Evaluation of linden fibre as a potential reinforcement material for polymer composites

Abstract

The aim of this study is to characterize linden fibres as a novel cellulose-based fibre to be used as a reinforcement material in composites and to investigate the adhesion property to unsaturated polyester. Up to now, there is no report regarding the potential usability of linden fibre in composite applications. Linden fibres were extracted from the stem of a plant of Tilia rubra DC. subsp. caucasica (Rupr.) V.Engl. Characterization of linden fibres was studied by Fourier transform infrared, X-ray photoelectron spectroscopy, thermogravimetric analysis, X-ray diffraction analysis, tensile and pull-out tests. Morphological properties of the fibres were observed through scanning electron and optical microscopes. Initial degradation temperature of the linden fibre was reported to be 238℃. The tensile strength and the Young’s modulus of the linden fibres were calculated to be 675.4 ± 45.7 MPa and 61.0 ± 9.8 GPa, respectively. The interfacial shear strength of the linden fibre with unsaturated polyester matrix was computed as 26.15 ± 2.27 MPa via pullout test. This study offers an alternative and eco-friendly reinforcement material which may have usability potential in polymeric composites.

Keywords

Fibre composite characterization linden

Introduction

Currently, finite petroleum resources, the increasing concern towards environmental issues, moreover regulations about carbon dioxide emissions, the availability of improved data on the properties and morphologies of natural fibres and recyclability of the materials have led to an increase in the use of natural fibre reinforced or filled polymer composites in the many industries such as automotive and construction [1 –4].

Recent trends towards environmentally friendly polymer composites have been focusing on the use of cellulose-based natural fibres such as flax, jute, hemp, ramie, and sisal as reinforcement in the composites instead of glass fibres [5 –7]. The natural fibres have advantageous properties such as lightweight, high specific modulus, low density, low cost, non-toxicity, biodegradability, less health hazards, no abrasion during processing, and absorbing CO₂ during their growth [8,9]. The density of natural fibres is lower (1.2–1.6 g cm⁻³) compared to glass fibre (2.4 g cm⁻³). This ensures the production of lighter composites [10]. The use of large volumes of synthetic fibre-reinforced polymer composites in different industries despite their high cost has led to disposal problems [4]. They are also very abrasive materials that lead to an increased wear of processing equipment such as extruders and moulds. Glass fibres can cause problems with respect to health and safety. For example, natural fibres, unlike glass fibres, have less impact on the health of composite manufacturers (irritation of the skin, lung cancer) [11]. Glass fibres give skin irritations during handling of fibre products, processing and cutting of fibre-reinforced parts [6]. These advantages and the disadvantages make natural fibres a potential reinforcement material in polymer composites.

As a result, natural fibres have attracted growing interest for industrial applications, technical textiles, composites, pulp and paper, as well as for civil engineering and building activities [12]. Approximately 43,000 tonnes of natural fibres were utilized as reinforcement in composites in the European Union (EU) in 2003 [13]. The amount increased to around 315,000 tonnes in 2010, which accounted for 13% of the total reinforcement materials (glass, carbon and natural fibres) in fibre-reinforced composites. It is forecasted that about 830,000 tonnes of natural fibres will be consumed by 2020 and the share will go up to 28% of the total reinforcement materials [14].

Growing environmental awareness leads to focusing on new plant fibre for polymeric composites. A fibre which can be used as a reinforcement in composite manufacturing should have high tenacity, high flexural and impact strength based on the application area. The main scope of using fibres as a reinforcement material is to enhance mechanical performance of polymers. Additionally, the surface chemistry and the surface morphology of the fibres should be compatible with polymers to provide high interfacial adhesion in composite system. Thus, many natural fibres have been investigated. In recent times, the use of artichoke fibres extracted from the stem of artichoke plant was investigated as a potential reinforcement in polymer composites. The microstructure, chemical composition, and mechanical properties of artichoke fibres were studied and these fibres were represented as an alternative to synthetic fibres (i.e. glass) as reinforcement in composites [15]. Besides, the potential of okra fibres extracted from the stem of a plant of the Malvaceae family as reinforcement in polymer composites was also examined [16].

