Mechanical and acoustical evaluation of bio-based composites made of cork granulates for acoustic ceiling tiles

1. INTRODUCTION

Nowadays, the use of green and sustainable products in the building industry has a growing interest being a common practice today as an alternative to the traditional synthetic materials such as glass wool, foams or rock wool, widely used for noise control applications (1-121. Buratti, C.; Belloni, E.; Lascaro, E.; López, G.A.; Ricciardi, P. (2016) Sustainable panels with recycled materials for building applications: environmental and acoustic characterization. Energy Procedia. 101, 972-979. https://doi.org/10.1016/j.egypro.2016.11.123.
2. Guna, V.; Llangovan, M.; Hu, C.; Venkatesh, K.; Reddy, N. (2019) Valorization of sugarcane bagasse by developing completely biodegradable composites for industrial applications. Ind. Crop. Prod. 131, 25-31. https://doi.org/10.1016/j.indcrop.2019.01.011.
3. Ali, M.; Alabdulkarem, A.; Nuhait, A.; Al-Salem, K.; Iannace, G.; Almuzaiqer, R.; Al.turki, A.; Al-Ajlan, F.; Al-Mosabi, Y.; Al-Sulaimi, A. (2020) Thermal and acoustic characteristics of novel thermal insulating materials made of Eucalyptus Globulus leaves and wheat straw fibers. J. Build. Eng. 32, 101452. https://doi.org/10.1016/j.jobe.2020.101452.
4. Guna, V.; Ilangovan, M.; Rather, M.H.; Giridharan, B.V.; Prajwal, B.; Vamshi Krishna, K.; Venkatesh, K.; Reddy, N. (2020) Groundnut shell / rice husk agro-waste reinforced polypropylene hybrid biocomposites. J. Build. Eng. 27, 100991. https://doi.org/10.1016/j.jobe.2019.100991.
5. Raj, M.; Fatima, S.; Tandon, N. (2020) An experimental and theoretical study on environment-friendly sound absorber sourced from nettle fibers. J. Build. Eng. 31, 101395. https://doi.org/10.1016/j.jobe.2020.101395.
6. Samaei, S.E.; Mahabadi, H.A.; Mousavi, S.M.; Khavanin, A.; Faridan, M.; Taban, E. (2020) The influence of alkaline treatment on acoustical, morphological, tensile and thermal properties of Kenaf natural fibers. J. Ind. Text. 3, 1528083720944240. https://doi.org/10.1177/1528083720944240.
7. Taban, E.; Mirzaei, R.; Faridan, M.; Samaei, E.; Salimi, F.; Tajpoor, A.; Ghalenoei, M. (2020) Morphological, acoustical, mechanical and thermal properties of sustainable green Yucca (Y. Gloriosa) fibers: an exploratory investigation. J. Environ. Health Sci. 18, 883-896. https://doi.org/10.1007/s40201-020-00513-9.
8. Yun, B.Y.; Cho, H.M.; Kim, Y.U.; Lee, S.C.; Berardi, U.; Kim, S. (2020) Circular reutilization of coffee waste for sound absorbing panels: A perspective on material recycling. Environ. Res. 184, 109281. https://doi.org/10.1016/j.envres.2020.109281.
9. Aly, N.M.; Seddeq, H.S.; Elnagar, Kh.; Hamouda, T. (2021) Acoustic and thermal performance of sustainable fiber reinforced thermoplastic composite panels for insulation in buildings. J. Build. Eng. 40, 102747. https://doi.org/10.1016/j.jobe.2021.102747.
10. Samaei, S.E.; Berardi, U.; Taban, E.; Soltani, P.; Mousvi, S.M. (2021) Natural fibro-granular composite as a novel sustainable sound-absorbing material. Appl. Acoust. 181, 108157. https://doi.org/10.1016/j.apacoust.2021.108157.
11. Taban, E.; Amininasab, S.; Soltani, P.; Berardi, U.; Abdi, D.D.; Samaei, S.E. (2021) Use of date palm waste fibers as sound absorption material. J. Build. Eng. 41, 102752. https://doi.org/10.1016/j.jobe.2021.102752.
12. Yuvaraj. L.; Jeyanthi, S.; Yogananda, A. (2021) An acoustical investigation of partial perforation in jute fiber composite panel. Mater. Today Proc. 37, 665-670. https://doi.org/10.1016/j.matpr.2020.05.632.
). These show high efficiency in noise reduction or thermal insulation. However, these are expensive to produce and, in addition, environmental factors or health hazards must be considered.

