1-s2.0-S0266353800001688-main

Eects of corona discharge and UV treatment on the propertiesof jute-fibre epoxy composites

Jochen Gassan a,*, Voytek S. Gutowski b

aInstitut fur Werkstotechnik, University of Kassel, GermanybC.S.I.R.O. Polymer Surface Engineering Group, Melbourne-Highett, Graham Road, Victoria 3190, Australia

Received 4 April 2000; received in revised form 13 July 2000; accepted 1 August 2000

Abstract

In this paper, tossa jute fibres were corona discharge and ultraviolet (UV) treated to improve the mechanical properties of nat-ural-fibre/epoxy composites. Corona-treated fibres exhibited significantly higher polar components of the free surface energy with

increasing treatment energy output. Owing to the diculties in eective treatment of three-dimensional objects with corona dis-charge, the increase of polarity of treated yarns is relatively small, in comparison to the results achieved with single fibres. Fur-thermore a decrease in the yarn tenacity was observed with increasing corona energy level. The UV treatment of the single fibresand yarns led to significantly higher gains in polarity in comparison with those observed in relation to corona-treated materials.

Increasing treatment time at a constant bulb-sample distance or alternatively decreasing the distance significantly increased thepolarity and decreased yarn tenacity. To improve the overall mechanical properties of jute/epoxy composites, an appropriate bal-ance needs to be achieved between increased polarity of fibre surface and the decrease of fibre strength subsequent to excessive

surface oxidation by corona discharge or UV radiation. At the optimum treatment conditions an increase in the composite flexuralstrength of about 30% was achieved. # 2000 Elsevier Science Ltd. All rights reserved.

Keywords: A. Polymer-matrix composites; B. Fibre/matrix bond; B. Interface; B. Mechanical properties; B. Surface treatment

1. Introduction

Composite materials made of cellulose-based fibres suchas natural fibres demonstrate remarkable environmentaland economical advantages. It is very well known, forexample that the mechanical or other physical properties ofcomposites depend on the properties of the individualcomponents and their interfacial compatibility. Cellulose-based fibres are intrinsically polar owing to the presenceof hydroxyl and carboxylic groups in their structure.Despite this inherent polarity, composite materialsmade with unmodified natural fibres frequently exhibitunsatisfactory mechanical properties. To overcome this,in many cases a surface treatment or compatibilizers needto be used prior to composite fabrication [1,2]. An over-view of chemical modifications of the interface in naturalfibre-reinforced plastics is given in earlier papers [2,3].

This article deals with the eects of corona dischargeand ultraviolet (UV) treatment on natural fibres, and theinfluence of these treatments on the physical propertiesof the fibres and their composites.Belgacem et al. [1] showed that the use of a powder of

high-alpha cellulose, hardwood fibres as reinforcingmaterial for polypropylene significantly improves theyield stress (approximately 25%) and elastic modulus(approximately 50%) of compression moulded compo-sites. The extent of improvement depends on the coronatreatment conditions, such as treatment time (up to 3min) and corona current (up to 35 mA). No significantdierences in the yield stress between the compositesmade of treated cellulose and untreated or treatedpolypropylene was found. Dong et al. [4] found adecrease in the apparent melt viscosity of cellulose filledpolyethylene composites if one (or both) of the constituentswas corona treated. Bataille et al. [5] used corona treatmentas a kind of pre-treatment to activate the cellulose toimprove the eciency of a grafting process.It has been reported that corona treatment of the

surface of wet pulp sheets improved the ply-bond

0266-3538/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.PI I : S0266-3538(00 )00168-8

Composites Science and Technology 60 (2000) 28572863

www.elsevier.com/locate/compscitech

* Corresponding author at present address: Fraunhofer-Institut fur

Kurzzeitdynamik, Ernst-Mach-Institut (EMI), Eckerstrasse 4, 79104

Freiburg, Germany. Fax: +49-761-2714-316.

