Synergistic Effect between a Novel Char Forming Agent and

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Synergistic Effect between a Novel Char Forming Agent and Ammonium Polyphosphate on Flame Retardancy and Thermal Properties of Polypropylene Nana Tian, Xin Wen, Zhiwei Jiang, Jiang Gong, Yanhui Wang, Jian Xue, and Tao Tang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie401058u • Publication Date (Web): 19 Jul 2013 Downloaded from http://pubs.acs.org on July 20, 2013

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Industrial & Engineering Chemistry Research

Synergistic Effect between a Novel Char Forming Agent and Ammonium Polyphosphate on Flame Retardancy and Thermal Properties of Polypropylene Nana Tian†, Xin Wen†, Zhiwei Jiang†, ‡, Jiang Gong†, ‡, Yanhui Wang†, Jian Xue†, Tao Tang†*

†

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

‡

University of the Chinese Academy of Sciences, Beijing 100039, China

* To whom correspondence should be addressed. Tel: +86 (0) 431 85262004 Fax: +86 (0) 431 85262827 E-mail: [email protected] 1 ACS Paragon Plus Environment


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ABSTRACT: The performances of a novel char forming agent containing spiro and caged bicyclic phosphate (SBCPO) and its mixture with ammonium polyphosphate (APP) were investigated. The flame retardancy of PP/SBCPO/APP system was evaluated by LOI, UL-94 test and cone calorimeter test. The maximum LOI value of PP/IFR could reach to 30.5 and passed UL94 V-0 rating. An obvious synergistic effect could be observed between SBCPO and APP by TGA, DSC and SE study, which remarkably improved the flame retardant properties of PP. The FTIR analysis indicated that APP could accelerate the decomposition of SBCPO due to the interaction between them. The thermal degradation kinetics results of PP/IFR system by Kissinger method and Flynn-Wall-Ozawa method showed that the apparent activation energy (Ea) increased with the increase of content for flame retardants.

KEYWORDS: char forming agent; polypropylene; flame retardancy; thermal properties; kinetics

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1. INTRODUCTION Polypropylene (PP) has been widely used in many fields such as packaging, textiles, automobiles and electric industry due to its low cost, easy processing, low corrosion, excellent mechanical properties, and so forth.

1, 2

However, its inherent flammability and melt dripping problems have

restricted its widespread applications. Therefore, it is important and necessary to improve the flame retardancy of PP. 3, 4 In the earlier study, it was found that halogen-containing flame retardants with antimony trioxide as a synergistic agent were very effective flame retardant for PP. 5, 6 However, the biggest drawback is the generation of large amounts of smoke and toxic gases from these systems during the burning process, which leads to the environmental pollution. With the increase of the environmental awareness, intumescent flame retardants (IFRs), including some advantages such as low smoke, low corrosion and antidripping, have been more and more popular as replacements for the halogen-containing flame retardants in recent research. 7-11 A traditional IFR system is usually composed of three components, i.e. an acid agent, a char forming agent and a blowing agent. 12, 13 However, there are also some drawbacks of the traditional IFR additives. One of the obvious problems is that the use of polyols such as pentaerythritol as char formers in intumescent formulations for polyolefin is associated with moisture sensitivity because of the hydrophilic properties of hydroxyl group in char forming agent. The effective method to solve this drawback is to develop the new charring agent based on the pentaerythritol derivatives. 14-19

Sprio phosphates

20-22

and caged bicyclic phosphates

23-25

compounds are two typical

pentaerythritol derivatives and have aroused lots of interest due to their good thermal stability and excellent flame retardant efficiency. In our previous work,

26

we have synthesized a novel char forming agent, 3,9-Bis-(1-oxo-2,6,73 ACS Paragon Plus Environment


