Definition: The “Fifth Plastic”
Polyurethane (PU) is a polymer containing repeating urethane groups (-NHCOO) in its main chain. It is an emerging organic polymer material, renowned for its exceptional versatility and performance, and is widely recognized as the “fifth plastic” after polyethylene, polypropylene, PVC, and polystyrene.
The synthesis of polyurethane primarily involves two key raw materials: organic polyisocyanates and hydroxyl-terminated compounds (polyols). Depending on the type of polyol used, PU is classified into polyester type and polyether type. Polyester-type PU (from diisocyanate and hydroxyl-terminated polyester) offers higher strength and hardness; polyether-type PU (from diisocyanate and hydroxyl-terminated polyether) provides better flexibility and hydrolysis resistance.
Key Raw Material Classifications
Understanding the different types of polyisocyanates and polyols is essential for selecting the right PU product for specific applications.
1. Polyisocyanates
Polyisocyanates contain two or more NCO groups. They are classified by the number of NCO groups (diisocyanates, triisocyanates, polymeric types) and by molecular structure (aromatic, aliphatic, cycloaliphatic).
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TDI (Toluene Diisocyanate)
TDI is widely used. It has two isomers (2,4 and 2,6). Commercial grades include TDI-100 (100% 2,4-isomer), TDI-80 (80% 2,4, 20% 2,6), and TDI-65. TDI is commonly used in flexible PU foams, cast elastomers, coatings, and adhesives.
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MDI (Methylene Diphenyl Diisocyanate)
MDI has three isomers: 4,4-MDI, 2,4-MDI, and 2,2-MDI. Pure MDI (MDI-100) is mainly used in spandex fibers and elastomers. Polymeric MDI (often called “black material”) is a mixture of pure MDI and polymethylene polyphenyl polyisocyanate, widely used in rigid PU foams (refrigerators, insulation panels, automotive parts).
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HDI (Hexamethylene Diisocyanate)
HDI is an aliphatic isocyanate. Because it contains no benzene ring, it offers excellent yellowing resistance and flexibility. HDI is reactive, volatile, and toxic, so it is often converted into biuret or trimer for use as a curing agent in coatings.
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IPDI (Isophorone Diisocyanate)
IPDI is a cycloaliphatic isocyanate. It provides outstanding light stability and chemical resistance. IPDI is used in high-end products such as UV-curable resins and premium coating curing agents.
2. Polyols
Polyols form the soft segment of PU and greatly influence final properties. They are mainly divided into polyether polyols and polyester polyols.
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Polyether Polyols (PPG)
These contain ether bonds (-R-O-R-) and terminal OH groups. For flexible foams, polyether polyols have long chains, low functionality, and average molecular weights of 2000–6500. For rigid foams, they have lower molecular weights (300–400) and higher hydroxyl values (350–600 mg KOH/g). -
Polyester Polyols
These contain ester bonds (-COO-) or carbonate bonds (-OCOO-), offering high polarity, high cohesive energy, and excellent adhesion. They improve oil and heat resistance but are more susceptible to hydrolysis. Aliphatic polyester polyols are used in PU inks, elastomers, shoe soles, adhesives, and coatings. Aromatic polyester polyols, containing benzene rings, enhance strength, heat resistance, and flame retardancy, making them suitable for rigid foams, adhesives, and elastomers.
3. Auxiliary Materials
PU formulations often include chain extenders (e.g., 1,4-butanediol), catalysts (e.g., tertiary amines, organotin compounds), surfactants, crosslinkers, and blowing agents (e.g., water, physical blowing agents). These additives control reaction kinetics, cell structure, density, and final product properties.
Advantages: Why Choose Polyurethane?
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Excellent mechanical properties – High abrasion resistance, tear strength, and load capacity. Cast PU elastomers surpass ordinary rubber in tensile strength and elongation.
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Wide hardness range – From Shore A10 (very soft) to Shore D80 (very hard), with adjustable resilience.
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Superior thermal insulation – Rigid PU foams have thermal conductivity as low as 0.018-0.028 W/m·K, ideal for refrigeration and building insulation.
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Chemical and oil resistance – Polyester-type PU resists oils, fuels, and solvents, suitable for seals and rollers.
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Strong adhesion – PU adhesives bond well to metals, plastics, wood, glass, and ceramics.
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Lightweight and eco-friendly potential – Foaming reduces density; bio-based polyols and water-blown technologies lower environmental impact.
Application Scenarios
| Category | Examples |
|---|---|
| Flexible Foam | Furniture cushions, car seats, mattresses, acoustic panels, cosmetic sponges |
| Rigid Foam | Refrigerator insulation, cold storage panels, building insulation, automotive headliners, pipe insulation |
| Elastomers | Cast PU (CPU), thermoplastic PU (TPU), millable PU (MPU) – used in seals, rollers, conveyor belts, tires, shoe soles, medical devices |
| Coatings & Adhesives | Automotive clearcoats, wood finishes, structural adhesives (e.g., wind turbine blades), footwear adhesives |
| Fibers & Synthetic Leather | Spandex (elastic fiber), PU artificial leather for clothing, bags, upholstery |
In the automotive industry, a typical car contains 15–25 kg of PU (seats, headliners, sound insulation). In appliances, a 500-liter refrigerator uses approximately 5–8 kg of rigid PU foam, cutting energy consumption by over 30%. In footwear, more than 30% of soles are made from PU.
Conclusion & Call to Action
Polyurethane’s ability to be tailored from soft foams to rigid plastics, elastomers, and adhesives makes it indispensable across industries. Whether you need raw materials (polyisocyanates, polyols), formulation optimization, or process support, ASSEMBTEK offers expert guidance, material selection, and production assistance.
👉 Contact ASSEMBTEK Today for Polyurethane Solutions
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