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FACTA UNIVERSITATIS  
Series: Mechanical Engineering Vol. 21, No 4, 2023, pp. 631 - 656 

https://doi.org/10.22190/FUME230905046P  

© 2023 by University of Niš, Serbia | Creative Commons License: CC BY-NC-ND 

Review article 

AGRO-WASTE NATURAL FIBER SAMPLE PREPARATION 

TECHNIQUES FOR BIO-COMPOSITES DEVELOPMENT: 

METHODOLOGICAL INSIGHTS 

Resego Phiri1, Sanjay Mavinkere Rangappa1, Suchart Siengchin1, 

Dragan Marinkovic2,3 

1Natural Composites Research Group Lab, Department of Materials and Production 

Engineering, The Sirindhorn International Thai-German Graduate School of Engineering 

(TGGS), King Mongkut’s University of Technology North Bangkok (KMUTNB), Thailand 
2University of Niš, Faculty of Mechanical Engineering, Serbia 

3TU Berlin, Department of Structural Analysis, Berlin, Germany 

Abstract. In today's engineering industries, there is a growing focus on sustainable and 

eco-friendly products due to their recyclability, abundant availability and property 

variability. One key aspect of this sustainability effort is the development of eco-friendly 

materials, particularly bio composites derived from agricultural waste residues. The 

physical, mechanical and thermal properties as well as suitability of these fibers depend 

greatly on the methods used for extraction, processing, chemical modification and 

physical treatments. Understanding these processes comprehensively is essential for 

obtaining desired natural fibers/fillers from agricultural waste for creating effective bio 

composites to meet specific application demands. This study aims to provide a 

comprehensive assessment of the various extraction and modification techniques employed 

for natural fibers. It offers an in-depth review of diverse extraction processes, ranging from 

the initial harvesting to the decortication stage. It has been revealed that the choice of 

extraction methodologies depends on climate, water resources, local traditions, and the 

desired fiber quality. Additionally, the paper explores chemical and physical treatments, 

highlighting how each method influences the structure and properties of natural fibers. 

Overall, this review offers practical insights into the steps taken to transform agro-waste 

biomass into desired natural fiber and in turn biocomposite material, while enhancing 

product quality and performance. 

Key words: Natural fibers, Extraction processes, Chemical treatments, 

Physical/surface modifications, Bio-composite development 

 
Received: September 05, 2023 / Accepted November 28, 2023   
Corresponding author: Sanjay Mavinkere Rangappa  

Natural Composites Research Group Lab, Department of Materials and Production Engineering, The Sirindhorn 

International Thai-German Graduate School of Engineering (TGGS), King Mongkut’s University of Technology 
North Bangkok (KMUTNB), Thailand 

E-mail: mavinkere.r.s@op.kmutnb.ac.th 



632 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

1. INTRODUCTION 

In response to the escalating global apprehension and consciousness regarding 

environmental issues, numerous industrial sectors have progressively shifted their focus 

towards the adoption of sustainable and regenerative materials. Natural fibers, being 

inherently renewable and readily accessible within the natural ecosystem, have emerged as 

a pivotal avenue in this pursuit. This inclination is largely attributed to their array of 

advantages, including but not limited to, cost-effectiveness, lightweight nature, inherent 

renewability, biodegradability and an array of distinctive high-specific properties [1–3]. 

With the rise of agriculture, deliberate cultivation of fiber-yielding plants became established. 

Early civilizations explored plant fibers, particularly in textile weaving, long before 

perfecting practices like metalworking or pottery. Various types of fibers held significant 

importance for their versatility. Some were used for crafting ropes and yarns, essential for 

activities like hunting, fishing, netting, climbing, and transportation, while others found their 

place in the intricate art of textile weaving [4]. Comprised mainly of lignocellulosic materials, 

natural fibers encompass a variety of resources, including wood, ramie, jute, hemp, kenaf, 

sisal, coir, flax, bamboo and various fruit fibers. Effectively integrating natural fibers into 

composite materials is crucial for their practical application. Substituting synthetic materials 

with natural fibers aligns with the principles of environmental stewardship, reflecting the 

contemporary shift towards sustainability. Table 1 presents a comparison of attributes 

between natural and synthetic fibers. While natural fibers generally exhibit lower tensile 

strength and modulus compared to synthetic counterparts, they offer significant cost and 

ecological advantages. Variations in fiber structure and growth conditions contribute to the 

diverse mechanical properties observed in natural fibers. Through precise treatment 

procedures, these fibers can be tailored to serve as reinforcing fillers with various 

mechanical characteristics. Moreover, careful application of specific treatments can even 

impart mechanical properties similar to those of synthetic fiber composites. 

Table 1 Assessment of key properties of major synthetic and natural fibers [5,6]. 

 Fibers Density 

(g/cm3) 

Tensile 

Strength 

(MPa) 

Elastic 

Modulus 

(GPa) 

Decomposition 

Temperature (°C) 

Cost 

(Euros/Kg) 

 Carbon 1.8-1.9 2090-5200 525 3647 33-166 

 Kevlar 

(Aramid) 

1.4 2380-3100 124 496 16.7 

Synthetic Zirconia 5.6 700 100 2497 41.7 

 E-glass 2.5 1625-3400 72 756 1.6-2 

 C-glass 2.5 2800 69 756 9.3-16 

 Sisal 1.4 507-855 24 270 0.7 

 Flax 1.4 250-1000 12-100 280 0.15-0.76 

Natural Cotton 1.5-1.6 350 11 246 0.35 

 Banana 1.3 791 30 320 0.7-0.9 

 Jute 1.3-1.5 187-540 3-55 270 0.7 

The environmental significance of natural fibers lies in their seamless integration with 

agricultural waste resources, promoting a holistic approach to waste management and 

valorization. This convergence taps into the untapped potential of discarded agricultural 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  633 

by-products, granting them secondary utility and enhancing the sustainability of resulting 

composites. The global surge in waste production, fueled by rapid urbanization, population 

growth, and industrialization, has reached alarming levels. This surge encompasses various 

waste streams, including agricultural residues. Table 2 provides an overview of the promising 

agricultural product resources and their respective countries of origin. These residues, once 

regarded as mere by-products, hold the significant potential to be transformed into valuable 

natural fibers. Leveraging these residues addresses the dual challenge of waste reduction and 

resource optimization, offering a pathway to mitigate environmental impact. Agricultural 

residues fibers encompass a diverse array of chemical substances, predominantly composed 

of cellulose, hemicellulose, lignin and smaller quantities of pectin [7]. The composition and 

proportions of these components can vary depending on the type of plant species, climatic 

conditions and the method of extraction and fiber preparation approaches employed. 

Numerous comprehensive studies have evaluated the properties of agricultural waste-

based composites [8–10]. These investigations include the isolation and characterization of 

agro-waste materials, involving analyses of their physical, mechanical and chemical 

attributes. However, a notable research gap exists in the exploration of agro-residues and their 

associated extraction and preparation strategies employed prior to their deployment into bio 

composites. This gap underscores the need for a more comprehensive understanding of the 

crucial pre-processing steps involved in optimizing the utilization of agricultural residues in 

biocomposite materials. Achieving optimal performance in bio composites materials requires 

a comprehensive understanding of the interplay between fiber characteristics and processing 

techniques. By examining the techniques for fiber extraction, cleaning and modification, 

along with their impact on agro-waste fibers and their composite performance, this paper 

seeks to shed light on the intricate relationship between preparation methods and the final 

properties of the resulting composites. By understanding the fiber preparatory techniques, 

merits and obstacles, researchers, engineers and practitioners will be better prepared to fully 

exploit the capabilities of materials founded on natural fibers.  

