Dynamic Surface Tension Controls in High Build Acrylic Varnish Jetting
Dynamic surface tension at sub-five millisecond timescales controls droplet breakup, satellite suppression, and dome retention in high-build acrylic jetting.
Substrate Surface Energy
Interfacial Property
Total excess free energy per unit area on a solid material surface determines the wetting, spreading, and adhesion of liquid coatings, inks, and adhesives. The metric of substrate surface energy derives from unsatisfied molecular bonds at the boundary phase, expressed in millinewtons per metre or dynes per centimetre. On uncoated paper and board, this property depends on the orientation of hydrophilic cellulose hydroxyls versus hydrophobic wood resins, internal sizing agents, and surface starches.
Synthetic plastic films, such as polyethylene, polypropylene, and polyethylene terephthalate used in extrusion coatings and laminations, present low polar surface forces. High surface energy allows liquid inks to overcome cohesive forces, spreading completely across the solid to achieve spontaneous wetting. When the surface energy of the solid falls below the surface tension of the applied liquid, the coating beads into discrete droplets, resulting in poor adhesion and pinholes.
Vector Partition
Surface energy breaks down mathematically into polar and dispersive components according to Young-Dupré and Owens-Wendt models. Dispersive forces represent London van der Waals interactions common to all molecular structures. Polar forces include hydrogen bonding, acid-base interactions, and dipole-dipole attractions critical for aqueous ink anchoring.
Contact angle goniometry using multiple probe liquids measures these constituent vectors by tracking droplet profiles on flat substrates. Highly sized paperboards display minimal polar energy, resisting water-based coatings until mechanical surface treatments or surfactants alter the thermodynamic balance. Corona discharge, flame treatment, and atmospheric plasma introduce oxygen-containing polar functional groups onto polyolefin surfaces, raising surface energy immediately prior to printing or extrusion.
Adhesion Limits
Printing press performance requires the substrate surface energy to exceed the fluid ink or coating surface tension by at least five to ten dynes per centimetre. If this thermodynamic margin is absent, flexographic and gravure inks suffer crawling, fisheyes, and inadequate mechanical transfer. Thermal lamination also depends on surface energy thresholds to prevent delamination along polymer-paper interfaces during carton creasing and folding.
Extrusion coatings applied to low-energy board fail to form durable chemical or physical bonds, peeling under light shear stresses. Storage time and atmospheric contamination degrade corona-treated films via post-oxidation decay, requiring inline testing with dyne pens before running high-speed converting operations.