The introduction of plastic packaging films more than 50 years ago required surface treatment systems that would run at normal production speeds. in general, plastic films have chemically inert and nonporous surfaces with low surface energy, causing them to be nonbonding to other substrates, inks, coatings, and Adhesives.pe and pp have the lowest surface energy of commonly used plastics and are most often subjected to surface treatment to improve bonding. surface treatment can be used to improve the bonding of virtually all plastics and some nonplastic materials such as foil and paper. following is a review of the methods.acid etchingchemical treatment of a film involves cleaning, etching, and rinsing steps. the cleaning removes any surface contaminants.
The etching involves the use of an acid or oxidizing agents, such as nitric acid nho3 or potassium chromate k2cr2o7, to change the polymer surface chemically. finally, the film is rinsed clean of the etching chemicals and dried.This process usually is done following film manufacturing, significantly adding to the final cost of the film. this method often is slow and creates waste disposal issues.priming often is done in conjunction with corona treatment to increase surface energy and improve adhesion of a coating, ink, or adhesive. a primer is chosen that will provide a high surface energy for good adhesion to the film. some primers bond chemically to the substrate. an example would be polyethyleneimine, which is a cationic chemical and bonds strongly with treated film surfaces, inks, and coatings that are anionic.
Flame treatment exposes a moving film surface to a gasfired flame at a high enough temperature to create a plasma. the plasma reacts chemically with the film surface, which adds polar functional groups and increases surface energy.corona discharge converts the substrate surface from a nonpolar to a polar state. ozone is generated during the process.
It consists of a highvoltage electrical discharge across a fixed air gap between an electrode and a dielectric, usually a roller for web treatment applications. this discharge forms a corona in the gap between the electrode and the dielectric roller, thus treating the film surface toward the electrode. Corona treatment often is done during film manufacturing and again inline with a secondary converting process such as printing to increase the film surface energy, often by 10 dynes and more. this treatment can deteriorate over time or become masked by additives, especially slip additives, migrating to the surface.If treatment is too high, it can cause blocking of the wound substrate roll if too low.
It can cause adhesion and wetting problems in converting.atmospheric plasma is similar to corona treatment. like corona, plasma is the electrical ionization of a gas. in contrast, the plasma glow discharge creates a smooth cloud of ionized gas with no visible electrical filaments. at this time plasma is more expensive, but benefits compared to corona include higher and longlasting treatment levels no backside treatment elimination of ozone production and longerlasting treatments.corona is the most widely used surface treatment method, but developments in plasma are continuing.
This method may find increased use in the future.as a matter of fact, the tag and label industry is the largest segment for the synthetic market. &ldquothese include wraparound labels, inmold labels, tags, and a multitude of pressuresensitive products. Inmold labeling iml is a good example of what synthetic papers bring to the table. with iml, the labels are positioned on the containers at the time they are being formed either blowmolded or injectionmolded.
They have to withstand the rigors of the high heat of molten plastic, without burning and without distorting. synthetics have proven a more reliable substrate than paper.in addition, when paper substrates are used in these applications, problems can arise in the recycling stream. the paper label must be removed from the container to allow the plastic container to be recycled. the reality is that there ends up being a certain amount of paper residue in postconsumer resin that inevitably causes a &ldquoleaker&rdquo when the residue burns a pinhole in the wall of a newly formed container
