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Article

Development of Lignocellulosic-Based Insulation Materials from Agave fourcroydes and Washingtonia filifera for Use in Sustainable Buildings

by
Felipe Rosas-Díaz
1,
David Gilberto García-Hernández
2 and
Cesar A. Juárez-Alvarado
1,*
1
Facultad de Ingeniería Civil, Universidad Autónoma de Nuevo León, San Nicolás de Los Garza 66451, Mexico
2
Facultad de Ciencias Biológicas, Universidad Autónoma de Nuevo León, San Nicolás de Los Garza 66451, Mexico
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(13), 5455; https://doi.org/10.3390/su16135455
Submission received: 27 May 2024 / Revised: 20 June 2024 / Accepted: 21 June 2024 / Published: 27 June 2024
(This article belongs to the Special Issue Build It Green—Sustainable Structural and Environmental Studies)
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Abstract

:
The objective of this work was to develop two prototypes of insulating materials based on natural plant fibers from Agave fourcroydes and Washingtonia filifera, available in Mexico, for their potential use in buildings. For the development of the prototypes, the raw materials were characterized by physical, chemical, and microstructural methods. The samples were prepared by a pulping process after boiling the fibers in a sodium hydroxide solution. We worked with a Taguchi experimental matrix of four variables in three levels, defining as response variables the sample’s thermal conductivity, density, and flexural strength. The results show that the henequen-based insulation obtained a density of 69.8 kg/m3 and a thermal conductivity of 0.0367 W/mK; on the other hand, the palm-based insulation obtained a density of 45.06 g/cm3 and a thermal conductivity of 0.0409 W/mK, which in both cases are like the conductivity values reported by conventional insulating materials, such as expanded polystyrene or mineral wool, and therefore both optimized prototypes are promising as thermal insulators with a high potential to be used in sustainable buildings in Mexico, reducing the energy consumption of air conditioning and the environmental impact associated with the production of materials.

1. Introduction

The energy consumption of buildings and pollutant emission patterns have become a matter of international concern, prompting the consideration of various energy-saving policy measures in numerous countries [1,2]. Energy, as a fundamental resource, underpins the economic growth and social dynamics of any city or region. Consequently, sustainable housing codes are gaining traction in Europe, North America, and Latin America. Studies indicate that the efficient use of energy has implications for energy use at different urban scales and in different communities [3]. Globally, buildings account for 45% of primary energy resources [4]. It is estimated that 10% to 20% of the energy goes into the manufacturing of all materials used in construction, while 80% to 90% of the total energy is used in the operational phase of the life cycle [5,6]. With air conditioning consuming the largest amount of energy in this phase (48%), it is evident that the operational phase of the life cycle is a significant contributor to the overall energy consumption of buildings. However, these values are highly variable depending on the geographical location where the building is located, resulting in different energy use patterns [7,8].
Reducing energy use necessitates an examination of the building’s various components, including materials, structures, their relationship with the environment, and the behavior of its occupants. This entails introducing the physics of buildings to the external environment, as they behave as open thermodynamic systems in a non-equilibrium situation capable of exchanging mass/energy with their environment, which is always in a dynamic situation. Consequently, the technologies currently selected in buildings have immediate consequences concerning energy consumption and their emission patterns [1].
One of the most effective ways to obtain energy savings is to improve the thermal insulation of buildings. This is achieved by reducing losses through the thermal envelope [9,10,11]. However, conventional building materials do not have good thermal insulation properties, which is why additional thermal insulation materials are used [12]. A material is considered a thermal insulator when its thermal conductivity is less than 0.07 W/mK [13]. Some of these materials reach 0.035 W/mK [14]. Based on this property, the processing of organic materials gave rise to the first insulating panels in the 19th century. This led to an increasing range of artificial materials that raised concerns about their excessive use and probable depletion [6]. Natural insulation materials offer numerous advantages over other materials and are therefore the most promising for construction. In this context, the shift from non-renewable materials (petroleum-derived resources) to renewable ones (biomass-derived resources) is becoming a current line of research regarding cleaner production and revaluation of lignocellulosic biomass [15,16,17,18].
The agro-industry is a sector that meets the demand for food supply, which, once processed, leaves residues that can be used in the construction industry [14,19,20]. These residues provide a solution to the problem of poor management of their final disposal, reducing energy losses in buildings and reducing the environmental impacts associated with the manufacturing of synthetic materials [21,22,23]. Natural fibers can be found in a wide variety of morphologies, and their surfaces can be easily modified to have a more hydrophilic or hydrophobic character [23]. The principal advantages of their use are cost-effectiveness and availability. They have low specific gravity, are renewable and biodegradable resources, consume less energy for their production and therefore have low CO2 emissions, require simple and environmentally friendly processing methods, have excellent electrical resistance, and have relatively high thermoacoustic insulation characteristics [24,25]. This is due to their low bulk density and cellular structure, which is formed by fibers of varying length, thickness, and orientation [23,26,27,28,29]. The cost and availability of natural fibers are highly dependent on location, region, import markets, and competing applications [9,27,30].
Natural fibers that are commonly available and exploited for application in building materials include hemp, straw, flax, wood, coconut, sisal, and sunflower, among others [12,23,31,32,33]. Those obtained from agricultural by-products present additional advantages from an economic standpoint. The reported values of thermal conductivity vary considerably, with values as low as 0.035 W/mK for pineapple leaves [34] and as high as 0.182 W/mK for a prototype based on banana fiber and polypropylene [35]. The considerable range of results observed for thermal conductivity and densities is dependent on several variables that have been identified as influencing the production process of the prototypes that have been studied. These include the intrinsic properties of the raw materials, the binder used, the dosage, the processing of the fibers, and several other factors [5,9,14,28,31,36,37]. Although there is a wide range of fibers that have been studied for the development of materials, it is still necessary to study fibers endemic to each region to diversify raw materials, reduce transportation costs, and strengthen the local industry. Two natural fibers that have not been extensively studied in Mexico for these applications are palm fiber and henequen. It is estimated that each palm tree produces approximately 20 kg of dry leaves per year as residue, which are considered waste and are therefore collected and dumped in established sites [36]. In contrast, henequen is a widely exploited resource in Mexico, with an estimated 6461 hectares of cultivation in the Yucatan and Tamaulipas areas, which yields 12,813 tons of fiber for various uses [38].
The objective of this research was to develop two prototypes of insulating panels based on Washingtonia filifera and Agave fourcroydes fibers for consideration as construction materials for housing. The utilization of these fibers resulted in the production of environmentally friendly materials and improvements in the energy performance of the buildings.

