and Richarz, 1977). Therefore, it can be assumed that the catalytic amination on similar copper oxide catalysts gives good results for most long chain aliphatic alcohols. Acknowledgment The authors thank Mr. W. Caprez for his help with preparation of the English manuscript. Literature Cited Arnold, H. R., U.S. Patent 2 078 922 (1934). Baiker, A., Richarz. W., Tetrahedron Lett., 1937 (1977). US. Patent 3 723 311 (1973). Baron, S., Fallstad, H. T., Rheineck, A. E., Dak, F. N., U.S. Patent 3 223 734 (1965). Farbenindustrie Aktiengesellschaft, French Patent 780 028 (1935).
Fine Organics Inc., French Patent 2 168 716 (1973). Gulf Research 8 Development Co.. Neth. APPL 6 507 514 (1965). Jefferson Chemical Co., British Patent 1 074 603 (1967). Kagan, Y . E., et al., Neftekhimiya, 7 (4). 619 (1967). Koebner. A,. Potts. H. A,. British Patent 1 067 762 (1967). Kraiman,’E.,’Grove, C., Austin, J., U.S. Patent 3 4 0 i 203’(1968). Markiewitz, K. H., U.S. Patent 3 278 598 (1966). Schmidt, W., Albrecht, H., German Patent 61 1 283 (1935). Schrauth, W., German Patent 611 924 (1935). Shinzo Otsuka, Yukagaku, 15, 416 (1966). Smeykal, K., German Patent 637 731 (1936). Takamitsu Nagaaki et al., Japanese Patent 75 32 113 (1975). Yakushkin, M.I., Khim. Prom., 42, 493 (1966).
Received for review April 4, 1977 Accepted April 27, 1977
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Solvents in Water Borne Coatings Charles M. Hansen Scandinavian Paint and Printing Ink Research Institute, 14 Odensegade, 2 100 Copenhagen, 0, Denmark
Certain types of solvents are frequently used in water-borne coatings. These solvents contribute to system stability, surface tension reduction, and ultimate coalescence of an otherwise dispersed polymer species. The fundamental reasons for their function are discussed.
Introduction Water-borne coatings are currently receiving exceptional interest because of their environmental desirability. Lower levels of organic solvents are nevertheless frequently required for these products to function properly. The favored organic solvents for this purpose are the ether alcohols, exemplified by ethylene glycol monobutyl ether (EGMBE), and the straight or branched chain alcohols. TLis brief report describes some physical properties of these types of solvents which account for their widespread use in water-borne coatings. Coupling Action A primary requirement for an organic solvent in a waterborne coating is to promote stability. I t is recognized that various types of surfactants having very limited true solubility in a system can promote stability, even a t low concentrations, but the ultimate in stability requires true solution of the components. T o measure the stabilizing potential (coupling ability) of various organic solvents, equal volumes of mineral spirits (Kauri-butanol value 29) and water were titrated with various organic solvents to the point of true mutual solubility (taken as the lack of visual turbidity). Table I lists the volume fractions of the titrating solvents a t this point where the hydrocarbon solvent and the water are “coupled”. The solvents were selected based on solubility parameters for solubility in both water and mineral spirits. It must be noted that higher alcohols and some ether alcohols which can be or are used in water-borne coatings were not included on this basis, but the principles described below are also applicable, in a general sense, to these solvents. High on the list in Table I are solvents frequently used in water-borne coatings. Figure 1shows a triangular plot of these data with the complete phase diagrams for water/mineral spirits/EGMBE and water/mineral spirits/DPM (dipropylene glycol monomethyl ether). 266
Ind. Eng. Chem., Prod. Res. Dev., Vol. 16, No. 3, 1977
An interesting exercise to interpret this data is given in Figure 2, where solubility parameter values for the liquids involved show that the coupling solvents have values intermediate between those of the mineral spirits and water. They are in fact, what might be termed boundary solvents for both, a fact which might have been expected. It must be noted that in water-borne coatings a totally miscible situation is not present and the coupling solvent will distribute between the phases present (Washborne, 1973;Hoy, 1973). This situation has been interpreted from a solubility parameter point of view (Hoy, 1973) where higher values of the hydrogen-bonding parameter correlate with higher concentrations of the organic liquid in the aqueous phase. The fact that these solvents can also orient a t an interface in a manner similar to surfactants is significant. Alcohol groups will attempt to place themselves in the aqueous phase while the hydrocarbon chains prefer organic media or the air surface. This leads to interfacial activity and presumably greater system stability. In some water-borne systems too much organic solvent can lead to instability since it softens the dispersed phase and interferes with the stabilizing action of the surface-active species already at the surfaces in the system. S u r f a c e Tension Reduction A measure of interfacial activity is the degree to which the interfacial (or surface) tension is changed by varying t,he concentration of the active material. For present purposes an indication of this activity for the solvents of interest is given in Figure 3. Here surface tension (against air-saturated vapor) was measured by the ring pull method for various solvent/ water mixtures. The data in Figure 3 show that those liquids appearing highest in Table I are the same as those providing the greatest surface tension reduction for their mixtures with water. These solvents promote both solubility and surface
