Degradation Behavior of Polypropylene–Organically Modified Clay

Article pubs.acs.org/IECR

Degradation Behavior of Polypropylene−Organically Modified Clay Nanocomposites K. J. Singala,†,‡ A. A. Mungray,*,† and A. K. Mungray*,† †

Department of Chemical Engineering, Sardar Vallabhbhai National Institute of Technology, Surat 395007, Gujarat, India Reliance Industries Limited, Hazira Manufacturing Division, Surat 394510, Gujarat, India

‡

ABSTRACT: The degradation mechanism of polypropylene (PP)−clay nanocomposites is investigated in the present study. The PP−clay nanocomposites are prepared through a solution process with 4 and 8 wt % incorporation of nanoclay in a polymer matrix. Nanoclay with two types of organic modifications (25−30 wt % methyl dihydroxyethyl hydrogenated tallow ammonium (TA) and 25−30 wt % octadecylamine (OA)) is used with isotactic PP, atactic PP, and ethylene propylene copolymer with ∼20 wt % rubber. Characterization of nanocomposites is done through X-ray diffraction (XRD) analysis while melting and crystallization behavior is studied through differential scanning calorimetry (DSC). XRD and DSC analysis shows formation of intercalated type nanocomposites with improved barrier properties. Degradation behavior of nanocomposites is studied through thermogravimetric analysis (TGA). Weight loss curves are further analyzed through the Friedman technique to find the rate of degradation. Nanoclay incorporation increases the lifetime of nanocomposites, calculated using an equation derived by Toop [IEEE Trans. Electr. Insul. 1971, 6, 2−14]. The peak rate of degradation in nanocomposites with isotactic homo PP and ethylene propylene copolymer is found to increase up to 1.5 times. Atactic homo PP and its nanocomposites show only a marginal improvement in properties as well as degradation rate. It is also concluded that the rate of degradation increased with clay loading due to increase in residual impurity.

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INTRODUCTION Polypropylene (PP) and polyethylene (PE) are the most widely used commodity polymers. PP has an advantage over PE due to its lower bulk density i.e. lower weight to mass ratio;1 moreover, its properties can be altered by altering the side chain methyl group in the polymer chain.2 Also, incorporation of a third component such as glass fiber, mica, clay, etc.3 improves the properties of polymer. Montmorillonite (MMT) type layered silicate clay is now being widely used in preparation of PP−clay nanocomposites.3−6 The wide usage is due to its unique properties such as large surface area to volume ratio, high interfacial reactivity, nano-sized layered structure, etc.7−10 Nanoclay can be used without any modification11 or can be organically modified with a suitable modifier such as octadecylamine7 or stearylammonium8−10 to reduce surface tension. The use of unmodified clay is limited to hydrophilic polymers such as polyethylene oxide and polyvinyl alcohol. Organic modification is done to reduce hydrophobicity and to increase the interlayer spacing of clay layers.12 Intercalated and exfoliated types of nanocomposites are formed when PP and nanoclay are mixed. The intercalation type of nanocomposite is said to be formed when there is partial retention of the layered structure of nanoclay in a nanocomposite. Exfoliation is said to be formed when the layered structure of the nanoclay is completely destroyed, and individual layers are uniformly dispersed in the polymer matrix.13 Different processes can be adopted for preparation of polymer clay nanocomposites i.e., in-situ polymerization, solution intercalation, and simple melt mixing. A melt mixing process was discovered by Vaia et al.14 where nanoclay and polymer are mixed in a melt extruder. The extent of mixing © 2012 American Chemical Society

depends on the type and number of screws used in the extruder. This is the most economical process, as it does not involve any solvent. This process is also industrially accepted, as all the manufacturers employ melt extruders for palletization of resin. Hence, nanocomposites can be prepared without hardware modification. An in-situ polymerization15,16 process was first employed for the synthesis of polyamide 6 nanocomposites by the Toyota research group. In this process polymer formation takes place between the silicate layers of nanoclay giving a highly exfoliated nanocomposite.5 But for a PP−clay nanocomposite, the in-situ polymerization process is less popular as nanoclay tends to deactivate the catalyst, which is used for propylene polymerization. In the solution blending/ intercalation process, nanoclay is dispersed in suitable solvent. Polymer is also dissolved in the same solvent in a separate vessel. Then slowly, these two solutions are mixed under predefined temperature, pressure, and mixing conditions. In this process, polymer chains are slowly adsorbed in between the clay layers. The residual solvent is evaporated to get the nanocomposites. Nanoclay, due to its versatile properties such as large surface area and layered structure, gives better mechanical, fire resistant, and barrier properties to nanocomposites.7−10,17,18 This is achieved with minimum clay loading as low as 5 wt %. Lower clay loading also helps in maintaining optical clarity in the final product. Silicates also act as nucleating agents19,20 improving crystallization kinetics. It also contributes to better Received: Revised: Accepted: Published: 10557

