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