Today we have become engulfed with products and materials made from materials extensively modified from their original, natural state. These modifications are often done in chemically irreversible ways. We want the products to be durable over their useful life but we also want them to be returned to nature when we no longer need them. We hope that their disposal will not cause pollution. We need our water to be pure, our air to be safe to breathe, and our soil to be uncontaminated.
Conflicts abound. If we are to resolve them and continue to use synthetic materials responsibly, we must plan carefully and gain a complete understanding of how materials will perform and degrade. In particular we must be able to understand how materials weather, what the by-products of weathering are and how materials can be transformed into non-polluting entities either through recycling or natural disposal. Terms such as "life cycle assessment", "recyclable", "biodegradable" and "lifetime warranty" slip easily off our tongues. We need to bring weathering testing to the point at which reliable testing and investigative studies can enable us to use these and related terms with complete confidence.
In spite of the efforts of research groups, standardization organizations and industry, there is much to be done to bring weathering testing to the level that will allow the results to predict the life of materials. There must be a willingness among the involved parties to cooperate and a comprehensive body of information to support their efforts.
This book is a contribution to the information base to assist the scientific efforts aimed at improving the knowledge of weathering.
One aim of this book is to provide a critical overview of methods and findings based on experimental work. Another is to create an awareness of the effect of the combined action of all the weather variables on materials under study.
The introductory chapter outlines experimental design techniques and equipment selection and emphasizes the importance of selecting the basic parameters of weathering including:
temperature of the specimens
rainfall and condensed moisture
The book is structured to illustrate the importance of these parameters on weathering studies. Throughout the book, the authors attempt to show that weathering is not only dependent on UV radiation but that the overall effect depends on the interplay of all parameters which create a unique sequence of events that will change if the parameters are changed. The lack of correlation between laboratory and outdoor exposure is frequently caused by combinations of factors among which the improper selection of laboratory conditions is prime.
After the introduction we discuss the choices available for outdoor weather testing. This relates laboratory tests to tests outdoors so that there may be correlation with natural conditions. The importance of precise control of both UV spectral intensity, temperature and heat flow is demonstrated in Boxhammer's careful use of available equipment and by studies done on automotive components.
The recent availability of the CIRA filters and the continued use of borosilicate filters now permits accurate duplication of solar radiation. The chapter by Summers and Rabinovitch shows how radiation wavelength impacts the performance of several polymers. The manufacturers of weathering equipment can perfectly simulate the solar spectrum. Researchers now must take advantage of these developments. We show that failure to duplicate the solar spectrum invalidates the experiment. The failure is caused by energy input, temperature, moisture, and radiative effects. These parameters should not differ in the experiment from that of natural exposure.
We compare the two most common artificial light sources - xenon arc and fluorescent lamps. The automotive, textile, polymer and stabilizer industries use xenon arc which gives the full spectrum of solar radiation (UV, visible, and near infrared). The use of fluorescent lamps, which lack the spectral range of the xenon arc, should be discouraged except in special cases where the known mechanisms for degradation are triggered only by radiation between 295 nm to 350 nm. Several industries report problems stemming from studies done with fluorescent lamps which fail to correlate with actual outdoor exposure.
Water spray during weathering studies has often been neglected. The reported work on co-polyester sheeting shows how complex material changes can be in the presence of water. More work is urgently needed to determine how humidity and condensation influence material degradation. Two contributions from the Edison Welding Institute have been included to demonstrate the effect of infrared energy and how different materials absorb this energy differently. In particular, the inclusion of pigments complicates infrared absorption. The chapter by Hardcastle shows how an evaluation of performance requirements helps to define a method of predicting the maximum allowable service temperature of vinyls based on measurements of their solar reflectance.
Products in service operate under mechanical stress due both to residual stresses developed during the forming process and to external stress in use. It has long been recognized that stress affects weathering but little has been done to evaluate the effect. Two chapters by White et al. propose methods of evaluating the effects of stress in weathering studies. These effects are complex since the initial stress distribution changes during exposure and this requires a knowledge of the kinetics of these changes. A similar situation exists with respect to the effects of pollutants. We know they influence weathering but there are few studies that assess their influence. Paterna et al. examine gas fading of automotive components in the presence of nitrous oxides. More elaborate techniques must be developed to evaluate the combined effects of UV radiation, moisture, temperature and pollutants on products to simulate outdoor applications. It is unrealistic to study these influencing factors independently.
Two studies on the effects of high energy radiation have been included to demonstrate well defined projects which evaluated material failures and determined the activation energies of the degradation process for many materials, explained why degradation occurred in industrial sterilization, and determined how such degradation might be prevented.
Assessment of automotive clearcoats and nanocomposites show that current test methods are sufficiently accurate, sensitive and suitable to detect degradation at an early stage of exposure. This is another area where more investigative work is needed. The benefit of this approach lies in gaining information early in the product development process using the equivalent of natural conditions without depending on the use of high energy radiation, often employed in accelerated testing, which causes degradation mechanisms which would not normally occur.
