INDUSTRIAL AND ENGINEERING CHEMISTRY
1088
LITERATURE CITED
Adkins, H., Frank, R. L., and Bloom, E. S., J . Am. Chem. SOC., 63, 549 (1941). Berman, N., and Howard, H. C., Fuel, 29, 109-11 (1950). Biggs, B. S.,J . Am. Chem. Sor., 58, 484 (1936). Biggs, B. S.,and Weiler, J. F., Ibid., 59, 369 (1937). Bone, W. A., Horton, L., and Ward, S. G., Proc. Rov. SOC. (London), 1278, 508 (1930). Ibid., 148A,521 (1935). Clar, E., “Aromatisohe Kohlenwasserstoffe,” p. 170, Berlin, Springer, 1941. Fieldner, A. C., and Davis, J. D., U. S.Bur. Mines, Monograph 5 11934). - -, Fisher, C. H., U. S. Bur. Mines, Bull. 412 (1938). Franke, N. W., and Kiebler, M. W., Chem. Inds., 58, 580 (1946). Gillam, A. E., and Hey, D. H., J . Chem. Son., 1939, 1170. Harris, E. E., D’Ianni, J., and Adkins, H., J . Am. Chem. Soc., 60, 1467 (1938). Hicks, D., and Xing, J. G., (Brit.), Dept. Sei. Ind. Research, Fuel Research Tech. Paoer 34 11931). --
Vol. 44, No. 5
ed.. PP. 737, 745, chap. 19, New York. John Wiles & Sons, I ~ c . ;i945. Kinney, J. R., J. Am. Chem. SOC.,69, 284 (1947). Le Claire, C. D., Ibid., 63, 343 (1941). Orlov, N. A., and Belopolsky, M. A., Ber., 62, 17524 (1929). Ruof, C. H., and Howard, H. C., Division of Gas and Fuel Chemistry, AM. CHEM.Soc., Pittsburgh Divisional Meeting, May 9 and 10, 1949. (21) Selvig, W. A., Pittsburgh Station, U. S. Bur. Mines, private communication. (22) Spielmann, P. E., “Constituents of Coal Tar,” p. 137, London, Longmans, Green and Co., 1924. (23) Travers, M. W.,J . Inst. Fuel, 6, 253 (1932-33). (24) Washburn, E. W., Bur. Standards J . Research, 2, 476 (1929). (25) Waterman, H. I., J . Inst. Petroleum Technol., 21, 661, 707 (1935). (26) Weiler, J. F., “Chemistry of Coal Utilization,” H. H. Lowry, ed., chap. 8, New York, John Wiley & Sons, Inc., 1945. (27) Ibid., chap. 10, p. 382. (28) Whitmore, Frank C., “Organic Chemistry,” p. 6, New York, D. Van Nostrand Co., Inc., 1937. (17) (18) (19) (20)
RECEIVEDfor review August 1, 1951. ACCEPTED December 17, 1951. Presented as part of the Symposium on t h e Nature of Bituminous Materials before t h e Division of Colloid Chemistry and the Division of Gas, Fuel, and Petroleum Chemistry a t t h e 118th Meeting of t h e AMERICANCHEMICAL SOCIETY, Chicago, Ill., September 1950.
Prevention of Mold Growth in Optical Instruments PANAMA CANAL ZONE EXPOSURE LEONARD TEITELL AND SIGMUND BERK Pitman-Dunn Laboratories, Frankford Arsenal, Philadelphia, Pa.
I
T I S well known that cotton and leather goods can be dam-
aged by molds (mildew). Deterioration caused by molds is not limited only to cotton and leather, for even optical instmments, such as microscopes, binoculars, and telescopes, are affected. Molds are found growing on the polished glass that is used for line optical equipment. Molds require dampness for growth and thrive best in warm (20’ to 30” C.) places. Tropical climates are warm and humid, and optiral instruments that are stored or used in the tropics tend to become moldy. The air inside the instruments becomes as humid as the surrounding tropical air and the instrument acts as an incubation chamber for the molds. Molds also grow inside optical instruments stored in temperate or even arctic climates, if the storage conditions are warm and humid. However, during the war moldy binoculars and telescopes n-ere encountered mostly in tropical areas, such as New Guinea, islands of the Southwest Pacific, the Philippines Burma, Malaya, and the Panama area. The damage caused by molds growing on the internal parts of binoculars and other optical equipment was known to people who lived in the tropics and used these instruments before the war. Prevention at that time was mostly a matter of special storage under dry conditions If an instrument became moldy, it was returned t o the manufacturer for cleaning. Before the war several companies had started investigations on mold growth in binoculars. However, with the advent of a global war large numbers of binoculars, telescopes, cameras, etc., were sent to tropical areas. Storage conditions were poor and binoculars became moldy so fast that there were insufficient personnel to repair and clean them. I n one of the earlier surveys on tropical deterioration, Magee, Hansen, and Grant (9) reported that the presence of fungal colonies in optical equipment in New Guinea was the rule rather
than the exception, and that of hundreds of items observed, the few instruments that were free from molds were either recently received in New Guinea or recently cleaned in workshops. Hutchinson (14) estimates that 70 to SOYo of the repairshop work in the Panama Canal Zone was primarily a result of mold growth. During the course of the investigation reported here, visits were made to repair shops a t Fort Gulick and Coroaal in the Canal Zone, and more than half of the binoculars that were in for repair had mold growth on the lenses. The problem was considered so serious that it was attacked almost simultaneously by investigators from three countries-by the British (b), who used Nigeria, West Africa, as a test eite; by the University of Melbourne staff ( 1 , ,$?I),who used New Guinea as a test area; and in the United States by the National Defense Research Committee of the Office of Scientific Research and Development (10-18), who used Barro Colorado Island in the Canal Zone as a testing area. Optical instruments present a unique problem in mold growth. Small amounts of mold, which on materials like cotton or leather would probably go unnoticed and certainly would do little if any damage, create a serious problem in optical instruments since the mold is in the optical path close to the viewer’s eye. Apparently the optical glass does not act as a food for the molds but provides a surface on which the molds can grow and water vapor can condense. Some mold spores seem to require liquid water for germination. The mold growth that takes place is initiated by sorption of water from a condensed droplet or from vapor in the air. The reserve food present in the mold spore, along with the sorbed water, allows sufficient mold growth to interfere with function of the instrument. More luxuriant mold growth occurs on a lens surface if accessory food, such as dust, debris, or small dead arthropods, is present on the glass.
