A blowing agent or foaming agent is a chemical that can add to plastics and generate inert gases when heated. This blowing or expansion of gases causes the plastic to expand, thus forming a foam. The foamed structure can vary by the type of foaming agent used, the type of gas it gives off and its solubility, the compounding method used, the temperatures and pressures involved in processing, and the viscosity of the melt.
The blowing agents favor the decrease in density to make the finished products lighter. The expanders also improve the backwash and mold fill phenomena. Backwashing is a phenomenon that occurs during the molding phase and causes incomplete filling of the mold; as a result, the printed article appears “empty”. To solve this type of problem, blowing agents are used which, by releasing gas inside the mold, compress the molten material, favoring its filling in a homogeneous way. The use of these masters reduces the mechanical characteristics of the product.
There are basically two groups of blowing agents: physical blowing agents and chemical blowing agents.
Physical expansion agents
Physical blowing agents change from one form to another during processing. Compressed gases and volatile liquids are the two main types. Compressed gases can dissolve under pressure in the resin and produce a foam when the pressure is released. The use of nitrogen in injection molded foam products is common. Volatile liquids foam resin as they change from a liquid to a gaseous state at high processing temperatures. The main materials in this area are fluorinated aliphatic hydrocarbons (chlorofluoromethanes or chlorofluorocarbons). These blowing agents have been widely used in both rigid and flexible polyurethane foams. Flexible polyurethane foams are blown with water, chlorofluorocarbons, or methylene chloride.
Chemical expansion agents
Chemical blowing agents are solid compounds that decompose at processing temperatures to develop the gas that forms the cellular structure. Both open and closed cell structures are possible. Chemical blowing agents can be used in almost any thermoplastic or thermoset polymer, and can be organic or inorganic.
The factors that determine the formation of a fine-cell plastic foam with a regular structure are the particle size of the blowing agent, the dispersion properties of the plastic processing machine used, the decomposition rate of the blowing agent, and the melt viscosity of the resin being processed.
The most common inorganic chemical blowing agent is baking soda. Chemical blowing agents are primarily hydrazine derivatives. Chemical blowing agents can also be subdivided into two main categories: endothermic and exothermic.
Exothermic blowing agents release energy during decomposition, whereas endothermic blowing agents require energy during decomposition.
In general, endothermic chemical blowing agents generate carbon dioxide as the main gas. The commercially available exothermic types primarily produce nitrogen gas, sometimes in combination with other gases. Nitrogen is a more efficient expansion gas due to its slower diffusion rate through the polymer compared to carbon dioxide.
Chemical blowing agents can be supplied in powder form that can be drum blended with granule resins, dry blended with powdered resin and Banbury, as is the case with PVC or polyethylene for rotational molding, or blended by extrusion. Blowing agents are also available in liquid concentrates or in granules that can be more convenient to handle than powder. Liquid concentrates, typically 50% chemical blowing agent in a compatible inert carrier, are added during processing down the throat of the machine, usually with a pump system. Granule concentrates are also available for most resins.
Foaming agent Malaysia is used in plastics for several reasons: weight reduction, cost and material savings, and obtaining new properties; which include heat and noise insulation, different surface appearance, improved stiffness, better quality (elimination of bumps in injection molded parts) and better electrical properties.