The Effect of Soluble Ammonium Polyphosphate on the Properties of Water Blown Semirigid Polyurethane Foams

Soluble ammonium polyphosphate (SAPP) is employed to prepare flame retardant semirigid polyurethane foam (SPUF) using water as blowing agent. The flame retardant property of SPUF is evaluated by limiting oxygen index (LOI) and horizontal burning test. Also the thermal degradation mechanism is studied by TG and Fourier transform infrared (FTIR). The results show that, with the increase of the content of SAPP, flame retardant property of SPUF improves obviously as the LOI value increases and the horizontal burning rate decreases. And residual char is increased up to 20% with 19wt% SAPP. Moreover, the mechanical property of SPUF is enhanced dramatically.


Introduction
Polyurethane foam is regarded as a versatile polymeric material for its comparatively excellent properties such as low density, high specific strength, great insulation, large specific surface area, and good sound-absorbing performance. Polyurethane foam is more easily burned compared to other foams since there are many easily decomposing urea bonds in it [1]. Thus, it is necessary to improve the flame retardant property of polyurethane foam [2,3].
Ammonium polyphosphate (APP), as inorganic phosphorus flame retardant with nitrogen-phosphorus synergistic intumescent effect [17][18][19][20], has the advantages of thermal stability and lasting effect. APP can also improve the mechanical properties of the material, so it is often used with other flame retardants [21][22][23], and the most common APP flame retardant studied by researchers is form II, of which the polymerization degree is greater than 1000 [24][25][26][27]. In this paper, the water blown SPUF is synthesized only with soluble ammonium polyphosphate (SAPP) with a low polymerization degree. Our aim is to study the effect of SAPP on the thermal degradation, the flame-resistant, and the mechanical properties of the SPUF.

Thermogravimetric Analyses (TG)
. TG were carried out at 10 ∘ C/min under nitrogen flow (flow rate: 40 mL/min) with a NETZSCH STA 449F3 (Germany) thermogravimetric analyzer. In each case, the mass of the sample powder was 2 to 5 mg, and the samples were heated from 25 ∘ C to 800 ∘ C at the rate of 10 ∘ C per min.

Fourier Transform Infrared.
Fourier transform infrared (FTIR) spectra were obtained by TENSOR 27 FTIR spectrometer (Germany Bruker) in the range of 600-4000 cm −1 with transmission infrared ray test.    According to the results of horizontal burning tests, the pure SPUF does not show self-extinguishing until the content of APP reaches up to 7 wt%. SPUF/APP 15∼19 achieves UL-94-HF-1 rating and the highest LOI reaches 24.5% in SPUF/APP 19 . These results illustrate that SAPP improves the flame retardant properties of SPUF effectively. However, the addition of SAPP will constantly increase the viscosity of the reaction system and extend the period of foaming time dramatically. So the content of SAPP should be no more than 19%. A serious dropping phenomenon was observed during the burning process of SPUF. It produced a combustible flow of tar. And the dropping phenomenon had appeared before the polymer formed a charred layer. APP as an effective intumescent fire retardant with the synergistic effect of nitrogen-phosphorus plays the role of acid and gas source in the process of foam combustion [18], promoting the formation of porous carbonaceous chars and preventing the droplets formation. The flame retardant mechanism can be explained where the thermal decomposition products of APP, ammonia, and water act as a physical barrier diluting the concentration of combustible gases and oxygen to exert a flame retardant effect [20]. Figure 1 shows TG and DTG curves of SPUF/APP 19 , pure water blown SPUF, and APP. APP has four stages, the exact temperature ranges of these stages are as shown in Table 2. The first stage is corresponding to the release of ammonia, carbon dioxide, and polyphosphoric acid. The second occurs around 235∼ 275 ∘ C in which the weight loss rate is up to 40%, and the third stage is approximately within 275∼360 ∘ C; both of them are caused by the dehydration and continual cross-link reaction of polyphosphoric acid. The last stage within 360∼440 ∘ C is caused by the cross-linked structure of polyphosphoric acid and thermal decomposition of other residual products. The residual char of APP is about 25%. There are two thermal decomposition stages of SPUF, corresponding to the decomposition of amino formic acid ester at the C-O bond and polyol, respectively [28]. Temperature of maximal weight loss rate ( max ) is at 370 ∘ C roughly, and residual char is 15% at 800 ∘ C. Compared to SPUF, the initial decomposition temperature ( in ) and max of SPUF/APP 19 decrease, resulting from the fact that phosphoric acid catalyzes the degradation of ammonia ester bond [29,30]. The catalytic reaction between APP and PU is shown as (1) in Figure 2. The residual char is increased to 20%. It illustrates that APP can promote the formation of residual char. The char layer is nonflammable, which plays a role of good insulation and prevents the oxygen from entering the underlying unburnt material, and inhibits the combustion to some degree. The flame retardant property of water blown SPUF improved remarkably with the increase of the mass of residual char [1,23].

