December, 1946
ANALYTICAL EDITION
obtained. If the pH varies by 10.01 unit at 3.60 the alkalinity will vary by 10.5%. This same percentage variation results from a change of 1 3 ’ or a change of +2.5% in the value offH+. .-ln error in measuring the volume of the standard acid of 10.05 ml. and an error of 6 2 parts in 1000 in the normality of the standard acid produce a change of +0.257,, while an error of h O . 1 ml. in measuring the volume of the sample produces an error of only * 0 . 1 5 ~ oin the alkalinity values. At this laboratory the acid is measured with calibrated automatic pipets and the sample bottles are usually prepared before the ship is under way. The sample is measured using a measuring pipet described by Thompson and Anderson (4). The pH values usually encountered are around 3.6. Under abnormal conditions they may rise to 4.0 and higher, but this can be remedied by adding more acid, Tvhich is a desirable thing to do, as this brings the fa+ values into the region where they are more nearly constant. The method described for determining the alkalinity of sea ,water routinely should be applicable to a number of other systems, such as wash waters, effluent from manufacturing opera.tions, urines, blood, and other biological fluids. If the procedure is to be applied successfully to other systems, the test solutions must be fairly uniform as to ionic strength .and the concentration of the unknown material. Then by using a standard size sample and a fixed amount of reagent, the relationship between the constituent whose concentration is being determined and the pH, or any electrode e.m.f. that is related t o the test substance, can be determined. Once the relationship has been established, the laborious titration process can be eliminated. Obviously, where only an occasional determination is to be made, no benefits are found. However, where large numbers of samples are examined in routine industrial or clinical work, a considerable saving of t,ime is effected.
Table
769
IV.
Sample
Summary
pH of Treated Sample
A
3.05 3 . 67 67 4.. . 1.. 3 4.37
B
3.01 3.39 3.91
C
2 98 3.47 3.65
of Calculated Alka’linity Values Alkalinity, Milliequivalents per Liter Calculated B y titration
Percentage Error
2.35 2.33 2.33 2 :. 3 2 Av. 2 . 3 3
2.35
1
Av.
1.74 1.72 1.70 1.72
1.71
0 5
av.
1.22 1.22 1.21 1.22
1.21
1
.
ACKNOWLEDGMENT
The authors are especially grateful to T. G. Thompson for his contribution to the preliminary phases of this investigation and to E. C. Tingafelter for his very valuable suggestions and criticisms. LITERATURE CITED
(1) Dole, Malcolm, “ T h e Glass Electrode”, New York. J o h n Wiley & Sons, 1941. (2) Mitchell, P. H., and RakeAtraw, N. W., Biol. Bidl., 65, 437-45 (1933). (3) Sverdrup, H. U..Johnson, M . W., and Fleming, R. H., “ T h e Oceans, Their Physics, Chemistry, and General Biology”, p. 193, New York, Prentice-Hall, 1942. (4) Thompson, T. G., and Anderson, D. H., J . Maririe Research, Sears Foundation, 3, 224-9 (1940). (5) Thompson, T. G., and Bonnar, R. U.,IND.k h ~ CHEX.. . ANAL. ED.,3, 393-5 (1931).
Accelerated Ozone Weathering Test for Rubber JAMES CRABTREE AND A. R. KEMP Hill, N. J.
Bell Telephone Laboratories, Inc., Murray
Light-energized oxidation and cracking b y atmospheric ozone are the agencies chiefly responsible for the deterioration of rubber outdoors. Since these processes are separate and distinct, i t is proposed to distinguish between them in the evaluation of rubber
for resistance to weathering. A n accelerated test for susceptibility to atmospheric ozone cracking is discussed. Apparatus for conducting the test and for measurement of ozone in minute concentration i s described in detail.
