Featured Rubber Research – November 2023


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 November, the featured papers include:

Assessing Children’s Potential Exposures to Harmful Metals in Tire Crumb Rubber by Accelerated Photodegradation Weathering: This study investigates the secretion of toxic metals from the surface of rubber crumb used in children’s playground flooring.

Usage of Eggshell as Potential Bio-Filler for Arcylonitrile Butadiene Rubber (NBR) Latex Film for Glove Applications: A feasibility study for the use of powdered waste eggshells as a sustainable filler in nitrile butadiene rubber (NBR) rubber gloves.

Intelligent Eucommia ulmoides Rubber/Ionomer Blends with Thermally Activated Shape Memory and Self-Healing Properties: A study into the shape memory and damage-repair mechanisms of natural rubber-based composites.

Uptake and Biotransformation of the Tire Rubber-derived Contaminants 6-PPD and 6-PPD Quinone in the Zebrafish Embryo (Danio rerio): A look into the various bio-transformation pathways of 6PPD that cause hazardous effects on aquatic life.

Read the full features below, complete with citations and links to read the original research online.

Featured Research Papers

Assessing Children’s Potential Exposures to Harmful Metals in Tire Crumb Rubber by Accelerated Photodegradation Weathering

One common use for end-of-life tires is as crumb rubber for playground flooring. With high shock absorption properties, these soft and bouncy floors are an obvious choice for children’s or sports flooring, reducing the likelihood of physical injuries. As a major pathway for recycled tyres, flooring tiles and mulch already accounted for nearly 50% of all tyre crumb rubber (TCR) usage in the US by 2017. This figure is expected to grow as manufacturers, consumers and producers continue to pivot to more sustainable practices that reduce the need for virgin fossil-fuel-derived materials.

However, while there is ample supply and growing demand for TCR products, there is also increasing concern over the secretion of harmful chemicals, such as organic contaminants and heavy metals, from sources of ageing rubber. On average, more than half of children visit a playground at least once a week for around 45 minutes. Potential exposure pathways to harmful chemicals include dermal contact, ingestion or inhalation.

While the constituent parts of rubber are specified when formulated, the makeup of many waste tyres is not always known at the point of recycling. Therefore, many chemical substances that have been proven to be harmful and are currently prohibited are still in circulation via recycled tyre crumb.

Of greatest concern are polycyclic aromatic hydrocarbons, phthalates, vulcanization additives and metal elements. While organic contaminants can degrade over time via oxygen or ozone exposure, UV light radiation, heat and water (through moisture or rain), metals are non-degradable and are found to accumulate over time.

Researchers measured weather-dependent metal surface release of TCR in children's flooring tiles to compare against Safe Drinking Water Act guidelines. Commercially available tiles underwent accelerated ageing to replicate 2 years of outdoor use. Tests revealed that chromium was the only element exceeding safe limit at installation, and after 3 years, antimony was the only metal exceeding the limit. The difference in type compound formulation caused variation in the dataset.

During the study, the researchers found that lead is the only element that has regulatory standards for environmental safety concerning bulk and surface contents. Although all the tiles tested in the study had lead contents below the threshold, it is important to note that other harmful chemicals may be present. This means that products can still be considered safe, despite containing dangerous levels of other toxins. Therefore, the researchers concluded that it is necessary to urgently review the regulatory standards for the surface content of toxic metals. Such a review is essential to ensure that children's playgrounds remain safe.

Citation:  Winz, R., Yu, L. L., Sung, L. P., Tong, Y. J., & Chen, D. (2023). Assessing children’s potential exposures to harmful metals in tire crumb rubber by accelerated photodegradation weathering. Scientific Reports, 13(1), 13877.

DOI: https://doi.org/10.1038/s41598-023-38574-z

Usage of Eggshell as Potential Bio-Filler for Arcylonitrile Butadiene Rubber (NBR) Latex Film for Glove Applications

Usage of Eggshell as Potential Bio-Filler for Arcylonitrile Butadiene Rubber (NBR) Latex Film for Glove Applications How do you like your eggs in the morning? While many of us enjoy eating eggs for breakfast, only a small fraction of eggshells are currently recycled at home as soil conditioners and fertilisers, while the majority are sent directly to landfill.

Eggshells are rich in calcium carbonate, but their protein membrane makes recycling difficult. Effective repurposing can reduce pollution and landfill space while lowering disposal costs for food manufacturers.

Eggshells are a promising sustainable bio-filler for polymers as they are rich in calcium carbonate. They offer an eco-friendly alternative to ground calcium carbonate, which is made from non-renewable limestone sources. Eggshells have already been shown to enhance mechanical, thermal, and processing properties when incorporated into composites.

This study focusses on nitrile butadiene rubber (NBR) rubber gloves, an industry that has expanded rapidly and continues to grow year on year. While many studies already document the incorporation of various fillers into NBR, very few investigate compounding during the latex phase. This is mainly due to concerns around latex phase stability during compounding.

In this study, researchers added eggshells at three levels of loading (2, 4 and 6 phr) to a NBR latex film. The composite was prepared through latex compounding and curing methods using sulphur crosslinking ingredients. Tensile properties, crosslinking density, viscosity, swelling and chemical degradation were all investigated to measure the effect of the bio-filler on the complex crosslinking and stability of the composite. The results showed that the irregular shape of the eggshell powder increased the interfacial and physical interaction between the eggshell and NBR, improving the ultimate tensile strength.

The macromolecular chains were shown to be tightly packed, indicating an increase in crosslink density that improved resistance to some forms of chemical degradation, an important consideration for gloves regularly exposed to chemical reagents. One of the main findings was that the bio-filler did not destabilise the latex phase and showed no apparent change in viscosity.

