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published online 1 March 2010Journal of Reinforced Plastics and CompositesM K Yakubu, M S Gumel, A Umar and R Metelerkamp

RubberPhysico-mechanical Effects of Surface-modified Sorgum Stalk Powder on Reinforced

  

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Physico-mechanical Effectsof Surface-modified Sorgum StalkPowder on Reinforced Rubber

M. K. YAKUBU*Department of Textile Science and Technology, Ahmadu Bello University, Zaria

M. S. GUMEL AND A. UMAR

Department of Pure and Industrial Chemistry, Bayero University, Kano

R. METELERKAMP

Materials Science and Manufacturing, Centre for Scientific Research (CSIR),Port-Elizabeth, South Africa

ABSTRACT: Stalk of sorghum was crushed into fine particle size and given three surface treat-ments: with groundnut oil, alkali, and 3-chloro-2-hydroxyl propyl trimethylammonium chloride. Itwas presumed that the oil treatment will preclude any negative surface potential that cellulose isknown to acquire in aqueous environment. The alkali treatment was to remove the natural surfaceadmixtures but to preserve the negative surface potential, and the tertiary ammonium salt was toconfer positive potential to it. The four samples including the untreated fiber were used to producefiber-reinforced natural rubber composite at varying filler loadings. The pycnometric specific densitymeasurement indicated the following decreasing trend: cationized filler, untreated, causticized, andfinally the oil treated. In each case, the density increased with filler volume. The tensile strength ofthe composites also varied such that both the cationized and alkali-treated fillers recorded highervalues than the untreated filler, the oil-treated filler was the least. However, the oil-treated filler didbest with the elongation at break for filler loading higher than 40 phr; for loadings lower than 40 phr,the alkali-treated filler did better. The cationized filler recorded low values but was better than theuntreated filler. The impact tests demonstrated critical loading effect for all the different samples; thetrend showed increase from 10 to 40 phr and then declined, the untreated and the oil-treated didbetter than the causticized and cationized which recorded the least value. The hardness index did notshow much difference between the untreated, causticized, and cationized fillers both of which didbetter than the oil treated. This work has shown some general improvements arising from cationiza-tion of cellulosic filler as reinforcing material for natural rubber.

KEY WORDS: sorghum powder, cationization, causticization, oil treatment, tan (delta), storagemodulus.

INTRODUCTION

THE TRADITIONAL FILLERS for natural rubber and indeed the synthetic ones espe-cially carbon black has fallen out of favor in recent years. This is not only because of its

Author to whom correspondence should be addressed. E-mail: koko80001@yahoo.com

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exhaustible petroleum base, the uncomfortable fluctuating prices of which has been a sourceof serious concern as a business commodity, but more because of the environmental concept[1,2]. Short-fiber rubber-reinforced composites are increasingly taking over due to theirrelatively cheap cost and processing advantages over the traditional types [3,4]. They arelighter, more amenable to processing variables, non-abrasive, less hazardous, and muchmore flexible. Several trials have indicated their superior green strength, creep and agingresistances, and dimensional stability during fabrication and in service [13]. Different typesof agro-based materials have been used as fillers for rubber; some of the widely reportedones include rice husk [34], coconut shells [33], pineapple fibers [13], coir fibers [5], andmanyothers [5,7]. Sorghum stalk is estimated to have an annual yield of 252,000 tons; its cellulosecontent is around 27%, hemicelluloses 25%, and has lignin content of 11% [8]. Nothing hasbeen reported on its use as filler in rubber processing. This paper has discussed some of thephysico-mechanical properties of rubber reinforced with sorghum stalk powder.

MATERIALS AND METHODS

Crumb natural rubber which conformed to technically specified rubber (TSR) wasobtained from Integrated Rubber Factory Sapele Road, Benin City, Edo state, Nigeria.The sorghum stalk containing the fiber was obtained from Doko, Garki local governmentarea, Jigawa state.

SAMPLE PREPARATION

The fiber was obtained from the stem of the sorghum plant by removing the bark of thedry stems manually. This exposed the stalk which contained the cellulose fiber and naturaladmixtures such as lignin and pectins. The dry sorghum stalk was ground using a cleanand dry grinding machine to particle size of 1mm.