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trioxa-1-phospha-bicyclo[2.2.2]oct-4-ylmethoxy)-2,4,8,10-tetraoxa-3,9-diphospha-spiro[5.5]undecane 3, 9-dioxide, (SBCPO, Scheme 1) containing sprio and caged bicyclic phosphate functional group at the same time. This compound is hydrophobic in nature and insoluble in many common organic solvents such as ethyl acetate, acetone, ethanol, etc. It was found that the novel intumescent flame retardants consisting of SBCPO and melamine showed excellent flame retardancy and dripping resistance for PP. The purpose of this study is to evaluate the flame retardancy of the IFRs system of APP in combination with SBCPO used as char source in PP. To investigate the possible interactions between APP and SBCPO, the thermal degradation behaviors and the products were analyzed by TGA and FTIR, respectively. 2. EXPERIMENTAL 2.1. Materials. Polypropylene (PP, isotactic, Mw=3.0×105 g/mol, polydispersity=3.45) powder was supplied by Daqing Petrochemical Co., Ltd. Ammonium polyphosphate (APP) was purchased from Zhenjiang Xingxing Flame-retardant Co., Ltd (P wt %: 32%). 2,6,7-Trioxa-1-phosphabicyclo-[2,2,2]octane -4-methanol (PEPA) was supplied by Hunan MintChem Development Co., Ltd. SBCPO was synthesized by ourselves according to the literature. 26 The synthesis route is shown in Scheme 1. 2.2. Synthesis of SBCPO. To a three-neck 1 L glass flask with a thermometer, a reflux and a nitrogen inlet were charged PEPA (0.6 mol), POCl3 (0.6 mol) and 500 mL of acetonitrile. The reaction mixture was maintained at 80 oC for 20 h and then cooled to room temperature. After filtration and rotary evaporation, the intermediate was obtained. Then a mixture of pentaerythritol (0.3 mol) and the intermediate dissolved in 300 mL acetonitrile was added into a three-neck 1 L round bottom flask equipped with a reflux condenser and a constant pressure funnel. 0.6 mol Et3N was slowly dropwise added to the mixture over a period of 0.5 h at the room temperature and heated to reflux. Thereafter, the reaction was kept for 20 h at the same temperature. Successively, the 4 ACS Paragon Plus Environment

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reaction mixture was cooled to room temperature, then poured the mixture into distilled water and filtered. The white solid was washed with acetonitrile and distilled water. It was dried at 100 oC under vacuum to constant weight. The final product was obtained with a yield of 48.2 wt %. 2.3. Preparation of sample. All samples were prepared via melt compounding at 180 oC in a Haake batch intensive mixer (Haake Rheomix 600, Karlsruhe, Germany) with a rotor speed of 100 rpm; the mixing time was 10 min for each sample. The mixed samples were transferred to a mold and preheated at 185 oC for 5 min, then pressed at 14 MPa and then successively cooled to room temperature while maintaining the pressure to obtain the mixture sheets for further measurements. Before mixing, all the components were dried in a vacuum oven at 80 oC for at least 12 h. Samples were thermally treated at a heating rate of 10 oC/min in a muffle furnace in air. The solid residues were obtained when heated to the designed temperature, and maintained at each temperature for 10 min. Then the residues were mixed with KBr powder and pressed into discs for FTIR analysis. 2.4. Analysis. The limiting oxygen index (LOI) values were measured on an HC-2C oxygen index mater (Jingning Analysis Instrument Company, China) with sheet dimensions of 130 mm × 6.5 mm × 3 mm according to ISO 4589-1984. The vertical burning tests were tested according to the UL-94 test standard (ASTM D 3801) with the test specimen was 130 × 13 × 3 mm3. Cone calorimeter tests were performed using an FTT, UK device according to ISO 5660 at an incident flux of 50 kW/m2, and the size of specimens was 100 mm × 100 mm × 6.0 mm; all samples were burned in triplicate and the data were the average of three replicated tests. Thermogravimetric analysis (TGA) was carried out with a Q600 thermal analyzer (TA Co., New Castle, USA) from 50 to 800 oC at a heating rate of 10 oC/min under nitrogen with a flowing rate of 100 mL/min. The mass of the sample used is 5-10 mg. The calculated TGA and DTG curves were 5 ACS Paragon Plus Environment


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summed up by the TGA and DTG curves of the mixture ingredients weighted by their contents. 27 n

Wcal (T ) = ∑ xiWi (T )

