Polymer Coating
Molten resin application deposits a continuous thermoplastic layer onto moving fibre substrates to establish moisture barriers for corrugated packaging stocks. Polyethylene extrusion bonds low density or high density polymers directly to unprimed paperboard through thermal oxidation at the die exit. Substrate moisture content must remain below five percent to prevent steam blistering beneath the polymer film during nip roll consolidation.
Die temperature profiles dictate melt strength and neck-in behavior, influencing film thickness uniformity across the running web width. Chill roll surface finish transfers either a gloss or matte texture to the cooling plastic face. Edge trim scrap gets recycled directly into pelletizers for regrind integration within secondary extrusion layers.
Barrier Performance
Permeation resistance against water vapor and grease depends heavily on coating weight consistency across the paperboard substrate. Polyethylene extrusion eliminates pinholes through precise gap control between the extrusion die and the nip point. Pinholes degrade the barrier capacity of liquid packaging cartons, causing liquid ingress into the underlying cellulose fibres.
Low density grades deliver superior heat sealing windows on high speed vertical form fill seal machinery. High density variations offer enhanced stiffness and puncture resistance for heavy duty industrial sacks. Converting lines monitor pinhole counts through continuous dye penetration tests on random web samples.
Adhesion Mechanics
Molten polymer oxidation at high temperatures generates polar carbonyl groups that anchor the plastic film to untreated cellulose. Polyethylene extrusion requires precise air gap exposure to promote surface oxidation without inducing thermal degradation or smoke generation. Corona discharge treatment on the incoming substrate surface improves mechanical anchoring for higher line speeds.
Poor oxidation leads to delamination during subsequent crewing and folding operations on carton converting lines. Thermal degradation produces odorous volatile compounds that contaminate food products packaged inside the finished carton. Bonding strength testing verifies compliance with end use specifications through peel tests executed at strict ambient temperatures.
