Polyurethane. The name might not sound exciting, but this versatile material is a hidden powerhouse, shaping our world in countless ways. From the comfy cushions on your furniture to the insulation in your walls, polyurethane is everywhere, and its fascinating chemistry is even more impressive. Let’s dive into the amazing world of polyurethane!
The Chemistry Magic:
At the heart of polyurethane lies a fascinating chemical reaction: the interaction between isocyanates and compounds containing active hydrogen (like alcohols and amines). This reaction creates long chains of molecules, forming the backbone of polyurethane materials. The magic lies in controlling this reaction: tiny adjustments in ingredients and processes can dramatically change the final product’s properties.
Foams: Lightweight Wonders:
One of the most common forms of polyurethane is foam. Think of those comfy mattresses, the insulation in your home, or even the cushioning in your car seats. The secret to foam formation is the introduction of a blowing agent, which creates tiny gas bubbles within the polyurethane structure, resulting in a lightweight yet surprisingly strong material. The type of blowing agent, the carefully controlled reaction, and even the temperature and humidity all play a role in determining the final foam’s properties—from soft and yielding to rigid and insulating.
Elastomers: Stretchy and Strong:
Beyond foams, polyurethane also creates incredibly versatile elastomers—stretchy, durable materials used in everything from shoe soles to automotive parts. These elastomers get their amazing properties from the precise balance of hard and soft segments in their molecular structure. The ratio of these segments, along with the type of polyol and isocyanate used, determines the final elastomer’s strength, elasticity, and resistance to wear and tear.
A World of Applications:
The versatility of polyurethane is truly astounding. Its ability to be tailored for specific applications makes it a star performer in a wide range of industries:
- Furniture: Comfortable cushions and supportive seating.
- Construction: Insulation, roofing, and coatings.
- Automotive: Seating, dashboards, and other interior components.
- Footwear: Durable and comfortable shoe soles.
- Medical: Implants, foams, and other specialized applications.
The Future of Polyurethane:
As research continues, polyurethane’s potential seems limitless. Scientists are exploring new ways to enhance its properties, making it even more sustainable, durable, and versatile. From innovative bio-based polyurethanes to self-healing materials, the future of this amazing material is as exciting as its past. So next time you sink into a comfy chair or admire the sleek design of a car, remember the incredible science and engineering behind the versatile world of polyurethane!
Types of active hydrogen compounds and NCO reactivity
Aliphatic NH2>aromatic NH2>primary OH>water>secondary OH>phenolic OH>carboxyl>substituted urea>amide>carbamate. (If the electron cloud density of the nucleophilic center is larger, its electronegativity is stronger, its reaction activity with isocyanate is higher, and the reaction speed is faster; otherwise, the activity is low.)
Effect of hydroxyl compounds on their reactivity with isocyanates
The reactivity of hydroxyl compounds to active hydrogen compounds (ROH or RNH2) that are reactive with isocyanates is related to the nature of R. When R is an electron-withdrawing group (low electronegativity), it is difficult to transfer hydrogen atoms, and active hydrogen compounds are The reaction of NCO is more difficult; if R is an electron-donating substituent, the reactivity of active hydrogen compounds and NCO can be improved.
What is the use of the reaction between isocyanate and water?
It is one of the basic reactions for preparing polyurethane foam. The reaction between them first generates unstable carbamic acid, and then decomposes into CO2 and amine. If the isocyanate is excessive, the generated amine will react with the isocyanate to form urea.
In the preparation of polyurethane elastomer
The moisture content of polymer polyols should be strictly controlled
It is required that there should be no air bubbles in elastomers, coatings and fibers, so the moisture content in raw materials must be strictly controlled, usually less than 0.05%.
Differences in the catalytic effects of amine and tin catalysts on isocyanate reactions
Tertiary amine catalysts have high catalytic efficiency for the reaction between isocyanate and water, while tin catalysts have high catalytic efficiency for the reaction between isocyanate and hydroxyl groups.
Why polyurethane resin can be regarded as a block polymer
What are the characteristics of chain segment structure?
Because the chain segment of polyurethane resin is composed of hard segments and soft segments, the hard segment refers to the segment formed by the reaction of isocyanate, chain extender, and cross-linking agent on the main chain of the polyurethane molecule. These groups have larger cohesion energy. , larger space volume and greater rigidity. The soft segment refers to the carbon-carbon main chain polymer polyol, which has good flexibility and is a flexible segment in the polyurethane main chain.
