Main Raw Materials and Foaming Processes for Sponge Production

Polyurethane Foam, commonly referred to as PU foam, is a type of polymer foam material. Flexible polyurethane foam features a porous cellular structure that provides excellent softness, elasticity, water absorption and water resistance.

It is widely used in sofas, mattresses, garments, flexible packaging, footwear materials and many other applications.

1. Main Raw Materials

1.1 Polyether Polyols

Polyether diols and polyether triols are commonly used in flexible polyurethane foam production. They generally have relatively low functionalities (2–3), low hydroxyl values and high molecular weights.

The molecular formulas can be represented as:

CH₃-CHO(C₃H₆O)m(C₂H₄O)nH

CH₂O(C₃H₆O)m(C₂H₄O)nH

1.2 Organic Isocyanates

Toluene diisocyanate (TDI) is commonly used in flexible polyurethane foam production.

TDI mainly consists of two isomers: 2,4-TDI and 2,6-TDI. A commonly used TDI grade for flexible foam production contains approximately 80% 2,4-TDI and 20% 2,6-TDI.

1.3 Water

Water is an important reactive component in polyurethane foam production.

It reacts with TDI to generate carbon dioxide (CO₂), which acts as a blowing gas and contributes to the formation of the cellular foam structure. Water also participates in the polymer chain-growth process.

1.4 Catalysts

Catalysts used to promote the reaction between polyether polyols and isocyanates include stannous octoate and various dibutyltin-based catalysts.

Catalysts such as triethanolamine, triethylenediamine (TEDA), and triethylamine may also be used to promote crosslinking reactions and the reaction between isocyanates and water.

1.5 Auxiliary Blowing Agents

Low-boiling fluorocarbon compounds, such as trichlorofluoromethane (CFC-11/F-11), were traditionally used as auxiliary blowing agents.

However, due to environmental concerns, these blowing agents have been heavily restricted or phased out.

In modern production, suitable alternative blowing systems can be selected according to the formulation, target foam density and processing requirements.

For foams that do not require extremely low densities, the formulation can also be adjusted to produce the foam without an auxiliary physical blowing agent.

1.6 Foam Stabilizers / Silicone Surfactants

Silicone-based foam stabilizers, also known as silicone surfactants, are commonly used in polyurethane foam production.

Their main function is to stabilize the cellular structure and control cell size and uniformity, thereby improving the overall structure and performance of the foam.

The dosage can be adjusted according to the specific formulation and processing requirements.

2. Synthesis Principles of Polyurethane Foam

The synthesis of flexible polyurethane foam mainly involves chain growth, foaming and crosslinking reactions.

These reactions are influenced by factors such as the molecular structure, functionality and molecular weight of the raw materials, formulation ratios and processing conditions.

2.1 Chain-Growth Reaction

Isocyanate groups react with the hydroxyl groups of polyether polyols to form urethane linkages, resulting in the continuous growth of polyurethane polymer chains.

By controlling the ratio of isocyanates to polyols, the molecular structure and final properties of the polyurethane system can be adjusted.

2.2 Foaming Reaction and Chain Growth

During polyurethane foam production, water reacts with isocyanates such as TDI to generate carbon dioxide (CO₂).

The generated gas forms cells within the reacting mixture, causing the material to expand and eventually develop a porous cellular foam structure.

At the same time, the amines generated during the reaction further react with isocyanates to form urea linkages, contributing to further polymer chain growth.

2.3 Crosslinking Reactions

Crosslinking reactions play an important role in the formation and final properties of polyurethane foam.

If crosslinking occurs too quickly or too slowly, it may adversely affect the cellular structure, dimensional stability and overall quality of the finished foam.

2.3.1 Crosslinking of Multifunctional Compounds

Multifunctional polyether polyols react with isocyanates to form a three-dimensional polymer network.

In general, increasing the crosslink density increases the hardness and structural strength of the foam, while potentially reducing its softness and certain elastic properties.

2.3.2 Urea and Biuret Crosslinking

The reaction between water and isocyanates generates amines, which subsequently react with additional isocyanate groups to form urea linkages.

Under suitable conditions, these structures can undergo further reactions and contribute to the formation of a three-dimensional crosslinked network.

2.3.3 Allophanate Crosslinking

Under suitable conditions, urethane groups can further react with isocyanate groups to form allophanate structures, increasing the degree of crosslinking within the polyurethane polymer network.

3. Production Process

Flexible polyurethane foam can be manufactured using different production methods, including continuous slabstock foaming and batch box foaming.

In a one-shot box foaming process, the raw materials are accurately metered according to the specified formulation, thoroughly mixed at high speed and then rapidly poured into a foaming or molding box.

A typical production process includes:

Raw Material Metering → High-Speed Mixing → Pouring → Creaming → Foam Rise → Gelation & Crosslinking → Curing → Maturation → Cutting & Processing

During this process, chain growth, foaming, crosslinking and curing reactions occur almost simultaneously, eventually producing flexible polyurethane foam with a uniform cellular structure.

This production method offers several advantages, including a relatively simple process, high production efficiency, flexible equipment configuration and suitability for a wide range of foam densities.