(1)

i =1

Where xi is the content of compound i and Wi is TGA or DTG curve of compound i. The thermal degradation kinetic of PP mixture was investigated in the temperature range of 50-700 oC under a nitrogen flow with a heating rate of 5, 10, 20, 30 oC/min, respectively. The mass of the samples used is 5-10 mg. Based on the original mass loss dependence of the temperature, the first derivative data, i.e., DTG were according obtained. FTIR analysis of solid residues collected at different steps of thermal treatment was recorded on a Vertex 70 FTIR spectrometer (Bruker, Germany) with KBr pellets. Spectra in the optical range 400-4000 cm-1 were obtained by averaging 16 scans at a resolution of 4 cm-1. The morphology of the residual char obtained from cone calorimetric test was examined by means of field emission scanning electron microscopy (XL303SEM). The surface of residual char was sputter-coated with gold layer before examination. Differential scanning calorimeter (DSC) was carried out in a nitrogen atmosphere by means of NETZSCH STA-449F3 analyzer from 25 oC to 700 oC at a heating rate of 10 oC/min. The nitrogen flowrate was 100 mL/min. 3. RESULTS AND DISCUSSION. 3.1. Fame retardancy. A novel charring-foaming agent (SBCPO) containing sprio and caged bicyclic phosphate group in this work was synthesized according to our recent report, 26 then a new intumescent flame retardants (IFRs) consisting of SBCPO and APP was developed to improve the flame retardancy of PP. To evaluate the flame retardancy of PP containing SBCPO and APP mixtures at different mass 6 ACS Paragon Plus Environment

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ratio, the LOI and vertical burning tests were conducted. The detailed results are summarized in Table 1. From the data listed in Table 1, it can be seen that using APP or SBCPO alone with 25 wt. % loading shows very poor flame retardancy for PP, and PP/APP (Sample PP1) or PP/SBCPO (Sample PP9) could not pass the UL-94 vertical burning test, which demonstrated that SBCPO and APP using alone showed low efficiency in improving flame retardancy of PP. However, when SBCPO and APP were mixed in a certain proportion, the LOI values of PP/IFR systems were remarkably enhanced. When the weight ratio of SBCPO to APP is 1:1, the LOI value of PP/IFR system reached the highest value (30.5). This composition of IFR showed the best flame retardant effect in PP. Furthermore, the results of vertical burning rate demonstrated that all of PP/IFR systems could pass UL-94 V-0 rating when the mass ratio of SBCPO to APP was between 1:4 and 4:1, in which the content of IFR was 25 wt. %. We selected the most effective intumescent flame retardant system, which consisted of 50 % APP and 50 % SBCPO according to the above results. With the decrease of IFR loadings, LOI values of PP/IFR systems decreased. When the content of the IFR reached to 20 wt. %, the LOI value was 27.9 and UL-94 testing passed V-2 rating (Sample PP11). However, LOI value of PP/IFR system was 26.4 when the content of the IFR was 17.5 wt. %, and this system was no rating. According to the above analysis, it could be concluded that if the ratio of SBCPO to APP was proper, the PP/IFR systems would have excellent flame retardancy. 3.2. Cone calorimeter study. Cone calorimeter is one of the most effective bench-scale methods to evaluate the flammability performance of materials and provides a wealth of information on combustion behavior of the material. The results of cone calorimeter test correlate well with those obtained from large fire tests and can be used to predict the behavior of materials in a real fire. Furthermore, it can provide quantified fire parameters such as the peak heat release rate (PHRR), 7 ACS Paragon Plus Environment


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the total heat release (THR), average heat release rate (AHRR) and average mass loss rate (AMLR). The experimental data and the flammability performance are presented in Table 2 and Figure 1, respectively. As shown in Figure 1(a), it can be found that neat PP burnt very fast once the material was ignited with a PHRR value of 1284 kW/m2. When 25 wt. % APP or SBCPO was added into PP, the HRR of the mixture decreased sharply. The PHRR of PP1 and PP9 decreased to 537 and 480 kW/m2, respectively, the values of which were reduced by 58% and 63% compared to that of neat PP. It is worth noting that there are two obvious peaks in the HRR curve of PP/IFR, which is a typical character of intumescent systems. 28 Moreover, the PHRR further decreased when both APP and SBCPO with the mass ratio of 1:1 were added simultaneously into PP. The PHRR for PP5 was the lowest among all the samples, and it was decreased by 75%. The experimental results indicated that the synergistic effect between APP and SBCPO could enhance the flame retardancy. The THR is another important parameter for the flame retarded materials. Figure 1(b) displays the curves of the THR for PP and its mixture. The flame spread for PP5 decreases significantly in comparison with other samples and the value of its THR is 161 MJ/m2 which is reduced by 25%. Figure 1(c) shows the change of normalized mass loss of the samples and the residues mass are also presented in Table 2. Clearly, the incorporation of flame retardant could significantly slow down the average mass loss rate and leave bigger residual mass at the end of burning. Pure PP burned almost completely and the AMLR was 0.112 g/s, while PP5 displayed 18.5 wt. % mass residues and the AMLR decreased to 0.051 g/s. The above results demonstrate that the intumescent system based on SBCPO and APP can effectively improve the flame retardancy for PP, because it can significantly reduce the PHRR, THR and AMLR during combustion. Figure 2 presents the digital photos for the residues of PP1, PP5 and PP9 after cone calorimeter 8 ACS Paragon Plus Environment

