The effect of layered double hydroxides on the vulcanisation and properties of elastomer compounds

Abstract

Elastomers such as natural rubber (NR) and solution styrene butadiene rubber (SSBR) are widely used, especially in the tyre industry. These elastomers are crosslinked, most often via sulphur vulcanisation, in order to achieve the desired mechanical properties. In recent years the use of ZnO as a crosslinking agent in sulphur vulcanisation has been cause for concern due to the fact than Zn can be harmful to aquatic life. As such, alternatives to ZnO and methods to reduce the amount of Zn present in the rubber are desirable. In this work, layered double hydroxides (LDHs) were investigated as alternative crosslinking agents to ZnO in order to either reduce or eliminate Zn in the vulcanisation formulation.

Three different stearate modified LDHs, namely CaAl-St, CaZnAl-St and CaFeAl-St were synthesised via one-step coprecipitation and added to NR, synthetic isoprene rubber (IR) and four different SSBR grades (one unfunctionalised and three functionalised grades) at different concentrations in the place of ZnO and stearic acid. The trimetal LDHs were synthesised by making a 10 mol % substitution of Ca with either Zn or Fe. The LDH synthesis was successful. XRD showed the characteristic LDH peaks and FTIR supported these results. In the case of the stearate modified LDHs, XRD showed that there was a clear shift in the primary peak to a lower 2θ value compared to the unmodified LDH, indicating that the basal spacing was increased and that the stearate anions were successfully intercalated into the LDH. SEM showed the presence of hexagonal platelets characteristic of LDH. Amorphous material was also present.

In the NR and IR study, it was found that these LDHs are not satisfactory crosslinking agents. Rheology data showed that the samples cured with LDH did not reach nearly the same level of crosslinking as the samples cured with ZnO and stearic acid, even when Zn was present in the LDH. The results indicated that a higher concentration of Zn is required in these elastomers for sufficient crosslinking to take place. The mechanical properties also reflected the low crosslink density. The LDH-cured samples had tensile properties similar to that of NR and IR cured with only sulphur and an accelerator.

In SSBR the LDH behaved differently. It was found that even 3 phr CaZnAl-St was sufficient for crosslinking, since rheology data showed that those samples reached similar levels of cure compared to the samples cured with ZnO and stearic acid. This equates to approximately 54 times less Zn in the vulcanisation formulation compared to the standard. Increasing the LDH concentration slightly increased crosslink density. In two of the functionalised grades, it is also possible to completely eliminate Zn, as the samples cured with CaFeAl-St reached similar levels of cure as the reference sample. In one of the functionalised grades CaAl-St also reached a similar level of cure. This indicates that there is potential for further investigation on how Zn-free LDH could be used to crosslink some SSBR grades. The mechanical property data showed that the LDH cured samples had tensile behaviour similar to the reference samples, indicating that the presence of LDH does not negatively affect the mechanical properties, nor does it affect properties such as glass transition temperature significantly.

This work therefore highlights the potential of using CaAl-based LDHs as alternatives to ZnO in SSBR, thereby reducing the amount of Zn required for sulphur vulcanisation. Further work is required to study the effect of the LDHs in SSBR filled with reinforcing fillers such as silica, which could enable the use of these LDHs in a commercial tyre formulation in future.