Mercerization of Ramie Part I Copy

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http://trj.sagepub.com/content/59/8/478The online version of this article can be found at:

DOI: 10.1177/004051758905900807

1989 59: 478Textile Research JournalLenore Cheek and Laura Roussel

Mechanical, and Accessibility CharacteristicsMercerization of Ramie: Comparisons with Flax and Cotton : Part I: Effects on Physical,

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Mercerization of Ramie: Comparisons with Flax and Cotton

Part I: Effects on Physical, Mechanical, andAccessibility Characteristics1

LENORE CHEEKAND LAURA ROUSSEL

LouisianaAgricultural Experiment Station, Louisiana State UniversityAgricultural Center,Baton Rouge, Louisiana 70803, U.S.A.

ABSTRACT

The effects ofslack and tension mercerization on selected physical and accessibilitycharacteristics of ramie were compared to the effects of the same treatments on flaxand cotton. Slack mercerization of ramie and flax resulted in considerable losses inyam strength, while tension mercerization resulted in increased strength. Increasedyarn strength was seen in both slack and tension mercerized cotton yarn. The effectsof mercerization on ramie fiber cross-sectional parameters included increases in pe-rimeter and area with no change in circularity. Little apparent change in fiber size or

shape occurred in flax fibers, while the primary effect on cotton was increased circu-larity. Increases in fiber accessibility were seen in all mercerized fibers, but the mag-nitude of change in flax was approximately half of that in ramie and cotton. For allthree fibers, the extent of swelling and modification in accessibility was greater underslack conditions.

With the recent revival ofinterest in ramie as a com-mercial apparel fiber, research to evaluate the responseof this fiber to conventional preparation and dyeingoperations is desirable. Several discussions on the

preparation and processing of ramie fiber, includingdegumming and spinning developments, have beenpresented [ 3, 4, 6, 8 ] , but little work has been reportedon the preparation and dyeing oframie yarns and fab-rics since the early work of Jefferson [7] ] and Marshall[9~]. Mercerization is one ofthe most commonly usedpreparation steps in the dyeing and finishing ofcottonyarns and fabrics. While several studies have focusedon the effect ofsodium hydroxide on the fine structureof ramie, there has been limited attention paid to themodification of physical and dyeability characteristics

of mercerized ramie fiber and yarn.Batra [2] ] presented an overview ofthe physical and

chemical characteristics of ramie.Among the naturalcellulosic fibers, ramie has the highest degrees of bothpolymerization and birefringence. The high strengthof ramie is attributed to its highly ordered structure.Purified ramie fiber is composed of a single cellulosewall with an S-spiral 6 to the axis [ 14, 17 ] . Using x-ray methods, workers have estimated the degree ofcrystallinity ofthe native cellulose in ramie at 70-74%.Ray and co-workers studied the fine structure of de-

gummed ramie and demonstrated that the crystallineareas increase in lateral size and decrease in crystalliteorientation as gum is removed from the intercrystallineareas [ 12 ] .

X-ray studies have shown that mercerization of ra-mie results in a complete conversion from cellulose Ito cellulose II and in a decrease in the degree of crys-tallinity to 50% [2]. Warwicker compared the effectsof mercerization on fibers of ramie and cotton by ex-

amining the changes in x-ray orientation that occurwhen each fiber is slack swollen in sodium hydroxide.He found that whereas the orientation ofthe crystallitesin cotton is increased, that of ramie is decreased. Heattributed this decrease to intracrystalline lateral swell-ing unrestrained by a restrictive primary wall f 171.

Since ramie is often blended with cotton to improveits performance and hand, and since ramie and flaxare often used in the same fabrications, informationon specific changes that occur to each ofthe three fibersas a result of common conditions of mercerizationshould be of interest. In this paper, we report on themodifications in fiber cross-sectional size and shape,fiber accessibility, and physical and mechanical yamcharacteristics that occur as a result of slack and tensionmercerization.

Materials and Methods

Premeasured hanks ofdegummed, bleached ramie

yarn ( 16/2), bleached flax yarn (20/2), and bleached

LouisianaAgricultural Experiment Station manuscript number

88-25-2341.

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cotton yarn ( 16 / 2 ), obtained from a handweavingsupply house, were slack or tension mercerized in 20%sodium hydroxide. The yarns were prewet in 0.05%Triton QS-44 (Rohm & Haas), immersed in NaGHfor five minutes, rinsed, neutralized in 2% acetic acid,and rinsed again with deionized water until the rinse

water was neutral. Slack treatment involved immersionof the hanks in NaOH solution without restraint. For

the tension treatment, the yarn was wrapped arounda stainless steel frame and held at constant length dur-ing treatment. The yarn was removed from the frameafter the final rinsing, and the hanks were allowed toair dry without restraint.Yam tex was calculated following the procedure of

ASTM D 1059-87 [ 1 ] . Yam strength and elongationwere measured according toASTM D 2256-80, usinga constant rate-of-elongation tensile testing machine

(Instron model 4201 ). In preparation for fiber cross-sectional measurement, yarns were sectioned using a

Hardy hand microtome. Photomicrographs were takenat a magnification of 250X and enlarged to IOOOX.Following the procedure described by Hebert el al. [ 5 ],an electronic digitizer (Numonics model 1224) wasused to determine fiber perimeter and area. Fiber cir-

cularity was calculated using the formula suggested bySkau [ 15 ] . The extent of mercerization was evaluated

by calculating barium activity numbers according to .AATCC 89-1980 [ 16 ] . Modifications in moisture re-

gain were determined according toASTMD

2654-76.A statistical analysis of differences occurring as afunction of treatment condition within fiber groupswas done using a one-way analysis ofvariance followedby a Newman-Keuls range test for comparison of in-

dividual pairs of means.

