Manufacturing and recycling of sisal-polypropylene composites

The current study focuses on manufacturing, recycling and mechanical testing of thin (1.5 mm) sisal-PP extruded composite sheets. An 3-factor two-level experimental design based on the Taguchi method was applied in the manufacturing of the composite sheets to maximise their mechanical properties. The sisal-PP composite sheets obtained by setting the factors predicted by the Taguchi analysis were mechanically recycled by pelletising the sheets and feeding through the extruder. The effects of recycling on crystallinity, fibre length, mechanical properties and stress-relaxation were evaluated. By selecting a fibre mass of 30%, polymer MFI of 1.3 g/10 min and 1% of lubricant, ∼12%, ∼20% and >100% increases in tensile strength, impact strength and tensile modulus respectively were observed. The fibre lengths dropped from 7 mm before extrusion to 6mm and under after extrusion, and under 5 mm after recycling. A marginal change (± 5%) in modulus values was observed in both directions after recycling, the ultimate tensile strength of the recycled sisal-PP specimens dropped down from 41.4 ± 2.5 to 36.4 ± 0.75 MPa along the machine direction, and increased from 19.2 ± 0.5 MPa to 21.4 ±0.1 MPa transverse to the machine direction mainly due to the decrease in the reinforcing fibre lengths. The recycled composites exhibited greater relaxation compared to the sisal-PP composites; this is because, at higher temperatures, the polymer matrix is in a softened state and the bonding between the fibre and matrix is expected to be weaker and the short fibres behave like polymer rich areas and fail to share the imposed load, thus exhibiting greater relaxation at elevated temperatures.