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measurements. It is clearly seen that the residue of the system with 25 wt. % APP (Figure 2a) is not expanded at the end of the cone calorimeter experiment. Only a smooth char layer with some apparent holes is observed. From Figure 2(c), for PP/25wt. % SBCPO system (PP9), a thick and cohesive residual char with many cracks is formed. However, the digital photo for the sample PP5 with the presence of APP and SBCPO (1:1) is significantly different from the other two samples after cone calorimeter test. As shown in Figure 2b, it could be observed that there are some swollen like “island” of residual char which are continuous and no obvious cracks. SEM is widely used as a tool to observe the morphology of the residual chars. Figure 3 and Figure 4 displays the SEM images of the surface and inside residual char for different samples after cone calorimetric test with a 50 kW/m2 external heat flux, respectively. As for PP1 with 25 wt % APP, the surface of residual char appears relatively smooth, thin and fragile sheets (Figure 3a and 3b). Meanwhile, many obvious cracks can be observed, and it is unable to swell and form intumescent char layer, due to the lack of a charring agent. So, the flame retardant property of PP1 is very poor. From Figure 4a and 4b, it can be seen that there are many bubble-like sheets inside the structure, which results from the production and release of ammonia by APP during combustion. In contrast, the char morphology is quite different when SBCPO is added alone. The surface of char layer of PP9 is a network structure connected by some particles with the size of is about 5 µm (Figure 3e and 3f). There are few holes on the surface. For the inside structure of PP9, many spheroidal particles are tightly packed together as shown in Figure 4e and 4f. Compared to the former two kinds of char layers, the residual surface of PP5 is more uniform and dense (Figure 3c and 3d). The size of the particles in the surface is obviously smaller than that of PP9. Furthermore, there are many holes with the diameter of 5 µm in the inside of residue, indicating that the intumescent structure is formed (Figure 3c and 3d). This kind of char layer has more advantages as 9 ACS Paragon Plus Environment


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a thermal barrier which could prevent diffusion of oxygen and heat transfer. The formation of such structure should result from the combination between APP and SBCPO. The formation of the efficient char layer for PP5 can reduce heat transfer between the flame zone and the substrate, and thus protect the underlying materials from further burning and pyrolysis.

29

In conclusion, the

synergistic effect between APP and SBCPO for improving flame retardancy mostly resulted from the formation of a more perfect char layer during combustion. 3.3. FTIR study. In order to further understand the interactions between SBCPO and APP, the thermal oxidative degradation residues at different temperature were investigated by FTIR. The changes in the FTIR spectra obtained were shown in Figure 5 and Figure 6. From the room temperature to 250 oC, the intensity of the absorptions of C-H (2960, 2911 and 1474 cm-1) and P=O (1196 and 1159 cm-1) increased, which indicated the degradation of P-O-C bond for phosphate esters and then formed the hydrocarbons and olefins products.

30

When the

temperature reached 250 oC, the cyclic structures of SBCPO began to decompose. The absorption bands at 850 cm-1 (P-O-C) and 1309, 1292 cm-1 (P=O) disappeared at 360 oC, indicating the complete degradation of SBCPO. Meanwhile, the broad peak ranged from 1250 to 1000 cm-1 demonstrated the formation of phosphoric acid, pyrophosphate and polyphosphates.

31

There were

three peaks (1243, 1016 and 928 cm-1) as shown in the spectrum at 650 oC, further confirming that the residue was mainly composed of P=O and P-O-P groups. In addition, the new peak at 486 cm-1 signified the formation of O=P-OH.

