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Chemical Properties of Flax Fiber

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 Chemical Properties of Flax Fiber
Md. Nazmul Islam
Department of Textile Engineering
Daffodil International University 
 



Introduction:

Flax, the source of Flax and technical bast fibers, is a versatile crop that can be grown in a variety of climates throughout the world. Its scientific name, Linum usitatissimum L., translated as “Flax most useful,” aptly describes this versatility. The commercially useful fibers of Flax are bast fibers (fibers produced in the cortical region of the plant between the outer cell layers and lignified inner core tissues of the stem), which are used for textiles, composites, and specialty paper and pulp. In addition to the bast fibers, the Flax plant is the source of linseed, from which is derived a highly prized industrial drying oil used in paints and varnishes and also a nutraceutical oil with high levels of 3-omega linolenic acid.
Figure —Scanning electron micrographs of Flax stems
Chemical Properties of Flax:
Flax is a natural cellulosic fiber and it has some chemical properties. Chemical properties of the Flax fiber are given below:
  1. Effect of Acids: Flax fiber is damaged by highly densified acids but low dense acids does not affect if it is wash instantly after application of acids.
  2. Effects of Alkalis: Flax has an excellent resistance to alkalis. It does not affected by the strong alkalis.
  3. Effects of Bleaching Agents: Cool chlorine and hypo-chlorine bleaching agent does not affect the Flax fiber properties.
  4. Effect of Organic Solvent: Flax fiber has high resistance to normal cleaning solvents.
  5. Effect of Micro Organism: Flax fiber is attacked by fungi and bacteria. Mildews will feed on Flax fabric, rotting and weakling the materials. Mildews and bacteria will flourish on Flax under hot and humid condition. They can be protected by impregnation with certain types of chemicals. Copper Nepthenate is one of the chemical.
  6. Effects of Insects: Flax fiber does not attacked by moth-grubs or beetles.
  7. Dyes: It is not suitable to dye. But it can be dye by direct and vat dyes
Description of Chemical Properties on Flax Fiber:
Effect of Acids
A better understanding of the chemical composition and surface adhesive bonding of natural fiberis necessary for developing natural fiber-reinforcedcomposites. The composition of natural fibers includes cellulose, hemicellulose, lignin, pectin,fat, waxesand water soluble substances . The composition may differ with the growing condition and test methods even for the same kind of fiber. 
 
Cellulose is a semicrystalline polysaccharide with a large amount of hydroxyl group in cellulose,giving hydrophilic nature to natural fiberwhen used to reinforce hydrophobic matrices; the result is a very poor interface and poor resistance to moisture absorption.Hemicellulose is strongly bound to cellulosefibrils presumably by hydrogen bonds. Hemicellulosic polymers are branched, fully amorphous and have a significantly lower molecular weight than cellulose. Because of its open structure containing many hydroxyl and acetyl groups,hemicellulose is partly soluble in water and hygroscopic.Ligninsare amorphous, highly complex,mainly aromatic, polymers of phenylpropane units but have the least water sorption of the natural fibercomponents.
 
The natural fiberexhibits a high hydrophilicity due to attraction or interaction between the hydroxylgroups of fibercomponents and water molecules. The interactions between fiberand water originate from the non-crystalline region and extend to thecrystalline region. The uptake of water by hygroscopic substance such as cellulose and hemicellulose is a hydration process involving accessible hydroxyl groups. Water molecule absorbed by cellulose molecule form cellulose hydrate and the reaction is exothermic, which provide the driving force for further absorption.The high moisture sensitivity of lignocellulosic fibercauses the dimensional instability and limits the use of fiberas reinforcement in composite materials. Low interfacial properties between fiberand polymer matrix often reduce their potential as reinforcing agents due to the hydrophilic nature of natural fibers;chemical modifications are considered to optimize the interface of fibers. 
 
