Polymer Degradation and Stabilization

The development of polymers as an important class of material was inhibited at the first by the premature failure of these versatile compounds in many applications. The deterioration of important properties of both natural and synthetic polymers is the result of irreversible changes in composition and structure of polymers molecules. As a result of these reactions, mechanical, electrical and/or aesthetic properties are degraded beyond acceptable limits. It is now generally recognized that stabilization against degradation is necessary if the useful life of polymers is to be extended sufficiently to meet design requirements for long-term applications. Polymers degrade by a wide variety of mechanisms, several of which affect all polymers through to varying degree. This monograph will concentrate on those degradation mechanisms which result from reactions of polymers with oxygen in its various forms and which are accelerated by heat and/or radiation. Those stabilization mechanisms are discussed which are based on an understanding of degradation reaction mechanisms that are reasonably well established. The stabilization of polymers is still undergoing a transition from an art to a science as mechanisms of degradation become more fully developed. A scientific approach to stabilization can only be approached when there is an understanding of the reactions that lead to degradation. Stabilization against biodegradation and burning will not be discussed since there is not a clear understanding of how polymers degrade under these conditions.

The development of polymers as an important class of material was inhibited at the first by the premature failure of these versatile compounds in many applications. Polymers degrade by a wide variety of mechanisms, several of which affect all polymers through to varying degree.

A) Introduction.- References.- B) Polymer Degradation.- I) Thermal Degradation in the Absence of Oxygen.- 1) Pyrolysis Mechanisms.- 2) Pyrolsis Reactions of Selected Polymers.- a) Pyrolysis of Polyvinyl Chloride.- b) Pyrolysis of Polyoxymethylene.- c) Pyrolysis of Polypropylene.- d) Pyrolysis of Copolymers and Polymer Blend.- II) Oxidative Degradation An Historical Introduction.- 1) Thermal Oxidation.- 2) Photooxidation.- a) Outdoor Weathering.- b) Mechanisms for Photooxidation.- 3) Miscellaneous Degradation Reactions.- III) Recent Developments in Oxidative Degradation.- 1) Photooxidation of Polyethylene.- 2) Photooxidation of Polypropylene.- 3) Photooxidation of Blends of Polyethylene and Polypropylene.- 4) Thermal and Photooxidation of Polystyrene.- 5) Degradation of Bisphenol-A-Polycarbonate.- 6) Thermal and Photooxidation of Polyacetals.- IV) Degradation by Hydrolysis.- References.- C) Stabilization Against Non-oxidative Thermal Degradation.- I) Stabilization by Structural Modification.- II) Stabilization by Copolymerization.- III) Stabilization by Crosslinking.- IV) Stabilization with Additives.- References.- D) Stabilization Against Thermal Oxidation.- I) Hydrocarbon Polymers.- 1) Short-term and Long-term Antioxidants.- 2) Chain-breaking Antioxidants.- a) Free Radical Traps.- b) Labile Hydrogen Donors.- 3) Preventative Antioxidants.- a) Hydroperoxide Decomposers.- b) Metal Deactivators.- 4) Antioxidant Combinations.- a) Additive Effects.- b) Antagonistic Effects.- c) Synergism.- 5) Non-migrating Antioxidants.- a) Carbon Black as a Thermal Antioxidant.- b) Bound or Grafted Antioxidants.- 6) Stabilization by Structure Modification.- 7) Stabilization Against Burning.- II) Non-hydrocarbon Polymers.- References.- E) Stabilization Against Degradation by Radiation.- I) Stabilization Against Ultraviolet-Induced Degradation.- 1) Light Screens.- 2) Ultraviolet Absorbers.- 3) Radical Traps.- 4) Quenchers.- 5) Stabilizer Combinations.- 6) Stabilization by Polymer Modification.- II) Stabilization Against Ionizing Radiation.- References.- F) Stabilization Against Degradation by Ozone.- I) Stabilization by Waxes.- II) Stabilization by Antiozonants.- III) Stabilization by Structure Modification.- References.- G) Test Procedures.- I) Methodology.- II) Properties Used to Evaluate Degradation.- III) Test for Thermal Degradation.- 1) Methods for Measuring Nonoxidative Thermal Oxidation.- 2) Test Methods for Measuring Thermal Oxidation.- a) Test for Measuring Stability During Processing.- b) Tests for Measuring Long-term Stability.- IV) Weatherability Tests.- 1) Outdoor Weatherability Tests.- 2) Indoor Weatherability Tests.- V) Miscellaneous Test Procedures.- References.- H) Future Trends.