The development of efficient green flame retardants is an important way to realize more sustainable epoxy thermosets and downstream materials. In this work, a monoepoxide is synthesized through O-glycidylation of eugenol, and then reacted with DOPO (9,10-dihydro-9-oxa-10-phosphophenanthrene-10-oxide) to obtain a new bio-based flame retardant, DOPO-GE. DOPO-GE is blended with a bisphenol A epoxy prepolymer exhibiting good compatibility and DDS (4,4′-diaminodiphenylsulfone) is used as the curing agent to afford epoxy thermosets. Although DOPO-GE leads to the reduced glass transition temperature of the thermosets, the storage modulus increases considerably. The DOPO-GE-modified thermosets exhibit the high thermal stability with the onset thermal decomposition temperature in nitrogen and air exceeding 300°C. When the phosphorus content in the thermoset is 1.0%, the residual yield of the thermosets at 750°C in nitrogen increases from 13.9% to 30.6%, due to the increased charring ability. More interestingly, when the phosphorus content is only 0.5%, the limiting oxygen index is as high as 30.3% with UL94 V0 achieved. Cone calorimeter results reveals the significantly decreased heat release rate, total heat release, mass loss and total smoke production. Furthermore, DOPO-GE can notably improve the flexural strength, flexural modulus and fracture toughness, whereas the shear and impact strength are reduced to varied extents. In short, DOPO-GE can be obtained via a facile way, and shows the good flame-retardant effect on the epoxy thermosets with an application potential.
Epoxy resins are widely used in many fields mainly because of their excellent mechanical properties, insulation properties and good processability. Bisphenol A-based epoxy resin (DGEBA) is the most commonly used epoxy prepolymer, accounting for more than 80% market share of the whole epoxy resin market. However, DGEBA is flammable and easy to burn in air. Therefore, improving the flame retardancy of DGEBA is a very important issue for a number of applications, and thus becomes a hot research topic in academia and industry. Many methods have been used to improve the flame retardancy of DGEBA. For example, brominated epoxy resins have high intrinsic flame retardancy, but they produce highly toxic gases in combustion. Therefore, halogen-free flame retardants for epoxy is of particular importance. For example, aluminum hydroxide, magnesium hydroxide and other inorganic fillers are added to the epoxy systems to improve flame retardancy, but such inorganic flame retardants usually need a high loading, resulting in problems regarding processing and compatibility. Therefore, developing organic flame retardants with better compatibility and higher efficiency provides an alternative solution to the above problems. In particular, organic flame retardants based on DOPO (9,10-dihydro-9-oxa-10-phosphophenanthrene-10-oxide) have the advantages of high flame retardancy, low volatilization, low moisture absorption, good transparency, good compatibility, and reasonable costs, so that DOPO functions as the very useful building block to develop many new and efficient flame retardants for polymeric materials including epoxy thermosets [
In order to reduce the impact on environment and relieve resource shortage, the development of renewable epoxy resin and related flame retardants has attracted more and more attention. In recent years, many studies on the synthesis of epoxy thermosets and related flame retardants by replacing bisphenol A with bio-based phenols have been reported extensively, such as cardanol [
In this paper, eugenol is undergone O-glycidylation to introduce a highly reactive terminal epoxy group, and then reacted with phosphorus hydrogen bond of DOPO to obtain a DOPO-modified eugenol-based flame retardant (DOPO-GE). DOPO-GE is used to modify commercially available DGEBA, and 4,4′-diaminodiphenylsulfone (DDS) is used as the curing agent to obtain the thermosets. We systematically study the dynamic mechanical properties, thermal decomposition behavior, flame retardancy, mechanical properties and fracture behavior of the thermosets. It is found that DOPO-GE is very effective to improve the flame retardancy and can increase the rigidity of the obtained epoxy materials simultaneously.
Eugenol was purchased from a commercial source, and decolorized by vacuum distillation before use. Fresh-distilled eugenol is a colorless liquid with a purity of +99% (GC). Epichlorohydrin, sodium hydroxide, benzyltriethylammonium chloride and methanol were purchased from Sinopharm. 4,4′-Diaminediaminodiphenyl sulfone (99%) was obtained from Energy Chemical. Commodity bisphenol A epoxy resin (E54TM) with epoxy value of 0.556 mol/100 g was used in this study. Unless there were other special specifications, all the chemicals and materials were used as received.
As shown in
DOPO (62.30 g, 0.28 mol) and glycidyl ether of eugenol (62.40 g, 0.28 mol) were charged into a 500 ml flask and heated in an oil bath (170°C) with stirring to obtain a clear solution. Then triphenylphosphine (0.31 g) was added and reacted for 28 h to obtain a viscous liquid without further separation as the bio-based flame retardant (DOPO-GE) in a quantitative yield.