What are the factors that affect the performance of polyurethane materials?
Cohesive energy of the group, hydrogen bonding, crystallinity, degree of cross-linking, molecular weight, hard segments, and soft segments.
What raw materials do the soft and hard segments on the main chain of the polyurethane material molecule come from?
The soft segment is composed of oligomer polyols (polyester, polyether glycol, etc.), and the hard segment is composed of polyisocyanate or a small molecule chain extender.
How do soft segments and hard segments affect the performance of polyurethane materials?
Soft segment: (1) Molecular weight of the soft segment: Assuming that the molecular weight of the polyurethane is the same, if the soft segment is polyester, the strength of the polyurethane increases with the increase in the molecular weight of the polyester diol; if the soft segment is polyether, the strength of the polyurethane The strength decreases as the molecular weight of the polyether glycol increases, but the elongation increases. (2) Crystallinity of soft segments: It contributes greatly to the crystallinity of linear polyurethane segments. Generally speaking, crystallinity is beneficial to improving the performance of polyurethane products, but sometimes crystallization reduces the low-temperature flexibility of the material, and crystalline polymers are often opaque.
Hard segments: Hard segments usually affect the softening, melting temperature and high-temperature properties of polymers. Polyurethane prepared from aromatic isocyanate contains rigid aromatic rings in the hard segment, which increases the cohesion strength of the hard segment. The material strength is generally greater than that of aliphatic isocyanate polyurethane, but its resistance to ultraviolet degradation is poor and it is prone to yellowing. Aliphatic polyurethane does not yellow.
Polyurethane foam classification
(1) Hard foam and soft foam, ⑵ high density and low density foam, ⑶ polyester type, polyether type foam, ⑷ TDI type, MDI type foam, ⑸ polyurethane foam and polyisocyanurate foam, ⑹ one-step method and Prepolymerization production, ⑺ continuous and intermittent production, ⑻ block foam and molded foam.
Basic reactions for foam preparation
It refers to the reaction of -NCO with -OH, -NH2, H2O. When reacting with polyol, the “gel reaction” in the foaming process generally refers to the formation reaction of urethane. Because the foam raw materials use multi-functional raw materials, a cross-linked network is obtained, which allows the foaming system to gel quickly.
In the foaming system in the presence of water, a foaming reaction occurs. The so-called “foaming reaction” generally refers to the reaction in which water reacts with isocyanate to form substituted urea and releases CO2.
Bubble nucleation mechanism
The raw materials react in the liquid or rely on the temperature of the reaction to produce gaseous substances and volatilize the gas. As the reaction proceeds and a large amount of reaction heat is generated, the amount of gaseous substances produced and volatilized continues to increase. After the gas concentration increases beyond the saturation concentration, sustained bubbles begin to form and rise in the solution phase.
The role of foam stabilizer in the preparation of polyurethane foam
It has an emulsifying effect, which enhances the mutual solubility between the various components of the foam material; after adding the silicone surfactant, because it greatly reduces the surface tension γ of the liquid, the increased free energy required for gas dispersion is reduced, making it dispersed in the raw materials The air in the mixture is more likely to nucleate during the mixing process, which helps to generate fine bubbles and improves the stability of the foam.
Foam stabilization mechanism
The addition of appropriate surfactants is beneficial to the formation of fine bubble dispersions.
The formation mechanism of open-cell foam and closed-cell foam
Open-cell foam formation mechanism: In most cases, when the maximum pressure is generated in the bubble, the cell wall formed due to the gel reaction is not strong enough and cannot withstand the stretching of the wall membrane caused by the increase in gas pressure, and the bubble wall membrane is torn apart. Gas escapes from the break, forming an open-cell foam.
Formation mechanism of closed-cell foam: For hard foam systems, due to the use of multi-functional, low-molecular-weight polyether polyols and polyisocyanates to react, the gel speed is relatively fast, and the gas in the cells cannot squeeze through the bubble walls, thus forming a closed-cell foam system. Closed-cell foam.
Foaming mechanism of physical foaming agent and chemical foaming agent
Physical foaming agent: Physical foaming agent means that foam pores are formed through changes in the physical form of a certain substance, that is, through the expansion of compressed gas, the volatilization of liquids, or the dissolution of solids.
Chemical foaming agents: Chemical foaming agents are compounds that release gases such as carbon dioxide and nitrogen when decomposed by heating, and form pores in the polymer composition.
