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Polypropylene‐based composites were prepared containing non‐treated and various treated cotton fibre and wood flakes. A correlation was observed among the fibre treatment and compounding parameters, mechanical and discoloration properties. The structural changes in fibres were demonstrated by Raman spectroscopic and DSC measurements. The possibility for forming cellulose fibre containing flame retardant composites was also investigated. The efficiency of various treatments on compounding, discoloration and mechanical properties enhance in the following order: no treatment < non ionic surfactant < reactive silicone segment containing non ionic surfactant < special silylation treatment. The best results obtained with the special silylation treatment were explained with the more organophilic character and by the thermal stability of the treated fibres. Cellulose fibre as a polyol‐charring component and ammonium‐polyphosphate together constitute a high performance intumescent flame retardant system in the PP matrix.

The principle of multilayer interphases was proposed earlier for modifying the mechanical properties and UV stability of various multicomponent polymer systems. This paper focuses on the applicability of this principle for improving the performance of intumescent flame‐retardant systems using melamine‐treated ammonium polyphosphate, silicone modified polyol + ammonium polyphosphate, and silicone modified nanoparticles in polypropylene. The structure–property relationship of the formed systems was studied. A melamine layer of 1.45 nm thickness was formed around ammonium polyphosphate in order to improve the hygrothermal stability, but this layer was not shear‐resistant enough. An interphase formed using a special silicone additive is more stable and acts with the intumescent flame‐retardant system synergistically. The advantageous interfacial structure is quite complex in this case: polyphosphate particles are surrounded with a macromolecular layer formed from polyol, silicone, and reactive surfactant in order to ensure good stability, efficiency and compatibility. AFM, XPS and a Cone Calorimeter were used for determining the structure and flame‐retardancy of these systems. Nanocomposites combined with silicone‐containing intumescent system were developed in order to avoid dipping at ignition in the vertical position. SAXS and µ‐TA methods were used for determining the structure of this material. Copyright ­© 2003 John Wiley & Sons, Ltd.