1 Introduction

Noise pollution is a significant environmental problem that affects living beings. It has an adverse effect on the ecosystem and can even impact the mental health of living beings [1]. Noise pollution is a significant environmental problem that affects living beings. It has an adverse effect on the ecosystem and can even impact the mental health of living beings [2]. Recent studies have revealed the promising potential of utilizing aerogels and gas-filled panels for acoustic and thermal insulation applications. These acoustic materials are classified into granular, cellular, and fibrous types. However, most commercial acoustic materials are synthetic, causing pollution and hazardous effects during their manufacturing, use, and disposal, leaving a massive carbon footprint [3]. This environmental impact has driven industries, researchers, and scientists to explore the use of eco-friendly materials for acoustic and thermal insulation applications.

In recent years, there has been a growing interest in eco-friendly materials obtained from natural resources for acoustic and thermal insulation applications. Zahra et al. [4] investigated the acoustic and thermal insulation properties of pure and cross-bred sheep wool and found that the animal fibers’ thermal insulation and sound absorption properties were comparable to traditional materials, such as rock wool and glass fibers. Further research has demonstrated that the performance properties of these eco-friendly materials can successfully compete with traditional and synthetic materials. Sustainability in raw material supply is a key factor in the mass production of building materials, and natural fibers obtained from plants can meet this demand. These fibers have remarkable properties, such as being eco-friendly, having low density, a porous structure, and being readily available and processable [5]. Research has shown that these natural fibers can be potentially used for acoustic and thermal insulation applications. In addition, natural fibers obtained from plants have excellent thermal insulating properties and hence they can be potentially used in thermal insulation applications. Natural fibers obtained from plants are of two types primary and secondary fibers. Primary fibers are fibers, where the plant is solely cultivated for its fibers, while secondary fibers are the fibers that are the byproduct of industries. For example, coir fibers with a thickness of 20 mm showed a good sound absorption coefficient of 0.8 and could absorb frequencies above 1 kHz [4]. Said et al. reviewed the acoustic and thermal properties of bio-based materials derived from sustainable biological resources such as natural fibers extracted from the date palm, reeds, olive tree, ramie, kapok, jute, milkweed, cotton, sisal, fig tree, etc., and animal resources, such as sheep wool and chicken feathers. Standard techniques such as impedance tube and thermal conductivity tests were used to analyze the sound absorption coefficient and thermal properties of these materials. The results showed that most materials exhibited a decent sound absorption coefficient in the range of 0.6–0.9 for medium frequencies, while the thermal conductivity ranged between 0.044 and 0.091 W/m K. These values were comparable to the acoustic and thermal conductivity of synthetic materials, such as glass wool (0.95, 0.044 W/m K) [6].

In a study by Shahrzad et al. developed sound-absorbing and thermal insulation material using bagasse waste fibers. It was found that the developed composite exhibited a thermal conductivity of 0.034 and 0.042 W/m K and a sound absorption coefficient of 0.26–0.64. The bagasse was considered a suitable material for thermal insulation and acoustic material due to its good acoustic properties and its availability [7]. In another study, Samanta et al. investigated jute fiber composites and found that the felt with a density of 500 g/cc exhibited thermal insulation up to 341 m2K/kW, which was comparable to glass wool (320 m2K/kW), polystyrene (381 m2K/kW), and nitrile (320 m2K/kW), respectively [8].

The current study focuses on developing eco-friendly composite materials for acoustic and thermal insulation applications. While there are various natural fibers available and identified for these applications, they are limited to specific geographic locations. Therefore, there is a continuous demand for the identification of new sustainable resources. The water hyacinth plant, is considered a notorious weed in Thailand due to its rapid growth and depletion of oxygen from water causing aquatic problems [9, 10]. Although there are various disadvantages, the fibers obtained from the water hyacinth plant have some advantages owing to the voids in the fiber structure [9, 11]. These properties make the fibers obtained from water hyacinth more suitable for acoustic applications, despite their various disadvantages.