Ethnobotanically, linden fibres have been used for centuries in Turkey. Traditionally, it is used especially for rope making due to durability properties. In the Northeast part of Turkey, many villagers occupy with traditional beekeeping and they used big carved wooden barrel as a hive. They use linden ropes for carrying these heavy hives. Scientifically, there has been no report on the Linden fibres. Therefore, the purpose of this research is to characterize and to investigate the mechanical performance of the Linden fibre as a novel cellulose-based fibre to be used as a reinforcement material for composite materials. According to this scope, the linden fibres were characterized by thermogravimetric analysis (TGA), Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The chemical composition and the fibre tensile properties were determined and also pull-out test was performed. Additionally, surface morphology of the linden fibres was observed by using scanning electron (SEM) and optical microscopes.

Material and methods

Materials

Linden tree, which is a member of Tiliaceae family, grows naturally in many regions of Turkey. The used materials were collected from Efeler Village of Artvin district located in the northeast part of Turkey. Plant materials were identified according to Flora of Turkey and East Aegean Island [17]. Linden fibres (close, general and detailed views) used in this study were extracted from Tilia rubra DC. subsp. caucasica (Rupr.) V.Engl (Figure 1a–c). Polipol polyester 383-T (specific gravity: 1.11 g/cm³, viscosity brookfield: 950 cP), which is isophthalic acid type resin, was used as resin in the pull-out test.

Figure 1.

The picture of the plant: (a) a close view, (b) a general view, (c) a detailed view.

Extraction process

The linden fibres are held together in the stems of the plant and in order to free the fibres, fermentation process called water retting was applied [18]. After collecting the fresh plant, the stems of the plants were retted for six weeks in tap water without any enzyme. The fibres extracted from the stems of the plants were washed with distilled water, combed, and dried in open air for about one week after the retting process. The average L/D ratio of the retted fibres was 25/0.13 (mm/mm). The wettability of the fibres was determined by water drop test. Average disappearance time of water drops of linden fiber is 43.85 ± 3.54 s.

TGA experiment

TGA was conducted by using Perkin–Elmer Diamond TG/DTA analyzer from 30℃ to 600℃ at a heating rate of 10℃/min under dynamic nitrogen flow.

FTIR analysis

FTIR analysis was carried out by using Perkin Elmer Spectrum BX-II. Prior to analysis, KBr was dried at 80℃ for 1 h. In order to prepare pellets for FTIR analysis, 1 mg of linden fibres was mixed with 100 mg of KBr. After drying at 80℃ for 2 h, pellet was produced.

Determination of chemical composition

The fibre samples were oven-dried at 105℃ for 4 h and kept in a desiccator before the chemical analysis for cellulose, hemicelluloses, and lignin. The details of methods are given elsewhere [19,20].

Tensile testing

In order to determine the tensile properties (the tensile strength, the Young’s modulus, elongation at break) of the linden fibre, single fibre tensile tests were performed by using a universal testing machine (Shimadzu AUTOGRAPH AG-IS Series) with 100 N loadcell at a contact speed of 0.1 mm/min. The single fibre having a gauge length of 20 mm was mounted on cardboard end tabs via a quicksetting polyester adhesive. The test was repeated eight times and the results were averaged.

SEM observation

The surface morphology of the linden fibres was examined via SEM analysis by using Quanta FEG 250 SEM (at an accelerating voltage of 7 kV). Before observation, the fibres were gold-coated via Emitech K550X automatic sputter coater.

XPS analysis

It is known that XPS is utilized to investigate the surface chemistry of materials. In order to determine distribution of chemical elements and the amount of functional groups on the surface of linden fibres, XPS analysis of the linden fibres was carried out by using Techno Scientific Al-Kα X-Ray Photoelectron Spectroscopy. The device was calibrated according to gold 4f_7/2 and a 10⁻⁸ mbar of vacuum was applied. Pass energy and energy step size were determined as 150 eV and 1 eV, respectively.

