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Thermal Treatment and Degradation of Cross-Linked Ethylene Vinyl Acetate−Polyethylene−Azodicarbonamide−ZnO Foams. Complete Kinetic Modeling and Analysis J. A. Reyes-Labarta* and A. Marcilla Chemical Engineering Department, University of Alicante, Apdo. 99, Alicante E-03080, Spain S Supporting Information *

ABSTRACT: The cross-linking and foaming processes of ethylene vinyl acetate (EVA) copolymer with different concentrations of polyethylene (PE), cross-linking agent (α-α′-bis(tert-butylperoxy)-m/p-diisopropylbenzene), and azodicarbonamide (as foaming agent and using additionally ZnO as accelerating agent) have been studied. The extent of the reactions has been evaluated by gel fraction and density measurements. The different transitions involved in the thermal processing and decomposition have been studied by DSC and TGA and compared for all the mixtures prepared. Mechanistic pseudokinetic models have been suggested and applied involving all the fractions susceptible of undergoing transitions or reactions, including complex, overlapped peaks and apparent heat capacities. The models applied can be of great interest in understanding the phenomena involved, as well as in modeling the heat effects in the whole processing or pyrolysis of this type of multicomponent products.

1. INTRODUCTION When mixtures of ethylene vinyl acetate (EVA) and polyethylene (PE) are adequately processed in the presence of cross-linking and foaming agents, low-density microcellular compounds are obtained. These compounds have a large field of commercial applications where the density reduction can be turned directly into material/cost savings and more valuable products (with closed-cell structure, resistance to oils and gasoline, low rates of heat transmission, and small water absorption). Typical examples are the soles of sport shoes, bicycle helmets, toys, nautical buoys and parts imparting buoyancy to boats, gymnasium floors, hygienic stable floors, thermal insulation in refrigerators, portable insulated chests, cushions for furniture and automobiles, and uses in the building industry (sandwich structures or hollow structural unit reinforcing). The cross-linking process consists of the formation of chemical bonds (cross-links) between adjacent molecular chains to form a three-dimensional network.1 Also, there exist several ways to produce cross-linked polymers such as chemical methods, using mainly peroxides2 or silanes,3 and high-energy irradiation (electronic-beam) techniques.4 Thermochemical cross-linking involving organic peroxides is widely used because of their controlled decomposition rates, minimal side products, and economical process.5 Figure 1 shows the general scheme of the cross-linking mechanism of EVA using peroxides to generate free radicals. Apart from the beneficial effects that the polymer cross-linking provides on the mechanical properties (even for long periods of time) leading to harder, stiffer, stronger, and tougher products that are needed for modern applications,6−10 another of the usual applications of the crosslinking process is to improve (or even to allow) the characteristics of foamed materials that by themselves, without previous cross-linking, have not many industrial applications. The foaming process consists in the internal generation of a gas © 2012 American Chemical Society

Figure 1. General scheme of the cross-linking mechanism of EVA using peroxides: (A) thermal decomposition of posibble peroxides; (B) initiation; (C) cross-linking.

which should be retained inside the sample. For this reason, the generation of this gas has to take place to a concrete temperature, i.e., when the polymeric matrix presents adequate characteristics, such as viscosity. On the other hand, it is important to remark that nowadays, in order to provide a response to the current environmental situation, the tertiary or chemical recycling of polymeric materials mainly by pyrolysis has received renewed attention to remove plastic wastes, taking advantage of them, due to the possibility of getting solid, liquid, and gaseous products with higher value as fuel or chemicals, such as hydrocarbons for the petrochemical industry, and contributing to environmental protection, as well.11−14 Received: Revised: Accepted: Published: 9515

March 15, 2012 May 7, 2012 June 11, 2012 June 20, 2012 dx.doi.org/10.1021/ie3006935 | Ind. Eng. Chem. Res. 2012, 51, 9515−9530

Industrial & Engineering Chemistry Research

Article

Table 1. Technical Data for LDPE PE003 and EVA PA-539 ALCUDIA (Repsol YPF) property

a

PE

EVA

melt flow index (g/10 min) softening pointa (°C) crystallinity (%) density at 23 °C (g/cm3) melting temperature (°C)

2 92 44 0.920 113

2 64 18 0.937 90

chemical formula

[ ̵ CH 2−CH 2 ]n̵

[ ̵ CH 2−CH 2]n − [CH 2−CH(OCOCH3)]m̵

Temperature at which a flat-ended needle penetrates the material to the depth of 1 mm under a specific load.

Table 2. Technical Data for Peroximon F-40 (Rusimont Group-Elf Atochem) property

value

α-α′-bis(tert-butylperoxy)-m/p-diisopropylbenzene (TBPPB) (wt %) inert charge (CaCO3 precipitated) (wt %) active oxygen (%) specific gravity at 20 °C (g/cm3) density (g/cm3)

40 60 3.78 1.63 0.71

chemical formula

CH3−C(CH3)2 −O−O−C(CH3)2 − C6H4−C(CH3)2 −O−O−C(CH3)2 −CH3

bis(tert-butylperoxy)-m/p-diisopropylbenzene, TBPPB, provided by Rusimont Group-Elf Atochem (Peroximon F-40, Table 2). Azodicarbonamide (Unicell-D supplied by LEDEX, Table 3) has been used as a foaming agent because it is the one

