Thermal degradation of silicone sealant

Transcription

1 Plasticheskie Massy, No. 3, 2011, pp Thermal degradation of silicone sealant E.V. Bystritskaya, 1 O.N. Karpukhin, 1 V.G. Tsverava, 2 V.I. Nepovinnykh, 2 and M.Yu. Rusin 2 1 N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow 2 Tekhnologiya Scientific Production Enterprise, Obninsk Selected from International Polymer Science and Technology, 38, No. 7, 2011, reference PM 11/03/47; transl. serial no Translated by P. Curtis Summary The kinetics of thermal degradation of a polyorganosiloxane sealant was studied by the thermogravimetric analysis (TGA) method at three constant heating rates (2, 5, and 20 deg/min). An investigation was also made of the change in shear strength of adhesive joints in the course of heat ageing at temperatures ranging from 250 to 340 C. The data obtained were processed by the non-linear regression method. The process of thermal degradation proceeds in two stages. The first stage has an activation energy of about 140 kj/mol and is accompanied with increase in the strength of the adhesive layer. The second stage with a higher activation energy (roughly 230 kj/mol) causes a sharp fall in strength of the adhesive layer. A possible mechanism of the thermal degradation process is discussed. INTRODUCTION Silicone sealants of the Viksint family, based on polydimethylsiloxane, are widely used in different sectors of industry, including the construction, aviation, and rocket industries and electronic engineering. One of the main advantages of these materials is their high heat resistance, with working temperatures ranging from -60 to 250 C. Investigation of the kinetics and mechanism of thermal degradation of silicone sealants is of considerable interest for determining the efficiency and service lives of various articles in which they are used under different service conditions. The laws governing the thermal degradation of linear polyorganosiloxanes have been studied by various authors [1 6]. For these polymers, in the absence of oxygen, thermal degradation comprises a single-stage depolymerisation process that takes place with siloxane bond rupture and the formation of low-molecular-weight cyclosiloxanes containing up to nine silicon atoms. The kinetic parameters of the process are strongly influenced by the nature of the terminal groups of the polymer molecules [1]. Thus, for polydimethylsiloxane (PDMS) with terminal hydroxyl groups, Andrianov et al. [1], using data of thermogravimetric measurements in vacuum, obtained an estimate of the activation energy of depolymerisation that amounted to kj/mol. Thomas and Kendrick [2] used the same method and the same conditions to study the thermal degradation of PDMS with trimethylsiloxyl groups at the ends of macromolecules and obtained an activation energy of 179 ± 12 kj/mol. Andrianov et al. [1] also studied the degradation of crosslinked PDMS with different contents of hydroxyl groups, which takes place in two stages. Those authors believe that the first stage, beginning at a temperature of 300 C, consists of depolymerisation by the action of hydroxyl groups. In parallel with this process, a reaction occurs that leads to the formation of crosslinks, again with the participation of hydroxyl groups. Above 430 C, the second stage of degradation, no longer involving hydroxyl groups, is intense. In the present work we used the TGA method to investigate the thermal degradation of sealant Viksit U-2-28 NT, which in the cured state comprises filled, crosslinked PDMS containing free hydroxyl groups. The data obtained are compared with the kinetics of degradation of a cured sealant under isothermal conditions at different temperatures, which was studied earlier [6, 7], and also 2012 Smithers Rapra Technology T/51

2 with data on the change in strength of an adhesive joint in the course of degradation. The mechanism of thermal degradation of the sealant is discussed. EXPERIMENTAL Two-component industrial sealant Viksit U-2-28 NT was cured at room temperature in the form of flat sheets of 3 4 mm thickness. The cured sealant comprises crosslinked, branched, zinc-oxide-filled polydimethylsiloxane. All measurements were conducted no earlier than 7 days after the manufacture of the sheets. From these sheets, specimens weighing mg were made for thermogravimetric analysis (TGA), and also specimens weighing g for investigating the kinetics of weight loss under isothermal conditions. Measurements by the TGA method were conducted on a Du Pont 1090 instrument in a nitrogen flow at a heating rate of 2, 5, and 10 deg/min in the temperature range C. Heating at constant temperatures of 250, 280, 300, and 320 C was carried out in an oven (type Sh-005, 2.5 kw power) without forced ventilation of the working volume. The weight of the specimens was taken on a VLR-200 balance with an accuracy to the fourth decimal place. Specimens for measuring adhesive joint shear strength (t sh ) comprised a composite in the form of rectangular prisms of non-porous (P < 1.0%) lithium aluminosilicate glass ceramic of mm size, bonded by sealant to Invar sheets measuring mm. The roughness of the sheets and ceramic prisms before bonding was determined using a profilometer and on average amounted to 0.85 and 0.45 µm respectively. To improve adhesion, in accordance with the corresponding technology [8], an adhesive sublayer based on organosilicon compounds was applied beforehand to the surfaces of the glass ceramic and metal specimens that were to be bonded. The area of bonding was 300 mm 2. The thickness of the adhesive joint ranged from 0.2 to 0.6 mm, with a mode of 0.3 mm. The shear strength of the adhesive joints of the specimens was determined on an R-0.5 tensile testing machine at room temperature, on the basis of recommendations [9]. The heat ageing of specimens before their failure was carried out at temperatures of C. The ageing time was varied from 20 to 2000 h. The measurement results were processed by the non-linear regression method using the Kinetic Trunk program [10]. RESULTS AND DISCUSSION TGA thermograms of the cured specimens of sealant, taken at three heating rates, are presented in differential Figure 1. Differential TGA thermograms of cured specimens of sealant at heating rates of 2 deg/min (1), 5 deg/min (2), and 10 deg/min (3). Lower graph second derivative of thermogram form in Figure 1. On the differential curves there are two peaks: a relatively small peak in the C temperature region and a more pronounced high-temperature peak. This indicates a two-stage mechanism of the thermal degradation process. In recent years, for quantitative processing of TGA data and determination of the kinetic parameters of degradation, primarily the activation energy, over ten different methods have been used. In the present work we used the three most popular methods: the Kissinger method (KS) [11], the Coats and Redfern method (CR) [12], and the Flynn, Wall, and Ozawa method (FWO) [13, 14]. Furthermore, the TGA thermograms were processed by the non-linear regression (NR) method used previously by us [15 17]. In the KS method, to determine the activation energy, use is made of the dependence of the temperature of the TGA differential curve peak on the heating rate. It is assumed that the degradation process proceeds by a first-order kinetic law, with the rate constant depending on temperature according to the Arrhenius law, i.e.: T/52 International Polymer Science and Technology, Vol. 39, No. 7, 2012

