DentoFit A S

Roskilde, Denmark

DentoFit A S

Roskilde, Denmark
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De Souza J.A.,Federal University of Rio de Janeiro | Goutianos S.,Technical University of Denmark | Skovgaard M.,DentoFit A S | Sorensen B.F.,Technical University of Denmark
Journal of the Mechanical Behavior of Biomedical Materials | Year: 2011

The fracture resistance (R-curve behaviour) of two commercial dental composites (Filtek Z350® and Concept Advanced®) were studied using Double Cantilever Beam sandwich specimens loaded with pure bending moments to obtain stable crack growth. The experiments were conducted in an environmental scanning electron microscope to (a) accurately measure the applied energy-release rate for crack initiation, (b) measure the early (rising) part of the R-curve, and (c) provide direct microscopic evidence of the toughening mechanisms ahead of and/or in the wake of the crack tip. The two tested composites displayed distinctly different R-curve behaviours. The difference was related to different toughening mechanisms as the two composites had markedly different microstructures. Contrary to common experience, the composite with the finer microstructure (smaller particles), the Concept Advanced®, showed significantly higher fracture resistance than the composite with the coarser microstructure. The fracture properties were related to the flexural strength of the dental composites. The method, thus, can provide useful insight into how the microstructure enhances toughness, which is necessary for the future development of such materials. © 2011 Elsevier Ltd.


Skovgaard M.,DentoFit A S | Skovgaard M.,Technical University of Denmark | Almdal K.,Technical University of Denmark | Sorensen B.F.,Technical University of Denmark | And 2 more authors.
Journal of Composite Materials | Year: 2011

A problem with dental resin composites is the polymerization shrinkage, which makes the filling loosen from the tooth or induces crack formation. We have developed an expandable metastable tetragonal zirconia filler, which upon reaction with water, is able to counter the polymer shrinkage. The shrinkage of the composite was calculated from density measurements using Archimedes method. The rate of the phase transformation in resin was measured by determining the volume fraction of monoclinic zirconia (vm). The composite had a vm of 0.5 after 8 h of water storage. The overall shrinkage of the composites was reduced from 3.2% (initially) to 1.7%. © The Author(s) 2011 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.


Skovgaard M.,DentoFit A S | Skovgaard M.,Technical University of Denmark | Ahniyaz A.,Swedish Institute for Surface Chemistry | Sorensen B.F.,Technical University of Denmark | And 2 more authors.
Journal of the European Ceramic Society | Year: 2010

For the first time, the effect of microscale shear stress induced by both mechanical compression and ball-milling on the phase stability of nanocrystalline tetragonal zirconia (t-ZrO2) powders was studied in water free, inert atmosphere. It was found that nanocrystalline t-ZrO2 powders are extremely sensitive to both macroscopic uniaxial compressive strain and ball-milling induced shear stress and easily transform martensitically into the monoclinic phase. A linear relationship between applied compressive stress and the degree of tetragonal to monoclinic (t→m) phase transformation was observed. Ball-milling induced microscale stress has a similar effect on the t→m phase transformation. Furthermore, it was found that even very mild milling condition, such as 120rpm, 1h (0.5mm balls) was enough to induce phase transformation. Surfactant assisted ball-milling was found to be very effective in de-agglomeration of our nanocrystalline porous ZrO2 particles into discrete nanocrystals. However, the t→m phase transformation could not be avoided totally even at very mild milling condition. This suggests that the metastable t-ZrO2 is extreme sensitive to microscale shear stress induced by both mechanical compression and ball-milling. The findings presented in this work are very important in further understanding the stress-induced phase transformation of nanocrystalline t-ZrO2 powders in a water free atmosphere and their further stabilization in industrially relevant solvents. © 2010 Elsevier Ltd.


