Converting Magazine

Eldridge M. Mount

Measuring treatment, Part 2
January 17, 2008

The next method of surface-treatment measurement is that of the Contact Angle: This is a more time-consuming test to determine the treatment level of treated films but is more reproducible from the aspect of operator reading of the results. Contact angle may be determined in several ways and has been described in great detail in many texts and publications. In the equilibrium sessile drop method, a controlled volume, stationary drop of liquid is placed on a flat surface and the angle between the solid, the saturated vapor or air and the liquid is related to the height, h, and base diameter, d, of the drop which are measured.

                Tan (theta/2) = 2 h/d                                  Equation 1

The  results may be reported as the contact angle in degrees, theta or Cos (theta), the cosine of the contact angle theta. The apparent contact angle may be affected by the chemical composition variation and the roughness of the surface. These effects may be checked, by measuring the advancing and receding contact angle. These tests are performed, by making the drop edge move across the surface by slowly increasing or decreasing the volume of the liquid drop. Then the drop is held at constant volume, and the contact angle is measured after the drop edge has come to rest. The difference between the two measurements, the hysteresis, can give information about the relative surface roughness. The angle is measured with a telescope to magnify the drop edge on the surface. The angle is measured with a goniometer

The Wilhelmy plate or tensiometric methods also give the equilibrium, advancing and receding contact angles by calculating the force required to lower or raise a sample into a fluid using a micro-balance. This test is gaining more use as it is less subjective in its measurement due to instrumentation used and gives several measurements with a single sample setup.

Solid Surface Free Energy: There is no direct, reliable method for directly determining the surface tension of a solid[i]. However there are several indirect empirical and semi-emperical methods based on contact angle measurements. The solid surface tension is important to know, because of its potential for determining the potential for adhesion between materials. It has been suggested that the surface free energy of a solid surface is composed of several intermolecular forces comprising, dispersive, polar and hydrogen bonding. It is postulated that two interacting materials, which shared these intermolecular forces should adhere together and the work of adhesion would also be comprised of these three intermolecular forces.   Measuring the contact angle of the solid surface with various liquids, allows the estimation of the solid surface energy, gamma (g_s), which can be shown to be composed of two parts, the polar part, g_s^p, and the dispersive part, g_s^d    These parameters are thought to be important in that they can be used to estimate the work of adhesion between a solid and another 
material. The polar portion is made up of the combination of the dipolar interactions and the hydrogen bonding attractions between two surfaces. Hydrogen bonding is the intermolecular attraction of an alcohol hydrogen with a molecular dipole such that a weak bond-like attraction is formed between two molecules keeping them in close association. In treated polyethylene surfaces the auto adhesion between two films has been shown to be due to hydrogen bonds between –C-OH groups on one surface and C=O functions on the other surface[ii]. For Polyester the auto adhesion is between terminal phenols in one surface and carbonyls in the other[iii].   Equation 2 is used to determine the dispersive and polar solid surface energies by measuring the contact angle with two different fluids and solving the set of two simultaneous equations for g_s^p and g_s^d

g_LV (1+cos(theta)) = 2(g^d_S g^d_LV) _^1/2 + 2 (g^p_Sg^p_LV) ^1/2                       Equation 2

Values for g_LV are tabulated , for well-characterized liquids and a combination of any two sets can be used for the determination[iv]. Often times water / methylene iodide or water / formamide are the liquid pairs used.

In the next posting we will discuss the measurement of surface oxygen concentrations and how it is used to determine the oxygen functionality on the surface. This is important as the chemistry of the oxygen on the surface will control the adhesion between the treated surface and inks, metal and adhesives.

Posted by Eldridge M. Mount on January 17, 2008 | Comments (0)