Alexei Volkov
The Mohs scale, developed by German mineralogist Friedrich Mohs in 1812, is a qualitative ordinal scale that assesses the scratch resistance of various minerals by determining whether a harder material can scratch a softer one. The scale ranges from 1 to 10, with talc at the lowest end and diamond at the highest. While the Mohs scale was originally designed for minerals, it has since been used to evaluate the hardness of other materials, including plastics. The Rockwell and Shore methods are the two mainstream techniques for measuring the hardness of plastics, with the Shore D scale being the preferred option for testing hard plastics.
| Characteristics | Values |
|---|---|
| Mohs scale developed by | Friedrich Mohs |
| Year of development | 1812 |
| Scale range | 1 to 10 |
| Material at the lowest end of the scale | Talc |
| Material at the highest end of the scale | Diamond |
| Plastic hardness on the Mohs scale | 1 |
| Rockwell hardness scale for plastics | E, M, and R |
| Shore hardness scale for plastics | A and D |
| Polycarbonate hardness on the Mohs scale | 3 |
| Acrylic hardness on the Mohs scale | 2.5 |
| Nylon hardness on the Mohs scale | 2.5 |
| Rockwell R scale hardness for PP plastic | 92 |
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What You'll Learn
The Rockwell Hardness method
Plastic typically ranks low on the Mohs scale of hardness, with a score of 1. This scale was developed in 1812 by German mineralogist Friedrich Mohs and is used to rank the hardness and scratch resistance of natural minerals.
The Rockwell hardness test is a method used to measure the hardness of plastics and other materials. It determines a material's ability to resist permanent indentation by applying a load with a ball indenter, typically made of steel. The depth of the resulting indentation is measured and used to calculate the surface area of the indent, which indicates the hardness of the material. This test is performed in two stages to minimise errors caused by irregularities in the material's surface and incorrect measurements due to backlash from the created indentation.
The preload force applied to the indenter is typically 9.8N, and the diameter of the ball indenter varies depending on the test method used. Different Rockwell hardness test scales employ different-sized steel balls and loads. The scales are listed alphabetically, with the most common scales for soft materials like plastics being E, M, R, and L.
The results of the Rockwell hardness test are written as a numeric hardness value followed by the letters "HR," indicating "Hardness according to Rockwell." For example, a result may be written as 45HR.
The Rockwell hardness test is widely used in industries such as automotive component manufacturing, where understanding the hardness of materials is crucial for assessing scratch resistance, strength, abrasion, and wear.
The Shore Hardness Method
Another popular method for measuring the hardness of plastics is the Shore hardness method. This method uses a spring-loaded needle that is pressed into the plastic until it reaches its maximum depth. The depth of the indentation is then measured and is proportional to the hardness of the plastic. The Shore hardness scale includes Shore A for testing very soft gels and rubbers and Shore D for testing hard rubbers, semi-rigid plastics, and hard plastics.
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Scratch resistance
Plastic ranks low on the Mohs scale, which means it is relatively soft and can be scratched by many other materials. However, not all plastics are created equal, and some are harder than others. The hardness of plastics can be increased by adding fillers, applying coatings, or using hardening processes.
The Mohs scale is a relative measure of the hardness and scratch resistance of minerals, created by German mineralogist Friedrich Mohs in 1812. It ranks ten natural minerals on a scale from 1 to 10, with 1 being the softest and 10 being the hardest. Plastic, which is not a natural mineral, is considered softer than talc (ranked 1 on the Mohs scale) and can be scratched by it.
The hardness of plastics is typically measured using the Rockwell hardness scale or the Shore hardness scale. The Rockwell method involves smacking a plastic sample with a hard steel ball and measuring the resulting indentation. The Shore hardness scale, on the other hand, uses a spring-loaded needle to measure the depth of indentation, which is proportional to the hardness of the plastic.
While plastic generally ranks low on the Mohs scale, some plastics are harder than others. Polycarbonate, for example, is known for its high scratch resistance and is used in screen protectors and eyeglass lenses. Scratch-resistant plastics are also used in electronics, automotive parts, and furniture to enhance durability and maintain an appealing surface.
To increase the scratch resistance of plastics, several methods can be employed:
- Fillers: Adding fillers such as glass fibers or nanoparticles increases the hardness and abrasion resistance of the plastic.
- Coatings: Applying a scratch-resistant coating to the surface of the plastic provides protection against scratches and wear.
- Hardening: Processes like crosslinking or hardening can reinforce the plastic's structure, making it less prone to scratches.
- Polypropylene (PP): A new approach to creating scratch-resistant PP involves producing an ultra-hard, highly crystalline surface, which has proven successful in laboratory settings.
By employing these techniques, plastics can be made more resistant to scratching and wear, improving the longevity and aesthetic appeal of products across various industries.
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Plastic's versatility and adaptability
Plastic is a highly versatile and adaptable material, with a wide range of applications and benefits. It is used in packaging, food storage, clothing, transport, and many other industries. Its versatility stems from its unique properties, such as its high strength-to-weight ratio, stiffness, toughness, ductility, corrosion resistance, and outstanding durability.
Plastics are also adaptable in terms of their production. They can be made from any feedstock containing carbon and hydrogen, with fossil fuels being the most common source. However, plastics can also be made from renewable resources such as sugar and corn, showcasing their adaptability to different raw materials.
The development of plastics has come a long way since the nineteenth century, with the synthesis of new polymers and the creation of biodegradable plastics. Traditional plastics are designed to last with chemicals that prevent decay, but biodegradable plastics contain additives to facilitate the decay process. However, biodegradable plastics may not always break down into harmless materials, leaving behind chemicals that can be harmful to the environment.
