Influence of ammonium polyphosphate microencapsulation on flame retardancy, thermal degradation and crystal structure of polypropylene composite

Accepted Manuscript Influence of ammonium polyphosphate microencapsulation on flame retardan‐ cy, thermal degradation and crystal structure of polypropylene composite Kun Wu, Yankui Zhang, Wenguang Hu, Jintian Lian, Yuan Hu PII: DOI: Reference:

S0266-3538(13)00141-3 http://dx.doi.org/10.1016/j.compscitech.2013.03.018 CSTE 5467

To appear in:

Composites Science and Technology

Received Date: Revised Date: Accepted Date:

22 December 2012 16 March 2013 19 March 2013

Please cite this article as: Wu, K., Zhang, Y., Hu, W., Lian, J., Hu, Y., Influence of ammonium polyphosphate microencapsulation on flame retardancy, thermal degradation and crystal structure of polypropylene composite, Composites Science and Technology (2013), doi: http://dx.doi.org/10.1016/j.compscitech.2013.03.018

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Influence of ammonium polyphosphate microencapsulation on flame retardancy, thermal degradation and crystal structure of polypropylene composite Kun Wu 1, Yankui Zhang 1, Wenguang Hu 1, Jintian Lian 1, Yuan Hu 2 1

Key Laboratory of Cellulose and Lignocellulosics Chemistry, Guangzhou Institute of

Chemistry, Chinese Academy of Sciences, Guangzhou 510650, Guangdong, China 2

State Key Laboratory of Fire Science, University of Science and Technology of

China, Hefei 230026, Anhui, China Abstract Microencapsulated ammonium polyphosphate (EPAPP) with shell of epoxy resin (EP) is prepared by in situ polymerization. EP is hydrophobic, so EPAPP has better water resistance and compatibility in polypropylene (PP) composite compared with ammonium polyphosphate (APP). Due to the reaction between APP and EP under heating, EPAPP can form a residue char with good thermal stability which prevents underlying materials from further destruction during a fire. Because of the interaction and alignment of the PP polymer chains at the surface of additives, microencapsulation affects the crystal structure and spherulitic morphology of PP composite remarkably. PP/EPAPP didn’t show characteristic peaks for β-form crystal, while APP could act as an effective β-nucleating agent in PP. Keywords: A. Polymers; A. Hybrid composites; B. Thermal properties; D. X-ray diffraction 1. Introduction In recent years, the flame retardation and thermal behavior of polypropylene (PP) composites has been a growing interest issue [1, 2]. The introduction of intumescent



Corresponding author, Tel (Fax): + 86-20-85231925; E-mail address: [email protected] 1

flame retardant into PP had been of great interest as a cost-effective route aiming to improve flame retardant properties of PP [3-5]. An intumescent flame retardant system is usually composed of three active ingredients, i.e. an acid agent, a carbonization agent and a blowing agent [5]. One of the conventional acid sources is ammonium polyphosphate (APP). Unfortunately, intumescent flame retardant system containing APP is not permanent due to the weak water resistance and poor compatibility of APP in polymers. Microencapsulation of APP in a polymeric shell could overcome above problems. In previous study by Liu et al., microcapsules containing APP with various shells, such as urea-formaldehyde, melamine-formaldehyde, nylon-6, etc., for intumescent flame retardant systems were reported [6-8]. The results show that after microencapsulation, flame retardancy and water resistance of APP in polymer composites were improved remarkably. Much work has been reported on microencapsultion of intumescent flame retardant, and the influence of microencapaulation on water resistance, flame reatrdancy of PP composites were discussed. However, few work focused on effect of polymeric shell on the crystal structures, spherulitic morphology and crystallization behavior of PP/APP composites. In this paper, APP was microencapsulated with epoxy resin, and its effect on the flame retardancy, thermal degradation, crystal structures and spherulitic morphology of PP composites were studied. 2. Experimental 2.1 Materials Diglycidyl ether of biphenol A (DGEBA, E44, epoxy equivalent = 0.44mol/100

