A study of the mechanical properties of short natural-fiber

reinforced composites

P.J. Herrera-Franco*, A. Valadez-Gonzalez

Centro de Investigacion Cientıfica, de Yucatan A.C., Division de Materiales, Calle 43 # 130, Col. Chuburna de Hidalgo, C.P. 97200 Merida, Yucatan, Mexico

Received 10 January 2005; accepted 10 April 2005

Available online 22 June 2005

Abstract

The degree of fiber–matrix adhesion and its effect on the mechanical reinforcement of short henequen fibers and a polyethylene matrix was

studied. The surface treatments were: an alkali treatment, a silane coupling agent and the pre-impregnation process of the HDPE/xylene

solution. The presence of Si–O–cellulose and Si–O–Si bonds on the lignocellulosic surface confirmed that the silane coupling agent was

efficiently held on the fibres surface through both condensation with cellulose hydroxyl groups and self-condensation between silanol groups.

The fiber–matrix interface shear strength (IFSS) was used as an indicator of the fiber–matrix adhesion improvement, and also to determine

a suitable value of fiber length in order to process the composite with relative ease. It was noticed that the IFSS observed for the different fiber

surface treatments increased and such interface strength almost doubled only by changing the mechanical interaction and the chemical

interactions between fiber and matrix.

HDPE-henequen fiber composite materials were prepared with a 20% v/v fiber content and the tensile, flexural and shear properties were

studied. The comparison of tensile properties of the composites showed that the silane treatment and the matrix-resin pre-impregnation

process of the fiber produced a significant increase in tensile strength, while the tensile modulus remained relatively unaffected. The increase

in tensile strength was only possible when the henequen fibers were treated first with an alkaline solution. It was also shown that the silane

treatment produced a significant increase in flexural strength while the flexural modulus also remained relatively unaffected. The shear

properties of the composites also increased significantly, but, only when the henequen fibers were treated with the silane coupling agent.

Scanning electron microscopy (SEM) studies of the composites failure surfaces also indicated that there is an improved adhesion between

fiber and matrix. Examination of the failure surfaces also indicated differences in the interfacial failure mode. With increasing fiber–matrix

adhesion the failure mode changed from interfacial failure and considerable fiber pull-out from the matrix for the untreated fiber to matrix

yielding and fiber and matrix tearing for the alkaline, matrix-resin pre-impregnation and silane treated fibers.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: (C)Resin; Fiber–matrix

1. Introduction

Cellulosic fibers, like henequen, sisal, coconut fiber

(coir), jute, palm, bamboo, wood, paper in their natural

condition, as well as, several waste cellulosic products such

as shell flour, wood flour and pulp have been used as

reinforcement agents of different thermosetting and thermo-

plastic resins [1–8].

During the decortication of the henequen leaves, and also

during the transformation of the raw fibers into cordage,

1359-8368/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.compositesb.2005.04.001

* Corresponding author.

E-mail address: pherrera@cicy.mx (P.J. Herrera-Franco).

approximately a 10% of waste is produced. These waste

fibers could be profitably used in the manufacture of short-

fiber polymer reinforced composites because they posses

attractive physical and mechanical properties [9]. These

waste fibers are composed of w60% of cellulose pulps

which are easily obtained and could also be used to

reinforce polymeric materials.

Unlike the traditional engineering fibers, e.g. glass and

carbon fibers, and mineral fillers, these lignocellulosic fibers

are able to impart the composite certain benefits such as low

density, less machine wear during processing than that

produced by mineral reinforcements, no health hazards, and

a high degree of flexibility. The later is especially true

because these fibers, unlike glass fibers, will bend rather

than fracture during processing. Whole natural fibers

Composites: Part B 36 (2005) 597–608

www.elsevier.com/locate/compositesb

Table 1

Nomenclature used for the different fibre surface treatments

Keyword Description

Fib Fiber without treatment

FIBNA Fiber treated with a NaOH aqueous solution

FIBPRE Fiber pre-impregnated with dissolved HDPE

FIBNAPRE Fiber treated with a NaOH aqueous solution and then

pre-impregnated with dissolved HDPE

FIBSIL Fiber treated with a silane coupling agent

FIBNASIL Fiber treated with a NaOH aqueous solution and then

with a silane coupling agent

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608598

undergo some breakage while being intensively mixed with

the polymeric matrix, but this is not as notorious as with

brittle or mineral fibers. Also, natural fibers impart the

composite high specific stiffness and strength, a desirable

fiber aspect ratio, biodegradability, they are readily

available from natural sources and most important, have a

low cost per unit volume basis. It should also be mentioned

that the hollow nature of vegetable fibers may impart

acoustic insulation or damping properties to certain types of

matrices.

