Review on the influencing factors towards improving properties of composite insulation panel made of natural waste fibers for building application

Abstract

The low performance of insulation panels has received great attention from many researchers and engineers to improve their properties as it has become a limiting factor that minimize their usage specifically as insulation building material. Recently, there has been an increase in interest in the development of composite insulation panels made from natural waste fiber. Due to the abundance of natural waste fiber generated by the agricultural industry, it has made them suitable to be utilized as raw material for composite insulation panels. Having improper waste disposal management might result in these massive quantities of valuable natural waste fiber having a detrimental effect on our ecosystem and contributing to various environmental pollution. A composite insulation panel made from natural waste fibers has unique and versatile features in terms of thermal, physical, and mechanical properties. Seeing such potential, this paper presents a review on the influencing factors towards improving comprehensive properties of composite insulation panels. Factors that influence the properties of the panel, such as the physico-chemical properties of fiber, fiber geometrical structure, and fiber treatment were discussed. Previous and recent research efforts related to factors influencing the properties of composite insulation panels were also compiled. Last but not least, the suitable criteria, limiting factors, and condition of the fiber used in composite insulation panels were also all highlighted.

Keywords

Composite building insulation panel fibers natural waste physico-chemical properties

Introduction

The application of insulating panels is currently in the spotlight worldwide where the enhancements towards systems of thermal performance are necessary to meet the pressing global demands.¹ Generally, the insulation panel can be defined as a product capable of resisting and reducing heat. The insulation panel has a wide range of applications, whereas in tropical climate countries it is commonly used as part of building materials to provide better thermal comfort for an indoor space.² A prior study has explored the thermal performance of insulation panels for construction applications in hot, dry climates, but this research is still in its infancy and has to be expanded in several ways.^3,4 The use of adequate insulation panels in the building envelope is one of the most efficient techniques to minimize the rate of heat transmission and energy consumption to cool and heat buildings.⁵ The most common examples of insulation panels used in tropical climate countries are polystyrene panels⁶ and gypsum panels.⁷ This is because they have a high load-bearing capacity at a low weight, greater water resistance and vapor barrier, have airtight in controlled settings, have a long shelf life, require little maintenance, and are economical to manufacture.⁸ Recently, with the growing significance of climate change, the building industry is under increasing pressure to minimize its environmental effect, including the use of insulation panels as a building material. Keeping this in mind, efforts are being undertaken to produce eco-friendly insulating panels that reduce the environmental impact of buildings over their life cycle by decreasing waste and greenhouse gas emissions. This includes the development of alternative materials such as natural waste fiber into composite insulation panels.⁹

Studies indicated that natural waste fibers have excellent insulation behavior in terms of properties and thermal performance making them suitable to be used as raw material for composite insulation panels.¹⁰ When utilized as composite panels, the unique properties of natural waste fiber are the result of the formation of an air-void structure that leads to the effective enhancement of thermal performance for insulation purposes.¹¹ In addition to enhancing thermal performance, the presence of air voids decreases the mechanical and physical properties of the composite insulating panel. Yet, according to Tsalagkas et al.¹² the composite insulating panel does not necessary to have great mechanical properties as long it has adequate thermal performance for the heat transfer mechanism. Due to these reasons, the majority of existing insulating panels are typically made of low-density materials with air-void structures. Due to the air void structure and low durability, it has been reported that its usage in insulation applications requiring high durability, curved surfaces, and greater susceptibility to impact damage is limited.¹³

Despite these conditions, it should be noted that the mechanical and physical properties of common insulation panels should be able to withstand the least amount of load possible during their handling, installation, and maintenance processes.¹⁴ Additionally, the thermal performance of the insulation panel can be easily deteriorated following damage and impact accidents during the installation process or when utilized in heavy condition applications.¹⁵ Therefore, both the properties of composite panels are critical and should not be compromised in the insulation application.

In this study data and information on composite insulation panels from earlier studies that enlighten on the influence in improving the properties of the composite panel have been critically evaluated. Evaluation and comparison of an acceptable fiber condition and treatment approach is necessary throughout the composite insulation panel production process to acquire superior composite properties. This review was aimed to discuss the important characterization parameters of fiber physico-chemical properties such as chemical composition, particle size, and thermal degradation temperature which are considered necessary when selecting fiber to fabricated composite insulation panel. The effect of geometrical structure in terms of fiber length and density on improving composite insulation properties by increasing fiber networks in composite structure was explained. This review also explored the impact of fiber physical treatment via the fibrillation process to enhance morphological structure of fiber, improving the fiber bonding and as well increasing properties of composite insulation.

Composite panel

A composite panel is a combination of two or more component materials that have been mixed to create a new material with enhanced characteristics or properties.¹⁶ The enhancement of the composite panel properties is dependent on the use of an appropriate integrated process or materials mixtures in the composite panel.¹⁷ In recent years, several techniques have been described for the production of composite panels made of eco-friendly materials such as natural waste fibers for a variety of purposes, including insulation panels.¹⁸ Numerous studies have indicated that there are promising potentials in utilizing natural waste fibers into composite insulation panels due to their superior mechanical properties, low cost, low density, low environmental impact, and most importantly, superior thermal properties.¹⁹

In composite research, binder is one of the components usually utilized as an adhesive to bind fiber particles together and form a dense fiber network in composite panels.²⁰ Typically, the binder retains the fiber by serving as a load carrier and smoothly absorbing the created stress in the composite panel.²¹ Several studies have shown that the physical and mechanical properties of a composite panel have greatly improved when there is more interaction occurred between the binder and the fiber particles.²² In addition, research has demonstrated that a sufficient amount and a homogeneous combination of binder with fiber particles are capable of filling the fiber particle bonding gap or void structure in composite panel cavities, hence enhancing their physical and mechanical properties.²³ The advantages and disadvantages of binder were listed in Table 1. In general, the composite panel using a binder may be split into two categories: synthetic binder and natural binder. Polymers such as polypropylene, polyethylene, polyvinyl chloride, epoxy resin, polyester, and polyurethane are often employed as synthetic binders.²⁴ Among all types of binder, urea-formaldehyde is considered the most selected synthetic binder used in the composite panel industry due to its high bonding performance and low-cost production.²⁵ Despite this, the use of urea-formaldehyde in composite panels was prohibited in a number of nations owing to its detrimental impact on the emission of carcinogenic poisonous gas and was deemed unsuitable for use as a construction material.²⁶ Thus, there is a significant need for natural binders such as polylactic acid²⁷ and starch²⁸ in the composite panel.²⁹

Table 1.

