The automotive industry usually validates material lifetimes by simulating ageing in climatic chambers. This process can take several months and is expensive, and time to market is a key issue in the auto industry.
The durability of polymeric materials used as body parts in the car industries is currently assessed either in devices like Weather-Ometer (Wom), which simulates environmental physical and chemical stresses in non-accelerated conditions, or in devices like Sepap 12-24, which offer a medium level of acceleration. In the latter case, the acceleration factor between the lifetimes in accelerated and real use conditions is deduced through a mechanistic approach. In this approach, which was recently approved by the ISO/TC61-SC2N committee, the prediction of the lifetime of polymeric materials weathering outdoors is based on the analysis at the molecular scale of the complex chemistry that controls physical degradation. Renault currently uses these two approaches in its ageing tests specifications, and these tests are correlated with a two-year natural ageing test performed in Bandol, Southern France (fig 1).
The time taken by validation tests has become an obstacle to innovation in areas like polymer material formulation. Renault has followed two distinct approaches to shorten material validation durations:
- The development of an “ultra-accelerated” photo-ageing device, whose exposure conditions are harsher but also four/five times faster than current devices such as Wom or Sepap 12-24.
- The mathematical and numerical modelling of the combination of chemical and physical phenomena which lead to the alteration of their appearance properties; in this perspective, the complex photothermal oxidation process has been divided into a series of elementary chemical and physical mechanisms in order to perform simulations in very different ageing conditions.
A Renault-supported team has been considering such an approach for a long time in the case of the thermal oxidation of unstabilised isotactic polypropylene (iPP) and is currently applying it to the iPP photothermal ageing in the framework of PhD researches started in December 2010 in association with Ensam Paristech. The team also intends to extend this model to stabilised iPPs in a near future. It is claimed that this tool could be used to improve the reliability of weathering test programmes.
Strategy in building the kinetic model
The strategy of building a full thermal-photo-oxidation kinetic model of stabilised blends of iPP and ethylene-propylene copolymers fillers starts with the simplest model. Its complexity gradually increased according to two axes: exposure conditions and formulation (fig 2). Parameters must be determined step-by-step owing to their multiplicity.
Kinetic modelling of photo-thermal oxidative ageing
In the kinetic modelling approach, the approach is to attempt to provide a relevant description of the system changes at the chemical scale. Basically, oxidation spreads by pull-out of the hydrogen atoms linked to carbon atoms ends by bimolecular coupling of different radicals, leading to inactive products in a short-chain radical mechanism.
Given the complexity of the phenomena involved, this purpose seeks to identify the main oxidation path. This is closely involved with the critical role of hydroperoxides which are at once the main propagation products and the species responsible of the oxidation initiation by either thermolysis (in pure thermal oxidation) or photolysis (in photooxidation). The sequence of chemical reactions is schematised in figure 3 in the general case of iPP photothermal oxidation.
This closed-loop character enables the accurate description of the autoaccelerated behaviour of oxidation; its monitoring is based on carbonyl species as secondary oxidation products (see fig 4). Two oxidation indicators are commonly defined: the induction time, which applies to the period where oxidation results in a negligible accumulation of oxidation products; and the oxidation rates (velocity) determined from the slope of the dramatic oxidation spreading. Induction time and oxidation speed values are characteristic of the different phases of material oxidation.
The induction time is characteristic of the initiation steps and can be noticeably lengthened thanks to stabilisers that protect the polymer against the deleterious effects of heat and UV-light. But when the quantity of stabilisers becomes insufficient, there is a short hydroperoxides accumulation period, immediately followed by the auto-catalytic hydroperoxides decomposition which generates carbonyl products. In contrast, the oxidation rate results globally from the competition between propagation and termination of peroxy radicals. Similarly, the reactions of stabilisers chemical consumption and their inhibiting or retarding role on the oxidation kinetics could be introduced into the model. The effect of temperature on oxidation kinetics is introduced by the Arrhenius laws which govern the kinetic rate constants.
If kinetic modelling is applied to photo-thermal-oxidation it will be necessary to differentiate ketones from the other carbonyl groups and to take into account the strong absorptivity of photo-reactive groups. The intensity and polychromatic character of light radiation as well as daily and seasonal variations of weathering conditions must also be incorporated. A model adapted to thick material would also have to take into account different physical phenomena including:
- depth of light penetration and attenuation into the polymeric material;
- oxygen diffusion from the surface to the core of the sample;
- migration of stabilisers from the core to the surface.
PhD research – main results
The existing thermal oxidation kinetic model has been improved and can now be used with any type of PP matrix. It can be considered reliable in temperatures ranging from ambiant to 140°C.
In order to extend the study to photothermal oxidation, the effects of the intensity and the polychromatic character of UV-light has been defined by calculating the energy that is absorbed by photosensitive species (namely hydroperoxides and ketones) and effectively initiating degradation (fig 5).
This calculation has been performed for each chromophore from the overlap of its absorptivity and the emission spectrum of the light source. The full procedure for calculating the resulting criterion, denoted “spectral overlap integral J” and accounting for the amount of light which can be absorbed by chromophores, is provided elsewhere.
This is to be used to define the spectral distribution of absorbed photons.
From the kinetic modelling approach, it has been possible to draw an abacus (fig 6) describing the competitive effects of UV-light and temperature on the iPP oxidation for different testing methods. Two types of iso-curves are depicted:
– an iso-damage curve, directly linked to a lifetime criterion: the induction time. The switch from one iso-curve to its neighbour shows an acceleration factor of 10;
– an iso-representativity curve, which represents the percentage of photochemical initiation vs thermal initiation.
This abacus is very useful in comparing different testing methods, regardless of the light source. For instance, it enables the improvement of the reliability of an accelerated testing method by choosing a couple of exposure conditions (temperature and UV-light energy), which are representative of those in outdoor weathering.
When applied in numerical tools, the kinetic modelling approach, unstabilised, allows the direct simulation of the oxidation kinetics and its subsequent changes of molecular masses (fig 8) which govern the mechanical properties.
In building the kinetic model of photo-thermal oxidation, iPP photo-thermal oxidation has been considered as an extension of the thermal oxidation case, by introducing additional photolysis steps of the main photo-reactive groups.
A project aiming at implementing the modelling approach to the ageing of stabilised materials is currently in progress. Once the model is considered to be reliable enough, it could be extended with the action of fillers (such as talc) and pigments. The objective is to be as close as possible to the PP formulation used for bumpers or side protection moulding.
The model can simulate any kind of light, in a wide range of temperatures. However, replacing an ageing test by the use of a kinetic model of PP thermal-photo-degradation requires knowledge of the exact formulation of the polymeric material; obtaining this information depends on the co-operation of raw material suppliers.