What is polyurethane foam? Consumers and manufacturers alike may want to know the answer to this question. Are you a polyurethane foam technician, a plant manager, or the owner of the foaming plant itself? Do you want a stronger foundational understanding of how polyurethane flexible foaming actually works?
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This article will detail the fundamental elements of polyurethane foaming, particularly as it applies to continuous flexible foaming.
At its most basic, polyurethane foam does two things in the factory. From the liquid stage it:
The liquid first expands as air bubbles are introduced, then a secondary reaction gels, or hardens the material at some point in that expansion.
So how does it expand and gel?
We have two foundational chemicals involved in the foaming of PU. Isocyanates, by definition, have the functional group R&#;N=C=O. Polyols, by definition, contain multiple hydroxyl, or &#;OH functional groups.
(Functional groups are basically ways to recognize huge molecules in organic chemistry. Certain combinations of certain elements react in specific and predictable ways, e.g. a molecule with an &#;OH group will have a high boiling point.)
In the world of foaming, the two most common isocyanates are TDI (for flexible and semi-rigid foaming) and MDI (for rigid foaming). Let&#;s use the shorter terms &#;TDI&#; and &#;polyol&#; for the rest of this article.
There are plenty of articles out there that break down the hard chemistry of this reaction. For our purposes, let&#;s just take note of the fact that when TDI and polyol react, and they both have multiple functional groups per molecule (the &#;DI&#; stands for diisocyanate, or two isocyanates, and polyol obviously contains multiple hydroxyl groups), they form branched or cross-linked polyurethane. This is the start of our polyurethane polymer.
A polymer is a big, or macromolecule composed of multiple subunits. If unit A links with another unit A links with another unit A, they make a linear, unbranched polymer. If unit A can link at once to two other unit A&#;s, they make branched and cross-linked structures. To compare their functional difference, just imagine climbing a rope versus climbing a ladder. The rope may be strong, but is structurally susceptible to bending and reshaping. Conversely, a ladder has multiple points of tension that allow for weight distribution. This manifests as elasticity, where the material can be twisted and stretched to certain degrees and still return to its original shape.
Alright, so TDI + polyol has made our polyurethane polymer, which now can be expanded to make foam. To do that, we add water. Isocyanate is highly reactive, and produces two things with water: urea linkages/urethane, and carbon dioxide (CO2) gas.
This is part two of our basic process: the gelling, or gellation of our material. This means the polyurethane is secured to a definite shape.
The CO2 is considered our primary blowing agent. As a gas, it blows little air pockets into the PU to form the foam. Just like blowing soap bubbles though, there is a point when the liquid casing cannot withstand the pressure of the air inside and bursts.
Here&#;s where gellation comes in, and why it&#;s so important. To successfully make material foam, the polyurethane must gel into shape once air has been blown to it. It&#;s harder than it sounds, and it&#;s also why foam technicians can make or break your factory production. The art of foam-making revolves around this sensitive balance between expansion, or blowing, and gelling.
Once they&#;re formed inside the polyurethane, we&#;re going to call the air bubbles cells. If the foam expands and gels before any air bubbles burst, you get closed-cell foams, which are semi-rigid (not truly rigid like foam made of MDI) since the material doesn&#;t bend as easily. To go back to our ladder metaphor, closed-cell foams are like wood ladders, whereas open-cell foams are like rope ladders. If some or all cell walls are allowed to rupture before gellation, you get a much more flexible material that bends, twists, and even breaks easier.
So what is polyurethane foam? Basically: TDI, polyol, water, polymer, open versus closed-cell. That&#;s the baseline, and now we&#;ll introduce the additives.
Let&#;s break down PU foaming additives by function. One of the most important additives is the catalyst, which can affect the basic reactions in several ways. It can speed the expansion, speed the gelling, cool the reaction (so you have less of a fire hazard on your hands), etc. There are also curing agents, which include chain-extenders and cross-linking agents. Chain-extenders, like their name suggests, extend polymer chains, which increases material flexibility. Cross-linking agents promote and strengthen cross-linkages, increasing structural integrity for more rigid foams.
Think of surfactants like emulsifiers. Oil and water on their own do not mix, but once you add some dish soap, they can be emulsified into a uniform mixture. Surfactants work like the soap. A more uniform mixture means a smoother reaction, and you get more even cell sizes, steadier reaction speeds, and finer control between gellation and foam collapse.
(The reason they&#;re called surfactants is because they reduce the surface, or interface tensions between two compounds. As in, the oil doesn&#;t just sit neatly on top of the water&#;surfactants blend that interfacing surface between them.)
Remember that CO2 gas from the reaction with water acts as a blowing agent? Well, other blowing agents may also be used or added. The main inconvenience of water blowing is the high temperature of the reaction, making PU foaming a fire hazard. Physical blowing agents (additives that physically encourage the expansion of cells instead of that initial CO2, which is chemically blown) reduce that fire hazard.
A similar class of additives is fillers. They come as particles or fibers. Particulate fillers can reduce flammability and add weight to foam (good for cushioning foams). Fibrous fillers reinforce cell structure. All fillers function to 1) add physical properties like tensile or compressive strength to foam, and 2) save on costs by reducing the amount of liquid chemicals used per batch.