Advantages and disadvantages of binder in composite.^32,34.

Type of binder	Name of binder	Advantages	Disadvantages	References
Natural	Starch	-Biodegradable	-High brittleness	³⁰
		-Biodegradable	-Lack compatibility
		-Renewable	-High hydrophilic nature
		-Non toxic	-Poor processing quality
Natural	Mycelium	-Water resistance	-Slow manufacturing process	³¹
		-Fire resistance
		-Biodegradable
		-Low thermal conductivity	-Easily contaminated
		-Low cost
		-Non toxic
Natural	Natural latex	-Mould resistance	-Strong odour	³²
		-Biodegradable	-Strong odour
		-Low thermal conductivity	-Expensive
		-Non toxic	-Expensive
Natural	Animal glue	-Water soluble	- Easy to get at room temperature	³⁰
Natural	Animal glue	-Non-toxic	-Poor mechanical strength	³⁰
Natural	Polylactic acid	-Biodegradable	-Low degradation rate	^{33, 34}
		-Biodegradable	-low cell adhesion
		-Bio compatibility	-Biological inert
		-Non toxic	-Acid degradation by product
Natural	Resorcinol adhesive	-High mechanical properties	-Expensive	³⁵
		-Water resistance	-Poor void filing capacity
		-Biodegradable	-Short shelf life
Synthetic	Melamine-formaldehyde	-Fire resistance	-Low storage stability	³⁶
		-Fire resistance	-High hardness
		-Low thermal conductivity	-Brittle and brittleness
		-Highly durable	-Formaldehyde emission
Synthetic	Epoxy resin	-High mechanical properties	-Long curing time	³⁷
		-Good temperature resistance	-Expensive
		-Chemical resistance	-Brittle
Synthetic	Polyurethane	-Durable	-Flammable	³⁸
		-Tough material	-Flammable
		-Chemical resistance	-Poor weatherability
		-Low cost	-Attacked by most solvent
Synthetic	Urea formaldehyde	-High mechanical properties	-Emit toxic gas	³⁹
		-Low cost	-Not biodegradable
		-Colourless
		-Fast curing
		-Water solubility

Binders, on the other hand, are less desirable when making composite panels for use as insulation. Existing research has shown that the use of binders results in the elimination of void structure and a thermal bridge reaction, which reduces the insulation effectiveness of the composite panel.⁴⁰ Technically, the establishment of a void structure will hinder the ability of the air to conduct heat. In the meantime, the elimination of void structure in composite panels will commence a conducting effect between fiber particles. It is worth to be mentioned that, material that inhibits heat convection of air tends to obtain lower thermal conductivity value. Consequently, the existence of void structure is crucial for reducing the heat transfer rate of composite panels and enabling their usage as insulation panels.⁴¹ In addition, research indicates that the lack of binder in composite panels not only increased thermal efficiency, but also contributed to the creation of a healthier indoor environment.⁴² It should be noted that the existence of a void structure has also led to the formation of a weak spot in the composite panel, which may have detrimental effects on its mechanical and physical properties.⁴³ It has been reported that relying on fiber entanglement using suitable fiber particle size or reinforcement material could be a potential approach to replace the use of binder and improve the properties of composite insulation panels while preserving the formation of void structures within the panel.⁴⁴

In general, the fabrication of composite can be carried out using variety types of methods and different types of material. Among of different types of materials, natural waste fiber is among of potential materials that can be used to fabricate insulation panels owing to their properties. The addition of a binder will considerably improve the physical and mechanical properties of composite insulation panel, but it may diminish its thermal performance. Apart of usage of binder, it was indicated that the performance of composite insulation panels also dependent on the properties of fiber itself. In the fabrication of composite insulation panels, longer fibers are seen more suited as raw materials. This is due to the fact that it can improve the reinforcing properties of fiber networks without removing the void structure. This factor is crucial as it will provide a significant impact on the thermal performance of the composite panel. The composite production flowchart is represented in Figure 1.

Figure 1.

Composite manufacturing process flowchart.

Properties of composite panel

Generally, the term properties in science definition is refer to a feature or attribute that may be used to characterize matters or materials by observation, measurement, or a combination of these. Properties of composite panel are defined as a measurement of composite panel properties in order to meet a standard requirement for a specific application.⁴⁵ The main purpose in evaluating the composite properties based on their standard was to ensure the panels able to maintain their performance efficiency during used its application including thermal insulation. According to previous studies, there are many types of thermal insulation standards that can be used to validate the properties of composite insulation panel, such as JIS A5905, ASTM C203, ISO 1209, and EN 12,089-B. Commonly, the standard tests for insulation application are i) flexural strength, ii) flexural modulus, iii) tensile strength,⁴⁶ iv) water absorption,⁴⁷ v) thickness swelling and vi) impact strength.

These standard mechanical properties for insulation application can be achieved in many ways such as, by down select the suitable composite raw material according to their physico-chemical properties, incorporating reinforcement material and optimizing the composite material mixture ratio. It has been stated that the physico-chemical properties such as chemical composition,⁴⁸ particle size,⁴⁹ and thermal degradation temperature⁵⁰ are among the important parameters affecting physical and mechanical properties of composite panel particularly made of natural fibers and or natural waste fibers. These physico-chemical properties are important and should be characterized before composite fabrication process.⁵¹

In a different approach, it has been emphasized that the properties of composite panels also can be improved by using reinforcing fiber or by selecting an appropriate mixture of fiber ratios during the fabrication of composite panels.⁵² For another approach in composite studies, enhancement of composite properties can be achieved by reducing the particle size and addition of binder. Nevertheless, neither method is ideal for the fabrication of composite insulating panels. Technically, reinforcement fibers with suitable physical structures in terms of fiber length, fiber diameter, and morphological structure can offer an adequate reinforcing mechanism without significantly impacting the insulating performance of the composite panel.⁵³

