Question: What happens when the foam cools?
About the article "Popcorn", section "Popping mechanism".

Candidate passages:
Polyurethanes, especially those made using aromatic isocyanates, contain chromophores which interact with light.
This is of particular interest in the area of polyurethane coatings, where light stability is a critical factor and is the main reason that aliphatic isocyanates are used in making polyurethane coatings.
When PU foam, which is made using aromatic isocyanates, is exposed to visible light it discolors, turning from off-white to yellow to reddish brown.
It has been generally accepted that apart from yellowing, visible light has little effect on foam properties.
This is especially the case if the yellowing happens on the outer portions of a large foam, as the deterioration of properties in the outer portion has little effect on the overall bulk properties of the foam itself.
It has been reported that exposure to visible light can affect the variability of some physical property test results.
Higher-energy UV radiation promotes chemical reactions in foam, some of which are detrimental to the foam structure. 
When a headliner fails, it sags or falls from the roof.
Repairs may involve tacking or replacing the entire headliner.
Heat, humidity, and time rapidly degrade the bond that the polyurethane foam has on the face fabric.
As the foam oxidizes and yellows, the fabric-foam bond fails.
This failure is more common in cars produced in the 1970s.
Replacing the entire headliner fabric is usually necessary as the oxidized foam can prove nearly impossible to remove from the sagging fabric.
To do this one would purchase a headliner replacement kit, headliner adhesive, and a scrubbing brush.
The general process as outlined by most repair guides:1) Carefully remove the current headliner board.<br>2) Peel the old headliner fabric.<br>3) Brush off the old adhesive foam from the headliner board.<br>4) Glue new headliner fabric and reinstall back into the vehicle.
<br>This also happens to similar vinyl headlining in boats, but with less predictability. 
Coring happens when a heated alloy, such as a Cu-Ni system, cools in non-equilibrium conditions.
This causes the exterior of the material to harden faster than the interior.
Coring causes the centers of the grains to retain more of the higher melting temperature element.
In this case, the dendrite arms formed from the exterior have a different composition than the alloy in the inner regions, resulting in a local compositional difference. 
There are two defects in this category: "hot tears" and "hot spots".
Hot tears, also known as , are failures in the casting that occur as the casting cools.
This happens because the metal is weak when it is hot and the residual stresses in the material can cause the casting to fail as it cools.
Proper mold design prevents this type of defect.
Hot spots are areas on the surface of casting that become very hard because they cooled more quickly than the surrounding material.
This type of defect can be avoided by proper cooling practices or by changing the chemical composition of the metal. 
Water has long been a universal agent for suppressing fires, but is not best in all cases.
For example, water is typically ineffective on an oil fire, and can be dangerous.
Fire-fighting foams were a positive development in extinguishing oil fires.
In 1902, a method of extinguishing flammable liquid fires by blanketing them with foam was introduced by the Russian engineer and chemist Aleksandr Loran.
Loran was a teacher in a school in Baku, which was the main center of the Russian oil industry at that time.
Impressed by the terrible and hardly extinguishable oil fires that he had seen there, Loran tried to find such a liquid substance that could deal effectively with the problem.
So he invented fire-fighting foam, which was successfully tested in several experiments in 1902–1903.
In 1904 Loran patented his invention and developed the first foam extinguisher the same year.
The original foam was a mixture of two powders and water produced in a foam generator.
It was called chemical foam because of the chemical action to create it.
In general, the powders used were sodium bicarbonate and aluminium sulfate, with small amounts of saponin or liquorice added to stabilise the bubbles.
Hand-held foam extinguishers used the same two chemicals in solution: To actuate the extinguisher, a seal was broken and the unit inverted, allowing the liquids to mix and react.
Chemical foam is a stable solution of small bubbles containing carbon dioxide with lower density than oil or water, and exhibits persistence for covering flat surfaces.
Because it is lighter than the burning liquid, it flows freely over the liquid surface and extinguishes the fire by a smothering (removal/prevention of oxygen) action.
Chemical foam is considered obsolete today because of the many containers of powder required, even for small fires.
In the 1940s, Percy Lavon Julian developed an improved type of foam called Aerofoam.
Using mechanical action, a liquid protein-based concentrate, made from soy protein, was mixed with water in either a proportioner or an aerating nozzle to form air bubbles with the free-flowing action.
Its expansion ratio and ease of handling made it popular.
Protein foam is easily contaminated by some flammable liquids, so care should be used so that the foam is applied only above the burning liquid.
Protein foam has slow knockdown characteristics, but it is economical for post-fire security.
In the early 1950s, high-expansion foam was conceived by Herbert Eisner in England at the Safety in Mines Research Establishment (now the Health & Safety Laboratory) to fight coal mine fires.
Will B. Jamison, a Pennsylvania Mining Engineer, read about the proposed foam in 1952, requested more information about the idea, and proceeded to work with the US Bureau of Mines on the idea, testing 400 formulas until a suitable compound was found.
In 1964, Walter Kidde & Company (now Kidde) bought the patents for high expansion foam.
In the 1960s, National Foam, Inc. developed fluoroprotein foam.
Its active agent is a fluorinated surfactant that provides an oil-rejecting property to prevent contamination.
In general, it is better than protein foam because its longer blanket life provides better safety when entry is required for rescue.
Fluoroprotein foam has fast knockdown characteristics and it can also be used together with dry chemicals that destroy protein foam.
In the mid-1960s, the US Navy developed aqueous film-forming foam (AFFF) .
This synthetic foam has a low viscosity and spreads rapidly across the surface of most hydrocarbon fuels.
A water film forms beneath the foam, which cools the liquid fuel, which stops the formation of flammable vapors.
This provides dramatic fire knockdown, an important factor in crash rescue fire fighting.
In the early 1970s, National Foam, Inc. invented Alcohol-Resistant AFFF technology.
AR-AFFF is a synthetic foam developed for both hydrocarbon and polar-solvent materials.
Polar solvents are combustible liquids that destroy conventional fire-fighting foam.
These solvents extract the water contained in the foam, breaking down the foam blanket.
Therefore, these fuels require an alcohol- or polar-solvent-resistant foam.
Alcohol-resistant foam must be bounced off of a surface and allowed to flow down and over the liquid to form its membrane, compared to standard AFFF that can be sprayed directly onto the fire.
In 1993, Baums Castorine of Rome, NY developed a fire fighting foam that is effective on both Class A and Class B foams.
Their patented formula was awarded the 1998 Presidential Green Chemistry Award as a bio-degradable alternative to AFFF and AR-AFFF and other foams and wetting agents.
Originally marketed under the name Pyrocool F.E.F., it is now sold under the name Novacool UEF (Universal Extinguishing Agent).
Novacool UEF is used at 0.4% for Class A fires and only 0.5% for Class B fires, 1/6th to 1/12th the amount required by other Class B foams and wetting agents.
Novacool extinguishes polar solvent fires such as ethanol and alcohol and also extinguishes 3D fires.
Working at the ionic level, Novacool is completely organic and bio-degradable and has also been proven to be effective at extinguishing Class D and Class K fires.