Table I. Volume Fraction of Organic Solvent Required to “couple” Equal Volumes of Mineral Spirits and Water VOl. fraction Solvent 1. Ethylene glycol monobutyl ether 2. tert-Butyl alcohol 3. Ethylene glycol monoisobutyl ether 4. Diethylene glycol monobutyl ether 5. Isopropyl alcohol 6. Ethylene glycol monoisopropyl ether 7 . Tripropylene glycol monomethyl ether 8. Dipropylene glycol monomethyl ether 9. Ethylene glycol monoethyl ether 10. Tetrahydrofurfuryl alcohol 11. Cyclohexanol 12. Diacetone alcohol 13. Hexylene glycol 14. Propylene glycol monomethyl ether 15. Ethylene glycol monomethyl ether 16. Diethylene glycol monoethyl ether
54.5 60.1 68.1 70.2 71.4 76.7 77.2
80.8 81.9 82.8 85.2 85.3 85.9 89.6 90.4 90.9
tension reduction to a greater degree than others. Both of these effects are desirable in water-borne coatings. Coatings having surface tensions lower than those ascribable to the substrate will not crawl or have other related defects (Hansen, 1972, 1976). A factor contributing to surface activity is limited solubility or a tendency toward limited solubility. Figure 4 demonstrates the tendency toward limited solubility of EGMBE in water. When the temperature is raised for the mixture containing 20% EGMBE and 80%water, for example, phase separation occurs a t 50 “C. For the case of tert-butyl alcohol which is water miscible, it can be noted that its close relative, n-butyl alcohol, is only partly soluble in water. Here again there is a tendency toward limited solubility where tert- butyl alcohol would also be a boundary solvent on the solubility parameter plot for the miscibility of organic liquids in water. An additional factor of limited solubility is that these solvents are also located near the boundary of solubility in the case of typical acrylic polymers. This enhances interfacial activity for these solvents in water-borne coatings in the organic phase as well. Surface tension data of the type shown in Figure 3 were also generated for a water-borne acrylic system with the same general trends, although the surface tensions were higher with the polymer phase present. Concentrations of the solvent in the aqueous phase were reduced because of a preferential migration into the organic phase, thus leading to lower solvent concentrations a t the air interface. This type of data could presumably be used to study distribution coefficients for the distribution or organic solvent between the phases. This was not attempted. Evaporation Phenomena Azeotropes have been reported for the (mono-)methyl, ethyl, and butyl ethers of ethylene glycol, for example, a t the normal boiling points. Immiscibility frequently leads to azeotropes. Evaporation of water/organic solvent mixtures from Petri dishes a t room temperature was studied in a manner similar to that of Ellis and Goff (1972). Analysis of residual volatiles was easily and rapidly done with refractive index measurements. No azeotropes were found to exist for the three liquids listed above. In fact, the composition as a function of percent evaporated could be estimated rather well using the simplest assumption of activity coefficients equal to unity and the procedure outlined by Sletmoe (1970). For these purposes the evaporation rate of water was measured as 45 relative to n butyl acetate equal to 100, and the evaporation rates of the organic solvents were taken from the literature (Doolittle, 1954). The results for EGMBE and water evaporation from
’
lmmiicible region
i‘\
Figure 1. Miscibility diagram for mixtures of mineral spirits (A), organic solvent (B), and water ((2)-volume fraction basis. (EGMBE, ethylene glycol monobutyl ether, DPM, dipropylene glycol monomethyl ether).
0
5
10
6,
15
20
l ~ a l ~ c m ~ i ~ ’ ~
Figure 2. Location of solvents on a solubility parameter diagram showing miscibility regions for water and mineral spirits. Petri dishes are shown in Figure 5 . The experimental data are in good agreement with that reported elsewhere (Shell Chemical Co., 1974). Spot checks for evaporation from thin films applied to metal substrates also agreed with these results where the “films” are somewhat thicker. Evaporation of water and organic solvents from latexes and water-borne coatings have been studied by a member of investigators (Andrews, 1973; Bieneman and Stromberg, 1967; Hansen, 1974; McEwan, 1973; Shell, 1974; Stromberg and Wind, 1968; Sullivan, 1975). The details of these studies are beyond the scope of the present report, but the general conclusion is that relative evaporation rates for the neat solvents are a useful formulating tool for predicting evaporation behavior. Therefore, Petri dish type (or evaporometer) experiments have some general relevance to practical behavior. Solvents having evaporation rates greater than that of water will tend to evaporate more rapidly than water, and conversely, solvents with lower evaporation rates than water will concentrate during drying and presumably be effective coalescing agents if the solubility parameter relations are in order. For water-borne coatings systems, room temperature azeotropes appear to be the exception rather than the rule (the system n-butyl alcohol/water is an exception, however (Ellis and Goff, 1972)). Ind. Eng. Chem., Prod. Res. Dev., Vol. 16,No. 3, 1977
267
73
i 103
J. 0
20
43 wt
X
In
l!n
gr
60
n,o
i
431 M
yi Vel
%
EGMBE
~n
m
103
wafw
Figure 4. Miscibility of water and E G M B E as a function of temperature.
