Description:
FIELD OF THE INVENTION
The present invention relates to one package protective coatings and, more particularly, to single container, zinc rich organo-silicate coatings adapted for the protection of ferrous surfaces. The present invention also relates to one package, zinc rich protective coatings having reduced gas evolution achieved by incorporating in the coating vehicle an appropriate alkyl or aryl nitro-compound, a cycloketone, or a lead oxide compound.
DESCRIPTION OF THE PRIOR ART
For many years zinc rich protective coatings have been extensively used to protect ferrous surfaces against corrosion. The metallic zinc in the coating provides cathodic protection to steel, the corrosive reactions taking place selectively on the zinc which places higher than iron in the electromotive series. For optimum protection, a high concentration of zinc dust in the dry coating is required, thus the binder used must have sufficient toughness and binding capacity to form a strong adherent coating even when present in relatively low weight percentage of the dry coating. Silicates form excellent binders for metallic zinc particles, and such zinc rich silicate coatings have found widespread commercial acceptance. Invariably however, the products heretofore available commercially have been of the two or three package variety, with the vehicle including the silicate source in one package and the zinc, and for some types, a curing agent for the silicate in separate packages. When the contents of the packages are mixed, the resultant coating must be applied immediately lest gelling of the mixture and/or irreversible settling or packing of the zinc occur.
The difficulties inherent in producing a commercially acceptable, single container zinc rich silicate coating may be appreciated by a consideration of various prior art approaches.
The use of separately packaged silicate source, zinc dust, and chemical curing agent is typified by U.S. Pat. No. 2,952,562. This coating utilizes zinc in a first container, a silicate packaged in a second container, and a curing agent comprising a solution of a salt capable of yielding an acid radical packaged in a third container. In a preferred embodiment the curing agent comprises a substantially non-aqueous solution of an organic amine salt of phosphoric acid, sulfuric acid or the like. The same curing agent is utilized for the coating taught in U.S. Pat. No. 3,287,142; in this patent a vehicle formulation including sodium silicate, water, and a borate source such as boric acid is employed. For both coatings, the silicate and zinc are mixed and immediately applied to a surface with subsequent application of the curing agent to affect insolubilication of the silicate binder. Clearly, a three package system is required, first and second packages containing the silicate and zinc, a third package the curing agent. Should the zinc and silicate vehicle be packaged together, gelling of the mixture would take place and an unusable product result.
A second approach of the prior art to provide a satisfactory zinc rich silicate coating is characterized by U.S. Pat. Nos. 3,130,061 and 3,320,082, each assigned to American Pipe and Construction Company, owner of the present invention. These coatings are known commercially under the trade name "Dimetcote". The first of these coatings utilizes a film-forming binder consisting of lithium hydroxide and alkali stabilized colloidal silica, while the second utilizes alkali stabilized colloidal silica in water together with lead oxide and a water miscible organic amine. Since zinc dust reacts to form salts in the presence of water, the zinc must be packaged separately from the aqueous portion of the coating, the zinc and the vehicle being mixed together immediately prior to application.
Yet another approach of the prior art involves utilization of organic silicates as the binder source material for zinc rich coatings. Typifying this approach is U.S. Pat. No. 3,056,684, wherein the vehicle comprises tetraethyl orthosilicate (TEOS) partially hydrolyzed to form siloxane polymers having an average chain length of 5 to 10 silicon atoms. When mixed with zinc powder and applied to a surface, the resultant coatings set up or dry in a matter of a few hours. However, should the zinc be added to the vehicle at the time of packaging, the reactivity of the zinc and vehicle cause gellation within a matter of hours, resulting in an unusable product.
Thus the achievement of a single package zinc rich silicate coating has been impossible using prior art vehicle systems. This severe, commercially unsatisfactory shortcoming of the prior art is overcome by the inventive zinc rich protective coatings, wherein use of a novel vehicle permits zinc dust to be pre-mixed to form one package coatings having long shelf life with no irreversible packing of the zinc.
In certain formulations of zinc rich protective coatings, a secondary problem of gas evolution has been experienced. In such formulations, a gas, apparently hydrogen, is generated in the container when stored at room temperature for periods exceeding about two months.
Of course, one approach to the solution of gas evolution problem in a one package zinc rich coating is to package the material in a container having a valve permitting hydrogen escape, or utilizing a gasket permitting gas leakage. However, such deviation from standard packaging procedures is undesirable, requiring specialized containers and complicating the manufacturing procedure. Moreover, such approach involves a safety hazard, since evaporating flammable solvents also may escape through the valve or gasket.
Prevention of gas evolution in zinc dust paint has been studied in the past only in conjunction with organic coating utilizing linseed oil or other resin binders. Thus, in the article entitled "Gas Evolution in Zinc Dust Paint" by W. J. Lantz, printed in the Mar., 1961, issue of Paint and Varnish Production, approaches to the prevention of gas evolution in zinc rich paints having linseed oil, phthalic alkyd resin and phenolic resin vehicles were discussed. In particular, Lantz pointed out that a small amount of water present in the resin based coating solution would react with the zinc dust according to the following equation:
Zn + 2H 2 O➝Zn(OH) 2 + H 2 1.