The present paper completes a systematic study of the effects that the different individual components of the multicomponent commercial EVA−PE foams have on the foaming, cross-linking, and pyrolysis reactions of the main polymeric matrix.15−20 In this article, we have investigated in the global industrial multicomponent mixture, the interactions among the different components all together, i.e., the polymeric matrix (EVA−PE), the cross-linking agent (CA), and azodicarbonamide (ADC). We analyze different concentrations to establish the effect of the overall cross-linking and foaming processes on the thermal treatment and degradation of the EVA−PE foams. First of all, thermal properties such as specific melting enthalpy, melting and degradation temperatures of un-crosslinked−nonfoamed and cross-linked−foamed samples, and their dependence on the PE, cross-linking, and foaming agent concentration were studied by differential scanning calorimetry (DSC) and by dynamic thermogravimetric analysis (TGA). The cross-link degree of the samples was associated with their gel content (GC), determined by a solvent-extraction method. Also, the density evolution of the cross-linked−foamed samples was determined. Finally, a mechanistic and pseudokinetic model is suggested and applied to the analysis and simultaneous correlation of all the corresponding DSC and TGA data (varying the PE, CA, and ADC contents), including complex or overlapped peaks and apparent heat capacities. The analysis presented in this paper allows for better knowledge of the processes involved in the thermal industrial processing and degradation of these cross-linking and foamed multicomponent compounds. Thus, the quantification of the heat involved in the cross-linking and foaming phenomena yields very useful information for the control, design, and optimization of the equipment, process, cycles, and formulations to be used.2,3,10,21−29

Table 3. Technical Properties of Azodicarbonamide UnicellD (LEDEX) property

value

size (μm) ADC (wt %) decomposition temperature (°C) ash (%) gas yield (isothermal at 210 °C during 15 min) (cm3/g) particle size (μm) density (g/cm3)

91.1 187 5.85 160 7 1.65

frequently used in the production of PVC and EVA−PE foams, as its decomposition liberates a high volume of gas, which is trapped in the melt.30−33 The decomposition of these two agents occurs after the melting of the PE and EVA, respectively, i.e., in the molten polymer, favoring the cross-linking reactivity and following foaming process (Figure 2). In order to move forward the foaming process in relation to the cross-linking reaction, i.e., to reduce the thermal decomposition temperature of the azodicarbonamide, ZnO (supplied as fine powder by Panreac) was added as an accelerating agent (kicker).17,32,34 This agent is commonly used in the industry due to the absence of color in the residue and its quality/price ratio. 2.2. Sample Preparation. Multicomponent mixtures of EVA with different concentrations of PE (5, 10, and 15 phr, i.e., 4.76, 9.10, and 13.04 wt %), TBPPB (0.75, 1.5, and 3 phr, i.e., 0.74, 1.48, and 2.91 wt %), ADC (1, 2, and 4 phr, i.e., 0.99, 1.96, and 3.85 wt %), and ZnO (1.5 phr) were studied. The concentrations of the agents were selected close to typical values used in industrial applications. The mixtures were prepared, prior to experiments, in a Brabender Plasticorder PL 2000 extruder at 398 K with a speed

2. EQUIPMENT AND EXPERIMENTAL PROCEDURE 2.1. Materials. The polymers used were low-density polyethylene (LDPE) PE003 and EVA PA-539 ALCUDIA copolymer, both supplied by Repsol YPF. Table 1 shows the properties of these two polymers according to the supplier. The cross-linking agent was a suitable peroxide frequently used in the production of EVA and PE foams, such as α-α′9516

dx.doi.org/10.1021/ie3006935 | Ind. Eng. Chem. Res. 2012, 51, 9515−9530


Article

Figure 2. DSC results (first and second runs) for pure polymers and pure cross-linking (TBPPB) and foaming (ADC) agents.

of 20 rpm, using a single screw. These conditions were chosen to achieve a good homogenization of the mixture, but avoiding the thermal decomposition of the peroxide and ADC.35,36 After the extrusion die, the mixture discharged was cut into pellets, immersed in a water bath, and, after that, dried at room temperature. 2.3. Thermal Analysis. 2.3.1. Differential Scanning Calorimetry (DSC). DSC tests were performed on a PerkinElmer DSC 7 controlled by a PC AT compatible system. Samples of 8−9 mg were encapsulated in aluminum pans and treated at a heating rate of 10 K/min, bearing in mind that the evolution of the decomposition heats of ADC may be only slightly influenced by the heating rate when the rate exceeds 3 °C/min.37 The atmosphere used was nitrogen with a flow rate of 45 mL/min (STP). To analyze the influence of the crosslinking and foaming agents before and after their corresponding decompositions, two consecutive runs for each sample were performed. Initially, a first experiment in the temperature range 313−573 K was run. The sample was then cooled until 293 K and a second run was performed at the same heating rate, in order to analyze the effect of the cross-linking and foaming agents not only before and during the cross-linking and foaming processes (first run), but also on the final cross-linked and foamed polymer (second run), i.e., to characterize the postprocessing product. 2.3.2. Thermobalance (TGA). The thermogravimetric experiments were carried out using a Netzsch Thermobalance TG209 controlled by a PC under the Windows operating system. The tests were performed in a nitrogen environment with a flow rate of 45 mL/min (STP). Samples of 5 mg were heated at 10 K/min from 303 to 873 K. In both techniques, the experiments were replicated in order to determine their reproducibility, showing very good results with a maximum deviation between the repeated runs of about 2%. 2.4. Cross-Linking and Foaming Samples. To obtain the cross-linked and foamed product, a hydraulic hot plates press MECAMAQ DE-200 was used. The temperature in both plates was 448 K, and the residence time was 10 min. With these