E-mail address: [email protected] (J. Gassan).

strength of paperboards [6]. Sakata et al. [6] showed intheir results that the contact angle of a droplet of urea-formaldehyde resin on a wood surface decreases byincreasing the degree of corona treatment. In this case,the improved wettability was reported to lead to higherbond strength.Corona treatment, however, is dicult to use on

three-dimensional objects [7]. This limitation applies notonly to 3-D shaped products, but also to fibrousmaterials,particularly in the form of woven fabrics or strands. Theawareness of the existence of these limitations has high-lighted the need for alternative, more 3-D compliantsurface modification techniques such as ozone and/orultraviolet radiation (e.g. UV) light. Investigations onthe UV triggered aging of wood surfaces showed a sig-nificant increase in carbonyl groups [8]. Almost everychemical constituent (cellulose, hemicellulose and lig-nin) of wood and natural fibres is sensitive to UVradiation with a consequential deterioration eect. Ofthese chemical constituents, lignin (as the surface con-stituent of a natural fibre [2]) appears to be oxidized anddegraded very rapidly by UV light. However, because ofthe strong ultraviolet-absorbing characteristic of lignin(due to the phenolic nature of its macromoleculararchitecture), it may also perform the function of ashielding agent to protect cellulosic components fromexcessive photo-degradation [9,10].

Daneault et al. [19] used ozone as a sole treatment togenerate more hydroxyl groups on the surface of woodfibres with the aim of improved grafting eciencythrough this activation.

2. Experimental

2.1. Materials and fibre treatment

Commercially available tossa jute fibres (J. SchilgenGmbH and Co., Germany) with a twist of 277 t/m andfineness of 13.5 cN/tex were used throughout the study.Their physical properties, composition and morphologyhave been quantified in detail and reported elsewhere[11]. An epoxy resin, Araldite F, with hardener HY 956(mixing ratio 100:25 wt.%), both from Ciba-Geigy, wasused as matrix system for the composites. The compositeswere cured at 60C for 12 h. The volume fibre contentfor all composites was approximately 0.30.The fibres were treated in a corona treatment unit

HV200 from Tantec EST Systems. Treatment condi-tions used were as follows:

. power output range from 40 to 100 Watts;

. treatment speed varies from 10 to 88 cm/min

For the UV treatment, a Fusion UV Curing bulb withUV emission spectrum according to Fig. 1 and a focalpoint 53 mm away from the bulb was used. The energyoutput at a wavelength of 254 nm, which is the mostimportant wavelength regarding ozone generation, isapproximately 80 W.Fibres and yarns were dried at 60C for 2 h before

any further treatment and processing of composites.

2.2. Surface energy determination

2.2.1. Single fibresContact angle measurements for the single fibres

(treated as single fibres) were carried out using a CAHN322 dynamic contact angle analyser. Single fibres weresubmerged into test liquids at a speed of 20.4 mm/s to adepth of typically 3 mm. At this depth, the fibres were heldshortly before being pulled out of the liquid. Distilledwater and diiodomethane were used as polar and non-polar test liquids (Table 1).The surface free energy components of the fibres were

calculated using the geometric mean [Eq. (1)] based onOwens and Wendt [12].

1 cos L 2 dLdS 1=2 pLpS 1=2h i 1

with

i di pi for i L; S:

Nomenclature

E0 storage modulus of compositesE0untr storage modulus of composites with

untreated fibresK geometric factorm masst timeT tenacity at max. of yarnsTuntr tenacity at max. of untreated yarns

L free surface energy of test liquid

S free surface energy of fibre

dL non-polar component of free surface

energy of test liquid

dS non-polar component of free surface

energy of fibre

pL polar component of free surface energy of

test liquid

pS polar component of free surface energy of

fibreL viscosity of test liquid contact angleL density of test liquidf flexural strength of compositesf;untr flexural strength of composites with

untreated fibres

2858 J. Gassan, V.S. Gutowski / Composites Science and Technology 60 (2000) 28572863

2.2.2. YarnsContact angle measurements for the yarns (treated as

yarns) were carried out using the capillary rise method(CAHN 322). The experimental procedure used hasbeen described in detail earlier [13]. By measuring thetime dependence of the liquid absorption in the fibrebundle, and subsequent liquid column rise, the contactangles will be calculated by using the Washburn equa-tion [Eq. (2)] [14].

m2=t K2LLcos

= 2L 2

The geometric factor K can be estimated using com-pletely wetting liquids (cos 1) such as hexane. Thesurface energy components of the yarns were calculatedusing Eq. (1).

2.3. Mechanical properties

Tenacity at maximum was measured by using yarntensile test according to DIN 53 834 with an eectivespecimen length of 50 mm (clamp distance) and a testspeed of 2 mm/min at room temperature (RT). Thevalues given are the means from at least five individualsamples.Flexural strength according to DIN EN 63 in 3-point

bending mode was determined with a test speed ofabout 2 mm/min and a support prisms span of 51 mm atRT. The geometry of the samples was 80151.5 mmin each case. The strength values given are the meansfrom five individual samples.Dynamic mechanicalthermal properties of composites

were determined through DMTA procedure carried outwith the use of Rheometrics Solid Analyser RSA II, in atemperature range between 50 and 120C. The samples

(53131.5 mm) were tested with a frequency of 10 Hzin three point bending mode.

3. Results and discussion

3.1. Free surface energy

3.1.1. Corona discharge treatmentThe corona treatment of the jute fibres leads to an

increase in the polar component of the free surfaceenergy (Fig. 2), mainly due to increasing the content ofcarboxyl and hydroxyl groups. Sakata et al. [6] reportedon the oxidation of hydrophilic constituents of the surfaceof dierent types of wood through corona treatments.However, the chemical changes induced by coronatreatment were restricted to the substrate surface anddid not appear to aect the bulk properties of treatedwood [6].In our experiments, the non-polar (dispersive) com-

ponent of the free surface energy for the jute fibres (dS )was approximately 23 mJ/m2 and, as anticipated,retained this constant value for both untreated and allcorona discharge treatment conditions. In contrast to theseresults, Belgacem et al. [1] found for single (hardwood)cellulose fibres an increase in the dispersive componentof the free surface energy for increasing corona currentsand constant treatment times, i.e. increasing corona energy.This unusual result, since d should remain constant,can be attributed to a degradation of the material,exposed to destructively excessive treatment energy.For the untreated jute fibres a contact angle of water

of 75 was determined, which is similar to the watercontact angle of lignin. In comparison, the water contactangle on cellulose is only about 33 [15]. This may indi-cate that lignin may be the predominant constituent ofthe surface layer of natural fibres such as jute. Thisobservation conforms to physical models of the structureof these types of fibres (the total lignin content of thejute fibre used is about 10 wt.%) [11].As known from other papers [16,17], the contact

angle of liquids on the surface of cellulose based fibres/materials is influenced by the penetration (absorption)

Fig. 1. UV emission spectrum of the UV bulb used (Fusion UVCuring).

Table 1

Free surface energy, specific density and viscosity of the test liquids

used [12]

Test liquids Viscosity

(mPas)

Density

(g/cm3)

dL(mJ/m2)

L(mJ/m2)

n-hexane 0.29 0.6595 18.4 18.4

Water 1.0 0.9982 21.8 72.8

Diiodomethane 2.6 3.3254 48.5 50.8

Fig. 2. Influence of the corona discharge energy on the polar compo-

nent of the free surface energy of treated single fibres and yarns.

J. Gassan, V.S. Gutowski / Composites Science and Technology 60 (2000) 28572863 2859

of the test liquid into the fibre. Similarly to the resultsreported earlier by Lee et al. [16] for lignin films andToussaint et al. [17] for cellulose films, the contact angleof highly polar distilled water in contact with jute fibresdecreases with total contact time [16,17]. In our experi-ments, this was observed through consecutive immersioncycles (Fig. 3). For a predominantly non-polar diiodo-methane the contact angles on the jute fibres are initiallydecreasing and become constant after the third cycle.Similar behaviour was observed by Toussaint et al. [17]after 25 minutes for the cellulose films. Based on theseobservations related to liquid absorption by porousmaterials, Kawasaki (as published earlier in Ref. [16]),has derived an expression relating the wettability of anorganic and absorbent solid to the diusion coecientof the liquid into the solid.Considering the above-mentioned dynamic character

of interfacial phenomena for a liquid in contact with aporous/absorbent solid, the contact angles and thecomponents of the free surface energy, respectively, canonly be successfully used to compare the influence ofdierent types of surface treatments (Fig. 2) understrictly controlled test and processing conditions, e.g.the same equilibrium moisture content of fibres andidentical time of exposure to test liquid. These weremaintained constant through our experiments.Data presented in Fig. 2 show that the average oxi-

dation eects of corona discharge are lower for thewhole yarn than for single fibres (fibre bundles). This isattributed to the diculties with eective use of coronatreatment on three dimensional objects [7], or those withundertreatment of the underneath side not directlyexposed to the bombardment by ionised species presentin the discharge zone.

