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he haze of a film is defined as the fraction of (nor-mal incident) transmitted light that deviates fromthe directly transmitted beam by more than 2.5 (1).The contrast of an object seen through a film de-creases with increasing haze. The resolution of detail,a second criterion of transparency, is usually referredto as clarity. From a light scattering point of view,haze corresponds to larger scattering angles than lossof clarity. Note that a reduction in haze not necessarilyleads to improved clarity. Haze due to surface rough-ness is in many cases inversely related to gloss, whichis another important optical property.Both bulk inhomogeneities and surface roughnessmay contribute to haze. The latter effect usually domi-nates for polyethylene (PE) films. The typical scale ofthe roughness relative to the wavelength of light ( )determines the scattering characteristics. The heightvariation of the surface is usually reported to follow aGaussian distribution (2), and optical surfaces areoften characterized by the root-mean-square deviation( ) from the mean surface height. Roughness values, , in the range 20–600 nm are typically reported forPE films, i.e. . The haze increases monotonouslywith increasing roughness. Various techniques havebeen used for characterizing the surface profiles.Atomic force microscopy (AFM), has been applied in anumber of recent studies (3–6).Many types of PE are available for film blowingprocesses. The three main types, low-density (LDPE),linear low-density (LLDPE) and high-density (HDPE),all have their advantanges and drawbacks, also whenit comes to optical properties. Early experimental liter-ature (up to 1981) on the haze of PE blown films wasreviewed by Willmouth (1). The main findings can besummarized as follows. Two main sources of surfaceroughness have been identified: 1) flow-induced irreg-ularities originating from the die, and 2) protrudingcrystalline structures (lamellae, stacks of lamellae orspherulites). The former is a well-known rheologicaleffect, and the resulting extrusion-induced haze isoften observed to increase with increasing melt elas-ticity, e.g. due to more long-chain branching (LCB) or broader molecular weight distribution (MWD). The melt elasticity of LDPE (containing LCB) can be |
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