51. The basic reaction of foam preparation
A: It refers to the reaction of -NCO with -OH, -NH2 and H2O. When reacting with polyol, the “gel reaction” in the foaming process generally refers to the formation of carbamate. Because the foam raw material is multifunctional, the cross-linked network is obtained, which makes the foaming system can gel rapidly.
In the foaming system where water is present, the foaming reaction is generated. The so-called “foaming reaction” generally refers to the reaction between water and isocyanate to produce substituted urea, and the release of CO2.
52. The nucleation mechanism of bubbles
The reaction of the raw material in the liquid or the temperature of the reaction production produces gaseous substances and makes the gas volatilize. As the reaction proceeds and a large amount of reaction heat is generated, the amount of gaseous substances occurring and volatilized increases. After the gas concentration increases beyond the saturation concentration, a dimensional bubble begins to form and rise in the solution phase.
53. The role of foam stabilizers in the preparation of polyurethane foam
Answer:It has emulsifying effect and makes the mutual solubility between the components of foam materials enhanced; after adding organosilicon surfactant, because it greatly reduces the surface tension of liquid γ , the free energy required for gas dispersion is reduced, so that the air dispersed in the raw material is easier to nucleate in the mixing process of stirring, which helps the generation of fine bubbles and improves the stability of foam.
54. Stabilization mechanism of foam
A: The addition of appropriate surfactants helps to generate fine bubble dispersions.
55. Formation mechanism of open cell foam and closed cell foam
A: The formation mechanism of open cell foam: Most of the time, when the maximum pressure is generated in the bubble, the strength of the bubble wall formed by the gel reaction is not high, and it cannot withstand the stretching of the wall film caused by the increase of gas pressure, so the bubble wall film is pulled, and the gas escapes from the rupture, forming open cell foam.
Closed cell foam formation mechanism: For hard foam system, due to the use of multifunctional, low molecular weight polyether polyol and polyisocyanate reaction, the gelation speed is relatively fast, the gas in the bubble pore can not squeeze through the bubble wall, so as to form a closed cell foam.
56. Physical foaming agent and chemical foaming agent foaming mechanism
A: physical blowing agent: physical blowing agent is the foam pore is formed by a change in the physical form of a substance, that is, through the expansion of compressed gas, liquid volatilization or dissolution of solids.
Chemical blowing agents: Chemical blowing agents are those compounds that can release gases such as carbon dioxide and nitrogen after decomposition by heating and form fine pores in the composition of the polymer.
57. Preparation methods of flexible polyurethane foam
A: One-step method and prepolymer method
Prepolymer method: That is, the prepolymer containing free NCO group is made by the reaction of polyether polyol and excess TDI first, and then mixed with water, catalyst, stabilizer, etc. to make foam. One-step method: Various raw materials are mixed directly into the mixing head through calculation to make foam in one step, which can be divided into continuous type and intermittent type.
58. The characteristics of horizontal foaming and vertical foaming
A: Horizontal foaming characteristics: Side film lifting method: This method adds upward traction side paper device on the basis of the original horizontal foaming machine, so that the foam edge and the middle rise synchronously foaming, so as to make a foam block close to the flat top. Balanced platen method: characterized by the use of top paper and top cover. Overflow trough method: characterized by the use of overflow trough and conveyor landing plate.
Vertical foaming features: can use a smaller flow rate to get a large cross-sectional area of foam blocks, while usually with horizontal foaming machine to get the same cross-sectional blocks, the flow level is 3 to 5 times larger than vertical foaming; due to the large cross-sectional foam blocks, there is no upper and lower epidermis, the side skin is also thinner, thus greatly reducing the cutting loss; equipment covers an area of small, plant height of about 12 ~ 13m, plant and equipment investment costs than Horizontal foaming process of low; can be easily replaced by the hopper and model, can produce cylindrical or rectangular foam body, especially for the production of rotary cut round block bubble blanks material.
