Microencapsulation of ammonium polyphosphate with melamine-formaldehyde-tris(2-hydroxyethyl)isocyanurate resin and its flame retardancy in polypropylene

Abstract

Microencapsulated ammonium polyphosphate with a novel melamine-formaldehyde-tris(2-hydroxyethyl)isocyanurate resin (MFT resin) was prepared by in situ polymerization. This microencapsulated ammonium polyphosphate was named MFT–APP. The MFT resin and MFT–APP were characterized by FTIR, XPS, TGA and SEM. Polypropylene (PP)/MFT–APP and PP/MFT–APP/THEIC were prepared. Their thermal stability and flame retardancy were investigated through TGA, limiting oxygen index (LOI), vertical burning tests and cone calorimeter tests. SEM was used again to observe the morphology of char residues. The results illustrated that MFT–APP and MFT–APP/THEIC had excellent flame retardant properties on PP. Furthermore, a water resistant test demonstrated that PP/MFT–APP/THEIC had better flame durability than PP/APP/THEIC. TGA-FTIR and XPS results revealed that the efficient flame retardancy of MFT–APP/THEIC was mainly attributed to the formation of a compact and thermostable intumescent char on underlying materials during a fire.

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1. Introduction

With the development of materials science, polymer materials have received high speed development. As one of the widely used polymer materials, polypropylene (PP) has outstanding features such as low price, low density, non-toxicity, easy processing and excellent mechanical properties, so it can be used in electrical, building, transport applications and as general household materials.^1,2 However, its inherent high flammability and melt dripping drawbacks limit its application range. Therefore, it is essential to endow PP with good flame retardancy.³

In recent years, intumescent flame retardants (IFRs) have drawn much attention due to their halogen-free, low toxicity, low smoke production and anti-dropping characteristics.^4,5 Generally, a typical IFR system is composed of three components: an acid resource, a charring agent and a blowing agent.^6–8 The acid resource is generally inorganic acid or precursor of acid, for example, ammonium polyphosphate (APP).⁹ During the combustion process, APP can release the NH₃ and the inorganic acid, and then there is a esterification reaction between the inorganic acid and the charring agent. At last it will form a intumescent charred layer cover on the surface of the polymer materials.^10–12 During materials processing, workers find that APP is easily exuded to the surface of materials, resulting in a decrease of flame-retardant efficiency.^13,14 Furthermore, APP has a low compatibility with polymer materials, affecting mechanical properties of polymeric composites.^15,16 Modification of APP with surfactants or microencapsulating with water-insoluble polymers are effective methods to improve water resistance and the polarity of APP.^17,18 Melamine-formaldehyde (MF) resin is widely used as coating materials for modification. For example, Wu¹⁹ studied the flame retardancy of microencapsulated ammonium polyphosphate with MF resin on PP, he found that the particles' size and water absorption of MFAPP was decreased. Yang²⁰ studied the thermal stability and the flame retardancy of PP/MF-APP/pentaerythritol (PER) system, he found that MF-APP was beneficial for improving the thermal stability and flame retardancy of PP/MF-APP/PER composites. However, researchers also found that APP or MF-APP used alone in PP cannot reach UL-94 V-0 rating at 30% dosage.^19,21 Incorporating an effective charring agent into the coating materials will be a promising route.

