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Characterization of Pressure Sensitive Adhesives by Rheology
午睡起来头晕Fred A. Mazzeo, Ph.D.
TA Instruments, 109 Lukens Drive, New Castle DE, USA
亮剑台词ABSTRACT
Three properties, shear resistance, tack and peel strength, generally characterized pressure nsitive adhesives (PSA). The properties are directly related to the PSA’s respon to the application of stress and may be measured using rheology. For example, tack describes the ability to spontaneously form a
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bond to another material under light pressures within a short application time. As
the contact time increas, higher shear resistance and peel strength properties (related to a materials long time flow behavior) are found. A single rheological
test is described to directly determine the respon of a PSA to varying deformation times, related directly to its shear resistance, tack and peel strength behavior.
INTRODUCTION
An adhesive is usually a polymeric material applied between two solid layers that form a bond with cohesive strength (1). Most adhesives exhibit viscoelastic behavior. Rheology, using small amplitude oscillations, may be ud to test adhesives throughout the whole viscoelastic profile. Applying small amplitudes caus the shear stress to be proportional to the shear strain, a necessary condition for linear viscoelasticity. In linear viscoelasticity, the dynamic modulus (G[ω]) is the ratio of shear stress to shear strain and is independent of the shear amplitude. Dynamic modulus may be parated into elastic (storage) modulus (G’) and the viscous (loss) modulus (G”). The ratio of the G” to G’ is equal to the tangent of the pha angle between them: tan δ = G”/G’.
The ability to form a bond and resist debonding from a substrate determines how appropriate a particular PSA is for a certain application. Oscillatory frequency sweeps are well suited for characterizing the bonding and debonding behavior of a PSA. In an oscillatory frequency sweep, a constant sinusoidal amplitude is applied to a material, within the linear viscoelastic region, while vary
ing the frequency of the oscillation. Low rates of deformation (i.e., low frequency) characterize the bond formation, while high frequency (i.e., high rates of deformation) is ud to characterize debonding behavior. The latter describes a materials shear resistance and the former relates to PSA’s tack and peel strength. Chiu describes how the modulus values at high frequency are related to peel or quick stick tests, and at low frequencies, to shear resistance of adhesives (2).
Further information can be obtained by using the Cox-Merz rule (3) to predict shear viscosity ()
η&) from oscillatory frequency sweep measurements. The Cox-Merz
(γ
rule is an empirical relationship that describes the relationship between the steady state
shear viscosity (plotted against shear rate) and the magnitude of the complex viscosity (plotted against angular frequency). The complex viscosity, |η*|, is defined by |η*| = |G*| / ω where G* is the complex modulus and ω is the angular frequency.
Table 1 shows a summary of the rheological properties that indicate specific adhesive behavior using dynamic mechanical properties. Pressure nsitive adhesives and other polymeric materials (becaus
e of their viscoelastic nature) exhibit temperature and time (frequency) dependent behavior during deformation and flow. Such data can be treated using time-temperature superposition (TTS) theory, overcoming the difficulty of extrapolating limited laboratory tests at shorter times to longer term, real world, conditions. TTS treatment is well grounded in theory (4, 5, 6) and may be applied to the rheology data obtained from oscillation experiments. The underlying bas for time/temperature super-positioning are (a) that the process involved in molecular relaxation or rearrangements in viscoelastic materials occur at accelerated rates at higher temperatures and (b) that there is a direct equivalency between time the stress is applied (the frequency of measurement) and temperature. Thus, oscillatory frequency sweeps are commonly ud. The time over which the process occur can be reduced by conducting the measurement at elevated temperatures and shifting the resultant data to lower temperatures. The result of this shifting is a "master curve" where the material property of interest at a specific end-u temperature can be predicted over a broad time/frequency scale.
Table 1 – Viscoelastic Properties Related to PSA Characteristics
Tack–Low tan δ peak and Low G’
–Low cross-links (G”>G’) @ ~ 1 Hz
⇒ High tack
Shear resistance –High G’ modulus @ low frequencies <0.1 Hz
–High Viscosity at low shear rates
⇒ High shear resistance
Peel Strength –High G” @ higher frequencies (~>100 Hz)
⇒ High peel strength
Cohesive Strength –High G’ and low tan δ
⇒ High cohesive strength (Bulk property)
Adhesive Strength –High G” and high tan δ
任洁玲⇒ High adhesion strength with surface
EXPERIMENTAL
For illustrative purpos, three examples of hot melt, pressure nsitive adhesives are characterized using an AR2000 Advanced Rheometer.