Meanwhile, the fiber ratio used in the composite should be manipulated in accordance with specific insulation application requirements to obtain the best, optimal and desirable properties and thermal performance.⁵⁴ It has been suggested that the manipulated fiber ratio during reinforcement fiber incorporation into composite insulation panels should not exceed the amount of primary fiber in the composite, as this will cause the composite panel to exhibit undesirable properties such as a higher moisture absorption rate, poor dimensional stability, and an unnecessary high mechanical strength.⁵⁵ Concerning this, it has been indicated that the addition of reinforcement material in the composite insulation panel would be suitable within the range of 10–50% of the overall weight of the panel.⁵⁶

Following that, fiber treatment is another method for improving the mechanical and physical properties of composite insulation panels, and it can be done in a variety of ways, including biological treatment,⁵⁷ chemical treatment,⁵⁸ and physical treatment.⁵⁹ Nevertheless, in terms of an environmentally friendly approach, the fiber treatment via physical method is much more preferable due to its feature as “green” approach, and it requires less time and involves low cost processing.⁶⁰ In composite studies, physical fiber treatment is frequently used to modify the morphological structure, such as by reducing the length or diameter of the fiber.⁶¹ Recent studies have reported that, the influence of modifying morphological fiber structure can contribute to improving the fiber distribution and interfacial fiber bonding in composite panels.⁶² As a result, the composite panel will obtain more excellent physical and mechanical properties.⁶³

In addition to the physical and mechanical properties of composites, the thermal conductivity of composite insulation is one of the most significant factors in the development of composite insulation panels. The thermal conductivity of a composite panel may be determined based on the amount of heat that passes through a unit thickness of the panel when a temperature difference is established between the faces.⁶⁴ In general, thermal conductivity is a critical parameter for estimating the rate of heat transfer through a composite panel. A high thermal conductivity number indicates that the composite panel is an excellent conductor, whereas a low thermal conductivity value indicates that the composite panel is an inefficient heat conductor suitable for use as insulation.⁶⁵ In this context, choosing an insulation panel with a low thermal conductivity rating reduces the rate of heat flows through the panel into a building.⁶⁶ It has been addressed that the thermal conductivity of composite panels is dependent on several parameters such as fiber morphology, composite density, and homogeneity of fiber distribution.⁶⁷

Several morphological studies have demonstrated that composite panels composed of fibrous structure material can have lower thermal conductivity values as a result of the formation of void structures inside the composite panel cavities. Due to the existence of void structures, the composite panel will experience a combination of heat conduction (heat transfer through solid particles) and convection (heat transfer through air). Due to the fact that some of the heat is transferred partially through the air in the void structures, the thermal conductivity value of the composite panel is reduced. Another parameter impacting the thermal conductivity of composite panels is their density variation. In general, increasing the density of a composite panel increases its thermal conductivity value. In comparison to the impact of particle size, it has been reported that density has a greater influence on thermal conductivity.

When the density of the composite panel is raised, the fiber particles create a greater degree of fiber bonding and surface contact, resulting in the elimination of void structures. This situation has shifted the heat transfer mechanism from convection to conduction. Finally, the thermal conductivity of composite panels can be influenced by the uniformity of fiber distribution and packing in the composite. When smaller fiber particles are used during the composite manufacturing process, the uniformity of fiber distribution and packing in the composite would be enhanced. Although this condition may improve the physical and mechanical properties, the surface contact between particles may initiate the conduction effect, leading to an increased thermal conductivity value. In relation to the evaluation of composite properties, it should be carried out to ensure the developed composite panel has achieved the required standard allowing it to be efficiently used during heat insulation application.

In this section, it can be concluded that, the enhancement of composite properties can be carried out by selecting suitable natural fiber based on their physico-chemical properties, optimizing the fiber ratio and using a suitable reinforcement material in composite. Although the physical and mechanical properties of composite are important, it should be reminded that the thermal performance of the composite insulation panel is the most crucial. Therefore, the thermal conductivity value of composite panels should take precedence and then followed by physical and mechanical properties.

The influence of fiber physico-chemical properties

The physico-chemical properties of fiber or natural waste fiber in composite panel typically consist of several characteristics such as chemical composition, density,⁶⁸ surface area, particle size,⁶⁹ moisture content, moisture sorption, elemental composition,⁷⁰ thermal degradation temperature, and crystalinity.⁷¹ These properties play an important role in the development of composite panels for insulation applications. Characterization study of these properties is critical to understand the suitability of a material for the development of composite in order to be used in thermal insulation applications. It should be mentioned that when fabricating a composite panel for a given purpose, the physico-chemical properties of fiber are critical, since the information data will be utilized to determine the fabrication method and properties of the composite insulation panel. Thus, the selection of composite material aligned with appropriate physico-chemical properties based on the composite panel requirements for insulation application should be prioritized.⁷² It has been reported that three main physico-chemical properties of fiber should be concerned in fabricating composite insulation panels which are chemical composition,⁷³ particle size,⁷⁴ and thermal degradation temperature.⁷⁵

Chemical composition of fiber

Chemical composition plays an essential role in determining the properties of composite made of natural waste fibers.⁷⁶ Many natural waste fiber sources such as sugarcane bagasse,⁷⁷ empty fruit bunch fiber, rubber wood fiber and spent mushroom substrate fiber⁷⁸ have been reported as the most common raw material to be utilized in the fabrication of composite insulation panels due to their substantial chemical composition in terms of cellulose, hemicellulose, and lignin. These compositions are crucial in selecting the suitable natural waste fiber used in the composite fabrication process, as they are responsible for providing mechanical strength to the fiber particles. Degradation on of these composition may led to the reduction on composite physical and mechanical properties. According to Hospodarova et al.⁷⁹ fourier transform infrared spectroscopy (FTIR) is common analysis non-destructive method used to identify the chemical composition in fibers or natural waste fibers such as cellulose, hemicellulose, and lignin which is based on the presence of specific functional groups. The FTIR characterization is carried out by identifying the functional groups based on peak intensity presence at each wavelength number, as shown in Table 2.

Table 2.

