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Preparation, characterization, and properties of novel ultraviolet-curable and flame-retardant polyurethane acrylate
Progress in Organic Coatings 129 (2019) 309–317
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Preparation, characterization, and properties of novel ultraviolet-curable and flame-retardant polyurethane acrylate
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Yongxia Rena, Yunsheng Donga, Yali Zhua, Jing Xub, , Youwei Yaoa, ⁎⁎
a b
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Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China College of Chemistry and Material Science, Shandong Agriculture University, Tai’an 271018, PR China
ARTICLE INFO
ABSTRACT
Keywords: Ultraviolet curing Polyurethane acrylate Surface drying time Flame retardancy 9,10-dihydro-9-oxa-10-phosphaphenanthrene10-oxide Hexa-phenoxy-cyclotriphosphazene
A series of flame-retardant polyurethane acrylate oligomer (FRPUA)–nhexa- phenoxy-cyclotriphosphazene (HPCP) (n = 0, 10, 15, and 20, where n is the amount of HPCP added to a 100 g mixture of FRPUA and other additives) ultraviolet (UV)-cured samples have been prepared. FRPUA was synthesized by the reaction of poly (glycidyl methacrylate) (an unsaturated polyether), a derivative of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) (2DOPO–4,4′- dihydroxy-benzophenone (DHBP)), and isophorone diisocyanate. As a control sample, the polyurethane acrylate (PUA) oligomer and cured PUA sample were also prepared. Surface drying of the PUA and FRPUA–nHPCP curable mixtures required a few seconds under UV irradiation (400 W lamp). The effects of 2DOPO–DHBP and HPCP on the flame retardancy and thermal stability of the PUA system were also investigated. For the FRPUA–nHPCP series, as n increases from 0 to 20, the limiting oxygen index (LOI) increases from 21.1% to 23.3% and the total heat release (THR) slowly decreases from 32.4 to 26.4 kJ/g, while the PUA cured sample possesses the LOI and THR values of 19.0% and 29.2 kJ/g, respectively. Besides, the PUA system can achieve better flame retardant properties when using both 2DOPO-DHBP and HPCP. Thermogravimetric analysis indicates that the amount of char residue of the FRPUA–nHPCP (n = 0, 10, 15, and 20) cured samples ranges from 15.1 to 21.9 wt.%, which are much are higher than 10.5 wt.% for the PUA cured sample. Improving the flame retardancy of the PUA system will expand its applications, such as for paper manufacturing and architectural coatings, while simultaneously retaining the feature of fast surface drying.
1. Introduction In recent decades, environmental regulations have proposed more stringent requirements for emission of volatile organic compounds (VOCs) in chemical industries, which has prompted interest in environmentally friendly products. As a new green technique, the ultraviolet (UV)-curing processing has wide applications in coatings, adhesives, printing inks, dental composites, and so forth. The advantages of UV-cured products are widely recognized. They are usually summarized as follows: low VOC emissions, low energy consumption, fast curing at ambient temperature, and short cycle time. All of the above demonstrate the necessity for research of UV-cured coatings [1–4]. UV-curable coatings consist of an oligomer, a reactive diluent, and a photoinitiator. The properties and polymerization kinetics of the coatings mainly depend on the oligomer structure and its concentration in the formulation. Polyurethane acrylate (PUA) [5–9], one of the most commonly used oligomers, is used in coating for automobiles, leather,
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printing, inks, and so forth, because PUA combines the excellent mechanical properties of polyurethane (PU) [10] and the weatherability, water resistance, and solvent resistance of polyacrylates [11,12]. However, compared with epoxy resins, PUA is not only more expensive, but it also possesses slower curing speed and lower photopolymerization activity. Some studies have attempted to achieve faster curing by changing the composition and curing conditions. For example, Qin et al. [13] synthesized PUAs modified by saturated alcohols with different chain lengths. They found that for 40-μm-thick PUA films, when C]C conversion reaches around 85%, the curing time is at least 1 min. Xu designed double-bond controllable UV-cured hyperbranched urethane acrylic (HBPUA) resins. When HBPUA was dissolved in a mixed solvent of ethanol and butyl acetate and mixed with 1 wt.% content of Irgacure 1173, the best curing time was 15 s under 1 kW UV irradiation [14,15]. There have been few reports about synthesizing a novel PUA oligomer for faster curing.
Corresponding authors at: Division of Energy and Environment, Tsinghua Campus, The University Town, Room 1510, Shenzhen, 518005, PR China. Corresponding authors at: Dai Zong Road No.61, Taian, Shandong, PR China. E-mail addresses: [email protected] (J. Xu), [email protected] (Y. Yao).
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https://doi.org/10.1016/j.porgcoat.2018.11.009 Received 29 August 2018; Received in revised form 15 October 2018; Accepted 6 November 2018 0300-9440/ © 2018 Published by Elsevier B.V.
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A significant drawback of many synthetic polymers is their inherent flammability [16,17]. Generally, PUA is flammable, which restricts its applications, such as for leather processing, textile and paper manufacturing, and architectural coatings [18]. There are usually two ways for flame retardant modification of polymers: blending and using a reactive-type flame retardant. Some researchers have used additiontype flame retardants, such as huntite and hydromagnesite, microcapsulated red phosphorus [19], aluminum hydroxide [20], expandable graphite [21,22], and ammonium polyphosphate [23,24], to modify the flammability of various polymers because of the easy operation of the blending method. Other researchers have used the reactive-type flameretardant method, which improves the compatibility between the flame retardant and the matrix, to achieve the same aim. 9,10-Dihydro-9-oxa10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives are commonly used as flame retardants because of their excellent flame retardancy in both the gas phase and condensed phase [25–32]. Liu [33] prepared a new diepoxide (2DOPO–PhOH) and a new diamine (2DOPO–A) and used both as monomers to prepare cured epoxy resins with high phosphorus contents. All of the as-prepared epoxy resins exhibited high thermal stability, with 5% weight loss temperatures (T5wt.%) above 316 °C, and excellent flame retardancy, with limiting oxygen index (LOI) values of 37–50%. As another highly efficient flame retardant, hexa-phenoxy-cyclotriphosphazene (HPCP) is usually added to epoxy resin owing to its good gas-phase flame-retardant effects [34–37]. Qian et al. [38] prepared flame-retardant rigid PU foams with HPCP/expanded graphene (EG) by box foaming. The LOI index, total heat release (THR), and temperature of the peak of the heat release rate (PHRR) of the neat PU foam are 23.3%, 19.8 MJ/m2, and 304.9 kW/m2, respectively. When 10 wt.% EG was added (PU/10 EG), the LOI value increases to 31.2%, the THR value increases to 23.1 MJ/m2, and the PHRR value is 141.0 kW/m2. Furthermore, adding HPCP to PU/10 EG drastically enhances the flame retardancy. Taking PU/10 EG/15HPCP as an example, the PHRR value is 88.5 kW/m2, the THR value is 7.4 MJ/m2, and the LOI value is 33.3%. However, there are few reports about the flame retardancy and UV curing activity of HPCP in PUA systems. In this study, we prepared a novel flame-retardant PUA (FRPUA) oligomer. A series of FRPUA–nHPCP cured samples or curable mixtures were obtained using FRPUA and other composites. All of the curable mixtures have fast surface drying speeds. Taking the FRPUA–20HPCP curable mixture as an example, surface drying of 1 mm of the mixture requires about 2 s under UV irradiation (400 W lamp). Using both 2DOPO–DHBP and HPCP in the PUA system can achieve better flame retardant performance. Namely, there is a synergistic effect between HPCP and 2DOPO–DHBP in the PUA system.
