VOCs Emissions and Structural Changes of Polypropylene During Multiple Melt Processing

VOCs Emissions and Structural Changes of Polypropylene During Multiple Melt Processing Q. Xiang, M. Xanthos*, S. Mitra and S. H. Patel* Department of Chemical Engineering, Chemistry and Environmental Science *Also: Polymer Processing Institute, GITC Building, Suite 3901 New Jersey Institute of Technology, Newark, NJ 072-1982 Abstract Polypropylene, as a commodity recyclable thermoplastic, is studied in this research to evaluate the potential environmental impact resulting from volatile organic compounds (VOCs) emitted during multiple reprocessing. Unstabilized commercial polypropylene (PP) grade was processed several times by injection molding. Samples were examined after each cycle for total VOCs emissions with a flame ionization detector (FID) and cumulative VOCs emissions were obtained after each processing step. Corresponding structural changes were investigated with Fourier Transform Infrared (FTIR) Spectroscopy and results were correlated with rheological data that showed decreasing viscosity particularly after the 7 th processing cycle. Introduction Polypropylene is widely used in commodity as well as engineering applications, particularly when reinforced. Due to its high production volume, it is recycled for both economic and environmental purposes. For instance, PP scrap from molding operations can be mixed with virgin material at appropriate ratios, and then remolded into new parts. From an environmental point of view, with the increasing cost of landfill space and the growing concern about waste materials, an increasing number of techniques for recycling and reusing waste PP and other plastic products have been developed (1). With respect to recycling/reuse, a number of potential environmental problems related to emissions have been of concern to the Environmental Protection Agency (EPA) and the Occupation Safety and Health Administration (OSHA). Among the emissions which can be as high as 5% of the actual tonnage of resins processed annually in the U.S., (2), volatile organic compounds (VOCs) are receiving much attention. Many of these VOCs are hazardous air pollutants, which can cause serious health problems, and environmental effects such as the formation of smog and ground-level ozone. Therefore, they are regulated by the Clean Air Act Amendments (CAAA) of 1990. Title V of this Act also established a permit program for emission sources to ensure an eventual reduction in volatiles (3). As a result of long thermal exposure, oxidative and mechanical degradation and the presence of moisture and contaminants, reprocessing of scrap plastics may generate a significant amount of VOCs emissions. These degradation mechanisms may also result in structural changes with concomitant effects on properties. For PP, there exists a number of reports on chain scission and the corresponding changes in molecular weight, structure and mechanical properties as a result of degradation due to multiple melt reprocessing of stabilized or unstabilized grades (1,4-6). However, no work has been published to our knowledge on the effect of multiple melt reprocessing on the VOCs emissions from polymers. This research examines the total VOCs emissions generated from PP processed several times by injection molding. The measurement of total VOCs was carried out with an analytical system based on the non-isothermal heating of the polymer followed by a direct analysis with a FID (flame ionization detector). 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. Consequently, this method can measure the true harmful environmental impact level from polymer recycling. Another advantage of this system, based on its very low detection limit for organics, is its higher sensitivity versus traditional thermal analysis techniques such as TGA and DSC. The variations in the total VOCs were compared for PP s processed several times, while the changes in chemical structure were analyzed with FTIR and correlated with rheological data. Materials Experimental Commercial grade unstabilized polypropylene (Profax 6501, Montell) was injection molded up to cycles in a Toyo Ti-90G injection molder. Experimental conditions were: barrel temperature profile 240-220 o C, nozzle

