Page 166 - Handbook of Surface Improvement and Modification

10.3 Application data 161

10.3 APPLICATION DATA

Compositions and coatings with reduced ice adherence and accumulation may include
film leveling agents to reduce film defects such as crawling, fish-eyes, cratering and the
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like. Examples of suitable film leveling agents include dimethyl cyclohexyl phthalate,
dibutyl sebacate, aqueous dispersions of oleochemical compounds, and polyethyle-
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neimine.
It has been observed that the leveling speed of the liquid film is dependent on the
thickness of the film, since the solid substrate surface creates viscous drag against the lev-
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eling flow of the liquid film. Molecules of the liquid film that are closer to the solid sub-
strate experience a greater viscous drag, thus it is especially difficult for thin films to level
themselves, and the leveling speed slows down exponentially as the film thickness
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decreases. As the film leveling speed becomes very slow, the film cannot level in a prac-
tical range of time before the viscosity of the liquid film increases (during the drying pro-
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cess) to such an extent that further leveling is inhibited. The drying process may cause
the liquid film to become solid before suitable leveling has been achieved, resulting in
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non-uniform films.
A composition for protecting a surface of an inorganic substrate, such as concrete,
terrazzo, or ceramic tile, includes a silicate (i.e., an alkali metal polysilicate or a colloidal
silica), a siliconate (e.g., a metal siliconate, such as an alkali metal methyl siliconate, etc.),
acrylic latex, a silane coupling agent, leveling agent (tris(2-butoxyethyl) phosphate having
the chemical formula C18H39O7P), and a solvent, such as ethylene glycol monobutyl
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ether.
Flow and leveling have a significant influence on the performance and appearance
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characteristics of a coating. For example, an inadequate flow of a coating leads to
defects such as craters and pinholes as a result of the coating not completely covering the
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substrate. Incomplete coverage of the substrate as a result of inadequate flow also leaves
parts of the substrate exposed and prone to corrosion, thereby affecting performance. 19
The flow and leveling of an applied powder coating are governed by viscosity and
surface tension, with the appearance of a coating determined largely by viscosity and by
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the time available for leveling. The viscosity of an uncured powder compositions
changes during cure. Initially, a powder composition will melt and flow over the substrate
as the temperature increases beyond the glass transition temperature of the composition
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and until the composition reaches its minimum viscosity. When the composition begins
to cure, the viscosity will increase and, therefore, the ability of the powder to flow and
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cover the substrate surface will decrease. The viscosity of a composition relates to its
reactivity − a highly reactive composition may never reach its minimum viscosity and,
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therefore, never demonstrate adequate flow. Therefore, it is important to balance viscos-
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ity and the reactivity of a composition in order to control flow. Conventionally, the flow
of a powder is lowest when the minimum viscosity of the powder composition is high-
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est. The high minimum viscosity limits the composition from flowing over the surface,
and if not optimized, leads to voids or pinholes in the cured coating. 19
It is believed that the surface tension affects the ability of a powder composition to
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wet a substrate. If surface tension is too high, poor wetting occurs, and defects such as
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craters are formed when the powder cannot flow over the substrate. On the other hand, if
surface tension is too low, other defects such as sagging and poor edge coverage will be