hydroxide (KOH) were supplied by Shanghai Maclin Biochemical Technology Co., Ltd. Ethyl alcohol was supplied by Guangdong Guanghua Sci-Tech Co., Ltd. 2,4,6-Trimethylbenzoyldiphenyl phosphine oxide (TPO) and tripropyleneglycol diacrylate (TPGDA) were purchased from Energy Chemical (China). All of the above reagents were of analytical reagent grade. HPCP (industrial grade) was purchased from Hubei Marvel-Bio Co., Ltd. All of the chemicals were used as received. 2.2. Synthesis of the unsaturated polyether To prepare poly(glycidyl methacrylate) (PGMA), 1,3-propanediol (7.4290 g, 0.0976 mol) and KOH (0.4000 g, 0.0009 mol) were added to a 500 mL three-necked flask equipped with a mechanical stirrer and a reflux condenser and then stirred for 1 h at 80 °C under vacuum to remove water. GMA (56.86 g, 0.4000 mol) with 4-methoxyphenol (0.0586 g, 0.0004 mol) was then slowly dropped into the system under a nitrogen atmosphere. When the reaction mixture became slightly yellow or thick, a certain amount of deionized water was added to the reaction system to stop the reaction. Three steps were then needed to obtain the unsaturated polyether: (1) addition of a certain volume of phosphoric acid (1 mol/L) until the pH of the product was 7 and filtered out the salt, (2) washing the product with acetone and deionized water about three times, and (3) further purification by rotary evaporation under vacuum at 80 °C. The synthesis process is shown in Fig. 1. 2.3. Synthesis of the 2DOPO–DHBP flame retardant monomer DOPO (6.2800 g, 0.0291 mol) was added to a 250 mL three-necked flask equipped with a mechanical stirrer and a reflux condenser and then stirred at 180 °C under a nitrogen atmosphere. When DOPO completely melted, DHBP (1.0711 g, 0.0050 mol) was added and the reaction was continued for 3.5 h. The reaction temperature was then regulated to 100 °C and toluene (40 mL) was added to the flask. The product was filtered under vacuum, washed with ethyl alcohol twice, and dried at 130 °C for 2 h. The solid was then dissolved in tetrahydrofuran for 1 h, repeatedly filtered and washed with ethyl alcohol, and dried for 48 h at 80 °C to give the flame retardant monomer. The synthesis process is shown in Fig. 2. 2.4. Synthesis of FRPUA The synthesis process is shown in Fig. 3. 2DOPO–DHBP (1.0000 g, 0.0016 mol), IPDI (0.8519 g, 0.0038 mol), and two to three drops of DBTDL were added to a 500 mL three-necked flask equipped with a mechanical stirrer and a reflux condenser, and the mixture was then stirred and reacted for 24–36 h. PGMA (5.0000 g, 0.0099 mol) dissolved in acetone was slowly dropped into the mixture and the reaction was not finish until the –NCO content was below 0.5 wt.%. Finally, the reaction mixture was dried by rotary vacuum evaporation at 45 °C for 3 h to remove acetone, and FRPUA was obtained.
2. Materials and methods 2.1. Materials
2.5. Preparation of the PUA and FRPUA–nHPCP UV-cured samples
Isophorone diisocyanate (IPDI), DOPO, 1,3-propanediol, glycidyl methacrylate (GMA), and DHBP were purchased from Aladdin Reagent (China). 4-Methoxyphenol, dibutyltin dilaurate (DBTDL), and potassium
First, the oligomer (70 wt.%), TPGDA (30 wt.%), and TPO (5 wt.%) were mixed with a mechanical stirrer for 12 h, and HPCP was then
Fig. 1. Reaction scheme for synthesis of PGMA. 310
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Fig. 2. Reaction scheme for synthesis of 2DOPO–DHBP.
Fig. 3. Reaction scheme for synthesis of FRPUA. Table 1 Reactant contents of the PUA/FRPUA curable mixtures. Samples\ Ingredients
PUA
PUA-10HPCP*
PUA-20HPCP*
FRPUA-0HPCP
FRPUA-10HPCP
FRPUA-15HPCP
FRPUA-20HPCP
2DOPO-DHBP/g IPDI/g PGMA/g TPGDA/g TPO/g HPCP/g
– 0.7333 5.0000 2.4570 0.4095 –
– 0.7333 5.0000 2.4570 0.4095 0.8599
– 0.7333 5.0000 2.4570 0.4095 1.7199
1.0000 0.8519 5.0000 2.9365 0.4894 –
1.0000 0.8519 5.0000 2.9365 0.4894 1.0000
1.0000 0.8519 5.0000 2.9365 0.4894 1.5000
1.0000 0.8519 5.0000 2.9365 0.4894 2.0000
* The samples were only used in the research of fire retardant properties.
added to the mixture. The cured samples were obtained by casting the mixtures into poly(tetrafluoroethylene) molds and then curing with a UV lamp for 15 min (main wavelength 365 nm, lamp power 400 W, distance between the mixtures and the center of the UV lamp 20 cm, and typical intensity 67.5 mW/cm2). The reactant contents of the PUA/ FRPUA samples are given in Table 1.
2.6. Characterization The Fourier transform infrared (FTIR) spectra of PGMA and 2DOPO–DHBP were obtained between 400 and 4000 cm−1 at 4.0 cm−1 resolution over eight scans with a PerkinElmer FTIR spectrometer using KBr pellets.
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The 1H NMR and 31P NMR spectra of 2DOPO–DHBP were recorded with an AVANCE 400 Bruker spectrometer using dimethyl sulfoxide (DMSO)-d6 as the solvent and tetramethylsilane as the internal reference. The high-resolution mass spectroscopy (HRMS) data of 2DOPO–DHBP were collected with a QSTAR Elite mass spectrometer with DMSO as the solvent. For the surface drying time tests, the PUA and FRPUA–nHPCP curable mixtures were placed on a tinplate sheet and irradiated by a UV lamp (main wavelength 365 nm, lamp power 400 W, distance between the thin sample and the center of the UV lamp 20 cm, and typical intensity 67.5 mW/cm2). When there was no trace by pressing a finger on the mixtures, the time of UV lamp irradiation was the surface drying time. The sizes of the PUA curable mixtures were 120 mm × 25 mm × 15 μm. For the FRPUA–nHPCP curable mixtures, the sizes were 120 mm × 25 mm × 1 mm. For the gel content tests, the cured PUA and FRPUA–nHPCP cured samples were cut into 1 cm × 1 cm squares, their weights (W0) were determined, and they were then immersed in acetone for 60 h. The rest weights (W) of the cured samples were obtained after drying for 36 h at 60 °C. The gel content G was calculated with the following formula:
Fig. 4. FTIR spectra of GMA and synthesized PGMA.
G=
W × 100% W0
Thermogravimetric analysis (TGA) was performed with a Mettler Toledo TGA/DSC1 analyzer. The cured samples (about 9–12 mg) were placed in alumina pans and heated from 25 to 600 °C at a heating rate of 10 °C/min in an Ar atmosphere (60 mL/min). The LOI values were obtained for 130 mm × 6.5 mm × 3.2 mm cured samples with an HC-2 oxygen index meter (Jiang Ning Analysis Instrument Co., China) according to the standard oxygen index test ASTM D2863. The UL-94 tests of the cured samples were performed for 130 mm × 13 mm × 3.2 mm cured samples with a CZF-II horizontal and vertical burning tester (Jiang Ning Analysis Instrument Co.) according to the ASTM D3801-1996 standard. The microscale combustion calorimetry (MCC) tests were performed with a Govmark MCC-2 microscale combustion calorimeter (Govmark, Farmingdale, NY, USA). In the tests, 3–6 mg cured samples were heated to 700 °C at a heating rate of 1 °C/s in a stream of nitrogen flowing at 80 mL/min. The pyrolysis products in the nitrogen gas stream were mixed with a 20 mL/min stream of pure oxygen prior to entering a 900 °C combustion furnace. The heat release rate dQ/dt (W) and the
Fig. 5. FTIR spectra of (a) DHBP, (b) DOPO, and (c) 2DOPO–DHBP.
