Foaming agent is a kind of material that can make the target material into pores. It can be divided intoChemical foaming agentandPhysical foaming agentandsurface active agent Three categories.The chemical foaming agent can release carbon dioxide, nitrogen and other gases after being heated and decomposedpolymerCompounds forming pores in composition;Physical foaming agent is a compound formed by the change of physical form of a certain substance through expansion of compressed gas, volatilization of liquid or dissolution of solid.The foaming agents all have high surface activity, which can effectively reduce the surface tension of the liquid, and arrange the double electronic layers on the surface of the liquid film to surround the air, forming bubbles, and then forming a foam from a single bubble.[1]
Chinese name
foaming agent
Classification
Chemical foaming agent, physical foaming agent, surfactant
Organic foaming agents mainly include the following categories:
1. Azo compound;
2. Sulfonyl hydrazides
3. Nitroso compound[2]。
Inorganic foaming agents mainly include the following categories:
Carbonate: an inorganic foaming agent, mainly usedcalcium carbonate、Magnesium carbonate、sodium bicarbonate。Sodium bicarbonate is a white powder with a specific gravity of 2.16.Decomposition temperature is about 100-140 ℃, and part of CO is releasedtwo, lose all CO at 270 ℃two。Soluble in water but insoluble in alcohol.
water glass: sodium silicate, when mixed with glass powder and heated to about 850 ℃, reacts with the glass and emits a large amount of gas, while strengthening the compressive and tensile strength of the material, which is mainly used as a foaming agent for preparing foam glass.
silicon carbide: At present, the main foaming agent used in the production of foam glass industry emits a lot of gas when sintered at 800-900 ℃.
carbon black: It is also a very useful foaming agent, which emits CO by heatingtwoThe foaming effect is good, but the disadvantage is that the price is high.
Common physical foaming agents have low boiling pointsalkaneandFluorocarbon。The physical foaming agent is a compound physical foaming agent, which not only has the advantages of high foaming times, good foam stability, low bleeding, but also can play a certain role in modifying magnesite cement and reducing the probability of halogen return and frost bleeding of products.Physical foaming agents have been widely used in the production of foamed magnesite products, such as fireproof boards and lightweight partition boards, and play a key role in the success or failure of some products (magnesite fireproof door cores, etc.).
Physical and chemical performance indexes of physical foaming agent Magarto:
When the anionic surfactant aqueous solution is introduced into the air by mechanical force, a large amount of foam will be producedPaper faced gypsum board、Foamed concreteA large number of applications in the field。
The essence of foaming agent is its surface activity.Without surface activity, it can not be foamed and can not become a foaming agent. Surface activity is the core of foaming.
Broad sense foaming agent and narrow sense foaming agent
Foaming agent has two concepts: broad sense and narrow sense.There are some differences between the two concepts, which can distinguish between non applied foaming agents and applied foaming agents.
① In a broad sense, foaming agents refer to all surfactants or surfactants whose aqueous solution can produce a large amount of foam when air is introduced.Because most surfactants and surfactants have a large amount of foaming ability, the generalized foaming agent includes most surfactants and surfactants.Therefore, the generalized foaming agent has a wide range and many kinds, and its performance and quality vary greatly, with a very wide range of selectivity.
There are no strict requirements for the technical performance of the generalized foaming agent, such as foaming ratio (foaming capacity), foam stability (usability), which only means that it has a certain ability to produce a large number of foam, and whether the foam produced can be used in practice is not defined。
② Foaming agents in a narrow sense refer to those that can not only produce a large amount of foam, but also have excellent foam performance, which can meet the technical requirements for foaming of various products, and can really be used to produce actual surfactants or surfactants.The biggest difference between it and generalized foaming agent is its application value, which is reflected by its excellent performance.Its excellent performance is shown by its strong foaming ability, large foam yield per unit volume, very stable foam, long time non defoaming, fine foam, good compatibility with the use medium, etc。The foaming agent in the narrow sense is the foaming agent used in industry, and the foaming agent commonly referred to refers to this kind of foaming agent in the narrow sense.Only the foaming agent in a narrow sense has the value of research and development.Foam concrete foaming agent belongs to the narrow sense of foaming agent.
