Viscosity, bubbly messes with PMMA, learning from mistakes

From:Gayle Callis


No apologies needed, just wanted to not confuse people.  Plastics are
difficult at times, hmmm more than we can say ------ 
>As for viscosity -- my favorite PMMA mixture for undecalcified bone was as
thin as any JB-4 GMA mixtures I used.  Over time and learning PMMA methods,
the progression of PMMA recipes was with mixtures thicker than molasses (S
van de Velde, early J of Histotechnology publication on technic) - very
difficult to mix, pour, handle, with extremely slow infiltration time,
never heated to polymerize, it sat on a sunny window sill for weeks, slowly
getting harder and set up like clear glass concrete. Viscosity will depend
on the molecular weight of prepolymerized PMMA powder/beads used whether
from BDH or Aldrich.  
>Using same number of gm, a low MW prepolymerized PMMA powder results in a
thick, gooey mixture;  medium molecular weight was thinner,  but 996,000
high MW powder results in a thin, easy to handle mixture with improved
infiltration and embedding. High MW PMMA powder was wonderful (Sterchi, J
of Histotechnolgy publication/private communication with this PMMA bone
expert). Monomer was not washed to remove hydroquinone, and PMMA powder
probably helps jump start polymerization. 
>Heat IS a factor for causing bubbles i.e. too rapid PMMA polymerization.
If we went to next higher temperature or from 37C to 40 - 45C (using
waterbath to provide even heating) - bubbles happened, even with high MW
powder mixture embedding media. One lab visited used used an 45C oven to
polymerize PMMA, and every block came out as an oozing, bubbled mass, the
fumes were unbelievable! They had beakers of unusable bone blocks. Another
factor is polymerizing large volume of PMMA at a time, that is where
layering embedding is needed with large bones.  When it goes, it blows -
bubbles that is! 
>Laziness and in a hurry with final 60C curing of tacky, incompletely
polymerized PMMA on top of a block resulted in turning block top into a
frothy, bubbly mess - rather pretty, but ugly to work with, had to be
ground off unless bubbles reached down to bone, disaster.  This never
happened if RT polymerization continued to very top of block.  It was
prudent to avoid any high temperatures for polymerization steps after
ruining/recovering/chipping away PMMA bubbles from blocks.   
>Don't recall having bubbly messes with GMA, except single bubble under
metal or plastic chuck. If that occured, GMA was added to fill in gap, or
did double embedding - I think this was more from retraction of polymerized
GMA under chuck. Metal chucks minimized this by dispersing heat evenly,
plastic chucks were worse. 
>Enough --

Gayle Callis
Research Histopathology Supervisor
Veterinary Molecular Biology - Marsh Lab
Montana State University - Bozeman
S. 19th and Lincoln St
Bozeman MT 59717-3610

406 994-6367 (lab with voice mail)
406 994-4303 (FAX)


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