An experimental study was carried out to investigate the phenomenon of interfacial instability in multilayer flat‐film coextrusion. For the study, a sheet‐forming die with a feed block was used to coextrude three‐and five‐layer flat films. Polymers coextruded were: (a) low‐density polyethylene with polystyrene, and (b) high‐density polyethylene with polystyrene. It was observed that, for a given polymer system, there is a critical value of wall shear stress at which an irregular (i.e., unstable) interface between the layers sets in, giving rise to a pattern similar to that usually found in a wood panel. Once the instability sets in, the severity of interfacial instability is found to depend on both the total volumetric flow rate (hence wall shear stress) of the combined streams and the ratio of the individual layer thicknesses. An attempt is made to correlate the critical conditions for the onset of interfacial instability in terms of the layer thickness ratio, and the viscosity and elasticity ratios of the two polymers being coextruded.

Multilayer flat film coextrusion was studied, both experimentally and theoretically. For the experimental study, a sheet-forming die with a feedblock MGIS designed, and plastic films of three and five layers were coextruded. The die was provided with three pressure transducers in thc axial direction in order to determine the pressure gradient in the die, allowing the dc+ermination of the reduction in pressure drop when different combinations of two polymer melts were coextruded. Polymers used for coextrusion were: (1) low density polyethylene and ethylene-vinyl acetate; (2) low density polyethylene and high density polyethylene; (3) low density polyethylene and polystyrene. For the theoretical study, the z-component of the equations of motion for steady fullydcucloped flow were solved using a power law non-Newtonian model. (hnpirisons were made ljctween the experimental ;ind the thcorctically predicted volumetric flow rates. Predictions ofthe velocity distriljutions, shear rate profiles, arid shear stress distrihutions n w c x made :is fiinctions of the processing conditions and the rheological properties of the individual polymers concerned

Background:Dentine hypersensitivity is a transient condition that often resolves with the natural sclerotic obturation of dentinal tubules. A potent topically applied in-office desensitizing treatment is indicated as the choice of treatment when dentine hypersensitivity is localized to one or two teeth.Aim:The present study aimed to evaluate and compare the clinical efficiency of CPP-ACP F, sodium fluoride, propolis, and distilled water that was used as placebo in treating dentinal hypersensitivity.Materials and Methods:120 patients aged 20–40 years reporting with dentinal hypersensitivity in relation to canine, premolar and molars with erosion, abrasion, and gingival recession were randomly assigned to four groups of 30 patients each. Response to air jet and tactile stimuli were measured using visual analogue scale initially on 1st, 7th, 15th, 28th, 60th, and final assessment was done on the 90th day.Statistical Analysis:A statistical analysis was done using Anova test (Fischer's test) and Tukey HSD test for multicomparison.Results:The teeth treated with the test group showed decrease in the mean hypersensitivity values compared to control group, over a period of three months. The results showed propolis to be most efficient in treating dentinal hypersensitivity and CPP- ACPF showed to be the least efficient.Conclusion:All test groups were effective in reducing dentinal hypersensitivity, although they differed in rapidity of action over the period of 3 months. Further studies can be done using advanced materials and techniques. Multiple therapeutic modalities have been developed to treat dentinal hypersensitivity including products that impede nerve conduction of pain stimulus, products that mechanically occlude dentinal tubules, and calcium containing products designed to create plugs in the tubules utilizing a demineralization mechanism.

An experimental study was carried out to investigate the effect of processing variables on unstable flow behavior in tubular film blowing. For the study, a laboratory blown film apparatus was constructed, and low-and highdensity polyethylenes were used in producing tubular films. After the process had been stabilized, disturbances in either the take-up speed or air pressure inside the bubble were introduced, and motion pictures of the disturbed bubble were taken. In order to investigate the effect of the rheological properties of the polymer melt on flow instability, melt temperature was varied. The present study indicates that a disturbance in the take-up speed influences bubble stability much more than a disturbance in air pressure inside the bubble does, and that a decrease in melt temperature tends to stabilize the bubble after it has been disturbed. A rheological interpretation of the experimental results is presented to explain flow instability in tubular film blowing.

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