1. Introduction
Agro-waste materials are available in huge amounts and are biodegradable, sustainable, eco-friendly, and natural. These wasted materials have to be recycled efficiently; otherwise, they create a burden on the environment. The Organization of Agriculture and Food [
1] has reported that Saudi Arabia is one of the largest countries in terms of date production, producing 1.2 million tons/year. The huge amount of waste that can be produced from the date palm trees has many useful applications, such as pulp paper production and as composite materials using fibers [
2,
3,
4]. Furthermore, 20 kg of leaves can be produced from each date palm tree per year as waste. Moreover, atmospheric pollution could occur if such waste is burned in the air, as usually happens as a common practice in some areas of the world [
5]. Therefore, good utilization of such wastes will have a good environmental impact in addition to economic benefits. Eleven million tons of agricultural waste per year can be produced in Saudi Arabia, and most of them belong to date palm trees. Those wastes have valuable benefits from an economic point of view [
6,
7]. Date palm spikelet and date palm fiber have been used with the bricks to improve their thermal insulation characteristics by Belatrache et al. [
8]. Their results showed that the thermal conductivity coefficient of the new bricks was 0.106 W/(m K) when 1.36% of date palm fiber was used. Raza et al. [
9] have developed a new composite made of date palm surface fiber and polystyrene as a new insulation material for buildings. Their new composite with 20% date palm surface fiber had a low thermal conductivity coefficient of 0.053 W/(m K). Four samples of date palm surface fibers were developed at different densities for thermal insulation by Raza et al. [
10] using polyvinyl alcohol as a binder. Their results showed an average thermal conductivity coefficient of 0.038–0.051 W/(m K). Ali and Abdelkareem [
11] have reported new thermal insulation materials extracted from date palm surface fibers. The thermal conductivity coefficient range of their produced sample boards was between 0.0475 and 0.0697 W/(m K) using cornstarch resin as a binder. Ali et al. [
12] have developed natural insulation materials as a composite between date palm tree leaves and wheat straw fibers. The thermal conductivity of their boards was in the range of 0.045–0.065 W/(m K) at temperatures of 10–60 °C, respectively, using wood adhesive as a binder. Alabdulkarem et al. [
13] have developed new experimental thermal insulation materials made as a hybrid between Apple of Sodom fibers and date palm surface fibers with different compositions using wood adhesive, corn starch, and white cement as binders. Their boards had average thermal conductivity coefficients in the range 0.04234–0.05291 W/(m K), and the absorption coefficient of the boards was also determined to be greater than 0.5 at high frequency.
On the other hand, the solid waste, which is a by-product of pineapple industries, is in the range of 40–50% from the peelings, crown, and core (Buckle, [
14]). Adhika et al. [
15] have reported that the sound absorption coefficient of pineapple fiber with an epoxy composite is greater than 0.5 at high frequencies, and it was affected by the density and applied pressure of the sample. Pineapple leaves were reported as good thermal insulation materials by Tangjuank [
16]. He used natural rubber latex as a binder for boards with different densities, and the measured thermal conductivity was in the range of 0.035 W/m. K to 0.043 W/m K. The same binder is used by Kumfu and Jintakosol [
17] in producing a thermal insulation board with a density of 338 kg/m
3 from pineapple leaves with a thermal conductivity coefficient of 0.057 W/(m K). Hybrid of pineapple fibers and polyester using a needle-punching technique was used in developing nonwovens by Thilagavathi et al. [
18]. Their product had better thermal insulation and sound-absorbing characteristics compared to pure pineapple fibers. Aerogel composites were made of cotton waste and pineapple leaf by Do et al. [
19]. That aerogel was tested as a thermal insulation, and its thermal conductivity coefficient was found in the range of 0.039–0.043 W/(m K). Pineapple leaf fibers with paper waste composites were examined for their sound absorption as an alternative to synthetic fiber by Sari et al. [
20]. Their results showed that the sound absorption coefficient increased as the pineapple leaf fibers increased in the composites at the expense of the impact strength. Suphamitmongkol et al. [
