TEXTILE BIOCOMPOSITES

Chapter 1

Meclzanical and Dielectrical Studies of Woven Sisal Fabric Reinforced Natural Rubber Textile Biocomposites

Abstract

Textile biocomposites were prepared by reinforcing natural rubber with woven

sisal fabric. Sisal fabric was subjected to various chemical modifications like

mercerization, sitanation and heat treatment and its influence on the mechanical

properties was analyzed. Tensile strength was seen to decrease with all chemical

modifications except for composites prepared with heat-treated sisal fabric.

Equilibrium swelling experiments were carried out to assess the extent of

interfacial adhesion. The hardness of the textile composites was analyzed. The

fracture topology of the composites was examined. The dielectric characteristics

.of the composites with reference to loading and chemical modification have also

been investigated.

Putt of ilre results in t1ti.s clrnpter l tm beet1 ctccuptetl for publicc~lion it? Jorrrnal of

3.1 .I INTRODUCTION

The interest in lignocellulosic fibres in the past few years has increased

dramatically and they are now in great demand because of their attractive

properties. They have become the focus of attention of scientists worldwide as they

exhibit a combination of high strength with low specific gravity. Plant fibre reinforced

composites are proof that it is possible lo construct high performance materials with

environmental friendly resources. While the history of natural fibre reinforced

composites dates back to several thousand years, modern need of an

environmentally friendly system has renewed the interest in this area and a new

insight is being discovered.

The development of textile technologies such as weaving, knitting and

braiding has resulted in the formation of textile biocomposites that have superior

mechanical properties, as continuous orientation of fibres is not restricted at any

point. Textile biocomposites have been investigated mainly from two points of view:

development of three dimensional textile fabrication techniques and evaluation of

mechanical properties.

Textile structural composites are findirig use in various high performance

applications recently'. The increased interest in textile reinforcements is due to the

enhanced strength, lower production cost and improved mechanical properties,

which they offer, compared to their non-woven counterparts. Additionally, textile

structural composites are associated with near net shape and cost effective

manufacturing process. Another special feature of textile reinforcement is the

interconnectivity between adjacent fibres. This interconnectivity provides additional

interface strength to supplement the relatively weak fibre-resin interface. In addition,

woven fabric composites may be more damage tolerant in the case of a

delamination. Formation of different textile performs is an important stage in

composite technology.

The development and application of advanced textile composites were

recently reported? Three-dimensional fi bre-reinforced polymer composites made by

the textile processes of weaving, braiding, stitching and knitting were found to have

tremendous potential for improving the performance of composite structures and

reducing their cost of manufacture. The current applications of three-dimensional

composites, including examples in the aerospace, maritime, automotive, civil

infrastructure and biomedical fields were also enumerated.

Twisted yarns have been reported to increase lateral cohesion of the

filaments as well as to improve the ease of handling,. In fact, fibre twist induces

normal forces between fibres and increasing in ter-fibre friction gives yam cohesion.

However, by twisting yarns, possible micro damages within the yarn can be

localized, leading to possible decrease in the failure strength of the yarn. Whatever

be the fibre material, fibre architecture has been found to influence the composite

properties based on the morphological and structural parameters4.

Xue and Ca05 developed an integrated micro- and macro-constitutive

model to predict the mechanical properties of woven composites during large

deformation based on the microstructure of composites, i .e., the dimensions of

fibres, yarns and unit cell, the material properties of composite constituents, as well

as the orien tation of yams. The proposed integrated microlmacro-model showed

excellent agreement with the experimental data and the 3D finite element results.

The mechanical properties and fracture surface morphology of woven date

palm fibre (DPF) reinforced polyester resin composites were investigated by

Wazzan6. Laminates with different orientation and volume fraction of reinforcement

were prepared using resin transfer molding (RTM) processing technique. The

woven DPF reinforced composites recorded a tensile strength of 76.9 MPa. D'

Amat07 recently proposed a numerical approach for simulation of nonlinear

behavior of textile composites. The results showed that nonlinear effects were

mainly caused by the variation of the waving of fibres under loading.

The effect of fibre surface treatments (silane and permanganate

treatments) on tensile strength and modulus of sisal textile reinforced vinyl-ester

resin composites was investigated by Li et aP. Chemical modification of fabric was

found to have not much improvement in tensile properties. Bledzki and Zhang have

reported on the usage of jute fabrics as reinforcement for the preparation of

composites? The mechanical properties of threedimensionally braided carbon fibre

epoxy composites were investigated by Wan et al.lO. The authors observed that

flexural strength and modulus were found to be dependent on the braiding angle

and are related to the presence of axial reinforcement but the shear and impact

strengths were found to be independent of axiai reinforcing fibres.

The tensile properties of jute fabric reinforced polyester composites was

analysed by Gowda et al.". They observed that there was increase of tensile

strength and flexural properties upon reinforcement with jute fabric. The hardness

of the composites was found to decrease due to the enormous differences in

hardness of jute fabric and polyester resin. The impact properties were also found

to improve considerably.

In another study concerning jute fibre, Mohanty et a1.12 have studied the

influence of chemical modification of jute fabric reinforced polyester amide

corn posi tes. They observed that mechanical properties increased as a result of

treatments and that alkali treatment and cyanoethylation resulted in superior

properties. The scanning electron microscopic investigations revealed that fibre

pull-out was minimized by chemical modification.

The tensile strength of ramie-cotton hybrid fabric reinforced polyester

composites was investigated by Junior et al, 13 The authors observed that tensile

behaviour was dominated by volume fraction of ramie fibres aligned in the test

direction. The fabric and diameter of the thread was found not to have any role in

tensile characteristics. Cotton fabric was found to have minor reinforcement effect

due to weak cottonlpolyester interface. Similar studies were performed by

Mwaikambo and Bisanda'4 on kapok- cotton fabric reinforced polyester composites.

Pothen et al.15 conducted tensile and impact studies of woven sisal fabric

reinforced polyester composites prepared by RTM technique. It was found that the

weave architecture was a crucial factor in determining the response of the

composites. Researchers have studied the micromechanics of moisture diffusion in

woven composites~6. The weave pattern of the fabric was found to have a profound

effect on the water uptake of the composites. They observed that woven

composites exhibited quicker diffusion than that of a unidirectional laminate with the

same overall fibre volume fraction. The plain weave with a lenticular tow and large

waviness was seen to exhibit the quickesl diffusion process.

Novolac type phenolic composites reinforced with jutelcotton hybrid woven

fabrics were fabricated and its properties were investigated as a function of fibre

orien talion and rovinglfabric characteristics17. Results showed that the composite

properties were strongly influenced by test direction and rovingslfabric

characteristics. The best overall mechanical properties were obtained for the

composites tested along the jute rovings direction. Composites tested at 45 and 90"

with respect to the jute roving direction exhibited a controlled brittle failure

combined with a successive fibre pullout, while those tested in the longitudinal

direction (0") exhibited a catastrophic failure mode. The researchers are of the

opinion that jute fibre promotes a higher reinforcing effect and cotton avoids

catastrophic failure. Therefore, this combination of natural fibres is suitable to

produce composites for lightweight structural applications.

The thermal diffusivity, thermal conductivity and specific heat of jutelcotton,

sisallcotton and ramielcotton hybrid fabric-reinforced unsaturated polyester

composites were investigated by Alsina el alla. The thermal properties of the

fabrics, i.e. without any resin, were also evaluated and were used to predict the

properties of the composites from the theoretical series and parallel model

equations. The effect of fabric pre-drying on the thermal properties d the

composites was also evaluated. The results showed that the drying procedure used

did not bring any relevant change in the properties evaluated.

Studies on woven sisal fabric reinforced natural rubber composites have

not been performed till date. This chapter deals with the analysis of mechanical

properties of woven sisal fabric reinforced natural rubber composites. Sisal fabric

was subjected to different chemical treatments and its influence on tensile

properties was also investigated. Swelling experiments were conducted to gel an

idea about the interfacial adhesion between rubber and sisal la.bric. The influence of

chemical modification on the dielectric properties has also been investigated.

