Surface modification of polypropylene by additives

Polypropylene (PP) is highly apolar material, therefore a modification aimed at the creation of a more polar surface is an important issue considering, for example, wettability, adhesion, barrier properties or dyeability. 

The modification of PP surfaces can be done by various means, such as grafting more polar monomers onto the surface, controlled oxidation or plasma treatment. Incorporation of a suitable component into the bulk polymer is one of the possibilities commonly considered [1, 2], 

Various low molecular organic compounds are used for the modification of PP in bulk. The list of effective modifiers includes fatty acids, their salts, unsaturated dicarboxylic acids and their anhydrides, plasticizers, surfactants, organic peroxides and ketones. Most of the relevant data can be found in patent literature [e.g. 3, 4]. 

The operative mechanism is the physical adsorption of the long aliphatic chain in the hydrophobic PP phase, while the polar part of the additive penetrates the surface and increases its polarity. Hence, suitable polar functional moieties can be bound onto the surface by chemisorption. A significant increase in adhesion is observed so that even cohesive destruction of the adhesive joints has been reported in some cases. 

When studying the compounds with the same length of aliphatic chain, the values of mechanical work of adhesion of modified PP towards several more polar polymers was found to decrease in the order sodium stearate oleic acid stearic acid. 

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Chodak, I. Novak, I. (1999). Surface modification of polypropylene by additives. An A-Z Reference. Karger-Kocsis, J. ed. Kluwer Acad. Publ Dordrecht, pp. 790-793, ISSN 2080-8259. 

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