Structural Performance of Chemically Modified Natural Jute Yarn for Strength Demanding Composite Applications


 Manufacturing natural based high-performance composites is becoming a greater interest to the composite manufacturers and to their end users due to their bio-degredability,low cost and availability. Yarn based textile architecture is commonly used in manufacturing these composites due to their excellent formability. However, for using natural based yarn as a reinforcing architectures in high load bearing structural composite applications, a significant improvement in mechanical performance is required. Particularly, jute fibre yarn suffers with poor mechanical properties due to the presence of fibrillar network, polysacharides and other impurities in the fibre. For achieving this, we use aqueous glycine treatment (10%, W/V) on alkali(0.5 %, W/V) and untreated jute yarns for the first time. The glycine treatment on alkali treated jute yarns (ATG) shows a huge improvement in tensile strength and strain values by almost ⁓105% and ⁓50 % respectively compared to untreated jute yarns (UT) because of the strong interactions and bonds developed between glycine, alkali and jute yarns. It is believed that the newly developed glycine treated jute yarns will be helpful to promote jute yarns in composite industries where load-bearing is primary requirement and replace their synthetic counterparts.


Introduction
Fibre-reinforced composites have gained significant interests in recent years, due to their design flexibility, durability, chemical resistance, and relatively higher strength and stiffness at low weight ratio. Traditionally, fibre reinforced composites are composed of synthetic fibres including glass, carbon and aramid as reinforcements in a polymer matrix ).
However, synthetic fibres are not environmentally friendly as they are manufactured from fossil fuels, associated with relatively higher energy consumption and carbon emission. Natural fibres reinforced composites can be environmentally sustainable alternative to their synthetic counterpart, due to lower environmental impacts including less carbon emission, less energy consumption and biodegradability (Karim et al. 2014(Karim et al. , 2015Pickering et al. 2015). The most promising natural plant fibres are jute, hemp, ramie, sisal, flax, and bamboo which could potentially replace synthetic fibers for various applications. Arguably, jute is the most attractive alternative amongst other natural plant fibres, due to its abundance, low production cost, lower density and high individual fibre length, as well as reasonable mechanical properties (Pandey et al. 2010;Sarker et al. 2018). In addition, Jute is the second most produced natural fibre in the world after cotton (~3.63 million tons), and at least ~50% cheaper than flax and other similar natural fibres. The use of jute for various applications could boost the farming economies of developing countries such as Bangladesh and India, where it is mostly produced.
The mechanical properties of jute are related to its relatively higher degree of crystallinity (~58%), and higher cellulose content (~70%) (Table S1, Supporting Information) (Defoirdt et al. 2010). Jute fibres have a 'fringed fibril model', where inter-fibrillar matrix contains hemicellulose within ultimate cells and middle lamella contains lignin between ultimate cells (Hearle 1963). The viscoelastic and tensile behaviour of jute fibre depend on such polysaccharides (e.g. hemicellulose, lignin and pectin) and their relative proportion, because they create links between the cellulosic microfibrils and are responsible for stress transfer 4 among them (Mukherjee et al. 2008). It has been reported in the previous study that the removal of polysaccharides eliminate the microvoids present between the ultimate cell and middle lamella of the fibre. As a result, microfibrils present in the fibre become more parallel and homogeneity of the fibre improves which results in improvement of the failure stress, failure strain and stiffness of jute fibres (Mukherjee et al. 2008). The alkali treatment is the most commonly used method to remove polysaccharides i.e. hemicellulose, lignin, pectin, which improves the load bearing capacity of jute fibre as a reinforcing material for fibre reinforced composites (Mukherjee et al. 2008;Sarker et al. 2018). However, jute fibre still contains microvoids among the fibrils which limit their load bearing capacity and create a weak fibre/matrix interface (Sarker et al. 2018). Previous studies have reported the removal of such defects via chemical, Saha et al. 2010;Arfaoui et al. 2017) physical (Militký and Jabbar 2015) and nanomaterials modifications (Sarker et al. 2018(Sarker et al. , 2019Dang et al. 2019) of jute fibres, which are time consuming and expensive. Additionally, there are concerns with nanomaterials safety and their potential carcinogenic nature to health. The alkali treatment is the commonly used chemical medication for natural fibres as it removes surface waxes and affects hemicelluloses. The alkali treatment (0.5 wt.-%) of jute fibres with the prolonged exposure into the alkali solution is considered to be the most effective way of removing hemicelluloses without affecting lignins significantly (Roy et al. 2012;Sarker et al. 2018).
