Coconut Coir Fiber Composites for Sustainable Architecture: A Comprehensive Review of Properties, Processing, and Applications

Abstract

The growing need for sustainable materials in architecture has sparked significant interest in natural-fiber-based composites. Among these, coconut coir, a by-product of the coconut industry, has emerged as a promising raw material owing to its abundance, renewability, and excellent mechanical properties. The promise of coir-based composites in architecture is highlighted in this review, which also looks at their problems, advantages for the environment, manufacturing processes, and mechanical, thermal, and acoustic performances. The fibrous shape of the coir provides efficient thermal and acoustic insulation, while its high lignin concentration guarantees stiffness, biological resistance, and dimensional stability. Fiber-matrix adhesion and durability have improved owing to advancements in treatment and environmentally friendly binders, opening up the use of cement, polymers, and hybrid composites. In terms of the environment, coir composites promote a biophilic design, reduce embodied carbon, and decrease landfill waste. Moisture sensitivity, inconsistent fiber quality, and production scaling are obstacles; however, advancements in hybridization, grading, and nanotechnology hold promise. This review provides comprehensive, architecture-focused review that integrates material science, fabrication techniques, and real-world architectural applications of coir-based composites. Coir-based composites have the potential to be long-lasting, sustainable substitutes for conventional materials in climate-resilient architectural design if they are further investigated and included in green certification programs and the circular economy.

1. Introduction

The global architecture and construction industry is gradually shifting towards sustainability in response to increasing environmental concerns, resource scarcity, and stricter building regulations. As demand for eco-friendly, low-carbon, and renewable materials grows, natural fiber-reinforced composites (NFRC) have gained significant attention as viable alternatives to synthetic and mineral-based construction materials due to their environmental benefits and sustainability [1,2,3,4,5]. These natural fibers offer advantages over synthetic options, being renewable, eco-friendly, and biocompatible, and are increasingly used in sectors such as automotive, aerospace, and medical industries. The life cycle of natural fiber reinforced composites has been illustrated in Figure 1a. It emphasizes the coir’s role in a circular materials economy, where natural fibers can be returned safely to the environment or be reused after their service life.

A major challenge for NFRCs is poor interfacial adhesion between hydrophilic natural fibers and hydrophobic polymer matrices, along with issues like high moisture absorption and low thermal stability, which can negatively affect mechanical properties [6]. To address these issues, various physical and chemical modification techniques, such as alkali treatment, acetylation, and silanization, have been employed to enhance interfacial bonding and reduce hydrophilicity [1,7,8,9]. Despite these challenges, NFRCs demonstrate excellent biodegradability and are increasingly used in automotive parts, construction materials, and packaging, highlighting their potential as sustainable alternatives to traditional materials [10]. Among various plant fibers, coconut coir derived from the husk of the coconut fruit has emerged as a particularly promising reinforcement because of its abundance, low cost, biodegradability, and remarkable mechanical durability [11,12]. The adaptability and sustainability of coir and other natural fibres in polymer composites are further highlighted by recent studies. Coir-reinforced polypropylene composites have been shown to have enhanced mechanical strength and thermal stability, confirming their potential for various applications [13] as depicted in Figure 1b. Additionally, because coir naturally resists moisture, fewer chemical treatments are required, which further minimizes embodied carbon [14]. By offering thermal insulation, lower-density coir boards also help buildings use less energy, and may even reduce emissions from air conditioners [15].

Coconut coir is known for its high lignin content, which provides rigidity, resistance to degradation, and dimensional stability features essential for materials exposed to varying thermal and moisture conditions in buildings. Its fibrous structure offers effective thermal and acoustic insulation, along with natural pest and rot resistance, making it suitable for both structural and nonstructural uses in architecture. Using coir also helps reduce agricultural waste and addresses disposal issues. However, a major challenge in using natural fibers is their difficulty in adhering to a hydrophobic matrix due to their hydrophilicity. They are typically treated chemically to enhance their adhesion to the matrix. Using agricultural waste such as coconut husks as a key raw material, coir-based composites show remarkable resource efficiency and reduce reliance on virgin wood resources [16]. Because larger coir pith particles increase the strength and dimensional stability without the need for extra binders, their application improves mechanical qualities while reducing material waste [14]. Furthermore, by using locally available coir fibers, coir fiber-cement boards lower transportation-related emissions and promote a circular economy [17]. By consuming less resin (16.7%) than UF-bonded alternatives (20.4%) and offering better mechanical performance, PF-bonded boards further optimize the resource consumption [14].

Figure 1. (a) Schematic of the life cycle of natural fiber reinforced composites (NFRCs), highlighting the circular economy of coir; (b) Applications of coir fibers in various industries. Reproduced from [18] under Elsevier.

Figure 1. (a) Schematic of the life cycle of natural fiber reinforced composites (NFRCs), highlighting the circular economy of coir; (b) Applications of coir fibers in various industries. Reproduced from [18] under Elsevier.

This review highlights the unique properties of coconut coir reinforced composites and their structural, thermal, and aesthetic uses in architecture, while critically assessing implementation challenges, material performance, fabrication methods, and environmental effects. Through the integration of innovative material science and sustainable architecture, this study establishes a foundation for further investigation and widespread use of coir-based composites in contemporary climate-responsive building systems.

2. Properties of Coconut Coir

2.1. Physical and Chemical Properties

Coconut coir fibers (Figure 2a) possess distinct physical and chemical properties that make them suitable for various applications. The composition of the fiber is contingent on the source from which it has been extracted. Coir fibers are primarily composed of cellulose, lignin, and hemicellulose [11]. They typically contain 40–50% lignin, 27–45% cellulose, and 15–20% hemicellulose, along with 1.3% ash and 9–11% moisture, as indicated in Figure 2b. The higher lignin content of coir, compared to other natural fibers, contributes to its hardness and natural rigidity. They are characterized by being strong, stiff, thick, and coarse and exhibit excellent hard-wearing capability, high hardness, and good acoustic resistance. Coir is also non-toxic, rot-resistant, and moth-resistant, and acts as a heat insulator. It has superior moisture resistance and can withstand salty water and heat exposure. The density of coir fibers ranges from approximately 1.15 to 1.45 g cm⁻³, with an elastic modulus of 4 to 7 GPa, and a strength of 54 to 250 MPa, along with 3 to 40% elongation at break, depending on factors like type, origin, nature, and processing [11,19]. Alkaline treatment modifies the coir pith’s physical and chemical properties, enhancing its suitability as a reinforcement material by improving surface characteristics and controlling water absorption, which is critical for its effective use in polymer matrix composites [20].

