ANALYTICAL SYSTEM FOR MEASURING THERMALLY GENERATED VOC EMISSIONS FROM POLYMERS

Transcription

1 ANALYTICAL SYSTEM FOR MEASURING THERMALLY GENERATED VOC EMISSIONS FROM POLYMERS Q. Xiang, S. Mitra, S. Dey*, M. Xanthos* Department of Chemical Engineering, Chemistry and Environmental Science & *Polymer Processing Institute New Jersey Institute of Technology Newark, NJ Abstract An analytical instrument and procedures were developed to investigate the thermally generated VOC emissions from different polymers with a flame ionization detector (FID). This system was applied to estimate the upper limit of VOC emissions from recyclable 100% carpet residue by exaggerating its thermal exposure at a predetermined temperature. The pattern of VOC emissions was also studied for the 100% carpet residue as well as its composite (80% carpet residue + 20% virgin ), and virgin resins ( and ). Introduction Volatile organic compounds (VOCs), which include a variety of organics with different functional groups, are emitted during plastics processing: extrusion, injection molding, thermoforming and other operations. The types and amounts of these VOCs depend on operational parameters and machine type. Normally, the highest temperatures reached during processing of most plastics are well below their decomposition temperatures. However, some resins may decompose at the high end of processing temperatures. In addition, VOCs may be emitted at fairly low processing temperatures from additives including antioxidants, stabilizers and lowmolecular-weight plasticizers(1). Many of VOCs are hazardous air pollutants, which can cause serious health problems and environmental effects not only by themselves, but also by the formation of smog and ground-level ozone(2). Therefore, the Clean Air Act Amendments (CAAA) of 1990 mandated the reduction of various pollutants (including VOCs) released to the atmosphere and established a permit program for emission sources to ensure an eventual reduction in emissions. When applying for a state operating permit, processing plants are first required to establish a baseline of their potential emissions, or may be required to have special permits and pollution control methods if their emissions are beyond threshold levels. This is a new challenge facing the plastics industry(1,3). In addition, the plastics industry is encountering other environmental problems related to the solid wastes generated during the manufacture and consumption of products. With the increasing cost of landfill space and the growing concern about waste materials incineration, researchers have been led to pursue the development of techniques for recycling and reusing waste plastics. Of all post-consumer resins, PET has the highest recycling rate, followed by PE,, PS and PVC. Commingled products such as carpets are discarded at a rate of more than 1.8 million tons per year in America(4). Apart from being recycled as raw materials for new carpet or industrial flooring, the waste can be remanufactured to be reinforcement for concrete, engine air-cleaner housing, low-cost plastic products and others(5,6). Since post-consumer plastics generally contain more contaminants and undesirable components, significant emissions may be generated during the recycling and remanufacture processes so that more attention should be paid to the emissions, compared to virgin resins. In response to the needs of regulatory and environmental issues, plastics industry has already performed studies to determine the emission factors for many resins, such as PE,, EVA, EMA, ABS, etc(3). In a recent review, detailed information related to the types of volatiles emitted from injection molding machines and extruders as well as analytical methods for their measurement were tabulated (1). Studies on the pyrolysis of polymers have also been widely performed for virgin resins (such as, PE and PVC)(7), but few papers report on the analysis of VOC emissions generated from recycled plastics. In this work, an analytical system was developed to investigate the thermally generated VOC emissions from polymers. Different from the two most common methods thermal gravimetric analysis (TGA) and differential thermo -gravimetric analysis (DTA), which measure the weight

