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J.w. Nicholson - One of the best experts on this subject based on the ideXlab platform.

  • Water Transport in Resin-modified Glass-ionomer Dental Cement
    Journal of Biomaterials Applications, 2008
    Co-Authors: Audrey Percq, Denis Dubois, J.w. Nicholson
    Abstract:

    Water uptake and water loss have been studied in a commercial resin-modified glass-ionomer Cement, Fuji II LC, under a variety of conditions. Uptake was generally non-Fickian, but affected by temperature. At room temperature, the equilibrium water uptake values varied from 2.47 to 2.78% whereas at low temperature (12°C), it varied from 0.85 to 1.18%. Cure time affected uptake values significantly. Water uptake was much lower than in conventional glass-ionomer restorative Cements exposed to water vapor. Loss of water under desiccating conditions was found to be Fickian for the first 5 h loss at both 22 and 12°C. Diffusion coefficients were between 0.45 and 0.76 × 10 cm/s, with low temperature diffusion coefficients slightly greater than those at room temperature. Plotting water loss as percentage versus s allowed activation energies to be determined from the Arrhenius equation and these were found to be 65.6, 79.8, and 7.7 kJ/mol respectively for 30, 20, and 10 s cure times. The overall conclusion is that the main advantage of incorporating HEMA into resin-modified-glass-ionomers is to alter water loss behavior. Rate of water loss and total amount lost are both reduced. Hence, resin-modified glass-ionomers are less sensitive to water loss than conventional glass-ionomers.

  • The effect of storage in aqueous solutions on glass-ionomer and zinc polycarboxylate Dental Cements
    Journal of Materials Science: Materials in Medicine, 2000
    Co-Authors: J.w. Nicholson, A. D. Wilson
    Abstract:

    Cylindrical specimens (dimensions 6 mm diameter×12 mm height) of glass-ionomer and of zinc polycarboxylate Dental Cement have been stored in aqueous solutions for periods of 24 h, 1 week and 1 month. The solutions were of varying composition and affinity for water, and storage in them resulted in fluctuations in mass of the Cements, an effect which was attributed to differences in the partitioning of water between the solutions and the Cement specimens. Unlike the zinc polycarboxylate, the glass-ionomer gained mass in most of the solutions examined (except Na_2SO_4), showing it to have a much greater affinity for water than the zinc polycarboxylate. Despite the fluctuations in water uptake by the glass-ionomer, and loss of water by the zinc polycarboxylate, no statistically significant differences in compressive strength were recorded in any solution at any storage time. This contrasts with results reported previously for zinc polycarboxylates using smaller specimens, showing that specimen size has an influence on the interaction of Cements with storage solutions. ©2000 Kluwer Academic Publishers

  • Studies in the setting of polyelectrolyte Cements: Part VII The effect of divalent metal chlorides on the properties of zinc polycarboxylate and glass–ionomer Dental Cements
    Journal of Materials Science: Materials in Medicine, 1998
    Co-Authors: J.w. Nicholson
    Abstract:

    A study is reported in which a zinc polycarboxylate and a glass polyalkenoate Dental Cement, were prepared from aqueous solutions of divalent metal chlorides, namely ZnCl2, CaCl2, MgCl2 and SrCl2, all at 1.0 mol dm-3 concentration, as well as from pure water. Calcium chloride was employed at additional concentrations, i.e. 2.0, 0.5 and 0.1 mol dm-3. As was previously found for monovalent salts, setting of the zinc polycarboxylate was speeded up and water uptake generally enhanced by the presence of the divalent metal salts. However, the divalent salts were found to reduce the compressive strength at 24 h (from 86 MPa to about 60 MPa). The glass polyalkenoate showed broadly similar effects to those observed in the presence of monovalent salts, with the setting time being increased, water uptake inhibited and compressive strength at 24 h reduced; however, by contrast, the working time was generally reduced. These results occur because the rate of the neutralization process is increased by the divalent salts, a consequence of the reduced pH of the poly(acrylic acid) caused by these salts. Infrared spectroscopy demonstrated interactions between the metal chlorides and poly(acrylic acid), with various chelate structures being apparent from the position of the asymmetric carbonyl stretch. © 1998 Chapman & Hall

