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Oil palm microcomposites: processing and mechanical behavior.

INTRODUCTION

Natural fibers are becoming a viable alternative to those who use glass fibers as reinforcement in the composites (1). It gained more importance because of its renewable nature, low cost, easily availability, low density, biodegradable nature, etc (2). Short fiber-reinforced rubber composites are of tremendous importance both in structural applications as well as in the area of research and development (3). Composites in which short fibers are oriented uniaxially in an elastomer have good strength and stiffness attributed from the fibers and matrix. Application of these composites in automobile structures. V-belts, hoses, and complex-shaped articles can significantly reduce the product weight (4), (5), In this 21st century, designing for recycling or "ecodesign" is becoming a philosophy that is applied more to composite materials (6).

Cellulose, the main component in natural fiber, is a linear homopolymer (polysaccharide) in which D-glucopyranose rings are connected with one another by [beta]-l,4-gly-cosidic linkage with a syndiotactic configuration (see Fig. 1). The crystal modulus of cellulose is comparable with those of high-perfomance synthetic fibers and higher than those of glass fiber and aluminum (7). In the fibers, cellulose chains aggregate to form microfibrils, long thread-like bundles of molecules, stabilized by hydrogen bonds between hydroxyl groups and oxygen of adjacent molecules (8). These microfibrils consist of monocrystalline cellulose domains with the microfibril axis parallel to the cellulose chains. Each microfibril can be considered as a string of polymer whiskers, linked along the microfibril by amorphous domains. During acid hydrolysis, transverse cleavage of the microfibrils occurs due to structural defects of amorphous regions. Several works were reported on the usage of fillers as reinforcement in elastomers. For example, the role of lignin as filler in stabilization of natural rubber (NR)-based composites were studied by Kosikova et al. (9). It was shown that the usage of lignin improved the physicomechanical properties of the rubber vulcanizates. Martins et al. (10) investigated the mechanical and fractographic behavior of NR/cellulose II composites. Incorporation of agricultural waste products as filler in NR has been analyzed by Okieimen and Ima-nah (11). Arayapranee et al. (12) highlighted the importance of the application of rice husk ash as fillers in NR industry. Jacob et al. (13) studied the effect of concentration and modification of fiber surface in sisal/oil palm hybrid fiber-reinforced NR composites. They concluded that an increase in fiber loading can create a reduction in tensile and tear strength but an increment in the modulus of the composites. Effect of NR on wood-reinforced tannin composites were done by Mosiewicki et al. (14). The modified materials exhibit substantial increase in mechanical properties. Jong (15) conducted the characterization of soy protein concentrate (SPC) in elastomer composites. Here, the addition of SPC increased the mechanical properties of the composites. Effects of bonding agents on the curing characteristics and mechanical properties of alkali-treated grass/NR composites have been studied by Debaish et al. (16). Thomas and coworkers (17), (18) showed that short coir, sisal, and banana fiber can be used as reinforcement for NR, polypropylene, and thermoset resins. A number of studies have been reported on natural fiber/NR composites; no systematic study has yet been done on microcomposites consisting of NR and cellulose microfibrils. Studies on oil palm microfibrils are attractive because of their easy availability in South India. Oil palm fibers have high elongation at break than many other natural fibers. In addition, they have higher fungal, bacterial, and weather resistance because of their high lignin content.

[FIGURE 1 OMITTED]

In this work, more detailed studies have been done on the processability, physical, and mechanical properties of a new microcomposite developed by reinforcing NR with oil palm empty fruit bunch (OEFB) cellulose microfibrils. The effects of microfibril loading, orientation, and chemical treatments were studied.

EXPERIMENTAL

Materials

Oil palm fiber was obtained from Oil Palm India, Kottayam, Kerala, India. The properties of OEFB fiber used in this study is given in Table 1. NR used for the study was ISNR-5 (light color) grade obtained from Rubber Research Institute of India, Kottayam, Kerala, India (Table 2). All other rubber ingredients such as vulcanizing agents, accelerators, and activators were of commercial grade.
TABLE 1. Characteristics of oil palm empty fruit bunch fiber.

Property                            Percentage

Chemical constituents (%)
  Cellulose                         65
  Hemicellulose                     -
  Lignin                            19
  Ash                               2
Physical properties
  Diameter ([micro]m)               150-500
  Density (g/cc)                    1.4
  Tensile strength (MPa)            248
  Young's modulus (GPa)             3.2
  Microfibrillar angle ([degrees])  46
  Elongation at break (%)           14

TABLE 2. Properties of natural rubber.

