Effect of coupling agent content and water absorption on the mechanical properties of coir-agave...

Effect of Coupling Agent Content and Water Absorptionon the Mechanical Properties of Coir-Agave FibersReinforced Polyethylene Hybrid Composites

Aida A. P�erez-Fonseca,1 Mart�ın Arellano,2 Denis Rodrigue,1 Rub�en Gonz�alez-N�u~nez,2

Jorge R. Robledo-Ort�ız3

1Department of Chemical Engineering and CERMA, Universit�e Laval, Quebec City, Quebec G1V 0A6,Canada

2Departamento de Ingenier�ıa Qu�ımica, Universidad de Guadalajara, Blvd. Gral. Marcelino Garc�ıa Barrag�an# 1451, Guadalajara, Jalisco 44430, M�exico

3Departamento de Madera, Celulosa y Papel, Universidad de Guadalajara, Carretera Guadalajara-Nogales km15.5, Las Agujas, Zapopan, Jalisco 45510, M�exico

In this study, high-density polyethylene/agave-coircomposites with two fiber contents (20 and 30 wt%)and different coir-agave fiber ratios (1–0, 0.8–0.2, 0.6–0.4, 0.4–0.6, 0.2–0.8, and 0–1) were produced in a two-step process using twin-screw extrusion followed byinjection molding. The effect of mixing two differentnatural fibers and the addition of coupling agent onwater absorption, mechanical properties, and morphol-ogy is reported. The rule of hybrid mixture was used topredict the properties of the composites, showing agood agreement with the experimental data. Theresults obtained showed that the combination of differ-ent fibers produces composites with unique character-istics as coir fibers absorb less water than agavefibers, while at the same time increase more tensileand flexural strengths. On the other hand, agave fiberswere found to improve the impact strength of coircomposites. Also, the effect of water absorption onthe mechanical properties was studied. Finally, the useof a coupling agent had a positive effect on mechani-cal properties, while lowering water uptake. POLYM.COMPOS., 00:000–000, 2015. VC 2015 Society of PlasticsEngineers

INTRODUCTION

Composites made from natural fibers have gained

increasing interest during the past decade. The addition of

natural fibers to polymer matrices reduces the consumption

of oil-based material and decreases density, abrasiveness,

cost, dermal, and respiratory irritation, as well as enhanc-

ing energy recovery compared to glass fiber composites

[1]. Therefore, manufacturing industries, especially pack-

aging, building construction, automotive, and furniture,

have been encouraged to use natural fibers in their applica-

tions instead of nonrenewable reinforcing materials [2].

Lignocellulosic fibers such as flax, jute, hemp, sisal, kenaf,

and bamboo have been extensively studied as replacement

for glass and carbon fibers [3]. The fact that these fibers

are obtained from natural resources makes them more

attractive in terms of sustainability and environmental

awareness [4]. However, the hydrophilic character of natu-

ral fibers results in poor interfacial bonding with most

polymer matrices limiting their applications.

Moisture absorption of composites containing natural

fibers has several adverse effects on their properties and

affects their long-term performance; i.e. increasing mois-

ture content is known to decrease mechanical strength

and change their dimensions. Numerous efforts have been

made to address this issue [1, 2, 5]. An alternative route

to overcome these disadvantages is the addition of cou-

pling agents which provides better fiber/matrix adhesion.

Chemical grafting using maleic anhydride is one of the

most popular, not only to modify the fiber surface,

exploiting the hydroxyl groups which are abundantly

available in cellulosic natural fibers, but also to entangle

with the polymer matrix achieving better interfacial bond-

ing resulting in better mechanical properties of the com-

posites [6].

In recent years, the development of composites with

reinforcing agents of more than one type and shape was

proposed to compensate their shortcomings, resulting in a

positive hybrid effect [7]. Hybridization using two types

Correspondence to: J.R. Robledo-Ort�ız; e-mail: [email protected]

DOI 10.1002/pc.23498

Published online in Wiley Online Library (wileyonlinelibrary.com).