Several contributors emphasize other complexities which must be dealt with in weathering studies. The materials themselves are complex. Many contain additives which interact with the host, the substrates and one another in a weathering situation. Conclusions may err if they are based on an inaccurate knowledge of the real composition of the material under study. Even the manufacturer may be unaware of the true composition as composite additives may have proprietary compositions which are not disclosed. Many fundamental studies are needed to investigate the interactions of multi-component systems and to unravel the effects of processing aids which may be added without knowledge of their effects or interactions. Such practices may lead to unexpected and possibly, catastrophic, failures which would remain undetected in routine research and quality control operations.
The stabilizer manufacturers have, as an industry, made a significant contribution to weathering testing methods. There are several chapters from these sources. They show that their reports to their customers are meticulous in relating the results of evaluations to the conditions of the test. Their approach is conservative in selecting both equipment and test conditions. The tests are expensive. They must relate to the real conditions of use and results should be comparable to those of prior tests.
The book concludes with an example of the type of ground work and planning that is required before routine analysis begins. Using work on automotive clearcoats, we demonstrate how information must be analyzed and categorized to provide a rationale for testing, defining performance requirements, exposure conditions, mechanisms of degradation and how best to observe and measure the changes in specimens. Information gleaned from field performance is used to determine the appropriate laboratory simulations. If this preparatory work is not done the subsequent testing efforts are unlikely to yield useful data and be of little use in predicting future product performance.
One final comment. Manufacturers must operate to meet economic goals. Industry as a whole is becoming increasingly competitive and is continually seeking ways to rationalize production methods to improve economics. Materials from different industries compete for the same markets. Durability has become one of the most important characteristics. The product is either made from an inherently durable material or it receives an external coating which gives the required durability. The first approach is more consistent with recycling processes which generally have difficulty in dealing with multi-component mixtures. As the understanding of weathering increases we may learn how to more frequently select a durable substrate which will not require the complication and cost (initial and recycling) of a surface coating. The economic answer would seem to lie in making the investment in weathering research to avoid the costs of material replacement and material failures.
• Basic Parameters in Weathering Studies
• Choices in the Design of Outdoor Weathering Tests
• A Comparison of New and Established Accelerated Weathering Devices in Aging Studies of Polymeric Materials at Elevated Irradiance and Temperature
• Current Status of Light and Weather Fastness Standards—New Equipment Technologies, Operating Procedures and Application of Standard Reference Materials
• Weatherability of Vinyl and Other Plastics
• Aging Conditions' Effect on UV Durability
• Molecular Weight Loss and Chemical Changes in Copolyester Sheeting with Outdoor Exposure
• Fourier Transform Infrared Micro Spectroscopy: Mapping Studies of Weather PVC Capstock Type Formulations
II: Outdoor Weathering in Pennsylvania
• Effects of Water Spray and Irradiance Level on Changes in Copolyester Sheeting with Xenon Arc Exposure
• Hot Water Resistance of Glass Fiber Reinforced Thermoplastics
Surface Temperatures and Temperature Measurement Techniques on the Level of Exposed Samples during Irradiation/Weathering in Equipment
• Infrared Welding of Thermoplastics: Characterization of Transmission Behavior of Eleven Thermoplastics
• Infrared Welding of Thermoplastics
• Colored Pigments and Carbon Black Levels on Transmission of Infrared Radiation
• Predicting Maximum Field Service Temperatures from Solar Reflectance Measurements of Vinyl
• Residual Stress Distribution Modification Caused by Weathering
• Residual Stress Development in Marine Coatings under Simulated Service Conditions
• Balancing the Color and Physical Property Retention of Polyolefins Through the Use of High Performance Stabilizer Systems
• Activation Energies of Polymer Degradation
• Failure Progression and Mechanisms of Irradiated Polypropylenes and Other Medical Polymers
• Chemical Assessment of Automotive Clearcoat Weathering
• Effect of Aging on Mineral-Filled Nanocomposites
• The Influence of Degraded, Recycled PP on Incompatible Blends
• Interactions of Hindered Amine Stabilizers in Acidic and Alkaline Environments
• Interactions of Pesticides and Stabilizers in PE Films for Agricultural Use
• The Influence of Co-Additive Interactions on Stabilizer Performance
• New High Performance Light Stabilizer Systems for Molded-in Color TPOs: An Update
• Stabilization of Polyolefins by Photoreactive Light Stabilizers
• Effect of Stabilizer on Photo-Degradation Depth Profile
• New Light Stabilizer for Coextruded Polycarbonate Sheet
• Ultraviolet Light Resistance of Vinyl Miniblinds
• Reaction Products Formed by Lead in Air
• Case Studies of Inadvertent Interactions between Polymers and Devices in Field Applications
• Automotive Clear Coats