1. Introduction
Natural fiber-reinforced polymer composites are attracting the attention of researchers due to their low environmental burden imposed by their manufacture and disposal. Most of the natural fibers used in new material development are residues (waste) from agro-industrial processes [
1,
2,
3]. Natural fiber reinforced composites have lower mechanical performance to glass fiber reinforced plastics (GFRP), but exhibit low thermal conductivity, biodegradation in natural environments, and high acoustic absorption [
2,
4].
An insulating material is defined by its high thermal resistance, which means that it limits heat exchange between two materials or environments with a temperature differential [
5]. Material thermal conductivity should not exceed 0.08 W/mK for insulation applications [
6]. Some other relevant parameters include resistance to water vapor diffusion, water absorption, moisture content [
7], low thermal transmittance (U in W/ m²K) [
5], and adequate resistance to bending, tension [
8], compression, fungal growth, and flame retardancy [
9,
10,
11]; this last is referred to material’s behavior under fire conditions or fire-proofing property, while the mold growth test verify the effectiveness of adding borax or other element, in the decrease of fungal growth [
12,
13,
14,
15,
16,
17].
Thermal insulation is essential for the transport and storage of temperature-sensitive products: for the prevention of overheating in certain areas of electronic devices and for indoor climate control in buildings [
18,
19,
20]. Air conditioning of buildings accounts for more than 10% of global energy consumption [
21]. Efficient thermal insulation based on renewable materials is an essential part of the technological change needed to mitigate climate change [
7,
22,
23,
24,
25,
26,
27]. Renewable and bio-based materials, such as wood chips and recycled paper, were widely used for thermal insulation before the introduction of fossil fuel-based foams, but their insulating performance is relatively low [
28,
29]. However, looking for alternatives to reduce the environmental impact generated by the production of commercial insulators [
30,
31], the use of vegetable and animal origin thermal insulators have been implemented again [
32].
Materials such as cellulose-containing newsprint have demonstrated good thermal properties [
33,
34,
35]. Cellulose is a material that stood out for being used as a thermal insulator since the beginning of the 20th century [
29], in North American and European countries. Rice husk, coconut tow and newspaper have also been presented as natural fibers with potential to be used in thermal insulation applications, due to their thermal conductivity properties [
36,
37,
38]. In the study developed by Benallel et al., 2021, four different types of agricultural by-products available in the Drâa-Tafilalet region, Morocco (reed, alfa, fig branches and olive leaves) with different mass fractions of cardboard waste (20%, 30%, 40%, 50% and 60%) were analyzed separately. The lowest thermal conductivity value obtained was 0.072 W/mK and the highest value was 0.98 W/Mk [
39].
A review on cellulose fiber insulation, covering its manufacturing, installation, and performance, revealed that while the typical thermal conductivity value for cellulose averages 0.040 W/mK. Its properties and performance may vary slightly based on the fabrication and installation methods [
12]. The difference in where the cellulose is sourced from can affect the thermal insulation efficiency [
40,
41].
The water absorption test evaluated the material's ability to resist water absorption [
42]. High water absorption can decrease efficiency and promote mold growth. Studies have highlighted the relevance of these tests in the characterization of insulators based on natural materials [
43]. In the water absorption test performed by Penjumras et al., 2015, the results indicate that all composites showed a percentage of water absorption greater than 40% after 24 hours of water immersion. It was highlighted that water absorption of composites is a critical aspect, as it can influence the durability and stability of materials, especially in construction applications [
44]. Water absorption depends on several factors, such as the chemical composition of the fibers, the size and surface area of the fibers, the density of the composites, and the intra and inter porosity characteristics [
16,
45]. Although all composites were observed to have water absorption above 40%, it was mentioned that rice husk composites showed the lowest water absorption (42%) after 24 hours of immersion, compared to wood fiber (63%) and textile fiber (60%) composites. This low water absorption percentage is attributed to the high level of porosity of rice husk compared to wood and textile fibers [
46].