Fig. 6. HRMS spectrum of 2DOPO–DHBP.
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Fig. 7. 1H and
32
P NMR spectra of 2DOPO–DHBP.
sample temperature as a function of time at a constant heating rate were measured during the test [39]. The specific heat release rate (HRR, W/g) was got by dividing dQ/dt at each point in time by the initial sample mass. The heat release capacity (HRC, J/(g K)) was obtained by dividing the maximum value of the HRR by the heating rate in the test [40].
vibration absorption peaks of the −CH3 and −CH2 groups are observed at 2958 and 2931 cm−1. The peak at 1170 cm−1 is ascribed to the stretching vibration of CeOeC in the ester group. All of above peaks also exist in the spectrum of PGMA. In the GMA spectrum, the peak at 909 cm−1 corresponds to characteristic absorption of the epoxy bond, while there are no homologous absorption peaks in the PGMA spectrum. The appearance of the stretching vibration absorption peaks of CeOeC in the ether bond at 1122 cm−1 and −OH at 3507 cm−1 confirm that the ideal structure of the target product PGMA was successfully obtained.
3. Results and discussion 3.1. Structure characterization 3.1.1. Characterization of PGMA PGMA was synthesized from GMA and 1,3-propanediol catalyzed by KOH. The FTIR spectra of GMA and PGMA are shown in Fig. 4. For GMA, the peaks at 1723 and 1638 cm−1 correspond to stretching vibration absorption of C]O and C]C, respectively. The stretching
3.1.2. Characterization of 2DOPO–DHBP The chemical structure of 2DOPO–DHBP was subjected to FTIR, 1H and 32P NMR, and HRMS analysis, and the results are shown in Figs. 5–7. The signal assignments of the FT-IR, 1H NMR, and 32P NMR spectra have been summarized by Liu [33]. In Fig. 5, the stretching 313
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3.2. Gel content and surface drying time The surface drying times of PUA curable mixtures with different TPO contents are given in Table 2. The PUA and FRPUA–nHPCP cured samples were prepared by UV irradiating for 15 min. The gel fractions were then measured, and they are given in Table 3. From Table 2, the PUA curable mixtures cure fast regardless of the amount of photoinitiator. As the TPO content increases from 2 to 5 wt. %, the surface drying time decreases from 4 to 1 s. Considering that flame retardant monomers will be added later, we chose 5 wt.% TPO content for the follow-up study. To investigate the effect of addition of flame retardant on the surface drying time, we chose the FRPUA–15HPCP and FRPUA–20HPCP curable mixtures to prepare about 1-mm-thick thin mixtures and recorded the surface drying time by the same method. When the UV-curable FRPUA–15HPCP and FRPUA–20HPCP mixtures were irradiating for 2 s, there were no trace on the surfaces. The results show that the FRPUA–nHPCP system has fast surface drying rates. The gel contents of the PUA and FRPUA–nHPCP cured samples are given in Table 3. The gel content of the PUA cured sample is 95.4%. For FRPUA–0HPCP, the gel content decreases to 87.6%, which shows that 2DOPO–DHBP is partly synthesized in the matrix and can partly exist as an addition-type retardant. After mixing with HPCP, the gel content decreases, which is in agreement with expectations because of formation of no new chemical bonds after adding HPCP.
Fig. 8. FTIR spectra of IPDI (curve a), PGMA (curve b), and PUA (curve c). Table 2 Surface drying times of PUA resins with different TPO contents. TPO content(wt.%)
Surface drying time(s)
2 3 4 5
4 4 2 1
3.3. Thermal stabilities of the PUA and FRPUA–nHPCP samples The thermal stabilities of 2DOPO–DHBP, HPCP, the PUA cured sample, and the FRPUA–nHPCP cured samples were investigated by TGA in a nitrogen atmosphere. The TG and differential TG (DTG) curves are shown in Fig. 9, and the related data are summarized in Table 4. For HPCP, the temperature at 5% weight loss (T5wt.%) is 371.5 °C, the temperature at the maximum degradation rate (Tmax) is 466.6 °C, and there is a large amount of char residue at 600 °C (40.5 wt.%). These data all confirm that HPCP has good heat resistance and high thermal stability. This can be attributed to the unique nitrogen–phosphorus organic heterocyclic polymer structure, and the results also show that HPCP may possess excellent flame retardant properties. For 2DOPO–DHBP, T5wt.% = 388.6 °C, Tmax = 477.4 °C, and there is 31.1 wt.% char residue at 600 °C, which indicate that 2DOPO–DHBP has high thermal stability. It has been reported that DOPO and its derivatives have better chemical stability and flame retardant performance than general organophosphates because of the biphenyl ring and a phenanthrene ring structure, especially when cyclic O = PeO bonds are introduced into the side phosphorus groups. For the PUA cured sample, there is continuous slight weight loss from the beginning of heating with T5wt.% = 288.7 °C. In addition, Tmax1 = 324.1 °C for the urethane bonds in the hard segments and Tmax2 = 406.9 °C for the soft polyester segments. The remaining char residue at 600 °C is 10.5 wt.% in a N2 atmosphere. For the FRPUA–0HPCP cured sample, compared with the PUA cured sample, T5wt.% increases to 332.1 °C and the char residue at 600 °C increases to 15.1 wt.%, while there is a slight decrease of Tmax to 396.1 °C. These results can be attributed to the high thermal stability of the reactive flame retardant 2DOPO–DHBP. However, with addition of HPCP, T5wt.% and Tmax decrease, which may originate from the fact that HPCP has lower T5wt.% and Tmax than 2DOPO–DHBP. For the FRPUA–20HPCP cured sample, T5wt.% = 269.6 °C and Tmax = 362.9 °C, which are lower than those of the PUA cured sample, showing that decomposition of the FRPUA–20HPCP cured sample is faster than that of the PUA cured sample. Wu et al. [42] reported that with increasing addition of bis(4aminophenoxy)phenylphosphine oxide, Tmax gradually decreases, which indicates that the phosphorus-containing chain extender accelerates degradation of waterborne PU molecule chains. We speculate
Table 3 Gel contents of the PUA and FRPUA–nHPCP cured samples. Samples
Gel content(%)
PUA FRPUA-0HPCP FRPUA-10HPCP FRPUA-15HPCP FRPUA-20HPCP
95.4 87.6 80.3 71.4 72.2
vibration absorption peak of C]O at 1629 cm−1 in the spectrum of DHBP and the stretching vibration absorption peak of PeH at 2430 cm−1 in the spectrum of DOPO are not present in the spectrum of 2DOPO–DHBP. In addition, the peaks of P–O–Ph at 1178 and 935 cm−1 and the stretching vibration absorption peak of −OH at 3246 cm−1 are not present in the spectrum of 2DOPO–DHBP. These results show that the ideal structure of the flame retardant monomer was successfully obtained. In the HRMS spectrum (Fig. 6), the m/z values of 629.1287 and 651.1097 are related to the molecular weight of 2DOPO–DHBP. The m/ z values of 199.0300 and 413.0913 are fragment ions. In the NMR spectra (Fig. 7), the data agree with the values in the literature [33,41]. 3.1.3. Characterization of the PUA oligomer In this study, we synthesized the PUA oligomer by reacting PGMA with IPDI. The chemical structures of PGMA, IPDI, and PUA were characterized by FTIR spectroscopy (Fig. 8). For IPDI (curve a), the peak at 2256 cm−1 is the characteristic absorption peak of –NCO. For PGMA (curve b), the characteristic peak position of −OH is 3507 cm−1. For PUA (curve c), the typical peak of –NCO at 2256 cm−1 has completely disappeared and the strong characteristic peak of the stretching vibration of –NH at 3394 cm−1 is clearly observed, which indicates that we obtained the target product.