Generally, chemical coupling agents are molecules possessing two functions.The first function is to react with hydroxyl groups of cellulose and the second is to react with functional groups of the matrix.The moisture absorbed by the fibers can be reduced by chemical modifications of fibers such as acetylation, methylation, cyanoethylation, benzoylation,permanganate treatment, acrylation etc.Acetylation of natural fibers is a well-known esterification method causing plasticization of cellulosic fibers. 
 
Polymer hydroxyl groups of the cell wall with acetyl groups, modify the properties of these polymers so that they become hydrophobic which could stabilize the cell wall against moisture,improving dimensional stability and environmental degradation.In addition, acetylation is one of the most studied reactions of lignocellulosic materials. Cellulose acetate was discovered in 1865, and partially acetylated cellulose products were commercialised as acetate rayon fibers and cellulose acetate plastics in the early 1900s. The first attempt of acetylating wood flour and sawdust took place in 1928. The principle of the method is to react the hydroxyl groups (–OH) of the fiberconstituents with acetyl groups (CH3CO–). The reaction is known to proceed to full esterification of all the three hydroxyls of anhydro-D-glucose when it is carried out in a homogeneous phase (i.e. when cellulose is dissolved),but in the case of fibers and wood where the reaction is heterogeneous. So it is thought to esterify all hydroxyl groups of the fiber. Hence, a highly non-uniform product may be obtained. In these cases, it is necessary to use catalyst speeding up acetylation process. There are large number ofcatalysts that have been used in the past, including sulphuric acid, pyridine, potassium and sodium acetate, gamma-rays, etc. 
 
However, the use of catalysts poses many problems. Strong mineral acids or acid salts are known to cause hydrolysis of cellulose resulting in damage of the fiberstructure.So, selection and optimization of catalyst is important for the acetylation of lignocellulosic fiber. The hydroxyl groups that react are those of the minor constituents of the fiber, i.e. lignin, hemicelluloses,and those of amorphous cellulose. That is because the hydroxyl groups in crystalline regions with close packing and strong interlock bonding are completely inaccessible.Reduction of about 50% of moisture uptake for acetylated jute fibers and of up to 65% for acetylated pine fibers has been reported in the literature.Seena et al. investigated the effect of acetylation in banana fiberreinforcedphenol formaldehyde composites and reported that the tensile strength, tensile modulus and impact strength are found to improve compared to non treated banana fibercomposites. Liu et al. studied the effect of acetylation in natural fibercomposites (cotton, rayon, wood with polystyrene as matrix) and they showed, by using the micro-debonding test that acetylated fibers had increased interfacial shear strength. 
 
Furthermore,they reported that acetylation increased the surfacefree energy of thefibers.Zafeiropoulos et al. investigated aectylation of flax,hemp and wood fiberand resulted in a removal of non-crystalline constituents of the fibers, altered the characteristics of the surface topography,changed the fibersurface free energy and improved the stress transfer efficiency at the interface.In the present work a detailed investigation has been carried out on the effect of acetylation on the flax fiberpropertiesin terms of fibercomposition,fibersurface, degree of crystallinity, degree of polymerisation, moisture absorption and thermal stability, as well as, a detailedinvestigation has been investigated on the effect of acetylation of flax fiberon the flaxfiberreinforced polypropylene composites properties.

This study inspected the effect of acetylation of flaxfiberon fiberproperties and its reinforced polypropylene composites properties-
  • Small amount of catalyst content has a significant effect on reaction rate and degree of acetylation.
  • Acetylation of flax fibers resists up to 50% moisture absorption properties depending on degree of acetylation.
  • Flax fibermorphology and components have been changed due to acetylation.
  • Thermal stability increased till certain range due to acetylation.
  • Highest tensile and flexural strength were scrutinized at 18% degree of acetylated flax fibercomposites and about 25% improvement on strength properties was observed compare to untreated fibercomposites.
  • With the addition of MAH, the improvement of tensile strength of composites was observed 20to 35% considering degree of acetylation.
Effects of Alkalis
Many efforts have been spent in order to develop fully biodegradable eco composites by combining natural fibers as reinforcements with biodegradable polymers. These composites are environmentally friendly, compostable, and sustainable.Natural lignocellulosicfibers i.e. flax, jute,hemp represent an alternative to conventional reinforcing fibers such as glass, carbon and aramide. Advantages of natural fibers over traditional ones are their relatively low cost, high toughness, better specific strength, reduced abrasiveness to processing equipments, enhanced energy recovery, CO2-neutrality when burned, and biodegradability. However, the main disadvantages of natural fibers in composites are the poor compatibility between fiber and matrix, which reduces their potential as reinforcing agents in composites.
 
