Impression Materials – Non-aqueous Elastomers

pain armchair dentist suffering
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WHAT IS NON-AQUEOUS ELASTOMETRIC IMPRESSION?

They are synthetic polymers with rubber properties after setting. Used to make the final impressions for indirect restorations and implants. 

All teeth in the arch and the soft tissues immediately surrounding the tooth preparation must be reproduced in the impression. They will allow the cast to be accurately articulated (to check occlusion) and will contribute to proper contouring of the planned restoration.

If it becomes necessary to store the impression before a cast will be made, the polyethers and addition silicones are preferable because they exhibit sufficient long-term dimensional stability; the other materials, particularly the reversible hydrocolloids, must be poured immediately.

 

Requirements of an impression for a cast restoration:

1. It should be an exact duplication of the prepared tooth, including all of the preparation and enough un-cut tooth surface beyond the preparation to allow the dentist and the technician to be certain of the location and configuration of the finish line.

2. Other teeth and tissue adjacent to the prepared tooth must be accurately reproduced to permit proper articulation of the cast and contouring of the restoration.

3. It must be free of bubbles, especially in the area of the finish line and occlusal surfaces of the other teeth in the arch.

 

Ideal Properties:

  • High accuracy (very small contraction <0.5%)
  • Biocompatibility
  • High dimensional stability
  • Compatibility to stone
  • Multiple pouring within 24 Hours after making impression
  • High tear strength
  • High elastic recovery
  • Long shelf life
  • Ease of use
  • Cost

 


 

Polysulfide

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  • Commonly called rubber base
  • Hydrophobic
  • Comes as two tubes of base and catalyst
  • Will begin to shrink after one hour from removal
  • Should be poured immediately

Although it is the least expensive elastomer, it is not well-liked by patients because of its unpleasant sulfide odor and long setting time in the mouth (about 10 minutes).

 


 

Condensation Silicones

DM_099

  • Can have pronounced shrinkage (poor dimensional stability)
  • Dies produced from this material can be undersized
  • The above occurs due to the evaporation of the by- product of the condensation reaction. (H2O for polysulfides and ethanol for Condensation Silicones)
  • Should be poured immediately
  • Hydrophobic
  • Silicone impression material is available in a variety of viscosities.

 


 

Polyether

DM_100

  • No volatile by-product is formed which results in excellent dimensional stability.
  • Hydrophilic in nature.
  • Pouring can be delayed to produce accurate casts even more than a day after the impression has been made.

 


 

Polyvinyl Siloxanes (Addition Silicone)

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  • Dimensional stability is quite high
  • Least affected by pouring delay of any material (due to no volatile by-product formation)
  • Surfactants have been added to the material to decrease it hydrophobicity
  • Comes in different viscosities (consistencies)

 

Pouring should be delayed with some of the earlier products. If this is not done, a generalized porosity of the cast surface caused by gas from the impression material will develop.

Newer products contain “scavengers” that prevent the escape of gas at the polymer-cast interface.

Addition silicone that contains scavenger material can be poured immediately.

 


 

Impression Trays

A) Stock trays

Stock trays can be used with these impression materials. Retention is provided by perforations, rim-locks, and/or tray adhesives.

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B) Custom trays

A custom tray improves the accuracy of an elastomeric impression by limiting the volume of the material. Stops are needed in the tray to maintain even space for the impression material. These are placed on non-centric cusps of teeth that are not to be prepared.

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Impression Making

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A) Single Mix (mono-phase) Technique

The impression Material for the mono-phase technique has one viscosity for tray and syringe.

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  1. Usually used with polyether
  2. Some of the material is applied to the syringe while the rest is applied to the tray
  3. Inject the material by the syringe around the prepared tooth
  4. Air drying to spread the material
  5. Place the tray
  6. Shorter working time is needed

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B) Double Mix Technique

Common methods for making crown and bridge impressions are:

1. A simultaneous, dual viscosity technique

2. Putty-wash technique

 

1. A simultaneous, dual viscosity technique (Heavy Body-Light Body Combination)

  • It’s a 4-handed technique
  • Light body is injected by a syringe
  • Heavy body placed by the tray
  1. On separate pads (one for the tray and one for the syringe material), disperse equal amounts of base and accelerator. Blend the two pastes thoroughly.
  2. THE LIGHT-BODIED MATERIAL IS THOROUGHLY SPATULATED.DM_107
  3. Mixture should be free of streaks and bubbles
  4. MEANWHILE, AN ASSISTANT MIXES THE HEAVY-BODIED MATERIAL.DM_108
  5. Low-consistency material is injected with a syringe into critical areas.DM_109
  6. Seat the tray loaded with the tray material (heavy-bodied), Tray must remain immobile while the material undergoes polymerization (6 to 12 minutes, depending on the material). Otherwise distortion of the impression can occur when it is removed.DM_110

 

– Automix Technique

Some manufacturers offer impression material in prepackaged cartridges with a disposable mixing tip attached.

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Advantages:

  • Elimination of hand mixing
  • Produces void-free impression

The base and catalyst are extruded into the mixing tip, where mixing occurs as they progress to the end of the tube.

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– Machine Mixing Technique

An alternative method for improving impression mixing is to use a machine mixer.

This system is convenient and produces void-free impressions.

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2. Putty-wash technique

Silicone impression material is available in a variety of viscosities. It is a two-step impression procedure whereby a preliminary impression is taken in high or putty-consistency material.

  1. Washing Hands Is A Must! If the putty is used, it should be not be dispensed or mixed while wearing latex gloves. Because the sulphur granules in the powder will retard the setting reaction.
  2. Accelerator is added to the putty.DM_118
  3. Putty is placed in the tray.DM_119
  4. Spacer is placed over the putty. This allows room for a thin wash of light-bodied material, which makes the impression.DM_120
  5. Take the impression by putty material.
  6. Remove or trim some of the putty material by scalpel; or place a spacer to provide even thickness e.g. celluloid paper.
  7. Mix the light body material and place it in the tray space and re-seat the tray in mouth.
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Final impression

 

– Simultaneous Putty Wash Technique

  1. Placement of a dimple in the putty, filled with wash, before tray seating.
  2. Wash material capturing margins which is free of voids and tears.
  3. A good bond between putty and wash (i.e. no separation).

Manufacturers add colouring agents to the accelerator and/or base as an aid in determining the thoroughness of the mix.

Normally a different colour is used for each consistency of a particular product line so one can distinguish the wash (low) consistency from the tray consistency in the set impression.

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 Lingual View Of Impression

The impression material cervical to the margin is termed “flash”.

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The more flash cervical to the impression, the easier it is to trim the dies during the laboratory phase of any project.

 

Less Ideal Impression

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All of the margin has been captured on the facial surface Far less flash is present on the facial making that portion of the die far more difficult to trim.

 

Ideal Impression

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Marginal clarity around each of the four anterior teeth . This was accomplished with good tissue management.

 

Evaluation of the Impression

  1. No air bubbles or voids especially at the margins.
  2. Intact uninterrupted cuff of the impression material should be present beyond every margin.
  3. Homogenous colour of the material. Presences of streaks indicate poor mixing.
  4. Good blend between heavy body and light body materials.
  5. No part of custom tray shown in the impression.
  6. The impression should not be separated from the tray.

 

The impression must be inspected for accuracy when it is removed.

If bubbles or voids appear in the margin, the impression must be discarded.

  • Voids on the margin of a preparation compromise the fit and function of the final restoration.
  • Voids on occlusal surfaces make articulation of stone models difficult.

 

 

Streaks of base or catalyst material indicate improper mixing and may render an impression useless.

A complete information about the impression material used is to be provided to the dental laboratory.

 

Disinfection:

When they are removed from the patient’s mouth, it must be assumed that all impression materials have been in contact with body fluids and therefore should be disinfected.

Disinfection is an essential step for preventing cross-infection and exposure of laboratory personnel. If it is performed properly, disinfection will not affect the accuracy or surface reproduction of the impression material.

  • After being removed from the patient’s mouth, the impression is immediately rinsed with tap water and dried with an air syringe.
  • Suitable chemicals should be used, such as glutaraldehyde solutions 2% (10 minutes soak time) or iodophor sprays.

Note:

Because polyether is a hydrophilic material (has an affinity for water), it is better to spray it rather than to soak it.

 


Guides For Selection of Appropriate Disinfection Methods for Impressions Transported to Dental Laboratory

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– end –

Impression Materials – Agar

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Agar is an aqueous impression material used for recording maximum details; for example, as in the production of dies for fixed restorations.

Agar is also known as a reversible hydrocolloidal impression material. It gives good detail reproduction than any other material. However, it has been replaced by rubber-based impression materials because of the costly armamentarium required and prolonged chair time.

