This dissertation focuses on the molecular design and development of advanced intumescent flame retardant (IFR) systems to simultaneously improve the flame retardancy, mechanical properties, and multifunctional performance of polypropylene (PP) composites. Through a combination of molecular aggregation control, covalent copolymerization, and metal–ligand coordination strategies, three classes of efficient IFR systems were constructed, namely nitrogen/silicon-based macromolecules (MNSi-n), a phosphorus/nitrogen/silicon-containing copolymer (PNSi-co-MP), and an aluminum-based hybrid macromolecule (Al-PAP). The relationships between molecular structure, dispersion behavior, char formation, and flame retardant mechanisms were systematically investigated to establish rational design principles for high-performance PP composites.

In the first part, a series of nitrogen/silicon-based macromolecules (MNSi-n, n = 1, 2, 3) were synthesized by inserting piperazine/phenyl-silicon charring skeletons (NSi-n) with varying aggregation degrees into a hydrogen-bonded melamine cyanuric acid (MCA) framework. These macromolecules were combined with ammonium polyphosphate (APP) to form (MNSi-n/APP)/PP composites. The effect of NSi-n aggregation on flame retardant efficiency and char formation was examined through glow-wire (GW), limiting oxygen index (LOI), UL 94, and cone calorimeter tests. Neat PP showed a glow-wire flammability index (GWFI) of only 725 °C, while PP composites containing 28 wt.% (MNSi-n/APP) achieved the highest classification of over 960 °C. The GWFI increased with the aggregation degree of the NSi-n structure, confirming that the clustered piperazine/phenyl-silicon framework was crucial for enhancing flame retardancy. The LOI value of neat PP was 18.6%, whereas 22%(MNSi-3/APP)/PP reached 25.7%. All 28 wt.% (MNSi-n/APP)/PP composites achieved LOI values above 28% and UL 94 V-0 rating. In contrast, MCA/APP/PP (without NSi-n) failed the UL 94 test, highlighting the synergistic role of the NSi-n skeleton with APP. Cone calorimeter results revealed remarkable fire resistance. The peak heat release rate (PHRR) of neat PP was 1591 kW·m-², while 25%(MNSi-n/APP)/PP decreased to approximately 400 kW·m-². The fire performance index (FPI) exceeded 0.055 s·m²·kW-¹ for all MNSi-n systems, compared to 0.018 s·m²·kW-¹ for neat PP, while the fire growth index (FGI) decreased by more than 40%. MNSi-3 exhibited the lowest heat release rate (HRR) and total heat release (THR), as well as the highest FPI, confirming that a higher aggregation degree of NSi-n promoted superior intumescent efficiency. Thermogravimetric analysis (TGA) showed that (MNSi-n/APP)/PP decomposed earlier than (MCA/APP)/PP, facilitating rapid formation of a protective char layer.

At 500-600 °C, 25%(MNSi-n/APP)/PP retained over 16% residue, while neat PP left nearly none. Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) indicated that MNSi-n/APP produced dense, continuous char layers rich in phosphorus and silicon. The 25%(MNSi-3/APP)/PP sample exhibited the highest carbon (7.58 wt.%) and silicon content, indicating enhanced Si-P synergism. Infrared thermal imaging further verified the strong barrier effect: (MNSi-n/APP)/PP maintained structural integrity after 60 s of flame exposure, while (MCA/APP)/PP collapsed severely. Gas-phase analysis using Fourier-transform infrared spectroscopy (FTIR) confirmed that MNSi-n/APP suppressed hydrocarbon fragment emissions (CH₃, CH₂, C=C) and released phosphorus oxides (POx· radicals), leading to reduced combustion efficiency and enhanced gas-phase inhibition. In summary, incorporating small amounts of piperazine/phenyl-silicon charring skeletons into the MCA framework generated nitrogen/silicon-based macromolecules with tunable aggregation. The optimized (MNSi-3/APP)/PP composite exhibited significantly improved LOI, GWFI, FPI, and residue yield, and reduced HRR, THR, and FGI values. This work demonstrates that controlling molecular aggregation within charring structures is an effective strategy to design high-performance, low-loading IFR systems for polyolefins.

The second part focused on a novel dendritic copolymer (PNSi-co-MP) containing phosphorus (P), nitrogen (N), and silicon (Si), synthesized via random copolymerization of a PNSi monomer with melamine phosphate (MP). This copolymer served as an efficient IFR for PP. Copolymerization disrupted monomer regularity, altering crystallinity and thermal behavior compared with the self-polymerized MPNSi/melamine polyphosphate (MPP) system. PNSi-co-MP displayed glass transition and thermal softening behavior during PP processing, improving dispersion and interfacial adhesion with the PP matrix. Morphological analysis revealed that PNSi-co-MP particles transformed from plate-like shapes (5-10 μm) into smaller spherical or rod-like particles (200-300 nm), enhancing stress transfer and energy absorption. Consequently, PNSi-co-MP/PP composites exhibited ductile fracture behavior, while MPNSi/MPP/PP fractured brittly. The 16PNSi-co-4MP/PP composite showed a 278% increase in elongation at break and a 33.3% improvement in impact strength compared with 16MPNSi/4MPP/PP, while maintaining similar tensile strength, demonstrating excellent balance between strength and toughness. Flame-retardant testing confirmed superior performance. The 20 wt.% 12PNSi-co-8MP/PP composite achieved a GWFI above 960 °C, a glow-wire ignition temperature (GWIT) of 875 °C, an LOI of 30.2%, and UL 94 V-0 rating. Cone calorimeter results showed reductions of 93.6% in PHRR and 87.2% in peak smoke production rate (PSPR) compared with neat PP. Combustion was effectively inhibited within 17 s after ignition, forming a dense Si/P/O/C/N-rich char layer that acted as an efficient thermal barrier. Raman spectroscopy indicated enhanced graphitization (lower ID/IG ratio), while X-ray photoelectron spectroscopy (XPS) confirmed the presence of Si-O-Si, P-O-C, and P-N bonds, demonstrating a synergistic network that reinforced char stability. The residue yield of 12PNSi-co-8MP/PP reached 61.4%, while THR and total smoke release (TSR) decreased by 57.6% and 61.9%, respectively, compared with MPNSi/MPP/PP. The flame retardant index (FRI) reached 42.23, indicating excellent overall fire safety. Mechanistic analyses using thermogravimetry coupled with FTIR (TG-FTIR) and pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) showed that copolymerization promoted early char formation and synchronized degradation-carbonization behavior, leading to fewer volatile species and faster condensed-phase reactions. The integration of P, N, and Si within one molecular backbone not only improved condensed-phase charring but also reduced gas-phase combustion by forming stable crosslinked structures. Overall, this study demonstrated that molecular copolymerization effectively balanced mechanical reinforcement, processability, and flame retardancy in PP composites. The PNSi-co-MP copolymer provides a promising pathway to high-performance IFRs with combined toughness and fire safety.

