Study on flame retardancy and interfacial properties of plant fiber reinforced epoxy resin composites

Abstract

With the exacerbation of environmental deterioration and resource crisis, reducing energy consumption and mitigating environmental impacts have become a consensus aims for ecological industry and sustainable social development. As representative products of green manufacturing technology, green composites not only have a reputation for their renewable fabrication and environmental friendliness, but also feature advantages such as low price, lightweight, and high specific strength, which make them promising candidates for application in various important end-uses including aerospace, construction, and transportation with a growing trend. Unfortunately, plant fiber-reinforced epoxy resin composites are beset by poor interfacial compatibility between the plant fabric and epoxy resin due to the hydrophilicity of plant fiber and the hydrophobicity of epoxy resin. The phenomenon of lamination and fracture is easy to occur when the material is subjected to load under the poor interfacial compatibility, which restricts the further development and application of the plant fiber reinforced epoxy resin composites. Additionally, the high flammability, due to the presence of flammable fibers and epoxy resin, is another obstacle in the development of the composites and limits their applications. Consequently, studying on how to effectively improve the flame retardancy of the composites has become a goal pursued by industry and researchers. Regrettably, there are difficulties in establishing a balance between flame retardancy and interfacial performance.
In this way, multi-functional modifier that can simultaneously improve mechanical and thermal performance is significant for composite production. On the basis of reviewing the research progress on flame retardants and interface modification of epoxy resin composites, and aiming at addressing the fire risk and poor interface compatibility of plant fiber/epoxy resin composites, this thesis designed and synthesized three kinds of modifiers for flame retardant plant fibers from the perspective of compound design. The effects of different modifiers on the flame retardancy and mechanical properties of the composites were studied, with the aim of achieving plant fiber-reinforced epoxy resin composites with both flame retardant properties and satisfactory mechanical performance. In addition, TG-IR, SEM, and Raman spectroscopy were used to study the flame retardancy mechanism. The application of the three modifiers to the flame retardancy and interfacial properties of plant fiber-reinforced epoxy resin showed remarkable innovation. The specific information for each part of the work is as follows.
To begin with, inspired by the chemistry of mussel, an interfacial modifier named FPD was designed and synthesized through one simple step, which were attached by three functional groups (i.e., catechol, N-H bond and DOPO). Due to the innate properties of each functional group FPD played multiple roles: adhere to the ramie fiber through catechol, cure with the epoxy resin from -NH-, provide antiflaming property from DOPO. Moreover, the compatibilizer between ramie fiber and epoxy resin was also improved by changing the polarity of ramie fiber. All of the above functions were proved by means of water contact angle (WCA), atomic force microscope (AFM) and scanning electron microscopy (SEM) etc. After solidification, the ramie fiber-epoxy composites demonstrated superior performances in terms of good mechanical properties and excellent flame retardant property. With the addition of 30 wt.% FPD, the tensile strength and modulus of the ramie/epoxy composite showed an improvement of 37.1% and 60.9%, flexural strength and modulus of the composite were improved by 8.9% and 19.3% comparing with no addition composite. Moreover, the composite could achieve the goal for V-0 rating in the UL-94 test and limiting oxygen index (LOI) value was 34.6% when the addition of FPD reached 30 wt.%. This work provided us with efficient methods for fabricating nature fiber/epoxy composites with good properties.
Secondly, a combined modifier consisting of a water-soluble benzoxazine (HGB) with high carbonization and phytic acid (PA) containing phosphorus elements and acid groups was devised to treat the regenerated cellulose fabric (SF). The treated fabric (F-SF) not only could realize self-extinguishing from the fire which helps to avoid wick effects, but also exhibited an optimized interfacial property of composites. After being combined with an intrinsic flame-retardant epoxy resin (F-EP), the composite material (F-SF/F-EP) achieved a V-0 rating in the UL-94 test and an excellent LOI value of 33.8% due to the flame-retardant effect in both the gaseous phase and condensed phase. Simultaneously, benefiting from the improved interfacial compatibility, both tensile modulus and impact strength were enhanced from 9.82 GPa and 6.58 MPa to 11.29 GPa and 7.21 MPa, respectively, compared with untreated SF/EP composite, especially the interlaminar shear strength (ILSS) dramatically increased by 21.7% Finally, in view of the damage of phytic acid to ramie and Suncell fabric found in the last chapter, imidazole phytate was synthesized via a simple one-step strategy to replace phytic acid. Imidazole phytate retained the high phosphorus content of phytic acid, changed the strong acidity of phytic acid, and caused less damage to the mechanical properties of plant fiber. At the same time, as the accelerator of epoxy resin curing, imidazole can reduce the curing temperature and curing time of epoxy resin, shows high thermal stability and good compatibility with EP matrix. Like last chapter, PAIM and arbutin benzoxazine to form an expansive flame retardant in order to make the composite material achieve flame retardant effect and maintain its mechanical properties to a certain extent. The modified fabric was further used to reinforce DGEBDB composites through a vacuum bag-assisted compression molding process, which could improve interfacial adhesion and fire safety of the regenerated cellulose fabric/epoxy composite in the simultaneous. After combining with an intrinsic flame-retardant epoxy resin (F-EP) and benefiting from the improved interfacial compatibility, both the tensile modulus and impact strength were enhanced from 9.82 GPa and 6.58 MPa to 11.33 GPa and 7.26 MPa compared with untreated SF/EP composite, especially interlaminar shear strength (ILSS) dramatically increased by 19.7%, from 11.41 to 13.66 MPa. Simultaneously, the composite material (F-SF/F-EP) achieved a V-0 rating in the UL-94 test and an excellent LOI value of 34.3% compared to pure composits. The pHRR of F-SF/F-EP achieved about 834.9 kW/m2 , which is decreased by 44.9% relative to 459.9 kW/m2 of SF/EP due to the flame-retardant effect both in gaseous phase and condensed phase.
This work provided an integrated strategy for synchronously enhanced interfacial compatibility and fire-proof performance of plant fiber-reinforcement composite.

Date of Award	Nov 2024
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Xiaosu Yi (Supervisor)