JUNE, 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
exposure varies from a few minutes up to 3 hours, depending upon the nature of the stocks and upon the degree of frosting desired. Measurements show a temperature of 127" t o 130' F. and a relative humidity of 70 to 75 per cent while the box is operating RS de-
scribed. Lowering the temperature and humidity slows the
teat, and if there is insufficient humidity, frosting will not take place.
Reaction of Ozone and Rubber Frosting occurs almost exclusively in warm, humid weather. Apparently the activity of the ozone or the rubber is increased by moisture and warmer temperatures. Under such weather conditions. windows are onen and the outside air circulates a great deai more indoors than at other seasons.
715
rubber which is not under tension. Harry L. Fisher found that a sample of unstressed pure gum stock in ozonized oxygen gained 0.2 per cent of its weight in 6 hours at room temperature. After the final weighing it was bent but showed no cracks, even under microscopic exnmination. Fisher atso found that xulcanized rubber swollen by benzene cracked in ozonized oxygen; the strain prodnced by solvent absorption evidently had the same effect as tension. The action of o~oneis not restricted to soft vuleani&rubber, Fisher found that bent samples of hard rubber, 0.5 m. thick with a coefficient of Twlcanization of 43, cracked in less than 2 -.:-~~L..
'IIIIIUU!S
:~~ .-~~.:--> "*.Y&C:L,. 111 "Z",llYliU ~
Ozone Cracking and Frosting Rubber behaves like a supercooled liquid with colloidal properties. Liquids exhihit surface tension, and this property is retained in supercooled liquids and solids (6) although it is not so easily demonstrated in the latter. Evidence of the surface tension of nibber is that the surface of unvulcanized rubber flows when its viscosity is reduced by heating. This seems to explain the glossy finish on air-cured rubber. The surface of rubber is therefore under tension even though the mass of the rubber is not strained. The surface is thus reactive to ozone. When exposed to ozonized air, the surface is disintegrated into a inaze CES ( x70) of minute cracks. This microscopically FICUHE 2. CmaoSTED (left) *ND FllOSTED ( l i g h f ) R r n B E n Sr:ar,n cracked surface designated as frosting is distinppiahed from atmosphcric cracking of stretclied rubber principally in t,he size and nnmher of the Air is known to contain a minute anmint of ozone, esticracks. mated by Reynolds (4) at 5 to 12 volumes per billion volumes The surface of vulcaiiized compounded rubber is essentially of air. It is supposed to be formed in the upper atintisphere a continntms transparent fihn of rubber which transmits or by ultraviolet light and reaches the lower atmospliere hg reflects light wiihout diffusion. The appearance and color diffusion and convection currents. Tlic eoncent.ration is of a pigmented ruhber composition depend upon the selective undoubtedly variable and tends to a maximuor in the spring. ahsorption arid reflection of light by the pigment dispersed An attempt was made to determine atnrospherir: oaone by the best chemical methods described ( I ) and nitli all possible in the rubber. Disintegration of the surface rubber film vrecantions. The maximum amount. found was 1.9 X IO--' gram per liter of air or about 0.10 p, p in. by volume of air. This detemiination was made in June and July, 1936. Ozone was first shown by Williams (8) in 1'326 to be the cause of the cracking of riibber under tensiorr. This was further establislwd by Van Rossem (7) who proposed the term "atmospheric cracking" to describe this effect. Ozone att,acks the double bonds of rubber to forni an ozonide ( 3 ) . The primary or secondary reaction product of oeone 011 rubber is a v i w m liquid lacking in physical strength and therefore unable to withstand a stress. It appears that where ozone attacks strct,cb~lrubber it F~CURE 3. A DROPOF DILUTE HmHocaLoaic ACID ox (left) AX UNFROSTED AND (right) A FllosTED RUBBEESUEFACE C O N T M X l X G WAITIXC breaks, and thus relieves the tension inadjacent areas and ilecrc~sesits reactivity to ozone. The strain is illcreased in the crack, a i d CRUSPS it t,o reflect, iliffuseii light. directly, and the color of the underlying pigment is obscured. ozone continues to react a t that point, and cause the crack tn grow. The surface strains in a piece of rubber are unFigure 2 (left) shows a photomicrograph of the surface of a doubtedly very nonuniform and result. in a nominiforin reglossy black rubber compound. The clear blackness results activity to ozone. irom the absorption of light by the black pigment uiider the It is not generally recognized that oaone will react with continuous, transparent rubber surface. The same surface is
VOL. 31, NO. 6
INDUSTRIAL AND ENGINEERING CHEMISTRY
716
0.35 ,&ne
art of 4,4‘4amin~iiphanylper 100 parts of rubber.
0.50 pYt oi Bntiirosting
Air-cured rubber oontrol. no Pmteoti”* *g*,nt.
