|Transparency is a highly valued property in plastic end products and increases their aesthetic value. Amorphous polymers provide transparency while crystalline polymers do not, particularly in thicker products such as injection molded end products. Crystallinity causes refraction of light ray. For crystalline polymer to get clarity, the ends products are either very thin like film and should be oriented so that size of crystals becomes smaller. Smaller crystals do not cause refraction of light rays.
As per Michael Sepe in plasticstoday, the light transmittance of some semicrystalline polymers can be improved through additive technology. PP injection molded product can be made clearer by incorporation of clarifying agents. The very low-density polyethylene that can be made with metallocene catalysts will also exhibit improved clarity due to a decrease in the degree of crystallinity. PP, due to its slower crystalline growth provides better clarity. PE on the other hand crystallizes very quickly. Clarifying agents do not improve the clarity of PE products even at lower thickness. Similarly, PET is one of those materials that typically are slow to crystallize unless additives are incorporated to nucleate the polymer. Amorphous PET is clear and tough and will soften at the glass transition temperature, usually near 80�C. However, if the material is heated to temperatures of 120-130�C, the product will begin to develop a haze that is caused by crystal formation.
The reasonably clear semicrystalline grades of nylon 6 are not to be confused with amorphous nylons, where the chemistry of the polymer chain has purposely been altered to prevent crystallization from occurring. Amorphous nylons are truly transparent and will not crystallize under normal molding conditions. The �clear� semicrystalline nylon 6 materials rely on rapid cooling rates and thin-walled designs. If the walls exceed thickness of more than 1-1.5 mm or if the mold temperature is raised, these materials will begin to exhibit the haze associated with crystal formation. Polymethyl-pentene, a little-known semicrystalline resin, can be molded with very low haze levels due to the extremely low density of the polymer.
But amorphous materials are inherently clear, and users of the materials expect good light transmittance and negligible haze levels to be part of the property profile. When these materials are molded into parts that exhibit haze, it is considered a defect and is usually attributable to contamination. Though the loss of desired clarity is usually obvious, but the amount of contamination needed to reduce light transmittance and create haze can be extremely small. This can make detection difficult despite the obvious nature of the defect. Loss of clarity can come from several causes. First, the contaminant itself may be translucent or opaque. If polycarbonate or general-purpose polystyrene (GPPS) becomes contaminated with even a very small amount of polyethylene (PE), the molded part will become hazy. A less obvious cause is the mixing of two clear materials. GPPS mixed with acrylic produces a nearly opaque part because of the dissimilar refractive indices of the two materials.