Pull-out testing

In order to characterize fibre–matrix adhesion, the bonding strength between matrix and fibre was determined via pull-out test using Shimadzu AUTOGRAPH AG-IS Series universal testing machine. Pull-out test was performed after embedding of the fibres in the matrix for 180 min at room temperature. The gauge length was kept as 10 mm with a cross-head speed of 0.1 mm/min. The tests were carried out for five times and an average value was taken into consideration. The debonding force F_max, the diameter d, and the embedded length of the fibres l_e were measured and the interfacial shear strength (IFSS) τ_d was calculated from the following equation [21]

τ_{d} = \frac{F_{max}}{d π l_{e}}

(1)

Optical microscopy studies

Longitudinal and cross-sectional views of the linden fibres were taken by using Olympus CX 21 optical microscope under 400 × magnification to examine the morphology.

XRD analysis

XRD analysis of the linden fibres was performed by Rigaku D/MAX 200 XRD using CuKα radiation. The diffraction intensity of the powdered sample was recorded between 3° and 90° (2θ) at a scan speed of 4^o/min. The crystallinity index of the linden fibres was calculated by following Segal empirical method [22,23]

CI % = \frac{(I_{002} - I_{am})}{I_{002}} \times 100

(2)

where I₀₀₂ indicates the maximum intensity of diffraction of the (002) lattice peak at a 2θ angle between 22° and 23°, which represents both crystalline and amorphous materials. I_am is assigned to the intensity of diffraction of the amorphous material, which is taken at a 2θ angle between 18^o and 19^o where the intensity is at a minimum value [22,24].

Results and discussion

TGA

Thermal decomposition profile for the linden fibres is given in Figure 2. TGA parameters are obtained by using TG, DTG, and D²TG curves, as revealed by Yao et al. and Grønli et al. [25,26]. The weight loss up to 110℃ due to water evaporation is less than 1% due to which linden fibres contain low water content. It is known that hemicellulose in the fibres is mainly responsible for moisture absorption; but accessible cellulose, amorphous cellulose, and lignin also contribute to this process [27]. From the chemical analysis, it is observed that hemicellulose content of the Linden fibre is quite low. The initial degradation temperature (T_initial) is presumed to correspond to a solid mass fraction equal to 0.975 [26].

Figure 2.

TGA curves; weight loss, 1st and 2nd derivative of the weight loss vs. temperature for the linden fibre.

Initial degradation temperature for linden fibre is obtained to be 238℃. As can be seen from DTG curve of the linden fibres, a shoulder around 300℃ is seen (weight loss is 17% until 300℃). This may mainly correspond to the thermal depolymerization of hemicellulose and the cleavage of glycosidic linkages of cellulose [16]. It is probable that decomposition of high content of lignin (34%) in the linden fibres takes place within the whole temperature range. The temperature T_peak (T_p), at which the maximum decomposition rate is reached, is determined as 337℃. From TG curve, it is obtained that the weight loss percentage corresponding to T_p is 42%. The extrapolated onset temperature of decomposition (T_o) is described by extrapolating the slope of the DTG curve (up to zero level of the DTG axis) in correspondence with the first local maximum in D²TG curve [25,26]. Final decomposition temperature was obtained to be 425℃. Besides, weight loss corresponding to major decomposition step until 600℃ is about 64%. This step is mainly due to decomposition of α-cellulose available in the linden fibres. Table 1 shows a comparison of thermal properties of some cellulose-based fibres. The linden fibres exhibit higher extrapolated onset temperature of decomposition than jute, hemp, bamboo, kenaf, cotton stalk, bagasse, and wood-pine fibres. Besides, the Linden fibres also have a greater maximum decomposition temperature than the other fibres as presented in Table 1. Weight loss value for the linden fibre is relatively low in comparison with those of the other fibres in Table 1. In terms of residual char of the fibres in Table 1, the linden fibre has the greatest value. As a conclusion, it is worth reporting from TGA that the Linden fibres are stable until around 240℃.

Table 1.

Thermal characteristics of the selected fibres and the linden fibre.

