Microencapsulation of ammonium polyphosphate with PVA-melamine-formaldehyde resin and its flame retardance in polypropylene

POLYMERS FOR ADVANCED TECHNOLOGIES Polym. Adv. Technol. 2008; 19: 1914–1921 Published online 2 September 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/pat.1231 Microencapsulation of ammonium polyphosphate with PVA–melamine–formaldehyde resin and its flame retardance in polypropylene Kun Wu1, Lei Song1, Zhengzhou Wang1,2* and Yuan Hu1* 1 2 State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, P.R. China School of Materials Science and Engineering, Tongji University, Shanghai 200092, P.R. China Received 30 April 2008; Accepted 17 June 2008 With a shell of PVA–melamine–formaldehyde resin, microencapsulated ammonium polyphosphate (VMFAPP) is prepared by in situ polymerization and characterized by FTIR and XPS. Microencapsulation gives VMFAPP better water resistance and flame retardance compared with APP in PP. Thermal stability and fire resistance behavior have been analyzed and compared. The LOI value of the PP/VMFAPP composite is higher than that of the PP/APP composite. The UL 94 ratings of most of the PP/VMFAPP composites are V-0, whereas PP/APP gives no rating at the 30% additive level. The water resistant properties of the PP composites are studied. Results of the cone calorimeter experiment show that VMFAPP is an effective flame retardant in PP compared with APP. The thermal degradation behaviors of APP and VMFAPP have been studied using TG and dynamic FTIR. Copyright # 2008 John Wiley & Sons, Ltd. KEYWORDS: microencapsulation; ammonium polyphosphate; intumescent flame retardation; thermal degradation; PVA; melamine– formaldehyde; PP INTRODUCTION There has been enormous loss of materials and high rates of human mortality due to the weak flame resistance properties of polymers. Intumescent flame retardants (IFRs) have attracted considerable attention in recent years because they are more environmentally friendly than the traditional halogen-containing flame retardants. IFR systems contain three active ingredients: an acid source (e.g. ammonium polyphosphate (APP), etc.), a carbonization agent (e.g. pentaerythritol, polyurethane, etc.), and a blowing agent (e.g. melamine). Bourbigot and co-workers have carried out extensive studies on the APP IFR system in polyolefins1–3 and have reviewed recent developments of IFR systems in great detail.4 But these systems are not durable due to the weak water resistance and low compatibility with organic materials of APP. In order to overcome this problem, microencapsulation is a good choice. Melamine–formaldehyde (MF) resin is commonly used in microencapsulation of red phosphorus,5 n-octadecane,6 etc. Poly(vinyl alcohol) (PVA) is used for the microencapsulation of di-ammonium hydrogen phosphate (DAHP) using coacervation and the interfacial polymerization technique, and the shell can be used as a charring agent in the flame retardance of polymers.7,8 PVA, with many O–H groups, *Correspondence to: Z. Wang and Y. Hu, State Key Laboratory of Fire Science, University of Science and Technology of China, Anhui 230026, P.R. China. E-mail: wwang@uste.edu.cn and yuanhu@ustc.edu.cn readily reacts with low molecular weight compounds and various functional polymers can be obtained with relative ease.9 Our group has recently microencapsulated APP with a MF and urea–melamine–formaldehyde (UMF) resin shell.10,11 Compared with APP, the microencapsulated APP (MCAPP) coated with MF or UMF resin can be dispersed well in the PP matrix and has lower water solubility. Because of the scarcity of the carbonization agent in MCAPP, there are no ratings in the UL 94 test for the PP composites containing MCAPP although the LOI values of the PP composites containing MCAPP are higher than those of the PP composites containing APP at the same content. To solve the problem, we developed a different approach to microencapsulate APP with a PVA modified MF resin shell. Our aim is to synthesize microcapsules which contain the three ingredients of a typical IFR system: acid source (APP), carbonization agent and blowing agent (PVA–melamine–formaldehyde (VMF)). The