VC 2015 Society of Plastics Engineers

POLYMER COMPOSITES—2015

of fibers having different lengths and diameters offers

some advantages over the use of each fiber alone in a

polymer matrix. Several studies on hybridization were

performed using combinations of natural fibers with glass

fibers [8, 9] to obtain materials with slightly lower

mechanical properties than using glass fibers alone, but

leading to composites with lower costs and weight.

Recently, the development of hybrid composites using

only natural fibers increased, showing that the combina-

tion of two natural fibers is a great option to get materials

with lower costs, friendlier to the environment and main-

taining good mechanical properties.

Zainudin et al. [10] prepared hybrid composites with

oil palm empty fruit bunch and coir fibers and found that

composites with 50:50 oil palm: coir fibers presented

optimum mechanical properties. Fernandes et al. [11] pre-

pared hybrid polyethylene cork/sisal composites and

found that addition of sisal in the presence of 2 wt% cou-

pling agent (based on maleic anhydride) improved the

tensile and flexural properties of the composites. P�erez-

Fonseca et al. [12] prepared polyethylene-pine/agave

composites and found that addition of agave to

polyethylene-pine composites in the presence of coupling

agent (based on maleic anhydride) increased tensile, flex-

ural, and impact strength, while the addition of pine to

polyethylene-agave composites reduced water uptake.

As a result of the advantages shown by previous stud-

ies with hybridization, hybrid composites based on high-

density polyethylene were prepared using two natural

waste fibers (agave and coir). Agave fiber is a waste of

the Tequila production process and has limited applica-

tion for this discarded fiber. On the other hand, coir

fibers are well known because of their high resistance

and low water absorption ability, but they are very abra-

sive to the processing equipment. In this sense, combina-

tion of these two fibers can produce composite materials

with unique properties at excellent costs. In particular,

the effect of water absorption and coupling agent addi-

tion on the composite mechanical properties is reported.

In particular, the effect of water absorption and coupling

agent addition on the composite mechanical properties is

reported.

EXPERIMENTAL

Materials

High-density polyethylene 60120U (PADMEX, Mexico)

with a melt flow index of 19 g/10 min (190�C/2.16 kg) and

density of 0.96 g/cm3 was used as the polymer matrix.

Coupling agent, maleated polyethylene Fusabond M603

(MAPE), was provided by Dupont Packaging and Indus-

trial Polymers (USA). Agave fibers (Agave tequilanaWeber var. Azul) were obtained from a local tequila com-

pany in Jalisco, Mexico. Coir fibers were provided by

Agrocoir S.A. de C.V., Colima, Mexico. The chemical

composition of the fibers is presented in Table 1. These

properties were determined according to TAPPI standards

T-204cm-97 (extractives), T-222-om-98 (lignin) and the

Jayme-Wise method (holocellulose).

Composites Preparation

The agave and coir fibers received a previous treat-

ment before being used. The fibers were soaked in water

for 24 hours and then passed through a Sprout-Waldron

refiner (D2A509NH) with two 30 cm diameter discs, one

fixed and the other rotating at 1,770 rpm to separate the

pith from the fibers. The fibers were then placed in a cen-

trifuge to remove excess water and finally dried outdoors.

Both fibers were milled and sieved to keep particles

between 50 and 70 mesh for agave and 100 and 140

mesh for coir. Two fiber contents (20 and 30 wt%) were

used in the formulations with different coir–agave fiber

weight ratios (1–0, 0.8–0.2, 0.6–0.4, 0.4–0.6, 0.2–0.8, and

0–1). The composites were prepared with and without

MAPE (3 wt% with respect to the total fiber content).

The materials were first processed in a twin-screw

extruder Leistritz Micro 27 GL/GG 32D with a tempera-

ture profile set to 140/150/150/160/160/160/170/170/

160�C. The extruder had three circular holes, each one

being 2 mm in diameter. The screw rotational speed was

set at 120 rpm giving a total flow rate of 4 kg/h.