List of the functional groups represents the cellulose, hemicellulose, and lignin in the fiber or natural waste fiber based on the FTIR characterization.^80,81

Wavenumber (cm⁻¹)	Functional group		References
3200 to 3600 cm⁻¹	O-H	• Stretching effect by back bone of cellulose and hemicellulose	^80,81
2800 to 3000 cm⁻¹	C-H	• Stretching in methyl and methylene group of cellulose
1650 to 1740 cm⁻¹	C = O	• Stretching in unconjugated ketones, carbonyl and in ester groups of hemicellulose
1371 to 1427 cm⁻¹	C-H	• Bending from cellulose and hemicellulose
1442 to 1612 cm⁻¹	C = C	• Stretching from aromatic ring of lignin
1000 to 1244 cm⁻¹	C–O–C	• Glycosidic bond vibration of cellulose and hemicellulose

Nevertheless, it should be emphasized that the FTIR analysis only analyzes the chemical composition content based on wavelength intensity and it is incapable of determining the actual lignocellulose component composition of the fiber. To quantify the exact percentage of lignocellulose in the fiber, an employment of Soxhlet extraction method is often utilized and described in a number of prior publications.⁸² The chemical composition such as cellulose, hemicellulose, lignin and extractives based on the Soxhlet extraction is summarized in Table 3.

Table 3.

Chemical composition of selected natural waste fibers using different Soxhlet extraction methods.^83,84,85

Fiber	Cellulose (%)	Hemicellulose (%)	Lignin (%)	Extractive (%)	Ash (%)	Types of methods	References
Coconut fiber	46.1	19.8	62.9	9.0	2.9	TAPPI standard	⁸³
Sugarcane bagasse	28.3–51	20–36.3	12.5–24	5.83–7.82	1.3–5	TAPPI standard	⁸⁴
Date leaves	26	18	16	12	5.5	TAPPI standard	⁸⁵
Pineapple leaves	45.3	17.4	25.6	3.9	4.6	ASTM standard	⁸⁶
Empty fruit bunch fiber	37.7–65.0	14.6–33.5	13.1–31.6	1.34–7.19	1.2–6.69	TAPPI standard	^87,88
Banana waste fiber	60–85	5–8	5–10.53	4.44–25	7–21	ASTM standard	⁸⁹
Rubber wood sawdust	57.12	19.1	13.12	10.66	2	Goering and Van Soest	⁹⁰
Canola	44	17.79	19.21	13	6	TAPPI standard	⁹¹
Typha	37.3–40.12	19.74–21.5	17.9–24.31	11.12–11.54	14.2	TAPPI standard	^92,93

In general, composite insulation panels were purposely fabricated at low density to produce a large number of voids within the material structure. As these voids were filled with air, the insulation panel will have a high tendency to initiate a heat convection effect and provide a lower thermal conductivity value. Nevertheless, the presence of these voids in composite insulation panels could also pose a disadvantage by degrading the mechanical strength of the panel. It should be reminded that, sufficient mechanical strength in composite is crucial to ensure the panel becomes more durable and is able to prevent damage during the storing or installation process. Therefore, a raw material with a higher amount of cellulose content is considered most suitable to be fabricated into an insulation panel. This is because, cellulose is a long chain sugar polymer that contributes to providing mechanical strength to individual fiber structures compared to hemicellulose and lignin. It was expected that, having a substantial amount of cellulose in the fiber could contribute to enhancing the mechanical strength of the composite insulation panel.

Particle size of fiber

Based on previous studies, it has been indicated that the high proportion of small fiber particle size can improve the fiber packing and reduce void formation in composite panels, which enhances its properties as an insulation panel. This condition has contributed to increasing the effective stress transfer between fiber particles, which has resulted in an improvement in the mechanical and physical properties of composite insulation panels. The effect of particle size on fiber distribution and packing has been widely reported in numerous studies as one of the crucial parameters that is responsible for providing an improvement in the properties of composite insulation panels.⁹⁴

In the development of composite insulation panels, past research has addressed the fact that reducing the size of the void structure caused by fiber particle size reduction is one of the key factors in enhancing thermal performance in terms of thermal conductivity value.⁹⁵ This claim is consistent with findings from a prior study, which found that the presence of fine void structure in composite cavities did contribute to a lower thermal conductivity value. Meanwhile, it has been reported that, a fiber particle size that is too fine could lead to the elimination of void structure and an increase in fiber interaction. As a result, this condition will initiate heat convection into heat conduction, which could reduce insulation performance due to an increased thermal conductivity value.⁹⁶ Therefore, determination of suitable fiber particle size in the fabrication process of composite panels towards insulation application is necessary.

For particle size research, there is no specified size for the material utilized in the fabrication of composite insulation panel. Various material sizes have been reported for use in composite insulation investigations. Nonetheless, the focus of this part is to justify where the size of composites basic materials must be optimized. In composite insulation investigations, there is a strong correlation between the fiber particle size towards the improvement of composite properties and thermal performance. Commonly, smaller particle size is used to improve the physical and mechanical characteristics, whilst bigger particle size is better suited to develop void structure in composites in a bid to improve thermal performance. Therefore, a particle size analysis is crucial in the composite fabrication process. Table 4 outlines the types of composite insulation panels created from a variety of natural waste fiber sources, as well as their characteristics, including particle sizes.

Table 4.

