I am working on formulation of adhesives based on Polyurethane chemistry. some of my formulations get cloudy after 24 hours. What can cause this problem?
it should be mentioned that with 2,4 MDI and 4,4 MDI 50-50 mixture the prepolymer is transparent but with the carbodiimide modified MDI the prepolymer gets cloudy after 24 hours.
Did you try to heat the sample up a bit? Typically with MDI at room temperature you are close to crystallisation. You might face a SLE at that temperature. Then you get clouds. The 50-50 mixture of 2,4 and 4,4 MDI has the lowest melting point due to the eutecticum. Please also have a look at the Information about clouds and temperatures given in the attached technical data sheet of Desmodur CD-S, a typical carbodiimide based on 4,4 MDI.
Dear Klaus Konejung
Thank you so much for the kind answer. Actually I tried to heat up the sample upper 60C. But the cloudy vision did not change. The MDI is ONG3800 which I am sure that you know. Here I do not have access to corvesto products.
Can you completely exclude water residues from your system? It may cause side reactions if the system is not dry enough.
Dear Gyorgy Banhegyi
It is a possible to have cloudy prepolymer with water contamination. But the same formulation with 2,4 and 4,4 MDI 50-50 is transparent. So, if the water is problem, then we should have same problem with MDI 50.
Another possibility is that the MDI contains dimers. This you can find out either by heating to 55°C and cooling to 20°C, after 24h the liquid will turn either turbid or a precipitate form of the uretdion. You have to make sure that the MDI is free of the dimers, they will in any case lead to turbidity. Further, if the carbodiimide containing MDI is > 6 months old uretonimines may form which as well lead to turbidity. Please, use fresh MDI and store is at 0°C for decresing the dimerization. The dimers will not split on reaction below 170°C.
Dear Gerhard Behrendt
Thank you so much for your comprehensive answer.
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