2.1. Plasma treatment of plant fibers

Chapter -2 Literature Review

School of Chemistry 19

The surfaces of plant fibers can be modified by conventional/chemical treatments and

environment friendly methods for better binding between fibers and matrix for a wide

range of applications. Chemical treatments include mercerization, acetylation,

peroxide, benzoylation, coupling agents and polymer grafting. Environment friendly

methods include treating fibers with plasma, enzyme, fungi and coating with

nanocellulose. This chapter consists of review of literature regarding biological

methods for natural fibers surface modification and their further applications in

different fields.

2.1. Plasma treatment of plant fibers

There are different types of plasma sources available to treat fibers or textile material

with plasma (Schutze et al., 1998). Low-pressure plasma treatment is used to modify

the surface of plant fibers by generating plasma discharge at atmospheric or under

vacuum conditions (Kalia et al., 2009). A major advantage of employing low-pressure

plasma treatment is that such plasma can be generated at low power output, i.e. 60–

100 W. By using smaller output, it is possible to minimize the thermal damage caused

by the plasma treatment on the plant fibers, thereby preserving the properties of plant

fibers (Yuan et al., 2004). Other advantages include uniform glow, low breakdown

voltages, high concentration of reactive species and generation of non-thermal

plasma. But being a batch process, low-pressure plasma does not meet the

requirements of continuous processing of textiles. Moreover, it requires sustaining the

vacuum leading to limitations on machine productivity. This limitation leads to the

development of the atmospheric pressure plasma technology to fulfill the need of the

textile industry (Samanta et al., 2006). Jeong et al. (1998) have developed the APPJ

for etching materials at atmospheric pressures and between 100 and 275 ⁰C. This is

different from thermal torches, arcs (>10,000 ⁰C), corona (>500 ⁰C), etc. because it

produces uniform plasma at low temperature that may be used for materials

processing on relatively large substrates. Wolter et al. (2009) have used the APPJ for

the treatment of temperature-sensitive materials with melting points below 150 ⁰C.

Their work was focused on the formation of HMDSO-based coating on polymethyl

methacrylate (PMMA) substrates using APPJ with a maximum substrate temperature

measured to be around 55 ⁰C. The influence of AGD plasma treatment on the



morphology, wettability, and fine structure of jute fibers and its impact on the

interfacial adhesion of jute fibers/unsaturated polyester have been investigated (Kim

et al., 2008). Plasma treatment also results in rough surface morphology and

degradation of fiber due to an etching mechanism and the development of

hydrophobicity in fibers. However, among all treated fiber composites, the flexural

strength of composites prepared with fibers treated for 10 min only showed an

improved mechanical strength of approximately 14% in comparison to raw fiber

composites (Sinha and Panigrahi, 2009).

2.2. Pretreatment with bacterial nanocellulose

This method involves the addition of new material onto the surface of plant fibers.

This type of modification involves the deposition of nanosized cellulosic materials

onto the surface of plant fibers to enhance the interfacial adhesion between the fiber

and the matrix (Lee et al., 2011). By culturing cellulose-producing bacteria such as A.

xylinum in the presence of plant fibers in an appropriate culture medium, bacterial

cellulose is preferentially deposited in situ onto the surface of plant fibers. The

introduction of bacterial cellulose onto plant fibers provides new means of controlling

the interaction between plant fibers and polymer matrices. Coating of plant fibers with

bacterial cellulose not only facilitates good distribution of bacterial cellulose within

the matrix, but also results in an improved interfacial adhesion between fibers and the

matrix. This enhances the interaction between plant fibers and the polymer matrix

through mechanical interlocking.

Figure 2.1. Photos of sisal fibers: (a) before and (b) after 2 days of bacterial

treatment. (c) SEM micrograph of sisal fiber surface after culture in fermentor,

showing that the sisal surface is partially covered by bacterial cellulose.

Reprinted from (Pommet et al., 2008), with permission from American Chemical

Society.



Figure 2.1 displays how bacterial cellulose-coated plant fibers introduced

nanocellulose at the interface between the fibers and the matrix, leading to increased

stiffness of the matrix around the plant fibers (Juntaro et al., 2007; Juntaro et al.,

2008; Pommet et al., 2008). Sisal fibers were successfully modified by culturing

cellulose producing bacteria in the presence of fibers in an appropriate culture

medium (Pommet et al., 2008). Many researchers have worked on the optimization of

the conditions that are necessary for the proper cultivation of bacterial cellulose.

Many researchers (Panesar et al., 2009; Embuscado et al, 1994; Pourramezan et al.,

2009; Masaoka et al., 1993; Neelobon et al., 2007; Keshk and Sameshima, 2005)

have optimized the carbon sources, nitrogen sources, pH, temperature and incubation

time for the maximum microbial cellulose yield production. Panesar et al. (2009)

have concluded that among the carbon sources (sucrose, fructose, D-galactose, D-

glucose, lactose, mannitol and ethanol) mannitol and glucose are the best carbon

sources while among the nitrogen sources (ammonium sulphate, ammonium nitrate,

riboflavin, glycine, peptone, sodium nitrate and methionine), methionine and peptone

are found to be the best sources for the maximum production of the cellulose by

Acetobacter aceti at pH 6.5–7.0 and 28 ⁰C temperature conditions. While Embuscado

et al. (1994) optimized the conditions for A. xylinum, i.e. fructose is best carbon

source at pH 4.5 and temperature 30 ⁰C. Pourramezan et al. (2009) have found

sucrose as best carbon source at pH 7 and 30 ⁰C conditions for the Acetobacter sp. by

which production reaches up to 11.98 g/LBC. Panesar et al. (2012) have optimized

the various process parameters for the production of cellulose by A. aceti (MTCC

2623). Response surface methodology (RSM) was applied to optimize the process

parameters during production. RSM is a collection of statistical techniques for

experiments designing, building models, evaluation of factor and finding of the

optimum conditions for desirable results. RSM can identify and quantify the various

interactions among different parameters and it has been applied for optimization of

medium and process conditions in bioprocesses (Bogar et al., 2003; Panesar, 2008).

Maximum cellulose production (1.73 g/L) and sugar utilization (99.8 %) was obtained

at 2.25 % (w/v) glucose concentration, 1.16 % (w/v) sodium nitrate concentration,

27.5 ⁰C temperature, 159 h of incubation time and 7.0 pH (Figure 2.2). Figure 2.2a

shows that the increase in pH and incubation time resulted to an increase in cellulose

production. Minimum cellulose production was observed at lower pH and shorter



incubation time, however longer incubation time and higher pH increases the

cellulose production. This indicates that alkaline pH is favorable for cellulose

production because of minimum conversion of glucose to gluconic acid (Masaoka et

al., 1993). Cellulose production also increased with longer incubation time but

decreased after 168 h because of the attainment of stationary phase by bacteria

beyond this time period. With increase in glucose and sodium nitrate, there was

increase in cellulose production (Figure 2.2b). However, excess sugar in fermentation

media reduced the cellulose yield because of conversion of excess of glucose into

gluconic acid (Vandamme et al., 1998).

Figure 2.2. Effect of: (a) incubation time and pH on cellulose production and (b)

glucose concentration and sodium nitrate concentration on cellulose production.

Reprinted from (Panesar et al., 2012), with permission from Elsevier.

