Lets face it folks: the urethane grouts are classified as a chemical grout. Anytime you find a product who's "chemical" name can not be pronounced without taking a deep breath and stutter, you need to exercise caution. You need to read the MSDS first and explain your workers the risks involved. It is not because a chemical grout is classified as "certified for potable water" (that is another story), that the product is safe to use and you think that you can start using it to brew your morning coffee. For those who are getting ideas, please read the disclaimer notice NOW! So what is the deal with TDI and MDI: TDI is Toluene Di Isocyanate (broke it up on purpose). This beauty has two isomers, you have the 2,4 TDI and the 2,6 TDI. The commercial version is a mixture of both isomers. It would be out of scope to go into detail on the different mixture ratios. Anytime I mention TDI, I make reference to the mixture of these two isomers. What are Isomers?? I am not about to explain that, you should have paid more attention in your chemistry class. For now just imagine them as 2 identical TDI twins, one with a birthmark on the right hand and the other on the left. I think you got the point. Pure TDI is classified as poison. Without the proper PPE (Personal Protective Equipment), chances are that you will get more then a headache 15 years from now. What about the MDI you might ask. First of, MDI stands for: 4,4' Diphenylmethane Di Isocyanate. I bet you had some trouble reading that one, right? The same as with TDI, many (not all of them) MDIs are mixtures of MDI variations (higher oligomers of MDI) etc.. Anytime I mention MDI, I make reference to the mixture. MDI is classified as harmful. Although less volatile then TDI, without the proper PPE, still as potent as TDI. The next statement and remarks only apply to the PYCOSA urethane grouts, because I do not know the other manufacturers their production procedures. At PYCOSA we produce prepolymer urethanes. What that means is that we take our different ingredients mix them in certain ratios and heat them. The result is, that a chemical reaction occurs that alters the chemical formula of the ingredients. We are no longer talking about a polyol or TDI only, but about a new product with new properties. Does it have a new chemical name? Sure, and because it is a prepolymer, the structure of it can change from batch to batch. Lets make an analogy: Take a raw egg, flour and baking soda, mix them together and apply heat. The result is a new product with new properties. The ingredients by itself will all taste nasty, but the final product could be a pretty good cake (of course depending who bakes it). Somewhere at the beginning of this article I wrote that TDI is a mixture of 2,4 and 2,6 TDI. Remember the "DI" in the name? Well "DI" means two, so TDI has 2 isocyanate groups in its structure. The isocyanates are the reactive parts of this beauty. The numbers 2,4 and 2,6 are positions on the chemical structure of TDI. The reactive isocyanate group located on position number 4 is more reactive than the number 2 position and so is also the number 6. I am not about to explain how that happened, just believe me this time around! Knowing all this, you can put one and one together, right? No?? Let me help you on the way; with the proper formulation, we can have all TDI molecules react with an OH-group. Remember the number 4?? All the OH-groups want to cling on to number 4. Once all the 4s are gone, the OH-groups will chase the number 6 and the 2s are chosen for last. Stick with me, because now it will get a little complicated. Because the 4 and 6 have been reacted with the OH-groups, the TDI molecule is now part of the prepolymer structure. We are not talking about single raw TDI anymore. Does this mean there is no free TDI in the prepolymer grout? You mean "no free" like zip, nada, nothing? No, the prepolymer probably contains some traces of free TDI. Live is not always perfect. The keywords here are: careful formulation. A chemist can calculate how many isocyanates will be snatched up by the OH-groups. So where does the hazardous part come in play? Well if you would add more TDI to the prepolymer after the prepolymer reaction is finished, you would end up with free TDI in the grout. Or if you would have not enough OH-groups to snatch up all 4s and 6s. That portion of the TDI that still has 4s and 6s left, is free TDI. Okay, are you still awake? Let's wrap this pup up. Are the PYCOSA urethanes based on TDI safe to use? Absolutely!! We do not add extra TDI and those festy OH-groups snatch up all 4s and 6s. What about the MDI you might ask? Same thing, different animal of course, but the MDI is also incorporated into the chemical structure of the prepolymer. Because nobody has any beef with MDI, I am not going to expand and waste more real estate space on this web page. Remember if you have any questions or comments, just e-mail them to faq@pycosa.com