Bonding and Curing systems
Adhesives are widely used in many industry sectors, where their use is essential in manufacturing thousands of everyday products. There are many kinds of formulations and methodologies capable of performing bonding or joining various substrates. The adhesive material "must be capable of wetting the surface to which it is applied, at least for an instant, be used in a relatively thin layer, so that it forms a joint capable of transmitting stress," and be both strong and lightweight.
The adhesives industry has many diverse applications for particle size and shape analysis, stability and rheological characterization.
If you are responsible for formulating a new adhesive or improving the performance of an existing product, control of the particle size and shape, formulation stability and rheology are essential.
It is well understood that viscosity is importance to measure to control the flow of the adhesive during coating operations. Specifically, it is usual to measure viscosity profiles over a wide range of shear rates to simulate flow under actual coating conditions. For some adhesives, measurement at high shear rates can demonstrate if a base or modified emulsion is stable or unstable under these conditions.
Also, rheological measurements at high shear rates on pressure sensitive adhesives can help explain variations in transfer properties during gravure coating. Although an increase in solids content increases viscosity, at high solids content, shear thickening may occur at high shear rates causing viscosity to increase and flow to decrease.
A smaller particle size generally increases the low shear viscosity due to colloidal interactions. Viscosity is often independent of particle size at higher shear rates, as hydrodynamic forces dominate. A wide particle size distribution usually gives a lower viscosity than narrow due to better particle packing. In addition, various fillers with different particle shapes (e.g. rods, plates, grains, spheres, etc.) can significantly affect rheological properties. For example, higher aspect ratio materials reduce flexibility.
Thixotropic (thickening) agents such as fumed silica, talc, colloidal silica and silicates are used in adhesives to impart properties such as flow, extrusion control, physical stability (anti-settling), anti-sag, and reinforcement of cured adhesives to improve tear strength. Tack and peel are important properties that can often be measured by measuring the viscoelastic properties of an adhesive over a range of frequencies.
During storage the adhesive must be stable under a variety of conditions, such as changes in temperature. Monitoring the particle size will indicate whether changes are taking place that will limit the shelf life.
Formulating a stable product can be assisted by measuring the zeta potential. Ensuring that the particles in the adhesive have a high zeta potential will mean that they resist aggregation. The zeta potential of the particles can depend on a number of parameters of the formulation including the pH and concentration of any ions present, and studying how the zeta potential changes with different formulations can lead to a more robust product.