Durability of Hand Wool Carpets

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Durability of Handmade Wool Carpets: AReviewS. K. Guptaa, K. K. Goswamib & A. Majumdarca Uttar Pradesh Technical University, Sitapur Road, Lucknow, UttarPradesh, Indiab Indian Institute of Carpet Technology, Chauri Road, S. R. N.Bhadohi, Uttar Pradesh, Indiac Department of Textile Technology, Indian Institute of Technology,Hauz Khas, New Delhi, IndiaPublished online: 24 Aug 2015.

To cite this article: S. K. Gupta, K. K. Goswami & A. Majumdar (2015) Durability of Handmade WoolCarpets: A Review, Journal of Natural Fibers, 12:5, 399-418, DOI: 10.1080/15440478.2014.945226

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Journal of Natural Fibers, 12:399418, 2015Copyright Taylor & Francis Group, LLCISSN: 1544-0478 print/1544-046X onlineDOI: 10.1080/15440478.2014.945226

Durability of Handmade Wool Carpets: A Review

S. K. Gupta,1 K. K. Goswami,2 and A. Majumdar31Uttar Pradesh Technical University, Sitapur Road, Lucknow, Uttar Pradesh, India

2Indian Institute of Carpet Technology, Chauri Road, S. R. N. Bhadohi, Uttar Pradesh, India3Department of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi, India

This paper presents various aspects related to the durability characteristics of handmade carpets. Variouskinds of materials and knots used in handmade carpet have been explained at the beginning. Thenthe mechanism of carpet durability has been explained with respect to mechanical and appearanceaspects. The various techniques for measuring the mechanical and appearance related durability char-acteristics have also been presented in this article. Finally, the influence of wool fibre, yarn, and carpetstructural parameters on the durability has been explained. The shortcomings in terms of durability,characterization of carpets, and scope of further research have been presented at the end of the review.

Keywords: carpet appearance, carpet constructional parameters, carpet durability, carpet manufacturingtechniques, handmade carpets, types of knots

INTRODUCTION

In the field of floor covering, carpets are very popular due to its comfort, thermal and resilienceproperties. Carpet is having a use-surface composed of textile material. Here use-surface means thatpart of a textile floor covering directly exposed to traffic. Carpets are classified into two groups,namely pile carpets and without pile carpets (IS: 11205 1984). There are several carpet manufactur-ing methods such as knotting, tufting, weaving, knitting, braiding, needle felting, fusion bonding,and flocking. Handmade carpets are manufactured in three different ways: knotted (Persian, Tibetan,etc.), flat weave (broad loom carpets, saggy, durry, etc.) and tufting (hand tufting, needle tufting).Knotting is an extensively used method for carpet manufacturing. The raw materials commonly

Address correspondence to S. K. Gupta, Research Scholar, Uttar Pradesh Technical University, Institute of Engineeringand Technology Campus, Sitapur Road, Lucknow, Uttar Pradesh 226 021, India. E-mail: [email protected]

Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/wjnf.

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400 S. K. GUPTA, K. K. GOSWAMI, AND A. MAJUMDAR

used in manufacturing of handmade carpets are wool, silk, polypropylene, and nylon as pile fibers.Cotton, jute, polyester, and polypropylene fibers are used as carpet backing. Wool in pile yarn isextensively used in handmade carpets because of excellent properties like hand, durability, stain-resistance, dyeability, flame resistance, insulation, static generation, and biodegradability (Goswami2009). There is a strong influence of carpet aesthetics in the form of color, design, and texture onpurchase decision-making process. However, customers always try to maintain a balance betweenaesthetics and performance of handmade carpets. Fiber qualities, carpet constructional parameters,backing techniques, etc., have strong influences on carpet performance (Cegielka 1988). Poor wear(shedding) and abrasion performance of carpet is still the most common problem. Carpet durabilityis defined as the wear life of a carpet in given situations. The wear performance of carpets can bedivided in two aspects, i.e., appearance retention (relatively more important in early stages of wearlife of a carpet) and durability (relevant in the later stages of wear life). There are several tests formeasuring carpet durability, such as compression and recovery characteristics, thickness loss underdynamic loading, thickness loss, and recovery after prolonged heavy static loading, surface pile massdensity factor, abrasion resistance, tuft bind, appearance retention, etc. It is possible to predict carpetdurability with the help of these parameters. This review focuses on the durability characteristics ofhandmade wool carpets.

HANDMADE CARPETS AND TYPES OF KNOTS

Generally carpets are divided into two groups on the basis of its manufacturing techniques:handmade and machine-made carpets. Handmade carpets are manufactured in three different ways:knotted, flat weave, and tufting. The manufacturing techniques of hand-knotted carpets have notchanged greatly over centuries because it consists of independent knots and a complex mechanismto tie these knots. There are no machines that can create knots in the same way as human fingersdo. A handmade carpet consists of two parts, the first one is the carpet backing manufactured bywarp and weft threads and the second one is the carpet pile formed by knotting threads. The tex-ture of hand-knotted carpets is formed by the independent knots. Turkish or Ghiordes, Persian orSehna, Tibetan, Spanish, and Kiwi knot structures are used in handmade carpet industry. Amongthese, Turkish and Persian knots are extensively used in handmade carpet sector (Crawshaw 2002;Goswami 2009; Liu et al. 2002; Topalbekiroglu et al. 2005). The Turkish knot is created by wrappingthe tuft around two adjacent warp threads by an angle of 3/2 radians each, as shown in Figure 1.

