Thermal degradation and combustion of a ... - Springer Link

3 downloads 0 Views 305KB Size Report
Abstract—The specific features of thermal degradation and combustion of polypropylene nanocomposites based on organically modified layered aluminosilicate ...
ISSN 0965-545X, Polymer Science, Ser. A, 2006, Vol. 48, No. 1, pp. 72–84. © Pleiades Publishing, Inc., 2006 Original Russian Text © S.M. Lomakin, I.L. Dubnikova, S.M. Berezina, G.E. Zaikov, 2006, published in Vysokomolekulyarnye Soedineniya, Ser. A, 2006, Vol. 48, No. 1, pp. 90−105.

DEGRADATION

Thermal Degradation and Combustion of a Polypropylene Nanocomposite Based on Organically Modified Layered Aluminosilicate1 S. M. Lomakin*,2, I. L. Dubnikova**, S. M. Berezina**, and G. E. Zaikov* * Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia ** Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 119991 Russia Received February 21, 2005; Revised Manuscript Received July 15, 2005

Abstract—The specific features of thermal degradation and combustion of polypropylene nanocomposites based on organically modified layered aluminosilicate were studied. On the basis of thermogravimetric analysis data, a kinetic model that takes into account the diffusive character of the thermal degradation process for the PP nanocomposite was proposed. The basic flammability parameters of the nanocomposite were determined with the use of a cone calorimeter. The influence of diffusion constraints in the charred nanocomposite layer on the maximum heat release rate as a principal parameter of flammability was considered. DOI: 10.1134/S0965545X06010111

INTRODUCTION

hydrogen atom transfer plays an insignificant part in PP degradation [4]. The values for the apparent activation energy of thermal degradation of PP were earlier reported to be in the range 220–270 kJ/mol [4–6]. For example, Chan and Balke [7] used thermogravimetric analysis to describe the kinetic model of thermal degradation of PP. They substantially simplified the kinetic model of thermal degradation by taking the apparent reaction order equal to unity [7]. The obtained data allowed the thermal degradation of PP at T < 404–421°C (depending on the heating rate) to be represented as a one-step first-order reaction with an activation energy of 98.3 ± 3.1 kJ/mol [7]. At T > 420°C, the kinetic model of thermal degradation of PP was also categorized by these authors as a one-step first-order reaction with an activation energy of 327.9 ± 8.6 kJ/mol, which is close to the value of the C–C bond energy [7]. Chan and Balke suggested that these two regions were separated by a relatively narrow transition zone. The results obtained in their study provided an additional piece of evidence for the principal fundamental assumption that the thermal degradation of polyolefins, in particular PP, is distinctly two-step in character. The process in the low-temperature region with a low activation energy is mainly initiated by the scission of “weak bonds,” whereas the same process in the high-temperature region is determined by the random rupture of C–C bonds in PP macromolecules [7]. In contrast to previous studies, Gao et al. [8] showed that the first-order reaction model is inapplicable to the description of PP thermal degradation. The plot of con-

Polypropylene is the most widespread synthetic polymer and is extensively used in many areas of human activity. Unfortunately, relatively poor thermal stability and fire resistance of PP seriously limit the conditions and range of its practical application. An analysis of published data shows that systematic investigation into the thermal degradation of PP began in the middle of the 20th century [1–3]. Although the general concept of the mechanism of this process has not undergone substantial changes, its investigation techniques are continuously improving [4–8]. In a recent review, Bockhorn et al. [4] considered the radical processes of thermal degradation of PP including individual initiation, propagation, and termination steps, as well as calculated the apparent parameters of the overall process of thermal degradation of PP under isothermal conditions from mass spectrometric data. On the basis of obtained results, they proposed a schematic mechanism for the thermal degradation of PP; this scheme suggests that, after chain scission into the primary and secondary radicals, tertiary radicals are formed via rearrangement reactions. Subsequent β-scission leads to short (lowmolecular-mass) secondary radicals and macromolecular fragments with a terminal double bond, and these short secondary radicals are saturated via intramolecular hydrogen transfer to result in alkanes. However, owing to the low observed concentration, the process of 1 This

work was supported by the Russian Foundation for Basic Research, project no. 04-03-32052. [email protected]