Table 2 Production statistics of agricultural products that can be used as potential natural 

fiber resources [11] 

Production (million tons) 

Countries Banana Pineapple Coconut Sugarcane Rice Kenaf Jute Oil palm 

India 24.87 1.46 11.93 341.2 159.2 0.12 1.98 - 

China 10.55 1 0.25 125.54 203.29 0.08 0.17 0.67 

Philippines 9.23 2.4 15.35 31.87 18.44 - 0.002 0.47 

Brazil 6.9 2.48 2.82 0.74 11.76 14.2 26.7 1.34 

Indonesia 6.19 1.78 18.3 33.7 71.28 4.35 0.007 120 

Thailand 1.65 2.65 1.01 100.1 38.79 1.3 0.06 12.8 

Vietnam 1.56 0.54 1.31 20.08 44.04 8.2 0.02 - 

2. AGRO-RESIDUES AS SOURCES OF NATURAL FIBERS 

On a global scale, roughly 140 gigatons of biomass are discarded as waste annually, 

contributing significantly to environmental challenges such as the release of greenhouse 

gases and the deterioration of soil-water quality [12].  As the world's population continues 



634 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

to grow and debates surrounding the allocation of resources for food and fuel intensify, immense 

pressure is placed on the agricultural sectors to ramp up their agro-food production. This 

heightened demand and supply of agricultural products have consequently led to a surge in 

the disposal of biomass residues. Given these circumstances, the recycling and the 

secondary utilization of this waste have become a critical issue with notable social and 

economic implications. To address this, the strategic use of agricultural residue waste as 

natural fiber resources can be approached through three primary avenues: incorporating 

them as additives to improve compatibility among principal components, reinforcing them 

within plastic matrices to enhance structural integrity and utilizing them as cost-effective 

and renewable filler materials. Agricultural residues can be categorized into two distinct 

groups: field residues, which are remnants left in agricultural fields after crop harvesting 

(examples include pineapple leaves, cornhusks, cotton seed pods, kapok seed pods and 

cotton stalks), and process residues, which are materials generated after processing crops 

into usable resources (examples include bagasse, pea peel, wheat straw and rice straw). 

Table 3 provides an overview of some major agricultural residues harnessed for their 

natural fiber attributes [13].  

Table 3 Overview of prominent agro-residues utilized for natural fiber resources. 

Agro-residues Synopsis Fiber source Utilization 

Rice straw Rice, a staple food for most of the 

world's population, contributes not 

only to sustenance but also to a 

significant agricultural residue known 

as rice straw. The leftover rice straw 

presents a valuable opportunity as a 

source of natural fibers with diverse 

applications. About 731 million 

metric tons of rice straw are generated 

globally each year. 

Rice straw comprises 

the stalks and leaves of 

rice plants left behind 

after the grains are 

harvested. The primary 

fiber source within rice 

straw is the vascular 

bundles located within 

the stem. 

Used for 

making paper, 

packaging 

materials and 

handicrafts. 

 

Wheat straw 

 

This is a by-product resulting from 

the collection of wheat grains, 

consisting of segments between 

nodes, nodal points, foliage, husks, 

and central stem parts of wheat. 

Around 600 million metric tons of 

wheat straw are generated globally 

annually. 

Wheat straw 

encompasses the plant 

material that remains 

after wheat grains have 

been collected. Main 

part is the vascular 

bundles found in the 

stem. 

 

Primarily used 

in paper 

production and 

as packaging 

material. 

 

Corn cob and 

husks 

These maize/corn cultivation residues 

are typically removed and discarded 

before consuming the corn. Corn 

husks and cobs contribute to a 

substantial portion of agricultural 

waste, but specific statistics may vary 

by region. 

The corn cob is the 

innermost part of the 

corn plant, while the 

corn husks is the outer 

layer of green leaves 

that envelop the corn.  

Used for crafts, 

textiles and 

cellulose for 

paper 

production. 

 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  635 

Sugarcane 

bagasse 

Sugarcane bagasse is the aftermath of 

the sugarcane crushing process to 

extract the sugary juice. What is left 

behind is the fibrous material that 

comprises the stalks' outer layers, 

mainly consisting of the fibers and 

pulp that provided structural support 

to the plant. Global sugarcane 

production yields about 1.9 billion 

metric tons of bagasse annually. 

Fibrous residue left 

after sugarcane juice 

extraction. 

 

Used for paper 

production, 

packaging and 

as a biomass 

source for 

energy 

generation. 

 

Coconut husks This refers to the coarse and fibrous 

exterior layer that encases the coconut 

fruit. This protective covering acts as 

a natural shield for the inner seed of 

the coconut. The roughness and 

texture of the husk are due to the 

dense network of fibers that make up 

its composition. Coconut husks 

contribute to a significant portion of 

waste from coconut processing, with 

specific quantities varying based on 

coconut production levels. 

Outer fibrous shell of 

coconut. 

 

Used for ropes, 

mats, brushes, 

geotextiles, and 

horticulture 

substrates. 

 

Pineapple leaves Pineapple leaves, an often-overlooked 

component of the fruit-bearing plant, 

hold remarkable value beyond their 

decorative appearance. These leaves, 

situated at the crown of the pineapple 

plant, are a source of natural fibers 

known as piña fibers. Pineapple 

cultivation gives rise to a notable 

quantity of plant waste, about 250 

metric tons per hectare. 

Leaves of pineapple 

plants. 

 

Used for 

delicate textiles 

like piña cloth, 

as well as 

crafts. 

 

Fruits and 

vegetable waste 

Fruits and vegetable waste refers to 

the discarded or unused parts of fruits 

and vegetables that are generated 

during the harvesting, processing, or 

consumption of these food items. 

Estimates of global food waste, 

including fruits and vegetables, have 

suggested that roughly one-third of all 

food produced for human 

consumption is lost or wasted. 

Peels, skins, stems and 

other parts not 

consumed as food. 

 

Used as 

growing 

mediums for 

plants, helping 

improve soil 

structure, water 

retention, and 

aeration in 

horticultural 

settings. 

3. NATURAL FIBER EXTRACTION PROCESSES 

Natural fiber extraction serves as the fundamental and pivotal initial phase in the 

broader realm of fiber processing. This process encompasses a series of techniques aimed 

at isolating fibers from various plant sources [14]. Achieving this separation requires the 

delicate task of disrupting the connections or bonds between the robust stem cores and the 

delicate fiber bundles surrounding them. The broader context of the fiber extraction process 



636 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

extends beyond just straw; it encompasses processes of liberating fibers from diverse plants 

components. These methods play a crucial role in enhancing the quality, durability and 

usability of these fibers. In the context of agricultural waste, the residues undergo 

preparatory steps to make fiber extraction more efficient. Depending on the type of residue, 

this might involve harvesting, cleaning, drying and removing any unwanted materials like 

dirt or impurities. Figure 1 provides an overview of the key processes involved in the 

extraction of natural fibers for the development of bio composites.  

