T h e left and right photographs in Figure 13 indicate how the addition of 1% Cab-0-Si1 affects the flow properties of Carbowax 6000. The powder in the left photograph trickles down rather slowly, while that in the right photograph flows more like a liquid than a powder (evidence of splashing against the sides of the beaker in the right view). Summary
Tests for measuring shear strength, bulk tensile strength, bulk density, and electrostatic charge of powders were used to determine effects of Cab-0-Si1 on the physical properties of Carbowax 6000. O n the basis of shear strength tests, optimum concentration of Cab-0-Si1 was found to be approximately 1% by weight. This amount of Cab-0-Si1 reduces shear strength of powdered Carbowax 6000 by about 30%, reduces bulk tensile strength by about 60%, increases bulk density by about 3070, and neutralizes electrostatic charge.
literature Cited
(1) Athv. L. F.. Bull. Am. Assoc. Petrol. Geoloeists 14. 1 11930) (2) Craik, D. J:, J . Phurm. Pharmacol. 10, 73 '(1958). (3) Craik, D. J., Miller, B. F., Ibtd., 10, 136T (1958). (4) Irani, R. R., Callis, C. F., Liu, T., Ind. Eng. Chem. 51, 1285-8 (1959). (5)' Nakajima, E., Yakugaku Zasshi 81, 712-16 (1961). (6) Shaxby, J. H., Evans, J. C., Trans. Faraday SOC.19, 60 (1923). (7) Spencer, R. S., Gilmore, G. D., Wiley, R. M., J . A$$. Phys. 21, 527 (1950). (8) Walker, E. E., Trans. Faraday SOC.19, 83 (1923). I
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RECEIVED for review October 26, 1964 ACCEPTED March 30, 1965 Division of Industrial and Engineering Chemistry, 144th Meeting, ACS, Los Angeles, Calif., April 1963. Work sponsored by the U. S. Army Chemical Research and Development Laboratories, Army Chemical Center, Md., under Contract D.A-18-108-405CML-824 with General Mills, Inc., monitored by the Colloid Branch, Physicochemical Research Division. September 11, 1963, the Aerospace Engineering and Research Departments of General Mills Electronics Division were purchased by Litton Systems, Inc.
HEAT-RESISTANT COATINGS BASED ON ZINC OXIDE AND ORGANIC PHOSPHORUS COMPOUNDS R. W.
L I G G E T T , S. E. M I L E S K I , A.
E. RAEUBER, A N D
E
A
V E R C H O T
Southern Research Institute, Birmingham, Ala. A.
M . M A L L O Y
Bureau of Naual Weapons, United States Department of the Navy, Washington, D . C.
A cold-setting ceramic paint has been developed, based on the controlled hydrolysis of organophosphorus compounds in the presence of zinc oxide pigment. A zinc phosphite or phosphate coating i s formed that adheres well to steel or aluminum alloys, i s hard and smooth, i s resistant to wet abrasion, and withstands heating to 1000" F. and quenching in cold water. The most successful formulation contains dimethyl phosphonate, zinc oxide, and colloidal silica. Maintenance of a high humidity of the environment during the early cure of the paint i s necessary in order to develop optimum water resistance and heat resistance in the final cured coating. HE rapid advances in aero and space technology in recent 'years have imposed drastically increased requirements on the serviceability of structural materials a t high temperatures. T h e research described here was undertaken with the purpose of developing protective coatings for metals that could be applied without baking and that would protect the metals during exposure to temperatures of 1000" F., as well as to normal atmospheric conditions. T h e systems chosen for study were based on the cold-setting ceramics of the type that is made from zinc oxide and phosphoric acid and used as a dental cement (7). I t was reasoned that the combination of a phosphate, phosphonate, or phosphite ester with a metal oxide, such as zinc oxide, might be usable as a coating formulation, if, after application to the surface the ester were hydrolyzed by atmospheric moisture to the acid, which would react with the metal oxide. I n this way, a cold-set ceramic coating would be produced, with the cold-setting reaction delayed until the coating had been formed. I n a series of preliminary experiments, it was determined that some phosphorus compounds, such as ethyl acid phosphate (a commercial mixture of ethyl dihydrogen phosphate
