Polymer processing flame treatment

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. 

Flame treatment is a commercial process to render polyolefins and polyethylene terephthalate adherable. The polymer article (e.g., film) is passed over an oxidizing flame formed by of an oxygen-rich (relative to stoichiometry) mixture of hydrocarbon gas. Variables affecting the extent of oxidation include the flame characteristics (e.g., excess oxygen) and the speed of the article movement. Gas flame... 

Liquid etchants can be used for chenucal modification or dissolving surface contamination. Etchants effectively treat irregularly shaped objects that are difficult to treat by other adhesion-promoting processes such as corona or flame treatment. A number of etching solutions and procedures have been developed for specific polymeric surfaces. The choice of the liquid etchant depends on the polymers. Polyolefins are usually treated by oxidizing acids such as chromic, sulfuric, nitric, or mixtures of these. Fluorocarbons are usually treated by sodium-napthalene etching solution. 

Surface Treatment. Most film surfaces require surface treatment for use in subsequent steps of coating, printing, lamination, or metallization. This treatment results in the chemical modification of the polymer at the surface. The most widely employed surface modification is oxidation of the polymer to create a polar smface (43). Processes that are widely employed are corona and flame treatment. Wet chemical oxidative treatments are seldom used and are restricted to low speed operations. A new surface oxidation technique has been recently developed which uses an atmospheric plasma generated in a hollow cathode with helium or helium gas blends (44). 

Surface modification refers to the modification that occurs only on the surface of a polymer material without further internal modification. Surface modifications of polymeric materials include surface chemical oxidation, corona surface treatment, surface flame treatment, surface heat treatment, surface plasma treatment, surface metallization processing, ion implantation, and surface grafting polymerization. Because surface modification occurs only on the surface of materials, the performance does not change uniformly. 

Important applications of XPS pertain to the characterization of polymer surfaces. These applications include the detection of oxidation products and impurities, and observations of molecular rearrangements and chemical modifications of polymer chains. Moreover, XPS is an appropriate tool to follow up flame and plasma treatments of polymers. In polymer processing, XPS can help to optimize surface properties while retaining desirable bulk properties. 

As indicated above, the surface treatment process may be responsible for the removal of such material and exposing the bulk of the polymer underneath the segregated layer. Such a situation can be illustrated by the XPS spectra of Fig. 10.4 fi om PVC in its extruded form and after flame treatment. The extrudate is protected by a thin film of low density polyethylene (LDPE), and upon removal of this film the spectrum (O Fig. 10.4a) is dominated by carbon and oxygen with significant contributions from PVC-spedfic elements such as chlorine, lead, and zinc. On flame treatment (O Fig. 10.4b), the carbon level is reduced to a certain extent... 

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