May 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
Mold growth on the interior of a binocular is not restricted to the glass surfaces. Upon disassembly, small amounts of mold are found on metal surfaces, but this growth does not show when using the binoculars. Since each telescope of a binocular contains ten internal optical surfaces (not counting the reticle), a slight amount of mold on each surface will have an accumulative effect, and the telescope will appear to be full of mold when viewed through the objective. Military binoculars are used for the measurement of small angles as well as for observation; therefore, there is usually a reticle in one of the telescopes of a binocular. When using the binoculars the reticle is in focus and is magnified by the eyepiece; consequently, defects on the reticle are serious.
1089
most frequently encountered. Richards (19) reported species of Alternaria, Bassisosporium, Cladosporium, and Pullularia isolated from the optical elements of microscopes. M O D E S O F INFECTION
Mold growth in binoculars starts from spores which have found their way onto the lens surfaces. These spores are microscopic in size, usually less than 10 microns in diameter, and do not interfere with vision. Mold spores are found in the air of factories where binoculars are assembled as well as in the repair shops. The spores can easily lodge on the lens surfaces before assembly. Also, if the optical instrument has any small openings, Bpores may be sucked into the instrument when the temperature is lowered. It is not possible to state when the initial spores entered a moldy instrument, since the types of molds that are found growing on the lenses are ubiquitous. Both Hutchinson (14) and Gray (1'7) have reported that mites are vectors of mold spores. Mites are mycophagous and may carry spores on their bodies or legs. The mites can enter binoculars through a small opening or crawl along the screw threads of the diopter movement. A dead mite can act as an excellent source of food for molds, and several examples of luxuriant growth radiating from a dead mite on an optical surface have been seen during this investigation. PRINCIPAL PREVENTIVE METHODS
Figure 1. Test Sign Photographed through Moldy Binocular (50 in Table I) Test sign in lower right eorner photographed through a nonmoldy binocular Fungus filaments on reticle are in focus
Mold growth on the lenses of an optical instrument can interfere with function in two ways. Firstly, growth on the reticle (Figure 1)or on the edges of the field lens will appear in focus and can be mistaken for, or obscure, the target. Secondly, a considerable amount of mold growth on lens surfaces, especially on several surfaces, may reduce light transmission or produce a blurred field. Mold growth on the lens surface can usually be removed by wiping with lens tissue. In more difficult cases paper wet with ethanol aids in fungus removal. A few cases have been seen in which, after removal of the mold, the outline of the growth remains which resists cleaning procedures and requires repolishing with an abrasive. This damage to glass by fungi has also been reported by the Australian investigators (1)and Hutchinson (11). Since binoculars and telescopes are precision instruments and the internal optical elements are not easily accessible, the reassembly of the instruments after removal of any mold growth from lenses requires specially trained personnel and a considerable amount of time. Mold growth should not be allowed t o remain for a long period of time because mold stain or etching occurs and the lens has to be repolished or replaced. T Y P E S OF MOLDS
The species of molds that are found growing on the optical glass are common forms that are found regularly in both temperate and tropical areas. Magee, Hansen, and Grant (9) reported finding the genera Aspergillus, Penicillium, and Oospora in New Guinea. Other Australian workers (1)have identified species of Trichoderma and Mucor as well as Aspergillus and Penicillium growing on optical components. Hutchinson ( 6 ) found that in instruments that became moldy in the Canal Zone Monilia crassa and species of Aspergillus and Penicillium were the organisms
Numerous ways have been investigated for preventing mold growth in optical instruments. The principal methods that have been used or tried are periodic cleaning, sterilization, dehumidification, fungus inhibitors, and miscellaneous preventive methods. PERIODIC CLEANING.Periodic cleaning is very simple and is used by many storekeepers in tropical regions to keep their leather goods free from mold growth. Every day or two the slight amount of mold growth that accumulates is wiped off. This method is of value in optical instruments only where the lens surfaces are easily accessible; it can be used for some types of cameras. STERILIZATION.A well-sealed instrument which is sterile after assembly-Le., does not contain any viable mold sporesshould remain free from mold even under severe exposure conditions. This would assume that the sealing is sufficient t o prevent the introduction of mold spores after the instrument is put together. This is not true of all military optical instruments. Sterilization by heat is not practical since cements, sealing compounds, and lubricants would be affectedby the high temperature necessary, Ultraviolet radiation is known t o kill mold spores, and the assembly of binoculars under ultraviolet light was tried in this investigation. DEHUMIDIFICATION. Since molds require moisture for germination and growth, a low relative humidity inside an optical instrument will prevent growth. Galloway (6)found that the minimum relative humidity permitting growth varies from 75 to 95% for different species of molds. Instruments that can be stored in well-constructed hot closets ( 1 3 ) or in air-conditioned rooms will present no fungus problem since the relative humidity can be maintained below 75%, the minimum level required for mold growth. However, there are 'many instances in which instruments cannot be given the benefit of ideal storage conditions. For instance, the instrumentb may be stored in hastily constructed shelters, they may be mounted on other equipment, or they may be in actual combat use. Under these conditions it is most important that the instrument be serviceable. The instruments, therefore, must be prepared to withstand mold-producing conditions. The atmosphere inside an instrument can be kept low by filling it with dry air and then hermetically sealing the instrument, This has been practical for height finders which are kept under a positive pressure of heliuni. There are difficultiesin hermetically
INDUSTRIAL AND ENGINEERING CHEMISTRY
1090
TABLE I. ItESULTs O F EXPOSCRE Test Telescope NO. 1 2 3 4 6 6 7 8 9 10 11
12 13 14 15 16 17 18 19
20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
41 42 43 44 45 46
Model No. 313 M3 1\16 316 M6 M6 316 h17 h17 M7
Treatment None None None Xone Sone None None None Sone None None None Sone Sone h-one None None None None None None
If9 nf 9 hI9 R19 n1i3 RI13 M13 3113 1113 M13 Elbow telescope AI17 n13 n13 313 316 XI 6 XI6
Cresatin Cresatin Cresatin Cresatin Cresatin Cresatin Cresatin Cresatin Cresatin Cresatin Cresatin M.T.S. c M .T.S. h1.T.S. h1.T.S. iv1 .T.S , M.T.S. M.T.S. M ,T.S. M.T.S. M .T.S.
