Research and Application of UV-cured Silicone-modified Polyurethane Coatings

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Since its emergence in the 1940s, polyurethane (PU) has been widely used in coatings, elastomers, foam plastics, and adhesives. It is a multifunctional polymer material and one of the fastest-growing polymer materials. Polyurethanes contain the characteristic urethane bond (—NH—CO—) in their structural units, and their chains consist of alternating soft and hard segments, which results in a multiphase structure in their aggregated state. This is what gives polyurethane coatings their excellent properties such as wear resistance and flexibility. However, single polyurethane coatings are not ideal in terms of water resistance, gloss, hardness, etc.; modification can enable them to achieve superior overall performance. There are two ways to modify polyurethanes: one is by combining two or more resins with complementary properties using simple physical methods ； Another approach is to use chemical methods to endow the product with the properties of two or more systems. Silicone materials possess excellent properties such as resistance to high and low temperatures, weathering, ozone, electrical insulation, flame resistance, non-toxicity, non-corrosivity, and physiological inertness; therefore, they are ideal materials for polyurethane-modified products. The use of silicone in the modification of polyurethanes overcomes the performance deficiencies of polyurethane materials, and it represents an important approach to expanding the application areas of polyurethanes. This article explores various approaches to silicone-modified polyurethane coatings and provides a brief overview of their applications. 1 Solvent-based silicone-modified PU coatings: Solvent-based coatings are still widely used in high-end coating applications such as luxury cars, aircraft skins, and precision instruments. Silicone-modified polyurethane motorcycle coatings, as studied by Sun Daoxing, Liu Xianglan and others, exhibit significantly improved resistance to saline solutions, acids and alkalis, as well as greater flexibility. Tian Jun, Xue Qunji, and others studied the blended modification in toluene solvent of polydimethylsiloxane with terminal hydroxyl groups and alcoholysis castor oil-modified polyurethane prepolymers. After the copolymer forms a film, the silicone segments in its molecular structure tend to aggregate and orient at the surface, while the polyurethane segments face the inner layer; this improves the mechanical properties of the copolymer film, such as adhesion, hardness, and curing speed ； At the same time, it has a low surface energy, and its heat resistance has also been improved. Coatings composed of polyurethane prepolymers, aminosilanes or siloxanes, polyorganosiloxane tackifiers, hydrogen-containing siloxanes, organic solvents, etc., cure into a film at (150–200)°C under the catalysis of chloroplatinic acid. The resulting film is smooth, heat-resistant, and wear-resistant, and it exhibits good adhesion to silicone rubber that has not undergone any surface treatment. Polyurethanes are modified in a solvent using silicone oils whose side chains contain polyamino functional groups. During the synthesis of polyurethanes, these side chains participate in the reaction, with the siloxane chains hanging from the main chain of the polyurethane; this facilitates the migration of silicon atoms to the surface. A small amount of amino-functionalized silicone oil is sufficient to improve the surface properties of the polyurethane. Wu Di, Guo Li, and others added the synthesized TDI polyurethane to an ethyl acetate solution of silicone resin in a certain ratio, then added dibutyltin dilaurate and stirred thoroughly. The resulting polyurethane-silicone varnish can cure at room temperature, without compromising its excellent heat resistance, weather resistance, and electrical insulation properties. Moreover, the coating’s resistance to various acids, bases, salts, and chemical reagents is significantly improved. Solvent-based coating systems are better suited to meet the requirements of practical applications, such as use as special damping materials, coating for optical fibers, high-temperature resistant insulation coatings, and camouflage coatings for military purposes. They are also difficult to replace, offering broad prospects for development. However, both organic solvents and residual isocyanates are toxic and flammable, causing severe environmental pollution. With the establishment of environmental protection regulations in various countries and an increasing awareness of environmental issues, the emission of volatile organic compounds (VOCs) in solvent-based polyurethane systems is being increasingly restricted. As a result, there is growing interest in the development of water-based, environmentally friendly silicone-modified PU coatings. 2 Water-based silicone-modified PU coatings Driven by environmental protection requirements, research into water-based polyurethanes, photocurable polyurethanes, low-viscosity high-solid-content polyurethanes, and powder-based polyurethane coatings is very active. Water-based polyurethane coatings use water as the dispersion medium, with little or no organic solvents present in the system; this makes them more suitable for the increasingly stringent environmental regulations. As a result, the development of water-based coatings has received considerable attention, and there are many reports on this topic abroad as well. Janusz reported that water-based coatings made from copolymers containing polysiloxanes and polyurethanes possess the capability of moisture-induced vulcanization. Cooper et al. introduced the cyanoethyl (–CH2CH2CN) group into polymethylsiloxane to obtain siloxanes with –NH2 as end groups and –CH2CH2CN on the side chains (PCEMS), which were used as soft segments to synthesize polysiloxane-polyurethane block copolymers. Wang Wusheng et al. further cross-linked water-based polyurethane microgels using epoxy-functionalized epoxysiloxanes, thereby improving the mechanical properties of the coating film. Using polyether (polyester) polyols, silicone oligomers (PDMS), polyisocyanates, and chain extenders as the main raw materials, silicone copolymer-modified polyurethane emulsions are prepared, which exhibit good stability and improved water resistance. Siloxane segments can accumulate on the surface of the emulsion film, exerting a significant surface-modifying effect on PU materials, with little change in their mechanical properties in the bulk. As a topcoat, it possesses excellent overall performance. Polymerized as polysiloxane-polyurethane block copolymers using polysiloxane as the soft segment, these materials combine the excellent properties of both polysiloxane and polyurethane. They exhibit good low-temperature flexibility, dielectric properties, surface affinity, and excellent biocompatibility. They overcome the drawback of poor mechanical properties associated with polysiloxane and address the issue of poor weather resistance in polyurethane, thus holding great promise for further development. Introducing urea bonds into polysiloxanes can improve the compatibility between soft and hard segments. The system features both hydrogen bonding between soft segments and hydrogen bonding between the two phases, which significantly enhances the mechanical properties of such materials. Additionally, forming mixed soft segments from polysiloxane and polyether is also an effective way to improve the mechanical properties of polysiloxane-polyurethane block copolymers. 