CHAPTER - 5 COTTON INTRODUCTION otton is the oldest fibre used for textile purpose. In the tropical countries, it is the most important fibre. India was the centre ‘for world’s cotton industry as well as variety of fine fabrics till 1600 A.D. The date of origin of cotton is unknown. A digest of ancient laws by Manu contains references to cotton plant and cotton textile material. China and Japas introduced cotton from India, only in 800 A.D. but extensive cultivation was started from 1300 AD. England manufactured cotton from 1600 A.D. At the same period, coton was cultivated in America.Now cotton is cultivated in almost every country in the world having a mild climate, suitable for cotton cultivation. American cotton dominates world market. Other countries, large producers of cotton are India, Brazil, Mexico, Egypt and China. In tropical and sub-tropical regions, vast tracts of land are cultivated for the production of cotton. These areas lie between lines drawn 45 degree north and 35 degree south of equator. Apart from the principal cotton growing areas i.e., USA, India, China, Brazil, Egypt, Mexico, cotton is cultivated in Turkey, Argentina, Spain, Pakisthan, Syria, Peru, Greece and Colombia. CULTIVATION Cotton plantation takes place just during the month of March and April and it takes one full year. The methods of cultivation vary considerably in different parts of the world according to the nature. Suitable soil and climate are absolutely essential for the growth of the plant. Of course, cultivation and application of essential fertilisers help to obtain improved fibre. In the preparation of the ground for planting, ridges should be formed with spaces of 1.5 - 2.0 metre between them. Then seeds are sown. When the plants appear on the ground, they should be thinned out leaving only the strongest plant growing at suitable interval along the ridges. During growth, precaution should be taken for its resistance towards insects and diseases. The flowers appear in June. This lasts only a day or so. After disappearance of the flower, the seeds become gradually surrounded by a soft fibrous substance. By this time, the flower pod appears and it grows to full size in August. In between this time, the fibre growth continues until they mature. The enlargement of the seed causes the pod to burst and a ball of whitish fibre is brought to light. In this period, it should be immediately collected for cotton fibre extraction. It should be remembered that cotton is a seed hair and it gets out of the pod as soon as the seed is mature. The fibre helps the seed to fly in the air and gets carried away by air flow to other places. So before the seed gets out of the pod, the cotton ball should be plucked. After plucking, the cotton with the seed is transferred to the ginning factory for the separation of the fibres from the seeds. YIELD The possibilities of improving any cotton plant for a better production per acre are almost unlimited. The improvement in yield to achieve a higher72) ioisleialpaioine ecg seem SOPRANO A text book of fibre science and technology production for a constant area can be due to the following factors : (a) timely planting, (b) addition of soil, (c) utilisation of healthy and better seed, (d) addition of fertiliser in appropriate time, (e) weed control, (f) controlling. insects by chemicals, (f) watering by irrigation, (g) defoliation and (h) timely harvesting of the cotton balls. The proper selection of land for best yield is another most important factor. The yield per acre in the world is approxiamtely 320 Kg. The yield per acre in India is only 170 Kg. DEVELOPMENT OF COTTON FIBRE IN THE SEED Cotton fibres grow as a dense cover over the seed surface. From botanical aspect, cotton is classified as a seed hair as it is an overgrowth in the form ofa single cell emerging from the epidermis or outer layer of the cotton seed. The development of the cotton fibre on the seed consists of two phases. In the first phase, there is sprouting of epidermal cells and a continued growth of the fibre. This growth is unidirectional for a period of 25 days from flowering. During this stage, the fibre is bounded by the primary wall and cuticle on its surface. In this period, cotton fibre consists of a hollow tube with a thin wall of primary cellulose. The tube is enclosed in the protective cuticle that contains the natural fats and waxes. In the second stage, the thickening of the fibre occurs layer by layer due to deposition of the secondary wall on the interior surface of the primary wall. This decreases the size of the lumen until it becomes very small. During the growth,the fibres are cylindrical. But on maturity, the cross- section collapses and forms a flat ribbon like fibre with a spiral twist or convolution. This thickening stage continues for a period of 35 to 40 days. These periods can be different due to different environmental conditions and fibre varieties. DAmace BY INSECTS AND DISEASES The yield of the cotton cultivation can be maximised by proper growth with adequate control of damages caused by insects and other diseases. Damages by insects : Cotton plant has a large list of insect enemies including most destructive pests. Insects such as cotton boll weevil, boll worm, pink boll worm, lygus, alphids are generally attack cotton. Whenever the insects start attacking cotton, insecticides like DDT, benzene hexa chloride, aldrin, endrin, toxaphene etc should be used to contro! for an efficient production.COMMON. cece eee eereerencceseererereerssstenscoes . 