Effects of Reprocessing on Additives in ABS Plastics, Detected by Gas Chromatography/Mass Spectrometry

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1 Effects of Reprocessing on Additives in ABS Plastics, Detected by Gas Chromatography/Mass Spectrometry Effects of Reprocessing on Additives in ABS Plastics, Detected by Gas Chromatography/Mass Spectrometry X. Bai 1 *, B.K. Stein 2, K. Smith 3, and D.H. Isaac 4 1 School of Material Science and Chemical Engineering, China University of Geosciences, Wuhan , China 2 EPSRC National Mass Spectrometry Service Centre, Swansea University, Swansea SA2 8PP, UK 3 School of Chemistry, Cardiff University, Cardiff CF10 3AT, UK 4 Materials Research Centre, School of Engineering, Swansea University, Swansea SA2 8PP, UK Received: 23 March 2011, Accepted: 15 August 2011 SUMMARY Acrylonitrile butadiene styrene (ABS) plastic is a widely used engineering thermoplastic. ABS is also an interesting material for recycling. In the present study, two pieces of ABS plastic from waste computer equipment were reprocessed. A torque rheometer was used to simulate reprocessing. To identify the additives used in the recycled ABS plastics and understand the effect of reprocessing on the recycled plastics from several aspects, gas chromatography/ mass spectrometry (GC/MS) was used to analyse the extracts from recycled ABS plastics before and after reprocessing. The effects of reprocessing temperature and reprocessing cycle on some additives, mainly antioxidants and lubricants, were investigated. Plastic processing led to additive loss because of volatilisation, decomposition, or conversion, particularly at high temperatures and during multiple processing cycles. Keywords: GC/MS; Antioxidant; Lubricant; Reprocessing temperature; Reprocessing cycle INTRODUCTION Additives are introduced during the initial processing of polymeric products to confer specific properties, such as long-term light and oxidative stability, fire Smithers Rapra Technology, 2012 Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1,

2 X. Bai, B.K. Stein, K. Smith, and D.H. Isaac retardancy, plastification, processability, and toughness, upon the final plastic products. These additives may be consumed, suffer chemical transformation, or even migrate to the environment during the polymer s lifetime. The study of the residual presence of these additives in polymeric waste and recyclates is of vital importance in the recycling industry. Methods for the determination of polymer additives usually combine an extraction step prior to analysis, although some extraction-free methods using spectroscopic and spectrometric techniques have been developed for determining the concentrations of additives. Both gas and liquid chromatography can be employed for extract analysis [1]. Gas chromatography in combination with mass spectrometry (GC/MS) is a powerful tool for the identification of trace components in complex mixtures without isolation. Many studies have been conducted on volatile emissions during the thermal treatment of polymers [2 4] and processing of plastics [5]. The technique has also been used to analyse the extracts from plastics. A wide range of compounds, from polymer segments to additives, have been identified by GC/MS [6 9]. Different low-molecular-weight compounds, such as residues of polymerisation, degradation products and additives, have been identified and relatively quantified using microwave-assisted extraction and further analysis by GC/MS. The real recycled samples may contain contaminations and fragments from additives included in their original formulations [10]. Acrylonitrile butadiene styrene (ABS) is an engineering thermoplastic that consists of an amorphous, heterophasic polymer with very good mechanical properties, especially high impact resistance. Poly(styrene-co-acrylonitrile) (SAN) copolymer forms the continuous phase. The second phase consists of dispersed polybutadiene or butadiene copolymer. These particles have a layer of SAN grafted onto their surface, which makes the two phases compatible [11]. ABS is an engineering thermoplastic widely used in the automotive industry, telecommunications, business machines and electric/ electronic casings. ABS is also an interesting plastic material for recycling. There are many studies on the effect of reprocessing on ABS resins [12 17], while we studied the effect of reprocessing on recycled ABS plastics from waste computer casings [18]. To identify the additives used in the recycled plastics and understand the effect of reprocessing on the recycled plastics from various aspects, GC/MS was used to analyse the extracts from waste ABS plastics before and after reprocessing. A torque rheometer was used to simulate reprocessing. The effects of recycling on some flame retardants have been studied [19 21]. In the present study, the effects of reprocessing on some other additives, mainly antioxidants and lubricants, were investigated. 2 Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1, 2012