One difficulty that has prevented a more extended

utilization of the henequen fibers is a lack of a good

adhesion to most polymeric matrices. The hydrophilic

nature of natural fibers adversely affects adhesion to a

hydrophobic matrix and as a result, it may cause a loss of

strength. To prevent this, the fiber surface has to be modified

in order to promote adhesion.

Several methods to modify the natural fiber surface such

as: graft copolymerization of monomers onto the fiber

surface, the use of maleic anhydride copolymers, alkyl

succinic anhydride, stearic acid, etc. have been proposed. It

is also known that the use of coupling agents such as silanes,

titanates, zirconates, triazine compounds, etc. also improves

fiber–matrix adhesion [10–14]. Furthermore, is has been

proven that pre-impregnation of the fiber with the polyolefin

solution will also improve fiber–matrix adhesion [15]. The

mechanical properties of a fiber-reinforced polymer com-

posite depend not only on the properties of constituents, but

also on the properties of the region surrounding the fiber

known as the interphase. Stress transfer from the matrix to

the fiber takes place at such interphase and, therefore, it is

important to characterise its properties to better understand

the performance of the composite. In this paper, the degree

of mechanical reinforcement for a thermoplastic matrix that

can be achieved with short-natural fibers by modifying the

fiber surface by different treatments and the optimization of

the composite material properties is studied. The chemical

surface modification of henequen fibres was carried out

using an organofunctional silane coupling agent in a

methanol/water medium. Heat treatment (curing) was

applied after reaching the equilibrium adsorption of the

pre-hydrolysed silane onto the lignocellulosic substrate. The

modified fibers were then characterised by diffuse reflec-

tance infrared spectroscopy (DRIFT).

1.1. Characteristics of the henequen fibers (Agave

fourcroydes)

Depending on their origin, natural fibers can be grouped

into bast (jute, banana, flax, hemp, kenaf, mesta), leaf

(pineapple, sisal, henequen, screw pine) and seed or fruit

fibers (coir, cotton, oil palm). Cellulose is the main

component of the henequen fibers and the elementary unit

of a cellulose macromolecule is anhydro d-glucose, which

contains three hydroxyl (OH–) groups. These hydroxyl

groups form hydrogen bonds inside the macromolecule

itself (intramolecular) and between other cellulose macro-

molecules (intermolecular).

The characteristics of henequen fibers have been studied

by several authors. The chemical analysis of the fibers

indicated that their main component is cellulose (60%),

hemicellulose (28%), lignin (8%), extractives (4%). The

structural arrangement of these components in the fiber, as

in other natural hard fibers, is in such a way that lignin acts

as a cementant matrix for the cellulosic fibers, which posses

their own structure. The mechanical properties of the

henequen fibers are a consequence of such structural

arrangement. The mechanical properties of the henequen

fibers are: tensile strength, 500G70 MPa, strain at break,

4.8G1.1% and Young’s modulus, 13.2G3.1 GPa.

2. Materials and experimental procedures

High density polyethylene, HDPE (Petrothene) extrusion

grade, was supplied by Quantum Chemical Inc. A melt flow

index (MFI) for the HDPE of 0.33 g/10 min was determined

following the ASTM standard D-1238-79 at 190 8C and

using a weight of 2160 g. A density for the HDPE of

0.96 g/cm3 was determined following the ASTM standard

D-792-86, using benzene as an immersion liquid. The

melting point (135 8C) was determined in a DSC-7 Perkin–

Elmer calorimeter.

Henequen fibers with an average diameter of 180 mm,

approximately, were used in the form of short fibers (6 mm

long) and it was supplied by Desfibradora Yucateca, S.A.

(DESFIYUSA Co.) of Merida, Yucatan, Mexico.

Sodium hydroxide and xylene, reagent grade from

Tecnica Quımica S.A., were used for the various surface

treatments. As a coupling agent, vinyltris (2-methoxy-

ethoxy) silane (Silane A-172) from Union Carbide was

used. Dicumyl Peroxide from Polyscience was used as

catalyst to the reaction between the silane coupling agent

and the polymer.

2.1. Fiber surface treatments

The nomenclature for the different henequen fiber

surface treatments used is shown in Table 1.

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608 599

2.2. Treatment with NaOH

The fibers were treated with a NaOH aqueous solution

(2% w/v) for 1 h at 25 8C, then, they were washed with

distilled water until all the sodium hydroxide was

eliminated, that is, until the water no longer indicated any

alkalinity reaction. Subsequently, the fibers were dried at

60 8C for 24 h.

2.3. Treatment with a silane coupling agent

For the fiber surface treatment with the coupling agent,

1% silane and 0.5% dicumyl peroxide (Polyscience), weight

percentage with respect to the fiber, were dissolved for their

hydrolysis in a mixture of methanol–water (90/10 w/w) at

25 8C. The pH of the solution was adjusted to 3.5 with acetic

acid with continuous stirring for 10 min. Then, the fibers

were immersed in the solution and left for 1 h under

agitation. Afterwards, the fibers were dried at 60 8C for 24 h

and at 120 8C for 2 h.

