Adhesive bonding of polypropylene

Adhesive bonding by definition is tiie joining of materials with an organic adhesive as an interlayer which adheres on the substrates without macroscopical changing of the material s state. 

The basic problem of adhesive bonding of polypropylene (PP) is its low surface tension, in the range between 29 and 35 mN/m. An absolute value is not given here, because the surface energy and also the surface state of PP depends, like the surface tension of all materials, on the history of surface creation. Surface energies of adhesives are normally higher than 35 mN / m and can go up on to the surface energy of water (72 mN/m). As a consequence, normally PP in an untreated state is not wettable by adhesives and in an untreated state not bondable. The only 

Three ways exist to create improved adhesional properties of a PP surface. 

The second type of surface treatment is in principle an improving of the chemical reactivity of the substrates by chemical or physical treatment methods. All the systems known lead to oxidizing effects of the nonpolar substrate and create carbonyl, carboxyl and hydroxyl groups at the surface which are polar and partly chemically reactive. The existence of these groups can be measured for example by Fourier transform infrared (FTIR) spectroscopy using the attenuated total reflexion (ATR) technique. 

The oldest treatment is an etching process in highly concentrated chromic-sulfuric acid with a temperature near 80°C. Highest adhesional strength between PP and, for example, a two-component epoxy (EP) adhesive is reached after etching times between 120 and 240 s. The recipe of the etching solution is 5078 parts of sulfuric add (density 1.82g/ml), 120 parts demineralized water and 75 parts potassium dichromate. 

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