Ambient Processing of Polymeric Web: Advanced Diagnostics and Applications

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Polymeric web materials are ubiquitous in today's society, with demand set to increase in areas as diverse as plastic electronics and biodegradable or compostable packaging. However, the need for greener technologies, reduced energy usage and lower material usage is clearly at the forefront of all future global manufacturing requirements, and any new products must meet these criteria. Key to determining the properties and performance of a polymer are its surface functionalities. For applications such as packaging, these can include the ability to prevent moisture and air ingress spoiling the products (i.e. barrier properties) or the ease with which labelling information can be printed onto the packaging (printability). These surface functionalities are now being modified through atmospheric pressure plasma processing in a number of industries, particularly using a family of discharges known as dielectric barrier discharges DBD's. In simple terms DBD's consist of a pair of parallel plates separated by a small gap, with at least one plate covered with a dielectric material.The replacement of conventional polymer web processing methods (such as vacuum-based technologies) with DBD plasma processing provides opportunities cleaner, more efficient processing and points the way ahead for many applications. The DBD geometry is ideally suited to web processing and clearly has the potential to make a major impact in this field. For example, polypropylene film coated by DBD technology could replace the current chlorinated polymer products for food packaging. These materials provide a transparent barrier layer, but the use of chlorinated polymers is under pressure from environmental legislation and alternatives are now required.In industry it is important that any web processing is performed uniformly across the polymer without detrimental damage to the surface. This would ideally require a homogenous discharge. However, dielectric barrier discharges usually operate in a filamentary mode, often resulting in non-uniform and small scale inhomogeneous treatment, and partial thermal degradation of the treated films. However, until very recently it has not proved possible to achieve reliable and controllable plasma discharges to deliver the desired surface functionalities over large areas. This is in part due to a lack of understanding of the fundamental processes of the discharge and their relationships to process stability and outcomes, which has limited large-scale system development. This proposal seeks to undertake a detailed investigation of the physics and chemistry of DBD's specifically designed to replicate key elements of an industrial scale reel-to-reel atmospheric plasma processing system. We will concentrate on two polymer substrates; polypropylene and cellulose, which find a range of commercial applications. We will focus only on process gases and precursors likely to deliver specific surface functionalities e.g. printability, barrier, etc. Thus, we will study a series of 'model systems' on the laboratory scale. Key novel elements of these studies will be the first use of molecular beam mass spectrometry to probe the DBD systems in addition to new power supply designs, incorporating user defined pulsed waveforms. These measurements will be complemented, time-resolved optical emission spectroscopy OES and 2-D filtered optical imaging will be used to identify and map out the key emitting species (ionic and neutrals) in the bulk discharge. Combining the results from the surface chemistry and plasma composition studies we shall endeavour to produce a comprehensive picture of the surface chemical routes in this discharge and the interplay between the plasma state and the substrate during the process. The information gained on these 'model systems' will then be transferred to an existing 2m long reel-to-reel industrial scale processing system through reengineering design at our collaborators, Innovia Films Ltd.