In December 2021, we received an inquiry from Mr. Hairun in Malaysia. Mr. Hairun is a mattress manufacturer in need of producing rebonded foam. He had limited knowledge about machine usage and selection and had no prior experience with the production process. Therefore, he required guidance from experts who could assist him from the ground up.



We systematically explained the principles of foam production to Mr. Hairun, along with the necessary materials and equipment. We also took him on a tour of our factory to provide a clear understanding of the entire production process.



After understanding Mr. Hairun's preferences for the rebonded foam, including density, softness, and market prices, we offered him the most suitable foam production solution. We also provided him with information on foam production costs and compared raw material prices for his reference.



Based on the client's needs, budget, and existing factory layout, we devised a cost-effective machine configuration and layout plan for his facility, including an assessment of startup costs.



Once the machines were successfully installed, our team of engineers provided Mr. Hairun with one-on-one foam production training. When he successfully produced the foam he desired for the first time, he called us and said, "I am happy with crying, thank you very much!" Afterward, he purchased a batch foam machine from us and continued to reorder foam chemical materials from our company.

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.

Beginners are concerned that if the settling plate is not adjusted properly, the liquid flowing out of the nozzle may cause front surging or back surging, affecting the foaming process. Within two minutes after starting the machine, the reaction speed gradually increases, sometimes requiring adjustments to the settling plate. Adjustments to the settling plate are more critical in low-density and high-moisture-content (MC) formulas.



TDI (Toluene Diisocyanate) flow rate can be calculated to correspond to the scale value, but it is recommended to actually measure the TDI flow rate during the first foaming. Flow rate is too important; if the flow rate is not accurate, everything else will be a mess. It's best to rely on the simplest and most intuitive method of measuring the flow rate.



When mixing powders, the mixed stone powder should be left overnight and production should start the next day. For ingredients containing melamine and stone powder, it is recommended to first mix melamine with polyether for a period of time before adding the stone powder.



Foam machine formulas with long mixing chamber in the machine head or more teeth on the stirring shaft usually have less amine and lower material temperature. Conversely, foam machine formulas with short mixing chamber in the machine head or fewer teeth on the stirring shaft usually have more amine and higher material temperature.



For the same formula, when switching between dual-spray swivel heads and single-spray swivel heads with similar nozzle cross-sectional areas, the requirements for mesh thickness and layers are similar.



For the calibration of minor material flow, one method is to measure the return flow of the minor material, and the other is to calibrate it by dividing the total amount used by the foaming time. When there is a significant difference between the two calibration methods, rely on the data from the second calibration method.



Formulas for high-quality soft foam are usually within an unstable range, such as a low TDI index, low water-to-MC ratio, low T-9 dosage, and low silicone oil dosage.

In annual fire incidents, a significant proportion of ignitions are caused by foam, including sofa fires and various ignitions from soft packaging. These incidents occur far too frequently. How can we fundamentally eliminate or reduce such events?



One effective approach is to start from the source materials, much like treating the root cause of an illness. Adding flame retardants to polyurethane foam can effectively prevent ignition.



Now, let's understand flame-retardant foam:



Flame-retardant foam, also known as fire-resistant foam, has a chemical name of polyurethane foam material, which is divided into soft foam (mainly used for furniture) and rigid foam (mainly used for insulation). Generally, it is a fireproof material synthesized by adding various flame retardants to polyurethane.



The product's fire-retardant effect meets the requirements of ASTM Standard 117 and national standards. The usage method is the same as regular foam.



The combustion of polymers is a very complex and intense oxidation reaction. The process occurs as the polymer is continuously heated by an external heat source, initiating a free radical chain reaction with oxygen in the air. This releases some heat, further intensifying the degradation of the polymer, generating more flammable gases, and making the combustion more severe.



There are two methods for the flame retardancy of fire-resistant foam:



One is to chemically introduce flame-retardant elements or groups containing flame-retardant new elements into the molecular structure of the foam. The other method is to add compounds containing flame-retardant elements to the foam. The former method uses flame-retardant substances called reactive flame retardants, while the latter method uses substances called additive flame retardants.