2. Materials and Methods

2.1. Materials

Two types of natural fibers were selected for this study: Agave fourcroydes fiber, commonly known as henequen, and Washingtonia filifera leaf fiber, commonly known as palm. Henequen fiber was procured from local suppliers in the Yucatan area, where this resource is extracted from the plant’s stalk and sold as raw material for various uses (Figure 1a–c). The palm leaf fiber was collected manually in the city of Monterrey in the state of Nuevo Leon as soon as it fell from the tree naturally. Both fibers were washed superficially, dried in an oven at 80 °C, and stored in airtight bags (Figure 1d–f) [39].

2.1.1. Chemical Characterization

The chemical characterization of the natural fibers was conducted by the TAPPI procedures (Technical Association of the Pulp and Paper Industry, 2007) and experimental methods described by Wise L.E. and Rowell [39,40]. The samples were ground and the fraction between 0.4 and 0.25 mm was selected [41]. Ash content [42], ethanol−toluene extractables [43], and acid insoluble lignin content [44] was determined by TAPPI procedures, the holocellulose content was determined by Wise’s method [45], and the α-cellulose and hemicellulose content were determined by Rowell’s method [46].

2.1.2. Physical Characterization

Fiber density was determined using a Quantachrome Instruments Multipicnometer, with N2 serving as the purge gas. Thermal conductivity was measured by the “Transient Line Heat Source” method, employing the TEMPOS (Meter Environment) thermal property analyzer that adheres to the specifications of IEEE 442-2017 [47] and ASTM D5334-00 standards [48]. The heating and cooling cycle was defined as 10 min [39,40,49].

2.1.3. Microstructural Characterization

An X-ray diffraction (XRD) analysis of the fibers was performed in the 2θ range between 10 and 80° using a PERT Pro MRD X diffractometer with CuKα λ = 1.5405 Å radiation. The data were obtained with a rotation speed of 15 rpm and a pitch size of 0.0508714°. Furthermore, the percentage of crystallinity (Crl) was calculated using Equation (1), where I002 corresponds to the maximum intensity observed in the graph and Iam is the intensity recorded at 2θ = 18°. The chemical composition of the fibers was obtained by mass-energy dispersive X-ray fluorescence analysis under a helium atmosphere with PANalytical-Epsilon 3 equipment [50]. FTIR was performed using an Agilent Cary 630 spectrometer with the Diamond ATR sample interface. The resolution of the spectrophotometer was set at 4 cm−1, and 140 scans were obtained in the region between 4000 and 400 cm−1 [23,39,51]. In addition, thermogravimetric analysis was conducted on samples of each fiber at temperatures ranging from 25 to 900 °C at a heating rate of 10 °C/min in a N2 atmosphere [39,51,52]. The fibers were subsequently visualized by SEM (model JSM-6510LV; JEOL, Tokyo, Japan) to analyze their morphology and were mounted on carbon-adhesive tape supports [51,53].
C r l   [ % ] = ( I 002 − I a m ) I 002 × 100

