Epoxy thermoset with enhanced flame retardancy and physical-mechanical properties based on reactive phosphaphenanthrene compound

Journal Pre-proof Epoxy thermoset with enhanced flame retardancy and physical-mechanical properties based on reactive phosphaphenanthrene compound Shanglin Jin, Zhen Liu, Lijun Qian, Yong Qiu, Yajun Chen, Bo Xu PII:

S0141-3910(19)30391-X

DOI:

https://doi.org/10.1016/j.polymdegradstab.2019.109063

Reference:

PDST 109063

To appear in:

Polymer Degradation and Stability

Received Date: 7 October 2019 Revised Date:

20 December 2019

Accepted Date: 28 December 2019

Please cite this article as: Jin S, Liu Z, Qian L, Qiu Y, Chen Y, Xu B, Epoxy thermoset with enhanced flame retardancy and physical-mechanical properties based on reactive phosphaphenanthrene compound, Polymer Degradation and Stability (2020), doi: https:// doi.org/10.1016/j.polymdegradstab.2019.109063. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Epoxy thermoset with enhanced flame retardancy and physical-mechanical properties based on reactive phosphaphenanthrene compound Shanglin Jin a,b,§, Zhen Liu a,b a

§

, Lijun Qian a,b,*, Yong Qiu a,b, Yajun Chen a,b, Bo Xu a,b

School of Materials Science and Mechanical Engineering, Beijing Technology and

Business University, Beijing 100048, China; b

Engineering Laboratory of non-halogen flame retardants for polymers, Beijing 100048,

China. * Corresponding author: Lijun Qian §

These authors contributed equally to this work

E-mail: [email protected] Address: Gengyun Building No.516, Fucheng Road No.11, Haidian District, Beijing, China. Telephone Numbers: 0086 10 68984011 Fax numbers: 0086 10 68984011

1

Abstract A phosphaphenanthrene flame retardant, DCAD, was synthesized by 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), p-phenylenediamine and bio-based material cinnamaldehyde. After being incorporated into epoxy thermosets, DCAD can impose better flame retardancy and physical-mechanical properties to epoxy thermosets. 4%DCAD/EP achieved a LOI value of 35.6% and passed the UL94 V-0 rating, obviously decreased peak value of heat release rate, and generated the compact, stable and foam-like char layer during combustion. Through tracking by the gas-phase real-time infrared spectrum and observing the microscopic structures of residue, the higher flame retardancy of DCAD was attributed to the effectively quenching effect in gas phase and the high-quality charring effect in condensed phase. In addition, the N-H structures in reactive-type DCAD molecule can react with the epoxy groups of epoxy resin and two C=C bonds in DCAD can form elastomer-like crosslink, resulting in that 4%DCAD/EP owned a raised impact strength of 25.74 kJ/m2 and did not decrease the glass transition temperature obviously in contrast to neat epoxy resin. Keywords: Flame retardant; DOPO; High toughness; Epoxy resin

2

1. Introduction Since excellent electrical insulation, corrosion resistance, and adhesive properties, epoxy resins are widely used in coatings, adhesives, electrical and electronic insulation materials, advanced composite substrates and other fields [1-4]. However, the flammability of epoxy resin limits its application, especially in electrical and electronic engineering field. Therefore, flame retardant modification of epoxy resins is necessary for manufacture advanced commercial materials [5-8]. Adding flame retardant is the most main method for flame retardant modification, and it is also easy way to endow flame retardancy to epoxy resin [9-12]. From several decades before to nowadays, halogen-containing flame retardant brominated epoxy resin was always mainly applied in epoxy resins to meet the requirements of electrical and electronic engineering materials [13-15]. In recent decade years, more and more halogen-free flame retardants, especially phosphorus-containing flame retardants [16-19] and phosphorus/nitrogen-containing flame retardants [20-22], had received extensive attentions

and

were

numerously

researched.

