By Eman Abdallah Kamel
Eman is an engineer and writer. She received her Bachelor of Applied Arts in Textile Sciences from the Faculty of Applied Arts, Egypt.
This article covers the stages of the hemp manufacturing process from cultivation to spinning. The features of cotton and hemp blend dyeing with ultrasonic pretreatment.
Hemp
Hemp is a soft, light-colored, lustrous, strong bast fiber from the hemp plant. The hemp plant (Cannabis sativa) is a hardy and sustainable crop. Hemp has many uses, including textiles, paper, food products, and furniture.
Hemp is known for its rapid growth and ability to thrive in several climates, making it an attractive option for sustainable agriculture. Hemp contains low levels of tetrahydrocannabinol (THC), making it suitable for industrial applications. In recent years, demand for hemp has increased, leading to its cultivation in many countries.
In 2021, hemp fiber was grown in roughly 21 countries, according to databases from the FAO, USA, and Turkey. These included the following nations: France, Germany, Greece, Italy, Japan, Lithuania, the Netherlands, Poland, the Republic of Korea (South Korea), Romania, the Russian Federation, Spain, Turkey, Ukraine, Austria, Bulgaria, Chile, China, the Czech Republic, the Democratic People’s Republic of Korea (North Korea), and the United States.
The top hemp producers include China, Canada, and the United States, while leading importing nations encompass Germany, the United Kingdom, and Australia, reflecting a growing global interest in the plant’s economic and ecological potential.
Meaning of Bast Fibers
Bast fibers are obtained from the outer cell layers of the stems of some plants. Flax, jute, hemp, ramie, and kenaf are the main plants that provide bast fiber.
Manufacturing Process
Hemp production involves the following steps:
- Cultivation,
- Retting,
- Breaking,
- Scutching,
- Carding,
- Roving,
- Spinning.
1. Cultivation
Hemp adapts to well-drained soil with a pH between 6.0 and 7.0. Hemp does not grow well in wet soils or soils with a high clay content. Hemp is sensitive to soil crust formation and compaction, which can occur in heavy clay soils. Varieties with low tetrahydrocannabinol (THC) levels should be chosen.
In some countries, such as the United States, the THC level in industrial hemp is less than 0.3. Medium to tall varieties are preferred in countries where there are no herbicides labeled for hemp.
Since cannabis cultivation and growth rely on healthy soil, it is best to plant in areas that were once grass-legume fields or reclaimed agricultural land. Areas with substantial erosion are not advised for cultivation.
After planting, effective management techniques must be maintained. Set up a cropping system that preserves soil productivity while reducing negative effects on soil quality.
Hemp can be grown for seed in rows, with a grain drill, or by no-till methods, using burn-in herbicides to control existing weeds. Industrial hemp seeding rates depend on the variety. Generally, a seeding rate of 25 to 35 pounds per acre is recommended. Planting depth should be ½ to ¾ inch. A dense, dug bed should provide some weed control. Planting depth should be ½ to ¾ inch. A dense, dug bed should provide some weed control.
By following the correct steps for growing cannabis, starting with choosing the right soil and watering methods, cannabis grows quickly, and within a few weeks, the plant will reach several feet in height.
2. Retting
Retting is a microbial process that breaks the chemical bonds holding the trunk together and separates the bark fibers from the woody core. Many methods for retting include dew, water, enzyme, physical, and chemical retting.
1. Dew Retting: The least expensive technique is field retting, which also has the benefit of using no water and little energy. Because field retting leaves behind naturally degradable residues, it improves soil fertility. The inability to regulate process variables like temperature and air humidity, which affect both the process’s duration and the fiber’s production and, consequently, its quality, is a drawback of field retting. After two weeks of soaking, the fungi’s activity causes the pectin-rich cells to break down, making it simple to separate the bast fibers and extract them from the intricate stem cell structure.
What does the duration of Retting mean?
The duration of retting means that the duration of action of the microorganisms on the fibers affects the fineness of the peeled hemp threads: a longer retting (soaking) time ensures a lower linear density of the fibers. After six weeks of soaking, the decrease in the linear density of the fibers compared to unsoaked fibers is about 50%.
2. Water Retting: The oldest method of retting is water retting (soaking). In this method, bark fiber stalks are soaked in artificial water tanks or natural water reservoirs such as rivers or lakes for five to seven days, where anaerobic bacteria cause the pectin to decompose. In artificial water tanks, variables such as water temperature and specific microorganisms can be managed to achieve maximum efficiency. After soaking, the stems are exposed to the sun to dry and turn white in the field. Sun drying is a long process that depends on weather conditions. Drying the stems in dryers is characterized by high energy consumption and the need for additional solutions, which have negative environmental and economic impacts. The time required for adequate retting under water is 1-2 weeks.
Did You know?
Long fibers extracted from the stems using the water-soaking process have high fineness, good mechanical properties, and high-quality spinnability.
3. Enzyme Retting: Enzymes are proteins that act as biological catalysts, speeding up chemical reactions. Enzymes degrade the pectin and gum in the stem, causing them to degrade within a short time, between 12 and 24 hours. Specific fiber properties for different applications can be achieved by varying the soaking time and the type of enzymes used. Disadvantages of enzymatic retting include reduced fiber strength and the high cost of enzymes.
