Network degradation

Novolac network degradation mechanisms vary from tiiose of resole networks due to differences in crosslinking metiiods. Nitrogen-containing linkages must also be considered when HMTA (or other crosslinking agent) was used to cure novolac networks. For example, tribenzylamines, formed in HMTA-cured novolac networks, decompose to cresols and azomethines (Fig. 7.50). 

Other transitions such as degradation and phase separation may be also observed during the formation of the polymer network. Degradation is usually present when high temperatures are needed to get the maximum possible conversion. Phase separation may take place when the monomers are blended with a rubber or a thermoplastic, to generate rubber-modified or thermoplastic-modified polymer networks. In these cases, formulations are initially homogeneous but phase-separate during the polymerization reaction. This process is discussed in Chapter 8. 

On the other hand, water uptake is a continuous problem for polyimides and particularly for polynadimides [133]. Dynamic mechanical spectrometry (viscoelastic measurements) have been used to investigate the network degradations due to hydrolytic process [134]. It was shown on a special PMR resin (AFR 700 B) that network reformations are possible through post-curing. In addition, for fluorinated systems, 19F NMR can be used to follow the hydrolysis of the im-ide groups [135]. 

Yushanov, S.P. Isayev, A.I. Levin, V.Yu. Percolation simulation of the network degradation during ultrasonic devulcanization. J. Polym. Sci. Phys. Ed. 1996, 34, 2409. 

For the general case of network degradation, both Mc and M will change. To see this, consider that Mc = pile and M = 2 p/[ends], where [ends] is the concentration of chain ends. Inverting and taking derivatives of these expressions gives... 

In the contact zone with air, the network degrades by the oxidation of primary and secondary amines. In the presence of water, only primary amines are oxidized. Consequently, the amines are consumed by the ongoing cure and by oxidation. These mechanisms complete the picture of the degradation mechanisms discussed in the hterature for low aging temperatures. Hydro-thermal aging at the presence of both types of stainless steel increases the oxidative modification in EPl. 

The advantage of this addition vulcanization is that no revision occurs because no byproducts are formed which might interfere with the network in terms of a reversible network degradation. Furthermore, although this vulcanization reaction is considerably accelerated at elevated temperatures. For a given receipe, the curing characteristics at different temperatures are shown in Table 4. 

This probe has tended to be replaced by thiolamines because of practical difficulties in its use and because of evidence that it may cause network degradation other than the cleavage of expected crosslinks. 

Keywords Interlayer adhesion loss Epoxy coatings Weathering Erosion mechanism UV absorber Humidity Water Crosslinking Network degradation Oxygen diffusion and solubility Photoproducts Oxidation zone Rain Lag time UV radiation degradation... 

---