Foaming process
DATE：2022/7/5 8:45:34 / READ： / SOURCE：This station
Foaming process
Foaming process can be described as dispersing a gas in a flame, and then cooling and shaping the polymer to obtain products with bubble structure. Coating composite production line www.handern com① There are many ways to dissolve gas into the melt, such as injecting a low boiling point liquid. Convert it into gas by raising the temperature or produce gas by chemical reaction. Coating composite production line www.handern com
② Bubble expansion, bubble diffusion or bubble mixing will increase the bubble size, so that the foam structure will expand. If the cellular membrane remains intact, a closed cell foam will be formed; If the bubble membrane breaks, foam with open cell structure will be formed.
③ The formation of bubbles can be described by two methods: self nucleation and nucleation. Self nucleation occurs when the system is a single liquid, and nucleation occurs when the system is two-phase. Coating composite production line www.handern com
④ The success of foaming process and foaming products is affected by the viscosity and viscoelasticity of melt, the gas permeability of polyolefin and the properties of foaming agent (such as steam pressure, solubility and decomposition temperature). Coating composite production line www.handern com
⑤ The force of the machine head decreases, and the dissolved gas separates from the melt to form bubbles. The bubble expands continuously until the equilibrium between steam pressure, surface tension, melt viscosity and the saturation degree of dissolved gas is reached. If the melt still contains gas, the nucleation will continue, causing the bubble to continue to grow, leading to the surface fracture of the extrusion.
Polyolefin foaming can form semi-rigid foam. In most cases, people want to get foamed plastics with closed cell structure. The closed cell foam products can be used in packaging and other fields requiring products to be elastic or resilient. Foamed PP has many properties of non foamed PP, such as the hardness of homopolymer, impact strength of copolymer and general chemical resistance. The density of foamed plastics is also an important parameter to determine its physical properties. Coating composite production line www.handern com
Generally speaking, foaming methods are divided into physical foaming and chemical foaming. The selected foaming agent determines the degree of foaming, that is, the product density and the types of equipment that can be used. Coating composite production line www.handern com
Physical foaming physical foaming is to inject a volatile liquid such as pentane, methyl chloride or chlorofluorocarbon into the polymer melt under a certain pressure. When the melt passes through the machine head, the pressure drops, the liquid vaporizes, and the melt expands to form foam.
The physical foaming of polyolefin requires special equipment, and the use of gas to form foam is its own disadvantage. Pentane and other hydrocarbon foaming agents are very flammable and easy to be compatible with pp. therefore, a cell nucleating agent (such as a chemical foaming agent) or minerals (such as calcium carbonate or talc) must be added to the melt as the center of cell formation. The use of chlorofluorocarbons is restricted by environmental protection regulations. The use of methyl chloride is also limited. When using chlorinated warp and hydrocarbon foaming agent for foaming, appropriate exhaust system is also required in the production area. At the same time, strict safety requirements must be met. The compatibility of gas foaming agent in the melt is poor, and an extrusion system with excellent mixing effect is required. In addition, the melt pressure and the pressure drop during extrusion also affect the physical foaming process. Compared with chemical foaming process, the density of physical foaming products is generally low. The foaming agent used for physical foaming must have the following properties: good compatibility with polymers; Low diffusivity; The vaporization speed is fast to ensure the low concentration of residual gas in the cell wall.
The advantage of chemical foaming is that the specificity of its equipment is not strong. The foaming agent can be added to the polymer in the form of masterbatch or directly measured. Chemical foaming depends on the thermal decomposition temperature and gas volume of foaming agent. Foaming agent is generally a solid, which releases the gas required for foaming during decomposition. Chemical blowing agents are divided into two categories: exothermic blowing agents, which generate heat when decomposed; Endothermic foaming agent needs to be connected with heat input to promote decomposition reaction. ① Exothermic foaming agent. The characteristic of this kind of foaming agent is that the decomposition reaction begins when the heat generated during its decomposition reaches the decomposition temperature, and the heat generated by the reaction makes the decomposition reaction continue, which may cause uncontrollable chain reaction. Most of these blowing agents are azo blowing agents. Some solid decomposition products incompatible with PP will be produced in the reaction, which will cause fouling or overflow at the machine head. Coating composite production line www.handern com
If this foaming process is used to produce plates with smooth and uniform surface, the foaming agent must be completely decomposed before the melt flows out of the machine head, otherwise the melt surface will crack. The foam structure produced with this foaming agent is relatively rough and loose. ② Endothermic foaming agent. This foaming agent is generally a mixture of citric acid, sodium bicarbonate and sodium carbonate. The reaction product is a necessary gas, and there will be no scaling problem. The foaming agent absorbs heat during decomposition, and the reaction can be controlled greatly, and the foaming structure can be well controlled. Coating composite production line www.handern com
Author：admin