Compositions: coating or plastic – Coating or plastic compositions – Corrosion inhibiting coating composition
Reexamination Certificate
1999-01-19
2001-01-23
Green, Anthony (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Corrosion inhibiting coating composition
C106S410000, C106S493000, C106S498000, C106S472000, C106S476000, C252S389100, C252S389200, C252S389520, C252S389530, C252S394000, C252S395000, C252S396000, C252S510000
Reexamination Certificate
active
06176907
ABSTRACT:
The invention relates to a lead- and chromate-free formulation for anti-corrosion coating materials.
BACKGROUND OF THE INVENTION
Metallic articles can be protected against corrosion by covering them with a metallic, inorganic or organic protective coat. The organic protective coats, in particular, are provided with specific pigments and/or fillers in order to increase their corrosion protection capability. These additives include red lead, zinc chromate, zinc phosphate, talc, graphite and mica. Alternatively, it is possible to employ organic compounds as anti-corrosion pigments, alone or in combination with inorganic pigments and fillers. Examples of such organic compounds are benzidine phosphate, benzidine molybdate, benzidine hexacyanoferrate, organic phosphonic and arsonic acids and both aromatic and aliphatic carboxylic acids and their salts, such as benzoates and laurates.
Owing to their toxic and/or carcinogenic properties, the highly effective lead pigments and chromate pigments can no longer be used for corrosion protection. The pigments used to date in their place, zinc phosphate and zinc tetraborate, are of only low activity in comparison. Zinc salts initially require substrate corrosion, as for example, in the case of iron:
Fe→Fe
2+
2e
−
½O
2
+H
2
O+2e
−
→2OH
−
and are then intended to form basic complexes of low solubility with the OH
−
ions formed. These complexes are intended to either adhere firmly to the substrate surface or to be precipitated into faults in an anti-corrosion primer and so plug these faults. A prerequisite for this is, firstly, that the corresponding zinc salt is present in an adequate pigment volume concentration and has not meanwhile been leached out owing to its solubility in water, and, secondly, that there are no other complexing species in the coating or in the adjacent corrosive medium. As a result, the zinc salt pigments frequently fail, and/or are markedly inferior in their action to the classical active pigments red lead and zinc chromate.
It is known from DD 281 427 that metal phthalocyanines show very good results as anti-corrosion pigments in coating materials on iron. They exert their effect both as pure pigment and in combination with a conductive carrier.
An improved formulation is the subject of EP 0 675 173, consisting of metal phthalocyanine, a conductive component, a component which binds hydroxide ions, and platelet-shaped pigments. A disadvantage of this combination is that the platelet-shaped pigment causes the coating material to be highly porous. The consequence is that the water which contaminates the coating has unhindered access to the metal surface, thereby accelerating the corrosion.
There is therefore a need for lead- and chromate-free pigment formulations which can be used for primer coats on corrosion-susceptible metals and which develop an anti-corrosion action which is equal to the protective effect of lead and chromate pigments.
SUMMARY OF THE INVENTION
The invention is based on the object of providing a formulation which can be incorporated into coating formulations based on customary binders and which, as a primer on a wide variety of metal substrates, especially on surfaces of ferrous materials, possesses anti-corrosion properties which are comparable with the protective effect of lead and chromate pigments. This pigment formulation is required to possess pronounced anti-corrosion properties not only under atmospheric exposure but also in aerated aqueous media.
This objective is achieved in accordance with the present invention by a formulation comprising
(i) 3-30% by mass of a monomeric and/or polymeric, metal-free or metal-containing chelate-forming compound of the general formula I or II,
in which
A and B independently at each occurrence are an aromatic or cycloaliphatic radical which may also contain heteroatoms such as S, Se, O and N and also aryl, alkyl or halo groups or oxygen-, nitrogen- or sulfur-containing groups as additional substituents,
R
1
, R
2
, R
3
and R
4
are H atoms or alkyl radicals, and
Me is Cu, Fe, Ni, Co, Mn, Bi, Sn, Zn or H
2
, (ii) 10-80% by mass of a material which binds hydroxide ions, and (iii)5-65% by mass of a conductive pigment based on carbon.
Chelate complex compounds (i) employed are the abovementioned compounds of the general formulae I and II, preferably phthalocyanines, tetraarylporphyrins and tetraazaannulenes. Among the phthalocyanines, preference is given to metal phthalocyanines and especially iron phthalocyanine. In the compounds I and II alkyl is preferably straight-chain or branched alkyl of 1-18 C atoms, in which one or more CH
2
groups can also be replaced by —CO—, —O—, —S—, —COO—, —O—CO— in such a way that no two O atoms are adjacent. Halogen is preferably bromine or chlorine.
The problem of high preparation costs caused by the metal phthalocyanine can be countered by applying this active component to conductive carrier materials, such as graphite, and thereby achieving an equal or even greater anti-corrosion effect with much less of the actual metal phthalocyanine active substance.
The chelate complex compounds are present in the formulation of the invention in a proportion of from 3 to 30% by mass, preferably from 15 to 25% by mass. The chelate complex compounds reduce oxygen, which, dissolved in water, penetrates by way of continuous pores and faults in the coating down to the metal substrate, and thereby passivate the exposed metal substrate. At the same time, the reduction of the oxygen produces hydroxide ions by the following equation:
O
2
+2H
2
O+4e
−
→4OH
−
These hydroxide ions are bound by component (ii) of the formulation of the invention.
As materials which bind hydroxide ions, (ii), it is preferred to employ phosphates, especially metaphosphates, bi- and triphosphates, silica gels, silicates, alumosilicates, calcite and all sparingly soluble metal salts which form sparingly soluble basic salts or complex compounds with OH
−
ions. Thus, for example, Ca[SiO
3
] takes up hydroxide ions to form Ca
3
(OH)
2
[Si
4
O
10
].
It is also possible to use those compounds which at their surface form a buffer system which holds the pH of the adjacent aqueous medium in the range 6<pH<8.5, which is considered unhazardous for the elamination of organic coatings on steel substrates:
R—COO—+H
2
O&rlarr2;R—COOH+OH—
Preference is given to the use of calcium metaphosphate, which binds the hydroxide ions formed when the oxygen is reduced.
The material that binds hydroxide ions is present in the formulation of the invention in a proportion of from 10 to 80% by mass, preferably 40 to 60% by mass.
In a further embodiment, the formulation of the invention additionally comprises from 5 to 65% by mass, preferably from 15 to 55% by mass, of a conductive pigment based on carbon. Either the conductive pigment consists of a carrier material which is covered with a conductive, carbon-containing coat, or the pigment is formed by the conductive material alone, such as, for example, in the case of carbon black, graphite or fluorine-doped graphite. Suitable carrier materials are mica, barium sulfate, glass flakes, silica or titanium dioxide.
Suitable conductive coats are carbon-containing metal oxide coats. The conductive pigments preferably have a particle size <30 &mgr;m, the particle morphology being unimportant. The conductive pigments are preferably platelet-shaped or spherical. In the formulation of the invention, the conductive pigment may also comprise a mixture of two or more different pigments.
The conductive pigment goes further towards ensuring the electronic conductivity of the formulation of the invention, which is required for the catalysed reduction of oxygen.
The binding of the hydroxide ions that are produced in the course of oxygen reduction prevents the delamination of the coating from the metal substrate, so that there is no subfilm corrosion (in the case of ferrous materials, underrusting).
The formulation of the invention i
Green Anthony
Merck Patent Gesellschaft mit beschrankter Haftung
Millen White Zelano & Branigan P.C.
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