I
light-colored goods. Susceptibility can be readily gaged by measurement of the oxygen absorbed when the material is irradiated in air or oxygen under controlled conditions. The present obstacle to formulation of a standard test is the lack of a constant and reproducible light, source simulating sunlight. This article is principally concerned with the ozone factor. In the case of rubber goods containing appreciable loadings of carbon black, such as insulated cable jackets and hose, by far the most, important cause of deterioration is the familiar cracking at atressed locations by t,he ozone of the atmosphere, popularly and erroneously referred t o as “light cracking”. Many workers have attempted to use ozone-cracking in t h e laboratory as a measure of outdoor-cracking, only t o abandon the method because of failure to duplicate outdoor ratings. The probable cause of this failure has been found by the authors to be the use of too high concentrations of ozone. Small additions of wax, for example, which would confer substantial protection to cracking at atmospheric concentrations of ozone, are quite Jyithout effect in most cases at concentrations one hundred times greater. If ozone concentrations approximating that, of the atmos-
T H.\S been previously shown by the authors ( 1 ) that
tTyo
separate and distinct factors are responsiblc for the changes which rubber compounds undergo when exposed outdoors: a light-cuergized oxidation and attack by ozone which is normally prescmt in the atnmpliere in minute concentration. The former acts independently of stress in the rubber, the latter only when the material is stretched. Since sunlight, ozone, and temperature vary hourly, daily, seasonally, and with locality, the over-all result t o the exposed material will depend on t,he balance between the causative factors and n-ill be unique, since any given combination of such factors will not be duplicated. I t is therefore essential that the susceptihilities of a given compound t o damage by these two factors be dcterminrd separate1 The information thus furnished will then permit of an es ate of the durability of the compound under any givcn set of conditions. Damage by light-energized oxidation can be mitigated to any great cxtent only by physical protection from light, which can be partially achieved by incorporation of carbon black or ferric oxide. I t is likely to be the major factor in the case of
INDUSTRIAL AND ENGINEERING CHEMISTRY
770 Table
I. Effekt of Temperature on Time Taken to Crack
Compound
Wax
%
GR-S cable jacket
0 1 2 34 5
Natural rubber cable jacket
0
1 2 3 4 5
Natural
a
b e
Temperature, Degrees Fahrenheit 110 120 130 140 150 Hours Hours Hours Hours Hours 3 1 1 1 1 5 2 1 1 1 3: 6 2 1 1 1 8 5 2 1 a 0 60 20 3 2 0 a 240 60 8 3
80 Hours 10 . 7 2 402
1
1
1 1 1
60
1 1 1 1
85
2
3 100 300 450
1.5 2 4 40
b b
1 1
.. .. ..
.. ..
..
.. .. .. .. .. ..
rubber
Good after 6 months. Good after 3 months. Good after 1 month.
E F F E C T OF W A X
ELONGATION. 259"
Vol. 18, No. 12
is examined under magnification of 7 to 10 times, which not only gives results more quickly but greatly reduces errors due to any visual defects of the observer. Ozone concentrations up t o 100 parts per 100,000,000 can be satisfactorily used in many cases of highly weather-resistant compounds but are not permissible for general use. Unfortunately in ozone at 25 parts per 108, the time taken to crack such compounds at room temperature is too long to be practicable for an accelerated test; so resort is taken to the effect of increased temperature. I n the absence of protective wax, increase in temperature results in more but much finer cracks which in many cases can be recognized only under magnification. In the presence of wax, cracks when formed are few but large and well defined. Raising the temperature reduces progressively tlie effects of the n-axes in current usage till a point is reached at which they are ineffectual. For a given wax this temperature increases with its concentration and is higher for GR-S compounds then for natural rubber compounds. For example, a GR-S cable jacket, a natural rubber cable jacket, and a natural rubber gum compound containing various additions of a certain wax, at 25% elongation in an ozone atmosphere of 25 parts per 100,000,000 gave the results shown in Table I in hours to crack. These values, plotted in Figures 1, 2, and 3 on a semilog basis, reveal a distinct connect,ion between temperature and \%-axcontent. As the temperature rises a progressive decrease in cracking time occurs, the gap between low and high wax additions being gradually closed, more rapidly in the case of natural rubber compounds than in GR-S compounds. It is this feature that is suggested as the basis of accelerated testing of susceptibility to atmospheric ozone cracking. The temperature of test chosen for a given t,ype of compound is such that the highest economically practicable addition of protective wax can be made to fail in a reasonable time, say 3 or 4 days. This temperature is obviously lower for natural rubber than for GR-S, 70" to 80" and 110" F. being suggested for the former and 120" or 125' for the latter. The proposed concentration of ozone is 25 to 30 parts per 100,000,000 and the degree of elongation 20 or 257,. The criterion of E F F E C T OF W A X
I
I
ELONGATION, 25%
I 1 I \I
I
I
I I
TEMPERATURE IN DEGREES F.
Figure 1. Effect of Temperature on Time Taken to Crack a GR-S Cable Jacket Compound Containing Different Amounts of Protective Wax O z o n e concentration 25 parts per 108, elonsatlon 25%
phere be used (around 3 parts per 100,000,000 by volume) outdoor exposures can be closely duplicated. Rubber compounds designed principally as vire and cable insulations for outdoor use have been rated in this way in these laboratories for some time with considerable success. The nature and degree of cracking of a given compound on exposure to dilute ozone depend on the concentration, the temperature, the degree of elongation, and whether this is static or dynamic (1). TEST CONDITIONS
For accelerated aging in the laboratory the authors have currently adopted a concentration of 25 * 2 parts of ozone per 100,000,000 of air by volume as the test atmosphere and the expiration of time for the first appearance of cracks in the stressed sample at the chosen degrees of elongation (usually 20 and 30%) and temperature as the measure of performance. The specimen
T E M P E R A T U R E IN DEGREES F.
Figure 2. Effect of Temperature on Time Taken to Crack a Natural Rubber Cable Jacket Compound Containing Different Amounts of Protective Wax O z o n e Concentration 95 pa& per
lo%,elongation 25%
ANALYTICAL EDITION
December, 1946
performance is the time taken for the first, sign of cracking to appear. It is not suggested that the ratio of performance between low and high wax additions remains t'he same with change in temperature, so that the lower temperature life may be determined by extrapolation from the higher. Compounds of known weathering performance must be uscd as controls. It is considered that as data accumulate over a wider range of compounds it will be possible to establish definite performance requirements under specified conditions similar t o those outlined above. The room t,emperature test is intcndtd t o cover compounds of lo\\. n-eathering resistance and to d(~tc*rt anonialitls, as, for rxamplc, a certain
EFFECT O f
WAX
7i'l
wax examined by the authors which is actually highly efficient a t 120" F. but performs poorly at room temperature (Figure 4). TEST APPARATUS
Essential to the test are means of furnishing and maintaining uniforrply an atmosphere of ozone of the required concentration. This is readily and most conveniently accomplished by passing air or oxygen over a mercury vapor lamp having an envelope of quartz or glasq transmitting short-wave ultraviolet light. Ozone is formed from the oxygcn by the short-wavr ultraviolet light. 0
1
E L O N G A T I O N , 25'10
l~
I
df
JP
_
80
90 IO0 110 T E M P E R A T U R E I N DEGREES F
liC
Figure 5.
Figure 3. Effect of Temperature on Time Taken to Crack a Natural Rubber Gum Compound Containing Different Amounts of Protective Wax O z o n e concentration 25 parts per 106, elongation 25%
0 r E M P E R A T U R E IN OEGREESF.
Figure 4. Anomalous Behavior of an Experimental Wax-Natural Rubber Gum Compound O z o n e 2 5 parts per
lo", elongation
25%
/
t
ko
i
Schematic Drawing of Test Chamber
For laboratory use modifications of t h e apparhtus shown in Figure 16 of the previous paper ( 1 )are used. With this arrangement a flow ranging up to 1 cubic meter of air per hour can be supplied a t concentrations from 5 to several hundred parts of ozone per 108. The higher the flow the loner the concentration. The ozone produced will decrease rather rapidly a t first but more slowly after a hundred hours or so, permitting a very uniform output, providing temperature and voltage are kept uniform and the humidity of the air supply has no extreme variations. The lamps should burn continuously. I-sed in this way they function for several months. I n the past the authors have made coniiderable use of the G.E. germicidal lamp as ozone generator, since the yield was much less and easier to'control than with the quartz tube. However, the nature of the glass in this lamp has recently been modified so aa to reduce the ozone output, making it practically useless as an ozone generator. The lamp in current use for the purpose is the Hanovia Safe-t-aire lamp, a mercury vapor discharge tube in quartz. This lamp (Catalog No. 2851) has normally a 30-cm. (12-inch) column and even when operated a t as low a voltage as possible generates far too much ozone for easy control. The output can, however, be regulated nicely by covering part of the column with aluminum foil or other opaquing means. Only about 5 cm. (2 inches) of exposed column are needed for a laboratory generator. The tube is held concentric by waxed or shellacked corks or rubber stoppers in a glass tube '2 or 3 inches in diameter provided with side tubes for inlet and outlet of air. This lamp operates a t a low temperature and has a very long life. [Currently (August, 1946) a small 1'2-volt lamp recently developed by the Westinghouse Electric Corp. (Sterilamp WL-794) is being tested. This lamp, of automobile headlight bulb size and intended for use in household refrigerators, is a most convenient source for a laboratory generator delivering from 0.5 to 1.0 cubic meter per hour. Enough experience has not been gained to determine its useful life.] The flow of ozonized air from these generators is conducted to the base of a laborat,ory-type oven maintained at the tempera-
