How to Achieve Optimum Cohesive Strength in Pressure Sensitive Adhesives?

If we consider the mechanical response in terms of the storage modulus (G') for a typical natural rubber-based PSA as a function of frequency. The storage modulus represents the elastic deformation of a material. It is a measure of the hardness at a given temperature and frequency. 

For a good PSA with high cohesive strength, the storage modulus (G') at room temperature has a value of 5x10⁴ to 2x10⁵ Pa.² The approximate strain rates encountered in typical operations associated with the manufacture and end use of a common pressure-sensitive tape adhesive can be calculated. These application rates correlate well with the frequency applied in an oscillation experiment. (See Figure 1)
 

• Low-frequency testing (e.g., low-deformation rates) characterizes bond formation and describes shear resistance.
• High frequencies are associated with debonding behavior and relate to tack and peel strength.

Figure 1: Storage Modulus (G') for a Typical Natural Rubber-based PSA as a Function of Frequency.

 

Tack refers to the ability of an adhesive to form a bond in short contact times and at low pressure.³ Bond strength, a major performance parameter, is tested using a peel or tack test. The bond strength is not measured directly, but assessed by measuring the force while breaking the bond after the adhesion step.⁴

The cohesive strength of a PSA can be measured in continuous shear; a creep experiment is the most appropriate test. In practice, the damping behavior tan δ obtained in an oscillation test is used. Tan δ is the balance between the loss modulus (G''), associated with the energy dissipation or viscous deformations in a material and the storage modulus, and correlates well with cohesive strength. A PSA has a good cohesive strength for low tan δ values.

 

In an oscillation test, the frequency dependence of a PSA can be measured easily and quickly in a range from 0.01 to 100 Hz. The modulus in this frequency range at application temperature relates directly to the wetting and flow properties during application. 
 

• If the modulus becomes too high, the ability to wet the substrate disappears.⁵
• The viscosity must be low enough for the adhesive to spread and flow into the substrate to make good contact.

Low surface tension, low modulus or low viscosity are necessary for good wetting and flow. An adhesive is not a good adhesive unless its bonds are strong enough to resist separation.

The experimental frequency range can be extended by making use of the t-T superposition (TTS) to create a master curve at the desired reference temperature. The procedure consists of measuring the material's properties in the same frequency range at different temperatures. The TTS is based on the following facts:
 

• At higher temperatures, molecular relaxation and rearrangements in viscoelastic materials occur at accelerated rates.
• A direct equivalence between temperature and time or frequency effects (thermo-rheological simple material) exists.
   

If a material is thermo-rheologically simple in the frequency range of interest, the data of the isothermal frequency scans can be shifted until they overlap. The result is a master curve over a much wider frequency range at a free-selectable reference temperature.⁶

Figure 2 illustrates the master curve for two PSA samples⁷ measured at temperatures from -90 to 130°C and shifted to a reference temperature of 20°C. Relevant to the PSA performance is the frequency range from the glass transition to the flow region.
 

• Sample A has a higher storage modulus G' and a lower tan δ, thus it has higher cohesive strength.
• Tack, which is related to the ability to form good contact with the substrate, is higher for Sample B.
• The higher tan δ, the higher G' is, i.e., the higher the energy dissipation associated with viscous deformation, the better the flow into the substrate.
• At low frequency, the modulus G' of Sample B drops faster than Sample A. This indicates that Sample A has a higher shear resistance than Sample B.

Figure 2: Master Curve of PSA Samples A and B Measured at Temperatures from -90 to 130°C and Shifted to a Reference Temperature of 20°C.

Instead of analyzing the frequency dependence of the dynamic mechanical properties of polymers, it is often more meaningful to look at temperature dependence. Many PSAs are block copolymers such as styrene-elastomer-styrene copolymers.

The dynamic mechanical properties of a styrene-isoprene-styrene copolymer and a natural rubber-based PSA are plotted in Figure 3. 
 

• The elastic modulus (G') exhibits a plateau region between the two domain glass transitions.
• The height of this plateau is governed mainly by the shape and size of the glassy domains of the polystyrene.
• Two glass-transition temperatures (Tg) are seen due to the two-phase structure of the block copolymers.
   

Figure 3: Temperature-dependent Dynamic Mechanical Properties of a Styrene-isoprene-styrene Copolymer and a Natural-rubber-based PSA.

