When using a batch foam machine for polyurethane soft foam foaming, have you encountered the following situations?

1.Uneven and numerous foam pores,

2. Rough foam texture.

3. Chaotic pore sizes across the entire foam surface, with slight signs of large pores.

Issues like these are quite common. The main reason for the first issue is that the distance between the mixing impeller of the foam machine and the bottom of the mixing barrel is too great; the second issue is that the mixing blades are too short and narrow: the third issue is that the angle of the mixing blades is too large.

Many manufacturers who design and produce foam machines only understand the principles during the design process, without understanding the significant relationship between a different design in foam production and product quality. A reasonable and perfect mechanical design can only be gradually improved in actual work, and only experienced foamers can achieve this.

Here are some experiences we have had with machine modifications and upgrades, hoping they will be helpful:

First, the installation position of the mixing wheel should be as low as possible, closer to the bottom of the mixing barrel is better. In general, the distance between the lowest point of the mixing blade and the bottom of the mixing barrel should be around two centimeters

Second, the shape of the mixing blade should be fan-shaped, with a moderately wide edge. The advantage of being wide is that it increases the contact area with the liquid material, providing sufficient power and also balances the liquid material.

Third, the length of the mixing blade should also be as long as possible, leaving about three to four centimeters from the baffle inside the mixing barrel.

Fourth, the two edges of the mixing blade should be sloped, with the angle of inclination based on the width of one end and two centimeters difference on both sides. After the mixing blade is modified, proper operation is also crucial, especially the mixing speed. Most batch foam machines nowadays are equipped with high-speed timing frequency conversion devices. However, in actual production, this device is often unnecessary. The operating speed mainly depends on the amount of material in the mixing barrel. If there is a lot of material, the speed should be appropriately faster, and if there is less material, then the speed should be lower.

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.

1. Basic Reactions

The formation of polyurethane foam involves two basic reactions: foaming reaction and polymerization reaction (also called gel reaction).

Foaming reaction: Isocyanate reacts with water to produce a reaction of di-substituted urea and carbon dioxide. The reaction equation is as follows:

2R-N=C=O + HOH → R-NH-CO-NH-R + CO2↑

The released carbon dioxide acts as the bubble core, causing the reaction mixture to expand, resulting in foam with an open-cell structure.

Polymerization reaction: The hydroxyl group in the polyether undergoes a stepwise polymerization reaction with isocyanate to form an aminoformate. The reaction equation is as follows:

R=N=C=O + R′-OH → R-NH-COO—R′

2. Polyols

Domestic block foam production uses 3-functionality, molecular weight 3000 (hydroxyl value 56) or 3500 (hydroxyl value 48, less commonly used) soft foam polyethers.

3. Polyisocyanates

The main polyisocyanate used is toluene diisocyanate (TDI). There are three main types of TDI industrial products: pure 2,4-TDI (or TDI100), TDI80/20, and TDI65/35. TDI80/20 has the lowest production cost and is the most widely used variety in industrial applications.

The molecular weight of TDI is 174, with two isocyanate groups (-N=C=O) having a molecular weight of 84. Therefore, the isocyanate content in TDI is 48.28%.

The amount of TDI used has a significant impact on foam properties. In foam formulations, the excess of TDI is expressed as the isocyanate index, which is the ratio of actual usage to theoretical calculated amount. When producing soft foam, the index is generally 105-115 (100 is equal to the theoretical calculated amount). Within this range, as the TDI index increases, the foam hardness increases, tear strength decreases, tensile strength decreases, and elongation at break decreases. If the TDI index is too high, it can lead to large and closed cells, long maturation times, and foam burning; if the TDI index is too low, it may lead to cracking, poor rebound, low strength, and significant compression permanent deformation.

4. Blowing Agents

Water reacting with TDI to produce carbon dioxide is the main blowing agent used in soft foam foaming. Increasing the amount of water in the formulation will increase the urea content, increase foam hardness, decrease foam density, and reduce foam load-bearing capacity. However, TDI reacts with water to produce a large amount of heat. If the water content is too high, it can cause the foam to burn or ignite.

Methylene chloride is a physical blowing agent with a boiling point of 39.8°C. It is a non-flammable gas that can vaporize during foaming, reducing foam density and hardness. The amount of methylene chloride added should prevent the foam from burning while ensuring that too much does not remove too much heat, affecting foam curing. The amount of methylene chloride used is limited.

5. Catalysts

The main role of catalysts is to adjust the speed of foaming and gel reactions to achieve a good balance.

