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Polymer Degradation

Topics Covered

Introduction

Accelerated Testing

Temperature Testing

Natural Weathering

Predicting Degradation Rates

Ranking of Materials

Summary

Introduction

Almost 50% of failures of engineering plastics result from environmental degradation. The increasing utilisation of plastics in more exacting applications and the pressure for increased life, without uneconomic over design, are imposing a requirement for improved characterisation of the performance of polymeric materials. The major challenge is to predict long term behaviour from short term laboratory or field exposures.

The most common mode of environmental degradation is caused by exposure of engineering polymers to chemicals, resulting in environmental induced stress cracking. The steps in the degradation process involve stress enhanced absorption and concentration of the chemical molecules at susceptible microstructural sites. Localised plasticisation then ensues, leading to crazing and subsequent crack development.

Failure may often be associated with exposure to secondary fluids, such as cleaning agents or lubricants, rather than the primary design environment, and with residual moulding stresses. Improved awareness of environment induced stress cracking can reduce the incidence of a failure but more data are required characterising the cracking resistance of plastics to a range of common fluids. More generally, there is a need to provide a methodology for life prediction involving testing and predictive models.

Accelerated Testing

It is essential to balance the need for accelerated testing with the inherent time dependence of the chemical and physical processes involved. The timescale for chemical ingress into the plastic or for leaching of mobile additives is important, and misleading results in ranking the aggressiveness of different chemicals, or the resistance of different materials to a specific chemical, are conceivable if chemical diffusivity is not considered.

Temperature Testing

The adoption of higher temperatures is commonly used for accelerated testing of the resistance of plastics to environment induced cracking, but there is an increasing trend towards simulating practical environments more realistically and deducing long term behaviour from short term measurements.

Natural Weathering

Natural weathering of plastics is also a significant cause of degradation. The interaction of UV radiation, oxygen, atmospheric pollutants, alternate wetting and drying cycles, and temperature variations can result in complex photomechanical reaction processes leading to a degradation of physical, mechanical and chemical properties. Failure may take the form of fracture as a consequence of impact, design or residual stresses, but more commonly loss of transparency, gloss or colour render the material unfit for purpose.

In conducting meaningful accelerated durability tests for weathering, whether for plastics, organic coatings or adhesives, the range of variables and their time and spatial variation pose intrinsic difficulties in devising standard tests of general applicability. When coupled with the complexity of the reaction processes in polymers, establishment of predictive models or damage functions is a formidable challenge. There is a prevalent view that there is no correlation between natural and artificial weathering. This view is expressed even in some of the standards. However, the term correlation is often used without clear definition.

Work at (National Physical Laboratory, UK) NPL has gone into evaluating the effectiveness of accelerated laboratory testing in predicting long term performance of plastics in natural weathering conditions, using a range of indices of degradation including microhardness, carbonyl index, colour and gloss.

Predicting Degradation Rates

Predicting degradation rates: The quantitative prediction of long term performance in natural conditions depends on the ability to predict the rate of degradation in natural weathering. It is insufficient to demonstrate that 2 months in a weatherometer with a specified weathering cycle is equivalent to 2 years, say, in natural conditions, for which data may be known as a reference point, unless there is a framework for predicting the extent of degradation at 15 years for example. The problem is illustrated in Figure 1 by the weathering performance of UPVC window profiles, using microhardness as an index of degradation. For this system and for many others, the rate of degradation changes with time in a complex way due to the development of a modified surface layer. The intrinsic dependence of the development of the degraded layer on exposure conditions means that it can be inherently difficult to establish rate equations which can be transferred meaningfully from artificial weathering tests to natural exposure.

Figure 1. Microhardness comparison for naturally and artificially weathered UPVC window profiles. 10GJm-2 is equivalent to 0.78 years of laboratory testing and 5.4 years of natural exposure in NE England. The accelerating factors were the increased temperature (65°C black standard) and continuous UV exposure for the laboratory tests.

Ranking of Materials

For ranking of materials it is necessary simply to demonstrate that the relative performance of the different materials, in terms of the extent of degradation, is consistent with that observed in natural conditions. However, work at NPL has shown that UPVC window-grade materials exhibiting a relatively high propensity to yellowing and to production of carbonyl degradation products can be more resistant to mechanical degradation and erosion of the surface. Hence, in ranking materials it is important to focus on the property of most practical relevance for the specific application, or to measure a range of properties.

Summary

The objective in laboratory testing is to establish a combination of artificial weathering parameters which can simulate the impact on material properties of natural weathering. It should not be expected that a specific set of artificial weathering parameters can be established that simulate temperature conditions, for example, for all materials, but for classes of material this would appear feasible. In this context, all artificial weathering tests would involve testing reference standards of the class of material in parallel. This would provide a basis for quality control on the test conditions and, if appropriate, natural exposure data for reference samples could be obtained, for relative ranking. In the latter context, there is a considerable virtue in collaborative programmes on an international basis so that a world wide ‘degradation map’ for classes of material could be established.

Considerable progress in artificial weathering testing in the laboratory is being made through adoption of sources of UV radiation capable of matching the solar spectrum, such as xenon arc and fluorescent tubes. Depth profiling of the material properties at NPL, using techniques such as nanoindentation for modulus and hardness measurement and FTIR with photoacoustic cell for chemical analysis, is revealing more details of the properties of the degraded surface layer and enabling quantification of the rate of material degradation as a function of time and proximity to the surface. Hence, the laboratory tools available now are increasingly more effective. There is considerable research to be done in relation to the adoption of dark periods in the exposure cycle and the role of pollutants and of stress. The complexity of weathering suggests that co­operative effort on an international basis will provide the most effective return on individual investment.

 

Primary author: A. Turnbull

Source: Materials World, vol. 3, pp. 182-83, 1995.

 

For more information on this source please visit The Institute of Materials

 

Date Added: Mar 12, 2001 | Updated: Jun 11, 2013
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