  • Studies on the setting of polyelectrolyte Cements: Part VI The effect of halide salts on the mechanical properties and water balance of zinc polycarboxylate and glass–ionomer Dental Cements
    Journal of Materials Science: Materials in Medicine, 1998
    Co-Authors: J.w. Nicholson, F Abiden
    Abstract:

    A study is reported in which a zinc polycarboxylate and a glass polyalkenoate Dental Cement, respectively, were prepared from aqueous solutions of NaCl, KCl, KBr and KI, all at 1 mol dm3 concentration, as well as from pure water. For the zinc polycarboxylate, setting as determined by oscillating rheometry was speeded up and water uptake was enhanced by the presence of the salts. Conversely, compressive strength at 24 h was unaffected. On the other hand, for the glass polyalkenoate, the setting reaction was slowed down, water uptake inhibited and compressive strength at 24 h reduced (from 94.3 MPa with pure water to 59.8 MPa with NaCl, 65.8 MPa for KCl, 67.0 MPa for KBr and 81.1 MPa for KI). Previous work with polyelectrolytes in aqueous solution suggests that the halides probably enhance the rate of the neutralization process. For the zinc polycarboxylate, this leads to a more rapid setting reaction. By contrast, for the glass polyalkenoate, it results in slower setting and weaker Cements. This result is attributed to inhibition of the secondary setting reaction, involving the formation of the silicate/phosphate network, by enhanced neutralization, a process which is consequently concluded to occur earlier in the overall setting of these Cements than had been assumed previously. © 1998 Chapman & Hall

  • Studies in the setting of polyelectrolyte Cements
    Journal of Materials Science: Materials in Medicine, 1994
    Co-Authors: H. M. Anstice, J.w. Nicholson, N. L. Bubb
    Abstract:

    Specimens of zinc polycarboxylate Dental Cement have been prepared with methanol or methanol/water mixtures as solvent. Such Cements set at a reduced rate compared with those activated by water alone, and final materials are about 50% weaker in compression. Methanol reduces the dielectric constant of the solvent system compared with water alone and such a reduction inhibits ionic reactions and causes the polyacid to adopt a coiled configuration in solution. Both of these effects are assumed to contribute to the observed reduction in setting rate. The reduction in setting rate was confirmed using infrared spectroscopy.

John W. Nicholson - One of the best experts on this subject based on the ideXlab platform.

  • The History and Background to Glass-Ionomer Dental Cements
    Glass-Ionomers in Dentistry, 2016
    Co-Authors: John W. Nicholson
    Abstract:

    This chapter provides a historical perspective and an insight into how the glass-ionomer Cement was invented following a long series of studies on Dental Cements, beginning with the now obsolete Dental silicate Cement. It reviews the experiments on the predecessor materials and also the early studies of the glass-ionomer Dental Cement. Glass-ionomer Cements emerged from research on the former Dental silicate Cement and the zinc polycarboxylate Cement. Dental silicates were poorly understood materials in the early 1960s when studies were started at the Laboratory of the Government Chemist in the UK. These studies showed for the first time that Dental silicates were acid–base materials that set to form a matrix of metal phosphates containing unreacted glass filler. From this, the role of the glass was understood for the first time and, in particular, the importance of its alumina/silica ratio in controlling basicity. Following this discovery, the means of producing a practical glass-polyacrylate Dental Cement was clear and was achieved by altering the alumina/silica ratio of the glass to increase its basicity and balance the reduced acidity of the poly(acrylic acid). The original glass capable of forming a practical Cement, known as G200, was high in fluoride and hence fairly opaque compared with modern ionomer glasses. Consideration of the role of fluoride led to the concept of chelating additives to control the setting reaction which led to the discovery of the effect of tartaric acid. This allowed glass-ionomer Cements of good translucency for clinical use to be developed. These inventions led on to the pioneering work described in this chapter in which the setting reactions were elucidated, the role of water established, the release of fluoride studied and the factors affecting strength determined. This knowledge informed early ideas of how these materials might be used in dentistry, and the chapter concludes with a review of these early clinical applications.

