Physical and chemical properties of flax fibres from stand-retted crops desiccated at different stages of maturity

Abstract

Dessication of flax at the mid-point of flowering, followed by stand-retting and high-speed decortication, yielded finer, stronger fibres with lower acid-insoluble lignin content and lighter colour than desiccation at later timings of two weeks after mid-flower and six weeks after mid-flower. Although fibres from all desiccation timings could be carded and blended with wool for spinning on woollen or worsted spinning systems, only the fine fibre from the earliest desiccated flax was suitable for carding, drawing, blending with cotton and spinning on cotton-processing systems.

It was concluded that the lower lignin content at mid-flower allowed more complete retting of fibre bundles to give a higher yield of fine elementary fibres. This cotton-compatible fibre was produced at a competitive cost compared with cotton. It is suggested that further optimisation of desiccation timing may permit increased yields of cotton-compatible fibre.

Introduction

The world market for textile fibres is of the order of 55 million tonnes per annum and, in recent years, has been growing at a rate of about 2.5% per annum through the effects of population growth and the increasing per capita consumption of textile fibres in developing economies (Coker, 1990, Johnson, 1996). Cotton accounts for about 50% of world fibre production and although some further increase is still possible, especially through more widespread exploitation of G.M. cotton, this will not meet demand indefinitely. In the future there will, therefore, be an increasing need for alternative fibres that are compatible with existing, cotton-based, textile manufacturing technologies and that are competitive on price and quality.

In the short-to-medium-term, this need will continue to be met by polyester fibre (from petrochemical feedstocks) and regenerated cellulosic fibres (from wood cellulose). In the longer term, however, a continual increase in the production of these alternatives is regarded as environmentally unacceptable, and other fibres, from renewable sources, are already under consideration.

Fibres that have the potential to meet the needs of the cotton-based textile industry include those from ramie (Boehmeria nivea) and flax (Linum usitatissimum), but whereas ramie cultivation requires a temperate or sub-tropical climate and is particularly exhausting to the soil, flax can be grown satisfactorily under almost any climatic conditions and requires very low inputs of fertilizers. Thus, in Europe flax is the most widely grown fibre crop, and in recent years flax acreages have been increasing (after many years of decline) in response to EU subsidy schemes designed to reduce the over-production of food crops by making profitable, alternative use of arable land (Lennox-Kerr, 1998).

The long, high-quality “line” fibre, traditionally extracted from dew-retted flax stems for spinning into linen yarns, costs about 1.2–3.0 €/kg to produce (about twice the price of cotton) and is not compatible with cotton or cotton-spinning machinery because of its excessive length and stiffness (Truevsev et al., 1995a, Truevsev et al., 1995b, Truevsev et al., 1995c).

Flax line can, however, be broken down into smaller fibre bundles using a variety of mechanical, chemical and biochemical methods to yield “cottonised” flax, which is spinnable in cotton blends containing up to 50% flax (Akin and Rigsby, 1999, Akin et al., 2001; Truevsev et al., 1995a, Truevsev et al., 1995b, Truevsev et al., 1995c). These additional processes necessarily make cottonised line fibre even more expensive (than traditional line) and its blends with cotton, or other fibres, are therefore suitable only for speciality, premium-priced products.

On the other hand, short flax fibre (known as “tow”), which is produced in similar quantities to line fibre during traditional flax scutching (decortication) and hackling (combing) processes is much cheaper (0.3–1.0 €/kg). Flax tow is widely used in low-cost, biodegradable fibre-reinforced composites and in high-quality paper, but it can also be cottonised satisfactorily. Mechanical drawing and carding produces fibre that is compatible with cotton in blends with up to 30% flax (Chigaeva, 2001). Further improvements in fibre fineness, achieved by secondary chemical or enzymatic retting, yield a high proportion of ultimate flax fibres (still at a competitive price) that can be blended at up to 50% with cotton and spun on modern high-speed, cotton-spinning machinery.

Since the market for flax line is relatively small, it is not cost-effective to produce greatly increased quantities of both line and tow by traditional methods, when the long-term need is for a high-tonnage, cotton-compatible fibre of consistent quality at a price no greater than that of raw cotton. The aim of the EC-funded project (FAIR-CT98-9774, 1997) was, therefore, to assess the feasibility of processing the whole fibre yield of flax crops to give a single quality of low-cost, cotton-compatible fibre.

A further reason for increased interest in the production of high tonnages of good-quality, cottonised flax is that in some European countries such as Poland and in the Russian Federation and several of the former Soviet republics, flax can be grown and processed into short fibre at a cost significantly less than that of imported cotton. Textile manufactures in these countries can therefore make substantial savings in foreign exchange, as well as increasing profits on finished goods, by replacing with flax a proportion of the cotton in 100% cotton and polyester/cotton yarns. The extent to which this can be achieved, using existing cotton spinning machinery, depends on the fineness and fibre length distribution of the flax (Truevsev et al., 1995a, Truevsev et al., 1995b, Truevsev et al., 1995c).

Two key elements in reducing the cost of flax fibre production have been:

• to replace the relatively labour intensive practice of “dew retting” by using crop desiccation and stand-retting methods (Easson and Long, 1994) and

• to replace the traditional scutching and hackling processes by a novel, high-throughput decortication and fibre-cleaning system (Smith-Keary, 2001).

Although much previous work has been reported on the desiccation and stand-retting of flax, this has usually been in the context of traditional line and tow fibre production with the main evaluation criteria being related to the yield and quality of line fibre (Easson and Long, 1994). Consequently, the two types of herbicide most often used for flax desiccation have been re-evaluated in the context of EC-FAIR-CT98-9774 (1997). A variety of fibre samples were produced from randomised, small-plot trials (carried out by the School of Agriculture, De Montfort University, Lincoln, UK) in which (i) fully mature crops (with ripe seed) were sprayed with a bipyridinium, “diquat” type herbicide (Reglone ex Monsanto) and (ii) immature crops were sprayed with a glyphosate (N-phosphono-methyl glycine) herbicide (Touchdown ex Zeneca) either at the mid-point of flowering (MPF) or at two weeks after MPF (Booth, 2001).

The progress of desiccation and the subsequent colonisation by micro fungi and retting of the standing crops was closely monitored (Booth, 2001). When retting was judged to have reached the ideal stage for decortication (Booth, 2001) the crops were combine-harvested and sent to “Industrial Crop Partnership” (Tamlyn) Ltd. for decortication and primary fibre cleaning. An untreated (no desiccant applied) mature crop was also harvested for processing at the same time as the stand-retted, Reglone-treated crop. The samples of decorticated flax fibre received for evaluation in the present work were from flax cultivar “Laura” and were as listed in Table 1. The production cost for these samples was 0.70–1.0 €/kg, which was within the target, cotton fibre range (Smith-Keary, 2001).