Surface molecular characterisation of different epoxy resin composites subjected to UV accelerated degradation using XPS and ToF-SIMS

Epoxy resin composites reinforced with E-glass (E), 3D glass (3D) and carbon fibre (CF) were subjected to an intense UV and high temperature accelerated degradation environment. X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) were used to provide a molecular characterisation of the surface of the degraded composites. The response at the surface of the epoxy resin composites to oxidative degradation is influenced by the composite reinforcement type and characteristics. XPS results indicate that 3D resin composites exhibit more surface oxidation as a result of the accelerated degradation in comparison with E and CF composites. Principal components analysis (PCA) of the ToF-SIMS positive ion spectra showed that E and 3D resin composites suffered chain scission while CF composites suffered chain scission and cross-linking reactions as a result of the intense UV exposure. The extent of the surface oxidation, cross-linking/condensation reaction and loss of low molecular weight (lower than C₄H_x) aliphatic hydrocarbons may be indicated using PCA of both the ToF-SIMS positive and negative ion spectra. PCA also provides insight for proposing epoxy resin chain scission and oxidation reaction mechanisms.     

Application of isoconversional and multivariate non-linear regression methods for evaluation of the degradation mechanism and kinetic parameters of an epoxy resin

The thermo-oxidative degradation of an epoxy resin obtained by curing of an industrially produced DGEBA mixture with 4,4′-methylene-dianiline (MDA) and used as electric insulator has been investigated by TG + DTG + DSC simultaneous analyses performed in static air atmosphere, at five heating rates. TG, DTG and DSC curves showed that, in the temperature range 25-900 °C, a glass transition followed by three thermo-oxidative processes occur. The processing of the non-isothermal data corresponding to the first process of thermo-oxidation was performed by using Netzsch Thermokinetics - A Software Module for Kinetic Analysis. The dependence of the activation energy, evaluated by isoconversional methods, on the conversion degree and the relative high standard deviations of this quantity show that the investigated process is a complex one. The mechanism and the corresponding kinetic parameters were determined by multivariate non-linear regression program and checked for quasi-isothermal data. It was pointed out that the first process of thermo-oxidation of the investigated resin consists in four steps, each step having a specific kinetic triplet. The obtained results were used for prediction of the thermal lifetime of the material corresponding to some temperatures of use and the end point criterion 5% and 10% mass loss.     

Synthesis, characterization, thermal properties and flame retardancy of a novel nonflammable phosphazene-based epoxy resin

Hexakis(4-hydroxyphenoxy)-cyclotriphosphazene (PN-OH) was synthesized through nucleophilic substitution of the chloride atoms of hexachlorocyclotriphosphazene and reduction of the aldehyde groups, and its chemical structure was characterized by elemental analysis, ¹H and ³¹P NMR, and Fourier transform infrared (FTIR) spectroscopy. A new phosphazene-based epoxy resin (PN-EP) was successfully synthesized through the reaction between diglycidyl ether of bisphenol-A (DGEBA) and PN-OH, and its chemical structure was confirmed by FTIR and gel permeation chromatography. Four PN-EP thermosets were obtained by curing with 4,4′-diaminodiphenylmethane (DDM), dicyandiamide (DICY), novolak and pyromellitic dianhydride (PMDA). The reactivity of PN-EP with the four curing agents presents an increase in the order of DDM, PMDA, novolak and DICY. An investigation on their thermal properties shows that the PN-EP thermosets achieve higher glass-transition and decomposition temperatures in comparison with the corresponding DGEBA ones while their char yields increase significantly. The PN-EP thermosets also exhibit excellent flame retardancy. The thermosets with novolak, DICY and PMDA achieve the LOI values above 30 and flammability rating of UL94 V-0, whereas the one with DDM reaches the V-1 rating. The nonflammable halogen-free epoxy resin synthesized in this study has potential applications in electric and electronic fields in consideration of the environment and human health.     

Influence of oxidation state of phosphorus on the thermal and flammability of polyurea and epoxy resin

The focus of this study is an investigation of the effect of oxidation state of phosphorus in phosphorus-based flame retardants on the thermal and flame retardant properties of polyurea and epoxy resin. Three different oxidation states of phosphorus (phosphite, phosphate and phosphine oxide) additives, with different thermal stabilities at a constant phosphorus content (1.5 wt.%) have been utilized. Thermal and flame retardant properties were studied by TGA and cone calorimetry, respectively. The thermal stability of both polymers decreases upon the incorporation of phosphorus flame retardants irrespective of oxidation state and a greater amount of residue was observed in the case of phosphite. Phosphate was found to be better flame retardant in polyurea, whereas phosphite is suitable for epoxy resin. Phosphite will react with epoxy resin by trans-esterification, which is demonstrated by FTIR and ³¹P NMR. Further, TG–FTIR and XPS studies also provide information on flame retardancy of both polymers with phosphorus flame retardants.     

