polyethylene green composites reinforced with cellulose fibers (coir and palm fibers): effect of...

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Polyethylene green compositesreinforced with cellulose fibers (coirand palm fibers): effect of fiber surfacetreatment and fiber contentRungsima Chollakup a , Wirasak Smitthipong a , Wuttinan Kongtuda & Rattana Tantatherdtam aa Kasetsart Agricultural and Agro-Industrial Product ImprovementInstitute (KAPI), Kasetsart University , 50 Ngam Wong Wan Rd,Chatujak, Bangkok , 10900 , ThailandPublished online: 10 Aug 2012.

To cite this article: Rungsima Chollakup , Wirasak Smitthipong , Wuttinan Kongtud & RattanaTantatherdtam (2013) Polyethylene green composites reinforced with cellulose fibers (coir andpalm fibers): effect of fiber surface treatment and fiber content, Journal of Adhesion Science andTechnology, 27:12, 1290-1300, DOI: 10.1080/01694243.2012.694275

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Polyethylene green composites reinforced with cellulose fibers (coir andpalm fibers): effect of fiber surface treatment and fiber content

Rungsima Chollakup*, Wirasak Smitthipong, Wuttinan Kongtud and Rattana Tantatherdtam

Kasetsart Agricultural and Agro-Industrial Product Improvement Institute (KAPI), Kasetsart University,50 Ngam Wong Wan Rd, Chatujak, Bangkok 10900, Thailand

(Received 15 December 2010; final version received 3 July 2011; accepted 14 May 2012)

Coir and palm fibers from agricultural waste were investigated as reinforcement for lowdensity polyethylene (LDPE). The effect of fiber preparation with alkaline treatment andwith/without bleaching on fiber physical properties was also an objective of this study. Thechemical composition and FTIR (Fourier transform infrared spectroscopy) results confirmedthat palm fibers had less impurity than coir fibers. This could be the reason for a greaterfiber-matrix interfacial interaction of the palm fibers as compared to that of coir fibers,which was in good agreement with the estimation of surface free energy of the dispersioncomponent. Moreover, fiber bleaching improved the single fiber pullout stress. Compositeswith both alkaline treated and bleached fibers, at different fiber contents (5, 10, 15, and 20wt.%), were manufactured using a compression molding machine. Addition of both fibersin the LDPE matrix resulted in composites with a higher Young’s modulus compared tothat of homopolymer. The Young’s modulus of the composites increased with the effect ofeither fiber content or fiber bleaching. Differential scanning calorimetry (DSC) showed thatcomposites reinforced with both types of fibers had a single melting temperature peak, indi-cating the existence of only one type of crystalline species. Moreover, there were no signif-icant differences in the melting temperatures for the fiber reinforced composites and thehomo-LDPE. The heat of fusion decreased in the case of fiber reinforced composites.

Keywords: cellulose fiber; coir fiber; palm fiber; polyethylene composite; fiber pullout test;mechanical properties; thermal properties

1. Introduction

In the past two decades, multicomponent materials and, in particular, polymer compositesreinforced with natural fibers have received widespread attention around the world. Naturalfibers, such as kenaf, flax, jute, palm, hemp, pineapple, and especially coir and palm oilfibers, which are the waste products from oil production, are abundant as biomass. These nat-ural fibers have many advantages such as low density, low cost, high stiffness and toughness,less abrasiveness, reduced dermal and respiratory irritations, good thermal properties, greaterdeformability, and biodegradability compared to traditional glass and organic fibers [1,2].There are many applications of fiber/polymer composites for industrial consumption such asautomotive, geotextile, and packaging [1].

Literature reviews about the usage of coir and palm fibers for polymer composites havebeen reported only in tropical countries such as Indonesia, Malaysia, India, Thailand, etc.Fiber preparation using alkaline and acidic treatment has been investigated elsewhere [3–5].

