On the electron beam-induced degradation of vinyl ester thermosets

• Published in: Polymer degradation and stability Vol. 211; p. 110307
• Main Authors: González-López, Lorelis, Kearney, Logan T., Janke, Chris, Wishart, James F., Naskar, Amit K., Kanbargi, Nihal, Bateman, Fred B., Al-Sheikhly, Mohamad
• Format: Journal Article
• Language: English
• Published: United States Elsevier Ltd 01-05-2023; Elsevier
• Subjects: polymers; radiation chemistry; RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
• ISSN: 0141-3910; 1873-2321
• DOI: 10.1016/j.polymdegradstab.2023.110307

•In the absence of organic solvent and water, despite the presence of radiation-protective phenyl groups in the polyester backbone, phenyl polyester networks can be scissioned and ultimately degraded by electron beam irradiation via the radiation-induced scissions of the C-O-C bonds.•EPR results show the chemical structures of the radiolytically produced C-centered radicals and alkoxyl radicals.•In the presence of water, the degradation was enhanced in terephtalic polyester resin due to the presence of •OH radicals, which play a crucial role in inducing C-O-C scission.
We have demonstrated that electron beam radiolysis induces scissions of the C-O-C bonds along the backbone of the chains of unsaturated polyester thermosets of different compositions based on dicyclopentadiene, isophthalic acid, epoxy vinyl ester, and terephthalic acid. The radiolysis is imminent irrespective of the degree of crosslinking in the thermosets both in neat resins and in the presence of solvents. Electron Paramagnetic Resonance (EPR) results show the formation of the alkoxyl radicals and C-centered radicals as the primary intermediate products of the C-O-C scissions. While the alkoxyl radicals of these resins exhibit very good stability even six months after the irradiation, the C-centered radicals decay very rapidly via their reactions with oxygen that is available either as adsorbed in the resins, dissolved in solvents or from the environment. The radiolytically produced •OH radicals in the unsaturated-ester aqueous solutions play a major role in inducing scissions on the backbone of the polymer chains. The solvated electrons (es) from organic solvents, such as dimethyl sulfoxide and isopropyl alcohol, also induce direct scission of the C-O-C bonds, giving rise to the formation of alkoxyl radicals and C-centered radicals. However, a considerable fraction of es is scavenged by the dissolved O2 to produce O2−. Despite the radiation-induced scissions, irradiation of the resins at a dose level of 1000 kGy results in an increase of the glass transition temperature, Tg. This is due to the simultaneous radiation-induced polymerization of the vinyl monomers, toluene, and styrene that are present in the resins. The increase in Tg is observed in all the resins except for the terephthalic polyester resin in aqueous solution, in which the absence of these monomers results in Tg decreasing sharply with irradiation. This work demonstrates that ionizing radiation triggers continuous free radical-chain reactions that lead to the formation of recyclable oligomers.