Cotton Cellulose 1, 2, 3, 4 Buthanetetracarboxylic Acid (BTCA)

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Textile Research Journal

DOI: 10.1177/0040517508096222 2009; 79; 780 Textile Research Journal

Olivera Sauperl, Karin Stana-Kleinschek and Volker Ribitsch Physical�chemical Methods

Cotton Cellulose 1, 2, 3, 4 Buthanetetracarboxylic Acid (BTCA) Crosslinking Monitored by some

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Textile Research Journal Article

Textile Research Journal Vol 79(9): 780–791 DOI: 10.1177/0040517508096222 www.trj.sagepub.com © 2009 SAGE PublicationsLos Angeles, London, New Delhi and Singapore

Cotton Cellulose 1, 2, 3, 4 Buthanetetracarboxylic Acid (BTCA) Crosslinking Monitored by some Physical–chemical Methods

Olivera Šauperl1 and Karin Stana-KleinschekUniversity of Maribor, Faculty of Mechanical Engineering,

Laboratory for Characterization and Processing of Polymers, Smetanova 17, 2000 Maribor, Slovenia

Volker RibitschUniversity of Graz, Institute of Chemistry, Rheology & Colloid Science, Heinrichstraße 28, 8010 Graz, Austria

In general, fibers are formed of microfibrils which consistof an assembly succession of crystallites and intermediate dis-ordered amorphous regions [1–3]. The amorphous regionsand inner surface area of voids significantly influence reac-tivity and sorption properties of fibers.1

The natural crystal is made up from metastable celluloseI with all the cellulose strands parallel and no inter-sheet

hydrogen bonding. This cellulose I (that is, natural cellulose)contains two coexisting crystal phases, cellulose Iα (triclinic)and cellulose Iβ (monoclinic), in varying proportions depend-

Abstract In this research, the influence of thealkaline modification of cotton cellulose on thecrosslinking with 1, 2, 3, 4 buthanetetracarboxylicacid (BTCA) is investigated. In order to antici-pate changes after alkaline modification the crys-tallinity change was evaluated using wide angle X-ray diffraction (WAXD), iodine adsorption meas-urements, and the Knecht method. Tensiometry,the methylene blue method, and the streamingpotential method enable hydrophilic/hydrophobiccharacter estimation, carboxyl group content deter-mination, evaluation of dissociation/sorption fibercharacteristics, as well as electrokinetic proper-ties characterization. All these parameters definethe changes at the accessible polymer surfaces andtherefore reflect the relationship between thechanged crystallinity and the incorporation of theBTCA molecules into accessible regions of cottoncellulose. It has been concluded that the crystal-linity degree characterizes crosslinking effective-ness of cotton cellulose crosslinked with BTCA.Mercerized materials show after crosslinking highercontact angles and retain a higher number of car-boxyl groups. Electrokinetic properties are in cor-relation with carboxyl group amount. It has beenconfirmed that the physical–chemical methodswhich were used in this research are suitable meth-ods for the crosslinking efficiency evaluation.

Key words crystallinity, cotton cellulose, mer-cerization, Knecht method, iodine adsorption,BTCA, crosslinking, tensiometry, methylene bluemethod, zeta potential

1 Corresponding author: tel +386 2 220-7889; fax.:+386 2 220-7990; e-mail: [email protected]



Cotton Cellulose 1, 2, 3, 4 Buthanetetracarboxylic Acid (BTCA) Crosslinking Monitored O. Šauperl et al. 781 TRJ

ent on its origin, Iα being found more in algae and bacteriawhilst Iβ is the major form in higher plants [4].

Crystalline structure of native cotton cellulose is classi-fied as cellulose I; it can in addition adopt other polymor-phic crystal structures. When native cellulose is treatedwith relatively strongly alkaline solutions (e.g. NaOH), cel-lulose adopts a modified crystal structure called “hydratecellulose,” “mercerized cellulose,” “regenerated cellulose”or, more generally, cellulose II [1,5,6]. Besides cellulose Iand cellulose II other polymorphic lattice structures (e.g.cellulose III and IV) are known [1,6].

Cellulose II can be obtained in the mercerization proc-ess of cotton, which proceeds via the formation of sodiumcellulose by interaction of the polymer with aqueous sodiumhydroxide and subsequent decomposition of this intermedi-ate by neutralization or washing out of the sodium hydrox-ide. In general, the process of transformation of cellulose Ito cellulose II is considered an irreversible reaction [7].The modification of cotton cellulose with sodium hydrox-ide (16–24%) causes significant changes of supermolecularstructure determinable by X-ray diffraction. Alkaline mod-ification (mercerization) changes the cellulose at themolecular and macromolecular level [8]. Because of thechanges in the fine structure of the fibers (cellulose Ichanges to cellulose II) the sorption properties of the mer-cerized fibers are altered [9,10].

