dear all,
i'm trying to reticulate a copolymer HEMA/MMA filled with hydroxyapatite by thermal polymerization, but i obtain samples with air bubbles, even after vacuum treatment (before the reticulation)
does someone have any idea ?
thank you.
Did you try to use ultrasonication before applying vacuum? What is the approximate viscosity of the monomer mixture you are using? Did you try to use evacuation before adding the monomer mixture to the filler?
I can give you the following answer:
please use a AIBN as initiator at around 45° C , overnight
this should work very well .
I agree with Helmut Ritter.
I think, temperatures higher than 50°C can make some bubbles. So you have to work at lower temperature with a suitable initiator (you can use redox initators that work even at 10°C)
Dear Gyorgy
the viscosity is very high (i can't give you a value but it's closer to honey than water) because i also add powders of PMMA,
sorry but i did not understand what do you mean with evacuation.
thank you
dear Helmut
i'm using benzoyl peroxide as radical initiator, what difference does AIBN makes ?
thank you
Dear Farzad
what type of redox initiator do you suggest, and how it would take for the polymerization ?
thank you
You may try to use vacuum on the filler and then to let the momomer mixture in.
If you combine benzoyl peroxide with so-called accelerators (usually aromatic teritary amines) the reaction will proceed at lower temperature.
you can use redox systems containe oxidants (persulfates or hydroperoxides) and reducing agents [ascorbic acid, formaldehyde sulfoxilate (SFS), tetramethyl ethylene diamine (TMEDA), Bruggolit 6 and 7 (FF6 and FF7), and sodium metabisulfites].
More detail is availabe in the following article:
Redox initiator systems for emulsion polymerization of acrylates
http://onlinelibrary.wiley.com/doi/10.1002/pola.23362/abstract
DBP decomposes at rel. high temperatures please use an azo initiator like AIBN. also a radical photo initiator at room temperature may work nicely: good luck
could you describe your process for the samples? fillers Mixing, reticulation of the copolymer, time and temperature vacuum etc. You are doing what mechanical test?It is normalized?
thanks
With my experience, decomposition of AIBN creates nitrogen bubbles and get trapped in the polymer. Therefore, you can consider a initiator which does not creates a gas or use milder condition for polymerization. If you have to use AIBN then, initiate the reaction at 60C and the decrease the temperature to 40C and leave it for a long enough time for reaction to complete.
I did similar composite specimens for mechanical test in such way: monomer was mixed with co-initiator (N,N-Dimethyl-p-toluidine), DBP was added to the filler mixture (polymer powder, mineral filler, etc.). After monomer liquid was mixed with filler, the viscous mass was placed for 30 sek in vacuum to evacuate bubbles, than was cured in the mold at room temperature. It cures in about 3-5 min, but you have only 30-40 sek to evacuate bubbles. Good luck!
My experience indicates that dibenzoyl peroxide (DPO) can solve the problem. It decomposes at higher temperatre than AIBN but does not produce nitrogen bubles.
You need to be careful how much vacuum you apply as if it is too high, it will pull monomer out of solution which will form bubbles. As previously suggested, ultrasonication would be appropriate, preferably a probe.
We have good experience with a stepwise addition of AIBN. We add 2/3 of the total amount of AIBN, stirr at 90°C for 20 minutes, cool it down to 35°C and add the remaining 1/3 of the AIBN under constant stirring. If AIBN is solved completly, the reaction mixture is cured at 60°C for 4 hours. Additionally, you can add a defoamer.
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