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Toxicity of Pyrolysis Products of Thermal-Resistant Plastics Including Polyamide and Polyester

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Toxicity of Pyrolysis Products of Thermal-Resistant Plastics Including Polyamide and Polyester 〔450〕 Toxicity of Pyrolysis Products of Thermal-Resistant Plastics Including polyamide and polyester Yasuhisa Yoshida, Koichi Kono, Akira Harada, Shuzo Toyota, Misuzu Watanabe and Kin Iwasaki Department of Hygiene and Public Health, Osaka Medical College, Osaka INTRODUCTION A certain kind of thermal-resistant plastics such as polyamide and polyester have been used widely among various industries or houses because of those excellent thermal stability, desirable combination of plasticity, lubricity and chemical inertness. When these plastics are used under crucial circumstances, particulary under a high temperature beyond these heat-proof limits for many hours or at incinerations of used plastics, dangerous gases and other substances are suspected to be produced. It is a vital subject for occupational health care and administration to analyze the de- gradation products, identify the influence upon the human body and establish adequate preventive measures. As a matter of fact, because of the properties of visibility and cleanness these plastic films are often used as wrapping materials of meat including poultry for oven cooking at heat-resistant limit temperature in many houses recently, so that the above-men- tioned purpose of the research also has a significance related to health care in the house- keepings. Many investigations about the toxicity of pyrolysis products of plastics have been reported. Cornish and Abar1) pyrolyzed polyvinyl chloride polymers in a stream of air at 600℃. They investigated and found that most deaths were due to carbon monoxide. Zapp2) studied the inhalation toxicity of pyrolysis products of polyurethane foam. Experimental animals revealed pulmonary congestion and edema after the exposure period. Coleman et al.3) observed particulate matter which was produced by pyrolysis of fluorinecontained thermal-resistant polymer (polytetrafluoroethylene) at 500 to 700℃, and found it to be the cause of particular polymer fume fever. On the other hand, Okamura4) stated that the major lethal factor derived from hydrofluoric acid in the case of thermal decomposition of tetrafluoroethylene. Kono5) and Yoshida et al.6) pointed out that hydrofluoric acid was also the main toxic product in pyrolysis of fluorocarbon elastmer and polycarbon monofluoride. Little investigation dealing with the toxicity of the pyrolysis products of polyamide and a few investigations about that of polyester have been described. Macfarland and Leong7) obserbed that the slowly developing pulmonary edema caused death of animals exposed to pyrolysis gases of Nylon products. In this report, we made an extensive studies that include toxicological investigations of decomposition products of polyamide and polyester analyzing the principal toxic factors in order to establish the industrial and environmental safety standards. The same analytical methods that we studied in case of the toxicity of pyrolysic products of fluorocarbon elastmer and polycarbon monofluoride have been used to compare the toxicity in the same level. MATERIALS AND METHODS Some thermal-resistant plastic films which were on the market for oven cooking and VOL. 33, NO. 2, JUNE 1978〔451〕 Fig. 1 An arrangement of thermal decomposition apparatus for polyamide and polyester sterilization in Japan and England were analyzed by means of infrared spectrophotography. Two materials were determined, one was polyamide (Nylon 6 and Nylon 6-6), the other was polyester (polyethylene terephthalate). These two kinds of plastics were thermally decomposed in a stream of air at the com- plete pyrolyzable temperature (550℃). The pyrolytic system is shown in Figure 1. The pyrolysis apparatus employed consisted of an 18-8 stainless steel vessel of 2.18 litre capacity, fitted with a removable lid on which inlet and outlet ports were set. The weighed sample was placed into the vessel. The bottom temperature of the vessel at pyrolysis experiments was thermo-electrically controlled by using the recording potentiometer (Yokogawa Recording Potentiometer M60) with a relay box and a solenoid valve to control the fuel gas. The air was introduced into the pyrolysis vessel at the rate of 2.5litre/min. through a gas meter. The pyrolysis gases from the outlet port were collected into a polyethylene gas bag and analyzed by a gas detector tube, gas chromatograph and GC mass spectrograph, and then used for the animal exposure tests. The Drager gas tube detector was utilized for preliminary analysis of main pyrolytic gases. Gaschromatographic analysis was performed with the Shimadzu Model GC-5A gas chromatograph using a flame ionization detector or a thermal conductivity detector. Analytical columns were 1.5 metre length of stainless steel tubings which were packed with 50/80mesh Porapak Q or Molecular Siebe 13X. Nitrogen and helium were used as a carrier gas at a flow rate of 60ml per minute. The column temperature was set from 30 to 160℃ at a heat elevation rate of 20℃ per minute. The Hitachi Model RMU-6MG GC mass spectrograph was also used for identification of gaseous components. Temperature of the Rhyhage type enricher was 240℃, the ionization voltage of mass unit was 20-50kV, ionization current was 250-300μA and acceleration voltage was 3-9kV. The grease-like substance resulting from pyrolyzed polyamide and the white powder com- ponents from that of polyethylene terephthalate were collected on the electric dust sampler (Shibata Model SK-E 100). These components were analysed by the element analysis using the CHN corder (Yanagimoto Model MT2) and infrared spectrophotograph (Hitachi Model P1-S2). 