Environmentally friendly fire-resistant epoxy resins based on a new oligophosphonate with high flame retardant efficiency

Abstract

Advanced flame retardant epoxy resins, environmentally friendly, with different contents of a new oligophosphonate (PFR), were prepared using dicyandiamide as a hardener and 1,1-dimethyl-3-phenylurea as an accelerator. PFR, with a high phosphorus content, was synthesized by polycondensation reaction of phenylphosphonic dichloride with a phosphorus-containing bisphenol, namely bis((6-oxido-6H-dibenz[c,e][1,2]oxaphosphorinyl)-(4-hydroxyaniline)methylene)-1,4-phenylene. The bisphenol was prepared by reacting 9,10-dihydro-oxa-10-phosphaphenanthrene-10-oxide with an imine bisphenol resulting from the condensation of 4-aminophenol with terephthalaldehyde. The structure and morphology of cured epoxy resins were evaluated by Fourier transform infrared spectroscopy and scanning electron microscopy (SEM) analysis, respectively. Differential scanning calorimetry analysis revealed that cured epoxy resins containing PFR possessed slightly higher glass transition temperatures than phosphorus-free cured epoxy resin. Thermogravimetric analysis and limiting oxygen index values indicated that the incorporation of PFR into epoxy resin substantially enhanced the thermal stability and flame retardancy of the char layer at high temperature. The surface morphology of the char residue was studied by SEM measurements. The kinetic processing of thermogravimetric data was carried out using Friedman and Vyazovkin methods. The lifetime prediction analysis established that the fire-resistant phosphorus-containing epoxy resins could be used at a constant temperature of 200 °C up to 620–630 minutes. The new PFR can be successfully used as a very efficient flame retardant for improving the fire-resistance properties of epoxy resins.

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Introduction

Thermosetting epoxy resins have been commercially available for more than half a century due to their attractive properties, such as excellent heat, moisture and chemical resistance, good electrical and mechanical properties, adhesion to many substrates, excellent processability and low cost.¹ They are excellent materials widely used for surface coating, casting adhesives, high-performance composites, insulating materials, packing for electronics and many other areas. However, the common epoxy resins possess high flammability and low thermal stability at elevated temperatures, which limit their use in applications where high flame retardancy is required, for example, in the electronics or aerospace industries.^2,3

Composite materials based on epoxy resins having improved properties, such as high thermal conductivity, good mechanical properties, increased dielectric constant and dielectric loss, were reported. Epoxy resin composites with enhanced mechanical properties were prepared using as filler β-silicon carbide micro particles modified by silane coupling reagent of γ-glycidoxy propyl trimethoxy silane.⁴ Also, composites based on epoxy resin and boron nitride as thermal conductive filler exhibited very high thermal conductivity, five times higher than that of native epoxy resin, and relatively good mechanical characteristics. The dielectric constant and dielectric loss of the composites increased with the increasing content of boron nitride.⁵

Halogen-containing epoxy resins were developed to improve the fire resistance, but the use of these systems has some disadvantages such as low glass transitions and low thermal stability. More than that, they cause major environment and health issues, during decomposition or incineration processes, due to the generation of corrosive and toxic gases (halogenated dibenzodioxins and dibenzofurans with severe toxicity) and high amounts of smoke.⁶ Thus, the ecological and health concerns are critical, governing the general tendency to avoid the use of halogenated fire-retardants. An alternative approach is the use of organophosphorus compounds as flame-retardants for epoxy resins. During combustion, phosphorus-containing compounds generate less toxic gases and smoke than the organohalogen ones.^7,8 They exhibit effective flame retardant capability, low corrosivity and lower release of toxic gas in flame when blended with epoxy resins and other polymers. Also, they can be utilized as additives or incorporated into the epoxy resin during its polymerization, and are known to be active in the condensed and/or gaseous phase, depending on the chemical nature and thermal stability of the additives as well as the host polymer matrices.⁹ While burning, the phosphorus-containing resins tend to yield carbonaceous char rather than CO and CO₂ and the carbonaceous char forms a protective surface layer.^10,11 This acts as a physical and thermal barrier to further combustion by impeding heat transfer to the underlying layers of polymer and, afterwards, minimizing the release of further flammable volatiles.

An attractive synthetic method to improve the properties, especially the flame retardant characteristics, is the utilization of monomers containing phosphorus, such as 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) which possess a polar P=O group and a bulky structure. The introduction of DOPO groups into the macromolecular chains leads to polymers having improved flame retardancy, thermal oxidative stability, solubility in organic solvents, good adhesion and low birefringence.^12–15 The use of DOPO and its derivatives as flame retardants for polymers has been widely investigated due to its high thermal stability, good oxidation and hydrolysis resistance.¹⁶ Phosphorus-containing composites were synthesized starting from bisphenol A epoxy resin and DOPO using a phenolic aldehyde as curing agent, 2,4,6-tri(phenol-methylene-amide)triazine, and a curing accelerator which is 2-methylimidazole.¹⁷ The flame-retardant and thermal stabilities of the composites were improved with the increasing phosphorus content, but other characteristics such as the flexural and impact behavior, dielectric constant and dielectric loss, glass transition temperature decreased.

