Plasma treatment of wool fibres

Excerpt: Plasma processes can be grouped into two main classes low- density and high-density - according to their electron temperature versus electron density.

WOOL PEER REVIEWED

Plasma treatment of wool fibres


P.Senthilkumar1, N.Sandhya2, R.Saravanakumar2
PSG College of Technology, Peelamedu, Coimbatore – 641004



ABSTRACT

In order to improve certain properties of fibres and textiles, they are modified by physical treatment. Physical modifications result in specific changes in the fibre's physical microstructure and its surface properties changing the functional and aesthetic features of fibres. One such physical modification is plasma treatment. Plasma is an ionized form of gas and can be created using a controlled level of AC or DC power and an ionizing gas medium. Plasma processes can be grouped into two main classes low- density and high-density - according to their electron temperature versus electron density. The advantage of such plasma treatments is that the modification turns out to be restricted in the uppermost layers of the substrate, thus not affecting the overall desirable bulk properties. Plasma treatment has an explosive increase in interest and use in industrial Applications as for example in medical, biomedical, automobile, electronics, semiconductor and textile industry. This has resulted in an increasing knowledge of the possibilities of this process regarding demands as wet ability, shrinkage resistance of wool, dye ability, printability, coating and wash ability of conventional and technical textile. This paper gives a brief overview on the various modifications incorporated in the cotton textile materials by the plasma treatment.

Key words: surface modification, plasma, wool fibre, anti-shrink, dyeability


Introduction

THE textile industry is searching for innovative production techniques to improve the product quality. Plasma technology is suitable to modify the chemical structure as well as the topography of the surface of the material[1] . The advantage of such plasma treatments is that the modification turns out to be restricted in the uppermost layers of the substrate, thus not affecting the overall desirable bulk properties[2] . Plasma is an ionized form of gas and can be created using a controlled level of AC or DC power and an ionizing gas medium. Plasma processes can be grouped into two main classes low-density and high-density - according to their electron temperature versus electron density.

The surface energies of the treated materials increased substantially (without any backside treatment), thereby enhancing their wet ability, printability and adhesion properties. Plasma treatments have been investigated for producing hydroscopicity in fibres, altered degradation rates of biomedical materials (such as sutures), and for the deposition of anti wear coatings. Plasma treatment has an explosive increase in interest and use in industrial Applications as for example in medical, biomedical, automobile, electronics, semiconductor and textile industry. This has resulted in an increasing knowledge of the possibilities of this process regarding demands as wet ability, shrinkage resistance of wool, dye ability, printability, coating and wash ability of conventional and technical textile. The morphology of wool is highly complex; this is not confined to the fibre stem but extends to the surface as well. Cuticle cells are overlapping each other to create a directional frictional coefficient. Moreover, the very surface is highly hydrophobic. As the surface is oxidized, the hydrophobic character is changed to become increasingly hydrophilic. The chemical and physical surface modification results in decreased shrinkage behaviour of wool top; the felting density of wool top (before spng) decreases from more than 0.2 g/cm3 to less than 0.1 g/cm3[3] .

What are plasmas?

The coupling of electromagnetic power into a process gas volume generates the plasma medium comprising a dynamic mix of ions, electrons, neutrons, photons, free radicals, meta- stable excited species and molecular and polymeric fragments, the system overall being at room temperature. This allows the surface functionalisation of fibres and textiles without affecting their bulk properties. These species move under electromagnetic fields, diffusion gradients, etc. on the textile substrates placed in or passed through the plasma. This enables a variety of generic surface including surface activation by bond breaking to create reactive sites, grafting of chemical moieties and functional groups, material volatilisation and removal (etching), dissociation of surface contaminants/layers (cleaning/ scouring) and deposition of conformal coatings. In all these processes a highly surface specific region of the material (< 1000 A) is given new, desirable properties without negatively affecting the bulk properties of the constituent fibres[4].


Fig.1: Schematic of gas and its comparison with plasma[4]

Plasmas are acknowledged to be uniquely effective surface engineering tools due to:

  • Their unparalleled physical, chemical and thermal range, allowing the tailoring of surface properties to extraordinary precision.
  • Their low temperature, thus avoiding sample destruction.
  • Their non-equilibrium nature, offering new material and new research areas.
  • Their dry, environmentally friendly nature.
Plasma reactors


Fig. 2: States of matter[5]

Different types of power supply to generate the plasma are:

  • Low-frequency (LF, 50–450 kHz)
  • Radio-frequency (RF, 13.56 or 27.12 MHz)
  • Microwave (MW, 915 MHz or 2.45 GHz)

The power required ranges from 10 to 5000 watts, depending on the size of the reactor and the desired treatment.