Tris(2-hydroxyethyl)isocyanurate (THEIC) was mainly used to prepare the polyester-based heat-resisting insulating paint. THEIC also could be a modifier for MF resin which could improve the flexibility and cohesiveness of MF resin.^22–24 As a derivative of triazine compound, THEIC and its derivatives could be served as the charring agent. Bourbigot²⁵ found that there was a synergistic effect between APP and THEIC. Chen²⁶ studied the advantages and disadvantages of THEIC when it was served as a charring agent. Yuan²⁷ and Chen²⁸ synthesized a novel charring agent tris(2-hydrooxyethyl)isocyanurate terephthalic acid ester, this charring agent exhibited high flame retardant efficiency on polylactide (PLA) and PP. Theoretically, introducing THEIC into MF resin as the coating materials for APP will be a novel route for improving the water resistance, thermal stability and flame retardancy of APP. In this work, MFT resin was prepared by in situ polymerization and it was characterized by Fourier transform infrared (FTIR). Then MFT resin was used as the coating material for APP. The structure and thermal property of MFT–APP were characterized by FTIR, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and scanning electron microscope (SEM). The MFT–APP and MFT–APP/THEIC were added into PP, measurements such as TGA, LOI, vertical burning test (UL-94) and cone calorimeter test were used. The morphology of char residues were observed by SEM. Water resistance of the flame retardant composites was investigated. TGA-FTIR and XPS were used to analysis the thermal degradation of flame retardant composites.

2. Experimental section

2.1. Materials

Polypropylene (PP, isotactic, T30 s, melt-flow rate: 2.3 g/10 min) was supplied by SINOPEC Maoming Company, China. Ammonium polyphosphate (APP) with average particle size 10 μm was purchased from Shandong Shi'an Chemical Co., Ltd. THEIC was purchased from Changzhou Lantian Chemical Co., Ltd. Melamine was obtained from Sinopharm Chemical Reagent Co., Ltd. Formaldehyde solution (mass fraction 36%) was obtained from Meryer (Shanghai) Chemical Technology Co., Ltd. Anhydrous sodium carbonate (Na₂CO₃) was obtained from Tianjin Zhiyuan Chemical Reagents, China. Acetic acid was purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received.

2.2. Preparation of microencapsulated APP

Preparation of the MFT resin pre-polymer. 37% formaldehyde solution and melamine (with a mole ratio of 3 : 1) were added into a four-neck bottle with a stirrer. The mixture was adjusted to pH 8–9 with 10% Na₂CO₃ solution, heated to about 95 °C and kept at that temperature for 0.5 h. When the mixture turned into transparent, the suitable amount of THEIC was put into the mixture and kept at 95 °C for another 1 h. The MFT pre-polymer solution was prepared. Fig. 1 shows reaction scheme.

Fig. 1 The reaction scheme of MFT resin pre-polymer.

Preparation of microencapsulated APP. APP (100 g) was first dispersed in 200 ml ethanol. Then suitable amount of MFT pre-polymer solution (coating rate is 30%) was added into the suspension, the pH of the mixture was adjusted to 4–5 with acetic acid. The resulting mixture was heated at 80 °C for 2 h. Then, the mixture was cooled to room temperature, filtered, washed with distilled water, and dried at 100 °C. The MFT–APP powder was finally obtained.

2.3. Preparation of flame-retardant PP samples

All samples were prepared via melt compounding at 180 °C in an internal mixer (SU70, Suyan Technology Company, China) with a rotate speed of 20 rpm for 8–10 minutes for each sample. The resulting composites were dried in an oven, and then molded into testing bars with plate vulcanization machine for fire property characterization.

