Porosity In Denture Resins,Tissue Conditioners

Porosity In Denture Resins, Tissue Conditioners

The heating process used to control polymerization is called as curing or polymerizing cycle. This process should be well controlled to avoid effect of uncontrolled temperature rise, such as boiling of monomer and denture base porosity.’

The curing cycle is the technical name for the heating process employed to control the initial propagation of polymerization in the denture mold. For heat-cure acrylic resins, the curing cycle used is of great importance in the quality of the final denture since the polymerization reaction is strongly exothermic.

The following two methods have been suggested to obtain non-porous dentures:

1. Slow curing cycle.
2. Fast curing cycle.

1. Slow Curing Cycle:

• It involves the processing of denture base resin in a constant temperature water bath at 74°C for 8 hours or longer with no terminal boiling treatment.
• The lower the constant temperature, the longer the time is needed for curing.
• If the bulk of the acrylic is more, better to cure at low temperatures,s and longer time is required for curing.
• Slowly raise the temperature of the water bath to about 74°C in about half an hour and maintain the temperature constant for 8 hours or more.

2. Fast Curing Cycle:

• It involves heating the resin at 74°C for approximately 1½ to 2 hours and then increasing the temperature of the water bath to 100°C and processing for 1 hour or more.
• Slowly raise the temperature of the water bath to about 74°C in about half an hour and maintain the temperature constant for one and a half hours.
• During this time, more than 95% of monomer is converted to polymer.
• In order to reduce the amount of residual monomer or to ensure complete polymerization, the water in the bath can be boiled for about 1 hour.
• This technique enables a denture to be fabricated in a shorter time.

Porosity means the presence of voids or pores within a structure.
When the porosities are present on the surface of an acrylic denture base, it:

• Makes the appearance of a denture base unsightly.
• Proper cleaning of the denture is not possible, thus oral hygiene suffers.
• Weakens the denture base.

Types Of Acrylic

Porosities are classified as:

On their Presence on Denture Base Surface:

• Internal porosity:
  • Shrinkage or underpacking or contraction porosity
  • Irregular voids
  • Gaseous or boiling monomer porosity
  • Subsurface porosity
  • Air bubbles
• External porosity

Based Upon the Causes:

• Gaseous
• Granular
• Contraction
• Air inclusion

Description of Acrylic Properties

Internal Properties:

Acrylic External porosit

It can occur due to two reasons:

• Lack of homogeneity: If the dough is not homogenous at the time of polymerization the portion containing more monomer will shrink that localized shrinkage results in voids. Resin appear white.
  • Avoided by: Using proper powder/liquid ratio and mixing it well. Packing should be done in dough stage.
• Lack of adequate pressure: Lack of pressure during polymerization or inadequate amount of dough in mould during fial closure cause bubble which are not spherical.
  • Avoided by: Using adequate amount of dough. Also check for excess or flsh during trial closure. Flash is indicative of adequate material.