*Corresponding author. Email: [email protected]

Journal of Adhesion Science and Technology, 2013Vol. 27, No. 12, 1290–1300, http://dx.doi.org/10.1080/01694243.2012.694275

� 2012 Taylor & Francis

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Besides, surface treatment of the coir and palm fibers has been found to improve bonding atthe fiber-matrix interface. These results showed an increase of mechanical properties of thecomposites [6,7]. The effect of coir and palm fibers on fiber reinforcement in the polymermatrix was investigated using the extrusion process for fiber composite preparation [7,8]. Thefiber lengths used in these techniques are limited to a maximum of 10mm. However, therehave been few reports on the composite preparation from cellulose fiber (>10mm length)with the heat press method and the interfacial interaction between the polymer matrix andfiber is not well understood.

In the present work, coir and palm fibers were chosen from the abundance of waste generatedby consumers and from oil production in Thailand. Especially for palm oil, it has been considereda prospective feedstock for biodiesel production [9,10]. These fibers are used to reinforce polyeth-ylene. Compression method is used to prepare the polyethylene composite because only a fewpieces of equipment are needed which minimizes machine costs [11]. The objective of this workwas to investigate the effect of fiber surface preparation and fiber content on the mechanical andthermal properties of coir or palm fiber-reinforced polyethylene composites.

2. Experimental

2.1. Fiber preparation

The raw material used for coir fiber was obtained from waste coconut shells from local mar-ket. But palm fruit bunch from an oil production factory was used for palm fiber. Firstly, theywere boiled in 25 wt.% NaOH at 100 °C for 3 h at a liquor ratio of NaOH to fiber 15:1.Then, the alkaline treated coir fibers (ACF) were bleached at 100 °C for 2 h using 50% hydro-gen peroxide (which was diluted to a 40% concentration), 1% sodium silicate, 0.5% magne-sium sulfate, and 1.5% NaOH at a liquor ratio of solution to fiber 30:1. The bleached coirfibers (BCF) were dried overnight in a hot air oven at 80 °C. The alkaline treated palm fibers(APF) were bleached following the above-mentioned method, but using less hydrogen perox-ide (only 30%). Then, the bleached palm fibers (BPF) were dried overnight in a hot air oventhe same way as BCF. All bleached fibers were beaten manually and then only fine fiberswere selected.

2.2. Composite preparation

The coir and palm fibers were arranged randomly using a mini carding machine to form afiber nonwoven mat. The fiber weights per unit surface area were controlled at 50–200 g/m2

of fiber mat (Figure 1), corresponding to fiber weights (5–20wt.%).

LDPE sheet

LDPE sheet

Fiber mat

Controlled frame

LDPE sheet

LDPE sheet

Fiber mat

Controlled frame

Figure 1. Composite preparation between the LDPE sheets and fiber mat.

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Low density polyethylene (LDPE) composites were prepared by a heat press machine at atemperature of 180 °C, with a constant pressure of 12.5MPa. Two layers of LDPE with afiber mat in the middle were heat pressed with a controlled frame of 0.25mm thickness(Figure 1).

2.3. Fiber and composite characterization

The chemical composition of fibers with alkaline and with/without bleaching treatment wasanalyzed following the standard Tappi method [12]. The fibers were characterized for ashcontent and total extractives in ethanol and benzene following the standard Tappi methodsT222 and T204-cm-97, respectively. All fibers underwent Soxhlet extraction with ethanol/ben-zene at a ratio of 1:1 (v/v) for 6 h before analyses for lignin (T222-om-02), pentosan (T223-cm-84), and holocellulose were conducted [13]. The determination of α-cellulose was per-formed after extraction of holocellulose with alkali following the standard method T203-cm-99. Surface analysis of all fibers was carried out using an ATR-FTIR (Fourier transform infra-red spectroscopy) spectrophotometer (Thermo Nicolet 380, Thermo Electron Corporation,USA) equipped with a hemispherical diamond crystal that had a 42° angle of incidence.Spectra were obtained using 32 scans at a resolution of 4 cm�1. After each treatment, fiberdiameter was measured using a light microscope with at least 50 samples tested.