The composites were cooled in a water bath and then

pelletized. The pellets were then oven-dried for 24 h at

65�C to be injection molded on a NISSEI ES 1000 with a

mold temperature of 30�C and a screw temperature pro-

file of 130/170/185/195�C. All the samples were prepared

in a rectangular mold cavity with dimensions of

8034032.5 mm3.

Morphology

Samples were submerged in liquid nitrogen and then

fractured. Micrographs of the exposed surfaces were

obtained by a scanning electron microscope HITACHI

TM-1000 to characterize the morphology; i.e. the state of

fiber adhesion/dispersion in the matrix before and after

water immersion.

TABLE 1. Characterization of agave and coir fibers.

Fiber Extractives (%) Cellulose (%) Lignin (%) Hemicellulose (%) Fiber length (mm) Fiber diameter (mm) L/D

Agave 6–7 58–65 21–24 10–13 1.49 0.18 8.27

Coir 4–6 38–46 37–41 10–15 1.10 0.12 9.16

2 POLYMER COMPOSITES—2015 DOI 10.1002/pc

Water Absorption Test

Water absorption tests were conducted according to

ASTM D570. Before testing, the weight of each specimen

was measured. Then, five specimens were submerged in

distilled water at 30 6 0.5�C for 160 days. The samples

were removed and weighted after specific time periods to

get water absorption kinetics curves. Each time, the sur-

face water was wiped off with a dry cloth and the sample

was immediately weighed before being returned in the

water bath. The amount of water absorption (Mt) was cal-

culated as

Mt5wi2w0

w0

3100 (1)

where wi is the weight of the sample after immersion and

w0 is the initial weight before water immersion.

Mechanical Properties

Impact strength of the composites was determined by a

Tinius Olsen model 104 impact tester. The specimens

were prepared according to ASTM D6110 and each value

represents the average of 10 notched (ASN automatic

sample notcher from Dynisco) samples. Flexural and ten-

sile properties were evaluated using a universal machine

Instron model 4411. Flexural tests were carried out

according to ASTM D790 and sample dimensions were

80312.732.5 mm3 with a span length of 41 mm. Six

samples per composition were tested at a crosshead speed

of 10 mm/min. Tensile tests were carried out using a load

cell capacity of 1 kN, and the specimens were tested

according to ASTM D638 (type V specimen). Testing

was performed at a crosshead speed of 5 mm/min with

seven specimens per composition. All the tests were per-

formed at room temperature (23�C), and all composites

were tested before and after water absorption to determine

the effect of water ageing.

Prediction of Properties by the Rule of Hybrid Mixture

The rule of hybrid mixture (RoHM) was used to pre-

dict the results of water absorption and mechanical prop-

erties of the hybrid composites. The base equation,

neglecting the interaction between two systems can be

written as (modified by Mirbagheri et al. [13]):

FIG. 1. SEM micrographs of HDPE-fiber composites with 30% of total fiber content. Agave: (a) 0% and

(b) 3% MAPE. Coir: (c) 0% and (d) 3% MAPE.

DOI 10.1002/pc POLYMER COMPOSITES—2015 3

rHc5rc1Xc11rc2Xc2 (2)

where rHc, rc1 and rc2 are the properties of HDPE/

coir-agave (hybrid), HDPE-agave and HDPE-coir compo-

sites respectively, while Xc1 and Xc2 are the relative fiber

mass fraction given by

Xc11Xc25Wf 1

Wf ;T1

Wf 2

Wf ;T51 (3)

where Wf,T is the total reinforcement mass fraction

(5 Wf1 1 Wf2).

RESULTS AND DISCUSSION

Morphology

The effect of coupling agent on agave and coir fibers

in the composite was observed via SEM micrographs.

Figures 1a and b present a fractured surface of HDPE-

agave composites without and with MAPE, respectively.

A clear reduction in the fiber–matrix interfacial space

because of coupling agent addition was observed (which

could be associated to better adhesion) as a result of the

reaction between the maleic anhydride groups of MAPE

and the hydroxyl groups of the fibers forming an ester

link. Furthermore, the nonpolar part (PE) of MAPE is

compatible with the HDPE matrix leading to macromo-

lecular entanglements [14]. Figures 1c and d show a frac-

tured surface of HDPE-coir composites without and with

MAPE, respectively. In these micrographs, similarly to

agave composites, it can be seen how MAPE improves

the polymer-coir fibers compatibility.

The effect of water absorption on composite morphol-

ogy is presented in Figs. 2 and 3. It can be observed that

agave composites were more affected by water uptake

than coir composites as Fig. 2b and d show higher agave

fiber damaged, with cracks on the fiber surface. This

behavior can be related to the chemical composition of

each natural fiber; i.e. the cellulose content of agave

fibers is higher than for coir fibers (see Table 1) leading

to more –OH groups available to form hydrogen bonds

with water molecules and higher water uptake. Water

absorption ageing may lead to the degradation of natural

fibers by a hydrolysis mechanism and its resulting effect

contributed to the loss of compatibilization between the

fibers and matrix. This phenomenon leads to fiber

debonding and weakening of interfacial adhesion.

FIG. 2. SEM micrographs of HDPE-agave composites with 30% of total fiber content before water uptake

with: (a) 0% and (b) 3% MAPE; and after water uptake with: (c) 0% and (d) 3% MAPE.


Furthermore, the degradation process occurs when swel-

ling of cellulose fibers develop stress in interfacial

regions leading to microcracks in the matrix around the

swollen fibers promoting capillarity and water transport

[15]. Since coir fibers have higher lignin content, which

is a hydrophobic compound, this leads to less fiber degra-

dation because of water uptake.

Water Absorption

Figure 4 shows the kinetics of water absorption for

hybrid composites where it can be seen that equilibrium

was reached at around 4000 hours and that composites

with higher fiber contents absorbed more water. For a

better comparison, equilibrium moisture content of the

hybrid composites was plotted in Fig. 5. It can be

observed that agave fiber composites absorbed more

water than coir fibers. When the fiber content is 30%,

HDPE-agave composites absorbed 6.7% of water, while

HDPE-coir composites absorbed only 3.8%. This behavior

is again related to the chemical composition of each fiber

as mentioned above: agave fibers have more –OH groups

which can interact with water molecules. It is known that

the macromolecular structure, and in particular the

hydroxyl groups, of polysaccharides are highly hydro-

philic which can attract water molecules via hydrogen

and van der Waals bonds. All these effects are contribut-

ing to higher water storage capacity of the composites

[16].

Moisture penetration into the composites can occur via

different mechanisms. The main process consists of water

molecules diffusion inside the microgaps between poly-

mer chains. Other common mechanisms are capillary

transport into the gaps and flaws at the interfaces between

fibers and matrix because of incomplete wettability and

impregnation, as well as transport by microcracks in the

matrix formed during compounding [17]. In this study, it

was observed that higher cellulose content in the fibers

enhanced these mechanisms. For composites with 30% of

agave fiber, the maximum amount of water absorbed was

6.7%, while the amount was reduced to 4.4% when using

MAPE. The use of coupling agents plays an important

role in this phenomenon because of the esterification

reaction between hydroxyl groups and maleic anhydride,

which causes a reduction in interfacial tension and

increases interfacial adhesion between the polymer and

fibers [18]. Also, this reaction reduced the number of

OH-group available to interact with water molecules. For

FIG. 3. SEM micrographs of HDPE-coir composites with 30% of total fiber content before water uptake

with: (a) 0% and (b) 3% MAPE; and after water uptake with: (c) 0% and (d) 3% MAPE.


coir composites, the effect of MAPE addition was less

significant (water absorption decreased from 3.8% to

3.4%) since the fibers are less hygroscopic.

Water absorption results showed that mixing coir with

agave fibers in a polymer matrix produced hybrid compo-

sites which absorb less water than composites with agave

fibers alone. For example, hybrid composites with a fiber

content of 30% and 0.4–0.6 (coir-agave) ratio absorbed

28% less water than composites made with agave fibers

alone. A comparison of the experimental and predicted

moisture content values using the RoHM equation is also

presented in Fig. 5 where good agreement is observed.

Tensile Properties

Tensile strength results are shown in Fig. 6. It can be

seen that using agave fibers alone, tensile strength

decreases with increasing fiber content. This is related to

poor interfacial adhesion between agave fibers (highly

hydrophilic) and the polymer matrix. On the other hand,

the fact that coir fibers are less hydrophilic can explain

why tensile strength loss was lower when the fiber con-

tent increased. Hybrid composites prepared with agave

and coir fibers without MAPE showed that for 20% fiber

content, the maximum tensile strength was achieved with

higher agave contents (22 MPa), but in the case of 30%

fiber content, tensile strength was more increased with

higher coir contents because the maximum was achieved

for composites using coir fibers alone (23 MPa). For all

hybrid composites, the tensile strength was lower than the

neat HDPE (28 MPa) because fiber surfaces have waxes

FIG. 4. Water absorption kinetics for hybrid composites: 20% of total fiber content (a) without and (b)

with MAPE and 30% of total fiber content (c) without and (d) with MAPE.

FIG. 5. Equilibrium moisture content of HDPE/coir-agave composites

with different fibers contents. (M) denotes composites prepared with 3%

MAPE and the lines are predictions using Eqs. 2 and 3.


and other noncellulosic substances such as hemi-cellulose,

lignins and pectin, which create poor adhesion between

the matrix and the fibers. Agave fibers contain more cellu-

lose than coir fibers and it is known that fiber tensile

strength increases with cellulose content [19]. Moreover,

when fiber content increases there is more fiber agglomer-

ation leading to lower tensile strength. Table 1 shows that

agave fibers were longer than coir fibers and this increased

the probability of fiber–fiber contact, resulting in higher

agglomeration and void formation lowering interfacial

stress transfer. For both fibers, it was found that the addi-

tion of MAPE increased tensile strength: 16% (26 MPa)

and 19% (25 MPa) for composites using coir or agave

fibers, respectively. The use of coupling agents based on

maleic anhydride (MA) increases tensile strength because

of the esterification reaction between the fibers hydroxyl

groups and the anhydride part of MA leading to interfacial

tension reduction and improved interfacial adhesion

between the matrix and the fibers [11].

It was observed that water uptake decreased tensile

strength at 30% fiber content without MAPE (from 21 to

19 MPa), while at higher coir fibers content this property

was maintained. Composites with MAPE had similar ten-

sile strength before and after water uptake at 20% fiber

content (around 22 MPa), while for composites at 30%

the values decreased with the amount of agave fibers in

the composite (from 25 to 23 MPa). Azwa et al. [15]

reported that lower mechanical properties are because of

debonding at the fiber/matrix interface, as well as fiber

degradation during water ageing. Swelling of natural fill-

ers can also develop shear stress at the interface which

facilitates debonding of the fibers from the matrix leading

to tensile strength reduction [18].

The tensile modulus results (Fig. 7) were very similar

for all hybrid composites prepared with and without

MAPE (around 400 and 500 MPa for 20 and 30% fiber

content) before water uptake. But after water uptake,

composites with MAPE maintained their modulus values

while decreases were observed without MAPE: 15% in

composites with higher amount of agave fibers (510 to

431 MPa), as well as tensile strength for coir composites

for which the tensile modulus was similar (around 475

MPa). Nevertheless, all modulus values of hybrid compo-

sites before or after water uptake were higher than the

neat HDPE tensile modulus (295 MPa). It has been

FIG. 6. Tensile strength of HDPE/coir-agave composites before and

after water absorption with (a) 20% and (b) 30% of fiber content. (M)

denotes composites prepared with 3% MAPE and the lines are predic-

tions using Eqs. 2 and 3.

FIG. 7. Tensile modulus of HDPE/coir-agave composites before and


denotes composites prepared with 3% MAPE and the line are predictions

using Eqs. 2 and 3.


reported that decreases in tensile modulus because of

water absorption are related to the amount of water

absorbed. Tamrakal et al. [20], working on

polypropylene-wood composites, reported that tensile

modulus was not affected when the composites absorbed

around 4–5% of water, while composites absorbing more

than 8% presented important decreases in tensile modulus

because of more microcracks formation.