The Persian knots is created by wrapping the tuft around one warp thread at an angle of 2 radiansand then around another adjacent warp thread at an angle of radians, as depicted in Figure 2.

The Turkish knot appears to be more secure, but the Persian knot is more suitable for finer carpetconstructions. To create fullness in backing, one of a pair of weft may be pulled tightly to straightenit and to make the other weft crimped. This arrangement also changes the levels of alternate warpends, so that a knot may be inclined to the left or the right, depending on which pair of warp ends

FIGURE 1 Turkish knot.

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DURABILITY OF HANDMADE WOOL CARPETS: A REVIEW 401

FIGURE 2 Persian knot.

FIGURE 3 Tibetan knot (Source: Goswami 2009).

FIGURE 4 Spanish knot.

is selected for knotting. It is also known for there to be one, two, or three weft shots between eachrow of tufts, and this can affect the levels of warps and therefore the detailed geometry of the tufts.The Jufti system of knotting, devised to accelerate production, involves looping the pile around fourwarp ends rather than two. It can be applied equally to Turkish or Persian style knots. Clearly, theJufti knot is unsuitable for producing carpets with fine design, but the lack of quality is usuallycompensated by providing a thick pile.

The Tibetan knot incorporates a remarkable combination of Persian and Turkish knots and asimple U-tuft. A steel rod is placed in front of the warp and a piece of pile yarn, which may befairly long depending on the requirements of the design, is looped around two warp ends above therod, over the front of the rod and then around the same two warp ends, but below the rod. At thebeginning of each row, and before each color change, a special, more secure knot is tied which wrapsone of the warp yarns once and the other, twice. When the rod is full, a knife is run along it to cutthe pile; the rod is removed and the pile is beaten down. Tibetan carpets, shown in Figure 3, aretypically thick and heavy.

The Spanish knot is created by wrapping the tuft at 3 radians around one warp thread, asdepicted in Figure 4. This type of knot is rarely used today.

The Kiwi knot, as shown in Figure 5, is created by a total wrapping angle of 3 radians for the tuftaround the two warp threads, in which the tuft wraps one warp thread by an angle of 3/2 radianswhereas an adjacent warp thread is wrapped by an angle of radians by one leg of the tuft and byan angle of /2 radians by the second leg of the tuft.

Hand-knotted carpets demonstrate good dimensional stability because tufts are highly securedand the carpet is densely woven. Thus, carpet backing is not required.

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FIGURE 5 Kiwi knot.

FIGURE 6 The Structure of India knot (Source: Goswami 2009).

A new knot was developed, known as India Knot (Goswami and Bhar 2008). This knot uses thedoup mechanism to permit movement of pile yarn from one side to the other, as shown in Figure 6.

MANUFACTURING TECHNIQUES

Handlooms for carpet manufacturing are either horizontal or vertical. The parts of horizontal loomsare pegged to any convenient depth of ground. The drawback of this type of loom is that thecarpet length is restricted by the reach of the weaver and, therefore, relatively small rugs can bemanufactured on such looms.

The vertical loom, as shown in Figure 7, consists of two posts which are supported with horizontalrollers (one considered as let-off roller and other take-up roller). The let-off roller is holding warpwhich are stretched between it and the take-up roller. When the carpet is manufactured, it is takenup by the take-up roller and the warp is released from the let-off roller. In industries, the loomis generally kept as 1218 feet in height to accommodate the full size of carpet and the warp istensioned accordingly. In such cases, several weavers are working together on the same carpet andthey move from the base of the loom to work on simple gantries as the carpet grows. The Reedis used to maintain a uniform spacing and to prevent entanglement of warp ends. The sheddingmechanism consists of two wooden rods inserted horizontally through the warp. The warp ends passalternately in front of or behind one rod and on the reverse side of the other. By moving one of therods relative to the other, a shed is formed through which weft can be inserted to create the backingstructure. A reverse shed is created by pulling harness yarns linked to the alternate warps that are atthe back (away from the weaver) when the original shed is formed.

A new carpet loom known as cross-bar horizontal has been developed, shown in Figure 8. Theadvantages of this type of loom are better productivity and easier working conditions for the weaverdue to improved loom design (Goswami et al. 2006).

The sequences of operations for manufacturing of hand knotted, hand tufted, and loom-madecarpets (broad loom techniques) were described (Goswami 2009). The process sequences for man-ufacturing of hand-knotted carpets involves prewarping operations, warping on rods, postwarpingoperations, on loom operations, carpet weaving preparatory on loom, operations of knotting on

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FIGURE 7 Vertical loom (Source: Goswami 2013).

FIGURE 8 Cross-bar horizontal hand-knotted carpet loom (Source: Goswami 2009).

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FIGURE 9 (a) & (b): Operational views of hand-tufted carpet (Source: Goswami 2013).

loom, measurement and control of pile height, knot density, waste dimension, etc., binding andinspection, deloom of carpet, carpet washing, and finishing.

To produce hand-tufted carpet, pile yarn is inserted into primary backing fabric with the help oftufting gun, as shown in Figure 9 (a) & (b). The process sequences for manufacturing of hand-tuftedcarpets are: framing of primary fabric, inspection and analysis of backing fabrics (primary,secondary, and tertiary) and yarn used for tufting, maintenance of tufting gun, design tracing overprimary fabric, tufting over primary fabric, application of latex on carpet backing and then dryingof carpets. Chauhan (1997) also documented equipment and processes related to winding, tufting,chemical coating (latex), embossing, packing, and quality control for manufacturing of hand-tuftedcarpets.