 

Fig. 1 Processes involved in the extraction of natural fiber from plant biomass to use as 

reinforcement phase in bio composites. 

3.1. Harvesting 

The process of harvesting natural fibers from various agricultural crop residues 

involves intricate systems that are often tailored to local conditions and practices. These 

systems are designed to effectively extract fibers from plants while considering factors 

such as the type of crop, the quality of the fibers needed and the prevailing agricultural 

practices in the region. As a result, harvesting methods can vary significantly depending 

on these factors. The development of harvesting systems is not a one-size-fits-all approach 

[15,16]. Rather, it is a dynamic process that considers the unique characteristics of each 

crop, the resources available and the expertise of local farmers. This localized approach 

ensures that the methods used are efficient, sustainable and suited to the specific agricultural 

context. One of the primary distinctions in harvesting methods lies in the choice between 

manual and mechanized approaches as shown in Figure 2. Manual harvesting is prevalent 

in many regions, particularly where labour is readily available and cost-effective. Skilled 

workers harvest fibers by hand, employing traditional techniques that have been refined 

over generations.  In contrast, mechanized harvesting involves the of technology and 

innovation. This harvesting method have gained popularity in areas where labour costs are 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  637 

higher or where the demand for efficiency is paramount. Specialized machinery is used to 

streamline the process, reducing labour requirements, and increasing overall output. 

  
a) b) 

Fig. 2 Comparison of manual harvesting and machine harvesting techniques in the context 

of natural fiber extraction from agricultural crops; (a) Manual harvesting, showcasing 

skilled workers meticulously handpicking fibers from the plants, (b) Machine harvesting, 

where specialized machinery efficiently gathers fibers from the plants. 

Moreover, within harvesting systems for natural fibers, two distinct methods often come 

into play: whole stem harvesting and cut stem harvesting. These methods are indicative of the 

intricate decision-making processes involved in optimizing the extraction of fibers from 

various agricultural produce. The whole stem harvesting approach involves gathering the 

entire plant, including the stalk, leaves and sometimes even the roots to extract fibers [17]. 

This method can result in minimal waste, as nearly all parts of the plant are used in some 

way. It can also simplify the processing steps, especially when the fibers are distributed 

along the entire length of the stem. In contrast, cut stem harvesting involves severing only 

the part of the plant that contains the desired fibers, often leaving behind leaves and other 

non-essential portions [18]. This method is frequently used for crops like sugarcane and 

bamboo. 

3.2. Fiber Retting 

Natural retting involves the controlled decomposition of non-fibrous materials that bind 

the fibers to the plant's stem. Through retting, this plant structure matrix is deliberately broken 

down, effectively loosening the fibers and making them accessible for further processing. A 

comparison between retting methods is shown in Table 4 [19–21]. The outcome of this 

transformative process has dual effect. First, the liberated fiber bundles, now freed from their 

binding matrix, emerge in a state that allows for their separation with greater ease. This sets 

the stage for subsequent processing steps, such as scutching and hackling, that refine the 

fibers and align them in a more uniform manner. Secondly, the residual fibers that remain 



638 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

after retting become enriched in cellulose content, a crucial factor contributing to the high-

strength properties of the extracted fibers [22].  

Numerous extensive inquiries and studies have been dedicated to comprehending the 

intricate influence of retting processes on the characteristics of natural fibers and their 

respective composites. Stawski et al. [23] examined the thermal and mechanical attributes 

of Okra fibers acquired through water retting and dew retting methods. These fibers were 

subjected to different processing techniques for the removal of non-fibrous components. 

Yu et al. [24] conducted a study involving the use of warm water retting as a pre-treatment 

method for banana stems. The research aimed to assess how varying durations of retting 

treatment impact both the physical and chemical properties of both the banana stems and 

the fibers derived from different parts of the plant. The findings revealed that longer retting 

periods were associated with a reduction in the presence of fiber impurities. However, this 

prolonged retting time also led to a decline in the strength and toughness of the resulting 

fibers. In their study, Narkpiban & Poonsawat [25] explored the impacts of hydrothermal 

pre-treatment on the characteristics of kenaf fibers subjected to water and enzyme retting. 

This involved treating the fibers in a high-pressure reactor at temperatures of 160, 180, and 

200°C. The findings revealed that both water and enzymatic retting led to an increase in 

cellulose content within the fibers.  

The implications of retting processes are not confined solely to the fibers themselves. 

The interactions between retted fibers and the matrix materials used in bio composites are 

a matter of considerable interest. For instance, De Prez et al. [26] conducted a study 

focusing on the mechanical properties of composites that were strengthened using fibers 

extracted after undergoing enzymatic treatments. The investigation also delved into factors 

such as fiber fineness, moisture absorption in the composites and the analysis of fracture 

surfaces. The findings indicated that all the enzymatic treatments exhibited encouraging 

results in terms of tensile strength and stiffness. Moreover, Rozyanty et al. [27] examined 

how different retting methods (water and mechanical retting) for kenaf bast fibers impact 

the tensile strength and water absorption properties of unsaturated polyester (UPE) 

composites reinforced with these fibers. The study revealed that water-retted kenaf bast 

fibers possess surfaces characterized by cleanliness, smoothness, minimal fiber detachment 

and fewer gaps compared to the mechanically extracted samples. 

Recent practices have witnessed the integration of advanced technologies and pre-

treatment methods into the retting process, exemplified by a study conducted by Ruan et 

al. [28]. In their research, they employed Radio Frequency (RF) pre-treatment to effectively 

break down non-cellulosic components present in the material. This pre-treatment process 

was particularly advantageous as it prompted the rapid absorption of water and expansion 

of the polar constituents within the flax phloem. As a result, this action effectively loosened 

the connections between individual fibers and fiber bundles, creating a more accessible 

pathway for enzymes to penetrate the material. The outcomes of their investigation 

revealed that the implementation of RF pre-treatment had a positive impact on the overall 

retting process. The enhanced breakdown of non-cellulosic materials and the improved 

water absorption and swelling characteristics contributed to an increase in retting 

efficiency. The choice between the retting methodology depends on factors such as climate, 

available water resources, local traditions and the desired quality of the fibers. Proper 

timing and monitoring are crucial in all methods to ensure that the fibers are neither under-

retted which can result in insufficient fiber separation nor over-retted leading to fiber 

damage. Réquilé et al. [29] conducted a study that affirms how the desired fiber quality in 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  639 

hemp can be ensured through the retting process by adjusting the duration of the retting 

process based on various weather conditions, such as the strength of solar radiation, the 

amount of rainfall and the surrounding temperatures. In situations where there is a shortage 

of rainfall or the air temperature is low, it becomes necessary to extend the retting period 

to allow for the attainment of an adequate level of retted biomass, which is crucial for 

achieving the desired fiber quality. Retting not only facilitates fiber separation but also has 

environmental implications. Improper disposal of retting water, which contains decomposed 

plant material, can lead to pollution of water bodies. Therefore, responsible retting practices are 

essential to minimize environmental impact. 

Table 4 The comparison between the natural fiber retting techniques. 