Different DOPO-containing bisphenols were prepared by the addition of DOPO to the imine linkage. Thus, two novel halogen-free flame retardants were synthesized by the electrophilic addition reaction of DOPO to the imine linkages of some bisphenols resulted from the condensation reactions of 3-methoxy-4-hydroxybenzaldehyde with 4,4′-oxydianiline and 4,4′-diaminodiphenylsulfone, respectively.¹⁸ Other two flame-retardant bisphenols were obtained by the condensation of 4-hydroxybenzaldehyde with 4-aminophenol and 4,4′-diaminodiphenylsulfone, respectively, followed by electrophilic addition reaction of DOPO to the resulting imine linkages.¹⁹ These bisphenols were used, together with 4,4′-diaminodiphenylmethane, as hardeners for epoxy resins. The epoxy thermosets exhibited high flame-retarding performance at a relatively low addition amount of DOPO-containing bisphenols and had excellent thermal stability and high glass transition temperature.

Low molecular weight flame retardant additives tend to slowly leach out of the polymer matrix, which restricts their long-term use in some applications, such as electric and electronic appliances. A wide range of phosphorus-containing oligomers or polymers were reported to overcome this shortcoming.^20–22 Aromatic polyphosphonates attracted the interest of specialists due to the particular characteristics of these compounds, such as nonflamability, thermal stability and high melting points. The most important method of synthesis is the diol esterification with phosphonic acid dihalides.²³ The introduction of phosphorus atoms both into the main and side chains leads to an increase of phosphorus content, thus improving the fire resistant properties.^24,25 These polyphosphonates can be used as very efficient flame retardants for improving the fire resistance of common polymers such as epoxy resins.

The uncured epoxy resin can be converted into a crosslinked macromolecule by using different kinds of curing agents. Dicyandiamide (DICY) is one of the widely used latent curing agents for epoxy resin. The latent curing agents have a long pot life in epoxy resin at ambient conditions, whereas they can cure epoxy resin quickly under the high temperature.^26,27 A main disadvantage of DICY is that a high curing temperature of 160–180 °C has to be applied that limits the wide use of this curing agent.^28–30 To reduce the curing temperature, accelerators are usually used for some curing agents such as DICY.^31–34 Accelerators in curing of epoxy resins decrease the curing temperature and time and enhance its rate. The curing mechanism of an epoxy system based on DICY is largely dependent on the accelerator, the reaction temperature and the ratio of epoxy/amine used. Various amounts of DICY, two grades of epoxy resins, i.e. Epiran 06 and Epikote 828, and three different accelerators including benzyl dimethyl amine, 3-(4-chlorophenyl)-1,1-dimethyl urea and 2-methyl imidazole were used in curing of DICY/epoxy resin system.³⁵ The optimum concentration of DICY for curing of epoxy resins was obtained based on the glass transition temperature of the cured epoxy/DICY formulations. The maximum glass transition temperature was obtained at a ratio DICY to epoxy of 0.65.

In a continuing effort to develop halogen-free flame retardants for practical applications we prepared in our laboratory DOPO-containing polyphosphonates and used some of them as flame retardant additives for epoxy resins. Thus, an oligophosphonate containing phosphorus both in the main and in the side chains was synthesized by solution polycondensation of 1,4-phenylene-bis(6-oxido-6H-dibenz[c,e][1,2]oxaphosphoryl)carbinol with phenylphosphonic dichloride.³⁶ The oligophosphonate, as flame retardant for epoxy resins, showed excellent compatibility with epoxy resins and remarkable efficiency in improving the fire resistant of the thermosetting epoxy polymer. A content of 1% phosphorus increased the value of limiting oxygen index with about 30%. Herein, a phosphorus-containing bisphenol, namely bis((6-oxido-6H-dibenz[c,e][1,2]oxaphosphorinyl)-(4-hydroxyaniline)-methylene)-1,4-phenylene was synthesized and used for the preparation of a new oligophosphonate. Fire resistant semi-interpenetrating polymer networks were prepared based on epoxy resin, the synthesized oligophosphonate, DICY as hardener and 1,1-dimethyl-3-phenylurea as accelerator. Their morphology, thermal and fire resistant properties were discussed in correlation with the oligophosphonate content. Non-isothermal kinetic study was performed using Friedman and Vyazovkin methods.

Experimental

Materials

9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was purchased from Chemos GmbH, Germany, and dehydrated before use. Diglycidyl ether of bisphenol A (D.E.R. 331, epoxy equivalent 182–192 g mol⁻¹) (EP) was supplied by DOW Chemical Company. Terephthalaldehyde, 4-aminophenol, dicyandiamide (DICY), 1,1-dimethyl-3-phenylurea (DPH) and phenylphosphonic dichloride were provided by Aldrich. 4,4′-Terephthalylidene–bis(p-hydroxyaniline), 1, was synthesized from 4-aminophenol and terephthalaldehyde, following a method previously reported.³⁷ All other reagents were used as received from commercial sources or purified by standard methods.