A single fiber pullout test was performed following a modified method [14] to character-ize the interfacial interaction between fiber and LDPE. Fibers having a length >40mm wereused as test samples by sandwiching individual fibers between two LDPE sheets (20 � 20 �0.25mm) and hot pressing at a temperature of 180 °C. The fiber was kept straight and passedthough a controlled frame (20 � 20 � 0.5mm) having a hole with a diameter of 1mm. Thesamples of single fiber bound in LDPE sheets were analyzed by pullout test using an Auto-graph (AGS-J 50N, Shimadzu, Japan) at a crosshead speed of 1mm/min (Figure 2). Thegauge length was 25mm which was included the intended fiber within the LDPE sheet andthe extended fiber outside the LDPE sheet as seen in Figure 2. At least 10 samples weretested. Pullout stress was calculated from the pullout force divided by the fiber cross-sectionalarea in which fiber diameter was measured under a light microscope.

The LDPE composites used were characterized for their mechanical properties, accordingto ASTM 638 (type II) using an Autograph (AGS5kN, Shimadzu, Japan). The gauge lengthand crosshead speed were 60mm and 100mm/min, respectively. At least eight samples were

15 mm

10 mm

Fiber

LDPE sheet

Gauge length

Figure 2. Sample preparation for single fiber pullout test.

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tested. From tensile tests, the stress–strain curve was obtained and the initial slope of thecurve was used to calculate Young’s modulus.

A scanning electron microscope (SEM) was used to investigate the morphology of thefibers and the tensile fractured surface of composites. The cellulose fibers after treatment(with/without bleaching) were sputtered directly with gold to observe the fiber surface. Forthe fiber cross section, the cellulose fibers were fractured in liquid nitrogen before sputteringwith gold. In the case of composites, the tensile fracture surfaces were also sputtered withgold and analyzed with a SEM (JEOL JSM-5310, England) at 10 kV acceleration voltage.

In addition, the crystallization, melting temperatures, and heat of fusion of the compositeswere determined by differential scanning calorimetry (DSC822, Mettler Toledo, USA). Sam-ples were first run in a condition [15] to remove the thermal history. Then, the samples werecooled to room temperature at 10 °C/min to record the crystallization behavior and reheatedto 200 °C at 20 °C/min to trace the melting behavior.

3. Results and discussion

3.1. Fiber characterization

Table 1 shows the chemical compositions of the coir and palm fibers before and after chemi-cal treatments. The coir fibers (raw material: RM) contained high lignin content (29.8%) withα-cellulose 39.3%. Total extractives contained fatty acid, wax, volatile matter, etc. which werehigh (28.5%) due to the waxy layer coated on the fibers. For the raw palm fibers, they con-tained lower both lignin (23.1%) and total extractives content (20.8%) but higher cellulosecontent (47.8%) compared to that of coir fibers. As a result, the palm fibers were softer andhad a smaller amount of waxy layer than those on coir fibers. After alkaline treatment, thepalm fibers had higher α-cellulose but lower lignin content. After bleaching treatment, the lig-nin and pentosan contents were slightly decreased in both types of fibers. These chemicalcompositions of both fibers were in good agreement with literature values [1,7].

Figure 3 shows the diameters of the coir and palm fiber bundles. The fiber diameters didnot show significant differences between the two types of fibers due to broad distribution ofdiameters. However, the average fiber diameter of the coir fibers was less than that of thepalm fibers. After bleaching, there were no significant differences between treatment and fibertypes. However, after the beating process was applied, the fiber diameter decreased signifi-cantly for both types of fibers. Using either alkaline or bleaching treatment had no effect onthe separation of fiber bundles to individual fibers. It needed a mechanical force (beating) toseparate the fiber bundles after lignin was removed by chemical treatment.

Table 1. Chemical composition (%) of coir and palm fibers.