2.2. Experimental Design

2.2.1. Experimental Matrix

The present study employed an experimental matrix developed using an L9 orthogonal arrangement for each fiber in triplicate (27 samples per fiber). The matrix included four variables at three levels, as detailed in Table 1. The objective was to generate prototypes with optimized properties, exhibiting an appropriate balance between density and thermal conductivity while maintaining sufficient consistency to be utilized as a construction material in the fabrication of vertical construction elements [14,39].
To assess the significance of the entry variables in the insulation density and thermal conductivity for palm fiber and henequen fiber, an analysis of variance (ANOVA) was performed using R [54]. First, four linear models were constructed to predict density and thermal conductivity for each fiber using four dependent entry variables: fiber length, boiling time, NaOH concentration, and blending time. This was achieved using the function lm from the stats package. Subsequently, an ANOVA with 95% confidence was conducted for each model using the anova function. Finally, to ascertain the relative contribution of the variables, the calc.relimp function from the relaimpo package [55] was employed.

2.2.2. Prototype Manufacturing

In the previous step of the manufacturing process, 70 g of the fiber was previously dried in an oven at a temperature of 80 ± 5 °C for 24 h. Subsequently, the fiber was cut into lengths of 3, 6, or 9 cm depending on the arrangement corresponding to the variable. Subsequently, the samples were introduced into a sodium hydroxide solution (at a concentration of 0.5, 1, or 1.5%, as appropriate) and subjected to a specified boiling time (30, 45, or 60 min) with a Cimarec hot plate magnetic stirrer. Subsequently, the fibers were then blended to a pulp (3, 6 or 9 s, depending on the case). Finally, they were washed until a neutral pH was obtained. They were then poured into circular stainless-steel molds with a diameter of 8 inches and dried in an oven at a temperature of 80 ± 5 °C for 24 h. The process is described in Figure 2.

2.3. Response Variables

2.3.1. Density and Porosity

The bulk density was determined in accordance with the specifications set forth in EN 160 [56], which stipulates that the value be obtained by measuring the sample on a balance with an accuracy of ±0.1 g. The volume was calculated by measuring the thickness with a vernier according to EN 823 [57] and using the known diameter of the circumference (8 inches). Subsequently, the bulk density ( ρ b ) was calculated using Equation (2) [5], [28].
ρ = m v  
where m is the mass of the prototype in grams and v is the volume in cm3. The real density ( ρ r ) was determined using a Quantachrome Instruments Multipicnometer. Finally, the percentage of porosity (P) of each prototype was calculated using Equation (3) [5].
P   [ % ] = 100 × 1 − ρ b ρ r

2.3.2. Thermal Conductivity

The thermal conductivity was determined by averaging at least five measurements on each sample (taken at a temperature of 21 °C and a relative humidity close to 40%) using the transient line heat source method with Tempos equipment of the Meter Group Brand [14,39]. The KS-3 probe was utilized, collecting data over a 10 min measurement cycle. The procedure conformed to the stipulations of IEEE 442-2017 [47] and ASTM D5334-00 [48].

2.3.3. Flexural Strength

The flexural strength properties were studied according to ASTM C578-19 [58], which stipulates the procedure for testing polystyrene samples. The results were then compared with those of a commercial material. In this test, six measurements were performed to obtain the maximum bending stress at 5% deformation [39]. The test site conditions were 20 ± 1 °C and 40 ± 5% relative humidity.

