Morphological and acoustical characterization of UV-irradiated foam composites from cooking oil and wood flake
Authors:
Anika Zafiah M. Rus, Hanani Abd Wahab, Yazid Saif, Noraini Marsi, M. Taufiq Zaliran, M. Hafizh Alamshah, Ita Mariza, Shaiqah M. Rus, Sami Al-Alimi, Wenbin Zhou
The article presents a detailed study on the morphological and acoustical characterization of foam composites derived from cooking oil and wood flakes, subjected to UV irradiation. It compares the performance of bio-epoxy (BE) and synthetic epoxy (SE) foams, focusing on their mechanical properties, bonding, and acoustical performance under varying UV exposure durations. The research underscores the potential of BE foams as sustainable alternatives for sound absorption applications, highlighting their superior performance in low-frequency sound absorption and better resistance to UV degradation compared to SE foams. The study also emphasizes the need for further research to enhance the UV resistance of bio-based polyurethane foams, ensuring long-term durability and sustainability.
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Abstract
Polymer foam composites for sound absorption with eco-friendly attributes have gained significant attention in sustainable materials research. This study investigates the impact of ultraviolet (UV) irradiation on the morphological, mechanical, and acoustical properties of bio-epoxy (BE) and synthetic epoxy (SE) foam composites, incorporating wood flakes as fillers at varying loadings (0–20 wt%). BE, derived from waste cooking oil, demonstrated superior resilience to UV exposure compared to SE, maintaining better pore structure, mechanical stability, and sound absorption performance. The results show that after 6000 h of UV exposure, BE composites retained 12–18% higher sound absorption coefficient (α = 0.62–0.78) than SE composites (α = 0.50–0.66) at 3000 Hz after 6000 h of UV exposure, demonstrating superior UV resilience. At 6000 Hz, SE outperformed BE (α = 0.45 vs. 0.35) as a result of structural degradation in BE at higher frequencies, attributed to the natural stabilizing properties of bio-based additives. This study proves that BE foam composites offer improved durability and acoustic performance under prolonged UV exposure, positioning them as promising materials for sustainable acoustics applications.
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Introduction
The rising demand for sustainable, high-performance materials has driven significant interest in sound absorption applications, where polymer foam composites play a crucial role. However, their durability under ultraviolet (UV) radiation exposure remains a key challenge, especially in the face of growing environmental concerns. Developing excellent sound-absorbing materials is critical for creating comfortable environments in situations such as offices, schools, healthcare facilities, and households [1]. Polymer foam composites have emerged as a popular choice due to their beneficial characteristics, low cost, and simple manufacturing procedure [2]. Initiatives aimed at improving noise control through enhanced sound absorption technologies offer intriguing opportunities for integrating sound-dampening techniques into a variety of porous materials. Many research efforts are focused on enhancing the acoustic capabilities of these materials in order to improve human quality of life [3‐5]. Synthetic polyurethane (PU) foams, which are commonly utilized in human-centric engineering, particularly for sound attenuation, have a long history of practical use. While prior work explores bio-foams derived from different vegetable oils, including castor oil [6], palm oil [7, 8], sunflower oil [9, 10], canola oil [11], and soybean oil [12], few addressed UV durability. Our study bridges this gap by comparing waste-cooking-oil-derived BE with SE, emphasizing UV-induced acoustic performance degradation as a critical factor for outdoor applications. The major goal is to develop environmentally friendly bio-based PU foams [13‐15], polymers derived from vegetable oils are also considered a feasible alternative [16]. Petroleum-based PU foam characteristics can be customized by carefully selecting ingredients and manufacturing procedures to match specific application needs. The increased environmental consciousness about the usage of petroleum-derived products has encouraged substantial attempts to generate materials derived from sustainable sources, such as vegetable oils [17]. Recently, there has been an increasing emphasis on employing eco-friendly polymer foam, prompting research into developing renewable plastics and chemicals to find environmentally favorable alternatives. A considerable shift towards bio-based foam products is an important step towards increased sustainability.
In particular, natural cooking oil has become an important player among renewable sources, showing promise as a newly created polyol source for making polyurethane (PU). Exploring alternative polyols for PU production has become a topic of significant interest [18, 19]. Developing polymers sourced from renewable origins poses a significant challenge for scientists. The increasing demand for sustainable and renewable alternatives stems from economic and environmental considerations, given the reliance on petroleum-based resources. To enhance the sound absorption capacities of composite materials, various additives are introduced. The utilization of natural fibers in producing PU foams has garnered considerable interest.
