Introduction to Polyurethanes
Polyurethane (PU) is a group of polymers formed by the reaction of a polyol (a compound with reactive hydroxyl groups -OH) and an isocyanate (a compound with reactive -NCO groups). The resulting chemical bond is called the urethane bond (carbamate group), after which the entire class of materials is named.
Polyurethanes were discovered by chemist Otto Bayer and his team at IG Farben in 1937. Bayer was looking for a way to compete with DuPont's newly developed polyamides (nylon), and while experimenting with the reaction of diisocyanates and diols, he came across a material with a unique combination of strength, elasticity, and durability. During World War II, PU coatings and foams were used in military aviation, but mass commercialization did not arrive until the 1950s and 1960s.
Today, polyurethanes are among the most versatile polymers of all — global production exceeds 25 million tons per year. Their strength lies in the fact that, by choosing the type of polyol, the type of isocyanate, catalysts, and additives, extremely different materials can be produced from practically the same chemical platform:
Flexible foams
mattresses, seats, automotive upholstery
Rigid foams
thermal insulation for buildings, refrigeration, sandwich panels
Elastomers & TPU
automotive parts, wheels, hoses, shoe soles
Coatings & varnishes (2K PU)
protective coatings for wood, metal, floors
Adhesives & sealants
construction, woodworking industry, sandwich panels
Fibers (spandex/elastane)
textile industry, elastic fabrics
Key Definition
Polyurethane is formed by polyaddition (not polycondensation) — no by-product such as water is formed in the reaction of a polyol with an isocyanate. The reaction is highly exothermic, and both its rate and the resulting material properties are governed by the choice of catalysts, polyol functionality, and the NCO index.
Polyurethane Reaction Chemistry
The basis of all PU chemistry is the reaction of an isocyanate group (-N=C=O) with a hydroxyl group (-OH) of a polyol. This forms a urethane bond (-NH-CO-O-):
This reaction is exothermic (releases heat, approximately 24 kJ/mol) and proceeds without the release of a by-product. The reaction rate can be controlled with catalysts — without them, it would proceed too slowly at room temperature for industrial use.
Reaction with Water — the Source of CO2 for Foaming
A key side reaction in foam production is the reaction of isocyanate with water. Water reacts with the -NCO group to form an unstable carbamic acid, which immediately decomposes into an amine and carbon dioxide (CO2):
The released CO2 creates bubbles that inflate the reaction mixture — this is called the blowing reaction, while the main polyol-isocyanate reaction is called the gelling reaction. The resulting primary amine further reacts with additional isocyanate to form a urea bond and create a polymer with so-called hard segments. The balance between the rate of the gelling and blowing reactions is key to the quality of the resulting foam — too fast a blowing reaction leads to foam collapse, too slow leads to over-densification.
NCO Index and Stoichiometry
The NCO index expresses the ratio of the actually dosed amount of isocyanate to the theoretical stoichiometric amount needed for a complete reaction with all hydroxyl (and, in foam systems, water) groups, multiplied by one hundred. An index of 100 means an exact stoichiometric ratio.
| NCO Index | Typical System | Effect |
|---|---|---|
| 90–100 | Flexible foams (softer) | Lower hardness, more unreacted OH groups |
| 100–110 | Flexible foams (standard) | Balanced mechanical properties |
| 105–115 | Rigid foams, elastomers | Higher crosslink density, harder material |
| 115–130 | Systems with biuret/allophanate crosslinking | Increased thermal resistance and hardness |
In practice, isocyanate is always dosed in a slight excess over stoichiometry, since part of it reacts with atmospheric moisture and impurities. Precise stoichiometry is one of the most important parameters a formulator controls — an error in the ratio of just a few percent significantly changes the mechanical properties of the resulting material.
Polyols — Types and Properties
The polyol forms the so-called soft segment in the final material and largely determines flexibility, chemical resistance, and hydrolytic stability. The two main groups — polyether and polyester polyols — have fundamentally different properties and uses.
Polyether Polyols
Produced by polymerizing propylene oxide (PO) and/or ethylene oxide (EO) on an initiator (glycerin, propylene glycol, sorbitol, sucrose). They make up approximately 90% of global polyol consumption for foams thanks to their low cost, good hydrolytic stability, and low viscosity that facilitates processing.
