The presented work deals with epoxy (EP) resins and the optimization of their properties by using modifiers such as tougheners, phosphorus based cross-linkers and flame retardants (FR’s). A particular focus was on the synthesis of new FR’s, their synergistic application in selected epoxy resins and the mechanisms of action. Possible applications for these optimized systems are epoxy matrix resins in fiber reinforced lightweight materials used by the aviation industry or printed circuit boards (PCB) which are used in the electronics industry.
The two main research topics were the following: In the first part of this work the structure-property relationship of EP-systems containing different EP-resins and curing agents were systematically examined. To facilitate this, two groups of EP-systems with different glass transition temperatures (Tg), which are suitable for injection technologies such as RTM (resin transfer molding), were studied. By adding modifiers, established characteristic parameters for those systems are reached and structure-property relationships were investigated. All EP-systems were based on bisphenol A diglycidylether (DGEBA), the use of materials with toxicological concerns were deliberately renounced. In the second part of this work new phosphorus-based flame retardants for EP-systems were synthesized and tested, synergistic formulations developed, their present mechanisms of action elucidated. These were compared with literature and commercially available references.
The Tg of the first EP-systems suitable for injection technologies are in the range of 190°C-200°C, cured at max. 180°C. Their material properties were compared with the reference system “RTM 6”. At the beginning DGEBA was combined with different multifunctional epoxy resins and aromatic amines (curing agent). The glass transition temperature of the cured samples was investigated by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA, Tg (max. tanδ)). DSC measurements revealed the influence of the different curing agents on the reactivity of the uncured EP-systems. In addition, the viscosity was determined by dynamic measurement between 30°C and 200°C and isothermal measurements at 90°C. Furthermore, the influence on fracture toughness KIC of the cured samples was investigated. The following newly developed systems should be highlighted: „EXA 30“ and „HP 20“, both contain DGEBA and one on the multifunctional epoxy resins; Exa-7250 (based on salicilaldehyde) and HP-4710 (based on naphthalene). As curing agent the aromatic diamine 4,4′-Methylenebis(2,6-diethylaniline) was used. Both systems exhibit a Tg in the range of 190°C (Tg (DSC) and Tg (DMA, max. tanδ)) with a fracture toughness comparable to the reference RTM 6 (KIC ~ 0,60 MPa m½). The system EXA 30, Tg DSC = 194°C, shows a 5°C higher Tg compared to HP 20, further investigations regarding the curing process were completed. For this purpose, model free kinetics were applied and the Kissinger equation was used. The investigation revealed a reaction enthalpy for RTM 6 of 438 J/g, which is 38% higher and the activation energy of 60 kJ/mol which is 5 % higher compared to EXA 30. Therefore, the reference system is slightly less reactive compared to EXA 30, which had already been indicated by the viscosity measurements. The measurement curves were qualified with the phenomenological model of Kamal and Sourour (autocatalytic model) and the corresponding kinetic parameters determined. Investigations for EXA 30 regarding the water uptake of the cured samples revealed a behavior comparable to EP-systems known from literature and to the reference RTM 6. At the beginning Fick’s second law can be applied, the diffusion coefficient D for the reference and EXA 30 are comparable provided that the measurement error is taken into account.
The fracture toughness of certain systems could be systematically improved. Especially the addition of the toughener polyetheramide (PEI) led to an increased fracture toughness (KIC = 0,90 MPa m½). The present mechanisms of action were elucidated by utilizing a scanning electron microscope (SEM), revealing the formation of a second phase in the cured EP-resin, hereby several previously published effects were proven, which explained the high efficiency of PEI as toughener. Also the effects of different cross-linkers such as the phosphites diethylphosphit (DEP), 5,5-dimethyl-1,3,2-dioxaphosphorinan-2-on (DDPO) and phosphonates including Poly[DMPAc-2-THIC], on material properties of the cured system EXA 30 were investigated by DSC, DMA, viscosity and KIC -measurements. A systematic increase of the Tg of EP-systems, cured with different aromatic diamines was shown. The cross-link reaction which often needs a post-curing step at 200°C or 215°C, had been already previously investigated with the special focus on cycloalliphatic amines containing EP systems and only a few containing aromatic amines.
For EXA 30 combined with DDPO, 1 wt% of phosphorus (1 wt% P), an increase of Tg by 20°C (DMA, max. tanδ) could be achieved, with a simultaneous decrease of KIC -value due to increased cross-link density. This cross-link process which is based on transesterification processes of the phosphites and phosphonates with the EP-matrix, was also investigated by nuclear magnetic resonance (NMR) and dynamic rheology measurements.
Due to a minor impact on viscosity characteristics, two cross-linkers demonstrated a performance particularly suitable for injection technologies. On the one hand the macromolecular phosphonate Poly[DMPAc-2-THIC], whose flame retardant properties were also investigated in the second part of this work. By adding a weight equivalent corresponding to1 wt% P, the Tg (Tg (DMA, max. tanδ)), could be increased from 193°C to 201°C, accompanied by a decrease of KIC to 0,52 MPa m½. On the other hand, by adding DEP (0,5 wt% P) the Tg was increased to 207°C with a KIC of 0,56 MPa m½.