Moreover, the disposal of waste palm oil is another pressing environmental issue. Although palm oil is widely consumed as a food ingredient worldwide, its improper disposal has severe and hazardous impacts on the environment, contaminating both land and water [12]. To address this issue, this study proposes to use waste palm oil as the polyol substrate of a polyurethane adhesive, which will serve as the binder for a composite sheet made of water hyacinth fibers. Previous studies conducted by our group have demonstrated the effectiveness of palm oil-based polyol as a substrate for polyurethane adhesive [9, 11, 13]. Based on this knowledge, using used palm oil can be modified and extended for use as a substrate for adhesive and binder for water hyacinth fiber sheets used in sound absorption and heat insulation board in housing materials. To serve that statement, this research study has been established.

Therefore, the objectives of the work were to synthesize the bio-based polyurethane from the used palm oil by epoxidizing and ring opening method to minimize the use of petroleum-based polyurethane. Waste palm oil and WHF were used to develop the dual functional engineering material for acoustic and thermal insulation. The utilization of bio-derived materials is the best solution for sustainability, engineering, and the environment. The dual functional composites were developed using the biomass fibers extracted from the water hyacinth plant and polyol-based waste palm oil as a binder. Various composites were developed by varying the density and weight per volume of the mold. The performance characteristics of the developed composites were analyzed through impedance tests, thermal conductivity, mechanical, and water absorption.

2 Experiment

2.1 Materials

The precursor for recycled palm oil (RPO) was used palm oil (UPO), which was purchased from a local market. The UPO had an acid value of 1.41 mg KOH/g and an iodine value of 40.1 m I2/g. Before chemical modification, the UPO was filtered. Ajax provided the hydrogen peroxide (H2O2) and sodium hydrogen carbonate (NaHCO3), while Fisher Chemical provided the formic acid (HCOOH) and RCI Lab-Scan Limited provided the ethyl acetate. Polymeric diphenylmethane diisocyanate (P-MDI, 31.5% NCO content, functionality = 2.7) was purchased from BASF. The dried water hyacinth was purchased from Supanburi province, Thailand. Commercial polyurethane foam, polyurethane foam/glass fiber, cotton litter foam, and polyester foam were purchased from the HomePro store in Nonthburi province, Thailand.

2.2 Methods

2.2.1 Preparation of Recycled Palm Oil (RPO)-Based Polyol

The used palm oil (UPO) was utilized to synthesize recycled palm oil (RPO)-based polyol. The epoxidation and ring-opening reactions were carried out simultaneously in a single step. Before the reaction, 250 g (0.3 mol) of UPO was filtered and dried at 70 °C for 8 h. In a 2 L reactor, 57.71 mL (1.5 mol) of 98% formic acid was added to the UPO, followed by 50.25 mL (0.75 mol) of 35% hydrogen peroxide. The mixture was continuously stirred for 4 h at 70 °C with a controlled speed of 200 rpm. The RPO was then washed with ethyl acetate, saturated NaHCO3, and NaCl solutions. To obtain pure UPO-based polyols, the RPO was evaporated under a vacuum in a rotary evaporator at 40 °C. The iodine value, acid value, and hydroxyl value of RPO were analyzed. The physical appearance of purified UPO and RPO is shown in Fig. 1. The results of the purified UPO and polyol-based RPO were fairly summarized. Furthermore, all results were clearly described in our previous work [9, 11, 14].

Fig. 1
figure 1

Chemical structure and properties of prepared UPO and RPO

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2.2.2 Preparation of Water Hyacinth Fiber (WHF)

Starting with a 1 cm length of dried water hyacinth ground with a double-blade blender, WHF was obtained (Thai grinder, Thailand). After grinding, the fine fiber with a mesh less than 20 were sieved out. All sieved fiber samples were vacuum-dried for 12 h at 80 °C before use.

2.2.3 Preparation of RPO-Based PU/WHF Composite Sheet

Preparation of RPO-based PU/WHF composite sheet involved a compression molding process, as shown in Fig. 2. Initially, RPO-based polyol was mixed thoroughly with polymeric diphenylmethane diisocyanate (P-MDI), which served as a precursor for polyurethane (PU) adhesive. The fiber to PU adhesive ratio of 60–40, which was determined to be the optimum ratio based on our previous work [13], was used in this study. The adhesive precursor was then added dropwise to the chamber of WHF and the components were well-mixed before being transferred to a mold of size 15 × 15 × 0.5 cm3. The samples were then compressed at 110 °C for 30 min under a pressure of 10 MPa. The weight of WHF per volume of mold varied between 40%, 50%, 60%, and 70%. The recipes of RPO-based PU/WHF composites are listed in Table 1.

