Cavitation as a Tool for Sustainable Packaging Design

Cavitation involves the creation of small voids in a film by incorporating incompatible particles followed by orientation (typically machine direction orientation (MDO) or biaxial orientation). Voids diffract light, increasing the opacity of the film, which is important for reducing ink usage and providing a light barrier. Cavitation also reduces the film density, increasing film yield, reducing material consumption, and enabling floatability of conventionally dense polymers in recycle separation systems. In recent years many film converters have installed blown film lines with inline MDO units to improve mechanical properties at thinner gauge. Many of these enhanced properties are maintained with cavitation. Chemical and physical foaming also produce voids, but these techniques have limited applicability in thin film due to the relatively large size of the cavities created, and the processing complexity they introduce. While inorganic cavitation agents (ICAs) such as CaCO3 have also been used for cavitating BOPP and in MDO breathable PE films, they typically require addition at greater than 10 wt.% to reach high opacity, which exceeds most PE recycling guidelines. This paper describes new polymeric cavitation agent (PCA) technology that is engineered to overcome these limitations. Combined with the property enhancements from the orientation process, cavitated films using PCAs allow downgauging while maintaining package performance and enable recycling in the store drop-off PE recycle stream. In a case study, alternatives to white cavitated BOPP flow-wrap packaging, often used for snack foods, were produced on a commercial-scale blown film line with inline MDO. While BOPP is unsuited for the PE recycle stream, using high levels of ink, TiO2, or ICAs to boost opacity in a PE-based alternative may also be problematic. Excessive ink affects the aesthetics of the recycled product. The use of TiO2 or ICAs often exceeds film density guidelines set to maintain floatability in separation systems, especially at concentrations needed for thin films. In this study, combining TiO2 with the PCA technology in a MDO HDPE/LLDPE blown film met the opacity requirements at a density of 0.83 g/cc. The physical properties of the film also met the application requirements, although MD modulus is less than the BOPP control. The work shows that cavitation using engineered PCAs and optimum MDO conditions enable PE-alternative structures that meet recycling guidelines.