Physical properties               Percentage

Dirt content by mass              0.03
Volatile matter by mass           0.50
Nitrogen by mass                  0.30
Ash by mass                       0.40
Initial plasticity, [P.sub.o]     38
Plasticity retention index (PRI)  78


Preparation of Microfibrils--Steam Explosion Method

Steam explosion method (STEX) is used as pretreatment to separate lignin hemicellulose, wax, and pectin from lignocellulosic biomass. The high temperature during the process softens the material and mechanical action during the high-pressure results in microfibril separation (19). A schematic representation of the microfibril preparation is given in Fig. 2. The diameter of microfibrils used in our study varies from 5 to 16 [micro]m (Table 3), and the average aspect ratio is 50-100. After STEX, the microfibrils showed three major changes: (a) an increase in the surface area due to its fragmentations, (b) an increase in fiber porosity due to lignin redistribution and surface becomes rougher, and (c) porosity is also caused by the hydrolysis and removal of hemi cellulose. When compared with short fibers, microfibrils appear to be finer, microrange size; fiber bundles separate and shrink, which causes high aspect ratio.

[FIGURE 2 OMITTED]
TABLE 3. Diameter of the microfiber.

Fiber                    Diameter ([micro]m)

Macro                    194-279
Micro                    9-21
Benzoylated micro fiber  6-14
Silanized micro fiber    5-12


Microfibril Surface Modification

Mercerization. The microfibrils were dipped in 5% solution of NaOH for 1 h with stirring. After that, the microfibrils were washed with distilled water containing little acetic acid and dried.

Silane Treatment. Vinyl tris(2-ethoxy methoxy silane) coupling agent was used for the treatment. The respective silane (0.4%) was prepared by mixing with an ethanol/water mixture in the ratio 6:4 and allowed to stand for 1 h. The pH of the solution was maintained between 3.5 and 4 with the addition of acetic acid. The microfibrils were immersed in 1% NaOH solution and then dipped in the silane solution and allowed to stand for 1.5 h. The ethanol/water mixture was drained out and the fibers were dried in air and then in an oven at 70[degrees]C.

Benzoyl Chloride Treatment (Benzoylation). About 35 g of microfibrils were soaked in 5% NaOH for 1 h, filtered, and washed with water. The treated microfibrils were suspended in 5% NaOH solution and agitated well with 50 ml of benzoyl chloride for 15 min. The isolated microfibrils were then soaked in ethanol for 1 h to remove the unreacted benzoyl chloride, finally washed with water, and dried.

Fabrication of Microcomposites. The formulation of the mixes is given in Table 4. The composite materials were prepared in a laboratory, two roll open mixing mill (150 X 300 [mm.sup.2]). The nip gap, mill roll speed ratio (1:1.25), time, temperature of mixing, number of passes, and sequence of addition of ingredients during mixing were kept as same for all mixes. NR was initially masticated in the mill for 2 min and then the additives were added. Microfibrils were added at the end of the compounding process. Then the samples were milled for sufficient time to confirm the uniform microfibril distribution. The rolling direction was kept constant because it is very important for microfibril orientation.
TABLE 4. Formulations of the mixes.

Ingredients (a)        NR   NRM10          NRM20          NRM30

Natural rubber       100          100            100            100

ZnO                    5.0            5.0            5.0            5.0

Stearic acid           1.5            1.5            1.5            1.5

Resorcinol               -              -              -              -

Hexa                     -              -              -              -

Silica                   -              -              -              -

TDQ (b)                1.0            1.0            1.0            1.0

CBS (c)                0.6            0.6            0.6            0.6

Sulfur                 2.5            2.5            2.5            2.5

Oil palm                 -           10.0           20.0           30.0
microfibrils

Treatments             Mercerization  Mercerization  Mercerization

Ingredients (a)        NRM40          NRMB30         NRMS30  NRMH30

Natural rubber                 100           100     100     100
ZnO                              5.0           5.0     5.0     5.0
Stearic acid                     1.5           1.5     1.5     1.5
Resorcinol                         -             -       -     7.5
Hexa                               -             -       -     4.8
Silica                             -             -       -       2
TDQ (b)                          1.0           1.0     1.0     1.0
CBS (c)                          0.6           0.6     0.6     0.6
Sulfur                           2.5           2.5     2.5     2.5
Oil palm microfibrils           40.0          30.0    30.0    30.0
Treatments             Mercerization  Benzoylation  Silane   HRH

a Parts per hundred rubber (phr).
b 2,2,4-trimethyl 1.2-dihydroxy quinolone.
c N-cyclohexyl-2-benzothiazyl sulphenamide.


Characterization of Microfibrils and Composites. The modified fiber surface was characterized by Fourier Transform Infrared Spectroscopy (FTIR) (Shimadzu IR-470) spectrophotometer. Powdered fiber that was pelletized with KBr was used for recording the spectra.