Retting 

method 

Process Description Merits Limitations Process 

duration 

Water 

retting 

The process of water retting 

involves immersing plant 

stems in water and 

monitoring them at regular 

intervals. As the water 

seeps into the central part 

of the stem, it causes the 

inner cells to swell, leading 

to the rupture of the 

outermost layer.  

▪ Remarkable 

degree of 

consistency and 

excellence in their 

overall quality.  

▪ Resulting fibers 

are characterized 

by an even and 

uniform level of 

retting throughout 

their structure. 

▪ Associated with 

significant pollution 

stemming from 

anaerobic bacterial 

fermentation. 

▪ Produces foul-

smelling odors. 

▪ The process is 

resource intensive. 

7-14 days 

Dew  

retting 

Plant stalks are evenly 

distributed across fields, 

allowing them to be 

exposed to ample sunlight, 

atmospheric air, and dew. 

This exposure facilitates the 

colonization of fungi, 

which in turn leads to the 

decomposition of cellular 

stem tissues and adhesives. 

▪ Minimal water 

requirement, 

making it a 

preferred option 

in regions facing 

water scarcity.  

▪ Relies on natural 

atmospheric 

conditions. 

▪ Reduced energy 

consumption and 

operational costs. 

▪ It relies heavily on 

weather conditions, 

making it less 

predictable and 

potentially slower 

than other retting 

methods.  

▪ It is highly susceptible 

to contamination with 

soil. 

2-10 weeks 

Enzymatic 

retting 

Involves the application of 

specific enzymes to 

facilitate the breakdown of 

non-cellulosic components 

that bind the fibers to the 

plant material. Enzymes 

facilitate the breakdown of 

gum and pectin materials 

present in the stem through 

a process known as 

hydrolysis. 

 

▪ Precise control 

over the retting 

process.  

▪ Reduced 

processing time 

compared to 

traditional retting 

methods. 

▪ More potential to 

produce high-

quality fibers with 

specific 

characteristics 

▪ High costs, as 

enzymes used can be 

expensive to acquire 

and apply.  

▪ Requires high level of 

expertise and training 

for the appropriate 

selection and 

implementation of 

enzymes.  

▪ Requires meticulous 

monitoring to achieve 

optimal conditions for 

enzyme activity. 

1 -2 days 



640 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

Retting 

method 

Process Description Merits Limitations Process 

duration 

Chemical 

retting 

Chemical retting is a fiber 

separation method that 

involves the use of 

chemical agents to break 

down the non-cellulosic 

components binding plant 

fibers to the stalks. These 

chemicals target and 

degrade the pectins, lignin 

and other adhesive 

substances, thus releasing 

the individual fibers. The 

plant material is typically 

immersed in the chemical 

solution for a specific 

period, allowing the 

agents to catalyze the 

breakdown of the binding 

materials. 

▪ Chemical agents 

enable 

manufacturers to 

finely regulate 

retting conditions, 

resulting in 

consistent and 

uniform fiber 

quality. 

▪ It is generally 

faster compared to 

some natural 

retting methods, 

significantly 

reducing 

processing times 

and increasing 

overall efficiency. 

▪ Allows for greater 

flexibility in 

adapting to 

various fiber 

types, making it 

applicable to a 

wide range of 

plant materials. 

▪ Necessitate careful 

management and 

treatment to prevent 

contamination of 

water sources. 

▪  The use of chemicals 

can lead to the loss of 

certain properties in 

the fibers, affecting 

their color, texture, 

and overall quality.  

▪ The reliance on 

chemicals also 

introduces an 

additional cost factor, 

as procuring and 

applying the 

necessary agents can 

contribute to higher 

production expenses. 

1-2 hours 

Microbial 

Retting 

Microbial retting 

harnesses the action of 

microorganisms to break 

down the non-cellulosic 

components binding plant 

fibers to the stalks. The 

harvested plant material is 

exposed to a controlled 

environment conducive to 

microbial activity. The 

microorganisms, 

including bacteria and 

fungi, produce enzymes 

that target and degrade 

pectins, lignin and other 

adhesive substances 

present in the plant 

material thereby 

loosening the fibers. 

▪ Relies on natural 

processes 

involving 

microorganisms 

rather than 

chemical agents 

hence presents a 

more sustainable 

approach aligning 

well with 

ecological ideals. 

▪ Challenging to control 

consistency as the 

process relies heavily 

on factors such as 

temperature, humidity 

and the availability of 

suitable 

microorganisms. 

▪ Microbial retting also 

results in longer 

processing times. 

▪ The process produces 

bad odors during the 

decomposition 

process. 

10-30 days 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  641 

3.3. Decortication 

In situations where fibers are tightly bound to the plant stem, a process called decortication 

is employed. This process entails the mechanical removal of the non-fibrous outer layers of 

the plant material to expose the fibers. Historically, hand decortication was the norm and 

involved pounding the leaves and scraping away the pulp with a knife, but this approach is 

labour-intensive and time-consuming [30]. Nowadays, mechanical decorticators offer an 

efficient alternative. These devices utilize rotating wheels with blunt knives to crush and beat 

the leaves, leaving behind only the fibers. Figure 3 illustrates how mechanical forces are applied 

to break the bonds between the fiber and its core. Various types of decorticators, such as 

crushing rollers, ball mills, hammer mills and drop weights, have been used for fiber processing. 

For high-value applications, additional treatment is often required to reduce unwanted non-

cellulosic remains in the mechanically extracted fibers. However, mechanical decortication 

can disrupt fiber cell wall structures, leading to defects that may negatively impact mechanical 

properties and composite performance [31]. The extent of such defects depends on factors 

like previous retting treatments [32]. Nevertheless, some research indicates that high-quality 

fibers can be obtained through mechanical decortication, especially in modern mills designed 

to maintain the integrity and length of the fibers by disentangling and aligning them [33]. The 

performance of a decorticator is influenced by several parameters, and understanding their 

effects can help improve current industrial decortication processes, even if there's a trade-off 

between fiber quantity and quality, depending on the chosen parameters and the condition of 

the natural fiber source. 

 

Fig. 3 Illustrations of mechanical decorticators in action; (a)automated decortication by 

hammering wherein drums with outward-extending hammers that repeatedly strike 

straw to break it down are employed and (b) automated decortication by roller 

milling wherein cylindrical rollers are used to break the biomass [36]. 

Following the initial decortication process, a crucial step known as post-decortication is 

carried out to refine the fiber extraction. This step involves the separation and cleaning of 

detached fibers from a mixture containing fiber/core-bound components and fine particles. 

Various methods are employed for this process. One common method is scutching, which 

involves further beating or mechanical action to separate the fibers from other components [34]. 

Another approach involves sieving, utilizing differences in particle size to isolate the fibers. 

These techniques contribute significantly to the overall quality improvement of the fibers 

extracted after decortication, making them more suitable for a wide range of applications across 



642 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

industries. Following the extraction of fibers through any of the methods, it is crucial to initiate 

a comprehensive washing process to remove any residual impurities. Subsequently, the fibers 

need to undergo a meticulous drying procedure. This is vital because the moisture content 

within the fibers can significantly impact their overall quality. Notably, artificial drying allows 

for better control over drying conditions, ensuring that the fibers are dried uniformly and without 

the risk of overexposure to sunlight, which can lead to bleaching and degradation [35]. In 

instances where natural drying is chosen, it is important to ensure that the fibers are protected 

from direct sunlight, they should be dried under shade to prevent them from becoming 

discolored or weakened due to prolonged exposure to sunlight. 