3 dy/dt = k 0 e E RT y where y = m/m 0 is the ratio of the current weight of the specimen to its initial weight, t is time, k 0 and E are the pre-exponential function and the activation energy of the rate constant of degradation respectively, R is the universal gas constant, and T is absolute temperature. In this case, during heating at a constant rate, the following equation holds: (1) ln ν 2 T = ln k R 0 E m E RT m (2) where n is the heating rate and T m is the temperature of the peak on the TGA differential curve. In the corresponding coordinates, this equation gives a straight line, from the slope of which the activation energy is determined. In the CR method, a zero order of the degradation process is assumed, i.e. the rate of the reaction is not dependent on the degree of transformation. This condition is normally met only at small conversions, up to 30 40%. In this case the kinetics of degradation with a constant heating rate can be described by the linear equation: this value is achieved on the heating rate. The results of such calculation (the so-called degradation profile) for the investigated sealant are given in Figure 3. It can be seen that, in the range of degrees of transformation of , the calculated activation energy does not change regularly, and only varies about a mean value, which is evidently connected with experimental spread. All three methods described regard thermal degradation as a single-stage process with a single activation energy. During processing by the method of non-linear regression, we managed to take into account the two-stage nature of degradation and to obtain estimates of the activation energy for each stage. It was assumed that each of the two stages proceeds independently and obeys a first-order kinetic law. As the investigated sealant contains a mineral filler, the weight of the specimen during thermal degradation does not fall to zero, and kinetic equation (1) acquires the form: ln 1 y T 2 = ln k R 0 E νe RT (3) TGA data for the investigated sealant in the coordinates of equation (3) are presented in Figure 2. In the temperature range C, at all three heating rates, linear dependences are actually observed, from the slope of which it is possible to determine the activation energy. In contrast to the two preceding methods, the FWO method does not assume a definite kinetic law for the degradation process. In this method, use is made of a generalised kinetic equation of the form: Figure 2. TGA data for cured specimens of sealant in the coordinates of equation (3) at heating rates of 2 deg/ min (1), 5 deg/min (2), and 10 deg/min (3). Temperature range C dy/dt = k 0 e E RT f(y) (4) where f(y) is a derivative function. As an approximate solution of this equation, the authors proposed the expression: lgf(y) = lg k E 0 lg(ν) E R RT (5) Using equation (5), it is possible for each given value of the degree of transformation y to calculate the activation energy from the dependence of the temperature at which Figure 3. Dependence of the activation energy calculated by the FWO method on the degree of transformation for cured specimens of sealant 2012 Smithers Rapra Technology T/53

