Understanding Tack Tests with Kinexus

General

A tack test measures adhesion and/or cohesion forces. Cohesion is an attraction between molecules of the same substance, while adhesion refers to an attraction between different molecules; for example, between a substance and a substrate.

Measurement Conditions

Tack tests are very common in the adhesive industry to quantify the stickiness of different products. In the following, the Kinexus rotational rheometer was used to perform a tack test on hair wax. To this end, a commercial hair wax strip was fixed on the lower plate and held at 25°C. The upper plate (8-mm diameter) was lowered until a normal force of 30 N was reached on the wax. After 5 s of contact, the gap was linearly increased at a speed of 30 mm·s-1.

Measurement Results

Figure 1 depicts the resulting normal force vs. time obtained for the test carried out at 25°C. As soon as the upper plate goes up, the force increases, reaching a maximum value at the point where the bond breaks.

Typical results of the tack test are:

• The peak area, related to the adhesive or the cohesive strength, dependent on the type of test.
• The force at peak maximum, called tack.
• The time for the force to decrease by 90%, or failure time.

The higher these values, the stronger the adhesiveness or cohesiveness.

The tack test can be performed to compare the bonding abilities of different materials. It can also be used to investigate the influence of the following parameters on stickiness:

• Temperature
• Applied axial force
• Contact duration
• Speed of the upper plate

Here, the test was carried out at different temperatures in order to determine how the tack capacity of the hair wax is affected if it is stored in a fridge or in the desert. To this end, temperatures of 8°C and 50°C were selected. Figure 2 and Table 1 depict the results. They show that temperature greatly influences the stickiness of hair wax. The tack differs by a factor of more than 20 between the measurements at 8°C and 50°C.

Table 1: Tack, cohesive strength and failure time

Conclusion

The temperature has a significant influence on the tack test results. As expected, the higher the storage temperature, the stickier this hair wax. Storage in the fridge (typical temperature: 8°C) must be avoided, as this hair wax would become very cohesive, and would not stick properly to the skin and hair.

It should be noted that the parameters selected for the tests highly influence the results. Thus, a comparison of the stickiness of different materials requires identical measurement conditions, particularly the sample temperature, the normal force applied and speed of the upper plate.