colour & masterbatch | Pigments
Specifiers need to be aware of what pigments really contain
range, and high-end types, which differ according to their thermal stability and light-fastness (and price). So compounders producing PVC plastisols at temperatures well under 200°C have the option of using lower-cost diarylide types that do not have high thermal stability. Similarly, polyethylene film makers running at around 190°C can use the less stable types, but injection moulders producing thin wall packaging on high-speed machines will require different grades. Companies processing engineering thermoplastics at 300°C have their own even more demanding requirements. Clariant’s organic alternatives to lead chromates include the PV Fast types, which for example, show virtually no migration in PVC, and Graphtol grades, which tend to have lower performance but better economics with often high thermal stability.
The colour strength of organic
pigments can be several times higher than inorganic pigments
Transparency can be good and bad Organic pigments do not have identical properties to lead chromate pigments. They are, for example, more transparent. This can be an advantage or a disadvan- tage, depending on the application. In food packaging films, and in rigid plastics packaging designed as an alternative to glass, for example, transparency is often an advantage. In applications where the organic pigment is being used as a straight replacement for an inorganic one, such transparency may not be desirable. On the other hand, the colour strength of organic pigments can be several times higher than inorganic pigments. Yellow 83, which is an organic pigment, has a colour strength up to 12 times that of Yellow 34 lead chromate-based pigment, for example. So depending on the application, coloration costs for organics may not be all that dissimilar from those for lead chromates. Alternatively, another additive may have to be used to ), for
achieve the same opacity: titanium dioxide (TiO2
example. But users of organic pigments do need to be wary of using them with TiO2
, has an , since it can have a negative
effect on the light-fastness of the organic pigment. Violet 19, for example, when used without TiO2
accelerated weather resistance of 10,000 hours. But if just 0.5% of TiO2
is added, that figure falls to around
6,000 hours. So when possible, it may be advisable to use an alternative to TiO2
ment Yellow 53 or Pigment Brown 24 for example. In some cases when using organic pigments as
replacements for lead chromates, metamerisim may be an issue. This is when two different pigments provide the same colour when viewed in one type of light, but appear different under light with a different wavelength. This is an issue that can be mitigated but not resolved completely by using a mix of pigments based on different chemistries.
There is a price to pay Unlike the situation with low-halogen pigments, organic alternatives to lead chromate pigments do have a premium attached to them. Depending on the formula- tion, users may have to pay anywhere between 1.5 and five times as much (possibly even more) than they did in the past. This is something that pigment suppliers are acutely aware of, and are working on. But for the foreseeable future it is clear that pigment
buyers will be paying more. It is simply impossible at this time to produce high performance organic alterna- tives to lead chromates at the same cost. So it is very much a case of “buyer beware.” More than ever, specifiers need to ensure that they are aware of what the pigments are buying actually contain.
About the authors Stefan Wannemacher is head of technical marketing plastics at Clariant. Philippe Lazerme is head of strategic marketing plastics at Clariant. ❙
www.clariant.com
36 INJECTION WORLD | April 2015
www.injectionworld.com , such as nickel titanate – Pig-
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