Pressure sensitive adhesives (PSAs) are composed of two components, namely a tackifier component to make the adhesive sticky and a latex component to allow the tackifier to flow.
They are complex colloidal systems involving the use of several additives to adjust their wet properties, to ensure their stability during storage, and their ability to mix and coat the substrate surface.
The compounding of PSAs involves the mixing of many different component parts together. Mixing of the tackifier emulsion and the aqueous latex with other components is performed to form the adhesive for coating.
It is necessary to perform rheological characterization for each component to identify its pump-ability. Moreover, it is necessary to characterize the whole PSA to measure the pumping and filtering properties.
The following expression can be used to calculate the shear rate experienced during processing:
Where, Q = Volumetric flow rate and r = Pipe radius
Viscosity measurement at selected shear rates a little above and below the estimated value allows for the generation of a relevant portion of the flow curve. The resulting data can be fitted with a power law model to determine the values of ‘k’ and ‘n’ so that the flow behavior can be described. The power law model is expressed as follows:
Where, k = Consistency, n = Power law index, σ = Shear stress, and ý = Shear rate.
The unit of consistency is Pasn. However, consistency is numerically equivalent to the viscosity at a shear rate of 1s-1. The range of power law index starts from ‘0’ for very shear thinning materials to ‘1’ for Newtonian materials.
This experiment measured and compared three PSAs. A Kinexus rotational rheometer featuring a Peltier plate cartridge and a 40 mm/1° cone-plate measuring system was employed to perform rotational rheometer measurements at 25°C, utilizing standard pre-configured sequences in the rSpace software.
The use of a standard loading sequence ensured the application of a consistent and controllable loading protocol on the samples.
The calculation of the relevant shear rate for flow in the pipe was performed automatically as part of the test sequence, using input values of volumetric flow rate, pipe radius and length.
This was followed by performing a shear rate table using a start value of (calculated shear rate/2) and an end value of (calculated shear ratex2). The resultant flow curve was then fitted with a power law model.
From the results presented in Figure 1, the highest viscosity is observed for Adhesive 3, followed by Adhesive 2 and then Adhesive 1. Therefore, it will be more difficult to pump Adhesive 3 compared to other two samples.
However, the ‘n’ value for Adhesive 3 is lower than other samples and therefore can be pumped more easily at higher shear rates.
Figure 1. Flow curves for the three PSA’s and their corresponding power law indices
Pumping problems can be minimized by lowering the viscosity of the sample through increased pumping shear rate. This approach is highly efficient with a smaller shear thinning index (n <<1). Pumping a high viscosity sample such as Adhesive 3 will be more difficult than a low viscosity sample unless the shear thinning index is small.
It is possible to analyze formulations and assess their pump-ability and mix-ability before plant trials. In addition, it is also possible to test the formulations to identify the ideal combination of additives for optimizing the sample for mixing and pumping.
This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.
For more information on this source, please visit Malvern Panalytical.