Rheological Testing of Tack

Materials that are tacky or sticky are easily identified by touch. This is normally done by holding the material between the forefinger and thumb, and feeling how sticky and difficult it is to remove the material from the fingers. Tack refers to the material’s ability to stick to a solid surface when a slight pressure is applied. However, the adhesive bond formation could not be determined directly, but rather measured by breaking bonds.

Tack should never be confused with peel. The latter refers to the separation of bonds of a rigid material and a flexible material, or of two flexible materials, by pulling the material from the joining surfaces. Contact time is the difference between peel and tack. The test must be designed in such a way that the tack probe is able to make contact with the adhesive for a brief period of time. However, in peel tests, the adhesive must make contact with the substrate for an extended period of time. A number of tests are available to determine tack such as the peel test, the probe tack test, rotating wheels and rolling ball. However, these tests cannot be compared with one another.

It appears that liquids will be more or less tacky within a certain range of viscosity. Usually, liquids that are moderately high in viscosity are tacky. As the liquids’ viscosity increases or decreases, the tack decreases in both the cases. As a result, tack will reach a maximum within a specific range of viscosity. Chewing gum, honey, syrups, and oils are extremely tacky. In order to ensure good contact, the viscosity should be sufficiently low to flow over the substrate’s surface. While fluidity and surface tension are required to ensure good flow and wetting, they are not the only conditions for producing high tack.

For pressure sensitive adhesives or PSA, the adhesive’s viscoelasticity must be taken into account. One could ask the questions, to what extent does elasticity and shear viscosity control the PSA’s tack? To resolve this question, a better understanding of tack mechanism is required.

Adhesive or cohesive failure or a combination of both can result in breakage of the adhesive bond. It is a well-known fact that rheology of viscoelastic materials relies on the rate and amount of deformation as well as on the mode of deformation or kinematics.

Both removal and attachment of a PSA from a substrate takes place in tension and compression modes, and shearing is introduced to a lesser extent. The PSAs’ rheology is calculated in shear to measure both their viscosity and elasticity in their linear viscoelastic regions. This is because when compared to extensional measurements, shear measurements can be easily carried out.

Nevertheless, researchers have correlated the shear rheology with peel and tack of PSA, especially the storage modulus G’ has been associated with these PSA characteristics. The Dahlquist’s Criterion considers compliance J, which is the inverse of shear modulus. There could be an issue when correlating linear viscoelastic rheological data to bulk mechanical deformations, particularly when the deformation modes are completely different. However, retardation spectrum analysis in the short time period of the tack measurement could provide a better understanding regarding the structural components controlling tack. There is still no clear understanding whether the tack is controlled by bulk rheology or surface tension, and whether shear rheology can be correlated to the PSA’s probe tack. These issues can be addressed by evaluating three unsupported, free film adhesives. These film adhesives have different tack properties and measure roughly 0.15mm thick. Measurement of shear creep that leads to retardation spectra detects the bulk viscoelastic characteristics of these adhesives. The tack properties of these adhesives are determined through the tack probe test.

Experimental Procedure

The adhesive is first fixed to a flat bottom plate measuring 19mm. The probe tack test device, made of steel, is flat and measures 5mm in diameter. Tack measurements are performed with the ARES rheometer. The unsupported PSA measuring 0.15mm thick is fixed to the bottom plate and the probe is mounted roughly 0.05mm above the adhesive. The tack test was programmed to ensure that the probe decreases at 0.1mm/s for a period of 2s, and then the direction is reversed at the same speed. Subsequently, the load was determined as a basis of time. The SR5 controlled stress rheometer was used to perform the creep tests, and all measurements were performed in the linear viscoelastic region. Tests were carried out at temperatures of 0°C , 25°C , 60°C  and 80°C by means of 25mm parallel plates. The sample‘s temperature was controlled with the peltier.

Results and Discussion

Extensive studies have been made to establish the link between the shearing rheology and the tack and peel adhesion of PSAs. Chu reported on the dynamic mechanical properties and 180° peel, Quick Stick, and Polyken Tack.

