Preparation and properties of modified ammonium polyphosphate synergistic with tris(2-hydroxyethyl) isocynurate for flame-retardant LDPE

2.1 Materials

LDPE: Grade 3518CB, relative viscosity 3.5 g/10 min, purchased from China Petroleum and Chemical Co.; ammonium polyphosphate (APP, type II, DP >1,000): average particle size 15 μm, purchased from Jinan Taixing Fine Chemical Co., Tris(2-hydroxyethyl) isocynurate (THEIC) was purchased from Changzhou Lantian Chemical Co., bis (2,2,6,6-tetramethyl-4-piperidyl) sebacate (HALS770) was kindly provided by Liansheng Technology Co. Ltd., China. Ethanol, acetic acid, emulsifier OP-10, vinyl triethoxysilane (A-151), TEOS, and triethanolamine: purchased from Sinopharm Chemical Reagent Co., China.

2.2 Synthesis of urea-formaldehyde resin prepolymers

About 5.0 g of urea and 10.0 g of formaldehyde solution were first placed in a 100 mL three-necked flask and then 50 mL of water was added. Next, the pH of the mixture was adjusted to 8.5 with a 10% solution of triethanolamine. The mixture was then immediately mechanically stirred in an oil bath at 80°C until it became clear. Finally, the clear prepolymer solution was cooled to room temperature to obtain the urea-formaldehyde resin prepolymer (UF).

2.3 Synthesis of UF-SiAPP

A continuous sol–gel method was employed to prepare APP encapsulated by A-151 and SiO₂ (Si-MAPP). The procedure was as follows: 50.0 g APP, 100 mL ethanol, and 50 mL deionized water were added to a 500 mL three-necked round-bottom flask, and the pH was adjusted to 10.0 with triethanolamine. Then, 1.0 g OP-10 was added, and the temperature was increased to 40°C. The reaction was stirred at 500 rpm for 10 min, and 12.5 g TEOS was added to keep stirring for 4 h. Then, 2.5 g A-151 was added, and the reaction was carried out at 60°C for 1 h. Next, the UF prepolymer solution was added dropwise to the suspension, and the pH was adjusted to 5.0 with acetic acid, and then the reaction was carried out at 80°C for 2 h. Finally, the mixture was cooled to room temperature, filtered, and washed three times with ethanol, and dried under vacuum at 100°C to obtain a white solid powder named UF-SiAPP. As a control, APP encapsulated in urea-formaldehyde resin (UF-APP) was also prepared (Figure 1).

Figure 1

Schematic illustration of the encapsulated process by in situ polymerization.

2.4 Preparation of flame-retardant LDPE composites

According to the formulation of IFR LDPE in Table 1, different samples of IFR LDPE were prepared by mixing APP, UF-APP, UF-SiAPP, THEICR, and LDPE in a mixer at 20 rpm for 15 min at 165°C. And then, the samples were hot pressed on a plate vulcanizer at 150°C for 10 min to obtain standard strips for testing.

2.5 Characterization

Field emission scanning electron microscopy analysis: A SIGMA-500 Field Emission Scanning Electron Microscope (Zeiss, Germany) was used to observe the surface morphology of the samples. Prior to analysis, the samples were dried and placed on a sample stage with conductive adhesive and gold sputtering treated under a vacuum.

FTIR analysis: The characterization was carried out using a WQF-520A Fourier Transform Infrared Spectrometer (Riley, North Branch, China). The tests were conducted at room temperature, mixing 50 mg KBr powder and an appropriate amount of sample by grinding and pressing method, with a scanning wave number range of 400–4,000 cm⁻¹, 32 scans, and a resolution of 4 cm⁻¹.

XPS analysis: The elemental composition of the flame-retardant particles was determined using an Escalab Xi + XPS (Thermo Fisher, USA).

Thermogravimetric analysis (TGA): The samples were tested using a TA Q-50 thermogravimetric analyzer (USA TA Instruments). Characterization was performed under a nitrogen atmosphere with a test temperature from 30°C to 800°C and a heating rate of 10°C·min⁻¹.

Contact angle test: The samples were tested using a JC2000D2 contact angle measuring instrument (Shanghai Zhongchen Digital Technology Equipment Co., Ltd., China).

