Engineering, Environment

Mechanical properties of thermoplastic composites reinforced with Entada Mannii fibre

Oluwayomi BALOGUN^1,2*, Joseph OMOTOYINBO^1,2, Kenneth ALANEME^1,2 and Caroline KHOATHANE³

¹ African Materials Science and Engineering Network (A Carnegie-IAS RISE Network)

² Department of Metallurgical and Materials Engineering, Federal University of Technology, Akure, PMB 704, Nigeria

³ Department of Chemical, Metallurgical and materials Engineering (Polymer Engineering division), Tshwane University of Technology, Staatsartillerie road, Pretoria, South Africa

E-mail(s): ¹yomdass@yahoo.com; ²ajibadeomotoyinbo@yahoo.com; kalaneme@yahoo.co.uk; ⁴khoathanec@tut.ac.za

^* Corresponding author, phone: +2348036362432

Received: November 30, 2016 / Accepted: June 13, 2017 / Published: June 30, 2017

Abstract

The mechanical properties and fracture mechanisms of thermoplastic composites reinforced with Entada mannii fibres was investigated. Polypropylene reinforced with 1, 3, 5, and 7 wt% KOH treated and untreated Entada mannii fibres were processed using a compression moulding machine. The tensile properties, impact strength, and flexural properties of the composites were evaluated while the tensile fracture surface morphology was examined using scanning electron microscopy. The results show that reinforcing polypropylene with Entada mannii fibres resulted in improvement of the tensile strength and elastic modulus. This improvement is remarkable for 5 wt% KOH treated Entada mannii fibre reinforced composites by 28 % increase as compared with the unreinforced polypropylene. The composites reinforced with Entada mannii fibres also had impact strength values of 70 % higher than the unreinforced polypropylene. However, the polypropylene reinforced with 5 and 7wt% KOH treated fibres exhibited significantly higher flexural strength and Young’s modulus by 53% and 52% increase as compared with the unreinforced polypropylene. The fracture surface of the polypropylene composites reinforced with untreated Entada mannii fibres were characterized by fibre debonding, fibre pull-out and matrix yielding while less voids and fibre pull-outs are observed in the composites reinforced with KOH treated Entada mannii fibres.

Keywords

Entada mannii; Polypropylene; Natural fibres; Polymer matrix composites; Fibre chemical treatment; Mechanical properties

Introduction

Over the past two decades, the use of plant fibres as reinforcements of polymer has been a topical issue which has continued to attract interest among researchers. They have been successfully utilized for technical applications in the automobile, construction, and aerospace industries [1-2]; and have reliably served as substitutes for synthetic fibre reinforcements [3]. Among natural fibres which have been investigated for suitability and competence as reinforcement in polymer matrix composites (PMCs) are jute, kenaf, flax, ramie, sisal and hemp [4]. These natural fibres are reported to possess good mechanical properties, low density, renewability and biodegradability [5]. They are also cheaply processed and have a high degree of flexibility compared to conventional synthetic fibres such as glass and carbon fibre [6]. Notwithstanding the potentials of natural fibres, fibre strength and strong adhesion of the fibres to the polymer matrix, and fibre degradability are areas where more work needs to be done. Natural fibre properties are reported to be influenced by many factors, including plant type and variety, growth conditions, the method used to extract the fibre bundle, and chemical treatment applied on the fibre [7]. The presence and distribution of lignocellulosic constituents in natural fibres are reported to also affect the fibre properties [8]. By nature, natural fibres are hydrophilic while the matrix is hydrophobic, thus creating an adhesion problem between both. As a result, optimal benefit of the goal of reinforcement may not be obtained if the fibres have poor adhesion to the matrix [9-10].

In order to improve the affinity between natural fibres and polymer matrices, use of either chemical or physical modifications on the surfaces of fibres and/or the matrix have been proposed [11-13]. The level of improvement in adhesion and interface bonding between fibre and matrix from these treatments are observed to show significant variation depending on the type of fibre. Hence several studies on the subject are still ongoing with different plant based fibres under exploration to ascertain the best suitable for reinforcement in polymer matrix composites (PMCs).

