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 degradation 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 components 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 observation after the exposure was utilized to determine the ALC values. Figure 2 shows the arrangement 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 carboxyhemoglobin 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 components 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 substance 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. The intensity of band at 730cm-1 strongly sug- gested δ-CH2. From the element analyses and the infrared spectra of these two pyrolysis residue and pure original plastics, they have very similar composition but the pyrolysis lead to deteriora- tion in degree of the polymerization. The approximate lethal concentration (ALC) in a one-week observation of the mice produced by a single one hour exposure to the pyrolysis gases was 30.5g of polyester per 1000 litre of air and 79.6g of polyamide. The main contents of these gases were 2700ppm of CO, 700ppm of ammonia and a small amount of hydrocarbon such as propylene in the pyrolysis of polyamide and 3000ppm of CO, 1000ppm of acetaldehyde and a small quantity of benzene in the pyrolysis of polyester. With experimental animals during one hour exposure, dyspnea and jumping action ap- peared in early stage, and then rapid breathing and loss of co-ordination occurred in ten to fifteen minutes. These symptoms occurred early and strongly related to the concentrations of the pyrolysis gases. The body weight of mice exposed to the gases of the ALC range decreased slightly in twenty-four to fourty-eight hours. Half of the animals died mainly in this period, but some of them had died within one hour exposure period. All animals died in the exposure test when 53.6g of polyester per 1000 litre of air and 134.0g of polyamide were pyrolyzed. In the blood pictures, using Coutler Counter Analyzer Type ZB-1, no specific changes were observed in the number of erythrocyte and leucocyte and hemoglobin, hematocrit and VOL. 33, NO. 2, JUNE 1978 〔455〕

MCV values in the 7-day observation. The content of CO-Hb in blood was calibrated from the saturated CO-Hb obtained by the passing of pure CO gas through the blood for about five minutes. The higher level of CO-Hb in blood which was above fifty five per cent was proved from the exposed mice, and the animals died during a single one hour exposure period. Figure 7 shows the dissociation curve of CO-Hb level in the blood after exposure to polyamide pyrolytic gases of the ALC, which contained 2700ppm of carbon monoxide. In two hours after exposure the content of CO-Hb of blood recovered to almost normal level. From the analyses of pyrolysis products of polyamide and polyester and the inhalation tests of the animals, it was proved that carbon monoxide was the main lethal factor and other pyrolytic products such as ammonia and acetaldehyde integrate the toxicity. In the pathological findings, slight changes Fig. 7 Dissociation curve of carboxy- were observed in the lung specimen prepared hemoglobin level after one hour immediately after the exposure to the pyrolysis exposure to polyamide pyrolysic gases of the ALC of polyamide and polyester. gases of the ALC The walls of the bronchi, bronchioles and alveoli were congested slightly, but hemorrhages, etc. were not yet noted. After twenty four hours, congestion of the alveolar blood vessels developed and moderate fluid exudate and edema in the alveoli were observed.

DISCUSSION

The results of the studies on polyamide and polyester have shown that the pyrolysis products of both materials could be lethal to mice when the high concentration of gaseous components were inhaled. Many investigations have been carried out about correlation between decomposition temperature and fluctuation of pyrolysis products of polyamide and polyester in air, nitrogen and vacuum atmosphere. When these plastics are used under 200℃, there are no weight loss and no pyrolysis products, so that, it is not necessary to discuss about the toxicity of pyrolysis

products. The melting points of Nylon-6 and Nylon 6-6 are 218℃ and 257℃8) and that of polyester is 250℃.9) In the case of polyamide, Straus and Wall 10) described that carbon dioxide and a small amount of hydrocarbons were obtained when it was decomposed at 310℃ and 380℃ in vacuum. Pohl11) decomposed polyester at 307℃ in vacuum and detected acetaldehyde, carbon dioxide and water in pyrolysis products. On the other hand, Marshall and Todd12) investigated the presence of carbon monoxide, carbon dioxide and water in the thermal degradation products of polyester at 283℃ to 302℃ in nitrogen atmosphere. The toxicological investigation in pyrolysis studies will depend on the method. This includes many factors such as presence of oxygen and water vapour during pyrolysis and kinds of animals to be exposed. In this report, apart from the strict physical properties of plastics, it was taken that such analytical and exposure method as the cases of the toxicological investigations of pyrolysis products of fluorocarbon elastmer5) and polycarbon mono- fluoride6) which were conducted by the authors. It was possible to compare the toxicity with other plastics at a same level in the oxydizing atmosphere which was more practical. 〔456〕 JAPANESE JOURNAL OF HYGIENE