The tensile strength and elongation at break were tested according to GB/T1040.2–2006, with a sample size of 2.0 mm × 4.0 mm (dumbbell shape) and a running speed of 5.0 mm·min⁻¹.

The LOI test was conducted using a 5801A fully automatic oxygen index tester (Suzhou Yangyi Volch Testing Technology Co., Ltd., China) according to the American National Standard ASTM D2863-97, with a sample size of 120.0 mm × 6.5 mm × 3.2 mm; the UL-94 vertical combustion test was conducted using a JT-UL94 model (Dongguan Jinte Instruments Co., Ltd., China), according to the American National Standard ASTM D635-77 with a sample size of 125.0 mm × 12.5 mm × 3.2 mm; the conical calorimetry test was carried out by using an FTT-iCone mini conical calorimeter (Occidental Instruments China Co., Ltd.) with a sample size of 100.0 mm × 100.0 mm × 4.0 mm and the sample was exposed to a radiation cone (45 kW·m⁻²), and the heat release rate (HRR), total heat release (THR), total smoke production (TSP), and smoke temperature (ST) were recorded.

3.1 Characterization of modified APP

The APP, UF-APP, UF-SiAPP, before and after modification were characterized by SEM, FTIR, XRD, and TGA, respectively.

It can be seen from Figure 2 that the surface of the unmodified APP is slightly rough and has obviously granular protrusions. However, the surface of the UF-APP becomes significantly smoother after being encapsulated by UF or co-encapsulated by UF and TEOS. In particular, a small number of silicon spheres can be found attached to the surface of UF-SiMAPP after being enclosed by TEOS.

Figure 2

SEM images of (a) APP, (b) UF-APP, and (c) UF-SiAPP.

Figure 3 shows the hydrophobicity of APP, UF-APP, and UF-SiAPP. The results imply that the unmodified APP and UF microencapsulated UF-APP are significantly hydrophilic, while the contact angle of UF-SiAPP is 135°, indicating that the hydrophobicity of UF-SiAPP after encapsulation with UF resin and TEOS and modified with A-151 is significantly increased. This is due to the fact that the surfaces of APP and UF-SiAPP contain a large number of NH 4 + and OH⁻ hydrophilic groups, whereas after being modified with TEOS and A-151, the surfaces exhibit a lot of hydrophobic silane groups and C═C double bonds; thus, the hydrophobicity is significantly enhanced, and the flame retardant has better compatibility with the LDPE matrix.

Figure 3

Contact angle test of (a) APP, (b) UF-APP, and (c) UF-SiAPP.

Figure 4 shows the FTIR spectra of APP, UF-APP, and UF-SiAPP. Among them, the characteristic peak at 3,106 cm⁻¹ is associated with the stretching vibration of N–H, the peak at 1,020 cm⁻¹ is attributed to the stretching vibration of PO₂ and PO₃, and the peak at 1,253 and 1,079 cm⁻¹ correspond to the symmetric stretching vibration of P═O and P–O, respectively. Furthermore, the asymmetric stretching vibration of P–O and P–O–P is illustrated by the peak at 886 and 802 cm⁻¹. All these are typical peaks of APP. After microencapsulation with UF resin, UF-APP has absorption peaks of 1,570, 1,340, and 3,208 cm⁻¹, which are attributed to the simultaneous stretching vibrations of NH 4 + and OH⁻. In addition to the characteristic absorption peaks of APP, when UF-APP was modified by TEOS and A-151, new peaks of 1,440, 1,106, and 1,250 cm⁻¹ are representative of the typical characteristic peaks of C═C, Si–O–Si, and Si–O–C, respectively, indicating the successful preparation of UF resin and the core-shell flame retardants microencapsulated and modified by TEOS and A-151.

Figure 4

FTIR spectra of APP before and after modification.

The thermal degradation behavior of APP, UF-APP, and UF-SiAPP in the N₂ atmosphere was analyzed by TGA. It can be seen in Figure 5 that the residual weight of UF-APP and UF-SiAPP increased compared with that of APP. The reason is that APP modified by TEOS and A-151 is encapsulated by silica gel, while SiO₂ cannot decompose during heating, thus contributing to the increase of residual weight. According to the TGA results, the introduction of TEOS and A-151 can effectively inhibit the thermal decomposition of APP.

