Ethan Clarke
Mechanical properties are the physical attributes a material exhibits when a force is applied. The mechanical properties of plastics are significant for any plastic professional wanting to make quality products. The fundamental mechanical characteristics of plastics include strength, stiffness, hardness, and toughness. The strength of a plastic is its resistance to external stress, while stiffness is its resistance to deformation. Hardness is the resistance of a material to deformation under a concentrated compressive load, and toughness is the energy absorption capacity of a material during impacts. Other mechanical properties of plastics include tensile strength, elongation, compression strength, flexural strength, and elasticity.
| Characteristics | Values |
|---|---|
| Strength | High |
| Toughness | Low |
| Ductility | High |
| Tensile Strength | High |
| Flexural Strength | High |
| Compression Strength | High |
| Impact Strength | High |
| Hardness | High |
| Stiffness | High |
| Elasticity | High |
| Lightweight | Yes |
| Heat Resistance | Yes |
| Electrical Resistance | Yes |
| Corrosion Resistance | Yes |
| Modulus-to-Weight Ratios | High |
| Electrical Conductivity | Yes |
| Transparency | Yes |
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What You'll Learn
Strength and toughness
When it comes to strength, there are several types to consider. Tensile strength, for instance, measures how much stress a plastic can withstand without breaking when stretched or pulled. Some plastics with high tensile strength include PEEK, Nylon (6 cast), PAI, and VESPEL. Flexural strength, on the other hand, measures how well a plastic resists deformation under load. PPS, PEEK, and PAI are examples of plastics with good flexural strength.
Toughness, often referred to as impact strength, is measured by how much energy is absorbed when a material fractures upon collision. The Izod impact test is a common method to determine this, where a small rectangular rod is struck by a pendulum at high speed. Some plastics with good impact strength include polycarbonate, LPDE, UHMW-PE, Polypropylene, PAI, and VESPEL.
It is important to note that strength and toughness are often at odds with each other. A material that is strong may not be very tough, and vice versa. For example, rigid plastics like polystyrene, poly(methyl methacrylate), and polycarbonate can withstand a lot of stress but are not very tough as they do not withstand much elongation before breaking.
To achieve a balance between strength and toughness, polymers with different properties can be combined to create a new material with a blend of characteristics. This can be done through copolymerization, blending, or the creation of composite materials.
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Stiffness and hardness
The stiffness of plastics is sometimes referred to as flexural strength or tensile modulus. However, the strength of a plastic is typically referred to as tensile strength or ultimate strength, which is how much stress a plastic can withstand without breaking when stretched or pulled. This distinction is important because it can be confusing when the terms are mixed.
Some plastics with high stiffness values include:
- Ultem® – PEI resins such as Ultem® offer high stiffness and strength, superior elevated thermal resistance, and broad chemical resistance. With a flexural modulus of 22,000 psi, Ultem® offers predictable stiffness and strength up to 200ºC/392ºF.
- PEEK – PEEK combines high stiffness (24,000 psi) and strength (14,000 psi) with outstanding chemical resistance and a continuous service temperature of 500ºF.
- Homopolymer acetal (Delrin®) is extremely stiff, with a flexural modulus of 14,300 psi, and provides superior wear and abrasion properties as well as excellent chemical resistance.
- PPS – PPS has a flexural modulus of 21,000 psi and maintains its stiffness and strength over wide temperature ranges.
Hardness is the measure of a material's resistance to deformation under a concentrated compressive load. Glass fibre and carbon fibre-filled materials exhibit the highest surface hardness.
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Stress/strain behaviour
The stress/strain behaviour of plastics is a crucial aspect of their mechanical properties, influencing their performance and suitability for specific applications.
Plastics are known for their strength, toughness, and ductility. Strength refers to a plastic's resistance to external stress, which can be tensile strength (resistance to stretching) or compressive strength (resistance to deformation under load). Tensile strength is particularly important in brittle materials, while ductile materials like thermoplastics may deform without clear breakage, making elongation an important consideration. Rigid plastics like polystyrene, poly(methyl methacrylate), and polycarbonate can withstand significant stress but have limited elongation before breaking. On the other hand, fibres like Kevlar, carbon fibre, and nylon exhibit high strength but low toughness, as they don't deform much under tensile stress.
