Investigation on Preparation, Thermal Property and Flame Retardancy of Poly (lactic acid) / Nickel-containing Compounds Nanocomposites

Abstract

The raw material of poly (lactic acid) (PLA), lactic acid can be produced from renewable resources. Therefore, PLA has a wide development and applications. PLA has good properties, such as thermoplastic, high strength, high modulus, high melting index, high transparency and ease of fabrication. However, because of its inherent chemical composition and molecular structure, PLA has poor thermal stability and it is easy to burn with serious dripping, which limit its applications and development in many fields. Therefore, research on the low loading and high flame retardant efficiency of additives is still an important task. Based on current research on the thermal stability and flame retardancy of PLA, a variety of nickel-containing compounds were synthesized. Then combined with new halogen-free flame retardants were used in preparing PLA composites. The thermal property and flame retardancy of PLA composites were researched in detail. The interaction of flame retardant compositions and effect mechanism of additives in PLA matrix were discussed.

The main research work is as follows :
1. Three different nickel-containing layered double hydroxides (NiFe, NiAl and NiCr LDHs) were synthesized by one-step co-precipitation method. The structure, morphology and property of these LDHs were investigated. They were used to prepare PLA/LDHs nanocomposites. The thermal property of PLA/LDHs composites was researched and compared. TGA results showed that PLA/NiCr LDH composite had the highest thermal stability. DSC suggested that PLA/NiAl LDH composite had the lowest melting enthalpy. SEM presented that the PLA/NiFe composite char residue was densest, and the largest ratio of the ordered carbon was presented in LRS. TEM and XRD images of PLA composites illustrated that all the LDHs had a good dispersion in PLA matrix, and exhibited intercalation or exfoliation. Owing to trivalent cations effect, NiFe, NiAl and NiCr LDHs played different roles in improving the thermal stability of PLA composites.

2. NiFe, NiAl and NiCr LDHs were first combined with Hexaphenoxycyclotriphosphazene (HPCP) to prepare PLA/HPCP/LDHs nanocomposites. The morphology, thermal property and flame retardancy of PLA/HPCP/LDHs composites were researched and compared. For PLA composites containing LDHs, TGA measurements confirmed that PLA/HPCP/NiCr LDH composite had slightly more char residue. DSC levels suggested that PLA/HPCP/NiFe LDH composite presented in the best case only a low crystallinity, and SEM images demonstrated that its char residue was densest. In addition, XPS revealed that this composite exhibited the highest P/C and N/C ratios. The largest ratio of ordered carbon for PLA/HPCP/NiAl LDH composite was evident by LRS. All the composites were given UL-94 V0 rating at the low loading. TEM images showed that all the LDHs had a good dispersion in PLA composites. Thus, NiFe, NiAl and NiCr LDHs played different better effect in improving the thermal stability and flame retardancy of PLA composites.

3. A new organic IFR formulation constituted by microencapsulated ammonium polyphosphate with silane (APP203) and pentaerythritol phosphate (PEPA) was added in PLA matrix. NiAl LDH had good lamellar barrier effect and catalytic charring, which was chosen as flame retardant synergist agent to improve the thermal stability and flame retardancy of PLA/IFR composites. Corn starch (CS) as a bio-carbon source was used to form another organic IFR composites with APP203. Poly(butylenesuccinate) (PBS) was used in PLA matrix, which had the toughening effect. PLA/PBS/APP203/CS/NiAl LDH composites were also prepared by melt blending method. The measurement results showed that NiAl LDH and the above two IFR systems could improve the thermal stability and flame retardancy of PLA composites apparently and resolve the combustion dripping problem. The effect mechanism of additives was mainly based on the condensed phase flame retardant mechanism.

4. The nanometer-scale nickel ferrite (NiFe2O4) particles were synthesized using the hydrothermal method, and ZnFe2O4 and CoFe2O4 nanoparticles were synthesized by the same method for comparative research. These metal ferrites (MFe2O4) were combined with an inorganic IFR constituted by ammonium polyphosphate (APP) and expandable graphite (EG) to added in PLA matrix. The flame retardancy and mechanical property of PLA composites were investigated and compared. The LOI value of PLA composites was increased significantly after adding IFR and MFe2O4. UL-94 V0 rating was obtained with 13 wt% additive loading. It was no dripping when composites was burned. MCC results showed that PHRR, HRC and THR value of PLA composites had a reduction of 30.7%, 32.8% and 17.5% respectively, comparing to pure PLA. Fire security was further improved when 1 wt% MFe2O4 was added. The mechanicl property of PLA composites was improved at some extent. NiFe2O4 effect was the best among the above three MFe2O4 nanoparticles. The promoting mechanism between MFe2O4 and APP/EG was caused by formation of the stable char layer, slowing heat and mass transfer between the condensed phase.

5. Nickel molybdate (NiMo) nanorods with uniform morphorlogy were prepared by the hydrothermal technique, and nickel molybdates doped with Ce (NiMoCe) nanorods were synthesized for comparative research. Polystyrene (PS) was used for enhancing PLA. Polyethylene glycol (PEG) was used as a toughening reagent. PLA/PS/PEG/naorod composites were prepared. The thermal stability of PLA composites, enhancing and toughening effect of additives were investigated. PLA composites with the addition of nanorods had more char residue and higher content of the ordered char layer. Therefore, nanorods were useful to improve the thermal stability of PLA composites. The results of mechanical property measurements indicated that PS and PEG had enhancing and toughening effect. With a just-right formulation, the tensile strength of PLA composites was improved with NiMo nanorod loading increase. NiMo nanorods effect was better than NiMoCe.

Date of Award	28 Aug 2014
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Siu Ming LO (Supervisor)