The density test can directly influence the material’s thermal insulating capacity and its mechanical properties [
47]. In density test performed by Muthuraj et al., 2019 [
46], it was noted that all composites produced showed low densities, within the range of 378 to 488 kg/m
3. These densities are comparable to those of wheat husk-based and fully biodegradable sugarcane bagasse-based particleboard composites, according to previous studies [
48,
49,
50]. The density is directly related to the porosity of the material and thus to its thermal conductivity [
51]. The relationship between density and thermal conductivity in porous materials is a combination of conduction through the solid part and convection through the gaseous part (air in this case). The porous structure of the material contributes to its low thermal conductivity, which is desirable for an insulating material [
52].
Composite materials based on natural fibers require the use of additives that slow down both, the material decomposition, and the speed at which the composite material burns [
53]. One of the most used additives is borax [
54,
55]. Borax is a derivative of the element boron and the proportions to be used in the composite material vary according to the manufacturer, matrix and fibers type used [
56]. The recommended proportion of boric salts (borax) and boric acid is between 15% to 20% composite material mass. The recommended ratio of borax to boric acid is 1/8, for a dosage of 16% to prevent combustion [
12]. Boron included in cellulose was found to have a sporocidal effect on five of the most common types of fungal spores, even when subjected to a high concentration of fungi. For untreated fibers exposed to fungal samples, moisture content and relative humidity were found to have an influence on the rate of mold growth on cellulose insulation [
57].
In a fire resistance study performed on hemp composites, following EN ISO 11925-2, the samples were exposed to a direct flame source inside a test chamber. The ignition time, the presence of burning droplets and whether the flames reached a specific mark within a predetermined time were recorded. It was observed that hemp composites showed higher fire resistance, while flaxseed composites exhibited better mechanical performance. Taken together, these results support the idea that bio-composites could be effectively used in the manufacture of end products for the construction sector, offering improved fire-proofing property compared to conventional materials [
58,
59]. Research could be oriented in materials that do not spread a fire or that they could extinguish themselves.
The building sector contributes significantly to energy consumption and greenhouse gas emissions [
60]. Conventional construction insulation materials are mostly manufactured from petrochemical sources [
61]. Their production and use contribute to a higher carbon footprint, pollution air, land, and water [
15,
62]. Natural fibers and recycled materials are increasingly attractive in construction sector, due to their sustainability and low environmental impact. Cellulose, obtained from recycled newspapers and rice husks, considered an agricultural by-product, emerge as promising elements for sustainable building materials [
63]. In a previous work [
20] it was found that the composite consisting of newspaper (14%), rice husk (9%), borax (15%) and polyvinyl acetate-based glue (62%), obtained a thermal conductivity of 0.042 W/mK, demonstrating promised insulation characteristics. Also, tensile, and compressive stress tests were performed, obtaining as results 1.74 and 20.5 MPa, respectively. The developed material exhibits competitive thermal conductivity and mechanical properties compared to various natural and recycled insulation materials, which could constitute a viable option for applications demanding efficient thermal insulation capacity [
24,
64], especially in hot-humid regions. In this context, this study focuses on determining, in laboratory-scale, water absorption percentage, density, fungal growth, and fire resistance of developed composite.
This study addresses the environmental and waste management challenges in Panama, where it is estimated that significant amounts of newsprint (15 000 tons) and rice husks (87 060 hectares) are disposed of in municipal landfills annually, causing waste accumulation and environmental deterioration. This reality highlights the need to develop innovative and sustainable solutions that take advantage of local resources and reduce dependence on conventional materials with a high environmental impact [
3,
4,
41].
3. Results and Discussion
3.1. Water Absorption
Table 2, presented below, compiles the dry mass and wet mass data of three replicates (S1, S2, and S3) of the composition detailed in the methodology (rice husk 9%, newspaper 14%, 15% borax and 62% of polyvinyl acetate). From these data, the percentage of water absorption is calculated. Additionally, an average value is provided, reflecting the average absorption of the tested samples.
Sample S2 shows the highest percentage of water absorption (73.77%), followed by sample S1 (65.63%) and sample S3 (64.79%). This variability in the results may be due to several reasons, such as possible fluctuations in the environmental conditions during the test or variations in the internal structure of the samples.