Skovgaard M.,DentoFit A S | Skovgaard M.,Technical University of Denmark | Almdal K.,Technical University of Denmark | Van Lelieveld A.,DentoFit A S
Journal of Materials Science | Year: 2011

Metastable tetragonal zirconia nanocrystallites were studied in humid air and in water at room temperature (RT). A stabilizing effect of different surfactants on the tetragonal phase was observed. Furthermore, the phase stability of silanized metastable tetragonal zirconia nanocrystallites was tested by prolonged boiling in water. The samples were analyzed with X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Changes in the monoclinic volume fraction in the samples were calculated. A number of surfactants were screened for their ability to stabilize the tetragonal phase upon exposure to humidity. Only silanes and phosphate esters of these were able to stabilize the tetragonal phase in water. Even as small amounts of silanes as 0.25 silane molecule per nm 2 are able to stabilize the tetragonal phase in water at RT. Aminopropyl trimethoxy silane and c-methacryloxypropyl trimethoxy silane were even capable of preventing phase transformation during boiling for 48 h in water. © Springer Science+Business Media, LLC 2010.


Skovgaard M.,DentoFit A S | Skovgaard M.,Technical University of Denmark | Almdal K.,Technical University of Denmark | Van Lelieveld A.,DentoFit A S
Journal of Composite Materials | Year: 2012

This study searches for small molecules, which can be generated by photoacid generators (PAGs) capable of inducing the tetragonal-to-monoclinic transformation in zirconia nanocrystals. Metastable tetragonal zirconia nanocrystals were exposed in alcohol suspension. X-ray diffraction analysis showed that water, HCl, HF, and NH3, all initiate phase transformation of tetragonal zirconia at room temperature, whereas NBu4Cl and NBu4OH do not. 2-(4-Methoxystyryl)-4,6-bis(trichloromethyl)-1,3,5-triazine) was the most efficient (monoclinic volume fraction reached 0.57) out of the four tested PAGs. For dispersion in a dimethacrylate matrix together with zirconia crystals, a monoclinic volume fraction of 0.19 was observed after 2-min of exposure to light, increasing to 0.6 after 30-min. © The Author(s) 2012 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav.


Skovgaard M.,DentoFit A S | Skovgaard M.,Technical University of Denmark | Almdal K.,Technical University of Denmark | Van Lelieveld A.,DentoFit A S
Journal of Materials Science | Year: 2010

The effect of phosphates and sulfates on the stability of metastable tetragonal zirconia in water and ambient atmosphere at RT was examined. All chemicals were supplied by Sigma-Aldrich Inc. and highly porous nanocrystalline tetragonal zirconia powders were synthesized. In the inert atmosphere of a glovebox, 1 g metastable tetragonal zirconia was stirred for 150 min with 10 ml anhydrous methanol and 1 ml of either 99.99% phosphoric acid or concentrated sulfuric acid. The polymer matrix prevents the tetragonal crystals from undergoing phase transformation. Such samples are termed matrix dispersed zirconia. The cured samples were then subjected directly to the X-ray diffraction (XRD) measurement for phase analysis. The stabilization is either a result of hindered reaction of water on the surface or a result of change in surface energy, which favors the tetragonal phase.


Skovgaard M.,DentoFit A S | Skovgaard M.,Technical University of Denmark | Almdal K.,Technical University of Denmark | Van Lelieveld A.,DentoFit A S
Journal of Materials Science | Year: 2011

Water is known to initiate a tetragonal to monoclinic phase transformation in zirconia particles. Carbonates on the zirconia surface react with water molecules and hence reduce the transformation rate. This study investigates the possibility of inhibition of the reaction between surface carbonates and water in order to increase the transformation rate in the zirconia crystals. It was found possible to limit the reaction by reacting the surface carbonates with alcohols, a thiol and a primary amide prior to reaction with water. It was also concluded that di- and trialcohols are able to stabilize the tetragonal phase, probably as a result of induced lattice strain. © 2011 Springer Science+Business Media, LLC.

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