2 E-mail:

72

THERMAL DEGRADATION AND COMBUSTION

version versus the maximum rate of PP degradation measured under dynamic heating conditions yielded an apparent reaction order of 0.35. They confirmed the validity of the obtained data by the coincidence of the Arrhenius parameters of PP degradation calculated for the isothermal and dynamic heating modes [8]. The approach to the enhancement of thermal stability and fire resistance based on the use of polymer nanocomposites has been extensively developed in recent years [9–11]. Intercalation in polymer melts is widely used as a simple method for the synthesis of corresponding materials [12]. In these studies, it was found that polymer composites based on layered silicates exhibit lower flammability as compared with the parent polymer, a result which can be achieved by introducing a small amount (≤5 wt %) of an inorganic silicate ingredient [10–12]. Zanetti et al. [13] studied the thermal degradation of a PP nanocomposite based on organically modified silicate using isothermal and dynamic TGA. They assumed that the thermooxidative degradation of the PP nanocomposite, which is accompanied by a considerable oxygen absorption, leads to effective charring as a result of catalysis at the polymer/layered silicate interface. The result of catalytic processes on nanoscale silicate layers is the reactions of oxidative dehydrogenation and subsequent crosslinking, aromatization, and condensation. On the basis of obtained data, Zanetti et al. [13] concluded that temperature-resistant carbon—a protective ceramic layer as an effective barrier to heat and mass transfer processes on the surface of the material—is formed on the surface during the hightemperature thermooxidative degradation of the PP nanocomposite. The effect of the surface of layered silicate on the thermal degradation and combustion of a PP–montmorillonite microcomposite was considered by Qin et al. [14]. They showed that the microcomposite also exhibited enhanced thermal stability and had a lower maximum heat release rate than neat PP. In addition, it was noted that the admixture of montmorillonite could catalyze the thermal degradation of PP and, hence, facilitate its ignition in combustion experiments [14]. The objective of this study was to investigate the specific features of the kinetics of thermal degradation and combustion of a polypropylene nanocomposite based on organically modified layered aluminosilicate. EXPERIMENTAL Materials and Methods As a polymeric matrix, we used isotactic PP (Moscow Refinery) with a melt flow index (MFI) of 0.7 g/10 min. The filler used to prepare nanocomposites was organically modified layered silicate Cloisite 20A (obtainable from Southern Clay), which constituted naturally occurring Na+-montmorillonite (MMT) modified with the quaternary ammonium compound dimethyldialkyl– POLYMER SCIENCE

Series A

Vol. 48

No. 1

2006

73

ammonium chloride, where the unsaturated alkyl substituent consists of ~65% C18 , ~30% C16 , and ~5% C14 groups. As a polar compatibilizer for PP, we used maleated PP (MPP) of the Licomont AR 504 brand (Clariant) having Mn ~ 2900, an amount of maleic anhydride of 4 wt %, and Tm ~ 156°C. Preparation of Nanocomposites PP–MPP–MMT compositions were prepared by the melt mixing of the components in a double-rotor laboratory (Brabender) mixer in two steps. In the first step, the two polymers PP and MPP were blended in a 5 : 1 ratio for 2 min; after that, an MMT powder was added in an amount of 7 wt % (2.7 vol %). The mixing of components at the second step lasted for 10 min at a temperature of 190°C and rotor speeds of 60 rpm. Samples designed for cone calorimeter testing in the form of 70 × 70 × 3 mm plates with a mass of 14.0 ± 0.1 g were prepared by molding at 190°C and 10 MPa followed by cooling under pressure at a rate of 16°C/min. Investigation Techniques XRD. X-ray diffraction analysis was performed in the reflection mode over the angular range of 2θ = 5°−40° on film samples at room temperature using a DRON-2 diffractometer (CuKα radiation) with modified collimation. Interlayer distances in MMT were determined from the angular positions of base reflections (001) of MMT in the diffraction patterns of composites. The degree of polymer intercalation was determined from a change in the interlayer distance in MMT. Transmission electron microscopy (TEM). The filler dispersion in the composites was studied by the TEM technique with a Philips EM-301 electron microscope (the Netherlands) at an accelerating voltage of 80 kV. Thin sections of film samples for TEM examination were prepared with an LKB Ultratome III® ultramicrotome (Sweden). AFM. A MultimodeTM nanoscope IIIaTM scanning probe microscope (Digital Instruments/Veeco Metrology Groups, USA) was used. Measurements were performed in the tapping mode with silicon probes having a spring constant of 40 N/m and a tip radius of 10 nm. To obtain a flat surface for AFM examination, the samples were preliminarily cut by means of low-temperature microtomy with a diamond blade at a temperature of –80°C using an MS-01 instrument (MicroStar Inc., USA). TGA. The thermal characteristics of the samples were studied with an MOM Q 1500 differential thermal analyzer (Hungary) at heating rates of 3, 5, and 10°C/min in air. A kinetic analysis of the thermal degradation of composites was performed with the use of the NETZSCH-Gerätebau Thermokinetics software (Germany). The algorithm of the kinetic analysis program was based on the calculation of regression by the

74

LOMAKIN et al.

4.2 nm

A

2

B

2.4 nm

B

B 1 3

6

9 2θ, deg

A

Fig. 1. X-ray diffraction patterns of (1) MMT and (2) the PP–MPP–MMT nanocomposite.

fifth-order Runge–Kutta method using the dedicated Prince–Dormand formula for automatic optimization of the number of significant digits [15]. Combustibility characteristics (cone calorimeter). Ignitability tests were performed according to the standard procedures ASTM 1354-92 and ISO/DIS 13927 using a cone calorimeter [16].

