Composites Part B

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Development of a biocomposite based on green epoxy polymer and natural cellulose fabric (bark cloth) for automotive instrument panel applications

Samson Rwawiire

^a^,^b^,^*

, Blanka Tomkova

^a

, Jiri Militky

^a

, Abdul Jabbar

^a

, Bandu Madhukar Kale

^a

aTechnical University of Liberec, Department of Material Engineering, Studentska 2, 461 17 Liberec, Czech Republic

bBusitema University, Department of Textile and Ginning Engineering, P.O Box 236, Tororo, Uganda

a r t i c l e i n f o

Article history:

Received 9 March 2015 Received in revised form 15 May 2015

Accepted 29 June 2015 Available online 15 July 2015

Keywords:

A. Fibers

A. Polymer-matrix composites (PMCs) B. Mechanical properties

D. Mechanical testing

a b s t r a c t

Naturalﬁber reinforced composites have attracted interest due to their numerous advantages such as biodegradability, dermal non-toxicity and with promising mechanical strength. The desire to mitigate climate change due to greenhouse gas emissions, biodegradable resins are explored as the best forms of polymers for composites apart from their synthetic counterparts which are non-renewable. In this study biodegradable bark cloth reinforced green epoxy composites are developed with view of application to automotive instrument panels. The optimum curing temperature of green epoxy was shown to be 120C.

The static properties showed a tensile strength of 33 MPa andﬂexural strength of 207 MPa. The dynamic mechanical properties, frequency sweep showed excellent ﬁber-matrix bonding of the alkali treated fabric with the green epoxy polymer with glass transition temperature in the range of 160Ce180C.

Treatment of the fabric with alkali positively inﬂuenced the mechanical properties of the fabric reinforced biocomposites.

1. Introduction

Interest infiber reinforced composite materials is on a gradual increase due to the fact that the addition offibers to polymer resins increases the mechanical strength of the resulting materials. Man- madefibers such as carbon and glass whose feedstock is fossil fuels are faced with global concerns with regard to greenhouse gas emissions which are responsible for climate change [1e3]. Ac- cording to the report on Global Natural Fiber Composites Market 2014e2019: Trends, Forecast and Opportunity Analysis[4], it was shown that by 2016, the natural fiber composites market is expected to be worth US 531.2 million with an expected annual growth rate of 11% for the nextfive years. Currently, natural fibers account to over 14% share of reinforcement materials; however, the share is projected to rise to 28% by 2020 amounting to about 830,000tonnes of naturalfibers[5].

Syntheticfibers whose feedstock is fossil fuel are the leading causes of environmental degradation due to the toxicity of the fumes emitted, demanding energy for production and non- biodegradability whereas natural fibers have advantages such as biodegradability, low cost, non-toxicity, sound absorption properties etc.[6]. Furthermore, the Intergovernmental Panel on Climate Change most recent report recommends cutting of greenhouse gas emissions by 70% and an increase of the use of clean green energy by 2050 respectively. Effective strategies such as utilization of sustainable biodegradable materials instead of synthetic materials can contribute to lowering greenhouse gas emissions thus combating climate change. In reference to European Union guide- line 2000/53/EG issued by the European Commission, 95% of the weight of a vehicle have to be recyclable by 2015[7]. Most plastics and syntheticfibers are faced with disposal concerns due to their resistance to microbial attack; piles of the disposed products which are ignorantly burnt in under-developed countries lead to increase in greenhouse gases and also a health risk to the consumers in the developing countries.

In the quest for a cleaner environment, waste materials can be re-used as reinforcement in engineering composites[8]. This has

* Corresponding author. Technical University of Liberec, Department of Material Engineering, Studentska 2, 461 17 Liberec, Czech Republic. Tel.: þ420 776427047;

þ256 776369920.

E-mail address:rsammy@eng.busitema.ac.ug(S. Rwawiire).

Contents lists available atScienceDirect

Composites Part B

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growing the crops. The life cycle analysis ofﬁbers has shown that ligno-cellulosic ﬁbers have an edge in terms environmental friendliness[17].

