Ultrasonic Hemp Fibre Processing
- Ultrasonic retting of fibrous materials such as hemp and flax fibers allows for a fast and efficient fiber modification.
- Ultrasonically processed bast fibers are fibrillated and show a significantly higher specific surface, increased tensile strength and flexibility.
- Ultrasonic fiber processing is a fast and easy-to-use processing technology for industrial production.
Ultrasonic Retting
Ultrasonic retting is a fast, efficient and green alternative to traditional wet- or dew-retting. Acoustic cavitation, generated by high-intensity, low-frequency ultrasound, breaks up cellular structures of bio-materials such as non-wood, vegetable fibres which include bast fibre such as flax, hemp, nettle, wheat straw, rice straw, jute,as well as leaf-derived fibres (e.g., sisal, manilla hemp, abacá) and fruit-derived fibre like coir from coconut shells.
Ultrasonic disentangling transforms microfibers (approx. 3-5µm) into nanofibers (≥100nm). Furthermore, ultrasonic processing induced degradation of pure xyloglucan and xylan in solution, demonstrating the ultrasounds ability to degrade hemicellulose.
Although ultrasonic retting is mainly used in an aqueous solution, it is possible – depending on the raw material and the targeted outcome – to combine the ultrasonic process with an alkali treatment. Solutions of NaOH, H2O2 and H2SO4 can be used for alkalization to obtain cellulose nanofibers in a short processing time. By ultrasonic treatment, a fibrillation of cellulose microfibers can be easily achieved. The ultrasonically produced fibers show a specific morphology in which the nanofibers (≥ 100nm) are distributed across the entire surface of the microfibers (3-5µm).
Scanning electronic microscopy analysis on flax, hemp and coir fibers with or without ultrasonic processing.
source: Renouard et al. 2014
UIP4000hdT (4kW) industrial ultrasonic processor for fiber processing
Ultrasonic Hemp Fiber Processing
With the growing market for hemp seeds and phyto-cannabinoids comes an increasing production of hemp straw. As a by-product, hemp straw and its fibers are mainly used for the production of paper or geo-textiles, the reinforcement in composite materials as well as building material.
Dried and cut bast straw can be used as raw material for ultrasonic treatment, however for superior ultrasonic process output the use of (partially) decorticated shives is recommended. The bast material is wetted in water (aqueous solution) so that a pumpable slurry is obtained, which can pass the ultrasonic flow-through cell. The sonication process takes only a short period of time (approx. 30-60 sec.). Scientific research has shown that ultrasonication improves the extraction of hemicellulose and lignin from lignocellulosic materials. Additionally, sonication degrades cellulose and pectin. Ultrasonic processing of hemp and flax also improves the flexibility and tensile strength of the fibers, which are valuable characteristics for textile and composite manufacturing.
- reduction of lignin content
- micro- and nano-fibrillated fibers
- increased fiber flexibility
- higher tensile strength
- rapid process
- easy to operate
Ultrasonic-alkali treatment of hemp fiber (Ferreira et al. 2019)
Ultrasonically Modified Hemp Fiber
Ultrasonically fibrillated bast fiber (e.g., hemp, flax) is particularly suitable as a reinforcement for polymeric resins, thermoplastic, and thermoset composites.
Hemp bast fibers are a valuable source from which cellulose nanocrystals (CNCs) can be extracted. Cellulose nanocrystals are characterised by their high surface area and their extraordinary stiffness and tensile strength. CNCs’ tensile strength excels the strength of glass or aluminum. Cellulose nanocrystals are a quite cheap and are thereby a competitive nano-additive, when it comes to price, availability, toxicity as well as sustainability.
Sonication is an easy-to-use, fast and green technique, which allows to produce high-quality cellulose nanocrystals.
Sosiati et al. 2014 show the benefical effects of sonication on fiber processing.
High Performance Ultrasonicators for Fiber Processing
Hielscher Ultrasonics manufactures high-performance ultrasonic equipment for heavy-duty applications. Our ultrasonic systems can by used for batch or continuous inline processing. All Hielscher industrial ultrasonic processors can deliver very high amplitudes. Amplitudes of up to 200µm can be easily continuously run in 24/7 operation. For even higher amplitudes, customized ultrasonic sonotrodes are available. However, the capability of very high amplitudes alone is not enough to run a successful ultrasonic fiber process, such as retting or fibrillation. Depending on the raw material and on the targeted outcome, the process parameters – namely, amplitude, pressure, temperature, and time – must be exactly controllable and adjustable.
Hielscher’s digital ultrasonic processors record automatically all process data on an integrated SD-card, so that process results are reproducible. Amplitude and processing intensity can be precisely adjusted and controlled from very mild to highly intense sonication conditions. This gives you the opportunity to process various materials to optimum output.
