Oxidation induced shrinkage for thermally aged epoxy networks

doi:10.1016/S0141-3910(03)00094-6

Polymer Degradation and Stability

Volume 81, Issue 2, 2003, Pages 239-248

https://doi.org/10.1016/S0141-3910(03)00094-6 Get rights and content

Abstract

The matrix oxidation of composite materials involves a weight loss and a density increase and as a consequence, a shrinkage of the skin layer of the material. To simulate this behaviour, we chose a kinetic model of radical chain oxidation coupled with the equation of oxygen diffusion. This model predicts the concentration profile of oxidation products, the weight loss profile and the shrinkage profile in a thick part. When the stress field induced by shrinkage reaches a critical value (the ultimate stress of the oxidized polymer), a “spontaneous” cracking appears in the skin layer of thick parts. In order to predict these critical conditions, we determined the mechanical properties of the oxidized layer of thick parts by studying the ageing of matrix thin films for which oxidation is not controlled by the oxygen diffusion.

Introduction

The thermal ageing of organic composite materials is essentially induced by matrix oxidation, which is limited to superficial layers [1], [2], [3], [4]. Scola et al. [4] showed that the oxidation is favoured in the direction parallel to fibres. For several years, ENSAM has developed kinetic models of oxidation coupled with the equation of oxygen diffusion in order to determine the concentration profile of oxidation products in the sample thickness [5], [6], [7]. The oxygen graft to the polymer induces an increasing density in the surface layer of the material and as a consequence, shrinkage, tension stresses and “spontaneous” cracking of the sample. This work deals with the relationship between the shrinkage and the oxidation conversion ratio in order to forecast the critical conditions for which cracks induced by oxidation occur in the sample. The industrial part is a structural part of the future European supersonic aircraft ESCT. It is made of a composite material, an epoxy matrix reinforced by carbon fibres. The supposed lifetime corresponds to about 20 000 thermal cycles between −55 and 120 °C with a total length of the plateau at 120 °C of 80 000 h (10 years). The main cracking causes are:

•
Matrix oxidation which is the aim of this study
•
Differential thermal expansion of fibres and matrix when the temperature increases or decreases at each cycle, a kind of thermal fatigue which induces some shear stresses at the interface matrix-fibres.
•
Functional loadings of the part with some damage induced by mechanical fatigue

Concerning the matrix oxidation, there is first the build-up of a surface oxidised layer with oxygen grafting to the macromolecular network and some chain breaking reactions with the diffusion of volatile compounds out of the part. The elimination of the volatile compounds produces a weight loss and oxygen grafting leads to an increasing density of the surface layer. Decreasing weight and increasing density result in shrinkage, which leads to the build-up of a strain field. From the knowledge of the elastic properties, one can calculate the stress field and from the knowledge of ultimate properties, one can forecast the critical conditions for crack initiation.

Section snippets

Material

The polymer is made of a mixture of aromatic epoxy (triglycidyl amino phenol-diglycidyl ether of bisphenol F) crosslinked by an aromatic diamine, the diamino diphenylsulphone. There is 30% by weight of a thermoplastic polyether sulphone for increased impact strength. Samples are cured 3 h at 150 °C+2 h at 180 °C, and then post-cured under vacuum during 1.5 h at 210 °C to achieve a complete cross linking reaction. The initial glass transition temperature is 190 °C±3 °C and the initial density is

Modelling

For the theoretical calculation of the weight loss and density increase, we started from a closed loop oxidation mechanistic scheme [8]. The main difference from the standard mechanistic scheme of oxidation is the initiation step: the main source of alkyl radicals is the hydroperoxide decomposition.

POOH+2 PH	→	2 P°+H₂O+ν V	k₁	Initiation
P°+O₂	→	PO₂°	k₂	Propagation
PO₂°+PH	→	POOH+P°	k₃	Propagation
P°+P°	→	Inactive products	k₄	Termination
P°+PO₂°	→	Inactive products	k₅	Termination
PO₂°+PO₂°	→	Inactive products+O₂	k₆

Results and discussion

The gravimetric curves of samples whose thickness varies between 70 μm and 4 mm, exposed in a ventilated oven at 150 °C and atmospheric pressure are shown on Fig. 1. Fig. 2 displays the weight loss rate versus the shape factor, which is equal to the surface/volume ratio. From these results one can determine the critical thickness above which the oxidation is controlled by oxygen diffusion; at 150 °C, it is equal to 93 μm (shape factor F=218 cm⁻¹). The influence of the partial pressure of oxygen

Conclusions

The failure of composite parts exposed in air at 120 °C for a great part of their life is expected to result at least partly from the embrittlement of surface layers induced by thermal oxidation. This latter leads to a density increase and a weight loss; both processes contribute to the shrinkage of the superficial layers, which generates tensile stresses and possibly “spontaneous” cracks. We tried to model this process on the basis of the following hypothesis:

1.
Oxidation proceeds by a branched

Acknowledgements

Special thanks to “Ministère de la Recherche” who supported financially the study and EADS Company who supplied samples.

References (10)

X. Colin et al.
Kinetic modelling and simulation of gravimetric curvesapplication to the oxidation of bismaleimide and epoxy resins
Polym. Degrad. Stab
(2002)
F. Magendie et al.
Thermal stability comparison of ceramic and carbon fiber BMI matrix composites
Proceedings of International SAMPE
(1990)
H. Parvatareddy et al.
An evaluation of chemical aging/oxidation in high performance composites using the Vickers micro-indentation technique
J. Compos. Mater
(1996)
M. Madhukar et al.
Thermo-oxidative stability and fiber surface modification effects on the in-plane shear properties of graphite/PMR15 composites
J. Comp.os Mater
(1997)
Scola D. Proceedings of the Joint US–Italy Symposium on Composite Materials Capri, Italy, 15–19 June 1981. New York:...

There are more references available in the full text version of this article.

Cited by (117)

Thermo-oxidative aging induced multi-scale shrinkage and damage in 3D angle-interlock woven composites
2024, Composites Science and Technology
The deterioration of resin and interface in composites deteriorate during high-temperature service, would impact both the thermal stability and overall performance significantly. This paper focused on the experimental characterization of multi-scale damage and resin shrinkage within composites induced by thermo-oxidative aging using Micro-CT, SEM and Confocal Interferometric Microscopy (CIM), and exploring the coupling mechanism between them. The results show that damage distribution and propagation are affected by the hierarchical structure of 3-D angle-interlock fabric. Macro-CIM shows that maximum resin shrinkage depth depends on the resin width and aging time. Resin shrinkage profile evolution demonstrated the accelerating effect of resin crack on shrinkage. Micro-CIM shows that macro yarn cracks form through the accumulation of micro resin cracks in resin-rich areas and fiber/resin interface debonding. The formation of fiber/resin interface crack is tied to the resin shrinkage state, only occurring when the surrounding resin shrinkage slope exceeds a specific threshold value.
Comparison of the effect of thermal and hygrothermal sub-Tg aging on the durability and appearance of multilayered filament wound composite structure
2024, Polymer Testing
The study conducts an experimental investigation to compare the effect of accelerated aging on a glass fiber/vinylester matrix filament wound composite. Two static aging conditions, thermal and hygrothermal, were chosen. Samples were aged at temperatures below the glass transition temperature (Tg) of vinylester for 224 days. Mechanical test – ILSS and flexure testing and physicochemical assessments – DMA, FTIR and colorimetry were conducted to gauge the aging effects on long-term durability and appearance. Results are presented as retained property values over aging durations and environments. Notably, coupons aged below 60 °C under both conditions exhibited residual curing, enhancing mechanical properties. Time-temperature coupling influenced behavior, with higher temperatures and longer durations leading to degradation. Hygrothermal aging at 80 °C prompted earlier degradation than thermal aging, highlighting moisture's impact. DMA revealed interfacial degradation for hygrothermally aged coupons, verified by fractography. Conversely, damping factor analysis indicated residual curing for thermally aged coupons. Results were confirmed through color change and FTIR, showcasing evolving chemical patterns with aging.
Thermal oxidation of diallylbisphenol A - bismaleimide networks. Comonomer effect and kinetic modelling
2024, Polymer Degradation and Stability
Pure BMI resin based on 4,4′-bismaleimidodiphenylmethane (BMI) and its copolymers with various quantities of 2,2′-diallylbisphenol A (BMI-DABPA 2–1, BMI-DABPA 1–1, BMI-DABPA 1–2) were thermally aged. Ageing at 160 °C was monitored by FTIR and showed the greatest reactivity of diallyl groups which accelerate the degradation of BMI-DABPA copolymers. It was confirmed by a gravimetric study of ageing under nitrogen or oxygen at 350 and 375 °C. A co-oxidation kinetic model was proposed for describing the oxidation of BMI and BMI-DABPA systems. The effect of the main kinetic parameters was discussed from a parametric study. The main rate constants were estimated from the fitting of experimental curves and comparable gravimetric data from literature.
Multi-scale dynamic behaviors of oxygen entering carbon fiber/epoxy resin interfaces under thermal environment
2023, Composites Science and Technology