 

DO YOU KNOW THE MEANING OF ‘AGAR’?

The word “agar” comes from the Malay word agar-agar – meaning” jelly”.

Historically and in a modern context, it is chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium for microbiological work.

* Wikipedia, the free encyclopaedia.

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Properties:

  • Agar is a hydrophilic colloid extracted from certain types of seaweed.
  • It is a complex sulfated polymer of galactose units.
  • It is a mucilaginous substance that melts at high temperature (about 100 °C) and solidifies into a gel at low temperature (about 36 °C).

 

Composition:

  1. Agar (8-15 %) – The basic constituent
  2. Borates – Improves strength but acts as a gypsum retarder
  3. Potassium sulfate – Acts as a gypsum hardner
  4. Fillers – Hard waxes which improves strength
  5. Plasticizer – e.g. glycerine and thymol
  6. Alkyl-benzoates (0.1 %) – Preservative that increases shelf-life.
  7. Colouring and flavoring agents
  8. Water (> 80%) – Dispersion medium

 

Usage:

1. Primary use of agar impression material: It is used to make secondary/final impression in dentolous patients requiring removable and fixed partial dentures.

For impression material usage, it comes in the form of:

  • Gel in tubes (tray material)
  • Number of cylinders in a glass jar or cartridges (Syringe material)

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2. Widely used at present for cast duplication (during fabrication of cast RPD).

For laboratory duplication material usage:

  • Bulk containers

 

Advantages:
1. Detail reproduction is very good.
2. Can record undercut areas correctly.
3. Distortion on removal is prevented due to elastic recovery.
4. Well tolerated by the patient.
5. Can be re-used.

 

Disadvantages:
1. Cannot be electroplated.
2. Thin sections of impression tears easily.
3. Multiple models cannot be poured like elastomeric impression materials.
4. Special armamentarium required.
5. Gypsum hardener required.
6. Sterilization of impression is difficult.

 

WHAT IS GELATION?

Gelation is a sol-gel transformation.

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Liquefaction Temperature:
Agar gels have fibrils held together by weak forces. They break at increasing temperature to the from sol.

This increased temperature is called liquefaction temperature and is 71 to 100 °C.

 

Gelation Temperature:
the sol can be converted to gel by decreasing the temperature below liquefaction.

It is called gelation temperature and it varies from 36 to 43°C.

 

WHAT DO YOU NEED TO KNOW ABOUT HYSTERESIS?

Hysteresis is the temperature lag (slow) between Liquefaction Temperature & Gelation Temperature. This helps in using agar as an impression material. This time is the manipulation time of the material.

The gelation temperature is critical when:

  • If too high: Injury to oral tissues due to increased temperature.
  • If too low: Temperature for gelation would be made difficult or impossible to attain.
  • Exactly at mouth temperature, surface stress may develop causing syneresis later.

 


 

Manipulation

Armamentarium:

i. Conditioning unit has 3 chambers

  • Liquefying chamber: Boiling (100 °C) for 10 minutes. For reused material, additional 3 minutes is required.
  • Storage chamber: Stored at about 65 °C.
  • Tempering chamber: Tempering at 45 °C for 3 minutes after it has being placed in the tray.

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ii. Water-cooled rim-lock tray

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Impression Making:

  1. The syringe material is taken from the second chamber and injected into the prepared tooth.
  2. The tray material is taken from the third chamber and the tray is seated in position.
  3. Cool the tray with water at 13 °C for 3 minutes.
  4. The tray should be held with little pressure and should not be distributed until gelation is complete (convert sol to gel).
  5. The impression is removed with a single stroke along the long axis of the tooth. Twisting or torquing should be avoided.

 

Dimensional Change:

Depends on water content of the hydrocolloid,
Decrease water content = Shrinkage = Syneresis – an exudation of fluid onto surface of set gel

Increase water content = Expansion = Imbibition – absorption of water

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Disinfection:

it is done to prevent contamination of gypsum models by viruses like AIDS, Hepatitis B, etc.

It can be done through the following steps:
i. Thoroughly rinse the impression under tap water to remove any blood or saliva.

ii. Disinfect the impression by:

  • Submerging it for 10 minutes in a fresh 0.5 % solution of sodium hypochlorite or glutaraldehyde.
  • Spraying with antimicrobial agents.
  • Wrapping the impression in the disinfectant soaked paper towel and placing it in a sealed plastic bag for 10 minutes.

 

Storage of the impression:

After disinfection, pour the model with stone immediately. Only in unavoidable conditions, storage for short period is done.

Methods of storage include:

  • Impression may be wrapped in a water soaked towel.
  • Placed in a plastic bag which is convenient for storing impressions under humid conditions.
  • It may be placed in a humidor (100% humidity) or 2% potassium sulfate solution.

 

Precautions:

  • Even under proper storage conditions the cast should be poured within an hour.
  • The impression should not be wrapped too tightly which incorporates stress.
  • Excess water in the towel may lead to imbibition.

 

Removal of the cast:

  • The contact of stone with the impression should be for 60 minutes before the cast is removed.
  • A chalky stone surface may be produced, if the cast is allowed to remain in contact with the impression overnight as set stone absorbs water from the impression.

 


 

Laboratory Duplication Procedure

In the construction of partial dentures and orthodontic appliances it is often necessary to produce more than one cast. It is not always possible or advisable to pour two or more casts from one impression and in such cases the first cast is duplicated using a reversible hydrocolloid duplicating material.

The duplicating hydrocolloid, which is normally thinner in consistency than the impression hydrocolloids.

The agar materials are widely used as laboratory duplicating materials.

For this application their main advantage is that the material can be reused.
A significant factor in this application where the products are used in relatively large bulk.

The technique for duplication involves standing the cast surrounded by a metal
duplicating flask which is designed to allow an even thickness of material all round.

DM_093
Duplicating Flask

Manipulation:

  1. The duplicating material is liquefied at 70 °C to 100 °C, tempered to 50 °C and then poured through an opening and filled.
  2. When gelation is complete, the master cast is removed with a rapid movement rather than by easing it away, in order to optimize elastic recovery within
    the gel.
  3. The duplicate cast should be poured immediately after removal of the master cast in order to avoid dimensional changes in the hydrocolloid.

    DM_095
    Dentures are suspended in the metal duplicating flask. Then, molten agar is being poured.
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The mold space after removal of the denture.
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Auto-polymerized acrylic resin is then poured in the mold space to produce template dentures for modifications.

 

– end –

Impression Materials – Alginate

DM_077

Alginate is classified as irreversible hydrocolloid. It is 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.

It is supplied in powder form and mixed with water. When set, the material is a flexible gel resembling rubber.The impression is made directly in the patient’s mouth producing a negative replica, then poured in dental stone,  producing a positive cast.

These materials are elastic enough to be withdrawn from the undercuts without permanent deformation or distortion. It is extremely accurate in tissue details when handled properly.

 

 

Other elastic impression materials:

  1. Agar hydrocolloid
  2. Alginate
  3. Polysulfide
  4. Condensation silicone
  5. Addition silicone
  6. Polyether

 

Composition:

The basic components are a soluble alginate (either potassium alginate or sodium alginate) and a reactor (calcium sulfate), which causes the alginate to gel.

 

Mixing the alginate:

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  1. Use the specific measuring devices (water & powder) provided by the manufacturer for mixing.
  2. Add the powder into the water. This ensures the powder particles are wet evenly.
  3. If mixed in reverse (the water is added to the powder) the chemical reaction will start early with some particles setting faster than others.

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Setting Time of Alginate:

Temperature is a major factor in the setting time. The colder the temperature of the water the longer it takes to set.

 

Dimensional Stability

Alginate has a tendency, after it sets to lose (syneresis) or absorb (imbibition) water, depending on the atomosheric conditions surrounding it.

If conditions are dry it loses water & shrinks (syneresis) ; if immersed in water, it imbibes moisture or swells. (imbibition)

 

Advantages:

  • It makes an accurate impression if handled properly
  • Elastic to be withdrawn from undercuts without distortion
  • Easy to work with, not time consuming
  • Inexpensive

 

Disadvantages:

  • Subject to dimensional changes if not stored properly before pouring (Syneresis or Imbibition)
  • Cannot be added to (add another layer)

 

Disinfection:

The disinfection of alginate impressions must be carried out with regard for the dimensional instability of the impression material.

Excessive immersion in aqueous solutions may cause swelling of the material, and should be avoided.

Immersing alginates in glutaraldehyde solutions for a relatively short term (approximately 10 minutes) can be effective without causing undue dimensional change.

