Preparation of Polyurethane Microencapsulated Expandable Graphite

ARTICLE pubs.acs.org/IECR

Preparation of Polyurethane Microencapsulated Expandable Graphite, and Its Application in Ethylene Vinyl Acetate Copolymer Containing Silica-Gel Microencapsulated Ammonium Polyphosphate Bibo Wang,† Shuang Hu,‡ Kuimin Zhao,† Hongdian Lu,§ Lei Song,*,† and Yuan Hu*,†,‡ †

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, People’s Republic of China ‡ Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, People’s Republic of China § Department of Chemical and Materials Engineering, Hefei University, 373 Huangshan Road, Hefei, Anhui, 230022, People’s Republic of China ABSTRACT: Polyurethane microencapsulated expandable graphite (PUEG) is prepared by in situ polymerization, and its structure is characterized by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The results indicated PUEG is successfully prepared. The microencapsulated expandable graphite (EG) leads to an increase in the thermal stability and the expanded volume. The research compares the influence of an EG/ ammonium polyphosphate (APP)/ethylene vinyl acetate copolymer (EVA) system and a PUEG/silica-gel microencapsulated ammonium polyphosphate (MCAPP)/EVA system on the flame retardancy and mechanical properties. The results indicated that EVA/MCAPP/PUEG composite possesses the same LOI value and UL-94 rating as those of EVA/APP/EG composite. However, the EVA/MCAPP/PUEG system can still pass a UL-94 V-0 rating after being treated with 70 °C water for 168 h, indicating excellent water resistance. Because of good interfacial adhesion between fillers and the EVA matrix, the EVA/MCAPP/PUEG composite shows better mechanical and dynamic mechanical thermal properties than those of the EVA/APP/EG composite. Moreover, the electrical property and the combustion behavior of EVA composites are investigated.

1. INTRODUCTION Ethylene vinyl acetate copolymer (EVA) is widely used in many fields, especially in the cable industry as an excellent insulating material, because of its good physical and mechanical properties. However, similar to the majority of organic materials, EVA is so combustible that its application is particularly limited. Thus, it is necessary to treat it with flame retardants, among which halogenated compounds are the most widely used. Unfortunately, there has been much concern worldwide over the use of halogen-type flame-retardant polymeric materials, because they give rise to toxic gases and smoke that choke the people in the toxic and acidic fume and damage costly equipment.1 So, currently, more and more attention has been paid to study halogen-free flame-retardant (HFFR) materials. In the search for halogen-free flame retardants, intumescent flame retardants (IFRs) are remarkable for their low toxicity, low amount of smoke, halogen-free nature, and high efficiency.2,3 As a typical intumescent additive, expandable graphite (EG) is an intrinsical graphite inserted by sulfuric acid or nitric acid between the graphite layers, which is widely used as a blowing agent and suppressor for smoke.4 6 When exposed to heat, EG expands and generates a voluminous insulative layer thus providing fireretardant performance to the polymeric matrix.7 Recently, much literature has reported on the research of flame-retardant treatment with EG.8 13 However, with the inclusion of EG, the mechanical properties of the composites are sacrificed, because of the poor compatibility.14,15 r 2011 American Chemical Society

In order to improve the interfacial adhesion between the EG particles and the polymer matrix, one possible approach is to modify the surface of EG with microencapsulation technology. Hopefully, the encapsulation or grafting of polymers onto the EG surface can improve the dispersibility and compatibility of the EG in polymer matrix, which will enhance the mechanical properties. The process of microencapsulation can be divided into three categories: physical,16 phase interfacial reaction,17 19 and in situ reaction coating.20 22 Among these methods, the in situ reaction is a widely used process for the preparation of polymers and has been adapted for the encapsulation of a variety of inorganic particles in polymers. So far, much work has been done on the encapsulation of inorganic materials such as silica,23 ferrite,24 calcium carbonates,25 and carbon black.26 In our previous work, microencapsulated ammonium polyphosphate (MCAPP) with melamine formaldehyde, polyurethane, and silica-gel resin with in situ polymerization method were investigated and reported.27 29 The results suggested that the MCAPP can significantly increase the water resistance of APP and the interfacial adhesion between APP particles and the polymer matrix. Li et al.30 reported EG microencapsulated by poly(methylmethacrylate) (PMMA) through in situ emulsion polymerization; the pEG-PMMA particles (core shell structure) with COOH Received: April 25, 2011 Accepted: September 11, 2011 Revised: September 11, 2011 Published: September 12, 2011 11476

