Many factors affect the foaming process and final product quality when manufacturing polyurethane flexible foam. Among these, natural environmental factors such as temperature, air humidity, and atmospheric pressure play crucial roles. These factors significantly influence foam density, hardness, elongation rate, and mechanical strength.

1. Temperature:

Polyurethane foaming reaction is highly sensitive, with temperature being a key control factor. As material temperature rises, the foaming reaction accelerates. In sensitive formulations, excessively high temperatures can pose risks like core burning and ignition. Generally, it's essential to maintain consistent temperatures for polyol and isocyanate components. Increasing temperature leads to a corresponding decrease in foam density during foaming.

Particularly in summer, elevated temperatures increase reaction speed, resulting in decreased foam density and hardness, increased elongation rate, yet enhanced mechanical strength. To counter hardness reduction, adjusting the TDI index is advisable. Manufacturers must adjust process parameters according to seasonal and regional temperature variations to ensure product quality stability. 

2. Air Humidity:

Air humidity also affects the foaming process of polyurethane flexible foam. Higher humidity causes reactions between isocyanate groups in the foam and airborne moisture, leading to reduced product hardness. Increasing TDI dosage during foaming can compensate for this effect. However, excessive humidity can raise curing temperatures, potentially causing core burning. Manufacturers need to carefully adjust foam process formulations and parameters in humid environments to ensure product quality and stability.

3. Atmospheric Pressure:

Atmospheric pressure is another influencing factor, especially in areas at different altitudes. Using the same formulation at higher altitudes results in relatively lower foam product density. This is due to atmospheric pressure variations affecting gas diffusion and expansion during foaming. Manufacturers operating in high-altitude regions should take note of this and may need to adjust formulations or process parameters to meet quality requirements.

In conclusion, natural environmental factors significantly impact the foaming process and final product quality of polyurethane flexible foam. Manufacturers must adjust process parameters based on seasonal, regional, and environmental conditions to ensure stable foam density, hardness, and mechanical strength, meeting customer demands and standards.

1. Polyether

Polyether, as the main raw material, reacts with isocyanate to form urethane, which is the skeletal reaction of foam products. When the molecular weight increases with the same functionality, the tensile strength, elongation, and resilience of the foam increase, while the reaction activity of similar polyethers decreases. With the same equivalent value (molecular weight/functionality), an increase in functionality accelerates the reaction, increases the cross-linking degree of polyurethane, raises foam hardness, and reduces elongation. The average functionality of polyols should be above 2.5; if it is too low, the recovery of the foam body after compression is poor.

If the amount of polyether used is high, equivalent to a reduction in other materials (TDI, water, catalysts, etc.), it is easy to cause foam products to crack or collapse. If the amount of polyether used is low, the foam product tends to be hard, with reduced elasticity and poor touch.

2. Foaming Agent

Generally, when producing polyurethane blocks with a density greater than 21, only water (chemical foaming agent) is used as the foaming agent. Low-boiling compounds such as methylene chloride (MC) are used as auxiliary foaming agents in low-density formulas or ultra-soft formulas.

Auxiliary foaming agents reduce the density and hardness of the foam. Since they absorb some of the reaction heat, curing is slowed down, requiring an increase in the amount of catalyst. By absorbing heat, the danger of core burning is avoided.

The foaming capacity can be expressed by the foaming index (the number of parts of water or equivalent number of water used for 100 parts of polyether):

IF = m (water) + m (F-11) / 10 + m (MC) / 9 (100 parts of polyether)

Water, as a foaming agent, reacts with isocyanate to form urea bonds and releases a large amount of CO2 and heat. It is a chain-growth reaction. Excess water reduces foam density and increases hardness. However, it also reduces the size and strength of foam pores, reducing their load-bearing capacity, making them prone to collapse or cracking. Increased TDI consumption leads to more heat release and a higher risk of core burning. If the water amount exceeds 5.0 parts, physical foaming agents must be added to absorb some of the heat and prevent core burning. Less water means a corresponding reduction in the amount of catalyst used, but it increases density.

3. Catalyst

Amine: A33 is generally used, which promotes the reaction between isocyanate and water, adjusting foam density, bubble opening rate, etc., mainly promoting foaming reactions.

Too much amine: The foam product cracks, and there are holes or bubbles in the foam; Too little amine: The foam shrinks, closes pores, and the bottom of the foam product becomes thick.

Tin: Typically, Tin(II) octoate (T-9) is used; Tin(IV) oxide (T-19) is a highly active gel reaction catalyst, mainly promoting the gel reaction, i.e., the later stage reaction.

Too much tin: Fast gelation, increased viscosity, poor resilience, poor air permeability, leading to closed-cell phenomenon. Properly increasing its dosage can obtain good open-cell foam plastics with relaxation, further increasing the dosage makes the foam gradually become denser, leading to shrinkage and closed cells.

Too little tin: Insufficient gelation, resulting in cracking during foaming. There may be cracks on the edges or tops, with burrs and poor consolidation. Reducing amine or increasing tin can increase the strength of the polymer foam film when a large amount of gas is generated, thereby reducing hollow or cracking phenomena.

Whether polyurethane foam plastics have an ideal open or closed cell structure mainly depends on whether the gel reaction rate and gas expansion rate are balanced during foam formation. This balance can be achieved by adjusting the types and amounts of tertiary amine catalysts and foam stabilizers in the formula.

4. Foam Stabilizer (Silicone Oil)

Foam stabilizers are a type of surfactant that disperses polyurea well in the foaming system, acting as "physical cross-linking points" and significantly increasing the early viscosity of the foam mixture, preventing cracking. On the one hand, it has an emulsifying effect, enhancing the miscibility between foam material components. On the other hand, the addition of organic silicon surfactants can reduce the liquid's surface tension, reduce the free energy required for gas dispersion, make the dispersed air in the raw materials nucleate more easily during stirring and mixing, facilitate the production of fine bubbles, adjust the size of foam pores, control the foam cell structure, and improve foaming stability. It prevents collapsed or burst bubbles, makes the foam walls elastic, controls the pore size and uniformity of the foam. Generally, the more foaming agent and POP used, the greater the amount of silicone oil used.

High usage: Increases the elasticity of foam walls in the later stage, making them less likely to burst, resulting in smaller pores and closed cells.

Low usage: Foam bursts, collapses after foaming, larger pore size, and easy co-foaming.

5. Temperature Influence

The foaming reaction of polyurethane accelerates with an increase in material temperature, which can pose a risk of core burning and ignition in sensitive formulations. Generally, the temperatures of the polyol and isocyanate components are kept constant. When foaming, the foam density decreases as the temperature of the material increases. With the same formula, if the temperature remains the same but the ambient temperature is high in summer, the reaction speed increases, leading to a decrease in foam density and hardness, an increase in elongation, and an increase in mechanical strength. In summer, the TDI index can be appropriately increased to correct the decrease in hardness.

6. Influence of Air Humidity

With increasing humidity, the isocyanate in the foam reacts with the moisture in the air, causing a decrease in hardness. Therefore, when foaming, the TDI amount can be appropriately increased. Excessive humidity can cause the curing temperature to rise too high, leading to core burning.

7. Influence of Atmospheric Pressure

With the same formula, foaming in high-altitude areas results in lower foam product density.

Chemical change is the process of producing new substances after the molecular groups of various reactants interact with each other. Many properties of substances are determined by their molecular structures, and understanding the molecular and group structures in polyurethane reactants is instructive for production.

The main indicators of British standard flame retardancy are generally threefold: thermal weight loss (the mass lost when the specified size of sponge is heated at a specified temperature for a specified time, with smaller values indicating better thermal stability); smoke density (the amount of smoke generated when the foam burns, indicating the ease of light passing through the smoke, with smaller amounts of smoke being better); and ease of combustion (the more difficult it is to ignite, with further subdivisions based on ignition time and burning rate).