Types of plasma
  • Low-pressure plasmas
  • Atmospheric pressure plasmas

Low-pressure plasmas

Low-pressure plasmas are a highly mature technology developed for the microelectronics industry. However, the requirements of microelectronics fabrication are not, in detail, compatible with textile processing, and many companies have developed technology of low pressure reactors to achieve an effective and economically viable batch functionalisation of fibrous products and flexible web materials.

A vacuum vessel is pumped down to a pressure in the range of 10-2 to 10-3 mbar with the use of high vacuum pumps. The gas which is then introduced in the vessel is ionised with the help of a high frequency generator. The advantage of the low- pressure plasma method is that it is a well controlled and reproducible technique.

Atmospheric pressure plasmas

The most common forms of atmospheric pressure plasmas are described below.

Corona treatment

Corona discharge is characterised by bright filaments extending from a sharp, high-voltage electrode towards the substrate. Corona treatment is the longest established and most widely used plasma process; it has the advantage of operating at atmospheric pressure, the reagent gas usually being the ambient air. Corona systems do have, in principle, the manufacturing requirements of the textile industry (width, speed), but the type of plasma produced cannot achieve the desired spectrum of surface functionalisations in textiles and nonwovens. In particular, corona systems have an effect only in loose fibres and cannot penetrate deeply into yarn or woven fabric so that their effects on textiles are limited and short- lived. Essentially, the corona plasma type is too weak. Corona systems also rely upon very small inter-electrode spacing (~1 mm) and accurate web positioning, which are incompatible with 'thick' materials and rapid, uniform treatment[4] .

Dielectric barrier discharge (Silent discharge)

The dielectric barrier discharge is a broad class of plasma source that has an insulating (dielectric) cover over one or both of the electrodes and operates with high voltage power ranging from low frequency AC to 100 kHz. This results in non-thermal plasma and a multitude of random, numerous arcs form between the electrodes. However, these micro discharges are non-uniform and have potential to cause uneven treatment.


Fig. 3: Scheme and layout of the apparatus for DBD treatment of textile materials[6]

Glow discharge

Fig. 4: Structure of wool

Glow discharge is characterised as a uniform, homogeneous and stable discharge usually generated in helium or argon (and some in nitrogen). This is done, for example, by applying radio frequency voltage across two parallel-plate electrodes. Atmospheric Pressure Glow Discharge (APGD) offers an alternative homogeneous cold- plasma source, which has many of the benefits of the vacuum, cold-plasma method, while operating at atmospheric pressure[4].

Effect of plasma on fibres and polymers

Textile materials subjected to plasma treatments undergo major chemical and physical transformations including (i) chemical changes in surface layers, (ii) changes in surface layer structure, and (iii) changes in physical properties of surface layers. Plasmas create a high density of free radicals by disassociating molecules through electron collisions and photochemical processes. This causes disruption of the chemical bonds in the fibre polymer surface which results in formation of new chemical species. Both the surface chemistry and surface topography are affected and the specific surface area of fibres is significantly increased. Plasma treatment on fibre and polymer surfaces results in the formation of new functional groups such as —OH, —CKO, —COOH which affect fabric wettability as well as facilitate graft polymerisation which, in turn, affect liquid repellence of treated textiles and nonwovens[4] .

Modification of polymer and fibre surfaces by plasma treatment presents many important advantages and offers a great potential as an established means of fabric processing. In the plasma treatment of fibres and polymers, energetic particles and photons generated in the plasma interact strongly with the substrate surface, usually via free-radical chemistry. Four major effects on surfaces are normally observed. Each is always present to some degree, but one may be favoured over the others, depending on the substrate and the gas chemistry, the reactor design, and the operating parameters. The four major effects are surface cleaning, ablation or etching, cross-linking of near-surface molecules and modification of surface-chemical structure. All these processes, alone or in synergistic combination, affect adhesion.