The Tg of the second series of EP-systems suitable for injection technologies is in the range of 130°C-150°C, (Tg (DSC)), cured at max. 130°C. At the beginning, EP-resins cured with cycloaliphatic amines were investigated. To reduce the reactivity of the curing agents, and to prolong the possible processing time, complexes consisting of cycloaliphatic amines including isophorone diamine (IPDA) or 1,2-diaminocyclohexane (1,2-DC) combined with Zinc neodecanoate (ZnNeo2) were produced. As expected, DSC measurements of the uncured systems (complex + EP) revealed a decrease of reactivity as desired.
In the second approach the reactivity of an EP-system, using the aromatic diamine M-DEA as a hardener, was increased by utilizing triethylamine (TEA) and salicylic acid (SAS) as accelerators. Besides EP-systems cured by a polyaddition mechanism (amine hardener like M-DEA), also systems cured by the anionic homopolymerization of EP-resins were investigated. An EP-system using a combination of curing agents was used, consisting of the imidazole 1-Ethyl-3-methylimidazolium acetate (EMIM Ac, homopolymerization initiator) and 2-Cyanoguanidine (DICY) as co-curing agent (EMIM Ac-DICY). By adding a core-shell toughening agent the fracture toughness KIC could be increased from 0,50 to 0,69 MPa m½. Furthermore, the mode of action of this toughening modifier was investigated as previously, again revealing the formation of a second phase in the cured EP-system.
In the second part of this work, which focuses on phosphorus containing FR’s for EP-resins, a reactive oligomeric and phenolic hydroxyl groups bearing commercially available FR based on 9,10-Dihydro-9-oxy-10-phosphaphenanthren-10-oxid (DOPO) and salicylaldehyde, was modified. The Atherton-Todd reaction and nucleophilic substitution was applied by reacting the phenolic hydroxyl groups with DOPO or 2-Chloro-5,5-dimethyl-1,3,2-dioxaphosphinane-2-oxide, obtaining new oligomeric FR’s with higher P-content.
Analogous to these modified oligomeric products, other new monomeric compounds based on salicylaldehyde and DOPO or phosphorus chlorides were synthesized and structurally characterized. Three of these compounds were analyzed by X-ray crystallography.
Furthermore, by applying the Phospha-Michael addition, the trifunctional acrylate 1,3,5-Tris(2-hydroxyethyl) isocyanurate (THIC) was modified with DOPO and/or dimethylphosphite (DMP). The intermediate product, containing on average one acrylate group per molecule, was polymerized by radical polymerization. The following three macromolecular compounds were produced: Poly[DOPAc-2-THIC], Poly[DOPAc-1-DMPAc-1-THIC] and Poly[DMPAc-2-THIC]. Compared to the molecular analog known from the literature, DOPAc-3-THIC, Poly[DOPAc-2-THIC] shows a 84°C higher thermal stability (2 wt% weight loss TGA = 376°C). A positive effect on Tg was found for the two DMP-units containing products, which act as cross-linkers, after applying a post-curing step. The Tg (DSC) could be increased from 224°C to 240°C and 248°C, without deterioration of the fracture toughness.
Subsequently the influence of new and established FR’s and synergistic FR mixtures on two different literature relevant EP-systems with different Tgs, often used for printed circuit board (PCB) applications, was investigated. The first one based on DGEBA and the second system based on an epoxy novolac resin (D.E.N. 438), both cured with DICY and Fenuron. The minimum amount of FR (wt% P) for passing UL-94-V-0 was determined.
In agreement with the literature, the more gas phase active FR's show a clearly increased performance in higher crosslinked and more aromatic units bearing EP systems such as the epoxy novolac system used. In contrast, for the less crosslinked DGEBA system, the more condensed-phase active FR’s show a better performance by increasing the amount of the char formed.
In the following, the influence on the flame retardant performance was investigated by adding the synergists melamine polyphosphate (MPP) or AlO(OH) (Boehmite) to the formulation. In general, a negative influence on the Tg as observed for the addition of the neat FR’s to the resin, could be prevented, which was tremendous for some systems not containing the synergists. The amount of the non-synergist FR could be reduced significantly, showing superior flame retardant efficiency.
Especially by adding MPP (15 wt%) to the epoxy novolac system, only 0,2 wt% P of additional FR had to be applied. Via TGA measurements, an interaction of MPP with the EP-matrix was demonstrated, which relates to the increased char residue. Further investigations applying cone calorimetry gave a more detailed insight into the present mechanisms of action of newly developed and referenced FR’s, singly and in combination with MPP and boehmite. These findings were complemented by SEM measurements of the remaining char residue, resulting in a better understanding of the present mode of action and interactions of the epoxy resin and the flame retardants during combustion. The superior flame retardant properties are evoked by the formation of dense char combined with gas-phase action of the organophosphorus compounds. | English |