Synthesis of high temperature resistant materials
INTRODUCTION In order to make a polymeric material useful and servicable at elevated temperatures-say above 300°C-several conditions have to be met. First of all, the material must maintain its mechanical properties, such as rigidity, tensile strength and impact strength as a matter of instantaneou.s response, which means essentially, that it must not melt or drastical{Y seiften in that temperature range. If somewhat Ionger periods of service are involved it also becomes necessary that the creep of the polymer under the inftuence of the existing external forces, such as shear stress, torques or pressure should be sufficiently slow in ordertopermit its proper functioning. At stilllonger times chemical phenomena start to interfere, which may concern the polymer alone or which may involve other ingredients such as oxygen, acids, bases, organic reagents and also active radiation from the near ultraviolet to the cosmic rays. These processes may ( and do) change radically the chemical structure of the material and, as a consequence, make it lower softening, increase its rate of creep, render it more brittle and reduce its mechanical strength to a very marked degree. In a certain (somewhat oversimplified) way one can, therefore, classify the conditions which are necessary for outstanding polymer stability: