The Role Of Linear Low Density Polyethylene Resin Properties In LLDPE/CaCO3 Microporous Films
Patricia L. Roberts1, Leah A. Leavitt2, Mahin Shahlari3, and Sunggyu Lee3. (1) Dow Chemical Company, Freeport, TX 77541, (2) NIC Industries, Inc., 7050 6th Street, White City, OR 97503, (3) Department of Chemical & Biological Engineering, University of Missouri-Rolla, 143 Schrenk Hall, Rolla, MO 65409-1230
The study of particulate-polymer composites had an established role in the field of polymer engineering for many decades. One field in which the demand is growing is the area of breathable films. Microporous LLDPE/CaCO3 breathable films first emerged in the personal care market during the 1980s, and since that time the use of these films grew into the medical, automotive and construction industries. Accompanying this expansion to new markets was the desire to tailor the properties of the breathable films to specific performance requirements. One area that has room for improvement is optimizing the breathability while maintaining the mechanical integrity of the films. The focus of this research is to develop a systematic study designed to elucidate the role of LLDPE resin properties on the final film characteristics. The results of this research show that the melt index (MI) of the carrier and let-down resins has a significant role on the film properties. The properties of the LLDPE carrier resin are critical in determining the water vapor transmission rate (WVTR) of the films. The properties of the LLDPE let-down resin govern both the stretching ability and mechanical integrity of the films, in addition to contributing to the WVTR. A method for developing an optimal LLDPE resin formulation is presented. The average density for seven different let-down resins and three different carrier resins was investigated. A factorial design with three factors was used to determine the main and interactive effects of LLDPE let-down resin density and LLDPE carrier resin density. An average overall LLDPE resin density in the range of 0.910-0.913 g/cm3 is optimum for achieving a highly stretchable film. Also, for a given LLDPE carrier resin, a complimentary LLDPE let-down resin density may be selected to achieve a stretchable film. The optimum LLDPE/CaCO3 breathable, microporous film results from the combination of the highest density LLDPE carrier resin coupled with a LLDPE let-down density in the range of 0.901-0.904 g/cm3. The stress-strain properties of the films were also investigated. The Young's modulus, tensile stress, and elongation at break were primarily dependent on the let-down resin density and to a lesser extent the carrier resin density. Statistical analysis shows that both LLDPE carrier resin and LLDPE let-down density are significant effects. A high let-down density results in a higher tensile stress, Young's modulus and WVTR. The work presented here helps establish a method for controlling the mechanical properties of breathable films during manufacture.