Investigations of Flame-Retarded Thermoplastic Poly(imideâurethane

Article pubs.acs.org/IECR

Investigations of Flame-Retarded Thermoplastic Poly(imide− urethane)s with Intumescent Flame Retardants Qiheng Tang, Rongjie Yang, Yun Song, and Jiyu He* School of Materials Science and Engineering, Beijing Institute of Technology, National Engineering Research Center of Flame Retardant Materials, Zhongguancun South Street 5, Haidian District, Beijing 100081, P. R. China ABSTRACT: A series of flame-retarded thermoplastic poly(imide−urethane)s (TPIUs) with either ammonium polyphosphate (APP) alone or APP and dipentaerythritol (DPER) have been prepared. The fracture strength and strain of TPIU composites decrease with increasing amount of intumescent flame retardant (IFR), as shown by tensile tests. Thermogravimetric analysis shows that the initial decomposition temperature (T5%) of TPIU composites decreases with increasing IFR loading. The flame retardancies of these TPIU composites have also been tested according to the limiting oxygen index (LOI) and UL-94 standards. The results have indicated that the LOI value of TPIU/APP is a little higher than that of TPIU/(APP + DPER) with the same loading. Furthermore, 50% APP alone or APP + DPER incorporation into TPIU allowed attainment of the UL-94 V-0 standard. Details of fire behavior, such as time to ignition, heat release rate, peak of the heat release rate, total smoke release, and total heat release, have been tested by means of a cone calorimeter. Moreover, the char residues of TPIU composites after cone calorimetry have been investigated in detail by scanning electron microscopy, Fourier transform infrared, and energy-dispersive X-ray reflectometry.

1. INTRODUCTION Thermoplastic polyurethane (TPU) is one of the most versatile of the engineering thermoplastics known as thermoplastic elastomers. It is synthesized by the reaction of polyether- or polyester-based diols and diisocyanates, followed by the introduction of a chain extender to form macromolecules.1 TPUs have wide applications in a number of different industrial sectors, such as construction, automobiles, biomaterials, and infrastructure cables because of their excellent physical properties, such as exceptional processability, durability, and ability to withstand external stresses, providing water, chemical, and solvent resistance as well as ease of fabrication and relatively low cost.2−6 It is well-known that conventional TPUs exhibit poor heat resistance, thermally degrading at temperatures above 200 °C.7−9 Polyimides are important heterocycle-based polymers with superior durability and remarkable thermal stability.10−14 Therefore, in this work, the thermal stability of TPU has been improved by introducing thermally stable heterocyclic imide groups into the main chains through copolymerization to synthesize thermoplastic poly(imide−urethane)s (TPIUs). However, the applications of TPIUs are limited by their flammability. Therefore, emphasis on the development of technologies to promote flame retardancy and create flameretardant materials has recently increased. Presently, the most popular approach for improving the flame retardancy of TPUs is the incorporation of flameretardant additives into the matrix by simple mechanical mixing at the compounding stage. Generally, flame-retardant additives are mostly based on halogens (e.g., bromine or chlorine), phosphorus, and nitrogen. Halogen-based additives act as very effective flame retardants for TPUs. These halogen compounds impart flame retardancy through vapor-phase mechanisms based on free-radical scavenging.15,16 However, halogen-based flame retardants have several disadvantages, such as a tendency © 2014 American Chemical Society

for bioaccumulation, the potential for corroding metal components, and the ability to generate toxic and corrosive hydrogen halides during combustion.17 Thus, the European Community has proposed to restrict the usage of halogen-based flame retardants. As a consequence, more attention has been paid to developing new environmentally friendly halogen-free flame retardants to replace the halogenated ones. Nowadays, some of the most effective halogen-free flame retardants are intumescent flame retardants (IFRs) containing phosphorus and nitrogen. Recently, both the academic and industrial communities have become interested in halogen-free IFRs for their multifold advantages, including low toxicity, low smoke, low corrosion, no corrosive gas, and no dripping during a fire.16,18−21 Generally, intumescent systems consist of three main substances: an acid source (e.g., a phosphorus-containing substance), a carbon source (e.g., a polybasic alcohol), and a gas source (a nitrogen-containing substance). During thermal degradation or combustion, a phosphorus-containing substance in an intumescent system can promote the formation of an expanded carbonized layer on the surface of a TPU.17 At high temperature, the char layer formed on the surface provides resistance to heat and mass transfer, imparting good heat insulation to the underlying TPU. Additionally, the char layer hinders diffusion of air/O2 to the TPU itself, that is, the site of combustion. Nitrogen-containing substances can produce nitrogen oxides, providing an inert diluent in the flame.17,22,23 Therefore, the underlying material is protected from the flame. Received: Revised: Accepted: Published: 9728

