Effect of Kenaf Fibres on the Properties of Natural … · Egypt as the third commonest fibre plant after cotton and flax. ... mechanical properties of natural rubber vulcanizates

549Polymers & Polymer Composites, Vol. 9, No. 8, 2001

Effect of Kenaf Fibres on the Properties of Natural Rubber Vulcanizates

± Corresponding author

INTRODUCTION

Fillers play an important role in enhancing theproperties of elastomers. The chemistry of the fillersurface has an important role in the initial stepspreceeding the actual reinforcement of rubber as wellas in the crosslinking rate itself1. In our previousstudies we have reported that Egyptian kaolin offersimprovements in the physico-mechanical propertiesof natural rubber (NR) and shows stability towardsoxidative ageing2. On the other hand, fumed silicaimproved the rheological properties of rubber mixes(NR&SBR) and showed a reinforcing effect similar tothat of commercial fillers e.g. Hisil3.

Reinforcement of rubber compounds with short fibreshas become necessary in many products, especiallyconstructing and bitting.

One advantage of short fibre elastomer composites isthat they combine the elastic behaviour of a rubberwith the strength and stiffness obtained fromreinforcing fibres4. The mechanical properties suchas modulus (stress at 100% strain), tensile strengthand ultimate elongation depend on the type and

amount of the fibres5-8. Other factors affecting themechanical properties of these composites are theaspect ratio of fibres after mixing8, the orientation ofthe fibres in the composites 9, the state of dispersionof fibres10-11 and the degree of adhesion of the fibresto the rubber matrix12. The adhesion of syntheticfibres to different types of rubber has attracted theattention of several researchers because of theiradvantages in mechanical properties, ready dispersionand adhesion to the rubber matrix13-16. The adhesionbetween many types of short fibres and mostelastomers has been mastered by the discovery of thetricomponent system (HRH) using hydrated silica,resorcinol and hexamethylene tetramine5-7,17.

Various types of short fibre such as carbon, polyester,kevlar, silk and polyamide were used to reinforcerubbers.

Setara and De18, found that NR and polychloroprene(CR) have higher critical cut length (1/C) values thanNBR which does not undergo strain crystallisation.The addition of short fibres was found to cause anincrease in (1/C) in all cases. The increase is moreprominent in the case of NBR than for NR and CR. Thecritical cut length values for the fibre-reinforcedcomposite at a higher temperature (100oC) remainedunchanged, but dropped in the case of unfilledvulcanizates.

Effect of Kenaf Fibres on the Properties of Natural RubberVulcanizates

S.H.El-Sabbagh*, D.M. El-Hariri** and M.A. Abd El-Ghaffar*±

*Department of Polymers and Pigments and ** Field Crops Research Department, National ResearchCentre, Cairo, Egypt

Received: 22nd August 2000; Accepted: 21st September 2001

SUMMARY

The mechanical properties and swelling behaviour of natural rubber (NR) vulcanizatesloaded with kenaf fibres were studied using (hydrated silica, resorcinol and hexamethylenetetramine) as the adhesion system and compared with those of NR vulcanizates loadedwith synthetic polyester short fibres (viscose).The effect of fibre content on the afore mentioned mechanical properties of NR vulcanizateswas also studied before and after ageing. The kenaf fibres showed a higher reinforcingeffect than that of synthetic polyester fibres and improved the rheological properties.Scanning electron microscopy was used to investigate the surface texture of unreinforcedand reinforced vulcanizates.

550 Polymers & Polymer Composites, Vol. 9, No. 8, 2001

S.H.El-Sabbagh, D.M. El-Hariri and M.A. Abd El-Ghaffar

Setara19 has also studied the processing, rheologicalcharacteristics and physical properties of rubberscontaining particulate fillers and various vulcanizingsystems reinforce short silk with fibres. The rubberswere NR, NBR, CR and SBR.

