Polymethylmethacrylate - Lucy Dennell 1/9/2003
Biochemistry
Acrylics and acrylic resins are common names for polymers of acrylic and methacrylic acid derivatives.
1920’s PMMA was one of the first plastics used industrially in its strong transparent form as ‘plexiglas’.
1,2 propenoic acid =acrylic acid, 2 methyl 1,2 propenoic acid = methacrylic acid
acid + alcohol = ester, methacrylic acid + alcohol =methylmethacrylic acid, its polymer is PMMA
The reaction is exothermic promoted by free radicals. Free radicals are generated from peroxide
Powder
PMMA spheres10-40um 90%
Peroxide initiator dibenzoyl peroxide 10-2%
Ba SO4 or Zirconium dioxide 4-8%
Heat stable antibiotic powder Gentamicin,Tobramycin, erythromycin, Vancomycin or Cephalosporin
Methylene blue or chorophyll to give colour
Liquid
MMA 85%
comonomers 10-15%
NNdimethyl-p-toluidine 1-2% (accelerator sets off the release of free radicals at room temperature)
hydroxyquinone stabiliser 50-100 ppm
Dough time 2-3 mins+ working time 5-8 mins = setting time7-11 mins
Factors which affect cement during mixing
Manual vs mechanical improves the uniformity of the cement mass
Vacuuming decreases voids
Centrifugation
evidence not strong either way
Speed of mixing/length of mixing time – too fast or too long can promote the loss of monomer
Temperature increases shorten dough and setting times by 5% per centigrade
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