A large telecommunications company authorized a study to determine which insulation materials to employ on wiring in its central offices. In addition to typical criteria related to electrical, mechanical, and environmental exposure characteristics, an analysis and comparison was made of the toxic gases given off by heating or burning of two materials, polyvinyl chloride (PVC) and polyethylene (PE).
This analysis was based primarily upon reviews of information available in technical literature sources, which provided data on the nature of the emitted gases, various toxicity thresholds, and the results of animal experiments with PE and PVC.
PE, first introduced commercially in the early 1950s, is a semi-crystalline polymer. It is composed of chains of alkene groups. PE, which is flammable unless a flame retardant such as a halogen compound is added, is used extensively as jacketing over telephone cables. Individual conductors may be covered with PE insulation, and sometimes the PE jacket is covered with a layer of PVC. PE is also used in a foamed form in coaxial communications cables.
Most PE compounds are classified as either low density, high density, or vulcanized (cross-linked). Cross-linking is accomplished by the use of organic peroxides, irradiation, or chemical grafting, followed by exposure to water to induce hydrolysis. Vulcanized or cross-linked PE has a high molecular weight and low density, and is capable of operating up to about 220 degrees Fahrenheit or higher without melting or dripping. Low-density PE (LDPE) is used for communication and power cable jackets, but its application is limited to temperatures below 170 degrees Fahrenheit because of its low softening temperature. The high-density variety (HDPE) is similar electrically to low-density PE except for its greater hardness, which increases its tendency to crack.
PE begins to decompose when exposed to a temperature of 627 to 892 degrees Fahrenheit, and has a self-ignition temperature of 660 degrees Fahrenheit. It gives off gases starting at about 392 degrees Fahrenheit. The gases include ketones and aldehydes, whose combustion becomes self-sustaining at higher temperatures. If PE burns with poor ventilation, both carbon monoxide (CO) and carbon dioxide (CO2) are produced.
Polyvinyl chloride (PVC) consists of a very long chain of alkenes that are double-bonded with a monomer. Flexible PVC insulation is used extensively for wire and cable at power voltages up to 600 volts, and control circuit voltages up to 5 kV, at temperatures up to 221 degrees Fahrenheit. PVC is used on individual telephone conductors, and also as an extruded layer over the outer PE jacket that covers telephone wire bundles. Test requirements include burning time in a vertical flame test. Standard specifications for PVC at 140 degrees Fahrenheit and 167 degrees Fahrenheit appear in ICEA and UL Standards.
PVC is readily ignitable when a plasticizer such as a phlalate is used: however, the flame retardant properties of PVC are retained with a plasticizer such as a phosphate ester. With the addition of a flame-retardant, PVC's low temperature performance is compromised, (i.e., brittleness is increased, and impact resistance is decreased). It decomposes in the range of 302 to 572 degrees Fahrenheit; flash ignites at 735 degrees Fahrenheit, and self-ignites at 850 degrees Fahrenheit. Up to about 446 degrees F, hydrogen chloride is given off in a white mist and CO is given off mainly above 482 degrees Fahrenheit. Between 752 and 1112 degrees Fahrenheit, ethylene, benzene, naphthalene, and other hydrocarbons are produced. When these products are burned with sufficient oxygen, hydrogen chloride, carbon monoxide, and carbon dioxide are produced.
Before this review and analysis began, it became apparent that it is extremely difficult to extract from the general literature information concerning toxic gases given off by PVC or PE formulations intended for specific electrical applications, (i.e., including degradation products of plasticizers, stabilizers, or other materials that are added to the basic polymer). For example, certain organo-metals which are used as stabilizers are powerful poisons. The results presented here do not include such possible effects.
Animal experiments were conducted to determine the toxic effects of the degradation products of rigid cellular PVC, flexible cellular PVC, solid PVC, and PE. Data on time-to-death of experimental animals were analyzed, and generally indicated shorter times to death during PE-emitted gas exposure, as compared to PVC. The CO concentrations existing at the times of death were also determined and in all cases it was concluded that CO could have been the sole intoxicant leading to death, based upon these concentrations and the known toxic effects of high concentrations of this gas.