Further to alkali treatment it is necessary to modify jute fibre to remove the flaws (microvoids) due to removal of lignins from the skin of fibre may also generate stress concentration. As a result, further chemical modification is necessary to solve this issues. However, the treatment should be cost effective, environmentally friendly and easy to scale up. At present nano surface modification is becoming popular in modifying natural fibres in composites applications.
However, dealing with these materials are health hazardous and costly.
For fibre-reinforced composites (FRC), yarn-based multi-axial textile architectures offer better mechanical properties including impact, compression after impact damage and interfacial strength, than the most popular unidirectional yarn architectures (De Albuquerque et al. 2000;Khondker et al. 2005). Textile architectures are mainly induced with plain, twill, sateen, and knitted derivatives which are mainly manufactured from yarns. Therefore, there exist a growing demand for FRC composite preforms comprised of woven fabric with multi-axial yarn architectures. However the use of the jute yarn for structural FRC applications is limited due to it poor performance properties. The tensile strength for jute yarn was reported ~42-45 MPa only. Such a lower tensile strength is mainly due to the fibre impurities and the twist imparted to the fibre during spinning into yarn. (Sharif Ullah et al. 2017;Bensmail et al. 2019). In addition, the strain to failure of jute yarn was found to be limited ~6.0-7.5 % with a large scattering in the value in those studies (Sharif Ullah et al. 2017;Bensmail et al. 2019). Recently, few studies have been carried out on the nano-modification of natural jute yarns to increase the strength and interfacial performance of the composites (Foruzanmehr et al. 2015;Li et al. 2015). It was found that the lower value of strain % is responsible for the amorphous phase's viscoelastic shear deformation present in the cellulose of the fibre. (Perremans et al. 2018) Such viscoelastic shear deformation can be avoided by improving the crystallinity of fibres. In addition, the tensile strength can be improved by aligning the microfibrils in the parallel directions with the loading axis Perremans et al. 2018). However, the improvement of mechanical properties of natural jute yarns are still limited, and not yet well understood.
Nevertheless, it is well established that the deformation of natural fibres largely depends on the interphase of elementary fibres in the yarn. Recently, glycine-based protein materials have been used in cellulosic materials to improve strain to failure and wettability of cotton fibres. Glycine was first applied on cotton fibres to improve tensile properties. It was found that the glycine treatment improved the strain% of fibres by 36% (Remadevi et al. 2018). This is mainly due to the interactions between the amino functional group of glycine and carboxylic groups present in the amorphous region of the cellulosic fibres that creates a strong chemical bond; thus, improves mechanical properties of cotton fibres. As jute has similar cellulosic structures like 6 cotton, glycine is believed to have similar effects to improve the mechanical properties of Jute fibres, which has not been reported yet to the best of our knowledge. In addition, glycine is an environmentally friendly simplest form of protein-based amino-acid which has been extensively used in the drug industry. The low price and nonhazardous feature of glycine and the presence of amino-functional group has widen the scope of using this material in many applications (Bose et al. 2012).
Here, we report for the first time the improvement of tensile properties of jute yarns including the tensile strength and strain via a combination of alkali and aqueous glycine treatment. We optimize the processing conditions and parameters for such treatment to achieve the best mechanical properties of jute yarns. The untreated jute fibres were treated with a lower concentration (0.5 wt.-%) of alkali treatment, followed by a subsequent glycine treatment on the alkali treated jute yarns. Surface topography of the treated and untreated jute yarns were examined using an optical and a scanning electron microscopes (SEM). In addition,, changes in the diameter of the yarns were recorded using a optical microscope, tensile properties were assessed via single yarn testing for 50 samples. We used Weibull statistics to analyse fibre failures after tensile tests. Chemical and thermal analysis were performed using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy(XPS) and Thermogravimetric analysis (TGA), respectively.