4. NATURAL FIBER CHEMICAL MODIFICATIONS 

One of the primary challenges encountered in utilizing natural fibers as reinforcements in 

polymer composites is the disparity between the hydrophilic nature of the fibers and the 

hydrophobic nature of the polymer matrix [37]. This incongruity can lead to poor interfacial 

adhesion and suboptimal composite properties. Addressing this issue involves reducing the 

inherent hydrophilicity of natural fibers through chemical treatment and surface modification 

techniques. Recent research endeavors have explored a range of chemical treatment processes 

targeted at enhancing the surface properties of natural cellulosic fibers. These treatments are 

aimed at refining the performance of these fibers across diverse applications.  

4.1. Mercerization 

This treatment procedure involves immersing the fibers in a solution containing strong 

alkaline substances like sodium hydroxide (NaOH). The mechanism of alkali treatment of a 

typical cellulose based natural fiber is exemplified in Figure 4. Alkaline agents break down the 

hemicellulose and lignin components present in the fiber, leaving behind a more refined form 

of cellulose. This treatment enhances the adhesion between the fiber and the matrix in composite 

materials through a two-fold process: Firstly, it diminishes the water-attracting characteristics 

of natural fibers and secondly, it clears away any impurities on the fiber's surface, resulting in a 

textured surface [38].The enhanced roughness texture of the natural fiber surface from this 

treatment facilitates a stronger connection between the fibers and the biopolymer matrix, 

achieved through mechanical interlocking. Another benefit arises from the reduction in the 

fiber’s hydrophilic nature, which curtails the interaction between the fiber's hydroxyl groups 

and water molecules [39]. Consequently, issues like fiber swelling, void formation and 

debonding in biopolymer composites are mitigated. This leads to the production of biopolymer 

composites exhibiting markedly advanced mechanical properties.  

 

Fig. 4 Illustration of a typical Alkali treatment reaction of cellulose-based fibers with NaOH 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  643 

Alkali treatment brings about a range of alterations in the characteristics of the natural 

fibers. The treatment's effects can encompass an elevation in the fiber's tensile strength, 

elongation at break and stiffness. The degree of these changes can be regulated by factors 

such as size and morphology of the fibers, soaking duration and concentration of the alkali 

solution [40]. The duration for which natural fibers are immersed in an alkali solution have 

been found to be a crucial factor for natural fiber performance. In a study conducted by 

Setyayunita et al. [41], the structural and functional characteristics of natural fibers exhibited 

noticeable variations when exposed to varying soaking durations. In the experiment 

conducted by Zin et al. [42] pineapple leaf fibers were immersed in alkali solution for 

durations of 1 and 3 hours. Their findings suggested that the most favorable tensile strength 

was achieved after a 3-hour alkaline treatment. In essence, the duration of the treatment 

significantly impacts the inherent properties of the fibers, leading to observable distinctions 

in the resulting behavior and attributes of the composite materials they contribute to. 

The concentration of the alkaline treatment has also been identified as a significant 

factor that profoundly affects both the performance of natural fibers and the properties of 

the resulting composite materials. Studies have consistently highlighted the substantial 

impact of the alkaline solution's concentration on the performance of natural fiber performance. 

Among the notable contributions in concentration effects in mercerization of natural fibers is 

the work by Boumaaza et al. [43], where the flexural properties of treated natural sisal, flax 

and jute fibers with respect to varying concentrations of NaOH (1.5%, 2%, and 4%) were 

investigated. In the study conducted by Cai et al. [44], abaca fibers were subjected to various 

concentrations (5%, 10%, and 15%) of aqueous NaOH solution. The research highlighted 

that elevated alkaline concentrations led to reductions in both tensile strength and Young’s 

moduli as a result of fiber breakage and complete removal of fiber binding materials. Table 

5 provides a synopsis of other investigations that have explored the alkali treatment of natural 

fibers for utilization in composites reinforced by natural fibers. 

Table 5 Examples of recent investigations related to alkaline treatments of natural fibers 

used in natural fiber reinforced composites. 

Fiber - matrix Chemical treatment procedure Outcome Ref. 

Abutilon indicum – 

Epoxy (L12) 

 

Initially, the stems were harvested and 

left to soak in water for approximately 

21 days, facilitating the extraction of 

the fibers. Following this soaking 

period, the fibers were cleaned and 

subjected to treatment with a 5% 

NaOH solution. The alkali-treated 

fibers then underwent another round of 

washing using purified water, then 

dried at 80°C to eliminate any 

remaining moisture content. 

The application of alkali 

treatment to alter the surface of 

the fibers led to a notable 

enhancement in the mechanical 

properties of the composite 

samples. Furthermore, the 

thermal stability of the 

composite samples treated with 

alkali exhibited an increase 

when compared to the untreated 

composites.  

[45] 

    



644 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

Fiber - matrix Chemical treatment procedure Outcome Ref. 

Dichrostachys 
Cinerea Bark fibers 
(DCFs) - Epoxy 

 

The initial process involved 
immersing the untreated DCFs in a 
solution of 5% (w/v) NaOH at room 
temperature. The immersion durations 
ranged from 30 to 120 minutes. 
Subsequently, the treated fibers were 
cleaned thoroughly until the solution 
was neutralized. Hydrochloric acid 
solution was added then final rinse 
with demineralized water was carried 
out before the fibers are dehydrated in 
an oven at 80°C for a duration of four 
hours. 

The alkali treatment exhibited a 
noteworthy impact on the 
crystalline index of the 
optimally treated DCFs due to 
the reduction of amorphous 
components. Furthermore, the 
cellulose degradation 
temperature of the optimally 
treated DCF improved 
significantly. Moreover, the 
mechanical characteristics of the 
chemically treated samples 
significantly outperformed those 
of the untreated raw samples. 

[46] 

Pineapple leaves - 
Araldite Epoxy resin 
 

Initially, the pineapple leaves were 
cleaned and dried for 24 hours in an 
oven at 70°C. Fibers were then cut 
into smaller lengths, approximately 2–
5 mm. Following this, the fibers were 
subjected to sodium hydroxide 
solution of concentrations of 3%, 5%, 
7% and 10% for an hour. Thorough 
cleaning with deionized water was 
carried out to ensure the removal of 
any excess alkali chemicals that may 
have adhered to the surface of the 
fibers during the treatment. 

The alkaline treatment 
demonstrated a multifaceted 
enhancement in the fiber's 
attributes, influencing their 
strength, thermal behavior and 
various compositional 
characteristics. 

[47] 

 

Salacca Zalacca fiber 
(SZF) - High Density 
Polyethylene 
(HDPE) 
 

The treatment of SZF involved 
immersing it in a 5% NaOH solution, 
with varying immersion times of 1, 2, 
3, 4, and 5 hours. The fiber was then 
washed with 1% acetic acid solution 
until a neutral pH value was achieved. 
Subsequently, the SZF was subjected 
to a rinse with distilled water, after 
which it was left to air dry at room 
temperature for approximately 3 days. 
A further dehydration process was 
undertaken using an oven set to a 
temperature of 35°C for 8 hours. 

