literature review of coconut coir in polymer composites

C o c o n u t C o i r | 1

LITERATURE REVIEW

COCONUT COIR AS NATURAL FIBERS FOR POLYMER COMPOSITES

Faruq bin LuqmanSchool of Material Engineering, University Malaysia Perlis (UniMAP),

02600, Arau, Perlis, Malaysia

The chemical composition of plant components varies between different species of

plants, within the same species of plants from different geographic locations, ages, climate and

soil conditions, as well as within different parts of the same plant (James S. Han, 1997). In line

with this, Craig M. Clemons and Daniel F. Caulfield (2005) stated that the structure and

chemical make-up of natural fibers varies greatly and depends on the source and processing

variables.

Natural fibers can be described as complex, three dimensional, polymer composites made

up primarily of cellulose, hemicelluloses, pectin and lignin (Craig M. Clemons, 2005) (Rowell,

1992, cited in Craig M. Clemons, 2005). James S. Han and Jeffrey S. Rowell (1997) provided

detailed information about the general chemistry and structures of the chemical components of

natural fibers.

Craig M. Clemons and Daniel F. Caulfield (2005) gave a simple but sufficient

explanation of the main chemical components of natural fibers. Cellulose is a highly crystalline,

linear polymer of anhydroglucose molecules with a degree of polymerization of around 10 000

(Craig M. Clemons, 2005). Cellulose is the major framework component of natural fibers that

gives it strength, stiffness and structural stability while showing the least variation of chemical

structure (Craig M. Clemons, 2005). Hemicelluloses are branched polymers containing five-and

six-carbon sugars of varied chemical structure, the molecular weights of which are well below

those of cellulose (Petterson, 1984, cited in Craig M. Clemons, 2005). Lignin is an amorphous,


cross-linked polymer network consisting of an irregular array of variously bonded hydroxy-and

methoxy-substituted phenylpropane units (Petterson, 1984, cited in Craig M. Clemons, 2005).

Lignin is less polar than cellulose and acts as a chemical adhesive within and between fibers

(Craig M. Clemons, 2005). Pectins are complex polysaccharides, the main chains of which

consist of a modified polymer of glucuronic acid and residues of rhamnose (H. Lilholt, 2000,

cited in Craig M. Clemons, 2005). Pectin chains are often cross-linked by calcium ions,

improving structural integrity in pectin-rich areas (H. Lilholt, 2000, cited in Craig M. Clemons,

2005). The lignin, hemicelluloses, and pectins collectively function as matrix and adhesive,

helping to hold together the cellulosic framework structure of the natural composite fiber (Craig

M. Clemons, 2005). Craig M. Clemons and Daniel F. Caulfield (2005) had also stated that

pectins are important in non-wood fibers but did not clarify the reason for its importance or the

form of its importance.

According to Craig M. Clemons and Daniel F. Caulfield (2005), natural fibers contain

small amounts of additional extraneous components, including low molecular weight organic

components (extractives) and inorganic matter (ash). Extractives are a group of cell wall

chemicals mainly consisting of fats, fatty acids, fatty alcohols, phenols, terpenes, steroids, resin

acids, rosin, and waxes (James S. Han, 1997). Extractives can have large influences on properties

such as colour, odour, and decay resistance (Petterson, 1984, cited in Craig M. Clemons, 2005).

Ash content is defined by James S. Han and Jeffrey S. Rowell (1997) as the residue remaining

after ignition at 575° ± 25° C (1067° ± 5°F) for 3 hours, or longer if necessary, to burn off all the

carbon. It is, however, understood that it does not have to quantitatively equal the remaining

residue but rather it is just a measure of it. Ash content can be quite high in plants containing

large amounts of silica (James S. Han, 1997), e.g. rice hulls, where their abrasive nature should

be of concern (Craig M. Clemons, 2005).

