Basic knowledge about polyurethane

1. Isocyanate index: indicates the degree of excess of isocyanate in the polyurethane formulation, usually indicated by the letter R.

2. Hard segment: a segment formed by the reaction of isocyanate, chain extender and crosslinker on the main chain of the polyurethane molecule. These groups have larger cohesive energy, larger space and greater rigidity.

3, soft segment: carbon-carbon backbone polymer polyol, good flexibility, in the polyurethane backbone is a flexible segment.

4, urethane ester solvent: the choice of solvent for the production of polyurethane should consider the solubility, evaporation rate, but the solvent used in the production of polyurethane should focus on the weight of the NC0 base in polyurethane. A solvent such as an alcohol or an ether oxime which reacts with the NCO group cannot be used. The solvent can not contain impurities such as water and alcohol, and cannot contain alkali substances, which will deteriorate the polyurethane.

Ester solvents are not allowed to contain moisture, nor contain free acids and alcohols, which react with NCO groups. For the ester solvent used in polyurethane, a "urethane ester solvent" having a high purity should be used. That is, the solvent was reacted with an excess of isocyanate, and the amount of unreacted isocyanate was measured by dibutylamine to check whether it was used in combination. The principle is that the consumption of isocyanate is not suitable because it indicates that the total value of the isocyanate is consumed by the water, alcohol and acid in the ester. If the number of grams of solvent required to consume the leqNCO group is expressed, the numerical value is good. .

An isocyanate equivalent of less than 2,500 or less is not used as a urethane solvent.

The polarity of the solvent has a great influence on the reaction of the resulting resin. The higher the polarity, the slower the reaction, such as the difference between toluene and methyl ethyl ketone by 24 times. The solvent molecule has a large polarity and can form a hydrogen bond with the hydroxyl group of the alcohol to make the reaction slow.

Polyurethane solvents are preferably selected from aromatic hydrocarbon solvents, and their reaction speed is faster than esters and ketones, such as xylene. In the double-bond polyurethane construction, the ester and ketone solvents can be used to extend the service life. In the production of coatings, the "urethane-grade solvent" mentioned above is advantageous for the storage of the stabilizer.

The ester solvent has strong dissolving power, moderate volatilization speed, low toxicity and high use, and cyclohexanone is also used frequently. The hydrocarbon solvent has low solid solubility, and is used less separately, and is used in combination with other solvents.

5. What types of commonly used isocyanates are structurally considered?

A: Aliphatic: HDI, alicyclic: IPDI, HTDI, HMDI, aromatic: TDI, MDI, PAPI, PPDI, NDI.

6. What kinds of commonly used polymer polyols are there?

A: Polyester polyol, polyether polyol

32. What are the main industrial production methods for polyester polyols?

Answer: A, vacuum melting method B, carrier gas melting method C, azeotropic distillation method

33. What are the special structures on the molecular backbone of polyester and polyether polyols?

A: Polyester polyol: a macromolecular alcohol compound having an ester group in a molecular main chain and a hydroxyl group (-OH) at a terminal group. Polyether polyol: A polymer or oligomer having an ether bond (-O-) and a terminal group (-OH) or an amine group (-NH2) in a molecular main chain structure.

4. What kinds of commonly used polyurethane catalysts are there, and which common varieties are included?

Answer: Tertiary amine catalysts, commonly used varieties are: triethylenediamine, dimethylethanolamine, N-methylmorpholine, N,N-dimethylcyclohexylamine metal alkyl compounds, commonly used varieties are: organotin Catalysts can be divided into stannous octoate, stannous oleate, dibutyltin dilaurate.

5. Effect of hydroxy compounds on its reactivity with isocyanates

Answer: The reactivity of active hydrogen compounds (ROH or RNH2) is related to the nature of R. When R is an electron withdrawing group (low electronegativity), the transfer of hydrogen atoms is difficult, and the reaction of active hydrogen compounds with NCO is difficult; When R is an electron-donating substituent, the reactivity of the active hydrogen compound with NCO can be improved.

6. When preparing polyurethane elastomer, the water content of polymer polyol should be strictly controlled.

A: There is no requirement for bubbles in elastomers, coatings and fibers, so the water content in the raw materials must be strictly controlled, usually less than 0.05%.

7. Why can polyurethane resin be regarded as a kind of block polymer? What are the characteristics of the segment structure?

Answer: Because the segment of polyurethane resin is composed of hard segment and soft segment, hard segment refers to the segment formed by the reaction of isocyanate, chain extender and crosslinker on the main chain of polyurethane. The cohesive energy of these groups Larger, larger in space and more rigid. The soft segment refers to a carbon-carbon backbone polymer polyol, which has good flexibility and is a flexible segment in the polyurethane backbone.

8. What are the factors that affect the performance of polyurethane materials?

A: The cohesive energy of the group, hydrogen bonding, crystallinity, degree of crosslinking, molecular weight, hard segment, soft segment.

9. What are the raw materials from the soft and hard segments of the molecular chain of the polyurethane material?

Answer: The soft segment consists of oligomer polyol (polyester, polyether diol, etc.) and the hard segment consists of polyisocyanate or its small molecular chain extender.