2.4. Characterization

FTIR spectra of the samples were recorded on a Bruker-Vertex 70 FTIR spectrometer in the range of 400–4000 cm⁻¹ by using KBr pellets. The thermogravimetry analysis (TGA) tests were carried out on Diamond TG/DTG (PE, America) at a heating rate of 10 °C min⁻¹ from room temperature to 800 °C with air atmosphere, sample weight was kept within 10 mg. The X-ray photoelectron spectroscopy (XPS) spectra were recorded with a VG ESCALAB MK II spectrometer using Al Kα excitation radiation (hν = 1253.6 eV). Water contact angles were measured with a JC2000C2 contact angle goniometer (Shanghai Zhongchen Powereach Company, China) by the sessile drop method. Limiting oxygen index (LOI) was carried out in an HC-2 oxygen index meter (Jiangning Analysis Instrument Co., China). The vertical burning tests were conducted according to the UL-94 test standard (ASTM D3801) with a test dimensions of 130 × 13 × 3 mm³. The morphology of the residual char obtained from the LOI test was examined by means of field emission scanning electron microscopy (JSM-6360). The surface of residual char was sputter-coated with a gold layer before examination. The combustion properties were evaluated by a cone calorimeter. All samples (100 × 100 × 3 mm³) were exposed to a Stanton Redcroft cone calorimeter under a heat flux of 50 kW m⁻² according to ISO-5660 standard. Tensile tests were completed in accordance with the procedures in GB/T 1040-1992 at a crosshead speed of 50 mm min⁻¹. Flexural properties were carried out in accordance with the procedures in GB/T 9314-2000 at a crosshead speed of 2 mm min⁻¹. Thermogravimetry-Fourier transform infrared spectroscopy (TGA-FTIR) was performed using a SDT Q600 V20.5 Build 15 instrument that was interfaced to a Nicolet IS10 FTIR spectrometer. About 5.0 mg of the sample was put in an alumina crucible and heated from 40 to 800 °C. The heating rate was set as 20 °C min⁻¹ (air atmosphere).

3. Results and discussion

3.1. Characterization of MFT–APP

3.1.1. Chemical structure of MFT resin. FTIR spectra of MF resin and MFT resin were shown in Fig. 2. For MF resin, the absorption peak at 3332 cm⁻¹ was attributed to the stretching vibration of N–H and O–H, or coupled absorption of hydrogen bonds. The peak at 2943 cm⁻¹ was attributed to the C–H stretching vibration of –CH₂ groups. The absorptions of 1589, 1500, and 1360 cm⁻¹ were due to the ring vibration of MEL. The spectrum of MFT resin revealed not only well-defined absorption peaks of MF resin, but also the characteristic bands of THEIC. There were two obvious absorption peaks at 1713 cm⁻¹ and 766 cm⁻¹, which could be found in the spectrum of MFT resin. They were assigned to the –C=O stretching vibration and triazine ring skeleton vibration. The appearance of these characteristic peaks in THEIC indicated the successful incorporation of THEIC into the MF resin.

Fig. 2 FTIR spectrum of MF resin and MFT resin.

3.1.2. Thermal stability of MF and MFT resin. The TG and DTG curves of MF and MFT resin are shown in Fig. 3. MF and MFT resin show a similar decomposing behavior. There were two main decomposing processes for them in air atmosphere: 50–100 °C and 350–400 °C. We supposed the first weight loss is primarily due to water vaporization. The second weight loss is correspond to the structural decomposition of the resins. The maximum weight loss rate of MF resin is larger than MFT resin. The DSC results for MF and MFT resin are shown in Fig. 4. For them, there is a main endothermic peak at 90 °C, which is correlated with the water vaporization. With the rising of temperature, endothermic situation is declining. Though there is a some small up and downs, we think the lowest point (about 150 °C) may be the curing temperature for MFT resin.

Fig. 3 TG and DTG curves of MF and MFT resin.

Fig. 4 DSC of MF and MFT resin.

3.1.3. Chemical structure of MFT–APP. The FTIR spectra of APP and MFT–APP were shown in Fig. 5, the spectrum of MFT resin was added as a contrast. The typical absorption peaks of APP included 3223 cm⁻¹ (N–H), 1256 (P=O), 1069 cm⁻¹ (P–O asymmetric vibration), 1020 cm⁻¹ (symmetric vibration of PO₂), 881 cm⁻¹ (P–O asymmetric stretching vibration), and 800 cm⁻¹ (P–O–P). For the MFT–APP, the main absorption peaks appeared at 3204, 1554, 1419, 1255, 1069, 1020, 880 and 800 cm⁻¹. The typical absorption peaks of APP were well verified in the structure of MFT–APP. Meanwhile, the peak at 1554 cm⁻¹ and 1419 cm⁻¹ were assigned to the ring vibration of MEL from the MF resin. It was clear that not only the absorption peaks of MFT resin but also the characteristic bands of APP appeared on the FTIR spectra of MFT–APP. Above results proved that APP was well coated by the MFT resin.

