Moisture embrittlement

Much the same happens if the prepolymers contain basic (tertiary) amino groups—e.g., diols based on primary amines extended with propylene and ethylene oxide and similar materials. The amine nitrogen reacts with the oxidizer, releasing ammonia, and is itself converted to the ammonium ion. Ensuing ionic interaction raises the viscosity of the batch to the point where it becomes unmixable (see also later section on moisture embrittlement). 

Moisture embrittlement must be distinguished from other forms of embrittlement. Brittleness of propellants at low temperatures is normally caused by binders which stiffen excessively with decreasing temperatures, owing to partial crystallization of the binder matrix (e.g.9 copolymers of butadiene with acrylonitrile, some hydrogenated polybutadienes, etc.). Such propellants are brittle at low temperatures whether they have been exposed to moisture or not. 

Another form of embrittlement, akin to moisture embrittlement, is encountered where the oxidizer (e.g., LiC104) possesses marked solubility in the binder phase even without the aid of water. Such systems also exhibit a comparatively poor processability (high mix viscosity). 

Its influence appears to result primarily from suppression of the embrittling effects of moisture in air (75). The role of chromium, on the other hand, is to reduce the embrittling effects of oxygen at temperatures above about 500°C (76). 

The polyphosphates have also been widely studied. Here the phosphate ion is found as a high polymer. The molecular weight of the polymer ranges from 250,000 to 2,000.000. The polyphosphates are water soluble and form fibers. No uses have been found for this class of materials. They hydrolyze slow ly in atmospheric moisture and also embrittle on standing. 

---