Diana Serbezeanu

A straightforward, eco-friendly and cost-effective approach towards flame retardant epoxy resins

Modification of epoxy resins with organophosphorus compounds, either as reactive co-reactants or additives, is the key to achieving non-flammable advanced epoxy materials. Herein, through a straightforward and cost-effective approach, epoxy thermosets were prepared by simply mixing a new phosphorus flame retardant (13.5 wt% phosphorus) with a bifunctional bisphenol-A based epoxy polymer, followed by thermal curing in the presence of an aromatic aminic hardener. It was proved that a very low content of phosphorus led to composites exhibiting remarkable improved flame retardancy. 1 wt% phosphorus was enough to increase the limiting oxygen index value to about 30%. Furthermore, the peak of the heat release rate was reduced up to 45%, depending on the content of the flame retardant additive introduced into the epoxy matrix. UL-94 V-0 materials were obtained when 2 and 3 wt% phosphorus were added into the epoxy matrix. Thermogravimetric data showed that the incorporation of the flame retardant additive significantly increased the char yield and thermal stability of the gradually forming phosphorus-rich carbonaceous layer at elevated temperatures.

Preparation and characterization of thermally stable polyimide membranes by electrospinning for protective clothing applications

The aim of the present study was to obtain high-performance materials for heat and flame protective clothing. Therefore, hybrid membranes were prepared using Kevlar as support and aromatic polyimide nanofibers as a protective coating. The exceptional performances of the prepared membranes were highlighted by selected indicators: high thermal stability, fire resistance and improvement in air permeability without modifying drastically the water vapor transmission rate properties. Attenuated total reflectance Fourier transform infrared and 1H nuclear magnetic resonance spectroscopy were employed to confirm polyimide formation. The ability of polyimide to form fibers was investigated by rheological measurements and scanning electron microscopy, respectively. Thermal degradation was studied using thermogravimetric analysis and a microscale combustion calorimeter. The transport properties of the materials were examined by air permeability, water vapor transmission rate and water resistance. It was shown that transport properties of the modified Kevlar membranes could be controlled by varying the spinning time of polyimide solution. Moreover, by annealing the modified Kevlar weavings at 260℃, the structural integrity and transport properties were not affected, whereas a higher resistance to water was found.

Environmentally friendly fire-resistant epoxy resins based on a new oligophosphonate with high flame retardant efficiency

Advanced flame retardant epoxy resins, environmentally friendly, with different contents of a new oligophosphonate (PFR), were prepared using dicyandiamide as a hardener and 1,1-dimethyl-3-phenylurea as an accelerator. PFR, with a high phosphorus content, was synthesized by polycondensation reaction of phenylphosphonic dichloride with a phosphorus-containing bisphenol, namely bis((6-oxido-6H-dibenz[c,e][1,2]oxaphosphorinyl)-(4-hydroxyaniline)methylene)-1,4-phenylene. The bisphenol was prepared by reacting 9,10-dihydro-oxa-10-phosphaphenanthrene-10-oxide with an imine bisphenol resulting from the condensation of 4-aminophenol with terephthalaldehyde. The structure and morphology of cured epoxy resins were evaluated by Fourier transform infrared spectroscopy and scanning electron microscopy (SEM) analysis, respectively. Differential scanning calorimetry analysis revealed that cured epoxy resins containing PFR possessed slightly higher glass transition temperatures than phosphorus-free cured epoxy resin. Thermogravimetric analysis and limiting oxygen index values indicated that the incorporation of PFR into epoxy resin substantially enhanced the thermal stability and flame retardancy of the char layer at high temperature. The surface morphology of the char residue was studied by SEM measurements. The kinetic processing of thermogravimetric data was carried out using Friedman and Vyazovkin methods. The lifetime prediction analysis established that the fire-resistant phosphorus-containing epoxy resins could be used at a constant temperature of 200 °C up to 620–630 minutes. The new PFR can be successfully used as a very efficient flame retardant for improving the fire-resistance properties of epoxy resins.

Phosphorus-containing polyimide fibers and their thermal properties

Phosphorus-containing polyimide was synthesized by solution polycondensation reaction of bis(3-aminophenyl) methyl phosphine oxide with 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride). The chemical characterization of polyimide was done using FTIR and NMR spectroscopy. Also, the influence of different parameters on the rheological and thermal behavior of the polyimide was investigated. We were able to prepare uniform submicron- or nano-sized fibers by electrospinning highly viscous polyimide solutions. The diameters of the electrospun fibers increased from 58 nm to 347 nm as the concentration of the polyimide solution was raised from 10 to 24 wt%. Isoconversional kinetic analysis of the thermal decomposition of the polyimide was performed using Friedman and Ozawa–Flynn–Wall methods. The thermal decomposition mechanism of polyimide was evaluated and proposed using thermogravimetric analyzer-Fourier transform infrared spectroscopy (TGA-FTIR) and pyrolysis gas chromatography mass spectroscopy (Py-GC-MS). The results showed a complex degradation process of the main chain, which started with the scission of weaker bonds such as ether, alkyl, imide and some aromatic groups. The char residue of the polyimide fibers from TGA experiments were analyzed using scanning electron microscopy (SEM). It exhibited a dense structure, with a homogeneous dispersion of phosphorus atoms.