The decrease in crystallinity and the increase of amor-phous fraction caused by alkaline treatment (mercerization)lead to an increased accessibility of the reactive hydroxylgroups of cellulose fibers which act as active sites where dif-ferent reactions (e.g. etherification, esterification) occur.The differences between the lattice structures of cellulose Iand cellulose II are shown in Figure 2.

Accessible hydroxyl groups of cotton cellulose take anactive part in crosslinking reactions which can be used togain/introduce mechanical properties modification [11].Some reagents used for crosslinking of cotton cellulosereact with hydroxyl groups of cellulose fibers in such a waythat a crosslinked structure is formed. Applying this treat-

ment, certain exploitable and preservative properties ofthese materials are obtained [11,12]. Polycarboxylic acidsappear as very promising non-formaldehyde crosslinkingagents to replace the traditional, mostly formaldehyde-based, compounds [13–17]. Among these acids 1, 2, 3, 4,buthanetetracarboxylic acid (BTCA) has shown the bestresults. Crosslinking results from the esterification of thehydroxyl groups of cellulose fibers with carboxyl groups ofBTCA. The following mechanisms are proposed: accord-ing to Yang et al. a cyclic anhydride is formed first which inthe next phase forms an ester with the hydroxyl group ofcellulose fibers (Figure 3) [13,14,18,19].

According to the second mechanism (Figure 4) [15] amixed linear anhydride is formed in the presence of thecatalyst (NaH2PO2.H2O).

In this study the influence of the changed crystallinity andsubsequently changed sorption properties on the cotton cel-lulose crosslinking is investigated. Crystallinity change wasmonitored using different techniques: wide angle X-ray dif-fraction (WAXD), the Knecht method, and the iodineadsorption measurements [20–22]. The hydrophilic/hydro-phobic properties (hydrophilicity/hydrophobicity) of cottoncellulose were determined using the Powder Contact AngleMethod, which is based on measuring the changed sample’smass during adsorption of liquids. The contact angle ϕbetween the solid phase (fibers) and the liquid phase(water) was calculated using the modified Washburn equa-tion on the basis of measured capillary velocity [23–27].

Figure 1 Supermolecular structure of cellulose fibres –schematically [1].

Figure 2 Comparison of crystallographic unit cell of (a)cellulose I and (b) cellulose II [1].



782 Textile Research Journal 79(9)TRJTRJ

Considering the crosslinking mechanism we can sup-pose that the additional free BTCA carboxyl group whichis accessible in the cellulose polymer reflects the effective-ness of cotton cellulose crosslinking.

A comparison of accessible carboxyl group amount intocrosslinked mercerized and alkaline unmodified (raw) cot-ton cellulose can be made using the methylene bluemethod, where the adsorption of methylene blue dye onthe cellulose material is monitored spectroscopically. Themethylene blue method is based on the ion-exchangecapacity of the cellulose polymers. The dye methylene bluebinds to acid carboxylic groups of fibers by the principle ofion exchange. Since the treatment is carried out in an alka-line medium (pH = 8.5), the carboxyl groups of the boundBTCA are present in the carboxylate form. Another expla-nation is that the dye methylene blue can be also adsorbednon-specifically on the fiber surface [28].

Based on the methylene blue method one can also predicthow the structural changes of sodium hydroxide treated cot-ton cellulose influence the accessible carboxyl group amountof the crosslinked cellulose fibers [7,29].

The effectiveness of crosslinking was also studied on thebasis of the electrokinetic properties of cotton cellulosecrosslinked with different mass fractions of BTCA (w(BTCA)= 1%, 3%, 5%, 7%) in the reaction solution. Electroki-netic properties describe the electrical potential of a solidsurface, if this is moved in respect to the surrounding liquidphase. It is described by the zeta potential (ζ), the surfacepotential at the border line between the stationary andmobile liquid phase. It can be assumed that the surface

potential influences strongly different kinds of solid–liquidinteractions. Free carboxylic groups of BTCA which arepresent in the crosslinked material have an influence onthe electrokinetic character of the materials. The stream-ing potential of mercerized and afterwards crosslinked cot-ton cellulose was measured as a function of the pH and thezeta potential ζ was determined [30,31].

