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Influence of Kapok Hollowness on Its Liquid Retention

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Vpliv votlosti kapoka

na sposobnost zadrževanja tekočin

Influence of Kapok Hollowness on Its Liquid Retention

Capacity

Izvirni znanstveni članek

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3FDFJWFEOctober 2009 r"DDFQUFENovember 2009

Vodilni avtor/corresponding author:

dr. Tatjana Rijavec

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Kapok (Ceiba pentandra) is a natural cellulose fibre with an extraordinary lumen and round- to-oval cross-section. It is one of the most effi- cient fibres for oil absorbers where it even out- performs synthetic fibres. The mechanism of oil sorption into kapok fibres has not been entirely researched yet. Excellent sorption capacities are attributed to the retention of oils in the kapok fi- bre lumen. The paper is going to present the re- sults of measurements of the kapok fibres hollow- ness, and the capacity of kapok fibers to retain liquids after having been soaked for 3 hours and centrifuged, as well as the mechanism of water and oil surface adsorption. Geometrical indices of raw kapok were measured or calculated based on the measured parameters of the raw fibres cross-sections: thickness of the cell wall 1.01 μm, the lumen diameter to fibre diameter ratio (d/D) 0.85, percentage of the hollowness 73.08%, vol- ume of the raw kapok lumen 2.1 cm3g, densi- ty of fibres 0.3968 g/cm3, specific surface area per volume unit 0.2324 µm/µm2 and per weight unit 0.6678 m2/g resp. It has been noticed that at contact of dry kapok fibres with liquid, water is spreading slowly over the surface of fibres where-

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1 INTRODUCTION

Hollow fibres belong to heterophyllic fibres of the sheat/core type with the core representing a hollow part of the fibre which is usually filled with air [1]. They are characterized by geomet- rical parameters among which the size of the hollow part as well as the external and internal surfaces are the most important [2, 3]. The hol- lowness of fibres has a significant influence on physical properties of fibres and end products, such as density, thermal and acoustic insula- tion and liquid retention capacity.

Man-made melt-spun hollow fibres are most- ly used as the stuffing for winter clothes, fake fur and upholstery. The fineness of these fibres, which are typically made of polyester and poly- propylene, is about 6 dtex for pillows stuffing, about 15 dtex for upholstery, and 40 dtex for filtration [4]. Solution-spun hollow fibres have porous semi-permeable wall and are most- ly used for liquid and gas filtration, e.g. seawa- as oil spreads very quickly. Water starts to pene- trate into the fibre lumen as soon as it comes into contact with kapok fibres. Oil penetrates into the fibre lumen at a slower rate than water. In the first few minutes only a very low amount of oil penetrated into the kapok fibre lumen, however, after a longer period of time, oil filled the kapok fibre lumen very well. The mean volume of a ka- pok fibre lumen is 2.1 cm3 per 1 gram of abso- lutely dry fibres, which represents the capacity of fibres to retain liquid in their lumen. The meas- ured mean amount of the retained liquid in ka- pok fibres after the fibres have been soaked for 3 hours and centrifuged was 1.03 g in the case of water, 1.32 g in the case of cooking oil and 1.07 g in the case of paraffin oil per 1 g of absolutely dry fibres. The quantity of oil retentioned in ka- pok after centrifugation was lower than capacity of fibres lumen which presents only few percent- ages of the whole quantity of retentioned oil in kapok fibres before centrifugation. Centrifuga- tion process enables a highly percentage of oil re- generation and reusage of kapok filters.

Keywords: kapok, fibre hollowness, geometrical indices of hollow fibres, amount of retained liq- uid

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ter desalination, pharmaceutical and biotechni- cal separation, and blood detoxination [5]. The example of such fibres are cupro fibres (CUP) used for dialysis membranes [6].

Vegetable elementary fibres are the remains of prosenchymatous, in the longitudinal axis ex- tended plant cells. When a cell dies out, proto- plasm, which is located in the central part of the plant cell, dries up and leaves an air-filled hollow space, the so-called lumen. Lumen runs along- side fibre; in some kinds of fibres (e.g. cotton fi- bres) it collapses when the pressure of protoplasm on the cell wall slackens. The size and shape of lumen highly differ from one type of vegetable fi- bre to another [7]. They depend on the develop- ment degree of a secondary cell wall as well as on tensions acting inside fibres during the plant cells dying out when protoplasm is drying.

Kapok (Ceiba pentandra) is a hollow seed mono- cellular fibre. It has thin but rigid secondary wall, which does not collapse during drying of proto- plasm. This is why most kapok fibres keep tubu- lar shape which is prone to deformations, such as flexures, kinks, etc. [8]. The morphological prop- erties of kapok can be compared with those of milkweed (Asclepias syriaca), which belongs to invasive plants in the territory of Slovenia.

Kapok is an extremely effective fibre for biode- gradable absorbents [9]. In the last decade sev- eral researches have been carried out about pos- sible use of kapok for oil filters and for removal of oil spills from the surface of rivers, seas and oceans. Investigations of the kapok’s filtration capacity in a 5, 10 and 15% water emulsion of Diesel and hydraulic oil have shown that more than 99% of oil has been removed [10]. Kapok is capable of absorbing even more than 40 g of oil per 1 gram of fibres from the oil suspension with fresh or seawater [9].

