1. Dental impression materials
Libyan International Medical University

2. Introduction, types, classification
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3. What is a dental impression?
It’s a negative likeness of the teeth and/or other tissues of the oral cavity.
The impression materials must be in a plastic or fluid status which become hard or set while in contact with oral tissues.
Impression tray is a device used to carry and control impression material while making an impression.
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4. Ideal requirements of impression materials
Biocompatible, non toxic and non irritant.
Should have acceptable taste, colour and smell.
Adequate shelf life for storage.
Easy to manipulate.
Suitable working and setting time.
Should be elastic to allow removal from undercuts without permanent deformation.
7. Adequate strength so that will not tear or break on removal from the mouth.
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5. 8. Should give accurate reproduction of tissue details.
9. Compatible with cast or die materials.
10. Adhere to the impression tray.
11. Should have good flow.
12. Dimensionally stable.
13. Can be disinfected without any changes of properties.
14. Economic or not expensive.
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7. Classification of impression materials
Non-elastic
imp. Compound, imp. Plaster, zinc oxide eugenol and imp. Wax.
Must not have undercuts
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8. 2. Elastic……can be used in undercut Hydrocolloids
Reversible (agar)……sol.↔ gel (temp)
Irreversible (alginate)…..sol.→ gel
b. Elastomers
Polysulfide (PS) set by polymerization
Silicon rubber (AS and CS)
Polyethers (PE)
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9. Impression techniques
Full dentures
Partial dentures
Fixed prosthodontics
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11. What's important? Patient Neutral taste and odour Short setting time
Small tray
Easily removed
No retakes
Non-toxic
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12. What's important? Dentists Easily mixed Short working time
Good quality impression
Low cost
Easily disinfected
Simple procedure
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13. Ease of mixing Low viscosity less viscous is easier
silicones are easiest to mix followed by polyether and polysulphides
Automatic mixing
reduces mixing time and bubbles
use correct mixing tip
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14. Auto-mixer
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15. Working and setting time
Dentist's time requirements must be balanced with patient's comfort.
A total working and setting time of 4 min is adequate for most crown and bridge procedures.
Using auto-mixing device for direct injection reduce working time to one minute.
A fast setting material can be removed from the mouth in about 3 min.
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16. Working and setting time
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17. Easy of removal Flexible. depends on the elastic modulus
Minimal distortion.
depends on degree of permanent deformation.
Sufficient tear strength.
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18. Flexibility of Impression materials
Easy
Moderate
Hard
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19. Resistance to Tearing
High
Medium
Low
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20. Accurate reproduction of surface details
Viscosity
High- muco-compressive
Low- muco-static
Wettability…..ability of liquid to cover the surface of the substrate completely
Hydrophobic
Hydrophilic
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21. Why do we want good wetting?
Fewer voids
Less entrapment of oral fluids
Bubble-free dies and models
Fewer retakes
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22. Wettability of Impression materials
Excellent
Good
Fair
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23. Wettability of Impression materials
Displace water and other oral fluids from the surfaces in the mouth so as to prevent voids in the impression and consequent bubbles in the casts.
Improve adaptation of gypsum so as to prevent voids in the casts.
Disinfection procedure!
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24. Dimensional accuracy and stability
Choice and use of tray
Setting shrinkage
Permanent set
Storage stability
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25. Choice and use of trays Avoid flexible trays Use perforated trays
Make sure the trays is clean
Apply only a thin layer of adhesive
Wait appropriate time before taking impression
Do not mix adhesives and impression materials
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26. Dimensional change of impressions
High
Medium
Low
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27. Oversized impression space results in an oversized die
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28. a. Elastic material, b. plastic material, c. viscoelastic material
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29. a. Impression in place before removal, b
a. Impression in place before removal, b. during removal- the impression is subjected to both compressive and tensile stresses.
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30. View of a material being used to record an impression of a crown with subgingival shoulders. The thin areas of impression material, which are most prone to fracture are arrowed.
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31. Imp wax
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32. Impression plaster
Rarely used these days as they are brittle and fractures very easily.
