How to find Equivalent Weight of Triacetin

[jelanier]

I am using Triacetin as a plasticizer. It will be mixed with R45 resin and cured with an isocyanate. It is easy to calculate the amount of curative when the EW is known for the substances. I can't seem to find any such info on Triacetin. I can't find EW, hydroxyl number, hydroxyl value or %OH. Is there a way to calculate it?

Thanks,

Jim

 

[chemisttree]

If triacetin is being used as a plasticizer, it is a reactant? The chemical structure of triacetin is the triacetate ester of glycerine, BTW. Ideally it has no hydroxyl number but we both know that isn't always the case, right?

 

[jelanier]

Yes you are correct. I was thinking that it would probably react with the isocyanate that was intended for curing the R45 resin (HTPB). If it does not react, then being suspended in the tracetin minimizes the effective curative available to the htpb polymer. In other words, how much more isocyanate should I add to make up for that reaction/suspension ? I did a couple of experiments. In one I used only enough curative to react with the HTPB while using 30% triacetin (61.6% R45/8.4% isocyanate/ 30%Triacetin). In the other I used more isocyanate and "pretended" the HTPB+30%Triacetin was equal to HTPB (58% R45 30% Triacetin and 12% isocyanate). They both worked fine although experiment one was a little sticky on the surface. They are both rubbery as intended. FYI without plasticizer it would be 88% R45 and 12% isocyanate. I will probably go somewhere in between on the mix. Does that seem reasonable to you? Thanks for the reply.

 

[chemisttree]

If it does not react, then being suspended in the tracetin minimizes the effective curative available to the htpb polymer.

No. Both the polymer resin and the isocyanate can dissolve in the triacetin. Triacetin does absolutely nothing to affect the reaction stoichiometry unless it is contaminated with a hydroxyl species (water, partial hydrolysis of triacetin, etc...) so nothing at all needs to be done, IMO. It is safer to use excess alcohol, though. Don't want any excess isocyanate hanging around, y'know.

In other words, how much more isocyanate should I add to make up for that reaction/suspension ? I did a couple of experiments. In one I used only enough curative to react with the HTPB while using 30% triacetin (61.6% R45/8.4% isocyanate/ 30%Triacetin). In the other I used more isocyanate and "pretended" the HTPB+30%Triacetin was equal to HTPB (58% R45 30% Triacetin and 12% isocyanate). They both worked fine although experiment one was a little sticky on the surface. They are both rubbery as intended. FYI without plasticizer it would be 88% R45 and 12% isocyanate. I will probably go somewhere in between on the mix. Does that seem reasonable to you? Thanks for the reply.

I'm not familiar with the specifics of the R45 resin you are using but you will need to know the effective molecular weight to determine the amount of hydroxyl to use for stoichiometry. The isocyanate also decomposes on contact with moisture in the air and can be fairly contaminated, so you need to know something about the purity of that species as well. You also have other considerations to look at like syneresis, reaction rate, emulsion stability, gel point and so forth. I would look at this set of experiments in terms of reaction rate, gel point and syneresis rather than improper stoichiometry provided you use an excess of the hydroxyl species (HTPBD). You want enough pot time to completely mix the ingredients intimately but you also want a fast enough reaction rate so it is unlikely that the mixture has a chance to segregate, emulsion break and one of the reactants preferentially move to the surface. This could disrupt the stoichiometry if separation occurs.

 

[jelanier]

Thanks for the reply. I think you are saying that I should only calculate the amount of curative based on the R45 because triacetin does nothing. I understand the delicate balance on cure time. BTW, the R45 resins have EW between 555 and 1370. I use MDI curative with EW of 131. The calculation is very simple for the correct amount. This liquid mixture is being mixed with 82% solids, so a longer pot time is required. (18% binder 82% solids) It takes a while to get a good homogeneous mix. I could always add a drop of dibutyltin near the end of my mixing to accelerate the cure when I am done.

Thanks again,

Jim