conditions the cross-linking and foaming processes were complete. 2.4.1. Density Measurement. The density of the different mixtures was measured after processing using a glass pycnometer at 298 K. The variation coefficient for the measured densities was estimated to be 0.005 g/cm3. 2.4.2. Gel Content Measurement. The degree of crosslinking can be estimated through the gel fraction (i.e., insoluble fraction). The gel content of the cross-linked and foamed samples was determined gravimetrically (according to UNE 53381-89) by a 16 h Soxhlet extraction cycle using THF (tetrahydrofuran) at 339 K (where pure EVA and PE are totally soluble and insoluble, respectively) and decalin as solvent at 458 K (where pure PE and EVA are totally soluble and insoluble, respectively). Approximately 0.3−0.4 g of the crosslinked and foamed polymer (m0) was cut into small pieces. After the extraction cycle, the sample was dried to a constant weight (m1) at 353 K. The gel fraction was calculated as the percentage ratio of the final weight of the polymer to its initial weight, taking also into account the initial insoluble fraction ( f) of the sample. The variation obtained for repeat measurements was lower than 5%. ⎛ ⎞ m1 GC = ⎜1 − ⎟ ·100 m0(1 − f ) ⎠ ⎝

(1)

3. KINETIC MODELS The pseudokinetic study of the process would be a very important aspect to quantify and simulate the evolution of the different species present in the sample compounds (with temperature or time), and also to determine the amount of gases evolved, and it may help in the optimization of industrial processing. It is widely accepted that cross-linking reactions go through free radical mechanisms. In general, the free radicals generated on thermal decompositions of peroxides can attack the molten polymer chains (abstracting hydrogen atoms to produce alkyl radicals) and cross-linking of the polymer chains may occur. 9517



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fractions susceptible of undergoing thermal reactions in DSC and TGA experiments (Table 4), respectively:

Examples of radical reaction mechanisms of cross-linking are proposed in the literature.38−40 In the case of ADC, the reactions involved in its thermal decomposition are complex and depend on the process conditions and the state of the product. Thus, it is assumed that primary decomposition takes place by reactions (i) and (ii) in Figure 3 with a secondary reaction (iii) taking place as cyanic

dQ mDSC dT

= wSCP ,S +

DSC dQ EVA

DSC dQ CA

+

dT

dT

ϕEVA, m +

ϕCA, m +

DSC dQ ADC

dT

DSC dQ PE

dT

ϕPE, m

ϕADC, m

+ (1 − wS)CP ,M TGA TGA TGA dwmTGA dwCA dwEVA dwPE = ϕEVA, m + ϕPE, m + dt dt dt dt TGA dwADC * ϕCA, m + ϕADC, m dt

(2)

(3)

where ADC*, in the case of the TGA, refers to a global thermal decomposition process that joins all the ADC decomposition processes. In order to obtain better correlations to fit all the experimental data of the whole DSC curve, due to the existence of a strong baseline variation, the contribution of the apparent heat capacities (CP) of the solid (wS) and melted (1 − wS) fractions with the temperature have also been included in the model as a second degree polynomial for each global fraction (solid or melted): CP = (aT2 + bT + c) (J/g K). In previous papers, Marcilla et al. suggested and applied a methodology and different pseudokinetic models to model and explain the thermal transitions and fusions involved in the thermal treatment of polymers (such as PE and EVA) prior to their decomposition,15,16 and the thermal (and catalytic) pyrolysis of PE, PP, and EVA polymers.44−48 These previous models use n-order kinetics and Arrhenius type behavior of the rate constant, and they can be developed and applied in an equivalent way for our analysis to model the different individual contributions in the multicomponent samples studied, following the corresponding scheme of reactions (Table 4). The total number of parameters that have to be fitted depends on the thermal treatment studied (number of peaks). Thus, in the DSC experiments, using four parameters per single reaction peak (ΔH, kref, Ea, and n), for an EVA−PE−CA−ADC mixture the model has 39 parameters (8 reactions + 2 baseline corrections + 1 unknown fraction). In the case of the TGA, the

Figure 3. Decomposition reactions for azodicarbonamide.

acid (HNCO) becomes available.31−34,41 The products of these reactions are solids (urazole and hydrazodicarbonamide) and a gaseous mixture of nitrogen, carbon monoxide, cyanic acid, and ammonia. This liberation of ammonia restricts the use of azodicarbonamides in polymers or materials sensitive to degradations or corrosions produced by this gas. Typical parameters for the commercial azodicarbonamides provided by the suppliers are the purity and the amount of gases (gas yield) evolved in an isothermal process at 210 °C for 15 min collected in DOP (diisooctyl phthalate).42,43,37 The aim of the present study is to evaluate the effects that the cross-linking and foaming processes produce on the global pseudokinetic constants of the different thermal transitions and degradation of the systems studied. Therefore, the following pseudokinetic models, which start from a linear combination of the effects that the different components cause considering the concentration of each compound of the multicomponent EVA foams (ϕi,m), have been suggested and applied involving all the

Table 4. General Scheme of Reactions of the Materials Studied (ADC, CA, EVA, and PE) in DSC and TGA Experiments DSC and TGA ADC Thermal Decomposition 2H4N4C2O2 → H6N4C2O2 + 2HNCO + N2

(r.1)

2H4N4C2O2 → H3N3C2O2 + 2HNCO + NH3 + N2

H4N4C2O2 + 2HNCO → H4N4C2O2 (HNCO)2 * H4N4C2O2 (HNCO)2 * → H6N4C2O2 + N2 + 2CO H3N3C2O2 → Gr.5

(r.5)

