1.5 Resistance to atmospheric agents and UV rays
In most cases, ELESA plastic Standards are used for “indoor” applications. In any case, due to the properties of the materials and the measures taken during the design stage, these products may also be used for "outdoor applications", where they are exposed to particular atmospheric conditions:
- rapid changes in temperature: within the working temperature range recommended for each product, rapid changes in temperature do not create problems due to the impact strength of the materials used;
- the presence of water or moisture may result in processes of hydrolysis and the absorption of a certain percentage of the water/moisture until a state of equilibrium is reached. This may alter some of the material’s mechanical properties.
Examples of materials that absorb water include polyamides (PA), transparent polyamides (PA-T, and PA-T AR) and duroplasts (PF).
Products made of these materials may undergo slight changes in size due to the absorption of water, which may affect dimensional tolerances. During the design stage, ELESA normally takes these possible variations into account in order to minimise their effects and to guarantee compliance with the technical specifications. The absorption of water results in a significant increase in impact strength.
The following polymers do not absorb water: polypropylene (PP), thermoplastic elastomers (TPE), and acetal resin (POM).
Occasional contact with rainwater followed by “drying” does not generally pose any problems in terms of the strength of the product. When used in “outdoor” applications, it is advisable to prevent water accumulating on the product by adopting suitable assembly conditions.
- Exposure to the sunlight and UV rays in particular.
Specific resistance tests have been carried out using specific equipment for accelerated ageing testing, in accordance with the ISO 4892-2 standard, and setting the following parameters:
- radiation power: 550 [W]/[m]2;
- internal temperature (Black Standard Temperature, BST): 65°C;
- OUTDOOR fi lter that simulates exposure to the open air, with low shielding against UV rays;
- relative humidity: 50%.
The relation between the hours of testing and the hours of actual exposure to an outdoor environment (“Equivalent Hours”) obviously depends on the weather conditions of each geographic area. Taking the Average Radiant Exposure per Day (ARED) as a basis for comparison, the reference values adopted on an international scale include:
- Miami Equivalent Hours = high intensity exposure, typical of countries with a tropical or equatorial climate (ARED = 9.2 MJ/m2);
- Central Europe Equivalent Hours = mean intensity of exposure, typical of continental climates (ERMG=2 MJ/m2).
At the end of prolonged tests carried out at the ELESA laboratories, the variation in mechanical strength was measured (tensile/compression breaking, and impact breaking) was measured. In general, the results show that the mechanical strength of polyamide (PA), polypropylene (PP) and Duroplast (PF) products is not significantly reduced by exposure to UV rays. As to the aesthetic appearance of samples exposed to the action of the UV rays, in some cases a slight variation in the surface appearance of the product was found, on completion of the tests. For further details on UV ageing tests on specific products, contact the ELESA Technical Department.