3.1.2 RESULTS AND DISCUSSION

3.1.2.1 Sisal fabric

In this particular analysis, a unidirectional type of fabric weave having a

count of 20 is used (See Figure 3.1.1). The properties of sisal fabric are given in

Part I, Chapter 2, Table 1.2.3. The sisal fabric - natural rubber textile composites were

prepared by sandwiching a single layer of sisal fabric between two layers of pre-

weighed rubber sheets which was then compression moulded at 150" C for 8 minutes.

The formulation of different composites is given in Part I, Chapter 2, Table 1.2.5.

Figure 3.1.1 Weave pattern of sisal fabric

///// I / / / / / / / / /// / / //,/

+- Rubber Matris

t-- Woven Sisal Fabric

Figure 3.1.2 Schematic sketch of rubber-sisal fabric-rubber composite

3.1.2.2 Chemical modification

L~nocellulosic fibres are amenable to chemical modification due to the

presence of hydroxyl groups. The hydroxyl groups may be involved in the hydrogen

bonding within cellulose molecules thereby activating these groups or can introduce

new moieties that form effective interlocks within the system. Surface characteristics

such as wetting, adhesion, surface tension, porosity can be improved upon

modification. An enormous amount of work has been conducted in the field of fibre

modification. In a recent review Eichorn et al.19 have looked into the latest research [hat

is going on in the field of lignocellulosic fibre and composites. In the case of textile

composites, fibre surface treatment is found to have a profound influence on

mechanical properties. The role of surface treatment in textile composites has been

examined by Nakai et al.2olt was found that glass fibres when treated with excessive

binding agent decreased the adhesion of the glass I epoxy interface.

The tensile strength of a composite material is mainly dependent on the

strength and modulus of fibres, the strength and chemical stability of the matrix and

effectiveness of the bonding strength between matrix and fibres in transferring

stress across the interface. The variation of tensile strength with chemical

modification is presented in Figure 3.1.3. It can be seen that the reinforcement of

natural rubber with woven sisal fabric has resulted in an increase of tensile

strength. The reinforcement of lignocellulosic fibres in natural rubber has been well

documented by Jacob et al.21. Generally it has been seen that incorporation of

fibres in natural rubber results in a decrease of tensile strength. This has been

attributed to the strain induced crystallization of natural rubber at high elongations.

In the case of hybrid systems, we have observed that the addition of sisal-oil palm

hybrid fibre reinforced in natural rubber resulted in a decrease at low fibre loading

and then an increase as fibre loading increased.22 In the case of woven fibre

reinforced rubber composites as the woven fabric is tightly bound they exhibit low

deformation at break and hence the composites exhibit high strength. Another

reason is due to the stretching nature of fabric. Strands in the fabric break at

different times as each fibre can stretch independently and break individually when

reaching their breaking stress.

Another interesting observation is that chemical modification of sisal fabric

has resulted in lowering of tensile strength. Alkali treated fabric composite (TBA)

exhibits minimum tensile strength. The mechanics of textile composites is different

from that of short fibre composites. The major contribution to strength in textile

composites is the alignment of yarns in warp and weft direction. Chemical treatment

results in the partial unwinding of yarns (as hemicellulose dissolves off) and hence

the alignment gets antagonized. This results in lowering of strength of composites.

Another reason is that as sisal fabric is composed of thick strands and knots, the

alkali and silane coupling agents did not penetrate into the fabric and therefore the

interfacial properties between the sisal fabric and rubber matrix has not been

improved enough. It can be seen that the lowest tensile strength is exhibited by

alkali treated composite while the highest values are exhibited by thermally trealed

composites. This could be attributed to the fact that upon heat treatment al 150°C,

the crystallinity of cellulose increases due to the rearrangement of molecular

structure at elevated temperatures.23 The thermal treatment also resulls in moisture

loss of the fabric thereby enhancing the extent of bonding between fabric and

rubber. Composites prepared with silane treated sisal fabric exhibits intermediate

tensile strength values.

T-Untreated Tf- Thermal TB- Bonding agent !

TBMS- Silane A174 TBAS- Silane A1 1 OOi - TBA- 4 O h NaOH I

TB Gum TBMS TBAS TBA

Figure 3.1.3 Variation of tensile strength with chemical modification

The variation of Young's modulus of the composites with chemical

modification is given in Figure 3.1.4. Here, one can observe that heat-treated

samples show maximum modulus indicating that the stiffness of these

composites is maximum. Composites containing alkali treated fabric exhibited

lowest values while the composites containing silane lreated composites

exhibited intermediale values.

T-Untreated TT- Thermal TB- Bonding agent TBMS- Silane A174 TBAS- Silane A1 100 TEA- 4 % NaOH

Figure 3.1.4 Variation of Young's modulus with chemical modification

Figure 3.1.5 presents the variation of elongation at break with chemical

modification. The values are found to increase with chemical treatment. Chemical

modification of the fibre results in the composite b'ecoming harder and stiffer. This

will reduce the composites resitience and toughness leading to higher elongation at

break. The composite containing the alkali treated sisal fabric shows higher

elongation at break values indicating that the composite has low stiffness value.

T-Untreated TT- Thermal TB- Bonding agent TBMS- Silane A174 TBAS- Silane A l l 0 0 TBA- 4 % NaOH

Figure 3.1.5 Variation of elongation at break with chemical modification

The variation of tear strength with chemical modification IS presented in

Figure 3.1.6. The composite containing thermally treated sisal fabric exhibils

highest strength while the composite containing alkali treated sisal fabric shows the

lowest value. In the case of the former composite, the bonding between sisal fabric

and rubber is good due to the presence of a strqng rubber -fabric interface.

Figure 3.1.6 Variation of tear strength with chemical modification

T-Untrealed TT- Thermal TB- Bond~ng agent

Figure 3.1.7 depicts the variation of hardness of the composites with

chemical modification. While the incorporation of bonding agent has resulted in a

slight increase in hardness; the composites containing silane treated and heat-

treated fabric show significantly higher hardness values. This indicates the better

interaction between the fabric and matrix. It is interesting to note that among the

chemically modified composites, the composite containing alkali treated fabric show

low hardness values.

I 1

T TBAS TBMS TB TB A

TBMS- Silane A1 74 TBAS- Silane A1 100

~ T B A - 4 % NaOH

l s l . l ' ! - I 1

t

t

T-Untreated lT- Thermal TB- Bonding agent TBMS- Silane A174 TBAS- Silane A1 100 TBA- 4 % NaOH

TBAS TB T TBA TBMS

Figure 3.1.7 Variation of hardness with chemical modification

The fabric-matrix adhesion in textile biocomposites can be further understood

by examining the fracture topology of fracture surfaces of tensile specimens.

Scanning electron microscopy is a common method to analyze the level of fibre-

matrix adhesion. Enormous amount of studies have been conducted to evaluate the

bonding between matrix and fibre. The tensile fracture surfaces of polypropylene-

sawdust composites were investigated by means of scanning electron microscopic

studies.24 The authors observed that the adil ion of higher maleated propylene

content to the composites produced better adhesion of saw dust to polypropylene

matrix. The interfacial adhesion as a function of fibre loading of sisal-oil palm hybrid

reinforced natural rubber composites was analyzed by Jacob et a1.Z. The authors

found that at low and high levels of fibre loading, the interfacial adhesion was quite

poor while at intermediate levels of loading interfacial adhesion was found to be good

as the population of fibres was just right for uniform stress transfer.

Mcclrnnicol nn(i Dielcciriml S ~ I I ~ ~ ~ L ' S o f Woven Sistrl.. . 31 5

Figure 3.1.8 (a,b,c,d,e and f) presents the tensile fracture surfaces of the

various chemically modified composites. In Fig 3.1.8 (a) one can see the presence of

cavities. This is due to the fact that individual strands of fibre are pulled out of the

rubber matrix upon the application of stress. This indicates that the adhesion between

rubber matrix and sisal fabric is poor due to the absence of bonding agent or

chemical modification. Figure 3.18 (b) exhibits the SEM of the composite in presence

of a bonding agent. Here there is the clear evidence of fibre breakage. The better

bonding between fabric and rubber matrix results in fibres being broken off rather

than unwinding itself from the matrix. One can clearly see broken ends of fibres

protruding from the matrix.