Polyester Polyols
Produced by the esterification of dicarboxylic acids (adipic, phthalic) with glycols. They provide higher mechanical strength, better resistance to oils and solvents, and better adhesion, but have poorer hydrolytic stability and a higher price. They are used primarily in elastomers, CASE applications, and high-quality coatings.
| Property | Polyether Polyols | Polyester Polyols |
|---|---|---|
| Price | Lower | Higher (2–3×) |
| Hydrolytic stability | High | Lower (prone to hydrolysis) |
| Mechanical strength | Medium | High |
| Oil/solvent resistance | Low | High |
| Viscosity | Low | Higher |
| Typical use | Flexible and rigid foams | Elastomers, CASE, coatings |
Molecular Weight and Functionality
Two key polyol parameters determine the properties of the resulting polymer: molecular weight (usually 200–8,000 g/mol for flexible foams, lower for rigid systems) and functionality (number of OH groups per molecule, typically 2–8).
- →Higher polyol molecular weight → longer soft segments → higher flexibility and lower hardness
- →Higher functionality → denser crosslinking → higher stiffness and thermal resistance (typical for rigid foams, functionality 3–8)
- →Low functionality (2) → linear structure → typical for elastomers and TPU
Practical Tip
For flexible foam, choose a polyether triol with an MW of 3,000–6,000 g/mol. For rigid insulation foam, choose a polyol with high functionality (4–8) and low molecular weight (250–700 g/mol) — this ensures the dense network structure needed for low thermal conductivity.
Isocyanates — Types and Safety
Isocyanates form the so-called hard segment of the polyurethane and are responsible for stiffness, thermal resistance, and mechanical strength. Industrial practice is dominated by two aromatic isocyanates — MDI and TDI — supplemented by aliphatic isocyanates for UV-stable applications.
MDI (Diphenylmethane Diisocyanate)
Lower vapor pressure than TDI, hence lower inhalation exposure risk. It exists in two forms: monomeric 4,4'-MDI (pure, for elastomers and CASE applications) and polymeric pMDI (a mixture of oligomers, higher functionality, for rigid foams). It is also used in modified/prepolymer variants with adjusted reactivity and viscosity.
4,4'-MDI (monomeric)
Elastomers, TPU, CASE, microcellular foams
pMDI (polymeric)
Rigid foams, insulation panels, spray foam
TDI (Toluene Diisocyanate)
Produced as a mixture of the 2,4-TDI and 2,6-TDI isomers, most commonly in a ratio of 80:20 (TDI-80) or 65:35 (TDI-65). Higher vapor pressure means higher volatility and stricter ventilation and respiratory protection requirements. Dominates flexible foam production thanks to lower cost and a favorable reaction profile.
TDI-80 (80/20)
Standard flexible foams — mattresses, seats
TDI-65 (65/35)
Higher reactivity, special foam profiles
Aliphatic Isocyanates — HDI and IPDI
HDI (hexamethylene diisocyanate) and IPDI (isophorone diisocyanate) do not contain an aromatic ring, which makes them resistant to yellowing from UV radiation. They are used exclusively where long-term color stability is required — automotive clear coats, outdoor industrial coatings, and UV-stable elastomers. They are significantly more expensive than MDI/TDI.
Safety When Working with Isocyanates
All diisocyanates are classified as respiratory and skin sensitizers. Repeated exposure can lead to the development of isocyanate asthma, which is irreversible even after exposure ends. Occupational exposure limits (OELs) in the EU range from a few to tens of µg/m³ (e.g., typically 0.05 mg/m³ as TWA for MDI). Essential measures include: closed dosing systems, vapor extraction, respirators with protection against aerosols and vapors, isocyanate-resistant protective gloves, and regular medical examinations of exposed workers.
Catalysts for PU Systems
Without catalysts, the polyol-isocyanate reaction would proceed too slowly at room temperature for industrial processing. Catalysts are divided into two main groups depending on which reaction they primarily accelerate.
Amine Catalysts
Tertiary amines such as DABCO (triethylenediamine, TEDA), dimethylcyclohexylamine (DMCHA), or bis(2-dimethylaminoethyl) ether accelerate both the gelling and blowing reactions, with varying selectivity. The choice and combination of amine catalysts determines the ratio between the rate of CO2 formation (blowing) and urethane bond formation (gelling/crosslinking) — this balance is key to the structure and stability of the foam during formation.
Organometallic Catalysts
Tin catalysts, primarily DBTDL (dibutyltin dilaurate) and stannous octoate, strongly favor the gelling reaction and are used for rapid curing of elastomers, coatings, and adhesives. Due to growing concerns about the toxicity of organotin compounds (some are classified as reprotoxic), the market is increasingly shifting to bismuth catalysts (bismuth neodecanoate) and zirconium alternatives, especially in consumer and healthcare applications.
| Catalyst | Type | Primary Effect | Use |
|---|---|---|---|
| DABCO / TEDA-33 | Amine | Gelling and blowing | Flexible and rigid foams |
| DMCHA | Amine | Blowing | Rigid foams, spray foam |
| DBTDL | Organotin | Gelling | Elastomers, coatings, adhesives |
| Cínatý oktoát | Organotin | Gelling (fast) | Flexible foams (older systems) |
| Bismut neodekanoát | Tin-free alternative | Gelling | Consumer and healthcare applications |
A practical formulation usually combines an amine catalyst (to control the reaction profile — cream time, rise time) with a small amount of a metal catalyst (to achieve the required hardness and curing speed). The exact ratio is tuned empirically for each specific system.
