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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.
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Phases in foam preparation
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Reactions taking place
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Blending of components
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Polyol, isocyanate, catalyst, surfactant, blowing agent
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Reaction of isocyanate
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Reaction with polyol or with water or trimerization begins, increase
in MW
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Increase in temperature in blend
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Vapor pressure of blowing agent builds, increase in temperature
counteracts to some extend the increase in viscosity due to higher MW.
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Nucleation in mixture
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Bubble start to form
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Growth of bubbles
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Gas diffuses to the formed bubbles as new bubbles form
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Increase in viscosity
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Conversion of reactive groups leads to a higher MW polymer
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Gel point
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Depending on functionality the gel point of the formulation is reached
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a crit.
= 1/(f-1) f = average functionality of reactants,
a crit. = gel point.
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Gas continues to diffuses into bubbles
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Change in solubility parameter of polymer reduces the solubility of
the blowing agent in the now solid phase of the foam
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Foam stops to grow in volume
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MW continues to grow, phase separation of the
urea/urethane/isocyanurate rigid phase.
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Pressure continues to built up in the cells.
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Foam formation completed, not all the reaction are over
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Foam Density
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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.
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Foam starts to cool
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Collapse of same cells. A part of the blowing agent might remain as a
liquid in the polymer.
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Nucleation
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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.
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Growth of bubbles
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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.
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Increase in viscosity
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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.
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Gel
point
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At the gel point some molecules reach
infinite molecular weight, viscosity starts to increase exponentially,
and flow becomes elastic.
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Foam
Density
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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.
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