Finally, we have the additive that most people know about: flame retardants. First, to combat flammable foam products, countries added flame retardant requirements to PU production. However, several wide-used flame retardants have proven to have negative health effects on consumers, so countries then changed flame retardant regulations. As it stands, different countries have different sets of regulations on additive types, whether PU products have to pass an open-flame or a char test, etc. Different regions also have different degrees of access to types of flame retardants. We have an upcoming article that will go further into detail about this very debate, but for now, suffices to say that this is an element of foam quality that can greatly impact your consumer market.
(Final aside: a class of additives we won&#;t get into detail about here is colourants, because simply, they just add color to your foam.)
Let&#;s have a concrete example. Here at Sunkist, this is our prototypical flexible foaming recipe:
Isocyanate: TDI
Polyol: polyol
Blowing Agents: water, methylene-chloride (MC)
Catalysts: amine, tin
Surfactant: silicone
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Now we understand what each element does during the foaming process. TDI + polyol begins the creation of polyurethane. A mixing head first injects small amounts of air into the liquid mix to kickstart the foaming process. TDI + water chemically produces the CO2 gas that blows the liquid into foam. In addition, we add MC so that less water is used in the initial reaction and the overall reaction temperature is lower, all while cell expansion is retained.
Meanwhile, the amine additive is doing multi-purpose catalysis (speeding the reaction) and tin provides a stable gelling catalyst, increasing foam structural elasticity. Silicone smooths and steadies the entire blowing process, maintaining cell structure evenness until gellation occurs.
And there we have it! Sunkist&#;s answer to the question &#;what is polyurethane foam,&#; laying out every basic element of the recipe. The chemist remains the true expert on ingredient quantities. However, if every foam technician, machine operator, and even factory owner has a foundational understanding of what&#;s actually going on in a foaming machine? Your plant will have built-in, well-informed quality-control in every step of the production process.
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by Nick Gromicko, CMI® and Ben Gromicko
Polyurethane spray foam is a versatile insulation material that is sprayed into building cavities where it quickly expands and molds itself to its surroundings. It is available in "closed-cell" and "open-cell" varieties, each of which offers advantages and disadvantages, depending on the requirements of its application. The following guide briefly explains the differences between these insulation options.
Closed-Cell Polyurethane Foam
Closed-cell polyurethane foam (CCPF) is composed of tiny cells with solid, unbroken cell walls that resemble inflated balloons piled tightly together. The cells are inflated with a special gas selected to make the insulation value of the foam as high as possible. Like the inflated tires that hold up an automobile, the gas-filled bubbles, when dried, create a material that is strong enough to walk on without major distortion. Wall-racking strength can be enhanced when CCPF is applied, and its strength makes it preferable for roofing applications. The high thermal resistance of the gas gives CCPF an R-value of approximately R-7 to R-8 per inch, according to the U.S. Department of Energy (DOE), which is significantly better than its open-cell alternative. It also acts as a vapor barrier, making it the product of choice if the insulation is likely to be exposed to high levels of moisture. Its density is generally 2 lb/ft3 (32 kilograms per cubic meter [kg/m3]).
Over time, the R-value of CCPF can drop as some of the low-conductivity gas escapes and is replaced with ordinary air, a process known as thermal drift. Research performed by the DOE revealed that most thermal drift occurs within the first two years after the insulation material is applied, but then the foam remains relatively unchanged unless it is damaged.
Foam Is a Fire Hazard
Semi-permeable rigid foam insulation and spray foam insulation (foam plastic) on the inside of basement foundation walls is often found during an inspection of the full-basement foundation of a house. Its use could be a good strategy for a moisture-resistant finished basement. However, fire and smoke characteristics of this type of insulation require that it be covered with a fire-resistant layer, such as gypsum wallboard (drywall).
Sometimes this requirement works fine when the basement is being finished. This requirement of having spray foam insulation to be protected by a thermal barrier is found in the International Residential Code (
IRC
) Section R316. In most cases where spray polyurethane foam insulation is installed, the foam should be separated from the interior living spaces by an approved thermal barrier of at least 1/2-inch gypsum wallboard (drywall), 23/32-inch wood structural panel, or a material tested to meet the acceptable criteria from NFPA. There are a few exceptions to this requirement, including flame spread index ratings.
If a basement will only be insulated and not finished, a fire-rated foam panel or similar fire-rated covering needs to be used. Because the above-grade portions of the basement wall can dry to the outside, fire-rated insulation on these surfaces may be of an impermeable type. For example, it can have a foil facing. But insulating approaches that restrict the drying potential of below-grade portions of the foundation wall toward the inside should be avoided.
In attics, a thermal barrier is not required when several conditions exist. Those conditions are listed within the IRC Code Section R316, and they include the attic access is required, the attic space is entered for only maintenance and when repairs are needed, and the foam insulation has been tested or the foam insulation is protected against ignition using a listed barrier material.
Packages and containers of spray foam insulation (or foam plastic) should be labeled and identified if they are delivered to a building site.
Open-Cell Polyurethane Foam
Open-cell polyurethane foam (OCPF) is a soft, flexible, spongy insulation with broken cell walls that permit air to fill them. They typically have a density of 0.5 lb/ft3 (8 kilograms per cubic meter [kg/m3]), which is significantly less than closed-cell insulation, as well as having a reduced R-value per inch, although OCPF still has excellent thermal-insulating and air-barrier properties. The foam is weaker and less rigid than closed-cell foams, too. It will require trimming and disposal of excess material as it expands to over 100 times its initial liquid size.
Builders often choose open-cell foam for the following advantages it affords, including:
In summary, polyurethane foam is available in two varieties that are suited for different applications.
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