Owing to their unique structural composition and high lignin content, coconut coir fibers offer a desirable combination of mechanical strength, durability, and environmental resilience. They are appropriate for a variety of uses, ranging from composite material reinforcement to building and furnishing, owing to their natural resistance to moisture, heat, and biological deterioration, as well as their strong acoustic and thermal insulation properties. In addition, they are renewable and biodegradable; coir fibers are positioned as a sustainable substitute for their synthetic counterparts, meeting the increasing need for environmentally friendly materials in contemporary industries.

2.2. Mechanical Properties

Coir fiber’s structure is characterized by its lignified cellulosic composition, multi-fibrillar arrangement, and the presence of non-cellulosic impurities. These structural attributes, particularly the high lignin content and relatively low crystallinity, distinguish coir from other fibers and influence its mechanical properties and applications [21]. Coir’s high lignin content contributes to its slower biodegradability compared to many other natural fibers. This property makes it a durable and long-lasting material for applications where longevity is prioritized, but it also highlights the trade-offs between durability and rapid decomposition when selecting materials for different ecological applications [22].

The stiffness and durability of coir fibers under stress are demonstrated by their moderate tensile strength (54–250 MPa), elongation at break (3–40%), and elastic modulus (4–7 GPa). Their hydrophobic nature guarantees good compatibility with resins and their low density makes them ideal for lightweight composites. The type, origin, processing, and chemical makeup of the fiber affect its mechanical properties, including tensile strength, elasticity, and elongation [11]. The modulus of elasticity (MOE) and modulus of rupture (MOR) are useful indices for assessing the quality and structural potential of biomass particleboard [19]. Coir fiber edge trims increase the compressive and flexural strengths of cement-bonded particle boards (CBPBs), demonstrating their superior mechanical performance compared to traditional materials [23]. Coir-reinforced concrete exhibits a better compressive strength (CS). A study was conducted to assess the effect of fiber length (FL) and fiber volume fraction (FVF) on the compressive strength of coir-reinforced concrete. The findings revealed that the highest CS of 34 N/mm² was achieved with 10 mm FL and 4% FVF. However, an increase in the fiber content led to reduced workability. A year later, FTIR and XRD tests confirmed the durability of the coir fibers in concrete, showing no significant alterations in their crystalline, thermal, or functional properties. This indicates the potential of coir fibers as environmentally friendly reinforcing materials [24].

The incorporation of coconut fiber (CF) into eco-friendly bricks reduces the construction costs, structural weight, and environmental impact. Tests for compressive strength, water absorption, efflorescence, and hardness were conducted with CF added at 5%, 10%, and 15% brick volume. The findings revealed that bricks containing 15% CF (test specimen 3) were the most effective, exhibiting the highest compressive strength (8.45 N/m²), lowest water absorption (8.39%), and minimal efflorescence. These lightweight, sustainable bricks not only decrease transportation expenses but also enhance workability, plaster finishes, and resistance to saline water. The above observations recommend the application of coconut fibers for manufacturing eco-friendly bricks [25]. Chemical treatments, particularly those involving NaOH, are highly effective for enhancing the intrinsic mechanical properties of coir and improving its compatibility with various polymer matrices. Alkali treatment effectively removed impurities and cementing materials like hemicellulose and lignin, leading to a cleaner fiber surface and better mechanical interlocking [26,27]. Physical treatments, such as thermal and plasma treatments, also play a role, primarily by altering the surface characteristics to improve interfacial bonding within the composites [28]. These treatments often reduce the hydrophilic nature of fibers by removing hydroxyl groups, leading to enhanced mechanical strength and dimensional stability.

The mechanical performance of coconut coir fibers, characterized by their balance of strength, flexibility, and durability, makes them a promising reinforcement material for various construction materials. Their ability to enhance compressive, flexural, and tensile properties, while reducing structural weight and environmental impact, highlights their value in sustainable building practices. Applications such as coir-reinforced concrete and eco-friendly bricks demonstrate not only improved mechanical performance but also added benefits in terms of workability, resistance to environmental degradation, and cost efficiency. These findings underscore the potential of coir fibers as eco-friendly alternatives to conventional reinforcement materials, aligning strength and functionality with sustainability goals [7].

2.3. Thermal and Acoustic Properties

The coconut coir possesses good thermal insulation owing to its low thermal conductivity (0.038–0.042 W/mK). Its porous structure also provides excellent sound absorption, making it ideal for use in acoustic panels and insulation materials in buildings. Thermogravimetric analysis (TGA) is a useful technique for examining the weight loss of biocomposite materials at various temperatures. The temperature-sensitive structural makeup of coir fibers, lignin, cellulose, and hemicellulose influences their thermal breakdown [29]. The breakdown behavior and composition of the biocomposites (coir and matrix) were examined using TGA. The interactions between constituents in the composite systems as the temperature varied were further revealed by derivative thermogravimetric (DTG) analysis. A typical TGA curve for polypropylene (PP) composites reinforced with coir fibers, for instance, demonstrates early mass loss (25–150 °C) as a result of moisture evaporation [30]. Coir fibers begin to break down at 190.18 °C, whereas coir/PP composites begin to break down at 211.2 °C, suggesting that PP addition improves thermal stability. Hemicellulose (200–260 °C), cellulose (240–350 °C), and lignin (280–500 °C) are among the substances that degrade at certain temperatures [30,31]. Mass losses of 43.89% (316.9–475 °C) and 23.95% (190.2–316.9 °C) were noted, demonstrating a steady deterioration trend. Thermal stability can be further improved by pretreating the coir fibers [30]. For example, Singh et al. created coir/carbon fiber/epoxy composites with a coir content of 10–30%. They observed that while weight loss increased with increasing coir content, thermal stability was enhanced by carbon fiber and treated coir [32]. The lower heat conductivity of biomass-based boards improves their insulation [19]. Coir-reinforced CBPBs are ideal for interior use due to their low thermal conductivity and enhanced acoustic insulation [23].