10. How do soft and hard segments affect the performance of polyurethane materials?

Answer: Soft segment: (1) Molecular weight of soft segment: Assuming that the polyurethane has the same molecular weight, if the soft segment is polyester, the strength of the polyurethane increases with the molecular weight of the polyester diol; if the soft segment is a polyether, then The strength of the polyurethane decreases as the molecular weight of the polyether diol increases, but the elongation increases. (2) Crystallinity of the soft segment: a large contribution to the crystallinity of the linear polyurethane segment. In general, crystallinity is advantageous for improving the properties of polyurethane articles, but sometimes crystallization reduces the low temperature flexibility of the material, and crystalline polymers are often opaque.

Hard segments: Hard segments generally affect the softening melt temperature and high temperature properties of the polymer. Polyurethane prepared from aromatic isocyanate has a rigid segment containing a rigid aromatic ring, so that the cohesive strength of the hard segment is increased, and the material strength is generally larger than that of the aliphatic isocyanate polyurethane, but the ultraviolet degradation resistance is poor and the yellowing is easy. Aliphatic polyurethanes do not yellow.

11. Classification of polyurethane foam

Answer: (1) hard foam and soft foam, (2) high density and low density foam, (3) polyester type, polyether type foam, (4) TDI type, MDI type foam, (5) polyurethane foam and polyisocyanurate foam, (6) one step Production by method and prepolymerization, (7) continuous and batch production, (8) slabstock foam and molded foam.

12. The role of foam stabilizers in the preparation of polyurethane foams

Answer: It has an emulsification effect to enhance the mutual solubility between the components of the foam material. After adding the silicone surfactant, since it greatly reduces the surface tension γ of the liquid, the increased free energy required for gas dispersion is reduced, and the dispersion is dispersed. The air in the raw material is more likely to nucleate during the stirring and mixing process, contributing to the generation of fine bubbles and improving the stability of the foam.

13. Method for preparing flexible polyurethane foam

A: One-step method and prepolymer method

Prepolymer method: First, a prepolymer having a free NCO group is prepared by reacting a polyether polyol with an excess of TDI, and then mixing with water, a catalyst, a stabilizer, etc. to form a foam. . One-step method: various raw materials can be directly mixed into the mixing head by calculation, and the foam can be manufactured in one step, which can be divided into continuous type and intermittent type.

14. Influence of environmental conditions on the physical properties of slabstock foam

A: The effect of temperature: The foaming reaction of polyurethane accelerates as the temperature of the material rises, which will cause the danger of burning core and fire in sensitive formulations. Effect of air humidity: As the humidity increases, the hardness of the foam decreases and the elongation increases due to the reaction of the isocyanate groups in the foam with the moisture in the air. The tensile strength of the foam increases due to the increase of the urea group. Increased. The effect of atmospheric pressure: For the same formulation, when foaming at higher altitudes, the density is significantly reduced.

15. Characteristics and use of polyurethane microcellular elastomer

Answer:Characteristics: Polyurethane elastomer is a kind of block polymer. It is generally composed of a flexible long chain of oligomer polyols. The hard segment is composed of diisocyanate and chain extender. The hard and soft segments are alternately arranged to form a repeat. Structural units. In addition to the urethane group, hydrogen bonds can be formed in and between the molecules of the polyurethane, and the soft and hard segments can form microphase regions and produce microscopic phase separation.

16. What are the main performance characteristics of polyurethane elastomers?

A: Performance characteristics: 1, high strength and elasticity, can maintain high elasticity in a wide range of hardness (Shore A10 ~ Shore D75); generally do not need plasticizer to achieve the required low hardness, Therefore, there is no problem caused by plasticizer migration; 2. Under the same hardness, it has higher bearing capacity than other elastomers; 3. Excellent wear resistance, its wear resistance is 2 to 10 times that of natural rubber; 4. Resistance Excellent grease and chemical resistance; aromatic polyurethane resistant to radiation; excellent in oxygen resistance and ozone resistance; 5, high impact resistance, fatigue resistance and vibration resistance, suitable for high frequency flexing applications; Good flexibility; 7, ordinary polyurethane can not be used above 100 °C, but with a special formula can withstand 140 °C high temperature; 8, molding and processing costs are relatively low.

17. Polyurethane elastomers are classified according to polyols, isocyanates, manufacturing processes, etc.

Answer: 1. According to the oligomer polyol raw material, the polyurethane elastomer can be divided into polyester type, polyether type, polyolefin type, polycarbonate type, etc., and the polyether type can be classified into tetrahydrofuran type according to specific varieties. , polyoxypropylene type, etc.; 2. According to different diisocyanates, can be divided into aliphatic and aromatic elastomers, subdivided into TDI type, MDI type, IPDI type, NDI type, etc.; from the manufacturing process, the traditional The polyurethane elastomer is divided into three types: casting type (CPU), thermoplastic (TPU), and mixing type (MPU).

18. What are the factors affecting the performance of polyurethane elastomers in terms of molecular structure?

Answer: From the molecular structure point of view, polyurethane elastomer is a kind of block polymer, which is generally composed of flexible long chain of oligomer polyol and soft segment. Diisocyanate and chain extender form hard segment, alternating hard segment and soft segment. Arrange to form repeating structural units. In addition to the urethane group, hydrogen bonds can be formed in and between the molecules of the polyurethane, and the soft and hard segments can form microphase regions and produce microscopic phase separation. These structural features make polyurethane elastomers excellent in wear resistance and toughness, and are known as "wear rubber."