To a preheated (135°C) mold sprayed with releasing agent, a quantitative mixture sample of E54, DDS, and DOPO-GE were filled and degassed under reduced pressure. The following curing procedures were carried out: 180°C for 2 h, 200°C for 4 h, and 220°C for 4 h, and then cooled to room temperature. Disassemble the mold and machine the cured thermosets into desired dimensions for further testing. The typical formulations of the epoxy thermosets are listed in
E54/g | DDS/g | DOPO-GE/g | P/wt% | |
---|---|---|---|---|
E54/DDS | 35.0 | 12.0 | 0 | 0 |
E54/DDS/0.5%P | 35.0 | 12.0 | 3.49 | 0.5 |
E54/DDS/1.0%P | 35.0 | 11.9 | 7.71 | 1.0 |
E54/DDS/1.5%P | 35.0 | 12.0 | 12.6 | 1.5 |
E54/DDS/2.0%P | 35.0 | 12.0 | 18.4 | 2.0 |
Infrared spectroscopy (FT-IR). A spectrophotometer (PerkinElmer Spectrum Two UATR) was used to register IR spectra. Potassium bromide was used as the supporter and wavenumber range was 4000–500 cm−1.
Nuclear magnetic resonance (NMR). A nuclear magnetic resonance spectrometer (400 MHz, JEOL) was used to acquire the 1H NMR spectra of the samples with deuterated chloroform (CDCl3) as the solvent and tetramethylsilane as the internal standard, and the number of scans was 8 times.
Thermogravimetric analysis. A thermal analysis system instrument (Q600, TA Instruments) was used to analyze the thermal decomposition of the cured epoxy samples. An appropriate amount (4–5 mg) of the cured epoxy thermoset was heated from room temperature at a heating rate of 20 K/min to 800°C in an air and a nitrogen atmospheres, respectively.
Dynamic mechanical thermal analysis (DMA). A DMA analyzer (Q800, TA instruments) was used to analyze the viscoelasticity of the cured epoxy thermosets. The sample (60 mm × 10 mm × 2.6 mm) was fixed on the dual-cantilever beam, the amplitude was 25 μm, the frequency was 1 Hz, and the heating rate was 3 K/min from room temperature to 300°C.
Shear strength. A universal testing machine (RGM-2010, Regal Instrument, Ltd., China) was used to measure the shear strength according to ISO 527-2012 with the tensile speed of 10 mm/min. The epoxy adhesive was coated on a steel plate with bonding surface of 12.5 mm × 25 mm, and cured at 180°C for 6 h.
Flexural properties. Testing of flexural properties of the cured epoxy was conducted on the universal testing machine. According to the national standard GB/T 9341-2008, the polished specimens were measured at room temperature with the crosshead speed of 5 mm/min.
Impact strength. An impact tester (XJJD-5, Chengde Jinjian Testing Instrument, China) was used to determine impact strength of V-notched specimens according to GB/T 2571-1995. The impact speed was 2.9 m/s and the impact energy (2 J).
Fracture toughness and fracture energy. The universal testing machine was used to measure the fracture toughness and fracture energy according to ASTM D5045. The processed samples (length 60 ± 0.02 mm, width 10 ± 0.02 mm, thickness 5 ± 0.02 mm, and notch 4 ± 0.02 mm) were tested at the crosshead speed of 10 mm/min in bending mode.
Limiting oxygen index (LOI). An oxygen index tester (JF-5, Fujian Survey Instrument and Equipment, China) was used to determine the LOI values in terms of ISO 4589-1996. UL 94 vertical burning test was carried out on the cured epoxy samples. A cone calorimeter (Fire Testing Technology) test was used to study flame retardancy the samples (100 mm × 100 mm × 4 mm) at a fixed heat flux of 35 kW/m2 according to ISO 5660.
Scanning electron microscope (SEM). A SEM equipment (SU3500, Hitachi, Japan) was used to check the surface morphology of the residue of the samples after burning in the air. Gold was sprayed on the surface of the samples and the accelerating voltage applied was 5 KV.