Foam Systems
Foams form the largest segment of polyurethane consumption by volume. They are divided into flexible, rigid, and semi-rigid based on cell structure and mechanical properties.
| Foam Type | Density | Cell Structure | Typical Application |
|---|---|---|---|
| Flexible foam | 20–60 kg/m³ | Open cells | Mattresses, seats, car upholstery |
| High-resilience flexible (HR) | 35–60 kg/m³ | Open cells, high resilience | Premium seats, orthopedic mattresses |
| Rigid foam | 30–50 kg/m³ | Closed cells (>90%) | Insulation panels, refrigeration, spray foam |
| Semi-rigid foam | 40–80 kg/m³ | Mixed structure | Bumpers, dashboard panels |
| Integral (skin) foam | proměnná (jádro-povrch) | Compact surface, porous core | Steering wheels, armrests, shoe soles |
Blowing Agents
The history of blowing agents reflects the evolution of environmental legislation. Originally, CFCs (chlorofluorocarbons), such as CFC-11, were used due to their excellent insulating properties. The 1987 Montreal Protocol mandated their phase-out due to ozone layer depletion. HCFCs followed (less harmful, but still regulated), and today the following are used primarily:
- →Water — reacts with isocyanate to form CO2 (see the chapter on reaction chemistry), the most common for flexible foams
- →Pentane (cyclopentane, isopentane) — a physical blowing agent for rigid insulation foams, zero ozone depletion potential
- →HFO (hydrofluoroolefins) — a new generation with low GWP (global warming potential), replacing older HFCs in spray foam applications
Trend in Insulation Foams
The EU F-Gas Regulation progressively restricts HFC blowing agents due to their high GWP. Rigid foam manufacturers are increasingly switching to HFO or pentane/CO2 combinations, which combine good insulating performance with low environmental impact.
Elastomers and CASE Applications
The acronym CASE (Coatings, Adhesives, Sealants, Elastomers) denotes non-foam applications of polyurethanes, where their mechanical strength, abrasion resistance, and chemical resistance are utilized.
Cast Elastomers
Produced by reacting a prepolymer (isocyanate pre-reacted with polyol) with a chain extender, poured into a mold. They are used for rollers, wheels, seals, conveyor belts — anywhere a combination of high tensile strength, abrasion resistance, and vibration damping is needed. Hardness can be specifically tuned across a wide range, from Shore A 60 to Shore D 75.
Thermoplastic Polyurethanes (TPU)
TPUs are linear block copolymers (without chemical crosslinking) that can be repeatedly melted and shaped by injection molding or extrusion like a common thermoplastic. They combine the elastic properties of rubber with the processability of plastic. They are used for hoses, cable sheathing, sports footwear, protective films, and 3D printing of flexible parts.
Two-Component (2K) PU Coatings
Consist of component A (polyol or acrylic polyol) and component B (isocyanate hardener, typically based on HDI biuret or isocyanurate for UV stability). After mixing, they cure to form a highly resistant coating film with excellent chemical resistance, hardness, and gloss. They are used for automotive clear coats, industrial coatings for steel structures, and flooring systems.
Key Mechanical Parameters of Elastomers
Shore A Hardness
40–95
Shore D Hardness
45–85
Tensile Strength
up to 60 MPa
Additives and Auxiliary Substances
Silicone Surfactants
- →Stabilize growing foam cells during foaming and prevent their coalescence
- →Control the size and uniformity of the cell structure (cell structure control)
- →Essential for achieving fine, uniform foam without cracks and collapse
Flame Retardants
- →Phosphorus compounds (TCPP, TEP) — the most widespread, reducing flammability
- →A trend toward halogen-free variants due to concerns about the toxicity of combustion products
- →Mandatory for construction insulation foams under fire standards (EN 13501)
Chain Extenders
- →1,4-Butanediol (BDO) — the most common extender for elastomers and TPU
- →MOCA (4,4-methylenebis(2-chloroaniline)) — provides excellent hardness, but is classified as a category 1B carcinogen and its use is strictly restricted in the EU
- →Ethylene glycol, hydroquinone bis(hydroxyethyl) ether (HQEE) as alternatives to MOCA
Fillers
- →Calcium carbonate — reduces cost, increases stiffness
- →Glass fibers — increase the modulus of elasticity and dimensional stability
- →Carbon black — UV protection and electrical conductivity in antistatic applications
Legislation and Safety
Working with isocyanates is one of the most strictly regulated areas of the chemical industry in the EU. A recent amendment to the REACH Regulation is a key milestone.