To improve thermal comfort in low-income homes in Cartagena de Indias, Colombia, this study assessed the application of mortars modified with coconut fiber (coir) as a facade coating. For 39 days, a standard 42 m² home was used to measure changes in humidity and temperature. A guarded-hot-cartridge device and differential scanning calorimetry (DSC) were used to examine the thermal characteristics of the coir-modified mortars. Applying a coir-modified mortar layer lowered indoor temperatures by 0.5 to 1.5 °C during peak solar hours (29 to 34 °C outdoors), according to simulations conducted with Energy Plus TM. This upgrade could result in a 16% reduction in annual cooling energy expenses, making it a sustainable and reasonably priced method to increase thermal comfort in low-income tropical dwellings [33]. Furthermore, the acoustic properties of the coir-reinforced composites were investigated. They have been reported to provide a better sound absorption ability. Figure 3 demonstrates that coir/jute (C/J) hybrid composites, particularly those with a higher coir content, such as S4 C/J, show promising sound absorption capabilities, especially for high-frequency sound waves. The blend ratio significantly influences the acoustic performance of a material [34]. The significant impact of coir fiber thickness, presence, and orientation of the glass fiber layer on the composite’s sound absorption characteristics ultimately emphasized the superior acoustic performance of natural fibers over synthetic alternatives [35,36].

Coconut coir is a very good natural insulator for heat and sound due to its low thermal conductivity, good thermal stability, and porous structure. It has the potential to lower cooling loads and increase indoor comfort, especially in hot areas, as evidenced by its ability to improve building energy efficiency in practical applications, such as facade-modified mortars. Furthermore, coir is a great material for acoustic optimization in architectural spaces because of its outstanding sound absorption, particularly in hybrid composites. All these characteristics work together to highlight the coir’s worth as a sustainable, multipurpose material that can meet the needs of contemporary architecture for both thermal and acoustic performance.

Table 1. Comparison of physicochemical, mechanical, thermal, and acoustic properties of various natural fibers.

Category	Properties	Coconut Coir	Jute	Sisal	Hemp	Flax	Reference
Physical	Density (g/cm3)	1.1–1.5	1.46	1.33	1.48	1.4	[11,37,38]
	Moisture content (%)	9–11	12	11	9	7
	Fiber length	15–35 cm	750–1500	800–1200	700–900	2500 (long hemp)
Chemical	Lignin (wt%)	40–50	12–13	9.9	10	2.2	[37]
	Hemicellulose (wt%)	15–20	14–20	12	15	18.6–20.6
	Cellulose (wt%)	27–45	61–71	65	68	71
Mechanical	Tensile Strength (MPa)	220	400–800	600–700	550–900	88–1500
	Youngs Modulus (GPa)	2–8	10–30	38	70	60–80
	Elongation at break (%)	15–25	1.8	2–3	1.6	1.2–1.6
Thermal and acoustic properties	Thermal Conductivity (W/(m·K))	0.04–0.1	0.036	0.038	0.028–0.110	0.033–0.12	[13,34,39,40,41,42]
	Onset degradation temperature (°C)	200	240–270	200–275	150–207	250–270
	Sound absorption coefficient (NRC)	0.8	0.9	0.4–0.7	0.9	0.5–0.8

summarizes the physicochemical, mechanical, thermal, and acoustic properties of natural fibers.

The comparison emphasizes the distinctive positioning of coconut coir among natural fibers: although it has lower cellulose content (27–45%) and higher lignin levels (40–50%) than fibers like jute, sisal, hemp, and flax, this composition grants coir superior durability, resilience, and resistance to microbial and water damage, though it comes at the cost of tensile strength and stiffness. Its relatively high elongation at break (15–25%) and lower modulus (2–8 GPa) indicate more flexibility compared to other fibers, which tend to be stronger and stiffer but more brittle. Coir’s density (1.1–1.5 g/cm³) is slightly lower than most other fibers, making it lightweight, although its shorter fiber length limits its use in continuous reinforcement applications. Thermally, coir has similar conductivity (0.04–0.1 W/m·K) to other bast fibers and begins degrading moderately at around 200 °C, suitable for most composite manufacturing processes. Additionally, coir provides excellent acoustic absorption (NRC ≈ 0.8), comparable to jute and flax, making it ideal for soundproofing and thermal insulation panels. Overall, despite being less strong and stiff than bast fibers, coir’s high lignin content, flexibility, sound absorption abilities, and durability in moist environments make it a valuable choice for sustainable composite and architectural uses.

3. Production of Coconut Coir-Based Composites

3.1. Source of Coconut Coir

Coir, a natural lignocellulosic fiber derived from the mesocarp (outer husk) of coconut (Cocos nucifera L.), is globally recognized for its durability, biodegradability, and versatility in textiles, geotextiles, composites, and construction materials [43]. The coconut fruit consists of an outer exocarp, a thick fibrous husk, a hard protective endocarp (shell), and an inner white albuminous endosperm (coconut meat) with sweet liquid (coconut water). Coir fibers are primarily derived from the husk, which constitutes 30% by weight of the husk material, a byproduct of coconut processing [44]. As a byproduct of coconut processing, it contributes significantly to the global coconut industry, which produces over 62 million tons of coconut per year, with major production hubs in India, Sri Lanka, Indonesia, the Philippines, and Vietnam [43,45]. As shown in Table 2, India and Sri Lanka have led the global coir export market with around 80–90% of the total coconut coir production globally, with Sri Lanka standing out for its high-quality bristle fibers. Figure 4a–d represents the source of coir.

Coir fibers are classified into three categories based on their length, thickness, and end-use compatibility [46]. Another important difference is between brown and white coirs, which is determined by the age of the coconut. Brown coir was derived from completely grown coconuts. The coarser, thicker, and greater lignin content (~45%) increases its stiffness and resistance to microbial degradation [47]. This makes it ideal for use in mats, brushes, sacks, upholstery padding, erosion control blankets, and packaging materials [48]. White coir is derived from young coconuts and is finer, softer, and more flexible owing to its reduced lignin and high cellulose content [11]. It has traditionally been used to spin yarn, weave fishing nets, and make high-quality handloom mats, and has also been utilized in the automobile industry [49]. In industrial practice, coirs are further classified as mattress fibers (short and flexible fibers for stuffing and cushioning), mat fibers (medium-length fibers for matting and moderate-strength woven products), and bristle fibers (longest, stiffest fibers with superior tensile properties used in heavy-duty ropes, brushes, and geotextiles) [46,50]. Traditionally, fiber extraction relies on retting, which involves immersing husks in brackish water for up to 10 months to allow anaerobic microbial action to destroy pectins and separate the fibers. It is especially used to produce high-quality white coir in Sri Lanka and parts of India. After retting, the fibers were manually washed, dried, and cleaned. Modern mechanized techniques shorten retting to approximately five days, after which the husks are crushed, drum-processed, washed, and brushed, resulting in constant fiber quality. Recent improvements include microbial enzyme-based retting, which is environmentally safe, faster to process, and allows bleaching or dyeing without the use of harsh chemicals [43].

