Nucleation unit for continuous sandwich panel production lines
POFI-Engineering concept
Polyurethane foam is primarily a cellular plastic assembly. The properties of this matrix depend primarily on two factors:
structure
and composition. The cell structure is achieved through the action of blowing agents. The finer and more homogeneous the structure, the better the mechanical properties of the polyurethane foam. The chemical composition also plays a significant role in its mechanical strength. A stoichiometrically homogeneous mixture gives polyurethane foam its optimal mechanical properties. Nucleation (air, nitrogen, or CO2) is a still little-used part of mechanical expansion. When physical agents are used to form the cells, the gas phase of the cellular plastic is chemically identical to that of the blowing agent. This technique has encountered implementation difficulties, recently resolved by adding the miscibility of gases in a liquid via a set of pressure variations combined with a high-performance mixing and homogenization system.
Direct effects of nucleation:
Action on chemical composition:
The principle of nucleation is to refine the cellular structure of the foam by a set of combined actions carried out on the Polymix (mixture of polyol and additives). Better mixing the polyol with its additives and integrating the air in very fine particles increases its reactivity, which considerably improves the quality of the isocyanate / polyol mixture and therefore optimizes the stoichiometry of the reaction. This function thus allows better crosslinking which improves the mechanical characteristics of the polyurethane foam. It is therefore possible to reduce the quantity of catalyst required for the reaction.
Action on cell structure:
The strength of the matrix structure is also a function of the fineness of the bubbles and their homogeneous distribution. Nucleation, by its concept, combines several physical actions causing the bubbles to burst into very fine and precise particles and thus considerably improves the structure of the matrix.
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Importance of air nucleation in rigid foam:
The production of rigid polyurethane foam requires two main liquid components: a polyisocyanate and a POLYMIX (polyol and a blowing agent). The blowing agent is typically added to the polyol along with other auxiliary components, such as activators (reaction accelerators), foam stabilizers, and flame retardants.
The reaction occurs when the two components are mixed together. During the reaction, a considerable amount of heat is released and used to evaporate the blowing agents present in the polyol. This evaporation, combined with the chemical reaction, forms the foam. Water is normally added to the polyol in varying amounts. The water reacts with the isocyanate to form polyurea and carbon dioxide, which serves as a co-blowing agent. As the primary blowing agent, some air is included in the Polymix. In fact, the polymerization reaction produces solid polyurethane, and it is by forming gas bubbles within the polymerization mixture, often referred to as “blowout,” that the foam is made.
The individual cells in the foam are insulated from each other by thin polymer walls, which effectively prevent the flow of gases through the foam. These materials offer good structural strength relative to their weight, combined with excellent thermal insulation properties. The cells contain a mixture of gases, and depending on their nature, the dimensions and proportions of the foams have different thermal conductivities. To maintain long-term performance, it is necessary for the low-thermal-conductivity gases to remain within the cells; therefore, more than 90 percent of the cells must be closed.
This demonstrates that good foam is the result of two components: structure and composition.
The composition is developed by the raw material supplier. We will focus on the mechanical component, the foam matrix.
There are several theories about foam development. Most are based on nucleation during the development phase. It appears that all the cells present in the finished foam are already present in the early development phase, when the raw materials are mixed in the mixing head; the reaction triggers the appearance of nucleating air bubbles present in the Polymix.
The dispersed gas bubbles grow due to the expansion of the expanding gas. This process continues until the spherical cells are more compacted in the liquid matrix. When the spherical cells are in contact with each other, they convert into polyhedral cells. The foam reaches its final structure and good mass distribution at the end of the wire-forming time.
The more homogeneous and finer the structure, the better the mechanical and insulation properties of the polyurethane foam.
Today, the advantages of air nucleation are still rarely used for mechanical expansion, with approximately 8 to 12% air in the Polymix. When physical agents are used to form the cells, the gas phase of the foamed plastic is chemically identical to that of the blowing agent. This technique has encountered implementation difficulties, recently resolved by adding the miscibility function of gases in a liquid via a set of pressure variations associated with a high-performance mixing and homogenization system. With this system we can add an amount of air nucleation around 65% without cavitation of the high-pressure pump; the result is a more regular matrix and a more homogeneous foam.
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Do not hesitate to contact us for further information.
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