Compositions: coating or plastic – Coating or plastic compositions – Corrosion inhibiting coating composition
Reexamination Certificate
2000-05-26
2002-10-22
Green, Anthony J. (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Corrosion inhibiting coating composition
C106S014120, C106S286800, C106S287100, C106S600000
Reexamination Certificate
active
06468336
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to water-borne zinc silicate shop primer compositions including a suitable amount of zinc, micaceous iron oxide and optionally a second filler.
BACKGROUND OF THE INVENTION
In marine and industrial construction, it is usually desirable to pre-paint steel with a zinc-containing primer before fabrication, and many such coating compositions known as shop primers or pre-construction primers are known, see e.g. Maltanti, Protective Coatings Europe, Jun. 16-22, 1998.
Shop primers, or preconstruction primers, are paints intended for short-term protection of steel during fabrication and before application of the full paint system. It must protect steel from rusting in usually aggressive environments and provide a sound surface for subsequent coats.
Many of such compositions include zinc powder in an organic resin such as an epoxy resin, an epoxy ester, a polyurethane, a polystyrene resin or a silicone resin. Coatings based on these organic binders are not well suited for coating steel that must ultimately be welded because the binder tends to decompose from the heat of the weld, resulting in pores in the weld seams.
In view of the compositions based on an organic resin, compositions based on a silicate-based binder system have been developed. Such compositions can be divided into two separate classes, namely water-borne or solvent-borne compositions.
One early example of an alternative to coatings based on organic binders was a zinc-containing coating based on inorganic binders, Schutt (U.S. Pat. No. 3,620,784). The coating composition contains zinc powder carried in a silicate vehicle. The particular formulation disclosed by Schutt is said to avoid many of the problems commonly encountered with inorganic coatings: weather wear, cracking, crazing and non-uniform adherence before and after baking. However, even if one surmounts the problems associated with the physical characteristics of the applied silicate coating, as Schutt claims to have done, one is still faced with the problem of welding steel that has been coated with zinc powder primer in a silicate vehicle. During welding, the vaporised substances can penetrate the weld root and become entrapped, creating a porous weld. It is sometimes possible to partially overcome the porosity problem by reducing the welding speed sufficiently to allow the gas to escape ahead of the weld. Even when this technique is successful, however, it results in increased fabrication costs due to reduced welding speed.
Among the solvent-borne compositions can be mentioned WO 88/06177 which discloses shop primer composition comprising zinc powder, fillers and pigments including a certain proportion of conductive fillers and pigments, anti-settling agents, optionally thickening agents, a silicate-type binder, and a solvent; and Zimmermann, Eur. Coatings. J, 1991, 1, 14-19, which discloses solvent-borne compositions in which a portion; of zinc has been replaced by fillers, e.g. micaceous iron oxide.
The disadvantage of using solvent-borne compositions are the environmental problems with handling of the composition and disposal of solvents evaporating from the object coated with the composition. Thus, water-borne compositions which differentiates from the solvent-bases systems have been developed.
Among the known water-borne compositions, it is known that a portion of zinc in zinc-containing primers can be replaced by iron phosphide and/or nickel phosphide of suitable particle size. The resulting zinc-coated steel, while retaining its anti-corrosive properties, can be more readily and efficiently welded.
As an example of this approach, Makishima et al. (U.S. Pat. No. 4,011,088) have proposed an anti-corrosive coating composition comprising 5 to 80% of a binder, which is either potassium silicate or ammonium silicate, and 20 to 95% of a pigment mixture of zinc powder and iron phosphide and/or nickel phosphide. It is stated that the ratio between zinc powder and phosphide should be in the range of 8:2 to 2:8 as “if the proportion of zinc is smaller than this range, the electrochemical anti-corrosive effect by zinc powder is insufficient, and if the proportion of phosphide is smaller than the above range, the weldability of the resulting coating is reduced.” In the embodiment in which potassium silicate is used as the binder, Makishima et al. indicate that the ratio of silicate to potassium should be between 2.5 and 4.0. If the molar ratio is smaller than 2.5, the film forming property is said to be insufficient and if the molar ratio is larger than 4.0, the stability of the binder is said to be reduced. In the examples, the average particle size of the zinc powder and the phosphide is generally 5 &mgr;m. Inclusion of other fillers is not suggested. Furthermore, it is described by Makishima et al. that useful properties are only found for the combination of the phosphide fillers and inorganic silicate binders, i.e. water soluble silicate binders.
Falberg (U.S. Pat. No. 5,580,371) has proposed yet another example of a zinc-containing primer that provides a durable, corrosion-resistant coating, combined with suitable weldability. The primer comprises zinc, iron phosphide and an aqueous potassium silicate solution wherein the ratio of silicate to potassium is between 4.1 and 6.0. It appears to be crucial that the amount of zinc in the pigment/filler part of the composition is 35-90% by weight and that the amount of iron phosphide in the pigment/filler part is 10-65% by weight. It is stated that the most preferred particle size for zinc as well as for iron phosphide is in the range of 3-8 &mgr;m. It is stated that “other pigments and fillers may be added to the composition of the invention as long as the percentages of zinc and iron phosphide are maintained within the stated range.” It should be noted that iron phosphide is obtained from limited natural sources, thus it can be envisaged that the potentially increasing prices for iron phosphide will be prohibitive for it's use in industrial products.
WO 98/58028 describes water-borne protective coatings (i.a. preconstruction primers) comprising zinc dust, Group IA metal silicates, colloidal silica and a carbonate-containing internal hardener. It is mentioned that the composition may comprise pigments and fillers (among which micaceous iron oxide is mentioned as an example). The role of the pigment and fillers is apparently only to make a film of the coating non-transparent as it is stated that “[t]he amount of pigment and filler that is used to form the composition is understood to vary, depending on the particular composition application, and can be zero when a clear composition is desired”. This is confirmed by the examples where no filler is used in the example illustrating a pre-construction primer.
Although certain technologies have been suggested within the field of water-borne zinc silicate shop primers, Maltanti (Protective Coatings Europe, June 1998, p 16-22) states that “In spite of paint manufacturers' efforts to develop and formulate [water-borne coatings], the goal seems to be far away. A lot of work remains in research and testing to produce a suitable and effective water-borne shop primer”.
Thus, In view of the above there is a need for alternative approaches for water-borne zinc silicate shop primers as metal phosphide filler types are rather expensive and sometime not fully efficient as predominant constituents of the pigment mixture of water-borne zinc silicate shop primers.
SUMMARY OF THE INVENTION
The applicants have found that water-borne zinc silicate shop primers comprising zinc, a well-defined amount of micaceous iron oxide, and optionally a portion of an additional filler are cost-efficient and provide excellent properties with respect to weldability, corrosion resistance for a sufficient period of time, etc.
The present invention thus provides a water-borne shop primer composition comprising:
(a) 5-40% by volume of a pigment mixture,
(b) 1.4-10% by volume of one or more water soluble silicates (based on the dry volu
Fiedler Helle
Hansen Ulla Kirsten Neestrup
Jensen Dorte Jeanette
Musaeus Gruska Elisabeth Folkmann
Birch & Stewart Kolasch & Birch, LLP
Green Anthony J.
J.C. Hempel's Skibsfarve-Fabrik A/S
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