The stability of polyurethane soft foam sponge foaming refers to whether the foam breaks, closes pores, collapses, and also includes product hardness, density, elasticity, tensile strength, pore size, and other aspects that meet customer requirements. To achieve these, it is necessary to standardize raw materials, formulations, and operating parameters, and to control the complex and diverse chemical reactions in different environments.

Density: Density is measured in kilograms per cubic meter or grams per cubic centimeter. For irregularly shaped small products, it is not easy to measure the cross-sectional area. One can use graph paper with small squares (such as graph paper with 2-millimeter square sides) to draw the cross-sectional area of the product being measured and calculate the density by counting squares. During the production process, the formulation density, flow rate, conveyor belt speed, and foam width have been determined. Measuring the foam height will reveal the foam density. For example, if a sponge reaches a height of 95 centimeters, the density is 20 kilograms per cubic meter. Density is related to the formulation and is also affected by the reaction rate. There is a density difference between the top and bottom of the same foam.

Hardness: Sponge hardness can be divided into two types. One reflects the surface hardness of the product, used for shoe materials, while the other reflects the overall hardness of the product, used for furniture sponges. The hardness of the foam is related to the hard segments, heat, and raw material content during the reaction, corresponding to the materials TDI, MC, and POP. The hardness of the foam is also affected by the degree of cross-linking. As the density of the sponge decreases, it becomes difficult to increase the amount of POP. For low-density, high-hardness foam, the focus is on how to increase POP and TDI in the formulation to reduce MC. For medium-high-density, high-hardness foam, the focus is on maximizing the hardening effect of POP and TDI.

Elasticity: Elasticity is primarily related to the molecular weight of polyether. The higher the molecular weight, the higher the product's resilience. Secondly, it is related to the formation of side chains during the sponge reaction; the fewer the side chains, the better the elasticity. Reducing the TDI index can reduce the formation of side chains, and reducing the heat inside the foam can also reduce the formation of side chains. However, if there are too few side chains, the tolerance of the formulation is not high, and the foam is not stable. Sponge elasticity is also related to the balance of the formulation. When ordinary foam sponge closes its pores, the elasticity drops sharply. High-hardness foam does not have good elasticity, but foam that is too soft also does not have high resilience.

Tensile Strength: Furniture sponges are mainly used for sitting and leaning, so the tensile strength requirements are not too high. The tensile strength of the sponge is related to the NCO content and cross-linking degree in the meridians. Increasing the TDI index and increasing the heat inside the foam can strengthen the NCO content and cross-linking degree. Increasing MC reduces the increase in tensile strength in many cases. The amount of TDI that a formulation can "accommodate" is related to the foaming method, such as high-pressure machines, low-pressure machines, and manual foaming, which are different. A sponge with a high elongation rate does not necessarily have a high tear strength. For products that emphasize tensile strength, adding a small amount of stone powder can greatly reduce the tensile strength without losing the original.

Pores: Foam with very good pores often becomes mid-to-high-end foam, and the price also rises significantly. Pore formation is a comprehensive problem, and to obtain uniform, delicate, and defect-free pores, one must have a deep understanding of the machinery, raw materials, formulations, and parameters. The formation of pinholes and pockmarks is generally caused by excessive air entrainment in the raw materials during stirring at the machine head or during the movement of the raw materials. It may also result from poor raw material quality or contamination. The theory that air leaks in pipes causing pinholes is not tenable. During foaming, the pressure inside the pipe is higher than the atmospheric pressure outside the pipe. Only the raw material flows out of the pipe, and air from the outside cannot enter. 

14. Poor Rebound

A. Raw materials: high activity polyether polyols, low molecular weight, highly active silicone oil.

B. Process formulation: high silicone oil content, excessive tin, high water content with the same tin usage, high TDI index, large amount of white oil and powder.

15. Poor Tensile Strength

A. Raw materials: excessive low molecular weight polyether polyols, low functionality hydroxyl value.

B. Process formulation: insufficient tin, poor gelation reaction, high TDI index with the same tin usage, low water content, low crosslinking.