Coir fiber Palm fiber

RM ACF BCF RM APF BPF

Lignin 29.8 28.5 26.8 23.1 12.6 10.9α-cellulose 39.3 48.9 48.8 47.8 68.2 67.2Pentosan 16.1 22.0 16.4 16.2 22.1 20.7Holocellulose 73.4 77.1 68.5 67.0 82.7 83.3Ash 4.8 1.8 2.1 6.5 1.3 2.2Total extractives 28.5 3.1 3.4 20.8 2.8 3.7

Notes: RM: raw material; ACF: alkaline treated coir fibers; BCF: bleached coir fibers;APF: alkaline treated palm fibers; and BPF: bleached palm fibers.


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The FTIR spectra of the coir and palm fibers are presented in Figure 4. The spectra of thetwo fibers after treatment were similar and in good agreement with published results [3]. Thespectra of the alkaline treated and bleached fibers were dominated by the peaks at 3200–3600and 1033 cm�1 which were due to the stretching vibrations of O–H and C–O of main cellu-lose, respectively [16]. In addition, the peaks at 2900 and 1730 cm�1 were the C–H stretchingof cellulose and C=O stretching of hemicellulose [17], respectively. After alkaline and

ACF

BCF

0.020.040.060.080.100.120.140.160.180.200.220.240.260.280.30

Wavenumber (cm-1)

Wavenumber (cm-1)

0.140.16

Abso

rban

ce

0.020.040.060.080.100.120.140.160.180.200.220.240.260.280.30

0.140.16

Abso

rban

ce

500 1000 1500 2000 2500 3000 3500 4000

500 1000 1500 2000 2500 3000 3500 4000

BPFAPF

(a)

(b)

Figure 4. FTIR spectra of (a) coir fibers (BCF: bleached coir fiber and ACF: alkaline treated coirfiber) and (b) palm fibers (BPF: bleached coir fiber and APF: alkaline treated palm fiber).

0

100

200

300

400

500

NaOH treatment BleachingProcess

BeatingFi

ber d

iam

eter

(um

)

Coir fiberPalm fiber

Figure 3. Diameters of the coir and palm fibers after fiber preparation.


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bleaching treatments, the peaks at 2900 and 1730 cm�1 decreased due to the removal ofhemicellulose and lignin. This FTIR result was in good agreement with that of the chemicalcomposition.

SEM micrographs of the coir fibers after treatment are shown in Figure 5. The fiber crosssection was round or oval with fiber diameter from 50 to 300 μm (Figure 5(a)). The nativecoir fibers contained cylindrical cells oriented in a cross section with each cylindrical celldiameter 10 μm [14]. These fibers had a central hollow part running along the fiber axiscalled lacuna. The globular particles, which are normally embedded in the fiber surface calledtyloses, were partly removed after alkaline treatment. Thus, the fiber surface showed a largenumber of regularly holes called pits (Figure 5(b)). These were also possibly from removal oftyloses on the surface due to alkaline treatment, which has been reported elsewhere [3,18].After bleaching, the impurities were removed, resulting in more porous cylindrical cells (Fig-ure 5(c)). Moreover, reduction in the number of pits and intercellular gaps was also observed(Figure 5(d)). This treatment made the fiber rougher.

The diameter of palm fiber was larger than that of the coir fiber as seen in the SEMmicrographs (Figure 6). After alkaline treatment, the cross section of palm fibers wasmostly oval and larger than that of the coir fibers (Figure 6(a)). The palm fibers also con-tained multicellular fibers as did the coir fibers. The surface of the palm fibers showed alarge number of pits (Figure 6(b)). After bleaching treatment, more porous cylindrical cellsat cross section were observed (Figure 6(c)). Also more gaps and pits could be seenbetween fibrils as shown in Figure 6(d). The porous surface morphology of both fibers

Figure 5. SEM micrographs of coir fibers after alkaline treatment (a and b) and after bleachingtreatment (c and d).


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could be useful for better mechanical interlocking with the polymer matrix for the com-posite process [19].