Advantages and disadvantages
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Botanic
The vegetable cement foaming agent is made from imported high-quality natural plant macromolecule materials through a series of complex chemical reactions, and the product is a pure, transparent, light yellow oily liquid[3]。It is insensitive to hard water, non-toxic, tasteless, and sediment free.The pH value is close to neutral, which is non corrosive to cement and metal, and does not pollute the environment.The foaming agent is resistant to storage, not easy to deteriorate, has excellent foaming ability, rich foam, and moderate foam stability.The foaming agent has the characteristics of low consumption, low cost and stable quality. Each barrel of foaming agent (200kg) can produce about 500 cubic meters of foaming cement, so it is widely used in floor heating and roof insulation projects, which can significantly reduce the construction cost.
Disadvantages: 1. Because the internal pores of the foamed cement made with the foaming agent are connected, the foamed cement has good air permeability and poor permeability resistance, and cannot be used in water seepage resistance projects.
2. Under the same density, compared with foamed cement made of animal foaming agent, its thermal insulation performance and compressive strength are slightly damaged, so its application in some projects with special requirements is limited.
3. Due to the restriction of foam stability of vegetable foaming agent, the thickness of foamed cement poured at one time is generally not more than 200mm, otherwise surface collapse is likely to occur.
Animality
Animal cement foaming agent is made from selected animal (cattle, sheep) keratin as the main raw material through a series of hydrolysis reactions, heating, dissolution, dilution, filtration and high-temperature shrinkage[3]。The foaming equipment and manufacturing process are adopted for production, and strict quality management is carried out in the production process. The product is dark brown viscous liquid, with low impurity content, light pungent smell, uniform quality, good quality consistency, good foaming property and excellent foam stability. It is a new generation of animal cement foaming agent.Because the foam surface strength produced by the foaming agent is very high and the foam is extremely stable, the bubbles of the foamed cement produced by the foaming agent are in an independent closed state, and the impermeability of the foamed cement products that are not connected between bubbles is very good.
1. At the same density, compared with foamed cement made of vegetable foaming agent, it has better sealing and heat preservation properties and high strength.
2. Because of its excellent foam stability, the thickness of its one-time pouring foam cement can reach more than 1.5 meters without collapsing, so it is suitable for the production of foam bricks, lightweight panels, and lightweight walls.
Disadvantages: 1. Due to the complex production process, the product cost is high.
2. The foaming property of the foaming agent is not as good as that of the vegetable foaming agent, and the consumption of foaming cement is relatively high.
3. It has a certain pungent smell and can disappear only after being made into foamed cement.
Select key points
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The problems of organic foaming agent are: (1) the gas generation is small, generally only 15% - 30% of that of inorganic foaming agent, and the residue is large, including water and volatile substances, sometimes with odor and surface frost;(2) The decomposition heat of foaming is large and difficult to remove, especially when the thickness of the product wall is large, the internal plastic will overheat and decompose, reducing the performance;(3) Foaming agents are almost flammable, with low decomposition temperature. Care should be taken in storage and use.
The ideal chemical foaming agent shall meet the following requirements:
1. The temperature range of gas generated by decomposition should be narrow
2. The gas release speed should be fast and the gas generation rate should be controllable
3. The released gas should be non-toxic, non corrosive, and non combustible. It is better to use nitrogen than carbon dioxide. You can also try nitrogen dioxide, but the nitrogen dioxide gas is corrosive, and it is better not to use it
4. The chemical foaming agent should be easy to disperse evenly, and it is best to mix with the plastic.
5. Low price, stable storage, non-toxic.
6. The decomposed residue shall be non-toxic, colorless, odorless, and will not affect the physical, chemical and mechanical properties of the plastic. It is chemically inert and will not affect the melting rate and curing of the plastic.
