Simultaneously enhancing the flame retardancy and toughness of epoxy by lamellar dodecyl-ammonium dihydrogen phosphate

Abstract

Organic/inorganic intercalated dodecyl-ammonium dihydrogen-phosphate (C₁₂ADP) has been prepared by a simple one-pot method. The incorporation of C₁₂ADP into epoxy could improve the flame retardancy and toughness of the obtained C₁₂ADP/EP composites, simultaneously, i.e. improved formed char quality, significantly reduced heat release rate and decreased total heat release, and enhanced impact toughness of EP composites as well.

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Epoxy (EP) is being widely used in construction, electronics, aerospace, and many other industrial fields, due to its low curing shrinkage, excellent adhesion to many substrates, resistance to chemical corrosion, electrical insulating property, and great freedom in formulation design.¹ However, due to its inherent structure, neat EP is very flammable and brittle, which hinders its application in broader fields and needs to be addressed.^2,3

In terms of flame retardancy, halogen-containing flame retardants are highly effective for improving the fire retardancy of EP. However, the obtained flame retarded EPs produce toxic and corrosive gases during combustion, seriously endangering the safety of life and polluting the environment.^4,5 Therefore, the application of halogenated flame retardant was already restricted by WEEE and RoHs. As a result of this, exploiting halogen-free fire retardants has attracted growing attention.^6–15 Phosphorus-containing flame retardants, as one type of halogen-free alternative flame retardants, have been investigated widely,^9–15 whereas, the introduction of phosphorus-containing flame retardant, usually, compromised the mechanical properties, such as impact strength, of EP matrix due to the poor compatibility.^14,15 Modification of phosphorus flame retardant with organic moieties, for example microencapsulation, could enhance the compatibility of phosphorus flame retardant with matrix, but the improvement was limited.¹⁵ Meanwhile, intensive research studies have shown that the organic/inorganic hybrid layered compounds, such as layered silicates,^16,17 metal chelates,¹⁸ layered double hydroxide,^19,20 and zirconium phosphates,^21,22 could toughen EP by introducing long flexible alkyl chain.

Alkyl-ammonium (di)hydrogen phosphates^23,24 are layered organic–inorganic intercalation compounds possessing phosphorus, nitrogen, and flexible alkyl chain as well, which suggests that this kind of compounds is potential promising EP modifier acting both as impact toughener and flame retardant. At here, we will describe a simple synthetic protocol of this series of compounds and exploit their application as promising EP modifiers focusing on the impact of dodecyl-ammonium dihydrogenphosphate (C₁₂ADP) on the thermal stability, flame retardancy, and impact toughness of the obtained composites.

Alkyl-ammonium (di)hydrogen phosphates were synthesized via a simple one-pot protocol by mixing and stirring alkyl amine, phosphoric acid and water at room temperature for 48 h. The crystal structure of alkyl-ammonium (di)hydrogen phosphates can be easily tuned by adjusting the molar ratio of alkyl amine to phosphoric acid (cf. small-angle powder XRD patterns of synthesized compounds in Fig. S1†) for all amines used. Generally, mixtures of monolayer and bilayer structures were obtained when the molar ratio of amine to acid equaled to 2 : 1. Once the molar ratio deviated from 2 : 1, one can get the pure compounds of bilayer structure or monolayer structure for the molar ratio larger or smaller 2 : 1, respectively. The interlayer space, d, of layered compounds were calculated according to the Bragg equation (2d sin θ = nλ), and plotted as a function of the carbon number of alkyl chain, n_C. Fig. 1a and b show the correlation between d and n_C, d = 0.30 + 0.24n_C and d = 0.45 + 0.14n_C (nm) for monolayer structure and bilayer structure by linear regression, respectively, where 0.24 and 0.14 are the slopes. Taken the C–C bond length of 0.154 nm and C–C–C bond angle of 123.9° for alkyl group into consideration,²⁴ the schematic presentations of monolayer and bilayer structure are proposed and shown in Fig. 1a and b, respectively. The alkyl chains of monolayer structure are nearly perpendicular to the inorganic layers, while the alkyl chains of bilayer structure tilt a little bit with respect to inorganic layer.

