Design and Construction of Flame Retardant Layer by Layer Coatings on Typical 2D Cotton Fabric and 3D Flexible Polyurethane Foam and Study on Their Properties and Mechanism
Student thesis: Doctoral Thesis
Related Research Unit(s)
Cotton fabrics and flexible polyurethane (FPU) foams are typical two- and three-dimensional polymer materials, respectively. Both materials are widely used in houseware, such as in curtains, mattresses, upholstered furnitures, and bedding bags. However, their poor thermal stability and high flammability limit their further application. Therefore, improving their flame retardancy is necessary. The layer-by-layer (LbL) assembly technique is an effective method for fabricating flame retardant coating on the surface of polymer materials. Thus, we develop this method to impart flame retardancy to cotton fabrics and FPU foams. The experiments focused on how to effectively fabricate a flame retardant LbL coating onto the cotton fabric surface and FPU foams.
In this dissertation, fully bio-based intumescent-like LbL coating and LbL coating filled with one-dimensional (1D) nanotube materials were initially fabricated separately onto the cotton fabric surface using the traditional bilayer approach. Significant flame retardant efficiency was observed for the cotton fabrics treated with these LbL coatings. Then, 1D nanotube-filled LbL coatings were fabricated onto the FPU foam surface using the traditional bilayer approach and the developed trilayer approach. The trilayer approach has been proven to be an effective method for fabricating LbL coating onto FPU foams. A hybrid bilayer approach was developed to fabricate GO-filled LbL coating and its thermally reduced product (RGO-filled coating) onto the FPU foam surface to simplify the dipping operation process in the trilayer approach. Finally, two binary hybrid-filled LbL coatings based on GO/β-FeOOH nanorods and GO/silica nanoparticles were fabricated separately onto the surface of the FPU foams using a developed hybrid trilayer approach to improve the flame retardant efficiency.
1. Two fully bio-based intumescent-like LbL coatings made from chitosan (CH) and phosphorylated polysaccharides were deposited separately onto the cotton fabric surface using the traditional bilayer approach (CH/PCL and CH/PT). Altering the concentration of phosphorylated polysaccharide (PCL and PT) modified the final coating loading for the cotton fabrics. The 2 wt% concentration of phosphorylated polysaccharide (PCL or PT) resulted in more coating loading than that of 0.5 wt%. The ATR-FTIR spectrum and EDX analyses directly confirmed that the intumescent-like LbL coatings were successfully deposited onto the cotton fabric surface. In the vertical flame test, the cotton fabrics assembled with 20 bilayers at the higher concentration (2 wt%) could extinguish the flame, indicating high flame retardant effect. The MCC result showed that the cotton fabrics treated with intumescent-like LbL coatings exhibited remarkably reduced peak HRR and THR compared with pure cotton fabric. The TGA result showed that the char residues of the cotton fabrics treated with intumescent-like LbL coatings were enhanced at temperatures from 400°C to 700°C. Consequently, cotton fabrics treated with intumescent-like LbL coatings had high char formation and low flammability. This study provided two self-extinguishing LbL coatings based on fully biodegradable polysaccharide, namely, CH and phosphorylated polysaccharide, on cotton fabric to enhance its flame retardancy and thermal stability.
2. The LbL coating based on 1D titanate nanotubes was deposited onto the cotton fabric surface using the traditional bilayer approach (CH/titanate nanotubes) to improve the flame retardancy of the fabric. First, titanate nanotubes were prepared by hydrothermal method. The coated cotton fabrics were then prepared by alternately submersing cotton fabrics into the CH solution and titanate nanotube suspension. The structure of the coating on cotton fabric surface was tailored by altering the number of bilayers and the concentration of the titanate nanotube suspension. The XPS result confirmed that the titanate nanotube-filled coating was successfully deposited onto the cotton fabric surface. Furthermore, the titanate nanotubes assembled a randomly oriented and entangled network structure, as shown by the images from the scanning electron microscopy (SEM). The TGA result indicated that the thermal and thermal-oxidation stability of all coated cotton fabrics improved at high temperatures, from 330 °C to 700 °C. The MCC result showed that all coated cotton fabrics exhibited reduced peak HRR and THR compared with that of pure cotton fabric. Moreover, the reduction was dependent on the concentration of titanate nanotube suspension and number of bilayers. The improved flame retardancy could be ascribed to the protective effect of the formed titanate nanotube network structure.