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Manipulation of Alginate Impression

  • Should not be exposed to air (dehydration).
  • Should not be immersed in water (imbibition).
  • Should be poured immediately   OR
  • Should be stored in a humid atmosphere by wrapping in a damp paper towel or in 100% relative humidity (humidor).
  • No separating medium is needed for pouring.

 

Forming the Cast (without boxing)

Procedure:

  1. Remove the alginate impression from the damp paper towel and shake out any moisture.
  2. Add gypsum product (powder) into water “according to manufacturer’s instructions” and spatulate. Place the bowl on the vibrator to escape the entrapped air.
  3. Hold the impression tray against the vibrator and add a small amount of mixed gypsum. Continue to add small increments until impressions of the teeth have been filled.DM_079
  4. Continue to add gypsum in larger portions until the impression is completely filled.
  5. Let the gypsum reach its initial set.
  6. The base of the cast can then be formed with a new mix of gypsum. The base should be at least 15 mm thick.DM_080

 

Forming the Cast (with boxing)

Procedure:

  1. Apply beading wax to the periphery of the impression. It should be placed 2-3 mm from the borders of the impression and should be 4 mm wide.DM_081
  2. Box the impression with boxing wax sheet.
  3. Hold the boxed impression on a vibrator and add mixed gypsum product in small increments until completely filled.DM_082
  4. After the gypsum has completely set & the exo-therm completed, peal off the boxing wax and remove the beading wax.DM_083
  5. Remove the impression gently off the cast with a plaster knife. For the border molded impression, the cast should be dipped in warm water for easy removal.DM_084

 

The sides of the cast are trimmed to be parallel, any stone nodules are carefully removed. The base can be trimmed for either orthodontic specifications (for a record cast) or to remove excess stone only ( for a master or a working cast ).

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– end –

Impression Materials – Non-elastic

DM_071These impression materials are rigid and therefore exhibit little or no elasticity. Any significant deformation produces a permanent deformation. They are used mainly for edentulous patient (complete denture) cases.

 

Non-elastic impression materials:

  1. Impression plaster
  2. Impression compound
  3. Zinc oxide eugenol
  4. Impression waxes

 


 

Impression Plaster / Plaster of Paris

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Composition:

  • Plaster of Paris
  • Potato starch – Makes the plaster more soluble and facilitates the separation of impression from the cast
  • Accelerator and retarder
  • Colouring agents and flavor

 

Advantages:

  • Good surface detail
  • Excellent dimensional stability

 

Disadvantages:

  • Cannot be added to (add another layer)
  • Properties affected by operator handling technique
  • Taste and roughness may cause the patient to vomit
  • Heat evolved during setting

 

Manipulation of Impression Plaster

As the impression is removed from the mouth it is common for pieces of plaster around the periphery of the impression to fracture off. These pieces should be retrieved and glued back onto the impression before it is cast.

Long narrow strips of wax are then fit around the periphery of the impression just below where it ends. This is called beading.

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The impression is then coated with a thin layer of separating medium and cast in fresh plaster.

The beading provides a clear indication of where the impression ends, this prevents over-trimming and over-extension.

 

 


 

Impression Compound

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Composition:

Dental compound contains several ingredients:

  • Natural resins, which comprise about 40% of the formulation, make the compound thermoplastic. Shellac is often used.
  • Waxes (about 7%) also produce thermoplastic properties.
  • Stearic acid (about 3%) acts as a lubricant and plasticizer.
  • Fillers and inorganic pigments account for the remaining 50% of the formulation.

 

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Dental Impression Compound (Types I and II)

There are two types of dental compound:.

Type I  is used for impression taking, impressions of partially or completely edentulous jaws.

Type II  is used for tray preparation. Impression trays in which a final impression is taken with another material.

Impression compound is available in either cakes or sticks in various colours from a number of manufacturers.

 

Thermal and Mechanical Properties

Dental compound is thermoplastic; it is used warm (45°C) and then cooled to oral temperature (37°C), at which it is fairly rigid.

The setting mechanism is therefore a reversible physical process rather than a chemical reaction.

 

Precaution with using Impression Compound

Care must also be taken to prevent overheating and burning of tissues. Also, cooling water should not be too cold, in order to prevent thermal shock.
Advantages:

  • Non-irritant and non-toxic
  • Reusable
  • Can be reheated and readapted
  • Good shelf life

 

Disadvantages:

  • Poor dimensional stability
  • Poor surface detail

A delay in preparing the stone cast may cause distortion. The cast should be poured as soon as possible after the impression has been removed from the mouth.

To ease separation of the die stone, the impression should first be softened by immersion in warm water.

 

Disinfection:

Dental impression compound can be disinfected by immersion in a proper disinfectants. For example: sodium hypochlorite, iodophors, or phenolic glutaraldehydes.

The manufacturer’s recommendations for proper disinfection should be followed.

 

 


Zinc Oxide Eugenol Impression Paste

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Zinc oxide eugenol’s main use as an impression material is for dentures on edentulous ridges with minor or no undercuts.

 

Composition:

This material is commercially available as two pastes.

  • One paste, called the base contains zinc oxide (ZnO), oil, and hydrogenated rosin.
  • The second paste, the accelerator, contains about 12% to 15% eugenol, oils, rosin, and a filler such as talc or Kaolin.

These two pastes have contrasting colours so it can be determined when the pastes are thoroughly mixed. The setting time is shortened by increases in temperature and/or humidity. The set material does not adhere to set dental plaster or stone.

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Impression materials are classified as hard and soft-set. The hard-set material sets faster (in about 10 minutes, compared to 15 minutes for the soft-set material).

Non-eugenol pastes containing carboxylic acids in place of eugenol are available to avoid the stinging and burning sensation experienced by some patients.
The model or cast should be made from gypsum-type plaster or stone. After the stone has set, the impression is immersed in warm water (60°C) to ease its removal from the cast.
Advantages:

  • Good surface detail.
  • Good dimensional stability.
  • Can be added to with fresh zinc oxide eugenol
  • Stable on storage and good shelf life.
  • Inexpensive

 

Disadvantages:

  • Variable setting time due to temperature and humidity.
  • Eugenol allergy in some patients.

 

Disinfection:
Zinc oxide-eugenol impressions can be disinfected by immersion in a proper disinfectants. The manufacturer’s recommendations for proper disinfection should be followed.

 


 

Impression Waxes

Waxes are thermoplastic materials, which flow at mouth temperature and are harden at room temperature.

Normally used to correct small imperfection (e.g. air bubbles) in other impressions, especially zinc oxide impressions.

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Composition:

They consist of a combination of a low melting paraffin wax and beeswax.

A cast should be poured up immediately after taking the impression to avoid distortion which easily occurs in wax.

These materials are not used to take impression, just to correct impression.

 

– end –

Impression Materials

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WHAT ARE IMPRESSION MATERIALS?

Impression materials are used to make replicas of oral structures.

All impression materials must be in a plastic or fluid state while the replica is being made.

 

A model or cast material (eg, plaster of Paris or high-strength stone) is poured into the impression and, upon setting, produces a positive impression of the tissues of interest.

Usually the impression material is carried to the mouth in an unset (plastic) condition in a tray and applied to the area under treatment. When the impression material has set, it is removed from the mouth with the tray.

The cast is made by filling the impression with dental stone or other model material.

The accuracy, detail, and quality of this final replica are of greatest importance.

Sometimes impression materials are used to duplicate a cast or model that has been formed. when more than one positive reproduction is required.

Such impression materials are referred to as duplicating materials.

 

Types of Impressions

A) Preliminary impressions 

i) Taken either by the dentist or an expanded-function dental assistant.

ii) Used to make a reproduction of the teeth and surrounding tissues.

iii) Used to make (1) diagnostic models, (2) custom trays, (3) temporary coverage, (4) orthodontic appliances, and (5) pretreatment and post‑treatment records.

 

B) Final impressions

i) Taken by the dentist.

ii) Used to make the most accurate reproduction of the teeth and surrounding tissues.

iii) Used to make indirect restorations, partial or full dentures, and implants.

 

C) Bite registrations

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i) Taken by the dentist or dental assistant.

ii) Make a reproduction of the occlusal relationship between the maxillary and mandibular teeth.

 

The requirements of impression materials

1. Accuracy and detail reproduction

2. Dimensional stability

  • Compatible with model materials
  • Resistant to disinfectant solutions

3. Good handling properties – easy to prepare/mix, adequate working and setting times

4. Acceptable for a patient – non-toxic, non-irritant, tasteless, Cost effective

 

Classification of impression materials

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Non-elastic impression materials:

  1. Impression Plaster
  2. Impression compound
  3. Zinc Oxide Eugenol
  4. Impression Waxes

These materials are rigid and therefore exhibit little or no elasticity. Any significant deformation produces a permanent deformation. They are used mainly for edentulous patient (Complete Denture) cases.