126 116 1\17
317 317 M3 h13 113 XI3 h13 n23 PMkII 21IkII PhIkII 2MkII 2MkII 2MkII Elbow telescope nfi7 Elbow telescope M17 M3 RI6 n16 R.16 hI7 >I7 h17 M7 &I6 "46 316 M9 R.I 9 M13
M.T.S.
fv1.T.S. Thxnite Thanite
No. of Other Half of Binocular 33 37 29 48 28 27 55 32 54 52 12 11
14 13
16 15 18 17 20 19
OF
TESTINSTRUYENl'S
Exposure Conditionn A A
B .4
i A
kB C
C C C C C C
C
C
..
36 36 34
3 ,51 53 8 1 24 22 23 2 47 40 39 42 41
43 43
11 days 0 0 0 0 0 0 0 0 0 0
.. .. ... ... 0 0 0
0 A -4
B
i 4 A B B B A B C C
5C
C A
A
16 weeks
++ ++ +0 ++ ++ ++ ++ 0
CANliL Z O S E Fungus Growthb 19 8 weeks months
.. '6
.. ..
..
..
..
0 0 0 0 0 0 0 0 0 0 0 0 0
.. ,.
,.
++ 0 0 0 0 0 0 0 0 0 0 0
+0 +O+
,. ,.
..
++ ++ ++ ++ ++ ++ ++0 +f
0 0 +0+ 0 0 0 0 0
..
.. .. ..
.
.. .. ..
.. .. ..
.. .. ..
..
.. .. .. .. ..
..
..
..
, .
..
0
t
, .
++ + +++ +. .
0
-I-
..
..
..
..
++ ++ t
0
0 0
0 0
0 ++ 0
,.
+0+
..
..
..
*... .. .. ~. ..
..
17 inonthc
..
..
.. .. I
131/z months
.. ..
..
0
0 .. ..
0
50 4Q
!
___
IN THE
Vol. 44, No. S
'd
0
0
0 0 0
0
Silica gel 38 B 0 0 .. 47 4 Silica gel A .. 0 .. 48 26 Silica gel A 0 . . .. 49 ++ Silica gel 25 .. 0 .. 50 Silica gel 30 0 .. 51 10 Silica gel B 0 0 52 Silica eel 31 B 0 ,. 0 .. 53 Silica iii 9 .. B 0 0 54 Radium foil 7 0 .. 55 57 Radium foil .. .. 56 None 56 A 57 Ultraviolet lamp 59 C .. .. 0 58 Ultraviolet lamp 58 C .. 0 59 61 Ultraviolet lamp C ~. 0 60 GO Ultraviolet lamp Iv113 .. 0 C .. 61 63 Ultraviolet lamp M13 C 0 62 Ultraviolet lamp h413 62 C 0 83 Ultraviolet lamp 65 M13 C 0 64 64 Ultraviolet lamp M13 C 0 65 67 Ultraviolet lamp hI9 C 0 66 Cltraviolet lamp AI9 66 C 67 a Condition A 16 weeks in forested area of F t . Sherman; condition B, 16 weeks i n forested area of Iit. Sherman, 19 weeks in warehouse a t E't. Guliok, and g more months i; t h e Ft. Sherman location: and condition C , 19 weeks in yarehouse a t F t . Gulick a n d 9 months in forested area a t Ft. Sherman. = a n appreciable amount of fungus = fungus growth on a t least one internal surface of t h e instrument; and -Ib Code: 0 = no fungus growth; growth on more than one surface. 0 E t h y l mercurithiosalicylate.
+ ++ ++
i
.
i
%
+++
++
.. ..
I
..
++ ++ ++ ++ ++ +
..
