This article considers three complementary analytical techniques that provide the information needed for optimal latex adhesive performance, dynamic light scattering, zeta potential and rotational rheometry.



Effectively formulating a latex adhesive requires knowledge of the material’s rheological response during normal use. In storage, the adhesive is subject only to gravitational force, a very low-stress condition. During application, however, high shear rates are applied. To meet performance targets, formulators tailor the adhesive’s viscosity profile (how viscosity changes with shear) by varying particle size, size distribution or solids loading. An optimal profile delivers a stable product with excellent transfer properties. This article considers three complementary analytical techniques that provide the information needed for optimal performance: dynamic light scattering, zeta potential and rotational rheometry.

Defining Optimal Performance

The first step in optimizing an adhesive’s formulation to obtain the best possible flow properties is to define viscosity limits that will enhance the performance of the chosen coating method. At low shear rates, high viscosity improves particulate sedimentation resistance during storage - an important feature for all adhesives. In addition, high viscosity allows a roller to pick up more adhesive during roller coating. However, too high a viscosity prolongs the time it takes for a coating to absorb into a substrate.

Performance at high shear rates can indicate an adhesive’s transfer properties and mechanical stability. A lower viscosity at a high shear rate will give better transfer between rollers or a thinner coating under a blade. Unfortunately, with some adhesives, viscosity rises under high-shear conditions due to aggregation or shear thickening. The propensity of a material to exhibit this behavior is a function of variables such as particle size distribution, solids loading and zeta potential.

Figure 1. The Potential of a Particle at Increasing Distance from the Surface.

Factors that Influence Adhesive Behavior

While the target viscosity profile may be clear, reaching it requires knowledge and the successful manipulation of many interrelated parameters. Each variable fits together and provides a thorough understanding of the stability and coating performance of an adhesive. Following are four key factors.

1. High Shear Viscosity
High shear viscosity, a measure of resistance to flow at the high shear rates imposed by coating processes, is a defining performance parameter. Therefore, subjecting an adhesive to high shear testing is the best way of assessing sample stability for spray-, blade- or roller-coating applications; it is also the ideal starting point for rheological analysis. If a sample displays inadequate stability for the preferred coating method, particle-size distribution and zeta potential measurement can provide more information about the underlying causes of instability.

A high shear rate is usually achieved using a rotational rheometer that features parallel plate geometry with a very narrow gap (typically 100 µm or less). Selecting an instrument capable of applying very high shear rates (up to around 4 x 105s-1) is critical for simulating the shear conditions generated by routine coating processes. A narrow gap provides accurate high shear rate measurements and ensures that the flow field remains within the stable regime (e.g., avoiding Taylor flow, or turbulence). The viscosity/shear rate profile from such an analysis provides a clear indication of whether and when a sample starts to aggregate or agglomerate.

2. Solids Loading
Solids loading is defined as the volume percentage of solid material in a sample. Adhesive manufacturers often seek to maximize the solids content of coatings, as a high solids content enables coatings to dry rapidly with minimal energy input. However, the viscosity of a sample rises markedly with increased particle loading, especially as it tends toward the maximum packing fraction.

Samples with solids loadings of 50-60% are shear thinning at low to moderate shears; however, at high shear rates (typically more than 104s-1), shear thickening can occur due to hydrocluster1formation. Hydroclusters1are aggregates that trap some of the continuous phase, thereby increasing viscosity. The shear thickening effect is usually temporary, as the molecules are separated by a lubricating solvent layer; however, it can be permanent where shear-induced aggregation occurs.2

3. Zeta Potential
The charge on a colloidal particle depends on both the chemical nature of the particle’s surface and also the dispersion medium. This may contain oppositely charged ions that will adhere to particle surface. The first layer of ions, the Stern Layer, strongly adheres to the surface. Outside of this layer is a less tightly bound layer of ions that form part of a diffuse cloud of oppositely charged ions. As the particle diffuses or sediments, there will be a plane, known as the slipping plane, which defines the junction of this cloud and the continuous phase. The net potential at this plane is known as the zeta potential, and it defines the effective charge on the particle (see Figure 1).