A natural bast fibre with potentials to serve as reinforcement of PMCs is Entada mannii plant fibres. It belongs to the family of Olive. Tisserant leguminous mermosae, liana plant. The plant is 2 to 3m high semi-climber which grows mainly in the tropical forests of Nigeria, Gabon and Madagascar [14].  Its erect trunk is comprised of anticlockwise- twisted pleats while its climber part comprises of hammock-like, twisted, woody stems [15]. The Entada structure also has its canopy spread from one support tree to another forming a long, leafless, cable-like stems (stolon’s) that navigates aerially approximately 15 m above the ground [16]. There are currently very limited literatures on the use of Entada mannii fibre as reinforcement in materials systems hence its consideration in this work.

This research work is aimed at assessing the mechanical properties and fracture mechanisms of thermoplastic composites reinforced with Entada mannii fibres suitable for interior of automobile construction.

Material and method

The materials utilized for this research are: Entada mannii fibre which was obtained from Ikare Akoko, Ondo state, Nigeria; Polypropylene-Homopolymer obtained from Safripol, South Africa; Maleic anhydride polypropylene MAAP which served as compatibilizers to improve the fibre-matrix adhesion; and Teflon sheet which was used as the releasing agent.

Extraction of fibre

Figure 1 show the Entada mannii plant and fibres extracted from the bark bundles by hand stripping. The hand stripped fibres were sun dried for a day after which they were dried in oven at 65⁰C for 2 days to remove moisture from the fibres.

(a)

(b)

Figure 1. (a) Distribution of Entada mannii plant containing the fibre, and (b) Extracted Entada mannii fibres

Fibre surface treatment

The Entada mannii fibres were immersed in 5% KOH solutions (which served as the chemical treatment solution) and placed in a shaker water bath at 50⁰C for 4 hours. The insoluble residue was DE lignified at pH 3 and washed with distilled water in order to remove mineral traces. In order to remove moisture from the fibres, they were dried in an oven at 60⁰C for 2 days. The treatment with KOH was for the purpose of removing hemicellulose, waxes, impurities and lignin from the surface of the fibres [17]. Some untreated Entada mannii fibres were also retained for control experimentation.

Composite Fabrication

Entada mannii fibres (treated and untreated) were chopped into 5 mm length as shown in Figure 2(a) and mixed with homo polypropylene which served as the matrix for composite production. An industrial granulator was used to process the mixture before drying in an oven at 65⁰C for 48 hours. The chopped Entada mannii fibre were weighed in 1, 3, 5 and 7 % and mixed with polypropylene .The mixture was compounded with 5% MAPP using a Twin-Screw extruder at processing temperature of range 190-230⁰C. A screw speed of 60 revolutions per minute was used to feed through the hopper of the extruder. The extrudate after cooling was granulated in an industrial granulator into pellets dimensioning 3 to 5 mm. The pellets were bagged in a plastic bag and placed in an oven to dry at 65⁰C for 2 days; after which they were compounded by hot a compression moulding machine. The compression of the composite was performed at 190⁰C and at pressure of 100 Mpa for 10 minutes. This process was necessary to help improve the dispersion of the fibres in the matrix. The composites produced were immediately transfer to the a cold press to cool down the composite at pressure of 50 Mpa for 15 min and the resultant sheet were of 150 mm by 150 mm by 2 mm in thickness for both untreated and treated fibre reinforced polypropylene composites (Figure 2b).

(a)

(b)

Figure 2. (a) Crushed treated Entada mannii fibres, (b) Entada mannii fibre reinforced composite

Tensile testing

Tensile test were performed on the composites produced using a universal tensile testing machine operated at a strain rate of 10 mm/min with 10 KN load cell. The sample preparation, testing procedure and determination of the tensile strength and tensile modulus were in accordance with ASTM D638 [18]. In each case six samples were tested and the average values were recorded to guarantee the reliability of the results obtained.