MODIFICATION OF THE SORGHUM FIBER

Causticization (Alkali Treatment)

The ground sorghum fibers were modified (causticized) by treatment with 20% OWS ofsodium hydroxide solution. The fiber (500 g) was placed in a solution of 200 g/L of NaOHdiluted by adding 7L of distilled water in a plastic bucket with periodic agitation.The mixture was kept at room temperature for 24 h; the wet fibers were then squeezedto remove excess water. The causticized fibers were washed repeatedly to remove excessNaOH until residual washing tested negative with methyl orange solution. The wet fiberswere put in an oven at 80�C for 5 h to dry.

Oil-treated Sorghum Fiber

Five hundred grams of the ground fibers was mixed thoroughly with 400mL of puregroundnut oil to give a homogenous mixture. The resultant fiber was used as the oil-treatedfiber in the composites preparations.

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Cationized Sorghum Fiber

The alkali-treated fibers were treated with a coupling agent, 3-chloro-2-hydroxylpropyltrimethylammonium chloride (65%) at 10% OWS [9].

The causticized fibers (500 g) were placed in plastic bucket, which contained a solutionof 77mL coupling agent, 123mL NaOH(10.92% w/v), and made up to 8L by addingabout 7800mL of distilled water. The mixture was kept at room temperature for 24 h,maintained at pH 12 by periodic addition of 10.92% w/v NaOH solution with constantstirring. The cationized fibers were then washed repeatedly to remove the excess NaOHand the coupling agent. The wet fibers were put in an oven at 80�C for 5 h to dry. Themechanism of the cationization is as shown:

CH CH2

CH3

CH3

N+ CH3

OH

Cell OCH2

CH2 CH

O

CH2

CH3

CH3

N+ CH3 OH–

Cell O–

CH2 CH CH2

CH3

CH3

N+ CH3

OHCl

OH–

MOISTURE CONTENT DETERMINATION

The percentage of moisture content of the three different forms of the sorghum cellulosefiber was determined using the following procedure.

1 g each of the three different forms of the fiber was weighed and put in a watch glass ofknown weight and kept at room temperature for 48 h. The weight of the watch glass andthe fiber was the initial weight (w1). The watch glass was put in an oven and maintained at100�C, the weight of the watch glass was determined at regular intervals of 30min to aconstant weight (w2). Triplicates were run for the four samples including the control.Finally, the moisture content was calculated using the relation;

% moisture ¼w1 � w2

w1� 100%

where w1 is the initial weight and w2 is the final weight.The results are as shown in Table 1.

COMPOUNDING

The recipe used in the formulation of the natural rubber compound (composites) isgiven in Table 1. Mixing was carried out in a laboratory two-roll mill in accordancewith the method described in the American Society for Testing of Materials (ASTMD3184-80). The cured composites (vulcanizates) were prepared by compression in an

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electrically heated press at 140�C for 40min. Appropriate specimens were cut and theproperties were evaluated.

DETERMINATION OF PHYSICO-MECHANICAL PROPERTIES

Density

The relative densities (RP) of the 29 prepared composites were determined gravimetri-cally using the density bottle. To calculate the relative density of each composite, thefollowing relation was used:

RP ¼Wr �Wb

Ww �Wbð Þ � Wrw �Wrð Þ;

where Wr¼weight of the composite, Wb¼weight of the density bottle, Ww¼weight ofbottle with water, and Wrw¼weight of bottle with water and sample.

The results are shown in Figures 1 and 2.

Swelling Test

The swelling property was determined using the method described in ASTMD3010. The29 cured samples including the blank were cut out into 3mm thickness mould and weighed(w1) and immersed in airtight containers containing toluene for 24 h at room temperature.The samples were then removed from the bottles wiped dry with tissue paper and weighedimmediately (w2). The change in weight of the samples was expressed as percentageswelling.

% swelling ¼w2 � w1

w1� 100%

The result is shown in Figure 3.

Tensile Strength

Tensile properties of the cured composites were measured with Instron UniversalTester at cross-head speed of 60mm/min using dumb bell test specimens as contained

Table 1. A typical formulation of rubber compounds.