dx.doi.org/10.1021/ie200886e | Ind. Eng. Chem. Res. 2011, 50, 11476–11484

Industrial & Engineering Chemistry Research

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Scheme 1. Schematic Diagram of Reaction Steps Involved in the Preparation of PUEG Microcapsules

groups can react with the R NCO groups of isocyanurate to synthesize pEG-PMMA/RPUF with favorable compatibility between the fillers and the matrix. The objective of this work is to provide a formulation for the HFFR EVA with excellent properties through combining microencapsulated EG with silica-gel microencapsulated ammonium polyphosphate (MCAPP). In this study, first, the surface of EG is modified with polyurethane by microencapsulation technology. PUEG and MCAPP then are used as IFRs for EVA. The flaming property is studied using standard test procedure UL-94 before and after water treatment, and the electrical and mechanical properties of EVA composites are discussed.

2. EXPERIMENTAL SECTION 2.1. Material. Pentaerythritol, toluene-2,4-diisocyanate, 1,4dioxane, polyoxyethylene octylphenol ether (OP-10), and dimethyl sulfoxide were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, PRC). The commercial products ammonium polyphosphate (APP) (phase II, degree of polymerization of >1000) was a product of Shandong Shian Chemical Engineering Corporation. Silica-gel microencapsulated ammonium polyphosphate (MCAPP) was synthesized in our laboratory. Expandable graphite (EG) was supplied by Qingdao Tianhe Graphite Co., Ltd. Its main properties are given as follows:

ash moisture volatiles pH value particle size expanded volume

1.0% 1.0% 15% 3.0 325 mesh 150 mL/g

EVA copolymer containing 28 wt % vinyl acetate was supplied by Samsung Total Petrochemical (Korea). All these commercial materials were used without further purification. 2.2. Preparation of Polyurethane Microencapsulated Expandable Graphite (PUEG). A 500-mL oven-dried threenecked round-bottomed flask is prepared and equipped with a machine stirrer and a condenser. To this system, 1.36 g of pentaerythritol (0.01 mol) is dissolved in 20 mL of dimethyl sulfoxide at 40 °C, and then that mixture is poured into 3.64 g of toluene-2,4-diisocyanate (0.02 mol) and 100 mL of 1,4-dioxane. After 25 min, 40 g of EG is added, followed by 100 mL of 1,4-dioxane, and 0.4 g (1 wt % of EG) of OP-10. The mixture then is heated to 85 °C, and kept for 8 h. When it cools to room temperature, the solution is filtrated, and a bright suspension is obtained. The suspension is washed with water and dried at 80 °C. The PUEG is composed of 1/9 (%) of PU and 8/9 (%) of EG. The schematic diagram of PUEG microcapsules is shown in Scheme 1. 2.3. Preparation of Flame-Retarded EVA Composites. All flame-retarded (FR) EVA composites are prepared in a Brabender-like apparatus at a temperature of ∼140 °C for 15 min. After mixing, samples are hot-pressed at ∼140 °C under 10 MPa for 10 min into sheets of suitable thickness for analysis. The formulations are given in Table 1. 2.4. Characterization. Expanded Volume Testing. The expansion multiple of different particles is measured after they were expanded at 950 ( 10 °C for 15 s, according to GB 10698-89 (China standard).30 Fourier Transform Infrared Spectra. Powders were mixed with KBr powders, and the mixture was pressed into a tablet. The Fourier transform infrared spectra (FTIR) spectra of samples 11477

dx.doi.org/10.1021/ie200886e |Ind. Eng. Chem. Res. 2011, 50, 11476–11484


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Table 1. Formulation of the EVA Composites and Related Limiting Oxygen Index (LOI) and UL-94 Test Results sample

EVA (%)