February 14, 2014 May 5, 2014 May 12, 2014 May 12, 2014 dx.doi.org/10.1021/ie500651t | Ind. Eng. Chem. Res. 2014, 53, 9728−9737

Industrial & Engineering Chemistry Research

Article

ray spectrometer (EDXS EX-350) in the scanning electron microscope. 2.3. Preparation of TPIU Composites. The right amount of PTMG was added to a flask containing quantitative melt MDI, and the reaction was stirred for 1 h at 80 °C. Later the appropriate amount of chain extender (PMDA) was dissolved in DMF and then added to the above flask. The mixture was rigorously stirred under a nitrogen atmosphere for a further 15 h. Finally, APP or APP/DPER was added to the mixture and stirred for another 1 h. The liquid mixtures were poured into a mold and the mold put into an oven to evaporate DMF. The molar ratio of MDI, PTMG, and PMDA is 2:1:1, and the total loadings of IFRs are shown in Table 1.

To date, however, there have been few reports on the use of IFRs in TPIUs. Therefore, in the present study, fire-resistant TPIU formulations containing different amounts of IFRs have been studied. The mechanical properties, thermal stabilities, and fire performances of the prepared TPIUs and composites have been evaluated by means of tensile testing, thermogravimetry, and combustion tests, namely, limiting oxygen index (LOI), UL-94 vertical burning, and cone calorimetry tests. Additionally, the macroscopic and microscopic structures of the residues formed after cone calorimetry tests have been examined visually from digital photographs and by scanning electron microscopy (SEM) and spectroscopically by energydispersive X-ray spectroscopy (EDXS) and Fourier transform infrared (FTIR) spectroscopy to obtain insight into the mechanism of fire resistance.

Table 1. Compositions of the TPIU/APP/DPER Composites

2. EXPERIMENTAL SECTION 2.1. Materials. 4,4′-Diphenylmethane diisocyanate (MDI) was provided by Nippoh Polyurethane Industry Co. Ltd. and was used as received. Poly(tetrahydrofuran) [PTMG; numberaverage molecular weight (Mn) = 2000 g/mol] was purchased from Aladdin Reagent Co. Ltd. (China). Pyromellitic dianhydride (PMDA) was purchased from Sinopharm Chemical Reagent Co. Ltd. Ammonium polyphosphate (APP) was purchased from Zibo Saida Flame Retardant New Materials Co., Ltd. Dipentaerythritol (DPER) was provided by Jiangsu Liyang Ruiyang Chemical Co., Ltd. Dimethylformamide (DMF) was purchased from Beijing Chemical Reagents Co. PTMG were dried in a vacuum oven at 110 °C for at least 2 h prior to use. 2.2. Characterization and Measurements. FTIR spectroscopy analysis was conducted with a Nicolet 6700 IR spectrometer. The spectra were collected through 32 scans with a spectral resolution of 4 cm−1. The thermogravimetric (TG) analysis was performed with a Netzsch 209 F1 thermal analyzer at a heating rate of 10 °C/min under a nitrogen atmosphere, and the temperature ranged from 40 to 600 °C. The dumbbell-shaped specimens of TPIU were cut through using a metallic die and then stored at 23 °C and 50% relative humidity for at least 12 h before testing. The specimens had a width of 10 mm in the neck and a thickness of 1−2 mm. Tensile tests were carried out with a cross-head deformation speed of 500 mm/min on a Universal Testing Machine Tension, according to the ASTM D 638-91 method. For each plaque of PIUs, at least five specimens were tested; the average tensile strength was with at least a 95% confidence level for statistical significance. Vertical burning tests were performed according to the UL94 standard with samples of dimensions 125 × 12.5 × 3.2 mm3. In this test, the samples were classed as V-0, V-1, and V-2 or unclassified according to their behavior (dripping of the burning material and burning time). Combustion experiments were performed with a cone calorimetry device (Fire Testing Technology Co., Ltd.). The samples with dimensions 100 × 100 × 3 mm3 were exposed to a radiant cone (50 kW/m2). SEM experiments were performed with a Hitachi S-4800 scanning electron microscope. Samples (in char analysis section) for SEM were the residue after the cone calorimetry test and sputtering the surface with Au. The C, O, P, and N elements in the residue were verified by an energy-dispersive X-

