Oxidative embrittlement

First, it has been considered that embrittlement occurs when the endlife criterion is reached in the superficial elementary layer. For polymers predominantly undergoing chain scission during their oxidation, embrittlement corresponds to a critical number of chain scissions Sp 16), But, for polymers undergoing both chain scission and crosslinking, such as PE, the problem remains totally open. 

Hieh Solids. Great expectations are held for high-solids formulations, for industry s first coating was, in reality, a 100% solids system. Natural drying oils flowed easily, they pigmented easily, and they did not require baking in order to cross-link. The problem was chemical the oxidatively dried unsaturated species were not resistant to water, oxidative embrittlement, yellowing, and the like. 

In long term exposure at high temperatures, there is the possibility of gradual oxidative embrittlement. Such applications should make use of a heat-stabilized grade of resin. 

The best non-oxide fibers have good creep resistance but are susceptible to degradation by formation of an amorphous silica layer upon oxidation. This layer offers some resistance to further oxidation, but prolonged exposure to oxidizing environments results in oxidative embrittlement of the composite. 

The implementation of CMCs in spacecraft and uninhabited vehicles is attractive because material life requirements are much less demanding than the requirements for aircraft and industrial applications (Table 2-4). Because the operating environment in space is often non-oxidizing, life limiting oxidative embrittlement of non-oxide ceramics will not occur. Consequently, materials based on SiC and C are viable choices and have already been well established from the perspectives of performance and manufacturing characteristics. These limited volume high value applications could add to the production base of CMCs. 

Non-oxide CMCs are susceptible to degradation by oxidation embrittlement that operates at intermediate temperatures, between 500 and 900°C. This is referred to as pest phenomenon by a few authors. The matrix cracks created upon loading become pathways for the ingress of oxygen into the material. The pyrocarbon interphase is consumed (oxygen reacts to form gaseous products) and the SiC fibers are degraded by oxidation,... 

Oxidation is a chemical phenomenon that develops progressively with time. Certain chemical species such as moisture have been reported to accelerate the oxidation process and increase the rate of the oxidation-induced mechanical degradation of SiC/BN/SiC CMCs . Realizing the relative abundance of water vapor in the combustion environment of the burner rig as compared to the relatively dry furnace environment, the propensity of an exposed BN interphase to oxidation will be higher in the burner rig leading to a faster oxidation embrittlement of the exposed fibers fora specimen... 

Many polymers that are claimed to be biodegradable are in fact bioerodable and degrade -at least initially - without the action of micro-organisms. This is also known as abiotic disintegration, and may include processes such as dissolution in water, oxidative embrittlement (heat aging), or photolytic embrittlement (UV aging). 

These are called Narish type I and II (NI and Nil) degradations. Develop the reaction mechanism. Show how an ester group could be formed. You can see that the photo-oxidation embrittles the polymer and makes the polymer hydrophilic also. 

---