Preparation method of soft polyurethane foam
One-step method and prepolymer method
Prepolymer method: First, the polyether polyol and excess TDI are reacted to form a prepolymer containing free NCO groups, and then mixed with water, catalyst, stabilizer, etc. to form a foam. . One-step method: Various raw materials are directly mixed into the mixing head through calculation, and foam plastics are manufactured in one step. It can be divided into continuous type and intermittent type.
Characteristics of horizontal foaming and vertical foaming
Features of horizontal foaming: Side film lifting method: This method adds an upward pulling side paper device on the basis of the original horizontal foaming machine, so that the edges and the middle of the foam rise and foam simultaneously, thereby producing a nearly flat-top foam block. Balanced platen method: It is characterized by the use of top paper and top cover. Overflow tank method: It is characterized by the use of overflow tank and conveyor belt landing plate.
Features of vertical foaming: Foam blocks with a large cross-sectional area can be obtained with a smaller flow rate, while usually using a horizontal foaming machine to obtain blocks with the same cross-section, the flow level is 3 to 5 times larger than that of vertical foaming; due to the foam The cross-section of the block is large, there is no upper and lower skin, and the edge skin is thin, so it is greatly Cutting loss is reduced; the equipment occupies a small area, and the height of the factory is about 12 to 13 meters. The investment cost of the factory and equipment is lower than that of the horizontal foaming process; cylindrical or rectangular foams can be produced easily by replacing the hopper and the model, especially It can produce round foam blanks for rotary cutting.
Basic points for selecting raw materials for soft foam preparation
Polyol: Polyether polyol used in ordinary block foam. The molecular weight is generally 3000 to 4000, mainly polyether triol. High-resilience foam mostly uses polyethertriol with a molecular weight of 4500 to 6000. When the molecular weight increases, the tensile strength, elongation and resilience of the foam increase; the reactivity of similar polyethers decreases. As the functionality of the polyether increases, the reaction is relatively accelerated, the degree of cross-linking of the polyurethane produced increases, the hardness of the foam increases, and the elongation decreases.
Isocyanate: The isocyanate raw material of polyurethane soft block foam is mainly toluene diisocyanate (TDI-80). TDI-65, which has relatively low activity, is only used in polyester polyurethane foam or special polyether foam.
Catalysts: Catalysts used for foaming lumpy soft foam can be roughly divided into two categories: one is organometallic compounds, with stannous octoate being the most commonly used; the other is tertiary amines, with bis(dimethylaminoethyl) ether being the most commonly used. .
Foam stabilizer: In polyester-type polyurethane block foam, non-silicon surfactants are mainly used, while in polyether-type block foam, silicone-alkylene oxide copolymer is mainly used.
Foaming agent: Generally, when manufacturing polyurethane soft block foam with a density greater than 21 kilograms per cubic meter, only water is used as the foaming agent; in low-density formulations, low-boiling point compounds such as methylene chloride (MC) are used as auxiliary foaming agents. agent.
Effect of environmental conditions on physical properties of block foam
Effect of temperature: The foaming reaction of polyurethane accelerates as the material temperature rises, which may cause core burn and fire hazards in sensitive formulations. Effect of air humidity: As the humidity increases, due to the reaction of the isocyanate group in the foam with the moisture in the air, the hardness of the foam decreases and the elongation increases; due to the increase in urea groups, the tensile strength of the foam decreases. increased. Effect of atmospheric pressure: For the same formula, when foaming at higher altitudes, the density decreases significantly.
The main differences between the raw material systems used in cold molded soft foam and hot molded foam
The raw materials used in cold-cured molding have high reactivity. No external heating is required during curing. Relying on the heat generated by the system, the curing reaction can be basically completed in a short time. The raw materials can be demoulded within a few minutes after injection. The raw materials for thermally matured molded foam have low reactivity. After the reaction mixture is foamed in the mold, it needs to be heated together with the mold. The foam products can be demoulded only after they are fully matured in the drying tunnel.
How does cold molded soft foam compare to hot molded foam?
① No external heat is required during the production process, which can save a lot of heat energy; ② The sag coefficient (sag ratio) is high and the comfort performance is good; ③ The rebound rate is high; ④ Foam without flame retardants also has certain flame retardant properties; ⑤ The production cycle is short, molds can be saved, and costs can be saved.
