Next Article in Journal
Editorial for the Special Issue on Additive Manufacturing of Advanced Composites
Previous Article in Journal
Biodegradable Alternatives to Plastic in Medical Equipment: Current State, Challenges, and the Future
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
J. Compos. Sci.
Volume 8
Issue 9
10.3390/jcs8090343
jcs-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Mechanical Properties of Polyurethane Foam Reinforced with Natural Henequen Fibre

by
Gloria E. Pech-Can
1,
Emmanuel A. Flores-Johnson
2,3,
Jose G. Carrillo
1,*,
Eral Bele
4 and
Alex Valadez-Gonzalez
1
1
Unidad de Materiales, Centro de Investigación Científica de Yucatán, Calle 43 No. 130 Col. Chuburná de Hidalgo, Mérida 97205, Yucatán, Mexico
2
Australian Nuclear Science and Technology Organisation (ANSTO), Lucas Heights, NSW 2234, Australia
3
School of Mechanical and Manufacturing Engineering, University of New South Wales (UNSW Sydney), Sydney, NSW 2052, Australia
4
Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, UK
*
Author to whom correspondence should be addressed.
J. Compos. Sci. 2024, 8(9), 343; https://doi.org/10.3390/jcs8090343
Submission received: 17 July 2024 / Revised: 25 August 2024 / Accepted: 28 August 2024 / Published: 1 September 2024
(This article belongs to the Section Fiber Composites)
Browse Figures
Versions Notes

Abstract

:
Polymeric foams are used in many applications, from packaging to structural applications. While polymeric foams have good mechanical performance in compression, they are brittle in tension and bending; fibre reinforcement can enhance their tension and flexural behaviour. This work reports a novel investigation of the mechanical properties of fibre-reinforced polyurethane (FRPU) foams with natural henequen fibres. Pull-out tests were performed with 10 mm fibres and various foam densities to identify the optimal density of 100 kg/m3. Thus, FRPU foams with this density and fibre contents of 1, 2 and 3 wt% were manufactured for mechanical testing. Compression tests showed an increase in the elastic modulus of the FRPU foam specimens compared to the unreinforced PU foam. The FRPU foams also exhibited higher yield stress, which was attributed to the reinforcing effect of the fibres on the cell walls. A maximum increase of 71% in the compressive yield stress was observed for the FRPU foam specimens with a fibre content of 2%. In addition, FRPU foam specimens absorbed more energy for any given strain than the unreinforced PU foam. Flexural tests showed the FRPU foams exhibited increased flexural strength compared to the unreinforced PU foam. A maximum increase of 40% in the flexural strength was observed for the FRPU foam with a fibre content of 1%. The findings reported here are significant because they suggest that FRPU foams incorporating natural henequen fibre exhibit promising potential as sustainable materials with enhanced mechanical properties.

Graphical Abstract

1. Introduction

Polymeric foams are widely used in many applications, ranging from packaging and insulation [1] to more sophisticated applications, such as the core of sandwich structures for construction applications [2] and in the automotive, aeronautical and aerospace industries [3,4]. Polymeric closed-cell foams are cellular solids made of a collection of thin-walled cells [5]. Most of their mechanical and thermal properties depend on the cells’ size and shape, the foam’s relative density, and the mechanical properties of the solid polymer of the cell walls [6]. These materials can be utilised for various applications, such as thermal insulators, packaging, structures, and energy absorbers. Polyurethane (PU) is a widely used foam material in the construction and automotive industries [7,8]. However, some of the applications of polymeric foams can be limited because of the degradation of their mechanical properties with elevated temperatures, particularly for temperatures approaching the glass transition temperature of the material [9]. In PU foams, the synthesis generally occurs through the formation of urethane bonds by the reaction of isocyanates with polyols [10,11,12]. It is well-known that while polymeric foams have good energy-absorbing performance in compression, they are brittle in tension and bending due to their porous microstructure; however, the use of fibre reinforcement can enhance the tension and flexural behaviour of cellular solids [13,14]. In this respect, investigations have been carried out addressing the enhancement of mechanical properties of PU foams reinforced with synthetic fibres [15]. For instance, Serban et al. [16] found that both the compression and flexural properties of PU foams improved with glass fibre reinforcement, while Yakushin et al. [17] found an improvement in the compressive strength of PU foams when adding milled carbon fibres. Nevertheless, there is a rising trend in utilising natural fibres for reinforcement. An increasing environmental awareness and a growing demand for eco-friendly materials with good mechanical properties, low density, and cost-effectiveness drive the increasing demand and interest in materials reinforced with natural fibres [18,19,20,21,22] and fillers [23,24,25].
Research on PU foams reinforced with natural fibres has been carried out to assess the effect of fibres on mechanical properties. Kuranska and Prociak [26] found that PU foam with a density of 40 kg/m3 and 5% flax fibre content exhibited increased compressive strength compared to unreinforced foam. Demiroglu et al. [27] investigated the mechanical properties of PU foam reinforced with nutshell fibres at volume fractions of 2.5 wt%, 5 wt% and 7.5 wt%. Their findings indicated that the maximum compressive strength enhancement was observed for a reinforcement of 2.5 wt%. In another study, Husainie et al. [28] found that PU foams with a density of around 130 kg/m3 reinforced with 1 wt% hazelnut shells exhibited improved tensile and shear strength compared to plain PU foam. Sair et al. [29,30] investigated the reinforcement of PU foams with 30–50 kg/m3 densities using hemp fibres with 5–30 wt% contents. They found that the highest enhancement of tensile and flexural properties was obtained with a fibre content of 15 wt%. Azmi et al. [31] found that with the addition of coir fibre, PU foams with a density of around 90 kg/m3 and fibre content of 5 wt% exhibited improved flexural properties compared to unreinforced PU foam. Abdel-Hamid et al. [32] investigated the reinforcement of microcellular PU with a density of around 600 kg/m3 using sisal fibre. They found an enhancement of the tensile properties of the reinforced materials, with fibre volume fractions of up to 3% compared to the unreinforced PU. In addition, they observed that the highest enhancement occurred when the sisal fibres were not chemically treated.
The abovementioned studies highlight the potential to produce greener PU foams with enhanced mechanical properties using natural and sustainable reinforcement resources instead of traditional synthetic fibres. Adding natural fibre reinforcement to the PU foams could produce more rigid cores with increased flexural strength, which could be used for more sustainable sandwich panels with enhanced structural properties for the construction industry [33]. Furthermore, the exploration of henequen (Agave fourcroydes Lem.) as a natural fibre for reinforcing cellular materials, especially PU foam, is notably limited compared to the research conducted on sisal (Agave sisalana Perrine) fibre as reinforcement [32,34,35]. The henequen plant, a close relative of the sisal plant, is the predominant Agave species cultivated for fibre production in the Yucatan peninsula in Mexico. In addition, henequen exhibits superior drought resistance compared to sisal and can thrive in semi-arid, rocky, and nutritionally poor soils [14]. To address this research gap, this study presents and discusses the mechanical characterisation of unreinforced PU foam and reinforced PU foam with 10 mm henequen fibres. Pull-out tests were performed to obtain an optimal foam density, which was then targeted to manufacture fibre-reinforced PU (FRPU) foams with different fibre contents. Uniaxial compression and flexural tests were then conducted, and the results were thoroughly analysed and discussed.
This work’s significance is that it contributes to understanding the mechanical behaviour of natural FRPU foams. The comprehensive study of the mechanical properties of FRPU foams, which includes compression and flexural tests performed on FRPU foams with a density of 100 kg/m3 and fibre volume fractions in the range of 1–3%, shows for the first time the potential of using the henequen fibre as reinforcement. Using low-cost natural fibres, could result in the development of sustainable materials for applications in the construction industry with improved mechanical properties.