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Natural fibers inherently possess biodegradability, and advancements in technology have made their processing more economically viable and ecologically sustainable. An overview addressed and examined the use of natural fibres in fibre-reinforced polymer matrix composites, analyzing their physical, chemical, thermal, and mechanical properties using XRD, FTIR, and SEM analysis [20]. Furthermore, the study addressed comprehensive studies for the use of natural materials in polymer composites, emphasizing how they affect mechanical, chemical, electrical, thermal, and physical properties. It emphasizes the potential for combining polymers and fillers in a variety of engineering and industrial applications, such as microelectronics, biomedicine, flexible electronics, flame retardant applications, food packaging, automotive, aerospace, and construction [21]. A wide range of natural fibers, such as wood dust fiber, coir fiber, sisal fiber, banana fiber, and pineapple fiber, is available for this purpose [22]. Wood fillers exhibit promising qualities as additives due to their sustainable and natural characteristics, complementing polymer materials [23]. As a structural element in plants and the most abundant biomass globally, wood fiber proves effective as a composite filler. Using wood dust fiber as filler offers numerous advantages, including minimal processing damage, cost-effectiveness, renewable sourcing, and lower density compared to mineral fibers [24]. Moreover, the disposal of wood fibers has minimal ecological impact. Given the escalating global attention on environmental issues, sustainability, and climate change, the utilization of wood fiber waste emerges as an optimal filler for diverse materials.
Previous studies suggest that BE shows more durability under UV exposure compared to traditional petroleum-based composites [25, 26]. This durability arises from the inherent UV resistance provided by the natural properties of the bio-based elements incorporated into the foam composites, which positively impact the overall properties and acoustic performance [27]. Jeyaguru et al. [28] have addressed the idea of combining natural fibre (hemp) with synthetic fibre (kevlar) to develop a hybrid composite. The hemp and Kevlar yarns were handloomed into fabrics with different architectures. In contrast, the composites were tested for mechanical properties, free vibration, and acoustic emission characteristics. The twill and basket weave hybrid composites showed better mechanical properties than plain weave hybrids, and the basket weave hybrid had the highest sound transmission loss value. Moreover, this study investigated whether weaving patterns affected the tensile, acoustic, and vibration properties of intra-ply Kevlar and pineapple leaf fiber (PALF) hybrid woven fabric reinforced epoxy matrix composites. Basket-type composites outperformed pure Kevlar and hybrid composites in terms of tensile strength, sound transmission loss, damping, and natural frequency [29].
This study explores bio-epoxy composites derived from waste cooking oil, reinforced with wood flakes, as a greener and more sustainable alternative to synthetic epoxies. Particular attention is given to their mechanical properties, structural morphology, and acoustic performance under prolonged UV exposure. Consequently, this study extensively examined and compared the influence of UV irradiation on acoustic behaviours. The aim of this work was to investigate the effect of UV irradiation exposure (2000, 4000, and 6000 h) on the morphological, mechanical, bonding, and acoustical performances of polymer foam composites. Polymer foam composites were produced by utilizing two distinct epoxies, namely bio-epoxy (BE) and synthetic epoxy (SE), incorporating varying loadings of wood filler flakes (0, 5, 10, 15, 20 wt%). BE (derived from waste cooking oil) and SE (from petroleum-based oil) were synthesized for this study.
Experimental setup
Materials
Bio-epoxy (BE) foam composites were developed by employing used cooking oil collected from small and medium-sized enterprises (SMEs). Flexible polyol (Aura 2104 H3) and flexible MDI (diphenylmethane 4,4-diisocyanate) were provided by Syarikat Saintifik Bersatu (M) SDN BHD in Johor, Malaysia. These materials were employed as crosslinkers to make synthetic epoxy (SE) foam composites. Moreover, the Meranti flakes wood fiber used as a filler were provided by Tukang Kayu A. Hamid SDN BHD, a furniture business in Johor, Malaysia.
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Synthesis of renewable BE
A renewable BE was made from waste cooking oil. It is made up of an acid-catalyst ring that opens up epoxy to make polyols. The waste oil conversion process into renewable monomer preparation started with the catalyst preparation in a glass reactor, and distilled water was added dropwise. Hence, the solution underwent heating at a temperature of 50 °C while being stirred continuously for a duration of 45 min until achieving a visibly apparent state. The obtained clear solution was cooled to ambient temperature, and concentrated acid (H2SO4) was poured into the solution. Subsequently, the introduced catalyst was combined with water, and the resulting mixture underwent a heating process at 70 °C. Stirring was maintained for a period of 6 h to facilitate the desired reaction.
Fabrication of polyurethane (PU) foam composite
The fabrication of PU foam was formulated using an open moulding technique that consisted of two different epoxies, which are bio-epoxy (BE) and synthetic epoxy (SE). Figure 1 shows the illustration of the fabricated foam composites, in which the mixing occurs mechanically between the monomer and crosslinker at a ratio of 1:0.5, respectively, at room temperature for 30 s before being added with different weights (g) of waste wood flakes filler (5%, 10%, 15%, and 20 wt%). Bubbles were formed as a result of the physical blending of air with the polymer foam mixture. Afterwards, the blend was swiftly poured into an open cylindrical mold and left to undergo curing for a duration of 6 h at ambient temperature. As the temperature increased during the forming process, bubbles emerged due to gas diffusion and the expansion of the gas phase. This process, characterized by heat generation, is known as an exothermic reaction. Subsequently, when one or more bubbles ruptured, a closed cell opened, referred to as the cell opening of the pore. Table 1 shows the list of samples obtained for both monomer-type BE and SE foam composites with different loadings of flakes of wood filler.
Fig. 1
Preparation of SE and BE foam composites
Table 1
List of the samples
Monomer
Filler loading (wt%)
Name of the sample
BE
0
BE0
BE
5
BE5
BE
10
BE10
BE
15
BE15
BE
20
BE20
SE
0
SE0
SE
5
SE5
SE
10
SE10
SE
15
SE15
SE
20
SE20
Characterization of SE and BE foam composites
Gelling time
The foaming time was recorded from the start to the end of the stirring, during which the bubbles were formed. At the same time, the gelling time was also recorded when the foam composites were expended until they no longer expanded. The BE foam formation was evaluated and compared to SE foam. Consequently, by maintaining uniform porosity within both the polyols and filler, a limitation was imposed on the loading of wood flake filler, restricting it to 20 wt% within the matrix. This decision was influenced by the extended reaction time between the monomer and crosslinker.
Moisture content
Water (H2O) was mixed with the monomer to produce a natural polyol containing a hydroxyl group (-OH), which was essential for reacting with isocyanate [28]. The moisture test for SE and BE foam composites was performed using the Moisture Analyzer (Adam Equipment PMB53). To ascertain the moisture content of the samples, the Loss on Drying (LOD) method was employed, a widely accepted test in these evaluations [29]. This involved utilizing the Loss on Drying (LOD) moisture meter technique, as illustrated in Eq. 1. The procedure involved weighing the sample, subjecting it to drying, and then reweighing. Determination of moisture content relied on comparing the difference in weight before and after drying to either the initial mass or the mass subsequent to drying.
An ultraviolet (UV) weatherometer test was performed to simulate the weathering conditions of the given material and analyze its properties using the QUV accelerated weathering tester (ASTM G154). It can simulate the sun, rain spray, and condensation, resulting in damage to the materials through exposure to sunlight with moisture and temperature. The temperature was set to 50 °C to simulate harsh environmental weather conditions. An array of UV fluorescent lamps emitting light in the range of 280 to 320 nm with a tail extending to 400 nm was used for UV irradiation exposure. UV exposure durations (2000–6000 h) were selected to simulate outdoor service in tropical climates based on ASTM G154 accelerated weathering standards. Filler loadings (0–20 wt%) were optimized to balance viscosity (foaming stability) and acoustic performance, as higher loadings (> 20%) caused phase separation in preliminary trials. The UV irradiation exposure hours are shown in Table 2.
Table 2
QUV accelerated weathering tester time exposure
QUV time
(minute)
QUV time
(hour)
Actual time
(hour)
250
4.17
2000
500
8.33
4000
750
12.50
6000
Scanning electron microscopy (SEM) via morphological analysis
The morphological characteristics of the PU foam composite were evaluated before and after exposure to UV radiation. Scanning Electron Microscopy (SEM) was used to study the internal structure of the added filler within the produced foams. This study aimed to investigate the influence of varying amounts of fiber fillers on the physical and mechanical attributes of PU foam composites. The focus was specifically on evaluating the internal pore structure, interconnected pore distribution, and strut diameter of the foam. A fine gold layer was applied to the foam surfaces using the Auto Fine Coater Machine, serving as insulation for electrically conductive foam samples to enable high-resolution electron imaging. The desired outcome of comparing pore structure images was to find changes in sample chemistry before and after UV treatment. The study was conducted using a Perkin-Elmer (UK) Spectrum One FTIR spectrophotometer to evaluate absorbance within a specific range. The carbonyl index (CI) has been calculated by determining the ratio of peak areas corresponding to (C = O) carbonyl group vibrations as well as the area associated with the (-CH2-) stretching vibration. Polymer degradation was assessed using SEM pictures taken from a Hitachi microscope under particular conditions. Furthermore, a greater carbonyl index suggested a deeper breakdown of the polymer.
Interconnected pore size
The interconnected pore size refers to the diameters of the paths or channels that connect nearby pores in the foam material. These linked pores contribute significantly to the foam’s permeability, allowing fluids or gases to move through the substance. This characteristic is significant in sound absorption materials, where linked pores aid in the dissipation of acoustic energy, reducing noise and contributing to the overall performance of the materials.
The effect of interconnected pore size
The acoustic, filtration, and thermal insulation qualities of the material are directly impacted by the connected pore size. The potential of the foam to absorb and release acoustic energy increases by larger interconnected pores, which enhance the effective surface area for interactions with sound waves. The pore size of the foam determines how well it enables fluid or heat passage while maintaining structural integrity in filtering and insulation applications.
Interconnected pore size measurement
SEM was used to assess the interconnected pore size, which was supplemented with Mercury Intrusion Porosimetry (MIP). SEM produces high-resolution pictures, enabling for qualitative analysis of pore connectivity and size. MIP assesses pore size distribution and connectivity by measuring the pressure required to push mercury into the foam’s porous network. The combination of these methodologies results in a thorough understanding of the foam’s microstructure and its impact on material properties.
Fourier transform infrared spectroscopy (FTIR) via bonding analysis
Changes in the sample’s chemical composition were examined using Fourier transform infrared (FTIR) analysis simultaneously before and after UV irradiation using the Perkin-Elmer (UK) Spectrum One FTIR spectrophotometer. The carbonyl index (CI) was computed by dividing the stretching vibration of the (-CH2-) group, which is represented by the area between 2950 and 2850 cm−1, by the (C = O) carbonyl group vibrations, which are reflected by the peak areas at 1680 and 1800 cm−1. An increase in the carbonyl index can signify a higher rate of polymer degradation.
Mechanical response via dynamical mechanical analysis (DMA)
The TA Instruments DMA Q800 was used to perform dynamic mechanical analysis (DMA) and determine the mechanical characteristics of the PU foam composite samples before and after exposure to UV irradiation. Rectangular specimens, measuring 40 mm × 10 mm × 5 mm, were prepared for testing. For the synthetic epoxy (SE) foams, DMA was executed with a ramp rate of 4 °C/min, reaching temperatures up to 180 °C. The tests were conducted in a single-cantilever bending mode at a frequency of 1 Hz. The parameter setting is shown in Table 3.
Table 3
Parameters for the DMA machine of a single cantilever
Parameter
Value
Heating rate
4 °C/min
Temperature range
25 °C – 180 °C
Frequency
1 Hz
Dimension shape
Rectangular (40 cm × 10 cm × 5 cm)
Acoustical properties measurement
The sound absorption of PU foams and their composites was assessed within the frequency range of 0–6000 Hz. Samples with different thicknesses (10 mm, 20 mm, and 30 mm) were evaluated for their acoustical properties. Both before and after UV irradiation exposure, the acoustical characteristics of the PU foam composite samples were analyzed. Figure 2 illustrates the schematic setup for the sound absorption test, including the low-frequency tube test (a) and the high-frequency tube test (b). The acoustical test utilized the Tube Impedance Kit Types SSC 9020 B/K and SSC 9020 B/TL, following international standards (ASTM E1050) for acoustic material properties measurement, as depicted in Fig. 2 (c). The acoustic material properties measurement system included an adjustable filter, a propagation tube, a large sample tube with a 100 mm diameter for the low-frequency range (100 to 2000 Hz), a small sample tube with a 28 mm diameter for the high-frequency range (2000 to 6000 Hz), and two microphones with a digital frequency analysis system. The normal incidence sound absorption coefficient and typical specific acoustic impedance ratios of materials were evaluated using this setup.
Fig. 2
Schematic setup of sound absorption (a) low frequency (100–2000 Hz), (b) high frequency (2000–6000 Hz), and (c) tube impedance kit types SSC 9020 B/K and SSC 9020 B/TL (ASTM E1050)
The equations include variables defined to clarify their physical significance. The sound absorption coefficient (α) represents the fraction of sound energy absorbed by a material upon impact, ranging from 0 to 1, with 1 signifying total absorption. Meanwhile, the absorbed energy (Eα) denotes the amount of sound energy taken in by the material during interaction. The incident energy (Ei) refers to the sound wave energy striking the material’s surface. Additionally, reflected energy (Er) is the portion of sound energy that bounces off the surface, while transmitted energy (Et) passes through the material to the opposite side. Equation (2) represents the sound absorption coefficient (α) as the ratio of the absorbed sound energy (Eα) to the incident sound energy (Ei). This equation gives a fundamental understanding of how much sound energy the material absorbs out of the total incident energy.
$$\alpha =\frac{{E}_{\alpha}}{{E}_{i}}$$
(2)
Equation (3) applies energy conservation concepts to redefine the sound absorption coefficient (α). It shows how the energy from an incident sound wave (Ei) is divided into three parts: energy absorbed (Eα), energy reflected (Er), and energy transmitted (Et). Therefore, the absorbed energy can be determined as the portion of incident energy not compensated for by the reflected and transmitted energy.
The sum of the energy that has been reflected, absorbed, and transmitted can be defined as the total sound energy. By converting energy, the equation is formulated as shown in Eq. (4) below.
$${E}_{i}={E}_{r}+{E}_{\alpha}+{E}_{t}$$
(4)
where \({E}_{\alpha}\) represents absorbed energy.
Results and discussion
Moisture content measurement
Figure 3 shows the moisture content of BE and SE foam composites with different flakes of filler loading. It is observed that the average moisture of BE foam composites is higher compared to SE foam composites. One of the reasons is the synthesis reaction using water as the reaction medium below 100 °C, which involves the transesterification of triglycerides in the waste cooking oil to produce the BE, also known as biopolyols [30]. When water is used in the reaction mixture, the process could lead to the hydrolysis of the ester bonds in triglycerides. Hydrolysis is a chemical reaction that breaks down ester bonds with the addition of water. In terms of waste cooking oil transesterification, hydrolysis can result in the formation of glycerol and fatty acids, which are both hydrophilic compounds [31]. These hydrophilic components contribute to the higher moisture content in the fabricated BE foam composites.
Fig. 3
Average moisture content of BE and SE foam composites
Another reason that can contribute to higher moisture content is the residual water in the biopolyols of the foaming process, where the water is trapped within the foam structure and produces a high moisture content. Typically, it is already known that BE possesses water in the form of a hydroxyl group (-OH) either from the impurities of waste cooking oil (moisture), the synthesized process, or the residual water from the foaming process that is expected to react with isocyanate to form the urethane linkages that are crucial for the fabricated foam’s structure and performance [32]. Meanwhile, the SE foam composites are typically synthesized without the use of water as a reaction medium, they are often manufactured using chemical processes that do not involve hydrolysis reactions, thus resulting in lower moisture content.
Foaming and gelling time measurement
Figure 4(a) and (b) show the results of foaming and gelling time for the reaction between polyols (SE and BE) that act as monomers and methylene diphenyl isocyanate (MDI), which acts as a crosslinker with wood filler (flakes). As presented, both foaming and gelling times for SE foam are faster than for BE foam. These results are due to a synthesis reaction using water as the reaction medium. As mentioned, when the water is in the reaction medium, the process could lead to hydrolysis, which can result in the formation of glycerol and fatty acids, which are both hydrophilic compounds [31]. These hydrophilic components contribute to the higher moisture content, which can potentially affect the reactivity, viscosity, and cure characteristics [33]. Moisture may act as an accelerator that interferes with the curing reaction and influences both foaming and gelling time. Although the synthesis reaction is complete, it is possible that some residual moisture could remain. In this case, moisture plays a significant role in influencing the foaming and gelling times.
Fig. 4
(a) Foaming time and (b) Gelling time for BE and SE foam composites
The pattern of the graph is also observed to increase along with the flakes of wood filler loading. These increments are due to the reinforcing fillers, which tend to retard the foaming process due to the increased viscosity of the mixture. Viscosity is known to be the key parameter in controlling the stabilization of the expanding structure [34]. Higher viscosity slows down the diffusion and collision of the molecules, which reduces the rate of the chemical reaction and thus increases the gelling time [35].
UV irradiation exposure to PU foam composites
For further investigation, the fabricated BE and SE foam composites undergo UV irradiation exposure in order to characterise their performance in terms of morphological, bonding, mechanical, and acoustical performance. The usual starting point of foam degradation is the outer surface of biopolymer foam that is exposed to UV irradiation. As shown in Fig. 5(a) and (b), discoloration is observed for all samples after the UV exposure. BE PU foam composites show significant impact after UV exposure, where the samples turn orange-red in colour while SE PU foam composites turn yellowish. These colour changes are due to the photochemical degradation that involves the breaking of the urethane bonds, and as they break, the molecular structure of the PU changes, which leads to an alteration in the sample’s properties, including its color [36]. Another factor is due to the central CH2 group between the aromatic rings within the PU structure being susceptible to photo-oxidation. The oxidation process introduces oxygen atoms into PU, thus, leading to changes in chemical composition and potentially the development of chromophores responsible for the discoloration [36].
Fig. 5
Discoloration of (a) BE and (b) SE foam composites before and after UV irradiation exposure up to 6000h
Morphological PU foam composites
Figure 6 shows the surface morphology for both BE and SE foam composites, respectively. The SEM shows some obvious differences in cellular structure between both PU foam composites. As observed, both foam composites produced an open-cell structure, while SE foam composite showed a uniform structure. From Fig. 6(b), the open cells in a homogeneous open-cell foam are evenly distributed, creating a consistent and regular pattern throughout the material, different from the BE foam composite shown in Fig. 6(a), where the open-cell has various sizes of foam structure. It is known that open-cell foams are effective at absorbing sound energy within a space. When sound waves enter an open-cell foam, they penetrate the material and encounter resistance and friction, leading to the conversion of sound energy into heat. This helps in reducing sound reflections and improving the acoustic quality [37]. Plus, open-cell foams are often used in environments where controlling echo or background noise is a concern [38].
Fig. 6
BE PU foam composite (a) Non-filler, (c) Flakes filler; SE PU foam composite (b) Non-filler, (d) Flakes filler
Meanwhile, Fig. 6(c) and (d) show the presence of wood flakes in the PU foam composite for both BE and SE. The use of wood flakes as filler in foam material can have a specific impact on the acoustic performance of the foam composite. For example, the wood flakes are able to enhance the porous structure of the foam material, thus improving their sound absorption capabilities [39]. The open-cell structure, in combination with the additional porosity of the flakes, enables for effective sound absorption. Furthermore, the flakes used in foam composites adhere to environmentally friendly and sustainable processes. A further benefit is that the flakes could provide rigidity and impact resistance.
Figure 7(a) shows the pattern graph of the average mean pore size for both BE and SE foam composites before and after UV irradiation exposure. It is observed that the pore size increases for all samples. The highest mean pore size is recorded at 6000 h of UV exposure. This pattern of graph is expected because the UV radiation could potentially affect the pore size of the foam composite. In a previous report, prolonged exposure to UV radiation can lead to the expansion or enlargement of pores within foam composites, especially foam materials with open-cells structure [28]. The UV radiation can break down chemical bonds within the foam, causing the material to weaken and expand [40]. In addition, the UV-induced degradation of the surface layers of foam composites can alter the surface pores, making the pore larger and more irregular in shape, as shown in Fig. 7(b) and (c) for both BE and SE foam composites [41].