Fig. 2
figure 2

Overall preparation of WHF/RPO-based PU composite sheet

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Table 1 Formulation of RPO-based PU/WHF composite sheet
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2.3 Characterization

The iodine values of RPO were determined by titration to assess the number of double bonds in the structures according to ISO3961-2009. The acid values were analyzed to determine the free fatty acids in the oils by titration according to ISO660-2009. The number of OH units in the structures was determined by titrating the OH values in accordance with ISO14900-2001.

The chemical structures of UPO and RPO were investigated using 1H-NMR on a Bruker 400 Fourier transform spectrometer at 400.13 and 100.62 MHz. TMS was used as an internal standard, and CDCl3 was used to dissolve all samples. Fourier Transform Infrared (FTIR) spectra were collected using a Nicolet Avatar 370 DTGS FTIR spectrometer in the 4000–400 cm−1 range.

Size exclusion chromatography (SEC) with a Shodex GPC KF-806M column was used to determine the molecular weights of the UPO and RPO samples.

The density of the sample was determined using the ISO4590-2002 test, and the exact dimensions were measured using a Vernier caliper. The density of the specimens was calculated using the equation density = mass/volume.

The moisture absorption of the samples was evaluated in accordance with ASTM D5229. Five specimens, each measuring 100 × 100 mm2, were tested and the average value was calculated. The specimens were subsequently dried and placed in a desiccator to cool. Thereafter, the specimens were weighed using an analytical balance. The specimens were then transferred to a humidity-controlled chamber with 60% relative humidity. Over a period of 7 days, the specimens were removed from the chamber daily, sealed in a container, allowed to equilibrate to room temperature, and weighed. The moisture absorption rate was determined using the following equation:

$$A \, \left( \% \right) \, = \, \left[ {\left( {W_{f} - \, W_{i} } \right)/W_{i} } \right] \times \, 100$$
(1)

where A (%) is the absorption percentage, Wi is the initial weight of the sample, and Wf is the weight after obtaining the moisture of the sample.

The morphology of the green composite board was examined using a scanning electron microscope (SEM) equipped with a QUANTA 450 (Thermo Fisher Scientific, Waltham, MA, USA) under high-vacuum and high-voltage settings at 20.00 kV. Prior to imaging, all of the samples were coated with gold.

Bending strength, a mechanical property, was determined by three-point bending on a universal testing machine in accordance with ISO 14125, with a crosshead speed of 1 mm/s and a span distance of 25 mm. The specimens were 60 × 15 × 2 mm3 in size. For one formulation, five specimens were tested, with the average value reported.

The acoustic properties of the sample were investigated in terms of the Sound Absorption Coefficient (SAC) according to ASTM E1050 90, using Kundt’s tube, which included an impedance tube, two microphones, and a frequency analyzer (Impedance Measurement Tube Type 4206) (Brüel&Kjr, Naerum, Denmark). The foam samples were cut to 25 mm diameter and 15 mm thickness before being tested with a working frequency range of 500–6000 Hz. In this study, commercial polyurethane foam, polyurethane foam/glass fiber, and polyester foam were compared.

The heat insulation properties of the sample were determined using the conductive constant method according to ASTM C518, with the Heat Flow Meter Instrument HC-074. The sheet sample was cut to a size of 150 × 150 × 15 mm3. The heat insulation was measured over a working range of 20–50 °C. The heat insulation properties were calculated using the following equation:

$$S \, = \, C \times \left( {T_{h} - T_{c} } \right)/\left( {E_{1} - E_{2} } \right)$$
(2)

where λ is the thermal conductivity, (\(\mathrm{W}/\mathrm{m}\bullet \mathrm{K}\)), C is the thermal conductance, (\(\mathrm{W}/{\mathrm{m}}^{2}\bullet \mathrm{K}\)),\(\Delta T\) is the temperature difference across the specimen (K), S is the calibration factor of the heat flux transducer, E is the heat flux transducer output (V), \({T}_{h}\) is the temperature of the hot plate surface (K), and \({T}_{c}\) is the temperature of the cold plate surface (K).

