ACCURACY OF UK OPENROAD FORECASTS
5.3 SURFACE PROPERTIES
5.3.2 SURFACE ENERGY
Introduction
Surface energy is a measure of the work required to increase the surface of a material by unit area. Surface energy is important because liquid and solid surfaces have varying energies due to their structural differences. These result in varying degrees of intermolecular attraction. Surface energy is commonly used to predict adhesion between two materials and is dependant upon the surface energies of the two substances which meet to form an interface, see Figure 5.12.
Figure 5.12 Adhesive failure of two materials and creation of new surface area (Bhasin 2006)
Surfaces with the least affinity for another substance are typically very low in surface energy.
These surfaces are often described as hydrophobic surfaces as they have poor affinity to water, see Figure 5.13. Creation of a low energy surface is an approach widely investigated for de-icing purposes on ships, leading edges of airplane wings and overhead telephone wires.
Figure 5.13. Influence of contact angle on surface energy, Rame Hart website (2009)
In contrast hydrophilic surfaces are attracted to water and the water will spread over large areas of the surface. A surface is typically described as hydrophilic if the contact angle is less than 90o. Measuring the values of a solid is less straightforward because direct measurement of the solid surface energy is not possible. Surface energy measurements of solids are typically determined indirectly and reflect the thermodynamics of a fluid (liquid or gas) and solid interaction.
Test Procedure
The surface energy of the icing chemicals can be used to predict adhesion between the de-icing chemicals and a second material such as bitumen or ice.
Measurement of the surface energy of fine powders, like fillers and chemicals is relatively complex. Two methods were employed to characterise the powders included within this study.
These included Dynamic Vapour Sorption and the Capillary Rise Technique.
Dynamic Vapour Sorption (DVS) is a technique developed to measure the surface energy of high surface area solids, such as fillers (Levoguer et al., 2003). In a vapour sorption measurement the surface tension of the liquid is considerably less than the energy of the solid surface and spreading occurs. The resulting contact angle is 0o and the magnitude of the spreading pressure significant. This relationship forms the basis of the vapour sorption approach of measuring surface energy. can be determined directly. The use of two (or more) further gases of differing polarity allows the calculation of surface energy components of the solid.
Samples of the fillers were prepared and surface energy measurements determined using Dynamic Vapour Sorption. The test liquids used in the Dynamic Vapour Sorption experiment were Octane, Chloroform and Ethyl Acetate. The surface energy components of the test liquids used in the vapour sorption tests are presented in the Table 5.7.
Liquid Total Surface
Table 5.7 Test liquids used in Dynamic Vapour Soprtion test
The DVS technique provided a suitable measure for the limestone filler, however complications were experienced when using the probe liquids selected in the test procedure on sodium formate and sodium silicate. This was because of significant variations in the recorded mass. In particular a reduction in the mass of the test specimen indicating a possible chemical reaction or swelling resulting in potential inaccuracies.
An alternative sorption technique of conducting Surface Energy measurements of the chemical compounds was achieved by using the Washburn technique. Testing of this element was subcontracted to the specialist chemistry laboratory Kruss GmBH, whom performed contact angle measurements using a K100 tensiometer and surface free energy calculations using Labdesk software.
Determination of the contact angles of the sodium formate and sodium silicate was achieved by the capillary rise of a pure liquid into a porous medium (which in this case was the powder). This
implies the replacement of the solid- T
ses the structure of the powder
bed using a totally wetting liquid such as siliconoil M3. This liquid perfectly wets out the surface of the powder without interacting by processes such as dissolution, swelling, or chemical interaction. This value can then be used to calibrate the subsequent test liquids assuming a contact angle of 0o.
Surface free energy of the samples was determined using the Van Oss model, inputting the contact angle data from the liquids dimethylesulfoxide, ethylenglycol, diiodomethane,
formamide and water. All measurements were performed at room temperature (T=23°C.)
