Ind. Eng. Chem. Prod. ~ e sDev. . 1983, 22, 1-5
1
SYMPOSIA SECTION
I.
Symposium on Inorganic Coatings M. Wismer and J. A. Seiner, Chairmen 183rd National Meeting of the American Chemica Society Las Vegas, Nevada, March 1982 (Continued from December 1982 Issue)
Reactive Pigments in Inorganic Silicate Coatings Lesley S. Dent Glasser,' Eric E. Lachowski, and Louise W. Murray Department of Chemistry, University of Aberdeen, Meston Walk, OM Aberdeen AB9 2U€,Scotland
The curing of sodium silicate paint films with both reactive and inert pigments has been studied by measurements of water loss, scrub tests, and analytical electron microscopy. Reactive pigments such as Cu20 and ZnO promote the formation of a silica gel-like matrix by reducing the water content and pH of the silicate solution. In the films studied, the matrices had ca. 6-7 Cu or Zn atoms/l00 Si atoms. Films made with an inert pigment (TiO,) had greater water retention and lower durability than those made with Cu,O, although water loss was greater than for unpigmented films. The silicate matrix resembled dried sodium silicate and had a very low Ti content.
Introduction Paints and coatings based on soluble inorganic silicates such as those of sodium and potassium, particularly when formulated in conjunction with inorganic pigments, have many attractive features. They are nontoxic and fire-resistant;their use helps to conserve petrochemical resources; they are relatively cheap, and the price is less sensitive to fluctuations in world oil prices than for paints based on organic vehicles. Unfortunately, they also have drawbacks. They tend to be brittle, which limits their suitability for coating metals. They are normally supplied as a twocomponent pack, and once mixed have a limited pot-life. Many formulations are not self-curing, so that either a further curing coat must be applied or the coating must be cured by baking. Such post-treatments, especially the latter, impose severe limitation on the use of these paints. However, in view of their other desirable properties, effort expended on overcoming some of these drawbacks would be well worthwhile. In some formulations, the pigment itself acts as a curing agent, at least in part, and the aim of the present paper is to discuss the ways in which this can happen. A better understanding of these processes is essential if progress is to be other than merely empirical. Inorganic silicate paints fall into two main categories. Those in which the pigment is a metal-typically zincnormally also have a very high loading, up to 95% pigment. Where an inorganic pigment-typically an oxide-is used, loadings are generally very much lower, perhaps 2&30% by weight of pigment. Before discussing how such pigments might react with silicate solution, we must consider briefly the nature of the latter. Figure 1,which is modified from the diagram of S t u " et al. (1967),shows how the solubility of silica depends on 0196-4321/83/1222-0001$01.50/0
pH. Below and to the right of the line, true solutions are obtained. At very low concentrations, and in very alkaline solutions, the silicate is present as monomeric species, from H4Si04to HSi04*, depending on pH. At higher concentrations and moderately high pH, polymeric species occur, becoming increasingly important as the upper right-hand part of the solubility curve is approached. Above and to the left of the curve all systems are unstable with respect to precipitation of amorphous silica: apparently clear solutions lying in this region invariably prove to be sols. The distinction between these two regions has important practical consequences. In the area of true solutions, the polymeric species are rather small, and often based on rings of three tetrahedra (Harris et al., 1981); they are quite labile and if the pH or concentration is changed the equilibrium between them rapidly readjusts. However, once the boundary into the instability region is crossed, the situation is quite different. After changes in pH or concentration, reequilibration is slow and alterations in properties can be followed for days, weeks, or even years. The polymeric species formed are large, at least of colloidal dimensions, and appear to be based on rings of four, five, or more tetrahedra; they are much less labile than the species in true solution, and changes can be reversed only slowly. If the concentration of silica is sufficiently high the system may solidify to a gel. Commercial solutions with mole ratios of silica to alkali of around 3.0-3.5 lie more or less on the solubility curve at points such as R; any treatment that lowers their pH brings them into the instability region. It should be noted that some degrees of dilution can have this effect, as indicated by the dashed line. (This is the reason why such diluted solutions frequently deposit amorphous material on storage.) The solubility curve thus sets one limit to the 0 1983 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 1, 1983 I 2
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INSTABILITY REGION 4
SOLUBILITY OF
V
AMORPHOUS SILICA
MONOMERIC SPECIES 1
'2
3
L
5
6
7
i
i
,
l
I
8
9
10
11
12
i
i
' 3 1 4
PH
Figure 1. Behavior of a soluble silicate shown as a function of concentration and pH.