Impact Strength

The impact strength of the Entada mannii fibre composite was evaluated using an Izod impact test machine. The sample preparation and testing procedure were in accordance with ISO180 standard [19]. All the composite specimen were notched and the test specimen supported by a cantilever beam. Hammer head of 7.5 J was released with impact velocity of 2m/ to strike the face and break the notch specimens. Six specimens were tested at room temperature and the values were recorded.

Flexural strength

    Flexural testing commonly known as three-point bending test was also carried out on the composites using a universal tensile testing machine. The sample preparation and testing procedure were in accordance with ASTM D790 [20]. The test was performed by supporting the composite specimens on a beam and load was applied at the centre. The test was carried out at temperature of 23⁰C with a cross speed of 2 mm/min at a strain rate of 10 point/sec. 6 samples were tested and the results was documented.

Morphology analysis (Scanning Electron Microscope analysis)

The surface and fracture morphology of the treated and untreated Entada mannii fibre reinforced polypropylene composites were examined using a JEOL JSM-7600F model scanning electron microscope. The sample were placed in vacuum chamber, air dried and coated with 100 A thick irradium in JEOL sputter ion coater at 15KeV.

Results and discussion

Tensile Properties

The effects of KOH treatment and Entada mannii fibre wt% on the tensile properties and Young’s modulus of the composites produced are presented in Figure 3.

Figure 3a. Comparison of the tensile properties of the treated and untreated composite

From Figure 3(a), it is observed that reinforcing polypropylene with Entada mannii fibres resulted in significant improvement of the tensile strength in comparison with the tensile strength value of the unreinforced polypropylene. It is also observed that irrespective of the wt % of the fibre, the KOH treatment resulted in improved tensile strength compared with the composites reinforced with the untreated Entada mannii fibres. This can be attributed to the removal of lignin, hemicelluloses, and wax which are reported to weaken fibre/matrix interface bonding. Poor interface bonding facilitates fibre pull-outs from the matrix during tensile loading resulting in strength levels lower than expectations [7]. With 5wt% the results show that the tensile strength increases with increase in fibre wt % by 32% than unreinforced composites an attaining an optimum with 5wt% Entada mannii fibre. Further increase in fibre content to 7 wt % resulted in a drop in the tensile strength of the composite. This drop in tensile strength may be linked to increased fibre density resulting in reduced fibre/ matrix contact, poor fibre-fibre interaction, and less satisfactory dispersion of the fibres in the matrix.

The elastic modulus, a measure of the resistance of the composite to elastic deformation (Figure 3b) is observed to follow a trend similar to the tensile strength values of the composites (Figure 3a).

Figure 3b. Comparison of the Young’s modulus of the treated and untreated composite

It is observed that reinforcing polypropylene with Entada mannii fibres resulted in significant improvement of the elastic modulus with the effect more remarkable for KOH treated Entada mannii fibre reinforced composites. The results also show that the elastic modulus increases with increase in fibre wt% attaining an optimum with 5wt% Entada mannii fibre. The increase of the elastic modulus of the composites is attributed to the high elastic modulus of the fibres which on account of the good fibre/matrix bonding impacts relatively higher stiffness (resistance to elastic deformation stresses) on the composites compared to the unreinforced polypropylene [21].

Tensile strength and Young’s modulus for both treated composites and pure PP are presented in Table 1 and 2.

