SURFACE ACTIVE MATERIALS FROM PERFLUOROCARBOXYLIC AND PERFLUOROSULFONIC ACIDS R. A. GUENTHNER AND M . L. V I E T O R Chemical Division, Minnesota Mining 3 Manufacturing Co., St. Paul, M i n n .
Commercialization of the 3M Simons electrochemical fluorination cell has led to the development of a new and unique group of surface active materials derived from the perfluorocarboxylic and perfluorosulfonic acids. These surfactants possess highly unusual properties and their preparation and applications are discussed. They are characterized b y high surface activity in water, electrolytes, and organic media; their chemical and thermal stabilities are excellent. The perfluoro materials have been utilized in fields of development where most conventional surfactants do not perform adequately. A number of these applications are outlined.
ESEARCH
in the expanding field of fluorine chemistry
R has led to the development of a unique series of fluorinecontaining surface active compounds. The surfactants derived from the perfluorocarboxylic and perfluorosulfonic acids are of main interest commercially and deserve further attention from the standpoint of their preparation, properties, and applications. Some of these materials and their properties have been discussed in previous publications. The surface tension data of the perfluorocarboxylic acids are well known (7, 6 , 8 ) . Their activity in acid media (70), use as evaporation inhibitors ( 2 ) , adsorption on various substrates (9), and the wettability of low energy solids have been disclosed (7). Information has appeared on the perfluorosulfonic acid derivatives (3) as in the chrome plating systems (5).adsorption of a substituted acid on metals (7), and the surface tension data of the perfluorosulfonates in strong and oxidizing media (77). Derivatives of these acids have found commercial utilization, mainly in the field of emulsifiers in polymerization of fluorinated olefins and in plating systems, and as wetting and leveling agents and corrosion inhibitors. As the number and variety of the materials become greater, these uses will continue to expand. The unique properties and effects obtained with these materials will continue to fit applications not now feasible with conventional materials. Preparative Chemistry
The materials described here are all derived from basic products of the 3M Simons electrochemical fluorination cell.
The Ri (fluorocarbon) may be varied in length. The perfluoro-octane sulfonyl fluoride is used for illustration. Direct hydrolysis leads to 1 or 2. The two sulfonamides, 3, are derived by reaction with a primary or secondary amine. These are then used to produce a sulfonamido aliphatic acid and its salts, 4; the alcohols, 5, or nonionics by addition of ethylene oxide, 5. Quaternization of the tertiary amino sulfonamide gives the quaternary cationics, 6 ; a lactone leads to an amphoteric betaine, 7, or neutralization to the salt, 8. The alcohol, 5 , by further reaction, leads to the normal sulfate or phosphate esters, 9 and 10. Thus, through conventional chemistry a wide variety of materials can be obtained by varying the fluorocarbon (R,) or hydrocarbon (RH)portion of the molecule. Changes in any of the R groups can give the desired physical state or property. VOL. 1
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Lengthening the RH decreases water solubility and increases organic solubility, especially in the sulfonamides (3) and their derivatives. Although in the quaternaries (6),carbon chain length of the quaternizing agent gives the desired water or oil solubility or insolubility, the anion, X, seems to have little effect on properties. The nonionics (5) show a low water solubility below n = 6 to 8; where n reaches 20 or higher, the products do not show a cloud point up to 100' C . All the derivatives described are extremely surface active in water. Even though the alcohol (5) has a very limited solubility in water, the surface tension at saturation is below 20 dynes. The sequence of reactions in the perfluoracarboxylic series may be illustrated as follows:
I
J
O.0MI
0.00,
0.0,
W E l r Y l PERCENT SOLIDL
Figure 1.
8.0
0.)
Surface tension of aqueous solutions at
10.0
25" C.
The R,is shown here by C,FW; and as before, the R, and and the R a may be chosen to give the desired properties for the desired effect. Surface Activity in Water
w
+ Y
2 0.WOI
0.001
0.01
0.1
1.0
WEIGHT PERCENT SOLIDS
Figure 2.
