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Manufacturing sustainable bast yarns

Dec 01, 2020
Manufacturing sustainable bast yarns

Interest in bast fibres such as flax or hemp has recently increased as environmental movements have gained great popularity. Bast fibres are very versatile and valuable for textile and non-textile applications. Rieter offers tailor-made, economical solutions for processing bast fibres in short staple fibre spinning.

Bast fibre is a type of plant fibre that can be collected from the inner bark of plants such as flax, hemp or ramie. Linen (made of flax) is one of the oldest textiles developed, dating back nearly 10 000 years.

With today’s increasing environmental awareness, textiles made of bast fibres are being rediscovered for everyday use as well as for luxury fabrics. Bast fibres are very sustainable. For the cultivation of flax, for example, very few pesticides are used and the water requirement is low (Fig. 1).

Flax – a fibre with a difference
Especially in summer, the advantages of linen clothing are obvious. The fabric absorbs moisture from the air and exchanges it with the ambient air. Thus, the fabric has a cooling effect and is still dry. An additional benefit of this water absorption is the antistatic effect. The linen fibre is very tear-resistant, so the fabric is hard-wearing and extremely durable. As a result, a linen garment can last for many years without damage.

Cottonization of bast fibres
Flax is used as an example to explain the spinning process for bast fibres. Flax is unique in the fact that different types of fibres can be extracted from the same raw material. Some of these fibres are processed via the traditional wet spinning process which, however, is very cost intensive. Other extracted fibres are well suited to be shortened which is a precondition for manufacturing yarns economically using cotton spinning technologies. The process of reducing the flax fibres to short staple fibres and giving them the same characteristics as cotton is called “cottonisation”. This works in a very similar way with other bast fibres, like for example hemp.

From flax tow to short fibres
To prepare the flax tow for short-staple fibre spinning, Rieter is cooperating with the German company Temafa. They are a global expert on blending and opening, recycling, air engineering and natural-fibre extraction. The so called Rieter-Temafa-concept prepares the flax tow in such a way that high-quality yarns can be produced from it using the rotor spinning process. The flax tow is progressively relieved of shives and dust – without a cutting process (Fig. 2). The raw material is refined in different opening and cleaning stages using machines from Temafa and Rieter. The cottonised material then passes into a baling press.

In this process, the fine cleaner UNIflex B 60 is responsible for giving the fibre material the same length, fineness, purity and spinning characteristics as cotton (Fig. 3). The desired fibre length is set by adjusting the nipping point. This also reduces the short-fibre content and thick, non-fibrillated fibres are removed. The variable opening intensity opens the fibre bundles into individual fibres. Intensive cleaning is performed at the same time by precisely setting the cleaning intensity. The quantity of waste is controlled via the VARIOset function. Using VARIOset and the integrated dedusting unit reduces trash accumulation at the card.

Further cottonisation with the web card C 75
The web card C 75 sets new benchmarks in quality and productivity for cottonizing bast fibres. Compared to conventional cards, the card technology with a 1.5-m working width achieves a reduction in process-related waste, which results in better raw-material utilisation. The card is adapted to suit the flax fibres using appropriate carding elements. The focus is on the fibre’s refinement, length and purity. The one-roller licker-in module is used to open the fibres gently.

The mote knife on the licker-in module ensures that any remaining shives, unopened flax bundles and dust are removed efficiently. The efficient removal of these components and the short fibres is continued by the carding elements in the pre- and post-carding zones. The main carding zone facilitates the separation and removal of short fibres, contamination and fibre neps. The cottonised web sliver passes into a baling press. The bales are then ready for spinning preparation.

Optimum fibre and spinning preparation
The specific structure of the bast fibres calls for suitable preparation technology. The automatic bale opener UNIfloc A 12 copes well with the heterogeneous flax fibres. It opens material uniformly from the bale and prepares the optimum tuft size for the subsequent machines.

When it comes to mixing, the UNImix B 72 with its three-point mixing principle is the ideal machine. The large storage capacity provides for an appropriate dwell time for the material and thus ensures a trouble-free production flow. Depending on the yarn counts to be spun, an integrated autoleveler draw frame module after the card is sufficient for 100% flax.

For blends with cotton or man-made fibres the spinning preparation process is different and involves the blender UNIblend A 81. Depending on the quality requirements, carding is followed by one or two draw frame passages.

Decisive advantages with rotor spinning of bast fibres
Rotor spinning machines offer advantages for the spinning of bast fibres. The incoming draw frame sliver is opened into single fibres in the spinning box, whereby the fibre material undergoes further cleaning and particles can be removed. In the case of blended yarns, the blend is improved by the re-doubling in the rotor grove. With the spinning nozzles in rotor spinning the yarn characteristic can be additionally influenced.

Yarns made of flax blends or of 100 per cent flax are manufactured efficiently on the Rieter rotor spinning machines R 70 (fully automatic) (Fig. 4) or R 37 (semi-automated).

These machines offer unique advantages in this context: The R 70 with its well-established Rieter spinning box allows an optimised trash extraction using the optimised BYpass for exact setting in combination with an adaptor part. For spinners using a semi-automated rotor spinning machine, the R 37 offers a special trash channel to make sure the heavy bast fibres are kept in the spinning process and are used optimally.

Depending on raw-material quality, yarn counts of Ne 4 to Ne 10 can be spun from 100 per cent flax. The finer the yarn count the higher the amount of fine fibres required. With blends yarn counts up to Ne 20 are possible.

Flax yarns produced on rotor spinning machines offer advantages in the end product compared to traditional ring spinning. The yarns have higher elongation, lower hairiness, a lower shive and dust content and significantly better downstream processing behavior. In addition, the conversion and equipment costs are low. Blended yarns are suitable for producing easy-care garments. The blends offer a wide scope for designing fashion items and thus enable a considerable expansion of the product range. The woven fabrics and end products offer good abrasion resistance and higher dye retention.

Pic captions:
Fig. 1: Flowering flax field
Fig. 2: Preparing the flax tow for the spinning mill on a Temafa machine
Fig. 3: The fine cleaner UNIflex B 60 gives the flax fibre material the same length, purity and spinning characteristics as cotton.
Fig. 4: Yarns of 100 per cent flax or blends can be produced economically on a rotor spinning machine.

For further information, please contact:
Rieter Management AG
Media Relations
Relindis Wieser
Head Group Communication
T +41 52 208 70 45
F +41 52 208 70 60

About Rieter
Rieter is the world’s leading supplier of systems for short-staple fibre spinning. Based in Winterthur (Switzerland), the company develops and manufactures machinery, systems and components used to convert natural and man-made fibres and their blends into yarns. Rieter is the only supplier worldwide to cover both spinning preparation processes and all four end-spinning processes currently established on the market. Furthermore, Rieter is a leader in the field of precision winding machines. With 16 manufacturing locations in 10 countries, the company employs a global workforce of some 4 570, about 21% of whom are based in Switzerland. Rieter is listed on the SIX Swiss Exchange under ticker symbol RIEN.