and diethyl hydrogen phosphate, containing some polyphosphates), reacted too rapidly with zinc oxide to yield a coating and that others, such as triethyl phosphite, reacted too slowly. However, some compounds-e.g., dimethyl phosphonatereacted at an intermediate rate that permitted the formation of coatings. For more extensive studies, formulations were prepared containing combinations of zinc oxide and dimethyl phosphonate, as an example of that group of phosphorus compounds. Some formulations also contained ethyl acid phosphate, as an example of a fast-acting compound that could be used to accelerate the setting of the ceramic. A wide variety of metal oxides other than zinc oxide were found to react in a similar manner with the phosphorus compounds, but since none appeared superior to zinc oxide, their reactions were not studied in detail. Experimental Details
Materials. The metal panels used as substrates in the coating studies were Alclad 2024-T3 aluminum and mild steel. The panels were cleaned with acetone and lightly sanded before coating. Commercial-grade phosphorus compounds and pigments were used. VOL. 4 NO. 2
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Preparation a n d Application of Coatings. Coating mixtures were prepared by mixing in a Waring Blendor for 45 to 90 seconds. The thinner (toluene) plus any acceleratore.g., ethyl acid phosphate-was added during the last 15 or 20 seconds of mixing. The mixtures were sprayed or doctored onto metal panels within 10 minutes after mixing. Coating thicknesses were usually limited to about 1 mil, since thicker coatings tended to blister when exposed to heat. The coatings were cured 24 hours under various conditions, as explained below. Coating Evaluation. Hardness was determined by scratching a coating With a fingernail. A good coating could not be scratched. Bonding was determined by bending a coated panel over a Bell Telephone Laboratories conical mandrel. A well-bonded coating showed no loss of coating a t a 0.5-inchdiameter bond. A coating was judged water-resistant if it was not affected by wet rubbing and heat-resistant if it was not damaged by exposure to 1000' F. for 5 minutes followed by quenching in cold water. Coatings that were heat-resistant by this test generally withstood repetitions of the test but were damaged by exposure to temperatures a few hundred degrees higher. Results and Discussion
Zinc Oxide-Dimethyl Phosphonate-Ethyl Acid Phosphate System. The composition of this system that gave coatings with the best combination of desired properties contained 10 parts of zinc oxide, 12 parts of dimethyl phosphonate, and 1 part of ethyl acid phosphate. Coatings could be prepared from this composition that had good bonding, but good thermal resistance and good water resistance could not be obtained simultaneously. The presence of a limited amount of water in the curing atmosphere was necessary for satisfactory curing of the coating. The amount of water present during the cure affected the thermal and water resistance in opposite ways. For example, when the coating was cured a t 100' F., a critical relative humidity for obtaining water resistance was about 40%. At curing temperatures below 75' F., results were erratic. Attempts to obtain a satisfactory coating by the addition of water to the formulation were unsuccessful. Curing of this composition was substantially complete within 2 hours. The pot life of the coating mixture was about 10 minutes. Zinc Oxide-Dimethyl Phosphonate-Colloidal Silica System, The composition of this system that gave the best combination of properties in coatings contained 10 parts of zinc oxide, 12 parts of dimethyl phosphonate, and 1 part of an aqueous dispersion of colloidal silica containing 33y0 solids (Ludox LS). Coatings of this composition cured at 72' F. and 75y0 R H were resistant to both heat and water. Cured a t 72' F. and a t or below 65% RH, the coatings were neither heat-resistant nor water-resistant. Curing a t 72' F. and 80% R H produced water resistance but not heat resistance. Coatings cured a t 60' F. and 90 to 99% RH were heat-resistant but not water-resistant. Coatings cured at 