In

these

researches,

9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) [23-25] and their phosphaphenanthrene derivatives have been proven to have superior flame retardant effects. Phosphaphenanthrene group also can combine with different flame retardant groups to form novel compounds with flame retardant group synergistic effect, such as phosphaphenanthrene/triazine [26,27], phosphaphenanthrene/phosphazene [28,29], phosphaphenanthrene/borate

[17],

phosphaphenanthrene/siloxane

[30-33],

and

phosphaphenanthrene/phosphate [34,35], and so on. These bi-group compounds usually 3

possessed higher flame retardant efficiency when they were used in epoxy thermosets. Some of the DOPO derivatives are additive-type additives, and some are reactive-type ones. Usually, the additive-type DOPO derivatives are easily applied via mixing process without reaction. The reactive-type ones need a reaction process for linking them to matrix. However, most of additive-type DOPO flame retardants, often decreased the glass transition temperature of epoxy resin because the flame retardants usually reduced the crosslinking density of epoxy resins [36,37]. This method improved flame retardancy but sacrificed some physical-mechanical properties, and it is contrary to our original intention to develop advanced polymer materials with better comprehensive properties. Recently, more and more reactive-type flame retardants were designed and prepared for both improving flame retardant properties and physical-mechanical properties of epoxy resin simultaneously [31, 38-40]. In this work, to obtain the flame retardant with imposing better flame retardancy and physical-mechanical properties to epoxy thermosets, a DOPO derivative flame retardant (DCAD), was synthesized using DOPO, p-phenylenediamine and bio-based material cinnamaldehyde. Then, the flame retardant and physical-mechanical properties of epoxy thermosets with different DCAD loadings were investigated. 2. Experiment 2.1. Materials DOPO was purchased from Shanghai Hanfeng Industry Co. Ltd, China. P-phenylenediamine and cinnamaldehyde were acquired from Shanghai Macklin Biochemical Co. Ltd, China. The epoxy resin, diglycidyl ether of bisphenol-A 4

(DGEBA), was obtained from Blue Star New Chemical Material Co. Ltd, China. 4,4'-diaminodiphenylmethane (DDM) was purchased from Sinopharm Chemical Reagent Co. Ltd, China. 2.2. Synthesis of DCAD. Dissolve cinnamaldehyde (13.22 g, 0.10 mol) and p-phenylenediamine (5.41 g, 0.05 mol) in absolute ethanol (500 ml) with a 1000 ml three-necked glass flask, and stir the mixture at room temperature for 4 h, to prepare the intermediate CAD. Then, introduce DOPO (25.94g, 0.12 mol) into flask and stir the mixture under 65 °C with nitrogen (N2) protection for 8 h. After that, filter the precipitate, and wash three times to remove the excess DOPO in ethanol. Finally, DCAD powder was obtained after drying process in 75 °C vacuum oven for 12 h. The yield of DCAD was above 90%. The synthesis route of DCAD was shown in Fig. 1. FTIR spectrum (KBr, cm-1), 3310 (N-H); 3059 (Caromatic-H); 3028 (C=C-H); 1622 (C=C); 1607, 1594, 1582, 1559 and 1514 (aromatic skeleton); 1430 (P-Caromatic); 1207 (P=O); 921 (P-O-Caromatic); 754 (o-R1-Ph-R2). 1H NMR (dimethyl sulfoxide-d6, ppm), 8.18, 8.14, 7.94, 7.74, 7.52, 7.18, 6.59 and 6.44 (Caromatic-H in phosphaphenanthrene); 7.38, 7.27 and 7.25 (Caromatic-H in monosubstituted benzene); 7.29 (Caromatic-H in para-disubstituted benzene); 6.37 and 6.25 (C=C-H); 5.38 and 5.07 (NH); 4.60 and 3.32 (CH).

31

P NMR (dimethyl

sulfoxide-d6, ppm): 29.75 and 31.81 (phosphaphenanthrene linked to chiral carbon).