2

g) was provided from Jiangsu Wuxi Resin Plant (Wuxi, China) and were used directly without further purification. The epoxy curing agent (Aradur3225, based on aliphatic amines, active hydrogen equivalent=75 g/eq) was obtained from Huntsman, Inc. The commercial products APP (phase II, the degree of polymerization >1000) was a product of Hangzhou JLS Flame Retardants Chemical Corporation, China. Ethanol was purchased from Shanghai Chemical Reagent Corporation. PP (F401) with a melt flow index (MFI) of 2.3 g/10 min (230 oC/2.16 kg) was provided by Yangzi Petroleum Chemical Company, China. 2.2 Microencapsulation of APP with epoxy resin A 500 ml three-necked round-bottomed flask was prepared and equipped with a machine stirrer and a condenser. 40.0 g APP and 10.0 g E44 prepolymer were dispersed in 120 ml of ethanol at room temperature with a stirrer. After 15 minutes, the mixture was heated to 60 oC, then 3.3 g Aradur3225 curing agent which was dissolved in 30 ml ethanol was added slowly, the process lasted 30 minutes. The resulting mixture was heated to 80 oC, and kept for 3.5 hours with stirring. After that the mixture was filtered, washed with ethanol and distilled water, and dried at 65 oC under vacuum, and then microencapsulated APP (EPAPP) powder was finally obtained. 2.3 Preparation of PP composites Flame retarded PP composites were prepared in a Brabender-like apparatus at a temperature of about 180 oC for 15 min. After mixing, the samples were hot-pressed at about 180 oC under 10 MPa for 10 minutes into sheets of suitable thickness for analysis. The mass percentage of APP or EPAPP in PP composites was 30 wt%.

3

2.4 Measurements Scanning electron microscopy (SEM) The morphologies of flame retardants and the surfaces of PP composites were obtained with a scanning electron microscope XL30 (PHILIPS). The flame retardants were sprinkled onto a double-sided tape and sputter coated with a gold layer. The composites were cryogenically fractured in liquid nitrogen, and then sputter coated with the conductive layer. Solubility in Water Sample (about 10 g) was put into 100 ml distilled water at a different temperature and stirred at that temperature for 60 min. The suspension was then filtered. 50 ml of the filtrate was taken out and dried to a constant weight at 105 oC. Solubility of samples in water can be calculated. Microscale Combustion Calorimeter (MCC) The heat release rate (HRR) was measured in a MCC (MCC-2, GOVMARK) for studying the molecular-level fire behaviors of materials. About 5 mg samples were heated at a heating rate of 1 K/s in nitrogen stream flowing at 80 cm3/ min. The volatile, anaerobic thermal degradation products in the nitrogen gas stream are mixed with a 20 cm3/ min stream of pure oxygen prior to entering a 900 oC combustion furnace. Limiting Oxygen Index (LOI) LOI was measured according to ISO4589. The apparatus used was an oxygen index meter (JF-3, Jiangning Analysis Instrument Company, China). The specimens

4

used for the test were of dimensions 100×6.5×3 mm3. Thermogravimetry (TG) TG was carried out under air atmosphere at a heating rate of 10 oC /min using a thermogravimetric analyzer (TG209 F3 Tarsus, NETZSCH, Germany). In each case, 3-10 mg of the sample was examined in an open Pt pan under an air flow rate of 6×10 －5

m3/min.

Thermogravimetry- Fourier transform infrared spectroscopy (TG-FTIR) The TG-FTIR instrument consists of TG analyzer (TG209 F3 Tarsus, NETZSCH, Germany) coupled with FTIR spectrometer (TENSOR 27, Bruker, Germany) and the transfer line. The investigations were carried out under nitrogen atmosphere at a flow rate of 35.0 ml/min for TG, with heating rate of 10 oC/min. In order to reduce the possibility of gases condensing along the transfer line, the temperature in the gas cell and transfer line were set to 230 oC. X-ray diffraction (XRD) XRD patterns of the samples were recorded on an X-ray diffractometer (D/MAX-1200, Rigaku Corporation, Japan), using Cu (Kα) radiation (λ=0.154 nm) at room temperature in the range of 1.5o to 60o by steps of 0.02o. The relative content of β-phase, Kβ, was calculated according to equation (1), suggested by Turner-Jones [9]: Kβ=Iβ1/ (Iβ1+Iα1+Iα2+Iα3)