Figure 4. Sources of coconut coir: (a) coconut tree with ripe fruit, (b) opened coconut fruit, (c) coconut husk, and (d) Coconut coir substrate. Reproduced from [51] under CC BY 4.0.

Figure 4. Sources of coconut coir: (a) coconut tree with ripe fruit, (b) opened coconut fruit, (c) coconut husk, and (d) Coconut coir substrate. Reproduced from [51] under CC BY 4.0.

Table 2. Distribution of global coconut production by country, highlighting key contributors [52,53].

Rank	Country	Production (metric tons)	Global Share (%)
1	India	586,686	44.00%
2	Sri Lanka	161,791	27.10%
3	Vietnam	390,541	11.20%
4	Philippines	60,983	4.80%
5	Indonesia	52,932	4.10%
6	Thailand	64,098	1.30%
7	Malaysia	12,361	1.00%
8	Mexico	11,981	0.90%
9	Guyana	9583	0.80%
10	Bangladesh	5072	0.40%

Table 2. Distribution of global coconut production by country, highlighting key contributors [52,53].

Rank	Country	Production (metric tons)	Global Share (%)
1	India	586,686	44.00%
2	Sri Lanka	161,791	27.10%
3	Vietnam	390,541	11.20%
4	Philippines	60,983	4.80%
5	Indonesia	52,932	4.10%
6	Thailand	64,098	1.30%
7	Malaysia	12,361	1.00%
8	Mexico	11,981	0.90%
9	Guyana	9583	0.80%
10	Bangladesh	5072	0.40%

In addition to fiber manufacturing, coir pith, which is traditionally considered waste, is now a widely traded horticultural substrate, replacing peat moss because of its water retention, porosity, and renewable origin. This valorisation emphasizes the coir’s expanding importance in the circular bioeconomy, which contributes to sustainable material procurement across numerous industries.

The market for natural FRP composites is projected to experience substantial growth, with an estimated to have annual growth rate (CAGR) exceeding 9% through 2027. The building and construction sector is anticipated to be the largest market for these composites, followed closely by the automotive industry. Natural fibers are increasingly becoming a crucial component in composites, replacing synthetic fibers due to their significant contribution to a less polluted environment through sustainable material consumption [9]. The global coir fiber market is experiencing robust growth driven by rising demand for sustainable and renewable materials. A recent Data Bridge Market Research report places the coir market at USD 685.29 million in 2024, projecting a rise to USD 1.287 billion by 2032, with a CAGR of 8.2% between 2025 and 2032 [53]. Another industry forecast by Expert Market Research values the market at USD 385.03 million in 2024, with expectations to reach USD 838.98 million by 2034, growing at a CAGR of 8.1% [53].

Key sectors driving this expansion include horticulture, geotextiles, construction, and automotive composites, where natural fibers like coir offer eco-friendly alternatives to synthetic materials. Specifically, horticulture leads demand—coir pith and substrates are increasingly used to replace peat moss in Europe and North America due to environmental restrictions. Additionally, coir geotextiles are gaining traction in infrastructure applications such as soil stabilization and erosion control, thanks to their biodegradability and effectiveness. Coir composites are also finding applications in the automotive sector for lightweight interiors and in construction as reinforcement in sustainable building materials [53]. Economically, coir contributes significantly to employment and exports in major producing countries. For instance, India supports over 700,000 workers in the coir sector, encompassing cultivation, processing, and manufacturing, and remains a leading exporter of processed coir products [54]. Sri Lanka and Indonesia similarly contribute substantially to global supply chains, especially for horticultural-grade coir. Figure 5 depicts the environmental impacts that need to be considered during the life cycle assessment of composites.

3.2. Fibre Extraction and Processing

Coir fiber is traditionally collected from the fibrous mesocarp of coconuts (Cocos nucifera L.) using processes that vary in efficiency, environmental impact, and fiber quality. These processes can be roughly classified into three categories: conventional retting (as depicted in Figure 6), mechanical extraction, and current enzyme-based processing, with additional chemical treatments used to enhance fiber performance. Traditional fiber extraction entails immersing husks in brackish or saltwater for 8–10 months, during which anaerobic fermentation degrades pectins, permitting fiber separation via beating and washing [56]. This process yields high-quality white coir fibers suitable for spinning, dyeing, and weaving. However, it is labor-intensive, time-consuming, produces a large amount of coir pith waste, and can pollute water via effluent discharge [56]. Despite these disadvantages, traditional retting is still commonly used in Sri Lanka and parts of India for high-quality goods [43].

Furthermore, to overcome the limitations of traditional retting, mechanical techniques use crushers, decorticators, and spinning drums to reduce the retting time to 5–7 days [11]. The husks were presoaked for a short duration before being mechanically beaten, cleaned, dried, and brushed. This technology is speedier and more scalable, allowing for industrial-scale manufacturing; however, it creates coarser fibers than traditional retting, making it unsuitable for fine yarns and high-end fabrics [11].

Modern enzyme-based decortication uses microbial or plant-derived enzymes to weaken the husk structure, lowering the retting time to 3–5 days while retaining the fiber quality [47,56]. This approach also improves the surface morphology and increases dye receptivity and adhesion in composite materials [26]. Enzyme retting is considered environmentally beneficial because it uses less water and produces biodegradable effluents. Alkali treatment is a standard method for improving the coir performance in composite applications. Treating coir with 5% NaOH for 48 h at a fiber length of 10 mm improves the tensile strength, interfacial adhesion, and surface roughness, resulting in greater load transmission in the composites [29]. It has been reported that optimum NaOH concentrations (2–5%) result in considerable gains in tensile strength of up to 42% [50,57]. However, severe alkalization (>6%) can harm the fiber microstructure and reduce mechanical performance [57]. Figure 7a shows the treatment of long coir fibers in an alkali solution, and Figure 7b illustrates the processing of long and short coir fibers in acetic anhydride [22,58].