Triethylenediamine (A33, a 33% solution of diisopropyl ether or dipropylene glycol) is the most important tertiary amine catalyst in soft foam production. It is 60% effective in promoting the reaction between isocyanate and water, i.e., foaming reaction, and 40% effective in promoting the reaction between hydroxyl and isocyanate, i.e., gel reaction.

Dibutyltin dilaurate (A-1) is a general-purpose tertiary amine catalyst for soft foam. It is 80% effective in promoting foaming reaction and 20% effective in promoting gel reaction. It is often used in combination with triethylenediamine.

Improper use of amine catalysts can have a significant impact on the product. Too much amine can cause:

(1) Short reaction time, rapid increase in initial viscosity, and excessive smoking during foaming.

(2) Foam cracking. Too little amine will result in slow initiation speed, affecting foam height.

Dibutyltin dilaurate is the most commonly used organic tin catalyst, which is very easy to hydrolyze and oxidize in the presence of water and tertiary amine catalysts in polyether mixtures.

The lower the foam density, the narrower the adjustable range of dibutyltin dilaurate. The effect of tin dosage on foam is as follows:

Too little dosage: Foam cracking.

Too much dosage: Rapid increase in viscosity, foam forming closed cells and shrinking, forming skins on the top and sides.

6. Foam Stabilizers (also called Silicone Oils)

Foam stabilizers reduce the surface tension of the foam system mixture, thereby stabilizing the bubbles, preventing foam collapse, and controlling the size and uniformity of voids.

Increasing the amount of silicone oil from the minimum amount to an appropriate level can produce well-opened foam plastics. When the amount is too high, the closed-cell rate of the foam increases.

7. Other Influencing Factors

In addition to the formulation, process parameters, and environment also have a certain impact on foam properties.

Raw material temperature: Under relatively normal ambient temperatures (20-28°C), the raw material temperature is controlled at 25±3°C, preferably within a range of ±1°C. It can also be controlled within the range of 28-30°C.

The effect of temperature increase or decrease on the speed of foaming and gel reactions varies. An increase in temperature results in a much greater increase in polymerization reaction compared to foaming reaction. Catalysts need to be adjusted for temperature changes.

For the same formulation, using the same amount of blowing agent, foam density is also related to altitude. In high-altitude areas, foam density noticeably decreases.

The formula for slow rebound sponge is similar to regular sponge with minor differences. Apart from the major difference in the main ingredient polyether, some additives can be interchangeable. However, for the production of high-quality products, careful consideration and selection of additives are still necessary.

1. Choice of Amine

The most classic amine used for slow rebound sponge is Dabco33-LV from the American Air Products. The dosage is generally 0.3-0.8 parts of the total polyether. It is formulated with 33% triethylenediamine and 67% di(propylene glycol) (DPG). The reason for recommending this product is because it uses di(propylene glycol) as a solvent. Some may ask, is a solvent of this kind important? The answer is yes. Looking at the ability to dissolve triethylenediamine, there are many alcohol compounds that can be used as solvents: such as propylene glycol, diethylene glycol, ethylene glycol, 1,4-butanediol, etc. Among these small-molecule alcohols, di(propylene glycol) has the largest molecular weight and the lowest hydroxyl value. As we all know, low molecular weight alcohols can be used as chain extenders or crosslinking agents. This means that these small-molecule alcohols can consume TDI, resulting in: on one hand, it reduces the TDI index, and on the other hand, it easily causes closed-cell sponges.

2. Choice of Tin

Craftsmen who have experienced with regular sponges often like to use stannous octoate (T-9) to create slow rebound, but the author suggests using dibutyltin dilaurate (D22, T-12, also known as K-19 domestically). Stannous octoate is suitable for creating medium to low density sponges. Its characteristic is that it is quick to stick initially but lacks strength later on. It is not good for post-curing when used with high-density sponges. T-9 is prone to hydrolysis, and slow rebound itself has a slow start (generally controlled to start at around 160 seconds), so it is in contact with water for a long time, leading to some hydrolysis, affecting curing. Dibutyltin dilaurate does not hydrolyze, and its initiation, gelation, and curing are stable with good post-curing properties.

Some peers have mentioned that the sponge's tensile strength is not good. The author suggests using dibutyltin dilaurate, and the feedback received was that the tensile strength improved. If using T-9, the dosage is between 0.1-0.4 parts. If using dibutyltin dilaurate, the dosage can be controlled between 0.03-0.05. For slow rebound on assembly lines, the dosage can be reduced to 0.001-0.01 parts. For export orders that restrict the use of tin catalysts, the author suggests using bismuth carboxylate to replace tin.