(1)

where Iβ1 is the diffraction intensity of the β[300] planes at diffraction angle 2θ= 16.02o, and Iα1, Iα2 and Iα3 are the diffraction intensities of the α[110], α[040] and α[130] planes at diffraction angles 2θ=14.06o, 16.76o and 18.52o respectively. [10]

5

Polarized Optical Microscopy (POM) Pure PP and its composites were placed between two microscope cover glasses on a heating stage, and heated to 200 oC to allow complete melting. Then, the samples were cooled down to 130 oC at a cooling rate of 10 oC/min and maintained at that temperature. Crystal morphologies of PP and its composites at the temperatures of 200 oC and 130 oC were recorded using a POM (Orthoplan, Leitz, Germany) equipped with a digital camera (Nikon-4500). Differential Scanning Calorimeter (DSC) DSC measurements were performed on a DSC Q200 differential scanning calorimeter. Samples of about 5-10 mg crimp-sealed in aluminum pans were heated at a heating rate of 20 oC/min in nitrogen atmosphere. When the temperature reached 200 oC, samples were maintained for 10 minutes to eliminate the thermal history and then cooled at a rate of 20 oC/min. The crystallization exotherm curves of the samples during cooling were recorded. 3. Results and discussion 3.1 SEM analysis Fig. 1 shows the morphologies of APP and EPAPP. The shape of APP particles is like a rod and exhibited smooth surface. After microencapsulation, EPAPP particles (Fig. 1b and c) present rough surfaces and cling to each other. It is proposed that, during the process of microencapsulation, single APP particle is coated first, and then particles collide and cling to each other in the further process. Above results reveal that APP was microencapsulated well.

6

3.2 Water solubility The water solubility of APP at 25 oC and 75 oC are 0.45 and 2.38 g/100ml H2O, respectively, indicating that the water solubility of APP rises quickly with the increase of temperature. The reason is that APP particles are easily attacked by the water molecules when exposed in water surroundings, especially at high temperature. After microencapsulation with EP shell, the solubility of EPAPP decreases to 0.038 g/100ml H2O at the temperature of 25 oC. Moreover, the change of temperature shows less effect on the solubility of EPAPP (1.22 g/100ml H2O at 75 oC) compared with neat APP. It is because that APP is coated by a layer of EP which is hydrophobic and prevents APP from being attacked by water. 3.3 Flame retardancy and water resistance Heat release rate (HRR) results of PP and its composites in MCC test are shown in Fig. 2. The presence of the flame retardant additives in PP decreases the HRR values when compared to the pure PP. In the case of the PP/APP composite, its peak HRR value is 815 W/g, which is lower than that of the pure PP (942 W/g). The peak HRR value of PP/EPAPP composite is 787 W/g, which is slightly lower than that of PP/APP. Considering the error range of MCC, it seems that microencapsulation show little effect on the HRR value of PP/APP in this test. In order to investigate the influence of microencapsulation on water resistance and LOI value of PP composites, the LOI values of PP composites before and after hot water treatment (50 oC, 24 h) are studied. The LOI value of PP/APP composite is 20.0%, whereas the value of treated sample is 17.5%. The value of PP/EPAPP

7

composite is 23.7%, and the value of treated sample just drops down by 0.7%. The LOI value of PP/EPAPP is higher than that of PP/APP, meaning that EP shell show synergic effect in this flame retardant system. Moreover, EP shell outside EPAPP particles is hydrophobic, which lead PP/EPAPP better water resistance compared with PP/APP. The fractured surface of PP/APP and PP/EPAPP composites before and after water treatment was observed by SEM as shown in Fig. 3. From Fig. 3a, it can be seen that APP particles are distributed unevenly in PP matrix. A clear interfacial line can be observed, which is attributed to the poor compatibility between APP and PP. Comparatively, EPAPP is enchased so well in PP that interfacial line is barely seen from Fig. 3c. After water treatment, many hollows were left on the surface of PP/APP (Fig. 3b), while the same thing does not occur on the surface of PP/EPAPP (Fig. 3d). It is speculated on the poor compatibility and water resistance of APP compared with EPAPP in PP. The results were consistent with that of LOI testing. 3.4 TG analysis TG curves of APP and EPAPP are illustrated in Fig. 4. The initial decomposition temperature (T0) and solid residue left at 700 oC were obtained from the TG curve; the temperature of the maximum mass loss rate (Tmax) of samples was obtained from the derivative thermogravimetric (DTG) curve. The thermal degradation of APP is composed of two steps according to the two DTG peaks (310 oC and 603 oC) and its T0 is about 270 oC. The first stage corresponds to a weight loss of 12.0%. The first stage mainly evolves in the elimination of