It has been noted that the complex interactions between the components of water-borne paints can influence the absolute evaporation rates (Sullivan, 1975). Evaporation of water and solvent is controlled by a combination of surface resistance and internal diffusion resistance (Hansen, 1964, 1965, 1967, 1970). Diffusion controlled the last 1%loss in some cases, and the presence of mixtures of organic solvents can lead to results difficult to predict in advance (Sullivan, 1975). Likewise, the inhomogeneous structure of latex coatings formed by individual particles sintering together can lead to easier access to the surface than would be found if the film were uniform. This would decrease diffusional resistance within the film. In general, however, organic solvents are not retained for exceptionally long times as they frequently are in exclusively organic systems. The diffusional resistance, if significant, will depend on the film structure and will be higher if the glass transition temperature of the polymer species is higher. Polymers used in water-borne coatings generally have glass transition temperatures which are lower than those used in conventional solvent systems. Therefore, internal diffusional resistance is also lower and surface resistance to solvent loss becomes more important on a relative basis. Linear and smaller molecules diffuse more rapidly than branched and bulky molecules (Hansen, 1964, 1965, 1967, 1970). The linear ether alcohols and low molecular weight alcohols appear to combine the best of circumstances for relatively rapid loss of solvents from industrial coatings while trade sales formulators have determined that organic liquids with still lower evaporation rates are required for most of their products. 268
P
u)
B) fraporaf.d
a
103
lwt/
Figure 5. Calculated and experimental data for evaporation from EGMBE mixtures with water.
The incorporation of organic solvents in water-borne coatings is frequently a result of processing requirements during polymer synthesis and handling. The requirements placed on the solvents do not differ greatly from those for solvent-based systems up to the point where significant amounts of water enter the system. Among the first to report data on phase immersion with the addition of water to "solvent-borne'' systems was Osse (1973). The effects during phase immersion are a combination of those described above and viscosity reduction by the given solvent in the given system. Amine neutralization of acid group containing polymers is, of course, a primary factor in the stability and viscosity of water-borne coatings but is beyond the scope of the present article.
t
Immtuibk
a
O
I
Figure 3. Surface tension of water/solvent mixtures as a function of solvent concentration.
0
I
Ind. Eng. Chern., Prod. Res. Dev., Vol. 16,No. 3,1977
Conclusions The principles controlling desirable functions of organic solvents in water-borne coatings have been described. In the systems tested, organic solvents can promote stability, reduce the surface tension of the system, and evaporate in a planned manner to allow the desired coalescence of the film from dispersed particles of polymer. Future developments can possibly further reduce or eliminate the need for organic solvents in coatings, but the requirements of processability, storage stability, suitable application properties, and final film properties will still have to be met. Acknowledgment The author wishes to thank PPG Industries for permission to publish the data generated in this study. Literature Cited Andrews, M. D., J. Paint Techno/., 46, No. 598, 40 (1973). Shell Chemical Co., Technical Bulletin SC:23-74. "Miscibility and Evaporation Data for Solvent-Water Combinations". Part 1, Shell Chemical Co., 1974. Bieneman, R. A., Stromberg, S. E., J. Paint Technoi., 39, No. 508, 290 (1967). Doolittle, A. K., "The Technology of Solvents and Plasticizers", Wiley, New York, N.Y., 1954. Ellis, W. H.. Goff, P. C., J. Paint Technol., 44, No. 564, 79 (1972). Hansen, C.M.. FargochLack, IO, No. 7, 169 (1964). Hansen, C. M.. Off. Dig., 37, No. 480, 57 (1965). HanSen, C. M., Doctoral Dissertation, Technical University of Denmark, Danish Technical Press, 1967. Hansen. C. M.. Ind. Eng. Chem., Prod, Res. Dev., 9, 282 (1970). Hansen, C. M.. J. Paint Techno/., 44, No. 570, 57 (1972). Hansen, C. M., Ind. Eng. Chem., Prod. Res. Dev., 13, 150 (1974). Hansen. C. M., FargochLack, 22, No. 11, 373 (1976) Hoy, K. L., J. Paint Technol., 45,No. 579, 51 (1973). McEwan, I. H., J. Paint Techno/., 45,No. 583, 33 (1973). Osse, P., Defazet, 27, No. 8, 365 (1973). Sletmoe, G.M., J. Paint Techno/., 42, 246 (1970). Stromberg, S.E.,Wind, G. J., J. Paint Technol., 40, No. 525, 459 (1968). Sullivan, D. A., J. Paint Technoi., 47, No. 610, 60 (1975). Washborne, R. N., Preprint from 7th Congress of the Scandinavian Paint and Varnish Chemists' Assoc., Sandefjord, Norway, Oct 1-3, 1973.
Received for review March 2, 1977 Accepted May 12,1977
