Synthesis of a novel DOPO‐based polyphosphoramide with high char yield and its application in flame‐retardant epoxy resins

A novel and highly effective flame retardant (FR), DOPO‐TPMP oligomer, was synthesized by a simple condensation of 4‐(hydroxymethyl)‐2,6,7‐trioxa‐1‐phosphabicyclo[2.2.2]octane‐1‐oxide and phosphorus oxychloride followed by a polycondensation reaction with 6‐(2,5‐dihydroxyphenyl)‐6H‐dibenzo[c,e][1,2]oxaphosphinine‐6‐oxide. The chemical structure of DOPO‐TPMP was well characterized using Fourier transform infrared and NMR spectra. DOPO‐TPMP was used as an additive‐type FR for epoxy resin (EP). The FR properties of the resultant EP composites were investigated by limiting oxygen index (LOI) test, UL‐94 vertical burning test and cone calorimeter measurements. Specifically, the EP composite containing 10.0% DOPO‐TPMP achieved a LOI value of 36.1%, V‐0 rating in the UL‐94 test and a 58% reduction in peak heat release rate. Further mechanism analysis attributed the enhanced flame retardancy to the increased char yield on the addition of DOPO‐TPMP. © 2019 Society of Chemical Industry     

Investigation on the spring-in phenomenon of carbon nanofiber-glass fiber/polyester composites manufactured with vacuum assisted resin transfer molding

Process-induced residual stress arises in polymer composites as a result of mismatched resin contraction and fiber contraction during the cure stage. When a curved shell-like composite part is de-molded, the residual stress causes the spring-in phenomenon, in which the enclosed angle of the part becomes smaller than the angle of its mold. In this paper, a new approach is presented to control and reduce the spring-in angle by infusing a small amount of carbon nanofibers (CNFs) together with liquid resin into the glass fiber preform using vacuum assisted resin transfer molding (VARTM) process. The experimental results showed that the spring-in angles of the L-shaped composite specimens were effectively restrained by the CNFs. An analytical model and a 3-D FEA model were developed to predict the spring-in phenomenon and to understand the role of CNFs in reducing the spring-in angle. The models agreed with the experimental results reasonably well. Furthermore, the analytical model explains how the CNF-enhanced dimensional tolerance control is accomplished through the reductions in the matrix’s equivalent coefficient of thermal expansion and linear crosslinking shrinkage.     

Effect of Initial Microstructure on High-Temperature Dynamic Deformation of Ti-6Al-4V Alloy

One of the attractive properties of Ti-6Al-4V alloy is control of microstructure through heat treatment to vary the mechanical properties. In this study, three different microstructures, Lamellar, Widmanstätten, and Martensitic morphologies, were created through heat treatment at a post-β transus temperature followed by cooling at different rates. With faster cooling rates, the microstructures evolved finer lamellae, smaller colony sizes, and thinner grain boundary layers. High-temperature dynamic compression was conducted on these specimens at a strain rate of 1000 s⁻¹ and temperatures in the range of 23 °C to 1045 °C. Flow stresses decreased linearly with colony size and grain boundary layer thickness, but increased with inverse square root of lamellar thickness. This strong correlation of flow stress to several microstructural feature sizes indicated multiple modes of deformation. All three microstructures showed identical thermal softening. The softening rate was intensified at elevated temperatures due to hcp → bcc allotropic phase transformation. Gangireddy modification to Johnson–Cook model could account for this augmented softening and the modified J–C model predicted the three microstructures to follow a similar thermal softening coefficient m = 0.8. The kinetics of phase transformation appear to be very rapid irrespective of the microstructural differences in the Ti-6Al-4V alloy.

     

In situ microscopy observation of liquid flow,zirconia growth,and CO bubble formation during high temperature oxidation of zirconium diboride–silicon carbide

The oxidation of ZrB₂–SiC composites at 1450–1650 °C was directly observed with in situ optical microscopy. Video frames showed the flow of silicate liquids, the formation of zirconia deposits, and the growth and collapse of gaseous bubbles on the oxide surface. Contrast in the incandescence of in situ images is analyzed as spatial variations in hue and intensity and related to differences in emissivity of the oxide scale surface features by comparing these hot images with room temperature images. Above 1450 °C, gaseous bubbles were observed to grow and collapse causing perturbations in the liquid oxide on the surface. The bubbles are associated with the evolution of CO from SiC oxidation and the onset is related to the critical temperature where the partial pressure of CO under the oxide scale exceeds atmospheric pressure.     