8

ammonia and the formation of a highly cross-linked polyphosphoric acid (PPA). The second stage occurs above 500 oC, which corresponds to PPA evaporation and/or dehydration to P4O10 which sublimes [11]. Moreover, there is about 4.2% residue left at 700 oC. EPAPP has two decomposition steps with the Tmax at 316 and 591 oC, respectively. In the first step, EPAPP decomposes earlier and faster than APP due to the reaction between APP (core) and EP (shell). The main degradation of EPAPP is similar to APP, but its maximum weight loss rate in the step is lower than that of APP. It is speculated on the formation of a char which has a better performance to resist the further degradation. Furthermore, the residue of EPAPP at 700 oC is 5.3%, which is a little higher than that of APP. The TG curves of PP and its composites are shown in Fig. 5. The pure PP begins to decompose at about 260 oC and has almost completely decomposed at 400 oC. The Tmax for its decomposition is 363 oC. Thermal decomposition of PP/APP includes three steps. Its initial decomposition temperature is a bit lower than that of PP. The T0 of PP/APP is 251 oC, which is lower than that of PP. The second step of mass loss is the main decomposition process of PP/APP, and the Tmax for this step was 359 oC. The third step occurs at above 500 oC due to further decomposition of the char. For PP/EPAPP, its initial decomposition temperature is similar to that of PP/APP. Moreover, due to the reaction between the acid source (core, APP) and carbonization agent (shell, EP), PP/EPAPP decomposes faster in comparison with PP/APP. The TG curve of PP/APP moves to a higher temperature in the range of 310-546 oC. Above 546 oC, PP/EPAPP is more stable than PP/APP. The increase in the weight of the

9

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/EPAPP are 301 and 360 oC, respectively. The third step is the decomposition of the char, and the Tmax value for this step is 574 oC. From the above results, it can be concluded that EPAPP is better than APP in improving the thermal stability of the PP composite at high temperature. 3.5 TG-FTIR of PP/EPAPP TG-FTIR is an effective method for analysis of the gaseous products during thermal decomposition, used here to study the thermal degradation behavior of PP/EPAPP. The characteristic FTIR spectra of the volatile pyrolysis products evolved at different temperature are shown in details in Fig. 6. It is clear that there is very little gas products produced below the temperature of 340 oC. At the beginning, the release of NH3 was detected at 346 oC, which is aroused by the decomposition of EPAPP. EPAPP releases NH3 and polyphosphoric acid (PPA), and accelerates the decomposition of EP shell and PP. So, when it comes to 450 oC, a lot of gas products are released. The typical absorptions for such products are reported with CH2/CH3 stretching (2965 cm-1, 2920 cm-1), C=C stretching (1661 cm-1), asymmetric CH3 bending (1458 cm-1), symmetric CH3 bending (1380 cm-1), NH3 (968, 933 cm-1) and P-O (880 cm-1), etc. [12, 13] Correspond well to the evolved gas reported for pure PP, the main decomposition products of PP/EPAPP were saturated and unsaturated hydrocarbons typically in the C1-C7 range [14, 15]. Moreover, there is a little absorption of P-O appears at 450 oC, and disappears upon 546 oC. The reason