32

Moreover, another new absorption band at 1598 cm-1 from

the residues obtained at above 360 oC corresponds to the stretching mode of C=C, implying the formation of aromatic structures. 33 The FTIR spectra for the mixture of SBCPO and APP at different degradation temperatures are shown in Figure 6. The peaks at 3208 and 1689 cm-1, assigned to the symmetric stretching and 10 ACS Paragon Plus Environment

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deformation of N-H for APP, respectively, decreased from 200 oC and disappeared on further heating to 360 oC. The difference between SBCPO and SBCPO/APP during the thermal degradation process is obvious. SBCPO opens its cyclic structure at 250 oC and decomposes completely up to 360 oC, while the complete degradation of the mixture of SBCPO/APP is at 310 oC. It could be found that the decomposition of SBCPO/APP is earlier compared to that of SBCPO, indicating that APP can accelerate the decomposition of SBCPO. Owing to the synergistic effect between P and N element,

34

we obtained the intumescent residual char when SBCPO/APP was thermally treated at

360 oC, while SBCPO turned into black powder at the same temperature. 3.4 Thermal decomposition behavior of the flame retardants. In order to further investigate the synergistic effect of SBCPO and APP, the thermal decomposition behavior of APP, SBCPO and SBCPO/APP (mass ratio=1:1) are investigated by TGA. Figure 7 shows the thermal degradation behaviors of APP and SBCPO. Meanwhile, the experimental and calculated TG curves of their mixture (mass ratio=1:1) are compared. In nitrogen atmosphere, there are two weight loss steps on the TG curves of SBCPO with Tmax1 at 364 oC and Tmax2 at 515 oC. During the first stage, the weight loss with a fast loss rate before 380 oC is

attributed to the scission of phosphate ester bonds and the second step is responsible for the formation of residual char. The weight loss rate of SBCPO notably slows down after 600 oC, and the residual char is 20.8% at 800 oC. Compared to SBCPO, the thermal degradation behavior of APP is quite different. The first stage is in the temperature range of 250-540 oC with Tmax1 at 318 oC, which is related to the elimination of ammonia and water. Then the fragmentation to volatile P2O5 happens after 540 oC and the stable solid residue at 800 oC is about 13.9%. From the calculated TG curve of their mixture (mass ratio=1:1), there are also two decomposition steps with Tmax1 at 350 oC and Tmax2 at 530 oC. The calculated residual char is 17.4%. However, 11 ACS Paragon Plus Environment


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from the experimental TG curve, a wider decomposition temperature range (from 300 to 600 oC) is detected and the residual char at 800 oC is 26.7%, which is 9.3% higher than the calculated residue. According to the above results, a conclusion could be drawn that there is a strong interaction between SBCPO and APP during the degradation process under N2 atmosphere because the enormous difference between the experimental and calculated TG curves are detected. This interaction eventually leads to the significant increase of the residual char. 3.5 Differential scanning calorimetry (DSC) study. The DSC results for pure SBCPO, pure APP and mixture of SBCPO and APP (mass ratio 1:1) are shown in Figure 8. It can be seen that there is a sharp heat release peak at 368 oC of pure SBCPO (Figure 8(a)), which is correlated to its thermal decomposition (see Figure 7). In the case of APP, there are two endothermic peaks at 314 o

C and 591 oC in the DSC curve (Figure 8(b)), which is well corresponding to that of the weight

loss in the entire temperature region (Figure 7). Compared to the pure SBCPO and pure APP, the number of peaks for mixture of SBCPO and APP has changed not significantly, but the position of peaks has been obviously changed. The exothermic peak of SBCPO shifts to 342 oC and become broad. This result may be due to the catalysis of APP for the decomposition of SBCPO, which is consistent with the results of FTIR study. Two endothermic peaks of APP also appear earlier. These results further demonstrate that there is a synergistic effect between SBCPO and APP, and the formation of a dense char layer is the key factor for obtaining good flame retardant properties. 3.6 Synergistic effectivity (SE) study. Synergistic effectivity (SE)

35

is designed as a general

tool for comparing synergistic systems, which is defined as the ratio of the FR effectivity (EFF) of the additive plus the synergist to the EFF of the additive alone. EFF is as the increment in OI for 1% of the element on which the FR additive is based. In the case when the synergism is between two or more FR agents the SE is the ratio of the measured OI of the mixture to the calculated value for it 12 ACS Paragon Plus Environment