The researchers were able to achieve the desired results using eggshell waste. However, it's important to note that they had to manually remove the membrane from the egg powder, which is a time-consuming process. This manual preparation presents a significant challenge in terms of using eggshell waste on a large scale and must be addressed before it can be considered commercially viable.

Citation: Munusamy, Y., & Kchaou, M. (2023). Usage of eggshell as potential bio-filler for arcylonitrile butadiene rubber (NBR) latex film for glove applications. Ain Shams Engineering Journal, 14(11), 102512.

DOI: https://doi.org/10.1016/j.asej.2023.102512

Intelligent Eucommia ulmoides Rubber/Ionomer Blends with Thermally Activated Shape Memory and Self-Healing Properties

Intelligent materials are a new group of functional materials that can sense and respond to external stimuli in a manner that mimics the operation of living systems. These so-called smart materials display unique characteristics, such as shape memory and damage-repair mechanisms, that have driven the rapid pace of research into high-end material innovation.

Shape memory is defined as the ability to reversibly transform to and from a temporary shape under external environmental conditions. Examples of environmental stimuli include temperature, humidity, light, pH level, and magnetic and electric fields. Self-healing polymers are also responsive, with the capability to repair damage by the action of chemically reversible bonds.

Polymers are ideal for smart materials due to their adjustable mechanical properties, lightweight, and biocompatibility. However, producing self-healing rubber is often complex and expensive, coupled with low efficiency. Therefore, there is a need for simple composite materials with excellent smart properties and economic feasibility.

Eucommia ulmoides is a hardy rubber tree native to China that can recover its original shape when exposed to external tensile or compressive forces. Blended with Surlyn resin, researchers investigated the mechanical, curing, thermal, shape memory and self-healing properties of the composite blends.

Four different composites were prepared with varying loading ratios. The curing characteristics were measured using an oscillating disc rheometer (ODR) and differential scanning calorimetry (DSC) techniques were used to determine the level of crystallinity.

The team used a dynamic mechanical analyser (DMA) for shape memory tests to determine the shape fixity and shape recovery ratios. To test self-healing properties, samples were cut, heated, and left to stand for five days. The tensile strength of the healed samples was then measured to calculate healing efficiency.

Overall, the composites showed good shape memory qualities across a large thermal range (25°C to 100°C) with a high degree of sensitivity. For self-healing properties, the blends demonstrated an impressive 87.41% self-healing efficiency, which is higher than that of other covalent-bonded composites.

The researchers concluded that their results may lead to the expanded use of Eucommia ulmoides in smart material applications, such as medical devices, sensors and actuators.

Citation: Wang, Q., Li, Y., Xiao, J., & Xia, L. (2023). Intelligent Eucommia ulmoides Rubber/Ionomer Blends with Thermally Activated Shape Memory and Self-Healing Properties. Polymers, 15(5), 1182.

DOI: https://doi.org/10.3390/polym15051182

Uptake and Biotransformation of the Tire Rubber-derived Contaminants 6-PPD and 6-PPD Quinone in the Zebrafish Embryo (Danio rerio)

PPDs are antioxidants used in tyre production to prevent rubber degradation. Of these, 6PPD is the most commonly used PPD, but it is controversial due to its harmful impact on aquatic life.

Toxic exposure happens when tyre wear particles, expelled due to friction between rubber tyres and roads, accumulate with other road surface materials and enter water supplies via rainwater runoff.

Despite having a short half-life, the downstream effects of 6PPD are already being felt. Researchers have already identified 6PPD-quinone (PPDQ), the transformation product of 6PPD, as being responsible for the unexplained acute mortality of coho salmon in the Pacific Northwest of the US. Further studies have also shown fatal sensitivities in rainbow and brook trout species. Before death, the fish experience increased ventilation, gasping, spiralling and loss of equilibrium.

However, other fish species, such as arctic char, sturgeon and zebrafish, demonstrated significantly higher tolerance to 6PPDQ. The discrepancy between the sensitivity of individual fish species to 6PPDQ and other transformative products is yet to be fully understood.

In this recent study, researchers exposed zebrafish embryos to 6PPD and 6PPDQ, discovering 22 and 12 transformation products respectively. This study sheds light on the nature of these biotransformations.

After initial exposure, 6PPD and 6PPQQ were significantly taken up. However, after 96 hours, over 50% and 95% were detoxified, respectively. This observation may explain the variation in tolerance between species due to their biotransformation pathways. Further comparative studies with sensitive and tolerant fish are needed to understand their species-specific sensitivity.

This study contributes to the growing body of knowledge on the toxicity of PPDs to fish of commercial, cultural, and ecological importance at relevant concentrations. This work is essential for completing environmental risk assessments for this emerging contaminant.

Considering the potential widescale environmental harm caused by 6PPD, its future in passenger tyres remains uncertain. However, as an important antioxidant that prevents rubber degradation, such as cracking, there are limited alternatives available that offer the same level of effectiveness. Therefore, until the environmental impact is fully understood, there is unlikely to be any regulation regarding its usage.

Citation: Grasse, N., Seiwert, B., Massei, R., Scholz, S., Fu, Q., & Reemtsma, T. (2023). Uptake and Biotransformation of the Tire Rubber-derived Contaminants 6-PPD and 6-PPD Quinone in the Zebrafish Embryo (Danio rerio). Environmental Science & Technology, 57(41), 15598-15607.

DOI: https://doi.org/10.1021/acs.est.3c02819


This month, Prescott Instruments has featured four recent scientific research papers concerning the world of rubber. November’s research topics include antimicrobial rubbers, eggshells as an alternative to silica filler, smart rubbery materials and the negative effects of 6PPD on aquatic life.

If you would like to see your research featured, or to suggest any further topics, contact us online.