The difference in the composition of the samples, because of the manufacturing process (small variations in the proportion of constituent materials), may have contributed to the observed disparities in water absorption levels. For example, the presence of different amounts of polyvinyl acetate, borax, or the distribution of cellulose and rice husk fibers could influence the ability of the samples to absorb water.
Comparing the water absorption capacity of the manufactured material (9% rice husk, 14% newsprint, 15% borax and 62% polyvinyl acetate), with the absorption capacity of other materials reported in the literature, such as cardboard, newsprint, egg trays, Ecovio (composites reinforced with rice and wheat husks), as well as textile and wood fibers. Ecovio emerges as a leading polymeric material in the search for sustainable alternatives to conventional plastics [
46]. Ecovio's manufacturing process is based on renewable raw materials, such as corn starch and polylactic acid (PLA), making it an environmentally friendly option. The results reported in the literaure for Ecovio composites reinforced with rice husks exhibited the lowest water absorption, with an average value of 43%. On the other hand, materials reinforced with wood fibers showed the highest water absorption, with an average of 65%. These variations in water absorption can be attributed to differences in the structure and chemical composition of the raw materials used [
46].
Paper-based materials, such as cardboard, wastepaper, and newsprint, exhibited extremely high levels of water absorption, with values more than 300%. This finding underlines the importance of considering moisture resistance when selecting materials for specific applications [
74].
Additionally, the fabricated composite for the present study, consisting of shredded newspaper, ground rice husk, polyvinyl acetate (Grip Bond 2), and borax (20 Mule Team Borax), showed a water absorption capacity of 68.06%. This indicates an intermediate performance in terms of moisture resistance compared to other materials presented in the reviewed literature.
Comparing the percentage of water absorption in natural and recycled insulating materials mentioned in the reviewed literature [
46,
74] with the developed composite, it was found to have an average water absorption (68.06%). This value is higher than other materials specifically designed to prevent water absorption, such as wheat husk/Ecovio and rice husk/Ecovio, with 43% [
46]. The developed composite is prone to absorb a significant amount of water when exposed to humid conditions. However, the incorporation of rice husk and borax may provide benefits such as improved moisture resistance compared to other cellulite insulators, which may be advantageous in specific climates [
75]. The water absorption exhibited by the proposed composite may indicate that it is susceptible to damage and deformation when exposed to wet conditions. Although despite the aforementioned, the studied composite compared to materials such as used paper and egg tray, it absorbs 4 times less water [
74].
3.2. Density Test
Table 3 shows the results obtained from three replicates (S1, S2, and S3) of the composition mentioned in the methodology (rice husk 9%, newspaper 14%, borax 15%, and polyvinyl acetate 62%).
Sample S2 shows the highest density (0.82 g/cm³), followed by sample S3 (0.79 g/cm³) and sample S1 (0.78 g/cm³). The differences in density between the samples could be related to the distribution and concentration of the constituent materials, as well as to the presence of possible porosities or irregularities in the structure of the composite.
Density is a fundamental property in materials characterization, especially in engineering applications where the ratio of mass to volume is crucial to the performance and functionality of the final product. In this discussion, we compare the density of a variety of polymeric and natural materials.
The results show a wide range of densities among the materials evaluated. Polymeric materials, such as low-density polyurethane (40 kg/m³), low density polyethylene (920 kg/m³) and high-density polyethylene (980 kg/m³), exhibited relatively low densities compared to natural and composite materials. Among the natural materials, cork showed a density of 120 kg/m³, while filter wool exhibited a density of 200 kg/m³. These density values are lower compared to polymeric materials, suggesting a higher porosity and less dense structure in these natural materials. On the other hand, wood showed a significantly higher density of 840 kg/m³, making it a denser and heavier material compared to the polymeric and other natural materials evaluated in this study. Natural rubber also showed a relatively high density of 910 kg/m³, making it suitable for applications where strength and durability are required [
76].
As for the fabricated material, which consists of a combination of shredded newsprint, ground rice husk, polyvinyl acetate, and borax, it exhibited a density of 657 kg/m³. This intermediate density indicates a compact structure and a balanced composition among the materials used in its manufacture. The proposed material has a lower density than silicone rubber, natural rubber, PEHD and PELD. This may be advantageous in some applications where a lighter material is required. On the other hand, the density of the proposed material is higher than cork, fiberglass, wool filter, and foamed polyurethane. However, it is still lighter than wood materials [
11,
76].
