Maya Ellison
Thermoplastics, or thermosoftening plastics, are plastic polymer materials that become pliable or moldable at high temperatures and solidify upon cooling. This is because the polymer chains are associated by intermolecular forces, which weaken rapidly with increased temperature, yielding a viscous liquid. Therefore, warm plastic is softer than cold plastic. At low temperatures, amorphous or non-crystalline thermoplastics are stiff and glassy, but they soften when warmed up. The glass transition temperature is the temperature interval below which a polymer behaves like glass and above which it becomes flexible or elastic. Polyethylene, polypropylene, and polystyrene are examples of commonly used thermoplastics.
| Characteristics | Values |
|---|---|
| Plastic temperature | Above the glass transition temperature (Tg), plastic is soft and pliable. Below Tg, it is hard and brittle. |
| Plastic molding | Plastic becomes easier to mold and shape when heated. |
| Plastic melting | When heated enough, plastic melts and can be poured, stretched, injected, or blown into molded shapes. |
| Plastic deformation | Plastic resists deformation better than elastomers but can be stretched and molded. |
| Soft-feel coatings | PU, silicone rubber (SR), and polyacrylic acid (PAA) coatings can be applied to plastic to create a warm, soft, and matte finish. |
| Vulcanization | The vulcanization process for natural rubber uses sulfur to cross-link chains and produce a more thermoset material. |
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What You'll Learn
Thermoplastics become softer when heated
The word "plastic" means pliable, and plastics are materials that can be easily shaped and moulded. They become easier to mould or shape when they are hot, and they melt when they get hot enough. This is why plastics are called thermoplastics. Thermoplastics are materials that can be melted and remoulded by heating.
The glass transition temperature is the most important factor in determining the physical properties of amorphous thermoplastics. This is because the state of a thermoplastic depends on the temperature and the time allowed to measure its physical properties. At room temperature, some plastics are below their Tg and are therefore hard, while others are above their Tg and are soft.
Additives can also be used to make plastics softer and more pliable. However, it is important to note that while plastics become softer and easier to deform when heated, they still require a significant amount of energy to stretch or deform compared to other materials like elastomers or fibres. This makes plastics useful in applications where resistance to deformation is desired.
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Glass transition temperature (Tg)
The glass transition temperature (Tg) is a property of amorphous polymers or amorphous regions of semi-crystalline polymers. It is the temperature at which these materials transition from a hard and brittle state to a soft and rubbery state. This process is known as the glass transition. The Tg is different for each plastic, and it is always lower than the melting temperature of the crystalline state of the material. For example, the Tg of polystyrene and poly(methyl methacrylate) is around 100 °C (212 °F).
Above the Tg, polymers behave like rubbery materials, and their molecular mobility increases. Below the Tg, polymers are relatively rigid, and their molecules have little mobility. This transition is reversible, and the Tg value depends on various factors such as the strain rate, cooling or heating rate, and testing conditions. The glass transition temperature is an important consideration when designing plastic parts, as it affects the material's strength and capabilities in a given application.
The glass transition can be understood through recent models that describe the behaviour of the liquid matrix that makes up the polymer. As the temperature decreases, the fluctuating input of thermal energy causes disturbances in the oscillations of the matrix, creating temporary cavities ("free volume") between the elements. The size of these cavities depends on the temperature, and as the temperature approaches Tg, the molecular matrix becomes more inert, delaying the setting of thermal equilibrium.
Techniques such as dynamic mechanical analysis and differential scanning calorimetry (DSC) can be used to measure the glass transition temperature. However, it is important to note that the definition of the glass transition and the glassy state is still a subject of ongoing research, with various models and definitions proposed over the years.
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Plastic is pliable when heated
The word "plastic" means pliable, referring to materials that can be easily shaped and moulded. All plastics have a certain temperature, known as the glass transition temperature or Tg, above which they are soft and pliable, and below which they are hard and brittle. This temperature range is called the glass transition temperature region. At low temperatures, thermoplastics are stiff and glassy, but when heated, they soften and become easier to mould and shape. This is because heat weakens the molecules that make up the material.