temperature 240 o C and mold temperature 25 o C. The 1 st cycle was carried out with the virgin powder polymer and the remaining 9 cycles with granules produced by grinding ASTM specimens obtained in the subsequent injection molding steps to less than 20 mesh size. Total VOCs emissions were measured on the virgin resin and materials obtained from a select number of reprocessing cycles. Total VOCs measurement A schematic diagram of the analytical system is shown in Fig. 1. The reactor is about 5"(length) x 1/4" (O. D.) stainless steel tubing blocked with glass fiber on both ends. About 2.5 mg of PP 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 36 o C to 180 o C, at a fixed ramp rate of 40 o C/min. To estimate the possible maximum amount of total VOCs, the sample was kept at the final temperature for an additional three minutes period. The VOCs emissions were measured with the FID as evolution profiles whose area could be determined by integrating at different time periods. Since this area is proportional to the amount of VOCs emitted from the polymer sample, it was calibrated with standard propane gas to calculate the total amount of VOCs emitted as a function of time. FTIR analysis Polypropylene films from each cycle were prepared by compression molding at 2 o C and 2268 kg (5,000 lb.) force. The thickness of the films ranged from 0 to 125 µm. A Perkin Elmer Spectrum One FT-IR Spectrometer was used to obtain the IR spectra of the films in the range of 4000-400 cm -1, with a resolution of 4 cm -1 and averaging 25 scans. Melt Flow Index (MFI) MFI was measured in a Tinius Olsen Extrusion Plastometer under standard PP conditions (2.16 kg, 230 o C) on samples containing additional antioxidant to prevent further degradation. Results and Discussion VOCs emissions from multiple processed PP Fig. 2(a) shows the thermally generated VOCs emissions from PP samples, selected among the virgin resin (0), 1 st, 3 rd, 4 th, 6 th, 8 th and th processing cycle. The two sets of data including averages of 3-6 determinations and standard deviations represent the emissions obtained when the sample was heated from 36 o C to 180 o C at a rate of 40 o C/min, and those emanated after the sample was held at 180 o C for three additional minutes. For both sets of data, and within the standard deviation range, the average value of total emissions does not appear to vary significantly among cycles. Standard deviations appear to increase after the first two cycles, (0 and 1), with the highest values shown for the th cycle. This may be attributed to equipment contamination that occurred during the time intervals between the multiple reprocessing and grinding steps of the samples produced in this work. The trends shown in Fig. 2(a) are not unexpected, since the data obtained here correspond to the possible maximum amount of VOCs generated under our particular experimental conditions. These values may not be equal to the VOCs amounts emitted during each processing cycle and lost in the atmosphere, or partly retained in the reprocessed granules. In addition, our experimental heating times, (in the absence of shear), are different from the residence times in the injection molder, (under shear). For example, it took 3.6 min to heat the sample from 36 o C to 180 o C at 40 o C/min, whereas in real processing, heating rate was higher, heating time shorter and processing temperature higher (240 o C). Experiments are under way to simulate more accurately times and temperatures between our static experiments and the actual processing conditions. Correlation between the VOCs emissions and the rheological data Fig. 2(b) presents the cumulative VOCs emissions as a function of processing cycle based on the subtotal of all the previous cycles. As the data in Fig. 2(a) represent the possible maximum amount of VOCs independent of the cycle number, Fig. 2(b) shows an increasing tendency of the maximum VOCs with increasing number of cycles. This trend of cumulative maximum VOCs versus number of cycles may be related to a similar trend of Melt Flow Index (MFI) versus number of cycles shown in Fig. 3. With increasing number of cycles, the increase in MFI indicates a decrease in melt viscosity, a corresponding reduction of MW and tendency towards further degradation. Comparison of Fig.2 (b) and Fig. 3 shows that both the cumulative VOCs and extent of degradation increase linearly up to the 6th cycle, and more rapidly thereafter. The enhanced increase of MFI and VOCs after the 7 th cycle is believed to be due to the combination of further degradation, higher contamination and changes in processing conditions, particularly cooling time, required to accommodate materials with increasingly lower melt viscosity. Changes in chemical structure of multiple processed PP Fig. 4(a) shows the FTIR spectra (wavenumber 2000-1500 cm -1 ) of films obtained from reprocessed PP. With