The coir pith, a by-product of fiber extraction, is currently an important material in horticulture, replacing peat moss because of its high water retention, porosity, and renewability [14,59]. Innovatively, whole coconut husks can also be processed directly into high-density, high-strength boards without using chemical binders, offering a cost-effective and environmentally sustainable raw material option [16].

Figure 7. Treatment of Long coir fibers and short coir fibers (a) Long coir fibers treated in alkali solutions. Reproduced from [60] under CC BY 4.0 (b) Treatment of long coir and short coir fibers composites in alkali solution (Alkali treated Long coir Composite (ALC), Alkali treated short coir (ASC)) and acetic anhydride solution (acetic anhydride long coir fiber composite (AALC), acetic anhydride treated short coir (AASC). Untreated long coir (ULC) and short coir fiber (USC) represent the control groups. Reproduced from [12] under CC BY 4.0.

Figure 7. Treatment of Long coir fibers and short coir fibers (a) Long coir fibers treated in alkali solutions. Reproduced from [60] under CC BY 4.0 (b) Treatment of long coir and short coir fibers composites in alkali solution (Alkali treated Long coir Composite (ALC), Alkali treated short coir (ASC)) and acetic anhydride solution (acetic anhydride long coir fiber composite (AALC), acetic anhydride treated short coir (AASC). Untreated long coir (ULC) and short coir fiber (USC) represent the control groups. Reproduced from [12] under CC BY 4.0.

3.3. Fabrication Techniques—Coir Reinforced Composites

Various fabrication techniques have been employed for coir fiber-reinforced composites, including compression molding, hot compression [61], extrusion molding, injection molding, and resin transfer molding (RTM), hand-lay-up method [62]. Compression molding is a widely used method for high-volume composite parts, suitable for both short and long fibers, and can be performed at room temperature or at high temperatures (with heat and pressure). Extrusion molding utilizes a screw extruder at specific speeds and temperatures, allowing for further molding after cooling and enhancing the mechanical strength and stiffness of the thermoplastic polymer-reinforced composites. Injection molding facilitates diverse processing for high-volume production, offering excellent dimensional stability with shorter cycle times, although it requires lower molecular weight polymers for adequate viscosity and is less influenced by the fiber length or processing temperature. The RTM method is known for producing high-quality finishes and dimensional accuracy, transferring thermoset polymeric resins into a closed mold at low temperatures and pressures, and is advantageous for its ecological, economic, and technological benefits. Additionally, open molding is an economical method for thermoset polymer-reinforced composites cured at ambient temperature in an open mold, although it may involve longer curing times and manual labor. These techniques are crucial for manufacturing biocomposites, with specific process parameters such as fiber volume, temperature, and pressure that influence the success of the final product [11,47].

Alkali treatment improves the fiber–matrix adhesion and mechanical properties [63,64,65]. The hand lay-up method, combined with surface modification, optimizes coir-polymer compatibility [66]. Binders such as UF, PF, and IC create insulating particleboards, whereas castor oil-based polyurethane resins produce eco-friendly, high-strength boards [67,68]. Innovative methods, such as the flowchart process, yield sustainable panels from coconut fibers and PET [69]. Coir-rubber-polyurethane composites excel in sound absorption [70]. When properly cured, PF resin produces particleboards with superior strength and moisture resistance compared to UF resin [14,59]. Green binders such as BST20 offer high density, MOR, and water resistance as sustainable alternatives to UF resin [71]. A higher resin content improves sound absorption in coconut husk-based boards [72]. Cement bonding creates durable coir fiber boards for construction [17], whereas density variations enable tailored applications, such as lower density for sound absorption and higher density for thermal insulation [15]. Coir fibers match the traditional wood strength in multi-layer particleboards, offering a sustainable alternative [73]. Several manufacturing processes that use agricultural waste for particleboard production, such as board pressing and bonding techniques, aid in comprehending material processing [19]. Coir fiber and GGBS combinations in cement-bonded particleboards demonstrate hybrid matrix techniques [23].

3.4. Selection of Matrix Materials

The choice of matrix material significantly affects the coir composite performance. Natural rubber with resorcinol-hexamethylenetetramine improves fiber-matrix adhesion [74]. Synthetic resins, such as urea-formaldehyde (UF) and phenol-formaldehyde (PF), are widely used for their mechanical properties and compatibility with coir fibers in particleboards and composite fiber boards [75]. Eco-friendly alternatives, such as castor oil-based polyurethane resins, provide strong reinforcement and meet mechanical standards [68]. PF resin outperforms UF resin in mechanical strength, moisture resistance, and dimensional stability, making it ideal for high-performance particleboards [14,59]. Green binders, such as BST20, provide high MOR and water resistance, offering a sustainable alternative to UF resins. Increased resin content enhances sound absorption in coconut husk boards, improving acoustic performance [71,72]. Cement serves as an effective matrix for durable coir fiber boards, meeting construction standards [17], whereas UF resin enables tailored applications, with density variations influencing sound absorption and thermal insulation properties [15]. Grain granulated blast furnace slag (GGBS) has been used as a sustainable cement substitute in coir-reinforced boards, resulting in strong bonding and environmental benefits [23].

Table 3. Mechanical properties of coir-reinforced composite materials.

Composite System	Matrix/Polymer	Tensile Strength (MPa)	Flexural Strength (MPa)	Notable Observations	Reference
Coir/Polypropylene (PP)	PP	25–32	38–50	Alkali/chemical treatment improves interfacial adhesion and strength	[7,71]
Coir/Polyethylene (PE)	PE	22–28	35–45	Surface modification enhances dispersion and mechanical stability	[7]
Coir/Poly(butylene succinate) (PBS)	PBS (biodegradable)	20–30	30–42	Alkali treatment significantly improves tensile and flexural strength	[62]
Coir/Polylactic Acid (PLA)	PLA	38–52	65–80	Exhibits good tensile and thermal stability; multifunctional properties highlighted	[30,72]
Coir/Epoxy (with hybrid glass fiber)	Epoxy	45–60	70–95	Hybridization with glass fiber shows synergistic improvement in flexural performance	[73]
Coir/Polyester	UPR	30–45	50–65	General literature consensus shows moderate strength; dependent on fiber treatment	[59]
Coir/Polymer composites (general fabrication)	PP/PE (various)	–	–	Hot compression molding shown to produce uniform dispersion & improved bonding	[58]
Coir/rubber	rubber	2.509	29–300	Coir improves post-cracking toughness and flexural performance	[74,76]
Coir/Glass		37	63	The optimal composition was 10 wt% coir at 15 mm fiber length, showing the best tensile and flexural strength, while 20 mm fibers enhanced hardness but reduced strength.	[37]

gives a comprehensive insight into the mechanical properties of various coir composites.

