Fibers and Polymers 2014, Vol.15, No.5, 1035-1041

1035

Mechanical and Thermal Properties of Josapine Pineapple Leaf Fiber (PALF)

and PALF-reinforced Vinyl Ester Composites

A. R. Mohamed, S. M. Sapuan1*, and A. Khalina

2

Faculty of Engineering, International Islamic University Malaysia, 53100 Jalan Gombak, Kuala Lumpur, Malaysia1Faculty of Engineering, University Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

2Institute of Tropical Forest and Forest Products, University Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

(Received September 24, 2012; Revised October 29, 2013; Accepted November 14, 2013)

Abstract: Although the pineapple leaf fibers (PALF) are long known as domestic threading material in Malaysia, they arecurrently of little use despite being mechanically and environmentally sound. This study evaluated some selected propertiesof Josapine PALF and PALF-vinyl ester composites as well as the effects of simple abrasive combing and pretreatments onfiber and composite properties. Using PALF vascular bundles extracted from different parts of the leaves did not significantlyaffect PALF-vinyl ester composite mechanical properties. At low weight fraction and consolidating pressure, PALF fibersregardless of diameters and locations performed equally well in enhancing composite flexural properties under static loading.Finer bundles enhanced PALF-vinyl ester composite toughness indicated by tests at higher speeds. Abrasive combingproduces cleaner and finer bundles suitable for reinforcing composites for applications not requiring high toughness.

Keywords: PALF, Abrasive combing, Thermal stability, Mechanical properties, Vinyl ester

Introduction

Pineapple leaf fibers (PALF) have been used traditionally

as a domestic threading material for a long time in Malaysia.

Unlike in the neighboring countries where PALF are used to

make numerous items especially clothing, they are currently

agricultural waste [1]. Pineapple leaves are burned or

composted in plantations causing environmental pollutions

including haze. Despite being environmentally sound and

mechanically excellent [2,3], PALF are the least studied

natural fibers especially as reinforcement in polymer composites

[3-6]. Rather than using PALF in textile applications involving

elaborate processes, their use as reinforcement in polymer

composites may still be explored. In a preliminary study [7],

PALF-reinforced vinyl ester composites produced using

liquid compression molding was shown to have potentials in

terms of good mechanical and other properties if optimized.

Though the presence of two different types of PALF in a

pineapple leaf was described by Bartholomew et al. [8], no

other studies were found differentiating and characterizing

them in terms of their physical, mechanical and thermal

properties. No reports were found studying their respective

performance as reinforcement in polymer composites. Previous

works generally utilized fine PALF bundles despite the

presence of largely vascular bundles [9] which the authors

determined to be approximately 75 wt% of the fiber content

in the leaves. Either categorically stated or inferred from

their literature, most workers used PALF with diameters

smaller than 100 µm [3,10-13]. George et al. [14] used those

within 50-150 µm while only Mukherjee and Satyanarayana

[15] were reported to characterize PALF of 45-205 µm in

diameter without separating them into vascular bundles and

fine fiber strands.

It is also known that natural fibers like PALF may vary in

their dimensions and properties even due to their locations in

a single plant. With respect to reducing variability, it is

beneficial and necessary to qualify whether PALF may be

utilized randomly or only from a certain portion from the

leaves. Like other natural fibers, PALF are susceptible to

rotting during storage and their reinforcing capability may

be compromised especially in hot and humid Malaysian

climate. It is therefore constructive to evaluate any possible

loss in PALF reinforcing efficiency after long storage period.

