Shot peening increases resistance to cyclic fatigue fracture of endodontic files

The objective of this study was to assess the resistance to fatigue fracture of conventional nickel–titanium files after undergoing shot peening. Forty NITIFLEX endodontic files, number 30, were divided into two groups; one was submitted to shot peening treatment and the other was not. All instruments were tested for fatigue fracture in simulated canals with a TRI-AUTO ZX endodontic motor. One file of each group was subjected to a residual stress analysis by XRD. Finally, the fractured surface was observed and elemental analysis performed by means of SEM and EDX. Roughness analysis was made by focal variation microscope. The shot peening group showed greater resistance to fatigue fracture; there was no difference in the length of the fractured fragments. XRD results showed the presence of residual compression stresses in the file submitted to shot peening, a decrease in the interplanar spacing, and an increase in the full-width-at-half-maximum and the microstrains. SEM and EDX showed a ductile fracture with zones of fatigue and an equiatomic ratio between the nickel and titanium. Surface roughness increased after the file was subjected to the shot peening procedure. In conclusion, shot peening increases the resistance to fatigue fracture due to the presence of residual compression stresses in files manufactured from a conventional nickel–titanium alloy.

Influence of structural physiognomies of pawpaw fiber–glass fiber hybrid–based green composites on mechanical properties and biodegradation potential of epoxy composites

The demand for durable and sustainable eco-friendly materials in recent times has caused many researchers to consider the use of plant fibers in composite development. In this research, the suitability of treated pawpaw fiber as a substitute for glass fiber was considered. The pawpaw fiber was extracted from the plant stem by dew retting and treated before been incorporated into the epoxy matrix. Two distinct fiber structures in linear and network forms were identified, separated, and used for the development of the composites. The composites were produced by incorporating a fixed amount of pawpaw fiber with a varied amount of glass fiber within 3–15 wt% in epoxy-based polymer matrix after which mechanical and biodegradation tests were carried out on the developed samples. Fractured surface morphology was also observed using a scanning electron microscope. The results revealed that the fiber structures influence the properties of the material. While mechanical properties were mostly enhanced by treated linear structure pawpaw fiber, biodegradation was highly promoted by treated network structure pawpaw fiber. Tensile (except for strain), hardness, and flexural properties were enhanced by the linear-structured treated pawpaw fiber, while biodegradability, impact, and tensile strain were improved by the network-structured treated pawpaw fiber compared to the control sample.

INFLUENCE OF KRAFT LIGNIN ON THE PROPERTIES OF RUBBER COMPOSITES

The influence of lignin content on reclaimed rubber (RR)/natural rubber (NR) blend composite properties has successfully been studied. Scanning electron microscopy (SEM) were used to understand morphology. Fourier-transform infrared spectroscopy (FTIR) for the possible chemical interaction, whereas thermogravimetric analysis (TGA) and tensile tester were used to predict strength and elongation for possible practical applications. The results indicated that the presence of lignin forms cavities which seemed to arise from complex interactions of the blend with the lignin. Those cavities dominated tensile fractured surface and the increase in lignin indicated inconsistencies of interfacial interactions. Lignin RR/NR blend composites revealed a drop in tensile strength and shift in glass transition temperature, except for the highest lignin containing blend composite. More active interactive constituent of the blend appeared to be NR. The interaction has not favored the thermal stability and crosslinking density.

Thermal properties of polybenzoxazine exhibiting improved toughness: Blending with cyclodextrin and its derivatives

Polybenzoxazines are emerging as a class of high-performance thermoset polymers that can find their applications in various fields. However, its practical application is limited by its low toughness. The cyclic β-cyclodextrin and a newly synthesized derivative (β-cyclodextrin-MAH) were separately blended with benzoxazine to improve the toughness of polybenzoxazine. The results revealed that the maximum impact strength of the blend was 12.24 kJ·m−2 and 14.29 kJ·m−2 when 1 wt.% of β-Cyclodextrin and β-Cyclodextrin-MAH, respectively, were used. The strengths were 53% and 86% higher than that of pure polybenzoxazine. The curing reaction, possible chemical structures, and fractured surface were examined using differential scanning calorimetry, Fourier transform infrared spectroscopy, and scanning electron microscopy techniques to understand the mechanism of generation of toughness. The results revealed that the sea-island structure and the presence of hydrogen bonds between polybenzoxazine and β-cyclodextrin and β-cyclodextrin-MAH resulted in the generation of toughness. Furthermore, the curves generated during thermogravimetric analysis did not significantly change, revealing the good thermal properties of the system. The phase-separated structure and the hydrogen bonds present in the system can be exploited to prepare synergistically tough polybenzoxazine exhibiting excellent thermal properties. This can be a potential way of modifying the thermoset resins.