Types of composite panels made of various natural waste fiber sources towards insulation and their characteristics.⁴¹

Type of composite panel made of natural waste fibers	Density (kg/m³)	Particle size (µm)	Thermal conductivity (W/m.k)	References
Washingtonian palm leaf rachis fiber	687.1–812.2	250–8000	0.079–0.089	⁴¹
Black locust tree bark	185.8–548.3	1000–8000	0.132–0.158	⁶⁶
Wood waste fiber	117–167	1000–4000	0.0568–0.0629	⁹⁷
Spent mushroom substrate fiber and sawdust fiber	190–200	600–2000	0.06–0.07	⁹⁸
Sawdust fiber	430.8	100–2500	0.087	⁹⁹
Wood fiber	180–200	500–1000	0.0091–0.0217	¹⁰⁰

Thermal degradation temperature of fiber

Thermal degradation analysis on fiber gives information, ideas, and guidelines, especially on the degradation temperature of fiber to avoid severe fiber burn and damage during fabrication, which could later affect the properties of the composite.¹⁰¹ Most existing studies in the literature have used thermogravimetric analysis (TGA), as a standard method to evaluate the thermal degradation temperature of each chemical composition in fiber. Technically, the evaluation mechanism of TGA is based on weight loss at each degradation stage with increasing heating temperature.¹⁰² In general, there are three major degradation stages that will be obtained in the natural waste fibers of composite insulation panels during thermal degradation analysis, which is cellulose, hemicellulose, and lignin.¹⁰³ Therefore, these three major weight losses will be the main focus of investigation in composite research. By understanding the nature and thermal degradation temperature of fiber, it is possible to monitor the conditions and methods performed throughout the composite fabrication process. In addition, this thermal degradation temperature technique gives data that may be utilized to prevent the composite fiber from exceeding its thermal degradation limit during the heating process. List of thermal degradation temperatures for chemical compositions and TGA graph of fiber (cellulose, hemicellulose, and lignin) in natural waste fiber are described in Table 5, and an example of a thermal degradation graph is illustrated in Figure 2.

Table 5.

List of thermal degradation temperatures for chemical compositions of fiber (cellulose, hemicellulose and lignin) in natural waste fiber.^104,105,106

Chemical composition	Thermal degradation temperature (°C)	Degradation of chemical composition	References
First stage	32–119	Corresponding to moisture absorb by hydroxyl group attached on polysaccharides	¹⁰⁴
Second stage	220–370	Correspond to degradation hemicellulose and cellulose	¹⁰⁵
Third stage	390–633	Correspond to lignin	¹⁰⁶
Third stage	390–633	Decomposition of hydrocarbons formed during hemicellulose degradation	¹⁰⁶

Figure 2.

Example of TGA and derivative TGA graph of empty fruit bunch fiber (EFB).¹⁰⁷

In the manufacture of the composite insulation panel, a thermogravimetric investigation was conducted to determine the thermal decomposition temperature of the natural fiber. This analysis has been published in several composite studies, since it is frequently used to investigate the thermal characteristics of fibers depending on their composition and to identify an appropriate hot press temperature during composite production. Technically, if the composite raw material is heated above its thermal degradation temperature, the properties and thermal performance of the panel are likely to degrade. Therefore, it is essential to conduct this research at the beginning of every material selection phase throughout the production of composite insulation panels.

The influence of fiber geometry structure

Fiber geometry is define as the shape, size, length, and diameter of a natural fiber, which can be changed or improved by using fiber treatment processes.¹⁰⁸ Different sorts of fiber geometries will have distinct effects on the properties of composite insulation panels. A few parameters, such as type of treatment methods, the condition of each treatment, and the species of wood, can have a substantial impact on the fiber geometrical structure.¹⁰⁹ Technically, the geometrical structure of fiber determines the arrangement, distribution, and packing of the fiber particle in a composite insulation panel.¹¹⁰ In some composite studies, the use of fiber particles with a fine geometric structure is significantly more advantageous. Utilizing a fine fiber geometrical structure in composite panels, especially for insulation applications, is essential for reducing the size of void structures and enhancing the interfacial interaction between fiber particles. In addition, the fine fiber geometric structure results in a smooth surface, allowing direct painting and lamination of the composite panel for its intended application as an insulating material.¹¹¹

Nevertheless, past research has shown that the utilization of a fine fiber geometrical structure is only advantageous to the composite if adequate binder is applied.¹¹² According to Das et al.¹¹³ when forming a bond between fibers, the binder plays a crucial function in generating a vast connection of fiber network, which contributes to the transmission of load between fiber particles in the composite. Consistent with a previous study, their findings indicated that fibers with a fine geometrical structure that lacked a binder suffered a significant disadvantage due to their inability to undergo particle bonding, resulting in a reduction of load distribution properties throughout the composite panel.¹¹⁴

In contrast, a research found that the use of a binder affected the elimination of void structures, resulting in a decrease in the thermal performance of the composite insulation panel. Concerning this, it was proposed that the usage of binder should be excluded, yet incorporating reinforcing material with an appropriate fiber geometry structure to achieve the same fiber networking effect in the composite insulating panel. The reinforcement material can be classified into fibers, whiskers, and particles.¹¹⁵ Conversely, in composite insulation panels, the reinforcing material with longer fibers is preferred because it may generate a better fiber network through particle overlap and recover the influence of the binder. On the other hand, the incorporation of reinforcing material in composite insulation panels through varying fiber ratios is significant and has been reported in a few previous studies as being able to improve the panel properties.¹¹⁶

From the literature, it can be determined that the geometrical structure of the fibers is one of the most essential aspects in defining the properties of a composite material for its intended applications. For heat insulation applications, the formation of a void structure is crucial to improving the properties and thermal performance of the panel. Therefore, the use of small fiber particles or the addition of a binder that can result in a compact fiber structure and the elimination of voids in composites is undesirable. As previously noted, composite properties can be enhanced by using reinforcing material. By following a similar method with the addition of a binder, the reinforcing material can contribute to the improvement of fiber network development and stress distribution areas in composites without removing the void structure.

Length of fiber

To acquire greater performance from the composite insulating panel, an appropriate fiber-length reinforcing material is required. Previous research indicated that reinforcing material with longer fiber lengths and higher fiber aspect ratios (ratio of fiber length to fiber diameter) might substantially improve the mechanical properties of composite insulating panels.¹¹⁷ Longer fiber lengths are more capable of creating a greater number of fiber particles that overlap, which leads to a higher degree of fiber network formation in the composite insulation panel. This is necessary for the material to perform its purpose as a reinforcement successfully.¹¹⁸ When the composite insulating panel is mechanically strained, the presence of fiber with a longer fiber length can improve its performance by serving as a load receiver and initiating a wider sliding field between particles.¹¹⁹