The robustness of Hielscher’s ultrasonic equipment allows for 24/7 operation at heavy duty and in demanding environments.
The table below gives you an indication of the approximate processing capacity of our ultrasonicators:
| Batch Volume | Flow Rate | Recommended Devices |
|---|---|---|
| 1 to 500mL | 10 to 200mL/min | UP100H |
| 10 to 2000mL | 20 to 400mL/min | UP200Ht, UP400St |
| 0.1 to 20L | 0.2 to 4L/min | UIP2000hdT |
| 10 to 100L | 2 to 10L/min | UIP4000hdT |
| n.a. | 10 to 100L/min | UIP16000 |
| n.a. | larger | cluster of UIP16000 |
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High-power ultrasonic processors from lab to pilot and industrial scale.
Literature/References
- Diana P.Ferreira, Juliana Cruz, Raul Fangueiro (2019): Chapter 1 – Surface modification of natural fibers in polymer composites. Green Composites for Automotive Applications. Woodhead Publishing Series in Composites Science and Engineering 2019, Pages 3-41.
- Sullivan Renouard, Christophe Hano, Joël Doussot, Jean-Philippe Blondeau, Eric Lainé (2014): Characterization of ultrasonic impact on coir, flax and hemp fibers. Materials Letters 129, 2014. 137–141.
- H. Sosiati, M. Muhaimin, P. Abdilah, D. A. Wijayanti, Harsojo, K. Triyana (2014): Effect of the chemical treatments on the
characteristics of natural cellulose. AIP Conference Proceedings 1617, 105 (2014). - M. Zimniewska , R. Kozłowski, J. Batog (2008): Nanolignin Modified Linen Fabric as a Multifunctional Product. Molecular Crystals and Liquid Crystals Vol. 484, Issue 1, 2008.
Facts Worth Knowing
Hemp Fiber
Hemp is a multipurpose crop used for the hemp seeds and subsequently seed oil, terpenoids and cannabinoids (i.e. CBD, CBG, etc.) and hemp straw, which can be processed to valuable fiber material. In regards to hemp fiber quality, there is distinguished between the so-called tow fibers, which are not-aligned, short fiber bundles and the so-called line fibers, which are long (longitudinal aligned) fibers.
The short fiber bundels are also called technical fiber and are mainly used in the automotive industry, for the production of paper and for bio-based composites. Long hemp fibers are used for textile and high-value applications such as high-performance composites and bio-composites.
Hemp fiber production:
Fiber hemp (hemp which is grown for fiber production)is ideally harvested before flowering. This early cropping results in higher fiber quality because quality declines if flowering is allowed. Generally, fiber hemp is harvested 70-90 days after seeding. To harvest the hemp, the plants are cut 2-3cm above the soil and then dried for a few days. After harvest, the hemp is retted. Retting is a process that uses moisture and microbes to break down the plant pectins, which chemically bonds the hemp stem together. Traditionally, hemp stalks would be water-retted or dew-retted before the fibers are scutched. The retting process facilitates the subsequent separation of the bast from the so-called hemp hurd or shiv (which is the woody core of the hemp stems). After retting, the hemp stalks are dried (to a moisture content of less than 15% and bailed.
To obtain hemp fibers, which can be used for manufacturing and as additives, the fibers must be separated in a process known as “scutching”. During the scutching process the hemp straw is mechanically processed to beak down the hemp plant, e.g., using a hammer-mill. In this mechanical process the hemp is beaten against a screen until hurd, smaller bast fibers, and dust fall through the screen. Modern high-speed kinematic decortication machines are capable of separating hemp into three streams; bast fiber, hurd, and green microfiber.
The cellulose content in hemp is approx. 70-77%. Hemp fibers are an excellent substitute for wood cellulose fibers
Advantages of Hemp Fibers
- cost-effective
- high tensile strength and stiffness
- ideally suited for needle-punched nonwoven products
- effective replacement for glass fibre
- reduces moulding time
- weight reduction in finished part
- easy to process and recycle
- can be customised to meet a variety of specifications and different manufacturing systems
- consistent quality and availability of supply is possible
Fibrous Bio-Materials
When straw fibres are extracted from flax straw, the non-fibre parts of the stem, not including the seed, are normally referred to as shives or hurds. For example in oilseed flax, shives comprise approx. 70 – 85% of total straw weight, which makes shives the main by-product of flax straw processing.
Ultrasonically produced, nano-structured lignin is used to make multifunctional linen fabrics. By padding linen textiles with nano-lignin, multifunctional textiles can be created. Those multifunctional textiles offer the additional properties of UV barrier, antibacterial, and antistatic properties.