Figure 5

Thermal degradation behavior of APP before and after modification in N₂ atmosphere.

Figure 6 shows the surface elemental analysis results of APP, UF-APP, and UF-SiAPP. The peaks located at 130.51 and 187.14 eV are attributed to P2p and P2s of APP, respectively. In UF-APP and UF-SiAPP, the intensity of these peaks is noticeably weaker, proving that APP is properly encapsulated. In contrast, the new peak at 102.05 eV is ascribed to the Si2p of TEOS and A-151. These changes in the above spectral peaks are mainly caused by the outer shell layer formed on the APP surface by UF, TEOS, and A-151. Furthermore, the elemental analysis of the three samples in Table 2 reveals that UF-SiAPP has large levels of C, N, and O as well as 1.67% Si, all of which are necessary to improve the flame retardancy of the composite material.

Figure 6

XPS Spectra of APP, UF-APP, and UF-SiAPP.

3.2 Flame-retardant properties of the LDPE composites

To evaluate the flammability of LDPE composites, the LOI and UL-94 vertical combustion tests were carried out. As can be seen from Table 3, pure LDPE is flammable, with many molten droplets during combustion, an LOI of 17.0% and no ranking in the UL-94 test. However, IFR can promote the formation of a carbon foam layer on the LDPE surface during combustion, which plays the role of heat insulation, oxygen barrier, smoke suppression, and anti-dripping, with a substantially higher LOI value. When the APP content reaches 30.0 wt%, the LDPE composites have a LOI value of 19.8, but still cannot achieve a V-0 level in the UL-94 test. This should be attributed to the poor carbon formation capacity due to the lack of carbon-forming agents in the IFR system. At the same content, the LOI of LDPE with UF-APP or UF-SiAPP is higher than that of APP and increases from 19.8% to 21.3% and 21.5%, respectively. Especially, the LDPE composites with UF-SiAPP attain a V-1 level in the UL-94 test, which is because the UF resin and silica layer cover the APP surface and play important roles: the former produces NH₃ was released at high temperatures to dilute the combustible gases and the oxygen in the air, and the latter promotes the formation of the carbon layer acting as an oxygen inhibitor and temperature barrier. Therefore, UF-APP and UF-SiAPP effectively improve the flame-retardant properties of the LDPE composites.

To further enhance the flame retardancy of the LDPE composites, APP and THEIC were used in combination to prepare the flame-retardant LDPE. As can be seen in Table 3, the LOI of the LDPE composites is further increased after adding THEIC. When the content of UF-SiAPP and THEIC is 20.0 and 10.0 wt%, respectively, the LDPE composites eventually have an LOI of 30.5 and reach a V-0 level in the UL-94 test. The reason is that the phosphoric acid decomposed by APP during the combustion process can promote the rapid formation of carbon layer by THEIC, thus improving the LOI of the LDPE composites, showing the synergistic effect between APP and THEIC, which plays a significant role in improving the flame-retardant properties of the LDPE composites.

3.3 Mechanical properties of the LDPE composites

Figure 7 shows the tensile strength and elongation at the break of the LDPE composites. Compared to pure LDPE, the mechanical properties of the LDPE composites have variedly diminished. This is due to the poor compatibility of IFR with the polymer, which causes a deterioration in the mechanical properties of the LDPE composites.

Figure 7

(a) Tensile strength and (b) elongation at break of the LDPE composites.

Compared to APP, the tensile strength of UF-APP and UF-SiMAPP modified LDPE shows a slightly enhanced, increasing from 11.45 to 12.65 MPa, and 11.76 MPa, respectively. And the elongation at break increases from 19.0% to 25.0%, and 24.0%, respectively. This is because UF-APP and UF-SiAPP modified by UF, TEOS, and A-151 are more compatible with the polymer and their dispersion in the LDPE matrix is better than that of APP. With the introduction of THEIC, the tensile strength of the LDPE composites is further improved. Compared with the APP/THEIC/LDPE composites, the tensile strengths of both the UF-APP/THEIC/LDPE and UF-SiAPP/THEIC/LDPE composites are decreased from the previous 14.28 to 12.15 MPa, and 13.55 MPa, respectively. And the elongation at break is decreased from 270% to 200%, and 250%, respectively, which is due to the antagonistic effect of UF and THEIC.