Currently, the vast majority of flame retardants used in foam are additive flame retardants, while reactive flame retardants are mainly used in thermosetting resins such as epoxy resins and polyurethanes. The primary function of flame retardants is to interfere with the three basic elements required for combustion: oxygen, heat, and fuel. This can generally be achieved through the following means:



Flame retardants can produce heavier non-flammable gases or boiling liquids that cover the surface of the foam, interrupting the connection between oxidation and fuel.



By absorbing heat through decomposition or sublimation, flame retardants reduce the surface temperature of the polymer.



Flame retardants generate a large amount of non-flammable gases, diluting the concentration of flammable gases and oxygen in the combustion area.



Flame retardants capture radical free radicals, interrupting the chain reaction of oxidation.

Polyurethane flexible foam, also known as sponge, usually undergoes a liquid-to-solid transformation process within two minutes. Compared to other fields of polyurethane, the short and intense reaction time of flexible foam increases the difficulty in achieving fine and orderly cell structure, as well as molecular arrangement.



Research and development (R&D) involve introducing new materials to give products new properties. New materials can be categorized into two types based on their reaction characteristics: one type consists of raw materials that conform to the chemical reactions of polyurethane, such as polyether, isocyanate, silicone oil, and catalysts with different properties and activities. These materials do not fundamentally disrupt the chemical reaction process. The other type includes raw materials (fillers) that do not conform to the chemical reactions of polyurethane, such as adding non-foaming substances like special metal powders, specially formulated inorganic or organic powders with unique properties, ultrafine inorganic powders (with a fineness of over 5000 mesh), special antibacterial agents, etc. These materials sometimes completely disrupt the reaction process, leading to new reaction patterns. Some fillers cause the original formula to collapse completely, while others cause a significant change in the density of the finished product by 30% to 50%. Some fillers drastically alter the rise time of the reaction in the formula, and some fillers cause severe damage to the foam structure after being added.



R&D usually keeps certain raw materials on hand to deal with common problems encountered during the process. These raw materials typically require a wide range of physical and chemical properties. For example, amines such as A33, A-1, A-210, A-230, A-260, 33-LV, CS-90, 9717, and 9727 are commonly used in production but may not all be kept on hand for R&D purposes because their reactivity does not have a fundamental difference. In extensive R&D, the above-mentioned amines can be classified as materials in the same category. For instance, if adding a particular substance causes significant collapse when using A33 for foaming, then the remaining eight types of amines might be deemed ineffective.



One common issue encountered in R&D is foam collapse, which is the least efficient result. It is often said that analyzing and adjusting can only be done effectively once the foam structure is stable. Many cases of foam collapse cannot be resolved by adjusting the formula alone; either the foam properties change drastically, rendering it impractical, or the foam remains unstable. To address this, crosslinking agents are introduced. Crosslinking agents, commonly used as additives in R&D, generally come in two types: linear structured agents, where 1,4-butanediol represents the raw material and urea formation occurs through linear chain growth, and bulk structured agents, such as diethanolamine and glycerol, which are closer to polyether chain growth patterns.



For example, in one R&D case, adjusting the formula could not prevent foam collapse. Experience revealed that strong amine activity caused imbalance. The amine content was reduced from 0.2 parts per hundred polyether to 0.05 parts, and the high cyanate content in the formula seemed odd. The overall reaction rate also slowed significantly with the reduction in amine content, but the collapse persisted. Later, switching to a weaker amine resolved the issue. Hence, sometimes the activity of an amine cannot be compensated for by its concentration alone. It should be noted that the addition of certain substances may result in significant differences in performance between small-scale and large-scale foam production. In such cases, the performance of the last large-scale foam produced should be the main reference point for adjustment.



Sunlight, air, and water—the most ordinary and often overlooked things—are the most precious. Mastering foam production, which seems simple and lacking content at first glance, is actually quite challenging. As a colleague in R&D often says, "I have too little knowledge (one regrets having too little knowledge when it's needed)" and "How long will we keep doing this? (There's pressure)." R&D and learning foam production involve using known points to understand unknown areas, while production and teaching foam production involve using known areas to operate unknown points. Although these two aspects seem similar, bridging the gap between them is quite difficult.