The mechanical properties of select natural fibers (refer to Table 1) are good, but not as

good as those of synthetic fibers such as glass (Craig M. Clemons, 2005). The select natural

fibers aforementioned did not include coir. However, from the investigations of A.G. Kulkarni et

al. (1980), the mechanical properties of coir (refer to Table 2) are at a comparable range to that

of those select natural fibers. Furthermore, based on the sugar analysis (refer to Table 3) as


shown by R.M. Rowell et al. (2000), it can be inferred that coir, being a natural fiber obtained

from coconut shells, has a chemical composition similar to that of common natural fibers, and

thus can be assumed to share some commonality with regard to mechanical properties.

Table 1: Mechanical properties of selected fibers (Craig M. Clemons, 2005)

Table 2: Properties of retted coir fibers at different strain rates with a gauge length of 0.05 m (A.G. Kulkarni et al., 1980)


Table 3: Sugar analysis of some agro-fibers (R.M. Rowell et al., 2000)


Beckwith (2008) provides a more direct comparison between coir, other natural fibers

and synthetic fibers. It is suggested that albeit having relatively lower strength properties, natural

fibers do provide a wide range of workable strength and stiffness properties when compared with

E-glass fiber, which is a common synthetic fiber (Beckwith, 2008). Based on the table (refer to

Table 4), it is noted that coir has a low density, low tensile strength, low tensile modulus, and

high range of elongation compared to the other fibers.

Table 4: A few typical mechanical and physical properties of natural fibers compared to their commercial and aerospace counterparts (Beckwith, 2008)

Obtaining accurate date for the chemical composition and physical properties of coir

were somewhat difficult, as there are many factors that make this data variable in so many

aspects. James S. Han et al. (1997) explains that although there are many reports on the chemical

composition of plant material, the analysis methods used were not standardized, thus in many

cases it is not suitable to make comparisons of data.

However, based on a review by Saira Taj et al. (2007), data on the chemical composition

(refer to Table 5) and mechanical properties (refer to Table 6) of coir agrees with those presented

heretofore.


Table 5: Chemical composition, moisture content, and microfibrillar angle of vegetable

fibers (Saira Taj, 2007)


Table 6: Mechanical properties of natural fibers as compared to conventional reinforcing fibers (Saira Taj, 2007)

Natural fibers have relatively low density and are usually available at a low cost. It is the

balancing of significant reinforcing potential at low cost and low density that makes natural

fibers attractive to industries (Craig M. Clemons, 2005). On the downside, the major chemical

constituents of natural fibers contain hydroxyl and other oxygen containing groups that attract

moisture through hydrogen bonding (Rowell, Penetration and Reactivity of Cell Wall

Components, 1984, cited in Craig M. Clemons, 2005). This is one of the difficulties faced in

composite fabrication and in the performance of the end product (Craig M. Clemons, 2005).

Hence, natural fibers must be dried before or during processing, otherwise processes that are

insensitive to moisture must be used (Craig M. Clemons, 2005).

Craig M. Clemons and Daniel F. Caulfield (2005) places emphasis on the effects of

moisture content on the performance of end products that uses natural fibers. It has been

suggested that even small quantities of absorbed moisture can affect performance by

plasticization of the fiber and the reduction in the fiber-matrix adhesion due to volume changes

associated with the moisture sorption (S. Peyer, 2000, cited in Craig M. Clemons, 2005).


Natural fibers change dimensions with changing moisture content because the cell wall

polymers contain hydroxyl and other oxygen-containing groups that attract moisture through

hydrogen bonding (Rowell, Opportunities for Lignocellulosic Materials and Composites, 1990).

According to Rowell (1990), the component that is mainly responsible for moisture sorption is

himicellulose, but added that “the accessible cellulose, non-crystalline cellulose, lignin, and

surface of crystalline cellulose also play major roles” (Rowell, Opportunities for Lignocellulosic

Materials and Composites, 1990: 20). In the paper, Rowell (1990) explains that moisture swells

the cell wall, expanding it until it is saturated with water. After being saturated, extra moisture

will exist as free water in the void structure and does not contribute to further expansion (Rowell,

Opportunities for Lignocellulosic Materials and Composites, 1990). It is also stated that this

process is reversible; shrinkage will occur as moisture is lost (Rowell, Opportunities for

Lignocellulosic Materials and Composites, 1990).