:I ++ ++
+
+
sealing other instruments since military telescopes contain diopter movements, and there must be a practical means of opening the instrument for repairs. Instruments which are not hermetically sealed but which are sealed sufficiently to allow only a slight exchange of gases with the outside could be kept dry by adding a desiccant, such as silica gel, to the interior. Silica gel is widely used as a desiccant for packaging. However, it is necessary that the instrument be well sealed or the silica gel recharged frequently. If moisture enters the container, eventually there will be sufficient water present so that the silica gel will give up moisture and produce a more humid condition inside the instrument than the ambient air. FUNGUS INHIBITORS. Various investigators have tested many materials in the vapor phase for toxicity to fungi in connection with the treatment of fruits, seeds, a n i other agricultural products. Recently there have been a few investigations of volatile
+
materials for the treatment of optical instruments. Hutchinson ( 1 4 ) tested about 60 chemicals for fungistatic effectiveness and found m-cresyl acetate, known commercially as Cresatin, to be the best material for application to telescopes. Hutchinson ( 1 4 ) also tested Cresatin in binoculars exposed on Barro Colorado Island in the Canal Zone and found that this material prevented mold growth on the lenses. A group of Australian investigators ( 1 ) reported that binoculars internally coated with a paint containing about 0.2% sodium ethylmercurithiosalicylate had no fungus grovth on the optics, even when a nutrient medium was placed on the optics and inoculated with mold spores. Turner, McLennan, Rogers, and Matthaei (31 ) state that dry sodium ethylmercurithiosalicylate is scarcely volatile but that it will decompose in the presence of water to give a fungicidal vapor. The sodium ethylmercurithio-
May 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
salicylate treatment for binoculars was satisfactory when tested in New Guinea b y the Australian workers (1,21). Gray (16) reported that the addition of 2% fenchyl thiocyanoacetate (Thanite) to the sealing compound, lubricants, and an interior lacquer coating, prevented the fungus growth in optical instruments. These three compounds, Cresatin, sodium ethylmercurithiosalicylate (or its decomposition products), and Thanite, are therefore reported to inhibit fungus growth in the vapor phase and are sufficiently volatile to be used in instruments.
Figure 2.
Jungle Site where Optical Instruments Were Exposed
Vicklund ( 2 2 )reported that mold growth could be prevented in transit telescopes by placing narrow metal foils containing radium salts around each lens. The alpha particles emitted during the radioactive disintegrations inhibited the mold growth. Alpha particles from radium and its decay products have a maximum range in air of 6.95 em. However, for practical purposes a range of about 4 em. is considered a maximum. The radium source must be located in the instrument so that the alpha particles can im‘ pinge directly on the lens surface without striking any interposing object. The total amount of radium in an instrument must be kept very low, as the hazardous gamma radiation that is also produced easily penetrates the body of the instrument. An advantage of radioactive foil is its permanence. The radium will easily outlast any optical instrument. The vapor phase fungicides have to be replenished periodically, depending on the volatility of the chemical and the degree of sealing the instrument. MISCELLANEOUS PREVENTIVE METHODS. Numerous other methods of preventing fungus growth in instruments have been suggested and a few have been tried at least experimentally. Some of these suggested methods are the incorporation of fungicides in the glass, the coating of a thin film of a fungicidal material on the lens surface, coating the interior with a radioactive lacquer, use of ‘(breathing” devices which would prevent entrapment of moist air, use of internal heaters, use of high-frequency currents, etc. Besides the chemical compounds already mentioned, many others could be considered for fumigation or volatile fungicides. EXPERIMENTAL WORK
At the time this investigation was started the most promising methods for preventing mold growth in optical instruments, except where storage was the only problem, were the use of silica gel desiccant, m-cresyl acetate, fenchyl thiocyanoacetate, sodium ethylmercurithiosalicylate, and radium foil. Representative military instruments were treated with these
1091
five materials, A sixth method, assembling binoculars under an ultraviolet germicidal lamp, was added. All but fenchyl thiocyanoacetate were used in binoculars. The fenchyl thiocyanoacetate treatment was tested in M17 elbow telescopes. The instruments and treatments used are listed in Table I. Each telescope of a binocular was considered as a test unit although the binoculars were exposed with the attached telescopes. The various binoculars are similar. Binoculars M3, M6, M9, and M13 are 6 X 30’s and binoculars M7 are 7 X 50’s. Binoculars M7 have no reticle but the other binoculars have a reticle in one of the two telescopes. Binoculars M13 are waterproof and have a synthetic rubber gasket under the cover plate. TROPICAL EXPOSURE CONDITIONS.All the test instruments were treated during May to July 1945, except for the 2 Mark I I Australian binoculars and the M17 elbow telescopes. For these the treatment history is not known. The test instruments were shipped b y air to the Panama Canal Zone. The binoculars and telescopes were exposed under a shed in a forested section of the Fort Sherman Reservation, Figure 2. Because of administrative changes a t the field site, the exposure was not continuous a t the same location. Initially all the instruments were exposed for 16 weeks {July 27, 1945, to November 16,1945) in the Fort Sherman jungle, except for the M9, M13, and Australian binoculars, which were received later. At the completion of the initial 16week exposure, some of the test instruments were shipped back to Frankford Arsenal for laboratory examination. The remaining instruments (and the M9, M13, and 2 Mark I Z binoculars which were received in the Canal Zone at that time) were stored for 19 weeks (November 16, 1945, to March