Plasma treatment can be used with great effect to improve the bond strength of polymer to fibre and polymer to polymer combinations. In these cases, the improved adhesion results from both increased wettability of the treated substrate and the modification of surface chemistry of the polymer[4] . Modified wettability is one of the most apparent results of plasma treatment and the method used for characterising the modification is to measure the advancing and receding contact angles against specific liquids. Plasma produced polar groups increase the surface free energy, γ, of the fibre and decreases the contact angle, θ, usually correlating with better bonding of adhesives; θ has often been used as an estimate of bonding quality. Plasmas offer uniquely effective surface engineering tools due to their unparalleled physical, chemical and thermal range, their low temperature (avoiding sample destruction), their dry, environmentally friendly nature, and their non equilibrium nature offering new material and new research areas[4].

Surface modification of wool textile substrate

Wool material

Wool is one of the oldest raw materials for clothing, this cannot be regarded as an old fashioned textile fibre. It has highly complex chemical and physical structure which is responsible for superior natural fibre properties.

The wool fibre exhibits a typical core–shell structure consisting of an inner protein core, the cortex, which is covered by overlapping cuticle cells with scale edges pointing in the direction of the fibre. The cortex is built up of spindle- shaped interdigitated cells, consisting mainly of ortho and denser Para cells, which divide the stem of a fine fibre into two halves. This bilateral asymmetry results in a natural crimp of the fibre being jointly responsible for crease resistance and, due to permitting enmeshed air, for insulation against loss of heat or protection from heat. The cortex contains macro fibrils formed by fibril-structured α-helical keratin proteins embedded in a cystine- rich protein matrix. Whereas the α- helical material is responsible for the fibre resilience, a diamino acid, cystine, cross-links the protein chains and thus stabilises the wool fibre towards environmental influe nces. Cystine is also responsible for the high wet strength, moderate swelling and insolubility of the fibre. Wool is hygroscopic and the amount of water taken up corresponds to the relative humidity and temperature of the surrounding air[7].

However, only the interior of the wool fibre is able to absorb water vapour to an extent of up to 30% o.w.f., whereas the fibre surface is water repellent due to the hydrophobicity of the outer surface of the cuticle. This apparently contradictory behaviour results in an intelligent moisture management system being responsible for the well-known wearing comfort of wool.


Fig. 4: Structure of wool[8]

The hydrophobic nature of the wool fibre is caused by covalently bound, branched fatty acids, which form the outer cuticle layer. Both the hydrophobic nature of the fibre surface and the high cross-linking density of the protein layer ortho and immediately below the lipid layer act as a natural diffusion barrier. This can complicate wool finishing processes and can necessitate a modification of the fibre surface by applying special auxiliaries and, in some cases, acid chlorination[7].

Modifications of wool material by plasma treatment

Improving plasma on different wool dyeing systems


Fig. 5: Chain structure of wool fibre[9]

Wool fibres were treated with oxygen plasma and three types of dyeing systems that are commonly used for wool dyeing, namely: (i) acid dye, (ii) chrome dye and (iii) reactive dye; were used in the dyeing process. The dyeing rate of three dyeing systems was increased significantly, but in the cases of acid and chrome dyeing, the exhaustion percentage at equilibrium did not show any significant change[10] . Such a slight change in the exhaustion percentage at equilibrium could be explained by the fact that the depth of penetration and etching caused by LTP was not sufficient to alter the internal structure of the fibre or to induce any new dye sites in the fibre. However, in the case of reactive dyeing, where LTP treatment has been done previously to the reactive dyeing, it has introduced oxygen functional groups in the fibre surface. These oxygen functional groups induced in the fibre surface enhance the dye-fibre reaction between reactive dye and wool fibre. As a result, the available dye sites were increased and thus the final dye bath exhaustion and dye fixation on fibre rates were increased accordingly. For the chrome dyeing system, it was noted that both the dyeing and after chroming processes were improved after LTP treatment[10] .

Improving the properties of wool treated by low-temperature plasma

An analysis of IR transmission spectra reveals that the greatest changes occur in the area of absorption by groups containing the C-H oscillator. This means that these groups are oxidised during LTP treatment. The SEM analysis of surface morphology reveals slight changes which occur on the surface of wool fibres as a result of plasma modification. The rising parameters of LTP treatment (time and power, or the total energy) lead to a slight increase in these changes causing a rounding of scales, micro cracks, recesses and tiny grooves, all caused by the 'etching' of the material. LTP damages an ultra- thin hydrophobic layer on the protective surface of the fibre [11] [12] . This process occurs only on the surface, and does not damage the inner structure of keratin. Removing the barrier layer results in increased sorption of humidity and dye and improved wettability of the knitted woollen fabrics. These properties are advantageous for textiles, and in particular they improve the comfort of wearing clothes[11] [12] .