Materials
The jute yarns were kindly donated by UMC Jute Mills Ltd., Narsinghdi, Bangladesh. Jute (Corchorus Olitorius) plants were harvested in Bangladesh via traditional methods, and then processed in jute mills. The fibres used in the manufacturing of jute yarn were pretreated with vegetable oil to fibrillate the technical jute fibres to improve the spinnability of the fibre ( Figure   1a and Figure S1a, Supporting Information). As supplied jute yarns have a linear density of 7 ~9.5 lbs/spindle and a TPI of ~4.09 (Table S2, Supporting Information). Sodium hydroxide (EMSURE® ISO, >99%) pellets were supplied by Merck (Germany). Glycine (Amino Acetic Acid, NH2CH2COOH 99.88% a white crystalline powder, Laboratory Reagent LR grade) used in this study was supplied by Fine Chemical Industries, India.

Alkali treatment
The untreated jute yarn (UT) were treated with 0.25%, 0.5%, 1% NaOH at room temperature for 24 hours, and 2% NaOH at room temperature for 2 hours at material to liquor ratio (M: L)

Glycine treatment
Alkali treated (AT) and untreated jute jute yarns were treated with aqueous glycine at different concentrations (5, 10, 15 and 20 g/L) at 100 ˚C and pH 7 for 1.5 hour with a M:L ratio of 1:20 in an infrared lab dyeing machine ( Figure 1d and Figure S2a-b, Supporting Information). In addition, four pH levels (3, 5, 7 and 11) were also selected for 10 g/L aqueous solution of glycine to see their effect on alkali treated jute yarns. All of the treated samples were thoroughly washed with distilled water and dried at 50 o C for at least 5 hours in the oven. Here, untreated jute yarn with glycine treatment is labelled as UTG, while alkali-treated jute yarn with glycine treatment was identified as ATG.

Optical and scanning electron microscope (SEM)
An optical microscope (

Chemical and thermal characterization
The surface chemical composition of untreated and treated jute yarns were analyzed using a Kratos axis X-ray photoelectron spectroscopy (XPS) and a Fourier transform infrared spectroscopy (FTIR). The thermal decomposition of untreated and glycine treated jute yarn was analyzed using a TA instrument (TGA Q50, UK) from room temp to 600 °C in a nitrogen atmosphere at a 10 °C/min heating rate

Tensile Test
For the tensile testing, yarn samples were taken randomly from the spool of alkali (UT, AT) and glycine-treated (UTG, ATG) jute yarns, and conditioned in a standard laboratory atmosphere (55% relative humidity and 20±2 o C) for 24 h before the final testing. An Universal Strength Tester (Testometric Model-M250-3CT, UK), was used with a load cell capacity of 25 KG to measure the breaking force and elongation at break of jute yarn. Single yarns were tested using a 50 mm gauge length at a cross-head speed of 2 mm/min as reported in the previous work. 1 The yarn was set on the machine using special pneumatic yarn gripper supplied by Testometric, which ensures no slippage during testing ( Figure S4 a-b, Supporting Information).
Tensile modulus of the yarns was calculated from the slope of 0.1-0.3 % strain of the yarn.

Weibull statistical analysis
The tensile properties of natural fibres (strength, modulus and strain %) are often described by the weakest link theory which is based on the statement that the materials are made of small elements and the elements are linked together. A material is considered to have failed if any of these small elements has failed. (Wang et al. 2015) The cumulative probability of failure for tensile and interface properties are given by the following formula: Where the cumulative probability of failure is related to the tensile strength ( ), tensile modulus (E), tensile strain ( ) , the Weibull modulus (m) and σ o, are the scale parameters of strength, tensile modulus of the fibre and tensile strain of the yarn.