Physical and thermal properties of poly(ethylene terephthalate) fabric coated with electrospun polyimide fibers

Two analogous polyimides (PIs) containing flexible isopropylidene units were prepared. One was based on 4,4′-oxydiphthalic anhydride and 4,4′-(4,4′-isopropylidenediphenyl-1,1′-diyldioxy) dianiline and the other was based on 4,4′-(4,4′-isopropylidenediphenoxy) bis(phthalic anhydride) and bis(3-aminophenyl) methyl phosphine oxide. The ability of these two PIs to form uniform nanoscaled fibers was investigated by electrospinning technique. At optimal spinning conditions, PI fibers were electrospun onto the surface of woven poly(ethylene terephthalate) (PET) support to form a bilayer composite structure. These new fabric systems were analyzed regarding morphology, air permeability, wetting properties, and thermal stability. It was expected that the new PET/PI mats would possess enhanced properties compared with the initial woven PET fibers due to the excellent properties of PIs. Experimental results showed that PET woven substrate coated with electrospun PI fibers had improved values of air permeability, water contact angle and thermal stability when compared with the initial woven PET fibers.

Thermal Degradation and Kinetic Studies of New Flame-Retardant Phosphorus-Containing Polymers with Liquid Crystalline Properties

In this study, two new thermotropic liquid crystalline phosphorus-containing polymers were prepared by polycondensation reaction of 1,4-phenylene-bis((6-oxido-6H-dibenz[c,e][1,2]oxaphosphorinyl)carbinol) with two different phosphonic dichlorides, namely ethyl dichlorophosphate and phenyl dichlorophosphate. The polymers were characterized by means of FT-IR, 1H NMR, differential scanning calorimetry, polarized light microscopy, wide-angle X-ray diffraction, and thermogravimetric analysis. Thermogravimetric analysis was performed in nitrogen atmosphere by heating the samples over a range of temperatures from 25° to 700°C using five different heating rates. The kinetic processing of data was achieved using the Freeman-Carroll, ASTM E1641, and Kissinger methods.

Synthesis and Characterization of New Phosphorus-containing Poly(ester imide)s

New phosphorus-containing poly(ester imide)s were prepared via polycondensation in solution of 1,4-bis[N-(4-hydroxyphenyl)phthalimidyl-5-carboxylate]-2-(6-oxido-6H-dibenz<1,2>oxaphosphorin-6-yl)-naphthalene with different aromatic dicarboxylic acids using a SOCl2/pyridine condensing agent. The polymers showed high thermal stability having a decomposition temperature above 375 °C and char yield at 700 °C in the range of 54—56%. The glass transition temperature was in the range of 190—226 °C. Solutions of the polymers in N,N-dimethylformamide showed fluorescence having maximum emission wavelength in the range 405—425 nm. The polymers exhibited thermotropic liquid crystalline behavior.

Preparation and characterization of new hydrogels based on poly(vinyl alcohol)/phosphoester — chondroitin sulphate

New polymer hydrogels based on partially phosphorylated poly(vinyl alcohol) (PVA/Phosphoester) have been prepared. Fourier transform infrared spectroscopy (FT-IR) was employed to confirm PVA/Phosphoester formation. Contact angle measurements were performed to evaluate the surface characteristics of the hydrogels. The PVA/Phosphoester hydrogels were co-networked with Chondroitin sulfate (CS) in various ratios by chemical crosslinking. The synthetic-natural mixed resulted semi-IPN hydrogels were structurally and morphologically investigated by ATR — FT-IR spectroscopy and scanning electron microscopy, respectively. The swelling behavior and dynamic moisture sorption capacity of the PVA/Phosphoester (p-methyl-phenyl phosphonic dichloride) (P3)-CS semi-IPN hydrogels were followed. It was found that the performance of the semi-IPN hydrogels was influenced by the CS. By kinetic studies, it has been shown that the swelling processes occurred by an anomalous transport mechanism.Graphical abstract

Synthesis and characterization of novel main chain bisazobenzene polyphosphonates

Preparation and experimental studies on the phase transition and morphology of novel phosphorus-containing copolymers, having bisazobenzene units and variable molar ratio of aliphatic segments in the main chain, were performed. The chemical structure of the novel compounds has been confirmed by Fourier transform infrared and 1H-NMR spectroscopy. The thermal stability was investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) while the mesomorphic behavior was closely observed with polarized light microscope (PLM) and by X-ray diffraction measurements, structure-properties relationships being established and discussed.