Surface tension of aqueous solutions a t
25' C.
The effect of the solubilizing group in water is illustrated in Figure 1, which shows the surface tension curves in water of some of the Cs sulfonic acid derivatives. The free acid and its salts do not give the surface tension depressions obtained with the derivatives, since these are a function of soluhility and surface orientation. Surface tensions below 40 dynes can be obtained at concentrations of 10 p,p.m, with the nonionic salts, the amido amines with a long-chain quaternizing agent, and the anionic salts of the substituted sulfonamidocarboxylic acid. Minima with all derivatives are below 20 dynes per cm. Figure 2 shows curves far the perfluorocarboxylic acids and derivatives. Surface Activity i n Strong Electrolytes
Figure 3.
Surface tension of electrolyte solutions
R
containing 0.01 166
70CsFnSO2N(C*HnO),H
l & E C P R O D U C T RESEARCH A N D D E V E L O P M E N T
In the strong electrolytes the same general lowering of surface tension appears. But, as might be expected, the salts and acids are the preferred depressants, especially in strong mineral acids or bases. Here the sulfanics are preferable over the carboxylic series, if volatility of the surfactant is a factor. A number of anionics used in concentrations from 0.01 to 0.1% by weight will give a surface tension of 20 to 25 dynes in 20'% aqueous sodium chloride. This is a reduction of over 50 dynes. The surface tension of a 25% sodium hydroxide solution can be lowered to about 25 dynes at the same cancentration range. The salts of the perfluorosulfonic acid are surface active in mineral acids at all concentrations. Figure 3 shows the remarkable surface tension reduction in electrolytes obtained with the nonionics. These measurements were taken after 24 hours at room temperature. The bar represents the surface tension of the electrolyte solution, the shaded area the reduction obtained.
Welling Power
The work of Zisman and coworkers has shown the excellent wetting power of fluorine-containing derivatives for low energy surfaces, such as polyperfluorotetraethylene ( I ) . This wetting power and ability to lower liquid surface tensions at extremely low concentrations have provided new concepts in the use of surfactant leveling agents (4). In liquid floor polishes, for example, a proper degree of leveling is obtained without undesirable side effects such as water sensitivity of the dried film. This sensitivity accompanies normal use of high concentrations of conventional surfactants. In dip coating operations, thinner, more uniform films can he produced because of the low surface tensions obtained. In the same manner, “drag-out” losses of expensive plating chemicals can he reduced through a more rapid drain-off of the films left on the plated pieces as they are removed from the bath. Foaming Power
The foaming power of fluorochemical surfactants varies greatly with structure. Foam heights in water measured by the Ross-Miles test vary from a few centimeters for the SUIfonamidophosphate used in a concentration of 0.1% hy weight (saturation) to in excess of 600 cm. for the betaine used in the low concentration of 0.005%. In general, foams are obtained even with low energy methods of foam generation. Electrolytes are less effective in depressing the foaming power of fluorochemicals than of analogous hydrocarbon surface active materials. Stable foams have even been prepared with portland cement-sand slurries and have provided a new method for producing low density structural materials. The effect of appropriate fluorochemical surfactants in producing stable foams in chrome plating baths is another example of their adaptability for unusual applications. Organic liquids may be easily foamed with some fluorochemicals. Stabilizing effects in foamed plastics have been noted, such as those based on the reaction of toluene diisocyanate with palyols. Emulsion and Dispersions
Perfluorocarhoxylic acids have been used as emulsifiers in a variety of vinyl-type polymerizations. Now with a wider range of materials available, the use of the perfluoro surfactants can he extended to an emulsion wherever a lower surface tension is beneficial. This is also true in the field of dispersions, where milling and suspension of solids can be facilitated through the inclusion of a surfactant.
