Omar Khalid
When we talk about the memory of formed plastic, we are referring to its ability to return to its original shape after being deformed. This is known as shape memory, and the materials that exhibit this property are called shape-memory polymers (SMPs). SMPs are smart materials that can transition between two or three shapes when triggered by an external stimulus, such as a change in temperature. The shape memory property of plastics is essential in various applications, including surgical sutures, where it helps avoid tissue damage and supports healing. In this context, the plastic remembers its original shape and can be repaired with a heat source, such as a hair dryer.
| Characteristics | Values |
|---|---|
| Memory in plastic materials | Ability to return to its original shape after being deformed |
| Tensile strength | Measurement of the stress required to stretch the part |
| Tensile elongation | Ability to be stretched without bouncing back |
| Creep resistance | Avoidance of permanent deformations over time |
| Crystalline materials | More rigid molecular structures |
| Amorphous materials | More random molecular structures |
| Shape-memory polymers (SMPs) | Return from a deformed state to their original shape when induced by an external stimulus |
| Mnemosynation | Controlled imparting of memory on amorphous thermoplastic materials using radiation-induced covalent crosslinking |
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What You'll Learn
Plastic deformation
In ductile metals, tensile loading causes elastic deformation, where each increment of load leads to a proportional increase in extension. When the load exceeds the yield strength, the extension increases rapidly, and some deformation remains when the load is removed. This is the transition point from elastic to plastic deformation, or the yield point, which can be measured using tensile strength testing equipment.
In crystalline materials, plastic deformation occurs when planes of atoms slip past each other, resulting in a permanent change of shape. Dislocations, or defects in the crystal structure, increase the likelihood of these slip events. However, in amorphous materials like polymers, the absence of long-range order allows for rearrangements and the formation of fibrils within regions of high hydrostatic stress, leading to plastic deformation.
To exhibit good memory properties, a plastic material should avoid permanent deformations over time. Materials with excellent creep properties, such as crystalline structures, tend to have better memory characteristics. The key factors influencing memory are high tensile elongation at yield, high tensile strength at yield, and good creep resistance. These properties determine the material's ability to return to its original shape after being deformed or held in a flexed position.
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Tensile strength
When we talk about the memory of plastic materials, we are referring to their ability to return to their original shape after being deformed. This deformation can occur when a part is flexed or bent, causing the outside surface to stretch under a tensile load.
The tensile properties of plastic are influenced by several factors, including the type of plastic, its molecular structure, and the processing conditions during production. Polymers with higher molecular weights tend to have higher tensile strength because they form stronger intermolecular bonds. Temperature and pressure can also affect tensile strength; for example, stretching plastic during manufacturing can increase its tensile strength in the direction of the stretching force while reducing it in other directions.
Some plastics with high tensile strength include Polyamideimide (PAI), with a tensile strength of 21,000 psi, and Ultem (PEI), with a tensile strength of 15,200 psi. Nylon is also notable, with a tensile strength of 12,400 psi, and excellent abrasion, chemical, and heat resistance.
While plastic is versatile, cost-effective, and easy to manufacture, it tends to break down in extreme weather conditions, and its tensile strength may diminish over time. Additionally, the presence of vacuum voids or trapped air inside a plastic part can be a sign of internal stress and may impact the material's performance.
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Creep resistance
Creep is the tendency of a solid material to undergo deformation while under persistent mechanical stress. It is a time-dependent deformation that occurs as a result of long-term exposure to high levels of stress that are still below the yield strength of the material. Creep is more severe in materials subjected to heat for long periods and generally increases as they near their melting point.
The rate of deformation is determined by the material's properties, exposure time, exposure temperature, and the applied structural load. The deformation may become so large that the component can no longer perform its function. For example, creep in a turbine blade could cause it to contact its casing, resulting in failure.
Creep occurs in stages: primary creep, secondary creep, and tertiary creep. During primary creep, an object deforms quickly before slowing down as it adjusts to a new shape imposed by external forces. Secondary creep is when most of the deformation occurs steadily, as the stress put on the object equals the resistance against permanent change within the material. Tertiary creep is when the object starts to break apart because it is so worn out.
The molecular structure of plastics, such as chain length, greatly affects their resistance to creep. The molecular weight of a polymer affects its creep behaviour, with increasing molecular weight tending to make the polymer more creep-resistant. Aromatic polymers are even more creep-resistant due to the added stiffness from their rings. Both molecular weight and aromatic rings add to the thermal stability of polymers, increasing creep resistance.
Materials with higher melting points generally have a higher lifetime and are more resistant to creep. Ceramic materials, for example, have high melting points, which is why they are often used in creep resistance applications.