5

CHO H2N

NH2

+

2

P-phenylenediamine

N

Cinnamaldehyde

N

+

2 O P O H

CAD

DOPO N2

O P O NH

HN

O P O

DCAD

Fig. 1. Synthesis route of DCAD. 2.3. Preparation of flame retardant epoxy thermosets As a reactive flame retardant, DCAD was introduced in DGEBA and the mixture was stirred evenly under 160 °C for 6 h. This process was carried out under N2 to protect the imino group. Then stop heating and cool the mixture to 110 °C. The curing agent DDM was added to the mixture with fully stirring. After that, the mixture was degassed in a vacuum oven for 3 min at 120 °C. Finally, the mixture was cured for 2 h at 120 °C and then for 4 h at 170 °C in polytetrafluoroethylene molds. One control sample, 4%DOPO/EP sample, was also prepared in the same method except adding DOPO instead of DCAD. Another control sample, EP sample, was also obtained with the same preparation process but without adding DCAD or DOPO and without N2 6

protection. All the formulations of each epoxy thermoset were listed in Table 1. Table 1 Formulae of epoxy thermosets. DCAD

DOPO

EP

DDM

(g)

(g)

(g)

(wt.%)

(g)

(wt.%)

EP

100

25.30

--

--

--

--

2%DCAD/EP

100

25.10

2.55

2

--

--

4%DCAD/EP

100

24.41

5.17

4

--

--

4%DOPO/EP

100

24.41

--

--

5.17

4

Samples

2.4. Instruments and characterization Nuclear magnetic resonance (NMR): 1H NMR and 31P NMR spectra were detected by a Bruker Avance III HD 700 MHz NMR spectrometer using dimethyl sulfoxide-d6 as deuterated solvent. Fourier transform-infrared spectroscopy (FTIR): The Fourier transform infrared spectroscopy was collected by a Nicolet iN10MX-type spectrometer over a frequency range from 500 to 4000 cm-1. The samples were ground into powder and mixed with KBr evenly, then pressed the mixture into pellets for testing. Limiting oxygen index (LOI) measurement: LOI of all epoxy thermosets was evaluated using an FTT (Fire Testing Technology, UK) Dynisco LOI instrument according to ASTM D2863. The size of all samples was 130.0 mm×6.5 mm×3.2 mm. Vertical burning test: UL94 combustion level of all samples was measured using an FTT0082 instrument based on ASTM D 3801. The dimension of all samples was 125.0 mm×12.7 mm×3.2 mm. Cone calorimeter test: Fire behavior of all samples was characterized using an FTT 7

cone calorimeter according to ISO 5660 under an external heat flux of 50 kW/m2. The size of the samples was 100 mm×100 mm×4 mm and every data was the average of twice measurements. Thermogravimetry-Fourier transform infrared spectroscopy (TGA-FTIR): The thermal decomposition process of all samples was analyzed on a STA 8000 simultaneous thermal analyzer, which produced by Perkin Elmer. The DCAD is powder-like specimen and the epoxy thermosets are piece-like specimens. With specimen was placed in an alumina crucible, the heat procedure was conducted at a rate of 20 °C/min from 50 °C up to 700 °C under N2 atmosphere. When testing under N2 atmosphere, a Frontier FTIR spectrometer, which also produced by Perkin Elmer, was used to collect the real-time FTIR spectra of evolved pyrolysis gases simultaneously. The weight of the samples was kept within 5-10 mg. The FTIR specimen cell was kept at 280 °C and the transfer line was also kept at 280 °C. Scanning electron microscope (SEM): Not only the residues from epoxy thermosets after vertical burning test but also the impact fractured surface, their micro-morphology was observed via a Phenom ProX scanning electron microscope produced by Phenom World under vacuum conditions with a voltage of 10 kV. Energy dispersive spectrometer (EDS): The elemental analysis of charring residue from vertical burning test was explored by energy dispersive spectrometer, which combined with the Phenom ProX scanning electron microscope mentioned above, produced by Phenom World. Unnotched charpy impact test: The impact strength of epoxy samples was 8

measured on a XJF-5 compound impact test machine in accordance with ISO 179-1 with a 5.5 J pendulum. The size of all samples was 80 mm × 10 mm × 4 mm and every data was the average of five times measurements. Differential scanning calorimetry (DSC): The glass transition temperature (Tg) and thermodynamic behavior below the decomposition temperature of specimen were observed under a N2 atmosphere via a Q20 differential scanning calorimeter produced by TA Instruments. During the measurement, each specimen was first subjected to thermal history elimination, and then a process of cooling and reheating was performed at a rate of 20 °C/min with 5 min isothermal time at each heating/cooling transition point. 3. Results and Discussion 3.1. Structural characterization To confirm the successful synthesis of the target phosphaphenanthrene derivative, the specific chemical structure of DCAD was characterized and identified in sequence, via FTIR, 1H NMR, and