Synthesis of bis((6-oxido-6H-dibenz[c,e][1,2]oxaphosphorinyl)-(4-hydroxyaniline)-methylene)-1,4-phenylene, 2

Compound 2 was prepared by reacting 1 with DOPO. Compound 1 (14.62 g, 0.0462 mol), DOPO (20 g, 0.0926 mol) and dried ethanol (103 mL) were introduced into a round flask equipped with a condenser and a magnetic stirrer. The mixture was stirred at 50 °C for 12 h under nitrogen atmosphere. The resulting precipitate was filtered, washed with ethanol and dried under vacuum (yield: 90%).

FTIR (KBr, cm⁻¹): 3265 (NH), 3060 (C–H aromatic), 1477 (P-Ar), 1218 and 1142 (P=O), 1043 (P–O–C), 914 (P–O-Ar), 753. ¹H NMR (400 MHz, DMSO-d₆, δ, ppm): 8.50 (m, 2H), 8.17 (m, 4H), 7.88 (m, 2H), 7.68 (m, 2H), 7.56 (m, 2H), 7.42 (m, 2H), 7.34 (m, 4H), 7.18 (m, 2H), 6.54 (m, 8H), 6.1 and 5.6 (m, 2H, N–H), 5.4 and 4.9 (m, 2H, CH–P).

Preparation of oligophosphonate, PFR

PFR was obtained by solution polycondensation reaction of equimolar amount of DOPO-containing bisphenol 2 with phenylphosphonic dichloride. In a flask equipped with a reflux condenser, magnetical stirrer and nitrogen inlet and outlet, were introduced and mixed bisphenol 2 (7.48 g, 0.01 mol), N-methyl-2-pyrrolidone (NMP) (30 mL) and triethylamine (3 mL). After a homogeneous solution was obtained phenylphosphonic dichloride (1.95 g, 0.01 mol) was added under stirring, during 0.5 h. The reaction flask was then immersed in an oil bath at 50 °C and the mixture was stirred vigorously for 8 h. The resulting solution was then cooled to room temperature and poured into methanol. The obtained solid was filtered and re-dissolved in NMP. The oligomer was isolated by precipitation in water, washed several times with water, and dried at 60 °C in a vacuum oven for 24 h to give a powdery solid (yield: 94%).

FTIR (KBr, cm⁻¹): 3295 (NH), 3060 (C–H aromatic), 1474 (P-Ar), 1212 and 1142 (P=O), 1046 (P–O–C), 925 (P–O-Ar), 753. ¹H NMR (400 MHz, DMSO-d₆, δ, ppm): 8.48 (d, 2H), 8.18 (m, 4H), 8–7.5 (m, 6H), 7.5–7.05 (m, 13H), 6.52 (m, 8H), 6.1 and 5.7 (m, 2H, N–H), 5.35 and 4.9 (m, 2H, CH–P).

Curing procedure of epoxy resins

Epoxy resin (EP) was cured using DICY as hardener and DPH as accelerator. The formulations of the pre-curing mixtures of EP, DICY, DPH and PFR are listed in Table 1. First, different quantities of EP were mixed with PFR under continuous stirring at 130 °C until complete dissolution was achieved, and then DICY and DPH were added when the solutions were cooled to 80 °C. The products were cured at 90 °C for 1.5 h, at 120 °C for 2 h and then at 155 °C for 2.5 h. The resulting thermosets were cooled slowly to the room temperature to prevent cracking.

Table 1 Preparation of cured epoxy resins EP-0, EP-0.5, EP-1 and EP-2

Sample	EP (g)	DICY (g)	DPH (g)	PFR^a (g/%)	Phosphorus (%)
a =% (w/w) PFR in the system.
EP-0	18.80	1.000	0.200	0/0	0
EP-0.5	13.00	0.676	0.134	0.624/4.33	0.5
EP-1	12.08	0.640	0.128	1.248/8.34	1.0
EP-2	12.18	0.645	0.129	2.850/18.03	2.0

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Measurements

The infrared spectra were recorded on a FTIR Bruker Vertex 70 Spectrophotometer at frequencies ranging from 4000 to 400 cm⁻¹. Samples were mixed with KBr and pressed into pellets.

¹H NMR (400 MHz) spectra were obtained at room temperature on a Bruker Advance DRX spectrometer, using DMSO-d₆ as solvent.

The molecular weights and their distribution were determined by gel permeation chromatography (GPC) with a PL-EMD 950 evaporative mass detector instrument. Two poly(styrene-co-divinylbenzene) gel columns (PLgel 5 μm Mixed-D and PLgel 5 μm Mixed-C) were used as stationary phase while N,N-dimethylformamide (DMF) was the mobile phase. The eluent flow rate was 1.0 mL min⁻¹. Polystyrene standards of known molecular weight were used for calibration.

Microscopic investigations were performed on an Environmental Scanning Electron Microscope type Quanta 200, operating at 10 kV with secondary electrons in low vacuum mode (LFD detector). For scanning electron microscopy (SEM) studies, the samples were fracturated, and the cross-section surface was examined. The Quanta 200 microscope is equipped with an Energy Dispersive X-ray (EDX) system for qualitative and quantitative analysis and elemental mapping.

Differential scanning calorimetry (DSC) was performed on a Mettler Toledo DSC822e. Approximately 10 mg of sample were tested applying a heating rate of 10 °C min⁻¹ from 20 to 200 °C, in air (50 mL min⁻¹). Heat flow vs. temperature scans from the second heating run were plotted and used for reporting the glass transition temperature. The mid-point of the inflexion curve resulting from the typical second heating was assigned as the glass transition temperature of the respective polymers.