H6N4C2O2 → Gr.6

(r.6)

(r.2) (r.3) (r.4)

kD,CA

CA ⎯⎯⎯⎯⎯→ sD,CA R D,CA + (1 − sD,CA )G D,CA DSC kT,EVA

TGA kDP1

kM,EVA

EVA ⎯⎯⎯⎯→ sDP1EVA* + (1 − s DP1)G DP1

EVA ⎯⎯⎯⎯⎯⎯→ EVA(T) ⎯⎯⎯⎯⎯⎯⎯→ EVA(M)

kDP2

kM,PE

EVA* + PE ⎯⎯⎯⎯→ sDP2 R DP2 + (1 − sDP2)G DP2

PE ⎯⎯⎯⎯⎯→ PE(M) 9518



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Table 5. Composition, Code, Density (g/cm3) and Gel Content (% of Insoluble Fraction in THF at 339 K and Decalin at 458 K) of Studied EVA−PE−CA−ADC−ZnO Mixturesa composition (phr) EVA

PE

CA

gel content (%) ZnO

code

density (g/cm3)

THF

decalin

2 2 2 2 2 1 4

1.5 1.5 1.5 1.5 1.5 1.5 1.5

EVA PE I II III IV V VI VII

1 2 4 2

1.5

1 2 4 2

1.5

0.938 0.923 0.363 0.392 0.429 0.319 0.610 0.498 0.287 0.935 0.932 0.930 0.938 0.944 0.958 0.404 0.337 0.205 0.361 0.935 0.942 0.952 0.450 0.351 0.282 0.375 0.944 0.943 0.941 0.938 0.949 0.323 0.341 0.368 0.429 0.215 0.361

0 100 95.0 92.6 93.1 89.9 95.4 93.4 92.4 4.1 8.2 11.3 92.5 95.2 97.8 0 0 0 0 − − − − − − − 97.1 96.3 95.2 92.4 99.0 4.1 8.2 11.3 8.2 8.2 95.2

100 0 88.9 87.9 87.5 84.2 93.6 86.7 87.3 − − − − − − − − − − 65.2 79.6 90.1 0 0 0 0 95.9 94.4 93.8 90.2 97.5 − − − − − −

ADC

100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100

100 100 100 100 100 100 100 100 100 100 100 a

100 5 10 15 10 10 10 10 5 10 15

1.5 1.5 1.5 0.75 3 1.5 1.5

0.75 1.5 3

100 100 100 100 100 100 100 5 10 15 10 10 5 10 15 10 10

0.75 1.5 3

1.5 1.5 1.5 0.75 3

1.5

2 2 2 1 4 2

EP(5)C(1.5)A(2)Z(1.5) EP(10)C(1.5)A(2)Z(1.5) EP(15)C(1.5)A(2)Z(1.5) EP(10)C(0.75)A(2)Z(1.5) EP(10)C(3)A(2)Z(1.5) EP(10)C(1.5)A(1)Z(1.5) EP(10)C(1.5)A(4)Z(1.5) EP(5) EP(10) EP(15) EC(0.75) EC(1.5) EC(3) EA(1) EA(2) EA(4) EA(2)Z(1.5) PC(0.75) PC(1.5) PC(3) PA(1) PA(2) PA(4) PA(2)Z(1.5) EP(5)C(1.5) EP(10)C(1.5) EP(15)C(1.5) EP(10)C(0.75) EP(10)C(3) EP(5)A(2) EP(10)A(2) EP(15)A(2) EP(10)A(1) EP(10)A(4) EC(1.5)A(2)

Pressed samples at 175 °C during 10 min. Results for binary and ternary samples are included. 2 ⎡⎛ ⎞ ⎛ dY ⎞ ⎤ dY ⎟ ⎢ ⎥ ⎜ ⎜ ⎟ OF = ∑ ∑ − ⎢⎝ dX ⎠exp ⎝ dX ⎠calc ⎥⎦ m=1 i=1 ⎣

total number of parameters to be optimized is 16 (4·kD, 4·(EaR), 4·n, and 4 yield coefficients sj). This number of parameters may appear too high, but the number of separate processes/peaks analyzed must be considered, taking into account the evolution of the apparent heat capacities and that no linearization has been made. (Table S1 in Supporting Information presents a resume of the kinetic equations used.) In the case of the correlation of the second DSC run, where the samples are already cross-linked and foamed, no more CA or ADC is present in the sample and the corresponding CA and ADC terms in eq 2 are removed. 3.1. Mathematical Treatment. The pseudokinetic parameter unknowns of the kinetic models proposed in this work have been optimized using the tool “Solver” included in the spreadsheet software Excel for Windows. In all the calculations the objective function considered was

N

4

(4)

where m represents the different samples simultaneously fitted (with different formulations), i represents the experimental data at temperature Ti and at time ti, N is the number of experimental points, (dY/dX)exp represents the experimental heat or mass derivative with respect to the temperature or time, as obtained from the DSC or TGA apparatus respectively, and (dY/dX)calc is the calculated value from the corresponding proposed kinetic model (eq 2 or 3). In order to compare different kinetic models, a relative standard deviation or variation coefficient is introduced:

RSD (%) =

9519

OF N−P

Dexp av

· 100 (5)



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Table 6. DSC Results (First and Second Runs) of Pure Polymers and Studied EVA−PE−CA−ADC−ZnO Mixtures EVA

PE

CA

ADC

DSC run

sample code

ΔHmelt. (J/g)

Ttrans (K)

