Effect of water retting on the physical and mechanical properties of extracted pine needle fibers

Due to the negative impact of synthetic fibers on the environment, natural fibers are being increasingly used in composite preparation. The hilly area of Uttarakhand state in India has an abundance of pine needle fibers (PNFs). This study focuses on extracting the PNFs using water retting, varying the retting duration (2 weeks, 4 weeks and 6 weeks) and measuring its physical and mechanical properties. After 4-weeks of retting, the extracted PNFs exhibited the lowest moisture content, resulting in improved hydrophobicity and rendering them suitable for reinforcement in composites. The tensile strength and Young’s modulus of the 4-weeks retting PNFs were 52.89 ± 12.77 MPa and 1.48 ± 0.39 GPa, respectively. Mechanical characteristics of extracted fibers have been assessed using Weibull’s statistical method in terms of tensile strength and Young’s modulus. Based on the findings, PNFs are predicted to serve as a reinforcement in environmentally friendly composites.


Introduction
Glass, carbon, and aramid fibers are popular reinforcing materials in composites.These synthetic materials, however, need a lot of energy to process and may be dangerous to workers and users.Additionally, due to environmental regulations and concerns, their non-biodegradable nature restricts their usage.As a result, plant based natural fiber are increasingly used as substitutes for synthetic materials [1].Nowadays, there is a growing focus on using natural fibers in composite fabrication, as they offer numerous benefits, such as being biodegradable, renewable, lightweight, non-abrasive, innocuous, and affordable [2].At present, the industry's demand for cellulosic fibers exceeds the current production capacity.To address this issue, researchers have introduced various natural fibers like cymbopogon flexuosus fiber, cocania grandis, acacia arabica, grand naine penduncle, mulberry barks fiber, kusha grass and althaea officinalis L. fiber to serve as reinforcement materials for composite [3].The local people in the Himalayan region utilize pine needles in various ways, such as for decorative items, animal bedding, and energy feedstock [4].In mountainous regions across the globe, there are a lot of pine needle trees.Environmental pollution occurs throughout the summer when the forest catches fire and decimates a large number of pine trees [5].Pine needles from pine trees can be utilised in composite fabrication, preventing wasteful burning and reducing the consumption of synthetic fiber.The qualities of a biocomposite are governed by the properties of its fibers, matrix, and interfacial bonds [6].Thus, it is vital to understand the fiber's qualities before constructing a green composite, which majorly depends on its extraction process.Plant-based natural fibers can be extracted using a variety of methods, including mechanical, chemical, water retting, and steam explosions, each process have their own advantage and disadvantage [7].Water retting involves the separation of individual fibers from the stem or leaf by eliminating amorphous material attached to 1291 (2023) 012017 IOP Publishing doi:10.1088/1757-899X/1291/1/012017 2 the fibers.This process requires the removal of pectin and other cementing compounds that bind the fibers, resulting in the production of single fibers.The success of the process depends on the condition and duration, which determine the yield and quality of the fibers [8].The retting process involves using enzymatic activity to remove non-cellulosic materials from the fiber bundle.In the case of water retting, anaerobic bacteria fermentation is employed to produce enzymes that break down the components which bind the fiber together [6].This method is simplest and eco-friendly in all, hence selected for the current study.
This study aimed to examine the effect of various water retting periods on the physical and mechanical properties of the extracted PNFs.Pine needles were collected from Pauri Garhwal, Uttarakhand, and their physical properties, such as average diameter, density and moisture content, were evaluated.Furthermore, single fiber tensile strength and Young's modulus were determined for different retting periods and subsequently compared.Minitab 16 software was utilized to examine the Weibull distribution of mechanical properties in the extracted samples.

PNFs extraction
Natural fibers like kenaf, hemp, and jute are obtained through conventional water retting.If the process is under-retted or over-retted, it can negatively impact the quality of the fiber [7].So, the optimal duration for water retting is a crucial factor in achieving high-quality fiber.The pine needle was taken from the Pauri Garhwal area in Uttarakhand, India (Figure 1(a)).After collecting the Pine needles from the pine tree, the leaves are separated from stem (Figure 1(b)).The needles were then dipped in water for microbial degradation (Figure 1(c)).Microorganisms are main cause in water retting which separates the plant-based fiber's layers, producing high-quality fibers without causing any physical damage [6].Three different times were chosen for water retting after lots of trials.The duration for the retting was selected as 2 weeks (fiber retted for two weeks in water), 4 weeks (fiber retted for four weeks in water) and 6 weeks (fiber retted for 6 weeks in water).After completion of water retting, the fiber was separated with the help of a wire brush [9].