INDUSTRIAL A N D ENGINEERING CHEMISTRY
112
Vol 18, No. 12
Specimrns for exposure are mounted as shown m the photograph of Fwn"rgu 6. Sheets are mounted 84 strips stapled to a waxed hoard, a, a t elongations chosen, usually 20, 25, 30, and 50%, though GR-S compounds usually break at the staple m t h 50% elongation. Alternatively strips are mounted bent around a mandrel of a. diameter calculated to grve the required elongatlon, the ends secured between 2 wooden strips, b. W m s , e, are wound around waxed dowels or glass rods of the necessary dirtmeter and cables, d , similarly wound on glass tubing. Speoimena muat not be exuosed immedmtely after mountmg, but must &and for a uniform time of at least 24 hours to allow healing of any wax film that ~ V ~~~been disruoted bv the stretchinn. "R have Comparison between sample &d control shoun be on the hasis oi identical mountings. Sheets should not be cdmpared with wires, for example. ~~~~
~
~
~
MEASUREMENT OF OZONE CONCENTRATION
The above procedure is of little value without a means of determining the concentration of the ozone in the atmospheres in the expob 5 d sure chambers. In the rout,ine aperat,ion. of Figure 6. Methods of Mounting Stretched Specimens for Test several of the aocelemted weathering devices described, a 'simple and rapid method of estimationhad t o bedevised, becsusealthoughwith careful attention to air flow, voltage, and temperature, uniform ture at which the specimens of ruhher are t o be exposed. Suitable baffles should he installed to ensure uniform flow a t all Parts can be maintained, osone is an unstable substance of the oven, and the inside should be well varnished or waxed, and assurauce of a uniform concentration can be had Only by resince rapid deoomposition of ~ c c u rst~ metal surfaces, peated checking st reasonably close intervala, at least daily. Even so, the concentration in the oven will he less than that from Moreover, the answer must he obtainable within a relatively the generating apparatus and should always be measured a t the location of the specimens. short time, not more than an hour. The ideal method would he by measurement of ultraviolet light absorption in the region of the oeone absorption band having a peak a t 2550 A. However, at Where large numbers of specimens are to be exposed a different . the concentrations of ozone in use this would call for a path at arrangement is used, depicted schematictieally in Figure 5. least 50 feet long to give 8. practicable measure and is thus inconvenient. This coneists of a drum or tank, 2 feet in diameter and 3 feet The method currently used reverts to the classical method of high on legs, bpen a t the bottom and closed a t the top, with a loosely fitting lid furnished with a short chimney. The inside is by absorption by a solution of potassium iodide in divided into 2 compartments by baffle B, 12 inches from the botwater and estimation of the iodine liberated, T~ furnish tam. This baffle is 2 inches less in diameter than the drum the cient iodine far measurement in the short time allotted a large tmro rings shown acting ligh&loek to preventlight volume of air must be dealt with. To ensure absorption of the reaching the upper chamber. Another simiIar baffle is mounted just below the lid. The lower chamber houses an aluminum ozone, t,he *ir t o be measured is made to gcnerate a fine spray 01 plate on which is mounted a single Hanovia No. 2851 Safe-t-aire iodide so~utian, this way tm surface lamp operated, after aging far a few hundred hours a t approxisolution is furnished far the reaction. The apparatlls is showr.rr in coneenmately 750 volts alternating current. oEone schematically in Figure 7 and illustratcd in Figure 8. tration is thus generated in this chamher. This air rises into the upper chamber through the opening between the edges of baffle B, and the sides of the d m . escaping to the outer air around biffle BI and through the chimney, Cy The m o u n t and concentration of air thus circulating can be readily controlled by n,diiist,ine the size of the omnine. 