⚠ Mandatory Training for Working with Diisocyanates Since August 2023
The REACH Regulation (Annex XVII, entry 74) introduced, effective August 24, 2023, a requirement that all industrial and professional users working with diisocyanates (MDI, TDI, HDI, IPDI, and others) at a concentration above 0.1% must complete appropriate safe-handling training before starting work. The training must be documented and presented to supervisory authorities upon request. Without valid proof of completed training, diisocyanates may no longer be used in the EU — suppliers are required to flag this obligation on the packaging and in the safety data sheet.
REACH (ES 1907/2006), příloha XVII, položka 74
Mandatory training for users of diisocyanates above 0.1% concentration, effective since August 2023. Covers both manufacturers of PU systems themselves and end processors (e.g., spray foam insulation applicators).
CLP (ES 1272/2008)
Diisocyanates are classified as category 1 respiratory sensitizers (H334) and category 1 skin sensitizers (H317). They require appropriate hazard pictograms and precautionary statements on the packaging.
Expoziční limity (OEL)
National exposure limits for MDI and TDI range from around 0.005–0.02 ppm (TWA) to 0.02–0.07 ppm (STEL). Exact values vary by member state — in the Czech Republic they are set by a government regulation on health protection at work.
MOCA a restrikce chain extenderů
MOCA is classified as a category 1B carcinogen (H350), and its use is subject to authorization under REACH. Many manufacturers are switching to alternative chain extenders (HQEE, ethylene glycol).
Nařízení o F-plynech (EU 517/2014)
Restricts the use of HFC blowing agents with high GWP in foam systems, accelerating the transition to HFO and hydrocarbon alternatives.
Industrial Applications
Automotive Industry
- →Seats and upholstery: flexible and HR foams, density 45–60 kg/m³
- →Headliners: semi-rigid composite foams
- →Bumpers and panels: integral and microcellular foams, RIM elastomers
- →Steering wheels and armrests: integral foam with a compact surface
Construction
- →Sandwich insulation panels: rigid foam between steel or aluminum sheets
- →Spray polyurethane foam (SPF): on-site application, closed-cell
- →Sealing foams (PU gun foam): installation and insulation sealing of joints
- →2K PU floor coatings and screeds: chemically resistant industrial floors
Furniture Industry
- →Mattresses: flexible foam 20–40 kg/m³, viscoelastic (memory) foams
- →Furniture upholstery: standard and HR flexible foams
- →Structural furniture parts: rigid integral foams imitating wood
Footwear Industry and Adhesives
- →Shoe soles: microcellular PU foam or TPU, abrasion resistance and flexibility
- →Adhesives for wood and sandwich panels: 1K and 2K PU adhesives with high bond strength
- →Construction sealants: PU sealants with good elasticity and adhesion
How to Choose the Right PU System
Choosing the optimal polyurethane system requires a systematic approach based on the requirements of the final application, not just the price of raw materials.
1. Define the Density
Flexible upholstery: 20–60 kg/m³. Rigid insulation: 30–50 kg/m³. Integral foams: core and surface density differ within a single part.
2. Determine the Required Hardness
Shore A for soft elastomers and flexible foams (40–95 Shore A), Shore D for rigid elastomers and compact materials (45–85 Shore D).
3. Verify the Reaction Profile
Cream time (reaction onset, visual whitening of the mixture), gel time (onset of gelling), tack-free time (surface stops being sticky) — must match your production line and mold time.
4. Choose the Processing Method
Pour-in-place for furniture and panels, spraying for on-site insulation, RIM (Reaction Injection Molding) for precise automotive parts.
5. Consider Chemical and UV Resistance
For outdoor applications exposed to UV radiation, choose aliphatic isocyanates (HDI/IPDI) instead of aromatic ones (MDI/TDI), which yellow.
6. Verify Legislative Requirements
Ensure your workers have valid training under REACH Annex XVII for working with diisocyanates, and check the current SDS.
7. Run a Trial Batch
Before series production, test the reaction profile, mechanical properties, and dimensional stability on a smaller trial batch.
Frequently Asked Questions
What is polyurethane and how is it formed?+
What is the difference between MDI and TDI?+
What does the NCO index mean in a polyurethane formulation?+
Why has training for working with isocyanates been mandatory since August 2023?+
What is the difference between flexible and rigid PU foam?+
How do I choose the right polyurethane system for my application?+
Looking for a Specific Polyurethane Raw Material?
GCG Group supplies polyols, isocyanates (MDI, TDI), catalysts, and special additives for the production of foams, elastomers, and CASE applications. Samples on request, technical support, individual pricing.