31

P NMR tests. As the synthesis route shown in Fig. 1, the

Schiff-base intermediate DCA was firstly prepared via the classical condensation reaction between cinnamaldehyde and p-phenylenediamine. Then, the target phosphaphenanthrene derivative DCAD was achieved, by driving the addition reaction between the characteristic C=N bond in Schiff-base DCA and the active P-H bond in DOPO. As shown in Fig. 2, the absorption peak at 3310 cm-1 represents the vibration of N-H bond in secondary amine, which belongs to the feature structure for the product of 9

the addition reaction C=N and P-H bonds. Therefore, it can be concluded that, the addition reaction between DCA and DOPO has been proceeded as expected. Moreover, the appearance of the peaks for Caromatic-H (3059 cm-1), C=C-H (3028 cm-1), C=C (1622 cm-1), aromatic skeleton (1607, 1594, 1582, 1559 and 1514 cm-1), P-Caromatic (1430 cm-1), P=O (1207 cm-1), P-O-Caromatic (921 cm-1), and ortho-disubstituted benzene (754 cm-1) further clarified the structural features of DCAD. These absorption peak characteristics indicate that, the target DCAD has been synthesized.

Fig. 2. FTIR spectrum of DCAD. Meanwhile, the 1H NMR spectrum of DCAD in Fig. 3 disclosed that, the peaks at 8.18, 8.14, 7.94, 7.74, 7.52, 7.18, 6.59, and 6.44 ppm corresponded to the Caromatic-H in phosphaphenanthrene; the peaks at 7.38, 7.27, and 7.25 represented the Caromatic-H in monosubstituted benzene; the peak at 7.29 ppm was assigned to the Caromatic-H in para-disubstituted benzene; the peaks at 6.37 and 6.25 ppm belonged to the C=C-H structure; the peaks at 5.38 and 5.07 were from secondary N-H group; the peaks at 4.60 10

and 3.32 were for tertiary C-H. Roughly, according to the integral results of peak area, the H atom molar ratio of aromatic and unsaturated moieties/ tert-C-H/ secondary N-H in synthesized DCAD was basically consistent with its calculated value. This result also verified target DCAD has been prepared successfully. Besides, as for the

31

P NMR in

Fig. 4, the peaks at 29.75 and 31.81 ppm both belonged to the phosphorus-containing structure in phosphaphenanthrene group. The two closed peaks should be caused by the different chiral chemical environment of P atoms.

Fig. 3. 1H NMR spectrum of DCAD.

11

Fig. 4. 31P NMR spectrum of DCAD. 3.2. LOI measurement and UL94 vertical burning test Flammability of thermosets was first evaluated by LOI and UL94 tests and the results were listed in Table 2. The combustion performance of pure EP sample was exhibited by a 26.4% of LOI value and unrated of UL94 test. After adding DCAD, the flame retardancy of epoxy thermosets was significantly improved, regardless of LOI value or UL94 rating. 2 wt.% DCAD imposed the LOI value of 32.0% and UL94 V-1 rating to epoxy resin, and 4 wt.% DCAD further enhanced the LOI value to 35.6% and increased UL94 rating to V-0, which implied that the flame retardancy of epoxy thermosets was improved with increasing the addition amount of DCAD. As a typical commercial control flame retardant, 4 wt.% DOPO also endowed epoxy thermosets with the LOI value of 36.8% and UL94 V-1 rating due to the determined flame retardancy of DOPO [23,39]. Compared with 4%DOPO/EP sample, 4%DCAD/EP sample slightly decreased the LOI but increased the UL94 rating to V-0. In contrast to DOPO, DCAD can more stably extinguished fire and kept the after-flame time (t1+t2) of 12

all the specimens below 10s. The bio-based nitrogen-containing phosphaphenanthrene derivative DCAD can impart excellent self-extinguishing performance to epoxy resin, thereby improving the flame retardant property of epoxy thermoset. In subsequent cone calorimeter test, the more positive results from DCAD in flame retardancy would be further exhibited. Table 2 LOI value and UL94 rating of epoxy thermosets. Vertical burning test LOI Samples (%)

After-flame time

UL94 Dripping

—

—

t 1 (s)

t 2 (s)

rating

EP

26.4

83.0a

--

Unrated

No

2%DCAD/EP

32.0

12.0

8.1

V-1

No

4%DCAD/EP

35.6

2.8

3.4

V-0

No

4%DOPO/EP

36.8

2.9

5.6

V-1

No

a

The after-flame of specimen burned to the clamp in the first flame application.