Typically, if 0.007 pounds of water were present in one gallon of linseed oil based coating, and this water were all to react with the zinc dust according to equation (1), 0.16 cubic feet of gas at standard conditions would be formed. If this gas were confined in the 9 cubic inch void space of a filled gallon can, a pressure in excess of 4,000 P.S.I. would develop.
The Lantz article suggested that such gas evolution in a resin based zinc dust paint may be inhibited by incorporating a water scavenger, tetraethyl orthosilicate (TEOS), and a fatty acid neutralizer such as diethylene triamine or calcium oxide. Of course, in a zinc rich organic polysilicate coating such as that described in this application, wherein TEOS may be used as the silicate source, the further addition of small quantities of TEOS (as suggested by Lantz) would not significantly effect the rate of gas evolution. Moreover, since such a silicate based zinc dust coating contains no fatty acids, the addition of a neutralizer such as diethylene triamine or calcium oxide likewise would be ineffective.
SUMMARY OF THE INVENTION
In accordance with the present invention a single container zinc rich protective coating is obtained by utilizing zinc dust in a vehicle comprising an organosilicate, a hydroxyl source and a solvent, the vehicle having a volume average dielectric constant below approximately 14. It has been found that by maintaining the vehicle dielectric constant below this value the zinc dust will not settle and pack into a hard mass, but rather the zinc will be readily redispersible even after prolonged storage in a closed container. In certain embodiments, incorporation of an alkyl or aryl nitro-compound, a cycloketone, or a lead oxide compound in the coating vehicle significantly reduces or eliminates gas evolution in the packaged zinc rich coating.
In a preferred embodiment, the inventive protective coating comprises fine zinc dust, an ethyl polysilicate, a hydroxyl ion source such as diamylamine which itself is non-reactive with the silicate, and enough solvent such as xylol of sufficiently low dielectric constant to insure that the average dielectric constant of the entire vehicle, as calculated on the basis of volume, is below about 14. The coating can be stored for extended periods of time in a single moisture-tight container before use. When applied to a ferrous or other surface, moisture from the atmosphere reacts with the amine to produce OH ions, which in turn cause hydrolysis and polycondensation of the silicate. The resultant protective film has a high zinc pigment volume concentration, the silica forming a tough, adherent binder for the zinc dust.
Thus it is the primary object of the present invention to provide a one package zinc rich protective coating.
Another object of the present invention is to provide a single package zinc rich organic coating utilizing a silicate binder.
Yet another object of the present invention is to provide a zinc rich protective coating utilizing a vehicle having characteristics which permit zinc dust to be pre-mixed therewith to form a one package system.
It is another object of the present invention to provide a one package protective coating utilizing zinc dust, an organic poly-silicate, a hydroxyl ion source to catalyze hydrolysis of the silicate, and a solvent, the dielectric constant of the vehicle being below approximately 14, this value being sufficiently low so as to permit continued suspension of the zinc dust.
Another object of the present invention is to provide means for inhibiting gas evolution in a single package zinc rich silicate coating.
It is a further object of the present invention to provide a one package protective coating for ferrous surfaces, including fine particle metallic pigment, an organic binder, a catalyst, and sufficient solvent so that the average dielectric constant of the binder, catalyst and solvent combination is below approximately 14.
It is another object of the present invention to provide a zinc rich protective coating having a vehicle which permits zinc dust to be premixed therewith to form a one package system, and which incorporates an ingredient to prevent gas evolution in the packaged coating.
Yet another object of the present invention is to provide a one package protective coating utilizing zinc dust, an amine or like hydroxyl source and a solvent, the volume average dielectric constant of the vehicle being sufficiently low so as to permit continued suspension of the zinc dust, the coating further incorporating sufficient quantity of a compound selected from the class consisting of alkyl and aryl nitro-compounds, cycloketones and lead oxide compounds so as to inhibit gas evolution in the packaged zinc rich coating.
A further object of the present invention is to provide a one package zinc rich silicate coating incorporating an alkyl or aryl nitro-compound which produces an amine as a by-product of reaction with hydrogen, the nitro-compound serving both to control gas evolution in the packaged coating, and to enhance effectiveness of the coating.
It is a further object of the present invention to incorporate in a one package zinc rich organic silicate coating a sufficient amount of a cyclic ketone so as to prevent hydrogen gas evolution in the packaged coating.
Yet another object of the present invention is to provide a one package zinc rich organic silicate coating incorporating a lead oxide compound which serves both as color pigment for the coating and for the control of gas evolution in the packaged formulation.
Still another object of the present invention is to provide various formulations for metallic particle protective coatings characterized by the utilization of a silicon ester binder, a catalyst to hydrolyze the ester when exposed to moisture, and sufficient solvent to lower the dielectric constant of the vehicle sufficiently to permit pre-mixing of the metallic particles with the ester into a one package, long shelf-like material.
Still other objects, features and attendant advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description of the preferred embodiments construed in accordance therewith.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention there is provided a one package zinc rich protective coating utilizing a vehicle comprising an organic silicate binder, a hydroxyl ion source and a solvent, the vehicle having a volume average dielectric constant below approximately 14, thereby insuring that zinc dust premixed with the vehicle will not irreversible settle or pack even after long shelf life.