Table 1. Comparison table of tensile strength of KOH treated fibre composites and pure PP

Fibre variations	1	3	5	7
KOH fibres	32.1	33.3	37.6	32.6
Pure PP	25.5	25.5	25.5	25.5
% Increase	20	23	32	22

Table 2. Comparison table of tensile strength of KOH treated fibre composites and pure PP

Fibre variations	1%	3%	5%	7%
KOH fibres	1368.1	1425.5	1425.5	1490
Pure PP	1157.7	1157.7	1157.7	1157.7
% Increase	15	19	23	22

            It is revealed that tensile properties and Young’s modulus of the composites were improved by adding Entada mannii fiber. However addition of 5% wt fibre increases the tensile strength by about 32% as compared to the Pure PP and Young’s modulus increased by about 23% than the pure PP. When compare tensile strength and Young’s modulus of the composites, Tensile strength is more significantly increased than Young’s modulus by adding Entada mannii fibre. This is due to the fact that there is an improved interfacial adhesion between the fibre and the matrix and thus improve the tensile strength at the elastic region of the composites but fracture at higher extension near the ultimate strength.

Impact strength

The impact strength of the polypropylene composites reinforced with Entada mannii fibber is presented in Figure 4.

Figure 4. Comparison of the Impact strength of the treated and untreated composite

Impact strength is the capability of the material to withstand the suddenly applied loads in terms of energy. It measures the impact energy required to fracture a sample [22].The variation of impact strength of the unreinforced polypropylene, KOH treated and untreated Entada mannii fibre reinforced composites are presented in Figure 4. It is again observed that the composites reinforced with Entada mannii fibres had impact strength values significantly higher than the unreinforced polypropylene. The improvement in impact strength was more remarkable for the KOH treated Entada mannii fibre reinforced composites. The general improvement in impact strength is due to removal of surface impurities such as hemicellulose and lignin from fibre surface. It is also observed that peak impact strength was achieved with the use of 5 wt % KOH treated Entada mannii fibre as reinforcement just as was the case with the tensile properties of the composites (Figure 3).

Flexural strength and Flexural modulus

The Flexural strength and modulus results for the treated and untreated polypropylene reinforced composites and the unreinforced polypropylene are presented in Figure 5.

Figure 5a. Comparison of the Flexural strength of the treated and untreated composite

From Figure 5(a), it is observed that there is marginal difference in the flexural strength of the unreinforced polypropylene and polypropylene reinforced with 1 and 3 wt % treated and untreated Entada mannii fibres. However, the polypropylene reinforced with 5 and 7 wt % KOH treated fibres are observed to exhibit significantly higher flexural strength compared with the unreinforced polypropylene. For all wt % fibre reinforcement, there was no noticeable difference in the flexural strength of the unreinforced polypropylene and the untreated Entada mannii fibre reinforced composites. The invariance in flexural strength with increase in wt % of untreated Entada mannii fibre in the composite may be attributed to presence of surface impurities and flaws within the composite. Related observations have been reported by Munirah et al. [23].

 Figure 5b. Comparison of the Flexural modulus of the treated and untreated composite

The flexural modulus variation (Figure 5b) did not follow a consistent pattern although it can be observed that the KOH treated fibre reinforced polypropylene on the average had the best flexural modulus with peak value obtained with the use of 5 wt % KOH treated Entada mannii fibre.

SEM micrograph images of Tensile fractured samples

Figure 6. SEM micrographs for the unreinforced polypropylene

b

a

d

c

Figure 7. SEM micrographs for 1% (a), 3% (b), 5% (c) and 7% (d) untreated Entada mannii composite

a

b

dd

c

Figure 8. SEM micrographs for 1% a, 3% b, 5% c and 7% d treated Entada mannii composite

The SEM secondary electron images of the unreinforced polypropylene and the fracture surfaces of the composites subjected to tensile loading to failure are presented in Figures 6-8. Figure 6 shows the surface morphology of the unreinforced polypropylene which was used as control sample in the investigation. The structure shows the matrix phase is continuous and homogeneous. In the case of Figure 7, it is observed that the fracture of the polypropylene composites reinforced with untreated Entada mannii fibres are characterized by fibre debonding, fibre pull-out and matrix yielding. It is also noted that the fibre debonding is more pronounced for the 7 wt % fibre reinforced composite Figure 7(d). Fibre pull-out and debonding is attributed to the presence of fibre constituents such as lignin, hemicelluloses and wax on the fibre surface. These constituents contribute to poor wetting and interfacial bonding between the fibre and the polypropylene matrix by decreasing the interfacial area of contact between the fibre and matrix [24, 25]. Hence tensile strength of the Entada mannii untreated fibre composite was found to be lower than the composites reinforced with KOH treated Entada mannii fibres. Similar observation was reported by Venkateshwaran et al. [22].