production of soluble silicates; the composition of more alkaline commercial solutions, for example those with ratios around 1.5-2.0, is limited rather by viscosity, which increases dramatically with total solids content when this is greater than about 50%. At solids contents of around 60-70% the material becomes semirigid and could be described as a hydrous glass. Hardening of Silicate-Based Paints From what has been said in the previous paragraph, it will be seen that a silicate-based paint could harden through loss of water, or through changing conditions to bring it into the instability region, or through a combination of both. Stoving or baking to remove excess water is plainly an example of the first of these; post-curing with phosphoric acid, the second. In fact, the best cures seem to be obtained when both factors operate; reactive pigments, as we shall see, appear to assist in this. It should be remarked, in passing, that systems corresponding closely to real paint formulations are very intractable from the point of view of the academic inorganic chemist, who is frequently driven to examining simplified model systems in order to try to understand the underlying chemistry. I t is salutory to bear in mind that results obtained with relatively pure materials, perhaps studied in bulk and often diluted, may not be directly applicable to thin films of concentrated viscous material prepared from technical grade reagents. Conversely, the paint technologist, juggling with a large number of variables in order to achieve the best formulation as quickly as possible, ought not to be disappointed if the academic fails to produce a theoretical basis for the empirical results. With this reservation, let us consider some of the chemistry that underlies the curing process. Zinc Dust. If coarse zinc dust is mixed with sodium silicate solution, hydrogen is evolved Zn + 2H20 + 20HZn(OH)42-+ H2
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This reaction is indeed one of the few hazards associated with such paints, which are approved for use in aviation fuel storage tanks: it is important to ventilate during application! (With modern hyperfine zinc dusts this gassing does not occur: reaction is limited to the layer of ZnO on the surface (Wright et al., 1970) and this will be considered below.) The obvious consequence of the reaction, other than the production of hydrogen gas, is to lower the pH of the solution and increase the degree of polymerization of the silicate +Si-OH + -0-Sif +Si-O-Si+ + OHWhen the pH is sufficiently lowered, particles of amorphous silica are produced, the system having entered the instability region. In addition, a certain amount of water is used in the reaction, and more is swept out with the
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evolved hydrogen gas, so that the system becomes more concentrated and hence more viscous. Hardening is thus due to both of the mechanisms discussed earlier. A protective coating of zinc particles bonded by amorphous silica seems, at first sight, to be an ideal anticorrosion barrier, but in fact there are snags. In the first place, it has been observed that the protective potential is lower than for a similar coating based on ethyl silicate (Kuzin et al., 1977);this is perhaps the result of adsorption of silica onto the surface of the zinc particles. In the second place, the amorphous silica produced is quite different in structure from a silica or silicate glass. It appears to be particulate in nature, much like a silica gel (Dent Glasser et al., 1978) and is consequently rather weak. It is indeed best regarded as a sodium-containing hydrous silica gel: what it is not, unfortunately, is a continuous network of silicate tetrahedra strongly bonded in three dimensions, such as is found in glasses. Oxide Pigments. A variety of oxide pigments has been tried in inorganic paints (Pass and Meason, 1965) including titanium dioxide, zinc oxide, chromium(II1) oxide and copper(1) oxide. Sodium silicate paints pigmented with ZnO and Ti02 are used for providing decorative and protective coatings to building panels, while cuprous oxide has been applied in antifouling paints for ships' bottoms (Rischbieth and Marson, 1963). Of the above pigments, zinc oxide might be expected to react most extensively with sodium silicate ZnO
+ H 2 0 + 20H-
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Zn(OH),2-
lowering the pH and consuming water but (unlike Zn) not evolving hydrogen. However, our studies (Dent Glasser et al., 1978) on a rather dilute model system (1g of ZnO in 1L of silicate solution diluted to 1.0 M SO2)suggested that reaction was extensive only in the immediate vicinity of the oxide particles; very little zinc appeared in the bulk of the matrix. Consequently, we were inclined to expect that, bearing in mind the less amphoteric nature of copper, very little reaction indeed would occur with Cu,O. At the same time, it was decided to compare the effect of Cu20 with that of Ti02, which is accepted as being an inert pigment. Experimental Section The sodium silicate solution used (Pyramid No. 1, Crosfield Chemicals) had a Na20:Si02mole ratio of 1:3.41 and a solids content of 38.1 % . Pigments were CuzO (BDH Laboratory reagent) and TiOz (Tioxide International); the latter was supplied as a wet paste which was dried and calcined at 1000 "C before using. Both pigments were sieved and the 200-300 mesh fractions were used so as to minimize surface area effects. Zinc oxide (Zincoli 360, Amalgamated Oxides (1939) Ltd.; specific surface 0.3 m2 g-' corresponding to a slightly smaller particle size than the other pigments) was used for comparison in the electron microscopy studies. The effectiveness of CupOand TiOz in producing a cure was assessed by practical tests. For these, 2-4 g of pigment was mixed with 10 g of silicate solution by kneading in sealed polythene bags so as to avoid water loss during the mixing process. Films of both pigmented and unpigmented silicate solutions were spread evenly on cleaned steel sheets and allowed to dry in air at room temperature; the water loss was monitored by weighing at intervals. The insolubility of the paint films after drying in this way for three days was assessed by subjecting the weighed coated steel sheets to 100 scrubs with a stiff nylon brush under cold running water and then reweighing. This rather rough-and-ready version of a standard testing technique
Ind. Eng. Cham. Rod. Res. h.. Vol. 22. No. 1. lB83 9
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.llnm I , l n l + C
Figure 2. Rate of drying of paint films,tbickneas 0.17 mm: 2 g of pigment (where used) to 10 g of sodium silicate solution.
proved to be quite effective. Cured paints were examined with a Kratos (AEI) CORA analytical electron microscope. For this purpose, films were prepared as described above, but they were coated on used photographic films instead of steel sheets; this allowed them to be detached easily. Initially, we tried to prepare specimens by ion-beam thinning,hoping to be able to examine the pigment-vehicle interface. Unfortunately, the pigment appeared to be removed selectively by the ion beam, and the method was eventually abandoned. The most satisfactory specimens were prepared by crushing the dried film and dispersing the particles obtained on electron microscope grids using isopropyl alcohol. Results Typical results for the weight loss from films are shown in Figure 2. Water is lost most rapidly from the film pigmented with Cu,O, and least rapidly from the unpigmented sodium silicate film, with the Tiorpigmented film lying in between. Moreover, even after 3 days,when curing appears to be complete, the amounts of retained water follow the same sequence. It appears that the presence of an inert pigment promoted loss of water from the film-perhaps by providing an escape route, or through a sort of “wick” effect, i.e., a “physical” as opposed to a ‘chemical” cure. A reactive pigment, such as CuzO, however, is even more effective. That reaction does occur is readily seen: sodium silicate solution of this ratio left in contact with CuzO becomes faintly, but distinctly, green. This distinction between reactive and unreactive pigments is supported by viscosity measurements (Rischbieth and M m n , 1961); the Viscosity of silicate solutions containing particles of zinc or copper(1) oxide increased steadily with time, while that of solutions containing inert pigments did not. Resulta of the scrubbing teats are shown in Figure 3, and they demonstrate strikingly the superiority of the cure using Cu,O. The percentage loss of weight is small for both thicknesses of film studied, and moreover it appears to be independent of pigment loading. This strongly suggests that chemical curing is, in this case, more important than any physical effect. The results for the unpigmented films and those containing TiO, are rather more difficult to explain. A t least in the thin films, TiO, appears to have improved the resistance of the coating, perhaps through the type of physical cure discussed above. With both of these types of film it was noticed that, after drying, the scrubbed films were more prone to flaking than those containing CuzO,
O”
I,lnl+
Figure 3. Durability of paint films as measured by loss of weight on scrubbing (see text for details). Pigment loading (g/10 mL of silicate solution) and film thicknw given below diagram.
a
b
Figure 4. Electron microgrsphs of the product of reaction of Cu20 and sodium silicate solution: (a) t y p i d areas of amorphous matrix. showing on average I atoms of Ca:100 atoms of Si (b) matrix of different appearance. very high in copper and apparently including minute crystallites.
a
b
1
Figure 5. The products of reactions of ZnO with sodium silicate solution, showing analyses of typical area8 of matrix. The speckled or grainy nature of the matzir is particularly apparent in (a). which also shows two typical particles of sodium silicate glass (A).
again supporting the idea that the latter produces a chemical cure. Electron Microscopy. Figures 4, 5, and 6 show the appearance of films pigmented with Cu,O, ZnO, and TiO,, respectively, together with some analyses of typical areas
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Ind. EW. Chem. Rod. Rea. Dev.. Vol. 22. NO. 1. 1983
P i " 6. The product of miring TiOPwith d i m Silicatemlution showing analysea of typical arena of matrix. Note the difference in appearance of the latter for similar areas in the other micrographs.