Experimental

MaterialsThe investigations were carried out on raw (unmercerized)and alkaline treated (mercerized) 100% cotton fabric. Thesurface mass (167 g/m2), warp density (47 threads/cm), fill-ing density (24 threads/cm) and the weave (3-way basictwill) were the same in both types of fabric. Pre-treatmentincluding alkaline modification (mercerization) was car-ried out in the firm MTT Tekstil d.o.o. Maribor accordingto its technological procedure. For the alkaline modifica-tion (NaOH, w(NaOH) = 24%) a classical technological pro-cedure was used [12].

Crosslinking ProcedureSamples were crosslinked following the procedure of drycrosslinking. This included impregnation with BTCA (Fluka),drying at T = 100°C, and crosslinking at 170°C. BTCA was

Figure 3 The mechanism of crosslinking of hydroxyl groups of cellulose with BTCA as a result of the formation of cyclicanhydride [18].

Figure 4 The mechanism of cros-slinking of hydroxyl groups of cel-lulose with BTCA as a result of theformation of linear mixed anhy-dride [15].




used in different mass fractions with NaH2PO2.H2O as cat-alysts. The amount of applied BTCA is expressed as aweight percentage based on the original weight, so calledwet pickup. The conditions of crosslinking are shown inTable 1.

After crosslinking the samples were rinsed at room tem-perature until the conductivity of distilled water (0.4 (µs/cm))was reached.

MethodsWide Angle X-ray DiffractionWith WAXD the influence of the alkaline modification oncotton cellulose crystallinity change was estimated. Thecrystallinity of raw and mercerized cotton cellulose was cal-culated using Hermans/Weidinger and Ruland/Vonk equa-tions. WAXD analyses were performed in the ChemicalInstitute Karl Franzens, University of Graz. Two circlegoniometer (A. Paar), positional sensitive detectors andspherical stops were used. Measurements were carried outwith X-ray generator Philips PW 2253/11 under accelerat-ing voltage 50 kV and electric current 45 mA. As X-raysource Cu-anode was used (CuKα: λ = 1.542 Å). Experi-mental curves were corrected for: absorption, incoherentdiffraction, and polarization. In such a way, the mathemat-ical correction of experimental curves was processed.

Iodine Sorption MeasurementAs proposed by Schwertassek the iodine sorption method isan empirical method to detect the extent of alkaline modi-fication on the basis of the amount of adsorbed iodine.Using iodine sorption value (ISV) (equation (1)) the crys-tallinity change can be calculated by comparing theamount of adsorbed iodine. The method procedure is toapply 1.2 ml of a concentrated potassium iodide solution(5 g I2, 40 g KI, 50 ml water) to 0.2 g of cotton cellulose, mixwell, let it stand for 3 min and transfer it into 100 ml satu-rated sodium sulfate decahydrate (Na2SO4x10H2O) solu-

tion. After standing for 1 hour with frequent shaking, thesupernatant solution can be analyzed for the iodine con-tent. The quantity of iodine taken up by the cotton celluloseis calculated as milligrams of iodine per gram of absolutelydry cotton cellulose from the difference in titer of thesupernatant solution and of a blank solution [32,33]. Fortitration 0.01 N sodium disulfate (IV) and a starch solutionas an indicator was used:

ISV = (mg/g) (1)

where ISV is the iodine sorption value (mg/g), a is ml ofsodium disulfate (IV) solution equivalent to initial iodinein the aliquot of the sample solution, b is ml of sodiumdisulfate (IV) solution for the aliquot of supernatant fil-tered from the sample, f is the aliquot factor, and m is themass of the absolutely dry cotton cellulose sample.

The Knecht MethodThe adsorption of the substantive dye on the fiber was fol-lowed using the Knecht method [11]. The raw and mercer-ized samples were dyed using the substantive dye TobazolScharlach 4BS (Cinkarna Celje). The conditions for dyeingare shown in Figure 5.

The remission values of the dyed samples were meas-ured using a spectrophotometer, type Datacolor Spectra-flash SF600. A numerical assessment of the dye differencebetween raw and mercerized samples was carried out inaccordance with the system CIELAB (∆Eab* = 6.04),where color difference was calculated according to [34]

∆E* = (2)

where ∆L* represents the difference in lightness (L* refer-ence sample – L* sample), ∆a* the difference on the axisred/green (a* reference sample – a* sample) and ∆b* thedifference on the axis yellow/blue (b* reference sample –b* sample) and ∆E* the color difference.

