Preparation and Applications of Carbon Nanotube Coatings to Reduce the Flammability of Polymeric Materials - II

In the April-May issue of Turkcoat Magazine, within the scope of the aforementioned study, which was presented with a general abstract, introduction and experimental section, single-walled carbon nanotubes were first functionalized to reduce the flammability of foams. Functionalized carbon nanotubes were deposited onto polyurethane foam. Additionally, a polyethyleneimine layer supported the interaction between the carbon nanotube and polymer layers, enabling the achievement of a uniform, durable and thick coating (Figure 1).

Figure 1. SWCNT / polymer coating process.

Characterization of the coatings was performed using techniques such as thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and others. For this purpose, the coated sample was kept in a desiccator and the mass change in the sample (before and after coating) was determined based on the fact that it would be the mass of the LbL (layer-by-layer coating) coating. The SWCNT mass concentration in the coating was measured using Thermogravimetric Analysis (TGA) at 800°C and heating to 800°C at a rate of 10°C/minute in a nitrogen atmosphere. SEM was used to visualize SWCNT coatings. Using these images, the coating thickness was calculated approximately and the distribution of nanoparticles and the overall coating quality of the LbL coating were evaluated qualitatively. Finally, the fire performance of uncoated and SWCNT-coated PUF was evaluated in these respects.

Results and Discussion SWCNT Coating Properties

After several trials, it became clear that SWCNT-based coatings on PUF would not create a fire-retardant coating. In the aqueous deposition solution, SWCNTs precipitated immediately after stirring was stopped. Coating attempts with SWCNT solution created a non-homogeneous light grey substrate, indicating that SWCNTs with poor distribution resulted in a low-concentration coating. During rinsing, the washing solutions turned light grey (due to SWCNTs) and the substrate returned to its original dirty white color. This demonstrated that SWCNTs were easily removed from the coating with insufficient adhesion. To increase the stability of SWCNT in water and its adhesion to the substrate, PEI or sodium deoxycholate surfactant was added to the SWCNT deposition solution. However, both methods were ineffective, although PEI showed promise for covalent bonding to SWCNT. The SWCNT-PEI prepared in this study contained approximately 10% PEI by mass (Figure 2), and this value was quite good compared to the literature [1-36]. The high PEI content is likely related to the higher surface area generated on the SWCNT used in this study. SWCNT-PEI remained stable in water for approximately 2 weeks, forming a suspension. The homogeneous dark grey color formed in SWCNT-coated PUF produced using SWCNT-PEI solutions indicates that the use of SWCNT-PEI increased both the SWCNT retention rate and the degree of distribution. However, the fact that washing solutions with water still had a slight grey tint indicated that unbound SWCNT was released. In the coating, a PAA monolayer accumulates on a SWCNT-PEI monolayer. During PAA deposition, the PAA solution turned grey and the substrate became lighter grey, showing SWCNTs passing into the PAA solution. This not only reduced the SWCNT concentration on the substrate but also restricted PAA accumulation by altering the pH of the solution. After four coatings, the substrate was homogeneous grey, but the color was significantly lighter than previously reported for SWCNT-based LbL coating on PUF.

Figure 2. Thermogravimetric results of pure SWCNT, PEI-SWCNT and PEI.

SEM images showed that SWCNTs were distributed across the substrate but there was a significant amount of SWCNT mass not covered by PAA (Figure 3). The coating thickness was found to be less than 80 nm. Rather than focusing on improving SWCNT-PEI and PAA adhesion, the SWCNT-PEI retention problem was addressed by depositing an additional PEI monolayer between the SWCNT-PEI monolayer and PAA monolayer, which showed that this method significantly increased the growth rate of the coatings.

Figure 3. SEM images of SWCNT coating.