Tantratian et al. (2005) have studied the effect of dissolved oxygen on cellulose

production by Acetobacter sp. and concluded that at high rotation speed, the dissolved

oxygen in the medium also increased which ultimately increased the gluconic acid

content and reduced the BC production. The addition of carboxymethyl cellulose to

the culture medium decreased the oxygen content resulting to reduced BC yield. So

this indicated that there was an optimum amount of oxygen, that when dissolved in

the culture medium would produce the higher cellulose yield. Gelin et al. (2007) have

characterized the water in the bacterial cellulose (A. xylinum) using dielectric

microscopy and electron microscopy. Freeze fracture transmission electron

microscopic indicated that the bulk-like water is not forming a continuous phase

throughout the gel rather it limited to the „„lakes‟‟. Water desorption isotherms



suggest that these „„lakes‟‟ decrease in size with increasing oxygen concentration used

during the biosynthesis process of the gels. So, the optimization of oxygen content is

also an important aspect in the biosynthesis of bacterial cellulose. Figure 2.3 shows

the arrangement of microfibrils in ribbons that supports the „„lake‟‟ assumption. The

microfibrils are clearly woven together in a network that leaves some open space

where water may likely exist in bulk phase.

Figure 2.3. Freeze fracture TEM images of disintegrated BC20. Reprinted from

(Gelin et al., 2007), with permission from Elsevier.

2.3. Pretreatment with bacterial cellulase

Biodegradation of cellulose to glucose is the principal reaction in any application

involving the treatment of plant fibers with bacterial cellulase, e.g. wood fiber

modification, biomass conversion or even textile biopolishing. Depending on the

process objective, the extent of cellulase degradation and the properties of the

resulting products can be controlled by adjusting the treatment parameters such as

treatment time, enzyme loading and the composition of cellulase mixture (Table 2.1)

(Esteghlalian et al., 2002). Park et al. (2006) reported the change in surface and pore

structure as a result of enzymatic hydrolysis and characterized these changes using

various test methods for high (600 U/g) and low (60 U/g) dosage of cellulase

treatment. For high dosage treatment swelling capacity, concentration of small pores

increased and there was a significant change in the length of fibers. A decrease in

polymer adsorption and an increase in the crystallinity index were also observed. It

was found that the amorphous portion of the cellulose is more readily hydrolyzed than

the crystalline region (Table 2.2).



Table 2.1. The role of major process variables in the treatment of natural fibers

with cellulase. Reprinted from (Esteghlalian et al., 2002), with permission from

Elsevier.

Application Desirable outcome Time Treatment parameter

Time cellulase

composition

Cellulsae

loading

Fiber

modification

Reduced cell wall

thickness, more

flexible and

collapsible fibers

Short monocomponents low

Cellulose

hydrolysis

to glucose

Full conversion of

cellulose to glucose

Longer (12-

24 h)

Complete (endo,

exo and β-G)

Relatively

high

Textile

biopolishing

Depilling and ageing

of fabric

Medium (20

min to 8 h)

Complete, EG or

EG-rich

Low

Table 2.2. Characteristics of cellulase treated fibers. Reprinted from (Park et al.,

2006), with permission from John Wiley and Sons.

Untreated Low dosage High dosage

Hydrolysis time (min) 0 60 240 60 240

Fiber length (mm) 2.56 2.49 2.54 1.76 0.32

Fiber width (μm) 30 30.1 30.4 31.2 33.4

Fiber content (%) 3.02 2.90 3.13 4.83 33.4

Degradationa (%) 0.00 0.92 1.78 25.7 46.3

Polymer adsorptiona

(μequiv./g)

40.6 36.2 40.12 35.8 23.8

Crystallinity indexb (%) 52.8 - - 54.8 54.3

aSamples for 30 and 120 min treatment were not measured.

bSamples for low dosage treatment and 30 and 120 min high dosage treatment were not measured.

Saikia et al. (2009) performed microbial degumming of ramie fibers and observed a

remarkable decrease in the residual gum content in 3–8 days treatment. The

percentage of residual gum content after 3–8 days was recorded as 13.50%, 10.40%,

10.0%, 9.70%, 8.20% and 7.10%. Tensile strength reduces with the increase in period

of treatment. The effect of temperature and pH was also studied and observed. The

optimum conditions were at 35 ⁰C and pH 7.5 for Bacillus subtilis, Aspergillus sp.



and Curvularia sp. bacteria. Kalia and Sheoran (2009) have hydrolyzed the cellulose

present in ramie fibers with Streptomyces albaduncus. It was observed that there was

a remarkable change in the morphology of modified ramie fiber as comparison to the

unmodified and chemically modified samples. Fiber surface became soft and bright

due to the extracellular protuberant structures, although there was a diminishing effect

on the thermal stability and crystalline structure of the fibers. Kalia and Vashishta

(2012) have treated the sisal fibers with Brevibacillus parabrevis and reported the

smoothened shiny surface, enhanced thermal stability and crystallinity of modified

fibers. Bacterial degumming was observed to be a better option than any other method

for plant fiber modification. Wang et al. (2007) have obtained the plant Apocynum

venetum cellulose fibers from different degumming methods such as by hand (fiber-

H), by machine (fiber-M), as well as the bacterial degumming method (fiber-B) and

characterized these fibers by various techniques, i.e. FTIR, SEM, XRD, etc. In case of

crystallinity, the degree of orientation and the mechanical properties, fiber-B has only

slight differences with those of fiber-H and fiber-M. This suggests that bacterial

degumming method has more industrial applications due to its high efficiency, low

cost and especially environmentally benign nature than the chemical degumming

methods. The surface of the treated fibers (Figure 2.4 b–d) became smoother as

compared to that of untreated materials. The treated fibers (10–25 mm widths) can be

directly used in textile industry and other applications.

Figure 2.4. SEM micrographs of the four A. venetum samples: (a) the bast of A.

venetum (b) Fiber-M (c) Fiber-H (d) Fiber-B. Reprinted from (Wang et al., 2007),

with permission from Elsevier.



2.4. Fungal treatment of natural fibers

Treatment of plant fibers with the fungi is also one of the alternate methods to the

chemical methods. Fungal treatment is an ecofriendly and efficient method. Fungi

cause the removal of lignin from plant fibers. It reduces the hydrophobicity of the

fiber by increasing the solubility of hemicelluloses. Fungal treatment causes the

formation of holes (pits) on fiber surface, which provides roughness to the fiber

surface and ultimately increases the interfacial adhesion between fiber and matrix

(Kabir et al., 2012; Jafari et al., 2007). Fungal treatment starts with the sterilization of

fibers at 121 ⁰C for 15 min. Then it is followed by the addition of fungi to the fiber.

After incubation of the culture set for 2 weeks at 27 ⁰C, the fibers were then washed

and dried. The use of white rot fungus in the treatment of plant fibers can be found in

literature (Pickering et al., 2007). Pickering et al. (2007) have treated hemp fibers

with five different white rot fungi. The species used were Phanerochaete sordid

(D2B), Pycnoporus species (Pyc), and Schizophyllum commune (S. com) of the

basidiomycetes group, Ophiostoma floccosum (F13) of the ascomycetes group, and

Absidia (B101), a zygomycete. An extra feature observed for D2B treated fibers was

pits in the surface of the fiber, believed to be where fungal hyphae had grown.

Standard BET analysis was carried out to assess the effect of treatment on surface

area. D2B treated fiber was found to have an effective area of almost twice that of

untreated fiber (3.13 versus 1.7 m2/g). Fungal treatment resulted in higher crystallinity

index as compared to the untreated fibers. This is a direct result of the fungi‟s ability

to remove non-cellulosic compounds such as amorphous lignin, thereby increasing the

crystallinity index. Cavaco-Paulo (1998) has studied the mechanism of fungi cellulase

action in textile processes using the Trichoderma reseei, fungi belonging to the class

ascomycetes, which produces the cellulase. Yu and Yu (2007) have carried out a

study on the microbe retting of kenaf fiber, a process of the separation of bast fibers

from its non-fibrous components. They have optimized the conditions for the proper

fungal growth so as to carry out best retting. The optimum retting conditions were: 32

⁰C culture temperature, initial culture medium pH of 6.0, 24 h cultivation time, 21 h

retting time and 25% inoculation size. Kenaf was retted under optimal conditions, and

then the constituents in retted fiber were tested as shown in Table 2.3. Pectin is



removed followed by hydrotrope and hemicellulose while the removal of lignin is

almost inconspicuous. After microbe retting of kenaf under the optimal condition, the

resultant fiber has only 14.46% of the original gum. The removal of pectin is 91.31%.