Macfarland and Leong7) stated that the deaths of animals exposed to pyrolysis gases of Nylon products were due to the slowly developing pulmonary edema, but they did not clarify the detailed degradation nature of the products and the lethal factors. The gaschromatographic analysis of pyrolysis products of polyamide and polyester showed that the major toxic component was carbon monoxide with minor existence of other compounds such as hydrocarbons. The single ALC of carbon monoxide in a one-hour exposure is 1200 to 4000ppm13), that of ammonia is 2.500 to 5000ppm and benzene is 3000 to 7500ppm. The concentration of carbon monoxide in the pyrolysis gases at the range of ALC was 2700ppm of polyamide and 3000ppm of polyester. Other components such as ammonia, acetaldehyde and benzene resulted in lesser amounts than the lethal concentration coefficient. As the signs in the experimental animals exposed to the gases in the ALC, dyspnea, jumping action and rapid breathing pattern were observed in early stage, and then slowed respiration, loss of co-ordination and loss of righting reflex followed. These syndrome can be observed distinctly in case of exposure to a single carbon monoxide. These results presumably indicate that the major cause of deaths by two kinds of plastics depends on the presence of relatively large amounts of carbon monoxide in the pyrolysis gases. This assumption could be confirmed by CO-Hb in the animals dying during the exposure period. Carboxyhemoglobin level in these animals exposed to the pyrolyzate in the ALC proved to be more than 55per cent. The blood CO-Hb levels correlated well with the concentrations of carbon monoxide in the pyrolysis gases and the amounts of pyrolyzed plastics. The gaschromatographic method14) for the determination of blood CO-Hb in this study was effective for a small amount of blood samples and could assure the cause of death definitely. The GC mass spectrometer has been of great value in analyzing the composition and structure of mixtured materials such as that found in pyrolysis gases of plastics. For example, octafluoroisobuthylene15) (C4F8, M/e 200) which is generated during the pyrolysis of polyfluoroethylenepropylene and has severe toxicity to the body, and octafluorocyclobutane16) (C4F8, M/e 200) which has no harmful effects under any biological conditions are distinguished precisely with GC mass spectrography17) Since Harris18) described polymer fume fever, the particulate products from the pyrolysis of plastics is the subject of interest among many investigators in environmental and occupational health because of those particular influenza-like symptoms. The particulate matters resulting from pyrolyzed polyamide and polyester had a diameter of approximately over 100μm and easily conglomerated immediately on the pyrolysis vessel and the gas bag. Even if these particles are inhaled into the body, on account of its size, the particulate matter will be rejected directly from the respiratory tract without invading into the alveoli and causing harmful influences. Therefore, it was considered that the grease-like substance and white powder component resulting from pyrolysis of two kinds of the plastics did not have any critical effects on the body. From the results obtained, it should be said that the main lethal factor of pyrolysis products of polyamide and polyester could be that carbon monoxide and other components were less toxic in the period of the acute exposure. As yet, relating the phenomena of the animals after exposure, because of the slowly developing pulmonary edema and reduction in the body weight etc., it is considered that the other components had some extent of toxicity and might be a second cause of death in this stage. Therefore, attention has to be paid to this point of view for care of the actual cases of poisoning. It may therefore be indicated that the necessity of the use of protective devices against the toxic gases and effective ex- haust system are essential where these plastics are used under a high temperature, and VOL. 33, NO. 2, JUNE 1978 〔457〕 continual measures and administration from the medical standpoint are also indispendsable.

SUMMARY

Polyamide and polyester have been used widely because of those excellent thermal stability and chemical inertness. When these plastics are utilized under a high temperature beyond those heat-proof limits for long hours, toxic products are suspected to be generated. It is of importance to study the mechanism of the toxicity of pyrolysis products in order to establish a safety standard. Having pyrolyzed these plastics, the authors analyzed those components with gas chromato- graphy, GC mass spectrography, infrared spectrography and gas detector tube method, and examined the principal lethal factors by means of the biological exposure technique. The gaseous products, yielded on pyrolyzing processes of those polymers, mainly consisted of carbon dioxide, carbon monoxide and hydrocarbons such as methane, ethylene, propylene and benzene. The concentration of carbon monoxide in the pyrolyzate was estimated at a dominant amount. On the contrary, those of hydrocarbons and ketons were lesser amounts than the lethal factors. The approximate lethal concentration (ALC) value in a 7-day observation of mice exposed one hour to pyrolysis gases of polyamide in air showed 79.6g per 1000 litre of air and 30.5g of polyester. More than 55% of blood CO-Hb level was confirmed from the animals which died during a single one-hour exposure period at the range of ALC, so that carbon monoxide proved to be the principal lethal factor and other components were less toxic in the exposure period. The particulate matters resulting from pyrolyzed polyamide and polyester had a diameter of over 100μm and were not impregnated in any acidic group. Even if these particulates are inhaled into the body, the particles will be rejected directly from the respiratory tract without harmful influences. Based on this study the authors could draw the conclusion that the exposure of the pyrolysis products of polyamide and polyester produced a toxic syndrome mainly caused by the effects of carbon monoxide in the acute stage. Futhermore, attention has to be paid following pulmonary edema in the actual cases. A part of this study was presented at 47th Annual Meeting of the Japanese Society for Hygiene held in Tokyo, April, 1977.

REFERENCES

1) Cornish, H. H. and Abar, E. L.: Toxicity of pyrolysis products of vinyl plastics, Arch. Environ. Health, 19, 15-21 (1969). 2) Zapp, J. A.: Hazards of isocyanates in polyurethane foam plastic production, Arch. Ind. Health, 15, 324-330 (1959). 3) Coleman, W. E., Sheel, L. D. and Gonski, C. H.: The particles resulting from polytetrafluoroethylene (PTFE) pyrolysis in air, Amer. Ind. Hyg. Assoc. J., 29, 54-60 (1968). 4) Okamura, T.: Studies on the toxicity of thermal decomposition products of fluorocarbonresin, Jap. J. Ind. Health, 12, 109-153 (1970). In Japanese. 5) Kono, K.: Toxicity of products of fluorocarbon elastomer pyrolysis, Jap. J. Ind. Health, 17, 288-301 (1975). In Japanese. 6) Yoshida, Y., Okamura, T., Harada, A., Kono, K., Watanabe, M., Toyota, S. and Iwasaki, K.: Acute toxicity of polycarbon monofluoride, Bull. OMS, 23, 14-32 (1977). 7) MacFarland, H. N. and Leong, K. J.: Hazards from the thermodecomposition of plastics, Arch. Environ. Health, 4, 591-597 (1962). 8) Watson, M. T. and Armstrong, G. M.: A new polycycloamide engineering plastics, S. PE. J., May, 〔458〕 JAPANESE JOURNAL OF HYGIENE

475-479 (1965). 9) Lee, H., Staffy, D. and Neville, K.: New Linear Polymers, McGraw Hill Book Co., New York (1967). 10) Straus, S. and Wall, L. A.: Pyrolysis of polyamide, J. Research Natl. Bur. Standards, 60, 39-45 (1958). 11) Pohl, H. A.: The thermal degradation of polyesters, J. Am. Chem. Soc., 73, 5660-5661 (1951). 12) Marshall, L. and Todd, A.: The thermal degradation of polyethylene terephthalate, Trans. Faraday Soc., 49, 67-78 (1953). 13) Sax, N. I.: Dangerous Properties of Industrials Materials, p. 576-577, Reinhold Publishing Co., New York (1963). 14) Arimoto, H. and Morri, T.: Method for quantitative determination of carbon monoxide hemoglobin in human blood, Japan Analyst, 21, 1608-1613 (1972). In Japanese. 15) Harris, D. K.: Some hazard in the manufacture and use of plastics, Brit. J. Ind. Med., 16, 221-229 (1959). 16) Clayton, J. W., Delaplane, M. T. and Food, D. B.: Toxicity studies with octafluorocyclobutane, Amer. Ind. Hyg. Assoc. J., 21, 382-388 (1960). 17) Majer, J. R.: Mass spectrometry of fluorine compounds, Advances in Fluorine Chemistry. 2, p. 55- 103, Butterworth, Washington (1961). 18) Harris, D. K.: Polymer fume fever, Lancet, 261, 1008-1011 (1951).

耐熱性プラスチック, ポリアミドおよびポリエステルの熱分解毒性について

吉田康久, 河野公一, 原田章, 豊田秀三, 渡辺美鈴, 岩崎錦

大阪医科大学衛生学・公衆衛生学教室

耐熱性の目的でポリアミド, ポリエステルが近年比較的多量に用いられているが, その限界温度以上での使用または廃棄時に毒性のある熱分解成分が生成する可能性がある。本研究はこれを解明する目的で空気中熱分解ガス等を, ガスクロマトグラフ, 同マススペクトログラフにより分析するとともに, また, 動物試験としてマウスについて急性暴露を行って, その主要死因を確かめたものである。熱分解ガスとしてポリエステルでは一酸化炭素, エタン, プロピレン, アセトアルデヒド, ベンゼン等が, また, ポリアミドではこれらの成分に加えてアンモニア, ペンテン等が検出されたが, 毒性並びに発生量よりみて一酸化炭素が優勢な成分として推定された。

動物試験では, 単一, 一時間暴露, 一週間観察による概算的致死濃度がポリアミドで79.6g, ポリエステルで 30.5g/1000l空気であり, この場合一酸化炭素濃度が2700～3000ppmと認められた。この一酸化炭素が主要死因であることは, 血液の一酸化炭素飽和度をガスクロマトグラフにより分析した結果, その値が55%を越えることから確認することができた。フユーム, またはグリース状物質として発生する熱分解成分は, 生成後直ちに凝集してその大きさ100μm以上に達し, また, 赤外分光分析により原試料に近似した構造を示すので, ある種のフッ素樹脂の熱分解における特異なポリマーフユーム熱の原因にはなり得ないと考えられる。動物試験ではまた暴露後48時間程度に肺水腫等の傾向が認められて死亡する場合も多いので, 一酸化炭素につぐ比較的毒性の大なる第2の致死因子が存在することも確実である。したがって, もし実際の症例においては, 予後における十分な観察と早期の治療が必要なものと指摘される。(受付1977年11月30日)