16. Smoke during Foaming

Excessive amine release a large amount of heat during water and TDI reaction, causing evaporation of low-boiling substances and smoke. If not core scorching, smoke mostly consists of TDI, low-boiling substances, and monomer cycloalkanes in polyether polyols.

17. Foam with White Streaks

Fast foaming and gelation reaction but slow transfer in continuous foaming, resulting in a dense layer due to localized compression, causing white streaks. Increase transfer speed or lower material temperature, reduce catalyst usage.

18. Brittle Foam

Excessive water in the formulation leads to excess biuret formation, which does not dissolve in silicone oil. Poor use of tin catalyst, insufficient cross-linking reaction, high content of low molecular weight polyether polyols, high reaction temperature causing ether bond breakage and decreased foam strength.

19. Foam Density Lower than Set Value

Foam index is too high due to inaccurate measurement, high temperature, low pressure.

20. Foaming with Skin, Edge Skin, and Bottom Air

Excessive tin, insufficient amine, slow foaming rate, fast gelation rate, low temperature during continuous foaming.

21. Elongation Rate Too High

A. Raw materials: high activity polyether polyols, low functionality.

B. Process formulation: low TDI index, insufficient cross-linking, high tin.

22. Uncontrolled Foam (Small bubbles rapidly moving beneath surface)

A. Low-pressure foaming machine: increase mixing head speed, decrease gas injection.

B. High-pressure foaming machine: increase mixing head pressure.

23.Milky Moving Lines

A. Increase conveyor speed

B. Adjust cushion plate inclination.

C. Reduce amine catalyst usage

24.Inserted Material Backflow

A. Increase conveyor speed.

B. Adjust cushion plate inclination.

C. Increase amine catalyst usage.

25.Moon Pits

A. Low-pressure foaming machine: reduce mixing head speed and gas injection.

B. High-pressure foaming machine: increase mixing head pressure.

C. Silicone oil quality issue.

D. Increase amine amount while reducing tin amount to ensure adequate cell opening.

26.Slow Curing,Sticky Surface

Polymer strength increases too slowly, resulting in soft, sticky foam that is difficult to cut.

Foam blocks appear unstable when exiting channels.

Increase catalyst usage, check polyol, water, and TDl measurement accuracy.

1. Basic Reactions

The formation of polyurethane foam involves two basic reactions: foaming reaction and polymerization reaction (also called gel reaction).

Foaming reaction: Isocyanate reacts with water to produce a reaction of di-substituted urea and carbon dioxide. The reaction equation is as follows:

2R-N=C=O + HOH → R-NH-CO-NH-R + CO2↑

The released carbon dioxide acts as the bubble core, causing the reaction mixture to expand, resulting in foam with an open-cell structure.

Polymerization reaction: The hydroxyl group in the polyether undergoes a stepwise polymerization reaction with isocyanate to form an aminoformate. The reaction equation is as follows:

R=N=C=O + R′-OH → R-NH-COO—R′

2. Polyols

Domestic block foam production uses 3-functionality, molecular weight 3000 (hydroxyl value 56) or 3500 (hydroxyl value 48, less commonly used) soft foam polyethers.

3. Polyisocyanates

The main polyisocyanate used is toluene diisocyanate (TDI). There are three main types of TDI industrial products: pure 2,4-TDI (or TDI100), TDI80/20, and TDI65/35. TDI80/20 has the lowest production cost and is the most widely used variety in industrial applications.

The molecular weight of TDI is 174, with two isocyanate groups (-N=C=O) having a molecular weight of 84. Therefore, the isocyanate content in TDI is 48.28%.

The amount of TDI used has a significant impact on foam properties. In foam formulations, the excess of TDI is expressed as the isocyanate index, which is the ratio of actual usage to theoretical calculated amount. When producing soft foam, the index is generally 105-115 (100 is equal to the theoretical calculated amount). Within this range, as the TDI index increases, the foam hardness increases, tear strength decreases, tensile strength decreases, and elongation at break decreases. If the TDI index is too high, it can lead to large and closed cells, long maturation times, and foam burning; if the TDI index is too low, it may lead to cracking, poor rebound, low strength, and significant compression permanent deformation.