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Fig. 9. (a) and (c) TG and (b) and (d) DTG curves of the PUA and FRPUA–nHPCP cured samples. Table 4 TGA data of the 2DOPO–DHBP, HPCP, PUA, and FRPUA–nHPCP cured samples.
Table 6 Detailed results of the MCC tests for the different samples.
Samples
T5wt.% (°C)a
Tmax1 (°C)
Tmax2 (°C)b
Residues (wt.%)c
Samples
HPCP 2DOPO-DHBP PUA FRPUA-0HPCP FRPUA-10HPCP FRPUA-15HPCP FRPUA-20HPCP
371.5 388.6 288.7 332.1 324.1 295.6 269.6
— — 324.1 — — — —
466.6 477.4 406.9 396.1 370.2 364.1 362.9
40.5 31.1 10.5 15.1 18.1 21.9 19.8
HRC (J/g·K)
PHRR (W/g)
THR (kJ/g)
Tmax (°C)
PUA PUA-10HPCP PUA-20HPCP FRPUA-0HPCP FRPUA-20HPCP
380 578 846 330 534
341.2 518.5 760.4 294.9 475.2
29.2 28.4 29.2 32.4 26.4
405.6 413 400.2 401.6 388.5
a b c
Temperature at 5% weight loss. Maximum degradation rate temperature. Char yield at 600 °C.
3.4. Fire retardant properties of the PUA and FRPUA–nHPCP cured samples The minimum oxygen concentration by volume for maintaining the burning of a material, is commonly used to evaluate the flame retardancy of materials. The UL-94 vertical burning test is used to characterize the ease of the ignition of polymeric materials using small burner. It is another indicator that ranks the flammability of materials. For PUA and FRPUA-nHPCP cured samples, the results of the LOI and UL-94 tests are given in Table 5. In order to investigate whether there is a synergistic effect between HPCP and 2DOPO–DHBP in the PUA system, we added a certain amount of HPCP to the PUA/FRPUA system and then recorded their LOI indexes and related parameters in UL-94 tests, which are also given in Table 5. For the PUA cured sample, the LOI index is 19.0%, indicating the intrinsic flammability of the PUA cured sample. For the FRPUA–0HPCP cured sample, the LOI value increases to 21.1%. The LOI indexes of PUA-10HPCP and PUA-20HPCP cured samples are 21.1% and 22.1%, respectively, which are higher than that of PUA cured sample. When adding HPCP into the FRPUA-0HPCP system, the LOI values increase with increasing HPCP content. The maximum LOI value 23.3% can be
Table 5 LOI and UL-94 data for the PUA and FRPUA cured samples. Samples
LOI(%)
UL-94 Classification
Dripping
PUA PUA-10HPCP PUA-20HPCP FRPUA-0HPCP FRPUA-10HPCP FRPUA-15HPCP FRPUA-20HPCP
19.0 21.1 22.1 21.1 22.4 22.9 23.3
No No No No No No No
N N N N N N N
Rating Rating Rating Rating Rating Rating Rating
P(wt.%)
N(wt.%)
0.000 1.218 2.233 0.969 2.063 2.544 2.986
1.074 1.528 1.905 1.044 1.489 1.683 1.862
that HPCP can also accelerate degradation of PUA chains, leading to the earlier complete decomposition for FRPUA. In addition, the char residue at 600 °C shows an increasing trend with increasing addition of HPCP, which indicates that HPCP may reflects a certain degree of flame retardancy. 315
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Fig. 10. HRR versus temperature (a) and HRR versus time (b) curves of PUA cured sample, FRPUA-0/20HPCP cured samples and PUA-10/20HPCP cured samples.
obtained with the highest HPCP content, namely for FRPUA-20HPCP. The P and N content for PUA-20HPCP is 2.233 wt.% and 1.905 wt.%, much higher than 2.063 wt.% and 1.489 wt.% for FRPUA-10HPCP. However, the LOI values for PUA-20HPCP is slightly lower than 22.4% for FRPUA-10HPCP. Comparing the LOI value and P, N content for PUA-20HPCP and FRPUA-10HPCP, HPCP alone did not significantly increase the flame retardancy. Namely, both 2DOPO- DHBP and HPCP in PUA system may achieved better fire retardant properties. However, no ratings in the UL-94 tests are observed for all samples. Consequently, 2DOPO-DHBP and HPCP cannot improve the vertical burning rating of the system effectively. These phenomena are also reported previously [43,44]. MCC is one of the most effective methods to compare and evaluate the fire performance of polymeric materials. The characteristic data from this test include the peak of the heat release rate (PHRR, W/g), heat release capacity (HRC, J/g·K), total heat release (THR, kJ/g), and the temperature at the PHRR (Tmax, °C). THR is the total amount of heat released from the beginning to the end of combustion [45]. HRC is a good predictor of the flame resistance and fire behavior when only research quantities are available for testing. HRR is an important parameter to describe the degree of fire hazard. Decreases in these data normally indicate that flame retardancy is improved. The detailed MCC data are listed in Table 6. The HRR versus temperature and time curves of the PUA cured sample, the FRPUA–0HPCP cured sample, the FRPUA–20HPCP cured sample, PUA-10HPCP and PUA-20HPCP cured samples are shown in Fig. 10. From Table 6, the PUA cured sample is highly flammable with high PHRR and THR values of 341.2 W/g and 29.2 kJ/g, respectively. After combining with 2DOPO–DHBP, namely, FRPUA–0HPCP, the THR value increases to 32.4 kJ/g, the PHRR value decreases to 294.9 W/g, and Tmax increases from 405.6 to 401.6 °C. The decrease in PHRR means that incorporation of 2DOPO-DHBP could reduce degree of fire hazard. Unexpectedly, higher THR showed higher total energy released during combustion. The THR value of FRPUA-20HPCP was 26.4 kJ/g, which is lower than 29.2 kJ/g of PUA cured sample. Unexpectedly, the PHRR and HRC of FRPUA-20HPCP exhibit the higher values of 475.2 W/g and 534 J/g·K. The results showed higher degree of fire hazard during the use of FRPUA-20HPCP cured sample. The PHRR and HRC values of PUA-10HPCP cured sample were 518.5 W/g, 578 J/g·K, higher than that of PUA obviously. When the content of HPCP continuously increase, the PHRR and HRC continuously increase, which indicates the incorporation of HPCP alone into the PUA system significantly increase the degree of fire hazard. In a word, 2DOPO-DHBP or HPCP can produce a limited certain flame retardant effect in the PUA system and using them together can reached better flame retardant properties. As shown in Fig. 10, there is a sharp and high peak in the HRR curve of the PUA–20HPCP and PUA-10HPCP cured samples and the combustion time of PUA-10/20HPCP were shorter than that of PUA cured
sample. Consequently, HPCP may accelerate decomposition of the cured sample and greatly decrease the burning time, which is in agreement with the TGA tests. 4. Conclusion A series of FRPUA–nHPCP (n = 0, 10, 15, and 20) curable mixtures and cured samples have been prepared. For pure PUA, the surface drying time decreases from 4 to 1 s as the TPO content in the system increases from 2 to 5 wt.%. Even if the flame retardancy is later modified by combining 2DOPO–DHBP and HPCP, all of the mixtures have fast surface drying times within few seconds. FRPUA–nHPCP cured samples have better heat resistance and higher thermal stability than PUA cured sample. The PUA system using both 2DOPO-DHBP and HPCP can achieve better flame retardancy. In other words, there is a synergistic effect between HPCP and 2DOPO–DHBP in the PUA system. Data availability The raw data required to reproduce these findings are available to download from https://pan.baidu.com/s/12_M-ykTrMvRPFvZrP8qLHA. The processed data required to reproduce these findings are available to download from https://pan.baidu.com/s/1Y6ID8b09iWOedopPfiI9JA. Acknowledgments This work was supported by the Special Development Program of the Strategic Emerging Industries of Shenzhen City (JSGG20160331104855391). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Preparation, characterization, and properties of novel ultraviolet-curable and flame-retardant polyurethane acrylate
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Yongxia Rena, Yunsheng Donga, Yali Zhua, Jing Xub, , Youwei Yaoa, ⁎⁎
a b
