Tack force

Two important factors in the generation of tack forces in a printing nip are cavitation and ink rheology. 

The cavities thus formed will expand. This would suggest that the increase in total surface energy due to the creation of new surface would be a contributor to the tack force. However, in an analysis of cavity expansion within a polyisobutene oil, Hoffman and Myers— found that surface tension contributed only 7 percent and the viscosity contributed 93 percent to the work of separation of the oil film. 

Elasticity at high shear rates is also bound to influence tack force in the nip of an offset press. Besides shear rate the elasticity is Influenced by the molecular weight and molecular structure of the dissolved polymers, and interactions between the pigment particles, the vehicle, and the dissolved polymer molecules. Of particular interest is the effect of the molecular mobility of stiff-chain polymers such as polyindene on elasticity and tack force at the shear rates encountered in an offset printing nip. 

Tack forces generated during ink film splitting are not well defined or easily measured with existing instrumentation. However, it is suggested that ink tack is related to ink elasticity, or more fundamentally, to molecular mobility of stiff chain polymer additives to lithographic inks. 

The recorded force first increases, then decreases the maximum value, called the tack force is a measure of the adherence in this experimental condition the area under the curve, called the tack energy, is equal to the work GdA of the singular stress at the crack tip. Tackiness refers to the ability of an elastomer to adhere instantaneously to a solid surface, or to itself, after a brief time of contact under low pressure. Probe tack testing procedure can be analyzed by Eq. 14 and tack curves obtained by computer integration closely coincide with experimental ones (25). ... 

Fig. 9. Glass ball on polyurethane influence of withdrawal speed on the tack force (from ref. 25).

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