Kenaf fibre (Hibiscus cannabins L.) is an annualplant, belongs to the Malvaceae family. It is grown inmany countries as a bast fibrous crop. It is grown inEgypt as the third commonest fibre plant after cottonand flax. Farmers used to grow kenaf for makingropes or bags for agricultural uses as well as makinghedges around cotton fields. More attention has beengiven to the use of jute especially for coarse bags,sacks and other special purposes.

Kenaf fibre is similar to jute. It consists of cellulose,pectin, lignin, hemicellulose, waxes and minerals.The characteristics of the kenaf fibres compared withnatural and synthetic fibres are illustrated in Table 120.

The aim of the present study was to evaluate theinfluence of kenaf fibres on the processing andmechanical properties of natural rubber vulcanizatescompared with polyester short fibres (using the HRHadhesion system). The importance of kenaf fibresarises from the fact that they are of natural plant originand being environmentally friendly a non-textileapplication for the fibres would help reduceenvironmental pollutants.

EXPERIMENTAL

1. Material:

1.1 Rubber

Natural rubber “Ribbed smoked sheets Rss-1, Density0.913 g/cm3. Mooney viscosity ML(1+4) at 100oC =60-90r and glass transition Tg = -75oC.

1.2 Fillers

1.2.1. High abrasion furnace carbon black N-330(HAF): Mean particle size 40 nm and density 1.78-1.82 g/cm3.

1.2.2. Hi-sil (Hydrated silicone dioxide): Whitepowder, mean particle size 22 nm and density1.95 g/cm3.

1.3 Acceleators

N-cyclohexyl-2-benzothiazole sulphenamide (CBS):Pale grey, non hygroscopic powder, has a meltingpoint of 95-100oC and density 1.27-1.31 g/cm3.

1.4 Antioxidants

N-isopropyl N‘-cyclohexyl paraphenylene diamine(IPPD): Purple grey flakes have density 1.17 g/cm3.

1.5 Plasticizers

Naphthenic processing oil has density 0.94-96 g/cm3

and viscosity 80-90 at 100 oC.

1.6 Curing system

Sulpher: Density 2.04-2.06 g/cm3.

1.7 Fibres

1.7.1 Kenaf Fibres

They were obtained from Giza 3 variety where rettingwas carried out in PVC tubes in summer (August) for10 days. Then stems were air dried for 3 days. Bastfibers were separated by hand and combed with a finemetal comb to obtain long clean strands of fibres.Random samples were taken from the middle part oflong strands and addition to others from short strands.The average length of the long fibres was 100-150 mm,

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while short fibres were 50-80 mm. These sampleswere cut into small pieces.

1.7.2 Polyester Short Fibres (Viscose): Polyestershort fibers (PET) ( trade name viscose), mean length38 mm and 1.5 denier. It is a product of Misr Companyfor Artificial Silk - Kafre El-Dawar, Egypt.

1.8 Adhesion System

The HRH adhesion system consists of threecomponents:

a. Hydrated silica (Hi-sil).b. Resorcinol: White crystalline powder, m.p. 109-

110oC and density 1.27 g/cm3.c. Hexamethylene tetramine (HMT): White

rhombic crystals, it has density of 1.3 g/cm3.

2 Techniques

2.1 Mixing

All rubber mixes were prepared on a laboratory two-roll mill of 170 mm diameter and 300 mm workingdistance. The speed of the slow roller was 24 rpm,with a 1:1.4 gear ratio. Care was taken to ensure fibreorientation in the mill direction. The compoundedrubber was left over night before vulcanization.

2.3 Vulcanization

This was carried out on a heated platinum pressunder pressure of about 40K/cm2 (3.9 MPa) at atemperature of 142 + 1oC.

2.4 Physical Testing

- Rheometric characteristics: minimum torque(ML), maximum torque (Mn), optimum curetime (tc90), scorch time (ts2) and cure rate index(CRI) were determined according to ASTMmethod D-1646 (1994), using a MonsantoOscillating Disc Rheometer model-100.

- The mechanical properties16 (tensile strength,elongation at break, 100% modulus and Young’smodulus) were measured at room temperatureon an electric tensile testing machine (Zwick1425) according to ASTM D 412-98a (1998).

