materials feature | Colourants
These images show poor
dispersion of blue pigment (left), as seen in thin film samples,
together with a better disper- sion of pigment (right)
masterbatch for gas pipe, it is advisable to adjust for a higher let-down ratio – for example, from 2% (49:1) to 4% (24:1) or more. This is more common in thin-walled pipe, where
there is a greater the chance that the agglomerates will affect its ability to hold pressure. Large diameter pipes can be more forgiving, thanks to their thicker walls. In this case, agglomerates or poorly dispersed particles will form voids or ‘windows’ in the wall. This is almost impossible to circumvent, but they must be minimised in both size and number in order to reduce the chance of pipe failure. Studies confirm that dispersion quality is critical to both UV performance and the plastic’s mechanical properties. For pressure-rated yellow PE gas pipe, the let-down ratio should be no less than 2% (49:1) in order to ensure adequate dispersion. For non-pressure applications, such as coloured conduit, a 1% (99:1) mix ratio will suffice. To measure dispersion quality in the masterbatch
pellets, Ampacet uses a micro dispersion scale of 1-5 – where 1 is the highest quality dispersion. This is done by converting the masterbatch into a thin polyethylene film of about 1.5 mils (38 microns) thickness, then measur- ing the agglomerates in the film via a visual inspection. This gives the masterbatch producer confidence that the additives are adequately dispersed for the customer to make a pipe wall that is smooth and solid. Also, it provides a company with a traceable lot number in the event of a pipe failure. The dispersion rating is reported on the certificate of analysis for every lot produced. For critical applications – such as pressure pipe or ge- omembranes – compounders should keep records of each lot and, if necessary, make that data available to the purchaser or an independent third party.
Weatherability factors When formulating non-black concentrates for outdoor use, weatherability – a plastic’s ability to resist chemical change from elements such as sunlight, temperature, and moisture – is a primary concern. Environmental exposure, whether above ground or below ground, will
20 PIPE & PROFILE EXTRUSION | November/December 2016
weaken or damage the polymer chain, making it brittle – or cause the pipe to discolour over time. Proper pigment selection plays a vital role in
extending the service life of polyolefin pipe. Studies have shown that not all pigments act the same when exposed to the elements. Some pigments, depending on their chemical composition and particle density, enhance the properties of plastic – sometimes having a synergistic effect with UV stabilisers to protect resin from damaging UV rays. But the reverse can also apply, with many pigments having a damaging effect on polyethylene, degrading it at an accelerated rate. The topic is complex, requiring an understanding of what happens in two distinctly different scenarios: above- ground and below-ground environments.
UV and thermal protection For above-ground piping systems and conduit, exposure to solar radiation and high temperatures will cause both photo-degradation and thermal degradation, which can vary with season and geographic location. Most pipes need a degree of protection from the elements, due to the lengthy times they are stored at construction sites. When exposed to heat, cold, UV rays and moisture, the molecular chains of the polymer will break over time. Free radicals are produced, which cause the PE or PP to become brittle and lose its durability. This leads to environmental stress cracking (ESC), which accounts for some failures of plastic pipe and fittings. With the addition of carbon black, two mechanisms
work to impart Environmental Stress Crack Resistance (ESCR) and extend the service life of the piping system – protecting against UV degradation and thermal degradation. This is why more than 80 % of polyolefin pipe made in North America is black: no single ingredient provides more cost-efficient protection than carbon black. Many studies explain how carbon black morphology and particle size can mitigate the harmful effects of UV radiation. Most have concluded that a minimum of 2% carbon black in the pipe wall is necessary to protect the polymer over the long term
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