In the final part, a hybrid macromolecule (Al-PAP) containing aluminum phosphate and piperazine phosphate was synthesized via polycondensation and combined with melamine polyphosphate (MPP) to construct a hybrid IFR system (Al-IFR) for PP. The inclusion of aluminum ions (Al³⁺) within the phosphate backbone facilitated crosslinking among acidic, carbon, and gas sources, thereby improving the compactness and thermal stability of the char layer. This approach addressed the common trade-off between flame retardancy and thermal conductivity in IFR/PP systems. At only 14 wt.% loading, Al-IFR raised the LOI of PP to 28.3%, equivalent to that achieved with 20 wt.% conventional IFR. With 20 wt.% Al-IFR, the LOI further increased to 32.2%, GWIT reached 850 °C, GWFI exceeded 960 °C, and UL 94 V-0 ratings were achieved at both 3.2 mm and 1.6 mm thicknesses. Cone calorimeter tests revealed an 84.7% reduction in PHRR and a 72.2% decrease in PSPR compared with neat PP. The FPI increased by 4.7 times, and the FRI reached the “good” level (1-10), demonstrating high fire safety and smoke suppression efficiency. To further improve heat conduction, alumina (Al₂O₃), boron nitride (BN), and multi-walled carbon nanotubes (MWCNTs) were incorporated into 20Al-IFR/PP composites. Al₂O₃ exhibited the best synergy, simultaneously enhancing thermal conductivity and flame retardancy. The 5Al₂O₃/20Al-IFR/PP composite showed a 90.8% reduction in PHRR, an 88.2% reduction in PSPR, and increases of 27.0% in thermal diffusivity and 48.9% in thermal conductivity. In contrast, BN and MWCNT disrupted the intumescent char structure, resulting in inferior fire performance and higher smoke emission. Mechanistic studies using FTIR, Raman spectroscopy, and XPS indicated that Al³⁺ and Al₂O₃ catalyzed phosphate-carbon interactions, generating dense Si/P/O/C-based layers with high graphitization (low ID/IG ratio). TGA and Py-GC/MS confirmed that aluminum species suppressed volatile release and promoted thermally stable phosphate-carbon frameworks. The residue yield increased from 11.2% for 20Al-IFR/PP to 52.2% for 5Al₂O₃/20Al-IFR/PP, confirming excellent condensed-phase stability. Mechanical testing showed that the 5Al₂O₃/20Al-IFR/PP composite maintained high toughness, with elongation at break exceeding 400% and a non-notched impact strength of 30 kJ·m-². In summary, the introduction of the Al-PAP hybrid macromolecule, together with Al₂O₃ fillers, enabled simultaneous enhancement of flame retardancy, thermal conductivity, and smoke suppression, producing multifunctional PP composites with high fire safety and robust mechanical performance.

Overall, this dissertation establishes a comprehensive and systematic molecular engineering strategy for designing high-efficiency, multifunctional intumescent flame retardant systems for polypropylene. Through three representative design pathways, including controlled molecular aggregation (MNSi-n/APP system), covalent copolymerization (PNSi-co-MP system), and metal coordination (Al-IFR system), a clear structure-function-performance relationship has been demonstrated. The results reveal that fine-tuning the aggregation degree of nitrogen/silicon charring structures can promote compact Si-P-rich char formation and efficient gas-phase inhibition, while copolymerization of phosphorus-nitrogen-silicon monomers achieve molecular-level compatibility and balanced mechanical performance. Furthermore, the introduction of aluminum ion and alumina enhances both char integrity and thermal conductivity, achieving the long-sought combination of fire safety and functional performance in PP composites. Beyond achieving superior flame retardancy and mechanical toughness, the developed IFR systems also contribute to understanding the synergistic interactions among P, N, Si, and Al elements during combustion. The insights gained from this work provide a theoretical foundation for the rational design of next-generation flame retardants that integrate multiple functionalities, such as heat dissipation, smoke suppression, and structural reinforcement, into a single molecular framework. The strategies proposed herein are not limited to PP but can be extended to other polyolefins and engineering plastics, paving the way for high-performance, environmentally sustainable materials for advanced applications in electronics, automotive components, and building materials where fire safety and durability are simultaneously required.