Chemical Modification of Rubber for Enhanced Self–healing and Recycling

: (Alternative Format Thesis)

Abstract

Over the last 200 years, rubber has become a commonplace object in our everyday lives and there are few applications that do not need a rubber component. Materials research has focused on enhancing the properties of rubbers in order to extend product lifetime. This has resulted in an enduring product that only breaks down under large stresses or excessive wear. However, these rubbers are then difficult to break down at the end of their product life and therefore generate huge amounts of waste (286,000 tonnes, 2018, UK) that persist in the environment for a long time after the original application. This approach is contrary to the approach taken by natural materials. Natural materials compromise on endurance by failing at smaller material stresses and then heal the damage to create enduring materials. This is much more effective at minimising waste. In today’s society there is increasing emphasis being placed on smart materials and sustainable processes in order to reduce the amount of waste. As better understanding of how to mimic natural self-healing progresses, rubbers can be developed with both durable and self-healing characteristics. Additionally, chemical degradation is a growing area of interest in order to treat the rubber waste already produced to form new products and move towards a more circular economy. Chapter 1 discusses the relevant literature regarding self-healing in elastomers and chemical degradation of rubber and its applications.

In Chapter 2, the chemical modification of epoxidised natural rubber via acid-based cross-linkers and alcohol-based cross-linkers is examined. A number of different cross-linkers are synthesised and conditions are optimised with the help of limonene oxide, a molecular model for epoxidised natural rubber. Ionic self-healing in epoxidised natural rubber is demonstrated with an acid-based imidazolium species and recovery of the tensile strength up to 59 % is achieved.

In Chapter 3, the relationship between cross-linking density and self-healing is explored with a range of sulphur and dicumyl peroxide cross-linked epoxidised natural rubber and natural rubber compounds. By plotting self-healing against cross-link density, compounds with different cross-linkers can be compared quantitatively, leading to the estimation of the different contributions to the self-healing. Additionally, different rubbers can be contrasted using this method and the self-healing performance can be used to find the compromise between optimal self-healing and mechanical properties.

In Chapter 4, a novel chemical degradation method is developed that can significantly degrade liquid epoxidised natural rubber in 10 minutes and virgin epoxidised natural rubber in 24 hours. Kinetic analysis is performed but needs further repeats and a rate equation is proposed from the initial data. The degradation method is extended to cross-linked and carbon black filled rubbers and initial results for recovering carbon black from the degraded rubber are shown.

In Chapter 5, chemical modification of butyl rubber and bromobutyl rubber is tested using molecular models in Diels-Alder reactions. Thiol-ene click chemistry is demonstrated to be successful for modifying butyl rubber with furan-based thiols. Several cross-linkers and a range of conditions were examined in order to modify bromobutyl rubber with maleimide, but further research is still required.

Date of Award	12 Oct 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Matthew Davidson (Supervisor), Antoine Buchard (Supervisor), Chris Bowen (Supervisor) & Chris Norris (Supervisor)