Reducing Curl in Multilayer Blown Film Part 1: Experimental Results, Model Development and Strategies, 2001 Polymers Laminations & Coatings Conference Proceedings

Barry A. Morris--Multilayer films often curl or roll-up on themselves, making them difficult to be used in packaging equipment. Curl is particularly acute in asymmetric barrier blown films. Experimental results from simple two- and three-layer structures are described in an effort to understand the underlying mechanisms behind curl. A model from the literature based on inbeam theorylu is adapted to film applications. In this model, force and momentum balances are used to solve for curl as a function of each layer’s thickness, stiffness and shrinkage during fabrication. Of these inputs, differential shrinkage is the most difficult to determine. Pressure-Volume-Temperature (PVT) curves are introduced to relate differences in volume change among polymers during quenching. PVT data alone are not sufficient for a predictive model. Qualitative agreement, however, between the experimental results and model predictions are obtained, leading to several strategies for curl reduction. These include reducing crystallinity, matching thermal expansion coefficients, matching freezing points and increasing the rate of quenching. Another strategy is to change the thickness and distribution of layers in the film, allowing the stiffness of one or more layers to counter-balance the curl. Such an approach can be greatly enhanced by a truly predictive model and is the subject of Part II.