Feeling blue about going green

Increasing environmental awareness and concerns over the impact of inorganic pigments containing heavy metals has led to legislation outlawing many inorganic pigments, particularly those containing cadmium and lead. This has had repercussions for the plastics industry, which is now being compelled to use organic pigments as colorants.

Historically, inorganic pigments presented few problems either to the design engineer or to the plastics processor being thermally stable and having negligible effects on the dimensional stability of mouldings. In contrast, environmentally acceptable substitute organic pigments, e.g. phthalocyanine blue and quinacridone red, are less thermally stable and have a dubious reputation in the moulding industry for causing problems with shrinkage, warpage and, occasionally, poor product performance. This is despite being more attractive because of their low cost, excellent colouring ability and ageing resistance.

Recent work at the National Physical Laboratory (NPL) has shown that phthalocyanine blue is an "active" pigment, having a pronounced effect on the crystallisation behaviour of semi-crystalline polyolefins (polyethylene and polypropylene). This pigment acts as a nucleating agent changing the crystallisation temperature and the nature of the crystalline structures formed by the host polymer as a result of heterogeneous nucleation.

So, what are the implications of these dramatic changes in morphology on the dimensional stability and mechanical performance of mouldings manufactured from polyolefins? There is the potential that an increase in crystallisation temperature could lead to dimensional instability by freezing in greater levels of internal stress during the cooling phase of the moulding cycle.

Also it is known that changing the size and number of spherulites can change the mechanical properties of the moulded product because the spherulite boundaries represent discontinuities in the matrix. It seems likely therefore that crystallisation differences will be reflected in the shrinkage and mechanical behaviour of pigmented mouldings.

These issues are of particular importance for the commodity plastics polypropylene and high density polyethylene that are used extensively in the plastics packaging industry to manufacture tamper-proof bottle caps, closures and snap-fittings.

These products have to meet tight specifications with respect to dimensional tolerance and stability and maintain mechanical performance within the limits of the product design (appropriate failure of tamper-proof mechanisms and creep resistance of closures).

In view of this, the NPL has made an exhaustive study of the crystallisation behaviour of polyolefins with and without phthalocyanine blue. The results of these detailed studies can be found on the NPL web site at:

http://www.npl.co.uk/npl/news/press/pr13-2000.html

These findings could have significant implications for the packaging industry. Some problems have already been encountered in tamper-proof bottle caps manufactured using phthalocyanine blue which could be attributed to the morphological, dimensional and mechanical changes caused by the pigment.