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.

The internal temperature of foam is as indispensable as vitality is to a person. If the post-cure temperature of the sponge is too low, its physical properties will not be optimal, and there will be significant fluctuations in these properties.



Once the foam is well developed, its internal temperature rapidly rises to over 120 degrees Celsius due to the exothermic reaction occurring under poor heat dissipation conditions, becoming one of the fire hazard risks.



The internal temperature of the foam is crucial for forming its superior properties. Foam matured at specific external temperatures exhibits exceptionally superior physical properties like tensile strength. Some calculate the foam temperature through formulas, while others use software to input formulas and automatically calculate the internal temperature of the foam. So, what factors influence the internal temperature of the foam? Is it significant to know these factors? It's akin to how modern phone cameras are high-resolution, but does that render professional photography useless? Are adjustments like aperture, focal length, and exposure time pointless? To better control things, one must grasp more of the key variables of that thing. Let's start with basic principles to understand the changes in internal foam temperature.



First, let's grasp a few basic rules.



The temperature of a space is directly proportional to the amount of heat energy injected into that space and inversely proportional to its size.

For example, if 10 kilojoules of heat are distributed in an 8-liter space, the temperature of that space is 20 degrees Celsius. If the same 10 kilojoules of heat are distributed in a 4-liter space, the temperature becomes 40 degrees Celsius.

The amount of heat input is directly proportional to the heat input value and the speed of heat input.

For instance, if 100 kilojoules of heat are released at speed "v," the heat input is "A." If the same 100 kilojoules of heat are released at 2v speed, the heat input becomes 2A.

The size of a space is directly proportional to the absolute temperature.

For example, a 1-liter space at 0 degrees Celsius becomes 1.366 liters at 100 degrees Celsius because (273.15 + 100)/(273.15 + 0) = 1.366.

The size of a space is inversely proportional to atmospheric pressure.

The lag in methane vaporization needs to be considered.

Next, let's examine how fine-tuning the formula affects the internal foam temperature.



Since this is fine-tuning, we'll approximate that the surrounding environment remains unchanged before and after the adjustments. Let's consider the effects of adjusting water and methane on the internal foam temperature.

For example, if a formula increases methane by 5%, we can be certain that the internal foam temperature decreases because methane vaporization absorbs heat, reducing the heat input to the foam, and increasing the space to accommodate heat. Similarly, if the water content is increased by 5%, the added water releases heat upon injection into the foam, raising the heat input, and the reaction of the added water generates gas, increasing the space for heat. So, does the internal foam temperature increase or decrease in this case? Experience indicates that the internal foam temperature increases. This suggests that the increased heat input due to this change contributes more to the increase in internal foam temperature than the gas produced by water diluting the temperature.



The changes involving foam index, heat release, and heat dissipation all increasing can make it difficult to intuitively guess whether the internal foam temperature will rise or fall. One might need to insert a probe after foaming to compare internal temperatures or calculate to reach a conclusion.



For calculations, several formulas (algebraic expressions) derived from the earlier basic rules are needed, along with some data: the heat released when water reacts with TDI to form carbon dioxide in kilojoules per mole, the heat absorbed during methane vaporization in kilojoules per mole. To estimate the total foam internal temperature, one must know the heat released when forming amino methyl formate, urea methyl formate, urea, and biuret (polyurea), in kilojoules per mole, and the reaction rate (reaction time).



This also explains why the density calculated from the foam index drastically differs from the theoretical and actual values for foams without fillers at 50 densities. The lower the density, the more closely the theoretical and actual values of foam density match.

What is Polyurethane Rigid Foam?



Polyurethane rigid foam, often abbreviated as PU rigid foam, is one of the most commonly used polyurethane products, second only to polyurethane soft foam, in polyurethane applications. Polyurethane rigid foam is mostly a closed-cell structure, known for its excellent insulation, lightweight, high strength-to-weight ratio, ease of construction, as well as soundproofing, shock absorption, electrical insulation, heat resistance, cold resistance, solvent resistance, and more. It is widely used in the insulation layers of refrigerator and freezer boxes, cold storage rooms, refrigerated trucks, as well as insulation materials for buildings, tanks, and pipelines. A small amount is used in non-insulation applications such as wood imitation and packaging materials. Generally, lower-density polyurethane rigid foam is primarily used as thermal insulation material, while higher-density polyurethane rigid foam can be used as structural material (wood imitation).



Polyurethane rigid foam is typically foamed at room temperature, with a relatively simple molding process. It can be categorized into manual foaming and mechanical foaming based on the degree of construction mechanization; high-pressure foaming and low-pressure foaming based on the foaming pressure; and casting foaming and spraying foaming based on the molding method.



What is Polyurethane Soft Foam?



Polyurethane soft foam, also known as PU soft foam, is a type of flexible polyurethane foam with a certain degree of elasticity. It is the most extensively used polyurethane product among all polyurethane products.



Polyurethane soft foam is mostly an open-cell structure, characterized by low density, good elastic recovery, sound absorption, breathability, and insulation properties. It is primarily used as cushioning materials for furniture, mattresses, vehicle seat cushions, and also finds industrial and domestic applications as filter materials, soundproofing materials, shock absorption materials, decorative materials, packaging materials, and insulation materials. Based on the degree of softness and load-bearing capacity, polyurethane soft foam can be divided into ordinary soft foam, super soft foam, high load-bearing soft foam, high resilience soft foam, etc. High resilience and high load-bearing soft foams are generally used for manufacturing seat cushions and mattresses. According to the production process, polyurethane soft foam can be divided into block foam and molded foam. Block foam is produced by continuous process to form large volume foam which is then cut into required shapes, while molded foam is produced by direct injection of the mixture into molds to form foam products of desired shapes.



After understanding polyurethane rigid foam and polyurethane soft foam, the question arises: how do we differentiate between the two?



In fact, the classification can be based on the degree of hardness, dividing them into soft foam plastics and hard plastics. Soft foam plastics have a matrix polymer component above the crystalline melting point, or, if it is an amorphous polymer, it is above the glass transition temperature; hard foam, on the other hand, has its matrix polymer in a crystalline state or an amorphous state but below the glass transition temperature. Semi-rigid foam is a foam plastic between soft and hard foam. It is similar to soft foam, with an open cell rate above 90℃, but semi-rigid foam has higher density and higher compression strength. After compression deformation, semi-rigid foam takes much longer to recover, and its crosslinking density is much higher than soft foam but lower than hard foam.



Based on this classification of softness and hardness, most polyolefin foams, unplasticized polyvinyl chloride (PVC) foams, phenolic foams, polycarbonate foams, and polyphenylene ether foams are all classified as hard foams, while elastic polyurethane foams and some polyolefin foams and plasticized PVC foams are classified as soft foams.



According to the national standard, soft foam plastics are those that are flexible, have low compression hardness, return to their original state after stress is relieved, and have minimal residual deformation. On the other hand, hard foam plastics are inflexible, have high compression hardness, deform when stress reaches a certain level, and do not return to their original state after stress is relieved.



The American Society for Testing and Materials (ASTM) specifies that to differentiate between soft and hard foam plastics, at a temperature of 18-29℃, a rod with a diameter of 2.5 cm is rotated around it for one full rotation within 5 seconds. If it does not fracture, it is classified as soft foam plastic; otherwise, it is classified as hard foam plastic.



According to ISO standards, when the compression deformation reaches 50% and is then released, if the thickness decreases by no more than 2% compared to the original thickness, it is classified as soft foam plastic. If it decreases by more than 10%, it is classified as hard foam plastic. If the decrease is between 2-10%, it is classified as semi-rigid foam plastic.



If we use elastic modulus as the criterion, in a standard environment of 23℃ and 50% relative humidity, a foam plastic with an elastic modulus greater than 686 MPa is classified as hard foam, less than 68.6 MPa is classified as soft foam, and between 68.6-686 MPa is classified as semi-rigid foam. Although the elastic modulus of semi-rigid foam is higher than that of soft foam, its stress-strain behavior is closer to soft foam and significantly different from hard foam. Generally, soft foam plastics mostly have an open-cell structure, while hard foam plastics mostly have a closed-cell structure, but there are exceptions.

In polyurethane flexible foams, dichloromethane (MC) is often used to adjust the density and hardness of the foam. With a boiling point of only 40.4°C, during foaming, the reaction of water and TDI generates a large amount of heat, causing MC to evaporate into gas, thus expanding the foam body and reducing foam density.



The vaporization of MC consumes a lot of heat, which can affect the foaming process of the foam in some cases. The following two figures show the changes in the maximum foam temperature and the time to reach it after adding different amounts of MC to a specific formula.



From the charts, it can be observed that after adding MC, the maximum foam temperature decreases significantly, and the time to reach the maximum temperature also increases.



These are just changes in data, but how do they manifest during the actual foaming process? To understand this, let's briefly look at the polyurethane reaction process.



The main reaction in polyurethane foaming is the reaction of water and isocyanate to produce carbon dioxide and amine, and the reaction of polyether polyol and isocyanate to produce polyurethane. However, there are many secondary reactions, summarized as reactions generating urethane and reactions generating urea.



Secondary reactions change the molecular structure of the polymer from linear to cross-linked. Due to different reaction conditions and raw materials, the structure of polyurethane can vary greatly. In general, the more secondary reactions, the more complex the cross-linked structure, resulting in increased hardness and improved tear strength. Of course, the resistance to yellowing also improves, but that's another topic. Increasing the foaming index will strengthen secondary reactions.



Having said so much, what does this have to do with MC? Secondary reactions are all endothermic reactions, requiring heat absorption. However, the vaporization of MC also requires a large amount of heat, thus creating a competitive relationship. Adding a large amount of MC will significantly weaken secondary reactions, increasing the proportion of linear structures in the foam, making it softer, and decreasing thermal plasticity.



Even in colder temperatures during winter, attention should be paid to this issue. Properly increasing the water content in the formula to generate more heat helps maintain the physical properties of the foam without significant changes.

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.