Ramie ( Boehmeria nivea (L.) Gaud.) and Spanish Broom ( Spartium junceum L.) fibres for composite materials: agronomical aspects, morphology and mechanical properties

Abstract

The agronomic characteristics of Ramie and Spanish Broom were investigated for seven years in the pedoclimatic conditions of Central Italy. The chemical, physical and mechanical properties of these fibres were also examined in order to evaluate the feasibility to use them in composite materials. Results demonstrate that Ramie grown in the temperate environment can be harvested three times a year. The yield of green Ramie plants per annum was almost 10 000 g m −2. The fresh Ramie plant is composed, by weight, of 30% green leaves and 70% green stems. The yield dry fibre is almost 3% of the green stems, giving a total yield of 200 g m −2. Spanish Broom can reach a fresh biomass yield of 4000 g m −2 per annum, represented by 53% of long slender terete green branches which constitutes the economic products. The dry yield per annum was about 1800 g m −2 with a dry branchlets yield of 900 g m −2. Ramie and Spanish Broom cortical fibres are multiple elementary fibres (ultimates) arranged in bundles. In Ramie, the elementary fibres are bound by gums and pectins, while in Spanish Broom they are bound together by lignin. Both species showed a thick secondary cell wall indicating a high cellulose content. Ramie ultimate fibres are flattened and irregular in shape, while those of Spanish Broom are more regular in shape. The diameter of the ultimates varies from 10 to 25 μm in Ramie, while the Spanish Broom ultimates ranges from 5 to 10 μm; the diameter of the whole bundle is about 50 μm for both species. Ramie fibre showed a content of lignin, pentosans and extractives lower than Spanish Broom. Both fibres had a high content of cellulose which, on the base of X-ray analysis, was evaluated to be in excess of 70%. Ramie and Spanish Broom fibres had tensile strength of 950 MPa and 700 MPa, respectively. The elastic moduli were ≈65 and ≈20 GPa, respectively, which well compare with the modulus of E-glass fibres (70–90 GPa). The strength of the fibre-matrix interface was measured using the single filament fragmentation technique and an epoxy resin as the polymer matrix. Values for carbon and glass fibres in the same resin were also measured for comparison. The interface strength for the vegetable fibres was higher than that of carbon and glass, likely due to a mechanical lock mechanism. These values confirm both fibres as potential replacement for man made fibres in composite materials.