The effective retting fungus is also observed as one kind of filamentous epiphyte (Yu

and Yu, 2007).

Table 2.3. Ingredients of kenaf fiber before and after retting. Reprinted from

(Yu and Yu, 2007), with permission from Elsevier.

Ingredients

Cellulose Wax Hydrotrope Pectin Hemicelluloses Lignin Gum

Raw

kenaf (%) 53.18 0.46 14.99 8.86 14.33 8.18 35.17

Retted

kenaf (%) 75.81 0 2.03 0.77 13.33 8.06 14.46

Removal

rate (%) - 100 86.46 91.31 6.98 1.47 58.89

2.5. Enzymatic pretreatment of natural fibers

The use of enzyme is becoming increasingly substantial for the processing of plant

fibers. Currently, the use of enzymes in the field of textile and plant fiber modification

is also rapidly increasing. A major reason for using enzyme is the fact that its

application is environmentally friendly. The reactions catalyzed are highly specific.

Enzymatic treatment of plant fibers results in separation of fibers from its non-fiber

components (Bledzki et al., 2010). Treatments utilizing hydrolases and

oxidoreductases are well studied for the polymer modifications. Within the class of

hydrolases, the most frequently used enzymes for enzymatic synthesis and polymer

modifications are glycosidases, proteases and lipases, whereas in the class of

oxidoreductases; tyrosinase, laccase and peroxidase have been in use for years (Gulitz

and Paulo, 2003). Recently, laccase had been used for the bonding of fiber boards,

particle boards, paper boards and kraft-liner boards. It had also been reported that the

strength properties of laccase-bonded fiber boards are more than those bonded using

urea formaldehyde adhesive (Felby et al., 2002). An alkaline pectinase that removes

pectin without significant degradation of cellulose (Scourzyme, Products & Solutions,

2009) has also been utilized to treat hemp fibers (Lee et al., 2011).



2.5.1. Laccase-assisted biografting

There has been a growing interest in the development of green technologies for

surface functionalization of lignocellulosic materials. In this section, two important

green approaches for surface functionalization of lignocellulosics i.e. laccase-assisted

biografting and chemo-enzymatic grafting are discussed Laccase enzyme assisted

biografting is a new and green approach to graft antibacterial, hydrophobic and other

functional molecules to impart better and new properties to lignocellulosic biomass

and biografted lignocellulosics can find potential applications in for various fields.

Laccase-assisted grafting of functional molecules on lignocellulosic materials is

summarized in Table 2.4 (Chandra and Ragauskas, 2002; Chandra et al., 2004;

Schroder et al., 2007; Nyanhongo et al., 2010; Fackler et al., 2008; Chandra et al.,

2004a; Elegir et al., 2008; Kudanga et al., 2011; Kenealy et al., 2004; Liu et al., 2009;

Suurnakki et al., 2006; Kudnga et al., 2010; Kudanga et al., 2008, Kudanga et al.,

2009; Kudanga et al., 2010a; Kudanga et al., 2010b; Witayakran and Ragauskas,

2009; Hadzhiyska et al., 2006). Enzymatic activation of lignin in cellulosic fiber was

studied by Felby et al. (1997) in 1997 through the formation of phenoxy radicals,

which facilitated cross-linking of fibers during board manufacturing. Radical coupling

reactions have been used to attach low-molecular weight compound to lignin rich

cellulosic fibers and they develop increased interest as the key mechanisms behind the

concept of biografting (Chandra et al., 2004; Liu et al. 2009). Two-step processes for

enzymatic functionalization with phenolics were well proposed by Acero et al.

(2012). In first step hydrolysis of amide bond was carried out by polyamidase (from

Nocardia farcinica) which form a surface having amines and carboxylic acids. In

second step, ferulic acid was grafted on the surface of polyamide by laccase (from

Trametes hirsuta) (Scheme 2.1). Modification of surface properties of lignocellulosic

materials by functionalization, coupling and grafting could enhance the surface

characteristics for their better performance. Laccase treatments can improve the

physical and chemical properties of different fibers to obtain better performance or

new value-added products (Polak and Jarosz-Wilkolazka, 2012).



Table 2.4. Laccase-assisted grafting of functional molecules on lignocellulosic

materials. Reprinted from (Kudanga et al., 2011), with permission from Elsevier.

Lignocellulosic

materials

Functional molecule Source

of laccase

Improved

properties

References

Kraft paper, soft

woods pulp

4-Hydroxyphenylacetic

acid, 4-hydroxybenzioc

acid,gallic acid,syringic

acid,vanillic acid

T. villosa Improving strength

properties (increase

in burst, tear and

tensile indexes)

Chandra &

Ragauskas,2002;Chand

ra et al.,2002;Schroder

et al.,2007;Chandra et

al.2004;Chandra,2003;

Chandra & Ragauskas,

2001;Kenealy et

al.,2004

Flax fibers Hydroquinone,ferulic

acid,guaiacol,vanillin,methy

l-3-hydroxy-4-

methoxybenzoate,2-

methoxy-5-nitrophenol

Trametes

hirsuta

Improvement of

color and

antibacterial

properties

Nyanhongo et al., 2010

Wood (spruce)

chips

4-hydroxy-3-

methoxybenzylurea

T. villosa Increase in internal

bond

Fackler et al., 2008

Kraft pulp Methyl syringate Aspergillu

s oryzae

Improvement in wet

strength (wet tensile

index doubled)

Liu et al., 2009

Bleached and

unbleached

TMP handsheets

Lauryl gallate Trametes

hirsuta

Improvement of

hydrophobicity

Suurnakki et al., 2006

Unbleached

kraft liner fibers

Caffeic acid,4-

hydroxybenzoic acid,

isoeugenol

Trametes

pubescens

Improvement of

antibacterial

properties of

prepared

handsheets;oligomer

ic forms of caffeic

acid and isoeugenol

were most effective

against both gram

positive and gram

negative bacteria

Elegir et al., 2008

Beech wood 3-

hydroxytyramine,tyramine,

3-O-methyldopamine,4-

hydroxy-3-

methoxybenzylamine

T. hirsute,

T. villosa,

bacillus

SF

Antifungal agents

onto wood due to

creation of stable

reactive surface

Kudanga et al., 2010;


Kudanga et al., 2009

Beech veneers 4-

(trifluoromethoxy)phenol,4-

fluoro-2-methylphenol,4-[4-

(trifluoromethyl)phenoxy]p

henol

T. villosa,

T. hirsuta

Improvement of

hydrophobicity


Kudanga et al., (101)

2010

Beech veneers dodecylamine T. hirsuta Improvement of

hydrophobicity

Kudanga et al., (149)

2010

Pulp and paper dihexylamine T. villosa Improving strength

properties

Witayakran and

Ragauskas, 2009

Cotton cellulose 2,5-diaminobenzenesulfonic

acid and 1-hydroxyphenol

(catechol)

Trametes

sp.

Coloration of cotton

fibers

Hadzhiyska et al.,

2006



Scheme 2.1. Proposed reaction mechanism for two-step enzymatic grafting of PA

with phenolics. The reaction involves first the hydrolysis of PA with a

recombinant polyamide from N. farcinica followed by laccase-catalyzed grafting

with ferulic acid. Reprinted from (Acero et al., 2012), with permission from

Elsevier.