increasing processing cycle number, the absorption increased in the range between 1800 and 1675 cm -1, which represents the region of carbonyl group (>C=O) absorption. The appearance of carbonyl group indicates that thermal/oxidative degradation took place during multiple injection molding. However, only after the 7 th cycle, oxidative degradation became more prominent as shown by a large increase in the absorption of carbonyl group at the 8 th cycle. This is more evident in Fig. 4(b), where the spectra were processed (subtracted) against that of the 0 cycle, (virgin resin), in order to eliminate the effect of film thickness (sample concentration). This remarkable increase in the concentration of carbonyl group indicates enhanced oxidative degradation, and thus, can be related to the rapid increase in the VOCs and MFI after the 7 th cycle. Fig. 4(b) also illustrates that with increasing number of processing cycle, the dominant carbonyl group absorption shifted to a higher wavenumber. Before the 8 th cycle, the carbonyl group showed a relatively strong absorption at 1722 cm -1 ; then the absorption at 1736cm -1 increased rapidly. Since the bands at 1725-1715 cm -1 are assigned to aldehyde and ketone, and those at 1745-1735 cm -1 are due to ester group (5), it appears that severe, further thermal/oxidative degradation occurred after the 8 th cycle accompanied by the formation of new functional groups. This is in agreement with the measured MFI and cumulative VOCs data after the 7 th cycle. Conclusions Polypropylene was evaluated for total VOCs emissions generated during multiple melt reprocessing. Results show that the maximum amount of total VOCs from each individual cycle did not significantly change, while the cumulative VOCs increased with increasing processing cycle. After the 7 th cycle, significant changes in chemical structure were observed, accompanied by prominent oxidative degradation shown as a strong FTIR absorption of ester group. The trends on cumulative VOCs emissions and FTIR analysis versus cycle number were in good agreement with changes in the rheological data (MFI). 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). Special thanks are due to Mr. J. Guo of PPI/NJIT for the injection molding experiments. Thanks are also due to Dr. V. Tan, Dr. S. Dey and Mr. D. Conti of PPI for the lab assistance. References 1. Guerrica-Echevarria, G., et al, Polym. Degrad. Stab., 53 1-8 (1996). 2. Patel, S.H. and M. Xanthos, Adv. Polym. Technol., 14:1 67-77 (1995). 3. Adams, K., et al, J. Air & Waste Mange. Assoc., 49 49-56 (1999). 4. Canevarolo, S. V., Chain scission distribution function for polypropylene degradation during multiple extrusions, Polym. Degrad. Stab., in press. 5. Hinsken, H., et al, Polym. Degrad. Stab., 34 279-293 (1991). 6. Gonzalez-Gonzalez, V. A., et al, Polym. Degrad. Stab., 60 33-42 (1998). Key words Volatile organic compounds (VOCs), emissions, polypropylene, multiple processing, degradation, FTIR spectroscopy N 2 Mass flow Controller Switching Valve Temperature Controlled oven Flame Ionization Detector Calibration standard Mass flow Controller Reactor Sample Computer Data Acquisition Figure 1. Direct FID measurement of the VOCs emissions thermally generated from PP

VOCs emissions as weight percentage (%) 0. 3 0 0. 2 8 0. 2 6 0. 2 4 0. 2 2 0. 2 0 0. 1 8 0. 1 6 0. 1 4 0. 1 2 0. 1 0 0. 0 8 0. 0 6 0. 0 4 0. 0 2 0. 0 0 h e a t e d f r o m 3 6 o C t o 1 8 0 o C h e ld a t 1 8 0 o C f o r 3 m in 0 2 4 6 8 1 0 1 2 n u m b e r o f c y c le Figure 2(a). Effect of multiple injection molding on the VOCs emissions generated from PP when heated from 36 o C to 180 o C at the rate of 40 o C/min Cumulative VOCs emissions (%) 1. 5 1. 4 1. 3 1. 2 1. 1 1. 0 0. 9 0. 8 0. 7 0. 6 0. 5 0. 4 0. 3 0. 2 0. 1 0. 0 h e a t e d f r o m 3 6 o C t o 1 8 0 o C h e ld a t 1 8 0 o C f o r 3 m in 0 2 4 6 8 1 0 1 2 n u m b e r o f c y c le Figure 2(b). Cumulative VOCs emissions as a function of processing cycle

40 MFI (g/min) 30 20 0 0 2 4 6 8 number of cycle Figure 3. Melt flow index (MFI) of PP after multiple injection molding Cycle 0 Cycle 2 7 2 8 1736 1722 4 6 %T 7 8 %T 9 1722 1736 2000.0 1950 1900 1850 1800 1750 1700 1650 1600 1550 1500.0 cm-1 (a) 2000.0 1950 1900 1850 1800 1750 1700 1650 1600 1550 1500.0 cm-1 (b) Figure 4 (a). FTIR spectra of unstabilized PP after different injection molding cycles (b). Comparison of the carbonyl group after different injection molding cycles