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.

In modern industrial production, polyurethane flexible foam play an important role in various fields such as furniture, automotive seats, and shoe insoles. However, the key technical control points for producing high-quality polyurethane flexible foam plastic products cannot be overlooked. Here are several key technical points in the production process:

Control of Toluene Diisocyanate (TDI):

The optimal isomeric ratio of TDI is 80/20. If this ratio is exceeded, it can lead to the formation of large and closed cells in the foam, prolonging the curing time. Particularly in the production of large block low-density foam products, an excessive isomeric ratio can delay heat release, potentially causing the foam center temperature to remain high for a long time, leading to carbonization and even ignition. If the isomeric ratio is too low, the foam product's density and resilience will decrease, and fine cracks may appear on the foam surface, resulting in poor process repeatability.

Addition of External Blowing Agents:

External blowing agents (water) not only reduce the density of the foam but also improve the softness of the product and help remove reaction heat. To prevent center carbonization in the foaming process of large block foam products, a certain amount of water is usually added. However, as the amount of water increases, the amount of catalyst should also increase correspondingly; otherwise, it may prolong the post-curing time of the foam. Generally, for every 5 parts increase in water, 0.2 to 0.5 parts of silicone oil should be added.

Catalyst Ratio:

Organic tin and tertiary amine catalysts are commonly used to control the NCO-OH and NCO-H2O reactions. By adjusting the ratio of different catalysts, the growth of polymer chains and the foaming reaction can be controlled. Under certain product densities, choosing the appropriate catalyst ratio can control the foam's open-cell rate, cell size, and void load value. Increasing the amount of organic tin catalyst can generally produce foams with smaller cell sizes, but excessive use may increase the closed-cell rate. It is necessary to determine the optimal catalyst dosage through experiments to achieve the best performance of foam products.

Foam Stabilizers:

The role of foam stabilizers is to reduce surface tension of the material, making the foam film wall elastic and preventing foam wall rupture until the molecular chain growth and cross-linking reactions lead to material solidification. Therefore, foam stabilizers play a critical role in the production of one-step polyether sponge and must be strictly controlled in usage.

Temperature Control:

The foam generation reaction is highly sensitive to temperature, and changes in material and foaming temperature will affect foaming operations and physical properties. Therefore, temperature control is one of the important conditions to ensure stable foaming processes. The material temperature is generally controlled at 20-25°C.

Stirring Speed and Time:

The stirring speed and time affect the amount of energy input during the foaming process. If stirring is uneven, a large number of bubbles may appear on the foam surface, leading to defects such as cracking. During mixing of Component A, the speed is 1000r/min; after Component B is added to Component A, the high-speed stirring speed is 2800-3500r/min for 5-8 seconds.

In summary, the key technologies for producing polyurethane flexible foam include controlling TDI, adding external blowing agents, adjusting catalyst ratios, using foam stabilizers, temperature control, and controlling stirring speed and time. Proper control of these technical parameters can ensure the production of stable quality and high-performance polyurethane flexible foam plastic products.

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.

1. Adjust Formulation:

Control the amount of water to not exceed 4.5 parts, and if necessary, use low-boiling-point liquid compounds as auxiliary foaming agents to replace some water. Pay attention to the amount of water in the formulation, which must not exceed 5 parts. The highest safe temperature rise point for low-density foam is 160°C, and it must not exceed 170°C.

2. Strictly Control the Accuracy of Component Measurement:

During continuous block foam production, adjust the discharge speed of the mixing head material and the conveyor belt speed to coordinate them. Avoid phenomena such as under-foaming materials flowing into the bottom of already foaming materials due to slow conveyor belt speed or excessive discharge, which can prevent normal foaming, resulting in collapse. Collapsed materials are not easily able to produce localized "gas species," leading to localized heat accumulation and increased risk of scorching. In actual production, poor process parameters may result in small yellow scorching lines appearing at the bottom of foam blocks.

3. Avoid Compressing the Newly Produced Foam:

This is because compressing the foam before it is fully cured affects the foam network and structure. It also prevents heat accumulation due to compression, increasing the risk of self-ignition of new foam. Especially during the most sensitive stage of foam rising, any operational errors and vibrations, such as sudden movements caused by tight conveyor belt chains or excessive folding of isolation paper and belt shaking, can cause compression of immature foam, leading to scorching.