The organic silicate utilized in the present invention preferably comprises an ethyl polysilicate such as that available from Union Carbide Chemical Company under the trade name "Ethyl Silicate 40". The latter material, which has an SiO 2 content of about 40 percent, comprises polysilicates having an average of five silicon atoms per molecule. Although less desirable because of their lower per volume silica content, condensed ethyl silicate or tetraethyl orthosilicate also may be employed in the present invention. Ethyl polysilicates form an excellent binder for zinc and other pigments when hydrolyzed and polymerized.
To catalyze the silicate hydrolysis, the vehicle of the present invention also includes a hydroxyl source which itself is non-reactive with the organic silicate but which reacts with moisture to produce hydroxyl ions. By way of example, the hydroxyl source in the inventive protective coating may comprise one or more organic amines such as mono-, di- or triethanolamine, diamylamine, cyclohexylamine, piperidine, and the like. Alternatively, other organic or inorganic hydroxyl sources such as potassium hydroxide may be employed. Since the hydroxyl source is non-reactive with the organic polysilicate of the vehicle, the silicate binder will not be hydrolyzed when stored in a moisture-tight container.
When the coating is applied to a surface, the hydroxyl source reacts with atmospheric moisture, ionizing to yield OH -ions . The ethyl polysilicate then undergoes basic hydrolysis and polycondensation to form silica, the resultant alcohol by-product being lost by vaporization. The reaction mechanism presumably involves the following steps;
RNH 2 + H 2 O⇋RNH 3 + + OH - 2.
wherein R is any organic group. Note that a more basic hydroxyl source will provide more hydroxyl ions than a less basic source, thereby accelerating the curing of the coating. Moreover no separate curing agent is required.
To insure that zinc dust will not irreversibly settle when added to the silicate and amine mixture, it has been found that the vehicle volume average dielectric constant must be less than about 14. This vehicle dielectric constant may be achieved by selecting appropriate volumetric combinations of silicate, hydroxyl source and solvent, as described below.
The volume average dielectric constant of a vehicle in accordance with the present invention may be calculated in conjunction with the values of dielectric constant given in TABLE I below. In particular, designate the volume percentage of silicate in the vehicle as V Si , the dielectric constant of the silicate as E Si , the volume percentage of hydroxyl source as V B , the dielectric constant of the hydroxyl source as E B , the volume percentage of solvent as V S and the dielectric constant of solvent as E S . Then, the volume average dielectric constant E T of the total vehicle is given by
V Si E Si + V B E B + V S E S (=)E T 4.
To obtain satisfactory zinc suspension, E T should be less than about 14. In general, the lower the value of E T , the less zinc settling and packing experienced.
Very wide latitude is permitted in the amount of zinc dust used in the inventive coatings. Thus satisfactory coatings result when the volume of zinc dust is from 30 percent to 200 percent of the volume of silicate employed in the coating. In other words, for each 100 parts by volume of silicate, an amount of zinc dust in the range of from 30 parts by volume to 200 parts by volume may be included in the coating. Particularly good coatings have been achieved when the amount of zinc dust present in the coating is in the range of from 70 parts by volume to 125 parts by volume for each 100 parts by volume of silicate. ------------------------------------------------------------
--------------- TABLE I
DIELECTRIC CONSTANTS
Solvents ____________________________________________________________
______________ pentane 1.8 methyl propyl ketone 16.8 hexane 1.9 cycloheptanone 17* heptane 2.0 diethyl ketone 17.3 octane 2.0 n-butyl alcohol 17.8 cyclohexane 2.0 isopropyl alcohol 18.0 bicyclodecane-1-one 2.0* cyclohexanone 18.3 turpentine 2.2 methyl ethyl ketone 18.4 carbon tetrachloride 2.2 cyclopentanone 19* dipentene 2.3 acetone 21.4 benzene 2.3 1-nitropropane 23.2 xylene 2.3 2-nitropropane 25.5 dioxane 2.3 ethyl alcohol 25.7 toluene 2.4 nitrotoluene 27.4 naphthalene 2.5 nitroethane 30.0 ethyl benzene 2.5 methyl alcohol 33.7 trichloroethylene 3.4 nitro benzene 35.7 ethyl ether 4.3 ethylene glycol 37.7 n-butyl acetate 5.1 nitromethane 39.4 ethyl acetate 6.4 furfural 41.9 trichloroethane 7.1 glycerol 42.5 ethylene glycol dimethyl sulfoxide 42.6 monoethyl ether water 80.4
acetate 7.6 Hydroxyl Sources methylene chloride 9.1 trimethylamine 2.4 propylene oxide * triethylamine 2.4 t-butyl alcohol 11.4 cyclohexylamine 3.0* ethylene oxide 13.0 diamylamine 4.0* benzyl alcohol 13.1 dimethylamine 5.3 cyclohexanol 15.0 ethylamine 6.9 mesityl oxide 15.1 aniline 7.2 isobutyl alcohol 15.5 pyridine 12.3 ethylene diamine 14.2 ethanolamine 30.0* alcoholic potassium hydroxide (1N in ethyl alcohol) 30.0* diethanolamine 40.0* triethanolamine 45.0* Organic Silicates tetraethyl orthosilicate 15.0* condensed ethyl silicate 20.0* ethyl silicate 40 30.0* ____________________________________________________________
______________
In TABLE I, the dielectric constant values marked with an asterisk are assigned values based on emperical studies; no published dielectric constant values for these compounds are presently available. Note that the ingredients listed in TABLE I are examplary only, and the present invention clearly may use solvents, hydroxyl sources and/or silicates other than those tabulated.