2.5 Swelling Test21

Swelling tests in toluene were carried out at roomtemperature (approximately 25oC) for 24 hours, andthe percentage of soluble matter was determined.

- Thermal oxidative ageing: Accelerated ageingwas carried out in an electric oven at 90 ± 1oC fordifferent time periods according to ASTM-D573(1952).

- Fatigue property:It was determined using Monsanto FatigueFailure Testing Machine according to ASTM D430-73 (1973).

- The change of the surface texture of the rubbervulcanizate samples before and after additionof fillers was studied using scanning electronmicroscope JSM-T20 Japan JEOL.

RESULTS AND DISCUSSION

Effect of Kenaf Fibre Concentration andHRH System on the Properties of NRVulcanizates

Table 2 illustrates the ingredients of the various NRformulations and the rheometric characteristics oftheir vulcanizates reinforced with kenaf fibres.

The minimum torque (ML) was not affected byincreasing the fibre content, while the maximumtorque (MH) was increased. The addition of the HRHadhesion system led to a decrease in scorch time andan increase in cure time with respect to the controlsample M1, which indicated that the vulcanizationreaction started faster but with slower rate.

Effect of Kenaf Fibre Concentration on theMechanical Properties of NR Vulcanizates

It can be seen from Table 3 that, the adhesion systemled to a decrease in the tensile strength, elongation atbreak, equilibrium swelling in toluene and also no. ofcycles (fatigue life), but increased the Young’smodulus. On the other hand, this data represents thedependence of both longitudinal and transversaltensile strength on the fibre concentration. There is asharp decrease in tensile strength in both directionswith the initial loading of fibre followed by a slightincrease in tensile strength with the increase of fibreconcentration. It is also shown that, the increase oftensile strength in the longitudinal direction is higherthan that in the transversal one and the addition offibre with HRH system leads to a sharp decrease in theelongation at break in both directions. The dataillustrate that the transversal elongation is lower thanthe longitudinal one, which indicates that the fibresare well oriented in the rubber matrix in mill direction.On the other hand the Young’s modulus increasedwith increasing fibre content up to 25 phr in both



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directions and it also did when the HRH adhesionsystem was introduced. The equilibrium swellingdecreased with increasing the fibre content, especiallyafter the addition of the HRH adhesion system, wherea very sharp decrease occurred (formulations M7&M8).

Effect of Polyester Short Fibres (PET) on theMechanical Properties of NR Vulcanizates

The rubber formulations, rheometric characteristicsand mechanical properties of their vulcanizates areillustrated in Table 4.

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From this data, it is clear that the addition of polyestershort fibres increases the maximum torque andminimum torque, but there was virtually no effect oneither the optimum cure time or the scorch time. Themaximum torque in the case of kenaf fibres washigher than for polyester fibers. It is also clear thatpolyester short fibres led to a decrease in the tensilestrength, elongation at break and equilibrium swelling.These properties also showed a decrease on using theHRH adhesion system, while the Young’s moduluswas increased. On the other hand the number ofcycles to failure showed better (higher) values in thecase of kenaf fibres than for polyester short fibres asshown in Table 4.

Effect of Thermal Ageing on the FibreLoaded Vulcanizates

Vulcanizates containing different concentrations ofkenaf and polyester short fibers and 25 phr long kenaffibres were subjected to thermal ageing at 90oC forvarious periods up to 7 days. The mechanicalproperties of the aged vulcanizates were determinedand represented graphically in Figures 2 and 3. Thevulcanizates reinforced with 25 phr short Kenaffibers in the absence and presence of the adhesionsystem HRH showed vintually no change in tensilestrength during the ageing periods (Figures 2a, c). Onthe other hand the bonding between polyester fibresand the rubber matrix depends markedly on ageing.The tensile strength increased slightly for NRcomposites loaded with 35 phr fbres from 2 to 7 days(Figure 2c), while that loaded with 25 phr fibres didnot show any change in tensile strength. Theelongation at break showed a slight decrease withaging, which may be due to the polyester short fibers.Young’s modulus of the vulcanizates, Figure 4 showedslight increase with ageing, while the equilibriumswelling of the composition in toluene decreasedwith increasing ageing time, indicating good adhesionbefore and during ageing, (Figure 5).

