Meaning
Optical transmission testing quantifies the percentage of transmitted light that deviates by more than two and a half degrees from the incident beam direction when passing through a specimen. Within blown and cast film production, film haze defines the cloudiness or milky appearance of flexible packaging substrates. The property accounts for combined forward light scattering originating from surface topography roughness and internal crystalline lamellar morphology.
Standard testing according to ASTM D1003 or ISO 14782 isolates this scattering value from total luminous transmittance, terminating its scope where specimens become completely opaque to visible wavelengths.
Scattering Mechanisms
Optical scattering originates at two distinct physical boundaries across extruded polymer films. Surface haze arises from micro-scale surface roughness generated as the extruded web cools against air, chill rolls, or nip rollers. Melt fracture during extrusion die exit introduces micro-grooves that disrupt specular light transmission.
Internal haze stems from density differences between amorphous domains and spherulitic crystalline structures within semi-crystalline polyethylenes or polypropylenes. Large spherulites whose dimensions approach visible light wavelengths scatter light intensely through internal refraction. Rapid cooling reduces spherulite diameter, suppressing bulk internal scattering.
Extrusion Setting
Processing temperatures on blown film lines govern bubble frostline height, which directly sets optical clarity. Raising blow-up ratios thins the web while elongating crystalline structures, diminishing bulk scattering defects. Die gap geometry and melt temperature dictate surface shear stresses, where excessive shear creates sharkskin roughness that elevates haze readouts.
Moulders running cast film manipulate chill roll water temperatures, because lower roll temperatures quench the resin rapidly to suppress crystalline growth. Additive packages introduce mineral anti-block particles that protrude through the film skin, intentionally compromising haze values to prevent layer blocking during downstream winding.
Resin Specification
Datasheet haze ratings describe pristine monomembrane films produced under ideal laboratory extrusion conditions using narrow-molecular-weight virgin resins. Flexible packaging convertors require tight clarity limits to ensure barcode readability and product visibility on retail shelves. Virgin low-density polyethylene delivers consistent single-digit haze percentages, whereas post-consumer recycled polyethylenes cause immediate optical degradation.
Gel particles, unmelted high-density inclusions, and residual pigment contaminants within regrind act as diffuse light-scattering sites. Films containing post-consumer blends frequently double baseline haze metrics, forcing convertors to restrict recycled fractions to inner core layers of coextruded barrier structures.