Coating thickness was measured at 400 nm ± 41 nm as a result of ten measurements taken from each of five different SWCNT-PEI / PUF samples. The surfaces in cross-section views are consistent with images obtained from the surface. For example; all SWCNT-PEI is embedded within the coating. After characterization procedures, cone calorimetry was used to evaluate the flammability of SWCNT/PUF according to ASTM E1474. Data for SWCNT/PUF and uncoated PUF are given in Figure 4. The heat release rate (HRR) curves for both SWCNT/PUF and PUF consist of 2 peaks. SWCNT-PEI coating resulted in a 21 ± 10% delay in peak heat release rate (PHRR) for each peak.

Figure 4. Heat release rate (HRR) curves of standard PUF and SWCNT/PUF.

Comparison with CNF (Carbon Nanofiber) and Clay-Based Coatings The three nanoparticle coatings (CNF, clay and SWCNT) have distinctly different properties. More specifically, CNF-based coatings are rougher with an appearance more resembling a fibrous network. All coatings cover all surfaces of the PUF. However, CNFs tend to accumulate in clusters rather than as individual fibers, creating areas of high density and areas without CNFs. High accumulation regions contain fibers partially embedded in the polymer coating. CNF size is larger than SWCNTs, creating a coarser coating surface and more extensive CNF aggregation. Inconsistent coating thickness and large cracks from microscopic images suggest that the coatings would not grow beyond the reported thickness. In contrast, SWCNT and clay coatings completely and homogeneously cover all surfaces of PUF with sparsely distributed nanoparticle clusters. At lower magnifications, the coatings appear smooth and featureless. Adding excess polymer layers increases nanoparticle/polymer interactions and creates a much smoother surface. Although different methods are used for SWCNT and clay discussed in the previous section, this effectively enhances polymer/nanoparticle interaction and promotes exponential growth. Strong attraction between nanoparticle monolayers and polymer monolayers results in higher thickness and a smoother surface for both coatings. Despite significant differences in the physical properties of the coatings, the reduction in PUF flammability in CNF-based and SWCNT-based coatings is similar. Clay-based coatings exhibit somewhat lower performance compared to CNF-based and SWCNT-based coatings. More specifically, CNF-based and SWCNT-based coatings result in a 32% reduction in PHRR and an 18 ± 3% reduction in THR, while clay-based coating showed only a 14% reduction in PHRR and a 4% reduction in THR. All three systems are capable of forming fire-protective char layers that would melt and prevent dripping in a real fire.

Conclusion

It has been demonstrated that three-layer LbL coatings with SWCNTs functionalized with PEI reduce the flammability of PUF. With the process described here, PAA/SWCNT-PEI/PEI coatings 400 nm thick containing 51% SWCNT were produced with good and homogeneous distribution on all internal and external surfaces of porous PUF. Except for isolated/dilute SWCNT clusters and minor surface cracks, SWCNT-PEI/PUF coatings are smooth. For the LbL process, it is important to use SWCNTs functionalized with PEI and to deposit a PEI monolayer between SWCNT-PEI and PAA layers. This LbL coating significantly reduces the heat release rate, total heat release and total burning time of PUF. Functionalized SWCNT-based coatings provided significant reduction in PUF flammability at lower concentrations. SWCNT-based coatings prevent the formation of a melted foam pool in a real fire scenario and can reduce the fire threat of soft furniture in residential settings by up to 32%. This research presents another milestone in using LbL to apply coatings on foam with a series of nanoparticles and other performance-enhancing additives. These results provide additional evidence that this method can be used with different nanoparticles to effectively enhance nanoparticle/polymer interactions and the quality of thin films used for multiple applications beyond fire protection. Betül Şen / Master's Student Department of Biochemistry / Faculty of Arts and Sciences / Dumlupınar Üniversitesi Enes Demir / Master's Student Department of Biochemistry / Faculty of Arts and Sciences / Dumlupınar Üniversitesi Zeynep Daşdelen / Master's Student Department of Biochemistry / Faculty of Arts and Sciences / Dumlupınar Üniversitesi Assoc. Prof. Fatih Şen / Department of Biochemistry Faculty of Arts and Sciences / Dumlupınar Üniversitesi

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