Synthesis of phosphorus-containing epoxy monomer

P-containing epoxy monomers were synthesized from aromatic DGEBA and aliphatic PER by adduct formation with 9,10-dihydro-9-oxa-10-phosphaphenantrene-10-oxide (DOPO). These syntheses were based on the method previously published by Wang and Lin [107].

4.1.2.1. Synthesis of DGEBA-DOPO adduct

In order to form an aromatic P-containing epoxy monomer DOPO was reacted with DGEBA in 1:1 molar ratio (Figure 4.1.6) [189,190]. Prior to the reaction DOPO was kept at 85 °C for 12 h, in order to remove the traces of moisture. DGEBA was kept in under vacuum at 110 °C to remove air and traces of moisture, and after adding DOPO, the mixture was stirred at 160 °C for 5 h. After cooling

50 to room temperature a solid adduct was obtained, which was used for reactive flame retardancy of DGEBA (see 4.4.4).

Figure 4.1.6 Synthesis of DGEBA-DOPO adduct

4.1.2.2. Synthesis of PER-DOPO adduct

In order to form an aliphatic P-containing epoxy monomer, DOPO was reacted with PER in 1:1 molar ratio (Figure 4.1.7) [191]. Prior to the reaction DOPO was kept at 85 °C for 12 h, in order to remove the traces of moisture. The mixture of PER and DOPO was stirred at 160 °C for 8 h. After cooling to room temperature a solid adduct was obtained, which was used for reactive flame retardancy of PER (see 4.4.1).

Figure 4.1.7 Synthesis of PER-DOPO adduct

4.1.3. Synthesis of phosphorus-containing crosslinking agents

In this chapter, a simple, cost-effective and environmentally friendly method (patented by the author and co-workers [133]) was used for the synthesis of phosphorylated amines with high P-content, which can act as FR crosslinking agent in epoxy resins. Reaction of triethyl phosphate with an aliphatic diamine, ethylene diamine and two aromatic diamines, o- and m-phenylenediamine was carried out. Concerning the choice of amine reagents, it can be noted that short chained aliphatic amines are widely applied as crosslinking agents in epoxy resins and are produced in large quantities, and choosing them as a reactant in this synthesis, high P-content of the hardener and the epoxy resin system can be achieved. As the P-content is proportional to the FR effect, this was an important aspect. On the other hand although the P-content which can be achieved using aromatic amines is lower than in case of short chained aliphatic amines, the aromatic backbone offers numerous advantages as high char yield, higher thermal stability, more rigid structure

51 leading to higher glass transition temperature. The curing properties, glass transition temperature, thermal stability and FR performance of the synthesized amines were compared.

4.1.3.1. Synthesis of N,N’,N’’-tris(2-aminoethyl) phosphoric triamide (TEDAP)

Transamidation of triethyl phosphate (TEP) was carried out with ethylene diamine (EDA) according to Figure 4.1.8.

Figure 4.1.8 Synthesis of TEDAP

EDA and TEP were reacted in 10:1 molar ratio instead of the necessary 3:1 to shift the equilibrium in the direction of the required trisubstituted product. After TEP was added dropwise to EDA, the mixture was stirred at the boiling point of EDA, at 118 °C for 1 h. The excess of EDA was removed by vacuum distillation to give the liquid, yellowish brown product with amine number of 5005 mg KOH/g in 93% yield. The FTIR spectra confirmed the formation of P-N-C bonds [192]. According to MALDI-TOF spectra the product mainly contained monomers, but possible fragments of dimers, trimers and tetramers were also detected. For detailed results see [193].

4.1.3.2. Synthesis of N,N’,N’’-tris(3-aminophenyl) phosphoric triamide (TMPDAP)

An aromatic diamine, m-phenylenediamine was used in the transamidation reaction according to Figure 4.1.9.

Figure 4.1.9 Synthesis of TMPDAP

7.136 g (0.066 mol) of m-phenylenediamine was heated in a round flask until melting (64-66 °C).

When the amine was completely melted, 3.72 ml (0.022 mol) of TEP was added dropwise, and then stirred at 90-95 °C for 2 h. The formed ethanol was removed from the reaction mixture by vacuum evaporation to give the liquid, dark green product with amine number of 6555 mg KOH/g in 90% yield. For detailed results see [193].

