Kolsky Bar Testing of Pressure Sensitive Adhesives

Abstract

Pressure sensitive adhesives (PSAs) are highly viscoelastic rubber-like materials which are widely employed to bond a range of materials. They typically exhibit rubbery-plateau shear moduli on the order of 1 MPa and glass-plateau shear moduli on the order of 1 GPa with variable levels of strain-rate sensitivity within each rheological plateau. PSAs are frequently employed in applications which can result in high rate and large deformation loading; however, characterization frequently emphasizes small-strain viscoelastic properties as measured by dynamic mechanical analysis (DMA) with time-temperature superposition. Kolsky bar (split-Hopkinson pressure bar) testing can provide direct measurement of the high-rate, finite strain properties; however, the experimental challenges to obtain reliable data are well documented for soft materials. Specifically, soft material testing is sensitive to issues with sample stress equilibrium, radial inertia effects, and poor signal to noise ratio. These challenges are compounded in PSAs, which are apt to adhere to the bar surfaces and can chemically interact with typical lubricants, thus changing the properties.

In this work, we detail the development of appropriate test conditions to obtain high rate mechanical properties of PSAs. Two commercially available materials are evaluated at a range of strain rates in a series of three systematic studies of test conditions. First, the effect of sample geometry is considered. Several thickness to diameter ratios were evaluated as well as annular specimens. Results indicate that annular specimens are not necessary to manage radial inertia effects for this class of materials, and a range of different thickness and diameter choices provide various tradeoffs in signal to noise ratio versus maximum measurable strain. Next, the effect of lubricant is presented for three commercially available lubricants; furthermore, the effect of contact time between the lubricant and the PSA is explored. Finally, we explore the selection of pulse shaper to obtain stress equilibrium. Several conventional copper pulse shaping methods are considered alongside paper, polymeric, and elastomeric pulse shapers.

As a final validation of the presented test methods, measurements were compared to finite element predictions based on a constitutive model derived from quasistatic uniaxial tests combined with DMA. Good agreement was obtained, indicating that the presented Kolsky bar method yields mechanical properties consistent with other modes of testing.