Figure 3. Mold Growth on Lens of Untreated M7 Binocular (8 i n Table I) Photographed t h r o u g h a telescopic a r r a n g e m e n t
27, 1946) in a warehouse in the Canal Zone. The binoculars were then returned to the jungle site for an additional 9 months (March 27, 1946, to December 26, 1946). The Canal Zone exposure for each test instrument is listed in Table I. The longest exposure for any binocular was 17 months. At the completion of the tropical exposure the instruments were returned to Frankford Arsenal for internal examination. The warehouse was situated in Fort Gulick, only a few miles from Fort Sherman, and the outdoor weather conditions were not too different, although in the daytime the relative humidity was lower in the warehouse than in the Fort Sherman jungle. Fungus trouble was not uncommon in optical instruments that were regularly stored in this warehouse. The rainy season in the Canal Zone lasts from April to December, and the binoculars that were exposed for the full time passed through more than half the rainy season of 1945 and almost all of
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1092
the rainy season of 1946. The warehouse storage was almost entirely during dry season weather. The test instruments were observed a t widely spaced intervals. The presence of fungus on the optics was determined by viewing the lenses in reverse field, except that the reticle was examined by viewing through the eyelens. The results of each examination are listed in Table I. hfter the instruments were returned to A
B
Vol. 44, No. 5
moldy as the binoculars of earlier design, and mold growth was observed on the gasket. These M13 binoculars had a magnesium fluoride coating on the lenses. This roating did not deter a n y fungus growth. IKDIVIDUAL LESS CoawosENTs. When the instruments were taken apai t, each individual lens was observed for fungus groivth, including growth on the edges which could not always be seen by observing with the aid of the straight telescope. The observations are shown in Table 11. Only those binocular telescopes that were moldy and that were disassembled are listed in the table. From Table I1 i t can be seen that fungus growth usually appeared on more than one lens of a test instrument. The prisms were moldy in almost every case and the Pyelens and objective lens were frequently free from mold growth. However, each prism has three polished surfaces on which the fungus may lodge and grow, whereas the objective and eyelens have only one internal surface for growth to occur on. Figures 4 and 5 show that all types of lenses are susceptible to fungus growth EVALUATION O F TREATMENTS
Figure 4. Mold Growth on Eyelens and Reticle of Ocular Assembly A.
E.
Eyelens of u n t r e a t e d M6 binocular (6 in Table I) Reticle of 336 binocular treated w i t h a silica gel desiccator (49 i n T a b l e I)
Frankford Arsenal, each lens in the binoculars was examined before disassembling the instruments, using a straight telescope to focus on each lens surface. Some of the lens surfaces were photographed before disassembling the binoculars. An example of fungus growth that is observed by viewing binoculars in reverse field is shown in Figure 3. The organisms present on the optics were isolated from only a fen- of the instruments. The isolations were made after the iustruments were returned from the Canal Zone. All the molds that were isolated were either Aspergillz or Penicillia.
CHESATIS. A Cresatin taffy, consisting of 50% ethylcellulose and 50% m-cresyl acetate, was placed in aluminum tubing which was cut a t regular intervals. Each section had small slits a t t h e ends (Figure 6). Each capsule conhined about 0.5 grain of the Cresatin taffy. The aluminum capsules were attached to the inside wall of the telescope body beneath a cover plate and outside the optical path. An asphaltic compound (Shell Y-37) was used to fasten the capsules to the wall. Two capsules were placed in the treated telescope of the M7 binoculars, and one capsule wa8 placed in the treated telescope of the other binoculars. As can be seen from Table I, the only treatment that complet,ely prevent,ed mold growth in all the binoculars was the mcresyl acetat,e. Of the 11 binocular telescopes tested, five were exposed for the full 17 months. One objection that had been raised ( I O ) to Cresatin was that it. softened the balsam used to cement the lenses. Frager ( 4 ) Cound that Cresatin vapors will soften optical cements at 60 C. Therefore, before assembly of the binoculars a saran lacquer was applied to the edges of the doublet,s in half of the instruments fitted with Cresatin capsules. When the binoculars were disassembled 'careful attention was paid t o t,he condition of the cemented O
R E S U L T S OF TROPICAL E X P O S U R E
UNTREATEDBINOCULARS. It can be seen from Table I that the exposure conditions will produce fungus growth in untreated binoculars. Twenty of the 21 untreated binocular telescopes were moldy a t the completion of the test. AIost of these instruments were moldy within the first 16 weelis of the exposure.
TABLE 11. PRESENCE OF FUNGCS GROWTHON INDIVIDCAL OPTICALELEMENTS OF DISASSEMBLED BINOCELAR TELESCOP~CS Test Telescope No. 1 2 3 4 5 6
7 8 9 10 11 12 15
33 35 37 39 42
Figure 5. Mold Growth on UDDer Prism of Ergctor Assembly of Untreated-;\'f6 Binocular (6 in Table I)
44
18 50 51 54 55
Field lem
Reticle
Upper pnsin
Lower prism
+ -I-t ++ ++- ++ ++ ++ ++o+o ++ ++ ++o (+d ++- ++ ++o ++ ++ +% + % -- ++ ++ ++ ++ 4 ' ++ ++o -- ++ ++ ++ ++ 0 0 0
(+) (+)
(
0 0
0 0
0 0
+0
49 52
The M9 and iv13 binoculars were not moldy within the first 19 weeks' initial exposure period, but this storage was in the warehouse during the dry season. The M13 binoculars are waterproofed. They have a synthetic rubber gasket beneath the cover plate. However, these waterproofed M13 binoculars became as
I'ungus Growtha Eyelens
0
0 0
++o
+++-
0 0 0
-
-
0
+++0
0
0
++o
++ -
0
%
0
+
+I ++ ++ 0 ++ ++ ++ ++ ++ ++ ++ ++o % + +++ ++ ++ ++o ++ ++ ++ ++ ++ ++
Objective
+ -t 0
(+)
a
0
(+I ++ +%-1- +
ti0
+o 0
++00 0
+o
+%
Code: 0 = no fungus growth; = slight t o moderate amount of growth: = more than moderate amount of fungus growth, one surface usually completely covered by growth: 0 = growth on edges only; and - = reticle not present in this telescope.