Flat weave refers to a carpet weaving technique where knots are not used so that carpet surfacelooks flat. A flat weave carpet is manufactured by interlocking warp (vertical) and weft (horizontal)threads. The warp strands are used as the base and the weft stands are used both as part of thebase and to create the patterns. The weft strands are simply passing through the warp strands. Theoriental flat woven carpet includes kilim, soumak, plain weave, and tapestry weaves. The European

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flat woven carpets include Venetian, Dutch, damask, list, haircloth, and ingrain (double cloth or two-ply, triple cloth or three-ply). Flat woven rugs are generally less expensive than knotted rugs becauseits take less time to weave.

TEXTILE MATERIALS USED IN HANDMADE CARPETS

Wool, silk, polypropylene, acrylic, and nylon are used as pile fibers whereas cotton, jute, polyester,and polypropylene fibers are used in carpet backing. Silk fibers are used as pile yarns in valuablecarpets. Mostly cotton yarns are used as the warp and weft due to their good tenacity and low strain(Moghassem and Gharehaghaji 2008).

The 100% polyester spun yarns are also used in hand-knotted carpets as warp and thin weft(untwisted strands inserted slack in a carpet). Usually 6580% polyester fibers with cotton are usedas the warp yarns. These types of yarns improve strength, elongation, work of rupture and abrasionresistance warp and thin weft yarns. Therefore, the usage of polyester fibers in the warp and thin weftincrease the production and weavers convenience due to reduced yarn breakage. The disadvantageof using polyester fiber in thin weft and warp yarn is the surface deformation of carpets with theapplication of irregular heating. The bending length also increases in the warp and weft directionwhen the percentage of polyester fiber increases in comparison with 100% cotton warp and thin weftyarns (Kamali et al. 2005).

The most common fiber used for producing pile yarn is wool. In the carpet industry, the mixedcoarse and fine wool fibers are used as carpet pile yarns when their resiliency, dyeability, fiber length,number of crimps, vegetable trash percentages, tenacity, elongation, and fineness are compatible.Nowadays, slipe wool fiber is mixed with virgin wool fiber and used as carpet pile yarn (Mirzaliliand Sharzehee 2005).

CARPET DURABILITY

Wilding et al. (1990) reported various causes of appearance loss in tufted pile carpets during itswear. The factors are shading, loss of tuft definition, soiling, staining, fading, loss of pile heightthrough fiber damage, and other factors such as fiber cross-section, optical properties of fibers, carpetconstruction etc. A shaded cut pile carpet shows areas which are lighter or darker than the nearbycarpet pile. This difference is caused by the reflection of light from pile tufts which lie in differentdirections. The meaning of tuft definition in any carpet is the distinctness of individual tufts. Thereasons behind loss of tuft definition are yarn twist loss at tips, tuft partition into single yarns,disorientation near the base of pile through buckling, entangling, flattening or ballooning of yarn,uneven wear in blended piles, and loss of fiber crimp (complete or partial).

The carpet ability for maintaining a clean look depends considerably on three factors: the type ofdirt particles, its easiness to take away, and ability to conceal remaining particles. Staining involveschemical bonding of staining substances to carpet fibers. Fading can be very serious where someareas of a carpets surface are regularly exposed to sunlight. Fading problem can be reduced, tosome extent, by the correct choice of fiber type and dye material. Fiber shortening due to fatigue,fracture, and abrasion is the main cause behind loss of pile thickness. In addition to the direct effecton carpet appearance, it can also create fragments within the pile which contributes further to theloss of aesthetic quality.

Wood (1993) reported various parameters for characterizing the textural properties of new andworn carpets. These parameters are tuft definition, lightness, periodicity, directionality, coarseness,pile coherence, and the presence of localized and extended features for describing carpet pile texture.

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Carnaby and Wood (1989) also reported that factors such as abrasive wear, shedding, pile com-paction, development of surface faults, shading, soiling, color loss, and pattern clarity are affectingcarpet appearance change during wear. Numerous newly installed carpets, mainly with a cut pile,tend to lose apparently large amounts of fiber during the first few weeks on the floor. This effect iscalled shedding or fluffing. All carpets made from spun yarns with natural or man-made fibers tendto shed or lose fiber. This is a characteristic of the yarn and carpet manufacturing method rather thanof the fiber type. Cut pile carpets tend to shed more than loop pile carpets because cutting the yarnproduces a certain amount of short fiber at the ends of the tufts which are not held in the backing.Usually one or both ends of the fiber are held in loop pile carpets. For the same reason, high pilecarpets shed more than carpets with a short pile. Higher levels of twist in a yarn tend to reduceshedding. Denser carpets also shed less as expected than openly constructed carpets.

