Preparation and properties of epoxy resin modified with phosphorus and nitrogen flame retardants

Epoxy resins (EPs) require the addition of flame retardants to extend their application scope. High-efficiency flame retardants typically contain halogens, which can cause tremendous harm to humans and the environment. In this study, we investigated the syntheses of new phosphorus–nitrogen flame retardants (DIT) and flame-retardant EP. DIT is composed of 6-(2,5-dihydroxyphenyl) dibenzo[c,e][1,2] oxaphosphinine 6-oxide, 5-isocyanato-1-(isocyanatomethyl)−1,3,3-trimethylcyclohexane, and tris-(2-hydroxyethyl) isocyanurate. DIT chemical structure was analysed by Fourier-transform infrared spectroscopy. Several formulations of modified epoxy resins (DIT-EPs) were obtained by adding different quantities of DITs. We found that when the flame retardant content reached 25%, the limiting oxygen index of the DIT-EP was 29.0%. This finding expands the application range of EPs.


Introduction
Epoxy resins (EPs) are a class of widely used thermosetting resins with better thermal stability, electrical insulation, mechanical properties, and adhesion than thermoplastic resins [1]. However, the flammability of these polymer materials considerably limits their applicability [2]. Therefore, studies on the flame retardancy of EP have attracted considerable attention [3]. Generally, flame retardants are added or the structure of EP is modified to achieve flame retardancy [4]. In the 1980s, European countries and the United States of America discussed the toxicity and environmental issues posed by halogenated flame retardants, which promoted research on new flame retardants. The goals of low toxicity, halogen-free, and synergistic compounding have become the focus of flame-retardant research [5].
The flame-retardant dibenzo[c,e][1,2] oxaphosphinine 6-oxide (DOPO), which does not contain halogens like chlorine and bromine, is a phosphophenanthrene compound. As shown in figure 1, the structure contains a highly active P-H bond, which not only react with a variety of groups, such as epoxy groups and double bonds, but also displays high-efficiency flame retardant properties [6]. Therefore, it is typically used as a starting material to prepare DOPO-based derivatives [7]. DOPO derivatives are some of the most promising substances for use in EP flame retardants because of their good flame-retardant effect and compatibility [8]. DOPO derivatives are less likely to produce toxic gases and are considered one of the most promising alternatives to halogenated flame retardants [9].
Nitrogen-based flame retardants are a new type of environment-friendly flame retardant that do not contain halogens, are less corrosive, and emit less smoke [11]. Generally, nitrogen-based flame retardants act as gasphase flame retardants in polymer combustion systems [12]. Nitrogen-containing flame retardants generate non-flammable gases, such as nitrogen, ammonia, nitrogen oxides, and water vapour, during the combustion process that consume a large amount of energy and thereby inhibit or slow the thermal decomposition of polymer materials [13]. Simultaneously, the generated gas can dilute the O 2 concentration in the air and slow the combustion process [14].
In this study, a phosphorus-nitrogen flame retardant (DIT) was formulated from DOPO-HQ, 5-isocyanato-1-(isocyanatomethyl)−1,3,3-trimethylcyclohexane (IPDI), and tris (2-hydroxyethyl) isocyanurate (THEIC), and its structure was confirmed by Fourier-transform infrared (FT-IR) spectroscopy. It was added to the EP, Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
solidified, and formed. The combustion performance of the DIT-EP was studied using an extreme LOI instrument. Its mechanical properties and thermal stability were also assessed.

Synthesis of flame-retardant DIT
To a solution of DOPO-HQ (6.28 g, 0.0193 mol, 1 eq.) in DMF (20 ml), IPDI (4.28 g, 0.0193 mol, 1 eq.) and DBTDL (0.1 ml) were added and reacted at 110°C under N 2 for ∼16 h in a four-necked round-bottom glass flask [15]. THEIC (5.1 g, 0.0195 mol, 1.01 eq.) was then added to the mixture and stirred for 4 h. The mixture was carefully poured into water (80 ml), filtered, and the filter cake was washed with water (80 ml) three times. The crude product was dissolved in dichloromethane (80 ml) and concentrated under vacuum to give the product (14.7 g, 82% yield). The reaction equation of DIT is shown in figure 2.

Dynamic mechanical analysis (DMA) and characterization of DIT-EPs
The analysis was performed using a DMA 242 C analyser at a frequency of 1 Hz, a test temperature of 20°C-200°C , and a heating rate of 5 K min −1 . All samples were processed in compression mode.

LOI measurement
LOI measurements were conducted on an oxygen index instrument (UK FTT0077) using samples with sheet dimensions of 120 mm × 6.5 mm × 3.2 mm and tests were performed in accordance with ASTM D-2863-77.

Thermogravimetric (TG) analysis
TG analysis was performed using a Netzsch machineusing samples with sheet dimensions of 3 mm × 2 mm × 2 mm. The temperature was measured from 30 to 700°C in a nitrogen environment at a heating rate of 10°C min −1 .

Measurement of mechanical properties
The impact strength of the DIT-EPs was tested using a SUN-500 (GALDABINI, Italy) universal material testing machine. The sample notch of the impact spline had dimensions of 3 mm × 2 mm × 2 mm, and the pendulum pre-lift angle of the impact testing machine was 149.2°C.