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without the synergistic effect. In order to better estimate the synergism between APP and SBCPO, the FR effectivity (EFF) and synergistic effectivity (SE) are compared with APP and its mixture with SBCPO, respectively. The detail is listed in Table 3. It can be seen that the values of EFF using APP or SBCPO alone with 25 wt. % loading are just 0.31 and 0.85, respectively. The SE for PP9 is 2.3. However, the results are significantly improved when 1/2 of APP is substituted by SBCPO. In the case the values of EFF and SE are increased to 3.08 and 8.3, respectively. These values are much higher than those of traditional intumescent systems given from the data published in the literature.36 Thus, the obvious synergistic effect will improve the flame retardancy of PP. 3.7 Kinetics analysis of flame retardant PP systems. It is known that the flame retardancy of polymers not only depends on their thermal stability but also on their degradation rate, char-forming rate and char yield. 37 As a consequence, the thermal degradation details of polymers influence their flame retardancy to a considerable degree and deserve a deep investigation. TGA has been widely used to estimate the kinetic parameters of degradation processes, such as E (activation energies), n (apparent reaction order) and A (pre-exponential factor) which can be calculated using various kinetic models such as Kissinger, 38 Flynn-Wall-Ozawa, 39 Friedman 40 and Coats-Redfern methods. 41 3.5.1 Kinetics analysis using Kissinger method. Kissinger method can be used to calculate the

activation energy of the solid reaction, which is obtained from plots of the logarithm of the heating rate vs. the inverse of temperature at the maximum reaction rate at constant heating rate. Kissinger assumes that the product n(1-αmax)n-1 is independent of β and the following equation can be used:

ln(

β 2 max

T

 AR  Ea ) = ln + ln[n(1 − α max ) n −1 ] −  Ea  RTmax

(2)

Where β is the heating rate, Ea is the apparent activation energy, Tmax is the temperature corresponding to the inflection point of the thermal oxidative degradation curves which corresponds 13 ACS Paragon Plus Environment


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to the maximum reaction rate, A is the pre-exponential factor, αmax is the extent of conversion at Tmax, and n is the reaction order.

The inflection point temperatures and the TG curves of various samples are shown in Table 4 and Figure 9, respectively. According to Equation (1), plots of ln(β/T2max) against -1/Tmax produce the fitted straight lines as shown in Figure 10. The obtained slopes of various straight lines are used to calculate the apparent activation energy of samples. As can be seen, the relativity of the various fitted straight lines is very good indicating the feasibility of Kissinger method. Table 5 displays the apparent activation energy results of different samples. The apparent activation energy of neat PP is 216.8 kJ/mol. The apparent activation energy of PP1, PP10 and PP5 increase to 234.8, 237.1 and 241.6 kJ/mol, respectively, after adding APP/SBCPO (1/1) of 20%, 22.5% and 25% suggesting that addition of IFRs improves the thermal stability of PP and hence contributes to the improvement of the flame retardancy of PP. This behavior is well coincided with the results of LOI and UL94 test. 3.5.2 Kinetics analysis using Flynn-Wall-Ozawa method. Flynn-Wall-Ozawa method can also be

used to calculate the action energy of the solid-phase reaction. The primary difference between the Kissinger method and Flynn-Wall-Ozawa method is that the former only utilizes one point, i.e. the point of maximum rate, while the latter requires all points of the TGA curves. Flynn-Wall-Ozawa method, 42 using the Dolye’s approximation for the integration, has been expressed as:

log β = −

 0.457 Ea  AEa + log[ ] − 2.315 RT g (α ) R  

(3)

At a given conversion degree α, plot of logβ against -1/T makes a fitted straight line with a slope 0.457Ea/R. Obviously, the apparent activation energy Ea at the given α value can be calculated from the value of 0.457 Ea/R. Figure 11 shows the fitted results using Flynn-Wall-Ozawa method. The fitted straight lines obtained at different α values for pure PP or FP PP materials are almost parallel, 14 ACS Paragon Plus Environment

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indicating the applicability of Flynn-Wall-Ozawa method to our system. All of the calculated apparent activation energies are listed in Table 6. The activation energy values calculated by Flynn-Wall-Ozawa method are close to that calculated by Kissinger method, and follow the same order: PP5 > PP10 > PP11 > PP0. It can be seen that the apparent activation energy of PP5, PP10 and PP11 is much higher than PP0. It is mainly from the degradation and the reaction of APP and SBCPO, after that, some cross-linking structures can be formed, which can act as a barrier to slow down the heat and mass transfer. Therefore, the apparent activation energies of PP5, PP10 and PP11 increase with the increase of content for flame retardant.

