A study of the effects of processing parameters on the morphologies and mechanical properties of polyethylene blown films.

摘要

A study was conducted to determine the effects of key blown film processing parameters on crystalline and amorphous morphologies and in turn how those morphologies affect tensile properties. Another aspect of the study was to develop a model to characterize tensile modulus in PE blown films. Despite the fact that the predominant molecular orientation is in the machine, or draw, direction (MD) the tensile modulus in the transverse, or perpendicular, direction (TD) is usually greater.; Series of blown films were produced concentrating on three key parameters: take-up ratio (TUR), blow-up ratio (BUR), and frostline height (FLH). In the experiment, two of the above parameters were held constant, along with output rate and melt temperature, while the third was varied. After production, films were analyzed by small angle X-ray scattering (SAXS), infrared dichroism, thermal shrinkage, and brittle fracture stress to determine relative crystal and amorphous phase orientations. Differential scanning calorimetry (DSC) was used to determine percent crystallinity. Results from analysis indicate that the crystalline structure in PE blown films is a row nucleated structure. The extended chain core is oriented parallel to the MD while chain folded lamellae, grown radially from the extended chain core, are oriented with their long dimensions in the TD and plate normals parallel to the MD.; The films were also analyzed to determine tensile modulus and yield. Results of the study indicate, that unlike cold drawn materials, PE blown film tensile properties are not significantly dependent on amorphous phase orientation. Yield stress and strain values are more dependent on the orientation of the chain folded lamellae than amorphous orientation.; Composite theories were used to characterize the tensile modulus anomaly. In the MD, the extended chain core of row nucleated structures act as continuous fiber reinforcement in an alternating amorphous crystal matrix. A rule of mixtures was applied. In the TD, chain folded lamellae act as short fiber reinforcement in an amorphous matrix. Halpin - Tsai theories were applied. Calculated results agree with experimentally determined results for modulus, crystal content, and crystal dimensions.