Hemp fiber reinforced polypropylene composites

Significance Statement

Natural fibers are becoming more desirable reinforcement alternatives owing to their excellent mechanical properties, biodegradability, and low density. When used with thermoplastic polymers, they exhibit good recyclability, low abrasiveness, superior energy recovery, good damping, non-toxicity and high strength to weight ratio. These natural fibers offer potential to synthetic fiber replacement such as glass fibers in a number of applications such as, underbody panels, automotive structural elements, door inserts and instrument panels.

However, these natural fibers suffer a setback of poor binding to hydrophobic polymers such as polypropylene and affinity to moisture. Material treatments have been found to enhance the adhesion or bonding between the natural fibers and matrix. Among these, alkaline treatment or mercerization makes it to the top of the list.

Treating these natural fibers with aqueous solution of sodium hydroxide leads to the ionization of the hydroxyl group to alkoxides. This degrades a particular amount of pectin and lignin and this exposes the reactive OH groups on the surface of the fiber. This removal enhances the bonding between the polymer matrix and the fiber. Theresa Sullins, Selvum Pillay and Haibin Ning at University of Alabama Birmingham in collaboration with Alastair Komus from Canadian Composites Innovation Centre studied the effects of the material treatment, chemical treatment and polymer matrix treatment, on the mechanical characteristics of the hemp fiber polypropylene matrix composite. They also studied the change in length of the fiber. Their work is now published in Composites Part B.

The authors dried the hemp fibers before treatment. They soaked the pre-dried fibers in 5wt% and 10wt% sodium hydroxide at ambient temperature. The fibers were soaked in the aqueous solution for about an hour after which they were removed and rinsed with water to eliminate alkali traces on the surface. The rinsed samples were dried and 5wt% polybond 3200 added to a pre-weighed polypropylene with respect to 15wt% and 30wt% hemp fiber. Maleic anhydride grafted polypropylene was mixed with polypropylene prior to processing.

The authors performed flexural testing. Five test pieces were prepared and tested to determine the strain. Also, five dog-boned specimens were obtained from each material treatment variance and modulus and tensile strength computed. The fiber’s length after the twin screw compounding and compression molding process was analyzed.

Surface images of the untreated hemp fibers indicate non-uniform impurities deposits on the surface. Sodium hydroxide treated fibers indicated cleaner and a rough surface. The authors observed that an increase in the alkali concentration eliminated most impurities and yielded a cleaner and a rougher surface. Sodium chloride treatment increased the surface roughness by distributing hydrogen bonding in the polymer-matrix composite, therefore, providing more sites for mechanical interlocking. The treatment process also led to defibrillation thus, increased surface area. Increased surface area in the treated hemp fiber is responsible for increased adhesion with the thermoplastic polymer matrix.

It was observed that material treatment led to polymer-matrix composites with improved mechanical properties as compared to untreated composites. The researchers observed that the composite with the 5wt% maleic anhydride grafted polypropylene posted the best mechanical properties.

Reference

Theresa Sullins, Selvum Pillay, Alastair Komus, and Haibin Ning. Hemp fiber reinforced polypropylene composites: The effects of material treatments. Composites Part B, volume 114 (2017), pages 15-22.

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