Poly(lactic acid) (PLA) is a commercially available linear, semicrystalline, aliphatic, biodegradable polyesterthat can be produced from lactic acid by the fermentationof renewable sources such as whey, corn, potato, or molasses.The resulting polymer can be processed similarly as polyolefins and other thermoplastics. Properties of PLA can be modified through the use of lingocellulosic fibers that reduce the cost of the material withoutrestricting their biodegradability.
 
The interface of any composite is an important factor that has a strong influence on the overall mechanical properties of a reinforcement and matrix system. For fiber reinforced systems, the interface plays a major role in the stress transfer from thematrix to the fiber and thus behaves as reinforcement.This process requires a good bonding between the polymeric matrix and the fibers. Insufficient adhesion between hydrophobic polymers and hydrophilic fibers result in poor mechanical properties of then naturalfiber reinforced polymer composites. These properties may be improved by physical treatments (cold plasma treatment,corona treatment and alkali treatment) and chemical treatment (by maleic anhydride,organosilanes and isocyanates).Alkali treatment (or mercerization) of cellulose fibers,especially cotton is awell-known method to increased strength, to give better absorptive properties, and usually, a high degree of luster. Alkali treatment leads to fibrillation of the fiber bundle intosmaller fibers by removing the natural and artificial impurities. 
 
In other words, alkali treatment reduces the fiber diameter and as a result of that it increases the aspect ratio. It also increases surface roughness, resulting inbetter mechanical interlocking and the amount of cellulose exposedon the fiber surface.The development of a rough surface topology and enhanced aspect ratio yield better fiber–matrix interfacial adhesion, which can result in better mechanical properties.In addition, alkali treatment also affects the molecular orientation of cellulose crystallites due to the removal of lignin and hemicellulose.In this study, the influence of alkali (NaOH) treatment on the mechanical, thermal and morphological properties of eco-composites of short flax fiber/poly(lactic acid) (PLA) is investigated. Morphological, mechanical and thermal properties of flax/PLA composites have been examined by scanning electron microscopy (SEM),tensile tests and differential scanning calorimetry (DSC), respectively.

Effects of Bleaching Agents
The technical progress observed recently in the textile industry is intended to make manufacturing processes friendlier to the environment. Therefore, technologists and researchers are paying more and more attention to the possible use of enzymes in finishing processes, as these natural agents undergo complete degradation in effluents.By using enzymatic agents in finishing processes, one can obtain textile goods with an improved comfort of use (a soft handle or feel, decreased fabric weight, reduced tendency to pilling) and increased lustre, as well as reduced shrinkage in the case of woolfabrics.The major problem in using enzymatic agents is their correct selection from the point of view of the effects obtainable during treatment.
 
Chemically, enzymes are colloidal macromolecular compounds consisting exclusively of a homogeneous protein substance (mono-component enzymes) or two or three components –protein and non-protein substances known as the prosthetic group (bi-or multi-componentenzymes).Enzymes are mostly classified on the basis of the reaction they catalyse. Similarly, the names of enzyme groups or particular enzymes are derived from thename of the reaction or the compound which undergoes enzymatic effects. Therefore, the group of enzymes which catalyse the hydrolysis of substrates into simpler compounds with water is called hydrolases, while the enzymes which decompose ester bonds are called esterases, the enzymes catalysing pectin hydrolysis are pectinases, cellulose hydrolysis -cellulases, etc. The large dimensions of enzyme molecules and their spatial arrangement reduce their abilities to penetrate the complex fiberstructure. 
 