Figure 3.18 (c) shows the SEM of tensile fracture surface of composite

containing alkali treated fabric. Generally mercerization treatment results in removal

of lignin and hemicellulose; for this to take place il is essential that alkali penetrates

each and every nook of the fabric. In this particular study we can see from the SEM

that this has not occurred. This is because of the fact that woven sisal fabric is

composed of thick yarns and when it is subjected to mercerization the alkali does not

penetrate within the individual fibre strands. As a result, chemical treatment is not

uniform and effective. Hence composites prepared tom the alkali treated fabric show

impaired tensile properties.

The SEM of tensile fracture surfaces of composites containing silane treated

fabric is presented in Figure 3.18 (d & e). The individual strands of the fabric are seen to

have a smoother appearance. The silanyl groups from the silane coupling promote

better interaction between rubber matrix and fabric (Figure 3.18 d). In Figure 3.18 (e)

there is the presence of fibre breakage suggesting good bonding. Traces of silanyl

coupling agents can be seen to adhere to the strands of the fabric surface. In Figure

3.18 (e) one can see the broken ends of fibre projecting from the matrix indicating a

strong interface. Upon thermal treatment the hydrophilic nature of fibres decreases

while the crystallinity of fibre increases. As a result the adhesion between rubber matrix

and sisal fabric increases giving rise to a strong interface as seen in the Figure 3.18 (0.

Figure 3.1.8 Scanning electron micrographs of tensile fracture surfaces of chemically modified textile composites (a) untreated (b) composite containing bonding agent (c) composite containing fabric treated with 4 % NaOH (d) composite containing fabric treated with silane A1100 (e) composite containing fabric treated with silane A174 (f) composite containing thermally treated fabric.

Mc.clrt~trictr/ nrrtl Dic/ccfrictrl Sfurlics of JYov~tr Sisal. . . 317

3.1.2.3 Swelling index and crosslink density determination

Swelling index which is a measure of the swelling resistance of the rubber

compound, is calculated using Ihe equation given in Part II, Chapter 1, Eqn. 2.1.9.

The diffusion mechanism in rubbers is essentially connected with the ability

of the polymer to provide pathways for the solvent to progress in the form of

randomly generated voids. Generally it is seen that as the void formation decreases

with fibre addition, the solvent uptake also decreases. In the case of hybrid system

comprising of sisal and oil palm fibre reinforced in natural rubber, it was observed

that addition of two different lignocellulosic fibres resulted in a decrease of swelling

and consequently low swelling index values26.

The swelling index values of the various textile composites are given in

Table 3.1 . I . It can be seen that swelling index value is maximum for composiles

containing alkali treated sisal fabric. This indicates that swelling is maximum in

composite TBA suggesting the level of adhesion and extent of crosslinks between

sisal fabric and rubber matrix is less. One can also see swelling index value is

minimum for composite containing thermal treated sisal fabric indicating that

swelling is minimum. This is attributed to the fact that there is a great deal of

bonding between the matrix and fabric due to the presence of larger number of

crosslinks. This shows that thermally treated composites provide better adhesion

and a stronger interface than other chemical modifications. Therefore the swelling

experiments support the findings from mechanical studies.

Table 3.1.1 Swelling index values of composites

The crosslink density can be calculated from the basic equation given in

Par1 II, Chapter 1, Eqn. 2.1.10. The volume fraction of elastomer in the solvent

Samples

T (Untreated)

T B (Bonding agent)

TBA (4 % NaOH) 1

TEAS (Silane A1 100)

TBMS (Silane A174)

TT (Thermal)

swollen filled sample is obtained from the equation given in Part II, Chapter 1, Eqn.

2.1.5. Figure 3.1.9 presents the crosslink density values of the various mixes. It can

be seen that composite containing thermally treated fabric exhibits maximum

crosslink density value.

A comparison of crosslink density can be measured from the reciprocal

Swelling index parameter

386

382

538

349

51 9

307

swelling values 1IQ (apparent crosslink density) where Q is defined as the amount

I

of solvent absorbed by l g of rubber and is calculated from the equation given in

Part 11, Chapter 1, Eqn. 2.1.8.

T-Untreated TT- Thermal T8- Bonding agenl TEMS- S~lane A174 TEAS- S~lane A1 100 TEA- 4 % NaOH

Figure 3.1.9 Variation of crosslink density with chemical modification

Table 3.2.2 presents the apparent crosslink density values of the

various mixes. It can be seen that composite containing thermally treated fabric

exhibits the maximum crosslink density indicating thal the extent of interaction

is maximum in this composite. One can also see that minimum crosslink density

is exhibited by composite containing alkali treated fabric. This is in justification

of our earlier results.

Table 3.2.2 Apparent crosslink density of composites

3.1.2.4 Dielectric properties

Samples

T (Untreated)

T B (Bonding agent)

TBA (4 % NaOH)

TBAS (Silane A1 100)

TBMS (Silane A174)

TT (Thermal)

Though textile composites have garnered the attention of the scientific

community, dielectric studies of textile biocomposites have been few and need to

be addressed in detail. Researchers have recently designed novel rubber

biocomposites by using a combination of leaf and fruil fibre in natural rubber27. The

incorporation of sisal and coir fibre in NR was seen to increase the dielectric

constant of the composites. These hybrid biocomposites were found to have

enormous applications as antistatic agents. In another interesting study, the

preparation of composites comprising of waste paper in natural rubber along with

boron carbide and paraffin wax, for radiation shielding applications, was

investigated?

Apparent crosslink density [ 1/Q]

0.195

0.1813

0.1 278

0.1984

0.1333

0.226

The dielectric parameters such as dielectric constant (E'), volume resistivity

(p) and dissipation factor, tan 6 are given in Part II, Chapter 5, Eqns. 2.5.1,2.5.2

and 2.5.4 respectively.

Figure 3.1.10 presents the variation of dielectric constant as a function of

chemical modification. It can be seen that dielectric constant decreases with

increase in frequency due to decrease in orientation polarization. Another

observation is that chemical modification of sisal fabric results in lowering of

dielectric constant. This is due to the decrease of orientation polarization of

composites containing treated fabric. Chemical modification results in reduction of

moisture absorption capacity of sisal fabric due to the reduction in interaction

between polar -OH groups and water molecules. The resultant decrease of

hydrophilicity of the fabric leads to lowering of orientation polarization and

subsequently dielectric constant.

The variation of volume resistivity of the composites with chemical

modification is presented in Figure 3.1 . I 1. We can see that volume resistivity

decreases with frequency. Another interesting observation is that composite

containing thermally treated fabric exhibits the highest volume resistivity. This is

because thermal treatment results in lowering of moisture content and thereby

increased interfacial adhesion leading to increased resistivity values. The

composite containing sisal fabric treated with 4 % NaOH exhibits the lowest

resistivity values.

.... - *-.... . . .... *-....a 0,. --. \ ....

-4- Bonding agenl -r- 4 % NaOH -+-- Silane A1 100 --+- Silane A1 74 -b- Thermal

3.0 3 5 4 0 4.5 5.0 5 5 6.0 G 5 7 0

log frequency [Hz]

Figure 3.1.10 Variation of dielectric constant as a function of chemical modification

-0- Bonding agenl -4- 4 % NaOH + Silane A1 100 +.- Silane A174 -4- Thermal

4.0 4.5 5.0 5.5 6.0 6.5 7.0

log frequency [Hz]

Figure 3.1.11 Variation of volume resistivity as a function of chemical modification

Meclr (rrticcrl mrt (1 Dieleciriccrl S/rmdie.r of Woven Sias(~I. . . 323

The variation of dissipation factor with frequency as a function of fibre

loading is presented in Figure 3.1.12. It can be seen that composite containing

thermally treated fabric exhibits the highest dissipation factor. Chemical

modification of sisal fabric does not have much effect on the relaxation mechanism

of composites.