A high content of inorganic substances (1–2.5%) and acetyl groups in a primary cell wall and a waxy surface of raw kapok have a significant impact on high wetting capacity of kapok fibres with oils. Lim and Huang [11] have found that after the extraction of raw kapok with chloro- form when superficial waxes and substances sol- uble in organic solvents were removed, kapok still remained water unwettable and highly ole- ophyllic fibre. A contact angle between extract and oil was 117°. The outstanding wettability of

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Figure 1: Left: Kapok fibre cross-section scanned on SEM microscope at 2000× magnification with presented read parameters. (Author Mrs. M. Leskovšek.) Right: Method of reading fibres cross-section dimensions. Index vl designates the external ellipse, and index lu the internal ellipse; 2a – large axis, 2b – small axis

kapok enables that high amounts of oils are re- tained on the surface of fibres. They are quickly removed during squeezing or centrifuging fibres.

The liquid absorption rate and the amount of chemically bound liquid are dependent on mo- lecular and supramolecular structure of the veg- etable fibres cell wall. The morphological and su- pramolecular structure of kapok has not been explained in detail until recently [12]. The molec- ular structure of kapok has been well researched.

Kapok contains 19–20% of lignin and 22–28% of xylan. Oleophyllic absorption capacities of a ka- pok cell wall are the result of even 12–13% of acetyl groups bound on non-cellulose molecules, particularly to lignin, which are distributed in- side low-crystalline cellulose of type II the con- tent of which in raw kapok is 35–43% [9, 13].

Penetration of oil in kapok fibres by capillarity is described by Choi and Moreau[14] who in- vestigated this mechanism with a scanning elec-

tron microscope at room conditions. They found out that »oil on kapok fibre surface disappeared from the surface after a certain period of time, probably due to sorption by capillary action be- cause of hollow lumen«. The behaviour of lu- men during liquid absorption differs in depend- ence of the type of vegetable fibres. A dry raw cotton fibre has a collapsed lumen. In water so- lution of NaOH (at mercerisation) a secondary cell wall of cotton swells inwards as a result of too slow swelling of a primary cell wall which

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leads to the decrease of lumen. A kapok lumen is filled with liquids. Since there is no informa- tion available in literature about penetration of oils through a cell wall and about kapok cell wall swelling, the effect of oil on the size of lu- men is not known. As kapok is a vegetable fibre with a very compact primary cell wall [12], we assume that the capacity of a primary cell wall to swell in oil and water is limited.

Hori [9] who investigated the absorption of raw kapok fibres in a suspension of Diesel oil and water noticed under optical microscope that it was dyed Diesel oil which was mainly present in the kapok fibres lumen. He estimated that water could not penetrate into kapok fibres due to high surface tension between kapok and air captured in the fibres (7.2 ¨ 10–4 N/cm at 20°C).

Since pores in fibres are also considered the zones of liquids retention, the amount of ab- sorbed liquids is additionally increased. Porosi- ty of kapok fibres has not been researched yet.

The purpose of our research was to investigate the geometrical hollowness of kapok and to evaluate the capacity of kapok fibres to retain water and oils in their lumen.

2 Research methods

The Java kapok of mean length 18.8 mm and fineness 1.02 dtex was used in the research.

(FPNFUSJDBMJOEJDFTPGLBQPL Geometrical indices of kapok were investigat- ed based on measurements of the fibre cross- section which was scanned on the JSM-2 JEOL scanning electron microscope at 2000¨ magnifi- cation, acceleration voltage 10 kV, specimen in- clination 45° and work distance 12 mm. A fibre wad was tightly pushed through a hole in the flat holder, and protruding fibre ends cut off.

As a result of such method of preparation of the specimen, fibres mostly underwent deformation from round to oval or ellipsoidal shape (Fig. 1).

Prior to scanning, fibres were steamed with a layer of carbon (C) and a layer of the gold/pal- ladium (Au/Pd) alloy. On digital images of fi- bres cross-sections the thickness of the cell wall (dst) as well as the large (2avl; 2alu) and small (2bvl; 2blu) axis of the fibres external and inter- nal ellipse cross-section (Fig. 1, right) were read

(6)

6UFäOPTQFDJĕʊOPQPWSÝJOP 1

[N

TNPJ[SBʊVOBMJJ[SB[NFSKBNFE [VOBOKPQPWSÝJOP 1

z

JONBTPWMBLOB N

vl

QPFOBʊCJ

Table 1: Assembly of geometrical hollow fibre indices

(FPNFUSJKTLJJOEFLTJ[BWPUMBWMBLOB

[PLSPHMJNQSFʊOJNQSFSF[PN (FPNFUSJDBMJOEJDFTPGIPMMPXĕCSFTXJUI round cross-section

4JNCPM 4ZNCPM

debelina stene vlakna Thickness of the kapok fibres cell wall d

st

NBMBOPUSBOKBPT Small internal axis of elipse C

lu

NBMB[VOBOKBPT Small external axis of elipse C

vl

velika notranja os Large internal axis of elipse B

lu

velika zunanja os Large external axis of elipse B

vl

notranji obseg vlakna Fibre internal (lumen) circumference o

lu

zunanji obseg vlakna Fibre external circumference o

vl

QSFNFSMVNOB Lumen diameter d

QSFNFSWMBLOB Fibre diameter %

[VOBOKBQPWSÝJOBWMBLOB Fibre external surface area per unit length 1

z

OPUSBOKBQPWSÝJOBWMBLOB Fibre internal surface area per unit length 1

n

WPMVNFODFMJʊOFTUFOF Cell wall volume 7

st

WPMVNFOMVNOB Lumen volume 7

lu

WPMVNFOWMBLOB Fibre volume 7

vl

votlost Hollowness r

vl

QPWSÝJOBQSFʊOFHBQSFSF[BMVNOB Lumen cross-section surface area "

