In principle, annual growth agrofibers like flax and jute can, by means of their intrinsic mechanical properties, compete with glass fibers. This paper discusses reinforcement of polypropylene with flax bast fibers. Grades of maleic anhydride grafted polypropylene (MAPP) were screened for coupling efficiency. Three mechanisms appear to be of great importance in interphase modifying activity: · maleic anhydride content grafted for each polymeric polypropylene-chain; · molecular weight of MAPP; and · crystallinity of MAPP. The coupling efficiency of different MAPP grades, defined as the maximum increase in composite strength, has been investigated. It was found that the molecular weight of the compatibilizer was a more important parameter than the maleic anhydride content for the crystalline varieties. It appeared to be the other way around for the noncrystalline varieties. Furthermore, a maximum or optimum in MAPP efficiency can be found; adding more coupling agent to the system does not additionally increase the composite mechanical properties.
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