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Brief introduction of silicone rubber foam material and process


Brief introduction of silicone rubber foam material and process

1 Overview
The preparation of silicone rubber foam material is mainly obtained by adding appropriate foaming agent to the mixed silicone rubber, foaming and vulcanizing by heating. The key to the preparation of silicone rubber foaming materials is that the foaming rate and the vulcanization rate are basically the same. In the process of silicone rubber foaming, the internal pressure is often greater than the external pressure. In addition to the rational selection of the foaming agent formula which is the main factor to control the foaming effect, the foaming process and mold equipment are also the key to the preparation of the silicone rubber foaming material. In the early stage of the foaming process, the gas causes the green body to rise and form a porous structure, which expands until the shape is equal to the mold cavity. During the vulcanization process, controlling the gas volume and rate has a great influence on the uniformity of the cells and the shape control of the product.

2 Classification of foaming agents
According to the different gas production methods in the foaming process of the foaming agent, it is divided into physical foaming agent and chemical foaming agent. The physical foaming agent changes the physical form of the foaming agent under certain conditions to form bubbles; the chemical foaming agent undergoes a chemical decomposition reaction to generate one or more gases, thereby foaming the polymer matrix. Among them, the thermal decomposition type organic chemical foaming agent occupies an important position in the production of rubber plastic foam products.

1) Physical blowing agent
Physical blowing agents can also be called volatile blowing agents, including inert compressed gases, soluble solids, and volatile liquids with a boiling point below 110°C. Inert compressed gas is mostly suitable for plasticizing and molding gas foaming. Under pressure, the inert gas is absorbed by the matrix, and then expands under normal pressure to foam the material. The conventional physical blowing agent is a low boiling point aliphatic hydrocarbon, called petroleum ether in industry, non-toxic, cheap, but flammable. The development and research of halogenated hydrocarbons, butane, pentane, hexane and other gases and effective substitutes such as chlorine, CO2, water, and air are active.

2) Chemical blowing agent
Chemical blowing agents include inorganic blowing agents represented by bicarbonate and organic blowing agents represented by azodicarbonamide (AC) and dinitrosopentamethyltetramine (blowing agent H), Usually refers to a thermally decomposable chemical foaming agent with powdery characteristics. Chemical foaming agents have extremely strong specificity and a wide range of categories, reflecting the current situation of the foaming agent industry to a certain extent.

3 Choice of chemical blowing agent

1) Inorganic foaming agent
Inorganic blowing agents are the earliest chemical blowing agents used. Among them, the bicarbonate foaming agent has the characteristics of endothermic decomposition, good nucleation effect, safety, and non-toxic CO2 as the gas generated. However, due to its poor dispersibility in polymers and poor storage stability, its applicability is limited. However, as a bubble nucleating agent, inorganic foaming agents play an important role in stabilizing and miniaturizing bubbles. In recent years, with the progress of surface treatment, nano-granulation and other technologies, the application fields of inorganic foaming agents have been broadened.
2) Organic foaming agent
The development of organic foaming agents is relatively late, but the development speed is fast, and the current market share is quite high. In the development process of organic chemical foaming agents for more than half a century, more than a thousand chemicals have been explored, and only a dozen chemical structures have been verified by experiments and have been widely used, especially azodicarbonamide (foaming Agent AC), dinitrosopentamethylmethyltetraamine (blowing agent H) are the most common. Organic chemical blowing agents have the following characteristics:
a. Good dispersibility in the polymer, fine and uniform bubbles;
b. The decomposition temperature range is narrow and easy to control;
c. It mainly releases N2. Because N2 diffuses in the polymer at a low speed, it is not easy to escape from the foam, so the foaming efficiency is high and the foaming quality is good;
d. The organic foaming agent is an exothermic foaming agent, which decomposes sharply when reaching a certain temperature, and the amount of gas generation is relatively stable. The relationship between the amount of foaming agent and the expansion ratio can be calculated. When the heat release is too large, it is easy to cause the internal temperature of the product to far exceed the external temperature, which affects the application performance of the product and facilitates the processing of thicker products;
e. Organic chemical foaming agents are mostly flammable materials, so please pay attention to safety when using them.
Organic foaming agents also have shortcomings: less gas generation, only 15% to 30% of inorganic foaming agents, large amount of residue, sometimes smelly and surface blooming; high heat of decomposition, easy to cause core burning; flammable .
3) Foaming aid
The foaming aid can effectively reduce the decomposition temperature of the foaming agent, stabilize the gassing process, and reduce the generation of malodorous gas. Commonly used co-foaming agents in industry include the following categories:
a. Urea derivatives and amino compounds, such as urea, ethanolamine;
b. Organic acids such as stearic acid, lauric acid, benzoic acid, salicylic acid;
c. Compounds of alkaline earth metals, zinc oxide, lead oxide;
d. Polyols such as glycerol;
e. Silicone, such as polysiloxane-polyalkoxy ether copolymer (foaming spirit).
In order to improve the poor dispersion and moisture absorption of urea derivatives in polymers, glycols, fatty oils, metal soaps and inorganic salts are often used in combination. In addition, the urea should be fully ground before being added to the rubber compound to ensure its uniform dispersion in the rubber compound. The ground urea should not be exposed to the air for too long, so as not to absorb the moisture in the air and bring it into the rubber compound. Rubber performance.

