Filament breaking length – Experimental and numerical investigations

Stencil printing of adhesive-based fuel cell sealings: The influence of rheology on bubble formation during the separation step

“…The breaking length of adhesive filaments depends on a wide range of rheological properties, stretching kinematics and on the sample geometry. 35 A typical dimensionless parameter used to characterize the filament stretching behaviour of a fluid is the so-called maximum Hencky-strain, ε He , max , given by the following equation:…”

“…The maximum Hencky-strain can be experimentally measured using filament stretching tests on CaBER extensional rheometers or Texture Analysers, 42 which were not used for the rheological characterizations in this paper. In order to avoid additional measurements and to better understand the isolated influence of each rheological parameter on the adhesive tendency to stretch filaments, an empirical relation introduced by Faßbender et al 35 to estimate the maximum Hencky-strain was adopted instead:…”

“…Equation (4) was originally derived to predict the maximum Hencky-strain of high viscous fluids, which is measured at an axisymmetric plate-to-plate setup and very controllable conditions. 35,41 In contrast, the stencil mesh introduces a considerable geometrical complexity to the separation process, where additional interactions between the adhesive bridges on neighbouring mesh threads are involved. These interactions are likely intensified at higher speeds, which can justify the reduced capability of ε He , mat to precisely describe phenomena related to the mesh separation at speeds ≥10 mm/s.…”

“…It should be emphasized that ln ( Ca ) was calculated using the viscosity at a shear rate of 1 s −1 , as derived in Faßbender et al 35 If the capillary number had to be estimated independently of equation (4), the characteristic viscosity for the corresponding print conditions would be necessary. Since the investigated adhesive systems exhibited a shear thinning behaviour, the characteristic viscosity should correspond to the viscosity at a characteristic shear rate, which in this case is equivalent to the rate at which the adhesive bridges break.…”

“…Due to the presented process advantages, stencil printing was investigated as a technique to produce fuel cell sealings. The primary aims of this study are to enhance the understanding on physical mechanisms governing the adhesive detachment from the stencil mesh and to evaluate the applicability of an existing empirical relation 35 to assess the quantity and size of air bubbles formed during the separation process. To achieve these objectives, a commercial acrylic adhesive was mixed with three different weight fractions of fumed silica to expand the range of rheological properties for comparison with the resulting separation mechanisms and bubble characteristics.…”