1.2 Text Natural cellulosic fibres
The basis of the chemical composition of all vegetable fibres is cellulose，which is present to a greater or lesser extent.Apart from these vegetable fibres，some man-made fibres，such as viscose and cuprammonium rayon fibres also consist of cellulose.In order to distinguish them from the man-made cellulosic fibres，the vegetable fibres are called natural cellulosic fibres.
Natural cellulosic fibres are usually divided into four types：seed fibres（such as cotton and kapok），bast fibres （flax，jute and ramie），leaf fibres（sisal and pina）and fruit fibres （coir）.
1.Cotton
Cotton is by far the most important textile fibre and makes up nearly 50 percent of the total weight of fibres used in the world.[1] Cotton is obtained from the cotton plant that grows in warm climates in most part of the world.Cotton is single cell fibre and develops from the epidermis of the cotton seed.Cotton consists of cellulose and non-cellulosic materials.Non-cellulosic materials in cotton fibre include protein，wax，pectin，mineral substances，etc.They can range from 4% to 12% together and are referred to as impurities by the manufacturer of cotton goods.Table 1-2 shows the composition of mature cotton fibres.
Table 1-2 Composition of mature cotton fibres
Cellulose can be considered to be a condensation polymer formed from the glucose units.Initially，two glucose units combine to form a cellobiose molecule with the elimination of water；and these polymerize further to form a chain of cellobiose.The molecular structure of cellulose is shown as followed：
The molecular structure of cellulose
The helical reversal structure of natural cellulose shows the constantly recurring cellobiose unit，consisting of two glucose units each with six carbon atoms.The length of the cellobiose unit along the fibre axis is 1.03 nm.There are 3000-5000 C6 or glucose units joined together in natural cellulosic fibres.This corresponds to a molecular weight of 300000-500000.
The morphological structure of the cotton fibre is described as follows.The most outer layer of the cotton fibre is the cuticle covered with waxes and pectins，and this surrounds a primary wall，built of cellulose，pectins and proteinic material.[2] The inner part of the cotton fibre comprises the secondary wall，subdivided into several layers of parallel cellulose fibrils，and the lumens.The non-cellulosic material is mainly situated in the primary wall and the secondary wall mainly composed of cellulose.The smallest unit of the fibrillae is the elementary fibril，consisting of densely packed bundles of cellulose chains，for which highly ordered（crystalline）regions alternate with less ordered（amorphous）regions in a longitudinal direction.Inside the microfibrils a microcapillary system is developed.The capillary system is responsible for swelling and absorption process which are important for the wet treatment of cotton.When cotton fibres dry from their initial fully swollen state，the cell wall collapse to give a typical kidney-shaped cross-section.Due to its chemical composition and structure，cotton is hydrophilic.
2.Flax（linen）
Flax is a bast fibre obtained from the flax plant by a complicated process to separate fibre from the woody core.This process is known as retting.After retting is complete，the stack is bundled together and passed between fluttering rollers that break the outer woody covering into small particles.It is then subjected to scutching process，which separates the outer covering from the fibre.Flax fibres are multicellular，with each cell having tapering ends and a narrow lumen.[3] The fibres show longitudinal striations and nodes.The ultimate fibres are composed of elementary fibrillae，which are spirally arranged.The fibrillae are held together by a binding or gummy substances.Flax is an assembly of the fibrillae cemented together within the fibres and an assembly of these fibres into bundles.[4] Due to the existence of the gummy substances，flax is prone to discoloration.
Cellulose is the main ingredient of flax fibre.The unretted flax contains about 78% cellulose，16.7% hemicellulose，1.8% pectins，2% lignin and 1.5% fats and waxes.The polymer of flax consists with a degree of polymerization of about 18000 cellobiose units.
Flax has relatively high strength.It is resistant to alkalis but is easily damaged by acids.Flax is considered to be the oldest textile fibre in the western world.It had been used traditionally in the manufacture of cloths，tent fabric，sewing thread，fishing lines，table cloth for a long time，but today it is often used as a component of blends.