Extracting natural fibers like PALF may be carried out

physically, mechanically or chemically and these separation

and extraction processes have major influences on fiber

costs, yield and final fiber quality [16]. PALF continue to be

hand-separated as the use of machines is generally slower

than the manual process [17]. Mechanical processes induce

damage to natural fibers through breaking, scotching and

hackling actions leading to tensile strength of the elementary

fibers to be only marginally higher than that of the technical

fibers [18]. Considering the difficulty in achieving fiber

defibrillation [19], Mohamed et al. [9] experimented with

abrasive combing of PALF vascular bundles. This simple

technique produced bundles with 50.3 % lower mean diameter

(p=0.01) without much negative effects on fiber integrity as

tested by single fiber tensile tests. Abrasive-combed PALF

were used in this study to reinforce vinyl ester composites

and composite flexural and impact properties were evaluated

and compared with those composites reinforced with coarse

vascular bundles and fine fiber strands.

Various treatments and modifications have been applied

on natural fibers including PALF [6] in order to improve

their properties with respect to mechanical properties, hydro-

phillicity and fiber-matrix adhesion. Bleaching with aqueous*Corresponding author: sapuan@upm.edu.my

DOI 10.1007/s12221-014-1035-9

1036 Fibers and Polymers 2014, Vol.15, No.5 A. R. Mohamed et al.

solution of sodium hypochlorite (NaOCl) has been used in

all the treatments investigated by Bel-Berger et al. [20] in

which case 0.5 % NaOCl solution was used for 60 minutes.

Used in conjunction with sodium hydroxide (NaOH), this

simple treatment produced desirable natural fiber fabric with

minimal strength loss. There is a need therefore to evaluate

the effects of this pretreatment on the characteristics of

PALF and PALF-vinyl ester composites.

In this study, thermogravimetric analysis (TGA) of untreated,

pretreated and abrasive-combed PALF was carried out. Flexural

and impact properties of vinyl ester composites reinforced

with various PALF were also evaluated and discussed. The

results were used to improve the understanding of PALF and

their use as reinforcement in vinyl ester composites in the

efforts to utilize these excellent lignocellulosic fibers.

Experimental

Materials

Six-month old manually-separated Josapine PALF vascular

bundles and fine fiber strands were utilized in this study (see

Figure 1). Abrasive-combed PALF were produced by pulling

large vascular bundles between #100 sandpapers simulating

the action of combing and separating them into finer bundles

[9]. One percent NaOCl aqueous solution was used to clean

the fibers for 1, 2 and 4 hours. Water-soaking for 24 hours

was done on one set of the fiber specimens. Fiber samples

were rinsed using tap water followed by distilled water for a

few times. Leaf tissues obtained during combing of vascular

bundles were collected for thermogravimetric analysis.

Table 1 provides most of the designations and descriptions

of various PALF used in this study.

Specimen Preparation

PALF bundles were cut into 127 mm long and laid in the

cavity of a 3-piece aluminum mold. EPOVIA RF1001M

vinyl ester resin supplied by Cray Valley Resins (M) Sdn.

Bhd. was catalyzed with 1.0 part per hundred resins of

Syrgis Andonox KP-9 methyl ethyl ketone peroxide from

the same supplier and the mixture stirred for two minute

before pouring on the fibers. In all the samples 20 wt% of

PALF was used as higher fractions would require the use of

pressure and different mold. In one set of the specimens,

newly extracted PALF fine fiber strands were used to reinforce

the composites. Slight pressure was applied on the cover to

ensure consistent sample dimensions. All samples were left

to cure at ambient temperature for a minimum of 72 hours.

Neat vinyl ester resin samples were also fabricated for

comparison. Figure 2 shows an example of the composite

specimen prepared using the above process.

Testing

Thermogravimetric (TG) Analysis

Various fiber samples were tested for their thermophysical

properties using a Perkin Elmer Diamond TG/DTA analyzer.

Specimens were scanned from 30-580oC at a heating rate of

10 oC min-1 in a nitrogen flow of 80 ml/min.