The growing environmental awareness around the world is the main reason why many researchers have developed environment-friendly bio-composites, derived from the significant amount of waste generated globally. These wastes are ever-increasing, and their disposal is becoming extremely difficult worldwide. These developments have shown that a wide range of these by-products can be potentially applied in acoustics panels.

Buratti et al. (11. Buratti, C.; Belloni, E.; Lascaro, E.; López, G.A.; Ricciardi, P. (2016) Sustainable panels with recycled materials for building applications: environmental and acoustic characterization. Energy Procedia. 101, 972-979. https://doi.org/10.1016/j.egypro.2016.11.123.
) characterized sustainable panels made of recycled paper and other scrap materials, such as wool and nonwoven polyester fabric, to improve the acoustic comfort of a lecture room. Guna et al. (22. Guna, V.; Llangovan, M.; Hu, C.; Venkatesh, K.; Reddy, N. (2019) Valorization of sugarcane bagasse by developing completely biodegradable composites for industrial applications. Ind. Crop. Prod. 131, 25-31. https://doi.org/10.1016/j.indcrop.2019.01.011.
) developed new biodegradable acoustic panels made of sugarcane bagasse for their uses as ceiling tile. These panels were characterized for their flexural strength, and thermal, water and acoustic resistance. Ali et al. (33. Ali, M.; Alabdulkarem, A.; Nuhait, A.; Al-Salem, K.; Iannace, G.; Almuzaiqer, R.; Al.turki, A.; Al-Ajlan, F.; Al-Mosabi, Y.; Al-Sulaimi, A. (2020) Thermal and acoustic characteristics of novel thermal insulating materials made of Eucalyptus Globulus leaves and wheat straw fibers. J. Build. Eng. 32, 101452. https://doi.org/10.1016/j.jobe.2020.101452.
) developed biodegradable natural thermal insulating and sound-absorbing materials for building walls. In the work of Guna et al. (44. Guna, V.; Ilangovan, M.; Rather, M.H.; Giridharan, B.V.; Prajwal, B.; Vamshi Krishna, K.; Venkatesh, K.; Reddy, N. (2020) Groundnut shell / rice husk agro-waste reinforced polypropylene hybrid biocomposites. J. Build. Eng. 27, 100991. https://doi.org/10.1016/j.jobe.2019.100991.
), two agricultural residues, namely rice husk and groundnut shell, were used to develop bio-composites for green building materials. These bio-composites showed acoustic properties comparable to commercially used gypsum ceiling tiles. Raj et al. (55. Raj, M.; Fatima, S.; Tandon, N. (2020) An experimental and theoretical study on environment-friendly sound absorber sourced from nettle fibers. J. Build. Eng. 31, 101395. https://doi.org/10.1016/j.jobe.2020.101395.
) characterized the nettle fibers to obtain physical properties linked to their acoustic performance. Samaei et al. (66. Samaei, S.E.; Mahabadi, H.A.; Mousavi, S.M.; Khavanin, A.; Faridan, M.; Taban, E. (2020) The influence of alkaline treatment on acoustical, morphological, tensile and thermal properties of Kenaf natural fibers. J. Ind. Text. 3, 1528083720944240. https://doi.org/10.1177/1528083720944240.
) studied experimentally and theoretically the acoustical and thermal properties of composites made of kenaf natural fibers when these were treated chemically with an alkaline treatment. Taban et al. (77. Taban, E.; Mirzaei, R.; Faridan, M.; Samaei, E.; Salimi, F.; Tajpoor, A.; Ghalenoei, M. (2020) Morphological, acoustical, mechanical and thermal properties of sustainable green Yucca (Y. Gloriosa) fibers: an exploratory investigation. J. Environ. Health Sci. 18, 883-896. https://doi.org/10.1007/s40201-020-00513-9.
) characterized morphological, acoustical, mechanical and thermal properties from fibers extracted from the leaves of Yucca and treated chemically. Yun et al.(88. Yun, B.Y.; Cho, H.M.; Kim, Y.U.; Lee, S.C.; Berardi, U.; Kim, S. (2020) Circular reutilization of coffee waste for sound absorbing panels: A perspective on material recycling. Environ. Res. 184, 109281. https://doi.org/10.1016/j.envres.2020.109281.
) showed the possibility of using sound-absorbing materials made of coffee waste inside commercial spaces through a novel technique for recycling this type of waste. In the work of Aly et al. (99. Aly, N.M.; Seddeq, H.S.; Elnagar, Kh.; Hamouda, T. (2021) Acoustic and thermal performance of sustainable fiber reinforced thermoplastic composite panels for insulation in buildings. J. Build. Eng. 40, 102747. https://doi.org/10.1016/j.jobe.2021.102747.
), the acoustic and the thermal insulation performance of composites, made of jute, polyester and hybrid jute-polyester with polypropylene as the matrix, were studied. Samaei et al. (1010. Samaei, S.E.; Berardi, U.; Taban, E.; Soltani, P.; Mousvi, S.M. (2021) Natural fibro-granular composite as a novel sustainable sound-absorbing material. Appl. Acoust. 181, 108157. https://doi.org/10.1016/j.apacoust.2021.108157.
) developed and optimized fibro-granular composites with kenaf fibers and waste rice husk granules bonded with a biodegradable polymer, polyvinyl alcohol. Taban et al. (1111. Taban, E.; Amininasab, S.; Soltani, P.; Berardi, U.; Abdi, D.D.; Samaei, S.E. (2021) Use of date palm waste fibers as sound absorption material. J. Build. Eng. 41, 102752. https://doi.org/10.1016/j.jobe.2021.102752.
) studied the acoustical performance of low-cost sound-absorbing panels made of date palm waste fibers bonded with polyvinyl alcohol (PVA) solution, showing that, with their use, the acoustic performance enhanced. Yuvaraj et al. (1212. Yuvaraj. L.; Jeyanthi, S.; Yogananda, A. (2021) An acoustical investigation of partial perforation in jute fiber composite panel. Mater. Today Proc. 37, 665-670. https://doi.org/10.1016/j.matpr.2020.05.632.
) developed jute fiber composite panels undergone partial perforation that had significant sound-absorbing effects. The advantages of using these sustainable products in acoustic applications, like acoustic panels, were a combination of very light mass, high physical stability, low cost, and high values of acoustic absorption. For environmental performance, these types of panels can be considered more acceptable from the health point of view and better suited to operate in an aggressive environment.