The respective characteristics and uses of soft foam and hard foam
Features of soft foam: The cell structure of polyurethane soft foam is mostly open-celled. Generally, it has the properties of low density, good elastic recovery, sound absorption, breathability, heat preservation and other properties.
Usage: Mainly used as furniture, cushion materials, vehicle seat cushion materials, and various soft cushion laminated composite materials. Soft foam is also used as filter materials, sound insulation materials, shock-proof materials, decorative materials, etc. in industry and civil use. Packaging materials and thermal insulation materials, etc.
Features of rigid foam: polyurethane foam has light weight, high specific strength, and good dimensional stability; polyurethane rigid foam has superior thermal insulation performance; strong adhesion; good aging performance and long insulation service life; the reaction mixture has good fluidity and can Smoothly fill complex-shaped mold cavities or spaces; the raw materials for polyurethane rigid foam production are highly reactive and can be cured quickly, enabling high-efficiency, mass production in factories.
Usage: Used as insulation layer material for refrigerators, freezers, refrigerated containers, cold storage, etc., insulation layer for oil transportation pipelines and hot water transportation pipelines, insulation layer for building walls and roofs, insulation sandwich panels, etc.
Key points of hard foam formula design
Polyols: Polyether polyols used in rigid foam formulations are generally high functionality, high hydroxyl value (low molecular weight) polyoxypropylene polyols; Isocyanates: Currently the isocyanates used in rigid foam are mainly polymethylene polyphenyl. Polyisocyanate (generally called PAPI), namely crude MDI and polymerized MDI;
Foaming agent: (1) CFC foaming agent (2) HCFC and HFC foaming agent (3) Pentane foaming agent (4) Water; Foam stabilizer: The foam stabilizer used in polyurethane rigid foam formulations is generally polyurethane foaming agent. Block polymer of dimethylsiloxane and polyoxyalkylene. Currently, most foam stabilizers are mainly Si-C type;
Catalyst: The catalyst of hard foam formula is mainly tertiary amine, and organic tin catalyst can be used in special occasions; other additives: According to the different use requirements and needs of polyurethane rigid foam products, flame retardants and hole openings can also be added to the formula. Agents, smoke inhibitors, antioxidants, antifungal agents, toughening agents and other additives.
Preparation principle of whole-skin molded foam
Integral skin foam (ISF), also known as self-skinning foam, is a foam that generates a dense skin during manufacturing.
Characteristics and uses of polyurethane microcellular elastomer
Features: 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 are alternately 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.
What are the main performance characteristics of polyurethane elastomer?
Performance characteristics: 1. High strength and elasticity, and can maintain high elasticity in a wide hardness range (Shore A10 ~ Shore D75); generally no plasticizer is needed to achieve the required low hardness, so there is no need Problems caused by plasticizer migration; 2. Higher load-bearing capacity than other elastomers at the same hardness; 3. Excellent resistance to Wearability, its wear resistance is 2 to 10 times that of natural rubber; 4. Excellent resistance to grease and chemicals; aromatic polyurethane is radiation resistant; has excellent oxygen resistance and ozone resistance; 5. High impact resistance, Good fatigue and vibration resistance, suitable for high-frequency flexing applications; 6. Good low-temperature compliance; 7. Ordinary polyurethane cannot be used under 100 It is used above 140℃, but the special formula can withstand high temperatures of 140℃; 8. Molding and processing costs are relatively low.
Polyurethane elastomers are classified according to polyol, isocyanate, manufacturing process, etc.
1. According to oligomer polyol raw materials, polyurethane elastomers can be divided into polyester type, polyether type, polyolefin type, polycarbonate type, etc. According to the specific varieties, polyether type can be divided into polytetrahydrofuran type, poly Propylene oxide type, etc.; 2. According to the different diisocyanates, it can be divided into lipids Aliphatic and aromatic elastomers are subdivided into TDI type, MDI type, IPDI type, NDI type and other types; according to the manufacturing process, polyurethane elastomers are traditionally divided into casting type (CPU), thermoplastic (TPU), There are three categories of mixing type (MPU).
What are the factors that affect the performance of polyurethane elastomer from the perspective of molecular structure?
From the molecular structure point of view, polyurethane elastomer is a block polymer. It is generally composed of long flexible chains of oligomer polyols to form soft segments, and diisocyanate and chain extenders to form hard segments. The hard segments and soft segments are arranged alternately. 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”.
Differences in performance between ordinary polyester and polytetrahydrofuran ether elastomers
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.