EuroWire May 2023

Technical Article

Carbon black: polymer protector against sunlight Carbon black has two parameters that help us describe and characterise it. On the one hand, we have the structure of carbon black, which describes how the different particles that make it up are put together. Carbon black structures can be divided into the following parts: The aggregates are sets of carbon black particles that generally form ramifications. These structures are formed by the primary particles joined together by high electrostatic forces (Van der Waals type) that make it impossible to separate them, despite the mechanical work achieved in the twin screw extruders used by compounders of colour concentrates. The agglomerates are the set of aggregates joined together forming a branched tree structure that characterise carbon black. They can present complex structures with many ramifications that will provide more or less thermally or electrically conductive properties, or simpler structures with a smaller number, providing non-conductive properties and less structural reinforcement.

The first is to use a family of additives called UV filters, which have the ability to absorb the 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. On the other hand, there is another family of UV additives called HALS (hindered amine light stabiliser) additives that react to free radicals that appear after light has broken the covalent bond between carbon atoms, generating free radicals. These additives react with said free radicals, thanks to their high reactivity, and prevent them from continuing to react with the polymer chain and causing breakage and the consequent degradation of the appearance of the cable and its elastic properties and mechanical resistance. When ultraviolet light strikes and the free radical is generated, the HALS act to stop the degradation process. Synergistic combinations of these two families of additives are also used with very good results. Also, it is common to supplement the additive package with antioxidants to also prevent degradation of the polymer due to thermal stress. When a polymer is exposed to high temperatures, either because it is close to a constant heat source, extreme climates or due to the intensity of use, the conduc tor increases its temperature notably due to the Joule effect, and another type of degradation due to thermal stress appears. Antioxidant additives also play a relevant role, since they capture the (highly reactive) free radicals formed by these high tempera tures and prevent chain degradation.

On the other hand, the size of the particles that build this structure is very important. This size can range from 120nm in particle diameter to below 15nm in those most used in the plastics industry. This is the most important characteristic to define the effective- ness of carbon black as a protector of the polymer against sunlight. The recommended particle sizes for good protection are below 25nm in diameter.

Furthermore, the process can be stopped by using carbon black, which achieves a high absorption rate of ultraviolet radiation. The advantage of carbon black is that it captures energy and prevents it from reacting with the polymer, and is the most economical option for this protection, but with the drawback that the colour of the cable must necessarily be black and this is not always possible in all exterior cable applications.

The smaller the particle size, the greater the carbon black surface that will be able to absorb the received sunlight. If we find a structure that has large particles, the total surface area will be less than if we find smaller particles.

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May 2023