IngredientPhr (parts

per hundred)

Natural rubber 100Filler 0�70Zinc oxide 5.0Stearic acid 1.5Sulfur 2.5CBSa 0.5

aN-cyclohexyl-2benzothiazole-2-sulfenamide.

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in ASTM D-412-87 (method A). The tensile strength at break was calculated as aver-age load at break divided by the cross-sectional area. The results are shown in

Figures 4 and 5.

Hardness Test

Hardness of the prepared composites was measured in shore A by Durometer machine;

model Din 53505 EN ISO 868. The measurement was in accordance to ASTM D-2240.The results are shown in Figure 6.

0 20 40 60 80

1.15

1.1

1.05

1

0.95

0.9

Den

sity

(g/

cm3 ) Untreated

Oil-treated

Causticized

Cationized

phr

Figure 1. The curves of density with fiber loading for different surface treatments of the fibers.

1.1

1.05

1

0.95

0.9

0.85Spe

cific

vol

ume

(cm

3 /g)

0 20 40 60 80phr

Untreated

Oil-treated

Causticized

Cationized

Figure 2. The curves of specific volume with fiber loading for different surface treatments of the fibers.

Sw

ellin

g (%

)

0 20 40 60 80phr

Untreated

Oil-treated

Causticized

Cationized

500

400

300

200

100

0

Figure 3. The percentage swelling for different surface treatments of the fibers.

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Impact Test

Impact test was conducted in accordance with the British Standard BSEN ISO 6603-2.A Zwick/Roell impact test machine (IFW 413) was used for the test, which has adrop-tower system. Test samples were cut into circular shapes of diameter 4.0 cm fittedon the impactor at different heights and impacted with a bob of mass 0.868 kg to the pointof break. The results are shown in Figure 7.

12

10

8

6

4

2

0

Str

engt

h (N

/m2 )

0 20 40 60 80phr

Untreated

Oil-treated

Causticized

Cationized

Figure 4. The curves of tensile strength with fiber loading for different surface treatments of the fibers.

phr

900800700600500

400

300

200

100

0

Elo

ngat

ion

(%)

0 20 40 60 80

Untreated

Oil-treated

Causticized

Cationized

Figure 5. The effect of pretreatment on the elongation at break of the composites.

0 20 40 60 80phr

UntreatedOil-treated

Causticized

Cationized

1009080706050403020100

Har

dnes

s (S

hore

)

Figure 6. The hardness ratings of the samples.

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Thermogravimetric Analysis

The thermal stability of the samples (about 5mg) was investigated with a thermogravi-metric analyzer model Perkin Elmer TGA Pyris 1 under nitrogen gas from about 20�C to800�C at a heating rate of 10�C/min. The result is shown in Figure 8.

Dynamic Mechanical Analysis

The storage modulus and loss factor of the composite specimens were measured as afunction of temperature (28�100�C) using a Perkin Elmer DMA 8000 model equippedwith a single-cantilever bending fixture at a frequency of 1Hz and a constant heating rateof 2�C/min.

SEM TOPOGRAPHY

The SEM scanning of the composites samples were taken by immersing the sample inliquid nitrogen and cutting to expose the cross-section. FEI Quanta 200(Eindhoven,Netherlands) was used at an accelerating voltage of 20 kV.

0.35

0.3

0.25

0.2

0.15

0.1

0.05

0

Impa

ct s

tren

gth

(J/m

2 )

0 20 40 60 80phr

Untreated

Oil-treated

Causticized

Cationized

Figure 7. The curves of impact strength with fiber loading for different surface treatments of the fibers.

150

100

50

0

Wei

ght (

%)

0 200 400 600 800 1000Temperature (°C)

Untreated

Control

Oil-treated

Alkali-treated

Bonding agent

Figure 8. TGA thermograms of the samples.