3. Results and Discussion

3.1. Raw Materials

Natural fibers are composed of cellulose, lignin, and hemicellulose, in addition to other minor components, such as water, proteins, peptides, and inorganic compounds [59,60]. Table 2 presents the results of the chemical characterization of henequen and palm fiber. The results indicate that the primary component of each fiber is cellulose, with a percentage composition of 33.64% for henequen and 46.43% for palm fiber. The difference in results is related to the intrinsic characteristics of each species, the part of the plant being analyzed, as well as environmental aspects [61]. For example, the lignin content is higher in the palm leaf fiber, corresponding to 36.04%. This can be associated with a higher fiber stiffness, since the henequen filaments are extracted from the pith of the plant and therefore from the less stiff zone [14,39]. Furthermore, non-volatile compounds extractable with organic solvents were identified in 7.59% of the henequen filaments and 13.33% of the palm fibers.
Table 3 provides an overview of the most used fibers that have been subjected to chemical characterization. The cellulose content of henequen fiber is comparable to that reported for hemp [62,63], oats [30], and coconut [64]. In contrast, palm fiber has a percentage similar to that reported for flax [65,66], sunflower [63,65], wheat straw [63], and sisal [30,64].
The intrinsic thermal conductivity and density of both raw materials were measured to ascertain their suitability for use in the development of a thermal insulation material. As shown in Figure 3, the density of palm fiber was found to be 0.8289 ± 0.0046 g/cm3, while its thermal conductivity was determined to be 0.0409 ± 0.0071 W/mK. These values are comparable to those obtained by sunflower fiber [70,71] and coconut fiber [51,64,72,73,74]. In contrast, henequen fiber exhibited a density of 1.2890 ± 0.0581 g/cm3 and a thermal conductivity of 0.0255 ± 0.0011 W/mK, rendering it comparable to flax fiber [27,51,70,73,74,75]. The values observed in both fibers are suitable for their application in thermal insulation, as they are comparable with those reported for other fibers that have been investigated for similar applications, including sugarcane bagasse [13], rubber, recycled paper [73,74], reed [76], and sargassum [77]. In general terms, both values fall within the characteristic range of lignocellulosic fibers. However, the density is considerably higher than that of conventional insulating materials such as expanded polystyrene, which has a density range of 10 to 30 kg/m3. It is therefore necessary to conduct further research to determine whether this has any effect on thermal inertia [5].
Lignocellulosic fiber structures exhibit three distinct types of ordered and stable molecular conformations, with cellulose being the most prevalent [61]. Figure 4 illustrates the X-ray diffraction curves obtained for the two raw materials under study. Broad signals were identified which are associated with the cellulose crystals that are characteristic of natural vegetable fibers [52,78,79].
The graphs of henequen and palm leaf fiber demonstrate that the maximum peaks recorded are 196,880 u.a. and 198,430 u.a., respectively. The intensities at 2θ = 18° are 144,444 and 124,988, resulting in a percentage of crystallinity of 26.6355% for henequen and 37.0156% for palm leaf fiber [78]. The calculated values are relatively low in comparison with other fibers, such as hemp (87.9%) or sisal (75%), and can be attributed to the fact that they correspond to non-structural parts of the plant. However, for palm fiber, the result is consistent with that reported in the literature for a similar fiber (Borassus fiber with a percentage of crystallinity of 38.4%) [23,26]. The mineralogical composition indicates that both fibers exhibit a notable presence of sulfur trioxide (SO3), potassium oxide (K2O), calcium oxide (CaO), and iron oxide (Fe2O3). The values are presented in Table 4.
The primary components of plant fiber are α-cellulose, hemicellulose, and lignin, which were identified as being characteristic of plant fibers in the spectrum between 400 and 4000 cm−1 of the FTIR test, as illustrated in Figure 5.
The graphs obtained for both fibers are characteristic of lignocellulosic fibers; however, the magnitudes differ. The band recorded at 1027 cm−1 is indicative of C-O-C stretching in the xylan of hemicellulose [80]. The absorption band at 1700 cm−1 refers to a stretching of a C=O double bond, which is associated with acid-like carboxyl groups that are generally present in hemicellulose. The signal observed at approximately 2920 cm−1 corresponds to the stretching vibration of the C–H group [81]. Furthermore, a robust and extensive absorption band is observed between 3650 and 3000 cm−1, with a peak at 3350 cm−1. Additionally, a moderate peak at 1410 cm−1 corresponds to O–H stretching vibrations, which are indicative of the presence of hydroxyl groups in compounds such as cellulose, hemicellulose, and lignin [26,80].
The results of the TGA are presented in Figure 6, which depicts three principal mass losses associated with distinct processes as the temperature increases. The test indicates that the initial mass loss process occurs between room temperature and approximately 125 °C, with a loss of 6.1% for both fibers. This process is associated with dehydration and the loss of volatile substances [80]. In this temperature range, the fiber exhibits thermal stability associated with its chemical characterization (mainly lignin content) and its crystallinity index. Consequently, this value is lower than that reported for other fibers, such as blue agave, Brazilian banana, and sugarcane bagasse. However, it allows for the safe use of fiber in thermal insulation applications in residential buildings [52]. Subsequently, the most significant degradation occurs between 125 and 375 °C, which is associated with the elimination of biopolymers and the decarbonization of important substances, beginning with hemicellulose and continuing with α-cellulose. Finally, a gradual degradation occurs up to 900 °C, corresponding to the loss of fixed carbon, which is mainly associated with lignin [39,82].
Figure 7a–d corresponds to micrographs of the henequen fiber. As shown in Figure 7a,b, the cross-section reveals a highly compact cell wall structure with an approximate fiber diameter of 480 to 500 μm. Consequently, the low bulk density of the resulting composite is attributed to the interweaving of the strands, which allows for the formation of air cavities. The surface of the fibers (Figure 7c,d) exhibits a consistent degree of roughness, which would facilitate adhesion in its use as reinforcement in the manufacture of composites with a binder. It is therefore recommended to inquire into this type of fiber for multiple applications, and it also displays minor defects that are typically associated with the extraction and handling process [52,53,82,83]. Figure 7e–h depicts micrographs of palm leaf fiber, which exhibits a markedly distinct morphology, with a thickness ranging from 200 to 225 μm. The cross-section (Figure 7e,f) reveals the presence of minute cavities with diameters between 10 and 20 μm, which could serve as reservoirs for air bubbles, enhancing the fiber’s insulating capacity. In contrast, Figure 7g,h of the longitudinal plane (fiber surface) demonstrate a regular structure with minimal roughness and the presence of small impurities, which may be calcium oxalates, a common feature of natural vegetable fibers [53,82,83].

