Service Life Prediction of Organic Coatings. A Systems Approach

Service Life Prediction of Organic Coatings - A Systemic Approach (Hardcover)

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Thus few units will fail or degrade appreciably in a test of practical length at normal use conditions. Thus the requirements for rapid product development and higher reliability have increased the need for accelerated testing of materials, components, and systems.

This paper describes methods for analyzing and planning accelerated life tests with an application in the area of microelectronics. In this application, the purpose of the accelerated life test was to study the dominant failure mechanism of an integrated circuit. It was believed that a first-order chemical reaction would provide an adequate description of this failure mechanism. Similar methods can be used to do laboratory evaluations of the life of products like paints and coatings. We have also included a section describing methods for using laboratory tests to predict life in the field with variability in use rates as well a both spatial and temporal variability in environmental conditions.

An ultra-accelerated material exposure test facility that uses highly concentrated natural sunlight has been developed at the National Renewable Energy Laboratory NREL. By adequately controlling sample temperatures and demonstrating that reciprocity relationships are obeyed i. In terms of natural sunlight exposure, one year's equivalent of representative weathering can be accumulated in just days. Failure comes in a variety of different forms, depending on the system and the environment to which it is exposed. However, failure has attributes that are common to all forms of failure.

The attributes of failure are: 1 chemical and physical phenomena initiate small flaws in materials, 2 flaws grow under certain environmental conditions, 3 flaw growth rates can be used to estimate time of failure, 4 failure occurs when flaws grow to a critical size, or damage accumulates at a high enough level to cause failure, or the flaw can be observed by the unaided eye and considered a failure, 5 the number of observed failures in a population is not a continuous function of time, 6 the cumulative number of failures in a population is a continuous function of time, 7 the magnitude of cumulative failure in a population is influenced by the environment, and 8 cumulative failure data can be transformed into a linear form and the data extrapolated outside of its range within reason to estimate population failure level.

These attributes can be used to estimate long-term failure times and associated levels of failure using reliability statistics. Failure levels for aerosol containers can be estimated to within an order of magnitude of actual failure levels.

Continuous UVB radiation was used throughout. The Arrhenius-Weibull model adequately described the influence of weathering temperature on color change. The Weibull distribution may be less suitable as a lifetime distribution for the gloss data obtained in this work. Durability and weathering studies of coatings involve accurate monitoring and meaningful analysis of the coating's appearance. The currently available techniques that monitor and analyze coatings are oftentimes deficient in accurate characterization of coating appearance.

Furthermore, visual attributes that comprise the surface of interest are typically assessed by humans, making objective assessments laborious and difficult. Implementation of digital imaging in various scientific applications has been proven accurate and productive.

1st Edition

As regards to service life prediction SLP of organic coatings, meaningful visualization and precise quantitative surface measurement would undoubtedly further the effort to understand the complex kinetics of a material's chemical and physical property degradation, as well as mechanical performance changes. The solution of a problem in photolytic degradation, viz. Stochastic methods are used and contrasted with the classical methods of chemical kinetics. The former seem conceptually simpler, technically easier and they yield solutions in certain cases which are not available otherwise.

Most of the world activity on research, development and control of polymer durability is still based on empirical techniques developed in the early ages of polymer uses. Those techniques should be critically analysed considering the advances of the fundamental understanding of these complex phenomena. In the field of coating ageing, empirism is prevalent in the mode of application of environmental stresses in laboratory conditions and in the degradation criteria used. A more rational approach is described which is based on the recognition of the chemical evolution mechanisms.

Applications of the physico-chemical stresses and definition of the degradation criteria should be consistent with the identified mechanisms. Aliphatic and cycloaliphatic hydroxylated polyesters crosslinked with substituted melamins or condensed isocyanates are presented as examples of the "mechanistic approach" ; the identification of the two chemical routes which account for the mechanical detriment is based on FTIR and micro-FTIR spectrophotometric analysis, the later technique being used to observe specifically the chemical evolution of the elementary layers of the clear-coat and base-coat.

Techniques for obtaining chemical composition and component distribution depth profiles for automotive coating systems have been identified, developed and applied. Primary reasons for this work were to determine general system composition, as well as component or additive distribution, as a function of locus i.

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Correlations between changes in the macroscopic performance and in the microscopic properties experienced by weathered materials can be used to identify degradation mechanisms and suggest new formulations that exhibit improved durability. Of particular interest is the relationship between optical reflectance and interfacial composition of metallized polymer solar mirrors. A sample test matrix of silvered polymethyl methacrylate PMMA mirrors was prepared to systematically study changes in interfacial composition with accelerated exposure and how the chemical changes influence the reflectance of the mirrors.