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thermal stability imparted due to improved barrier properties toward oxygen and other volatiles. With improved properties, the life of the polymer will also increase. Almost all synthetic polymers are nonbiodegradable and consume huge amount of energy in recycling and reprocessing. Hence, an efficient mechanism needs to be developed by which the usable life of polymer increases but degradation period becomes shorter. In this paper, an attempt is made to demonstrate the degradation behavior of PP−clay nanocomposites prepared with nanoclay having two different types of organic modifications vis-à-vis its virgin counterpart.

all PP is dissolved. A 0.16 g portion of nanoclay (Montmorillonite (MMT) with 25−30 wt % TA) is added in the solution and further maintained at the same temperature with constant stirring for 120 min (the nanoclay is sonicated in TCB for 12 h prior to use in experimentation). The solution is cooled to room temperature to precipitate resultant nanocomposites. Further, the nanocomposites are washed with n-hexane and dried to remove traces of solvent. 2. NC−iH2−TA: The process as described in preparation of NC−iH1−TA is repeated with 0.32 g nanoclay instead of 0.16 g. 3. NC−aH1−TA: The process as described in preparation of NC−iH1−TA is repeated with 4 g atactic homo PP instead of isotactic homo PP. 4. NC−aH2−TA: The process of preparation of NC− aH1−TA is repeated with 0.32 g nanoclay instead of 0.16 g. 5. NC−iC1−TA: The process as described in preparation of NC−iH1−TA is repeated with 4 g ethylene− propylene copolymer instead of isotactic homo PP. 6. NC−iC2−TA: The process of preparation of NC−iC1− TA is repeated with 0.32 g nanoclay instead of 0.16 g. The procedure described above in 1−6 is followed except MMT with 25−30 wt % OA is used for the preparation of nanocomposites NC−iH1−OA, NC−iH2−OA, NC−aH1− OA, NC−aH2−OA, NC−iC1−OA, and NC−iC2−OA (Table 1). Characterization. X-ray diffraction (XRD) measurements are carried on Bruker AXS, D8 Advance X-ray diffractometer. The composite sample placed on the zero background, X-ray transparent sample holder. The samples then transferred to instrument. The step size in XRD measurements is 0.02° and the time per step of 12 s. Component analysis is performed using TOPAS v3.0 software by Bruker AXS. The melting points of nanocomposites measured by differential scanning calorimetry (DSC) with a Perkin-Elmer DSC-7 at a heating rate of 10 °C·min−1 from 25 to 230 °C (with a hold for 3 min at 230 °C), cooling rate of 10 °C·min−1 from 230 to 25 °C, and again heating from 25 to 230 °C at 10 °C·min−1. The thermal stability of nanocomposites is analyzed by thermogravimetric analyzer (TGA) with a Perkin-Elmer TGA-7 at heating rate of 20 °C·min−1 from 50 to 700 °C under oxidative conditions.

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MATERIALS AND METHODS Material. Commercial polypropylene (homo) having 3.0 MFR is dissolved in stabilized xylene at 160 °C. The solution is cooled to get precipitates of isotactic homo PP. The remaining solvent is then evaporated to get atactic PP. Ethylene propylene copolymer with 20% (wt) rubber content is used as received from the commercial plant (Reliance Ind. Ltd.). MMT, 25−30 wt % octadecylamine (OA), and MMT, 25−30 wt % methyl dihydroxyethyl hydrogenated tallow ammonium (TA) (Aldrich Chem. Co., USA), are used as received. Trichloro benzene (Aldrich Chem. Co., USA) and n-hexane (Labort, India) are used after distillation. All the experiments are carried out under a high purity nitrogen atmosphere. Nanocomposite Preparation. Polypropylene is dissolved in trichloro benzene (TCB). In the prepared PP solution, nanoclay is added under constant stirring at the same temperature and maintained for a fixed time interval. The nanocomposites thus formed are separated through precipitation, cooling, washing, and drying.21,22 Three types of PP are used in preparing nanocomposites, namely atactic homo PP, isotactic homo PP, and ethylene propylene copolymer. Nanocomposites with varying amount of clay loading are also prepared and studied. A total of twelve types of nanocomposites (NC) are prepared (Table 1) with three different types of polypropylene at 4% (wt) and 8% (wt) clay loading and two types of organic modifiers of clay (OA and TA) as per procedure21 given below; 1. NC−iH1−TA: In a 250 mL capacity round-bottom flask, 4 g isotactic homo PP is added in 100 mL TCB. The solution is heated to 160 °C under constant stirring until

