Using a Rotational Rheometer to Determine Pressure Sensitive Adhesion and Tack with Axial Measurements

In the context of material behavior, tack or tackiness is related to stickiness, and may result from cohesive forces in a material bridging two substrates or adhesive forces between two materials in contact.

Tack is an essential requirement for pressure sensitive adhesives such as labels and tapes, and describes their ability to create an adhesive bond to a substrate when exposed to slight pressure and brief contact. However, in certain materials and applications, tack may be an undesirable property. For instance, bone cements need to be tack-free in accordance with ISO5833 so that they can be shaped and applied without adhering to gloves or application aids.

The behavior and perception of consumer products may also be affected by tack, for instance, chewing or handling of sticky foods, and extrusion of thick viscoelastic products like toothpaste. In addition, surface properties can be assessed, and the cleanliness of the surface can be judged. Therefore, tack can be qualitatively assessed through touch or feel, but such evaluations are subjective, difficult to measure, and may be affected by other additional factors.

Tack Measurement

Screening, comparing, and quantifying 'tack' or 'stickiness', with the help of a simple objective test, can be useful for many research and development purposes. A number of such standard tests are available for the adhesives industry based on the type of products to be analyzed. Loop tack, quick stick and rolling ball tests are some of the standard tests.

This article discusses the inverted probe test, a widely used method in the adhesives industry. This test involves bringing an inverted probe into contact with an adhesive at a constant speed, pressure and contact time to measure the tack as the maximum force needed to cause failure of the resultant bond.

As shown in Figure 1, the peak in negative normal force (tension) is attributable to 'tack', the area under the force-time curve to adhesive or cohesive strength and the time needed for the peak force to decay by 90%, which is 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 measuring tack properties of Blu-Tack® by applying a range of contact forces (5 N, 10 N, 15 N and 20 N). A Kinexus rotational rheometer, equipped with a Peltier plate cartridge, was employed to perform the measurements at 25 °C, using a 20 mm upper plate and 65 mm lower plate (stainless steel), and a standard pre-configured sequence in the rSpace software.

After placing a 1.3 g ball of sample in the center of the lower plate, without applying any pressure, the upper plate was brought into contact with the specimen at a rate of 10 mm/s, until attaining the required contact force. After a brief contact period of 2 s, the gap was linearly increased at a rate of 10 mm/s, and the normal force was measured as a function of time.

Experimental Results

Figure 2 shows a gap and normal force profile for the Blu-Tack® under a normal force of 10 N, showing the approach of the upper plate to the specimen at a rate of 10 mm/s, and the corresponding increase in the normal force when the contact is made. When the gap is linearly increased after a contact period of 2 s, there is a corresponding decrease in compressive force, but a residual tensile force corresponding with tack and adhesion.

Normal force-time profiles for Blu-Tack® with 10N pressure applied.

Figure 2. Normal force-time profiles for Blu-Tack® with 10N pressure applied.

Figure 3 and Table 1 present the comparative results obtained from the application of different contact pressures, correlating only to the tensile (negative) force corresponding with tack and adhesion.

Tensile force-time profiles for Blu-Tack® with different contact pressures.

Figure 3. Tensile force-time profiles for Blu-Tack® with different contact pressures.

From the results, there is an increase in the residual tension or tack with applied force, especially up to 15 N. However, only a slight incremental increase was observed at the applied force of 20 N.

The area under the curves represents the adhesive/cohesive strength of the material, and there is an increase in the area with contact force up to 15 N. However, the area is decreased at 20 N, indicating the presence of an optimum contact force between 10 N and 20 N that provides optimum adhesion under these conditions.

Table 1. Analysis results based on Figure 3 for different contact pressures.

Sample Description Action Name Time (action)(s) Normal force(N) Gap (mm) Area result
5 N Peak normal force 0.3573 -1.677 7.8614
Time for Force to reduce by 90% of peak 0.7006 -0.1677 11.2888
Area under force time curve (N/s) 0.4799
10 N Peak normal force 0.3525 -3.492 6.6156
Time for Force to reduce by 90% of peak 0.6906 -0.3492 9.9909
Area under force time curve (N/s) 0.8353
15 N Peak normal force 0.3690 -4.220 6.0800
Time for Force to reduce by 90% of peak 0.7127 -0.4220 9.5118
Area under force time curve (N/s) 1.977
20 N Peak normal force 0.3105 -5.363 5.2124
Time for Force to reduce by 90% of peak 0.6522 -0.5363 8.6237
Area under force time curve (N/s) 1.280

For contact forces of 5 N, 10 N and 15 N, the time for the force to decay by 90% of its peak value is similar, but slightly lower for 20 N. This indicates a slightly quicker failure rate beyond 20 N of pressure.

Conclusion

The tackiness or cohesive/adhesive properties of pressure sensitive adhesives can be assessed using a Kinexus rotational rheometer with advanced axial test capabilities. This article discussed the assessment and comparison of such properties for a sample of Blu-Tack® under different contact forces. The results revealed that there is a maximum pressure between 10 N and 20 N, providing optimum adhesion under the test conditions.

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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