3.2. Cone calorimeter test Cone calorimeter test was applied to analyze the combustion behavior of the epoxy thermosets. The typical results, such as the peak value of time to ignition (TTI), heat release rate (pk-HRR), average effective heat of combustion (av-EHC), total heat release (THR), average carbon monoxide yield (av-COY), the average carbon dioxide yield (av-CO2Y), total smoke production (TSP) and residue production, were all listed in Table 3, while the curves of the heat release rate (HRR) were shown in Fig. 5. Table 3 Cone calorimeter data of epoxy thermosets. 13

av-CO2 Samples

TTI

pk-HRR

av-EHC

THR

TSP

Residue

av-COY

(kW/m2)

(MJ/kg)

(MJ/m2)

(m2)

(wt.%)

(kg/kg)

Y (kg/kg)

EP

50

1370

23.3

115.8

46.5

5.6

0.090

1.98

2%DCAD/EP

54

952

22.4

104.7

45.1

9.9

0.122

1.83

4%DCAD/EP

51

790

20.5

93.3

45.3

12.4

0.116

1.75

4%DOPO/EP

50

1126

21.7

99.0

42.3

12.0

0.125

1.74

Error range(±)

2

30

0.8

3.2

2.3

0.5

0.008

0.04

The heat release behaviors for each sample during combustion were evaluated by the HRR curves. As shown in Fig. 5 and Table 3, the burning intensity of 2%DCAD/EP and 4%DCAD/EP all showed a significant reduction compared with EP sample, and the burning intensity of 4%DCAD/EP was suppressed to an obviously lower level than that of 4%DOPO. It disclosed that DCAD possessed the better inhibition effect on combustion than DOPO although DOPO has higher phosphorus-containing contents. Furthermore, the pk-HRR of flame retardant samples appeared earlier than that of pure EP. It revealed that DCAD and DOPO played an effective flame retardant effect in the early combustion and inhibited the combustion intensity. Meanwhile, the flame of 4%DCAD/EP, 2%DCAD/EP, 4%DOPO/EP and pure EP extinguished in sequence according to the HRR curves, indicating that DCAD not only inhibited fully combustion at the early stage of combustion but also promoted early termination of combustion at the end of combustion. The ignition time of 2%DCAD/EP and 4%DCAD/EP was earlier than EP. The results showed that DCAD promoted the thermal decomposition of EP, which contributed to forming char layer in advance. 14

Whether DCAD or DOPO, the flame retardants inhibited the burning intensity and promoted the charring behavior of epoxy resin, which were mainly reflected in the lower pk-HRR, av-EHC, THR and higher residues for 2%DCAD/EP, 4%DCAD/EP and 4%DOPO/EP, compared with pure EP sample. With increasing the adding loadings of DCAD from 2wt.% to 4wt.%, the pk-HRR, av-EHC and THR values of 4%DCAD/EP were further declined, indicating that DCAD is an excellent flame retardant for epoxy resin. Further, with same adding loadings of DCAD and DOPO to EP, pk-HRR value, TSR value and av-EHC value of 4%DCAD/EP all lower than that of 4%DOPO/EP, implying DCAD exhibited better flame retardant performance than DOPO did and the flame retardant efficiency of DCAD is much higher than that of DOPO. The av-COY and av-CO2Y values disclosed the flame retardant effect of DCAD from another direction. As shown in Table 5, compared with EP, the av-COY values of DCAD/EP specimen increased, while the av-CO2Y values decreased, indicating that there is an obvious incomplete combustion in burning process. In contrast to DOPO, DCAD caused the lower av-EHC value, which meant better gas-phase flame retardant effect in epoxy resin. Additionally, the TSP and residue yields of 4%DCAD/EP were higher than those of 4%DOPO/EP. TSP was caused by the solid particles in gas phase, and the higher TSP value implied that DCAD promoted the formation of larger fragments and reduced the inflammable gas compared with DOPO. The higher residue yields from 4%DCAD/EP directly testified the higher charring-promotion ability of DCAD than DOPO. Therefore, it can be confirmed that DCAD compound not only possessed excellent gas-phase flame retardant effect but also 15