73 Cotton diseases: Cotton diseases are less problematic than insect pests The diseases can be affected more when the climate is unfavourable. The diseases can be grouped as (a) Seedling diseases related to damage or killing of young plant, (b) Root diseases related to spoiling of the roots and destruction of the plants due to unfavourable soil, (c) Leaf and stem diseases resulting a severe loss to yield, and (d) Boll rots due to exposure of the undried fibres to the atmosphere resulting biological deterioration of the fibre and more ginning lint. GRADING The cotton qualities from place to place and plant to plant differ; the difference in quality can be expressed in grading and staple length. Grade is generally determined from three factors i.e., (a) colour, (b) trash content and (c) ginning quality. Colour: Best cotton is only white in colour. But continued exposure to weathering and micro-organisms cause white cotton to lose its brightness. The colour groups present in cotton are : White, Light spotted (Lt Sp), Spotted (Sp), Tinged (Tg), Yellow stained (YS), Light grey (Lt Gy), and Grey (Gy). Trash Content: The trash includes such materials as leaf, stems, hulls, bark, seeds, shale, motes, grass, sand, oil and dust. Cottons which contain minimum amount of trash after ginning have highest spinning value. Depending upon the trash content, cotton can be graded as Strict Good Middling (SGM), Good Middling (GM), Strict Middling (SM), Middling (M), Strict low middling (SLM), Low middling (LM), Strict good ordianary (SGO) and Good ordinary (GO). Sometimes, depending upon the trash content, plus (+) can be given to any grade like SLM+ or SGM+. Quality of Ginning : Presence of neps and naps are two important factors to determine the quality of cotton. Neps are small tangled knots of fibre that are visible as dots. This type of cotton is known as neppy cotton. Naps are large clumps or matted masses of fibres that contribute to the rough appearance. This type of cotton is known as nappy cotton. In general, the grading indicates the trash and colour of the cotton like LM Tg, M Lt Gy etc. BOTANICAL CLASSIFICATION OF COTTON Cotton fibres belong to the botanical genus ‘Gossypium’. Further cotton is classified in four names as per the places of cultivation like :1) OR Seereeceon eece nsec A text book of fibre science and technology Gossypium Arboreum 2 India. Gossypium Herbaceum : Middle East, Egypt, India. Gossypium Barbadense : _ Peru, Egypt, Sea Islands. Gossypium Hirsutum : America, West Indies. COMMERCIAL CLASSIFICATION OF COTTON Cotton fibres are classified commercially according to the source they are obtained from as well as their staple length. There are six types of cotton available. Those are (a) Sea Island cotton, (b) Egyptian cotton, (c) Brazilian cotton, (4) American cotton, (e) Indian cotton and (f) China cotton. Sea ISLAND CoTTON Sea island cotton comes originally from Barbadoes. Hence it has the name ‘Gossypium Barbadense’. It is the most important cotton and is grown in USA, Carolina, Georgia and Florida. It is a long, fine, soft and silky fibre. This cotton is more uniform with minimum variation in length or twist. The colour is of a light creamy tint. The staple length is around 5 cm or more. EcyYPriaN COTTON This cotton is available in Egypt and middle east countries. These fibres are also long, fine, soft and silky like that of sea island cotton with slight inferior qualities. The staple length of these types of cotton is in between 3.7 em to 4.5 cm. BRAZILIAN COTTON This cotton is also known as Peruvian cotton as it was originated from Peru and are available in Peru and Brazil. The colour is generally dull white to cream but some cotton are dull golden. The fibres are harsh and wiry to touch but elastic. The staple length is in between 3 cm to 4 cm. AMERICAN COTTON American cotton is grown in USA and in the south of North America. The staple length of this cotton is between 2.5 cm to 3.5 cm. INDIAN COTTON This cotton is available in India. The quality is generally poor with lower staple and coarser diameter. This cotton is generally white in colour. The staple length is in the range of 2 cm to 3 cm. CHINA COTTON This type of cotton is available in China only. The quality of this cottonis the poorest and it cannot be used for finer variety of fabrics. The staple length is 1.5 cm to 2.cm only. MoRPHOLOGICAL STRUCTURE ye (Cotton is a seed hair. It is an outer growth in the form of a single cell emerging from the epidermis or outer layer of cotton seed. Each flower of cotton plant may produce 20-25 seeds enclosed in a green pod or boll. When the growth ceases, the boll splits. Thg fibres grow in tubular form, with a well developed wall enclosing the lumens running down the centre. When the boll splits, the moisture inside it evaporates. This deshapes the tubular form. As drying proceeds, the wall of the fibre shrinks, collapses, lumens become smaller and the fibre develops convolutions. Different types of morphology