Fig. 7
Average main pore size before and after UV irradiation at 2000, 4000, 6000 h for both BE and SE foam composites
Meanwhile, for the average interconnected pore size graph presented in Fig. 8, the size is increased for all samples, similar to the average mean pore size. There are studies that state the interconnected pore size of foam composites is less likely to be directly affected by UV radiation, where the interconnected pores are a fundamental and relatively stable aspect of foam materials [42, 43]. However, the increase in interconnected pore size might be caused by the use of wood flakes as fillers in PU foam. For information, wood flakes that are not protected or treated can be sensitive to UV radiation [44]. Prolonged UV exposure can lead to the degradation of the wood flakes component within the foam composite, causing it to shrink or break down, thus creating voids within the foam and enlarging the interconnected pores [44]. In addition, wood is hygroscopic and can absorb moisture from the environment. The combination of UV exposure and moisture absorption can cause the wood flakes to swell, potentially increasing the interconnected pore size as the wood swells within the foam [45].
Fig. 8
The average interconnected pore size before and after UV irradiation at 2000, 4000, and 6000 h for both BE and SE foam composite
For strut thickness, the value decreases after UV irradiation exposure, as shown in Fig. 9. This decrease in value is related to the interconnected pore size measurement; as the interconnected pore size increases, the strut thickness decreases. The same reason is applied for strut thickness, where wood flakes degradation and moisture absorption play a significant role in measuring the thickness. For wood degradation, the wood flakes break down and shrink due to UV exposure, and the voids left behind in the foam matrix can result in a decrease in strut thickness. A similar effect occurs for moisture absorption, where the combination with moisture absorption causes the wood flakes to swell and expend within the foam composite, thus leading to a decrease in the strut thickness by displacing the foam material [45].
Fig. 9
The average strut thickness before and after UV irradiation at 2000, 4000, and 6000 h for both BE and SE foam composites
Bonding analysis
Figure 10(a) shows the FTIR spectroscopy graph with different UV irradiation exposures for each BE and SE foam composite, Fig. 10(b) represents important peaks that provide insights into the composition and chemical bond within the material. Carbon chain analysis (Fig. 10d) shows synthetic epoxy has longer, more cross-linked chains, explaining its superior thermal stability (higher Tg/stiffness). Bio-epoxy’s shorter chains align with lower heat resistance but have an eco-friendly appeal. Filler interaction enhances both materials’ performance, favouring synthetic epoxy for high-temperature uses. The FTIR graph for all samples shows a similar pattern for each UV irradiation exposure. In particular, the presence of a peak at 3200–3600 cm−1 represents the hydroxyl group (OH) bond [46]. At 2800–3000 cm−1, a strong aliphatic (C–H) stretching vibration bond appeared, indicating the presence of aliphatic hydrocarbons found in polyurethane foam, and the carbonyl group (C = O) at 1700–1800 cm−1 represents various components, including polyurethane, epoxy, or cooking oil residues [47, 48]. At 900–1300 cm−1, (C–O–C) stretching vibration peak is observed, indicating the presence of epoxy groups and any change due to curing and crosslinking [49].
Fig. 10
(a) FTIR spectra of BE and SE foam composites before and after UV irradiation; (b) bond for selected peaks; (c) carbonyl index calculation; (d) crosslinking of BE/SE polymer foam
From the extracted FTIR graph, the carbonyl index, CI, is evaluated for all PU foam composite samples using the equation shown in Fig. 10(c). Two peaks are involved in calculating the CI, which are the carbonyl group (C = O) at 1700–1800 cm−1 and the aliphatic (C-H) stretching vibration at 2800–3000 cm−1 [50]. Table 4 represents the calculated average of CI before and after UV irradiation exposure. Basically, CI is calculated to quantify the effect of UV irradiation exposure on PU foam. The carbonyl group (C = O) is often a product of oxidative degradation, so its presence and intensity can indicate the degree of oxidation in the material [51]. Meanwhile, the aliphatic (C-H) stretching vibration is used as a reference as it is a stable and common feature of many organic compound [50]. By comparing the CI before and after UV exposure, it is possible to quantify the effect of the UV irradiation on PU foam, thus providing valuable information about resistance materials to UV degradation and their suitability for certain applications. From Table 4, the average CI is observed to increase after UV irradiation exposure. In terms of acoustic performance, a higher carbonyl index indicates an increased presence of a carbonyl group (C = O) in a material, which is not necessarily desirable [2]. Such changes have a negative impact, especially on the mechanical and acoustic properties of the material.
Table 4
The CI for BE and SE before and after being exposed to UV light
Name of sample
Absorbance value
Carbonyl index, CI
UV irradiation exposure (hours)
Carbonyl group (C = O), A1700–1800
Asymmetric stretching (C-H), A3000-2800
0
2000
4000
6000
BE0
0.2483
0.3116
0.7968
0.8262
0.8116
0.7878
BE5
0.2770
0.3692
0.7502
0.807
0.7921
0.8205
BE10
0.2655
0.3325
0.7986
0.8227
0.821
0.8561
BE15
0.3295
0.4624
0.7126
0.873
0.8296
0.8613
BE20
0.2649
0.3422
0.7740
0.8625
0.8796
0.816
Average CI
0.76644
0.83828
0.82678
0.82834
SE0
0.0821
0.1028
0.7990
0.9204
0.9186
1.3315
SE5
0.0654
0.0854
0.7657
0.8998
0.8486
0.7401
SE10
0.0515
0.0879
0.5853
0.8229
0.8865
0.8942
SE15
0.0474
0.0832
0.5695
0.4397
0.4628
0.6927
SE20
0.0738
0.0860
0.8574
1.0574
0.7652
1.0751
Average CI
0.77362
0.82804
0.77634
0.94672
Dynamic mechanical analysis (DMA)
In DMA, there are two fundamental parameters and two important parameters that provide insight into the mechanical behaviour of a sample. Two fundamental parameters are the loss modulus and storage modulus, while the imporatant parameters are the tan (δ) peak and Tg value. The Tan (δ) value represents the ratio of the loss modulus to the storage modulus. Figure 11(a) shows the average glass transition temperature, Tg, for both BE and SE PU foam composites as a function of UV irridiation exposure. Theoretically, Tg is the temperature at which a polymer changes from a hard, glassy state to a rubbery state. This transition is linked to an increase in the free volume of the material, which allows for greater molecular motion [52]. From the graph, it appears that the Tg value for BE PU foam composite is consistently higher than SE PU foam composite. This pattern shows that BE has a higher degree of crosslinking or chemical composition that results in a higher Tg. In this case, BE, which has different chemical compositions from SE, affects the crosslinking density and consequently the Tg value due to the same crosslinking and filler used to fabricate the PU foam composite [53]. Elsewhere, the graph also shows that the Tg value decreases with increasing UV exposure for both materials, suggesting that the material becomes more flexible and less rigid with increasing UV exposure time. As mentioned before, UV radiation can cause changes in the chemical structure of the samples, leading to degradation and a decrease in Tg.
Fig. 11
(a) Average glass transition, Tg (°C), and (b) average Tan (δ) peak for BE and SE PU foam composites before and after UV irradiation exposure
For Fig. 11(b), the Tan (δ) peak is a measure of the loss factor or damping factor of a material, which is, as mentioned, the ratio of the loss modulus to the storage modulus. From the graph, it is observed that the average Tan (δ) peak decreases with increasing UV irridiation exposure. This is due to the fact that UV radiation causes changes in the molecular structure of the materials, thus leading to a decrease in their loss and storage moduli. In more detail, prolonged UV exposure can lead to photochemical reactions that cause the breakdown of the molecular structure of the material, which result in the formation of new chemical groups, changes in the cross. For example, crosslinking density increment can increase the storage modulus, while degradation can decrease both loss and storage moduli [54]. The SE PU foam composite shows a higher average Tan (δ) peak compared to the BE PU foam composite, which suggests that SE PU foam possesses a higher loss modulus and storage modulus, making it more resistant to deformation and able to store more energy [55]. Another reason that can be connected to the lower average Tan (δ) peak for BE PU foam composite is due to the higher moisture content, as reported in the previous section. The higher moisture content of BE can act as a plasticizer, softening the samples and reducing their stiffness [56]. This can lead to a lower storage modulus and a higher loss Tan (δ) value in samples, thus indicating a more viscoelastic response [57]. Meanwhile, at the highest exposure level, there is a slight decrease that might be due to the overexposure to UV radiation, thus leading to the degradation of the materials.
Bio-epoxy has higher initial stiffness than synthetic epoxy, but synthetic epoxy shows greater filler-induced improvements, especially in thermal stability (Tg rising to ~ 130 °C vs. bio-epoxy’s ~ 100 °C). Flake filler enhances stiffness, energy dissipation, and heat resistance in both, but synthetic epoxy excels in high-temperature applications (e.g., structural durability), while bio-epoxy remains viable for moderate uses with eco-friendly benefits, more details of DMA will be provided in the Supplementary file.
Acoustical adsorption analysis
The sound adsorption coefficient (SAC) is a measure of a material’s ability to absorb sound at a given frequency. A higher SAC indicates better sound absorption. Figure 12 shows SAC using a 10 mm thickness of BE and SE PU foam composite with different filler loadings at different UV irradiation exposures. From the graph, the relationship between SAC and frequency appears to be non-linear for all samples. At 3000 Hz, the SAC increases but decreases as the frequency reaches 6000 Hz for all samples. Foam composites possess a porous structure that can effectively absorb sound waves [2]. At lower frequencies, the longer wavelength of the sound waves can be absorbed by the foam as they cause the air within the pores to vibrate [58]. As a result, a smaller amount of sound is reflected back when the sound energy is converted to heat [59‐61]. Unfortunately, the shorter wavelengths of sound waves may not interact with the foam in the same way as the frequency rises. They may be too short to significantly shake the air in the pores, which would reduce the amount of sound absorbed. Lastly, the physical properties of the foam composites—density and porosity, for instance—that were covered in the previous section may have an impact on this behaviour since they may be better at absorbing lower-frequency noises than higher-frequency ones.