The green strength of uncured 2-mm-thick sample was measured at a strain rate of 500 mm/min. The surface tack was eliminated by pressing the sample at 120[degrees]C for 1 min between two sheets of aluminum film in a hydraulic press.

Curing properties were measured by means of an oscillating Mosanto R-100 disc rheometer as per ASTM standard D-5289. The samples were vulcanized at 150[degrees]C in a hydraulic press to their respective cure times as obtained from the rheograph.

The stress--strain measurements were carried out by using Series IX automated materials testing system 138 by Instron Corporation model 4411 at a cross head speed of 500 mm/min. Tensile measurements of the composites were determined by using samples cut along (longitudinal) and across (transverse) the grain direction. Tensile strength, modulus, and elongation at break were determined according to ASTM D-412-68 and tear strength by ASTM D-624-54. Hardness was measured by using a shore-A hardness tester (Durometer-Mitutoyoshore-A meter) as per ASTM D-2240. A Zwick Din abrader was used to measure the abrasion resistance as per ASTM D-2228 and the samples were abraded of for about 3 min.

The surface modification of microfibrils and fracture morphology of the composites were observed by means of SEM (Zeiss FESEM supra 25). The fracture ends of the tensile specimens of the composites were mounted on aluminum stubs and gold coated to avoid electrical charging during examination.

RESULTS AND DISCUSSION

Physical Modifications: Fiber Surface Morphology

Figure 3a and b represents the Scanning electron micrograph (SEM) of raw and mercerized microfibril surface. The multicellular nature of the microfibrils, the porosity of the fibers, and their fibrilar structure were revealed from the SEM photographs. The porous surface morphology of fiber is useful to provide better mechanical interlocking with the matrix. Alkali treatment improves the microfibrils' surface adhesive characteristics by removing natural and artificial impurities thereby producing a rough surface topography. Moreover, alkali treatment leads to fibrillation of fibers, that is, breaking down of the fiber bundle into smaller nanofilaments called microfibrillated cellulose (MFC) whose diameter varies from 5 to 15 [micro]m with a high aspect ratio 50-100. MFC has excellent mechanical properties due to its crystal structure. As a result of treatment with alkali, benzoyl chloride, and silane, the fiber diameter decreases (Table 3) and irregular surfaces are formed.

[FIGURE 3 OMITTED]

Chemical Modification: IR Spectroscopy

The FTIR spectrum of the untreated fiber is given in Fig. 4a. The fiber shows peak at 3400 [cm.sup.-1] that corresponds to the hydroxyl groups present in the fiber as well as the absorbed moisture through the formation of hydrogen bonding between the hydroxyl groups in the fiber and moisture. The peak at 2906 [cm.sup.-1] is due to--CH stretching vibration and that at 1735 [cm.sup.-1] corresponds to the--C=O stretching vibration of carboxylic acid. The small peak at 1329 [cm.sup.-1] corresponds to -CH deformation. A peak at 895 [cm.sup.-1] observed arises from [beta]-glycosidic linkage. STEX followed by mercerization leads to an increase in the amount of cellulose nanofibrils. The important modification is the removal of hydrogen bonding in the network structure. From Fig. 4b an increase in the intensity of the--OH peaks at 3400 [cm.sup.-1] has been observed. Also on mercerization, a peak at 1735 [cm.sup.-1] assigned to C=O stretching vibration of carboxylic acid or ester in the spectrum of the fiber disappeared. This is due to the fact that a substantial amount of uranic acid, a constituent of hemicellulose (xylan), was removed from the fiber resulting in the disappearance of the peak (20). The reaction during the mercerization is given in Eq. 1.

[FIGURE 4 OMITTED]

Fiber - OH + NaOH [right arrow] Fiber - ONa.sup.+] + [H.sub.2]O (1)

The vibration peak at 1510 [cm.sup.-1] assigned to the benzene ring vibration of lignin also reduced. These results indicate that alkali treatment lead to the removal of lignin and hemicellulose.

The hydrophilicity of the microfibril reduced due to the reaction between the cellulosic -OH groups of the microfibrils and benzoyl chloride, which is given in Eqs. 2 and 3.

Fiber - OH + NaOH [right arrow] Fiber - [ONa.sup.+] + [H.sub.2]O (2)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

The FTIR spectrum of benzoylated fiber exhibited a number of characteristic observations (Fig. 4c). After benzoylation, as a result of esterification of the hydroxy] group, the intensities of the peak corresponding to the -OH stretching vibrations in the region 3400-3450 [cm.sup.-1] are found to be decreased. Absorption bands around 1630 and 659 [cm.sup.-1] indicate the presence of aromatic ring and peaks around 1725 and 1310 [cm.sub.-1] show the presence of ester group (17).