Craig M. Clemons and Daniel F. Caulfield (2005) provides several methods to solve this

potential problem which include adequately dispersing and encapsulating the fibers in the matrix

during compounding, limiting fiber content, chemically modifying the fiber, or by simply

avoiding exposure to moisture of the end product.

Preliminary work on the mechanical properties of coir was undertaken by A. G. Kulkarni

et al. (1980). In the introduction to the work, A.G. Kulkarni et al. writes: “detailed scientific

information is still lacking particularly in the case of natural fibres like coir fibre (cocos nucifera

linn)” (A.G. Kulkarni, 1980: 905). A.G. Kulkarni et al. (1980) studied about the mechanical

properties of coir fiber as functions of the retting process, the diameter and gauge length of the

fibers at various strain rates as well as using optical and scanning electron microscopy (SEM) for

structural studies of the observed results. The retting process was defined in the study as “a

bacterial process in which the husks are soaked in water for about 8-10 months and then beaten

to produce dry fibers” (A.G. Kulkarni, 1980: 906). The retting process is principally based on the

relative differences that exist in the susceptibility of different constituents of the husk to

microbiological decay (D.K. Salunkhe, 1995).


One of the drawbacks of the study by A.G. Kulkarni (1980) is that there is lack of

description on the unretted coir fiber material used. Having performed the experiments in the

study (A.G. Kulkarni, 1980) and obtaining the results as shown, the method of extraction and the

conditions underwent by both the retted and unretted coir fiber material are questionable and

should have been investigated and described. From the results (refer to Table 7) of the study

(A.G. Kulkarni, 1980: 906), there was found to be no significant differences between the

mechanical properties of retted and unretted coir fibers. Another weakness of the study (A.G.

Kulkarni, 1980) is that the experimental procedure lacked description and use of any proper

testing standards. On the basis of the time that the study (A.G. Kulkarni, 1980) was conducted, it

would be an acceptable argument that proper testing standards suitable for coir fiber were yet to

be tabulated. However, the testing methods used in the study (A.G. Kulkarni, 1980: 906) should

have been given a more detailed explanation.

Table 7: Mechanical properties of the retted and unretted coir fibers with a strain rate of 2.5 × 10-2 m min-1 and a gauge length of 0.05 m (A.G. Kulkarni, 1980: 906)

These results (refer to Table 7) were argued by A.G. Kulkarni et al. (1980) on the basis of

the internal structure of the coir fibers. A.G. Kulkarni et al. (1980) described the coir fibers as

being comprised of long chain molecules that form crystalline regions as well as non-crystalline

regions; the former being due to the cellulose content while the latter being due to the lignin

content. A.G. Kulkarni et al. (1980) further suggests the structure as being long crystals in the

form of helical spirals embedded in non-crystalline regions based on previous studies using X-

ray methods (A.G. Kulkarni, 1980: 909).


Based on the suggested spiral structure, A.G. Kulkarni et al. (1980) provides two possible

explanations to the behavior of coir fibers under tensile loading. The first explanation is that the

microfibrils along with the non-crystalline regions may elongate, and the second explanation is

that the microfibrils may simply uncoil like springs with bending and twisting. Both of these

explanations seem to be logical arguments that may very well explain the elongation of coir

fibers (refer to Table 4 and Table 6) which is much higher than other natural fibers as discussed

hitherto. It is enlightening to note that A.G. Kulkarni et al. (1980) considers the operation of both

suggested mechanisms in the case of coir fibers but without certainty on which of the two

mechanisms is more prevailing.