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RESULTS AND DISCUSSION

Moisture Absorption and Density

Effect of the various types of treatments on the absorption of moisture by the filler is

shown in Table 2. The cationization significantly reduced the moisture content of the fillerby about 30% and the alkali treatment by only 10%, while the oil treatment had no effect

on the moisture uptake. It had been reported that reduction in the moisture uptake of

short fibers in rubber compounding could lead to increased adhesion [13]. Reduction in

moisture content is a direct measure of decrease in hydrophilicity, which is believed to

influence adhesion to a hydrophobic matrix.Density of composites is a measure of the degree of compaction or adhesion that is

brought to bear between the elements. The higher the density, the lesser the void content of

the composite and as is reflected in Figure 2, the smaller the specific volume. As reflected

in Figure 1, the cationized filler produced the composite with the highest density followed

by the untreated fiber, the causticized fiber, and lastly the oil-treated one. It is also notedthat the density of all but the oil-treated composites increased with fiber content. This

increase may result if the density of the fiber is higher than that of the matrix, in this case

rubber [10].The oil-treated fiber composite indicated decrease in density with increase in

fiber loading. This decrease in density with increase in loading was reported to be due to

the presence of voids that originated at the interface, because of poor adhesion, or withinthe lumen of the fiber [11].

Swelling Result

The swelling indices of the samples are shown in Figure 3. All the composites as

expected depicted a trend of decreasing swelling with increasing loading. The decrease

in swelling for unfilled rubber is known to be due to the extent of crosslinking between

the curatives and the rubber molecules, Manroshan and Baharin [12]. Several factors caninfluence the equilibrium sorption of organic solvents by rubber and its composites; some

of the important ones are: filler dispersion of the composite, nature of solvent, and the type

and treatments done on the filler [33]. The untreated filler surprisingly gave the least

swelling indices at all loadings. The difference between the other three is not very signif-

icant for loadings upto 50 phr; thereafter the oil-treated sample gave slightly higher values.The plausible reason for this is the nature of the treatment reagents. The oil used to treat

the filler is certainly soluble in toluene, and by virtue of the openness in the matrix wit-

nessed by the low density, the solvent will hardly meet any impediment in its way through

the matrix. The cationized filler though has the highest density but the organic moiety

Table 2. Moisture content of the raw and modified fillers.

Fiber type Moisture content (%)

Dry fiber 8.00Causticized 7.20Cationized fiber 5.60Oil-treated fiber 8.00

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which constituted it may also be soluble in toluene and the moment it is wetted, penetra-tion into the rubber network will be easy.

The tensile strength of the composite shown in Figure 4 indicated strength decrease withincrease in fiber loading. Several investigators reported the same findings [13�15]. Thetensile strength of rubber-reinforced composites is not only governed by the introductionof crosslinking but also by the ability of the constituent particles to coalesce among them-selves. The rubber molecules are themselves internally crosslinked, possessing rather hightensile strength as a result of the so-called strain-induced crystallization [16]. However, theaddition of the fiber interfered with this natural tendency leading to disruption of theregular arrangement of the molecules, resulting in the loss of ability to crystallize andhence the observed decrease in tensile strength with increase in fiber loading. It has alsobeen proposed that the poor fiber�rubber bonding is responsible for the decrease in tensilestrength with loading. This is mainly due to the incompatibility of the hydrophilic fiberwith the hydrophobic rubber molecules [17,18]. This necessitated different approaches toimprove the interfacial bonding between the phases [19�22]. Some researchers presumedthat surface treatments that could lead to decrease in moisture content will also improvethe interface [23,24]; others concentrated on methods that would cause changes to thesurface topology of the fibers by removing natural admixtures [1]. Yet others looked formethods that would encourage covalent bonding between the fiber molecules and rubber[25]. We hereby attempt to give the surface different surface potentials as a means ofcontrolling interfacial phenomenon. It is known that cellulose acquires electronegativepotential in the presence of water; if the surface of the fibers can be covered with alayer of glyceride, the effect of this electronegative surface potential will be eliminated;the surface is also given a mild scouring to remove the admixtures as control while yetagain, an electropositive potential is given to the surface to study how this would impacton the properties of the composites.