The stiffness of a plastic is its resistance to deformation, and this is closely related to its hardness, which is its ability to withstand deformation under concentrated compressive loads. Toughness, meanwhile, refers to a plastic's energy absorption capacity during impacts. Toughness can be improved by blending polymers, creating materials that combine strength and toughness, such as high-impact polystyrene (HIPS).
The stress/strain behaviour of plastics can be visualised using stress-strain curves. These curves show how a plastic deforms under stress, with the slope of the curve indicating the force required to deform the material. For example, rigid plastics have steep slopes, requiring a lot of force to deform, while elastomers like polyisoprene have gentle slopes, making them easy to stretch or bend.
Additionally, the flexural modulus of elasticity, also known as Young's modulus, is an important mechanical property. It describes the relationship between stress and deformation along the same axis, providing insight into the material's elasticity. This modulus is determined using Hooke's law, which involves applying force to an object and observing its deformation pattern in the elastic deformation region.
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Tensile strength
In contrast, materials like Kevlar, carbon fibre, and nylon exhibit high tensile strength. These materials can withstand high levels of tensile stress without breaking and are often used in applications where strength is a priority. Nylon, for example, is used in clothing, rope, and reinforcement in rubber materials like car tires. It has a tensile strength of 12,400 psi and offers excellent abrasion resistance, making it a versatile material.
Some plastics offer exceptionally high tensile strength. For example, PAI (polyamideimide) boasts a tensile strength of 21,000 psi, the highest of any unreinforced thermoplastic. PAI is used in jet engines, internal combustion engines, and printed circuit boards due to its high strength, low flammability, and thermal stability. Another example is Ultem (PEI), a high-performance thermoplastic with a tensile strength of 15,200 psi. Ultem is popular in medical and chemical instrumentation for its heat, solvent, and flame resistance, as well as its strength and rigidity.
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Flexural strength
When it comes to the mechanical properties of plastics, one of the key considerations is flexural strength, which is the ability of a material to resist deformation under bending loads. This is distinct from tensile strength, which measures a material's resistance to stretching or tension. In the case of plastics, flexural strength is a critical factor in determining their suitability for various applications.
The flexural strength of plastics typically falls between 40 to 1000 MPa, showcasing significant variability depending on the specific type of plastic. Reinforced plastics, for example, exhibit high flexural strength due to the redistribution of stress from the less robust polymer matrix to more resilient fibres. This makes them ideal for construction applications, as they can withstand loads and improve load-bearing capacity while resisting bending or failure under stress. Additionally, reinforced plastics are used in high-performance vehicle components due to their strong, lightweight properties, high tensile strength, and chemical resistance.
Composites, which are made up of two or more different materials, also demonstrate unique flexural strength properties. The combination of materials in composites often results in high flexural strength, making them suitable for structural and load-bearing applications. FR4, a popular grade of glass-reinforced epoxy laminate, is a notable example with superior mechanical and electrical properties, making it widely used in the electronics industry.
The measurement of flexural strength is crucial in determining the suitability of plastics for specific applications. ASTM D790 and ISO 178 are standard methods for measuring the flexural properties of materials under bending strain. These tests provide precise data that facilitates the evaluation of material behaviour under bending forces, ensuring the right plastic is selected for the right application.
While flexural strength is essential, it is just one aspect of a plastic's mechanical properties. Other factors, such as tensile strength, toughness, and ductility, also come into play when designing and selecting plastics for different applications. The specific requirements of an application will dictate the most critical mechanical properties, and sometimes a trade-off between strength and toughness, for example, must be made.
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Frequently asked questions
Mechanical properties of plastics include toughness, reliance, high strength, modulus-to-weight ratios, corrosion resistance, electrical conductivity, and transparency.
Rigid plastics such as polystyrene, poly(methyl methacrylate), and polycarbonate can withstand a good deal of stress but are not very tough. Polycarbonate is the strongest plastic on the market, being 200 times stronger than other plastics.
Fibers like Kevlar, carbon fiber, and nylon are more strong than tough, but elastomers like polyisoprene, polybutadiene, and polyisobutylene exhibit high toughness.
Mechanical properties of plastics can be investigated through standardized test methods. Tensile strength, for example, can be determined by briefly applying a load in one direction with a tensile test. Impact strength can be measured by striking a small rectangular rod with a pendulum at high speed and measuring the energy absorbed.