3.Ramie
Ramie is also a bast fibre.Ramie plant has been grown in China for centuries，so ramie fibre is often referred to as China grass.The ribbons of ramie fibre are removed from the stalks of the ramie plant by the process of decortication.Due to the presence of a large quantity of gum，the ribbons must be treated to separate them into fibres for spinning by a degumming process.Commercial degumming is carried out by treating the fibres with caustic soda solution for several hours.The use of mixed bacterial cultures and enzymes is also reported to remove the gum from ramie fibre.[5]
Like other vegetable fibres，the main component of the purified ramie fibre is still cellulose，but ramie has high degree of both polymerization and orientation.The crystalline degree of ramie fibre is high，which impart a low molecular accessibility to the fibre.So ramie fibre is highly resistant to alkalis，microorganisms and insects.It is not easy to be damaged by cold acid and to be dyed.[6] The cell of the fibre is long and the cross-section is irregular in shape.The lumen narrows and disappears towards the end of the cell.Ramie fibre is white，lustrous，stiff and fairly coarse like canvas.It has high strength.This is attributed to its highly ordered structure.Ramie fibre is often blended with cotton for the use in clothing such as shirts，suitings，sweaters and blouses.The ramie adds strength and the cotton introduces softness.Ramie fibre absorbs water rapidly and fabrics made from it will launder easily and dry quickly.
4.Jute
Jute is a lignocellulosic bast fibre.Jute tends to be brown in colour due to about 20% lignin present in the fibre.The individual jute fibres are very short and they cannot be used for spinning.So Jute fibres are processed into the form of bundles of fibrous material held together by a gummy substance.The removal of the gummy substance causes massive loss of strength.
Jute is mainly grown in Bangladesh and India.It is herbaceous annual plant，which grows as high as 20 feet.Jute fibres are extracted from the stem of the plant by retting which usually consists of physical stage and microbial stage.The function of the physical stage is to swell and break down the outer cuticle layer of the stem；the microbial stage is to partially destroy some impurity components by fermentation through bacterial action and remove them from the jute fibres.
The cross-section of jute fibre is polygonal，usually with five or six sides.It has thick walls and a broad lumen of oval cross-section.Chemically，jute fibres are composed of mainly polysaccharides and lignin，although a number of minor components，such as waxes，pectins，inorganic salts，nitrogenous substances，coloring matter etc.，are found.It is a low-cost fibre but rather intensive labor.It has been used traditionally for clothes，carpet backing and packaging.
New words
1.kapok['keipɔk]n.木棉
2.sisal['sisəl]n.剑麻，西沙尔麻
3.pina['pi:nə]n.同pineapple，菠萝（纤维）
4.coir['kɔiə]n.椰壳纤维
5.non-cellulosic[nɔn,selju'ləusik]adj.非纤维素的
6.glucose['ɡluɡkəus]n.葡萄糖
7.cellobiose[,selə'baiəus]n.纤维二糖
8.morphological[,mɔ:fəlɔdʒikəl]adj.形态上的
9.lumen['lju:min]n.中腔，腔管
10.crystalline['kristəlain]adj.结晶的
11.amorphous[ə'mɔ:fəs]adj.无定形的
12.microfibril[,maikrəu'fɑibril]n.微纤维
13.microcapillary['maikrəukə'piləri]n.微型毛细管
14.capillary[kə'piləri]n.毛细管 adj.毛细管的，毛细作用的
15.kidney-shaped['kidni∫eipt]adj.肾形的
16.hydrophilic[,haidrəu'filik]adj.亲水的，吸水的
17.retting['retiŋ]n.沤麻，浸解
18.multicellular[,mʌlti'seljulə]adj.多细胞的
19.striation[strai'ei∫ən]n.条痕，条花，沟纹，缝隙
20.unretted[,ʌnretid]adj.未浸解的
21.hemicellulose[,hemi'seljuləus]n.半纤维素
22.fabric['fæbrik]n.织物，布，织品
23.ribbon['ribən]n.带状物，纤维条
24.decortication[di'kɔ:tikei∫ən]n.剥皮
25.enzyme['enzaim]n.酶
26.accessibility[,ækəsesi'biliti]n.接受性
27.canvas['kænvəs]n.帆布
28.sweater['swetə]n.厚运动衫，毛线衫
29.blouse[blauz]n.宽松上衣，女装衬衫
30.lignocellulosic[,liɡnəu'selju'ləusik]adj.木质纤维素的
31.Bangladesh[,bɑ:ŋɡlə'de∫]n.孟加拉国
32.herbaceous[hə:'bei∫əs]adj.草本的
33.microbial[mai'krəubiəl]adj.微生物的
34.fermentation[,fə:men'tei∫ən]n.发酵
35.polygonal['pɔliɡənl]adj.多角形的，多边形的
36.polysaccharide[pɔli'sækəraid]n.多糖，聚糖，多聚糖
37.low-cost[ləu-kɔst]adj.低廉的，低成本的
Phrases and expressions
1.primary wall 初生胞壁
2.secondary wall 次生胞壁
3.fluttering roller 抖动式滚筒
4.scutching process 打麻工艺
5.degumming process 脱胶工艺，精练工艺
6.caustic soda 烧碱，苛性钠，氢氧化钠
Notes
[1] Cotton is by far the most important textile fibre and makes up nearly 50 percent of the total weight of fibres used in the world.
