New Bound and Hybrid Composite Insulation Materials from Waste Wheat Straw Fibers and Discarded Tea Bags

Abstract

This study utilizes waste wheat straw fibers and discarded tea bags as novel raw materials for developing new thermal insulation and sound absorption composites. Wood adhesive (WA) is used to bind the polymer raw materials. Loose polymers and different composites are experimentally developed in different concentrations. Sound absorption and thermal conductivity coefficients are obtained for the developed boards. Bending moment analysis and the moisture content of the boards are reported in addition to a microstructure analysis of the straw fibers from wheat. The results indicate that as the wheat straw fiber’s percentage increases in the composite, the thermal conductivity coefficient decreases, the flexure modulus decreases, the sound absorption coefficient increases at some frequencies, and the moisture content increases. The range of thermal conductivity and the noise reduction coefficient are 0.042–0.073 W/m K and 0.35–0.6 at 24 °C for the polymer raw materials, respectively. The corresponding values for the composites are 0.054 and 0.0575 W/m K and 0.45–0.5, respectively. The maximum moisture content percentages for the polymers and composites are 6.5 and 1.15, respectively. The composite flexure modulus reaches maximum and minimum values of 4.59 MPa and 2.22 MPa, respectively. These promising results promote these polymer and composite sample boards as more convenient insulation materials for green buildings and could replace the conventional petrochemical thermal insulation ones.

1. Introduction

Many food residues and/or by-products of agricultural waste can be considered as novel raw materials for new industrial products. Wheat straw fibers are a biodegradable natural waste material. Yemis and Mazza [1] reported that six hundred and eighty-one million metric tons of wheat (grain) is produced annually. This amount produces 600–900 million metric tons of wheat straw as a by-product. Another recent study by Sajid et al. [2] showed that 1140.0 million metric tons of wheat straw is produced annually worldwide. In Saudi Arabia, 2.5 million tons is produced annually (Backer [3]). Another tremendous source of waste is discarded tea bags. Wang et al. [4] showed that the production of tea could reach 7.4 million tons by 2025, and current global consumption has reached 5.82 million tons worldwide. In addition, six million tons of tea was consumed in 2017 (Sel et al. [5]). These huge biodegradable disposable wastes, if not utilized efficiently, end up in landfills and increase negative environmental impacts and pollution. Therefore, the motivation of the current study is to manage the use of such waste raw materials in developing thermal insulation and sound-absorptive boards, which could be used in architectural applications and construction buildings.

Liu et al. [6] developed insulation materials from wheat straw fibers as an aggregate and geo-polymer as a resin with a variable fiber length and other parameters. The conductivity coefficient of these materials was 0.092–0.186 W/m K for a density range of 235–894.1 kg/m³. The thermal and mechanical characteristics of straw-based materials in buildings were reviewed by Tlaiji et al. [7]. They suggested developing a thermal model for straw fiber and improving its acoustics to increase its sound absorption coefficient. Thermal insulation boards were constructed from corn husk and wheat straw residual fibers by Rojas et al. [8]. The boards’ thermal conductivity coefficients ranged from 0.046 to 0.047 W/m K. Csanády et al. [9] reported the conductivity of wheat straw bulk raw material in the range of 0.0436–0.0496 W/m K for a density range of 80–180 kg/m³ at 10 °C, respectively. Soto et al. [10] studied a mixture of recycled paper and wheat straw and found that its conductivity was less than 0.041 W/m K for a range of density from 50 to 100 kg/m³. Hybrid composite boards of wheat straw and eucalyptus globulus leaves were developed on a laboratory scale using cornstarch resin by Ali et al. [11]. Their study reported that the conductivity coefficient of the leaves and wheat straw fibers was between 0.045 and 0.055 W/m K, and that of the composites was lower than 0.065 W/m K. The boards’ sound absorption coefficient had values of >0.5 in the frequency range from 500 to 1600 Hz. Ali et al. [12] constructed thermal insulation and sound-absorbing boards made of agave and wheat straw fibers. The average conductivity coefficient of the boards was 0.04555–0.06835 W/m K in a range of 10–60 °C, respectively. New insulation composite hybrid boards were made of date palm tree leaves and wheat straw fibers by Ali et al. [13]. The boards’ average conductivity coefficients were 0.045–0.065 W/m K using different binders. The range of sound coefficients was >0.6 for a frequency range greater than 900 Hz.