Bark cloth is believed to have originated in South China and the technology of extraction of bark cloth has been conﬁrmed by ar- chaeologists discovering grooved stones in Xiantouling site of Shenzhen similar to grooved hammers used today. It's believed that the extraction of bark cloth in ancient china spread to Taiwan, Philippines, Africa, Central America and Oceania. Bark cloth produced from Polynesia is derived from the bast of mulberry whereas in Uganda the felt is derived from Ficus natalensis, Ficus brachypoda and Antiaris toxicaria. Bark cloth has been in production in Uganda for over six centuries; the technology transfer of bark cloth production from the elderly to the youth has been impeded by rural to urban migration of the youth and inﬂuence to modernization. That notwithstanding, in 2005, UNESCO proclaimed bark cloth as a

“Masterpiece of the Oral and Intangible Heritage of Humanity”.

Bark cloth terracotta in colour, is composed of an array of cellulose microﬁbers, thermally stable below 200C just like other cellulosic

2.1. Materials

Bark cloth was extracted from F. natalensis using a method described by Rwawiire& Tomkova[18].Table 1shows the chemical composition of bark cloth. Biodegradable epoxy resin CHS-Epoxy G520 (viscosity ¼ 12.0e14.5 Pa at 25 C) and hardener Telalit 0600 supplied by Spolchemie, Czech Republic was used in composite sample fabrication. Green epoxy CHS-Epoxy G520 is a low molecular weight basic liquid epoxy resin containing no modiﬁers, certiﬁed by International Environmental Product Declaration Con- sortium (IEC).

2.1.1. Bark cloth extraction

Fig. 1shows the detailed process of production of bark cloth. The extraction of the naturally occurring non-woven starts with scraping off the surface layer of the trunk to expose the fresh raw bark using a sharp blade. The blade is held at an angle such that only the surface layer is removed and also avoids damaging the tree

Fig. 1. Extraction of Bark cloth nonwoven natural fabric: (a) Scrapping of tree outer layer. (b) Use of local wedged tool to peel off the bark. (c) Peeling of the bark. (d) Covering of the tree stem for environmental sustainability. (e) Pummelling under the shade. (f) Sun drying of the non-woven fabric.

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and fresh bark as shown inFig. 1a. A ring is then cut with a knife on both ends of the scrapped stem that reﬂected the length of bark cloth that is to be produced. At the same time, a vertical slit is made from the top of the stem to the bottom. With the help of a wedged tool locally known as ekiteteme, carved out of the innermost part of a banana stem, the bark is easily peeled off starting from the base slowly moving upwards (Fig. 1b and c).

For environmental sustainability, the debarked stem is wrapped with banana leaves, (Fig. 1d) which acts as bandages to prevent dehydration; these are usually removed after a week, giving way for growth of fresh bark. The extracted bark is then subjected to heat as a means of softening prior to pummelling process which utilizes different well designed wooden grooved hammers. Pummelling is usually done under a shade to prevent direct sunrays from creating creases in the bark cloth (Fig. 1e). After pummelling, the bark cloth is sun-dried for 3 h every day for 6days giving it a rich deep ter- racota colour and then re-pounded to smoothen the cloth surfaces.

Drying involves stretching the wet fresh bark cloth using heavy loads at its perimeter to retain its dimensions on drying (Fig. 1f).

2.1.2. Bark cloth alkali treatment

Bark cloth fabrics were treated in 5% alkali solution at room temperature for one hour. After treatment, theﬁbers were rinsed with running water and oven dried at 80C.

2.2. Bio-epoxy resin characterization

The bio-epoxy resin curing behaviour was characterized by using the Perkin Elmer Differential Scanning Calorimeter DSC6. Using thermal analysis, it was possible to deduce the optimum curing temperature where fast polymer cross-linking takes place[20]. A small drop of resin: hardener (100:32 weight%) weighing approximately 7 mg was placed in aluminium crucible and subjected to a heating rate of 10 C/min from 25 C to 400 C in an inert environment.

2.3. Composite fabrication

The composite specimens were prepared using the hand lay-up method. Bark cloth fabrics (Fig. 2a) were impregnated with green epoxy resin and placed in a fabricated mould (Fig. 2b). The mould was treated with a mould release agent and thereafter Teﬂon sheets Fig. 2. Biocomposites processing: (a) Bark cloth fabrics. (b) Fabricated composite

mould.

Fig. 3. Tensile composite specimens and tensile testing rig.

Fig. 4. Flexure test of the biocomposites.

Fig. 5. Curing behaviour of the green epoxy polymer.

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were applied to aid the fast removal of cured composite specimens.