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Table 1. PP−Clay Nanocomposites with 4 and 8 wt % Clay Loading in Atactic Homo PP, Isotactic Homo PP, and Ethylene−Propylene Copolymer with Two Different Organic Modifications in Nanoclay nanocomposite

nanoclay charged (mg)

polymer (g)

nanoclay in polymer wt %

NC−iH1−TA NC−iH2−TA NC−aH1−TA NC−aH2−TA NC−iC1−TA NC−iC2−TA NC−iH1−OA NC−iH2−OA NC−aH1−OA NC−aH2−OA NC−iC1−OA NC−iC2−OA

160 320 160 320 160 320 160 320 160 320 160 320

4 4 4 4 4 4 4 4 4 4 4 4

4.0 8.0 4.0 8.0 4.0 8.0 4.0 8.0 4.0 8.0 4.0 8.0

RESULTS AND DISCUSSION Preparation of Polypropylene Clay Nanocomposites. XRD is used to determine the extent of mixing of nanoclay in a polymer matrix. Figure 1a shows the characteristic peak of clay and nanocomposites prepared using isotactic homo PP. As shown in Figure 1b, the clay with organic modification of 25− 30 wt % octadecylamine has lower d001 spacing as compared to clay having organic modification, 25−30 wt % methyl dihydroxyethyl hydrogenated tallow ammonium. Further, nanocomposites prepared with OA as the organic modification in nanoclay have higher d001 values as compared to the corresponding nanocomposites prepared with TA as the organic modification in nanoclay. The microstructure of clay can be preliminarily determined in accordance with the position and intensity of characteristic diffraction peak of clay.The interlayer spacing is calculated by bragg’s law d001 = nλ/(2 sin θ). Where, n is an integer, λ is wavelength of the incident wave (1.5404 Å), d001 is the spacing between the planes in the atomic 10558

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Figure 1. XRD pattern of PP−clay nanocomposites prepared from different organic modifications and clay loadings: (a) with isotactic homo PP, (b) interlayer spacing of clay in nanocomposites, (c) with atactic homo PP, and (d) with ethylene propylene copolymer.

Figure 2. Crystallization and melting behavior of PP−clay nanocomposites prepared from two different organic modifications and clay loadings: (a and c) isotactic homo PP and (b and d) ethylene propylene copolymer.

lattice, and θ is the angle between the incident ray and the scattering planes. Further, as shown in Figure 1a, when prepared with isotactic homo PP, an intercalating type of composite is obtained. Nanoclay modified with OA has better

ability to intercalate as compared to nanoclay modified with TA at 4 and 8 wt % clay loading. Figure 1c shows the intercalation behavior of atactic PP with the type of clay at both 4 and 8 wt % loadings. In atactic PP also, nanoclay modified with OA has 10559