observed in cotton fibre is shown in Fig.5.1 606 Om ar OP @ Fig. 5.1 Different types of morphology observed in cotton fibre The morphological structure of the fibre consists of four parts. These are (a) Cuticle, (b) Primary wall, (c) Secondary wall and (d) Lumen. Cuncie The cuticle of the cotton fibre is a very thin layer tightly attached to the outside of the primary wall. More accurately, cotton fibre is enclosed in cuticle, which protects the fibre from any mechanical and chemical damages. The cuticle consists of cotton wax, a complex mixture of fats. waxes and oils. During initial stages of growth, at the time of attaining full length. the cuticle appears as an oily film. During the later stage, the cuticle becomes hard like a varnish.)eee se eeseess +A text boot of fidre scxnce and techasingy PRIMARY WALL - The primary wall is built up from cellulose. It also contains pectimeoms substances. The cellulose appears to concentrate from the growth period and increases proportionally during the later stage of cell elongation. Ou the surface, the molecular chains in the primary wall are arranged mma random manner without any orientation and definite order. However, cellulose present inside the pnmary wall is in the form of fine threads or fibrils, when observed through microscope. The fibrils are not paral! t the fibre axis but spiral at an angle of about 70° round the fibre axis. The spirals do not reverse in their direction; the spiral angle is greater af the tip and smaller at the base. The diameter of the cotton fibre is fairly constant throughout the length except at the base and the tip. The diameter of the fibre is in the order of 15-20 microns, whereas the primary wall is very thim and about 0.1-0.2 micron thick. Seconpary WALL It is composed mainly of cellulose and contributes most of the weight to the fibre. In general, within the primary wall, the bulk of the fibre consists of secondary wall. Like primary wall, it consists of concentric layers of fibrils in spiral formation. The outer layers of secondary wall, deposited near the primary wall is built up of fibrils at spiral angle of about 20 - 30°. re ‘The fibrils in the subsequent layers are finer than former and the spiralling angle is about 20 - 45°. The spiral angle changes | slightly in magnitude berween the outside and the inside. The spirals also change their direction of rotation at frequent intervals along the fibre length. At the G_% reversal point, they simply form acurve. ; Always the second set of fibril begins in the opposite direction. In all the layers, the fibrils tend to follow a closely similar ~~" pattem. Arrangement of chain molecules aS Xe in different parts is shown in Fig.5.2. At the centre of the growing fibre, Fig. 5.2 Arrangement of there is a lumen, which remains as chain molecules in different cylindrical void at maturity. The area is parts of cotton cellulose about 30 - 35% of the total area of crass a,section. The lumen contents evaporate after the boll splits. After drying and collapsing of the fibre, the area of lumen is reduced to about 5% of the total area. Of course, there is variation from fibre to fibre. In the dried state, lumen contains colouring matters apart from other impurities, which decides the colour of the fibre.) ConvoLuTions After bursting of the mature boll, the fibre wall shrinks and collapses On drying and collapsing of the fibre, the cylindrical cross-section is converted into a convoluted ribbon form with the flattening of the ribbon The structure of the wall allows shrinkage in perpendicular direction to the fibrils than parallel direction. Due to the spiral structure, the collapse results twisting of the fibre about its axis. If the convolution consists of a rotation through 180° of the whole cotton fibre, it is regarded as a flat ribbon. The direction of rotation of convolutions changes at irregular intervals along the length of the fibre at places determined by reversal points in the pattern of the fibrillar spirals in the secondary walls. The pitch of the convolutions depends on ribbon width and wall thickness. The convolutions arise in order to relieve internal stresses during - the drying and collapse of cotton fibre. The convolutions form as liquid water during early stages of growth. On drying, the water is lost and then be set into the structure by further drying and stress- relaxation. The convoluted ribbon ig. 5.3 uti ii is the natural state of cotton fibre eae convo fi aac ene free of stress. When the fibre is swollen in stronger swelling agents like caustic soda, most of the convolution of the fibre can be removed. This results in an approximately rounded cross-section. The convolution present in swollen cotton and native cotton is shown in Fig.5.3. CHEMICAL COMPOSITION OF COTTON Native cotton is the purest form of natural cellulose. However, it contains usual constituents of a vegetable cell. The contents are proteins, oil and wax, pectose and pectins, mineral matters and natural colouring matters.70 ere Grit sfotsieyalefstelslelsistere eis) A text book of fibre science and technology The percentage of impurities of cotton depends upon the origin 1.