200 nm

A

B B A B

RESULTS AND DISCUSSION Investigation of Nanocomposite Structure To characterize the structural features of nanocomposites, the joint use of XRD, TEM, and AFM techniques is widely practiced. Figure 1 shows the diffraction patterns of the initial MMT and the PP–MPP– MMT nanocomposite. It is seen that the diffraction pattern of the MMT displays a distinct base reflection corresponding to an interplanar distance of d001 = 2.4 nm. The shift of the base reflection toward smaller angles indicates an increase in the interplanar distance in the silicate structure of the PP–MPP–MMT nanocomposite from 2.4 to 4.2 nm by intercalation. The use of the XRD technique alone does not allow the complex structure of the PP nanocomposite to be fully characterized, since this method can establish either the existence of certain order in the layered structure from variation in principal interplanar distances (d001) in periodically distributed stacks (tactoids) of laminar silicate layers or its complete absence. It is obvious that the actual structure of the polymer nanocomposite (hybrid type) obtained from the melt is more complex. Along with intercalation regions (nanoclusters) with various architectures, individual, completely delaminated silicate monolayers can also be present in this structure. Additional information on the structure of the PP–MPP–MMT nanocomposite was obtained with the use of TEM (Fig. 2). TEM photographs at var-

A

100 nm

Fig. 2. TEM photographs of the PP–MPP–MMT nanocomposite at different magnifications: (A) zones with delaminated silicate structure and (B) the intercalation region.

ious magnifications clearly show regions of intercalated stacks (tactoids) containing ten or more laminar silicate layers (B), as well as zones with the delaminated silicate structure in which individual monolayers are present (A). The number of individual monolayers was counted and the statistical distribution in a certain selected region of a microphotograph 500 × 500 nm in size was estimated with the use of the Noesys VISILOG 6.3 software program. As a distribution criterion, we took the magnitude of the ratio of geometric dimensions of the monolayer length l to the mean monolayer thickness d (1.5 ± 0.5 nm) (Fig. 3). On the basis of the obtained data, it may be concluded that the majority of monolayers have the size ratio of l : d ~ 75, while the length of silicate layers in intercalated nanoclusters ranges within 300–500 nm. POLYMER SCIENCE

Series A

Vol. 48

No. 1

2006

Average number of monolayers per 0.25 µm2

THERMAL DEGRADATION AND COMBUSTION

75

10

5 B

50

100

150

200

A

250 l:d

200 nm Fig. 3. Distribution curve for silicate monolayers in a fragment of a TEM photograph (500 × 500 nm) of the delaminated structure of the PP–MPP–MMT nanocomposite.

Fig. 4. AFM image of the PP–MPP–MMT nanocomposite: (A) zones with delaminated silicate structure and (B) the intercalation region.

Structural investigation was also carried out by the AFM technique in the tapping mode (Fig. 4). This mode makes it possible to obtain high contrast in imaging of submicrometer-sized structures in homogeneous samples and to recognize different components in heterogeneous polymer systems without disturbing their surface structure. In this experiment, silicon probes with a spring constant of ~40 N/m and a resonance frequency of 150–170 kHz were used. To enhance phase contrast, the tip was scanned under conditions when the drive amplitude Asp was 0.5–0.8 of the amplitude of free probe oscillation of A0 = 25 nm. Under these conditions, the brightest areas in a phase image correspond to the most rigid component and vice versa. The AFM image displays regions of intercalated (B) and delaminated silicate structure in which single monolayers are present (A) (Fig. 4). On the basis of the above consideration, we may characterize the structure of the PP–MPP–MMT nanocomposite obtained by melt intercalation as a structure of a mixed hybrid type in which polymer–silicate intercalation nanoclusters coexist with delaminated-filling regions.

TGA measurements on PP and PP–MPP–MMT samples over the range 25–600°C in air showed that PP completely decomposes in air at a heating rate of 10°C/min, whereas the degradation of the PP–MPP– MMT nanocomposite leads to the formation of ~10 wt % carbon–ceramic residue, of which ~60% is made up by carbon (Fig. 5a). From Fig. 5b, it is seen that the temperature at which the degradation rate reaches a maximum value, Tmax , is 400°C for PP and Series A

Vol. 48

(a)

100 75 50 25

2 1 200

400

dm/dT

600 T, °C

(b)

0

–0.4

–0.8

Study of Thermal Degradation

POLYMER SCIENCE

Mass of residue, %

No. 1

2006

1 200

400

2 600 T, °C

Fig. 5. (a) TGA and (b) DTG curves for (1) PP and (2) PP–MPP–MMT nanocomposite samples measured during the thermal degradation of the samples in air at a heating rate of 10°C/min.

76

LOMAKIN et al. Heat, O2

Region of diffusion process

Phase boundary reactions

Char

Intact polymer

Fig. 6. Scheme of processes occurring during the thermal degradation of the PP–MPP–MMT nanocomposite in air.

50°C higher for the PP–MPP–MMT nanocomposite. The stabilizing effect, to the first glance, may be explained in terms of the barrier effect of silicate nanolayers, which impede the diffusion of oxygen and thus protect the polymer from contact with the gas [13]. Note that about 5% carbonaceous residue is produced via oxidative dehydrogenation of PP under the thermal degradation conditions in air at 450°C; this residue further completely decomposes before a temperature of 600°C is reached [17]. It is also interesting that there is an insignificant mass gain by the PP nanocomposite at the early steps of thermooxidative degradation (T < 300°C) due to oxygen adsorption on its surface (Fig. 5a). A formal conclusion that follows from the TGA data is that a part of PP macromolecules occurring inside silicate nanolayers, owing to close contact with catalytically active silicate surfaces and oxygen, transform into a condensed carbonaceous–ceramic residue, which is much more stable than ordinary carbonaceous char. The process of thermooxidative degradation of the PP– MPP–MMT nanocomposite is schematically shown in Fig. 6. Kinetic Analysis of Thermal Degradation of the PP–MPP–MMT Nanocomposite Kinetic studies of the thermal degradation of polymer materials by means of TGA are widely used to characterize their thermal stability and to predict their behavior over a wide temperature range. It is com-

monly accepted that the degradation of materials follows the base equation [18] dc/dt = –F(t, T, c0, cf ),

(1)

where t is the time, T is the temperature, c0 is the reactant concentration, and cf is the product concentration. The right-hand part of the equation (F(t, T, c0, cf ) can be represented by the two separable functions k(T) and f(c0, cf ): F ( t, T , c 0, c f ) = k ( T ( t ) f ( c 0, c f ) ) .