The following exemplary coating vehicle formulations exhibit the dielectric constant requirements of the present invention.
EXAMPLE I Ingredient V E VE Ethyl Silicate 40 0.10 30 3.0 Heptane 0.80 2 1.6 Diamylamine 0.10 4 0.4 E T =5.0
example ii ingredient V E VE Ethyl Silicate 40 0.30 30 9.0 Heptane 0.50 2 1.0 Isopropyl alcohol 0.10 18 1.8 Diamylamine 0.10 4 0.4 E T =12.2
example iii ingredient V E VE Ethyl Silicate 40 0.40 30 12.0 Isopropyl alcohol 0.50 18 9.0 Diamylamine 0.10 4 0.4 E T =21.4
zinc dust was mixed with each of the vehicles of Examples I, II and III, and the resultant mixtures stored in closed containers. The coating of Examples I and II, having vehicle volume average dielectric constants of 5.0 and 12.2 respectively, exhibited long shelf life. The zinc dust mixed with the vehicle of Example I showed little if any tendency to pack, while the zinc dust mixed with the Example II vehicle was readily redispersed even after extended storage. Conversely, zinc dust mixed with the vehicle of Example III rapidly settled and packed into an extremely hard mass which could not be redispersed even with vigorous stirring. Thus Examples I, II and III illustrate that the lower the value of E T , the more readily redispersed is the zinc dust. Examples I, II and III also indicate that the value of E T may be controlled by appropriate selection of the volume amount of solvent used, for like silicate and amine. The noted suspension characteristics were substantially unaffected by the volume of zinc dust present in the coating, so long as the volume of zinc dust was in the range of from 30 to 200 percent of the volume of silicate in the coating.
The following Examples IV, V and VI illustrate the wide variety of silicate and solvents which may be employed in conjunction with the present invention. Each formulation lies within the range of E T values which will permit premixing of zinc dust to provide a one package protective coating.
EXAMPLE IV Ingredient V E VE Tetraethyl orthosilicate 0.30 15 4.50 Methyl ethyl ketone 0.20 18.4 3.68 Toluene 0.40 2.4 .96 Ethylamine 0.10 6.9 .69 E T =9.83
example v ingredient V E VE Condensed ethyl silicate 0.30 20 6.0 Isopropyl alcohol 0.10 18 1.8 Cyclohexane 0.55 2.0 1.1 Diethanolamine 0.03 40 .2 Monoethanolamine 0.02 30 .6 E T =10.7
example vi ingredient V E VE Ethyl silicate 40 0.30 30 9 Heptane 0.65 2 1.3 Cyclohexylamine 0.05 3 .15 E T =10.45
again, long shelf life with no irreversible packing of zinc was obtained when fine zinc particles were mixed with each of the vehicle of Examples IV, V and VI and the resultant coating stored in closed containers.
The film quality obtained with each of the vehicles of Examples IV, V and VI was observed. Example IV gave a powdery, frangible film resulting from use of the highly volatile ethylamine hydroxyl source. Use of diamylamine instead of the ethylamine provided a film having improved hardness, resulting from the lower volatility of diamylamine. The films formed using the vehicles of Examples V and VI each were tough and adherent to the coated surface.
While Examples I-VI have included organic amine hydroxyl sources, the invention is not so limited. Thus other hydroxyl sources may be used, as illustrated by the vehicle formulation of Example VII below.
Example VII Ingredient V E VE Ethyl Silicate 40 0.20 30 6.0 Isopropyl alcohol 0.10 18 1.8 Xylene 0.60 2.3 1.4 KOH in alcohol (1N) 0.10 30 3.0 E T =12.2
satisfactory zinc storage characteristics were achieved with the vehicle of Example VII, however the film quality was not as good as that achieved using a vehicle similar to that of the example, but substituting a more concentrated solution of potassium hydroxide.
A high bacisity of the hydroxyl ion source, in conjunction with a polysilicate of high silica content appear to provide optimum film qualities. The following Examples VIII, IX and X illustrate coating vehicle formulations using the same volumetric amounts of Ethyl Silicate 40 and of the highly basic monoethanolamine. Although excellent films may be achieved with each formulation if applied immediately after combining the vehicle with zinc, only the formulation of Example VIII permitted long time storage in a single container. When zinc was mixed with the vehicle of Example X, hard packing of the zinc resulted, and the coating was unusable after a very short period of time. The formulation of Example IX also was unacceptable as a one package coating.