However, in the view of some authors, the action of enzymes on fibers results in the disturbance of fiberstructure which, depending on the treatment intensity, assumes the form of partial fibrillation or superficial defibrillation, due to the enzymatic hydrolysis of the fibermaterial.In the textile industry, flax fibersare mostly processed as technical or industrial fibers. The share of partly disintegrated complex fiber(cottonine) in textile processing is still relatively low. One should believe that the trends towards more ‘delicate’, thinner and softer fabrics, aswell as towards extending the range of blended fabrics of flax and other fibers, will promote the introduction of higher amounts of complex and even elementary flax fibersinto textile processing.The quality of the final technical flax fiber(either complex or elementary) will be affected by all the processes used in its isolation, in addition of course to the specific features of the raw material resulting from the plant’s botanical variety and the conditions of its growth. The quality of the final technical fiberalso depends on the structure of the elementary fibers, and their arrangement and combination in bunches and in the bundle of technical fibers.
 
Considering the effects of enzymatic treatments of cellulose fibers, one may as-sume that major changes will result from the removal of non-cellulose substances present in the fibers in various amounts as well as those resulting from cellulose decomposition and its partial elimination from fibers. These changes will be followed by the modification of thefiber’s morphological structure and properties (especially surface properties) which will be shown in different properties of the final products (above all, yarns). The aim of the present study was to examine the effect of enzymatic treatment on the morphological structure and properties of flax fiber, and the metrological parameters of the yarn made of this fiber.

Effect of Micro Organism
PAHs (Polycyclic Aromatic Hydrocarbons) are the largest class of carcinogenicchemicals in the environment. As burning the fossil fuel and refuseetc.,making the PAHs widely distributed in our environment. In the U.S. EPA list of prioritized control,itidentified 16 PAHs as priority monitoring pollutants in 1979. Studies have shown that the detection rate of small rings is generally higher than that of large ring in water,in which the highest one is the detection rate of naphthalene and the detection rate of phenanthrene ,pyrene ,fluoranthene ,fluorene ,and other compounds are lower. China's water pollution has been generally affected by PAHs and the content are higher than most of the foreign countries. The content of PAHs in water changes with the seasons ,which is usually significantly higher in lowwater season than in the flood season. Ming Xifound thatthe low-ring PAHs are detected in the Yangtze Estuary in each section and the concentration in summer is higher than those in median water period and low water season in autumn and winter. Wang Pingand a few others’ studies have shown that the 16 PAHs were all detected in water Yellow River in Lanzhou. Among those PAHS,the content of and pyrene are at higher levels,and the smaller molecular weight of PAHs has a larger proportion. Ju Lizhong’sstudies show that the total concentration of 10 PAHs was 989~96210ng / L in the surface water of Hangzhou.

The density of flax fiber,HRT and aeration rate have a significant effect on the degradation of PAHs,the higher the density of flax fiber,HRT and aeration rate,the betterthe effects of decontamination ,which means the water quality of effluent meets the GB5749-2006 standard that the total content of PAHs is less than 2000 ng·L-1.With the same density of flax fiber or HRT or aeration ,the degradation rate of large ring PAHs was significantly about 2%lower than the small ring. By selectingthe natural dielectric material density in water tank 12h HRT and 6h aeration the degradation effect is the best.There exist a lot of different types of indigenous micro-organisms in water body,many of which such asBacteria,fungi and algae all have the ability to degrade PAHs. When water contaminated by PAHs,after a naturalselection process,a number of specific indigenousmicroorganism induced by contaminants of PAHs and produced the enzyme to decompose the PAHs so as to realize the degradation and transformation of PAHs. The flax fiber media materials provide a good micro-environment for the indigenous microorganism biochemical reactions,therefore to achieve the objective of strengthening the natural degradation of PAHs.

Effects of Insects

Yellow-leaf is the most serious disease of harake and various fungal infections (moulds and spots) can affect the appearance of harakeke leaves. Harakeke is home to two native moths as well as mealy bugs and scale insects.