2. Materials and Methods

2.1. Materials

A commercially available two-part PU expanding foam liquid system consisting of the polyol component (Part A) and the isocyanate component (Part B) [36] was used to fabricate the PU foams. The mix ratio by weight of 1:1 (Part A–Part B) recommended by the manufacturer was used in this work. Natural henequen fibres obtained from the local producer Desfibradora La Lupita, located in Baca, Yucatan, Mexico, were used to reinforce the PU foams. The density of the henequen fibre has been reported as 1570 kg/m3 [18], while the reported tensile mechanical properties are an elastic modulus of 13–15 GPa [37] and a strength of ~500–600 MPa [38,39]. The average henequen fibre diameter, measured from scanning electron microscopy (SEM) images, was 270 μm. The henequen fibres were cut into shorter segments of ~10 mm to fabricate the reinforced PU foams. This length was selected based on initial preliminary tests, which showed that longer segments were entangled during the foam preparation and prone to bend, reducing their reinforcement effectiveness; in contrast, it was observed that shorter segments were difficult to cut homogeneously, and the shorter length limited the reinforcement effect. The fibres were not chemically treated, considering that existing hydroxyl (OH) groups on the surface of natural fibres react with the NCO groups of diisocyanate of the PU, resulting in a good interfacial fibre–matrix bonding [32]. Before preparing the fibre-reinforced specimens, the fibres were placed in a convection oven at 60 °C for 8 h to eliminate excess humidity that could influence the mixing process.

2.2. Preparation of Samples for the Pull-Out Test

As previously mentioned, a fibre length of 10 mm was used throughout this work. For this reason, the single-fibre pull-out test was performed to investigate the bonding between henequen fibres and PU foams of various densities. This pull-out test has been successfully used to assess fibre-matrix bonding in other fibre-reinforced cellular solids [40]. For this test, foam specimens with an embedded single 10 mm fibre were prepared by using a polypropylene tube with an inner diameter of 18 mm and a length of 10 mm as a mould (Figure 1a). The pull-out specimen diameter was selected based on the foam cell diameter; that is, the distance from the centre of the specimen to the sample edge was at least 15 cell diameters in length to avoid edge boundary effects; the average cell diameter of the PU foams was ~0.5 mm. On the other hand, the length of the sample was selected to be equal to an embedded length of 10 mm, which was the length of the fibres used to reinforce the PU foams. A similar approach was used in [40] to fabricate pull-out samples of polymeric foams with embedded hemp fibre.
The mould was covered on both sides with polyethylene terephthalate (PET) film and wooden covers, which had a hole in the middle to keep the fibre centred and in place. To fabricate the specimens, first, the top cover was raised to allow the pouring of the PU liquid mixture into the mould (Figure 1a). Subsequently, the top cover was moved down to cover the mould; both covers were then kept in place using four bolts (Figure 1b). Finally, the mould was uncovered after 24 h (Figure 1c) and the specimens were placed in a desiccator at room temperature. Figure 1d shows an image of the pull-out specimen. Samples of various densities (80, 90, 100 and 110 kg/m3) were fabricated. The different densities were achieved by varying the amount of PU liquid mixture poured into the mould.

2.3. Preparation of Fibre-Reinforced PU (FRPU) Foam Specimens

Fibre-reinforced PU (FRPU) foam specimens for compression and flexural tests were prepared using reinforcement contents of 1–3 wt% (Table 1). This range was selected based on initial preliminary tests, which showed poor fibre dispersion and reinforcement for percentages greater than 4 wt%. The specimens were prepared as follows: First, a mixture was prepared by mixing the 10 mm long henequen fibres and the liquid polyol component (Part A) in a container. The mixture was stirred manually using a spatula for approximately two minutes until the fibres were evenly distributed. Next, the isocyanate component (Part B) was added to the mixture, which was further stirred manually for 1 min and then poured into a square wooden mould with dimensions of 300 × 300 × 30 mm3; subsequently, the mould was covered with a PET film and a wooden cover, which was kept in place using eight bolts. Lastly, the mould was uncovered after 24 h, and the FRPU foam block was placed in a desiccator at room temperature. In addition, unreinforced PU foam blocks were also fabricated. In total, two FRPU and two unreinforced PU foam blocks were fabricated. It is noted that a FRPU target density of 100 kg/m3 was used for all specimens based on the results from the pull-out tests presented in Section 3.1. To qualitatively observe the distribution of the fibres in the PU foam, FRPU foam blocks were prepared using fibres coloured with a commercial powder dye, since the colour of the PU foam and the henequen fibres is very similar (yellowish colour). These blocks were prepared for illustration purposes and were not mechanically tested.

2.4. Pull-Out Test

The pull-out tests were performed using a steel frame built for this purpose, as shown in Figure 2a. The frame was then placed in a universal testing machine, Shimadzu AGS-X (Shimadzu, Kyoto, Japan), equipped with a 100 N load cell (Figure 2b), and the test was performed using a crosshead speed of 0.1 mm/min. As mentioned, specimens with an embedded length of 10 mm and various densities (80, 90, 100 and 110 kg/m3) were fabricated for this test. At least five specimens were tested for each density. The interfacial adhesion between the henequen fibres and the PU foam matrices with different densities was compared using the apparent interfacial shear strength (IFSS) τ i , which was calculated using Equation (1) [41,42]:
τ i = F π D L
where F is the maximum pull-out force, D is the fibre diameter, and L is the embedded fibre length. It is noted that while Equation (1) is usually used for solid matrices, it has been employed to estimate the apparent τ i between fibres and foam matrices [40].

