3.1 FTIR analysis and morphology
Scheme 1 presents the synthesis of PDMS and DOPO-DAM modified WPU (PD-WPU) using PDMS and DOPO-DAM as modifying agents. The FTIR spectra of PDMS modified WPU (P-WPU), DOPO-DAM modified WPU (D-WPU), PDMS and DOPO-DAM modified WPU (PD-WPU) are displayed in Fig. 1. It can be found that the FTIR spectra of all the samples display absorption at 3322 cm− 1 (N-H stretching vibration), 1536 cm− 1 (N-H deformation and C-N stretching vibrations), 1701 cm− 1 (C = O stretching vibration), signifying the formation of urethane groups[18, 19]. The absorption bands for -CH3 and -CH2- groups are confirmed by the peaks at 2946 cm− 1 and 2854 cm− 1. The peak at 1100 cm− 1 is assigned to the C-O-C bond in polyurethane. In PDMS modified WPU (P-WPU), it can be found that the characteristic absorption peak of -Si-CH3 in PDMS appears at 804 cm− 1[20]. The peak that appears at 750 cm− 1 in the FTIR spectrum of DOPO-DAM modified WPU (D-WPU) is related to the deformation vibration of the C-H bond of the benzene ring. At the same time, a small shoulder peak of the P = O bond is observed at 1257 cm− 1. Additionally, the characteristic absorption peaks corresponding to PDMS and DOPO-DAM can be observed in the spectrum of PDMS and DOPO-DAM modified WPU, indicating that PDMS and DOPO-DAM have successfully been embedded into the polyurethane chains.
TEM was used to observe the morphology of PDMS and DOPO-DAM modified dispersion particles. The TEM image in Fig. 2 shows that the shape of modified WPU dispersion particles is close to regular spherical. Furthermore, the diameter of modified WPU dispersion particles is around 70 nm.
3.2 Dispersion properties
The particle size of the dispersion is a key factor in the application and stability of the WPU. Figure 3 shows the average particle size and its distribution of WPU dispersions with different PDMS dosages. It is found that when the PDMS dosage is 3–6 wt%, the average particle size of PD-WPU dispersions are mainly concentrated between 65 and 100 nm, and the dispersions have a unimodal size distribution. However, when the PDMS dosage is over 6 wt%, the average particle size of the PD-WPU dispersion increased sharply. The reason is that the insertion of PDMS can decrease the ratio of hydrophilic to hydrophobic segments in polyurethane chains, which makes the polyurethane molecules in water difficult to disperse, reflecting the increase in particle size of the dispersion. In addition, the stability of the as-prepared WPU dispersion is determined by the centrifugation test. It can be found that when the PDMS dosage is 3–6 wt%, there is no sediment production in the dispersion. However, a small amount of precipitation appears in the dispersion when PDMS dosage is higher than 6 wt%.
Figure 4 displays the average particle size and its distribution of PD-WPU dispersions with different DOPO-DAM dosages. It is found that the particle size distribution of PD-WPU dispersions is unimodal distribution. Moreover, the average particle size increases with the addition of DOPO-DAM. While the DOPO-DAM dosage is lower than 6 wt%, the particle size of the dispersion increases slightly. However, when the DOPO-DAM dosage is higher than 6 wt%, the average particle size increases obviously. This is mainly because DOPO-DAM has a special biphenyl ring structure, which will produce greater steric hindrance and limit the movement of the molecular chain, thus making the polyurethane molecules difficult to disperse. Additionally, according to the centrifugation test, a small amount of precipitate appears in the dispersion when the DOPO-DAM dosage is greater than 6 wt%.
3.3 Mechanical properties
Mechanical strength is an important index to evaluate the comprehensive properties of materials. The tensile strength and elongation at break of PD-WPU films are shown in Fig. 5. When the DOPO-DAM dosage is less than 6 wt%, the tensile strength of the PD-WPU films is enhanced, which is attributed to the fact that the introduction of DOPO with rigid benzene ring structure in the polyurethane molecular chain increases the hard segment content and rigidity of the polymer chain. However, as the dosage of DOPO-DAM is more than 6 wt%, the tensile strength of PD-WPU films decreases with the increase of DOPO-DAM. The reason is that when excessive DOPO-DAM is introduced, the bulky and rigid DOPO units in its structure produce greater steric hindrance, which is unfavorable for regular arrangement and closely packed arrangement, leading to the decline of tensile strength. At the same time, the hardness of PD-WPU films increases with the addition of DOPO-DAM, resulting in a decrease in the breaking elongation of PD-WPU films.
3.4 Flame retardancy
The cone calorimeter test (CCT) has been widely used to evaluate the flammability performance of polymeric materials. To verify the flame retardancy of PD-WPU films in a real fire environment, a cone calorimeter was used to test the flame retardant properties of PD-WPU films. In this section, the obtained parameters from CCT, including peak heat release rate (PHRR), heat release rate (HRR), time to ignition (TTI), total heat release rate (THR), total smoke production (TSP), effective heat combustion (av-EHC), and carbon dioxide production rate (CO2P) are discussed to reveal the combustion behavior of PD-WPU films. The HRR, THR, EHC, and CO2P curves of PD-WPU films with different DOPO-DAM dosages are depicted in Fig. 6, and the corresponding data are listed in Table 1. It can be clearly seen from Fig. 6 and Table 1 that compared with pure WPU film, the introduction of 6 wt% DOPO-DAM into PD-WPU film reduces the PHRR, THR, TSP, av-EHC, CO2P to 317.41 kW/m2, 33.97 MJ/m2, 1.25 m2, 21.50 MJ/kg, 0.75% respectively, and increases TTI to 39 s. When the HFR-WPU film is ignited, the DOPO units at the side chain decompose at first and produce phosphoric acid or polyphosphoric acid. Under the action of phosphoric acid or polyphosphoric acid, an expanded carbon layer is formed to prevent the spread of heat and gas. Therefore, the formation of more stable and richer char promoted by DOPO-DAM can endow PD-WPU film with better flame retardancy. However, the flame retardancy of PD-WPU film decreases slightly with the DOPO-DAM dosage up to 8 wt%. The reason for this negative effect may be that the excess of DOPO-DAM creates a greater spatial site resistance, resulting in the precipitation of the dispersion and less DOPO-DAM content in the WPU molecular chain.
