Atomic force microscopy (AFM) has been leveraged to study the surface defects of natural rubber vulcanizates so that their structural−mechanical properties can be defined. This data is vital to improving the quality of rubber products.
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Natural rubber is considered to be one of the most important polymers in the modern world. It is a raw material that is used to produce over 40,000 different products that are important to a wide range of industries including automotive, aviation, healthcare, fashion, and consumer products.
The global industrial rubber market is growing, reflecting increased demand for natural rubber. In 2018, the market was valued at $29.8 billion and is expected to grow to reach $44.6 billion by 2026. Part of this growth is likely to come from sales of natural rubber vulcanizates, which are used to make products such as bowling balls, conveyor belts, erasers, insulation materials, rubber hoses, shoe soles, shock absorbers, tires, and toys, making natural rubber vulcanizates one of the most widely used rubber materials in the industry.
It is important that natural rubber vulcanizates are produced with the properties that are expected of them, such as strength, fracture resistance, and durability. To enhance the quality of natural rubber vulcanizates it is vital that we understand what conditions and factors compromise these properties so that they can be mitigated.
Determining Which Factors Predetermine Rupture
The impact of loading conditions on the surface morphology of natural rubber vulcanizates has been the subject of research for many years. As a result, much is understood about the nature of fractures and how they impact the material’s properties. However, until recently, factors that predetermine these fractures had been neglected by research.
To address this issue, in recent years a number of publications have emerged that have aimed to analyze the polymer state prior to rupture at the micro-and nanoscale. This research is helping to develop our understanding of how to strengthen natural rubber vulcanizates and enhance their resistance to fracture. AFM has been a key tool in these studies.
What is AFM?
Gerd Binnig, Calvin Quate, and Christoph Gerber developed the atomic force microscope in 1986. The invention was revolutionary; it allowed scientists to view surface objects accurately at the nanoscale, at resolutions far higher than were possible with the electron microscope. The non-destructive technique is used to take reliable and accurate measurements of various properties of the sample’s surface, including topographical, electrical, magnetic, chemical, optical, and mechanical, at incredibly high resolutions.
An AFM uses a scanning probe with a sharp tip that passes over the surface of a sample in a raster pattern. As it moves over the surface the tip moves, causing deflection of the cantilever as it engages with textures that change the height of the tip. A laser beam tracks the deflection of the cantilever and directs it into a position-sensitive photodetector.
Constant interaction with the sample surface is ensured by a feedback loop that manages the vertical extension of the scanner. Finally, a three-dimensional topographic image of the sample’s surface is generated by combining the coordinates produced by the tracks of the AFM tip during the scan.
Since its invention, AFM has been leveraged by a number of industries, often for the advantages it holds over electron microscopes. Research in the rubber industry has recently utilized the technique to gain a clearer picture of factors that influence the structural integrity of natural rubber vulcanizates.
Rubber Surfaces Under the Atomic Force Microscope
A study published in the journal Macromolecules in 2016 by Ilya Morozov describes how AFM was utilized to investigate the structural–mechanical properties of surface defects in natural rubber vulcanizates. Via the images produced by AFM, the study revealed cracks and strands known as elastoplastic microdefects on the surface samples at particular cutting conditions.
AFM also revealed that in stretched samples, matrix detachment was obvious at the poles of inclusions and that the propagation of longitudinal nanocracks was hindered by inclusions and cross-linking heterogeneities. The study also discovered further details about the conditions and factors that compromise the properties of natural rubber vulcanizates. The properties of defects–transverse cracks within the stretched rubber were found to be contingent on properties such as distance from the crack tip and filler concentration.
Another study, published in the journal Rubber Chemistry and Technology, showed how AFM images were able to reveal changes in ethylene propylene diene monomer (EPDM) morphology that were directly related to crosslinking and loading with fillers such as carbon black (CB) and silica particles, and oil. Using these AFM images, the study was able to conclude that the morphology of rubber vulcanizates was dependent on factors such as the ratio of components, degree of cure, and processing conditions.
It is likely that AFM will continue to be an important tool for enhancing our knowledge of the properties of rubber. As a result, the rubber industry may be able to produce higher wealth rubber products with more desirable properties.
More from AZoM: Why Use AFM for the Analysis of Biopolymers?
References and Further Reading
Morozov, I., 2016. Structural–Mechanical AFM Study of Surface Defects in Natural Rubber Vulcanizates. Macromolecules, 49(16), pp.5985-5992. https://pubs.acs.org/doi/10.1021/acs.macromol.6b01309
Yerina, N. and Magonov, S., 2003. Atomic Force Microscopy in Analysis of Rubber Materials. Rubber Chemistry and Technology, 76(4), pp.846-859. https://meridian.allenpress.com/rct/article-abstract/76/4/846/93064/Atomic-Force-Microscopy-in-Analysis-of-Rubber?redirectedFrom=PDF