(2)

According to the basic Arrhenius equation, k(T) = Aexp(–E/RT),

(3)

dc/dt = – A exp ( – E/RT ) f ( c 0, c f ) .

(4)

hence,

For one-step reactions, f(c0, cf ) reduces to the known form f(x), where c0 = 1 – x and cf = x (x is the degree of conversion). The complete separation of variables in Eq. (2) is possible only for one-step reactions. Thus, an analytical solution for differential equation (1) can also be obtained for one-step reactions. For complex multistage reactions, differential equation (1) leads to a system of differential equations that does not have a simple analytical solution. In this case, the nonlinear regression method is used, which allows a direct fit to experimental data [15]. Most multistep processes can be analyzed using the nonlinear regression method. However, nonlinear regression treatment proves benefiPOLYMER SCIENCE

Series A

Vol. 48

No. 1

2006

THERMAL DEGRADATION AND COMBUSTION

77

Table 1. Taken reaction models dc/dt = –Aexp(–RT)f(c0 , cf) Reaction type

f(c0, cf)

First-order reaction (F1)

c

Second-order reaction (F2)

c2

nth-order reaction (Fn)

cn

Two-dimensional phase boundary reaction (R2)

2c1/2

Three-dimensional phase boundary reaction (R3)

3c2/3

One-dimensional diffusion (D1)

0.5/(1 – c)

Two-dimensional diffusion (D2)

–1/ln(c)

Three-dimensional diffusion (Jander’s type) (D3)

1.5e1/3(c–1/3 – 1)

Three-dimensional diffusion (Ginstling–Brounstein type) (D4)

1.5/(c–1/3 – 1)

One-dimensional diffusion (Fick’s law) (D1F)



Three-dimensional diffusion (Fick’s law) (D3F)



Autocatalytic reaction described by the simple Prout–Tompkins equation (B1)

c0cf

Reaction with autocatalysis described by the expanded Prout–Tompkins equation (Bna)

c0 c f

First-order reaction with autocatalysis (C1 – X)

c(1 + KcatX)

nth-order reaction with autocatalysis (Cn – X)

cn(1 + KcatX)

Two-dimensional nucleation according to the Avrami–Erofeev equation (A2)

2c(–ln(c))1/2

Three-dimensional nucleation according to the Avrami–Erofeev equation (A3)

3c(–ln(c))2/3

n-dimensional nucleation according to the Avrami–Erofeev equation (An)

Nc(–ln(c))(n – 1)/n

cial to one-step processes as well since it provides better approximation as compared with multivariate linear regression. Kinetic analysis can be based on models that involve processes with one, two, three, or four stages and relates each individual step to independent parallel, competing, or consecutive reactions. Each step may be associated with one of the following types of classical homogeneous or heterogeneous reactions and the corresponding rate law f(c0, cf ) considered in the simulation (NETZSCH Thermokinetics) of thermal degradation (Table 1) [18]. The thermogravimetric analysis of degradation of PP and the PP–MPP–MMT nanocomposite was carried out in the dynamic mode at heating rates of 3, 5, and 10°C/min in air. At the preliminary analysis step, a model-free evaluation of the activation energy was performed for the choice of initial conditions and preliminary assessment of a model of the process using the Friedman method (Fig. 7) [19]. From Fig. 7, it is seen that the Friedman plots of activation energy versus conversion show relatively high values at the beginning of the degradation process; in addition, these energies increase at high conversions. This finding indicates the multistage character of the process. In accordance with the results obtained, we selected as a base model for POLYMER SCIENCE

Series A

Vol. 48

No. 1

2006

n a+

nonlinear analysis a two-step process (A x1 B x2 C) for PP and a more complex three-step process (A x1 B x2 C x3 D) involving the charring step for the PP–MPP–MMT nanocomposite [18, 20]. In multivariate linear regression as an analytical method, several dynamic measurements made at different heating rates are treated jointly. For the two-step consecutive thermal degradation reaction of PP A B C, the set of equations has the form E1 ⎞ da - , ------ = – f ( a, b ) A 1 exp ⎛ – -----------⎝ dt RT j, k⎠

(5)

E2 ⎞ da db ------ = – ------ – f ( b, c ) A 2 exp ⎛ – ------------ , ⎝ RT j, k⎠ dt dt

(6)

where c = 1 – a – b. The results of nonlinear regression analysis in terms of the set of reaction models f(c0, cf ) (Table 1) for the two-step process of thermal degradation of PP made it possible to calculate the values of apparent kinetic parameters that provide the best approximation of experimental TGA curves (Fig. 8a).