0.100 - -4-4 -4-..*-- , -4 -..A 4 4

--0- Bonding agent L 4 - 4 % NaOH p 0.075 - u r - - Silane A1 100 2 + Silane A174 C 0 .- - m . - .-a--"-=---=A .% 0 050 - UI .- 0

I 0.025 -

>--

- *-~-----

log frequency [Hz]

Figure 3.1.12 Variation of dissipation factor as a Function of chemical modification

3.1.3. CONCLUSIONS

Tensile strength of the woven sisal fabric reinforced composite was seen to

increase from that of the gum compound. The mechanical properties were seen to

decrease with chemical modification except for composites containing thermal

treated fabric. Thermally treated composites exhibited superior mechanical

properties because of increased crystallinity while composites containing .alkali

treated fabric exhibited impaired properties. This was attributed to the fact that alkali

did not penetrate uniformly within the thick strands of the fabric. Chemically

modified composites exhibited high hardness values. Swelling experiments

confirmed strong bonding in composites containing thermally treated fabric.

Scanning electron micrographs revealed the presence of a strong interface in

thermally treated composites. Chemical modification of textile composites resulted

in lowering of dielectric constant. Composite containing thermally treated sisal fabric

exhibited the highest volume resistivity.

References:

1. Pandita S.K.,Falconet D., Verpoest I., Comp. Sci. Tech. 62 11 13 2002

2. Bannister M.K., Proceedings of the 1 MECH E Part L J. Materia1s:Design and

Applications 21 8 3 253 2004

3. Naik N.K., Kuchibhotla R., Composites Part A 33 697 2002

4 . Bueno M.A., Renner M., Pac M.J., J. Mater. Sci. 37 2965 2002

5. Xue P., Cao J., Chen J., Comp. Struc. 70 1 585 2005

6. Wauan A.A., Inter. J. Polym. Mater. 54.3 21 3 2005

7. D'Amato E., Comp. Struc. ( in press)

8. Li Y., Mai L-W., Ye L., Comp. Interf. 12 1-2 141 2005

9. Bledzki A. K., Zhang W., J. Reinf. Plast. Comp. 20 14 2001

10. Wan Y.Z., Wang Y.L., Cheng G.X., Han K.Y., J. Appl. Polym. Sci. 85 1031 2002

11. Gowda T.M., Naidu A.C.B., Chhaya R., Composites : Part A 30 277 1999

Meclrrrrriccrl rrrrrl D;elecfric(rl Strrrlie.s of Woven Sbrrl.. . -- .- - . . . . . 32.5

12. Mohanty A.K., Khan M.A., Hinrichsen G., Composites Part A 31 143 2000

13. Junior C.Z.P., Carvalho L.H., Fonseca V.M., Monteiro S.N., Almeida J.R.M.,

Polym. Test. 131 2004

14. Mwaikambo L.Y., Bisanda E.T.N., Polym. Test. 18 181 1999

15. Pothen L.A., Thomas S., Li R.K.Y., Mai Y.W., In Proceedings of International

Conference on Textile Composites, IIT Delhi February 2002

16. Tang X., Whilcomb J.D., Li Y., Sue H-J., Comp. Sci. Tech. 65 817 2005

17. De Medeiros E.S., Agnelli J.A.M., De Carvalho L.H., Mattoso L.H.C.,Polym.

Comp. 26 1 1 2004

18. Alsina O.L.S. Carvalho L.H.O., Filho F.G.R., Almeida J.R.M.D,, Polym. Test.

24 1 81 2005

19. Eichorn S.J., Baillie C.A. Zafeiropoulos N., Mwaikambo L.Y., Ansell M.P.,

Dufresne A,, Enhvistle K.M., Herrera-Franco P.J., Escamilla G.C., Groom C.,

Hughes M., Hill C., Reals T.G., Wild P.M., J. Mat. Sci. 36 2107 2001

20. Nakai A., Osada T., Hamada H., Takeda N., Composites Part A 32 487 2001

21. Jacob M . , Thomas S., Natural Fiber Composites, Kluwer Academic Press,

Edited by Peijs T. & Berglund L. ( in press)

22. Jacob M., Thomas S., Varughese K.T., Comp. Sci. Tech. 64 955 2004

23. Rong M.Z., Zhang M.Q., Liu Y., Yang G.C., Zeng HM., Comp. Sci. Tech. 61

1437 2001

24. Suarez J.C.M., Coutinho F.M.B., Sydenstricker T.H., Polym. Test. 22 819 2003

25 Jacob M., Thomas S., Varughese K.T., J. Appl. Polym. Sci. 93 5 2305 2004.

26. Jacob M., Thomas S., Varughese K.T., In proceedings of USM-JIRCAS Joint

lnlernational Symposium; 20-22 March pp. 161-163 Penang, Malaysia 2001.

27. Haseena P., Unnikrishnan G., Proceedings from International Conference on

Advances in Polymer Blends and Composites (ICBC 2005) March 21-23 ,51,2005

28. Madani M; Basta A H; Abdo A E-S; El-Saied H., Prog. Rubb. Plast. Recycl.

Tech. 20 4 287 2004

Dynamic Mechanical & Thermal Analyses of Woven Sisal Fabric Reinforced Natural Rubber Textile Biocomposites

Abstract

The viscoelastic properties of the textile rubber composites were analysed at different

frequencies (0.1, 1 & 10 Hz). Sisal fabric was subjected to different chemical

modifications like mercerization, silanation and thermal treatment and their influence on

dynamic mechanical properties was analyzed. Storage modulus was found to increase

upon reinforcement of natural rubber with woven sisal fabric. Chemical modification of

sisal fabric resulted in a decrease of storage modulus. The damping factor was found to

decrease with chemical treatment and gum compound exhibited maximum damping

characteristics. Thermal properties of the cornposifes were also analyzed. Activation

energy was calculated and compared from three methods: Horowitz and Metzger

method, Coats-Redfern method and Kamal method.

;

>

'

3.2.1 INTRODUCTION

Developmenls in textile lechnologies such as weaving, knitting and braiding

has resulted in the formation of textile composites that have superior mechanical

properties, as no discontinuous orientation of fibres is entailed at any point. Woven

fabrics are altractive as reinforcements since they provide excellent integrity and

conformability for advanced structural applications. The driving force for increased

use of woven fabrics compared to their non-woven counterparts are excellent

drapeability, reduced manufacturing costs and increased resistance to impact

damage. The non delamination characteristics of three-dimensional braided

composites under ballistic impact also make them possess considerable potential in

ballistic protection applicationsl.

The engineering properties of new types of geocomposites using nonwoven

heat-bonded and woven geotextiles were recently reportedz. Changing the

geotextile type was seen to significantly alter the performance characteristics of the

geocomposites including filtration properties, transmissivity and flow properties,

interface shear strength, ply-adhesion strength and ultraviolet resistance. A number

of emerging applications of drainage geocomposites, such as bioreactor landfills,

pavements, green roof tops, where these new drainage geocomposites may fulfill

' the performance requirements better than the traditional nonwoven needlepunched-

based drainage composites have also been enumerated.

In an interesting study, the dynamic mechanical analysis of woven sisal

fabric reinforced polyester composites were reported by PothenJ. The impact

strength of the composites increased with the number of layers and fibre volume

fraction. Storage modulus registered a dramatic increase for composites with four

layers of the fabric. The hygroscopic behavior of a woven fabric carbon-epoxy

composite and its effect on the viscoelastic properties and glass transition

temperature was investigated by Abot et al4. The viscoelastic properties were not

affected during the moisture absorption process but plasticization effect was found

to be very pronounced.

Donnell et al.5 characterized the dynamic mechanical properties of natural

fibre reinforced and acrylated epoxidized soyabean oil resin composites which were

manufactured by vacuum assisted resin transfer moulding. The different natural

fibres used were flax, hemp, cellulose and recycled newspaper. The authors

observed that recycled newspaper reinforced resin composiles exhibiled a storage

modulus value that was almost five times greater than that of the resin. The

composites were also found to possess high damping characteristics making them

a probable choice as material for anti-vibration parts in automotive industry.

