SUMMARY OF THE RESULTS

Novel trifunctional bioepoxy monomers with high glass transition temperature were synthesized from a renewable and easily available starting material, D-glucose. The carbon fibre reinforced bioepoxy composites had better mechanical properties, than the mineral oil based ones, therefore they offer an alternative to the latter ones. It was shown that the combined gas and solid phase flame retardant mechanism is a key factor in efficient flame retardancy of epoxy resins by phosphorus-containing flame retardants. The complex gas and solid phase mechanism was reached both by the reactive flame retardant developed and by the combination of two additive flame retardants. It was demonstrated that cyanate esters can be used as multifunctional reactive modifiers acting as crosslinking agent, compensating the glass transition temperature decreasing effect of flame retardants and improving the thermal stability and mechanical properties of epoxy resins. It was found that the flammability of carbon fibre reinforced epoxy resin composites can be effectively reduced, maintaining the mechanical properties as well, with the formation of a multilayer composite consisting of a load-bearing reference composite core and an intumescent epoxy resin coating layer. The synergistic effect of the common application of phosphorus and silicone was demonstrated on the flammability of natural fibres. It was stated that for effective flame retardancy of natural fibre reinforced epoxy resin composites both the fibre and the matrix have to be flame retarded. Synergistic effect was found both in terms of flame retardancy and mechanical properties, when both the epoxy resin matrix and the natural reinforcement contained phosphorus.

The results on synthesis of polymer components (4.1.4), development and characterization of bio-based polymer matrices (4.2.3) and composites (4.3.3), as well as on flame retardancy of epoxy resins (4.4.6) and their composites (4.5.3) are summarized in more details at the end of each main chapter in the experimental results section, therefore this chapter focuses on the exploitation of these results, accompanied by the novel results summarized in the form of theses, and last but not least the further tasks are discussed.

5.1. Exploitation of the results

The results summarized in this work were achieved in the frame of Hungarian and international projects listed in chapter 6. During the work carried out the industrial applicability of the results was considered to be essential, both in the case of new polymer components developed and novel composite materials investigated.

134 There is a continuous demand on P-containing flame retardants and consequently novel and improved methods for their synthesis are also of great industrial importance. The novel simple, cost-effective and environmentally friendly method for preparation amine functional phosphoric amides, elaborated by the author and her co-workers was protected by PCT patent in 2009, after the Hungarian patent filed in 2007 [133]. The synthesis of the P-containing amine type hardeners was optimized and scaled up in a computer controlled 1L reactor. Among the P-containing amines synthesized using this patented method, N,N’,N’’-tris(2-aminoethyl)phosphoric triamide (TEDAP) found its use not only as FR and crosslinking agent for epoxy resins [211, 224], but also as base material for pH-reversible supramolecular hydrogels [232] and for preparation and stabilization of gold nano- and microcrystals [233].

As according to the literature [126,134], it is not trivial that the reaction between a tertiary phosphoric ester and diamines, in particular, in the case of TEDAP the reaction of triethyl phosphate and ethylene diamine would take place, the Fourier transform infrared (FT-IR) vibrational spectra of TEDAP were modelled to support the identification of this novel compound [192]. The molecular geometry and vibrational wavenumbers of TEDAP in its ground state have been calculated by using Density Functional Theory/B3LYP and Hartree-Fock functionals with 6-31++G (d,p) basis set. The obtained vibrational wavenumbers and optimized geometric parameters were seen to be in good agreement with the experimental data. The calculated results also serve as a basis for the identification of other amine functional phosphoric amide derivatives synthesized with this method.

Concerning the FR effect of TEDAP, a mathematical model was developed to describe the degradation of reference and flame retarded epoxy resins initiated by a constant heat flux under mass loss calorimeter test conditions [234,235]. The applied model describes both the heat and mass changes of a polymer layer with finite thickness and predicts the whole temperature and pressure profiles of the system. It is assumed that the polymer degrades to a fixed mass of char and volatile gas in an instantaneous step, at the moment when the temperature reaches a critical value. The most important heat transport mechanism is conduction, which dominates the temperature profile. The mass transport of gas is described by Darcy’s law, with a simplifying condition that the overall solid volume is constant during degradation. The transport processes have been modelled in one spatial dimension. Experiments in mass loss calorimeter and computer simulations have been carried out to establish the effects of critical parameters such as layer thickness, heat flux and material properties. The predicted ignition times and critical temperatures were in good agreement with the experimental data. Furthermore it could be concluded that the heat capacity of polymer does not have any effect on the temperature profile of the preheating

135 process, it determines the preheating time instead. The effect of re-radiated combustion heat was established and it has been found that the amount of absorbed pyrolysis heat is an important factor in the degradation model.