advantage of this is that it is possible to synthesize an intrinsic flame retardant which may have better flame retardance and higher water resistance in the polymer, compared with APP. In this paper, microencapsulated APP (VMFAPP) with a VMF resin shell was prepared by in situ polymerization and characterized by water solubility, Fourier transform infrared (FTIR), thermogravimetry (TG), and X-ray photoelectron spectroscopy (XPS). The use of VMFAPP as a flame retardant in PP is evaluated by limiting oxygen index (LOI), UL-94, TG, cone calorimetry, and scanning electron microscopy (SEM), Copyright # 2008 John Wiley & Sons, Ltd. Microencapsulation of APP with VMF resin 1915
1 and the results from VMFAPP and APP are compared. Moreover, the water resistant properties of the PP composites containing VMFAPP (or APP) are studied by LOI test and water leaching rate. The thermal degradation behaviors of APP and VMFAPP were also studied using TG and dynamic FTIR, and the results from both samples are compared. heating rate of about 108C min . Dynamic FTIR spectra were obtained in situ during the thermal degradation of the samples. EXPERIMENTAL Solubility in water Materials
1 PP (F401) with a melt flow index (MFI) of 2.3 g/10 min (2308C/2.16 kg) was provided by Yangzi Petroleum Chemical Company. APP with average degree of polymerization n > 1000 was purchased from Hangzhou JLS Flame Retardants Chemical Corporation. PVA (polymerization degree ¼ 1750, degree of alcoholysis ¼ 98–99%) was kindly supplied by Shanghai DongCang International Trading Co., Ltd. (China). Melamine and formaldehyde were purchased from Shanghai Chemical Reagent Corporation. XPS spectra The XPS spectra were recorded with a VG ESCALAB MK II spectrometer using Al ka excitation radiation (hn ¼ 1253.6 eV). The sample (about 10 g) was put into 100 ml distilled water and stirred for 60 min at different temperatures. The suspension was then filtered, 50 ml of the filtrate was removed and dried to constant weight at 1058C, permitting the calculation of solubility. SEM The SEM micrographs of the PP composites were obtained with a scanning electron microscope AMRAY1000B. The specimens were cryogenically fractured in liquid nitrogen, and then sputter-coated with a conductive layer. Preparation of microencapsulated APP Content of the VMF resin measurement Synthesis of prepolymer: PVA (6, 9, 12, 15, or 18 g), melamine (4 g), and distilled water (200 ml) were put into a threenecked bottle with a stirrer. The mixture was adjusted to pH 4–5 with acetic acid, heated to about 908C, and kept at that temperature for 1.5 hr. Then the pH was adjusted to pH 8–9 with 10% Na2CO3 solution, then 4 g melamine and 10 ml 37% formaldehyde solution were added into the system. The temperature was kept at 908C for 1 hr. The prepolymer solution was then prepared and it was ready for the next step. Preparation of microencapsulated APP: 40 g APP was first dispersed in 100 ml ethanol with a stirrer (1000 rpm, 5 min). The prepolymer solution obtained from the above step was added to the mixture, and the pH of the mixture was adjusted to pH 4–5 with sulfuric acid. The resulting mixture was heated at 808C for 3 hr with stirring (300 rpm). After that, the mixture was filtered, washed with distilled water, and dried at 808C, and the VMFAPP powder was finally obtained. The D50 value of the microcapsules was about 20 mm. Some APP or VMFAPP powder was dissolved in nitric acid at 1508C, and inductively coupled plasma atomic emission spectrometry (Atomscan Advantage, Thermo Jarrell Ash Corporation, USA) was used to measure the phosphorus content of APP or VMFAPP. The symbols PVMFAPP% and PAPP% represent the percentage of phosphorus in VMFAPP and APP, respectively. Assuming that the content of phosphorus remains constant in the process of the microencapsulation of APP, we obtain: Preparation of flame retarded (FR) PP composites All flame-retarded PP composites were prepared in 15 min in a Brabender-like apparatus at a temperature of about 1808C. After mixing, the samples were hot-pressed at about 1808C under 10 MPa for 10 min into sheets of suitable thickness for analysis. MEASUREMENTS FTIR spectra Powders were mixed with KBr powder, and the mixture was pressed into a tablet. The FTIR spectra of samples were recorded using a Nicolet MAGNA-IR 750 spectrophotometer. Real time FTIR spectra were recorded using the above spectrophotometer equipped with a ventilated oven having a heating device. The temperature of the oven was raised at a Copyright # 2008 John Wiley & Sons, Ltd. MAPP  PAPP % ¼ MVMFAPP  PVMFAPP % where MAPP is the content of APP used and MVMFAPP is the content of VMFAPP obtained. Therefore the percentage of the resin (Wresin, wt%) in VMFAPP can be expressed as follows: Wresin ðwt%Þ ¼ 1