sample designation TPIU TPIU/20% TPIU/30% TPIU/40% TPIU/50% TPIU/20% TPIU/30% TPIU/40% TPIU/50%

(APP (APP (APP (APP APP APP APP APP

+ + + +

DPER) DPER) DPER) DPER)

TPIU (wt %) 100.0 80.0 70.0 60.0 50.0 80.0 70.0 60.0 50.0

APP (wt %) DPER (wt %) 0.0 15.0 22.5 30.0 37.5 20.0 30.0 40.0 50.0

0.0 5.0 7.5 10.0 12.5 0.0 0.0 0.0 0.0

3. RESULTS AND DISCUSSION 3.1. Mechanical Properties. The mechanical properties of TPIU composites were evaluated by tensile testing. Data pertaining to the tensile strength and elongation at break are summarized in Figure 1, which give clear insight into the

Figure 1. Effect of the loading contents on the mechanical properties.

mechanical properties. It can be observed that the tensile strength and elongation at break show a significant variation compared with those of pure TPIU. Furthermore, they decrease monotonically with increasing IFR loading. Moreover, it should be noted that, at the same loading, both the tensile strength and elongation at break for TPIU/APP are higher than those for TPIU/(APP + DPER). In order to determine the different effects of APP alone and APP + DPER on the mechanical properties of TPIU composites, the microscopic structures of the samples were observed by SEM. The fracture surface morphologies of TPIU and its composites are illustrated in Figure 2. Representative SEM images of fracture surfaces of TPIU, TPIU/50% APP, and TPIU/50% (APP + DPER) are shown in Figure 2A−C. No exposed particles were observed at 9729

dx.doi.org/10.1021/ie500651t | Ind. Eng. Chem. Res. 2014, 53, 9728−9737


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Figure 2. SEM micrographs of TPIU and its composites: (A) TPIU; (B) TPIU/50% APP; (C) TPIU/50% (APP + DPER).

Figure 3. SEM micrographs of APP (A) and DPER (B).

observed that APP + DPER particles are sharply separated from the matrix and are not closely packed, giving rise to many fragile sites in the TPIU matrix. This accounts for the inferior mechanical properties of TPIU/(APP + DPER) compared to those of TPIU/APP. According to the above results, it can be concluded that the incompatibility between the filler and matrix is detrimental to the mechanical properties of the composites and that the greater the incompatibility, the poorer the mechanical properties. 3.2. TG Analysis. TG curves of pure TPIU, APP, and DPER as well as those of the TPIU/APP and TPIU/(APP + DPER) composites are presented in Figure 4. Relevant thermal decomposition data, including T5%, defined as the temperature

the fracture surface of TPIU (Figure 2A). However, for TPIU/ 50% APP (Figure 2B) and TPIU/50% (APP + DPER) (Figure 2C), many particles, with average diameters in the range 5−15 μm, were exposed at the fracture surfaces because of their poor compatibility with the TPIU matrix. This incompatibility has a detrimental effect on the mechanical properties of TPIU composites. To investigate the micromorphologies of APP and DPER particles, they are characterized by SEM and shown in Figure 3A,B. It can be seen that APP particles are grainy; however, DPER particles are sawdust-like. Compared with TPIU/50% APP, APP + DPER particles in TPIU/50% (APP + DPER) show inferior compatibility with a TPIU matrix, as is evident from Figure 2C. In TPIU/50% (APP + DPER), it can be 9730



