Understand the basic concepts related to Polyurethane
1. Hydroxyl value: The amount of hydroxyl (-OH) contained in 1 gram of polymer polyol is equivalent to the number of milligrams of KOH in mgKOH/g.
2. Equivalent: the average molecular weight occupied by a functional group.
3. Isocyanate content: The content of isocyanate in the molecule
4. Isocyanate index: Indicates the degree of excess isocyanate in the polyurethane formula, usually represented by the letter R.
5. Chain extender: refers to low molecular weight alcohols and amine compounds that can extend and expand the molecular chain or form spatial network cross-linking.
6. Hard segment: The segment on the main chain of the polyurethane molecule formed by the reaction of isocyanate, chain extender, and cross-linking agent. These groups have larger cohesion energy, larger space volume, and greater rigidity.
7. Soft segment: Carbon-carbon main chain polymer polyol has good flexibility and is a flexible segment in the polyurethane main chain.
8. One-step method: refers to a method in which oligomer polyol, diisocyanate, chain extender and catalyst are mixed at the same time and then directly injected into the mold and solidified at a certain temperature.
9. Prepolymer method: First, the oligomer polyol and diisocyanate are prepolymerized to generate an NCO-terminated polyurethane prepolymer. During casting, the prepolymer is reacted with a chain extender to prepare the polyurethane elastomer. method, called the prepolymer method.
10. Semi-prepolymer method: The difference between the semi-prepolymer method and the prepolymer method is that some polyester polyols or polyether polyols, chain extenders, catalysts, etc. are added to the prepolymer in the form of a mixture.
11. Reaction injection molding: Also known as RIM (Reaction Injection Molding), oligomers with small molecular weight are measured in liquid form, mixed instantly and injected into the mold at the same time, and react quickly in the mold cavity, and the material A process in which the molecular weight increases rapidly and a new polymer containing a new characteristic group structure is generated at an extremely fast speed.
12. Foaming index: The proportion of water equivalent to 100 parts of polyether is defined as the foaming index (IF).
13. Foaming reaction: Generally refers to the reaction in which water reacts with isocyanate to form substituted urea and releases CO2.
14. Gel reaction: generally refers to the formation reaction of carbamate.
15. Gel time: The time required for a liquid substance to form a gel under certain conditions.
16. Milky white time: When zone I is about to end, milky white phenomenon will appear in the liquid polyurethane mixture material. This time is called cream time in polyurethane foam production.
17. Chain extension coefficient: refers to the ratio of the amount of amino and hydroxyl groups (unit: mo1) in the chain extender component (including mixed chain extender) to the amount of NCO in the prepolymer, that is, the active hydrogen group and NCO Ratio of moles (number of equivalents).
18. Low unsaturation polyether: mainly developed for PTMG. The price of PPG and the degree of unsaturation are reduced to 0.05mol/kg, which is close to the performance of PTMG. It uses DMC catalyst and the main variety is Bayer’s Acclaim series products.
19. Urethane grade solvent: When selecting a solvent for the production of polyurethane, the solubility and evaporation speed must be considered. However, the solvent used for the production of polyurethane should focus on the heavy NC0 group in the polyurethane. Solvents such as alcohols and ether alcohols that react with NCO groups cannot be used. The solvent must not contain water, alcohol and other impurities, and it must not contain alkali substances, which will cause the polyurethane to deteriorate.
Ester solvents are not allowed to contain moisture, nor are they allowed to contain free acids and alcohols, which will react with NCO groups. The ester solvent used for polyurethane should be a high purity “urethane grade solvent”. That is, the solvent is reacted with excess isocyanate, and then dibutylamine is used to measure the amount of unreacted isocyanate to test whether it is suitable for use. The principle is that it does not apply to those that consume a lot of isocyanate, because it shows that the water, alcohol, and acid contained in the ester will consume the total value of isocyanate. If it is expressed in grams of solvent required to consume leqNCO groups, the one with the larger value will have better stability. .
Those with an isocyanate equivalent weight below 2500 are not used as polyurethane solvents.
The polarity of the solvent has a great influence on the reaction that produces the resin. The greater the polarity, the slower the reaction. For example, the difference between toluene and methyl ethyl ketone is 24 times. This solvent molecule is highly polar and can form hydrogen bonds with the hydroxyl group of the alcohol, making the reaction slow.
It is better to use aromatic hydrocarbon solvents as polyurethane solvents. Their reaction speed is faster than esters and ketones, such as xylene. When constructing double-button polyurethane, the use of ester and ketone solvents can extend its service life. When producing coatings, choosing the “urethane grade solvent” mentioned earlier is beneficial to the stability of storage.
Ester solvents have strong solubility, moderate evaporation speed, low toxicity and are often used. Cyclohexanone is also commonly used. Hydrocarbon solvents have low solid solubility and are rarely used alone. They are often used in combination with other solvents.
20. Physical foaming agent: Physical foaming agent means that the pores of the foam are changed through the physical form of a certain substance, that is, through the expansion of compressed gas, the volatilization or solidification of liquid.Elenic type, etc.; 2. According to the difference of diisocyanate, it can be divided into aliphatic and aromatic elastomers, which are further subdivided into TDI type, MDI type, IPDI type, NDI type and other types; according to the manufacturing process, traditionally Polyurethane elastomers are divided into three categories: casting type (CPU), thermoplastic type (TPU), and mixing type (MPU).
69. From the perspective of molecular structure, what are the factors that affect the performance of polyurethane elastomer?
Answer: From the molecular structure point of view, polyurethane elastomer is a block polymer. It is generally composed of flexible long chains of oligomer polyols to form soft segments, and diisocyanate and chain extenders to form hard segments. The hard segments and soft segments alternate. arranged to form repeating structural units. In addition to containing urethane groups, hydrogen bonds can be formed within and between polyurethane molecules, and soft and hard segments can form microphase regions and produce microscopic phase separation. These structural characteristics make polyurethane elastomers have excellent wear resistance and toughness, and are known as “wear-resistant rubber”.
70. Differences in performance between ordinary polyester and polytetrahydrofuran ether elastomers
Answer: Polyester molecules contain more polar ester groups (-COO-), which can form strong intramolecular hydrogen bonds. Therefore, polyester polyurethane has high strength, wear resistance and oil resistance.
Elastomers made from polyether polyols have good hydrolytic stability, weather resistance, low temperature flexibility and mold resistance.