MAPP =MVMFAPP ¼ ð1
PVMFAPP %=PAPP %Þ  100% if PVMFAPP % and PAPP% are measured, Wresin (wt%) can be calculated. LOI LOI was measured according to ASTM D2863. The apparatus used was an HC-2 oxygen index meter (Jiangning Analysis Instrument Company, China). The specimens used for the test were of dimensions 100  6.5  3 mm3. UL-94 testing The vertical test was carried out on a CFZ-2-type instrument (Jiangning Analysis Instrument Company, China) according to the UL-94 test standard. The specimens used were of dimensions 130  13  3 mm3. Water leaching rate of FR PP composites The specimens (marked Wa) were put in distilled water and the temperature was kept at 508C for 24 hr. The treated Polym. Adv. Technol. 2008; 19: 1914–1921 DOI: 10.1002/pat 1916 K. Wu et al. specimens were subsequently removed and dried to constant weight at 808C (marked Wc). The water leaching rate of the specimens can be expressed as (Wa–Wc)/Wa  100%. Each sample was examined under air flow (30 ml min
1) on a DTG-60H apparatus (Shimadzu Company) at a heating rate of 108C min
1. The weight of all samples was kept within 3– 5 mg in an open Al pan. Cone calorimeter C1s Relative Intensity (au) TG O1s The combustion tests were performed with a cone calorimeter (Stanton Redcroft, UK) tests according to ISO 5660 standard procedures, with 100  100  3 specimens. Each specimen was wrapped in an aluminum foil and exposed horizontally to 35 kW m
2 external heat flux. N1s VMFAPP APP P2pP2S 200 400 600 800 1000 Banding energy (eV) Figure 2. XPS spectra of APP and VMFAPP. RESULTS AND DISCUSSION Characterization of VMFAPP by FTIR and XPS The FTIR spectra of APP and VMFAPP are shown in Fig. 1. It is clear that for VMFAPP, the main absorption peaks appear at 3200, 1664, 1560, 1504, 1430, 1339, 1256, 1138, 1075, 1020, and 880 cm
1. The typical absorption peaks of APP include 3200 (N–H), 1256 (P ¼ O), 1075 (P–O symmetric stretching vibration), 880 (P–O asymmetric stretching vibration), and 1020 (symmetric vibration of PO2 and PO3) cm
1.12 The absorptions of 1560, 1504, and 1339 cm
1 are due to the ring vibration of melamine from the MF resin.5 The 1138 cm
1 band is representative of symmetric C–O–C of –CH2– O–CH2– between melamine groups.13 The peaks at 1430– 1458 are attributed to the O–H, C–H bending, and –CH2 deformation of PVA. The band at 1664 cm
1 is assigned the C– – O group in PVA, and the C –– O group was likely due to the absorption of the residual acetate group.14 The spectrum of VMFAPP reveals not only well-defined absorption peaks of VMF resin but also the characteristic bands of APP, indicating that the resin exists in the VMFAPP. Figure 2 shows XPS spectra of APP and VMFAPP. It can be seen that the peaks located at about 134 and 191 eV are Water solubility of VMFAPP Figure 3 shows the influence of PVA content in the prepolymer used on the water solubility of VMFAPP in the microencapsulation. From Fig. 3, it can be seen that the solubility of APP without microencapsulation at 25 and 808C is 0.45 and 2.4 g/100 ml H2O, respectively, indicating that APP can be easily attacked by moisture or water, especially at high temperatures. After the microencapsulation of APP with VMF resin, the solubility of VMFAPP decreases above 90% at 258C. As the content of the PVA in prepolymer increases further, the solubility of VMFAPP changes little. The trend of solubility of VMFAPP at 808C is similar to that of 1020 1256 1075 3200 VMFAPP 1430 o 80 C 2.5 Solubility g/100 ml H2O APP Transmittance(%) attributed to P2P and P2S of APP. For VMFAPP, the intensities of aforementioned peaks decrease sharply, meanwhile the intensities of the C1S and N1S peaks centered at about 285 and 398 eV, respectively, increase greatly. The changes of the above peaks are due to the coverage of the outside APP particles with the VMF resin, which indicates that APP was well coated by the resin. 