A study by Leng et al.¹²⁰ emphasized that the enhancement of a higher degree of fiber network might be impacted by the existence of a larger ratio of long fiber structure, which led to the improvement in the load transfer efficiency of composite panel. In addition, this condition may restrict the formation of cracks in the composite by keeping the fiber bonded within the cavity, therefore enhancing the stiffness of the panel.¹²¹ In addition, it has been demonstrated that the usage of fine fiber particles in composite insulation panels has a high tendency to create extra fiber ends, which ultimately serve as a point of failure for load concentration in the structure of composite panel.¹²² In spite of this, the utilization of a long fiber geometrical structure as a reinforcing material in order to recover the impact of the binder has resulted in the composite being afflicted by a few drawbacks. According to the findings of a number of research, composite materials with longer fiber lengths tended to exhibit a higher fiber entanglement effect.¹²³ As a consequence of this, the composite insulation panel may experience non-uniform load transmission due to inhomogeneous fiber distribution and a lack of intimate connection between the fiber particles.¹²⁴ In addition, previous studies demonstrated that a longer fiber might result in the production of a greater number of gaps and micro cracks in the composite insulation panel cavities.¹²⁵ As a result, the composite insulation panel may have poor physical properties because more water molecules are able to penetrate the void when exposed to wet conditions.¹²⁶ Yet, it should be reminded that, the existence of void structures in a composite panel is crucial for achieving optimum thermal performance in an insulation application.

For a conclusion, although longer fiber length has the potential to contribute to an improvement in the mechanical properties of a composite, it also presents a challenge in terms of contributing to a degradation on physical properties of composite panel. Creating an optimal fiber distribution in a composite material is more challenging when working with long fiber lengths. As a consequence of this, there may have been insufficient interfacial bonding between the fiber particles. This scenario may result in the formation of a large void structure, reducing the dimensional stability of composites especially when exposed to wet conditions. Hence, it is necessary to pick the optimal fiber length throughout the composite fabrication process.

Density and bulkiness structure of fiber towards compression effect

The compression effect may be described as the densification of the fiber structure in a composite insulating panel caused by a high pressing force and high temperature during the fabrication process.¹²⁷ According to the findings of a study, a dense composite panel that was manufactured using a greater compression effect has the ability to achieve outstanding mechanical properties.¹²⁸ The increasing compression effect in the panel may have facilitated a larger inter-particle contact, resulting in the improvement of its mechanical properties.¹²⁹ On a more technical level, the compression effect has produced an influence that is comparable to the density effect in terms of mechanical properties. Notably, the compression effect in the composite insulating panel is not influenced by the weight of fiber particles, but rather by fiber density and bulkiness structure.¹³⁰ By utilizing a fiber that has a low density and a more bulky structure than normal, it would be possible to generate a greater compression effect in the composite panel. In general, fiber that had a low density and a bulky structure required a bigger volume of fiber in order to be compressed into a panel at a particular thickness and density.¹³¹ Therefore, this condition may have led to a higher increase in fiber particle surface contact within the composite structure.¹³²

The primary compression effect in the composite insulation panel was initiated by the reduction of void structures between fiber particles.¹³³ Concerning the usage of composite insulation panels, it is possible that a significant compression effect might lead to the elimination of void structures, which would then result in a decrease in the thermal efficiency. At this stage, the mechanical properties of the panel can be improved due to the enhanced compression effect that occurred primarily on the composite surface.¹³⁴ When applied to certain types of composites, particularly those made with many layers of fiber mat, this compression effect may provide advantages. This is due to the fact that the multilayer composite may experience a greater compression effect at the surface and a lesser compression impact in the middle section of the composite panel. This results in some of the void structure responsible for thermal insulation being able to remain even after pressing.¹³⁵ Using this approach, the improvement in mechanical properties can be obtained without highly affecting the thermal performance of the composite insulation panel. In composite studies, it is standard practice to use reinforcing material that has a low density and a bulky structure. This is done in order to obtain the desired high compression effect. This strategy has been proposed in a number of earlier studies as a means for enhancing the properties of the composite through the use of the compression effects on the composite.¹³⁶ It should be noted, that an excessive proportion of bulky low-density fiber in the composite may also result in a disadvantage, where the fiber in the middle section of the composite insulating panel may suffer from insufficient compression.¹³⁷ This scenario is frequent when the composite is fabricated at a set thickness. During hot pressing, the compression action on fiber begins at the composite surface and continues until the composite reaches its desired thickness. As a result, the compression effect in the middle section may become insufficient, and the interfacial connection between fiber particles is reduced.

Contrary to mechanical properties, a study by Santos et al.¹³⁸ indicated that a high compression effect had resulted in an extreme physical drawback impact to the composite insulation panel when being exposed to wet conditions. According to Liao et al.,¹³⁹ a drastic increase in water absorption and thickness swelling of the composite insulation panel could be caused by the release of compression stress that is imparted during the hot pressing process. This phenomenon is termed as spring back effect, which only occurs when the composite panel is exposed to wet conditions. In addition, a study stated that the spring back force became extremely high when the composite panel was made of a high volume of bulky and low-density fiber.¹⁴⁰ On the other hand, for a binderless composite that is often used in insulation applications, the spring back force effect may occur extensively due to the breakdown of hydrogen bonds between fiber particles in the composite panel cavities. This condition would eventually lead to an increase in the size of void structure and the formation of a space between composite panel fiber particles. Consequently, when more voids and capillaries are generated, water absorption capacity of composite panels increases.¹⁴¹

Last but not least, it should be noted that the higher compression effect is not only limited by the implementation of low-density fiber with bulky structure but according to a research work by Benthien et al.¹⁴² higher compression effect could also be achieved by increasing the compression resistance in the middle part of the composite panel. To achieve this condition, previous research suggested that a few parameters might be applied and implemented in composite panels, such as raising the fiber ratio of fine fiber particle,¹⁴³ utilizing a high density fiber,¹³¹ increasing the amount of resin in fiber¹⁴⁴ and reducing the moisture content of fiber.