Results and Discussion
Surface Morphology Presence of impurities such as hemicelloses and lignins in the interfibrillar network of fibres increase fibre irregularity may cause the loose appearance of fibres in the yarn.
However, after the alkali treatment, a noticeable change in the fibre packing of AT yarn ( Figure   2b) was observed which could be related to the removal of hemicelluloses after alkali treatment.
As a result, yarn diameters were slightly reduced from ⁓0.92 mm to ⁓0.71 mm (Table S4, Supporting Information), which is in agreement with the results obtained in previous studies. (Fernandes et al. 2013;Orue et al. 2015;Sarker et al. 2018) Glycine treatment on untreated jute fibre (UTG) slightly improved the packing of technical fibre in jute yarn while no change in the diameter was observed for UTG jute yarn ( Figure S5a and Table S4, Supporting Information).
12 13 However, glycine treatment on alkali treated jute yarn (ATG) enabled a significant tight packing of the fibre in the jute yarn. As a result, a uniform reduction of yarn diameter (see Figure 2c and Table S4 Figure 2g shows the FTIR spectra of UT, AT, UTG and ATG jute yarns, which demonstrates four characteristics peaks for UT jute yarns. (Roy et al. 2012;Sarker et al. 2018) The peak located at ~3400 cm -1 is responsible for the stretching of hydrogen bond, which is originated 14 from the hydroxyl groups present in the cellulose, hemicellulose and lignin of jute fibres. The peaks between 2900 and 2700 cm -1 are for the C-H stretching of alkyl groups of cellulose, lignin and hemicelluloses present in jute fibres. In addition, FTIR spectrum of untreated jute fibres show peaks at ~1738 and ~1249 cm -1 band. The peak at ~1738 cm -1 is for C-O stretching of carboxylic and ester groups from hemicelluloses presents in the interfibrillar region of UT jute fibres. Furthermore, the band at 1249 cm -1 is for the C-O stretching of acetyl groups from lignins of untreated jute fibres. After alkali treatment, these peaks were disappeared which is in agreement with a previous study. (Sarker et al. 2018) The disappearance of such peaks clearly confirms the removal of hemicellulose and lignin from the bundle of technical jute fibres after alkali treatment, which is evident from the optical and SEM images of AT jute yarns ( Figure   2b,e) , and could be explained by the formation of Na-cellulose from the reaction between the cellulose and NaOH, Figure 3.

Chemical and Thermal Characterizations of Coated Jute Yarns
For glycine treatment on UT jute yarn, no additional absorption peaks were observed, which may be due to the presence of impurities in jute fibre that restricts the reaction between the Cellulose Sodium hydroxide Cellulose-Na Possible interaction between the jute fibre with sodium hydroxide and glycine glycine compound and jute fibre. However, ATG jute yarns show slightly extended peaks at ~1540 cm -1 and ~570 cm -1 , which is possibly due to the formation of amide bond between the amine groups of glycine and carboxylic acid groups of jute fibres. Such observation is further supported by the elimination of the carboxylic ester peak at ~1732 cm -1 after glycine treatment.
In addition, the peak at 570 cm -1 indicates the formation of N-C=O bending (Saroj et al. 2013) again possibly due to the interaction between amine functional groups of glycine and carboxyl groups of jute fibres, (Figure 2g) Furthermore, , the broadening of the peak at ~3200 cm -1 may be due to the formation of hydrogen bond between the functional groups of glycine and alkalitreated jute fibres.