¢. the type of cotton and its maturity, In general, immature cotton has more impurities than mature cotton. The impurities are mostly located on the outer side of the fibre. The approximate composition of a dry mature cotton fibre ig shown in Table 5.1. Table 5.1 Chemical composition of dry cotton Chemicals Composition (%) Cellulose 88.0 - 97.0 Protein 10- 2.0 Oil and wax 04- 15 Pectins 04- 15 Minerals 0.7- 16 Others 8.0 0.5 - Cellulose Native cotton contains maximum amount of cellulose in its purest form. Cellulose is composed of glucose molecules, which are arranged in stearic manner. In cotton fibre, cellulose is not combined with lignins or pectins. Cotton cellulose contains about 12,000 to 18,000 glucose residues in its macromolecule. The molecular weight of the cellulose present in cotton approximately 2 million. Protein The proteins in cotton fibre are of vegetable protein. As usual, the proteins are polypeptides and amino acids. The proteins are mainly concentrated in the primary wall and lumen of the fibre. The amount varies between 1-2%. The colour of the cotton may be due to its protein content. Oil and wax Waxes are hydrophobic substances acting as protective coatings on the surface of the fibre. These are basically very complex substances of high molecular weight glycerides and fatty acids present either in free or in esterified form. These are located mainly in the cuticle and in the primary wall of the fibre. The glycerides can easily be saponifiable. the melting point of waxes varies between 68° C to 80° C. Because of the waxes, it is difficult to wet cotton. Waxes are generally converted into soaps during scouring treatment in alkali solution.Coton... eee cere eee e eens eee eee eee rs 79 Pectins Cotton fibre contains about 0.4-1.5% pectin. These are primarily calcium magnesium and iron pectates with some free pectic acid and methyl pectate These are carbohydrates similar to cellulose and their esterified form is soluble in water. The free acid and its calcium and magnesium salts are not soluble in water but they will be converted into soluble product by alkali hydroxides or soda ash. So these can be removed during alkali boiling or scouring of the cotton material. Minerals Cotton contains about 0.7-1.6% mineral matter which is left as ash after cotton is burnt. Due to soil and atmospheric impurities and at the time of bursting of the pod, the mineral matter is added to the impurities on the outside surface of cotton fibre. The mineral matter consists of chlorides, carbonates and sulphates of potassium, calcium and magnesium. The total amount of mineral matter present in the fibre may be reduced by simply boiling the cotton fibre in water. PHYSICAL PROPERTIES OF COTTON StapLe LENGTH Staple length is one of the important primary properties of any textile fibre. The staple length of cotton varries from 1 cm to 8 cm for different classes. which is shown in Table 5.2. Table 5.2 Staple length of cotton Sea Island 5.0 emand more Egyptian 3.8cm-4.4em Brazilian American : Indian 2.0em-2.5 em China 1.5cm-2.0em FIBRE FINENESS The wall thickness of different types of cotton ranges from 3.5 micron to 10 micron. Ribbon width is said to range from 12 micron to 25 micron. The thickness part of a fibre is not at the base but it is at the middle. The tip end is usually gently tapered. The base end is slightly finer than the middleQDI Tstelare1s'o1c!sle\s sfulnielele s/elsievslels\ sie ‘A text book of fibre science and technology Fipre UNIFORMITY Cotton cannot be considered a uniform material even though sufficiently large number of fibres may have a characteristic average behaviour. Each fibre must be regarded as an individual with its own characteristic length, strength, fineness and other properties. For this reason, sampling methods are extremely important and test data must be handled by statistical method, It has been observed that longer cotton tends to become uniform in length than the shorter ones. The varying percentage of immature fibre also indicates non-uniformity of wall thickness for the same variety of fibres, Also, there are considerable differences between cotton grown from the same seed in the same location from time to time. Porosity Cotton fibre is porous and exhibits capillary effects to a higher degree, The fibrils themselves are dense as a result of the higher packing density of the molecules and so non-porous. This part of the structure constitute approximately 70% or more of the fibre. The arrangement of denser fibrils in the fibre may be visualised as analogous to the packing of fibres ina well made yarn. So the porosity of the unoccupied space in the fibre ranges from 20-40% of the fibre volume. The fine cottons are more compacted than the coarse variety. Also, the lumen is generally small, about one third of the unoccupied space. The pore space is largely between the fibrils as capillaries of small diameter. The pore arises from imperfections in the lateral packing of the microstructural elements. Pores of cotton fibre influence properties and reactivity of the fibre in presence of water. Pores are generally expressed in terms of average surface area. The surface area of dry cotton is 0.6 to 0.7 sq.m/gm. The internal surface area can be developed and can be made larger by immersing cotton fibre in water, acetic acid or ethanol. The surface area of cotton fibre in water is around 137 sq.m/gm. Lustre The natural lustre of cotton fibre is determined by two factors i... fibre shape and fibre polish. The lustre does not depend upon hair weight, length, diameter, fineness or convolutions. It depends upon the ratio of semi- major and semi-minor axes of the ellilptical fibre cross-section. If the ratio is be low, the lustre will be high. The highest lustre is noticed in the fibre with circular cross-section. So the dominating influence in lustre is the external fibre surface and the exact geometric shape is of secondary importance.Cotton. . . 