4. Blowing Agents

Water reacting with TDI to produce carbon dioxide is the main blowing agent used in soft foam foaming. Increasing the amount of water in the formulation will increase the urea content, increase foam hardness, decrease foam density, and reduce foam load-bearing capacity. However, TDI reacts with water to produce a large amount of heat. If the water content is too high, it can cause the foam to burn or ignite.

Methylene chloride is a physical blowing agent with a boiling point of 39.8°C. It is a non-flammable gas that can vaporize during foaming, reducing foam density and hardness. The amount of methylene chloride added should prevent the foam from burning while ensuring that too much does not remove too much heat, affecting foam curing. The amount of methylene chloride used is limited.

5. Catalysts

The main role of catalysts is to adjust the speed of foaming and gel reactions to achieve a good balance.

Triethylenediamine (A33, a 33% solution of diisopropyl ether or dipropylene glycol) is the most important tertiary amine catalyst in soft foam production. It is 60% effective in promoting the reaction between isocyanate and water, i.e., foaming reaction, and 40% effective in promoting the reaction between hydroxyl and isocyanate, i.e., gel reaction.

Dibutyltin dilaurate (A-1) is a general-purpose tertiary amine catalyst for soft foam. It is 80% effective in promoting foaming reaction and 20% effective in promoting gel reaction. It is often used in combination with triethylenediamine.

Improper use of amine catalysts can have a significant impact on the product. Too much amine can cause:

(1) Short reaction time, rapid increase in initial viscosity, and excessive smoking during foaming.

(2) Foam cracking. Too little amine will result in slow initiation speed, affecting foam height.

Dibutyltin dilaurate is the most commonly used organic tin catalyst, which is very easy to hydrolyze and oxidize in the presence of water and tertiary amine catalysts in polyether mixtures.

The lower the foam density, the narrower the adjustable range of dibutyltin dilaurate. The effect of tin dosage on foam is as follows:

Too little dosage: Foam cracking.

Too much dosage: Rapid increase in viscosity, foam forming closed cells and shrinking, forming skins on the top and sides.

6. Foam Stabilizers (also called Silicone Oils)

Foam stabilizers reduce the surface tension of the foam system mixture, thereby stabilizing the bubbles, preventing foam collapse, and controlling the size and uniformity of voids.

Increasing the amount of silicone oil from the minimum amount to an appropriate level can produce well-opened foam plastics. When the amount is too high, the closed-cell rate of the foam increases.

7. Other Influencing Factors

In addition to the formulation, process parameters, and environment also have a certain impact on foam properties.

Raw material temperature: Under relatively normal ambient temperatures (20-28°C), the raw material temperature is controlled at 25±3°C, preferably within a range of ±1°C. It can also be controlled within the range of 28-30°C.

The effect of temperature increase or decrease on the speed of foaming and gel reactions varies. An increase in temperature results in a much greater increase in polymerization reaction compared to foaming reaction. Catalysts need to be adjusted for temperature changes.

For the same formulation, using the same amount of blowing agent, foam density is also related to altitude. In high-altitude areas, foam density noticeably decreases.

Understanding the principles behind foam reactions is crucial. To master foaming, we must strive to establish a foam reaction model in our minds using the following four reaction equations. Through familiarity with the variations within the model, we cultivate sensitivity that allows us to comprehend the entire foam reaction process. This approach helps structure our knowledge base and professional skills in polyurethane foam. Whether actively studying foam reaction principles or passively exploring them during the foaming process, it serves as a vital means for us to deepen our understanding of formulations and enhance our skills.