MECHANICS OF CARPET DURABILITY

Carpet durability can be divided into mechanical and optical aspects. Mechanical properties are usedto explain walking comfort and abrasion-related wear and optical properties are used to explaincarpet appearance or aesthetics. Carnaby and Wood (1989) reported that the mechanical propertiesof carpet durability are classified into resilience (compression and recovery behavior of each pile),inelastic mechanism (flattening), and fatigue mechanism (abrasive pile loss). Resilience is the abilityof carpet pile to return to its original state after deformation. The reasons behind the carpet flatteningare frictional slippage between fibers inside yarns through yarn bending and between yarns as thepile yarns slip past each other and viscoelastic properties of pile yarns. There is production and lossof short fiber segments, which are not anchored to the carpet backing. These short fiber segmentsoccur when a fiber ruptures along its length. Fatigue mechanism in wool fibers is preceded by a seriesthat consists initially cuticle failure, followed by fibrillation loss along the cell-wall membranes andfinally fracture in the fibrillated region.

Dayiary et al. (2009) investigated pile yarn actions under a compressive load and they presenteda pile bend mechanism as follows. There are triple processes in cut-pile carpet deformation. In thefirst process, load is growing followed by deflection of pile while a bend occurs along its length(Figure 10). During second process, the pile changes from a bend shape to two sectional shapesincluding a straight part in top and a curved part in base (Figure 11). In the third process, the twosectional forms bend further leaning towards the base (Figure 12).

In optical properties of carpet durability, the appearance of a new carpet, and the changes through-out wear is observed. It depends on the way in which light interacts with the pile fibers. Light is eitherabsorbed or reflected by the carpet surface. Light transmitted by one fiber may be reflected, transmit-ted or absorbed by the next fiber until it is either totally absorbed or emerges outside as transmittedlight. When carpet absorbs more light, then it loses brightness. The effect of the reflected light on

FIGURE 10 Beginning of load: (a) New carpet. (b) Worn carpet.

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FIGURE 11 Pile deflection.

FIGURE 12 Final location (deformation in jamming mode).

carpet lustre depends on how it is reflected. It will be either diffusely or specularly reflected. In thecase of diffused light, a low level of light is returning in haphazard orders. Specular light is reflectedfrom fiber surfaces either externally or internally. When it is reflected from a plane face, the lightwill emerge as lustre. The size of the lustre point will depend on the size of the plane area that issloping to reflect incoming light toward the viewer (Werny 1985).

MEASUREMENT OF CARPET DURABILITY

Carpet durability measurements are based on wear performance testing, which measures the numberof steps to wear a carpet to its backing, the rate of thickness loss or weight change. Long-termdurability is certainly an essential property of a carpet. However, its appearance can be considerablychanged so that it loses its visual appeal and probably will be replaced regardless of its residualmechanical properties. In the carpet industry, it is frequently acknowledged that carpet does not wearout, it uglies out. It is, therefore, not illogical to assume that a carpets life is mostly determinedby its appearance retention rather than its long-term durability.

APPEARANCE RETENTION

There are subjective or objective techniques to assess carpet appearance retention by inspectingtextural changes between a control and a worn sample.

Subjective MethodIn this method a carpet or its picture is compared with standard samples or photographs to identify agrade standing for a class of appearance. A subjective method for testing service wear of textile floorcoverings is guided by ES ISO 9405 (2013). This International Standard describes the procedures for

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assessing the overall change in appearance of textile floor coverings caused by Vettermann drum andhexapod tumbler testers in accordance with ISO 10361 or other appropriate methods. The changein appearance of a specimen after a process of fatiguing is assessed by visual comparison withstandard digital image scales. The dominant factors (structure, roughness, color, and/or pattern) ofthe change are observed and recorded. At least three assessors grade the specimens independently.If the difference between the individual ratings within an assessing team is greater than one grade,then the number of assessors is increased by two. Overall change in appearance can be graded fromreference scales

Miller (2002) proposed quad analysis procedure to evaluate subjective properties. This is a com-petent way to carry out paired comparisons for quantifying such properties. He also introduced anexpansion of quad analysis to grade order the subjective properties of larger datasets by quad foldingof one quad design into another.

There is immense value of subjective evaluations on the whole description of textile products,but this is time-consuming, boring, complex, and costly. The responses are also highly divergent,so a large number of tests are usually essential to attain an acceptable level of statistical reliability(Slater 1997).

Due to above disadvantages of subjective evaluations, trustworthy instrumental methods havebeen developed to evaluate carpet appearance, which have vital implications for product characteri-zation and quality control.

Objective MethodThere are several instrumental methods to measure carpet appearance characteristics objectively.These are listed below:

Photography Microscopy Densitometry Goniophotometry Glass bead filling Photometer Image analysis

PhotographyWilding et al. (1990) inspected the control and worn carpet samples visually by photography meth-ods. A Nikon FE camera with a 55-mm macro lens was used for photographic evidence of carpetface exterior. Macro photography was also used for examining tuft profiles obtained by removingsections from the carpets.

MicroscopyWilding et al. (1990) used two types of microscopy namely, optical microscopy and scanning elec-tron microscopy (SEM) to examine control and worn carpet samples.The samples were initiallyinvestigated by an Olympus SZ stereo-zoom microscope at low magnification for finding out thegeneral appearance and array of the tufts within the pile and then the pile yarns and fibers wereviewed using an Olympus BHSP polarizing microscope at higher magnifications. Finally, individ-ual tufts were detached from carpet samples. The tufts were severed as close as possible to thebacking fabric and identification of tip and bottom part of each tuft was carefully preserved. Eachtuft was investigated to evaluate changes in yarn twist and tuft integrity at medium magnification

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besides checking the crimp loss and damage of fibers at higher magnification. SEM investigationsof detached tufts were also done with an ISI 100 A microscope. The tufts were removed in a similarmanner as for optical microscopy.

