Coatings continued soft-film

In practice there is a continuous range of coating hardnesses varying from very soft to very hard, and the hardness is determined not only by the binder but by the concentrations of molybdenum disulphide and other solid components. The definition of hardness is also relative to the service contact loads. A coating of intermediate hardness may behave like a hard film under low contact pressures or like a soft film under high contact pressures. As a result, it is very difficult to compare the quoted load-carrying capacity and wear lives of different films tested under different conditions by different research groups. 

Emulsion polymerization also has the advantages of good heat transfer and low viscosity, which follow from the presence of the aqueous phase. The resulting aqueous dispersion of polymer is called a latex. The polymer can be subsequently separated from the aqueous portion of the latex or the latter can be used directly in eventual appUcations. For example, in coatings applications-such as paints, paper coatings, floor pohshes-soft polymer particles coalesce into a continuous film with the evaporation of water after the latex has been applied to the substrate. 

Polypropylenes are available as molding powder, extruded sheet, cast film, textile staple, and continuous-filament yarn. They find use in packaging film molded parts for automobiles, appliances, and housewares wire and cable coating food container closures bottles, printing plates carpet and upholstery fibers storage battery cases crates for soft-drink bottles laboratory ware trays fish nets surgical casts and a variety of other applications. 

When the sucrose esters were evaluated as modifiers for cellulose acetate butyrate, the crystal-producing compounds gave brittle and sometimes hazy or opaque films as they continued to crystallize, even in the presence of the cellulose polymer (Table II). On the other hand, the liquid esters performed as plasticizers for cellulose acetate butyrate, producing soft and tacky films at 50% modification. Because of this softening action, sucrose esters, forming low-viscosity liquids, were eliminated from commercial consideration. It also was estimated that these esters could not compete costwise with the commonly used, coatings plasticizers. 

Erosion-corrosion arises from the eombined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion. Virtually all metal alloys, to one degree or another, are susceptible to erosion-corrosion. It is especially harmful to alloys that passivate by forming a protective surface film the abrasive action may erode away the film, leaving exposed a bare metal surface. If the coating is not capable of continuously and rapidly re-forming as a protective barrier, corrosion may be severe. Relatively soft metals such as copper and lead are also sensitive to this form of attack. Usually it can be identified by surface grooves and waves having contoms that are characteristic of the flow of the fluid. 

The surface chemistry, morphology, and mechanical properties may be important to the adhesion, film formation process, and the resulting film properties. The underlying bulk material can be important for the performance of the surface. For example, a hard coating on a soft substrate may not function well if, under load, it is fractured by the deformation of the underlying substrate. The bulk material can also influence the surface preparation and the deposition process by the continual outgassing and outdiffusion of internal constituents. 

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