EVA

100

EVA/EG

80

EVA/PUEG

80

EVA/APP

80

EVA/MCAPP

80

EVA/APP/EG

80

EVA/MCAPP/PUEG

80

PUEG (%)

EG (%)

MCAPP (%)

APP (%)

20 20 20 20 10 10

10 10

t1/t2 (s/s)a

dripping?

weight loss (g)b

LOI (%)

>30/

yes

1.42

18.5

NRc

4.6/

yes

1.35

28

NRc

4.5/

yes

1.33

28

NRc

5.4/

yes

1.26

25.5

NRc

4.4/

yes

1.32

26

NRc

UL-94 rating

1.8/2.5

no

0.12

32

V-0

1.4/1.6

no

0.08

32

V-0

The symbol “ ” means that the specimen burns completely and, therefore, t2 is not detectable. b Weight loss is the difference of weight before and after ignition. c No rating. a

were recorded using a Nicolet Model MAGNA-IR 750 spectrophotometer. X-ray Photoelectron Spectroscopy (XPS). The X-ray photoelectron spectroscopy (XPS) measurement was performed using an ESCALAB MK II (VG Co., Ltd., England) spectrometer, with Al Kα excitation radiation (hν = 1253.6 eV), under ultrahigh vacuum conditions. Scanning Electron Microscopy. The morphology of the MCAPP and after gold-sputtered were studies by Philips Model XL30E scanning electron microscopy (SEM) microscope. The specimens of EVA composites were cryogenically fractured in liquid nitrogen first, and then sputter-coated with a conductive layer. The accelerated voltage was 20 kV. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) was performed using a Q5000 IR thermogravimetric analyzer (TA InstrumentsWaters, China) at a linear heating rate of 20 °C min 1 in air. The temperature sensitivity and quality sensitivity of Q5000 are 0.1 μg and 0.001 °C, respectively. The weight of all the samples were kept within 5 10 mg. Samples in an open Pt pan were examined under an airflow rate of 150 mL min 1 at temperatures ranging from room temperature to 800 °C. Limiting Oxygen Index. The limiting oxygen index (LOI) was measured using a Model HC-2 oxygen index meter (Jiang Ning Analysis Instrument Company, China) on sheets with dimensions of 100 mm 6.7 mm 3 mm, according to the standard oxygen index test ASTM D2863-77. UL-94 Vertical Burning Test. The vertical burning test was conducted using a Model CZF-II horizontal and vertical burning tester (Jiang Ning Analysis Instrument Company, China). The specimens used had dimensions of 127 mm 12.7 mm 3 mm, according to the UL94 test (ASTM D3801-1996 standard). Cone Calorimeter. The combustion test was performed on the cone calorimeter (Stanton Redcroft, U.K.) 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/m2 external heat flux. For each formulation, the test was repeated three times and the experimental error was (5%. Determination of Water Resistance of EVA Composite. The specimens used for the flammability test were placed in 500 mL of distilled water at 70 °C and was kept at this temperature for various time periods. The specimens were subsequently removed, dried in the vacuum oven, and evaluated using burning tests (UL-94). Electric Properties. Volume resistivity of the composites was measured at room temperature by a high-insulation resistance

Figure 1. FTIR spectra of EG, PU, and PUEG.

meter (Shanghai Precision & Scientific Instrument Co., China). Square samples with an area of 100 mm 100 mm were used after they were cut from the molded sheets. Mechanical Properties. The mechanical properties were measured with a universal testing machine (Instron, Model 1185) at temperatures of 25 ( 2 °C. The crosshead speed was 20 mm/min. Dumbbell-shaped specimens were prepared according to ASTM D412. The tensile strength and elongation at break were recorded. Dynamic Mechanical Thermal Analysis (DMTA). Dynamic mechanical thermal analysis were measured (Model DMAQ 800, TA Instruments, USA). The dynamic storage modulus were determined at a frequency of 10 Hz and a heating rate of 5 °C/min over the range of 70 °C to 150 °C. The dimensions of the samples were as follows: ∼1 mm thickness, 20 mm length, and 5 mm width.