2.4. Surface pre-impregnation with a polyethylene dilute

solution

The henequen fibers were pre-impregnated with a

1.5% w/w HDPE/xylene solution as follows: HDPE in

powder form was dissolved in xylene at 110 8C in a Kettle

reactor by continuously stirring it with a magnetic bar. The

natural fibers were placed in a stainless-steel basket and

carefully immersed in the hot solution and stirred continu-

ously for 1 h. Then, the basket was removed and the lumps

of fibers were transferred to a flat tray and kept in an oven at

60 8C for 24 h to allow evaporation of the solvent. The

lumps of impregnated fibers were dispersed before blending

them with the matrix.

Fig. 1. Schematic representation of the micromechanical techniques used

for the determination of the interfacial shear strength measurement: (a)

pull-out test; and (b) single fiber fragmentation test.

2.5. Composites processing

A 20% v/v fiber content HDPE-henequen fiber compo-

site was chosen in order to determine the effect of the

different surface treatments on its mechanical properties.

The henequen fibers were incorporated into the HDPE

matrix, at 180 8C using a Brabender Plasticorder intensive

mixer, model PL330. The mixing process was performed in

the following order. First, one half of the HDPE was placed

inside the mixing chamber for about 1 min at 30 rpm; then

henequen fibers were added over a period of 3 min. Then,

the other half of the HDPE was placed inside the mixing

chamber and the mixing speed was increased to 60 rpm for

5 min. The total mixing time process was 10 min. The

resulting material was compression molded at a pressure of

1 ton using a Carver laboratory press and a temperature of

180 8C. The specimens for the mechanical test were

obtained from these laminates according to ASTM

standards.

2.6. Fiber–matrix adhesion characterization

2.6.1. Fiber pull-out

Is a direct method used for the characterization of the

fiber–matrix interphase and relies on the use of single fiber–

matrix adhesion and detection of failure modes. In this test

the fiber is pulled out of the matrix (Fig. 1), which can be

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608600

a block of resin, a disc, or a droplet [16]. In this case, one

end of the fiber was embedded in the middle plane of a plate

made from the resin. The fibers were first aligned on a plate

and a second one was used to complete the assembly. The

samples were made using compression molding at a

pressure of 1 ton using a Carver laboratory press and a

temperature of 180 8C. The load and displacements were

monitored continuously and upon fiber pull-out, the load (P)

registered at debonding was converted into an average

interfacial shear strength (t) by

t ZP

pdL

where d is the fiber diameter, L is the embedded length of

the fiber. The force was applied by holding one end of the

resin plate fixed and pulling from the free end of the fiber

using an Instron Universal Testing Machine Model 1125,

equipped with a 50 kg load cell, after conditioning at 25 8C

and a loading speed of 0.02 mm/min was used.

2.6.2. Single fiber fragmentation

In this method, a single-fiber coupon is loaded in tension

to determine the stress transfer efficiency at the fiber–matrix

interphase. Upon load application, the tensile stress on the

fiber (sf) increases and fragmentation occurs at points where

its tensile strength is reached. This fragmentation process

continues as the applied stress is continuously increased. At

some point, the fiber fragments become so short that stress

transferred to the fiber though the interface is no longer high

enough to cause any further fiber breakage. When this

occurs, it is said that a critical fiber fragment length (lc) has

been reached [16]. To a first approximation, the interface

shear strength (t) is calculated from a simple relationship

obtained from a force balance on a fiber fragment and the

recognition of the random nature of the fiber-fragmentation

process as:

t Zsf

2bG 1 K

1

a

� �

here G is the gamma function, a and b are Weibull shape

and scale parameters.

2.7. Mechanical properties

Both, tensile and flexural tests were performed using an

Instron Universal Testing Machine Model 1125, equipped

with a 500 kg load cell, after conditioning at 25 8C

according to ASTM standards D638 and D790, respectively.

The cross-head speed used for the type IV tensile specimens

was 5 mm/min. For the Flexural test (three point bending) a

specimen with nominal dimensions of 50!25!2 mm3, a

span of 32 mm and a cross-head of 1 mm/min were used.

The Iosipescu Shear Test was carried out following the

ASTM D-5379 standard using a Wyoming Shear Test

Fixture adapted to the Instron machine, after conditioning at

25 8C. The Iosipescu shear test specimens were cut from the

laminates obtained previously, and the dimensions of

specimens were 76 mm of length, 19 mm of wide, 2 mm

of thickness and the distance between the two 908 notches

was 12 mm. The cross-speed used was 0.5 mm/min.