3.4 SEM analysis of the LDPE composites

Figure 8 shows the cross-sectional morphology of the LDPE composite. It can be noticed that the size of the dispersed phase (APP) in LDPE is larger, and the two-phase interface is obvious, which is due to the poor compatibility between LDPE and APP. Compared to LDPE1, the components of LDPE2 and LDPE3 show better compatibility with each other, and the two-phase interface becomes more blurred, indicating a better modification by UF and A-151. In addition, the compatibility between the components of LDPE5 and LDPE6 becomes slightly worse again compared to LDPE4, with the two-phase interface becoming clearer, indicating that the introduction of THEIC reduces the interaction between LDPE and APP. These phenomena are consistent with the results of flame retardancy and mechanical properties.

Figure 8

SEM Images of (a) LDPE1, (b) LDPE2, (c) LDPE3, (d) LDPE4, (e) LDPE5, and (f) LDPE6.

3.5 Thermal degradation behavior of the LDPE composites

For LDPE0, it is almost completely degraded at high temperatures, just as shown in Figure 9a. However, when flame retardant is added, the carbon residue of the LDPE composites all increases. LDPE1 is an APP/LDPE composite, which formed significantly more carbon residues than LDPE at high temperatures. LDPE2 is a UF-APP/LDPE composite, in which APP has been modified by UF and microencapsulated, and it has significantly fewer carbon residues than LDPE1, which is due to the large amount of NH₃ produced by UF at high temperatures. LDPE3 is a UF-SiAPP/LDPE composite and has higher carbon residues formed at high temperatures compared to LDPE2 because SiO₂ is not decomposed during combustion. LDPE4, LDPE5, and LDPE6 are the APP/THEIC/LDPE, UF-APP/THEIC/LDPE, and UF-SiAPP/THEIC/LDPE composites, respectively. The carbon residues of LDPE4 and LDPE5 further decrease following the addition of THEIC. However, LDPE6 contains the highest amount of residual carbon formed at high temperatures when compared to LDPE4 and LDPE5.

Figure 9

Thermal degradation behavior of the LDPE composites in N₂ atmosphere: (a) TGA and (b) DTG.

As can be seen from Figure 9b, the thermal degradation of LDPE0 undergoes only one stage in the N₂ environment, whereas the LDPE composites with flame-retardant addition all experience a three-step degradation process. Compared to LDPE0, the matrix decomposition temperatures of LDPE1, LDPE2, and LDPE3 are all pushed back by 30°C, and the maximum thermal decomposition rates are all delayed by 40°C. In particular, the decomposition temperatures of LDPE5 and LDPE6 are pushed back more significantly than that of LDPE4. The reason is that APP is encapsulated by a silica gel shell layer containing UF resin and vinyl silicon gel, and SiO₂ remains stable and less prone to decomposition at high temperatures.

3.6 Cone calorimetric testing of the LDPE composites

The combustion performance of the LDPE composites was investigated using a cone calorimeter. Figure 10a–d show the HRR, THR, TSP, and ST values versus time for LDPE0, LDPE1, LDPE2, LDPE3, LDPE4, LDPE5, and LDPE6 at a heat flow rate of 50 kW·m⁻². As can be seen in Figure 10a, pure LDPE burns rapidly and its HRR quickly reaches a peak of 892 kW·m⁻². Compared to LDPE0, the PHRR (peak of HRR) of the LDPE composites is decreased when 30.0 wt% of APP, UF-APP, or UF-SiAPP is introduced. To be specific, the PHRR will decrease from 395 to 327 kW·m⁻² for both LDPE2 and LDPE3 compared to LDPE1. This is because the LDPE composites can form a silica layer on the surface during combustion when it is mixed with modified APP, thus reducing the HRR of the composites. After the introduction of THEIC, the PHRR of LDPE5 and LDPE6 even decreases from 216 to 195 kW·m⁻², and 129 kW·m⁻², respectively, compared to LDPE4. The reason is that the phosphoric acid decomposed by APP during combustion can promote the rapid formation of the carbon layer by THEIC. A similar trend of THR is observed as shown in Figure 10b. At the end of combustion, the LDPE0 reaches the THR value of 166 MJ·m⁻², while that of LDPE1, LDPE2, and LDPE3 is 127, 104, and 115 MJ·m⁻², respectively. And compared to LDPE4, the THR of LDPE5 and LDPE6 decreases from 115 to 98 MJ·m⁻², and 95 MJ·m⁻², respectively.