3.1.2. UV treatmentThe results of the dynamic contact angle measure-

ments for the UV treated fibres reveal significantincrease of the polar component of the free surfaceenergy (up to 88 mJ/m2) (Fig. 4) and that of fibrepolarity (max. 0.96) with the treatment time increase (up

to 15 min in our experiments). This increased polaritycan be attributed to an increase of concentration ofcarboxyl groups on the fibre surface, as shown in earlierinvestigations of the UV-aging of wood surfaces [8].Significantly, elevated temperatures (particularly duringlong exposure times combined with high energy of theUV radiation during the treatment) provide synergeticeects to a rapid increase of surface density of thesepolar groups [8]. Other scientific reports on UV oxida-tion of dierent types of thermoplastic materials such asPE, PEEK, and PP demonstrated (by the means ofcontact angle of water) that prolonged treatment timesup to 4, 5, 1560 min, respectively, are commonlyessential to reach maximum polarity on these surfaces[7,18], by the means of exposure to UV radiation otherthan excimer laser.As for all UV related surface treatments carried out in

ambient atmosphere (i.e. in absence of vacuum), apartfrom the treatment time, the distance between the sub-strate and the UV bulb has a significant influence on thedegree of eective oxidation of the fibre surface (see Fig.5). UV radiation emitted by high energy lamp results inthe formation of the atomic oxygen. In absence ofvacuum, as in our experiments, the atomic oxygen canrecombine or react with other gas molecules before itreaches the polymer surface [7]. Due to this phenomenon,

Fig. 3. Influence of test liquid penetration into the fibre on the contact

angle of test liquids on untreated jute fibres (the contact with the test

liquid during one cycle was approximately 2.5 min).

Fig. 4. Influence of the UV treatment time on the polar component of

the free surface energy and the polarity of jute fibres.

Fig. 5. Influence of the UV bulb-substrate distance on the polar com-

ponent of the free surface energy and the polarity of jute fibres treated

with UV radiation source.


the increase of surface density of oxygenated species suchas carboxylic and hydroxyl groups, as demonstrated byhigher value of polar component of the free surfaceenergy becomes smaller with the increasing distancebetween substrate and UV source. Regardless of theabove it has been observed (see Fig. 5) that increasingthe bulb-substrate distance up to 160 mm, the surfacepolarity values for the treated fibres are distinctly higherthan those for the untreated ones (Fig. 5).

3.2. Mechanical properties

3.2.1. Corona discharge treatmentThe corona treatment of the jute fibres with increasing

energy output leads to a remarkable decrease in thetenacity of the fibre (Fig. 6). Lower treatment speeds[which result in higher specific energy output (mJ/mm2)at fixed generator power] increased this tendency forreducing the fibre tenacity (T). It is well documented inthe literature (see, for example, the work of Sakata et al.[6]) that chemical changes through the corona treatmentonly happen on the wood surface. On the other hand,the surface ablation and subsequent weakening of thesuperficial region of the material is known to occur as aresults of excessive treatment of polymeric materials bycorona or plasma [20,21]. It has been observed in ourwork, that although the above observations are con-firmed by the decrease in fibre tenacity, the elasticitymodulus was approximately constant over the wholerange of energy output. These observations and resultson the influence of corona treatment on surface polarity,fibre tenacity and composite strength are consistent withthe underlaying surface chemistry and the materialsmechanical performance. The increased treatment levelon individual fibres does increase the fibre polarity. Theablation-related fibres surface roughness acts as asuperficial micro-crack initiator. This, in turn, do notadversely aect the elasticity modulus of the fibres (inthe linear part of the stress-strain characteristic). Theadhesion at the matrix-fibre interface may be increased

due to the rise of the number of OH and COOH groups,as demonstrated by the increase in the surface polarity. Theoverall strength and fracture performance of the compositematerial is, however, lowered when fibres are over-treateddue to the decrease of the strength and fracture energyof the individually treated (over-treated) fibres.In addition to the above, other factors require con-

sideration. As a result of warming up of the fibres by thehigh energy source, a degradation of, for instance, degreeof polymerisation or the crystallinity can be possible. Forexample, even short exposure times at 190C reduce thedegree of polymerisation and, as result, the strength ofthe jute fibres decreases distinctly [11].As already discussed above, the results of the contact