2.5.1.1. Biografting of phenolics

Antimicrobial packaging has attracted much attention in food-industry because of

costumers demand for less processed and preservative-free products. Various

researchers across the world worked in this direction and developed antimicrobial

packaging material. Hydrophilicity is one of the main disadvantages of lignocellulosic

materials, which makes it unsuitable for various industrial applications. There are

many chemical methods to reduce hydrophilicity but laccase-mediated reactions

found to be more interesting keeping in mind the concept of green chem-istry.



Laccase-assisted biografting of hydrophobicity enhancing molecules is an interesting

study to reduce the chances of growth of microorganisms that are responsible for

biodeterioration (Nyanhongo et al., 2011). Laccase-assisted biografting of phenolics

on lignocellulosic materials was found to enhance hydrophobic properties and

antimicrobial activities.

Laccase polymerization of unbleached kraft liner fibers in the presence of several

phenolic compounds was found to enhance the antimicrobial activity by radical

coupling between phenolic compounds and lignin. Laccase-assisted antibacterial

handsheet paper were fabricated in the presence of caffeic acid. This handsheet paper

showed strong bactericidal effect on Staphylococcus aureus at low monomer

concentration i.e. 4 mM, while high concentration required for killing Escherichia

coli. Among the various essential oils, isoeugenol found to be most effective against

Bacillus subtilis. Dopamine was effective against both gram positive and gram

negative bacteria (Elegir et al., 2008). Laccase-induced coating of flax fibers was

evaluated by obtained coloration and color depth. Pre-treated flax fiber were

incubated in citrate buffer at 50 ◦ C at different ratios, enzyme concentration and

incubation time. Coating with ferulic acid and hydroquinone resulted in enhanced

antimicrobial activity against B. subtilis and S. aureus, while methoxyphenols showed

different coloration. Ferulic acid found to be most active, guaiacol and vanillin

showed weak antibacterial activities while non-coated samples showed no

antibacterial activity (Schroeder et al., 2007). Laccase–FRC (ferulic acid) system was

investigated by Aracri et al. (2011). They carried out the functionaliza-tion of sisal

pulp fibers by laccase-catalyzed grafting of ferulic acid by treating a mixture of

sodium tartrate buffer and sisal pulp with different laccase and ferulic acid (FRC)

concentration in a reactor at 50 ◦ C for 1 h or 4 h. Effect of the laccase–FRC system on

the refined and unrefined pulp fibers was investigated. Increased grafting and

handsheets with improved strength properties were obtained with refining before the

enzyme treatment.

Analytical pyrolysis was first time used by Aracri et al. (2010) to study the

functionalization of sisal and flax pulp fibers by the laccase-induced grafting of

simple phenols like syringaldehyde, acetosyringone, p-coumaric acid,

coniferaldehyde, sinapaldehyde, ferulic acid and sinapic acid. Functionalization was

carried out in oxygen pressurized reactor containing fiber pulp, sodium tartrate buffer,



laccase and Tween 80 as surfactant. Pulp samples treated under same conditions

without phenolic compounds were used as control. Analytical pyrolysis is a powerful

and sensitive tool for the „in-situ‟ analysis of mechanism of laccase-induced coupling

of natural phenols on to the natural fibers (Del-Rio et al., 2001). Chandra et al.

(2004a) have studied the change in the physical properties of high yield kraft pulp by

treatment with laccase, syringic acid and vanillic acids. A mixture of kraft pulp and

phenolic acid was suspended in a Kapak bag and stirred in a water bath at 45 ◦ C

followed by addition of laccase. This treatment enhanced the attachment of acidic

groups on the fiber surface. Enhancement in the paper strength properties has also

been reported by Chandra et al. (2004b) by treating kraft pulp with laccase system in

the presence of gallic acid. Laccase-assisted grafting of gallic acid on kraft pulp was

performed in a Kapak bag under stirring conditions at 45 ◦ C. An improvement in

burst, tensile and wet strength was observed by grafting of kraft pulp in presence of

gallic acid by laccase. An innovative method has been developed by Garcia-Ubasart

et al. (2011) for analyzing the internal sizing capability of kraft pulp, by using laccase

from Trametes villosa and nine differ-ent products (ethyl gallate (r1), propyl gallate

(r2), octyl gallate (r3), lauryl gallate (r4), b-sitosterol (r5), a-tocopherol (r6), 4-[4-

(trifluoromethyl)phenoxy]phenol (r7), isoamyl salicylate (r8) and 2,4,6-tris(1-

phenylethyl)phenol (r9) containing hydrobhobic moieties. Fibers were suspended in a

mixture of buffer aqueous solution, respective reactant (r1–r9) and laccase under

shaking con-dition at 50 ◦ C for 1 h. Lauryl gallate have shown the strongest internal

sizing effect. Recently, Pei et al. (2013) have carried out the enzymatic

functionalization of unbleached kraft fiber in the presence of different phenolic

compounds (isoeugenol, butyl p-hydroxybenzoate, p-coumaric acid and ferulic acid)

to develop antimicrobial activities of the pulp. Enzymatic treatment was car-ried out

in a solution consists of kraft pulp, sodium tartrate buffer, laccase and phenolic

compounds. Handsheets with most signifi-cantly increased bacterialcidal effect were

produced using laccase in the presence of butyl p-hydroxybenzoate. It has been

observed from Figure 2.5 that control fibers having smooth surface while laccase-

treated fibers showing adhesion with adjacent fibers and surface became rough. In

last, agglutination can be seen to a large area which is contributed to the increased

kappa number and coarseness of fibers.



Figure 2.5. SEM images of unbleached kraft pulp fibers, showing: (a) control

fibers (b) laccase-treated and (c) laccase/BPH treated fibers. Reprinted from (Pie

et al., 2013), Open access.

Fillat et al. (2012) reported an enhancement in antimicrobial properties, when

unbleached flax fibers treated with the laccase (from Pycnoporus cinnabarinus) in the

presence of low-molecular weight phenols (syringaldehyde – SA, acetosyringone –

AS, p-coumaric acid – PCA). Grafting of flax pulp was carried out in a closed vessel

containing pulp, sodium tatrate buffer. P. cinnabarinus laccase, natural phenols and

Tween 80. Incubation was carried out under shaking conditions at 30 rpm, 50 ◦ C for 4

h. Widsten et al. (2010) have followed a complete environmentally friendly way to

impart the antimicrobial activity to the wood veneer and pulp. Succinate buffer

containing dissolved tannins and laccase were used for wood treatment. Reaction

mixture of kraft pulp, tannins, laccase and laccase mediators was stirred followed by

dilution of shaking beakers and preparation of paper handsheets. They have treated

the wood and pulp with tannins (natural polyphenols) in the presence or absence of

laccase. Tannic acid and laccase showed bacteriocidal effect on S. aureus and reduced

the growth of E. coli by 50%. Tannic acid without laccase only reduces the growth by

89%, which shows the effectiveness of Tannic acid–laccase system. Kudanga et al.

(2008) have successfully inserted the phenolic amines as anchor groups to the lignin

moieties on wood surface in the presence of laccase (Figure 2.6). 77% increase in the

coupling of propiconazole onto tyramine-functionalizes wood and 91% increase in

coupling of thiabendazole onto 3-(3,4-dihyoxyphenyl)-dl-alanine-functionalized wood

was observed. This was 42% and 58% increase in coupling of propiconazole and

thiabendazole respectively as compared to the coupling done by laccase only on the



wood pieces. Amine functionalization of the wood surface enhances the binding of

the functional groups (propiconazole, thiabendazole) which is responsible for the

antimicrobial activity.