possessed better condensed-phase flame retardant effect. DCAD presented better flame retardant effects both in gas phase and condensed phase than DOPO when they were used with the same amounts in epoxy resins.

Fig. 5. HRR curves of epoxy thermosets. 3.3. Real-time FTIR spectra of evolved pyrolysis gases Through the analysis of cone calorimeter test, DCAD partly carried out its flame retardancy in gas phase. Therefore, the real-time volatiles of flame retardant epoxy thermosets were traced by TGA-FTIR test to analyze the effect of DCAD on the volatiles released by epoxy resin. The real-time FTIR spectra of gas-phase decomposition products from typical samples at different weight loss points were shown in Fig. 6. In comparison, the spectra of all specimens were dominated by spectra of the gas-phase products from EP sample, because of the almost same characteristic peaks for free N-H (3736 cm−1), free OH (3652 cm−1), aromatic C-H (3034 cm−1), methylic C-H (2974 cm−1), lactones (1754 cm−1), benzene skeleton (1610, 1512 cm−1), C-N (1338 16

cm−1), phenolic C-O (1259 cm−1), aliphatic C-O (1178 cm−1), para-disubstituted benzene C-H (828 cm−1), and mono-substituted benzene C-H (747 cm−1) at different weight loss points. Although the spectra of gas-phase products from each specimen were qualitatively same, some differences also can be found from a quantitative view. On one hand, the green region (peak at 2974 cm−1) and the red region (peak at 1778 cm−1) represented the aliphatic C-H bond and the lactone structure, respectively, which were produced by thermal decomposition of epoxy resin thermoset and all were inflammable fuels. When the weight loss process proceeded to 20%-30%, the peak height sequence of these two regions were EP>4%DCAD/EP, indicating that the addition of DCAD inhibited the thermal decomposition process of epoxy resin, and the reduced fragments were released from thermoset matrix; on the other hand, the peak height sequence in blue region (peak at 746 cm−1) was 4%DCAD/EP>EP at the same weight loss points, which suggested a particular contribution from phosphaphenanthrene structure. The peak at near 750 cm−1 is caused by the decomposed o-substituted benzene ring from o-phenylphenol structure of DOPO [39,40] and determined the gas-phase flame retardant effect of DCAD on EP thermoset because of the quenching effect of phosphaphenanthrene fragments. In addition, by comparing with spectra of 4%DCAD/EP and 4%DOPO/EP, no significant difference was found between them. It showed that the flame retardant effect of DCAD in gas phase was similar to that of DOPO, which mainly from the quenching effect of decomposed phosphaphenanthrene fragments.

17

Fig. 6. FTIR spectra of gas-phase products of epoxy thermosets from TGA under N2 at different weight loss points. 3.4. Thermal decomposition process of epoxy thermosets Beside the real-time FTIR of volatiles, TGA data was another result obtained by 18

TGA-FTIR test were shown in Table 4. The thermal mass loss processes of all epoxy thermosets were analyzed and the TGA curves were shown in Fig. 7. In contrast to EP, the onset decomposition temperature (1% mass loss) of 4%DCAD/EP decreased significantly, indicating that DCAD led to the earlier decomposition of epoxy resin. At the end of thermal decomposition, the residual weight of 4%DCAD/EP was enhanced compared with EP. The results implied DCAD possessed better charring ability, thus enhancing condensed-phase flame retardant effect to some extent. Furthermore, the residual weight of 4%DACD/EP was also higher than that of 4%DOPO/EP, indicating that DCAD was more efficient than DOPO in promoting charring behavior of epoxy resin, which was consistent with the charring result in cone calorimeter test.

