Methacrylate-based composites are widely used in restorative dentistry. Such materials are composed of an organic matrix and silanized inorganic fillers. Typically, the organic phase is based on difunctional monomers such as 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)-phenyl]propane (BisGMA), bis-[(2-methacryloyloxyethoxy-carbonyl)-amino]-2,2,4-trimethylhexane (UDMA) and ethoxylated bisphenol A dimethacrylate (BisEMA), as well as reactive diluents such as 1,10-decanediol dimethacrylate (D3MA) and triethylene glycol dimethacrylate (TEGDMA). Besides monomers, the organic matrix of dental composites contains photoinitiators like bis(4-methoxybenzoyl) diethylgermane (Ivocerin®), camphorquinone (CQ)/ ethyl 4-(dimethylamino)benzoate (EDAB) and additives such as stabilizers.[1], [2] The inorganic phase usually consists of a mixture of various fillers such as radiopaque barium or strontium silicate glass fillers, aggregated silica nanoparticles, silica-zirconia mixed oxides and ytterbium fluoride (YbF3).[2] The surface of the silica based filler particles is typically functionalized with polymerizable silane coupling agents. The most common silane in dental materials is 3-methacryloyloxy-propyltrimethoxysilane (MPTS). The commercial MPTS contains two functional groups: A trialkoxysilane and a methacrylate moiety connected by a propyl spacer. The trialkoxysilane moiety reacts with hydroxy groups on the surface of silica particles.[3] The resulting organic layer on the particles increases the hydrophobicity of the surface and improves the wettability of such particles with rather hydrophobic methacrylate-based monomer mixtures.[4] Hence, the mixing of filler and monomer matrix is eased and the water uptake of the resulting cured restoration is reduced. Furthermore, filler particles that are functionalized with methacrylate-bearing silanes will be covalently incorporated into the dimethacrylate network during curing. The efficient coupling of both the organic and inorganic phases is essential to obtain proper mechanical properties (flexural strength and modulus) of cured dental composites after water storage.[5] Recently, a MPTS based derivative containing an octyl spacer called 8-methacryloxyoctyltrimethoxysilane (MOTS) was evaluated by Yon et al. and the effect of the variation in chain length on mechanical properties of the corresponding dental composites was discussed.[6] The results revealed that composites based on MOTS functionalized filler exhibited lower flexural modulus and microhardness values in comparison to composites containing MPTS functionalized filler. Nonetheless, a significant improvement in flexural strength was observed for composites containing MOTS functionalized filler.

In general, dental composites can be divided into two main categories, depending on their consistency: flowable and packable composites. Typically, the consistency of dental composites is mainly related to the filler content and to the nature of the incorporated particles (filler composition, particle size and surface functionalization). Packable composites are highly filled restorative materials (typically 80–83 wt%) with a stiff consistency.[2] The dentist can shape and sculpt such materials before light curing.[1] Owing to their high filler content, restorations made with packable composites exhibit high flexural modulus values. Unfortunately, voids or bubbles could be formed during placement of packable composites in the tooth cavity due to its stiff consistency.[7] As a result, a poor adaption of packable composites to the tooth cavity wall is often obtained.[8] Hence, the properties and the longevity of restorations depend on the skill level of the clinician and on the operation conditions as well.[9], [10] Such technique sensitive procedures like handling and placement of packable composites need to be adapted to obtain good marginal adaption and to minimize marginal leakage and the formation of secondary caries.[11], [12] As a workaround, packable composites are frequently used in combination with liners, which are typically flowable composites. As the name indicates, flowable composites are dental filling materials exhibiting a lower viscosity in comparison to packable composites. Such materials contain a lower amount of inorganic fillers (63–68 wt%).[2] For this reason, flowable composites exhibit lower flexural moduli and higher volumetric shrinkage in comparison to packable composites upon curing.[13] Nonetheless, owing to the low viscosity of flowable composites, the wetting of the cavity walls is improved and such materials enhance the marginal adaption.[14] Indeed, the consistency is a key property for composite handling.

To simplify this workflow, a composite, which could switch from a packable to a flowable consistency upon application of an external stimulus, would be highly advantageous. A proposed strategy consists in sonicating or heating the composite before filling of the cavity. Recently, Kerr developed a technology that enables a strong drop of the viscosity of a bulk-fill composite upon sonication (SonicFill).[15], [16] Meanwhile, VOCO released a new dental composite called VisCalor bulk exhibiting a strong reduction in consistency upon heating.[17] Preheating of composites is increasingly used in dentistry. By increasing the composite temperature to 37 °C, a reduction in viscosity was observed, which could potentially improve the composite adaptivity.[18], [19], [20] Furthermore, Loumprinis et al. showed that the consistency of packable composites can be significantly reduced by preheating up to 54 °C without increasing the stickiness of the composite.[21] Additionally, the preheated composites at 60 °C exhibit improved double bond conversion.[22] Though, an increase in volumetric shrinkage at elevated temperatures (37 °C and 44 °C) was reported as well.[23] Although a consistency change is clearly observed, the preheating of most the packable composites does not result in flowable like consistency and the viscosity reduction is only moderate. Recently, Elbishari et al. revealed that the size of barium glass particles (0.45 µm, 0.7 µm, 1.0 µm and 1.5 µm) has significant impact on the drop in consistency at 37 °C.[19] The larger the used glass particles in dental composites, the stronger the drop in viscosity upon heating. It seems that the higher volume to surface ratio for larger particles benefits this trend. Unfortunately, if larger particles are used in dental composites, the wear resistance and the polishability of the restoration decrease.[4], [24] Hence, the use of large particles is to avoid for the development of esthetic dental composites. There is therefore a need for a technology enabling a stronger consistency change of dental restorative materials upon heating.

The hypothesis of this work is that the nature of the silane coupling agent for the surface modification of the particles would strongly influence the rheological properties of preheated composites. It has already been reported in the literature that the nature of the silane has a direct impact on various physical properties such as handling characteristics, solvent sorption, flexural strength, and modulus.[25], [26], [27] To provide a strong drop in consistency at elevated temperatures, filler should be functionalized with coupling agents which can form bonds with the monomers that can dissociate upon heating. Particularly interesting is the incorporation of H-bond forming groups like urethane and urea moieties into methacrylate-based silanes. Such modified silanes should increase the interaction of the coupling agent with the urethane and glycidyl-based monomers at room temperature in comparison to MPTS. Furthermore, the formed H-bonds should dissociate upon heating as well.

Although the influence of the silane nature on the consistency at room temperature was already shown, its impact on the drop in consistency upon heating has not been studied yet. In this context, functional group containing silane coupling agents SI 1–4 and MOTS and DTS without functional groups in the spacer were selected in order to evaluate the influence of the following parameters on the consistency change of heated composites: spacer length between trimethoxysilane and methacrylate group, presence of a polymerizable moiety and presence of various H-bonding groups (urethane and urea). The selected silanes will be used to functionalize barium aluminum borosilicate glass filler and corresponding composites will be formulated. The consistency of the materials will be evaluated at various temperatures (23 °C, 30 °C, 50 °C and 60 °C). (Fig. 1). The influence of the silane nature on the flexural strength, flexural modulus, and double bond conversion (DBC) of cured composites will also be discussed.