3.5. Binder Materials

Green composite materials are considered to be environmentally sustainable [77]. The increasing use of natural fillers as reinforcement in polymer composites is primarily motivated by environmental concerns and the desire for cost reduction. These materials offer an eco-friendly alternative, contributing to the development of low-density materials with improved properties [78,79]. The increasing use of natural fibers in composites is driven by their environmental and economic benefits, with a clear upward trend in the market. These fibers offer unique properties like low density and good strength, finding applications in automotive, aircraft, marine, and construction industries [80]. Natural and synthetic binders are viable coir composites. The lignin of white coir acts as a natural binder under heat and pressure, yielding strong water-resistant boards under optimal conditions (220 °C, 160 kgf/cm², 4 min) [81]. Similarly, whole coconut husks heated above 140 °C release natural resins that facilitate the formation of high-strength binderless boards through a simple and eco-friendly method [16]. BST20 outperformed other binders with high strength and water resistance, making it a safer alternative to UF, whereas BST00 showed the weakest performance [71]. Coir–durian peel boards showed that density had a greater impact on strength and stability than binder type. Their low thermal conductivities make them suitable for insulation [67]. Ferreira et al. reviewed the integration of natural fibers and nanocellulose in the automotive sector, emphasizing advantages such as light weighting and sustainability, while also noting persistent challenges like moisture sensitivity and variability in fiber properties. In conclusion, natural lignin in coir offers a sustainable alternative to synthetic binders, and among green options, BST20 has emerged as the most effective and environmentally safe binder. Coir fibers and GGBS improve moisture resistance and reduce water absorption [23]. Hybridizing BSF with coir fiber also proved effective in creating composites with improved properties, suggesting their potential as wood substitutes [82]. The integration of natural fibers and nanocellulose in the automotive sector has emphasized advantages such as light weighting and sustainability, while also noting persistent challenges like moisture sensitivity and variability in fiber properties [78].

Table 4. Key insights and applications of various binder types used in coir-based composites.

Matrix/Binder Type	Type	Material Used	Key Insights	Applications	Reference
Urea-formaldehyde (UF)	Synthetic	Coir fibers, durian peels	Good bonding, cost-effective; density impacts thermal/acoustic insulation	Particleboards, acoustic/thermal panels	[15,67,75]
Phenol-formaldehyde (PF)	Synthetic	–	Superior strength, dimensional stability, better than UF	High-performance boards	[59]
Natural rubber	Natural	–	Improved fiber-matrix adhesion; elastic and eco-friendly	General coir composites	[74]
Polyurethane (castor oil-based)	Bio-based	–	Meets mechanical standards; sustainable with low VOC	Eco-friendly composite boards	[68]
Bst20 (green binder)	Bio-based	–	High MOR (11.61 MPa), water resistance; greener than UF	Sustainable fiberboards	[71]
Coconut husk with resin (varied %)	Composite	Coconut husk	Resin % affects sound absorption; tunable acoustic properties	Sound-absorbing boards	[72]
Binderless (natural lignin)	Binder less	White coir/Whole husks	Thermosetting lignin acts as a natural binder, eco-friendly, but has lower durability	Low-cost, simple composite boards	[16,68]
Cement	Inorganic	–	High durability, load-bearing, water-resistant	Construction-grade boards	[17]

provides a comprehensive analysis of the various natural and synthetic binders used in various applications.

4. Architectural Applications

In addition to utilizing its advantageous mechanical, thermal, and acoustic qualities, the incorporation of coconut coir into composite manufacturing is consistent with the more general concepts of sustainability and circular economy, whereby agricultural waste is converted into valuable materials, recycled, and eventually released back into the environment with little ecological impact [9,18]. Every stage of the process, from fiber extraction and mechanical or chemical treatment to the final composite applications, presents opportunities to reduce waste, use less energy, and add value. Coir composites can be recycled into new products or biodegraded into organic matter at the end of their useful life, which promotes crop regeneration and ensures the renewability of resources.

This regenerative approach underscores the appeal of coir-based composites not only for their structural performance but also for their role in reducing dependency on non-renewable resources and fostering climate-resilient material systems in architecture. Various applications of coir-reinforced composites in architecture, such as structural components, interior decorations, aesthetic furniture, soundproofing, and other biophilic applications, have been discussed in detail.

4.1. Structural Components

Coconut coir composites are suitable for non-load-bearing applications, such as panels and partitions, owing to their light weight and acceptable strength. Their adaptability allows their use in both rigid and flexible forms, depending on the fiber composition [83]. Coir fibers are used in civil engineering owing to their cost-effectiveness, ductility, and energy absorption; however, further study is required before they can be used in important structural components, such as walls, beams, and columns [84]. By enhancing damping and reducing structural deterioration, coconut fiber-reinforced concrete (CFRC) increases earthquake resilience. It performs best at a 5% fiber content and 5 cm fiber length [85]. For both structural and non-structural applications, coir fibers and epoxy resin exhibit enhanced mechanical qualities, making them lightweight and affordable substitutes for synthetic fibers [86]. These observations demonstrate the adaptability and promise of coir in structural and architectural applications, particularly in seismic-resistant and non-structural designs.

Binderless boards made from coconut coir fibers have gained popularity as eco-friendly alternatives to synthetic resin-based composites. The fundamental advantage of binderless technology is the absence of chemical adhesives, as the natural ingredients of coir particularly lignin and hemicellulose act as intrinsic binders when heated and compressed. Whole coconut husks can be converted into binderless high-density boards via thermo-compression, removing the requirement for synthetic resins. These panels, produced at a moisture content of 10–25% and densities of 900–1000 kg/m³, exceed the worldwide mechanical criteria for furniture and interior construction. The degassing phase during pressing reduces blistering and increases dimensional stability [87]. This method provides a low-cost and environmentally beneficial alternative to traditional wood-based panels [16].