8 To manufacure a lustrous yarn, apart from the lustre of fibre, the fibre length is another important factor. When two cottons of the same lustre are used, the longer fibre yields more lustrous yarn. Denstty Cotton fibre has a density of 1.54 gm/cc, which corresponds to a specific volume of 0.64 ce/gm. MOISTURE Cotton fibres are composed of an assembly of fibrils. Due to the imperfections in the packing of the fibrils, the fibre absorbs moisture. The moisture absurption takes place on the surface of the fibrils. Each anhydro- glucose unit in cellulose has three hydroxyl groups. Due to the fibrils and imperfections, about half of the hydroxyl groups are accessible. So in an average, one out of three hydroxyl groups on each glucose residue is a site for moisture absorption. The amount of moisture in cotton depends on the relative humidity and temperature of the air to which it is exposed. The moisture is more sensitive to relative humidity than temperature. At higher temperature, there is a small change in moisture and cottons retain constant moisture over small change in temperature. The moisture pick-up is about 5.8% at 40% humidity (R.H.), which can be increased to 12.0% at 90% R.H. and at 25°C. At 65% R.H. and 22°C, the moisture pick-up is around 8.3%. At higher humidity, the fibre absorbs more moisture as a result of breakage of hydrogen bonds in non-crystalline region and availability of more hydroxyl groups. Also, at higher relative humidities, the absorption occurs on top of the directly absorbed water. This is generally referred to as ‘indirect .usorption’ or ‘solid solution’. Also, at saturation, there is considerable swelling of the fibre. ‘STRENGTH The load required to break i.e., tensile strength of single cotton fibre varies widely. It depends upon the thickness of the wall, prior damage to the fibre and cellulose degradation. Matured fibres with coarse and heavy wall are the strongest fibres. Their strength ranges from 9 gm to 13 gm per fibre. The strength of the mature fibres of intermediate and fine types is between 4 gm to 9 gm per fibre. On the other hand, immature fibre strength can be as low as 0.5 gm to 1.0 gm per fibre. The strength of the fibre increases at higher humidity or at higher moisture. In general, the tensile strength increases upto a relative humidity of 60% and then it remains mostly constant. At higher humidity or moisture62peeea eee eee A text book of fibre science and techrilagy Pick-up, moisture or water penetrates inside amorphous region, breaks the inter molecular forces, also internal stresses and imporves its strength as well as deformability because of more uniform load transfer action. With the same cotton, yarn strengths can be augmented by changes in friction at fibre surface, by increasing the yarn density and by selection of uniform staple length of the fibres. Long staple cottons with their fineness and higher fibre bundle strength can be expected to make the yarn more stronger. ELoncation When load is applied, the length increases. The change in length with respect to the original length is defined as extension or elongation or strain. Average fibre elongation at break is about 5% to 10%, exactly around 6% to 8%. In the structure of the cotton fibre, the fibrils spiral round at an angle of about 20° to 30° to the fibre axis. In general, increasing the helix angle reduces the resistance for extension. Mobutus Modulus is generally related to the resistance to deformation. Uptoa certain limit of deformation, the stress and strain follow Hooke’s law ie, strain is proportional to stress. The proportionality between stress and strain is referred to as modulus or elastic modulus. In case of cotton fibre, the elastic modulus means little unless the exact history of the sample is known. The modulus of cotton fibre is about 500-525 g.wt/tex. The sress-strain relation for a single fibre is roughly a straight line when the fibre contains little moisture and in this case, Hooke’s law is valid upto the breaking point. TORSIONAL RIGIDITY The mean rigidity of cotton fibre is about 7.910 ~¢ g.wt.sq.cm.sq.tex. Rigidity varies with the shape, conditions of growth and wall thickness of the fibre. The high rigidity of thick walled fibres suggests why coarse cottons must be more highly twisted than fine cottons to produce yarns of the same size. CHEMICAL PROPERTIES OF COTTON The cotton fibre is an elongated cell, constructed from millions of cellulose molecules. Small amount of moisture, fatty materials, minerals are other constituents of cotton. So the chemical properties of cotton are mostly influenced by the chemical characteristics of cellulose.COUN ere wielersieisleroisie siatcleteilejleiwietr