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Randolph L.D.,Catholic University of Louvain | Randolph L.D.,Center for Research and Engineering on Biomaterials | Palin W.M.,University of Birmingham | Watts D.C.,University of Manchester | And 8 more authors.
Dental Materials | Year: 2014

Objectives. to complement our previous work by testing the null hypotheses that with shortcuring times and high DC, TPO-based resin composites would exhibit (1) higher polymer-ization stresses and consequently display (2) higher temperature rise and (3) higher flexuralmodulus, flexural strength and hardness, compared to a conventional CQ-based experimen-tal composite.Methods. Two experimental resin composites using either Lucirin-TPO or cam-phorquinone/DMAEMA as photoinitiators were prepared. Light curing was carried outusing spectral outputs adapted to the absorption properties of each initiator. Different irra-diation protocols were selected (0.5, 1, 3, 9 s at 500, 1000 and 2000 mW/cm2 for Lucirin-TPObased composites and 20 or 40 s at 1000 mW/cm2 for Lucirin-TPO and camphorquinone-based composites). Degree of conversion (DC) was measured in real time by means of FT-NIRspectroscopy. Pulpal temperature rise (ΔT) was studied in a tooth model. Polymerizationstress was monitored using the Bioman instrument. For cured specimens, flexural modulusand flexural strength were determined using a three point bending platform and Vickershardness was determined with a microhardness indentor on samples prior to and after24 h incubation in 75/25 ethanol/H2O. Premolars were restored with both materials andmicroleakage at the teeth/composite interfaces following restoration was assessed.Results. Lucirin-TPO-based composites irradiated at radiant exposures of 3 J/cm2 and moreexhibited significantly higher DCs, associated with increased flexural moduli and hardnesscompared to CQ-based composites. For an ultra-short irradiation time of 1 s at 1000 mW/cm2 ,TPO-composites displayed similar polymerization stresses compared to CQ-controls with yet a 25% increase for flexural modulus and 40% increase for hardness measured after EtOH/H2O sorption. Higher stress rates were however observed in all curing protocols compared to CQ-composites. Microleakage was similar between TPO and CQ-composites irradiated at 1000 mW/cm2 for 3 and 20 s respectively, while a significant increase was observed for TPO-composites irradiated for 1 s. ΔT measured through a 0.6 mm thick dentin layer were all below 5.5 °C; TPO-composites exhibited similar or lower values compared to controls.Significance. The use of Lucirin-TPO in resin composites along with appropriate curing conditions may allow for a major reduction of irradiation time while improving mechanical properties. The amount of stress observed during polymerization in TPO-based composites can be similar to those using CQ and the cohesion at the restoration-tooth interface was not affected by short curing times. Contrary to other studies, we found that the temperatures increases measured during polymerization were all well below the 5.5 °C threshold for the pulp. © 2014 Academy of Dental Materials.

Randolph L.D.,Catholic University of Louvain | Randolph L.D.,Center for Research and Engineering on Biomaterials | Palin W.M.,University of Birmingham | Bebelman S.,Catholic University of Louvain | And 9 more authors.
Dental Materials | Year: 2014

Objectives To test the null hypotheses that photoactive resin composites containing a Type I photoinitiator would exhibit reduced DC or increased monomer elution at substantially short curing times compared with materials based on a Type 2 ketone/amine system. Methods Two experimental resin composites were prepared, using either Lucirin-TPO or camphorquinone/DMAEMA. Specimens were light-cured using appropriate spectral emission that coincided with the absorption properties of each initiator using different irradiation protocols (0.5, 1, 3, 9 s at 500, 1000 and 2000 mW/cm2 for Lucirin-TPO based composites and 20 or 40 s at 1000 mW/cm2 for Lucirin-TPO and camphorquinone-based composites). Degree of conversion (DC) was measured by Raman spectroscopy, propagating radical concentrations were collected by means of electron paramagnetic resonance (EPR) and monomer leaching was characterized using high-performance liquid chromatography (HPLC). Results The null hypotheses were rejected, except for a single irradiation protocol (0.5 s @ 500 mW/cm 2). Lucirin-TPO-based composites could cure 20 times faster and release at least 4 times less monomers in comparison to camphorquinone-based composites. At 1000 mW/cm2, and 1 s irradiation time for curing times of 1 s, Lucirin-TPO based composites displayed 10% higher DC. The difference in polymerization efficiency of Lucirin-TPO compared with camphorquinone-based resin composites were explained using EPR; the former showing a significantly greater yield of radicals which varied logarithmically with radiant exposure. Significance Lucirin-TPO is substantially more efficient at absorbing and converting photon energy when using a curing-light with an appropriate spectral emission and otherwise a limitation noted in several previous publications. At concentrations of 0.0134 mol/L, Lucirin-TPO-based composites require a minimum light intensity of 1000 mW/cm2 and an exposure time of 1 s to provide significantly improved DC and minimal elution compared with a conventional photoinitiator system. The use of a wide range of curing protocols in the current experiment has realized the significant potential of Lucirin-TPO and its impact for clinical applications, in replacement to materials using camphorquinone. © 2014 Academy of Dental Materials.