3.2. Composite characterization

It is understandable that a better knowledge of the adhesion phenomena is required for practi-cal applications of multicomponent materials. However, the thermodynamic model of adhe-sion generally attributed to Schonhorn and Sharpe [20] is certainly the commonly usedapproach in adhesion science at present. In this theory, it is considered that adhesion betweentwo solids is due to interatomic and intermolecular forces established at the interface, pro-vided that an intimate contact is achieved. The most common interfacial forces result fromvan der Waals and Lewis acid–base interactions. The magnitude of these forces can generallybe related to fundamental thermodynamic quantities, such as surface free energies of bothentities in contact [21–25].

As described before, the coir and palm fibers are composed essentially of cellulose. Papir-er et al. [26] found a relationship between the dispersion component γD of the surface freeenergy and the crystallinity of cellulose fibers; γD decreases when the crystallinity of cellulosedecreases. Generally, the degree of crystallinity increased with increasing α-cellulose. Regard-ing Table 1, the α-cellulose of palm fiber was higher than that of coir fiber. Then, we sup-posed that the γD of palm fiber should estimate to be higher than that of coir fiber. Theresults of the single fiber pullout test in LDPE matrix are shown in Figure 7. We found that

Figure 6. SEM micrographs of palm fibers after alkaline treatment (a and b) and after bleachingtreatment (c and d).


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there were no significant differences among fibers before and after treatments. However, theaverage value of single fiber pullout stress tended to increase after bleaching for both fibertypes. This could be explained by lignin and impurities being removed by the bleaching treat-ment which induced better interaction of the single fiber in the matrix. Rjiba et al. found thatthe presence of waxes at fiber surface play a major role on the surface energy of cotton fibers[27]. The palm fibers, which are rich in α-cellulose (higher γD estimation), exhibited a greaterfiber-matrix interfacial interaction than coir fibers as shown by single fiber pullout stress test.

There has also been an interest to analyze the mechanical properties of fiber reinforcedcomposites. More precisely, most works in this field considered that good final mechanicalperformance or use properties of the resulting reinforced materials depend significantly on thequality of the interface which formed between both solids [28]. In regard to the mechanicalproperties of LDPE reinforced fiber (Figure 8), its Young’s modulus increased with increasingthe fiber content for both coir and palm fibers. Thus, the inclusion of the coir or palm fibersmade the LDPE matrix stiffer. With the heat press technique of composite preparation, it wasdifficult to prepare the homogenized fiber mat when adding >20% fiber content due to highfiber volume. This effect resulted in a variation of the Young’s modulus (data not shown). So,the maximum fiber content in this study was 20wt.%. Moreover, due to an effect of bleach-

050

100150200250300350400450500

0 5 10 15 20Fiber content (%)

Youn

g's

mod

ulus

(MPa

)

LDPE/ACFLDPE/BCFLDPE/APFLDPE/BPF

Figure 8. Young’s modulus of LDPE composites reinforced with coir and palm fibers at different fibercontents (ACF: alkaline treated coir fibers; BCF: bleached coir fibers; APF: alkaline treated palm fibers;and BPF: bleached palm fibers).

0

20

40

60

80

100

120

140

ACF BCF APF BPF

Fibers

Sin

gle

fib

er p

ullo

ut

stre

ss (

MP

a)

Figure 7. Single fiber pullout stress of coir and palm fibers in the LDPE matrix (ACF: alkaline treatedcoir fibers; BCF: bleached coir fibers; APF: alkaline treated palm fibers; and BPF: bleached palmfibers).


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ing treatment, it was found that an increase in the Young’s modulus of composites was signif-icant for incorporation of both fibers.