100' F. above 5570 RH were water-resistant but not heat-resistant, whereas coatings cured a t lower relative humidities were heat-resistant A similar relation was found a t 136' F., where the critical relative humidity was 40%. Curing of this composition also was substantially complete within 2 hours. The pot life of the coating mixture was about 15 minutes. As an alternative to curing under controlled temperature and humidity conditions, good coatings were also obtained from this composition by applying it at room temperature (approximately 75' F.) followed by brief spraying with a fine mist of water within 5 minutes after application and every 5 minutes thereafter for a total of 90 minutes. The coatings were then allowed to stand 24 hours a t room temperature before being evaluated. If the intervals between sprayings 146
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
were longer than 5 minutes, the coatings lacked water resistance. Coatings of this type applied to aluminum or mild steel protected the metal substrate and appeared unchanged after 100 hours in a salt-spray test [Method 6061, MIL-P-14105 (CE), Specification TT-P-1411. The coatings also appeared unchanged after accelerated weathering for 300 hours in an Atlas Twin-Arc Weather-Ometer. The coatings were not damaged by exposure to common solvents at room temperature, but they were destroyed by dilute acids or alkalies. The dielectric strength of the coatings was approximately 125 volts per mil, and the electrical resistance was about 3 X 109 ohms per sq. inch per mil. Systems with Other Phosphorus Compounds. Coatings made from compositions containing 12 parts of diethyl phosphonate, 1 part of ethyl acid phosphate, and 10 parts of zinc oxide were similar in properties to analogous coatings made with dimethyl phosphonate. Similar coatings based on bis(2-ethylhexy1)phosphonate also had good properties but did not cure as rapidly, requiring over 1 week for curing under ambient laboratory conditions. The following phosphorus compounds reacted so vigorously with zinc oxide that the mixtures did not give coherent coatings : orthophosphoric acid, polyphosphoric acid, methyl dihydrogen phosphate, ethyl acid phosphate, and diethyl acid pyrophosphate. The following phosphorus esters reacted so slowly with zinc oxide that the coatings did not cure easily: triethyl phosphite, tributyl phosphite, and tributyl phosphate. Phosphorus esters that reacted at an intermediate rate with zinc oxide to give coherent coatings were: dimethyl phosphonate, diethyl phosphonate, di-n-butyl phosphonate, trimethyl phosphite, n-butyl acid phosphate, and bis(2-ethylhexyl) hydrogen phosphate. Curing Reactions. In the zinc o x i d d i m e t h y l phosphonate system, the curing process evidently involves the hydrolytic liberation of methanol from the phosphonate with the formation of a polymeric zinc phosphonate or phosphite. A coating made from 10 Zn0-12 dimethyl phosphonate-1 colloidal Si02 (33% in HzO) attained constant weight in approximately 90 minutes when cured a t 72' F. and 57% RH, with a weight loss of approximately 37%. The weight-time curve was smooth, with no steps apparent. The calculated weight loss on the assumption of hydrolysis of the phosphonate is 35%. In another experiment, the volatile materials from the curing of a 12 ZnO-IO dimethyl phosphonate-1 ethyl acid phosphate coating were trapped in liquid nitrogen. The condensate was water and methanol, with a trace of ethanol. The optimum molar ratio of zinc oxide to dimethyl phosphonate is about 1, in accord with the formation of a polymeric zinc phosphonate or phosphite. The necessity of having moisture present during the cure agrees with the hydrolytic mechanism. The ethyl acid phosphate evidently serves as a curing accelerator. Acknowledgment
The assistance of F. D. Alexander, H. A. Kirk, and R. E. Lacey is gratefully acknowledged. This research was conducted a t Southern Research Institute under the sponsorship of the Bureau of Naval Weapons, United States Department of the Navy. literature Cited
(1) Kingery, W. D., J . Am. Cerarn. SOC. 33, 239 (1956). RECEIVED for review November 19, 1964 ACCEPTEDMarch 29, 1965