Mucostatic impression of edentulous ridges
High W/P ratio
Can be combined with impression compound
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33. Impression plaster advantages Easy to mix Working time 2-3 min
Setting time 2-3 min
Low viscosity-mucostatic
Good dimensional stability and accuracy
Cheap
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34. Poor abrasion resistance Rigid once set Dry sensation in the mouth
Disadvantages
Low strength
Rough surface finish
Poor abrasion resistance
Rigid once set
Dry sensation in the mouth
We need separator
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35. Impression compound Sheet primary impression of edentulous mouth.
close fitting special tray used with zinc oxide eugenol wash.
Stick
single crown impression (copper ring technique).
Border molding.
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37. Composition
Thermoplastic materials, natural or synthetic resin and waxes….47% characterizes the softening temp.
Fillers…..e.g. Talc or shalk 50%..gives body increasing viscosity of the softened materials, reduce thermal contraction.
Lubricant or acids 3% e.g. stearic acid to improve flow properties (plasticizer).
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38. Properties Softening temperature 55-60ºC….↑ flow by Temp.
High viscosity-mucocompressive…37ºC solid.
High coefficient of thermal expansion….
Do not over heat because it loose oils and altered characteristics
Rigid….↑ with rigid tray
Poor thermal conductivity….take time during heating or cooling.
Poured as soon as possible
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39. Contraction of imp. Compound from oral temp. to room temp. is 0.3%
Since pressure is applied during the formation of the impression, the residual stress occurs in the material storage in warm environments or extended storage results in warpage.
Contraction of imp. Compound from oral temp. to room temp. is 0.3%
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40. Zinc oxide eugenol Mucostatic impression of edentulous ridge.
Initial ST 3-6 min.
Final ST 10 min.
Two pastes, different colors, equal length, different size opening….
Not elastic.
Temp. and moisture ↓ ST….accelerate reaction…drop of water accelerate setting.
Stick to dry surfaces.
Eugenol irritant, cause burning sensation of lips.
Pour any time.
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41. Protect unsupported edges. Put Vaseline on patient lips.
No separator needs.
Protect unsupported edges.
Put Vaseline on patient lips.
If patient irritant to eugenol use rubbers imp. or eugenol free (phenols) zinc oxide impression.
Mucostatic
Hydrophilic….accurate reproduction of surface details
Dimensionally stable…. 0.1%
Compatible with dental stone….separated by softening the set paste in water at 60ºC.
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42. Normally used with close-fitting special tray.
Eugenol react with zinc-oxide to form salt of zinc eugenolate and unreacted zinc oxide.
Normally used with close-fitting special tray.
The thickness of paste used is normally around 1 mm.
This thin section ↓ the dimensional changes.
Low viscosity of the mixed paste gives accurate recorded details.
Defect sometimes arise on the surface of the impression but can be corrected by imp. Wax.
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43. The major restriction is their lack of elasticity.
The set material easily distort or fracture when removed from undercut.
Available in two types identified as hard set and soft set.
The mixed materials has adequate adhesion to acrylic or tray material, therefore adhesives do not need.
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44. Hydrocolloids Hydro=water kola=glue Oid=like
A solution is a homogenous mixture consisting of single phase.
A suspension is a mixture of two phases
A colloid is a heterogeneous mixture of two phases, where the two phases are not easily differentiated.
Agar and alginate impression materials.
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45. Reversible hydrocolloids …..agar Gel → Sol → Gel
Irreversible hydrocolloids…..alginate
Sol → Gel
heating
cooling
Chemical reaction
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46. Agar Sea weed Uses…..crown/bridge imp, cast duplicate-lab.
Composition; Agar 15%, Borax .2%, sodium sulphate 1%, anti fungal and water 85%
Good storage
Strengthens gel
Affect gypsum setting against? Why!!!
Accurate details
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47. Largely replaced by alginate and rubber impression materials.
The preference of alginate as result of minimum equipment required and rubber imp due to greater dimensional stability, high quality gypsum model and option of obtain metal or epoxy die.
It supplies as a gel in tooth paste like tube or syringe.
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48. This consists of three rooms each contain water.
Manipulation:
Agar is normally conditioned before use, using a specially designed conditioned bath.
This consists of three rooms each contain water.
The tube or syringe of gel is first placed in the 100ºC bath. This rapidly convert gel to sol and content of tube become very fluid (10 min).
The tube then transferred to the 65ºC bath where it is stored until required for use.
