A reproduction of upto 25 um (micrometers) is achievable with agar hydrocolloids.
Accuracy and Dimensional Change
Agar impressions are highly accurate at the time of removal from the mouth, but shrink when stored in air or 100% relative humidity and expand when stored in water. The least dimensional change occurs when the impressions are stored in 100% humidity (for not more than one hour). However, prompt pouring of plaster or stone models is recommended.
Working and Setting Time
The working time ranges between 7-15 minutes and the setting time is about 5 minutes. Both can be controlled by regulating the flow of water through the cooling tubes. Since the cooling tubes are on the periphery, the material sets from the periphery towards the teeth surfaces.
Tear and Compressive Strengths
Agar has a tear strength of 715 gm/cm2 and compressive strength of 8000 gm/
cm2. According to ADA sp. No. 11 for Agar, compressive strength should not be
less than 35.6 psi. The above values are for tray materials. The syringe materials have poorer mechanical properties.
LAMINATE TECHNIQUE: OR AGAR ALGINATE COMBINATION TECHNIQUE
Here, after injecting the syringe agar on to the area to be recorded, an impression tray containing a mix of chilled alginate that will bond with the syringe agar is positioned on it. The alginate gels by a chemical reaction, whereas the agar gels by means of contact with the cool alginate rather than the water circulating through the tray.
Advantage
1. The syringe agar gives better details than alginate. 2. Less air bubbles.
3. Water cooled trays are not required.
4. It sets faster than the regular agar technique.
WET FIELD TECHNIQUE
This is a relatively new technique which has become popular. The areas to be recorded are actually flooded with warm water. Then the syringe material is introduced
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quickly, liberally, and in bulk to cover the occlusal and/or incisal areas only. While the syringe material is still liquid, the tray material is seated. The hydraulic pressure of the viscous tray materials forces the fluid syringe hydrocolloid down into the areas to be recorded. This motion displaces the syringe materials as well as blood and debris throughout the sulcus.
CAST DUPLICATION
After the introduction of alginate, agar slowly lost its appeal as an impression material. However, it is still popular today as a duplicating material primarily because • When liquefied it flows readily, (like a fluid) over the cast to be duplicated. This
makes it an ideal mould material.
• Large quantities can be prepared relatively easily. • It is economical because it can be reused.
The agar is broken into small chunks and loaded into the liquefying machine (Fig. 5.1) where it is liquefied and stored. The liquid agar is poured into a mould former in order to create a mould. Later, investment is poured into this to create a refractory cast which is used in the fabrication of the cast partial denture framework.
IMPRESSION DISINFECTION
Since the impression has to be sent to the laboratory, the need to disinfect it is very important. Most manufacturers recommend a specific disinfectant. The agent may be iodophor, bleach or glutaraldehyde. Apparently little distortion occurs if the recommended immersion time is followed and if impression is poured promptly.
ADVANTAGES AND DISADVANTAGES OF AGAR HYDROCOLLOID Advantages
1. Accurate dies can be prepared, if the material is properly handled.
2. It has good elastic properties and reproduces most undercut areas correctly. 3. It has good recovery from distortion.
4. As it is not hydrophobic, it gives good model surface. 5. It is palatable and well tolerated by the patient. 6. It is cheap as compared to synthetic elastic materials.
7. It can be reused when used as a duplicating material (reuse is not recommended when used as impression material).
Disadvantages
1. Does not flow well when compared to newly available materials. 2. It cannot be electroplated.
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4. Tears relatively easily. Greater gingival retraction is required for providing adequate thickness of the material.
5. Only one model can be poured.
6. Extensive and expensive equipment is required.
7. A soft surface of the gypsum cast results unless a plaster hardener is used. 8. Although it can be reused, it is impossible to sterilize this material. Also with repeated use there may be contamination of the materials and a deterioration in its properties.
IRREVERSIBLE HYDROCOLLOID—ALGINATE
The word Alginate comes from ‘Algin’ which is a peculiar mucous extract yielded by certain brown seaweed (Algae). The term ‘Algin’ was coined by a chemist from Scotland at the end of the last century. Algin was used for many purposes.
In England 40 years later, another chemist, S. William Wilding received a basic patent for the use of algin as a dental impression material. Alginate was developed as a substitute for agar when it became scarce due to World War II (Japan was a prime source of agar). Currently, alginate is more popular than Agar for dental impressions, because it is simpler to use.
TYPES
Type I- fast setting. Type II- normal setting.
SUPPLIED AS
A powder that is packed
• In bulk containers (tins, bins or sachets) (Fig. 5.4). • In preweighed packets for single impression.
A plastic scoop is supplied for dispensing the bulk powder, and a plastic cylinder, is supplied for measuring the water.
Modified Alginates
Other forms are also available:
• In the form of a sol, containing the water. A reactor of plaster of Paris is supplied separately.
• As a two paste systems one contains the alginate sol, while the second contains the calcium reactor. These materials are said to contain silicone and have superior resistance to tearing when compared to unmodified alginates. They may be supplied in both tray and syringe viscosity.
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FIGURE 5.4: Alginate is available in bulk packing or as small sachets for individual impressions. The plastic measures provided by the manufacturers usually give satisfactory water powder ratio
Commercial Names
Zelgan (DPI), Jeltrate (Dentsply).
APPLICATIONS
1. It is used for impression making: — When there are undercuts.
— In mouths with excessive flow of saliva. — For partial dentures with clasps.
2. For making preliminary impressions for complete dentures. 3. For impressions to make study models and working casts. 4. For duplicating models.
COMPOSITION
Ingredients % Wt. Functions
1. Sodium or potassium or Dissolves in water and reacts with triethanolamine alginate 15% calcium ions
2. Calcium sulphate (reactor) 16% Reacts with potassium alginate and forms insoluble calcium alginate 3. Zinc oxide 4 % Acts as a filler
4. Potassium titanium fluoride 3 % Gypsum hardener 5. Diatomaceous earth 60% Acts as a filler
6. Sodium phosphate (retarder) 2 % Reacts preferentially with calcium sulphate
7. Coloring and flavoring agent Traces e.g. wintergreen, peppermint and anice, orange etc.