2.8. Infrared spectroscopy (FTIR)

The FTIR analysis was performed using a Nicolet model

Protege 460 Magna IR spectrometer. Diffuse reflectance

spectroscopy (DRIFTS), with a deuterated triglycine

sulphate (DTGS–KBr) detector, was used. In order to

obtain a good resolution spectra it was necessary to mill the

henequen fibers to an average length of 0.2 mm. The milled

fibers were then mixed with an analytical grade KBr and the

spectra were recorded with a 2 cmK1 resolution and 200

scans.

2.9. Scanning electron microscopy

The failure surfaces of shear for specimens will be used

to illustrate the effect of the different fiber surface treatments

and their effect on the effective properties of the composite.

This analysis was performed using a Zeiss DSM 940A

Scanning Electron Microscope (SEM). Fracture surfaces of

the composite samples were coated with gold and then

analyzed using the SEM operated at 20 keV.

3. Results and discussion

3.1. Surface treatments

3.1.1. Alkaline treatment

Alkaline treatments, similarly as with other lignocellu-

lose fibers, induced variation in the physicochemical

properties of henerquen fibers. Valadez et al. using [18]

FTIR and TGA pointed out that these lignocellulosic fibers

can experience a significant weight loss due to the partial

dissolution of hemicellulose, lignin and pectin. They clearly

identified that the band around 1730 cmK1, corresponding

to hemicellulose, disappears when the fiber is treated by a

dilute NaOH aqueous solution. The DRIFT spectra of FIB

and FIBNA and their subtraction in the region between 1500

and 650 cmK1 are shown in Fig. 2. It could be appreciated

that the strong band at 1240 cmK1 disappears whereas two

bands become evident, the doublet at 1200 and 1230 cmK1.

These behavior has been associated to the mercerization of

lignocellulosic fibres [19,20]. Mercerization removes waxy

epidermal tissue, adhesive pectins and hemicelluloses that

bind fiber bundles to each other and to the pectin and

hemicellulose rich sheats of the core [21]. In Fig. 3, features

of the fibers surfaces are shown. A comparison between the

native fiber and the alkaline treated one reveals topogra-

phical changes because of the removal of low molecular

weight compounds resulting in a formation of a rougher

1500 1350 1200 1050 900 7500.0

0.1

0.2

0.3

0.4

1600 1400 1200 1000 800 600

0.0

650

1370

700

1320

1430

1060

1111

1161

1200

1240

1230

895

Abs

orba

nce(

a.u)

Wavenumber(cm–1)

FIBFIBNA

700

124013

70

965

895

Abs

orba

nce(

a.u)

Wavenumber(cm–1)

1

Fig. 2. Fourier Transform Infrared (FTIR) spectra for untreated henequen

fibers (FIB) and a Na(OH) aqueous solution treated fibers (FIBNA).

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608 601

surface. One of the consequences of the topographical

change is an increment of the surface area.

Fig. 3. SEM photomicrographs of henequen fibers for (a) untreated

henequen fibers (FIB) and (b) Na(OH) aqueous solution treated fibers

(FIBNA), showing their surface features.

3.2. Silane treatment

The spectra in the 1500–600 cmK1 range, for FIBSIL

and FIBNASIL and their subtraction spectra are shown in

Fig. 3. The broad intense bands around 1200 was assigned to

the stretching of the –Si–O–cellulose and the absorption

bands at 700 and at 765 cmK1, were assigned to the –Si–O–

Si– bonds [18,19,23–25]. It should be noted that the

intensity of these bands was more evident after the curing

at 120 8C, suggesting that both the grafting of silane onto

cellulose and the intermolecular condensation between

adjacent adsorbed –Si–OH groups had been substantially

enhanced. The peaks near 1060 cmK1 are related to residual

unhydrolysed Si–ethoxy–methoxy groups and their small

intensity, compared to those of the spectrum of the

hydrolysed A-172 (not shown), suggested that most of the

silane adsorbed under our conditions had been hydrolysed.

These peak assignments are in agreement with those

reported in other studies dealing with glass surfaces treated

with the same coupling agents [26,27]. These findings

suggest that the condensation reaction between the hydro-

lysed silane and the henequen fiber was carried out in a

greater extent in FIBNASIL than FIBSIL (see Fig. 4).

Table 2

Fiber critical length and fiber aspect ratio obtained for the different surface

treatments

Fiber sur-

face treat-

ment

Critical

length (mm)

Fiber aspect

ratio (lc/df)

IFSS pull-

out (MPa)

IFSS SFFT

(MPa)

FIB 12.96 72 2.5 5.4

FIBPRE 11.40 63.33 4.2 5.0

FIBNA 9.25 51.38 3.5 6.0

FIBNAPRE 6.00 33.33 4.0 9.2

FIBSIL 5.45 30.27 3.75 11.9

FIBNASIL 3.50 19.44 5.0 16.0

3.3. Stress transfer at the interface between the short fiber

and the matrix

Generally, the best mechanical properties in a composite

depend mainly on fiber orientation, but the adhesion

between the fiber and the matrix is also important. This is

especially true for short fiber reinforced composites. The

fibers are loaded through the matrix and for good

performance, the load must be transferred effectively to

the fiber, and a strong fiber/matrix bond is required.