Thermal stability was analyzed by means of a Mettler Toledo TGA-SDTA851e derivatograph. The thermogravimetric (TG) and derivative thermogravimetric (DTG) curves were recorded in air, with a flow of 20 mL min⁻¹ in the temperature interval 25–900 °C and with the following heating rates: 7, 10, 13 and 16 °C min⁻¹. In nitrogen, TG and DTG curves were recorded in the temperature interval 25–700 °C, with the heating rate of 10 °C min⁻¹. Constant operational parameters were preserved for all the samples, which had the mass ranged between 1.8 and 5.3 mg, so as to obtain comparable data and, moreover, the recordings were repeated for the same heating rate, so as to verify their reproducibility. The curves were processed using the STAR software from Mettler Toledo in order to obtain the thermal and kinetic characteristics.

Limiting oxygen index (LOI) measurement was carried out using a LOI chamber Qualitest by measuring the minimum oxygen concentration required to support the candle-like combustion of samples according to the ISO 4589 standard protocol. Samples of 90 × 6.5 × 3 mm³ were burnt in a precisely controlled atmosphere of nitrogen and oxygen.

Results and discussion

Bisphenol 2 and oligophosphonate PFR

Bisphenol 1 containing imine groups is synthesized by condensation reaction of 4-aminophenol with terephthalaldehyde. DOPO-containing bisphenol 2 is prepared by the addition of DOPO on the imine linkages of 1 (Scheme 1). The chemical structures of 2 and PFR were identified by FTIR and ¹H NMR spectroscopy (Fig. S1 and S2 in the ESI†). The FTIR spectrum of 2 shows a sharp absorption band at 1477 cm⁻¹ due to the aromatic P–C stretching vibrations. The bands appearing at 1212 and 1142 cm⁻¹ are associated with P=O stretching vibrations. Characteristic bands appear also at 1043 (asymmetric stretching vibration of P–O–C link), 3080 (C–H aromatic) and 753 cm⁻¹ (deformation vibration of 1,2-disubstituted aromatic DOPO rings).³⁶ The structure of 2 was also characterized by ¹H NMR. Two secondary amine peaks at 6.1 and 5.7 ppm, and two aliphatic hydrogen peaks at 5.35 and 4.9 ppm confirm the existence of two diastereomers. This phenomenon was previously reported.^38–40 All these facts allowed us to conclude that bisphenol 2 was successfully synthesized.

Scheme 1 Synthesis of DOPO-containing bisphenol 2.

Polycondensation reaction of equimolar amount of bisphenol 2 with phenylphosphonic dichloride in NMP as solvent and triethylamine as acid acceptor yields PFR (Scheme 2). The oligomer PFR is soluble in polar aprotic solvents such as NMP, DMF, or N,N-dimethylacetamide. GPC was used to determine the molecular weights and their distribution. The oligomer has the number average molecular weight (M_n) of 5400 g mol⁻¹, the weight average molecular weight (M_w) of 6100 g mol⁻¹ and the polydispersity (M_w/M_n) of 1.13. The GPC curve displays narrow molecular weight distribution. The relative low molecular weight values are due to the presence of voluminous DOPO groups which lead to a decrease of functional group reactivity and to a hindered propagation of polycondensation reaction.³⁶

Scheme 2 Synthesis of PFR.

The semi-interpenetrating polymer networks were obtained by mixing a bisphenol-A type epoxy resin with different amounts of linear PFR under heating and stirring to reach a molecular level, followed by the curing procedure in the presence of DICY as cross-linking agent and DPH as accelerator (Scheme 3). These polymer networks are characterized by the penetration on a molecular scale by a linear polymer. This is distinguished from an interpenetrating polymer network because the constituent linear can, in principle, be separated from the constituent polymer network without breaking chemical bonds.⁴¹ The ratio of DICY to epoxy is 0.60. This value was considered after some preliminary experiments made in order to find the optimum conditions for the preparation of thermosets with the highest glass transition temperature values.

Scheme 3 Preparation of thermosetting polymers (EP-0, EP-0.5, EP-1 and EP-2) containing different concentrations of PFR.

The structures of the resulting thermosets are identified by FTIR spectroscopy. In the case of phosphorus-free EP-0 spectrum characteristic bands are observed at 2970 and 2930 (aliphatic C–H asymmetric stretching vibrations), 2875 (aliphatic symmetric stretching vibration), 2184 (CN group), 1610 and 1501 (aromatic C=C stretching vibration), 1240 and 1040 cm⁻¹ (–C₆H₄–O–CH₂-asymmetric and symmetric stretching vibrations, respectively). In the case of phosphorus-containing epoxy resins EP-0.5, EP-1 and EP-2 characteristic bands appeared due to the presence of PFR at 1476 (aromatic P–C stretching vibrations), 1240 (aromatic P=O stretching vibrations) and 751 cm⁻¹ (deformation vibration caused by 1,2-disubstituted aromatic DOPO rings).