 

Elastic impression materials:

  1. Agar hydrocolloid (reversible)
  2. Alginate hydrocolloid (irreversible)
  3. Polysulfide
  4. Condensation silicone
  5. Addition silicone
  6. Polyether

 

 

The non-elastic and elastic impression materials will be further discussed in the coming posts.

 

– end –

 

Gypsum – Investment Material

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The investment material forms the mould into which an alloy will be cast.

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Requirements of Investment Materials

  • The investment should be capable of reproducing the shape, size and detail recorded in the wax pattern. The accuracy of the casting can be no better than the accuracy of the mould.
  • Thermal stability: the investment mould should be capable of maintaining its shape, integrity and have a sufficiently high value of compressive strength at the casting temperatures.
  • Compensating expansion: the investment mould should compensate for the casting shrinkage, achieved by a combination of setting, hygroscopic and thermal expansion.

Selection of Investment Material

The main factors involved in the selection of investment material are:

  • The casting temperature to be used.
  • The type of alloy to be cast.

The investment which is best able to retain its integrity at the casting temperature and able to provide the necessary compensation for casting shrinkage is chosen.


Composition of Investment Materials

Basic Components

Investment materials consist of a mixture of:

1. Refractory material: Silica is the refractory material of choice, it is available in three crystalline forms quartz, cristobalite and tridymite.

  • It adequately withstands the temperatures used during casting.
  • It is responsible for producing much of the expansion which is necessary to compensate for the casting shrinkage of the alloy.

2. Binder material: which binds the refractory particles, and may provide additional expansion to compensate for the casting shrinkage of the alloy.

The nature of the binder characterizes the material:

  • Gypsum-bonded Investment material
  • Silica-bonded Investment material
  • Phosphate-bonded Investment material

Gypsum-bonded Investment Material Composition

These materials are supplied as powders which are mixed with water and are composed of a mixture of silica (SiO2) and calcium sulphate hemihydrate (gypsum product) with minor components including powdered graphite or powdered copper and various modifiers to control setting time.

The calcium sulphate hemihydrate reacts with water to form calcium sulphate dihydrate (gypsum) which effectively binds together the refractory silica.

Gypsum alone is not satisfactory as an investment for alloy casting since it contracts on heating as water is lost and fractures before reaching the casting temperature.

The magnitude of the contraction, which occurs rapidly above 320°C, is significantly reduced in investment materials by the incorporation of sodium chloride and boric acid.

The setting expansion of the calcium sulphate dihydrate, when mixed with water partially compensate for the shrinkage of the alloy which occurs on casting.

Further compensation can be achieved by employing the hygroscopic setting expansion.

Silica-bonded Investment Material Composition

These materials consist of powdered quartz or cristobalite which is bonded together with silica gel.

On heating, the silica gel turns into silica so that the completed mould is a tightly packed mass of silica particles.

The binder solution is generally prepared by mixing ethyl silicate or its oligomers with a mixture of dilute hydrochloric acid and industrial spirit (improves the mixing of ethyl silicate and water). A slow hydrolysis of ethyl silicate occurs producing a sol of silicic acid with the liberation of ethyl alcohol as a byproduct.

Stock solutions of the silicic acid binder are normally made and stored in dark bottles. The solution gels slowly on standing and its viscosity may increase noticeably after three or four weeks, when this happens it is necessary to make up a fresh solution.

(C2H5O)4Si + 4H2O → Si(OH)4+ 4COH2H5

The silicic acid sol forms silica gel on mixing with quartz or cristobalite powder under alkaline conditions achieved by the presence of magnesium oxide in the powder.

It is necessary to incorporate as much powder as possible into the binder solution to have sufficient strength at the casting temperature. This process is aided by a gradation of particle sizes such that small grains fill in the spaces between the larger grains. A very thick, almost dry mix of investment is used and it is vibrated in order to encourage close packing and produce as strong an investment as possible.

Phosphate-bonded Investment Material Composition

These materials consist of a powder containing silica, magnesium oxide and ammonium phosphate.

On mixing with water or a colloidal silica solution, a reaction between the phosphate and oxide occurs to form magnesium ammonium phosphate. This binds the silica together to form the set investment mould.

The formation of the magnesium ammonium phosphate involves a hydration reaction followed by crystallization. A small setting expansion results from the outward thrust of growing crystals.

The material is also able to undergo hygroscopic expansion if placed in contact with moisture during setting.

Moisture adversely affects the unmixed material and the container should always be kept closed when not in use.

On heating the investment prior to casting, mould enlargement occurs by both thermal expansion and inversion of the silica.

At a higher temperature some of the remaining phosphate reacts with silica forming complex silicophosphates.


Thermal Stability

One of the primary requirements of an investment is that it should retain its integrity at the casting temperature and have sufficient strength to withstand the stresses set up when the molten alloy enters the investment mould.

Gold alloys are cast at relatively low casting temperatures of around 900°C whilst some chromium alloys require casting temperatures of around 1450°C.

Phosphate and silica-bonded materials have sufficient strength at the high temperatures used for casting higher melting base metal alloys.

Gypsum-bonded Investment Material Thermal Stability

Gypsum-bonded investments decompose above 1200°C by interaction of silica with calcium sulphate to liberate sulphur trioxide gas. This not only causes severe weakening of the investment but would lead to the incorporation of porosity into the castings.

CaSO4 + SiO2 → CaSiO3 + SO3

Thus, gypsum-bonded materials are generally restricted to use with those alloys which are cast well below 1200°C. This includes the majority of the gold alloys and some of the lower melting base metal alloys.

Another reaction may occur on heating gypsum-bonded investments above 700°C is that between calcium sulphate and carbon (from the residue left after burning out of the wax pattern or graphite present in the investment). Further reaction can occur liberating sulphur dioxide.

The effects of these reactions and can be minimized by:

  • ‘Heat soaking’ the mould at casting temperature to allow the reaction to be completed before casting commences.
  • The presence of an oxalate in some investments reduces the effects by liberating carbon dioxide at elevated temperatures.

Phosphate-bonded Investment Material Thermal Stability

The use of colloidal solution of silica instead of water for mixing with the powder increase the strength of set material.

The cohesive strength of the phosphate investments is such that they do not have to be contained in a metal casting ring. The material is generally allowed to set inside a plastic ring which is removed before heating.

The formation of silicophosphates on heating cause a significant increase in the strength of the material at the casting temperature.

The higher strengths of the phosphate-bonded materials promote them becoming widely used for casting all types of alloys (precious, semi-precious and base-metal).

The wax burn-out temperature is varied to suit the type of alloy being cast. This temperature is normally held for 30 minutes for small moulds and 1 hour for larger moulds before the metal is cast. Burn-out times need to be extended when resin-based pattern materials are used.


Porosity

The gypsum-bonded and phosphate-bonded materials are sufficiently porous to allow escape of air and other gases from the mould during casting.

The silica-bonded materials are so closely packed that they are virtually porosity-free and there is a danger of ‘back pressure’ building up which will cause the mould to be incompletely filled or the castings to be porous. These problems can be overcome by making vents in the investment which prevent the pressure from increasing.


Compensating Expansion

The accuracy of  fit of a casting depends primarily on the ability of the investment material to compensate for the shrinkage of the alloy which occurs on casting.

The magnitude of the shrinkage varies widely but is of the order of 1.4% for most gold alloys, 2.0% for Ni/Cr alloys and 2.3% for Co/Cr alloys.

The compensating expansion is achieved by a combination of:

  • Simple thermal expansion.
  • Expansion caused by silica crystal inversion at elevated temperatures.
  • Setting expansion.
  • Hygroscopic expansion.

Thermal Expansion

The expansion is accomplished by a combination of simple thermal expansion coupled with a crystalline inversion which results in a significant expansion.

Quartz undergoes inversion at a temperature of 575°C from the so-called ‘low’ form or α-quartz to the so-called ‘high’ form or β-quartz.

For cristobalite, conversion from the low to the high form occurs at a lower temperature of around 210°C.

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The expansion is due to a straightening of chemical bonds to form a less dense crystal structure. The change is reversible and both quartz and cristobalite revert back to the low form on cooling.

The overall thermal expansion and inversion expansion of materials containing cristobalite is greater than those containing quartz.

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Hygroscopic Expansion

Hygroscopic expansion can be used to supplement the setting expansion of gypsum-bonded materials. This is also possible for phosphate-bonded materials but is rarely used in practice.

