Polyurethane is a versatile polymer composed of organic units joined by urethane (carbamate) links. It can exist in various forms ranging from flexible foams to rigid plastics and elastomers.
Preparation of Polyurethane
Polyurethanes are synthesized through a step-growth polymerization reaction between:
- Diisocyanates (or polyisocyanates) – e.g., Toluene diisocyanate (TDI), Methylene diphenyl diisocyanate (MDI), Hexamethylene diisocyanate (HDI)
- Polyols – Polyether polyols or Polyester polyols (compounds with multiple hydroxyl groups)
Main Reaction
| Common Industrial Isocyanates and Polyols | ||
|---|---|---|
| Type | Examples | Typical Use |
| Aromatic Diisocyanates | TDI (2,4- and 2,6-toluene diisocyanate) MDI (4,4'-methylene diphenyl diisocyanate) Polymeric MDI (PMDI) | Flexible foams (TDI), Rigid foams & CASE (MDI/PMDI) |
| Aliphatic Diisocyanates | HDI, IPDI, H12MDI | Weather-resistant coatings, light-stable elastomers |
| Polyether Polyols | PPG (polypropylene glycol), PTMEG | Flexible foams, elastomers |
| Polyester Polyols | Adipate-based, PCL, PCA | Higher strength elastomers, adhesives |
| Amine-initiated Polyols | Mannich-base polyols, ortho-toluene diamine polyols | Fast-reacting rigid foams (spray foam) |
Foaming Reaction (for foam production)
Water reacts with isocyanate to produce carbon dioxide gas, which acts as a blowing agent:
Step-1: R–N=C=O + H2O → R–NH2 + CO2↑
Step-2: R–NH2 + R–N=C=O → R–NH–C(=O)–NH–R (urea linkage) + heat
The overall stoichiometry with water:
2 R–NCO + H2O → R–NH–C(=O)–NH–R + CO2
Catalysts (amines, tin compounds), surfactants, and physical blowing agents (e.g., pentane, HFCs) control cell structure and foam properties.
Properties of Polyurethane
| Property | Description |
|---|---|
| Versatility | Can be soft foam, rigid foam, elastomer, coating, adhesive, or fiber |
| Density | From 6 kg/m³ (soft foam) to >1000 kg/m³ (solid elastomers) |
| Mechanical Strength | High tensile strength, tear resistance, abrasion resistance (especially in elastomers) |
| Thermal Insulation | Excellent (lowest thermal conductivity among common insulation materials) |
| Chemical Resistance | Good resistance to oils, solvents, and many chemicals |
| Elasticity & Resilience | Flexible foams recover shape; elastomers have high elasticity |
| Temperature Range | Typically –40 °C to +120 °C (special grades higher) |
| Water Resistance | Hydrophobic (especially closed-cell foams) |
Major Uses of Polyurethane
| Form | Applications |
|---|---|
| Flexible Foam | Mattresses, furniture cushions, car seats, carpet underlay, packaging |
| Rigid Foam | Thermal insulation in buildings, refrigerators, freezers, pipes, panels |
| Elastomers | Wheels, rollers, bushings, seals, gaskets, shoe soles, skate wheels |
| Coatings & Paints | Floor finishes, automotive clear coats, wood finishes, anti-corrosion coatings |
| Adhesives | Structural adhesives, woodworking, construction, footwear |
| Fibers (Spandex) | Stretchable clothing (sportswear, underwear, swimwear) |
| Binders | Rebonded foam (carpet padding), wood composites (OSB, particleboard) |
| Sealants | Construction joints, automotive windshields |
Summary
Polyurethane is one of the most versatile polymers available today. Its properties can be precisely tailored by varying the raw materials and formulation, making it indispensable in industries ranging from furniture and automotive to construction and textiles.