The influence of fiber physical treatment effect towards properties of composite insulation panel

Fiber treatment is a common technique used by composite panel researchers to modify fiber properties in terms of chemical and physical, which could later improve the composite panel properties.¹⁴⁵ According to previous studies, fiber chemical treatment could be used to improve the functionality of fiber and thus the compatibility of fiber and binder in composite panels.¹⁴⁶ Aside from chemical treatment, physical treatment is another approach used to modify the fiber geometrical structure in terms of fiber length and fiber diameter to improve interfacial bonding and enhance compatibility between fiber particles in composite panels.¹⁴⁷ As previously discussed, the incorporation of a binder in a composite insulating panel was deemed undesirable. Therefore, the physical treatment of fibers is preferable, as composite production requires just physical bonding and physical interlocking between fiber particles. In instance, the common fiber physical treatment that previously reported in composite panel studies were included grinding, ball milling,¹⁴⁸ and refining process.¹⁴⁹

Among the fiber physical treatments, the refining process is a technique capable of modifying the fiber geometrical structure into a small fiber diameter and producing a large number of fiber fibrils while reducing the fiber length significantly.¹⁵⁰ Generally the refining process is commonly used in the pulp and paper industries.¹⁵¹ According to Ahmad et al. the refining process, also known as beating process.¹⁵² Technically, the treatment process involves frictional force between two rotating steel parallels grooved disks.¹⁵³ During the process of refining, the fiber is loaded, aggregated, and trapped between the edges of the bar on the disk surface, as illustrated in Figure 3. As a consequence of this, the fiber is struck several times, which causes the structure of the fiber to be sheared and peeled into fragments of smaller fibers. This process is referred to as the fibrillation effect.¹⁵⁴

Figure 3.

Refining mechanism using the frictional force of bars on the fiber structure.⁶³

In addition, the settings of the refining process can be changed from lower to higher in order to generate a stronger frictional force, which in turn leads to the generation of a higher fibrillation degree effect on the geometrical structure of the fiber.¹⁵⁵ Existing studies state that, this higher condition can be achieved by reducing the gap between the refining disks in the refiner.¹⁵⁶ Even so, the studies also mentioned that increasing the refining process into extreme conditions might also cause an extreme frictional force, which not only generates small fibrils but also may result in a poor impact on the fiber structure. A higher refining condition may cause the fiber to have an extreme fiber shortening effect and more damage on fiber structure, both of which may eventually contribute to a reduction in the mechanical properties of the composite panel.¹⁵⁷

Technically, the composite structure was formed based on reinforcement and interfacial bonding between particles. In this relation, the mechanical properties of composite is highly influenced by the strength of reinforcement material and its bonding structure. As was previously stated, the enhanced refining conditions will damage the fiber structure and eventually decrease the strength of reinforcement material. As a result, the composite mechanical properties were decreased. The impact of the fiber shortening effect on the mechanical properties of composites has been the issue of several investigations in the past. Nevertheless, it was also determined that the short fiber has an additional benefit in that it is capable of forming a homogeneous fiber distribution and producing a stronger fiber interlock and fiber packing inside the composite structure. Moreover, the importance of the fiber shortening effect on improving the mechanical properties of composites increases since the short fibers tend to have a greater surface area, hence creating more bonding sites between fiber particles. As a result the mechanical properties of composite become increase. As previously mentioned, the influence of fiber shortening effect towards increasing composite mechanical properties could be achieved only when the binder was added into composite. But, the binder was not meant to be utilized in this study owing to the elimination of air voids in the composite, which might impair the insulation panel thermal performance. Therefore, the fiber bonding in composite insulation panels was formed and dependent only by weak bonding which is hydrogen bond. Increasing the greater number of fiber end through increasing fiber shortening effect will initiate more hydrogen bonding between fiber particles but also decrease reinforcement strength of fiber particles. As the result, the mechanical properties of composite become decreased.

In this section, it can be concluded that the fiber treatment is an initial step toward developing composite panel, especially for insulation applications. There are many types of fiber treatment that can be carried out either physical, biological and chemical, nevertheless it should be reminded that, the selection of fiber treatments are depending on composite targeted application. Unnecessary fiber treatment will lead to waste the resources and time.

Morphological structure of treated fiber

According to the findings of previous research, the morphological structure of the fibers plays a significant part in the process of increasing the properties of composite insulation panels.¹⁵⁸ In this regard, it has been reported that the refining process is one of the fiber physical treatment methods that produces the most optimal fiber morphological structure in terms of fiber diameter, surface area, fibrillation degree, fiber cutting, and fiber swelling.¹⁵⁹ Therefore, the treated fiber from refining process is much more desirable, as it contributed to a large fiber network formation to recover the effect of binder in composite.¹⁶⁰ As mentioned, during the refining process, the fiber is treated repeatedly by the shearing technique with a frictional bar in the refiner. This condition allows the fiber to be morphologically modified and undergo internal and external fiber fibrillation.¹⁶¹ Internal fibrillation could be defined as the delamination of the fiber wall structure caused by compressive refining process. The statement has a good agreement with a study by Mayr et al.¹⁶² where the friction force during the refining process had caused the fiber wall to be delaminated, collapsed, and torn out, resulting in altering the fiber structure. The internal fibrillation had also contributed to increasing fiber deformability and flexibility. These improvements on the fiber structure had caused the composite insulation panel to be able to form a higher compact structure and create a larger contact surface area between fiber particles, as well as enhance the physical properties of the composite.¹⁶³

In contrast, external fibrillation refers to a condition in which the fibrils attached to the fiber surface are peeled away. During the refining process, abrasion between the refining bar and fiber particles led to the development of this condition.¹⁶⁴ External fibrillation can contribute to the improvement of fiber bonding in composite panels by increasing the number of fiber interlocks, resulting in the enhancement of mechanical panel properties.¹⁶⁵ External fibrillation can also contribute to a higher number of free hydroxyl groups, fractionated by the fiber surface area. This condition could increase the fiber reactivity to form better fiber bonding between fiber particles in the composite panel, which later could contribute to improving the mechanical properties of the composite. The influence of internal and external fibrillation towards improving fiber morphological structure on composite insulation panels is shown in Figure 4. From the figure, it was indicated that the fibrillated fiber seems to have better deformability and flexibility, which is able to create a multiple fiber network formation through a larger contact surface area between fiber particles in composite.

Figure 4.

