IMPRESSION MATERIALS

HYDROCOLLOIDS

INTRODUCTION

IDEAL REQUIREMENTS TO

OBTAIN AN ACCURATE

IMPRESSION

COLLOIDS IN DENTISTRY

DIMENSIONAL EFFECTS

Syneresis

Imbibition

AGAR- AGAR- (REVERSIBLE

HYDROCOLLOID)

Composition

Gelation temperature

Liquefaction temperature

Gelation process

Viscosity of material

Manipulation

Making the Agar

impression

Laminate technique

(alginate – agar method)

ALGINATE (IRREVERSIBLE

HYDROCOLLOID)

Composition

Mode of supply

Classification of alginate

Gelation process

Manipulation

Removal of impression

Compatibility with gypsum

Biocompatibility

Applications

CONCLUSION

INTRODUCTION

An accurate impression plays a pivotal role in success of

a prosthesis. This accuracy develops through thorough

knowledge of impression materials and in the ability of the

operator to understand the material.

An impression is an “imprint” or negative likeness of the

teeth and/or edentulous areas, made in plastic material which

becomes hardened or set while in contact with the tissue

-Heartwell

An impression is the perpetual preservation of what already

exists and not the meticulous replacement of what is missing

- M.M.Devan

IDEAL REQUIREMENTS TO OBTAIN AN ACCURATE

IMPRESSION

Fluid enough to adapt to the oral tissues

Viscous enough to be contained in the tray

Adequate Setting time

Adequate tear resistance

Dimensionally stable

Biocompatible

Cost effective

COLLOIDS

A colloid is a substance that is microscopically dispersed

uniformly throughout another substance. This description

appears to be similar to that of a solution.

PHASES OF COLLOIDS:

A colloidal system consists of two separate phases:

THE DISPERSED PHASE- particles in a solution. molecules held

by primary or secondary forces

THE DISPERSION PHASE- solutions suspending the particles.

The sizes of the colloid particles ranges from 1 to 200 nm.

COLLOIDS IN DENTISTRY

Colloidal silica in resins

Fillers in impression materials

4th state of matter

Size of particle is 1 to 200nm.

TYPES OF COLLOIDS

Aerosol-liquids or solids in air.

Lyosol-liquids or solids in a liquid.

Foams-Gas, liquid and solids in solid.

HYDROCOLLOIDS

If the dispersion phase of a colloidal system is water, it is

called a hydrocolloid.

CHARACTERISTICS OF HYDROCOLLOIDS

During impression making, the material sets from a flowable

state to a solid state. This change of state associated with

hydrocolloids is called THE SOL-GEL TRANSFORMATION.

A SOL: is a colloidal dispersion of very small particles in a

continuous liquid medium

A GEL: is a suspension that behaves like an elastic solid.

If a hydrocolloid contains an adequate concentration of the

dispersed phase, the sol, under certain conditions, may change to a

gel.

In the gel state, the dispersed phase forms agglomerates in the

form of chains or fibrils, also called micelles.

The fibrils may branch and intermesh to form a brush-heap

structure, which can be envisioned as resembling the intermeshing of

tree branches or twigs in a brush pile. The dispersion medium is held

in the interstices between the fibrils by capillary attraction or

adhesion.

TYPES OF HYDROCOLLOIDS

Reversible Irreversible

(AGAR) (ALGINATE)

AGAR (REVERSIBLE HYDROCOLLOID)

Organic hydrophillic colloid which is a galactose sulphate

Polysaccharide extracted from seaweeds

Sulphuric ester of a linear polymer of galactose.

AGAR : Secondary bonds (weak) hold the fibrils together- break

at slightly elevated temperatures and become re-established as it

cools to room temperature

Thus, it is called a reversible hydrocolloid.

MODE OF SUPPLY:

As gel in collapsible tube

Number of cylinders in a glass jar

GELATION TEMPERATURE

Temp at which hydrocolloid material sets to gel.

If temp higher than oral temperature of 37oc,material may

injure soft tissue.

