Influence of field retting duration on the biochemical, microstructural, thermal and mechanical properties of hemp fibres harvested at the beginning of flowering

Abstract

The use of hemp fibres as reinforcements in polymer composites requires a thorough understanding of the hemp fibres transformation processes to obtain a constant quality. In this context, the upstream processing termed field retting is considered. Retting allows a subsequent fibre separation from the plant stems by degradation of cementing compounds by microorganisms. This operation depends on weather conditions and is currently empirically carried out in fields, so that a large variability in the hemp fibres quality (color, morphology, biochemical composition, thermal properties and mechanical properties) is resulting. Therefore, the present study aims to investigate the influence of different retting durations (up to 9 weeks) on hemp fibres properties when harvested at the beginning of flowering growth stage to survey their temporal dynamic. Various assessments were applied on fibres: color observations, morphological (optical microscope), surface (ESEM) and biochemical (gravimetry) analyses, spectrocolorimetric measurements (pectins content), thermogravimetric (TGA) analysis, and mechanical in tensile mode testings. The results reveal that increasing the field retting duration leads to a change of color characteristics from light green to grey due to the development of microbial communities (most probably fungal and bacteria) at the stem surface. A separation of the fibres bundle to elementary fibres occurs with the degradation of pectins during retting. An increase of thermal stability of the fibres is also observed. Both increase of cellulose fraction and crystallinity induce an enhancement in tensile properties.

A Comparative Study of the Effect of Field Retting Time on the Properties of Hemp Fibres Harvested at Different Growth Stages

“…Hemp (Cannabis sativa L., Cultivar 'Santhica 27') was sown at rate of 35 kg/ha on 6 May 2016 in the south of France (N 44.130673 • , E 4.315895 • ) by CIVAM Chanvre Gardois [22]. The hemp plants were harvested manually using a shear at three growth stages (beginning of flowering (BF), end of flowering (EF), seed maturity (SM)) ( Figure 1).…”

“…Table 1 presents the retting duration and corresponding samples for each selected growth stage. Since morphological feature, chemical composition and mechanical properties depend on hemp stem sections [1,23,24], only the middle part of the stem was investigated [22] in order to limit the dispersion of the results. Table 1.…”

“…The biochemical analyses were done using two methods described in details in our recent work [22], one based on solvent extractions to quantify cellulose, lignin and lipophilic extractives contents according to several ASTM test methods (ASTM D 1107-56, ASTM D 1104-56, ASTM D 1103-60, ASTM D 1106-56 and ASTM D 1102-84), the other based on spectrophotometry to determine the pectins content.…”

“…In Europe, the most widely used treatment is field retting (also known as dew-retting) because it is inexpensive and easy to be applied [18,19]. This treatment consists of cutting and leaving the hemp plants on the field so the microorganisms (fungi and bacteria) attack and colonize the stems, producing a wide range of enzymes, such as pectinolytic that lead to the removal of the components in the middle lamella (pectic substance) and allows the cortex fibres to be progressively separated [20][21][22]. Therefore, the field retting is a crucial step for the production of hemp fibres, but it is empirically realized in the field due to its dependence on the weather conditions (temperature, humidity) which could cause problems of the inconsistency of the hemp fibres quality.…”

“…In addition, they found after the comparison between unretted and 5 weeks-retted fibres that the prolongation of field retting under a very rainy retting period caused over-retted fibres with lower quality in terms of mechanical properties. In contrast, when the stems are retted in summer weather conditions, a long period of retting is required [13,22]. Thus, understanding the impact of field retting treatment on the fibres harvested at different initial states of hemp plant development is needed for getting composite materials with high performance.…”