more useful in some salutianPe.g., the carboxylic acid gives greater activity in the peroxides. The perfluoro-octane sulfonates also have excellent thermal stabilities. Decomposition temperatures range from 350° C. for the longer chain sulfonates to 425‘ C. for the trifluoromethane sulfonate. The exceptional stability of the perfluorosulfonates makes possible their use in strong acid and base cleaners, plating baths, peroxidation reactions, and rocket flyids. The stability of the perfluara acids and reduction of interfacial tensions can he used to increase rates of reaction in a heterogeneous system where a hydrocarbon surfactant would be destroyed. This has been demonstrated by hydi-olyses in refluxing concentrated hydrochloric acid. Ethyl diethyl malonate was completely hydrolyzed in 3 hours when perfluoro-octane sulfonic acid or its salts were added. The control hydrolysis without additive was incomplete after 8 hours. In the sulfonation of benzene with sulfuric acid, the perfluarasulfonic acid or its salts reduced sulfonation time by onethird. Adsorption on Solids
Surface active Auorachemicals can he either physically or chemically adsorbed on solids. The resulting films exhibit both oil and water repellency, increased lubricity, and the other effects associated with low energy films. Contact angles of water, methylene iodide, and hexadecane on a film of perfluoro-octane sulfonamidoacetic acid chemisorbed on aluminum, in the range of 110’ to 160°, are among the highest ever measured (7). The decyl bromide quaternary of the sulfonamide amine is an example of quaternaries which show strong adsorption on glass and silica surfaces. The adsorptive capacity of surface active fluorochemicals on metals suggests their function as dry-film lubricants, antiwear lubricant additives, and extreme pressure additives. Improvements in both point B values (for load us. wear and wear us. time) as measured in a Shell four-ball wear test have been exceptional. Lubricants using fluorochemical extreme pressure additives at 100 to 500 p.p,m. have been formulated which approach the efficiencies of the best extreme pressure lubricants known. Release properties have been demonstrated for fluorochemical surfactants under a variety of conditions. When the surfactant is properly oriented on a surface, the low energy film produced can reduce adhesion significantly. An example
T Stability
In addition to the surface activity obtained at low concentration, both chemical and thermal stability are outstanding features of the perfluoro materials. The perfluoro acids and salts are active in all concentration ranges of mineral acids and strong bases over a wide temperature range. The carbon-sulfur-nitrogen linkages in the sulfonic derivatives were stable to aqueous acids or bases when heated at 100” C. for one week. Examples of the variety of media in which the salts of the perfluorosulfonic acids are completely stable are shown in Figure 4. Surface activity is indicated by the reductions in surface tension as shown by the shaded areas. Maximum depression is not always obtained with the type of structure shown; other more specific materials may be
Figure 4.
Surface tension reduction, C8FnSO&i VOL 1
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of metals with brightening or etching compositions. As an example, the addition of 25 to 50 p.p.m. of a sulfonic acid salt to a conventional aluminum bright dip solution greatly improves brightening action of the solution. Smut is reduced when surfactants are used in alkaline cleaners for aluminum and with sulfuric acid pickles for steel. Surface Activity in Organic Media
S"9FACE TENSIONS (DYNES)
Figure 5.
CM.
Surface tension depressions in organic liquids
Because of the extremely low surface energy of the perfluorocarbon chain, the perfluoro materials exhibit a marked tendency for positive surface adsorption in organic liquids and can function to lower surface tensions significantly a t relatively small concentrations. Figure 5 shows surface tension depressions obtained with various fluorochemical surfactants in eight organic liquids a t concentrations below 0.1%. In most cases values of less than 20 dynes per cm. have been achieved. Selection of the proper solubilizing group is of paramount importance in developing maximum surface activiq. The solubilizing group must provide a just sufficient solubility and allow proper orientation of the molecule at the surface. Surface Barrier Effects
Figure
6.
