Two step foaming process

日期:2022/6/29 8:47:50 / 閱讀: / 來源:本站

Two step foaming process
The two-step process is divided into prepolymerization and semi prepolymerization: (1) the prepolymerization foaming process is usually applied to polyether foams, while polyester foams have higher viscosity due to the high viscosity of polyester itself, which makes it difficult to operate during foaming. Prepolymerization method is to first react isocyanate with polyol to generate prepolymer containing a certain amount of free isocyanate group, then add water, catalyst, surfactant and other additives into the prepolymer, and mix them under high-speed stirring to generate foamed plastics. The viscosity of prepolymer is an important index in the preparation of prepolymer. For the preparation of polyester prepolymer, due to the high viscosity of polyester itself, one-time feeding is usually used, that is, the polyester and isocyanate required in the formula can be added once to achieve the viscosity required by the prepolymer. Polyether prepolymer is usually synthesized by two steps. PVB film production line www.handern com
The first step is to react the polyether with a part of isocyanate to form a prepolymer with isocyanate group at the end and high relative molecular weight; The second step is to add the remaining isocyanate in the formula to the flow product of the polymerized foam production process for dilution, so that the isocyanate content of the final product is about 8%~10%. Compared with the one-time reaction method, the prepolymer prepared by this method has a larger molecular weight distribution. Due to the existence of higher relative molecular weight components, the viscosity of the prepolymer has been greatly improved. PVB film production line www.handern com
During the prepolymerization preparation, it is necessary to ensure that the ends of the reactants have isocyanate groups and obtain a certain viscosity, and at the same time to prevent excessive cross-linking of the materials and the formation of gel. Therefore, while strictly controlling the quality of raw materials, it is also necessary to control various factors that affect the viscosity and prevent gel, such as the total acidity in the reaction system, the proportion of nco/oh, the moisture content, the number of branched chains, and the reaction temperature and time. PVB film production line www.handern com
① Raw material control. In order to make the reaction repeatable in the prepolymerization preparation, it is necessary to strictly control the specifications of various raw materials, such as the moisture, hydroxyl value, pH value, metal ion content, etc. in the raw materials, as well as the acid value in polyether polyols and the unsaturated end group content in polyester polyols. Generally, a small amount of antioxidant is added to polyether polyol to prevent oxidation during storage, so as to avoid further generation of harmful carbonyl and increase peroxide value and acid value.
② Effect of isocyanate and hydroxyl ratio (nco/oh ratio). The relative molecular weight of the polymer is related to the ratio of hydroxyl and isocyanate. Theoretically, when the ratio of nco/oh is 1, the relative molecular weight of the polymer is the largest.
③ Effects of different polyols. The prepolymer used in soft foamed plastics generally adopts the mixture of polyether or polyester diol or polyol, and their ratio and yield prepolymer
The higher the nco/oh ratio, the smaller the viscosity of prepolymer
④ Effect of reaction temperature and time. The reaction temperature and time of prepolymer preparation can not only affect the viscosity of prepolymer, but also affect the chemical structure of prepolymer. When there is no catalyst and the temperature is lower than 100 ℃, carbamate chain is preferentially formed, and urea group is preferentially formed in the presence of water. When the temperature is higher than 100 ℃, urea formate and biuret will be further reacted to form branched chain reaction. Generally, the strength of the foamed plastics made from the prepolymer in the low-temperature reaction room is good. When the reaction temperature is greater than 100 ℃, the strength (especially the high-temperature strength) decreases, but the gel strength during foaming is large, which is conducive to the foaming operation. Therefore, during the preparation of prepolymer, the operation scheme is generally selected according to the needs. The reaction temperature and time directly affect the viscosity of prepolymer, and the viscosity of prepolymer has a great impact on the foaming process performance, But the effect on the final performance of the product is not significant. PVB film production line www.handern com
⑤ Influence of pH. Generally, in the preparation process of prepolymer, the acid condition is conducive to the chain growth reaction to produce carbamate and urea group, so as to obtain prepolymer with relatively low viscosity; The alkaline condition is conducive to the formation of branched chain reaction, the formation of urea formate biuret and trimerization, so as to obtain prepolymer with relatively high viscosity, and even gel phenomenon. Therefore, in the preparation of prepolymerization, the control of pH in the material is the key to the reaction.
⑥ Taking the polyester prepolymer foaming process as an example, polyester is added into a drying reactor with stirring, and then isocyanate is added while stirring at room temperature. Due to the exothermic reaction between isocyanate and polyester, the addition rate of isocyanate must be controlled according to the capacity of the reactor, so that the reaction temperature does not exceed 90 ℃. After feeding, continue to react at 90 ℃ for about 1H, and then put it into a drying container after cooling. In the production process, it is better to dry nitrogen to remove moisture in the system. PVB film production line www.handern com

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