Results and discussion
3.1. Infrared spectrum of DIT The FT-IR spectrum of DIT reveals where the -OH of DOPO-HQ and THEIC react with the -NCO group of IPDI (figure 3). The characteristic peak at 3490 cm −1 corresponds to the elastic vibration absorption bonds of the amide group, and the elastic vibration absorption peak of the C = O double bond is observed at 1642 cm −1 [17]. There was no characteristic peak of -NCO at 2205-2270 cm −1 , indicating that DIT had been successfully synthesised [18].

Infrared spectrum of DIT-EPs
The chemical structure of DIT-EPs is confirmed in figure 4; the characteristic peak at 913 cm −1 corresponds to the bending vibration of the epoxy group, and the peaks at 1642 cm −1 and 1400 cm −1 are the stretching vibrations of the group, and that at 1257 cm −1 and 1096 cm −1 are the stretching vibration of P=O [19], which indicates that the flame retardant DIT was successfully added to the EP. The absorption peak at 3017 cm −1 may result from the superposition of the -N-H and -OH absorption peaks [20].

DMA of EP with varying DIT content
The glass transition temperatures (T g s) of the DIT-EP-cured products are shown in figure 5. When the DIT content was 5%, the T g of the EP increased, which may be due to the rigid groups contained in DIT; the greater the internal friction of the DIT-EP molecular segment during movement, the more the movement of the segment fails to keep up with the change in external force. Moreover, both tan δ and T g increase. With increasing flame retardant DIT content, the number of groups that hinder the movement of the main chain in the system also increases and T g increases.

LOI analysis of EP with varying DIT content
LOI was performed on six groups of DIT-EPs with different DIT formulations, and the results are shown in figure 6. The LOI of EP increased with increasing DIT content. Compared with EP, the LOIs of the DIT/EP-5%, DIT/EP-10%, DIT/EP-15%, DIT/EP-20%, and DIT/EP-25% samples increased from 22.5% to 25.5%, 26.5%, 27.0%, 28.5%, and 29.0%, respectively, indicating that the DIT had a significant flame-retardant effect. This may occur because DIT flame retardants are thermally degraded to generate compounds like phosphoric acid, which fulfils the condensed-phase flame-retardant mechanism, promotes dehydration of the surrounding carbon, and forms a dense and heat-insulating carbon layer. The thermal decomposition of DIT also produces incombustible gases like N 2 , which dilute the oxygen around the EP system, thereby improving the flame-retardant performance of DIT-EP.

Thermogravimetric analysis of EP containing various DIT contents
Thermogravimetric analysis was performed on DIT-EPs with different flame-retardant DIT contents ( figure 7). The DIT-EPs generally had a lower thermal degradation temperature than the EP without flame retardant. The carbon residue was higher after complete degradation. When the flame retardant content was 0%, the initial decomposition temperature of the EP was 81.5°C. The temperature at 5% weight loss was 399.5°C; it decomposed rapidly in the range temperature range from 399.5°C to 459.5°C, and the total weight loss was 76%. At 484°C, the curve was flattened, decomposition was complete, and the compound had a carbon residue of 16.11%. The initial degradation temperature of DIT-EP was ∼77°C; the 5% weight loss temperature was ∼283°C. Decomposition was complete at 440°C, and the residue consisted of carbon. The residual mass was ∼25.57%.
This result may be explained by the preferential degradation of DIT after the EP containing DIT is heated; therefore, the initial degradation temperature of the EP containing DIT is lower, and the degradation of DIT generates organic acids and nitrogen, which play the role of a phosphorus-nitrogen synergistic flame retardant, promoting the dehydration of the material into carbon, absorbing the heat around the material, diluting the oxygen concentration around the material to form a denser and more stable insulation layer, and delaying or preventing the degradation of DIT-EP.   3.6. Impact strength of EP containing various DIT contents The impact strengths of the EP-cured products with different DIT contents are shown in figure 8. The impact strength increased with increasing DIT content. The DIT structure consists of a chain with rigid aromatic rings in its side chains. The phosphorus content associated with the increase in impact strength is believed to be due to this rigid side-chain structure.

Limiting oxygen index and char residue of EP modified by different methods
The oxygen indices of the EPs modified by different methods are shown in table 2. The overall limiting oxygen indices (LOIs), which are the volume fractions of oxygen in mixtures of oxygen and nitrogen when the polymer can only support its combustion, are shown in the table 2. A high oxygen index indicates that the material does not burn easily, whereas a low oxygen index indicates that the material burns easily. Generally, an oxygen index less than 22% indicates a flammable material, an oxygen index between 22% and 27% indicates combustible materials, and an oxygen index greater than 27% indicates refractory materials. The flame retardant epoxy resin obtained by different modification methods is shown in table 2, and their LOI values are mostly concentrated at 29%. In addition, the carbon mass after combustion of DIT-EPs is basically higher than that of other methods.

Conclusion
In this study, compounds containing DIT were added to an EP to provide flame retardancy. Various DIT-EPs containing DIT were melted and then solidified using a curing agent, DDS. In this study, the T g s, LOI values, impact strengths, and TG curves of the different DIT-EPs groups were studied. It was found that the T g of DIT-EPs was higher than that of the pure EP, but the overall change was not significant. The LOI value of the pure EP was 22.5%, and when the DIT content was 25%, the LOI value of the EP was 29.0% and the flame-retardant effect was improved to a certain extent. Simultaneously, the quantity of carbon remaining after the combustion of DIT-EPs was greater, and its thermal stability and impact strength were also improved. In summary, DIT-EPs extends the application scope of EPs and has practical value.