materials feature | Colourants
commonly found in soil. Their analysis found that diarylide pigments are generally resistant to ammo- nium sulphide and other sulphides in the earth. Some of those that were resistant, when combined with other reactive colours, can have a stabilising effect on the formulation as a whole. This kind of research is critical – because pipelines that are buried for long periods of time must demonstrate recognisable colour so that repair technicians can identify them many years later. When formulating colours specifically for pressure
Ampacet’s uses weatherom- eters to test samples of polyethylene pipe and film
products, Ampacet uses two different kinds of weath- erometer at its research facility in Terre Haute, Indiana. One is the fluorescent UV accelerated weathering
tester (ASTM G 154) often called by its brand name QUV. It does not attempt to reproduce sunlight entirely, but is designed to produce mainly ultraviolet (UV) rays – espe- cially the damaging effects that occur in the 300-400nm spectrum. The other is the Xenon arc test chamber (ASTM G 155) with lamps that provide the closest possible equivalent to natural sunlight, including UV, visible and infrared (IR) light from 295 to 800nm. It also uses heat and water cooling to control temperature, and simulate the effects of moisture with direct water-spray and humidity controls. As a general rule, 12 months of outdoor exposure in Miami can be simulated with Xenon arc weathering in about 2,000 hours or about 83 days.
Preserving buried colours For below-ground pipe, separate criteria apply. The pigments should have stability underground, because sulphides in the soil can severely discolour the pipe: if lead chromate pigments come into contact with ammonium sulphide, for instance, this forms lead sulphide – which over time can turn a yellow pipe black. To understand the chemical effects on buried pipe, Ampacet’s R&D facility in Messancy, Belgium, carried out a study on colours for underground use – testing more than 80 colour masterbatches for resistance to ammonium sulphide, a highly reactive element
applications, it makes sense to use as few additives as possible – as any one of them could interfere with dispersion and potentially weaken the pipe wall. In non-pressure applications, such as conduit, dispersion is not as critical. In extreme cases, too much additive masterbatch can cause ‘blooming’ – in which additives rise to the pipe’s surface. If severe, blooming can affect butt fusion – the process used to join sections of pipe together. For the additives that are essential, these should be
easily dispersible in a concentrate. In other words, there should be no agglomerations of pigments or other additives. Some pigments – such as molybdenum orange, copper-based greens and particularly phthalo- cyanine blue – can accelerate the degradation of a pipe structure over time. This is because their chemistries interfere with the polymer bonds, causing plastic to become brittle. This is termed a ‘Stage Three’ failure – which can also be caused by high levels of chemicals in the water such as free chlorine or chloramine. Four other criteria should be considered when
formulating colour masterbatches for pipe: 1) Only pigments that are light stable for at least two years should be used.
2) Only sulphide-resistant pigments should be used for yellow gas pipe.
3) Only pigments that are NSF-acceptable should be used for potable water applications
4) Only pigments that are stable to 500F should be selected.
Other factors In contrast to the use of colour throughout the pipe wall, companies that co-extrude a skin layer of yellow for gas pipe – or blue for potable water – will see significant cost reduction. This is because the amount of
Questions to consider before specifying a masterbatch for performance piping systems: l Will the pipe be used in a pressure or non-pressure application? l Will the pipe be installed indoors, outdoors above ground or outdoors below ground? l Does the pipe require ultraviolet protection? If yes, for what length of time? l Are heavy metal pigments OK? Or are non-heavy metal pigments required? l Will the pipe need to meet certain ASTM or ISO standards for OIT or other test criteria?
24 PIPE & PROFILE EXTRUSION | November/December 2016
www.pipeandprofile.com
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