4
0
Qr
ds
cu
ii
Ib
io
atomsIatam SI Figure 7. Analyses of areas of matrix similar to those indicated in Figurea 4a and 5 for Cu and Zn. Mean valuea and estimated standard deviations (a) are indicated.
of the amorphous matrix, expressed as atomic ratios. Pigment particles, b e i i crystalline, could be identified by their characteristic appearance and diffraction patterns, and they were readily avoided. The analpea of the amorphous matrix had considerable scatter, shown in Figure 7, even ignoring atypical areas such as that shown in Figure 4b (discussed below) This
is partly the resdt of the poor counting atatistics inevitably associated with low concentration. Both the Cu and the Zn results showed a bimodal distribution, but in view of the large scatter this is probably not significant. The values for both Cu and Zn in areas such as those shown in Figure 4a and 5 were measured for a number of particles; the mean values were about 6-7 atoms/loO atoms of Si in both cases. This is considerably higher than the value of 1:lOO that we found in an earlier study (Dent Glasser et al., 1978)of the ZnO system using the more dilute silicate solutions to which our studies were restricted before the analytical electron mieroacope became available to us. This illustrates the point made above conceming the danger of extrapolating from simplified or more dilute model systems. Those areas of amorphous matrix, when sufficiently thin to be transparent, showed a characteristic speckled a p pearance similar to that observed in silica gel, some samples of which are shown in Figure 8 for comparison. In contrast, hydrous sodium silicate glass is quite featureless when the water content is low (two particles can be seen at A in Figure 5a) and intumesces when the water content is high. The area in Figure 4b that shows high copper content is interesting, in that it appears to contain minute particles of crystalline material. These gave a faint electron diffraction paattem that could be identified as Cu20. These particles are very much smaller than the pigment particlea, and we believe that they have been reprecipitated from the sodium silicate matrix. Such behavior would be reasonably consistent with what is known of the chemistry of copper in alkaline solution: a copperhydroxyl complex appears to form, but is quite unstable and readily decomp e s to copper oxide on heating or if the pH drops a little. The Ti0,pigmented specimens,in contrast, showed only very small amounts of Ti in the amorphous matrix (Figure 6 ) confirming that this is not a reactive pigment. The matrix was relatively featureless and showed a pronounced tendency to intumesce, consistent with ita being largely unaltered sodium silicate. The effect of both ZnO and Cu20is thus to promote the formation of a silica gel-like material from the sodium silicate. It is of particular interest to compare their action with that of COP,used in the foundry industry. Although the latter reaction proceeds much more rapidly, since the setting agent is both gaseous and more acidic, there is plainly some analogy between the processes. The more particulate nature of the matrix formed in all these mes
a C b Figure 8. Variow siliea gels: (a) commercial (self-indicating) silica gel; (b) chromatographic silica gel: (e) silica gel produced by allowing sodium silicate solution to react with CO,. Note the similarity of this to Figure 5a.
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Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 5-8
suggests that modification of the structure of the sodium silicate into a material that is more like silica gel is an important part of the cure. Registry No. Cu20, 1317-39-1; ZnO, 1314-13-2;Ti02, 1346367-7; sodium silicate, 1344-09-8.
Literature Cited
Kuzln, V. A.; Orlov, V. A.; Kilmenko. N. S. Lakokras. Meter. Ikh Primen. 1077, 6 , 31.
Pass, A.; Meason, M. J. F. J . OIICo. Chem. Assoc. 1085, 48, 097. Rlschbieth, J. R.; Marson, F. Nature (London) 1981, 748. Rlschbieth, J. R.; Marson, F. J . Oil Co. Chem. Assoc. 1983, 4 6 , 499. Stumm, W.; HUper, H.; Champlln, R. I . Environ. Sci. Techno/. 1087, 1 , 21 1. Wright, M. 0. B.; Madge, J. W.; Bond, M.; Crowl, V. T. “Surface Characterlstlcs of Metallic Zinc Pigments”; International Lead Zinc Research Organisation: New York, Project ZC 162, 1970.
