【Frontiers in HaiChuan Chemical Technology】Investigation and Comparison of Hydrophilic and Hydrophobic Vapor-Phase Silica in UV Plastic Coatings
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As an efficient and environmentally friendly coating system, UV plastic coatings are widely used in the field of surface protection and decoration for plastic products. As an important additive, gas-phase silica, thanks to its unique nanoscale structure and excellent properties, can significantly improve various characteristics of UV plastic coatings; it plays a key role, in particular, in terms of viscosity thixotropy. This article will explore in detail the effect of the hydrophilic fumed silica HL-200 and its competitor A, as well as the hydrophobic fumed silica HB-151 and its competitor B, on the viscosity thixotropy of UV plastic coatings. Vapor-phase silica is amorphous silica obtained by the hydrolysis of volatile chlorosilanes in a hydrogen-oxygen flame, featuring nanoscale primary particles with particle sizes typically ranging from 7 to 40 nm. Their aggregate and agglomerate structures result in a highly developed specific surface area, ranging from 50 to 450 m²/g. This unique microstructure endows fumed silica with a range of special properties, such as high purity, high specific surface area, high activity, good dispersibility, and the ability to significantly influence the rheological properties of the system. Among them, the hydrophilic vapor-phase silica particles retain a large number of hydroxyl groups on their surface, which endows them with hydrophilicity, allowing them to be wetted by water and dispersed in it. In UV plastic coating systems, the silanol groups on the hydrophilic surface of fumed silica form a physical cross-linked network through interactions such as hydrogen bonds, thereby affecting the viscosity and thixotropy of the coating. Hydrophobic vapor-phase silica is obtained by surface modification of hydrophilic vapor-phase silica, with common modifiers including hexamethyldisilazane (HMDS) and dimethyldichlorosilane (DDS). After modification, the hydroxyl groups on the silica surface are reacted or shielded, resulting in hydrophobicity. In UV plastic coatings, hydrophobic fumed silica can influence the rheological properties of the coating not only through the hydrogen bonding of surface silanol groups but also via the entanglement of treatment agent molecules.
So, what effect does hydrophilic fumed silica have on the viscosity and thixotropy of UV plastic coatings? Adding hydrophilic fumed silica to UV plastic coatings can effectively increase the viscosity of these coatings. Hydrogen bonds form between the surface hydroxyl groups, creating a three-dimensional network structure that hinders the free movement of molecules in the coating, resulting in an increase in viscosity on a macroscopic scale. When subjected to external forces (such as the shear force during application by brushing or spraying), this hydrogen bond network structure is partially disrupted, resulting in a decrease in viscosity and enabling the paint to be applied easily. Once the external forces are removed, the hydrogen bond network re-establishes itself gradually, and the viscosity increases again, thereby providing a good thixotropic effect. This helps to prevent the paint from sagging during application, improves its storage stability, and prevents solid particles such as pigments from settling. Taking HuiFu Nano’s hydrophilic fumed silica HL-200 as an example, in UV plastic coating applications, when the addition level is 0.5%, there are minimal changes in the viscosity and thixotropic properties of the UV plastic coating. However, as the addition level increases from 0.5% to 2%, the viscosity can rise from 400 Pa·s to 3100 Pa·s at low shear rates, representing a increase of 675%; at high shear rates, the viscosity increases from 420 Pa·s to 1100 Pa·s, an increase of 161%. These changes meet the flow requirements of UV plastic coatings under different application conditions. The thixotropic value increased from 0.9 to 2.8, also showing an increase of around 210%. At the same time, Competitor A and HL-200 exhibit similar behavior in terms of thickening thixotropy, achieving the best results at an addition level of 1.5%.
Taking HuiFu Nano’s hydrophobic fumed silica HB-151 as an example, when the addition level is 0.5%, there are minimal changes in the viscosity and thixotropic properties of the UV plastic coating. However, as the addition level increases from 0.5% to 2%, the viscosity rises from 400 Pa·s to 2800 Pa·s at low shear rates, an increase of 600%; at high shear rates, the viscosity increases from 420 Pa·s to 1050 Pa·s, a rise of 150%. The thixotropic value increased from 0.9 to 2.8, also showing an increase of around 210%. At the same time, competitor B and HB-151 exhibit similar behavior in terms of thickening and thixotropy, achieving the best results at an addition level of 1.5%. In UV plastic coatings, hydrophobic fumed silica affects the rheological properties of the coating primarily through hydrogen bonding of the surface silanol groups, as well as through the entanglement of treatment agent molecules. Due to its hydrophobicity, it tends to interact more easily with non-polar organic molecules in the coating system, forming a loose network structure. Compared to the hydrophilic type, the hydrophobic type of fumed silica has a slightly weaker effect on increasing the viscosity of coatings. In UV plastic coating formulations that require high water resistance, HIFULL® hydrophobic fumed silica from Huifu Nano not only confers good thixotropy to the coating, effectively preventing sagging, but also significantly improves the water resistance of the coating after it forms a film, thereby reducing the adsorption and penetration of water molecules. Both hydrophilic and hydrophobic fumed silica can exert a significant influence on viscosity thixotropy in UV plastic coatings through unique mechanisms, thereby providing the coatings with good application properties and storage stability. Hydrophilic types rely primarily on hydrogen bonding between surface hydroxyl groups, while hydrophobic types depend on hydrogen bonding between surface silanol groups as well as the entanglement of treatment agent molecules. In practical applications, the type and amount of fumed silica should be selected appropriately based on the specific performance requirements of UV plastic coatings, in order to optimize the coating properties, meet the coating needs of different plastic products, and promote the development of the UV plastic coating industry.
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