PU Foaming Chemistry

2021-06-03 Author : SUNKIST CHEMICAL MACHINERY LTD.
You're a polyurethane foam technician, a plant manager, or the owner of the foaming plant itself, and you want a stronger foundational understanding of how polyurethane flexible foaming actually works. This is a smart move. Knowledge has always been power, and we here at Sunkist believe in empowering everyone at all levels of the industry through sharing our time-tested expert knowledge.

In this article, we will detail in simple language every fundamental element of polyurethane foaming, particularly as it applies to, specifically, continuous flexible foaming.


At its most basic, PU foam does two things in the factory. From the liquid stage it:
 
  • expands
  • and gels.

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?

Part I: Isocyanates and Polyols
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.

(And what are functional groups? They are basically ways to recognize huge molecules in organic chemistry. Certain combinations of certain elements react in specific and predictable ways, so when you know a molecule has an −OH group that definitely means the molecule will have a high boiling point, why bother identifying that molecule by its carbon and hydrogen and nitrogen and oxygen and sulfur etc. etc. content?)

In the world of foaming, the two most common isocyanates are TDI (for flexible & 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 (which they do–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.

Part II: Blowing the Foam
The CO2 is considered our primary blowing agent, since 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, we need to gel the PU to shape once air has been blown to it. It's harder than it sounds, and 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.

Instead of air bubbles, we're going to start calling them cells, once they're formed inside the polyurethane. If the foam expands and gellation quickly follows 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, TDI, polyol, water, polymer, open versus closed-cell: that's the baseline reaction you need to know for PU flexible foaming. The actual process of course is not this simple. Now, we’ll introduce the additives and what they do.

Part III: Additives

 
Forget the names of the ingredients for a second; let's focus on 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, increasing 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 (so, 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.
 
glass fiber filler
 
Finally, we have the additive with probably the most public exposure in the world of PU: 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 well, simply, they just add color to your foam.)

Part IV: For Example…
Let's have a concrete example, shall we? 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

Now we understand what each basic element does during the foaming process. TDI + polyol begins the creation of polyurethane. A mixing head begins by injecting 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, MC is added so that less water is used in the initial reaction and the overall reaction temperature is lower, while foam 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! A simple and straightforward breakdown of the PU flexible foaming process, laying out every basic element of the recipe. The chemist, of course, 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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Author:SUNKIST CHEMICAL MACHINERY LTD.
For over 40 years, Sunkist has steadily served the worldwide polyurethane foaming industry with our robust high-fidelity machines, all manufactured to order by us right here in Taiwan. Founder Ken Hwang established the principles of “Sunkist Quality” for our company’s products, spent decades hard at work building machines of every function precise to customer specifications, and cultivated a network of clients all over the globe. Because of this, we have become the only company to offer both high-end foaming and cutting machines, as well as products to serve all other functions in the foam plant. Our key markets have been foaming, mattresses, rubber, and nonwoven material in over 140 countries. We provide reliable, efficient, and durable machinery for our customers, and strive to continuously improve our product quality to match, if not become, the best in the world.
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