4 dy/dt = k 0 e E R(T 0 +νt) ( y y ) where y is the limiting degree of transformation at t = 0, T = T 0, and y = 1. Processing was carried out in two stages. To begin with, the initial section of the thermogram in the range C was processed by model (6). The theoretical degrees of transformation obtained for each value of T were subtracted from the experimental values, and the remainder were again processed by model (6) in the entire temperature range up to 650 C. The results are shown in Figure 4. It can be seen that the experimental data are in satisfactory agreement with the model used. The values of the activation energy of thermal degradation, obtained from TGA data by processing by different methods, are given in Table 1. The temperature range whose data were used during processing is also given in Table 1 for each method. It can be seen that all methods give different estimates of the activation energy. The lowest values are obtained by processing using the CR method, and here the magnitude of the activation energy falls with increase in the heating rate. This most likely means that a zero-order model describes the process taking place inadequately. (6) The highest value for the second high-temperature stage of degradation is given by the method of non-linear regression, and here this value coincides with the estimate obtained by Andrianov et al. [1] for the thermal depolymerisation of linear polydimethylsiloxane. It can be assumed that the lower estimates of the activation energy that were obtained by the other methods can be attributed to the contribution of the first high-temperature stage, which, within the framework of these methods, it is impossible to allow for. The scales of the first stage are small, and the limiting weight loss at this stage does not exceed 2% and is associated most likely with the desorption of moisture and other low-molecularweight impurities retained in the material. Although this stage practically ends at a temperature of 370 C, and processing by the KS, CR, and FWO methods was conducted at considerably higher temperatures (Table 1), its contribution during processing by these methods turns out to be sufficient to lower considerably the calculated value of the activation energy. The kinetic curves of thermal degradation of the cured specimens of sealant at three constant temperatures are given in Figure 5. These data were also processed by Figure 4. Processing of TGA thermograms of cured specimens of sealant at heating rates of 2 deg/min (1), 5 deg/min (2), and 10 deg/min (3) by the method of non-linear regression. Points experimental measurements; continuous lines result of processing Figure 5. Kinetics of weight loss under isothermal conditions in air at temperatures of 250 C (1), 280 C (2), and 300 C (3). Points experimental measurements; continuous lines result of processing Table 1. Values of the activation energy of thermal degradation of cured specimens of sealant, calculated from TGA data by different methods Processing method KS CR FWO NR n = 2 n = 5 n = 10 E 1 E 2 T, C E, kj/mol ± ± 4.1 T/54 International Polymer Science and Technology, Vol. 39, No. 7, 2012

5 the non-linear regression method by a two-stage model of the form: y = 1 a 1 1 e k 1 ( t ) a 1 2 e k 2 ( t ) where a 1 and a 2 are the contributions of the first and second stage respectively, and k 1 and k 2 are their rate constants, each of which depends on the temperature according to the Arrhenius law with its own characteristic activation energy. As can be seen in Figure 5, data of isothermal experiments are described by this method fairly well. The activation energy of the first stage is determined with low accuracy on account of the small scales of this process, E 1 = 39 ± 23 kj/mol. Nevertheless, this result, with account taken of measurement error, does not contradict the estimate of E 1 that was obtained from TGA data. The activation energy of the second stage E 2 = ± 2.8 kj/mol is in good agreement with the value obtained by processing TGA data by the NR method. Figure 6 shows the change in shear strength P of the adhesive joint on Viksit U-2-28 NT sealant during holding in air at four constant temperatures. It can be seen that, at temperatures below 300 C, the strength hardly changes for more than 1000 h, or even increases slightly, although the weight loss under these conditions should be significant. At temperatures of 300 and 320 C, a rapid fall in strength is observed, indicating breakdown of the network. It can be assumed that, in the adhesive layer, processes of crosslinking and degradation occur simultaneously, and here crosslinking predominates at lower temperatures. (7) Data on the change in strength were processed by the non-linear regression method using a kinetic model of two first-order successive reactions [15]. Here, it was assumed that the magnitude of the shear strength s is proportional to the concentration of crosslinks, which are an intermediate product, i.e. they are formed at the first stage and ruptured at the second: dσ dt = bq 1 e q 1 t q 2 σ where b is the limiting strength at the maximum degree of crosslinking, and q 1 and q 2 are the rate constants of the first and second stages respectively; each of the stages has its own activation energy; at t 0 = 0, s = s 0. The results of processing by this model are represented in Figure 6 by solid lines. The obtained estimates of the activation energy for the first and second stages amount to E q1 = 155 ± 66 kj/mol and E q2 = 232 ± 23 kj/mol. Thus, the first, relatively low-temperature process of degradation leads not to a fall but to an increase in the strength of the adhesive joint. It can be assumed that this process, already described by Andrianov et al. [1], proceeds with the participation of the hydroxyl groups contained in the sealant. Here, the terminal hydroxyl groups initiate the depolymerisation of the free ends of the macromolecules, which leads to weight loss but does not affect the active part of the network. At the same time, reactions of hydroxyl groups on active chains lead to the formation of crosslinks and to an increase in strength. The second process comprises network degradation itself, which is accompanied with a fall in shear strength. It proceeds with a much higher activation energy and possibly with the participation of oxygen. (8) CONCLUSIONs Figure 6. Change in shear strength P of the adhesive joint under isothermal conditions in air at temperatures of 250 C (1), 280 C (2), 300 C (3), and 320 C (4). Points experimental measurements; solid lines result of processing Comparison of data on the thermal degradation of silicone sealant, obtained by the dynamic TGA method and under isothermal conditions, indicates that the main process is the depolymerisation of the free ends of the polymer chains with the participation of terminal hydroxyl groups. The activation energy of this process that was calculated by the non-linear regression method coincides well for dynamic and isothermal regimes and amounts to about 140 kj/mol. Simultaneously with depolymerisation, the process of formation of crosslinks with the participation of hydroxyl groups located on the active chains of the network probably occurs. This process is accompanied with increase in the strength of the adhesive layer. Network degradation, leading to a fall in strength, proceeds with a higher activation energy of about 230 kj/mol Smithers Rapra Technology T/55

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