59. The basic points of raw material selection for soft foam preparation
A: Polyol: polyether polyol used for ordinary block foam, molecular weight is generally 3000 ~ 4000, mainly polyether triol. High resilience foam is mostly used in the molecular weight of 4500 ~ 6000 polyether triol. When the molecular weight increases, the tensile strength, elongation and resilience of the foam increases; the reactivity of similar polyethers decreases. If the functionalities of polyether increase, the reaction is relatively faster, and the cross-linkage of polyurethane is increased, and the hardness of foam is increased and the elongation is decreased. Isocyanate: The raw material of isocyanate of polyurethane soft block foam is mainly toluene diisocyanate (TDI-80). And the relatively low activity TDI-65 is only used for polyester type polyurethane foam or special polyether type foam. Catalyst: The catalyst for block soft foam is roughly divided into two categories: one is an organometallic compound, tin octanoate is most commonly used; the other is a tertiary amine, bis(dimethylaminoethyl) ether is commonly used. Foam stabilizer: In polyester polyurethane block foam, non-silicone surfactants are mainly used, and in polyether block foam, organosilicon – olefin oxide co-polymer is mainly used. Foaming agent: Generally, in the manufacture of polyurethane soft block foam with density greater than 21 kg per cubic meter, only water is used as foaming agent; only in low-density formulations are low-boiling compounds such as methylene chloride (MC) used as auxiliary foaming agents.
60. Influence of environmental conditions on the physical properties of block foam
A: Influence of temperature: The foaming reaction of polyurethane is accelerated with the rise of material temperature, which will cause the danger of burning core and fire in sensitive formulations. Influence of air humidity: As the humidity increases, the hardness of the foam decreases and the elongation increases due to the reaction of the isocyanate group part of the foam with the moisture in the air; the tensile strength of the foam increases due to the increase of the urea group. The effect of atmospheric pressure: for the same formulation, the density decreases significantly when foaming at higher altitudes.
61. The main difference between the raw material system used in cold molded soft foam and hot molded foam
A: The reaction activity of raw materials used in cold molding is high, no external heat is needed for maturation, and the heat generated by the system can basically complete the maturation reaction in a short time, and the raw materials can be demolded within a few minutes after injection. The reaction activity of the raw materials used in thermal molding foam is lower, and the reaction mixture is heated together with the mold after foaming in the mold, and the foam products are fully cooked in the baking channel before they can be released.
62. What are the characteristics of cold molded soft foam compared with hot molded foam?
A: ①No external heat is needed in the production process, which can save a lot of heat energy; ②High sag coefficient (indentation ratio), good comfort performance; ③High rebound rate; ④No flame retardant foam also has certain flame retardant performance; ⑤Short production cycle, which can save the mold and save costs.
63. The characteristics and uses of soft foam and hard foam respectively
A: Soft foam characteristics: polyurethane soft foam is mostly open pore structure. It generally has the properties of low density, good elastic response, sound absorption, air permeability, heat preservation, etc. Uses: mainly used as furniture, cushioning materials, transport seat cushioning materials, various soft padding laminated composite materials, industrial and civil also use soft foam as filtering materials, sound insulation materials, shockproof materials, decorative materials, packaging materials and thermal insulation materials, etc.
Rigid foam features: polyurethane foam has light weight, high specific strength, good dimensional stability; polyurethane rigid foam has superior adiabatic properties; strong adhesion; good aging performance, long adiabatic life; reaction mixture has good fluidity, can smoothly fill the complex shape of the mold cavity or space; polyurethane rigid foam production raw materials with high reactivity, can achieve rapid curing, can achieve high efficiency in the factory and High production volume.
Uses: Used as insulation layer material for refrigerator, freezer, refrigerated container, cold storage, etc., insulation layer for oil transmission pipeline and hot water transmission pipeline, insulation layer for building wall and roof, insulation sandwich panel, etc.