4. CONCLUSIONS The intumescent flame retardants consisting of SBCPO as a novel char source and APP as acid source showed excellent flame retardancy and dripping resistance for PP. A maximum LOI value of 30.5 and UL-94 V-0 rating were achieved when the mass ratio of SBCPO and APP was 1:1 with a total loading of 25 wt. %. The results of cone calorimetric test indicated that the IFR system with this formation could significantly improve the flame retardancy for PP. In addition, the microstructure observation showed that the char layer from the systems containing the mixture of APP and SBCPO was denser than that from the systems using APP or SBCPO alone. Meanwhile, the results from FTIR, TGA, DSC and SE analysis demonstrated that the synergistic effect between APP and SBCPO could enhance the flame retardancy of PP.



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ACKNOWLEDGEMENT This work is financially supported by the National Natural Science Foundation of China for the Projects (21204079, 50873099 and 51073149).


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(37) Chen, Y. H.; Liu, Y.; Wang, Q.; Yin, H.; Aelmans, N.; Kierkels, R. Performance of intumescent flame retardant master batch synthesized through twin-screw reactively extruding technology: effect of component ratio. Polym. Degrad. Stab. 2003, 81, 215. (38) Kissinger, H. E. Reaction kinetics in differential thermal analysis. Anal. Chem. 1957, 29, 1702. (39) Flynn, J. H.; Wall, L. A. General treatment of thermogravimetry of polymers. J. Res. Natl. Bur. Stand. A Phys. Chem. 1966, 70, 487.

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FIGURE CAPTIONS Scheme 1. Synthesis route of SBCPO. Figure 1. HRR (a), THR (b) and MLR (c) curves of PP0, PP1, PP5 and PP9 at a 50 kW/m2 external heat flux. Figure 2. Digital photos of residues after cone calorimeter test: (a) PP1, (b) PP5 and (c) PP9. Figure 3. SEM images of surface for residual chars: (a, b) PP1, (c, d) PP5 and (e, f) PP9. Figure 4. SEM images of inside for residual chars: (a, b) PP1, (c, d) PP5 and (e, f) PP9. Figure 5. FTIR spetra of SBCPO with different pyrolysis temperatures. Figure 6. FTIR spetra of SBCPO/APP with different pyrolysis temperatures. Figure 7. Experimental and calculated TG curves of SBCPO, APP and SBCPO/APP(mass ratio=1:1) under N2 atmosphere (10 oC/min). Figure 8. DSC curves of SBCPO (a), APP (b) and SBCPO/APP(mass ratio=1:1) (c) under N2 atmosphere (10 oC/min). Figure 9. TG and DTG curves of samples under nitrogen: (a) PP0, (b) PP5, (c) PP10 and (d) PP11. Figure 10. Kissinger method applied to TG data at different heating rates under nitrogen: (a) PP0, (b) PP5, (c) PP10 and (d) PP11. Figure 11. Plots of log β versus -1/T at various conversion values (α) in the range 10-80%: (a) PP0, (b) PP5, (c) PP10 and (d) PP11.


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Table 1. Effect of APP/SBCPO Composition on Flame Retardancy of PP. Components (wt %)

Samples

LOI

UL-94

0

18.2

No rating

25

0

20.9

No rating

75

20

5

27.2

V-0

PP3

75

18.75

6.25

28.1

V-0

PP4

75

16.67

8.33

28.8

V-0

PP5

75

12.5

12.5

30.5

V-0

PP6

75

8.33

16.67

29.9

V-0

PP7

75

6.25

18.75

29.3

V-0

PP8

75

5

20

28.4

V-0

PP9

75

0

25

22.6

No rating V-1

PP

APP

SBCPO

PP0

100

0

PP1

75

PP2

PP10

77.5

11.25

11.25

28.2

PP11

80

10

10

27.9

V-2

PP12

82.5

8.75

8.75

26.4

No rating

Table 2. Cone Calorimetry Analysis Data for Different Samples at 50 kW/m2. Samples PP0 PP1 PP5 PP9 a

tign (s)

PHRR (kW/m2)

38 34 31 35

1284 537 318 480

AHRR (kW/m2)

THR (MJ/m2)

AMLR (g/s)

510 319 188 298

214 177 161 168

0.112 0.082 0.051 0.085

Residual char (%) a 0.1 8.8 18.5 16.1

Residual char%: mass percentage left when testing finished.

