Chemical Foaming Agents Boost Foam Plastics Manufacturing!

Chemical blowing agents are a class of chemicals capable of generating gases, with common released gases including nitrogen, carbon dioxide, water vapor, and small amounts of other gases. Among these, nitrogen is the preferred gas due to its low permeability, odorlessness, and non-toxicity, enabling the production of high-performance foam products with uniform cell structures. During specific stages of plastic processing, chemical blowing agents release gas at appropriate times, facilitating the formation of cells within the plastic. The foaming process can generally be divided into four key stages.

In the first stage, the foaming agent must be completely and uniformly dispersed within the polymer, which is often in a liquid or molten state at this point. During this stage, the foaming agent either forms a true solution within the polymer or is evenly dispersed to create a two-phase system. The mixing of the foaming agent with the polymer material is commonly carried out using internal mixers or open mills. When selecting a foaming agent, special attention must be paid to the mixing temperature to ensure it is lower than the decomposition temperature of the foaming agent, thus ensuring the subsequent foaming process proceeds smoothly.

According to the Color Masterbatch Industry Network, in the second stage, when a large number of individual bubbles are formed, the entire system transitions to a state where gas is dispersed within the liquid. To promote the formation of a large number of small bubbles, nucleating agents are usually added to the system. Without nucleating agents, it is very easy to produce extremely uneven cell structures. Nucleating agents are generally very fine inert particles, such as calcium carbonate, kaolin, and carbon black, which can all serve this role and help form a uniform pore structure.

The core of the third stage is the process of bubble growth. The initially formed bubbles will continuously grow larger as more gas penetrates the polymer and enters the bubbles through diffusion. If this process lasts long enough, individual bubbles will come into contact and merge. Depending on the connectivity of the bubbles, foam can be classified into different types: open-cell foam, primarily formed through interconnected bubbles, and closed-cell foam, consisting of unconnected bubbles. It is important to note that if bubble fusion proceeds without restriction, the foam will eventually collapse.

The fourth stage is the stabilization and shaping stage of the foam. When the viscosity of the polymer increases and the blowing agent has almost completely decomposed, the cells can no longer grow, and the foam stabilizes to become a shaped product. There are various ways to increase the polymer viscosity; methods such as cooling and crosslinking can achieve this purpose. From a time perspective, the last three stages of the foaming process take a relatively short duration—ranging from as little as a fraction of a second to, at most, a few seconds.

The quality of foam plastics is closely related to the decomposition of the foaming agent and the gelation process of the polymer material (i.e., the viscosity change of the polymer material) during production. Generally, it is required that the decomposition process of the foaming agent should be synchronized with the gelation process of the polymer material, or that the gelation process should occur slightly earlier than the decomposition of the foaming agent. If these two processes cannot be matched, it is difficult to produce foam plastics with good performance. Specifically, if the gelation process lags behind the foaming process, the viscosity of the cell walls will be too low, resulting in overly soft foam, and in severe cases, the foam may collapse due to insufficient support. On the other hand, if the strength of the cell walls is too high, the internal pressure of the cells will become excessive, leading to tearing of the cell walls and, ultimately, large cracks appearing in the center of the foam.