During silanization, alkoxy silanes are able to form bonds with the hydroxyl groups. Silanes undergo hydrolysis and condensation at the bond formation stage. Silanols can form polysiloxane structures by the reaction with hydroxyl groups of the microfibrils. The reaction is given in Eqs. 4 and 5.

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (4)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

Here, in the FTIR spectra of the silane-treated microfibril (Fig. 4d), due to the reaction, peaks were observed at 710 [cm.sup.-1], which corresponds to the--Si--C--symmetric stretching band. The band around 1158 [cm.sup.-1] can be attributed to the asymmetric stretching of the--Si--O--Si--and/or to the--Si--O--C--bonds. The former bond is indicative of the polysiloxanes deposited on the fiber, whereas the latter points to a condensation reaction between the silane coupling agent and the fiber. The presence of a few residual--Si--OH bonds is revealed by the band around 898 [cm.sup.-1].

Extent of Fiber Orientation From Green Strength Measurements

An important advantage of short fiber-reinforced NR composites is their enhanced green strength, which facilitates shape retention prior to complete cure. This in turn leads to early demolding, which can reduce the molding cycle time. The green strength of short fiber-reinforced composites depends on the degree of fiber orientation, which can be expressed as % and is given in the Eq. 6 (21).

% of micro fibril orientation = [S.sub.L]/[S.sub.GL]/[S.sub.L]/[S.sub.GL] +[S.sub.T]/[S.sub.GL] (6)

where S is the green strength and the subscription G, L, and T represents gum, longitudinal, and transverse direction, respectively. The effect of microfibril loading on percentage orientation is given in Table 5. At the lower fiber loading (i.e., at 10 phr), the percentage of orientation is less and the value increases as a function of fiber loading. The maximum, is for the composites having 30 phr fiber loading, beyond that limit the value decreases. At lower level of microfibril loading, the fibrils could assume a multitude of alignment directions and so the freedom of movement was high. At 40 phr microfibril loading, the percentage of orientation decreases, indicating that the microfibrils could not orient themselves because of the entanglements and aggregation caused by the increased population of fibrils.
TABLE 5. Green strength measurements of the microcomposites.

              Green strength (MPa)

Composite  Longitudinal      Transverse  Fiber orientation

NR                 0.25            0.24                  -
NRM10              0.69            0.53               54.4
NRM20              0.90            0.44               66.1
NRM30              0.96            0.36               71.7
NRM40              0.73            0.47               60.8


Cure Characteristics

Figure 5 displays the cure characteristics of microfiber reinforced composites mixes as a function of fiber loading (Gum; 10, 20, and 30 phr). For all mixes, the torque initially decreases, then increases, and finally levels off. The initial decrease in torque was attributed to the softening of the rubber matrix and the subsequent increase to the crosslinking of the rubber (22). The leveling off can be taken as an indication of the completion of curing. The cure characteristics of the various composites are given in Table 6. Gum without any microfibrils, exhibits a low torque value and required a longer vulcanization time with conventional systems. The addition of fibers increases the torque of the NR composites due to the increase in the rigidity of the vulcanizates. This increment in torque is a function of the degree of cure and of fiber dispersion. Chemical links between the cellulose surface and the rubber network are also possible. From Table 6, it is clear that as the fiber concentration increases the difference between maximum and minimum torque increases. Among the untreated microfiber composites, 30 phr microfibril-reinforced composite shows a higher value. On treatment, maximum torque increases and maximum values are exhibited by silane-treated composite, indicating maximum crosslinking for silane coupling reaction. Cure time does not show any systematic trend on treatment of the microfibrils.

[FIGURE 5 OMITTED]
TABLE 6. Cure characteristics of various microcomposites.

Composite  [T.sub.max]  [T.sub.min]  [DELTA]T  [t.sub.90]

NR                56.0          7.0      49.0       14.20
NRM10             68.0          7.1      60.9       10.90
NRM20             73.0          7.0      66.0       11.10
NRM30             78.0          8.0      70.0       11.30
NRM40             75.6          7.5      68.1       11.00
NRMB30            79.0          8.1      70.9       11.50
NRMS30            80.0          8.3      71.7       11.60
NRMH30            76.0          7.0      69.0       11.30

[T.sub.max], maximum torque; [T.sub.min], minimum torque; [t.sub.90],
optimum cure time.