2.5. Compression Test

Compression tests were carried out on specimens with a density of 100 kg/m3 reinforced with 10 mm henequen fibres (1–3 wt%) and without reinforcement (Table 1) in accordance with ASTM D1621 standard [43]. Specimens with dimensions of 52 × 52 × 32 mm3 were used, where 32 mm is the specimen height. At least six specimens were tested for each foam configuration in the rise direction (Table 1), which were obtained from two different FRPU foam blocks. For this test, a universal testing machine, Shimadzu AGS-X (Shimadzu, Kyoto, Japan), equipped with a 5 kN load cell, was employed using a crosshead speed of 2.5 mm/min (Figure 3a). The compressive tests were stopped after the densification strain was reached and before reaching 80% of the load cell’s capacity. The compressive strain was calculated by dividing the change in length of the specimen (produced by the compression) by the original height of the specimen, while the compressive stress was calculated by dividing the compressive force by the original cross-section of the specimen [43]. The elastic modulus was calculated from the steepest straight-line portion of the stress–strain curves’ elastic region [43].

2.6. Flexural Test

The flexural properties of the foams shown in Table 1 were obtained using the three-point bending test in accordance with the ASTM D790 standard [44]. A Shimadzu AGS-X universal testing machine (Shimadzu, Kyoto, Japan) with a 1-kN load cell was used for this characterisation, using a crosshead speed of 5 mm/min. The specimens had a squared cross-section and dimensions of 25 × 25 × 150 mm3, where 150 mm is the specimen length. The distance between the supports was 100 mm. Figure 3b shows the three-point bending setup. At least six specimens were tested for each foam configuration (Table 1), which were obtained from two different FRPU foam blocks.

2.7. Scanning Electron Microscopy (SEM)

The morphological observations of the extracted fibres in the pull-out tests and fractured surfaces of specimens subjected to flexural tests were performed using scanning electron microscopy (SEM). The morphological characterisation was carried out using a JEOL JSM 6360LV SEM microscope (JEOL, Tokyo, Japan). The samples were coated with a thin gold layer for electron conductivity.

2.8. Chemical Characterisation by Infrared Spectroscopy

The chemical characterisation of fibres extracted after being subjected to the pull-out test was carried out using Fourier-transformed infrared (FTIR) spectroscopy with attenuated total reflectance (ATR) technique. The FTIR spectroscopy analyses were performed to determine the functional groups in the fibre impregnated with PU foam remnants. The tests were performed using a Nicolet 8700 spectrometer (Thermo-Scientific, Waltham, MA, USA) with an ATR accessory in the region of 500–4000 cm−1 with a resolution of 4 cm−1.

3. Results and Discussion

3.1. Pull-Out Tests

3.1.1. Mechanical Results

Figure 4 shows representative force–displacement curves from the pull-out tests for each PU foam density. It can be seen that for the first three densities (80, 90 and 100 kg/m3), a typical pull-out behaviour is observed, i.e., an initial elastic zone is observed, which is followed by an inelastic zone indicating the beginning of the interfacial debonding between the fibre and matrix until the peak load is reached, which means full debonding; this is then followed by a residual load resistance attributed to interfacial frictional forces. For the 110 kg/m3 density, the fibre could not be extracted from the PU foam due to the increased fibre–matrix interfacial adhesion produced by the increased density, resulting in fibre tensile failure, which is observed as a sudden drop in the force–displacement curve (Figure 4).
Table 2 shows the mean values (±standard deviation) for the peak force, the apparent IFSS, and the failure mode observed during the pull-out test. In addition, Table 2 shows the fibre tensile strength calculated for the 110 kg/m3 specimens where fibre failure occurred rather than fibre extraction. It is noted that for a density of 100 kg/m3, most of the specimens failed by fibre extraction; however, two samples failed by fibre tensile failure (Table 2), suggesting that the threshold density, at which the transition from fibre extraction to fibre tensile failure occurs, is close to 100 kg/m3. This observation also means that from the densities tested here, 100 kg/m3 is the optimal density for the fibre length of 10 mm used in this work, i.e., a density of 100 kg/m3 provides the maximum IFSS with fibre extraction. In comparison, for densities of 110 kg/m3 or higher, the fibre is not extracted and fails in tension, producing an inefficient stress transfer between the fibre and the matrix. For this reason, a density of 100 kg/m3 was selected to fabricate the fibre-reinforced foam specimens for the mechanical tests presented in the following sections.

3.1.2. SEM and Chemical Characterisation of Debonded Fibres

Figure 5 shows SEM images of the fibres extracted during the pull-out tests for specimens with densities of 80 and 100 kg/m3. The extracted fibre is completely covered with remnants of the PU foam material for both densities, indicating good adhesion between the fibre and the PU foam matrix. For the sample with a density of 100 kg/m3, the cell size of the PU foam remnant on the fibre surface is smaller than that of the sample with a density of 80 kg/m3, as expected, considering that an increase in density results in a reduction in cell size. It is noted that the thickness of the PU foam remnant layer surrounding the extracted fibre from the sample with a density of 100 kg/m3 is increased compared to the sample with a lower density. The increased thickness could be attributed to the fact that the increase in the density of the PU foam generates a more significant internal pressure in the mould during the expansion process, producing a more intimate interaction of the PU foam with the fibre.
Figure 6a shows the ATR-FTIR spectrum of the henequen fibre. The observed peaks at 3391 cm−1 and 3328 cm−1 are attributed to the stretching vibrations of the OH groups of cellulose. Also, peaks at 2918 cm−1 and 2850 cm−1 can be observed, which are related to the stretching of CH groups. In addition, a peak is observed at 1731 cm−1, corresponding to the stretching of the C=O groups of hemicellulose. Finally, peaks at 1241 cm−1 and 1026 cm−1 can be attributed to the stretching vibration of the C-O-C groups of the cellulose and hemicellulose [29,45].
Figure 6b shows the ATR-FTIR spectra for both PU foam and the fibre extracted during the pull-out test with PU foam remnants on its surface. Peaks at 3334 cm−1 and 3305 cm−1 corresponding to the stretching of NH groups are identified [46]. The peak at 2921 cm−1 corresponds to the CH groups. Finally, the peak at 1709 cm−1 corresponds to the C=O groups. It can be seen in Figure 6b that the peaks in the spectrum of the extracted fibre are similar to those observed in the PU foam spectrum, indicating good adhesion between the fibre and PU foam matrix. Additionally, a decrease in the intensity of the characteristic peaks of the NH and CH groups was observed in the extracted fibre compared to those in the PU foam spectrum. The reduction of the NH peak in the extracted fibre spectrum could be explained by the interaction between this group and the OH contained in the fibre, which again suggests a good bonding between the fibre and the PU foam [30,46].