SEM analysis is used to investigate the morphology of the fracture surfaces of the samples. Fig. 1 presents the SEM images of EP-0, EP-1 and EP-2. The SEM micrographs of the EP-0 surface indicate a glassy and homogeneous microstructure, suggestive for the brittle and poor strength of the cured epoxy resins. The surface is smooth with low ridges and shallow grooves along the axis of crack.

Fig. 1 SEM photomicrographs of the cross sections of EP-0, EP-1 and EP-2.

The incorporation of PFR into epoxy resin modifies the morphology of the samples. Thus, EP-1 fracture surfaces indicate extensive crazing. They are full of branches and fibrils disposed almost parallel. In the case of EP-2 some particles can be observed on the cross-section surfaces but these particles were likely resin shards resulting from the material fracture. From EDX mapping of EP-1 and EP-2, the phosphorus atoms were uniformly dispersed in the epoxy matrix. The amount of phosphorus increases with increasing content of PFR in epoxy matrix from 0.27 at% (EP-1) to 0.56 at% (EP-2) (Fig. S3 in the ESI†).

Thermal characterization

Thermogravimetric analysis was carried out in order to obtain information on the influence of the working atmosphere (inert gas or air) and the rate of heating on thermal stability of the analyzed samples. The main thermogravimetric characteristics that have been obtained: T_onset – the temperature at which the thermal degradation starts at every stage; T_peak – the temperature at which the thermal degradation is maximum; T_endset – the temperature at which the degradation process ends of each stage, W% – percent weight loss during each stage and the amount of residue are shown in Table 2. Fig. 2 presents comparatively the derivative thermogravimetric curves at a heating rate of 10 °C min⁻¹ under air (a) and nitrogen (b). In Fig. S4 in the ESI, † the corresponding TG curves are shown.

Table 2 Thermogravimetric characteristics at various heating rates, and the glass transition temperatures (T_g)

Sample	β (°C min^−1)	Work atmosphere	Stage	Tonset (°C)	Tpeak (°C)	Tendset (°C)	W (%)	Residue (%)	Tg (°C)
a Residue at 900 °C.b Residue at 700 °C.
EP-0	7	Air	I	301	339	362	36.31	4.65^a	127
			II	405	518	556	59.04
	10	Air	I	308	352	373	38.96	8.29^a
			II	453	540	588	52.75
	10	Nitrogen	I	335	412	443	83.76	16.24^b
	13	Air	I	306	352	374	37.78	4.86^a
			II	501	544	591	57.36
	16	Air	I	303	357	381	40.25	4.97^a
			II	486	549	595	54.78
EP-1	7	Air	I	286	328	355	33.97	6.82^a	130
			II	442	516	562	59.21
	10	Air	I	291	332	362	33.48	7.86^a
			II	449	526	571	58.66
	10	Nitrogen	I	308	378	411	70.01	29.99^b
	13	Air	I	294	339	367	33.69	6.09^a
			II	454	538	603	60.22
	16	Air	I	307	346	382	35.34	6.66^a
			II	462	549	615	58.00
EP-2	7	Air	I	285	314	363	28.31	15.78^a	131
			II	466	529	575	55.91
	10	Air	I	286	318	364	31.01	10.28^a
			II	479	531	584	58.71
	10	Nitrogen	I	318	366	450	72.75	27.25^b
	13	Air	I	288	326	366	30.29	9.62^a
			II	486	537	577	60.09
	16	Air	I	315	357	381	38.78	5.22^a
			II	491	548	597	56.00
PFR	7	Air	I	264	278	304	5.89	28.92^a	138
			II	304	373	467	12.49
			III	467	493	628	28.07
			IV	688	756	850	24.63
	10	Air	I	264	286	300	4.86	28.37^a
			II	349	381	468	16.63
			III	468	495	565	20.49
			IV	565	791	900	29.65
	10	Nitrogen	I	262	293	319	13.32	47.20^b
			II	367	382	403	13.16
			III	470	504	700	26.32
	13	Air	I	272	293	311	5.32	32.92^a
			II	358	382	473	13.83
			III	473	502	580	24.11
			IV	580	—	—	23.82
	16	Air	I	276	297	369	8.61	35.60^a
			II	369	384	474	13.26
			III	474	500	577	21.99
			IV	577	—	—	20.54

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Fig. 2 Comparative DTG curves (a) air and (b) nitrogen.

The thermal degradation of EP-0 is carried out in a single stage in a nitrogen atmosphere and in two stages in air atmosphere. In air atmosphere the amount of the residue is from 4 to 9%, and approximately 16% under nitrogen at a heating rate of 10 °C min⁻¹. The thermal degradation of PFR is carried out in four stages in air, and in three stages when performed in nitrogen atmosphere. The amount of residue that remains when degradation is performed under air is comprised from 29 to 36%, with higher values if the heating rate is over 10 °C min⁻¹. If it is performed under nitrogen, its amount is to about 47%. Flame-retardant epoxy resins EP-1 and EP-2 have a degradation mechanism similar to reference epoxy, degradation being performed in two stages under air and in one stage under nitrogen. Analyzing derivative thermogravimetric curves shown in Fig. 2, it is observed that temperature at which the thermal degradation is maximum (T_peak) from the first stage decreases with the increase of the phosphorus content, regardless of the working environment. The same behavior is found by analyzing the initial temperature values at which degradation from the first stage starts (Table 2), irrespective of working atmosphere or heating rate. This decrease of thermal stability together with the increase of the phosphorus content in the sample was reported also by other researchers.^42–46 Obtained thermogravimetric data indicate a decrease in weight loss rate from the first stage together with the increase of the phosphorus content and an increase by few percents of the amount of residue obtained both in inert and air atmosphere. This behavior can be explained by the decomposition of PFR to phosphoric acid that condenses quickly, producing pyrophosphate structures and eliminating water vapours that dilute fuel gas phases. Phosphoric and pyrophosphoric acids can catalyze the dehydration reaction of the OH groups of the epoxy resins leading to the formation of carbon–carbon double bonds.