TensiometryThe hydrophilic/hydrophobic character of the fibrous mate-rials can be measured using tensiometry. The Powder ContactAngle Method was used for the contact angle determina-tion. This method is based on measuring the sample’s masschanges during adsorption of liquids.

The contact angle ϕ between the fibers and water was cal-culated using modified Washburn equation (equation (3)) onthe basis of measured capillary velocity (mass2/t):

(3)

Table 1 The content of the BTCA in the reaction bath and the conditions of crosslinking.

Mass fraction of BTCA – w(BTCA) (%) 1, 3, 5, 7

Mass fraction of catalyst – w (NaH2PO2.H2O*) (%) 1, 3, 5, 7

pH 2,2

Liquor pick up (%) 100

Drying temperature Tdrying (°C) 100

Drying time tdrying (min) 10

Crosslinking temperature Tcrossl. (°C) 170

Crosslinking time tcrossl. (min) 3

a b.1,33–( ).f.2,5384m

---------------------------------------------------

L∗∆( )2a∗∆( )2

b∗∆( )2+ +

ϕcos mass2

t-------------- η

ρ2 γ c⋅ ⋅-------------------⋅=




where mass2 is the sample mass/g, t is the time (s), η is theliquid viscosity (m Pa s), ρ is the liquid’s density (g/cm3), γis the surface tension of the liquid (N/m), ϕ is the contactangle between the solid and liquid phases (°), and c is thematerial constant or c factor mass2 sample mass (g). Theconstant c was determined according to equation (4),where r is average radius for the pores/mm and nk numberof capillaries:

(4)

The constant c depends on the type of measured sampleand the measuring cylinder and is determined measuringthe wetting behaviour of the solid sample (fibers) using liq-uids providing complete wetting, such as hexane and hep-tane where the contact angle equals zero and cos ϕ is 1.When the material has the ability to be wetted with liquid,then the values of the contact angle are between 0° and 90°[24–27]. For each individual sample three parallel meas-urements were performed under identical experimentalconditions. The results were statistically evaluated with astandard deviation.

Spectrophotometric Methylene Blue MethodA weighted oven-dry cellulose sample (approximately 0.5 g)of known water content up to 0.5 g was suspended in 25 mlof aqueous methylene blue chloride solution (300 mg/l) and25 ml of borate buffer of pH = 8.5 for 1 h at 20°C in a 100 mlErlenmeyer flask and then filtered through a sintered-glassdisk. Five or 10 ml of the filtrate were transferred to a 100 mlcalibrated flask. Then 10 ml of 0.1 N HCl and subsequentlywater, up to 100 ml, were added and the methylene bluecontent of the liquid was determined spectrophotometri-

cally, employing a calibration plot. The total amount offree, i.e. non-adsorbed, methylene blue was calculated. Thedye methylene blue binds to acid carboxylic groups of fibersby the principle of ion exchange. Because the treatment iscarried out in an alkaline medium (pH = 8.5), the carboxylgroups of bound BTCA are found in carboxylate form:

B+A– + Cel—COO– Cel—COO– B+ + A–

where B+A– dye is in the form of salt.The dye methylene blue can be also adsorbed non-spe-

cifically on the fiber surface [28].The number of carboxyl groups relative to the mass of

the absolutely dry cellulose sample is calculated from theamount of unbound methylene blue dye as follows:

ρ(COOH amount) = (7.5 – A) x 0.00313 m (5)

where ρ is COOH groups amount (mmol/g), A is the partof unbound dye methylene blue (g), and m is the mass ofthe absolutely dry sample of cellulose (g) [7,29,35].

The concentration of the dye solution was determinedusing the Perkin Elmer Lambda 2 UV/VIS spectrophotome-ter. For each individual sample three parallel measurementswere performed under identical experimental conditions. Theresults were statistically evaluated with a standard deviation.

The Streaming Potential MethodThe streaming potential method is one of the possible waysto obtain information about the nature of charges of solide(fiber) surfaces. This technique enables the determinationof electrokinetic properties, i.e. the zeta potential (ζ), offiber systems, which is calculated from the streamingpotential (US) data using the Smoluchowski equation:

Figure 5 Dyeing conditions accord-ing to the Knecht method.