4. Strictly Observe the Curing and Storage Process of Foam:

For the production of polyurethane soft block foam, the curing process of new foam is a high-risk period for fire accidents. Due to the high internal temperature and long duration of heat dissipation in large block foams, the time to reach the highest internal temperature is usually about 30 to 60 minutes, and it takes 3 to 4 hours or longer for it to slowly decrease. During this time, the new foams have left the production line and entered the curing and storage phase, which is easily overlooked. Without proper monitoring measures, it can easily lead to fires. There have been reports that when producing block soft foam with a density of 22kg/? using a polyol with a molecular weight of over 5000, 4.7 parts of water, and 8 parts of F-11 with a TDI index of 1.07, a small amount of light yellow smoke was observed 2 hours later. Although the external temperature of the foam was not high, the interior was in a very dangerous initial stage of decomposition, with a temperature of around 200-250°C, already beginning to self-ignite.

5. To Prevent Self-Ignition of Foam:

Newly produced foam should be cured and stored, not exceeding 3 layers when stacked, with a spacing of more than 100mm between layers, preferably placed separately. The curing and storage phase should have dedicated personnel for enhanced monitoring, such as measuring the internal temperature of the foam every 15 minutes for at least 12 hours, or even longer, before normal storage. For foams that may generate high temperatures, large foam blocks should be cut horizontally (e.g., with a thickness of 200mm) to facilitate heat dissipation. When smoke or self-ignition is detected, use water spray or fire extinguishers, and do not move the foam or open doors and windows indiscriminately to prevent increasing airflow and exacerbating the fire.

Polyurethane foam often encounters various accidents and problems during actual foaming production, each of which is caused by multiple factors. In the analysis of accidents caused by complex factors, it is generally difficult to list all influencing factors and the main factors that actually play a role. Below are 15 frequently encountered problems and their causes, let's take a look together!

1. High closed-cell content

  a. Polyether polyols: high proportion of ethylene oxide, high activity, often occurs when switching to polyether polyols with different activities.

  b. Process formulation: excessive tin octoate usage, high isocyanate activity, high crosslinking degree, rapid crosslinking speed, excessive amines and physical blowing agents causing low internal pressure of foam, inability to open cells when foam elasticity is high, and high TDI index can also result in high closed-cell content.

2. Shrinkage (gelation speed greater than foaming speed)

  a. High closed-cell content, shrinkage during cooling.

  b. Process conditions: low air temperature, low material temperature.

  c. Process formulation: excessive silicone oil, excessive physical blowing agent, low TDI index.

3. Internal cracking

  a. Process conditions: low air temperature, high reaction center temperature.

  b. Process formulation: low TDI index, excessive tin content, high early foaming strength.

  c. High activity of silicone oil, small usage.

4. Top cracking (unbalanced gasification gelation speed)

  a. Process conditions: low air temperature, low material temperature.

  b. Process formulation: insufficient catalyst usage, small amine usage, poor quality silicone oil.

5. Bottom corner cracking (excessive amine usage, too fast foaming speed)

  Large pore surface: excessive physical blowing agent, poor quality silicone oil and catalyst.

6. Poor low-temperature performance of foam

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

7. Poor air permeability

  a. Weather conditions: low air temperature.

  b. Raw materials: high polyether polyols, high activity silicone oil.

  c. Process formulation: excessive tin or same tin usage, low water and amine content, high TDI index.

8. Poor resilience

  a. Raw materials: high activity polyether polyols, low relative molecular weight, high activity silicone oil.

  b. Process formulation: large amount of silicone oil, excessive tin content, more water at the same tin usage, high TDI index.

9. Poor tensile strength

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

  b. Process formulation: insufficient tin causes poor gelation reaction, high TDI index at the same tin usage, low crosslinking degree with less water.

10. Smoking during foaming

  a. Excessive amine causes a large amount of heat to be released from the reaction of water and TDI, evaporating low boiling point substances and causing smoking.

  b. If not charring, the smoke is mostly composed of TDI, low boiling point substances, and monomeric cycloalkanes in polyether polyols.

11. Foam with white streaks

  Fast foaming and gelation reaction speed, slow transmission speed in continuous foaming, local compression to form a dense layer, resulting in white streaks phenomenon. The transmission speed should be increased promptly, or the material temperature should be reduced, and catalyst usage should be decreased.

12. Brittle foam

  The formula has excessive water, resulting in many unreacted urea formations that are not dissolved in silicone oil, poor tin catalyst usage, insufficient crosslinking reaction, high content of low relative molecular weight polyether polyols, excessively high reaction temperature, and ether bond breakage which reduces foam strength.

13. Foam density lower than set value

  Foaming index is too large due to inaccurate metering, high air temperature, low air pressure.

14. Foam with skin, edge skin, bottom voids

Excessive tin and insufficient amine, slow foaming speed, fast  

15、High elongation at break

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

b. Process formulation: insufficient crosslinking due to low TDI index, excessive tin, and high silicone oil content.