3. RESULTS AND DISCUSSION 3.1. Characterization Polyurethane Microencapsulated Expandable Graphite (PUEG). 3.1.1. Structure and Component of PU Shell Microcapsule. Figure 1 presents the FT-IR spectra of

EG, PU and PUEG. Comparison of the FTIR spectrum of EG, after microencapsulation, with that of PUEG reveals the absorption peaks of polyurethane. The absorptions at 1730, 1530, and 1223 and 1065 cm 1, as well as that at 2850 cm 1, are assigned to carbonyl groups (CdO) stretching of urethane, amide (NH), and the ether group (C O C) in urethane ( NHCOO ), as 11478



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Figure 2. XPS spectra of EG and PUEG.

well as =C H stretching, respectively.31 The FTIR data also indicates the completion of the reaction between diisocyanate and PER by the disappearance of the NCO absorption band at 2270 cm 1. The above results illustrate that the PUEG powder contains PU. XPS is applied to determine the quantity of the elemental and chemical composition of the outermost portion for the PU microcapsules. Figure 2 shows the evolution of C 1s and N 1s XPS spectrum of EG and PUEG. The C 1s spectrum of EG reveals the peaks at 284.6 and 286.2 eV, corresponding to the C C and C O groups, respectively.32 EG has the N 1s spectra at 401.8 eV due to the quaternary nitrogen.33 After microencapsulation, the PUEG’s surface has the urethane structure ( NHCOO ), the C 1s peak at 289.1 eV can be assigned to the N CdO or C CdO group, and the N 1s peak at 399.9 eV is assigned to the C N group. The changes of the above peaks are due to the coverage of the EG particles’ surface with the polyurethane, which indicates that EG is well coated with polyurethane. 3.1.2. Morphology of the Microcapsule Shell. Figure 3 shows the morphologies of the original EG and PUEG. As presented in Figure 3a, the EG particles have a lamellar structure, and some exfoliated graphite flakes with much smaller sizes are observed at the surface of the particles. The EG surface is smooth, which is because of the regular layers of the graphite flakes. After microencapsulation, the PUEG (presented in Figure 3b) presents irregular shapes with rough surface, which is due to the EG is coated with polyurethane. 3.1.3. Thermogravimetric Analysis of EG and PUEG. In order to determine whether a potential increase or decrease in the thermal stability happens between EG and the PU shell, the

Figure 3. SEM images of (a) EG and (b) PUEG.

weight difference curves between experimental and calculated TG curves were computed as follows:34 • MEG(T): values of weight given by the TG curve of EG • MPU(T): values of weight given by the TG curve of PU • MPUEG(T): values of weight given by the TG curve of PUEG • MTheoretic curve(T): theoretical TG curve computed by linear combination between the values of weight given by the TG curve of both additives: MTheoretic curve(T) = MEG(T) (8/9) + MPU(T) (1/9). The TGA and DTG curves of EG, PU, PUEG, and the theoretical curve are shown in Figure 4, and related TGA data are listed in Table 2. The 5% weight loss temperature is considered to be the initial decomposition temperature. Compared with the theoretical curve, the PUEG demonstrates a higher initial decomposition temperature and higher thermal stability in the temperature range of 200 800 °C. This is because the PU shell has a higher initial decomposition temperature, 11479



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Figure 4. (a) TGA and (b) DTG curves of EG and PUEG.

Table 2. TGA Data of EG and PUEG Tmax (°C) sample

T

5 wt %

(°C)

T1,max

T2,max

residue at 800 °C (%) 16.4

EG

201.4

216.4

724.7

PU

228.8

324.6

586.7

2.0

PUEG

217.5

227.2

713.6

33.7

theoretical curve

204.6

216.4

725.1

14.6

Scheme 2. Reaction between Nitric (Sulfuric) Acid and Graphite at High Temperature

Figure 5. Heat release rate (HRR) curves of EVA and flame-retardant EVA composites.

