Discussion on Issues of Collapsing, Bursting, and Closed-Cell Formation in Polyurethane Foam

The reasons for collapsing foam are primarily due to either the lack of one or more components or a severely imbalanced formula. Generally, if small test foams do not collapse, they will not collapse during machine foaming either. However, caution is needed when adding mineral oil and stone powder to large low-density foams. In certain cases, small test foams that performed well—even when scaled up 4-5 times—can collapse or form closed cells when machine-foamed, although such cases are rare.

In continuous production foaming, foam heads sometimes collapse within half a meter, typically due to the distance of the polyether bulk material to the mixing chamber, the stirring conditions at startup, and the total flow rate of the foaming process. This is because the discharge of amine, tin, silicon, and methane depends entirely on the polyether propelling them into the mixing chamber. Some foaming machine manufacturers shorten the distance between the polyether bulk material and the mixing chamber and set programs like stirring at 60% of the set speed for the first three seconds after startup, then gradually accelerating to the foaming set speed. For high-pressure continuous foaming machines, since all high-pressure components must be atomized at 30-100 atmospheres and directly sprayed into the mixing chamber to be effective, the high-pressure component nozzles are installed around the mixing chamber, avoiding the aforementioned issues.

There are various forms of foam bursting. It is not always necessary to add a tin catalyst when bursting occurs. For bursting during the foam opening phase, insufficient gelation might be the cause, which can be solved by adding tin. However, in the following two scenarios, tin might not be the solution:

Some high-powder foams or hard low-density foams with added powder might have deep cracks at the top, possibly due to early reaction imbalance, which adding tin often cannot resolve.

Internal cracks where the foam surface appears normal, but cutting open reveals long internal cracks, which are also often caused by early reaction imbalance.

Understanding the outgassing conditions is crucial when discussing foam bursting. In practice, the following situations may occur: the outgassing point and foam peak are not synchronized, leading or lagging by 0.2-0.5 meters. Some foam tops and sides outgas evenly, some only on the sides, and some not at all. Some people judge permeability by blowing on freshly grabbed foam, which is clearly limited. The correct approach is to carefully observe the structure and brightness points (membranes) of the freshly grabbed foam. Generally, foam will show the following signs before bursting due to insufficient gelation during outgassing: decreasing brightness points inside the foam → losing gloss on the foam structure → severe loss of gloss accompanied by structural breakage → bursting. Insufficient tin is just one cause of bursting. Imbalanced formulas, overly intense stirring (speed and pressure), low raw material temperatures, uncoordinated sedimentation plates, and subsurface currents can all cause bursting. Observing the foam structure and brightness points can also quickly identify the cause of bursting.

Closed-cell foam primarily affects production safety. Increased membranes in closed-cell foam reduce permeability, making it difficult to dissipate heat. Closed-cell shrinkage is particularly dangerous as it causes localized density increases, leading to hot spots. Some people worry about burning when bursting occurs, but with the same formula, burst areas can be safer as they allow better heat dissipation.

Some foaming experts judge excess tin by observing the foam sides, but this is not always accurate. For example, in high-powder foam, reducing tin to eliminate "excess tin" on the sides can degrade properties or cause bursting. This is similar to the phenomenon where some foams show many brightness points when freshly grabbed but appear fewer the next day. Some foaming experts think that normal outgassing indicates no closed-cell shrinkage, but this is not always true. For high-resilience foam, even with strong top cover outgassing, closed-cell shrinkage can occur the next day. Experienced foaming experts consider other factors when working with high-resilience foam, especially those over 6000 molecular weight polyether.

People tend to oversimplify problems for easier handling: Is this issue caused by adding too much of one material? What caused this issue? This questioning method is flawed, reflecting limited understanding.

Assessing a foaming expert's overall capability is not about handling exotic products but rather their holistic view (grasping key points), understanding of foaming (concepts), flexibility (quickly finding the most suitable formulas and parameters), and overall control ability (effectively guiding the reaction and production process). These skills are the hardest to master, requiring refined technique and extensive practice.