Abstract of Special Research Report (RR-26)

National Institute of Occupational Safety and Health, Japan

High Temperature IR on Autoxidation of iso-Polypropylene in Wet Oxygen

RR-26-1

Takashi KOTOYORI

: High temperature infrared techniques were applied to examine the influence of water vapour on the rate of thermal autoxidation of isotactic polypropylene film in pure oxygen of normal pressure at 97°C.
For this purpose a Hitachi Model 285 grating (5,000 line per mm) infrared spectrophotometer was used together with an oxidation cell and an oxygen humidifier (Fig.1, 2). The unstabilized, purified isotactic polypropylene (PP) powder employed in this work was furnished from Mitsubishi Petrochemical Co. Ltd. ; mol. wt. 1.7 × 10⁵ ; isotactic index 96% ; ash content less than 0.003%.
    The powder was pressed to give a 0.2 mm thick film by means of a vacuum press, under a pressure of about 10^-5 mmHg at 198 °C for 2 min, between polyethylene terephthalate sheets 0.025 mm thick, together with a Teflon spacer 0.2 mm thick with an inside diameter of 30 mm. Owing to the crystallizing nature of PP, the preparation of specimens of reproducible oxidation behaviour requires the strict control of press conditions. The film was mounted on a horizontal KBr disc. The temperature of the disc itself was 97 °C at the center. Thus this temperature was assumed to be the reaction temperature throughout this study. A close contact between the film and the disc was always minded to get reproducible results. Moreover precautions were always taken in handling the powder and films to minimize exposure to light, air and skin.
    The humidifier was thermostated at 35 °C. Therefore a partial pressure of water vapour, which corresponds to the saturated vapour pressure of water at 35 °C, can be added to the oxygen stream when it passes through the humidifier. The interfering water vapour absorptions were essentially eliminated by virtue of the double beam feature of the instrument. Further the condensation of water vapour on the KBr windows or on the inner wall of the cell happened to be avoided due to the hot state of the cell.
    Infrared spectra of a 0.2 mm PP film before and after oxidation is shown in Fig.3. Major changes in the infrared absorption spectra upon oxidation are indicated to occur in the hydroxyl (-OH) region between 3,600 and 3,200 cm^-1, and in the carbonyl (CO) region between 1,800 and 1,600 cm^-1. Thus the measurements were undertaken in an attempt to detect any difference in the rates of formation of the -OH and CO groups during the thermal oxidation of PP films in dry and wet oxygen (Fig.6, 7 and 8). Subtle, but significant differences were observed in these runs. Films in wet oxygen were found to undergo slower thermal degradation reactions than those in dry atmosphere. In further detail, the intensity of the -OH bands in wet oxygen shows a slightly faster increasing rate during the first 3 hrs, then it begins to level off, and eventually it becomes weaker, compared to that in dry atmosphere ; whereas that of the CO bands in wet oxygen continues to increase slower in the whole course of oxidation than the counterpart in dry atmosphere.
    These phenomena are thought to mean that the decomposition of hydroperoxides in the polymer matrix, attended with the evolution of water, is suppressed by the existence of water vapour in the gas phase in terms of chemical equilibrium. The initial slight increase of -OH concentration in wet oxygen can be attributed to a little accumulation of hydroperoxides as a result of the suppression of its decomposition. But, as the oxidation progresses, the concentrations of products containing a variety of -OH groups become gradually higher in dry oxygen, in which the decomposition of hydroperoxides is naturally faster than in wet atmosphere.
    However, the thermal oxidation of actual polymers in the market may undoubtedly be complicated hy the presence of trace impurities such as metal components. Therefore there would be certain cases in which oxidation reactions are rather enhanced by the coexistence of moisture.

Safety for Electrical Equipment under Artificial Environments (3rd Report) --Ignition of Flammable Solid Materials by Low Voltage Capacitive or Resistive Sparks--

RR-26-2

Ryuji TANAKA and Kenji ICHIKAWA

: In the previous report, the authors described the result of ignition limits of the flammable solid materials by inductive sparks in oxygen-enriched atmospheres.
    In this work the ignition limits by capacitive or resistive sparks have been determined.
    The experimental technique used was essentially the same as already reported, except that the spark producer was the Intermittent Mk-II apparatus.
    Oxygen and the mixture of oxygen/nitrogen or oxygen/helium, each pressurized up to 1.3 MPa, were used as the test atmosphere.
    In the capacitive circuit, the minimum igniting voltages were measured by varying the circuit capacitance between 0.34 μF and 50 μF : in the resistive circuit, the determination of the minimum igniting currents were made with the supply voltage of 15 to 300 volts.
    Almost throughout the experiment, the flammable solid material was the Japanese paper.
    The effects of percent oxygen and total pressure of the test atmospheres on the ignition limit, have been investigated in different circuit conditions.
    Combination of oxygen concentration and total pressure of oxygen/nitrogen or oxygen/helium mixture, which gives the same ignition limit as that in atmospheric pure oxygen, is also illustrated.