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pure TPIU is 425 °C, which is higher than that of this kind of conventional TPU (Tmax = 407 °C).24 The mechanism of degradation of pure APP has been widely investigated.25−27 The degradation of APP consists of two main steps. The first step with 5% weight loss is at 312.5 °C. The volatile products in this step are mainly NH3 and H2O, and at the same time, a highly cross-linked poly(phosphoric acid) (PPA) layer is formed. The second step is the main decomposition process, which occurs above 500 °C. PPA begins to evaporate and/or dehydrate to P4O10, which sublimes. For a more intuitive interpretation, Scheme 1 gives the main thermal degradation of APP. TPIU composites with APP showed much lower T5% and Tmax values than those of pure TPIU, indicating that APP has an obvious influence on the thermal stability of TPIU. In addition, the initial thermal stabilities (T5%) of TPIU/APP composites were seen to decrease with increasing APP content. This is due to the fact that acid produced by APP decomposition can catalyze and accelerate decomposition of polyurethane, resulting in faster degradation of TPIU,30−32 and the degradation mechanism is shown in Scheme 2. Furthermore, TPIU composites with APP + DPER showed much lower T5% and Tmax values than those of TPIU composites with APP alone. This is because of the inferior thermal stability of DPER. From Figure 4 and Table 2, it can be seen that T5% of DPER is 267 °C, which is lower than those of TPIU and APP, thus resulting in faster thermal decomposition of TPIU/(APP + DPER). From Figure 4, it can be observed that, up to 550 °C, the residue increased with the amount of additive, whereas above 550 °C, the char showed the opposite trend. According to the aforementioned analysis, it can be known that TPIU can react with PPA to make an intumescent char. Increasing the amount of additive in the composite means that less TPIU can react with PPA and then form a smaller char layer. At the moment, excess APP can only decompose to volatile components. Thus, the higher the APP content, the greater the observed weight loss. 3.3. Flame-Retardant Properties. 3.3.1. Cone Calorimetry. Cone calorimetry is an effective approach for evaluating the combustion behavior of flame-retardant polymeric materials by investigating parameters such as the heat release rate (HRR), peak of the heat release rate (p-HRR), total heat release (THR), time to ignition (TTI), total smoke release (TSR), and mean specific extinction area (mean SEA). In order to investigate the effect of APP alone or APP + DPER on the

Figure 4. TG and DTG curves of pure TPIU, APP, DPER, and TPIU composites.

at 5% mass loss, Tmax, defined as the temperature of the maximum mass loss rate, and the char residues at 600 °C, are given in Table 2. Table 2. Thermal Decomposition Data of APP, DPER, Pure TPIU, and TPIU Composites sample designation pure TPIU APP DPER TPIU/30% TPIU/50% TPIU/30% TPIU/50%

(APP + DPER) (APP + DPER) APP APP

T5% (°C)

Tmax (°C)

char at 600 °C (%)

348 312 267 296 283 308 300

425 587 367 349 345 351 354

11.4 35.9 0.8 27.0 26.2 23.9 19.6

T5%: defined as the sample temperature at 5% weight loss. Tmax: defined as the decomposition temperature of the maximum weight loss rate.

In a nitrogen atmosphere, pure TPIU undergoes degradation between about 300 and 450 °C, showing a rapid weight loss of 80%, and the residue at 600 °C is 11.4%. This result shows that pure TPIU has a high charring yield compared to the conventional TPUs synthesized by MDI, PTMG, and 1,4butanediol (the residue at 600 °C is 1.9%).24 Moreover, Tmax of Scheme 1. Mechanism of Degradation of APP28,29

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Scheme 2. Possible Flame-Retardant Mechanism of TPIU with APP

5B shows the THR value for each of the samples, from which it is apparent that the THR value of TPIU was higher than those of TPIU/30% APP and TPIU/30% (APP + DPER). Evidently, the IFR not only reduced the p-HRR value but also decreased the THR value. This phenomenon can be attributed to two factors. The first is the dilution effect of ammonia and other noncombustible gases derived from decomposition of APP on the oxygen concentration surrounding the TPIU composite;31,33,34 the second is the abundant char residue formed during combustion of TPIU composites with the IFR. Parts A1, B-1, and C-1 of Figure 6 are digital photographs of the residues from pure TPIU, TPIU/30% APP, and TPIU/30% (APP + DPER). The photographs demonstrate that the pure TPIU resin was almost burnt out, whereas more coherent and dense chars were formed with the addition of IFRs. On the basis of the SEM images, there is an obvious difference between the morphologies of the residues from TPIU and its composites. As shown in Figure 6A-2, the char layer of pure TPIU had many holes and showed no expansion; this structure is conducive to gas diffusion and heat transfer, which makes the sample burn easily. On the contrary, for TPIU composites, a dense and compact char layer on the surface of the burning material creates a physical protective barrier. This barrier can prevent heat transfer between the flame zone and burning substrate and also limits oxygen diffusion to the substrate or ensures a less disturbing low volatilization rate, thereby