c 12---.π2.r2.nk

2=




(6)

where ζ is the zeta potential, Us the streaming potential, ∆pthe hydrodynamic pressure difference across the plug, ηthe liquid viscosity, ε the liquid permittivity, ε0 the permit-tivity of free space, L the length of the plug, Q the cross-sectional area of the plug, and R the electrical resistanceacross the plug. Equation (6) is adequate for most practicalsystems. The term (L/Q) consists of two parameters nei-ther of which can be easily measured. In the Fairbrotherand Mastin approach the term (L/Q) is replaced by (Rs χs),where Rs is the electrical resistance of the plug when themeasurement cell is filled with an electrolyte whose spe-cific conductance, χS, is accurately known. Thus, equation(6) becomes:

(7)

For most practical systems equation (7) is perfectly ade-quate and was used in this study. If the surface conduct-ance has to be taken into account a 0.1 n KCl is usedinstead of the standard electrolyte. Electrokinetic proper-ties of uncrosslinked samples and crosslinked samples withdifferent mass fractions of BTCA in the reaction bath(w(BTCA) = 1%, 3%, 5%, and 7%) were monitored withthe streaming potential measurements. The swelling of fib-ers strongly influences zeta potential values, thereforebefore the zeta potential was determined the samples werepre-treated for 2 hours in the electrolyte solution. Thestreaming potential measurements were performed using0.001 n KCl as an electrolyte solution. The pH of the elec-trolyte solution was first adjusted to pH 10 using 0.1 nNaOH and afterwards decreased step-wise with 0.1 n HCl.

The zeta potential/pH function was determined in thisway; each ζ value presented is the mean of five measure-ments at opposite flow directions calculated according tothe Fairbrother–Mastin equation (equation (7)). Measure-ments were performed using the Electrokinetic Analyzer,EKA, A. Paar KG, Austria.

Results and Discussion

Crystallinity Changes of Mercerized Cotton CelluloseTable 2 shows the degree of crystallinity of raw and mercer-ized cotton cellulose following the methods of Hermans/Weidinger (wc,x (HW)) and Ruland/Wonk (wc,x (RV)).

It is well known that crystallinity of mercerized cottoncellulose is reduced compared with raw cotton cellulose(without alkaline modification). WAXD confirms that the

lower degree in mercerized cotton cellulose is caused dueto changes in morphology. During alkaline modification,the cellulose fibers swell. In the swelling action interfibril-lary and intrafibrillary processes occur. In more concen-trated alkali solutions, the swelling becomes intrafibrillarywith the participation of higher lateral order parts of thefiber. This is consistent with the decreased size of alkalihydroxide hydrates when the concentration increases; thusthe hydrate NaOH.nH2O appears in a noticeable propor-tion. In contrast to hydrated ion parts, solvated dipolehydrate is able to enter the crystalline parts of cellulosefiber [20]. This causes the changes of the crystallinity. Asshown in Table 2 the degree of crystallinity of raw cottoncellulose calculated with Hermans/Weidinger methodamounts to 0.567 ± 0.029 and for mercerized cotton cellu-lose to 0.503 ± 0.033.

Following the methods of Ruland/Vonk one obtains thesame decreasing trend of crystallinity degree by alkalinemodification of raw cotton cellulose. In the case of mercer-ized cotton cellulose the degree of crystallinity is lower(0.469 ± 0.052) in comparison with the Hermans/Weidingerapproximation. This difference is caused by the fact that theRuland/Vonk method includes a so-called disorder factorwhich leads to the differences in calculated values. However,both methods show identical influence of alkaline modifica-tion on the degree of cotton cellulose crystallinity.

The ISV displays the ability of cotton cellulose to adsorbiodine and it deceases after alkaline modification, indicatingalso the reduced degree of crystallinity [1]. The fiber modifi-cation leads to an increased amount of accessible hydroxylgroups. Iodine is incorporated into the cotton cellulose fib-ers by interaction with the cellulose hydroxyl groups. It isarranged in a monomolecular layer in comparison withwater, which is bound in multimolecular layers. Thereforethe amount of adsorbed iodine is first of all influenced bythe amount of accessible hydroxyl groups located in theamorphous phase. Figure 6 shows the degree of crystallin-ity (xC (%)) determined using WAXD and iodine adsorp-tion (xISV (%)). The iodine sorption method correlateswith the degree of crystallinity.

Both methods show the decrease of the cotton cellulosecrystalline phase causing higher accessibility for iodine as aconsequence of changed supermolecular structure (cellu-lose I transformation to cellulose II).

With the Knecht method the increased reactivity of themercerized cotton cellulose was also confirmed. From theremissive value of raw and mercerized samples and the cal-

ζUs

p∆------ η

ε ε0⋅------------ L

Q---- 1

R----⋅ ⋅ ⋅=

ζUs

p∆------ η

ε ε0⋅------------

Rs.χs

R-------------⋅ ⋅=

Table 2 Degree of crystallinity of raw and mercerized cotton cellulose.

