Polyurethane and Polyurethane Foam

Polyurethane is a very useful polymer because it can be polymerized into so many different forms.  In general, a polymer is an assembly of relatively small organic molecules called monomers.  “Poly” means many, and “mono” means one or singular, so a polymer is “many” monomers.  There are a very wide range of means that one assembles the urethane polymers to create all sorts of different material properties.  Urethanes can be extremely stiff, rigid, and strong; light, elastic, and pliable and most anything in between.  Some are solid and some are foamed or “blown” to produce foam.  Examples of the breadth of urethane permutations are:

Other applications are:

Gaskets

In the surfing industry we are especially interested in polyurethane foams, because polyurethane foam is a light, strong, low cost foam core substrate which is tolerant to polyester resin, the preferred low cost and strong glassing resin.  Polyurethane foams can be roughly grouped into two types called “TDI” and “MDI” foams.  The bulk of the surfboard business uses “TDI” based foams; however the bulk of the rest of the commercial industry tends to prefer “MDI” foams.  The recent issues pertaining to Clark foam suggest that polyurethane and particularly TDI polyurethane may have some health and environmental concerns. 

This is a simple discussion of some of the differences between TDI and MDI based polyurethane foams. 

Surfboard Foam

The standard foam for most surfboards is“blown” rigid white polyurethane foam.  Foam with low voids and good mechanical properties for shaping are desirable.  Polyurethane foam is preferred over polystyrene foam (or expanded polystyrene, EPS) because polyurethane does not de-polymerize (or liquefy or fall apart) when exposed to polyester resins.  Polyester resins are the standard glassing resin due to low cost; relatively ease of use, and a relatively quick curing process.  White foam is especially attractive because it permits a simple low cost board to be made by simply shaping the foam core and overlaying the foam with a clear glass/polyester resin shell yielding functional and aesthetically attractive products.  White foam also provides an ideal template for over-painted colors and images.    Surfboard foams range in densities where density of the mass per volume of the material.  Nominal foam densities are on the order of 2 pound per cubic foot with some foam at higher and lower densities depending on the application.   

Other foams are also used to make surfboards like EPS and polyethylene (PE).  EPS is commonly used with epoxy resins because polyester resins dissolve polystyrene and PE foam is a material used for soft board products like boogie boards and soft surfboards. 

Polyurethane Polymerization

In general polyurethanes start as some sort of hydrocarbon chain, a polyol, and during the polymerization process the ends of the polyol chain are terminated with a diisocyanate compound.  The method and means of selecting the polyol and the method and means of the end termination provide several parameters for controlling the mechanical properties of the polyurethane.  In addition, a blowing agent can also be used to produce gas voids inside the polyurethane and hence produce foam. 

The most popular diisocyanate compounds used to produce polyurethane foam are:

• Methylene diphenyl diisocyanate or also know as MDI
• Toluene diisocyanate or also know as TDI

MDI makes a light tan/yellow foam and TDI produce a white foam.  TDI is the preferred material in making surfboard polyurethane foam. 

Some people claim that TDI is a health and environmental hazard and some chemical suppliers will only carry MDI and will not sell TDI.  TDI appears to have some features which are less attractive.  What makes TDI different than MDI? 

Health Hazards of Dissocyanates

Diisocyanates are not the friendliest of compounds.  As the name suggests they contain a “cyanate” which describes a chemical bond which is similar to cyanide.  Cyanide is well known to be unhealthy.  All diisocyanates share a fundamental health issue by causing industrial occupational asthma.  The level of exposure to the chemical to cause asthma is modest and can occur at the threshold below the olfactory limit.  Or simpler said, you can be exposed to a toxic dose of dissiocyanate which will cause asthma but you will not be able to smell the chemical.  These kinds of chemical exposures are insidious because you can be exposed yet not know until later on when one develops the physical damage (i.e. asthma).  If one works with diisocyanates, it would be imperative to ensure that the level of exposure is understood and controlled to prevent inadvertent exposure with resulting long term health consequences.

TDI and MDI both have the ability to produce this occupational asthma, in addition to other health and environmental effects.  It appears that the practical physical properties of TDI would encourage greater exposure to the diisocyanate compound and hence greater risk of personal health risk. 

A comparison of the physical properties of the two diisocyanates seems to bear this out. 

One sees that the molar mass (or molecular mass or the molecular weight) of TDI is less than MDI.  Chemical that have lower molecular weights tend to volatize (i.e. vaporize) more readily produces more vapors or fumes from the surface of an exposed liquid.  So one would guess that TDI would produce more fumes than MDI at the same temperature based on a comparison of the molecular weights (molar mass). 

The melting point, boiling point and flash point for TDI are all lower than for MDI.  This also suggests that TDI is more likely to volatize or vaporize at lower temperatures and produce more chemical fumes than MDI. 

Therefore, TDI appears to be more likely to produce chemical vapors and fumes in comparison to MDI and would likely be a more dangerous chemical to handle in regards to inhalation health hazards.