High frequency storage modulus G’ (100 rad/s) and low frequency storage modulus G’ (0.1 rad/s) correlate well with peel and tack, respectively. Giordano also demonstrated the link between G’ and tack, and Dale and others also reported excellent correlations with tan delta.

Low G’ PSAs are characterized by higher tack. Dahlquist carried out creep measurements and ultimately discovered that retardation and compliance spectra contribute to the tack or stick of PSA. Dahlquist found that a PSA should have creep compliance greater than 10^-8 1/Pa for a suitable tack. This is referred to as the Dahlquist’s Criterion. Retardation and creep spectra were acquired to measure the PSAs’ viscoelastic properties. Crosby and Shull used a probe test to study the PSAs and investigate the failure mechanism of a filled and unfilled adhesive. They studied the rate of energy release as well as the relative effects of interfacial and bulk processes.

Zosel analyzed the rate of PDMS cross-linking of PDMS on the tack behavior and discovered that the highest tack strength takes place when the adhesive is not strongly cross-linked. This structure results in a stress-strain curve with a large shoulder followed by the stress maximum and a high strain to break. From high speed photography the formation of fibrillar structures within the adhesive were observed. Good and Gupta examined the mode of separation, that is, adhesive against cohesive failure, and the mechanism of filament elongation. It should be noted that in this analysis the rheological information was achieved in the time scale of the probe tack test. The PSA compliance information was acquired in its linear viscoelastic region, but the probe tack information occurs in its non-linear viscoelastic regions over a strain of 10%, corresponding to 0.2s. Based on the Newtonian viscosities, different flow behavior should be seen in the samples upon contacting the steel tack probe. Viscosities of 5x10⁷, 3.4x10⁵ and 4.8x10⁴ Pa are found in samples A, B, and C, respectively.

Figure 1. Typical data from a tackprobe test V.

A stranded probe tack test result is shown in Figure 1. The 0.15mm adhesive is fixed to a flat steel plate, and is compressed and released at 0.1mm/s rate. The same is squeezed for 1 to 2s and drawn in tension; 1.53N is the highest release force, 7.15N s is the area under the force vs time plot, and 31s is the separation time. This correlates to 3.1mm separation distance and 2100 % strain at separation.

Figure 2. Tack probe data of sample C at 25°C.

The force-time plot of Sample C pulled by the tack probe is shown in Figure 2. At 0.2s, the sample was strained 10% and at 1.2s, the sample yielded when strained was at 80%. It started flowing where fibrils are created. The sample was stretched further than 2000% without separating. The tack curves for the three PSA samples A, B and C are shown in Figure 3. All samples were tested at room temperature.

Figure 3. Comparison of tack probe data of sample A, B, C at 25°C.

It can be seen that Sample C has the maximum tack as defined by the longest separation time and the largest area under the tack curve, while Sample B is intermediate in its amount of tack and exhibits a yield force between Sample C and Sample A. Both failed cohesively. With an adhesive failure, Sample A does not elongate upon separation.

Figure 4. Creep compliance of sample A, B, C.

The compliances of the three samples tested at 25°C are shown in Figure 4. In order to establish their linear viscoelastic regions, Samples A, B, and C were tested in creep at different levels of stress for individual temperature.

Figure 5. Expanded view of the creep recovery curve of sample B tested at 60°C

Figure 5 illustrates the short time strain for Sample B examined at 60°C temperature during the course of the creep experiment. The strain corresponding to 0.2s is 10%. This corresponds with time and strain of the force-time plot of the tack probe test. The tack probe test data for all the measurements are summarized in Table 1. Also shown are areas measured from the force-time plot, the yield forces, and break strains where the sample isolates from the probe. Samples separating in the range of 100g or less are considered as failing adhesively. Bigger area correlates to yielding processes and leads to cohesive failure.

Table 1. Yield forces and break strain for samples A, B, C at various temperatures

Let us take the retardation spectra of three samples. In Figure 6, spectra of Sample A, B and C are seen to go further than the time period of the tack probe test. The time taken to deform at a strain of 10% is 0.2s. This strain represents the upper limitation of the linear viscoelastic region in elongational as well as shearing modes of deformation. This is very critical given that the elastic or viscous response is controlled by the Deborah number during the adhesive separation process and/or cohesive failure process.

