DENTURE BASE RESINS (DENTAL MATERIALS)

DENTURE BASE RESINS
The following list indicates the requirements for a clinically acceptable denture base material:
1- High strength, stiffness, hardness, toughness, and durability.
2- Good thermal conductivity.
3- Processing accuracy and dimensional stability.
4- Chemical stability (unprocessed as well as processed material).
5- Insolubility in and low sorption of oral fluids.
6- Absence of taste and odor.
7- Biocompatible.
8- Natural appearance.
9- Color stability.
10- Adhesion to plastics, metals, and porcelain.
11- Ease of fabrication and repair.
12- Moderate cost.
13- Accurate reproduction of surface detail.
14- Resistance to bacterial growth.
15- Radiopaque.
16- Easy to clean.
ACRYLIC DENTURE BASE RESINS

Types of acrylic resins:
1- Heat cured resin.
2- Cold cured resin.
3- Visible light cured resin.
4- Microwave activated resin.
ACRYLIC RESINS
(HEAT CURED ACRYLIC & COLD CURED ACRYLIC)

COMPOSITION
POWDER
1- Poly (methy1 methacrylate), (prepolymerized phase) it may be modified with small amounts of ethyl, butyl, or other alkyl methacrylates to produce a polymer somewhat more resistant to fracture by impact.
2- Initiator such as benzoyl peroxide to initiate the polymerization of the monomer liquid after being added to the powder.
3- The pigment such as cadmium sulfate is used to obtain the various tissue-like shades.
4- Titanium oxides are used as opacifiers.
5- Nylon or acrylic fibers are usually added to simulate the minute blood vessels of oral mucosa.


LIQUID
1- Methyl methacrylate monomer: it is clear, colorless, low viscosity liquid, boiling point is 100.3°C, and distinct odor exaggerated by a high vapor pressure at room temperature Care should be taken to avoid breathing the monomer vapor. Animal studies have shown that the monomer can affect respiration, cardiac function, and blood pressure.
2- Hydroquinone inhibitors are added to give the liquid adequate shelf life. The inhibitor is a chemical material added to prevent polymerization during storage and in order to provide enough working time.
3- Plasticizers are sometimes added to produce a softer, more resilient polymer. They are relatively low-molecular weight esters, such as dibutyl phthalate.
4- If a cross-linked polymer is desired, organic compounds such as Ethylene glycol dimethacrylate (EGDMA) are added to the monomer, using cross-linking agents (chemical bonds between different chains) provides greater resistance to minute surface cracking, termed crazing, and may decrease solubility and water sorption.
5- With chemical cured acrylic an accelerator is included in the liquid. These accelerators are tertiary amines (N,N-dimethyl-para-toluidine). These acrylics also called self-curing, cold-curing, or autopolymerizing resins.
POLYMER/MONOMER INTERACTION
The liquid placed in clean, dry mixing jar followed by slow addition of powder, allowing each powder particle to become wetted by monomer. After mixing the powder with liquid the mixture is left until it reaches a consistency suitable for packing. During this period, a lid should be placed on the mixing jar to prevent evaporation of monomer.

The polymer-monomer mixture, on standing, goes through several PHYSICAL STAGES, which may be qualitatively described as:
(1) Sandy stage.
The polymer gradually settles into the monomer forming a fluid, incoherent mass.
(2) Stringy or sticky stage.
The monomer attacks the polymer by penetrating into the polymer. The mass is sticky and stringy (cobweb like) when touched or pulled apart.
(3) Dough stage.
As the monomer diffuses into the polymer, it becomes smooth and dough like. It does not adhere to the wall of the jar. It consists of undissolved polymer particles suspended in a plastic matrix of monomer and dissolved polymer. The mass is plastic and homogenous and can be packed into the mold at this stage.
(4) Rubbery stage.
The monomer disappears by further penetration into the polymer and/or evaporation. The mass is rubber like, non-plastic, and cannot be molded.
(5) Stiff stage.

Processing for heat-polymerized plastics
The curing temperature must be maintained close to 74° C, because the polymerization reaction is strongly exothermic. The heat of reaction will be added to the heat used to raise the material to the polymerization temperature.

Because of the excessive temperature rise, porosity will more likely occur in thick sections of the denture. Porosity also results when insufficient pressure is maintained on the flask during processing.

FOLLOWING ARE THE RECOMMENDED CURING CYCLES:
• LONG CYCLE: heat the flask in water at 60-70 °C for 9 hours.
• SHORT CYCLE: heat the flask in water at 74 °C for 90 minutes, then boil for 1 hour for adequate polymerization of the thinner portions.

Other problems associated with rapid initial heating of the acrylic dough above 74°C is production of internal stresses, warpage of the denture after deflasking, and checking or crazing around the necks of the artificial teeth.

LIGHT CURED DENTURE PLASTICS

It consists of:
1- Urethane dimethacrylate matrix.
2- Acrylic copolymer.
3- Microfine silica filler.
4- Camphoroquinone-amine photo initiator system.

It is supplied in premixed sheets having clay like consistency. It is provided in opaque light-tight packages to avoid premature polymerization. The denture base material is adapted to the cast while it is in a plastic state. It is polymerized in a light chamber (curing unit) with blue light of 400-500 nm from high intensity quartz-halogen bulbs. The denture is rotated continuously in the chamber to provide uniform exposure to the light source.