When only one specimen is used, the thermal conductance of the specimen can be calculated using the following equation:

$$C \, = \, S \times E/\Delta T$$
(3)

In addition, where applicable, calculate the thermal conductivity, as the following equation:

$$\lambda = \, S \times E\left( {L/\Delta T} \right)$$
(4)

3 Results and Discussion

3.1 Chemical Characteristic

Figure 3 shows the FTIR spectra of UPO, RPO, RPO-based PU adhesive, WHF, and WHF sheets to verify their chemical structure. It can be observed that the chemical structure of UPO and RPO are mostly similar, but the difference is that the double bond disappeared and formed a hydroxyl group on the RPO structure. The main vibration bands of the RPO sample were observed at 1,150, 1,600, 2800–3000, and broad peaks at 3300–3500 cm−1, which were attributed to the C–O–C, C=O, C–H, and O–H functional groups, respectively. The appearance of a double bond of UPO was found at a wavenumber of about 3000 cm−1. The hydroxyl group on the RPO structure was noted to be one of the active groups that interacted with NCO active group from isocyanate-based PMDI. To confirm that the RPO polyol and MDPI can be used as PU adhesive as well, they were mixed and cured for 5 min and checked the structure. After polymerization, the chemical structures of the obtained PU adhesive showed new peaks at 3302, 1707, 1602, 1538, and 1224 cm−1, assigned to –N–H stretching, –C=O ester stretching, N–H bending of urethane position, –N–C=O vibration of polyurethane linkage, and C–O acyl stretching of urethane linkage, respectively, which was consistent with other research works [9, 11, 15, 16]. Another spectrum fiber sheet, WHF, demonstrated the significance of functional group as cellulose at 1061, 1373, and 3300–3500 cm−1 attributed to C–O–C, C–H and O–H stretching of cellulose structure, respectively [17, 18]. The spectra of the WHF composite sheet showed mixed spectra from RPO-based PU adhesive and cellulose structure from WHF. In conclusion, RPO-based PU adhesive and WHF composite sheets were successfully synthesized from fully waste palm oil or RPO as a precursor and WHF.

Fig. 3
figure 3

FTIR spectra of UPO, RPO, RPO-based PU adhesive, WHF, and WHF sheet

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3.2 Physical Property of Fiber Sheet

3.2.1 Density and Physical Appearance

The density of the WHF/RPO-based PU composite material was a crucial property in this study, since the effect of the weight of the composite on the volume of the mold ratio on the sound absorption and heat insulation properties was investigated. Figure 4 shows the physical appearance and density of the WHF/RPO-based PU composite sheet series. The density values of 50-WHF/V, 60-WHF/V, and 70-WHF/V were 0.391, 0.493, and 0.547 g/cm3, respectively. The density increased with the weight of the composite to mold volume ratio. This was because the amounts of WHF and RPO-based PU composite of compressed components increased from 56.25 to 78.50 g, while the mold volume was fixed at 112.5 cm3, resulting in an increase in the density of the final composite sheets, as shown in Fig. 3.

Fig. 4
figure 4

Physical appearance and density of WHF/RPO-based PU composite sheet series

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3.2.2 Moisture Absorption

The moisture absorption properties of the WHF/RPO-based PU composite material were evaluated to determine their performance in actual situations, and the results are reported in terms of moisture uptake percentage. Figure 5 displays the moisture absorption of the composite materials with different weight-to-mold volume ratios at 60% relative humidity. The results showed that the moisture absorption increased with decreasing weight-to-mold volume ratio. At day 7, the moisture absorption of 70-WHF/V was the lowest, at 1.05%. However, when the ratio was increased from 50 to 60%, the moisture absorption at day 7 increased to 1.44%. On the other hand, the material with the lowest weight-to-mold volume ratio, 50-WHF/V, showed the maximum moisture absorption of 5.75%. This can be attributed to the density and cavity inside the materials. The material with the lowest weight-to-mold volume ratio, lowest density, and the greatest cavity interior can easily absorb moisture in the real system. The polar component of moisture (–OH group) can hydrogen bond with the hydroxyl group of the water hyacinth fiber surface and the urethane linkage (–C=O(NH)) of the polyurethane adhesive. The cavity of the materials allowed moisture to enter the substance and react with the active polar site. As a result, the material with the lowest density and greatest cavity content, 50-WHF/V, has the maximum moisture absorption over 7 days, whereas the material with the highest density and lowest cavity content, 70-WHF/V, has the lowest moisture absorption.