Effects of Microfibril Loading on the Mechanical Properties

The effect of microfibril loading and orientation on the tensile strength of oil palm microfibril-reinforced NR composites is given in Table 7. It is clear that the tensile strength in the longitudinal and transverse direction increases with increasing concentration of microfibril up to 30 phr and then declines. NR inherently possesses a high strength because of strain-induced crystallization. When microfibrils are incorporated into NR, the regular arrangement of rubber molecules gets disrupted; hence, the ability for crystallization is lost. Therefore, microfibril reinforced NR composites possess lower tensile strength than gum compounds (23). When these composites are subjected to load, the microfibrils act as carriers of load and stress is transferred from the matrix along the microfibrils. This leads to effective and uniform stress distribution, which results in good mechanical properties for the composites. The uniform distribution of stress is dependent on the population and the orientation of the fibers. At low level of loading, microfibril orientation is poor and the fibrils are not capable of transferring load to one another. Thus, stress gets accumulated at certain points in the composite, which leads to a low-tensile strength. At higher level of microfibril loading due to the increased population of microfibril, agglomeration occurs and stress transfer gets blocked. At intermediate levels of loading (30 phr), the population of microfibrils is sufficient for maximum orientation and the fibers actively participate in stress transfer. The tensile strength of the microcomposites in which the fibers are in the longitudinal direction is always greater than that of the transverse direction (Table 7). Figure 6a and b presents the SEM of longitudinally and transversely oriented microcomposites. Greater hindrance to the progress of the fracture front is experienced when the microfibrils are oriented longitudinally. This is because the microfibrils are aligned in the direction of the force and they can transfer stress uniformly. When transversely oriented, the fibrils are aligned perpendicular to the direction of load and cannot take part in stress transfer. The same trend is observed for the values of tensile modulus at 100% elongation (Table 7). The modulus value shows a steady increase with microfibril loading and is maximum for the composite with 30 phr loading. The value of elongation at break shows a reduction with increase in microfibril loading (Table 7). Increased loading in the rubber matrix results in stiffer and harder composites. This reduces the composites resilience and toughness and led to lower resistance to break.

[FIGURE 6 OMITTED]
TABLE 7. Properties of microfibril-reinforced NR composites.

Properties                                 O *  NR           NRM10

Tensile strength (MPa)                     L    21.1 (1.20)  7.94 (0.18)
                                           T    21 (2.32)    6.71 (0.38)
Tensile modulus (MPa) at 100%) elongation  L    1.42 (0.02)  3.02 (0.51)
                                           T    1.18 (0.22)  1.98 (0.07)
Elongation at break (%)                    L    1196 (0.61)  771 (0.51)
                                           T    1156 (0.21)  760 (0.49)
Tear strength (kN/m)                       L    21.2 (0.79)  28.2 (1.74)
                                           T    21.1 (0.09)  25.4 (0.31)
Hardness (shore A)                              48           71
Density (g/cc)                                  0.961        0.996
Relative volume loss (min)                      192          108

Properties                                 O *  NRM20

Tensile strength (MPa)                     L    9.62 (0.37)
                                           T    8.12 (0.87)
Tensile modulus (MPa) at 100%) elongation  L    3.36 (0.38)
                                           T    2.61 (0.23)
Elongation at break (%)                    L    721 (0.18)
                                           T    708 (0.26)
Tear strength (kN/m)                       L    32.5 (1.36)
                                           T    27.3 (0.09)
Hardness (shore A)                              75
Density (g/cc)                                  1.015
Relative volume loss (min)                      109

Properties                                 O *  NRM30

Tensile strength (MPa)                     L    10.13 (0.54)
                                           T    9.24 (0.49)
Tensile modulus (MPa) at 100%) elongation  L    3.81 (0.28)
                                           T    3.1 (0.09)
Elongation at break (%)                    L    695 (0.30)
                                           T    684 (0.57)
Tear strength (kN/m)                       L    35.2 (0.92)
                                           T    29.7(1.08)
Hardness (shore A)                              78
Density (g/cc)                                  1.013
Relative volume loss (min)                      107

Properties                                 O *  NRM40

Tensile strength (MPa)                     L    8.92 (0.04)
                                           T    7.83 (0.24)
Tensile modulus (MPa) at 100%) elongation  L    2.89 (0.09)
                                           T    1.95 (0.36)
Elongation at break (%)                    L    768 (0.61)
                                           T    730 (0.37)
Tear strength (kN/m)                       L    30.3 (1.62)
                                           T    26.6 (1.41)
Hardness (shore A)                              73
Density (g/cc)                                  0.99
Relative volume loss (min)                      111

The values given in parenthesis are standard deviation.
O *, orientation; L, longitudinal; T, transverse.