The micromechanical events that occur for a long fiber

reinforced composite are not the same as those observed for

a short reinforced composite. In a short fiber there are

variations in stress distribution along the fiber–matrix

interface, and end effects can be neglected in the case of

long fibers, but they can be very important in the case of

short fiber reinforced composites. Additionally, because the

fibers are not oriented parallel to the applied loads, fiber–

matrix adhesion should be studied more carefully. Table 2

0.80.60.40.20.00

50

100

150

200

250

300

PU

LL-O

UT

ST

RE

SS

(M

Pa)

STRAIN (mm/mm)

fibfibnafibnaprefibnasil

Fig. 5. Typical average interfacial shear stress from pull-out experiments

versus displacement curves for HDPE and henequen fibers subjected to

different surface treatments.

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608602

shows fiber critical lengths and fiber aspect ratios obtained

for the different fiber surface treatments, obtained using the

single fiber fragmentation test.

For all the surface-treated fibers there was a noticeable

decrease of the fiber critical length as compared to that

measured for the untreated fibers. For the untreated fibers

(FIB), an average critical length value of 12.96 mm was

obtained, 6 mm for the pre-impregnated fiber (FIBNAPRE)

and 3.5 mm for the silane treated fiber (FIBNASIL). The

dimension of the fiber critical length is important from

several points of view. First, it is convenient to select a

suitable fiber length in order to allow for an adequate flow of

the molten composite material during processing. This will

avoid the development of excessive temperature in the

composite. Especially during injection or extrusion, extre-

mely high pressures are developed, and consequently, very

shear stresses can produce fiber breakage or splitting.

The increase in the respective calculated IFSS values

were 5.4 MPa (FIB), 9.2 MPa for FIBNAPRE and 16.0 MPa

in the case of FIBNASIL. The initial treatment with the

aqueous solution of NaOH removed some lignin and

hemicelluloses from the fiber surface, thus, the fiber surface

area also increased. Such fiber surface increase resulted in a

larger area of contact between the fiber and the matrix.

Then, the hydroxyl groups on the cellulose fibers could

better react with the silane-coupling agent because a larger

number on possible reaction sites were available. It is

interesting to note that even though a pre-impregnation of

the fiber with a solution of the matrix ensures a better fiber

wetting, there was a decrease of the measured fiber critical

length, and the corresponding increase of the IFSS, does not

seem to improve considerably.

Typical curves of average interfacial shear stress versus

displacement are shown in Fig. 5. It can be noted that all the

curves shown in this figure exhibit the non-linear behavior

characteristic of a ductile matrix. However, once the load

1500 1350 1200 1050 900 750

0.0

0.1

0.2

0.3

0.4

1500 1350 1200 1050 900 750 600–0.1

0.0

0.1

1100

1061

1370 13

30

1240

840 70

0

765

Abs

orba

nce(

a.u)

Wavenumber(cm–1)

FIBSILFIBNASIL

1100

765

700

665

1240

1200

1330

965

1061

Abs

orba

nce(

a.u)

Wavenumber(cm–1)

Fig. 4. Fourier Transform Infrared (FTIR) survey spectra for (a) silane

coupling agent solution treated henequen fibers (FIBSIL) and (b) silane

coupling agent solution plus NaOH aqueous solution treated henequen

fibers FIBNASIL.

reaches its maximum value there are clearly significant

differences in the way of these curves drop. It can be seen

that during the pull-out process the debond force follows a

linear behavior but the slope differs between them

depending on the fiber–matrix interactions. For the

untreated and the alkaline solution treated henequen fibers,

it can be seen that after reaching the maximum force value,

then, there is a smooth transition and a decrease in a linear

fashion until the total embedded length of fiber is pulled-out.

This behavior agrees well with the behavior of a poor

interphase that results as a consequence of the incompat-

ibility of the hydrophilic fiber and hydrophobic matrix.

In the case of a combination of the alkaline solution and

the matrix-pre-impregnation treatment, the pull-out force

was higher than that observed for the previous fiber

treatments. After reaching a maximum the pull-out process

takes place more rapidly but some frictional effects could be

mentioned. That is, after failure of the interphase, the fiber

can be extracted in a controlled way and the friction

measured up to the final point [14]. In the case of the fiber

treated with the alkaline solution and the silane coupling

agent, the pull-out force was higher that the previous

treatments, but the interphase failed and pull-out process

occurred catastrophically.

