Antonio Apicella

Effect of adhesive layer properties on stress distribution in composite restorations—a 3D finite element analysis

OBJECTIVES Teeth, adhesively restored with resin-based materials, were modeled by 3D-finite elements analysis that showed a premature failure during polymerization shrinkage and occlusal loading. METHODS Simulation of Class II MOD composite restorations with a resin bonding system revealed a complex biomechanical behavior arising from the simultaneous effects of polymerization shrinkage, composite stiffness and adhesive interface strain. Due to a polymerization contraction, shrinkage stress increases with the rigidity of the composites utilised in the restoration, while the cusp movements under occlusal loading are inversely proportional to the rigidity of the composites. The adhesive layers strain also plays a relevant role in the attenuation of the polymerization and occlusal loading stresses. RESULTS The choice of an appropriately compliant adhesive layer, able to partially absorb the composite deformation, limits the intensity of the stress transmitted to the remaining natural tooth tissues. For adhesives and composites of different rigidities, FEM analysis allows the determination of the optimal adhesive layer thickness leading to maximum stress release while preserving the interface integrity. Application of a thin layer of a more flexible adhesive (lower elastic modulus) leads to the same stress relief as thick layers of less flexible adhesive (higher elastic modulus).

3D-finite element analyses of cusp movements in a human upper premolar, restored with adhesive resin-based composites

The combination of diverse materials and complex geometry makes stress distribution analysis in teeth very complicated. Simulation in a computerized model might enable a study of the simultaneous interaction of the many variables. A 3D solid model of a human maxillary premolar was prepared and exported into a 3D-finite element model (FEM). Additionally, a generic class II MOD cavity preparation and restoration was simulated in the FEM model by a proper choice of the mesh volumes. A validation procedure of the FEM model was executed based on a comparison of theoretical calculations and experimental data. Different rigidities were assigned to the adhesive system and restorative materials. Two different stress conditions were simulated: (a) stresses arising from the polymerization shrinkage and (b) stresses resulting from shrinkage stress in combination with vertical occlusal loading. Three different cases were analyzed: a sound tooth, a tooth with a class II MOD cavity, adhesively restored with a high (25 GPa) and one with a low (12.5GPa) elastic modulus composite. The cusp movements induced by polymerization stress and (over)-functional occlusal loading were evaluated. While cusp displacement was higher for the more rigid composites due to the pre-stressing from polymerization shrinkage, cusp movements turned out to be lower for the more flexible composites in case the restored tooth which was stressed by the occlusal loading. This preliminary study by 3D FEA on adhesively restored teeth with a class II MOD cavity indicated that Youngs modulus values of the restorative materials play an essential role in the success of the restoration. Premature failure due to stresses arising from polymerization shrinkage and occlusal loading can be prevented by proper selection and combination of materials.

Poly(Ethylene oxide) (PEO) and different molecular weight PEO blends monolithic devices for drug release

An interpretation of the drug release from monolithic water-swellable and soluble polymer tablets is presented. A convenient parameter, alpha, which compares the drug-diffusive conductance in the gel layer with the swelling and dissolving characteristics of the unpenetrated polymer was used to describe the release behaviour of beta-hydroxyethyl-theophylline (etofylline) from compression-moulded tablets of hydrophilic pure semicrystalline poly(ethylene oxides) of mol wt 600,000 and 4,000,000 and of two blends of the two molecular weights of poly(ethylene oxides). The water swelling and dissolution characteristics of two polymers and two blends were analysed, monitoring the thickness increase of the surface-dissolving layer and the rates of water swelling and penetration in the tablets. The drug diffusivities in the water-penetrated polymer gels were measured by carrying out permeation tests. Finally, drug release tests were performed to investigate the release kinetics of the different systems in an aqueous environment at 37 degrees C. The drug release from the high molecular weight poly(ethylene oxide) is principally related to the material swelling rather than polymer dissolution, leading to a progressive decrease of the drugs diffusive conductance in the growing swollen layer, and hence to a non-constant release induced by the prevailing diffusive control. Conversely, drug release from the low molecular weight poly(ethylene oxide) is strictly related to the polymer dissolution mechanism. The achievement of stationary conditions, in which the rate of swelling equals the rate of dissolution, ensures a constant release rate, even in the case of very low drug-diffusive conductance in the external gel layer. Intermediate behaviours were detected in the case of the two blends.

Effect of thermal history on water sorption, elastic properties and the glass transition of epoxy resins

Abstract The DGEBA/TETA (diglycidyl ether of bisphenol-A/triethylene tetramine) system was examined in postcuring conditions at 100°C for 3 and 6 days. Sorption kinetics and equilibria at different temperatures and mechanical tests have been performed on the two sets of samples. Thermal treatment of the epoxy resin containing sorbed water affects its subsequent water sorption characteristics, elastic modulus and glass transition temperature. Attention has also been given to the effects on solubility on thermal history in the presence of water. It was observed that the saturation values in such systems are determined once the higher temperature of the thermal cycle is defined. The differences in solubility of samples with different hygrothermal history are explained in terms of microcavities that can be formed by effect of crazing in the plasticized system exposed to high temperatures.

Hygrothermal history dependence of equilibrium moisture sorption in epoxy resins

Abstract Water uptakes in sorption experiments made on thin sheets of an epoxy resin have been followed by means of a quartz microbalance placed in a temperature and humidity controlled cell. Attention has been given to the humidity history dependence of the apparent water solubilities at constant temperature. The sorption behaviours under the same temperature and humidity conditions have been successively compared for samples with different hygrothermal histories. Additional sorptions have been carried out from liquid water at low temperatures (2 and 20°C). Equilibrium moisture sorptions have been found to be represented in the same temperature and humidity conditions both by linear and upward isotherms, depending upon the humidity history to which the system had been previously subjected. Such humidity history dependence, which is progressively lost as the test temperature is decreased, has been attributed to a microcavitational damage process in the form of localized solvent crazing.

Rheological behaviour of a commercial TGDDM-DDS based epoxy matrix during the isothermal cure

The change of the viscosity profile during the isothermal cure of a commercially available epoxy system commonly used as a matrix for high performance composites, TGDDM-DDS from CIBA, has been obtained by means of both a constant shear rate viscometer and a dynamic one. The range of temperature investigated varied from 120 up to 180°C. The increase of the molecular weight during the cure reaction is reflected on a macroscopic level in a progressive linear increase of the logarithm of the shear viscosity up to a critical point, near gelation, where an upturn is observed. The values of the time and viscosity at the critical point have been used to normalize the experimental data in a single generalized curve of the cure-viscosity profile. The William, Landel and Ferry equation has been found to adequately describe the temperature dependence of the viscosity for systems in the range of temperatures where the cure reaction did not occur. The apparent activation energy of the cure reaction, 19.7 Kcal/mole, obtained from the critical times, is consistent with calorimetric determinations.

Effects of orientation on the penetration, crazing, and dissolution of polystyrene by n-hexane

Abstract Extruded, drawn, and quenched samples of polystyrene were subsequently immersed in liquid n-hexane at temperatures between 35° and 55°C for various time intervals. Samples were removed from the immersion bath, quenched and fractured. Subsequent microscopic examination of the cross-sections revealed a distinct boundary between a crazed outer shell and an essentially unpenetrated central core. The time dependence of the depth of penetration of the advancing craze front was measured at various temperatures for several draw ratios. The initial rate of penetration increased monotonically with draw ratio (orientation) and the advance of the penetrant front was completely controlled by diffusion for drawn samples at 55°C. More complex kinetics, involving relaxations at the moving boundary, describe the penetration at lower temperatures; a slight systematic variation in the relative contribution of diffusion and relaxation was observed with increasing draw ratio. An activation energy of 23.7 k cal/g-mole characterized the temperature dependence of the initial penetration rate, independent of sample orientation. Gravimetric swelling experiments were confounded by sample dissolution in the case of the oriented samples. Intriguing swelling patterns, including discernable differences between the pronounced edge effects in the draw and tranverse directions, were apparent. Conversely, diffusion transverse to the orientation direction was accelerated by the orientation resulting in an increasing component of relaxation control in the penetration experiments and increased rates of dissolution in the oriented samples.

Recoil kinetics of uniaxially oriented polystyrene

SummaryRecovery experiments aboveTg have been performed on highly oriented polystyrene sheets obtained from extrusion and subsequent hot drawing. A shift procedure has been applied to recoil data using the WLF equation on two different types of commercial polystyrene. A variableλ defined as the fractional distance from equilibrium is able to reduce all the data obtained on sheets drawn at different draw ratios to a single master curve. The change in molecular weight affects only the value of the glass transition and the relaxation kinetics but has no influence on theC1 andC2 constants of the WLF equation. Moreover the molecular orientation does not modify theTg of the drawn polymers.ZusammenfassungEs werden Kriecherholungsversuche oberhalb der GlastemperaturTg an hochorientierten Polystyrolplatten durchgeführt, die durch Extrusion und nachfolgende Heißverstreckung hergestellt worden sind. Die Schrumpfdaten von zwei verschiedenen kommerziellen Polystyrolen werden mittels der WLF-Gleichung übereinander geschoben. Durch eine Variableλ, welche den relativen Abstand von der Gleichgewichtskonfiguration beschreibt, kann man sämtliche verschiedenen Verstreckungsverhältnissen zugeordnete Meßwerte auf einer einzigen Master-Kurve vereinigen. Der Unterschied im Molekulargewicht beeinflußt lediglich die Glastemperatur und die Relaxationsgeschwindigkeit, hat aber keinen Einfluß auf den Wert der KonstantenC1 undC2 der WLF-Gleichung. Darüber hinaus wird die Glastemperatur durch die molekulare Orientierung infolge der Verstreckung nicht verändert.

Stoichiometric and heating-rate effects on DSC-evaluated kinetics for a high-performance epoxy system

Abstract Dynamic DSC experiments on a high-performance amine-cured epoxy system have shown that the kinetic parameters determined assuming an overall reaction for the system are dependent on the heating rate of the scan. The high-performance system examined in this investigation is the TGDDM—DDS epoxy system. Changes in the number, location, and magnitude of the transitions in the thermograms which occur with changes in the heating rate were interpreted through the definition of two reaction regimes: one in which the primary-amine addition reaction predominates; and the other in which the secondary-amine addition, hydroxyl—epoxide, and homopolymerization reactions become significant. To elucidate these regimes, three compositions were examined: epoxy alone; excess epoxy; and excess amine. The activation energies for the two reaction regimes were determined to be 16.7 and 42.3 kcal mol − , respectively. A model proposed by Prime for the heating rate dependence of the kinetic parameters as determined by DSC is also analyzed for its applicability for the kinetic analysis of this reaction system. Further, analysis of isothermal data is considered as an integral part of the kinetic analysis for this system.

DIMENSIONAL STABILITY OF REINFORCED MATRICES

Particulate fillers are often used to improve the dimensional stability of polymeric items obtained through processing techniques such as injection molding, deep drawing, hot stamping, etc. In these operations the material undergoes large multiaxial deformations which result in molecular orientation and remain as frozen-in stresses during cooling of the material. Once the formed objects are exposed to sufficiently high temperatures, various degrees of spring-back take place and subsequent changes in shape and dimension occur (1).