78

LOMAKIN et al. E, kJ/mol 300

logA [s–1] (a)

16

12

220

8 140 4

logA [s–1]

E, kJ/mol 350 (b)

16

250

8 150

0 0

0.4 Conversion

0.8

Fig. 7. Model-free Friedman analysis of the thermal degradation of (a) PP and (b) its nanocomposite.

The obtained data support the earlier assumption on the two-step character of the process (consecutive reactions) during the thermal degradation of PP in air in the dynamic heating mode. The values of the apparent activation energy and the preexponential factor for the first step are 110.3 kJ/mol and 106.4 s–1, respectively, and the reaction order is close to unity (1.13). In the second step (T > 350°C), the process occurs with a higher activation energy (E2 = 151.6 kJ/mol, A2 = 109.9 s–1) and the apparent reaction order is greater than unity (n2 = 2.59). In a similar way, using multivariate nonlinear regression analysis, we calculated the kinetic parameters for the

thermal degradation of the PP–MPP–MMT nanocomposite in air in terms of the model of three-step process of consecutive reactions (A x1 B x2 C x3 D) (Fig. 8b, Table 2). A statistical analysis of the obtained parameters of the process makes it possible to select a model that describes most reliably the pattern of experimental curves. A preferred model for the thermal degradation of the PP–MPP–MMT nanocomposite in air consists of D1 Fn , where D1 two consecutive reactions Fn is one-dimensional diffusion and Fn is an nth-order reaction (Table 2). For the thermal degradation of the POLYMER SCIENCE

Series A

Vol. 48

No. 1

2006

THERMAL DEGRADATION AND COMBUSTION

79

Mass of residue, % 100 1 2 3

A→1→B→2→C 60

(a) 20

200

400

600 T, °C

Mass of residue, % 100

A→1→B→2→C→3→D

60 (b)

20 200

400

600 T,°C

Fig. 8. Simulation of the thermal degradation of (a) PP and (b) the PP–MPP–MMT nanocomposite at heating rates of (1) 3, (2) 5, and (3) 10°C/min by means of nonlinear regression analysis. The solid curves represent regression analysis data.

PP–MPP–MMT nanocomposite in air, the apparent activation energy in the first step is 113.3 kJ/mol and the reaction order approximately equals unity (n1 = 1.11). In the second step, which is described by the one-dimensional diffusion model (T > 400°C), E2 = 100 kJ/mol; in the third step, the burn-up of the carbonaceous residue, the activation energy increases almost by a factor of 2, to 199.8 kJ/mol, and the apparent reaction order remains close to unity (n3 = 1.17) (Table 2). Nonetheless, it should be noted that the statistical correlation suggests only a formal criterion for the selection of parameters. The possibility of description of the proPOLYMER SCIENCE

Series A

Vol. 48

No. 1

2006

cesses in terms of other model concepts also should not be excluded. In particular, the scheme Fn An Fn is of certain interest, where the intermediate step An represents the n-dimensional nucleation process in accordance with the Avrami–Erofeev equation, for which close values of the apparent kinetic parameters were obtained (Table 2). Flammability Characteristics of PP and PP-Based Nanocomposites The interest in char-forming silicate additives with a nanoscale particle size used to fill various polymers at

80

LOMAKIN et al. Mass loss, % 100

dm/dt, s–1

(a) 4

20

3

15

60

10

2 20

5

0.05

0.10

2

1 0.20

0.15

1 4

8

Time, min Mass loss, % 80

12 Time, s

Fig. 10. Simulated behavior of the mass loss rate on time under the conditions of isothermal degradation at 600°C for (1) PP and (2) the PP–MPP–MMT nanocomposite.

(b) 4

60

burns rapidly and completely. The high degradation rate of PP and a self-ignition temperature at the level of 570°C predetermine its flammability. Polypropylene nanocomposites have generated considerable interest due to an increased char yield in relation to fire safety. It is assumed that the carbonaceous–ceramic char presents an effective barrier to mass and heat transfer processes on the surface of a burning polymer [21–23]. Char-forming nanosized silicate additives are of particular interest because of their high efficiency at a concentration of 2–10 wt % [10, 23].

40

20

3

0.05

0.10

2 1 0.20

0.15

Time, min

Data on the isothermal pyrolysis of PP and the PP– MPP–MMT nanocomposite above 400–600°C make it possible to predict their behavior during combustion under exposure to external radiant heat. It is known that a temperature of 600°C corresponds to an incident heat flux of 35 kW/m2 , which is typical of a real-scale fire [16].