The resin to hardener ratio was maintained at 100:32 as per the manufacturer's speciﬁcations. Curing of the composites was done using a hot air oven for 45 min. For each set of composites, four untreated bark cloth plies with ply angles 90, 0,45, 45were utilized for biopolymer reinforcement.

2.4. Characterization methods

2.4.1. Morphology

The bark cloth surface and composite fracture morphology were investigated using a Vegas-Tescan Scanning Electron Microscope with accelerating voltage of 20 KV.

Fig. 6. (A) Fabric morphology. (B) Transverse section of fabric. (C) Biocomposites tensile fracture surface morphology. (D) Biocompositeﬂexure fracture surface morphology.

Fig. 7. Surface functional groups of fabric and biocomposites.

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2.4.2. Fourier transform infrared spectroscopy (FTIR)

Nicolet iN10 MX Scanning FTIR Microscope was used to provide the spectrum of the sample. The infrared absorbance spectrum of each sample was obtained in the range of 4000e700 cm¹.

2.4.3. Thermal properties

The Perkin Elmer Differential Scanning Calorimeter DSC6 was used. Samples weighing approximately 10 mg were placed in aluminium crucibles and sealed. The specimens were heated in nitrogen atmosphere from room temperature 25C to 400C at a heating rate of 10C/min.

2.4.4. Mechanical properties

Tensile strength properties of fabric-reinforced composite samples were characterized in accordance with ASTM D3039 on a Testometric (M500-25 kN) universal mechanical testing machine

operating at a crosshead speed of 4 mm/min (Fig. 3). Four specimens were tested to obtain average tensile properties of the composite.

Flexural test was conducted as per ASTM D790 using a Tiratest 2300 (Fig. 4). Four samples were tested using three point bending (3 PB) with a recommended speed of testing of 2 mm/min. The span length to thickness ratio was 32:1.

Four Charpy impact test rectangular specimens with dimensions 80 mm10 mm34 mm were tested according to ISO 179 using LaborTech 2.050 (maximum load of sensor: 5 N)

2.4.5. Dynamic mechanical properties (DMA)

The DMA was carried out on a DMA 40XT machine. The samples with dimensions 56 13 2.5 mm were tested in a 3 PB mode using a frequency scan from room temperature to 150 C at a heating rate of 3C/min.

Fig. 8. Thermal behaviour of fabric and biocomposites.

Fig. 9. Thermogravimetric behaviour of biocomposites.

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3. Results and discussion

3.1. Curing behaviour of the bio-epoxy

The curing behaviour curve of the bio-epoxy resin as obtained from DSC is shown inFig. 5. The selected temperature for the curing process of bark cloth biocomposites was 120C which is near the temperature peak of the curing curve.

The curve shows only one exothermic peak, which is attributed to the cross-linking reaction between the green epoxy polymer and the hardener. The reaction starts at a temperature of approximately 55C and ends at about 200C. The peak temperature of the curve indicates the maximum cross-linking temperature or fastness of the curing reaction that was obtained at 123C. For optimization of the curing of the resin, the oven temperature was therefore set at 120C and the samples baked for 45 min. The selected temperature of 120C ensures maximum cross-linking within a short period of time less than an hour. The 45 min was chosen based on the fact that from room temperature to the maximum cross-linking temperature obtained from the curing curve, the virgin resin-hardener mixture took 12 min, therefore introduction of the reinforcing fabric means that baking from 30 to 45 min is sufﬁcient for efﬁcient bond formation and cure of the resin. The exothermic peak at 123C, released heat of 373 J g¹which is higher than the petroleum based epoxy resins exothermicity of 200 J g¹. This implies that the curing of renewable green epoxy is higher than that of petroleum based epoxy resins at room temperature[20,21].

3.2. Morphology of biocomposites

The fabric morphology is made up of a dense network of microfibers that are naturally bonded and aligned at angles, (Fig. 6a). The inter-fiber bond structure gives the strength of the load bearing microfibers and damage is initiated through separation of the in- dividualfibers through failure of the inter-fiber bond and thence

absorbance peak of the biocomposite at 3372 cm and 2900 cm show that the green epoxy polymer effectively cross-linked with the OH-cellulose chains which are activated through alkali treatment of the bark cloth fabric.

The peaks at 2929 cm¹and 2873 cm¹ correspond to sym- metric and asymmetric stretching vibrations of CH and CH2groups.

