The use of unretted hemp fibre in composite manufacture

doi:10.1016/S1359-835X(00)00098-1

Composites Part A: Applied Science and Manufacturing

Volume 31, Issue 11, November 2000, Pages 1279-1283

https://doi.org/10.1016/S1359-835X(00)00098-1 Get rights and content

Abstract

The usefulness of unretted hemp as a source of fibre for composite manufacture is investigated. A decortication machine was used to remove fibre from retted and unretted stems using two different decortication procedures, defined as ‘normal’ and ‘pinning’. The later caused less fibre breakage. Composites manufactured with fibre from the different treatments, and epoxy resin, were subjected to tensile tests and the Young's Modulus and strength recorded. The pinning-decortication treatment resulted in stronger, stiffer, composites. Lack of retting did not significantly change the reinforcing capabilities of hemp fibre. Composites made with unretted, pinned, fibre at a volume fraction of 0.2 had a Young's Modulus of 8 GPa (S.D. 1.9) and strength of 90 MPa (S.D. 22). However in unretted fibre more Cuticle and epidermal tissue remained attached to the fibre tissue and it was shown that the interface between these two tissues is susceptible to breakdown over time. This could limit the useful life of the composites. It would therefore be an advantage if the processing of unretted hemp could be improved so that more epidermal tissue is removed.

Introduction

Long plant fibres have the potential to rival glass fibre in the manufacture of composite materials, using matrix materials such as polypropylene, epoxy and phenolic resins [1], [2], [3], [4], [5], [6], [7].

Hemp and flax are the only commercial sources of long natural fibres grown in the UK. Plant stems are processed by various mechanical methods to extract the fibre [8], [9], [10], [11]. To increase the ease of extraction the stems are retted before processing. The retting process is controlled degradation of plant stems to allow the fibre to be separated from the woody core. This stage is a serious barrier to the wider use of flax and hemp fibre, both economically and in terms of fibre quality [12], [13]. The problems caused by retting include microbiological contamination of the fibre, increased variability of fibre properties, insecurity of fibre supply owing to poor weather during field retting and delays in the planting of subsequent crops. These factors limit the competitiveness and range of application of the fibre. Alternatives to retting, such as steam explosion have been considered but most of these significantly increase the cost of the fibre [14]. If fibre of useful quality could be extracted from unretted hemp, the problems caused by the need to ret the stems could be avoided.

A prototype apparatus to extract fibre from unretted hemp has been developed at Silsoe Research Institute, but the properties of the fibre and its performance in higher value applications such as composites have not been established. It is likely that unretted fibre will be coarser because the cells will be more difficult to separate due to the presence of pectins and other natural polymers. However previous work has indicated that fibre cells do not need to be separated to provide good reinforcing capability [15]. In some instances strips of fibre-containing tissue can act as effective reinforcing elements for composite manufacture. However problems such as lack of adhesion between the resin and cuticle have been observed and these problems could be greater in unretted tissue. The research reported here aimed to establish the properties and performance in composites of fibre extracted from unretted hemp stems, so as to open the way to the exploitation of this UK-grown crop.

Section snippets

Methods

Hemp at normal maturity from the 1997 harvest was used for the initial work. Stems of hemp from a commercial source were collected from the 1999 harvest at intervals of approximately one week and dried without retting. This gave two samples younger than the normal commercial harvest maturity, one at normal maturity and one a week post mature. Each harvest was split into two batches. One batch was retted and dried before further processing and the other batch was just stored at room temperature.

Results

The unretted fibre (Fig. 1A and B) is coarser than the retted fibre and is composed largely of fibre bundles and narrow strips of fibre tissue with some epidermis and cuticle still adhering to the outside. The epidermis is composed of thin walled, slightly elongated cells and does not contain any fibres. The cuticle is a waxy layer secreted by the epidermal cells. Retted fibre on the other hand is composed largely of individual fibres and fibre bundles and there is much less epidermal tissue

Discussion

Lack of retting does not significantly change the reinforcing capabilities of hemp fibre. This is because in unretted tissue the fibre cells and bundles are well stuck together and strips of tissue act as effective fibres. Retting breaks down the interfaces between bundles and the decortication process then separates these bundles. Thus in retted, decorticated, tissue the effective fibres are bundles and individual cells, rather than strips of tissue. However given that the properties of the

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Citation Excerpt :
For retting, to improve the suitability of hemp fibre for current or future intended industrial applications (e.g. structural composites), various biotic or abiotic treatments (sometimes both) have been adopted on the fibres [105] and on the stems [96, 92]. Decortication of stems is made easier following retting – be it water-retting [106] or dew-retting. The underlying mechanism in action here is the decrease in the fracture energy at the interphases of the bast fibres with both shives [107] and the epidermis.
Given the vast amount of available research in the area of natural fibre composites, a significant step forward in the development of next-generation plant fibre-based products would be to devise a framework for rational design. The authors use a top-down approach, starting with an example final product to define the product specifications for high-performance hemp fibre-reinforced composites. Thereafter, all process steps are critically analysed: from textile preform and reinforcement yarn production, to fibre extraction and the agricultural process chain, to the microbiology of field retting, to cultivation and selection of crop variety. The aim of the analysis is to determine how far the current state of knowledge and process technologies are in order to use hemp fibres in high-performance composites. Based on this critical evaluation of the state-of-the-art, it can be stated that hemp will be found in high-performance composites in the short-to-medium term. There is, however, a need for performance optimisation especially through the selection of crop variety, best practices in retting, and effective fibre extraction methods to obtain more consistent fibre qualities suitable for reinforcement spinning and composite preform manufacturing processes.

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Short CommunicationThe use of unretted hemp fibre in composite manufacture

Abstract

Introduction

Section snippets

Methods

Results

Discussion

Enzyme Microb technol

Preparation and properties of polypropylene composites reinforced with wheat and flax straw fibres. Part I characterisation

J Mater Sci

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J Mater Sci

The nature of adhesion in composites of modified cellulose fibres and polypropylene

J Appl Polym Sci

Short Communication
The use of unretted hemp fibre in composite manufacture