lu

QPWSÝJOBQSFʊOFHBQSFSF[BWMBLOB Fibre cross-section surface area "

vl

WPMVNTLBTQFDJĕʊOBQPWSÝJOBWMBLOB Fibre specific surface area per unit of volume 1

[7

gostota vlakna Fibre density Ϯ

vl

VUFäOBTQFDJĕʊOBQPWSÝJOBWMBLOB Fibre specific surface area per unit of weight 1

[N

SB[NFSKFNFEQSFNFSPNMVNOBJOWMBLOB Lumen diameter to fibre diameter ratio E%

SB[NFSKFNFE[VOBOKPJOOPUSBOKP

TQFDJĕʊOPQPWSÝJOP External to internal specific surface area ratio 1

z

1

n

by means of a computer program. The measure- ments were carried out on 100 fibres.

The thickness of the kapok fibres cell wall (dst) can also be calculated by using Equation 1.

The read values of the ellipse large and small axes were used to calculate the circumference of the fibre cross-section (ovl) and the lumen (olu)

(7)

7QSFHMFEOJDJTP[CSBOJTJNCPMJJOOKJIPWQPNFO[BHFPNFUSJKTLF JOEFLTFWPUMJIWMBLFO[PLSPHMJNQSFʊOJNQSFSF[PN

2.2 Mehanizem sorpcije tekočin in količina zadrževane tekočine

.FIBOJ[FN TPSQDJKF WPEF JO PMK TNP QSPVʊFWBMJ W PQUJʊOFN NJ - LSPTLPQV0MZNQVT$9OBLBUFSFNTNPWMBLOBUVEJGPUPHSBĕSBMJ TQPNPʊKPEJHJUBMOFLPNQBLUOFLBNFSF0MZNQVT41QSJUSKF - OFOBUSFUKFNPLVMBSKVNJLSPTLPQB

,PMJʊJOP[BESäBOFUFLPʊJOF ,;5TNPEPMPʊJMJQPTUBOEBSEV%*/

<> 1SPVʊFWBMJ TNP TQPTPCOPTU [BESäFWBOKB EFTUJMJSB - OFWPEFKFEJMOFHBJOQBSBĕOTLFHBPMKBTVSPWFHBLBQPLBTVSPWFHB CPNCBäB ĕOPʊFEUFYJOWJTLP[OJIWMBLFO EUFY7[PSFD NBTF H TNP USJ VSF OBNBLBMJ W NM EFTUJMJSBOF WPEF P[J - SPNBPMKBWFSMFONBKFSJDJ[CSVÝFOJN[BNBÝLPN;BTVSPWLBQPL TNPNPSBMJ[BUFIUPWMBLFO[NBOKÝBUJOBHWMBLFOEBTNPKJI QPOBNBLBOKVWPMKVMBILPWMPäJMJWFQSVWFUFTBKTPTVSPWBWMB - LOBLBQPLBOBW[FMB[FMPWFMJLPLPMJʊJOPPMKB/BUPTNPWMBLOBPE - TFTBMJJO[BQFUNJOVUPCUFäJMJTLJMPHSBNTLPVUFäKP7MBLOBTNP nato kvantitativno prenesli v steklene epruvete, kot jih predpisu- KFTUBOEBSEMFUFQBWLPWJOTLFFQSVWFUFDFOUSJGVHF$FOUSJGVHJSB - MJTNPNJOVUQSJWSUMKBKJIOBNJOVUP1PDFOUSJGVHJSBOKV TNPWMBLOBUBLPKTUFIUBMJ4DFOUSJGVHJSBOKFNTNPPETUSBOJMJQPWS- ÝJOTLPBETPSCJSBOPUFLPʊJOPJOVHPUBWMKBMJ[BESäFWBOKFUFLPʊJOFW OPUSBOKPTUJWMBLFO,PMJʊJOP[BESäBOFUFLPʊJOFTNPJ[SBʊVOBMJQP FOBʊCJ

LKFSKFN

NBTBTVIJIWMBLFON

NBTBWMBLFOQPDFOUSJGVHJSBOKV JO,;5LPMJʊJOB[BESäFWBOFUFLPʊJOFJ[SBäFOBWPETUPULJI

6QPSBCJMJTNPQBSBĕOTLPPMKFQSPJ[WBKBMDB41IBSNBDIFN4VÝOJL +PäFTQ HPTUPUBHDN

WJTLP[OPTUN1BTQSJ¡$JO KFEJMOPSBĕOJSBOPPETUPUOPTPOʊOJʊOPPMKF4POʊOJDWFUQSPJ - [WBKBMDB(FBEE HPTUPUFHDN

QSJ¡$

3F[VMUBUJ[SB[QSBWP

3.1 Geometrijski indeksi kapoka

7QSFHMFEOJDJTP[CSBOJJ[NFSKFOJHFPNFUSJKTLJJOEFLTJQSFʊOFHB QSFSF[BLBQPLBWQSFHMFEOJDJQBJ[SBʊVOBOJHFPNFUSJKTLJJOEFLTJ QPFOBʊCBIo

*[SBʊVOBOJQSFNFSWMBLFOLBQPLB[PLSPHMJNQSFʊOJNQSFSF[PNKF ϪN1PWQSFʊOBJ[NFSKFOBEFCFMJOBDFMJʊOFTUFOFKFϪN NJOJNBMOB EFCFMJOB ϪN NBLTJNBMOB EFCFMJOB ˜N 1P HFPNFUSJKTLJILBSBLUFSJTUJLBILBQPLJ[TUPQBWQSJNFSKBWJ[WPUMJNJ TJOUFUJʊOJNJWMBLOJ[BQPMOJMBLKFSOQSOBUSHVEPTUPQOBWPUMBWMBL OBPLSPHMFHBQSFʊOFHBQSFSF[BEPTFHBKPQSFNFSFWFʊLPU˜N PCEFCFMJOJTUFOFPLSPH˜NJONBOK

of kapok by using Hudson’s Equations 2 and 3 for ellipse [16].

From the kapok fibre cross-section circumfer- ence the diameter of kapok fibres with round cross-section (D) (Eq. 4) and the diameter of the related lumen (d) (Eq. 5) were calculated.

The external and internal diameter are basic parameters which define hollow fibres. High- er is the ratio between them (d/D), hollower are the fibres, lower is the fibres density, high- er is the liquid absorbing capacity, better are the thermal insulation properties and lower is the mechanical rigidity of fibres.

External (Pz) and internal (Pn) surface areas per unit length (l) of 1 metre of hollow fibres have been calculated by using Equations 6 and 7.

The volume of fibre (Vvl), lumen (Vlu) and cell wall (Vst) for tubular non-collapsed fibre with round cross-section per length (l) of 1 metre of fibres has been calculated by using Equations 8, 9 and 10.

The hollowness of a kapok fibre (rvl) has been expressed as the ratio of the volume of lumen (Vlu) to the volume of entire fibre (Vvl); it equals the ratio of the lumen cross-section surface area (Alu) to the entire fibre cross-section surface area (Avl) or the ratio between their diameters (Eq. 11).Fibre specific surface area per unit vol- ume (Pz/V) is the ratio of external surface (Pz) to total fibre volume (Vvl) (Eq. 12).

Fibre mass has been calculated from the vol- ume (Vst) and cell wall density (ρst = 1.474 g/

cm3 [17]) by using Eq. 13. Fibre density has been calculated from the fibre mass (mvl) per to- tal fibre volume (Eq. 14).

Specific surface area per unit mass (Pz/m) has been calculated from the ratio of external surface (Pz) to fibre mass (mvl) by using Equation 15.

In a Table 1 the meanings of symbols for hollow fibres with round cross-section are assembled.

-JRVJETTPSQUJPONFDIBOJTN BOEBNPVOUPGSFUBJOFEMJRVJE The water and oils sorption mechanism was in- vestigated by using the Olympus CX2 optical microscope, on which fibres were photographed by using the Olympus SP-350 digital compact camera, fixed on the third microscope ocular.

The amount of retained liquid (KZT) was de- termined according to DIN 53 814 standard

(8)

Table 2: Measured geometrical indices of kapok fibres with ellipsoidal cross-section

(FPNFUSJDBMJOEJDFTPGLBQPLĕCSFT with ellipsoidal cross-section

.FBOWBMVF ϪN Standard deviation ϪN

$PFďDJFOUPG WBSJBUJPO

-BSHFFYUFSOBMBYJT B

vl

4NBMMFYUFSOBMBYJT C

vl

-BSHFJOUFSOBMBYJT B

lu

4NBMMJOUFSOBMBYJT C

lu

ćJDLOFTTPGĕCSFXBMM E

st

Table 3: Calculated geometrical indices of kapok fibres with round cross-section

Geometrical Index

D (μm)

d

(μm) d/D o

z

(μm)

o

n

(μm)

V

vl

(μm

3

)

V

lu

(μm

3

)

V

st

(μm

3

)

ρ (g/cm

3

) Value 16.81 14.31 0.85 52.78 44.93 227 110 165 970 61 140 0.3968

Geometrical Index

A

vl

(μm

2

)

A

lu

(μm

2

)

r

vl

(%)

P

z

(μm

2

)

P

n

(μm

2

) P

z

/P

n

P

z,V

(μm /

μm

2

)

P

z,m

(m

2

/g)

Value 227.11 165.97 73.08 52 780 44 930 1.17 0.2324 0.6678

-VNFOQPNFOJQPWQSFʊOPLBSPETUPULBLBQPLPWFHBWMBLOB 1PWPUMPTUJEPTFHBLBQPL[FMPWJTPLFWSFEOPTUJTBKJNBKPLFNJʊOB WMBLOB[BQPMOJMBQPOBWBEJWPUMPTUQPEPETUPULJ<>1SJQPW - QSFʊOJHPTUPUJLBQPLBHDN

JOWPUMPTUJPETUPULBKFOP - USBOKJWPMVNFOLBQPLB WPMVNFOMVNOBDN

NMOBHSBN WMBLFO,BQPLTFPEMJLVKFUVEJ[J[KFNOPWFMJLPTQFDJĕʊOPQPWS- ÝJOPLJKFQPNFNCOB[BQPWSÝJOTLPBETPSQDJKPUFLPʊJO/KFHPWB [VOBOKBTQFDJĕʊOBQPWSÝJOBKFN

/g WMBLFO1SJNFSKBWBLB - QPLBTTJOUFUJʊOJNJNJLSPWMBLOJLJKJIOQSVQPSBCMKBKP[BUJvʊV- EFäOFi ʊJTUJMOF LSQF W HPTQPEJOKTUWJI QPLBäF [B QPMPWJDP OJäKP TQFDJĕʊOPQPWSÝJOP N

