Evaluation of Ozone Protective Agents for Elastomers Apparatus for the Evaluation under Dynamic Conditions K. E. CREED, Jr., R. B. HILL, AND J. W. BREED Research Department, Monsanto Chemical Co., Nitro, W. Va. The widespread acceptance of synthetic elastomers for use in rubber products has greatly magnified the problem of obtaining satisfactory protection against weathering i n general and ozone attack in particular. Because the tentative ASTM method of evaluation was not entirely satisfactory, a dynamic apparatus has been developed which provides a n effective means for accelerated evaluation of ozone protective agents using a iiew mechanical arrangement for flexing the rubber, a new specimen shape, rigid control of the ozone concentration, and a new rating system. The apparatus consists of a series of rotating pulleys within a circhlar chamber. The test specimens, circular molded belts, are rotated around two pulleys, which gives them approximately 75 flexes per
minute with an elongation of 0 to 209Z~. The flexing provides a mobile surface, so that rigid waxy films cannot he formed, and the molded specimen eliminates the possibility of freak cracking and anomalies due to minute nicks or imperfections during dieing out. The larger surface available for examination facilitates interpretation of results. Air a t a rate of 1 cubic meter per hour passes through the chamber w-ith a controlled ozone concentration of 25 =k 5 parts per 100,000,000 a t 30” C. Uniformity is ensured by use of a fan in the chamber. The concentration of ozone is determined by potassium iodide method. By a simplified rating system a nontechnical person can rate a large number of specimens rapidly and without elaborate equipment.
E
XPOSURE cracking has been evidenced to various degrees
by the majority of elastomers under strain and has presented a serious service and appearance problem for many years. Its cause has been attributed to many factors-light, oxygen, ozone, heat, and moisture. Newton (7) clarified the situation and showed that ozone was the major cause of “exposure cracking.” With the widespread acceptance of GR-S for use in rubber goods, the need for a protective agent effective against ozone has become more pressing, as GR-S appears to be more susceptible to ozone attack than the natural rubber which it replaced ( 4 ) . \Taxes (1, 11) oxidized rubber ( I d ) , cellulose (S), and bituminous paint ( 5 ) , which prevented ozone from reaching the rubber surface, have been cited in theJiterature and in some cases employed commercially as a means of reducing this cracking in articles where static or even mild flexing conditions prevailed. For these, a static method of evaluation ( 2 ) appeared to be satisfactory. During the past few years, several compounds have been reported which provided protection under dynamic conditions; however, these were found to be unsatisfactory because of their toxicological or deleterious aging properties.
Originally, circles died from standard stress-strain sheets were employed. Considerable difficulty was encountered in the visual evaluation of the cracking because of the variation in the size of the cracks. A thick specimen was adapted after inspection of-a number of Ross flexer slabs from the Phillips Petroleum CO.’E Laboratories in Akron, Ohio. These slabs, 0.25 inch thick, offered a big advantage over the thin specimens, in that they showed a uniform sized surface crack, they were large enough so that the degree of cracking was easily determined by visual means, and
DEVELOPMENT B4CKGROUND
.At the time this program was instigated, two instruments were readily available for evaluating the surface cracking of stocks under dynamic conditions: a fleser developed by Throdahl (10) to study the surface cracking in GR-S exposed to ultraviolet light and a belt flexer utilized by the Monsanto Chemical Co.’s Technical Service Laboratories in Akron, Ohio, for studying ozone cracking. The belt flexer consisted of a canvas belt suspended over pulleys to which ‘/a X 2 X 1 inch test slabs were vulcanized. Flexing was obtained as the belt passed over the pulleys. This type of apparatus has been employed by the soling industry to evaluate the flex characteristics of shoe soles. For studying ozone cracking the belt mechanism was enclosed in an ozone chamber. .4 modification of the ultraviolet light flexer proved unsatisfactory because with the use of higher amounts of ozone, 25 & 5 parts per 1OO,OOO,OOO parts of air, the strips cracked so severely within a few hours that the sensitivity of the method was lost. The belt type of flexer was found to meet the needs and provided the additional advantage of simplicity of construction.