Fig. 1 (a) and (b) correlation of interlayer distance and number of carbon atoms of alkyl amines and the schematic presentation of organic–inorganic intercalation structure for monolayer and bilayer structures, respectively. (c) SEM image of C₁₂ADP, inset: small angle powder XRD pattern of C₁₂ADP.

C₁₂ADP was used as an example to demonstrate the impact of this series of compounds on the properties of EP composites. The schematic presentation of the synthesis process for C₁₂ADP is shown in Scheme 1. The morphology of C₁₂ADP determined by SEM (Fig. 1c) clearly shows that C₁₂ADP is of layered structure. The small-angle powder X-ray diffraction pattern of C₁₂ADP (see inset of Fig. 1c) is in good agreement with SEM observation and displays two diffraction peaks at 2θ = 4.09° and 2θ = 8.44°, suggesting an interlayer space of 2.17 nm.²⁴ In addition, ICP-AES results revealed that the weight percentages of phosphorus element and nitrogen element in C₁₂ADP were 10.89(3) wt% and 4.52(8) wt%, respectively, which gives the molar ratio of P : N ≈ 1 : 1. Therefore, the formula of C₁₂ADP could be C₁₂H₂₅NH₃⁺·H₂PO₄⁻, indicating a type of dihydrogen phosphate.

Scheme 1 Schematic presentation of synthesis process for C₁₂ADP.

The EP/C₁₂ADP composites were prepared by dispersing C₁₂ADP in EP resin and subsequent hot curing. Prior to mixing and hot curing, the as dried C₁₂ADP was ground down to 100% passing 0.074 mm to facilitate the dispersion of C₁₂ADP in EP matrix. XRD analysis on the obtained composites showed that only the characteristic patterns of EP were observed, which may indicate that C₁₂ADP could be dispersed in EP matrix at low loadings (Fig. S2†).

We examined the thermal stability behavior of EP/C₁₂ADP composites both in nitrogen and air (cf. Fig. 2). In N₂ atmosphere (Fig. 2a and c), EP/C₁₂ADP composites underwent single step thermal degradation, while two steps of weight loss were observed in air atmosphere (Fig. 2b and d), the first step is mainly due to the softening-pyrolysis-crosslinking and char formation of composites, and the second one is most likely attributed to the degradation of remaining char formed at the first step. The incorporation of C₁₂ADP into EP gave rise to composites with higher residue amount at 700 °C, which increased with increasing C₁₂ADP loadings. In N₂ atmosphere, residual chars increased from 14.9 wt% to 16.0 wt% to 20.8 wt% further up to 22.9 wt% as C₁₂ADP loading increased from 1.5 wt% to 10 wt%. The increase in residual chars mainly contributes to the residue of additive, and is related to the intumescent flame retardant characteristics of added C₁₂ADP. In air atmosphere, the influence of C₁₂ADP loading on the increase in residue amount showed a similar tendency, but the impact was not so significant as in N₂ atmosphere, the addition of 1.5 wt%, 5 wt% and 10 wt% C₁₂ADP into EP resulted in the increase in char residue of 0.4 wt%, 1.2 wt% and 1.9 wt%, respectively, in comparison with 0.2 wt% residual char for neat EP. On the other hand, the incorporation of C₁₂ADP into EP caused the earlier onset of the decomposition of the matrix, it is noted the temperature at 5% weight loss and maximum weight loss for composites decreased as the loading of C₁₂ADP increased in both nitrogen and air atmosphere. The thermal degradation behavior of composites could be explained well by the thermal properties of C₁₂ADP, since it is observed in Fig. 2b that: (1) C₁₂ADP lost significant weight at 200 °C due to the dihydrogenphosphate group and organic amine molecules which may lower initial decomposition temperature.^25–27 (2) The carbon residue of C₁₂ADP (18.8 wt% at 700 °C) is much higher than the residual carbon content for neat epoxy.

Fig. 2 Impact of C₁₂ADP loading on the thermal stability of EP/C₁₂ADP composites in nitrogen (a and c) and air atmosphere (b and d).

