ENHANCED ADHESION BETWEEN POLYPROPYLENE AND POLYAMIDE-6 - ROLE OF INTERFACIAL NUCLEATION OF THE BETA-CRYSTALLINE FORM OF POLYPROPYLENE

We present an investigation of the mechanisms of mechanical reinforcem ent at interfaces between polypropylene (PP) and polyamide-6 (PA6), as sociated with the incorporation of a small amount of maleic anhydride functionalized PP (PP-g-MA) which reacts with the NH2 groups of the PA 6 to form a copolymer in situ. In a previous study we have demonstrate d, for one molecular weight of PP-g-MA, that diblock copolymer molecul es were indeed formed at the interface, with an areal density Sigma, c ontrolled by the reaction temperature and the reaction time, and that the measured fracture toughness of the interface scaled as G(c) propor tional to Sigma(2), regardless of the reaction temperature, but for si milar sample cooling conditions. We report here the behavior of the sa me system for a higher molecular weight PP-g-MA: at a reaction tempera ture above 220 degrees C, very close to the melting point of the PA6, and above a given Sigma, the measured G(c) becomes 4 times higher than that for reaction temperatures below 220 degrees C, where the observe d G, values are identical to what has been measured for the low molecu lar weight PP-g-MA. G(c) is therefore no longer uniquely dependent on C. Crystallographic analysis on the PP side of the intel face showed a correlation between the presence of the PP beta-phase in the 20 mu m near the interface and a high toughness; this crystalline phase was no t present in the samples prepared at T less than or equal to 220 degre es C or with the low molecular weight PP-g-MA which always exhibited a low toughness even for samples prepared above 220 degrees C. It is ar gued that the presence of this beta phase of the PP is the main factor responsible for the very high fracture toughness, first evidence of t he influence of the crystallinity of a semi crystalline polymer on its adhesive properties.