HQSJ1"NJLSPWMBLOJIPLSPHMF - HBQSFʊOFHBQSFSF[BTQSFNFSPNϪN<>

3.2 Mehanizem sorpcije tekočin

7 QSJNFSKBWJ [ WPUMJNJ LFNJʊOJNJ WMBLOJ LJ JNBKP W OPUSBOKPTUJ FOPBMJWFʊLBQJMBS[PEQSUJOBNJOBPCFITUSBOFIWMBLFOTPLBQP- LPWBWMBLOBOBWSIV[PäFOF[BQSUFDFWLF TMJLBB[P[LPPEQSUJ - OP TMJLBCMFOBNFTUVLKFSTPCJMBQSJSBTMBOBTFNFOB

1SJ NJLSPTLPQTLFN PQB[PWBOKV QSPEJSBOKB UFLPʊJO W MVNFO LB - QPLPWJIWMBLFOTNPWMBLOBOBSF[BMJOBEPMäJOPoNNEBTNP PNPHPʊJMJQSPEJSBOKFUFLPʊJOFWWMBLOB1SJQSBWJMJTNPTVIFQSF - QBSBUF JO PQB[PWBMJ ÝJSKFOKF LBQMKJDF UFLPʊJOF QP QPWSÝKV WMBLFO 0CTUJLVTVIJIWMBLFO[WPEPTFKFMFUBQPʊBTJÝJSJMBQPQPWSÝKV

[18]. We investigated the retention capacity for distilled water, cooking oil and paraffin oil of raw kapok, raw cotton (fineness 1.8 dtex) and viscose fibres (1.7 dtex). The specimen with the mass of 0.4 g was soaked for 3 hours in 150 ml of distilled water or oil in the Erlenmeyer flask fitted with ground joint. In case of raw kapok the mass of fibres had to be reduced to 0.2 g in order to be inserted into test tubes after having been soaked in oil because raw kapok fibres ab- sorbed a very large amount of oil. After that fi- bres were gently squeezed and loaded with a 1 kg weight for 5 minutes. Fibres were then transmitted into glass test tubes in the quanti- ties prescribed by the standard. Test tubes were placed into metal test tubes of centrifuge. The process of centrifuging was carried out 30 min- utes at 3000 rpm. Immediately after the proc- ess of centrifuging process has stopped, fibres were weighted. By the centrifugation proc- ess the surface adsorbed liquid was eliminated and the quantity of retained liquid in the fibres was measured. The amount of the retained liq-

(9)

WMBLFOPCTUJLV[PMKFNQBTFKFMFUP[FMPIJUSPSB[ÝJSJMPQPQPWSÝ - KVLBQPLB3B[MPH[BUBLÝOPPCOBÝBOKFKFPMFPĕMOPQPWSÝKFLBQP- LPWJIWMBLFOLJPNPHPʊBEPCSPBEIF[JKP[PMKJJOTMBCPBEIF[JKP[

WPEP5PPCOBÝBOKFKFEPCSPEPÝMP[WJEJLBVQPSBCFLBQPLB[BPMK - OFĕMUSFLKFSTFNPSBPMKFIJUSPJOQSFEWPEPBETPSCJSBUJOBLBQP- LPWBWMBLOB

/BÝBPQB[PWBOKBTVSPWFHBLBQPLBWPQUJʊOFNNJLSPTLPQVTPQP - LB[BMBEBWPEBäFUBLPKPCTUJLVTLBQPLPWJNJWMBLOJ[BʊOFQSP - EJSBUJWMVNFOWMBLFO7[BʊFUOJGB[JQSPEJSBOKBTNPPQB[JMJÝUF- WJMOF[SBʊOFNFIVSʊLFWWMBLOJILJTPOBTUBMJ[BSBEJJ[QPESJWBOKB [SBLBJ[WMBLFO TMJLBB1PM[BQSUBPCMJLBWMBLFOWQMJWBOBTMBC - ÝPTQPTPCOPTUOBW[FNBOKBUFLPʊJOLFS[SBLLJHBQSFETFCPKJ[QP- ESJWBUFLPʊJOBLJQPLBQJMBSOFNNFIBOJ[NVQSPOJDBWMVNFOWMB - Figure 2: Thinned top end of kapok fibre (a) and its bottom end (b), which was attached to the seed, with a well visible opening. (200x magnification (Fig. a), 400x magnification (Fig. b). Scanned on Olympus optical microscope. Author: Mrs.T. Rijavec).

Figure 3: Scans of lengthwise appearance of kapok fibres after few minutes of soaking in water (a). Formation of air bubbles at the fibre top end of kapok fibre (b). (Scanned on Olympus optical microscope at 100x magnifi- cation (a). and at 400x magnification (b). Author: Mrs.T. Rijavec)

LFO TMJLBCMBILPJ[TUPQBJ[WMBLOBMFOBQPÝLPEPWBOJINFTUJI DFMJʊOFTUFOF

7OBTQSPUKV[WPEPPMKFQPʊBTOFKFQSPEJSBWMVNFOWMBLFO;FMP NBMPPMKBQSPESFWMVNFOLBQPLBW[BʊFUOJINJOVUBIPNBLBOKB /BTMJLJBQPTOFUJOFLBKNJOVUQPWMPäJUWJWMBLFOWPMKFWJEJNP a)

B

b)

C

uid has been calculated by using Equation 16, where m1 is the mass of dry fibres, m2 the mass of fibres after centrifuging, and KZT the quan- tity of the retained liquid expressed in percent- ages.