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Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, PR China College of Chemistry and Material Science, Shandong Agriculture University, Tai’an 271018, PR China
ARTICLE INFO
ABSTRACT
Keywords: Ultraviolet curing Polyurethane acrylate Surface drying time Flame retardancy 9,10-dihydro-9-oxa-10-phosphaphenanthrene10-oxide Hexa-phenoxy-cyclotriphosphazene
A series of flame-retardant polyurethane acrylate oligomer (FRPUA)–nhexa- phenoxy-cyclotriphosphazene (HPCP) (n = 0, 10, 15, and 20, where n is the amount of HPCP added to a 100 g mixture of FRPUA and other additives) ultraviolet (UV)-cured samples have been prepared. FRPUA was synthesized by the reaction of poly (glycidyl methacrylate) (an unsaturated polyether), a derivative of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) (2DOPO–4,4′- dihydroxy-benzophenone (DHBP)), and isophorone diisocyanate. As a control sample, the polyurethane acrylate (PUA) oligomer and cured PUA sample were also prepared. Surface drying of the PUA and FRPUA–nHPCP curable mixtures required a few seconds under UV irradiation (400 W lamp). The effects of 2DOPO–DHBP and HPCP on the flame retardancy and thermal stability of the PUA system were also investigated. For the FRPUA–nHPCP series, as n increases from 0 to 20, the limiting oxygen index (LOI) increases from 21.1% to 23.3% and the total heat release (THR) slowly decreases from 32.4 to 26.4 kJ/g, while the PUA cured sample possesses the LOI and THR values of 19.0% and 29.2 kJ/g, respectively. Besides, the PUA system can achieve better flame retardant properties when using both 2DOPO-DHBP and HPCP. Thermogravimetric analysis indicates that the amount of char residue of the FRPUA–nHPCP (n = 0, 10, 15, and 20) cured samples ranges from 15.1 to 21.9 wt.%, which are much are higher than 10.5 wt.% for the PUA cured sample. Improving the flame retardancy of the PUA system will expand its applications, such as for paper manufacturing and architectural coatings, while simultaneously retaining the feature of fast surface drying.
1. Introduction In recent decades, environmental regulations have proposed more stringent requirements for emission of volatile organic compounds (VOCs) in chemical industries, which has prompted interest in environmentally friendly products. As a new green technique, the ultraviolet (UV)-curing processing has wide applications in coatings, adhesives, printing inks, dental composites, and so forth. The advantages of UV-cured products are widely recognized. They are usually summarized as follows: low VOC emissions, low energy consumption, fast curing at ambient temperature, and short cycle time. All of the above demonstrate the necessity for research of UV-cured coatings [1–4]. UV-curable coatings consist of an oligomer, a reactive diluent, and a photoinitiator. The properties and polymerization kinetics of the coatings mainly depend on the oligomer structure and its concentration in the formulation. Polyurethane acrylate (PUA) [5–9], one of the most commonly used oligomers, is used in coating for automobiles, leather,
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printing, inks, and so forth, because PUA combines the excellent mechanical properties of polyurethane (PU) [10] and the weatherability, water resistance, and solvent resistance of polyacrylates [11,12]. However, compared with epoxy resins, PUA is not only more expensive, but it also possesses slower curing speed and lower photopolymerization activity. Some studies have attempted to achieve faster curing by changing the composition and curing conditions. For example, Qin et al. [13] synthesized PUAs modified by saturated alcohols with different chain lengths. They found that for 40-μm-thick PUA films, when C]C conversion reaches around 85%, the curing time is at least 1 min. Xu designed double-bond controllable UV-cured hyperbranched urethane acrylic (HBPUA) resins. When HBPUA was dissolved in a mixed solvent of ethanol and butyl acetate and mixed with 1 wt.% content of Irgacure 1173, the best curing time was 15 s under 1 kW UV irradiation [14,15]. There have been few reports about synthesizing a novel PUA oligomer for faster curing.
Corresponding authors at: Division of Energy and Environment, Tsinghua Campus, The University Town, Room 1510, Shenzhen, 518005, PR China. Corresponding authors at: Dai Zong Road No.61, Taian, Shandong, PR China. E-mail addresses: [email protected] (J. Xu), [email protected] (Y. Yao).
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https://doi.org/10.1016/j.porgcoat.2018.11.009 Received 29 August 2018; Received in revised form 15 October 2018; Accepted 6 November 2018 0300-9440/ © 2018 Published by Elsevier B.V.
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A significant drawback of many synthetic polymers is their inherent flammability [16,17]. Generally, PUA is flammable, which restricts its applications, such as for leather processing, textile and paper manufacturing, and architectural coatings [18]. There are usually two ways for flame retardant modification of polymers: blending and using a reactive-type flame retardant. Some researchers have used additiontype flame retardants, such as huntite and hydromagnesite, microcapsulated red phosphorus [19], aluminum hydroxide [20], expandable graphite [21,22], and ammonium polyphosphate [23,24], to modify the flammability of various polymers because of the easy operation of the blending method. Other researchers have used the reactive-type flameretardant method, which improves the compatibility between the flame retardant and the matrix, to achieve the same aim. 9,10-Dihydro-9-oxa10-phosphaphenanthrene-10-oxide (DOPO) and its derivatives are commonly used as flame retardants because of their excellent flame retardancy in both the gas phase and condensed phase [25–32]. Liu [33] prepared a new diepoxide (2DOPO–PhOH) and a new diamine (2DOPO–A) and used both as monomers to prepare cured epoxy resins with high phosphorus contents. All of the as-prepared epoxy resins exhibited high thermal stability, with 5% weight loss temperatures (T5wt.%) above 316 °C, and excellent flame retardancy, with limiting oxygen index (LOI) values of 37–50%. As another highly efficient flame retardant, hexa-phenoxy-cyclotriphosphazene (HPCP) is usually added to epoxy resin owing to its good gas-phase flame-retardant effects [34–37]. Qian et al. [38] prepared flame-retardant rigid PU foams with HPCP/expanded graphene (EG) by box foaming. The LOI index, total heat release (THR), and temperature of the peak of the heat release rate (PHRR) of the neat PU foam are 23.3%, 19.8 MJ/m2, and 304.9 kW/m2, respectively. When 10 wt.% EG was added (PU/10 EG), the LOI value increases to 31.2%, the THR value increases to 23.1 MJ/m2, and the PHRR value is 141.0 kW/m2. Furthermore, adding HPCP to PU/10 EG drastically enhances the flame retardancy. Taking PU/10 EG/15HPCP as an example, the PHRR value is 88.5 kW/m2, the THR value is 7.4 MJ/m2, and the LOI value is 33.3%. However, there are few reports about the flame retardancy and UV curing activity of HPCP in PUA systems. In this study, we prepared a novel flame-retardant PUA (FRPUA) oligomer. A series of FRPUA–nHPCP cured samples or curable mixtures were obtained using FRPUA and other composites. All of the curable mixtures have fast surface drying speeds. Taking the FRPUA–20HPCP curable mixture as an example, surface drying of 1 mm of the mixture requires about 2 s under UV irradiation (400 W lamp). Using both 2DOPO–DHBP and HPCP in the PUA system can achieve better flame retardant performance. Namely, there is a synergistic effect between HPCP and 2DOPO–DHBP in the PUA system.