Non-contact mechanical property measurements at ultrahigh temperatures

The “Electro-Magnetic Mechanical Apparatus”, a novel non-contact method of mechanical testing of ultrahigh temperature ceramics at high temperatures, was developed where a mechanical stress is applied using Lorenz forces on a sample heated to high temperatures with an electric current. The design of the apparatus and an analysis relating stress to magnetic flux density, electrical current, and specimen dimensions are presented here. Significant creep deformations were observed in ZrB₂–SiC samples deformed under 20 MPa of flexural stress resulting in 0.08% strain after 240 s at 1600 °C and 0.21% strain after 150 s at 1750 °C. A fatigue load of 6 MPa at 60 Hz frequency at 1700 °C in air increased the oxidation rate. This mechanical apparatus has potential application not only for high temperature creep and fatigue experiments but also fracture and elasticity. Though developed for ceramics, this technique can be used to study high temperature mechanical properties of any conducting material.     

Flexural creep of zirconium diboride–silicon carbide up to 2200 °C in minutes with non-contact electromagnetic testing

Flexural creep of ZrB₂–30 vol% SiC ultra high temperature ceramic composite was studied at 1700–2200 °C and 20–50 MPa using the novel method of electromagnetic Lorentz force loading of electrically heated specimens. Experiments were conducted in air and in non-oxidizing atmospheres. The apparent activation energy for creep was 344 ± 35 kJ/mol for non-oxidizing conditions. The stress exponent was 1.4 ± 0.4. The creep rate was slightly higher in air due to a decrease in the size of the load bearing substrate because of oxidation. There was no evidence of electric field effects. Creep experiments could be performed up to 2200 °C very quickly, with experiments conducted in a few minutes.     

Iron‐Catalyzed Direct Synthesis of Amides from Methylarenes

An efficient, green and first catalytic process has been developed for the direct synthesis of amides from readily available petroleum by‐products (methylarenes) and amines using an iron catalyst. In this new catalytic reaction, the methyl group of the methylarene is oxidized to the corresponding aldehyde through non‐directed C H oxidation followed by its oxidative amidation with N‐chloroamine, yielding the carboxylic amide. Oxidation with an iron catalyst, tert‐butyl hydroperoxide (TBHP) as sole oxidant, the synthesis of amides under mild reaction conditions and the utilization of methylarenes as starting material make this methodology novel and environmentally benign.

     

Epoxy-based multilayers for flame resistant flexible polyurethane foam (FPUF)

Flexible polyurethane foam (FPUF), a widely used cushioning and comfort component for homeware and automobile industries if tailored for flame resistance, can exhibit enhanced market value. With the view to preparing such protective films, multilayered coating composed of montmorillonite (MMT) and triphenyl phosphate (tPP) along with polyethyleneimine (PEI), poly[(o-cresyl glycidyl ether)-co-formaldehyde] (CNER), tPP solution and MMT suspension was deposited via layer-by-layer technique so as to suppress the flammability of FPUF. A comparative study applying “cone test” revealed that multilayered coatings having PEI(CNER/NH₂-MMT/PEI)_n and PEI(CNER/NH₂₋MMT/tPP)_n architectures, where “n” signifies the number of layer pairs, exhibited remarkable decrease in “peak heat release rate” (pHRR) of 20 and 25%, respectively, as compared to uncoated FPUF. Moreover, these coatings showed a 50% reduction in “fire growth rate index” (FIGRA) when compared to uncoated FPUF. This significant decrease in FIGRA after a specific combination of epoxy-clay features that such multilayered coatings can be used to protect the underlying FPUF due to the labyrinth effect offered by MMT and accelerated char formation by tPP. Furthermore, the evolution of volatile gases upon combustion of multilayered coated foam decreased due to the inhibition of oxygen permeation owing to the protective effect of the coating. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48890.     

Hemophagocytic lymphohistiocytosis: An unusual presentation of tuberculosis in hemodialysis patients

We report a series of three patients with end‐stage renal disease on maintenance hemodialysis presenting with hemophagocytic lymphohistiocytosis (HLH) as an unusual manifestation of extrapulmonary tuberculosis. All three patients were middle‐aged men. They presented with fever, pancytopenia, varying degrees of hepatosplenomegaly, abnormal liver function tests, coagulopathy, increased serum ferritin, and triglycerides. Tests for fever work‐up were negative. Bone marrow examination revealed hemophagocytosis and caseating granuloma. Acid fast bacilli were demonstrated in two patients. The HLH‐2004 diagnostic criteria suggested by the histiocytic society were followed to arrive at the diagnosis. All of them succumbed to death even before the definitive diagnosis could be made. We suggest that aggressive diagnostic work‐up must be done when hemodialysis patients present with fever and pancytopenia. Priority should be toward early diagnosis and appropriate treatment to improve the prognosis.