From the load-embedded length curve two character-

istics should be noted: the initial portion of the curve is

linear only in a very small portion and that the slope varies

for each surface treatment. The first is indicative of a ductile

interphase behavior and the second that the interaction

between fiber and matrix is different for each treatment.

Several authors [15,28,29] have pointed out that the

silanization of the natural fibers modifies the surface

properties and increases the fiber–matrix interaction. The

silane used in this study has two functional groups: an

ethoxy–methoxy group able to react with hydroxil groups of

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608 603

henequen fibers and a vinyl group which reacts with HDPE,

so a picture of this interphase is one with silane groups

chemically attached to fiber surface on one side and bonded

with some chains of the matrix in the other one.

3.4. Mechanical properties

In order to determine whether any statistically signifi-

cant difference existed between the values of mechanical

properties as a result of the application of one or more

different fiber surface treatments, a One Way Analysis of

Varianza (ANOVA), was performed. All pairwise multiple

comparison procedures were performed following a

Student–Newman–Keuls method. Also, all strength values

for the different fiber surface modifications are related to

the strength obtained for the untreated fibers.

3.5. Tensile test results

The tensile strength of the HDPE/henequen fibers

(80:20 v/v) composites is shown in Fig. 6. When the fiber

surface was modified with an aqueous NaOH solution

(FIBNA) or with the pre-impregnation process (FIBPRE),

the tensile strength of the composite did not seem to

improve noticeably. With the pre-impregnation process

(FIBNAPRE), the tensile properties of the composite

showed a small improvement. The tensile strength increases

11% resulting from the pre-impregnation process resulted in

an enhancement of the mechanical interlocking. However,

when the silane coupling agent (FIBSIL) is used, a 19%

increase in the tensile strength was observed. Furthermore,

when the henequen fibers are first treated with alkaline

solution and then with the silane coupling agent (FIB-

NASIL) a 30% increase in the tensile strength was observed.

This increment is attributed to chemical interactions in

Fig. 6. Tensile strength of a HDPE/henequen-fiber (80:20 v/v) composite,

plotted as a function of fiber surface treatment.

the case of FIBSIL and to both, mechanical and chemical

interactions in the case of FIBNASIL. No noticeable effect

on the elastic modulus was noticed from the fiber–matrix

interface improvements of the HDPE-henequen fibers

composites (Fig. 7). Bisanda et al. [22] showed that the

mercerization of sisal fibres (0.5 N NaOH-solution for 72 h)

greatly improved the resin pick-up or wettability of the

fibres, which resulted in a 21% rise in the compressive

strength of the composite. It is believed that this alkali

treatment results in an improvement in the interfacial

bonding by giving rise to additional sites of mechanical

interlocking, hence promoting more resin/fibre interpene-

tration at the interface.

It is observed in this study, that the ratio between values

of fiber–matrix interfacial shear strength for the different

fiber surface techniques with respect to the untreated fibers

is different from the ratio of material property values

obtained from the composites using treated fibers and the

untreated fibers. It is well known that the fiber reinforcing

effect is most efficient along the fiber axis orientation.

However, the processing technique will dictate the final

fiber orientation distribution, one of the most important

characteristics, which will determine the composite mech-

anical properties. The anisotropy expected from an injection

molded part is not the same as that found in extruded or

compression molded parts.

Furthermore, the mathematical prediction of a composite

stiffness is well established for common fiber orientation

distributions: (1) when all the fibers are oriented parallel to

the test direction, (2) when all the fibers are oriented in a

transverse direction, perpendicular to the test direction and

(3) when all the fibers are randomly oriented. The formulas

developed for these different scenarios have been developed

for fibers, which because of the high stiffness will remain

straight after processing. However, in the case of natural

Fig. 7. Elastic modulus of a HDPE/henequen-fiber (80:20 v/v) composite,

plotted as a function of fiber surface treatment.

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608604

fibers, they do not remain straight after processing because

they are highly flexible.

The effect of the fiber surface modifications on the micro

failure mode, the surface of fractured specimens were

examined using SEM. Fig. 8 shows representative photo-

micrographs of fracture surfaces of samples subjected to

tensile stresses, for the untreated, NaOH aqueous dilute

solution treated, and those which additionally were

subjected to a matrix pre-impregnation and to a silane

coupling agent. The surfaces of the untreated fibers (FIB)

are completely devoid of matrix material. This is a clear

indication of fiber–matrix interfacial failure followed by

extensive fiber pull-out from the matrix. Furthermore, the

matrix also shows considerable tearing. The fibers subjected

to the alkaline aqueous solution bath plus matrix pre-

impregnation also show interfacial failure but in this case,

there is not much indication of fiber pull-out, but rather, it

shows massive failure of both fibers and matrix. The failure

mode observed on the fibers shows fiber splitting and tearing

and it is attributed to a better interaction with the matrix, but

still such interaction is by friction and mechanical

interlocking. Also, when the fibers were pre-impregnated

with the matrix in a dilute solution form, there were clear

indications and many traces of matrix material still

surrounding the fibers, thus indicating a closer contact

between the fiber and the matrix and a better wetting of the

fibers. This is also an indication that was able to fully

penetrate into the fiber and upon failure of the laminate, it

showed tearing and shearing and this explains the presence

of traces of matrix material on the fibers surface. In the case

of the silane treated fibers, the failure surface indicates a

massive matrix failure and the fibers are completely coated

with the matrix, and there are no indications of fibers pull-

out.

