Epoxy network plasticization

The flexibility and extensibility of a crosslinked epoxy network are determined by the available glassy-state free volume. If the free volume is insufficient to allow network segmental extensibility via rotational isomeric changes then the brittle mechanical response of the epoxy glass is not controlled by the network structure but rather by macroscopic defects such as microvoids. For epoxies with sufficient free volume that allows plastic network deformation the mechanical response is controlled by the network structure. 

In Sect. 5, the macroscopic extension at break of the glassy networks considered was shown to be s, 4-6% and only weakly dependent on the network s chemical composition. However, local plasticity markedly depends on the chemical composition of the network. L( in the samples with an excexs of amine (P = 1.3) is about 10 times larger than in the samples with an excess of epoxy groups (P = 0.8). This is an additional argument in favour of the assumption of the keyrole of 3-linked chemical crosslinks in network plasticity (see Sect. 5 and Fig. 20). It is evident that the excess of amine in the initial mixture leads to a relatively high concentration of 3-type crosslinks in cured resins. 

Moisture is a well-known plasticizer for macromolecules 130-132. Specifically, water penetrates into an epoxy network and can lower the glass temperature of the resin 131 133 134. In this report, moisture has for the first time been utilized as... 

The Increase in magnitude of the low temperature tan 6 associated with the 0-transltlon peak can be rationalized due to a plasticization of the epoxy network. However, the significant differences in the tan 6 magnitudes which arise above the 25"C for both epoxies are not attributable to plasticization. This shift is in fact a tertiary u dynamic mechanical transition (3, 4). This ti) transition is highly sensitive to the presence of polar solvents. Chu and Seferls (6) obtained dynamic mechanical data which demonstrate formation of such a tertiary u transition in a T(3)DM-DDS epoxy system from residual amounts of acetone in the network. Keenan, et al (4) present data for the N-5208 epoxy system which indicate a slight drop in the temperature location of the u transition with Increased moisture content. Broadening of the peak was also observed. 

Transient dynamic mechanical data on the DGEBA-TETA and high performance M-5208 epoxy based systems have been obtained and compared with "equilibrium" data.. The transient data have demonstrated that moisture can act not only to plasticize an epoxy network but also to restrict and stiffen molecular chain movement. The behavior observed was explained by examining the synergistic effects that moisture and temperature have on the particular epoxy network structure. 

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. 

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). 

Of special interest are the results of studying jump-like creep on the micro-scale level when the controlled structural heterogeneities of the same sizes are present in polymers, namely, for epoxy networks with a globular sUiicture [311], for epoxy composites containing diabase microparticles [310], for POM plastics with different spherulite sizes [320], and for Pl-graphite composites [320], These materials can be considered as models for checking up the micro-plasticity vs sUiicture correlations. It was possible to compare directly the sizes of heterogeneities (solid microparticles or densely packed micro-domains of polymers) with the creep micro-jumps, and to draw the conclusions about their interrelationship. 

Park J and Jana S C (2003) Effect of plasticization of epoxy networks by organic modifier on exfoliation of nanoclay, Macromolecules 36 8391-8397. 

J. C. Hedrick, N. M. Patel, and J. E. McGrath, Toughening of Epoxy Resin Networks with Functionalized Engineering Thermoplastics, in Rubber Toughened Plastics, K. Riew (Ed.), American Chemical Society, Washington, DC, 1993. 

Fig. 8.4 Plots of relative change in electrical resistance against tensile deformation of a CNT/epoxy composite (a) shows the various characteristics of the piezoresistivity of nanocarbon networks linear resistance change in the elastic regime, nonlinear region after inelastic deformation and the permanent electrical resistance drop due to plastic deformation (image adapted from [30]) ...

The weight loss after the sorption/desorption cycle is 1.3% for the specimens irradiated to 10,000 Mrads. The low weight loss indicates limited degradation. The glass transition temperature should have returned to the value of the unirradiated epoxy (280°C) if degradation products are the only species plasticizing the network. 

The time and temperature dependent properties of crosslinked polymers including epoxy resins (1-3) and rubber networks (4-7) have been studied in the past. Crosslinking has a strong effect on the glass transition temperature (Tg), on viscoelastic response, and on plastic deformation. Although experimental observations and empirical expressions have been made and proposed, respectively, progress has been slow in understanding the nonequilibrium mechanisms responsible for the time dependent behavior. 

Phenolic, epoxy, urea, melamine, and polyester (alkyd) polymers are cross-linked (thermoset) plastics. They are solvent-resistant and are not softened by heat. Unlike the thermoplastic step reaction polymers, which are produced by the condensation of two difunctional reactants, these network polymers are produced from reactants at least one of which has a degree of functionality higher than two. 

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