〔452〕JAPANESE JOURNAL OF HYGIENE An acryl gas chamber of 1.5 litre capacity was used for the animal exposure tests. Three adult male mice of the dd-N strain weighing 17-20g were employed in each trial. The same exposure test was tried three times with an identical condition. One hour exposure period was chosen for all trails, and a 7-day observa- tion after the exposure was utilized to determine the ALC values. Figure 2 shows the arrange- ment of gas exposure technique. At autopsy, the mice were anesthetized with ethyl ether, and the blood was taken by heart puncture for the determination of carboxyhemo- globin level (CO-Hb), and other blood studies. The Shimadzu blood gas sampler (BGS-1A), in which carbon monoxide was dissociated from Fig. 2 A gas exposure apparatus CO-Hb with van-Slike reagents and directly for pyrolytic gases transferred into the gas chromatograph, was used to measure of CO-Hb in the blood of the experimental animals. Tissues were fixed in 10% formaldehyde and stained with hematoxilin and eosin for histopathological observations. RESULTS In the case of thermal decomposition of polyamide (Nylon 6 and Nylon 6-6) in air below 200℃, there was no weight loss of the samples during pyrolysis, and no gaseous components were detected. However, at 300℃, formation of the gas and grease-like substance were observed, At 350℃ and above, the samples were completely pyrolyzed in a short time. In Column -Porapak Q Column -Porapak Q 50-80 mesh 50-80 mesh 1.5m×2 3.5m×2 Carrier gas -N2 40ml/min. Carrier gas -N2 40ml/min. Column temp.-30-160℃ Column temp.-30-160℃ Detector -F.I.D. Detector -F.I.D. Fig. 3 Gaschromatographic analysis of Fig. 4 Gaschromatographic analysis of pyrolysis gases of polyamide at pyrolysis gases of polyethylene 450℃ terephthalate at 450℃ VOL. 33, NO. 2, JUNE 1978〔453〕 the case of polyester from 300℃, the pyrolysis began and the gases and white powder com- ponents which charred on further heating were produced and adhered to the inside wall of the vessel, outlet port and plastic bag. Figure 3 illustrates the example of gaschromatographic analysis of pyrolysis gases of polyamide at 450℃ temperature and qualitative analysis of each component by means of GC mass spectrography. Separations of such components as methane, ethylene, ethane, propylene, acetaldehyde, butane, benzene and other very small amounts of hydrocarbons were observed. The thermal degradation gas chromatogram of polyethylene terephthalate at 450℃ is shown in Figure 4. The constituents were also composed of small amounts of hydrocarbons; saturated, unsaturated and cyclic compounds such as methane, ethane, ethylene, propylene, acetaldehyde and benzene Carbon monoxide and carbon dioxide were also noted abundantly in both thermal decomposed gases using the gas detector tube and TCD gaschromatographic analysis. Figure 5 gives the mass spectra of acetaldehyde (C2H4O molecular weight 44, bp. 20.8℃) and benzene (C6H6, molecular weight 78, by. 80.1℃) which were taken in the pyrolysis of polyamide. Fig. 5 Mass spectra of pyrolysis gases of polyamide In the spectrogram of acetaldehyde, the relative intensity of the molecule-ion peak rose at mass 44, 43, 42 and 29. The peak at mass Table 1 Element analysis of original polyamide 29 is the largest in the spectrum. and grease-like substance Weight per cent of the grease-like sub- stance produced by pyrolysis of polyamide to the original sample was 17.6%. The results of the element analysis of the grease-like substance and pure polyamide are shown in Table 1. The residue indicates the following percentage composition: 66.7% for C, 8.7% for H and 11.13% for N. It is assumed that Table 2 Element analysis of original poly- the missing 13.45% composition is oxygen be- ethylene terephthalate and white cause the pyrolysis occurred in the air. powder component Table also shows the element analysis of the white powder component and that of pure polyester. Infrared spectra presented in Figure 6 were obtained from the pyrolysis residue which had been pelletized into a thin film in the KBr 〔454〕 JAPANESE JOURNAL OF HYGIENE Fig. 6 Infrared spectra of original films and solid decomposition products die, a thin film of pure polyamide and one of polyester. In Figure 6, the region between 3600cm-1 and 2500cm-1 displays very broad absorption. This could be due to the bonded OH stretching absorption. The band of frequencies in the region of 1700cm-1 is attributed to carbonyl (C=O) absorption and the band at 1275cm-1 indicates the presence of CO2, respectively.
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