Leprince J.G.,Catholic University of Louvain | Leprince J.G.,Center for Research and Engineering on Biomaterials | Palin W.M.,University of Birmingham | Hadis M.A.,University of Birmingham | And 4 more authors.
Dental Materials | Year: 2013

Objectives: This work aims to review the key factors affecting the polymerization efficiency of light-activated resin-based composites. The different properties and methods used to evaluate polymerization efficiency will also be critically appraised with focus on the developments in dental photopolymer technology and how recent advances have attempted to improve the shortcomings of contemporary resin composites. Methods: Apart from the classical literature on the subject, the review focused in particular on papers published since 2009. The literature research was performed in Scopus with the terms "dental resin OR dimethacrylate". The list was screened and all papers relevant to the objectives of this work were included. Results: Though new monomer technologies have been developed and some of them already introduced to the dental market, dimethacrylate-based composites still currently represent the vast majority of commercially available materials for direct restoration. The photopolymerization of resin-based composites has been the subject of numerous publications, which have highlighted the major impact of the setting process on material properties and quality of the final restoration. Many factors affect the polymerization efficiency, be they intrinsic; photoinitiator type and concentration, viscosity (co-monomer composition and ratio, filler content) and optical properties, or extrinsic; light type and spectrum, irradiation parameters (radiant energy, time and irradiance), curing modes, temperature and light guide tip positioning. Significance:: This review further highlights the apparent need for a more informative approach by manufacturers to relay appropriate information in order for dentists to optimize material properties of resin composites used in daily practice. © 2012 Academy of Dental Materials.

Randolph L.D.,Catholic University of Louvain | Randolph L.D.,Center for Research and Engineering on Biomaterials | Steinhaus J.,Bonn-Rhein-Sieg University of Applied Sciences | Moginger B.,Bonn-Rhein-Sieg University of Applied Sciences | And 7 more authors.
Dental Materials | Year: 2016

Objectives The use of a Type I photoinitiator (monoacylphosphine oxide, MAPO) was described as advantageous in a model formulation, as compared to the conventional Type II photoinitiator (Camphorquinone, CQ). The aim of the present work was to study the kinetics of polymerization of various composite mixtures (20-40-60-80 mol%) of bisphenol A glycidyl dimethacrylate/triethylene glycol dimethacrylate (BisGMA/TegDMA) containing either CQ or MAPO, based on real-time measurements and on the characterization of various post-cure characteristics. Methods Polymerization kinetics were monitored by Fourier-transform near-infrared spectroscopy (FT-NIRS) and dielectric analysis (DEA). A range of postcure properties was also investigated. Results FT-NIRS and DEA proved complementary to follow the fast kinetics observed with both systems. Autodecceleration occurred after ≈1 s irradiation for MAPO-composites and ≈5-10 s for CQ-composites. Conversion decreased with increasing initial viscosity for both photoinitiating systems. However despite shorter light exposure (3 s for MAPO vs 20 s for CQ-composites), MAPO-composites yielded higher conversions for all co-monomer mixtures, except at 20 mol% BisGMA, the less viscous material. MAPO systems were associated with increased amounts of trapped free radicals, improved flexural strength and modulus, and reduced free monomer release for all co-monomer ratios, except at 20 mol% BisGMA. Significance This work confirms the major influence of the initiation system both on the conversion and network cross-linking of highly-filled composites, and further highlights the advantages of using MAPO photoinitiating systems in highly-filled dimethacrylate-based composites provided that sufficient BisGMA content (>40 mol%) and adapted light spectrum are used. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Randolph L.D.,Catholic University of Louvain | Randolph L.D.,Center for Research and Engineering on Biomaterials | Palin W.M.,University College Birmingham | Leloup G.,Catholic University of Louvain | And 3 more authors.
Dental Materials | Year: 2016

