Abstract
Functional additives play a crucial role in polymer, serving to fulfil specific application requirements. Recently, driven by the increased use of bioplastics and heightened awareness of safety and environmental protection, a variety of sustainable functional additives derived from renewable resources are being gradually developed to replace traditional functional additives that are petroleum-based and toxic (such as phthalate plasticizers and halogenated flame retardants (FRs)). As one of the most promising bioplastics, polylactide (PLA) holds significant potential to replace traditional plastics in various applications. Plasticizers and FRs are essential for reducing the brittleness and flammability of PLA to meet specific end-use requirements. A variety of sustainable plasticizers and FRs have been reported to enhance the ductility and flame retardancy of PLA; however, some issues remain, primarily due to poor compatibility. Therefore, this thesis focuses on the compatibility and sustainability of functional additives, designing two molecular structures based on lactate groups and renewable monomers for PLA plasticizer and FR. The relationship between the effects and the structure is investigated.Firstly, geraniol-initiated lactide oligomers (GLOs) with 100 % bio-carbon were synthesized through the ring-opening polymerization of L-lactide. GLOs exhibited increased plasticization efficiency on PLA with a decrease in the number of lactic acid repeating units, because of the gradually strengthened interactions between the GLO and PLA chains. The elongation at break of PLA increases remarkably to the range of 271.4 %–514.5 %, with 10 wt% to 20 wt% loading of GLO4. The transparency and ductility of all plasticized PLA blends can be maintained after 90 days, indicating good durability and compatibility.
Secondly, tetrahydrogeraniol-initiated lactide oligomers (TLOs) were synthesized through bulk ROP without any solvent. TLOs exhibited similar effects to GLOs. When comparing the effects of TLOs with an ethanol-initiated lactide oligomer, a short lactide oligomer with a relatively long nonpolar terminal group seems better at interrupting the intermolecular forces between PLA chains and exhibits higher plasticizing efficiency.
Thirdly, HELCP was synthesized by reacting ethyl lactate with cyclotriphosphazene to balance the flame retardancy and mechanical properties of PLA. A small amount of 5.0 wt% HELCP endows PLA with a UL-94 V-0 rating. The mechanical properties and transparency of PLA/HELCP blends are almost the same as those of neat PLA. The flame-retardant mechanism was studied. The ethyl lactate part of HELCP undergoes ester exchange reactions with PLA and improves the compatibility between PLA and HELCP.
Fourthly, the roles of the alkyl chain length of the above structure on flame retardancy and mechanical properties of PLA were investigated by replacing the ethyl lactate with the methyl lactate (HMLCP) and n-butyl lactate (HBLCP). HMLCP with short alkyl lactate shows stronger flame retardancy and reactivity, but a weaker plasticizing effect on PLA, and can better balance the flame retardancy and mechanical properties of PLA. The flame-retardant mechanisms were also analysed.
Finally, the plasticizing effect of the hexa-lactate-cyclotriphosphazenes (HLCPs) was further studied by increasing the loading. At the same time, the combined impact of the HLCPs and the lactide oligomers (LOs) was also investigated. It was found that HMLCP and HELCP could be viewed as flame-retardant plasticizers for PLA. The physical combination of HLCPs and LOs sacrifices partial flame retardancy; however, the total amount can be reduced to achieve the same flame retardancy and ductility due to the plasticizing effect of HLCPs. These two additives can be used in combination to enhance the comprehensive properties of PLA, due to their good compatibility.
Overall, this thesis adds greener and more efficient choices to the sustainable plasticizers and FRs for PLA. With these sustainable functional additives, more potential applications of the modified PLA could be explored. The examination of the correlation between the LO and HLCP structure and the comprehensive effect on PLA is beneficial for the development of more efficient, sustainable, and multifunctional PLA additives.
| Date of Award | 15 Nov 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Kok Wong (Supervisor), Nai Yeen Gavin Lai (Supervisor) & Haibin Yu (Supervisor) |