The necessary physical properties (density, viscosity, total surface energy, dispersive component, polar component, the acid and base part) of these test liquids are summarised in
Table 5.8 Test liquids used in capillary rise test
Where:
SFT = Total Surface Energy - the amount of energy/work required to create a unit surface area of a material in vacuum.
Dispersive Component = Acid-Base component
Polar Component = Lifshitz-Van der Waals component
Acid Part = Lewis acid component of surface interaction
Base Part = Lewis base component of surface interaction
According to the Washburn theory there is a relationship between the wetting properties i.e.
the contact angle and the mass increase during the sorption process which can be described by:
(Eq. 5.5) Where m is the total mass, t the time, the density, the viscosity, the surface tension of the liquid and C the capillary constant of the powder sample in the sample holder. In order to determine the constant C, a measurement with a liquid which exhibits a contact angle of 0°
when in contact with the solid, is performed i.e. Siliconoil M3.
As a result of the experiment and the physical data of the test liquid, the contact angle can be easily measured for other liquids. By determining the slope of the square of mass versus time it is possible to calculate the contact angle, see Figure 5.14.
Figure 5.14 Example plot of Washburn calculation
An attempt to compare the surface energy measured by both the DVS and Capillary Rise test (section 5.3.2) for limestone filler was not possible. This is because a zero contact angle was recorded for multiple probe liquids, these included water, ethylene glycol, diiodomethane, benzyl alcohol, isopropanol, ethanol, toluene and glycerol. This was due to the relatively high surface energy of the limestone filler because any probe liquid with a surface energy less than that of the material to be tested will readily spread on the surface of that material. This either give a zero contact angle or makes it very difficult to obtain a true contact angle value.
Test Results
Overall a number of techniques have been used to measure the surface energy of the different powders being investigated.
The mean spreading pressure for each of probe liquids on limestone filler using the DVS technique are reported in Table 5.9.
The mean contact of probe liquids for sodium formate and sodium silicate using the Washburn technique are presented in Table 5.10.
Surface energy components of the reference materials were computed by applying the Van Oss theory. The total surface energy and the surface energy components are presented in Table 5.11.
The results produced using the different techniques are not directly comparable, however they do provide an indication of the potential surface energy characteristics of the chemicals/filler and confirm whether the reference material is likely to have a high surface energy or a low surface energy.
Mean Spreading Pressure (mJ/m2) for each Probe Liquid
Octane Ethyl Acetate Chloroform
Limestone Filler 26.09 87.82 75.2
Table 5.9 Mean spreading pressure of probe liquids for limestone filler using dynamic vapour sorption
Mean Contact Angle (o) for each Probe Liquid
Dimethylesulfoxide Ethyleneglycol Diiodomethane Formamide Water Sodium
Table 5.10 Mean contact of probe liquids for sodium formate and sodium silicate using the washburn technique
Table 5.11 Surface energy components of reference materials computed by applying the Van Oss theory
Discussion of Test Results
The chemicals sodium formate and sodium silicate have demonstrated very different physical and surface properties in comparison to limestone filler. These differences are likely to influence the mechanical properties and the anti-icing performance of asphalt.
In terms of the mechanical properties of asphalt:-
Sodium formate is coarser, less dense and more elongated than standard limestone fillers with significant surface roughness. Sodium formate has a low Rigden Voids and higher maximum packing fraction.
Sodium silicate is also coarser and less dense than standard limestone fillers, however the particles are spherical in nature and in a consistent manner to limestone filler. The particles are well rounded and smooth. Sodium silicate has a higher Rigden Voids and lower maximum packing fraction.
The combined effect of these materials may provide similar compactabilty characteristics.
In terms of the anti-icing performance:
Both sodium formate and sodium silicate have a much lower surface energy than limestone. This may reduce ice adhesion but may make the asphalt more sensitive to water.
Sodium formate and sodium silicate are both highly soluble at room temperature. The chemicals are likely to be transferred from asphalt pavement surface to promote anti-icing.