Jin-shu et al. [14] used waste tea leaves to form boards using urea-formaldehyde as an adhesive binder. The study’s objective was to reduce emissions from the formaldehyde adhesive boards by using waste tea leaves, which reacted with the formaldehyde to reduce such emissions. Tutuş et al. [15] produced paper and pulp from waste tea leaves using the kraft-anthraquinone method. They found an enhancement in the paper properties by adding Turkish pine pulp. To reduce clean clay consumption and make the brick body lighter, Ozturk et al. [16] used tea waste at various concentrations to enhance the properties of brick clay mixtures. It was found that using tea waste at 12.5% reduced the thermal conductivity coefficient of the brick by 42%. Bagheri et al. [17] showed that adding 5% of tea waste to nano clay and melted polypropylene enhanced the sound coefficient at a low frequency. If this percentage increased to 60%, then the improvement in the sound absorption coefficient occurred at 1000 Hz in the range of 2500–3000 Hz. Lightweight bricks were produced by Crespo-Lopez by adding tea waste at different percentages [18]. They found a reduction in the conductivity coefficient of these bricks due to a porous increase. Recently, Ali et al. [19] developed composite insulation samples of date palm tree surface fibers and wasted black tea bags. The conductivity coefficient of the composites was less than 0.07 W/m K at 24 °C and the noise coefficient was >0.37.

The present study reports new polymer and composite boards made of wheat straw and discarded tea bags. These biodegradable, eco-friendly boards have a low conductivity coefficient and a high noise reduction coefficient, which makes them suitable for thermal insulation and sound absorption in building applications. These boards could replace synthetic and other petrochemical thermal insulation materials, moving toward green buildings.

2. Materials

The two waste raw materials used were wheat straw fibers (WSFs) and discarded tea bags (TBs). Wheat straws were obtained from the nearby market area, and the TBs were collected from the households of the authors’ team and nearby cafes. The wheat straw fibers were cut into pieces between 10 and 15 cm. Both wastes were washed using running water to clean them of dust and other impurities. The raw materials were dried in an electric convection oven for 12 h at 90 °C. Figure 1a,b show samples of the used WSFs and TBs.

3. Method

Each loose raw waste material was gathered in a wooden box measuring 28.5 × 28.5 × 4.1 cm³ and 24.5 × 24.5 × 2.2 cm³ for the TBs and WSFs, respectively, to prepare for conductivity coefficient measurement. Wood adhesive (WA) (safe and non-toxic Polyvinyl Acetate Resin) was used as a binder for the raw materials. The resin solution consisted of one part binder to two parts water by mass. The TBs and WSFs were immersed in the binder solution. Then, the bound WSFs and TBs were moved to different stainless-steel boxes measuring 30 × 30 × δ cm³ and compressed to a specified thickness δ using a mechanical press at 1.9 kPa. After compression, the stainless-steel boxes were moved to the drying oven. The drying process took 72 h at 90 °C, until there was no more condensed water on the inner glass surface of the oven. The next step was to move the sample from the stainless-steel box to measure its conductivity inside the heat flow meter. The last step was to cut the samples into three pieces; two of them were used for the sound absorption coefficient measurement and the third for the bending moment test. Hybrid thermal insulation boards were made by mixing a specific dried known mass of both WSFs and TBs according to

Table 1. Specifications of the polymer and composite samples.

Material	Board Number
	TB Loose (# 1)	TB Bound (# 2)	WSF Loose (# 3)	WSF Bound (# 4)	Hybrid (# 5)	Hybrid (# 6)	Hybrid (# 7)
TB %	100	65.4	0.0	0.0	48.0	33.0	17.0
WSF %	0.0	0.0	100	62.0	16.0	33.0	53.0
Resin ratio to the total mass %	0.0	34.6	0.0	38.0	36.0	34.0	30.0
Thickness (mm), δ	41.0	18.0	22.0	51.0	30	34.0	50.0
The volume of the sample (cm3)	3330	1620	1500	4590	2700	3060	4500
Fig. #	Figure 1b	Figure 2a	Figure 1a	Figure 2b	Figure 2c	Figure 2d	Figure 2e
Density of dried samples (kg/m3)	132	472	78.0	177	291	250	158
Total dried mass (g)	439	765	117.0	810	786	764	711

. The mixed hybrid mass was immersed in the resin solution and then moved to a stainless-steel box in a similar way to the bound boards. After drying, the hybrid sample was moved from the box and scaled again to obtain the mass of the polymerized resin (shown in Table 1). The next step was to perform different laboratory tests on the hybrid dried sample boards, as shown next. Figure 2 shows the developed bound and hybrid composite samples.

4. Test Analyses and Characterization

4.1. Scanning Electron Microscopy Analysis (SEM)

The surface morphology of the loose polymer of wheat straw fibers (WSF L, # 3) at different magnifications was determined by SEM of type (JEOL; JSM7600F). The loose WSFs were initially coated with platinum in order to avoid any electrostatic charging during scanning. The TBs’ surface morphology was reported in our previous publication [19].

