By Eman Abdallah Kamel
Eman is a writer and engineer with a Bachelor’s degree in Textile Science from the Faculty of Applied Arts in Egypt.
The article covers the definition of hemp, its chemical composition, physical and chemical properties, and the advantages of a blend of wool and hemp fibers.
Hemp Definition
Hemp, or Cannabis sativa, is one of the strongest and toughest natural fibers and is also reasonably priced. Hemp can be blended with wool, silk, and cotton. For centuries, hemp has been used to make many products, including paper, ropes, sails, and textiles. Hemp wool is also used as insulation.
Hemp is native to India and Persia, although it has been cultivated in almost all temperate and tropical countries. In Europe and Canada, a specific type of hemp can be grown, and it must contain no more than 0.2% or 0.3% tetrahydrocannabinol (THC).
In recent years, hemp fibre has received increasing attention as a potential alternative to synthetic fibres. Due to its low density, high specific strength, and stiffness, it is a renewable resource, and the waste is 100% biodegradable. The most promising application of hemp fibre is as reinforcement in polymeric composites or through synthetic hybridization.
After two to three months of seeding, hemp is typically ready for harvesting. The largest producers of hemp fibre are Russia, France, Germany, Italy, Yugoslavia, Chile, China, Japan, and Peru.
Chemical Structure
The hemp fibers’ cross-section is not uniform throughout their length and has an uneven shape. The bark contains primary bast fibers, which consist of a bundle of fibers extending along the entire length of the plant stem and are composed of about 70-74% cellulose, 15-20% hemicellulose, 3.5-5.7% lignin, 0.8% pectin, and 1.2-6.2% wax.
Hemp fibers have a multicellular structure, which can be seen as a composite material with many cavities side by side. The cell wall of hemp fibers is multilayered and consists of the primary cell wall (the first layer deposited during cell growth) and the secondary (S) wall, which consists of three layers (S1-3). The primary fibers are bound together in the middle lamella by lignin. The highest cellulose concentration is in the S2 layer, at about 50%. S2 is also the thickest layer, and due to the higher cellulose concentration, it controls the properties of the fibers.
The hemp fiber structure has three main components:
- Cellulose,
- Hemicellulose,
- and Lignin.
The primary structural element of fiber is cellulose, which gives the fiber its strength, stiffness, and structural stability. Crystalline and amorphous regions characterize the cellulose structure of the fiber, while hemicellulose and lignin are amorphous. In the cellulose crystalline region, there are a large number of hydroxyl groups that are strongly bonded and difficult for other chemicals to penetrate. In the amorphous region, the hydroxyl groups are loosely bonded to the fiber structure and are relatively free to interact with other chemicals. Because of this freedom, the hydroxyl groups in the amorphous region can easily combine with water molecules from the atmosphere.
Amorphous hemicellulose and lignin’s hydroxyl groups first permit water molecules to pass through the fiber’s surface. The water molecules then combine with the hydroxyl groups in cellulose (in the amorphous region) and remain in the fiber structure, making the fibers hydrophilic and polar. During composite processing, hydrophilic fibers hinder the development of adhesion properties with many hydrophobic binders. This problem can be addressed by treating these fibers with suitable chemicals. Treatments can eliminate the lignin and hemicellulose coatings from the fiber surface, reduce the number of hydroxyl groups in the amorphous region, and reveal the cellulose structure for reaction with binder materials. Reactions resulting from alkalinization and acetylation are widely used to modify the fiber structures.
Properties
1. Physical Properties
- Moisture: At 23°C and 5% relative humidity, the moisture content of balanced hemp fiber is approximately 10%. This high moisture content can be a major factor in the relatively high void content in the composite made from these fibers.
- Pyrolysis: At a temperature above 150 C, hemp fiber undergoes pyrolysis. Cellulose breaks down at around 360 °C, while hemicellulose and pectin break down at around 260 °C.
- Weight Loss: According to the researchers, a sample of hemp fiber, cut from the prepared hemp fiber mat at 23°C and 50% relative humidity, was kept in a desiccant containing copper sulfate, and weight changes over time were recorded. The weight loss is rapid at first as the desiccant absorbs the moisture in the fiber but starts to stabilize after about 1500 minutes as the amount of moisture in the fiber starts to decrease. The fiber lost approximately 4% of its original weight after being kept for 7200 minutes in the desiccant.