0. of t,he chimnev or installine ,~~~~~ *-damper in the chimnef. A & ymdl d l fan rotating"s1owly on re: duced voltaae voltage may advantageously be mounted on baffle BIto ensure unif&mity~of uniformity of composition and temperature of the air in the upper chamber. The fan blades should be adjusted to generxt,e onlv a hariaontd disturbance. If the "device is to be operated a t an elevated temperature, a ring heater is mounted on the under side of baffle B,, regulated by a thermostat mounted in the upper chamber. A 500-watt heater will maintain a temperature of 110" F. and % 750-n.att heater a temperature of 120" F. When used a t these elevated temperatures the d m is insulated with a layer of thick felt or other suitable material. The speFimens to be exposed are suspended by wires from rail R around the inside walls of the upper chamber. The drum should preferably he of aluminum, which is less active 84 a decomposition catalyst of ozone than Copper, iron, or zinc. I n any event the inside should be painted with a thin layer of wax. I n the units used in these laboratories a full chimney opening gives a concentration of ozone in the upper chamber oi 25 parts per 108, using the lamp referred to. The opening is reduced in size as the activity of the lamp decreases with age. (I
.
&,
0
~~~~~~
~
~
~
~
A N A L Y T I C A L EDITION
December, 1946
reaching to within 0.5 inch of the bottom of the bottle. third neck serves to introduce and remove the reagent
The
k
phate and 0.025 fotassium d i h y d r o g e n phosphate). The solution is introduced into the titration flasks, the electrodes are inserted, and the liquid is swirled vigorously over them. Following an initial kick th? galvanometer spot will return to zeroif noiadine is present, because polarimtion of the electrodes will prevent passage of current. Presence of an oxidiaing agent such as iodine removes the polarizing hydrogen from the cathode and current flaws. Addition of thiosulfate (through the hole in the side of the flask) till the iodine is removed restores the polarieed state and returns the galvanometer deflection to aero. The reagent Will usually require the addition of 2 to 5 drops of 0.002 N thiosulfate, depending on the batch, to bring t h i s s h u t . After the ozone run the iodide solution containing the iodine is placed in t,he titration vessel and thiosulfate is added unt,il only a barely perceptible yellow remains; then the electrodes are inserted and thiosulfate is added drop by
phere whose ozone content is to be determined and the exit from
type is probably sufficient Figure 8. Ozone-Absorbing Device
l i d d is thk trap and the One ec.
represents 0.0112 cc. of ozone LKT.P.
Figure 7.
Ozone-Absorbing
Device
then returned through to rinse the apparatus titration is completed. of 0.001 N thiosulfate
A little difficulty may bc encountered a t first in identifying the end point to within one drop of thicmnlfate solution at this low concentration. It will be found easier if the titration is made to a small rtxidud defleotion of the galvanometer. Neither the form nor dimensions of the apparatus described are critical. Those given Itre of the apparatus in current use. Duplicate apparatus reproduces results within * 5 % , which is goad enough a t these low concentrations. It hits been found that reducing the concentration of the potassium iodide solution helow 20% gives low results. The system described passes about 0.3 cubic meter of air per hour, but this can he changed by changing the size of the air jet, C. With a given jet there may he considerahle leeway in the size of the capillary n o d e . The criterion i s a reaction vessel filled with a mist of reagent, The authors have compared reaction vessels ranging in size from 125 ml. to 12 liters and find a tendency t,o low results with a capacity of less than 500 ml. A larger vessel than this yields no advantage. A mast important point to remember is that potassium iodide in solution is photochemically oxidized to iodine in presence of light, even in neutral or alkaline solution. Therefore titration must not he conduoted in brigkt daylight and during the ozone run the whole of the absorption apparatus must be enclosed in 5 lighttight box as shown in Figure 9. Failure to observe this precaution will result in utterly erroneous findirrgs. The 0,002 N thiosulfate should be standardized a t frequent intervals, as such dilute solutions lose strength through oxidation. Since the iodine in solution has a vapor pressure, some will be carried away in the exhausted air; a correction must therefore be applied to the result obtained. The m o u n t of this has been determined in two ways: (1) by making determinations over a range of increasing periods of time, platting the ozone found against time, and extrapolating to zcro time which gives the true value; (2) by operating the apparatus with a stream of nitrogen
