Photostabilization of Polymers : Priciples and Application

During the last two decades, the production of polymers and plastics has been increasing rapidly. In spite of developing new polymers and polymeric materials, only 40-60 are used commercially on a large scale. It has been estimated that half of the annual production of polymers is employed outdoors. Increasing the stability of polymers and plastics towards heat, light, atmospheric oxygen and other environmental agents and weathering conditions has always been a very important problem. The photochemical instability of most of polymers limits them to outdoor application, where they are photo­ degraded fast over periods ranging from months to a few years. To the despair of technologists and consumers alike, photodegrada­ tion and environmental ageing of polymers occur much faster than can be expected from knowledge collected in laboratories. In many cases, improved methods of preparation and purification of both monomers and polymers yield products of better quality and higher resistance to heat and light. However, without stabilization of polymers by applica­ tion of antioxidants (to decrease thermal oxidative degradation) and photostabilizers (to decrease photo-oxidative degradation) it would be impossible to employ polymers and plastics in everyday use.

1 Introduction to the Photo-oxidative Degradation Mechanisms of Polymers.- 1.1 Definitions of Photostability of Polymeric Materials.- 1.2 General Mechanism of Polymer Photo-oxidation.- 1.3 Photoinitiation of Polymer Degradation.- 1.4 Chain Propagation.- 1.5 Chain Branching.- 1.6 Photo-reactions of Carbonyl Groups.- 1.7 Hydrogen Abstraction.- 1.8 Photodecomposition of Hydroperoxide Groups.- 1.9 Formation of Acids and Peracids.- 1.10 Formation of Unsaturated Groups.- 1.11 Termination Reactions.- 1.12 Phenyl Ring Opening Photo-reactions.- 1.13 Kinetics of Photo-oxidative Degradation of Polymers.- 1.14 Role of Polymer Morphology in Photo-oxidation.- 1.15 Role of Polymer Manufacturing.- 1.16 Role of Metals or Metallic Compounds in Thermal and/or Photo-oxidative Degradation of Polymers.- 1.17 Role of Processing in Thermal Oxidation of Polymers.- 1.18 Mechanisms Involved in the Photolysis of the Thermally Oxidized Polyolefins During Processing.- 1.19 Physical Processes in the Photo-oxidation of Polymers.- 2 Introduction to the Oxidative and Photo- stabilization of Polymers.- 2.1 Antioxidants.- 2.2 Photostabilizers.- 2.3 Stabilizers in a Polymer Matrix.- 2.4 Distribution of Stabilizers in a Polymer Matrix.- 2.5 Quenchers.- 2.6 Singlet Oxygen Deactivators.- 2.7 Photostability of Additives.- 2.8 Synergism and Antagonism between Additives.- 3 Antioxidants.- 3.1 Hindered Phenols.- 3.2 Hydroxygalvinol/Galvinoxy Radical System.- 3.3 Phenolic Sulphides.- 3.4 Monosulphides.- 3.5 Thiophenols and Dithiobisphenols.- 3.6 Sulphoxides and Sulphones.- 3.7 Synergistic Effects between Hindered Phenols and Sulphur- Containing Compounds.- 3.8 Dixanthogens and Thiophosphoryl Disulphides.- 3.9 Metal Dithiolates as Antioxidants.- 3.10 4-Alkyl-2-mercaptothiazolines.- 3.11 Sulphur Dioxide Role in Antioxidizing Systems.- 3.12 Role of the Antioxidant Structure in Stabilization Processes.- 3.13 Oven Ageing Antioxidant Performance.- 3.14 Polyquinones as Antioxidants.- 3.15 Lignin Derivatives as Stabilizers.- 3.16 Organophosphorus Antioxidants.- 3.17 Pentaerythrityl Diphosphites.- 3.18 Organoboron Antioxidants.- 3.19 Macrocyclic Ligands.- 3.20 Aromatic Amine Antioxidants.- 3.21 Nitroso Compounds and Nitrones.- 3.22 Antioxidant Properties of Nitrosoamines.- 3.23 Antioxidant Properties of Antistatic Agents (Tensides).