Omar Khalid
The Mohs scale is a mineral hardness scale that ranges from 1 to 10, with 10 being the hardest. It was introduced in 1812 by German geologist and mineralogist Friedrich Mohs. The scale is based on the ability of a harder mineral to scratch a softer one. While it was originally designed for minerals, the Mohs scale has also been used to assess the hardness of other materials, including plastics. Plastic materials are used in various industries, and their hardness is a key property that determines their suitability for different applications. For instance, polycarbonate, a common plastic with a Mohs hardness of around 3, is known for its hardness and impact resistance, making it ideal for applications requiring durability and transparency, such as eyewear lenses and protective barriers.
| Characteristics | Values |
|---|---|
| Plastic hardness on Mohs scale | 2 |
| Range of Mohs scale | 1 to 10 |
| Hardest material on the Mohs scale | Diamond |
| Softest material on the Mohs scale | Talc |
| Mohs scale introduced in | 1812 |
| Who introduced the Mohs scale | German geologist and mineralogist Friedrich Mohs |
| Use of the Mohs scale | To measure the scratch resistance of minerals and other materials |
| Use of the Mohs scale in the context of plastics | To determine the suitability of a plastic for specific industrial applications |
| Plastic with a Mohs hardness rating of around 3 | Polycarbonate |
| Plastic with a Mohs hardness of approximately 2.5 | Acrylic |
| Common plastic with a lower Mohs hardness, typically around 2 | Polyethylene |
| Plastic with a Mohs hardness of about 2.5 | Nylon |
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What You'll Learn
Plastic hardness and industrial applications
The hardness of a material is a critical factor in determining its industrial applications. This is especially true for plastics, where hardness is influenced by factors such as chemical composition, processing conditions, and post-processing treatments.
Plastic hardness is a measure of its resistance to permanent indentation by a stronger body. It is an essential property in plastics, as it determines the material's ability to withstand deformation, penetration, scratching, and indentation. A harder plastic will be more resistant to these external forces, while a softer plastic will be more susceptible to them. This is particularly evident when two materials are rubbed together, causing one to scratch or lose surface texture.
The Mohs scale, introduced in 1812 by German geologist and mineralogist Friedrich Mohs, is a widely used method for measuring scratch resistance. It assigns a numerical value to minerals based on their ability to scratch or be scratched by other minerals. On this scale, most polymers fall within the 2-4 range, with metals varying from 2 for soft metals like tin and lead, to around 7 for hardened steels.
While the Mohs scale is useful for identifying minerals, it is not always accurate for predicting the performance of materials in industrial settings. Other scales, such as the Rockwell R scale and the Shore hardness scale, are also used to measure the hardness of plastics. The Shore hardness scale, for example, is used to ensure that materials have the right amount of flexibility or hardness for specific applications. It is an important consideration for rubber and plastic components, as it helps determine the balance between flexibility and impact resistance.
In industrial applications, plastic hardness plays a significant role in material selection. For instance, calcium carbonate is a common filler for plastics due to its cheap cost and non-abrasive nature, which makes it suitable for appliances and automotive applications. Similarly, zinc sulphide (ZnS) pigments are used in plastics as they have low Mohs hardness, causing minimal wear on moulds and maintaining the mechanical strength of fibre-reinforced plastics.
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Scratch resistance
The Mohs scale is a mineral hardness scale that characterises the scratch resistance of minerals by observing which minerals can scratch others. It is a qualitative ordinal scale ranging from 1 to 10, with 10 being the hardest.
Plastic is not a natural mineral, so it does not have a Mohs hardness rating. However, certain minerals are added to plastics as fillers to improve their scratch resistance. For example, calcium carbonate is a cheap and non-abrasive filler used in plastics. It does not cause wear during processing and is available worldwide, making it the most important filler for plastics.
Another example is zinc sulphide (ZnS), which is used as a pigment in plastics. ZnS has a low Mohs hardness, so it does not cause wear on moulds and does not impair the mechanical strength of fibre-reinforced plastics.
Scratch-resistant plastic has many applications, especially in the automotive industry. For example, a household appliance OEM wanted to use polypropylene (PP) in vacuum cleaner housings due to its low cost and environmental friendliness. However, PP has inferior scratch resistance. A new approach to creating scratch-resistant PP involves making it with an ultra-hard, super high crystallinity surface.
Scratch-resistant plexiglass is also commonly used in display cases, shadow boxes, windows, and other applications requiring scratch protection and high optical clarity. It is more durable than standard glass and acrylic plastic, lighter and cheaper to produce and transport, and does not require additional films or coatings.
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Plastic versatility and adaptability
Plastic is a highly versatile and adaptable material that has revolutionised product design and functionality across various industries. Its unique properties, such as lightweight, malleability, and high strength-to-weight ratio, make it a preferred choice for engineers and designers.