  • Zinc polycarboxylate Dental Cement for the controlled release of an active organic substance: proof of concept
    Journal of Materials Science: Materials in Medicine, 2010
    Co-Authors: Mohammad Naseem Ali, Mark Edwards, John W. Nicholson
    Abstract:

    The potential of employing zinc polycarboxylate Dental Cement as a controlled release material has been studied. Benzalkonium chloride was used as the active ingredient, and incorporated at concentrations of 1, 2 and 3% by mass within the Cement. At these levels, there was no observable effect on the speed of setting. Release was followed using an ion-selective electrode to determine changes in chloride ion concentration with time. This technique showed that the additive was released when the cured Cement was placed in water, with release occurring by a diffusion mechanism for the first 3 h, but continuing beyond that for up to 1 week. Diffusion coefficients were in the range 5.62 × 10^−6 cm^2 s^−1 (for 1% concentration) to 10.90 × 10^−6 cm^2 s^−1 (for 3% concentration). Up to 3% of the total loading of benzalkonium chloride was released from the zinc polycarboxylate after a week, which is similar to that found in previous studies with glass-ionomer Cement. It is concluded that zinc polycarboxylate Cement is capable of acting as a useful material for the controlled release of active organic compounds.

  • Ion-release, dissolution and buffering by zinc phosphate Dental Cements
    Journal of Materials Science: Materials in Medicine, 2003
    Co-Authors: Beata Czarnecka, Honorata Limanowska-shaw, John W. Nicholson
    Abstract:

    The interaction of zinc phosphate Dental Cement with aqueous solutions has been studied in order to elucidate the relationship between pH change and ion release (dissolution). For each storage medium (deionized water, lactic acid at pH 2.7 and lactate buffer at pH 2.2) five cylindrical specimens of zinc phosphate Cement (6 mm diameter×12 mm height) were prepared and weighed. They were stored individually in 8 cm^3 of solution for a week, then the pH was determined and the specimens reweighed. The solutions were replaced and the specimens stored for a further week, then the pH and the weight were again measured. This was repeated for four weeks. For each storage solution at each time interval, the concentration of ions leached (Na, Mg, Al, Zn and P) were determined using ICP-OES. The lactate buffer was particularly erosive and reduced specimens to 4.1% (±0.9%) of their original mass after 4 weeks. The lactic acid was also erosive, but in water, specimens showed no significant mass change after 4 weeks. In all media, Na, Al, Mg, Zn and P ions were released, with mole ratios varying at each time interval. In all cases, the pH shifted towards neutral, but the relationship between ion release and solution pH was not straightforward. From the mole ratios of ions, estimates could be made of the relative proportions of attack at matrix to attack at filler, and this showed attack at filler predominated in most solutions at most time intervals.

E. A. Wasson - One of the best experts on this subject based on the ideXlab platform.

  • A study of the structure of zinc polycarboxylate Dental Cements
    Journal of Materials Science: Materials in Medicine, 1993
    Co-Authors: J.w. Nicholson, S. J. Hawkins, E. A. Wasson
    Abstract:

    Two examples of zinc polycarboxylate Dental Cement were studied, one of which was prepared from an aqueous solution of poly (acrylic acid) together with the zinc oxide powder, the other being prepared by adding water to a mixture of dried polyacid and zinc oxide powder. The changes in the properties of the resultant Cements with length of storage in various media were determined. In all cases the maximum strength was achieved fairly rapidly, usually at 1 week, after which there was little or no increase. Cements stored in water achieved the lowest compressive strengths, whereas Cements stored in highly desiccating conditions, over concentrated sulphuric acid, achieved very high (if variable) compressive strengths. There appeared to be very little difference between the water-activated and conventional Cements. These results confirm previous findings that zinc polycarboxylate Cements are relatively poorly hydrated compared with other polyelectrolyte biomaterials. This in turn implies that water does not play a structural role in these Cements.

Garry J.p. Fleming - One of the best experts on this subject based on the ideXlab platform.

