文档介绍：Chapter 1
Failure Mechanisms
Plastics Failure Due to Oxidative Degradation
in Processing and Service
Myer Ezrin, Amanda Zepke, John Helwig, Gary Lavigne and Mark Dudley
University of Connecticut, Institute of Materials Science, Storrs, CT 06269-3136, USA
INTRODUCTION
People cannot live without oxygen and water. But these are deadly enemies of polymers,
both in processing of plastics formulations and in service. Water is a problem mainly for
condensation polymers which degrade by hydrolysis. In this paper the focus is on oxidative
degradation.
Oxygen degrades polymers to lower molecular weight (MW) by reacting with polymer
free radicals to form peroxy free radicals (ROO•) and hydroperoxides (ROOH). Free radi-
cals have an unshared electron and react in any way they can to restore the atom or molecule
to a balanced structure. Often that leads to chain scission. As MW goes down most polymer
properties suffer. As little as 5-10% reduction in MW may cause failure. Avoiding contact
with oxygen and using an antioxidant (AO) as a free radical scavenger are means of pre-
venting degradation.
The high temperature required to process plastics is the major cause of degradation in
injection molding, extrusion, blow molding, etc. High temperature is needed to fuse poly-
mers and to reduce melt viscosity to a level that the machines can handle. Mechanical shear
of the melt and the presence of oxygen, even in small amounts, are major factors in degrada-
tion due to processing. The chain carbon atoms attached to a branch, such as methyl group
(CH3), tend to split off a hydrogen atom, creating a free radical at a tertiary carbon atom.
CH2 CH(CH3) CH2 C(CH3) + H
2 Plastic Failure Analysis and Prevention
Very little oxygen is needed to react with free radicals during processing. Polymer sup-
pliers usually have very little AO in the resin as sold to processors. Unless additional AO is
added, polymer is likely to degrade in process. Polyolefins, whic