3. lucrare_fibra de cocos

7/30/2019 3. Lucrare_fibra de Cocos

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Surface treatment of coir (Cocos nucifera) fibers

and its influence on the fibers physico-mechanical properties

M. Mizanur Rahmanaa, Mubarak A. Khanb

b

aDepartment of Applied Chemistry and Chemical Technology, University of Dhaka, Dhaka

1000, Bangladeshb

Nuclear Radiation and Polymer Chemistry Laboratory, Institute of Nuclear Science and

Technology, Bangladesh Atomic Energy Commission, P.O. Box 3787, Dhaka 1000, Bangladesh

Received 29 May 2006. Revised 21 December 2006. Accepted 19 January 2007. Available

online 31 January 2007.

Abstract

Coir, an important lignocellulosic fiber, can be incorporated in polymers like polyacrylate in

different ways for achieving desired properties and texture. But its high level of moisture

absorption, poor wettability and insufficient adhesion between untreated fiber and the polymer

matrix lead to debonding with age. In order to improve the above qualities, adequate surface

modification is required. In our present work, fiber surface modification by ethylene

dimethylacrylate (EMA) and cured under UV radiation. Pretreatment with UV radiation and

mercerization were done before grafting with a view to improve the physico-mechanical


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performance of coir fibers. The effects of mercerization on shrinkage and fiber weight losses

were monitored at different temperature and alkali concentration. We observed that, fiber

shrinkage is higher at low temperature and 20% alkali treated coir fibers yielded maximum

shrinkage and weight losses. It was found that higher shrinkage of the polymer grafted fiber

showed enhanced physico-mechanical properties. The grafting of alkali treated fiber shows an

increase of polymer loading (about 56% higher) and tensile strength (about 27%) than 50% EMA

grafted fiber. The fiber surface topology and the tensile fracture surfaces were characterized by

scanning electron microscopy and were found improved interfacial bonding to the modified

fibermatrix interface.

Keywords

Coir fiber; Mercerization; Photocuring; Surface pretreatment; UV radiation; Physico-mechanical

properties .

1. Introduction

Coir fibers can be incorporated in polymers to form a biodegradable matrix. But a high level of

moisture absorption and insufficient adhesion between untreated fibers and the polymer matrix

may lead to bio-composites having high water absorption characteristics that reduce their utility

in many applications since major restriction in the successful use of natural fibers in durable

composite applications is their high moisture absorption and poor dimensional stability

(swelling) [1]. Coir is a versatile lignocellulosic fiber obtained from coconut trees (Cocos

nucifera), which grow extensively in tropical countries. The chemical constituents and the

mechanical properties of coir fibers are included in Table 1. Because of its hardwearing quality,

durability and other advantages, it is used for making a wide variety of floor-furnishingmaterials, yarn, rope, etc. [5] and [6]. However, these traditional coir products consume only a

small percentage of the total world production of coconut husk. Coir is a cheap fiber, even

cheaper than sisal and jute [7]. Coir fibers are not as brittle as glass fibers, are amenable to

chemical modification, are non-toxic and possess no waste disposal problems, but unfortunately

the performance of coir as a reinforcement in polymer composites is unsatisfactory and not

comparable even with other natural fibers. This inferior performance of coir is due to various

factors such as its low cellulose content, high lignin content, high microfibrillar angle, and large

as well as variable diameter.


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2. Experimental

2.1. Materials

Coir fibers were collected from the local market of Bangladesh. The fiber was subjected to

various surface modifications like UV ageing, alkali treatment and grafting with acrylate

monomer (EMA) using UV radiation. The monomer ethylene dimethacrylate (EMA) having

molecular weight 198 was procured from Fluka Chemica (Switzerland). Photoinitiator, Irgacure-

500 was procured from Ciba-Geigy, Switzerland.

2.2. Experimental procedure and data analysis

Coir fibers were cut into small pieces (1015 cm in length). The coir fibers were washed in 2%(commercially available washing powder, Surf, Unilever, UK) detergent solution at 70 C for 1

h, then washed overnight with rinsing tap-water and finally thoroughly washed with distilled

water followed by a drying process in a vacuum oven at 70 C. The detergent washed fibers were

aged with UV radiation to measure the effect of radiation on coir properties. The fibers were then

grafted with different concentration monomer + MeOH solution at different grafting conditions

and cured under same UV light (model IST Technik, Germany) at 254313 nm wavelengths of


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radiation and 2 kW power at 50 A current. The UV radiation source contains a conveyor belt

which moves around the mercury lamp and after moving once to the lamp it was considered one

pass. The optimum grafting condition with respect to different physical properties was measured

in this method.

The coir fibers treated with EMA was immersed in acetone for 5 min t o remove unreacted

monomer. After drying in an oven the fiber samples were weighed and wrapped with stainless

steel net and finally put in a soxhlet. Homopolymers are removed through extraction in hot

acetone for 72 h in the soxhlet. In this process the homopolymer will remove from the surface of

the fiber and only the grafted molecules will remain. The amount of grafting was determined by

the following equation:

2.3. Fiber surface treatment

Coir fibers were soaked in 550% NaOH (before treatment with EMA) for about 0.5 h at

temperature ranges from 0 to 100 C in order to activate the OH groups of the cellulose and

lignin in the fiber. The appropriate concentration of NaOH solution used in mercerization before

each type of chemical treatment was completed in the initial work [14] indicated that a 1030%sodium hydroxide solution produced the best effects on natural fiber properties. The fibers were

then washed many times in distilled water and finally dried.


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Water absorption properties of treated and untreated fibers were determined by placing the

sample in a static water bath at room temperature, and water uptake for different swelling time

were measured by the formula:

2.4. Fiber surface morphology

In order to evaluate changes in the surface morphology, all investigated natural fibers were

analyzed by scanning electron microscopy (SEM) using a Hitachi S-2700 scanning electron

microscope (Nissei Sangyo GmbH, Rathingen, Germany). The excitation energy was 10 keV

with a beam current of 0.5 nA.

3. Results and discussion

Lignocellulosic fillers offer attractive properties, but are used only to a limited extent in

industrial practice [15]. Natural fibers are strongly hydrophilic materials and moisture absorption

leads to a significant deterioration of their mechanical properties. Furthermore, most polymers

are hydrophobic and due to this divergent behavior, the interface in natural fiber composites is

rather poor. Any alteration of the characteristics of the cell wall, either chemical or

morphological, has an effect on the mechanical properties of the fibers. By limiting the

substitution reaction to the fiber surface, the good mechanical properties are preserved and a

degree of biodegradability is maintained.

The tensile properties (tensile strength, elongation at break and Youngs modulus were

measured) of the untreated virgin raw fiber was measured and the average values for twenty

samples were; TS 117.5 MPa, Eb 14.8% and Youngs modulus (M) 628 MPa. Tensile strength

and elongation factors are the enhancement of these properties after polymer treatment with

EMA compared with the virgin raw fibers. Each of the data presented in this report were the

average value of at least five samples and the results obtained were with in the accuracy of 2%.

4. Effect of UV radiation on tensile properties of virgin coir fiber

Detergent washed virgin coir fibers were irradiated under UV radiation of different intensities

(represented by number of passes) to investigates its effect on the mechanical properties. The

values of tensile strength; TS (MPa), elongation at break; Eb (%) and modulus; M (MPa) were

graphically represented in Fig. 1 against the doses of UV radiation by which the raw coir fiber

was aged. These factors are the ratio between UV pretreated and untreated fibers. We observed


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significant change of mechanically properties after coir fibers were pretreated by UV radiation.

Increasing UV doses to the fiber also increases tensile strength, elongation at break and Youngs

modulus. Highest tensile strength (156.3 MPa) was achieved after the fiber was aged by 125

passes of UV radiation, which is 33% higher than the tensile strength of virgin fiber. This also

gives highest elongation (20.5%) which is about 39% higher than virgin coir fiber. Increase of

tensile properties with increasing radiation doses could be due to the inter-crosslinking between

the neighboring cellulose molecules occur under UV radiation and the decrease at higher

radiation doses could case due to the photo-degradation of cellulose backbone at higher UV

doses. During photo-degradation there will be loss in strength due to primary bond breakage in

the cellulose constituent.

Fig. 1. Tensile properties of UV pretreated coir fiber at various radiation intensities (tensile

properties) are represented as the ratio of the tensile properties of the UV pretreated coir fiber to

that of untreated one. Tensile strength of virgin raw coir fiber is 117.5 MPa. The elongation at

break and Youngs modulus are 14.8% and 628 MPa.

5. Treatment of coir fiber with EMA

5.1. Polymer loading and grafting


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The PL values of the coir fibers were calculated as the weight gain (%) after they were cured

under UV radiation. The PL values of EMA treated fibers at different radiation intensities are

given in Fig. 2a as a function of monomer concentration. The values are relatively low at low

monomer concentration and it increases with increasing monomer concentration up to 50% inmethanol. But at 70% EMA it showed reduction of PL values. PL increases with increasing UV

intensities up to a certain value and after attainment of maximum values it again decreases with

increasing radiation doses. The highest PL (19.12%) is observed for the coir sample treated with

50% EMA in methanol and at 125 passes of UV radiation.


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Fig. 2. Polymer loading (a) and grafting (b) of EMA treated coir fibers at different UV radiation

doses as a function of monomer concentration.


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The amount of grafted polymer was determined after the extraction of homopolymer through the

extraction of the EMA treated fibers in hot acetone for 72 h. This will gives exact percentages of

grafted polymer on coir fiber surface. The results of grafting efficiencies are graphically

represented against UV radiation doses as a function of monomer concentration and are plotted

in Fig. 2b. We have observed that grafting efficiencies are also increases with UV doses and the

highest efficiency (76.5%) was obtained for the coir sample grafted with 50% monomer and

cured at 125 passes of UV radiation. We have also found that at low monomer concentration and

at low UV doses the grafting efficiencies are low because at lower monomer concentration the

vinyl monomer promotes rapid propagation reaction with the help of photoinitiator leading to

network polymer structure through graft copolymerization reaction via their double bond [16].

As EMA concentration is increases the amount of residual unsaturation is also increased with the

consequence of faster rate of formation of three dimensional network structures causing

restricted mobility. After maximum value of PL the reduction of values at higher monomer

concentration may be caused by two factors. At higher EMA concentration radicalradical

recombination processes may be dominating creating more homopolymers rather than monomer-cellulose backbone reaction. Another reason could be the swelling of cellulose backbone with

MeOH is insufficient due to lower amount solvent; methanol. As a result monomer molecules

are incapable of penetrating the cellulose molecules in presence of low solvent which may cause

a smaller number of reacting sites at the cellulose backbone and thus continue to reduce the

active sites as MeOH amount reduces at higher EMA concentration.

5.2. Tensile properties

The chemical processing directly influences the cellulosic fine structure of plant fiber.

Consequently, the chemical treatments (grafting with monomers) and have a lasting effect on the

mechanical behavior of coir fibers, especially on fiber strength and stiffness. Mohanty et al. [17]

showed that the higher strength treated fibers compared to untreated fibers may be a result of the

removal of surface imperfections after the treatment. The increased uniformity of the fibers

would give an increase to strength, as points of nonconformity are removed during the treatment

and this changes the deformation. The tensile strength of coir fiber-reinforced composites is

determined both by the tensile strength of the fiber and by the presence of weak lateral fiber

bonds. The variations in the tensile strength at yield of the composites on different modification

were attributed to the changes in the chemical structure and bondability of the fiber. Many of the

modifications decrease the strength due to the breakage of the bond structure, and also because

of the disintegration of the non-cellulosic materials [14]. The reinforcing ability of the fibers didnot just depend upon the mechanical strength of the fibers but on many other features, such as

polarity of the fiber, surface characteristics and presence of reactive centers. These factors

control interfacial interaction. The improved stiffness of the fibers was attributed to the

crystalline region (cellulosic) of the fiber. The fiber also showed very good elongation

properties, with values increasing upon modifications. Lower elongation of the untreated fiber

may be due to the three dimensionally cross-linked networks of cellulose and lignin. Treatment


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broke this network structure giving the fiber higher elongation and lower strength properties.

Misra et al. [18] investigated the tensile properties of untreated, chemically modified and AN-

grafted sisal fibers. Chemically modified fibers showed an appreciable decrease in the tensile

properties. This decrease was attributed to the substantial delignification and degradation of

cellulosic chains during chemical treatment.