Table 1
Cone calorimetric test results of the PD-WPU films
DOPO-DAM dosage (wt%) | TTI (s) | PHRR (kW/m2) | THR (MJ/m2) | TSP (m2) | av-EHC (MJ/kg) | CO2P (%) |
0 | 28 | 460.49 | 48.87 | 5.67 | 34.26 | 0.95 |
4 | 36 | 436.02 | 39.32 | 4.18 | 28.48 | 0.94 |
6 | 39 | 317.41 | 33.97 | 1.25 | 21.50 | 0.75 |
8 | 38 | 329.01 | 36.56 | 3.40 | 27.50 | 0.77 |
The char residue of polymers obtained after combustion is an important parameter of flame retardants. The digital photographs of the residues of PD-WPU films with different DOPO-DAM dosages after the cone calorimeter test are shown in Fig. 7. It can be clearly observed from Fig. 7(a) that the pure WPU film is almost burnt out and only little residual char remains after burning. When the DOPO-DAM dosage increases to 6 wt%, a more rigid and compact char residue for HFR-WPU film is obtained, which can act as a shield to prevent heat, air and pyrolysis product transfer[21]. However, the residual carbon layer of PD-WPU film with 8 wt% DOPO-DAM decreases after combustion, indicating that its flame retardancy decreases, which was in accordance with the CCT results.
SEM spectra of the residues of the pure WPU film and PD-WPU film with 6 wt% DOPO-DAM after cone calorimeter test are shown in Fig. 8. It is clearly seen from Fig. 8(a) that the pure WPU film after combustion only has a small amount of carbon residue and cannot play an effective blocking role in the combustion process, resulting in poor flame retardancy. It is notable from Fig. 8(b) that PD-WPU film with 6 wt% DOPO-DAM forms an intumescent and continuous char layer that can be used as a barrier to protect the substrate. Simultaneously, the EDS images of the PD-WPU film with 6 wt% DOPO-DAM in Fig. 9 demonstrate that there are 15.53 wt% of P elements, 2.69 wt% of N elements and 5.23 wt% of Si elements uniformly distributed on the surface of the residual carbon layer. The better flame retardancy of PD-WPU film with 6 wt% DOPO-DAM is attributed to the synergistic effect of P, N, and Si during combustion. When PD-WPU film is ignited, the P element promotes dehydration and carbonization, the N element promotes the formation of a fluffy carbon layer and produces non-flammable gases, and the Si element produces a special insulating Si-O layer. Based on the above results, it can be concluded that the introduction of DOPO-DAM and PDMS endows PD-WPU with superior flame retardancy.
The flame retardant performance of PD-WPU coated polyester fabrics with different DOPO-DAM dosages are investigated by vertical burning and limiting oxygen index tests, as shown in Table 2 and Fig. 10. According to GB/T 17591 − 2006, there are two different classifications of flame retardant fabrics based on the vertical burning test: (1) B1 classification: damaged length ≤ 15 cm, after-flame time ≤ 5 s, after-glow time ≤ 5 s; (2) B2 classification: damaged length ≤ 20 cm, after-flame time ≤ 15 s, after-glow time ≤ 15 s[22]. It is clearly seen from Table 2 that as DOPO-DAM dosage increases from 0 wt% to 6 wt%, the after-flame time, after-glow time, damaged length and LOI value of PD-WPU coated polyester fabrics are changed from 26.2 s, 0.0 s, 15.8 cm and 20.6% to 7.0 s, 0.0 s, 8.5 cm and 26.2% respectively. Among them, the flame retardant performance of PD-WPU coated polyester fabric with 6 wt% DOPO-DAM is close to the B1 classification. This indicates that the addition of DOPO-DAM reduces the flammability of the polyester fabric. For PD-WPU coated polyester fabric with 8 wt% DOPO-DAM, the after-flame time, after-glow time, damaged length and LOI value are changed to 8.2 s, 0.0s, 9.2 cm and 24.18%. This is mainly because when the PD-WPU coated fabric is heated, the DOPO units produce phosphoric acid and polyphosphoric acid. The main function of these acids is to accelerate the fabric's dehydration and carbonization, forming a heat-resistant carbon layer. At the same time, element N decomposes into nonflammable gases to dilute the oxygen in the surrounding air. In addition, the Si element in PDMS produces a special insulating Si-O layer to promote the generation of a dense char layer. The foregoing results show that the PD-WPU coating can protect the underlying fabric from burning. This is consistent with the results presented by the CCT test.
Table 2
Vertical burning and limiting oxygen index test results of PD-WPU coated polyester fabrics with different DOPO-DAM dosages
DOPO-DAM dosage (wt%) | After-flame time (s) | After-glow time (s) | Damaged length (cm) | Ignition the cotton | LOI(%) |
0 | 26.2 | 0.0 | 15.8 | Yes | 20.6 |
4 | 20.1 | 0.0 | 13.4 | Yes | 21.6 |
6 | 7.0 | 0.0 | 8.5 | No | 26.2 |
8 | 8.2 | 0.0 | 9.2 | No | 24.1 |