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better ability to intercalate. The intercalation behavior of nanoclay with polymer can be correlated with the chain length of organic compound used for the modification of clay. The higher chain length in the case of TA may be causing hindrance to a polymer chain for intercalation. Nanocomposite prepared with ethylene−propylene copolymer (Figure 1d) also gives better intercalation with nanoclay with OA as the organic modification. As shown in Figure 1b, ethylene−propylene copolymer has the highest ability to penetrate between the clay layers. The spacing (d001) between clay layers in NC−iC1−OA (4.4 nm) and NC−iC1−TA (4.0 nm) is higher than NC−iH1−OA (4.2 nm) and NC−iH1−TA (3.7 nm), respectively. Nanocomposites prepared with atactic PP have the lowest ability to penetrate due to their lower bond strength of polymer chain. With an increase in clay loading from 4 to 8 wt %, the intercalation ability of the polymer decreases as the amount of clay available per unit volume is higher. At 4 wt % clay loading in NC−iC1−OA, the d001 spacing is 4.4 nm, which is decreased to 3.7 nm in NC−iC2−OA with 8 wt % clay loading. A similar trend can be seen in Figure 1b for all nanocomposites. Differential scanning calorimetry (DSC) measurements of isotactic homo PP, ethylene−propylene copolymer, and nanocomposites with 4 and 8 wt % clay loading are carried out to determine the effect of nanoclay on properties such as heat retardency, crystallization behavior, nucleation effect, etc. Figure 2a and b shows the crystallization behavior of the virgin polymer as well as nanocomposites. The crystallization temperature of virgin isotactic homo PP (V-iH) is 108.4 °C, which increases to 110.1 °C in NC−iH1−OA and 110.9 °C in NC−iH1−TA. Further, increasing the amount of nanoclay loading leads to increasing the crystallization temperature. At 8 wt % clay loading, the crystallization temperature is 111.8 °C for NC−iH2−OA and NC−iH2−TA. These results are in good agreement with the literature which describes an increase in crystallization temperature with the addition of nanoclay.19,23 As shown in Figure 2b, nanocomposites prepared with ethylene propylene copolymer also shows similar trends, where crystallization temperature increases with increase in nanoclay loading. Figure 2c and d show the melting behavior of the virgin polymer as well as nanocomposites. The heat barrier property is clearly reflected as the peak shift toward higher temperature with increase in the loading of nanoclay. The type of organic modification in nanoclay seems to play a very small role, as the peak temperature in NC−iH1−TA and NC−iH1− OA is the same (164.1 °C). A similar trend is observed in nanocomposites prepared with ethylene−propylene copolymer. Degradation of Nanocomposites. Thermogravimetric analysis (TGA) has been used extensively to throw light on the precise mode of thermal decomposition.24,25 TGA analysis is used to measure change in polymer weight as it is heated in presence/absence of oxygen at a predefined rate of heating. It can also be used as a kinetic approach to predict polymer lifetime under specific test conditions, i.e. the time period during which the polymer retains its physical properties. Figure 3 shows decomposition curves obtained at heating rate of 20 °C for virgin resin as well as nanocomposites. Nanocomposites prepared with isotactic homo PP and ethylene propylene copolymer show higher initial degradation temperature as compared to their virgin counterpart. Further, the initial degradation temperature increases from 459 to 479 °C with increases in clay loading from 4 to 8 wt % in isotactic homo PP and from 405 to 424 °C in ethylene−propylene copolymer.

Figure 3. TGA curves of PP−clay nanocomposites with nanoclay having two different organic modifications and clay loadings and (A) isotactic homo PP, (B) ethylene−propylene copolymer, and (C) atactic homo PP.

Palza et al.26 have studied PP−spherical silica nanocomposites for improvements in thermal behavior. TGA and DTG techniques are extensively used to analyze the nanocomposite. The peak degradation temperature is found to increase up to 70 °C, which indicates the thermal stability in the nanocomposite. The effect of the organic compound present in the clay is negligible on the initial degradation temperature for NC−iH1− TA and NC−iH1−OA which start degrading at the same temperature (459 °C). While NC−iC1−TA and NC−iC1−OA start degrading at 405 °C, NC−iH2−OA and NC−iH2−TA start at 479 °C. Also, NC−iC2−TA and NC−iC2−OA start at 424 °C. Leszezynska et al.,27,28 in their review of polymer− montmorillonite (MMT), described factors and mechanisms of thermal stability improvement. The “labyrinth” effect limits oxygen diffusion in the polymer from the outside and also restrains the gases evolved in the polymer bulk during degradation. In another effect, MMT catalyzes the degradation process leading to a decrease in thermal stability. Few hypotheses are proposed for the catalytic effect of MMT in the degradation such as decomposition of organic modifier leading to generation of acidic sites in the polymer bulk, adsorption of water in polyamides, polyesters leading to hydrolysis, etc. Nanocomposites prepared with atactic PP do not show much variation in initial degradation temperature. The degradation of virgin resin (V-aH) as well as nano10560