10

is that EPAPP degrades and a spot of micromolecule containing P-O is released. With the rise of temperature, it’s the further degradation of precursor char and it releases little gaseous species. From above analysis, it can be seen that, firstly EPAPP release PPA and stimulate the thermal degradation of shell and PP. In the range of 346-546 oC, the main degradation process of PP/EPAPP was aroused by the pyrolysis of PP main chain. As a result, a residue char with good thermal stability was formed which can act as a protective thermal barrier. 3.6 Crystal structure The microstructures of PP, PP/APP and PP/EPAPP crystals were investigated with XRD analysis. Fig. 7 shows the XRD patterns of PP and its composites. Their 2θ values of different crystal phases and Kβ are list in Table 1. In general, PP can form four crystal phases: α-monoclinic, β-hexagonal, γ-triclinic, and δ in different environmental conditions. The monoclinic α phase with a cross-hatched lamellar structure is predominantly formed, and it shows the best stability among these crystal systems. So, pure PP crystallized in the monoclinic α-form as shown in the presence of the reflections at 2θ = 13.90◦, 16.77◦, 18.46◦, 21.10◦, and 21.80◦ in Fig. 7 [16]. Besides the characteristic diffraction spectrum of α-formed PP, XRD curves of PP composites (PP/APP, PP/EPAPP) had two characteristic peaks located at about 14.79◦ and 15.57◦, which are due to the orthorhombic structure of APP [17]. It was obvious that there was no change in d-value whether the flame retardant

11

(APP or EPAPP) was added or not in the PP composites. However, there was some β-form (16.01◦) crystal in PP/APP and its Kβ is 17.9%. This indicated that APP could act as a β-nucleating agent for PP crystallization in the composite. Here it is interesting that PP/EPAPP didn’t show characteristic peaks for β-form crystal. It can be explained that through microencapsulation the surface properties of APP particles changed, and surface modification of the flame retardant may influence the crystal forms of PP composite. 3.7 Spherulitic morphology POM images of PP and its composites at the temperature of 130 oC and 200 oC are presented in Fig. 8. PP has a nice capability of crystallization. As shown in Fig. 8b, the pure PP’s spherulites were quite big and integrated with many Maltese crosses, as clearly shown. At 200 oC, PP shows no spherulites because PP was molten at the temperature. The addition of flame retardants in PP composites influenced the spherulitic morphology remarkably. For images of PP/APP composites at 200 oC, there are many bright spots about 20 μm in diameters which are due to the presence of APP. However, Fig. 8e (PP/EPAPP at 200 oC) shows only a few faint bright spots. The reason for the lack of bright spots is that APP was coated with EP resin. In Fig. 8d, the spherulite size decreased and became distorted with the presence of APP. It was possible because APP could act as heterogeneous nucleation agents and could form more nucleating points in PP composites. As a result, when plenty of crystal nucleuses grew simultaneously, the PP spherulites developed into smaller and

12

more integrate ones. After microencapsulation, EPAPP can’t act as effective heterogeneous nuclei. For PP/EPAPP (Fig. 8f), it shows few crystal. The reason is that the interface between polymer and additive plays an important role in the crystallization behavior of PP. Due to the interaction and alignment of the PP polymer chains at the surface of additives, microencapsulation affect the crystal behavior and spherulitic morphology of PP/EPAPP remarkably. 3.8 Crystallization behavior The DSC exotherms of PP, PP/APP and PP/EPAPP under non-isothermal conditions are shown in Fig. 9. From it, Tc (the temperature at the crossing point of the tangents of the baseline and the high-temperature side of the exotherm) and Tcp (the peak temperature of the exotherm) can be obtained. It is clear that the Tc and Tcp of the FR PP composites (PP/APP and PP/EPAPP) shifted to higher temperature compared with that of neat PP (Tc=116.6 oC, Tcp=110.5 o

C), indicating that the particles act as nucleating agents accelerating the

crystallization of the PP. Tc and Tcp values of PP/EPAPP (Tc=121.2 oC, Tcp=117.2 oC) are lower than that of PP/APP (Tc=123.6 oC, Tcp=120.5 oC). It is explained that APP is an effective nucleating agent compared with EPAPP. Moreover, the Tc-Tcp values of PP/APP (3.1) and PP/EPAPP (4.0) were smaller than that of neat PP (6.1), indicating that the addition of flame retardant into PP increased the crystal growth rate (CGR) of PP. The Tc-Tcp values were as follows: PP > PP/EPAPP > PP/APP, meaning that PP/EPAPP had higher CGR. In SEM picture (Fig. 1), EPAPP particles presented rough surface. So, it can be proposed here that the roughness of additive surface may