  • Application of analytical stress solutions to bi-axially loaded Dental ceramic-Dental Cement bilayers.
    Dental materials : official publication of the Academy of Dental Materials, 2008
    Co-Authors: Owen Addison, Garry J.p. Fleming
    Abstract:

    Abstract Objective Increased consideration of Dental ceramics and Dental Cements as a singular structural unit for in-vitro mechanical testing has resulted in the reporting of a wide range of analytical methods to calculate the failure stresses. Therefore a comparison of observations between studies is complicated by the use of dissimilar stress solutions despite the employment of a similar testing methodology and specimen geometry. Materials and methods Three analytical solutions to calculate failure stresses in bi-axially loaded Dental ceramic–Dental Cement bilayers were appraised for a commonly utilized testing geometry (ball-on-ring). Clinically relevant datasets were generated from the bi-axial flexure testing of uncoated and Dental Cement coated aluminous core ceramic exposed to differing ceramic surface preparations. A Weibull statistical approach was utilized in order to provide insight into the impact of the analytical method on both the scale ( σ 0 ) and distribution ( m ) of the failure stress data. Results Calculation of the bi-axial flexure stress utilizing Timoshenko's analysis resulted in an increase in σ 0 for the uncoated (6%) and Dental Cement coated (11–12%) aluminous core ceramic, when compared with the bilayered solutions reported by Rosenstiel and Hsueh. However, the shape of the failure distributions illustrated by the consistency of m and associated 95% confidence intervals was not influenced by the analytical stress solution employed. Significance The choice of the analytical method chosen to calculate failure stresses in bi-axially loaded Dental ceramic–Dental Cement bilayers will impact on the magnitude of the reported strength. Comparison between the failure stresses of uncoated and Cement coated Dental ceramics is more accurately represented by bilayer solutions, which account for the mismatch between the elastic constants of dissimilar materials. However, within the context of Dental Cement coated Dental ceramics of clinically relevant dimensions, the choice of solution is unlikely to impact on the interpretation of the observations previously reported in the Dental literature.

  • influence of powder liquid mixing ratio on the performance of a restorative glass ionomer Dental Cement
    Biomaterials, 2003
    Co-Authors: Garry J.p. Fleming, Ahmed A Farooq, Jake E Barralet
    Abstract:

    Abstract The influence of powder/liquid mixing regime on the performance of a hand-mixed restorative glass-ionomer Cement (GIC) was evaluated in terms of compressive strength, working characteristics and the porosity distribution. Mean compressive fracture strengths, standard deviations and associated Weibull moduli ( m ) were determined from series of 20 cylindrical specimens (6 mm height, 4 mm diameter) prepared by hand-mixing the relative proportions of the powder and liquid constituents. Working characteristics were assessed using an oscillating rheometer whilst scanning electron microscopy and image analysis were used to investigate the influence of the mixing regime on pore distribution. For a constant volume of liquid (1 ml) the mean compressive strength decreased from 102.1±23.1 MPa for 7.4 g of powder, to 93.8±22.9, 82.6±18.5 and 55.7±17.2 MPa for 6.66, 5.94 and 3.7 g of powder, respectively. A concomitant increase in both the working and setting times was also observed. GICs manipulated to a powder/liquid mixing consistency below the manufacturers’ recommend ratio, for a constant volume of liquid, resulted in reduced porosity levels in the Cement mass and extended working and setting times. Unfortunately, a reduction in the concentration of reinforcing glass particles in the set material below that specified by the manufacturers decreases the Cements’ load bearing capacity so that they fail at lower compressive stress levels in the posterior region of the mouth.