Applying alkali treatment to 
SZF leads to improvements 
across several key parameters, 
including density, crystallinity 
index, tensile properties and the 
interfacial shear strength 
between the fibers and the 
matrix. 

[48] 

 

Chestnut cupula fiber 
- recycled high-
density polyethylene 
(r-HDPE) 
 

The cupula fibers were carefully dried 
in a conventional air oven until their 
moisture content was reduced to less 
than 1%. Subsequently, the dried 
cupula fiber underwent treatment with 
a 5 wt% NaOH solution for around 24 
hours at room temperature. After the 
treatment, the fibers underwent 
multiple washes with distilled water to 
ensure the removal of excess alkali. 
The fibers were once again dried in an 
oven, this time at 60°C for a period of 
24 hours. 

The composites consisting of 
alkali-treated cupula fibers 
exhibited superior mechanical 
properties in comparison to their 
untreated counterparts. This 
improvement can be attributed 
to heightened surface roughness 
and enhanced interface 
adhesion. 

[49] 

 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  645 

Fiber - matrix Chemical treatment procedure Outcome Ref. 

Lagenaria Siceraria 

fiber (LSF) – Epoxy 

 

The fibers were washed and 

subsequently immersed in a solution 

containing NaOH with a concentration 

of 4% (w/v) at room temperature. 

Following this, the fibers were 

subjected to a rinsing process 

involving tap water. Afterwards, the 

fibers were washed with a solution 

consisting of 1% (v/v) acetone and 

distilled water. Fibers were then dried 

in an oven at 60°C for 24 hours. 

The compressive, flexural, 

impact, and tensile strengths of 

the composite made from 4% 

alkaline-treated LSF were found 

to surpass those of the untreated 

LSF composite. This 

improvement is attributed to the 

robust interlocking between the 

fibers and the matrix. 

[50] 

 

Kenaf - Polyvinyl 

alcohol (PVA) 

 

 This treatment was carried out by 

immersing the fibers in a pre-prepared 

NaOH solution with concentrations of 

4, 6, 8, and 10 weight percentage 

(wt.%) for a duration of 4 hours, all at 

room temperature. The fibers were 

subjected to rinsing using deionized 

water until a neutral pH was achieved. 

Treated fibers were then carefully 

dried in a drying oven set to a 

temperature of 60°C. 

The utilization of NaOH 

treatment effectively removed 

impurities, resulting in a cleaner 

and more textured surface for 

the Kenaf fibers. Additionally, 

the treatment led to substantial 

improvements in the tensile 

properties of the fibers. 

[39] 

4.2. Silane treatment 

This procedure involves the interaction of the fiber with a silane coupling agent followed 

by fiber drying and curing stages. The application of silane treatment yields a range of 

potential modifications on natural fibers. These modifications can involve enhancing the 

hydrophobic nature of the fibers, along with altering their surface morphology and chemical 

characteristics. The process of applying silane treatment to natural fibers involves three 

distinct reactions: hydrolysis, self-condensation, absorption and chemical bonding as 

illustrated in Figure 5 [51]. Silane molecules undergo hydrolysis when exposed to water and 

ethanol, releasing alcohol and silanol groups. Subsequently, the silanol groups initiate a self-

condensation process, which is deliberately moderated by maintaining an acidic pH 

environment. This control ensures the presence of smaller molecular-sized free silanol 

groups. Upon introducing fibers to the silane solution, the hydroxyl groups found in the 

cellulose/hemicellulose of the fibers absorb the silanol groups through hydrogen bonding. 

This interaction leads to the formation of robust polysiloxanes (-Si-O-Si-) structures which 

ultimately establish connections with the matrix through chemical bonding [52]. 

The degree of modification in natural fibers can be controlled by adjusting key factors: 

the concentration and duration of the silane treatment, as well as the specific type of silane 

coupling agent used. This nuanced control has been extensively explored by numerous 

researchers and scientists, leading to valuable insights. For instance, Youbi et al. [53] conducted 

a study evaluating the impact of silane treatment on surface modification of Raphia vinifera 

Fibers (RVF). In their research, fibers were subjected to various concentrations of silane 

solutions (1% and 5%) to gauge their effects. Alshahrani & Prakash [54] carried out 

investigations specifically focused on the effects of silane treatment on fibers and their 

subsequent impacts on composite properties. In their work, raw chopped fibers and peel-



646 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

dried particles were treated with amino silane using the acid hydrolysis method. In a 

separate study by Sabarinathan et al. [55], a novel cellulosic fiber extracted from the peduncle 

portion of the fish tail palm tree underwent treatment with different concentrations (1%, 5%, 

and 9%) of silane solution. Across these research works, a consistent trend emerged: silane-

treated fibers exhibited superior mechanical properties when compared to untreated fibers. 

Additionally, it was noted that higher concentrations of coupling agents and prolonged 

treatment durations led to more pronounced modifications in the properties of the fibers. In 

addition, certain investigations have delved into the impacts of various silane coupling agents. 

For instance, Wang et al. [56] conducted a study focusing on the mechanical properties and 

thermal behavior of bamboo fibers reinforced polypropylene composites under the influence of 

different silane treatments. This study involved the use of three distinct silane coupling 

agents: γ-Aminopropyltriethoxysilane, γ-Methacryloxypropyltrimethoxysilane and 3-

Glycidyloxypropyltriethoxysilane. The research found that the tensile strength and flexural 

strength of the silane-treated fiber composites displayed enhancements when compared to 

untreated fiber composites, this was attributed to an enhanced interfacial adhesion between the 

fibers and the matrix. This study underscored the importance of selecting the appropriate silane 

coupling agent to achieve desired improvements in the performance characteristics of 

composite materials. 

 

Fig. 5 The general mechanism underlying the formation between silanes (e.g. 3-

Glycidyloxypropyl trimethoxysilane shown here), and the surface of the fiber. 

4.3. Acetylation 

Acetylation, a widely employed chemical treatment, serves as a versatile method for 

altering the characteristics of natural fibers. This treatment revolves around the interaction 

of cellulose with an acetylating agent, such as acetic anhydride or acetic acid, usually in 

the presence or absence of a catalyst as illustrated on Figure 6. The fundamental concept 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  647 

behind acetylation is the substitution of hydroxyl groups (OH) present in natural fibers with 

acetyl groups (CH3CO), effectively rendering the fiber surface more hydrophobic [57]. 

