Analysing polymers

FTIR Spectroscopy may aid compliance with new RoHS directives

From July 2006 the RoHS (Restriction of Hazardous Materials) directive will restrict and regulate the use of certain substances in electrical and electronic equipment. Its objective is to protect human health, the environment and to reduce contamination by brominated compounds during the recycling of electronic wastes. Consequently, polybrominated biphenyls (PBB) and polybrominated diphenylethers (PBDE), currently used as flame-retardants in polymers in concentrations from 5% up to 10%, can no longer be used unless their concentrations are lower than 1,000 ppm. The RoHS will restrict the use of compounds such as tetrabrominated biphenyl A (TBBA), brominated polystyrene and brominated aromatic triazine.

Figure 1. Structural formulae of brominated biphenyls

According to RoHS, pentabrominated diphenyl ether (PentaBDE) and octoabrominated diphenyl ether (OctaBDE) are considered hazardous. OctaBDE has been used in polymers such as ABS and PS. Currently, decaBDE, which is largely being used as a flame retardant in PS, PE, ABS and polyester, has not yet been included in the directive. Commercial decaBDE, however, consists of a mixture of approximately 97% to 98% decaBDE and 0.3% other BDEs.  Therefore, when a polymer contains 10% decaBDE (containing 1% contamination of other brominated BDEs), the PBDE content will exceed the RoHS threshold value of 1,000 ppm.

FTIR spectroscopy – fast, non destructive, simple
To comply with the requirements of the RoHS directive, first the total bromine content of a sample is determined.  If this exceeds 5% after preliminary examination using the Shimadzu EDX systems, infrared spectroscopy is recommended to enable compound identification. This simple and non-destructive method can quickly lead to useful results. Compound identification has been possible as the flame-retardants were present in concentrations of higher than 5%. This level is still detectable in polymer mixtures using FTIR. However, concentrations that approach the detection limit must be measured using other analytical methods. In these cases, GCMS is highly suitable as all brominated compounds can be separated and detected down to the trace level.  GCMS, on the other hand, is more time consuming with respect to sample preparation and data analysis.

In general it is recommended pre-screening via energy-dispersive X-ray fluorescence (EDX) is carried out. Using this analytical method the total concentration of elemental bromine in the sample is detected, although it is not possible to distinguish which compound actually contains the bromine. When more than 5% of total bromine is detected, FTIR can be used for further identification of bromine compounds. When less than 5% bromine is detected, GCMS analysis can be implemented for separation and identification.

Fast and straightforward analysis of polymers is possible since brominated biphenyls exhibit their own very characteristic infrared spectra.  The polystyrene example (Fig 2) exhibits three spectra: DBDPE, PS with DBDPE and pure PS.  The range in the IR fingerprint, where DBDPE in PS identification is possible, is clearly discernable.   

Figure 2: Polystyrene spectra with and without flame-retardant as well as
the IR spectrum of the flame-retardant decabrominated diphenyl ether

Figure 3: IR spectra of polystyrene with several flame-retardants

Fast identification of brominated flame retardants
The fingerprint region of 1500 cm-1 to 1000 cm-1 is important for the identification of brominated flame-retardants, where clear differences between the spectra can be seen.  Based on this information, an analytical method for fast identification of brominated flame-retardants and polymers has been developed using Shimadzu’s FTIR 8400S in combination with a single-reflection accessory.

In the present example, a diamond ATR unit with KRS-5 crystal was used as a single –reflection accessory. A diamond is recommended as a sample surface because the polymer can be present in a flexible or solid state. The diamond surface enables the application of high pressures to ensure the sample is positioned tightly on the crystal so that optimum penetration of the sample by the beam is guaranteed.  The beam penetrates the sample surface to a depth of approximately 2 µm.

As RoHS specifies a homogeneous sample material, this depth of penetration is sufficient in order to completely characterise the sample. Using the measuring configuration, the spectrum is acquired within a very short time interval (approximately one minute) and is evaluated automatically according to RoHS guidelines.

To validate analysis results, the spectrum is compared with a library of polymer spectra. For polymer identification, a database already containing 41 polymers is used. This database contains logical associations and the Distinction Software tests for plausibility, for instance by evaluation of signal ratios.
 
Table 1 showing overview of 41 polymers and polymer mixtures in an expandable database.
The decision criterion includes warning messages that range from “identification of the polymer not possible” to “applied pressure not sufficient” and finally to the conclusion “OK” or “not OK”.  These FTIR analysis results can be combined with the pre-analysis from the EDX System.

Infrared spectrometry can therefore be regarded as a fast and simple alternative solution to the pre-selection of polymers.  Minimal sample pre-treatment is necessary and fast results are obtained via predefined methods.

By Shimadzu