The mechanism of hygroscopic expansion may be envisaged that water is attracted between crystals by capillary action and that the extra separation of particles causes an expansion.

Hygroscopic Expansion Techniques:

A) Water immersion technique:

The investment mould is placed into water at the initial set stage, this can result in an expansion of five times the normal setting expansion.

B) Water added technique:

A measured volume of water is placed on the upper surface of the investment material within the casting ring. This produces a more readily controlled expansion.

Hygroscopic expansion is further encouraged by lining the casting ring with a layer of damp asbestos which is able to feed water to a large surface area of the investment mould.

This technique is routinely employed even when no attempt is made to maximize hygroscopic expansion by immersing in water or adding water.

Gypsum-bonded Investment Material Compensating Expansion

The setting expansion of a typical gypsum-bonded material is of the order of 0.3% which may be increased to around 1.3% by hygroscopic expansion.

The magnitude of the hygroscopic setting expansion which occurs with gypsum bonded investments is greater than that which occurs with gypsum model and die materials.

If hygroscopic expansion has been used to achieve expansion it is likely that the magnitude of the thermal expansion required will be relatively small.

When thermal expansion is used as the primary means of achieving compensation a cristobalite-containing investment mould heated to around 700°C is required.

Three types of gypsum bonded investments can be identified as follows:

  • Type 1 thermal expansion type; for casting inlays and crowns.
  • Type 2 hygroscopic expansion type; for casting inlays and crowns.
  • Type 3 for casting complete and partial dentures.

Silica-bonded Investment Material Compensating Expansion

Silica-bonded investments undergo a slight contraction during setting and the early stages of heating due to loss of water and alcohol from the gel material.

Continued heating causes considerable expansion due to the close packed nature of the silica particles. A maximum linear expansion of approximately 1.6% is reached at a temperature of about 600°C.

The total linear expansion is therefore identical with the linear thermal expansion.

Phosphate-bonded Investment Material Compensating Expansion

For phosphate-bonded materials, the use of colloidal silica solution instead of water for mixing with the powder has the dual effect of increasing the setting expansion and thermal expansion of the material.

A combined setting expansion and thermal expansion of around 2.0% is normal, provided the special silica liquid is used with the investment.

Many manufacturers of phosphate-bonded investments supply instructions which enable the expansion to be varied,

by selecting the most appropriate liquid dilution the investment can be made to compensate for casting shrinkages of both base-metal alloys and gold alloys.

Special liquid : water

Expansion (%)

Neat liquid

1.9-2.1

3 : 1

1.7-1.9

1 : 1

1.5-1.7

1 : 3

1.3-1.5

The expansion reaches a maximum at 700°C and remains the same to 1000°C. The lowest permissible burn-out temperature for any particular alloy normally gives the best results so it is essential to follow the directions given for any particular alloy.

Two types of phosphate-bonded investment can be identified as follows:

  • Type 1 for inlays, crowns and other fixed restorations.
  • Type 2 for partial dentures and other cast, removable restorations.

Consideration of the relatively large casting shrinkages which can occur with some base-metal alloys in comparison with the compensating expansions possible with the investments may suggest that ideal compensation is not always possible.

It should be remembered, however, that further compensation may take place during other stages in the production of the casting. A small contraction of the impression, for example, may give the required compensation.


Applications

Investment Primary use
Dental plaster or stone Mould for acrylic dentures
Gypsum-bonded materials Mould for gold casting alloys
Silica-bonded materials Mould for base metal casting alloys (rarely used)
Phosphate-bonded materials Mould for base metal and gold casting alloys;

mould for cast ceramics and glasses

Refractory die for ceramic build-up

– end –

Gypsum – Plaster & Dental Stone

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Many dental restorations and appliances are constructed outside the patient’s mouth using models and dies which should be accurate replicas of the patient’s hard and soft tissues.

The morphology of the hard and soft tissues is recorded in an impression and models and dies are prepared using materials which are initially fluid and can be poured into the impression, then harden to form a rigid replica.

WHAT IS A MODEL? WHAT IS A DIE?

Model: is a replica of several teeth and their associated soft tissues or, alternatively, to an edentulous arch.

Die: is a replica of a single tooth.

Many materials have been used for producing models and dies but the most popular are the materials based on gypsum products.

Requirements of Model and Die Materials

The model and die materials ideally, should:

  • Dimensional accuracy: the dimensional changes which occur during and after the setting of these model materials should be minimal in order to produce an accurate model or die.
  • Fluid at the time it is poured into the impression so that fine detail can be recorded.
  • Minimize the presence of surface voids on the set model by encouraging surface wetting.
  • Strong to resist accidental fracture.
  • Hard enough to resist abrasion during the carving of a wax pattern.
  • Compatible with all the other materials with which it comes into contact.

Gypsum Products

Gypsum is a naturally occurring, white powdery mineral with the chemical name calcium sulphate dihydrate (CaSO4·2H2O).

Gypsum products used in dentistry are based on calcium sulphate hemihydrate (CaSO4)2·H2O. Their main uses are for casts or models, dies and investments.

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Types of Gypsum Products

The current ISO Standard for Dental Gypsum Products identifies 5 types of material as follows:

Type 1 Dental plaster, impression

Type 2 Dental plaster, model

Type 3 Dental stone, die, model

Type 4 Dental stone, die, high strength, low expansion

Type 5 Dental stone, die, high strength, high expansion

Chemical Composition

Gypsum products used in dentistry are formed by driving off part of the water of crystallization from gypsum to form calcium sulphate hemihydrate.

Gypsum                 →   Gypsum product (plaster or stone)   + water

2CaSO4·2H2O       →   (CaSO4)2·H2O         +  3H2O

Calcium sulphate dihydrate  →   Calcium sulphate hemihydrate  +  water

MANUFACTURE OF PLASTER aka PLASTER OF PARIS  “POP”

Plaster is produced by a process known as calcination. Gypsum is heated to a temperature of about 120̊ C in order to drive off part of the water of crystallization. This produces irregular, porous particles referred to as β-hemihydrate particles.

Overheating the gypsum may cause further loss of water to form calcium sulphate anhydrite (CaSO4), whilst underheating produces a significant concentration of residual dihydrate. The presence of both components has a marked influence upon the setting characteristics of the resultant plaster.

MANUFACTURE OF DENTAL STONE

Dental stones may be produced by one of two methods:

  • Gypsum is heated to about 125̊ C under steam pressure in an autoclave to form α-hemihydrate (more regular and less porous than β-hemihydrate).
  • Gypsum is boiled in a solution of a salt such as CaCl2. This gives a material similar to that produced by autoclaving but with even less porosity. Manufacturers normally add small quantities of a dye to dental stones to differentiate it from dental plaster.

Advantages

Gypsum model and die materials have the advantages of

  • Inexpensive and easy to use.
  • The accuracy and dimensional stability are good
  • They are able to reproduce fine detail from the impression, providing precautions are taken to prevent blow holes.

Disadvantages

  • The mechanical properties are not ideal and the brittle nature of gypsum occasionally leads to fracture , particularly through the teeth, which form the weakest part of any model.
  • Problems occasionally arise when gypsum model and die materials are used in conjunction with alginate impression. The surface of the model may remain relatively soft due to an apparent retarding effect which hydrocolloids have on the setting of gypsum products.

Applications

Stones are normally used when strength, hardness and accuracy are required. These materials are used when any work is to be carried out on the model or die as would be the case when constructing a denture on a model or a cast alloy crown on a die.

The cheaper dental plaster is used when mechanical properties and accuracy are not of primary importance. Thus, plaster is often used for mounting stone models onto articulators and sometimes for preparing study models.


Manipulation and Setting Characteristics

Chemical Reaction

Application of gypsum products in dentistry involves hydration of the Calcium sulphate hemihydrate with water to produce Calcium sulphate dihydrate.

Gypsum product (plaster or stone) + water   →   Gypsum

(CaSO4)⋅H2O        + 3H2O   →   2CaSO4⋅2H2O

Calcium sulphate hemihydrate + water   →  Calcium sulphate dihydrate

Water/Powder Ratio

Plaster and stone powders are mixed with water to produce a workable mix. The table illustrates Water/Powder ratios for gypsum model and die materials.

  Water (ml) Powder (g) W/P ratio (ml/g)
Plaster 50–60 100 0.55
Stone 20–35 100 0.30
Theoretical ratio 18.6 100 0.186

Excess water is absorbed by the porosities of gypsum particles.

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Armamentarium for hand mixing:

  • Clean, scratch free rubber or plastic bowl having a top diameter of about 130 mm.
  • Stiff spatula with a round-edged blade of around 20–25 mm width and 100 mm length.

