Introduction:
Hemp is a natural vegetable bast fiber which is obtained from the stem of the hemp plant. It belongs to the cannabis sativa species and cannabaceae family. The plant grows to a height of 5m . It is native to central Asia and grown mostly in Philippines, EU, central Asia and china. Hemp plants prefer to grow in semi -humid conditions with temperature between 14 to 27 ͦ c and rainfall of 30-40cm.
Processing of hemp fiber:
Harvesting:
Harvesting has to be done before the seeds set but after flowering. Because fibers become more coarse around the time of seed formation. It is done manually or with mechanical harvesters.
Retting:
There are 4 types of retting processes. Any one process can be followed to separate the fibers from the stalks.
Biological retting:
Dew retting, water retting and enzyme retting comes under biological retting.
In the dew retting, the stalks spread on the field and expose them to sun and rain for several weeks until the stalks begin to separate from the fiber naturally. But this method reduces the quality and strength of the fiber.
In the water retting process, the bundles of stalks are immersed in water of at least 60 cm in depth until the fibers separate from the stalks. This process takes around 2 weeks. Then the bundles were taken out of the water and left to dry.
In the enzyme retting process, fibers can be separated from the stalks by using enzymes like pectinase and xylanase. It is the safest and fastest process of retting.
Mechanical retting:
In this process decorticator is used to separate the bast and core fibers.
Stalks will be feeded by feeding mouth, then they will enter to beaters to crush the stalks. Then the crushed stalks which contains both bast fiber and core fiber will be pushed into delivery plate and will be separated manually.
Chemical retting:
In this process, the stalks are boiled with acids and alkalies such as sodium hydroxide, sodium benzoate and hydrogen peroxide for several hours at a specific temperature and then washed with clean water. Compared to other retting methods the fiber which is obtained from this method is usually felt and rough.
Physical retting:
It requires low processing time. It involves 2 steps which starts with modifying the surface by chemical treatment. Then the stalks will be subjected to steam explosion. In this the bast fibers will be separated from the core fibers by blowing apart. But this method only produces short fibers.
Decortication:
Removal of outer bark, central woody core, gums and waxes from the stems of ramie by hand or decorticating machine is called decortication. Stalks will be feeded by feeding mouth, then they will enter to beaters to crush the stalks. Then the crushed stalks will be pushed into delivery plate and will be separated manually. The quality of fiber depends on the condition of the plants and efficiency of the operator.
Washing:
It should be washed after the extraction of fiber.
Drying:
After washing, it should be dried to prevent the attack of mildew.
Chemical composition of hemp fiber:
Properties of hemp fiber:
Physical properties:
Length: 5-55mm
Density: 1.4gm/cc
Tensile strength: 550-1110Mpa
Young’s modulus: 30-70Gpa
Moisture content: 8%
Failure strain: 1.6%
Color: yellowish grey to deep brown
Standard moisture regain: 12%
Resistant to sunlight
Excellent resistant to degradation by heat
Chemical properties:
Damaged by cold concentrated acids and hot dilute acids
Resistant to alkalies
Resistant to insects
Damaged by fungi and bacteria
Resistant to organic solvents
Uses:
It can be used to make
Ropes, sacking, heavy duty tarpaulin, paper
Tapestry, hats, shawls, rugs, towel, household textiles
T-shirts, dresses, hoodies, underwear, socks, other forms of apparel
Carpets, canvas, ship cordage, sailcloth