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A new analytical solution for the biharmonic equation was developed for single screw extrusion cross-channel fluid mechanical flow. This analysis led to a quantitative model for residence time distribution when combined with the historic solutions of the drag and pressure flow in the rectangular channel in the single-screw extruder. The focus of the theoretical and experimental investigation here was to examine how closely the new analytical solution correlated with experimental residence time data for a free-helix extruder. This new extrusion device was operated as both a conventional extruder and a more positive displacement device by using only helix rotation as the pump. The Moffatt eddies that occur in the quiescent corners of the rectangular channel with screw rotation were found to have a strong effect on the residence time of the extruder. Because there were no quiescent corners for the free-helix flow there was essentially no residence time tail for this mode of extruder displacement. The theoretical results for a sheet of dye spanning the screw channel width and dye 'blobs' were compared with experimental results for both modes of operation. In all cases, the experiments and the theory predictions were essentially consistent. KEYWORDS biharmonic based theory, experimental data, residence time distribution, single screw extrusion, 1 | INTRODUCTION Single-screw plasticating extruders convert a feedstock of plastic pellets into a molten polymer at high enough pressure to form useful shapes of the polymer by extruding it [...]

Purpose: To assess effects of a novel hydrogen peroxide leave-on whitening emulsion on surface hardness, fracture susceptibility, surface erosion, and surface morphology of enamel and dentin. Methods: Human enamel and root dentin sections embedded in resin were leveled and polished for uniformity. A cycling treatment simulating overuse conditions (60 hours over 10 days), coupled with incubation in pooled human saliva at 37°C and two daily toothpaste treatments were used to evaluate the safety of a 3% hydrogen peroxide whitening emulsion treatment (Crest Whitening Emulsions). Controls included a no treatment group, three erosion controls (water, 0.25% citric acid pH 3.6, 1% citric acid pH 3.6), and a bleaching control (8.25% sodium hypochlorite). Color measurements (b*) were taken on select post-treated specimens to confirm bleaching activity. Effects on enamel and dentin physical properties were determined by surface microhardness, fracture toughness, erosion depth, and surface morphology by light and scanning electron microscopy. Results: The hydrogen peroxide emulsion b* value was significantly different versus water control (P< 0.05), confirming bleaching activity. Microhardness and fracture toughness results for hydrogen peroxide emulsions were not significantly different versus baseline (P> 0.2) and no treatment (P= 1.0), respectively. Erosion loss for the hydrogen peroxide emulsion was not observed on enamel (comparable to water) and significantly less than 0.25 % citric acid (P< 0.05) on dentin which was verified by microscopic visualization. [ABSTRACT FROM AUTHOR]

Purpose: To use non-inferiority statistical testing with simple microhardness measurements (SMH) as a prediction of potential erosive hard tissue damage of topical treatments on enamel. Methods: Three independent experiments of a simple acid cycling demineralization (ACD) model were used to screen softening effects of various commercial beverages on dental enamel. The cycling model consists of six repeated exposures of enamel slabs with alternating treatments of artificial saliva over the course of 6 hours. After six repeated cycles, effects on surface microhardness were measured. Softening effects of beverages were evaluated using a statistical non-inferiority test of the positive control (water) and negative control (1% citric acid). To confirm whether softening effects as evaluated by a noninferiority test translated to like differences in enamel erosion susceptibility, selected beverages then underwent niore complex erosion cycling model (ECM) evaluation where enamel blocks were cycled with beverages (vs. historically established citric acid) and pooled saliva over a period of 5 days. The ECM also incorporated dentifrice treatments, sodium fluoride (NaF, Crest Cavity Protection, negative control) and a positive control stannous fluoride dentifrice (SnF2, Crest Pro-Health Advanced), to confirm model performance against historically published results of in situ erosion protection benefits of SnF2. Results: There was a spectrum of softening properties of 16 commercial beverages in the ACD test, ranging from a ASMH of-22.6 to -316 vs. baseline. Four beverages were evaluated further in ECM testing. Despite a measurable change in SMH, Sprite and beer treatments in the ACD passed the statistical non-inferiority test and both were evaluated in erosion cycling, showing no enamel surface loss. Vinegar (-5% acetic acid) and Gatorade also showed measurable changes in SM H in the ACD, but they failed statistical non-inferiority testing. Both beverages subsequently showed significant enamel tissue loss (erosion) in further erosion cycling testing. This combined set of data suggests that simple surface microhardness evaluation may be used as a proxy for potential erosion surface loss if properly quantified. SnF2 dentifrice significantly reduced erosion from all erosive beverages with greater efficacy than NaF control dentifrice, consistent with prior clinical and in vitro evidence. [ABSTRACT FROM AUTHOR]