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have a positive effect on the CGR of the PP composites. 4. Conclusions In this paper, ammonium polyphosphate (APP) was microencapsulated with epoxy resin by in situ polymerization. After microencapsulation, microencapsulated APP (EPAPP) particles present rough surfaces and cling to each other. Because the shell outside is hydrophobic, EPAPP shows better water resistance compared with neat APP in PP. From TG and TG-FTIR analysis, it can be seen that, EPAPP release PPA and stimulate the thermal degradation of PP and shell at earlier stage and then a residue char with good thermal stability was formed which can act as a protective thermal barrier. As a result, PP/EPAPP show higher LOI value and lower HRR value compared with PP/APP. Due to the interaction and alignment of the PP polymer chains at the surface of flame retardant, microencapsulation affect the crystal form, spherulitic morphology and crystallization behavior of PP/APP remarkably. APP could act as a β-nucleating agent for PP crystallization in the PP composites, while PP/EPAPP didn’t show characteristic peaks for β-form crystal. The roughness of EPAPP surface may have a positive effect on the crystal growth rate of PP. Acknowledgements The financial supports from the National Natural Science Foundation of China (No. 51003123), Natural Science Foundation of Guangdong Province, China (No. 10451065004004230) and Zhujiang Science&Technology New-star Program of Guangzhou, China (No. 1217000264) are acknowledged. References

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a

b

c

Fig. 1 SEM images of APP (a) and EPAPP (b, C)

17

1000

PP PP/APP PP/EPAPP

1000 800

800

HRR (w/g)

600

600

450

500

550

400

200

0 100

200

300

400

500

600

Temperature ( C) o

Fig. 2 HRR curves of PP, PP/APP and PP/EPAPP

a

b

c

d

Fig. 3 SEM images of fracture surfaces of the composites: a) PP/APP; b) PP/APP (50 o

C, 24h); c) PP/EPAPP; (d) PP/EPAPP (50 oC, 24h). The scale bar represents 40µm.

18

100

APP

Weight (wt%)

80

EPAPP 60

40

20

0 100

200

300

400

500

600

700

Temperature ( C) o

Fig. 4 TG curves of APP and EPAPP

100

Weight (wt%)

80

60

PP/APP

40

PP/EPAPP

PP

20

0 100

200

300

400

500

600

Temperature ( C) o

Fig. 5 TG curves of PP, PP/APP and PP/EPAPP

19

700

2965 2920

o

750 C o

700 C o

600 C o 546 C 1458

1380

1661

880

o

450 C

933

968

o

346 C o

250 C o

200 C 4000

3500

3000

2500

2000

1500

1000

500

Wave number ( cm ) -1

Fig. 6 FTIR spectra of gas products for PP/EPAPP at different temperature

15.5

16.02 14.7

PP/APP

PP/EPAPP

PP 10

15

20

25

30

2 theta (degree)

Fig. 7 X-ray diffraction patterns of PP, PP/APP and PP/EPAPP

20

a

200 oC

b

130 oC

c

200 oC

d

130 oC

e

200 oC

f

130 oC

Fig. 8 Polarized optical micrographs of PP (a, b), PP/APP (c, d) and PP/EPAPP (e, f) at 200 oC and 130 oC

Endothermic heat flow

PP/EPAPP

PP/APP

Tc

PP

Tcp 40

60

80

100

120

140

160

180

Temperature ( C) o

Fig. 9 DSC crystallization exotherms of PP and its composites during cooling

21

Table 1 Main crystal parameters of PP, PP/APP and PP/EPAPP PP PP/APP PP/EPAPP Sample 14.06

2θ (°)

16.76 18.52 21.10 21.78

0.63

dhkl (nm)

0.53 0.48 0.42 0.41

100

I /I0 (I: intensity)

58 46 41 44

Kβ (%)

--

22

14.08 14.66 15.52 16.02 16.86 18.56 21.08 21.88 26.10 27.52 29.12

14.16 14.76 15.60

0.63 0.60 0.57 0.55 0.53 0.48 0.42 0.41 0.34 0.32 0.31

0.63 0.60 0.57

13 35 100 7 11 8 10 11 11 20 13

13 32 100

17.9

--

16.88 18.64 21.16 21.86 26.14 27.60 29.22

0.52 0.48 0.42 0.41 0.34 0.32 0.31

13 10 10 12 10 20 9