Tensile properties
Jute yarns, usually brittle, show a sudden decrease in load which corresponds to the failure strain of the yarn as reported by many other researchers who worked with natural fibres (Mukherjee et al. 2008;Roy et al. 2012;Fiore et al. 2016;Sarker et al. 2018). To analyze the tensile behavior of UT and surface treated-jute yarn, single yarn tensile test was conducted. Figure 5 shows a large variation in tensile properties which could be due to the variations in the fibre fineness. Therefore, 50 single yarn tests for each type of treated and untreated jute yarns were tested in this work. The values of tensile properties (tensile strength, modulus and strain%) were statistically analyzed using a two-parameter Weibull statistical distribution ( and the effects of pH for surface treatments of jute yarn. These results are listed in (Table S4, Supporting Information). For alkali treatment 0.5% concentration was found to the most effective in the case of 10 g/L glycine concentrations which is considered the best suited glycine concentration in this study (see Table S4, Supporting Information). The effects of glycine percentage on the alkali-treated jute fibre was studied and optimized as 10 g/L (Table S4, Supporting Information). The effect of pH on the tensile properties of optimized glycine-treated (10 gm/L) jute yarns was studied in order to understand the intensity of interaction between the glycine moieties and cellulose functional groups at different pH levels. The neutral pH (7) of glycine solutions was found to be good enough to improve the tensile properties of ATG jute yarns (Table S4, Supporting Information). Based on the obtained results, here we used 0.5% alkali concentration and 10 gm/L glycine solution with p H (7) for AT, UTG and ATG yarns.  In addition, the orientation of the elementary fibre located in the jute yarn can re-arrange themselves and parallel along the length of the yarn during tensile loading (Figure 2b). The alkali-treatment on natural fibre reduces the spiral shape of cellulose microfibrils that allow the re-arrangement of the cellulose chains and improve the tensile properties of the fibre. (Sawpan et al. 2011) Similarly, the alkali treatment of abaca fibre enabled higher tensile properties than the untreated one, due to the rearrangements of cellulose microfibril along the longitudinal axis. (Cai et al. 2016) In addition, the alkali treatment can make better arrangement of cellulose chain in the fibre which is responsible for the release of internal strain that leads to improve the strength and strain% of jute fibre.  Further glycine treatments on alkali-treated jute yarns improve tensile properties significantly (Table S5,  MPa and ~0.7% to ⁓86 MPa and 11.5 % for ATG yarns, respectively, which are almost ⁓105% and ⁓50 % increment in strength and strain values respectively compared to the UT yarn. The enhancement in the tensile strength is supported by the stress-strain curves for UT and ATG yarns, Figure 5(b, c). The possible reason for the improvement in the tensile properties of ATG yarn is related to the strong connection between the AT jute fibres in the yarn with the functional group of glycine via suitable chemical or physical bonding. The proposed reaction mechanism is provided in Figure 3. In addition, the abundance of oxygen functional group and possible formation of hydrogen bonds have been by XPS and FTIR analysis (Figure 4e and Figure 2g). This is in agreement with a previous study, (Remadevi et al. 2018) where they treated cellulosic cotton fibre with glycine and found a significant increment in the tensile properties due to the bonding between glycine and cellulosic fibres. In addition, glycine can form zwitterion, which The improved mechanical properties could also be described based on the SEM image observations found in Figure 2f of this study where an excellent fibre packing and the paralleled, fibrillated fibres interconnection were achieved for ATG yarns. As a result, there was no stress decay to print the fibre in parallel direction leading to improve the stress carrying capacity of ATG yarn during tensile loading applications. Between ATG and UTG yarns, ATG yarns showed better tensile strength and strain properties, because ATG yarns contain more hydroxyl groups for the alkali treatment which enables a better interaction with the functional groups of glycine compared to UTG yarns. However, ATG yarns exhibited lower tensile modulus value, this is possibly due to the new bonds formed in the ATG yarns which increases the the fibrillar cohesion to enhance the strain with respect to the increase of stress (see Figure 5e) . Stressstrain curves of ATG yarn showed more improvement compared to UT yarns (see Figure 5c).
Here, improvement in tensile properties of jute yarn is related with the better packing of fibrils in the fibre due to strong chemical interactions after glycine treatments. Beside this, we observe diameter of the yarns has changed after glycine treatment which also resulted in improving tensile properties of jute yarn is also shown in Figure 5g-i.This result can be supported from the SEM and optical image observations (Figure 2c&f)) that yarn with smaller diameter have reduced porosity and less impurities in the fibre. We conduct statistical analysis to validate the data obtained after tensile experiments (see Table S6, Supporting Information). Weibull statistical distribution were performed to evaluate the scale parameter (α) and shape parameter ln curve (Figure 6d-f). It is seen that this statistical model provided an excellent fitting of the data for tensile properties of the yarns. Moreover, Weibull distribution calculated a reasonable numerical prediction of the experimental data, provided in (Table S6, Supporting Information).
It was found that tensile strength and strain values (scale parameter) of the yarn were improved after introducing the alkali and glycine treatments on jute yarn (ATG). In this case, both the probability of failure and ln curves were seen to shift from left to right significantly when ATG yarns were compared with UT jute yarns. The Weibull modulus was obtained from the ln curve of the untreated and treated yarns as shown in (Table S6, Supporting Information). UT yarn showed relatively lower value in Weibull modulus (⁓3, 4.5 and 4.8, for tensile strength, tensile modulus and tensile strain respectively) due to the high scattering of UT yarns linked to the presence of impurities in the fibre located in the yarn. However, the Weibull modulus was increased to 6.1 for tensile strength, 5.9 for tensile modulus and 6.1 for failure strain of the ATG jute yarn which could be because of the better bonding between the jute fibre and the glycine, as we discussed in the earlier sections of this work. The higher value in Weibull modulus for the jute yarn were found in similar to synthetic fibres reported in the literature. Chawla et al. (Chawla et al. 2005) experimented the value of 4.6 for the Weibull modulus of ceramic fibre, whereas this study calculated the Weibull modulus of 6.1 of ATG yarns which confirmed the weakest link in the fibre caused by flaws present in the fibre was reduced significantly after introducing alkali and glycine treatment on jute yarns.

