Foaming method of polyurethane foam
日期：2022/6/23 8:43:41 / 阅读： / 来源：本站
Foaming method of polyurethane foam1. one step foaming process
One step foaming process is a widely used production process of polyurethane foam at present. It mainly adds polyether, ISOACID, water, chemical agent, foam stabilizer and other additives in one step, and then foams after mixing under the condition of high-speed stirring. Due to the use of high-efficiency catalysts such as organotin, the reaction speed is fast, the temperature is high during heat release, the curing is not required after foaming, and the silicone foam stabilizer is used. Therefore, even when the viscosity of polyether and other materials is low, the foam products with uniform cell can be obtained. In addition, the reaction device does not need prepolymer, so it has the advantages of simple process, less equipment investment, easy operation and management, etc. Therefore, at present, most of the production adopts one-step method. Coating compound production line www.handern com
(1) One step foaming process of polyester soft foam
① Raw materials. The polyester raw material generally adopts ethylene glycol glycol diacid polyester or other types of diacid polyester, and some also use a small amount of aliphatic dibasic acid. The isocyanate part usually adopts tdi80/20 and tdi65/35
② Foaming formula. Different raw materials and foaming formulas are used according to different process and product performance requirements. Typical formula of one-step foaming process for medium density polyester soft foam ③ foaming process. In the one-step foaming process, the above formula is generally divided into two or more components. From the storage tank, several groups of precision metering pumps are respectively sent into the mixing head of high-speed mixing in proportion. Under high-speed mixing, it is quickly mixed evenly, and injected into the belt conveyor or mold for foaming. The mixing time is usually 1~5s, the starting foaming time (i.e. whitening time) in the mold is 4~6s, and the foaming time is generally 40~80s. After the foam solidifies, it is a foamed plastic product that is cured at 100 ℃ for 2h. In addition, it can be stored at room temperature for one week, and it can also reach the predetermined strength.
④ Relevant process factors. Coating compound production line www.handern com
a. In theory, the di index should be 100%. In order to complete the reaction and ensure the product performance, the TDI index should be generally controlled between 103%~110%. If it exceeds 110%, it is easy to form a coarse pore structure in foam or cause the cracking of foam products. B. the ratio of TDI isomerism has an impact on the foaming process. Tdi65/35 is used in the same preparation. The foaming time is short and the heat release is large: if the heat release temperature of td165/35 is 100~102 ℃, then 100% 2 and 4-TDI are 87~92 ℃, which is mainly caused by the large steric resistance of 2.4-tdi. At the same time, the product is soft when the ratio of 2,4-td1 increases.
The softness of C foam products can be adjusted by the number of functional groups and relative molecular weight of polyester, or by controlling the crosslinking density in polymer molecules. Coating compound production line www.handern com
d. The density of foam products can be adjusted by isocyanate and water. The more the dosage, the smaller the product density.
e. In the foaming process, different mixing head forms, mixing shear force, mixing speed, residence time, back pressure of mixing head, outlet pore diameter of mixing head and other factors have a great impact on the pore diameter and pore structure of foam products and the performance of final products. Generally speaking, coarse holes, hollows or cracks will be formed if the mixing speed is slow or the residence time is not long enough. With the slow down of stirring speed and the increase of residence time, the pore diameter of the product gradually changes from coarse to fine. When these factors increase to a limit value, the bubbles will form coarse pores or collapse. The discharge diameter of the mixing head also has some influence on the pore diameter. The smaller discharge diameter is beneficial to the smaller pore size of foam, but too small discharge diameter may lead to the formation and cracking of foam. Coating compound production line www.handern com
f. In the process of mechanical mixing, a small amount of air entering the mixing head can make the pore size of foam smaller, but a large amount of air entering the mixing head is easy to make the foam form large pores. At the same time, mixing dust or dirt into the foaming system will also make the foam form macropores or pinholes.
(2) One step foaming process of polyether flexible foams
① Raw materials. It mainly includes polyether polyol, TDI (usually tdi80/20) silicone foam stabilizer, catalyst and water, and fillers or other additives can be added as required. Coating compound production line www.handern com
② Foaming formula. The foaming formula of polyether foams is basically similar to that of polyester foams. Because the viscosity of polyether is much smaller than that of polyester, high-efficiency foam stabilizer must be added. Polysiloxane polyolefin mosaic copolymer is usually used. At the same time, because the hydroxyl group at the end of polyether is mainly secondary hydroxyl group, and the reaction activity is small, high-efficiency composite catalysts such as stannous octanoate, dibutyltin dilaurate and triethylenediamine are generally used.