Pigment Levelling through Nucleation

Nucleating additives increase the polymer’s crystallization temperature and its rate of crystallization.  This can improve physical properties, control warpage and improve processing speed.  Clarifiers are nucleating additives that also generate optical property improvements.  Nucleants are commonly used, especially in PP, to confer those benefits.

All pigments consist of particles, which provide sites for polymer crystallite formation and tend to increase the crystallization temperature.  MD (machine direction) and TD (transverse direction) shrinkage are two important parameters. When compared to uncoloured polymer, inorganic pigments generally have little effect on overall properties or the MD to TD ratio.  Many organic pigments, especially phthalocyanines, can drastically alter shrinkage and the MD to TD ratio producing undesirable effects.  When we say that a pigment has a tendency to nucleate resins, this is not considered a good thing.  We are saying that it is likely to result in –

• The need to alter moulding conditions for every colour in a product range because each affects shrinkage differently.
• Warpage due to frozen-in stresses.
• Loss of impact strength or even stress-cracking due to frozen-in stresses.

These negative effects manifest most strongly in HDPE and PP Copolymer.  As a result, organic pigments that cause nucleation can’t be used in certain applications.  Some pigment manufacturers offer modified, low nucleation versions of their products.  These are generally weaker, incur a cost premium, and don’t always eliminate the problems.  Sometimes the only solution is to reformulate using a different class of pigments.  Often the new formulation is more expensive and can’t accurately replicate the original shade.

In theory, if the positive effects of a nucleating agent can outperform the nucleating effect caused by the various pigments, the negative nucleation effects will disappear and all coloured articles will behave in the same way.  Warpage and stress cracking will be eliminated and moulding conditions will not need to be altered for each colour.  Pigment levelling is the term commonly used to describe this effect.

The plastics colourist would greatly benefit from a nucleant or nucleant mixture capable of reliable pigment levelling in polyolefins, especially HDPE.  Simplified pigment inventory and ability to match a wider colour range at competitive prices are two obvious advantages.  Ideally, any pigment levelling nucleant should be effective in low concentrations, be easy to incorporate into concentrates and add little to the formulation cost.  We are not at the ideal stage yet, but edging closer.

There are many commercially promoted nucleating agents.  They create heterogeneous nucleation sites in the polymer melt so crystallization is initiated at higher temperatures. Some, like Sodium Benzoate, Talc, Silica, PTFE and Hyperform HPN-20E recently introduced by Milliken, remain solid at processing temperatures.  For maximum efficiency these should be present in nano form to generate the highest density of nucleation sites.  This increases costs.  Sorbitol derivatives and organic acid derivatives are examples of additives that partly dissolve in the polymer melt during compounding.  They then crystallize on cooling into a fibrillar network providing a huge density of nucleation sites.

Most nucleant research has focused on reducing cycle times, physical and optical property improvements, because such benefits are easy to sell.  PP and PET have benefited the most because they are relatively easy to nucleate.  Development of effective nucleant systems for HDPE has been slow. This is frustrating for the plastics colourist because pigment levelling nucleants are most useful in HDPE.

In my experience, Sodium Benzoate, which has been used for a very long time, deserves some respect as a pigment levelling nucleant.  It has solved warping problems for me in the past.  There are now many powerful nucleating agents to choose from and perhaps the colour industry should be conducting its own tests.  After all one can hardly expect Milliken or Adeka to be suggesting something other than one of their own products.  The most effective pigment levelling nucleant could turn out to be a blend of two or more components (several Adeka nucleants are blends rather than single chemicals).

If the pigment levelling nucleant adds extra costs, it will be much more readily accepted by the plastics processor, if it also adds value by increasing processing speed and improving physical properties of the finished item.  Perhaps the colour suppliers, whilst solving their problems, should aim to create a win-win scenario.