EXAMPLE VIII Ingredient V E VE Ethyl Silicate 40 0.30 30 9.0 Monoethanolamine 0.05 30 1.5 Ethyl benzene 0.65 02.5 1.6 E T =12.1
example ix ingredient V E VE Ethyl Silicate 40 0.30 30 9 Monoethanolamine 0.05 30 1.5 Trichloroethane 0.65 07.1 4.6 E T =15.1
example x ingredient v e ve ethyl Silicate 40 0.30 30 9 Monoethanolamine 0.05 30 1.5 N-Butyl alcohol 0.65 17.8 11.6 E T =22.1
another formulation of a one package zinc rich protective coating in accordance with the present invention is set forth in Example XI below. This formulation, utilized with about 70 volumes of zinc dust for each 100 volumes of ethyl silicate 40, provides a most satisfactory commercial product. ##SPC1##
It has been found that the amount of zinc dust employed for a given amount of silicate may vary widely. As noted above, zinc volume percentages of from 30 to 200 percent of the total silicate volume provide satisfactory coatings. Moreover, a wide variety of zinc particle size may be employed with satisfactory results, zinc dust having an average particle size of about 3 microns providing a good commercial product.
While the actual physical mechanism of the present invention is somewhat uncertain, it is hypothesized that fine zinc dust particles exhibit an electrostatic charge of the same polarity. If such particles are placed in a polar solution (e.g., a vehicle employing a polar solvent), the charge on the particles very rapidly dissipates into the solution. Adhesion, surface attraction or other mechanism then tends to cause the fine particles to agglomerate and clump into a solid mass. On the other hand, if such zinc dust particles are placed in a solution of low polarity (e.g., a vehicle employing a non-polar solvent), the electrostatic charge remains on the particles. Since all particles are like-charged, the electrostatic repulsion tends to counteract the tendency to adhere or agglomerate.
This mechanism is somewhat analogous to the zeta potential associated with sols or other colloidal solutions in which stability is achieved by appropriate control of the repulsion between charged colloidal particles.
Polarity of a solvent system is somewhat difficult to express in terms of numbers. However, the dielectric constant of a material is closely related to polarity of the material, and applicants have recognized that characterization of the coating vehicle in terms of volume average dielectric constant permits accurate determination of which vehicles may be pre-mixed with zinc dust. Thus it will be appreciated that if the vehicle volume average dielectric constant is below approximately 14, the corresponding polarity of the vehicle will be sufficiently low so that continued zinc suspension can be achieved. This unique characteristic has never been recognized in the past, and permits formulation of true one package zinc rich protective coatings.
Since the packing and hardening characteristics of the zinc dust are related only to the polarity of the vehicle, the particular solvents employed and the ratio of these solvents in the mixture are not important with respect to achieving zinc suspension in the coating. Further, from the theoretical considerations just discussed, there appears to be no reason why any combination of vehicle ingredients having the claimed dielectric constant would not perform equivalently.
Of course, there are certain practical limitations well known to those skilled in the coating art concerning the exact selection of solvents used. For example, it is well known that it is undesirable to use solvents having boiling points above about 300°; use of such solvents results in coatings having very long, commercially unacceptable, curing times.
Another practical consideration is whether sufficient silicate is present in the coating to provide satisfactory film hardness. The following Examples XII, XIII and XIV are illustrative:
EXAMPLE XII V E VE Tetraethyl orthosilicate 0.05 15.0 0.75 Pentane 0.94 1.8 1.69 Diamylamine 0.01 4.0 0.04 E T =2.48
example xii v e ve condensed ethyl silicate 0.05 20.0 1.00 Cyclohexylamine 0.01 3.0 0.03 Xylene 0.94 2.3 2.16 E T =3.19
example xiv v e ve ethyl silicate 40 0.09 30.0 2.70 Heptane 0.90 2.0 1.80 Cyclohexylamine 0.01 3.0 .03 E T =4.53
all of the above Examples XII-XIV exhibited very good zinc dust suspension characteristics, although coatings prepared from Examples XII and XIII had relatively poor film hardness and poor adhesion because of the low percentage of silicate in these coatings.
In general, vehicle formulations most useful in the inventive one package zinc rich protective coatings are seen to have volume average dielectric constant values below about 14. All such formulations provide coatings in which zinc dust will not settle irreversibly. While vehicle formulations having volume average dielectric constant values below about 5 exhibit very good zinc dust suspension, such formulations generally do not have sufficient silicate to provide satisfactory binding of the zinc dust. Further, optimum vehicle formulations include volume percentages of silicate (V Si ) which are within the range of approximately 0.1 to 0.3; volume percentages of hydroxyl source (V B ) within the range of approximately 0.04 to 0.1; and volume percentages of solvent (V S ) within the range of approximately 0.6 to 0.8.
