Forming principle of polyurethane foam
DATE：2022/6/15 8:44:54 / READ： / SOURCE：This station
Forming principle of polyurethane foam
The main characteristic of polyurethane foam is that it is porous, so it has low density and high specific strength. According to the different raw materials used and the change of formula, soft, semi-rigid and hard polyurethane foams can be made. If the polyols used are classified, they can be divided into polyester and polyether polyurethane foams; According to the foaming method, there are block molding and spraying polyurethane foam. Hollow plate production line www.handern comIn the forming process of polyurethane (PUR) foam, there are chemical reactions from beginning to end, and the reaction mechanism is complex. The following describes the raw materials and forming principle.
1、 Raw material hollow board production line www.handern com
The main raw materials for the production of polyurethane foams are isocyanates, polyols and other additives, which are described below.
1. isocyanate
Isocyanate is the main raw material for the production of polyurethane foam. Isocyanate is a kind of highly reactive compounds, which is determined by its own chemical structure. It has a lot of unsaturated groups -n-c-o. in addition to addition reaction with many compounds, it can also undergo self polymerization under heating or catalyst. There are many types of isocyanates. Toluene diisocyanate (TDI) is commonly used. Its chemical structure formula usually uses the mixture of these two isomers. For example, toluene diisocyanase 20 and 65/35 represent the proportion of 2.4 and 2.6 isomers, which is called isomer ratio,
Tdi80/20 and tdi65/35. Hollow plate production line www.handern com
Due to the effect of steric hindrance, the reactivity of 2.4-tdi is higher than that of 2.6-td1, so the higher the isomerization ratio, the faster the gel and foaming reaction of foam, and the cell structure tends to be closed; On the contrary, the lower the isomerization ratio is, the lower the reaction activity is. "The structure of" tends to open holes. D80/20 soft foams are often used in production, and tdi65/35 soft polyester foams are used. Hollow plate production line www.handern.com
2. polyols
Polyols are another important raw material for the production of polyurethane foam. Polyether polyols and polyester polyols are mainly used in industry. Polyether polyols are polyols with hydroxyl groups at the end, which are obtained by ring opening polymerization of oxidized olefins with low molecular polyols as the starting agent under the action of alkali catalyst. Polyester polyols are polyols with hydroxyl at the end, which are prepared by polycondensation of dicarboxylic polyols. Carboxylic acids include adipic acid, sebacic acid and phthalic acid, and polyols include ethylene glycol, propylene glycol and hexaerythritol. Hollow plate production line www.handern com
Generally, polyether or polyester used to produce flexible foams are linear or slightly branched long-chain molecules, each macromolecule has 2~3 hydroxyl groups, and the relative molecular weight is 2000~4000. The relative molecular weight of rigid foam is small, and it has branched chain structure, and the functionality is between 3~8. Polyester polyurethane foam has good properties, but its price is high. It is generally used for special purposes. At present, polyether polyols are mostly used to produce foamed plastics, which have a wide range of sources, low price, good processing performance and good performance. Hollow plate production line www.handern com
3. catalyst
In order to accelerate the formation reaction and foaming reaction of polyurethane, a catalyst should be added. Tertiary amines, such as triethylamine and triethylenediamine, are generally used in production. Among them, tertiary amine catalyst has strong catalytic effect on isocyanate alcohol hydroxyl and isocyanate water reaction, while tin catalyst has particularly obvious catalytic effect on isocyanate hydroxyl reaction. both
4. foaming agent
With proper mixing ratio, chain growth and foaming reaction balance can be effectively controlled. Carbon dioxide is the main foaming gas of polyurethane foam, which is formed by the reaction of isocyanate and water. Too much water will make the polymer chain have more polyurea structure, so that the foam is brittle, and a large amount of reaction heat will be generated at the same time. Due to the low thermal conductivity of the foam, the heat can not be released quickly, and the center of the foam will be discolored and charred; It is difficult to produce low-density foamed plastics when the amount of water is too low. Therefore, proper water volume must be controlled. In the production of rigid foams and low-density flexible foams, in order to reduce the density and the consumption of isocyanates, fluorocarbon HFC blowing agent and dichloromethane are often added as blowing agents. They absorb heat and vaporize during the foaming process, making the polymer foam and reducing the internal temperature of the foams. The addition of this kind of blowing agent makes the foamed plastics have the lowest thermal conductivity and better softness.
5. surfactant
Surfactants play the following important roles in the formation of polyurethane foam.
① With the function of emulsifier, all foaming components are evenly mixed to ensure that various reactions of foam formation are balanced.
② Nucleation makes bubbles form at low gas concentration.
③ Reduce the surface tension of the foaming system to facilitate the stability and uniformity of bubbles.
The commonly used surfactants include water-soluble silicone oil, sulfonated fatty alcohol, sulfonated fatty acid and other non-ionic surfactants.
In addition, flame retardant, anti-aging agent, reinforcing agent and colorant can be added according to the needs of foamed plastic products. Hollow plate production line www.handern com
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