Plasma treatment of wool to achieve shrink-resistance

The morphology of wool is highly complex; this is not confined to the fibre stem but extends to the surface as well. Cuticle cells are overlapping each other to create a directional frictional coefficient. Moreover, the very surface is highly hydrophobic. As a consequence, in aqueous medium, because of the hydrophobic effect, fibres aggregate and, under mechanical action, exclusively move to their root end. This is the reason for felting and shrinkage. Plasma treatment of wool has a two-fold effect on the surface. First, the hydrophobic lipid layer on the very surface is oxidized and partially removed; this applies both to the adhering external lipids as well as to the covalently bound 18-methyl-eicosanoic acid[1] . Since the exocuticle, that is, the layer below the fatty acid layer of the very surface (epicuticle), is highly cross-linked via disulfide bridges plasma treatment has a strong effect on oxidizing the disulfide bonds and reducing the cross-link density.

Again, due to the surface-directed activity of the plasma, the tenacity of the fibres is hardly influenced. As the surface is oxidized, the hydrophobic character is changed to become increasingly hydrophilic. The chemical and physical surface modification results in decreased shrinkage behaviour of wool top; the felting density of wool top (before spinning) decreases from more than 0.2 g/cm3 to less than 0.1 g/cm3 . With respect to shrink-resist treatment, this effect is too small as compared with the state-of-the-art treatment of wool top with acid aqueous chlorine solution, reduction with sulphite, and application of a thin resin (polyaminoamide) layer to the surface[1].

AFM studies of nano roughness effect were performed on single wool fibres extracted from fabric before DBD application. Analysis performed on precisely detected zone of single fiber has demonstrated the increasing of produced nano roughness with applied treatment energy. Not treated fibre presents characteristic “striations” structure .Treatment by air DBD with the energy of 60 J/cm2 produces nano roughness on fibres with RMS value of 10 nm[13] .

SEM images of fibres treated at the energy of 60 J/cm2 , which produce requested by Woolmark anti shrinking property on wool fabric, present no difference respect to not treated fibres. Subsequent application of higher energies on the same single fibre increase roughness to RMS 50 nm. Weight losses were measured on wool fabric. After post treatment conditioning (RH 65%, 24 hours), there were observed weight losses of 0, 5% and 1, 1%, correspondingly[13] .

Wool and wool/nylon blend dyed fabrics with LTP treated ones, and post treated with the biopolymer CHT and/or industrially softened, showing the compatibility of such processes with the present industrial process. The LTP based processes achieve the improvement of many properties of the dyed fabrics; all fabrics treated with LTP and post-treated are hydrophilic, do not cause any relevant colour alteration and show improved fastness to washing. In addition, LTP, LTP+CHT and LTP+CHT+ softening produce shrink- resistant wool and wool/nylon blend fabrics, although the handle is still not optimum for commercialisation and will have to be further improved in future studies[14].

Sorption properties of wool fibres

The hydrophobic nature of the cuticle and the high cross- linking density in the outermost fibre surface creates a nature diffusion barrier2 , which influences sorption properties complicates wool finishing processes, such as printing, dyeing or shrink-proofing. Surface modification plays an important role for many chemical finishing processes in textile industry[15] . The required surface modification is mainly accomplished by wet chemical processes using special auxiliaries which attack the cuticle by hard chemicals as for example NaClO.

The effects of atmospheric pressure plasma treatment on wool fabric were tested in this study. A Diffuse Coplanar Surface Barrier Discharge (DCSBD) has been used in this study. The operating frequency was 15 kHz, the power input 300 W. Wool fabric has been exposed by different times. Pure wool fabric has been exposed to different intensive plasma treatment (different exposure times at constant conditions). A conventionally-finished, plain-weave pure wool fabric (222 g m−2 manufactured from yarn of 2×19 tex) was used [15] .


Fig. 6: Lipoproteins in epicuticle[15]

The dye concentration increases with dye time and plasma treatment intensity. The plasma treatment for 10 second is exactly insufficient. The best dye sorption is obtained by plasma treated wool fabric for longest time (100 second). Experiments showed invasion of surface layer of cuticle by plasma. Plasma treatment wool adsorbs dye more intensive at lower temperature. Plasma treatment of wool can in future replace wet pre-treatment processes for wool dyeing and wool printing. The pre-treatment of wool with atmospheric plasma give an appropriate environmentally acceptable alternative to conventional treatments[15] .