棉花是迄今为止最重要的纺织纤维，占全世界所用纤维总量的近一半。 by far用于表示程度，意为“到目前为止、迄今为止”。
[2] The most outer layer of the cotton fibre is the cuticle covered with waxes and pectins，and this surrounds a primary wall，built of cellulose，pectins and proteinic material.
棉纤维的最外层是被蜡和果胶质覆盖的表皮，它包覆着由纤维素、果胶质和蛋白质构成的初生胞壁。
[3] Flax fibres are multicellular，with each cell having tapering ends and a narrow lumen.
亚麻纤维是多细胞纤维，每个细胞都有渐细的末端和一个狭窄的包腔。
这里，with后面加宾语，说明附带情况。
[4] Flax is an assembly of the fibrillae cemented together within the fibres and an assembly of these fibres into bundles.
在亚麻中，原纤胶结成纤维，纤维胶结成束。
[5] Commercial degumming is carried out by treating the fibres with caustic soda solution for several hours.The use of mixed bacterial cultures and enzymes is also reported to remove the gum from ramie fibre.
工业上普遍采用的脱胶方法是用烧碱溶液对其进行数小时的浸泡，也有采用培养细菌和酶的混合物来脱胶的事例。
第二句等同于“The use of mixed bacterial cultures and enzymes to remove the gum from ramie fibre is also reported”。
[6] It is not easy to be damaged by cold acid and to be dyed.
苎麻纤维不易被冷的酸破坏，也不易被染色。
此为两个并列句，后一句省略主语。完整形式是：It is not easy to be damaged by cold acid and it is not easy to be dyed。
Exercises
Ⅰ.Answer the following questions.
1.What is the definition of natural cellulosic fibres？
2.What are included in non-cellulosic materials of cotton fibre？
3.What is the morphological structure of the cotton fibre？
4.How do we obtain flax from the flax plant？
5.Why do ramie have high strength？
6.Which processes are included in retting？
Ⅱ.Translate the following sentences into Chinese.
1.Natural cellulosic fibres are usually divided into four types：seed fibres（such as cotton and kapok），bast fibres（flax，jute and ramie），leaf fibres（sisal and pina）and fruit fibres（coir）.
2.The inner part of the cotton fibre comprises the secondary wall，subdivided into several layers of parallel cellulose fibrils，and the lumens.
3.When cotton fibres dry from their initial fully swollen state，the cell wall collapse to give a typical kidney-shaped cross-section.
4.It had been used traditionally in the manufacture of cloths，tent fabric，sewing thread，fishing lines，table cloth for a long time，but today it is often used as a component of blends.
5.Due to the presence of a large quantity of gum，the ribbons must be treated to separate them into fibres for spinning by a degumming process.
6.The function of the physical stage is to swell and break down the outer cuticle layer of the stem；the microbial stage is to partially destroy some impurity components by fermentation through bacterial action and remove them from the jute fibres.
Reading material Degradation of cellulose
Cellulose can be degraded by the action of acids，alkalis，oxidizing agents，enzymes，heat or radiation.The study of degradation of cellulose has important implications for satisfactory dyeing and finishing，since even slight degradation during processing may be accompanied by unacceptable loss of strength and other undesirable effects.
Most of cellulosic fibres have poor resistance to aqueous acids.For example，when cotton is treated with a dilute solution of hydrogen chloride，the rate of hydrolysis is very rapid and the strength loss of the cotton is great.So it is not usually allowed to treat cellulosic fibres with acids，especially with strong acids and at high temperatures.
The glycosidic linkages in cellulose are not attacked by alkali at temperature lower than about 170℃.However，the preparation of cellulosic textiles for dyeing frequently includes hot alkaline treatments that cause greater weight loss than be accounted for by the removal of non-cellulosic substances.In this case degradation of cellulose is present.
Since each intermediate unit in a cellulose molecule contains three hydroxy groups，the number of possible oxidation products is considerable.These oxidations may damage textiles，it is，therefore，necessary to choose safe bleaching agent and maintain proper conditions.
Biodeterioration of cellulosic textiles is a serious problem and many efforts have gone into methods of prevention.If damp cotton is exposed to air，mildew may gradually develop on it，accompanied by staining that is difficult or impossible to remove.Prolonged exposure also causes a serious loss of tensile strength.In the biodeterioration process，enzymes play an important role.
Cellulose can be heated for many hours up to about 120℃ without any serious deleterious effect.In dry air at high temperatures，however，considerable depolymerization takes place，accompanied by the formation of carbonyl and carboxyl groups in the solid material，the evolution of water，carbon monoxide，and carbon dioxide，and a loss of tensile strength.