Rheology or dynamic mechanical analysis (DMA) has been widely used to study the correlation of viscoelasticity and pressure sensitive adhesive (PSA) properties, such as peel, tack and holding power, since 1980s. Almost all polymers including PSAs are viscoelastic materials possessing both viscous (energy dissipation) and elastic (energy storage) behaviors. These behaviors can be easily determined by a rheometer or dynamic mechanical analyzer. None of the other analytical instruments can evaluate viscoelasticity of materials.
Instrumentally, a rheometer is capable of applying a controlled oscillatory strain or stress to a material and determine the resulted complex torque * or complex modulus (G*), and phase angle between the applied and the resulted strain-stress sinusoidal oscillatory wave. Based on these detected parameters, some very useful rheological properties, such as G’ (elastic modulus), G” (viscous modulus), and Tan (the ratio of G” to G’) are calculated according to a selected test geometry and condition.
Rheology is a science for studying deformation and flow of matter.
Complex Modulus, G* is the vector sum of G’ and G”. It is determined directly from a rheometer.
Elastic Modulus, G’ represents how much energy a material stores through elasticity
Viscous Modulus, G” indicates a material’s ability to dissipate energy– often in the form of heat
Tan , Loss Factor, (G”/G’) is the relative importance of viscous and elastic behaviors for a material
G* = G’ + iG” or
(G*)2 = (G’)2 + (iG”)2
Tan = G”/G’
All rheological properties are temperature, oscillatory frequency, and time dependent. It is important to mention that the rheological property generated at a lower temperature is essentially equivalent to those determined at a higher frequency or a shorter time. Conversely, the rheological property resulted from a higher temperature is also equivalent to those determined at a lower frequency or a longer time. This is the well known time-temperature superposition principle.
Practically, it is very time consuming and unrealistic to determine a PSA at a very low frequency or spend an extremely long time. Based on the time-temperature superposition principle, most of rheological studies for PSA are determined based on a temperature ramp scanning under a fixed frequency. Depending upon the selected frequency, typically at 10 radians/sec (or 1.59 Hz), and a soak time of 0-3 min. to equilibrate the temperature of tested PSA, a temperature ramp testing for most HMPSAs is normally within three hours.
A huge number of articles discussing the correlation of rheological properties and PSA performances have been reported since 1980s. How to effectively utilize these correlations to design and develop optimum HMPSA formulations will be discussed topic by topic in the future articles
For more information call or email Pierce Covert,
Glue Machinery Corporation