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ELASTOMERIC IMPRESSION MATERIAL
ALGINATE
IT IS A NEGATIVE LIKENESS OR COPY IN REVERSE OF THE SUBJECT OR OBJECT.
EXAMPLE: AN IMPRINT OF THE TEETH AND ADJASCENT STRUCTURES FOR USE IN
DENTISTRY
WHAT IS AN IMPRESSION
ALGINATE
Alginate is classified as irreversible hydrocolloid. Hydrocolloid because it consists of particles of a
gelatinous (colloidal) state in water (hydro) and irreversible because once it has jelled it cannot be returned to a liquid solution.
The Word Alginate comes from the term “ALGIN”. It is a peculiar mucous extract yielded by certain
brown seaweed ( algae ) . The term „ALGIN‟ was coined by chemist from
Scotland at the end of 19th century.
TYPES
Type I – Fast setting. Type II – Normal setting.
MODES OF SUPPLY
It is supplied as a powder that is packed: a) In bulk or in tins or in sachets (or)b) In pre-weighed individual containers. A plastic scoop is supplied for dispensing the bulk
powder, & a plastic cylinder is supplied for measuring the water .
APPLICATIONS
1) Used for impression making : a) when there are undercuts b) In mouth with excessive flow of saliva c ) For partial dentures with clasps.
2) For making preliminary impressions for complete denture.
3) For orthodontic and study models.
4) For duplicating models.
COMPOSITION
Potassium or sodium alginate(15%)
Dissolves in water and reacts with ca+ ions Calcium sulfate(16%)
Reacts with the potassium alginate to form the gel.Sodium phosphate(2%)
Reacts preferentially with calcium sulphate/ retarder
Diatomaceous earth( 60 %)
filler that adds bulk to the material. Zinc oxide (4%)
Adds bulk to the material / fillerPotassium titanium fluoride(3%)
gypsum hardenerColoring and flavoring agent(traces)
Peppermint, orange, etc
PHYSICAL PHASES OF ALGINATE
The first phase is a sol (as in solution). In the sol phase, the material is in a liquid or semi liquid form.
The second phase is a gel. In the gel phase, the material is semisolid, similar to a gelatin dessert.
SETTING REACTION
Reaction occurs by a chemical reactionSoluble alginate reacts with calcium sulphate to
produce insoluble calcium alginate as a gel The production of calcium alginate is delayed by the
addition of a third soluble salt (tri sodium phosphate) to the solution, with which the calcium sulphate will react in preference to the soluble alginate to form an insoluble calcium salt.
In a nutshell , when powder is mixed with water to obtain a paste ,two main reaction occur during setting :
a. 2Na3P04 + 3CaSo4---> Ca3(Po4)2+3 Na2SO4
b. sodium alginate(Powder ) + CaSO4 + H2O --> Ca alginate + Na2SO4(Gel)
First , sodium phosphate reacts with the calcium sulphate to provide adequate working time.
Second ,after the sodium phosphate has reacted, the remaining calcium sulphate reacts with sodium alginate to form an insoluble calcium alginate which forms a gel with water.
WATER-TO-POWDER RATIO
An adult mandibular impression generally requires two scoops of powder and two measures of water.
An adult maxillary impression generally requires three scoops of powder and three measures of water.
MANIPULATION
Fluff or shake the powder to distribute the powder particles evenly
Use the specific measuring devices (water & powder) provided by the manufacturer for mixing
Follow the manufacturer's direction regarding the ratio of water to powder.
Use exact measurements
After the water and powder have been measured, place the water in a clean dry bowl. Shift the powder into the water. Adding the powder to the water ensures the powder particles are wet evenly. If mixed in reverse (the water is added to the powder) the chemical reaction will start early with some particles setting faster than others.
Mix the alginate for the specific amount of time and using a stiff spatula “swipe” the alginate mass against the sides of the bowl to avoid entrapment of air in the mix.
ALGINATE SETTINGS
Working time
The time allowed for mixing the alginate, loading the tray, and positioning the tray in the patient's mouth.