Studies on the effect of different levels of toughener and flame retardants on thermal stability of epoxy resin

A thermoplastic toughener, polyether sulphone (PES) and a number of different types of flame retardants were blended in different ratios with a commercial epoxy resin triglycidyl-p-aminophenol (TGAP) and 4,4-diamino diphenyl sulphone (DDS) a curing agent. The effect of type and levels of flame retardants (FR) and the toughening agent on the curing, thermal decomposition and char oxidation behaviour of the epoxy resin was studied by the simultaneous differential thermal analysis and thermogravimetric techniques. It was observed that the toughener slightly increases the curing temperature (by up to 20 °C) but had minimal effect on the decomposition temperature of the resin. Flame retardants, however affected all stages depending upon the type of flame retardant used. The curing peak for samples containing tougher and flame retardants although slightly changed depending upon the type of FR, was not more than ± 20 °C compared to that of samples containing toughener only. All flame retardants lowered the decomposition temperature of the epoxy resin. Phosphorus- and nitrogen-containing flame retardants reduced the char oxidation leading to more residual char, whereas halogen- containing flame retardants had less effect on this stage.     

Synthesis and effect of hyperbranched (3-hydroxyphenyl) phosphate as a curing agent on the thermal and combustion behaviours of novolac epoxy resin

A novel type of hyperbranched (3-hydroxyphenyl) phosphate (HHPP) with high functionality as a curing agent of epoxy resins was synthesized and characterized by FTIR, ¹H NMR and vapor phase osmometry (VPO). The cured epoxy resin with HHPP possessed improved glass transition temperature. The thermostability and flame retardancy of O-cresol novolac epoxy resin cured with different contents of HHPP were investigated by thermogravimetric analysis (TGA), limiting oxygen index (LOI) and cone calorimetry. The obtained results show that the samples containing a higher percentage of HHPP exhibit relatively lower thermostability at lower temperature while higher thermostability at elevated temperature and more char was formed compared with those containing a lower percentage of HHPP. The LOI value increased from 22.0 to 30.0 when HHPP, instead of 1,3-dihydroxybenzene, was used as a curing agent. The 25 wt% addition of HHPP in the curing agent complex effectively decreased the heat release rate and improved the char yield to the content nearly similar as those of the epoxy resin cured with pure HHPP.     

Effect of nanosilica on the kinetics of cure reaction and thermal degradation of epoxy resin

Nanocomposites from nanoscale silica particles(NS),diglycidylether of bisphenol-A based epoxy(DGEBA),and 3,5-diamino-N-(4-(quinolin-8-yloxy) phenyl) benzamide(DQPB) as curing agent were obtained from direct blending of these materials.The effect of nanosilica(NS) particles as catalyst on the cure reaction of DGEBA/DQPB system was studied by using non-isothermal DSC technique.The activation energy(E_a) was obtained by using Kissinger and Ozawa equations. The E_a value of curing of DGEBA/DQPB/10%NS system showed a decrease of about 10 kJ/mol indicating the catalytic effect of NS particles on the cure reaction.The E_a values of thermal degradation of the cured samples of both systems were 148 kJ/mol and 160 kJ/mol,respectively.The addition of 10%of NS to the curing mixture did not have much effect on the initial decomposition temperature(T_i) but increased the char residues from 20%to 28%at 650℃.     

A mesoscale study of thermal expansion behaviors of epoxy resin and carbon fiber/epoxy unidirectional composites based on periodic temperature and displacement boundary conditions