Fig. 5 FTIR spectrum of APP and MFT–APP.

The XPS spectrum of APP and MFT–APP was shown in Fig. 6. The peaks located at 134 eV and 192 eV were assigned to the typical chemical shifts of P2p and P2s in APP. For MFT–APP, the intensities of peaks mentioned above decreased sharply, meanwhile the intensities of the C1s peak increases greatly. The intensities of the N1s peak did not increase greatly like the C1s ones, which were attributed to the structure of THEIC in the MFT resin. The growth and decline relationship among these typical chemical shifts eloquently proved that APP was well coated by the MFT resin.

Fig. 6 XPS spectrum of APP and MFT–APP.

Fig. 7 shows the surface morphologies of APP and MFT–APP. The surface of APP particle was very smooth. After APP was microencapsulated by MFT resin, the MFT–APP presented a comparably rough surface. It was evidence that APP was well coated by the MFT resin. As we know, water contact angle tests could evaluate the surface hydrophobicity of solid powder. As shown in Fig. 7, the water contact angle of APP is only 9°. This indicated that pure APP is hydrophobic with high surface energy and easily attacked by the influence of water or moisture. However, due to the formation of the weak polar MFT resin microcapsules on the surface of APP, the hydrophile of MFTAPP is greatly decreased. In this condition, the water contact angle of MAPP reaches 45°. When MFT–APP particles are incorporated into polymer matrix, the compatibility between MFT–APP and polymer materials will be improved.

Fig. 7 SEM of APP and MFT–APP. (a1) APP × 200, (a2) APP × 500, (a3) APP × 3000. (b1) MFT–APP × 200, (b2) MFT–APP × 500, (b3) MFT–APP × 3000.

Table 1 shows the solubility of APP and MFT–APP. The solubility of APP was 0.55 g/100 ml H₂O at room temperature (25 °C). After APP particles were microencapsulated by the MFT resin, the solubility of MFT–APP was decreased to 0.3 g/100 ml H₂O. The solubility was decreased about 45.5% after APP was microencapsulated. The results proved that APP was well microencapsulated by MFT resin.

Table 1 Solubility of APP and MFT–APP

Sample	Solubility (100 g ml^−1 H2O)
APP	0.55
MFT–APP	0.30

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The TG and DTG curves of APP, MFAPP and MFT–APP were shown in Fig. 8 and the corresponding data were listed in Table 2. There were two decomposing processes for APP in air atmosphere: 220–460 °C and 460–800 °C. The temperatures with the maximum mass loss rates during the two steps are 330 °C and 607 °C, respectively. The weight loss of the first stage (220–460 °C) should be attributed to the elimination of NH₃ and H₂O in the thermal degradation process of polyphosphate. The weight loss of the second stage (beyond 460 °C) was attributed to the release of phosphoric acid, polyphosphoric acid, and metaphosphoric acid with APP decomposition. However, MFAPP and MFT–APP showed different degradation behavior compared with APP at the whole process. There were about three degradation stages for MFAPP and MFT–APP. The onset decomposition temperature (T_5%) and the first maximum weight loss temperature (T_max1) were shifted to lower temperature. At the first degradation stage, MFAPP and MFT–APP decomposed faster than APP, which can be attributed to the low thermal stability of the MF resin and MFT resin. The weight loss might be caused by the release of the nonflammable gases from the resin. For the second and third degradation stages, it was the results of joint action between APP and the resin. The residual of the resin and the phosphorus-containing compounds would inhibit APP further decomposition. From data listed in Table 2, it was clear that the thermal stability of MFT–APP was better than others in the high temperature. As mentioned above, THEIC can reacted with APP, introducing THEIC into the MF resin would be helpful for improving the thermal stability of APP. Moreover, MFT–APP had about 34.9% residue after 800 °C, whereas the residue for APP and MFAPP at this temperature was just 14.5% and 24.2%, respectively. It is clear that MFT–APP presented better thermal stability than APP and MFAPP in the high temperature.

