1
Atmospheric Pressure Plasma Treatment of Polymers to Enhance Adhesion

K. Lachmann*, M. Omelan, T. Neubert, K. Hain and M. Thomas

Fraunhofer Institute for Surface Engineering and Thin Films IST, Braunschweig, Germany

Abstract

In this chapter, we present an overview on recent research and development work in the area of atmospheric pressure plasma treatments (APPTs) to generate adhesion-promoting surfaces of polymers used in various applications in automotive, aerospace, packaging, and medical fields. In comparison to the classical "corona treatment" the APPTs provide access to a broader range of industrially interesting surface modifications that are normally better controlled with respect to their physicochemical nature. Thus, application of APPTs may become a superior option for preparing polymer surfaces for adhesive bonding, adhesive-free low-temperature bonding involving homogeneous and heterogeneous substrates, lacquering, or coupling of specific biomolecules, proteins, or cells. APPT technology is, however, not only flexible for tuning surface chemistry but also is flexible with respect to plasma source and equipment design. Versatility of the APPT technology facilitates its integration into a variety of process chains. An example presented here is a hybrid technology combining both APPT and additive manufacturing based on 3D printing processes using fused filament deposition. Inline treatment by quasi-simultaneous execution of printing and APPT can, for instance, increase the adhesion of 3D printed products in print direction (z-Axis) and thus increase the mechanical stability of the printed part. In the medical field such a technology may be attractive for cell growth by promoting treatment of internal surfaces of printed porous scaffolds. In future, products made from biobased or recycled polymers will become increasingly important. APPT technology could become an important enabler for meeting the technical requirements for the adhesion of such products.

Keywords: Atmospheric pressure plasma treatment, chemical groups, functional coatings, bonding, adhesive-free bonding, additive manufacturing, binding of biomolecules

1.1 Introduction

Polymers play an important role in packaging, automotive, medical, biomedical, and aerospace applications. Polymer market still experiences growth, driven by the increased use of polyolefins and also of new biobased and functional polymers. Most of these polymers exhibit inert surfaces that need to be treated to provide sufficient adhesion for lacquers, paints, and adhesive bonding. Treatment can be carried out using a wide range of surface-functionalizing processes including wet-chemical etching [1, 2], plasma [3-5], and laser modification [4, 6, 7], as well as ozone and UV treatment [4], many of which are nowadays well established in industry. In recent years, application of cold Atmospheric Pressure Plasma Treatments (APPTs) in industry has experienced very strong growth attributed to the high versatility in applications, low investment cost and the very good integration of the technology in existing process chains [8].

Changes in technical, social, and legal requirements will inevitably lead to changes in processes also in the adhesive bonding industry which are therefore the focus of this review. Adhesive bonding technology is used in a wide range of products as it offers various material combinations, longterm stability and safety while maintaining material properties and creating additional functions in the bonded product. Continuous inventions and innovations in the development of raw materials, adhesives and bonded products have enabled a dynamic development in the past decades.

Innovative bonding technology enables new, more environmentally-friendly and sustainable products and processes in many areas. For instance, improved resource efficiency can be achieved in the production of renewable energy power plant components where sealing of solar cells or joining of wind turbine rotor blades has become possible. Another example is the electromobility area with the sealing of battery cells, heat management of batteries with heat-conducting adhesives and hermetic sealing of fuel cells. In addition, adhesives with improved product properties can lead to savings, even in food packaging to increase shelf-life with resource-efficient materials.

In Europe the "European Green Deal" with its agenda responding to current socio-political changes will bring additional requirements and thus new challenges for bonding technology [9, 10]. The main point is the transition from a linear to a circular economy, minimizing the use of resources, generation of waste and emissions, and the inefficient use of energy. This will be achieved by considering resource needs, life extension, repair, refurbishment, recycling and closing energy and material loops.

The eco-friendly APPT technology with its innovations in combining suitably functionalized surfaces with bonding technology certainly has the potential to meet the new set of requirements that may result from implementation of the agenda of the European Green Deal.

These new challenges must be realized in terms of closed-loop systems along the value chain. Therefore, life cycle assessments (LCAs) enable a holistic view of adhesively bonded products with regard to ecological and economical improvements along the complete value chains [11, 12]. In this context atmospheric pressure plasma treatments (APPTs) of polymers and other materials like paper, rubber, fabrics, steel, glass and different composite materials have gained considerable importance in recent years due to their technological and economical capabilities and thus offer the potential to become the future leading technology for surface functionalization to improve adhesive bonding technology. The main advantages of APPTs are that they need no expensive equipment, are easy to handle, have a very good scalability and can simply be integrated in existing process lines.

Considering that in most applications materials are joined with other structural parts, the adhesion behavior is of great significance. The adhesion property is strongly affected by the chemistry and morphology of the surfaces. APPTs alter surface morphologies and chemical composition of different materials without affecting their bulk properties. The plasma treatment can also change the surface from hydrophobic to hydrophilic state and vice versa, depending on the type of monomer or process gas used for surface activation. Thus, plasma treatment can alter the inert polymer surface to have greater chemical/physical affinity by incorporating chemical functional groups. This leads to an increase in surface free energy and enlargement of the contact area and thus improved adhesion property. Different research groups have carried out a variety of APPT processes to improve the surface properties of materials [13-19].

These cold plasma treatments have gained increasing interest as they represent environmentally-friendly solution and can even be performed under atmospheric pressure conditions at relatively low cost [20-22]. The plasma treatment can also affect the morphology by exerting the etching effect on the surface and changing the surface roughness and wettability. This results in improved adhesion of metal/plastic compounds [23]. The cold plasma treatment of various plastics such as polypropylene, polyethylene, poly(ethylene terephthalate), poly(etheretherketone), etc. has been investigated [24-26].

In this chapter, we will focus on the use of APPTs to improve the adhesion by chemical functionalization for different applications. Firstly, we will give a short overview about the historical development of APPTs. The second section is about the establishment of functional surfaces with a high number of different chemical groups. In the third section, the adhesion improvement by APPT promoted joining of materials is discussed. In the fourth section we will show the relevance of APPTs for biomedical application for improving e.g. cell adhesion. Finally, we will give an overview on the use of APPTs to promote adhesion in additive manufacturing processes.

1.2 Historical Development of APPTs

The industrialization of APPTs started in 1951, when A. W. Eisby, a Danish engineer, invented a high-frequency treatment with a dielectric barrier discharge in air for polymer foils to improve the adhesion of inks [27]. This so-called "corona treatment" is nowadays commercially well established in the packaging sector for surface activation and cleaning. In the following decades different kinds of plasma sources for APPT working with air as process gas were developed, enabling the treatment of flat and complex surfaces [5]. In the meanwhile, APPTs have been established in the biological, pharmaceutical and medical fields, in automotive, aerospace, and electronics industries and have become important for the alternative energy sector, additive manufacturing as well as for the pretreatment of renewable and sustainable materials. Figure 1.1 shows the most common sources like dielectric barrier discharges, stabilized corona systems, and plasma jets or torches which can be operated using different types of excitations, e.g., alternating current (AC), pulsed direct current (DC), low-frequency (kHz), radio frequency (RF, 13.56 MHz) and microwaves (MW) with a frequency of 2.45 GHz.

Today APPTs can be used to functionalize a surface with...