21] have used pineapple leaf fiber (PALF) as a potential source of sound absorption and thermal insulation materials. They showed that the thermal characteristics of the composites made of PALF were better than those with PET and asbestos but comparable to the composites made of glass fiber. On the other hand, they found that the acoustic properties of PALF are better than those of glass fiber but lower than polyester fiber. Recently, Ali et al. [
22] have experimentally studied the effect of using natural polymers of PALF, sunflower seeds, and watermelon seeds and their hybrid composites as new thermal insulation and sound absorption materials. Their results showed that the average thermal conductivity for the composite of PALF and the sunflower seeds was 0.05921 W/(m K) and 0.06577 W/(m K) for the composite of PALF and the watermelon seeds. The sound absorption coefficient was found above 0.5 for most of the bound and hybrid composites. New bio-degradable composite foams made of pineapple stem starch and pineapple leaves were reported by Namphonsane et al. [
23]. Their results indicated that the flexural strengths of the composite foams ranged from 1.5 to 4.5 MPa. Furthermore, the sound absorption coefficient of natural fibers such as corn, sugar cane, coir, and dry grass was measured for different thickness samples by Fouladi et al. [
24] and found to be good alternatives for common building acoustic boards. Noise reduction and sound absorption coefficients were reported for hemp, kenaf, coconut, cork, sheep wool, cardboard, and cane by Berardi and Iannace [
25]. They have shown that these coefficients were thickness, density, and porosity-dependent, and they have been recommended for use in buildings.
Most of the literature mentioned above focused on using natural fiber polymers of DPSF and PALF only; however, bound or hybrid composites were not considered. Therefore, this study presents new novel bound and hybrid composite boards made of date palm surface fibers and pineapple leaf fibers as thermal insulation and sound-absorbing materials. Different densities and composition boards are made, and their thermal conductivity and sound absorption coefficients are found to be promising to use as applications in buildings and can be considered as good biodegrading ecofriendly materials in replacing the synthetic and petrochemical ones.
9. Results and Discussion
Figure 7a,b show the force-deflection profiles and the flexure stress–strain curves, respectively, for the bound composite numbers 2 and 4 and the hybrid ones, numbers 5, 6, and 7. The flexural sample’s dimensions are listed in
Table 2, and the calculated mechanical properties, such as flexural stress
σf, flexural strain
ϵf, and flexural modulus
Ef, are shown in
Table 3. These parameters are evaluated following Equations (1)–(3) above.
Table 3 presents the maximum
σf before the deviation from linearity [
36] (
Figure 7b), where the flexural strain
ϵf is obtained. It should be noted that the slope (m) is calculated from the linear straight line of the profiles in
Figure 7a.
It is worth mentioning that an enhancement is observed in both
Ef and
σf as the density of the specimen increases, which agrees very well with the results obtained by [
37,
38]. Therefore, specimen number 7 is the best among the hybrid composites, while number 2 is the best among the bound composite specimens.
Figure 8 compares the mechanical properties of the hybrid and bound samples in terms of bar charts with error bars. It should be noted that the compactness degree plays a very important role in enhancing the flexural modulus
, flexural stress
, and
ϵf. This compactness depends on the polymerized binder ratio and the density of the specimen. The error in measuring the length, slope, deflection, and load is ±1.0 mm, ±0.7 N/mm, ±0.001 mm, and ±1.0 N, respectively. A computer program is written to calculate the absolute uncertainty and its percentage following the procedure described by McClintock [
39] and Moffat [
40]. The maximum uncertainties of flexural modulus (
Ef), flexural stress (
σf), and flexural strain (
εf) are 13.4%, 8.9%, and 4.6%, respectively.
Figure 9a,b show a surface morphology comparison of the loose (Lo # 1) and bound composite (Bo # 2) of PALF.
Figure 9a shows the texture shape of the loose leaf at 2000 magnification, while
Figure 9b shows that after being ground in a blender, where the thickness of the leaves is in the range of 2.86–81.4 μm.
Figure 9c shows the composite after mixing and compressing with the binder. Red spots show some of the polymerized binders. It is also noticed that there are a lot of cavities between the skinny fibers, which in turn enhances the thermal conductivity and the sound absorption coefficient, as shown in the next sections.