Figure 2.6. Attachment of anchor groups (i.e. phenolic amines) to lignin moieties

of wood for further functionalization. Reprinted from (Kudanga et al., 2008),

with permission from John Wiley & Sons.

A new procedure for the hydrophobization of cellulose fiber from hardwood kraft

pulp using laccase and a hydrophobic phe-nolic compound (lauryl gallate) was

developed. This treatment resulted in the internal sizing of paper and also

significantly reduced water penetration in the handsheets and wettability of the paper

surface (Garcia-Ubasart et al., 2012). Laccase-catalyzed method of covalently

grafting hydrophobicity enhancing fluorophenols onto Fagus sylvatica veneers was

studied. Laccase-catalyzed grafting of fluorophenols 4-[4-(trifluoromethyl)

phenoxy]phenol (4,4-F3MPP), 4-(trifluoromethoxy)phenol (4-F3MP) and 4-fluoro-2-

methyl-phenol (4,2-FMP) resulted in a fluorine content of 6.39%, 3.01% and 0.26%,

respectively and 65.5%, 28.6% and 9.6%, respectively increase in hydrophobicity

when compared to treatments with the respective fluorophenols in the absence of

laccase (Kudanga et al., 2010). Dong et al. (2014) have been grafted the hydrophobic

dodecyl gallate onto jute fibers by laccase and observed that there is enhancement in

the hydrophobicity. Ferulic acid biografted by Rencoret et al., 2014 onto sisal fiber

surface by enzyme (Trametes villosa). They have characterized the biografting by

pyrolysis and 2D NMR methods.



2.5.1.2 Biografting of other functional molecules

In addition to phenolics, there are some other molecules which can be grafted onto

lignocellulose to enhance the antimi-crobial and hydrophobic properties. Kudanga et

al. (149) (2010) have carried out the lacasse catalyzed coupling of long chain alky-

lamines to lignocelluloses material, which resulted in an increase in hydrobhobicity.

Grafting of dodecylamine and dihexylamine onto beech veneers resulted in an

increase in contact angles which shows 53.8% and 84.2% increase in hydophobicity,

when compared to simple adsorption. Garcia-Ubasart et al. (2013) carried out laccase-

assisted coupling of short non-polar chains having aromatic groups onto flax fibers

and nanofibrillated cellulose to generate different levels of hydrophobicity (Figure

2.7).

Figure 2.7. Coupling of non-polar chains onto fiber surface. Reprinted from

(Garcia-Ubasart et al., 2013), with permission from American Chemical Society.

Coupling of dodecyl 3,4,5-trihydroxybenzoate was found to produce the highest

hydrophobicity and yielded water contact angles of 80–96◦, water absorption time

(drop tests) of ca.73 min. Ultra-thin films of nanofibrillated cellulose were also used

as substrates for enzyme-mediated hydrophobilization with dodecyl 3,4,5-

trihydroxybenzoate and water contact angle was achieved in the range of 87–104◦.

Laccase-catalyzed iodination of wood was carried out for enhancing the antimicrobial

activity. Norway spruce wood (Picea abies L.) was treated with laccase (Trametes

versicolor) in the presence of potassium iodide salt or some phenolic compounds

thymol and isoeugenol to impart antimicrobial property to wood surface. Laccase with

iodide found to show the bactericidal effect on E. coli, S. aureus and Saccharomyces

cerevisiae. This Method was found to be an efficient and eco-friendly method to

wood protection (Schubert et al., 2012). Chemical modification of cellulose material



was carried out by Hou et al. (2009) in the presence of triazine derivatives containing

multi-cationic benzyl groups. Modified cellulose biomaterial has both cationic and

long carbon-chain groups (Figure 2.8), which enhances the antibacterial activity.

Witayakran and Ragauskas (2009) have treated the softwood kraft pulp with laccase

and various amino acids (glycine, phenylalanine, serine, arginine, histidine, alanine,

aspartic acid) (Scheme 2.2). After optimization of the conditions, histidine found to

give the best yield of acidic groups on the pulp fibers and it was then used for the

prepa-rations of the handsheets for physical strength test. SEM images (Figure 2.9) of

the handsheet surface of the control, laccase treated, and laccase-histidine treated pulp

show that the laccase-histidine treated fibers are more collapse than control and

laccase-treated fibers, which led to form better bonding between fibers in hand-sheet

resulting in increased paper strength of laccase-histidine treated pulp.

Figure 2.8. Chemical structure of modified cellulose. Reprinted from (Hou et al.,

2009), with permission from Elsevier.

Scheme 2.2. Propose mechanism for the grafting treatment of linerboard pulp

with laccase and amino acids. Reprinted from (Witayakran and Ragauskas,

2009), with permission from Elsevier.



Figure 2.9. Scanning electron microscope (SEM) images of handsheets made

from: (a) control pulp (b) laccase-treated pulp (c) laccase-histidine treated pulp.

Reprinted from (Witayakran and Ragauskas, 2009), with permission from

Elsevier.

2.5.2. Chemo-enzymatic biografting

Cellulose is major constituent and percentage composition of lignin vary from 0% to

40% of total lignocellulose material (Jorgensen et al., 2007; Bledzki et al., 1996).

Biografting of organic molecules mainly targeted at lignin thereby leaving most of the

lignocellulosic surface unmodified. Therefore a new approach i.e. chemoenzymatic

grafting has been developed to functionalize the surface of lignocellulose material.

This technique is relatively non-selective and can be applied to all major constituents

of the lignocellulose material. This approach is considered as comparatively a more

effective way for surface functionalization of lignocellulose material. In chemo-

enzymatic method, completely new chemical functionalities can be grafted onto the

lignin-containing fibers (Gronqvist et al., 2006). Aracri and Vidal (2012) also

reported another method called laccase–TEMPO mediated systems for the oxidation

of sisal cellulose fibers. Biorefining potential of the enzyme mediator system was

found to be increased by increasing the pulp consistency chemo-enzymatically with

methyl syringate, p-hydroxybenzoic of the sisal pulp during the enhancement in the

effectiveness of laccase–TEMPO treatment. Contents of aldehyde group were

increased and carboxyl groups were increased after pulp refining. The biorefining

effect applied by the laccase–TEMPO system was observed in an increased

fibrillation degree of fibers after treatment. Also stronger compaction was observed in

oxidized pulp handsheets as comparison to the initial pulp, which results in increased

tensile strength. Increase in the kappa number and decrease in the brightness was

observed due to the formation of the chromophores groups as a result of oxidative

action of the enzyme treatment or the grafting of phenolic compounds onto the pulp.



TEMPO mediated oxidation is an efficient method for introducing carboxyl and

aldehyde functional groups into cellulose in aqueous media at room temperature.

Laccase–TEMPO system resulted in a modest increase in carboxyl groups in cellulose

fibers and as suggested by the viscosity results, wet strength improvement obtained

and the formation of a substantial amount of aldehyde groups that provide inter fiber

bonding through hemiacetal linkages (Aracri et al., 2011).

Surface properties of lignin-containing nanofibrillated cellulose were modified by

chemo-enzymatic derivatization method using the high redox potential T. hirsuta

laccase and hydrophobic dodecyl gallate as the derivatizing agent. Nanofibrillated

cellulose modi-fied with T. hirsuta laccase and dodecyl gallate showed decreased

hydrophilicity as compared to native nanofibrillated cellulose. A cross-linked product

with increased strength and stiffness was obtained by laccase treatment with and

without dodecyl gallate (Saasamoinen et al., 2012). Unbleached softwood kraft pulp

was functionalized acid, gallic acid and syringaledhyde. The wet strength of fibers

treated with methyl syringate, and syringaledhyde increased by 57.9% and 31.9%,

respectively. The dry strength of fibers treated with p-hydroxybenzoic acid, gallic

acid and syringaledhyde increased from about 64 N m/g to 68 N m/g. The

participation of phenolic compounds enhanced the reactivity of fibers to laccase in

varying degree and decreases the brightness and curl index of fibers (Liu et al., 2013).

