Adhesion immersion coatings

Immersion coating is a classic process for applying zinc onto steel. The object made of steel is briefly immersed into a bath of liquid zinc (boiling point 420 °C) and a zinc deposit forms whose thickness depends on the immersion time. At the Fe-Zn interface, a interdiffusion zone is created that ensures good adhesion. Other metals with low melting points, in particular aluminum, can be deposited in the same fashion. 

The presence of a sharply defined interface between a polymer solution and a solid wall leads to important modifications in the local polymer concentration with respect to the bulk concentration. These variations can be positive or negative depending on the sign of the interaction between the solid wall and the macromolecular chains immersed in the solvent. Attractive forces lead to adsorbed layers while repulsive forces lead to depletion layers. Due to their connection with important technological applications such as adhesives, protective coatings, microlithography, emulsion stabilizers, adsorbed polymer layers have been the subject of extensive theoretical studies. ... 

When plating any substrate less noble than copper, only a few mg/L of dissolved copper in the acid baths can adversely affect adhesion. Coatings can be too thin to be visible, yet contribute to poor adhesion. Small additions of thiourea have been used to prevent copper immersion, but it acts as a potent inhibitor, and work should be re-electrocleaned after the acid. Work should be exposed to the mildest acid treatments possible. Over-etching should be avoided. 

Knife test (KNF) The test is done by making two intersecting scratches through the paint film to the substrate with a sharp steel knife. Adhesive or cohesive failures are evaluated by peeling the coating from the intersection point and outwards. Common for the three adhesion evaluation methods are that the test is performed on immersed and non- immersed panel-half (referred to as respectively "wet" and "dry" adhesion). The type of rupture is reported, and the severity is judged on a scale from 5 (perfect) to 0 (poor). 

Organic coatings are commonly evaluated using salt water immersion, salt fog or spray, modified salt exposure tests (e.g., salt fog with added SO2), and various cyclic exposure tests. Humidity exposure and water immersion, and, for many applications, physical resistance tests (adhesion, impact resistance, etc.) are widely used preliminary tests. Standard methods for most of these tests are given in compilations of standard tests such as the Annual Book of ASTM Standards (16). Test methods have been extensively reviewed (e.g., 17-23). 

When a ferrous alloy is immersed in phosphoric acid, il initially forms a soluble phosphate. As the pH rises at the mclal/solutiun interface, the phosphate becomes insoluble and crystallizes epitaxially on Ihe substrate metal. The phosphate coating thus produced consists of a nonconduciivc layer nf crysinlx that insulates the metal from any subsequently applied film and provides a topography with enhanced tooth" for increased adhesion. The cry stals insulate microanode and microcathode centers caused by stress or imperfections in the metal surface. This greatly reduces Ihe severity of electrochemical corrosion. 

To determine the wet bond strength coated panels were immersed in distilled water for 1500 h, removed and discs 25.4 mm in diameter stamped from them. The surfaces were wiped with a dry tissue and bonded between two cylindrical test pieces using a polyamide cured epoxide adhesive and immediately placed in a sealed container at 100% RH for the adhesive to cure. After 16 h the specimens were broken on an Instron Universal Test Machine with minimum delay. Recovered values were measured after the panels had dried out at room temperature and humidity for 7 days. Clearly, it is unlikely that the values reported represent the minimum bond strengths, as some drying out is almost inevitable, but the values are directly comparable. 

Corrosion performance was evaluated by the scab corrosion test. The coated panels were scribed and subjected to 25 cycles as follows 15 min immersion in 5% NaCl solution, 75 min air-dry at room temperature, followed by 22.5 h exposure to 85% relative humidity (RH) and 60°C environment. The tested samples were examined visually for failure such as corrosion, him lifting, peeling, adhesion loss, or blistering. The distance between the scribe line and the unaffected coating was measured as the corrosion creepage. 

Metal specimens coated with the various blends were immersed in water in an accelerated test to determine the effect of high humidity. Cellulose acetate butyrate blends containing carboxylated polyesters prepared with hexahydroterephthalic acid [H(NPG), T50H(NPG)] were particularly susceptible to moisture and failed the adhesion test after immersion for only 0.5 hour similar blends containing T50I(NPG) extended with dianhydride Via or with PMDA passed the adhesion test after immersion for 16 hours. When coatings on cold-rolled steel of cellulose acetate butyrate (EAB-381-0.5) blends containing 1% of each of the... 

Other significant uses of PCBs included heat exchangers and hydraulic fluids. Prior to controls PCBs were also used in adhesives, coatings, plasticizers and inks for microencapsulating dyes for carbonless duplicating paper as extenders in pesticide formulations and catalyst carriers in olefin polymerizations to impart hydrophobicity to materials and surfaces in bactericide formulations (combined with insecticides), and in immersion oil for microscopes. Mixed with chloronaphthalenes, PCBs were also used in wire and cable insulation in the mine and shipbuilding industries (ref. 80, p. 455). 

Standard tests of water immersion, humidity resistance, corrosion, flexibility, adhesion, and weight are conducted on other specimens of the same coating system. 

Phosphate conversion coatings are formed on metal surfaces by immersion in aqueous phosphate solutions. These coatings are primarily intended to promote the adhesion of subsequently applied paints. Phosphate coatings typically consist of a compact mass of hydrated metal phosphate crystallites Mi(P04)2 H20. They differ from CCCs in that they are crystalline, do not attempt to provide... 

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