Flexural and Impact Tests

Flexural properties of the composite sheets were measured

using an Instron 3365 with a 5 kN load cell utilizing specimens

63 mm long, 12.7 mm wide and 3 mm thick and ASTM

D790 as the reference. A span to thickness ratio of 16 was

used and a crosshead speed of 2 mm/min was set. Un-

notched Charpy impact tests were carried out on specimens

having 63 mm length, 12.7 mm width and 3 mm thickness

using an Advanced Pendulum Impact tester (Dynisco Polymer

Test) and ASTM D256 as the reference. In all the above tests

Table 1. PALF sample designations and descriptions

Letter Description Letter Description

A Neat VER E Abrasive-combed whole vascular bundles

B Vascular bundles from middle portion of the leaves F Newly extracted middle technical fibers

C Vascular bundles from whole of the leaves G Bleached whole vascular bundles (1 % NaOCl – 1 hour)

D Water-soaked middle vascular bundles (24 hours) H Bleached middle vascular bundles (1 % NaOCl – 2 hour)

Figure 1. Manually-separated PALF used in this study. Figure 2. Composite specimen for flexural and impact tests.

Mechanical and Thermal Properties of Pineapple Leaf Fibre Composites Fibers and Polymers 2014, Vol.15, No.5 1037

five specimens were tested and the mean value and standard

deviation were reported.

Results and Discussion

It is obvious from Figure 3 that untreated PALF vascular

bundles (BDL10562) and untreated fine fiber strands

(FINE10872) were identical in nature as indicated by their

thermal stabilities. Pre-treating PALF with aqueous NaOCl

solution (BDL12482, AC116102 and FINE12192) reduced

PALF thermal stability due to fiber degradation as confirmed

by lower cystallinity indexes (see Table 2). The presence of

more amorphous PALF as a result of the bleach causing

greater moisture absorption may explain higher weight loss

between 100-200 oC. Shifting the curves by the average

values of weight loss between 100-200 oC revealed identical

pyrolytic rates for all five curves occurring between 200-

350 oC thus suggesting that hemicelluloses and cellulose

were still present in all samples. Furthermore, the presence

of relatively more lignin in PALF pre-treated with NaOCl

solution (BDL12482, AC116102 and FINE12192) was

clearly shown by the increase in final char products [21].

This confirms that aqueous NaOCl solution did not de-

lignify the PALF fibers. Comparing the five curves with that

of epidermal tissues strongly suggested that fine PALF

(bleached and unbleached) and abrasive-combed PALF have

less epidermal tissues compared to PALF vascular bundles

(bleached and unbleached) as reflected by relatively lower

final char products in the former cases. In addition to

separating PALF bundles, abrasive combing removed more

epidermal tissues from the fiber surfaces resulting in cleaner

fibers.

The similarity in thermal nature of PALF vascular bundles

and fine fiber strands accompanied the similarity in structure

as inferred from X-ray diffraction (XRD) analysis carried

out by Mohamed et al. [9]. Using the XRD data from this

study crystallinity indexes were calculated using equation (1)

and the results are provided in Table 2. The values of

crystallinity index of various PALF calculated were similar

to those reported by Mwaikambo [22] for non-treated jute or

treated with up to 0.08 % NaOH for 24 hours. The XRD

profiles and IXRD suggest that structurally 24-hour soaked

PALF, abrasive-combed or fine fiber strands are basically the

same. Only bleached PALF showed reduction in crystallinity

and IXRD decreased with pretreatment period. As the increase

in fiber crystallinity is the major factor contributing towards

natural fiber tensile modulus and strength and accompanying

reduction in elongation at break, this explains the reduction

in strength and ductility.

(1)

Figure 4(a) shows that adding 20 wt% untreated PALF

significantly improved vinyl ester (A) flexural strength (p=

0.05) unlike reported by Korte [19] in which significant

reduction in flexural strength was observed with similar

weight fractions of treated hemp fibers in epoxy. The

composite mechanical properties obtained clearly indicated

that the reinforcing capability of these fibers in composites

were comparable to those found in published data [12,23,24]

at similar fiber fractions. Calculating PALF fiber strength

and modulus from composite flexural strength and modulus

[19,25] gave slightly lower mean values compared those

obtained in single fiber tensile tests due to imperfect fiber-

matrix bonding but higher or close to those reported in some

literature [13,26]. The results clearly indicated that storing

PALF in hot and humid conditions did not significantly

reduce their reinforcing capability in thermoset composites.

