Recent advances in UV/thermal curing silicone polymers

Highlights

• UV curing silicone polymers integrating photosensitive groups are discussed.

• UV curing combined with thermal curing is contemplated in silicone polymers.

• UV/thermal self-healing silicone polymers with dynamic bonds are summarized.

• The frontal polymerization, a candidate for curing silicone, is presented.

Abstract

Silicone polymers exhibit various attractive properties such as outstanding biocompatibility, transparency, and stable chemical properties so that can be applied in coatings, additive manufacturing and flexible electronics. In these fields, the structural design of functionalized silicone polymers is the hot issue, especially silicones with dual-reactive photosensitive and thermosensitive groups. Indeed, some comments discussed thermal curing or ultraviolet (UV) curing silicone polymers, and the development of silicone polymers and copolymers. However, none of them have fully introduced the combination of thermal curing and UV curing for silicone polymers.

Herein, we discuss UV curing mechanisms and the influence of silicone polymers with photosensitive groups on the curing process. Then, the approaches of UV curing combined with thermal curing are summarized for the aim of replenishing energy for the curing process and further improving the degree of curing. To deal with accidental damage, the introduction of dynamic bonds in the system to construct UV/thermal curing self-healing silicone polymers is highlighted. Subsequently, we summarize additive manufacturing, which could well demonstrate the superiority of UV/thermal curing silicone polymers. Finally, the corresponding challenges and future opportunities are contemplated.

Introduction

The silicone polymers composed of Si–O backbones with functional groups exhibit various attractive properties such as excellent high-temperature resistance, stable chemical properties, and biological compatibility [1], [2], [3], [4], [5]. Generally, Si–O bonds endow the polymers with outstanding thermal and chemical stability, while the substituents bring the silicone polymers more potential. Due to these outstanding performances, silicone polymers have been widely applied in the protective coatings, films, additive manufacturing, and flexible electronics [6], [7], [8], [9], [10].

Typically, traditional silicone polymers are prepared by thermal curing, whereas photocuring is gradually integrated into the curing system of silicone out of consideration for environmental protection and energy conservation. Here, photocuring, mainly referring to UV curing, has been regarded as an eco-friendly technique owing to its distinct features, such as high efficiency, low energy consumption, solvent-free formulation, and mild reaction conditions [11], [12], [13], [14]. Under UV irradiation, many photopolymerization mechanisms (free-radical polymerization or cationic polymerization) can be realized through photoinitiators and various photosensitive groups.

However, the UV curing system relies on UV irradiation and cannot guarantee the curing degree of shadow areas. To overcome this shortcoming, thermal curing was utilized as an assistant function to promote further curing of the system, which can ensure that at least enough cure can be obtained in the shadow area [15], [16], [17]. Thus, UV/thermal curing silicone polymers occur, which can utilize UV and heat while curing, and broaden the concept of silicone polymers into new application scenarios.

In UV/thermal curing silicone polymer systems, well-designed structures with photosensitive and thermosensitive groups have become a hot issue. For the preparation of dual-reactive silicone polymers, various functional groups are utilized, such as acryloxy, vinyl, and thiol groups for UV curing, Si–H, and phenyl groups for addition polymerization, and epoxy and phenolic group-modified silicone polymers for thermal curing. Nevertheless, silicone materials inevitably suffer from external influences, and microcracks appear during practical use. To deal with this situation, emerging dynamic bonds are introduced into the system to endow materials with self-healing ability.

In this review, as shown in Fig. 1, we discuss UV curing silicone polymers primarily through UV curing mechanisms, emphasizing the curing effect of structural designs integrating photosensitive groups. Then, to further enhance the degree of curing and improve performance of polymers, methods to incorporate thermal curing into UV curing by tailoring the properties are contemplated. Due to the inevitability of material aging and failure, we summarize how dynamic bonds integrated into the UV/thermal curing system result in the development of self-healing silicone polymers. Subsequently, additive manufacturing based on UV/thermal curing silicone polymers is summarized according to the desired performance. Ultimately, the challenges and future development of UV/thermal curing silicone polymers are considered.