossil resources [34,36]. Typical direct DA/retro DA reaction is a promising candidate to handle this difficulty. When the polyurethane chains are cross-linked by DA covalent bonds, the break stress and Young's modulus are distinctly strengthened but it is still recyclable via retro DA/direct DA reactions [36,37]. Hence, the novel cross-linked 2K-WPU-DA-x is expected to exhibit not only interesting self-healing property but also fruitful recyclability.

Therefore, in this work, we are planning to prepare a novel two-component WPU system functionalized by reversible DA chemistry between furan modified WPU-F and maleimide modified WPU-M, and these environmentally friendly hybrid 2K-WPU-DA-x are anticipated to exhibit robust mechanical property, effective self-repairing capacity, and recyclable properties. In detail, furfuryl alcohol and dihydroxyl furan (F–2OH) are taken to modify the WPU-F component, and N-(2-Hydroxyethyl)-maleimide (M − OH) and dihydroxyl maleimide (M-2OH) are selected to decorate WPU-M component. By adjusting the proportion of WPU-M to WPU-F dispersions, the 2K-WPU-DA-x films with different cross-linking density resulted from DA bonds will be prepared, and their mechanical performance will be flexibly controlled and significantly enhanced. Relying on the retro DA/direct DA process, the new 2K-WPU-DA-x networks will display excellent self-healing and recyclable properties. The appearance and stability of WPU emulsions, the reversibility of retro DA/direct DA reactions in the polymer horizon, and the tensile properties are well studied through diverse measurements. In addition, the self-healing and recyclability of 2K-WPU-DA-x films are qualitatively and quantitatively researched.