Cork, obtained from the cork oak tree (Quercus suber L.), is a sustainable and natural raw material that grows extensively in countries with a Mediterranean climate, such as Portugal or Spain (1313. Maderuelo-Sanz, R.; Barrigón Morillas, J.M.; Gómez Escobar, V. (2014) Acoustical performance of loose cork granulates. Eur. J. Wood. Wood. Prod. 72, 321-330. https://doi.org/10.1007/s00107-014-0784-x.
). White cork granulate is a by-product obtained from stoppers for wine, beer or champagne (1414. Maderuelo-Sanz, R.; Barrigón Morillas, J.M.; Gómez Escobar, V. (2014) The performance of resilient layers made from cork granulates mixed with resins for impact noise reduction. Eur. J. Wood. Wood. Prod. 72, 833-835. https://doi.org/10.1007/s00107-014-0845-1.
). Most of the uses of white cork granulate, due to its acoustical and mechanical properties, are associated with material researchers for different applications, where their inclusion in a high number of composite materials for building applications are also numerous: as resilient layers made from cork granulates mixed with polyurethane and epoxy resins (1414. Maderuelo-Sanz, R.; Barrigón Morillas, J.M.; Gómez Escobar, V. (2014) The performance of resilient layers made from cork granulates mixed with resins for impact noise reduction. Eur. J. Wood. Wood. Prod. 72, 833-835. https://doi.org/10.1007/s00107-014-0845-1.
), as a core element in noise barriers (1515. Maderuelo-Sanz, R.; Barrigón Morillas, J.M. (2019) Loose cork granulates as possible new core element in noise barriers. Eur. J. Wood. Wood. Prod. 77, 1229-1231. https://doi.org/10.1007/s00107-019-01464-1.
), as a lightweight aggregate for cement-based materials (1616. Pacheco Menor, M.C.; Serna Ros, P.; Macías García, A.; Arevalo Caballero, M.J. (2019) Granulated cork with bark characterised as environment-friendly lightweight aggregate for cement based materials. J. Clean. Prod. 229, 358-373. https://doi.org/10.1016/j.jclepro.2019.04.154.
) or as a thermal insulation panel in buildings, both in facades and roofs (1717. Sierra-Pérez, J.; García-Pérez, S.; Blanc, S.; Boschmonart-Rives, J.; Gabarrell, X. (2018) The use of forest-based materials for the efficient energy of cities: Environmental and economic implications of cork as insulation material. Sustain. Cities Soc. 37, 628-636. https://doi.org/10.1016/j.scs.2017.12.008.
).

