Radical bulk polymerization of styrene in the presence of rubber particles from recycled tires: a kinetic study using DSC

Abstract

The kinetics of the radical bulk polymerization of styrene in the presence of rubber particles from recycled tires, commercially called ground tire rubber (GTR), have been studied using isothermal differential scanning calorimetry. Two different radical initiators, namely benzoyl peroxide and 2,2-azobis(2-methylbutyronitrile), were tested in a series of variable-composition experiments under three different polymerization temperatures, in order to selectively address the role of the presence of GTR on the course of the polymerization reaction. The variation of the overall effective kinetic rate constant and activation energy were also determined to quantify the effect of the presence of GTR in the reaction mixture. The results of the study demonstrate that the evolution of the monomer conversion versus time is significantly affected when the GTR content in the polymerization system is above 30 mass/%. The observed phenomena are consistent with previous results and may be primarily attributed to the chemical interactions taking place between the initiator and additives contained in the mixture.

Appendix

A simplified kinetic scheme of radical polymerization typically consists of a sequence of three steps, involving radical initiation (Eq. 6), propagation (Eq. 7) and termination (Eq. 9) reactions. To this classical kinetic scheme, one can include a generalized inhibition reaction (Eq. 8), when supporting indications or evidence exist. In the present work, Z designates, in a generalized manner, all those elements of GTR that may act as inhibitor or retarder in the course of the polymerization. Hence, the following kinetic scheme can be formulated:

$$\begin{aligned}&I \xrightarrow {k_{\mathrm{d}}; f} 2M\cdot \end{aligned}$$

(6)

$$\begin{aligned}&M\cdot +\, M \xrightarrow {k_{\mathrm{p}}} M\cdot \end{aligned}$$

(7)

$$\begin{aligned}&M\cdot + \,Z \xrightarrow {k_{\mathrm{z}}} MZ\cdot \end{aligned}$$

(8)

$$\begin{aligned}&2M\cdot \xrightarrow {k_{\mathrm{t}}} P \textit{ (or 2P)} \end{aligned}$$

(9)

where I, $MZ\cdot$ and P denote the initiator, inhibited radical and terminated polymer radical, respectively. f is the initiator efficiency while $k_{\mathrm{d}}$, $k_{\mathrm{p}}$, $k_{\mathrm{z}}$ and $k_{\mathrm{t}}$ are the respective kinetic rate constants of the initiator decomposition, radical propagation, deactivation (i.e., by inhibitor) and termination (e.g., by combination or disproportionation) reactions. On the basis of the postulated kinetic scheme (Eqs. 6–9), and under the assumptions of constant radical concentration and the LCH, Eq. (10) can be derived via direct integration of Eq. (2), to express the variation of the monomer concentration (or equivalently monomer conversion) with respect to reaction time:

$$\begin{aligned} -{\rm ln}(1-x)=k_{\mathrm{p}}[M\cdot ]t \end{aligned}$$

(10)

To express the concentration of radicals, $M\cdot$, the QSSA can be applied to yield the following equation:

$$\begin{aligned} \begin{aligned} 2fk_{\mathrm{d}}[I]-2k_{\mathrm{t}}[M\cdot ]^{2}-k_{\mathrm{z}}[M\cdot ][Z]=0 \end{aligned} \end{aligned}$$

(11)

Note that the reactivity of the radical $MZ\cdot$ is assumed here to be too low to further promote polymerization. Also, the decomposition rate constant, $k_{\mathrm{d}}$, corresponds to the overall decomposition rate of the initiator, including thermal homolysis and eventual redox decomposition. From the above expression it can be seen that, when all terms are important, the final expression of $M\cdot$ will be a root of the second-order algebraic equation whose generalized form cannot be simplified. In contrast, significant simplification can be achieved if one considers two extreme cases for the radical termination mechanism; the first one corresponds to the case where radical termination can be considered to occur primarily by bi-radical termination (Eq. 9), considering inhibition reactions (Eq. 8) as negligible. In this case, the expression of $M\cdot$ is reduced to the commonly employed :

$$\begin{aligned}{}[M\cdot ] = \bigg (f \frac{k_{\mathrm{d}}}{k_{\mathrm{t}}}[I]\bigg )^{1/2} \end{aligned}$$

(12)

The second case corresponds to the conditions where the radical inhibition reactions (Eq. 8) are considered as the dominant mechanism of radical termination, thus resulting in the following expression for $[M\cdot ]$:

$$\begin{aligned} {[}M\cdot ] = 2f\frac{k_{\mathrm{d}}}{k_{\mathrm{z}}}\frac{[I]}{[Z]} \end{aligned}.$$

(13)