Figure 10

CCT Analysis diagram of the LDPE composites: (a) HRR, (b) THR, (c) TSP, and (d) ST.

Furthermore, the TSP and ST values are very crucial parameters to evaluate flame-retardant properties. Figure 10c and d give the two values of the LDPE composites as a function of time during combustion. The TSP of pure LDPE can reach 16.3 m², while that of LDPE1, LDPE2, LDPE3, LDPE4, LDPE5, and LDPE6 all decreases. Compared to LDPE1, the TSP of LDPE3 decreases from 14.7 to 9.7 m². And when THEIC is introduced, that of LDPE4 decreases to 12.6 m². Further, by introducing UF, TEOS, and A-151 modified APP, that of LDPE5 and LDPE6 can decrease by 5.2 and 5.9 m², respectively. Figure 10d gives the ST value of the LDPE composites as a function of time during combustion. Compared to pure LDPE, all ST values of the composites are decreased. It should be noted that the introduction of UF, TEOS, and A-151 will further reduce the ST value. And in terms of the ST alone, the flame-retardant capacity of UF-SiAPP is more effective than that of UF-APP.

The residual carbon base of LDPE6 after conical calorimetric testing was explored using SEM. The results are shown in Figure 11. When UF-SiAPP and THEIC were added to the LDPE, the char layer obtained by combustion was very compact. There is a clear thin layer and no holes on the surface. The main reason for this phenomenon is that the flame-retardant releases NH₃, CO₂, N₂, and other non-flammable gases at high temperatures to expand the surface of the LDPE substrate, and the phosphoric acid and hypophosphoric acid produced by the decomposition of APP promote the dehydration into char and the silica gel covered by APP surface acts as a physical barrier to protect the substrate. It effectively prevents the exchange of oxygen and heat, restricts the diffusion of organic substances, and achieves a good flame-retardant effect.

Figure 11

Scanning electron micrograph of carbon residues of LDPE6 after conical calorimetric testing.

Table 4 and Figure 12 show the elemental content data, elemental diagrams, and EDS results of the carbon residue of LDPE6 after cone calorimetric testing. As shown in the figure, the char residue of LDPE6 after combustion contains C, O, Si, and P with 13.08, 51.13, 8.04, and 27.75 wt%, respectively. These elements were uniformly distributed on the surface of the LDPE6 carbon residue, and it is possible that UF-SiAPP released non-combustible gases such as NH₃ and CO₂ at high temperatures, so the content of C elements in the LDPE6 carbon residue decreased sharply and the content of N elements was 0 wt%. The presence of elements such as P and Si in the carbon residue once again proved that the flame retardant has a cohesive phase flame-retardant effect in LDOPE, and the phosphoric acid and hypophosphoric acid produced by the decomposition of APP at high temperatures promote dehydration into carbon as well as the silicon gel wrapped around the surface of LDPE on the surface of APP, thus playing a flame-retardant role.

Figure 12

Elemental map and EDS results of carbon residues of LDPE6 after conical calorimetric testing.

3.7 Flame-retardant mechanism

Based on the above experimental results, a possible flame-retardant mechanism was proposed, as shown in Figure 13. APP can produce NH₃ during the combustion of the LDPE composites and form a highly condensed polyphosphoric acid, which provides an acid source for the flame-retardant system and induces the carbonization of THEIC and partially decomposed LDPE to form foamed char, so as to achieve the flame retardancy of condensed phase. During the process, the volatile gases (NH₃, N₂) are acted as expansive agents for the formed char, which can play a gas phase flame-retardant role. The introduction of UF, TEOS, and A-151 into APP will not only supplement APP with a large amount of N₂ and produce enough NH₃ to dilute external combustible gases and O₂ in the air during combustion but will also cover the surface with a dense silica layer. The silica layer not only prevents or delays small molecules of combustible matter from entering the outside of the composites and coming into contact with the air to play a flame-retardant role but also promotes the formation of a char layer and increases its thickness.

Figure 13

Flame retardation diagram.