Tmelt. (K)

ΔHmelt. (J/g)

Tmelt. (K)

ΔHdecomp (J/g)

Tdecomp (K)

ΔHdecomp (J/g)

Tdecomp (K)

ΔHdecomp (J/g)

Tdecomp (K)

1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd 1st 2nd

EVA

57.3 52.7 − − 55.8 37.9 56.5 39.5 55.7 40.1 56.1 42.1 55.1 36.4 55.9 36.3 54.8 38.9

321.8 322.0 − − 323.8 322.5 322.1 323.0 322.5 322.5 322.0 323.0 322.3 323.0 322.5 322.5 322.1 322.0

345.3 345.3 − − 349.6 344.5 347.6 341.8 347.2 342.3 344.8 343.1 349.0 340.6 349.3 342.6 348.2 346.8

0.8 0.8 81.3 79.7 4.1 3.5 8.3 7.2 11.9 10.3 8.3 7.8 8.1 6.6 7.9 6.8 8.3 7.6

386.1 386.1 387.8 387.8 385.5 383.5 385.3 382.5 385.3 382.5 386.1 385.6 385.5 381.1 386.5 382.6 386.8 385.6

− − − − −4.9

− − − − 472.3

− − − − −12.4

− − − − 452.0

− − − − 3.6

− − − − 525.5

−4.8

469.6

−11.8

449.8

3.2

524.6

−4.6

474.8

−11.6

450.6

3.7

524.6

−2.6

470.3

−11.5

450.1

3.7

525.6

−8.9

471.0

−9.2

452.3

3.6

525.6

−7.5

471.0

−5.7

457.3

3.8

524.6

−3.1

470.8

−24.9

448.0

3.9

525.5

PE I II III IV V VI VII

Figure 4. DSC results (first runs) for the mixtures studied with various PE contents: 5, 10, and 15 phr.

where P is the number of parameters to be fitted and Dexp av is the average of the experimental derivatives. The integration of the kinetic equations was carried out using the Euler method.

and increase in ADC concentration produces a decrease in the final density of the sample as a consequence of a larger extension of the foaming process. The strong decrease of the density with the ADC concentration in the sample is reduced with the presence of CA, showing the negative effect of the higher melt viscosity of the cross-linked polymeric matrix in the gas evolution from the ADC decomposition. However, a small quantity of CA can help the encapsulation of the gas in the foaming process; thus the multicomponent sample IV with 0.75 phr CA presents a lower density than the corresponding sample without CA. On the other hand, the values of gel content indicate the extension of the cross-linking process increases with the concentration of CA, if we compare the corresponding gel content values of the pure polymer. However, these increases are lower than those corresponding to the equivalent binary or

4. RESULTS AND DISCUSSION In this section, the main characteristics and the thermal behavior of the multicomponent EVA foams with different amounts of their compounds (PE, cross-linking and foaming agents) is discussed. 4.1. Physical Properties of the Cross-Linked and Foamed Samples. The amounts of density and gel content for the cross-linked and foamed samples are shown in Table 5 to confirm the effect especially of the cross-linking and foaming reactions. Data of the pure polymers (PE and EVA) and some binary and ternary mixtures are also included as a reference in comparison with the multicomponent samples (with different amounts of PE, peroxide, and ADC). As expected, the presence 9520



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Figure 5. DSC results (first runs) for the mixtures studied with various CA contents: 0.75, 1.5, and 3 phr.

Figure 6. DSC results (first runs) for the mixtures studied with various ADC contents: 1, 2, and 4 phr.

ternary samples without ADC, showing the interferences that the foaming process introduces in the cross-linking reaction. 4.2. Thermal Properties and Analysis. 4.2.1. DSC Experiments. Table 6 shows the melting temperatures (or reaction temperature in the case of the cross-linking and foaming agents) and heats (i.e., area under the corresponding peak), determined by DSC measurements for all the samples studied including pure components, un-cross-linked and nonfoamed samples (first DSC run), and cross-linked−foamed samples (second DSC run). DSC for First Runs. Analyzing the first run for the multicomponent mixtures studied (Table 6 and Figures 4−6), the peaks corresponding to the thermal transitions of the polymeric matrix are similar (number of peaks, reaction temperatures, and heat) to those corresponding to the pure polymers used (Figure 2). After melting of the polymer matrix in the multicomponent samples, three more peaks appear in the first run of the DSC curves corresponding to the thermal degradation of the cross-linking and foaming agents. The first

two overlapped peaks correspond to the exothermic (primary) thermal decomposition of ADC and the decomposition of CA. The following endothermic peak corresponds to the secondary decomposition of ADC. Thus, the first two exothermic processes clearly observed in the case of the pure ADC (p1 and p2 in Figure 2) are now, in Figures 4−6, totally overlapped in a wide peak (p1 + p2) around 450 K due to the presence of ZnO,17 and that appears at lower temperatures (i.e., temperatures of maximum rate) than the second exothermic peak (p2) in the DSC of the pure ADC (Table 6 and Figure 2), taking also into account that the peak corresponding to the CA decomposition is also overlapped around 470 K (Figure 5). Nevertheless, this peak should correspond mainly to the second process of the ADC decomposition (i.e., the more marked in the DSC of the pure ADC). The peak temperature of this wide exothermic process in the DSC curves of the multicomponent samples decrease as the concentration of ADC increases (Figure 6). Additionally, the endothermic process (p3) appears 9521



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Figure 7. DSC results (second runs) for the mixtures studied with various PE contents: 5, 10, and 15 phr.