This work is a first study to evaluate new acoustic panels made of cork granulates coming from stopper by-products bonded with a water-based acrylic resin to be used as acoustic ceilings, providing sustainable and environmentally friendly alternatives for synthetic materials.

2. MATERIALS AND METHODS

Previously, granulate cork samples were dried in an oven at 70 °C for 24 h to eliminate the possible moisture. Subsequently, cork grains were separated using five calibrated sieve sets to extract grains with sizes between 3 and 7 mm, weighed and introduced together with the binder, 25% in mass, in an industrial mixer until a complete homogenisation was achieved. Two polymers, one water-based acrylic resin (AC) with a density of 1.00 g cm^-3 and another water-based epoxy resin (EP) with a density of 1.05 g cm^-3, both from Globalpaint Coating S.L., were used for manufacturing the acoustic panels. Once obtaining the homogeneous mixes, they were placed into different moulds, and in metallic nets for 24 h to let the binder percolate freely, obtaining samples with the highest porosity and avoiding the creation of impermeable layers on the bottom side of the sample (Table 1). Subsequently, the samples were placed in an oven at 60 ºC for 24 h to be cured. When the samples were cooled, they were removed from the moulds and cut to prepare the test samples.

The flexural test was used to measure the flexural strength when applying a load at the centre of the sample. Several parameters could affect this, such as bulk density or thickness sample. Three-point bending tests were carried out using an AUTOGRAPH AG-IS 20 kN universal testing machine from Shimadzu according to the norm ASTM D7264-2021(1818. ASTM International: Designation: D7264/D7264M-2021, Standard test method for flexural properties of polymer matrix composite materials.
). The sample was subjected to a quasi-static load at its centre at a speed range from 600 Ns^-1 to 1200 Ns^-1. A load-displacement curve was recorded using Shimadzu’s TRAPEZIUM software. The flexural stress, σ_f (MPa), and the flexural strain, ε_f (mm mm^-1), were obtained and recorded for any point on the load-deflection curve for each sample according to Equations [1] and [2], and the flexural modulus of elasticity, E_f (MPa), was obtained according to the Equation [3]:

where F is the applied force (N), L, b and h are, the length, the width and the thickness of the sample (mm), respectively, δ is the recorded deflection at the centre of the sample (mm) and m the slope of the force-deflection curve. Samples were processed using a rectangular mould (200 mm width, 100 mm length, 40 mm thickness). Due to the possible use of these bio-composites, as suspended acoustic ceiling panels, where it is necessary to allow some deformation of these panels, their deformations were also evaluated in this work. Five different bio-based composites were prepared and tested.

To provide further information about the mechanical properties of the bio-composites, the deformability of the samples was also analysed using the failure tensile energy (G) and the resistance coefficient to cracking propagation (R). The first is strongly dependent on the maximum flexural force and can be evaluated by the area below the force vs. the load-deflection curves. In the latter, the force component is not taken into account, and it can be defined by the relationship between the failure tensile having the maximum flexural force (R = G/F_max ).