  • encapsulated verses hand mixed zinc phosphate Dental Cement
    Biomaterials, 1999
    Co-Authors: Garry J.p. Fleming, A C Shortall, R M Shelton, P M Marquis
    Abstract:

    Zinc phosphate Cements are commonly supplied as two components, powder and liquid, and the proportions of the constituents are determined by operator experience. A capsulated system which is mechanically mixed has been marketed and this study investigated the performance of the encapsulated Cement system. The mean fracture strength, standard deviation and associated Weibull Moduli (m) of encapsulated Cements were determined by compressive fracturing 20 Cement specimens filled directly from the mixing syringe or from narrower Cement tubes. Pore distribution within the cylindrical specimens was determined using image analysis to assess the influence of the method of mould filling with the Cement. The strength data showed variation in magnitude and consistency ranging from 44.6+/-13.7 MPa (m = 3.18+/-0.71) for Cements filled directly from the syringe to 61.0+/-7.8 MPa (m = 8.35+/-1.87) for Cements filled from Cement tubes. Larger pores were found in specimens consolidated directly from the Cement syringe. Mechanical mixing of the encapsulated Cement resulted in air entrapment in the Cement mix which manifested itself as large pores (over 200 microm diameter) within the cylindrical specimens. The smaller orifice of the Cement tube compared with the syringe was considered to be responsible for eliminating the majority of the air entrapped in the Cement mass during mixing. Whilst mechanical mixing of encapsulated Cements is quicker and more convenient, the encapsulated specimens consolidated according to the manufacturers instructions from the syringe offered no significant advantage in terms of reliability or strength over hand-mixed Cements in this investigation.

  • the influence of clinically induced variability on the distribution of compressive fracture strengths of a hand mixed zinc phosphate Dental Cement
    Dental Materials, 1999
    Co-Authors: Garry J.p. Fleming, P M Marquis, A C Shortall
    Abstract:

    Abstract Objectives : Conventional approaches to comparing Dental Cements use standard property tests under manufacturers’ specified conditions. Zinc phosphate Cements are supplied in powder/liquid form and manipulation frequently involves mixing the components by eye so a range of mixing ratios will inevitably occur in practice. Unfortunately, the physical, chemical, biological and mechanical properties of Cements are known to be dependent on the mixing ratio. Methods : Forty Dental nurses prepared a series of three Cement samples to a luting consistency they considered acceptable for use in practice. Results : It was found that each Dental nurse produced consistent Cement mixes, although, the mixing ratios varied from 1.7 to 3.2 g/ml between nurses. The mean compressive strength, standard deviation and associated Weibull Moduli ( m ) of the Cements were determined as a function of this mixing ratio range and showed considerable variation ranging from 33.5±3.2 MPa ( m =11.0) at 1.7 g/ml to 71.4±8.4 MPa ( m =8.6) at 2.6 g/ml and 42.5±10.0 MPa ( m =5.0) at 3.2 g/ml. Significance : An analytical approach was adopted which facilitated an integrated analysis of the mixing ratio variability with the strength data. It was found for the test group of Dental nurses that 25% of Cement mixes produced would have achieved strengths below 40 MPa whilst strengths below the standard value were produced in at least 70% of mixes. These results indicate that a simple analysis of the properties of Cements manipulated under optimum conditions, provides little information on the material characteristics obtained in practice because of clinically induced variability.

A. D. Wilson - One of the best experts on this subject based on the ideXlab platform.

  • The effect of storage in aqueous solutions on glass-ionomer and zinc polycarboxylate Dental Cements
    Journal of Materials Science: Materials in Medicine, 2000
    Co-Authors: J.w. Nicholson, A. D. Wilson
    Abstract:

    Cylindrical specimens (dimensions 6 mm diameter×12 mm height) of glass-ionomer and of zinc polycarboxylate Dental Cement have been stored in aqueous solutions for periods of 24 h, 1 week and 1 month. The solutions were of varying composition and affinity for water, and storage in them resulted in fluctuations in mass of the Cements, an effect which was attributed to differences in the partitioning of water between the solutions and the Cement specimens. Unlike the zinc polycarboxylate, the glass-ionomer gained mass in most of the solutions examined (except Na_2SO_4), showing it to have a much greater affinity for water than the zinc polycarboxylate. Despite the fluctuations in water uptake by the glass-ionomer, and loss of water by the zinc polycarboxylate, no statistically significant differences in compressive strength were recorded in any solution at any storage time. This contrasts with results reported previously for zinc polycarboxylates using smaller specimens, showing that specimen size has an influence on the interaction of Cements with storage solutions. ©2000 Kluwer Academic Publishers