3 Effects of Reprocessing on Additives in ABS Plastics, Detected by Gas Chromatography/Mass Spectrometry EXPERIMENTAL Materials 1. Virgin ABS. Virgin ABS was purchased from Aldrich Chemical Company. The molar ratio of acrylonitrile/butadiene/styrene was 2:1:2, and the glass transition temperature of the SAN phase was 103 C. 2. Standards. Antioxidant butylated hydroxytoluene (BHT) in the form of white crystals was purchased from Aldrich Chemical Company. Its molecular weight (MW) was , its boiling point (b.p.) was 265 C, and its melting point (m.p.) was C. Antioxidant tris(2,4-di-tert-butylphenyl) phosphite (Irgafos 168) in the form of a white powder was provided by Ciba Specialty Chemicals Company. Its MW was 646.9, its b.p. was not applicable, its m.p. was C, and its density (20 C) was 1.03 g/cm 3. Lubricant methyl stearate in the form of fine white plates was purchased from Aldrich Chemical Company. Its MW was , its b.p. was C, and its m.p. was 38 C. Lubricant ethylenebisstearamide (EBS) was purchased from Aldrich Chemical Company. Its MW was , its b.p. was not applicable, and its m.p. was C. 3. Waste ABS Plastics. Two pieces of plastic from waste computer equipment were identified as ABS plastic by Fourier transform infrared spectroscopy (FTIR). Extraction Procedure Extraction of Waste ABS Plastics Before and After Reprocessing Samples were produced from the two waste ABS plastics. The first waste plastic, named ABS1, was reprocessed by a torque rheometer at 230 and 270 C for 10 min. The second waste plastic, named ABS2, was reprocessed at 230 C for 10 min for one, two, and three cycles. A sample of plastic (6.00 g) before reprocessing or after reprocessing was placed in a 100 ml round flask. Dichloromethane (40 ml) was added as the extraction agent. The flask was sealed, and the mixture was allowed to stand for 24 h, after which it was ultrasonicated for 2 h, and finally filtered through Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1,

4 X. Bai, B.K. Stein, K. Smith, and D.H. Isaac glass wool. The undissolved solid was recovered, and extraction was performed again, this time with dichloromethane (30 ml) and for 30 min in an ultrasonic bath. The two extracts were collected together, and methanol (80 ml) was added to precipitate any polymer. The solution was filtered through a glass fibre filter and then concentrated to 3 ml. After filtering once more through a glass fibre filter, the concentrated extract was analysed by GC/MS. Extraction of Virgin ABS With and Without Additives After Processing To confirm the GC/MS analysis results for the recycled plastics, standards of BHT and methyl stearate were dissolved in methanol and then injected into the GC/MS system under the same conditions as the ABS plastic extracts. Virgin ABS and standard Irgafos 168 and EBS were used to produce two different samples: (1) virgin ABS after processing at 270 C by a torque rheometer for 10 min; (2) virgin ABS with 3.5% lubricant EBS and 2% antioxidant Irgafos 168 after processing at 270 C for 10 min. After processing, the samples were extracted by solvent. Samples of processed virgin ABS (10.00 g) with and without additives were placed in 100 ml round flasks. Dichloromethane (60 ml) was added as the extraction agent. The mixture was allowed to stand for 24 h and then subjected to ultrasound in an ultrasonic bath for 2.5 h. Methanol (80 ml) was added to precipitate any polymer. The solution was filtered through a glass fibre filter and then concentrated to 2 ml. After filtering through a glass fibre filter, the concentrated extract was analysed by GC/MS. Identification of Extracts by GC/MS GC/MS was performed at the EPSRC National Mass Spectrometry Service Centre at Swansea University. A Fisons GC 8000 gas chromatograph with a Fisons MD 800 mass spectrometer (Thermoquest, UK) was used. The gas chromatograph was equipped with a fused silica capillary column with polydimethylsiloxane liquid phase (30 or 15 m 0.25 mm i.d.; film thickness 1.0 μm; Chrompack DB-1). Gas chromatograms with long retention times were obtained using a long column, whereas total ion current chromatograms with short retention times were obtained using a short column. The chromatographic conditions were as follows: initial temperature 40 C held for 1 min, then raised at 8 C/min up to 300 C and held for 20 min; carrier gas helium (head pressure 31 kpa); injection temperature 220 C; injection splitless; injection volume 1 μl; mass spectrometer source temperature 200 C; electron energy 70 ev; electron current 200 μa; GC/MS interface temperature 300 C. For identification of mass spectra, the NIST/EPA/NIH Mass Spectra Library (v.2.0) was used. 4 Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1, 2012