The thermal expansion behaviors of neat epoxy resin and carbon fiber/epoxy unidirectional (UD) composites were experimentally and numerically studied in this paper. The dynamic mechanical analysis (DMA), thermogravimetric analysis (TG), differential scanning calorimetry (DSC) and thermal conductivity measurement were used to measure the thermo-mechanical properties of epoxy resin at different temperatures. The dilatometer was used to measure the thermal strains and linear CTEs of neat epoxy resin and UD composites. In addition, a mesoscale finite element model based on the periodic temperature and displacement boundary conditions was presented to analyze the thermal expansion behaviors of UD composites. The resin-voids representative volume element (RVE) was used to calculate the thermo-mechanical properties of several kinds of resin-voids mixed matrix. From the results it can be found that the glass transition temperature of epoxy resin, porosity and fiber orientation angle have significant effects on the thermal expansion behaviors of UD composites. The mesoscale finite element analyses (FEA) have obvious advantages than various existing analysis models by comparing their predictive results. The distributions of thermal displacement, thermal stress and thermal strain were extracted between the carbon fiber, resin-voids mixed matrix and their interface, and also between the front and back surfaces of the loading direction, to further investigate thermal expansion structure effects of UD composites. This paper revealed that the mesoscale FEA based on periodic temperature and displacement boundary conditions can be also used for thermal expansion researches of other complex structure composites.     

An improved microbond test method for determination of the interfacial shear strength between carbon fibers and epoxy resin

An improved microbond method, with a corresponding testing device, was developed to measure the interfacial shear strength (IFSS) between carbon fibers and epoxy resin. Compared to other methods, this proposed approach is both highly efficient and easy to operate. As a case study for this new method, we measured the IFSS between carbon fibers and epoxy resin. Although the average IFSS obtained was only 7.08 MPa, which is much lower than values documented in several previous studies, the displacement-load curves demonstrate the strong reliability of this method. The lower IFSS could be explained by the highly inert surface of the carbon fibers, which was highly graphitized and had no sizing treatment. Therefore, this method has high potential in applications for screening the sizing agents of carbon fibers or optimizing the surface sizing processes.     

Chemical recycling of carbon fibre reinforced epoxy resin composites in subcritical water: Synergistic effect of phenol and KOH on the decomposition efficiency

The combination of phenol and potassium hydroxide (KOH) was used to chemically recycle carbon fibre reinforced epoxy resin cured with 4,4′-diaminodiphenylmethane in subcritical water. This combination had a synergistic effect on decomposing this kind of epoxy resin. The main decomposition products from the epoxy resin were identified by means of GC-MS, and a possible free-radical reaction mechanism for the decomposition of epoxy resin is proposed. The recovered carbon fibres were characterized using single fibre tensile tests, scanning electron microscopy and X-ray photoelectron spectroscopy. Compared to virgin carbon fibres after sizing removal, the surface compositions of the recovered carbon fibres had little change and the tensile strength of the recovered carbon fibres was well retained.     

Nano- and macro-geometrical structural change of caffeine and theophylline anhydrate tablets during hydration process by using X-ray computed tomography

The effects of nano- and macro-geometrical factors on the hydration kinetics of caffeine (CA) and theophylline anhydrate (TA) tablets at high humidity were investigated using X-ray high-resolution computed tomography (CT). Hydration profiles of CA and TA tablets obtained at 25 and 50 MPa, 96% relative humidity, and 20 °C were measured by weight and X-ray CT. The total tablet volume (G-V) and average tablet density (G-D) calculated based on the volume and weight of tablets, and tablet volume (CT-V) and tablet density (CT-D) were evaluated by X-ray CT. The hydration kinetics of CA and TA tablets followed two-dimensional growth of nuclei (Avrami-Erofee) and three-dimensional phase boundary equations, respectively. The increase in the G-V of TA tablets was initially more than, but later less than, that of CA tablets. The G-D of CA tablets varied extensively and was constant initially, whereas that of TA tablets decreased significantly in the initial stage. The CT-V of CA tablets gradually increased initially, but that of TA tablets increased significantly early on. The inter-granular volume (IG-V) of both tablets decreased initially with large fluctuations, but then increased. The CT-D of CA tablets decreased significantly, but that of TA tablets did not decrease. The hydration kinetics of CA and TA tablets was affected by changes in the geometrical structure of the tablets. X-ray CT is a powerful tool for evaluating dynamic changes inside tablets.     

An investigation of strontium nitrite and its role in the ageing of the magnesium-strontium nitrate pyrotechnic system using isothermal microcalorimetry and thermal analysis techniques

A sample of strontium nitrite has been synthesised and thermally characterised to investigate its role in the ageing of magnesium-strontium nitrate pyrotechnic compositions in the presence of water vapour. Studies by isothermal microcalorimetry show that the addition of strontium nitrite to a 50% magnesium-50% strontium nitrate composition eliminated the induction reaction normally observed in closed ampoule studies in air at 50 °C and relative humidities in the range 65-69%.