hydroxide (KOH) were supplied by Shanghai Maclin Biochemical Technology Co., Ltd. Ethyl alcohol was supplied by Guangdong Guanghua Sci-Tech Co., Ltd. 2,4,6-Trimethylbenzoyldiphenyl phosphine oxide (TPO) and tripropyleneglycol diacrylate (TPGDA) were purchased from Energy Chemical (China). All of the above reagents were of analytical reagent grade. HPCP (industrial grade) was purchased from Hubei Marvel-Bio Co., Ltd. All of the chemicals were used as received. 2.2. Synthesis of the unsaturated polyether To prepare poly(glycidyl methacrylate) (PGMA), 1,3-propanediol (7.4290 g, 0.0976 mol) and KOH (0.4000 g, 0.0009 mol) were added to a 500 mL three-necked flask equipped with a mechanical stirrer and a reflux condenser and then stirred for 1 h at 80 °C under vacuum to remove water. GMA (56.86 g, 0.4000 mol) with 4-methoxyphenol (0.0586 g, 0.0004 mol) was then slowly dropped into the system under a nitrogen atmosphere. When the reaction mixture became slightly yellow or thick, a certain amount of deionized water was added to the reaction system to stop the reaction. Three steps were then needed to obtain the unsaturated polyether: (1) addition of a certain volume of phosphoric acid (1 mol/L) until the pH of the product was 7 and filtered out the salt, (2) washing the product with acetone and deionized water about three times, and (3) further purification by rotary evaporation under vacuum at 80 °C. The synthesis process is shown in Fig. 1. 2.3. Synthesis of the 2DOPO–DHBP flame retardant monomer DOPO (6.2800 g, 0.0291 mol) was added to a 250 mL three-necked flask equipped with a mechanical stirrer and a reflux condenser and then stirred at 180 °C under a nitrogen atmosphere. When DOPO completely melted, DHBP (1.0711 g, 0.0050 mol) was added and the reaction was continued for 3.5 h. The reaction temperature was then regulated to 100 °C and toluene (40 mL) was added to the flask. The product was filtered under vacuum, washed with ethyl alcohol twice, and dried at 130 °C for 2 h. The solid was then dissolved in tetrahydrofuran for 1 h, repeatedly filtered and washed with ethyl alcohol, and dried for 48 h at 80 °C to give the flame retardant monomer. The synthesis process is shown in Fig. 2. 2.4. Synthesis of FRPUA The synthesis process is shown in Fig. 3. 2DOPO–DHBP (1.0000 g, 0.0016 mol), IPDI (0.8519 g, 0.0038 mol), and two to three drops of DBTDL were added to a 500 mL three-necked flask equipped with a mechanical stirrer and a reflux condenser, and the mixture was then stirred and reacted for 24–36 h. PGMA (5.0000 g, 0.0099 mol) dissolved in acetone was slowly dropped into the mixture and the reaction was not finish until the –NCO content was below 0.5 wt.%. Finally, the reaction mixture was dried by rotary vacuum evaporation at 45 °C for 3 h to remove acetone, and FRPUA was obtained.
2. Materials and methods 2.1. Materials
2.5. Preparation of the PUA and FRPUA–nHPCP UV-cured samples
Isophorone diisocyanate (IPDI), DOPO, 1,3-propanediol, glycidyl methacrylate (GMA), and DHBP were purchased from Aladdin Reagent (China). 4-Methoxyphenol, dibutyltin dilaurate (DBTDL), and potassium
First, the oligomer (70 wt.%), TPGDA (30 wt.%), and TPO (5 wt.%) were mixed with a mechanical stirrer for 12 h, and HPCP was then
Fig. 1. Reaction scheme for synthesis of PGMA. 310
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Fig. 2. Reaction scheme for synthesis of 2DOPO–DHBP.
Fig. 3. Reaction scheme for synthesis of FRPUA. Table 1 Reactant contents of the PUA/FRPUA curable mixtures. Samples\ Ingredients
PUA
PUA-10HPCP*
PUA-20HPCP*
FRPUA-0HPCP
FRPUA-10HPCP
FRPUA-15HPCP
FRPUA-20HPCP
2DOPO-DHBP/g IPDI/g PGMA/g TPGDA/g TPO/g HPCP/g
– 0.7333 5.0000 2.4570 0.4095 –
– 0.7333 5.0000 2.4570 0.4095 0.8599
– 0.7333 5.0000 2.4570 0.4095 1.7199
1.0000 0.8519 5.0000 2.9365 0.4894 –
1.0000 0.8519 5.0000 2.9365 0.4894 1.0000
1.0000 0.8519 5.0000 2.9365 0.4894 1.5000
1.0000 0.8519 5.0000 2.9365 0.4894 2.0000
* The samples were only used in the research of fire retardant properties.
added to the mixture. The cured samples were obtained by casting the mixtures into poly(tetrafluoroethylene) molds and then curing with a UV lamp for 15 min (main wavelength 365 nm, lamp power 400 W, distance between the mixtures and the center of the UV lamp 20 cm, and typical intensity 67.5 mW/cm2). The reactant contents of the PUA/ FRPUA samples are given in Table 1.
2.6. Characterization The Fourier transform infrared (FTIR) spectra of PGMA and 2DOPO–DHBP were obtained between 400 and 4000 cm−1 at 4.0 cm−1 resolution over eight scans with a PerkinElmer FTIR spectrometer using KBr pellets.
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The 1H NMR and 31P NMR spectra of 2DOPO–DHBP were recorded with an AVANCE 400 Bruker spectrometer using dimethyl sulfoxide (DMSO)-d6 as the solvent and tetramethylsilane as the internal reference. The high-resolution mass spectroscopy (HRMS) data of 2DOPO–DHBP were collected with a QSTAR Elite mass spectrometer with DMSO as the solvent. For the surface drying time tests, the PUA and FRPUA–nHPCP curable mixtures were placed on a tinplate sheet and irradiated by a UV lamp (main wavelength 365 nm, lamp power 400 W, distance between the thin sample and the center of the UV lamp 20 cm, and typical intensity 67.5 mW/cm2). When there was no trace by pressing a finger on the mixtures, the time of UV lamp irradiation was the surface drying time. The sizes of the PUA curable mixtures were 120 mm × 25 mm × 15 μm. For the FRPUA–nHPCP curable mixtures, the sizes were 120 mm × 25 mm × 1 mm. For the gel content tests, the cured PUA and FRPUA–nHPCP cured samples were cut into 1 cm × 1 cm squares, their weights (W0) were determined, and they were then immersed in acetone for 60 h. The rest weights (W) of the cured samples were obtained after drying for 36 h at 60 °C. The gel content G was calculated with the following formula:
Fig. 4. FTIR spectra of GMA and synthesized PGMA.
G=
W × 100% W0
Thermogravimetric analysis (TGA) was performed with a Mettler Toledo TGA/DSC1 analyzer. The cured samples (about 9–12 mg) were placed in alumina pans and heated from 25 to 600 °C at a heating rate of 10 °C/min in an Ar atmosphere (60 mL/min). The LOI values were obtained for 130 mm × 6.5 mm × 3.2 mm cured samples with an HC-2 oxygen index meter (Jiang Ning Analysis Instrument Co., China) according to the standard oxygen index test ASTM D2863. The UL-94 tests of the cured samples were performed for 130 mm × 13 mm × 3.2 mm cured samples with a CZF-II horizontal and vertical burning tester (Jiang Ning Analysis Instrument Co.) according to the ASTM D3801-1996 standard. The microscale combustion calorimetry (MCC) tests were performed with a Govmark MCC-2 microscale combustion calorimeter (Govmark, Farmingdale, NY, USA). In the tests, 3–6 mg cured samples were heated to 700 °C at a heating rate of 1 °C/s in a stream of nitrogen flowing at 80 mL/min. The pyrolysis products in the nitrogen gas stream were mixed with a 20 mL/min stream of pure oxygen prior to entering a 900 °C combustion furnace. The heat release rate dQ/dt (W) and the
Fig. 5. FTIR spectra of (a) DHBP, (b) DOPO, and (c) 2DOPO–DHBP.
