Evaluation of Bast Fibres of the Stem of Carica papaya L . for Application as Reinforcing Material in Green Composites

Aims: The production of green composites based on natural fibres is rising with regard to increasing environmental problems and declining fossil raw materials. Bast fibres of papaya (Carica papaya L.) accumulate on plantations at a large scale but remain an unused resource. The characterisation of the bast allows a first evaluation of the potential of papaya-fibres for use in composites. Study Design: Material testing. Place and Duration of Study: Institute for Botany, Technische Universität Dresden and Original Research Article Kempe et al.; ARRB, 6(4): 245-252, 2015; Article no.ARRB.2015.082 246 Fraunhofer Institute for Nondestructive Testing Dresden (IZFP-D), 2011 and 2013. Methodology: The anatomic structure of fibre cells and the microfibril angle of the cell walls were determined as well as a chemical analysis to determine the proportion of cellulose, lignin and hemicelluloses of the fibre cells. In addition, samples of fibres were subjected to static uniaxial tension tests revealing Young's modulus, tensile strength and breaking strain at different plant ages and of two origins. Results: Fibres of a two-year-old plant exhibited a Young's modulus of 10.7 GPa, a tensile strength of 101 MPa and a breaking strain of 1.2%, on average. Fibres from six-months-old plants, grown under greenhouse conditions had a mean Young's modulus of 4.4 GPa, tensile strength of 49 MPa and a breaking strain of 1.4%. Having one of the lowest fibre densities with ca. 0.85 g/cm3, papaya fibres exhibit noteworthy specific mechanical properties among all studied natural fibers. Conclusion: These data allow us a first estimation for a potentail use in green composites as reinforcing material.


INTRODUCTION
Composite materials are increasingly important for the production of light and stiff constructions for various applications.However, apart from the favourable mechanical properties, composites have some shortcomings.Primarily, the recycling of components turns out to be difficult and a large amount of composites end up in dumps or incinerators.Even though the energy value of the material is used, there is still a contribution to CO 2 emissions and environmental pollution [1].Apart from that, the non-renewable resource mineral oil has to be used for some matrices and fibres, such as polymer matrices or carbon fibres.Therefore, alternatives are being highly looked after in view of dwindling resources and the increasing ecological awareness.
Plant fibres offer some advantages over synthetic materials.They provide carbon neutrality when being burned, which is highly relevant with regard to ecological problems.At the same time, they are less abrasive to production machinery due to their flexibility and perform as good acoustic and thermal isolators.Plant fibres are usually of low density and the costs of the fibres (on a volumetric basis) are low [1,2].Still, there are disadvantages, especially the high variability of mechanical properties depending on the conditions of growth (nutrition, exposure to wind and sunlight) and the age of the plant [1,2].Furthermore, the hydrophilic character of natural fibres may severely effect the material properties.Due to swelling of fibres upon moisture uptake and a reduction of the binding of fibre and matrix can be reduced, stiffness may decrease or the overall geometry may be altered.Additionally a maximum processing temperature must not be exceeded to avoid damage to the fibre [1].However, there is a broad field of application for bio-composites, for example in the production of casing structures or in automobile and packaging industries, as not for all components the mechanical properties of advanced composites are needed [1].Precondition for the use of natural fibres in composites is a deep knowledge of their mechanical properties.The characterisation of fibres is the first step to evaluate their potential for use in composites.Knowledge of Young's modulus, strength and breaking strain allows the selection of suitable matrix material in order to achieve appropriate material properties of the composite.Mechanical properties of various natural fibres as flax, hemp, jute, sisal and more are already investigated and comprehensively summarised by Faruk et al. [2] and Muessig [3].
So far fibre properties of Papaya, another interesting resource of natural fibres, have not been studied yet.Papaya plants may reach an age of 20 years and do usually not branch [4].Characteristically, the secondary xylem remains completely parenchymatous [5,6,7].Despite the lack of wood, individual plants grow up to nine metres [5,7,8]; hence papaya is called a giant herb [4].Lignified fibres occurring in the bark (secondary phloem) are the only reinforcing structures interacting with the turgor pressure [9].The fibres form a complex lattice like mesh, which again is filled with parenchyma (Fig. 1).The small amount of fibres compared to the total weight of the stem arouses interest as a model for light-weight applications and structures.Papaya plants were cultivated primarily due to the palatable fruits.Main producers in 2012 have been India (5.16 million ton), Brazil (1.52 million ton) and Indonesia (0.91 million ton).The world's total yield in 2012 accounted for ca.12.4 million ton [10].Papaya may therefore be source for fibres as by-product after have been harvested, such as pineapple, palm and coir.Since the papaya fruit maximum in the first three years, replaced mostly after 3 -5 years.fibre material is available in a large usually discarded.Further processing material in composites could be source of income for the producer.only idea for an application was the use papaya fibres as biosorbent to remove metals from water [11,12].