The amount of the retained water was inves- tigated on the fibres of raw kapok, raw cotton, and viscose fibres. In addition to distilled water, paraffin oil produced by S Pharmachem Sušnik Jože s.p. (density 0.865 g/cm3, viscosity 186 mPas at 20 °C), and 100% cooking oil Sončni cvet produced by Gea d.d. (density 0.950 g/cm3 at 20 °C) were used.

3 Results with discussion

(FPNFUSJDBMJOEJDFTPGLBQPL The measured geometrical indices of the ka- pok fibre cross-section are presented in Table 2,

(10)

Figure 4: Scans of lengthwise appearance of kapok fibres after few minutes of soaking in paraffin oil (a) and after 48 hours of soaking in paraffin oil (b). (Scanned on Olympus optical microscope at 100x magnification.

Author: Mrs. T. Rijavec.)

B C

EBKFWFʊJOBWMBLFOLBQPLBÝFWFEOP[BQPMOKFOB[[SBLPN OFQSP- TPKOJI/BTMJLJCQPTOFUJQPVSBIOBNBLBOKBLBQPLBWPMKV KFPMKF[BQPMOJMPMVNFOQSJTLPSBKWTFIWMBLOJILBQPLBLJTP[BUP WJEFUJQSPTPKOB

3.3 Količina zadrževane tekočine

(MFEFOBJ[SBʊVOBOPWFMJLPTUMVNOBWLBQPLVDN

OBHSBNWMB - LFOKFLBQBDJUFUB[BESäFWBOKBUFLPʊJOFWMVNOVFOFHBHSBNBBCTP- MVUOPTVIJIWMBLFOHWPEFHKFEJMOFHBPMKBJOHQBSB - ĕOTLFHBPMKBPCQSFEQPTUBWLJEBUFLPʊJOBOFQPW[SPʊBOBCSFLBOKB DFMJʊOFTUFOFJOTUFNOFWQMJWBOBWFMJLPTUMVNOB

1PUSJVSOFNOBNBLBOKVWMBLFOWWPEJJOPMKJITNPTDFOUSJGVHJSB - OKFNPETUSBOJMJQPWSÝJOTLPWF[BOPUFLPʊJOP7SFEOPTUJ,;5 QSF - Table 4: Amount of retained liquid (KZT) at room temperature on raw kapok (KP

raw

), raw cotton (CO

raw

) and viscose (CV) fibres

-JRVJE Statistical

value

"NPVOUPGSFUBJOFEMJRVJE,;5 XU

,1

raw

CO

raw

$7

%JTUJMMFEXBUFS

x

T

$7

Cooking oil x

T

$7

1BSBďOPJM

x

T

$7

and the calculated geometrical indices by using Equations 1–15 are presented in Table 3.

The calculated diameter of a kapok fibre with round cros-section is 16.81 μm. The mean measured thickness of the cell wall is 1.01 μm, the minimum thickness is 0.70 μm, the maxi- mum thickness is 1.73 µm. By considering ge- ometrical indices kapok fibres surpass substan- tially hollow synthetic fibres used as stuffing materials; namely, commercially available hol- low fibres with round cross-section have dia- maters larger than 250 µm with the wall thick- ness about 13 µm and less.

(11)

HMFEOJDBQSJLB[VKFKPTLVQOPLPMJʊJOPLFNJʊOPWF[BOFUFLPʊJOF WWMBLOJIJOĕ[JLBMOP[BESäFWBOFUFLPʊJOFWMVNOVJOQPSBIWMB- LFO *[NFSKFOB LPMJʊJOB [BESäBOFHB PMKB QP DFOUSJGVHJSBOKV W TVSPWJI LBQPLPWJI WMBLOJI KF CJMB OBKWFʊKB [B KFEJMOP PMKF JO TJDFS PETUPULBLBSQPNFOJHKFEJMOFHBPMKBOBHSBNBCTPMVUOPTV - IJIWMBLFO5BLPMJʊJOBKFNBKIOBWQSJNFSKBWJTLBQBDJUFUPSBTUMJO - TLJIWMBLFOLJTLVQBKWWMBLOJIJOOBQPWSÝKV[BESäJKPWFʊLPUH PMKBHWMBLFO<>JOLBQBDJUFUPTVSPWFHBLBQPLBLJ[BESäJTLVQBK UVEJoHEJ[FMTLFHBPMKBOBHSBNWMBLFO<>,PMJʊJOBWLBQP - LPWJIWMBLOJIBCTPSCJSBOFHBKFEJMOFHBPMKBQPNFOJMFOFLBKPETUPU - LPWTLVQOFLPMJʊJOF[BESäBOFHBPMKBQSFEDFOUSJGVHJSBOKFN 3B[MJLBWLPMJʊJOJ[BESäBOFHBKFEJMOFHBJOQBSBĕOTLFHBPMKBKFQP - MFHSB[MJLFWHPTUPUJPMKQPTMFEJDBOKVOFSB[MJʊOFQPWSÝJOTLFOBQF - UPTUJ