Table 3. FR Effectivity (EFF) and Synergistic Effectivity (SE) of intumescent system on PP. Samples

FR

PP1 PP5 PP9 PP PP PP

APP APP SBCPO APP APP APP

Synerg. SBCPO Petol Melamine Petol+Melam

EFF

SE

0.34 3.08 0.85 1.7 0.92 2.4

8.3 2.3 5.536 3.036 7.736



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Table 4. Inflection Point Temperature of TG Curves for Different Samples at Different Heating Rates. Temperature inflection (oC)

Heating rate (oC/min)

PP0

PP5

PP10

PP11

5 10 20 30

444.86 457.36 472.13 479.55

461.24 474.16 486.01 494.70

461.00 473.38 486.70 494.58

459.90 473.24 486.01 493.86

Table 5. Calculated Activation Energies at Various Conversions Using Kissinger Method for Different Samples. Samples

E (kJ/mol)

Correlation coefficient (r)

PP0 PP5 PP10 PP11

216.8 241.6 237.1 234.8

0.9995 0.9994 0.9999 0.9998

Table 6. Calculated Activation Energies at Various Conversions Using Flynn-Wall-Ozawa Method for Different Samples. PP0 α 0.1 0.15 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Mean

E (kJ/mol) 207.6 211.3 207.6 211.3 209.9 209.6 208.7 208.8 206.8 209.1

PP5 r 0.9991 0.9999 0.9991 0.9999 0.9996 0.9995 0.9998 0.9997 0.9992

E (kJ/mol) 215.6 232.1 238.4 239.5 239.0 238.4 237.8 236.8 234.5 234.7

PP10 r 0.9694 0.9921 0.9974 0.9994 0.9998 0.9999 0.9998 0.9998 0.9996

E (kJ/mol) 205.8 219.7 228.5 233.9 235.8 235.8 235.6 235.6 235.9 229.6

PP11 r

0.9956 0.9978 0.9988 0.9996 0.9999 0.9999 0.9999 0.9998 0.9997

E (kJ/mol) 207.8 218.5 230.9 231.1 231.9 232.5 232.2 231.3 226.9 227.0

r 0.9942 0.9938 0.9987 0.9992 0.9994 0.9996 0.9997 0.9996 0.9990


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Scheme 1. Synthesis route of SBCPO.



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Figure 1. HRR (a), THR (b) and MLR (c) curves of PP0, PP1, PP5 and PP9 at a 50 kW/m2 external heat flux.


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Figure 2. Digital photos of residues after cone calorimeter test: (a) PP1, (b) PP5 and (c) PP9.



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a

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b

20 µm

c

5 µm

d

20 µm

e

5 µm

f

20 µm

5 µm

Figure 3. SEM images of surface for residual chars: (a, b) PP1, (c, d) PP5 and (e, f) PP9.


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a

b

5 µm

20 µm

c

d

20 µm

e

5 µm

f

20 µm

5 µm

Figure 4. SEM images of inside for residual chars: (a, b) PP1, (c, d) PP5 and (e, f) PP9.



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Figure 5. FTIR spectra of SBCPO with different pyrolysis temperatures.


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Figure 6. FTIR spectra of SBCPO/APP with different pyrolysis temperatures.



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Figure 7. Experimental and calculated TG curves of SBCPO, APP and SBCPO/APP(mass ratio=1:1) under N2 atmosphere (10 oC/min).


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Figure 8. DSC curves of SBCPO (a), APP (b) and SBCPO/APP(mass ratio=1:1) (c) under N2 atmosphere (10 oC/min).



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Figure 9. TG and DTG curves of samples under nitrogen: (a) PP0, (b) PP5, (c) PP10 and (d) PP11.


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Figure 10. Kissinger method applied to TG data at different heating rates under nitrogen: (a) PP0, (b) PP5, (c) PP10 and (d) PP11.



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Figure 11. Plots of log β versus -1/T at various conversion values (α) in the range 10-80%: (a) PP0, (b) PP5, (c) PP10 and (d) PP11.


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For TOC only


Synergistic Effect between a Novel Char Forming Agent and

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