The so-called chemical blowing agent refers to those substances that generate gas through chemical changes during the foaming process and then foam. There are generally two ways to generate gas: one is by-products of polymer chain extension or crosslinking; the other is by adding Chemical Blowing Agent to generate foaming gas.
For example, when preparing polyurethane foam, when the alkyd resin with carboxyl groups reacts with isocyanate, or when the polyurethane resin with isocyanate end groups reacts with water, CO2 gas will be released; ammonium bicarbonate can be used at a certain temperature Decomposition produces CO2, H2O and NH3.
Ideal Chemical Foaming Agents should have the following properties:
☑ The thermal decomposition temperature is constant or within a narrow range;
☑ The rate of the thermal decomposition reaction must be controllable and there must be an adequate rate of gas generation;
☑ The gas produced must be non-corrosive and easy to disperse or dissolve in the polymer system;
☑ must be stable in storage;
☑ The price is cheap and the source is sufficient;
☑ The decomposition residue should have no bad smell, low toxicity, colorless, and no pollution to polymeric materials;
☑ Decomposition should not generate significant heat:
☑ Does not affect vulcanization or melting rate;
☑ The decomposition residue does not affect the physical and chemical properties of the polymeric material; the decomposition residue should be compatible with the polymeric material, and no blooming of the residue occurs.
In addition, blowing agents can also be divided into two categories according to their chemical properties: organic blowing agents and inorganic blowing agents.
Among them, the common organic blowing agents mainly include azo (foaming agent AC, AIBN, etc.), hydrazine derivatives (BSH, TSH, OBSH Blowing Agent, etc.), semicarbazide (TSSC), tetrazole compounds (5- PT, etc.), nitroso compounds (DNPT, etc.), etc.
Common inorganic blowing agents include ammonium carbonate, ammonium bicarbonate, sodium bicarbonate, potassium bicarbonate, ammonium nitrite, potassium borohydride, sodium borohydride, hydrogen peroxide, etc.
Inorganic foaming agent has the advantages of safety, endothermic decomposition, and good nucleation effect, and the gas produced is CO2. Sodium bicarbonate is a typical inorganic endothermic foaming agent. It has been valued because of its large gas generation, easy availability of raw materials, and low price. However, its decomposition temperature is low and its decomposition temperature range is wide, and it will decompose and lose early in the plasticizing process, resulting in poor plasticizing effect, and its application range in polymers is limited. The exothermic foaming agent is mixed with sodium bicarbonate to obtain a composite foaming agent with a small calorific value, which is beneficial to the control of plastic processing conditions. The decomposition rate of ammonium carbonate is slow, which greatly limits the foaming rate.
Due to the poor dispersion in polymers, the application of inorganic blowing agents is limited to a certain extent, but with the advancement of technologies such as miniaturization and surface treatment, the application fields of inorganic blowing agents are gradually expanding.