LIQUIFACTION TEMPERATURE

Gel heated to a temperature to return to sol

Temperature is(70-100oc)

Higher than gelation temperature

Temperature lag is hysteresis

COMPOSITION

GELATION PROCESS

Gel Sol (70 – 100oC) liquefaction temperature

Sol Gel (37 – 50oC) gelation temperature

Temperature lag makes it possible to use agar as

dental impression material.

DISTORTION DURING GELATION

Some contraction occurs because of the physical change in

the hydrocolloid transformation from a sol to a gel. If the material is

held rigidly in the tray, the impression material will shrink toward the

center of its mass, thereby creating larger dies.

Since the sol is a poor thermal conductor, rapid cooling may

cause a concentration of stress near the tray where the gelation first

takes place.

Consequently, water at approximately 20 °C is more

suitable for cooling the impression than is ice water.

VISCOSITY OF MATERIAL

Plays an important role in impression material

Fillers and other modifiers like borate can decrease viscosity.

Triple tray technique

MANIPULATION

Preparation of material

Making of impression

Removal of impression

CONDITIONING UNIT

LIQUIFYING : Placed in boiling water for 10 min

STORAGE: Stored in sol condition at 65oC

TEMPERING : at 45oC for 3-10 minutes

MAKING THE AGAR IMPRESSION

It’s a syringe material, directly taken from storage

compartment - applied on to the prepared tooth.

Tray hydrocolloid is removed from the tempering basin,

outer layer is removed and impression made.

Gelation is accelerated by circulating cool water (18-

21oC) through the tray for 3-5 min.

Perforated water cooled tray to circulate water so that the

material gels fast.

Wet Field Technique

LAMINATE TECHNIQUE (ALGINATE – AGAR METHOD)

The tray hydrocolloid is replaced with a mix of chilled alginate

that bonds to the syringe agar.

Alginate gels by a chemical reaction.

Agar gels by means of contact with the cool hydrocolloid.

ADVANTAGES

Maximum detail is produced

As hydrocolloid and not alginate is in contact with prepared

tooth

SHORT COMINGS

Bond between agar and alginate is not always strong.

Higher viscosity alginate displaces the agar during seating.

Dimensional accuracy of alginate limits the use to single units.

PROPERTIES

FLOW sufficiently fluid to record details if correctly

manipulated.

GELATION TEMPERATURE agar sol should revert to

a gel at temp of 37.c(98.6 f).

FLEXIBILITY when removed from mouth flexibility

should be4-15%,when a stress of 14.2 psi is applied. on

an average flexibility of 11% is desired

ELASTICITY AND ELASTIC RECOVERY occurs to

extent of 98.8%.

ACCURACY AND DIMENSIONAL CHANGES least

dimensional changes occur when stored in 100%

humidity, prompt pouring is recommended.

Reproduction limit a reproduction limit up to 25um is

achievable.

WORKING AND SETTING TIME ranges between 7-15

min and setting time is 15 min.

TEAR AND COMPRESSIVE STRENGTH tear strength

tear strength of 4 psi and compressive strength of 116psi.

ADVANTAGES

Accurate dies can be prepared, if material is handled properly.

Produces most undercut areas correctly

Well tolerated by patients

Cheap as compared to elastic materials.

DISADVANTAGES

Cannot be electroplated

Painful during insertion/gelation.

Tears easily

Expensive armantarium required

Impossible to sterilize material

DIMENSIONAL STABILITY

After 1 hour shrinks in air

.15% in another one hour shrinks

Syneresis and imbibition make it dimensionally unstable

Storage in 100%humidity makes it dimensionally stable

DISINFECTION

Even though unsuitable with disinfectant solution, Powell

showed that IODOPHOR disinfectant prevents microbial

growth.

Other agents may be bleach or gluteraldehyde.

APPLICATIONS

Full mouth impressions without deep undercuts.

Quadrant impressions without deep undercuts and single

impressions.

Duplicating dental casts and models.

Crown and bridge impressions because of accuracy.

ALGINATE

(IRREVERSIBLE HYDROCOLLOID)

Developed as a substitute for the agar impression material

when its supply become scarce during World War II.