The effect of alkali treatment on the dynamic mechanical properties of

kenaf and hemp fibre reinforced polyester composites was analyzed by Aziz and

Ansell.6. The authors observed that the mechanical properties of a treated fibre

composites have higher storage modulus values* and lower damping parameter

indicating greater interfacial bond strength and adhesion between polyester resin

matrix and fibre and inferior impact properties compared to the untreated fibre

composites. They have also noticed a similar pattern of results when cashew nut

shell liquid was used as mattix7.

In a study concerning'hybrid bio-fibre systems, the dynamic mechanical

properties of sisal-oil palm hybrid fibre reinforced natural rubber composites was

analyzed by Jacob et al.8 We noticed that there was an increase of storage

modulus with fibre reinforcement while damping characteristics registered a .

decrease. Chemical modification of sisal and oil palm fibres resulted in an increase

of storage modulus.

This chapter deals with the viscoelastic properties of woven sisal fabric

reinforced natural rubber biocomposites. The effect of chemical modification and

frequency on the dynamic properlies was also analyzed. The thermal stability of the

textile biocomposites has also been investigated.

3.2.2 RESULTS AND DISCUSSION

3.2.2.1 Storage Modulus

Storage modulus provides valuable insight into the stiffness of a material

with reference to temperature. It measures the elastic response of a material.

Figure 3.2.1 shows the variation of storage modulus with temperature of the gum

and untreated samples. It can be seen that the composite containing untreated sisal

fabric exhibits an increase in storage modulus as compared to the gum compound.

Storage modulus mainly depends upon stiffness and rigidily of a composite. Any

factor that increases the stiffness of the system will result in an increase of storage

modulus. The gum compound comprising of only the rubber phase gives the

.material more flexibility resulting in a low degree of stiffness and hence low storage

modulus. When sisal fabric (which is tightly knit) is incorporated in the otherwise

flexible rubber matrix, the stiffness of the composite increases resulting in high

storage modulus. Also, the addition of woven fabric allows greater stress transfer at

the interface, which consequently increases the storage modulus. It can also be

observed that storage modulus is tremendously high at the glassy region.

It can be observed that chemical modification of sisal fabric has resulted in

a decrease of storage modulus. The mechanics of textile composites is expected to

be different from short fibre composites. Generally, it has been seen that chemical

treatments increase the properties of the composite, In fact we have seen that the

incorporation of mercerized sisal and oil palm fibres increased the tensile strength

of natural rubber compositesg. The major contribution to strength in textile

composites is the alignment of yarns in warp and weft direction. Chemical treatment

results in the partial unwinding of yarns (as hemicellulose dissolves off) and hence

the alignment gets antagonized. This results in lowering of strength of composites.

Another reason is that as sisal fabric is composed of thick strands and knots, the

alkali and silane coupling agents did not penetrate into the fabric and therefore the

interfacial properties between the sisal fabric and rubber matrix has not been

improved enough. Hence chemical treatment was ineffective due to which the high

stiffness obtained in the former case was antagonized when the fabric was

subjected to conventional chemical treatment of fabric.

Among the treated composites, composite containing thermally treated

fabric exhibits the highest storage modulus. 'This was attributed to decrease in

moisture content of fabric which in turn promotis better adhesion between the

fabric and rubber. Another factor is that crystallinity increases due to adjustment of

molecular structure at elevated temperatures. It was noticed in a previous study'o

that Ihe tensile strength and modulus of the composite containing alkali treated

fabric was lower compared to the untreated composite. Another interesting

observation was that composite containing thermally treated fabric had the highest

tensile strength and modulus. This is in justification of the above results.

4- Untreated 4- Bonding agenl --.A- 4 % NaOH --r-A1100 -4- A174 4- Thermal +- Gum '1

Temperature ["C]

Figure 3.2.1 Variation of storage modulus with temperature as a function of chemical modification [I Hz]

3.2.2.2 Loss modulus and damping characteristics

The loss modulus represents the viscous response of a material. Figures 3.2.2

presents the variation of loss modulus with temperature as a function of chemical

modification. It can be observed that .maximum loss modulus is exhibited by the

composite containing untreated sisal fabric and.upon chemical modification the loss

modulus value decreases. The height and area of the peak regions are also

indications of Ihe energy absorbed by the system. Compared to the untreated

composite, the peak height decreases for the composites comprising of chemically

modified sisal fabric.

Dyntrmic Mcc/~nr.ticnl At1nlys.i.r rrttd Tlrrrntnl Stutlies of.. . 333

--e- Bonding agent --+- 4 % NaOH U-- Silane A1 100 --t Silane A174 -4- Thermal -+ Gum

2 , . , . 7 , . , -100 -50 0 50 100 150

Temperature ["C]

Figure 3.2.2 Variation of loss modulus with temperature as a function of chemical modification [I Hz]

Damping is an important parameter related to the study of dynamic

behaviour of fibre reinforced composite structures. Tan 6 relates to the impact

resistance of the material. As the damping peak occurs in the region of the glass

transilion where the material changes from a rigid lo a more rubbery state, it is

associated with the segmental mobility within the polymer structure all of which are

initially frozen in. Therefore higher the tan 6 peak value, greater is the degree of

molecular mobility'.

Figure 3.2.3 presents the variation of tan 6 with temperature as a function

of chemical modification. It is observed that the gum compound exhibits maximum

damping characteristics and damping decreases upon reinforcement with sisal

, fabric. l ncorporation of fabric resulted in barriers being formed restricting the

mobility of rubber chains, leading to lower flexibility, lower degrees of molecular

motion and hence lower damping characteristics. Another reason for the decrease

is that there is less matrix by volume to dissipate the mechanical energy. When

compared to the untreated composite, we can see that chemically modified

composites exhibit lower damping characteristics.

Table 3.2.1 presents the T, values from loss modulus peak and tan delta peak.

It is evident that the incorporation of sisal fabric resulted in an increase of T,. This is

attributed to the lowering of mobility of the rubber-fabric system. It can also be seen that

T, values decrease to lower temperatures upon chemical treatment. This decrease is

due to the ineffective interfacial bonding which enhances the mobility of the molecular

chains resulting in the transition state being pushed to lower limits.

4 - Bonding agent -+- 4 % NaOH 4- Silane A1 100 -v- Silane A1 74 --a-- Thermal U Untreated -+-GUM , 1

Temperature 'C

Figure 3.2.3 Variation of damping factor with temperature as a function of chemical modification [I Hz]

Dvttrrttric Meclrrr~ticml Atrrrlvsis rrnd Tlterttrctl Slrrr1ic.s of.. . 335

Table 3.2.1 T, values from loss modulus peak and tan delta peak

3.2.2.3 Frequency dependence of textile composites

The mechanical behavior of viscoelastic materials is dependent on time (or

frequency) as well as on temperature. The variation of dynamic properties of sisal fibre

reinforced polypropylene with frequency has been ihestigated by Joseph et al.11. The

authors observed that storage modulus increased with frequency and this increase

was prominent at higher temperatures, Figures 3.2.4 and 3.2.5 represent the

variation of storage modulus of the untreated and thermally treated composite wilh

temperature at three different frequencies 0.1, 1 and 10 Hz. It can be clearly seen

that storage modulus increases with frequency and this increase is prominent only

at low temperatures. This can be attributed to the lesser mobility of the rubber

chains when the speed of cyclic stress is too fast to bring about deformation.

Temperature PC]

Figure 3.2.4 Variation of storage modulus of the untreated composite with temperature at different frequencies

l , . l . , . l . , , l . { -100 -80 -60 -40 -20 0 20

Temperature ["C]

Figure 3.2.5 Variation of storage modulus of the thermally treated composite. with temperature at different frequencies

3.2.2.4 Thermal properties

Generally it has been seen that the incorporation of plant fibres into

different matrices increases the thermal stability of the system. In an interesting

study, the thermogravimetric analysis of biodegradable composites comprising of

poly(Propy1ene carbonale)(PPC) and short, lignocellulose fibre Hitdegardia

populifolia was performed by Li et al.12 and the investigation revealed that the

introduction of the fibre led to a slightly improved thermo-oxidative stability of PPC.

