Sofia Martinez
Blowing agents are additives used in the manufacture of foamed plastics. They are reactive agents that release controlled levels of gases during the processing of thermoplastics. Blowing agents are used to reduce the weight of plastics and are distinguished by their thermal insulation properties. They are used in a wide variety of applications, including refrigerators, building insulation, automobiles, furniture, and packaging. Blowing agents can be divided into chemical blowing agents and physical blowing agents. Chemical blowing agents are solid organic or inorganic compounds that release a propellant gas, usually nitrogen and/or carbon dioxide, at a specific processing temperature. Physical blowing agents, on the other hand, undergo a change of state during processing and are usually compressed gases or volatile liquids.
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What You'll Learn
- Blowing agents are added to plastics to create foam structures and reduce density
- They can be chemical or physical blowing agents, or a mix of both
- Chemical blowing agents are solid compounds that release gases at specific temperatures
- Physical blowing agents undergo a change of state, while chemical blowing agents decompose
- Blowing agents can be dangerous, especially exothermic blowing agents
Blowing agents are added to plastics to create foam structures and reduce density
Blowing agents are additives that are used to create foam structures in plastics and reduce their density. They are commonly used in plastics with a wall thickness of more than 10mm, such as polyethylene and polypropylene. By adding blowing agents, the weight of the plastic can be reduced, and the stiffness can be increased. This is achieved by creating a cellular or foamed structure within the plastic, which results in a reduction in specific gravity and, consequently, weight.
There are two main types of blowing agents: physical blowing agents (PBAs) and chemical blowing agents (CBAs). PBAs are compressed gases or volatile liquids that undergo a change of state during processing. Nitrogen is a commonly used PBA as it is inert, non-flammable, and can be utilised at any processing temperature. It is injected at high pressure into the plastic melt, and as the pressure is relieved, it expands to form cells. However, PBAs can result in a coarser cell structure and poorer surface appearance compared to CBAs.
CBAs, on the other hand, are typically solids that undergo a decomposition reaction to form a gas during processing. Examples of CBAs include isocyanate and water for polyurethane, azodicarbonamide for vinyl, and sodium bicarbonate for thermoplastic foams. CBAs can produce a finer cell structure than PBAs, and nucleating agents can be added to further enhance this. The use of CBAs helps to balance the thermal energy released and absorbed during the foaming process, minimising temperature rise and preventing thermal degradation of the plastic.
The amount of blowing agent used can vary depending on the specific application, ranging from 0.1wt% for eliminating sink marks in injection-moulded parts to 5-15% for compression-moulded foam products. Blowing agents can also be mixed with other materials to create hollow spheres and porous particles, further enhancing the foamed structure. Additionally, the type of blowing agent used can influence various properties of the plastic, including physical, mechanical, electrical, and thermal conductivity.
In recent years, there has been a shift towards using zero ozone-depleting hydrocarbon blowing agents, such as n-pentane, isopentane, and cyclopentane, due to environmental concerns. These agents have raised questions about their long-term viability due to their high global warming potential (GWP). Overall, blowing agents play a crucial role in the plastic industry by enabling the creation of lightweight, stiff, and insulating foam structures with improved properties.
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They can be chemical or physical blowing agents, or a mix of both
Blowing agents are reactive additives that release controlled levels of gases during the processing of thermoplastics. They are added to plastics to create foam structures, which can reduce the density of the plastic and improve its thermal and acoustic insulation properties. The type and amount of blowing agent used will influence the properties of the foamed plastic. For instance, about 0.1% of a blowing agent is used for eliminating sink marks in injection-moulded parts, while 5 to 15% is used for compression-moulded foam products.
Blowing agents can be chemical or physical blowing agents, or a mix of both. Physical blowing agents (PBAs) undergo a change of state during processing, while chemical blowing agents (CBAs) are usually solids that undergo a decomposition reaction to form a gas. PBAs are compressed gases or volatile liquids. Compressed gases, usually nitrogen, are injected at high pressure into the plastic melt during processing. As the pressure is relieved, the gas becomes less soluble in the plastic melt and expands to form cells. Nitrogen is inert, non-flammable, and can be used at any processing temperature. It also leaves no residue, making plastic recycling easier.
Chemical blowing agents, on the other hand, are solid organic or inorganic compounds that release a propellant gas, usually N2 and/or CO2, at a specific processing temperature. They decompose by heat and mechanical stress to produce blowing gases, which can be trapped within the polymer melt to create a cellular or foamed structure. Examples of chemical blowing agents include isocyanate and water for polyurethane, azodicarbonamide for vinyl, hydrazine and other nitrogen-based materials for thermoplastic and elastomeric foams, and sodium bicarbonate for thermoplastic foams.
Mixed physical/chemical blowing agents are used to produce flexible PU foams with very low densities. Both types of blowing agents are used together to balance each other out in terms of thermal energy released and absorbed, minimising temperature rise. For example, in polyurethane systems, isocyanate and water (which react to form carbon dioxide) are used alongside liquid carbon dioxide (which boils to become a gas) in the production of very low-density flexible PU foams for mattresses.