SEM micrographs of the tensile fractured surfaces of LDPE/ACF and LDPE/BCF com-posites at 15% of fiber content are shown in Figure 9. Homo-LDPE is well known for ductilebreakage when tensile force is applied. Adding fiber in the LDPE matrix still provided theductile fracture of LDPE surface with fiber pullout from the matrix. For the alkaline treatmentof the coir fibers, the interaction between LDPE matrix and fiber was weak and hence itshowed fiber pullout from matrix and presence of hole indicating easy fiber pullout fromLDPE (Figure 9(a)). In the case of the LDPE/BCF composite, the impurities were removedfrom the fiber allowing a stronger interaction between fiber and matrix to take place. Thiswas evident from length reduction in fiber pullout from matrix and an existence of cracks atthe broken fiber ends suggesting failure occurred at the fiber due to stronger interactionbetween the fiber and LDPE matrix (as shown in Figure 9(b)). Morphology of the tensilefractured surfaces of the LDPE composite with the palm fibers also had the tendency to pres-ent the same appearance of the LDPE composite with the coir fibers (figure not shown here).

The crystallinity and melting temperature as well as the degree of crystallinity of the com-posites are summarized in Table 2. The degree of crystallinity of the composite was deter-mined from Equation (1).

vcð%Þ ¼ 100� �Hf

�H0f

ð1Þ

Table 2. Crystallization and melting temperatures (Tc, Tm), heat of fusion (Hf), and crystallinity degree(Xc) of the LDPE composites reinforced by coir and palm fibers.

Tc (°C) Tm (°C) Hf (J/g) Xc (%)

LDPE 108.9 125.0 119.0 41.0LDPE/BCF5% 107.9 126.0 104.6 36.115% 107.8 125.5 105.7 36.5LDPE/BPF5% 108.4 125.4 97.5 33.615% 108.3 125.8 100.0 34.5

Notes: BCF: bleached coir fibers and BPF: bleached palm fibers.

Figure 9. SEM micrographs of tensile fractured surface of: (a) LDPE/ACF composite and (b) LDPE/BCF composite at 15% fiber content (ACF: alkaline treated coir fibers and BCF: bleached coir fibers).


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where DH0f and ΔHf are the heat of fusion of the 100% crystalline polymer (290 J/g for

LDPE in [29]) and the measured heat of fusion, respectively.Table 2 shows that there were no significant differences in both crystallinity and melting

temperatures between fiber-LDPE composite and homo-LDPE. On the other hand, the heat offusion and crystallinity of the composites decreased in the case of fiber reinforced compositeswith no significant differences between the two types of fibers. The lowering of the LDPEcomposite crystallinity compared to that of homo-LDPE could be attributed to the reductionin the structure regularity and close packing ability of the polymer chains in the presence ofthe fibers [30,31]. Both LDPE/BCF and LDPE/BPF composites showed only a single meltingtemperature peak, indicating the existence of only a single type of crystalline species in allfiber-LDPE composites.

4. Conclusion

Utilization of the coir and palm fibers from agricultural waste for reinforcement of the LDPE tocomposites was investigated. Fiber preparation with alkaline and with/without bleaching treat-ment affected the fiber–matrix interaction and its mechanical properties. Lignin and impurityremoval in both types of fibers after bleaching treatment displayed a tendency for an increase ofsingle fiber pullout stress indicating a better fiber–matrix interaction in the composites. The sin-gle fiber pullout stress of palm fiber-LDPE was higher than that of coir fiber-LDPE. TheYoung’s modulus of the composites increased with increasing fiber content. Inclusion of fiber inthe LDPE matrix did not change the crystallinity and melting temperatures of the composite. Onthe other hand, the degree of crystallinity of composites decreased with fiber inclusion.

AcknowledgmentsThis work was supported by Kasetsart University (KU). The authors would like to deeply thank UnitResearch of Pulp and Paper Technology, KAPI, KU for fiber chemical composition analysis, Laboratoirede Photochimie Moléculaire et Macromoléculaire (LPMM), UMR CNRS-UBP 6505, Aubière, Francefor FTIR analysis and Dr Michel Nardin (Institut de Science des Matériaux de Mulhouse, CNRS,France) for scientific discussion.

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