So far, resol curing could be designated as the „work horse“ for the dynamic vulcanisation of EPDM/PP-based thermoplastic vulcanisates (TPVs). However, for thermoset vulcanisation of EPDM it is hardly used, because it is considered to be too slow and suffers from marching cure. Dr. Niels van der Aar, Head of Technical Service & Application Development at Keltan, explains: „Although emphasis of our studies is on the activation of various resol curing systems for EPDM rubber, it is also shown that this novel way of activation is applicable for resol cure of other types of rubber, including (X)IIR, CR, (H)NBR, BR, SBR and NR.“
Depending on the particular resol curing system and rubber under investigation, the cure rate is strongly increased. This means a reduction of scorch and vulcanisation times up to 75 per cent. Furthermore, the final degree of crosslinking is clearly enhanced – almost up to the double value. „Resol curing can now be considered as a realistic alternative to sulphur and peroxide curing systems making curing at lower temperatures possible and giving e.g. improved heat aging properties“, says van der Aar. „Finally, it will also be shown that zeolite activation is effective not only for the thermoset vulcanisation of EPDM, but also for the dynamic vulcanisation of EPDM-based TPVs, enabling a reduction of the resol content and, thus, improving the TPV colour characteristics.“

Digging Deeper: Interview with Dr. Niels van der Aar, Head of Technical Service & Application Development at Keltan, Lanxess

KGK Could you explain the readers, why the resol-zeolith system offers a higher curing speed and higher curing efficiency in comparison to established methods? Some figures about the degree of crosslinking?
van der Aar We carried out a study to optimize EPDM formulations for resol curing, which involved making the correct selection of compounding ingredients, i.e. ingredients that don‘t kill or retard the cure. In this objective we were very successful, and recognized an opportunity to replace peroxide in continuously cured extruded profiles, where the product required excellent heat aging performance. It is a feature of peroxide curing that products must be vulcanized in an environment where oxygen is excluded to avoid surface degradation and tackiness, so the use of liquid curing medium (salt bath) is common. This is not a problem experienced with resin curing. The product remains free of surface degradation when cured in the presence of oxygen. However, to reduce the risk of porosity forming within the section of an extruded profile during the curing process it is common practice to add a desiccant to the rubber compound. The most commonly used desiccant is calcium oxide, but we had already established that due to its basic nature it seriously retards a resol cure. We therefore had to find an alternative. It was while testing zeolite 5A as a possible desiccant that we recognized its ability to significantly activate the resol cure.
We have since carried out extensive studies in an attempt to understand why zeolite 5A increases the activity of resol cures, but as yet we cannot give any solid answer. It is certainly clear that it is important for the zeolite to have an extremely low moisture content when it is mixed into the rubber compound, suggesting that the mechanism is perhaps associated with moisture removal from the cure reaction, but our studies have not yet been able to prove this. The mechanism of resol-zeolite activation is therefore still not fully understood.

KGK Lanxess announced a reduction of vulcanisation time up to 75 per cent. Could give us some figures about?
van der Aar The extent of activation depends on the type of resol cure selected. There are different options for the primary activation of a resol cure, including the combination of a halogen donor with a metal oxide, the selection of a halogenated resin with a metal oxide or the use of a metal halide such as stannous chloride. A study carried out based on Keltan EPDM using a non-halogenated resin and SnCl2. 2H2O activation reduced the MDR t90 time at 180?°C from 10.2 minutes to 2 minutes when 10phr of dry zeolite 5A was added. It should also be noted that cross-link density as determined by MH-ML saw a 40?% increase, and the tendency towards a marching modulus, which is commonly seen with resol curing, was significantly reduced. Several patent applications have been made about the use of Zeolite as an activator of resol cure.

KGK Could you please give us an example concerning the economical aspects. What about the costs: chemicals, process engineering?
van der Aar Again this depends upon the type of resol cure system used, although it should be expected that it will be more expensive than typical sulphur or peroxide cure systems. The use of a halogen donor such as polychloroprene would be a cheaper option than stannous chloride, and the use of zeolite activation offers an opportunity to reduce the amount of resin and activator added to the compound, but these possible saving would be countered by the cost of the zeolite. Because it is important that the zeolite is extremely dry at the time of use, special precautions must be taken if using a zeolite 5A powder, such as weighing it immediately before it is used to avoid moisture absorption from the air, and possible even pre-drying the zeolite. Mixing can be carried out using conventional mixing techniques, but care should be taken to achieve a mixing temperature that is high enough to melt the resin, but not so high that the rubber cures in the mixer. This tends to give a processing window of between 95?°C and 120?°C.

KGK The new activation system is applicable for other types of rubber. Do you have any experiences and could tell us some facts and figures?
van der Aar We have conducted tests using zeolite activated resol cures in a wide range of compounds based general purpose polymers, including butyl, halobutyl, natural rubber, SBR, BR, NBR and polychloroprene. In all cases, to a greater or lesser extent, the use of zeolite 5A resulted in faster cures and higher cross-link densities.

KGK How can user benefit and which products are you focusing?
van der Aar Apart from increasing the cure rate and the state of cure, there are several other possible benefits of resol curing, one of which I have already mentioned, i.e. that it offers similar heat aging performance to a peroxide cure system, but it is possible to cure products in hot air without causing surface degradation. It has also been found that resol curing generates very low VOC levels, and can therefore be a good choice for low fogging applications. In the course of our studies we also found that resol curing offers extremely stable anaerobic heat resistance, with much better cross-link stability than peroxide, so this could offer opportunities in certain sealing and geothermal applications where oxygen is excluded.
A further benefit that we have found is the surface appearance of resol cured products. Both sulphur and peroxide systems can cause bloom, with sulphur cures also sometimes showing an effect known as iridescence. Resol cures give products with excellent surface esthetics.

KGK The zeolite activation is also applicable for TPV. It might improve the TPV color characteristic. Could you show that in an example?
van der Aar TPV products are commonly produced using resol cure systems, which unfortunately have a tendency to cause some discoloration of non-black compounds. Because zeolite increases the cure activity, it is possible to reduce the amount of resin used in the TPV, and this in turn reduces the level of discoloration „more white“ and the presence of particles „brown specs“.

KGK Are you and how are you supporting rubber and TPV industry to try the new system?
van der Aar Lanxess has an extensive global network of polymer technologists who provide technical support for all their rubber and TPV customers, and we regularly publish and present technical papers on a range of technical topics, many of which are available for Keltan customers on our online technical service website „Keltan Connect“. Resol curing technology and the use of zeolite 5A as a resol curing activator has been broadly presented around the world. The technical service provided to our customers includes working closely with them to assists in their product and process developments.

KGK What are the main obstacles for the new technology and how could they be overcome?
van der Aar One of the main issues is the necessity to use zeolite 5A in an extremely dry form. Even a few minutes exposure to normal ambient conditions will reduce the activity of the zeolite because it is highly hydrophilic and will draw moisture from the air. We have carried out studies, and continue to work with our sister company Rhein Chemie to develop a zeolite 5A masterbatch where the zeolite is bound within a polymer matrix. This significantly slows down moisture absorption from exposure to air, and therefore makes handling significantly easier. Work is currently underway to develop a commercial zeolite 5A masterbatch product.

Über den Autor

Dr. Etwina Gandert, Redaktion KGK