Page 167 - Handbook of Surface Improvement and Modification

162 Anti-cratering and Leveling

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seen. The surface tension and viscosity also influence the leveling behavior of a compo-
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sition. Therefore, to obtain a coating with optimized flow and leveling, it is necessary to
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balance the viscosity, surface tension, and reactivity or cure behavior of the composition.
The compositions of primer exhibit a much larger viscosity reduction as temperature
increases (typically greater than 50% drop in viscosity between 25 and 45°C compared to
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less than 30% for the prior art). Such rheological behavior permits primer to be applied
at an elevated temperature, provide excellent printing and leveling at an elevated tempera-
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ture while having limited penetration into the board. Such behavior is also illustrated by
the more robust gloss retention performance of the primer at different coating film weights
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compared to the commercial formulations. Polyacrylates are suitable leveling addi-
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tives.
The hydrophobically modified alkylene oxide urethane branched polymers provide
polymers with superior rheological properties which are useful in obtaining sag-leveling
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balance.
Compositions of polyether polyol resins (hydroxyfunctional oligo- or poly-ether)
comprising a mixture of α,α-branched alkane carboxylic glycidyl esters derived from
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butene oligomers permit the development of materials with excellent leveling properties.
A liquid crystal polyester-containing liquid composition which is capable of produc-
ing a prepreg and a resin film in which surface defects are remarkably reduced even in the
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case of drying at high temperature to remove the solvent in the production process. The
formulation contains a liquid crystal polyester, an organic solvent, and leveling agents. 23
The leveling agent is selected from the group consisting of a polyether-modified polydim-
ethylsiloxane, a fluorine-modified polymer, and a polyester-modified polydimethylsilox-
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ane (0.005 to 0.5 wt%).
The uneven ink distribution on the printing plate in the printing nip, caused by film
splitting in the nips (ink filaments) and inadequate ink film leveling, can be a likely reason
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for pinhole formation. The ink surface tension, gravitational forces, and surface-tension-
gradient effects (Marangoni flow, which arises during drying) determine leveling (ink film
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flatness). Leveling is a process of eliminating surface irregularities of a continuous ink
film under the influence of the ink's surface tension. It is an important step in obtaining a
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smooth, flat and uniform ink film. The factors that resist leveling are viscosity, elasticity,
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and surface tension gradient (responsible for upward flow). The process of leveling
depends on many parameters such as: the ink film thickness, extent and frequency of sur-
face irregularities, ink surface tension, viscosity, etc. but the surface tension and viscosity
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of the ink play the major roles. The leveling speed increases with increasing surface ten-
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sion of the ink and decreasing ink viscosity.
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