Haze - Thermoplastic polymers, elastomers and additives

Transparency in semicrystalline polymers is directly related to crystallinity. Spherulites are much larger than the wavelength of visible light (0.4-0.7 μm), and the refractive index of crystalline regions is higher than that of amorphous regions. As the rays of light pass from the amorphous to the crystalline regions, they encounter large spherulites, which produces a scattering of light; as a result, transparency is less and turbidity occurs. Due to its non-crystalline structure, amorphous materials have lower turbidity levels than semicrystalline ones, a decrease in crystallinity in a semicrystalline polymer improves clarity. Excessive reductions in crystallinity can result in unacceptable reductions in strength, rigidity and resistance to softening, so a compromise must be reached that is appropriate for the application.

Nucleating agents such as dibenzylidene sorbitol reduce the size of spherulite below a level that disperses visible light, resulting in a dramatic reduction in haze. The distribution of spherulite sizes is also reduced. A higher concentration of nucleating agent can result in greater clarity. Clarity can be optimized by using a resin with low crystallinity with an aggregated nucleating agent and rapidly cooling the molten polymer to accelerate the rate of crystallization.