THERMAL DEGRADATION of Pvac

Journal of the University of J.Chemical Blazevska-Gilev, Technology D. Spaseskaand Metallurgy, 40, 4, 2005, 287-290

THERMAL DEGRADATION OF PVAc

J. Blazevska-Gilev, D. Spaseska

Faculty of Technology and Metallurgy, Received 20 October 2005 Ss Cyril and Methodius University, Accepted 15 November 2005 P.O. Box 580, MK-1001 Skopje, Republic of Macedonia, E-mail: [email protected]

ABSTRACT

The kinetics of decomposition of plastics are interesting from different points of view, i.e. evolution of harmful substances during fires or waste incineration, recovering of chemical raw materials from plastic refuses and designing of recycling procedures. Non isothermal or dynamic thermogravimetry (TGA) has been used for kinetic study of the thermally activated process of polyvinyl acetate (PVAc), heating by two different rates up to around 723 K. As the easily measured weight changes of the samples in the defined thermal conditions are a suitable sensor for their structural and chemical changes, by means of some methods like Gropjanov-Abbakumov,s one, the useful information for identifying the formal kinetic parameters of the investigated processes taking place in the course of thermal treatment have been obtained. The thermal variation of the rate constant as well as the kinetic equations for the examined process depending on the investigated parameters have been derived. Keywords: polyvinyl acetate, thermal degradation, degradation kinetics.

INTRODUCTION mally degrades under vacuum at an appreciable rate at temperatures above 500 K. The mechanisms of ther- Polyvinyl acetate (PVAc) is known to be one of mal degradation of PVAc is very complex process in- the most important commercial polymers as a result cluding, among others, the following reactions: chain of its low cost and high performance of the products, fission, radical recombination, carbon hydrogen single combined with the wide range of properties. In spite bonds fission, hydrogen abstraction, mild-chain and of its enormous technical and economic importance, end-chain scission, radical addition, hydrogen transfer its degradation at high temperature is still intensively and disproportionate, conjugated double bonds forma- studied problem by many scientists [1-3]. Today, one tion, cyclisation, aromatization, fusion of aromatic ring, more reason for studying the thermal degradation of char formation and graphitization [4]. Nevertheless, PVAc is the indispensability of polymer reprocessing some scientists have reviewed the kinetic results of and recycling using enhanced temperatures. PVAc nor- PVAc thermal degradation, which have been usually

287 Journal of the University of Chemical Technology and Metallurgy, 40, 4, 2005

fitted in the literature to a potential kinetic model of K). The gas chromatograph analysis at the tree main tem- the order. peratures gave the results presented in Table 1. The main gas products released from PVAc at dá/dô = k f(á) = k f(á) exp(-E/RT) 0 the three characteristic temperatures are acetic acid fol- lowed by acetone, benzene and acetaldehyde, appeared McNeill [4] has revealed that PVAc degradation in vacuum is essentially a two-stage process, correspond- ing to the producing mainly acetic acid and volatile Table 1. The released gas products of the thermally products evolved during breakdown of the unsaturated treated PVAc. polymer backbone. In this paper it was derived the analytical shape Gas 620 K 673 K 723 K of the kinetical curves for the two temperature stages of mol % the investigated process by means of Gropjanov- 0,25methane Abbakumov,s method. ethene 0,55 1,82 0,48 eth ane 0,98 0,69 ethane propane 1,08 0,84 CH3CHO 1,14 EXPERIMENTAL butadiene 2,15 1,12 0,50 acetone 8,95 8,35 2,64 c-C5H6 1,53 1,20 Thermal degradation of poly(vinyl acetate) has acet.acid 72,99 81,50 86,96 benzene 10,25 4,75 5,65 been investigated during heating up to around 723 K at toluene 2,20 0,19 0,45 the rate: dT/dô = 4 and 10o/min using Cahn D-200 styrene 1,39 0,22 0,20 recording microbalances in a stream of inert gas at the flow rate 100 ml Ar/min. For the purpose of the identi- 100 fying the outgoing gases the isothermal analyses was used, - 4O C/min subjecting PVAc samples 10 min at the characteristic 80 - 10O C/min temperatures. The analysis has been made by automatic 60 sampling gas chromatograph Hewlett-Packard GC 5890 a with FID and TCD detectors and the Porapak P packed 40 column (i.d 2 mm, length 2 m). Simultaneously, the (100- )/20 % outgoing gas was analyzed continually on mass spec- trometer VG GAS Analysis LTD. 0 0 200 400 600

O RESULTS AND DISCUSSION T/ C Fig. 1. TG curves of PVAc in inert atmosphere. A standard PVAc was thermally treated in inert atmosphere, up to 723 K, using a heating rate of 4 100 and 10o/min. The thermal degradation occurs in two steps to nearly 100 % weight loss. In Fig. 1 the dynamic 80 TG curves of PVAc in Ar atmosphere (100á) is pre- O sented, but in Fig. 2, the dependences of the degraded 60 - 4 C/min

/% sample (á) on the temperature (T) is shown. O

a 10 C/min 40 - The first degradation step lasting up to 673 K, leads to a weight loss of about 72%, but the second one, occur- 20 ring up to 723 K, results in nearly 100 % weight loss. The liberated gas products as a result of the ther- 0 mal degradation have been determined by an isothermal 0 200 400 600 O treatment of the investigated PVAc, 10 minutes at about T/ C the characteristic temperatures (620 K, 673 K and 723 Fig. 2. Degradation polytherms of PVAc.