3. The 1D β-FeOOH nanorod-filled LbL coating was fabricated onto the FPU foam surface using the traditional bilayer approach (PEI/β-FeOOH) and the developed trilayer approach (PEI/β-FeOOH/SA). The ATR-FTIR spectra and SEM images confirmed that the trilayer approach can effectively fabricate the β-FeOOH nanorods network structure formed on the FPU foam surface compared with the bilayer approach. The cone test result showed that the FPU foams treated with β-FeOOH nanorod-filled coating by the trilayer approach had significantly reduced peak HRR and peak SPR, indicating significant improvement in the fire safety for FPU foams. These results confirm that the trilayer approach can effectively enhance the nanoparticle/polymer interactions and the quality of the LbL coating used for flame retardant FPU foams. Thus, another 1D nanoparticle-filled LbL coating based on titanate nanotubes was fabricated onto the FPU foam surface using the trilayer approach (CH/titanate nanotubes/SA). The ATR-FTIR spectra and SEM images confirmed that a homogeneous dispersion and random entanglement of titanate nanotube-filled coating successfully covered the surface of the FPU foam. The cone test result showed that the incorporation of titanate nanotube-filled coating onto the FPU foam can remarkably reduce its peak HRR, peak SPR, TSR, and peak CO production during combustion. Greatly reduced peak HRR (70.2%), peak SPR (62.8%), TSR (40.9%), and peak CO production (63.5%) were observed especially for the FPU foam with only 5.65wt% coating mass gain. The insulating barrier effect and adsorption effect of titanate nanotubes may be the reason for such an improvement.
4. The hybrid bilayer approach [PEI/(GO+SA)] was developed to fabricate the GO-filled LbL coating and its thermally reduced product [RGO- filled coating] onto the FPU foam surface; a comparative study of flame retardancy was also performed. In this stduy, the hybrid bilayer system comprised PEI and (SA+GO), in which the SA and GO component are held together by the combined electrostatic attraction and hydrogen bonding in a common solution. The TGA result showed that the RGO-coated FPU foams have higher thermal stability than the GO-coated FPU foams at high-temperature range (330 °C to 600 °C). The cone test result indicated that all coated FPU foams have lower peak HRRs, peak SPRs, and TSPs compared with pristine FPU foam. In addition, the ―delayed effect‖ in heat release rate and smoke production rate was observed for coated FPU foams with high bilayer number (>3), indicating the good physical barrier effect of graphene material-filled coating. A comparative study between GO- and RGO-coated FPU foams revealed that the GO-coated FPU foams have better flame retardancy and smoke suppression property, which can be attributed to the loss of high amount of RGO nanosheets during burning.
5. Two binary nanoparticle hybrid-filled LbL coatings based on GO/β-FeOOH nanorods and GO/KH-550-SiO2 were separately fabricated onto the FPU foam surface using the developed hybrid trilayer approach. In this study, the hybrid trilayer approach is a combination of the hybrid bilayer and the trilayer approaches. The comparative study in the cone test revealed that the binary hybrid-filled coatings had greater reduction in peak HRR and could eliminate the second peak HRR for FPU foams compared with the single component (GO or β-FeOOH nanorods or KH-550-SiO2)-filled coating. The complementary effect between GO nanosheets with β-FeOOH nanorods or KH-550-SiO2 enhanced the flame retardant effect for the FPU foams.