A chemist from Scotland perceived that certain brown seaweed

(algae) succumbs an atypical mucous extraction

anhydro-B-d-mannuronic acid or alginic acid (insoluble in water)

England chemist received patent for using alginate as dental

impression material.

United States-1945

Introduced alginate during World War II.

STRUCTURAL FORMULA OF ALGINATE

COMPOSITION:

DIATOMACEOUS EARTH?????

Diatoms are tiny, single-celled algae found in plankton.

Diatomaceous earth is a fine, white, crystalline powder made

up of the fossilized shells of diatoms.

Lightweight, gritty, and porous.

The structure of the fossilized diatoms in diatomaceous earth

resemble small sponges with many openings. With so many

places for particles to become stuck, diatomaceous earth makes

excellent filler material

Diatomaceous earth is the remains of microscopic

phytoplankton, similar to coral, found in prehistoric freshwater

sea beds.

DE is approximately

3% magnesium,

86% silicon, 5% sodium,

2% iron

and trace minerals such as titanium, boron, manganese,

copper and zirconium

MODE OF SUPPLY

• As powder

In bulk in tin or sachets

In preweighed individual containers.

CLASSIFICATION OF ALGINATE (Robert G. Craig)

• I. According to concentration of

sodium phosphate

• Fast set 1.25 – 2min

• Regular set 3 – 4.5 min

• II. According to concentration of filler

• Soft set

• Hard set

GELATION PROCESS

Soluble alginate + calcium sulfate Insoluble calcium alginate gel

trisodium phosphate (retarder)

• Na3Po4+3CaSo4-----Ca3Po4+3Na2So4

• Sodium alginate+CaSo4+H2o----Calcium alginate+Na2So4

Final set structure differs from agar in that these are held

together by primary bonds rather than intermolecular forces so no

effect of temperature.

The fibrils are formed by chemical action - irreversible

GELATION TIME

Amount of retarder added (manufacturer)

By altering the temperature of water.

Colder the water - longer is gelation time

Warmer the water - Shorter is gelation time

Cooling mixing bowl and spatula

Prolong the gelation time.

MANIPULATION

Preparing the mix

Loading the tray

Seating tray

Removal of impression

LOADING TRAY

Sufficient bulk of material to be inserted for.

Impression material should adhere to tray so that it can

be withdrawn from around teeth.

Thickness of alginate impression between tray and mouth

3mm atleast.

Adhesives used in plastic or rim lock trays.

SEATING THE TRAY

Saliva to be wiped off from palate

To be held in place firmly without any movement.

Strength increases after gelation

2-3 min after gelation tray to be kept in mouth

Tackiness lost during this time.

REMOVAL OF IMPRESSION

Impression to be removed with a jerk.

Gentle ,long continued pull causes tearing of impression.

Causes deformation.

Pouring as soon as possible.

PROPERTIES

• Permanent deformation : 1-1.5% for 10% strain

• Flexibility : 14% at a stress of 1000 g /cm2

• Strength : Compressive strength= 5000-9000 g/cm2

Tear strength= 380-700 g / cm

COMPATIBILITY WITH GYPSUM

To overcome surface softness of gypsum cast for waxing

procedures from a hydrocolloid impression material

Immersing impression in solution containing accelerator for

setting of gypsum

Incorporating plaster hardener in material

HARDENING AGENT PREVENTS

Syneresis

Retarding action of gel

Zinc sulphate,magnese sulfate, potash alum, titanium fluoride

DIMENSIONAL STABILITY AND SHELF LFE

AIR– looses water by evaporation and shrinks

WATER—Absorbs water and swells

If held in mouth for 6 to 7 min,distortion occurs

Short shelf lie

STORAGE

Should be poured at once, however storage if becomes

necessary then in

In wet paper towel

In plastic bag

Humidor

If storage is unavoidable, keeping in environment 100%

relative humidity results in least dimensional change.