Materials
In a botanical definition a single cell fibre when the length is much larger diameter.In fibre technology usually individual fibre cells are processed.simplification, the term fibre is used bundles of cells that in fact are lamellae.
We put emphasis on experimentally papaya fibre's Young's modulus, tensile and breaking strain by static tension for mechanical analyses were taken from the base of plant between 0 and 10 cm above an interesting after the plants pineapple, oil fruit yield is at a years, plants are years.At that point large scale that is processing of fibre an additional producer.So far, the suggestion to remove heavy

METHODS
cell is termed as larger than the usually bundles of processed.For used here for lamellae.
experimentally determining tensile strength tension tests.Fibres for mechanical analyses were taken from the base of plant between 0 and 10 cm above ground.The fibre mesh was isolated by maceration in water.After one month of immersion non-lignified cells could easily be removed from the fibre mesh.Subsequent fibre mesh was rinsed and cleaned with water and kept in water until testing.The mesh was cut in individual samples of ca. 3 cm in length and 0.1 -1.1 mm width using a scalpel and dried at room temperature before testing.Fibre have been compared from plants of sources: 1. a commercial plantation and 2. plants, grown in a greenhouse botany institute of the TU Dresden.samples were taken from a two year plantation (Carica papaya L. 'Red November 2011.The plant had a metres and a diameter of ca. 10 cm.Carica papaya L. 'Red Lady' plants from seeds in the greenhouses of April 2011.Samples for mechanical taken from two of these plants, aged and with heights of ca.one metre diameter of about one centimetre.another three plants were used investigations.

Anatomy
Fibre dimensions were analysed using Electron Microscopy (SEM).Fibre measured based on cross sections plants.Cell dimensions were calculated pixel number corresponding to cell width and a calibration image standardised scale with 1 µm GIMP, an open-source image editing Wide-angle X-ray diffraction (WAXD) used to measure average microfibril of the cell wall according to Lichtenegger [13].

Density, Contents of Hemicellulose and Lignin
The density of fibres was determined to DIN EN ISO 1183-1 using a precision and a pycnometer with an accuracy 0.05 ml respectively.Cellulose, and lignin content were determined Kuerschner [14], Poljak [15] and Details of the extraction process by Bremer [17].

Mechanical Tests
All specimens were subjected to using a dynamic mechanical analyser ground.The fibre mesh was isolated by maceration in water.After one month of lignified cells could easily be removed from the fibre mesh.Subsequently the fibre mesh was rinsed and cleaned with water and kept in water until testing.The mesh was cut cm in length and mm width using a scalpel and dried at Fibre properties of two different plantation in Nicaragua greenhouse at the Dresden.In Nicaragua, year old papaya Red Lady') in a height of two cm.In Dresden plants were grown the institute in mechanical testing were aged six months metre and with a centimetre.Samples of for anatomical using Scanning ibre cells were sections of three calculated from the cell length and image taken of a scaling, using editing program.(WAXD) has been microfibril angles (MFA) Lichtenegger et al.

Cellulose,
determined according precision balance accuracy of 0.01 g and hemicellulose, determined according to and Klason [16].are described to tension tests analyser (DMA TA instruments Q800) at the Fraunhofer Institute for Non-destructive Testing Dresden (IZFP-D).Experiments were recorded with an force sensitivity of 0.01 mN and a displacement accuracy of 1 nm.The samples were strained at a rate of 1 N/min to reduce speed dependent influences.Since the fibres have a sheet like cross sectional area, the width was measured using a microscope at 25 -50 fold magnification.The thickness was measured using a vernier calliper.Stress-strain diagrams were compiled using the program TA Universal Analysis 2000.Tension stiffness and Young's modulus were calculated from the slope of the initial linear part of the stress-strain curve.Additionally ultimate tensile strength (maximum stress that a material can withstand before breaking) and breaking strains (maximum strain when failure occurs) were determined.