The increase in the absorption peak of the biocomposites above that of the fabric is due to the combination of the CH and CH2

groups in the green epoxy polymer and the cellulose and hemicelluloses in the fabric[25]. The increase in the absorbance band at 1750 cm¹of the green epoxy polymer is attributed to the presence of carbonyl groups while benzene ring stretching of Lignin is at 1630 cm¹.

3.4. Thermal behaviour of biocomposites

Thermal degradation of naturalfiber components is dictated by the supramolecular structure of the cellulosic materials[26]. The composites and fabric behaviour as characterized by DSC (Fig. 8), shows an endothermic peak starting from 20C to 120C centred at around 52C. This peak is characterized by the removal of adsorbed moisture from the fabric. As seen fromTable 1, bark cloth is majorly made up of cellulose therefore; its affinity to moisture is high since cellulose is hydrophilic in nature. Studies with NMR have shown that moisture is concentrated in the amorphous or non-crystalline regions of cellulose[27]. Therefore the endotherm at 52C corre- sponds to the amorphous component of cellulose in bark cloth. The peak at 140C is attributed to the decomposition of paracrystalline molecules of pectin and hemicelluloses in the bark cloth[28]. The levelled behaviour of biocomposite confirms that the selected curing temperature of 120C for 45 min was sufficient for cure. TGA is a useful technique for the study of the thermal behaviour of composite materials. Thermal stability of the polymer and reinforcing materials is an important parameter because manufacturing of composites in most cases requires curing;

therefore the degradation behaviour of the reinforcingﬁbers helps in selecting the processing temperature and also the working temperature of the developed composite materials. Thermal stability behaviour of the reinforcing bark cloth, virgin resin and biocomposite was studied at a heat rate of 10C/min in a nitrogen atmosphere as shown in Fig. 9. As it is seen, there was no Fig. 10. Typical Load-Strain behaviour of the biocomposites.

Table 2

Mechanical properties of biocomposites.

Composites Polymer Tensile

strength [MPa]

Tensile modulus [Gpa]

Elongation at break [%]

Flexural strength [Mpa]

Flexural modulus [Gpa]

Impact strength [MJ]

Bark cloth biocomposites Green epoxy 33 3 2.1 207 1.4 5.73

Bark cloth composites[19] Synthetic epoxy 30 4.1 1.8 153 3.1 9.62

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degradation taking place until 290C, above this temperature, the thermal stability of the biocomposites tremendously decreased and eventually decomposition of the biocomposites occurred at 350C due to the decomposition of cellulose in bark cloth. The TGA curve of the green epoxy resin shows a gradual weight loss with increasing temperature which started around 300C. The thermal analysis has illustrated that bark cloth biocomposites are stable until 290 C, a crucial intrinsic temperature that is important if other serial production techniques such as compression moulding are to be used with thermoplastic resins.

3.5. Mechanical properties of biocomposites

Fig. 10illustrates the typical load-strain behaviour of the tested tensile samples andTable 2shows the tensile andﬂexural properties of the composites. All the composite samples had a similar behaviour which was nonlinear due to the highly anisotropic structure of bark cloth fabrics. The physical structure of bark cloth

fabric is composed of microﬁber, it's therefore important to have a stacking sequence that will be beneﬁcial for composite applications [16]. In this study, the composites had ply orientations of 90, 0,

45, 45due to the fact that in the investigation of the effect of layering pattern of bark cloth composites using a synthetic epoxy polymer, it was shown that the stacking sequence of bark cloth with orientation 90, 0,45, 45was ideal and had higher mechanical properties[19]. The modulus of a composite material is dependent on the reinforcingfiber properties, whereas the tensile strength is a function of the matrix properties[2]. Failure by tensile was through matrix failure and the disintegration of the non-woven structure through the tensile forces. The entangled microfiber web of the fabric has natural bonds holding the microfibers together. The biocomposite samples experienced three modes of failure: the brittle failure of the epoxy polymer matrix; matrix cracking and fiber fracture (Fig. 3c). Damage of the non-woven structure is triggered by the inter-fiber bond structure; re-arrangement of the fiber network and reloading and finally fiber fracture[29].

Fig. 11. Behaviour of biocomposite storage modulus.

Fig. 12. Variation of damping factor against temperature.

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The developed biocomposites had an average strength of 33 MPa higher than the strength obtained using synthetic epoxy.

