Surface coatings prevent dirt from coming into contact with the object surface and protect it from abrasion during handling and dusting. The coating should therefore have appropriate resistance to abrasion and this is indicated by its hardness. Hardness expresses resistance to deformation and is a complex property. When assessed by indentation or penetration methods, it involves factors such as elastic modulus, yield strength, plasticity and rate of stressing. It is standard practice to define hardness only in terms of the methods of measurement or apparatus used, all of which measure slightly different things. A commonly used scale is Mohs hardness. This is a qualitative scale of unequal intervals based on increasing scratch resistance of minerals with talc equal to one and diamond equal to ten. It is too broad for coatings because all organic polymers rate below three on Mohs scale (they can be scratched with cal-cite). Pencil hardness covers a much smaller range than Mohs and is more useful to describe the hardness of coatings. It is obtained by using graphite pencils of increasing hardness (6B-9H) until a definite scratch is produced. Sward hardness can also be used. This measures the damping effect of a film on the movement of a rocker placed on the surface. Soft films damp the motion of the rocker more than hard ones.
The hardness of a coating is related to the viscosity grade of that material and hence to the average degree of polymerization. The viscosity grade of a material is usually taken as the viscosity of its solution at 20% solids concentration in toluene (Feller and Curran, 1985). Some idea of the range of hardness of different materials and of hardness within a polymer series can be obtained from the data in Table 4.2.
Hardness of a coating is related to the Tg of its polymer material. PVAC of viscosity grade 9 will have a Tg of about 17 °C whereas PVAC of viscosity grade 80 will have a Tg of about 24 °C. The practical lower limit to the hardness of a surface coating is that if it is too soft it will tend to pick up and imbibe dirt falling on the surface, thus defeating its purpose. This can be tested by coating some of the material on to a white surface and leaving it for a time in a dirty area at the expected service temperature. If the polymer has a Tg much below the service temperature it will resist attempts to clean it with water, detergent and a soft cloth and will remain dark grey with the collected dirt. This tends to be a problem with low viscosity grades of PVAC and with poly (n-butyl methacrylate). Shellac wears well and can be an excellent coating where hardness is desired. A balance of hardness for protection yet flexibility to accommodate movement in the substrate is desirable.
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