As shown in
Formulation | E54/DDS | E54/DDS/1.0%P | E54/DDS/2.0%P |
---|---|---|---|
238.6 | 170.9 | 143.8 | |
Tan δmax | 0.797 | 0.876 | 1.026 |
187.7 | 226.0 | 293.3 | |
2339 | 2719 | 3421 | |
68.93 | 47.77 | 56.55 | |
1571 | 2179 | 2199 | |
45.95 | 85.83 | 172.3 | |
0.322 | 0.615 | 0.599 |
Note:
The effect of DOPO-GE on the thermal decomposition behavior of the cured epoxy resin in nitrogen and air is studied, see
Entry | Residual/% 750°C/N2 (air) | LOI/% | VL 94 | |||
---|---|---|---|---|---|---|
E54/DDS | 403 (399) | 397 (390) | 423 (411) | 13.9 (5.1) | 22.6 | Fail |
E54/DDS/0.5%P | 383 (379) | 366 (367) | 404 (391) | 21.8 (0) | 30.3 | V0 |
E54/DDS/1.0%P | 376 (374) | 358 (332) | 386 (386) | 30.6 (0) | 32.5 | V0 |
E54/DDS/1.5%P | 364 (369) | 346 (318) | 382 (383) | 20.0 (0) | 28.5 | V0 |
E54/DDS/2.0%P | 363 (367) | 324 (306) | 389 (382) | 9.5 (0) | 29.3 | V0 |
The flame retardancy of the epoxy materials is characterized by limiting oxygen index (LOI) and from UL94 vertical burning tests. See
By using a cone calorimeter to monitor combustion of the epoxy, the characteristic parameters of the thermosets containing the flame retardants with the 0.5%P content are studied in a more quantitative and comparable way.
E54/DDS | E54/DDS/0.5%P | |
---|---|---|
Time to ignition (s) | 157.5 ± 0.5 | 132.5 ± 5.5 |
Peak HRR (kW m−2) | 831 ± 50 | 659 ± 45 |
Total heat release (MJ m−2) | 95.6 ± 1.45 | 67.8 ± 1.8 |
Mass loss (%) | 86.75 ± 0.01 | 65.99 ± 0.01 |
Total smoke release (m2 m−2) | 4113 ± 95 | 3197 ± 190 |
Total smoke production (m2/Kg) | 36.4 ± 0.83 | 28.3 ± 1.78 |
In
The mechanical properties of the thermosets are evaluated, and the effects of DOPO-GE on shear strength, impact strength, flexural modulus and fracture toughness and energy are investigated. The results in
Entry | Flexural strength/MPa | Flexural modulus/GPa | Shear strength/MPa | Impact strength/kJ/m2 | ||
---|---|---|---|---|---|---|
E54/DDS | 183 ± 2 | 2.34 ± 0.03 | 11.9 ± 0.1 | 2.52 ± 0.12 | 2.4 ± 0.1 | 4.2 ± 0.9 |
E54/DDS/0.5%P | 234 ± 1 | 2.97 ± 0.09 | 10.9 ± 0.1 | 1.89 ± 0.28 | 3.2 ± 0.1 | 5.0 ± 0.4 |
E54/DDS/1.0%P | 241 ± 2 | 3.69 ± 0.10 | 9.38 ± 0.09 | 1.56 ± 0.02 | 3.8 ± 0.1 | 5.7 ± 0.2 |
E54/DDS/1.5%P | 251 ± 3 | 3.74 ± 0.13 | 8.53 ± 0.06 | 1.38 ± 0.19 | 4.7 ± 0.1 | 5.2 ± 0.1 |
E54/DDS/2.0%P | 281 ± 2 | 3.98 ± 0.13 | 7.73 ± 0.09 | 1.06 ± 0.02 | 4.9 ± 0.2 | 4.9 ± 0.2 |
By reacting glycidyl ether of eugenol with DOPO, a new bio-based halogen-free phosphorus flame retardant (DOPO-GE) was readily synthesized. Its molecular structure was characterized by FTIR and 1H NMR. DOPO-GE was used to modify a bisphenol A epoxy prepolymer (E54) with DDS as the curing agent. DOPO-GE significantly reduced the glass transition temperature of the resultant thermosets, but increased the storage modulus. The cured epoxy material had high thermal stability. The onset thermal decomposition temperature in nitrogen and air exceeded 300°C. When the phosphorus content was 1.0 wt%, the residual content at 750°C in nitrogen increased from 13.9% for the unmodified epoxy to 30.6%. Therefore, the charring ability of the epoxy was greatly enhanced. When the phosphorus content was as low as 0.5 wt%, the LOI value could reach 30.3%, and the thermosets could pass the UL94 test with V0 level arrived. When the content of phosphorous content was increased to 1.0 wt%, LOI could further increase to 32.5%. Cone calorimeter results showed that the 0.5 wt% phosphorus incorporation in the epoxy led to the significantly decreased HRR (heat release rate), THR (total heat release) and TSP (Total smoke production), by 15.9%, 25.5% and 22.2%, respectively. The results of mechanical properties showed that the addition of DOPO-GE could significantly improve the flexural strength, modulus, fracture toughness, but could decrease the impact and shear strengths. In short, DOPO-GE is an effective halogen-free flame retardant derivable from renewable eugenol. When its loading is low (0.5–1.0 wt%, phosphorus based), DOPO-GE can effectively improve flame retardancy of the epoxy thermosets, and can enhance the rigidity of the epoxy network at the same time.