3.2. Experimental Design

The sample manufacturing procedure involves boiling the fiber in a sodium hydroxide solution at varying concentrations, which results in a modification of the manner in which the hydrogen bond is removed from the network, as described by Equation (4) [35,84].
F i b e r − O H + N a O H → F i b e r − O − N a + + H 2 O + S u r f a c e   i m p u r i t i e s
The consequences of exposing the fiber to the alkaline solution are the removal of surface impurities such as waxes and other non-cellulosic materials, which generates an external, rougher structure. Moreover, the fiber bundles are fragmented into smaller components in a process referred to as defibrillation [84]. The chemical composition of the fiber has been reported to undergo a general reduction in the percentage contents of hemicellulose and lignin according to the duration of the treatment. Conversely, the cellulose content is observed to increase [85,86].
A Fourier transform infrared (FTIR) analysis was conducted on both fibers prior to and following their exposure to the alkaline solution (Figure 8). In their original state, both fibers exhibited an O-H stretching band at 3350 cm−1, which is indicative of O-H bonding. This bonding is primarily associated with cellulose vibrations and hydrogen bonds of the hydroxyl groups, as previously reported in the literature [39]. The band at 2920 cm−1 indicated the presence of C-H bonds of methyl and methylene groups. The peak at 1700 cm−1 indicates the presence of C=O stretching in the acetyl groups of hemicelluloses, which is subsequently absent in alkali-treated fibers. This observation is consistent with the removal of hemicelluloses, as mentioned [87]. A similar phenomenon occurs with the peak visible at 1244 cm−1, which is associated with C-O stretching in hemicelluloses. This peak was subsequently not visible, confirming the lack of hemicelluloses. The band at 1027 cm−1 exhibited a gradual decrease with alkaline treatment, indicative of a reduction in lignin content [61,83,84,86].
The removal of surface compounds from the fiber allows for the exposure of the cellulose fibrils, which can interact with each other. The alkaline solution depolymerizes the cellulose molecules, breaking intramolecular hydrogen bonds and facilitating the modification of these bonds. This modification allows for the formation of non-covalent interactions, which are susceptible to the presence of moisture [60]. This reaction allows for the agglomeration of the fiber without the incorporation of additional external substances and confers rigidity to the developed material. The degree of rigidity is dependent on the NaOH concentration, the boiling time, and the blending time.