4VSPWJCPNCBäJNBPEEPLSBUTMBCÝPTQPTPCOPTU[BESäFWB- OKBPMKLPULBQPL1PWSÝKFTVSPWFHBCPNCBäBKFQSFLSJUP[o VUWPTLPWOBNBTPTVIJIWMBLFO<>LBSPNPHPʊBQPWSÝJOTLP BEIF[JWOPTUEPPMKBJOQSFQSFʊVKFQSPEJSBOKFWPEFWTVSPWBWMBLOB ÀFNBOKÝBKFCJMBJ[NFSKFOBLPMJʊJOB[BESäBOJIPMKWWJTLP[OJIWMB - LOJILKFSTFPMKFMFĕ[JLBMOPWFäFTBNPOBTUSVLUVSJSBOJäMFCJʊB - TUJQPWSÝJOJJOWQPWSÝJOTLJIQPSBI#PMKWJTLP[OPQBSBĕOTLPPMKF TFKF[BESäBMPOBCPNCBäVJOWJTLP[OJIWMBLOJIWWFʊKJLPMJʊJOJLPU NBOKWJTLP[OPKFEJMOPPMKF

,BQPLKFJ[SB[JUPMJHOJOPDFMVMP[OPWBLOPTQPSB[EFMKFOJNIJESP - GPCOJNMJHOJOPNUVEJWOPUSBOKPTUJDFMJʊOFTUFOFWMBLFO4LVQBK[

WPTLBTUJNIJESPGPCOJNQPWSÝKFNOPUSBOKBTUSVLUVSBLBQPLBPWJ - SBQSPEJSBOKFWPEFTLP[JDFMJʊOPTUFOP/BQPEMBHJNJLSPTLPQTLJI PQB[PWBOKTNPVHPUPWJMJEBWPEBIJUSPQSPEJSBWMVNFOLBQPLB TLP[JPEQSUJOFWWMBLOJI*[NFSKFOBLPMJʊJOBHWPEFHTVIJI WMBLFOKFNBOKÝBPELBQBDJUFUFMVNOBWLBQPLPWJIWMBLOJI 4VSPWJ CPNCBä W WPEJ OBCSFLB QPSF JO MVNFO TF NV [BQPMOJ - KP[WPEP*[NFSKFOBLPMJʊJOB[BESäBOFWPEFWTVSPWFNCPNCBäV KFCJMBHWPEFHSBNTVIJIWMBLFOLBSKF[FMPCMJ[VLPMJʊJOF[

WPEPOBTJʊFOJICPNCBäOJIWMBLFOLJ[OBÝBPEEPHWPEF OBHSBNTVIJIWMBLFO<>

*[NFSKFOBLPMJʊJOB[BESäBOFWPEFWWJTLP[OJIWMBLOJIKFCJMBWFʊKB LPUWCPNCBäV7JTLP[OBWMBLOBTP[FMPWPEPWQPKOB[BSBEJOJ[LF LSJTUBMJOJʊOPTUJJOPSJFOUBDJKFUFSQPSP[OFTUSVLUVSF

4LMFQJ

1PĕOPʊJWMBLFOMBILPLBQPLPQSFEFMJNPLPUOBSBWOPNJLSPWMB - LOP;VOBOKJQSFNFSQSPVʊFWBOJIWMBLFOLBQPLBKFCJMQPWQSFʊOP

˜N1PWQSFʊOBJ[NFSKFOBEFCFMJOBDFMJʊOFTUFOFKFCJMB˜N -VNFOKFQPNFOJMLBSPETUPULPWWMBLOB

*[KFNOPWFMJLBTQFDJĕʊOBQPWSÝJOBLBQPLPWJIWMBLFON

/g JOOKFOBPMFPĕMOBOBSBWBTUBHMBWOBW[SPLBWFMJLFTQPTPCOPTUJTV - SPWFHBLBQPLB[BQPWSÝJOTLPBETPSQDJKPPMK

On average, the lumen represents even 73.1%

of a kapok fibre. Kapok has very high values of hollowness if it is considered that the hollowness of man-made fibres used for stuffing is usually under 50% [19]. With mean density of a kapok fibre 0.348 g/cm3 and hollowness 73.08% the internal volume of a kapok fibre (volume of lu- men) is 2.1 cm3 (2.1 ml) per 1 gram of fibres.

Kapok also boasts outstandingly high specific surface area, which is important for surface ad- sorption of liquids. Its external specific surface area is 0.6678 m2/gof fibres. The comparison of kapok with synthetic microfibres, which are used for the so-called “magic”cleaning wipes for households, reveals that PA 6 microfibres hav- ing round cross-section with diameter 10,6 μm have by 50% lower specific surface area (0.3291 m2/g) [20].

-JRVJETTPSQUJPONFDIBOJTN

In comparison with hollow man-made fibres, which have in their interior one or more capil- laries with openings on both sides of fibres, ka- pok fibres are closed tubes, thinned on the top end (Fig. 2 a) and with a narrow opening (Fig.

2b) only on the points on which they were at- tached to the seeds.

For the purposes of microscopic observation of liquids penetration into the kapok fibres lumen we cut fibres to the length of 1–2 mm to ena- ble penetration of the liquid into fibres. We pre- pared dry specimen and observed spreading of a liquid droplet over the surface of fibres. At contact of dry fibres with water, water spread slowly over the surface of fibres, but at contact with oil, oil spread very quickly over the kapok surface. Such behaviour is the result of the ka- pok fibres oleophyllic surface, which imparts good adhesion with oils and poor adhesion with water to the fibres. Such behaviour is advan- tageous when kapok fibres are used for oil fil- ters where oil must be absorbed by kapok fibres quickly and prior to water.