sie teipie(s selene nists 83 Action oF Heat Cotton fibre ignites easily and burns with an odour similar to that of burning paper. It burns with a bright flame, which continues even after the fibre is removed from fire. After the flame has been extinguished, the fibre continues to smolder and smoke. This is a typical test of cellulose. Cotton can be heated in a dry state to 150°C without any decomposition. But if heating continues, a brown colour on cotton develops gradually. A slight brown discolouration can occur at temperatures lower than 150°C, which does not deteriorate the fibre. However, it is sufficient to spoil the effects of bleaching. So care should be taken to control the temperature of drying. The temperature should not exceed more than 93°C. Prolonged exposure at high temperature to an atmosphere containing oxygen causes tendering due to the formation of oxycellulose. At about 170°C, cotton begins to scorch even in a short time. If cotton is heated out of contact with air, the cotton cellulose molecules break down to form gaseous hydrocarbons, methyl alcohol, acetic acid and carbon dioxide. The mechanism of thermal degradation of cellulose may be assumec to include two main reactions. One reaction consists of dehydration and other, scissions of C-O bond in the chain i.e., either in the rings or between the rings. The C-O bond is weaker than the C-C bond and so are more likely to be ruptured. Scission of C-O bond in the ring results in the disintegration of the ring as per the scheme given in Fig.5.4. Scissioning of the external C-O bond degrades the chain molecule with the formation of levoglucosan unit and another glucose unit with hydroxy! end. Action oF Lint Exposure to air in presence of sunlight for a long period will have an effect on cotton like that of heat. Oxycellulose is gradually formed accompanied by tendering because of atmospheric oxygen. The tendering effect by light and air is accelerated by traces of metals like copper. AcTION OF WATER Raw cotton is very hard to wet because the wax present on the surface of the fibre i.e., cuticle is difficult to wet. Wax can be removed by scouring. So unscoured cotton will not absorb water so easily as scoured cotton. Cold water swells cotton without any chemical damage. The swelling is accompanied by the disappearance of the natural twist i.e., deconvolution.“ _. eA text book of fibre science and technology I “ Oo H o— nf \/ ; ‘ o\e /™ c—c t I H OH Fig. 5.4 Scission of C-O bonds in cellulose, (a) scission of the ring C-O bond and (b) scission of external C-O bond. The irregular cross-section becomes more circular, which reappears on drying. Structurally, swelling is due to the intercrystalline areas, which means only amorphous regions are affected by swelling. Sea-water can sometimes degrade cellulose and form hydrocellulose. Acrnon oF Acips Cold dilute soiutions of mineral acids at boil have no effect on cotton cellulose, provided the acid are neutralised or washed out completely before drying. However, if traces of mineral acid like 0.01% be allowed to dry in, tendering soon becomes apparant due to formation of hydroceilulose.Boiling with dilute acids will ultimately hydrolyse cellulose to glucose. At low temperature, the action by acid is mild hydrocellulose forms. Cold concentrated sulphuric acid dissolves cellulose and forms cellulose hydrate. If this solution is poured in cold water, the cellulose hydrate is precipitated in a gelatinous form. This principle is used for parchmentising paper to give a transparency effect with higher strength, Hydrochloric acid affects cotton much more severely than sulphuric acid. Degradation is more rapid and severe in presence of hydrochloric acid than sulphuric acid. Nitric acid, on account of its oxidising action, differs from other acids in its behaviour towards cellulose. Immersion for a short time in concentrated nitric acid results partial shrinkage with higher tensile strength and affinity for dyestuffs. Prolonged action oxidises cellulose to oxycellulose and finally breaks it down to oxalic acid. The reaction rate is higher at higher temperature. If nitric acid is allowed to dry in cotton, the material will tender on storage in a similar manner like that of other mineral acids. ActTION OF ALKALIS One of the main advantages of cotton is its resistance to alkali solutions. Mild alkalis like sodium carbonate have no action on cotton in the absence of air either at low temperature or at high temperature. However, in presence of oxygen or air, oxycellulose is formed with gradual tendering of cotton On the other hand, the action of strong alkalis on cotton fibre is very interesting. Dilute solution of strong alkalis like sodium hydroxide with concentration of 2% - 7% can be boiled without least tendering in absence of air. Generally, dilute solution of sodium hydroxide is used for scouring i.e., removal of waxy and other impurities from cotton fibre. The scouring process purifies cellulose and imparts hydrophillic character and permeability to cotton fibre. In this range, the fibre will have moderate swelling depending upon concentration of alkali used. Strong alkalis with higher concentration induce structural and physical changes in cotton fibre. Sodium hydroxide as well as