Featured Rubber Research – October 2023
Introduction
Every month, Prescott Instruments features several recent scientific papers covering the world of rubber – from cultivation, production, processing, and testing to real-world applications, sustainability and novel concepts.
This October, the featured papers include:
Experimental Study and Modelling of Rubber Joints for Railway Vehicles using Magnetorheological Shear Stiffening Elastomers: The use of elastomers with magnetic field-dependent stiffness properties for use in high-speed railway joints.
Enhancing Eco-Friendly Natural Rubber Composites through the Incorporation of Spent Coffee Grounds: The novel use of spent coffee grounds as a reinforcement filler in natural rubber blends.
Viscoelastic Materials are Most Energy Efficient when Loaded and Unloaded at Equal Rates: A comparative analysis of the energy profiles during loading and unloading of an animal tendon and a synthetic elastomer.
Ozone Impact on the Rubber Contained in the Tip Cap of Medical Prefillable Syringes: A thesis exploring the effect of environmental conditions on the integrity and sterility of rubber syringe caps.
Read the full features below, complete with citations and links to read the original research online.
Featured Research Papers
Experimental Study and Modelling of Rubber Joints for Railway Vehicles using Magnetorheological Shear Stiffening Elastomers
While high-speed trains do improve transportation efficiency by increasing the flow of passenger capacity, they are also limited by their safety and stability at high speeds. This is because straight and curved sections of tracks have opposing stiffness requirements – on straight tracks, a hard primary longitudinal stiffness is essential, whereas, on a curved track, this high stiffness impairs the ability to safely traverse bends.
Therefore, it is necessary for railway joints to have a variable stiffness profile that reflects the demands of the high-speed track. Active and passive systems can be employed to regulate the stiffness but must also have fail-safe systems in cases of power losses or signal failures.
Magnetorheological shear stiffening elastomers (MSSE) are a group of materials whose stiffness can be controlled by both an external magnetic field and by speed of travel. As a semi-active system, the storage modulus can be increased on demand, whilst also maintaining a higher stiffness at higher frequencies by default. This means that even in the result of power failure, the inherent lower stiffness at lower frequencies will ensure fail-safe operation around the curved section of the tracks.
To achieve this, a shear stiffening gel was mixed with a vinyl methyl silicone rubber alongside carbonyl iron particles, a key component that gives rubber its magnetic properties. Using a rheometer, the shear stiffening properties of MSSE were by measuring the storage modulus as a function of frequency. The magnetic properties of the joint were also investigated, by altering the magnetic field intensity and measuring the subsequent effect on the storage modulus. Lastly, a Dynamic Mechanical Analyser was used to measure the effective stiffness of the joint under load with and without an applied current to stimulate the magnetic field.
Together, these experimental methods demonstrated that the novel MSSE joint demonstrated sufficient stiffness to navigate both straight and curved sections of tracks at high speeds, even in a result of power failure.
Citation: Gong, L., Gong, N., Wang, B., Yang, J., Du, H., Sun, S., … & Li, W. (2023). Experimental study and modeling of rubber joints for railway vehicles using magnetorheological shear stiffening elastomers. Smart Materials and Structures, 32(9), 095032.
Enhancing Eco-Friendly Natural Rubber Composites through the Incorporation of Spent Coffee Grounds
Coffee is one of the most widely consumed beverages in the world. With over 2 billion cups of coffee drunk every day, nearly 15 million tonnes of spent coffee grounds are generated annually. As a waste product, spent coffee grounds can be used as compost or even as a biofuel. An alternative use is as a reinforcement filler in a natural rubber composite.
Natural rubber composites have garnered a positive response due to their renewability, biodegradability and their overall lower environmental impact than traditional synthetic rubber composites. The addition of natural fillers, such as spent coffee grounds, are also popular because they are relatively inexpensive as well as being environmentally friendly.
Spent coffee grounds are particularly well-suited to being a reinforcement filler because they are renewable, lightweight, biodegradable, and low risk. They are also rich in natural compounds that can interact with the natural rubber matrix, potentially improving mechanical properties. The porosity and surface structure of the grounds also contribute to increasing interfacial bonding with the natural rubber.
In this study researchers mixed coffee grounds into a natural rubber composite, ranging from 0 – 25 phr. The cure characteristics, mechanical properties such as tensile strength and hardness, and odour absorption were all studied. Overall, the researchers found that the coffee grounds were evenly dispersed throughout the mix. While filler enhanced both the stiffness and hardness, the coffee grounds also reduced tensile and tear strength. The researchers also highlighted that the coffee grounds could undergo surface modification to further enhance the properties of the composite mix.