Results and Discussion

EFFECTSON YARN PROPERTIES

Because the yarns used in this study came from dif-ferent commercial sources and because

ramie, flaxand

cotton are spun on different systems, there was varia-tion between them in tex and yarn twist. Therefore,the evaluation of yarn dimensional properties as afunction of the two mercerization treatments across

fibers is relative rather than absolute.

As expected, slack mercerization resulted in consid-

erable yarn shrinkage, while tension mercerizationproduced minimal changes in yarn length. Slack mer-cerized yarns of all three fibers appeared thickened and

kinky, with no increase in luster, but the tension mer-cerized yams were visibly more lustrous than the con-trol yarns.After the slack treated yarns had been gentlyextended to their full length and released ten times.much of the elasticity was released and the yarn was

sufficiently stabilized in length to permit remeasure-ment. Yarn shrinkage was 32% for ramie, 27% for flax,and 20% for cotton. The revised measurements oftwist

and tex for the mercerized yarns are shown in TableI. Since the two-ply experimental yarns were not sep-arated into distinct strands for measurement, changesin yarn length reflect the effect ofunrestrained swellingon increases in the twist of the ply as well as contrac-tions in length ofthe fibers themselves.

Table I shows the tensile behavior ofthe control and

treated yarns. Since tenacity depends on linear density,the strength data have been calculated on the basis ofthe tex of each control and treated yarn and therefore

take into consideration differences in initial yarn tex

between the experimental yarns as well as the shrinkagethat occurred during the mercerization treatment.Afterslack mercerization, ramie and flax yarns showed 45%

and 49% decreases in strength, while cotton showed a26% increase in strength.After tension mefcerization,all three yarns showed increased strength, but the pro-portional increase was considerably lower for ramie

(7%) than for flax (22%) or cotton (38%).All yamsincreased in elongation after slack mercerization andmaintained their original degree of elongation aftertension treatment. The yarn elongation data have not

been adjusted for change in yarn tex, since elongation

TABLE I. Effect of mercerization on the characteristics of ramie. flax. and cotton yarns.

.Tex of treated yarns retlects yarn shrinkage.

bYam elongation data not adjusted for changes in tex.

cns No significant differenCt

from control (p s 0.05).



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is not affected by linear density. Changes in slack mer-cerized yarn elongation are a reflection of yam com-

paction.The higher initial strength of ramie yarn is areflec-

tion of the highly ordered structure of the fiber,. The

strength loss here for slack mercerized ramie yarn was

similar to that noted earlier by Jefferson [ 7 ] . The de-crease in slack mercerized ramie fiber strength has beenattributed to decreases in crystalline orientation as afunction of the unrestrained lateral swelling ofthe fibrils

[ 11, 13 ] . Warwicker noted that the differential responseof ramie and cotton to slack swelling with sodium hy-droxide is a function of fiber morphology rather than

differences in fine structure [ 17 ] . He demonstrated that

ramie has a simple structure with no primary wall toexert restraint on lateral swelling and consequent dis-orientation ofthe swollen fibril$. Since the morphology

of ramie and flax are similar [ 14 ], a similar explanationshould apply to the loss of strength we observed inslack mercerized flax. Under tension treatment, the

lateral movement ofthe polymer chains in ramie and

flax is limited, so strength losses do not occur as in theslack treatment. The increase in cotton strength underboth slack and tension swelling is well documentedand has been attributed to the release of internal strains

and to the orienting pressure exerted on the plasticizedswollen fiber by the restraining primary wall and S,layer oriented transverse to the axis [ 10, 11, 17 ] .

EFFECTS ON FIBER SWELLING

SEM photomicrographs showed unmercerized ramieas flat, broad fibers with occasional twists along their

length. The fiber surface appeared relatively smoothand uniform, except for the presence of longitudinalfissures in the cell wall. Traverse cross-striations, similarto those in flax, were visible along the length of thefiber (Figure I a).A cross-sectional view oframie yamshowed the individual fiber cells as rounded, elongated

polygons with pronounced fissures in the cell wall.There was considerable variation in fiber size and cell

wall thickness (Figure 2a ) .

After slack mercerization, ramie fibers appearedtwisted; after tension mercerization no twist was evi-dent. Photomicrographs ofthe surface ofswollen mer-cerized ramie fibers are shown in Figures 1 b and c. Thetraverse striations appeared somewhat less pronounced.Figures 2b and c illustrate the effect of mercerizationon the cross-sectional shape of ramie fibers. Most of

the fibers in the slack mercerized sample appeared

swollen. The tension mercerized samples showed lessuniform swelling: some ofthe fibers appeared swollen,while many did not.

FIGURE l. Photomicrographs of longitudinal view of ramie fibersshowing effects of slack and tension mercerization (750X): ( A ) con-trol, ( B ) slack mercerized, (C) tension mercerized.

The mean cross-sectional parameters of control and

mercerized ramie fibers are shown in Table II. Theperimeter measurements of untreated ramie fibers

ranged from 50-90 gm; fiber areas ranged from 126-470 ~m; circularity values ranged from 0.49-0.95.Slack mercerization resulted in mean increases of 31 %

in fiber perimeter and 73% in fiber area, with no changein circularity. This overall increase in fiber size, withno modification in fiber shape, indicated that an out-ward expansion of the fiber, with little restraining forceexerted by the outer fiber wall, had occurred. Tensionmercerization produced no significant increase in fiber

perimeter and a 40% increase in fiber area.As with theslack treated fibers, there was no significant change in

circularity.



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FIGURE 2. Photomicrographs of cross sections oframie yarn show-ing effects of slack and tension mercerization ( 113x ): (A) control,(B) slack mercerized, (C) tension mercerized.

FiGURE 3. Photomicrographs of cross sections of flax yam shown

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