5 Effects of Reprocessing on Additives in ABS Plastics, Detected by Gas Chromatography/Mass Spectrometry RESULTS AND DISCUSSION Analysis of Additives in Recycled ABS Plastics Figure 1 shows the total ion current (TIC) traces of the ABS1 extracts before and after reprocessing at 270 C. The dominant peak in the TIC chromatogram of ABS1 before reprocessing peaked at min (scan 874). The peak was an antioxidant butylated hydroxytoluene (BHT). At 24 magnification, several other peaks could be seen in the chromatogram. Using the library of mass spectra in the computer, it was found that most of the peaks were oligomers of acrylonitrile, styrene, or butadiene, or combinations of these monomers. In the TIC trace of ABS1 after reprocessing at 270 C, the BHT peak was significantly reduced. At the same time, some peaks (scans 1032, 1206, 1365, 1514, etc.) increased. The component giving rise to the mass spectrum of the ABS1 peak at min (scan 1032) after reprocessing at 270 C appeared from the library search to be 2,6-di-tert-butyl-4-methoxymethylphenol, a possible derivative product of BHT resulting from oxidation in the methyl group and subsequent reaction with methanol. However, there was also a significant new ion at m/z 100 in the mass spectrum, suggesting that the peak contained a second component after reprocessing. The new ion at m/z 100 may be an N-methylene morpholine radical that belongs to a decomposition product of morpholine fatty acid salt. Figure 1. TIC traces of ABS1 before and after reprocessing at 270 C Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1,

6 X. Bai, B.K. Stein, K. Smith, and D.H. Isaac The mass spectrum of the peak at min after reprocessing at 270 C (scan 1096) corresponded to 3,5-di-tert-butyl-4-hydroxylbenzaldehyde, a possible oxidation product of BHT. The mass spectra of ABS1 peaks at (scan 872), (scan 1032), and min (scan 1096) after reprocessing at 270 C are shown in Figure 2. In the TIC trace of ABS before reprocessing, the peak at min (scan 1235) was given by methyl palmitate, and the peak at min (scan 1388) was methyl stearate. Methyl palmitate and methyl stearate are lubricants commonly used in plastics. From Figure 1, after reprocessing at 270 C, the peaks of methyl palmitate and methyl stearate became smaller relative to other peaks in the spectra. These findings suggest that the contents of the two compounds decreased during reprocessing at 270 C. Comparing the TIC traces of ABS1 before and after reprocessing at 270 C, after reprocessing at 270 C, the peaks at min (scan 1206) and min (scan 1365) increased. Based on the mass spectra of the two peaks (Figure 3), the compounds may be N-alkyl morpholines. Their long-chain aliphatic group indicates that they are decomposition products of a lubricant. The unknown lubricant may be a morpholine fatty acid salt. During reprocessing at 270 C, the lubricant decomposes into lower molecules, which could be Figure 2. Mass spectra for ABS1 components eluted at 17.54, 20.21, and min after reprocessing at 270 C 6 Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1, 2012

7 Effects of Reprocessing on Additives in ABS Plastics, Detected by Gas Chromatography/Mass Spectrometry Figure 3. Mass spectra of ABS1 peaks at scans 1206 and 1365 Figure 4. Extracted ion chromatogram for ions at m/z 100 and the corresponding TIC trace of ABS1 after reprocessing at 230 C detected by the GC/MS system. From Figure 1, before reprocessing, these lower molecules already exist in the material, but their concentration is low. This suggests that the lubricant is fairly stable at room temperature and at low processing temperatures. However, it decomposes at high processing temperatures. The ABS1 extract reprocessed at 230 C was also analysed by GC/MS. The extracted ion chromatogram for the ion at m/z 100 and the TIC trace of the extract are shown in Figure 4. The shorter retention times Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1,

