**1. Introduction**

Dental resin-based composites (RBCs) for both direct and indirect dental restorations have been in use for the past 50 years [1]. These materials are composed of inorganic particles (conferring most of the mechanical properties to the final material), which are embedded into an organic matrix (consisting of a mixture of crosslinking monomers, a photoinitiator system and other additives, which form a dense cross-linked polymer upon a free-radical copolymerization) [1,2].

Bisphenol A glycidyl methacrylate (Bis-GMA) is the base-monomer most frequently used in the formulations of dental restorative materials since it reduces polymerization shrinkage and enhances both modulus and thermal stability of the resulting materials; however, it also exhibits a high viscosity, which yields usually a heterogeneous material and problems during handling and application of the product. Thus, in order to achieve high filler loading in dental resin composites, low-viscosity diluent

monomers such as triethylene glycol dimethacrylate (TEGDMA) are commonly used [3–5]. Other monomers have been also used to improve specific properties [1].

Monomers used in the commercial formulations of dental composites have remained largely unchanged, whereas the type, shape, size and distribution of inorganic fillers have undergone significant changes [1]. Recently, the incorporation of filler particles with nanometric dimensions into dental resins has attracted great attention, as it is possible to obtain materials with improved properties with it. Wear resistance, gloss retention, elastic modulus, flexural strength, diametral tensile strength and reduced polymerization shrinkage have been improved by the addition of nanoparticles to dental composites [5,6].

It is worth mentioning that these improvements are generally achieved through addition of a small amount of nanoclay (contrary to what has been observed in conventional inorganic macro- and micro-fillers as they constitute up to 75–85 wt.%). This fact is due to the extremely large interface area provided by the nano-size particles of the suspended filler [7–9].

At present, there is an increasing interest in the incorporation of nanoclays into dental composites. Montmorillonite (MMT) is the most commonly used clay, both in its natural form and organically modified. Its use has been reported in Cloisite® Na+ [6,7,9]; Cloisite® 10A [10]; Cloisite® 20A [6] and Cloisite® 93A [7], although Cloisite® 30B is the organoclay most employed [5–7,11].

In contrast, there are few studies in the literature involving the use of alternative clay minerals. Tian et al. and Zhang et al. reported the used of palygorskite (attapulgite) in dental resins, while Weidenbach et al. studied materials containing functional halloysite–nanotube filler [12–14].

Therefore, the aim of this work is to investigate the influence of nanoclay type at different loading levels (0, 2, 4, 6, 8 and 10 wt.%) on properties of the dental resins composites. Tetraethylene glycol dimethacrylate (TTEGDMA) was used as co-monomer instead of triethylene glycol dimethacrylate (TEGDMA) monomer, as Rüttermann et al. suggested that viscosities of both monomers are similar but the molecular mass of TTEGDMA is higher than TEGDMA; in consequence, it was expected to be more advantageous regarding shrinkage [15]. The TTEGDMA is often used not only in commercial resins but also in experimental dental restorative resins [15,16].

#### **2. Materials and Methods**

#### *2.1. Materials*

The monomers used for the preparation of the dental composites were Bisphenol A glycidyl methacrylate (Bis-GMA) and tetraethylene glycol dimethacrylate (TTEGDMA). Camphorquinone (CQ) and *N*,*N*-dimethyl aminoethyl methacrylate (DMAEMA) were used as photo-initiator and co-initiator, respectively. All reagents were purchased from Sigma–Aldrich Co. (Milwaukee, WI, USA) and used as received without further purification. The clay minerals used in this study were a commercial montmorillonite (Cloisite® 30B, abbreviated as MMT) modified with a quaternary ammonium salt (90 meq/100 g of clay) from Southern Clay Products (Gonzáles, TX, USA) and an HCl purified palygorskite (PLG) extracted from a mineral deposit in Chapab, Yucatán, México.

#### *2.2. Preparation of Dental Composite*

Bis-GMA was mixed manually with TTEGDMA in a glass container at 50:50 wt.% ratio. The photo-initiator CQ and the tertiary amine DMAEMA were then added to the Bis-GMA/TTEGDMA mixture (both materials at 0.5 wt.% to the monomer mixture). The container was covered with aluminum foil (to avoid premature curing) and refrigerated until its use. Monomers and initiator system were manually mixed, and the nanoclay (MMT or PLG) was slowly added at different loading levels in small portions to avoid the formation of agglomerates; formulations were mixed until no filler agglomerations were visually observed in the monomer mixture. Resin composites were cured with a light emitting diode unit (LED H, Woodpecker, Guilin, China) with a wavelength range of 420–480 nm

and a light intensity of 1000 mW/cm<sup>2</sup> . Table 1 summarizes the dental resin composites prepared in this study.


**Table 1.** Composition of dental composites.
