How to control migration of slip agents in polyethylene films

Plastic packaging systems are ubiquitous in manufacturing. They are an integral component of applications from food to industrial shipping, but their usefulness and efficiency depend critically on fulfilling a number of technical requirements. For example, the plastic films on high-speed packaging lines must show adequate machinability—i.e., avoid sticking to metals and other parts of the handling lines—a characteristic that is determined mainly by the level of friction between the film and equipment.1–4 An optimal amount of friction is commonly achieved through the use of so-called slip additives, which are added to the polymer matrix in molten stage and later migrate to the film surface. Some diffusion in this case is a good and necessary thing, as the slip agent must move to the surface where it should act. However, unpredictable migration caused by environmental and formulation factors can lead, for example, to excess oiliness and influence the downstream processes that packaging materials usually undergo, such as printing, sealing, and handling. The overall result is occasional loss of productivity on the packaging lines. Here we describe work aimed at investigating the migration process of erucamide, a common lubricant, as a slip additive and how this behavior is influenced by co-additives incorporated into standard film formulations of polyolefin plastics (e.g., polypropylene and polyethylene, PE).5 We studied three-layer films of PE produced on a film co-extrusion line. Layer A was made of 7:5 m-thick PE and layer B of 35 m-thick PE. Layer C was 7:5 m thick and consisted of PE that contained erucamide along with varying amounts of different types of surface modifiers. Following a two-month period of aging the samples at room temperature, the amount of erucamide that had diffused to the film surface of each system was determined by Fourier transform IRattenuated total reflection (FTIR-ATR) spectroscopy. In an initial phase of this work, the question was whether it was possible to change the migration behavior of erucamide when a hydroxylterminated co-oligomer of PE and polyethylene oxide (PEO) is added Figure 1. Erucamide content at the film surface as a function of the amount and composition of a co-additive. EO: Ethylene oxide. wt%: Weight percent.