Fig. 6. HRMS spectrum of 2DOPO–DHBP.
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Fig. 7. 1H and
32
P NMR spectra of 2DOPO–DHBP.
sample temperature as a function of time at a constant heating rate were measured during the test [39]. The specific heat release rate (HRR, W/g) was got by dividing dQ/dt at each point in time by the initial sample mass. The heat release capacity (HRC, J/(g K)) was obtained by dividing the maximum value of the HRR by the heating rate in the test [40].
vibration absorption peaks of the −CH3 and −CH2 groups are observed at 2958 and 2931 cm−1. The peak at 1170 cm−1 is ascribed to the stretching vibration of CeOeC in the ester group. All of above peaks also exist in the spectrum of PGMA. In the GMA spectrum, the peak at 909 cm−1 corresponds to characteristic absorption of the epoxy bond, while there are no homologous absorption peaks in the PGMA spectrum. The appearance of the stretching vibration absorption peaks of CeOeC in the ether bond at 1122 cm−1 and −OH at 3507 cm−1 confirm that the ideal structure of the target product PGMA was successfully obtained.
3. Results and discussion 3.1. Structure characterization 3.1.1. Characterization of PGMA PGMA was synthesized from GMA and 1,3-propanediol catalyzed by KOH. The FTIR spectra of GMA and PGMA are shown in Fig. 4. For GMA, the peaks at 1723 and 1638 cm−1 correspond to stretching vibration absorption of C]O and C]C, respectively. The stretching
3.1.2. Characterization of 2DOPO–DHBP The chemical structure of 2DOPO–DHBP was subjected to FTIR, 1H and 32P NMR, and HRMS analysis, and the results are shown in Figs. 5–7. The signal assignments of the FT-IR, 1H NMR, and 32P NMR spectra have been summarized by Liu [33]. In Fig. 5, the stretching 313
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3.2. Gel content and surface drying time The surface drying times of PUA curable mixtures with different TPO contents are given in Table 2. The PUA and FRPUA–nHPCP cured samples were prepared by UV irradiating for 15 min. The gel fractions were then measured, and they are given in Table 3. From Table 2, the PUA curable mixtures cure fast regardless of the amount of photoinitiator. As the TPO content increases from 2 to 5 wt. %, the surface drying time decreases from 4 to 1 s. Considering that flame retardant monomers will be added later, we chose 5 wt.% TPO content for the follow-up study. To investigate the effect of addition of flame retardant on the surface drying time, we chose the FRPUA–15HPCP and FRPUA–20HPCP curable mixtures to prepare about 1-mm-thick thin mixtures and recorded the surface drying time by the same method. When the UV-curable FRPUA–15HPCP and FRPUA–20HPCP mixtures were irradiating for 2 s, there were no trace on the surfaces. The results show that the FRPUA–nHPCP system has fast surface drying rates. The gel contents of the PUA and FRPUA–nHPCP cured samples are given in Table 3. The gel content of the PUA cured sample is 95.4%. For FRPUA–0HPCP, the gel content decreases to 87.6%, which shows that 2DOPO–DHBP is partly synthesized in the matrix and can partly exist as an addition-type retardant. After mixing with HPCP, the gel content decreases, which is in agreement with expectations because of formation of no new chemical bonds after adding HPCP.
Fig. 8. FTIR spectra of IPDI (curve a), PGMA (curve b), and PUA (curve c). Table 2 Surface drying times of PUA resins with different TPO contents. TPO content(wt.%)
Surface drying time(s)
2 3 4 5
4 4 2 1
3.3. Thermal stabilities of the PUA and FRPUA–nHPCP samples The thermal stabilities of 2DOPO–DHBP, HPCP, the PUA cured sample, and the FRPUA–nHPCP cured samples were investigated by TGA in a nitrogen atmosphere. The TG and differential TG (DTG) curves are shown in Fig. 9, and the related data are summarized in Table 4. For HPCP, the temperature at 5% weight loss (T5wt.%) is 371.5 °C, the temperature at the maximum degradation rate (Tmax) is 466.6 °C, and there is a large amount of char residue at 600 °C (40.5 wt.%). These data all confirm that HPCP has good heat resistance and high thermal stability. This can be attributed to the unique nitrogen–phosphorus organic heterocyclic polymer structure, and the results also show that HPCP may possess excellent flame retardant properties. For 2DOPO–DHBP, T5wt.% = 388.6 °C, Tmax = 477.4 °C, and there is 31.1 wt.% char residue at 600 °C, which indicate that 2DOPO–DHBP has high thermal stability. It has been reported that DOPO and its derivatives have better chemical stability and flame retardant performance than general organophosphates because of the biphenyl ring and a phenanthrene ring structure, especially when cyclic O = PeO bonds are introduced into the side phosphorus groups. For the PUA cured sample, there is continuous slight weight loss from the beginning of heating with T5wt.% = 288.7 °C. In addition, Tmax1 = 324.1 °C for the urethane bonds in the hard segments and Tmax2 = 406.9 °C for the soft polyester segments. The remaining char residue at 600 °C is 10.5 wt.% in a N2 atmosphere. For the FRPUA–0HPCP cured sample, compared with the PUA cured sample, T5wt.% increases to 332.1 °C and the char residue at 600 °C increases to 15.1 wt.%, while there is a slight decrease of Tmax to 396.1 °C. These results can be attributed to the high thermal stability of the reactive flame retardant 2DOPO–DHBP. However, with addition of HPCP, T5wt.% and Tmax decrease, which may originate from the fact that HPCP has lower T5wt.% and Tmax than 2DOPO–DHBP. For the FRPUA–20HPCP cured sample, T5wt.% = 269.6 °C and Tmax = 362.9 °C, which are lower than those of the PUA cured sample, showing that decomposition of the FRPUA–20HPCP cured sample is faster than that of the PUA cured sample. Wu et al. [42] reported that with increasing addition of bis(4aminophenoxy)phenylphosphine oxide, Tmax gradually decreases, which indicates that the phosphorus-containing chain extender accelerates degradation of waterborne PU molecule chains. We speculate
Table 3 Gel contents of the PUA and FRPUA–nHPCP cured samples. Samples
Gel content(%)
PUA FRPUA-0HPCP FRPUA-10HPCP FRPUA-15HPCP FRPUA-20HPCP
95.4 87.6 80.3 71.4 72.2
vibration absorption peak of C]O at 1629 cm−1 in the spectrum of DHBP and the stretching vibration absorption peak of PeH at 2430 cm−1 in the spectrum of DOPO are not present in the spectrum of 2DOPO–DHBP. In addition, the peaks of P–O–Ph at 1178 and 935 cm−1 and the stretching vibration absorption peak of −OH at 3246 cm−1 are not present in the spectrum of 2DOPO–DHBP. These results show that the ideal structure of the flame retardant monomer was successfully obtained. In the HRMS spectrum (Fig. 6), the m/z values of 629.1287 and 651.1097 are related to the molecular weight of 2DOPO–DHBP. The m/ z values of 199.0300 and 413.0913 are fragment ions. In the NMR spectra (Fig. 7), the data agree with the values in the literature [33,41]. 3.1.3. Characterization of the PUA oligomer In this study, we synthesized the PUA oligomer by reacting PGMA with IPDI. The chemical structures of PGMA, IPDI, and PUA were characterized by FTIR spectroscopy (Fig. 8). For IPDI (curve a), the peak at 2256 cm−1 is the characteristic absorption peak of –NCO. For PGMA (curve b), the characteristic peak position of −OH is 3507 cm−1. For PUA (curve c), the typical peak of –NCO at 2256 cm−1 has completely disappeared and the strong characteristic peak of the stretching vibration of –NH at 3394 cm−1 is clearly observed, which indicates that we obtained the target product.