Dent Glesser, L. S.; Gard,J. A.; Lachowski, E. E. J . Appl. Chem. Blotechnol.
Received for review March 9, 1982 Accepted August 27, 1982
1078, 28. 799. Harrls, R. K.; Knight, C. T.; Hull, W. E. J . Am. Chem. SOC.1081, 703, 1577.
I I.
Symposium on Fluoropolymers K. J. L. Paciorek, Chairman 183rd National Meeting of the American Chemical Society Las Vegas, Nevada, March 1982
Copolymerization Studies of Fluorinated Epoxides Kazlmlera J. L. Paclorek,’ Thomas I. Ito, James H. Nakahara, and Relnhold H. Kratrer Uttrasystems, Inc., 2400 Michelson Drive, Imine, California 927 15
Telomerizations of perfluoro-1,Bepoxyheptane and 4-chloro- and 4-bromoheptafluoro-l,2-epoxybutanes with hexafluoropropene oxide at 0 and -23 OC afforded as major products room temperature involatile copolymers of 3000 average molecular weight. At higher temperatures, substantial quantities of low molecular weight telomers were obtained. The nature of the products formed with respect to the arrangement of the epoxide units was determined by mass spectrometry. The investigations showed that true copolymers were formed.
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Introduction Pertluoroalkylethers belong to a class of low Tgthermally and oxidatively stable materials and as such present potential candidates for fluid and elastomer applications where extremes of temperatures are to be encountered. To date, these compositions were found to provide fluids and lubricants (Binaghi et al., 1973; Gumprecht, 1966, 1967; Lawson, 1970; Sianesi et al., 1971; Snyder and Dolle, 1976). A practical elastomer must possess a sufficiently high molecular weight and be amenable to cross-linking. The chemical inertness of the perfluoroalkyl ether chain precludes cross-linking without an introduction of functional sites. The current investigation was undertaken to determine the feasibility of copolymerizing hexafluoropropene oxide with oxides containing side chains other than trifluoromethyl with the ultimate objective to incorporate a bromine-terminated pendant group into the polymer chain. Results and Discussion In the telomerization reactions, hexafluoropropeneoxide and was copolymerized with perfluoro-l,2-epoxy-n-heptane 4-chloro- and 4-bromoheptafluoro-1,2-epoxybutanes.The experimental procedures were based essentially on the work of Anderson (19681, but in the current studies, higher temperatures were utilized. To optimize the polymerization conditions, homopolymerizations of hexafluoropropene oxide were also carried out. These experiments are summarized in Table I. It should be noted that of the four epoxides, hexafluoropropene oxide is most volatile, bp -28 “C (Sianesi et al., 1966), whereas the perfluoro0 196-4321 16311222-0005$01.50/0
heptene oxide (VPoec,18 mm) is the least volatile. The 4-chloroepoxybutane (VPooc,199 mm) (Ito et al., 1979) is significantly more volatile than its bromo analogue ( VPaoc, 99 mm) (Ito et al., 1979). The copolymerization of hexafluoropropene oxide with perfluoroheptene oxide resulted in the incorporation of approximately 50% of the available heptene oxide. Yet, the consumption of hexafluoropropene oxide was higher than that observed on its homopolymerization where up to 10% unreacted epoxide was recovered. This would indicate that, although perfluoroheptene oxide appears to be less reactive than hexafluoropropene oxide, it performs a solvent function for the telomerization process. The ratio of C7FI40to C3F60units in the resultant polymer was calculated to be 1:7.8 based on the recovered perfluoroheptene oxide. This value is in good agreement with the ratio of 1:7.5 obtained by NMR analysis; the NMR data also confirmed that a true copolymer was formed. The major portion of the research effort was centered on the copolymerizationsof hexafluoropropene oxide with 4-chloroheptafluoro-l,2-epoxybutane.The chloro moiety is not sufficiently reactive to provide a functional cross-link site; however, there is a close similarity between 4chloroheptafluoro-1,Zepoxybutaneand its bromo analogue to utilize the former to evaluate and optimize the reaction conditions. This approach was prompted by the relative availability of the precursor 1,1,1,5-tetrachloroperfluoropentane as compared to l,l,l-trichloro-5-bromoperfluoropentane (Ito et al., 1979). Examining the data in Table I, it is evident that the lower the temperature of polymerization and the lower the relative amount of cesium 0 1983 American Chemical Society