Fig. 12
SAC for 10 mm thickness at different UV irradiation exposures (a) 0 h, (b) 2000 h, (c) 4000 h and (d) 6000 h for BE and SE PU foam composites with different loading
Meanwhile, BE composites exhibit a higher sound absorption coefficient (SAC) at lower frequencies (α = 0.62–0.78 at 3000 Hz), which is consistent with natural fiber-reinforced composites. In this regard, Jeyaguru et al. [28] found that basket-weave hybrid composites of hemp and Kevlar had a sound transmission loss (STL) of 32 dB at 2000 Hz, highlighting the importance of filler architecture in acoustic performance. While STL and SAC measure distinct acoustic properties, both studies underscore the importance of structural design—whether through weave patterns [28, 29] or wood flake-induced pore networks in optimizing energy dissipation in this work. A noteworthy finding of this study is that after 6000 h of UV exposure, BE foams have a 12–18% greater SAC than SE foams. This contrasts with recent research that found UV-sensitive behaviour in untreated natural fibres [44]. The difference highlights the stabilizing effect of bio-epoxy lignin derivatives, which reduce UV-induced pore enlargement while preserving acoustic efficiency at lower frequencies. This result suggests that BE is more effective at absorbing lower-frequency sounds compared to SE, which might be due to the unique physical and chemical properties of BE that allow the foam to interact more effectively with lower-frequency sound waves [53, 60, 62]. As frequency increases, the difference between BE and SE decreases, and SE becomes dominant at 6000 Hz, which suggests that the SE PU foam composite is more effective at absorbing higher-frequency sounds. Meanwhile, for the effect of UV irradiation exposure, the SAC value seems to decrease as the radiation may degrade the sound absorption capabilities of the foam composites. However, the UV radiation does not change the pattern of the SAC graph involving all samples, as BE PU foam composite is still dominant for 3000 Hz of frequency compared to SE PU foam composite, which is dominant at the end of 6000 Hz of frequency, but the margin is not too significant.
As for 20 mm thickness, the graph from Fig. 13 shows a similar pattern where it is observed that SAC increases at 3000 Hz but then decreases as the frequency reaches 6000 Hz due to the same reason as 10 mm thickness foam composite. However, unlike the 10 mm thickness graph, this graph pattern shows that the frequency starts to drop right after the SAC reaches 3000 Hz for foam composites that are exposed to UV irradiation. It proves that UV irradiation can cause deterioration in the properties of both BE and SE foam composites where the radiation leads to short-term physical changes or long-term chemical changes in the polymer materials [44]. Meanwhile, the flakes filler inside the foam composites plays a less significant role where the foam composite sample looks scattered throughout the graph.
Fig. 13
SAC for 20 mm thickness at different UV irradiation exposures (a) 0 h, (b) 2000 h, (c) 4000 h, and (d) 6000 h for BE and SE PU foam composites with different loading
Figure 14 shows the SAC value for 30 mm thickness, where the pattern is quite the same for all samples, showing that 30 mm is the maximum thickness to observe the effect of UV irradiation exposure. The only difference is observed at 1000 Hz, where some samples produce an unstable frequency and a higher unstable frequency for samples with UV irradiation exposure. Elsewhere, no major changes occur throughout the graph.
Fig. 14
SAC for 30 mm thickness at different UV irradiation exposures (a) 0 h, (b) 2000 h, (c) 4000 h, and (d) 6000 h for BE and SE PU foam composites with different loading
Conclusion and future work
In conclusion, throughout this work, it was observed that BE PU foam composites possess better physical characteristics and properties, especially before UV irradiation exposure takes place, which affects the SAC performance. Starting with higher moisture from BE that affects the foaming and gelling time, it also affects the fabricated PU foam composite. As for mechanical analysis, BE PU foam composite has greater molecular motion but is less resistant to deformation and stores less energy, as proved by the average Tg value and Tan (δ) peak. All these properties have made the BE PU foam composite a better foam for absorbing sounds and thus producing better acoustical performances. Meanwhile, the exposure to UV irradiation makes the PU foam composite less efficient in terms of acoustical performance. The radiation has changed their characteristics, such as their pores, carbonyl index, and mechanical properties, especially SE PU foam composites. Although the properties were reduced, BE composites derived from waste cooking oil retained 12–18% higher SAC at 3000 Hz than SE after 6000 h of UV exposure, attributed to natural antioxidants delaying pore degradation. However, SE’s superior high-frequency performance (α = 0.45 at 6000 Hz) post-exposure suggests hybrid BE/SE foams could balance low- and high-frequency absorption.
In addition, further study could potentially improve the UV resistance of bio-epoxy (BE) polyurethane (PU) foam composites, ensuring long-term durability. Exploring techniques to improve UV stability in synthetic epoxy (SE) PU foam composites provides an opportunity for extensive, real-world research on their long-term performance and environmental effects. The emergence of hybrid materials or blends that strike a balance between UV protection, strength, and sound performance is an exciting idea. Practical applications of these improvements, notably in the construction and automobile industries, are worth investigating. Advanced approaches for analyzing minute structural changes following UV exposure can provide valuable information. Likewise, exploring introducing more bio-based elements to improve sustainability and performance is a possible area for future research endeavours. Future studies should explore hybrid fillers for synergistic UV/acoustic performance and industrial-scale lifecycle assessments to quantify CO2 savings versus synthetic foams.
Acknowledgements
The authors express gratitude to the Ministry of Higher Education (MOHE) and the Royal Society for their support of this research through the International Exchanges Scheme and the Fundamental Research Grant Scheme (FRGS/1/2020/STG01/UTHM/02/2). Also, thanks are given to the Advanced Manufacturing and Material Centre (AMMC) and the Faculty of Mechanical and Manufacturing Engineering (FKMP).
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Competing interests
The authors declare no competing interests.
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