which can protect the inside EG core from the heat at the initial decomposition stage. Moreover, the PU shell can react with intercalation agents H2SO4 and HNO3 to form intumescent char, which works as an insulating layer to reduce the heat transfer and protect residue of EG. Therefore, the PUEG has greater residue than that of the theoretic curve. Therefore, the PUEG has better thermal stability than that of the EG or theoretic curve. 3.1.4. Expanded Volume of EG and PUEG. The expanded volume of EG and PUEG are measured according to expanded volume testing: the expanded volume of PUEG (170 mL/g) is larger than that of raw EG (150 mL/g). Generally, the expansion mechanism of EG is based on a redox process between acid and the graphite, which generates blowing gases, CO2, SO2, NO2, and H2O, according to the reaction in Scheme 2. Meanwhile, the lamellar structure of EG particles is transformed to a vermicular structure by expansion and exfoliation along the c-axis of the graphite crystal.7 When PUEG particles are subjected to the heat, the escaping speed of the blowing gases from the edge of the graphite flakes slows. This is because EG is microencapsulated by the shields of PU and char that is generated by the reaction of acid

and PU shell at high temperature. Therefore, the PUEG possesses greater expanded volume. 3.2. Effect of Flame Retardant on the Properties of EVA Composites. 3.2.1. Cone Calorimeter Study. The cone calorimeter, based on the oxygen consumption principle, has been widely used to evaluate the combustion behaviors of materials and products since its development at the National Bureau of Standards (NBS) (now known as the National Institute for Standards and Technology (NIST)) in 1982. Although the cone calorimeter is a small-scale test, some of its results have been found to correlate well with those obtained from large-scale fire tests and can be used to predict the behavior of materials in real fires. Heat release rate (HRR) results of EVA and flame-retardant EVA composites are shown in Figure 5, and the related total heat release (THR), peak HRR (PHRR), time to ignition (TTI), time to peak HRR, average HRR, and average specific extinction area (SEA) are recoded in Table 3. It can be found that pure EVA burns out within 268 s after ignition. A very sharp HRR peak appears in the range of 50 286 s with a peak heat release rate (pk-HRR) of 1269.1 kW m 2. However, the flame-retardant EVA composites show a dramatic decline of the HRR peaks and 11480



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Table 3. Related Cone Date of Intumescent Flame Retardant (IFR) EVA Composites EVA/APP/PER

TTI (s) time to PHRR (s) PHRR (kW m 2) THR (MJ m 2) average HRR (kW m 2) average SEA (m2 kg 1) FPI (m2 s kW 1)

EVA

55

144

1269.1

93.8

363.2

710.2

EVA/APP/EG

49

80

151.6

46.4

90.9

582.5

0.043 0.323

EVA/MCAPP/PUEG

55

75

159.7

44.6

67.7

567.8

0.344

Table 4. UL-94 Test Results of Flame-Retardant EVA Composites after Being Treated by Water for 7 days at 70 °C t1/t2

weight

rating after

(s/s)

dripping?

loss (g)a

water treatment

EVA/APP/EG

3.5/14.6

no

0.26

V-1

EVA/MCAPP/PUEG

2.3/5.1

no

0.14

V-0

sample

a

Weight loss is the difference of weight before and after ignition.

prolongation of the combustion times. The pk-HRR value for the EVA/MCAPP/PUEG system is 159.7 kW m 2, which is slightly higher than that for the EVA/APP/EG system (151.6 kW m 2). The reason may be due to the fact that the actual amount of flame retardant after microencapsulation is lower than that of the one without microencapsulation. The HRR curve for the EVA/APP/ EG composite exhibits two peaks: the first peak is assigned to the ignition and to the formation of an expanded protective shield, while the second peak is explained by the destruction of the intumescent structure and the formation of a carbonaceous residue. However, the HRR curve for the EVA/MCAPP/PUEG composite decreases steadily after peaking. From the above discussion, it can be concluded that the microencapsualation can increase the intensity of expanded protective shield and avoid the rupture of shield. Therefore, minroencapsulation enhances the flame-resistance property of the EVA composite. The THR, average HRR, and average SEA results also demonstrate that EVA/MCAPP/PUEG has higher flame retardancy than that of the EVA/APP/EG composite. In order to judge the fire hazard more clearly, the fire performance index (FPI) was selected. The FPI is defined as the proportion of TTI and the peak HRR. It is reported that there is a certain correlation between the value of FPI of material and the time to flashover. When the value of FPI reduces, the time to flashover will be advanced. Thus, it is generally accepted that the value of FPI of a material is smaller and its fire risk is higher. Apparently, the EVA/MCAPP/PUEG composite has the greatest safety in a fire hazard. 3.2.2. Flammability Properties. The flame retardancy of the EVA composites were evaluated by the LOI and UL-94 tests, and the results, together with the composition of the EVA composites, have been shown in Table 1. It can be found that the LOI values of the EVA/EG, EVA/PUEG, EVA/APP, and EVA/ MCAPP systems were 28%, 28%, 25.5%, and 26%, respectively. The UL-94 test specimen burns to the clamp and serious dripping occurs during the second phase of ignition. No ratings in the UL-94 tests were observed for the systems. Consequently, a single additive did not improve the flame retardancy of the system significantly. However, when EG and APP are combined, the LOI value of EVA/APP/EG and EVA/MCAPP/PUEG composites increase to 30%. The combination of EG and APP or PUEG and MCAPP also affects the combustion kinetics, as perceived by measuring the time of burning after the first flame application (t1): indeed, t1 decreases (4.4 5.4 s, vs 1.8 and 1.4 s)