The following Table II indicates that the ranges recited are supported by the examples presented hereinabove. In this table, each example within the preferred dielectric constant range of from 5 to 14 is listed, together with the volume percentages of silicate (V Si ), hydroxyl source (V B ) and solvent (V S ) for each example. ------------------------------------------------------------
--------------- TABLE
II Example E T V Si V B V S ____________________________________________________________
______________ I 5 0.1 0.1 0.8 II 12.2 0.3 0.1 0.6 IV 9.83 0.3 0.1 0.6 V 10.7 0.3 0.05 0.65 VI 10.45 0.3 0.05 0.65 VII 12.2 0.2 0.1 0.7 VIII 12.1 0.3 0.05 0.65 XI 13.73 0.238 0.048 0.714 ____________________________________________________________
______________
it will be understood to one skilled in the protective coating art that various additives may be included together with the inventive one package zinc dust and vehicle system described hereinabove to produce a commercial product. For example, color pigments such as titanium dioxide, iron oxide and the like may be added to improve the appearance of the resultant films. Likewise, reinforcing pigments such as asbestos or mica may be added for reinforcement of the film, and thixotropizing agents such as modified bentonite clay may be added for viscosity adjustment. Moreover, other cathodically active metals such as magnesium or aluminum may be used in combination with the zinc dust.
It should be apparent from Equations 2. and 3. herein above that a more basic hydroxyl source will provide more hydroxyl ions than a less basic source, thereby accelerating curing of the coating. Thus, while a wide variety of coating vehicle formulations exhibiting the required dielectric constant may be utilized, formulations having a relatively high hydroxyl source basicity are preferred, since less basic coatings require longer curing periods. Particularly good results have been observed when utilizing hydroxyl ion sources having basicity equal to or greater than that of triethanolamine.
Coating formulations as described above and having hydroxyl ion sources of basicity equal to or higher than that of triethanolamine have been stored for many months with no irreversible settling of the zinc dust. In each case, the zinc dust was readily redispersible. However, with such formulations, gas evolution in the packaged container was noted after a shelf life of several months at room temperature. Gas evolution in a one package zinc rich coating having the formulation of Example XV is exemplary of that noted for other formulations having the vehicle dielectric constant and hydroxyl source basicity characteristics set forth hereinabove.
EXAMPLE XV V E VE Zinc Dust Ethyl Silicate 40 0.25 30 7.5 Xylene 0.6 2.3 1.4 Isopropyl Alcohol 0.1 18 1.8 Triethanolamine 0.05 45 2.25 E T =12.95
a coating having the formulation of Example XV was packaged in conventional metal paint containers and stored at room temperature. Sampling of the cans indicated that evolution of a gas, apparently hydrogen, began within the can after a period of approximately two months. After a shelf life of 4 to 5 months, gas pressure within the container became sufficiently great so as to cause dimpling, ballooning, and in some cases actual bursting of the can. Parenthetically, irreversible settling of the zinc dust did not occur during this period of time, and the zinc dust readily could be redispersed even after extended months of room temperature storage.
Experimentation indicated that if the cans were stored at a temperature of about 140°F, noticeable gasing occurred within a period of one week. In general, gas evolution in the packaged zinc rich silicate coating appeared to be a function of temperature, evolution being more rapid with increasing storage temperature.
While the exact mechanism of gas evolution in the packaged zinc rich silicate coating is not known, it is hypothesized that hydrogen is generated by the reaction of zinc dust with water present in the system, in accordance with equation 1 above. The source of water is conjectural, since not only is water not one of the constituent ingredients of the vehicle, but its presence is undesirable, since water would react with the hydroxyl source to catalyze hydrolysis and polycondensation of the ingredient polysilicate. Nevertheless, small amounts of water may be present in the packaged coating, possibly representing atmospheric humidity absorbed during the manufacturing process, or alternatively, representing raw material contamination. Thus, trace amounts of water may be present as a contaminant in the solvents incorporated in the system, or as a contaminant in the amines, which tend to be hygroscopic.
Gas evolution in the system is aided, despite the minute amounts of water present, by the very high reactivity of fine particulates. Thus, when using zinc particles of less than 10 micron average size, high reactivity results because of increased zinc surface area, and the tendency for zinc to adsorb water on its surface.
It has been found that incorporation in the vehicle of a reducing agent selected from the class consisting of alkyl or aryl nitro-compounds, cycloketones, and lead oxide compounds results in inhibition or significant reduction in gas evolution in the packaged coating. The following discussion sets forth examples of coating formulations incorporating these compounds, and suggest the nature of the reactions in each instance.
Particularly effective control of gas evolution in a one package zinc rich organic polysilicate coating has been achieved by incorporating in the vehicle between 1 weight percent and 10 weight percent (as based on the weight of the zinc dust incorporated in the coating) of an alkyl or aryl nitro-compound having a molecular weight below about 250. Particularly effective have been the following compounds: 1-nitropropane having the formula
2-nitropropane having the formula
nitrotoluene, nitroethane, nitrobenzene; and nitromethane.
The following formulas suggest the reactions involved:
Zn + 2H 2 O➝ Zn(OH) 2 + H 2 1.RNO 2 + 3H 2 ➝RNH.su b.2 + 2H 2 O 5.
wherein R comprises an alkyl or aryl group. Note that the alkyl or aryl nitro-compound reacts with the evolving hydrogen to produce water and an amine (RNH 2 ) by-product At first glance it would appear that incorporation of an alkyl or aryl nitro-compound is self-defeating, since the compound reacts with hydrogen to form additional water which in turn can react with zinc to evolve even more hydrogen. However, close inspection shows this not to be the case. Thus, equation 5. may be re-written as follows:
3Zn + 6H 2 O➝3Zn(OH) 2 + 3H 2 5.