Coir composites replace energy-intensive materials such as synthetic resins and traditional wood panels, greatly reducing the carbon footprint. Without the use of chemical binders, whole-coconut husk boards are as strong as commercial wood panels without the use of adhesives derived from fossil fuels [16]. In addition to meeting the JIS A 5908 criteria with a lower environmental impact, BST20, a green binder, reduces the dependency on urea-formaldehyde (UF) resins, which are linked to high volatile organic compounds (VOCs) [71]. Additionally, because coir naturally resists moisture, fewer chemical treatments are required, which further minimizes embodied carbon [14]. By offering thermal insulation, lower-density coir boards also help buildings use less energy, and may even reduce emissions from air conditioners [15]. Coir-based materials provide greater end-of-life sustainability because they are non-toxic and biodegradable, breaking down naturally, unlike synthetic composites, and decreasing landfill waste [72]. Because coir fibers maintain their useful qualities even after prolonged exposure, PF-bonded boards, despite their durability, can be recycled or composted after use [14]. Despite not being biodegradable, cement-bonded coir boards can be crushed and repurposed as aggregates, which is consistent with the circular building methods [17]. Despite being partially synthetic, hybrid coir-glass fiber composites lengthen product lifespans through improved durability and reduced waste by adding renewable content [73].

4.2. Interior Applications

Coir-based composites are increasingly being employed in decorative panels, ceiling tiles, and furniture, offering a sustainable alternative to wood and synthetic materials. Because of their practical qualities and sustainability, coir fibers are increasingly utilized in architectural interiors. The coir-reinforced composites showed better environmental resilience, such as weathering, moisture, pests, and UV rays, particularly in humid and outdoor settings, when compared to other natural fiber-reinforced composites, and have been recommended for use in panels and furniture, and help create silent settings in places such as classrooms and theaters [86]. Coir fibers improve the flexural strength, tensile strength, and hardness of polypropylene (PP) composites; they work best at 60% coir fiber content, which makes them appropriate for use in automobile interiors [88]. Coir and other natural fibers can be treated and modified to improve their mechanical qualities, making them eco-friendly substitutes for synthetic materials in furniture, insulation, and acoustic panels [89]. Coir fiber reinforced polypropylene has been considered to be a durable, recyclable, and lightweight alternative to existing materials, particularly for the automotive industry. It was observed that coir reinforced composites could be a viable material for designing an interior car door panel capable of absorbing impact energy and meeting human safety criteria [90].

The following observations demonstrate how a coir’s adaptability and sustainability can improve the usability and beauty of architectural interiors: To assess the potential of coir-based materials for interior applications, this study compared them with comparable natural fiber composites and examined their characteristics, uses, and limits. The longevity and environmental resilience of coir-based composites were critically examined, with particular attention paid to how well they withstand weathering, moisture, pests, and UV rays, particularly in humid and outdoor settings.

4.3. Thermal and Acoustic Insulation

Acoustic control is a critical aspect of interior design, aiming to provide noise-free environments in residential and commercial spaces to ensure peaceful living and working conditions. Because of their excellent insulation properties, coir composites are used in wall cladding, acoustic panels, and thermal insulation layers. In architecture, coir fibers are becoming increasingly popular for their ability to insulate against heat and sound [91]. Owing to their low heat conductivity, affordability, and sustainability, they are utilized in cement composites and particleboards, which makes them ideal for energy-efficient building insulation [67,75,92]. The thermal conductivity requirements of low-density insulating boards made from coconut husk and bagasse are similar to those of conventional materials such as mineral wool [93]. Alkali-treated coir fibers improve the mechanical qualities and thermal insulation of cement composites [94], and because of their low heat conductivity, they can be used to reinforce biopolymer composites, offering environmentally friendly insulation options [95]. Additionally, coir-based composites are becoming more popular in the field of acoustic insulation, helping create soundproof interior spaces [94]. These characteristics demonstrate how adaptable and sustainable coirs enhance the thermal and acoustic performance in an architectural setting. Multilayered coir fiber-reinforced PF resin composite panels with variable densities, demonstrating superior insulation properties and improved mechanical performance, particularly in medium-density panels. The findings suggest a strong potential for these bio composites as effective insulation materials for industrial applications [96]. A bio composite based on coconut fibers and sodium alginate was developed, which exhibited improved thermal and acoustic bio composite solution, based on coconut fibres and sodium alginate designed for building applications. The research also explored the impact of different surface geometries on acoustic performance, finding that surface cavities enhance sound absorption [97].

Novel tri-layered bio composite panels utilizing pretreated long coir fibers (LCF) and fibrous chips (CFC) with a melamine-urea-formaldehyde (MUF) adhesive. The overall investigation confirmed the successful formation of these bio composite panels with enhanced thermomechanical performance, making them feasible for industrial applications [60].

4.4. Biophilic Applications

Coir boards are sustainable and adaptable materials for various interior applications. They are ideal for interior wall panels and claddings in homes, workplaces, and hospitality areas because of their natural texture, V-0 flame resistance, and exceptional thermal stability, which provide both safety and visual appeal [15]. They work well as acoustic panels in classrooms, conference spaces, and auditoriums because of their fibrous structure, which offers superior sound absorption [13,15]. Coir boards provide excellent ceiling panels for eco-friendly interior spaces, such as cafés, health facilities, and eco-homes, because they are lightweight, fireproof, and aesthetically pleasing. They offer a reasonable level of strength for decorative partition panels and can be laser-cut into complex designs to add privacy and natural design aspects to interior spaces [13]. They are used as environmentally friendly materials for product panels, signage backing, and non-load-bearing display shelving in retail environments, particularly in establishments with a brand identity that is in line with nature. Coir boards combine plant life with stable, natural, and safe support materials, making them an excellent choice for use in interior green walls and vertical gardens. Finally, their biodegradability and eco-branding potential make them a perfect choice for temporary installations and exhibition booths for short-term but significant interior applications in trade exhibitions or themed events [13].

Composites made of coconut coir fibres have shown good potential in the structural, interior, thermal, acoustic, and biophilic areas of architecture. In terms of structure, cement boards, binderless boards, and coir-reinforced panels are lightweight, environmentally friendly alternatives to traditional wood and synthetic materials, with added advantages in durability and seismic resilience. Coir composites, which offer natural textures, recyclability, and enhanced environmental resistance, are being utilized more and more in interior applications such as furniture, partitions, ceiling tiles, and automobile interiors. They work well as wall cladding, acoustic panels, and insulation layers because of their low heat conductivity and porous structure, which improves indoor comfort and energy efficiency. Additionally, coir-based boards support applications in eco-friendly interiors, green walls, and decorative partitions by fusing natural aesthetics with practical performance, thereby promoting biophilic design.