Phosphate anions (pyro and polyphosphates) will participate in the formation of carbonaceous residue. The carbonaceous layer may isolate and protect the underlying sample against fire; it prevents the formation of new free radicals and isolates the polymer from the heat released during the combustion process.⁴²

Table 2 also presents the glass transition temperatures obtained by means of the differential calorimetric studies for the epoxy resins and PFR. The addition of PFR in the epoxy matrix resulted in a homogeneous mixture with a single glass transition suggesting that PFR was well dispersed in the epoxy (Fig. S5 in the ESI†). The glass transition temperatures prove that the introduction of oligophosphonate does not lead, as it was expected, to the decrease of the glass transition temperatures. Similar results were obtained by M. Ciesielski et al.⁴⁷ for an epoxy resin containing two DOPO groups.

Non-isothermal kinetic study

The determination of the kinetic parameters of degradation is important in order to correlate them with the presence of different elements in thermosetting polymers. If the rates of degradation can be accurately measured, then it becomes possible to predict material properties as they relate to the specific thermal environment. Thermo-oxidative stability is also a crucial factor in determining the processing and application of materials.

To measure the variation of activation energy to the degree of conversion, the isoconversional method suggested by Friedman was used⁴⁸ as it uses no approximations, therefore, it is considered to be more accurate than integral isoconversional methods.⁴⁹ The mathematical relationship by which the apparent activation energy was calculated based on the degree of conversion (α) is as follows:

Graphical representation of values in inverse relation to absolute temperature values (value pairs obtained at four heating rates β = 7, 10, 13 and 16 °C min⁻¹ in air) for constant α, led to obtaining the lines from the slopes of which the apparent activation energy E_α is obtained. Variation of the activation energy to degree of conversion, calculated by using the method of Friedman, is shown in Fig. 3.

Fig. 3 Variation of the activation energy to degree of conversion if the degradation is produced in air.

The correlation coefficients obtained in the graphic representation of the lines were higher than 0.88.

The obtained results confirmed that thermal stability decreases with the increase of the phosphorus content. Therefore, up to transformation degrees of approximately 0.1, the apparent activation energy increases in the series: EP-2 < EP-1 < EP-0. Similar results, respectively the decrease of the activation energy upon small conversion degrees once with the phosphorus content increase, were also obtained by other researchers.^46,50 Degradation mechanism similar to the analyzed epoxy resins has also been observed. The existence of two stages when the degradation is performed under air has been confirmed. In the second stage corresponding to thermo-oxidation processes, the value of apparent activation energy is approximately constant of around 100 kJ mol⁻¹. In the first stage, the activation energy has the highest value of α = 0.35 for reference epoxy resin, 0.3 for the one containing 1% P and 0.25 for the sample having 2% P.

To perform a stringent kinetic study which involves the inclusion of the nucleation process, the method proposed by Vyazovkin⁵¹ was used for each degradation stage of the analyzed samples. The activation energy variation once with the conversion degree for the two degradation stages of the epoxy resins is comparatively presented in Fig. 4 and 5. The correlation coefficients were higher than 0.9. In the first degradation stage, for a transformation degree equal to 0.3 the apparent activation energy has almost the same value, respectively 155 kJ mol⁻¹ both for the epoxy resin and for those containing 1% P and 2% P. Up to a transformation degree equal to 0.3, the apparent activation energy for the phosphorus-containing resins is smaller than for the reference epoxy resin. In values higher than 0.3 for the conversion degree, the apparent activation energies are higher for the phosphorus-containing epoxy resins than for the reference one. Activation energy increase once with the increase of the phosphorus content was also highlighted by T. S. Ho et al.⁴⁴ for phosphorus-containing dicyclopentadiene epoxy resins. In the previously carried out kinetic studies^52,53 for a range of phosphorus-containing polyesters and imide polyesters, it was established that at higher conversion degrees the activation energy tends to increase when the phosphorus is located in the main chain and is lower when the phosphorus is located in the lateral groups. A different behavior, respectively higher activation energies when phosphorus is located in the lateral groups as compared to the case when it is located in the main chain was noticed in smaller conversion degrees, confirming the thermal stability range established from the thermogravimetric data. The oligophosphonate used as flame-retardant agent in this paper contains phosphorus both in the main chain and in the lateral groups and probably this is the cause of the different behavior in the two fields: α < 0.3 and α > 0.3. The activation energy increase once with the conversion degree from the first stage can suggest a degradation mechanism through parallel reactions of the epoxy resins, according to the classification proposed by S. V. Vyazovkin and A. I. Lesnikovich.⁵⁴

Fig. 4 Variation of the apparent activation energy with the conversion degree in the first degradation stage of the epoxy resins.