In another study, the decomposition products of PE and PVC were predicted using a computer model.
If heated in air to about 480 degrees Fahrenheit, PE melts, sublimes, and to a degree decomposes without burning. The amount of CO given off during combustion, in the temperature range of 896 to 986 degrees Fahrenheit, depends on the oxygen to PE monomer ratio. At a ratio of 1:1, a max CO concentration of 9400 ppm (parts per million) was reached after the burning of 10 grams of PE in a 60-cubic-foot closed compartment. But for a ratio of 3:1 oxygen to PE monomer, the maximum CO concentration reached was only about 1410 ppm. Both were achieved in the range of 896 to 986 degrees Fahrenheit after about 20 min. It should be noted that the permissible exposure limit for CO at that time was 50v ppm, and the IDLH (Immediately Dangerous to Life or Health) level was 1500 ppm. In PVC tests, a maximum hydrogen chloride concentration of about 2100ppm was reached after burning of 10 grams of PVC for about 50 minutes, in the range of 500 to 570 degrees Fahrenheit. The permissible exposure limit for hydrogen chloride was 5ppm, and the permissible IDLH level was 100ppm.
It was also determined that the principal decomposition and combustion products of a 50 percent PVC, one-gram sample in a one-square-meter volume at 1020 degrees Fahrenheit were 220ppm of CO, zero chlorine, 360 ppm of hydrogen chloride, and 0.10 ppm of phosgene. Phosgene has stronger adsorption characteristics on the mucous membranes of the lungs than hydrogen chloride. Its IDLH level was 2 ppm. It is dangerous after one-half to one hour exposure at 12.5 ppm, and fatal in one-half hour at 25 ppm. Phosgene hydrolyzes (reacts with water ions) to hydrogen chloride and CO at the bronchiales and alveoli of the lungs, causing pulmonary edema and asphyxiation.
PE was also subjected to toxic gas analysis under three fixed conditions: thermo-oxidation in air at 660 degrees Fahrenheit; pyrolysis (chemical decomposition by heat) in helium at 750 degrees Fahrenheit, and combustion in air at a polymer temperature of 1110 degrees Fahrenheit. During long-term heating, initial thermal degradation was observed beginning at 390 degrees Fahrenheit. During thermo-oxidation, the principal gas products were aldehydes, representing about 48 percent, and ketones, about 2.8 percent. In addition to oxygen-containing compounds, about 25 percent olefins and 12 percent paraffins were present. After pyrolysis, the sum of the olefins was about 59 percent, and the total paraffins were about 35 percent. The percentage of aldehydes was about 31 percent, compared to 48 percent for thermo-oxidation. During combustion proceeding in air at about 1110 degrees Fahrenheit, the products of this degradation burned with flame. The percentage of aldehydes was about 46 percent. Acrylic acid was apparently the most toxic component of the PE combustion products.
A summary of these studies and tests is as follows:
- Under identical test conditions, laboratory test animals exhibited initial symptoms of muscular dystrophy - coordination after approximately the same times of exposure to PVC and PE heat - degradation products. However death generally occurred about 20-30 percent sooner when they were subjected to PE toxic gases, as compared to PVC toxic gases.
- All deaths were attributed exclusively to high CO concentrations.
- For an event leading to a limited build-up of heat to a level of about 570 degrees Fahrenheit, PVC emits a significant amount of a gas (hydrogen chloride) with a well-known long-term toxic effect on the lungs; whereas PE degradation products under identical circumstances would be negligible. Such a situation might arise within electrical equipment in which leakage current flowed over an extended period of time due to defective insulation, contamination, or insulation damage, without a fire occurring. However, in that circumstance, a relatively small amount of plastic material would generally be involved, compared to the combustion of a much greater amount of material in a large fire. In a large conflagration, burning of additional materials such as structural members, furnishings, and electrical components such as circuit boards renders the relative toxicity difference between PE and PVC less important.
Dr. MAZER is a consulting electrical engineer who currently specializes in electrical safety issues. His telephone number is (202) 338-0669, and e-mail address is firstname.lastname@example.org.