SELF-HEALING THERMOSET GREEN RESINS

Abstract

The goals of this research were to prepare different types of microcapsules containing green healant and evaluate the self-healing efficiencies and mechanical behaviors of green resins containing different loadings of these microcapsules. Soy protein isolate (SPI) exhibits high water solubility and great potential for crosslinking to improve the properties. Further, it is a fully sustainable plant-based protein that is commercially available in all parts of the world. Therefore, SPI was chosen as the resin as well as the healing agent in the present research. Glutaraldehyde (GA) was chosen as the SPI crosslinker in this research due to its easy availability and fast reactivity with SPI. Microcapsules were prepared using the evaporation technique. Poly(DL-lactide-co-glycolide) (PLGA) was used as the shell material to encapsulate the SPI healant. Two microcapsule-preparation techniques were developed and are discussed in this thesis. To produce uniform-sized microcapsules, a membrane emulsification technique2 was used. Instead of using Shirasu Porous Glass (SPG) membrane, which is frequently used in the preparation of monodispersed particles and microcapsules, syringe filters were used to reduce the production costs. Results showed that the diameters of the spherical microcapsules produced with the aid of the syringe filter ranged between 0.5 μm and 2 μm with an average diameter of 1.30 μm. In addition to spherical microcapsules with a narrow size distribution, elongated microcapsules with aspect-ratios greater than one were observed using the syringe filter emulsification technique. Microcapsules with various geometrical shapes including spherical, rod-like, dog-bone and elliptical, were prepared. It was found that the aspect-ratio of elongated microcapsules observed reached as high as 20. Even though polyvinyl alcohol (PVA) was introduced onto the surfaces of microcapsules to enhance their bonding with the resins, the results showed that the interfacial adhesion was still not strong enough as many unbroken microcapsules were observed at the fracture surfaces. This raised the possibility that cracks would go around the microcapsules instead of fracturing the microcapsules, adversely affecting the self-healing efficiency. To further improve the microcapsule/resin adhesion, a second microcapsule-production technique was developed by incorporating ground bacterial cellulose (GBC) into microcapsules. Due to its exceptional mechanical properties and morphology, GBC was used as a reinforcing agent as well as a surface modifier in this research. A spraying technique was used in the course of preparing microcapsules with GBC. A porous structure was constructed as a result of applying this technique. Another geometry made from the spraying technique was the elongated microcapsules with aspect-ratio as high as 50. The benefit of incorporating GBC in the microcapsules is to increase the surface roughness and to improve the self-healing efficiency by bridging the fracture surfaces. As the results have shown, microcapsules coated with GBC adhered to the resin tightly so that both tensile strength and fracture toughness were enhanced due to the presence of GBC. The structural resins embedded with the four kinds of microcapsules mentioned above demonstrate healing behaviors and displayed the highest self-healing efficiency (strength recovery) of about 47% compared to 14% for the control group (virgin resin) and 63% toughness recovery compared to 24% of the control group. With the help of self-healing properties, the application of green resins and composites can be expanded. This can help reduce petrochemical products and wastes generated at the end of their life and promote sustainability in modern society.