Figure 6. SEM micrographs of the surfaces of the EVA composites treated by water for 7 days at 70 °C ((a) EVA/APP/EG and (b) EVA/ MCAPP/PUEG) and the fracture surface ((c) EVA/APP/EG and (d) EVA/MCAPP/PUEG).

because a carbonaceous residue is quickly formed.34 The relative time (t2) decreases obviously and without dripping during the second phase of ignition. The UL-94 results show that the EVA/ APP/EG and EVA/MCAPP/PUEG systems give a V-0 rating. The weight loss of the EVA/APP/EG and EVA/MCAPP/PUEG systems are 0.12 and 0.08 g, respectively, which are obviously lower than that of the EVA/EG, EVA/PUEG, EVA/APP, and EVA/MCAPP systems (weight loss of >1 g). Therefore, it could be concluded that a significant synergistic effect existed between APP and EG when they were applied to EVA. 3.2.3. Flammability Properties of EVA Composites after Water Treatment. Table 4 shows the UL-94 test results of EVA composites after water treatment. After being treated by water at 70 °C for 7 days, the EVA/APP/EG system can only obtain a V-1 rating, whereas the EVA/MCAPP/PUEG system can pass the UL-94 test. This is because the hydrophobicity and good compatibility of the shell with the EVA matrix; the loss rate of MCAPP composites is only slightly affected by hot water with long time treatment. The SiO2 gel shell can prevent the APP core from the reaction and preserve the flame retardancy of the flameretardant (FR) EVA composites. The EVA/MCAPP/PUEG composite has better water resistance than that of the EVA/APP/EG composite, which also can be demonstrated by the SEM images. Figure 6 shows SEM micrographs of surface and fracture surface of the composites 11481



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Figure 7. (a) Tensile strength and (b) elongation at break of EVA composites.

Figure 8. The temperature dependence of (a) tan δ and (b) storage modulus of pure EVA and flame-retardant EVA composites.

treated by water for 7 days at 70 °C. As shown in Figure 6a, many particles in the EVA/APP/EG composite are exposed on the surface, which is due to its poor compatibility with the EVA matrix. Because of the corrosion of APP particles by water, the surface is porous. From Figure 6b, the surface of the EVA/ MCAPP/PUEG composite is smooth and has almost no holes after treatment, because the shell isolates the core from water and increase the compatibility with the EVA matrix. It can also be seen from Figure 6c that many little holes are present in the EVA/APP/EG fracture surface, which is attributable to corrosion of APP particles by water; big holes are gaps between EG particles and EVA matrix, which are due to poor compatibility. The SiO2 hydrophobicity gel can greatly decrease the water solubility of MCAPP; thus, many MCAPP particles still exist on the fracture surface after treatment with water (Figure 6d). The above results indicate that the microencapsulation has a positive effect on the water durability of flame-retardant EVA composites. 3.2.4. Mechanical Properties. The tensile strength and elongation at break of EVA composites are shown in Figure 7. As revealed in Figure 7, the microencapsulated flame-retardant-filled

