The causes of foam collapse mainly fall into two categories: the absence of one or more components or a severe imbalance in the formula. Generally, small-scale foam tests that do not collapse will also perform well when scaled up. However, for low-density foams with high proportions of white oil and calcium carbonate, caution is required. In rare cases, foam samples that perform exceptionally well during small-scale trials—even with raw material proportions increased four to five times—may still collapse or form closed cells when transferred to full-scale production.
In continuous production lines, foam can sometimes collapse within half a meter of the foam head. This is often linked to factors such as the distance between the main polyether feed and the mixing chamber, stirring conditions during machine start-up, and overall foam flow rate. The introduction of amine, tin, silicone, and methane into the system relies entirely on the polyether driving them into the mixing chamber. To address this, some machine manufacturers have minimized the distance between the polyether feed and the mixing chamber and incorporated preset programs—for example, setting the stirring speed to 60% of the target during the first three seconds and gradually increasing it thereafter. High-pressure foam machines, which operate with components atomized at 30–100 bar, avoid these issues by directly injecting components into the mixing chamber from nozzles installed around it.
Foam bursting can manifest in various ways, and adding tin catalysts is not always the solution. For bursting during the foam’s opening phase, insufficient gelation might be the cause, and tin can help. However, the following scenarios may not be tin-related:
1.Surface Cracks: In high-filler or low-density rigid foams, deep surface cracks may result from imbalances during early reactions, which tin catalysts cannot resolve.
2.Internal Cracks: These occur when the foam surface appears intact but internal cavities or fissures develop, often several centimeters long. These too can stem from early-stage reaction imbalances.
Understanding gas release is critical for addressing foam bursting. Operational issues often arise when gas release points do not align with the foam’s rise, leading or lagging by 0.2–0.5 meters. Some foams release gas uniformly from the top and sides, while others exhibit side-only or no release. Judging permeability by blowing air through freshly formed foam is limited. Instead, closely observing the foam’s cell structure and membrane density is more reliable. Typically, the process of bursting due to insufficient gelation before gas release follows this sequence:
-Bright points within the foam decrease
-Foam struts lose their sheen
-Severe dulling and breakage of foam struts occur
-Bursting ensues
While insufficient tin is one factor, other causes include unbalanced formulations, overly intense mixing (speed and pressure), low raw material temperatures, misaligned settling plates, and poor flow control. Observing the foam’s structure and membrane brightness can help identify specific causes.
Closed cells in foam primarily pose safety risks. The increased membrane density reduces airflow, making it harder for heat to dissipate. This is particularly dangerous in foams prone to shrinkage, where localized density increases can exacerbate heat retention. Despite common fears that bursting foams are more likely to overheat, the opposite is often true—bursting allows heat to escape more efficiently.
Judging tin levels by foam edge appearance is not always reliable. For example, reducing tin to eliminate visible edge issues in high-filler foams might compromise properties or cause foam bursting. Similarly, foams with high initial brightness may show reduced brightness by the next day. Experienced foam technicians understand that visible gas release alone does not rule out closed-cell shrinkage. For instance, high-resilience foams may exhibit robust gas release at the surface but still shrink internally the following day. Operators working with high-resilience polyether foams (above 6000 molecular weight) should monitor multiple indicators beyond just gas release.
People often oversimplify issues to identify a single cause, such as whether a specific material was overused. This approach reflects a lack of understanding of the complexities involved.
The competency of a foam technician is not measured by handling exotic products but by their ability to demonstrate:
1.Holistic Perspective: Focusing on key factors
2.Understanding of Foam Dynamics: Sound conceptual grasp
3.Adaptability: Quickly identifying optimal formulas and parameters
4.Overall Control: Effectively guiding the reaction and production process
These qualities emerge only through mastery and extensive practical experience.