The process of applying a fluid onto a substrate is a dynamic one whenever active surfactants are used, causing surface tension to vary as the speed of the application varies. Surfactants are used to reduce surface tension in water- and solventborne formulations. Waterborne coatings, inks, and similar formulations require alcohols and surfactants to lower their surface tension to acceptable levels, usually below the surface energy of what is being coated.

Surfactants used in fluid formulations tend to be surface active, and diffusion rates can vary significantly, depending on molecular weight and structure. It can take from several seconds to several minutes for surfactant molecules in the bulk solution to diffuse to an interface where they can lower the surface tension to its lowest level of “static” (or “equilibrium”) surface tension. Because of this surfactant migration time factor, the most important parameter in designing and measuring formulations is dynamic surface tension rather than equilibrium surface tension, since dynamic surface tension directly impacts the quality of spreading and adhesion. Formulators who know the principles of dynamic surface tension, and have suitable instruments to make dynamic measurements, can solve formulation problems and optimize their system coating operations.

Dynamic Surface Tension and Surface Age

Fluid surface tension is the intermolecular force of attraction between adjacent molecules. Surface tension dictates whether an ink, coating, or adhesive will wet and spread over, or retract from, a solid surface. It is expressed as force per unit of width: dynes/cm. At the instant an ink or coating is deposited on the surface of a substrate, the concentration of surface active molecules at the interface will be the same as in the bulk solution, equal to the surface tension of the pure solvent. The molecules then begin to diffuse to, and adsorb at, the newly created fluid/air interface and the fluid/substrate interface. The concentration and the activity of the surfactant determines by how much the surface tension is reduced, but the speed at which the diffusion takes place influences how fast wetting will occur.

Surface age is the amount of time a coating process allows the surfactant molecules in solution to migrate to any newly formed interface. Examples are a printing press roller picking up ink and depositing it on a substrate, or an atomized droplet from a paint gun. As surface age gets shorter, fewer molecules can migrate to the interface, resulting in higher dynamic surface tensions. Whether it is a droplet from a spray gun or in bulk from a brush stroke, when a coating is spread onto a surface, several interfaces come into play, both before and after the coating is applied. Initially, an air/fluid interface forms, to which the surfactant molecules from within the bulk solution will migrate. Upon coating of the surface, the fluid will form two interfaces: a solid/fluid interface on the substrate surface and an air/fluid interface on the fluid surface exposed to the air. At both of these surfaces, surfactant molecules are required to promote wetting, spreading, and adhesion (at the solid/fluid interface), and knitting and smoothing of the liquid surface (at the air/fluid interface).

Dynamic surface tension curves, using as little as three measurement points, provide dynamic comparisons of different formulations or different production batches. Figure 1 compares the curves of six formulation samples. In this specific example, #5 has a lower surface tension toward the equilibrium end of the curve than #4. Classical equilibrium surface tensiometers can provide this relationship. However, they can not show that #4 rises slower than #5 as surface age decreases, as they both transition into the dynamic zone. This suggests that #4 has increasingly better dynamic characteristics. Classical surface tension methods such as the DuNoüy Ring and Wilhelmy Plate can measure only equilibrium (static) values because they measure only the top fluid surface, and that surface has invariably had sufficient time to reach surfactant equilibrium.

Maximum Differential Bubble Pressure Method

The maximum bubble pressure method is ideal for determining dynamic characteristics of formulations since the method allows users to control the rate of bubble formation and, therefore, surface age. SensaDyne tensiometers use a patented maximum differential bubble pressure method. Two probes with different orifice sizes bubble within a test fluid at surface age rates set by the user, independent of immersion depth, and immune to surface contaminants or surface foam. Pressure differential between the bubbles is directly proportional to fluid surface tension. Patented software peak detection allows both laboratory and in-process monitoring with precise real-time surface age determination, ranging from a few milliseconds to several minutes. Many common surfactant solutions reach equilibrium in a number of seconds, while some can take a minute or longer.

