Surface pretreatments corona-discharge treatments

Conventional coating approaches to TPO, which rely on the use of substrate pretreatments such as flaming, corona discharge, plasma or chemical pre-treatment, have led to a substantial oxidation of the surface, which enhances adhesion. These pretreatments, however, have a limited service life before the surface reverts to its unoxidized, apolar condition. 

With the help of complementary surface analysis techniques such as XPS, Static SIMS and AES, we have been able to show how a short (23 msfilms leads to a slight oxidation of the surface as well as to the formation of N2 containing species. These modifications are necessary for the improvement of the adhesion observed with a scotch-tape test. However, the presence of oxygen is not the only factor responsible for a good adhesion, since the AES profiles of die deposited aluminium, show the same oxidized interface in the case of the non treated metallized polymeric film. The films are pretreated in a corona discharge configuration (hollow electrode-grounded cylinder) and the aluminium is deposited onto the film in situ. 

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. 

Polyolefins, such as polyethylene, polypropylene and polymethyl pentene, as well as polyformaldehyde and polyether, may be more effectively treated with a sodium dichromate-sulfuric acid solution. This treatment oxidizes the surface, allowing better wetting by the adhesive. Flame treatment and corona discharge have also been used. Table 7.20 shows the relative joint strength of bonded polyethylene and other plastic substrates pretreated by these various methods. 

Various surface treatment processes have been developed to ensure adhesion of adhesives, coatings, or print to polymeric surfaces. These treatments are both mechanical and chemical. They include abrasive cleaning, chemical etching, corona discharge, flame treating, plasma treatment, and laser pretreatment. (See also surface treatment.)... 

Blackman et al. [71] compared the use of conventional abrasion pretreatments with corona discharge and oxygen plasma treatments. Again, the need to optimise the surface polarity was confirmed. As in the previous work, both adhesive and matrix chemistry influenced the optimum plasma conditions. For carbon/PEEK... 

Many chemical pretreatments have been reported for polyolefins but the only one of commercial importance is etching in chromic acid. It is typically used for the treatment of irregularly shaped or large objects when it would be difficult to use either flame or corona-discharge (Section 4.2.7) treatments. The chromic acid etches the polyolefin surfaces and results in loss of material into solution. 

It must be reiterated that an atmospheric plasma glow discharge provides a treatment with high uniformity and homogeneity, along with the ability to micro-etch and functionalize the surface of films (in this case with carbon-based functionality). In the case of OPP film, the level of heat seal strength is 20% higher than the level obtained with corona pretreatment. 

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