Fig. 7. TGA curves of epoxy thermosets in N2 atmosphere. Table 4. The typical TGA data of epoxy thermosets Samples

Td,1% (oC) 19

Char yields at 700 oC (wt.%)

EP

363

14.8

DCAD

279

12.8

4%DCAD/EP

349

20.0

4%DOPO/EP

340

18.0

3.5. Microscopic morphologies of residues In order to explore the flame retardant mechanism of DCAD, the observation on the residue of epoxy thermosets after vertical burning test was performed by SEM, and the results were shown in Fig. 8. The char layer of EP was very fragile, loose and had no continuous morphological structure, resulting in that the heat, oxygen and combustible fragments exchanged unhinderedly and further aggravated the burning intensity of epoxy resin. The residue of 4%DCAD/EP was continuous and liquid corrugated, and many bubble structures was on the residue surface of 4%DCAD/EP, indicating that the sample produced viscous contents in residue during combustion. These viscous contents packed some decomposed gas to form foam-like char layer with stronger barrier effect, which weakened the thermal decomposition process of epoxy thermosets from fire and also blocked exchange of heat and inflammable gas, inhibiting the burning intensity significantly. In the sample of adding DOPO, the residue morphology of 4%DOPO/EP was relatively complete and dense, but there were some cracks and holes on the surface of char layer, indicating that the char layer from 4%DOPO/EP did not possess such strong barrier effect as that of 4%DCAD/EP. This also disclosed the charring and barrier effect from DCAD was the important factor in enhancing flame retardant effect.

20

Fig. 8. SEM photos of residues from vertical burning test (×500). 3.6. FTIR spectra and element analysis of residues To further research the enhanced flame retardant effect of DCAD, the FTIR spectra of residues after vertical burning test were observed and compared, and the results were shown in Fig. 9. The FTIR spectra of residues of 4%DCAD/EP, 4%DOPO/EP and EP were basically the same, except for some differences appearing at 1511 cm-1, 1243 cm-1, 1176cm-1 and 830cm-1. They were representative C=CAr (1511 cm-1), P=O (1243 cm-1), P-O-Ar (1176 cm-1) and Ar-H (830 cm-1) structures, respectively. Compared with EP, 4%DCAD/EP showed obvious peaks at the above absorption bands and 4%DCAD/EP exhibited the stronger intensities in these absorption bands than 4%DOPO/EP, indicating that 4%DCAD/EP reserved more aromatic and phosphorus-containing contents in residue. The more aromatic and phosphorus-containing contents in residue facilitated epoxy thermosets to generating the compact residue with the better barrier effect.

21

Fig. 9. FTIR spectra of residues after vertical burning test. In order to clarify the charring performance of DCAD, the elemental compositions of the residues from vertical burning test were analyzed via energy dispersive spectrometer, and the results were shown in Table 5. The phosphorus content of 4%DCAD/EP residue was 0.4% and the nitrogen content of 4%DCAD/EP was 9.48%, which were all higher than those of 4%DOPO/EP residue and also much higher than neat EP residue, indicating that DCAD promoted to reserve more nitrogen- and phosphorus-containing structures in condensed phase, which was beneficial to formation of a stable and dense barrier char layer [43,44]. In contrast to 4%DOPO/EP, more carbon contents were observed in residue of 4%DCAD/EP, implying that DCAD exerted the stronger locking effect on carbonaceous contents during combustion. Table 5 Elemental composition of residues from vertical burning test. Samples EP