Grafting of acrylamide was carried out onto lignin matrix catalyzed by fungal laccase

in combination with dioxane peroxide in aqueous organic solvent mixtures. It was

further reported that the reactivity of phenoxy radicals generated by laccase catalysis

was not sufficient to start side chain polymerization and alkoxy radicals generated by

reduction of peroxides were supposed to initiate homopolymerization of acrylamide.

Lignin graft copolymers were then formed by covalent bonding between radical end

of the growing, „living‟ polyacrylamide homopolymer and phenoxy radicals of the

lignin backbone (Mai et al., 2000). Laccase was not only able to polymerized lignin in

aqueous dioxane but also copolymerize lignin of a different origin with low-molecular

weight compounds (Milstein et al., 1994). It was reported that laccase caused free

radical copolymerization of acrylamide and lignin. In the presence of organic

peroxides particularly dioxane peroxide, tetrahydrofuran peroxide and t-

butylhydroperoxide. Dioxane peroxide was reported to be better than the other

organic peroxides in a laccase-mediated acrylamide-lignin copolymerization (Mai et



al., 1999). The variation of phenol and laccase concentration presents a means to

control the molecular weight of the copolymers under moderate reaction conditions.

Moreover, the application of simple lignin-like phenolics provides further insight into

the mechanism involved in the chemo-enzymatic grafting of lignin. Copolymerization

of acrylamide and acrylic acid and phenolics initiated by a Fenton-like reaction was

compared to the initiation of a phe-noloxidase/peroxide system. It was reported that

extent of phenol incorporation into the polymer chain was enhanced by the presence

of laccase in the reaction mixture and was significantly higher than in polymerization

initiated by a Fenton-like reaction (Mai et al., 2001).

Scheme 2.3. Proposed mechanism of chemo-enzymatically induced graft

copolymerization. Reprinted from (Mai et al., 2002), with permission from

Elsevier.



The mechanism of laccase-assisted chemo-enzymatic synthesis of lignin graft-

copolymers was investigated by Mai et al. (2002). As per the proposed mechanism,

the formation of a covalent bond between lignin sulfonate and the acrylic side chain

occurs through a termination reaction of growing (“living”) acrylic chains with the

unpaired electron of a phenoxy radical (step 4) (Mai et al., 1999). In addition to the

contribution in termination reaction, phenoxy radicals may also generate alkoxy or

peroxy radicals. Peroxides were reduced to alcoxy radicals whereas the phenoxy

radicals were oxidized to quinones in case of transferring an electron onto the

peroxide by phenoxy radicals (step 2a) (Mai et al., 1999). Further, peroxy radicals

might possibly be generated through the oxidation of peroxides (dioxane peroxide, t-

butylhydroperoxide) which were mediated by phenoxy radicals and then back-

reduced to phenols (step 2b) (Scheme 2.3).

2.6. Effect of environmentally friendly methods on properties of plant

fibers

Surface features and tensile strength of jute fibers treated with enzymes like pectinase,

hemicellulase, xylanase, laccase and cellulase were investigated. Lignin and

hemicelluloses constitu-ents were removed from the surface of the fiber by enzymatic

treatment and breaking strength of the treated fibers reduced by 15–25% (Joko et al.,

2002). Effect of white rot fungi cellulase enzyme and a mixture of enzymes (cellulase,

xylanase and pectinase) on the physical properties of jute fibers was investigated.

Enzyme treatment degraded the lignin polymers and a reduction in flexural rigidity

and tenacity was observed (Jayapriya and Vigneswaran, 2010). Pommet et al. (2008)

have used the A. xylinum to modify the surface of sisal and hemp fibers and found

deposition of the bacterial cellulose on the fiber surface due to the strong hydrogen

bonding between hydroxyl group present in the bacterial cellulose and lignocelluloses

in plant fibers, due to which 5–6% weight found to be increased of the treated plant

fibers (Figure 2.1). The modification did not affect the mechanical properties of sisal

fibers but significantly reduced the mechanical properties of the hemp fibers (Table

2.5).



Table 2.5. The mechanical properties of natural fibers modified with bacterial

cellulose nanofibrils. Reprinted from (Pommet et al., 2008), with permission from

American Chemical Society..

Natural fibers Young’s modulus

(GPa)

Tensile strength

(MPa)

Elongation at

break (%)

Neat sisal fiber 15.0±1.2 342±33 2.9±0.1

Bacterial cellulose-

modified sisal fiber 12.5±1.0 324±33 4.5±0.4

Bacterial cellulose

modified sisal fiber

with purificationa

12.0±0.9 310±32 4.1±0.5

Neat hemp fiber 21.4±2.0 286±31 2.0±0.2

Bacterial cellulose

modified hemp

fiber

8.8±0.7 171±11 2.9±0.2

Bacterial cellulose

modified hemp

fiber with

purificationa

8.0±0.6 130±12 2.9±0.2

apurification indicates the extraction of post-bacterial cellulose-modified sisal fiber with NaOH at

80 ⁰C.

Enzymatic hydrolysis caused an increase in the elastic modulus from 33.4 4.3 to 140.6

MPa, whereas the tensile strength increased from 1.09 0.39 MPa in the bacterial

cellulose nano-fibers (Woehl et al., 2010). Fungal treatment of hemp fibers have

reduced the tensile strength because it gave the roughness, striations and holes to the

fiber surface which resulted into the increased stress concentration and therefore

lowered the fiber strength (Pickering et al., 2007). Cotton fibers lost its strength when

treated with EG (endoglucanase) due to the splitting and the fibrillation of the outer

layer of the cotton fibers (Verenich et al., 2008). Rough surface appearance has been

observed on the ramie fibers when treated with plasma and plasma treated ramie

fibers showed 50% increase in interfacial shear strength (Zhou et al., 2011). Pine

fibers were modified with cellulase enzyme from T. reseei and a significant increase

in the crystallinity index was observed (Park et al., 2006). The lower crystallinity



index of ramie fibers after enzyme treatment was observed (Kalia and Sheoran, 2009).

On the other hand, an increase in the crystalline index was observed in case of sisal

fibers, because there was a decrease in the amount of amorphous and gel like

polysaccharide layer on the surface of fibers (Kalia and Vashista, 2012). Hemp fibers

showed an increase in the crystallinity when treated with the fungi as compared to the

untreated hemp fibers (Pickering et al., 2007). Kalia and Sheoran (2009) have treated

the ramie fibers with S. albaduncus that resulted into enhanced softness and

brightness due to the removal of gum materials from fibers. Similar morphology was

observed in the case of sisal fibers when treated with B. parabrevis. Verenich et al.

(2008) have subjected the raw cotton fibers to the high EG and observed the surface

of fiber faces with shallow holes. Hemp fibers subjected to the fungal treatment have

less glossy surfaces and striations became more visible along the fiber length as

compare to the untreated fibers (Pickering et al., 2007). Bledzki et al. (2010) have

studied the morphology of enzyme treated abaca fibers. The surface morphologies of

enzyme treated and untreated abaca fibers are shown in Figure 2.10. In Figure 2.10 a,

it was observed that the untreated fiber surface is rough, containing waxy and

protruding parts. The surface morphology of treated fibers is displayed in Figure 2.10

b and c. The surface becomes smoother after the removal of the waxy material and

cuticle. Fibrillation is also known to occur when the binding materials are removed

from the surface of the treated fibers. Fiber surface damage was also observed for

digested fibers which occur in plant digestion systems. Treatment of ramie fibers with

cellulase did not affect their thermal stability, while enhanced thermal stability was

observed in case of sisal fibers treated with cellulase (Kalia and Vashista, 2012).

