Thermo-oxidative stabilization

Phenolic networks are well known for their excellent thermal and thermo-oxidative stabilities. The mechanisms for high-temperature phenolic degradation include dehydration, thermal crosslinking, and oxidation, which eventually lead to char. 

Table I Thermo-oxidative stability (at 150 C) of PP (ICI.HF22) without and with 30X glass fibre (6F) in the absence and presence of antioxidants. Soxhlet extraction was with chloroform and acetone...

One important point should be stressed here the efficiency of any stabilizing system depends very much on the removal or depletion of the defect structures in the polymer. An example is shown in Figure 1 where the case 5 mechanism vide supra) of a synergistic mixture is theoretically depicted [7]. The line 2 shows that the effect of the synergistic mixture of two antioxidants on thermo-oxidation stability is negligible, if there occurs a relatively significant initiation of oxidation reaction in a way independent from the route taking place via hydroperoxides. 

The SSP behavior of co-polyesters with rigid or voluminous comonomers, such as the flame retardant additive 9,10-dihydro[2,3-di-9-oxa-(2-hydroxyethoxy)-carbonylpropyl]-10-phosphaphenanthrene-10-oxide, or the ionic compound, sodium 5-sulfoisophthalate, is inhibited. This also occurs in the melt phase and cannot be improved by the use of catalysts [56], The results of studies examining the influence of employed catalysts with respect to stability and quality of the polymer suggest the use of antimony catalysts. The thermal or thermo-oxidative stability is, however, reduced by the interaction of the catalyst with the carboxylic groups of the polymer [57],... 

TABLE IV. COMPARISON OF THEORETICAL AND EXPERIMENTAL THERMO-OXIDATIVE STABILITIES OF BCB/K-353 SYSTEMS (eSO F/AIR/aODh)... 

Poly(arylene ether)s generally exhibit high glass transition temperatures, resistance to hydrolysis, as well as an excellent thermo-oxidative stability. 

Polyimides have experienced tremendous growth since their conception over 30 years ago. Whereas the initial goal of these materials was high thermo-oxidative stability along with excellent mechanical properties, more recent applications... 

Poly[2,2 -(m-phenylene-5,5 -benzimidazole)] (PBI) is a very high glass transition temperature (Tg 430°C), commercially available material. It possesses excellent mechanical properties, but is difficult to process into large parts and has high moisture regain and poor thermo-oxidative stability at temperatures above approximately 260 °C. Polyimides, especially the thermoplastic polyimides, offer attractive thermo-oxidative stability and processibility, but often lack the thermal and mechanical characteristics necessary to perform in applications such as the matrix for high use-temperature (over 300 °C) structural composites (for example, carbon fiber reinforced) for aerospace use. The attempt to mitigate... 

Fig. 4. Effects of phosphoric acid (PA) treatment on the thermo-oxidative stability of PBI and PBI/PEI blend films aged in air at 315 °C...

Blend systems with significantly improved thermo-oxidative performance can be achieved through incorporation of carefully designed polyimide molecules. As shown in Table 1, a copolyimide containing the sulfone and 6F moieties which exhibits a Tg above 300 °C (see Fig. 5), as well as extraordinary short-term thermo-oxidative stability can be synthesized. 

The thermo-oxidative stability of a silane where silicon is linked to imide nitrogen by an aromatic radical (CA-25) was greater than that of a compound where silicon is linked to nitrogen by an aliphatic group (CA-11). Upon... 

Environmental demands and are as follows (a) improved fuel economy (reduced viscosities, reduced friction, special viscosity improvers), (b) reduced oil consumption (unconventional base oils, improved seal compatibility), (c) extended oil life (improved thermo-oxidative stability), (d) extended engine life (improved detergents and antiwear additives), (e) beneficial effects on emission/after treatment hardware (new additives), (f) technological and environmental sensitivity (no halogens, limited metal types/concentrations, new organic compounds), (g) recyclability (limitations the polycyclic aromatic hydrocarbons content and high chlorine levels (Havet et al., 2001 Waara et al., 2001). 

Segmented polyimide-polydimethylsiloxane copolymers have been successfully synthesized both in laboratory and industrial quantities to produce multiphase siloxane-modified polyimides. The siloxane detracts somewhat from the otherwise excellent thermo oxidative stability of the polyimide, but it does produce a number of important properties. These include multiphase behavior, improved adhesion to many substrates, improvements in fire resistance and enhanced gas and liquid separation membranes, where one wishes not only to maximize the contribution of the siloxane to permeability, but also to utilize the imide to re-... 

Table 4. Thermo-oxidative stabilities of modified PMRs...

It appears that the nadimide end-cap is the main contribution to the thermal degradation of crosslinked PMR. This is confirmed by a comparison between two similar fluorinated PMRs with two different end-capping groups nadimide or styrene. The styrene end-capped system exhibits the best thermo-oxidative stability. 

Semicarbon or oxidized polyacrylonitrile fibers, produced by thermo-oxidative stabilization of either viscose or acrylic fibers, have excellent heat resistance, do not melt or burn, and have excellent resistance to molten metal splashes. Panox (RK Textiles), Panotex (Universal Carbon Fibers), and Pyron (Zoltek Corp) are some examples, produced from acrylic fibers. 

LPD deposition of ceramic thin films is a potentially general approach to improving the abrasion resistance and thermo-oxidative stability of polymers. Dutschke et al.14,15 have deposited titania on variously treated polystyrene (PS). Continuous, adherent anatase films form on PS either after hydroxylation in... 

Generally, the thermal stability of the composites was improved by the addition of the CNTs, which may be attributed to the following combined effects (1) the uniformly dispersed carbon nanotubes presumably provided thermo-oxidative stability to the polymers in the vicinity of the tube surfaces (2) the enhanced thermal conductivity of the composite can facilitate heat transport and thus increase its thermal stability (81) (3) it is possible that the formation of compact chars of CNTs and polymer matrix during the thermal degradation is beneficial to the improvement of thermal stability of the composites (82). 

---