3.2. Fibre-Reinforced PU (FRPU) Foam Specimens

3.2.1. Fibre Distribution in the FRPU Foams

Figure 7 shows cross-sections of the FRPU foam specimens with a density of 100 kg/m3 for the three different percentages of fibre reinforcement content considered for the mechanical characterisation (1%, 2% and 3%). Figure 7a shows the specimen cross-section perpendicular to the rise direction, while Figure 7b shows the cross-section parallel to the rise direction. A qualitative observation of Figure 7 shows that, in all cases, the fibres are mainly randomly dispersed. It is observed that there are some areas containing a larger quantity of fibres than other areas where fewer fibres are observed; however, this distribution is not preferential, and fibres can be found across the entire length and diameter of the specimens. It can also be seen that by increasing the fibre content, areas of fibre agglomeration occur, as observed in the specimen with 3 wt% fibre content. It is noted that an optimal dispersion was not achieved, particularly for the higher fibre contents (Figure 7). This problem is mainly related to the nature of the natural fibres, which has been highlighted in other investigations [30,47]. Fibre dispersion could be enhanced by improving the mixing process, using shorter fibres, treating the fibres, and using coupling agents [47]; however, a comprehensive study to find the factors that could further improve fibre dispersion is beyond the scope of this work.

3.2.2. Compression Tests

Figure 8a shows representative stress–strain curves from compression tests of FRPU foams with different fibre content percentages. The observed mechanical response is characteristic of a closed-cell rigid polymer foam under compression, in which an initial elastic regime is observed until the yield stress is reached, followed by an inelastic regime depicting strain hardening until the densification regime is reached. The elastic regime in closed-cell foams is explained by the bending, stretching, and contracting of the cell edges and the stretching of the membranes which form the cell faces. In contrast, the inelastic regime response is characterised by pressure from compressed gas inside the closed cells and cell walls collapsing and crushing [6]. Finally, the densification regime happens when all the crushed cells interact, rapidly increasing compressive stress with further strain increase [6,48]. It can be seen in Figure 8a and Table 3 that the elastic modulus and compressive yield stress increase with an increase in fibre content of up to 2%. The increase in the elastic modulus compared to the unreinforced PU foam can be attributed to the stiffness of the reinforcing henequen fibre [49], while the increase in the compressive yield stress can be attributed to the reinforcing effect of the fibres on the cell walls and the debonding of fibres when cell walls are being crushed during compression. Compared to the unreinforced PU foam, a maximum increase of 71% in the compressive yield stress is observed for the PU-F (2%) specimens. However, for a fibre content of 3%, the increase is only 20%. This observation could be explained by fibre agglomeration at higher fibre content, producing cell wall embrittlement [27].
The determination of the densification strain employed a method based on the maximum of the energy absorption efficiency curve. In this approach, the energy efficiency, denoted as η ε , is defined as [50]:
η ε = 1 σ ( ε ) ∫ 0 ε σ ε d ε
where σ ( ε ) and ε are the compressive stress and strain, respectively. The densification strain ε D is then identified as the point corresponding to the maximum of the energy absorption efficiency curve. Table 3 shows the average ε D for all foam configurations. It can be seen that while ε D tends to slightly decrease when the fibre reinforcement is added compared to the unreinforced PU foam, the values of ε D are similar for the PU-NF, PU-F (1%) and PU-F (3%) cases; however, for the PU-F (2%) specimens ε D decreases by 18% compared to the unreinforced case.
The energy absorption of the different foams during compression was compared using the absorbed energy per unit volume W, which was calculated by integrating the compression stress–strain curves. Figure 8b shows W for the representative unreinforced PU foam specimen and specimens reinforced with different henequen fibre content. It is observed that for any given strain, the FRPU foam specimens absorb more energy than the plain PU foam; in addition, the PU-F (2%) foams absorb more energy than all the other specimens. Table 3 shows W at a compressive strain of 0.5 and also at ε D , which highlights the energy absorption enhancement of the FRPU foams compared to the unreinforced PU foam; in particular, the energy absorption of the PU-F (2%) specimens at 0.5 strain is 60% higher than that of the plain PU foam. In contrast, when W is compared at ε D (Table 3), the energy absorbed by the PU-F (2%) specimens is only 17% higher than the unreinforced PU foams. This observation is explained by ε D being the lowest for PU-F (2%) foams (Figure 8b).

3.2.3. Flexural Tests

Figure 9 shows the representative flexural stress–flexural strain curves of the unreinforced foam and FRPU foams with different fibre content subjected to three-point bending, while Table 4 shows the flexural properties of the foams obtained from those tests. Table 4 shows that the flexural modulus of the FRPU foams is higher than that of the unreinforced PU foam; however, this increase due to the fibre reinforcement is slight, indicating that the PU cell’s mechanical behaviour dominates the flexural modulus of all foams. In flexural tests, cellular solids are subjected to both compression and tension, and the mechanical response is related to the cell’s mechanical response. The mechanical response of the cells in tension is brittle, and edge bending and face stretching dominate the elastic response. In contrast, the stretching of the cells dominates the inelastic response [6].
It can be seen in Figure 9 and Table 4 that for all cases, the flexural strength of the FRPU foams is higher than that of the unreinforced PU foam, showing the advantage of adding fibres to the PU foams to improve the flexural performance. For instance, the flexural strength of the PU-F (1%) is 40% higher than that of PU-NF. However, the increase in flexural strength with respect to the unreinforced PU foam decreases with the increase in fibre content, indicating that, for the percentages tested here, the PU foam is not reinforced efficiently for percentages greater than 1%. This behaviour may be due to fibre agglomeration and not uniform fibre distribution throughout the matrix at higher fibre percentages. These observations are in accordance with the experimental results reported for PU foams reinforced with other natural fibres. For instance, Azmi et al. [31] observed an increase in the flexural modulus when PU foams were reinforced with a coir fibre content of 5 wt% compared to unreinforced foams; however, they observed a decrease in the flexural modulus compared to the unreinforced PU foams for higher fibre contents in the range of 10–20 wt%. A similar trend was observed for PU foams reinforced with hemp fibres [29].
Moreover, Table 4 shows that the flexural strain to failure is lower for the FRPU foams than that of the unreinforced PU foam and decreases with the increase in fibre content. This observation is also attributed to the increased fibre agglomeration throughout the matrix with increased fibre content, which could act as defects promoting cell wall cracking [51].
Figure 10a shows a representative PU-F (1%) specimen tested in bending. It can be seen that the specimen developed a single tension crack at the midsection of the bottom face. A similar failure mechanism was observed for the other FRPU foams and the unreinforced PU foam, i.e., a single tension crack developed at the same location. Figure 10a also shows fibres pulled out from the PU foam matrix. Figure 10b shows a SEM image of one of the extracted fibres during bending. It can be seen that the fibre is surrounded by PU foam remnants, which confirms good adhesion between the fibre and the foam matrix.
The findings presented in this study demonstrate the feasibility of utilising natural henequen fibre to produce FRPU foams with enhanced mechanical properties, thereby contributing to reduced environmental impact and increased sustainability. However, the results showed that for the fibre content of 3 wt%, the enhancement of compressive and flexural properties is lower than in the cases where 1 wt% and 2 wt% are used. This observation suggests that fibre dispersion should be improved; however, a comprehensive microstructural analysis and further microscopy studies should be performed to verify this. In addition, FRPU foams with different fibre lengths should be investigated to assess the effect of this parameter on the mechanical properties. Finally, the effect of surface treatments on the natural fibre should also be investigated to improve fibre matrix adhesion.