The thermograms and derivative thermograms of the untreated and treated

sisal fabric reinforced rubber composites are shown in Figures 3.2.6 & 3.2.7

respectively. Table 3.2.2 gives the peak temperatures and O/O weight loss of the

various composites. For the gum composite the peak at 3605°C corresponds to

the maximum degradation of rubber matrix. In the case of the composite containing

woven sisal fabric, the peak temperatures have decreased to 329.3 "C and a new

peak has come at 480.2 "C due to hemicellulose and a-cellulose degradation. For

the composite containing fabric treated with bonding agents and alkali the peak

temperatures have increased to 501.3"C ad 501.7" C. This means that

mercerization has resulted in greater thermal stability for the composites. For the

composites containing silane treated sisal fabric it can be seen that the peak

temperatures have shifted to temperatures 487.2 "C and 482.5"C indicating that

silane treated composites are thermally less stable than alkali treated composites.

Thermal stability is found to be maximum for the composite containing the thermally

treated sisal fabric which is evident from the peak temperature at 504°C. The

amount of residual char left at 700°C increases for the composite containing alkali

treated sisal fabric. A similar observation was reported by Ray et al.13 for, alkali

treated jute fibres who explained that mercsrization reduced the hemicellulose to a

considerable extent giving rise to a lignin-cellulose complex thereby making the

product more stable than the raw sample and this was reflected in the increased

amount of char lefl behind.

Table 3.2.2 Peak temperatures and O h weight loss of composites

Dvtrrrttric Mcclrn~ricr~I A~trlvsiv urirl Tlrcrrnol S/rrr/ies of. .. 339

0 100 200 300 400 500 600 700

Temperature rC]

Figure 3.2.6 Therrnograms of the composites

rn

l . l . l . , . l . l . l . i 0 100 200 3W 400 500 6W 700

Temperature ["C]

Figure 3.2.7 Derivative thermograms of the cornpasites

3.2.2.5 Kinetics of thermal degradation

In the present sludy kinetic parameters such as pre-exponential factor (A),

activation energy (E) and rate constant (k) for the decomposition of the various

composites have been determined using three different methods, viz. ( a) Horowitz

and Metzger method14 /Part II, Chapter 3, Eqn. 2.3.11 and (b) Coats- Redfern

methodl5 [Part II, Chapter 3, Eqn. 2.3.21 (c) Kamal method.

In the Kamal method, activation energy is obtained from the equation:

The order of decomposition reaction was determined from the best linear fit

of the kinetic curve that gives the maximum correlation coefficient. The activation

energy (E) was calculated from the slope that is obtained by a plot of

The kinetic parameters are presented in Table 3.2.3,

A look at the activation energy obtained from the Horowitz and Metzger

method reveals that the incorporation of woven sisal fabric in rubber matrix has

resulted in lowering of activation energy indicating greater stability. The chemically

modified composites show higher activation energy values when compared lo the

untreated composite. This same trend can be seen for the activation energy values .

obtained from Coats-Redfern method, On comparing these values obtained from

the two methods it can be seen that activation energy values obtained from Coats

Redfern method are higher. In the case of the values obtained from Kamal method

it cam be seen that thermally treated composites have the lowest activation energy.

Table 3.2.3 Activation energy and rate constant of composites

Samples

3.2.3 CONCLUSIONS

Storage modulus was found to increase upon reinforcement of natural

rubber with sisal fabric. This was attributed to the increase of stiffness to the rubber-

fabric network. Chemical modification of sisal fabric resulted in lowering of storage

modulus. The damping parameler registered a decrease upon chemical

Rate constant (s-')

Coats-Redfern method

16.12

8.38

10.86

12.93

10.06

9.271

8.85

Gum

T (Untreated)

TB (Bonding agent)

TBA (4 % NaOH)

TBAS (Silane A1 100)

TBMS (Silane A1 74)

TT (Thermal)

Activation energy (kJlmol)

Horowik and

Mefzger method

102.26

85.4 1

102.72

93.45

92.01

84.6

95.1 7

Kamal method

152

132.46

84.24

85.1 0

102.30

144.49

68.09

Coats- Redfem method

140

83.6

101

115

94.6

88.66

88.22

modification due to lowering of mobility of the polymer chains. Storage modulus

was also found to increase with frequency for the untreated as well as the treated

composites. Thermogravimelric analysis revealed that composiles containing

chemically treated fabric were thermally more stable than untreated composite.

Among the composites containing chemically treated fabric, thermally treated

composites were found to be more stable. Activation energy was calculated from

three different methods and the values were found to be in agreement.

References:

1. Gu B., Ding X., J. Comp. Mater. 39 8 685 2005

2. Ramsey B., Narejo D., Proceedings of the Sessions of the Geo-Frontiers

Congress 2005

3. Pothen L.A., Polschke P., Habler R., Thomas S., J. Comp. Mat. 39 1007

2005

4. Abot J.L., Yasmin A., Daniel I.M., J. Reinf. Plast. Comp. 24 2 195 2005

5. O'Donnell A., Dweib M.A., Wool R.P. C o i p . Sci. Tech. 64 1135 2004

6. Aziz S.H., Ansell M.P. Comp. Sci. Tech. 64 121 9 2004

7. Aziz S.H., Ansell M.P. Comp. Sci. Tech. 64 1231 2004

8. Jacob M., Thomas S., Varughese K.T., Macromol. Mat. Eng. (Communicated)

9. Jacob M., Thomas S., Varughese K.T., Comp. Sci. Tech. 64 955 2004 .

Dync~rtric Mecl~trtricrrl A nnlysis rmtl Tlrern~ trl Strrrlics of . . . 343

10. Jacob M., Thomas S., Varughese K.T., J. Comp. Mater. (in press)

11. Joseph P.V., Mathew G., Joseph K., Groeninckx G., Thomas S., Composites

Part A 34 275 2003

12. Li X.H., Meng Y.Z., Wang S.J., Rajulu A.V., Tjong S.C., J. Polyrn. Sci. Part B

- Polym. Phys. 42 4 666 2004

13. Ray D., Sarkar B.K., Basak R.K., Rana A.K., J. Appl. Polym. Sci. 85 2594 2002

15. Coats A.W., Redfern J.P., Nature 201 68 1964

Chapter 3

Sorption Studies of Woven Sisal Fabric Rein forced Natural Rubber Textile Biocomposites

I Abstract

The moisture uptake of the textile composites was investigated as a function of

chemical modification and temperature. The effect of chemical modification of sisal

fab"c on moisture uptake was also analyzed. Mercerization was seen to increase the

water uptake in the composites while thermally treated fabric reinforced composites

exhibited lower water uptake. The thermodynamic parameters of the sorption process

were also evaluated. The interaction of three different types of aromatic solvents,

namely, benzene, toluene and xylene with the textile rubber composites with reference

to chemical modification was analyzed. Swelling was found to be predominantly

dependent on the aromatic solvent used and chemical treatments. Uptake was found to

be maximum for textile composite containing sisal fabric treated with 4 % NaOH. This

was attributed to the weak interfacial adhesion due to partial disruption of the alignment

of yams in the fabric. ,. ., I ' i . ' i .: h,.' I.* . i , i < l , , + i < f 8 . * . . ' , t ' , .

P~rrt of tlre results itr tlris chapter Itas becrt conrnrrrtricnletl to Jnirrtrnl nfApp/ied Pnlvnrer Science

3.3.1 INTRODUCTION

Lignocellulosic fibres have become Ihe wonder materials of this era. Plant

fibre reinforced composites have grabbed the attention of the scienlific world for

their desirable properties like low specific gravity, enhanced mechanical properties

and biodegradability. While the history of natural fibre reinforced composites dates

back to several thousand years, modern need of an environmentally friendly system

has renewed the interest in this area and a new insight is being discovered.

