Some problems in polyurethane synthesis……
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First, although the water has been removed, there will be bubbles when the polymerization temperature is above 50 degrees. There are bubbles almost in the entire synthesis process. Even if it is water, it cannot be that much, right? Toluene was used as the solvent, and the temperature was still controlled at around 80°C. Second, liquefied MDI is put into the flask using toluene as the solvent, but it turns out to be turbid. Is liquefied MDI insoluble in toluene? Third, the PEG used is inherently solid. After synthesis, I thought it could destroy the crystallization and turn it into a liquid, but no, toluene and the product were separated after cooling... Fourth, in addition to using toluene as the solvent, can other polar solvents such as ethyl acetate be used as the solvent? There are many questions, so thank you in advance for your answers! This post was last edited by tingyi on 2008-11-22 20:55 ]
The water removal is not complete, the molecular weight of PEG is too large, and propylene glycol can also be used as a solvent.
PEG was dehydrated in a vacuum at over 100°C for two hours, and the solvent was also treated with molecular sieves. I wonder if so many bubbles would have a great impact on my experiments... I originally wanted to use Bingtong, but Bingtong gave up using Bingtong because the temperature was too low and the reaction time was too long... I don't know if it is okay to use ethyl acetate? (Actually, I don’t want to use so much solvent, and I’m afraid it will be too sticky, haha...) This post was last edited by tingyi on 2008-11-23 10:14 ]
You can consider the following aspects: 1. Is the dehydration time enough? 2. The ratio of MDI and PEG. The ratio is close to 1, the molecular weight is large, and it is easy to bubble. 3. The reaction speed, you can keep it at 50 degrees for a period of time, and then heat up the reaction.
The dehydration time may be a little short, and the dehydration time will be increased next time... The ratio is very large, so to rule out the problem of high degree of polymerization, my PEG is 6000. I don't know if the reaction is incomplete and MDI is not completely connected, causing the reaction to end or crystallization. The reaction speed control is indeed insulated at around 50°C. My purpose is to prevent the reaction from going too fast. I don’t know why littlepaint said it is insulated? Thank you!
What is the vacuum level during dehydration? The vacuum degree is too low and it will not work.
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Personal opinions for reference 1. Bubble problem: First of all, it is necessary to judge whether physical bubbles or chemical bubbles. Physical bubbles are mainly air mixed in during the feeding and stirring process, which can be removed by vacuuming. Chemical bubbles are carbon dioxide generated by the reaction between moisture and isocyanate. Moisture in any raw materials will cause bubbles, especially when you use For highly reactive liquefied MDI, in addition to the raw materials, it is best to fill the reaction process with dry nitrogen. 2. Crystallization issues and solvents. The product you get when you use PEG6000 to react with MDI is solid. If you want to turn it into a liquid, you need to use a large amount of solvents to dissolve it. Good solvents for polyurethane include ethyl acetate, butyl acetate, propylene glycol, and butyric acid.* * , trichlorethylene, dimethylformamide, etc., which can be selected according to the boiling point and your actual needs.
The vacuum degree should still be enough, it is -0.1 MPa. I spent three hours removing water today, and it should be fine... (But I made a gel today...depressed...). The reason for the gel may be that I lowered the ratio of NCO and OH. I will calculate the NCO content later, haha... What you are talking about is the question I asked in my last post about whether toluene and PEG can be used to remove water together (that is, toluene is still left in the system as a solvent in the end). What is very confusing is what is the principle of this? Can toluene bring out the water in PEG? So if the remaining toluene remains in the system, there will be no water? Thank you very much for your answer. Next time I can try vacuuming (I don’t know how to control the vacuum degree? Haha...) In addition, it seems that it is impossible to dissolve it. Moreover, it is useless for me to bring it up again after it has been dissolved. The main thing is the water solubility. I will try it tomorrow... Thank you again for your answers!
Dizzy... It turns out that the NCO content is too low. I didn't know... Sigh... I'm still a boss... Question: Can the solvent in the test be added while doing the test? Is there any impact? Are there any tips for adding solvent? What I did today was to add more as I was making it. No matter how much you add when gelling, it’s useless. I’m climbing the pole... Haha... This post was last edited by tingyi on 2008-11-24 20:24 ]