4.2. Thermal Conductivity Measurement

The conductivity coefficient (λ) of the constructed boards, either loose or composite, was determined using a disk top heat flow meter (HFM) (Lambda, HFM 436). The wooden box mentioned earlier in the method section was used to hold the loose polymers inside the heat flow meter, and composite samples measuring 30 × 30 × δ were used (before cutting them into specimens for the SAC and BM tests (Figure 2)). It should be noted that Lambda is designed to hold samples of any size, specified by 30 × 30 × δ cm³, where δ is the thickness (max. 10 cm). The device has a self-automated sensor for thickness detection. The device follows the ASTM-C518 [20] standard. The HFM has accuracies of ±1% to 3% W/(m K) and ±0.01 °C for conductivity and temperature measurements, respectively. The conductivity is reported at 20, 30, 40, 50, 60, 70, and 80 °C for all boards.

4.3. Sound Absorption Coefficient (SAC) Determination

The impedance tube test system provided by BSWA (BSWA Technology Co., Ltd., Beijing, China) is shown in Figure 3. This system consists of 10 cm and 3 cm diameter tubes used to measure the SAC at a wide range of frequencies from 63 Hz to 6300 Hz. Samples measuring 10 cm and 3 cm in diameter were prepared, as shown in Figure 2, marked by SAC. More details about the principle of operation and the microphone’s position can be found in [21]. The designed software VA-Lab IMP (Ver: V1.03) by BSWA for measuring the SAC follows the 10534-1 [22] and 10534-2 [23] ISO standards.

4.4. Bending Moment (BM) Test

A BM (three-point) analysis was performed on the composite sample boards. The specimens used for this test are shown in Figure 2, marked by BM. The test follows the ASTM D790-03 [24] standard. A universal testing machine of type (UTM, INSTRON 5984) is shown in Figure 4. This machine has a 2 mm/min crosshead speed. The deflection D, flexural strain ϵ_f, force F, and flexural stress σ_f were recorded by the machine. The mechanical properties are defined by Equation (1), where E_f is the elastic modulus.

$${\sigma }_f={\displaystyle \frac{3FL}{2b {\delta }^2}} , {ϵ}_f={\displaystyle \frac{6 D\delta }{L^2}} , E_f={\displaystyle \frac{L^{3}S}{4b {\delta }^3}}$$

(1)

where L, δ, and b stand for the span, thickness, and width of each specimen, respectively, and S presents the slope, which is determined from the force–deflection curve.

Table 2. Specimens’ dimensions used for BM.

Specimen No.	Thickness (δ) (mm)	Width (b) (mm)	Span (L) (mm)	Slope (S) (N/mm)
2	18	47.34	150.0	3.193
4	51	50.70	150.0	10.185
5	30	57.84	150.0	8.839
6	34	51.51	150.0	4.908
7	50	52.20	150.0	16.565

presents the dimensions of the BM samples.

4.5. Moisture Content

A small piece of each sample was dried at 100 °C in an oven for eight hours. Then, its mass was defined as m2. The piece was allowed to cool down in the laboratory under environmental conditions (relative humidity of 51.7% and temperature of 21.6 °C), where its mass was recorded every five minutes and named m1. The process continued until m1 reached a steady value. The amount of moisture absorbed by each small piece of mass was recorded as a percentage following Equation (2). The moisture content process follows the ASTM D2974-07A standard [25].

$$\% \mathrm{of} \mathrm{moisture}={\displaystyle \frac{m1-m2}{m2}} \times 100$$

(2)

5. Results and Discussion

Figure 5a,b,d present a wheat straw fiber’s cross-section at magnifications of 43 and 100 in (a) and (b), respectively, using SEM images. Figure 5a indicates that each fiber has a big hole at the center, surrounded by multicellular tubes. Figure 5d shows that the minimum and maximum diameters of the multicellular tubes are 32.1 μm and 65.4 μm, respectively. Figure 5c shows that the average outside diameter of the fiber is 1.98 mm. It should be mentioned that the presence of these tubular multicellular cells has a great reducing effect on the conductivity coefficient when the straw fibers are used for thermal insulation, as will be shown next.

Figure 6 presents the λ profiles for all samples in Table 1 at different temperatures ranging from 20 °C to 80 °C. Curve fittings through the data are presented by solid lines and their correlations are obtained by λ = (B × t) + A, where t is the temperature in degrees Celsius. Coefficient of determination (R²) constants A and B for each solid line fitting are shown in

Table 3. The constants and coefficients of determination for each sample fitting curve presented in Figure 6. (Different symbols refer to those that appeared in Figure 6).