5. Challenges and Limitations

5.1. Moisture Sensitivity

Coir fibers are moisture-sensitive, which affects their durability and strength. Treatment with NaOH improves surface roughness and limits water absorption, thereby improving fiber-matrix bonding [24]. However, coir fibers exhibit high moisture absorption, which can degrade their performance in composites, particularly under acidic or weathered conditions [98]. The high lignin content (30–45%) of coir fibers contributes to their moisture-absorbent nature and reduces their crystallinity, making them less stiff and more prone to water retention [21]. Additionally, coir-based materials, such as plywood, exhibit high water absorption, which limits their use in indoor applications [99]. Despite these limitations, treatments and hybrid composites with synthetic fibers can mitigate moisture sensitivity, improve mechanical properties, and expand potential applications [24].

5.2. Consistency in Quality

The mechanical qualities of coir-based products are controlled by the fiber source, processing method, and resin used. PF-bonded boards require precise curing for good MOR, whereas the coir pith size influences density and stability; larger particles improve performance but impede uniformity [14]. Although fiber-matrix compatibility is still a problem and requires chemical treatments for uniform bonding, hybrid composites (such as coir-glass) overcome this problem by combining fibers [100]. Systems for fiber grading and automated processing may improve the repeatability of industrial applications.

5.3. Cost and Scalability

Although coir is a low-cost byproduct, processing procedures, such as alkali treatment and resin bonding, increase manufacturing costs. Although PF resins perform better, they are more expensive than UF resins, limiting their affordability in low-income regions [14]. The eco-friendly binder BST20 exhibits potential but needs to be optimized to match the prices of traditional resins [71]. Large-scale manufacturing is hampered by labor-intensive decortication and fiber extraction; however, cost efficiency may be increased by innovations such as waste-to-material techniques [72] and whole-husk use [16]. The geographical availability of coconuts reduces global supply chain reliability, emphasizing the importance of localized manufacturing centers.

5.4. Flammability Properties

The high lignin concentration of coir improves fire resistance by producing protective char. Composites at 30 wt% attained a V-0 rating (UL-94) with reduced flame spread and enhanced thermal stability, as proven by TGA and SEM studies [101]. Coir composite boards require fire retardant (FR) treatment owing to their combustibility. Boron- and phosphorus-nitrogen-based FRs increase fire and decay resistance, whereas silicon-based FRs minimize smoke and toxicity. More research is required on self-extinguishing applications [102]. The high lignin concentration of coir produces protective char, which improves the fire resistance of coir/PP composites. At 30%, they reached UL-94 V-0, with a TTI of 78s. SEM validated the strong bonding, making it appropriate for fire-safe, non-load-bearing interior applications [101].

6. Future Prospects

With focused research and development, coir composites are promising materials. Nanotechnology (e.g., lignin nanoparticles) and bio-based resins (e.g., BST20) may improve the mechanical and moisture resistance [71]. Consistency problems can be resolved by standardizing the fiber processing and implementing AI-driven quality control [15]. Costs can be decreased by implementing circular economy concepts, such as incorporating hydroponic coir waste [103] into buildings. Market potential is further diversified by new uses of earthquake-resistant concrete [85] and acoustic panels [72]. Adoption in green building certifications can be accelerated by cooperative policies that promote coirs as carbon-negative materials.

Economic feasibility can be improved by embracing circular economic strategies such as valorizing hydroponic coir waste as an additional raw material stream for composite production. Expanding the scope of applications to earthquake-resistant concrete elements and high-performance acoustic panels can diversify the market opportunities beyond conventional insulation and panel products. Furthermore, aligning product development with green building certification frameworks (e.g., LEED and BREEAM) and implementing policy incentives for carbon-negative materials can accelerate adoption in both developed and emerging markets. Through these combined approaches, coir composites can transition from niche eco-materials to critical components in climate-resilient and resource-efficient architectural systems.

7. Conclusions

Coconut coir fiber composites hold significant promise as sustainable alternatives to conventional building materials, offering environmental, economic, and architectural advantages. They are a sustainable and locally viable option for building and interior use. Material consistency is ensured by sourcing from the nearest available suppliers, thereby supporting regional economies and minimizing transportation-related environmental impact. Coir’s inherent lignin allows for binder-less board manufacturing under heat and pressure, avoiding the need for toxic synthetic resins. Eco-friendly binders, such as BST20, exhibit high mechanical strength and water resistance; however, board density is more important in determining structural strength and insulation than binder type. PF-bonded coir boards have the same tensile strength, nail/screw holding, and MOR as traditional wood panels, with larger pith particles improving moisture resistance and dimensional stability, making them appropriate for humid locations. These composites also adhere to biophilic design principles, providing natural textures, excellent thermal and acoustic performances, and versatility in wall cladding, partitions, and ceiling panels. Denser boards provide better insulation, whereas lighter boards aid in sound absorption.

The high lignin concentration of coir enhances flame resistance, resulting in UL-94 V-0 and a slower flame spread. Fire retardancy can be increased by boron-, phosphorus-nitrogen-, or silicon-based additives, and research into self-extinguishing systems is currently ongoing. Coir composites are compatible with ordinary tools and offer an excellent balance of strength, safety, and design versatility, making them ideal for sustainable architecture and interior design. Their high lignin content, natural durability, and effective thermal and acoustic insulation properties make them suitable for a range of architectural applications, from wall panels and partitions to interior finishes and insulation systems. The integration of coir-based composites not only supports biophilic and climate-responsive design principles but also contributes to waste valorization and circular economy practices by transforming agricultural by-products into functional materials.

Despite these strengths, several limitations constrain their widespread adoption. Variability in fiber quality, coupled with challenges in achieving consistent mechanical performance, remains a barrier to large-scale industrial use. The hydrophilic nature of coir leads to moisture sensitivity, reducing durability under humid or acidic conditions. Furthermore, while binderless and eco-friendly composites show potential, issues related to scalability, cost competitiveness with synthetic alternatives, and long-term fire resistance require further research.