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49. A few minutes before the impression is recorded, the contents are cooled to 45ºC, prevent burn and pulp death.
If the material is maintained in 45ºC for long, it slowly convert to gel.
The conversion from sol to gel takes place slowly (many minutes) at mouth temp and it could be accelerator by cooled water spray onto the imp tray or by using water-cooled imp tray.
Agar is very sensitive to water uptake and loss (syneresis and imbibition).
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50. Water bath used for conditioned agar imp material
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51. After recording an imp it should be poured within 30 minutes.
Short storage, wet tissue in sealed bag.
Its possible to re-use agar, however, concerns a about cross infection, alteration of material properties and incorporation of small gypsum chips.
Expensive instruments.
hydrophilic
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52. Alginate
Widely used? Diagnostic casts, primary impression for CD, PD, opposing jaw for C&B, repairs of prosthesis, provisional restorations, flouride and bleaching trays, night guards, sport protector.
Not fine details and low flow.
Not C&B, inlays, onlays…..
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53. Advantages of alginate over other impression materials
Ease of mixing and manipulation
Minimum equipment
Flexibility of the set impression material
Accuracy if properly handled
Cost effectiveness
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54. Disadvantages
They restrict the choice of model and die materials…we can not make a metal die which have higher abrasion resistance.
They do not give fine details to gypsum die as Agar or rubber impression do.
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55. Tri Na phosphate retarder Glycols for dustless alg
P sulphate
Tri Na phosphate retarder
Glycols for dustless alg
Colour, disinfectants, flavour, filler
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56. Composition Sodium or potassium salt……..to dissolve in water.
Calcium sulfate……to react with dissolved alginate to form insoluble calcium alginate.
Sodium phosphate……react with calcium sulfate and act as retarder
Silicate powder……to control the consistency of mix and flexibility of the impression.
Potassium sulfate or potassium fluoride….to counteract the inhibiting effect of alginate on gypsum model or die.
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57. Chlorhexidine…..to provide self disinfection.
Organic Glycol…..to coat the powder particles to minimize dust during dispensing.
Chlorhexidine…..to provide self disinfection.
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58. Manipulation
When water and powder are mixed, a smooth plastic mass is formed which become an irreversible gel a few minutes after mixing.
Sodium alginate+CaSO4+H2O → Calcium alginate+Na+SO4+H2O.
The manufacturer controls the setting time by amount of sodium phosphate in the powder.
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59. Manipulation Powder and water-dispensing cup. Rubber mixing bowl
Spatula with a reasonably wide and flexible blade
Proper size of tray should be selected before the mixing is begun
Perforated or rim locks tray
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60. Properties
Mixing and setting times…….smooth, creamy consistency and free of graininess in less than 1 min.
Normal set usually set in 2 minutes that is about 4 minutes after the start of initial mix.
There are normal and fast set alginate
Increase water Temp will increase the speed of reaction and decrease the WT and ST.
Proportion of powder and water also affects the setting time
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61. 2. Permanent deformation
Most of commercially available alginate have about 1.5% permanent deformation.
Alginate is flexible but not perfectly elastic.
The amount of plastic deformation increased when the time that it is held under compression is increased and W/P ratio is increased.
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62. 3. Strength
The strength of alginate in compression and tearing are important, the tear strength is more critical.
Higher rates of removal (deformation) resulting in higher compressive and tear strengths.
The strength of alginate ↑ when mix is thick than thin.
The tear and compressive strength increased if the time of removal is delayed.
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63. 4. Dimensional change
Set alginate contain large quantities of water. It loses water and shrinks if stored in air and absorbs water and expands if stored in water.
Storage in 100% humidity results in the least dimensional changes (maximum one hour).
Syneresis and imbibition.
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64. 5. Disinfection
Sodium hypochlorite (10 min), iodophor (10 min), glutaraldehyde and phenylphenol (20 min) solutions has been used and some manufacturers have added disinfectant to the alginate powder.
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65. This technique elements the use of water cooled impression trays!
After removing the impression from pt mouth should be rinsed with water to remove ay blood and saliva……why?
Agar-alginate impression…..agar is injected around the prepared tooth or teeth, then tray contains alginate mix is placed over agar before it get jelly.
This technique elements the use of water cooled impression trays!
