A NEW RAPID ANALYSIS METHOD FOR FIRE RETARDANTS IN POLYMERS

Transcription

1 A NEW RAPID ANALYSIS METHOD FOR FIRE RETARDANTS IN POLYMERS P Baird, H. Herman, W. Mortimore and G. C. Stevens GnoSys UK Ltd, University of Surrey, Guildford, Surrey GU2 7XH, UK. ABSTRACT The reliable and rapid analysis of fire retardants (FRs) in polymers has become an imperative to support the growing need for improved management of recycled plastics, the auditing of FR containing materials and components and to support the formulation and production of virgin polymer compounds and masterbatches. Such requirements are being driven by European directives and also by the growing trend of green procurement and Ecolabels to avoid particular types of FR. The FR industry may also make use of such methods to demonstrate compliance and to support voluntary emission reduction schemes such as VECAP. However, to be effective it is important that the analysis methods provide reliable molecular identification of the FR system. Measurement should also provide quantitative information on the concentration of the FR present in the material or component in question. For recycling it is also valuable to be able to identify the host material and its salient characteristics. Finally, with the growing requirement to undertake measurements in the field, on the factory floor and during regulatory inspection or testing, it is essential that the techniques be fully portable and robust. The increasing use of coloured and black carbon-loaded engineering thermoplastic polymers for electronic equipment enclosures may cause problems for the recycling of these materials if the materials and FR systems used are not identified. This may lead to many materials waste streams being classified as hazardous due to uncertainty regarding their chemical composition and the potential presence of FRs that may be classed as hazardous. We have investigated a variety of molecular and elemental spectroscopies to provide both elemental and molecular analyses of engineering thermoplastics containing new and conventional brominated and organo-phosphorus FRs. In both cases gravimetrically calibrated specimens were used to construct predictive models to enable rapid and accurate measurement of concentration in addition to molecular identification of the FR system. The analytical problem of carbon black filled enclosure materials is significant as many remote analysis methods are completely overwhelmed by the lack of material reflectivity. We describe the development and application of a portable Raman method that is capable of identifying organic based FRs and most inorganic crystalline FRs even in the presence of carbon black. This includes FR systems that are generally difficult to identify and quantify, including brominated as well as such as phosphorous and inorganic based systems. We discuss the use of the Raman method in the rapid identification of the polymer, discrimination of closely related FRs and measurement of their concentration in a portable lightweight device. Further we describe the ability of the method to undertake rapid measurements in support of compounding of virgin materials and ongoing developments to support materials recycling. Page 1 of 14

2 INTRODUCTION There are three main applications requiring rapid analysis of flame retardants in polymeric materials: 1) for quality control purposes during the formulation and production of virgin polymer masterbatches, 2) for quality control during the production of electric and electronic equipment in order to comply with the RoHS Directive, and 3) in the characterisation of waste engineering thermoplastics in order to qualify them for material recycling or recovery processes [1]. As a result of the RoHS Directive prohibiting the use of polybrominated diphenyls (PBBs) and poly bromodiphenylethers (PBDE) at concentrations above 0.1% by mass, manufacturers of electrical and electronic equipment (EEE) have had to inspect large numbers of components in order to demonstrate compliance. Recycling of polymeric materials has increased in importance in recent years. As such, identification and sorting of materials according to FR and polymer type is necessary prior to recycling [2,3]. This will increasingly require differentiation of restricted or banned FRs in historic waste from currently acceptable FRs. Formulation additives as well as the polymer matrix itself can present significant interference challenges in the analysis of FRs in polymers. Conventionally, removal of these typically requires time-consuming destructive sample pre-treatment steps to extract the substances of concern from the matrix. We have previously examined several optical spectroscopic methods that are non-destructive, portable, rapid, and potentially provide identification of polymer matrix and FR [4]. This paper concentrates on Raman methods. RAMAN METHODS The use of infrared (IR) analysis to examine FRs at the 5 to 20% level in flame-retarded polymer systems is well known. However, the strong polymer matrix absorbances seen in the IR require the use of sample contact attenuated total reflectance methods (ATR), and thus the analysis tends to be limited to a few microns depth at the surface. Although this approach has utility, for example in looking at the FRs in low-density polyurethane foams, there are many examples where the surface is not representative of the bulk material. By using the near infrared spectral region (NIR wavelength range from 780 to 2500 nm) where the absorbances are at least two orders of magnitude weaker, the analysis is not skewed by surface effects [4]. However, as with infrared methods, highly halogenated FRs are not sufficiently distinguished to provide reliable concentration predictions and access to the low-frequency molecular carbon-chlorine and carbon-bromine vibrations would be desired for discrimination. Raman spectroscopy is a laser based spectroscopic scattering technique that is complimentary to infrared. The main advantages arise from operating in the visible and near infrared spectral ranges, where conventional fibre-optics can be used to guide the laser excitation and capture the scattered signal, and the ability to look at low frequency molecular vibrations, such as those seen in highly brominated systems [5]. It does not require sample contact. In other respects it provides information similar to that obtained from infrared methods though the spectral bands tend to be sharper, especially for crystalline and inorganic species. The technique can be swamped by fluorescence from degraded samples, but this can be minimised by a suitable choice of excitation wavelengths, preferably using lasers in the near infrared. Page 2 of 14

