PROPERTIES OF ACRYLIC DENTURE RESINS (DENTAL MATERIALS)

properties of acrylic denture resins
1- taste and odor
completely polymerized acrylic resin is tasteless and odorless. denture with porosity can absorb food and bacteria, resulting in an unpleasant odor and taste.
2- esthetic
the esthetic of acrylic is acceptable, because it is a clear transparent resin which can be easily pigmented and it is compatible with dyed synthetic fibers.
3- density
the polymer has a density of 1.19 gm/cm3.
4- strength
they have adequate compressive and tensile strength for complete or partial denture applications. ideally denture base resins should have high impact strength to prevent breakage when it is accidentally dropingped. cold cured acrylic has lower impact strength, but addition of plasticizers increase the impact strength.
the strength is affected by:
a- composition of the resin.
b- technique of the processing.
c- degree of polymerization.
d- water sorption.
e- subsequent environment of the denture.

5- hardness
acrylic resins have low hardness they can be easily scratched and abraded.
heat cured acrylic resin: 18-20 khn.
cold cured acrylic resin: 16-18 khn.
6- modulus of elasticity
they have sufficient stiffness (2400 mpa) for use in complete dentures. however, when compared with metal denture bases they are low. self-cured acrylic has slightly lower values.
7- dimensional stability
a well processed acrylic denture has good dimensional stability.
8- shrinkage
acrylic resins shrink during processing due to:
a- thermal shrinkage on cooling.
b- polymerization shrinkage.

9- polymerization shrinkage
however, in spite of the high shrinkage, the fit of the denture is not affected because the shrinkage is uniformly distributed over all surfaces of the denture the processing shrinkage is balanced by the expansion due to water sorption.
• volume shrinkage: 8 %.
• linear shrinkage: 0.53 %.
self-cured type has a lower shrinkage (linear shrinkage: 0.26 %).
10- water sorption
acrylic resin absorbs water and expands. this partially compensates for its processing shrinkage. this process is reversible. thus, on drying they lose water and shrinkage. however, repeated wetting and drying should be avoided as it may cause warpage of the denture.
11- solubility
acrylic is virtually insoluble in water and oral fluids. they are soluble in ketones, esters, and aromatic and chlorinated hydrocarbons. alcohol causes crazing in some resins.


12- thermal properties
stability to heat: poly methyl methacrylate is chemically stable to heat up to a point. it softens at 125 °c.
thermal conductivity: they are poor conductors of heat and electricity.
coefficient of thermal expansion: acrylics have a high coefficient of thermal expansion.
13- color stability
heat cured acrylics have good color stability. cold cured has lower color stability, due to oxidation of amine accelerator.

14- biocompatibility
completely polymerized acrylic resins are biocompatible. true allergic reaction to acrylic resins is rarely seen in the oral cavity. direct contact of the monomer over a period of time may provoke dermatitis.
15- residual monomer
the highest residual monomer level is observed with cold cured acrylic.
16- adhesion
the adhesion of acrylic to metal and porcelain is poor, and mechanical retention is required. adhesion to plastic denture teeth is good (chemical adhesion).
17- shelf life
acrylic resins dispensed as powder/liquid have the best shelf life. the gel type has a lower shelf life and has to be stored in a refrigerator.