Untreated polyolefin film

So far, emphasis has been placed on the treatment of polyolefins to improve printability, but the same treatments affect also other processes. As an example, while an untreated polyethylene surface will not retain an adhesive the treated and modified surfaces are receptive to adhesives. A field of increasing importance for packaging is the production of laminates of film with other materials and, as examples, suitably pre-treated polyethylene film can be bonded securely to substrates such as cellulose or polyester films and aluminium foil, using standard adhesives, whereas this cannot be achieved without pre-treatment. 

Polyolefins can be coated with other materials, such as poly(vinylidene chloride), in order to reduce the permeability by gases of the walls of blow mouldings or film (thus extending the shelf-lives of products contained in them) however, barrier coatings of this nature cannot be applied satisfactorily to the untreated surfaces. 

Bearing in mind the difficulties that may arise it is important to have available reliable and easily understood tests for assessing the level of treatment. Since an effect of treatment is to increase the wettability of the surface the most simple test is to apply a liquid of low surface tension, in order to compare treated with untreated film. If the liquid spreads uniformly the surface may be judged suitable for printing if, on the other hand, it coalesces into discrete droplets, the wettability is insufficient. There are available for such tests commercial surface tension fluids, which simply are applied to the polyolefin and their behaviour noted. 

Fig. 4.8 (a) Histograms of pull-off force values obtained with an unmodified Si3N4 tip on untreated and oxyfluorinated iPP films in ethanol. The total surface free energy y of the polymer film is shown, (b) Mean values of pull-off force measured with COOH-terminated tips on modified polyolefin surfaces (iPP, isotactic polypropylene LDPE, low-density polyethylene) in ethanol (top) and with OH-terminated tips on oxyfluorinated iPP in water (pH 3.8, bottom) as a function of cos 0 (contact angle measured with water). (Reprinted in part/adapted with permission from [26, 27]. Copyright 1998, 2000, American Chemical Society.)... 

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. 

Du Pont has three types of Tedlar film, Type A, with one side treated for bonding Type B, with both sides bondable and Type S, which is untreated and is used as a release film (www2.dupont.com). Type B is used in laminating to metals, plastics, wood, and other materials it requires no further surface preparation for adhesive bonding. The methods for preparing the untreated film for adhesive bonding are similar to those of polyolefins and lluoropolymers (see Chapter 6). 

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