The tensile properties represented by tensile strength factor (Tf) and elongation factor (Ef) at

different UV intensities are graphically presented in Fig. 3a and b as a function of EMA

concentration. Tf and Ef are ratio of tensile strength and elongation at breaking point of grafted

and ungrafted virgin fibers. We observed that both Tf and Ef increases with UV radiation doses

and after attainment of maximum values the tensile properties decreases with increasing UV

intensities. This decrease of Tf and Ef at higher radiation dose could be caused due to the

degradation of polymer at higher UV doses [19]. The tensile property increases with monomer

concentration and after the maximum values Tf and Ef decreases with increasing EMA

concentration. This phenomenon can be explained by the same reason as described in polymer

loading section. The highest tensile strength (Tf = 1.5) is given by the sample treated with 50%

EMA and at 125th pass of UV followed by the sample grafted with 30% monomer and after 50

passes of UV radiation only. The lowest tensile strength (Tf = 1.06) which is almost similar to

that of virgin fiber is given by the coir fiber treated with 5% monomer and at 125 passes of

radiation.


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Fig. 3. Tensile properties of EMA treated coir fibers at different UV radiation doses as a function

of monomer concentration; (a) shows tensile strength factor and (b) shows elongation factor.


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The enhancement of elongation up to breaking point of coir fiber samples were expressed as

elongation factor (Ef) and is depicted in Fig. 3b against the doses of UV radiation with respect to

different EMA concentration. As usually the highest elongation (Ef = 1.39) is observed for the

sample treated with 50% monomer and after 125 passes of UV radiation and the lowest by the

sample treated with 5% monomeric solution. This case highest value after 125 passes is almost

similar to the virgin fiber (Ef = 1.02)

6. Effect of mercerization (alkali treatment) on coir fiber

Alkali treatment of cellulosic fibers, also called mercerization, is the usual method to produce

high quality fibers. Alkali treatment improves the fibermatrix adhesion due to the removal of

natural and artificial impurities from the fiber surface as well as it change the crystal structure of

the cellulose [18]. Moreover, alkali treatment reduces fiber diameter and thereby increases the

aspect ratio. Therefore, the development of a rough surface topography and enhancement in

aspect ratio offer better fibermatrix interface adhesion and an increase in mechanical properties

[17]. Alkali treatment increases surface roughness resulting in better mechanical interlocking andthe amount of cellulose exposed on the fiber surface. This increases the number of possible

reaction sites and allows better fiber wetting. The following reaction takes place as a result of

alkali treatment:

Fiber-OH+NaOHFiber-O-Na++H2O

Consequently, mercerization or more general alkali treatment had a lasting effect on the

mechanical behavior of coir fibers, especially on fiber strength and stiffness [4]. Several other

studies were conducted on alkali treatment [14], [18] and [20]. They reported that mercerization

led to the increase in the amount of amorphous cellulose at the expense of crystalline celluloseand the removal of hydrogen bonding in the network structure.

The coir fiber was mercerized by different alkali concentration (550%) and at different

temperatures ranges from 0 to 100 C. The weight loss and shrinkage was measured before it

was treated with EMA solution and was graphically presented in Fig. 4a and b. We observed that

at low temperature shrinkage is higher. But as the temperature is increased shrinkage become

insignificant. The highest shrinkage (5.88%) is observed for the sample treated with 20% alkali

and at 0 C. When the fiber is soaked in strong caustic solution (NaOH), different Na-cellulose

complexes are formed. These transformations do not change the cellulose chain length but takes

a large amount of NaOH and water in the crystal structure and is there by swollen. Thus, the

fiber becomes shorter. So, shrinkage is proportional to swelling. But swelling is the result of ion-

exchange process where cellulose acts as a weak acid. The overall process of ion-exchange-

complex formation-phase transformation involves heat of hydration and neutralization so the

process is exothermic. By basic thermodynamics, lowering the temperature should favor the

equilibrium in the positive direction. So, lowering the temperature should produce increased


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swelling and therefore increased shrinkage. The weight loss due to alkali treatment is increased

with increasing temperature up to a certain value and then decrease after attainment of maximum

value. The highest weight loss (12.65%) is obtained at temperature 25 C and for 20% alkali

solution. The coir fibers treated with 20% alkali solution at 25 C were thoroughly washed and

dried. After drying the fiber was grafted with 50% EMA solution and cured under UV radiation

of different intensities. The results of PL and tensile properties at this condition was compared

with out alkali treated grafted coir fiber. The results clearly showed that alkali treated fiber

showed increased PL and tensile strength and alkali treatment increases about 56%PL to the fiber

and 27% of tensile strength. The elongation at break is slightly lower than untreated fiber and the

reduction amount is negligible.


