Transparents polymers - Thermoplastic polymers, elastomers and additives

From the point of view of their use, the most interesting optical properties of plastic materials are those related to their ability to transmit light, take color and have brightness, which provide manufactured objects with a high quality aesthetic visual appearance. The main phenomena are the transmission, reflection and absorption of light. It is said that a sample is transparent if it is possible to perceive objects through it and is defined as the fraction of light that is transmitted with a deviation less than 0.1 ° with respect to the normal incident ray to the surface.

The transparency of a material depends on the softness of the surface and the fundamental structure. It is defined in terms of two measures: transmittance and haze. A material with good transparency will have a high transmittance and little turbidity. Transmittance is the relationship between transmitted light and incident light. Reflectance, the relationship between reflected light and incident light, is the complementary measure. For an ideal material, the sum of transmittance and reflectance would be unity. For real materials, the difference between the unit and the sum of transmittance and reflectance represents the light absorbed. Haze is the proportion of incident light that passes through the sample that deviates within a given angle by forward scattering.

The plastics are divided into crystalline and amorphous, crystalline does not mean transparent, in fact the only crystal polymer 100% and transparent is the PMP. Some of the polymers are transparent; polystyrene and polymethyl methacrylate stand out for their optical clarity, even superior to that of many inorganic glasses. The main factor that mediates the transparency of a polymer is its amorphous / crystal structure (semicrystalline). Light is scattered at each boundary between the crystalline and amorphous phases, so the transparency depends directly on the size and concentration of the crystalline spherulites in the polypropylene. Random copolymers are more transparent than block copolymers and homopolymers. 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 greater 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 levels of turbidity than semicrystalline materials, and a decrease in crystallinity in a semi-crystalline 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.

Other polymers are translucent and whitish like polyethylenes although in films they are transparent and some, like phenolic resins and polyamides, have a yellowish color and are translucent or opaque. The optical properties are related to the chemical structure and the morphology of the material. The refractive index, n, is directly related to the electronic polarizability that depends on the polar moment induced by the radiation.

Polypropylene can exist in different morphological forms, depending on the tactic of the resin and the crystallization conditions, such as pressure, temperature and cooling rate. Different forms can coexist, and one polymorphic form can change to another as conditions change.

In atactic polypropylene, with its random (random) molecular structure, the molecules can not crystallize in an orderly fashion, and a polymer with low crystallinity is formed. The low crystallinity polymers consist of ordered crystalline regions surrounded by disordered amorphous material, similar to a spaghetti with entangled polymer chains. Amorphous polypropylene does not have a defined melting point.