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PhD Defence Gina Butuc | Novel Cyclic Polysulfide Based Blends. Elucidation of the Role of Zinc Oxide in Sulfur Crosslinking

Novel Cyclic Polysulfide Based Blends. Elucidation of the Role of Zinc Oxide in Sulfur Crosslinking

Due to the COVID-19 crisis the PhD defence of Gina Butuc will take place (partly) online.

The PhD defence can be followed by a live stream.

Gina Butuc is a PhD student in the research group Elastomer Technology and Engineering (ETE). Her supervisor is prof.dr. A. Blume from the Faculty of Engineering Technology (ET).

Rubber as a material of choice continues to expand and grow in areas such as transportation, household, medical and industrial applications. With a continuous increase of the number of applications and usage, comes the quest for new and improved rubber compounds. In general, new materials are used for new applications. But how about expanding existing materials’ use beyond the original intent? With a creative way of combining existing materials, the resulting blend can exhibit properties that are a novelty.

In this research project new rubber compound blends are proposed that have a common feature, they all contain cyclic tetrasulfide (CTS). CTS is a new development from the class of polysulfides, and its proposed usage is as a sulfur donor. In the rubber blends studied in this project, CTS is also a part of the gum stock in combination with another polymer. The binary blends are comprising of either a non-polar polymer or a polar polymer and CTS. For the non-polar polymer and CTS blends, ethylene propylene diene terpolymer (EPDM) or ethylene propylene copolymer (EPM) were used and for the polar polymer and CTS blends, nitrile rubber (NBR) or hydrogenated nitrile rubber (HNBR) were used. Showcasing the advantage of the blend concept in an EPDM and CTS blend indicates a superior performance in applications where EP(D)M does not fulfil the demand;  the resulting rubber compound exhibits a lower swell in hydrocarbon oil by 38%, when compared against an EPDM.

Dimensional stability, good mechanical properties and chemical resistance of rubber compounds are achieved by vulcanization or crosslinking. Most of the rubber compounds contain one crosslinking system, i.e. either a sulfur or an organic peroxide. It is therefore expected  that a selection of the rubber blends proposed in this study are crosslinked by sulfur, which is generated in situ from CTS. A dual crosslinking system, meaning both sulfur and organic peroxides is a highly unlikely occurrence in rubber compounding as it requires care to be exerted during compounding and crosslinking, as sulfur can deactivate the organic peroxide.

The ultimate goal of the study was the elucidation of the mechanism(s) of crosslinking for new binary polymer blends. Detailed proposed mechanisms of crosslinking provide an in-depth understanding of the chemical process taking place during rubber crosslinking i.e. vulcanization. The studies of the polymer blends in this work, represent the foundation of a new platform of CTS-based polymer blends. With a thorough understanding of the mechanisms of crosslinking, a customized approach can be taken in designing unique polymer blends based on CTS. In-depth analyses were conducted to explain the chemical process the CTS is undergoing during the crosslinking process.

The research project is structured in two paths, which are as follows:

  1. The first path is dedicated to rubber compounds development and data analysis. Theoretical calculations are provided in thermodynamic calculations and solubility parameter determination. Experimental data is collected by mixing and rheometer studies, stress-strain information at 25 ˚C and temperature sweep from -40 ˚C to 120˚C, and Payne effect studies. Polymer phase microstructure analysis by various microscopic analyses was performed to understand the phase distribution. (Chapter 3–6)
  2. Chemical model studies involving small molecules and molecular modelling studies represent the focus of the second path of the study. The purpose of this study is to gather information which is further used to understand the chemical process taking place in the rubber blends and to propose the mechanisms of crosslinking. NMR and various spectrometric techniques are used to characterize products of reaction between an olefin and various sulfur donors. Extensive microscopy, spectroscopy and XRD studies helped elucidate the role the ZnO crystal plays in sulfur crosslinking. (Chapter 7-8)

Both paths merge in Chapter 9, as a culmination of all information accumulated in the previous studies. In this chapter, mechanisms of crosslinking were proposed for the four polymer blends studied. A selection of the rubber compounds developed in this research project contain this dual cure system. The organic peroxide is added to the system, while the sulfur evolves from CTS.  Proposed mechanisms of crosslinking are detailed for both single and dual cure systems.

The ring opening reaction of CTS facilitated by the ZnO and stearic acid  opens the possibility of additional chemical interactions to take place. At the surface of ZnO after reacting with stearic acid a “template” or cavity is formed, where the sulfur crosslinking reaction takes place. The fragments resulting from the ring opening reaction of the CTS are facilitating the crosslinking reaction of the secondary polymer present in the rubber blends by forming a variety of sulfur bridges and long chain crosslinks. In addition, the CTS fragments are recombining to form an in-situ polymer, a linear polysulfide, within the polysulfide phase. By carefully crafting a rubber blend compound taking into account a decoupling of each individual cure type, successful rubber compounds can be created that are incorporating a dual cure system.

The versatility proven by CTS sparked additional research ideas which were proposed in Chapter 10.  Further investigation on creating new rubber compounds with unique properties, with an emphasis on rubber compounds which require high levels of sulfur are proposed in this chapter. In addition, further chemical studies are proposed for additional mechanistic evaluation of distinctive effects observed in some of the proposed rubber blends.

The studies of the polymer blends presented in this project represent the foundation of a new platform for CTS-based polymer blends.