4. Physical Retting: The steam explosion is a process in which cellulosic biomass is treated with hot steam at 180 to 240 °C under pressure of 1 to 3.5 MPa, then transferred from low to atmospheric pressure. This process causes the rigid structure of the fibers to break, changing the biomass into a dispersed fibrous solid. The abrupt pressure release causes the hydrogen bonds and glycosidic bonds between the glucose chains to hydrolyze, which produces the shear force. Hemp fiber can be effectively separated using stem explosion combined with alkaline hydrolysis, yielding about 92% of the primary fiber from the total fiber biomass.
5. Osmotic Degumming: The osmotic degumming of bast fibers is based on natural physical phenomena such as water diffusion and osmotic pressure. Water diffuses into the stem mass, causing it to swell. This increases the hydrostatic pressure inside the stem, breaking the epidermis longitudinally without shortening the fibers. The pectin becomes diluted and dissolves with other substances in the bark in the flowing water. The osmotic degumming process can provide long, soft, high-quality, well-cleaned, brightly colored fibers without the typical odor of water-soaked fibers. Ultrasound in a water bath facilitates the process and significantly increases the fibers’ ability to be degummed.
3. Breaking
Once the stalks are retted (soaked), dried, and tied, they are transported to a central processing location. The stalks are passed between corrugated rollers to crush and break the woody pulp into short pieces (hurds), separating some of the bark fibers.
4. Scutching
Scutching is the process used to separate the remaining fiber and hurds. The fiber bundles are held between rubber belts or chains and carried through rotating drums equipped with protruding rods that strike the fiber bundles, separating the hurds and short fibers from the remaining long fibers.
5. Carding
Carding is a process by which the fibers are opened, detangled, and cleaned. Short fibers, dust, and dirt can be removed from the fiber bundle to provide better performance for subsequent operations.
6. Roving
To increase strength, “the sliver” is further drawn out and twisted.
7. Spinning
Hemp can be spun on a flax spinning system. Hemp is spun from bleached or raw gray roving. To produce a finer yarn, the fibers are thoroughly moistened in a small bath of water as part of the spinning process, known as wet spinning. The fibers can also be spun dry, resulting in a coarser yarn. It can produce approximately 70-80 kg of fine, long hemp yarns from one ton of raw hemp straw.
The cotton spinning system for cotton hemp fibers (cottonization) makes it possible to produce pure hemp yarns or blends of hemp with other cotton-like fibers using a cotton ring spinning frame or rotary spinning. Wool-like hemp fibers can be blended with wool and processed on the spinning system of wool.
After that, textiles are produced through knitting, weaving, and nonwoven techniques.
Cotton and Hemp Blend Dyeing
Cotton cultivation requires the use of pesticides and a large amount of water. The use of pesticides and other fertilizers in cotton fields can affect the ecosystems in the surrounding areas and lead to water pollution, which has a significant negative impact on the environment as well as the health of workers.
Hemp has the potential to be a good addition to cotton due to its great properties. Compared to cotton, hemp fibers have a faster growth rate than other crops and require fewer pesticides and less water. Cotton-hemp blended fabrics have higher durability than 100% cotton and have higher tensile strength.
According to a study, natural indigo dye is used to dye a 50%:50% mixture of cotton and hemp fibers. The dyeing process was carried out using three pre-treatment methods: conventional scouring, ultrasound-assisted pretreatment without normal chemicals, and ultrasound-assisted pretreatment with normal chemicals. Conventional chemicals such as
- Lavotan DSU 100% is used to wash and clean the textile from impurities that can affect the coming processes on the textile. It can be used for bleaching textiles.
- Beixon AB 200% cleans the material and helps the fallout from the material stay in the bath during the process.
- Sodium hydroxide (NaOH) removes impurities such as fats and waxes. NaOH is alkaline; it is added into the scouring process as a neutralizer to neutralize the acidic bath.
What are Ultrasonic Waves?
Ultrasonic waves transmit mechanical energy through particle vibrations at frequencies greater than 20 kHz. Ultrasonic pretreatment contributes to improving the sustainability aspect of the textile industry by reducing energy consumption. Water, chemicals, time, and temperature can be reduced.
Remember
Ultrasonic scouring reduces energy waste because the molecules create great energy that can be carried out in a short time.
Ultrasonic pretreatment of 50:50 cotton/hemp blend yarns gives good mechanical results and color fastness to abrasion. Also, there is no need for bleaching to dye the yarns darker.
Uses
Hemp fibers are strong when compared to other natural fibers like linen and cotton. The fibers have been widely used to make ropes, sails, and nets.
Hemp fibers find many applications in textiles, such as clothing, jeans, sportswear, bags, hats, pillowcases, blankets, socks, accessories, yarn, carpets, and upholstery. Textiles are easy to produce, biodegradable, and have strong thermal properties. The fabrics are also antimicrobial, hypoallergenic, and mold-resistant.
Sources
- Industrial Hemp Planting and Production
- Harvesting, Retting, and Fiber Separation
- Growing Hemp for the Future
- Hemp Fibre Properties and Processing Target Textile: A Review
©Eman Abdallah Kamel, 2024
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