4 Silicone rubber foam forming process

1) Compression molding
The compression molding method is a long-standing method for simultaneous shaping and vulcanization in the production of silicone rubber products. That is, the blank is placed in the mold cavity, the mold is accurately clamped, and it is placed between the plate vulcanizer to continue pressure and heating under certain conditions. After the vulcanization is completed, the mold is opened and trimmed and then the product is obtained by secondary vulcanization. During the operation, the speed of mold closing should be controlled so that the rubber material in the mold cavity can be fully filled. For thick-walled products, dicumyl peroxide (DCP) and double 2,5 (DBPMH) and other vulcanizing agents with higher decomposition temperature are commonly used. The molding shrinkage rate of silicone rubber is 2% to 4%, and its linear expansion coefficient is 17 times that of iron; coupled with the influence of vulcanized volatile matter, its shrinkage rate is very large. It should be fully considered when designing the mold. The mold material is generally hard Stainless steel is chrome plated.
Molded vulcanized rubber products have been applied earlier. Due to the low thermal conductivity of silicone rubber relative to thermally conductive materials, the foaming and vulcanization of silicone rubber is prone to uneven foaming and cracking. A well-designed silicone rubber foam molding die plays a key role in the control of the cell structure.
2) Transfer molding
The transfer compression molding method is an improved compression molding method, also known as the shift mold method. The mold used for transfer molding is different from the ordinary mold. There is an injection chamber with a piston on its upper part. The unvulcanized blank is placed in the injection chamber, and the plunger is pushed by a press to make the blank enter the cavity through the gate for vulcanization. Since the injection chamber and the mold are respectively fixed on the press, the same injection chamber can be used to inject multiple molds. The advantage of this method is that the product has high density, the curing time is shorter than that of the ordinary molding method, and it is easy to manufacture products with complex shapes.

3) Injection molding
The injection molding method is a method in which the mixed silicone rubber is uniformly plasticized in a rubber barrel, and then pushed into the cavity of a closed mold by a plunger or a movable screw through a nozzle, a main flow channel, and a branch flow channel. This method is very suitable for the manufacture of complex shapes and the forming of insert products. The temperature of the barrel is generally controlled at room temperature, but it can be raised to 80°C to 90°C according to the type of vulcanizing agent; the forming time can be adjusted according to the shape of the part and other factors, generally controlled at 30s to 90s.
4) Extrusion
The extrusion molding method, also known as the extrusion molding method, is a method in which mixed silicone rubber is continuously formed through a die of a certain shape under the extrusion of the screw of the extruder. Mixed silicone rubber is generally retrained on an open mill first, or it can be extruded with a cold feed extruder. The extrusion process of silicone rubber is widely used, and the extrusion products of hoses, wire coatings, cable sheaths and various cross-sections are all formed by this method. Extrusion molding pay attention to: the expansion of the product after extrusion from the die and the deformation before vulcanization; in addition, the extruder should ensure that the cooling is controlled below 50 ℃ to prevent the scorching of the rubber and the failure of the vulcanizing agent; the rubber is being extruded Cannot stay while leaving the plane. The main vulcanization methods used in extrusion molding are air vulcanization (HAV), hot bath vulcanization (HLV), and vapor vulcanization (CA).

5) Secondary vulcanization
Secondary vulcanization is post-vulcanization. Silicone rubber generally needs to be vulcanized after vulcanization. Its main purpose is: 1. Remove the residual vulcanizing agent decomposition products in the product; 2. Give the product thermal stability during use. The secondary vulcanization is generally carried out in a hot-air circulating oven, and boiling treatment can also be used to achieve the above purpose according to the application. Generally, the secondary vulcanization is carried out by a temperature-programmed method.

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