As a complete reference source for surface modification of polymers, this book reviews traditional and conventional methods for improving the adhesion of inks, coating adhesives, metals, and other adherends to polymers and introduces new methods for molecular engineering polymer surfaces to enhance their adhesion to a wide range of materials. In addition, this work serves to turn the vast amount of disparate information regarding plastics surface modification from wide ranging sources into practical application knowledge. In order to make this information most useful for practitioners, consultative data is arranged in consistent formats.
When addressing the adhesion of polymers to interfacing materials, the primary and foremost challenge is to understand the fundamental driving forces which can initiate the development of adhesion strength between polymer-to-polymer, polymer-to-metal, polymer-to-ceramic, or polymer-to-inks coatings and adhesives. These interfaces also exist in multivariate environments, such as heat and humidity, which also must be examined. Ultimately, it is the polymer and the interface chemistry that determine adhesion. However, there can be adhesion failure between the polymer and an inorganic, such as a metal, due to an oxide layer that is weakly attached.
With the advent of readily available non-paper substrates (plastics and foils) in the mid-to-late 1950s, the requirement for reliable production speed surface treatment processes became apparent. Several different technologies were developed and evaluated, but only one, corona treatment, has become the primary surface treatment technology used across the converting and extrusion industries worldwide. The development of corona treatment as the leading surface treatment method will be traced, followed by detailing of what is the current state-of-the-art in atmospheric discharge surface modification equipment, control parameters, and applications.
It is well understood that the surface modification of polymers by air plasma processes involves the activation of a surface primarily by the incorporation of oxygen species. The main processing parameters of air plasmas are atmospheric pressure and discharge power. However, because the propensity of air plasma discharges to modify surfaces is also governed by dielectric barriers, which exist not only between the discharge device and the subject substrate surface, but also a potentially required ground plane opposite the substrate, a review of the options for applying these specific dielectrics with respect to substrate properties will better define the best discharge configuration for a particular application. Therefore, we will review the features and benefits of bare roll, covered roll, and universal roll-type air plasma discharge configurations for two-dimensional web materials, and blown arc and blown ion air plasma for three-dimensional parts.
Although the technology of producing ozone has been well known going back to the 19th century, there has been a recent renaissance in the development and utilization of ozone generation systems to assist primarily corona discharges in the promotion of surface adhesion, primarily with polyolefins, such as polypropylene and polyethylene, as well as smooth or gloss paper grades, which do not exhibit sufficient surface tension relative to their interface to ensure adequate adhesion.
The practicality of using flame treatment to create reactive surfaces has been known since the 1950s. Improvements in the surface adhesion properties of polyolefin films, foams, and paper-based materials have been realized by oxidizing and polarization effects. Flame treatment typically creates fixed levels of oxidized species on the surface of films, along with the formation of hydroxyl, carboxyl and carbonyl functionalities. Treatment (oxidation) depths vary by substrate, as does the generation of low molecular weight organic material at the surface.
Plasma is generally defined as a partially or completely ionized gas with an approximately equal number of positively and negatively charged particles. In this definition, charged particles must be close enough together such that each particle influences many nearby charged particles, rather than just interacting with the closest particle. There are also non-neutral plasmas, such as charged particle beams, where the plasma is composed of only a single charge species. However, our discussion will focus on the features, benefits, and treatment optimization strategies of applying two major categories of commercial plasmas for the purposes of surface adhesion preparation and organic cleaning – low pressure vacuum plasmas and atmospheric pressure plasmas.
Methods for surface modification will vary greatly depending on the material to be decorated, the decorating inks to be used, the decorating process applied, and the desired surface properties of the intended decoration. Many surfaces require preparation to establish minimum adhesion requirements for the decorating adherend. This step has been referred to previously as pretreatment and post-treatment, depending upon to time-phase of the decorating process and influenced by the base material surface receptivity for the decorating layer at the time of final processing. For example, pretreatment of a metal surface may require chemical-assisted etching, mechanical abrasion, annealing, or other contamination-stripping methods. For example, metal parts can be immersed in a solvents (dip tanks), solvents can be wiped or sprayed onto the parts, solvent vapor degreasing processes can be used, or various combinations thereof.
Coating processes are used to improve the physical, functional, and/or decorative properties of substrates. These coating processes bridge across a wide range of virgin and composite materials, adhesion of which to non-porous and low polarity surfaces such as polyolefins and fluoropolymers is most problematic. Successful coating of three-dimensional, sheet, and web materials depends largely on an understanding of the interaction between the substrate surface structure and coating chemistry to predict and avoid undesired surface effects. These might include functional/performance properties such as oxygen or moisture barrier, and physical/mechanical properties such as tensile strength, shear strength and flexibility/rigidity. Most fabrics, for example, rely on polymers such as polyamides, polyesters, polypropylenes, and polyethylenes to provide chemical resistance, hydrophilicity, hydrophobocity, conductivity, and barrier surface properties.
Plastic nanocomposites (PNC) are materials composed of nanometer(one-billionth of a meter)-size inorganic particles dispersed in a polymer-based matrix. These PNCs are gradually becoming integral to an increasing number of automotive parts, wide web packaging films, household appliances, and medical devices. PNCs contribute structural reinforcement to these applications by virtue of their aspect ratio (the ratio of particle length or thickness to that of its diameter), large surface area, and the molecular interaction between the nanoparticle and the matrix it is blended with.