From Figure 8, it is observed that a more homogeneous fibre-matrix composite structure is obtained by reinforcing polypropylene with KOH treated Entada mannii fibres. The SEM images show that there are less voids and fibre pull-outs in comparison with the composites reinforced with untreated Entada mannii fibres (Figure 7). This confirms that removal of impurities and fibre constituents by chemical treatment contributed to a strong interfacial bonding between the fibre and the matrix.

Conclusions

The mechanical properties of thermoplastic composites reinforced Entada mannii fibres was investigated. The results show that:

Reinforcing polypropylene with Entada mannii fibres resulted in improvement of the tensile strength and elastic modulus of the thermoplastic composites as compared with unreinforced and untreated composites. Impact strength was improved with the increasing fibre loading which is remarkable for the 5 wt % of the KOH treated fibre reinforced composites compared with unreinforced polypropylene. However, flexural modulus variation did not follow a consistent pattern although the KOH treated fibre reinforced polypropylene and on the average had the best flexural modulus.  Polypropylene reinforced with 5 wt % KOH treated Entada mannii fibre in all cases exhibited the best mechanical properties. The fracture surface of treated fibre reinforced composites revealed that fibres were well bonded to the polypropylene matrix while the untreated Entada mannii fibres reinforced were characterized by matrix yielding. Entada mannii fibre reinforced composites could be utilized for automobile interiors.

Acknowledgements

The authors wish to acknowledge the following organizations for their support: African Materials Science and Engineering Network (AMSEN), Regional Initiative in Science Education (RISE), Science Initiative Group (SIG); and Prototype Engineering Development institute (PEDI-NASENI), Ilesha, Osun State, Nigeria.

References

1. Ku H., Wang H., Pattarachaiya K.N., Trada M. A., A review on the tensile properties of natural fibre reinforced polymer composites, Composites Part B- Engineering, 2011, 42, p. 856.

2. Pickering K. L., Beckermann G. W., Alam S. N., Foreman, N. J., Optimizing Composites, Composites part A: Applied science and manufacturing, 2007, 38, p. 461

3. Murali M.R.K., Mohana R.K., Prasad, A.V.R., Fabrication and testing of natural fibre composites: Vakka, sisal, bamboo and banana. Materials and Design, 2010, 31, p. 508–513.

4. Saheb D.N., Jog J.P., Natural fiber polymer composites: a review. Advance Polymer Technology 1999, 18 (4), p. 351–63.

5. Beckermann G.W.,Pickering K.L., Engineering and evaluation of hemp fibre reinforced polypropylene composites: Fibre treatment and matrix modification, Journal of Composites: Part A, 2008, 39, p. 979–988.

6. Daiane R.H. Luiz O.J., Sandro C.A., Ademir J.Z., Preparation and characterization of Ramie-glass fibre reinforced polymer matrix hybrid composite. Engeharia de material Rod Washington. 2011, 4, p. 445-452.

7. Essabir H., Elkhaoulani A., Benmoussa K., Bouhfid R.,  Arrakhiz F.Z.,  Qaiss A., Dynamic mechanical thermal behaviour analysis of doum fibbers reinforced polypropylene composites, Materials and Design 2013, 51, p. 780–788.

 8. Lai S.M., Chen C.M., Preparation, structure, and properties of styrene ethylene – butylene–styrene block copolymer/clay nanocomposite: Part III. Effectiveness of compatibilizers. Euro Polymer Journal 2007, 43, p. 2254–64.