Effect of Ozone on the Resistance of the NRVulcanizates Reinforced with Kenaf andPolyester Short Fibres

The vulcanizates were subjected to an ultra-violetradiation lamp to study the effects of the irradiationdose on the performance of the exposed rubbervulcanizates. The vlcanizates showed highperformance after 240 h (10 days) exposure. Novisible cracks were observed, as illustrated in Figures6a and 6d, which may be attributed to the presence ofIPPD (N-isopropyl-N’-phnyl-p-phenylene diamine)in the formulation. IPPD enhances protection againstozone, but small cracks were formed after exposure.

Investigation of the NR VulcanizateSurfaces using Scanning ElectronMicroscopy

Scanning electron microscopy offers the simplestinvestigative procedure23, since it reveals surfacefeatures. The internal structure of the NR vulcanizateswas investigated by viewing the fracture surfacescreated at ambient temperature. Figure 7a shows themorphology of NR vulcanizates without any fibre.Lamellae were obtained resembling cavitieshomogeneously distributed all over the surface. In

Figure 1a Variation of tensile strength and elongation atbreak with volume loading of short kenaf fibres

Figure 1a Variation of tensile strength and elongation atbreak with volume loading of short polyester fibres



Figure 2a,b,c The longitudinal tensile strength vs. ageingtime at 90 °C, of NR vulcanizates unloaded and loadedwith kenaf and polyester fibres

Figure 3a,b,c The longitudinal elongation at break vs.ageing time at 90 °C, of NR vulcanizates unloaded andloaded with kenaf and polyester fibres



NR vulcanizates loaded with 25 phr short kenaf fibre,there was an increase in the number of fine flow linesand a reduction in the number of cavities and cracks.However, the fibres distributed in scattered planesseem to be perpendicular to or making angles with,the flow line indicating a smoother surface with areduction in matrix deformation as illustrated inFigure 7b. Photomicrographs Figure 7c show that theincorporation of an adhesion system to the NR loadedwith 25 phr short kenaf fibre decreased the average of

Figure 4a,b,c The dependence of the longitudinal Young's modulus on the ageing time

A

B

C

the number of fine flow lines. This may be due to thesilica filler or the adhesion system to increase meltviscosity of the rubber. For NR loaded with 25 phrkenaf long fibre with the adhesion system, the scanningelectron microscope shows holes and cracks on thesurface. This is due to poor adhesion between thefibre and rubber matrix as indicated in Figure 7d.After adding the adhesion system to NR and usinglong kenaf fibres, the microscopy showed nearlyuniform distribution of the fibre (i.e. the surface often



showed spots with highly charged fibruioragglomerates Figure 7e). The morphology seen byscanning electron microscopy of NR loaded with 25phr short polyester fibres is illustrated in Figure 7f.The sharp fine grains were distributed

homogeneously. Addition of the adhesion systemenhanced the adhesion forces between fibres andrubber matrix, thus improving the interface of both,and thus helped the grains forming islands to attacheach other as shown in Figure 7g.

Figure 5a,b,c The dependence of the equilibrium swelling in toluene on the ageing time



Figure 6a,b Photographs of natural rubber vulcanizates loaded with (a) kenaf fibre; (b) polyester fibre without and withthe adhesion system after 10 days exposure to ultraviolet radiation



Figure 7 Scanning electron micrographs for NR vulcanizates unloaded and loaded with short and long kenaf fibre andshort polyester fibres in the absence and in the presence of the HRH adhesion system

Figure 7e 500x NR vulcanizates loaded with 25 phr longkenaf fibres using the HRH adhesion system

Figure 7b 500x NR vulcanizates loaded with 25 phr shortkenaf fibre

Figure 7c 500x NR vulcanizates loaded with 25 phr shortkenaf fibres + HRH adhesion system

Figure 7f 500x NR vulcanizates loaded with 25 phr shortpolyester fibres

Figure 7d 500x NR vulcanizates loaded with 25 phr longkenaf fibres

Figure 7a 500x NR vulcanizates without fibre



CONCLUSIONS

1. The HRH adhesion system had an acceleratingeffect on the curing of the NR vulcanizatesreinforced with kenaf fibres or short polyesterfibres.

