Plastic materials and durability: weathering testing
We all have heard that plastics do not withstand sun, become white, more rigid and more fragile with time, etc. But, do they offer any guarantee over the years?
It is a real challenge for the plastics industry to predict a product’s durability depending on the aggressions it will suffer during its shelf life. Among the affected agents, we find plastic raw material manufacturers, equipment manufacturers, who simulate these aggressions by accelerated ageing test and finally testing laboratories. The last ones must comply with requirements many times, such as predicting the minimum time of proper functioning of a product. Apart from tensile stress, vibration or chemical attacks appearing with a continued use, a product may be subject to certain environmental conditions related to temperature, humidity and sunshine, what may endanger expected aesthetic, functional or safety requirements.
It seems logical to think on exposing products to the service conditions expected, or even better, to the worst conditions to what they must fight against, as a way of studying their behaviour. Although predicting a future product or material weathering or degradation due to isolated or combined factors (cool, heat, rain, etc.) over time is nothing but an estimate, there are certain tools to be used to predict how our products are standing the passage of time: weathering testing.
Climate chambers for accelerated weathering
These tests are standardized under documents detailing cool and heat cycles, damp heat cycles or temperature or humidity cycles. The duration of accelerated weathering may be also set in standards, although is a current practice adapting the test duration and aggressiveness in extreme temperatures based on the durability requirements and the final product demands. For instance, in the automotive sector each one of the car manufacturers has his own standards to simulate components’ behaviour that, incorporated inside the vehicle, are going to undergone extreme temperature and radiation changes.
In this kind of tests, the component’s final location inside the car’s cabin determines the extreme temperatures of the cycle, so that for components located over the waistline and therefore with higher sun exposure, they must pass a more demanding test than the component under the waistline or at ground level, such as feet fitted carpets in cars.
Do not forget that plastic materials are synthetic materials obtained by polymerization processes or carbon atoms’ multiplications in long molecular chains of oil-based organic compounds and other natural substances. The mobility of these long chains depends basically on the environment’s temperature and, of course, affects directly to the properties of plastics.
Given its nature, during weathering testing and ageing test, the movement of molecular chains or their breaking up affects the properties of plastics, almost always to make them worse. Let’s see how some weathering agents affect:
Cool: low temperatures, below material’s glass transition temperature, immobilize chains, so the material becoming fragile. This is because chains cannot loose shape to absorb forces.
Dry heat: high temperatures cause molecules’ mobility, thus causing crystalline structure changes and therefore in material’s properties. Furthermore, heat encourages migration phenomena, so effects such as plasticizers loss is accelerated since they migrate to the surface.
Heat + high humidity: polar polymers (PA, PET, PUR, etc.) are able to absorb a huge amount of water and cause different changes both at structural and final properties’ level. This absorption of water affects some properties:
It changes dimensional stability.
It reduces resistance and hardness and increase resilience.
It reduces electrical resistance
Water may act as a plasticizer (needed in cases such as polyamides) or be harmful, since it may cause hydrolysis of some polymers in combination with high temperatures, for instance in PET.
Cool-heat cycles: temperature and humidity changes generate stresses in the material favouring the appearance of cracks, breaks, dimensional changes, etc. All these factors and damages that weathering factors produce, either in a natural way or in an artificial way by accelerated weathering, must be assess and quantified if possible.
In Climate we have a huge number of equipments to carry out weathering testing, as well as devices to quantify the alterations that the structure, material or part’s surface side may be changed. A usual way to assess a material’s degradation is by measuring certain properties before and after subjecting the part or product to ageing test and checking the decrease of some properties at the end of the test. The most common and critic properties where important decreases are normally produced depending on test’s aggressiveness are: brilliance, colour, hardness, flexural or impact strength, adherence status of varnishes and paints at surface level and possible defects, such as blisters, delamination, cracks or breaks.