Fibres and Yarns | News & Insights

SUNN HEMP FIBRES

Published: May 10, 2024
Author: TEXTILE VALUE CHAIN
BY NCFA TEAM
1. Botanical information

Sunn Hemp (Crotalaria juncea L.) is a versatile plant native to tropical Asia and belonging to the legume family. Also known as Brown hemp, Indian hemp, or Madras hemp, it’s a short-lived crop growing up to 4 metres tall with a strong taproot system.

Sunn Hemp offers a variety of benefits:

  • Soil improvement: Grown as green manure, it enriches the soil with nitrogen and organic matter.
  • Animal feed: The leaves and stems can be used as livestock fodder.
  • Fibre source: The strong stems provide a source of natural fibre.
  • Weed suppression: Its dense growth shades out weeds, reducing their populations.

Interesting facts:

  • Sunn Hemp is a short-day plant, meaning it flowers when daylight hours are shorter.
  • Although once a popular source of fabric, Sunn Hemp cultivation has decreased compared to other fibres.
  • This cross-pollinated plant has 16 chromosomes.
  • Scientists are exploring ways to improve seed production through treatments like indole butyric acid and colchicine.
2. Area and Production

Sunn hemp is a widely cultivated crop, with major producers including China, South Korea, Pakistan, Bangladesh, Romania, Russia, and countries within the Commonwealth of Independent States (CIS). These regions account for nearly 92% of the global sunn hemp coverage and 83% of total production, with a combined area of 320,000 hectares and a yield of approximately 200,000 metric tons of fibre.

India is a significant player in sunn hemp cultivation, contributing 23% of global production and boasting 27% of the world’s dedicated area. Interestingly, sunn hemp finds use in various applications across the country, serving as a source of fibre, green manure, and even fodder. The total area under sunn hemp cultivation in India is around 45,000 hectares, with an annual production of 0.102 million bales (approximately 18,360 tons) of fibre. This translates to an average productivity of 409 kg per hectare.

Within India, several states take the lead in sunn hemp production for fibre purposes. These include Bihar, Madhya Pradesh, Maharashtra, Rajasthan, Orissa, and Uttar Pradesh, collectively accounting for nearly 87% of the country’s total sunn hemp cultivation area. Notably, Orissa stands out as the single largest producer, contributing 26% of India’s total sunn hemp output.

Drilling down further, specific districts within these states are particularly concentrated hubs for sunn hemp cultivation. In Uttar Pradesh, Varanasi and Jaunpur are the key players. Maharashtra relies heavily on Aurangabad and Jalna districts, while Orissa’s Ganjam district is another major contributor. Together, these five districts account for an impressive 20% of India’s total sunn hemp cultivation area and 23% of the nation’s production.

Finally, the highest-yielding regions for sunn hemp fibre are found in Orissa’s Balasore and Sundargarh districts, along with Rajasthan’s Chittaur district. These areas are followed closely by Ganjam and Kalahandi districts, also located in Orissa.

3. Agro Climatic Conditions for Cultivation

Sunn hemp is a versatile crop that thrives in a variety of conditions. It’s a champion weed suppressor thanks to its rapid growth. While well-drained soil is key, sunn hemp isn’t too picky. For green manure during the monsoon season, most any such soil will do. If you’re aiming for fibre, light, well-drained soils with a sandy loam or loamy texture is ideal. These soils should retain moisture well, with around 30% being optimal for germination.

Sunn hemp is a curious case when it comes to daylight. While it flowers under short days, it prefers long days for vegetative growth. It’s a tough customer, tolerating a wide range of rainfall (49-429 mm annually), temperatures (15-37.5°C annually), and even soil pH (5-8.4). However, a near-neutral pH (6-7) with high phosphate availability is the sweet spot.

This adaptable plant can handle both tropical and subtropical climates. In Brazil, it’s even grown in the Amazon under near-equatorial conditions. In India, the planting season varies. Summer or monsoon rains are ideal in the north, while the south utilises the milder winter for a “rabi” season planting. Consistent rainfall of at least 400 mm during the growing season is crucial. Interestingly, sunn hemp switches gears depending on the desired product. Long days promote fibre growth, while short days favour seed production.

When it comes to soil, sunn hemp avoids extremes. Acidic, alkaline, and waterlogged areas are a no-go. Well-drained loams or sandy loams are its happy place. A pH between 6.0 and 7.5 is essential for optimal growth and healthy root development. Phosphorus and calcium-rich soils are the cherry on top for sunn hemp. Preparing the land is relatively simple: one ploughing followed by a few harrowings creates a good seedbed. A slight slope for drainage is helpful, and ensuring adequate moisture at sowing time is vital for successful germination. Northern India under irrigation sees plantings in the second half of April, while rainfed areas sow with the monsoon’s arrival. Finally, southern India cultivates sunn hemp as a rabi season crop.

4. Fibre Pre-treatment and Extraction  

Retting is a crucial step in obtaining usable fibre from certain plants. It involves submerging plant stems in water to allow microorganisms to break down the material surrounding the desired fibres. The length of time required for retting depends on various factors like water temperature, season, and the type of plant. Different methods are used, but they all involve soaking the stems for several days.

The stalks are typically arranged in a single layer within a pool or tank of water. Weights are used to keep them submerged, but materials that release tannins should be avoided. The retting time can vary depending on factors like water temperature, with warmer water leading to a faster process. Once the fibres are loose enough to be separated from the stem easily, the retting is complete.

Extracting the fibres is a manual process that can differ slightly depending on the region. In some areas, the partially decomposed bark is first removed by hand. Then, the worker grasps the stalk and scrapes the fibres off using their thumb and fingers. Alternatively, the stalks might be dried first, then broken and scraped to remove the fibres. This method is said to produce higher-quality fibres.