DensitometryMukhopadhyay et al. (1993) described densitometry technique to investigate carpet faces texturalchanges due to wear. A computer controlled densitometer was used for this experiment. Unfilteredwhite (tungsten filament) light was formed into rays of rectangular cross-section, by means ofcollimating optics. This light scans the photographic negative under inspection. Output from theinstrument is processed for giving optical density variation. An adjustable slit controls the extentof the scanned area and resolution. A dark spot on the negative corresponds to a light spot on thecarpet. The darker areas on the carpet occur in regions between tufts, whereas the tuft tips are lighterareas. Therefore, each major peak on the graph represents a tuft. The distance between two mini-mum points (valleys) on the densitometer chart corresponds to the distance between two dark areaswith a bright area (or tuft) between them.

GoniophotometryJose et al. (1988) used a recording goniophotometer built by HunterLab to obtain reflectance read-ings on the actual carpet samples. A goniophotometer is used to measure the light reflectance ofcarpet surfaces by using a fixed angle of incidence and a varying angle of detection. This methodis more comprehensive than conventional reflectometers, colorimeters, and gloss or lustre meters,all of which fix the angular geometry of light source versus viewing detector. In actual condition,carpet appearance is judged over a whole range of viewing angles. The phenomenon of shading incut pile carpet is an obvious and important illustration of differences in carpet appearance caused bythe relative viewing and pile orientation angles.

Glass Bead FillingThis technique was earlier reported for the measurements of specific volume of yarns (Carnaby1974). Same technique was adapted to measure carpet texture during wear (Carnaby and Thomas1978). The degree of pile openness or closeness (the amount of void space within the pile), animportant carpet textural property, was considered. The bead fluid frictional resistance was used todetain the beads in the carpet voids between the individual tufts for testing purpose. In this regard,carpet was placed on an even plane and then excess of the beads were poured to overfill the voids.Then the carpet was sloped gradually until all the beads above the plane of the pile top drop. Theslope angle was not too steep so that the beads between the tufts were supported and stay put in thecarpet. The mass of beads retained by the carpet can be calculated by weighing the carpet beforeand after this process. This gives an assessment of the volume of the spaces between the tufts. Glassbeads graded to 40 meshes ( 370 m) had been found suitable for this testing. This technique isnot suitable for bend yarns or for carpets with little tuft densities as the gaps between the tufts arevery large, therefore there is too much beads loss during sloping of the carpets. The disadvantage ofthis method is its sensitivity.

PhotometerLamb et al. (1993) developed a simple and inexpensive instrument that directs a beam of light ontothe carpet face. The intensity of the scattered light was examined by a photometer. Mechanicalstresses due to traffic generate more curved filaments in the tufts, therefore light reflection will be

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greater than before. A halogen lamp with fiber optics was used to throw powerful light rays ontothe carpet surface. The incidence and detection angles were varied independently over a range of180o. The carpet specimen was set on a turntable, which was rotated constantly during investigation.Therefore, the light intensity sensed by the detector fluctuates as the carpet tufts are presented to theray at diverse angles.

Image AnalysisAmong all objective methods for examining carpet appearance, image analysis technique has shownimmense prospects. Parameters from digitized images can be calculated using a powerful PC withsuitable algorithms. These image parameters are capable to enumerate carpet appearance change(Steenlandt et al. 1996).

Ulcay and Altun (2006) reviewed various image processing techniques for measuring carpetappearance loss with use. They also classified various image operations into the following threecategories:

1. Point operations: These operations are employed on individual pixel and using the informa-tion connected with that pixel only.

2. Neighborhood operations: These operations alter the value of a pixel taking into relation ofthe neighborhood pixels.

3. Global operations: These operations alter the value of all image pixels.

Xu (1994) also reviewed various carpet characteristics and applied image analysis methods aspresented in Table 1.

The common approach in digital image processing is to recognize parameters that consistentlydistinguish dissimilar carpet textures due to different levels of wear. However, no single param-eter is competent to describe the appearance distinctiveness totally for any type of carpet (Wood1993).

Presley (1997) discussed the changes caused in texture periodicity of worn carpets using mor-phological covariance and compared the results with subjective human evaluation. The influences ofstructural variables, i.e., pile weight, ply twist, linear density, and wear level on carpet performancewere measured using this technique. Jose et al. (1986) analyzed real carpet samples and theirphotographs with computerized image analysis technique. The differences of carpet appearancewere assessed in terms of textural changes caused by pile/tuft definition or coherence. Appearancechanges, tuft spacing differences, and related carpet properties were connected with gray levelhistograms. As the spacing between tufts decreases, the frequency of pixels at the darker gray levelsdecreases whereas the frequencies of pixels at the lighter gray levels increase. This effect is strongerin actual carpet samples than their photographs. Wang and Wood (1994) reported a new method toinvestigate carpet texture changes with Fourier power spectra of control and worn carpets images.The algorithm was tested experimentally using a range of carpet types, together with those that havenot been easy to compute with other image analysis algorithms. The new measure agrees well withthe subjective sense of tuft texture change. Xu (1997) applied fractal dimension method for measur-ing carpets surface roughness to examine carpets appearance loss due to mechanical wear. Presley(1997) examined the texture of various nylon cut-pile carpets due to mechanical wear with covari-ance and co-occurrence techniques of image analysis and compared the results with the subjectivemethod.