By combining equations (1) and (5a), the overall reaction may be summerized as follows:
3Zn + 6H 2 O + RNO 2 + 3H 2 ➝RNH 2 + 2H 2 O + 3Zn(OH) 2 + 3H 2 6
which itself may be simplified as follows:
3Zn + 4H 2 O + RNO 2 ➝3Zn(OH) 2 + RNH 2 6a
It is apparent from equation 6a that the overall effect of incorporating an alkyl or aryl nitro-compound is to react with the zinc and the contaminate water to produce the innocuous by-product Zn(OH) 2 and an amine. As will be recalled from the discussion hereinabove, the coating vehicle itself should contain a hydroxyl source such as an amine. Hence, the by-product RNH 2 of reaction 6a simply adds slightly to the quantity of amine present in the vehicle, actually enhancing operation of the coating by providing an additional amine source of hydroxyl ions.
While the amount of alkyl or aryl nitro-compound included in the vehicle is relatively non-critical, optimum results have been obtained when using from between 1 weight percent and 10 weight percent of nitro-compound as based on the weight of zinc dust present in the coating formulation. With less than about 1 weight percent, some gas evolution still was noted in the packaged coating. With greater than 10 percent concentration, the zinc dust showed a greater tendency to settle in the packaged formulation than when less than 10 percent of the nitro-compound was present. Excellent results have been achieved using between 3 and 4 weight percent of the nitro-compound based on the weight of the zinc in the formulation. In any event, the overall dielectric constant of the coating vehicle, including the alkyl or aryl nitro-compound, should be less than approximately 14 to insure that the zinc dust will not irreversibly settle when the packaged zinc rich coating is stored for extended periods of time. Further, as discussed above, the volume amount of zinc dust used in the coating should be from 30 to 200 percent of the volume of silicate employed.
The following Examples XVI and XVII set forth typical one packaged, zinc rich organic polysilicate coatings in accordance with the present invention, and incorporating an alkyl or aryl nitro-compound to inhibit gas evolution in the packaged formulation. Examples XVI and XVII are illustrative only of the type of formulations encompassed by the present invention; the invention should in no way be interpreted as being limited only to the following formulations.
EXAMPLE XVI Ingredient V E VE Zinc dust Ethyl Silicate 40 0.25 30 7.5 Diethanolamine 0.05 40 2 2-Nitropropane 0.05 25.5 1.3 Xylene 0.6 2.3 1.4 Isopropyl Alcohol 0.05 18 .9 E T =13.1
example xvii ingredient V E VE Zinc dust Condensed ethyl silicate 0.2 20.0 4.0 Diamylamine 0.1 4.0 .4 Nitrobenzene 0.1 35.7 3.6 Heptane 0.6 2.0 1.2 E T =9.2
note that 2-nitropropane has a weight of about 6 pounds per gallon. Thus, in Example XVI, the 2-nitropropane represents 0.3 pounds per gallon of vehicle. In this case, the preferred amount of zinc dust would be about 8.5 pounds per gallon of vehicle (assuming 3.5 weight percent of 2-nitropropane based on the weight of zinc dust).
Note in each of the preceding Examples XVI and XVII that the total vehicle dielectric constant E T is less than 14. Actual tests with these formulations have shown them to provide excellent one package zinc rich coatings having extremely long shelf life. Thus, when the packaged formulations were stored for periods of many months, no irreversible settling or packing of the zinc dust occurred. Moreover, no noticeable gas evolution took place, even when the storage was at elevated temperature. When the coatings were applied to a ferrous surface, atmospheric water reacted with the amines to provide hydroxyl ions, thereby catalyzing polycondensation of the ethyl silicate to form a touch, adherent binder for the zinc dust. Such coatings provided excellent long term protection against corrosion of the ferrous surface.
A second approach to control of the evolution of hydrogen gas in a one package, zinc rich organic silicate coating involved incorporation in the coating vehicle of a saturated hydrocarbon cyclic ketone (i.e., a "cycloketone"). Particularly useful were the following ketones: cyclohexanone having the formulation
; cylcopentanone having the following formulation
; cycloheptanone having the following formulation
; and bicyclodecane-1-one having the following formulation
In addition, propylene oxide having the following formulation
and ethylene oxide have been found to be effective in inhibiting gas evolution in coatings such as those described. The preferred range for such gas inhibiting ingredient is from about 1 percent to about 10 percent by volume of the liquid components of the coating.
The apparent reaction involved is typified in the following reaction of cyclohexanone with hydrogen:
Several non-cyclic ketones such as methyl ethyl ketone (MEK) were added to the coating vehicle to determine if they too would react with the evolving hydrogen. However, such non-cyclic ketones appeared to have little reactivity with the hydrogen, and hence were not effective in inhibiting gas evolution.
The cyclic ketone cyclohexanone may be characterized by the following representation:
Note that the oxygen ion appears to project outwardly away from the main portion of the structure; this oxygen ion can easily be contacted by free hydrogen. Conversely, in a non-cyclic ketone such as MEK, the other ions in the compound tend at least partially to encircle the oxygen ion, making it more difficult for a hydrogen ion to attach itself thereto. This description is consistent with the known property that cycloketones generally exhibit lower steric hinderance than do aliphatic ketones.