angle measurements proved an increase in fibre polarityfor increasing treatment energies. The associated betterwettability of the fibre by the matrix material was thoughtto lead to improved composite properties. However, theloss in mechanical fibre strength as discussed, preventsthese higher composite strength properties (Fig. 6) to beattained.In contrast to the composite strength, the storage

modulus, E0, at 60C (see Fig. 7) increases with highercorona energy and follows the trend of increase of thepolar component of free surface energy (Fig. 2), due tothe unchanged fibre stiness. It can be seen from Figs. 2and 7 that the improved wettability at a corona energylevel of 155 mJ/mm2 leads to an increase of the storagemodulus, E0, at 60C of at least 15%.

Fig. 6. Influence of the corona discharge energy on the flexural

strength of the composites and the tenacity (at max) of the fibres:

symbols without parentheses are for fibres treated at the speed of 79

cm/min; symbols with parentheses are for those treated at 61 cm/min.

Fig. 7. Influence of corona discharge energy on the storage modulus,

E0, at 60C (treatment speed=79 cm/min).

Fig. 8. Influence of UV treatment time on the flexural strength of the

composites.


3.2.2. UV treatmentAs shown in Figs. 4 and 5, the UV treatment leads to

higher polarities of the fibre surface. Consequently, thewettability of fibres and the composite strength wereimproved. The trend observed in Fig. 8 shows, that inthe case of appropriately treated jute fibres an increaseof 30% in composite strength is possible, after a 10 mintreatment at a distance of 150 mm away from the UVlamp. This respectively relates to 97 mm distance for thefocal point to UV source. For a shorter distance of 120mm, the same trend can be observed.Increasing the substrateUV bulb distance for constant

treatment time (see Fig. 9 providing data for two treatmenttimes: 5 and 10 min) at first leads to improvements incomposite flexural strength. However, once the optimumdistance is exceeded (e.g. 150 mm for 10 min treatmenttime) the increase in the polarity (Fig. 5) is too small toenhance the composite properties. Excessive treatmentconditions, such as short distances or long treatmenttimes, result in the degradation of fibre tenacity as shownin Fig. 10. Mild treatment conditions, in comparison, asshown for 5 min treatment time, result in comparativelysmall, or no degradation eects on fibre, but are asso-ciated with a smaller increase in the fibres surfacepolarity.

For instance, a 5 min treatment at a distance of 135mm increases the polar component of free surfaceenergy up to 49 mJ/m2 but reduces the fibre tenacity by18%. A 10 min treatment at 150 mm improves thepolarity more eectively (67 mJ/m2) with an approxi-mately similar decrease in fibre tenacity (15%).Based on these results, a balance between an increase in

polarity of the fibre surface and decrease of the fibrestrength is necessary to successfully improve the compositestrength.

4. Conclusions

Corona discharge treated individual jute fibres exhibitsignificant increase in the polar component of the freesurface energy with increasing corona energy. For outputenergies of approximately 130 mJ/mm2 a maximum pSvalue of 46 mJ/m2 was reached, in comparison to pS 10 mJ/m2 for untreated fibres. In comparison with theresults relevant to individually treated fibres, the increaseof polarity of treated yarns is only small (maximum ofapproximately pS 26 mJ/m2), due to the diculties oftreating three dimensional objects with corona. Unfor-tunately, the oxidation of the fibre surface always ledsto a decrease in the yarn tenacity (decrease in fibre tensilestrength), which prevents higher composite strengthvalues to be easily achieved. Fibre stiness (modulus ofelasticity) is unaected by these corona energy levelsand an improved (by approximately 15%) of storagemodulus, E0, at 60Cwas achieved for optimised treatmentconditions.The UV treatment of the jute yarn led to higher

polarity (up to 200% increase) with increased treatmenttime and constant bulb-substrate distance. Furthermore,the treatment distance was showed to have a significanteect on the polarity as well as to the tenacity of theyarn. To improve the mechanical properties of jute-epoxy composites by the use of UV-oxidation of fibres abalance between increased polarity of fibre surface anddecrease of fibre strength is necessary. Under optimumtreatment conditions an increase of the composite flexuralstrength of about 30% was achieved.

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Fig. 9. Influence of UV bulbsubstrate distance and UV treatment

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Fig. 10. Influence of UV bulbsubstrate distance and treatment time

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