Sample wc,x (HW) wc,x (RV)

Raw cotton cellulose 0.567 ± 0.029 0.569 ± 0.034

Mercerized cotton cellulose 0.503 ± 0.033 0.469 ± 0.052




culation of dye difference ∆Eab*, which amounts to 6.04, aconsiderably larger absorption of mercerized cotton cellu-lose was noted (Figure 7).

The Knecht method confirms also the foreseen higheraccessibility of mercerized cotton cellulose for the substan-tive dye. Results are in good accordance with structuralchanges determined by WAXD analysis and ISV.

Changed Sorption Character of Mercerized and Subsequently Crosslinked Cotton CelluloseIt is obvious that the contact angle of crosslinked cottoncellulose increases with increasing BTCA mass fraction inthe reaction solution (Figure 8).

In the raw sample group the highest contact angle (43°)is observed with the uncrosslinked cotton cellulose sample.

A relatively high contact angle (33°) is also observed withuncrosslinked mercerized sample. The difference of 10° isdue to the more hydrophobic character of raw uncrosslinkedcotton cellulose compared with the uncrosslinked mercer-ized one. The latter has a higher number of reactive hydroxylgroups and therefore a more hydrophilic character.

In the case of raw cotton it is remarkable that the con-tact angle of the non-crosslinked fiber exceeds that of thecrosslinked fibers independently of the applied BTCA con-centration. It can be assumed that due to the raw cottoncellulose’s relative high crystallinity only a relatively smallamount of hydroxyl groups is able to react with the BTCAcarboxyl groups. A considerable part of these carboxyl groupsremains therefore uncrosslinked. These hydrophilic polargroups increase the hydrophilic character (hydrophilicity) ofthe crosslinked material and therefore the contact anglesof crosslinked raw samples are lower in comparison with

Figure 6 Crystallinity degree defi-ned using WAXD and iodine acces-sibility defined using iodine sorptionmeasurements.

Figure 7 Remissive curves of rawand mercerized cotton cellulosestained with the substantive dyeTobazol Scharlach 4BS.




the raw uncrosslinked one. The treatment of raw cottoncellulose with different mass fractions of BTCA in thereaction solution (1%, 3%, 5%, and 7%) gradually increasesthe contact angles. Nevertheless they are anyway lower incomparison with the raw uncrosslinked cotton cellulose.

In the case of mercerized cotton cellulose where theextent of less ordered regions is higher than that of rawcotton cellulose, the influence of the accessible hydrophilicpolar groups is clearly seen. In the case of crosslinking with1% and 3% BTCA mass fraction the contact angles arelower in comparison with uncrosslinked cotton cellulose. Itcan be estimated from the contact angle results that a satis-factory ratio between hydroxyl cotton cellulose groups andBTCA carboxyl groups, and therefore a satisfactory reac-tion of esterification, is reached using mass fractions ofBTCA in the reaction bath of 5% and more.

It can be concluded from the contact angle results thatmercerized cotton cellulose crosslinked with the samemass fraction of BTCA in the reaction bath shows a better

crosslinking effect than raw cotton cellulose. Mercerizedcotton cellulose treatment with 5% and 7% BTCA massfraction of BTCA in the reaction bath causes considerablyhigher contact angles. It is evident that in this examplemore carboxyl groups are included in the esterificationprocess. Tensiometry as a method suitable for the determi-nation of the degree of crosslinking confirms that the alka-line modification gaining more reactive cotton cellulosenoticeably improves the crosslinking effect.

Based on the results of the methylene blue method it isproven that the content of the carboxyl groups increases in allcases with the increased mass fraction of BTCA (Figure 9).

The diagram (Figure 9) illustrates the situation that themercerized samples have on average a 2.2 mmol/kg or 5%higher carboxyl group content than the raw cotton cellu-lose, regardless of the degree of crosslinking (amount ofBTCA in the reaction bath).

Figure 9 shows that the treatment with 1% mass frac-tion of BTCA does not affect the contents of the carboxyl

Figure 8 Contact angle as a func-tion of the BTCA content in thereaction solution for raw and mer-cerized cotton cellulose.

Figure 9 The content of the car-boxyl groups with dependence onthe concentration of the BTCA forraw and mercerized cotton cellu-lose. The mass fractions of BTCAin the reaction solution were 1%,3%, 5%, and 7%.




group. The biggest increase in the carboxyl groups appearswhen the mass fraction of BTCA is increased from 1% to3%. This amounts to 4 mmol/kg or 16% in mercerizedsamples and to 5 mmol/kg or 13% in raw samples.