Understanding the thermo-oxidative ageing behavior of carbon fiber/epoxy resin interfaces at multiple scales is crucial for structural optimization and durable design of composites. Here we report the effects of oxygen entering the interface on the 3D ageing crack evolutions, interface spatial structures, and multi-scale dynamic mechanisms for carbon fiber/epoxy composites under a thermal environment using experiments and molecular dynamics (MD) simulations. The dynamic behaviors under ambient temperature environments were studied for comparison. Ageing cracks first appear at the interface and then spread to the surrounding resin-rich region as the ageing time increases. Interface cracks provide channels for oxygen to enter the interior of the composite. The MD results show that interface systems with fewer oxygen molecules have serious interface cracks, weaker interaction energies and stronger diffusion capabilities than those with more oxygen molecules. The oxygen amounts entered the interface and the intermolecular interactions affect the interface dynamic behaviors. High temperature promotes the molecular movement, accelerates the interface cracking, weakens the interaction energy, and improves the diffusion capability. The diffusion of oxygen molecule from the interface to the epoxy matrix is a gradual process and is affected by the interface cracks.
Thermal oxygen coupling effects on multiple-layer degradation behaviors and full-field crack evolutions of 3D5d braided composites
2023, Composites Science and Technology
Mechanical behaviors of thermo-oxidative aged 3D braided composites are affected by matrix degradation and crack evolution. Here we report multiple-layer degradation behaviors, spatial crack propagations and mechanical reduction mechanisms of three-dimensional five-directional (3D5d) braided composites under thermal oxygen coupling environment with dynamic thermomechanical analysis, Fourier transform infrared spectroscopy and X-ray micro-computed tomography. Ageing behaviors of samples in pure thermal environment were analyzed for comparison. Storage modulus of thermo-oxidative aged resins in rubbery state increases and a new tanδ peak appears, while these phenomena disappear after continuous removal of the oxidized layer. Oxidized molecular chains and re-crosslinking reactions occur on the near-surfaces of the thermo-oxidative aged resins. The cracks in the thermo-oxidative aged composites initiate and propagate faster than those in the thermal aged composites. Yarn arrangements affect the crack growth rates. The synergistic effect of oxygen and crack continuously reduces the mechanical properties of the 3D5d braided composites.
Stress and embrittlement in organic coatings during general weathering exposure: A review
2022, Progress in Organic Coatings
Failure is defined by a coating's purpose, but a crack is a failure in almost every application. Cracks cause problems in service when they first arise, well before they are widespread. By itself, degradation of a polymer coating can cause cracks or adhesive failure, but several other sources of stress are possible. How do these stresses arise and how do coating properties change during service resulting in cracks? Moisture can produce swelling or blistering, entailing compressive stresses, but often tensile stresses also result at the same time. Environmental stress cracking causes embrittlement in polymers due to a combination of plasticisation and swelling caused by moisture. Seemingly small stresses, from thermal mismatch or bending, can initiate failure by repetition or due to a stress concentration. Polymers become brittle, manifested in several ways. Modulus and tensile strength may increase, but elongation and fracture surface energy diminish considerably. The Lake-Thomas theory allows estimates of how breaking bonds contributes to fracture surface energy but currently there is no settled model for calculating how mechanical dissipation contributes or changes. Degradation causes small defects to become more likely to initiate failure than in unweathered coatings and Poisson's ratio provides insight how 3-dimensional structure relates to potential cavitation. Not only do all these factors affect the overall coating film, but they also affect the internal integrity between the binder and other ingredients and thus failure can be initiated internally before it become apparent externally.

View all citing articles on Scopus

View full text

Oxidation induced shrinkage for thermally aged epoxy networks

Abstract

Introduction

Section snippets

Material

Modelling

Results and discussion

Conclusions

Acknowledgements

Polym. Degrad. Stab

Thermal stability comparison of ceramic and carbon fiber BMI matrix composites

Proceedings of International SAMPE

An evaluation of chemical aging/oxidation in high performance composites using the Vickers micro-indentation technique

J. Compos. Mater

Thermo-oxidative stability and fiber surface modification effects on the in-plane shear properties of graphite/PMR15 composites

J. Comp.os Mater