The acetylation process can yield a spectrum of modifications to the properties of natural 

cellulosic fibers, encompassing heightened resistance to water, improved dimensional 

stability and increased thermal stability. Furthermore, the treatment can induce alterations 

in the fiber's surface morphology and bring about changes in its chemical and physical 

attributes. The extent of modification achievable can be fine-tuned by adjusting the 

concentration and duration of the acetylation treatment. Oladele et al. [58] conducted 

investigations which revealed that employing higher concentrations of acetylating agents 

and extending the treatment duration result in more pronounced transformations in the 

natural fiber's properties. Beztout et al. [59] investigated how acetylation processes and 

cellulose content affect mechanical, thermal and rheological properties of PCL/cellulose 

composites. The treatment resulted in improved interfacial adhesion and dispersion of 

cellulose within the PVC matrix, leading to an enhanced surface morphology. Furthermore, 

Boustani et al. [60] conducted a comprehensive investigation into the impact of solvent-

free acetylation using acetic anhydride on fiber structures. In their study, they acetylated 

flax and wood pulp fibers at room temperature in a solvent-free system, employing acetic 

anhydride and sulfuric acid as a catalyst. The results revealed a direct relationship between 

the degree of fiber acetylation and the reaction duration. Additionally, Souza Da Rosa et 

al. [61] explored the feasibility of acetylation techniques applied to natural fibers derived 

from waste materials originating from pulp and paper mill operations. Their study 

encompassed four distinct acetylation methodologies, broadening the understanding of the 

potential applications of acetylation across various fiber sources. 

 

Fig. 6 Acetylation mechanisms, (a) with acid catalyst and (b) without acid catalyst. 

4.4. Bleaching 

Bleaching is a common chemical treatment employed to alter the characteristics of natural 

cellulosic fibers like cotton, bamboo and hemp. This treatment involves the use of various 

chemical agents, including hydrogen peroxide, chlorine dioxide and sodium hypochlorite [62]. 

The selection of a specific bleaching agent hinges on the unique requirements of the fiber in 

question and its intended application. Through the bleaching process, natural fibers undergo 

several alterations, such as increased luminosity, enhanced capacity for dye absorption and 



648 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

improved mechanical attributes. However, if not conducted properly, bleaching can also 

weaken the fibers. The extent of these modifications is influenced by factors like the type and 

concentration of the bleaching agent, the temperature, the pH conditions and the duration of 

treatment [63,64]. However, there are growing concerns regarding the ecological consequences 

of the chemicals utilized in conventional bleaching procedures. As a result, efforts are underway 

to develop more sustainable alternatives involving the use eco-friendly bleaching agents and 

enzymatic bleaching approaches. Many investigative studies in this regard have been explored. 

For instance, Pereira et al. [65] explored the influence of acetosolv and environmentally friendly 

bleaching treatment on orange bagasse fiber. Their findings suggest that all the treatment 

variables employed in their investigation significantly contribute to the fiber's characteristics. 

The use of this eco-friendly bleaching approach holds promise for employing orange bagasse 

fibers as reinforcement in polymer matrix applications. In a study conducted by Zhang et al. 

[66], an innovative and eco-friendly scouring and bleaching method for flax rove was 

investigated, utilizing supercritical carbon dioxide fluid instead of a traditional water-based 

medium. In comparison with the conventional water-based approach, this proposed 

supercritical bleaching process conserves substantial amounts of water and energy, establishing 

it as an eco-friendlier option for producing flax rove and other materials used in the textile 

industry. Furthermore, Sojka-Ledakowicz et al. [67] introduced a bleaching method that 

employs the hydrogen peroxide in the gaseous phase. Their results demonstrated that vaporized 

hydrogen peroxide-based bleaching is not only environmentally friendly but also operates at 

lower temperatures, saves energy and generates minimal waste compared to the conventional 

water- and energy-intensive bleaching process. Regarding the use of enzymatic bleaching Singh 

et al. [68], proposed the use of crude xylano-pectinolytic enzymes for bleaching ramie fibers 

intended for textile applications. Their conclusion highlighted the environmental safety of 

enzymatic bleaching, which efficiently removed non-cellulosic impurities, rendering the fibers 

more suitable for textile use compared to chemical bleaching. Similarly, Hassan et al. [69] 

conducted bleaching experiments using protease, xylanase and alkaline protease enzymes to 

evaluate the effects of enzymatic bio-scouring on the physiochemical and mechanical properties 

of fiber fabrics. Their enzymatic approach yielded results on par with the conventional chemical 

method. 

4.5. Grafting 

Grafting treatment is a surface modification process employed to alter the properties of 
natural cellulosic fibers through the introduction of polymer chains onto the fiber surface, 
as illustrated in Figure 7. It is achieved through the chemical bonding of monomers or 
polymers onto the fiber surface via covalent bonds, resulting in the creation of a polymer 
layer on the fiber surface [70]. Various techniques, such as radiation-induced grafting, 
chemical grafting, and enzymatic grafting, can be used for the grafting process. The choice 
of technique is determined by the specific properties required for the fibers and the intended 
application. Grafting can bring about diverse alterations in the properties of natural 
cellulosic fibers, including increased hydrophobicity, enhanced mechanical properties and 
improved thermal stability. The properties of the grafting layer can be finely tuned by 
adjusting the type and concentration of monomers or polymers utilized, as well as the 
reaction conditions. These variables have been thoroughly explored by researchers. In a 
study conducted by Tataru et al. [71], the radiation grafting procedure of flax natural fibers 
was investigated. The grafting of epoxidized plant oils onto flax fabrics was found to 
facilitate the formation of an interphase when cured with an epoxy-amine thermoset matrix. 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  649 

While the composite's tensile strength exhibited a significant improvement of 10-20% 
compared to untreated fabric composites, the study highlighted the potential of radiation 
grafting in enhancing composite properties. Additionally, research by Bao et al. [72] 
demonstrated the application of enzyme-initiated grafting for the modification of cellulose 
fibers. Besides its excellent environmental friendliness, this method showcased the ability to 
graft chains with well-controlled structures and lengths onto cellulosic fiber surfaces. As a 
result, this approach offers an environmentally friendly and controllable means for grafting, 
which can be effectively employed in the production of various functional cellulosic materials. 

 

Fig. 7 Illustration of the grafting process [73] 

5. SURFACE/PHYSICAL TREATMENT 

Physical treatments can also be used to modify the surface of natural cellulosic fibers 

to improve their compatibility with other materials or enhance their properties.  Physical 

treatments do not alter extensively the chemical composition of the fibers, and therefore 

the interface is generally enhanced via an increased mechanical bonding between the fiber 

and the matrix. One the most attractive feature of physical methods is the fact that they are 

‘clean’ since they do not involve chemicals. Some common physical treatment methods 

are outlined in Table 6. 

Table 6 Physical treatment processes for natural fibers. 

Surface 
treatment 

Description Benefits Drawbacks Sources 

Plasma 
treatment 

Plasma treatment is a 
process that enhances 
natural cellulosic fiber’s 
surface properties using 
low-pressure, non-
thermal plasma. This 
plasma is created by 
applying an electric field 
to a low-pressure gas 
(e.g., oxygen, nitrogen, or 
argon), generating 
reactive species. These 
reactive species interact 
with the fiber surface, 
introducing functional 
groups, surface etching or 
crosslinking. 

▪ Effectively modifies 
the surface of natural 
polymers without 
altering their core 
properties. 

▪ Increases the 
wettability of the 
fiber-matrix 
interface, without 
causing pollution. 

▪ The etching 
mechanism can 
lead to surface 
degradation and 
creates pits on the 
fiber surface. 

[74–76] 

  ▪  ▪   



650 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

Surface 
treatment 

Description Benefits Drawbacks Sources 

Corona 
treatment 

Corona treatment is an 
electrical process where 
high voltage is applied 
near the fiber surface, 
generating a corona 
discharge. This discharge 
produces reactive species 
like ozone, which 
introduce polar groups to 
enhance fiber surface 
energy and improve 
adhesion to polymer 
matrices. 
 