Wu et al. (1990) confirmed that color imaging with digital processing and enhancement tech-niques offers a great possibility to evaluate appearance changes in carpet objectively. This can beused to both new and worn carpets. They also confirmed the importance of tuft geometry dimensions

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as a reasonable successor to bulk histogram data. When the carpet was worn then definite trends intuft size distribution was seen as a result of changed appearance.

THICKNESS LOSS

Numerous types of forces such as axial compression, bending, flattening, and extension are acting onpiles throughout carpet wear. These forces are mostly produced by dynamic loading such as walkingor static loading by furniture, therefore carpet thickness will be reduced. One of the most essentialcarpet quality attributes is thickness loss through static and dynamic loads. Lower thickness lossimplies superior resilience and durability. The carpets compression properties can be calculated byfollowing test methods.

Carpet Thickness, Compression, and Recovery CharacteristicsBS 4098 (1975) specifies a method for measuring these carpet parameters as depicted in Figure 13.

Here t2 is the initial thickness at 2 kPa pressure (point A).t200 is the compressed thickness at 200 kPa pressure (point B).tr is the recovered thickness at 2 kPa pressure after loading to 200 kPa pressure (point E). Compression (t2 - t200): The change in thickness of the textile floor covering when the pressure

is increased from 2 kPa to 200 kPa. Work of Compression: The work done on the textile floor covering when the pressure is

increased from 2 kPa to 200 kPa, i.e. the area under the load-compression curves e.g. areaABC.

Percentage Thickness Recovery (100 tr/ t2):The thickness to which the textile floor coveringrecovers when the pressure is diminished from 200 kPa to 2 kPa, expressed as a percentage ofthe initial thickness.

Percentage Compression Recovery: The change in thickness when the pressure is diminishedfrom 200 kPa to 2 kPa, expressed as a percentage of the compression. Numerically it isexpressed as [(tr - t200)/ (t2 - t200)] 100.

Percentage Work Recovery: This is estimated by the ratio of the work of recovery to the workof compression, e.g., 100 area CDE/area ABC.

FIGURE 13 Typical thickness-pressure curve for textile floor coverings (Source: BS 4098 1975).

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Determination of Thickness Loss Under Dynamic LoadingIn this method, a weight-piece by means of two steel feet on its base is dropped on to the specimenrepetitively. Thus the specimen is subjected to cyclic loading-unloading action. The specimen isalso traversed gradually so that vertical shearing forces created by edges of the feet operate on thenecessary area of the specimen. The thickness of the specimen is calculated before and subsequentto treatment (BS/ISO 2094 1999).

Determination of Thickness Loss and Recovery After Prolonged Heavy Static LoadingIn this method, a test specimen is subjected to a prolonged heavy static loading treatment and thick-ness is calculated before loading and after various recovery periods. Static loading machine is ableto apply a pressure of 700 kPa (ISO 3416 1986).

TUFT-WITHDRAWAL FORCE

One aspect of durability is the risk of removal of cut-pile tufts or laddering of loop-pile construc-tions in wear. Tuft bind or ``pile-withdrawal strength is commonly measured on finished carpets(Harrison 2003). Tuft bind (or the tuft withdrawal force) is related to the manufacturing of the carpetand it has an effect on durability. Pulling out of tufts creates holes in the carpet and consequentlythe carpet may wear out prematurely. In loop pile carpets, ``laddering`` can reveal a line of missingloops. The WIRA Tuft Withdrawal Tensometer measures the requisite force for pulling out a singletuft or loop of pile from a carpet, i.e., the binding force between the carpet pile and backing. Theinstrument is used either on small samples of carpet in the laboratory or in chosen positions on bigpieces. The carpet sample is detained downward with a steel plate. A couple of surgical tongs areclamped to one end of the tuft to be tested or a hook threaded through one loop. The tongs or hookis connected to a dial balance which is raised at stable speed by a small electric motor. Therefore,tension on the tuft or loop is increased and the pointer on the balance indicates the maximum forcerequired for pulling out of tuft (BS 5229 1975). It is suggested by IWS test method 202 the mini-mum tuft-withdrawal force for woven carpets, tufted carpets (cut pile) and tufted carpets (loop pile)should be 3.5 N, 10.0 N, and 20.0 N, respectively (IWS/TM-202 2001).

SURFACE PILE MASS DENSITY

A durability parameter called surface pile mass density (P2/t) enables to predict the wear perfor-mance of wool carpets where P is the surface pile mass (in g/m2) and t is the pile thickness in mm.Usually higher the value of P2/t, larger the projected wear life of a wool carpet. This was validatedby the International Wool Secretariat (IWS) through wide-ranging floor trials and laboratory testing.IWS TM-234 and 142 specifies methods for measuring surface pile weight (per unit area) and pilethickness of textile floor coverings. Another predictor of wear life that has been used is P2/t dividedby a fiber factor called wear index (Crawshaw 2002). Fiber factors used in EN 1307 are given inTable 2. Fiber factor is used for calculating wear index of any carpet taking consideration of fiber asfactor. For example, same values of P2/t for Nylon BCF and Wool carpets; the wear index of NylonBCF carpet is 1.9 times greater than Wool carpets due to difference in values of fiber factor.