Correlation	λ = (B × t) + A
Sample #	Symbol	A	B	R2, %	Density, kg/m3
1 (TBs loose)	ο	0.049	0.0002	92.6	132.0
2 (TBs bound)	●	0.064	0.0004	98.7	472.0
3 (WSFs loose)	◊	0.038	0.0002	99.0	78.0
4 (WSFs bound)	♦	0.046	0.0002	99.5	177.0
5 (Hybrid)	■	0.052	0.0003	99.1	291.0
6 (Hybrid)	▲	0.053	0.0002	98.7	250.0
7 (Hybrid)	+	0.051	0.0002	93.2	158.0

. Figure 6 also shows that the percentage increase in λ with a temperature ranging from 20 °C to 80 °C is 21.6, 26.2, 24.7, 23.2, 30.0, 23.6, and 19.6 for all samples from 1 to 7 in a row, respectively. It is observed that the value of λ depends on the resin mass, the void volumes, and the density of the sample. Thus, λ decreases when the polymerized binder is less, the density is lower, and there are more voids in the sample. Furthermore, it is noticed that as the percentage of WSFs increases in the composite samples, keeping the binder ratio almost the same, the λ decreases, as shown for composites number 5, 6, and 7. Figure 7 is constructed using the data of hybrid composites number 5, 6, and 7 to confirm the effect of polymerized resin and density on the λ profiles. This figure shows that λ rises with an increasing density and polymerized binder at a constant temperature. In addition, Figure 8a,b show another example of λ increasing between the loose polymer and its bound composite at different temperatures. Figure 8a shows the loose polymer of TBs (sample # 1) and its bound composite (sample # 2), and Figure 8b shows the loose polymer of WSFs (sample # 3) and its bound composite (sample # 4). A higher binder content typically increases the composite’s density, rigidity, and λ values. It should be noted that each curve fitting in Figure 6 presents one polymer or composite sample, however, the dashed lines in Figure 7 and Figure 8 connect different composites (Figure 7) or connect between loose polymers and their bound composites (Figure 8).

Table 4. Comparison between the newly constructed composite boards and conventional insulation materials.

Materials	Density (kg/m3)	Thermal Conductivity Coefficient, λ (W/m K)	Ref.
TBs (# 1), loose	132	0.054–0.067	This article
TBs (# 2), bound	472	0.073–0.095	This article
WSFs (# 3) loose	78.0	0.042–0.054	This article
WSFs (# 4) bound	177.0	0.049–0.062	This article
(# 5), hybrid	291.0	0.059–0.076	This article
(# 6), hybrid	250.0	0.058–0.072	This article
(# 7), hybrid	158.0	0.054–0.065	This article
Commercialized recycled (PET)	15–60	0.034–0.039	[26]
Recycled glass fiber produced for commercial use	100–165	0.038–0.050	[26]
Polyurethane foam	30–80	0.02–0.027
Rock wool	40–200	0.033–0.040	[26]
Expanded polystyrene (XPS)	15–35	0.031–0.038	[26]
Extruded polystyrene (EPS)	32–40	0.032–0.037	[26]
Kenaf	30–180	0.034–0.043	[26]
Sheep wool	10–25	0.038–0.054	[26]
Recycled Polyethylene Terephthalate (PET)	30	0.0355	[27]

presents a comparison between the current developed composites and conventional thermal insulation materials. Although conventional materials have a lower λ, producing the current boards is more cost-effective. In addition, producing the current boards helps to lower the environmental impact, while manufacturing the conventional ones produces CO₂, since they are fossil fuel products and, therefore, pollute the environment and make it worse.

Figure 9a,b show the SAC in a frequency range of 100 Hz–6000 Hz on a linear-log scale. Figure 9a shows that bound samples 2 and 4 are composites of a single raw material and the binder. Sample 2 (TBs bound) has a good SAC above 0.4 for a frequency between 850 Hz and 2700 Hz and reaches a maximum of 0.9 at 1750 HZ. On the other hand, sample 4 (WSFs bound) has a better, wider range (above SAC = 0.4) from about 425 Hz to 6000 Hz and reaches a maximum of about ≈1 at 800 Hz and 4000 Hz. The reason that sample 4 has a better performance than sample 2 can be attributed to the fact that WSFs have a lot of void channels, as shown from the SEM picture in Figure 5a,b,d, which allow for sound wave absorption. Figure 9b presents the SACs for the hybrid composites # 5, 6, and 7. In the communication range, it is noticed that samples 5 and 7 have a good SAC above 4.0 starting at about 650 Hz. However, sample 6 starts to have a good SAC at 475 Hz. Furthermore, beyond the communication range, all composite hybrid samples exhibit a good SAC above 0.4. Moreover, samples 5, 6, and 7 have bell mouth peaks in the communication range at 1000 Hz, 850 Hz, and 950 Hz, respectively.