Fig. 5
figure 5

Moisture absorption of WHF/RPO-based PU composite sheet series at 60% relative humidity

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3.3 Morphology

The cross-sectional morphology of the developed composites was analyzed using scanning electron microscopy, and the images are presented in Fig. 6. The SEM images were captured at two different magnifications for better understanding of the morphology.

Fig. 6
figure 6

SEM images of WHF/RPO-based PU composite sheet series at a magnification of 84× and 500×

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In Fig. 6, the SEM images of the cross section of the developed composites were analyzed. Two different magnifications were used to present the morphology of the cross section. The SEM image of 50-WHF/V in Fig. 6 showed that the composite volume was loosely packed, and there were many voids. This resulted in a reduced density of the composite and had a significant impact on its stiffness and sound absorption property. Since there was a lot of space within the composite, sound waves could easily penetrate and cross the composite without dissipating within the composite structure.

For the 60-weight % fiber-loaded composite, it was observed that the composite volume was closely packed with water hyacinth fibers when compared to the 50-weight % fiber-loaded composite, as shown in Fig. 6. Hence, the composite density was increased with considerable space between the reinforced fibers. This phenomenon was ideal for sound absorption, where the sound waves entered the composites and got dissipated within the composite system because of the presence of spaces. Finally, from the cross-sectional SEM image of 70 weight per volume percent, it was observed that the reinforced fibers were tightly packed, and hence the space between the fiber reinforcement was significantly reduced. This phenomenon increased the modulus and density of the composite but lowered the sound absorption coefficient, because the sound waves were not able to pass through the composite due to the lower space between the reinforced fibers. In this case, some amount of sound energy was dissipated within the system, while the rest got reflected due to the higher composite density.

3.4 Mechanical Properties

Mechanical properties are important in assessing material performance. In this study, the mechanical properties of the composite boards were characterized using impact testing and flexural testing. The results were collected, calculated, and reported as means, as shown in Figs. 7 and 8.

Fig. 7
figure 7

Impact strength of WHF/RPO-based PU composite sheet series

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Fig. 8
figure 8

Flexural results—a Modulus, b strength and c strain of WHF/RPO-based PU composite sheet series

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3.4.1 Impact Properties

The impact properties of the developed composites are presented in Fig. 7. It was observed that the composite with 70-WHF/V exhibited the highest impact energy of 0.035 J/mm, which was 1.59 times higher than that of 60-WHF/V (0.022 J/mm) and 3.5 times higher than that of 50-WHF/V (0.010 J/mm). The higher impact energy for the 70% weight per volume composite was due to the composite’s ability to absorb more energy before failure. Moreover, during the application of the impact load, the material was able to absorb more energy and dissipate it within the system [19, 20]. Another reason is that the fiber structure is porous, having cavities that were able to absorb and dissipate the energy within it. In the case of 50 and 60 weight per volume %, the lower impact energy was due to a deficiency in the infill percentage resulting in lower energy dissipation [21, 22].

3.4.2 Flexural Properties

The flexural modulus and strength of the developed composites are shown in Fig. 8a, b, respectively. The composite with 50%w/v exhibited a lower flexural strength of 0.441 MPa. When the weight % of the fibers to mold volume was increased to 60%, the flexural strength of the composite increased up to 1.43 times (1.652 MPa). Furthermore, when the volume density was increased to 70%w/v, the flexural strength of the composite increased to 2.377 MPa, which was 5.39 times higher than the 50%w/v weight density. When the flexural load was applied to the composite, the top layer of the composite experienced a compression load, while the bottom surface of the composite experienced a tensile load [23, 24]. The higher flexural strength for the 70%w/v fiber loading was due to the increased bulk density when compared to the other composites. In addition, there was good stress transfer within the composite due to the increased weight of fiber loading, which allowed the composite to withstand more load before failure. (Nagaraja Ganesh and Muralikannan 2016; Georgiopoulos et al. 2018; Krishnan et al. 2020). The increased weight per volume % of fiber loading also had a significant impact on the flexural modulus of the developed composite. The flexural modulus of the 70-WHF/V was 179.977 MPa, which was 1.097 times higher than the 60-WHF/V (163.925 MPa) and 7.08 times higher than the 50-WHF/V (25.397 MPa). The higher modulus for 70%w/v of the fiber-reinforced composite was due to the increased density and stiffness of the composite. It was noted that the composite with 50%w/v of fiber reinforcement had the highest strain of 2.71%. The higher strain value was due to the lower stiffness offered by the composite during the application of flexural load. As the composite had only 50% of fiber weight per volume, it exhibited a lower young’s modulus and enabled elongation before failure. However, in the case of the composite with 70%w/v fiber reinforcement, the strain was reduced due to the higher stiffness and modulus of the composite. From the results, it was noted that the strain decreases with an increase in weight % of fibers, stiffness, and modulus [27]. Moreover, the higher fiber infill density has a significant effect on the sound absorption coefficient and thermal conductivity of the composites.