The maximum increase of carboxyl groups is gained ifcrosslinked with 7% BTCA; an increase of 16% in the caseof raw and 24% in the case of mercerized cotton celluloseis obtained. Again it was noted that the more reactive mer-cerized samples contain larger amounts of carboxyl groupsthan raw samples if identical BTCA mass fractions areused in the reaction solution.

The results of the methylene blue method also confirmthat the changes in crystallinity of cotton cellulose inducedby alkaline modification increases the effect of crosslink-ing, caused by the increased amount of the carboxyl groupcontent [20].

Polymer surfaces in aqueous (polar) solutions of electro-lytes create charged groups on the surface. The phenome-non of the surface charge is connected with the polymers’functional group dissociation as well as adsorption of ions(preferably -OH) onto the polymer surface. The represent-ative quantity ζ depends on ionic strength, and the electro-lyte pH and type. Since the process of crosslinking isperformed in an aqueous medium, knowledge of the poly-mers’ electrokinetic properties give us information abouttheir accessibility as well as their interaction ability. There-fore these properties should also offer the possibility to mon-itor the crosslinking efficiency and were applied to obtainadditional information about the extent of crosslinkingdependent on the BTCA mass fraction in reaction solutions.

Zeta potential measurements in 0.001 n KCl solutionswere performed as a function of the electrolyte pH. Theincrease in negative ζ with increasing pH is due to theincreased dissociation of acidic surface groups. In the case

of basic groups, the number of positively charged groupsincreases with decreasing pH [36]. Complete dissociation ofacidic or basic functional groups is related to the plateau inthe ζ–pH curve. The zeta potential (ζ)–pH functions of rawand mercerized cotton cellulose crosslinked with 1%, 3%,5%, and 7% of BTCA in the reaction solution are shown inFigures 10 and 11. ζ at the plateau of the totally dissociatedsurface groups above pH = 5 correlates with the amount ofaccessible carboxyl groups and with the crosslinking effi-ciency.

It is clearly displayed (Figures 10 and 11) that the zetapotential decreases with an increasing degree of crosslink-ing. This is more pronounced in the case of mercerizedthan in the case of raw samples.

The isoelectric point (IEP) is a direct measure of theacidity or basiticity of a solid surface if the dissociation ofsurface groups is the predominant mechanism of the for-mation of the electric double layer [37]. It describes the pHvalue, where a balance between negative and positive sur-face charge is reached. The isoelectric point of anionic sur-face groups can frequently not be obtained experimentallybecause the low pH value would chemically change thesample. Therefore a positive ζ is never reached. The IEP isusually obtained by extrapolation. In the case of ampho-teric materials both the negative and positive plateau(where total dissociation is obtained) are experimentallyaccessible. IEPs (ζ = 0) are shifted slightly to lower pH val-ues. In the case of raw uncrosslinked cotton cellulose sam-ples the IEP is shifted from pH 3.4 with 1% mass fractionof BTCA to 2.8 with 7% BTCA in the reaction solution.

In the case of mercerized cotton cellulose a similar trendis observed. The ζ plateau is shifted to even more negativevalues, from –12.8 mV for uncrosslinked to –46 mV of 7%BTCA.

Figure 10 Zeta potential as a func-tion of electrolyte pH of raw cottoncellulose crosslinked with 1%, 3%,5%, and 7% of BTCA in the reactionbath.




Isoelectric points of all mercerized samples are near pHregion 3.2 with the exception of crosslinking with 7%BTCA mass fraction in the reaction solution, where theisoelectric point could not be detected but is extrapolatedto a pH of 3.0.

Previous studies have shown [23] that crosslinking withthis relatively high BTCA mass fraction in the reactionsolution (7%) causes a relatively high extent of crosslinking(i.e. relatively high extent of ester connections between cot-ton cellulose and BTCA) as well as a relatively high amountof free BTCA carboxyl groups which remain uncrosslinked.Therefore, the ζ plateau is shifted towards very negative val-ues and the isoelectric point (ζ = 0) towards a lower pHvalue.