9. Arrakhiz FZ, Achaby M.E.I., Malha M., Bensalah M.O., Fassi-Fehri O.  Bouhfid R.K., Benmoussa K., Qaiss A., Mechanical and thermal properties of natural fibbers reinforced polymer composites: Doum/low density polyethylene, Materials and Design, 2013, 43, p. 200–205.

10. Alejandro M.P., Alex V.G., Pedro J., Herrera F., Effect of fibber surface treatments on the essential work of fracture of HDPE-continuous henequen fibber-reinforced composites, Polymer Testing. 2013, 32, p. 1114–1122.

11. Wielage B., Lampke T.H., Utschick H., Processing of natural-fibber reinforced polymers and the resulting dynamic-mechanical properties. Journal of Materials Processing Technology, 2003, 139, p. 140–6.

12. Karnani R., Krishnan M., Harayan R., Bio fibre - reinforced polypropylene composites. Polymer Engineering Science 1997, 37, p. 476–83.

13. Ngueho Y.M.C, Koubaa A., Cloutier A., Soulounganga P., Stevanovic T., Wolcott M.P., Effects of hot water treatment of raw bark, coupling agent, and lubricants on properties of bark/HDPE composites. Industrial Crop Production. 2013, 42, p. 50–6.

14. Niklas K.J., Plant Biomechanics, an Engineering Approach to Plant Form and Function, the University of Chicago Press, Chicago. 1992.

15. Salvatore R., Rita De P., Massimiliano F., Maria P.G., Rokia S., Antonio R., Micromorphological investigations on Entada africana Guill. et Perr. Mimosaceae, Il Farmaco 2001, 56, p. 361–363.

16. Ramesh M.K., Sankara R., Ramachandra T.V., Structural characteristics of a giant tropical liana and its mode of canopy spread in an alien environment. Current Science, 2009, 96, 1 p. 58-65.

17. Srinivasa C.V., Bharath K.N., Impact and Hardness Properties of Areca Fiber- Epoxy Reinforced Composites, India, Journal of Materials Environment. Science, 2011, 2(4), p. 351-356.

18. AlMaadeed M.A., Mabrouk O., Noorunnisa K.P., Effect of chain structure on the properties of Glass fibre/polyethylene composites, Materials and Design, 2013, 47, p. 725–730.

19. Khoathane C.M., The processing properties of natural fibre reinforced higher α-olefin based thermoplastics, submitted in partial fulfilment of the requirements for the degree, department of polymer technology Tshwane University of technology, Pretoria.2005, p. 5-30.

20. Asumani O.M.L., Reid R.G., Paskaramoorthy R., The effects of alkali–silane treatment on the tensile and flexural properties of short fibre non-woven kenaf reinforced polypropylene composites, Composites: Part A, 2012, 43, p. 1431–1440.

21. Mukherjee P.S., Satyanarayana K.G., Structure and Properties of Some Vegetable Fibres-Part 2 Pineapple Fibre (Anannus comusus), Journal of Materials Science, 1986 21, p. 51-56.

22. Venkateshwaran N., Elaya Perumal A., Arunsundaranayagam D., Fibre surface treatment and its effect on mechanical and visco-elastic behaviour of banana/epoxy composite, Materials and Design, 2013, 47, p. 151–159.

23. Munirah M., Abdul R.R., Azman H., Characterization and treatments of pineapple leaf fibre thermoplastic composite for construction application, Jabatan Kejuruteraan Polymer, Faculty Kejuruteraan Kimia dan Kejuruteraan Sumber Asli, University Technology Malaysia 2007, p. 2-60.

24. Saurav S., Mechanical behaviour of natural fibber based polymer composites, a thesis submitted in partial fulfilment of the requirements for the degree of bachelor of technology in mechanical engineering. 2011, p. 23-29.

25. Kabir M.M., Wang H., Lau K.T., Cardona F., Effect of chemical treatment on hemp fibre structure, Journal of applied surface science, 2013, 27(6), p 15-21.