In-Situ Reactions in Mixing Process of TESPT-Silanized Silica/NR tire tread
Compounds
Wisut Kaewsakul
1*, Kannika Sahakaro
2and Jacques W.M. Noordermeer
3 1*Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Hat Yai Campus, Songkhla 90110 Thailand
2
Department of Rubber Technology and Polymer Science, Prince of Songkla University, Pattani 94000 Thailand 3
Department of Elastomer Technology and Engineering, University of Twente, 7500 AE Enschede, the Netherlands Phone +66-74-288-383, Fax +66-74-288-395, e-mail: wisut.ka@psu.ac.th
Abstract
The change of dump temperature plays an important role on filler-filler and filler-rubber interactions of silica-filled natural rubber (NR) compounds with the use of bis-triethoxysilylpropyltetrasulfide (TESPT) as a coupling agent. By increasing dump temperature, Payne effect (i.e. filler-filler interaction) decreases, while bound rubber content (i.e. filler-rubber interaction) and Mooney viscosity substantially increase. The optimum properties are achieved when the
dump temperature reaches approximately 135oC. The results suggest that two significant reactions simultaneously
take place during mixing; 1) the silanization reaction between silica and silane, and 2) the coupling reaction of sulfur contained in TESPT towards rubber molecules. In addition, the inevitably rubber-rubber crosslink can occur due to the sulfur donation of TESPT. The cure characteristic of unfilled NR compound (without elemental sulfur) combined with TESPT and diphenylguanidine (DPG) as an accelerator confirms that free sulfur released from TESPT readily reacts
with NR at the temperature as low as 120oC.
1. Introduction
Mixing of rubber and silica with a silane coupling agent involves the reaction between silanol groups on the silica and ethoxy or hydroxyl groups of the silane, i.e. the silanization reaction. However, the efficient use of the silane as coupling agents in the silica-filled rubber compounds is rather limited, caused by side-reactions. Various mixing-processing conditions need to be optimized for silanized silica-filled compounds [1]. The temperature window for mixing silica compounds is rather limited by a too low silanization rate versus the risk of scorch. High temperatures improve the silanization rate due to the temperature dependence of the reaction and reduction of sterical hindrance of the silylpropyl groups of the coupling agent by increased thermal mobility [2]. Based on a study with
SBR/BR compounds [3], the dump temperature should be in the range of 145oC to 155oC to achieve good silanization
and to avoid pre-crosslinking. Furthermore, a mixing time of at least 10 minutes is needed for the silanization reaction during the first mixing step. In the present work, the influence of mixing temperature on technical properties of silica-reinforced NR compounds was investigated. Some insight understanding in respect to the reactions/interactions, simultaneously occurred during mixing process, and their effects on compound properties has been discussed.
2. Experimental
2.1 Compound preparation
The mixing was performed using an internal mixer with a mixing chamber of 500 cm3. The mixer was operated at a
fill factor of 70 % and a rotor speed of 60 rpm. Initial temperature settings of the mixer were adjusted in the range of
50-140oC. NR was initially masticated for 2 mins. Then, half of the silica and silane were added and mixed for half
the duration of the silica-silane-rubber mixing interval, i.e.: 5 mins, prior to adding the second half of silica and silane together with TDAE oil. The mixing was then continued to the full intervals, i.e.: 10 mins. Subsequently, the other ingredients: ZnO, Stearic acid, TMQ and DPG, except CBS and sulfur, were added and mixed for 3 mins. The
compounds were then dumped, sheeted out on a two-roll mill, and kept overnight prior to incorporation of CBS and sulfur on a two-roll mill.
2.2 Determinations of properties
Mooney viscosity of the compounds was performed according to ASTM D1646. Payne effect and chemically bound rubber content were measured using the same procedures as those described elsewhere [4]. The cure characteristics
were determined using a Moving Die Processability Tester at a temperature of 150oC, a frequency of 0.833 Hz and
2.79 % strain.
3. Results and discussion
0 10 20 30 40 50 60 70 80 90 90 110 130 150 170 190 ML (1+ 4) , 10 0 oC [ MU ] Dump temperature [oC] (a) 0 10 20 30 40 50 60 0.0 0.5 1.0 1.5 2.0 2.5 3.0 90 110 130 150 170 190 Ch em ic all y B d R [wt %] G'0 .5 -G'1 0 0 .0 [MP a] Dump temperature [oC] (b) ∆G' BdR 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.00 0.05 0.10 0.15 0.20 0.25 100 120 140 160 180 200 MH -M L of SBR+TE SP T (dN.m) MH -M L of NR+TE SP T (dN.m) Cure temperature [oC] NR SBR
4. Results and discussion
In Figure 1a, the increase of the silica-filled NR compound viscosities for dump temperatures in the range of 100-145oC
is to a large extent due to the silica-silane-NR coupling reaction that occurs during the mixing process, and this reaction
reaches its maximum at a dump temperature around 135-150oC. This premature crosslinking reaction dominates over
the breakdown of rubber chains and silica structures. The observation can be confirmed with the results of Payne effect and chemically bound rubber content as shown in Figure 1b. In addition, the free sulfur liberated from TESPT plays a significant role towards increased compound viscosity as the results shown in Figure 2.
5. Conclusions
With increasing dump temperature, Mooney viscosities of the silica-filled NR compounds with the use of TESPT as a
silane coupling agent significantly increase till the dump temperature of 135 – 150oC, and then gradually drop off.
This is due to the effects of filler-filler and filler-rubber interactions in the systems as proved by the results of Payne effect and bound rubber content. In addition to the silanization reaction, premature crosslinking reactions take place
since NR can start to react with sulfur coming from TESPT molecules at a temperature as low as 120oC.
References
[1] Dierkes, W.K., Noordermeer, J.W.M., Kelting, K., Limper, A., Rubber World. 229, 6 (2004).
[2] Mark, J.E.; Erman, B.; Eirich, F.R., The Science and Technology of Rubber, Elsevier Academic, US 2005.
[3] Reuvekamp, L.A.E.M., Ten Brinke, J.W., van Swaaij, P.J., Noordermeer, J.W.M., Kautsch. Gummi Kunstst. 55, 41 (2002).
[4] Kaewsakul, W., Sahakaro, K., Dierkes, W.K., Noordermeer, J.W.M., Rubber Chem. Technol., 85, 277 (2012).
Fig.1 Effect of dump temperature on: (a) Mooney viscosity; and (b)
Payne effect and chemically bound rubber (BdR) content, of TESPT- silanized silica/NR compounds.
Fig.2 Torque differences as a function
of cure temperature of gum NR- and SBR-based compounds in the presence of TESPT and DPG.