Data Analysis
The obtained data were analysed by Mann Whitney U-test comparing differences of morphological and mechanical results with regard to origin of the samples.P values less than 0.05 were considered significant.Spearman rank correlation was used to check any correlation of morphological and mechanical results, i.e. whether morphological differences cause mechanical differences.

Anatomy
Fibre bundles contained between 15 and 130 individual fibre cells.The cells are polygonal in cross section and had a maximum diameter of up to 30 µm and a mean length of 1.1 (±0.2) mm.Two cell-wall layers enclosed the lumen; the thin primary and the secondary cell wall, with a total thickness of ca.2.1 (±0.4) µm.Cross-sectional areas of the fibres differed significantly between 0.103 (±0.06) mm² for samples from the plantation and 0.075 (±0.04) mm² for those from the greenhouse.The microfibril angle (MFA) of the secondary cell wall of plantation and greenhouse samples accounted for 8° -10° and 9° -20° respectively.See Table 1.

Density, Contents of Cellulose, Hemicellulose and Lignin
The samples originated from the same plants that were used for mechanical tests.A total amount of 3 g (greenhouse) and 6 g (plantation) fibres were used.The density of fibres differed between 0.84 (±0.09) g/cm³ (greenhouse) and up to 0.86 (±0.07) g/cm³ (plantation) (see Table 1).Furthermore, 10 g per plant was the minimum to determine the contents of cellulose, hemicellulose and lignin.Fibres from plantation and greenhouse exhibited nearly identical contents of lignin, cellulose and hemicellulose: 20.3%, 50.5% and 29.4% (greenhouse) as well as 20.3% 52.8% and 29.1% (plantation).

