Retention properties

The definition of polymer thermal stabiUty is not simple owing to the number of measurement techniques, desired properties, and factors that affect each (time, heating rate, atmosphere, etc). The easiest evaluation of thermal stabiUty is by the temperature at which a certain weight loss occurs as observed by thermogravimetric analysis (tga). Early work assigned a 7% loss as the point of stabiUty more recentiy a 10% value or the extrapolated break in the tga curve has been used. A more reaUstic view is to compare weight loss vs time at constant temperature, and better yet is to evaluate property retention time at temperature one set of criteria has been 177°C for 30,000 h, or 240°C for 1000 h, or 538°C for 1 h, or 816°C for 5 min (1). 

Polyimides (PI) were among the eadiest candidates in the field of thermally stable polymers. In addition to high temperature property retention, these materials also exhibit chemical resistance and relative ease of synthesis and use. This has led to numerous innovations in the chemistry of synthesis and cure mechanisms, stmcture variations, and ultimately products and appHcations. Polyimides (qv) are available as films, fibers, enamels or varnishes, adhesives, matrix resins for composites, and mol ding powders. They are used in numerous commercial and military aircraft as stmctural composites, eg, over a ton of polyimide film is presently used on the NASA shuttle orbiter. Work continues on these materials, including the more recent electronic apphcations. 

Cross-linked polyester composites have a relatively low coefficient of thermal conductivity that can provide beneficial property retention in thick laminates at high temperatures as well as remove the need for secondary insulation. The coefficient of thermal expansion of glass-reinforced composites is similar to aluminum but higher than most common metals. 

The double bond present in the diene part of the elastomer is generally more susceptible to thermal and oxidative degradation. The selective hydrogenation of olefmic unsaturation in NBR imparts significant improvements in resistance to degradation and other properties, such as permeability, resistance to ozone and chemicals, and property retention at high temperature. 

For long-term heat ageing, property retention depends on the property and grades considered, notably the heat stabilizers used. For a heat-sensitive characteristic such as elongation at break, the values for a given polyethylene after ageing at 120°C are roughly ... 

For long-term heat ageing, property retention depends on the property and grades considered, notably the heat stabilizers used. 

Suitable compounds exhibit good property retention after six years of exposure in Arizona, for example ... 

PA 46 has better property retention when the temperature rises higher HDT and higher modulus at high temperature. 

Mrad, property retentions are about 100% for tensile strength and 86% for elongation at break... 

After exposure to gamma radiation at doses of 300Mrad at 50°C, property retentions are about ... 

The resistance to gamma radiation is good with acceptable degradation after 1000 Mrad exposure. For a given grade, property retentions are ... 

The property retention when the temperature rises is generally excellent but depends on the matrix, the nature and level of fillers and reinforcements, and the base properties. 

NASA is developing a special polyimide (PETI-5) for the High Speed Civil Transport (Mach 2.4) whose lifespan is more than 60 000 hours at 177°C with excellent property retention at this temperature. 

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