Aspholt-paper laminate stc
left. Stained by oil migrcltion Right. No stain, rnigr~tionstopped
is an 80% reduction in ice adhesion on aluminum wim a molecular surface film of the phosphate ester. Heat treatment of adsorbed films on metals such as steel, brass, aluminum, and copper often results in improvement in the properties of the film. For example, a phosphate ester without heat treatment is easily desorbed from copper by water. Mild heat treatment (200' to 400' F.) renders the film highly resistant to washing and more protective to corrosive environments such as medium strength mineral acids. Corrosion Inhibition
Combined properties of adsorption and stability make it possible for the fluamchemical surfactants to function effectively as corrosion inhibitors in a wide variety of environments. Some fluorochemicals can function as hoth solution inhibitors and protective surface film. The sulfonamidophosphate, for example, adsorbed on aluminum as a protective film, completely inhibits the surface attack of 3% hydrochloric acid at room temperature. Used as a solution inhibitor, this material a t 200 p,p.m. reduces the attack of dilute sodium hydroxide on aluminum by over 75%. Copper, brass, steel, etc., can be protected in the same manner hy the use of the phosphate and other fluorochemicals such as the quaternary ammonium salts and sulfonamide derivatives. The surface agents can also modify the reaction 168
l&EC PRODUCT RESEARCH A N D DEVELOPMEN1
The surface activity of fluorochemicals in organic media has extended to surface barrier effects. Fluorochemicals can function in hydrocarbon media in the same manner as cetyl alcohol in water, absorbing at the liquid-air interface to form a barrier film to the escape of bulk molecules, This has been demonstrated by the inhibition of gasoline evaporation (2). The addition of 0.003% fluorochemical can reduce the evaporation of the gasoline under static conditions to the paint of nonflammability Another use of the fluorachemical surface barrier effect is in the control of loss of plasticizer or other migratory components from one phase to another. A typical example is that of a paper-asphalt laminate (Figure 6 ) . The low molecular weight oils transfer from the asphalt phase of this paper construction into the paper, causing yellowing and black stains. With the addition of 100 to 300 p.p.m. of the proper fluorochemical surfactant to the asphalt, transfer is completely stopped, even at 140" F. I n general, fluorochemicals may find application whenever exudation of mobile components and/or extraction of such components by solvents from a solid phase normally occurs-for example, plasticizer loss or transfer from vinyl films. Micrabiocidal Activity
Surface active fluorocbemicals are interesting from the standpoint of microbiocidal activity. As a class most are inert; yet some show unusually high activity as bactericides and algicides. Therefore, the application of these materials in insecticides, herbicides, or fungicides can he viewed from either of two desirable approaches. Those which are inert may be used as wetting agents or dispersants in microbiocides where activity of the surfactant would be an undesirable side effect; or those with high activity can serve in a dual role as surfactant-biocide. The wide variety of applications and mes made possible through the unique properties of the perfluoro acid derivatives discussed include: Paints Polishes Metal cleaners Coatings
Electrochemical processes Corrosion inhibition Soaps and detergents Insecticides and herbicidrs Fuels Electroplating Leveling agent3
Release coatings Lubrication Foams Evaporation inhibition Plasticizer migration Emulsions
As the number and variety of these compounds become larger, even further uses-not now met by conventional materials-will be feasible. literature Cited
(1) Bernett, M. K., Zisman. W. .4.,J . Phvs. Chem. 63, 1912 (1959). ( 2 ) Blake, G. B., Ahlbrecht, .4.H., Bryce, H. G., “Perfluoroalkyl Surface Active Agents for Hydrocarbon Systems,” 126th Meeting, ACS, New York. 1954. Abstracts, p. l l Q .