Reaction 1

TDI + Polyether → Urethane

Reaction 2

TDI + Urethane → Isocyanurate

Reaction 3

TDI + Water → Urea + Carbon Dioxide

Reaction 4

TDI + Urea → Biuret (Polyurea)

01: Reactions 1 and 2 are chain-growth reactions, forming the main chain of the foam. Before the foam reaches two-thirds of its maximum height, the main chain rapidly elongates, with chain-growth reactions predominating inside the foam. At this stage, due to relatively low internal temperatures, reactions 3 and 4 are not prominent.

02: Reactions 3 and 4 are cross-linking reactions, forming the branches of the foam. Once the foam reaches two-thirds of its maximum height, the internal temperature rises, and reactions 3 and 4 intensify rapidly. During this stage, reactions 1 to 4 are vigorous, marking a critical period for the formation of foam properties. Reactions 3 and 4 provide stability and support to the foam system. Reaction 1 contributes to foam elasticity, while reactions 3 and 4 contribute to foam tensile strength and hardness.

03: Gas-producing reactions are termed foaming reactions. The generation of carbon dioxide is a foaming reaction and the primary exothermic reaction in polyurethane foam. In reaction systems containing methane, the vaporization of methane constitutes a foaming reaction and an endothermic process.

04: Reactions leading to the formation of foam constituents are known as gelation reactions, encompassing all reactions except for gas-producing reactions. This includes the formation of urethane, urea, isocyanurate, and biuret (polyurea) from reactions 1 to 4.

Have you ever wondered how polyurethane plastic foam is formed? In the previous article, we revealed the basic reactions behind it: isocyanates, polyether (or polyester) polyols, and water, all work together to create this magical substance. So, does this mean that in actual production, we only need these three raw materials? The answer is far from it. In our actual production process, in order to more precisely control the reaction rate and produce products with excellent performance, we often need to harness the power of various additives. These additives not only have wide-ranging applications but also can play a huge role in making our production process more efficient and stable.

Surfactants / Silicone Oil

Surfactants, also known as silicone oil, are also called foam stabilizers. In the production process of polyurethane foam, its role is crucial. The basic duty of silicone oil is to reduce the surface tension of the foaming system, thus improving the miscibility between components, adjusting the size of bubbles, controlling the bubble structure, and enhancing foam stability. Furthermore, it also bears the responsibility of preventing foam collapse. Therefore, it can be said that silicone oil plays an indispensable role in the production of polyurethane foam.

Catalysts

Catalysts play a crucial role in the synthesis process of polyurethane, mainly by accelerating the reaction between isocyanates, water, and polyols. This reaction is a typical polymerization reaction. Without the presence of catalysts, this reaction may proceed very slowly or even not at all. Currently, catalysts on the market are mainly divided into two types: amine catalysts and organic metal catalysts. Amine catalysts are compounds based on nitrogen atoms, which can effectively promote the polymerization reaction of polyurethane. Organic metal catalysts, on the other hand, are compounds that particularly affect the reaction between polyols and isocyanates in the formation of polyurethane, usually organotin compounds. The characteristic of these catalysts lies in their ability to precisely control the reaction process, resulting in a more uniform and stable final product.

Blowing Agents

Blowing agents are substances that generate gas during the polyurethane reaction and help form foam. Depending on the way gas is generated, blowing agents are usually divided into chemical blowing agents and physical blowing agents. Chemical blowing agents refer to substances that undergo chemical changes during the reaction, generate gas, and promote foam formation. Many common substances in our daily lives are actually chemical blowing agents, such as water. Physical blowing agents, on the other hand, are substances that generate gas through physical means. For example, dichloromethane (MC) is a common physical blowing agent.

Other Additives

Relying solely on basic raw materials is far from enough to make products have outstanding performance. In order to meet various needs, other additives are cleverly incorporated into the production process, and their roles should not be underestimated. For example, flame retardants can add flame resistance to products, crosslinking agents can enhance their stability, colorants and fillers can give products a more colorful appearance and texture, and various other additives with different functions are also playing their roles. It is these carefully selected additives that comprehensively improve the performance of the products and bring users a better user experience.