It was observed that the RoHM equation was able to

predict the tensile strength of hybrid composites knowing

the properties of the composites prepared with a single

type of fiber. Similar studies have shown that RoHM,

despite being a very simple model, can predict the

mechanical properties of hybrid composites compared to

more complex models [7, 13].

In the case of tensile modulus, RoHM showed differ-

ences between experimental and calculated values indicat-

ing that RoHM can have some limitations because it

neglects some factors. As fiber loading increases, more

particle–particle interaction is expected. Moreover, the

maximum packing fraction of each particle depends on

their size and shape distribution, which is not considered

in the RoHM [21].

Flexural Properties

Flexural strength results are presented in Fig. 8. Flex-

ural strength without MAPE was very similar for all

hybrid composite compositions: around 24 and 29 MPa

for 20% and 30% fiber content, respectively (higher than

22 MPa for neat HDPE). This is mainly attributed to the

reinforcing effect imparted by the fibers, which allowed a

uniform stress distribution from the continuous polymer

matrix to the dispersed fiber phase [22]. The hybrid com-

posites prepared with coupling agent showed that the

addition of MAPE increased the flexural strength of all

the composites (6–12%), the maximum being achieved

for HDPE-coir composites at 30% (32 MPa). As men-

tioned before, agave and coir fibers have different dimen-

sions and chemical composition, which explains why coir

properties were more enhanced by MAPE addition. El-

Sabbagh [6] obtained similar results with sisal, hemp, and

flax composites where he reported that sisal fibers had

the lowest improvement with maleated polypropylene

(MAPP) because of a lower number of available sites for

fiber surface–MAPP interaction because of their relatively

larger sizes.

The high agave ratio composites at 30% fiber content

were more affected by water exposure, with flexural

strength reduction from 29 to 25 MPa. Water molecules

penetrate into the composites through microcracks and

reduce interfacial adhesion between the fibers and the

matrix, causing fiber swelling creating cracks in the

matrix and eventually leading to fiber debonding [23].

The cellulose molecules are held tightly together inside

the fibrils by bonds between molecules lying closely

alongside one another. However, water can penetrate this

cellulose network and move into the capillaries and

spaces between the fibrils. In this case, water molecules

tend to force the cellulose molecules apart, reducing the

forces holding them together and lowering their rigidity

because water acts like a plasticizer and improves the

motion of cellulose molecules [1]. Consequently, cellu-

lose is softened and swelled leading to fiber dimensional

changes. Because of higher cellulose content, agave-based

composites are more susceptible to water uptake. Addi-

tion of coir to agave composites helped to prevent this

property loss after water uptake. In all cases, flexural

strength reduction (around 5%) for composites with

MAPE was the same for all hybrid combinations.

Flexural modulus is presented in Fig. 9 and the values

are very similar for all hybrid combinations, but change

with fiber content (566 MPa for neat HDPE, 900 MPa at

20% and 1100 MPa at 30%). It can be observed that fiber

content is an important parameter related to water absorp-

tion. For composites at 20% fiber content, the reduction

is less than 2%, while at 30% the reduction is 9%. The

addition of MAPE had a positive effect on flexural modu-

lus by lowering the effect of water absorption. Similarly

FIG. 8. Flexural strength of HDPE/coir-agave composites before and





to tensile properties, the RoHM was able to predict the

flexural strength and flexural modulus values before and

after water uptake.