3. Choice of Silicone Oil

A typical slow rebound silicone oil is B8002, with a dosage between 0.5-2 parts. It is used less for high-density sponges and more for low-density sponges. It is used more for manual foaming and less for machine foaming. In recent years, domestic silicone oil suppliers have developed many silicone oils for slow rebound, and their performance is also good. Some use L-580 for slow rebound, and in this case, the amount of silicone oil should be reduced, as L-580 is more active.

4. Use of Pigments

The use of pigments is basically the same as for regular sponge. Just be cautious when dealing with black sponges because the carbon black used to prepare black pigments is hydrophobic, which can affect the compatibility of various components in the formula and the efficiency of the catalysts. Many colleagues have encountered the phenomenon of easy cracking of black sponges, and the reason lies here. Therefore, when creating black sponges, proper adjustments should be made to the catalyst dosage. These are personal work experiences provided for reference only, and feedback and corrections from colleagues are welcome.

Polyurethane soft foam plastic is one of the important products in the polyurethane industry. Its production necessarily involves the use of organic amine catalysts, especially organic tertiary amine catalysts. This is because organic tertiary amine catalysts play a significant role in the main reactions of polyurethane foam formation: the reactions of carbon dioxide and molecular polymerization, promoting rapid expansion of reaction mixtures, increased viscosity, and sharp increase in polymer molecular weight. These conditions are essential for the formation of foam bodies, ensuring that soft foam plastics have advantages such as low density, high strength-to-weight ratio, high resilience, and comfort for sitting and lying. There are many types of organic amine catalysts that can be used for polyurethane soft foam plastics. Among them, the highly efficient catalysts recognized by various manufacturers are: triethylene diamine (TDEA) and bis(dimethylaminoethyl) ether (referred to as A1). These are also the most widely used organic amine catalysts in the world today, with the highest consumption among various catalysts.

Due to the molecular structural differences between TDEA and A1 catalysts, there are significant differences in their catalytic performance, particularly in their reactions to carbon dioxide gas and molecular polymerization. If the user does not pay attention to these differences in production, not only will they fail to produce qualified foam products, but it will also be difficult for foam bodies to form. Therefore, understanding and mastering the performance differences between these two catalysts in polyurethane foam production is of great significance. TDEA exists in a solid state under normal conditions, making its application less convenient. In actual production, low molecular weight alcohol compounds are commonly used as solvents, formulated into 33% solutions for ease of use, commonly referred to as A33. On the other hand, A1 is a low-viscosity liquid that can be directly applied. Below is a comparison of the catalytic performance differences between A1 and A33 in the production of polyurethane soft foam plastics.

A33 has a 60% catalytic function for the reaction with carbon dioxide gas and a 40% catalytic function for molecular polymerization. It has a low effective utilization rate of carbon dioxide gas, resulting in lower foaming height and higher foam density. Since most of the catalytic function is used for molecular polymerization reactions, it is easy to produce closed-cell foam bodies, which are stiff with low rebound, and the adjustable range of tin catalysts becomes narrower. To achieve the same catalytic function, the amount used is 33% more than A1. Both the bottom skin and outer skin of the foam body are thicker. Increasing the amount can increase the reaction speed, but the amount of tin catalyst must be reduced accordingly, otherwise closed-cell foam bodies will be produced.

A1 has an 80% catalytic function for the reaction with carbon dioxide gas and a 20% catalytic function for molecular polymerization. It has a high effective utilization rate of carbon dioxide gas, resulting in higher foaming height and lower foam density. Since most of the catalytic function is used for gas generation reactions, it is easy to produce open-cell foam bodies, which are soft with high rebound, and the adjustable range of tin catalysts becomes wider. To achieve the same catalytic function, the amount used is less than A33. Both the bottom skin and outer skin of the foam body are thinner. Increasing the amount can increase the reaction speed, but the amount of tin catalyst must be increased accordingly, otherwise over-foaming and cracking may occur.

In terms of overall performance between TDEA and A1, A1 has a higher comprehensive catalytic performance than triethylene diamine. Its actual application effects are also better, although not as convenient as triethylene diamine in terms of transportation and storage. Currently, the vast majority of mechanical continuous foam production facilities almost exclusively use A1, while all box-type foam production facilities use TDEA. However, this is not absolute. With a clear understanding of the differences between the two and appropriate formulation adjustments, they can be interchangeable and both can produce excellent foam products.