Cellulose functional groups (-OH, -COOH) dissociatein alkaline pH environments therefore the cellulose fibersare negatively charged and have negative ζ value. Accord-ing to Figures 10 and 11 zeta potential plateau values areshifted to the electronegative region in both raw and mer-cerized cotton cellulose with increasing BTCA mass frac-tion in the reaction solution. This phenomenon is connectedwith the cotton cellulose hydrophilicity (with increasingBTCA concentration in the reaction solution the cotton cel-lulose hydrophobicity gradually increases) and with theaccessible carboxyl group dissociation. As shown by otherauthors, the ζ plateau of differently pre-treated cellulosefibers can be between –4 and –14 mV [30]. Due to carboxylgroups which are present in the cellulose fibers the nega-tive ζ increases if crosslinked. As seen from the crosslink-ing mechanism [13], an average of two groups of BTCA arenot included in crosslinking reaction.

The zeta potential plateau difference at constant pH(8.5) is, for mercerized uncrosslinked cotton cellulose, 9%.

In the case of alkaline modification followed by crosslink-ing the zeta potential plateau increases between 7% and40% in the BTCA concentration range 1–7%.

The contact angle results coincide with zeta potentialresults (Figure 12). Increased hydrophobicity and higheramounts of accessible carboxyl groups which are enteredinto material by the crosslinking (confirmed with tensiom-etry) decrease the zeta potential plateau values to morenegative values (even to -46 mV in the case of 7% BTCAcrosslinking).

The methylene blue method has proven that crosslink-ing with a higher BTCA mass fraction in the reaction solu-tion produces an increased amount of accessible carboxylgroups. This trend correlates with calculated differences ofthe ζ plateau values (Figure 12). Both results indicate thata high BTCA concentration improves the crosslinking effi-ciency

Conclusion

In this study the influence of mercerization on crosslinkingefficiency of cotton cellulose crosslinked with 1, 2, 3, 4buthanetetracarboxylic acid (BTCA) was evaluated. Inorder to estimate how the structural changes (cellulose Iinto cellulose II) influence the crosslinking, several physi-cal–chemical methods were applied. The results werecompared to see if there is any correlation between the crys-tallinity and crosslinking efficiency. The aim of the pre-sented work is also to clarify whether the physical–chemical methods used in this research are suitable toolsfor crosslinking efficiency monitoring.

Figure 11 Zeta potential as a func-tion of electrolyte pH of mercerizedcotton cellulose crosslinked with1%, 3%, 5%, and 7% of BTCA in thereaction bath.




After mercerization the crystallinity changes were eval-uated using WAXD, iodine adsorption values and theKnecht method. All methods confirm that cotton cellulosealkaline modification (mercerization) reduces the degreeof crystallinity.

Using tensiometry, the methylene blue method, andelectrokinetic measurements the connection between cot-ton cellulose structure, hydrophilic/hydrophobic character(hydrophilicity/hydrophobicity), number of carboxyl groups,and dissociation–adsorption properties of crosslinked cot-ton cellulose were investigated.

With increasing BTCA mass fraction in the reactionsolution the contact angle increases almost linearly in bothraw and mercerized cotton cellulose. In comparison withraw crosslinked cotton cellulose crosslinked mercerizedcotton cellulose had a much lower sorption ability (highercontact angle), which means a much better esterificationeffect. The sorption ability increases if BTCA carboxylgroups remain uncrosslinked. This phenomenon is espe-cially perceivable in the group of raw cotton cellulosebecause of the relatively high crystallinity level. It seemsthat structural changes in mercerized cotton celluloseimprove the effect of crosslinking, particularly when thereaction solution contains a relatively high level of BTCA.

Another criterion describing crosslinking efficiency isthe number of free accessible carboxyl groups. Thesegroups were estimated using the spectroscopic methyleneblue method, which shows that the extent of free carboxylgroups increases with the increasing BTCA mass fractionin the reaction solution. The results of the methylene bluemethod also confirmed that alkaline modification favora-bly influences the effect of crosslinking.

Changes of the hydrophilic/hydrophobic character (cel-lulose hydrophilicity/hydrophobicity) and a changed amountof free carboxyl groups caused by the crosslinked processinfluence the surface dissociation/adsorption characteris-

tics of the accessible surfaces. Streaming potential meas-urements show that zeta potential of crosslinked cottoncellulose with a higher number of ester connections (mercer-ized) is shifted towards a more electronegative region incomparison with raw cotton cellulose, where the number ofester connections is smaller. This is indicated by the higherhydrophobicity of the crosslinked material.

In this research it was estimated that all physical–chem-ical methods used in this study are suitable tools to deter-mine the crosslinking efficiency of cellulose hydroxyl groupswith BTCA.

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Cotton Cellulose 1, 2, 3, 4 Buthanetetracarboxylic Acid (BTCA)

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