There was no significant difference between flexural

strength of composites reinforced with PALF taken from the

middle portion of the leaves (B) and those taken from the

whole length of the leaves (C). Though not statistically

significant (p=0.05), the flexural strengths of composites

reinforced with finer abrasive-combed bundles (E) and fiber

strands (F) however were relatively higher. Abrasive-combed

PALF (E) and fine fiber strands (technical fibers (F)) performed

equally well in this instance. Apparently, at 20 wt% PALF

and low molding pressure, PALF diameter did not influence

composite flexural strength significantly.

As shown in Figure 4(b), adding PALF increased the

composite bending stiffness significantly (p=0.01). As with

strength, addition of PALF taken from different locations in

IXRDI200 Iam–

I200------------------- 100×=

Figure 3. TGA curves for non-treated and bleached PALF.

Table 2. Crystallinity indexes of various PALF

Types of PALF Crystallinity index

24 hour water-soaked 73.68

Abrasive-combed 72.85

Fine fiber strands 73.38

1 % NaOCl (2 hours) 72.28

1 % NaOCl (4 hours) 70.97

1038 Fibers and Polymers 2014, Vol.15, No.5 A. R. Mohamed et al.

the leaves did not alter the stiffness. Similarly, it was clear

that fiber diameter (E and F) did not seem to influence the

stiffness of resultant composites differently. The bending

behavior was very similar to that reported by Mishra et al. [12]

in which case much finer PALF were used. This suggests

that at low fiber weight fraction and low consolidating pressure,

PALF vascular bundles and fine fiber strands performed

equally well as reinforcement in vinyl ester composites. Also,

untreated PALF may be used from different locations on the

leaves and used randomly without significantly influencing

flexural properties of PALF-vinyl ester composites.

Soaking PALF in water for 24 hours was not beneficial in

enhancing composite flexural strength and modulus as

indicated in Figure 5(a-b). The focus should be in removing

soil and dirt through adequate washing with water without

resorting to prolonged soaking thus saving time and cost.

Using PALF pre-treated in aqueous NaOCl solution resulted

in reduced composite strength mainly due to loss of fiber

strength and ductility. Sodium hypochlorite bleach used at

high concentrations or long soaking time has been known to

cause chain scission and consequently degradation of textile

fabrics. Increase in fiber stiffness and dramatic drop in

ductility of bleached PALF caused the significant increase in

the composite stiffness (H).

For fibers with similar diameters such as vascular bundles

(B and H), the concept of ‘normalized fiber strength’ was

found to be the deciding factor in composite flexural strength

(see Figure 6(a)). As flexural strength depends also on the

fiber-matrix interfacial shear stress, finer fibers (E and F)

resulted in higher interfaces generated thus higher flexural

strength (Figure 6(b)). Similar phenomenon was observed

with composite flexural modulus (see Figure 7(a-b)).

Figure 8(a) shows that adding 20 wt% of untreated PALF

into vinyl ester increased the impact strength by more than

four-folds with the mean value almost equal to that of 30 wt%

detergent-washed PALF-reinforced polyester composites [12].

Figure 4. (a) Flexural strength and (b) modulus of neat VER and PALF-reinforced VE composites.

Figure 5. (a) Flexural strength and (b) modulus of neat VER and PALF-reinforced VER composites.

Mechanical and Thermal Properties of Pineapple Leaf Fibre Composites Fibers and Polymers 2014, Vol.15, No.5 1039

Using PALF vascular bundles taken from different parts of

the leaves (B and C) however did not significantly alter the

impact strength of the composites thus confirming that

PALF fibers may be mixed and used at random. The results

strongly suggest that using PALF fiber strands (F) led to

better composite toughness than using vascular bundles, i.e.,

PALF fiber diameter does play an important factor in

determining the impact strength of PALF-reinforced vinyl

ester composites at low fiber weight fraction using hand lay-

up process through higher number of fiber-matrix interfaces

Figure 6. The influence on composite flexural strength by (a) normalized fiber strength and (b) fiber elongation at break and fiber diameter.