Then, it can be said from these failure modes that the

increase of the tensile strength of the composite is an effect

of the fiber–matrix interface increases. It can also be said

that a low fiber–matrix adhesion, results in a failure mode

dominated by fiber pull-out and matrix failure. As the

interfacial shear strength increased, the failure mode is more

like matrix tearing and flow and fiber tearing.

3.6. Flexural properties

The flexural strength of the HDPE-henequen fiber

composites plotted again as a function of the different

fiber surface treatments are shown in Fig. 9. Also, the

observations made earlier for the tensile strength on the

effect of fiber–matrix adhesion are also seen clearly here.

The fiber surface treatments had a marginal effect on the

flexural modulus, similarly to the observations made for the

tensile properties. The flexural strength improved by

Fig. 8. Photo-micrographs of a HDPE/henequen-fiber (80:20 v/v) compo-

site fracture surfaces of a sample subjected to tensile stresses, for (a) the

untreated, NaOH, FIB aqueous dilute solution treated, and those subjected

to a matrix pre-impregnation and to a silane coupling agent and (b) FIBNA;

(c) FIBNASIL and (d) FIBNAPRE.

"

0

400

800

1200

1600

2000

2400

TYPE OF FIBER SURFACE TREATMENT

FLE

XU

RA

L S

TR

EN

GT

H

FIB FIBNA FIBPRE FIBNAPRE FIBSIL FIBNASIL

Fig. 9. Flexural strength of a HDPE/henequen-fiber (80:20 v/v) composite,

plotted as a function of fiber surface treatment.

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608 605

approximately a 3% just by modifying the topography of the

fiber by an alkaline aqueous solution. The increment of

interfacial area of contact was beneficial for the flexural

strength (Fig. 9). Such relative increase was doubled when

the fibers were pre-impregnated with the matrix-liquid

solution even when the fiber had no surface modification by

any alkaline treatment. When these two surface treatments

are combined, and additional increase is observed for the

flexural strength. This indicates that a better contact and the

increase in area of contact between the fiber and the matrix

are improving the level of adhesion, probably by the

incorporation of a mechanical component of adhesion for

the matrix–fiber interfacial strength. When the fiber is

treated with the coupling agent a larger increase in the

flexural strength is also observed (13% approximately).

When the chemical and the mechanical components of the

adhesion are combined, a larger increase of strength is

observed (20%).

The failure modes are discussed to further clarify the

relationship between fiber–matrix adhesion and the flexural

strength. The specimens for FIB (Fig. 10(a) shows no trace

of matrix material on both between and around the fibers.

The fiber treated with NaOH and pre-impregnated with the

matrix (Fig. 10(b)) show traces of polymer still adhering

into and around the fiber. The fiber failure mode even shows

the cellulose microfibrils still surround by the polymer.

When the fiber was treated with NaOH and the coupling

agent, the failure mode changed to matrix failure and the

fibers are still held by the matrix (Fig. 10(c)). Considerable

amount of fibers tearing was also noticed.

Fig. 10. Photo-micrographs of a HDPE/henequen-fiber (80:20 v/v)

composite fracture surfaces of a sample subjected to tensile stresses, for

(a) the untreated, NaOH, FIB aqueous dilute solution treated, and those

subjected to a matrix pre-impregnation and to a silane coupling agent and

(b) FIBNA; (c) FIBNASIL and (d) FIBNAPRE.

"

0

2

4

6

8

10

12

14

16

18

20

TYPE OF FIBER SURFACE TREATMENT

SH

EA

R S

TR

EN

GT

H(M

Pa)

FIBNASILFIB FIBNA FIBPRE FIBNAPRE FIBSIL

Fig. 11. Shear strength of a HDPE/henequen-fiber (80:20 v/v) composite,

plotted as a function of fiber surface treatment.

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608606

3.7. Iosipescu shear strength

The shear strength of the HDPE-henequen fiber compo-

sites plotted again as a function of the different fiber surface

treatments is shown in Fig. 11. The shear strength is shown

again in a non-dimensionalized format. Similar obser-

vations made earlier for both the tensile and flexural

strength on the effect of fiber–matrix adhesion are evident

here. However, the effect of increased fiber surface area of

contact with the matrix seems to have a larger effect on the

shear strength. Fiber pre-impregnation by itself has no great

contribution to the shear strength. The largest increase in

shear strength is observed for the fiber treated with both the

aqueous alkaline solution and the silane coupling agent.