WIRA Abrasion TesterWIRA abrasion test is also extensively used for wool carpet durability measurement. Carpet samplesin the shape of small discs are rubbed against an abrasive fabric for a specified number of revolutions.

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TABLE 2Fiber factor of various pile fibers (Source: Crawshaw 2002)

Pile fiber Fiber factor Pile fiber Fiber factor

Nylon BCF 1.0 Wool 1.9Nylon staple 1.2 Cotton 2.0Polypropylene BCF 1.2 Modified viscose staple 2.2Polypropylene staple 1.4 Modacrylic staple 2.4Polyester staple 1.6 Blends Pro rataAcrylic staple 1.8

The earlier edition of the test used to determine the number of machine revolutions required by thecarpet sample to attain its end point where the backing is just noticeable as the tufts abrade away.At the present edition, the sample is abraded up to 5000 revolutions and the resulting loss of fiberfrom the pile is calculated (IWS/TM-283 2000).

A weight loss of approximately 55 mg per 1000 revolutions is considered to be acceptable byWools of New Zealand based on industrial experience whereas higher than 70 mg/1000 revolutionsindicates major fiber damage.

FACTORS AFFECTING CARPET DURABILITY

Wool Fiber ParametersInce and Ryder (1984) investigated the effect of various wool fiber properties formed by genetic-selection experiments in which fleece properties could be altered by breeding. The performanceproperties of yarns and carpets manufactured from these wools were examined. They reported thatfiber shedding from the carpet in the early stages of wear was due to higher percentage of short fiberin wools. The coarser and more medullated fibers manufactured a weaker carpet yarn. Improvedset yarns were manufactured from the bulky or crimpy wools and subsequently carpets having idealappearance were manufactured from these yarns. There was a significant relationship between fiberdiameter and carpet abrasion. The abrasion behavior (number of revolutions required to reach carpetbacking) found for carpets made from finer wools (nonmedullated New Zealand Romney Wool)showed better results than carpets made from coarser wools (Drysdale and British Mountain wools).They also reported that medullated wools enhanced carpet-compression characteristics. The colorof wool does not influence the carpet performance. However, color of wool plays a significant rolewhile the fibers are selected. It has been reported that an increase in percentage of slipe wool causesan increase in compression and matting and also reduction in elastic recovery of pile yarn (Mirzaliliand Sharzehee 2005; Moghassem and Gharehaghaji 2008). Gupta et al. (1998) also studied the effectof fiber diameter and medullation on mechanical processing and quality performance of carpets.They found a positive correlation among bulk of carpets with fiber diameter and medullation. Theyalso reported that more volume per unit mass of carpet produced if fibers have higher medullationand large diameter. The collective result of fiber diameter and medullation percentage had positiveinfluence on compressibility. The carpet resiliency significantly increases but thickness loss afterdynamic loading reduces with increase in average fiber diameter and medullated fiber content inthe blends. Shakyawar et al. (2006) evaluated 62 different carpet samples for Carpet AestheticValue (CAV) and Carpet Hand Value (CHV) subjectively by 10 different judges. They investigatedthe influence of fiber mix, spinning system, yarn number, constructional parameters, and finishingtreatment on CHV of carpet. They reported that carpets made from synthetic fibers were ranked

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very poor as compared to wool carpets because synthetic fibers do not have desirable attributes forcarpet construction.

Woollen Yarn ParametersNilgn et al. (2012) reported that the compressed thickness of carpets manufactured from thickeryarn is on higher side in comparison to that of carpet manufactured from finer yarn. This is due to thebulkier construction and lesser carpet compressibility in case of thicker yarn. Onions et al. (1967)reported that increase in yarn twist results in a lesser compression index because high-twist yarnsremain thick in elevated pressures. Arora et al. (1999) studied the influence of tuft constitution onthe functional and aesthetic properties of hand-woven carpets. They observed that carpets of variableweight per unit area obtained by changing the tuft constitution, pile density, and pile height. Theyalso reported that pile density increases by increasing the number of folds and plies in the yarn,which, in order, increases the resiliency and decreases the compressibility. Tuft withdrawal forceof carpet increases by increase in the number of plies and folds in yarn because of the improvedcohesion between the component yarns. There was less abrasion loss in carpets of more regular pilesurface (when using single ply yarn) than a reduced amount of regular pile surface (when usingplied yarn). Carpets manufactured from the plied yarn were favored from the handle point of viewbut carpet appearance was judged better produced from single ply yarn.

The carpet manufactured from Dref II spun yarn was not favored by judges than carpets producedfrom woollen spun yarns because of the presence of wrapper fibers in Dref spun yarns which dimin-ish pile compressibility. The carpet possesses higher CHV manufactured from finer yarns than fromcoarser yarn because of more number of piles per unit area which improves the resiliency and appealof the carpet (Shakyawar et al. 2006).

Handmade Carpet Constructional ParametersLiu et al. (2002) compared tuft withdrawal forces of hand-knotted carpets manufactured by fivetypes of knots. Tuft-withdrawal force varies with the style, complexity, and total tuft wrapped angleabout the warp threads by the knots. They found that tuft-withdrawal force of Persian and Turkishknots (``self-locking`` structures having more tuft security) are higher than that of Spanish knot(simpler structure).

Goswami and Bhar (2008) reported the geometry of various binding mechanism for pile yarns.A change of path that increases friction through varied geometric wrapping angles can result inincreased tuft withdrawal force values.

