In some applications (e.g. circuit board assembly), the part will move around several stations before the adhesive used is finally cured. The parts are held on to the substrate only through the paste-like behaviour of the adhesive. The consistency of this adhesive needs to be such that the parts do not move as the assembly moves through the various process stages. The ability of the adhesive to prevent parts moving is assessed in electronics assembly through ‘sledge’ tests—a board with several standard electronic components (of different sizes) slides down an inclined ramp until it hits a stop at the bottom of the ramp; the position of the components is measured to assess the wet strength of the adhesive. The physical properties of the adhesive needed to achieve a good wet strength are the same as those needed for slump resistance and it should be possible to quantify these through rheological measurements.
Wetting is the spreading over and intimate contact of a liquid (adhesive) over a solid surface (substrate). If sufficiently intimate contact is achieved between the two phases, a physical attraction develops causing the liquid to conform to the surface on a macro and micro scale, displacing air and thus minimising interfacial flaws. Good wettability of a surface is a prerequisite for ensuring good adhesive bonding, and hence considerable effort has been expended in developing simple wettability tests to assess surface energy/tension prior to bonding (see below) [40–42].
Surface energy is defined as the work necessary to separate two surfaces beyond the range of the forces holding them together and is given in energy per unit area. Surface energy is often referred to as surface tension and is often expressed in dynes/cm (1 dyne/cm is equivalent to 1 mJ/m2). It is dependent on the interfacial intermolecular forces.
The surface free energy of a solid can be indirectly estimated through contact angle measurements. The determination of contact angle at the solid/liquid phase boundary is one of the most sensitive methods for determining the surface energies of solid materials. Contact angles are closely related to wettability. A liquid (adhesive) will wet a solid (adherend) when its surface energy is lower than the solid surface energy. Force balance or equilibrium at the solid-liquid boundary is given by Young’s equation for contact angles greater than zero:
where q is the contact angle, and glv, gsv and gsl are the surface free energies of the liquid-vapour, solid-vapour and solid-liquid interfaces, respectively.
The lower the contact angle, the greater the tendency for the liquid to wet the solid, until complete wetting occurs at an angle q = 0 (i.e. cos q = 1). The surface tension of the liquid is then equal to the critical surface tension of the substrate. Large contact angles are associated with poor wettability.
Wilhelmy plate method for measuring dynamic contact angle
Two methods are available for measuring contact angles of solid surfaces: static (known as sessile drop or goniometry) [44, 46–48, 50–52] and dynamic (known as tensiometry or Wilhelmy method) [49]. The static method requires the observation of a drop of test liquid on a solid substrate, while the dynamic method measures the forces of interaction as the solid is immersed in a test liquid. Problems associated with the techniques include swelling of the solid surface, roughness and porosity. Cohesive hydrogen bonding within the test liquids and the solution pH may influence the measured surface energy. The surface tensions of many liquids (particularly water) change dramatically with the absorption of small quantities of surface-active impurities (from surfaces or the atmosphere). Care must be taken with the handling and storage of probe liquids. Water is normally freshly de-ionised or distilled before use. For porous materials, the observed contact angle will change with time as the probe liquid penetrates the surface.
Static Contact Angle can be measured by observation of a drop of test liquid on a solid substrate either on a goniometer or by projection, either directly or using video imaging. Determination of the contact angle can be carried out using automated image fit techniques. Commonly used probe liquids for measuring contact angle include distilled water, glycerol and methylene iodide (typical drop size is 2 to 20 m1). Analytical instruments can be used to measure the contact angle of a water droplet placed on the surface of a film [46]. In the packaging industry contact angle is measured according to Tappi Methods for papers [51] and for polymer film surfaces [115]. However, this technique is not commonly employed, as it is perceived as time consuming and requiring a degree of interpretation of the results.
Dynamic Contact Angle: can be measured using the force-balance technique, using rectangular Wilhelmy plate specimens [49]. The forces exerted as the solid/liquid phase boundary is moved along the sample surface are measured. The contact angle may be calculated from the forces of interaction, sample geometry and liquid surface tension. This technique involves placing the sample on a balance and zeroing at the solid weight. The liquid (of known surface tension) is raised to meet the solid and the point of contact is determined and recorded as zero immersion depth. The solid continues to be lowered into the liquid to a set depth and the forces on the balance are continuously monitored. The process is then reversed and the solid is raised from the liquid until the liquid film ruptures. Dynamic contact angles measured at low speeds should equal static contact angles.
Both the advancing (qadv) and receding (qrec) angles can be measured. The difference (qadv—qrec) is the contact angle hysteresis, which depends on surface heterogeneity and roughness since the local surface energy (or chemistry) and local physical topography act as barriers to the motion of the liquid contact line. Multiple cycles can be performed which yield information on adsorption. However, the sample geometry must be regular, have a constant perimeter over the entire length, have low mass, be suspended perpendicular to the surface of the liquid with the bottom edge parallel to the liquid surface and have the same surface treatment on all sides in contact with the liquid.
Quicker and simpler methods exist for fast inspection of the wettability of surfaces. These are the water break test and wettability (or dyne) pens.
Water Break Test: This qualitative (go/no-go) test involves the specimen (in the form of a flat plate) being either immersed in water or water brushed or water sprayed onto the surface [43]. The plate is then checked to determine the distribution of water on the surface (i.e. remains as a continuous film indicating good wettability or forms distinct droplets indicating poor wettability). Other liquids can be used for wetting break tests.
Wetting Pens: This semi-quantitative test [45, 53] involves marking the surface with a pen containing ink of a known surface tension and observing whether or not the initial continuous line breaks into distinct droplets (indicating that the surface energy is lower than the surface tension of the ink). ‘Dyne solutions’ (often referred to as Sherman pens) are available in sets of different surface tensions and systematic use can quickly provide an estimate of surface energy. Despite the widespread use of these solutions, the method has notable critics [54]. The shelf life of pens can be limited, particularly if in regular use, as transfer of contamination from surfaces to the ink may occur.
Next: References