3.2.1. Orthogonal Array

The study, conducted using the Taguchi L9 matrix, revealed that, for each raw material, the optimal prototype corresponded to the third arrangement, as detailed in Table 5.
The results demonstrated that, in regard to palm-based material density, only fiber length was a significant contributor, with 1.54% (p < 0.05; see Table 6a). For thermal conductivity for palm fiber, the fiber length contributed to 24.65% (p < 0.001), followed by blending time with 22.73% (p < 0.001), NaOH concentration with 13.42% (p < 0.001), and boiling time with 8.5% of contribution (p < 0.01; see Table 6b). In the case of henequen-based material density, the boiling time was the most significant contributor with 24.21% (p < 0.001), NaOH concentration 19.6% (p < 0.001), blending time 18.58% (p < 0.001), and fiber length 14.32% (p < 0.001; see Table 6c). Concerning the thermal conductivity of the henequen fiber, it can be observed that there was no significant contribution (see Table 6d).
Another crucial aspect pertains to the yield of the fiber introduced into the process in comparison to that which results from the manufacturing of the prototype. In the case of the optimal prototype based on henequen fiber, its yield is 84.47%. In contrast, the yield of the prototype based on palm fiber is 52.57%. The recorded loss is associated with the manufacturing process. On the one hand, there is the boiling phase in sodium hydroxide where hemicellulose is hydrolyzed, impurities are extracted, and lignin is solubilized from the fiber. On the other hand, material losses are associated with the manufacturing process [39].

3.2.2. Density, Porosity, and Thermal Conductivity

For both optimal prototypes, the bulk density was calculated to be 0.0698 ± 0.006 g/cm3 for the henequen and 0.04506 ± 0.00616 g/cm3 for the palm fiber prototype. The discrepancy in the values can be attributed to the inherent characteristics of each raw material, as well as the efficacy of the pulping process, which facilitates the alignment of the fibers to form the prototype. The results of the fiber density and the bulk density of the prototypes were employed to ascertain the porosity of each optimal sample, which impacts the insulating capacity of the material. In the case of the henequen-based insulating, the porosity was determined to be 94.5849%, while for the palm-fiber-based material, the porosity achieved was 94.5639%. These values, although relatively high, are advantageous in dissipating heat transfer due to the ability to contain airtight air, which has a thermal conductivity of 0.028 W/mK [88], resulting in a low weighted conductivity. This phenomenon can be attributed to the mechanisms of heat transfer in porous insulating materials. These mechanisms include heat conduction from the solid network (fiber) and the gas phase (air), convection of heat from moving air in the pores, and radiation between the fibers [89]. Consequently, the high porosity of the samples enables the attainment of thermal conductivity of the optimized sisal and palm prototypes of 0.0353 ± 0.0019 W/mK and 0.0367 ± 0.0012, respectively. The outcomes permit the establishment of a positive linear correlation between the density and thermal conductivity of the insulating materials, as illustrated in Figure 9. This is because, as the solid phase fraction increases with decreasing porosity, the contribution of solid conduction increases with increasing density [89]. The values obtained for both developed prototypes are comparable to conventional thermal insulation materials such as expanded polystyrene (0.0361 W/mK) and even lower than mineral wool (0.045 W/mK) [14,70,88]. On the other hand, when compared with other lignocellulose-based insulation materials, it can be observed that the thermal conductivity is similar to that of the insulator based on hemp, flax, and sheep wool. These materials have been widely used for this type of application and have lower thermal conductivity than insulators based on straw, sugar cane bagasse, and wood fibers. This suggests that these raw materials have great potential for use in construction in regard achieving aspects of sustainability and energy efficiency [5,27,51,70,73,74,75].

3.2.3. Flexural Strength

Figure 10 presents the outcomes of the bending test conducted on the optimized insulating material prototypes. The results indicate that the henequen prototype exhibits a value of 0.0129 ± 0.0006 MPa at 5% deformation, with a modulus of elasticity of 0.2486 MPa. The maximum stress observed for the palm fiber prototype was 0.01827 ± 0.0009 MPa, with a modulus of elasticity of 0.4059 MPa. These results are compared with those of a prototype of expanded polystyrene and a prototype of a eucalyptus-bark-based insulator, both of which have been previously reported in the literature [39]. Furthermore, it can be observed that the behavior of the natural insulators is quite similar, with a maximum stress value much lower than that reported for EPS (0.20 ± 0.01 MPa). Therefore, it is advisable to conduct a more detailed study to limit their applications in construction elements according to their intrinsic properties. Although the flexural properties are reduced, the material could be employed similarly to that of non-rigid insulators such as sheep wool or mineral wool.