Sample	T_o (℃)	T_p (℃)	Weight loss until T_p (%)	Residual char (wt) (%)	Reference
Jute	205	283	44	25.2^a	[25]
Hemp	205	282	38	24.6^a	[25]
Bamboo	214	286	45	20.5^a	[25]
Kenaf	219	284	43	22.4^a	[25]
Cotton stalk	222	293	50	17.1^a	[25]
Bagasse	222	299	54	20.4^a	[25]
Wood-Pine	230	312	59	14.9^a	[25]
Linden	243	337	42	36^b	In this study

Until 800℃.

Until 600℃.

FTIR analysis

The lignocellulosic fibre compounds containing cellulose, hemicelluloses, and lignin consist of some oxygen containing functional groups (ester, ketone, and alcohol), alkenes, and aromatic groups [16,28]. As shown in Figure 3, a strong absorption band was observed at 3418 cm⁻¹ due to OH stretching and hydrogen bonds. The bands at 3418 cm⁻¹, 1265 cm⁻¹ (due to C-O-C), and 1056 cm⁻¹ (C-OH stretching vibration of the cellulose backbone) indicate the polysaccharide components, largely cellulose. C-H bending and OH deformation bands of alcohol group of cellulose are located at 1377 cm⁻¹ and 1321 cm⁻¹, respectively [29 –31]. The band at 2926 cm⁻¹ and a shoulder at about 2850 cm⁻¹ correspond to C-H stretching vibration from CH and CH₂ in cellulose and hemicelluloses [32]. This weak shoulder confirms low content of hemicellulose in the linden fibres. C = O stretching vibration of linkage of carboxylic acid in lignin or ester group in hemicellulose is centered at 1736 cm⁻¹ [33]. C = C stretching of aromatic ring of lignin appears at 1506 cm⁻¹. The band at 1654 cm⁻¹ is assigned to the antisymmetric COO⁻ stretching or presence of water [30]. The band at 1428 cm⁻¹ may be attributed to the CH₂ symmetric bending in cellulose [34]. The absorption band at 1261 cm⁻¹ may be indicative of C-O stretching vibration of the acetyl group in hemicellulose [35,36]. A weak band detected at 1161 cm⁻¹ corresponds to the antisymmetrical deformation of the C-O-C band. β-glycosidic linkages between the monosaccharides show a band at 893 cm⁻¹ [16,35]. The aromatic C-H out-of-plane vibration in lignin is located at 833 cm⁻¹ [30]. The lateral crystallinity index of the fibres is obtained by using absorption values at 1428 cm⁻¹ and 893 cm⁻¹ attributed to the CH₂ symmetric bending mode and C₁ group frequency, respectively [30,37]. The baseline-corrected absorptions at 1428 cm⁻¹ and 893 cm⁻¹ correspond to crystalline and amorphous cellulose structures, respectively [32,38 –40]. The Lateral order index (LOI) shows the order of crystallinity rather than the amount of crystalline cellulose relative to the amorphous components [40]. LOI for the linden fibre was computed as follows

LOI = \frac{a_{1428 {cm}^{- 1}}}{a_{893 {cm}^{- 1}}}

(3)

where a_1428cm−1 and a_893cm−1 correspond to the absorbencies at 1428 cm⁻¹ and 893 cm⁻¹, respectively [37]. The LOI value for the linden fibre was calculated as 0.96. This value is greater than those of althaea and ferula fibres (althaea fibre: 0.79, ferula fibres: 0.70).

Figure 3.

FTIR spectrum for the linden fibre.

Chemical composition

The contents of basic constituents of the fibres such as cellulose, hemicelluloses, and lignin were obtained to be 61.8%, 4.2%, and 34.0%, respectively. Cellulose content of the linden fibres is comparable with those of abaca, agave Americana, and flax fibres [19,41,42] However, lignin content of the linden fibres is higher than those of the other cellulose-based fibres. The lignin contents of abaca, agave Americana, hemp, and jute fibres are revealed to be 4.9%, 8.5%, 4%, and 8%, respectively [19,20,41,43]. The high amount of lignin may explain moisture resistance behaviour of the Linden fibres [44]. This result is in accordance with TGA shown in Figure 2.

