Development of layer-by-layer assembled carbon nanofiber-filled coatings to reduce polyurethane foam flammability
Graphical abstract
Introduction
The estimated annual societal cost of soft furnishing (mattresses and upholstered furniture) fires to the United States economy is $5 billion [1], [2], [3], [4]. These are among the deadliest as they account for 5% of all residential fires annually, but are responsible for a disproportionately high portion of the fire losses (33% of the civilian fatalities, 18% of civilian injuries, and 11% of the property losses). Over the next decade, federal flammability performance regulations are expected to significantly reduce these fire losses [5], [6], [7]. Soft furnishing manufactures are and will likely continue to comply with these current and proposed flammability regulations by using fire blocking barrier fabrics. Despite this compliance, engineering and technical options to comply are quickly diminishing because of mandated sustainability regulations, such as REACH [8] and EcoLabel [9], for consumer products. Using Layer-by-layer assembly to create a fire resistant armor on the components in soft furnishings is being evaluated in this project as a novel technology to enable these manufacturers to comply with flammability and sustainability regulations.
Layer-by-layer (LbL) assembly has been extensively studied for the past 20 years as a methodology to create multifunctional thin films generally less than 1 micron thick [10], [11], [12]. These thin films were commonly fabricated through alternate deposition of a positively charged layer and negatively charge layer (called a bilayer, BL). By taking advantage of electrostatic [13], H-bonding [14], covalent bonds [15], and/or donor/acceptor interactions, these bilayers were assembled on the surface of substrates. The LbL process is quite flexible and robust, which allows it to be tuned for specific coating characteristics and for coating a range of substrate types. For example, altering the concentration, pH, and/or temperature of the LbL solutions can result in a 1 nm rather than 100 nm thick BL [16], [17].
LbL thin films have been used in an extensive breadth of applications, such as oxygen barriers [18] and sensors [19], and have useful properties, such as antimicrobial [20] and antireflection [21]. For more than a decade clay has been used as an additive in polymers since clay has shown to simultaneously improve the mechanical and fire performance attributes of polymers [22], [23], [24]. More recently, fabrication of clay containing LBL thin films has been studied [18], [25], [26]. Li et al. [12] focused on using LbL clay coatings (sodium exchanged montmorillonite) on cotton fabric to improve the fire performance characteristics of this textile, which is directly aligned with the research presented in this manuscript. Their results are exciting in that uniform high quality clay based coatings on cotton were achieved. In addition, the clay coatings resulted in a significant retention of fabric like char after conducting vertical burn tests and there was no (or less) ember afterglow when the flame was removed. These results suggest the coating may better prevent thermal and flame penetration from reaching and igniting the polyurethane foam (PUF), and therefore, a clay/cotton ticking or barrier may reduce flame spread in residential homes if used in soft furnishings.
Another additive filler that has gained increasing attention for improving the properties of polymeric materials is carbon nanofiber (CNF). Carbon nanofibers (CNFs) are cylindrical nanostructures constructed of stacked graphitic cones or cups. Compared to carbon nanotube (CNT), CNF can be at least an order of magnitude larger, with a diameter and length in the range of 5 nm–300 nm and 0.1 μm–1000 μm, respectively. Due to the intrinsic electrical, thermal, and mechanical properties of CNF, the thermal and electrical conductivity, tensile and compressive strength, ablation resistance, damping properties, and flammability of polymers [27] have been significantly altered with their incorporation [28].
Zammarano recently reported a reduction in PUF flammability by the incorporation of CNFs directly into the polyurethane matrix [27]. At a CNF loading of 4 mass fraction %, the CNFs formed a network structure that reduced the peak heat release rate (PHRR) in burning PUF by 35% and prevented melt dripping, which in a real fire scenario, could result in an additional 30% reduction in PHRR [29]. The approach of incorporating CNFs into the PUF has a few potential drawbacks. For example, commercialization may be difficult, as the foam manufacturing process is quite sensitive to small changes in recipe, especially the presence of solid particles, and the manufacturing conditions. Another potential drawback is based on the mechanism by which CNFs are believed to reduce polymer flammability [30]. This reduction in flammability is believed to result from the formation of a char at the surface that thermally protects the polymer and prevents volatilization of degradation products [31]. Since the CNFs are dispersed and distributed throughout the polymer matrix, the polymer has to burn for some time before enough of the polymer is pyrolyzed that a high enough concentration of CNFs can aggregate at the surface to form the protective char layer (armor).
The research presented here is unique in that it is the first published report of fabricating carbon nanofiber (CNF) based thin films/coatings using LbL assembly, of fabricating LbL coatings on foam (polyurethane foam, PUF), and of altering the fire performance attributes of foam using this thin film techniques. The large CNF dimensions are undesirable for typical applications of LbL coatings, as the coating thickness is generally comparable to the CNF dimensions and it facilitates aggregation both in the fabrication solutions and in the coatings. However, for reducing flammability, the larger dimensions may enable the formation of a CNF network armor that protects the foam. The thin coating approach is believed to be ideal for reducing the flammability of foam as it may more quickly form the char-like armor because the high concentration of CNFs is already at the surface rather than randomly mixed throughout the polymer. Provided are the details of fabricating CNF coated polyurethane foam (CNF/PUF) using LbL, the physical characteristics of the LbL CNF coatings on PUF, and the measured fire performance of PUF and CNF/PUF.
Section snippets
Materials
All materials were used as-received from the supplier unless otherwise indicated. Branched polyethylenimine (PEI, branched, mass average molecular mass = 25,000 g/mol) and poly (acrylic acid) (PAA, mass average molecular mass = 100,000 g/mol) were obtained from Sigma–Aldrich (Milwaukee, WI). PR-24-XT-PS carbon nanofibers (CNF, average diameter = 150 nm ± 100 nm, length was 65μm ± 30 μm) were obtained from Pyrograf Products Incorporated (Cedarville, Ohio). The standard (untreated) polyurethane
CNF coating characterization
According to QCM (Fig. 2) and profilometry (Fig. 3), the coating growth occurs in two stages. Up to three bilayers there is very little increase in mass or coating thickness. Interpolation of this data indicates less than 50 nm thickness and 12 μg/cm2 after three bilayers. At four bilayers the coating growth rate rapidly increases with each bilayer increasing the coating thickness by 87 nm ± 10 nm. This is consist with qualitative observations during the LbL process where the foam transitioned
Conclusions
For the first time, LbL assemblies made with CNFs are shown to improve the fire performance of polyurethane foam. The process described here generates thin film coatings that completely cover all internal and external surfaces of the porous polyurethane foam. Even though the CNF distribution is not completely uniform, and the CNFs are not completely embedded in the polymer coating, the coating significantly reduces the flammability of foam. This LbL coating significantly reduces the heat
Acknowledgements
The authors thank Professor Alexander Morgan at University of Dayton Research Institute for cone calorimeter testing.
References (35)
- et al.
Coordination Chemistry Reviews
(2009) - et al.
Thin Solid Films
(2008) - et al.
Polymer Degradation and Stability
(2003) - et al.
Composites Science and Technology
(2007) Total cost of fire in the United States
(2010)- et al.
Fire Technology
(1989) - et al.
2005-2007 residential fire loss estimates
(2010) Home fires that began with upholstered furniture
(2008)16 CFR 1632 Standard for the flammability of mattresses and mattress pads
(1991)16 CFR 1633 Standard for the flammability (open flame) of mattress sets
(2006)
16 CFR 1634 Standard for the flammability of residential upholstered furniture
Polyelectrolyte multilayers, an overview
Acs Nano
Langmuir
Liquid Crystals
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