The effect of microfibril loading and orientation on tear strength is presented in Table 7. The tear strength in the longitudinal direction is greater than that in the transverse direction. This is because when the microfibrils were oriented longitudinally, the greater obstruction caused by the microfibrils to the tear paths made propagation more difficult. As loading increases, the tear strength increases and the composites having 30 phr microfibril loading shows maximum value. At higher loading (40 phr) the presence of closely packed fibers leads to an increase in strain in the matrix and at the interfacial region. This increases the tendency for tearing and reduces the tear strength. Because of the close packing arrangement of microfibrils in NR matrix, the hardness of the composite as a function of microfibril content (Table 7). The abrasion resistance is measured in terms of relative loss of volume over a given time period. If the loss of volume is low, then the samples have high abrasion resistance and vice versa. Relative volume loss is given by the equation

Relative volume loss = Loss of weight/Density X [200/213.36] (7)

From Table 7, it is also clear that as the microfibril loading increased, the relative volume loss decreased, that is, the abrasion resistance of the microcomposite increased.

Effect of Treatment on the Properties of Microcomposites

The effect of various chemical treatments on the mechanical properties of oil palm microfibril-reinforced NR microcomposite is given in Table 8. The increase in the properties like tensile strength, tear strength, tensile modulus, hardness, abrasion resistance, etc. indicates that bonding of the NR to the treated microfibrils are stronger when compared with the untreated one. The benzoylation and silance treatment reduces the hydrophilic nature of microfibrils. This causes a better microfibrill/matrix interfacial adhesion; hence, increase in the mechnical properties. The modulus at 100% elongation is maximum for silane-treated microfibril reinforced composites. The formation of polysiloxanes and benzoyl group on the fiber surface reduces the hydrophilicity of the fiber considerably. The wettability of the polymer by microfibril depends on the viscosity of the polymer and surface thesion of both materials. For better wettability, surface tension of matrix should be lower than the surface tension of microfibril (24). Because of the silane coupling action, the adhesion improvement arises from the relatively high sruface aenergy of the microfibril, which results in the superior mechanical properties of silane-treated micro-composites.
TABLE 8. Properties of treated microfibril-reinforced NR composites.

Properties   O *  NRM30         NRMB30        NRMS30         NRMH30

Tensile      L    10.13 (0.54)  12.84 (1.20)  13.41 (1.101)  11.23
strength                                                     (1.81)
(MPa)

             T    9.24 (0.49)   11.63 (2.32)  12.25 (0.51)   9.87
                                                             (0.76)

Tensile      L    3.81 (0.28)   4.57 (0.02)   4.86 (0.08)    4.35
Modulus                                                      (0.14)
(MPa) at
100%
elongation

             T    3.1 (0.09)    3.56 (0.22)   4.02 (0.14)    3.48
                                                             (0.40)

Elongation   L    695 (0.30)    690 (0.61)    681 (0.82)     692
at break                                                     (0.31)
(%)

             T    684 (0.57)    660 (0.21)    634 (0.55)     686
                                                             (0.17)

Tear         L    35.2 (0.92)   37.65 (0.79)  39.16 (1.42)   36.85
strength                                                     (1.51)
(kN/m)

             T    29.7 (1.08)   33.55 (0.09)  35.68 (0.19)   31.62
                                                             (1.10)

Hardness          78            79            81             76
(shore A)

Density           1.013         0.961         0.971          0.985
(g/cc)

Relative          107           105           103            106
volume loss
(min)


SEM of the tensile-fractured ends of untreated and silane-treated microcomposites (Fig. 7a and b) gives a strong evidence for the improved microfibril/matrix adhesion. In the case of untreated fiber composites, the fracture surface shows several holes after the fibers were pulled out from the matrix. However, in the SEM of silane-treated microfibril-reinforced composites, the fracture surface contains broken fibers, since the failure occurs in the microfibrils as a result of the strong interfacial adhesion.

[FIGURE 7 OMITTED]

Theoretical Modeling

Several theories have been proposed to evaluate the experimental values obtained for mechanical properties by using different parameters such as fiber length, fiber orientation, fiber dispersion, fiber geometry, fiber loading, interfacial adhesion between fiber/matrix, and the properties of constituent components of the composites. These theories take account of the nature of the matrix and reinforcements. The reinforcingeffects of microfibrils on NR matrix can be evaluated using Series model, Hirsch's model, and modified rule of mixtures (MROM).

a. Series model is given by the equation (25)

[T.sub.c] = [T.sub.m][T.sub.f]/[T.sub.m][V.sub.f] + [T.sub.f][V.sub.m] (8)

where [T.sub.c] is the tensile strength of the composites, [T.sub.m] is the tensile strength of the matrix, [T.sub.f] is the ultimate strength of the microfibril, and [V.sub.f] is the volume fraction of microfibrils.

b. Hirsch's model is given by the equation (26)

