Significance and Use
Contact angle measurements may be used to study the relative sorbtive rates of uncoated sorbent papers, or the relative printing or writing characteristics of coated or sized printing and writing papers.
The complex interaction between a liquid and a surface may be looked upon as a combination of three different processes of wetting, absorption, and adsorption. Wetting is best explained with a drop of water on a plastic film. The liquid volume remains the same, the drop base diameter will increase, and the contact angle will decrease as a function of time. When the liquid volume is reduced as a function of time, the base diameter of the drop is studied. When this diameter remains constant, the absorption is dominating. When the drop is spreading across the surface (increasing base diameter), the interaction is based on adsorption.
For sized papers, an increase in feathering is likely as the rate of change in the contact angle with time increases, indicating a relative increased degree of liquid transport or penetration (absorption) into the paper.
For sorbent papers, the change in contact angle with time may be very rapid, with those papers showing the greater relative change per unit time having the fastest rate of sorption.
For hard sized papers, little change in contact angle with time may be seen, and for laminates or polymer coated and barrier papers, release papers, or other similar specialty grades, there may be no change in contact angle over the time interval of a typical test.
It is generally found that papers having contact angles with water-based inks in the range 90 to 110° work best in printing and writing applications. Feathering may be expected for contact angles less than 90°. Breaks in the flow of ink onto the paper may occur for contact angles greater than 110°.
Because of the wide range of paper coating possibilities and ink compositions, further generalizations are difficult. However, contact angle is a precise empirical tool for use in studying specific liquid/substrate combinations for product and process improvements.
In addition, contact angle measurements on films are used to determine printing and gluing characteristics of films with specific printing inks or adhesives. In such applications, the procedure may use a constant film substrate with various test liquids of significance to a specific end-use application. By measuring substrate surface free energy and then monitoring and controlling any surface treatment of the material using contact angle measurements, improved end-use performance in gluing or printing applications is possible.
1.1 This test method measures the contact angle of a test liquid in contact with a flat specimen of a film or a paper substrate under specified test conditions. This test method may be used with any liquid of interest which is compatible with the equipment used, particularly with regard to liquid viscosity, tackiness, and vapor pressure (evaporation). This test method may be used with any substrate of interest, which can be cut to dimensions compatible with the equipment used.
1.2 For materials which sorb the test liquid under the specified test conditions, the rate of change of the contact angle as a function of time may be significant, and may be determined using procedures described in this test method. It is also possible to evaluate the sorptive properties of a surface, as the remaining liquid volume on top of the specimen surface is measured as a function of time.
1.3 The conditions required in this test method specify reagent water as the test liquid when testing papers designed to be absorbent, such as absorbent tissue grades.
1.4 Conditions are specified for the testing of a wide range of papers considered to be of low absorbance or nonabsorbent, including release papers, sized, coated, or unsized papers designed for printing, writing, wrapping, and similar tasks where the paper surface interaction with aqueous or solvent based inks or other aqueous or nonaqueous liquids is important. In such cases, test liquids other than reagent water, including writing and printing inks, or organic liquids or mixtures of organic liquids may be used as the test liquid upon prior agreement of those involved in the testing, provided the liquid is compatible with the equipment used. Where test liquids other than reagent water are used, the actual liquid used is reported.
1.5 Conditions are also specified for the testing of polymer films, polymer-coated papers, paper laminates, felt, textiles and non-wovens, using water or other fluids compatible with the equipment and important to the end-use applications of the materials tested, including gluing and printing.
1.6 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.