From the results of the mechanical properties of the developed composite sheet based on biomass to be green industrial technology, the mechanical properties were summarized in Table 2.

Table 2 Summary of mechanical properties of WHF/RPO-based PU composite sheet series
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3.5 Sound Absorption Properties

The sound absorption properties are typically evaluated based on the sound absorption coefficient (SAC) measured across a frequency range of 50–6000 Hz. Figure 9 presents a comparison of the RPO-based PU/WHF sheet composites with various other materials, including commercial grade PU foam, PU foam/glass fiber, polyester foam, and cotton litter foam. The SAC spectra demonstrate that the RPO-based PU/WHF composite sheet possesses excellent absorption properties across all frequency ranges, with a SAC value greater than 0.5. According to the standard for sound absorption materials, materials with SAC values greater than 0.4 and covering the frequency range of 125–6000 Hz are considered to be sound-absorbing materials [11, 28]. Therefore, these findings suggest that the RPO-based PU/WHF composite sheet has great potential as a sound-absorbing material for industrial applications.

Fig. 9
figure 9

Sound absorption coefficient of RPO-based PU/WHF composite sheet with various weight of WHF to volume of mold and commercial sound absorption materials

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It has been previously reported that the majority of audible sounds that humans perceive fall within the frequency range of 250–6000 Hz [29]. The human auditory system is capable of perceiving three distinct frequency ranges: low-frequency sound waves, which reside at and below 300 Hz; middle-frequency sounds, which range from 300 to 2000 Hz; and high-frequency sounds, which extend beyond 2000 Hz. Of these ranges, the middle-frequency range is where most of the sounds that humans encounter on a daily basis can be found, including speech, animal vocalizations, and musical instruments [30]. Therefore, this study has focused its discussion on the results obtained within the middle-frequency range.

The sound absorption properties of RPO-based PU/WHF composite sheets were investigated across a frequency range of 50–6000 Hz in this study. Figure 9 shows a comparison of the SAC values of the composite sheets with those of commercial PU foam, PU foam/glass fiber, polyester foam, and cotton litter foam. The highest SAC value of 0.53 at 50-WHF/V was observed in the frequency range of 782 Hz, with the value exceeding 0.4 for all frequency ranges. The SAC value of 0.83 at 60-WHF/V was found to be the highest in the frequency range of approximately 1280 Hz, with a value greater than 0.5 for all frequency ranges. The highest SAC value of 0.76 in the frequency region around 1600 Hz was observed in the final specimen, 70-WHF/V. The prepared composite materials exhibited a multi-peak trend in the SAC values due to the presence of various pore size ranges in the composites, which is consistent with previous reports [31]. These findings suggest that sample density and cavity significantly impact sound absorption qualities. The SAC values of the RPO-based PU/WHF composite sheets indicate their potential for use as effective sound absorption materials in various applications.

The study findings suggest that although the sound absorption coefficient (SAC) value of the composite sheet increased with higher density, the trend observed was different. The presence of sample cavities was found to have a significant impact on the sound absorption of the composite sheets, with sound waves being introduced and absorbed by the cavity walls. Therefore, it is crucial to consider the optimal density and cavity structure of the sample. Morphological analysis, supported by SEM images, can offer valuable insight into the ideal structure for effective sound absorption.