t Figure 1. Isometric Sketch Showing Dimensions of iMolded, Circular, Belt-Type Test Specimens Used in Conjunction with Dynamic Ozonizer
they were easy to handle. Therefore, a thick, molded, circular, belt-type test specimen measuring I/? inch wide, inch thick, and 55/16 inches in diameter was developed. An isometric sketch of the test specimen is shown in Figure 1. DESCRIPTION OF DYNAMIC OZONIZER
A photograph of the dynamic ozonizer is shown in Figure 2. The operation of the apparatus is indicated in Figure 3. 241
ANALYTICAL CHEMISTRY
242
An ozone concentration of 25 + 5 parts per 100,000,000 parts of air is generated by a Hanovia. burner, 0. Baffles at the top and the bottom of the chamber protect the personnel from the ultraviolet light, but do not restrict the air flaw. The production of ozone is controlled manually by means of Variac V through a 15,000-volt transformer. The osonized air passes through duct D to the chamber. The circular test specimens, B, are supported on a series of 1-inch rotating stainless steel rads, R , whiah in turn are mounted on an iron frame hv means ofpulley hanpers, H. The rods are rotated by a 2.5-ihch pulley; G, bonnected hp. motor, M . As any portion of test through a V-heit to a specimen passes over the rolls, a momentary elongation through 8. ranee of 0 to ZOO/,is orovided. This Bexine is a t a rate of auproxihtely 75 fl&s per minute. The present apparatus consists of two such series of rolls, upon which 36 specimens may be exposed a t the mme time (Figure 4). Uniformity of the ozoneair mixture is ensured by bhe use of the fan, F , mounted on the frame. The rate of flow (35 cubic feet per hour or 1 cubic meter
rable 1. Ozone Concentration in Various Parts of the Dynamic Tester parts per A t s ~ m r , l i noutlet ~ side Opposite side TODbelow vent stack center of frame Cornparitesideand top
ioo,ow.ooo
26.3 26.6 29.4 23.4 29.4
is maintained in the chamber. The uniformity of the ozonized air is shown in Table I.
I II
AO/
Figure 2. Dynamio O mnieer Chamber io position and owne a o a l y sis s p p a m f Y P
An array plot of the oaone concentration over an extended period of time is shown in Figure 5. As may be seen, the addition of the stack and the vent fan has a marked effect in maintaining % uniform and consistent atmosphere. No attempt is made to control the air temperature, but an average temperature of 30' C. is usually maintained. The ozone concentration is determined twice daily by the conventional potassium iodide method of Crabtree and Kemp ($1. Samples of air are drawn from the chamber a t N by application of a vacuum (Welch Duo-Seal pump), P , tothe Woulff bottle, W ,and the flow is measured by a standard Porter-Fisher flowmtter, F M . The Woulff bottle and reagents are enclosed in a light-tight container, C, to prevent the photochemical decomposition of the potassium iodide, Other methods of ozone anal,& have been considered, but none provides the accurecv and practicability of the potassium iodide method.
Figure 4. Construotion and Position'of Rolls and M o u n t i n g of Test Specimens
V O L U M E 25, N O .
2, F E B R U A R Y 1 9 5 3
243 were obtained. The standard series for Hevea and GR-S tread stocks are shown in Figure 6. An arbitrary rating of 1 was assigned to the sample with no cracking and 6 to the one with extremely severe cracking. The others were given appropriate ratings in between these extremes. The experimental test speeimens were visually compared a t intervals (usually 24 hours) with the standards and were a8signed a rating which was indicative of the extent of cracking a t the time. This provided a measure of the effectiveness of a given material over a period of time and showed the approximate time for initial cracking. For the sake of clarity, t-ypioal examples of each step are shoNn.
. Q
a
22
k
a c
8
a
" I 0
S
EPT.
'
NOV
NUMBER
re 5.
NS IN SEPUENCE h a y Plot of Ozone .-unu;nrrarrun i n Chamber over an Extended Period of ~
'l'iinLe motersium iodide m ethod.
ASTM limits of 25
f
5 parts are designated
RATIN
