hello
My thesis is defined base on acrylated Polyurethan. I used HEMA (hydroxy ethylmethacrylate) as a reactive diluent and benzophenone (BP) as a UV initiator with methyl diethanolamine as an accelator. But my system is so viscous and needs heating for adding photo initiator and proper mixing. I use 45º c for adding photo initiator and mixing. but most of the time it stat to crosslinking from the bottom of the sample container. I mean before curing with UV lamp it start crosslinking.
I thought that its because of BP . Its a thermal initiator too. so I need a UV initiator can be used in higher temperature. Could you please introduce appropriate one?
You have written "my system is so viscous"!
What did you put in your reactive container besides HEMA, BP and diethanolamine ?
Form my point of view 45 deg. is a moderate temperature for a PI.
BP = benzophenone should be stable
BP = benzoyl peroxide is a thermal initiator
I suggest varying the system using DMHA or TPO-L (both are liquid and mixing a RT will be possible but they are Norrish type I one and to not need the cooinitiator). I think your problem is the inhomogeneity of the used heating procedure (more than 80deg. at the bottom) which induces thermal polymerization of the double bonds. (Alternatively use a three roller mill, a evaporable solvent, mixing plate...)
Within the link you find some initiators and most of them have a higher melting point (indicating a thermal stability). Norrish type I and II are explained one publication (ZnO). Information concerning TPO-L and DMHA can be found within the second publication (bulk).
https://www.rahn-group.com/en/rahn/download-document/dc9b4777-aac5-4365-9095-bf5515b30df4/rahn_product_guide_a4_112016.pdf
Article Synthesis and testing of ZnO nanoparticles for photo-initiat...
Article Method to analyse energy and intensity dependent photo-curin...
The polymerization that you are seeing can also be caused by the thermal self-initiation of HEMA, especially if the concentration of HEMA in the mixture is high. I had some problems before purifying HEMA when I tried to concentrate it under vacuum it polymerizes even at room temperature.
thank you all for your responses
I do consider all your comments .
I should say that I used water bath for uniform heating during mixing . also I am agree that it is likely because of self- initiation of acrylate monomers . also some of my samples start to cross linking during mixing with HEMA and photo initiator not all of them and I have had proper samples after mixing and also after UV curing.
The problem is faced in samples with higher molecular weight of acrylated polyurethane oligomers .
dear Dr Schmitt I will repeat preparing my samples with another photo initiator form type I .
thank you again
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