3 Raman data were acquired on several instruments, including a laboratory FT-Raman system [6] operating with an excitation wavelength of 1064 nm and a spectral resolution of 4 cm -1, and a number of portable dispersive systems [7] with excitation wavelengths around 785 nm and spectral resolution between 4 and 30 cm -1 ; an example is shown in Figure 1. Figure 1: A Portable Dispersive Raman System with Fibre Coupled Probe. Materials Analysed The materials analysed are shown in Table 1. Table 2: Phosphorus and Bromine-based FRs and Polymers Examined by Raman. Page 3 of 14

4 Application to Brominated FRs Figure 3 compares several base resins used in electrical and electronic equipment and FR (decabromodiphenyl ether). FR-1210 is characteristic of heavily brominated FRs, having very strong but unique bands relating to C-Br deformations in the 200 to 240 cm -1 region. Figure 3: Comparison of FR-1210, HIPS, ABS and PC. FR HiPS -100 ABS PC Energy / Raman Shift (cm-1) This FR can be distinguished from other brominated FRs, as seen in Figure 4 which compares several FRs with ABS. The Raman bands of brominated FRs are intense, Page 4 of 14

5 with a practical limit of detection around 0.01 to 0.1 wt%, for spectral acquisition times of 10 to 100 seconds using 785 nm excitation. It is worth noting that these strong bands do not coincide with any significant sharp bands relating to the polymer, and different brominated FR types can be distinguished due to slight variations in molecular structure. Some sharp bands do occur in this region due to the Sb-O vibrations of the antimony trioxide co-synergist in the polymer matrix, which is commonly added to the FR, but these bands are at significantly different frequencies to allow clear discrimination. Figure 4: Comparison of FR-1210, FR-245, FR-720 and ABS, all to the same scale FR FR FR ABS Energy / Raman Shift (cm-1) As with many additives in polymers, the spectra of the pure materials maybe changed in the polymer matrix, and this is illustrated for FR-1210 in Figure 5. Here most of the bands are due to antimony trioxide the spectrum of ABS in this region is almost featureless. The signal due to FR-1210 is somewhat degraded compared to its pure form, but still easily distinguished and measured down to a level of 0.1 wt%. Figure 5. ABS and 10% FR-1210 in ABS matrix, all to the same scale ABS with 10% FR ABS reference Energy / Raman Shif t (cm-1) Page 5 of 14

6 A comparison between gravimetric and spectrally determined concentrations is shown in Figure 6. Some of the apparent variance in the calibration is attributable to the non-homogeneity of the dispersal of FT1210 in the polymer matrix. FR-1210 concentration from spectra / % Figure 6: Calibration of FR-1210 in ABS matrix FR-1210 concentration / % As with the NIR approach used elsewhere [4], the optimum methodology for an automated system lies in first identifying the polymer matrix, and then the additive, before producing an estimated concentration. Application to Phosphate FRs Phosphate ester-based oligomers are used in thermoplastic blends such as PC/ABS. Raman spectra of RDP (resorcinol bis(diphenyl phosphate)) and BDP (bis-phenol A-bis(diphenyl phosphate)) are shown in Figure 7. Page 6 of 14

7 Figure 7: Raman Spectra of Fyrolflex RDP and BDP RDP 0 BDP Calibration samples using these FRs in a number of PC/ABS blends provide the data shown in Figure 8; visually there appears to be only weak information on the FR concentration. Figure 8: Raman Spectra of RDP and BDP in PC/ABS blends. However, the use of multivariate statistical methods allows for even apparently weak signals to be extracted and used in a reliable way to calibrate the methodology. Furthermore, this approach is not limited to simply obtaining a concentration. A number of gravimetrically formulated samples were used to construct a model that directly links the spectral data to polymer composition, phosphorus concentration and even the result of the UL-94 flammability tests, with the results shown in Figure 9. Page 7 of 14

