Surface microhardness and compressive strength are crucial parameters in fabrication of dental cast and dies to resist laboratory manipulative forces during indirect dental restoration construction. Moreover, the setting expansion (dimensional changes during setting) is a critical factor for the successful fabrication of accurate sized wax pattern and consequently dental restoration (Abdelaziz et al. 2002).
Several attempts have been performed to enhance the mechanical properties of dental plaster that include incorporation of various additives such as Arabic gum, silicone oxide, glass fibers, and calcium oxide (Razak et al. 2017; Akkus et al. 2018). In the current study, aluminum oxide nanoparticles have been used as fillers to reinforce the conventional dental plaster. The conventional dental plaster was used as negative control, while the improved stone was served as a positive control.
The set gypsum material final surface microhardness strength and setting expansion are prominently controlled by the crystal particle number and entanglement, and any substance that inhibits the crystal growth or decrease their number will affect their strength and abrasion resistance (Jayaprakash et al. 2014).
The gypsum product materials utilized for construction of final casts or dies should express a high resistance to scratching and sufficient strength that withstand the manipulated force during laboratory manipulation and wax pattern construction, which in turn changed to final indirect dental restorations.
After calcium sulfate hemihydrate (gypsum products) mixing with water, they began to set and undergo dimensional changes. This expansion mostly is due to growth in both number and size of the growing calcium sulfate dihydrate crystals. Entanglements of the growing crystals push against each other lead to a phenomena called “crystal outward thrusting action.” Expansion will occur as a result of occupying the crystals a larger space volume. Additionally, internal porosity in the set mass will be created due to the outward thrust action (Michalakis et al. 2012).
Minimization of the setting expansion of the gypsum products upon transformation of calcium sulfate hemihydrate to dihydrate to very little values is mandatory to get a proper fitting of the indirect dental restoration to tooth surface (Razak et al. 2017). Aluminum oxide nanoparticles are considered as a chemically inert ceramic, non-toxic, and white in color. It could serve as promising fillers into advanced dental materials. They have a great capability to produce act as reinforcing agent to increase surface microhardness and enhance the mechanical properties (Alhareb et al. 2017).
The null hypothesis was that there has no significant difference between the surface microhardness, compressive strength, and setting expansion between conventional and reinforced dental plaster with 10 wt.% aluminum oxide nanoparticles fillers.
The higher surface microhardness of the novel reinforced dental plaster with 10 wt.% aluminum oxide nanoparticle fillers may be due to the effect of incorporation of hard aluminum oxide ceramic nanoparticle fillers into gypsum products (Alhareb et al. 2017; Razak et al. 2017).
The increase in compressive strength of the reinforced dental plaster group may be due to two facts: the addition of strong aluminum oxide ceramic nanoparticle fillers into gypsum products, in addition to the reduction in calcium sulfate powder on expense of the added aluminum oxide nanoparticle filler that will lead to decrease in the formed calcium sulfate dihydrate crystals, subsequently decrease the formation of internal porosity in the set materials (Alhareb et al. 2017; Razak et al. 2017).
The little dimensional changes of the reinforced dental plaster group may be attributed to the lowest setting expansion values due to the addition of aluminum oxide nanoparticle fillers on expense of calcium sulfate dihydrate powder. Thus, reduction in the produced amount is the calcium sulfate dihydrate crystals with subsequent decrease in the resultant outward thrust action of the formed crystals, hence reduction in the linear expansion (Razak et al. 2017).
Moreover, the aluminum oxide in nanoparticle sizes has a very fine size and high surface area which may cause great interfacial interaction with matrix tha enables them to restrict the molecular motion of calcium sulfate dihydrate, thus decrease their expansion during setting (Safi 2014).
The requirement of the American Dental Association Specification No. 25 and ISO standard 6873:1998 for gypsum products that used for construction of final casts and dies included that the compressive strength values should be not less than 35 MPa. Moreover, the setting expansion should be not more than the range of 0.0–0.15 % (ADA 1985)(ISO 1998).
The null hypothesis was rejected as the innovative dental plaster reinforced by 10 wt.% aluminum oxide nanoparticle fillers achieved improvement in both surface microhardness and compressive strength with very little setting expansion.
Our novel reinforced dental plaster gets a compressive strength values higher than those obtained from other studies that depends on the addition of glass fibers, gum Arabic, ferric oxide, and calcium oxide (Razak et al. 2017)(A Hatim et al. 2007). Furthermore, it gives a low setting expansion than study that depends on epoxy resin nanoparticle and carbone nanostructure fillers (Kreve 2018). Moreover, it gets a higher compressive strength and surface microhardness and lower setting expansion values than those of our previous investigation of microsized aluminum oxide fillers (Hamdy, 2019). Our study gives a higher compressive strength and low setting expansion than that of improved stone which not studied before.