EVA composites have higher mechanical properties than that of raw flame-retardant-filled EVA composites. Because EG and APP are inorganic materials and contain some polar groups, the inorganic filler have bad distribution and compatibility in the EVA matrix, which can be confirmed from the SEM micrographs of the EVA composites. After microencapsulation, the shell materials can improve the interfacial compatibility between the filler and the EVA matrix and achieve a uniform dispersion of filler in the EVA matrix. Therefore, the mechanical property of EVA/MCAPP/PUEG improves greatly. 3.2.5. Dynamical Mechanical Behavior. The change in tan delta (tan δ) and storage modulus of pure EVA, EVA/APP/EG and EVA/MCAPP/PUEG with temperature are presented in Figure 8. The temperature at the maximum tan δ peak is considerd to be the glass-transition temperature (Tg). Figure 8a shows that the EVA/APP/EG system showed higher Tg values than that of pure EVA, which is due to the rigid filler system, which limited the mobility of the polymer chains. The EVA/MCAPP/PUEG shows relatively lower Tg values than that of the EVA/APP/EG composite, which is because EG is 11482



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Table 5. Volume Resistivity of Pure EVA and Flame-Retardant EVA Composites sample

volume resistance (Ω cm)

from the EVA matrix. In summary, the investigation provides a promising formulation for the halogen-free flame-retardant EVA with excellent properties.

EVA

6.87 1014

’ AUTHOR INFORMATION

EVA/EG EVA/PUEG

1.34 1014 1.01 1015

Corresponding Author

EVA/APP

1.23 10

EVA/MCAPP

1.38 1015

EVA/APP/EG

1.68 1015

EVA/MCAPP/PUEG

1.98 1015

15

microencapsulated by a flexible PU shell. As shown in Figure 8b, filled EVA composites at the glass-transition region have a higher storage modulus than that of pure EVA. It can be explained that the rigid filler has imparted stiffness behavior to the filled EVA composites. From 70 to 70 °C, the storage modulus of EVA/ MCAPP/PUEG have higher value than that of EVA/APP/EG, which indicates that microencapsulation enhance of the compatibility between the filler and the matrix system. 3.2.6. Volume Resistivity. Table 5 lists the electrical properties of pure EVA and flame-retardant EVA composites. As shown in Table 4, the volume resistivity of the EG-filled EVA composite is significantly reduced. After microencapsulation, all the microencapsulated flame-retardant-filled EVA composites have higher volume resistivity than that of raw flame-retardant-filled EVA composites, while the volume resistivity of the EVA/PUEG composite is almost one order magnitude higher than that of the EVA/EG composite. This is because EG is electrically conductive, and the addition of EG will cause a decrease of the bulk resistivity of EVA composites. When EG is microencapsulated by PU shell, the shell can be considered as an insulating shield which can isolate the EG from the EVA matrix. As a result, a whole conductive pathway cannot be generated. Therefore, the microencapsulated EG flame retardant EVA composite has higher volume resistivity.

4. CONCLUSION In this paper, expandable graphite (EG) is microencapsulated with polyurethane from pentaerythritol toluene-2,4-diisocyanate via an in situ polymerization method. It is found that the presence of the microencapsulated EG results in an increase in the thermal stability and expanded volume. In comparison with the ethylene vinyl acetate copolymer (EVA)/ammonium polyphosphate (APP)/EG composites, the EVA/microencapsulated ammonium polyphosphate (MCAPP)/polyurethane microencapsulated expandable graphite (PUEG) ternary composites at the same additive loading have the same limiting oxygen index (LOI) value and UL-94 rating. The EVA/MCAPP/PUEG composites can still pass the UL-94 V-0 rating after treatment by water for 7 days at 70 °C. Whereas, the EVA/MCAPP/PUEG composites can only maintain UL-94 V-1 rating. The cone calorimeter results indicate that the EVA/MCAPP/PUEG can increase the intensity of expanded protective shield and avoid rupture of the shield. Furthermore, the EVA/MCAPP/PUEG composites demonstrate higher mechanical, dynamical mechanical, and electrical properties than those of EVA/APP/EG composites. That is because the microencapsulated shell material not only can enhance the interfacial adhesion between the fillers and EVA matrix, but it can also act as an insulating shield to isolate the EG

*Tel./Fax: +86-551-3601664. E-mail addresses: yuanhu@ustc. edu.cn (Y.H.), [email protected] (L.S.).