++ 5
May 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
doublets. The Cresatin had not affected the cements of either the lacauered or unlacauered lenses. In recent tests of volatile materials for fungicidal action, Scheffer and Duncan ( d o ) and Clark and Leonard (3) found Cresatin to perform poorly in comparison with many other compounds tested. It therefore appears that further work should be done to see if any of these compounds that are superior to Cresatin in laboratorytestswiil also be better for protecting optical instruments.
Figure 6. Aluminum Capsules Containing Cresatin Taffy Actual size Ends of capsules are crimped
SODIUMETHYLMERCURITHIOSALICYLATE. The sodium ethylmercurithiosalicylattreated binoculars were modified by coating a black nitrocellulose lacquer containing 0.25% sodium ethylmercurithiosalicylate on all the internal metallic parts. The dried lacquer then contained about 1% of the mercury compound. The I Mark 11 binoculars (39 to 44 in Table I), which are also sodium ethylmercurithiosalicylate-treated binoculars, were treated in Melbourne, Australia, and were obtained for test through the Australian liaison office. These 8 Mark 11 binoculars are somewhat different from the U. S. military binoculars that were used, in that the cover plate is on the objective end rather than the ocular end, and the lens are attached to a center column and not to the body. The binoculars were treated with a lacquer containing sodium ethylm e r cu r i t h i o s a1icylate as described in the Australian recommendations for tropic proofing ( 1 ) The sodium ethylmercurithiosalicylate treatment was not effective. Of 12 binocular telescopes only three did not become moldy (Figure 7). All of the Australian 2 Mark 11 treated binoculars were somewhat moldy. Tests in the Canal Zone of binoculars treated with sodium ethylmercurit h i o s a1i c y 1a tle b y t h e National Defense Research Committee (10)also showed s o d i u m ethylmercurithiosalicylate did not prevent mold growth. On the other hand, Australian field exFigure 7, Growth on Posure tests ( 1 , 21) in New Prism of Sodium EthylGuinea showed that sodiumm e r c u ri t h i os a l i o v l a teethylmercurithiosal icy1 a t e Treated Binocular (37 i n controlled fungus growth. Table I) It has been suggested (10) that these contrasting results were due to differences i n applying the lacquer and sealing compounds containing the mercury compound, or to differencesin the instruments. Now it has been shown that the Australian type of binoculars treated in Australia with the sodium ethylmercurithiosalicylate will also become moldy in the Canal Zone. The difference in result8 may be attributed to
.
1093
the location of the test sites. The Canal Zone may be more favorable for producing mold growth in instruments than New Guinea. DESICCATOR MODIFICATIONS. The desiccant cartridges contained 2 grams of fresh silica gel, The cartridges, either metal or plastic (Figure 8 ) were attached to the binocular wall beneath the cover plate and outside the optical path. Silica gel cartridges were not at all effective in the M3, M6, and 1197 types of binoculars that were tested. A desiccant may be more effective in an instrument that is better sealed. The M17 binoculars have a gasket under the cover plate and are better sealed than the Model M3, M6, and M7 binoculars. Even in these fairly well-sealed instruments leakage will occur through the diopter movement, making silica gel impractical. However, there have been developed improved static and rotary seals for optical instruments, and dehumidification may be effective for newly designed optical instruments. The M17 elbow telescopes were received for tests THANITE. already treated with Thanite from the Eastman Kodak Co. Although details of the treatment are not known, it is believed that the telescopes were modified in accordance with the method recommended by Gray (15). A
Figure 8.
B
Containers for Silica Gel Attached inside Binocular Body A.
Plaatic container
B . Metal container
The Thanite treatment that was used on the two M17 elbow telescopes was ineffective. The eontrol telescope had a considerable amount of mold growth on every lens. The treated telescopes had almost as much growth except that the objective lenses were free from mold. The prisms were intensely moldy. Besides the lenses, the filters in the elbow telescopes became moldy. Binoculars for this test were disULTRAVIOLET TREATMENT. assembled, and the parts were exposed t o ultraviolet radiation in a cabinet. The cabinet contained a 30-watt General Electric germicidal lamp ( 7 )and was all enclosed, except for two openings for entrance of the hands and arms of the technician. The germicidal lamp was kept on while the binoculars were reassembled in the cabinet. Since radiation from the germicidal lamp will kill fungi (a), it was thought that sterile binoculars might be obtained by this method. During the Canal Zone exposure almost every one of t h e binoculars became moldy. If it is amumed that the instruments were sterile when reassembled, then they must have become infected later. However, it is very doubtful that the binoculars were sterile when removed from the ultraviolet eabinet, as there are numerous recesses in the instruments that the radiation probably did not reach. Also, the sealing compounds and lubricants that were used were not sterile. RADIOACTIVE FOIL. The radium teat binoculars were treated with strips of gold foil containing 15 micrograms of radium (as radium sulfate) per square inch. The strips were 1/18 inch wide and placed around the lens s u r f a m as described by Vicklund (92).