KES-F analysis of wool fabric

Low temperature plasma (LTP) treatment was applied to wool fabric with the use of a non-polymerising gas, namely oxygen. After the LTP treatment, the fabric mechanical properties, including low-stress mechanical properties, air permeability and thermal properties were evaluated. The low- stress mechanical properties were evaluated by means of the Kawabata Evaluation System for Fabric (KES-F), revealing that the tensile, shearing, bending, compression and surface properties were altered after the LTP treatment. The changes in these properties are believed to be closely related to the inter-fibre and inter-yarn frictional force induced by the LTP. The decrease in the air permeability of the LTP-treated wool fabric was found to be probably due to the plasma action effect on the increase in the fabric thickness and a change in the morphology of the fabric surface, which was confirmed by Scanning Electron Microscopy micrographs. The change in the thermal properties of the LTP-treated wool fabric was in good agreement with the above findings and can be attributed to the amount of air trapped between the yarns and fibres. This study suggested that LTP treatment can influence the

[16] final properties of wool fabric . The impact of corona modified fibres chemical changes on wool dyeing Corona/plasma treatment is an environmentally friendly process applied to wool fabrics. The main contribution of the present work was to study the impact of Corona on dye ability of wool fibres. First, the different chemical aspects of a woven wool fabric's surface were determined using two different analytical skills (XPS and polyelectrolyte titration).The results show that, low-temperature plasma treatment has ability to change wool fibre morphology which could have an impact on sorption properties. Fabrics were dyed with blue acid and blue metal-complex dyes, and dyeing behaviour was studied by [15] Fig. 7: DCSBD equipment

[17] means of on-line VIS spectrophotometer . Finally, dyed samples were calorimetrically evaluated and colour differences were calculated. The results provided evidence that the overall carbon content was decreased while oxygen and nitrogen atoms were increased when using ionized air for fabric modification. It has also been noted that the amount of positive-charged functional groups in various pH ranges are higher for Corona-treated wool fabric in comparison with the untreated sample. The surface performances of untreated and Corona-treated wool fabrics were studied both morphologically and chemically. Corona treatment is

confirmed as inducing chemical and physical changes on the surface such as oxidizing/removing external fatty-acid monolayer, enlarging positively charged functional groups, creating new dye sites, and therefore, improving the

[17] exhaustion rate during dyeing . Corona treatment applied in the pre-treatment stages of wool production can lead to an optimization of different dyeing procedures, implying lower dyeing temperatures and shorter dyeing time, achieving the same or even better colour exhaustion in comparison to conventional pre-treated wool fabric. For these reasons, the energy consumption can be

[17] reduced, thus also enhancing environmental protection . Improving anti-felting properties and physicochemical properties of wool Low-temperature plasma treatment, which offers an alternative to the existing treatment using chlorine, has been drawing worldwide attention as an eco-friendly technology for the anti-felting of wool. However, the reason why that plasma- treated wool does not shrink even after repeated aqueous laundry has still not been elucidated, though a number of related papers have been published. The aim of this study is to explore the reason by analyzing the surface of Ar-plasma treat wool using FT-IR and also XPS and by examining the changes of physicochemical properties of plasma-treated wool. It is suggested that intermediate cystine oxides(i.e., –S(O)-S- and - S(O)2-S- groups generated on the wool fibre surface) have a strong affinity with water. Subsequently, cohesive force is exerted between fibre surfaces and this result in a decrease of the flexibility of the individual fibre in assembly. Therefore, any entanglements between fibres are suppressed, and the felting is controlled. This is an important anti-felting property plasma treatment provided. In addition, carboxyl and sulphonic acid groups produced on the fibre surface play some part in the felting behaviour of wool probably due to an

[18] increase in the hydration of the fibre surface . As the FT-IR analysis could give no information to explain the anti-felting effect of plasma-treated wool. FT-IR analyses reveal that the plasma treatment has very little or no influence on the outer surface of wool fiber. The chemical change may be induced on the outermost layer of the fiber surface rather than the deeper part of the surface.