- Tensile Strength: Hemp fibers can be used as reinforcement in composite materials because of their high tensile strength. The researchers found that the hybrid-reinforced epoxy composites of hemp and flax fibers had the highest tensile strength, modulus of elasticity, and impact strength of 58.59 MPa, 1.88 GPa, and 10.19 kJ/m2, respectively. The hybrid-reinforced epoxy composites of jute and hemp fibers achieved the maximum flexural strength of 86.6 MPa.
- Fiber Length: Increasing the length of the hemp fibers and decreasing their diameter increases the fiber aspect ratio (length/diameter), which has a positive effect on the mechanical properties of polymeric composites. Reducing the fiber length beyond the critical length at which the fiber stress decreases from its maximum value reduces the stress transfer between the matrix and the fibers.
Did You Know?
Hemp and synthetic fiber-reinforced composites balance the cost of fibers, reduce moisture absorption, and enhance the performance and quality of the final composite materials. The effect of water aging on the mechanical properties of hemp and polyester hybrid composites reinforced with hemp/glass fiber was studied. The addition of glass fiber reduced the water absorption, which delayed the aging deterioration of the hybrid composite materials. The hybrid composites also showed superior strength retention.
2. Chemical Properties
- Alkali Effect (NaOH) : (Fiber-OH + NaOH → Fiber-ONa + H2O). Alkali treatment removes the chemical components of the fibers, such as hemicellulose, lignin, and pectin, increasing the surface roughness of the fibers, modifying the structure of the cellulose by increasing its crystallinity, improving surface shear strength and thermal stability, and reducing moisture absorption. The treatment also leads to improvements in the final properties of the composites, such as tensile and impact strengths, modulus of elasticity, and fracture toughness. At high concentrations of NaOH, the thermal resistance and strength of hemp fibers decrease due to the decomposition of cellulose.
- Acetylation: It is a method of replacing the hydroxyl groups of the fiber with an acetyl group (CH3CO). As a result, the hydrophilic nature of the fiber is reduced. The hemicellulose-lignin coating on the fiber surface is also removed by the treatment.
- Silanization: Silane is a multifunctional molecule used as a coupling agent to modify fiber surfaces. Silane molecules cover the surface of fibers as well. Several stages of hydrolysis, condensation, and bond formation occur during silane processing. Silanols are formed in the presence of moisture in the fiber. During the condensation process, silanols react with the hydroxyl group of cellulose and form chemical bonds with the fiber components.
Natural fibers suffer from biodegradation, which has multiple causes, such as the existence of microorganisms in the environment or the fiber’s tendency to absorb moisture. Bactericidal agents such as phenolic compounds (ferulic acid and syringaldehyde), flavonoids such as catechins, or mineral salts (Ag+) can protect hemp fibers. Milder oxidase enzymes, such as laccase and peroxidase, can also be used to increase the antimicrobial activity of hemp fibers.
Wool
Natural wool is a fiber obtained from sheep and other animals. It is a good heat insulator and fireproof. Wool fibers are very flexible; they increase about 30% of their length under simple tensile force and return to their normal state when the tensile force is removed.
Wool and Hemp Blend Features
According to the study, a semi-rigid acoustic thermal insulation panel was manufactured from wool, which partially acts as a binder, and hemp fibers. Both components retain their chemical and physical properties, and hemp fibers give the product a relatively high density due to the fiber’s tensile strength. The hemp used to produce the panels was cut and kept in the field for four months (October–February) for retting (soaking).
Wool that is unsuitable for use in the textile industry because of its dark color, poor quality, or irregular fiber length and thickness can be used in this process.
The process involves mixing sheep wool and hemp fibers, then treating them with a soda solution to make wool fibers to release some of the keratin protein, then removing the soda solution and drying the board in an oven.
The following is a summary of the features of the product:
- Moderate thermal insulation performance.
- It increased sound absorption performance.
- The process has reduced the environmental impact of the product.
- Gives higher rigidity.
- No waste of low-quality wool.
- Stimulates increased hemp cultivation.
Sources
- Industrial Hemp Fibers: An Overview
- A Study in Physical and Mechanical Properties of Hemp Fibres
- Effects of chemical treatments on hemp fiber structure
©Eman Abdallah Kamel, 2024
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