Sample A: high cohesive strength, low tack properties, moderate peel strength,
possible low temperature adhesive
Sample B: lower cohesive strength, higher tack, lower viscosity, low shear
resistance compared to A
Sample C: lower tack, higher cohesive strength, lower peel strength, lower shear resistance compared to A
讲道德
The Smart Swap Parallel Plate temperature stage with 8 mm plates is ud in conjunction with the Environmental Test Chamber to control the temperature from the glassy to the terminal region of each sample. Oscillatory frequency sweeps are conducted at different isothermal temperatures, ranging from –100 to 130 °C, stepping every 10 °C for each frequency sweep. The range of frequencies is 0.1 to 100 Hz at an oscillation amplitude of 0.025 % strain. The normal force is controlled to account for the thermal expansion of the sample throughout the test.
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RESULTS AND DISCUSSION
The modulus values in the frequency range of 0.1 to 100 Hz describe the wetting and creep behavior of PSA’s. When the modulus becomes too large, the ability to wet the substrate reduces (2). The unshifted TTS profile for sample A is shown in Figure 1 and the TTS master curve for A and B at 20 °C is shown in Figure 2. In the frequency range of 0.1 to 100 Hz of Figure 2, sample A posss a higher G’ and a lower tan δvalue than B, indicating that the PSA has a higher cohesive strength or low creep compliance. Sample B exhibits a lower G’ and a higher tan δ value than A. The lower modulus value indicates that the adhesive will deform more easily when in contact with the substrate.
For tack enhancement, tan δ should be greater than unity. That is, G” is greater than G’ indicating that the polymer dissipates energy through its own deformation. This allows the material to adhere and easily form good contact to the substrate. In Figure 2, tan δ is shown to be greater than unity for B and is greater than A, correlating well with the supplier’s obrvations.
Log [G' (Pa)]
Log [frequency (Hz)]
Figure 1 – Raw Data
Log [frequency (Hz)]
Figure 2 – TTS Master Curves (Samples A and B)
Rheological information at high frequency is related to the peel strength of the
materials. The rheological behaviors for samples A and B indicate that similar peel strength properties should exist. The low frequency information (<0.1 Hz) shows that sample B drops off in properties faster than sample A, directly related to its obrved low shear resistance behavior.
Figure 3 shows the master curves at 20 °C of samples A and B after being
transformed to steady shear via the Cox-Merz rule. At lower shear rates, sample B has lower viscosities than sample A, correlating to their obrved behavior. As the shear rate increas, both samples converge to have the same profile, as is expected.
Log [shear rate (1/s)]Log [viscosity (Pa.s)]
Figure 3 – TTS Master Curves with Steady Shear Treatment (Samples A and B)
Another way to treat the information from Figure 1 is to convert the rheological刮画作品
information vers frequency to that vers temperature. This is shown in Figure 4 and 5.The spread in the curves shows the dependence of the rheological properties with
frequency. Each t of connected data points reprents G’ as it changes with temperature at a fixed frequency of 0.1 to 100 Hz.
Low temperature limit for an adhesive is indicated by the glass transition. Figure 4 shows that, as expected, the elastic modulus within the glassy region (i.e., the transition near 0 °C) is independent of frequency. As the temperature is incread, a large dependence on frequency is en during the glass transition region and into the terminal region. Taking the peak of tan δ to reprent the glass transition temperature (Figure 5),
a value of T
g at 15 °C is obrved indicating that sample A is not suitable for low
temperature applications.
temperature (°C)
Log [G' (Pa)]
Figure 4 – Temperature Dependence of Modulus (Sample A and B)
temperature (°C)
Log [tan(delta)]
Figure 5 – Temperature Dependence of Pha Angle (Sample A and B)
Sample C exhibits high cohesive strength, low shear resistance and low viscosity. In Figure 6 sample C is compared to sample A showing similar values for modulus and a