XPS analysis

Table 2 gives the atomic concentrations and O/C ratio of the linden fibres with respect to XPS analysis. As can be seen in Table 2, C and O concentrations of the Linden fibres are 85.63% and 10.70%, respectively. O content of the Linden fibres is very low in comparison with luffa cylindrica and jute fibres which have O contents of 34.9% and 29.7%, respectively. O/C ratio of the linden fibres is calculated to be 0.13, which is lower than the other fibres such as flax (0.156) [45], jute (0.46) [20], and luffa cylindrica (0.54) [46]. A low O/C ratio can be used as an indicative of hydrophobicity of the fibres [47]. Therefore, moisture resistance property of the linden fibres can be confirmed by relatively low O/C ratio.

Table 2.

Surface chemical composition of the linden fibre.

	C (%)	O (%)	Ca (%)	O/C
Linden fibre	85.63	10.70	3.7	0.13

Deconvulation analyses of C1s and O1s were performed to determine the functional groups and their concentrations on the surface of the linden fibres. The peak assignments and the concentrations of related functional groups are given in Table 3 and their curves are presented in Figure 4. As can be seen in Table 3, the high proportions for the linden fibres belong to C-C/C-H and O-C groups. It is noteworthy that proportion of C = O groups is 36.2% and this value is relatively high as compared with jute (6.1%) and luffa cylindrica fibres (4.6%). Carbonyl groups mainly exist in lignin and also present in hemicellulose components in lignocellulosic fibres. This high concentration of carbonyl groups can confirm the high amount of lignin.

Figure 4.

XPS spectra showing the deconvoluted C1s (a) and O1s (b) envelope for the linden fibre.

Table 3.

The concentration of functional groups on the surface of the linden fibre [20,48,49].

	C-C, C-H	C-O	C = O	O-C	O = C	O-H
	C1s			O1s
eV	284.5	286.3	287.6	533.5	532.2	530.7
%	45.2	18.6	36.2	54.3	23.5	22.2

Tensile properties

Figure 5 shows tensile testing results, in terms of tensile strength versus strain, for linden fibre. Tensile properties of linden fibre were determined and a comparison with the other plant fibres was made, as shown in Table 4. In this study, the tensile strength and the Young’s modulus of the linden fibre were obtained as 675.4 ± 45.7 MPa and 61.0 ± 9.8 GPa, respectively. Besides, the elongation at break of the linden fibres was determined as 2.95% ± 0.20. It is interesting to note that, the linden fibres have close values to hemp fibres for related tensile properties. The tensile strength, tensile modulus, and elongation at break of hemp fibres have been given as 690 MPa, 70 GPa, 2.0–4.0%, respectively [50]. It can also be emphasized that the tensile strength and the Young’s modulus for the linden fibre are better than those for ferula communis and coir, as presented in Table 4.

Figure 5.

The stress–strain curve of the linden fibre.

Table 4.

Tensile properties of linden fibre and other natural fibres.

Fibre	Tensile strength (MPa)	Tensile modulus (GPa)	Elongation at break (%)	Reference
Ferula	475.6 ± 15.7	52.7 ± 3.7	4.2 ± 0.2	[22]
Althaea	415.2 ± 11.5	65.4 ± 7.2	3.9 ± 0.1	[51]
Luffa	385 ± 11	12.2 ± 1.0	2.65 ± 0.05	[49]
Jute	400–773	10–30	1.5–1.8	[52]
Pineapple	180–748	25–80	1.6–3.2	[53]
Ramie	400–938	61.4–128	3.6–3.8	[22,52]
Flax	500–1500	27.6	2.7–3.2	[54]
Kenaf	930	53	1.6	[55]
Hemp	690	70	2.0–4.0	[56]
Banana	700–800	27–32	2.5–3.7	[52]
Abaca	756	31.1	2.9	[57]
Sisal	511–635	9.4–22	2.0–2.5	[58]
Piassava	76.9	2.93	10.45	[59]
Coir	95–220	2.5–6.0	13.7–51.4	[60]
Linden	675.4 ± 45.7	61.0 ± 9.8	2.95 ± 0.20	In this study

Interfacial shear strength

The properties of fibre-matrix interface affect the mechanical properties of composite materials due to the role of fibre-matrix interface in transferring stress between the fibre and the matrix. In order to transfer the load from matrix to fibre, which is also required for good performance, a strong fibre-matrix adhesion is necessary [61]. Interfacial shear strength of the linden fibre-unsaturated polyester matrix was obtained to be 26.15 ± 2.27 MPa. Seki et al. [62] have determined IFSS value of Ferula Communis/unsaturated polyester as 16.21 MPa. The IFSS value obtained in the study is about 1.6 times greater than that of Ferula Communis/unsaturated polyester.

