Dr Reach - When is a substance a polymer?

Q. Does a substance meet the definition of a polymer under REACH?

Over the last few months, there have been a number of queries relating to polymers. This month’s Dr Reach column focuses on the issue again in a hope to ensure that the key aspects of this challenging subject have been highlighted

REACH defines a polymer as:

1. a simple weight majority of molecules containing at least three monomer units which are covalently bound to at least one other monomer unit or other reactant;
2. less than a simple weight majority of molecules of the same molecular weight.

When there is particular possibility that a product may rquire registration as a substance rather than having to meet the relevant requirements for a polymer, then the implications of both the following scenarios must be carefully analysed and presented:

1. what if it is a substance under usual REACH registration, and,
2. what if it is a polymer under REACH thereby triggering the registration rules for its monomers/reactants CW European Business Briefing June 2008.In either case, the registration and chemical safety assessment must sufficiently cover potential risks to health and the environment

The characteristics of a polymer also hold implications for any additives present as REACH offers reduced requirements if they are appropriately incorporated into a relevant material matrix.

OECD TG 118

To help companies with their analyses and subsequent justification of their decisions, I am pleased to offer some advice from my colleague Dr Steve Holding, Smithers Rapra principal consultant on polymer characterisation:

"The guidance on polymers from the European Chemicals Agency refers to gel permeation chromatography (GPC) as the preferred method for determining whether a substance falls under the definition of a polymer and further makes reference to the OECD document TG 118. However, OECD TG 118 describes how to do GPC, not how to relate the results to REACH.

There are inevitable fundamental problems in estimating the proportion of a polymer below a given molecular weight. These arise due to different detector selectivity at low molecular weight. This was raised some years ago with regard to food contact polymers at the European Committee for Standardization (CEN).

OECD TG 118 only provides relative molecular weight data – it is correct when the polymer type of interest is the same as the calibrant but there is a very limited range of calibrant polymer types. Frequently molecular weights will be expressed as the ‘polystyrene equivalent’ molecular weights.

Results are therefore relative rather than absolute. And when identifying appropriate conditions with more unusual solvent systems, compliance with OECD TG 118 can be particularly difficult.

Polymer end groups

For REACH, the main interest resides in the short-chain polymers or oligomers. The chemistry (and hence behaviour) of end-group (‘other reactant’) plus one repeat unit, is likely to be very different to end group plus two repeat units – as the repeat units increase, the behaviour of the ‘polymer’ will become uniform. But this is not the case at low molecular weight.

For instance, the differential refractive index (dn/dc) for the substance in the solvent: the dn/dc for monomer, dimer, trimer etc. can be very different. There are situations where dn/dc is negative for monomer, around zero for dimer and positive for trimer. Given that dn/dc is the main method for measuring concentration in GPC, this means that the quantification of oligomers will normally be incorrect.

There are similar complications with other detectors used for GPC and other measurement techniques – the key issue being that the end-group heavily influences all properties of a short chain ‘polymer’.

Caveats

When identifying alternative methods to GPC, one must consider that these usually relate to number average molecular weight rather than quantification of the contributions from the shortest chain molecules.

Measuring the proportion of a polymer with a molecular weight less than 1,000 (or any other arbitrary figure) is therefore far from routine. Even if strictly in accordance with OECD TG 118, the results will inevitably be accompanied by a lot of caveats. These must always be identified and clearly reported."

Dr Holding has been responsible for many years for the polymer molecular weight characterisation provided by Smithers Rapra, based on gel permeation chromatography and systems that cover nearly all polymer types. www.rapra.net

Send questions to Dr Steffen Erler and read his previous answers http://chemicalwatch.com/dr_reach