The presence of gypsum residues in the mixing bowl can noticeably alter the working and setting characteristics of a fresh mix and so the need for cleanliness is emphasized.

Mixing Steps

  1. The requisite amount of water is added to a moist bowl and the powder added slowly to the water over about 10 seconds.
  2. The mix is allowed to soak for about another 20 seconds.
  3. Then mixing/spatulation carried out for around 60 seconds using a circular stirring motion.
  4. The material should be used as soon as possible after mixing since its viscosity increases to the stage where the material is unworkable within a few minutes.
  5. After the material has been mixed and used, the mixing bowl should be thoroughly cleaned before the next mix is performed.

Air Porosity

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Considerable quantities of air may be incorporated during mixing and this may lead to porosity within the set material.

Air porosity may be reduced either by:

  • Vibrating the mix of plaster or stone in order to bring air bubbles to the surface
  • Mixing the material mechanically under vacuum
  • Both

Setting Process

The setting process begins rapidly after mixing.

  1. The water becomes saturated with hemihydrate, which has a solubility of around 0.8% at room temperature.
  2. The dissolved hemihydrate is then rapidly converted to dihydrate which has a much lower solubility of around 0.2%, since the solubility limit of the dihydrate is immediately exceeded it begins to crystallize out of solution.
  3. The process continues until most of the hemihydrate is converted to dihydrate.
  4. The crystals of dihydrate grow from specific sites called nuclei of crystallization. These may be small particles of impurity, such as unconverted gypsum crystals, within the hemihydrate powder.

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Setting Time

Two stages can be identified during setting.

A) Initial setting time

The time at which the material develops the properties of a weak solid and will not flow readily. At this time, it is possible to carve away excess material with a knife.

B) Final setting time

The time taken to reach a stage when the models or dies are strong and hard enough to be worked upon. The term is misleading since it implies that the material has reached its ultimate strength, which is reached several hours later.

Type 1 Type 2 Type 3 Type 4 Type 5
Initial setting time (min) 5 – 10 5 – 20 5 – 20 5 – 20
Final setting time (min) 4 20 20 20 20

Factors Controlling Setting Time

A) Factors controlled by manufacturers

  • The concentration of nucleating agents in the hemihydrate powder: a higher concentration of nucleating agent, produced by ageing or from unconverted calcium sulphate dihydrate, results in more rapid crystallization.
  • Addition of chemical accelerators or retarders to dental stones: Potassium sulphate is an accelerator which act by increasing the solubility of the hemihydrate. Borax is a widely used retarder, the mechanism by which it works is not clear.

B) Factors under the control of the operator

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  • Temperature variation has little effect on the setting time. Increasing the temperature accelerates the dissolution of hemihydrate but retards the crystallization of dihydrate.
  • Increasing the W/P ratio retards setting by decreasing the concentration of crystallization nuclei.
  • Increasing mixing time accelerates setting by breaking up dihydrate crystals during the early stages of setting, producing more nuclei on which crystallization can be initiated.

Exothermic Setting Reaction

The setting reaction is exothermic, the maximum temperature is reached during the stage when final hardening occurs. Temperature rise is negligible at the time of the initial set.

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Temperature–time profile for a gypsum material during setting. Points I and F correspond to the initial set and final set points indicated by indentors

The magnitude of temperature rise depends on the bulk of material used and can reach 30̊ C at the centre of a mass of setting material. This may be maintained for several minutes due to the thermal insulating characteristics of the materials.

This marked rise in temperature can be used to good effect when flasking dentures since it softens the wax of the trial denture and enables it to be easily removed from the mould.

Setting Expansion

A small expansion caused by the outward thrust of growing crystals. The maximum rate of expansion occurs at the time when the temperature is increasing most rapidly. The expansion is only apparent since the set material contains a considerable volume of porosity.

Type 1 Type 2 Type 3 Type 4 Type 5
Setting expansion (%) 0-0.15 0-0.30 0-0.20 0-0.15 0.16-0.30

In order to produce an accurate model or die it is necessary to maintain the setting expansion at as low a value as possible.

Accelerators or retarders which are added by manufacturers to dental stones in order to control the setting time also have the effect of reducing the setting expansion and are sometimes referred to as antiexpansion agents.

Alterations in W/P ratio and mixing time have only a minimal effect on setting expansion.

Hygroscopic Expansion

If the material is placed in water at the initial set stage, considerably more expansion occurs during setting.

This increased expansion is sometimes used to increase the setting expansion of gypsum-bonded investment materials.


Properties of the Set Material

A) Compressive Strength

The strength of gypsum depends on:

  • The porosity of the set material.
  • The time for which the material is allowed to dry out after setting.

The porosity, and hence the strength, is proportional to the W/P ratio. Since stone is always mixed at a lower W/P ratio than plaster it is less porous and consequently much stronger and harder.

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Although a gypsum model or die may appear completely set within a relatively short period its strength increases significantly if it is allowed to stand for a few hours.

The increase in strength is a function of the loss of excess water by evaporation. It is thought that evaporation of water causes a precipitation of any dissolved dihydrate and that this effectively cements together the crystals of gypsum formed during setting.

Type 1 Type 2 Type 3 Type 4 Type 5
Compressive strength 1h (MPA) 6 12 25 40 40
Compressive strength 24h (MPA) 24 70 75 75

B) Flexural Strength

Gypsum is a very brittle material.

Plaster is fragile with very low value of flexural strength. Stone is less fragile but must be treated with care if fracture is to be avoided. It is relatively rigid but has a poor impact strength and is likely to fracture if dropped.

Type 1 Type 2 Type 3 Type 4 Type 5
Flexural strength 24h (MPA) 1 1 15 20 20

C) Dimensional Stability

The dimensional stability of gypsum is good.

Following setting, further changes in dimensions are immeasurable and the materials are sufficiently rigid to resist deformations when work is being carried out upon them.

D) Solubility

Set plaster is slightly soluble in water.

Solubility increases with the temperature of the water and if hot water is poured over the surface of a plaster cast, as happens during the boiling out of a denture mould, a portion of the surface layer becomes dissolved leaving the surface roughened.

Frequent washing of the surface with hot water should therefore be avoided.

E) Detail Reproduction

The ability of dental gypsum products to reproduce surface details of hard or soft tissues either directly or from impressions is central to their suitability as model and die materials.

Types 3, 4, and 5 stones are capable of recording greater fine detail than type 2 plaster material.

Type 1 Type 2 Type 3 Type 4 Type 5
Detail reproduction (µm) 75 75 50 50 50

Dental Waxes

clear glass container with coconut oil
Photo by Dana Tentis on Pexels.com

WHAT IS WAX?

The definition of a wax is a thermoplastic molding material that is solid at room temperature. By implication, heating a wax will convert it to a liquid phase and make it much more easily moldable.

WHAT IS DENTAL WAX?

Dental waxes may be composed of natural and synthetic waxes, gums, fats, fatty acids, oils, natural and synthetic resins, and pigments of various types. The particular working characteristics of each wax are achieved by blending the appropriate natural and synthetic waxes and resins and other additives.

Dental waxes are composed of 3 major components –

  • BASE wax (that is almost paraffin),
  • MODIFIER waxes (to contribute properties such as increased hardness, stickiness, or brittleness), and
  • COLORANTS (which represent only about 1% of the composition in general).

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WHAT ARE DENTAL WAXES USED FOR?

IN CLINICS IN LABORATORIES
Bite registration Boxing techniques
Alterations and adaptation for impression trays Baseplate for complete and partial dentures
Direct waxing for cast restorations Hold components before articulation
_______________ Indirect pattern for casting

 

Types of wax

A) Natural

  1. Mineral
  2. Plant
  3. Animal (insect)

 

B) Synthetic

 


Natural wax

1) Mineral origin

Paraffin waxes are obtained principally from the high-boiling point fractions of petroleum. The presence of oils in the wax, however, lowers the melting temperature; paraffin waxes used in dentistry are refined waxes and have less than 0.5% oil.

Microcrystalline waxes are similar to paraffin waxes, except they are obtained from the heavier oil fractions in the petroleum industry and, as a result, have higher melting points. These waxes are tougher and more flexible than paraffin waxes.

 

2) Plant origin

Carnuba wax and candilia wax are not true waxes; they are chiefly fats.

 

3) Animal (insect) origin

Beeswax is the primary insect wax used in dentistry. It is used to modify the properties of paraffin waxes, and is the main component in sticky wax.


Synthetic wax

Although the use of synthetic waxes and resins is increasing, it is still limited in dental formulations, and the natural waxes continue to be the primary components.

Synthetic waxes are complex organic compounds of varied chemical compositions.