Another important parameter in the acoustic analysis is the noise reduction coefficient (NRC), which is obtained as the one-third octave value of the SAC at 250, 500, 1000, and 2000 Hz following [28,29]. Figure 10 shows NRC bar charts for all composite samples 2, 4, 5, 6, and 7. Sun et al. [30], Sharma et al. [31], and Zhang et al. [32] reported that materials with an NRC of >0.2 are considered sound-absorbing ones and that those with a value greater than 0.4 have practical value. Based on these classifications, samples 4, 5, 6, and 7 have practical value since their NRCs are >0.4, as shown in Figure 10. Also, sample 2 represents a good absorbing material with an NRC = 0.35.

Figure 11a,b show the force–deflection and stress–strain profiles for all composites, respectively. The slope of each sample is calculated from the linear region shown in the force–deflection profiles in Figure 11a. Figure 12a–c present the calculated values of the flexural elastic modulus E_f, flexural stress σ_f, and flexural strain $ϵ$_f for each composite sample, respectively.

It is worth mentioning that E_f increases as the density of the sample increases, either for the raw bound composites (2 and 4) or the hybrid ones (5, 6, and 7), which is in line with the results of [33,34]. Furthermore, increasing the percentage of polymerized resin in the sample and reducing its thickness (high degree of compactness) has a better enhancement effect on the flexural elastic modulus. This observation is clear when comparing the raw bound composite sample numbers 2 and 4, where number 2 has a high density and a high degree of compactness (small thickness) with almost the same percentage of binder. The same behavior could be applied to composite hybrid samples 5 and 6 compared to sample 7.

Figure 13a shows the steady-state moisture profile test for raw polymer materials and Figure 13b shows the same for the bound and hybrid composites. The moisture profiles in Figure 13a indicate that wheat straw fibers (# 3) absorbed 6.5% moisture, while TBs (# 1) absorbed 4%. This is due to the higher porosity found in the wheat straw fibers, as shown by the SEM morphology in Figure 5a,b,d, which allows for the absorption of more moisture to fill the void channels. Figure 13b clearly shows that as the percentage of WSFs increases in the composite, the moisture content absorbed by the composite increases. This increase in moisture content is solely because of increasing the WSFs in the composite, since the binder ratio is in the order of 30% for all composites. Therefore, bound composite samples # 2 (TBs bound) and # 4 (WSFs bound) have a moisture content of about 0.75% and 1.15%, respectively. Moreover, hybrid composite samples number 5 and 7 have about 0.9% and 1.1% moisture contents, respectively. Nevertheless, all polymers and composites have moisture content percentages well below the safe 16%, consistent with Bainbridge [35] for similar natural fibers. It should be noted that increasing the moisture content has a worse effect on thermal conductivity and, hence, increases its value. Therefore, the water content in the sample should be kept to a minimum to obtain better insulation materials. Figure 14 shows a moisture content bar chart comparison of all polymers and composites.

6. Conclusions

Agro-waste by-products of wheat straw fibers and discarded tea bags were used as raw materials in developing eco-friendly materials to regulate inside temperature and reduce noise level. The thermal conductivity coefficient was in the range of 0.042–0.052 W/m K, 0.072–0.095 W/m K, and 0.052–0.077 W/m K for loose wheat straw fibers, loose TBs, and different hybrid composites in a temperature range of 20–80 °C, respectively. The SAC was above 0.4 for the composite hybrid boards above 650 Hz, for frequencies of >400 Hz for bound WSFs, and for the frequency band of 850–2500 Hz for bound TBs. The flexure modulus for composite samples had maximum and minimum values of 4.59 MPa and 2.22 MPa, respectively, and decreased as the percentage of WSFs increased in the composites. The composite samples exhibited almost hydrophobic behavior, as they had a reduced porosity (low moisture content). These conclusions show an enhancement in thermal conductivity coefficients, acoustic characteristics, mechanical properties, and the ability to absorb moisture, which promotes the new composites for use in the construction of green buildings. These composites could be used as sandwich wall panels and for ceiling sound absorption, replacing the conventional insulation materials derived from fossil fuel products. In addition, these developed composite boards are biodegradable, have a low environmental impact, have a low cost compared to rock and glass wool insulations, and are available as renewable natural resources in tremendous amounts.