It has been found that when using a cycloketone to inhibit gas evolution in a one package, zinc rich organic polysilicate coating, at least one percent of the liquid composition should comprise a cycloketone. However, it is desirable that as little of the cycloketone as necessary to prevent gas evolution be used, because addition of a cycloketone tends somewhat to slow down the curing rate of the coating. This is so because the ketones have a tendency to react slightly with the amines, thereby reducing the basicity of the vehicle.
The following Example XVIII is illustrative of a one package, zinc rich coating in accordance with the present invention wherein a cycloketone (herein cyclohexanone) is used to prevent gas evolution in the packaged formulation.
EXAMPLE XVIII Ingredient V E VE Zinc dust Ethyl silicate 40 0.25 30 7.5 Diamylamine 0.1 0.1 0.4 Cyclohexanone 0.1 18.3 1.8 Heptane 0.55 2.0 1.1 E T =10.8
coatings in accordance with the formulation of Example XVIII were stored for long periods of time in closed containers. No evidence of gas evolution in the containers was noted. Moreover, although some zinc settling did occur after storage for several months, the zinc dust readily could be redispersed by stirring. When the coating was applied to a ferrous surface, the vehicle cured to form a tough, adherent binder for the zinc dust, the curing being only slightly slower than in formulations not employing a cycloketone.
As a third approach, experimentation has indicated that the addition of red lead (Pb 3 O 4 ) in quantities from between 5 to 14 percent by weight of zinc dust in the vehicle formulation significantly reduced or completely eliminated gas evolution in a one package, zinc rich organic silicate coating. The red lead, while imparting a red pigment or color to the applied coating, did not effect either the zinc settling characteristics in the packaged material, or the curing characteristics of the material when applied to a ferrous surface.
In some formulations an insoluble percipitate, presumably a lead silicate, did occur as a by-product of the reaction between the red lead and the evolving hydrogen. Incorporation of a surface active agent such as imidazole together with the red lead reduced the tendency of the insoluble percipitate to clump, thereby providing a commercially acceptable product.
In addition to red lead, other lead oxides similarly were found to inhibit gas evolution in a one package zinc rich organic silicate coating. Thus, any of the lead oxide compounds listed in the following TABLE III may be used in accordance with the present invention: ------------------------------------------------------------
--------------- TABLE III
Name Chemical Formula Pigment Color ____________________________________________________________
______________ Litharge PbO Light yellow Red Lead Pb 3 O 4 Red Lead Chromate PbCrO 4 Orange Basic Lead Chromate PbCrO 4 . PbO Yellow Lead Dichromate PbCr 2 O 7 Yellow ____________________________________________________________
______________
In each case, preferred concentrations of the lead oxide range from between 5 and 14 percent by weight of the zinc dust used. A lead oxide concentration on the order of 5 percent appeared to be optimum. Note that excessive lead oxide concentration tends to reduce the corrosion resistance provided by the zinc. On the other hand, too little lead oxide may be insufficient to prevent gas evolution in the packaged coating. With the lead oxide or chromate present in the preferred concentration, its presence may be neglected in calculating the vehicle dielectric constant, which dielectric constant should be less than about 14 to insure prevention of irreversible zinc dust settling.
The following Example XIX is typical of one package, zinc rich organic silicate coating formulations in accordance with the present invention wherein a lead oxide is sued to control gas evolution in the packaged product.
EXAMPLE XIX Ingredient V E VE Zinc dust Ethyl silicate 40 0.2 30.0 6.1 Diethanolamine 0.05 40.0 2.0 Triethanolamine 0.05 45.0 2.25 Xylene 0.7 2.3 E T =11.85 Bentone 34 (thixotropic agent) Red Lead Surface Active Agent
Other metal oxides were investigated to determine if they could be used in place of lead oxide or lead chromate in the present invention, however, satisfactory results generally were not obtained. For example, antimony oxide, titanium oxide and cuprous oxide all were found not to function effectively in preventing gas evolution. As another example, calcium oxide was tried, but was unsatisfactory since in the basic medium it tended to activate the zinc or to react with the ethyl silicate causing gellation thereof.
In review, a one package, zinc rich protective coating for ferrous surfaces is provided by combining zinc dust, an organic polysilicate, an amine or other hydroxyl source and a solvent, the volume average dielectric constant of the vehicle being below approximately 14. This vehicle characteristic insures that the zinc dust will not pack or settle into a hard mass, and will be readily redispersible even after prolonged storage in a closed container. By incorporating in the vehicle an ingredient selected from the class consisting of alkyl or aryl nitro-compounds cycloketones and lead oxide compounds, gas evolution in the packaged formulation, resulting from reaction of contaminant water with the zinc, may be eliminated. The resultant one package coating has very long shelf life, and when applied to a ferrous surface provides a zinc rich silicate coating which insures long term corrosion protection to the surface.
While the invention has been described in terms of various preferred embodiments and formulations, it is clearly to be understood that the foregoing is by way of example and illustration only, and that various changes and modifications may be made without departing from the spirit and scope of the invention, the invention being limited only by the terms of the appended claims.