Coating processes – Applying superposed diverse coating or coating a coated base – Synthetic resin coating
Reexamination Certificate
1997-07-14
2001-01-16
Sellers, Robert E. L. (Department: 1712)
Coating processes
Applying superposed diverse coating or coating a coated base
Synthetic resin coating
C138S146000, C427S386000, C428S035900, C428S416000, C523S466000, C525S481000
Reexamination Certificate
active
06174569
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to three-layer metal pipe coating compositions based on a powder primer containing epoxy resins and phenolic crosslinking agents, on a thermoplastic hard adhesive and on a polyolefin cladding. Furthermore, the invention relates to a process for the exterior coating of metal pipes by a three-layer method.
BACKGROUND OF THE INVENTION
Metal pipes are used worldwide for the transportation of, for example, petroleum, petroleum products, gas, water and solids from the place where they are found or produced to the consumer. Metal pipes must be protected on the one hand against corrosion and on the other hand against mechanical defects, in order to ensure the operational reliability of the lines over a long period. Exterior coatings of metal pipes must possess a high impact strength, compressive strength and abrasion resistance, and it is necessary to prevent damage to the exterior coatings during the transportation, storage, laying and operation of the metal pipes. In addition, exterior coating compositions for metal pipes should have a good aging resistance, good weathering resistance during the interim storage of the pipes, an outstanding resistance to external influences, good dielectric properties and a low permeability to water vapor and to gases.
One-coat systems based on epoxy resin are known for the exterior coating of metal pipes, but because of the low mechanical strength of the epoxy resin coatings these systems are disadvantageous when subjected to impact stress.
It is also known to coat the exterior of metal pipes using a three-layer method which involves first applying a powder primer based on epoxy resin to the metal substrate, applying a hot melt adhesive to the powder primer, and subsequently applying a polyolefin cladding. Three-layer coatings of this kind for metal pipes are described in, for example, EP-A-57 823, GB 1 542 333, DE-A-22 22 911, DE-A-22 57 135, DE-A-19 65 802, DE-A-29 44 809 and DE-A-32 30 955. These three-coat systems comprising epoxy resin primer, thermoplastic hard adhesive and polyolefin cladding material go a long way to fulfilling many of the requirements. They are markedly superior to the one-coat systems. The three-coat systems combine the positive properties of the thermosetting primer layer (good adhesion to steel) with the thermoplastic polyolefin cladding (toughness, long-term resistance, good dielectric properties, low gas permeability).
The use of powder coatings based on epoxy resins, dicyandiamide curing agents and crystalline or amorphous silicic acids as powder primer in the three-layer exterior coating compositions for metal pipes is known. Also known are powder primers comprising epoxy resins, phenolic curing agents and amorphous silicic acid fillers.
However, the disadvantages of the three-layer systems known up to now consist in an inadequate peel strength after storage in hot water, in which partial detachment of the powder coating from the substrate occurs during peeling after storage in hot water. Furthermore, the properties with regard to submigration during cathodic delamination are a continual target for improvement.
The object of the present invention was therefore to provide multi-layer coatings for the coating of metal pipes, which coatings do not have the disadvantages described above. The intention was thus to increase, in particular, the peel resistance of the coatings after storage in hot water, and also to obtain excellent results in respect of cathodic delamination. The coatings were to be firmly adhering and resistant and were to provide a high degree of protection against corrosion.
SUMMARY OF THE INVENTION
The object of the present invention is achieved by three-layer metal pipe coating compositions based on a powder primer containing epoxy resins and phenolic crosslinking agents, on a thermoplastic hard adhesive and on a polyolefin cladding. The metal pipe coating compositions are characterized in that the powder primer contains epoxidized novolak resins, phenolic crosslinking agents and fillers based on crystalline silicic acid modifications.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The epoxy resin primer, which is in powder form, contains epoxy novolak resins. In these novolak resins, the phenolic hydroxyl groups are etherified with alkyl or aryl groups or the like. Epoxide groups are incorporated into the molecule by reacting the phenolic hydroxyl groups with epichlorohydrin. The so-called epoxy novolak is formed on the basis of novolaks. The epoxidized novolaks are related in structure to bisphenol A resins. The basic model for the epoxidized novolaks is the diglycidyl ether of bisphenol F. Epoxidized novolaks can be prepared by epoxidization of novolaks which are composed, for example, of from 3 to 4 phenolic rings interconnected via methylene bridges. Other novolak resins which can be used are alkyl-substituted phenols which are reacted with formaldehyde. Epoxidized novolak resins have a higher functionality than bisphenol A resins. It is preferred to use epoxidized novolak resins having an average functionality in the range from 2.4 to 2.8 and an epoxide equivalent weight in the range from 600 to 850.
Suitable epoxidized novolak resins are commercially available, for example, under the designation DER 672U (Manufacturer: Dow Chemicals), Epikote 2014 (Manufacturer: Shell) and GT 7220 and GT 7255 (Manufacturer: Ciba Geigy).
In order to cure the epoxidized novolak resins, the powder primer contains phenolic crosslinking agents. These may, for example, be any phenolic resin, provided it has the methylol functionality which is necessary for reactivity. Preferred phenolic resins are reaction products of phenol, substituted phenols and bisphenol A with formaldehyde, which products are prepared under alkaline conditions. Under such conditions the methylol group links to the aromatic ring in either the ortho or para position. The present invention gives particular preference to employing, as phenolic crosslinking agents, hydroxyl group-containing bisphenol A or bisphenol F resins having a hydroxy equivalent weight in the range from 180 to 600, particularly preferably in the range from 180 to 300. Such phenolic curing agents are prepared by reacting bisphenol A or bisphenol F with components which contain glycidyl groups, for example the diglycidyl ether of bisphenol A. Phenolic crosslinking agents of this kind are available, for example, under the trade name DEH 81, DEH 82 and DEH 87 from Dow, DX 171 from Shell and XB 3082 from Ciba Geigy.
The epoxy resin primer contains one or more catalysts which are suitable for the epoxy resin/phenolic resin curing. Suitable catalysts are phosphonium salts of organic or inorganic acids, imidazoles and imidazole derivatives, quaternary ammonium compounds and amines. The catalysts are generally employed in proportions of from 0.001% by weight to about 10% by weight, based on the overall weight of the epoxy resin and the phenolic crosslinking agent.
Examples of suitable phosphonium salt catalysts are ethyltriphenylphosphonium iodide, ethyltriphenylphosphonium chloride, ethyltriphenylphosphonium thiocyanate, ethyltriphenylphosphonium acetate/acetic acid complex, tetrabutylphosphonium iodide, tetrabutylphosphonium bromide and tetrabutylphosphonium acetate/acetic acid complex. These and other suitable phosphonium catalysts are described in, for example, U.S. Pat. Nos. 3,477,990 and 3,341,580.
Examples of suitable imidazole catalysts are 2-styrylimidazole, 1-benzyl-2-methylimidazole, 2-methyl-imidazole, 2-butylimidazole. These and other imidazole catalysts are described in, for example, Belgian Patent No. 756,693.
Some commercially available phenolic curing agents already contain catalysts for the epoxy resin/phenolic resin crosslinking.
The powder primer contains crystalline silicic acid modifications as filler. These modifications are conventionally employed in a proportion of from 10 to 50% by weight, based on the overall weight of the powder primer. In some cases, it is also possible to have proportions of more than 50% by w
Blomer Werner
Rademacher Josef
Reiter Udo
BASF Coatings AG
Sellers Robert E. L.
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