The formulation of a stable adhesive can be greatly assisted by the measurement of zeta potential, as for small particles (nominally < 1 µm); it is the zeta potential that determines whether or not a sample is likely to be stable. A strongly positive or negative zeta potential, such as > ±30 mV, usually indicates that a sample will resist agglomeration and be more stable at both low and high shear rates.

Zeta potential is measured by applying an electric field to the dispersion, causing particles with a zeta potential to migrate toward the electrode of opposite charge. The velocity at which the particles move is proportional to the magnitude of the zeta potential and can be measured using light scattering techniques.

4. Particle Size Distribution
Particle size distribution (PSD) and particle shape influence the ability of particles to pack together. For a given solids content, a suspension with a bimodal (or broader) PSD has a lower viscosity than a monomodal dispersion because smaller particles fill the voids between larger ones, giving the system better packing3properties and lubricating interparticle movement. As the particle size of adhesive constituents often lies in the sub-one micron range, light scattering techniques are a good choice for accurate measurement.

Figure 2. Viscosity vs. Shear Rate Profiles of Samples A (Green), B (Red) and C (Blue).

Case Study: Characterizing the Performance of Three Adhesives

Dynamic light-scattering techniques and rotational rheometry together quantify the factors that strongly influence adhesive performance, providing information to optimize formulation. Analysis of three samples of styrene butyl acrylate latex adhesive with different high shear stabilities illustrates this point. A Gemini rheometer (Malvern Instruments) with a 40 mm parallel plate (PP40) geometry was used for the high shear rheology measurements, and a Zetasizer Nano dynamic light scattering particle characterization instrument (Malvern Instruments) was used for the particle size and zeta potential measurements.



Figure 3. Close-up of High Shear Viscometry Data 5,000-200,000 s-1. Samples A (Green), B (Red) and C (Blue).

Viscosity profiles for these adhesives (see Figure 2 and 3) show that Samples A, B and C are stable up to around 147,000 s-1, 112,000 s-1and 75,700 s-1, respectively (see Table 1). Samples B and C both have a bimodal PSD (see Figure 4 and Table 1) and display correspondingly lower viscosities at low to moderate shear rates, despite having a comparable solids loading to Sample A. It is worth noting that both absolute values and the ranking of viscosities at lower shear rates are very different from those at high shear rates. This finding suggests that a viscometer would have missed vital data in this analysis, as such instruments tend only to measure at low to moderate shear rates.

Figure 4. Particle Size Results Measured by Dynamic Light Scattering. Samples A (Green), B (Red) and C (Blue).

The relatively low mechanical stability of Sample C - as shown by the sudden drop in viscosity at high shear rates, when the sample thickened so much that it came out of the instrument (see Figure 3) - is attributable to an insufficiently high zeta potential. Samples A and B both have a higher zeta potential (see Table 1) and, therefore, greater resistance to shear-induced aggregation.2At very high shear rates where hydrodynamic effects dominate, Sample A shows the greatest stability, mainly due to its high zeta potential and possibly also the higher water content (which is necessary to give it a sufficiently low viscosity). Here, zeta potential appears to be the most important factor in determining stability, particle size distribution having exerted a marked but less-critical impact at lower shear rates.

Table 1. Zeta Potential and Particle Size Results.

Conclusion

The key properties affecting the performance of latex adhesives can be measured using modern rheometers and dynamic light-scattering instrumentation. It is the combination of zeta potential, particle size and rheological data that offers a powerful predictive insight into how samples will behave in pilot or full-scale trials and, most importantly, when used by the customer. In new product development, the use of these analytical techniques eliminates the need for wasteful trials and facilitates the identification of optimal formulations at minimal cost. The experiment described here demonstrated that relevant information can be obtained even by testing small batches of formulation (as low as 10 ml of each sample in this instance), making it easier to test products at the very earliest stages of the development cycle.

For more information on testing, visit www.malvern.com.

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