114
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 18. No. 12
and simulating the ozone reaction by adding small amounts of N iodine a t frequent intervals over the desired period and in amount equivalent to the ozone concentration studied. Comparison of the iodine remaining in solution with the amount added gives the error. These two methods checked each other within * 5 % a t any concentration between 3 and 25 parts of ozone per 0.001
is felt that t.his method is well within the over-all securtLcy possible in weathering values. It is planned t o cheek tho method further against light-absorption determinations. An apparatus far the continuous recording of omne concentration, employing the above principles, has been described by Gluckauf et al. (4). The output of such a n apparatus could readily be arranged to maintain a constant concentration of ozone in a test chamber by adjustment of the voltage to the ultraviolet lamp. It is possible, however, that the effort involved in maintenance of such apparatus would exceed that called for by intermittent control in the procedures described in this paper. I n the measurement of ozone concentrations ai the atmosphere by the pot,assiumiodide reaction, the criticism is often advanced that other oxidiaing agents known t o be present also liberate iodine from potassium iodine. These are nitrogen oxides, hydrogen and other peroxides, and chlorine. However, Gluckauf etal. (4) point out that the reaction producingiodinc
aetermlnatlons were made Over a penod of only 2 hours a t the highest concentration, since measurement time in practice neve, exceeds 1 hour. With decreasing concentration the period was increased progressively up to 8 hours for the lowest. The curve applies only to tho particular conditions employedVis., & flow of 0.27 to 0.30 cubic meter per hour and a vacuum of 60 em. of mercury. For other conditions t,ho correction must be determined experimentally. The absorption of ozone is assumed to be complete, since increasing the concent.ration of potassium iodide up to 80% gave no higher values. Further, dilution experiments, in which omne of concentrations up t o 100 parts per million NBS diluted with a known volume of air and the dilution simultanoausly measured, when carefully conducted, checked within -5%. This is not a definite proof but a reasonable assumption. It might be thought that the loss of iodine could be reduced by cooling the reaction vessel in ice. The reduction has proved on trial t o be insignificant. Expausion of the air a t the nozzle and mme evaporation of %he water results in cooling the reagent t,o around 6 0 " F., depending on the duration of the run. Whom room temperatures are high as in summer months, howevrr, immersion in ice would ensure more uniform conditions. The principal merit of this method lios in the rapidity a i t h which ozone concentrations a t the level suggested for use in the accelerated weathering methods described can be made. Since variations in the rubber compounds tested and the porsond errors
NO2
+ 2HC + 21- +H*O + NO + I,
proceeds only to a very limited extent a t low concentrations and 7.0, because of the lack of hydrogen ions and prove this by experiment. The present authors have found by trial that nitrogen peroxide even in concentration three- or fourfold that of the atmosphere (8, 5 ) does not liberate iodine from potassium iodide by the procedure described. As for hydrogen and other peroxides, the authors have made repeated attempts t o trap suspended particles of organic peroxides in a water Bpray and to freeze out hydrogen peroxide in traps cooled by a dry ice-acetone bath. The liquids so obtained from several cubic meters of air have invariably failed to liberate titratable amounts of iodine from buffered potassium iodide solution. Free chlorine spparently is absent from the atmosphere a t the location of these laboratories. Nevertheless, in this connection the possibility of the presenec of these or other oxidizing agents in the atmosphere of industrial locations should he borne in mind.
at pH
LITERATURE CITED
(1) Crabtree, J., and Kemp., A. R.. IND.ENQ.CHEX..38,278 (1946). (2) Edgar, J. L., and Paneth, F. A,. J . C h m . Soo., 1941,519. (3) Foulk, C. V., and Bowden, A. T.,J. Am. Ghem. Soc., 48, 2046 i144fA ~.".-,.
E., Ned, H. G.. Marten, G . R.; and Paneth, F. A,, h .Soc., 1944, 1. (5) Reynolds. W. C..J. Soc. Chem. I%&, 49, 168T (1930). (4) GlueJtauf,
J. C