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Chemical blowing agents are solid compounds that release gases at specific temperatures
Blowing agents are substances that produce a cellular structure in materials that harden or undergo a phase transition, such as polymers, plastics, and metals. They are typically applied when the material is in a liquid state. The cellular structure in a matrix reduces density, increases thermal and acoustic insulation, and increases the relative stiffness of the original polymer.
Chemical blowing agents (CBAs) are solid compounds that undergo a decomposition reaction during processing, resulting in the formation of a gas. They are usually organic or inorganic compounds that release a propellant gas, such as nitrogen (N2) or carbon dioxide (CO2), at a specific processing temperature. The decomposition temperature should match the processing temperature of the polymer to be foamed. CBAs provide a controlled release of gas, enabling a more uniform cellular structure, which is critical for applications where mechanical strength and consistency are essential.
The type of blowing agent used influences the physical, mechanical, electrical, and thermal conductivity properties of the foamed plastic. For example, in polyethylene and polypropylene, blowing agents are used to reduce weight and cycle time. In engineering plastics, they can be used to achieve maximum weight reduction, greater stiffness, and control over warping, collapse, and shrinkage.
Some common chemical blowing agents include isocyanate and water for polyurethane, azodicarbonamide for vinyl, hydrazine and other nitrogen-based materials for thermoplastic and elastomeric foams, and sodium bicarbonate for thermoplastic foams. These agents undergo decomposition when heated, releasing gases like nitrogen, carbon dioxide, or water vapour.
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Physical blowing agents undergo a change of state, while chemical blowing agents decompose
Blowing agents are substances that act as a source of gas in the production of foamed materials. They are used to produce gas and generate cells or gas pockets in plastics. The two main types of blowing agents are physical blowing agents (PBAs) and chemical blowing agents (CBAs).
Physical blowing agents undergo a change of state during processing. They are compressed gases or volatile liquids. Compressed gases, usually nitrogen, are injected under high pressure into the plastic melt during processing. As the pressure is relieved, the gas becomes less soluble in the plastic melt and expands to form cells. Nitrogen is inert, nonflammable, and can be used at any processing temperature. No residue is left in the foamed plastic, so recycling the plastic part is easy. Examples of PBAs include carbon dioxide, nitrogen, butane, ethane, pentane, and isobutane.
On the other hand, chemical blowing agents (CBAs) undergo a decomposition reaction during processing that results in the formation of a gas. CBAs are usually solids that decompose at various processing temperatures to form a gas. The decomposition temperature should match the processing temperature of the polymer to be foamed. Examples of CBAs include azodicarbonamide, sodium bicarbonate, isocyanate, and water.
The choice between physical and chemical blowing agents can depend on various factors such as the desired properties of the final product, environmental impact, recyclability, and cost. For instance, physical blowing agents may have a lesser environmental impact compared to some chemical agents, and they do not leave a residue, making the recycling process easier.
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Blowing agents can be dangerous, especially exothermic blowing agents
Blowing agents are substances that produce a cellular structure in a matrix through a foaming process. They are used to reduce the density of plastics and increase their thermal and acoustic insulation. However, some blowing agents can be dangerous, particularly exothermic blowing agents.
Excessive exothermic heat from a physical blowing agent can cause thermal degradation of a developing thermoset or polyurethane material. This is why chemical and physical blowing agents are often used together to balance the thermal energy released and absorbed, minimising the temperature rise. For example, in the production of very low-density flexible PU foams for mattresses, isocyanate and water are used in combination with liquid carbon dioxide to avoid thermal degradation.
Ozone-depleting blowing agents, such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), have been banned due to their negative impact on the environment. While second-generation blowing agents like hydrochlorofluorocarbon (HCFC)-141b were introduced as replacements, they still contain chlorine atoms and act as ozone-depleting substances (ODSs). This led to the US Environmental Protection Administration (EPA) issuing a 'Use Ban' regulation on HCFC-141b in 2005.
Some blowing agents, such as tetramine and tetrazole, have limited use in plastics due to their high exotherm of decomposition and the unpleasant odour of the residue. Hydrocarbons, such as pentane, isopentane, and cyclopentane, are alternative blowing agents with zero ozone-depleting potential. However, their long-term viability is questionable due to their high global warming potential (GWP).
In summary, blowing agents, particularly exothermic blowing agents, can be dangerous due to their potential for thermal degradation, environmental impact, and decomposition byproducts. It is important to carefully select and use blowing agents to balance thermal energy and minimise negative consequences.
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Frequently asked questions
Blowing agents are additives used in the manufacture of foamed plastics. They are usually chemical or physical blowing agents that create fine and regular cellular structures during polymer processing.
Blowing agents are added to plastics to reduce weight and cost. They also provide thermal insulation.
Some examples of blowing agents include isocyanate, water, azodicarbonamide, sodium bicarbonate, nitrogen, and carbon dioxide.
Blowing agents release controlled levels of gases during the processing of plastics, which create a cellular or foamed structure. This structure reduces the density and weight of the final product.
One disadvantage of using blowing agents is that they can be dangerous to health and the environment if not handled properly. Additionally, they may require painting to hide visible traces on the surface of the product.