Pile height and knot density are two most important constructional parameters of handmade car-pet which affect the carpet durability. Research indicated that the extent of carpet thickness loss inthe initial months of use was more in comparison to that occurred in later months. A linear relation-ship was found between the thickness and logarithm of the number of impacts on the carpet or thenumber of people walking over the carpet (Cusick and Dawber 1964; Ince and Ryder 1984; Noonanet al. 1975; Onions 1967). However, a dense carpet and short pile height will give less compressionand less loss of thickness after recovery. Loss of pile height after recovery increased with increase inpile height (Cusick and Dawber 1964; Onions 1967). Studies show that the variation in pile heightand density per unit area result in a change in resilience and elastic recovery of the pile (Dunlop andJie 1989; Mirzalili and Sharzehee 2005). An increase in pile height improves the compressibilityof the carpet, but there is no observable change in its elastic recovery. Having more pile densityimproves the piles elastic recovery, reduces the thickness variation of the carpet and increases thestability factor.

Moghassem and Gharehaghaji (2008) investigated the effects of knot density, pile height andpercentage of slipe wool on the performance of hand woven carpet. They found that compression

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and matting of the pile yarn decreased and its elastic recovery increased with the increase in knotdensity. Increase in pile height caused an increase in the degree of variation for carpet samples.

CHV increases with the increase in knottage upto 2300 knots/dm2 and increase in pile height upto12 mm then starts decreasing. Carpet washing is an important finishing process. The objectives ofthis process are to remove loose fiber, any surface debris picked off or singeing residues. Generally3-types of washing practices used in carpet industries are normal washing, herbal washing, andantique washing. Normal washing requires heavy rubbing and uses chemicals such as bleachingpowder, caustic soda, acetic acid, and softening paste. Herbal washing uses different natural productslike henna and ritha. Antique washing gives a particular shine to the carpet similar to many years offoot traffic. Normal wash carpets showed higher CHV than the herbal and antique wash due to littlepile damage (Shakyawar et al. 2006).

Shakyawar et al. (2008) developed software for predicting abrasion loss and CHV of hand-knotted carpet with the help of C language. Fiber characteristics such as average fiber diameterand medullation, construction parameters such as pile height, pile density, and carpet thicknessare required to predict these carpet performance parameters. They also reported that abrasion lossmainly depends on fiber diameter and number of medullated fibers present in the yarn but CHVdepends on carpet constructional parameters such as pile height, pile density, and carpet thickness.The software can predict abrasion loss and CHV within the range of error value.

LIMITATION OF MEASURING AND PREDICTING DURABILITY: SCOPEFOR FURTHER RESEARCH

Carpets should have high durability during use as it is costly home textile materials. There havebeen difficulties in correlating laboratory tests with real carpets durability performance as well asdissimilarity regarding key durability characteristic. Various carpet durability tests have been devel-oped (e.g., surface pile mass density, WIRA abrasion tester, tuft bind, etc.) which illustrate very poorcorrelation with real floor wear when diverse types of pile fibers are compared.

Ince and Ryder (1984) determined durability factor (D) for carpets made from experimentalwools. Durability factor classifies wool carpets in accordance with their end-uses. This is an indexand its not has any unit. The equation to calculate the durability factor is as follows:

D = 19 R + 1.4 P2/t

1000,

where R = number of rubs on the WIRA Carpet Abrasion Machine and P2/t = Surface pile massdensity.

In the above durability factors, researchers had included only two durability parameters, i.e.,WIRA abrasion-resistance test and surface pile mass density. Further research is needed to developa model by including more parameters so that the durability of handmade carpets can be predictedon the basis of their uses.

CONCLUSION

The handmade carpet segment has immense scope of standardizing the product quality. The wearlife of handmade carpets depends mainly on its manufacturing technique and material and construc-tion parameters. Fiber properties, yarn parameters, knots/tufts density, pile height etc. are some ofthe parameters which largely influence the durability and cost of the handmade carpet. With the

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advent of modern optical and image processing technologies, it has become possible to appraise theappearance change of carpets objectively. Although a large number of research work has been pub-lished in the area of carpet durability measurement, laboratory test methods often fail to correlatewell with the actual use trials. Research work to correlate the durability of handmade carpets withits structural and materials parameters is therefore needed.

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ABSTRACTHANDMADE CARPETS AND TYPES OF KNOTSMANUFACTURING TECHNIQUESTEXTILE MATERIALS USED IN HANDMADE CARPETSCARPET DURABILITYMECHANICS OF CARPET DURABILITYMEASUREMENT OF CARPET DURABILITYAPPEARANCE RETENTIONSubjective MethodObjective MethodPhotographyMicroscopyDensitometryGoniophotometryGlass Bead FillingPhotometerImage Analysis

THICKNESS LOSSCarpet Thickness, Compression, and Recovery CharacteristicsDetermination of Thickness Loss Under Dynamic LoadingDetermination of Thickness Loss and Recovery After Prolonged Heavy Static Loading

TUFT-WITHDRAWAL FORCESURFACE PILE MASS DENSITYWIRA Abrasion Tester

FACTORS AFFECTING CARPET DURABILITYWool Fiber ParametersWoollen Yarn ParametersHandmade Carpet Constructional Parameters

LIMITATION OF MEASURING AND PREDICTING DURABILITY: SCOPE FOR FURTHER RESEARCHCONCLUSIONREFERENCES

Durability of Hand Wool Carpets

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