4. Conclusions

Fibre-reinforced polyurethane (FRPU) foams with 100 kg/m3 density were reinforced with 10 mm natural henequen fibres using 1, 2 and 3 wt% fibre contents and subjected to compression and flexural tests. The following conclusions can be drawn from this study:
  • Compression tests showed an increase in the elastic modulus of the FRPU foams compared to the unreinforced PU foam, which was attributed to the stiffness of the reinforcing henequen fibre.
  • An increase in the compressive yield stress was also observed for the FRPU foams, which was attributed to the reinforcing effect of the fibres on the cell walls.
  • The maximum increase of 71% in the compressive yield stress was observed for the FRPU foam specimens with a fibre content of 2% compared to the unreinforced PU foam.
  • Flexural tests showed that the FRPU foams exhibited increased flexural strength in all cases compared to the unreinforced PU foam.
  • The FRPU foam with a fibre content of 1% showed a maximum increase in flexural strength of 40%.
  • The flexural strain to failure was lower for the FRPU foams than that of the unreinforced PU foam, which was attributed to the increased fibre agglomeration with increased fibre content.
  • The findings are significant because FRPU foams incorporating natural henequen fibre exhibit promising potential as sustainable materials with enhanced mechanical properties.
  • However, this work is limited to a single foam density and a single fibre length, and further research is imperative to gain a comprehensive understanding of these materials’ mechanical performance.