Figure 4 showed that the tensile strength of the composites filled with causticized andcationized fibers was higher than the composites made with untreated fibers at mostloadings especially for loadings of 20�50 phr. The effect of causticizing on the propertiesof short fiber composites has been investigated by several workers [11,26]. It is claimedthat the treatment removes the admixtures and exposes the surface of the fibers for betteradhesion with the rubber molecules and at the same time creates a rough surface formechanical bonding [10]. However, the strengths of the cationized samples were onlymarginally higher than the causticized samples. The deleterious effect of admixtures onmechanical properties is depicted by the response of the oil-treated samples to the testwhich clearly gave the lowest strength values even as compared with the untreated sam-ples. This has further given credence to the assertion that surface smoothness occasionedby the presence of glycerides and lignin on the surface negatively affects adhesion. Both theuntreated and the oil-treated samples also indicated sharp decreases in strength. Thestrengths of the untreated composite samples decreased from 9.62 to 4.96MPa for 10and 20 phr respectively, while that of the oil-treated samples decreased from 5.68 to2.69MPa for 20 phr and 30 phr, respectively. It has been explained elsewhere that thesesharp decreases indicate the loading at which the filler bears more of the stress, and insome way it also indicates the limiting levels of loading [13,27]. Preiss et al. [28] whileexplaining the decrease in strength of an experimental cis-1,4 isoprene with increase infiller loading even as the crosslink density had increased, adduced it to the increasinginability of the rubber latex particles to coalesce and cooperate fully with increasing cross-link; other workers also gave the same explanation [29].

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Neither the causticized nor the cationized filler demonstrated such sharp falls in strengthwith fiber loading apart from the one shown with respect to the raw rubber itself. Thisobservation could once again be explained from the point of view of the adhesion thatimproved as a result of treatment with caustic soda and the bonding agent.

The effect of the treatments on the elongation at break is given in Figure 5. The generaldecrease in elongation at break with loading has severally been reported and discussed.It has been linked to the loss in resilience accompanying stiffening of the backbone of therubber matrix [30�32]. The oil-treated samples gave best results in this regard especiallyfor loadings higher than 30 phr compared with all the other samples. The causticizedsamples recorded better elongation at break values than the cationized ones for almostall the loadings. However, for both samples there appeared a sharp drop of elongation atbreak after the 40 phr loading. The elongation at break for the causticized and cationizedcomposites at 40 phr are 234.5% and 118.5%, respectively, which dropped to 72% and45%, respectively, at 50 phr loading. This indicated the maximum level of the fillers atwhich they are able to carry the stress, being less extensible than the matrix, the elongationat break reduced phenomenally. The untreated filler sample showed this phenomenon at alower loading level � 30 phr loading; the elongation at break at this loading was 317.6%compared to 62.41% at 40 phr loading.

Impact Energy and Hardness

The hardness result in Figure 6 of all the different samples increased with increasingfiber content. This is in consonance with the reduced elasticity as a result of the reinforce-ment in the rubber molecules. Egwaikhide et al. [33] credited this increase in hardness withfiber loading to the increase in rigidity as the elasticity of the virgin rubber decreases,Sae-oei et al. [34] attributed this phenomenon to the incorporation of more rigid fillerparticles into soft elastic rubber. It is observed that the cationized fiber-reinforced samplegave higher hardness ratings at loading levels less than 40 phr. The untreated fiber gavesuperior values at loadings above 40 phr. The hardness rating of the causticized samples isintermediate between these two; the oil-treated fibers produced composites with the leasthardness values at all loading levels. The level of compactness as is evidenced from thedensity values of the composites in Figures 1 and 2 had earlier conferred the cationizedsamples more denser status than the rest. The elasticity result depicted in Figure 5 alsoshowed that at loading levels less than 30 phr, the cationized samples are the least elastic,with all these factors in mind, the superior hardness rating of the cationized sample atloading levels less than 40 phr is clearly established. All the aforementioned factors rele-gated the oil-treated sample behind the rest in hardness index; it is most elastic especiallyfor loadings above 40 phr and the least dense of the rest. The superior hardness of theuntreated sample at loading levels higher than 40 phr can also be inferred from its poorelasticity; it showed the poorest elasticity at loading levels higher than 12 phr.

