Microstructure and Mechanical Properties of Poly (Vinyl Chloride) Modified by Silica Fume /Acrylic Core-Shell Impact Modifier Blends

This research explored replacing acrylic core-shell impact modifier （AIM） by silica fume to toughen PVC. 100%, 75%, 50% and 25% of AIM （8 phr） were substituted by silica fume in PVC respectively, and then processed by dry blending and twin-screw extrusion. Severe silica fume agglomeration was observed by scanning electron microscope （SEM） in the PVC matrix when 8 phr pure silica fume was used and processed by screw speed of 20 rpm. Its tensile strength was thereby reduced by 38% comparing to unmodified PVC. The silica fume was successfully dispersed while the screw speed was slowed down to 10 rpm to give a stronger screw torque and a longer melt residential time in the extruder. The tensile strength was ’recovered’ to a level comparable to unmodified PVC. Impact test were performed on all formulations extruded at 10 rpm screw speed and synergetic toughening effect was found with 50% substitution and it had the impact strength that was comparable to 8 phr pure AIM toughened PVC.

Mechanical Properties and Microstructure of Blended Cement Containing Modified Quartz Tailing

The amount of inert quartz tailing used in concrete construction is limited due to the low strength development of cementitious materials that may be caused by the quartz tailing. We manage to increase the strength of blended cement by modifying quartz tailing through solid-phase reaction of quartz tailing with carbide slag at high temperature. The mineral composition and morphology of the modified quartz tailing were examined by X-ray diffraction（XRD） and scanning electron microscopy（SEM）. The mechanical properties and microstructure of blended cement mortars containing modified quartz tailing were investigated. Results showed that the strengths of blended cement mortars containing modified quartz tailing were close to those of the corresponding blended cement mortars containing quartz tailing at early age of 3 d, but increased significantly to be similar to that of plain Portland cement mortars at late ages of 90 d. This is attributed to the microstructure densification and the enhancement of interface between quartz tailing and cement paste due to the hydration of b-C_2 S surface layer on modified quartz tailing.

Phase Specific Comparisons of High and Low Vertical Jump Performance in Collegiate Female Athletes

Introduction Countermovement vertical jump testing has become a staple in athlete assessment protocols.As the popularity of jump testing has grown,a need and interest has also grown in identifying the factors that underpin high-level outputs.As jump height alone as a variable in evaluating vertical jump performance has been questioned in athletic populations,other variables such as the reactive strength index modified(RSIm)allow for not only evaluating the outcome,but the strategy used in obtaining that outcome.Purpose Thus,the purpose of this investigation was to examine the differences in high and low vertical jump performances,as determined by the RSIm in female collegiate athletes.Methods Thirty NCAA Division I female volleyball and basketball athletes performed countermovement vertical jump tri­als on a force platform.The sample was then broken into two groups as determined by median RSIm values.Independent sample t-test were then used to compare groups.Results High RSIm group displayed greater jump heights(P<0.05).Additionally,the high performing group displayed lower eccentric duration times(P<0.05).No differences between groups were seen in kinetic variables.Conclusion The high performing group displayed faster eccentric times which translated to lower values in time to take-off though not statistical significant.The higher RSIm values appear to be a result of both greater jump heights and reduced time to take off.Thus,focus being placed on the speed of the movement during training would be of benefit in improving RSIm values.