The flame retardancy of EP/C₁₂ADP composites was evaluated by cone calorimetric test, which is regarded more close to real fire scenarios. The curves of heat release rate (HRR) were displayed in Fig. 3. The major combustion parameters including time to ignition (TTI), total heat release (THR), peak of heat release rate (pHRR), mean specific extinction area (SEA) mean mass loss rate (MLR), as well as total smoke production (TSP) data were summarized in Table 1. Improvement in TTI could be observed for EMP-1.5 in comparison with neat EP, suggesting that the addition of a small amount of C₁₂ADP could reduce the ignitability of EP/C₁₂ADP composites. However, the TTI value of composites dropped to close to that of neat EP as the C₁₂ADP loading increased. The tendency of TTI value at different C₁₂ADP loadings could be related to the nature of phosphate and long alkyl amine. In the presence of P and N elements, the as-formed char layer could prevent heat and mass transportation, therefore the ignitability of composites reduced. On the other hand, the aliphatic amine used was unstable against heat, therefore the TTI value of composites compromised as reported in literature for layered organo-silicate filled polymers.¹⁷ It is noted that the incorporation of C₁₂ADP into EP led to slightly decreased TSP, which gradually decreased with increasing C₁₂ADP loadings, this might suggest that C₁₂ADP plays a role of smoke suppression agent.

Table 1 Some detailed data from cone calorimetry measurement

Sample	TTI (s)	THR (MJ m^−2)	pHRR (kW m^−2)	MLR (g s^−1)	SEA (m^2 kg^−1)	TSP (m^2 m^−2)
EP	55.5	92.3	1008	0.090	710	26.2
EMP-1.5	63.5	84.7	938	0.078	865	23.2
EMP-5	60.0	79.1	593	0.063	702	21.1
EMP-10	56.0	83.6	366	0.054	664	20.9

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HRR curves (Fig. 3) clearly show the introduction of C₁₂ADP into EP substrate significantly reduced the pHRR, the decrease in pHRR shows a positive correlation with the C₁₂ADP loading, and is in a good agreement with MLR value shown in Table 1. As the adding amount of C₁₂ADP increased from 1.5 wt% to 10 wt%, the pHRR of corresponding composite reduced by 7.0%, 41.1%, and 63.7%, respectively, which suggests the high efficiency of C₁₂ADP in reducing pHRR. From the above discussion, it is clear that low amount of flame retardant C₁₂ADP could delay the ignition time of EP, reduce the amount of smoke emission, and effectively reduce the heat release rate.

Fig. 3 HRR curves of EP/C₁₂ADP composites in comparison with that of neat EP.

In general, the presence of hydroxyl groups from EP or trace amount of water may catalyze the decomposition of the C₁₂ADP to generate a series of acids, such as phosphoric, metaphosphoric, and polyphosphoric acids, which could catalyze and involve in the formation of protective layer, and the quantity and quality of char layer greatly influences the flame retardant property of materials.^28,29 Hence, photography and SEM were used to study the char morphology of samples after limiting oxygen index (LOI) test in macroscopic and microscopic scales, respectively. The LOI value of neat EP was 25.0% while those of the obtained composites with increasing C₁₂ADP loadings increased from 25.0% to 27.5%. Fig. 4 shows the digital photos and SEM images of the char morphology of LOI test samples. From Fig. 4a, it is easy to identify that more char residue was retained as more C₁₂ADP was dispersed in epoxy matrix. The structure of char changed from bend and split one for neat EP to a more compact, stable and rigid ones for EP/C₁₂ADP composites. SEM images in Fig. 4b and c provide more detailed information. Two types of char structures have been observed for EP with and without C₁₂ADP. One is sand-like structure illustrated in Fig. 3b, this is the main structure of char from burned neat EP. The other is foam-like structure illustrated in Fig. 4c, which is generated owing to the incorporation of C₁₂ADP into EP. The foam-like char structure is typical for intumescent flame retardant polymer³⁰ and could hinder the transportation of oxygen and heat so that the combustion reflection could be inhibited.

