In normal procedure of MF preparation, the complete procedure needs ~ 3.5-4 h. I want to know why recently the reaction times for both first and second step decreased to 0.5 h (totally 1 h)? we control the pH and temperature of solution in all steps. As I know at lower pH the reaction could be faster, but we try to keep it at basic condition during the reaction. During the reaction some times the pH increased to ~10, however sometimes it drops to ~8 and we try to maintain the pH by addition of Na2CO3 or NaOH.
Procedure of MF preparation: A predetermined pH value of the initial formaldehyde was adjusted by the addition of solution NaOH or Na2CO3 (pH~9), at room temperature (~30 °C). Melamine powder were then added to the reaction vessel and heated gradually to 95-99 °C, under continuous mixing, and the mixture was kept at this condition to form a clear and transparent liquid. During this step the cloud points test was conducted to find the end point for the first step of reaction, and usually this step was completed at 1-1.5 h. After this point, the reaction temperature reduced to 85 °C and the polymerization reaction was conducted for around 2 hours under constant agitation to obtain higher degree of conversion. The end point of the reaction was checked regularly by water tolerance and cloud point methods.
The pH of the reaction is adjusted as needed to maintain the pH at a desired level (~ 9.2 - 9.3).
The miscibility is strongly affected by salt content so time to reach your end point may be affected by formic acid or methanol content in your formaldehyde, tap water vs distilled water or type of base you use to adjust pH. Maybe usin a little amine (TEA) to buffer pH instead of carbonate would help. You can also check if your melamine source has changed since contaminants are mostly less soluble compounds. Also, adding melamine should be done in best conditions for dilution. Heat your system to 65-70C before adding melamine and bring it fast to reflux after checking pH.
Dear Dr. Pierre,
thanks for your answer. Actually we have used NaOH to control pH, but later we changed it to Na2CO3. then it works well for 5-6 batches, but later again the reaction time decreased and the produced MF became gel. I will check the effect of TEA, and preheating of formalin to see the result. Is it possible to have this problem because of the presence of some small amount o Urea in the formalin even in ppm level?? however the formalin is produced by the company also and we did not change any thing in the formalin production, but if it was due to the presence of Urea, it must be same problem in all batched! Do you have any other suggestion?
Regards,
Dear Dr, Which one exactly you mean by TEA? Triethanolamine or Triethylamine? If I'm not mistaken you mean Triethanolamine which is strong base.
Sorry for the acronym, yes I referred to Triethanolamine. Its just a suggestion to help keeping pH in line with your target, nothing more (NaOH is cheaper and its perfectly feasible to make a MF resin relying solely on it). Measuring pH is a must, even if upon cooling your sample, it turns into a slurry or a gel. I cannot give you advices like how to cook since I'm working for the industry. But I can give you common knowledge like answering you question about urea, which has nothing to do with your problem, specially at ppm level. From the information you provided, you have more chances to solve your problem by carefully looking at what is going on (with pH and temperature) a little before you start adding melamine until the resin turns clear. You don't need to go to reflux dissolve melamine (not safe an not required). But you need good initial conditions before adding melamine. Melamine, unlike urea, has a very low water miscibility. It is methylolation that makes this molecule more soluble. You want to get good initial methylolation rate to solubilize the melamine as fast as possible before condensation starts being the leading reaction. With the usual tools, you don't learn much from step growth reactions before large molecules starts being formed, which happens when the reaction is about 90% completed. All you can control is in which condition molecules are formed (pH, Temperature, Concentration of chemicals). Hope it helps!
Regards
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