The differential maximum bubble pressure method is ideal for determining dynamic characteristics of most fluid formulations since the method allows users to control the rate of bubble formation and surface age from the millisecond range to several minutes. Both bubble interval and surface age are measured and displayed in real time. Since bubbling is continuous, this system is also easily adaptable to online, continuous measurement, and process control using surface tension as the controlling parameter. For example, this system has been successfully implemented to improve surfactant usage in latex polymerization production tanks, operating at high temperatures and pressures.

Instruments are calibrated using two standards of known surface tension values, such as deionized water and alcohol. SensaDyne’s newest fully automatic tensiometer, the PC500-LV (shown in Figure 2 with a dispenser), features fully automatic dynamic surface tension measurement with user-programmed surface age range. It also has automatic viscosity compensation where increasing hydrodynamic forces that resist the formation of bubbles in increasingly viscous fluids are automatically canceled out. These instruments can also measure fluids, Freons, Freon replacements, aerosols and liquefied gases under pressures up to 225 PSI (1500 kPa) by using a pressure vessel with a high pressure sample injector in place of the dispenser.

Dynamic Critical Micelle Concentration (CMC)

Many surface tension related properties such as detergency, foaming, emulsification, dispersion and wetting are believed to either maximize or minimize at the surfactant critical micelle concentration (CMC), the surfactant concentration beyond which surfactant molecules in solution self-assemble into aggregates called micelles. CMCs have been commonly measured using classical DuNoüy Ring or Wilhelmy Plate instruments, which are limited to equilibrium measurements. A dynamic surface tensiometer, however, with or without an automatic dispensing system, can generate both static and dynamic CMCs, where optimum surfactant effectiveness can be determined when correlated to actual process coating speeds, which are not necessarily at equilibrium CMCs. Users can improve transfer, spreading and adhesion by determining the best surfactant concentrations by way of dynamic CMC determination, and by choosing surfactant and additive combinations that provide the best dynamic surface tension profiles for their specific coating applications.

Figure 3 illustrates a series of dynamic CMC curves generated for different concentrations at varying surface ages. The theoretical intersection of the two straight legs of each curve (point of change in slope) defines the CMC point. The shift in the dynamic CMC is toward increasing surfactant concentration as surface age is reduced. As with surface tension, the relevant parameter in choosing surfactant concentrations is dynamic CMCs rather than equilibrium CMCs.

Automatic Dispensing System

The dispenser system (Figure 2) comes with a selection of barrels/pistons and precision dispensing tips to cover a variety of dispensing ranges. The adjustable barrel stand integrates the dispensing pistons and tips with most standard measurement beakers that are used with the tensiometer probe assembly stands. When setting up an automatic dispensing run, the Base Sample Amount, the Initial Dispensed Amount and the Initial Dispensing Time is entered into the software program’s “Edit Setup” dialog box, illustrated in Figure 4, along with the Maximum Concentration, the Increments Concentration and the Stirring Time. The computer calculates the required time for each dispensing cycle.

During automatic operation, the software program turns on the integrated laboratory stirrer, stirs the contents for a predetermined period of time, stops the stirrer and pauses for a preselected time period to let the fluid settle. The program then initiates a surface tension measurement, captures the value in a user-named data file, and repeats the cycle, until the run is completed. Data from automatic dispensing runs is compiled and stored in user-named files, and can be displayed immediately in chart and spreadsheet format, or can be converted to ASCII files for export to other graphing and spreadsheet programs.

The automatic dispensing system is a valuable time saving accessory for users who do a lot of formulation or CMC studies with various surfactants or additives. For CMCs, the added benefit is that curves can be run over different diffusion times by changing the surface age. This generates data that shows how the CMC shifts with changes in surface age.