Fig. 8 The TG and DTG curves of APP, MFAPP and MFT–APP.

Table 2 The data of TG and DTG test of APP, MFAPP and MFT–APP

Sample	T5%/°C	Tmax1/°C	Tmax2/°C	Tmax3/°C	Umax%/min	Residual
						600 °C/%	700 °C/%	800 °C/%
APP	320.4	330	—	607	−8.13	49.2	23	14.5
MFAPP	199.8	310	405	549	−3.23	36.9	29.2	24.2
MFT–APP	199.8	308	391	524	−3.19	42.2	37.6	34.9

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3.2. Flame retardancy and burning behaviors

3.2.1. LOI and UL-94 tests. The LOI and UL-94 tests were listed in the Table 3.

Table 3 The LOI and UL-94 test results of the samples^a

Sample	PP (%)	APP (%)	MFAPP (%)	MFT–APP (%)	THEIC (%)	MFT–APP:THEIC	LOI (%)	UL-94
a N.R = no rating.
PP0	100	0	0	0	0	0	17.8	N.R
PP1	70	30	0	0	0	0	21	N.R
PP2	70	0	30	0	0	0	30.5	N.R
PP3	70	0	0	30	0	0	32	V-0
PP4	70	0	0	15	15	1	—	—
PP5	70	0	0	20	10	2	35	V-0
PP6	70	0	0	22.5	7.5	3	36	V-0
PP7	70	0	0	25	5	5	35.5	V-0
PP8	70	0	0	27	3	9	34	V-0
PP9	70	22.5	0	0	7.5	0	34.3	V-0
PP10	75	0	0	12.5	12.5	1	—	—
PP11	75	0	0	16.7	8.3	2	32.5	V-0
PP12	75	0	0	18.75	6.25	3	33	V-0
PP13	75	0	0	20.83	4.17	5	32	V-0
PP14	75	0	0	22.5	2.5	9	31	V-0

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It could be found that pristine PP (PP0) was highly combustible and could not reach any UL-94 rating. After 30 wt% APP and MFAPP was added into the PP (PP1 and PP2), the LOI value increased to 21% and 30.5% respectively. However, PP1 and PP2 still could not pass the UL-94 V-0 test owing to the scarcity of charring agent. When 30 wt% MFT–APP was added to PP (PP3), PP3 successfully passed V-0 level of UL-94 test. The reason may contain two aspects, one was the MFT resin released water vapor and NH₃ gases which would reduce the concentration of air and make the char swell to form protective char. The other reason can be attributed to the structure of THEIC in the MFT resin, which would enhance the synergistic effect between the phosphorus and nitrogen. Though the MFT–APP/PP could pass the UL-94 V-0 test, the LOI value still be low. In order to further improve the flame retardancy, THEIC was added into the PP/MFT–APP composites, which was served as a charring agent. From the data listed in Table 3, it was clear that the LOI values of the ternary composites were increased greatly. At the same additive level (30%), the highest LOI value could obtain 36%. When the total addition of the MFT–APP and THEIC was decreased into 25%, all ternary composites still could obtain UL-94 V-0 rating. These results demonstrated that MFT–APP and MFT–APP/THEIC would remarkably improve flame retardant performance of PP, and THEIC showed a good synergistic effect with MFT–APP.

3.2.2. Thermal stability of flame retardant PP. The thermal degradation behavior of PP0, PP2, PP3 and PP6 was shown in Fig. 9 and the corresponding data were listed in Table 4. It could be seen that only one step weight loss (300–450 °C) for pure PP (PP0) and the maximum mass loss rate occurred at about 322.5 °C. There was a similar thermal decomposition behavior between PP2 and PP3, they had an approximate onset decomposition temperature (T_5%) and similar decomposing stages. The thermal stability of PP3 was better than PP2. When THEIC was incorporated into the PP/MFT–APP, the thermal decomposition behavior of the flame retardant composites changed greatly. Before temperature 630 °C, PP6 had a lower thermal stability than PP2 and PP3, which could be attributed to the low stability of THEIC. At 287 °C, the first maximum weight loss was occurred for PP6. The first maximum weight loss rate of PP6 was larger than PP2 and PP3. The reason might be THEIC could promote the decomposition of MFT–APP. After temperature 630 °C, the thermal stability of PP6 was better than PP2 and PP3. The reason could be attributed to the synergistic effect between THEIC and MFT–APP. The synergistic effect between THEIC and MFT–APP was beneficial for forming a swell char layer, and this swell char layer would protect the below materials from further decomposition.

Fig. 9 The TG and DTG curves of PP0, PP2, PP3 and PP6.

Table 4 The data of TG and DTG test of PP0, PP2, PP3 and PP6

Sample	T5%/°C	Tmax1/°C	Tmax2/°C	Tmax3/°C	Umax%/min	Residual
						600 °C/%	700 °C/%	800 °C/%
PP0	257.6	322.5	—	—	−16.95	0	0	0
PP2	261	276	370	611	−9.75	16.3	2.69	0.88
PP3	258.5	266	357	607	−7.77	19.8	6.72	5.42
PP6	250.9	287	339	569	−7.5	16.7	8.44	6.5

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The residual charred layer was important for the flame retardancy of polymer materials. The images of residual chars for PP1, PP2, PP3 and PP6 were shown in Fig. 10. As seen in Fig. 10a, no residual chars could be seen on the surface of PP1. It was attributed to lack of char formation during combustion. Heat and volatiles could easily penetrate the surface during the burning process. For PP2 and PP3, a typical intumescent charring layer still could not be seen, which was due to insufficient charring agent introduced into the system. Though PP3 could pass the UL-94 V-0 test, the incorporation of THEIC into the MF resin was helpful for improving the thermal stability of the MFT–APP. The situation was improved greatly when THEIC served as a charring agent. For PP6, a compact and swelling structure could be observed. Furthermore, there were lots of folds on the surface, which could act as a skeleton to strengthen the surface layer.^29,30

Fig. 10 SEM image of the char from PP1, PP2, PP3 and PP6. (a1) PP1 × 500, (a2) PP1 × 1000. (b1) PP2 × 500, (b2) PP2 × 1000. (c1) PP3 × 500, (c2) PP3 × 1000. (d1) PP6 × 500, (d2) PP6 × 1000.

This result provided further evidence that THEIC had a good synergistic effect with the MFT–APP, homogeneous and stable intumescent char layer formed, leading to better flame retardancy.

The digital images of the residual chars for PP0, PP3 and PP6 were shown in Fig. 11. Nothing was left for PP0 which was coincided with inherent properties of flammability and melt dripping of PP. However, situation was improved to some extent after 30 wt% MFT–APP was added into the PP. For PP3, some swollen residual char could be seen, though the degree of the expansion was not ideal. After THEIC was added into the flame retardant composites, a compact and dense char layer was observed on the external surface of PP6. This swollen charred layer could be effectively prevented the inside pyrolysis products from transmitting into the flame zone.

Fig. 11 Digital photographs of residues for PP0, PP3 and PP6 after cone calorimeter test.

