Thermal-oxidation of epoxy/amine followed by glass transition temperature changes

Thermal oxidation of three epoxy resins differing by the nature of prepolymer (bisphenol A diglycidyl ether and 1,4-butanediol diglycidyl ether) and hardener (isophorone diamine and 4,7,10-Trioxa-1,13-tridecanediamine) was studied by monitoring changes in glass transition temperature using DSC. Results were discussed using the DiMarzio's approach in which parameters are estimated from an additive group contribution. This theory allowed a fair assessment of Tg values for unaged networks. During oxidation, epoxy networks were shown to undergo chain scissions occurring in great part in hydroxypropyl ether and isophorone groups. However, the exploitation of Tg changes showed the coexistence and even the predominance of crosslinking in materials having linear aliphatic segments. The DiMarzio's approach was used to discuss the possibility of intramolecular cyclization or intermolecular crosslinks which were shown to predominate. Crosslinks were tentatively justified from a mechanistic point of view and quantified depending on experimental conditions

Prediction of stress induced by heterogeneous oxidation: Case of epoxy/amine networks

A methodology to predict the formation of superficial stress during the diffusion-limited oxidation of thick epoxy/amine samples is proposed. This quantitative methodology is based on the understanding of mechanisms responsible for this stress lead to superficial micro-cracks. The first step consists of simulating the homogeneous oxidation on the surface of thick samples. The extent of oxidation is assessed through the concentration of oxidation products as amide groups measured using Fourier-transform infrared spectroscopy (FTIR). These experimental results are compared to simulations obtained from a kinetic model based on a close loop mechanistic scheme in chain oxidation. Shrinkage and tensile behavior changes are correlated with an oxidation tracer, being amide groups in our case. The second step considers diffusion-limited oxidation in thick samples where the gradients of oxidation products characterized through thickness by micro-FTIR are simulated by coupling the previous kinetic model with oxygen diffusion. Finally, the gradient of strain and stress induced by oxidation of a thick epoxy/amine sample is simulated by inserting the mechanical behavior modifications and shrinkage gradient associated with the amide gradient in finite element code (Abaqus®). As a result, it is shown that it is possible to predict the superficial stress level as a function time/temperature of exposure and sample geometry.ANRT (CIFRE No 2013/0356

Origin of epoxies embrittlement during oxidative ageing

International audienceThermal oxidation of three epoxy resins prepared from flexible or rigid prepolymers and hardeners was studied by monitoring epoxy mechanical and physical changes. The physical changes were followed by mass measurements, glass transition temperature using DSC and sub-glass β transition using DMA. It was put in evidence that embrittlement is not directly associated to Tg or mass loss changes since epoxy network based on isophorone diamine (IPDA) hardeners were shown to undergo mainly a chain scission at the beginning of exposure process whereas epoxy network based on trioxatridecane diamine (TTDA) hardeners exhibits a crosslinking process with a significant mass loss. The only common feature for both epoxy systems to understand embrittlement is the drop of amplitude of β transition with oxidation

Thermal oxidation of epoxies: Influence of diamine hardener

International audienceThis work reports the thermal oxidation of DGEBA resins totally cured with cycloaliphatic (isophorone diamine) or with linear aliphatic (trioxa-tridecanediamine or TTDA) diamine hardener. Chemical changes (carbonyl build-up) were monitored by FTIR and macromolecular changes (chains scission and crosslinking) from Tg variations measured by DSC. Exposition at several temperatures and under several oxygen pressures showed that DGEBA/IPDA has systematically the highest oxidation rate. It was also observed that DGEBA/IPDA undergoes mainly chain scissions whereas DGEBA/TTDA undergoes a predominant crosslinking under air being partially suppressed under enhanced oxygen pressure. Results were discussed on the basis of the possible influence of the structure of hardener

Rigid amorphous fraction in different molecular weight polylactic acidafter cold crystallization from the glassy state

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Rigid amorphous fraction in different molecular weight polylactic acidafter cold crystallization from the glassy state

International audienc

Rigid amorphous fraction in different molecular weight polylactic acidafter cold crystallization from the glassy state

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Influence de la Fraction Amorphe Rigide sur les Propriétés Mécaniques d'un Composite Carbone/Polysulfure de Phenylène

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Effect of Molecular Weights on the Physical Properties and Microstructure of Poly(lactic acid)

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Influence de la Fraction Amorphe Rigide sur les Propriétés Mécaniques d'un Composite Carbone/Polysulfure de Phenylène

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