1. Introduction
The recent increase in greenhouse gas emissions has led to a rapid escalation of global warming. Surface temperatures on Earth have shown a swift rise since 1970, resulting in significant climate change. In response, the international community has adopted low-carbon policies to minimize greenhouse gas emissions. Enhanced thermal insulation in buildings is anticipated to contribute to a reduction in the use of fossil fuels, thus making it a key objective of low-carbon policies.
Furthermore, the global spread of COVID-19 has significantly shifted towards non-face-to-face lifestyles, resulting in a sharp increase in parcel delivery services [
1]. In 2020, the number of parcels delivered reached 3.37 billion, marking a 20.9% increase from 2.798 billion in 2019. Recycling paper, commonly disposed of after use, can save 17 trees per ton (907 kg). Research is underway to utilize cellulose, a component of paper, for producing construction materials, aiming to achieve both low-carbon emission policies and recycling objectives.
The main components of paper are cellulose, hemicellulose, lignin, and various additives. Cellulose constitutes about 40% to 60% of paper and serves as the primary structural element, providing strength and durability. Hemicellulose accounts for approximately 10% to 30% and consists of shorter fibers compared to cellulose. Lignin makes up about 15% to 30% and binds cellulose and hemicellulose together, enhancing their strength [
2].
Traditional insulation materials used in buildings primarily aim to prevent energy loss. Depending on the materials used, traditional insulation can be classified as shown in
Figure 1.
According to a study by Han-Hsi, L., et al., the toxicity indices of organic foaming materials and polyurethane, as well as polyethylene-based building materials, evaluated using the NES-713 assessment method, were found to be higher than 10, indicating a high risk of smoke toxicity [
3]. Despite their efficient energy storage capabilities, organic insulation materials are susceptible to fire hazards. Consequently, in the event of a building fire, the expansion of the fire and the emission of toxic gasses can result in significant casualties.
As awareness of the shortcomings of organic insulation materials grows, there has been an increasing use of inorganic insulation materials. However, inorganic insulation materials are characterized by a lower insulation efficiency compared to organic ones and pose challenges in installation. Hence, research on new technology materials with excellent insulation efficiency is in demand.
Aerogel is a lightweight and porous insulating material characterized by its nanoporous structure. Aerogels are composed primarily of silica, with oxygen arranged in a dual-structure, double-bonded form on both sides. Silica finds application in various fields such as nanocomposites, construction, and chemical industries. Historically, the high production cost of aerogel has hindered its widespread application as a conventional material. However, recent advancements in mass production technology have lowered production costs, leading to its utilization in both research and consumer goods.
In 1931, Steven Kistler introduced aerogel, which is lightweight and porous. Silica, a basic component of soil and rocks, including granite and basalt, is the most abundant mineral on Earth and serves as the fundamental material for aerogels [
4]. Silica finds application in various fields such as nanocomposites, construction, and chemical mechanical polishing [
5]. Research on silica-based aerogel manufacturing methods for commercialization is ongoing [
6]. Silica aerogel exhibits outstanding insulation properties due to its ability to transmit only about 1/100th of the heat compared to ordinary glass [
7]. With pore sizes ranging from 5 nm to 70 nm, silica aerogel contains a variety of cells filled with air, which block heat transfer and enhance insulation performance [
8]. The thermal conductivity of silica aerogel, a porous material, is approximately 14 mW/m·K [
9].
Initially, aerogel was mainly used in advanced chemical products for aerospace and chemical industries, with limited application in construction materials due to high production costs [
10]. However, in the early 2000s, aerogel materials began to be used in buildings, primarily as aerogel blankets [
11]. Aerogel-based construction materials, such as aerogel blankets (thermal conductivity around 15 mW/m·K) and aerogel boards (thermal conductivity around 16~19 mW/m·K), are utilized in buildings [
12]. In addition to insulation performance, the sustainability of construction materials is crucial for increasing the lifespans of buildings [
13].
Sambucci et al.’s study identifies the emerging use of fiber-reinforced aerogel blankets (FRABs) as an alternative insulation material for cryogenic tanks used in liquefied natural gas (LNG) transportation. They found that compared to the traditionally used back-filled perlite-based system for transporting liquefied natural gas (LNG), it allows for a thinner outer shell, stores more material, and reduces the weight of LNG transportation semitrailers [
14].