2. Reinforcement of the NR with kenaf fibresenhanced the mechanical properties better thanshort polyester fibres.

3. Kenaf fibre reinforced NR vulcanizates possessgood thermal stability.

4. Reinforcement of NR with kenaf and polyesterfibres decreased the equilibrium swelling onageing.

5. NR vulcanizates reinforced with knaf andpolyester short fibres showed high resistance toUV irradiation for 10 days (240 h) especially inthe presence of the adhesion system.

REFERENCES

1. D.H. Soleomon and D.G. Hewthorne; Chemistryof Pigments and Fillers, John Wiley and Sons,New York, (1983)

2. F.M. Helaly, S.M. El-Sawy and M.A. Abd El-Ghaffar, “Physico-mechanical properties of SBRfilled with Egyptian Kaolin”, J. of Elastomers &Plastics V.26, 355 (1994)

3. F.M. Helaly, A.A.. Ahmed and M.A. Abd El-Ghaffar, “Silica fume as a new filler for naturaland styrene butadiene rubber vulcanizates”,Proceeding of the 4th Arab International

Conference on Polymer Science and Technology,Cairo, September 8-11 (1997) Vol.(1) p79-93

4. A.Michio and N.Toru, J.Appl. Polym.Sci., 29,661, 670 (1981)

5. J.E.O’Connor, Rubber Chem. Technol., 50, 945(1977)

6. D.k.Setara and S.K.De, Rubber Chem.Technol.,56, 804 (1983)

7. V.M.Murty and S.k.De, Rubber Chem.Technol.,55, 287(1982)

8. K.Boustany and A.Y Coran, Ger, Offend. 2, 233,011 ( 1973) (US Appl. 159, 955 (1971). C.A.80(2),4655s (1974)

9. S.R Moghe, Rubber Chem. Technol. 49, 1160(1976)

10. G.C.Derringer, Rubber world 165,45(1975)

11. T.J.Leo and A.H. Johansson, U.S. Pat. 4, 263,184 (to Wyrough and Loser Inc.) (1981).C.A 95(20), 17078k (1981)

12. D.D. Dunnom, Hi Sil Bulletin, PPG Ind.Inc.No.35(1967)

13. J.M Capell. Prog. Rubber Technol. 11,13,(1978)

14. U.S. Pat. 3, 376, 669 (1972) (to Monsanto’Co.)

15. K.Boustang and R.L. Arnold. J.Elastoplast, 8,160(1976)

16. A.Y.Coran, P.Hamed, and L.A Goettler, RubberChem. Technol. 49,1167 (1976)

17. A.F.Younan, M.N.Ismail, and A.A.Yehia, J.Appl. Polym. Sci., 45, p1969-1971(1992)

18. D.K.Setara and S.K.De, J.Appl. Polym. Sci.20,2653 (1985)

19. D.K. Setara, Polym. Mater. Sci. Eng. 52, 246(1985)

20. G.Mackie, “Flax and competitive fibers- theirposition in world market”, Eurofflax Newsletter,INF, Coordination Center of FAB Flax Network,No1 (5) 1996

21. A.Shvarts, Kautchuki Rezina 7,31 (1957)

22. A.F.Younan, M.N.Ismail and A.I.Khalaf,Polymer Degradation and Stability, 48(1), 103-109(1995)

23. J.R.While and E.L. Thomas, Rubber Chem.Technol., 57, 457 (1984)

Figure 7g 500x NR vulcanizates loaded with25 phr shortpolyester fibres using HRH adhesion system

Documents

Effect of Kenaf Fibres on the Properties of Natural … · Egypt as the third commonest fibre plant after cotton and flax. ... mechanical properties of natural rubber vulcanizates