To address the environmental concerns associated with plastics, there is a growing focus on recycling and reusing plastic materials. Europe, for example, has made significant strides in transitioning towards a circular and resource-efficient society, reusing around 31% of 26 million tonnes of plastic waste in 2016. Additionally, there is innovation in conventional plastics, with efforts to reduce volume and weight, and the emergence of bioplastics made from biodegradable or natural recyclable materials.
In terms of hardness, plastic is ranked as 1 on the Mohs scale, indicating that it is relatively soft and can be scratched by materials of a higher hardness ranking, such as glass (6-7) or diamond (10). On the Rockwell hardness scale, plastics typically fall within the R scale, with PP, for instance, having a hardness of 92. The Shore hardness scale is also used, with Shore A for testing very soft gels and rubbers, and Shore D for testing harder materials, including semi-rigid plastics.
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Industrial applications
Plastic is a versatile material used in various industrial applications due to its adaptability and unique properties. The hardness of plastic is a critical factor in determining its suitability for specific tasks, and this can be measured using the Mohs scale.
The Mohs scale, developed by German mineralogist Friedrich Mohs in 1812, is a qualitative ordinal scale that ranks materials based on their scratch resistance. The scale ranges from 1 to 10, with talc being the softest at 1 and diamond being the hardest at 10. While the scale was initially designed for minerals, it has since been applied to other materials, including plastics.
In the context of plastics, the Mohs scale is particularly useful for determining the suitability of a specific type of plastic for an industrial application. For example, polycarbonate, with a Mohs hardness of around 3, is often used in applications requiring durability and transparency, such as eyewear lenses and protective barriers. Acrylic, with a Mohs hardness of approximately 2.5, is valued for its clarity and ease of fabrication, making it a popular choice for signage, aquariums, and display cases. Nylon, another plastic with a Mohs hardness of about 2.5, is known for its strength and wear resistance. These properties make nylon ideal for mechanical parts such as gears, bearings, and bushings, where its hardness and low friction ensure longevity and reliability.
The automotive industry has also benefited from high-performance plastics. Materials like polyamide and polyoxymethylene, with their excellent mechanical properties and resistance to wear and chemicals, are now commonly used in engine components, fuel systems, and interior parts. Additionally, the hardness of plastics can be modified by layering high-hardness resins on low-hardness plastic components, a method particularly suitable for extrusion products like sheets, films, pipes, and plates.
In summary, the Mohs scale is a valuable tool for assessing the hardness of plastics and determining their suitability for specific industrial applications. By understanding the hardness of different plastics, industries can select the most appropriate materials for their specific requirements, ensuring optimal performance and longevity.
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Other hardness scales
Plastic ranks at 1 on the Mohs scale of hardness. This scale was developed in 1812 by German mineralogist Friedrich Mohs and is used to test the hardness and scratch resistance of minerals.
There are several other hardness scales used to test the hardness of materials, including plastics. Here are some of the most common ones:
Rockwell hardness scale
The Rockwell hardness scale is likely the most common hardness scale used today. It is a highly accurate and fast method of testing hardness and is used across a number of metrological settings. This scale was first developed by Paul Ludwik in 1908, and a machine called the Rockwell hardness tester was patented in 1914 by Hugh and Stanley Rockwell. The Rockwell hardness scale is defined by the standard American Society for Testing and Materials (ASTM) E18 and is approved for measuring commercial shipments.
The Rockwell hardness method involves smacking a sample with a hard steel ball and applying a small pre-stress. Then, a larger load is applied and removed, and the remaining indentation is read from the scale. Different steel balls and weights are used depending on the material and scale. The Rockwell R scale is commonly used to measure the hardness of plastics.
Vickers hardness scale
The Vickers hardness scale uses a square-based pyramid-shaped diamond to make an impression on the material. The size of the impression is then measured to determine the hardness of the material. One benefit of this scale is that the width of a square impression is much easier to measure than a circle, allowing for the use of the same formula and indenter for all material types.
Brinell hardness scale
The Brinell hardness scale is one of the older hardness scales and may be reliable to use, but it has disadvantages such as the slowness to complete the test.
Knoop hardness scale
The Knoop hardness scale is suitable for testing thin, small, or delicate materials as it utilizes less force than some of the other hardness scales.
Shore hardness scale
The Shore hardness scale is used to determine the indentation hardness of materials such as rubbers and thermoplastics. Shore A is chosen when testing very soft gels and rubbers, while Shore D is preferred for testing hard rubbers, semi-rigid plastics, and hard plastics. The hardness of plastics is measured using a device called a Durometer.
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Frequently asked questions
The Mohs scale is a qualitative ordinal scale that lists the hardness of minerals from weakest to strongest. It was developed in 1812 by German geologist and mineralogist Friedrich Mohs. The scale ranges from 1 to 10, with higher numbers indicating a higher level of scratch resistance.
The Mohs scale determines the hardness of a material by finding the hardest material that the given material can scratch, or the softest material that can scratch the given material. For example, if a material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would be between 4 and 5.
Plastic hardness on the Mohs scale is determined through a series of scratch tests. A plastic material is scratched with a reference set of minerals or other materials of known hardness. A higher Mohs rating indicates greater scratch resistance, while a lower rating suggests higher flexibility and impact resistance.
In addition to the Mohs scale, other common scales for measuring plastic hardness include the Rockwell hardness scale and the Shore hardness scale. The Rockwell hardness scale is widely used for plastics and involves applying loads to a sample using different steel balls and weights. The Shore hardness scale, on the other hand, is employed for measuring the indentation hardness of thermoplastics.