Green Secondary Plasticizer/Thermal Stabilizer in PVC Processing

Replacing 10 php DOP plasticizer by ESO has slightly increased tensile properties of PVC. A variety of media were used to study the leaching property of PVC and the results showed that the presence of ESO as a secondary plasticizer improved the leaching characteristic. There was slight difference in the hardness of the PVC samples with and without the ESO. In general, the PVC material became softer when DOP was partly replaced by ESO. The hardness of both samples became smaller after soaking in water, 30 wt.% acetic and 10 wt.% KOH solution and much bigger in other media. After immersion in n-hexane, the 60/0 sample blistered and became very hard, while the hardness of 50/10 sample increased only 12%. The morphology of fractured surface of the samples after soaking in n-hexane has clearly demonstrated this result. The replacing 10 php DOP by ESO has been also improved remarkably migration characteristic, thermal properties, and movement and volatilization in hot air. That means, ESO could be used as a secondary plasticizer/thermal stabilizer in PVC formulation.

Resistance to fracture due to cyclic fatigue of stainless steel manual files and its association to surface roughness

The aim of this study was to evaluate the possible association between the roughness of 5 brands of stainless steel endodontic files and their resistance to fracture due to cyclic fatigue. The study included five different brands of stainless steel endodontic files: SybroEndo Triple-Flex Files (Kerr, Glendora, USA), Ready Steel K-Flexofile (Dentsply Sirona, Ballaigues, Switzerland), Mani Flexile Files (Mani, Tochigi- Ken, Japan), FKG K-Files (FKG, La Chaux-de-Fonds, Switzerland) and Zipperer Flexicut Files (VDW, Munich, Germany). Twelve files per brand (total 60 files) were evaluated. File surface roughness over an area (Sa) was quantified using a focus variation microscope. Then the files were subject to a cyclical fatigue test to determine the number cycles to fracture due to fatigue and length of fractured fragment. Finally, fractographic analysis was performed using a scanning electron microscope. The electropolished Ready Steel K-Flexofile® files had the highest roughness according to Sa parameters, though they also had the highest resistance to fracture due to cyclic fatigue and the longest fractured fragment. Moderate positive correlation was found between fractured fragment length and roughness. The fractured surface showed characteristics of ductile fracture with cracks and plastic deformation. The electropolished stainless steel Ready Steel K-Flexofile® files were the most resistant to fracture due to cyclic fatigue even though they had highest surface roughness.

Investigation on the flexural properties of nanofillers loading on the Jute/Carbon/PLA nanocomposites

Presence of fibers and fillers in a composite can be an efficient way to arrest crack either at macro or micro levels. In this work, woven jute and carbon fibers were arranged alternately in PLA (Polylactic Acid) nanocomposite. Graphene or nanoclay was embedded into PLA matrix to make polymer nanocomposite. Fiber reinforced polymer nanocomposites were prepared by varying the concentration of graphene or nanoclay in the PLA matrix and alternate woven jute/carbon fibers was then bind with the PLA nanocomposite. Influence of graphene or nanoclay concentration and presence of woven fibres in the composite was quantified by flexural analysis. Flexural strength and flexural modulus were found to increase at 3wt% of nanofiller concentration for both graphene/jute/PLA and nanoclay/jute/PLA nanocomposites with increment up to 37% and 31%, respectively. FTIR was used to determine the interaction between PLA and nanofillers. Morphology observation by Scanning Electron Microscopy (SEM) was done to investigate the fractured surface of the hybrid jute/carbon fibres reinforced PLA nanocomposite.

Multi material 3D printing of PLA-PA6/TiO2 polymeric matrix: Flexural, wear and morphological properties

The poly-lactic acid (PLA), bio compatible polyamide (PA6) and TiO2 has established bio-medical applications especially in 3D printing of scaffolds. But hitherto little has been reported on improving the performance of multi-material matrix for PLA-PA6/TiO2 especially in 3D printing application of biomedical scaffolds. The anti-bacterial properties of PA6/TiO2 make it worthy to be explored with PLA matrix in multi layered fashion on the platform of fused deposition modeling (FDM) being low cost 3D printing technology for in house development of scaffolds. In this work an effort has been made for in-house development of feedstock filaments of PLA and PA6/TiO2 based polymeric composite matrix on twin screw extrusion (TSE) machine. Further the feedstock filament wires were used on FDM to establish the flexural, wear and morphological properties of multi-material 3D printed functional prototype. The results of the study suggest that for flexural strength, infill speed: 90mm/s; infill pattern: triangular and layer combination as 5 consecutive layers of PLA and 5 consecutive layers of PA6/TiO2 are the optimized conditions for FDM printing. The wear testing results suggest that the composite of PA6/TiO2 held low wear rate (823 µm) in comparison to PLA (wear rate: 1092 µm). Further porosity testing (based upon optical photomicrographs) at ×100 and fractured surface analysis at ×30 supported the observed trends for flexural and wear testing. The photomicrographs of fractured surface were 3D rendered to predict the role of surface roughness (Ra) profile for flexural properties. The mechanical and morphological observations are also supported with scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis.