Corona-pretreated film

Fig. 8-22 Reaction of a two-component aqueous lamination adhesive (acrylate dispersion + water-dispersible triisocyanate) with corona-pretreated films.

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. 

Polyurethane dispersions adhere strongly to a broad range of corona-pretreated plastic films. Achievable adhesion levels with PU dispersions are in many cases higher than with acrylates. As mentioned above, PU can be crosshnked for use in... 

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. 

The main results obtained for the growth kinetics, the adhesion, and the structure and morpholy of aluminum thin films deposited on bi-axially stretched PET films are the following. (l)For a set of experimental conditions (flux, temperature, and polymer surface), the aluminum film is discontinuous up to 10 nm (island formation) and then become continue. The grain size always increases with the thickness of the aluminum Elm as opposeted to the adhesion which remains rather constant. (2)When the aluminum film is continuous and for one thickness of deposition, the adhesion coefficient increases when the grain size decreases. (3)We found an increase of the adhesion coefficient when the skin of the polymer is "semi-crystalline and when the polymer is pretreated with a corona discharge. (4)The best results (for the adhesion of Al/PET) are found for a polymer treated in a fluorine atmosphere and when the deposition of the aluminum on polymer is done by sputtering. 

Containers are normally pretreated by flaming and films by corona treatment. Pretreatment can be checked by wetting the surface and observing the affinity of the surface to water (untreated surfaces retain the water as droplets) or actually measuring the angle of contact, which is reduced when the surface is oxidised. Pretreated surfaces effectively last for 3-6 months. Surface treatment may also enhance the effectiveness of some of the constituents which operate at the surface of the container, e.g. antistatic slip and anti-slip. 

Certain surfaces require some form of pretreatment prior to printing. Foil usually requires primer wash, and polyethylenes and polypropylenes need to have the surface oxidised. The corona process is almost invariably used for films and flame treatment for bottles. Whether a surface has received treatment or not can be detected by immersing it in water and observing whether or not the water runs off. An oxidised surface has a lower wetting angle. If the surface is not printed soon after treatment another treatment may be necessary. Inks will not key onto non-oxidised PE and PP and will be removed when a self-adhesive tape test is employed. 

Nowadays, the flaming process is rarely used in the lining of containers with polyethylene and in the printing of bottles (Kreidel process). Among the chemical pretreatments, immersion in chromosulfuric acid (for a few seconds to a few minutes), followed by careful rinsing with water has proved successful. Thin plastic layers for the production of laminated films for use as packaging materials are best pretreated by corona discharge. 

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