3 Powdered organic silicon-modified PU coating: Polyurethane powder coating is a powdered mixture composed of powdered polyurethane resin, curing agents, pigments, fillers, and additives. It is typically produced by high-speed mixing of these components, followed by extrusion, cooling, crushing, and screening. In addition to having the properties of conventional powder coatings, it also boasts excellent physical and mechanical properties as well as anti-corrosion capabilities. Its coating is shiny and full-bodied, resistant to wear and scratches, resistant to solvents, has good leveling properties, and strong adhesion; it is widely used in home appliances such as refrigerators, washing machines, and air conditioners, as well as in industries involving pianos, high-end furniture, cars, and motorcycles. First, an end-capped polymer is prepared, which is then depolymerized at high temperatures and reacted with hydroxyl-containing compounds in the system to form powder coatings with excellent properties. Such coatings not only possess high decorative value and excellent physical and mechanical properties, but also have advantages such as good hardness, flowability, chemical resistance, weather resistance, UV resistance, and particularly low tendency to yellow. Currently, there are few reports on the use of silicone-modified polyurethanes as powder coatings. One of the development trends is to create powder coatings with a low film-forming temperature, no volatile by-products generated during film formation, and performance comparable to that of similar two-component solvent-based PU coatings. In addition, powder coatings are also evolving in directions such as ultra-finement, thin-film formation, and supercritical fluidization. 4. UV-curable silicone-modified PU coatings: The traditional curing methods for polyester coatings include moisture-curing and heat-curing. Due to the advantages of UV curing technology, such as fast curing speed, energy savings, resource conservation, and no solvent pollution, UV-curable silicone polyurethane coatings have attracted considerable interest since their introduction, showing great potential for development. The polyurethane coating with a polysiloxane-type photosensitive prepolymer as its soft segment incorporates Si—O bonds, which significantly improves the heat resistance, weather resistance, and adhesion of the photocured coating film. The silicone/acrylate/polyurethane resin synthesized by Tao Yonghong and Tang Kai, when used in UV-curable coatings, exhibits improved resistance to yellowing, while also maintaining good wear resistance and adhesion. Qi Yusong, Zeng Zhaohua, and others synthesized a silicone-modified polyurethane acrylate (Si-PUA) prepolymer using hydroxyethyl methacrylate, isophorone diisocyanate, and bis(γ-trioctysilanylpropyl)amine as raw materials; the photopolymerized films resulting from this synthesis exhibit excellent electrical and thermal properties. Zhao Shilin, Chen Guoxin, and others utilized the hydrolysis reaction of the silane coupling agent KH-570 under acidic conditions to synthesize alcohol-soluble organosilicon resins in situ; simultaneously, nano-SiO2 was dispersed within these resins to create an environmentally friendly nanometric UV-blocking transparent coating. This coating can be used as a topcoat for building exterior walls and wooden surfaces, improving the coating’s UV-blocking efficiency and transparency while also extending its service life. 5 Other silicone-modified PU coatings: In addition to the aforementioned types of silicone-modified polyurethane coatings, Chen Jinghua, Liu Weiqu and others have reported the synthesis of a series of silicone-polyurethane prepolymers under solvent-free conditions using polypropylene glycol (PPG), aminoethylaminopropylpolydimethylsiloxane (amino silicone oil, AEAPS), and toluene diisocyanate (TDI) as raw materials. The silicone-polyurethane materials obtained by curing these prepolymers with dimethyltoluidine DADMT exhibit excellent mechanical properties, heat resistance, and surface hydrophobicity. Furthermore, there are also many reports on composite silicone-modified PU coatings using ternary or polymeric copolymer systems that incorporate third components such as epoxies and acrylates, in addition to silicone-modified polyurethane coatings. The propyl polysiloxane-polyurethane-epoxide tercopolymer synthesized by Li Yongqing et al. exhibits good tensile strength, hydrophobic properties, and a low surface tension. Mosleh et al. reduced the friction coefficient of coatings obtained by adding single-crystalline silicon to an aqueous polyurethane dispersion, by controlling the thickness of the coating film, thereby improving the surface properties of the coating film. Yang et al. used the conventional sol-gel method to prepare a coating by blending silicon-containing components and inorganic fillers with polyurethane; this coating was applied to aluminum surfaces and subjected to pore corrosion tests. It was found that slight corrosion occurred on the aluminum surface initially, but the formed alumina along with the existing SiO2 created a stable layer that prevented further pore corrosion. The ability of coatings prepared by adding SiOX, as a special light stabilizer, to polyurethanes to prevent photooxidation was **enhanced** by Meng and Klenchuk et al. Zhou and Wu et al. incorporated nano-silicon into acrylic/polyurethane, resulting in significant improvements in the hardness, wear resistance, and scratch resistance of the coatings. Moreover, as more nano-silicon was added, the tensile strength and Young’s modulus of the coatings also increased. 6 Conclusion The application of silicone-modified polyurethane coatings is very widespread; it has become one of the main methods for modifying polyurethane materials, and it still holds potential for further development. It is believed that as research progresses further, silicones will play an even greater role in the development of polyurethane coatings, enabling the creation of composite materials with improved properties.