[T.sub.c] = x([T.sub.f][V.sub.f] + [T.sub.m][V.sub.m]) + (1-x)[T.sub.m][T.sub.f]/[T.sub.m][V.sub.f] + [T.sub.f][V.sub.m] (9)

where x determines the stress transfer between microfibril and matrix and it varies between zero and one.

c. According to modified rule of mixtures (27),

[T.sub.c] = [T.sub.m] x (1 - [V.sub.f]) + [T.sub.f][V.sub.fe] (10)

where [T.sub.c] is the ultimate strength of the composites, [T.sub.m] is the strength of matrix at the failure, [T.sub.f] is the ultimate strength of the microfibril, [V.sub.f] is the microfibril volume fraction, and [V.sub.fe] is the effecitive microfibril volume fraction. The effective microfibril volume fraction is given in terms of the microfibril volume fraction and the ratio of real contribution as follows.

[V.sub.fe] = [V.sub.f](1 - P) (11)

where P is the degradation parameter for the effective microfibril volume fraction lying between zero and one. P can be calculated from the microgeometry of the composite component and depends only on the microfibril volume fraction because the microgeometry is intimately related to the microfibril volume fraction under identical manufacturing conditions. P can be calculated from the equation

P = [DELTA][T.sub.c]/[T.sub.f][V.sub.f] (12)

where [DELTA][T.sub.c] is the difference between the experimentally measured strength and the strength predicted by the rule of mixtures.

Figure 8 shows the comparison of theoretical and experimental tensile strength values of longitudinally oriented composites with microfibril loading. It can be seen that for all the models theoretical value increases regularly with microfibril loading but experimental value increased up to 30 phr loading and then decreases. The Series model shows significant deviation from the experimental curve. The Hirsch's model also shows agreement with the experimental values when the value of x is 0.35. However, the curve of MROM model shows great similarity with the experimental value. At lower fibril loading, there is effective and uniform stress transfer which results in good agreement between experimental and theoretical values. When the microfibril loading increases, deviation from theorietical value is greater due to the increased population of microfibrils which leads to agglomeration and in poor stress transfer. The nonuniform shape of the microfibril cross sections also accounts for the deviation of the tensile properties from teh theoretical predictions. This is not accounted for any of the models.

[FIGURE 8 OMITTED]

Equilibrium Swelling Studies

Rubber-Fiber Interactions. The extent of interaction between rubber and fiber can be analyzed using Kraus equation (28). The equation is

[V.sub.ro]/[V.sub.rf] = 1 - m [f/1 - f] (13)

where [V.sub.ro] is the volume fraction of rubber gum vulcanizates, f is the volume fraction of fiber, and m is the polymer-fiber interaction parameter

[V.sub.rf] is the volume fraction of elastomer in the solvent swollen filled sample and is given by the Ellis and Welding equation (29).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (14)

where, d, deswollen weight of sample; f, volume fraction of fiber; w, initial weight of sample; [P.sub.p], density of polymer; [P.sub.s], density of solvent; [A.sub.s], amount of solvent absorbed; [V.sub.ro], volume fraction of elastomer in solvent swollen unfilled sample.

As Eq. 13 is in the form of a straight line, a plot of [V.sub.ro]/[V.sub.rf] versus f/l--f should give a straight line, whose slope (m) will be a direct measure of reinforcement of the fiber. The ratio [V.sub.ro]/[V.sub.rf] represents the degree of restriction of the swelling of the rubber matrix due to the presence of fiber. According to the theory by Kraus, reinforcing fillers will have a negative higher slope. In the present case, it is observed that as the fiber loading increases, the solvent uptake of the sample decreases. This causes an increase in [V.sub.rf] values, which will decrease the ratio of [V.sub.ro]/[V.sub.rf] as [V.sub.ro] is a constant. It can be seen from the graph (Fig. 9) that [V.sub.ro]/[V.sub.rf] decreases with fiber loading. This behavior leads to a negative slope indicating the reinforcement effect of the oil palm microfibers. As given in Table 9, the value of [V.sub.ro]/[V.sub.rf] decreases in the case of treated composites. This indicates a strong interaction between rubber and treated microfibers in the composites. The value is minimum for silane-treated microfiber composites showing strong interfacial adhesion in silane-treated composites.

[FIGURE 9 OMITTED]
TABLE 9 [V.sub.ro]/[V.sub.rf] values.

Composite  [V.sub.ro]/[V.sub.rf]

NRM10                       0.97
NRM20                       0.94
NRM30                       0.91
NRMB30                      0.88
NRMS30                      0.86
NRMH30                      0.89


Swelling Index and Crosslink Density Determination. The swelling index, which is a measure of the degree of swelling of the rubber compound, is calculated using the equation:

Swelling index (%) = [A.sub.s]/W x 100 (15)

where [A.sub.s] is the amount of solvent absorbed by sample and W is the initial weight of sample before swelling.