Impact Strength

Impact strength results (Fig. 10) show that agave fiber

composites have higher impact strength compared to coir

fiber composites. HDPE-agave composites at 30% fiber

content had an impact strength of 53 J/m, HDPE-coir

composites had only 37 J/m, while neat HDPE has an

impact strength of 46 J/m. It has been reported that natu-

ral fibers can improve the impact behavior of polymers,

especially when long fibers were used [24] as it was the

case of agave fibers. Oksman et al. [25] studied the effect

of different fibers on the mechanical properties of poly-

propylene (PP) composites and found that sisal fibers

increased more significantly the impact strength of PP

composites than other fibers like flax and jute, showing

that sisal fibers were the toughest (agave tequilana is

from the same gender of sisal). Wambua et al. [26] men-

tioned that impact strength of fiber-reinforced polymers is

governed by the fiber–matrix interfacial bonding and the

individual properties of both fiber and matrix. When the

composites undergo an impact, the energy is dissipated

by the combination of fiber pullouts, fiber fracture, and

matrix deformation/cracking. For example, Alomayri

et al. [23] prepared composites of poly(lactic acid) and

reported a substantial impact strength improvement

because of the addition of cotton fibers. They concluded

that this increase was related to the higher elongation at

break of cotton fibers. High cellulose content in fibers

generally produce higher elongations at break. Since cot-

ton has a cellulose content around 88–96%, elongation at

break and impact strength were directly correlated.

In our case, it was observed that hybridization of coir

composites with agave fibers produced composites with

better impact strength. For example, a combination 40–

60% (agave-coir fibers) increased this property (compared

with coir alone) by up to 18% (from 37 to 44 J/m) with-

out MAPE and by up to 10% (45 to 49 J/m) with MAPE.

With respect to MAPE, its addition produced an impact

strength improvement only for composites at 30% fiber

content (between 5 and 7 J/m), while at 20% a negligible

effect was observed. Similar results were reported by

P�erez-Fonseca et al. [12] where a more important

FIG. 9. Flexural modulus of HDPE/coir-agave composites before and




FIG. 10. Impact strength of HDPE/coir-agave composites before and





coupling agent effect was observed at higher fiber con-

tents (30%) increasing the impact strength of HDPE/

pine-agave hybrid composites. Also, Fig. 10 shows that

after water uptake the impact strength of hybrid compo-

sites increased substantially, the maximum obtained was

for composites at 30% of agave fiber alone and MAPE

after water immersion (73 J/m). Nevertheless, it was

found that impact strength improvement was around 20%

and 30% for all compositions regardless of fiber type.

Once the moisture penetrates inside the composite, the

fibers tend to swell. The matrix structure can also be

affected by water uptake processes such as chain reorienta-

tion and shrinkage [4]. These results are similar to those

found by Alamri and Low [2] and Karmaker [27]. Lin

et al. [17] evaluated the effect of water absorption time

and temperature on the mechanical properties of wood

composites and found that impact strength increased with

exposure time. They explained that swelling of wood fibers

made it difficult to pull the fibers out of the matrix (more

contact leading to more friction). Once again, the RoHM

equation could properly represent the experimental impact

strength values for all the hybrid composites prepared.

CONCLUSION

In this study, it was shown that a combination of coir

and agave fibers in HDPE is a good option to produce

hybrid composites with better properties than those using

coir or agave fibers alone. While agave fibers enhanced

impact strength, coir fibers enhanced flexural and tensile

strength and decreased water uptake. Composite proper-

ties with both fibers were also improved by the addition

of a coupling agent (MAPE), obtaining materials with

higher impact, tensile, and flexural strength. In general,

the tensile and flexural properties of the composites

decreased after moisture uptake because of the effect of

water molecules modifying the structure and properties

of the fibers, matrix, and mostly their interface. Addition

of MAPE also helped to decrease water uptake limiting

composite degradation when exposed to water. On the

other hand, impact strength was increased after water

absorption for all the hybrid composites because fiber

swelling made it more difficult to pull the fibers out of

the matrix. Finally, the simple rule of hybrid mixture

(RoHM) was found to predict with good precision the

effect of fiber ratio on tensile, flexural, and impact prop-

erties of hybrid composites with or without MAPE, as

well as before and after water uptake.

ACKNOWLEDGMENTS

The authors gratefully acknowledge Agrocoir S.A. de

C.V. (Colima, M�exico) for supplying coir fibers. One of the

authors (A.A.P.F.) acknowledges the financial support of

the Mexican National Council for Science and Technology

(CONACyT #266256) for a scholarship.

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Effect of coupling agent content and water absorption on the mechanical properties of coir-agave...

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