Organic protective coatings represent the most versatile and economical technology available for protection of metals against atmospheric corrosion. Corrosion inhibitive primer coatings are solid composites, comprising finely divided, usually multi-component, inorganic pigment phases dispersed in continuous organic polymer phases, which provide strong adherence to the protected metal substrates.
Although the physical characteristics of primers are reinforced by the dispersed inorganic pigment phases, they nevertheless remain permeable to O.sub.2, H.sub.2 O and air-borne pollutants. As a consequence, thin organic coatings do not prevent atmospheric corrosion of metals, unless they are specifically formulated with active corrosion inhibitor constituents, included in the pigment phase.
Contrary to appearances, a metal substrate protective primer, in equilibrium with its environment is a dynamic medium which accommodates several concurrently occurring physical and chemical processes, in which water, always present due to the resin phase's permeability, plays a critical role.
Water content affects the physical integrity of organic coatings in several ways. Most importantly, in this specific sense, it dissolves all soluble components (inclusive of pigments) to saturation concentrations and supports in situ diffusional transport processes of dissolved constituents. Water accumulates preferentially at metal-coating interfaces, causing loss of interfacial adhesion and more specifically supporting the electrochemical processes of metal corrosion. Paradoxically, in actively pigmented organic coatings water supports the inhibition of metal substrates' atmospheric corrosion, as well, by in situ solubilization of active pigments. Corrosion inhibitor pigment constituents of protective primers can be regarded as built in reservoirs of corrosion inhibitive species.
Commercially available chemically active corrosion inhibitor pigments used for Fe, Al, or Cu protection, belong exclusively to only a few classes of inorganic compounds, such as chromates, phosphates or polyphophates, molybdates, borates, silicates and phosphites of Zn, Ca, Sr, Ba, Al, Mg, Pb, Cr, Fe, or various combinations of these anionic and cationic species. Chromates and some of the latter cationic species, particularly Ba, Pb and Cr are known to be toxic. Transition metal derivatives of hydrogen cyanamide, particularly ZnNCN are also known for pigment grade application, limited, however, to special mirror backing coatings intended for Ag protection.
Generally, the active inhibitor species are the anionic constituents. Cations present, however, determine important physical properties of pigments such as solubility. Chromates, and specifically SrCrO.sub.4, are the standard of the industry, being the most versatile, applicable on all metal substrates such as Fe and specifically Al alloys and being highly effective, although toxic, pigment grade corrosion inhibitors. It is the redox activity of chromate species, accountable for their inhibitive efficiency which provides interference with corrosion processes in both, anodic and cathodic environments.
In contrast with chromates, all other active corrosion inhibitive pigments, more specifically, the anionic constituents of phosphates, molybdates, silicates, borates, phosphites and cyanamides are "redox inactive" under usual metal corrosion conditions. The consequences of "redox inactive" inhibitive mechanisms are significant. It can be generally stated that non-chromate based corrosion inhibitor pigments are less effective and qualitatively inferior, but non-toxic alternatives of chromates. As a consequence, for aircraft coatings used for aluminum protection and for coil coatings, where top corrosion inhibitive performances are required, SrCrO.sub.4 has no equally effective non-chromate alternatives. Development of effective and non-toxic alternative to chromates remains one of the major objectives of contemporary corrosion inhibitor chemistry.
As is well known, there is a large arsenal of organic corrosion inhibitors employed in industrial practice, limited, however, to gas phase and liquid medium applications. Logically, it would be expected, that the same arsenal would be appropriate for paint and coating applications as well, which, paradoxically, is not the case. The apparent contradiction is understandable by considering that in paint and coatings related applications, in addition to corrosion inhibitive efficiency, "pigment grade" qualities are required, as well.
Besides general quality requirements, "pigment grade" quality is defined by an additional, quite limiting set of parameters. Most important of these are non-volatility, solid consistency, specific gravity of 2.5-5.0, effective but limited solubility in water, virtual insolubility in organic solvents, absence of deleterious effects on coating's mechanical properties and, notably, no interference with curing processes. It will be apparent in this sense, that coatings related applications are not compatible with physical properties such as volatility, excessive solubility in water or organic media, which are however, critical requirements of gas phase or liquid medium related applications of organic corrosion inhibitors, respectively.
