Polyether Polyol: Hydroxyl Value 36, Primary Hydroxyl > 65%, 60%.

Polymeric Polyol: Hydroxyl Value 28, Copolymer 20%, 40%.

Water: 3%.

80TDI and Polymeric MDI (Viscosity 300mpa): 80:20.

T12: 0.025%.

A33: 0.4%.

HR-3 Silicone Oil: 1%.

HA-1 Crosslinker: 6%.

Di(b-dimethylaminoethyl) Ether: 0.15%.

Manufacturing polyurethane filter foam typically involves two key stages. The first stage involves preparing open-cell or partially open-cell polyurethane foam according to the desired porosity. If the foam is closed-cell, it needs to be subjected to roller compression to rupture the cell walls, thereby creating the necessary porous structure. The second stage is to remove all the cell membranes to form a reticulated structure.



To produce polyurethane filter foam, the size of the pores and the structure of the network of the foam are largely determined by the catalyst, foaming agent, and surfactant.



In practical operations, there are two common methods for forming the network:



First is the alkali hydrolysis method. The steps of this method involve immersing the soft polyester-type polyurethane foam obtained from the first stage in a 10% sodium hydroxide solution at 50 degrees Celsius for about 10 minutes. Then it goes through processes such as washing with water, neutralization with acetic acid, another round of water washing, and drying, resulting in the final product of polyurethane filter foam.



Another method is the combustion method, also known as the explosion method. This method requires placing the soft polyether-type or polyester-type polyurethane foam obtained from the first stage into a sealed container. The container is then evacuated to 13.3 Pa, followed by the introduction of oxygen and natural gas (in a volumetric ratio of 2:1), bringing the internal pressure of the container to a certain level (which increases with the porosity). Next, the gas inside the container is ignited using a spark plug. The heat generated from the combustion process will burn off or melt the cell membranes without damaging the cell struts. Finally, the resulting product after combustion is cleared with air, and the filter foam is removed from the container.



Both of these methods are effective ways to prepare polyurethane filter foam, and the specific choice between them depends on the material of the foam and the desired structural characteristics.

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.

I. Advantages of Polyurethane On-site Foaming Technology:



The method of on-site foaming, spraying (or pouring) polyurethane foam insulation layer, has the surface as a whole without seams, reducing heat loss, with high construction efficiency, easy to meet quality requirements, reducing construction procedures, and eliminating the need for anti-corrosive coatings on pipe surfaces.



II. Polyurethane On-site Foaming Construction Process Principle:



The principle of polyurethane foam plastic foaming and spraying, pouring process is that polyether isocyanate can undergo a polycondensation reaction to form amine methacrylate, which can generate the required polyaminomethyl ethyl, commonly known as polyurethane foam plastic. Catalysts, crosslinking agents, foaming agents, foam stabilizers, etc., are simultaneously added during the reaction to promote and perfect the chemical reaction.



These raw materials are divided into two groups, fully mixed, and then pumped into a special spray gun by metering pumps in proportion. They are fully mixed and sprayed on the surface of pipelines or equipment in the spray gun or pouring mixer, react, foam, and form foam plastic within 5-10 seconds, which then cures and solidifies.



III. Polyurethane On-site Foaming Construction Methods:



Spraying Method: According to this formula, two groups of solutions are stored in two barrels respectively. The materials are filtered to the metering pump, driven by a pneumatic motor, and input into the gun body through the material tube. Compressed air regulates the material into the mixing chamber, mixed, and then sprayed onto the pipeline or equipment to foam and form.



Pouring Method: The prepared two groups of solutions are stored in barrels, filtered to the metering pump, driven by a pneumatic motor, and input into the pouring mixer through the material tube. Compressed air is introduced into the pouring motor, driving the stirring shaft to mix the two groups of materials, which are then injected into the mold for foaming and forming.



Precautions for Polyurethane On-site Foaming Construction:



Stir the material at room temperature to mix and react, then quickly pour it into the space that needs to be formed. During construction, control the reaction foaming time so that the mixed material after stirring is in a liquid state when poured into the gap. During the foaming process, significant expansion forces will be generated, so proper reinforcement should be made to the pouring interlayer or mold.