Acid degradation of thermoset materials

• Main Author: Lim, Jacob Song Kiat
• Other Authors: Gan Chee Lip
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Material testing and characterization; Engineering::Materials::Testing of materials
• Online Access: https://hdl.handle.net/10356/136945
• Institution: Nanyang Technological University

Thermoset polymers are a class of materials known for its insolubility in solvent due to its crosslinked network, which consequently impart upon it a high degree of chemical resistance. However, the degradation of thermoset materials upon exposure to strongly acidic conditions are not well understood by the scientific community due to a surprising lack of established research work in this topic. Often, it is presumed that acid degradation of polymer is chemical in nature, and thus the physical diffusion aspect tends to be overlooked. This research work intends to study and understand both the chemical and physical degradation behaviour of thermoset materials under strong acids, thus facilitating the identification of acid resistant or susceptible moieties, which may then either enable the design of an acid resistant thermoset material or allow for the development of a process to recycle thermosets. A new methodology based on dielectric spectroscopy to quantify acid degradation of thermoset polymers was developed, in order to relate the observed degradation phenomena with quantitative data and thus substantiate the proposed degradation mechanism with evidence. Amine-cured epoxy swollen with HCl acid was observed to possess very interesting dielectric properties, which warranted further investigation in this Chapter. The unique dielectric features including negative permittivity and resonant tan δ peaks indicative of specific binding interactions were observed, which was thoroughly investigated by dielectric spectroscopy. Oscillatory Bode plots and spiral Nyquist plots from impedance analysis of HCl-swollen TGAP-NBDA provided insights into the charge transfer mechanism within such systems. The mechanism of acid degradation in thermoset composite materials and various contribution from composite morphology and filler content was studied in an attempt to quantify the influence of the interface. the mass uptake behaviour of epoxy/silica composites with different filler morphology, particle size and filler content in HCl acid immersion was thoroughly investigated to study the role of the interface/interphase region in the acid degradation mechanism. Dielectric analysis was performed to elucidate insights into the state of the polymer network between the inert silica fillers. The role of the interface was determined to the most dominant factor governing the acid degradation of epoxy/silica composites, regardless of the filler shape morphology, size or content level. The relevance of this research topic and impact justification was demonstrated through the utilisation of the fundamental principles of acid degradation uncovered in this thesis to develop an innovative approach for carbon fibre composite recycling. The acid delamination process has the ability to transit polymer thermal degradation in air from an anaerobic mode to oxidation dominated decomposition simply by increasing interfacial exposure to air, which could significantly improve the economics of the carbon fibre recovery process. This two-step acid and oxidation method can recover carbon fibre from CFRP waste without the need for significant size reduction pre-treatment, hence enabling the recovery of entire carbon fibre textile and long fibres, which is more suitable for more applications than the short fibres recovered by current techniques.