The effect of process and polymer variables on light transmission properties of polypropylene tubular blown films

The direct light transmission factors of polypropylene (PP) tubular blown films are measured as a function of film preparation conditions such as blow-up ratio (BUR), draw down ratio (DDR), cooling rates (frost line height, FLH) and polymer variables such as molecular weight, molecular weight distribution (MWD) and nucleating agent additive.

The contribution of light scattering from the film surface is investigated by separating the overall direct light transmission factor in to components due to transmission through the bulk phase and through the surface phase of films. It was found that overall direct transmission factor is controlled by the surface phase transmission factor. The bulk phase of film was further investigated by small angle light scattering which confirmed the spherulitic morphology for PP blown films. The size of spherulites is correlated to bulk phase transmission factors.

The surface phase transmission factor is separated in to inside and outside surface phase transmission factors. Light transmission through inside surface was found to be higher than that from outside surface. Profiles of surface irregularities of the films were obtained using a mechanical surface profiler with a diamond stylus and were characterized statistically. The standard variation of height deviation of the surface irregularities correlate well with the surface phase transmission factors.

Overall light transmission factors were higher for films prepared at high BUR/DDR and low FLH. Films made from high molecular weight resin with narrow MWD and nucleating agent additive exibit highest values of light transmission factors. However, narrow MWD resins showed increased bubble instabilities.