A Study on the Time Intervals between Industrial Accidents

RR-26-3

Shigeo HANAYASU

: In many industrial areas, the accident frequency rate (the number of injuries per million employee-hours of exposure) is often used as a measurement of safety performance. However this measurement has been thought neither stable nor sensitive to changes in system input and also has dubious reliability.
Another important drawback is that this measurement requires accidents to occur before it can be calculated.
    n routine accident control it is necessary for a manager or safety committee to have the earliest possible indication of a significant change in the accident situation.
    This paper deals with the time intervals between industrial accidents in the hope of establishing its usefulness as a measurement of safety performance and calculating the probability of whether the accidents will occur or not at a given time T for one industrial place having a certain accident risk.
    If some events are taking place at random in time and the expectation of events per unit time is constant, then the frequency of occurrence of events in fixed intervals of time have Poisson distribution and the time intervals between events are exponentially distributed.
    In the previous paper, the author described that the frequency distribution of fatal accidents in one day in the Tokyo Metropolitan area in the year of 1973 had distributed as Poisson distribution so that Exponential distribution could be used as the time intervals between industrial accidents.
    In calculating the probability of exponential distribution, the parameter λ of exponential distribution in Equ.1 (the reciprocal number of the expectation per unit time) may be estimated using accident frequency rate in the form Equ.2. Then the probability whether an accident will occur or not within the given time t for a certain accident frequency rate could be calculated by Equ.3 or 4. (See Fig.1 & 2) Also tests may be applied to discover whether there is any significant tendency for changing accident situation in succeeding intervals of time by the help of Fig.3.
    It may be shown that if the time intervals are exponentially distributed, the sum of k intervals becomes Gamma distribution (Equ. 7). Then the probability whether k accidents will occur or not within the particular time T for a certain accident frequency rate can also be analyzed similarly as exponential distribution. (Fig.5 - 12)
    Consider the problem that when an industrial area has some industrial groups in which each group has its own proper accident rate, what shall the collective time intervals between accidents become ? (Fig.15) In conclusion, the collective time intervals result in the same form exponential distribution as shown in Equ.14 to 16. In these equations the parameter as a whole is required to total together all the groups accident frequency rates. All of the graphs arid tables used up to this point could be used for this problem in the same manner.
    When the information concerning accidents such as the number of occurrences and the time the accidents taking place are given, interval estimation of accident frequency rate can be conducted. Namely the mean of k intervals, say t', comes to be Erlang distribution (Equ.29, Fig.20, 21) and it follows from Erlang distribution that 2kλt' are distributed as χ² distribution with the degree of freedom φ = 2k. This indicates an existing χ² distribution table may be employed in accident interval analysis. As shown in Equ.31 or 32, t' may be used to provide confidence intervals for an unknown value of A (frequency rate) as well as λ. (Fig.19, 22) If the accident frequency rate of an industrial area which is already known and denned as an a priori frequency rate, say Ac, is given then the possibility that the frequency rate at the time when an accident happens will be smaller or larger than Ac could be calculated by Equ.24 and 25 respectively. (Fig.17, 18)
    As conclusions in this study, the time intervals between industrial accidents seemed to be useful as a measurement of safety performance. They can be used especially as an earlier indication of changes of the accident situation. The fact that interval estimation of frequency rate using time intervals could be conducted would probably be the most useful for safety management.

Direction Initiation of Spherical Gaseous Detonations Initiated by a Planar Detonation in the Tube

RR-26-4

Hidenori MATSUI

: The present paper describes some results for the critical energy for direct initiation of spherical detonations in acetylene-air, fuel (H₂, CH₄, C2H2, C2H4, C3H8 individual)-oxygen and fuel (H₂, C2H4, C3H8 individual)-oxygen-nitrogen systems initiated by a planar detonation from a linear tube into the larger volume of the same explosive gas.
    The critical diameter for direct initiation at atmospheric pressure was determined by extrapolation of the experimental relation between critical initial pressure for direct initiation and the tube diameter. The energies for direct initiation were calculated from the critical diameter data and the detonation properties using simple initiation model.
    The results lie between 10^-3 and 10⁴ joules within the whole experimented gas systems and compositions. Comparisons with existing data from exploding wire and electrical spark indicate that the present results are more than one order smaller than those of exploding wire or electric spark. This may arise from the fact that the critical energies using exploding wire or electric spark is not based on the effective energy but on the total stored energy (E _T = 1/2 CV ² ). However, the present data show a good agreement with the results obtained from detonator based on the chemical energy. This suggests that the energy transfer mechanism for these two types of igniter is almost same.
    For hydrocarbon mixtures it is indicated that the critical energy takes on minimum value that are widely displaced from the stoichiometric composition for combustion to CO₂ and H₂O ; instead they are fairly close to-composition corresponding to combustion to CO and H₂O.
    Minimum critical initiation energy for direct initiation of spherical detonation in fuel-air mixtures without acetylene are estimated at above 10⁴ joules.

Systematization of Spontaneous Ignition Temperatures of Organic Compounds (1st Report) --Spontaneous Ignition Temperatures of Alkyl Alcohols--

RR-26-5

Shozo YAGYU

: Ignition temperatures of combustible substances have so far been determined mainly by experiments. However, only a limited number of substances was taken up in this connection in the past, among a wide variety of combustible substances. So, still more sufficient experimental data must be accumulated from the standpoint of safety engineering.
    The author has intended from some years ago to systematize the ignition temperatures of combustible substances, which are mainly organic compounds, to estimate the ignition temperature of a compound without experimental determination. For this purpose, it is necessary in the first place to investigate the relationship between ignition temperatures and molecular structures of organic compounds of various kinds.
    Now, a relationship between the ignition temperature and the average carbon chain length has been already reported by Zabetakis for the paraffin hydrocarbon homologs¹⁾. In this paper is described the result of application of his method to alkyl alcohol homologs, as the first report of this Series of study.
    As a result, a similar relationship to that in the case of paraffin hydrocarbon was obtained also in alkyl alcohol homologs. Further, a very similar relationship to both mentioned above was also found between the boiling points and the ignition temperatures of alkyl alcohol homologs.

JNIOSH