The environment negatively affects the useful life of cables. The exposure to sunlight, temperature fluctuations or humidity make up what’s referred to as ‘weathering’: an irreversible chemical process that causes discolouration and loss of the mechanical properties of the polymers. Therefore, avoiding these effects on cables exposed to outdoor conditions is essential to extend their life expectancy.
The cable market offers a range of designs and applications intended for outdoor use. We see it in medium voltage cables, photovoltaic cables for solar panels, power cables for buildings, the increasingly popular aerial fibre optic cables or those required for electric car chargers. Several highly relevant sectors require cables capable of resisting photodegradation.
We are talking about a photochemical process produced by sunlight that is cumulative, which means that it worsens over time and is irreversible. Degradation of the polymer by ultraviolet (UV) radiation influences its physical properties and therefore its life expectancy.
Thus, UV radiation causes a photo-oxidative degradation that results in the breaking of the polymer chains, producing free radicals, which start a chain reaction that causes the deterioration of its mechanical properties. Consequently, there is a degradation of the visual appearance, of the brightness and of the surface, as well as a loss of the mechanical properties of the cable, limiting its useful life.
It should be remembered that when a plastic is exposed to solar and climatic conditions, there are two independent phenomena taking place: polymeric degradation and photodegradation of the chromophore groups present in the pigments, responsible for the final colour of the cable.
Mitigate polymeric photodegradation
There are three ways to mitigate polymeric photodegradation. One of them is using additives called UV filters, which have the ability to absorb ultraviolet light that causes degradation. In this way, when sunlight comes into contact with these additives, they react and prevent the light from reaching the polymer, as if it were a parasol.
It is also possible to mitigate through HALS (Hindered Amine Light Stabilizer)—a family of additives that reacts to free radicals, blocking them and preventing the initiation of polymer chain breakage. When UV light strikes and the free radical is generated, the HALS act to stop the polymer degradation process.
On the other hand, by means of black pigment (i.e., Carbon Black) a high capacity for absorbing UV radiation from light is obtained, since it traps this energy thus preventing it from reacting with the polymer. The most effective grades are those with the smallest particle size, since they present a greater surface to trap solar radiation.
And what about the colour? There is no additive that protects the colour of the cable as it does the polymer. The solution in this case is to formulate the colour using pigments prepared to withstand outdoor conditions. In other words, it is necessary from the beginning to choose the correct pigment or combination of pigments that are lightfast. The in-depth study of the behaviour of the pigments in the different types of polymers used in the cable industry (PVC, HFFR, TPU, XLPE, EPDM, etc.) under outdoor conditions is key to ensuring the colour stability of the cable during its useful life outdoors.
‘Weathering’ and ‘lightfastness’
The behaviour of pigments outdoors can be measured with a test that accelerates the state of exposure of plastics under different sunlight conditions. In other words, there are effective ways to know what kind of behaviour to expect from outdoor cables, which facilitates the selection of pigments and, therefore, the prediction of the life expectancy of the plastic.
In this way, it facilitates decision making and it contributes to the installation of solid cables capable of withstanding weathering and conditions that cause lightfastness. And thus the historical concept of lightfastness, which takes into account the incidence of light, combines with that of weathering, which in addition to light takes into account factors such as temperature and humidity.
In fact, the pigments that have good lightfastness behaviour may not work properly in different temperature ranges, under solar radiation, and are also affected by the polymer used in the definition of the cable. Both concepts join together to make up the problem faced by outdoor cable designers, but fortunately there is a solution.
A good understanding of pigment behaviour in the different types of polymers used in the cable industry, such as XLPE, PVC, HFFR, TPU, TPE-E, EPDM, is essential to be able to propose pigment formulations that obtain the required colour with excellent colour stability when faced with outdoor conditions, not only due to the aggressive action of light, but also due to all the aforementioned weathering factors.
The most critical colours that we can find in outdoor cables are those that must be designed with red and yellow pigments, since they are the most sensitive to light and weather effects. That is why it is essential to have tests that help us predict the behaviour of these pigments in the different polymeric bases and with the necessary concentrations. These tests must be done for these critical pigments using Suntest (a laboratory apparatus that under standardized conditions allow us to make these predictions).
If these tests are not carried out with the critical pigments, it is possible to have degradation of the chromophore groups of these pigments, resulting in discoloration of the cables in much shorter times than desired, with the attendant problems related to the repair and control of electrical circuits and data cables in outdoor installations.