Fig. 9. Theoretically calculated time dependence of conversion under isothermal degradation conditions for (a) PP and (b) the PP–MPP–MMT nanocomposite at T = (1) 450, (2) 500, (3) 550, and (4) 600°C.

the level of 2–10 wt % is due to the improved fire resistance of the nanocomposite material [10, 21–23]. Owing to its completely aliphatic structure, pure PP

The kinetic parameters calculated in this study for the thermal degradation of PP and the PP–MPP–MMT

Table 2. Kinetic parameters for the thermal degradation of the PP–MPP–MMT nanocomposite in air as obtained by multivariate nonlinear regression analysis of the model process (A X1 B X2 C) logA1, s–1

E1, kJ/mol

n1

logA1, s–1

6.9

113.4

1.11

4.7

6.7

114.7

0.97

4.7

6.3

113.4

1.16

8.8

E1, kJ/mol Fn 100.0 Fn 97.7 Fn 150.9

n1 D1

logA1, s–1

E1, kJ/mol

n1

Correlation coefficient

12.0

199.8

1.17

0.9991

11.9

200.0

1.26

0.9981

11.5

198.5

0.78

0.9974

Fn –

An

Fn –

Fn

Fn 2.46

POLYMER SCIENCE

Series A

Vol. 48

No. 1

2006

THERMAL DEGRADATION AND COMBUSTION

nanocomposite were used to predict the behavior upon isothermal heating in the range of 400–600°C (Fig. 9). From the integral curves of mass loss under isothermal heating conditions at 600°C, the corresponding plots of the mass loss dm/dt were constructed (Fig. 10). From Fig. 10, it is seen that, during isothermal pyrolysis at 600°C, the pattern of the time dependence of the thermal degradation rate for the PP–MPP–MMT nanocomposite considerably differs from that for pure PP. It is obvious that the intermediate diffusion step in the thermal degradation of the PP–MPP–MMT nanocomposite has an effect on the character of the process; as a consequence, the PP–MPP–MMT nanocomposite decomposes at a lower rate than the parent PP. As has been noted above, the thermal-degradation kinetics of polypropylene A B C is described by a set of two differential equations and one mass balance equation dc E A, 1⎞ n1 – --------A = A 1 exp ⎛ – --------- c , ⎝ RT ⎠ A dt

(7)

81

Concentration, % (a)

100 75

C

A A

50

B

C

25 B Concentration, % (b)

100 A

75

B

C

D

B D

E A, 1⎞ n1 E A, 2⎞ n2 dc --------B = A 1 exp ⎛ – --------- c A – A 2 exp ⎛ – --------- c , ⎝ RT ⎠ ⎝ RT ⎠ B dt

50

(8) A

[ A ] + [ B ] + [ C ] = [ A ] 0 = 1,

25

and a similar process of the thermal degradation of the PP–MPP–MMT nanocomposite A B C D is rationalized by a system of three equations: dc E A, 1⎞ n1 – --------A = A 1 exp ⎛ – --------- c , ⎝ RT ⎠ A dt

(9)

E A, 1⎞ n1 E A, 2⎞ dc --------B = A 1 exp ⎛ – --------- c – A 2 exp ⎛ – --------⎝ RT ⎠ A ⎝ RT ⎠ dt

dc E A, 2⎞ --------C = A 2 exp ⎛ – --------- × 0.5/ ( 1 – c B ) ⎝ dt RT ⎠ (11)

[ A ] + [ B ] + [ C ] + [ D ] = [ A ] 0 = 1. On the basis of the calculated kinetic parameters of thermal degradation, we plotted kinetic curves for the conversion and buildup of the components [A], [B], [C], and [D] during the isothermal (600°C) heating of the sample over 30 s (Fig. 11). The buildup curve for the component [C] in Fig. 11b, which characterizes the formation and consumption of the carbonaceous residue in the pyrolysis of the PP–MPP–MMT nanocomposite, is interesting. POLYMER SCIENCE

Series A

Vol. 48

No. 1

10

20

30 Time, s

Fig. 11. Consumption and buildup rate curves for the components [A], [B], [C], and [D] under the conditions of isothermal degradation of (a) PP and (b) PP– MPP–MMT nanocomposite samples at 600°C.

(10)

× 0.5/ ( 1 – c B ),

E A, 3⎞ n2 – A 3 exp ⎛ – --------- c , ⎝ RT ⎠ C

C

2006

The choice of the test temperature of 600°C is not accidental since this temperature corresponds to an incident heat flux of 35 kW/m2 that has been used in working tests on the ignitability of samples with a cone calorimeter [24]. The tests made it possible to evaluate important combustibility characteristics, such as heat release rate, mass loss rate, specific heat of combustion, specific extinction area, and carbon monoxide yield. The calculation of the heat release rate as a fundamental parameter measured by the cone calorimeter was based on the oxygen absorption principle. According to this principle, the heat released during the burning of a material is proportional to the amount of oxygen required for its combustion. For solid materials, the consumption of 1 kg of oxygen for their combustion is basically accompanied by evolution of 13.1 MJ of heat [24]. One of the tasks of this study was the correlationbased evaluation of the heat release rate under the cone calorimetry test conditions and the rate of mass loss under the conditions of isothermal pyrolysis in air.

82

LOMAKIN et al. VH , kW/m2

. . . . . Qtot = Qex + Qfl – Qrad – Qconv

(a) 2000

1

Cone calorimeter

. Qfl

. Qex

. Qrad

1500

1000

2

500

3

50

δ

150

250 Time, s

VM , g/(s m2) . Qconv

(b)

45

1

Fig. 12. Heat balance scheme in cone calorimeter tests; δ is the sample thickness. 30

In the general form, the scheme of heat balance for the cone calorimetry testing of samples is given in Fig. 12. The total heat flux Q˙ tot is composed of an incident heat flux from an external heater Q˙ ex , a heat flux from the flame on the surface of the material Q˙ fl , and heat losses due to convection Q˙ conv and radiation to the surroundings Q˙ rad . A phenomenological equation relating the mass loss rate to the net heat flux during combustion may be represented by [25] Q˙ tot Q˙ fl – ( Q˙ rad + Q˙ conv ) m˙ = ------- + --------------------------------------------- , Lg Lg

(12)

where Lg (kJ/g) is the heat of gasification related to the total heat flux on the surface. In practice, Lg is determined from the slope of the plot of the mass loss rate as a linear function of external heat flux [25]. The basic equation relating the mass loss rate to the heat release rate during combustion is as follows: 2 Q˙ tot ( kW/m ) = χ∆H comb m˙ .