Figure 8. DSC results (second runs) for the mixtures studied with various CA contents: 0.75, 1.5, and 3 phr.

Obviously, the peak area (J/gsample) corresponding to the thermal decomposition of PE, cross-linking agent, or foaming agent increases lineally with the concentration of this component in the mixture, but this increase does not necessarily mean any variation of its specific decomposition enthalpy (J/gCA) as will be shown in the quantitative kinetic analysis. However, another aspect that is worth mentioning is the different extensions of the reactions of the ADC decomposition in the DSC of the EVA−PE−CA−ADC−ZnO mixtures when varying the ADC content (Figures 4−6). Whereas the heat evolved (J/gsample) in the first wide exothermic processes (p1 + p2) of the samples increases almost lineally with the ADC content (Table 6), the heat corresponding to the last endothermic process (p3) remains almost constant regardless of the ADC content. Figures 4 and 5 show very similar peaks for the thermal decompositions of CA and ADC. This fact indicates a slight effect of the variation of the PE and CA contents on these

at higher temperatures in the mixture with the higher ADC content (4 phr). The displacement of the peaks associated with the ADC thermal decomposition in the multicomponent samples compared with the pure ADC might be explained as a result of two different effects: 1. The polymeric matrix has a negative effect on the heat transfer inside the sample that contributes to the delay of the peaks in the multicomponent samples as compared with pure ADC. 2. According to the azodicarbonamide degradation reactions shown in Figure 3, the heterogeneous reaction (iii) could introduce an autoaccelerating effect30,31,36,49 when the concentration of ADC in the sample increases, due to a higher probability of reaction (higher contact time) between the nonreacted ADC and the HNCO gas generated from reactions (i) and (ii) in Figure 3. This reaction can accelerate the global decomposition of ADC and produce the observed shift to lower temperatures. 9522



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Figure 9. DSC results (second runs) for the mixtures studied with various ADC contents: 1, 2, and 4 phr.

Figure 10. Experimental TGA curves for pure components: CA (TBPPB), ADC (azodicarbonamide), EVA, and PE.

to a larger viscosity of the melted and cross-linked polymeric matrix. DSC for Second Consecutive Runs. If the second consecutive heating run of the multicomponent mixtures is considered (Figures 7−9 and Table 6), we can observe that in all the samples (including the pure EVA) the first peaks corresponding to the polymer transitions present a decrease in height, peak temperature, and total area, compared with the first run, that depends mainly on the initial CA content of the sample, and therefore is due to the negative effects of the crosslinking process in the final crystallinity of the cross-linked foams.2,3,50 Furthermore, the third peak corresponding to the melting of the ethylene domains of the EVA copolymer and PE presents a much lesser variation than the previous peaks (corresponding

processes. On the other hand, as can be seen in Figures 5 and 6, the presence of the cross-linked agent and especially the foaming agent also produces a relative and progressive increase of the slope of the final baseline (at high temperatures) that means an increase of the apparent heat capacities of the sample when the composition of these agents in the sample increases. This phenomenon can be explained in terms of the heat transfer within the sample. When the ADC content increases, a higher amount of the degradation products is obtained (solid residues and encapsulated gases), yielding a progressively more foamed sample which can decrease the apparent thermal conductivity and increase the apparent heat capacities. The increase of the cross-linked agent also produces a relative increase of the final baseline slope (at high temperatures) due 9523



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Figure 11. Experimental TGA curves for mixtures studied with various PE contents: 5, 10, and 15 phr.

Figure 12. Experimental TGA curves for mixtures studied with various CA (TBPPB) contents: 0.75, 1.5, and 3 phr.

to the vinyl acetate domains), due to the fact that the acetate groups of the EVA copolymer help the formation of radicals to a larger extent than in the case of the PE domains.50−52

On the other hand, almost no changes in the peak temperature and area for the melting process of the polymeric matrix are observed in the multicomponent mixtures when 9524



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Figure 13. Experimental TGA curves for mixtures studied with various ADC contents: 1, 2, and 4 phr.

varying the PE content or at low ADC contents; in the case of the sample with the highest ADC content (4 phr), the effect of the cross-linking reaction seems to be reduced (Figure 9). The baselines after the polymer peaks, for both runs of each sample, are very similar. Therefore, the baseline slope should depend mainly on the cross-linking and foaming processes, i.e., on the gas that continues to be encapsulated in the sample and the inert components of ADC. The fact that this slope also increases with the content of the foaming and cross-linking agents, as noted previously in the analysis of the first DSC runs, certifies the new closed-cell structure formed in this kind of foamed product. Finally, it can be also observed that for all the second run experiments no more CA or ADC decomposition peaks appear, indicating that these agents were consumed completely during the first runs. 4.2.2. TGA Experiments. Figure 10 shows the TGA curves for the commercial pure components. As expected, CA presents a thermal degradation with only one step, around 450 K, and a final inorganic solid residue corresponding to 60% of the initial weight, while ADC presents a complex thermal decomposition with multiple reactions from 425 K. Figures 11−13 represent the TGA curves for the studied samples. The general shape of the thermal degradation of the EVA−PE polymeric matrix of the multicomponent samples studied is similar to that corresponding to pure polymers:15 a first peak corresponding to the vinyl acetate loss and a second peak corresponding to the decomposition of the polyolefin chain, resulting from the first process, and the decomposition of the PE present in the samples. A magnification of the range of temperatures where the CA and ADC undergo decomposition