Objective: The mechanical properties of dental resin-based composites (RBCs) are highly dependent on filler characteristics (size, content, geometry, composition). Most current commercial materials are marketed as "nanohybrids" (i.e. filler size <1. μm). In the present study, filler characteristics of a selection of RBCs were described, aiming at identifying correlations with physico-mechanical properties and testing the relevance of the current classification. Methods: Micron/sub-micron particles (> or <500nm) were isolated from 17 commercial RBCs and analyzed by laser diffractrometry and/or electron microscopy. Filler and silane content were evaluated by thermogravimetric analysis and a sedimentation technique. The flexural modulus (Eflex) and strength (σflex) and micro-hardness were determined by three-point bending or with a Vickers indenter, respectively. Sorption was also determined. All experiments were carried out after one week of incubation in water or 75/25 ethanol/water. Results: Average size for micron-sized fillers was almost always higher than 1μm. Ranges for mechanical properties were: 3.775wt%) were associated with the highest mechanical properties (Eflex and σflex>12GPa and 130MPa, respectively) and lowest solvent sorption (∼0.3%). Significance: Mechanical properties and filler characteristics significantly vary among modern RBCs and the current classification does not accurately illustrate either. Further, the chemical stability of RBCs differed, highlighting differences in resin and silane composition. Since Eflex and sorption were well correlated to the filler content, a simple and unambiguous classification based on such characteristic is suggested, with three levels (ultra-low fill, low-fill and compact resin composites). © 2016 The Academy of Dental Materials.

Leprince J.G.,Catholic University of Louvain | Leprince J.G.,Center for Research and Engineering on Biomaterials | Leveque P.,Catholic University of Louvain | Leveque P.,Center for Research and Engineering on Biomaterials | And 7 more authors.
Dental Materials | Year: 2012

Objectives: To demonstrate that determination of the depth of cure of resin-based composites needs to take into account the depth at which the transition between glassy and rubbery states of the resin matrix occurs. Methods: A commercially available nano-hybrid composite (Grandio) in a thick layer was light cured from one side for 10 or 40 s. Samples were analyzed by Vickers indentation, Raman spectroscopy, atomic force microscopy, electron paramagnetic imaging and differential scanning calorimetry to measure the evolution of the following properties with depth: microhardness, degree of conversion, elastic modulus of the resin matrix, trapped free radical concentration and glass transition temperature. These measurements were compared to the composite thickness remaining after scraping off the uncured, soft composite. Results: There was a progressive decrease in the degree of conversion and microhardness with depth as both properties still exhibited 80% of their upper surface values at 4 and 3.8 mm, respectively, for 10 s samples, and 5.6 and 4.8 mm, respectively, for 40 s samples. In contrast, there was a rapid decrease in elastic modulus at around 2.4 mm for the 10 s samples and 3.0 mm for the 40 s samples. A similar decrease was observed for concentrations of propagating radicals at 2 mm, but not for concentrations of allylic radicals, which decreased progressively. Whereas the upper composite layers presented a glass transition temperature - for 10 s, 55 °C (±4) at 1 mm, 56.3 °C (±2.3) at 2 mm; for 40 s, 62.3 °C (±0.6) at 1 mm, 62 °C (±1) at 2 mm, 62 °C (±1.7) at 3 mm - the deeper layers did not display any glass transition. The thickness remaining after scraping off the soft composite was 7.01 (±0.07 mm) for 10 s samples and 9.48 (±0.22 mm) for 40 s samples. Significance: Appropriate methods show that the organic matrix of resin-based composite shifts from a glassy to a gel state at a certain depth. Hence, we propose a new definition for the "depth of cure" as the depth at which the resin matrix switches from a glassy to a rubbery state. Properties currently used to evaluate depth of cure (microhardness, degree of conversion or scraping methods) fail to detect this transition, which results in overestimation of the depth of cure. © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

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