Howdy Nigel,
My career has been in “plastics” with most of that time spent providing engineered solutions involving fluoropolymers to the medical and aerospace industires. The past year, however, has been spent developing polyurethane foam as well as a production system for manufacturing surfblanks, so I most definitely am NOT an impartial observer.
My previous posts on this thread were strictly by way of clarifying some common misconceptions regarding polyurethane foam. We held many of these same misconceptions at the beginning of our research so I thought the thread might benefit from the clarification. The posts were not intended as propaganda and my apologies if you feel hoodwinked.
Since I am not an impartial third party I am not in a postion to give you an objective response to your question regarding the differences between TDI and MDI, but maybe it would help if I explain why we chose to pursue an MDI based polyurethane. In the early stages of our R&D program we were running both a TDI and an MDI track and had the option of going either way. After making our evaluation we dropped the TDI program and concentrated on the MDI program.
There were two main reasons we chose to go with an MDI formulation: The increased safety of our workplace and the physical properties of the resultant foam.
Looking at the environmental issue we found MDI benefited us in three main areas: Employee safety, product handling and air quality management. MDI has a much lower volatility then TDI. TDI is listed on the EPA list of Extremely Hazardous Chemicals reflecting the hightened danger associated with its use or its release into the environment. It is easier for us to provide a safe workplace by eliminating the use of TDI and opting instead for MDI. Likewise the regulatory requirements (EPA, OSHA, AQMD, Fire, etc.) associated with use of MDI based polyurethanes are less onerous then the requirements surrounding the use of TDI based foam due to issues of volatility and toxicity. On the environmental side all signs pointed towards pursuing MDI and not TDI. (As you mentioned it should be noted that although MDI poses a lower threat that does not make it a “green” material, it just makes it safer to use.)
As for the properties of the foam we felt there were more benefits to the MDI formulations then the TDI formulations. Although TDI did have better compression set, we found with MDI we were able to achieve better tensile strength, tear resistance, elongation and resiliancy which relate closely to flexibiity, a property we have been focused on in our R&D.
It should be noted that the safety issues are more pertinent to the manufacturers of the foam and suppliers of the chemicals then they are to the end users of the foam. In other words, the decision to use either TDI or MDI has a huge impact on the surfblank manufacturer’s workplace but does not have much impact on the safety of the shaper. When the MDI or TDI (the isocyanate) is added to the polyol (polyester, polyether, soy, etc.) and the other components (blowing agent, chain extender, surfactant, antioxident, etc) it all reacts to form foam. Once the foam is fully cured the MDI or TDI should no longer be present so the shaper, glasser, rider, garbageman should not be exposed to the raw chemical. However, if the manufacturer does not balance the reaction correctly (i.e there are not enough molecules for the isocyanate to react with) or does not fully cure the blank then there is some danger of exposure to the unreacted isocyanate for someone handling the foam.
Nigel, I don’t know if any of this helps but here it is anyway.