Only two binocular telescopes were tested that contained radium foil around the lenses. Both became moldy within 16 weeks and one waa intensely fogged. However, in the moldy instruments all the growth waa restricted to the prism surfaces beneath the prism shields. These prism surfaces were not encircled by the foil
INDUSTRIAL AND ENGINEERING CHEMISTRY
1094
as the shields lie over the p r i m faces. Since the prism surfaces that were encircled by foil did not contain any mold growth, i t appears from this test that the alpha particles from the radium foil did inhibit fungus growth. Vicklund (t%) used a small transit telescope with only a few lenses for his tests. The total amount of radium in the telescope was about 21 micrograms. The binocular telescopes tested in the Canal Zone each contained about 16
Val. 44, No. 5
micrograms of radium. I t is estimated that another 8 micrograms would be required to protect the four prism faces covered by the shields. Therefore, each telescope would need 24 or 48 micrograms of radium for a complete 6 X 30 binocular and more for a 7 X 50 binocular Even though the concentration of radium in the foil is very low, optical instruments of complex design would require so much of the foil that this preventive method would become a health hazard to users of the equipment, as well as create serious problems for storage where many instruments would be in the same location
A few of the untreated instruments had slight corrosion of the shields, which was of a nonpowdery nature. The binoculars with the silica gel cartridges resembled the untreated ones with respect to corrosion. The sodium ethylmercurithiosalicylatetreated binoculars did not have any corrosion on the lacquered parts; the under sides of the prism shields which were not coated sometimes were slightly corroded, but the products m r e adherent to the metal (Figure 9). I n the binoculars treated with Cresatin severe corrosion of all the prism shields occurred (Figure 9). White powdery corrosion products accumulated, which fell on the prism surfaces and seriously affected the function of the instruments much as a very dirty lens would. In some cases the prisms were pitted where the corrosion products had stuck to the glass. The corrosion caused by the Cresatin was limited to the prism shields. These shields are made of a copper alloy, usually gilding metal, that is blackened by chemical or electrochemical means ( U . S.Army Specification 51-70-1). Frager (4)exposed metallic materials for 1 month to atmospheres saturated with moisture and containing Cresatin in one part per 10,000, and he found that copper oxide-blackened brass was only slightly affected although zinc was severely corroded. Aluminum n‘as unaffected. Frager also reported that the Cresatin hydrolyzed during the exposure, and the acetic acid formed by the hydrolysis reduced the p H of the water in the vessels to 2.8. I n the test binoculars the formation of the powdery corrosion products was worse than the fungus growth that the Cresatin was intended to prevent. Since all the other metallic parts were in good condition, substitution for the copper alloy shields by a Cresatin-resistant material, such as blackened aluminum, blackened stainless steel or suitable plastic, would permit the use of Cresatin.
FOGGING AYD FILMING
CONCLUSIONS
Besides determining the effectiveness of these treatments in preventing mold growth, other forms of deterioration were looked for, such as fogging. formation of films on lenses, and corrosion of metallic parts. Fogging and filming are distinguished from each other in that fogging is a transitory condition of moisture condensation on the lenses, while filming is a less volatile formation that appears either as a thin layer or as discontinuous droplets. The exact nature of these films is unknown, but they may have been volatile fractions of lubricants or other organic materials that condense on the optics. Fogging of the instruments was noticed a t various times during the tropical exposure. Fogging occurred in control telescopes and in telescopes with the various treatments. With the exception of the radium-treated telescopes, there were no indications that any of the fungicides increased or decreased fogging. The radiumtreated telescopes became badly fogged very quickly. However, when the binoculars were returned to the drier air a t Frankford Arsenal, moisture condensation disappeared from all the test instruments. Filming also occurred in haphazard distribution in the binoculars, and there were no indications that any of the fungicides influenced filming. After disassembly all the films were easily removed b y wiping the lenses with ethanol. Filming is not a form of deterioration that is found principally in wet climates as is fogging and fungus growth. The problem of filming of lenses in binoculars occurs in instruments stored throughout the United States.
m-Cresyl acetate (Cresatin) vapor was the only treatment that completely prevented mold growth in binoculars. The Cresatin vapor, or its hydrolysis products, is very corrosive to the cupric oxide-blackened copper alloy prism shields. Treatment of binoculars Kith sodium ethglmercurithiosalicylate, as recommended by the Australian investigators, perniitted some mold growth, while silica gel desiccators that were used in M3, M6, and M7 binoculars kept the humidity low- for too short a period to be of practical value in preventing mold growth. An oiganic coating containing fenchyl thiocyanoacetate (Thanite), used to treat elbow telescopes, did not prevent mold growth. The use of ultraviolet germicidal lamps to sterilize the interiors of binoculars was not a practical method for preventing mold growth. The waterproofed M13 binoculars became moldy about as readily a3 the relatively poorly sealed M3 and RI6 binoculars. The magnesium fluoride-coated lenses were subject to mold growth.
B
A
D
C
Figure 9. A. B. C. D.
.
I’risni Shields from Test Binoculars
Untreated (2 and 6 in Table I) Containing s i l i c a gel (47 a n d 49 i n Table I) S o d i u m ethylmercurithiosalicylatetreated (35 in Table I ) Cresatin-treated (27, 23, a n d 25 in Table I)
CORROSION
The interiors of the binoculars were not appreciably corroded, except for the prism shields in some of the instruments (Figure 9).
-
ACKNOW LEDGMEhT
Appreciation is expressed to C. C. Fawcett and E. R. Rechel of the Pitman-Dunn Laboratories and to the Ordnance Corps for permission to publish this paper. Special thanks are due to RIav Frager for his helpful comments in the revievv of the paper. LITERATURE CITED
(1) Australian Ministry of Munitions, Scientific Instruments, and Optical Panel, “The Tropic Proofing of Optical Instruments. Part I. The Value of Merthiosal its an Internal Fungicide,” Melbourne, July 1944. (2) Campbell, I. G., “Fungi in Optical Instruments under Tropical Conditiona and Possible Control,” DAIE, War Office, Great Britain, December 1944.