Author Contributions

Conceptualisation, J.G.C. and G.E.P.-C.; methodology, G.E.P.-C., J.G.C., E.B. and A.V.-G.; validation, A.V.-G. and E.A.F.-J.; formal analysis, G.E.P.-C., J.G.C. and E.A.F.-J.; investigation, G.E.P.-C. and J.G.C.; resources, J.G.C. and E.B.; data curation, G.E.P.-C. and E.A.F.-J.; writing—original draft preparation, G.E.P.-C. and J.G.C.; writing—review and editing, A.V.-G., E.B. and E.A.F.-J.; visualisation, G.E.P.-C., J.G.C. and E.A.F.-J.; supervision, J.G.C., A.V.-G., E.B. and E.A.F.-J.; project administration, J.G.C. and E.B.; funding acquisition, J.G.C. and E.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partly supported by the Consejo Nacional de Humanidades Ciencias y Tecnologías de Mexico (CONAHCYT) through the project Ciencia Básica 2017–2018 (Grant No. A1-S-8864) and by the Royal Society International Exchanges Scheme 2020 (Grant No. IES/R1/201026). The author, G.E.P.-C., was supported by the CONAHCYT scholarship (No. 947692).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank Santiago Duarte Arana for his assistance with the SEM analysis and Alejandro May Pat and César Martín Barrera for their assistance with the mechanical testing.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Wu, G.; Xie, P.; Yang, H.; Dang, K.; Xu, Y.; Sain, M.; Turng, L.-S.; Yang, W. A review of thermoplastic polymer foams for functional applications. J. Mater. Sci. 2021, 56, 11579–11604. [Google Scholar] [CrossRef]
  2. Mazzuca, P. Flexural behaviour of GFRP sandwich panels with eco-friendly PET foam core for the rehabilitation of building floors. Structures 2024, 60, 105815. [Google Scholar] [CrossRef]
  3. Flores-Johnson, E.A.; Li, Q.M. Experimental study of the indentation of sandwich panels with carbon fibre-reinforced polymer face sheets and polymeric foam core. Compos. Part B 2011, 42, 1212–1219. [Google Scholar] [CrossRef]
  4. Chen, Y.; Das, R. A review on manufacture of polymeric foam cores for sandwich structures of complex shape in automotive applications. J. Sandwich Struct. Mater. 2021, 24, 789–819. [Google Scholar] [CrossRef]
  5. Flores-Johnson, E.A.; Li, Q.M.; Mines, R.A.W. Degradation of Elastic Modulus of Progressively Crushable Foams in Uniaxial Compression. J. Cell. Plast. 2008, 44, 415–434. [Google Scholar] [CrossRef]
  6. Gibson, L.J.; Ashby, M.F. Cellular Solids: Structure and Properties, 2nd ed.; Cambridge University Press: Cambridge, UK, 1997. [Google Scholar]
  7. Ates, M.; Karadag, S.; Eker, A.A.; Eker, B. Polyurethane foam materials and their industrial applications. Polym. Int. 2022, 71, 1157–1163. [Google Scholar] [CrossRef]
  8. Sheikhi, M.R.; Hasanzadeh, M.; Gürgen, S.; Li, J. Enhanced anti-impact resistance of polyurethane foam composites with multi-phase shear thickening fluids containing various carbon nanofillers. Mater. Today Commun. 2024, 38, 107991. [Google Scholar] [CrossRef]
  9. Mazzuca, P.; Firmo, J.P.; Correia, J.R.; Garrido, M. Mechanical behaviour in shear and compression of polyurethane foam at elevated temperature. J. Sandwich Struct. Mater. 2022, 24, 1429–1448. [Google Scholar] [CrossRef]
  10. Gama, N.V.; Ferreira, A.; Barros-Timmons, A. Polyurethane Foams: Past, Present, and Future. Materials 2018, 11, 1841. [Google Scholar] [CrossRef] [PubMed]
  11. Cordero, A.I.; Amalvy, J.I.; Fortunati, E.; Kenny, J.M.; Chiacchiarelli, L.M. The role of nanocrystalline cellulose on the microstructure of foamed castor-oil polyurethane nanocomposites. Carbohydr. Polym. 2015, 134, 110–118. [Google Scholar] [CrossRef]
  12. Shivakumar, N.D.; Deb, A. Dependence of the mechanical properties of rigid PU foam on density. J. Reinf. Plast. Compos. 2022, 41, 355–363. [Google Scholar] [CrossRef]
  13. Castillo-Lara, J.F.; Flores-Johnson, E.A.; Valadez-Gonzalez, A.; Herrera-Franco, P.J.; Carrillo, J.G.; Gonzalez-Chi, P.I.; Agaliotis, E.; Li, Q.M. Mechanical behaviour of composite sandwich panels with foamed concrete core reinforced with natural fibre in four-point bending. Thin-Walled Struct. 2021, 169, 108457. [Google Scholar] [CrossRef]
  14. Castillo-Lara, J.F.; Flores-Johnson, E.A.; Valadez-Gonzalez, A.; Herrera-Franco, P.J.; Carrillo, J.G.; Gonzalez-Chi, P.I.; Li, Q.M. Mechanical Properties of Natural Fiber Reinforced Foamed Concrete. Materials 2020, 13, 3060. [Google Scholar] [CrossRef]
  15. Dzulkifli, M.H.; Zainal, S.Z.; Majid, R.A.; Yahya, M.Y. Industrial Rockwool Fiber Wastes as Reinforcement in Rigid Polyurethane Foam: Preparation and Characterization. Fibers Polym. 2023, 24, 329–336. [Google Scholar] [CrossRef]
  16. Şerban, D.-A.; Weissenborn, O.; Geller, S.; Marşavina, L.; Gude, M. Evaluation of the mechanical and morphological properties of long fibre reinforced polyurethane rigid foams. Polym. Test. 2016, 49, 121–127. [Google Scholar] [CrossRef]
  17. Yakushin, V.; Stirna, U.; Bel’kova, L.; Deme, L.; Sevastyanova, I. Properties of rigid polyurethane foams filled with milled carbon fibers. Mech. Compos. Mater. 2011, 46, 679–688. [Google Scholar] [CrossRef]
  18. Agaliotis, E.M.; Ake-Concha, B.D.; May-Pat, A.; Morales-Arias, J.P.; Bernal, C.; Valadez-Gonzalez, A.; Herrera-Franco, P.J.; Proust, G.; Koh-Dzul, J.F.; Carrillo, J.G.; et al. Tensile Behavior of 3D Printed Polylactic Acid (PLA) Based Composites Reinforced with Natural Fiber. Polymers 2022, 14, 3976. [Google Scholar] [CrossRef] [PubMed]
  19. Malekzadeh, H.; Md Zaid, N.S.B.; Bele, E. Characterization and structural properties of bamboo fibre solid foams. Cellulose 2021, 28, 703–714. [Google Scholar] [CrossRef]
  20. Chiacchiarelli, L.M.; Cerrutti, P.; Flores-Johnson, E.A. Compressive behavior of rigid polyurethane foams nanostructured with bacterial nanocellulose at low and intermediate strain rates. J. Appl. Polym. Sci. 2020, 137, 48701. [Google Scholar] [CrossRef]
  21. Tengsuthiwat, J.; Limrungruengrat, S.; Sukhawipat, N. A Green Dual Function Engineering Material—Sound Absorption and Heat Insulation Based on Water Hyacinth Fiber and Waste Palm Oil. Fibers Polym. 2023, 24, 3355–3368. [Google Scholar] [CrossRef]
  22. Oliveira, M.; Pickering, K.L.; Gauss, C. Hybrid Polyethylene Composites with Recycled Carbon Fibres and Hemp Fibres Produced by Rotational Moulding. J. Compos. Sci. 2022, 6, 352. [Google Scholar] [CrossRef]
