The surface chemistry of a film is one of the biggest secrets that a substrate has. In general, this will be determined by the polymer that is on the surface of the film and any subsequent process steps that are applied to it, such as priming or surface treatment.

For instance, corona treatment of films is widely used to improve the adhesion of ink, coatings, metal, etc., in the conversion process. It works by inserting oxygen molecules into the polymer at the surface of the film, and the surface energy of the film will be a function of the amount of oxygen inserted and the type of oxygen bond produced.

But what is the real nature of the surface of the film? If the film is an oriented polypropylene (OPP), will it have the same surface chemistry as a corona-treated cast polypropylene? To answer this, we have to know the secret of the surface for both films.

In the case of OPP, it is very unlikely that the treated surface is a homopolymer polypropylene, (PP), rather it is likely to be a coextruded surface of copolymer or terpolymer polypropylene, or even a polyethylene. However, the cast film may indeed be a homopolymer surface or a homopolymer/copolymer blend depending on its structure and intended end use. Additionally, the orientation will affect the way the corona interacts with the film surface and, even if the polymers are the same, the chemistry of the two film surfaces is likely to be different. This will be true, even if they have the same surface energy (treatment level).

If we are comparing flame and corona treatments, then the surface chemistry will be very different on the same substrate, as the treatment mechanisms are different, and we get very different results in terms of surface energy vs. levels of surface oxygen. It's possible to determine what type of treatment, corona or flame, was used by a study of the surface-treatment level and the amount of oxygen in the surface. While it is generally felt that the higher the treatment the better, this is oftentimes not the case because the integrity of the polymer surface is destroyed when treated to very high levels with typical treatment systems.

The optical properties of a substrate are controlled by its internal and external structure. This appears in clear substrates as haze, which is due to light scattering.

The haze is composed of two parts: surface haze and internal haze. Surface haze is due to surface roughness, which exceeds about 1/6 the wavelength of light. Internal haze is due to the scattering of small particles in the film, as well as variations in density (amorphous and crystalline rejoins) within the substrate.

In amorphous polymers such as polystyrene, there is no internal haze due to scattering at the crystal/amorphous interface because the polymers contain no crystals, and the polymer is very clear. This is in contrast to high-density polyethylene (HDPE), which is highly crystalline and has high haze values due to the crystals in the substrate film.

The components of a substrate's haze (surface and internal) can be separated and measured to determine the source. 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 to cover the surface irregularities. The oil will fill in the surface irregularities and create a smooth surface which will not scatter 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 polyolefins have a refractive index of about 1.5 and can be oiled with mineral oil. In a pinch, the oil on your face can be used to get a fast determination of surface haze. Spread the oil from the sides your nose on the film surface and see if the film becomes clearer, and if so, then there is some surface haze.

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 re-measure the haze. The difference between the two measurements is the surface haze, and the difference between the surface and total haze is the internal haze.

 Figure 1
The impact of surface roughness as it affects haze, where the line separating no effect and an effect, can be illustrated in the Rayleigh Model for a wavelength of 550 nm (red light) (see Figure 1). Here the wavelength (lamda) is divided by 8sin(angle) which is where the approximate 1/6 of the wavelength of light rule-of-thumb is derived. If we want to make a low surface haze, we want a smooth surface, but if we want a hazy surface (matte), this allows us to estimate the size of the bumps needed in the film surface to make it hazy.

Generally, 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 polymer blend. In this case, the viscosity ratio of the various polymers can be used to control the particle diameter. Broadly, for OPP films, the surface layer is a blend of PP, HDPE and perhaps copolymer PP. There are many patents for matte-film blends, and it's hard to decipher what the real blends are, but generally they are the three polymers as described here.

The source of internal haze in substrates composed of a single semi-crystalline polymer is lamellar crystals which form larger semi-crystalline aggregates called spherulites. The spherulites are composed of the lamellar crystals embedded in amorphous polymer chains.

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 defracted, increasing haze. Second, there can be form scattering from the spherulites if the size of the spherulite is about that of the wavelength of the light passing through. If the spherulites are small relative to the light's wavelength, or if they are large relative to the light's wavelength, then they will not increase light scattering.

This is how nucleation works to lower haze, by forming many small spherulites. In amorphous polymers such as polystyrene (PS), polycarbonate (PC) and polymethyl methacrylate (PMMA), there are no crystals. The polymers are crystal clear and any internal haze present would be due to small particles (catalysts) 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 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.

Why be concerned with film optical properties? In some cases, the relative importance of any of these properties will rightly depend on the substrate's application. Surface haze is very important for printing and metallization if a bright, glossy metal appearance or excellent printing is required. Internal haze is important if the film will be reverse-printed and printing will be viewed through the film. Conversely, internal haze would not be important for an opaque, metallized film.

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 a paper-like appearance to polymer substrates. Internal haze could also help give a flatter appearance, but if it is combined with a smooth surface, it might not look so much like paper.

To some degree what appears “paper-like” is somewhat a matter of taste to the target market, in the same way as color tinting and what “white” is, cold with a blue tint or warm with a reddish tint.

Film-manufacturing methods definitely affect a substrate's optical properties. In particular, quenching conditions can have a profound impact.

 Figure 2
In blown-film applications, the formation of the solid film from the melt is oftentimes delineated at the “frost line” (see Figure 2). Below the frost line, the film is clear because it is molten, and above the frost line, it is hazy because it is frozen, and the crystals are scattering light as previously described. The limiting factor in blown-film production is usually the quenching rate because the film is cooled by moving air and the heat transfer coefficients are somewhere in the range of 15-20 btu/hr/ft2/ 0F (85.2 to 114 W/m2/0C) for high-velocity air.

In comparison, a cast-film line with its higher quenching rate of approximately 250 btu/hr/ft2/ 0F (1,420 W/m2/0C) from the cooling drum will generally give better film clarity because crystal morphology is different from those of blown film, and this affects the internal haze of the film. A frost line can also be found on a cast line at the point where the film solidifies, but it may also be confused with the point where the film releases for the chill roll.

Choosing a blown or cast line can at times be difficult because there are capital-cost and throughput differences as well as film properties. But, if you are looking for the best film optics, then the improved quenching obtainable from a cast line could be an important consideration.