Figure 10a,b show the same configuration but for the DPSF, where
Figure 10a shows the size of the loose rough fiber between 15.2 μm and up to 0.54 mm outside diameter. Red spots show the polymerized binders hugging the fibers, leaving some void cavities. It should be noted that the lower corner of
Figure 10a shows the small fiber size with larger magnification taken from another photo and montaged here to conserve the number of figures.
Figure 11a–c show similar surface morphology of the hybrid composite samples 5, 6, and 7, respectively. It should be mentioned that the red arrows, ellipses, and rectangles denoted some of the textures of the PALF, DPSF, and binder, respectively.
Figure 12a compares the thermal conductivity coefficient (K) of both loose date palm surface fiber (DPSF, Lo, # 3) and pineapple leaf fibers (PALFs, Lo, # 1) with their bound samples (Bo, # 2 and # 4). Adding binders increases the thermal conductivity coefficient since most of the little porous void spaces in the loose samples get filled with the polymerized binder [
22,
41,
42]. Solid lines present the best curve-fitting through the data. The vertical dashed line at an ambient temperature of 24 °C shows that K for all samples is below 0.06 W/(m K), which indeed promotes these discarded waste polymer and composite materials as good thermal insulation for buildings.
Figure 12b compares the loose samples of both fibers to that of hybrid composite numbers 5, 6, and 7 at different compositions, as shown in
Table 1. It should be noted that K depends on the amount of polymerized binder used since more binders mean more void porous pores will be filled, which tends to increase K.
Furthermore, the degree of compactness also tends to reduce those pores, which also increases K. In addition, for the same material, increasing the density tends to increase K for the same reason. Moreover, the thermal conductivity depends on the type of materials used. It is also observed that the percentage of increasing the thermal conductivity for the temperature range of 20 °C to 80 °C is 23%, 33%, 23%, 27%, 24%, 29%, and 25% for samples Lo #1, Bo #2, Lo # 3, Bo # 4, Hy #5, Hy # 6, and Hy # 7, respectively. This figure also shows that at an ambient temperature of 24 °C, they have a low thermal conductivity coefficient below 0.06 W/(m K). Solid lines present the linear regression of the data in the form of
Table 4 shows the constants C1 and C2 that appear in Equation (4), the coefficient of determination R
2 of the correlation, the thermal conductivity coefficient at room temperature, and the density of each sample.
Figure 13a shows the effect of density on the thermal conductivity coefficient for the same material when it is loose (with no binder) or bound at different temperatures. This figure ensures that for constant density, K increases as the temperature increases. Furthermore, adding a binder (bound composite sample) increases the density and, in turn, increases K at all temperatures. On the other hand,
Figure 13b presents the variation of K with the density but for hybrid composite sample numbers 5, 6, and 7 at different temperatures. It should be noted that each curve presents three different samples, each of which may have a different polymerized binder ratio in addition to the different composition of the raw materials at each hybrid composite sample. In this case, the K profile trend looks different than that of
Figure 13a due to the different compositions and the ratio of the binder of each sample; therefore, this figure summarizes the relation between K and the density at different temperatures for each hybrid composite sample.
Table 5 shows a comparison between the obtained thermal conductivity and those found in the literature for similar materials.
The low thermal conductivity coefficient of the bound and hybrid composites, which is below 0.07 W/(m K) at all temperature ranges up to 80 °C, promotes their use as insulation materials for buildings and other engineering applications.
Figure 14 shows the sound absorption coefficient (SAC) for the bound composite sample numbers 2 and 4 and the hybrid composite ones, numbers 5, 6, and 7, for a frequency range up to 6000 Hz. In the communication range for frequencies up to 2000 Hz, it is noted that hybrid numbers 6 and 7 have SAC greater than 0.4 at a frequency of 1000 Hz and increases until 0.65 at 2000 Hz with a bell shape reaching 0.9 between 1000 and 2000 Hz. Hybrid sample number 5 has an even better SAC at the same frequency range mentioned for the other hybrid ones. On the other hand, the bound sample number 4 has the best SAC in the lower frequency range from 250 to 1000 Hz, which corresponds to SAC in general greater than 0.5. The bound sample number 2 has a lower SAC of about 0.1 up to 2000 Hz. In general, all samples exhibit high SAC at frequencies greater than 2000 Hz. The noise reduction coefficient (NRC) is determined by calculating the average value from the one-third octave values of the SAC at frequencies of 250, 500, 1000, and 2000 Hz and rounding the result to the close 0.05 following [
46] and [
25], as shown in
Table 6.