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°C. Hence, it can be said that the nanoclay imparts a temperature barrier due to its unique layered structure in the initial phase of the polymer life cycle, but it acts as a catalyst in the degradation process which enhances the rate of degradation of polymer in later stage of its life. The enhanced rate of degradation can be due to the higher amount of residual impurity in the polymer matrix or because of acid active centers generated between clay layers.29 Ethylene−propylene copolymer and its nanocomposites also show similar behavior (Figure 4b), where the peak rate of degradation increases from 44 wt %/min in virgin resin to 74 wt %/min in OA type clay with 8 wt % loading. Similarly the virgin resin of atactic homo PP (Figure 4c) shows a peak rate of degradation at 38 wt %/min, which increases to 43 and 40 wt %/min in TA and OA types of clay, respectively, with 4 wt % clay loading. On further increasing the clay loading to 8 wt %, the peak rate of degradation increases to 47 and 63 wt %/min for TA and OA types of clay, respectively. The kinetic parameters of degradation such as activation energy (E), order of decomposition (n), frequency factor (ln(Z)) are determined through the Friedman technique.30

composites starts between 360 and 365 °C. This is because in atactic PP, interchain bonding is much weaker, which is further affected by the addition of nanoclay. The increase in initial degradation temperature of nanocomposites indicates that the life cycle is enhanced to a certain extent with the introduction of nanoclay, but the degradation process also becomes faster in nanocomposites as compared to virgin resin. Figure 4 shows

⎛ dα ⎞ E ln⎜ ⎟ = ln(Z) + n ln(1 − α) − ⎝ dt ⎠ RT

(1)

Where, α is the conversion at time t. R is the gas constant (8.314 J·mol−1·K−1) and T is the absolute temperature (K). The plot of ln(dα/dt) vs 1/T is linear with the slope E/(R). From the slope of the linear plot of ln(1 − α) vs 1/T, E/(nR) is obtained, which is used to calculate n. Figure 5 shows the plot of ln(dα/dt) vs 1/T. The plot of isotactic homo PP and its nanocomposites (Figure 5a) shows a straight line with slope ranging from 780 to 787 for different nanocomposites. Activation energy (E) remains almost constant with clay loading in the polymer matrix (Table 2), as the clay loading in the polymer matrix increases. The activation energy for virgin isotactic homo PP is 94 J/mol, which is similar to 94.5 and 94.3 J/mol in TA and OA types of clay, respectively, with 4 wt % loading. As the clay loading is further increased to 8 wt %, the value of E is marginally increased to 95.2 and 95.1 J/mol in TA and OA types of clay, respectively. A similar trend is observed in nanocomposites with ethylene propylene copolymer (Figure 5b). The virgin resin shows an activation energy of 87.3 J/mol, which is equivalent to 88.7 and 88.3 J/mol in TA and OA types of clay, respectively, with 4 wt % clay loadings and 89.5 and 89.2 J/mol when clay loading is increased to 8 wt % in TA and OA types of clay, respectively. Nanocomposites with atactic PP show altogether different behavior. Due to weak bonding strength in the polymer matrix, the weight loss starts at a much lower temperature ∼300 °C; hence, the DTG plot is also bimodal (Figure 4b) and the plot of ln(dα/dt) vs 1/T is parabolic (Figure 5c) instead of a straight line. Further values of n and ln(Z) are also tabulated in Table 2. The values of n range from 0.52 to 0.76 in nanocomposites prepared from isotactic homo PP, while it ranges from 0.48 to 0.75 in nanocomposites prepared from ethylene−propylene copolymer. With an increase in clay loading from virgin resin to 4 wt % and further to 8 wt %, the n values also increase, which indicates an enhanced order of degradation. Frequency factor ln(Z) is calculated for each temperature as per eq 1. The average values are presented in Table 2. The life cycle of virgin PP and nanocomposites is calculated using Toop’s equation.31

Figure 4. DTG curves of PP−clay nanocomposites with nanoclay having two different organic modifications and clay loadings and (A) isotactic homo PP, (B) ethylene−propylene copolymer, and (C) atactic homo PP.