The degree of anti-felt shrinkage greatly increased after Ar- plasma treatment, but the value decreased when the plasma- treated wool was immersed in a dilute solution of sulfite. This finding suggests that the intermediate cystine oxides produced by the oxidation of disulfide bonds were decomposed by sulfite ions and, therefore, the partially oxidized cystine groups play an important role in anti-felting shrinkage. According to the result of XPS analyses, cystine disulfide K-S-S-K groups subjected to the oxidative degradation may be transformed into sulfur oxides, K-S(=O)-S-K, K-S(=O)2-S-K (165 ! 168ev) and KSO3H ( ! 167e " ). Accordingly, the surface energy increased, and the fiber to fiber cohesion may increase because of water molecule, which is combined with sulfur oxide groups through hydrogen bonding. Thus, the anti-

[18] shrinkage effect can be achieved . There was no meaningful difference in the surface structure between plasma-treated and untreated wool fibers. Any morphological changes such as lifting of the scale edge and wrinkle of the surface were not found on the plasma- treated fiber by using a 3D profile microscope. On the SEM image, however, polymer-like substance produced by the decomposition of lipid matter was detected on the surface of

[18] the plasma-treated fibres . The plasma treatment removes the outermost hydrophobic lipid layer of the cuticle cells, and produces carboxyl and

[18] sulfonic acid groups on the fiber surface . These anionic groups play some part in the felting behaviour of wool probably due to an increase in the hydration of the fiber surface. Improving the total hand value and adhesion properties of bi-fabrics of fused interlining and wool fabrics

In this study plasma effects on adhesive strength between face and interlining fabrics are measured objectively. Fabric mechanical properties are investigated, and adhesion properties of bi-fabrics structure of fused interlining and wool fabrics are also discussed. Three types of face fabric are treated by argon-plasma and fusible interlining is adhered on them respectively. Interlining used in this study is polyester plain weave with polyamide adhesive resin. Interlining and face fabrics are bonded together using Flatbed type press machine. Face fabric samples used in this experiment are basket weave, twill and hound's-tooth wool fabrics. Bending, shearing and compression properties and air permeability are measured by KES measurement instruments. Peeling strengths of the bonded fabrics were measured using KES FB1 tensile

[19] tester . AFM evaluation of wool modified by low-temperature plasma

AFM images indicate that plasma treatment causes a significant superficial modification of wool fibres. The influence of plasma treatment on the surface etching effect depends on the gas applied and treatment time used. Oxygen plasma treatment leads to a progressive surface etching effect whereas, in the case of argon plasma, the re-deposition effect takes place. This study highlights that the effectiveness and uniformity as well as severity of plasma wool treatment can be successfully assessed by the AFM technique. Accordingly, a useful control of the treatment conditions could be carried out in order to avoid excessive fibre damage. AFM also can be used as an accurate and quantitative means of measuring wool fibre scale height. With further improvements and breakthrough in instrumentation and in resolution level, AFM will become an extremely effective and powerful tool for characterisation of wool fibre surface topography, surface friction properties as well as the effectiveness of treatments. In this way, AFM will play a significant role as a control tool in the development of

[20] improved wool surface treatments . Plasma treatment on tailorability and thermal properties of wool fabrics Dielectric barrier discharge type of plasma reactor was used

for the low-temperature plasma (LTP) treatment of the wool fabrics. Air was used as the non-polymerizing gas for the plasma treatment at different time intervals. Low-stress mechanical properties of the treated and untreated wool fabrics were evaluated using Siro-fast technique which revealed that the tensile, bending, compression, shear, dimensional stability and surface properties were altered after the LTP treatment. Other properties such as thermal conductivity, thermal resistance and pilling propensity were also evaluated. The surface topographical changes of the wool fibres after LTP treatment were analysed by scanning electron microscopy. The changes in these properties are supposed to be related closely to the interfibre and inter yarn frictional force and increased surface area of the fibres

[21]

induced by the etching effect of plasma . Surface morphological changes were found to be dependent on treatment duration. The dimensional stability and thermal resistance were found to be improved after LTP treatment (for all treatment times) compared to the untreated fabric. The shear rigidity and bending rigidity of the LTP- treated fabrics were higher than the untreated fabric but extensibility was found to be lower for both warp and weft of the LTP-treated fabrics. As the LTP treatment time was increased, the dimensional stability, thermal resistance, resistance to pilling, shear rigidity and bending rigidity increased linearly whereas the thermal conductivity and extensibility for both warp and weft decreased linearly with

[21]

increase in LTP treatment time .

References
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Dyeing of silk with raw natural colours

(Contd. from page 43)

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