Gabriela Lisa

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.

Thermal behaviour of polystyrene, polysulfone and their substituted derivatives

Abstract The thermal stabilities of polystyrene (PS), polysulfone (PSF), poly(p-chloromethyl) styrene (CMPS), chloromethylated polysulfone (CMPSF), and their corresponding derivatives substituted with 2-(N,N-dimethylamino)ethanol (HAPS, HAPSF) or triethylamine (APS, APSF) were investigated. The influence of the chemical structure on the thermal stability was assessed by thermogravimetric analysis in the dynamic conditions of temperature and atmospheric air. The substituted compounds undergo a thermal degradation involving a complex mechanism with three steps. These steps are studied from both kinetic parameters and thermogravimetric characteristics points of view. The chloromethylation reaction and subsequent substitution reaction of chlorine atom with alkyl and hydroxyalkylamines led to a significant decrease of the thermal stability of both PS and PSF polymers. The following series was established: PS>CMPS>HAPS>APS (for PS series), and PSF>CMPSF>HAPSF>APSF (for PSF series, respectively).

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.

Polyimide‐Polydimethylsiloxane Copolymers. Evaluation of the Thermal and Electrical Properties

Poly(dimethylsiloxane‐amic‐acid)s have been prepared starting from a fluorinated dianhydride, namely 4,4′‐(hexafluoroisopropylidene)diphthalic anhydride, and a mixture of an aromatic diamine and bis(aminopropyl)‐oligodimethylsiloxane of controlled molecular weight, in different ratios. A solution imidization procedure was used to convert them quantitatively to the corresponding polyimides. The polymers, easily soluble both in polar (N‐methylpyrrolidone) and less polar (chloroform) solvents, were prepared for film‐forming by casting from solution. The thermal behavior in dependence on composition was evaluated by thermogravimetric analysis and differential scanning calorimetry. The kinetic processing of thermogravimetric data was carried out using the Flynn‐Wall‐Ozawa and Kissinger methods. Electrical insulating properties of some polymer films were evaluated on the basis of dielectric constant and dielectric loss tangent and their variation with frequency.

New highly thermostable aromatic polyamides with pendant phthalonitrile groups

AbstractA phthalonitrile-containing diamine monomer, 4,4′-diamino-4″-(3,4-dicyanophenoxy)-triphenylmethane, 1, was prepared, and its cure behavior was investigated. The diamine 1 was thermally polymerized at 250 °C even in absence of curing additives. The resulting resins exhibited exceptional high thermal and thermo-oxidative stability, with the 5% weight loss being above 500 °C and the char yield at 700 °C being over 62%, in air or inert atmosphere. New soluble aromatic polyamides derived from diamine 1, 2,2-bis[4-(4-aminophenoxy)phenyl]propane and terephthaloyl chloride were synthesized. They showed good solubility in organic solvents, film forming properties, and high thermal stability, with the decomposition temperature being above 400 °C. The kinetic processing of thermogravimetric data was carried out using differential and integral methods. Electrical properties of the polymer films were evaluated on the basis of dielectric constant and dielectric loss and their variation with frequency and temperature. The influence of the phthalonitrile content on the polymer properties was studied.

Polyimides containing cycloaliphatic segments for low dielectric material

New polyimides have been synthesized by the reaction of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) or/and 5-[2,5-(dioxotetrahydrofuryl)]-3-methyl-3-cyclohexene-1,2-dicarboxylic anhydride (DOCDA) with 4,4′-methylene dianiline (DDM). Polycondensation reactions were carried out in N, N-dimethylformamide solution and the resulting polyimides gave flexible free-standing films. The introduction of alicyclic moieties in the polyimide backbone improved the solubility of the resulting polymers and facilitated their processing while maintaining a high thermal stability. The dielectric constant values (measured at 106 Hz at room temperature), were found in the range of 3.14–2.83 and decreased with increasing of the alicyclic units content.

Thermal Degradation Kinetics of some Aromatic Poly(1,3,4-oxadiazole-ether)s

Poly(1,3,4-oxadiazole-ether)s containing isopropylidene, hexafluoroisopropylidene, cyclohexylidene, fluorene, phthalide or phenylquinoxaline groups have been prepared by nucleophilic substitution polymerization technique of 2,5-bis( p-fluorophenyl)-1,3,4-oxadiazole with various aromatic bisphenols. The thermal decomposition behavior of the polymers was studied using thermogravimetric analysis in air, at several heating rates: 6, 10 and 14° C min . The polymers showed high thermo-oxidative stability with decomposition temperature corresponding to 5% weight loss above 380°C. The kinetic processing of data was carried out using the Coats—Redfern, Reich—Levi, Flyn—Wall and Kissinger methods.

Hockey Stick Liquid Crystals Based on a 2,5-Asymmetric Disubstituted [1,3,4]Oxadiazole Core

The article describes the liquid-crystalline properties of some 2,5-asymmetric disubstituted [1,3,4]oxadiazole derivatives containing an azo and an ester linkage obtained through esterification of 2-(4-methoxyphenyl)-5-(4-hydroxyphenyl)-[1,3,4]oxadiazole with a series of 4-(4-alkoxyphenyl)-benzoic acids containing 6–10 and 18 aliphatic carbon atoms. All reported compounds present liquid-crystalline properties, evidenced by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM), with nematic and smectic C type structures, with very large range of stability of mesophases (between 130°C and 198°C on heating and 134°C and 214°C on cooling).

Evaluation of the thermal stability and set recovery of thermo-hydro-mechanically treated lime ( Tilia cordata ) wood

The effect of the thermo-hydro-mechanical (THM) densification in a closed system on the set recovery and thermal stability of lime (Tilia cordata) wood was evaluated. The THM densification process consists of four steps: plastification, densification, post-treatment and cooling. The THM-densified wood samples were subjected to three different post-treatment temperatures, and for each one, three different periods were used. Comparison between THM-densified wood sample without and with post-treatment at different temperatures and times was made. The set recovery tests showed that the post-treatment at higher temperatures and times improves the dimensional stability of the samples suggesting a permanent fixation of the transversal compression, but also reduces their thermal stability, by having lower temperatures corresponding to the maximum decomposition rate, lower values for the mass loss at the end of decomposition stage and lower values for the kinetic parameters. The samples with post-treatments at lower temperatures showed high set recovery values and exhibited higher values for the temperatures corresponding to the maximum decomposition rate, lower values for the mass loss at the end of the stage and higher activation energies and reaction orders.

Synthesis and characterization of N-(2-aminoethyl)-2-acetamidyl gellan gum with potential biomedical applications

N-(2-aminoethyl)-2-acetamidyl gellan gum (GCM-EDA) was prepared by carboxymethylation (via nucleophilic substitution of primary hydroxyl groups of the β-D-glucose unit of gellan gum, in the presence of alkali and chloroacetic acid) and reaction with tert-butyl N-(2-aminoethyl) carbamate (N-Boc-EDA) using 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDAC) as an activator, followed by deprotection with trifluoroacetic acid. The structural confirmation and characterization of N-(2-aminoethyl)-2-acetamidyl gellan gum was performed by spectroscopic, rheological and thermogravimetric analysis, and in vitro tests showed a lack of cytotoxicity which is indicative of the potential of this material to be used in biomedical applications.