Plastics are synthetic polymers that can be categorised into two main types: thermoplastics and thermosets. Thermoplastics, which account for about 80% of global plastic consumption, can be melted and reshaped multiple times without altering their fundamental properties. This makes them ideal for applications where flexibility and reworkability are required. On the other hand, thermosets, which make up the remaining 20%, are characterised by their irreversible setting after moulding, resulting in strong and rigid structures.
The versatility of plastics extends beyond their physical properties. They can be produced from a wide range of feedstocks, including fossil fuels, sugar, and corn. This flexibility in raw materials contributes to their adaptability and accessibility. Additionally, advancements in plastic engineering have led to the development of biodegradable plastics, addressing environmental concerns associated with conventional plastics.
Plastics have found numerous applications in industries such as aerospace, automotive, and electronics. In aerospace, plastics are used for components that must resist high temperatures and pressures. In the automotive sector, plastics contribute to lightweighting, enhancing fuel efficiency and reducing carbon emissions. Plastics are also essential in electrical and electronic devices, providing insulation from high voltages and temperatures.
The adaptability of plastics is further exemplified by their use in innovative medical devices. Certain plastics, like Vespel® Polyimide, exhibit exceptional stability and resilience, making them suitable for medical applications where durability and heat resistance are crucial. The ability to mould plastics into complex shapes, coupled with their cost-effectiveness, reinforces their prominence in modern engineering and design.
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Determining plastic suitability
The Mohs scale is a mineral hardness scale that ranges from 1 to 10, with 10 being the hardest. It is used to determine the scratch resistance of minerals by observing which minerals can scratch others. However, it is not an accurate predictor of how materials perform in an industrial setting.
Plastics do not feature on the Mohs scale as they are not minerals, but their suitability for specific applications can be determined through various tests:
Melt Flow Testing
Melt flow testing is a common and essential method for testing plastics. A small sample of thermoplastic is heated, melted, and forced through a die, and its weight and volume are recorded to determine the melt flow rate (MFR) and melt volume rate (MVR). This test helps understand how the polymer will behave at different temperatures and processing techniques, as well as for batch comparison.
Impact Testing
Impact testing involves dropping a dart of different weights onto a plastic sample from different heights and velocities. This measures the resistance of the plastic and how much energy it can absorb.
Rheology Testing
Rheology tests are conducted when a polymer is in its melt phase to understand how stress and applied force relate to the deformation of the material. This helps optimise the product and production process, reducing waste and improving cost efficiency.
Thermal Testing
Thermal testing measures the effects of ageing processes, additives, and varying production conditions on a polymer.
Compression and Bond Strength Testing
This type of testing ensures that a plastic product, such as a water bottle, can be minimised in size without breaking and leaking.
These tests help determine the suitability of a specific plastic for its intended application, ensuring product functionality, health and safety, and customer satisfaction.
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Mohs scale testing
To conduct the test, a specimen of unknown hardness is placed on a tabletop and held firmly in place with one hand. A reference specimen is then placed against a flat, unmarked surface of the unknown specimen. The reference specimen is pressed firmly against the unknown specimen and dragged across its surface. This process is repeated with reference specimens of increasing hardness until the unknown specimen is scratched. The hardness of the unknown specimen is then determined to be between the hardness of the reference specimen that scratched it and the one that did not.
The Mohs hardness scale is commonly used in mineral identification procedures in the field, classroom, or laboratory. It is also used in manufacturing processes to confirm that hardening treatments have been applied correctly. However, it is not an accurate predictor of how materials will perform in an industrial setting. The scale is also used by field geologists to roughly identify minerals using scratch kits.
While the Mohs hardness scale is widely used, it has some limitations. For example, it may be difficult to determine the hardness of a material that is a mixture of multiple substances, as the hardness of each mineral component may vary. Additionally, the Mohs scale is a qualitative ordinal scale, which means that it provides only a relative comparison of scratch resistance, rather than a precise quantitative measurement. Other hardness tests, such as the Vickers hardness scale, provide more precise measurements by estimating the size of indentations microscopically.
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Frequently asked questions
The Mohs scale is a mineralogy-derived hardness scale that was introduced in 1812 by German geologist and mineralogist Friedrich Mohs. It characterises the scratch resistance of minerals by observing which minerals can scratch others. The scale ranges from 1 to 10, with talc at the lowest end and diamond at the highest.
The Mohs scale works by testing whether a mineral can scratch another mineral. If an unidentified mineral can scratch a known mineral, it is harder than that mineral. If it cannot, then it is softer. This process is repeated with harder and softer minerals until the hardness level of the unidentified mineral is determined.
While the Mohs scale was originally designed for minerals, it has been used to assess the hardness of other materials, including plastics. Common plastics such as acrylic and nylon typically have a Mohs hardness rating of around 2.5, while polycarbonate is harder, with a rating of around 3.
Understanding the hardness of plastics on the Mohs scale is important for determining their suitability for specific industrial applications. For example, a plastic with a higher Mohs rating would be more scratch-resistant, making it ideal for surfaces that experience frequent contact. Conversely, plastics with lower Mohs ratings might be more suitable for applications where flexibility and impact resistance are more important.
