dental02.jpg (11342 bytes)Fundamentals of
Dental Materials

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Restorative Materials

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a. Equipment. The equipment required for mixing zinc phosphate cement consists of a glass mixing slab, a stainless steel spatula, and a matched set of powder and liquid. See figure 1-4.

Figure 1-4. Setup for mixing zinc phosphate cement.

b. Powder and Liquid. The first step in preparing a mix of zinc phosphate cement is to measure the desired amount of liquid and powder onto the surface of a clean, cool, dry, glass-mixing slab. The amount of each ingredient depends upon the amount and the consistency desired. Experience gained in usage of desired consistencies enables the dental specialist to estimate accurately the amount of powder used according to the number of drops of liquid dispensed. The estimated powder is placed on one end of the slab. The powder is divided into quarters. Then, the first quarter (only) is divided in half (into eighths), and the eighth portion (only) is further divided in half (into sixteenths). When the process is completed, a total of six portions of powder is readied for mixing. See figure 1-5. An additional small amount of powder is often placed on the corner of the slab for use if the estimated powder is insufficient for the desired mix. The liquid is dispensed with the dropper supplied by the manufacturer. The required number of drops of liquid is dispensed from the dropper bottle in accordance with the manufacturer's instructions. The drops are dispensed over a wide area. The close estimation of powder, according to type of mix and number of drops, and the small increments will enhance the slow blending of powder. This slows the setting reaction and enables the user to blend the maximum amount of powder to attain desired consistency. This will help obtain a cement with optimum physical properties.

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Figure 1-5. The division of powder into standard portions.

c. Mixing. Mixing is done by the slow blending of the segments one at a time. This procedure aids in neutralizing the acid and achieving a smooth consistency. A considerable portion of the slab is used. Mixing is done with a moderate circular motion of the spatula blade held flat against the slab. The spatula should be rotated occasionally to blend the material that collects on the top of the blade. A good rule is to spatulate the first three segments for about 15 seconds. (See figure 1-5.) The next two segments should be spatulated for about 20 seconds. The final segment should be spatulated for 30 to 35 seconds. If this is done, the mixing time is not critical and completion of the mix will take about 1 1/2 minutes. It is important to reach the desired consistency by using more powder and not to allow a thinner mix to stiffen by crystallization.

d. Characteristics of Completed Mixes. When a mix is ready for use, it should be similar to the consistency of melted ice cream or liquid glue (adhesive rubber). When the spatula is placed on the slab and the spatula is raised to one inch, the mix will cling to the spatula in a thin thread (peak) for one or two seconds before it breaks and then gradually spreads.

e. Precautions. The following precautions should be observed.

(1) Prevent loss or gain of moisture in liquid cement by keeping bottles tightly stoppered.

(2) Dispense drops only when ready to mix.

(3) Use a cool, dry glass slab (65 to 75 F).

(4) Use the same brand of powder and liquid.

(5) Add increments of powder slowly.

(6) Use the maximum amount of powder to obtain the desired consistency. (To incorporate the most powder, the material should be mixed with a moderate circular motion over a large area of the slab, turning the spatula often.)

David L. Heiserman, Editor

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Revised: May 22, 2017