Surface energy flame treatment

While polymeric surfaces with relatively high surface energies (e.g. polyimides, ABS, polycarbonate, polyamides) can be adhered to readily without surface treatment, low surface energy polymers such as olefins, silicones, and fluoropolymers require surface treatments to increase the surface energy. Various oxidation techniques (such as flame, corona, plasma treatment, or chromic acid etching) allow strong bonds to be obtained to such polymers. 

Some physical techniques can be classified into flame treatments, corona treatments, cold plasma treatments, ultraviolet (UV) treatment, laser treatments, x-ray treatments, electron-beam treatments, ion-beam treatments, and metallization and sputtering, in which corona, plasma, and laser treatments are the most commonly used methods to modify silicone polymers. In the presence of oxygen, high-energy-photon treatment induces the formation of radical sites at surfaces these sites then react with atmospheric oxygen forming oxygenated functions. 

Evenness of treatment depends on factors such as the designs of the chamber and of the electrode, the levels of energy and pressure, and the period of time of exposure once established at satisfactory values, these parameters can be controlled fairly easily. The surface chemistry is similar to that with simple flame treatment but the use of a gaseous reactant means that even treatment of quite complex shapes is practicable. Besides even and repeatable results the method offers the additional advantage that adhesion promoters are not necessary—with associated savings in costs and enhanced environmental implications. 

Polymer surface modifications are omnipresent in applications where the surface properties of materials with favorable bulk properties are insufficient. By altering the surface characteristics using physical or chemical modification the desired surface properties may be achieved. Such treatments are required e.g. to enhance printability of films, the adhesion of paints, metal or other coatings, biocompatibility, protein resistances/reduced biofouling, etc. The diverse approaches met in practice include, among others, wet chemical and gas phase chemistry, plasma or corona, UV/ozone and flame treatments. In most cases surface chemical modification reactions take place that alter the surface energy in a desired way. For example,... 

A second relevant example is the surface oxidation of polyolefins, such as LDPE, which is carried out frequently for thick specimens using an oxidizing flame treatment. For instance, while untreated LDPE surfaces are non-polar and thus fairly hydrophobic, flame-treated LDPE possesses a much higher surface energy and therefore improve the binding of these surfaces with other substances, such as adhesives, printing inks, paints, and various metal surfaces. 

In furnaces and other high-temperature equipment, where radiation is particularly important, the usual objective is to obtain a controlled rate of net heat exchange between one or more hot surfaces, called sources, and one or more cold surfaces, called sinks. In many cases the hot surface is a flame, but exchange of energy between surfaces is common, and a flame can be considered to be a special form of translucent surface. The following treatment is limited to the radiant-energy transfer between opaque surfaces in the absence of any absorbing medium between them. 

Untreated, many polymers, in particular polyolefins, are hydrophobic and therefore tend to repel water and water-based substances. The oxidation of the film through flame surface treatment increases the surface energy of the film, making the material less repellent of water, improving film wettability. Increasing wettability is a necessary component of many manufacturing processes that require water-based inks and adhesive materials to be applied to the polymer film surface. 

Adhesion can be modified by chemical or physical methods to increase the surface free energy. The following procedures can be used for modifying the surface chemical treatment, ultraviolet treatment, flame treatment, and plasma treatment. 

Flame treatment is a suitable technique for the improvement of the surface energy of many types of polyolefins. However, it has been exploited to a minor extent in comparison with the corona treatment. Although improvements in safety conditions and in some technical aspects were observed, this approach is especially used in by industrial sector that historically lagged behind in using this treatment technique. The mechanism of firee radical degradation is characteristic for the... 

To enhance the adhesive strength, the surface properties of substrate are very important. The surface treatments are often used to enhance the adhesive strength between the coating and the substrate, as well as to enhance the surface energy of substrate - improving wettability of coating. If adhesive problem occurs, check out the surface treatment processes such as flame, plasma, or corona. Corona treatment does not persist permanently, therefore it should be done in-line, the corona is often applied directly before the coating station. 

Adhesives manufacturers are continually trying to develop adhesives to meet the needs of industry. One group of plastics that have been difficult to bond are polyolefins and related low-energy substrates (see Surface energy). They could not be bonded without elaborate surface preparation such as Flame treatment or Plasma pre-treatment, Corona discharge treatment or oxidative chemical methods. 

To obtain good results on these plastics, it is necessary to modify the surface of the plastic, to cross-link the weak layer into the plastic below and to oxidize it, thus creating polar groups and a higher energy surface. The preferred treatments for doing this are corona discharge (for plastic films) and flame treatment (for moulded articles). 

Keywords contact angle, corona treatment, flame treatment, functionality, surface energy, surface modification, wettability, XPS. 

The surface energy of the support may be increased by oxidizing it, such as in a flame (flame treatment) or in an electrical discharge (corona treatment). Flame treatment is permanent, but for many polymers corona treatment is labile—after a matter of hours or days the surface energy decreases toward its original value. 

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