  23. Maia, L.S.; Zanini, N.C.; Camani, P.H.; Medeiros, S.F.; Rosa, D.S.; Mulinari, D.R. PU Foams Resistance Against Natural Weathering Aging: The Effect of Coffee Husk Residues in Different Contents. J. Polym. Environ. 2023, 31, 2073–2092. [Google Scholar] [CrossRef]
  24. De Luca Bossa, F.; Santillo, C.; Verdolotti, L.; Campaner, P.; Minigher, A.; Boggioni, L.; Losio, S.; Coccia, F.; Iannace, S.; Lama, G.C. Greener Nanocomposite Polyurethane Foam Based on Sustainable Polyol and Natural Fillers: Investigation of Chemico-Physical and Mechanical Properties. Materials 2020, 13, 211. [Google Scholar] [CrossRef]
  25. Amena, B.T.; Altenbach, H.; Tibba, G.S.; Hossain, N. Investigation of Mechanical Properties of Coffee Husk-HDPE-ABS Polymer Composite Using Injection-Molding Method. J. Compos. Sci. 2022, 6, 354. [Google Scholar] [CrossRef]
  26. Kuranska, M.; Prociak, A. Porous polyurethane composites with natural fibres. Compos. Sci. Technol. 2012, 72, 299–304. [Google Scholar] [CrossRef]
  27. Demiroğlu, S.; Erdoğan, F.; Akin, E.; Karavana, H.A.; Seydibeyoğlu, M.Ö. Natural Fiber Reinforced Polyurethane Rigid Foam. Gazi Univ. J. Sci. 2017, 30, 97–109. [Google Scholar]
  28. Husainie, S.M.; Khattak, S.U.; Robinson, J.; Naguib, H.E. A Comparative Study on the Mechanical Properties of Different Natural Fiber Reinforced Free-Rise Polyurethane Foam Composites. Ind. Eng. Chem. Res. 2020, 59, 21745–21755. [Google Scholar] [CrossRef]
  29. Sair, S.; Oushabi, A.; Kammouni, A.; Tanane, O.; Abboud, Y.; El Bouari, A. Mechanical and thermal conductivity properties of hemp fiber reinforced polyurethane composites. Case Stud. Constr. Mater. 2018, 8, 203–212. [Google Scholar] [CrossRef]
  30. Sair, S.; Oushabi, A.; Kammouni, A.; Tanane, O.; Abboud, Y.; Hassani, F.O.; Laachachi, A.; El Bouari, A. Effect of surface modification on morphological, mechanical and thermal conductivity of hemp fiber: Characterization of the interface of hemp–Polyurethane composite. Case Stud. Therm. Eng. 2017, 10, 550–559. [Google Scholar] [CrossRef]
  31. Azmi, M.A.; Abdullah, H.Z.; Idris, M.I. Properties of polyurethane foam/coconut coir fiber as a core material and as a sandwich composites component. IOP Conf. Ser.: Mater. Sci. Eng. 2013, 50, 012067. [Google Scholar] [CrossRef]
  32. Abdel-Hamid SM, S.; Al-Qabandi, O.A.; NAS, E.; Bassyouni, M.; Zoromba, M.S.; Abdel-Aziz, M.H.; Mira, H. Fabrication and Characterization of Microcellular Polyurethane Sisal Biocomposites. Molecules 2019, 24, 4585. [Google Scholar] [CrossRef]
  33. Kuranchie, C.; Yaya, A.; Bensah, Y.D. The effect of natural fibre reinforcement on polyurethane composite foams—A review. Sci. Afr. 2021, 11, e00722. [Google Scholar] [CrossRef]
  34. Pan, L.; Ban, J.; Xu, T.; Liu, R.; Lu, S. A study on sisal-based polyurethane foam with multi-shape memory properties. New J. Chem. 2021, 45, 13579–13584. [Google Scholar] [CrossRef]
  35. Jena, S.; Khatri, V.N.; Nainegali, L.; Dutta, R.K. Study on Physical, Mechanical, Morphological, and Crystallographic Properties of Chemically Treated Sisal Fibers. Fibers Polym. 2024, 25, 2517–2534. [Google Scholar] [CrossRef]
  36. Poliformas, Espuma de Poliuretano para Vaciado (Polyurethane Xxpanding Foam). Technical Data Sheet (In Spanish). 2018. Available online: https://poliformas.mx/front/files/pdf/Ficha_ESPUMADEPOLIURETANOVACIADO88677.pdf (accessed on 10 September 2023).
  37. Yasin, P.; Venkata Ramana, M.; Krishna Vamshi, C.; Pradeep, K. A study of continuous Henequen/Epoxy composites. Mater. Today Proc. 2019, 18, 3798–3811. [Google Scholar] [CrossRef]
  38. Valadez-Gonzalez, A.; Cervantes-Uc, J.M.; Olayo, R.; Herrera-Franco, P.J. Effect of fiber surface treatment on the fiber–matrix bond strength of natural fiber reinforced composites. Compos. Part B 1999, 30, 309–320. [Google Scholar] [CrossRef]
  39. Tarrés, Q.; Vilaseca, F.; Herrera-Franco, P.J.; Espinach, F.X.; Delgado-Aguilar, M.; Mutjé, P. Interface and micromechanical characterization of tensile strength of bio-based composites from polypropylene and henequen strands. Ind. Crops Prod. 2019, 132, 319–326. [Google Scholar] [CrossRef]
  40. Galzerano, B.; Formisano, A.; Durante, M.; Iucolano, F.; Caputo, D.; Liguori, B. Hemp reinforcement in lightweight geopolymers. J. Compos. Mater. 2018, 52, 2313–2320. [Google Scholar] [CrossRef]
  41. Herrera Franco, P.J.; Drzal, L.T. Comparison of methods for the measurement of fibre/matrix adhesion in composites. Composites 1992, 23, 2–27. [Google Scholar] [CrossRef]
  42. Asprone, D.; Durante, M.; Prota, A.; Manfredi, G. Potential of structural pozzolanic matrix–hemp fiber grid composites. Constr. Build. Mater. 2011, 25, 2867–2874. [Google Scholar] [CrossRef]
  43. ASTM D1621-16; Standard Test Method for Compressive Properties of Rigid Cellular Plastics. ASTM International: West Conshohocken, PA, USA, 2016.
  44. ASTM D790-17; Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials. ASTM International: West Conshohocken, PA, USA, 2017.
  45. Aguilar-Rios, A.; Herrera-Franco, P.J.; Martinez-Gomez, A.d.J.; Valadez-Gonzalez, A. Improving the bonding between henequen fibers and high density polyethylene using atmospheric pressure ethylene-plasma treatments. eXPRESS Polym. Lett. 2014, 8, 491–504. [Google Scholar] [CrossRef]
  46. Cinelli, P.; Anguillesi, I.; Lazzeri, A. Green synthesis of flexible polyurethane foams from liquefied lignin. Eur. Polym. J. 2013, 49, 1174–1184. [Google Scholar] [CrossRef]
  47. Saheb, D.N.; Jog, J.P. Natural fiber polymer composites: A review. Adv. Polym. Technol. 1999, 18, 351–363. [Google Scholar] [CrossRef]
  48. Flores-Johnson, E.A.; Li, Q.M. Indentation into polymeric foams. Int. J. Solids Struct. 2010, 47, 1987–1995. [Google Scholar] [CrossRef]
  49. Alonso, M.V.; Auad, M.L.; Nutt, S. Short-fiber-reinforced epoxy foams. Compos. Part A 2006, 37, 1952–1960. [Google Scholar] [CrossRef]
  50. Tan, P.J.; Harrigan, J.J.; Reid, S.R. Inertia effects in uniaxial dynamic compression of a closed cell aluminium alloy foam. Mater. Sci. Technol. 2002, 18, 480–488. [Google Scholar] [CrossRef]