288 J. Blazevska-Gilev, D. Spaseska

as a result of decomposition and the recombination of the gas products in two steps of the process. a1 The description of the degradation process is dif- ficult, because of the complexity of the chemical reactions carried out at the same time. In this work, the characterization of the rate of weight loss, is made by overall kinetic parameters determined on the basis of the weight loss curves The determination of PVAc degradation kinetics is commonly carried out by mathematical treatment of the TG curve, or series of curves obtained under different temperature-ramped conditions. a2 In order to obtain the formal kinetic parameters which explained two steps of the underlying process comprising TG curve, we have adopted Gropjanov- Abbakumovs method which could analyze groups of reactions in the separated steps, by using two different heating rates. Using the mentioned method, the data of Fig. 1 and 2 as well as the equation b

dá/dô = K f(á) = K f(á) exp(-E/RT) 0 where: dá/dô - percentage of the degraded sample in unite time, K - entropy factor independent on temperature, 0 f(á) - function of the decomposition degree, depending on the control mechanisms of the reaction, E - energy of activation, R - gas constant, T - temperature, the energy of activation can be determined [5]:

á/dô/exp(-E/RT) α α / τ α / τ Fig. 3. Dependence of ( d ) on f( ) for the first E = R[ln( d d )2 - ln(d d )]1 (a - 1, 2) and the second (b) region 1 – f(α) = (1-α)4 ; 2–f(α) = 1/α. (1/T1) - (1/T2) In our case the value of the activation energy for ordinate are calculated by the two relationships, con- o the temperature region up to around 673 K is 133,520 cerning the two different heating rates, 4 and 10 C/min: -18 kJ/mol, but for the region starting from 673 up to 723 1,367/exp( - 240830/RT) ·10 and 3,357/exp(- 240830/ K, the value is 240,830 kJ/mol. The values dá/dô have RT) ·10-18, respectively. been determined by using the relationship: dá/dô=( dá/ For the first region, by the heating rate of 4o/ min, there are two values for the magnitude K : 2´1012 dT) x( dT/dô) 0 The value Ko is determined as tg (a) from the and 5´1013 as a result of the validity of the two model liner plots of (dá/dô) / exp(-E/RT) versus f(á)which relationships for f(á): 1/á and (1á)4. For the heating rate of 10o/min, the two values of K are: 5´1012 and passes through the origin of coordinates.(Fig. 3. a, b). 0 Concerning the first temperature region, the val- 1,33´1014 depending on the model relationships valid- ues on the ordinate is calculated by the relationship: ity: 1/á and (1 á)4. In the temperature region starting -13 o from 673 up to 723 K, the values of K depending on 9,375/exp(-133520/RT)·10 for the heating rate of 4 / 0 min, but for the heating rate of 10o/min, the valid rela- the heating rates of 4 and 10o/min are: 2,15´1022 and tionship is as following: 27,06/exp( - 133520/RT) ·10- 1,66´1022. The analytical shape of the kinetic curve 13. For the second temperature region, the values on the f(á) is: (1 á)4.

289 Journal of the University of Chemical Technology and Metallurgy, 40, 4, 2005

Table 2. The values of Ko and the equations of Kf(á) By means of the thermogravimetry and for two thermal regions. Gropjanov-Abbakumovs non-isothermal method for the investigation of the formal kinetic of PVAc Temperature interval Ko Kf(á) thermal degradation, the classical kinetics parameters like energy of activation and up to 673 K õ = 4o/min 1/á 2.1012 2.1012 .exp(-133520/RT)·1/á preexponential factor for the characteristic tem- õ = 4o/min (1–á)4 5.1013 5.1013 .exp(-133520/RT)·(1–á)4 perature regions have been determined. The analytical model of the kinetic õ = 10o/min 1/á 5.1012 5.1012 .exp(-133520/RT)·1/á curves for the two characteristic regions have õ = 10o/min (1–á)4 1,33.1014 1,33.1014 .exp(-133520/RT)·(1– á)4 been given. The adequacy of distinct equations in up to 723 K õ = 4o/min (1–á)4 2,15.1022 2,15.1022 .exp(-240830/RT) ·(1–á)4 different temperature regions has been deter- õ = 10o/min (1–á)4 1,66.1022 1,66.1022 .exp(-240830/RT) ·(1–á)4 mined.

REFERENCES Depending on the mechanism of the degradation process the analytical shape of the kinetic curve f(á) for 1. G. Camino, Flame retardants: intumescent systems, the temperature interval up to around 673 K is pre- in Plastics additives, Pritchard G, ed. Chapman & sented with the model equations f(á)=(1á)4 and 1/á, Hall, London, 1998, p. 297-307. but for the temperature region from 673 up to around 2. M. Le Bras, G. Camino, S.Bourbigot and R.Delobel, 723 K, is given by the model equation: f(á) =(1á)4 Fire retardancy of polymers. The use of intumescence. (Table 2). The Royal Society of Chemistry, Cambridge, 1998. The values of the pre-exponential factor K , depen- 3. L.Costa, M.Avatanco, P.Bracco, V.Brunella, Char for- 0 dent on the slope of the degradation curves, are expressed mation in polyvinyl polymers I. Polyvinyl acetate, different for the two examined steps of degradation. Polym. Deg. Stab., 77, 2002, 503-510. 4. I.C.McNeill & R.C.McGuiness, The Effect of Zinc CONCLUSIONS Bromide on the Thermal Degradation of Poly (vinyl Acetate), The Polym. Deg. Stab. 5, 1983, 303- The thermal degradation of PVAc was analyzed 316. in this paper on the basis of a phenomenological model 5. V.M. Gropjanov, V.G. Abbakumov, Neizotermiceski that describes the kinetics of the two separate tempera- metod issledovanija kinetiki termiceski aktiviruemih ture stages of the process. processov, Him. i him.tehnol. 18, 2, 1975, 202.

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