The components of a substrates haze, surface and internal haze, can be separated and measured to determine the source of the haze. Because surface haze is principally due to roughening of the surface, it can be measured by eliminating or replacing the surface with another surface. This can be done by coating the substrate surface with a liquid, such as paraffin or silicone oils, to cover the surface irregularities. The oil will fill in the surface irregularities and create a smooth upper liquid surface which will not scatter the light making the surface haze disappear. The liquid should match the refractive index of the film surface to remove any remaining light scattering between the surface and the oil. Most polyolefin polymers have a refractive index of about 1.5 and can be oiled with mineral or paraffin oils. In a pinch for many films, the oil on your face [next to your nose] can be used to get a fast determination of surface haze. Spread the oil from beside your nose on the film surface and see if the film becomes clearer, and if so then there is some surface haze.
Therefore, to determine the relative amount of surface and internal haze, you first measure the total film haze, then you place an oil layer on the film surface, either one surface at a time or both surfaces and then measure the haze again. Sometimes this is done with the oil and film placed between glass slides. The difference in haze between the two measurements is the surface haze and the difference between the surface and total haze is the internal haze.
Sources of surface haze are due to processing conditions or additives to make the film surface matte and we will explore the causes and trouble shooting of surface haze in the days ahead.
Continuing with the subject of surface haze we have the figure 1 showing the impact of surface roughness to the affect of haze where the line separating no effect and an affect is shown as the Rayleigh Model for a wavelength of 550 nm, which is red light. Here the wavelength (lambda) is divided by 8sin(angle) which is where the approximate 1/6 the wave length of light rule of thumb comes from.
Now if we want to make a low surface haze we want a smooth surface, but if we want a hazy surface (matte surface) this allows us to estimate the size of the bumps we need to put in the film surface to make it hazy.
In general matte films are produced by coextruding a thin layer onto a substrate where the outer skin is filled with mineral particles or by using an incompatible blend of polymers. In this case the viscosity ratio of the various polymers can be used to control the particle diameter. Generally for oriented PP films (OPP) the surface layer is a blend of PP, HDPE and perhaps copolymer PP. There are many patents for the matte film blends and because the blend range descriptions are so complex, it is hard to decipher what the real blends are, but generally they are three polymers as described here
The attached figure 2, showing light passing through a polymer containing crystalline regions is designed to show the source of internal haze in substrates composed of a single semi crystalline polymer. In the polymer there are crystalline regions called lamellar crystals which form larger semi crystalline aggregates called spherulites. The spherulites are composed of the lamellar crystals embedded in amorphous polymer chains. The light scattering causing the internal haze is from two primary sources. First, at the transition from the amorphous to crystalline phases there is a density change which causes some of the transmitted light to be diffracted or scattered causing an increase in haze. Second there can be form scattering from the spherulites if the size of the spherulite is about that of the wave length of the light passing through the sample. If the spherulites are small relative to the lights wavelength, or if they are large relative to the lights wavelength then they will not increase light scatting. This is how nucleation works to lower haze, by forming many small spherulites. In amorphous polymers such as polystyrene (PS), polycarbonate (PC), polymethyl methacrylate (PMMA)there are no crystals and the polymers are crystal clear and any internal haze present would be due to small particles (such as catalyst) in the polymer.
The way the polymer substrate is manufactured will affect the level of the internal haze depending on how it affects level and size of the crystalline regions as well as the density of the amorphous phase. Orientation, especially solid state orientation will increase the average density of the amorphous phase and decrease light scattering at the crystal/amorphous phase boundary and decrease the haze. Casting heat transfer conditions of semi crystalline polymers will affect crystal nucleation and growth rates and affect internal haze. We will spend more time on these aspects of substrate manufacture in the coming days to better explain the impact of processing on substrate optical properties.
Interfacial instabilities in coextruded substrates as well as the density jump between layers can also impact internal haze and the quality if an image transmitted through the substrate and will be addressed in upcoming posts.
Importance of optical properties:
We have discussed some of the principle sources of substrate optical properties, and in particular haze. So why should we be concerned with these film optical properties? Well in some cases we may not need to be concerned as the relative importance of any of these properties will rightly depend on the applications in which the substrate will be used. Surface haze is very important for printing and metallization if we desire bright, glossy metal appearance or excellent printing. Internal haze is important if we intend to reverse print and view the printing through the film. However, it is hard to imagine why internal haze would be important for a metallized film as it will be opaque.
However, if we want a matte or paper like appearance to a polymer film then surface haze is an important feature to add to the substrate, either on the printing surface or the surface through which the ink will be viewed. In this instance a smooth, high gloss printing surface combined with a rough, matte viewing surface may be the best combination to give paper like appearance to polymer substrates. Internal haze could also help give a flatter appearance but if it is combined with a smooth surface might not look so much like paper. To some degree what appears “paper like” is somewhat a matter of taste to the target market. Much as with color tinting and what “white” is, cold with a blue tint or warm with a reddish tint. As we expand on various substrate manufacturing methods we will comment on the impact that the manufacturing methods have on the optical properties as well as other surface and bulk properties.