Fig. 5 Variation of the apparent activation energy with the conversion degree in the second degradation stage of the epoxy resins.

In the second stage (Fig. 5) the activation energy decreases once with the conversion degree increase, tending to a constant plateau of almost 100–110 kJ mol⁻¹, suggesting a degradation mechanism through successive reactions.⁵⁴ In this stage, the activation energy has smaller values for the phosphorus-containing epoxy resins and at conversion degree values higher than 0.65, they are the same regardless of the phosphorus percentage of the sample content.

For the first degradation stage (Fig. 6) the thermal decomposition model is characteristic to the random nucleation with the observance of a kinetic law characteristic to the 1st order monomolecular reactions (F1 model) for the reference epoxy resin and that with a content of 1% P and 2nd order reactions (F2 model) for the resin containing 2% P. V. Pistor et al.,⁵⁵ A. Motahari et al.⁵⁶ and L. Xia et al.⁵⁷ also identified a F1 model for the decomposition of the epoxy resin diglycidyl ether of bisphenol A.

Fig. 6 Variation of the g(α) function with the conversion degree for the first degradation stage of the epoxy resins.

In the second stage of thermal decomposition for the reference epoxy resin the degradation takes place through reactions controlled by the two-dimension displacement of the interface (R2 model), and for the ones containing 1% P, respectively 2% P, the degradation takes place through the three-dimension displacement of the interface (R3 model) (Fig. 7).

Fig. 7 Variation of the g(α) function with the conversion degree for the second degradation stage of the epoxy resins.

The integral kinetic equation for these models has the expression given by the relation:

g(α) = k₀t (2)

where k₀ is the rate constant and t is time.⁵⁸ The expressions of the g(α) conversion functions for the obtained models, enforcing the method proposed by Vyazovkin⁵¹ are presented in Table 3. The standard deviation was calculated for each obtained model, with the following relation:where n represents the number of experimental data and p the number of parameters. The obtained values of the standard deviations are presented in the g(α) function variation diagrams with the conversion degree. Based on the kinetic parameters determined through the method proposed by Vyazovkin,⁵⁰ we assessed the lifetime prediction (t_f) for the epoxy resins, an extremely important parameter, taking into account the potential practical applications for these materials. Table 4 presents the values for t_f, at conversion degrees between 0.01 and 0.1 and the following temperatures: 200, 250 and 300 °C. As compared to the reference epoxy resin t_f decreases up to 8 times in the phosphorus-containing resins, but they have the advantage that, during the thermal decomposition process appears a carbonaceous residue which can insulate and protect the polymer against the flame, respectively which prevents the formation of new free radicals and insulates the polymer from the heat discharged during the burning process.

Table 3 Expressions of the g(α) conversion functions

Model	g(α)
Power law – P2	α^1/2
First-order – F1 (random nucleation with one nucleus on the individual particle)	−ln(1 − α)
Second-order – F2 (random nucleation with two nuclei on the individual particle)
Contracting cylinder – R2 (phase boundary controlled reaction)	1 − (1 − α)^1/2
Contracting sphere – R3 (phase boundary controlled reaction)	1 − (1 − α)^1/3

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Table 4 Thermal lifetime values estimated by means of Vyazovkin kinetic methods

Sample	α	tf (min)
		200 °C	250 °C	300 °C
EP-0	0.01	1400	50	2.7
	0.05	2000	60	3.3
	0.10	3300	90	4.5
EP-1	0.01	180	8.7	0.7
	0.05	330	13.5	1.0
	0.10	630	23	1.5
EP-2	0.01	250	8.3	0.6
	0.05	375	12.5	0.75
	0.10	620	19	1.1

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The thermal decomposition of PFR as we noticed in the thermogravimetric data presentation is performed through a complex mechanism in four stages.

By enforcing the method proposed by Vyazovkin,⁵⁰ the variation of the activation energy with the conversion degree was calculated for each stage. The obtained results are represented in Fig. 8. On the diagram only the pairs of values (E_a, α) for which the correlation coefficients were higher than 0.85 were presented. In the first stage, the average value of the activation energy is of 170 kJ mol⁻¹, in the following stages the value increases to 202 kJ mol⁻¹, respectively to 398 kJ mol⁻¹ and in the last stage corresponding to the thermooxidation process it reduces to 114 kJ mol⁻¹.

Fig. 8 Variation of the apparent activation energy with the conversion degree for PFR.

According to the results presented in Fig. 9, the beginning of the PFR degradation takes place through controlled reactions of three-dimension displacement of the interface (R3 model) and continues in the second stage through the random nucleation with the observance of a kinetic law characteristic to the 1st order monomolecular reactions (F1 model). In the third stage takes place the two-dimension displacement of the interface (R2 model) and in the last stage a model type power law (P2) is observed. The standard deviations obtained by comparing the values of the g(α) functions calculated with proposed models with the experimental ones are smaller than 0.132 and are presented in Fig. 10, and the expressions of the g(α) conversion functions are presented in Table 3.

Fig. 9 Variation of the g(α) function with the conversion degree for PFR.