There are several high-performance plastics that exhibit good creep resistance:
- PEEK (Polyether Ether Ketone) is a high-performance engineering plastic that combines excellent mechanical strength and dimensional stability with outstanding resistance to harsh chemicals and superior creep resistance.
- PPS (Polyphenylene Sulfide) is a high-performance thermoplastic that exhibits excellent mechanical properties and is resistant to thermal degradation.
- PTFE (Polytetrafluoroethylene) is another high-performance thermoplastic with excellent mechanical properties.
- ECTFE (Halar) has greater strength, abrasion resistance, stiffness, and creep resistance than softer fluoropolymers such as PTFE.
- PAI (Torlon) is an extremely strong, stiff, dimensionally stable plastic with superior creep resistance.
- Vespel is a highly durable polyimide used in demanding applications that require excellent strength and impact resistance, low wear and/or low friction rates.
- Polycarbonate is an engineering plastic with better creep resistance than standard plastics due to its more ordered molecular structure and better thermal stability.
- Acetal, nylon, and PBT are crystalline materials with better creep resistance than amorphous materials like polycarbonate.
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Crystalline vs amorphous plastics
When we speak of memory in plastic materials, we are referring to their ability to return to their original shape after being deformed. Materials with good memory properties can resist permanent deformations over time.
Some plastic materials are crystalline, while others are amorphous. Crystalline plastics have a highly ordered molecular structure, with strong intermolecular forces. They are considered to have a more rigid molecular structure and tend to have better creep properties. Common crystalline materials include PP, PE, Nylon, Acetal, and PBT.
On the other hand, amorphous plastics have a randomly ordered molecular structure, lacking a sharp melting point. They soften gradually as the temperature increases. Amorphous materials are often compared to a plate of spaghetti, with no set order. Examples of amorphous plastics include PC, Acrylic, ABS, and polystyrene. Amorphous materials are more likely to be clear.
Amorphous plastics are easier to thermoform and possess better dimensional stability than crystalline plastics. They are less likely to warp and offer superior impact strength. Amorphous thermoplastics also have excellent resistance to hot water and steam, good chemical resistance, and good stiffness and strength.
However, crystalline plastics have advantages in certain applications. They perform extremely well in wear, bearings, and structural loads. Crystalline plastics also provide excellent chemical resistance, while amorphous materials do not. Crystalline plastics have very good stiffness and strength, good toughness, and a low coefficient of friction.
In summary, both crystalline and amorphous plastics have unique properties that make them suitable for different applications. Amorphous plastics are ideal for applications requiring high dimensional accuracy and stability, with a transparent appearance, in environments with low mechanical abuse and chemical contact. Crystalline plastics, on the other hand, are better suited for environments with repeated cyclic loading, chemical contact, or high levels of mechanical abuse.
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Plastic memory applications
The ability of plastics to return to their original shape after being deformed is referred to as plastic memory. This is achieved through a process called mnemosynation, which involves the controlled imparting of memory on amorphous thermoplastic materials using radiation-induced covalent crosslinking.
Shape-memory polymers (SMPs) are an innovative application of plastic memory, with a wide range of potential uses. SMPs are smart materials that can change from a temporary, deformed shape to their original, permanent shape when exposed to external stimuli such as heat, light, or an electric or magnetic field. This shape-memory effect (SME) is particularly useful in various industries, including medicine, robotics, and automotive.
In medicine, SMPs can be used for less invasive implants, tissue scaffolds, and medical devices. For example, SMP-based catheters can soften at body temperature, reducing the risk of tissue damage during surgical procedures. SMPs can also be used in robotics to provide a soft grip for object handling, and in the automotive industry for self-repairing car parts.
Additionally, SMPs have been proposed for use in aircraft, with the potential to morph during flight, and in smart fabrics, self-deployable sun sails for spacecraft, and self-disassembling mobile phones. The development of SMPs continues to expand the range of applications and improve existing products.
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Frequently asked questions
Plastic memory refers to a plastic part's ability to return to its original shape after being deformed.
A plastic's memory is determined by its ability to avoid permanent deformations. Plastics with excellent "creep" properties tend to have better memory properties.
Creep properties refer to a material's ability to withstand permanent deformation over time. Crystalline materials tend to have better creep properties due to their more rigid molecular structures.
Common crystalline plastics with good memory properties include PP, PE, Nylon, Acetal, and PBT.
Plastic memory is useful in various applications such as surgical sutures, compression garments, and self-repairing structural components. For example, a dented car fender made of shape-memory polymer can be repaired by heating it, allowing it to return to its original shape.