▪ Swiftly and 
efficiently enhances 
the bonding between 
natural fibers and 
diverse polymer 
matrices. 

▪ Enhances wettability 
by increasing surface 
polarity, promoting 
compatibility 
between hydrophilic 
fibers and 
hydrophobic 
matrices. 

▪ Extended 
treatment 
durations could 
lead to the 
development of a 
considerably 
uneven fiber 
surface. 

▪ Surface ablation 
and etching from 
the process 
diminishes fiber 
strength. 

[77–79] 

UV/Ozone 
treatment 

UV/Ozone treatment of 
natural fibers is a surface 
modification technique 
that involves exposing the 
fibers to ultraviolet (UV) 
light in the presence of 
ozone. This process 
induces photochemical 
reactions on the fiber's 
surface, leading to the 
formation of oxygen-
containing functional 
groups. These newly 
introduced polar groups 
enhance the fiber's 
wettability and adhesion 
properties, making it 
more compatible with 
polymer matrices and 
other materials in various 
applications. 

▪ Enhanced fiber 
surface properties by 
introducing polar 
functional groups, 
resulting in improved 
wettability and 
enhanced adhesion to 
polymer matrices.  

▪ Highly versatile and 
can be applied to a 
wide range of natural 
fibers, making it 
adaptable to various 
industries. 

▪ Fibers can be 
subjected to UV 
degradation. 

▪ The treatment's 
effectiveness may 
vary depending on 
the type of natural 
fiber. 

▪ While the process 
is environmentally 
friendly, its energy 
consumption may 
be a concern in 
terms of overall 
sustainability. 

[80,81] 

Thermal 
treatment 

Thermal treatment of 
natural fibers is a surface 
modification process that 
involves subjecting the 
fibers to elevated 
temperatures in a 
controlled environment. 
During this treatment, the 
fibers are exposed to heat 
in an inert gas 
atmosphere. The high 
temperature induces 
chemical changes on the 
fiber's surface, which can 
include the removal of 
impurities, moisture and 
hemicellulose, resulting 

▪ Improved thermal 
stability of the fibers, 
this is beneficial 
where fibers may be 
exposed to elevated 
temperatures. 

▪ Enhanced flame 
resistance of natural 
fibers, reducing their 
flammability and 
making them less 
susceptible to 
catching fire or 
sustaining 
combustion. 

▪ Increased 
crystallinity of 

▪ If not controlled 
precisely and 
subjected to long 
exposure, natural 
fibers mechanical 
properties become 
compromised. 

▪ Heat treatment of 
the fibers can also 
make them more 
brittle and reduce 
their flexibility, 
impacting their 
suitability for 
certain 
applications. 

[82–84] 



 Agro-waste Natural Fiber Sample Preparation Techniques for Bio-composites Development...  651 

in enhanced fiber purity 
and increased cellulose 
content. 
 

natural fibers, higher 
crystallinity often 
leads to improved 
mechanical 
properties. 

Electron 
radiation  

Electron radiation 
treatment of natural fibers 
is a surface modification 
technique that involves 
exposing the fibers to 
high-energy electrons. 
During this process, the 
electrons penetrate the 
fiber's surface, causing 
molecular changes and 
enhancing its surface 
properties. The energy 
from the electrons 
initiates chemical 
reactions on the fiber's 
surface, leading to the 
formation of new 
functional groups and 
increased wettability. 
 

▪ This treatment is 
highly controllable, 
allowing for precise 
adjustments to 
achieve desired 
modifications. 

▪ The exposure to 
irradiation-
induced 
degradation will 
lower the peak 
thermal 
decomposition 
temperature of 
materials with 
high cellulose 
content. 

[85,86] 

Dielectric 
barrier 

Dielectric barrier 
treatment of natural fibers 
is a surface modification 
method that employs a 
high-frequency electrical 
discharge in a controlled 
gas environment to 
enhance the fiber’s 
surface properties. This 
process involves placing 
the fibers between two 
electrodes separated by a 
dielectric material, 
creating a barrier that 
prevents direct electrical 
discharge contact with the 
fibers. When high-voltage 
alternating current is 
applied, it ionizes the 
surrounding gas, forming 
a plasma. Reactive 
species generated within 
the plasma chemically 
modify the fiber surface, 
introducing polar 
functional groups and 
increasing surface energy. 

▪ Reduces water 
absorption, enhances 
chemical resistance 
and fosters stronger 
interfacial bonding, 
ultimately increasing 
the overall 
performance and 
versatility of natural 
fibers in a wide range 
of applications. 

▪ It requires 
specialized 
equipment and 
expertise to create 
and maintain the 
necessary 
electrical 
discharge 
conditions. 

▪ The process may 
not be suitable for 
all types of natural 
fibers, and 
achieving uniform 
treatment across 
large fiber 
surfaces can be 
challenging. 

[87,88] 
 



652 R. PHIRI, S. M. RANGAPPA, S. SIENGCHIN, D. MARINKOVIC 

6. CONCLUSIONS 

In response to the growing global concern for environmental issues, many industrial 

sectors have increasingly shifted their focus towards adopting sustainable and eco-friendly 

materials. The use of agro-waste residues as natural fibers for biocomposite development has 

emerged as a promising alternative, primarily because these resources are inherently renewable, 

readily available in the natural ecosystem and help address waste management challenges. 

Despite the advantages these materials offer, they face challenges in commercialization due to 

issues such as poor compatibility with polymer matrices, limited wettability, high moisture 

absorption, and reduced durability. These limitations are closely tied to both the nature of the 

fibers themselves and the extraction and preparation processes involved in transforming them 

into final products. Therefore, it is crucial to comprehend the impact of fiber extraction methods 

and fiber modification procedures to tailor fiber properties to specific application requirements. 

This review provided a comprehensive understanding of the preparation and processing 

methods of agro-waste residues into suitable natural fibers for composite development. It was 

highlighted that the selection of suitable harvesting, retting and decortication method plays a 

crucial role in the physical properties of the fibers. Additionally, it was deliberated that the 

choice of retting method depends on factors such as climate, water resources, local traditions 

and desired fiber quality. Furthermore, various chemical treatments approaches used for natural 

fibers offer a range of properties by chemically altering the chemical composition of the fiber 

thereby enhancing compatibility with matrices and modifying hydrophilicity. Physical 

treatment approaches also play a vital role in modifying the surfaces of natural fibers without 

changing their chemical compositions. Various physical treatment techniques have been 

assessed to determine their merits and drawbacks in the modification of natural fiber surfaces. 

Overall, it is important to understand the holistic process of fiber preparation, starting from 

its origin in agricultural fields to the production of the final fibers or fillers that are essential 

for effectively incorporating into a sustainable and eco-friendly composite material. 

Acknowledgement: This research was supported by King Mongkut’s University of Technology 

North Bangkok (KMUTNB), Thailand under Grant No. KMUTNB-PHD-66-02. This research was 

also funded by National Science, Research and Innovation Fund (NSRF), and King Mongkut’s 

University of Technology North Bangkok with Contract no. KMUTNB-FF-67-A-01. 

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