1339 880 1138 pure APP 2.0 1.5 1.0 o 25 C 0.5 pure APP 1664 4000 3500 3000 2500 1504 1560 1256 2000 1500 1000 0.0 -2 500 0 2 4 6 8 10 12 14 16 18 20 Content of PVA in prepolymer (g) -1 Wave number cm Figure 1. FTIR spectra of APP and VMFAPP. Copyright # 2008 John Wiley & Sons, Ltd. Figure 3. Solubility of APP and VMFAPP versus content of PVA in prepolymer. Polym. Adv. Technol. 2008; 19: 1914–1921 DOI: 10.1002/pat Microencapsulation of APP with VMF resin VMFAPP at 258C. It is interesting to find that there is a great difference in solubility of APP at 25 and 808C. However, the difference in solubility of VMFAPP at 25 and 808C is small. This is because the VMF resin outside APP is hydrophobic, leading to a decrease in the solubility of APP. Flame retardation of PP composites The influence of PVA content in prepolymer used in the microencapsulation on the LOI value of PP/VMFAPP composite is shown in Fig. 4. VMFAPP is blended with PP at the mass percentage of 30%. From the figure, it can be seen that with the increase of PVA content, the LOI value of PP/ VMFAPP increases. It is proposed that a suitable phosphorus/nitrogen/carbon ratio in the IFR system is very important for the flame retardant action of IFR in polymers. So when the dosage of PVA in the prepolymer is above 15 g, the LOI values of FR PP composites containing VMFAPP show essentially no change. Moreover, it should be noticed that the UL 94 results of most of the PP/VMFAPP composites can reach V-0. As a result, the prepolymer containing 15 g PVA was selected for the microencapsulation; from the equation of content of the VMF resin, it can be calculated that this VMFAPP sample is coated with 34.8% resin. The LOI value of the composite containing 30% VMFAPP (coated with 34.8% resin) is 32, whereas the value of the PP/ APP composite at the same additive level is only 20%. The explanation for the increase may be the formation of an intumescent char which can protect the underlying materials from burning. When the PP composites containing VMFAPP are heated, the resin in the coating layer of the microcapsule releases water vapor and NH3 gas which would reduce the concentration of air and make the material (formed mainly by the esterification between APP and PVA) swell to form an intumescent char. The above results illustrate that APP used alone in PP does not have good flame retardancy (no ratings in the UL-94 test), due to the scarcity of the carbonization and blowing agents. Due to the presence of VMF resin outside APP, VMFAPP is an effective flame retardant in PP in comparison with APP. 1917 Water resistance of FR PP composites The change of LOI values of the PP composites containing APP or VMFAPP (microencapsulated with prepolymer containing different PVA contents) after the hot water treatment (508C, 24 hr) is shown in Fig. 4. For the PP/APP binary composite at 30% additive level, the LOI values are about 20% before the treatment and the values decrease by 2.5% after the hot water treatment. The LOI value of PP/ VMFAPP composite at a 30% loading is 31%, whereas the value is still as high as 30% after the treatment. In spite of the decrease in the LOI values of the PP/VMFAPP composites after treatment, the good UL 94 ratings are maintained (most are still V-0 rating). Therefore, it can be concluded that the water resistance of VMFAPP is much better than APP in PP composites. Water leaching rates of PP/APP and PP/VMFAPP versus content of PVA in prepolymer are shown in Fig. 5; it can be seen that by microencapsulation, the leaching rate of FR PP composites are reduced from 9.81 to 0.22% when the percentage of VMFAPP is 30%. Here it is hypothesized that the VMFAPP particle’s low solubility in comparison with APP causes this reduction. Due to the hydrophobicity of the VMF resin with an increase of PVA content used in