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. As the void formation decreases with fiber addition, the solvent uptake also decreases. Table 10 shows the swelling index values of composites at different fiber loadings and treatments. The swelling index values are lower for the treated composites as the strong interface restricts the solvent uptake.
TABLE 10. Swelling index, [V.sub.rf], apparent crosslink density values
of microcomposites.

Composite  Swelling index  [v.sub.rf]  I/Q

Gum        411                         0.1399
NRM10      354             0.1132      0.1432
NRM20      337             0.1168      0.1473
NRM30      332             0.1206      0.1502
NRMB30     328             0.1236      0.1552
NRMS30     322             0.1251      0.1585
NRMH30     330             0.1221      0.1536


The extent of crosslinking can be established by determining the [V.sub.rf] values of the composites. Table 10 shows the [V.sub.rf] values of the various microcomposites. A comparison of crosslink density has also been made from the reciprocal swelling values 1/Q, where Q is defined as the amount of solvent absorbed by 1 g of rubber. The apparent crosslinking value of different composites is given in Table 10. The reinforcing effect of the fibers can be supported by the crosslink density values. It can be seen that as the fiber loading increases, the crosslink density values also increases. This effect can be explained using the basic equations used for swelling:

[M.sub.c] = - [rho.sub.r][V.sub.s][V.sub.rf.sup.1/3]/In (1 - [V.sub.rf]) + [V.sub.rf] + [[chi].[V.sub.rf.sup.2]] (16)

where [M.sub.c], molecular weight of polymer between crosslinks; [P.sub.r], density of polymer; [V.sub.s], molar volume of solvent; [chi], interaction parameter; [V.sub.rf], volume fraction of elastomer in the solvent swollen filled sample.

[chi] is given by Hildebrand equation

[chi] = [beta] + [V.sub.s]([[delta].sub.s] - [[delta].sub.p]).sup.2/RT (17)

where [beta] lattice constant; [V.sub.s], molar volume; R, universal gas constant; T, absolute temperature; [[delta].sub.s], solubility parameter of solvent; [[delta].sub.p], solubility parameter of polymer.

As loading increases, the amount of solvent absorbed by sample decreases that leads to an increase in [V.sub.rf] values and this in turn increases crosslink density values. At lower fiber loading, the crosslink density value is lower and it absorbs a maximum quantity of solvent indicating poor reinforcement. In the case of treated fiber composites, there is strong interaction between treated microfibers and matrix. Therefore, higher [V.sub.rf] values results in higher apparent crosslink density values of treated composites.

CONCLUSIONS

The study of oil palm microfibril-reinforced NR composites showed that variation in microfibril orientation, loading, and treatment affects the processability and mechanical properties of the composites. Incorporation of microfibrils to NR offered a good reinforcing effect there by and an improvement in the mechanical properties. The extent of microfibril orientation calculated from green strength measurements showed that fibril orientation was the greatest in the composites having 30 phr loading. After the treatment, due to the reduction in the concentration of polar components on the fiber surface, the extent of adhesion between microfibril and rubber matrix increased. FTIR spectra indicated the presence of nonpolar functional group on to the surface of the microfibril after the treatment. SEM provided strong evidence for the physical and microstructural changes to the microfibril surface during chemical modification and strong microfibril/matrix adhesion. Except the Series model all other models showed good agreement with the experimental tensile strength up to 30-phr fiber content. Finally, the swelling measurement and crosslink density values supported that treatment increased the fiber/matrix interaction.

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Shaji Joseph, (1) Sreekumar P.A., (2) Jose M. Kenny, (3) Debora Puglia, (3) Sabu Thomas, (4) Kuruvilla Joseph (1)

(1) Department of Chemistry, St. Berchmans' College, Changanacherry, Kottayam, Kerala 686 101, India

(2) Polymer Technology Laboratory, National Institute of Technology Calicut, Calicut, Kerala 673 601, India

(3) University of Perugia, Loc. Pentima Bassa, Terni 21-05100, Italy

(4) School of Chemical Sciences, Mahatma Gandhi University, Kottayam, Kerala 686 560, India

Correspondence to: Kuruvilla Joseph; e-mail: kjoseph.iist@gmail.com

DOI 10.1002/pen.21699

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Author:Joseph, Shaji; P.A., Sreekumar; Kenny, Jose M.; Puglia, Debora; Thomas, Sabu; Joseph, Kuruvilla
Publication:Polymer Engineering and Science
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Date:Sep 1, 2010
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