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Title: Boron and phosphorus-based flame retardants for poly (lactic acid)
Authors: Tawiah, Benjamin
Degree: Ph.D.
Issue Date: 2019
Abstract: The pursuit of comfort and improved living conditions has resulted in the massive use of polymers in all spheres of human endeavor. Meanwhile, most polymers are highly susceptible to flames with just a little spark of fire. Polymers serve as the primary source of fuel in most fire accidents with damages in billions of dollars annually around the globe. PLA is an aliphatic polyester with enormous potential to replace the petrochemically derived plastics due to its biodegradability and biocompatibility. On the other hand, PLA is also highly flammable, often accompanied by severe melt dripping, toxic fumes, and comparatively lower mechanical strength. This thesis proposes the application of boron and phosphorus-based additives for reducing the flammability, melt-dripping, toxic fumes production and improve the mechanical properties of PLA for engineering applications. Our first study on synergistic azo-boron-BPA (AZ-A) and polydopamine (PDA) was conjectured based on the environmentally benign nature of boron compounds and their ability to induce glassy char formation during combustion. Azo bond introduced to boron compound was meant to serve as nitrogen source and to make the boron compound more effective. The AZ-A compound was then combined with PDA due to its radical scavenging ability in flames and the propensity to induce stable chars. Nuclear Magnetic Resonance (NMR), Fourier transform infrared (FTIR), and Raman spectra confirmed the successful synthesis of AZ-A and PDA. The AZ-A and PDA compounds were compounded with PLA using the solvent mixing and compression molding approaches. Their FR properties were investigated by thermogravimetric analysis (TGA), vertical burning test, Limiting Oxygen Index (LOI) and Cone Calorimeter test (CCT). TGA showed that synergistic AZ-A / PDA was generally efficient in reducing the degradation rate of PLA with appreciably higher char residue compared to the individual FR components. The synergistic AZ-A / PDA achieved a V-0 rating with an LOI value of 23.7% at 10 wt% loading in the ratio 1:1 whereas the individual FR components could only give a V-2 rating. The AZ-A / PDA synergy also resulted in a minimal reduction in peak heat release rate (PHRR ~19%) and evolved pyrolysis gaseous products, but total smoke release (TSR) was still high. However, the tensile strength and the elongation at break were compromised. To improve the tensile strength and reduce the TSR of PLA, graphene oxide (GO) was modified with azo-boron (AZOB) and reduced/intercalated in-situ with NaBH4/SMB. In this way, the glassy charring of boron, the lamellar blocking effect of RGO, in addition to the production of nitrogen oxides by the azo group, and the release of water by hydrated sodium metaborate (SMB) could reduce the flammability, melt dripping, and the smoke production of PLA. Based on this premise, RGO-AZOB/SMB hybrid was synthesized and characterized by FTIR, X-ray powder diffraction (XRD), Scanning electron microscopes/energy-dispersive X-ray (SEM/EDS), and X-ray photoelectron spectroscopy (XPS) to validate the structure and morphology. The hybrid was compounded with PLA at 2 wt% maximum via solvent mixing and compression molding approaches. CCT showed improved FR performance by substantial reductions in PHRR ~ 76.5%, total heat released (THR) ~ 76.9%, TSR ~ 55.6%, peak CO produced (PCOP) ~25.9% and peak CO2 produced (PCO2P) by ~ 78.6%. The fire performance index (FPI) also increased by 84.2%, indicating enhanced fire safety for RGO-AZOB/SMB/PLA nanocomposites. A V-0 rating was attained in the UL-94 test with a higher LOI value of 31.2%. TG-IR study showed considerable reductions in pyrolysis products consisting of hydrocarbons, CO, CO2, and carbonyl compounds at the maximum decomposition temperature. The tensile strength and Young's Modulus were improved by 49.1 and 34.9%, respectively.
Due to the commercial significance of phosphorus-based FRs and their perceived high efficiency in a wide range of polymers, further studies was conducted to improve the FR and the mechanical properties of PLA at a very economical loading. Inspired by the inherent FR properties of polybenzoxazole (PBO), a similar PBO-like compound was synthesized as additive FR for PLA. The compound - phenylphosphonic bis(2-aminobenzothiazole) (P-ABZT) was characterized by FTIR, 1H, 13C, 31P NMR. P-ABZT was incorporated into PLA using the solvent mixing and the compression molding approaches; and the crystalline, FR and mechanical properties of the PLA/P-ABZT composites were investigated. TGA showed improved thermal stability and high char yield. CCT demonstrated substantial reductions in PHRR by ~ 50%, THR ~ 37%, average effective heat of combustion (AEHC) ~ 31%, PCOP ~ 60.4 %, and PCO2P ~ 31 % with 3% P-ABZT loading. The LOI improved to 32.4% whilst a V-0 rating was attained in the UL-94 vertical burning test. Substantial reductions in pyrolysis gaseous products were achieved. However, the tensile properties of PLA were compromised slightly with 3 wt% P-ABZT loading. To surmount the apparent challenge of deteriorating mechanical properties and melt dripping of PLA, a similar PBO-like phosphorus-based FR - phenylphosphonic 3(2-aminobenzothiazole) (P-TAB) was synthesized with higher benzothiazole content and combined with recycled short wool fibers (WF) as mechanical reinforcement and anti-dripping remedy. P-TAB was characterized and compounded with PLA and WF similar to the previous methods. Considerable reductions in PHRR, THR, CO and CO2 produced were attained with 3 wt% P-TAB and various WF loading. Meanwhile, the PHRR of the composites with WF increased with higher WF loading compared to the sample with only P-TAB. A general improvement in FPI (~ 38.2%) and fire growth index (FGI ~ 48.1%) was attained, and a reduction in FR performance similar to the trend observed in PHRR occurred. The composite achieved a V-0 rating and an LOI value of 28.5% at 20 wt% WF loading. Significant improvement in tensile strength and Young's Modulus was realized with the increasing content of WF in the matrix. TG-IR results showed substantial reductions in evolved gaseous products. Furthermore, a novel additive phosphorus FR that could serve as mechanical reinforcement with high FR efficiency was designed. A cyclo-phosphorus-nitrogen FR - hexaphenyl (nitrilotris(ethane-2,1-diyl))tris(phosphoramidate) (HNETP) was synthesized, characterized, and incorporated into PLA using similar approach in the previous systems. CCT results showed significant reduction in PHRR ~51.3%, THR 43.1%, PCOP ~ 46.5%, PCO2P ~ 51.3% with 3 wt% HNETP loading. Unlike the previous studies, the FPI increased by 65.8 % while the FGI decreased by 56.7% with a V-0 rating and an LOI of 32.5%. The tensile strength and Young's Modulus improved by ~ 67.4 and 87.8% respectively. Insignificant changes were observed in the glass transition temperature, melting temperature, and the degree of crystallization for PLA in the FR composites. The various FR systems presented in this thesis appear to be effective and economical for reducing the flammability of PLA. Significant improvements in mechanical properties were obtained in some of the FR systems, which can be very beneficial for engineering applications. In cases where the mechanical properties were compromised, it was only marginal. The high smoke production by the phosphorus systems remains a challenge. To surmount this challenge, a combination of the phosphorus-based FRs developed with GO and other known smoke suppressants is recommended.
Subjects: Hong Kong Polytechnic University -- Dissertations
Fireproofing agents
Polymers -- Flammability
Pages: 245 pages : color illustrations
Appears in Collections:Thesis

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