the differential thermogravimetric curves, where the peak rate of degradation is calculated in virgin resin and its nanocomposites. The nanocomposites with isotactic homo PP with 4 wt % clay loading has peak rate of degradation at 49 and 56 wt %/min for TA and OA types of clay, respectively. The virgin resin of isotactic homo PP has peak rate of degradation at 43 wt %/min. On increasing the clay loading from 4 to 8 wt %, the peak rate further increases to 57 and 76 wt %/min for TA and OA types of clay. It is interesting to note that the peak rate of degradation increases from 43 wt %/min in virgin resin to 76 wt %/min in nanocomposites with 8 wt % clay loading with a very narrow margin of temperature range between 490 and 500 10561

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Figure 5. Plot of ln(dα/dt) vs 1/T for the virgin resin and nanocomposites (A) isotactic homo PP, (B) ethylene−propylene copolymer, and (C) atactic homo PP.

Figure 6. Lifetime of PP−clay nanocomposites: (A) isotactic homo PP, (B) ethylene− propylene copolymer, and (C) atactic homo PP.

Table 2. E, n, and ln(Z) Values Calculated from Friedman Equation sample

E

n

ln(Z)

V-iH NC−iH1−TA NC−iH1−OA NC−iH2−TA NC−iH2−OA V-iC NC−iC1−TA NC−iC1−OA NC−iC2−TA NC−iC2−OA

94.0 94.5 94.3 95.2 95.1 87.3 88.7 88.3 89.5 89.2

0.52 0.62 0.65 0.77 0.76 0.48 0.63 0.66 0.74 0.75

9.4 10.7 10.5 11.5 11.6 9.0 9.6 9.3 10.4 10.1

ln tf =

⎤ ⎡ E E + ln⎢ P(X f )⎥ ⎦ ⎣ βR RTf

composites, with a varied amount of clay loading. Virgin polymer of isotactic homo PP (Figure 6a) shows a lifetime of 4.7 × 107 min at 260 °C, which decreases to 1.0 × 105 min as the temperature increases to 450 °C. On incorporation of 4 wt % nanoclay, the lifetime increases to 4.9 × 108 and 4.2 × 108 min for TA and OA types of clay, respectively, at 260 °C. As the amount of nanoclay incorporation is further increased to 8 wt %, the lifetime of PP is increased to 9.8 × 1010 and 1.2 × 1011 min in TA and OA types of clay, respectively. The increase in the lifetime of nanocomposites with an increase in clay loading is obvious as the nanoclay imparts better barrier properties toward environmental conditions. The lifetime of ethylene−propylene copolymer virgin resin (Figure 6b) is 1.2 × 106 min, which increases to 1.0 × 109 min, when clay incorporation is increased. Figure 6c shows the lifetime of nanocomposites prepared from atactic homo PP. The atactic PP itself having very much less strength and poor mechanical properties does not show significant improvement in lifetime with incorporation of nanoclay. The virgin atactic PP shows a lifetime of 5.6 × 103 min, which is only marginally increased (8.8 × 103 min) at 8 wt % clay loading.

(2)

Where, tf is estimated as the time of failure (min), E is activation energy (J/mol), Tf is the failure temperature (K), R is the gas constant (8.314 J/mol K), P(Xf) is a function whose values depend on E, and β is heating rate (°C/min). Figure 6 shows the lifetime of different types of PP and its nano10562