Fourier Transform
Infrared. FTIR spectra of pure SPUF and SPUF/APP 19 calcined at 350 ∘ C, APP at room temperature, are shown in Figure 3. The (c) spectrum shows the typical characteristic peaks of 1176 and 3180 cm −1 assigned to N-O and N-H bonds of APP. By comparison, the (a) 350-SPUF/APP 19 spectrum does not show the N-O and N-H bonds and three new characteristic peaks 1253, 1012, and 880 cm −1 assigned to P=O symmetric stretching vibration peak, P-O symmetric stretching vibration peak, and P-O asymmetric stretching vibration peak appear. It shows that APP has degraded at 350 ∘ C producing phosphoric acid and other products. By analyzing (b) 350-pure SPUF and (a) 350-SPUF/APP 19 , it can be seen that (b) spectrum shows the characteristic peaks 1541 and 1125 cm −1 of polyurethane; however, (a) spectrum only has the 1610 cm −1 stretching vibration peak of aromatic ring C=C; it concludes that 350pure SPUF does not degrade while 350-SPUF/APP 19 appears with the thermal decomposition due to the catalysis of APP pyrolysis product such as phosphoric acid. The analysis in this section also confirms the results of TG. Figures 4 and 5 show the digital pictures and SEM images of SPUF/APP 19 and SPUF after LOI test. The residual char structure of SPUF shrinks more severely and there is more foam uncovered by char, as shown by the red circle in Figure 4. As a result, the interphase is burned more severely for pure SPUF. It is also observed from SEM images that the residual char structure of SPUF sticks together, while the SPUF/APP 19 remains at the original state basically. Figure 6 shows the SEM micrographs of SPUF/APP 19 and SPUF. As for SPUF, the size of bubble holes is uniform, cell-walls are smooth, and there are few ruptures. As for the SPUF/APP 19 , SAPP is in a compatible state with the foam system, serving as the bubble nucleuses [17]; thus, the number of bubble holes increases and the size of bubble holes reduces. With generation and growing of the bubble holes, APP scatters inside the cell-walls and turns into the support structure as shown in Figure 6.

Mechanical Properties. Water blown SPUF contains both
closed and open cells. The apparent density can reflect cellular structure in some degree. Table 3 illustrates that the apparent density of SPUF/APP increases when W APP is more than 11%. The Shaw hardness increases with increasing of APP. The compression strength and modulus of compression improve slightly compared with SPUF, when W APP is more than 15%; both of them increase significantly. It can be explained that APP acts as the bubble nucleus; a small part scatters onto the cell-walls, resulting in a growing number of bubble holes and reducing size of bubble holes. When W APP is more than 15%, the filling effect of APP dominates the interaction, and APP acts as the receptor of the external load, resulting in the enhancing of the compression strength dramatically.  Advances in Materials Science and Engineering

Conclusions
The flame-retardant of SPUF is enhanced by soluble ammonium polyphosphate with low molecular weight. The SPUF becomes self-extinguished as the content of APP is higher than 7 wt%. SPUF/APP 15∼19 achieves UL-94-HF-1 rating. The highest LOI reaches 24.5% when the content of APP is 19 wt%. The decomposition temperature of SPUF decreases by the catalytic reaction between phosphoric acid of APP and imino group of PU, while the residual char reaches up to 20%. The physic-mechanical properties analysis shows that a few agglomerate particles of APP have certain damaging influence on morphology of bubble holes. The apparent density of SPUF/APP 5-11 remains unchanged first, and then a gradual uptrend appears when W APP is more than 11%. The Shaw hardness presents an increasing trend with the addition of APP. Both the compression strength and modulus of compression present an increasing variation with the addition of APP compared to SPUF, when W APP is more than 15%; both of them increase significantly.

Conflicts of Interest
The authors declare that they have no conflicts of interest.