the synthesis of the prepolymer, the water leaching rates of PP/ VMFAPP show little change. So, upon exposure to water, the comparatively better water resistance of VMFAPP in the PP matrix would prevent the flame retardant from being exuded, and a certain flame retardation of FR composite can still be maintained. The fractured surface of PP/APP and PP/VMFAPP composites before and after water treatment was observed by SEM, shown in Fig. 6(a), 6(b), 6(c), and 6(d). Before water treatment, many APP grains are exposed on the surface due to the relatively great polarity of APP. So when the composites are exposed to water, the water molecules will absorb on the surface of the material, and some APP grains on the surface will first dissolve in the water, leaving some defects on the surface. On the other hand, it can be seen that in Fig. 6(d) after the PP/VMFAPP composite was treated with water for 24 hr, there are still some VMFAPP grains left 32.5 V-0 V-0 32.0 V-0 LOI 31.5 V-0 V-0 31.0 V-0 V-1 V-0 V-0 30.5 V-1 Water Leaching Rate (wt%) 10 water treated without treated 8 PP/APP 6 4 2 0 30.0 6 8 10 12 14 16 18 Content of PVA in prepolymer (g) Figure 4. LOI values of PP/VMFAPP before and after water treatment (508C, 24 hr) versus content of PVA in prepolymer. Copyright # 2008 John Wiley & Sons, Ltd. -2 0 2 4 6 8 10 12 14 16 18 20 Content of PVA in prepolymer (g) Figure 5. Water leaching rate of PP/APP and PP/VMFAPP (508C, 24 hr). Polym. Adv. Technol. 2008; 19: 1914–1921 DOI: 10.1002/pat 1918 K. Wu et al. Figure 6. SEM micrographs of fracture surfaces of the composites (1500), (a) PP/APP; (b) PP/APP (508C water treated for 24 hr); (c) PP/VMFAPP; (d) PP/ VMFAPP (508C water treated for 24 hr). This figure is available in color online at www.interscience.wiley.com/journal/pat in the PP matrix. The results indicate that microencapsulation has a remarkable effect on the water resistance of APP in the PP matrix. Thermal analysis The TG and DTG curves of APP and VMFAPP are shown in Fig. 7. APP has two main decomposition processes, beginning at about 2708C. The evolved products in the first step are mainly ammonia and water (about 20% mass loss), and crosslinked polyphosphoric acids (PPAs) are formed simultaneously.15 The second stage occurs in the range of 500–7008C, which is the main decomposition process of the APP, and weight loss is about 78%. The temperatures of maximum mass loss rate (Tmax) for the two steps are 326 and 6258C. The residual weight of APP is 0.6% at 8008C. In the first process, VMFAPP decomposes faster than APP due to the esterification between APP and PVA. Above 6168C, VMFAPP is more stable than APP. The Tmax for three steps of VMFAPP decomposition are 309, 391, and 5708C, respectively. Moreover, VMFAPP after decomposition at 8008C left about 19.6% residue, which is much higher than that of APP. The TG and DTG curves of PP and the PP composites are shown in Fig. 8. It is clearly seen that the pure PP begins to decompose at about 2408C and has almost completely Copyright # 2008 John Wiley & Sons, Ltd. decomposed at 3608C. The Tmax for the decomposition is 2998C, as shown in Fig. 8 (B). Thermal decomposition of the PP/APP composite includes three steps. Its initial decomposition temperature is a bit higher than that of PP. The composite PP/APP decomposes initially at about 2508C, which is caused by the decomposition of APP. The second step of mass loss is the main decomposition process of the composite, and the Tmax for this step was 3668C. The third step occurs at above 5008C due to further decomposition of the char. For PP/VMFAPP, its initial decomposition temperature is similar to that of PP/APP. Moreover, due to the esterification between the acid source and carbonization agent, PP/ VMFAPP decomposes faster in comparison with PP/APP; the TG curve of the sample PP/APP moves to a higher temperature in the range of 415–5958C. Above 5958C, PP/ VMFAPP is more stable than PP/APP. The increase in the weight of the residue at high temperature may be due to the formation of a more thermally stable carbonaceous char. The Tmax values for the first two decomposition steps of PP/ VMFAPP are 274 and 3718C, respectively. The third step is the decomposition of the char, and the Tmax value for this step is 6188C. From the above results, it can be concluded that VMFAPP is better than APP in improving the thermal stability of the PP composite at high temperatures. Polym. Adv. Technol. 2008; 19: 1914–1921 DOI: 10.1002/pat Microencapsulation of APP with VMF resin 1919 Figure 7. TG (A) and DTG (B) curves of (a) APP; (b) VMFAPP. Figure 8. TG (A) and DTG (B) of (a) PP; (b) PP/APP; (c) PP/ VMFAPP. Cone calorimeter study HRR ¼ 228 kW m