8 Figure 9: Multivariate Model results for PC:ABS ratio, sample phosphorus content and UL-94 score. Predicted PC:ABS ratio PC:ABS ratio Predicted Phosphorus content / % Phosphorus Content / % Predicted UL-94 Score UL-94 score The precision of the method is 0.2 for the PC:ABS ratio, 0.06% for the phosphorus content, and 2.4 units for the UL-94 score. This latter variable was heavily influenced by an inverse correlation with the PC:ABS ratio and that the pass samples were all assigned a 30 value in the UL test. Nonetheless, it demonstrates that a relatively quick and simple method can provide good predictions of properties that are difficult and laborious to measure. Distinguishing PBDE FRs One of the most pressing needs for recyclate materials is the ability to determine the presence of polybrominated diphenyl ethers and to distinguish the penta-, octa- and deca-substituted additives. Until recently deca-bde was still an accepted FR. The Raman spectrum of the three PBDEs is shown in Page 8 of 14

9 Figure 10, where the spectra are also compared with theoretical predictions using Gaussian 03 with no adjustable parameters. Although the band positions are 40 to 50 cm -1 higher than observed the relative intensities are very well reproduced which is a little surprising given that PBDE is a mixture of congeners. Page 9 of 14

10 Figure 10: Raman of Deca-, Octa- and Penta-BDE Model Overlay. These three PBDEs are apparently easily distinguished using the Raman method. However, once in a polymer matrix, such as ABS, the spectral bands change somewhat (as seen in Figure 4). Even with band changes, there is still discrimination although the limits of detection are degraded to about 0.2 wt % for a reasonable analysis time. Carbon-loaded samples The current vogue for black housings in consumer electronics means that there is a large amount of carbon loaded polymer being formulated, incorporated into components and discarded at end of product life. The presence of carbon black makes spectroscopic analysis more difficult, especially when it is required to analyse additives at low concentrations in the polymer matrix as could be required for recycle. Methods which rely on reflection of visible or near-infrared light (including Raman) have difficulty due to the strong absorbing characteristics of carbon black. The additional presence of lacquer coatings on many black samples causes further issues for surface sensitive method such as Raman. Surface coatings were analysed and found to be a thin inorganic material which contributed to loss of signal and poor discrimination. With this removed through simple abrasion to reveal the bulk material underneath yielded a typical signal for Raman analysis. Examples of HIPS samples from used TVs are shown in Page 10 of 14

11 Figure 11. Page 11 of 14

12 Figure 11: Samples of HIPS from used TV enclosures surface coating removed. There is some gradation of black across these samples, and hence different levels of interference from the carbon colouring. If no precautions were taken the dwelling laser spot of the Raman instrument heats up and damages the black sample surface to a degree that prevents any practical measurement, and therefore the laser spot has to be moved during the data acquisition. This is not necessary for non-black colouring. Using these sample preparation and measurement techniques, a reasonably clean Raman spectrum can be obtained from these materials, as shown in Figure 12. Figure 12: Raman spectrum of black HIPS sample with surface coating removed. Carbon black Page 12 of 14