3.3. Flammability Test
Figure 3 shows the photographs of the six specimens (3 specimens without borax and 3 specimens with borax) after performing the flammability test.
The addition of borax to the composite proved to be effective in significantly improving its fire resistance compared to samples lacking borax. During the tests, it could be observed that the samples with borax presented less fire propagation, and superficial damage of the specimen. On the other hand, in the specimens without borax, the fire spread more rapidly over the entire surface and affected the specimen internally, showing a lower fire resistance capacity. These results indicate that borax in the material created is a fireproofing agent, which is why it plays a fundamental role if it is to be placed or used in a place prone to catch fire, such as buildings, vehicles, electronic devices, among others.
3.4. Fungi Growth Test
Table 4 shows the results of the Simplate technique performed on three samples with borax and three samples without borax.
Table 4 shows the fungal and yeast counts for samples of material with borax and without borax, in different test tubes and dilutions. In the samples of mate-rial with borax, it is observed that there was no microbial growth in the three dilutions performed (D, 10
-1, 10
-2), indicated by "<1" or "<1" or "<1" Colony Forming Units (CFU) per mL. This suggests that the presence of borax in the material may have an inhibitory effect on the growth of fungi and yeasts.
In contrast, in samples of material without borax, a significant growth of fungi and yeasts was observed at all dilutions. The CFU counts per mL are higher, indicating a higher concentration of microorganisms compared to the samples of material with borax.
It is important to note that the detection limit of microorganisms for this technique is higher than the technique used in this study. This means that it is possible that the CFU counts are underestimated and that the actual presence of fungi and yeasts is higher than reported (only for the case of samples without borax).
Overall, these results suggest that the presence of borax in the composite material may have an inhibitory effect on the growth of fungi and yeasts. However, it is necessary to consider that these results were obtained using a specific technique and that there could be differences in the sensitivity and specificity of other microbiological counting techniques.
These findings are consistent with previous research that has pointed out the antimicrobial potential of borax [
12,
57,
77]. However, additional studies are required to better understand the exact mechanisms of inhibition and to evaluate the safety and efficacy of the composite material in relation to microbial growth.
The method used, the Simplate Technique for enumeration of fungi and yeasts according to the official AOAC 2002.11 method, is a total enumeration technique that does not focus on identifying specific species of fungi and yeasts. Instead, it is used to determine the presence and total amount of these microorganisms in the analyzed sample. Since our objective was to evaluate composite behavior with and without borax to the general presence of fungi and yeasts, to obtain a total count, rather than to identify individual species.
4. Conclusions
The water absorption exhibited by the studied composite may indicate that it is susceptible to damage and deformation when exposed to wet conditions. Although this material is prone to absorb a significant amount of water when exposed to humid conditions, it absorbs 4 times less water compared to materials such as used paper and egg tray. The incorporation of rice husk and borax may provide benefits such as improved moisture resistance compared to other cellulite insulators.
The average density obtained was 0.66± 0.06 g/mL, which indicates an intermediate density compared to other polymeric and organic materials. This characteristic can be advantageous in applications where a balance between strength and lightness is required. This value is higher than other materials specifically designed to prevent water absorption.
The addition of borax provides an effective barrier against fire and holds promising behavior in terms of fire resistance. These findings support the potential application of the composite as thermal insulation of walls, for its fireproofing property. The material with borax exhibits inhibition of fungal and yeast growth, while the material without borax shows significant growth of these microorganisms. These results provide relevant information for the microbiological characterization of the composite material and may be useful in future research and industrial applications.
The developed material has an intermediate density and water absorption compared to other polymeric materials used as insulators. This characteristic presents advantages in applications that demand a balance between strength and lightness. This research aims to promote the use of materials considered as agro-industrial waste in our country, demonstrating their potential and applicability in different areas, across various engineering domains, including the production of lightweight components, construction panels, sustainable packaging, and others.
It is important to address the impact and contribution of this kind of studies, as an innovative and sustainable solutions that take advantage of local resources and reduce dependence on conventional materials with a high environmental impact.