When compared to commercial polyurethane (PU) and PU/glass fiber materials, the newly developed PUF/WHF composite materials demonstrated superior sound absorption performance, with SAC values exceeding 0.4 for all frequency ranges. Although commercial cotton litter foam exhibited a SAC value of 0.5 at high frequencies, polyester foam only absorbed sound with a SAC value of over 0.8. Given the practical applications, the middle-frequency range of sound absorption was the focus. Therefore, the PUF/WHF composite materials hold promise for use in various sound absorption applications, including sound absorption walls and other engineering materials designed for sound absorption.

After evaluating the SAC efficiency, it was determined that the PUF/WHF composite with a ratio of 60-WHF/V was the most appropriate, as it demonstrated the highest SAC value across all frequency ranges. As a result, the RPO-based PU/WHF composite sheet produced in this study exhibited superior sound absorption performance, making it a promising choice for the production of environmentally friendly industrial materials.

3.6 Heat Insulation Properties

This study investigates the heat insulation property of the composites and how air pockets and density affect this property. The results are presented in terms of reversed thermal conductivity values, where a higher value indicates lower thermal insulation. The thermal conductivity values for composites with 50% WHF/V, 60% WHF/V, and 70% WHF/V are 0.0111, 0.0116, and 0.0112 W/m K, respectively. Figure 10 displays the thermal conductivity results, and a summary is provided in Table 3.

Fig. 10
figure 10

Thermal conductivity of RPO-based PU/WHF composite sheet series and commercial materials

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Table 3 Sound absorption and thermal conductivity values of the materials
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The results suggest that increasing the weight percentage of WHF in the composite does not significantly affect its thermal insulation properties. This can be attributed to the presence of air pockets within the composite, which act as insulators by impeding the flow of heat through the material. The density of the composite also plays a crucial role in determining its thermal insulation properties, with lower densities leading to better insulation.

When compared to commercial materials, such as polyester foam, cotton litter foam, PU foam, and PU foam/glass fiber, which have thermal conductivity values of 0.0426, 0.2345, 0.1486, and 0.0021 W/m K, respectively, there is no significant difference in heat absorption values between the prepared WHF/UPO-based polyurethane composites. However, commercial materials have greater thermal conductivity values, indicating lower thermal insulation properties. Glass fiber is shown to have a significant impact on thermal insulation, as the thermal insulation value of PU foam/glass fiber is approximately 70 times that of PU foam.

Despite having better thermal insulation values than the prepared materials, PU foam/glass fiber is made from petroleum-based substances and glass fiber, both of which can have harmful effects on human health. Therefore, a natural-based material made from waste such as the PUF/WHF composite is a promising and desirable option for engineering applications, particularly for heat insulation and thermal insulation in buildings, to minimize potential health impacts.

The fully waste-based engineering sheet material, composed of WHF and UPO, was developed through in situ polymerization. This involved mixing UPO-based polyol with MDPI as the PU adhesive precursor, which was then compressed at 110 °C for 30 min to ensure complete curing. The study suggests that these fully waste-based engineering sheet materials have the potential to be utilized for both heat insulation and sound absorption applications. A summarized result and possible model of this work is illustrated in Fig. 11.

Fig. 11
figure 11

Summary results and possible model of green composite board made of RPO and biomass-based WHF—a 50-WHF/V, b 60-WHF/V and c 70-WHF/V

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4 Conclusion

This study successfully developed a fully waste-based material utilizing water hyacinth fibers and used palm oil through in situ polymerization. The mechanical results of the material showed that the density, stiffness, modulus, and strength of the composite increased with an increase in the reinforcement weight percentage. The composite with 70 weight per volume percent exhibited the highest flexural strength and modulus. In addition, the material exhibited excellent sound absorption and thermal insulation properties, with the composite containing 60% w/v showing the highest sound absorption coefficient and heat insulation compared to 50 and 70% w/v. These properties are attributed to the lower density and ample space between the reinforced fibers, allowing for the dissipation of sound, heat, or energy within the material. The findings of this study demonstrate the potential for this material to be used in sound absorption and thermal insulation applications, particularly as sound and thermal lining for walls and roofs. This fully waste-based material not only offers a sustainable and eco-friendly alternative to petroleum-based materials but also provides a solution for managing waste products, specifically water hyacinth, in regions, where it is considered an environmental hazard. Overall, the development of this material provides a promising step towards creating more environmentally friendly and sustainable materials in the future.