13 Various features are observed, in addition to carbon black, which are characteristic of HIPS. There is enough detail to distinguish between polymer types in the presence of carbon black. Detection of high level additives is also possible, though the limit-of-detection compared to non-black samples is degraded by a factor of ten at the current stage of development. DISCUSSION It is clear there is no one technique capable of providing discrimination and quantitation of FRs across all the different consumer polymers in the market. Infrared spectroscopy, even when used in the laboratory with some fairly laborious sample preparation, is generally limited to surface analysis (1-2 microns) using a contact ATR method. Past experience has shown that the surface is not necessarily indicative of the bulk material. Both Raman and near infrared methods have much greater penetration and provide answers that correlate with gravimetric determinations. The presence of carbon black causes difficulties for all the molecular spectroscopy techniques: NIR methods are overwhelmed, IR depth penetration is further reduced, and the Raman method requires that the analyser spot is kept moving over the sample surface, otherwise sample damage obscures the analysis. Poor quality carbon-black can also fluoresce under the laser beam, limiting the analysis. However, because of the sensitivity to carbon, the Raman method could provide information on carbon quality. The Raman method is the only one out of the three primary techniques that provides for a reasonable amount of spatial resolution typical spot size from portable systems is about 200 microns whereas the other methods analyse between 1 to 400 mm 2. This may be of use for dispersion studies in high concentration systems. CONCLUSIONS Robust molecular spectroscopy methods to identify and quantify FRs in a polymer matrix must first identify the matrix and then assign the FR before undertaking a concentration determination. This can be undertaken using a number of vibrational spectroscopy methods sensitive to polymer matrix composition, additive identity and additive concentration, most of which require a multivariate statistical approach to extract this information. These methods can be implemented on portable instruments that can be used in the field or integrated into the production processes. In the quest for in-situ measurements with no sample preparation, none of the possible methods is universal. NIR methods are insensitive to highly brominated FRs without hydroxyl groups present and cannot cope with materials with a high level of carbon black filler; Infrared methods are biased by the sample surface and do not see highly brominated FRs. In contrast, Raman methods can be affected by fluorescence from degraded materials or very poor quality carbon-black. However, Raman appears to cover most masterbatch and compounded materials systems well and it will handle many but not all recyclate materials in its current state of development. The resulting spectra are information-rich and can be used not only to discriminate between polymer matrices and additives but it can quantify additives when present at greater than 0.1 wt%; this includes most FRs with few exceptions. The technique can also be correlated with other materials properties, such as copolymer composition, melt flow rate, density, mechanical properties etc., and the prospects for predicting UL-94 test performance is good and is likely to extend to total heat release. The work presented here shows that the Raman method can be used to identify and measure absolute concentrations of FRs in compounding and masterbatching applications, with rapid 10 to 20second measurements replacing many hours of conventional analysis. These applications are is viable and Page 13 of 14

14 robust for both low and very high additive concentration formulations. The small analysing area will reveal and quantify variation in additive and FR dispersion. This can be averaged by examining more samples, as for instance on a conveyor or at the exit of an extruder or pelletiser. This is a valuable addition to the tools available to formulating high concentration materials where dispersion and solubility are key metrics for reliable production. For materials recycling the discriminating powers of the Raman method tend to be better than the other vibrational spectroscopy methods and thus producing the expert system to automatically discriminate, identify and quantify the matrix and FRs will be more robust. Calibration models have been built for the major FRs in the large volume polymer matrices, but the constantly evolving nature of FR development for different polymers and blends means that there will invariably be some samples that will not be identified unless the models have been established. Judicious use of multivariate methods will enable such difficult to analyse materials to be identified and managed accordingly. Development and refinement of the Raman method for general recycling applications is therefore ongoing. ACKNOWLEDGEMENTS We wish to thank ICL-IP for the manufacture and supply of gravimetrically determined samples, and for conducting the UL-94 tests. REFERENCES [1] Sabine Kemmlein, Dorte Herzkeb and Robin J. Law, Brominated flame retardants in the European chemicals policy of REACH Regulation and determination in materials, J. Chromat. A, 2009, 1216(3), [2] G C Stevens, H Herman, R Mason and P J Baird, The rapid assessment of electronic product enclosure plastics for manufacturing support and end of life management, Sustainable use of materials 2006, Oxford, September [3] G C Stevens, H Herman and P J Baird, Rapid assessment of electronic product enclosure plastics for improved resource management, Polymers in Electronics 2007, Munich, [4] P.J. Baird, H. Herman and G.C Stevens, Rapid assessment of electronics enclosure plastics, Chapter 9 of Issues in Environmental Science and Technology, 27, Electronic Waste Management, eds. R E Hester and R M Harrison, RSC, [5] S. Kikuchi, K. Kawauchi, S. Ooki, M. Kurosawa, H. Honjho, and T. Yagishita, Non-destructive Rapid Analysis of Brominated Flame Retardants in Electrical and Electronic Equipment Using Raman Spectroscopy, Anal, Sci., 2004, 20(8), [6] P Baird, I Finberg, P Georlette, H Herman, W Mortimore and G Stevens, Flame Retardants 2008, London, [7] P. Baird, I. Finberg, P. Georlette, J. Leopold, H. Herman, W. Mortimore and G. C. Stevens, A new rapid analysis method for flame retardants in polymers, Fire and Materials 11 th International Conference and Exhibition 2009, San Francisco, Page 14 of 14