Using a Rotational Rheometer to Assess Adhesion and Tackiness

The tack, or tackiness, of a material is related to its stickiness, and is caused by cohesive forces in a material bridging two substrates, or by the adhesive forces between two materials in contact. Tack is an essential aspect of pressure sensitive adhesives such as labels and tapes, as it defines their ability to create an adhesive bond with a substrate when exposed to mild pressure and brief contact.

However, there are some materials and applications where tack may be an undesirable property. For instance, bone cements must be tack- free, in accordance with ISO5833, so that they can be shaped and applied without using gloves or application aids. Tack may also have an impact on the behavior and perception of consumer products, for instance, chewing or handling of sticky foods and extrusion of thick viscoelastic products like toothpaste from tubes.

Tack is also helpful in evaluating the surface properties and confirming whether the surface is clean or not. Therefore, tack can be qualitatively assessed simply through touch or feel, but the results are subjective and difficult to measure, due to the possibility of influence of other additional factors.

Tests for Tack Measurement

Screening, comparing, and measuring 'tack' or 'stickiness' by means of a simple objective test can be useful for many research and development activities. Several such standard tests are available for the adhesives industry, based on the type of products. Loop tack, quick stick and rolling ball tests are some of the standard tests.

Another test used in the adhesives industry is the inverted probe test, where the adhesive is contacted by an inverted probe at a fixed speed, contact time, and contact pressure. The tack is then measured as the maximum force needed to break the resultant bond.

While an equivalent test can be performed with a Kinexus rotational rheometer, this article describes a more general test that can be applied to a wide variety of materials. In this method, the force needed to separate two parallel plates containing a prescribed volume of material between them from a stationary position is measured without applying any initial pressure.

The peak negative normal force (tension), shown in Figure 1, is attributable to 'tack', the area under the force-time curve to adhesive or cohesive strength, the time required for the peak force to decay by 90% – a comparative measure of failure rate or time.

Annotated normal force-time profile showing key features for assessing adhesive/cohesive properties under tension.

Figure 1. Annotated normal force-time profile showing key features for assessing adhesive/cohesive properties under tension.

Experimental Procedure

This experiment involved the measurement and comparison of the tack properties of three different commercial products: hair wax, toothpaste and honey. A Kinexus rotational rheometer, equipped with a Peltier plate cartridge, was employed to perform tack measurements, using matched 20 mm upper and lower plates and 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 working gap employed was 100 µm, and the sample was trimmed flush with the plate edge. By automatically adjusting the working gap before performing tack measurements, the applied force was ensured as zero at the start of the measurement. In the pull away test, the gapping speed used was 25 mm/s, and all measurements were carried out at 25 °C.

Experimental Results

Figure 2 and Table 1 show the tack testing results from the three samples. The hair wax showed a peak normal force of -10.32 N. This makes it the tackiest product of the three samples, compared to the toothpaste with a peak normal force of -7.97 N and the honey with -6.7 N peak normal force.

Normal force-time profiles for hair wax, toothpaste and honey

Figure 2. Normal force-time profiles for hair wax, toothpaste and honey

The gap relative to this peak force is associated with the critical strain of the material, describing the deformation behavior of the material before the onset of failure when taking into account the initial gap. Here, the most ductile material is the hair wax, reaching a gap of 0.348 mm. The gap for the toothpaste and the honey is 0.326 mm and 0.27 mm, respectively.

Table 1. Analysis results based on Figure 2 for hair wax, toothpaste and honey.

Sample Description Action Name Time (action)(s) Normal force(N) Gap (mm) Area result
Hair Wax Area under force time curve (N.s) 2.557
Time for Force to reduce by 90% of peak 0.4744 -0.9868
Peak normal force 0.1388 -10.32 0.3476
Honey Area under force time curve (N.s) 1.536
Time for Force to reduce by 90% of peak 0.4646 -0.6289
Peak normal force 0.1251 -6.644 0.2704
Toothpaste Area under force time curve (N.s) 1.766
Time for Force to reduce by 90% of peak 0.4684 -0.7540
Peak normal force 0.1297 -7.967 0.3260

The area under the curves represents the adhesive/cohesive strength of the material, and the hair wax is the strongest material, followed by the toothpaste and then the honey. Although the materials show differences in tack and strength, their force decay profiles are very similar, taking similar times for the force to decay by 90% of its peak value in the region of 0.47 s.

Conclusion

The tackiness or cohesive/ adhesive properties of a material can be evaluated by monitoring normal force (tension) during a pull away test, using a Kinexus rotational rheometer. This article discussed the evaluation and comparison of such properties for hair wax, toothpaste and honey.

This information has been sourced, reviewed and adapted from materials provided by Malvern Panalytical.

For more information on this source, please visit Malvern Panalytical.

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