Haze is the scattering of light by a film that results in a cloudy appearance or poorer clarity of objects when viewed through the film. More technically, haze is the percentage of light transmitted through a film that is deflected more than 2.5° (degrees) from the direction of the incoming beam. This property is used to describe transparent and translucent films, not opaque films.
Relevance to performance
Film clarity is a highly desirable feature for most clear packaging and label applications. It tends to symbolize quality and contributes to the positive visual display of the product. Compared to other flexible plastic films, clear OPP has one of the lowest haze values, generally less than 3%.
What determines the haze level of a clear film
Haze is greatly influenced by material selections and product design. Resin characteristics, such as crystallinity and molecular weight distribution, have a key impact. Copolymers are generally hazier than homopolymers. Additives and coatings usually contribute to increased haze. All other things being equal, thicker films will be hazier than thinner films. Additional variables, like process temperatures in the different stages of film-making, can further affect haze, so they are tightly controlled.
As represented in Figure 1, a unidirectional light beam is directed onto the film specimen. After it enters an integrating sphere, a photo detector measures the total light transmitted by the film and the amount of transmitted light that is scattered more than 2.5°. Haze is the percentage of total transmitted light that is scattered by more than 2.5°.
|Figure 1: Haze measurement|
Commercial hazemeters are typically used for this testing, but ASTM D 1003 also allows the use of a spectrophotometer, provided it meets the procedure requirements. ExxonMobii uses the SKY-Gardner XL-211 Haze-gard and Haze-gard plus hazemeters consistent with ASTM guidelines. The hazemeter may also be set up to measure and display light transmission.
Light transmission: Light transmission is the percentage of incident light that passes through a film, which is represented by the following equation.
|Light Transmission (%) =||Tt||x 100|
Translucent: Translucent describes material that transmits light, but also diffuses it, so that objects cannot be seen clearly.