Fig. 4 (a) Digital photos of the remaining of EP/C₁₂ADP composites after combustion, from up to down are EP, EMP-1.5, EMP-5, and EMP-10, respectively. (b) SEM images of the remaining char of EP, showing the sand-like char, scale bar 100 μm, the scale bar of inset is 1 μm. (c) SEM images of the remaining chars of EP/C₁₂ADP composites (EMP-1.5, EMP-5, and EMP-10), exhibiting the foam-like structure, scale bar as 50 μm.

Finally, we assessed the influence of C₁₂ADP loading on the impact toughness of EP/C₁₂ADP composites, the un-notched Charpy impact test was performed and the experimental results were illustrated in Fig. 5. In comparison with the impact toughness of neat EP, composites of EPM-1.5 and EPM-5 showed 17.8% and 25.1% increase in impact toughness, respectively. However, as the loading of C₁₂ADP increased further to 10 wt%, the impact toughness did not improve further. The reason could be: low amount of C₁₂ADP could be easily dispersed in EP and resulted in the improvement of the impact toughness of composites, however, high C₁₂ADP content may lead to the stack of C₁₂ADP layers that would cause a sharp increasing stress and lower the energy for cracks to bypass the obstructions under external stress.³¹

Fig. 5 (a) Impact toughness of EP/C₁₂ADP composites with different C₁₂ADP loadings. (b) and (c) SEM images of the fractured surfaces of neat EP and EP/C₁₂ADP composite with 5 wt% C₁₂ADP, respectively, scale bar as 20 μm.

The fractographic analysis of the fractured surfaces after un-notched Charpy impact test was performed to have a better understanding on the improvement in impact toughness by adding C₁₂ADP into EP (see Fig. 5b and c). The fractured surface of neat EP is smooth and without any sign of deformation (see Fig. 5b), showing a typical brittle fracture. By introducing 5 wt% C₁₂ADP into EP, the fractured surface (see Fig. 5c) became rougher and the channel of crack propagation deflected, which accounts to the improved impact toughness.³² The effective improvement of impact toughness could be mainly attributed to two issues: (1) the rigid inorganic sheets in C₁₂ADP acting like nail and hindering the pathway of crack propagation, (2) the flexible alkyl chains in C₁₂ADP consuming and absorbing the impact energy. However, it is worth of further analysis on the effect of C₁₂ADP on epoxy curing process (kinetics and thermodynamics) and the mechanical properties to elucidate acting fracture failure mechanism.

In conclusion, organic/inorganic intercalated alkyl-ammonium (di)hydrogen phosphates containing phosphorus, nitrogen, and flexible alkyl chain were synthesized in one-pot. Introducing 5 wt% C₁₂ADP into EP achieved composites with 20% increase in impact toughness compared with neat EP. Additionally, the introduction of C₁₂ADP into EP delayed the ignition time and effectively reduced the heat release rate, total heat release, as well as total smoke release. It suggests that C₁₂ADP as a single-component additive could simultaneously enhance the flame retardancy and impact toughness of EP at low loadings. Further study on the effect of C₁₂ADP on the curing process (kinetics and thermodynamics), as well as the acting flame retardant mechanism, will be performed to deepen the understanding of the role of C₁₂ADP.

Acknowledgements

The authors would like to thank Mr C. Xie for LOI measurements, Mr B.-B. Xu for TEM measurements, Prof. Dr L.-Z. Dai, Prof. Dr J.-X. Mi and Dr D.-H. Li for valuable discussion, as well as Dr G. Aufermann and Ms A. Völzke at MPI CPfS in Dresden of Germany for elemental analyses. This work was supported by the Natural Science Foundation of Fujian Province of China (No. 2010J0130, No. 2014J06021), Scientific and Technological Innovation Platform of Fujian Province of China (2014H2006), and the National Natural Science Foundation of China (No. 21201144 and 21233004).

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Footnote

† Electronic supplementary information (ESI) available: Details of the sample preparation and characterization techniques used. Fig. S1: small-angle powder XRD patterns of synthesized epoxy modifiers. Fig. S2: powder XRD patterns of modified epoxy composites in comparison with that of epoxy modifier. See DOI: 10.1039/c5ra18358h

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