combustion of TPIU, the results for the composites TPIU/30% APP and TPIU/30% (APP + DPER) were taken as representative, and the relevant parameters are reported in Table 3. Table 3. Cone Calorimetry Data for TPIU and Its Composites sample designation TTI (s) p-HRR (kW/m2) THR (kW/m2) TSR (m2/m2) mean SEA (m2/kg) mean CO yield (kg/kg) mean CO2 yield (kg/kg)

TPIU

TPIU/30% (APP + DPER)

TPIU/30% APP

55 700 147 924 249 0.031

27 445 72 1314 483 0.052

26 464 98 1050 403 0.050

2.7

1.6

2.5

The HRR curves and p-HRR values of TPIU without and with different loadings are presented in Figure 5A and Table 3. It can be observed that pure TPIU burned rapidly from ignition to 85 s and then the curve developed into a slow rise with a pHRR value of 700.4 kW/m2. When APP or APP/DPER was added to TPIU, the p-HRR values of the flame-retarded TPIU composites were much lower than that of pure TPIU. Figure

Figure 5. Curves of the HRR (A) and THR (B) values for TPIU and its composites with time. 9732



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Figure 6. Digital photographs and SEM micrographs (×100 magnification) of char residues after cone calorimetry tests: (A) TPIU; (B) TPIU/30% APP; (C) TPIU/30% (APP + DPER).

protecting the underlying material from further burning and retarding pyrolysis of the polymer. The results indicate that IFRs impart excellent flame retardancy to TPIU. In particular, compared to those of TPIU/30% APP, the pHRR and THR values of TPIU/30% (APP + DPER) were reduced by 4.11% (from 464.1 to 445.0 kW/m2) and 27.0% (from 97.8 to 71.4 kW/m2), respectively. According to Figure 6B,C, it should be noted that the char layer from TPIU/30% APP exhibited different features compared to that from TPIU/ 30% (APP + DPER). The char from TPIU/30% APP appeared as a continual layer and was composed of many fibrous strips. However, the char layer from the TPIU/30% (APP + DPER) sample appeared to be more compact and rugged and was covered in small bumps because of esterification between PPA and DPER, which is consistent with the literature and confirmed by EDXS analysis in section 3.4.2. The possible esterification mechanism is shown in Scheme 3. This made it more efficient in protecting the substrate from fire, and hence this composite has a lower p-HRR value. This result confirmed the anticipated benefit of the synergistic effect between APP and DPER during char formation.

Moreover, the slope of the THR curve can be assumed to be representative of fire spread.36 It is clear that the flame spread of the samples is in the order TPIU > TPIU/30% APP > TPIU/30% (APP + DPER). Therefore, it can be inferred that the rate of flame spread is dependent on the density of the carbon layer, indicating that APP/DPER can remarkably enhance the flame-retardant properties of TPIU. This result further explains why the p-HRR value of TPIU/30% (APP + DPER) is lower that those of TPIU/30% APP and TPIU. TTI is used to determine the influence of a flame retardant on ignitability. The TTIs of TPIU/30% APP and TPIU/30% (APP + DPER) were measured as 26 and 27 s, respectively, considerably shorter than that of pure TPIU (55 s). There may be two reasons for this: (1) some small volatile molecules are produced from decomposition of APP;37 (2) decomposition of APP catalyzes and accelerates decomposition of pure TPIU and then produces some small volatile molecules. These small molecules result in the shorter TTI. The major requirement of a flame retardant is to prevent or delay flashover from the surface of a combustible material. Therefore, a flame retardant is not designed to prevent the polymer from ignition but to 9733