potassium hydroxide form different hydrated forms in association with water as shown in table 4.2. The diameter of these hydrated forms depend on the concentration of the alkali used. For small concentration of alkali i.e., less than 5%, the diameter of hydrated ions is too large. So it cannot penetrate into the structure of cotton. As the concentration of alkali increases, the number of water molecules per molecule of alkali decreases for the formation of smaller hydrates. Thus the diameter of the hydrated form of alkali decreases. Thea elel se oreralarteleteieee eects sete) ‘A text book of fibre science and technology penetration of the hydrated molecule inside the structure i.e.. amorphous and/or crystalline phase depends on the diameter of the hydrates. Because of penetration, the fibre swells, When cotton fibre is treated with 7% - 15% sodium hydroxide solution, the swelling increases to a maximum value with deconvolutions. For alkalis with a concentration of 15% - 23%, swelling remains constant. The maximum swelling occurs at 13% sodium hydroxide, and is related to the penetration of the hydrate to the amorphous region only, which is generally referred to as “inter-crystalline swelling’ Cotton is attacked and degraded by strong hot alkalis. The rate of degradation varies depending upon presence or absence of air. The degradation is slow and in a stepwise manner in the absence of air. However, the degradation is very serious in presence of air. The oxidative degradation of cotton results im chain scission, weight loss, lowers the molecular length of cotton and forms oxycellulose. The extent of degradation is higher at higher temperature MERCERISATION Cotton can be mercerised by treating with or without tension in a strong solution of alkali ike sodium hydroxide The properties can be improved like - shrinkage in yarn or cloth due to swelling, improvement in lustre with a silky look, ~ improvement in tensile strength, - improvement in dyeability and uniformity in dyeing, - improvement in dimensional stability, - improvement in clasticity and stretchability Mercerisation is generally defined as *subject of cotton, linen and other vegetable fibres, either in fibre form or any other stage of its manufacture to the action of caustic soda, caustic potash, dilute sulhphuric acid or zine chloride of a temperature and strength sufficient to produce a new effect. The changes that take place during mercensation is a physical one, though there 1s some chemical change. Solvated dipole hydrates form is this alkal: concentration. The hydrate penetrates inside cotton fibre. Twoto three hydrated molecule combines with the hydroxyl groups of comes cellulose In this manner, cellulose, sodium hydroxide and water form & ternary complex, generally referred to as “soda cellulose’. During washing, soda cellulose decomposes with the formation of cellulose I. Washing is also an equally important process for the decomposition of soda celtalose.Cotton. ACTION OF MICRO-ORGANISMS Many micro-organisms attack cotton. Numerous fungi cause mildew. The mildew discolours, rots and weakens the fibre. Most fungi reproduce by means of spores, largely present in air and are attracted to cotton, wherever found. Certain bacteria also cause micro-biological rotting, and appear to be its main cause under water logged conditions. MobDIFICATION OF COTTON Cotton fibres have the following inherent drawbacks : (a) Poor solubility in common solvents which restricts the improvement in fibres and yarns. (b) Poor crease resistance, which makes garments made from cellulosic fibres crumple easily during wear. (c) Lack of thermoplasticity, which is a requirement for heat-setting and shaping of garments. r (d) Poor dimensional stability, resulting in distortion of the garments during laundering and ironing. These drawbacks have directed attention towards improving the properties of cotton by modification of its physical and chemical structure. The physical structure of the fibre can be changed either by swelling or by regeneration. Cotton or cellulose can be swollen ina suitable swelling agent and then partially deswollen by removal of the swelling agent. Change in physical structure enhances strength, lustre and reactivity. The chemical structure of cotton can be modified in several ways like substitution, cross-linking and grafting reactions. SusstiruTion The cellulose hydroxyls in cotton can be substituted.In this process, hydroxyl groups in cellulose molecules are altered through introduction of side groups by an etherification or by an esterification reaction. Some of the draw backs of cellulose or cotton such as flammability, susceptibility to rot and mildew, swellability ete can be eliminated or reduced. Chemical reaction rate of cellulose hydroxyls is generally slow and non-uniform. So conditions for chemical modification must prevent the decrease in molecular mass or chain length. Acetylation involves treatment of cellulose with a mixture of ac anhydride, acetic acid and a catalyst such as sulphuric acid or perchlorBB sees A text book of fibre science and technology acid. The reaction of cellulose with acetic anhydride may be written as: Cell-OH + (CH, CO),0 —-> Cell-0-CO-CH, + CH, COOH Acetylated cellulose possesses several useful