Enhanced acoustic damping in flexible polyurethane foams filled with carbon nanotubes
Introduction
The inclusion of carbon nanotubes (CNTs) in polymer matrices has already been shown to improve their mechanical, electrical and thermal properties [1], [2]. The reinforcement of delicate systems where conventional fibres cannot be physically accommodated, such as polymer films [3], [4], [5], [6], [7], [8], [9], [10], [11], fibres [6], [12], [13], [14], [15] and foams [11], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], offers unique opportunities. However, most studies on nano-filled foams have focused on nanoclays [11], [16], [17], [18], [19], [21], [25], [26], [27], [29], [32], [36], [37], whereas the properties of carbon nanofibre (CNF) or CNT-reinforced polymeric foams have received only limited attention [28], [30], [31], [33], [34]. These existing studies have, nevertheless, highlighted the potential benefits on both the foaming process and mechanical behaviour of the final nanocomposite foams.
Polymeric foams are important and versatile materials due to their outstanding strength-to-weight ratio, their resilience, and their electrical, thermal, and acoustic insulating properties, amongst other characteristics [38]. Polyurethane is one of the largest and most versatile families of polymers. The control of parameters such as functionality, chemical composition, and molecular weight of the different reactants, produces a wide set of materials with significantly different properties [39], [40]. This versatility has led to the use of polyurethanes as foams, elastomers, coatings, sealants, and adhesive-based products. However, the main market for PU is in polymeric foams, which can be flexible or rigid. While rigid PU foams are mainly used for thermal insulation, the flexible varieties are used as cushioning materials in furnishings, transportation and packaging applications. These foams offer degrees of comfort, protection, and utility not matched by any other single cushioning material. By selecting the reactants and manufacturing process appropriately, PU foam can be made to satisfy a wide-range of applications (see [40] for a full list).
Free-rise polyurethane foams are generally obtained from the simultaneous reaction of a polyisocyanate with a polyol matrix and water. The reaction of the polyol with isocyanate produces urethane bonds. Meanwhile, the reaction of the isocyanate with water generates CO2 which drives the foam expansion. The polymer structure must build up rapidly to support the fragile foam, i.e. to form a stable cellular structure, but not so fast as to stop bubble growth. These two competing reactions are balanced by the addition of catalysts and surfactants.
Historically, carbon black has mainly been introduced into rigid PU foams to improve thermal insulation properties by increasing the infrared absorption of the cell walls and edges (US5373026A1, US5604265A1) and as a fire retardant particle (WO2005073267A1). Recent studies have examined PU foams loaded with different types of nanofillers, including nanoclay [16], [32], titania nanoparticles [23], [24] and silica [20]. A CNT/PU rigid foam has also been developed as the core of a sandwich structure for radar absorbing panels [28]. However, to the authors’ knowledge, no straightforward, CNT-filled, reactive, flexible, polyurethane foam has been developed previously. In this study, we report the fabrication and properties of CNT-filled reactive PU (PUR) foams.
Section snippets
Materials and sample preparation
Both isocyanate and polyol used in this study were supplied by Bayer Group. The isocyanate was a modified diphenylmethane-4,4′-diisocyanate (MDI), Desmodur TP:PU 1805 (NCO content: 28 wt%) and the polyol was polyether-based, Bayfit TP.PU 40IK05 (OH value: 248.8). The catalyst used is a tertiary amine (Dabco 33-LV) and the surfactant is a silicone glycol (Dabco DC2585) from AirProducts and Chemicals.
Aligned multi-walled carbon nanotubes (MWCNTs) were grown by the CVD injection method based on
Results and discussion
CNT-filled PU flexible foams were produced with loading fractions up to 0.2 wt%. Both as-produced and oxidised CNTs were introduced initially. As-produced CNTs were reasonably dispersed in the system, but some agglomerates were visible both during the mixing stage and in the final foam. On the other hand, oxidised CNTs readily yielded a better dispersion with no visible agglomerates. The different outcome can be attributed to both the length reduction suffered by the CNTs during the oxidative
Conclusions
The present work demonstrates a straightforward route to the production of CNT-filled flexible PU foams through a direct reaction process. This system has potentially wide relevance as polyurethanes have amongst the top six sale volumes of all polymers worldwide. Here, an initial study has shown that even very low CNT loading fractions can deliver a significant increase in the sound absorption capabilities, one of the critical properties for such systems, as well as improving the
Acknowledgement
R.V. thanks EPSRC for funding.
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Present address: Institute of Polymer Science and Technology, Consejo Superior de Investigaciones Cientificas (CSIC), 28006 Madrid, Spain.