The windower and the notcher
Harakeke is home to two native moths that are a great nuisance to weavers. The caterpillar of one, the ‘looper’ or ‘windower’, chews narrow strips on the underside of the leaves, exposing the fiber, which soon decays. The other caterpillar chews notches on the side of the leaves.
'Looper' or 'windower' damage
Both caterpillars are sensitive to light. They eat at night and shelter during the day in rolled up dead leaves and dry debris. They cannot withstand prolonged immersion in water. Harakeke grew naturally along rivers and in wetlands, and when flooding occurred, great numbers of pests were destroyed.
'Notcher' damage
It’s not easy to control these pests. Insecticides that kill caterpillars can be used, combined with a sticking agent to make sure the spray sticks to the leaves. Spray needs to cover the leaves very thoroughly and penetrate the leaf bases. Sprays are best applied in December.Sprays are not always a practical or desirable option. The best preventative measure is to keep the ground clear of hiding places by getting rid of dead leaf tubes and other debris, and keep the bushes well trimmed and opened up to the light. Encourage bird life too.

Mealy bugs and scale insects

Mealy bugs are commonly found where the leaves sheath together at the bottom of the fan and on the rhizome. Often they are living amongst masses of whitish-grey powder. Generally, they do not affect the health of the plant, but if an infestation is bad enough, the harakeke leaves develop a red discolouration.

Scale insects form crusty, white, woolly looking patches on the underside of the leaves, especially where the leaves are shaded. The insects suck the sap, discolouring and weakening the leaves, which are then more susceptible to disease. Large infestations can be very damaging to the health of the bush.

Scale can be effectively controlled with oil sprays, available from any plant nursery. Severely affected leaves should be removed.

Fungal infections
Various fungal infections (moulds and spots) can affect the appearance of harakeke leaves. Some weavers like the effect of pinkish red patches on woven articles. However, fiber is discoloured, weakened and broken by these infections. Remove and burn badly diseased leaves.

Yellow-leaf disease
Yellow-leaf is the most serious disease of harakeke, and is caused by a phytoplasma, a specialised bacterium, transmitted by the native flax plant hopper, Oliarus atkinsoni. It occurs mainly in the North Island, and is unlikely to be found below the north of the South Island.
Yellow-leaf disease
Yellow-leaf disease is characterised by abnormal yellowing of the leaves, stunted growth and premature flowering. The rhizome rots, and leaves collapse. In a normal healthy fan, one or two outer leaves yellow and die off as they age, but with yellow-leaf disease the dying back is much more pronounced. The affected leaves are actually more orange than yellow. There is a big accumulation of recently dead leaves about the base of the fan.

Dyes:
Flax fabric dyeings are not as simple as other cellulosic fibers because they are greatly influenced by the presence of noncellulosic content. The dyeability of flax fabric increases with the progressive removal of noncellulosic impurities with direct dye. The exhaustion equilibrium of direct dyeing on flax fabric is obtained at the specific level of RNC (9.37%) vis-à-vis hemicellulose (7.35%). The fastness to washing, etc, of direct dyes – as on other cellulosic fibers – is average to poor on flax fiber but can be improved by suitable aftertreatments. The dye structure also plays an important role in the dye exhaustion of direct dye on flax, ie. dye having a low molecular weight and/or linearity of structure shows higher exhaustion, and vice-versa.

References:
  1. http://www.astm.org/SNEWS/SEPTEMBER_2005/akin_sep05.html
  2. http://textilelearner.blogspot.com/2012/01/linen-fiber-characteristics-of-linen.html
  3. www.engr.usask.ca/societies/csae/PapersCSAE2003/CSAE03-337.pdf
  4. Flaxhttp://en.wikipedia.org/wiki/Flax
  5. www.expresspolymlett.com
  6. http://kubanni.abu.edu.ng:8080/jspui/handle/123456789/3449
  7. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2989356
  8. https://www.dora.dmu.ac.uk/handle/2086/534 
 
Published by
S.M. Hossen Uzzal
B.Sc. in Textile Technology
Monno Fabrics Ltd. Manikgonj

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