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Home , Contact angle, Wetting

performance products CHEMICAL FABRICS AND FILM ASSOCIATION > PERFORMANCE PRODUCTS DIVISION

SURFACE TENSION

INTRODUCTION less than 90 degrees tend to wet surfaces. A contact is a phenomenon that we see every day. It angle of 0 degrees indicates a will completely may be a rain on a leaf or soup on a spoon. It is cover a surface, while a of 180 degrees also seen in how an ink, or spreads indicates the liquid beads up on a surface. on a film. Wetting, in simple terms, is the ability of a liquid to spread across the surface of a to MEASUREMENT produce a uniform, continuous surface. How a solid The of a solid film cannot be is wetted by a liquid is measured by the surface measured directly. Practical measurements of solid tension of the liquid relative to the surface tension of surface tensions utilize the interaction of solid the solid. For a coating, adhesive or ink to be usable surfaces with test . it must a) wet and adhere to the substrate, b) cure, and c) exhibit excellent enduse properties. The ASTM D724 describes one method to measure surface tension of a solid has important influence on surface tension. Contact angle of a test liquid is these properties. A solid surface with intimate measured on a specified surface. This procedure uses coverage of a liquid is necessary to produce a strong projection instruments and microscopes or a uniform adhesive, coating or ink bond. reflective Goniometer to measure the resultant contact angle. The procedure is more appropriate for THEORY laboratory measurements. Lower contact angles indicate better wetting. Surface Tension The interactions at the interface between a liquid and Another commonly used method employs a series of solid are complex but have practical importance. For liquids of varying surface tensions to measure critical a drop of liquid to spread across the surface of a solid surface tension based on the principles detailed in (wet the surface) work occurs to change the shape of ASTM D2578. Critical surface tension of a solid is the liquid drop. Surface tension is the work per unit the highest surface tension liquid which just wets its area to reshape the liquid. It is measured in dynes/cm. surface. Test liquids and application kits are available from a number of sources. (See Appendix) For a liquid to wet the surface of a solid, the surface tension of the liquid must be lower than the solid Application kits or “dyne pens” provide useful but surface tension. not precise measurements of surface tension. A CFFA member study indicated that surface tension Contact Angle measurements for individual technicians are A useful measurement of the ability of a liquid to wet consistent, but modest differences exist between a surface is contact angle. Contact angle is the angle testing labs. In the study, dyne pens are shown to be between the solid surface and the tangent to the liquid directional indicators of significant differences in the surface at the angle of contact. surface tension, capable of identifying “good” and “bad” printing surfaces. Contact Angle (Fig. 1) Liquid APPLICATION Solid Surface tension measurements are useful to predict surface wetting, however, they are not the only If the contact angle is greater than 90 degrees, the criteria for good print acceptance or . Each liquid tends to bead up. Liquids having contact angles > coated fabrics and film

1300 Sumner Avenue | Cleveland, OH 44115-2851 | (P) 216-241-733 | (F) 216-241-0105 www.chemicalfabricsandfilm.com | www.cffaperformanceproducts.org 2 printing, coating, or adhesive application is unique increase surface tensions and improve ink or and is affected by many process and material coating acceptance. variables. Several factors may affect any individual ƒ Moderate heating of the film can increase application. surface tension and receptivity. ƒ Surface contamination or surface exudate Climatic Conditions will adversely affect ink and coating ƒ and humidity affect how a acceptance. Cleaning the surface with solid and liquid interact. Surface wetting solvent or light mechanical roughing can characteristics may be significantly different improve receptivity to ink and coating. It is winter to summer or dependent on humidity. also important to consider sources of contamination which may affect surface Liquid Consideration tension during printing and coating ƒ High surface tension liquids have more operations. difficulty wetting the surface of than ƒ Exudation or surface bloom can indicate low surface tension liquids. Waterborne inks material problems. Ingredient quantity and and have higher surface tensions selection in the material formula must be than most solvent systems. and adjusted to the application. Plasticizer, cosolvents are useful in lowering surface lubricant or stabilizer can be optimized to tensions. the particular application with suppliers. ƒ The , drying time and solvency ƒ Surface tension of a solid surface must be (“bite”) of the liquid will affect how an ink higher than the liquid applied to achieve or coating wets a surface. Changing the acceptable wet out. solvent in the ink or coating can improve surface wetting. SUMMARY ƒ Heating the ink or coating can favorably Surface tension is a key measure of how a solid affect application. surface and liquid will interact. Intimate wetting ƒ It is important to remember that the surface promotes good print acceptance and adhesion. tension of an ink, coating, or adhesive must be lower than the surface to which it is Surface tension measurements are good indicators of applied to achieve good wet out. Working wetability but do not produce precise numbers which with the ink or coating supplier, many guarantee good adhesion or print acceptance. Each additional suggestions may be obtained to application is unique. Surface tension testing which improve surface wetting. has been correlated to good product experience is useful benchmarking. Surface tension requirements Solid Considerations will indicate significant changes from these baseline ƒ Surface tension is affected by material values. Good ink and coating acceptance and composition. Selection of plasticizer, adhesion are results of many variables at play in each lubricants, stabilizer, and process aids application; surface tension is one of many. contributes to surface tension. Material selection and application information should APPENDIX be reviewed with suppliers for each Test liquids and application kits employing the application. principle detailed in ASTM D2578 are available from ƒ Surface roughness can affect ink and coating a number of sources, including but not limited to: wetting. Rougher surfaces allow ink and coating “dive in” and promote mechanical Sherman Treaters, Inc. Diversified Enterprises bonding. 964 Westport Crescent 91 Main Street-Suite N ƒ Surface tension can decrease with material Mississauga, Ontario, Claremont, NH 03743 age. Wiping the surface with a solvent to Canada L5T 1G1 800.833.4644 905.670.9117 freshen the surface or a light mechanical roughening with an abrasive can improve UV Process Supply, Inc. Entercon Industries Corporation surface receptivity. 1229 West Cortland Street P.O. Box 773 ƒ Surface treatments, chemical washes, corona Chicago, IL 60614-4805 Menomonee Falls, WI 53052 treatment, sanding, or coats can 800.621.1296 414.255.6070 > coated fabrics and film

1300 Sumner Avenue | Cleveland, OH 44115-2851 | (P) 216-241-733 | (F) 216-241-0105 www.chemicalfabricsandfilm.com | www.cffaperformanceproducts.org