( 3 ) Brice, 7’. J., Trott, P. W., L. S. Patent 2,732,398 (1956). (4) Geen, H. V., Zbid., 2,937,098 (1960). (5) Hama, G. M., Frederick, W. G., Millage. D., Brown, H.: A m . Ind. Hyg. Assoc. Quart. 15, 3 (1954). (6) Klevens, H. B., Raison, M., J . Chem. Phys. 51, 1 (1954). (7) Ryan, J. P., Kunz, R . J., Shepard. J. W., J . Phys. Chem. 64, 52s (1960). (8) Scholberg, H. M., Guenthner, R. A , , Coon, R. I., J. A m . Chem. SOC.57, 923 (1953). (9) Shepard, J. W., Ryan, J . P., J . Phys. Chem., 60, 127 (1956). (10) Talbot, E. L., “Effect of pH and Ionic Strength upon the Surface Tension of Certain Perfluoroalkyl Acids and Their Salts,” 124th Meeting, ACS, Chicago: Ill., 1953, Abstracts. p. 41. (11) Talbot, E. L., J,Phys. Chem. 63, 1666 (1959).
RECEIVED for review Sovember 24, 1961 A C C E P T E D June 19, 1962 Division of Industrial and Engineering Chemistry, Fluorine Symposium. 140th Meeting, .4CS. Chicago. Ill.. Septrmber 1961.
INCOMPATIBILITY IN EXPLOSIVE MIXTURES Detection of Thermal& Hazardous Explosives Mixtures R A Y M O N D
N.
ROGERS
Uniuerstty of Caltfornta, Lor Alamos Sctentz’jc Laboratory. Los Alarnos, .V
id.
A thermal initiation test is proposed to detect instability in explosive systems. Materials are termed thermally incompatible with an explosive when their addition lowers the thermal stability of the explosive. The degree of hazard associated with the handling of a given system under conditions that may cause temperature excursions is estimated from its thermal initiation curves.
of explosive mixtures for specific purposes, two different but closely related requirements must be met. First, the explosive system must yield the desired result and retain its properties through various terms and conditions of storage. Second. the system must not be unduly hazardous to compound or to handle. When normally inert materials or other explosives are mixed with familiar explosives. the properties of the mixture cannot be inferred from the properties of the components. The various components of the mixture may be found to be “incompatible” with one another-i.e.. the system will not operate as desired. or the mixture is hazardous. Prediction and/or diagnosis of these tlvo different types of incompatibility has led to some confusion. Incompatibility of the first type may be caused b!- secondary- chemical reactions. or mobility of residual solvents. gases, or plasticizers, leading to unexpected modifications of mechanical: physical. or electrical properties. Incompatibility of the second type appears as an unexpected increase in sensitivity or decrease in thermal stability, and ma! be caused by any of the foregoing phenomena. It is, however. primarily a problem in chemical kinetics, caused by chemical interaction between explosive components, or between an explosive and a material that is normallv considered to b? ineri.
I
N THE D E v E L o P M E m
Assuming ideal conditions and complete knowledge of the components of a mixture, incompatibility of the first type may be detected or predicted by use of tests designed to look at the vapor phase over test mixtures; e.g., the vacuum stability test ( 7 . 4>7 ) : pyrolysis ( 6 ) ,Taliani ( 9 ) .or gas chromatography (2). In addition, unexpected reactions and phase changes can be detected by use of the differential thermal analysis technique (DTA) (5: 8 ) . However, the interpretation of the results of such tests usually has to be done in a relatively subjective manner. and the correctness of the final decisions should be checked by long term: full scale surveillance tests. KO explosives work should be attempted without access to complete sensitivity testing equipment. and incompatibilities leading to sensitivity hazards should be the primary concern in the preliminary testing of any system. Sensitivity testing is. unfortunately. an extremely complex problem. and it is outside the scope of this paper to summarize tests and methods. Thermal instabilities in mixtures containing explosives, leading to hazards in handling and fabrication, can be detected in a reliable. sensitive manner. The unexpected appearance of phase transitions-e.g., the appearance of a liquid phase (eutectic, or minimum melting solid solution) with associated rapid liquid phase decomposition kinetics, and lor the appearance of exothermal secondary reactions can be detected with a VOL. 1
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