BROMINATED POLYSTYRENE: A QUALITATIVE ASSESSMENT OF BROMINE CONTENT BY SEM-EDX

Brominated polystyrene (for short PS-Br or BPS) is one of the most common self-extinguishing additives used in polyamide-based compounds.

One of the key characteristics of this type of additive is, clearly, the bromine content. The bromine content is indeed the factor that determines the self-extinguishing level of the final compound.

To date, polyamide-based compounds with PS-Br remain among the most suitable for obtaining high values ​​of GWIT (Glow Wire Ignition Temperature), according to IEC 60695-2-13, making these compounds widely used in the household appliance industry.

Being a fundamental characteristic, the bromine content is always reported on the PS-Br technical sheet and this content can be measured with various techniques (also developed internally by the various PS-Br producers).

For a number of reasons (for example quality problems or incoming material control) it could still be useful, or necessary, to get an idea of ​​the bromine content value of a used PS-Br (value possibly obtained by a third party).

In this short study we will focus on the SEM-EDX analysis (https://rdlab137.it/en/laboratory-tests/electron-microscopy-sem-edx.html) for the evaluation (although an approximated one) of the bromine content of three commercial PS-Brs (two supplied by one manufacturer and one by a second manufacturer) respectively named PS-Br 1, PS-Br 2, PS-Br 3.

All three analysed PS-Br are in the form of granules / flakes (image 1).

Image 1: physical form of brominated polystyrene (here is depicted the PS-Br called PS-Br 3).

The SEM-EDX analyses were performed on several granules (flakes) of the various available PS-Br and, for each of these, in different positions (with 1000X magnification).

Table below (Table 1) summarizes the results found:

 Table 1: average bromine content measured and reported on technical data sheet for the considered PS-Brs.

Although there is a certain variability in the measured bromine content for the single measurement, linked to the fact that it is a punctual analysis (on a microscopic scale), the average values obtained are reasonably centred on the average nominal value.

Clearly, in order to have a more significant average value, more measurements are needed (in this case fifteen measurements have been made for each PS-Br considered).

An example of element microanalysis for a measurement on PS-Br 2 is shown in Table 2.

Table 2: elements content for one of the measurements carried out on PS-Br 2.

The mapping of the elements (including bromine, in yellow) for a measurement again carried out on the PS-Br 2 is shown in image 2.

Image 2: SEM image and elements mapping for a measurement on PS-Br 2

It can be noted, as mentioned, that the average value of the bromine content (calculated on all the granules and positions) is close to that declared by the manufacturers and reported on the technical data sheet.

For PS-Br 2, the bromine content value is also practically identical to the average value obtained from the values ​​determined by the manufacturer on twenty-four production batches of this PS-Br (average value of 59.96% against a measured average value of 60%).

Furthermore, on the other hand, if it is known that a compound contains PS-Br (and does not contain any other relevant source of bromine) through an evaluation of the bromine content it is possible to make a reasonable assumption on the PS-Br content in the compound (this is useful both as a control of the bromine content that the material should have based on the defined PS-Br content, and as an evaluation, although complementary to other analyses, of the PS-Br content in case of compounds with an unknown composition).

For example, some compounds containing known percentages of PS-Br were analysed and the following table shows the results obtained:

Table 3: average measured and nominal bromine content for some compounds containing specific PS-Br. Measurements carried out on the central (internal) part of a plate moulded with the analysed compound.

In this case, even taking into consideration the fact that the analysed compounds contain many components and that the quantities of these components (including PS-Br) are in any case fluctuating with respect to the nominal value (and that the compounds also have imperfect homogeneity , also due to the production and / or transformation process), the analysis on a microscopic scale (for a limited number of measurements) yields values ​​that are more different from the nominal value than those found for PS-Br as such, although in line with nominal values ​​as order of magnitude.

Clearly if the analysis is made on a part (or specimen) obtained for example by injection moulding, between the surface of the part and the internal zone there may be a variation in the punctual concentration of elements linked to specific additives, such as PS -Br, due to effects related to the transformation of the compound.

The SEM-EDX analysis can also be useful to verify the presence of metallic impurities present in the PS-Br and, from the mapping of the elements, it is also possible to have an idea (albeit qualitative) of the homogeneity of the material, for example of the homogeneous presence of bromine in the polymer matrix (important for the self-extinguishing behaviour of the material).

In RDLab137 we can support for evaluations of this type, or for other morphological analyses, and mapping of the elements, through SEM analysis with EDX probe for microanalysis of the elements (the instrumentation is available in RDLab137).

Ing. Luca Ciceri - RDLAB137 srl

Last revision: 28/04/2021