2 loss of samples and have been intensively employed to study the thermal degradation of polymers(7), this analytical system measures the organic compounds emitted from polymers with a flame ionization detector (FID). Since FID does not respond to H 2 O, CO and CO 2, this method gives a more accurate measure of VOCs emitted from polymers. Another advantage of this system is that it is more sensitive than TGA and DTA because FID has a very low detection limit for organics. Moreover, this system can be easily modified to perform on-line monitoring of VOC emissions generated during processing. In this paper, the developed system was used to study the thermally generated VOC emissions from recyclable carpet residue, which is being evaluated as a potential feed stock for building applications(5). By exaggerating the thermal exposure time of the carpet residue at predetermined temperatures, the resulted maximum amount of VOC emissions may represent its upper limit. The system was also utilized to study the pattern of thermally generated VOC emissions from different polymers, including virgin resins (, ) and recycled plastics (carpet residue and its composite). As mentioned earlier, the quantity of VOCs emitted during actual processing is significantly influenced by a great number of variables(1,3), so it would be ideal to measure the emissions directly from each individual process. A more sophisticated system referred to as the C-NMOC (Continuous Non-Methane Organic Carbon) analyzer has been developed(8) and will be employed to perform online monitoring in the future. Materials Experimental Carpet residue and its composite, and commercial grades of virgin and were used in this study. The related information about these samples is listed in Table 1. Analytical system and Procedures A schematic diagram of the developed system is shown in Figure 1. The reactor is about 5" (long) x 1/4" (OD.) stainless steel tubing blocked with glass fiber on both ends. About 2.5 mg to 100 mg of polymer sample was placed in the reactor. After the system reached a steady state, with a flow of N 2 (carrier gas, ~30ml/min), the reactor was heated in a temperature-controlled oven from room temperature to final ones. For the estimation of maximum VOC emissions, the sample was heated at fixed ramp rate (40 o C/min) and kept at the final temperatures for over 30 minutes, while the emission pattern study was carried out by heating the polymers at varied ramp rates (1-20 o C/min). The VOC emissions were measured with a FID, which was connected to a computer. The detector output was recorded as VOC evolution profile (referred to as FID profile), which can be integrated at different time periods to get the area under it. Since this area is proportional to the amount of VOCs emitted from the polymer sample, one can calculate the evolution profile of the amount of total VOC emissions as a function of time. The area was also calibrated with standard propane gas to get the calculated amount of total VOCs emitted as a function of time. For the pattern study, many researchers have scrutinized a wide range of virgin resins. However, most of them concentrated on the high-temperature pyrolysis (typically over 300 o C), in which thermal decomposition of polymers occurs(7). Since the transition and typical processing temperatures of most common polymers are usually lower than 300 o C (as shown in Table 2), this study is focused on the pattern of VOC emissions generated from polymers in the normal processing temperature range. Results and Discussion VOC emissions as a function of time The amount of VOC emissions and the FID profile of 8.3 mg of 100% Carpet residue at 150 o C are shown in Figure 2 as a function of time. It can be seen that the VOC evolution rate is not detectable during the first 3 minutes, when the sample is heated from room temperature to 150 o C at the ramp rate of 40 o C/min. After that, the VOC evolution rate abruptly increases to the maximum (in less than one minute) then decreases relatively slowly to approach the bottom of the peak asymptotically. With the extension of heating time to over 30 minutes, the rate tends attain a stable value. This means that the amount of VOC emissions from the 100% carpet residue is negligible at room temperature or slightly elevated temperature (the first 3 minutes). However, significant amount of VOC emission is possible at processing temperature (150 o C). The rate of the emission decreases on prolonged heating. For the carpet residue, the possible maximum amount of VOC emissions was found to be 0.22% as a weight fraction with the exaggerated thermal exposure to 40 minutes. After the system reached 150 o C, about 40% of the total VOCs emitted within the first 3 minutes, which is the characteristics time period for thermal exposure of the processing of polymers in practice. 90% of the VOCs, part of which could be the decomposition product of the carpet residue due to long heating period, emitted in 30 minutes.