The impact testing result is shown in Figure 7. All the curves indicated a sharp decreasewith the introduction of the filler followed by increase upto the 40 phr loading beforeslumping again to very low values. Senadi et al. [35] explained that the impact strengthof agro-fibers’ reinforced thermoplastics generally decreases compared to the main matrixmainly because their incorporation creates regions of stress concentration that easily givesin under pressure. The insensitivity of surface treatment on impact strength was reportedby Chuai et al. [36].Wan et al. [37] discovered that the impact strength of fiber-reinforced

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PLA composites increased to a maximum value at 30 vol.% fiber, but the control has the

highest impact strength.Thermal and dynamic mechanical analyses of 40 phr of the composites with the control

were carried out and given in Figures 8�10; the summary of these are shown in Table 3.

The Tg obtained from the tan d plots indicated that the oil-treated filler has the lowest

value: �55.1�C and the untreated filler is next: �51.8�C; cationized sample: �49.3�C,causticized: �48.8�C and the control: �44.3�C in this order. Storage moduli at �60�Cand �50�C closely followed similar trend reflected by the Tg. All of these values are in

consonance with the tensile property results earlier seen. The thermal degradation of the

2.000E+00

1.500E+00

1.000E+00

5.000E–01

0.000E+00

Tan

(δ)

–80.0 –60.0 –40.0 –20.0 0.0Temperature (°C)

Untreated

Oil-treated

Causticized

Cationized

Control

Figure 10. Tan � curves of the samples.

5.000E+10

4.000E+10

3.000E+10

2.000E+10

1.000E+10

0.000E+10

Sto

rage

mod

ulus

(N

/mm

3 )

–80.0 –60.0 –40.0 –20.0 0.0

Temperature (°C)

Untreated

Control

Oil-treated

Causticized

Cationized

Figure 9. Storage modulus of the samples.

Table 3. TGA and DMA data of 40 phr of the composites.

Storage modulus(GPa) Thermal degradation temperatures (8C)(TGA)

Sample �608C �508C Tg(8C) Onset 20% 50% 80%

Untreated 9.480 0.449 �51.8 303 380 436 494Control 37.310 16.100 �44.3 405 412 433 465Oil-treated 4.570 0.784 �55.1 387 393 428 471Causticized 17.900 4.450 �48.8 333 365 423 477Cationized 19.680 3.360 �49.3 320 371 422 473

Surface-modified Sorgum Stalk Powder 11

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samples indicated that the onset of degradation for the unfilled rubber is highest; 405�C,

about 18�C higher than the next sample � the oil-treated composite. The untreated sample

recorded the least onset temperature of 303�C.

Sample Topology

It could be observed from the SEM micrographs (Figure 11) that the cationized fillers

gave best filler dispersion and this was closely followed by the unmodified filler. Both the

causticized and the oil-treated fillers indicated poor and non-uniform dispersion. The fact

that the oil-treated filler produced least modulus is therefore very evident from the

(a) (b)

(d)(c)

(e)

Figure 11. SEM photomicrographs of the samples: (a) control, (b) untreated, (c) oil-treated, (d) causticized,(e) cationized.

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micrographs, and the superiority of the cationized filler over the causticized filler can alsobe inferred from the micrographs.

CONCLUSION

The treatment of cellulosic agro-based fillers for rubber reinforcement with alkali, gly-ceride oil, and cationic agents indicated that glyceride oil further inhibited adhesionbetween the filler and the rubber matrix. This is seen from the very low density, strength,and hardness which it gave. The alkali treatment is aimed at cleaning the surface of thecellulosic filler free of its natural admixtures such as lignin, pectins, and some fats and oils;it is believed that some surface roughening will also be imparted to encourage mechanicalbonding. To a greater extent, the objectives of this treatment have been met: the tensilestrength, hardness, and density all improved over the untreated. The cationic treatment isdesigned to confer a positive potential to the filler as a prelude to trigger electrostaticattraction with rubber. This objective seems to have been met; the cationic filler compositeis the densest, and for some low loadings, has the highest strength and hardness. It alsorecorded the lowest elongation at break for low loadings; all of these are indicative ofimproved interaction between rubber and the filler.

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