In terms of the bonding that exists within the coir fibers, A.G. Kulkarni et al. (1980) lists

three types which are the bonding of the fiber surface by a gum type material called ‘cuticle’,

bonding of the cells in the fiber by a resinous material (Fig. 1) and the embedment of the

microfibrils in the cells in a non-crystalline matrix. A.G. Kulkarni et al. (1980) suggests that the

breaking of the fibers may be due to either one or all of the three bondings breaking or either the

crystalline fibril of the non-crystalline matrix breaking. This was evidenced in the metallographic

examination conducted whereby no separation of the cells near the periphery of the fiber from

the cuticle (Fig. 2) and no breaking of the bonding material between the cells or cracks along the

intercellular region were observed (A.G. Kulkarni, 1980: 910-911). A.G. Kulkarni et al. places

his view in that: “the final sudden break of the coir fiber is due to the fracture of the cell

themselves, which in some cases is accompanied by the uncoiling of the fibrils” (Fig. 3 & 4)

(A.G. Kulkarni, 1980: 911).

A major section of the study done by A.G. Kulkarni et al. (1980) discusses about the

effect of the size of fiber. An increase in the size of the fiber diameter means an increase in the

total number of cells in the fiber and is accompanied by a decrease in the number of cells per unit

area (A.G. Kulkarni, 1980). According to A.G. Kulkarni et al. (1980), there may be an increase

in defects and decrease in the crystalline component as the size of the fiber increases. This may

seem contradictory to the results of the study (refer to Table 7), as it can be observed that the

strength (Fig. 5) and percent elongation of the fibers increases up to a certain diameter, after

which it remains rather constant (A.G. Kulkarni, 1980: 907). On the other hand, the initial


modulus (Fig. 6) gradually decreases with an increase in the diameter of the fibers in the entire

range investigated (A.G. Kulkarni, 1980: 907).

However, further discussion provided by A.G. Kulkarni et al. (1980) states that the

strength of the fiber increases with the number of cells but decreases with increase in

microfibrillar angle, decrease in cellulose content and increase in defects. It was mentioned that

microfibrillar angle increases with the increase in the size or coarseness of plant fibers (A.G.

Kulkarni, 1980: 911). A.G. Kulkarni et al. (1980) presents the argument that up to a certain

stage, “the effect of increase in total number of cells is more predominant that the other factors

which affect the strength value of the fiber” (A.G. Kulkarni, 1980: 912).

Besides its relatively low density and low cost, the use of natural fibers in polymer

composites is attractive due to environmental concerns. It is a well known fact, that natural

fibers, being taken from plants, are biodegradable. Rowell (1990) explains that lignocellulosics

are degraded biologically because organisms have specific enzyme systems capable of

hydrolyzing the carbohydrate polymers in the cell wall into digestible units (Rowell,

Opportunities for Lignocellulosic Materials and Composites, 1990). Lignocellulosics are

substances that contain both cellulose and lignin (Rowell, Opportunities for Lignocellulosic

Materials and Composites, 1990) such as the natural fibers discussed here. The strength of

lignocellulosics, and hence natural fibers, will decrease as the cellulose polymer undergoes

degradation through oxidation, hydrolysis, and dehydration reactions (Rowell, Opportunities for

Lignocellulosic Materials and Composites, 1990).

Rowell (1990) also states that exposure to ultraviolet light will cause photochemical

degradation of lignocellulosics, primarily in the lignin component which is responsible for the

characteristic colour changes. Lignin is the component that holds the cellulose together (Rowell,

Opportunities for Lignocellulosic Materials and Composites, 1990). Rowell (1990) explains that

once this component degrades, “the poorly bonded carbohydrate-rich fibers erode easily from the

surface, which exposes new lignin to further degradative reactions” (Rowell, Opportunities for

Lignocellulosic Materials and Composites, 1990: 20).


Craig M. Clemons and Daniel F. Caulfield (2005) mentions about natural fibers, having

relatively low thermal stability, can burn and release volatile components. Rowell (1990)

explains that lignocellulosics burn due to pyrolysis reactions of cell wall polymers with

increasing temperature to give off volatile, flammable gases (Rowell, Opportunities for

Lignocellulosic Materials and Composites, 1990: 21). Craig M. Clemons and Daniel F. Caulfield

(2005) recommends basic precautions such as avoiding high processing temperatures, using

well-ventilated equipment, and eliminating ignition sources (Craig M. Clemons, 2005).


