Impression Material

Manipulation

1. Selection-based on strength for models, casts, or dies

2. Mixing
(1)Proportion the water and powder 
(2) Sift powder into water in rubber mixing bowl
(3) Use stiff blade spatula to mix mass on side of bowl
(4) Complete mixing in 60 seconds

3. Placement

(1) Use vibration to remove air bubbles acquired through mixing
(2) Use vibration during placement to help mixture wet and flow into the impression

POLYCARBOXYLATE CEMENT 

Use:. The primary use of polycarboxylate cement is as a cementing medium of cast alloy and porcelain restorations. In addition, it can be used as a cavity liner, as a base under metallic restorations, or as a temporary restorative material. 

Clinical Uses

Polycarboxylate cement is used in the same way as zinc phosphate cement, both as an intermediate base and as a cementing medium. 

c. Chemical Composition. 

(1) Powder:. It generally contains zinc oxide, 1 to 5 percent magnesium oxide, and 10 to 40 percent aluminum oxide or other reinforcing fillers. A small percentage of fluoride may be included. 
(2) Liquid. Polycarboxylate cement liquid is approximately a 40 percent aqueous solution of polyacrylic acid copolymer with other organic acids such as itaconic acid. Due to its high molecular weight, the solution is rather thick (viscous). 

d. Properties. 

The properties of polycarboxylate cement are identical to those of zinc phosphate cement with one exception. Polycarboxylate cement has lower compressive strength. 

e. Setting Reactions: 

The setting reaction of polycarboxylate cement produces little heat. This has made it a material of choice. Manipulation is simpler, and trauma due to thermal shock to the pulp is reduced. The rate of setting is affected by the powder-liquid ratio, the reactivity of the zinc oxide, the particle size, the presence of additives, and the molecular weight and concentration of the polyacrylic acid. The strength can be increased by additives such as alumina and fluoride. The zinc oxide reacts with the polyacrylic acid forming a cross-linked structure of zinc polyacrylate. The set cement consists of residual zinc oxide bonded together by a gel-like matrix. 

Precautions. 
The following precautions should be observed. 
o    The interior of restorations and tooth surfaces must be free of saliva. 
o    The mix should be used while it is still glossy, before the onset of cobwebbing. 
o    The powder and liquid should be stored in stoppered containers under cool conditions. Loss of moisture from the liquid will lead to thickening. 
 

DISTORTION OF THE PATTERN

Distortion is dependant on temperature & time interval before investing .
To avoid any distortion ,
Invest the pattern as soon as possible .
Proper handling of the pattern .

PREREQUISITES
Wax pattern should be evaluated for smoothness , finish & contour .
Pattern is inspected under magnification & residual flash is removed .

PHYSICAL PROPERTIES OF MATERIALS

Definite and precise terms are used to describe the physical properties of dental materials.

a. Hardness. Hardness is the measure of the resistance of a metal to indentation or scratching. It is an indication of the strength and wearability of an alloy or metal.

b. Ductility. Ductility is the measure of the capacity of a metal to be stretched or drawn by a pulling or tensile force without fracturing. This property permits a metal to be drawn into a thin wire.

c. Malleability. Malleability is the measure of the capacity of a metal to be extended in all directions by a compressive force, such as rolling or hammering. This property permits a metal to be shaped into a thin sheet or plate.

d. Flexibility and Elasticity. These terms differ in their technical definition but they are very closely related. Flexibility is the characteristic of a metal, which allows it to deform temporarily. The elasticity of a metal is used when it returns to its original shape when the load or force is removed.

e. Fatigue. Fatigue is the property of a metal to tire and to fracture after repeated stressing at loads below its proportional limit.

f. Structure (Crystalline or Grain Structure). Metals are crystalline and many of their physical properties depend largely upon the size and arrangement of their minute crystals called grains.

(1) Grain size. The size of the grains in a solidified metal depends upon the number of nuclei of crystallization present and the rate of crystal growth. In the practical sense, the faster a molten is cooled to solidification, the greater will be the number of nuclei and the smaller will be the grain size. Generally speaking, small grains arranged in an orderly fashion give the most desirable properties.

(2) Grain shape. The shape of the grains is also formed at the time of crystallization. If the metal is poured or forced into a mold before cooling, the grains will be in a flattened state. Metal formed by this method is known as cast metal. If the metal is shaped by rolling, bending, or twisting, the grains are elongated and the metal becomes a wrought wire.

g. Crushing Strength. Crushing strength is the amount of resistance of a material to fracture under compression.

h. Thermal Conductivity. Thermal conductivity is defined as the ability of a material to transmit heat or cold. A low thermal conductivity is desired in restorative materials used on the tooth whereas a high thermal conductivity is desirable where the material covers soft tissue.