The cone calorimeter test was considered to be an effective way to simulate the actual fire burning behavior and the measurements provided several useful burning parameters.^31–33

The HRR and THR curves of PP0, PP3 and PP6 were shown in Fig. 12 and the interrelated data were listed in Table 5. The HRR of PP0 showed a significant increase after ignition up to the peak value (pHRR) and decreased sharply toward the end of burning. When 30 wt% MFT–APP or MFT–APP/THEIC were added into PP, the HRR of the mixture decreased sharply. The pHRR of PP3 and PP6 decreased to 375 kW m⁻² and 232 kW m⁻², respectively. The values were reduced by 69.7% and 81.2% compared to that of pure PP, respectively. The times to pHRR (t_pHRR) of PP3 and PP6 were 169 s and 192 s, respectively. They were significantly delayed in comparison with PP0. For PP0, the largest THR value was 123.7 MJ m⁻². In contrast, the curves of PP3 and PP6 showed a much lower value in THR plot, about 116.4 MJ m⁻² and 100.7 MJ m⁻², respectively. For pSPR and TSP, the relative relationship among PP0, PP3 and PP6 was similar to that of pHRR and THR. At the end of burning, the residual weight of PP3 and PP6 still had 17.4% and 20.7%, respectively. The above results demonstrated that MFT–APP and the intumescent system based on MFT–APP and THEIC could effectively improve the flame retardancy of PP.

Fig. 12 Cone calorimeter test curves of PP0, PP3 and PP6 (a) heat release rate curves (HRR) (b) total heat release curves (THR).

Table 5 Combustion parameters obtained from cone calorimeter

Sample	TTI (s)	pHRR (kW m^−2)	tpHRR (s)	THR (MJ m^−2)	pSPR (m^2 s^−1)	TSP (m^2)	Residual (%)
PP0	33	1238	149	123.7	0.154	15.78	0
PP3	24	375	169	116.4	0.058	14.45	17.4
PP6	28	232	192	100.7	0.04	13.38	20.7

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In order to investigate water resistance of the flame retardant composites, PP/APP/THEIC and PP/MFT–APP/THEIC were soaked in water at 70 °C for different time and their flame retardancy properties were tested, including LOI and UL-94 tests. The results were listed in Table 6. PP/APP/THEIC only obtained a V-2 rating after 24 h, with melt dripping occurred during combustion process. After 48 h, no rating could be obtained for it. However, PP/MFT–APP/THEIC could still obtain V-0 rating after 96 h soaking time. The results indicated MFT–APP/THEIC has a good water resistant property, the flame retardant durability of PP/MFT–APP/THEIC was improved greatly compared with PP/APP/THEIC composite.

Table 6 Effect of water soaking time on LOI value of PP/IFR composites

Water treated time	PP/APP/THEIC		PP/MFT–APP/THEIC
	UL-94 V-rank	LOI	UL-94 V-rank	LOI
24 h	V-2	30.2%	V-0	33%
48 h	NR	28.5%	V-0	32%
72 h	NR	27.5%	V-0	32%
96 h	NR	26.9%	V-0	31%

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The mechanical properties of flame retardant composites are presented in Fig. 13, including tensile properties and flexural properties. It can be seen that the addition of APP into PP decreases sharply the tensile strength and flexural strength of PP1, with a falling range about 35.1% and 21.4% respectively. When MFT–APP is added into PP, the falling range is not so much. The falling range for PP2 is only 18.9% and 0% respectively. So there is a improvement of compatibility when MFT–APP is added into PP compared with APP. When THEIC is added into the composites, the situation between PP6 and PP9 is not so obvious. The falling ranges of tensile strength and flexural strength of PP9 are 43.2% and 14.3% respectively. The results for PP6 is 29.7% and 14.2%. We think the reason for the above situation between PP6 and PP9 is that THEIC can react with APP or MFT–APP in the processing. So the superiority of MFT–APP in compatibility can not be shown.

Fig. 13 The mechanical properties of PP0, PP1, PP2, PP6 and PP9.