Optical microscopy studies

Figure 6 shows the longitudinal and the cross-sectional optical views of the linden fibres. It can be seen from Figure 6 that impurities and non-cellulosic materials are present on surface of the linden fibres. In addition, the linden fibres exist as a bundle of elementary fibres or cells because one elementary fibre seems to be separated from the bundle in longitudinal view of the linden fibres in Figure 6. The linden fibres have polygonal cross-sectional shape which can vary notably from irregular shape to reasonably circular.

Figure 6.

Optical microscopy images of the linden fibres (400×): (a) the longitudinal image, (b) the cross-sectional image.

SEM observation

SEM micrographs of the linden fibres are depicted in Figure 7(a and b). Figure 7(a) shows that the structure of the linden fibres includes several elementary fibres (or cells) bonded with non-cellulosic materials along the fibre axis, which is also determined in optical images of the fibres. It is clearly seen that surface impurities are present on the surface of the linden fibres (Figure 7a). The diameter values of the linden fibres vary in the range of 40–300 µm (Figure 7b). The average diameter of the linden fibres were roughly measured as 131.75 µm. This result is a consequence of the measurement of several ultimate fibres that exist in a bundle of the linden fibres. As can be seen from SEM micrograph of the fracture surfaces of the linden fibre (Figure 7c–d), the fibre showed brittle fracture behavior. The tensile strength versus strain curve (Figure 5) shows brittle behavior of the linden fibre, with a low strain (3–5%), similar to other fibres, such as sisal [63], pineapple [64], and banana [65].

Figure 7.

SEM micrographs of the linden fibres: (a and b) general view and (c and d) fracture surfaces.

XRD analysis

The XRD pattern of the linden fibres is shown in Figure 8. As can be seen from Figure 8, the major crystalline peak of the linden fibres occurred at 2θ = 16.7°. It is also found that the linden fibres have diffraction peaks at 13.9°, 14.8°, 22.0°, and 25.2°, which can be assigned to the typical diffractions of cellulose I. The crystallinity index of the linden fibres is 53% (Figure 8). As compared with the other commonly used cellulose-based fibres, this value is higher than wrighitia tinctoria seed fibre (49.2%) but lower than coir fibres (57%), ramie (58%), cotton (60%), jute (71%), flax (80%), and hemp fibres (81%) [22,35]. The cystallinity index of the linden fibres is also lower than recently developed potential plant fibre, Althaea fibres (68%) [51]. This may be due to the high content of non-cellulosic materials as compared with the other cellulosic-based natural fibres [50].

Figure 8.

XRD pattern of the linden fibres.

Conclusion

The contents of cellulose, hemicelluloses, and lignin of the linden fibres were obtained to be 61.8%, 4.2%, and 34.0%, respectively. LOI value for the linden fibres, obtained from IR analysis, was calculated as 0.96. XRD analysis showed that the crystallinity index of the linden fibres was calculated as 53%. O/C ratio of the linden fibres was obtained to be 0.13 from XPS analysis. It may be said that the surface of linden fibres contains non-polar components. The tensile strength and the Young’s modulus of the linden fibres were 675.4 ± 45.7 MPa and 61.0 ± 9.8 GPa, respectively. Interfacial shear strength of the linden fibre-unsaturated polyester matrix was computed as 26.15 ± 2.27 MPa. Although the maximum decomposition temperature of the linden fibres is obtained to be 337℃, the linden fibres are stable until around 240℃. It is probable that linden fibres may be used as a reinforcement material for unsaturated polyester due to relatively good adhesion properties and high thermal stability.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship,and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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