EuroWire May 2023

Technical Article

A good metaphor to understand this behaviour would be to imagine a pool filled with basketballs that would have a certain surface area. If we convert basketballs to tennis balls (a smaller particle size), the surface area of carbon black capable of capturing carbon black radiation would be much larger and therefore the protection of the polymer would be much more efficient. The quantity is also very important in order to ensure that the ultraviolet rays of sunlight will not be allowed to degrade the polymer. Various studies confirm that between 2 and 2.5 per cent of these types of carbon blacks with small particle size, in polyolefins, achieve non-degradation of this at least for periods of more than 15 years, depending on the thickness of the sheath. For those that are normally found in insulation, this amount is enough. And what about the colour? How do we ensure that the colour of the cable remains stable throughout the useful life of the outdoor cable? There is no other additive that protects the colour of the cable like polymer does. The solution in this case is to formulate the colour with pigments prepared to withstand outdoor conditions. In other words, we must properly select the lightfast pigment or pigment combination from the start. In order to understand the colour behaviour of an outdoor cable, it is important to introduce the concept of lightfastness or resistance to discolouration of the pigments used in the manufacture of a masterbatch. We define the lightfastness of a pigment as a quality specification of exposure of the pigment to daylight. Among sunlight compositions, the most destructive part for pigment lightfastness is ultraviolet (UV) light. In the lightfastness of the pigments we are discussing, we only evaluate the quality specifications of the pigment’s tolerance to light environment in the outdoor environment. The pigment’s lightfastness index, which excludes other external environmental factors, can help us provide a meaningful and reproducible objective assessment of an outdoor cable’s colour stability. Pigment weatherability indicators can be measured by outdoor exposure experiments, or indoors using artificial weathering equipment to simulate outdoor performance. Another possibility is to subject the pigments to exposure in the open air. In these cases, specific locations are usually selected as experimental sites, which are often areas with very extreme climatic conditions (strong exposure to the sun, highly polluted industrial atmospheres, etc). The most well-known outdoor exposure test site is Florida in the USA. The test sample is generally placed facing south five degrees and exposed to outdoor exposure for 12 months or more. The lightfastness of the pigments can vary both by the type of polymer used to manufacture the cable and its concentration. It has been scientifically proven that, despite using high-resistance pigments, if the concentration is not high enough, discolouration of the cable can occur. Guaranteed resistance with behavioural acceleration tests

In order to know what the behaviour is at different concentrations of the same pigment, it is very important to study the behaviour of pigments at different concentrations in outdoor acceleration chambers. These tests are carried out both with pigment-free titanium dioxide and with the same pigment with the addition of a known amount of titanium dioxide. In this way, it is possible to better know what the behaviour of the designed colour will be in the cases that titanium dioxide is required in the formulation. The choice of this pigment is very common in most RAL colours used in the cable industry, and it is also widely used in the case of very low thicknesses in the insulation layer of cables that require sufficient opacity to cover the colour of the cable conductor, whether it is copper or aluminium. Typically, test chambers are used with humidity cycles that simulate rain and dry cycles, with temperature control, and with xenon light exposure that radiates a spectrum of light at wavelengths of sunlight and is more complete than UV lamps, which cover a smaller range in the light wavelengths. In the cable industry there are multiple regulations and tests that describe both the hours of exposure to light and the humidity and temperature cycles. At the same time, it is necessary to study the behaviour of pigments in depth for the different types of polymers used in the cable industry (PVC, HFFR, TPU, XLPE, EPDM, etc) under outdoor conditions. Having this experience and in-depth knowledge is key to guaranteeing the stability of the colour of the cable throughout its useful life outdoors. Hence, it is important to work with collaborators and partners capable of providing advice and solutions.

Delta Tecnic https://deltatecnic.com

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May 2023