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Scheme 3. Possible Reaction Mechanism of Char Formation for APP and DPER28,31,35

reduce polymer flammability through their ability to form gaseous intermediates that scavenge flame-propagating free radicals (HO • and H • ), thereby inducing incomplete combustion. Products of incomplete oxidation are mainly responsible for smoke emission.39,40 For TPIU/30% (APP + DPER), the TSR and SEA values are lower than those for TPIU/30% APP. The synergistic effect between APP and DPER was further verified by the cone calorimetry results. The results of further investigations of the releases of CO2 and CO during combustion using a cone calorimeter are presented in Figure 8. Pure TPIU gave the highest CO2 production rate value but the lowest CO production rate value among the samples. This result corresponds well with the concept that phosphorus-based flame retardants can inhibit complete combustion of the polymer to CO2 and thus produce more CO.39,40 In the case of TPIU/30% (APP + DPER), the peak CO production rate value was higher than that of TPIU/ 30% APP. Moreover, from Table 3, it can be seen that the mean CO yield of TPIU/30% (APP + DPER) was higher than that of TPIU/30% APP, whereas the mean CO2 yield showed the opposite trend. This result further confirmed that the flame retardancy of APP combined with DPER was much better than that of APP alone. 3.3.2. LOI and UL-94 Vertical Burning Test. The LOI values and UL-94 vertical burning classifications for TPIU and its composites are listed in Table 4.

minimize the flame spread rate and prevent sustained burning.38 The TSR curves of TPIU and its composites are shown in Figure 7, and the relevant data are shown in Table 3. It can be seen that TPIU/30% APP and TPIU/30% (APP + DPER) produced more smoke than did pure TPIU. It has been reported that flame retardants containing phosphorus can

Figure 7. Curves of the TSR values for TPIU and its composites with time. 9734



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Figure 8. CO (A) and CO2 (B) production rates of TPIU and its composites with time.

Table 4. Results of LOI and UL-94 Tests for TPIU and Its Composites sample designation pure TPIU TPIU/20% (APP + DPER) TPIU/30% (APP + DPER) TPIU/40% (APP + DPER) TPIU/50%(APP+DPER) TPIU/20% APP TPIU/30% APP TPIU/40% APP TPIU/50% APP

UL-94 (3.2 mm)

LOI (%)

no, t1 > 30 s no, t1 > 30 s

none none

19.5 22.2

no, t1 > 30 s

none

23.4

no, t1 > 30 s

none

24.6

no, t1 = 1.0 s, t2 = 2.1 s no, t1 > 30 s no, t1 > 30 s no, t1 > 30 s no, t1 = 1.1 s, t2 = 3.8 s

V-0 none none none V-0

27.8 22.5 24.0 26.0 28.0

dripping (yes or no)

Figure 9. FTIR spectra of the char residues from pure TPIU, TPIU/ 30% APP, and TPIU/30% (APP + DPER) after cone calorimetry tests.

atoms at ν = 1620 cm−1, along with a broad peak at ν = 1116 cm−1, attributable to C−O−C stretching vibrations, and a new absorption at ν = 954 cm−1. The latter absorption may be assigned to a P−O−phenyl stretching vibration, as has been reported previously.41,42 P−O−phenyl structures in the char help to enhance its thermal stability. These results indicate that APP exhibits excellent flame-retardant effects not only in the gas phase but also in the condensed phase. In Figure 9, the most notable feature is the FTIR spectrum of the residue from TPIU/30% (APP + DPER), which shows several significant changes compared with that of the residue from TPIU/30% APP. The CC stretching vibration of bonds between polyaromatic C atoms at ν = 1620 cm−1 is clearly enhanced. This indicates that the use of a mixture of APP and DPER resulted in more aromatic compounds remaining in the char. It may be speculated that esterification between APP and DPER occurred in the condensed phase, which helped to retain more aromatic compounds containing C−O and P−O in the char. Consequently, this interaction could enhance the thermal stability of the products in the condensed phase during combustion of the flame-retarded TPIU composites. The superior thermal stability of the char could explain the good flame retardancy of TPIU/30% (APP + DPER) in the cone calorimetry test. 3.4.2. EDXS Analysis of the Char Residue. In order to confirm FTIR analysis of the condensed-phase products, exterior and interior layers of the chars derived from TPIU, TPIU/30% APP, and TPIU/30% (APP + DPER) were investigated by EDXS analysis. The concentrations of C, O, P, and N in these char layers are listed in Table 6. For pure TPIU and its composites, both the exterior and interior chars contained abundant C and O. For the composites, there was much P in the char, consistent with FTIR analysis.