The promotion of the electrocatalyticethanol oxidation reaction(EOR) on extended single-crystal Pt surfaces and dispersed Pt nanoparticlesby Sn under acidic conditions is well known. However, the correlationof Sn coverage on Pt nanoparticle electrocatalysts to their size hasproven difficult. The reason is that previous investigations havetypically relied on commercially difficult to reproduce electrochemicaltreatments of prepared macroscopic electrodes to adsorb Sn onto exposedPt surfaces. We demonstrate here how independent control over bothSn coverage and particle size can yield a significant enhancementin EOR activity in an acidic electrolyte relative to previously reportedelectrocatalysts. Our novel approach uses electroless nanoparticlesynthesis where surface-adsorbed Sn is intrinsic to Pt particle formation.Sn serves as both a reducing agent and stabilizing ligand, producingparticles with a narrow particle size distribution in a size rangewhere the mass-specific electrocatalytic activity can be maximized(ca. 1–4 nm) as a result of the formation of a fully developedSn shell. The extent of fractional Sn surface coverage on carbon-supportedPt nanoparticles can be systematically varied through wet-chemicaltreatment subsequent to nanoparticle formation but prior to incorporationinto macroscopic electrodes. EOR activity for Pt nanoparticles isfound to be optimum at a fractional Sn surface coverage of ca. 0.6.Furthermore, the EOR activity is shown to increase with Pt particlesize and correlate with the active area of available Pt (110) surfacesites for the corresponding Sn-free nanoparticles. The maximum area-and mass-specific EOR activities for the most active catalyst investigatedwere 17.9 μA/cm2Ptand 12.5 A/gPt, respectively, after 1 h of use at 0.42 V versus RHE in an acidicelectrolyte. Such activity is a substantial improvement over thatof commercially available Pt, Pt–Sn, and Pt–Ru alloycatalysts under either acidic or alkaline conditions. [ABSTRACT FROM AUTHOR]

Objective: The purpose of this study was to compare the abrasivity of commercial dentifrices by two techniques: the conventional gold standard radiotracer-based Radioactive Dentin Abrasivity (RDA) method; and a newly validated technique based on V8 brushing that included a profilometry-based evaluation of dentin wear. This profilometry-based method is referred to as RDA-Profilometry Equivalent, or RDA-PE., Methods: A total of 36 dentifrices were sourced from four global dentifrice markets (Asia Pacific [including China], Europe, Latin America, and North America) and tested blindly using both the standard radiotracer (RDA) method and the new profilometry method (RDA-PE), taking care to follow specific details related to specimen preparation and treatment., Results: Commercial dentifrices tested exhibited a wide range of abrasivity, with virtually all falling well under the industry accepted upper limit of 250; that is, 2.5 times the level of abrasion measured using an ISO 11609 abrasivity reference calcium pyrophosphate as the reference control. RDA and RDA-PE comparisons were linear across the entire range of abrasivity (r2 = 0.7102) and both measures exhibited similar reproducibility with replicate assessments. RDA-PE assessments were not just linearly correlated, but were also proportional to conventional RDA measures., Conclusion: The linearity and proportionality of the results of the current study support that both methods (RDA or RDA-PE) provide similar results and justify a rationale for making the upper abrasivity limit of 250 apply to both RDA and RDA-PE.

Objective: This paper describes the development and standardization of a profilometry-based method for assessment of dentifrice abrasivity called Radioactive Dentin Abrasivity - Profilometry Equivalent (RDA-PE)., Methods: Human dentine substrates are mounted in acrylic blocks of precise standardized dimensions, permitting mounting and brushing in V8 brushing machines. Dentin blocks are masked to create an area of "contact brushing." Brushing is carried out in V8 brushing machines and dentifrices are tested as slurries. An abrasive standard is prepared by diluting the ISO 11609 abrasivity reference calcium pyrophosphate abrasive into carboxymethyl cellulose/glycerin, just as in the RDA method. Following brushing, masked areas are removed and profilometric analysis is carried out on treated specimens. Assessments of average abrasion depth (contact or optical profilometry) are made., Results: Inclusion of standard calcium pyrophosphate abrasive permits a direct RDA equivalent assessment of abrasion, which is characterized with profilometry as Depth test/Depth control x 100. Within the test, the maximum abrasivity standard of 250 can be created in situ simply by including a treatment group of standard abrasive with 2.5x number of brushing strokes. RDA-PE is enabled in large part by the availability of easy-to-use and well-standardized modern profilometers, but its use in V8 brushing machines is enabled by the unique specific conditions described herein., Conclusion: RDA-PE permits the evaluation of dentifrice abrasivity to dentin without the requirement of irradiated teeth and infrastructure for handling them. In direct comparisons, the RDA-PE method provides dentifrice abrasivity assessments comparable to the gold industry standard RDA technique.