Various organic compounds with --SH functionality, such as thiols, derivatives of dithiocarbonic, dithiocarbamic and dithiophosphoric acids, are known to exhibit corrosion inhibitive activity. For example, diverse thio-organic compounds, such as N-containing heterocyclic mercapto derivatives, i.e, 2-mercaptobenzothiazole (MBT), are well known corrosion inhibitors employed practically exclusively in dissolved form, as a functional component of aqueous, polar organic or hydrocarbon based liquid systems. Typical applications of organic corrosion inhibitors in water, polar organic solvents or hydrocarbons include heat exchangers, anti-freeze systems, steam condensers or hydraulic oils, metal cutting liquids, and lubricants. Water soluble related Na or K salts (for example Na-MBT) or "thio" compounds in their more hydrocarbon soluble acidic form (such as MBT) are preferred in the former or the latter applications, respectively.
U.S. Pat. No. 4,329,381 shows the use of toxic Pb and Zn salts of selected five or six membered nitrogen-containing heterocyclic mercapto derivatives, notably Zn(MBT).sub.2, as corrosion inhibitor components of organic coatings, more specifically by incorporating such compounds as finely divided, distinct solid component phases into paint or coating systems, more specifically for protection of Fe.
A significant limitation of this concept is that technical grade Zn(MBT).sub.2, when produced according to the procedures described in the '381 patent contains high amounts of unreacted MBT. Thus, when formulated as a finely divided, distinct low specific gravity component in paint systems, these ingredients interfere with and inhibit the curing process of oil alkyd resin based coatings. This is a significant limitation, because it is estimated that about 60% of all primers intended for metal protection are oil alkyd resin based.
Two known procedures for synthesis of MBT derivatives and specifically of Zn(MBT).sub.2 are described in the '381 patent:
A. Conversion into Zn(MBT).sub.2 of aqueous mixed suspension containing ZnO (or alternatively, basic zinc carbonate) and MBT performed with extensive agitation and heating at approximately 100.degree. C., in the presence of catalytic amounts of acetic acid, according to: EQU xZnO+2xMBT.fwdarw.(1-x)Zn(MBT).sub.2 +2xMBT+xZnO+(1-x)H.sub.2 O(1)
B. By double decomposition or precipitation, using aqueous solutions of NaMBT and zinc salts according to: EQU 2NaMBT+Zn(X).sub.2 +H.sub.2 O.fwdarw.(1-x)Zn(MBT).sub.2 +2xMBT+xZn(OH).sub.2 +2NaX (2)
where x&gt;0, X=Cl(-), NO.sub.3 (-), etc.
As will be demonstrated in Comparative Example 1, neither procedure is suitable as disclosed, because of inability to yield reasonably pure Zn(MBT).sub.2 necessary for paint applications, but rather a mixture containing quite high amounts of unreacted MBT and ZnO is produced. Notably, this is true as to all commercially available technical grade Zn(MBT).sub.2 of diverse origin. For example, the commercial product offered by the Bayer Corporation under the trade name Vulkacit ZM contains in excess of 13% of free MBT.
It is important to observe that unreacted MBT content in technical grade Zn(MBT).sub.2 has significant adverse consequences with respect to suitability of such products in paint-coating applications. Specifically, if added as a finely divided distinct solid component into paint systems, technical grade Zn(MBT).sub.2 inhibits the curing process of oil alkyd resin based coatings. Conversely, as it was also discovered that Zn(MBT).sub.2 purified by solvent extraction does not display any cure inhibitive activity even at considerably higher concentrations in medium oil alkyds. (See Comparative Example 1).
As for the chemical mechanism which causes the curing inhibition, free MBT reacting with Co(II) or Pb(II) species (the active constituents of driers typically used in oil alkyd paint formulations), is hypothesized. Consequently, with respect to interference with curing processes, similar behavior should be considered typical for all thio-organic compound families with --SH functional groups, as well as for related zinc salts of technical grade.
