There are many reasons for the cracking of polyurethane foam, including both chemical and mechanical factors. During production, it is important to stop and inspect, making sure to eliminate the phenomenon of cracking. Below are some insights we have summarized based on years of production experience, hoping to inspire everyone.

1. Sudden Changes

Sudden changes in the speed of the conveyor, variations in catalyst usage, or irregular operation of the conveyor can cause the foam to crack. Carefully and gradually adjust the above situations. A significant change in the speed of the conveyor can lead to large cracks in the foam blocks.

2. Polyethylene Film

If the film stops moving for some reason, the foam will come into contact with the static surface, leading to cracking. If this situation occurs, check the winding roller of the rewinder and examine the separation of the film in the curing area.

3. Foam Forming Around Certain Substances

Foam tends to form around certain substances, especially in the feed chute, which can easily cause cracking.

Before production, ensure there are no residues or debris left in the assembly of the mixing head, feeding hose, and discharge chute.

4. Excessive Amine

When there is insufficient stannous octoate, excessive amine can lead to the same effect and accelerate the rise time. Therefore, the dosage of amine must be reduced.

5. Insufficient Stannous Octoate

Due to the faster foaming reaction compared to the polymerization reaction, some foam may flow, leading to cracks. Therefore, the amount of stannous octoate must be increased.

6. Silicone Oil

Foaming becomes unstable and prone to rupture, potentially forming "depressions" on the foam, so it is necessary to increase the dosage of silicone oil.

7. Structure of Small Air Bubbles

Thin foam walls due to foaming can cause cracking. To address this comprehensively: reduce the air content, decrease the speed of the mixing head, or increase the pressure of the mixing head. It is also necessary to change the dosage of stannous octoate catalyst.

1. Core Scorching (Center temperature exceeding material's oxidation temperature)

A. Poor quality polyether polyols: excessive moisture, high peroxide content, high boiling point impurities, elevated metal ion concentration, improper use of antioxidants.

B. Formulation issues: high TDI index in low-density formulas, improper ratio of water to physical blowing agents, insufficient physical blowing agent, excessive water.

C. Climate impact: high summer temperatures, slow heat dissipation, high material temperatures, high humidity leading to center temperature surpassing oxidation temperature.

D. Improper storage: Increased TDI index leading to accumulation of heat during post-curing, resulting in elevated internal temperature and core scorching.



2. Large Compression Deformation



A. Polyether Polyol: Functionality less than 2.5, propylene oxide ratio greater than 8%, high proportion of low molecular weight components, unsaturation greater than 0.05 mol/kg.

B. Process Conditions: The reaction center temperature is too low or too high, poor post-curing, incomplete reaction, or partial scorching.

C. Process Formula: TDI index too low (controlled within 105-108), excess silicone oil stannous octoate and silicone oil, low foam air content, high closed-cell content.



3. Soft Foam (Decreased hardness at same density)

A. Polyether polyols: low functionality, low hydroxyl value, high molecular weight.

B. Process formulation: insufficient T9 octoate, slow gelation reaction, lower water content with the same amount of tin catalyst, higher amount of physical blowing agents, high dosage of highly active silicone oil, low TDI index.



4. Large Cell Size

A. Poor mixing: uneven mixing, short cream time; increase mixing head speed, reduce mixing head pressure, increase gas injection.

B. Process formulation: silicone oil below lower limit, insufficient or poor quality octoate tin, slow gelation speed.



5. Density Higher than Set Value

A. Polyether polyols: low activity, high molecular weight.

B. Process formulation: silicone oil below lower limit, low TDI index, low foam index.

C. Climate conditions: low temperature, high pressure. A 30% increase in atmospheric pressure increases density by 10-15%.



6. Collapsed Cells and Hollows (Gas evolution rate greater than gelation rate)

A. Polyether polyols: excessive acid value (affects reaction rate), high impurities, low activity, high molecular weight.

B. Process formulation: excess amine, low tin catalyst (rapid foaming and slow gelation), low TDI index, insufficient or ineffective silicone oil.