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1952
(3) Clark, W.L.,and Leonard, J. M., Naval Research Laboratory, Rept. C-3114 (July 1947). (4) Frager, M., Philadelphia, Frankford Arsenal, R e p t . R-537 (July 1944). (5) Galloway, L. D., J. Testile Inst., 26, T123-9 (1935). (6) Hutchinson, W. G., Scientific Monthly, 43, 165-77 (1946). (7) Luckiesh, M.,“Applicationa of Germicidal, Erythemal, and Infrared Energy,” pp. 123-6, New York, D. Van Nostrand Co., Inc., 1946. ( 8 ) Ibid., pp. 250-3. (9) Magee, C. J., Hansen, C. T., and Grant, C. K., “Report on the Condition of Service Materiel under Tropical Conditions in New Guinea,” Scientific Liaison Bureau, Melbourne, Australia, October 1943. (10)Sational Defense Research Committee, “Tropical Deterioration of Equipment and Materials,” Vol. 1, Summary Technical Rept. of Tropical Deterioration Administrative Committee, 1946.
1095
(11) Office of Scientific Research and Development, Washington, D. C., OSRD Rept. 1833 (July 1943). (12)Ibid., 3803 (June 25, 1944). (13)Ibid., 3952 (October 1945). (14) Ibid., 4118 (September 1944). (15) Ibid., 4371 (December 1944). (16) Ibid., 5684 (Oct. 31, 1945). (17)Ibid., 5767 (April 1945). (18)Ibid., 6055 (Oct. 15, 1945).
(19)Richards, 0.W., J. Bact., 58,453-5 (1949). (20) Scheffer, T. C.,and Duncan, C. G., IND.END.CHEM.,38,619-21 (1946). (21) Turner, J. S.,McLennan, E. I., Rogers, J. S., and Matthaei, E., Nature, 158, 469-72 (1946). (22)Vicklund, R. E., IND. ENG.CHEM., 38, 774-9 (1946). RECEIVEDE or review July 28, 1951.
ACCEPTED December 13, 1951.
Oxidation of General-Purpose Polvethvlene Resin J
J
C . S . MYERS Development Laboratories, Bakelite Co., Division of Union Carbide and Carbon Corp., Bound Brook, N . J .
0
XIDATIVE degradation of high polymers under the influence of light and heat is well known. Polyethylene resin, no exception in this respect, is a good subject for fundamental studies of oxidative degradation phenomena because of relative simplicity of its chemical structure. Considerable data have been published (6, 9,11, 14) on changes in electrical loss characteristics and alteration in chemical structure of polyethylene resins imparted by exposure to light and heat. Increases in carbonyl oxygen concentration and, to a lesser extent, increases in other oxygenated groups are the major chemical effects previously reported. This paper presents quantitative data on dielectric loss properties and corresponding chemical effects caused by thermal oxidation which permit a definition of the kinetics of the oxidation reaction. TIIEORETICA L CON SIDERATION S
Recent chemical researches ( 1 7 ) in the field of hydrocarbon oxidations have demonstrated that hydroperoxides ( R O O H ) appear as one of the first products of reaction. Kinetic studies have shown that subsequent oxidation of the parent hydrocarbon is autocatalyzed by the decomposition of hydroperoxides, which produces free-radical initiators for the chain reaction. The decomposition of hydroperoxides also leads to the formation of secondary oxidation products, such as ketones, aldehydes, acids, alcohols, water, and carbon dioxide. One of the simplest and best delineated autoxidation mechanisms is the one proposed by Bolland and Gee ( 1 ) for the oxidation of ethyl linoleate, C1,HalCOOCzHs. At sufficiently high pressures of oxygen and in the range where a sufficiently high concentration of hydroperoxide has been built up so that hydroperoxide decomposition is the important chain initiation step, they proposed the mechanism ki BROOH-R,
+ R O O . + other decomposition product Oz
ROO.
+ R. +RH
kz +ROO. k3 +ROOH
(I)
(2)
+ Re
(3)
ROO.
lie + R O O ---+ROOX + Oz
(4)
where R H represents ethyl linoleate nnd the H in question is the one that detaches most easily, because of the activating influence of the double bonds. It is commonly accepted that antioxidants, although stable in atmospheric oxygen, operate by interrupting the above chain reaction whereby the inhibitor is oxidized to stable compounds. Some theories on the mechanism of antioxidant action postulate that the stabilizer is reformed, to some extent, during the series of reactions. However, experimental measurements show that the antioxidant is destroyed or becomes ineffective, ultimately. I n the absence of inhibitors, Equations 1t o 4,inclusive, demonstrate the formation and destruction of free radicals a t steady reaction rates, IC1, k,, 123, etc. Stable secondary oxidation products created by the decomposition of hydroperoxides result in an increase in concentration of ketones, aldehydes, acids, alcohols, water, etc. Designating RO as stable oxygenated hydrocarbon, the secondary reaction is assumed schematically as kr ROOH+RO
+ H20 + other decomposition products
(5)
Since R O O H is being formed. continuously, increase in concentration of R O is expected to be defined by dt
=
h7(RO)
(6)
Integration of Equation 6 and conversion to the common log form gives log ( R O ) =
-+ B
k7t 2.303
(7)
Therefore, concentration of stable combined oxygen, (0), is exponential with time, t , and limited by the concentration of R O O H present which is decomposing at a rate, k,. On the basis of this hypothesis, determination of concentration of polar oxygen groups in polyethylene as a function of milling time and temperature should supply suitable data for defining the kinetics of the oxidation reaction. Power factor measurements at high fre-