The Deborah number refers to the ratio of the typical retardation or relaxation time to the stimulus time. In event the Deborah number is higher than one, the deformation would be elastic. For a standard viscoelastic material, the time required for the molecular rearrangement to occur is similar to the time period of the experiment.

Figure 6. Retardation spectra of samples A, B, C tested at 25°C.

Figure 7. Retardation spectra of sample A, tested at 25 and 8°C.

Figure 8. Retardation spectra of sample B tested at 25 and 60°C.

Samples B and C will behave similar to elastic solids if the deformation rate is relatively fast. If the Deborah number is close to one, the material will be viscoelastic.

The retardation spectra in Figure 6 reveal typical plateau, transition, and terminal zones. For Sample A, the retardation spectrum displays merely a plateau, while Sample C reveals all the three zones, i.e., plateau, transition, and terminal zones. Only plateau and transition zones are seen in Sample B. Figures 7 and 8 show the retardation spectra of sample A, tested at 25°C and 8°C, and retardation spectra of sample B tested at 25°C and 60°C, respectively.

Sample A remains steady across the quantified retardation times between 0.001 and 100s. It exhibits the lowest compliance in the stimulus range of the tack probe tester. This is not favorable to huge deformations as observed from the result of the tack probe test. A wide rubbery plateau suggests that the sample could be slightly cross-linked, and hence, will deform elastically across a range of release rates. On account of its high cohesive strength, the sample does not elongate, thus failing adhesively.

PSA C has all three zones, rendering it an excellent balance of cohesion and adhesion. It will have poor tack at rapid release rates and shorter times of less than 0.03s. This is because the compliance is very low as per the Dalhquist’s Criterion. However, it will be more fluid-like and flow easily because the compliance increases at the longer times with respect to the terminal zone. Since the adhesive flows and filaments are created, elongation at separation would be quite high.

Sample B exhibits the plateau and terminal zones of the spectrum and displays a wide plateau. Due to this aspect, its tack behavior and viscoelastic will be more rate dependent when compared to adhesives having a flatter plateau in their retardation spectrum. In addition, Sample B will behave solid-like below 0.04s and turns relatively compliant at longer times, but fails to elongate like sample C as the time period of the terminal zone is quite long for the stimulus time of the tack probe test.

The tack of Sample A was enhanced by increasing the temperature. It was heated to 80°C temperature and tested in tack and creep. For unknown reason, adhesive failure occurred as the area reduced to 1 N s. When Sample B was tested at 60°C temperature, the yield force reduced from 7.16 to 1.53N, but when tested at 0°C, the highest adhesive force was found to be 3.38N. The area measured just 0.5N s, which showed that yielding does not occur.

Figure 9. Plot of log force against log 1/5 second creep compliance for all samples

The rubbery plateau still remained between 25°C  and 80°C and the decreased compliance at the long time of Sample A is unrealistic. Upon comparing the retardation spectra of Sample B at 0°C , 25°C  and 60°C, the consistent trend is shown where the retardation times are changed to lower compliance with longer times and decreasing temperature.

A linear relationship between log of 1/5s compliance and logarithm force is shown in Figure 9. Samples that have areas of work of separation greater than 0.981 N s were fitted to a linear least squares plot. These data correspond to cohesive failures. The correlation coefficient of 0.982 shows the agreement is good. The other two data sets are not included because the are adhesive type failures

Conclusion

Tack is a property of “quick stick” determined by the nature of separation, irrespective of whether it is the result of cohesive or adhesive failure. When the compliance of the adhesive is over the 10^-8 1/Pa, the probe test can be estimated from shear creep information, if measurements are performed in the same period of time. This relates to 0.2s in this analysis. A log-log linear link was observed between the tack probe maximum force and the 1/5s compliance.

This information has been sourced, reviewed and adapted from materials provided by TA Instruments.

For more information on this source, please visit TA Instruments.