Automatic Viscosity Compensation

In measuring surface tension of coatings using the maximum bubble pressure method, as the viscosity of the coating increases, the hydrodynamic resistance of the fluid against the moving bubble also increases. This causes a measurement error sometimes referred to as the viscosity effect. Though negligible for coatings of small viscosity, it can be several tens of dynes/cm (mN/m) for highly viscous coatings. Stokes law quantifies this viscosity effect: the difference between the measured value of dynamic surface tension and its real value. Single probe bubble pressure tensiometers require correction factors in order to deal with this viscosity effect problem. Even if correction values are used, this means that each test sample must be measured accurately for viscosity in order to apply the proper correction factor.

Two probe differential tensiometers can be set up to generate viscosity compensated surface tension values. A thorough study and understanding of the viscosity effect and the inherent limitations of single transducer tensiometer instruments led to the development of an automatic viscosity compensated instrument. The Sensa- Dyne PC500-LV tensiometer uses a patented, multi-probe differential method that facilitates automatic viscosity compensation.

Instead of two probes, the PC500-LV uses three probes: one 0.5 mm, one 1.0 mm and one common 4.0 mm. The 4.0 mm probe acts as the control probe shared with both of the smaller orifices. This allows a separate differential pressure signal to be generated by each small orifice (0.5 and 1.0) and the common 4.0 large orifice. The maximum bubble pressure peak values from each of these two differential signals are then electronically subtracted by the software program to give a final differential in which the hydrodynamic components cancel.

Results obtainable by the automatic viscosity compensated tensiometer can be illustrated by comparing surface tension measurements of a low- to medium-viscosity oil sample (1000 Centipoise) tested using a standard two probe automatic PC500-L tensiometer against results using an automatic viscosity compensated PC500-LV tensiometer. In Figures 5-6, ST is surface tension, T is temperature, SA_A is the small orifice surface age, and BI_A is the small orifice bubble interval. The large orifice, in the PC500-L test, was set to equal the small orifice surface age.

Figure 5, using the PC500-L appears to give a classic dynamic surface tension curve, with surface tension increasing as surface age is decreased. One would assume from this curve that there are some surfactants that contribute to the classic dynamic shape; as the surface age decreases, less surfactant molecules migrate to the interface and the resultant surface tension is higher. However, this sample is surfactant free, and the “curve” that we see is totally due to the increasing viscosity effect at decreasing surface age. As the surface age decreases the increasing hydrodynamic resistance to bubble formation causes higher pressure readings that increase the “surface tension” reading.

The same fluid was tested using an automatic viscosity compensated PC500-LV tensiometer with it’s automatic mass flow controllers (MFCs) programmed to give a 1:2 (‘A” probe to “B” probe) surface age ratio. These results are shown in Figure 6. One additional column entry shows the calculated surface age ratio of the “A” (0.5 mm.) to “B” (1.0 mm.) orifice. This comes very close to the theoretic flat line that we should see for a non-surfactant containing oil whose readings are viscosity compensated. The slight “droop” at each end is attributed to the excursion from the ideal ratio of A:B; as the surface age of the “B” orifice decreases slightly and skews the results.

Formulation Problem Solving

Problems in coating applications can be solved or avoided by measuring formulations before they are used. Users can improve transfer, spreading, and adhesion by looking at the dynamic surface tension curves of their formulations over their entire projected coating range. Optimal surfactant concentrations can be determined by way of dynamic critical micelle concentration studies, and by choosing surfactant and additive combinations that provide the best dynamic surface tension profiles and CMCs for specific applications. Because many coatings have moderate to high viscosities as a rule rather than as an exception, it is advantageous to measure surface tension with an instrument that can give automatic viscosity compensating, dynamic, surface tension readings.

For more information on surface-tension measurement, contact SensaDyne Instrument Div., Chem-Dyne Research Corp., 2855 E. Brown Road, Suite 20, Mesa, AZ 85213; phone 480/924.1744; fax 480/924.1754.