Solventless Polyurethane Adhesive Wetting Kinetics on Treated Polyolefin Films
Solventless polyurethane wetting requires a 10 dyne surface energy margin and Capillary numbers below air entrainment limits to prevent micro-void mottle.
Flame Treatment
Oxidative Functionalization
Controlled thermal surface activation utilizes high-velocity hydrocarbon gas combustion to alter the chemical characteristics of non-polar substrate surfaces. In web processing, flame treatment oxidizes polyolefin films, plastic coatings, and synthetic barrier layers on paperboard by exposing the moving material to an open, air-rich combustion flame for several milliseconds. The thermal boundary layer strips loosely bound electrons and breaks carbon-hydrogen bonds along the polymer chains, allowing active oxygen radicals to form carbonyl, carboxyl, and hydroxyl groups.
These newly formed polar functional groups raise the critical surface tension, converting a low-energy surface into a receptive base for water-based inks, barrier varnishes, and functional adhesives. The physical process stops at the immediate molecular face, penetrating no deeper than five to ten nanometres into the bulk material. Unlike electrical corona systems, this gas-phase treatment avoids generating back-side pinholes, static accumulation, or ozone emissions inside the converting hall.
Combustion Dynamics
Precise operation requires a stoichiometric or slightly oxidizer-rich air-to-gas ratio, typically maintained between 1.05 and 1.15 to ensure an abundance of unreacted oxygen radicals within the flame envelope. Premixed natural gas or propane burns across specialized ribbon burners mounted directly over water-cooled backing rolls. The web passes through the luminous inner cone of the flame, where active radical concentrations peak before thermal recombination diminishes their chemical reactivity.
Line speeds must balance residence time, as dwell times exceeding twenty milliseconds cause film shrinkage, surface melting, or substrate scorching. Backing roll surface temperatures are regulated with chilled water circuits to pull heat continuously from the web and prevent board curling. Surface energy verification using dyne test pens or automated contact angle goniometers confirms treatment levels, with typical targets ranging between forty-four and fifty dynes per centimetre.
Operational Boundaries
Substrate composition dictates the longevity of the modified surface state, since polymer additives like fatty acid amides migrate back to the surface over time, causing dyne decay. Clay-coated boards demand tighter burner gap control than bare plastic webs to prevent heat blister formation within the mineral pigment matrix. Burner face distance must hold steady within two millimetres across the full deckle width, otherwise cross-web ink adhesion variations emerge during printing.
Ambient draft shielding around the burner hood prevents air turbulence from disturbing the laminar flame front. Regular burner slot cleaning removes carbon scale that would otherwise cast unoxidized streaks down the machine direction of the web.