is presented, clearly showing that this process is visually proportional to the amount of agent used. Additionally, it is possible to observe a progressive shift of all decomposition processes to higher temperatures when increasing the peroxide content in the samples; also, this effect is negatively affected by the presence of the foaming reaction. Peak temperatures for all the mixtures studied are shown in Table 7. 4.3. Kinetic Modeling Results. 4.3.1. DSC Kinetic Model Analysis. In the simultaneous correlation of the first DSC run for all the multicomponent EVA−PE−CA−ADC−ZnO samples studied, where no differences in the polymer peak temperatures have been observed, all the pseudokinetic Table 7. TGA Peak Temperatures (K) of Pure Polymers and Studied Samples cross-linking and foaming agents

polymeric matrix

sample code

peak CA

peak 1 ADC

peak 2 ADC

peak 3 ADC

peak 1 EVA

peak 2 EVA

peak PE

EVA PE I II III IV V VI VII

− −

− −

− −

− −

639 − 650 650 650 642 657 651 648

676 − 680 679 679 675 681 680 676

747 747 752 753 753 748 760 756 751

460a 460a 460a 455a 465a 475a 460a

a

All the thermal decomposition processes for the cross-linking and foaming agents appear overlapped.

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Figure 14. Experimental and calculated DSC curves (first run) with the different contributions of each component for the mixture EP(10)C(3)A(2)Z(1.5).

Figure 15. Evolution of CP contribution (for global solid and melted species) and ADC and CA decompositions in the DSC (first run) for pure EVA and mixtures EVA−PE(10)−CA(1.5)−ADC−ZnO(1.5), with different ADC contents: 1, 2, and 4 phr.

parameters (ΔH, kref′, Ea, and n) corresponding to polymer transitions or melting are constant (validating the linear combination of the individual behaviors of the components), except those corresponding to the CP contributions (for solid and melted species) that necessarily have been fitted independently for each multicomponent sample, indicating the nonlinear influence of the cross-linking and foaming agents and processes in the internal heat transfer (Table S2 in the Supporting Information). Only in the case of varying the PE content are all the parameters corresponding to the decomposition reactions of the cross-linking and foaming agents also constant. When varying the CA content, due to the differences observed in the

different peaks and commented previously, the following parameters have to be fitted additionally in a independent way for each of these samples: ΔHD,CA, ΔHD,ADC,r.1, and ΔHD,ADC,r.2. In the case of varying the ADC content the additional parameters to be fitted are ΔHD,CA, ΔHD,ADC,r.1, ΔHD,ADC,r.2, ΔHD,ADC,r.6, EaD,ADC,r.1, and EaD,ADC,r.2. In all cases, the parameters present an approximate linear dependence with the content of the corresponding agent. The different reaction heats decrease with the CA or ADC content, while EaD,ADC,r.1 and EaD,ADC,r.2 increase with the ADC content (Figure S1 in the Supporting Information), according to the qualitative analysis previously done. The parameters obtained for the EVA and PE peaks do not present significant differences from those 9526



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Figure 16. Experimental and calculated TGA curves for mixtures EVA−PE(10)−CA−ADC(2)−ZnO(1.5), with two different CA (TBPPB) contents: 0.75 and 3 phr.

corresponding to the pure polymers,15 and reactions r.3 and r.4 of the ADC decomposition (Table 4) can be negligible. Figure 14 shows an example of the deconvolution of the calculated curves and the satisfactory degree of the correlation obtained. In this figure, we can also observe the contributions of the different components and reactions to the global DSC curve. The contributions of CP,M and CP,S in the different samples increase when the agent contents increase, especially at high temperatures and with different ADC contents (Figure 15). In the case of the second DSC run, i.e., once the polymeric matrix is already cross-linked and foamed, there exists only a significant, progressive change in the location and area of the peaks associated with the melting processes of the corresponding polymers, with changing CA content (Figures 7−9). This feature shows that the linear combination no longer applies, and thus, it is necessary to allow the variation of the corresponding pseudokinetic parameters with the cross-linking agent concentration (apart from those corresponding to the CP contributions), in order to obtain a satisfactory fit. Thus, the parameters that have been fitted independently for each of these samples are ΔHT,EVA, ΔHM,EVA, ΔHM,PE, kref,T,EVA′, kref,M,EVA′, kref,M,PE′, EaM,EVA, and nM,EVA. In most cases, the parameters ΔH, log kref′, Ea, and n present an approximate linear dependence with the CA content, and only the parameter ΔHT,EVA presents a parabolic dependence (Table S3 and Figure S2 in the Supporting Information). As noted before, the parameter ΔH decreases with the CA content, while the rest of the parameters increase in accordance with the experimental evolution observed of these peaks (decrease in height, peak temperature, and total area). At this point, it is important to remark that the comparison between the pseudokinetics parameters must be carefully done since the parameters are highly interrelated. The order of reaction is related to the shape and symmetry of the peaks, and the activation energy and the pre-exponential factor are both related to the peak location and width, being the width of the peak more sensitive to Ea.16

