Importance of the air nucleation in rigid polyurethane 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 usually added to the polyol with further auxiliary components such as activators (reaction accelerators), foam stabilizers and flame retardants. The reaction occurs when both components are mixed together. During the reaction a considerable amount of heat is released which is used to evaporate the blowing agents included in the polyol. This evaporation added to the chemical reaction will form the foam. Various quantities of water are normally added to the polyol. The water reacts with the polyisocyanate to form polyurea and carbon dioxide, which serves as a co-blowing agent. As a first blowing agent, a part of air is included in the Polymix.
In fact, the polymerization reaction produces solid polyurethane and it is by forming gas bubbles in the polymerizing mixture, often referred to as ‘blowing’, that foam is made.
The individual cells in the foam are isolated from each other by thin polymer walls, which effectively stop the flow of gas through the foam. These materials offer good structural strength in relation to their weight, combined with excellent thermal insulation properties. The cells contain a mixture of gases and depending on their nature, size and proportions the foams will have different thermal conductivities. In order to maintain long-term performance it is necessary for the low thermal conductivity gases to remain in the cells, consequently more than 90 per cent of the cells need to be closed.
This will explain that a good foam is the result of two components, the structure and the composition. The composition is developed by the raw material supplier. We are going to concentrate on the mechanical part, the matrix of the foam.
There are various theories about the development of the foam. Most are based on nucleation in the development phase. It seems 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 activates the appearance of the air bubbles of nucleation present in the polymix.
The initially dispersed spherical gas bubbles grow due to the expanding of the blowing gas. This process continues until a certain time in which the spherical cells are most densely packed in the liquid matrix. When the spherical cells contact together it convert to polyhedral cells. The foam reaches its final structure and mass distribution at the end of fiber time.
More the structure is homogeneous and fine; more the mechanical and heat insulation characteristics of foam polyurethane are good.
Today the advantages of air nucleation enters the part still little used mechanical expansion, around 8 to 12% of air in the polymix. When physical agents are used to form the cells, the gas phase of the alveolar plastic is chemically identical to the inflating agent. This technique had problems of implementation, recently solved by the addition of the function of miscibility of gases in a liquid via a whole of variations of the pressures and flow speed associated with high performances mixing system for dispersion. With this system we can add an amount of air nucleation around 65% without cavitations in the high pressure pump; the result is a more regular matrix and more homogeneous foam.
Effects of this nucleation system: Better mix the polyol with its additives will increase its reactivity, better mixing quality of the couple Isocyanate/Polyol and of this fact an optimized stoichiometry of the reaction. It will give a better reticulation which improves the mechanical characteristics of foam polyurethane. Action on the structure of the cells: The resistance of the structure of the matrix is also function of the smoothness of the bubbles and their homogeneous distribution. Nucleator from its concept associating the bursting of the bubbles in very fine particles and several precise physical actions improves considerably of dispersion of the cells and this fact structure more finely the matrix.