After extraction, the fibres are washed thoroughly to remove any remaining impurities. Sun drying follows to remove moisture. Finally, the fibres are bundled and prepared for market. In some regions, the fibres are twisted into bundles, while others simply tie the ends together. The yield of usable fibre per hectare can vary depending on the plant and the effectiveness of the retting process.

5. Features of Fibre

Yarn strength is crucial for its quality. We measure this strength using a value called tenacity. Tenacity is calculated by dividing the fibre’s breaking force (measured in grams) by its fineness (measured in Tex). To assess this, we test either bundles of fibres (for overall strength) or individual filaments.

5.1 Physical Properties of Sunn Hemp Fibres

Under mechanical testing, the parameters that are pertinent and significant at every stage of harvesting and processing are listed in Tables 1 through 3.

  •  Fineness

Sunn Hemp fibres are relatively fine compared to other natural fibres like jute or mesta. This is measured by the Length to Breadth Ratio (LIB) of the ultimate cell. Sunn hemp boasts a much higher LIB (450) compared to these fibres (75-140), indicating a finer structure. However, some natural fibres like ramie, flax, and cotton have even higher LIBs (900-3500), making them even finer.

  • Tenacity

Sunn hemp demonstrates good strength when tested in bundles. Its tenacity (breaking force divided by fineness) falls within the same range as jute. Both sunn hemp and jute outperform other natural fibres like white jute, mesta, or coir in terms of bundle tenacity. However, ramie, flax, true hemp, and sisal exhibit even higher strength. It’s important to note that retting conditions can significantly impact fibre strength, so these comparisons might vary depending on processing methods.

  • Density

The density of sunn hemp fibres can be measured by calculating the bulk density. This involves measuring the volume of a specific weight of fibres arranged in a particular way under constant pressure. To achieve the most accurate “true” density, the fibres are submerged in an inert liquid that fills any air pockets. This method provides a more precise measurement of the fibre’s inherent density.

5.2 Chemical Composition of Sunn Hemp Fibre

The appropriateness of sunn hemp fibres for use in textiles or other applications is determined by analysing their chemical composition.

5.2.1 Chemical Composition of Sunn Hemp Fibre

Tables 4,5 and 6 provide the chemical composition of sunn hemp fibres about major, minor, and monosaccharide constituents.

5.2.2 General Observations on the Chemical Composition of Sunn Hemp
  • Cellulose

Sunn hemp boasts the highest alpha-cellulose content among bast and leaf fibres, second only to ramie. Its molecular weight, measured by viscometry, ranges from 132,840 to 182,250 depending on the processing method (17.5% vs 9.3% caustic soda). This exceptional cellulose content, combined with the long chain length (high degree of polymerization), makes sunn hemp a prime choice for speciality paper production.

  • Hemi-cellulose

Sunn hemp’s key hemicellulose is a 1,4-linked glucomannan. This means its backbone is a chain of glucose and mannose sugars connected in a specific order (1:2.1 ratio). Branch points linked by 1,6 bonds occasionally deviate from this main chain. The glucomannan chains are moderately long, with an estimated degree of polymerization of 49. Notably, most non-reducing ends (chain terminals) have glucose residues. While some sugar sequences might have adjacent glucose and mannoses, the primary structure features alternating sugar units.

6. Use of Sunn Hemp Retted Fibres

Sunn hemp shines in the realm of strong, saltwater-resistant cordage. Ropes, strings, and fishing nets made from sunn hemp surpass jute in strength, though falling short of true hemp. The coarse fibres, after retting, find use in marine caulking (filling gaps in boats).

Sunn hemp’s versatility extends beyond cordage. With adaptations to flax spinning machinery, it produces coarse fabrics ideal for hoses, belts, and canvas.  Across the UK and Belgium, sunn hemp takes on various forms: twines, cords, matting, sacking, tarpaulins, and even shoe soles and sandals.  Furthermore, sunn hemp graces our homes as rugs, carpets, webbing, and even table and bed linens.

7. Typical High-End Uses of Sunn Hemp Fibre
  • Personal wear like shirts and skirts
  • Home linen like towels, upholstery, bedding, and curtains
  • Gardening accessories such as mulch mats, weed barriers, and soil and tool bags.
  • Painting canvases for artists
  • Cloth baby diapers (due to their high water absorption)
  • It is used for making – Cordage, fishing nets, Soles of shoes, sandals, Ropes, tat Pattis, sacking, twines, canvas, floor mats, rugs, carpets, webbing, table and bed linen, currency notes, cigarette paper, tissue paper, carbon base paper, electrical condenser, handmade paper, airmail paper, and wax impregnated paper.
  • American hemp products included clothes, wagon covers, grain sacks, and ropes. Hemp waste found its way to oil mills where it was processed into hemp oil, which was then utilised to make paints, inks, varnishes, and lamp oil.
8. Bibliography
  1. https://www.textilecoach.net/post/sunn-fiber
  2. https://www.webmd.com/a-to-z-guides/what-to-know-about-hemp-fabric

iii.http://jafexpert.crijaf.icar.gov.in/Sunnhemp_expert/sunnhemp_cultivation.aspx#:~:text=In%20India%20traditionally%20it%20is,Pradesh%2C%20Tamil%20Nadu%20and%20Rajasthan.

  1. http://jafexpert.crijaf.icar.gov.in/Sunnhemp_expert/sunnhemp_cultivation.aspx
  2. J. Chaudhury, D.P. Singh, S.K. Hazra “Sunnhemp (Crotalaria juncea, L)” CRIJAF (ICAR); Barrackpore 743, 24-Parganas (North), West Bengal

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