Figure 1: Longitudinal section of coir fiber after tensile testing indicating that the crack has not propagated along the cell wall and bonding between the cells is intact (× 80) (A.G. Kulkarni, 1980: 910)

Figure 2: Fractured end of the coir fiber showing necking of the fiber (× 172) (A.G. Kulkarni, 1980: 910)


Figure 3: Fractured surface of the coir fiber showing pull-out of the cells and collapse of the cell walls (× 860) (A.G. Kulkarni, 1980: 911)

Figure 4: Fractograph of the coir fiber showing pull-out of the cells and collapse of the cell walls (× 1375) (A.G. Kulkarni, 1980: 911)


Figure 5: Plot of ultimate tensile strength against size of the coir fiber (A.G. Kulkarni, 1980: 907)

Figure 6: Relation between the initial modulus and size (fineness) of the fiber (A.G. Kulkarni, 1980: 907)


REFERENCES

A.G. Kulkarni, K. S. (1980). Mechnical behaviour of coir fibres under tensile load. Journal of Materials Science , 16 (1981), 905-914.

Beckwith, D. S. (2008). Natural Fibers: Nature Providing Technology for Composites. SAMPE Journal , 44 (3), 64-65.

Craig M. Clemons, D. F. (2005). Natural Fibers. In P. D. Xanthos, Functional Fillers for Plastics (pp. 195-206). Weinheim: WILEY-VCH Verlag GmbH & Co. KGaA.

D.K. Salunkhe, S. K. (1995). Handbook of Fruit Science and Technology: Production, Composition, Storage, and Processing. New York: Marcel Dekker.

G Satyanayarana, S. A. (1986). Composites, Fabrication and Properties of Natural Fibre-reinforced Polyester Composites. Kerala: Council of Scientific and Industrial Research.

H. Lilholt, J. L. (2000). Natural Organic Fibers. In T. Chou, Comprehensive Composites Materials, Vol.1: fiber Reinforcements and General theory of Composites (pp. 303-325). New York: Elsevier.

James S. Han, J. S. (1997). Chemical Composition of Fibers. In R. A. Roger M. Rowell, Paer and Composites from Agro Based Resources (pp. 83-135). Boca Raton: CRC Press, Inc.

Petterson, R. (1984). The Chemical Composition of Wood. In The Chemistry of Solid Wood (pp. 76-81). Washington DC: American Chemical Society.

Ritsuko Hori, J. S. (n.d.). Microfibril Angle in Wood and its Biological Significance. Retrieved from www.spring8.or.jp/pdf/en/res_fro/01-02/023-024.pdf

Roger M. Rowell, J. S. (2000). Characterization and Factors Effecting Fiber Properties. Sao Carlos: Embrapa Instrumentação Agropecuária.

Rowell, R. M. (1990). Opportunities for Lignocellulosic Materials and Composites. Emerging Technologies for Materials and Chemicals from Biomass. Washington DC: American Chemical Society.

Rowell, R. M. (1992). Opportunities for Lignocellulosic Materials and Composites. In T. P. R. M. Rowell, Emerging Technologies for Materials and Chemicals from Biomass. Washington DC: American Chemical Society.

Rowell, R. M. (1984). Penetration and Reactivity of Cell Wall Components. In R. Rowell, The Chemistry of Solid Wood (p. 176). Washington DC: American Chemical Society.

S. Peyer, M. W. (2000). Engineered Wood Composites fo Naval Waterfront Facilities. Washington: Yearly Report to Office Naval Research.

Saira Taj, M. A. (2007). Natural Fiber-Reinforced Polymer Composites. Proc. Pakistan Acad. Sci. , 44(2S):129-144.

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