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(9) Hasegawa, N.; Kawasumi, M.; Kato, M.; Usuki, A.; Okada, A. Preparation and mechanical properties of polypropylene-clay hybrids using a maleic anhydride-modified polypropylene oligomer. J. Appl. Polym. Sci. 1998, 67 (1), 87−92. (10) Zhang, J.; Manias, E.; Wilkie, C. A. Polymerically Modified Layered Silicates: An Effective Route to Nanocomposites. J. Nanosci. Nanotechnol. 2008, 8 (4), 1597−1615. (11) Aloui, M.; Soulestin, J.; Lacrampe, M. F.; Krawczak, P.; Rousseaux, D.; Marchand-Brynaert, J.; Devaux, J.; Quiévy, N.; Sclavons, M. A new elaboration concept of polypropylene/unmodified Montmorillonite nanocomposites by reactive extrusion based on direct injection of polypropylene aqueous suspensions. Polym. Eng. Sci. 2009, 49 (11), 2276−2285. (12) LeBaron, P. C.; Wang, Z.; Pinnavaia, T. J. Polymer-layered silicate nanocomposites: an overview. Appl. Clay Sci. 1999, 15 (1−2), 11−29. (13) Hambir, S.; Bulakh, N.; Jog, J. P. Polypropylene/Clay nanocomposites: Effect of compatibilizer on the thermal, crystallization and dynamic mechanical behavior. Polym. Eng. Sci. 2002, 42 (9), 1800−1807. (14) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Synthesis and properties of two-dimensional nanostructures by direct intercalation of polymer melts in layered silicates. Chem. Mater. 1993, 5 (12), 1694−1696. (15) He, A.; Hu, H.; Huang, Y.; Dong, J.-Y.; Han, C. C. Isotactic Poly(propylene)/Monoalkylimidazolium-Modified Montmorillonite Nanocomposites: Preparation by Intercalative Polymerization and Thermal Stability Study. Macromol. Rapid Commun. 2004, 25 (24), 2008−2013. (16) He, A.; Wang, L.; Li, J.; Dong, J.; Han, C. C. Preparation of exfoliated isotactic polypropylene/alkyl-triphenylphosphonium-modified montmorillonite nanocomposites via in situ intercalative polymerization. Polymer 2006, 47 (6), 1767−1771. (17) Lee, K. Y.; Goettler, L. A. Structure-property relationships in polymer blend nanocomposites. Polym. Eng. Sci. 2004, 44 (6), 1103− 1111. (18) Rodríguez-Llamazares, S.; Rivas, B. L.; Pérez, M.; Perrin-Sarazin, F.; Maldonado, A.; Venegas, C. The effect of clay type and of clay− masterbatch product in the preparation of polypropylene/clay nanocomposites. J. Appl. Polym. Sci. 2011, 122 (3), 2013−2025. (19) Prachum, Y.; Adam Strauss, R. H.; Kiatkamjornwong, S. The physical and mechanical properties of beta-nucleated polypropylene/ montmorillonite nanocomposites. J. Appl. Polym. Sci. 2011, 122 (2), 1066−1076. (20) Preschilla, N.; Rasheed, A. S. A.; Sahadevan, S.; Biswas, A.; Bellare, J. R.; Shyamroy, S. Study of layered silicate clays as synergistic nucleating agent for polypropylene. J. Polym. Sci. Part B: Polym. Phys. 2010, 48 (16), 1786−1794. (21) Schadler, L. S. Polymer-Based and Polymer-Filled Nanocomposites. In Nanocomposite Science and Technology: Wiley-VCH Verlag GmbH & Co. KGaA: 2004; pp 77−153. (22) Filippi, S.; Mameli, E.; Marazzato, C.; Magagnini, P. Comparison of solution-blending and melt-intercalation for the preparation of poly(ethylene-co-acrylic acid)/organoclay nanocomposites. Eur. Polym. J. 2007, 43 (5), 1645−1659. (23) Wang, K.; Liang, S.; Zhao, P.; Qu, C.; Tan, H.; Du, R.; Zhang, Q.; Fu, Q. Correlation of rheology−orientation−tensile property in isotactic polypropylene/organoclay nanocomposites. Acta Materialia 2007, 55 (9), 3143−3154. (24) Tabari, H. Z.; Nourbakhsh, A.; Ashori, A. Effects of nanoclay and coupling agent on the physico-mechanical, morphological, and thermal properties of wood flour/polypropylene composites. Polym. Eng. Sci. 2011, 51 (2), 272−277. (25) Li, X.-G.; Huang, M.-R.; Guan, G.-H.; Sun, T. Kinetics of thermal degradation of thermotropic poly(p-oxybenzoate-co-ethylene terephthalate) by single heating rate methods. Polym. Int. 1998, 46 (4), 289−297. (26) Palza, H.; Vergara, R.; Zapata, P. Improving the Thermal Behavior of Poly(propylene) by Addition of Spherical Silica Nanoparticles. Macromol. Mater. Eng. 2010, 295 (10), 899−905.