(13)

Here, χ is the combustion efficiency, ∆Hcomb is the heat of complete combustion, and m˙ is the rate of mass loss per unit surface. If ∆Hcomb is a constant value for PP and the PP– MPP–MMT nanocomposite (i.e., the silicate additive

2 15

3

50

150

250 Time, s

Fig. 13. Plots of (a) the heat release rate VH and (b) the mass loss rate VM versus time for (1) PP, (2) PP– MMT, and (3) PP–MPP–MMT samples as measured in cone calorimeter tests at an external radiant heat flux of 35 kW/m2 .

does not inhibit gas-phase processes in the flame and has no effect on the heat of combustion), the heat release rate linearly depends on the mass loss rate. In this case, the coefficient χ for the linear equation characterizing the combustion efficiency or the completeness of combustion directly depends on the amount and structure of the carbonaceous residue. The flammability characteristics were examined with a cone calorimeter at an external heat flux of 35 kW/cm2 for the samples having a standard surface area of 70 × 70 mm and identical masses of 14.0 ± 0.1 g. Figures 13 and 14 depict the plots of the basic ignitability characteristics—heat release rate, mass loss rate, and specific heat of combustion—versus time for PP, as well as for the PP–MPP–7 wt % MMT and PP– POLYMER SCIENCE

Series A

Vol. 48

No. 1

2006

THERMAL DEGRADATION AND COMBUSTION

83

Specific heat of combustion, MJ/kg

CO, kg/kg (a)

45 0.15

3

1 2 3

30

2

0.10

15 0.05

50

100

150

200 Time, s

70 SEA,

The obtained results lead to the conclusion that char formation plays a key role in the mechanism of flame retardation for nanocomposites. Vol. 48

No. 1

210

280

m2 /kg

1200

7 wt % MMT nanocomposites. The last sample differs from the PP–MPP–MMT nanocomposite in that it lacks maleated PP and represents a completely intercalated PP nanocomposite. From Fig. 13a, it is seen that the maximum heat release rate for PP is 2060 kW/m2 , whereas that for the intercalated PP–MMT nanocomposite and the hybrid PP–MPP–MMT nanocomposite is 990 and 844 kW/m2, respectively; thus, the peak heat release rate decreases by a factor greater than 2. A similar trend is observed in Fig. 13b, which illustrates the dependence of the mass low rate on the combustion time; Fig. 14 shows the time dependence for the specific heat of combustion, which is practically identical for all three samples. This resemblance confirms that the silicate agent does not inhibit the gas-phase combustion process and, thus, does not affect the heat of combustion of the samples; the effect of twofold reduction in the maximum rate of heat release may be explained in terms of formation of a protective char layer on the burning polymer surface. The value characterizing the average amount of released carbon monoxide remains practically unchanged over the entire set of test samples; however, a strong increase in the maximum CO yield for nanocomposite samples during short time intervals indicates the crossover of active combustion to the oxygen-deficient smoldering phase (Fig. 15a). It is noteworthy that, despite effective charring, the maximum level of smoke formation during the combustion of PP nanocomposites does not exceed the level of ordinary PP and its total yield is practically the same in all cases (Fig. 15 b).

Series A

140

Time, s

Fig. 14. Specific heat of combustion as a function of time for (1) PP, (2) PP–MMT, and (3) PP–MPP– MMT samples as measured in cone calorimeter tests at an external radiant heat flux of 35 kW/m2 .

POLYMER SCIENCE

1

2006

(b)

900

600

300

1

70

140

2

3

210 Time, s

Fig. 15. Plots of (a) the CO yield and (b) the specific extinction area (SEA) versus time for (1) PP, (2) PP– MMT, and (3) PP–MPP–MMT samples as measured in cone calorimeter tests at an external radiant heat flux of 35 kW/m2 .

Earlier, it was assumed that the basic reason behind the fire retardancy of polymer nanocomposites is the removal (burn-up) of the organic polymer ingredient from the surface of burning material and an increase in the concentration of the silicate ingredient. As a result, more hydrophilic silicate particles have a tendency toward separation from the polymer matrix with the subsequent aggregation into intercalated layered fragments, especially under the conditions of ebullience of the pyrolyzed polymer [26]. Although the density of main representatives of layered silicates is ~2–3 g/cm3 [27] and they are heavier than polymers by definition, accumulated char layers (floccules) filled with silicate particles constitute porous entities that are rather light to remain on the surface of the polymer melt during its burning up.

84

LOMAKIN et al.