A serious problem relating to the use of lignocellulosic fibres and fabrics in

composites is their affinity towards moisture. The presence of hydroxyl groups in

the cellulosic units in the fibres allows them to form hydrogen bonds with water

molecules and it is essential to examine the durability of fibre reinforced under wet

and humid conditions. Therefore it is quite imperative to realize the water sorption

characteristics of natural fibre reinforced composites.

Recently researchers reported on the water sorption behaviour of raw and

treated flax fibres'. The effects of conventional pretreatment processes on the

sorption ability of flax fibres were compared Lo environmentally friendly enzymatic

procedures. In another interesting report con~erning flax fibres, the influence of

absorbed water on the tensile strength of fibres was investigated by Baley et aI2.

The authors observed that drying of flax fibres resulted in modification of adhesion

between cellulose microfibrils and matrix. This modification was believed to be due

to evolution of components ensuring the transfer of load between microfibrils and

thus enhancing the strength of the cellular wall.

Sorp tior1 Strrdies of Wovcn Sisal Fuhric Reirlforcecl. . . 347

In an interesting sludy using braided synthetic libre composites, the

moisture absorption of threedimensionally braided carbon fibre epoxy composites

were investigated by Wan et al.3. The authors observed that moisture uptake

reduced the mechanical properties of the composites. Also the moisture uptake of

epoxy resin was found to be much higher than the reinforced composite as braided

carbon fibre did not absorb water. The experimental values were also found to be

higher than the theoretical data. This was attributed to the wicking effect of fibre-

matrix interfaces and micro-cracks within the composites.

The micromechanics modeling of moisture diffusion in woven composites

was reported by Tang et al4. The authors observed that woven composites

exhibited quicker diffusion than that of a unidirectional laminate with the same

overall fibre volume fraction. The plain weave with a lenticular tow and large

waviness was seen to exhibit the quickest diffusion process. Pothen et al.5

investigated the water sorption characteristics of woven sisal fabric reinforced

polyester composites. The authors observed that the diffusion mechanism in the

textile composites followed a fickian pattern.

While studies on water sorption behaviour of short fibre reinforced

composites are prevalent, those of lignocellulosic fabric reinforced composites are

limited and therefore need to be addressed in detail. This chapter explores the

moisture uptake characteristics of woven sisal fabric reinforced natural rubber

composites. The water uptake at three different temperatures viz. 30, 50 & 70°C

are analyzed. The thermodynamic parameters of the sorption process have also

been evaluated. The interaction of three different types of aromatic solvents,

348 Clt np fer 3

namely, benzene, toluene and xylene with the textile rubber composites with

reference to chemical modification was also analyzed.

3.3.2 RESULTS AND DISCUSSION

The details of sisal fabric and fabrication technique are elaborated in Part

Ill, Chapler 1, Section 3.1.2.1.

3.3.2.1 Moisture uptake in textile biocomposites

Figures 3.3.1, 3.3.2 and 3.3.3 give the sorption behaviour of untreated and

treated sisal fabric reinforced natural rubber biocomposites in distilled water at 30,

50 and 700C. The amount of water absorbed by specimen at 300C was closely

followed. It can be seen from Figure 3.3.1 that all the samples show a fickian mode

of diffusion i.e. the process takes place by simple diffusion with out any physical as

well as chemical reactionss.

It can also be seen that minimum water sorption is shown by sample

containing bonding agent alone [rB] and maximum uptake is exhibited by

composite containing alkali treated sisal fabric FBA]. Generally it has been seen

that chemical modification of fibres results. in lowering of water uptake in

composites. In fact, in a previous study7 it was found that composites containing

' alkali treated sisal and oil palm fibres exhibited minimum water uptake compared to

untreated composite. But in the present case, contrary to expectations, alkali and

silane treated textile composiles exhibit higher water uptake than the untreated

composite. Only the thermally treated composite VT] shows lower water uptake

than the untreated sample.

Sorp fiotr Sfrrtlics (,/ Woven Si.sl~f Frrbric Reittfiorcetl.. . 349 - .-

4 . 4 - 4 4 + . 4 444 ,,.d* -,-..-* .* *w .-,-e.:::: --I-. .. -. -. -... - . , , ~ ~ . ~ : ~ ~ m = t t = l = ¶ - ~ ~ 1 E t ¶ . .

. - 0- TT(tt1ermal) -- A- TB(bonding agent) .-r -- TBA(4% NaOH). +- TBAS(aminosi1ane) 4- TBMS(melhylsilane)

0.000; . , , , . , 1 , . I 0 50 100 150 200 250 300

Time ~rnin]'~

Figure 3.3.1 Sorption curves of untreated and treated sisal fabric reinforced natural rubber composites at 30°C

The higher uptake of water by samples TBA, TBAS and TBMS with respect

to untreated one has been analyzed and can be explained as follows. The

mechanics of water diffusion in short fibre composites is different from that of textile

composites. In the case of short fibre composites, chemical treatment improves the

fibre surface adhesive characteristics by removing natural waxy materials,

hemicellulose and artificial impurities and by producing a surface topographp. In

addition lo this, alkali treatment can lead to fibrillation i.e., breaking down of fibres

into smaller ones. All these factors provide a large surface area and give a better

mechanical interlocking between the fibre and matrix and thus reduce water

absorption. Besides the removal of hemicellulose and waxes, the treatment with

NaOH solution also promotes the activation of hydroxyl groups of cellulose unit by

. breaking the hydrogen bond.

In the case of textile composites, chemical treatment of sisal fabric results in

the partial unwinding of yams (as hemicellulose dissolves off) and hence the alignment

of the yams gets antagonized. Another aspect is that as sisal fabric is composed of

thick strands and knots, the alkali and silane coupling agents cannot penetrate into the

fabric and therefore Ihe interfacial properties between the sisal fabric and rubber matrix

has not been improved enough.9 As there is nol good bonding between fabric and

rubber matrix, water can easily penetrate into the voids and interfacial gaps that are

present in the interfacial area causing higher water uptake.

Another feature is that when the fibres are made into yarn form, the

hydrophilic character comparalively gets reduced. Due to the twisted struclure,

some of the hydrophilic -OH groups may not be much activated to form H-bonds

with water. This can further explain the lower water uptake shown by the untreated

composite. However, on treatment with NaOH, there may be a chance of activation

of all the hydroxyl groups which is now able to form ti-bonds with water and

increase the uptake of water significantly.

Aminosilane treated (TBAS) composite shows lower uptake than methyl

silane treated (TBMS) composite due to the strgng bond formed between the matrix

and fibre (See Part 11, Chapter 11, Scheme 2.2.1). Such a possibility does not exist

in TBMS due to the non-polar nature of methyl group; the bond formed between

fabric and matrix in this case is very weak.

From Figure 3.3.1, it can be seen that thermally treated sample shows a

lower uptake then untreated one. This could be attributed to the fact that upon

lhermal treatment the crystallinity of cellulose increases due to the rearrangement

of molecular structures at elevated temperatures. Thermal treatment also results in

Sorprion S111die.v of Wovetr Si.vd Fnhric Rein forcetl. . . 35 1

moisture loss of the fabric thereby enhancing the extent of bonding between fabric

and rubber leading to a more compact rubber-fabric network. This increases fabric-

matrix adhesion and leads to lower water uptake.

The minimum water uptake exhibited by sample TB can be attributed to the

improved adhesion between rubber and sisal fabric in the presence of bonding

agents. The same result has been observed from the calculation of diffusion

coefficient (See Table 3.3.1).