8 X. Bai, B.K. Stein, K. Smith, and D.H. Isaac compared with those referred to in Figure 1 resulted from the removal of part of the GC column. The extracted ion chromatogram for the ion at m/z 100 shows that this ion occurred in a number of the peaks, most abundantly in peak scan 475. However, the TIC trace shows that peak scan 475 was only a very minor component, which confirms that the degree of decomposition of the lubricant is low at low processing temperature. In Figure 1, the peak at min (scan 1514) increased after reprocessing at 270 C. Using the library of mass spectra, this peak was identified as a possible degradation product of ABS. The chromatograms of extracts from ABS2 after reprocessing at 230 C for zero, one, two, and three cycles are shown in Figure 5. The peaks at min (scans 840 and 841) and and min (scans ) grew with increasing number of reprocessing cycles. The peak at min, confirmed to be 2,4-di-tert-butylphenol by comparison with a standard, obviously increased after one cycle of reprocessing. This implies that, during reprocessing, some components in ABS2 decomposed to yield 2,4-di-tertbutylphenol. The compounds at and min could be hexadecanenitrile and octadecanenitrile respectively. This indicates that ABS2 may contain EBS [22]. EBS is used as a plastic lubricant mainly in the production of ABS Figure 5. TIC trace of the ABS2 extracts after reprocessing for different numbers of cycles 8 Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1, 2012

9 Effects of Reprocessing on Additives in ABS Plastics, Detected by Gas Chromatography/Mass Spectrometry copolymers [16]. The main component of EBS is ethylenebis(stearamide), which is produced from stearic acid and ethylenediamine (EDA). Technicalgrade stearic acid may contain other fatty acids, mainly palmitic acid, so EBS is not only composed of N,N -ethylenebis(stearamide) but may also contain N,N -ethylenebis(palmitamide) and ethylene-n-palmitamide-n -stearamide. When EBS decomposes, hexadecanenitrile and octadecanenitrile can form. Increasing the amounts of EBS could lead to decomposition at 230 C with increasing numbers of reprocessing cycles. The peak at min was identified to be palmitic acid. After three reprocessing cycles, the peak becomes smaller. Besides palmitic acid, some other compounds can be found in the material, e.g. the compound at min (scans ) appears to be stearic acid, but its content is much lower than that of palmitic acid. From Figure 5, the concentration of stearic acid increased after reprocessing for two cycles, and then decreased after reprocessing for three cycles. The stearic acid of ABS2 may have been a product formed from an acid scavenger, e.g. magnesium stearate [16]. The metal stearate decomposed during reprocessing, and the molecular weight decreased. Before the second reprocessing cycle, the rate of decomposition of metal stearate was faster than the volatilisation of the decomposition products. Therefore, the concentration of stearate acid detected by GC/MS increased. When ABS2 was reprocessed for a third cycle, the concentration of metal stearate was obviously reduced. The rate of decomposition of metal stearate was slower than the volatilisation of decomposition products, which led to the decreased concentration of stearate acid detected by GC/MS. Palmitic acid in ABS2 may originate from metal palmitate because metal stearate is always a compound of metal with a mixture of solid organic acids and chiefly consists of variable proportions of metal stearate and metal palmitate. Identification by Standards The TIC trace of standards BHT and methyl stearate in methanol is shown in Figure 6. The first peak (scan 359) is BHT; the second (scan 579) is methyl stearate. The TIC trace for ABS1 after reprocessing at 230 C is also shown in Figure 6. The peak of standard BHT and the peak (scan 333) in the trace of ABS1 had similar retention times. In addition, the mass spectra of the two peaks were the same. Therefore, ABS1 undoubtedly contains BHT. Comparing the TIC trace of ABS1 before reprocessing in Figure 1 and the TIC trace of ABS1 after reprocessing at 230 C in Figure 6, it can be seen that BHT is easily lost during reprocessing at 230 C. ABS1 contains methyl stearate. In Figure 6, the extracted ion chromatogram is shown. The selected ion was the molecular ion m/z 298 of methyl stearate. Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1,

10 X. Bai, B.K. Stein, K. Smith, and D.H. Isaac The retention time of the ABS1 peak (scan 558) was similar to that of standard methyl stearate (scan 579). Furthermore, the mass spectra of the two peaks were the same. Hence, material ABS1 definitely contains methyl stearate. Compared with the peaks of oligomers in ABS1, the concentration of methyl stearate obviously decreased after reprocessing at 230 C. Figure 7 shows the TIC traces for the extract from virgin ABS after processing at 270 C, the extract from virgin ABS with additives EBS and Irgafos 168 after processing at 270 C, and the extract from ABS2 after reprocessing Figure 6. Chromatograms from top to bottom: TIC traces of BHT and methyl stearate standards, extracted ion chromatogram for ABS1 after reprocessing at 230 C, and TIC trace for ABS1 after reprocessing at 230 C Figure 7. TIC traces of virgin ABS extracts after processing at 270 C, virgin ABS extracts with EBS and Irgafos 168 after processing at 270 C, and ABS2 extracts after reprocessing at 230 C for two cycles 10 Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1, 2012

