High Performance Dental Resin Composites Based On Novel Hybridized Tetrapod-like Zinc Oxide Whiskers

Dental resin composites（DRCs） were commonly used in dental restoration. One primary drawback of DRCs is that it is easy to accumulate bacteria on the surface, resulting in secondary caries and thus fracture failure of the materials. The effective solution for endowing DRCs with excellent antibacterial activity is adding antibacterial agents, but available antibacterial agents tend to degrade mechanical properties. The second drawback of DRCs is large polymerization shrinkage, which often brings marginal leakage and restoration failure. In order to get low polymerization shrinkage, the amount of inorganic fillers should be very high（more than 40 wt%） in conventional DRCs, such high loading of fillers usually sacrifices the process feature and mechanical properties of dental resins. Therefore, it is a significant subject to develop DRCs with low polymerization shrinkage, good antibacterial activity and high mechanical properties through using a relatively low content of inorganic fillers. This is the target of our project reported herein.Firstly, a series of novel SiO₂ hybridized tetrapod-like zinc oxide whisker（coded as SK-T-ZnOw） with different Si O2 loading were prepared through a two-step process, they are the preparation of silane-modified tetrapod-like zinc oxide whisker（coded as K-T-ZnOw） using 3-methacryloxypropyltrimethoxysilane（γ-MPS）, followed by coating K-T-ZnOw with SiO₂ through St?ber method. An orthogonal experiment was built to study the influences of four factors including amount of ammonia, amount of water, reaction time and dropping time of tetraethylorthosilicate（TEOS） on the SiO₂ loading. Results show that these four factors have different degrees of impact on the Si O2 loading, among them the reaction time and amount of ammonia have greater impacts than the amount of water and dropping time of TEOS. The longer the reaction time is, the greater the SiO₂ loading. An elevated amount of ammonia increases the Si O2 loading while an excess of ammonia would reduce the SiO₂ loading. Several means were used to characterize the structure of SK-T-ZnOw. Antibacterial tests show that all fillers have extremely similar antibacterial rates, indicating that γ-MPS treatment and SiO₂ hybrid do not affect the antibacterial activity of tetrapod-like zinc oxide whisker（T-ZnOw）.Secondly, SK-T-ZnOw and commercial dimethacrylate resin（UBT） were used to develop a series of composites（coded as SK/UBT）. The influences of the surface properties and the content of fillers on the structures and comprehensive performances（degree of conversion, polymerization shrinkage, antibacterial activity, mechanical properties, water absorption and solubility） of SK/UBT composites were systematically investigated. Results show that SK/UBT composites have not only excellent and durable antibacterial activity, but also higher mechanical properties and lower polymerization shrinkages, water absorptions and solubility compared with composites with T-ZnOw（T/UBT）. Especially, for the SK/UBT composite containing 20 wt% SK-T-ZnOw, its antibacterial rates before and after stored in distilled water at 37 °C for 20 days reach up to 98.1% and 93.7%, respectively, and its flexural strength, flexural modulus and Vickers hardness were severally 1.3, 1.1 and 1.4 times of the corresponding values of the T/UBT composite containing 20 wt% T-ZnOw; moreover, the polymerization shrinkage of the former is only 0.8 times of that of the latter, clearly demonstrating that the bottleneck on secondary caries, low mechanical properties and large polymerization shrinkage of previously reported DRCs have been dissolved. These excellent performances are derived from the unique structure of SK-T-ZnOw. The existence of vast tiny SiO₂ particles on whiskers surface not only endows the interface with additional riveting effect and leads to better reinforcing effect of whiskers, but also makes the whiskers be separated from each other and thus improved dispersion in matrix. In addition, the γ-MPS treatment gives filler surface C═C bonds which can react with UBT, this is also beneficial to improve the dispersion of fillers and the interfacial adhesion between fillers and UBT resin.