From previous observations of the tensile and flexural

properties for this fiber surface treatment combination,

increments of approximately a 20 and 30%, respectively,

were obtained. In the case of the shear strength, such

increase is of the order of a 25%.

Observation of failure surface of Iosipescu shear test

samples further clarifies the importance of fiber adhesion on

shear strength results. Fig. 12 shows a series of Scanning

Electron Microscope photographs for the composite for

20% v/v fiber content and tested under shear loads. In

Fig. 12(a), the untreated fibers appear to be free of any

matrix material adhering to them, thus indicating poor fiber–

matrix adhesion. In Fig. 12(b), the fibers treated with the

aqueous NaOH solution and then are pre-impregnated, more

tearing of the fibers could be observed, together with some

cavities left by the pulled-out fibers. Despite this fact, there

is some fiber pull-out but the fiber is coated with the matrix

Fig. 12. Photo-micrographs of a HDPE/henequen-fiber (80:20 v/v)

composite fracture surfaces of a sample subjected to tensile stresses, for

(a) the untreated, NaOH, FIB aqueous dilute solution treated, and those

subjected to a matrix pre-impregnation and to a silane coupling agent and

(b) FIBNA; (c) FIBNASIL and (d) FIBNAPRE.

"

P.J. Herrera-Franco, A. Valadez-Gonzalez / Composites: Part B 36 (2005) 597–608 607

polymer. Another feature of this fiber treatment is on the

matrix failure mode, because appreciable shear yielding

rather than tearing is observed. It can be inferred that

mechanical interlocking and friction are responsible for the

observed composite strength increment. Fig. 12(c) shows

the failure surface for the composite with fibers treated with

the aqueous NaOH solution and the silane coupling agent. It

can be observed that the fibers are still coated with the

matrix and that the matrix failed by shear yield flow and

tearing from the fiber. If the matrix failure mode of the

untreated fiber composite is compared to the pre-impreg-

nated fiber composite, it can be seen that it changes from

tearing mode to shear yielding. Then, from these matrix

failure modes, it can be said that there exists a higher force

transfer capability at the fiber–matrix interface in the

composites with fibers treated by pre-impregnation and a

silane coupling agent. It should be pointed out that the

matrix properties are an upper bound for the composite

strength, especially when the fiber–matrix adhesion is

strong, in this case, such upper bound is given by the

yield strength of the matrix.

4. Conclusions

The mechanical behavior of short natural fiber reinforced

HDPE was studied. The fiber–matrix interaction was

changed by modifying the surface properties of the fiber,

first to increase the area of contact and to further expose the

cellulose microfibrils, and to improve fiber wetting and

impregnation. Also, a chemical interaction was promoted by

using a silane coupling agent. The ratio of IFSS observed

between the different fiber surface conditions and the

untreated fibers was interesting because such interface

strength almost doubled only by changing the mechanical

interaction and the chemical interactions between fiber and

matrix. The micromechanical techniques allowed the

determination of an indicator of the fiber–matrix interface

strength and also a suitable value of fiber length, both to

improve fiber matrix force interactions and to be able to

process the composite with a relative ease. One important

fact resulting from the fiber surface treatment and improved

fiber–matrix adhesion was a considerable reduction of fiber

(l/d) ratio from 72 to w20.

The mechanical properties observed did not increase in

the same ratio as those observed for the single-fiber

interfacial shear strength. This was attributed to the fact

that the random fiber orientation resulted in lower properties

that those observed for uniaxial single-fiber reinforced

composites. The increase in the mechanical properties

ranged between 3 and 30%, for the tensile and flexural

properties. In the case of the shear strength of this

composite, such increase was of the order of 25%. From

the micro-photographs, obtained from failure surfaces from

the SEM, it was observed that with increasing fiber–matrix

interaction the failure mode changed from interfacial failure

to matrix failure. The interface failure was mainly a

frictional type failure, and only for the pre-impregnated

and silane treated fibers, matrix tearing and shearing was

observed. The silane surface treated fibers also showed a

layer of polymer covering the fibers even after failure.

The mechanical properties determined from the tensile,

flexural and shear tests exhibit a similar behavior for each of

the different fiber surface treatments but the effect of the

fiber surface treatment was more noticeable for the shear

properties.

Acknowledgements

The authors would like to express the support given by

the Consejo Nacional de Ciencia y Tecnologıa [Grant #

5116-A9406]. Also, the authors would like to thank Ms S.B.

Andrade-Canto for the SEM photomicrographs and M.V.

Moreno-Chulim for the FTIR analysis.

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