3 Pattern of VOC emissions Figures 3 and 4 exhibit the FID profiles of different polymers (100% carpet residue, 80% carpet residue + 20% composite, and PE) at the same ramp rate (10 o C/min). As expected, the FID profiles of the fairly stable and are much smoother than those of the 100% carpet residue and its composite. In the typical processing temperature range (lower than 200 o C), peaks (complete and incomplete ones) are found in the FID profiles of (one peak), (one peak), 100% carpet residue (three peaks) and the composite (three peaks), but only and the composite have a complete peak. Each peak represents the emission of one or more components in the polymers (such as monomer residue, stabilizers, additives), or organics generated from them (usually as decomposition products). These emission processes may be independent or may interact with other to give particular peak configurations. For each peak, the temperature at which the VOC evolution rate reaches a maximum is characteristic, and strongly influenced by many material related factors (such as the molecular structure of polymers, monomer residue, presence of stabilizer, etc.) and processing parameters of the polymers. Emission profiles from different polymers The VOC evolution profiles of the four polymers in Figures 3 and 4 are distinct from each other. First, the virgin resins and produce fewer emission peaks and fewer VOC emissions (based on the area under the evolution profile) than the complex recycled plastics (the 100% carpet residue and its composite). Second, a tiny peak, which may have resulted from certain stabilizers, is observed in the VOC evolution profile of the studied sample at a lower temperature range, compared to the sample. After the maximum temperature of this peak, the VOC evolution rate of the sample increases with an increase of the heating temperature, while that of the sample decreases to a minimum around 200 o C then goes up slowly. Third, compared with the 100% carpet residue in the low temperature range (<200 o C), the 80% carpet + 20% composite has the same number of peaks (three) and similar VOC evolution profile, but the maximum temperatures of all three peaks are shifted to lower values (the left of the X axis). This left-shifting is more remarkable at lower heating rate, such as 5 o C/min or 1 o C/min. In the high temperature range ( o C), the carpet residue composite has a quite smoother VOC evolution profile and a smaller evolution rate. These variations indicate that the VOC evolution profile of the composite has incorporated the attributes of its two major components (carpet residue and ), and the VOC emission characteristics have also changed in a favorable way especially in the high temperature range. Conclusions Different polymers including virgin resins and recycled plastics were studied with the developed system to investigate the mechanism of VOC emissions during the low temperature pyrolysis. The results show that the total VOCs evolution profile and the curve of the VOCs amount obtained by extending the thermal exposure period of the polymers provide helpful information on the thermally generated VOC emissions. However, an on-line measurement is desirable to quantify the exact amount of emissions produced during each individual process, and this will be performed in a future study. Acknowledgments Financial support for this work was provided by the Multi-Lifecycle Engineering Research Center (MERC) of NJIT, which is partially funded by the New Jersey Commission on Science and Technology (NJCST). Allied- Signal and Exxon provided the carpet residue, and respectively. Thanks are due to D. Conti of I for the lab assistance. References 1. Patel, S.H. and M. Xanthos, Adv. Polym. Tech., 14: (1995). 2. U. S. Environment Protection Agency, 40 CFR Part 59, Fed. Regist., 63: (1998). 3. Barlow, A., et al, J. Air & Waste Manag. Assoc., (1997). 4. Bisio, A.L. and M. Xanthos, How to Manage Plastics Waste, Carl Verlag, New York, (1994). 5. Yilmazer, U., et al, Proc. SPE 57 th ANTEC, (1999). 6. Proc. SPE Annu. Recycl. Conf., Proc. SPE Tech. Papers, Region. Tech. Conf., ( ). 7. Wu, C.H., et al, J. Envir. Eng., 124: (1998). 8. Mitra, S., et al, Proc. EPA/AWMA Conf. on Meas. Tox. Related Comp. in Air, Durham, NC, (May 1997). 9. F. Rodriguez, Principles of Polymer Systems, 4 th edition, Taylor & Francis, 55,75 (1996). Key words Volatile organic compounds (VOCs), emissions, recycling, decomposition

4 Table 1. Information about the samples used in this study 100% Carpet residue 80% Carpet + 20% composite Obtained from Allied-Signal (developmental Q product) as dark colored brittle flakes, containing 63-65% calcium carbonate, 15-18% and 12-15% SBR by weight [Ref. 5] Prepared by extrusion compounding [Ref. 5] General purpose homopolymer powder Commercial polymer pellets, cryogenically ground Table 2. Transition and processing temperatures of selected polymers [Ref. 9] Polymer Transition ( o C) Typical Processing ( o C) (T m ) 150 HDPE (T m ) (T m ) 200 PET (T g ) 265 (T m ) 290 PS 100 (T g ) 200 PVC (T g ) 180 PBT (T m ) 250 PC 150 (T g ) 250 PMMA 105 (T g ) 175 N 2 Mass flow Controller Switching Valve Controlled oven Flame Ionization Detector Calibration standard Mass flow Controller Reactor Sample Computer Data acquisition Figure 1. Schematic diagram of the system to investigate the thermally generated VOC emissions from polymers

5 FID profile Amount of VOC emissions Amount of VOC emissions as weight fraction (%) time (min) Figure 2. FID profile and VOC emissions of 8.3 mg of 100% carpet residue at 150 o C % carpet residue 80% carpet residue + 20% Figure 3. FID profiles of about 100 mg of,, 100% carpet residue and 80% carpet residue + 20% composite at the ramp rate of 10 o C/min (Arrows indicate incomplete and complete peaks)) % carpet residue 80% carpet residue + 20% Figure 4. Magnified FID profiles in Figure 3 to show the peaks (indicated with arrows) in lower temperature range or in lower scale of FID response