Diameter measurement
A total of 25 PNFs specimens were chosen randomly to calculate the mean diameter of 2, 4, and 6 weeks water-retted PNFs.An optical microscope (Radical-RSMr-3) with a 40X magnification was used to measure the fiber diameters.The average diameter of the 2, 4, and 6 weeks water-retted PNFs is provided in Table 1.
The density of PNFs was measured using pycnometer.For this arrangement, a liquid with a known density is utilised as a reference liquid (toluene = 866 kg/m 3 ) [10].Equation 1 calculates extracted PNFs density.
Where m1 = pycnometer weight, m2 = weight of pycnometer filled with PNFs, m3 = weight of toluene with the pycnometer, and m4= weight of pycnometer filled with toluene and PNFs.

Moisture Content
The oven-dry weight method was employed to determine the moisture content of PNFs, where initial and oven-dry weights of the fiber were utilized in the testing process [1].Three repetitions were carried out to ensure data accuracy, and the resulting mean values were reported.Equation ( 2) calculates PNFs moisture content.
% of moisture in PNFs = The weight of the fiber before drying is denoted by W1, whereas weight of the fiber after drying in a hot air oven is denoted by W2.

Tensile Test of PNFs
The mechanical properties of single PNFs were evaluated using the ASTM D 3822 standard test method [11].To maintain a tight and straight hold on the test clamp, the fiber was pasted onto a paper frame.A 25 mm gauge length was utilized with a paper frame as depicted in Figure 2 (a) to conduct the test.The paper frame was then clamped between the top and bottom jaws of the Universal Testing machine as shown in Figure 2(b).The Instron 5982 was used to measure the pine needle's tensile strength and Young's modulus, with a cross-head speed of 15 mm/min and a load cell of 5 kN.Just before testing, the paper was carefully cut down the middle.

Diameter, density and moisture analysis
The diameter, density and moisture content of 2, 4 and 6 weeks water-retted PNFs were analysed as shown in Table 1. Figure 3 (a), (b) and (c) shows the diameter measurement of PNFs.The PNFs average diameter of 2 weeks of retted fiber is higher than 4 weeks and 6 weeks of retted fiber (Table 1).The result indicated that the average diameter of the fibers was reduced with respect to increasing the duration of water retting.The fiber density of 6 weeks retted fiber shows the highest value of 1040 kg/m 3 followed by 2 weeks (923 kg/m 3 ) and 4 weeks (1011 kg/m 3 ) retted fiber which indicates that fiber from 6 weeks is highly dense, Senwitz reported similar result et al. [12] for bast fiber.However, all retted PNFs confirmed a lower density than synthetic fibers like E-glass (2500 kg/m 3 ) and S-glass (2500 kg/m 3 ), making them a potential substitute for lightweight applications [3].Natural fibers reinforced composites have a significant disadvantage of having a greater tendency to absorb moisture, resulting in dimensional instability of the composites and decreased mechanical properties [13].Moisture absorption in 4 weeks of extracted PNFs is less than in 2 weeks and 6 weeks of extracted PNFs.After 2 weeks of water retting, the PNFs contained 8.80% moisture, while PNFs retted for 4 weeks had a moisture content of 8.45%.However, PNFs that were retted for 6 weeks had a higher moisture content of 17.04%.

Analysis of mechanical properties
At each period of water retting, 25 sample fibers were randomly selected for mechanical properties testing  Tensile testing on a single lignocellulosic fiber to analyse natural fiber mechanical characteristics are difficult owing to the high degree of variation.This variety is caused by the fiber's source, plant's age, extraction method, and flaws.To address this issue, a two-parameter Weibull distribution was used to analyze natural fiber mechanical properties [21].By utilizing the Weibull distribution to plot the data, it becomes possible to estimate statistical parameters such as the shape and scale factors.Additionally, this method can help in validating the accuracy of the collected data [22].The probability distribution plot in Figure 5 represents the data obtained from 25 trials of fiber tensile strength and Young's modulus.In Figure 5, the values lie within the line and are a perfectly fit to the distribution, indicating that they closely correspond to the experimental outcomes.This study examined how the duration of water retting PNFs' physical and mechanical properties.The water-retted extracted PNFs after 2, 4, and 6 weeks have a lower density than synthetic fiber.Hence they can be used as reinforced material in lightweight composites.Compared to 2 weeks and 6 weeks of extracted PNFs, 4 weeks extracted PNFs absorb less moisture.The less moisture content could potentially improve the adherence of the fibers to the matrix in composites when used as reinforcement.The significant differences in tensile strength and Young's modulus, were seen in PNFs retted for 2, 4, and 6 weeks.The extracted fibers, after the retting process, offer good mechanical properties.Water retting process for 4 weeks gives the best tensile strength and Young's modulus compared to 2 weeks and 6 weeks water-retted fiber.The results above suggest that pine needle fibers retted for 4 weeks are the best suitable reinforcement for the composite fabrication.

Figure 1
Figure 1 (a) Pine Trees (b) Pine needles and (c) Pine needles in water

Figure 2
Figure 2 (a) Paper frame Specimen and (b) Specimen with tensile testing machine

Table 1
Comparison of physical and mechanical properties of PNFs with other natural fibers