Polymer processing corona treatment

Instead of the simultaneously extmded polymer film, a pre-extmded or laminated film can be laminated on the substrate (e.g., paper or paper board). In this case, the extruder can be used to extrude the polymer providing adhesion (e.g., Ecoflex ) in a thin layer between the polymer coating film or laminate and the substrate. Another option is to apply a dispersion or heat in the laminator (Fig. 10, 4) to obtain adhesion. After thickness measurement and second subsequent corona treatment, substrate and polymer coating are wound on a winder (Fig. 10, 6) in order to obtain rolls for transport to subsequent processes (e.g., printing, cutting, cup forming, box making). 

There are many different methods for modifying polymer surfaces to improve their adhesion and wetting properties. They include chemical etching and oxidation, ion bombardment, plasma treatments, flame treatment, mechanical abrasion and corona-discharge treatments (1.2). Especially flame and corona treatments are widely used for the modification of polyolefin surfaces to enhance, for instance, their printabilify. Despite the widespread use of such processes in industry, the understanding of the fundamental processes which occur at the polymer surface is very limited. This is undoubtedly due to the shallow depth to which the polymer is modified, typically 5 nm or less. 

In this paper some applications of static SIMS to a variety of modified polymer surfaces are described. They include plasma treatments in reactive and inert gases, corona treatment in air, as well as thermal and ion beam modifications of polymer-metal interfaces. The examples presented and discussed here primarily serve to illustrate the capabilities of static SIMS for the study of such surfaces and interfaces. More detailed discussions of the actual chemical processes that proceed in several of the systems cited will be published elsewhere. 

Plasma vs. Corona Treatment of Polypropylene (PP1. Corona treatments of polyolefins to modify their surfaces are very common in the polymer industry. The chemistry at such surfaces has been widely studied by XPS (4). It is generally assumed that corona treatments create abundant amounts of radicals which react with oxygen to form a hydroperoxide. This reacts further to eventually form crosslinks, oxidized products (ranging from hydroxyls to esters) with and without chain scission. The latter process is believed to lead to low-molecular weight material. There is some controversy over this material. Its role in determining the surface properties of the modified polymer is not completely understood. Its formation cannot be demonstrated directly by XPS, but only by comparing spectra before and after washing. 

In spite of these disadvantages, plasma treatment of polymers is an attractive process to produce the required surface modification. By using different types of gas, various chemical functionalities can be introduced on the surface. In general, more uniform surfaces are produced by plasmas than by flame and corona treatments. The modification is typically confined to the surface without changing the bulk physical and chemical properties of the pol)uner. 

Metallized polypropylene (PP) is used today in many different fields such as automotive, decoration, electrical. In order to obtain a good adhesion between the aluminium and the polymer a pretreatment of the film prior to the metallization is necessary. Indeed, the very extreme surface of the polymer has to be modified in order to prepare it to a good adhesion with the metal. Thus the polymer is placed in a low pressure plasma of nitrogen with a corona discharge configuration of electrodes, and the metallization is carried out in situ after the pretreatment in nitrogen. This process, which simulates an industrial polymer film treatment has proven a great efficiency for very short treatment times (23 ms) (1).However, the mechanisms responsible for the improvement of adhesion are not totally explained yet. 

The poling treatment uses a high electric field to reorient polarization alignment within the polymer material. And this electric field must be higher to drive domain wall motion. There are typical two systems for poling process namely electrode poling and corona poling (Ramadan et al., 2014 Crossley et al, 2014). 

Electrical ( corona ) discharge treatment of polyethylene and polypropylene film has been used for many years in order to render these surfaces printable and suitable for lamination or coating. In this process the polymer film is passed over a grounded metal roller covered with a "dielectric" (insulating) material. Separated from the film by mm is an electrode bar to which a high voltage is applied (typically 15 kV at 20 K Hz). Air in the film-electrode gap is ionised, the corona discharge thus formed is stable and this "treats" the film surface. 

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