Synthetic waxes include

  1. Polyethylene waxes
  2. Polyoxyethylene glycol waxes
  3. Halogenated hydrocarbon waxes
  4. Hydrogenated waxes
  5. Wax esters from the reaction of fatty alcohols and acids

 


 

Other Subtances Related to Wax

Gums

Many waxes obtained from plants and animals mresemble in appearance a group of substances described as gums. Many plants produce a variety of gums that are viscous, amorphous exudates that harden on exposure to air.

 

Fats

As a class of substances, waxes are harder and have higher melting temperatures than fats, but in some ways they resemble fat. Both are tasteless, odorless, and colorless in the pure form, and they usually feel greasy.

The fat may be used to increase the melting range and hardness of compounded wax. Fats have a pronounced effect on the properties of waxes.

 

Resins

In some respects natural resins resemble waxes in appearance and properties, although they form a distinct classification of substances.

Most natural resins are obtained from trees and plants; shellac, however, is produced by insects. Numerous natural resins are blended with waxes to develop waxes for dental applications.


 

Properties of Waxes

Physical Properties of Waxes are:

  1. Transition temperature
  2. Flow
  3. Thermal expansion
  4. Thermal conductivity

 

1. Transition temperature

The time-temperature cooling curve for a typical dental wax shows the two inflections, the upper inflection indicates the MELTING POINT and the lower inflection the TRANSITION TEMPERATURE.DM_036

  • i) At temperature above the melting point, the crystallites have melted and the wax is fully fluid.
  • ii) At temperature below the transition temperature the wax is rigid and cannot easily be molded.
  • iii) At temperatures between the melting point and the transition temperature, the wax is partly fluid and partly solid. i.e. it is VISCOELASTIC (viscous – flow; elastic – solid)

All molding of wax should, ideally, be carried out above the melting point, but at least above the transition temperature. However adapting wax at too high temperature may result in excessive thermal contraction and dimensional inaccuracy.

 

2. Flow

The flow of wax is the viscous response to an applied stress as, for example, in manually adapting hot wax to a particular shape. Usually, maximum flow is required while the adaptation is being carried out, while minimum flow is desirable when the process is complete.

The main factor which determines the extent of flow for a given wax is temperature. Flow is greatly increase as the melting point of the wax is approached.

 

3. Thermal expansion

Like other materials, waxes expand when subjected to a rise in temperature and contract as the temperature is decreased. This fundamental property may be altered slightly when various waxes are blended.

In general, dental waxes and their components have the largest coefficient of thermal expansion of any material used in restorative dentistry.

 

4. Thermal conductivity

The thermal conductivity of waxes is low which implies that these materials gain, and lose, heat very slowly.


 

Types of Dental Waxes

A) Pattern waxes

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  • Inlay wax
  • Casting wax
  • Baseplate wax

 

B) Processing waxes

  • Boxing wax
  • Utility wax
  • Sticky wax

 

C) Impression waxDM_049DM_050

  • Bite registration wax
  • Horseshoe-shaped wax

PATTERN WAXES

1. Inlay wax

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Inlay wax is used in gold inlays, crown and bridge units. They are formed by a casting process that uses the lost-wax pattern technique.

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Lost-wax Pattern Technique

 

Some inlay waxes are described as hard, regular (medium), or soft, which is a general indication of their flow. The flow can be reduced by adding more carnauba wax or by selecting a higher melting point paraffin wax.

> Composition:

  1. 60% Paraffin Wax = BASE Wax
  2. 25% Carnuba Wax = MODIFIER Wax
  3. 10% Ceresin = MODIFIER Wax
  4. 5% Beeswax = MODIFIER Wax
  5. <1% Colorants = COLORANT

 

Properties & Physical Characteristics:

The accuracy and ultimate usefulness of the resulting gold casting depend largely on the accuracy and fine detail of the wax pattern .

  • Type 1 wax is a soft wax used as an indirect technique wax. It shows greater flow than type 2 wax at temperatures both below and above mouth temperature.
  • Type 2 wax is a harder wax prescribed for forming direct patterns in the mouth, where lower flow values at 37 ̊ C tend to minimize any tendency for distortion of the pattern on its removal from the cavity preparation. Therefore it has lower flow
    and greater ease of carving.

 

2. Casting Wax

Casting wax is used to form the pattern for a partial denture framework in a dental casting alloy such as Cobalt-Chromium.

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Pre-formed moulds are available for the production of wax patterns for clasp arms, meshes etc. in thicknesses suitable for use with Cobalt-Chromium.

 

> Composition:

The composition of these sheet and shaped waxes, include ingredients similar to those found in inlay waxes, with various combinations and proportions of paraffin, ceresin, beeswax, resins, and other waxes being used.

 

> Properties:

Although casting waxes serve the same basic purpose as inlay waxes in the formation of patterns for metallic castings, their physical properties differ slightly.

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The characteristics most desired include a…

  • certain degree of toughness and strength
  • true gauge dimension
  • minimum of dimensional change with change in temperature, and the
  • ability to be vaporized completely from the investment mold.

>Physical Characteristics:

These waxes are available in the form of sheets, usually of 28- and 30-gauge (0.40 and 0.32 mm) thickness, ready-made shapes, and in bulk.

 

3. Baseplate Wax

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A common use of dental modeling wax is for making occlusion rims and for setting up artificial teeth during the fabrication of dentures.

The material is usually colored pink to simulate the natural mucosa so that a more meaningful visual impression of the prosthesis can be obtained.

> Composition:

Baseplate waxes may contain 70% to 80% paraffin-based waxes or commercial ceresin, with small quantities of other waxes, resins, and additives to develop the specific qualities desired in the wax.

 

> Properties:

The melting point is about 58 deg. and the transition temperature is about 50 deg. Thus, all manipulation of modeling wax should be carried out above the latter temperature to minimize stress relief.

There is residual stress within the baseplate wax that holds and surrounds the teeth of a wax denture pattern. This stress results from differential cooling, “pooling” the wax with a hot spatula, and physically manipulating the wax below its most desirable working temperature. The waxed-up denture should be invested as soon as possible to minimize stress-relief.

 

>Physical Characteristics:

Baseplate waxes are normally supplied in sheets 7.60 x 15.00 x 0.13 cm in pink or red. The manufacturer usually formulates two types of wax to accommodate the varying climates in which they will be used, because the flow of the wax is influenced greatly by the temperature.

The hard type is suitable for use in warm climates but tends to crack and flake at low temperatures. The medium type is suitable for use at low temperatures but flows excessively at high temperatures.


 

PROCESSING WAXES

1. Sticky Wax

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A suitable sticky wax for prosthetic dentistry is formulated from a mixture of beeswax, paraffin waxes and resins or other additive ingredients.

Such a material is sticky when melted and adheres closely to the surfaces on which it is applied. However, at room temperature the wax is firm, free from tackiness, and brittle.

Uses: It holds broken pieces of a denture together and assembles components of fixed partial dentures and wrought partial dentures in preparation for soldering.

Sticky wax should fracture rather than flow if it is deformed during soldering or repair procedures so that any loss of alignment is immediately obvious. Although this wax is used to assemble metallic or resin pieces in a fixed temporary position, it is primarily used on dental stones and plasters.

 

2. Boxing Wax

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Boxing wax is used to form a box around impressions of the mouth when making a cast (model). The boxing limits the flow of either plaster of Paris or stone gypsum material.

>Physical Characteristics:

Boxing wax is usually issued in red strips measuring 1 1/2 inches wide, 12 inches long, and 1/8 inch thick. Boxing wax is soft enough at room temperature to be formed into a desired shape without heating.

 

3. Utility Wax

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Utility wax is used to provide rim locks and otherwise to adapt impression trays for individual impressions, to build up post-dam areas on impressions, and to form a bead or border on preliminary and final impressions.


 

IMPRESSION WAX

Impression wax is a wax that is especially compounded so that when subjected to controlled pressure it will flow to some extent in the mouth.

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Dental Material – Biological Properties

red and white mouth plastic toy and food plastic toys

WHAT ARE THE BIOLOGICAL PROPERTIES OF DENTAL MATERIAL?

It is a primary requirement of any dental material that it should be harmless to the patient and to those involved in its manufacture and handling. Ideally, a material placed into a patient’s mouth should be:

Non-toxic

Non-irritant

No carcinogenic or allergic potential

 


Biocompatibility

It is defined as the ability of a material to elicit an appropriate biological response in a given application in the body.

Whether a material is biocompatible is therefore dependent on what physical function we ask of the material and what biological response we require from it.