A holistic perspective of coatings technology

Most polymers must be stabilized against the impact of the environment. Antioxidants and ultraviolet UV stabilizers are of vital importance to protect polymeric materials from deterioration. To be effective UV absorbers, these compounds must also have the capability to dissipate the energy without causing damage. Long range and efficient protection of organic polymeric materials from the environment requires carefully designed and stabilizing systems including polymerbound UV stabilizers. Dynamic mechanical analysis DMA and chemical stress relaxation determinations indicate that automotive topcoats undergo significant changes in glass transition temperature and crosslink density during exposure to UV light and moisture in weatherometers designed to mimic but accelerate the effects of outdoor exposure.

The resulting changes in physical properties are believed to contribute to failure by clearcoat cracking or by delamination at various interfaces between paint layers in the auto coating system. Topcoats are frequently still glossy when failure by some other mode occurs. Cracking and delamination occur abruptly whereas gloss loss, observed with older technologies, occurred gradually. The benefits of using service life prediction SLP methods are well documented, but application to auto coating durability will require consideration of abrupt versus gradual failure modes.

Property change data may be important for understanding failure mechanisms, but more work is needed to effectively integrate property change data with SLP methodology. The sources of stress in complete automotive paint systems have been identified and measured as a function of weathering.

The main sources of stress are thermal expansion coefficient mismatch, humidity expansion mismatch, and densification of the clearcoat. Stresses generally increase during weathering due to a slow densification of the clearcoat and increasing water absorption and desorption stresses. Finite element analysis FEA was used to compute the stress distribution in full paint systems.

Stresses are typically in-plane and highest in the primer and clearcoat. Stresses approaching those required to propagate cracks can be attained in weathered paint systems. The presence of flaws, either cracks or incipient delaminations, will lead to large stress concentrations that can give rise to peeling forces not present in coatings without cracks. An important factor in coated metal systems is the polymer-metal interface. This paper focuses on this interface. It is discussed how it controls the overall corrosion rate, and how it can be engineered to improve the service life of the entire coated system.

Examples are given of coated cold-rolled steel, galvanized steel and Galvalume, where the interface was modified and the performance of the system increased. We have studied two methods for interface modification, viz. EIS and accelerated corrosion tests measured the corrosion rates of the coated systems.

The main conclusions of this work are that pretreatments of metals based on plasma or silane treatments could be developed that would improve the service life of the coated metal systems. In large part, paint and coatings determine the esthetic appeal of a vehicle. Long-term customer satisfaction with paint is determined by how well the paint protects the body and by how well the paint maintains its overall appearance. The average age of vehicles has been steadily increasing. Customers expect reasonable maintenance of paint appearance for 10 years or longer.

Catastrophic changes in appearance peeling, cracking, etc. A critical goal of weatherability testing is to be able to predict the risk of catastrophic paint failure in service. Paint performance is a function of the intrinsic capability of the particular paint system and the environment into which it is placed.

It is important to note that the in-service time-to-failure has to be described by a distribution function. To predict this distribution function, it is necessary to determine the distribution functions for both the environment harshness and the paint system capability.

This paper describes how to develop paint failure models that are capable of predicting the distribution of in-service failure rates. By using mechanistic failure models, test time can be greatly reduced. The development of the computer models necessary to support the prediction of service life of complicated products is proceeding rapidly. Not only is the available computer power growing continuously, but also our understanding of the behavior of engineering materials on a nano- and micro-scale has increased significantly.

Use of Uniformly Distributed Concentrated Sunlight for Highly ... - NREL

However, the robustness of prediction is critically affected by the quality of data used in modeling. Many of these data are quite complex and depend on a large number of independent variables. Data must be drawn from many different sources, often incompatible in terms of storage formats, nomenclature, and ancillary information. Prediction of the service life of organic coatings is just now becoming an area of great interest and in the future will rely considerably on modeling. This paper addresses issues related to capturing and maintaining high quality materials properties data and databases necessary to support service life prediction of organic coatings, specifically assessing data quality, development of formats for computerizing these data, and the role of formal and informal standards in accepting these formats.

Emphasis is placed on sharing and exchanging data across scientific disciplines, a major demand of service life prediction. This paper addresses materials data needs from the users' perspective, and focuses on a the types and characteristics of information most often needed, notably numeric data, b the uses of the information, and c the delivery mechanisms available. Experience has shown that most end-user searchers for material data want very specific numeric data, which are relatively complex in requiring units, may involve many orders of magnitude, and have many delimiting parameters that define their range of usefulness and applicability.

Useful databases must address these characteristics of the data while providing search software that provides the flexibility and versatility to enable the searcher to get specific, well-documented answers. The breadth of types of delivery systems for material property data are described, and the advantages and limitations of each type are provided.

In this paper, we will address materials data needs from the users' perspective. This will not deal with the. Database tools have been developed for managing and tracking of automotive OEM paint system resistance to environmental acid etch. The flat file environment includes use of line drawings e. The automobiles we currently produce and use globally are coated with protective and decorative layers.

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