Citation: Somdee, P., Shettar, M., Prasoetsopha, N., Detsunhnoen, S., Matnok, S., & Ansari, M. A. (2023). Enhancing Eco-Friendly Natural Rubber Composites through the Incorporation of Spent Coffee Grounds.
Viscoelastic Materials are Most Energy Efficient when Loaded and Unloaded at Equal Rates
In nature, animals use biological springs like tendons to run, jump and hop impressive distances. Depending on the movement, these biological springs can either be used to conserve energy or perform ultra-fast motion. Energy-conserving motions involve cyclic, repeating locomotive actions such as running. Meanwhile, fast spring action is characterised by a rapid release of energy, such as the movement of jumping frogs.
The key difference between these types of actions is how the rate of deformation during loading compares to that during unloading. Typically, cyclic motion involves equal rates of deformation during loading and unloading, whereas spring action requires unequal rates, with loading rates far exceeding those of unloading.
The difference between these symmetrical and asymmetrical loading rates presents as differences in energy efficiency. To express this difference quantitively, resilience is defined as the ratio of energy released divided by the energy stored during a single cycle of loading and unloading.
To study the effect of the loading/unloading profile on resilience, researchers compared the mechanical response of an American bulldog plantaris tendon with that of a synthetic elastomer. Using a DMA to generate bespoke triangle waveforms to simulate asymmetrical loading profiles, the researchers investigated which profile resulted in the best resilience for each material.
Then they compared the results against mathematical viscoelastic models to explain how these differences in mechanical efficiency are related to rates of loading and unloading. Overall, both the synthetic elastomer and the tendon experienced the highest resilience of around 90% with equal loading and unloading rates.
With unequal rates of loading and unloading, the resilience of both dropped by up to 30%. What’s more, even at higher strains where non-linear effects become more apparent, the resilience showed the same significant decrease with asymmetry and slightly lower resilience overall.
Citation: Tsai, L., Navarro, P., Mendoza, E., Azizi, E., & Ilton, M. (2023). Viscoelastic materials are most energy efficient when loaded and unloaded at equal rates. arXiv preprint arXiv:2308.14955.
Ozone Impact on the Rubber Contained in the Tip Cap of Medical Prefillable Syringes
While countless items use rubber seals, it is often overlooked how much behind the scenes design and testing goes into developing these tiny parts to ensure that they are safe and fit for purpose. One such example is medical syringes, which use rubber tip caps to maintain a sterile environment and prevent unintended needle pricks.
With a long shelf life, these syringes must be tested to the extremes of their likely environmental conditions to pass quality control checks. One of the most important sources of environmental degradation to study is UV and ozone radiation, which over time can cause cracking and eventual material failure. While packaging can be made opaque to combat UV exposure, there are currently no known ozone-blocking packaging materials.
A recent thesis has investigated the effect of ozone on rubber in medical syringes. This detailed report includes a theoretical review and experimental verification of all the external factors than can affect the performance and longevity of the rubber syringe cap.
Firstly, as stress can be a strong contributory factor to ozone cracking, the configuration of the syringe caps was modelled using FEA to study areas of stress concentration. Then, further material characterisation tests included tensile strength, swelling and dynamic mechanical analysis. In these tests, both the impact of stress and packaging were investigated. Lastly, a full suite of functionality tests was conducted on the syringes.
While this amount of investigation may appear excessive for such a tiny part, this level and scope of analysis is common for parts that are safety critical, as material failure could lead to costly product recalls. Additionally, as rubber is a material that is not only made but designed to meet a specification, it is necessary to complete thorough testing to understand how the material will perform in its intended application.
This is particularly important as rubber behaves dynamically according to its environment so a thorough test regime is required to fully understand the material response.
Citation: Ge, Eliot. “Ozone impact on the rubber contained in the tip cap of medical prefillable syringes." (2023).
DOI: Link
Conclusion
This month, Prescott Instruments has featured four recent scientific research papers concerning the world of rubber. October’s research topics include elastomers controlled by magnetic fields, spent coffee grounds used as filler, the loading and unloading energy efficiency of elastomers and the impact of ozone on the stability of syringe cap lids.
If you would like to see your research featured, or to suggest any further topics, contact us online.
Discover More Featured Rubber Research From Prescott Instruments:
September 2023 Featured Research
October 2023 Featured Research
November 2023 Featured Research