The Tg varies for different plastics, and some plastics are naturally above or below their Tg at room temperature. Additives can also be included to make plastic softer and more pliable. For example, the plastic around electronic cables is typically soft, while the plastic keys on a keyboard are hard.
The process of softening plastic with heat is utilised in the creation of products like drinking glasses and toys, where plastic is heated and then poured, stretched, injected, or blown into moulded shapes.
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Additives can make plastic softer
The word "plastic" means pliable, and plastics are materials that can be easily shaped and moulded. They become even easier to mould and shape when heated, and they melt when they get hot enough. This is why plastics are called thermoplastics.
Additives can be used to make plastic softer. Additives are added to polymers to modify and improve their properties. Plasticizers are one such additive, and they are used to lower the glass transition temperature of a polymer, making it softer. Blowing agents are another additive that forms gases in the plastic when heated, producing a foam material that is softer than the original plastic. Lubricants are yet another additive that can make plastic softer by reducing friction and making parts smoother so they slide easily.
Other additives can be used to improve the strength, appearance, safety, and degradability of plastics. For example, fillers or extenders are usually mineral-based additives that increase the overall bulk of the plastic while lowering its cost. Light and heat stabilizers are used to make plastic parts resistant to heat and light, which can cause them to wear down over time. Anti-fog additives, such as optical bleach, are used to improve the appearance of plastic products by reducing discoloration and making colours brighter and more vibrant. Anti-stick additives are added to packaging to prevent clumping, which can make it difficult to open packages and insert products.
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Plastic resists deformation better than elastomers
The word "plastic" means pliable, and plastics are materials that can be shaped and moulded easily. They become easier to mould and shape when heated, and they melt when they get hot enough. This is why plastics are called "thermoplastics". However, it takes a lot of energy to deform plastics into shapes, and they resist deformation better than elastomers.
Elastomers are a subset of polymers with specific elastic properties. Polymers are organic compounds formed by many smaller subsets or monomers. Elastomers are typically considered amorphous, lacking a crystalline or ordered structure. They are flexible and can undergo reversible deformation, returning to their original shape and size after exposure to extreme stress or other forces. This is due to their unorganized cross-linked structure. Elastomers are less reactive than other elastic materials, with their reactivity limited to certain conditions and circumstances.
Plastics, on the other hand, can be more varied in their chemical format than elastomers. They can be polymers that are sintered but lack chemical cross-links, or they can be cross-linked materials that are below the glass-transition point and do not behave as elastomers. The properties of plastics can change substantially at transition points, affecting characteristics such as extrusion resistance. Some plastics offer high strength and can compete with metals on a strength-to-weight basis, making them attractive for structural components.
Plastics are also more resistant to harsh environments than elastomers, which can deteriorate over time when exposed to harsh conditions or mechanical stress beyond their limits. Elastomers are more flexible at low temperatures, but plastics can be used across a wider temperature range, including both higher and lower temperatures than elastomers. Additionally, some plastics, such as PTFE, have a very wide chemical resistance, although this varies depending on the individual material.
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Frequently asked questions
Plastic is a pliable material, meaning it can be shaped and moulded easily. When heated, plastic becomes softer and easier to mould and shape. This is because the intermolecular forces that hold the polymer chains together weaken with increased temperature, turning the plastic into a viscous liquid.
The glass transition temperature is the temperature range at which amorphous thermoplastics become softer and less stiff. Below this temperature, thermoplastics are stiff and glassy.
A thermoplastic is a plastic polymer material that becomes pliable or mouldable at elevated temperatures and solidifies upon cooling. Thermoplastics differ from thermosetting polymers, which do not melt when heated.
Polyethylene is a common thermoplastic that is used in a variety of applications, such as milk jugs, laundry detergent bottles, and artificial joints. It is flexible at room and low temperatures and can be heat-sealed.