The influence of fibrillation effect on morphological structure of natural waste fiber.⁶³

Fibrillation degree of treated fiber

The fibrillation degree is defined as the numerical value derived based on changes in the morphological structure of the fiber before and after refining, taking into consideration the effect of internal and external fibrillation on the fiber.¹⁶⁶ It has been observed that the fibrillation effect on fiber morphological structure has had a significant impact on the enhancement of the mechanical and physical properties of composite insulating panels.¹⁶⁷ At higher fibrillation degrees, the fiber particle size tends to decrease, causing a significant improvement in the fiber distribution and fiber packing systems in the composite insulation panel. Since small fiber particles may fill the spaces and form a bridge between the long fiber particles that are distributed throughout the composite structure. It indicated that small fiber particles could be used to strengthen the composite.¹⁶⁸ Furthermore, fibrillation occurred after refining not only reduced particle size but also collapsed the fiber structure, resulting in greater fiber flexibility. This condition has enhanced the fiber capacity to establish intimate contact, resulting in the generation of entanglement, a stronger fiber network, and fiber interlock, resulting composite insulating panel to obtain a strong fiber bonding. In addition, the decrease in particle size at a higher degree of fibrillation resulted in an increase in fiber surface area, which also promoted stronger fiber bonding in the composite structure. Throughout the composite manufacturing process, all of these aspects have contributed to the improvement of fiber structure compactness, as well as the improvement of composite panel insulating properties.¹⁶⁹

In contrast, when the particle size difference between fibers in a composite panel is higher, the compatibility of fiber bonding in composite structures is lowered. For mix type composite panel, low particle size difference between the two types of fiber particles resulted in a considerable improvement in interfacial bonding between fiber particles in the composite panel. When the composite panel was subjected to mechanical stress, the low particle size difference between the particles also contributed to the uniform stress and strain distribution throughout the fibers.⁶² The findings were consistent with a study by Belini et al.¹⁷⁰ where, the large difference in fiber particle size may result in poor compatibility between fibers in each layer, increasing the possibility of delamination and lowering sandwich composite panel performance. Therefore, raising the fibrillation degree on fiber through a refining process is regarded as a viable method for reducing particle size variation and enhancing fiber particle compatibility in composite panel.

Additionally, it has been reported that the improvement on fiber distribution in the composite insulation panel at a higher fibrillation degree results in an improvement in the water resistance through a reduction in the formation of large void structures. For information, a study by Ariawan et al.¹⁷¹ pointed out that the presence of huge void structures in the composite insulation panel was deemed undesirable since it allowed water molecules to readily infiltrate the panel. Therefore, a lesser number of void structures or a smaller size of void structure is required in composite insulation panels in order to limit this water penetration impact. Consequently, it is important to note that the elimination of void structures and the reduction in size of the void structures were not only influenced by the reduction in fiber particle size, but were also influenced by the higher fiber flexibility that occurred on fiber at a higher fibrillation degree.¹⁷² Technically, greater fiber flexibility will cause the composite to achieve greater compaction during the hot pressing process and eventually reducing the formation of void structure.¹⁷³

Fibers with a higher fibrillation effect or smaller particle size can enhance surface area, remove kinks, and contribute to the generation of fine fiber particles. These physical alterations to the fiber structure occurred as a result of the breakdown of the intra-fiber bond and the delamination of the fiber wall structure during the refining process, which was triggered by a frictional force.¹⁷⁴ These fiber physical changes contributed to the composite insulation panel obtaining better dimensional stability and a low spring back force effect due to a reduction in the accumulated pressure between fiber particles during the hot press process. Notably, strong dimensional stability capabilities and a low spring back force effect are necessary for the composite insulation panel to keep its form and thickness, hence reducing composite thickness swelling when exposed to wet conditions.

Future perspective

Future studies should be carried out to further investigate the performance of composite panels for insulation applications. There are several recommendations for improving the next phase of research on the utilization of natural fiber into composite insulation panels, such as i) investigation on the performance of the composite insulation panel with commercial insulation panel based on laboratory or full-scale conditions that include test chamber simulation of an insulated indoor space; ii) performance study on the composite insulation panel for different applications in buildings, such as wall, ceiling, and floor; and iii) detailed thermal performance study including heat and mass transfer based on numerical, simulation, and experimental approaches.

Conclusions

This review examines significant theoretical literature based on previous research. Several types of physicochemical characterization of composite insulation panel raw materials are addressed in this article. The fiber must possess a substantial chemical composition, an acceptable particle size, and a high thermal degradation temperature in order to be considered as a suitable raw material. In addition to discussing the characterization of raw materials, this article examines methods for improving the properties of composite insulation panels. It was discovered that employing fiber with a low density can result in a larger compression impact, increase fiber particle densification, and improve the properties of composite panels. Conversely, higher fiber length and great bulkiness may decrease fiber particle bonding in composites and cause excessive spring back force when composite insulation panels are in wet conditions. Furthermore, it was revealed that by employing a refining process able to improve the morphological structure of fiber, resulting in a substantial enhancement in the interfacial bonding between fiber particles in composite insulating panels. Nevertheless, despite the fact that enhanced fiber distribution and fiber packing might enhance the properties of composite insulation panels, they may also lead to poor panel thermal performance due to the elimination of void structure in the composite structure. At the conclusion of this study, the findings can be used to get a better understanding of fundamental and applied knowledge in material science and technology related to the fabrication of composite panels from natural waste fiber. It may serve as a starting point for further investigation into the use of natural waste fiber as a raw material in the fabrication of composite panels for insulation purposes. Also, the findings in this research are beneficial in the fields of insulation panel technology and engineering, notably in the building industry.

Footnotes

Author contribution

Conceptualization, MI., MR, YY; formal analysis, MAS, NKR; investigation, MAS, NKR; writing—original draft preparation, MAS, NKR; writing—review and editing, MIA, YY, MR, AMA, HAS, MAH; supervision, MIA, MR, YY, MAH; funding acquisition, MIA, MR.

Declaration of conflicting interests

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

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The first author would like to thank the Fundamental Research Grant Scheme (FRGS/1/2019/TK10/USM/02/8) from the Ministry of Education, Malaysia for the financial support of his graduate research assistant scheme associated to this research.

ORCID iD

Mohd Rafatullah

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