Coatings made of paints, clear coats and other related products are used both indoors and outdoors where they are exposed to direct solar radiation for long periods of time. For this reason, it is very important to study the effects that solar radiation, heat, humidity and other climatic stresses may have on the properties of polymers.
This deterioration in coatings can be observed by the loss of properties such as:
- Discoloration and/or color change
- Loss of gloss
- Blistering
- Oxidation
- Cracking
- Peeling
- Chalking
The speed of this deterioration varies depending on the sensitivity of the material and the spectrum of light that is incident on it, i.e. a plastic substrate will not be affected in the same way by solar radiation as a resin or a pigment.
To understand the underlying reason for the degradation of coatings exposed to solar irradiation, we must first understand how this solar radiation is formed. Find out what the effects of solar radiation are in this article.
Therefore, one way of assessing the degradation of coatings is by external exposure to solar radiation. A disadvantage of these types of tests is that it can take long periods of time, even years, to observe the defects produced by solar radiation. In contrast, it is often necessary to determine more quickly the effects of light, heat and moisture on the physical, chemical and optical properties of coatings. These types of tests can be achieved by means of accelerated artificial aging tests.
These types of tests are reproduced under more controlled conditions than those that can be reproduced in natural environments, with the aim of accelerating the degradation of the coating and causing its failure. To perform them, we will need accelerated artificial aging chambers.
There are two different types of these chambers:
- Artificial aging chamber using xenon arc light (Q-sun).
- Artificial aging chamber using UV lamps (QUV).
The two types of chambers reproduce the damage caused by sunlight and rain by means of two different mechanisms. On the one hand, there is the Q-sun chamber, with which we can perform accelerated artificial aging tests simulating the entire solar spectrum. On the other hand, the QUV chamber will allow us to carry out accelerated artificial aging tests simulating only the irradiation corresponding to ultraviolet light, which is the most aggressive irradiation of the entire solar spectrum.
Today we will focus on xenon arc light (Q-sun) testing.
Basic principle
With the xenon arc artificial aging chamber, we can perform an accelerated test on the degradation of all types of coatings to the exposure of solar irradiation and rain; this is done by varying the temperature and humidity conditions to make the tests as aggressive as possible, allowing us at the same time to observe the failures that would occur to the coating in a long period (years) in a non-accelerated environment.
In order to achieve these accelerated artificial aging conditions, we must take into account that we must resort to climatic conditions that would not be reproduced in a natural environment, that is to say:
- We must perform the test using xenon arc lamps that emit a spectrum similar to the solar spectrum but not the same. In order to be able to emit a spectrum of irradiation that resembles that of the sun as closely as possible, we must make use of light filters.
- In order to accelerate the tests, we must simulate the most aggressive effect of solar radiation, over a constant and long-lasting period. This period of maximum solar irradiation occurs during the summer solstice at noon. Using the Q-sun operating conditions, we can translate this particular period to be tested continuously over long periods of time. Through this process, we can evaluate how coatings degrade over the long term, sacrificing correlation in natural environments due to variations in solar irradiation throughout the year or geographic location.
Structure
The Q-sun method has all the necessary tools to achieve the following accelerated artificial aging conditions:
- Light source: This is the part in charge of emitting the necessary irradiation to degrade the coatings. In the case of the Q-sun, it will consist of 3 xenon arc lamps that emit radiation from a minimum wavelength of 270 nm extending across the entire visible spectrum into the infrared. Although it is the light source that most accurately simulates the broad solar spectrum, it requires light filters to reproduce it more accurately.
- Light filters for radiation from lamps: They are used to achieve irradiation conditions that resemble the solar spectrum as closely as possible. The most recommended filter type for testing materials and coatings designed for outdoor use are daylight filters. These filters produce spectra nearly equivalent to direct summer sunlight at noon and allow for the highest correlation between xenon arc light and natural outdoor exposures.
- Water spraying system: This system is in charge of providing moisture to the samples to accelerate their degradation. This system is associated with a humidity sensor that continuously ensures that the humidity is within the corresponding ranges.
- Coating surface temperature controllers using the black surface temperature sensor: By means of this type of sensor, which absorbs practically all the radiation, we can control the maximum temperature that the surface of a coating can reach during a test.
- Thermometer to measure the ambient temperature inside the chamber during the test.
- Support for samples: This is the flat support where the samples with the applied coatings to be tested are placed.
Evaluation of results
All tested products are periodically evaluated for the following parameters:
- Discoloration or color change using a color spectrophotometer to compare the tested samples with reference samples.
- Loss of gloss of the tested coating compared to reference samples using a glossmeter.
- Surface defects that may appear due to coating degradation such as blistering, oxidation, cracking, peeling or chalking.
Conclusions
- By means of accelerated artificial aging tests we can reproduce, in the short term, the degradation of all types of coatings that would take long periods of time (years) to appear under natural exposure conditions.
- Accelerated artificial aging tests using xenon lamps and daylight filters provide the irradiation spectrum most similar to the solar spectrum.
- The longer the test duration, the more susceptible the coating is to degradation and, consequently, to discoloration or color change, loss of gloss or surface defects such as blistering, oxidation, cracking, peeling or chalking.