properties such as heat resistance, mildew resistance and rot resistance. Modification of cellulose by cyano-ethylation results in cyano-ethylated cotton. In this procedure, cotton is first impregnated with dilute sodium hydroxide solution and then treated with acrylonitrile at 55°C, followed by rinsing with dilute acetic acid and further washing with water. The reaction between cellulose hydroxyl and acrylonitrile is as follow Cell-OH + CH,=CH-CN—-> Cell-O-CH,-CH,-CN Cyano ethylated cotton possess improved resistance to rot, heat and damage by acids and abrasion. Also, these fibres are better dyeable. Carboxyrethylation is one of the common methods of chemical modification. Partially carboxymethylated cotton may be prepared by padding the material with monochloro acetic acid or its sodium salt, followed by padding it with sodium hydroxide. The reaction is as follows Cell-OH + NaOH ——> Cell-O-Na + H,O Cell-O-Na + CI-CH,- COO-Na ——-> Cell-O-CH, - COO-Na + NaCl Carboxymethylated cotton shows improvement in properties like moisture regain, water absorbency, water permeability, dyeing, soil resistance and soil removal. . Cellulose reacts with acrylamide in an alkaline medium to give carbamoyl ethylated cellulose. Cell-OH + CH, =CH-CO-NH, —> Cell-O-CH,-CH,-CO-NH, The carbamoyl ethylated cellulose possesses better rot resistance and heat resistance properties. Cellulose phosphate can be prepared by reacting with suitable phosphorylating agents like phosphoric acid, ammonium phosphate. Phosphorylated cotton shows better fire resistance and soil resistance qualities. CROSS-LINKING Cotton can be reacted with bi- or poly functional compounds. This results in the production of cross-linked or resinification products andCotton........ ee eee eee ee eee reer errr rrrrery seeeeeeee 89 stabilises its structure. The interaction of cellulose and formaldehyde at higher temperature leads to the formation of methylene ether cross-links with secondary hydroxyl groups of cellulose. The cross-linking reaction takes place in highly disordered regions. In prescnce of swelling agents, the cross-linkages appear to be located in the ordered regions i.¢., crystalline regions Also, formaldehyde condensates with urea, phenol or melamine and gives rise to urea-formaldehyde, phenol-formaldehyde and melamine- formaldehyde resins. These resins are generally applied and cross-linked with cotton as well as other cellulosic material to obtain crease-resistance and low shrinkable fabrics Cell-OH + HO-CH,-NH-CO-NH-CH,OH + HO-Cell —> Cell-O-CH,-NH-CO-NH-CH,-O-Cell Cell-OH + HO-CH,-Ph-CH,-OH + OH-Cell ——> Cell-O-CH,-Ph-CH,-O-Cell o% CellO-CH.NNH-CH,OH + HO-Cell —+ I CH, ~ CH, CO. oN Cell-OH + HO-CH-N N-CH,-0-Cel CH— CH, In a similar manner, epoxy resins are cross-linked with cotton and other cellulosic materials to retain fabric feel in addition to the crease- resistance and anti-shrink properties Cell-OH + CH,——CH, - R - CH, CH, + HO-Cell —~> Rp Re Cell-O-CH,-~CH-R-CH-CH-CH-CH,-O-Cell | | OH OHA text book of fibre science and technology GrRarmnc Cotton can be grafted to prepare a branched cellulose chain in combination with synthetic polymers. This process is known as grafting, usually done by modifying the cellulose molecules through creation of branches of synthetic polymers. Grafting confer certain desirable properties on the fibre without destroying its intrinsic properties. Grafting on cellulose is a heterogeneous reaction because of the structure of cellulose or cotton. Grafting can be done by (1) chain transfer, (2) activation of the macromolecule or (3) introduction of functional groups. Chain transfer reaction involves the interaction of reactive centres in the middle of cellulose macromolecule with the grafting agent. Activation of cellulose consists of introduction of active centres in the macromolecules by thermal, mechanical, chemical, photo-chemical or radiation energy like gamma or UV rays. The third method consists of introducing groups into the macromolecule which will decompose to form free radicals. These free radicals induce grafting. The mechanisms proposed for graft are graft initiation, propagation i.e., chain growth and termination. Mostly vinyl and acrylic monomers are used for grafting. Initiation consists of formation of free radicals on the cellulose backbone from the initiator and addition of a monomer molecule to the cellulose free radical. This results in formation of a covalent bond between cellulose and monomer unit with a free radical on the newly formed branch. This is followed by subsequent additions of monomer molecule to the initiated chain, thereby propogating the chain. Termination occurs by reaction with impurities, initiator or activated monomer or by a chain transfer process. There are different types of polymers used for grafting cotton or cellulose for different applications. Some of the examples are : Flammability Poly(methyl vinyl pyridine), Phosphorous containing vinyl polymers like poly(vinyl pyrolidone), and Phosphorous containing monomers like vinyl phosphate monomers, phosphonate acrylamide. Waterproofing Fluorine containing polymers; Poly(isoprene) ; and Polyolefins. ( Moth and mildew Silver or copper salts of poly(acrylic acid) or poly(methyl methacrylate), Poly(acrylonitrile) .