Evaluation of various monofunctional monomers for the development of fracture tough dental materials exhibiting a low crosslink density

Abstract

3D printing of fracture tough dental materials that additionally exhibit excellent mechanical properties is challenging. Nowadays, most of the 3D printing dental materials contain a mixture of highly reactive dimethacrylates. The corresponding printed materials exhibit a high crosslink density and are particularly brittle. They are therefore not suitable for additive manufacturing of fracture tough denture bases. Recently, an efficient technology based on the combination of a urethane dimethacrylate macromonomer with a monofunctional monomer and a poly(ɛ-caprolactone)-polydimethylsiloxane-poly(ɛ-caprolactone) triblock copolymer was developed. Materials exhibiting a low crosslink density, excellent mechanical properties, and high fracture toughness were obtained. In this article, further developments of this highly efficient technology are described. A wide range of monofunctional monomers (both methacrylates and acrylates) was evaluated in this system. It was shown that the structure of the selected monofunctional monomer has a strong influence on the mechanical properties (flexural strength and modulus) as well as on the fracture toughness of the light cured materials. Thanks to the formation of nanostructures, a strong increase of fracture toughness was obtained upon addition of the toughening agent (poly(ɛ-caprolactone)-polydimethylsiloxane-poly(ɛ-caprolactone) triblock copolymer). The most promising materials were the ones based on the following monofunctional monomers: 2-phenoxyethyl methacrylate, (octahydro-4,7-methano-1H-indenyl)methyl acrylate, isobornyl acrylate, tetrahydrofurfuryl methacrylate and 4-tert-butylcyclohexyl acrylate. Indeed, these materials exhibited excellent mechanical properties (90.0 ± 3.8 MPa < flexural strength < 102.6 ± 4.7 MPa; 2402 ± 90 MPa < flexural modulus < 2714 ± 68 MPa) combined with high fracture toughness (1.89 ± 0.06 MPa m1/2 ≤ maximum stress intensity factor (Kmax) ≤ 2.18 ± 0.08 MPa m1/2; 418 ± 14 J m−2 ≤ work of fracture (Wf) ≤ 591 ± 25 J m−2). The measured Kmax and Wf were even significantly higher than the values reported for Probase Hot (Kmax = 1.44 ± 0.18 MPa m1/2; Wf = 270 ± 30 J m−2), a clinically proven and well-established PMMA based denture base material from Ivoclar. The successful DLP 3D printing of a monoblock denture using the most promising formulation confirmed the great potential of this technology for the development of 3D printing fracture tough denture bases.