TDI (toluene diisocyanate) has one benzene ring, MDI (diphenylmethane diisocyanate) has two benzene rings, and crude MDI has multiple benzene rings. Benzene rings are very stable substances, requiring a large amount of energy (bond dissociation energy) to break. As the number of benzene rings increases, the thermal stability of the foam increases (crude MDI > MDI > TDI), making it less likely to decompose when heated. With more benzene rings, there are more carbon atoms in the molecule, resulting in more smoke when incompletely burned (crude MDI > MDI > TDI). From the above, it can be concluded that when one formula decreases the amount of TDI and increases the amount of MDI, the thermal stability of the foam will be enhanced. The thermal weight loss index is likely to pass the British standard test, but the smoke density, which is not easy to pass, will increase. At this point, it is advisable to appropriately increase the amount of melamine cyanurate to reduce smoke density.

The higher the molecular weight of the polyether, the worse the thermal stability, but the better the fire resistance. In the production of high-rebound flame-retardant foam, the amount of flame retardant added is only two-thirds that of regular-density flame-retardant foam, yet the flame retardancy remains very good and does not ignite. However, high-rebound flame-retardant foam is more difficult to pass the British standard test than regular foam (thermal weight loss is difficult to pass).

Flame retardants are not very stable when heated. Since the British standard test emphasizes thermal weight loss, the amount of flame retardant in the formula is the minimum required to pass the flame retardancy test.

When both TDI and water content in the formula decrease while methane content increases, the foam is less likely to ignite. The decrease in intrinsic properties due to the reduction of hard segments results in decreased thermal stability, thus reducing the ability to pass the thermal weight loss index.

When the foam density decreases, the TDI content increases, and both smoke density and thermal stability increase.

Inorganic materials like calcium carbonate and barium sulfate do not decompose when heated during British standard tests, but their addition does not improve the foam's properties, so they are not used in the British standard formula.

Besides selecting raw materials, achieving a balance is also crucial when meeting British standards. For example, both TDI and flame retardants, if given too much or too little, make it difficult to pass the test. Foaming is a balanced science, adjusting the formula is about seeking balance, and selecting raw materials is also about seeking balance.

The reaction of PU foam is based on two main chemical components: polyether polyols and isocyanates, along with other additives including water, dichlorodifluoromethane, foam stabilizers, and catalysts. These materials are instantly and vigorously mixed, reacting to form foam, a process that generates a considerable amount of heat.

Foam plastic is a porous material with a large surface area. While the heat generated at the edges of the foam can dissipate, the heat in the central part, due to the insulation effect of the foam, is more difficult to remove. In a typical reaction, the heat released raises the temperature of the center of the foam block to achieve curing. It has been observed that within 2 to 6 hours after foaming, temperatures can rise to 140-160°C, and sometimes even higher, around 180°C. If the temperature continues to rise, it can lead to core burning, smoking, and even spontaneous combustion.

Additionally, prolonged exposure of polyurethane foam to sunlight can trigger an auto-oxidation reaction, causing polymer degradation, discoloration, embrittlement, and a decrease in physical properties, rendering it unusable. Since the industrialization of polyurethane, core burning and aging have been hot topics of research and concern in the polyurethane industry.

Antioxidants are crucial additives in polyurethane foam production. Proper antioxidants prevent the decomposition of polyols, reduce the formation of by-products, decrease the risk of core burning, and can delay thermal oxidative aging during product use, thereby extending its lifespan. Commonly used antioxidants for PU foam are typically liquid and fall into three categories: aromatic amines (such as 5057), hindered phenols (such as 1135), and phosphite esters (such as PDP). For applications with low color requirements, a combination of aromatic amines and hindered phenols is generally used, while applications with higher color requirements may use a combination of hindered phenols and phosphite esters.

Furthermore, if products are frequently exposed to sunlight, a certain amount of UV stabilizers should be added to improve lifespan and resistance to yellowing. UV stabilizers mainly consist of UV absorbers and hindered amine light stabilizers (HALS). UV absorbers, such as benzotriazoles, benzophenones, and triazines, absorb harmful UV radiation and convert it into heat through intramolecular hydrogen bond transfer or cis-trans isomerization. HALS refers to amines with two methyl groups on each α-carbon atom, which, after photooxidation, transform into nitroso radicals. These radicals are considered stable components that can capture free radicals, regenerate nitroso radicals by reacting with peroxide radicals. UV blocking agents include carbon black, zinc oxide, titanium dioxide, and other pigments, which are used as colorants. These agents utilize their high dispersibility and covering power to reflect harmful UV radiation, protecting the polymer.

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.