BIOCOMPATIBILITY

Fine silica particles become airborne from the can when lid is

removed silicosis and pulmonary hypersensitivity

Incorporation of glycerin to agglomerate alginate particles to

reduce dusting

ADVANTAGES

Easy to mix

Minimum equipment

Flexibility of set impression

Low cost

Comfortable to patient

DISADVANTAGES

Not accurate for crown and bridge impressions

Cannot be corrected.

ACCURACY

Agar is among the most accurate of impression materials.

Most alginate impressions are not capable of reproducing the finer

details observed in impressions with other elastomeric impression

materials.

Nevertheless, alginate materials are sufficiently accurate that

they can be used to make impressions for removable partial dentures.

RECENT ADVANCES IN ALGINATE

HIGH TEAR STRENGTH ALGINATES

SILICON REINFORCED 2 PASTE SYSTEM

CHROMATIC ALGINATE

Sodium Silico Fluoride --PH Controller

DUST FREE ALGINATE

organic glycol---- dedusting agent

ALGINATE WITH DISINFECTANT SOLUTION

SOFT SET AND HARD SET-according to filler content.

ALGINATE WITH DISINFECTANT SOLUTION

Alginate compatible with disinfectant solutions such

as sodium hypochlorite,

gluteraldehyde chlorhexidine and organic peracetic acid

Musil reported peracetic acid disinfestation for 1 min did not

disturb the qualities of impression

A quaternary ammonium salt has been added to alginate powder,

BLUE PRINT ASEPT. And it provides high degree of

disinfection when compared with plain material.

BDJ.(1990,169:83)

F GHANI,J.A KIRK,M,.WILSON

APPLICATIONS

Impression making

For duplicating models.

Making impressions for orthodontic and study models

Preparation of athletic mouth protectors for sports people.

when undercuts are present

in mouth with excessive flow of saliva

For partial dentures with clasps

Preliminary impressions for complete dentures.

For duplicating models.

Making impressions for orthodontic and study models

Preparation of athletic mouth protectors for sports people.

CAUSES TO REMAKE HYDROCOLLOID IMPRESSIONS

EFFECT AGAR ALGINATE

Grainy

material

1. Inadequate

boiling

2. Storage time too

long

3. Improper mixing

4. Undue gelation

Separation of

tray and

syringe

material

Water soaked

layer of tray

material not

removed

Not applicable

tearing 1. Inadequate bulk 3. Inadequate bulk

2. Premature

removal from

mouth

4. Moisture

contamination

External

bubbles

Gelation of syringe

material preventing

flow

Air incorporated

during mixing

Irregularly

shaped voids

Material too cool

or grainy

1. moisture or

debris on tissue

Rough or

chalky stone

model

1. Inadequate

cleansing of

impression

2. Excess water in

impression

3. Improper

manipulation of

stone

4. Premature

removal of

model

distortion 1. Impression not

poured

immediately

2. Moment of tray

during gelation

3. Impression not

poured

immediately

4. Premature

removal from

mouth

PROPERTIES OF HYDROCOLLOID IMPRESSION

MATERIALS

The quality of a stone cast depends on the properties of the

hydrocolloid impressions before pouring and the potential

interaction between gypsum and alginate during setting of the

gypsum.

DUPLICATING MATERIALS

Both types of hydrocolloids are used in the dental laboratory

to duplicate dental casts or models for the construction of prosthetic

appliances and orthodontic models.

Agar hydrocolloid is more popular in the lab because it can

be used many times. In addition, with intermittent stirring, agar

hydrocolloid can be kept in a liquid form for 1 or 2 weeks at a

constant pouring temperature. These factors make the cost of

reversible impression materials quite reasonable.

The hydrocolloid-type duplicating materials have the same

composition as the impression materials but their water

content is higher.

Consequently, the agar or alginate content is lower,

which influences their compressive strength and percent

permanent set.

These property requirements are identified in ANSI/ADA

Specification No. 20 (ISO 14356; Dentistry—Duplicating material).

CONCLUSION

The need to understand and evaluate the composition,

properties and handling characteristics of impression materials is

vital as it helps us use materials in a better and more rational manner

thereby eliminating causes of failures.