The dynamic modulus of elasticity E_d was determined by a non-destructive methodology, the fundamental resonance frequency method, based on the standard EN 14146-2004 (1919. European Committee for Standardization: EN 14146-2004 Natural stone test methods. Determination of the dynamic modulus of elasticity (by measuring the fundamental resonance frequency).
). This methodology was performed using a Zeus ZRM 2005 test machine, with the Longitudinal Resonance Frequency procedure. Measuring the transit time measurements for longitudinal ultrasound pulses, the resonance frequency of the first fundamental mode was obtained. The dynamic E_d tests were performed on five specimens for each bio-composite configuration and subsequently averaged. E_d was determined using the following expression:

The acoustic properties of the bio-based composites, sound absorption coefficient at normal-incidence, were carried out according to ISO standard 10534-2 (2020. ISO 10534-2:1998 Acoustics-Determination of sound absorption coefficient and impedance in impedance tubes. Part 2: Transfer function-method. International Organization for Standardization, Geneva.
). Figure 2 shows the experimental set-up according to (2020. ISO 10534-2:1998 Acoustics-Determination of sound absorption coefficient and impedance in impedance tubes. Part 2: Transfer function-method. International Organization for Standardization, Geneva.
). The loudspeaker is located at the beginning of the tube. The sample holder is located at the end of the tube, in a rigid termination. The surface of the sample is faced normal to the direction of the incoming sound waves. Random pink noise is generated into the impedance tube through the loudspeaker. Two microphones separated 5 cm, measure the sound pressure level inside the impedance tube.

A Brüel & Kjær impedance tube type 4206T and two 1/4” Condenser Microphones Type 4187 were used. Signals were analysed with a portable Brüel & Kjær PULSE System with four input data channels (type 3560-C). This tube kit consisted of one low-frequency and one high-frequency impedance tube with diameters of 100 and 29 mm, respectively. The first was employed for measurements over the range of frequencies from 100 Hz to 1600 Hz, and the latter over the range of frequencies from 500 Hz to 6400 Hz. Previously to start any measurement, and after the sample positioning, the influencing environmental parameters (atmospheric pressure, air temperature and relative humidity) were measured and introduced in the software. The temperature inside the impedance tube needed to be stabilized. Sound velocity, c₀ , and air density, ρ_a , were evaluated according to the next equations:

where T is air temperature in Kelvin, ρ₀ = 1.186 kg m^-3, p₀= 101.325 kPa, T₀ = 293 K and p_a the atmospheric pressure. Microphones calibration was accomplished by using a Brüel & Kjær calibrator type 4231 at 94 dB level at 1 kHz. The evaluation of the signal-to-noise ratio was carried out in order to obtain at least 10 dB of difference between them. Finally, the transfer function calibration for the channels phase displacements was evaluated. For each acoustic panel, five different samples were measured and subsequently averaged.

This method shows advantages and disadvantages in its use (2121. Oldham, D.J.; Egan, C.A.; Cookson, R.D. (2011) Sustainable acoustic absorbers from the biomass. Appl. Acoust. 72, 350-363. https://doi.org/10.1016/j.apacoust.2010.12.009.
). As an advantage that it is a practical method because the apparatus itself is small, and only small samples are required for the tests. Two acoustical properties can be measured, the surface impedance and the absorption coefficient. As for a disadvantage, these acoustics properties are only measured for sound at normal-incidence to the sample, although it is possible to obtain approximated values of the random incidence absorption coefficient from the surface impedance values, applying a correction. When the materials are heterogeneous, some uncertainties can be introduced. It is due to the pore structure of the samples that may vary considerably when samples are taken from different regions of a large sample. Two different tubes, and samples having different sizes, were used to obtain values in a wide range of frequencies.

To evaluate the sound absorption capability of the samples, single number grading methods, which are independent of frequencies, were used. This index is useful for a practical evaluation of the performance of sound porous absorbers. For this purpose, the ASTM C423-2017 norm (2222. ASTM International: Designation: C423-2017 Standard test method for sound absorption and sound absorption coefficients by the reverberation room method.
) defines the Sound Absorption Average (SAA). SAA is defined as the average of the sound absorption coefficients for 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000 and 2500 Hz, rounded off to the nearest 0.01. Six samples were tested, and the SAA averages were obtained for each grain group.