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



7. High Closed-Cell Ratio

A. Polyether polyols: high epoxy ethane ratio, high activity, often occurs when switching to polyether polyols with different activity levels.

B. Process formulation: excessive octoate tin, high isocyanate activity, high crosslinking degree, high crosslinking speed, excessive amine and physical blowing agents leading to low foam pressure, high foam elasticity resulting in poor cell opening, excessively high TDI index leading to high closed-cell ratio.



8. Shrinkage (Gelation rate greater than foaming rate)

A. High closed-cell ratio, shrinkage during cooling.

B. Process conditions: low air and material temperature.

C. Process formulation: excessive silicone oil, less amine, more tin, low TDI index.

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



9. Cracking

A. "八" shaped cracks indicate excess amine, single line cracks indicate excess water.

B. Mid and bottom cracks: Excessive amine, fast foaming rate (excessive physical blowing agent, poor silicone oil and catalyst quality).

C. Top cracks: Unbalanced gas-evolution gelation rate (low temperature, low material temperature, insufficient catalyst, less amine, poor silicone oil quality).

D. Internal cracks: Low air temperature, high center temperature, low TDI index, excessive tin, high early foaming strength, highly active silicone oil in small quantities.

E. Side middle cracks: Increase tin dosage.

F. Cracking throughout the process may be due to discrepancies in dropping plate and foaming reaction, or premature foaming, or incorrect plates. Apart from formulation, it also relates to the smoothness of the base paper; if the base paper is wrinkled, it can divide the liquid into several parts, causing cracks.



10. Blurred Cell Structure

A. Excessive stirring speed.

B. High air injection volume.

C. Inaccurate metering pump flow.

D. Clogged material pipelines and filters.



11. Bottom Edge Cracks (Excessive amine, fast foaming rate)

Surface large pores: excessive physical blowing agent, poor silicone oil and catalyst quality.



12. Poor Low-Temperature Performance

Poor inherent quality of polyether polyols: low hydroxyl value, low functionality, high unsaturation, low TDI index with the same tin usage.



13. Poor Ventilation

A. Climate conditions: low temperature.

B. Raw materials: high polyether polyol content, highly active silicone oil.

C. Process formulation: excess tin, or low tin and amine content with the same tin usage, high TDI index.

When establishing a polyurethane foam factory, careful consideration of the site selection and construction conditions is crucial for its success. Several principles guide the selection of the factory site:

Firstly, the principle of optimizing and reorganizing existing resources of project units is essential. This ensures that the factory can make the best use of available resources without unnecessary duplication.

Secondly, the principle of saving land and reducing investment is vital. By selecting a site that is efficient in its land use, the factory can minimize costs and maximize efficiency.

Thirdly, the principle of facilitating transportation and reducing product production costs is significant. A location that allows for easy transportation of raw materials and finished products helps in lowering overall production costs.

Lastly, the principle of preventing urban pollution and protecting the environment is paramount. Choosing a site away from densely populated areas helps in reducing the impact of factory operations on the city's environment.



In addition to these site selection principles, various factors related to construction conditions must also be considered:

Geographic location and transportation conditions play a crucial role. An ideal location would have good access to transportation networks, such as highways or railways, facilitating the movement of goods.

Resource status and social conditions are important factors. This includes assessing local service supporting facilities, availability of labor resources, and government policies that might affect the factory's operations.

Natural conditions, such as climate, geological factors, and seismic considerations, should not be overlooked. Understanding these factors helps in planning for any potential risks or challenges during construction and operation.

Factory construction conditions such as water supply, drainage, power supply, and heating are essential for the smooth functioning of the facility. Adequate provisions for these utilities must be ensured during the planning stages.

In conclusion, the successful establishment of a polyurethane foam factory hinges on a thoughtful analysis of both site selection principles and construction conditions. By adhering to these considerations, the factory can be set up in an optimal location with the necessary infrastructure for efficient and sustainable operations.

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.

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.