2. These data are consistent with the results of flame retardation of PP composites. Moreover, addition of VMFAPP leads to a delay in the time to ignition and strongly prolongs the process of combustion compared with APP in PP. From the above results, it can be concluded that the microencapsulation can remarkably enhance the flame retardant properties of APP in PP. Cone calorimetry is an effective approach to compare the combustion behavior of FR polymers. Heat release rate (HRR) results are shown in Fig. 9. The presence of intumescent systems in PP decreases the HRR values significantly compared with the pure PP (the HRR peak value of PP is 1177 kW m
2). In the case of PP/APP composite, its HRR peak is behind that of pure PP, and its peak value is a little lower (1064 kW m
2) compared with PP. However, it is noted that the ignition time (IT) of the PP/APP composite (24 sec) is less than that of PP (44 sec). The reason may be due to the fact that APP decomposes earlier than pure PP after the cone heater irradiated the surface of the composite, and some small volatile molecules are produced from the decomposition of APP. It can be seen in Fig. 9 that the HRR curve of PP/VMFAPP is very flat and the values of HRR decrease sharply compared with that of PP/APP. The HRR curve of the PP/VMFAPP is typical of IFR systems. The HRR curve exhibits two peaks. The first peak is assigned to the ignition and to the formation of an expanded protective shield; the second peak is explained by the destruction of the intumescent structure and the formation of a carbonaceous residue.4 Associated data for the PP/VMFAPP are: IT ¼ 39 sec, peak Copyright # 2008 John Wiley & Sons, Ltd. Figure 9. Heat release rate of PP and FR PP composites. Polym. Adv. Technol. 2008; 19: 1914–1921 DOI: 10.1002/pat 1920 K. Wu et al. Figure 10. Residues at the end of cone calorimeter tests of (a) PP/APP; (b) PP/ VMFAPP. This figure is available in color online at www.interscience.wiley.com/ journal/pat To study the flame retardant mechanism of APP or VMFAPP in polymers, we used dynamic FTIR to evaluate the thermal degradation of the flame retardant. For APP (Fig. 11), below 2508C, the spectra of APP present the same peaks as the pure products taken at room temperature; no modification of the chemical structure is observed. Above 2508C, the bands that correspond to –NH4 (1434 cm
1)16 disappear; this may be related to the elimin- ation of NH3. As the pyrolysis temperature increases from 25 to 3008C, the 1256 cm
1 peak (P – – O) moves to a higher wave number. The reason for the movement may be the scission of P–O–N and the elimination of NH3. These results demonstrate that the evolved products in the first step are mainly ammonia and water, and crosslinked PPA are formed simultaneously. Moreover, the 1019 cm
1 spectral peak corresponds to the symmetric vibration of PO2 and of PO316 which allows us to determine the stability of the PPAs. From 5508C upwards the change of the peaks at 1019 cm
1 can be explained by the further decomposition of PPA, which is in agreement with the changes in the absorption range (3100–3300 cm
1) of the O–H or N–H.12,16 For VMFAPP, its thermal degradation process is more complicated than that of APP. It can be seen from Fig. 12 that the strong absorption around 3200 cm
1 gradually disappears below 3008C; this may be caused by the elimination of NH3 of APP and dehydration of APP and PVA. This viewpoint can be verified by the movement of 1080 cm
1 peak (P–O–C).5,16 Moreover, we should notice the absence of the absorptions of 1560, 1504, and 1339 cm