’ ACKNOWLEDGMENT The work was financially supported by the Program for Specialized Research Fund for the Doctoral Program of Higher Education (200803580008), the youth innovation fund of USTC, the Program for Science and Technology of SuZhou (SG-0841), and the joint fund of Guangdong province and CAS (No.2010A090100017). ’ REFERENCES (1) Sen, A. K.; Mukherjee, B.; Bhattacharya, A. S.; Sanghi, L.; De, P. P.; Bhowmick, K. Preparation and characterization of low-halogen and nonhalgoen fire-resistant low-smoke (FRLS) cable sheathing compound from blends of functionalized polyolefins and PVC. J. Appl. Polym. Sci. 1991, 43, 1673–1684. (2) Heinrich, H.; Stefan, P. The importance of intumescent systems for fire protection of plastic materials. Polym. Int. 2000, 49, 1106–1114. (3) Bourbigot, S.; Le, B. M.; Duquesne, S.; Rochery, M. Recent advances for intumescent polymers. Macromol. Mater. Eng. 2004, 289, 499–511. (4) Duquesne, S.; Le, B. M.; Bourbigot, S.; Delobel, R.; Vezin, H.; Camino, G. Expandable graphite: A fire retardant additive for polyurethane coatings. Fire Mater. 2003, 27, 103–117. (5) Lyon, R. E.; Speitel, L.; Walters, R. N.; Crowley, S. Fire-resistant elastomers. Fire Mater. 2003, 27, 195–208. (6) Qu, B. J.; Xie, R. C. Intumescent char structures and flameretardant mechanism of expandable graphite-based halogen-free flameretardant linear low density polyethylene blends. Polym. Int. 2003, 52, 1415–1422. (7) Ye, L.; Meng, X. Y.; Liu, X. M.; Tang, J. H.; Li, Z. M. Flameretardant and mechanical properties of high-density rigid polyurethane foams (RPUF) filled with decabrominated dipheny ethane (DBDPE) and expandable graphite (EG). J. Appl. Polym. Sci. 2009, 111, 2372–2380. (8) Li, Z. Z.; Qu, B. J. Flammability characterization and synergistic effects of expandable graphite with magnesium hydroxide in halogen-free flame-retardant EVA blends. Polym. Degrad. Stab. 2003, 81, 401–408. (9) Kuan, C. F.; Yen, W. H.; Chen, C. H.; Yuen, S. M.; Kuan, H. C.; Chiang, C. L. Synthesis, characterization, flame retardance and thermal properties of halogen-free expandable graphite/PMMA composites prepared from sol gel method. Polym. Degrad. Stab. 2008, 93, 1357–1363. (10) Meng, X. Y.; Ye, L.; Zhang, X. G.; Tang, P. M.; Tang, J. H.; Ji, X.; Li, Z. M. Effects of Expandable Graphite and Ammonium Polyphosphate on the Flame-Retardant and Mechanical Properties of Rigid Polyurethane Foams. J. Appl. Polym. Sci. 2009, 114, 853–863. (11) Kandare, E.; Chukwudolue, C.; Kandola, B. K. The use of fireretardant intumescent mats for fire and heat protection of glass fibrereinforced polyester composites: Thermal barrier properties. Fire Mater. 2010, 34, 21–38. (12) Kandare, Everson.; Chukwunonso, A. K.; Kandola, B. K. The effect of fire-retardant additives and a surface insulative fabric on fire performance and mechanical property retention of polyester composites. Fire Mater. 2011, 35, 143–155. (13) Zhu, H. F.; Zhu, Q. L.; Li, J. A.; Tao, K.; Xue, L. X.; Yan, Q. Synergistic effect between expandable graphite and ammonium polyphosphate on flame retarded polylactide. Polym. Degrad. Stab. 2011, 96, 183–189. 11483


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Preparation of Polyurethane Microencapsulated Expandable Graphite

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