Fractographic study of jute fibre yarn
We investigated the fracture specimen of different treated jute yarns using SEM. In this investigation we observed that bundles of micro-fibrils are present in yarn which can be seen in Figure 6g-i. In the case of broken specimen from UT yarn, a very uneven fracture of jute fibre bundles with fibre pull-out from the skin of UT yarn is visible in Figure 6g. This might be due to the presence of impurities into the interfibrillar network of UT yarn which is also supported by other studies (Mwaikambo 2009;Sarker et al. 2018).. Fibre splitting was observed as the dominant fracture feature with a small amount of fibre pull-out for AT yarns (see Figure   6h). The dominance of fibre splitting might be related to the improvement in the crystallinity of AT jute fibres and removal of the hemicelluloses which act as the stress concentration points of jute fibres. Brittle fracture was occurred in the transverse direction of the UTG yarns at some extent for the glycine treatment whereas in ( Figure S6, Supporting Information). ATG yarn showed a vivid brittle fracture surface without any fibre pull-out (see Figure 6i). For ATG yarns, fibrils in the yarn were broken in the same order along the transverse direction indicating that the improved packing of microfibrils created with the both alkali and glycine treatments evenly distributed along the length of the fibre.

Comparative study with the literature
A comparison was tried to make in (Table S7,

Conclusions
In this study, aqueous glycine treatment was applied on untreated and alkali treated jute yarns, and their influences on the chemical, thermal, morphological and mechanical properties of jute yarn were evaluated. The results indicate an extremely positive effects of glycine treatment on jute yarn towards structural properties improvement. Glycine treatment on alkali treated jute 25 yarns (ATG) brought a remarkable improvement by almost ⁓105% and ⁓50 % increment in tensile strength and strain properties respectively compared to untreated jute yarns. The significant improvements in the mechanical properties of newly developed glycine and alkali treated jute yarns (ATG) achieved in this work will be helpful to develop the use of jute yarn based woven or multi axial textile architectures as reinforcing elements in natural plant-based composites for load bearing and structural applications.