Figure 7. The influence on composite flexural modulus by (a) normalized fiber modulus and (b) fiber elongation at break and fiber diameter.

Figure 8. Impact strengths of (a) neat VER and various untreated PALF-reinforced VER composites and (b) treated PALF-VER composites.

1040 Fibers and Polymers 2014, Vol.15, No.5 A. R. Mohamed et al.

[27]. It is expected that with higher fiber weight fractions

and higher consolidating pressures, even greater enhancement

may be achieved through even higher number of fiber-

matrix interfaces.

Although the use of fine abrasive-combed PALF bundles

(E) did not seem to negatively affect the flexural properties

of PALF-reinforced vinyl ester composites and they performed

favorably with fine fiber strands (F), its use did reduce the

composite toughness (see Figure 8(a) and Figure 9). This

behavior may be explained by the fact that abrasive combing

introduces defects on the fibers and the negative effects of

these defects were not detectable during low speed fiber and

composite flexural tests. This study has shown that poor

fiber integrity may be detected through testing at higher

speeds and fiber mechanical properties must therefore be

tested and compared at both low and high test speeds.

However, this shortcoming may be expected to decrease or

even disappear at higher fiber fraction and with higher

consolidating pressure.

The toughness of composites reinforced with bleached

PALF (G and H) was significantly reduced due to the highly

brittle fibers (see Figure 8(b)). Longer pretreatment period

did not reduce the toughness further however which may be

explained by the lack of further degradation of PALF. This

phenomenon may be observed as the plateau in the PALF

strength-treatment data calculated and reported by Mohamed et

al. [9] and the reduction in dramatic decrease of PALF

elongation at breaks observed therein.

More studies are needed in order to study the effects of the

abrasive combing on the performance of PALF in PALF-

vinyl ester composites. However, the above results suggest

that this simple extraction method may be used to produce

finer PALF bundles with reasonable quality.

Conclusion

It can be concluded from the present study that the

reinforcing capability of PALF is not degraded after long

storage in hot and humid local conditions. Flexural and

impact properties of PALF-reinforced vinyl ester composites

are not affected by the presence of some epidermal tissues

on the fibers and by the PALF location in the leaves.

Adequate washing of PALF is necessary while prolonged

soaking as well as pretreating PALF with dilute household

NaOCl solution are not beneficial in terms of enhancing

PALF mechanical and thermal properties as well as improving

PALF-vinyl ester adhesion. Under low consolidating pressure,

flexural strength and modulus increase with increasing PALF

volume fraction while fiber diameter does not significantly

affect the flexural property of PALF-reinforced vinyl ester

composites. PALF diameter does affect significantly PALF-

reinforced vinyl ester composite toughness. Abrasive combing

produces cleaner and finer bundles suitable for reinforcing

composites for applications not requiring high toughness

and thus may be further investigated for its potential in

PALF extraction. The study also indicates that the PALF-

reinforced vinyl ester composites can be used to make

products with reasonable properties for applications such as

interior automotive and household use.

Acknowledgements

The researchers thank Mr. Zulkafli, A. from Sepang,

Selangor, Malaysia for supplying the pineapple leaves and

Ms. Normalely, O., Mr. Luqman, H.M. and Ms. Fathimah,

Z.M. for extracting the PALF. Special thanks are due to Mr.

Ibrahim, R., Mr. Syamsul Kamal, A., Mr. Rahimie, A.A and

Mr. Mohd. Hairi, M.R. from the Faculty of Engineering,

International Islamic University Malaysia and Mr. Wildan

M.I.M.G. from the Faculty of Engineering, Universiti Putra

Malaysia for assistance in carrying out the tests.

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