Fig. 10 SEM photomicrographs of the char residue at 700 °C of PFR, EP-0 and EP-2.

The LOI measurements were performed in order to evaluate the flame retardancy of the thermosets. The LOI values were 24% for EP-0, 26% for EP-0.5, 27% for EP-1 and 30% for EP-2.

Taken into accounts that a material can be considered as fire resistant when LOI value is higher than 26, it can be concluded that the samples containing 1 and 2% phosphorus (EP-1 and EP-2 samples) correspond from this point of view. The highest LOI value was obtained for EP-2 having the highest concentration of phosphorus of 2%. These results show that the novel PFR is an effective flame retardant for epoxy resins. The quantitative contribution of PFR on the improved flame retardancy can be given by the increased values of LOI with about 8.0, 12.5 and 25.0% for EP-0.5, EP-1 and EP-2 having 0.5, 1 and 2% phosphorus, respectively.

Char, a carbonaceous porous residue, results from the thermal degradation of the material being pyrolyzed. It can act as an insulating barrier between the external heat source and the unpyrolyzed material under the char. The flammability of polymers can be reduced by decreasing the rate of production of combustible gases while increasing the rate of char production in the solid phase. The char formation usually reduces the production of smoke and other products or incomplete combustion.⁷ In the most phosphorus-containing compounds the decomposition leads to phosphorus-rich char layer. The presence of phosphorus reduces the production of combustible gases while increasing the char yield in the solid phase. SEM images of the pyrolyzed solids of PFR, EP-0 and EP-2 obtained by heating the samples up to 700 °C in nitrogen, with the heating rate of 10 °C min⁻¹ are presented in Fig. 10. The char of PFR is dense and compact while the char of the phosphorus-free sample EP-0 is loose and porous. With the introduction of PFR to epoxy resin the charring properties were improved. The chars undergo a process from loose to compact. Thus, the chars of EP-2 are dense and compact suggesting that they can better prevent the heat transfer between the flame zone and the burning substrate in comparison with that of EP-0 char. Also it can inhibit the release of flammable or toxic volatiles, resulting in an enhancement of flame retardancy. This trend observed by SEM corroborates the LOI results obtained: EP-0 < EP-1 < EP-2.

The high concentration of phosphorus in the chars of PFR and EP-2 was evidenced by EDX analyses (Fig. S6 in the ESI†). The maximum phosphorus concentration on the residue surface was observed in the case of PFR. But high phosphorus content was observed also in the case of EP-2. In order to investigate the atom distribution into the char yield surface a mapping technique was used. Fig. 11 presents the EDX mapping of PFR, EP-0 and EP-2. It can be observed that phosphorus atoms are uniform dispersed in the char yield surface. These results which are in accordance with the TGA measurements and LOI testing, demonstrate that phosphorus-containing PFR has an important role in improving the fire resistant of epoxy resins.

Fig. 11 EDX mapping of the char residue of PFR (a), EP-2 (b) and EP-0 (c) (C: carbon, O: oxygen; N: nitrogen; P: phosphorus).

Conclusions

Flame retardant semi-interpenetrating polymer networks based on a bisphenol A-epoxy network and an oligophosphonate PFR having high phosphorus content were prepared by thermally crosslinking in the presence of DICY as curing agent and DPH as accelerator. The morphological analyses revealed a good compatibility between the epoxy matrix and PFR. The thermogravimetric and derivative thermogravimetric analysis established that the initial temperature when the degradation begins and the temperature at which the thermal degradation is maximum from the first stage decrease once with the increase of the phosphorus content, regardless of the working atmosphere. The residue quantity at 700 °C increases once with the increase of the phosphorus content in both working atmospheres, but to a larger extent in inert atmosphere. The flame-retardant epoxy resins containing 1% P, respectively 2% P have a degradation mechanism similar to the reference epoxy resin, the degradation being performed in two stages in the air and in one stage only in nitrogen. The reference epoxy resin and the flame-retardant ones have a smaller thermal stability in the air than in nitrogen, if the stability criterion is the initial decomposition temperature or the temperature at which the degradation rate is maximum in the first degradation stage. In the case of oligophosphonate the thermal stability is not influenced by the working atmosphere (air or nitrogen). The differential calorimetric studies have shown that oligophosphonate introduction does not lead to the reduction of the glass transition temperatures. The non-isothermal kinetic studies proved the complexity of the degradation mechanism of the epoxy resins and of oligophosphonate. The variation of the activation energy with the conversion degree was obtained; for each degradation stage were established the expressions of the g(α) conversion functions for the models which correlate the experimental data with the smallest standard deviations, by enforcing the method proposed by Vyazovkin. The lifetime prediction (t_f) analysis established that the flame-retardant phosphorus-containing epoxy resins can be used at a constant temperature of 200 °C up to 620–630 minutes. LOI values of the thermosets ranged from 24% in the case of phosphorus free sample EP-0 to 30% for the sample containing 2% P (EP-2). SEM measurements of the char residues showed a more compact structure for the samples containing phosphorus.

Acknowledgements

Dr Corneliu Hamciuc is thankful to the European Union's Seventh Framework Programme (FP7/2007–2013), under grant agreement no. 264115–STREAM, for the financial support offered to investigate the flame retardant properties of the materials.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27451f

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