Table 3.3.1 Diffusion coefficient, Sorption coefficient and Permeability coefficient of composites

TBA (4 % NaOH)

TBAS (Silane A1 100)

TBMS (Silane A1 74)

TT (Thermal)

50 70 30 50 70 30 50 70 30 50 70

4 595 x 103 1,687 x 10." 5.5 x 10.'" 1.46 x 10-9 1.188 x 10-9 7.713 x 1 0 ' 0 1.2017 x 10.9 1 .I726 x 10-8

2.64 x 1O10 8.324 x 10-9

7.4172 x 10-8

0.34801 .76307

0.20218 0.2023 1.2025

0.21286 0.29589 1.0574

0.16885 0.21831 0.9739

1.5226 x lo-* 2.995 x 10-l1 1.1122 x 1010 2.9535 x 10." 1.4292 x 1011 2.1286 x 101° 2.3504 x lo-" 1.2399 x 4.457 x 10-11 2.2704 x 10-10 7.2236 x'10-12

Figure 3.3.2 presents the water uptake of textile composites at 50°C. The

maximum water uptake is exhibited by TBA sample while minimum water uptake is

shown by TB sample. An interesting feature in this graph is the anomalous

behaviour shown by TT sample which shows higher water uptake than untreated

composite. This can be explained as follows. At higher temperatures like 50°C,

there may be chance of breakage of the additional H-bonds created in the cellulosic

network and this paves way for diffusion through these additional sites. Thus, the

diffusion through the normal sites as well as through the additional sites increases

the uptake of water compared lo all other samples.

The results are also apparent from the diffusion coefficient calculated

i.e., diffusion increases with increase in temperature. The rate of diffusion of water

is time and temperature dependent. As temperature increases, activation of

diffusion increases and hence uptake of water also increases. Diffusion is related to

the velocity of the diffusing molecules by the equation given in Part 11, Chapter 4,

Eqn. 2.4.1. Since the mean velocity increases with lemperature, diffusion also

increases with temperature.

In the case of amino silane treated (TBAS) and methyl silane treated

samples (TBMS), the water absorption becomes lower than that of untreated

composite. Also the uptake of water in TBMS is lesser than TBAS which is reverse

to the sorption curve of the sample at 300C. This may be due to weaker fabric-

matrix adhesion in TBMS. Water will easily penetrate into the interfacial gaps in the

initial stages. This causes the yarns to swell, reducing the distance between the

fibre bundles or interstitial positions which will gradually result in decreased water

uptake by the sample.

Somtiort Strrrlics of Woven Sisrrl fihric Rrinfi)rccrl.. . 353

0 . . , , . ! . , . , I

0 50 100 150 200 250 300

Time [min]'"

Figure 3.3.2 Sorption cuwe of untreated and treated sisal fabric reinforced natural rubber composite at 50°C

Figure 3.3.3 represents the water uptake at 70°C. It can be seen that

maximum water uptake is exhibited by mercerized sample while minimum water

uptake is shown by the thermally treated sample. Another interesting observation is

that none of the samples have any tendency to attain saturation point.

In Ihe case of mercerized sample, as explained earlier, the -OH groups

in TBA are highly activated. Due to prolonged exposure, the swelled fibres will

have a tendency to escape from the yarn structure and there may occur fabric-

matrix debonding. The weave architecture will be completely destroyed resulting

in the formation of a large number of interstitial gaps which can increase water

uptake abnormally. I 1- /

Another aspect is that the sisal fabric-matrix adhesion decreases at

elevated temperatures which further facililate the water flow through the hydrophilic

fabric and there may be a chance of fabric-matrix debonding leading to abnormal

water uptake. Thus, uptake of water increases due to the fibre debonding from the

matrix as exposure time increases. Moreover, there may also be a chance of

degradation of the sample either by osmotic cracking or by destruction of the weave

architecture.

Time [min]'"

Figure 3.3.3 Sorption curve of untreated and treated sisal fabric reinforced natural rubber composite at 70°C

3.3.2.2 Kinetic parameters

The thermodynamic parameters of sorption process can be calculated from

diffusion data which are given in Part 11, Chapter 4, Eqns. 2.4.3,2.4.4 and 2.4.5.

Activation energy can be calculated from the equation 2.4.2.

3.3.2.2.1 Diffusion coefficient

Diffusion coefficient explains the rate at which a diffusion process takes

place. It is the rate of transfer of the diffusing substance across unit area of section

divided by the space gradient of concentration. Diffusion coefficient characterizes

the ability of water molecules to diffuse into the fibre.

Diffusion coefficient can be calculated by the equalion 2.4.3

3.3.2.2.2 Sorption coefficient

Sorption coefficient (S) is calculated by the equation 2.4.4. It gives a

measure of the extent of sorption.

3.3.2.2.3 Permeability coefficient

Permeability coefficient gives an idea about the amount of water

permeating through uniform area of the sample per second. The permeability

coefficient is given by equation 2.4.5.

The calculated diffusion and permeability coefficient are in agreement with

the results obtained by analyzing the graph (Table 3.3.1).

Thermodynamic functions AS, AH and AG were calculated by linear-

regression analysis using the Van't Hoff equation which is given in Part II, Chapter

4, Eqn. 2.4.6.

The thermodynamic parameters are presented in Table 3.3.2. The enthalpy

and entropy of sorption are positive indicating that the process is endothermic. It

can also be seen that for the composites containing chemically treated fabric, AS

and AH values are lower. The free energy values are found to be negative for all

the systems indicating that diffusion process is a spontaneous reaction due to the

presence of hydrophilic sisal fabric.

Table 3.3.2 Thermodynamic parameters of composites

3.3.2.3 Solvent uptake in textile biocomposites

Figure 3.3.4 presents the variation of toluene uptake of untreated and

treated composites at room temperature. Contrary to expectations, it can be seen

that composites containing chemically modified fabric show higher solvent uptake

than untreated, indicating that poor interfacial adhesion is present in the

composites. The minimum solvent uptake is exhibited by composite containing

thermally treated sisal fabric. This suggests that there is better interfacial bonding in

the composite due to removal of water and increased crystallinity of fabric.

The variation of solvent uptake in different organic solvents (benzene,

toluene and xylene) is depicted in Figure 3.3.5. Here also we can see that uptake is

maximum when the solvent used is benzene and minimum uptake occurs when

xylene is used as the solvent. Diffusion is related to the velocity of the diffusing

molecules by the equation given in Part 11, Chapter 4, Eqn. 2.4.1. Since velocity

decreases with size of penetrating molecules, diffusion also decreases upon using

high molecular weight solvents.

j # p4-C'° 1 -I-- Untreated -*- Bonding agent

Time fmin]''

Figure 3.3.4 Variation of toluene uptake of untreated and treated textile composites

I Benzene 1

Untreated Bonding 4 % A l I00 A 174 Thcrmal agent NaOlI

Figure 3.3.5 Variation of solvent uptake in different organic solvents (benzene, toluene and xylene) of untreated and treated textile composites

CONCLUSIONS

Water sorption experiments revealed that uptake was mainly dependent on

the properties of the woven fabric. The mechinism of diffusion was found to be

fickian for the composites. Among the chemically treated composites, mercerized

samples exhibited the maximum sorption while the composite containing bonding

agent alone showed minimum water uptake. Thermodynamic parameters were

evaluated; it was found that free energy of the textile composites were found to be

negative indicating that sorption is a feasible reaction. Solvent uptake was found to

higher for composites containing chemically treated sisal fabric. Uptake was found

to be maximum for textile composite containing sisal fabric treated with 4 % NaOH.

References:

--

1. Kreze T., lskrae S., Smole S.S., Stana-Kleinschek K., Strnad S., Fakin D., J.

Nat. Fib. 2 3 23 2005

2. Baley C., Morvan C., Grohens Y., Macromol. Symp. 222 195 2005

3. Wan Y.Z., Wang Y.L., Cheng G.X., Han K.Y., J. of App. Polym. Sci. 85 1031

2002

4. Tang X., Whitcomb J.D., Li Y ., Sue H-J., Comp. Sci. Tech. 65 81 7 2005

5. Pothen L.A., Potschke P., Habler R., Thomas S., J. Comp. Mat. 39 1007

2005

6. Liao K., Schultheisz C.R., Hunston D.L., Composites Part B 30 485 1999

7 . Jacob M., Varughese K.T., Thomas S., Biomacromolecules 6 (6) 2969-2005

8. Bledzki A.K., Gassan J., Prog. Polymer Science 24 221-274 1999

9. Jacob M., Varughese K.T., Thomas S., J. Comp. Maier. ( in press)