3. RESULTS AND DISCUSSION

Table 2 shows σ_f , εf and E_f , obtained at the maximum load for each sample. It should be noted that the samples with acrylic resin showed the highest values for E_f , ranging from 0.59 to 0.71 MPa, while samples with epoxy resin showed lower values ranging from 0.33 to 0.66 MPa. These values were according to the results obtained for similar bio-based composites showed in others works (2323. Binici, H.; Aksogan, O.; Demirhan, C. (2016) Mechanical, thermal and acoustical characterizations of an insulation composite made of bio-based materials. Sustain. Cities Soc. 20, 17-26. https://doi.org/10.1016/j.scs.2015.09.004.
, 2424. Khalaf, Y.; El Hage, P.; Mihajlova, J.D.; Bergeret, A.; Lacroix, P.; El Hage, R. (2021) Influence of agricultural fibers size on mechanical and insulating properties of innovative chitosan-based insulators. Constr. Build. Mater. 287, 123071. https://doi.org/10.1016/j.conbuildmat.2021.123071.
). As expected, dynamic E_d values were slightly higher than the static E _f values. In the case of the rupture energy, R, the results for samples made of acrylic resin are higher than samples made of epoxy resins due to the stiffness of the epoxy resin once the latter is dried. These results ranged between 6.02 to 10.66 mm for the first, while for the latter, this range was 3.55 to 6.53 mm. As is well known, the larger the R, the larger the energy needed to produce micro-cracking in the bio-based composite, so the less probable that evolution is. For the samples under analysis, the sample AC_3 has the higher failure tensile energy and the higher resistance coefficient to cracking propagation, showing an increase of ductility just before failure. Samples made with acrylic resin revealed the highest influence on the G and R coefficients, incrementing these coefficients in comparison with the samples made of epoxy resin.

Sound absorption spectra at normal incidence for acrylic and epoxy samples, having 2 cm (solid line) and 4 cm (dash line) in thickness, are shown in Figures 3 and 4, respectively. It should be noted that a displacement of the first absorption maximum to lower frequencies can be observed due to the increasing thickness of the sample. Moreover, for smaller grain sizes, the values of the sound absorption coefficients were slightly greater than for larger grains. The values of the sound absorption coefficients for the first maximum ranged from 0.42 to 0.65 for samples having 2 cm in thickness and 0.53 to 0.76 for samples having 4 cm in thickness, in the case of the acrylic samples. In the case of the epoxy samples, these values ranged from 0.46 to 0.59 for samples with 2 cm in thickness and 0.67 to 0.82 for samples with 4 cm in thickness.

The effect of the air gap on the sound absorption coefficient was also investigated. This technique is already known in an acoustic absorber to obtain better sound absorber performance for thin thicknesses. Figure 5 shows the effect of the air gap on the sound absorption coefficient of the bio-based composites in the case of the sample AC-3. The air-back cavity between the sample tested and the back-rigid wall also led to an improvement in the sound absorption performance, mainly at low frequencies. This improvement can be observed in the absorption coefficient spectra where the first absorption maximums were shifted to lower frequencies while the values of the sound absorption coefficient become reduced. Table 3 shows the Sound Absorption Average (SAA) obtained for the samples. The air-back cavity between the sample and the back-rigid wall also led to an improvement in the sound absorption performance. This improvement can be observed in the SAA value. This is a very useful effect to improve the sound absorption at low frequencies, instead of increasing the absorber thickness. The SAA differences, in percentage, between bonded samples (BS) and bonded samples with an air-gap of 3 cm (BS-3) and 6 cm (BS-6) can be observed in Table 3. The lower the thickness of the samples, the greater the difference is. This is more evident in samples having 2 cm in thickness where differences are up to 36 % and 52% for 3 cm and 6 cm of air-gap respectively, and more pronounced in samples having grains with lower size (samples EP_3 and AC_3).

4. CONCLUSIONS

This first research explores new avenues on using cork granulate, deriving from cork stopper processes, into acoustic panels. With the configurations used in this work, bio-based composites showed good acoustical performance, mainly at medium to high frequencies. In the case of low frequencies, the sound absorption coefficient is low. The samples reached good sound absorption average values with low thicknesses when introducing an air-gap behind the sample, leading to a considerable decrease in material quantity to be used, and therefore, in weight. Moreover, mechanical properties obtained showed acceptable values for the purpose as they were studied. Therefore, these types of bio-based panels could be used as an alternative product to the traditional materials (glass wool, foams or rock wool) used for suspended acoustic ceiling panels inside commercial spaces, such as closed entertainment areas.