{"id":13758,"date":"2022-10-26T06:05:30","date_gmt":"2022-10-26T12:05:30","guid":{"rendered":"https:\/\/www.ulprospector.com\/knowledge\/?p=13758"},"modified":"2023-03-07T14:54:54","modified_gmt":"2023-03-07T20:54:54","slug":"pc-fundamentals-of-wetting","status":"publish","type":"post","link":"https:\/\/ulprospector.ul.com\/13758\/pc-fundamentals-of-wetting\/","title":{"rendered":"Fundamentals of Wetting"},"content":{"rendered":"

\"photoOne of the most critical properties in the design of a paint system that determines the ease of processing and ultimate performance properties is the wettability<\/strong> of substrates, pigments and other ingredients. Major factors in determining \u201cwettability\u201d are surface tension and surface energy. For example, if the surface tension<\/em> of a liquid is lower than that at the interface (boundary) of the two materials then wetting will occur. Surface tension considerations impact: pigment dispersion, paint flow and leveling, sprayability (ease of atomization of a liquid), latex coalescence, powder coalescence and adhesion. A liquid with a high degree of cohesive forces as a result of polarity or other intermolecular attractive forces will have a higher surface tension than a liquid with a lower degree of cohesive forces.<\/p>\n

For example, the degree of wetting of a given solid by a liquid is determined by\u00a0the balance of forces between cohesive and adhesive forces of that liquid:<\/p>\n

\"Chart<\/p>\n

Surface tension<\/strong> ( \u213d<\/strong> ) of a liquid is the tendency of a liquid at rest to shrink into the minimum possible surface area. For example, versus less polar liquids, liquid water has a higher level of cohesive forces and thus a high surface tension.<\/p>\n

\"surface<\/p>\n

On a molecular level, segments of molecules that minimize surface tension tend to orient at the surface. For example, a general relationship of increasing surface tension gradient<\/strong> of molecular segment types follows:<\/p>\n

\"surface<\/p>\n

Surface tension also increases as the temperature<\/strong> is decreased. Dynamic surface tension <\/strong>is more representative of the interactions when a wetting agent is utilized for incorporation into a paint system. Dynamic surface tension <\/strong>may be defined as the time required to reach equilibrium after agitation has stopped. The time required to reach this equilibrium is dependent on multiple factors including the paint composition, viscosity and temperature. For example, small more flexible molecules reach equilibrium faster. Also when there are large differences in the surface tension of components such as in waterborne coatings, equilibrium occurs faster. Lastly, since solvents generally have lower surface tension than polymers, as solvent evaporates during film formation surface tension increases decreasing flow.<\/p>\n

\"surface<\/p>\n

As lower surface tension fluids wet higher surface tension surfaces, per Tables 1 and 2, none of the liquids in Table 1 would wet PTFE as the intermolecular attractive forces within the liquid are\u00a0greater than the adhesive forces to the substrate to enable flow. On the other hand, MEK and toluene would be able to develop sufficiently high adhesive forces to wet aluminum and steel.<\/p>\n

To improve the wettability of a high surface tension liquid such as water, wettability can be improved by the addition of a water-miscible solvent with a lower surface tension than that of water such as ethyl cellosolve or with the use of a wetting agent <\/strong>such as a compatible nonionic, anionic, zwitterion and cationic surfactant.<\/em><\/strong> For additional information on surfactant and defoamer technology, there are a number of articles posted on Prospector Knowledge Center<\/a>, the most recent was written by Marc Hirsh.<\/p>\n

A wetting agent is an all-inclusive name that can encompass:<\/p>\n

    \n
  1. Surfactants<\/a><\/li>\n
  2. Flow and leveling agents<\/a><\/li>\n
  3. Defoamers<\/a><\/li>\n
  4. Pigment dispersants<\/a><\/li>\n
  5. Emulsifiers<\/a><\/li>\n<\/ol>\n

    Flow and leveling agents<\/h3>\n

    Flow and leveling agents are utilized to overcome surface defects that may be caused by airborne contaminants. Such contaminants can cause defects such as craters, fisheyes, pinholes and trapped air. Flow and leveling agents that are utilized correctly help to promote a smooth defect-free surface.<\/p>\n

    The most common flow and leveling agents follow:<\/p>\n