1 which are due to Figure 11. Dynamic FTIR spectra of APP with different pyrolysis temperatures. Figure 12. Dynamic FTIR spectra of VMFAPP with different pyrolysis temperatures. The appearance of FR PP composite residues at the end of cone calorimeter tests is shown in Fig. 10. It is clear that there is almost no residue left at the end of the cone calorimeter test for the PP/APP composite. On the other hand, the surface of the PP/VMFAPP residue is covered with an expanded char network. The residue left by PP/VMFAPP is mainly formed of thick black char, and the char is better than that of PP/APP in protecting the underlying materials. The results indicate that a good and coherent char can prevent the heat transfer and flame spread, and thus protect the underlying materials from further burning. Thermal degradation of APP and VMFAPP Copyright # 2008 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2008; 19: 1914–1921 DOI: 10.1002/pat Microencapsulation of APP with VMF resin 5 the ring vibration of melamine groups at about 3508C; this can be explained by the fact that melamine is disassociated and evaporates at higher temperatures. It is interesting to find that between the range of 300 and 6008C, the shapes of the spectra show few changes. This may be related to the formation of stable structures containing P–O–P and P –– O (1080, 1020, 880 cm
1) complexes. Between the same temperatures, it is clear from Fig. 11 that APP decomposes further. These dynamic thermal degradation data give positive evidence for the flame retardant mechanism: the microcapsule with a shell composed of VMF can form a more stable charred layer in the condensed phase compared with APP during burning of polymer materials. This char layer slows heat and mass transfer between the gas and condensed phases and prevents the underlying polymer from further degradation. From these results, it can be concluded that the shell can be used as a carbonization agent and blowing agent. These results are in agreement with the data of TG, LOI, UL 94, and cone calorimetry. CONCLUSIONS In this work, APP was microencapsulated with VMF resin by an in situ polymerization method. Microencapsulated APP (VMFAPP) decreases its water absorption and increases its water resistance in PP matrix. The LOI values of the PP/ VMFAPP composites increase compared with those of the PP/APP composites at the same loading. It has been found that APP used alone in PP does not reach the UL 94 V-0 rating and VMFAPP used alone in PP can reach V-0 at the additive level of 30%. Moreover, after water treatment at 508C for 24 hr, the composites containing VMFAPP could still maintain good flame retardant properties. These results show that microencapsulation gives better water resistance and flame retardance compared with APP in PP. The cone calorimeter results show that VMFAPP is an effective flame retardant in PP compared with APP owing to the shell which can be used as blowing and carbonization agent. It is observed from the TG and dynamic FTIR study that VMFAPP can form a more stable charred layer which can prevent the underlying polymer from further combustion in the condensed phase compared with APP in PP. Acknowledgments The financial support from the National Natural Science Foundation of China (No. 20776136), the program for New Century Excellent Talents in University, and National 11th Five-year Program (2006BAK01B03, 2006BAK06B06, and 2006BAK06B07) is acknowledged. Copyright # 2008 John Wiley & Sons, Ltd. 1921 REFERENCES 1. Le Bras M, Bourbigot S, Delporate C, Siat C, Le Tallec Y. New intumescent formulations of fire-retardant polypropylene— discussion of the free radical mechanism of the formation of carbonaceous protective material during the thermooxidative treatment of the additives. Fire Mater. 1996; 20: 191–203. 2. Almeras X, Le Bras M, Hornsby P, Bourbigot S, Marosi G, Keszei S, Poutch F. Effect of fillers on the fire retardancy of intumescent polypropylene compounds. Polym. Degrad. Stab. 2003; 82: 325–331. 3. Almeras X, Le Bras M, Poutch F, Bourbigot S, Marosi G, Anna P. Effect of fillers on fire retardancy of intumescent polypropylene blends. Macromol. 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