An alternative mechanism of char formation is related to the transfer of silicate particles to the surface of a burning polymer by upward convection flows in the polymer melt: the migration of silicate particles driven by diffusion or surface tension forces is not a process that determines the mechanism of charring in this case [28]. The sample surface coated with a composition of silicate particles and the heat-resistant organic ingredient of char is a very effective barrier to flame propagation over the surface. The ideal structure of the protective layer containing silicate particles and organic char is a densely crosslinked network structure and possesses considerable mechanical strength sufficient for the protective layer to remain intact during the burning up of the polymer from the surface. CONCLUSIONS In this study, we considered the basic features of thermal degradation and combustion of PP nanocomposite. A kinetic scheme for the thermal degradation process was proposed, which takes into account the diffusion character of formation of the solid-sate structure of char. The reason for the reduction in the flammability of the PP nanocomposite is the formation of heat-resistant char on the surface of the burning material. On the basis of the given model mechanisms of suppressing the flammability of polymeric nanocomposites, it may be assumed that one of the promising lines of research in the flame resistance of polymer nanocomposites will be the simulation of the dynamic behavior of particles with different morphologies and relation between geometric dimensions under the conditions of convective flows of viscous polymer melts. On the other hand, it is of interest to develop studies on the barrier properties of nanodispersed polymer composites. From the formal standpoint, it is obvious that the layered morphology is especially effective as compared with other forms of fillers because of the maximal increase in the length of the diffusion range of individual molecules in the composite medium. Research in this area will aid in answering the question as to whether microintumescence in the surface layer of the burning polymer is responsible for the slowing of the diffusion of low-molecular-mass degradation products. ACKNOWLEDGMENTS We are grateful to Prof. R. Kozlovsky (Institute of Synthetic Polymer Fibers, Poznan, Poland) for testing flammability with a cone calorimeter. REFERENCES 1. L. A. Wall, J. Polym. Sci. 17, 141 (1955).

2. N. Grassie, Chemistry of Vinyl Polymer Degradation (Butterworth, London, 1956). 3. Yu. M. Moiseev and M. V. Neiman, Vysokomol. Soedin. 3, 1383 (1961). 4. H. Bockhorn, A. Hornung, U. Hornung, and D. Schawaller, J. Anal. Appl. Pyrolysis 48, 93 (1999). 5. L. Ballice and R. Reimert, Chem. Eng. Process. 41, 289 (2002). 6. M. V. S. Murty, P. Rangarajan, E. A. Grulke, and D. Bhattacharyya, Fuel Process. Technol. 49, 75 (1996). 7. J. H. Chan and S. T. Balke, Polym. Degrad. Stab. 57, 135 (1997). 8. Z. Gao, T. Kaneko, I. Amasaki, and M. Nakada, Polym. Degrad. Stab. 80, 269 (2003). 9. E. Giannelis, Adv. Mater. (Weinheim, Fed. Repub. Ger.) 8, 29 (1996). 10. J. W. Gilman, T. Kashiwagi, M. R. Nyden, et al., in Chemistry and Technology of Polymer Additives, Ed. by S. Ak-Malaika, A. Golovoy, and C. A. Wilkie (Blackwell, Malden, 1999), Chap. 14, p. 249. 11. M. Zanetti, S. Lomakin, and G. Camino, Macromol. Mater. Eng. 279, 1 (2000). 12. Y. Kojima, A. Usuki, M. Kawasumi, et al., J. Mater. Res. 8, 1185 (1993). 13. M. Zanetti, G. Camino, P. Reichert, and R. Mulhaupt, Macromol. Rapid Commun. 22, 176 (2001). 14. H. Qin, S. Zhang, C. Zhao, et al., Polym. Degrad. Stab. 85, 807 (2004). 15. J. Opfermann, Rechentechnik/Datenverarbeitung 22, 26 (1985). 16. V. Babrauskas, Fire Mater. 19, 243 (1995). 17. N. Grassie and G. Scott, Polymer Degradation and Stabilization (Cambridge Univ. Press, Cambridge, 1985; Mir, Moscow, 1988). 18. J. Opfermann, J. Therm. Anal. Cal. 60, 641 (2000). 19. H. L. Friedman, J. Polym. Sci., Part C 6, 175 (1965). 20. J. Opfermann and E. Kaisersberger, Thermochim. Acta 11, 167 (1992). 21. S. M. Lomakin and G. E. Zaikov, Modern Polymer Flame Retardancy (VSP International Scientific, Utrecht, 2003). 22. J. W. Gilman, Appl. Clay Sci. 15, 31 (1999). 23. G. W. Gilman, C. L. Jackson, A. B. Morgan, et al., Chem. Mater. 12, 1866 (2000). 24. V. Babrauskas and R. Peacock, Fire Saf. J. 19, 255 (1992). 25. A. Tewarson, in Handbook of Fire Protection Engineering, Ed. by P. J. DiNenno (National Fire Protection Association, Quincy, 1988), Sect. 1, Chap. 13, p. 178. 26. R. D. Davis, J. W. Gilman, and D. L. Van der Hart, Polym. Degrad. Stab. 79, 111 (2003). 27. R. Krishnamoorti and E. P. Giannelis, Macromolecules 30, 4097 (1997). 28. T. Kashiwagi, R. H. Harris, Jr., Zhang Xin, et al., Polymer 45, 881 (2004). POLYMER SCIENCE

Series A

Vol. 48

No. 1

2006