Handmade mesh sponge foaming tutorial
Mesh sponge is a special type of sponge product. It started late and appeared in the Chinese market around 1995. The industry generally calls it: mesh cotton, blasting cotton, and coarse-pore cotton. It can be specifically described as: a permeable sponge with a certain pore size and a pentahedral main pore connected by veins. Because it is “hollow” and has the strength and ductility of sponge, it can be widely used in:
① Air filtration
② Water-based filtration
③ Silence
④ Shock wave buffer
⑤ Foam metal substrate
⑥ Metallurgical filter base material
and other fields.
The production of a real mesh sponge should be completed in two steps: foaming and hole breaking. The pore size of the mesh is controlled during foaming, so we now focus our discussion on the “foaming” step.
Before 2008, I had been making hard foam products. I had only been exposed to the molding type of soft foam products, and I had only been exposed to the theoretical level of large block foam products. Through several years of exploration, testing and production verification, we have summarized some unusual processes and adjustment ideas. Fortunately, I have been doing hard soaking for more than ten years.
Thanks to all the brothers and sisters who have helped me in this field: He Shenxiang, Xu Dongming (The Fifth Element), Zou Hanjiang, He Qibin, Qiu Minghua, Wang Jianrong, Yang Zhicheng, Teacher Wei Haidong, Teacher Liu Shouping, Teacher Xiu Yuying, He Xulin, Zheng Fashan , Gong Zhilin, Chen Yinghuai, Liu Yunzhen…
【Principle of Foaming Reaction】
Like ordinary sponges, our main reactions here are still two:
①The reaction between the hydroxyl group of macromolecular polyol and TDI isocyanate (hereinafter can be abbreviated as -OH, -NCO):
②Reaction between water and TDI:
Here, we can’t see any relationship between the change in sponge pore size and the main reaction. It’s normal. They are all born to the same parents. The key is the acquired growth environment.
Many brothers who have made ordinary sponges all know that there is a component called “silicone oil” in the sponge raw material. In fact, it plays the role of a “mediator”, that is:
1. Responsible for integrating polyether polyol, water, catalyst and TDI with each other.
2. After CO2 is generated, it can stabilize the foam airbag wall and make it elastic, so that it will not be weak and break when touched by the wind (otherwise it will collapse without discussion).
3. It should not be too stable. During the foam growth process, when the pressure inside the air bag reaches a certain level, the bag wall diaphragm will be broken by the wrapped gas (when the whole foam is still a viscous fluid, it will be a “burst hole”. “, the later stage of foam molding is “deflation”) to maintain the stability of the system.
The first step in the production of handmade mesh is to produce cotton with a certain pore size. Its production characteristics are characterized by: the raw materials are evenly (but not necessarily “fully” evenly) mixed, and have obvious “starting” “, “burst holes”, “elongate”, “gel”, “deflate” and “set” the foaming process. The key control is “hole bursting”. All factors that can affect the degree of “hole bursting”: the nature of raw materials, mixing methods, temperature control, air pressure difference, use of additives, etc. are the goals of our research.
(The “burst” phenomenon here is the bursting phenomenon when the microbubbles that can be seen burst when the liquid surface is exposed to light after the polyether, additives, and TDI are formally mixed and poured into the foaming box. Generally, it will appear at the beginning Occurs within 30 to 40 seconds after the stirring timer, and the duration will last for 8 to 15 seconds)
[Factors of TDI]
You can safely use 80/20 type TDI, that is: 2.4-toluene diisocyanate accounts for about 80%, and 2.6-toluene diisocyanate accounts for 20%. The activity of -NCO at position 2.4 is different, and the sequence of their reaction with -OH and reaction with water is also different. Don’t underestimate this difference in activity. The activity of emitting CO2 gel (rapid chain growth) is the starting point.” The difference in phase will directly lead to the sponge body being easy to open holes.Cyclohexylamine (cyclohexylamine for short) is a very exciting variety. The pore diameter it produces is larger than that of A-33, and the pore size distribution uniformity is obviously better. I performed mass production for a while.
The disadvantages are: the smell is too strong (bitter in the throat) and the onset speed is too fast. It is recommended to use A-33 in combination with automatic line production.
④ Benzylamine
It was also my favorite during my hard foam years. I recommend it because it has the same effect as hard foam: smooth rise and gel, good cell size distribution, and the pore size is smaller than A-33. .
⑤ Triethanolamine
A frequent visitor in hard bubbles, it participates in the reaction and has a trimerization function. It can improve the compression strength of the cotton body, but the only drawback is that it cannot be used with high energy.
【Silicon oil】
This is an important point. Generally speaking, when making coarse-pore sponges, we will give priority to making them “rough” first. In fact, this is not the case, because we have to break holes in the foamed coarse-porous cotton. There will be specific requirements for the hole diameter, vein structure, and remaining film amount after the holes are broken.
Everyone hopes that the finished mesh will look pleasing to the eye, with uniform pore size, no film residue, and no spots or other defects. As we all know, silicone oils have different levels of activity. As a broad-spectrum L-580, it can be applied to high, medium and low density formulations. Its pore diameter is not the thickest, but it is the most uniform. Although 8123, 8110, 4900, 5933, etc. have the advantage of being “coarse”, their pore size distribution is unsatisfactory and there are many residual films. It is recommended to use them in combination.