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Fig. 4. Effect of alkali treatment on shrinkage and weight losses of coir fibers at different alkali

(NaOH) concentration as a function of solution temperature; (a) shows percent shrinkage and (b)

shows percent weight loss at different alkali concentration.

7. Water absorption property

Water uptake of treated (EMA grafted and alkali treated followed by polymer grafted) and

untreated coir samples were monitored at 25 C in a static water bath at different time interval

(30 s60 min). We have observed that all the samples attained maximum water uptake within

initial 15 min and then soaking of water stops for treated sample while the untreated sample still

continues to soak water slowly. The untreated samples showed the highest water uptake of 61%

and the grafted sample yielded a value of 31% while the alkali treated fiber showed only 26% of

water absorption. The reduced water uptake by the treated samples is due to the fact that the

monomer reacts with the OH group of cellulose through graft co-polymerization reaction and

hence reduces the hydrophilic nature of the coir fiber and also polymer fills the void space of the

treated fiber. By replacing some of the hydroxyl groups on the cell wall polymers with bondedchemical groups, the hygroscopicity of the lignocellulosic material is reduced.

8. Fiber surface morphology (scanning electron microscopy)

Scanning electron microscopic analysis examined the surface topology of untreated and treated

fibers. The removal of surface impurities on plant fibers is advantageous for fibermatrix

adhesion as it facilitates both mechanical interlocking and the bonding reaction due to the

exposure of the hydroxyl groups to the chemicals used in treatment. Fig. 5 shows the SEM

images of coir fiber surfaces before and after alkali treatment as well as alkali treated followed

by polymer grafted in (a), (b) and in (c). A porous structure is observed for untreated fibers.These images indicate that after chemical treatment, the surfaces of the fibers became rougher

enhancing the mechanical interlocking with resins. It is observed that grafting with EMA gave

surface coating to the fibers, and surface features of fibers were not clearly visible. Since coir

fibers exhibited micropores on their surface, the monomer penetrated into the pores and formed a

mechanically interlocked coating on their surface. Smooth fiber surface is observed due to the

substances deposited on the surface of the fiber. The surface topography is entirely modified

after treatment with monomer followed by UV curing. Therefore, the modification by EMA

treated coir fibers develops into changes in fiber surface (as shown in Fig. 5c). Comparison of

Fig. 5b with Fig. 5c showed that in the intercellular gaps as well as on the surface of the unit

cells, grafted EMA has been deposited. Again, the surface of EMA grafted coir seems to be

uniform and smooth in comparison with alkali treated coir fiber. These changes will effectively

result in improved surface tension, wetting ability, swelling, adhesion and compatibility with

polymeric materials [21]. EMA grafting on to coir is targeted with a view to improve surface as

well as the bulk mechanical properties for its potential use as a reinforcing fiber for polymer

composites.


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Fig. 5. SEM images of untreated and treated coir fibers; (a) virgin coir fiber, (b) coir fiber treated

with 20% alkali solution and (c) coir fiber grafted with EMA.

9. Conclusion

Renewable fibers like coir can be used as reinforcing materials for low cost composites, due tothe economic and environmental advantages of such materials. However, coir fiber is hydrophilic

due to the presence of hydroxyl groups from cellulose and lignin. The results of the present study

show that a useful composite with good strength could be successfully developed using coir

fibers as a reinforcing agent for the polymethacrylate matrix. A significant increase in the

strength of the composites was observed after treatment of the fibers. UV pretreatment of the coir

showed an increase of tensile properties and this increase of tensile properties with increasing

radiation doses could be due to the inter-crosslinking between the neighboring cellulose

molecules occur under UV radiation. The best improvement was observed for the 20% alkali

treated followed by grafting with 50% EMA. Alkali treatment of coir fiber also reduces the

hydrophilicity of the fiber and significant increase of tensile properties of the grafted fiber wasobserved.

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Compos Part A: Appl Sci Manufact, 31 (2) (2000), p. 143Article | PDF (1161 K) | View Record in Scopus | Cited By in Scopus (92)

Corresponding author. Present address: Department of Applied Chemistry and Biochemistry, Kumamoto University,

2-39-1 Kurokami, Kumamoto 860-8555, Japan. Tel.: +880 2 8126522/81 96 342 3662; fax: +81 96 342 3662.

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