Figure 15 shows a comparison of the NRC of the samples in terms of bar charts. The SAC and the NRC indicate that in the communication range of frequency, the hybrid composite samples, and bound number 4 have good acoustic characteristics, which promote their use as sound-absorbing materials in buildings and other applications. On the other hand, at a frequency greater than 4000 Hz, all samples have good SAC, which means they have the potential to be used for protection against noise emitted by different ultrasonic devices [
47]. Moreover, it has been reported [
48,
49] that materials with SAC ≥ 0.4 can be classified as effective sound-absorbing materials and could be used for absorbing sound in engineering applications.
Figure 16 shows the profiles of degradation and decomposition of PALF (raw material) through the thermogravimetric analyses (TGAs) and their differential thermogravimetric analysis (DTGA) tests. This figure indicates that the PALF is stable up to about 218 °C, where the material loses its moisture content and its mass decreases by only about 10 percent (■), which corresponds to the starting of its first major degradation (
■) in the DTGA profile (left). The material loses 50% of its mass (●), which is shown in the DTGA as (
●) at about 315 °C. It is noted that the TGA profile has an inflection point at 372 °C (♦), where the material lost about 58% of its mass with a changing decomposition rate, where the second major degradation starts (
♦), as shown on the DTGA profile. The material reaches a char at about 550 °C, where it loses about 86% of its mass (▲). This thermal characteristic and behavior of the PALF indicate that it is thermally stable up to 218 °C, which promotes its suitability for thermal insulation in buildings and other thermal insulation applications.
Figure 17 presents the TGA and its DTGA for the date palm surface fibers (DPSFs). This figure indicates that the DPSF is thermally stable up to 232 °C, where the fiber loses about 8.5% of its mass (■). This point is shown as (
■) in the DTGA profile, which indicates the start of the major degradation that continues up to 475 °C (
♦) or (♦) on the TGA profile. The 50% degradation temperature of the fiber is approximately 364 °C (● on TGA and
● on DTGA), and the fiber reaches a char at about 1192 °C at 22% of its mass (▲). Comparison between
Figure 16 and
Figure 17 confirms that DPSF is a little more thermally stable than PALF. Nevertheless, both can stand high thermal temperatures above 200 °C.
Figure 18a–c show the TGA and the DTGA for the bound composite sample (2), bound composite sample (4), and hybrid composite sample (5), respectively. They have similar profiles to that of
Figure 16 and
Figure 17; however, each composite has its own stability, degradation, and char formation temperature, as shown in
Table 7.
Figure 18d shows a comparison of the TGA profiles of the bound and hybrid composites.
Table 7 indicates that the bound or hybrid composites are more thermally stable than the loose fiber polymers since their thermally stable temperatures are 272.8 °C, 275.8 °C, and 287.8 °C for sample numbers 2, 4, and 5, respectively, higher than that for the loose PALF and DPSF. This observation is due to the binders.
The bound and hybrid composites are thermally stable at higher temperatures above 200 °C, which gives them the potential to be used as safe insulation materials for buildings and other engineering applications.
Figure 19a shows the moisture content profiles for the loose PALF (Lo # 1) polymer, bound (Bo # 2), bound of DPSF (Bo # 4), and hybrid composite sample numbers 5, 6, and 7 until they reach a steady state condition. This figure ensures that the PALF has a low percentage of about 4% moisture content. It is also observed that all the bound and hybrid composites have much lower moisture content (less than 2%) since most of the void porous spaces of the loose fibers are filled by the binder and hence reduce their ability to absorb more moisture. Therefore, these low moisture contents are much below the 16% that presents safe moisture content, as suggested by Bainbridge [
50] for similar natural straw fibers. The moisture content for the loose DPSF polymer (Lo # 3) is presented in
Figure 19b since it reaches a steady state at a longer time of about 2.5 hours.
Figure 20 shows a bar chart of moisture content percentage for all samples at steady-state conditions for easier comparison.