 

The PSA performance is dependent upon the glass-transition temperatures of the adhesive constituents relative to the expected use temperature. For room-temperature PSAs, a Tg of about -15°C to 5°C offers good adhesive performance. The temperature of the glass transition defines the lowest use temperature for an adhesive. 

The elastic modulus G' is approximately 1x10⁵ Pa for the SIS block copolymer adhesive. It is independent of temperature in the range from 15 to 80°C, thus providing a high and consistent cohesive strength over a range of use temperatures.

For most synthetic elastomers, the storage modulus at room temperature is higher than for natural rubbers. By adding compatible tackifying resin, the storage modulus (G') at use temperature is reduced. But at the same time, the low-temperature rubber transition is shifted to a higher temperature. The parameter limiting the addition of resin is the low-temperature glass transition. 

Optimum performance for a standard household adhesive tape is found for tan δ peaks between -10°C and +10°C.⁸ Storage modulus can further be adjusted by adding low-molecular-weight oil, which only reduces the adhesive modulus.

Figure 4: Application window showing the procedure to formulate any rubber into a PSA by adding tackifying resin and low-molecular-weight plasticizer.
 

Figure 4: Procedure to Formulate Any Rubber into PSA by Adding Tackifying resin and Plasticizer

 

According to the procedure shown in Figure 4, most rubber compounds can be formulated to become a PSA. This is done by adding resin and a low-molecular-weight component. The two key parameters characterizing the performance of a PSA are:
 

• the glass transition temperature (Tg) and
• the modulus at application temperature

An adhesive performing over a wide temperature range (e.g. deep freeze label) needs a low tan δ peak and a constant storage modulus value of around 10⁵ Pa over a range of use temperatures. If the modulus becomes higher than 10⁸ Pa approaching the glass transition, the adhesion becomes weak and the bond breaks. The shear resistance at use temperature can be correlated to the elastic modulus at higher temperatures. 

According to the t-T superposition principle, the high-temperature behavior measured at 1 Hz corresponds in effect to the low-frequency (rate) region at use temperature. The drop-off temperature of the storage modulus (G') correlates very well with shear resistance at use temperature. The higher the drop-off temperature, the higher the shear resistance.

 

A pressure-sensitive adhesive adheres instantaneously to most solid surfaces upon the application of light pressure to the adhesive film. This behavior is a consequence of the adhesive being "tacky." A variety of performance tests have been developed to measure tack directly. These methods include ASTM D 2979-95, ASTM D 3121-94, PSTC-6, PSTC-5, etc.

 

Probe tack test

An important method of measuring PSA tack is the probe-tack test defined by ASTM D 2979-95.
 

• This test employs a cylindrical steel probe with a flat surface of 5.0 mm in diameter.
• The probe is brought in contact with a PSA film at a fixed stress (usually 10 kPa) for a short time (typically 1 s) and then removed at a constant rate.
• The maximum force required to pull the probe away is recorded. This maximum force is defined as the sample's tack.

Force vs. distance curves

A more complete analysis of tack would require the evaluation of force vs. distance to failure curve as a function of contact time, pressure, etc.⁹ The test consists of two steps: the application of the pressure (force and time are variable) followed by the withdrawal of the probe at a constant pull-off speed.

Figure 5 shows the force-distance curves obtained for three different PSAs.¹⁰ The pull-off speed used is 0.1 mm/s.
 

• Sample A shows adhesion failure, and the adhesive does not elongate when the probe separates.
• Samples B and C fail cohesively. The area under the curve, proportional to the energy to failure, has been evaluated. Sample C has the largest energy to failure and the longest separation time.
• Sample B is in the middle with an intermediate degree of tack and a yield force between samples A and C.
   

The failure energy - the area under the stress-strain curve - is a much more sensitive measure of tack compared to the height of the peak (maximum force) in the simple ASTM test.¹¹ It also correlates well with the loss modulus (G") measured at around 1 Hz in an oscillation experiment.

 

Rheological measurements provide useful information for the processing, product development and application of adhesives. Practical parameters, such as flow, wetting, tack, cohesive strength and shear resistance, can be related to rheological properties. Comprehensive tack tests can be performed on any rheometer with linear test modes and the energy to failure can be calculated for a better evaluation of the tack performance of an adhesive.