Quantitative determination of unreacted MBT content of Zn(MBT).sub.2 can be conveniently carried out gravimetrically, by repeated extraction in acetone, or it can be estimated by IR spectroscopy. Intense absorption bands situated at 1496 and 1425 cm(-1) of the related spectrum, are characteristic for MBT. (See Comparative Example 1)
An additional limitation of the concept of the '381 patent relates to the fact that the specific gravity of Zn(MBT).sub.2 (1.5-1.7), and generally that of other mercapto derivatives, is quite low in comparison with such values typical for other components of a paint system's dispersed inorganic phase, which range from 2.5 to 5.0, or with density values of cured coatings of about 2.0. Notably, also, the zinc and lead salts of mercapto derivatives form ordinary mixtures with other components of the dispersed pigment phase.
As is known, shelf-stable paint systems' dispersed solid phases, usually multi-component ordinary mixtures of the constituents of the same, tend to segregate by "flooding", if they contain components with appreciably different specific gravities. Due to convective processes related to solvent evaporation, flooding occurs during the curing of freshly applied coatings, resulting in predominant accumulation, at the coating-air interface, of the dispersed solid phase's low specific gravity components and ultimately, in an anisotropic composition, and consequently, a reduced protective capacity of the resultant coatings. These phenomena are well known with respect to paint applications colored by organic pigments.
Since low specific gravity values are typical for zinc salts formed by relatively "bulky" organic moieties, such as Zn(MBT).sub.2, segregation by flooding has relevance to the application of such compounds as paint additives and constitutes a considerable shortcoming thereof.
There are essentially three different structural states in which two (or more) microcrystalline or amorphous component phases of distinctively different chemical composition can co-exist as constituents of multi-phase and finely divided solid systems: ordinary physical mixtures, micro-composites and solid solutions.
Solid solutions, although formed spontaneously, are not commonly found in multi-phased solid systems. In some cases, however, distinct combinations of three or more anionic and cationic constituents, which ordinarily form two or more solid phases of distinct chemical composition, in special conditions form unified solid phases of complex chemical composition. Such unified phases are characterized by uniform distribution at molecular level of all constituent ionic species.
Ordinary mixtures of finely divided (and normally polydispersed) solid phases of different chemical composition are constituted of distinct and separable microparticles of the distinct component phases, mixed and uniformly distributed in the system. Ordinary mixtures can be prepared by simple mechanical procedures. However, they are often formed spontaneously in chemical processes as well, such as during concurrent or subsequent formation by precipitation of two or more solid phases in aqueous systems.
Conversely, micro-composite (multi-phase and finely divided) solid systems are constituted of microparticles, containing distinctly identifiable, but physically inseparable component phases of different chemical composition which form common interfaces and are held together by chemical forces. In some cases composite microparticles possess a structural configuration of a coating-core type, often formed in some heterogenous chemical processes, such as described in my U.S. Pat. No. 5,176,894, wherein a finely divided suspension of an essentially insoluble solid reactant is reacted with a dissolved component reactant of a liquid phase and consequently is converted into a finely divided suspension of a solid reaction product, which is insoluble in the reaction medium. During the course of the reaction the suspended solid phase consists of composite micro-particles of a coating-core configuration, in which both solid component phases, i.e., the reaction product coating and the reactant core are simultaneously present, separated and bound together by an interface which in many cases is of a generally spherical shape, but also may consist of other configurations such as lamina, etc. This mechanism, obviously, implies the reagent species' continuous diffusion through the coating phase toward the reactant core and generally, the reaction is diffusionally impeded. Thus, by preventing completion of the reaction, the solid product's micro-composite structure is preserved.
Notably, there are also numerous examples known to the art regarding preparation of microcomposite multi-phase solid systems by, in comparison, a quite different process and mechanism. Known as "particle encapsulation" procedures, widely used in the pigment manufacturing industry, are the formation by precipitation of chemically inert coatings of diverse chemical composition on the surfaces of finely divided solids in aqueous suspension.
The microcomposite structures of multi-phase solids prepared in aqueous processes, commonly are preserved throughout typical pigment manufacturing operations, which include filtration, dehydration and grinding. Pertinent experimental data are presented in Comparative Examples 2.1 and 2.2.
Another significant limitation of the concept of patent '894 is, that zinc salts of thio-compounds, such as Zn(MBT).sub.2, in ordinary mixtures with zinc phosphate, the inhibitor pigment specified in the patent, do not form synergistic systems and do not display synergistically enhanced corrosion inhibition performance.
In view of all of these considerations, a need has existed for improved corrosion inhibitor systems.