Influence of testing conditions on the performance and durability of polymer stabilisers in thermal oxidation
Introduction
Technical and economic problems arising from negative influences of the environment on polymer durability have been studied since the commercial introduction of polymers. The properties of most polymers are subject to changes as a result of degradation, ultimately to the point where the material is no longer able to fulfil its expected function safely. The longevity of plastics, rubbers or coatings is of crucial importance. Commodity and engineering polymers degrade during all stages of their service life. The mode, extent and/or mechanism of degradation are strongly dependent on the intensity and duration of the physical and chemical stresses, to which the polymer is exposed, including environmental pollutants, heat, light etc., which may be experienced singly or, more often, concertedly.
Classification of the effects of single attacking stresses is rather simple. In contrast, concerted or cyclic attack of several influences on a polymer may result in a rather complicated rating of degradation and stabiliser efficiency due to synergistic or antagonistic effects which are often unpredictable. Stabilised polymer articles or engineering components are stressed by exposure to oxidizing atmospheres at various temperatures and by different weather factors (solar radiation, moisture, trace atmospheric pollutants). Moreover, mechanical stress or pressure (leading to fatigue, creep or abrasion) and contact with extracting fluids such as oils, cooling media or acid rain affect degradation.
Polymer lifetime is controlled by both oxidation and mechanical degradation, mostly proceeding independently [1]. The degree of material change is influenced by the intrinsic sensitivity of the polymer to individual effects, by inhomogeneities in the polymer structure or the presence of adventitious impurities, and by the (photo)chemistry of additives [2], [3], [4]. Each of the changes has a characteristic induction period (IP), during which there is gradual stabiliser consumption (Fig. 1). At the end of the IP there is rapid oxygen uptake and build-up of oxygenated structures in the polymer, both relatively difficult to monitor. The length of the IP and the slope of the post-IP curve, indicating the progress of degradation, are both influenced by the severity of the environmental attack and by the stabiliser efficiency and durability [3], [4], [5]. It follows that increasing severity of the testing methods in depleting stabilisers chemically or physically, together with acceleration of polymer degradation is reflected in shorter IP. Among accelerated tests only oven tests allow direct monitoring of the principal failure criteria providing information on changes in material properties.
The search for materials having better properties, including durability in harsh environments, has been reflected in the development both of new materials and of more effective additives. The increasing resistance of stabilised polymeric materials to degradation requires the use of accelerated testing methods to obtain reproducible information in an acceptably short time, on resistance to processing degradation, thermal oxidation (long-term heat ageing, LTHA) or weathering under the expected exposure conditions. Reliable accelerated tests allowing lifetime prediction are essential to tailor material properties and additive formulations to fit commercial requirements [7], [8] It is rather surprising that aspects of the influence of the testing conditions and of the polymer matrix on stabiliser durability and performance have been rather neglected in development of accelerated tests, particularly where they are expected to have a value for correlation with natural ageing and correct prediction of the lifetime of stabilised plastics, rubbers or coatings.
Exploitation of the knowledge of the mechanisms of action of stabilisers, the chemistry of their transformations (sacrificial and depleting consumption), and the influence of radiation intensity or atmospheric pollutants is necessary. Useful data have been reported for phenolic antioxidants [3], aromatic and unsaturated heterocyclic amines [3], [9], thiosynergists [3], organic phosphites [10], UV absorbers [4], hindered amine stabilisers (HAS) [9], [11], antacids [12], heat stabilisers [13], pigments and dyes [4], [14]. The chemical transformations leading to consumption of stabilisers under foreseen exposure conditions of stabilised polymers are outlined. Activity limitations of some stabilisers due to ceiling temperatures (e.g., in HAS [15] or high-intensity irradiation [4] are well documented, but seem to be neglected in commercial testing.
Various accelerated tests have been developed to monitor ageing of plastics [6], [7], [8], rubbers [8], [16], [17] and coatings [18]. Some have been recommended as standards [5], [7], [8], [16] and their severity is reflected in both accelerated degradation of polymers and increased sacrificial consumption or depletion of stabilisers.
Development of polymer failure with exposure time is typically monitored using criteria relevant to the intended application. Conventional methods used to determine changes in material properties of stabilised polymers after processing, LTHA and weathering include mechanical properties (elongation at break, stress at break, tensile impact strength, Charpy impact, time to embrittlement); physical properties (colour (yellowness index, YI), haze, loss of gloss, craze and crack formation, weight loss, surface roughness, electrical properties, changes in morphology etc) and chemical properties [changes in molecular weight (MW, Mn, MZ), crosslinking, build-up of carbonyl and hydroxyl groups, hydroperoxides, changes in unsaturation CC, volatile fragments, formation of radical intermediates, loss of stabilizers]. A complex view on property changes can be reached by additional more specialised methods providing information on super-molecular changes such as chemical recrystallization due to annealing [19], [20], [21], changes in free volumes in semicrystalline polymers [22], [23], degradation gradients [24], [26] or stabiliser migration [25], [26], particularly relevant to preferential surface protection of polymers (mainly important for antiozonants and UV absorbers).
Determination of stabiliser consumption during the lifetime of polymers needs appropriate analytical methods [27]. The analyses are rather complicated, in rubbers in particular, and mostly limited to monitoring the consumption of originally added stabilisers. Analyses of stabiliser transformation products require the use of model compounds. Hence, determination of stabilisers and/or their principal transformation products is not part of routine testing. However, the data are often useful for assessment of negative influences of testing severity on the fate of stabilisers.
Interpretation of experimental data obtained in polymer degradation, in particular changes in physical and mechanical properties, including super-molecular changes, is the basis of information used for explaining why data on stabiliser efficiency obtained in molten polymers may be invalid, and helps to avoid misuse of over-accelerated test methods. Literature sources believed to be reliable, combined with practical experience are exploited in a critical overview on assessment of thermal methods allowing development of tailor-made and durable stabiliser recipes optimally adjusted to a particular polymer system. For stabilisers mentioned in the text, see Appendix.
Section snippets
Processing stability
Tests of the melt stability of stabilised polyolefins are used for assessment of stabiliser performance under the conditions of limited oxygen access and high shear loading typical of processing. They are mostly performed at 200–280 °C by multiple extrusion in devices matched as closely as possible to the production conditions, i.e. without significant acceleration [5], [28]. Factors generally used to characterise stabiliser performance in protecting polymers from processing degradation include
Long-term heat ageing, thermal oxidation
Assessment of the efficiency of hindered phenols, thiosynergists and hindered amine stabilisers (HAS) in protecting polyolefins, rubbers and various engineering polymers against thermal oxidation requires accelerated testing methods to get results in commercially useful times. Exposure of polymer articles to heat in air causes irreversible chemical changes and ultimately results in loss of mechanical strength, decrease in mechanical toughness and/or onset of embrittlement. Oven ageing allows
Conclusions
To get appropriate information on the efficiency of stabilisers in protecting polymers against thermal oxidation by selected failure criteria, testing under service conditions is essential. However, development and testing of modern highly-effective stabilisers under these conditions needs excessively long testing times. Hence, accelerated tests are necessary to get the desired information in acceptable times. Selection of the tests depends on the character of the required information on the
Acknowledgements
Financial support by grants CZE 01/28, KONTAKT ME-543 and KONTAKT ME-558 from the Ministry of Education, Youth and Sports of the Czech Republic, Grant No. 203/02/1243 from the Grant Agency of the Czech Republic and Grant No. S4050009, Program for the support of task research and development of Academy of Sciences of the Czech Republic are gratefully appreciated. The authors thank Mrs D. Dundrová for technical cooperation.
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