materials feature | Colourants
Solar radiation varies widely according to geographic location, which can cause a problem for pipe manufac- turers
from the effects of UVA and UVB rays. There is also evidence, from a study performed by Cabot Corporation, that the dispersion of carbon black in the polymer can positively influence the expected life of plastics. When pigments other than black are used, UV
inhibitors, light stabilisers and anti-oxidants must be added to give equivalent outdoor protection. Without UV inhibitors and anti-oxidants, UV radiation will break down the chemical bonds of the polymer. Depending on the geographic location of the piping system, the degradation can accelerate quite rapidly. Coloured HDPE pipe left outdoors for any length of time will be subject to embrittlement, chalking, fading and loss of tensile strength – which ultimately leads to ESC.
Different colours are
used to identify various piping systems and conduit lines
UV inhibitors and anti-oxidants Hindered Amine Light Stabilisers (HALS) are by far the best light stabilisers. Their effectiveness in scavenging free radicals also makes them the most cost-effective. For coloured pipe, it is advisable to use the high-perfor- mance polymeric HALS, as they are less migratory than most others. This, together with their long-term anti-oxidative effect, makes them the products of choice for the extended outdoor exposures of pipe and conduit. Within certain limits, they are also NSF acceptable for potable water applications. Heat from the sun also increases the temperature of
exposed pipe – making it as much as 60F higher than ambient, depending on colour. This causes thermal degradation. Such extreme temperature fluctuations over time will also affect the polymer bonds. In addition, it should be noted that chemical reaction rates increase exponentially as temperature increases. So, it is important in some cases to add anti-oxidants or heat stabilisers into colour masterbatch formulas to prevent thermal degradation of the pipe or conduit. Two kinds of anti-oxidants – primary and secondary
– are generally used in colour masterbatches as thermal stabilizers. Primary AOs are chemicals that
22 PIPE & PROFILE EXTRUSION | November/December 2016
www.pipeandprofile.com
slow the thermo-oxidation of the polymer outdoors over time, and extend the functional lifespan of the plastic; secondary anti-oxidants are used in pipe production to inhibit the thermo-mechanical effect of processing the polymer, which degrades it for as long as it resides at high temperature in the extruder zone. Secondary anti-oxidants are especially important when a polymer is exposed to two or even three heat cycles – such as when regrind is added to pipe. Without enough secondary AO, the thermo-mechanical degrada- tion of a mixture containing regrind could cause the compound to stick to the extruder. This happens particu- larly on the inside of the pipe, where more heat is contained in the process, and can result in surface fracture and even tearing on the inside surface. Formula- tors must take this into consideration, and add enough AO to prevent this type of surface fracture from occurring. These protective anti-oxidants improve ESCR and
protect against Slow Crack Growth (SCG), but they also increase cost. Add colour to the mix, combined with UV inhibitors, and it will be even more expensive per foot of pipe extruded. As a result, the cost of using a colour is generally two to five times that of black concentrates.
Accelerated weather testing To determine the optimum amounts of UV and AO necessary for a given application, accelerated weathering tests can be performed on pieces of pipe. With the right laboratory equipment, accelerated weathering can be conducted by exposing pipe samples to controlled cycles of intense energy, light, heat and simulated rain spray. To gauge and verify the weather resistance of customers’
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