Main Raw Materials and Foaming Process for Foam Production

Foam is a type of polyurethane foam plastic and belongs to the category of flexible polyurethane foam. Due to its porous, honeycomb-like structure, it has good softness, elasticity, water absorption, and water resistance, and is widely used in industries such as sofas, mattresses, garments, flexible packaging, and footwear materials.

1. Main Raw Materials

1.1 Polyether Polyol

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

The molecular formulas can be represented as:

CH₃-CHO(C₃H₆O)m(C₂H₄O)nH

CH₂O(C₃H₆O)m(C₂H₄O)nH

1.2 Organic Isocyanate

Toluene diisocyanate, commonly abbreviated as TDI, is widely used. TDI mainly has two isomers: 2,4-TDI and 2,6-TDI. In commonly used TDI for foam production, 2,4-TDI accounts for approximately 80%, while 2,6-TDI accounts for approximately 20%.

1.3 Water

In foam production, water is an important reactive component. Water reacts with TDI and releases carbon dioxide (CO₂), which provides the blowing effect while also participating in the polymer chain-growth process.

1.4 Catalysts

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

Catalysts used to promote crosslinking reactions and the reaction between isocyanates and water include triethanolamine, triethylenediamine, triethylamine, and others.

1.5 Auxiliary Blowing Agents

Traditionally, low-boiling fluorocarbon compounds were used as auxiliary blowing agents, such as trichlorofluoromethane (F-11). Due to environmental concerns, these blowing agents are now strictly restricted or have gradually been phased out.

In modern production, suitable alternative blowing systems can be selected according to the formulation, product density, and process requirements. If ultra-low-density foam is not required, production can also be carried out without auxiliary blowing agents by adjusting the proportions of the main raw materials.

1.6 Foam Stabilizer (Cell Regulator)

Silicone-based foam stabilizers, also known as cell regulators, are commonly used in foam production. Their main function is to stabilize the cell structure and regulate cell size and uniformity, thereby improving the overall structure and performance of the foam.

The specific dosage can be adjusted according to the product formulation and production process.

2. Synthesis Principles of Foam

During the synthesis of polyurethane foam, the main reactions include chain growth, foaming, and crosslinking. These reactions are influenced by factors such as the molecular structure, functionality, molecular weight, formulation ratio, and reaction conditions of the raw materials.

2.1 Chain Extension Reaction

Isocyanates react with hydroxyl groups in polyether polyols to form urethane bonds, promoting continuous growth of the polymer molecular chains.

By controlling the ratio of isocyanate to polyol, the molecular structure of the polyurethane system and the properties of the final product can be adjusted.

2.2 Foaming Reaction and Chain Growth

During foam production, water reacts with isocyanates such as TDI and generates a large amount of carbon dioxide (CO₂). The gas forms cells within the reaction system, causing the material to expand and eventually form a porous foam structure.

At the same time, the amines generated during the reaction continue to react with isocyanates to form urea bonds, accompanied by further polymer chain growth.

2.3 Crosslinking Reaction

Crosslinking reactions are very important to the formation of the foam structure and the final product performance. If the crosslinking rate is too fast or too slow, it may affect the cell structure, dimensional stability, and final product quality.

2.3.1 Crosslinking by Multifunctional Compounds

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

In general, an increase in crosslink density can improve the hardness and structural strength of the foam, but may also reduce softness and some elastic properties.

2.3.2 Urea Bond and Biuret Crosslinking

After water reacts with isocyanate, amines are formed. These amines further react with isocyanates to form urea bonds. Under suitable conditions, these structures can continue to participate in reactions and form a three-dimensional crosslinked system.

2.3.3 Allophanate Crosslinking

Under certain conditions, urethane structures can further react with isocyanates to form allophanate structures, thereby increasing the degree of crosslinking in the polymer system.

3. Production Process and Flow

At present, flexible polyurethane foam can be produced using different processes, such as continuous foaming and box foaming.

In a one-step box foaming process, the various raw materials are accurately metered according to the specified formulation, thoroughly mixed under high-speed agitation, and then quickly poured into a foaming box or molding box.

A typical production process is:

Raw Material Metering → High-Speed Mixing → Pouring into Molding Box → Foaming Initiation → Foam Rise → Gelation & Crosslinking → Curing → Aging → Cutting & Processing

Throughout the production process, chain growth, foaming, crosslinking, and curing reactions occur almost simultaneously, ultimately forming flexible polyurethane foam with a uniform porous structure.

This process features a relatively simple production flow, high production efficiency, flexible equipment configuration, and a wide applicable density range.