In order to obtain highly transparent isotactic polypropylene (PP) sheets using an industrial process, various factors contributing to transparency were analyzed. 

At first, the observation was experimentally conducted by using a lot of different screws with various geometries. The transparency of the melted web was obtained through screw geometry so that the specific energy consumption was small. 

Then the higher order structure of the higher tacticity PP sheets was investigated. After the heat treatment of quick quenched sheets, the matrix was transformed from a smectic structure to the α-monoclinic crystal phase. The density and refractive index differences between spherulite and matrix were decreased, and the transparency was drastically improved. 

Additionally, the influence of the isotacticity, molecular weight distribution, crystallization control material and the multilayer extrusion process were analyzed. The lower tacticity PP generated fewer and smaller spherulites and showed improved behavior in regards to transparency by heat treatment. In the case of the addition of L-LDPE with a specific density to PP, the transparency was markedly improved by heat treatment. This phenomenon could be explained by the refractive index difference of the PP matrix and the fine distribution particles of L-LDPE. 

The shear stress was reduced by laminating resin with low melt viscosity on both surfaces, and therefore it can be surmised that the stress induced crystallization was restrained. 

APPLICATIONS: Thanks to this research, the highly transparent sheets which have a haze value of 2% are being produced at a rate of 6,500ton/year. The sheets are known as Super Purelay® and Purethermo®. Super Purelay® is a clear PP sheet having unprecedented transparency, a brilliant surface and high stiffness and Purethermo® is a highly transparent PP sheet for thermoforming clear food containers.