CONCLUSIONS Three types of PP, namely, atactic homo, isotactic homo, and ethylene−propylene copolymers are used in the present study. An aluminosilicate layered type nanoclay with two different organic modifications (25−30 wt % methyl dihydroxyethyl hydrogenated tallow ammonium and 25−30 wt % octadecylamine) is used. The virgin polymer and nanocomposites with 4 and 8 wt % clay loading are subjected to stability and degradation studies. The result indicates that PP−clay nanocomposites bear higher mechanical and barrier properties than virgin PP. Initiation of degradation is also delayed as revealed in TGA studies. Furthermore, the lifetime of nanocomposites is increased with an increasing amount of clay in the polymer matrix. Further analysis of the weight loss curve through differential thermogravimetry (DTG) techniques revealed interesting results. The peak rate of degradation is almost 1.5 times more in nanocomposites with 8 wt % nanoclay incorporation as compared to its virgin counterpart. This indicated that as the amount of nanoclay incorporation in the polymer matrix increased, the rate of degradation also increased. This can be explained as the degradation in the polymer starting from the point where the impurity is present in the matrix. The oxidation/photo-oxidation process in the polymer matrix starts around the catalyst or any other type of residual impurity, and slowly, the cracks develop in the matrix, which leads to disintegration of polymer chains, therefore degradation. With incorporation of silicate type nanoclay, the residual impurity in the polymer matrix is increased; hence, oxidation/photo-oxidation starts from multiple sites leading to faster degradation of nanocomposites as compared to its virgin counterpart.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.A.M.); [email protected] (A.K.M.). Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Kumar, A. P.; Depan, D.; Singh Tomer, N.; Singh, R. P. Nanoscale particles for polymer degradation and stabilizationTrends and future perspectives. Prog. Polym. Sci. 2009, 34 (6), 479−515. (2) Zigler, K. Consequences and development of an invention. Rubber Chem. Technol. 1965, 38, 22−41. (3) Pavlidou, S.; Papaspyrides, C. D. A review on polymer−layered silicate nanocomposites. Prog. Polym. Sci. 2008, 33 (12), 1119−1198. (4) Lee, E. C.; Mielewski, D. F.; Baird, R. J. Exfoliation and dispersion enhancement in polypropylene nanocomposites by in-situ melt phase ultrasonication. Polym. Eng. Sci. 2004, 44 (9), 1773−1782. (5) Kiliaris, P.; Papaspyrides, C. D. Polymer/layered silicate (clay) nanocomposites: An overview of flame retardancy. Prog. Polym. Sci. 2010, 35 (7), 902−958. (6) Lonkar, S. P.; Morlat-Therias, S.; Caperaa, N.; Leroux, F.; Gardette, J. L.; Singh, R. P. Preparation and nonisothermal crystallization behavior of polypropylene/layered double hydroxide nanocomposites. Polymer 2009, 50 (6), 1505−1515. (7) Kato, M.; Usuki, A.; Okada, A. Synthesis of polypropylene oligomerclay intercalation compounds. J. Appl. Polym. Sci. 1997, 66 (9), 1781−1785. (8) Kawasumi, M.; Hasegawa, N.; Kato, M.; Usuki, A.; Okada, A. Preparation and Mechanical Properties of Polypropylene−Clay Hybrids. Macromolecules 1997, 30 (20), 6333−6338. 10563

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Industrial & Engineering Chemistry Research

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(27) Leszczyńska, A.; Njuguna, J.; Pielichowski, K.; Banerjee, J. R. Polymer/montmorillonite nanocomposites with improved thermal properties: Part I. Factors influencing thermal stability and mechanisms of thermal stability improvement. Thermochim. Acta 2007, 453 (2), 75−96. (28) Leszczyńska, A.; Njuguna, J.; Pielichowski, K.; Banerjee, J. R. Polymer/montmorillonite nanocomposites with improved thermal properties: Part II. Thermal stability of montmorillonite nanocomposites based on different polymeric matrixes. Thermochim. Acta 2007, 454 (1), 1−22. (29) Qin, H.; Zhang, S.; Liu, H.; Xie, S.; Yang, M.; Shen, D. Photooxidative degradation of polypropylene/montmorillonite nanocomposites. Polymer 2005, 46 (9), 3149−3156. (30) Paik, P.; Kar, K. K. Kinetics of thermal degradation and estimation of lifetime for polypropylene particles: Effects of particle size. Polym. Degrad. Stab. 2008, 93 (1), 24−35. (31) Toop, D. J. Theory of life testing and use of thermogravimetric analysis to predict the thermal life of wire enamels. IEEE Trans. Electr. Insul. 1971, 6 (1), 2−14.

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dx.doi.org/10.1021/ie3007616 | Ind. Eng. Chem. Res. 2012, 51, 10557−10564