Generally speaking, the dosage of silicone oil is “large dosage, small cells”, what about “small dosage”? There must be a limit, 0.4% is the lower limit. There are exceptions. Some varieties have rougher pores the more they are used, but the type of sample I got has too many side effects and cannot be used in mass production.
[Other additives]
A lot of effort has been spent on this. Generally speaking, external additives are nothing more than defoaming agents, pore opening agents, etc. If the uniformity of pore size distribution is not considered, there are a large number of types and varieties of additives. agent can be used.
We firmly do not support the addition of powders. “Even dispersion” is a problem, except for colorants.
【Temperature factor】
Foaming temperature: Everyone used to like the 20-23°C range for manual foaming. I also like it, but it is limited to the foaming environment temperature.
If the material temperature is too high, the starting time will be advanced, and the manual mixing time will be greatly affected. Therefore, the material temperature must be lowered below 16°C.
It is not that the lower the material temperature, the better, because 1. TDI will crystallize into a solid below 14°C; 2. The viscosity of polyacid alcohols will increase significantly, making stirring difficult. The specific temperature control depends on your actual situation. The situation is determined.
【Stir】
The horizontal mixer has a disadvantage. There is a dead space at the bottom of the central area of the mixing barrel and on the back flow surface of the baffle. As we all know, the purpose of stirring is to mix the materials evenly. The coarse-pore cotton requires the mixing speed to be as low as possible, and to avoid the vortex mixing with a large number of air bubbles causing “bean eyes”. Therefore, it is not suitable to use paddle-type mixing on a horizontal mixer. It is recommended to use a disc type bucket with holes for exchanging upper and lower materials.
I prefer to use a vertical mixer. There is no dead space such as the base in the mixing barrel. I can also use the “slightly inclined” method to deliberately create eccentricity to minimize the possibility of mixing dead space.
[Other factors]
① Low pressure foaming
I have done this kind of test. The change in pore diameter at -0.03Mpa is at least two levels (normal pressure 40PPI, negative pressure 25PPI), but the operability is not good. Not to mention the time-consuming process of pouring and stirring, just reducing the area of the foaming box to -0.03~-0.05Mpa within 10 seconds is enough to cause a headache. The repeatability must not be good, and it is very difficult to adjust the formula and process. big.
② Air temperature and air pressure
The greater the humidity, the more difficult it is (referring to repeatability);
The lower the air pressure (sultry heat), the more difficult it is to do.
③ Mold and temperature
It is best to use wooden molds, especially for the bottom. If the temperature of the base plate is high in hot weather, the cells will become thinner and the height will be difficult to control. The same goes for cold weather, the cells will become thicker, the height will be difficult to meet the ideal requirements, and the bottom layer will be thicker.
④ Zombie
If the foaming barrel is not cleaned properly, or if debris is mixed into the material, the foam will produce random flowers and spots.
[Density Requirements]
Low-density mesh is easy to make and easy to understand. The nucleation number at the beginning of gasification is closely related to stirring. After initiation and bursting, the number of vesicles per unit weight has been fixed, and each vesicle grows fat according to its own “expansion multiple”. With the continuous release of CO2, the balloon expands due to the continuous increase in material temperature. After the final release and finalization, the more gas generated must be the coarse holes.
But for medium and high-density sponges, the water consumption in the formula must be reduced, and the gas production will inevitably decrease accordingly. The only way we can control the coarse pores is to ① reduce the stirring speed, ② the silicone oil must be reduced to the limit dosage, ③ The amount of amine should be as high as possible, ④ the material temperature should be as low as possible, ⑤ use negative pressure foaming when conditions permit, ⑥ and the second one is to increase the TDI index.
【Automatic line foaming】
The biggest problem with manual foaming is that the gap between the upper and lower holes of the foam body is too large. The pore diameter is largest at 2/3 of the height, followed by the top, and the pore diameter becomes smaller as you go down.
If automatic line (flat pull foam) foaming is used, the drop plate will reduce the resistance difference between the upward and downward growth of the sponge during foaming, and the size difference between the upper and lower pores will be very small. The size of the foaming pores is completely controlled by the pressure of the mixing head.
The principle: similar to popcorn, the mixing head is pressurized and stirred evenly, and the pressure is quickly released from the outlet of the spray head. The newly nucleated microbubbles will quickly complete their first expansion and bursting. With the continuous generation of CO2 and the phase separation of the system, the second burst was successfully achieved.
The disadvantage is: the plate vibration of the flat cable will cause disturbance and bubbles after the second bubble, so it is difficult to mass-produce 8-15PPI products with large holes on automatic lines.
The outlet of the material head comes out and the pressure is quickly released, and the newly nucleated microbubbles will quickly complete their first expansion and bursting. With the continuous generation of CO2 and the phase separation of the system, the second burst was successfully achieved.
The disadvantage is: the plate vibration of the flat cable will cause disturbance and bubbles after the second bubble, so it is difficult to mass-produce 8-15PPI products with large holes on automatic lines.