- 4 Pigments.- 4.1 Photostabilizing Properties of Pigments.- 4.2 Titanium Dioxide.- 4.3 Zinc Oxide.- 4.4 Metal Salts Coatings.- 4.5 Carbon Black.- 5 Photostabilizers.- 5.1 Alkyl-Substituted p-Hydroxybenzoates.- 5.2 Phenyl Esters of Benzoic Acid.- 5.3 Salicylanilides and Oxanilides.- 5.4 Diketones.- 5.5 Benzylidene Malonate.- 5.6 Esters of ?-Cyano-?-phenylcinnamic Acid.- 5.7 Hydroxybenzophenones.- 5.8 Chromanone and Tetralone Photostabilizers.- 5.9 Hydroxybenzotriazoles.- 5.10 Organic Metal Photostabilizers.- 5.11 Metal Stearates.- 5.12 Metal Salts of Organic Acids.- 5.13 Metal Oxime Chelates.- 5.14 Nickel(II) Mono-oxime (Hydrazone) Chelates.- 5.15 Metal Salicylate Chelates.- 5.16 Nickel(II) Bis(stilbene dithiolate) Chelates.- 5.17 Pyrazole Chelates.- 5.18 Nickel(II) Thiopicoline Anilide Chelate.- 5.19 Nickel(II) 2,2?-Thiobis(4-tert.-octylphenolato)-n-butylamine.- 5.20 Nickel(II) Bis(o-butyl-3,5-di-tert.-butyl-4-hydroxybenzyl)-phosphonate.- 5.21 Metal Dithiolates as Photostabilizers.- 5.22 Metal Chelates as Pro-oxidants and Photoinitiators.- 5.23 Dithiocarbamate Iron Complex Acting as Photoinitiator of Polymer Degradation.- 5.24 Metal Acetylacetonates.- 5.25 Metal Deactivators.- 5.26 Organotellurium Compounds as Photostabilizers.- 6 Hindered Amine Light Stabilizers (HALS).- 6.1 Mechanisms of HALS Reactions.- 6.2 Mechanisms of Nitroxy Radical Reactions.- 6.3 Transformations of Hindered Amines and Nitroxy Radicals During Processing of Polymers.- 6.4 Stabilizing Properties of HALS in Commercial Polymers.- 6.5 Synergistic/Antagonistic Effects of HALS with Other Additives.- 6.6 Novel HALS.- 7 Polymer-Bound Additives.- 7.1 Properties of Polymer-Bound Additives.- 7.2 Polymeric Antioxidants.- 7.3 Polymeric Photostabilizers.- 7.4 Hindered Amine (HALS) Monomers and Polymers.- 7.5 Polymers with Hindered N-oxy Group in the Main Chain.- 8 Industrial Aspects of Polymer Stabilization.- 8.1 Industrial Methods of Incorporating Additives.- 8.2 Toxicity of Photostabilizers.- 8.3 ‘World Index of Polyolefine Stabilizers’.- 8.4 Industrial Application of Antioxidants.- 8.5 Photostabilization of Low-Density Polyethylene for Agricultural Applications.- 8.6 Commercially Produced Polymeric HALS.- 8.7 Photostabilization of Polypropylene Tapes and Fibres.- 8.8 Photostabilization of Rubber-Modified Plastics.- 8.9 Photostabilization of Poly(vinyl chloride) by Surface Photografting.- 8.10 Photostabilization of Coatings.- 8.11 Additives in a Space Environment.- 9 Experimental Methods in Polymer Degradation and Stabilization.- 9.1 Preparation of a Test Specimen.- 9.2 Determination of the Additive Content after Polymer Photo-oxidative Degradation.- 9.3 Quantitative Determination of Antioxidants.- 9.4 Determination of Effectiveness of Antioxidants.- 9.5 Measurement of Evaporation of Additives from Polymer Samples.- 9.6 Absorption Spectroscopy.- 9.7 Chemiluminescence Measurements.- 9.8 Photoacoustic Spectroscopy.- 9.9 Electron Spin Resonance Spectroscopy.- 9.10 Electron Spectroscopy for Chemical Analysis (ESCA).- 9.11 Analytical Methods for the Determination of Hydroperoxide Groups.- 9.12 Studies of the Kinetics of Oxidation by Oxygen Uptake Measurements.- 9.13 Determination of the Quenching of Singlet Oxygen.- 9.14 Radioactive Labelling Techniques.- 9.15 Viscosity Measurements.- 9.16 Determination of Chain Scission.- 9.17 The Gloss Loss Test.- 9.18 Testing Methods of Thermo-oxidative Degradation and Stability of Polymers and Plastics.- 9.19 Mechanical Testing of Polymer (Plastic) Samples.- 9.20 Testing of Weathering in Polymers.- 9.21 Computer Modelling of Photo-oxidation and Photostabilization.- References.- Appendices.