The results achieved in the field of bioepoxy composites were used in prototype development for Dassault Aviation in the relevant Clean Sky EU7 project. As composite sandwich structures play important role in aeronautical applications, especially in case of indoor elements (e.g. aircraft floors, ceilings, sidewalls and storage compartments), our aim was to compare the properties of sandwich composite structures prepared from polymethacrylimide foam, a core material preferred in aeronautical applications, using DGEBA and GFTE as matrices, cured with AR917 anhydride type curing agent. Jute fibre reinforcement was used to reach as high as possible renewable ratio in composite sandwich structures. In case of indoor elements the flexural strength and modulus are the most important properties; therefore three point bending tests were carried out on the sandwich composites.

According to the test results [204] both in the case of 6.5 and 20 mm thick core, after the bending stress value reached a maximum, it started to decrease due to the failure of the upper composite layer of the sandwich composite structure. After that two different phenomena took place: In the case of the 6.5 mm core, the bending stress decreased further, while with 20 mm core, the bending stress increased until the breaking of the specimen. Two different types of failure occurred: in the case of the 6.5 mm core, when the sandwich structure broke, the specimen stayed together (non-catastrophic failure), while in the case of the 20 mm core, the failure of the specimens was catastrophic. In the case of the 6.5 mm core, the thin core had higher force intermediary and low damping properties, than the 20 mm core. It lead to a decreasing force, resulting in non-catastrophic failure at the end of the bending tests. GFTE sandwich composites with 6.5 mm core had significantly better average flexural properties than the DGEBA composites (flexural strength: GFTE – 55.07±1.20 MPa; DGEBA – 44.22±3.94 MPa, flexural modulus: GFTE – 33.93±0.10 GPa; DGEBA – 3.67±0.09 GPa). In the case of the GFTE composites, the average flexural strength was 24%, and the average flexural modulus was 7% higher than with DGEBA, respectively.

With 20 mm core the DGEBA composite had 15% higher flexural strength, the modulus values were in the same range (flexural strength: GFTE – 13.48±0.36 MPa; DGEBA – 15.90±0.32 MPa, flexural modulus: GFTE – 0.68±0.02 GPa; DGEBA – 0.70±0.03 GPa).

The reason behind these results is the different polarity of DGEBA and GFTE epoxy matrices. As the applied polymethacrylimide core material is polar, the more polar epoxy resin leads to better impregnation at the phase boundary of the foam. The polarity of the epoxy resins was quantified by the topological polar surface area (TPSA) method according to Ertl et al. [236]. This method was

136 developed for quantitative characterization of the polarity of potential drug candidate molecules in order to predict if they can pass the blood-brain barrier, however, it proved to be suitable for the comparison of the polarity of polymer components as well. According to TPSA calculations GFTE has twice as much TPSA as DGEBA (92.99 vs. 43.52), therefore in the case of the thinner core material with better load transfer capabilities, the better impregnation lead to better flexural properties in the case of the GFTE sandwich composite.

Based on these results, the bio-based GFTE can replace the DGEBA EP component in the sandwich composite structures, with jute fibre reinforcement, especially in the case of the thinner cores, for example the 6.5 mm one. No interfacial failure between the composite layers and the core material was observed, which indicates a good adhesion between the composite resin system and the core foam.

The prepared GFTE sandwich composites were used for Falcon business jet cabin applications by Dassault Aviation (Figure 5.1.2).

Figure 5.1.2 Aircraft cabinet prototype prepared by Dassault Aviation using GFTE bioepoxy sandwich composites

5.2. Theses

The new scientific achievements of the experimental work, categorized into new bio-based epoxy resins and composites, and new green flame retardancy solutions, are the followings: