Blowing Agent

Foaming agents, also known as blowing agents, can reduce the surface tension of the liquid and produce a large amount of uniform and stable foam by mixing with pressurized air. Ingredients of a foaming agent include surfactants, foam stabilizers, and so on. The molecules of surfactants contain two parts, hydrophilic cluster and hydrophobic cluster, which tend to accumulate at the interface between liquid and gas in the solution, forming a thin molecular film to reduce the surface tension of the liquid and make the solution capable of foaming.

The main function of the foam stabilizer is to slow down the dissipation of the foam and stabilize the foam. According to the effect, foaming agents can be categorized as general-purpose foaming agents or dispersing foaming agents. General-purpose foaming agents are used mainly for soils with low clay content, and dispersing foaming agents are used mainly for soils with high clay content.

The foaming agent is formulated into a solution based on a certain concentration, and a large amount of foam can be generated through foam generators. The generated foams, once mixed with the soil, can improve the performance of the soil for shield tunnelling.

The conditioning effects of foams on the soil are mainly reflected in the following aspects: The foams work as lubricating agents after being injected into the soil, can significantly reduce the internal friction angle and improve the workability of the soil, facilitate shield soil removal, effectively establish the soil pressure in the excavation chamber to balance earth and water pressure on the excavation surface, and reduce the energy consumption of the shield machine.

Because the foams fill the pores between the soil particles, the impermeability of the conditioned soil can be significantly improved. The foam-conditioned soil can form a cushion layer in the excavation chamber, which is similar to an impervious but compressible “sponge,” resulting in improved compressibility of the soil.

When the muck discharging rate by screw conveyor changes suddenly, the sensitivity of the soil pressure in excavation chamber is reduced, owing to the cushion effect, and it helps to maintain the stability of the excavation face . A dispersive foaming agent can also reduce the adhesion between the particles, thereby preventing flocculation or agglomeration of the soil and reducing the possibility of muck clogging on the cutterhead in the soil chamber.

Chemical foaming agents (alias thermal decomposable foaming agents) are a class of foaming agents that are solids or liquids at room temperature and, on heating, decompose at a particular temperature, releasing a gas such as nitrogen, carbon dioxide, or carbon monoxide.

Typically, they are used by mixing a chemical foaming agent with a solid polymer and heating the mixture while maintaining the mix under pressure. Heating fluidizes the polymer and causes the foaming agent to release a gas through a chemical reaction. The gas forms bubbles or cells in the fluid polymer. The mixture is maintained under pressure until it is placed into a mold where the gas expands, and in turn causes the fluid polymer to expand.

On solidifying, the cell structure remains. Different foaming agents release gas at different temperatures, and the choice of foaming agent depends on the processing conditions, type of polymer, and the size of the desired gas cells. Chemical foaming agents have the advantages over physical (gaseous) foaming agents in that they can be added to a solid polymer before heating. Physical foaming agents, such as nitrogen and some fluorocarbons, must be injected into the already fluidized polymer.