FIooaE 4. EFFECT OF
ymts
ANTIFROSTING
w-
of rubber.
per 100
CHEMICALS
shown at the right after it has been frosted. The granular appearance is due to the reflected light from the disintegrated and cracked surface. The larger objects on the surface are dust or pigment particles picked up during handling. I n severe frosting the ozone disintegration of the surface exposes some filler particles which further interfere with the transmission of light and increase light diffusion. If a frosted rubber stock contains whiting, it can easily be demonstrated that the pigment particles are exposed by treating with dilute acid and observing the evolution of carbon dioxide. Zinc and other elements extractable by aqueous acids can be detected in an acid extract of the frosted surface. In an unfrosted or bloomed surface, the pigment particles are coated by a film of rubber and are not so easily attacked by dilute acids. Extraction of a pigment from a frosted surface does not restore the original appearance. Figure 3 (left) shows a drop of dilute hydrochloric acid on an unfrosted surface. At the right a similar drop of acid is shown on the same rubber sample after frosting. This rubber compound contained whiting and the gas bubbles visible in tbe drop of acid are carbon dioxide being evolved from the frosted surface. The acid is prevented from r e acting with the pigment in the unfrosted sample by the continnous rubber surface.
aldehyde-amine and ketone-amine antioxidants are sometimes slightly effective in inhibiting frosting. Most of the usual types of antioxidants and antiflex-oracking agents are entirely without effect. A limited number of chemicals have been found which are of any value. The most effective of these are all members of one class, the diprimary aryl imines, represented by p-phenylene diamine, benzidine (4,4’-diamincdiphenyl), and 4,4’-diamioodiphenylmethane. $-Phenylene diamine and benzidine are toxic chemicals unsatisfactory for commercial use. 4,4’-Diaminodiphenylmethane has some antioxidant properties and gives some activation of the cure for which allowance should be made. It has been in commercial use for a number of years and is far superior to other known antifrosting chemicals. The effect of these antilrosting agents is shown in Figure 4. These samples were subjected a t the same time to ozonized air in the frosting chamber with the lower half of each sample covered with tin foil. The protective action of the added chemicals is clearly demonstrated. This discussion of frosting should not be completed without emphasizing that it is only one of the causes for objectionable changes in the finish of vulcanized rubber products but i t should be suspected in any case not obviously of another type.
Prevention of Frosting
Literature Cited
Rubber vulcanized with aldehydeamine accelerators seems to be more resistant to frosting than rubber vulcanized with the thiazoles, dithiocarbamates, and guanidines which are now more widely used. It is believed that frosting was not encountered until recent years hecause the accelerators formerly used produced vulcanized rubber less sensitive to ozone than is obtained with the present acoelerators. Frosting can obviously be prevented by any protective fim which excludes ozone from contact with the rubber. Varnished and lacquered surfaces do not frost. Also chemical treatments which reduce the chemical reactivity of the surface will reduce its tendency to frost. Treatments with chlorine, bromine, or sulfur monochloride are effective. . Frosting is also inhihited by incorporating a small proportion of certain high-melting special petroleum waxes which bloom to the surface and form a protective film. Casper wax (E) from a Wyoming petroleum field is most effective. Ordinary paraffin is without effect. Excessive wax bloom may affect the adhesive qualities of the unvulcanized rubber or the finish after vulcanization. One half per cent of wax on the rubber can generally be used without a noticeable bloom hut this is not sufficient to give complete protection against frosting and atmospheric cracking. A large number of chemicals have been investigated for inhibiting frosting by the laboratory frosting test. It i n noteworthy that the chemicals which have been found to inhibit frosting in the laboratory accelerated test have confirmed this result under natural frosting conditions. Some
(1) Allen, IND. EN^. CHEM.,Anal. Ed., 2. 55 (1930). (2) Bradley and Mason, U. S. Patent 1,832,964(NOT. 24, 1931). (3) Memmler. “Science of Rubber”. Am. Ed., p. 210 (1934). (4)Reynolds, J . Soc. C h m . Id.,49,168-72T (1930). (5) Sheehan and Camody, IND. ENO.CHEM.,Anal. Ed., 9,8 (1937). (6) Taylor, “Treatise on Physical Chemistry”, p. 1286, New York, D. Van Nostrand Co., 1925. (7) Van Rossem and Tale”, Rubbei Chem. Tech.. 4,490 (1931). (8) Williams. IND. ENG.CZEY., 18.367 (1926). P x e s ~ w ~ sat n the Sard Meating of the Arnericsn Chernioal Society. Chapel Hill, N. C
Rotameter Flow Rates-Correction I n our paper “Correlation of Rotameter Flow Rates” which appeared on pages 451-6 of the April, 1939, issue, we find that some confusion has arisen as to units employed. I n the master plot of calibration data (Figure 3) the ea acity of the meter, 9, was expressed 8 8 cubic inches/minute. taking slo es from this plot for correlation purposes, the dimensions of tge slopes were converted from square inches/minute/rninute to square inches/second/second by dividing by 3600; the handling of extremely large numbers was thus avoided. The value of ZQL (423) was determined from Q = 386.4 inches/second/seoond, and L &s inches; therefore 423 was added to the above determined slope to give the constant C. C‘ (I Cp) was determined from this C; p and all other dimensions were 8.9 stated in the nornen-
8,
clature.
PR,NC.*ON
J. C. WEITWELL I). S. PLUMB UNlvensITY
Pnmc*mn. N. s.
