Ambient Processing of Polymeric Web: Advanced Diagnostics and Applications

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.

This research programme deals with both fundamental and applied science and engineering. Our quest is to understand and enhance the plasma treatment and coating of plastic webs in atmospheric conditions for applications in packaging and plastic electronics.

The large international academic and business communities involved in plasma processing will be the main beneficiaries of the programme through the acquisition of new and important results, namely enhanced understanding of the plasma chemical pathways present in industrially relevant atmospheric pressure dielectric barrier discharges, and methods for advanced discharge excitation and control. Consequently, we envisage that this project will have a significant positive impact in areas such as the UK food packaging industry and producers of mobile electronic consumer goods, with important potential spin-offs towards biosensors, printing and imaging technology, coating and surface engineering businesses.

In particular, the involvement of Innovia Films Ltd, a world-leading plastic packaging company, provides the project with 'real world' products and applications against which we can benchmark the impacts of our research.

Innovia Films Ltd is ranked as the world's No. 1 producer of cellulose and BOPP films for overwrap, labels and security products. It is the world's No. 2 in BOPP speciality packaging films. They recognise the importance of research and development into new technologies in order to maintain their position in the world. They are one of very few UK manufacturers of polymeric web materials and are the major employer in their region (980 employed in Wigton, Cumbria).

This proposal, therefore, contributes significantly to the success of the existing UK base in this area. The project is multi-disciplinary in nature; we will utilise sophisticated plasma diagnostic techniques, some for the first time in this field; surface analysis and characterisation of polymers will be required at levels down to the nano-scale; and new power supplies will be designed and built. Thus, we will be helping to maintain the health of the UK's research efforts in plasma physics, surface engineering, surface science, polymer science and electrical engineering.

In addition, the project also provides a platform for Kentech Instruments Ltd, a UK leading SME specialising in the development of fast electronics, to design new high voltage, high power nanosecond pulsed power supplies for atmospheric pressure discharge operation, particularly industrial scale DBD operation. This extension in their activity has the potential to benefit the company financially after the end of the project with the establishment and marketing of a new product range. Prior to commencement of the project all partners will sign a formal collaboration agreement, covering disclosure and IP exploitation.

The main dissemination channels for the outputs of the research to the global community will come through publication in high-quality, peer-reviewed journals, presentations at major international conferences and national meetings, through open access web publishing, the holding of a one-day stakeholder workshop and interaction with relevant European networks such as the COST Actions.

An important part of this programme is the recruitment and training of research workers. Liverpool and MMU offer excellent training environments, with significant infrastructure, supported by experienced staff. This project provides many "world-class" academic challenges and an opportunity for the PDRA's to develop as future scientific leaders. This will benefit the UK as a whole through strengthening its research base, and bringing forward new talent.

Both PIs will work with their respective Public Engagement (PE) officers to seek out opportunities to undertake PE activities, such as lectures to teachers, visits to schools, participation in IOP schemes and science societies.