Aging epoxy networks

Apparently, there is less free volume in the aged polymer network. Any water coming into the aged network would tend to swell the polymer because there are simply less vacant sites in the distribution of free volume. In short, there is more polymer-solvent interactions as water diffuses into an aged epoxy network. 

Decelle, Huet, N., Bellenger, V. Oxidation induced shrinkage for thermally aged epoxy networks, Polymer Degradation and Stability, 81 (2003), p. 239 -248... 

The aging of hulk epoxy networks has been studied by various techniques such as FTIR spectroscopy [1-4], gravimetric analysis [1, 3, 5, 6], mass spectroscopy [1], DSC [2], or XPS [6]. The aging conditions applied vary broadly from thermo-oxidation at elevated temperature (100-250 °C) [1-3, 5, 6] or photo-oxidation [1, 2] to humid or environmental conditions [4, 7]. FTIR investigations [1-4] show the rise of new IR bands at 1660-1670 cm and 1720-1730 cm in an oxidized region near to the sample surface. The bands are explained by amide and carbonyl formation due to radical oxidation mechanisms initiated by the elevated temperature or UV irradiation [1, 2, 4]. Also, backbone cleavage is held responsible for the observed decrease in glass transition temperature [2]. 

The results show that the crosslinking state and the degradation processes in the epoxy network depend not only on the type of curing agent and the aging regime. The properties of the stainless steel also change these properties within the epoxy interphase at the contact. 

This paper reports on the aging behavior of an RT-cured and of a hot-cured epoxy network under thermal and hydro-thermal loading in the bulk and in layers on stainless steel. During a period of 100 days, the chemical and stractur-al changes have been observed by IR spectroscopy and differential scanning calorimetry (DSC). 

At selected aging times t, samples are taken for experimental characterization. After three months of immersion, some samples are re-dried under thermal aging conditions at 40 °C and 60 °C, respectively. With the dry samples, the effect of aging in the epoxy network is studied without artifacts from evaporating water during the experiments. 

It is assumed that the irreversible plasticization is caused by the breaking of bonds under the influence of water. This hydrolytic aging process should reduce the crosslink density of the epoxy network. The hydrolysis does not deteriorate the entire network, however, because the irreversible plasticization comes to an end within the period of HTA we investigated. 

Moisture is a well-known plasticizer for macromolecules (14). Specifically, water penetrates into an epoxy network and can lower the glass temperature of the resin (15). In this report, moisture has for the first time been utilized as a probe to characterize denslfIcatlon process during epoxy aging. Also, using the same rationale, heavy water diffused Into the epoxy resin Is used to study the Interactions of moisture with the aging polymer by hydrogen-2 (deuterium) NMR spectroscopy. 

Earlier we reported that physical aging affects the "swelling efficiency" and diffusivity of epoxy as moisture is transported into the network ( ). In conjunction with this earlier communication ( ), we can now summarize the findings for interactions between moisture and aging epoxies ... 

For the first time. It Is now possible to characterize the molecular aggregation during physical aging In network epoxies by spectroscopic technique. Other Important findings are summarized as physical aging of the polymer network glass proceeds,... 

M. Aboulfaraj, C. G Sell, D. Mangelinck, and G. B. McKenna, Physical Aging of Epoxy Networks after Quenching and/or Plastic Cycling , J. Non-Cryst. Solids 172-174, 615-621 (1994). 

In a study of aluminum-epoxy adhesive joints, Dillingham and Boerio" used XPS analysis to determine that, after humid aging, the locus of fracture was within the metal oxide. They further proposed that catalysis during setting by acidic hydroxyls on the oxide caused the epoxy near the oxide to be more crosslinked than the bulk epoxy network. Another researcher" showed that fracture progressed within a weak boundary layer when a polyvinylidene fluoride-nylon bond was ruptured. 

Numerous studies have been reported on the effect of diffusion of water into epoxy resins 177 178>. It is generally agreed that moisture acts as a plasticizer which lowers the Tg of the resin uo 134). Very little work, however, has been reported on the effect of physical aging on the diffusion behavior of water into network epoxies. This Section of the review summarizes the first attempt to study such an effect on TGDDM-DDS epoxy/water interactions. 

Cook et al. also recently reported studies on the yielding of model epoxy thermosets. They focused first on the effects of strain rate and composition [169] and then on the effects of physical aging [170]. Their results are consistent with the trends summarized above. Networks cured with aromatic amines had higher Gcy than those cured with aliphatic amines. The value... 

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