Setting time
The time required for the chemical action to be completed.
Normal set alginate Working time of 2 minutes and a setting time of up to 41/2 minutes
after mixing. Fast set alginateWorking time of 11/4 minutes and a setting time of 1 to 2 minutes.
SETTING TIME OF ALGINATE
Since alginate reacts or gels chemically, temperature is a major factor in the setting time. The colder the temperature of the water the longer it takes to set; conversely, the higher the water temperature the faster it sets.
Ideal water temperature is 68 degrees or room temperature.
LOADING OF TRAY
A perforated tray is used so that the material is forced out slightly through the holes in the tray during loading, thereby locking itself mechanically into the tray
The surface of the alginate in the tray may be smoothened out by moistening the finger with water and running it over the surface of the alginate.
SEATING THE TRAY
Before making the upper impression, saliva should be wiped off from the palatal region
In order to prevent the material from flowing along the throat and causing nausea to the patient, the posterior portion may be seated first and then the anterior portion seated properly
Since the material sets from tissues towards periphery, any movement during gelation may result in distortion.
So once the tray is seated ,it must be held in place firmly without any movements.
REMOVAL OF THE IMPRESSION
An alginate impression when set develops a very effective peripheral seal
This seal should be freed by running the finger around the periphery.
The impression must be removed suddenly ,with a jerk.
After removal from the mouth, washed with water to remove saliva. cast should be poured as soon as possible.
MODIFIED ALGINATES
Dust free alginates
De-dusting agent (glycerin or glycol) Siliconized alginates
silicon polymers are added to strengthen the material Alginates containing disinfectants
eg :quaternary ammonium salts or chlorhexamine are added to alginate powder.
Hard and soft set alginates
by adjusting the amount of fillersChromatic Alginates
Alginates which change color on setting
PROPERTIES
FLEXIBILITY
It is about 14% at a stress of 1000g/cm2. Lower w/p ratio results in lower flexibility.
ELASTICITY AND ELASTIC RECOVERY
Alginate are highly elastic but less when compared to the agar. 97.3% elastic recovery occurs.
Permanent deformation is less if the impression is removed from the mouth quickly.
STRENGTH
The compressive strengths of alginate gels range from 0.5 TO 0.9 MPa
The tear strengths vary from 0.4 to 0.7 MPa
Both of these properties are time dependent with higher values being obtained if the time of removal is delayed.
DIMENSIONAL STABILITY :
Alginate impression loses water by evaporation and shrinks on standing in air. If it is placed in water it absorbs water and swells. Therefore ,cast should be poured immediately after making the impression.
BIOLOGICAL PROPERTIES
No known chemical or allergic reaction have been identified. However , avoid breathing in Silica particles after fluffing alginate powder.
ADHESION
Alginate does not adhere well to the tray, retention is obtained by mechanical locking features in the tray or by applying an adhesive.
SHELF LIFE
Alginate impression material have a short shelf life. They deteriorate rapidly when stored at elevated temperatures in a humid environment. Therefore ,it is better not to stock more than 1 year supply .
CAUSES FOR DISTORTION AND DIMENSIONAL CHANGE OF ALGINATE
IMBIBITION
If an alginate impression is stored in water or in a very wet paper towel, the alginate will absorb additional water and expand. This condition is called imbibition.
SYNERESIS
If an alginate impression remains in the open air, moisture will evaporate from the material, causing it to shrink and distort. This condition is called syneresis.
ADVANTAGES OF ALGINATE
It is easy to mix and manipulateMinimum requirement of equipmentsFlexibility of the set impressionAccuracy if properly handledLow costComfortable to the patientGives a good surface detail even in the presence of
saliva
DISADVANTAGES
Can’t be corrected. Cannot be electroplated so metal dies are not
possiblePoor dimensional stabilityPoor tear strengthIt is not recommended where higher degree of
accuracy is required, example: for RPD, FPD, Crowns etc.
THANK YOU