Our observations of raw kapok under optical microscope have revealed that on contact with fibres, water immediately starts to penetrate into the fibres capillaries. During the initial stage of penetration the formation of lots of air bubbles in the fibres can be noticed, which is the result of displacement of the air from fibres (Fig.

(12)

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3a). The semi-closed shape of fibres degrade their liquid absorption capacity, because the air which is displaced by the liquid penetrating into the fibre lumen by capillarity (Fig. 3b) can leave the fibre only through the points of mechanical damage in a cell wall.

Oil penetrates into the fibres lumen at a slower rate than water. Very small amount of oil pene- trates into the kapok fibres capillaries in the first minutes of wetting. It is also evident in Figure 4a where most kapok fibres are still filled with air (opaque). In Figure 4b, taken after 48 hours of soaking in oil, it is seen that oil has filled almost all capillaries so that the fibres look transparent.

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Based on the calculated size of the kapok lumen, i.e. 2.1 cm3 per 1 gram of fibres, the capacity of liquid retention in the lumen of 1 gram of abso- lutely dry fibres is 2.1 g of water, 2.0 g of cook- ing oil and 1.8 g of paraffin oil by assuming that liquid does not induce swelling of a cell wall and has therefore no impact on the size of lumen.

After soaking fibres in water and oils for 3 hours, we removed the surface bound liquid by centrifuging. KZT values (Table 4) show total amount of chemically bound liquid in the fibres and physically retained liquid in the fibres lu- men and pores.

The measured amount of retained oil in raw ka- pok fibres after centrifuging was the highest for cooking oil, i.e. 131.6%, which means 1.32 g of cooking oil per 1 gram of absolutely dry fibres.

This amount is low if compared with vegetable fibres which are capable of retaining more than 30 g of oil per 1 gram of fibres inside fibres and on the surface altogether [14] as well as if com- pared with raw kapok, which is capable of re- taining a total amount of even 36–45 g of Die- sel oil per 1 gram of fibres [11]. The amount of cooking oil absorbed in kapok fibres represents only few percents of the total amount of re- tained oil prior to centrifugation.

The differences in the amount of retained cook- ing and paraffin oils are attributed to different surface tensions as well as different densities of both oils.

Raw cotton has 3–5 times lower oil retention capacity than kapok. Since raw cotton is cov- ered with 0.4–1.0% by weight of waxes per dry

(13)

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fibres mass [21], its surface is adhesive for oils and impermeable for water. Even lower was the measured amount of retained oils in viscose fi- bres where oil was only physically bound to the structured grooved surface and in superficial pores. More viscous paraffin oil was retained on cotton and viscose fibres in a higher amount than less viscous cooking oil.

Kapok is a distinctively ligninocellulose fi- bre with hydrophobic lignin distributed also in the interior of a fibre cell wall. The kapok in- ner structure together with a waxy hydropho- bic surface impedes the penetration of water through a cell wall. Based on microscopic ob- servations we have found that water penetrates quickly into the kapok lumen through the open- ings in fibres. The measured amount of 1.03 g of water per 1 gram of dry fibres is lower than the capacity of the kapok fibres lumen.

Raw cotton swells in water, its pores and lumen get filled with water. The measured amount of retained water in raw cotton was 0.41 g of wa- ter per 1 gram of dry fibres, which is very close to that of water saturated cotton fibres, which is between 0.43 and 0.52 g of water per 1 gram of dry fibres [21].

The measured amount of retained water in vis- cose fibres was higher than that in cotton fibres.

Viscose fibres are highly water absorbent due to low crystallinity and orientation as well as po- rous structure.

4 Conclusions

If fineness of fibres is considered, kapok can be defined as a natural microfibre. The exter- nal diameter of the investigated kapok fibres was about 16 µm and the cell wall thickness was only about 1.0 µm. Lumen represents even 73% of fibre.

Extraordinary large specific surface area of ka- pok fibres, i.e. 0.6678 m2/g, and its oleophyllic nature are the main reasons or high capacity of raw kapok for oil surface adsorption.

Absorbed liquids, which remain in kapok fi- bres after centrifuging, are retained chemical- ly bound in amorphous zones of a cell wall and physically bound in the fibres pores and lumen.

In the amount of retained water kapok outper- forms vegetable and regenerated cellulose fi-

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bres. Despite hydrophobic surface and inferior wettability, kapok retained even 104% of water with regard to the mass of dry fibres after hav- ing been soaked in water for 3 hours and centri- fuged. Water was mostly retained in the lumen.

Raw kapok is distinctively oleophyllic fibre.

It binds oils particularly by surface adsorp- tion and in the lumen into which it can pene- trate through the openings at the end of fibres or at the points of mechanical damage. Raw ka- pok retained 132% by weight of cooking oil and 107% by weight of paraffin oil after soaking and centrifuging, raw cotton retained only 24%

by weight of cooking oil and 29.7% by weight of paraffin oil, and viscose fibres even less, i.e. 16

% by weight of cooking oil and 24% of paraffin oil at room temperature.

The amount of retained oil, which remains in kapok after centrifuging, is very low (1.07–1.32 g of oil per 1 g of fibres) and represents only few percents of the total amount of oils on fibres prior to centrifugation, which is higher than 30 g of oil per 1 g of dry fibres. Centrifugation of kapok fibres creates the opportunity for a high percentage of oils regeneration and for reuse of filters.

Reference

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