Liu et al. studied the thermal conductivity of specimens as the mass fraction of expanded graphite increased from 2 wt% to 10 wt%. The measured results indicated a linear increase in thermal conductivity. Specifically, the measured value increased from 0.2055 to 0.5218 W/m·K. The thermal conductivity of the mixture with 10 wt% expanded graphite added was 3.28 times higher than before the addition of expanded graphite. The increase in thermal conductivity was found to correspond to a decrease in thermal insulation performance [
15].
This study produced specimens by pulping paper to extract its main component, cellulose, and adjusting the content of porous aerogel. Specimens mixed with paper and porous aerogel were classified as the pure test group. The composite test group was also created by combining porous aerogel with 30 wt% ceramic binder and 40 wt% expanded graphite to delay fire spread [
16]. The thermal insulation performance of the fabricated specimens was evaluated using the ISO 22007-2 method, through which their thermal conductivity and thermal diffusivity were measured.
4. Discussion
Thermal conductivity is an intrinsic property of a material that indicates its ability to transfer heat. It is typically measured in W/m⋅K and denoted by symbols such as
k, λ, or κ. For instance, at 1 atmosphere and 293 K (20 °C), the thermal conductivity of air is approximately 0.025 W/m⋅K, while that of water is around 0.5918 W/m⋅K. Formula (2) is commonly used as the basic formula for thermal conductivity, with the most significant factor being the heat transfer quantity, denoted as Q. As this value increases, indicating better heat transfer, the thermal conductivity of the material increases accordingly. A higher
K value signifies greater heat loss, as materials with higher thermal conductivity allow more heat to pass through.
where
K = thermal conductivity (W/(m·K)),
Q = heat transfer (W),
A = heat transfer area (m
2), and Δ
T = temperature difference (K or °C)
According to a study by Cha et al., the thermal conductivities of conventional building materials measured using the Heat Flow Meter 436 (HFM 436) were found to be 0.1254 W/m·K for reinforced flooring, 0.2021 W/m·K for fire-resistant gypsum board, and 0.0415 W/m·K for polystyrene [
24]. Through our research, the composite specimens based on recycled paper and aerogel showed the best thermal conductivity of 0.1616 W/m·K when the aerogel content was 1000 mL. This value is comparable to those suitable for flooring materials such as laminate flooring and gypsum board.
The
R-value (thermal resistance) is a commonly used metric for evaluating the performance of insulation materials, expressed in units of m
2·K/W. Formula (3) expresses the method for calculating the
R-value.
Table 6 presents the
R-value values of the experimental specimens.
where
R = thermal resistance (m
2·K/W),
L = thickness (M), and
k = thermal conductivity (W/(m·K)).
According to a study by Acharya et al., the
R-value of architectural materials composed of aerogel was reported to be 0.26 m
2·K/W.
Figure 9 shows the
R-value values of conventional architectural insulation materials and the mixed insulation materials used in this study [
25]. For the pure experimental group specimens with recycled paper and 1000 mL aerogel added, the
R-value was confirmed to be 0.127 m
2·K/W. Furthermore, in the case of the composite experimental group specimens under the mixing conditions of ceramic binder and expanded graphite, the specimen with 200 mL aerogel showed the highest
R-value of 0.137 m
2·K/W.
5. Conclusions
Firstly, adding aerogel, a porous material, to recycled paper to enhance thermal insulation performance resulted in the highest value of 0.1616 W/m·K at the highest concentration of 1000 mL. This value indicates a 16.66% improvement in thermal insulation performance compared to the control group. The architectural material incorporating 1000 mL aerogel into recycled paper showed values suitable for use as flooring materials and gypsum boards in conventional architectural materials.
Secondly, the thermal conductivity value of the composite control group was confirmed to be 0.2157 W/m·K. Additionally, among the composite control group specimens, the specimen with 1000 mL of added aerogel showed a thermal conductivity value of 0.1791 W/m·K. The reduced value demonstrates a 17.06% improvement in thermal insulation performance compared to the control group.
Lastly, in terms of R-value, the specimen mixed with 1000 mL aerogel in the composite experimental group showed an R-value of 0.137 m2⋅K/W. The R-value values of EC-1 and A-1000 were lower than those of Styrene Foam.
In this study, consistent with prior aerogel research, no significant increase in thermal conductivity was observed. This can be attributed to the chosen specimen preparation method involving compression, resulting in high specimen density. It is speculated that this high density impedes the formation of air gaps crucial for thermal insulation. Nevertheless, this study quantitatively confirms the potential for further development and commercialization of cellulose-based porous aerogel composite building materials through additional research.