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Fig. 9. (a) and (c) TG and (b) and (d) DTG curves of the PUA and FRPUA–nHPCP cured samples. Table 4 TGA data of the 2DOPO–DHBP, HPCP, PUA, and FRPUA–nHPCP cured samples.
Table 6 Detailed results of the MCC tests for the different samples.
Samples
T5wt.% (°C)a
Tmax1 (°C)
Tmax2 (°C)b
Residues (wt.%)c
Samples
HPCP 2DOPO-DHBP PUA FRPUA-0HPCP FRPUA-10HPCP FRPUA-15HPCP FRPUA-20HPCP
371.5 388.6 288.7 332.1 324.1 295.6 269.6
— — 324.1 — — — —
466.6 477.4 406.9 396.1 370.2 364.1 362.9
40.5 31.1 10.5 15.1 18.1 21.9 19.8
HRC (J/g·K)
PHRR (W/g)
THR (kJ/g)
Tmax (°C)
PUA PUA-10HPCP PUA-20HPCP FRPUA-0HPCP FRPUA-20HPCP
380 578 846 330 534
341.2 518.5 760.4 294.9 475.2
29.2 28.4 29.2 32.4 26.4
405.6 413 400.2 401.6 388.5
a b c
Temperature at 5% weight loss. Maximum degradation rate temperature. Char yield at 600 °C.
3.4. Fire retardant properties of the PUA and FRPUA–nHPCP cured samples The minimum oxygen concentration by volume for maintaining the burning of a material, is commonly used to evaluate the flame retardancy of materials. The UL-94 vertical burning test is used to characterize the ease of the ignition of polymeric materials using small burner. It is another indicator that ranks the flammability of materials. For PUA and FRPUA-nHPCP cured samples, the results of the LOI and UL-94 tests are given in Table 5. In order to investigate whether there is a synergistic effect between HPCP and 2DOPO–DHBP in the PUA system, we added a certain amount of HPCP to the PUA/FRPUA system and then recorded their LOI indexes and related parameters in UL-94 tests, which are also given in Table 5. For the PUA cured sample, the LOI index is 19.0%, indicating the intrinsic flammability of the PUA cured sample. For the FRPUA–0HPCP cured sample, the LOI value increases to 21.1%. The LOI indexes of PUA-10HPCP and PUA-20HPCP cured samples are 21.1% and 22.1%, respectively, which are higher than that of PUA cured sample. When adding HPCP into the FRPUA-0HPCP system, the LOI values increase with increasing HPCP content. The maximum LOI value 23.3% can be
Table 5 LOI and UL-94 data for the PUA and FRPUA cured samples. Samples
LOI(%)
UL-94 Classification
Dripping
PUA PUA-10HPCP PUA-20HPCP FRPUA-0HPCP FRPUA-10HPCP FRPUA-15HPCP FRPUA-20HPCP
19.0 21.1 22.1 21.1 22.4 22.9 23.3
No No No No No No No
N N N N N N N
Rating Rating Rating Rating Rating Rating Rating
P(wt.%)
N(wt.%)
0.000 1.218 2.233 0.969 2.063 2.544 2.986
1.074 1.528 1.905 1.044 1.489 1.683 1.862
that HPCP can also accelerate degradation of PUA chains, leading to the earlier complete decomposition for FRPUA. In addition, the char residue at 600 °C shows an increasing trend with increasing addition of HPCP, which indicates that HPCP may reflects a certain degree of flame retardancy. 315
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Fig. 10. HRR versus temperature (a) and HRR versus time (b) curves of PUA cured sample, FRPUA-0/20HPCP cured samples and PUA-10/20HPCP cured samples.
obtained with the highest HPCP content, namely for FRPUA-20HPCP. The P and N content for PUA-20HPCP is 2.233 wt.% and 1.905 wt.%, much higher than 2.063 wt.% and 1.489 wt.% for FRPUA-10HPCP. However, the LOI values for PUA-20HPCP is slightly lower than 22.4% for FRPUA-10HPCP. Comparing the LOI value and P, N content for PUA-20HPCP and FRPUA-10HPCP, HPCP alone did not significantly increase the flame retardancy. Namely, both 2DOPO- DHBP and HPCP in PUA system may achieved better fire retardant properties. However, no ratings in the UL-94 tests are observed for all samples. Consequently, 2DOPO-DHBP and HPCP cannot improve the vertical burning rating of the system effectively. These phenomena are also reported previously [43,44]. MCC is one of the most effective methods to compare and evaluate the fire performance of polymeric materials. The characteristic data from this test include the peak of the heat release rate (PHRR, W/g), heat release capacity (HRC, J/g·K), total heat release (THR, kJ/g), and the temperature at the PHRR (Tmax, °C). THR is the total amount of heat released from the beginning to the end of combustion [45]. HRC is a good predictor of the flame resistance and fire behavior when only research quantities are available for testing. HRR is an important parameter to describe the degree of fire hazard. Decreases in these data normally indicate that flame retardancy is improved. The detailed MCC data are listed in Table 6. The HRR versus temperature and time curves of the PUA cured sample, the FRPUA–0HPCP cured sample, the FRPUA–20HPCP cured sample, PUA-10HPCP and PUA-20HPCP cured samples are shown in Fig. 10. From Table 6, the PUA cured sample is highly flammable with high PHRR and THR values of 341.2 W/g and 29.2 kJ/g, respectively. After combining with 2DOPO–DHBP, namely, FRPUA–0HPCP, the THR value increases to 32.4 kJ/g, the PHRR value decreases to 294.9 W/g, and Tmax increases from 405.6 to 401.6 °C. The decrease in PHRR means that incorporation of 2DOPO-DHBP could reduce degree of fire hazard. Unexpectedly, higher THR showed higher total energy released during combustion. The THR value of FRPUA-20HPCP was 26.4 kJ/g, which is lower than 29.2 kJ/g of PUA cured sample. Unexpectedly, the PHRR and HRC of FRPUA-20HPCP exhibit the higher values of 475.2 W/g and 534 J/g·K. The results showed higher degree of fire hazard during the use of FRPUA-20HPCP cured sample. The PHRR and HRC values of PUA-10HPCP cured sample were 518.5 W/g, 578 J/g·K, higher than that of PUA obviously. When the content of HPCP continuously increase, the PHRR and HRC continuously increase, which indicates the incorporation of HPCP alone into the PUA system significantly increase the degree of fire hazard. In a word, 2DOPO-DHBP or HPCP can produce a limited certain flame retardant effect in the PUA system and using them together can reached better flame retardant properties. As shown in Fig. 10, there is a sharp and high peak in the HRR curve of the PUA–20HPCP and PUA-10HPCP cured samples and the combustion time of PUA-10/20HPCP were shorter than that of PUA cured
sample. Consequently, HPCP may accelerate decomposition of the cured sample and greatly decrease the burning time, which is in agreement with the TGA tests. 4. Conclusion A series of FRPUA–nHPCP (n = 0, 10, 15, and 20) curable mixtures and cured samples have been prepared. For pure PUA, the surface drying time decreases from 4 to 1 s as the TPO content in the system increases from 2 to 5 wt.%. Even if the flame retardancy is later modified by combining 2DOPO–DHBP and HPCP, all of the mixtures have fast surface drying times within few seconds. FRPUA–nHPCP cured samples have better heat resistance and higher thermal stability than PUA cured sample. The PUA system using both 2DOPO-DHBP and HPCP can achieve better flame retardancy. In other words, there is a synergistic effect between HPCP and 2DOPO–DHBP in the PUA system. Data availability The raw data required to reproduce these findings are available to download from https://pan.baidu.com/s/12_M-ykTrMvRPFvZrP8qLHA. The processed data required to reproduce these findings are available to download from https://pan.baidu.com/s/1Y6ID8b09iWOedopPfiI9JA. Acknowledgments This work was supported by the Special Development Program of the Strategic Emerging Industries of Shenzhen City (JSGG20160331104855391). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
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