64. Key points of rigid foam formula design
A: polyol: polyether polyol for hard foam formulation is generally high functional, high hydroxyl value (low molecular weight) polyoxypropylene polyol; isocyanate: is currently used in hard foam isocyanate is mainly multi-methylene multi-phenyl polyisocyanate (generally known as PAPI), that is, crude MDI, polymeric MDI; blowing agent: (1) CFC blowing agent (2) HCFC and HFC blowing agent (3) pentane Foam stabilizer: the foam stabilizer used in polyurethane hard foam formulations is generally a block polymer of polydimethylsiloxane and polyolefin oxide, and most foam stabilizers are mainly Si-C type; catalyst: the catalyst of hard foam formulations is mainly tertiary amine, and organic tin catalyst can be used in special occasions; other additives: according to the requirements and needs of different uses of polyurethane hard foam products, fire retardant, flame retardant, polymeric MDI can also be added to the formulation. According to the requirements and needs of different applications of polyurethane rigid foam products, we can add flame retardant, porosity-opening agent, smoke inhibitor, antioxidant, mildew inhibitor, toughening agent and other additives into the formula.
65. Principle of preparing whole-skin molding foam
A: Integral skin foam (ISF), also known as self-skinning foam, is a foam that produces a dense skin when it is manufactured.
66. Characteristics and uses of polyurethane microporous elastomer
A: Characteristics: Polyurethane elastomer is a block polymer, generally composed of oligomer polyol flexible long chain to form a soft segment, diisocyanate and chain extender to form a hard segment, hard and soft segments alternately arranged to form repeated structural units. In addition to containing urethane groups, polyurethane molecules can form hydrogen bonds within and between molecules, and the soft and hard segments can form microphase regions and produce microphase separation.
67. What are the main performance characteristics of polyurethane elastomers?
A: Performance characteristics: 1, higher strength and elasticity, can be in a wide range of hardness (Shore A10 ~ Shore D75) to maintain high elasticity; generally no plasticizer can be achieved without the required low hardness, and thus no plasticizer migration problems; 2, in the same hardness, higher than other elastomers bearing capacity; 3, excellent wear resistance, its wear resistance is 2 to 10 times that of natural rubber; 4, resistance to Excellent grease and chemical resistance; aromatic polyurethane radiation resistance; excellent oxygen and ozone resistance; 5, high impact resistance, fatigue and vibration resistance, suitable for high-frequency flexing applications; 6, low-temperature flexibility; 7, ordinary polyurethane can not be used above 100 ℃, but the use of special formulations can be resistant to 140 ℃ high temperature; 8, molding and processing costs are relatively low.
68. Polyurethane elastomers are classified according to polyol, isocyanate, manufacturing process, etc.
A: 1. According to the oligomeric polyol raw materials, polyurethane elastomer can be divided into polyester, polyether, polyolefin, polycarbonate, etc., and polyether can be divided into polytetrahydrofuran, polypropylene oxide, etc. according to specific varieties; 2. According to the different diisocyanate, it can be divided into aliphatic and aromatic elastomer, and subdivided into TDI, MDI, IPDI, NDI and other types; from the manufacturing From the manufacturing process, polyurethane elastomers are traditionally divided into three categories: cast (CPU), thermoplastic (TPU), and mixed (MPU).
69. What are the factors affecting the performance of polyurethane elastomers from the viewpoint of molecular structure?
A: In terms of molecular structure, polyurethane elastomer is a block polymer, generally composed of oligomeric polyol flexible long chains to form soft segments, diisocyanate and chain extender to form hard segments, hard and soft segments are arranged alternately to form repeated structural units. In addition to containing urethane groups, polyurethane molecules can form hydrogen bonds within and between molecules, and the soft and hard segments can form microphase regions and produce microphase separation. These structural features give polyurethane elastomers excellent abrasion resistance and toughness, and they are known as “abrasion resistant rubber”.
70. Performance differences between ordinary polyester and polytetrahydrofuran ether elastomers
A: Polyester molecules contain more polar ester groups (-COO-), which can form strong intramolecular hydrogen bonds, so polyester polyurethane has higher strength, wear resistance and oil resistance.