  51. Członka, S.; Strąkowska, A.; Kairytė, A. The Impact of Hemp Shives Impregnated with Selected Plant Oils on Mechanical, Thermal, and Insulating Properties of Polyurethane Composite Foams. Materials 2020, 13, 4709. [Google Scholar] [CrossRef] [PubMed]
Jcs 08 00343 g001
Figure 1. Pull-out specimen preparation.
Figure 1. Pull-out specimen preparation.
Jcs 08 00343 g001
Jcs 08 00343 g002
Figure 2. (a) Schematic of the pull-out test; (b) pull-out test setup.
Figure 2. (a) Schematic of the pull-out test; (b) pull-out test setup.
Jcs 08 00343 g002
Jcs 08 00343 g003
Figure 3. (a) Compression test setup; (b) flexural test setup.
Figure 3. (a) Compression test setup; (b) flexural test setup.
Jcs 08 00343 g003
Jcs 08 00343 g004
Figure 4. Representative force–displacement curves of specimens with different densities subjected to the pull-out test.
Figure 4. Representative force–displacement curves of specimens with different densities subjected to the pull-out test.
Jcs 08 00343 g004
Jcs 08 00343 g005
Figure 5. SEM images of the fibres extracted during the pull-out test of samples with densities of (a) 80 kg/m3 and (b) 100 kg/m3.
Figure 5. SEM images of the fibres extracted during the pull-out test of samples with densities of (a) 80 kg/m3 and (b) 100 kg/m3.
Jcs 08 00343 g005
Jcs 08 00343 g006
Figure 6. ATR-FTIR spectra of (a) henequen fibre; (b) PU foam and fibre extracted during the pull-out test with PU foam remnants on its surface.
Figure 6. ATR-FTIR spectra of (a) henequen fibre; (b) PU foam and fibre extracted during the pull-out test with PU foam remnants on its surface.
Jcs 08 00343 g006
Jcs 08 00343 g007
Figure 7. Cross-sections of FRPU foams with different percentages of fibre content; (a) cross-sections perpendicular to the rise direction; (b) cross-sections parallel to the rise direction.
Figure 7. Cross-sections of FRPU foams with different percentages of fibre content; (a) cross-sections perpendicular to the rise direction; (b) cross-sections parallel to the rise direction.
Jcs 08 00343 g007
Jcs 08 00343 g008
Figure 8. (a) Compression stress–strain curves and (b) absorbed energy per unit volume–strain curves of representative unreinforced PU foam specimens and specimens reinforced with different henequen fibre content (the point in the curves corresponding to a strain equal to ε D is indicated with the large markers).
Figure 8. (a) Compression stress–strain curves and (b) absorbed energy per unit volume–strain curves of representative unreinforced PU foam specimens and specimens reinforced with different henequen fibre content (the point in the curves corresponding to a strain equal to ε D is indicated with the large markers).
Jcs 08 00343 g008
Jcs 08 00343 g009
Figure 9. Representative flexural stress–flexural strain curves of the unreinforced foam and FRPU foams with different fibre content subjected to three-point bending.
Figure 9. Representative flexural stress–flexural strain curves of the unreinforced foam and FRPU foams with different fibre content subjected to three-point bending.
Jcs 08 00343 g009
Jcs 08 00343 g010
Figure 10. (a) Failure mode of PU-F (1%) specimen subjected to three-point bending; (b) SEM image of extracted fibre during three-point bending.
Figure 10. (a) Failure mode of PU-F (1%) specimen subjected to three-point bending; (b) SEM image of extracted fibre during three-point bending.
Jcs 08 00343 g010
Table 1. Formulation of FRPU foams.
Table 1. Formulation of FRPU foams.
MaterialFibre Content
(wt%)		PU Content
(wt%)		Density
(kg/m3)		Nomenclature
PU foam0100100PU-NF
Henequen/PU foam199100PU-F(1%)
298100PU-F(2%)
397100PU-F(3%)
Table 2. Mechanical results from the pull-out tests.
Table 2. Mechanical results from the pull-out tests.
Density
(kg/m3)		Peak Force
(N)		Apparent IFSS
(MPa)		Fibre Tensile Strength
(MPa)		Failure Mode
805.35 ± 1.010.62 ± 0.12-Fibre
extraction
906.14 ± 1.130.75 ± 0.14-Fibre
extraction
1008.39 ± 0.630.98 ± 0.07-Fibre
extraction
10010.11 ± 3.75 *-168.58 ± 62.55 *Fibre tensile
Failure *
11010.69 ± 1.18-178.23 ± 19.68Fibre tensile
failure
* Only two samples were used to calculate these values.
Table 3. Mechanical properties from the compression test of unreinforced PU foam specimens and specimens reinforced with different henequen fibre content.
Table 3. Mechanical properties from the compression test of unreinforced PU foam specimens and specimens reinforced with different henequen fibre content.
MaterialE
(MPa)		 σ y
(MPa)		 ε D
(mm/mm)		W at 0.5 Strain (kJ/m3) W   at   ε D  (kJ/m3)
PU-NF6.83 ± 1.450.35 ± 0.050.62 ± 0.02221.5 ± 16.8311.3 ± 18.0
PU-F (1%)9.30 ± 0.600.44 ± 0.010.60 ± 0.04274.0 ± 5.4362.6 ± 8.8
PU-F (2%)12.53 ± 0.540.60 ± 0.030.51 ± 0.07354.3 ± 12.5365.1 ± 13.4
PU-F (3%)9.15 ± 1.290.42 ± 0.030.61 ± 0.02262.6 ± 18.1355.3 ± 16.6
Table 4. Flexural properties of the unreinforced foam and FRPU foams with different fibre content subjected to three-point bending.
Table 4. Flexural properties of the unreinforced foam and FRPU foams with different fibre content subjected to three-point bending.
MaterialFlexural Modulus
(MPa)		Flexural Strength (MPa)Flexural Strain to Failure
(mm/mm)
PU-NF19.3 ± 3.41.24 ± 0.130.23 ± 0.02
PU-F(1%)21.4 ± 0.91.74 ± 0.100.18 ± 0.01
PU-F(2%)21.8 ± 2.41.60 ± 0.160.17 ± 0.01
PU-F(3%)21.5 ± 1.11.37 ± 0.020.16 ± 0.01
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Pech-Can, G.E.; Flores-Johnson, E.A.; Carrillo, J.G.; Bele, E.; Valadez-Gonzalez, A. Mechanical Properties of Polyurethane Foam Reinforced with Natural Henequen Fibre. J. Compos. Sci. 2024, 8, 343. https://doi.org/10.3390/jcs8090343

AMA Style

Pech-Can GE, Flores-Johnson EA, Carrillo JG, Bele E, Valadez-Gonzalez A. Mechanical Properties of Polyurethane Foam Reinforced with Natural Henequen Fibre. Journal of Composites Science. 2024; 8(9):343. https://doi.org/10.3390/jcs8090343

Chicago/Turabian Style

Pech-Can, Gloria E., Emmanuel A. Flores-Johnson, Jose G. Carrillo, Eral Bele, and Alex Valadez-Gonzalez. 2024. "Mechanical Properties of Polyurethane Foam Reinforced with Natural Henequen Fibre" Journal of Composites Science 8, no. 9: 343. https://doi.org/10.3390/jcs8090343

Article Metrics

Back to TopTop