The increase of the apparent heat capacity shows the effect of the new structure (with greater viscosity and molecular weight) generated by the cross-linking and foaming reactions, decreasing the thermal conductivity inside the sample. The averages of the relative standard deviations obtained are 0.135 and 0.175 for all the DSC curves (1200 points), first and second runs, respectively. It is important to remark that the comparison of pseudokinetic parameters must be carefully considered since the parameters are highly interrelated.15 Thus, the order of reaction is related to the shape and symmetry of the peaks, being almost symmetric for first order reactions. The activation energy and the pre-exponential factor are both related to the peak location and width, being the width of the peak more sensitive to Ea. Additionally, in the case of the melting heat, the values obtained from the kinetic analysis do not include the contributions of the heat capacities, which are included in the values of the melting heat obtained by the direct integration of the DSC curves. 4.3.2. TGA Kinetic Model Analysis. As noted before, a progressive displacement in the decomposition temperatures of the polymeric matrix to higher values can be observed when increasing the cross-linking degree of the samples. This variation causes, necessarily, the variation of all the pseudokinetic parameter corresponding to the polymeric domain decompositions compared to the pure EVA. Additionally, in order to obtain a simultaneous and satisfactory fit of all the cross-linked−foamed samples studied, the pre-exponential factors of the polymeric domain decompositions (kref,DP1 and kref,DP2) that are directly related to the temperature of maximum degradation (and sDP1, when varying the CA content in samples II, IV, and V) have also been optimized independently for each sample, while Ea and n have been kept constant for each series of samples. The results obtained show that the logarithm of the pre-exponential factors corresponding to the thermal degradations of the polymeric domains decreases and presents an almost linear tendency with the CA and ADC contents of the 9527



■

processed samples corresponding to the progressive shift of the decomposition process to higher temperatures with increasing peroxide content, while the parameter sDP1 presents a parabolic dependence with the CA content (Table S4 and Figure S3 in Supporting Information). As an example, Figure 16 presents the experimental and calculated TGA curves for the multicomponent samples with 0.75 and 3 phr CA showing an excellent degree of coincidence. The average of the relative standard deviations obtained is 0.510 for all the TGA experiments (1679 points).

dT

= −ΔHj = ΔH

dwj dT

=−

ΔHj dwj νH

dt

⎛ −Ea, j ⎛ 1 1 ⎞⎞ wjni exp⎜⎜ ⎜ − ⎟⎟ Tref ⎠⎠ νH ⎝ R ⎝T

k ref, j

DSC = differential scanning calorimetry dwj/dt = mass derivative with respect to the time of process j Ea,j = activation energy of reaction j EVA = polyethylene vinyl acetate copolymer Gj = gas produced in the thermal decomposition j ΔHj = constant latent heat of reaction j kref,j = pre-exponential factor of reaction j at Tref (373 K) kref,j′ = kref,j/νH M = melt state N = number of experimental points N = reaction order OF = objective function P = number of parameters to be fitted PE = polyethylene or polyethylene domains in EVA phr = parts per hundred of resin PP = polypropylene R = ideal gas constant Rj = solid residue produced in the thermal decomposition j RSD = relative standard deviation S = solid state sj = yield coefficients for the solid residue produced in the thermal decomposition j TBPPB = α-α′-bis(tert-butylperoxy)-m/p-diisopropylbenzene T, Ti = temperature (at a given time) t, ti = time (s) TGA = thermogravimetric analysis Tp = peak temperature wm = mass fraction of nontransformed polymer (or nonreacted material) (g/gsample) wS = global solid fraction (g/gsample)

ASSOCIATED CONTENT

S Supporting Information *

Summary of the pseudokinetic equations used in the DSC and TGA modeling of the multicomponent mixtures EVA−PE− CA−ADC−ZnO studied, and the corresponding parameters obtained, are listed in Tables S1−S4. Additionally, Figures S1− S3 present the variation of the pseudokinetic parameters with the CA or ADC contents. Figures S4−S6 show the calculated DSC curves, with the contributions of the different components and reactions, and also the evolution of the CP contributions and the ADC and CA decompositions for different mixtures studied. This material is available free of charge via the Internet at http://pubs.acs.org.

■

NOTATION a, b, c = parameters of the heat capacity second degree polynomial ADC = azodicarbonamide (foaming agent) CA = cross-linking agent (Peroximon) CP = heat capacity of the solid (S) or melted (M) fractions (J/g K) dQj/dT = heat derivative with respect to the temperature of process j, with dQ j

5. CONCLUSIONS The analysis of the thermal properties of un-cross-linked and nonfoamed multicomponent samples showed that the melting point, heat of fusion, and crystallinity of pure EVA are not significantly modified by the presence of PE, peroxide, and azodicarbonamide, before the cross-linking and foaming processes (DSC first run). In the cross-linked and foamed samples, mainly the increase of the cross-linking degree produces a decrease of the melting point, heat of fusion, and crystallinity (second DSC run) and, on the other hand, an increase of the corresponding gel content and the thermal decomposition temperature (TGA). Therefore, cross-linking produces a delay or a stability effect on the main decomposition process of the multicomponent EVA foams studied. As expected, the presence of ADC provokes an important decrease of the density and a reduction of the effects of the cross-linking process. The models proposed satisfactorily fit the different thermal processes that occur in the different unprocessed and crosslinked−foamed materials studied by the DSC and TGA techniques, even when complex and overlapped peaks are present. These models especially include the evolution of the apparent heat capacities, the decrease of the melting point, heat of fusion, and crystallinity in the processed samples, and the increase of the thermal decomposition temperature. These results can be used to control simultaneously the thermochemical cross-linking and foaming processes (including the heat capacity contribution), to optimize the energy requirements and the properties of cross-linked foams, and to simulate their thermal degradation.

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Greek Symbols

■

νH = constant heating rate ϕ i,m = weight percentage of component i in the corresponding sample m γ = vinyl acetate fraction in EVA copolymer

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel.: (34) 965 903 789. Fax: (34) 965 903 826. E-mail:ja. [email protected]. Notes

The authors declare no competing financial interest. 9528



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