Powder to liquid ratio is important for workability and dimensional stability. MMA monomer undergoes 21% shrinkage by volume on conversion into polymer. For workable mix and low shrinkage, powder / liquid ratio of 2.5:1 by weight needs to be used in order to ensure that the required characteristics are produced for the denture base.
When the monomer is polymerized to form the polymer the density of the material changes that causes volumetric shrinkage (21% as JP mentioned above. Linear shrinkage can also effect the dimensional stability of a denture. Water absorption could be another factor as polymethyl methacrylate absorbs small amounts of water in aqueous environments which inevitably causes slight expansion of the acrylic - estimated that 1% increase in weight from absorption can cause 0.23% linear expansion. (Phillips, 2008: 742).
Distortion of the denture can occur if the flask is cooled and unpacked too quickly. The denture flask should be allowed to cool at room temperature (bench cooling) for more than 30 min, the cooling process can then be accelerate by submerging the clamp under water. The pressure of the clamp should be relief after the denture flask is jugged cool enough. (please correct if wrong...)
1. Co-efficient of thermal expansion with regards to the gypsum investment 2. Pressure exerted during processing 3. The mixing ratio of the polymer and monomer 4. Water absorption 5. Polymerisation temperature 6. Polymerisation Technique 7. the materials used
Water absorption could be another factor as polymethyl methacrylate absorbs small amounts of water in environments which causes slight expansion of the acrylic and the water powder ratio is important for the stabilty of the denture base materials, also see that the polymere and monomere mix is the correct and measured amounts to get good mix with out persoity
factors: expansion and contraction of the mould. thermal expansion of the dough polymerization shrinkage of the resin cooling down contraction may not uniform due to uneven thickness of the denture shape and size of the denture the pressure applied during curing the curing cycle used the rate of cooling after the curing process
internal stresses: 1 the different coefficients of thermal expansion of the various materials(methyl methacrylate, plaster of paris, etc) produces stresses in the denture. 2 thermal contraction 3 internal stress lessens the resistance to fracture and will cause warpage if the temperature approaches the softening point of the material. 4 stress are greater when high compressive forces are used during packing.
granularity: the loss of monomer via: 1 evaporation during mixing 2 evaporation during the trial closure 3 loss of monomer to the gypsum because of the lack of seal
crazing: 1 small cracks can appear on the surface after a period of time 2 often seem to radiate outward fro the necks of the teeth and metal inserts 3 the outer layer is stretched setting up tensile stress on the surface.
Powder to liquid ratio is important for workability and dimensional stability.
ReplyDeleteMMA monomer undergoes 21% shrinkage by volume on conversion into polymer.
For workable mix and low shrinkage, powder / liquid ratio of 2.5:1 by weight needs to be used in order to ensure that the required characteristics are produced for the denture base.
When the monomer is polymerized to form the polymer the density of the material changes that causes volumetric shrinkage (21% as JP mentioned above. Linear shrinkage can also effect the dimensional stability of a denture.
ReplyDeleteWater absorption could be another factor as polymethyl methacrylate absorbs small amounts of water in aqueous environments which inevitably causes slight expansion of the acrylic - estimated that 1% increase in weight from absorption can cause 0.23% linear expansion. (Phillips, 2008: 742).
Distortion of the denture can occur if the flask is cooled and unpacked too quickly. The denture flask should be allowed to cool at room temperature (bench cooling) for more than 30 min, the cooling process can then be accelerate by submerging the clamp under water. The pressure of the clamp should be relief after the denture flask is jugged cool enough. (please correct if wrong...)
ReplyDelete1. Co-efficient of thermal expansion with regards to the gypsum investment
ReplyDelete2. Pressure exerted during processing
3. The mixing ratio of the polymer and monomer
4. Water absorption
5. Polymerisation temperature
6. Polymerisation Technique
7. the materials used
Water absorption could be another factor as polymethyl methacrylate absorbs small amounts of water in environments which causes slight expansion of the acrylic and the water powder ratio is important for the stabilty of the denture base materials, also see that the polymere and monomere mix is the correct and measured amounts to get good mix with out persoity
ReplyDeletefactors:
ReplyDeleteexpansion and contraction of the mould.
thermal expansion of the dough
polymerization shrinkage of the resin
cooling down contraction may not uniform due to uneven thickness of the denture
shape and size of the denture
the pressure applied during curing
the curing cycle used
the rate of cooling after the curing process
internal stresses:
1 the different coefficients of thermal expansion of the various materials(methyl methacrylate, plaster of paris, etc) produces stresses in the denture.
2 thermal contraction
3 internal stress lessens the resistance to fracture and will cause warpage if the temperature approaches the softening point of the material.
4 stress are greater when high compressive forces are used during packing.
granularity:
the loss of monomer via:
1 evaporation during mixing
2 evaporation during the trial closure
3 loss of monomer to the gypsum because of the lack of seal
crazing:
1 small cracks can appear on the surface after a period of time
2 often seem to radiate outward fro the necks of the teeth and metal inserts
3 the outer layer is stretched setting up tensile stress on the surface.