 


 

ADVERSE EFFECTS FROM DENTAL MATERIAL

A) Toxicity

  • Materials may be capable of releasing substances into a patient’s body that can cause overt toxicity
  • Of all the biological responses to materials, the first screening test used for almost all materials is a toxicity test.

 

B) Inflammation

  • The contribution of dental materials to inflammatory reactions is important because pulpal and periodontal diseases are largely chronic inflammatory responses to long-term infections.

 

C) Allergy

  • Some materials, such as latex and nickel, cause allergy directly by activating antibodies to the material.
  • Non-allergic inflammatory response = be DOSE-DEPENDENT and reactions are proportional to the amount of the substance.
  • Allergic response = individual’s immune system recognizes a substance as foreign, thus not all individuals will react to that substance = DOSE-INDEPENDENT

 

D) Mutagenicity

  • Mutagenic reactions results when the components of a material alter the base-pair sequences of the DNA in the cell. These alterations are termed mutations.
  • Mutagenicity does not imply carcinogenicity, no dental material has been shown to be carcinogenic.



 

Allergy Exposures

i) Latex

Latex material is used when dental personnel wear gloves to reduce the risk of disease transmission and during the use of rubber dam.

Latex hypersensitivity is an increasing problem and the reactions vary from localized rashes and swelling to more serious anaphylaxis. Dermatitis of hands (eczema) is the most common adverse reaction.

 

ii) Beryllium and Nickel

Precautions should be taken to avoid exposure to metallic vapor, dust, or grindings containing beryllium and nickel.

Physiological responses may range from contact dermatitis to severe chemical pneumonitis. Therefore efficient local exhaust and filtration systems should be used when casting, finishing, and polishing these beryllium-containing alloys.

The presence of nickel is of greater importance because it is a known allergen. The incidence of allergic sensitivity to nickel has been reported to be from 5 to 10 times higher for females than for males, with 5% to 8% of females showing sensitivity.

However, no correlation has been found between the presence of intraoral nickel-based restorations and sensitivity.

 


 

 

Two key factors that determine a material’s biocompatibility

 

A) Biological environment in contact with the material

The key point is that the biocompatibility of the material depends to a large degree on the degradation process. It is determined not only by a material’s composition but also by the biological environment in contact with the material.

 

B) Surface characteristics

The surface may negatively affect the biological response. A rough surface promotes corrosion. If the corrosion products have adverse effects, then roughness is not desirable.

Roughness may also promote the adherence of bacteria and promote periodontal inflammation or decay in teeth.

 

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Dental Material – Physical Properties

photo of woman looking at the mirror

WHAT ARE THE PHYSICAL PROPERTIES OF DENTAL MATERIAL?

The physical properties that will be discussed are

  1. Optical properties
  2. Thermal properties
  3. Electrical properties
  4. Other physical properties

 


Optical Properties

About colour and light…

COLOUR

The perception of the Colour of an object is the result of a physiological response to a physical stimulus (light), either a reflected or a transmitted beam of white light or a portion of that beam.

LIGHT

It is an electromagnetic radiation that can be detected by the human eye. It can be seen that the visible electromagnetic radiation is in the range from 400-700 nanometers.

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A) Color parameters

i) Hue: It is the dominant wavelength. It represents the color of the material, i.e green, red and blue.

ii) Chroma: or excitation purity. It represents the strength of the color or degree of saturation of the color (color intensity).

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iii) Value: or luminous reflectance. It represents the lightness or darkness of color. BLACK = 0 ; WHITE = 100

 

B) Properties of materials in relation to light transmission and absorption

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i) Opacity: is a property of the material that prevents the passage of light. Opaque material = absorbs all light = black

ii) Translucency: is a property of the material, which allows the passage of some light but disperses the light. So objects cannot be clearly seen through them. E.g. Ceramics, resin composites, denture plastics

iii) Transparency: is a property of a material, that allows the passage of light in such a manner that little distortion takes place. So that objects can be clearly seen through them. E.g. Glass, pure acrylic resin
C) Factors affecting color appearance and selection.

i) Surface finish and thickness: This determines the relative amount of light reflected from the surface. rough surface appears grayer than a smooth surface ; thickness of a restoration can affect its appearance. Opacity increases as the thickness increases.

ii) Metamerism: It is the change of color matching of two objects under different light sources. if possible, color matching should be done under two or more different light sources.

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iii) Fluorescene: It is the emission of light by a material when a beam of light is shone on it. It makes the teeth bright and vital, as it increases the brightness. *Anterior teeth restoration*

 


 

Thermal Properties

A) Thermal Conductivity

  • It is the amount of heat in (calories or joules) per second passing through a body 1cm thick, with a cross section 1cm2  when the temperature difference is 1°C.

 

Clinical importance in Dentistry: * Metallic filling materials.

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  • Gold or amalgam filling or crown in proximity to the pulp may cause the patient considerable discomfort when hot or cold foods produce temperature changes, cements are relatively poor conductors and insulate the pulp area.

 

B) Thermal Diffusivity

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  • It is a measure of transient heat-flow. Denture-base material should have a high value of thermal diffusivity in order that the patient keep a satisfactory response to hot and cold stimuli in the mouth.

 

WHAT IS THE COEFFICIENT OF THERMAL EXPANSION?

The change in length per unit length of the material for a 1°C change in temperature is called the linear coefficient of thermal expansion(α).
Clinical importance in Dentistry:

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  • Close matching of the coefficient of thermal expansion (α) is important between: the tooth and the restorative materials to prevent marginal leakage.
    This can lead to: Recurrent caries. Discoloration. Hypersensitivity.
  • Such expansions and contractions may break the marginal seal of the filling in the tooth, particularly if the difference between the coefficient of expansion of the tooth and the restorative material is great.
  • For the filling material, the most ideal combinations of properties would be a low value of thermal diffusivity combined with a coefficient of thermal expansion value similar to that for tooth substance.

 


 

Electrical Properties

A) Electrical Conductivity and Resistivity

  • The ability of a material to conduct an electric current may be stated either as specific conductance or conductivity, or, conversely, as the specific resistance or resistivity.
  • Resistivity is important in the investigation of the pain perception threshold resulting from applied electrical stimuli and of displacement of dentinal fluid in teeth caused by ionic movements.
  • The electrical resistance of normal and carious teeth has been observed to differ, with less resistance offered by the carious tissue. Sound enamel is a relatively poor conductor of electricity, whereas dentin is somewhat better.

 

B) Galvanism

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  • The presence of metallic restorations in the mouth, results from a difference in potential between dissimilar fillings in opposing or adjacent teeth.
  • When two opposing fillings contact each other, the cell is short-circuited, and if the flow of current occurs through the pulp, the patient experiences pain.
  • The galvanic currents developed from the contact of two metallic restorations depend on their composition and surface area.

 

C) Electrochemical Corrosion

  • It is also referred to as wet corrosion, since it requires the presence of water or some other fluid electrolyte.
  • Metals undergo chemical or electrochemical reactions with the environment resulting in dissolution or formation of chemical compounds.
  • In most cases corrosion is undesirable. However, in dental practice, a limited amount of corrosion around the margins of dental amalgam restorations may be beneficial, since the corrosion products tend to seal the marginal gap and inhibit the ingress of oral fluids and bacteria.

 


 

Other Physical Properties

A) Tarnish and discolouration – is a surface discoloration on a metal , or a slight loss or alteration of the surface finish or luster.

B) Water sorption – represents the amount of water adsorbed on the surface and absorbed into the body of the material during fabrication or while the restoration

C) Abrasion resistance – is the ability of a material to withstand mechanical action that tends progressively to remove material from its surface.

D) Creep & Flow – Creep is the time-dependant plastic strain of a material under a static load or constant stress. Flow is a measure of material’s potential to deform under small elastic load.

E) Dimensional stability

  1. Syneresis: It refers to a loss of water by evaporation due to exposure to the air. The result is shrinkage in dimension.
  2. Imbibition: It may occur when a substance takes on additional water. The result is  swelling in the material.
  3.  Viscosity: The ability to flow. Thick or viscous liquids flow poorly, whereas thin liquids flow easily. gypsum product (plaster).DM_034
  4. Adhesion: It is the interaction between two materials at an interface where they are in contact.
What is the difference between adhesive and cohesive?

 

F) Mechanism of bonding

DM_035

  1. Physical bonding: The forces involved may be primary (ionic and covalent) or secondary (hydrogen bonds, dipole interaction, or van der Waals) forces.
  2. Chemical bonding: chemical bonding to the inorganic component (hydroxyapatite) or organic components (mainly Type I collagen) of tooth structure.
  3. Mechanical bonding: Mechanical interlocking of the adhesive with irregularities in the surface of the adherent.

 

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