7. It shall not release a lot of heat during decomposition, which will not affect the melting and curing of plastics.
8. The decomposed residue has good compatibility with the resin, and there is no residue frosting or dialysis.
9. The decomposition temperature of foaming agent shall be consistent with the melting temperature of resin.
The above are the key points of foaming agent selection, which shall be selected according to the required conditions。
Type selection
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Foamed magnesite cement is different from ordinary magnesite cement, and its bulk density is generally controlled at 500kg/mthreeBelow, some products, such as fire door core plate, require lower unit weight, which is generally controlled at 250~340kg/mthreeabout.Therefore, foamed magnesite cement has high requirements for foaming agent.There are many kinds of foaming agents on the market, and the quality is uneven, leading to the producers often at a loss in choosing foaming agents.The selection of foaming agent in foamed magnesite cement is briefly introduced.
Fully understand the types of foaming agents: different types of foaming agents have different foaming power and foam stability, so there are certain limitations in the use process.For example, the foam stability and foaming ratio of rosin foaming agent are not very good, so it can only be used for bulk density greater than 600kg/mthreeMore than 500 kg/m ³ of foamed magnesite cement can not be used for low-density foamed magnesite cement;Anionic synthetic surfactant foaming agent has fast foaming, large foam volume, poor foam stability, and low density (500 kg/mthreeThe following) foamed magnesite products are difficult to produce;The foaming power and foam stability of protein foaming agents are relatively good, but the foam viscosity produced is large, which is not conducive to mechanized production;Compound foaming agents have good foaming power and foam stability, but the specific technical indicators of foaming agents are also different.
By adding some fillers to the plastic, some properties of the plastic can be improved, which is more suitable for some special purposes.In order to reduce the density and hardness of the plastic, or enhance its heat insulation or sound insulation, the most ideal filler is the gap.Plastics containing voids or bubbles are classified as foamed plastics.With the degree of foaming, that is, the difference in the volume fraction of foam caused by voids, the performance of foamed plastics may be quite different from that of basic plastics.Foaming agent is a kind of chemical that can be added to the plastic. At the proper time of processing, it will release gas to form bubbles in the plastic.
The formation of plastic foam can generally be divided into four stages.In the first stage, the foaming agent must be completely and evenly dispersed in the polymer, which is usually in a liquid or molten state.At this time, the foaming agent can form a real solution in the polymer, or it can only be evenly dispersed in the polymer to form a two-phase system.In the second stage, after a large number of single bubbles are formed, the system is transformed into a system where gas is dispersed in the liquid.At this time, nucleating agent is often added to promote the formation of a large number of small bubbles.Nucleating agents are usually very fine inert particles, which provide sites for the formation of new gas phase.
In the third stage, the initially formed cell is expanding, because more gas diffuses and enters the cell through the polymer.If this time is long enough, the individual bubbles will contact each other.If the wall separating a single cell breaks, a larger cell will be formed by this coalescence.If the foam is mainly formed through the interconnection of bubbles, it is called open cell foam.If the foam is formed by disconnected bubbles, it is called closed cell foam.If bubble coalescence is allowed to proceed without limitation, the foam will collapse, because all gases are automatically separated from the polymer.
In the fourth stage, when the viscosity of the polymer increases and the cell can no longer grow, the foam will stabilize.The polymer viscosity can be increased by cooling, cross-linking or other methods.In terms of time, the last three stages of the foaming process can be as short as a fraction of a second, and the longest is not more than a few seconds.The formation of foam requires the polymer to be liquid.For this purpose, the polymer can be dissolved or plasticized by heating.The production process of foamed plastics is almost the same as any ordinary plastic production process, usually through extrusion, rotational molding and injection molding, as well as plastisol processing and thermoforming.For the same reason, basically any kind of plastic can be made into foam.Polyvinyl chloride (both hard and soft), polystyrene, polypropylene, ABS and polyethylene have been made into foam plastics on an industrial scale.The same is true for heat-resistant engineering plastics and thermosetting polymers.
Development prospect
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At present, the most common and easy to use common physical foaming agent is the foam stabilizing agent which is made from the combination of anionic surfactant and foam stabilizing products, or the excellent compatibility of animal and plant surfactants. The anionic surfactant takes fatty alcohol polyoxyethylene ether sodium sulfate, sodium dodecyl sulfate, etc. as the mainstream, and the foam stabilizing system is mainly silicone polyether emulsion.