52 4.1.3.3. Synthesis of N,N’,N’’-tris(2-aminophenyl) phosphoric triamide (TOPDAP)

The transamidation reaction was also carried out with another aromatic diamine, o-phenylenediamine according to Figure 4.1.10.

Figure 4.1.10 Synthesis of TOPDAP

To the solution of 70 ml toluene and 7.136 g (0.066 mol) of o-phenylenediamine 3.72 ml (0.022 mol) of TEP was added and the mixture was stirred at the boiling point of toluene, at 110 °C for 7 h. After cooling, the dark violet solid product with amine number of 3075 mg KOH/g was filtered out, with a yield of 90%. For detailed results see [193].

4.1.3.4. Preliminary testing of the synthesized phosphorus-containing amines Curing properties of the synthesized amines

For investigating the applicability of the synthesized P-containing amines as curing agents for epoxy resins, DSC measurements were carried out (Table 4.1.4). The pentaerythritol-based model epoxy monomer (PER) and the synthesized amines, as well as the reference cycloaliphatic diamine (T58), were mixed in an appropriate ratio. The highest enthalpy of curing was measured in the case of the latter one. The difference between the measured values of the two P-containing aromatic amines (TMPDAP and TOPDAP) is negligible; the peak of curing appears at somewhat higher temperature in case of the o-phenylene diamine-based molecule, which can be explained by steric hindrance of the amine groups in ortho position compared to that of the meta one. The aliphatic P-containing TEDAP showed the lowest curing enthalpy, so the least exothermic reaction, which can be beneficial at large scale curing. As for the glass transition temperatures (Tg), the cycloaliphatic reference hardener had the highest value, as the rigidity of the cycloaliphatic rings hinders the segmental movements in the crosslinked resin. In the case of the two aromatic, P-containing amines, the Tg is somewhat lower, as the rings are bound together via the more flexible N-P-N bonds compared to the one atom distance between the cycloaliphatic rings. TEDAP showed the lowest glass transition temperature among the investigated resin systems, as both the epoxy component and the hardener have flexible aliphatic chains, which allow easy segmental movements and thus relatively low Tg.

53 Table 4.1.4 Curing behaviour and glass transition temperature of the synthesized amines

curing agent onset temperature decrease at 295 °C, and a very high decomposition rate (Table 4.1.5). The residue at 500 °C is less than 10%, as no charring agent is present in the system. When the P-containing hardeners are applied, the decomposition starts at lower temperatures, which can be explained by the evolved PO radicals at the early stage of the degradation slowing down the further degradation steps [173]. The degradation of the TOPDAP-cured resin shows a two-step curve, with almost the same decomposition rates. This double degradation can be explained by the lower stability of the amine starting material itself due to the –NH2 groups in o-position. The residue of the TOPDAP-cured resin is less than that of its stereoisomer, TMPDAP: 28.9% compared to 41.8%. The TMPDAP-cured system shows elongated and relatively slow degradation. The highest decomposition rate appears at 280 °C, between the values of the two steps of TOPDAP. The amount of charred residue for the aliphatic P-containing hardener (26.6%) is somewhat lower than that for the aromatic ones, which is related to the beneficial effect of the aromatic rings in char formation. Also the rate of decomposition is higher in the case of TEDAP; however this maximum is reached at higher temperature.

Table 4.1.5 TGA results of PER cured with different amines

curing agent T_-5% measurements were carried out (Table 4.1.6). The P-containing hardeners result in decreased flammability compared PER, LOI values are above 30 V/V% in all cases, which indicates the beneficial effect of P in terms of decreasing the ignitability of the resins. The reference system

54 reached only HB UL-94 classification with a relatively high flame spreading rate (32 mm/min), while the P-containing resins passed the horizontal test, no flame spreading rates could be measured. The significance of the P-content is doubtless: the TMPDAP-cured resin contains less than 2% P and reaches only V-1 classification (the difference between the P-content of the two aromatic amines is caused by the difference of their amine values requiring different mixing ratios for achieving the same level of crosslinking), while the samples with more than 2% P-content could reach the best, V-0 classification.

Table 4.1.6 Comparison of LOI and UL-94 results of reference and FR epoxy resin matrices

curing agent P-content [%)

LOI [V/V%]

UL-94*

reference 0 23 HB (32 mm/min)

TEDAP 2.8 33 V-0

TMPDAP 1.7 31 V-1

TOPDAP 3.0 30 V-0

* in parenthesis the horizontal burning rate is showed, where measurable