4. Conclusions

The high consumption of resources by the construction industry has generated significant interest in the development of strategies for sustainability and energy efficiency. The objective of this study was to develop two prototypes of insulating materials based on residual natural fibers available in Mexico with potential applications in buildings that would result in energy savings over the life cycle of a construction project. Two optimized prototypes based on henequen fiber and palm leaf fiber were developed and their thermal conductivity values were determined. The thermal conductivity values for the two prototypes were 0.0485 and 0.0409 W/mK, respectively. These values are associated with the nature of the raw materials and the high porosity of the samples. These values are comparable to those reported for conventional insulating materials used in construction, such as expanded polystyrene, polyurethane, and mineral wool. At the same time, they are lower than the values of insulating materials developed based on natural fibers, such as hemp, flax, and wheat straw. This demonstrates the high thermal insulation performance of the developed prototypes. The flexural strengths of both prototypes are lower than those reported for rigid insulating panels; however, the values achieved are sufficient for their handling, transportation, and application as construction insulation material. This research opens the door to further study of similar applications for diverse local agro-industrial resources, which are currently not well defined and are treated as waste. It also allows for the reduction in energy consumption in housing and the strengthening of the local industry, thereby advancing sustainability in the field of construction in Mexico.

Author Contributions

F.R.-D. originated the idea and conceptualization, led the research design, and write-up of the paper, and contributed to research execution; D.G.G.-H. contributed to writing—review and editing; C.A.J.-A. contributed to research design, write-up, and contributed to research execution. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Convocatoria de Ciencia Básica y/o Ciencia de Frontera. Modality: Paradigms and Controversies of Science 2022, grant number 320051.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to express their gratitude to the technical, professional, and human staff of the Facultad de Ingeniería Civil of the Universidad Autónoma de Nuevo León for their invaluable assistance in executing this research.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a–c) Henequen Plant (Agave fourcroydes); (d–f) Palm Tree (Washingtonia filifera).
Figure 1. (a–c) Henequen Plant (Agave fourcroydes); (d–f) Palm Tree (Washingtonia filifera).
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Figure 2. Prototype manufacturing process. (a) Obtaining and cutting the sample, (b) boiling the fiber in NaOH solution, (c) fiber drying in molds, (d) demolding prototypes of natural fiber-based insulators.
Figure 2. Prototype manufacturing process. (a) Obtaining and cutting the sample, (b) boiling the fiber in NaOH solution, (c) fiber drying in molds, (d) demolding prototypes of natural fiber-based insulators.
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Figure 3. Densities and thermal conductivities of different natural fibers.
Figure 3. Densities and thermal conductivities of different natural fibers.
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Figure 4. XRD of Henequen and Palm fiber.
Figure 4. XRD of Henequen and Palm fiber.
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Figure 5. FTIR of Henequen and Palm Fiber.
Figure 5. FTIR of Henequen and Palm Fiber.
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Figure 6. Thermogravimetric analysis of natural fibers.
Figure 6. Thermogravimetric analysis of natural fibers.
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Figure 7. Micrographs of natural fibers. (a–d) Henequen fiber. (e–h) Palm fiber. (HT: henequen transversal; HL: henequen longitudinal; PT: palm transversal; PL: palm longitudinal).
Figure 7. Micrographs of natural fibers. (a–d) Henequen fiber. (e–h) Palm fiber. (HT: henequen transversal; HL: henequen longitudinal; PT: palm transversal; PL: palm longitudinal).
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Figure 8. FTIR of the fiber before and after exposure to NaOH solution. (a) Henequen fiber and (b) palm fiber.
Figure 8. FTIR of the fiber before and after exposure to NaOH solution. (a) Henequen fiber and (b) palm fiber.
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Figure 9. Thermal conductivity and density results of natural fibers.
Figure 9. Thermal conductivity and density results of natural fibers.
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Figure 10. Prototypes flexural test.
Figure 10. Prototypes flexural test.
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Table 1. Study variables.
Table 1. Study variables.
VariablesLevel 1Level 2Level 3
Fiber length [cm]369
Boiling time [min]304560
NaOH concentration [%]0.511.5
Blending time [s]3610
Table 2. Results of chemical characterization of the fibers.
Table 2. Results of chemical characterization of the fibers.
TestAgave fourcroydesWashingtonia filifera
Moisture analysis2.7461 ± 3.3908%9.7856 ± 2.7482%
Ash analysis3.7284 ± 0.5952%18.8786 ± 2.8753%
Ethanol−Toluene extractables7.5916 ± 0.8028%13.3333 ± 0.7217%
Acid insoluble lignin12.1453 ± 2.5991%31.8416 ± 1.7471%
Holocellulose content18.1001 ± 3.6485%36.0441 ± 2.6094%
α -cellulose content33.6363 ± 2.3945%46.4297 ± 1.6553%
Hemicellulose content15.5362 ± 1.4369%10.3856 ± 4.1917%
Table 3. Chemical composition of different natural fibers.
Table 3. Chemical composition of different natural fibers.
SpeciesCellulose [%]Hemicellulose [%]Lignin [%]Source
Sugar cane bagasse572019[67]
Pine84.915.3ND[67]
Eucalyptus79–89210.5[64,67]
Hemp35–529–2717–28[62,63]
Flax43–4724–2621–23[65,66]
Sunflower42.129.713.4[63,65]
Wheat44.533.222.3[63]
Oats31–3727–3816–19[30]
Coconut35–6015–2820–48[64]
Sisal43–889–273.8–9.9[30,64]
Blue agave73.6ND21.1[52]
Agave lechuguilla17.7217.157.32[68,69]
Henequen77.68–2077.8[30]
ND: not determined.
Table 4. Mineralogical composition of natural fibers obtained by XRF.
Table 4. Mineralogical composition of natural fibers obtained by XRF.
FiberSO3ClK2OCaOFe2O3SiO2Al2O3
Henequen4.300%0.188%8.768%83.227%1.738%NDND
Palm13.817%0.433%4.425%15.814%1.240%59.623%0.622%
ND: not determined.
Table 5. Optimum value of variables of the orthogonal array.
Table 5. Optimum value of variables of the orthogonal array.
VariableFiber LengthBoiling TimeNaOH ConcentrationBlending Time
Optimum value3 cm60 min1.5%10 s
Table 6. Analysis of variance.
Table 6. Analysis of variance.
VariableSum SqMean SqF ValuePr (>F)Contribution [%]
(a) Palm Fiber Insulation Density
Fiber length0.000221192.21 × 10−46.23330.01806 *1.54
Boiling time0.000005175.17 × 10−60.14570.705273.59
NaOH concentration0.000000065.90 × 10−80.00170.967744.1
Blending time0.000110951.11 × 10−43.12660.086877.71
(b) Palm fiber insulation thermal conductivity
Fiber length4.59 × 10−54.59 × 10−524.90000.00002193 ***24.65
Boiling time1.58 × 10−51.58 × 10−58.59930.0062713 **8.5
NaOH concentration2.50 × 10−52.50 × 10−513.57460.0008708 ***13.42
Blending time4.24 × 10−54.24 × 10−522.98750.00003867 ***22.73
(c) Henequen fiber density
Fiber length1.87 × 10−30.001872719.08400.0001299 ***14.32
Boiling time3.16 × 10−30.003164832.30000.000003067 ***24.21
NaOH concentration2.56 × 10−30.002562726.11500.00001566 ***19.6
Blending time2.43 × 10−30.002429624.75900.00002302 ***18.58
(d) Henequen fiber thermal conductivity
Fiber length3.58 × 10−53.58 × 10−53.91000.056959.99
Boiling time1.44 × 10−51.44 × 10−51.57020.219554.01
NaOH concentration1.41 × 10−51.41 × 10−51.54000.224443.92
Blending time9.98 × 10−69.98 × 10−61.09000.304772.78
Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’
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Rosas-Díaz, F.; García-Hernández, D.G.; Juárez-Alvarado, C.A. Development of Lignocellulosic-Based Insulation Materials from Agave fourcroydes and Washingtonia filifera for Use in Sustainable Buildings. Sustainability 2024, 16, 5455. https://doi.org/10.3390/su16135455

AMA Style

Rosas-Díaz F, García-Hernández DG, Juárez-Alvarado CA. Development of Lignocellulosic-Based Insulation Materials from Agave fourcroydes and Washingtonia filifera for Use in Sustainable Buildings. Sustainability. 2024; 16(13):5455. https://doi.org/10.3390/su16135455

Chicago/Turabian Style

Rosas-Díaz, Felipe, David Gilberto García-Hernández, and Cesar A. Juárez-Alvarado. 2024. "Development of Lignocellulosic-Based Insulation Materials from Agave fourcroydes and Washingtonia filifera for Use in Sustainable Buildings" Sustainability 16, no. 13: 5455. https://doi.org/10.3390/su16135455

APA Style

Rosas-Díaz, F., García-Hernández, D. G., & Juárez-Alvarado, C. A. (2024). Development of Lignocellulosic-Based Insulation Materials from Agave fourcroydes and Washingtonia filifera for Use in Sustainable Buildings. Sustainability, 16(13), 5455. https://doi.org/10.3390/su16135455

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