11 Effects of Reprocessing on Additives in ABS Plastics, Detected by Gas Chromatography/Mass Spectrometry twice at 230 C. Comparing the TIC traces of virgin ABS and virgin ABS with additives after processing at 270 C, three large peaks (scans 353, 499, and 572) appeared in the chromatogram of virgin ABS with additives. The two samples were prepared using the same virgin ABS, and the polymer processing conditions, solvent extraction conditions, and GC/MS operation conditions were all the same. The differences were clearly induced by the addition of EBS and Irgafos 168. Based on their mass spectra, the peaks at scans 499 and 572 were decomposition products of EBS, and the peak at scan 353 was a decomposition product of Irgafos 168. In Figure 7, the virgin ABS (with additives) peak at scan 353 after processing at 270 C is 2,4-di-tert-butylphenol, the decomposition product of Irgafos 168. The ABS2 peak at scan 354 had the same retention time and mass spectrum as this peak. This finding means that some large molecules, particularly some antioxidants, can decompose and produce 2,4-di-tert-butylphenol. In other research, two degradation products of Irganox 1076 (n-octadecylb-(4-hydroxy-3,5-di-tert-butyl-phenyl)-propionate) had been found: 2,4-ditert-butylphenol and 2,6-di-tert-butyl-p-benzoquinone [23]. The degradation products of six antioxidants were studied by liquid chromatography combined with atmospheric pressure photoionisation mass spectrometry, and loss of tert-butyl groups was observed at elevated temperatures [24]. However, the specific chemicals in the recycled material cannot be determined by GC/MS. Other techniques may be used to identify them [25, 26]. Figure 8 shows the mass spectra of virgin ABS (with additives) peaks at scans 499 and 572 after processing at 270 C, and ABS2 peaks at scans 497 and Figure 8. Mass spectra of virgin ABS (with additives) peaks at scans 499 and 572 after processing at 270 C, and ABS2 peaks at scans 497 and 570 after reprocessing at 230 C for two cycles Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1,

12 X. Bai, B.K. Stein, K. Smith, and D.H. Isaac 570 after reprocessing at 230 C for two cycles. Comparing the retention times and mass spectra of these peaks, ABS2 more likely contains EBS. However, this should be further confirmed using other techniques [22]. CONCLUSIONS In the present study, we found that reprocessing ABS plastics leads to loss of some additives because of volatilisation, decomposition, or conversion. During reprocessing, antioxidant BHT is easily lost, Irgafos 168 decomposes, and 2,4-di-tert-butylphenol is yielded as a decomposition product. Fatty acids and their derivatives are lost during reprocessing, while EBS decomposes. To maintain good processability and service characteristics, it is necessary to add new additives during reprocessing. Reprocessing at a relatively low temperature can reduce the loss of additives in plastics. REFERENCES 1. F. Vilaplana and S. Karlsson, Quality concepts for the improved use of recycled polymeric materials: a review. Macromol. Mater. Eng., 293 (2008) M.M. Shapi and A. Hesso, Gas-chromatographic mass spectrometric analysis of some potential toxicants amongst volatile compounds emitted during large-scale thermal degradation of poly(acrylonitrile butadiene styrene) plastic. J. Chromatogr., 562 (1991) P. Kusch and G. Knupp, Headspace-SPME-GC-MS identification of volatile organic compounds released from expanded polystyrene. J. Polym. Environ., 12(2) (2004) F. Vilaplana, M. Martínez-Sanz, A. Ribes-Greus, and S. Karlsson, Emission pattern of semi-volatile organic compounds from recycled styrenic polymers using headspace solid-phase microextraction gas chromatography mass spectrometry. J. Chromatogr. A, 1271 (2010) J.C. Arnold, S. Alston, and A. Holder, Void formation due to gas evolution during the recycling of Acrylonitrile Butadiene Styrene copolymer (ABS) from waste electrical and electronic equipment (WEEE). Polym. Degrad. Stab., 94(4) (2009) C. Nerín, J. Salafranca, C. Rubio, and J. Cacho, Multicomponent recycled plastics: considerations about their use in food contact applications. Food Addit. Contam., 15(7) (1998) W. Camacho and S. Karlsson, Quality-determination of recycled plastic packaging waste by identification of contaminants by GC-MS after 12 Progress in Rubber, Plastics and Recycling Technology, Vol. 28, No. 1, 2012

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