The Polyurethane Foam Book

Chapter One: What is Polyurethane Foam?

by David B. South
Illustrations by Merrisa Carter

What is polyurethane foam? If you want to know absolutely, unequivocally, technically, all of the things that make up polyurethane foam (urethane for short), I refer you to the manuals produced by the chemical companies. I suggest you talk to them. From a nontechnical, practical point of view, I will attempt to define and describe rigid urethane foam. First we must understand a little about plastics. There are two major groups of plastics: 1) thermal plastics; 2) thermoset plastics. Each group is made using different processes.

Thermal Plastics

Thermal plastics are everywhere. Polyethylene, polystyrene and thousands of others meet the classification of thermal plastics. They can be heated, molded, allowed to cool and retain a new shape. For example, when a cube of butter is warmed it can be poured into a container. When cooled, it has a new shape. We can reheat it, pour it into a cube mold, let it cool and obtain the cube shape again. Examples of thermal plastics are milk jugs, polyethylene sheathing, styrene cups, polyester, etc.

Thermoset Plastics

Thermoset plastics, on the other hand, are more complex. Their molecules permanently join in a process called cross linking. Thermosets cannot be transformed back to their original shape simply by heating. An egg is a good example of thermosetting material. Once an egg is boiled, no amount of heat can make it runny again or make it look like a fried egg. Most thermoset plastics have two components. Both groups of plastics have great application in our everyday life.

Polyurethane foam is a thermoset plastic. When it is allowed to combine and catalyze, it takes on a permanent shape. Urethane foam can be burned, but it burns in place. The by-products of that burning are ash and chemicals combined as vapors let into the air as gases.

A block of wood has the same burning process. You can never heat wood and make it melt. But thermal plastics will melt and run. In a freezer application, this is one of the fire hazards of EPS (expanded polystyrene -- a thermal plastic). When heated, thermal plastics melt and molten materials can run down conduits, holes and cracks in the structure to start additional fires in other areas. In other words, as it runs and spreads fire, it acts like jellied gasoline.

Remember: polyurethane (urethane) is a thermoset and expanded polystyrene (EPS) is a thermal plastic. These two rigid insulations are often compared to each other.
When we examine a piece of rigid urethane and a piece of EPS foam using a microscope, we find that each has approximately a million little cells per cubic inch, (I don't know who counted them all). These cells have a cell wall made of the respective plastics. Each of the cells are filled with a gas. Expanded polystyrene (EPS) is made by heating the thermal plastic material until it becomes liquid. Steam and air are bubbled through the hot liquid plastic, creating the froth that makes the cells. The plastic froth is then allowed to cool, capturing the gases (H20, N2, O2, etc.) used to create the froth. In other words, the cells are filled with molecules of oxygen, nitrogen, carbon dioxide, as well as steam molecules (water vapor).

To illustrate: Think of each cell of this EPS insulation as being the size of a large room and molecules within the cell as being the size of a bat or a mouse. This room is filled with all of these creatures, and they move relatively freely within the cell(room). If we apply heat to one side of the cell, the creatures against the heated side become warm; they decide to move to another area. They cross to the other side of the cell and lean up against the other wall where they deposit some of the heat they gained from the hot part of the cell. Therefore, by rapidly rotating from the hottest to the coolest part of the cell, they transfer heat. This convection within the cell allows for a substantial amount of the heat to be transferred across the cell itself. Total heat transfer is relatively slow, as the heat must travel from cell to cell.

The urethane foam cell is also created by the bubbling action of the gases that fill the cell. These gases are giant chained molecules. Technically, they are a type of hydrogenated dichloratrifloramethene. Again, consider the cell as the size of a large room, but the molecules themselves the size and shape of large dragons. Those dragons have their heads leaning against one wall and their tails leaning against the other. As the cell fills with these dragons, it becomes an unwieldy mass through which the dragons cannot easily move. If we apply heat to one side of the cell, the dragons' tails or heads become warmed, but their bodies are such that they have a very difficult time moving to the cooler side of the cell. They can twitch their heads or tails, but it takes them a long time to roll over and transfer the heat to the cell's other side. Urethane foam is much more insulating than EPS because of this sluggishness of the large chained molecules.

The cell walls of either of these materials are full of tiny little holes. When I say little, I mean they are so small, a water molecule as a liquid cannot enter; so they are literally water tight. On the other hand, water vapor molecules that are much smaller (as they are detached from each other) can and will seep through the holes, as will oxygen, nitrogen and other chemicals in our natural air.

As these molecules infiltrate in and out of the EPS, they don't change the mix very much. On the other hand, when they infiltrate into a urethane foam cell, they add another element. There is virtually no migration of the dragons out of the cell; they are too large. But as the little critters get inside, they transfer heat from one side to the other, somewhat faster than the large dragons do. Some sources claim a loss of up to 10% of thermal efficiency occurs as urethane ages. But my anecdotal evidence tells me the loss is minimal. We have had foam tested that has been exposed to the weather for a considerable length of time, and it tests virtually as well as freshly sprayed foam.

This aging and devaluing of urethane insulation is, in my opinion, a sales gimmick used by competitors of the urethane industry. The urethane foam industry was hammered badly in the 1970s; consequently, it's scared to death to make a claim it can't absolutely, definitely back up. In reality, if urethane insulation is a little less than what the book says it is, what does it matter? It is still so much better than the next best insulation that there is no comparison.

In any case, let's get back to the urethane. If we put it in a situation where we can keep other gases from migrating into the cells, so much the better. That can be done by putting a vapor barrier over it, or a vapor barrier on both sides, as is done with aluminum foil on an Isocyanate board (polyurethane board).

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