C

N

O

P

79.43

6.39

14.17

--

22

4%DCAD/EP

75.31

9.48

14.79

0.40

4%DOPO/EP

72.03

9.08

18.52

0.36

3.7. Unnotched charpy impact test Except for the flame retardant properties, physical and mechanical properties also affected the application performance of epoxy resin. The impact strength was tested for evaluating the mechanical property of DCAD flame retardant sample, and the results were listed in Table 6. Compared with EP, 4%DCAD/EP had the obviously increased impact strength 25.74 kJ/m2, which could be caused by three reasons: 1) the two C=C bonds crosslinked to form ruble microparticles in thermosets; 2) the N-H group of the reactive-type flame retardant DCAD reacted with the epoxy groups of epoxy resin and linked itself in epoxy matrix; 3) the polarity of phosphaphenanthrene groups strengthened the intermolecular interaction with matrix. The microscopic morphology of impact fracture surface was observed by SEM to obtain more acting traces and the results were shown in Fig. 10. Unlike the flat laminar fracture characteristic of the impact fracture surface in EP, the impact fracture surface of the 4%DCAD/EP exhibited massive filamentous structure and more intricate fracture paths, which can absorb more impact energy, making the fracture process of 4%DCAD/EP more difficultly proceed. The control sample, 4%DOPO/EP showed an impact strength value of 15.56 kJ/m2. This comparison disclosed that DCAD can better improve mechanical properties in epoxy composites than DOPO did. Table 6 Impact strength of epoxy thermosets. Impact strength (kJ/m2)

Samples 23

EP

10.60±3.62

4%DCAD/EP

25.74±3.89

4%DOPO/EP

15.56±2.89

Fig. 10. Microtopography for the impact fractured surface of epoxy thermosets (SEM500×). 3.8. Glass transition temperature Besides the impact strength, Tg, which directly affected the application temperature range of materials, was also an important parameter of physical and mechanical property of epoxy resin. The Tgs of the specimens were determined by DSC test, and the results were shown in Fig. 11. Compared with 166.12 °C Tg value of EP, that of 4%DCAD/EP had almost no change, was 165.48 °C, whereas that of 4%DOPO/EP was decreased to 152.18 °C. It can be explained by that the reactive-type flame retardant DCAD reacted with epoxy group with N-H groups acting as curing agent, and sustained the crosslinking density of epoxy thermoset as similar as that of neat EP. Therefore, the Tg value of 4%DCAD/EP were not decreased and the DCAD flame retardant epoxy resin can be used in a relatively wide temperature range.

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Fig. 11. DSC curves of epoxy thermosets. 4. Conclusion A DOPO derivative, DCAD, was synthesized by DOPO, p-phenylenediamine and bio-based material cinnamaldehyde. When it was applied in epoxy thermosets, 4%DCAD/EP exhibited a LOI of 35.6%, passed the UL94 V-0 rating, obviously decreased the pk-HRR value. In contrast to DOPO, DCAD facilitated the epoxy thermosets to generate compact, stable and foam-like char layer, which hindered the heat exchange, weakened the thermal decomposition of matrix accordingly reduced the release of inflammable gas. DCAD stably inhibited the burning intensity and heat release at a lower level during combustion. The results disclosed that DCAD exhibited higher free radical quenching effect in gas phase and better charring effect in condensed phase. Simultaneously, the impact strength of 4%DCAD/EP increased from 10.60 kJ/m2 to 25.74 kJ/m2 and did not reduce Tg compared with pure EP. The improved flame retardant effect and physical-mechanical properties all benefited from the reactive-type 25

characteristic of DCAD, which bonded DCAD to the crosslinked network of epoxy resin through its imine structure without decreasing the crosslinking density of thermosets. Acknowledgement This work was supported by National Natural Science Foundation of China (No. 51973006), the Project of Great Wall Scholar from Beijing Municipal Commission of Education (CIT&TCD20180312) and Beijing Talents Project (No. 2018A39). References [1] P. Garra, B. Graff, G. Schrodj, F. Morlet-Savary, C. Dietlin, J.P. Fouassier, J. Lalevée. Ultrafast

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Highlights · An epoxy resin had excellent flame retardancy and physical-mechanical properties. · The excellent properties were caused by bio-based DOPO derivative DCAD. · Two C=C bonds in DCAD formed elastomer-like crosslink and led to high toughness. · Epoxy resin with only 4 wt.% DCAD can effectively inhibit the burning intensity. · DCAD in epoxy resin caused the high-quality charring effect in condensed phase.

S.L. J., Z. L. and L.J. Q. jointly conceived research, designed research plans. S.L. J. and Z. L. conducted experiments and data analysis and prepared manuscript. All authors discussed the results and implications and commented on the manuscript at all stages. L.J. Q. revised manuscripts and agreed to contribute. In addition, S.L. J. and Z. L. contributed equally to this work.

Declaration of Interest Statement We declare that we do not have no conflict of interest.