Bridging the Modulus Gap for Films Used in HDSS Applications, 2001 Polymers Laminations & Coatings Conference Proceedings

K. Williams--Developing blown film resins for the production of Heavy Duty Shipping Sack (HDSS) films that exhibit the toughness properties of LLDPE with the stiffness and tensile characteristics of HDPE has historically been difficult to achieve for both resin producers and film converters. Resin producers have attempted to push the density higher for LLDPE film resins and lower for HDPE film resins. Separately, film producers are continuously searching for the same hybrid film through blending low-density and linear low-density polyethylenes with medium and high-density polyethylenes. Producing successful film blends for heavy- duty applications with densities over 0.918 g/cc usually results in a high performance LLDPE film resin blended with low levels of a MDPE or HDPE. As the level of MDPE or HDPE increases, loss of critical toughness properties such as machine direction tear (MD tear) and dart impact strength is observed.

Now there is a new approach to blending to achieve a higher stiffness film without the same historical losses in tear and dart impact properties. This approach utilizes a new family of high molecular weight, high-density polyethylene film resins. Historically, HMW-HDPE resins were kept within a relatively tight density range near 0.950 g/cc. That density range has been significantly expanded in the last few years to include film resins that have densities as low as 0.938 g/cc. With the increased level of comonomer incorporation, or short chain branching (SCB), the blendability with LLDPE is suitable over a full range. This allows films to be made from 0.918 to 0.938 g/cc with excellent retention of dart impact and MD tear strength with improvements in MD modulus, yield stress and break stress.