DISCUSSION
The obtained results serve as a first characterisation of the mechanical properties of papaya phloem fibres.A comparison of papaya fibres with those of other plant derived fibres shows, that Young's moduli of fibre samples from plantations (almost 11 GPa) are in the range of abacá (12 GPa), curauá (11.8 GPa) or bamboo (11 -17 GPa) [2].These values are distinctly lower than synthetic fibres or flax  and hemp (17 -70 GPa) fibres [2,18,19,20].The tensile strength of papaya fibres (101 MPa) is rather low in comparison to other plant fibres.Similar low values of tensile strength could be found for bamboo (140 -450 MPa) and coir (131 -250 MPa) [2,21,22,23].One reason for low breaking stresses can be short fibre cell lengths.
Both bamboo and coir have fibre cell lengths of 2.9 mm and 0.8 mm respectively [24,25], compared to papaya fibre cells with ca 1.1 mm.Indeed, fibre cells with greater lengths; for example flax ca.20 mm and hemp ca 23 mm [26] show higher breaking stresses [27,28].Also values of breaking strain (1.2%) are lower than other natural fibres.The nearest value is 1.6% for hemp, jute and kenaf, respectively [2].The Young's modulus of fibres from greenhouse plants (ca 4.4 GPa) is comparable to coir (4 -6 GPa) and oil palm fibres (3.2 GPa) [2,22].Tensile strength (49 MPa) is well below all other fibres while the breaking strain (1.4%) again corresponds to hemp, jute and kenaf [2].
Even though it seems that papaya fibres represent a comparatively weak material, we found that the density of papaya fibres is low with about 0.85 g/cm³.High Young's modulus in fibres is connected to a much higher density as shown for hemp and jute 1.3 -1.5 g/cm³ (Table 2).Considering now the specific Young's modulus of these fibres we notice that papaya fibres range in the same sphere of lowest given values of specific modulus of high-modulus fibres.
Because of this particular low density papaya fibres could be of interest for applications in green composites.Calculating a specific tensile strength, papaya fibres remain still below average (Table 2).
Comparing fibres with regard to the origin of our investigated plants shows that each of the mechanical properties, Young's modulus, tensile strength and elongation at break, varies significantly.The plants, however, grew under completely different environmental conditions regarding wind load, solar radiation, and day length or soil quality and volume.Especially due to the lack of external loads such as wind in a greenhouse and the absence of fruits, the difference in mechanical properties is hardly surprising.However, the variation appears most likely with increasing age.The greenhouse plants had an age of six months, whereas the Nicaraguan plants were about 2 years old.One important criterion is the microfibril angle (MFA), which is known to change with the age of a plant -for trees.Higher MFAs lead to lower Young's moduli and larger breaking strains whereas lower MFAs have the opposite effect [29].Fibres of young individuals which need to be flexible often have larger MFAs [30].Older plants need to be stiffer to upright their organs and therefore possess fibre cells with smaller MFAs [30].Our observations indicate that MFA of C. papaya adapts likewise, since Young's moduli increase with age of plants.
Lignin, cellulose and hemicellulose contents were almost identical when comparing the fibre origin.A high variability of mechanical properties can thus be a result of irregular cell geometry and fibre cross-sectional area [31].Natural fibres have a lumen, which is usually not subtracted from the cross-sectional area of the fibre.Fibre diameter and lumen size are linked, so that the higher the fibre cross-sectional area, the higher the discrepancy in the calculation of Young's modulus and tensile strength.As a consequence the calculated Young's modulus and tensile strength is lower than the actual value would be.
That is demonstrated by the fact that Young's modulus and cross-sectional area as well as tensile strength and cross-sectional area correlate significantly in our results (Table 3).
We investigated the potential of papaya fibres for use in green composites, fibre boards, etc.only from the view of mechanical properties of the fibres.Other attributes such as processing, response to moisture, thermal or insulating properties might be subject of further investigations.The compatibility of fibre and matrix material concerning the elastic and fracture behaviour is crucial for a composite.In case of tension loads of the composite, the ratio of breaking strain of the fibres to that of the matrix is recommended to be 1:3 at least.Since tension forces are taken up mainly by fibres, the respective Young's moduli should be larger than those of the matrix.For a first estimation considering fibres derived from plantation plants, Young's modulus of the matrix should be smaller than 11 GPa and breaking strain should be larger than 3.6%.For example vinyl ester resin (Young's modulus = 3.4 MPa, breaking strain = 3.5 -7.0%) or epoxy resin (Young's modulus = 2.8 -3.6 MPa, breaking strain = 6 -8%) would represent suitable matrix materials [32].
Characteristic values such as Young's modulus, tensile strength and breaking strain exhibit a high variability of materials properties.However, biological materials characteristically show a broad variation in acquired properties with regard to location-dependent conditions, such as temperature, wind exposure or nutrient supply, as well as age of used plants.It should be the aim of further research to verify the impact of these factors.Plantations are renewed every 3 -5 years for economic reasons, so that age of plants already is limited.Reasonable applications for these composites might be components subject to low mechanical stresses, e.g.interior parts in automotive industries (panels in car doors) or in package industries (housings).The use of entire fibre mats as design elements emphasizing its peculiar fibre arrangement (see Fig. 1B) is conceivable, too.Since papaya plants were cultivated for fruits fibre material accumulates in a huge amount on plantations, which is still unused.A rather conservative estimate reveals 1.2 million tons of fibre material in 3 -5 years.A current area of commercial plantations summing up to about 406,000 ha in 2011 [10] is taken as basis, calculated with one plant per 10 m² (estimated) and ca. 3 kg fibres per stem estimated from 5% fibre content per stem [9].Although not all of the accruing fibres are usable, the remaining material would represent a substantial amount bearing in mind that the world production of jute, sisal, and flax are 3.6, 0.4, and 0.2 million tons, respectively, in 2011 [10].

CONCLUSION
A high variability existed in the investigated material properties Young's modulus, tensile strength and breaking strain.Plant age, growth conditions and the mode of fibre extraction most likely account for that.Compared to mechanical properties of other natural fibres Young's modulus, tensile strength and breaking strain are average.Otherwise papaya fibres exhibit noteworthy specific Young's modulus due to low fibre density.Fibre material accumulates in a huge amount on plantations, which could be an easy accessible and cheap source for further utilisation.Based on investigation of material properties papaya fibres provide a new opportunity of application in the field of green composites.For this purpose pursuing research is necessary dealing with fabrication and testing of first composite prototypes made of papaya fibres.

Fig. 1 .
Fig. 1.Detail of papaya stem's cross section (A) and fibres after removal of parenchyma in longitudinal view (B)