In Table 4, for the two series of TPIU composites, the LOI values are seen to show an increasing trend with increasing additive content. Furthermore, melt dripping was not observed during any of the combustions. Subjected to the UL-94 test, only with 50 wt % APP alone or APP/DPER, the UL-94 classification could attain a V-0 rating. Generally, because APP/DPER serves as an IFR, and TPIU serves as a source of gases, it might have been expected to show a more efficient flame-retardant effect. In order to investigate the abnormal phenomenon, the LOI values of TPIU with different mass ratios of APP and DPER (the total loading of APP/DPER was 20% from APP:DPER = 1:1 to 5:1) were determined, and the results are shown in Table 5. It can be Table 5. Effect of Different Molar Ratios of APP/DPER on the LOI Values molar ratio LOI

1:1

1:2

1:3

1:4

1:5

21.8

22.0

22.2

22.5

22.5

observed that the LOI values increased with the APP content, indicating that APP used alone in TPIU is more beneficial in improving the LOI value than APP combined with DPER. 3.4. Mechanistic Discussion. 3.4.1. FTIR Analysis of the Char Residue. The FTIR spectra of the char residues obtained after the cone calorimetry tests are shown in Figure 9. The only absorbance of the residue from pure TPIU is a broad peak at approximately ν = 1620 cm−1, which indicates the formation of polyaromatic C atoms. Similar FTIR spectra were recorded for the residues from TPIU/30% APP and TPIU/30% (APP + DPER), featuring the same absorbance due to polyaromatic C 9735



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Notes

Table 6. EDXS Results of the Chars from TPIU, TPIU/30% APP, and TPIU/30% (APP + DPER)

The authors declare no competing financial interest.

■

element concn (%) pure TPIU

TPIU/30% APP

TPIU/30% (APP + DPER)

element

exterior

interior

exterior

interior

exterior

interior

C O P N

76.06 23.94 0.00 0.00

76.45 23.55 0.00 0.00

16.80 57.46 25.73 0.00

29.99 50.92 19.09 0.00

23.05 58.29 18.67 0.00

34.91 46.75 18.33 0.00

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For the char from TPIU/30% APP, the exterior layer had a higher P content than the interior char. For TPIU/30% (APP + DPER), however, the P contents in the exterior and interior chars were almost the same. This result is attributed to esterification, which could inhibit the migration of P-containing compounds to the surface. This inhibitory effect further contributes to protecting the inner matrix from fire, giving rise to better flame retardancy in the cone calorimetry test. It should be noted that no N was detected in either the exterior or interior char layer. Therefore, it can be concluded that the flame-retarding effect of N is only exerted in the gas phase.

4. CONCLUSION IFRs have been incorporated into TPIU as flame retardants in order to study their effect on the mechanical and thermal properties as well as their flame-retardant effects. The mechanical and thermal properties of TPIU composites have been investigated by tensile testing and TG. Gradual decreases in the yield stress and strain were observed with increasing IFR loading. SEM micrographs of TPIU composites suggested that the IFR filler was dispersed with particle sizes of 5−15 μm but that these particles were sharply separated from the TPIU matrix. Furthermore, the thermal stability of TPIU composites decreased with increasing IFR loading because APP catalyzes and accelerates decomposition of TPIU. Analyses of the TTI, HRR, p-HRR, and THR values for TPIU composites have indicated that the IFR imparts excellent flame retardancy to TPIU. Moreover, according to the HRR and THR curves, it can be concluded that APP combined with DPER has a better flame-retardant effect on TPIU composites than APP alone. This may be due to the stronger and denser char layer formed during combustion of TPIU/(APP + DPER). With the same loading, the LOI values of TPIU/APP composites are a little higher than those of TPIU/(APP + DPER) composites, although they have the same vertical flame rating. The chars from TPIU composites after the cone calorimetry tests were investigated by FTIR and EDXS. The results confirmed the formation of polyaromatic, C−O−C, and P−O−phenyl structures in the char layers. Moreover, the changes in the concentrations of C, O, and P indicate that esterification between APP and DPER may lead to more P being retained in the char. This would increase both the quantity and thermal stability of the char. These results provide insight into the flame-retardancy mechanisms of IFRs in the condensed phase.

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