The percentage elongation of the biocomposites was higher than the synthetic composites. This is attributed to the green epoxy polymer properties, however the variability of the reinforcing material is observed with the low modulus of the biocomposites owed to the treatment with alkali that dissolved impurities. Sapuan and Abdalla[30]showed that a tensile strength of atleast 25 MPa is needed for composites developed for car dashboard panels. Bark cloth reinforced green epoxy composites are therefore an alternative material for secondary indoor automotive structures such as car dashboard panels. Theflexural strength of the developed green epoxy biocomposites was 207 MPa higher than the untreated and synthetic composites. This was due to the effectivefiber to matrix adhesion owed to the alkali treatment. During the three points bending, the upper and lower laminate surfaces are loaded with tension and compression forces respectively, whereas the axisym- metric plane is subjected to shear. Therefore failure duringflexure is achieved when theflexural and shear stress reach a critical value [31]. Charpy impact tests yield 5.73 MJ of energy which is much lower than the value obtained with synthetic epoxy polymers.

3.6. Dynamic mechanical properties

In order to assess the performance of structural applications, the dynamic mechanical properties help in material evaluation so as to understand the viscoeslastic behaviour of the material against temperature, time and frequency. Two parameters: storage modulus (E⁰) and damping factor (tan d) were obtained over a temperature range from 30C to 250C and were used to predict and control composite processing behaviour[32].

Fig. 11shows the variation of the storage modulus with temperature at three scan frequencies of 0.1, 1 and 10 Hz. The storage modulus shows the stiffness of the composites against temperature. It's observed that the storage modulus generally decreases with increasing temperature. Addition of reinforcement to the epoxy polymer greatly enhanced the dynamic mechanical properties increasing the storage modulus from 2.6 GPa of virgin resin to 5.1 GPa of biocomposites at 30 C. The high value of storage modulus of biocomposites is attributed to the reinforcement. Under

loading, the polymer chains move about and are re-arranged, with the addition of the bark cloth, the mobility of the polymer chains is greatly reduced. A sudden fall of the modulus of the composites was observed at 130C which is marked by a sharp decrease in the storage modulus until to around 450 MPa at 225C. As the composite approaches the glass transition temperature, there's a sudden decrease in the storage modulus attributed to the free molecular movement of the polymer chains. Polymer viscoelastic behaviour is a function of time, frequency and temperature. A frequency scan showed that the storage modulus increases with increase in the frequency. So the modulus at 10 Hz (Short time) is higher than the modulus at 0.1 Hz (long time).

The variation of tand against temperature, (Fig. 12) aides in obtaining the glass transition temperature. It's observed that the Tg obtained by the damping factor curve was 163C, 170C and 185C for 0.1 Hz, 1 Hz and 10 Hz respectively. The Tg increases to a higher temperature as the analysis frequency increases[33]. Beyond the glass transition temperature, the biocomposite transitions from glass to rubbery state due to the high mobility of the polymer molecular chains. The virgin epoxy polymer exhibited a sharp and intense peak centred at 175C because there is no restriction to the polymer molecular chains at the glass transition temperature. The source of crack initiation is usually weak ﬁber to matrix bond interface; therefore higher energy is dissipated than strong in- terfaces. The high tandpeak therefore shows green epoxy polymer is viscous when loaded compared to the reinforced composites [34,35].

The dynamical mechanical properties have therefore shown that from 30C, the optimum temperature range of application of biocomposites is up to130C. Beyond 130C, the composites enter into a rubbery state and the performance is diminished.

4. Conclusions

For the ﬁrst time, biodegradable bark cloth reinforced green epoxy biocomposites have been developed for the possible application in interior automotive panels. The static mechanical properties show that alkali treated biocomposites had a tensile strength 33 MPa and modulus of approximately 4 GPa. Theﬂexural strength of the composites was 207 MPa. The biocomposites exhibited glass Fig. 13. Scheme of processing of bark cloth biocomposites for automotive applications.

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transition temperature in the range of 163Ce185C depending on the frequency. The developed biocomposites with an average strength of 33 MPa higher than the 25 MPa threshold strength needed for car instrument or dashboard panels make bark cloth reinforced green epoxy composites an alternative material for interior automotive panels (Fig. 13).

Acknowledgements

Theﬁrst author is grateful to God for life given to carry out the research work. He is also grateful to Association of African Uni- versities for“Small Theses & Dissertation Grant” through Busitema University. The authors acknowledge Ing. Jana Mullerova, Ph.D., of the Institute for Nanomaterials, Advanced Technologies and Inno- vation, TUL, for providing the FT-IR spectra.

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