Natural cork agglomerate enabled mechanically robust rigid polyurethane foams with outstanding viscoelastic damping properties
Graphical abstract
Introduction
Rigid polyurethane (PU) foams are often shaped in a three-dimensional and cross-linked polymer-based cellular solid with a closed-cell structure. The rigid PU foams have been widely used in various applications since they possess outstanding thermal and electrical insulation properties, low apparent density, chemical resistance, and affordable price [[1], [2], [3]]. In particular, thanks to the lightweight along with low cost, they are so popular in structural applications including building construction, insulation, electrical appliances, and sandwich composite structures, and so on [[4], [5], [6]]. Significant efforts have been made to improve the performance of rigid PU foams for the use of structural applications. SANTIAGO-CALVO et al. reported the effect of nanoclay and nanosilicas on the structure and mechanical properties of rigid PU foams [7]. The chemical groups on the surface of nanoparticles affected the blowing and enhanced the mechanical properties with increased urea groups. ZHANG, Zhiyong, et al. studied on expanded graphite reinforced rigid PU foams, which can engaced flame retardancy and mechanical properties [8]. Sylwia et al. synthesized rigid polyurethane composite foams with silanized walnut shells as cellulosic fillers (1–5 wt%). The silanized walnut shells improved the mechanical properties, such as compressive strength (~15%), tensile strength (~9%) impact strength (~6%) [9]. In addition, a number of reinforcements and including fibers [[10], [11], [12]] (e.g., cellulose, lignocellulosic fiber, glass fiber, and carbon fiber), nanocarbon materials [7,13,14], natural materials [10,15,16] have been explored in order to enhance the material properties of polyurethane. However, the researches on rigid polyurethane foam were mainly focused on flame retardancy and mechanical properties. The study on viscoelastic damping properties of composite foams based on rigid polyurethane is not sufficient despite various uses of rigid polyurethane foams.
Cork is an outer bark of Quercus suber L., and it is peeled from the cork oak trees, which commonly live more than 200 years, periodically for every 9 years [17,18]. Cork has been well reported for its extraordinary and remarkable mechanical properties [4,17,19]. Because of its combination of aligned prismatic closed-cells, chemical composition (suberin, lignin, cellulose, etc.) and their unique structural arrangement, the natural corks possess very unique and intriguing properties, such as super-compressibility without any failures, nonlinear elasticity, and outstanding dimensional recoverability, which eventually can result in excellent energy-absorbing capability. In addition to its mechanical properties, other attractive features include excellent thermal insulation properties and impermeability to gases or liquids [17,20]. Motivated by the natural cork's extraordinary properties, several investigations on solid polymers with the addition of the cork agglomerates have been carried out [[21], [22], [23], [24]]. REIS, P. N. B., et al. studied the static viscoelastic properties of epoxy composites filled with cork powder [25]. For the stress relaxation, the composites filled with cork powder exhibited higher stress decreases than neat resin when any fiber filler was applied to the composites. However, to the best of our knowledge, there seems to hardly exist a report on the influence of cork particles for viscoelastic damping properties, not only mechanical properties [26,27].
Here, natural cork particles were employed as reinforcement in the rigid PU foam in this study. The Cork/PU composite foams were fabricated with four different loading fractions of the cork particles (0, 1, 3 and 5 wt%) and then characterized for their uncovered material properties, including the mechanical and viscoelastic properties. Their cellular microstructural morphology-properties relationship was systematically investigated in this study.
Section snippets
Materials
For synthesis of polyurethane foams, polyol, polymeric diphenyl diisocyanate (PMDI), catalysts, and blowing agents were used and supplied by BASF Co. Polyol was a mixture composed of a sucrose base polyol (hydroxyl value: 490 mgKOH/g), glycerine base polyol (hydroxyl value: 560 mgKOH/g), and ester base polyol (hydroxyl value: 300 mgKOH/g). The PMDI with 2.5 average functionality of NCO and 31 ± 1 wt% of NCO content was selected for isocyanate. Tertiary amine-type catalysts,
Reactivity of foam formation
The reactivity of the foam formation with respect to the cork loading fraction was investigated by measuring the cream and gel times. The cream time was identified when the color of mixed reactants turns to bright ivory color resulting from the blowing. The gel time was measured at the time when it takes for the foam to start for consolidation by the reaction of urea and urethane linkage and/or crosslinking. Expectedly, with the increase of the cork particle loading fraction in the cork/PU
Conclusion
The Cork/PU composite foams were fabricated with four different loading fractions of cork particles (0, 1, 3, and 5 wt%). Cork, a natural and renewable material, affected the composite synthesis process's reactivity with its outstanding thermal insulating property. This study clearly showed that cork loading fraction has linear relation with reaction rate, cell size, and FRD of Cork/PU composite. In the compression test, compare to the pristine rigid PU foam, the 5 wt% Cork/PU composite foam
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work has been financially supported by the Air Force Office of Scientific Research under FA2386-19-1-4029 and FA2386-19-1-4082. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2018R1A2B2001565).
This work was supported by the Technology Innovation Program (20013794, Center for Composite Materials and Concurrent Design) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). AMORIM for kindly providing us with
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Both authors contributed equally to this work.