The object of the invention is polyamide powders for the coating of metals.
The state of the prior art of producing polyamide powders may be ascertained by reference to U.S. Pat. No. 2,698,966; 2,742,440; and 2,975,128 of Louis L. Stott; U.S. Pat. No. 3,203,822 of Junker et al; U.S. Pat. No. 3,410,832 of Griehletol; 3,299,009 of Stephen D. Bruck; and the Kirk-Othmer "Encyclopedia of Chemical Technology," 2nd Ed., Vol. 16 (1968), under the section "Polyamide (Plastics)", pages 88-105, particularly page 92 - polylauryllactam (nylon-12), and polyundecanamide (nylon-11), page 101 Solution Processes, and Powder Processing, pages 101-102, the disclosures of which are incorporated herein.
U.S. Pat. No. 2,698,966 discloses physical mixtures of different types of nylon powders produced by dissolving the nylon in organic solvents and precipitating the powdered particles. In Example 14 of U.S. Pat. No. 2,742,440, the solution of epsilon caprolactam in alcohol-water solution and its precipitation as a powder is disclosed. The dispersion of Teflon powder in a solution of epsilon caprolactam and the coprecipitation of the powders is disclosed in U.S. Pat. No. 2,975,128.
The flame spraying and fluidized bed coating of nylon on a metal base is disclosed in U.S. Pat. No. 3,203,822. U.S. Pat. No. 3,299,009 discloses the n-methoxymethylation of nylons, and U.S. Pat. No. 3,410,832 discloses the preparation of polymers and copolymers of lauryllactam.
It is known that one can use polyamide powder for the production of lacquer-like coatings of metals. The coating process may be carried out by fluidized bed, by flame spraying, or by electrostatic deposition. The polyamide powder is produced by precipitation of the powdered polyamide from solution (German Application laid open to the public S 28 05 39b22/04) or by grinding polyamide granules. Polylauryllactam can also be produced in the same manner and can be used in the same coating methods. Coatings produced by polylauryllactam exhibit serious faults, especially when relatively thick walled, sharp edged articles are coated, because of bulges of the coating on both sides of the sharp edges, and at the same time the edges themselves are insufficiently coated or not coated at all.
There is also a method known for the production of polylauryllactam in which the resin is precipitated from a solution of the same, together with 0.1 - 5 percent by weight with respect to the amount of polylauryllactam of a known plasticizer hydroxyl group containing compound and/or phosphoric acid ester. This powder produces substantially improved coatings on metals except that the edge coatings of thick metal articles are not entirely satisfactory. See German Patent (DT-AS) 1,669,821.
It is further known to include additives to polyamide powders such as monomers or oligomers containing isocyanate groups which react with the resin in the molten state to improve coating adherency. These additives do not, however, lead to adequate coatings for sharp edges or edges having a very small radius of curvature. See German Pat. (DT-AS 1,271,286).
It has been suggested to coat metals at high temperature using a copolyamide consisting of 30 - 90 percent by weight polylauryllactam and 70-10 percent by weight of a copolyamide of lauryllactam. This powder permits better edge covering of metal parts but must be employed in a narrow temperature range. See Patent Application Ser. No. 283,966, filed Aug. 28, 1972.