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Foams are solids or liquids which have a gas dispersed in their structure. The individual gas bubbles can be separated from each other by a cell wall. These foams are called closed cell foams. The individual cells can also be connected, these foams are called open cell foams. Most urethane foams contain both closed cell and open cell structures. Closed and open cell foams are being used in different applications. Foams can be prepared by an number of different techniques and out of many raw materials. The process to prepare a foam will depend on the materials used to prepare a foam.

Polyurethane foams are normally prepared by reacting a low MW polyol with a di or polyisocyanate. Many different techniques can be used to incorporate the gas bubbles and many gases can be used as blowing agent. In the simplest case air or a gas can be incorporated into to a foam by mechanical mixing the gas during the foam preparation. Most polyurethane foams are being made by creating the gas by evaporation of a low boiling liquid or by chemical reaction. It is also possible to dissolve a gas at high pressure in the components used to make a foam under pressure and to release the pressure during mixing of the components. The different steps in formation of a foam are greatly influenced by the blowing agent, surfactant and catalysts. The rate of the competing reactions has a substantial effect on the properties of a foam. Below are the typical steps one might find during the formation of a foam with a low boiling liquid as a blowing agent.

There are a number of requirements for the successful formation of a foam. 


Phases in foam preparation Reactions taking place
Blending of components Polyol, isocyanate, catalyst, surfactant, blowing agent
Reaction of isocyanate  Reaction with polyol or with water or trimerization begins, increase in MW
Increase in temperature in blend Vapor pressure of blowing agent builds, increase in temperature counteracts to some extend the increase in viscosity due to higher MW.
Nucleation in mixture Bubble start to form
Growth of bubbles Gas diffuses to the formed bubbles as new bubbles form
Increase in viscosity  Conversion of reactive groups leads to a higher MW polymer
Gel point Depending on functionality the gel point of the formulation is reached

a crit. = 1/(f-1)   f = average functionality of reactants, 

a crit. = gel point.

Gas continues to diffuses into bubbles Change in solubility parameter of polymer reduces the solubility of the blowing agent in the now solid phase of the foam
Foam stops to grow in volume MW continues to grow, phase separation of the urea/urethane/isocyanurate rigid phase.
Pressure continues to built up in the cells. Foam formation completed, not all the reaction are over
Foam Density Assuming the foam is stable and does not collapse during the blowing stage, the foam volume generated depends on the molecular weight of the blowing agent, the amount of blowing agents, temperature and amount of blowing agent evaporated.
Foam starts to cool Collapse of same cells. A part of the blowing agent might remain as a liquid in the polymer. 

Nucleation
For a gas bubble to form in a solution, the surface tension of a small bubble has to be overcome. Nucleation in a solution can be increased by heterogeneity in the solution such as surfaces or solids. For the uniform formation of cells in a foam nucleation in many places has to take place at the same time. This requires fast changes in the vapor pressure of a solvent or fast heating. Fast heating is accomplished by high reaction rates.  
Growth of bubbles
The blowing agents gas pressure is above the atmospheric pressure. Diffusion of gas to the already formed bubbles is taking place. It takes a high pressure in a liquid to nucleate a new bubble. Diffusion of gas to already formed bubbles is in equilibrium with nucleation of new bubbles. High viscosity will reduce diffusion rate and favor nucleation, so will low surface tension and heterogeneity. Fast increase in temperature or high reaction rates will favor nucleation, low temperatures or slow reaction will favor growth of bubbles. Diffusion of surfactants can have a large effect on bubble growth. Silicone surfactants diffuse during the growth of the bubbles to the surface of the bubbles, they raise the surface viscosity and lower surface tension. Higher surface viscosity reduces the diffusion of gas into already formed bubbles.
Increase in viscosity
Reaction of the isocyanate with the polyol increases the MW and thus the viscosity of the polyol. The introduction of hydrogen bonds into the polymer also increases the viscosity of the polymer formed due to hydrogen bonding. In water blown foams the reaction of the water with the isocyanate eliminates a diluent from which will also increase the viscosity. Initially as the polymer is formed, the polymer might still be homogenous, but as the conversion proceeds the higher content of urethane or urea groups will lead to phase separation. In high functional polymers the increase in MWn is faster than in lower functional monomers.
Gel point  
At the gel point some molecules reach infinite molecular weight, viscosity starts to increase exponentially, and flow becomes elastic.
Foam Density  
In foams blown with blowing agents the  density depends on the MW of the blowing agent, the amount of blowing agents used, the boiling point of the blowing agents. The foam density can be approximately calculated. 

Last edited on:

November 14, 2006

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