Hemp fibers showed increased thermal stability after fungal treatment due to reduced

non-cellulosic material and increased crystallinity (Pickering et al., 2007).

Figure 2.10. Micrograph of abaca fiber surface morphology: (a) unmodified (b)

plant system digestion (NDS) modified and (c) fibrillation. Reprinted from

(Bledzki et al, 2010), with permission from Elsevier.



2.7. Applications of modified natural fibers

2.7.1. Reinforcement in composite materials

Plant fibers modified by the green methods can be used as a reinforcing material to

form composites with green image, i.e. ecofriendly biocomposites. Biocomposites are

the combination of plant fibers such as wood fibers or non-wood fibers (e.g. wheat,

kenaf, hemp, jute, sisal and flax etc.) with polymer matrices from both of renewable

and nonrenewable resources. Biocomposites have a wide range of applications in

various fields such as building industry, medical aerospace, automobiles, electronic

industries, etc. (Kalia et al., 2014). Pommet et al. (2008) have modified the surface of

the sisal and hemp fibers with cellulose producing bacteria A. xylinum. These

modified fibers were then incorporated into the PLLA and CAB (cellulose acetate

butyrate) matrices to obtain a new class of model hierarchical composites, which are

better options than polypropylene-based composites to be used in the automobile

industries because of its enhanced mechanical properties and green image. The

interfacial adhesion between the modified fibers and the polymer matrices was

quantified using the single fiber pull-out test. The IFSS (interfacial shear strength) of

the bacterial cellulose modified fibers increased significantly as compared to the

unmodified sisal (Table 2.6).

Table 2.6. Interfacial Shear Strength (IFSS) between fibers and renewable

matrices. Reprinted from (Pommet et al., 2008), with permission from John

Wiley & Sons, and (Juntaro et al., 2008), with permission from American

Chemical Society.

Treatment

IFSS To CAB

(MPa)

IFSS To

PLLA31

(MPa)

Natural Sisal Fiber (Sisal-N) 1.02 ± 0.06 12.1 ± 0.5

Sisal Fiber Modified With Bacterial

Cellulose (Sisal-NBC) 1.49 ± 0.03 14.6 ± 1.2

Acetone-Treated Sisal Fiber 9.5 ± 0.7

Acetone-Treated Sisal Fiber,

Modified With Bacterial Cellulose Internal Failure

Natural Hemp Fibers (Hemp-N) 0.76 ± 0.06

Hemp Fiber Modified With Bacterial

Cellulose (Hemp-NBC) 1.83 ± 0.12



SEM images (Figure 2.11b) of both the pulled-out fiber fragment and the cavity

matrix showed that the outer layer of the bacterial cellulose modified sisal fibers

remained adhered to the matrix. In contrast, all the other fibers exhibited clean,

smooth surfaces after pull-out (Figure 2.11a).

Figure 2.11. SEM micrographs of: (a) bacterial cellulose-modified sisal (b)

acetone-treated and bacterial cellulose-modified sisal fibers and the

corresponding CAB matrix cavities after single fiber pull-out testing. Reprinted

from (Pommet et al., 2008), with permission from American Chemical Society.

Figure 2.12. Single fiber pullout results for hemp and sisal fibers in CAB matrix

(&) plant sisal fiber (Sisal-N) (*) sisal fiber modified with bacterial cellulose

(Sisal-NBC) (^) plant hemp fiber (Hemp-N) (~) hemp fiber modified with

bacterial cellulos (Hemp-NBC). Reprinted from (Pommet et al., 2008), with

permission from American Chemical Society.



To determine the average IFSS between the modified fibers and the polymer, the

maximum pull-out force was plotted as a function of the embedded fiber area. Figure

2.12 displays the exemplary pull-out data for the hemp and sisal fibers from CAB

(Pommet et al., 2008). Pickering et al. (2007) have investigated the use of fungi to

treat hemp fibers to create better bonding characteristics between plant fibers and

composites. They observed 22% improvement in the composite strength than

untreated fiber reinforced composites. Composite fracture surface supported the

improved interfacial bonding having high degree of influence on strength. The

improvement in interfacial bonding was due to the production of holes in the fiber

walls.

Kim et al. (2008) have used the atmospheric glow discharge (AGD) to modify the

surface properties of wood powder as well as plant fiber to improve the compatibility

between the fiber surface and PP matrix. The AGD surface modification process

allows the constituents to disperse evenly within the matrix with a strong interfacial

bonding between the fiber and the polymer matrix (Figs. 2.13 and 2.14). The tensile

strength of the wood powder/PP composites, coir/PP composites and jute/PP

composites has been examined. The tensile strength of plasma treated Jute/PP

composites increased by 50 and 114% and for Coir/PP increased by 22 and 92%

compared with NaOH treated and raw fibers. Effect of APPJ treatment on ramie fibers

was observed by Zhou et al. (2011) and polypropylene (PP) composites reinforced

with modified ramie fibers were also prepared. APPJ treatment has increased the

roughness of the fiber surface in comparison to the untreated fibers which helps in the

enhancement of the interfacial adhesion between the two phases and hence the

mechanical properties (Figure 2.15). Foulk et al. (2006) have compared the dew-

retted flax fibers reinforced high-density polyethylene (HDPE) compo-sites with the

enzyme-retted flax fiber reinforced (HDPE) composites. Flax fibers were extracted

with Viscozyme L. (Novozyme, Bagsvaerd, Denmark) and a chelator, mayoquest 200

(chelator that contains 36–38% sodium EDTA and 40% total dissolved salts).

Chelators are used to improve the efficiency of enzyme activity. It helps in removing

pectin and calcium, therefore imparting enhanced interfacial adhesion between the

fibers and the matrix. Dynamic mechanical and thermal properties of polyester



composites reinforced with enzyme-treated and NaOH-treated jute fabrics were

investigated. Storage modulus of the composites increased by using modified jute

fibers in comparison to untreated fibers. Thermal stability of the composites decreased

due to the removal of thermally more stable lignin polymers by NaOH and enzyme

treatments (Karaduman et al., 2013). Karaduman et al. (2013) improved the fiber–

matrix interface and mechanical properties of jute fiber-reinforced polyester

composites by enzymatic treatments. Jute fabrics were treated with pectinase, laccase,

cellulase and xylanase enzyme solutions with varying enzyme mixtures and treatment

time. Dong et al. (2014) have bografted the jute fiber with hydrophobic dodecy

gallate by laccase which resulted into the increased hydrophobicity, these modified

jute fibers were novely used for the composite preparation. Jute fiber/polypropylene

composites were observed to have increased breaking strength, regular and neat

fracture section due to laccase assisted grafting with dodecyl gallate.

Figure 2.13. SEM of 50 % jute fiber/PP composites. Reprinted from (Kim et al.,

2008), with permission from WIT Press.

Figure 2.14. SEM of 50 wt % of wood powder within PP. The wood powder is

circled. (Kim et al., 2008), with permission from WIT Press.



Figure 2.15. SEM of ramie fibers: (a) control (b) plasma treated for 24 s with

ethanol pretreatment. Reprinted from (Zhou et al., 2011), with permission from

Elsevier.

2.7.2. Textile industry

Sun and Stylios (2006) have studied the low temperature plasma treatment of wool

and cotton fabric specimens and found increased hydrophilicity, improvement in the

scouring and dyeing processing by nearly 50%. They concluded that the exposure of

cotton and wool fabrics to low temperature plasma treatment could produce rough

surface as well as smooth surface (Figure 2.16).

Figure 2.16. SEM images of: (a) untreated wool (b) C2F6 plasma treated wool (c)

O2 plasma treated wool (d) untreated cotton (e) C2F6 plasma treated cotton (f)

O2 plasma treated cotton. Reprinted from (Sun and Stylios, 2006), with

permission from Elsevier.



A rougher surface fabric is a good padding cloth and is important in the conventional

aqueous textile finishing and dyeing processing because of the higher rate of liquid

uptake. They also observed that this treatment could bring changes in the chemical

composition of the fabric. In processing of textile fibers, enzymes can be used in order

to develop environmentally friendly alternatives to chemical processes. There are

already some commercially successful applications of enzymes in textile processing

such as amylases for desizing, cellulases and laccases for denim finishing, and

proteases incorporated in detergent formulations (Casal and Cavaco-Paulo, 2008).

2.7.3. Antimicrobial activities

Researchers have studied the antimicrobial activities of modified plant fibers. Fabrics

(cotton) having a smoother surface as a result of the low temperature plasma

treatment can be used in antibacterial treatments (Figures 2.17) (Sun and Stylios,

2006). The anti-microbial activity of the enzyme pre-treated cotton fabrics was

assessed after processing with neem leaf extract. The treated fabrics showed 100%

bacterial reduction against Staphylococcus aureus and 98% against Escherichia coli

organism (Nithya et al., 2012). A number of chemicals have been employed to impart

antimicrobial activity to textile materials. Most of them are toxic to humans and are

difficult to degrade within the environment. The textile industry continues to search

for eco-friendly processes as substitutes for toxic textile chemicals. An increasing

interest has been noticed in the functionalization based on environmentally friendly

and biodegradable reagents (Gao and Cranston, 2008). Plasma treatment of fibers can

be used for the surface activation of plant fibers and for chitosan adsorption (Strnad et

al., 2010). The effect of plasma treatment on the antimicrobial capacity of chitosan

treated fabrics was studied. It was reported that increased number of active functional

groups on the cellulose surface due to plasma activation enabled the adsorption of

higher amounts of chitosan and, consequently, a higher amount of amino groups

responsible for antimicrobial activity (Lim and Hudson, 2004; Kumar, 2000; Zitato et

al., 2003). The increased amino groups in plasma-activated–chitosan-treated samples

increase the probability that a protonated amino group met the bioplasm of the

bacteria and resulted in a greater bacterial reduction capacity (Zitato et al., 2003).

Mulberry bast fibers were obtained by alkali-assisted micro-wave and biological

enzymatic pretreatment technique. The morphology, microstructure, physic-

mechanical, and antibacterial properties of the mulberry bast fibers were investigated.

Pretreat-ment of fibers results in excellent antimicrobial activities against S. aureus.

So mulberry bast fibers are suitable for the new textile materials and they are

environmentally acceptable and renewable substitutes (Qu and Wang, 2011).



Figure 2.17. Surface roughness of different plasma gases treated wool (left) and

cotton (right). Reprinted from (Sun and Stylios, 2006), with permission from

Elsevier.

Several authors have suggested that laccase treatments can improve physical

properties of different fibers by producing phenoxy radicals in the lignin matrix that

undergo cross-linking reactions (Gronqvist et al., 2003; Wong et al., 2000; Felby et

al., 2002). Many phenolic compounds such as eugenol, thymol, carvacrol etc.

extracted from natural sources have been shown to exhibit antimicrobial activity

against a wide spectrum of microorganisms due to phenolic acids present in these

extracts (Nohynek et al., 2006; Kubo et al., 2003; Rodriguez et al., 2007; Almeida et

al., 2006; Alakomi et al., 2007). These phenolic structures can react to different extent

with laccase and other enzymes to generate radicals and therefore potentially be

grafted on the lignocellulosics to develop covalently bound antimicrobial

lignocellulosic materials (Elegir et al., 2008). Antibacterial activities have been

developed by grafting of phenolics compounds onto flax fiber surface (Fillat et al.,

2012), kraft pulp (Pie et al., 2013), lignocellulosic model surfaces (Widsten et al.

2010; Schroeder et al., 2007).

2.8. Comparison of advantages and disadvantages of chemical and

green methods

Surface modification of plant fibers can be carried out by a number of methods such

as physical methods, chemical modification and some green approaches for their

better use in different applications. Every method has its advantages and

disadvantages. Some of advantages and disadvantages of these methods are

summarized in Table 2.7 (Kalia et al., 2013).



Table 2.7. Advantages and disadvantages of some pre-treatment methods. Reprinted from (Kalia et al., 2013), with permission from

Elsevier.

Sr. no. Method Types Effects Advantages Disadvantages References

1 Chemical

treatment

Mercerization,

acetylation, peroxide

treatment, permanganate

treatment, isocyanate

treatment polymer

grafting, silanization,

benzoylation, etc.

Cleaned fiber surface,

chemically modify the surface,

stop the moisture absoption

process, increased surface

roughness, good interfacial

adhesion, improved fiber

strength

Easy processability, useful and

acceptable in diversified

applications, implemented at

industrial scale, most

commonly used methods

Use of hazardous

chemicals, proper

handling, solvent waste

disposal, increased cost of

final product

Kalia et al., 2009; John

and Anandjiwala, 2008;

Li et al. 2007; Lee et

al., 2011; Bogoeva-

Gaceva et al., 2007;

Cicala et al., 2010

2 Plasma Atmospheric pressure

plasma, low pressure

plasma, etc.

Removal of weakly attached

surface layers, formation of

new functional groups,

positive impact on the

mechanical properties,

enhanced hydrophobicity,

improved interfacial adhesion

No need of hazardous

chemicals or solvents,

minimized environmental

impact, low operating cost,

short treatment time, greater

flexibility

Low-pressure plasma

required a well-designed

plasma reactor system

along with expensive

vacuum systems, only

batch process is possible

Lee et al., 2011;

Schutze et al., 1998;

Bismarck, 2006; Tsai et

al., 1997; Noeske et al.,

2004., Shenton and

Stevens, 2001; Viol et

al., 2013

3 Nanocellu

lose

coating

Bacterial or microbial

cellulose

Addition of new material on

the surface of fibers, improved

interfacial adhesion

Environment friendly,

inexpensive and easiest

process

Hydrophilic nature of

bacterial cellulose, loss of

fiber strength in some

cases

Keshk and Sameshima,

2005; Juntaro et al.,

2007; Pommet et al.,

2008; Goelzer et al.,

2009; Lee et al., 2012

4 Enzymes Hemicellulase, laccase,

xylanase, pectinase,

cellulase etc.

Separation of fibers from non-

fiber components, improved

interfacial adhesion, cleaned

fiber surface, improved

appearance and color

brightness

Environment friendly, hig

quality fibers, well-controlled

environment treatment, low

fermentation waste

Expensive and limited to

pilot scale

Pallesen, 1996;

Buschle-Diller et al.

1999; Kenealy et al.

2003; Dam, 1999

5 Fungi Fungi belonging to

basidomycetes group,

ascomycetes group and

zygomycetes group

Removal of non-cellulosic and

amorphous compounds,

enhanced crystallinity index

Low cost, highly efficient and

ecofriendly

Proper care in choosing

the treatment time, long

treatment time may affect

the strength of fibers

Pickering et al. 2007;

Gutierrez et al. 2001;

Pickering and Farrell,

2009; Gulati and Sain,

2006

2.1. Plasma treatment of plant fibers

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