3.3. Thermal degradation analysis

TGA-FTIR spectra were used to analyze the gas produced during the thermal degradation process of polymers. The three-dimensional TGA-FTIR spectrum and characteristic spectra obtained at different temperature of PP6 is shown in Fig. 14. In Fig. 14a, the small band observed at 3400–4000 cm⁻¹ can be attributed to the O–H vibration of H₂O; the intense bands at 2800–3100 cm⁻¹ are assigned to saturated hydrocarbons and saturated alkane; another intense bands at 2300–2350 cm⁻¹ are assigned to CO₂; the bands at 2250–2300 cm⁻¹ are assigned to CO which is transformed into CO₂ with the rising of temperature; the bands at 1600–1900 cm⁻¹ are due to the structures containing aromatic rings; the peak near 1370 cm⁻¹ is assigned to gas containing the P–O–C structure; the sharp peaks at 930–960 cm⁻¹ referred to the N–H vibration of NH₃. In Fig. 14b, FTIR spectra of volatile pyrolysis products evolved at different temperature are shown. There is almost no infrared signal below 200 °C, indicating that the MFT–APP/THEIC mixture do not decompose under this temperature, thus it is thermostable for melting-blend with PP (170–210 °C). With further increasing the temperature, the release of H₂O, CO₂, and NH₃, etc. can be detected. The reason for these gases production is that MFT–APP can release NH₃ to form poly(phosphorus acid) and these acid can react with THEIC to release H₂O. When the temperature increased to 400 °C, a maximum signal intensity is observed. These can be explained by that, with increasing temperature, a swollen char is formed and prevented the polymer from decomposing. Afterward, the signal intensity of the pyrolysis products decreased gradually, indicating that the decomposition rate of the composite is slowed down by the promoted char formation. When the temperature above 500 °C, the intensity of CO₂ is increasing gradually. The reason may be the promoted char layer is decomposing.

Fig. 14 TGA-FTIR spectra of pyrolysis products during the thermal degradation of PP6.

In order to further analyze chemical structure of the char layer, XPS spectra of C1s, O1s for PP6 after combustion is studied. The XPS spectra are shown in Fig. 15. The C1s spectrum is presented in the Fig. 15a. The peaks at 284.6 eV and 285.8 eV are assigned to C–H and C–C in aliphatic species and C–O in ether, hydroxyl groups, C–O–P in hydrocarbonated phosphate, and/or C–N in heterocyclic compounds, respectively. For the O1s spectrum (Fig. 9b), two peaks are found at around 532.5 and 533.5 eV. It is impossible to distinguish inorganic and organic oxygen because the O1s band is structureless. The peaks at 532.5 and 533.5 eV are assigned to =O in carbonyl or phosphate groups and –O– in C–O–C, C–O–P, or C–OH groups, respectively. The char in form of these structures for PP6 can effectively prevent the substrate materials from burning, thus leading to an efficient flame retardancy.

Fig. 15 (a) C1s and (b) O1s XPS spectra of the residue of PP6.

4. Conclusions

A microencapsulated ammonium polyphosphate with melamine-formaldehyde-tris(2-hydroxyethyl)isocyanurate resin was successfully prepared by in situ polymerization. The water resistance and thermal stability of MFT–APP was improved greatly compared with APP. When 30 wt% MFT–APP was added into PP, UL-94 V-0 rating was achieved with a LOI value of 32%. To further improve the flame retardancy, THEIC was added into the PP/MFT–APP composites, a remarkable improvement of flame retardation could be found. The maximum LOI values increased to 36% and an excellent swell char layer was formed. The results of cone calorimeter test indicated that the addition of MFT–APP or MFT–APP/THEIC significantly decreased the HRR, pHRR, THR, pSPR and TSP. The results of water resistance test illustrated that MFT–APP was beneficial for improving the flame durability of PP/IFR composites. The TGA-FTIR and XPS results indicated that PP/MPP/TBM could form a compact and thermostable char residue, which could effectively protect the substrate material from burning. All the results illustrated that microencapsulating APP with this MFT-resin was a promising way to endow the PP/IFR with good water resistance and flame retardancy.

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