What Are the Raw Materials of Flexible Polyurethane Foam?-Part 1

Flexible polyurethane foam is a polymer material widely used in furniture, mattresses, and other fields. Its excellent elasticity and comfort make it an important material in modern industry. To understand the preparation of flexible polyurethane foam, we must first clarify its raw material composition. The main raw materials of flexible polyurethane foam include:

1.Isocyanates: Aliphatic, cycloaliphatic, aromatic

2.Oligomer Polyols: Polyester polyols, polyether polyols (propylene oxide polyether polyols, tetrahydrofuran polyether polyols, other polyether polyols), other polyols

3.Chain Extenders (Crosslinkers): Amine-type chain extenders, alcohol-type chain extenders

4.Catalysts: Tertiary amine catalysts, organometallic compounds

5.Additives: Flame retardants, antioxidants, UV absorbers, colorants, plasticizers

Among these, toluene diisocyanate (TDI) is the most commonly used isocyanate. It reacts with polyols to form the polyurethane backbone and, with the aid of blowing agents and other additives, produces a soft, porous material. This article first introduces TDI.

Toluene Diisocyanate (TDI)

Molecular Weight: 174.2

Equivalent Weight: 87.1

Functionality (f): 2

Density: 1.2244 g/cm³

TDI is the earliest developed, most widely used, and largest-volume diisocyanate monomer. Depending on the positions of the two isocyanate (–NCO) groups on the benzene ring, it is divided into 2,4-TDI and 2,6-TDI.

At room temperature, TDI is a colorless or slightly yellow transparent liquid with a strong irritating odor. It is commercially available in three grades:

*T-65: A 65%/35% mix of 2,4-TDI and 2,6-TDI.

*T-80: An 80%/20% mix (most widely used and cost-effective, common in the coatings industry).

*T-100: >95% 2,4-TDI with minimal 2,6-TDI (more expensive, for high-performance uses).

Reactivity: Due to steric hindrance from the –CH₃ group, the 4-position –NCO is more reactive than the 2-position –NCO. At 50°C, reactivity differs by about 8 times, equalizing around 100°C. This can be used to design well-structured polymers.

Drawbacks: TDI has a high vapor pressure, is volatile, and toxic. It is often used as an oligomer to reduce hazards. TDI-based polyurethane products can yellow over time due to photochemical reactions of aromatic groups, producing color-causing compounds like quinones or azo structures.

For flexible foam, TDI 80/20 is typically used. The 2,4 isomer reacts quickly, while the 2,6 isomer contributes to foam curing. Catalysts mainly adjust reactivity. Higher 2,4 content increases foaming time and improves softness within limits.

Properties of TDI

Relative Density (20/4°C): 1.2244

Freezing Point: TDI-65: 3.5–5.5°C; TDI-80: 11.5–13.5°C; TDI-100: 19.5–21.5°C

Flash Point (Open Cup): 132°C

Boiling Point: 251°C

Refractive Index (20°C): 1.569

Storage: Sealed with inert gas (argon/nitrogen), stored in cool, dry, and dark conditions

Hazard Classification: Class 6.1 toxic substance

UN Number: 2078

Physical State: Colorless liquid, pungent odor, darkens upon light exposure

TDI reacts with water to form unstable carbamic acid, which decomposes into amines and CO₂ gas, producing foam. More gas lowers foam density. Urea structures formed can increase foam hardness and strength, especially in low-water formulas.

TDI Specifications and Applications

T-65: Mainly used for flexible polyurethane foam

T-80: Most used TDI, for foam and other PU products

T-100: High-purity TDI for elastomers, coatings, and fibers

TDI-80 Comparison from Different Manufacturers

Structural Characteristics of Isocyanates

Isocyanates are compounds containing –N=C=O groups. Their general formula is:

R–(NCO)ₙ, where R can be alkyl, aryl, or cycloalkyl, and n ≥ 2 for PU synthesis.

Isocyanate Classification

1.By Number of NCO Groups

Diisocyanates: TDI, MDI, HDI, NDI, PPDI, IPDI, XDI

Polyisocyanates: PAPI

2.By Structure

Aliphatic: HDI

Cycloaliphatic: IPDI, HTDI, HMDI

Aromatic: TDI, MDI, PAPI, PPDI, NDI

Reaction Mechanism

TDI reacts with water to generate unstable carbamic acid, which quickly decomposes into amine and CO₂. The amine reacts with excess isocyanate to form urea, producing gas and forming foam. Urea contributes to the rigid segments in the polymer, affecting foam density, hardness, and load-bearing capacity.

TDI Dosage Formula

TDI dosage = [0.1554 × (PPG hydroxyl value + acid value) + 9.667 × water] × TDI index

or = (8.68 + water × 9.67) × TDI index

Typical Applications:

Notes:

Typical TDI index range: 95–135. Higher index increases foam hardness within limits.

The second part is about Catalysts, please click the following link to read: https://www.sabtechmachine.com/what-are-the-raw-materials-of-flexible-polyurethane-foam-part-2.html

The third part is about other chemicals, please click the following link to read: https://www.sabtechmachine.com/what-are-the-raw-materials-of-flexible-polyurethane-foam-part-3.html