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66. Alginate Dust free powder Cheap Limited shelf life
Well controlled working and setting time
Mucostatic
Hydrophilic
Poor surface reproduction
Poor storage stability…Imbibition and syneresis
Low tear strength
Permanent deformation
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67. Elastomeric impression materials
Polysulphides
Polyethers
Silicones
condensation cured
addition cured
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68. Polysulfide rubber impression material
The first rubber based impression material was a polysulfide.
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69. Classification and Uses:
•. Supplied as a two paste system one tube labeled Catalyst or Accelerator and the other marked Base .
•. They are classified into three as:- Light, Regular, or Heavy-bodied depending on their viscosity and how easily they flow under load.
.•. The light bodied class is used as a syringe material in combination with a tray material, and the regular material is used alone. The light material is also used alone in case of a custom-made tray (special tray) for the preparation of dentures.
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70. Composition
Base:
•. 80% low-molecular weight organic polymer, containing reactive Mercaptan (-SH) groups, and 20% reinforcing agents, such as Titanium Dioxide, Zinc Sulfate, Copper Carbonate, or Silica.
Accelerator/Catalyst:
•. This tube contains a compound that causes the Mercaptan groups to react to form a Poly sulfide Rubber.
An inert oil such as Dibutyl or Dioctyl Phthalate carries the catalyst.
• The catalyst is Lead Dioxide, with or without Manganese Dioxide; using it results in the paste, being dark brown to gray.
Another catalyst system is Copper hydroxide and when mixed with the white base paste a blue green color results.
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71. Setting Reaction
The overall setting reaction involves the Oxidation of the Mercaptan groups from two different molecules in the presence of the catalyst that joins the molecules together, forming Disulphide bonds.
Mercaptan + Lead Oxide Polysulfide + H20
The reaction is very sensitive to moisture and temperature and increase in either of these would lead to an acceleration in the setting reaction
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72. Properties of clinical interest are:
Toxicity
Color of the base and accelerator
Time required for mixing
Working time
Consistency
Permanent deformation during removal
Dimensional stability
Flow after setting
Flexibility
Reproduction of detail
Compatibility with die and model materials
Deterioration during storage of unmixed material
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73. Although most accelerator pastes contains Lead Dioxide, proper use does not result in any toxicity.
Manufacturer supplies the base paste as white in color and the other as in a contrasting color. When properly mixed the mix is free of steaks and this can be done in about 45secs or no less than I min should be required.
In general, the working time for poly sulfide rubber impression materials decreases as the consistency becomes stiffer (from light -to heavy -bodied). The working time is an indication of the maximum time allowed before the impression material should be in the mouth, with typical values being 5 to 7 mins. The final setting time is usually in the range of 8 to 12 minutes form the start of mixing.
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74. The permanent deformation values, indicate that polysulfides are not perfectly elastic and that compression during removal of the impressions should be kept to a minimum. The permanent deformation of polysulfide rubber is slightly greater than that for Hydrocolloid Impression materials.
Polysulfide rubber impression materials shrink 0.3% to.4% during the first 24hrs and thus models and dies should be prepared promptly. It is shown that the accuracy is slightly increased if the models and dies are made in less than 30mins.
The flexibility of polysulfides varies considerably with the heavy bodied materials having lower values.
Polysulfide rubber provides excellent reproduction of surface detail and is readily capable of reproducing fine lines 0.025mm wide.
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75. The polysuifides are highly compatible with model plaster and high strength stones~ an impressions are easily metallized by electroforming (electroplating). Silver plating is easier than copper plating~ although their use requires special care because of their poisonous nature.
The polysulfide materials have excellent shelf life but should not be stored in a warm location but is best if refrigerated.
The resistance to tearing is about eight times that reported for Hydrocolloid impression materials.
The strength of permanent deformation increases with time and a few extra minutes in the mouth will considerably increase the characteristics. Also the time before the preparation of dies should be balanced as the continuation chemical reaction causes shrinkage.
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76. Manipulation: Two-Paste Type:
Equal lengths of the base and accelerator are extruded onto a paper pad. The accelerator should not be extruded onto the base as this may result in a non-homogenous mix. A tapered stiff bladed spatula is recommended.
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77. If the material is a light-bodied, it is loaded into a syringe before injecting into the cavity preparation.
If the material is a regular or heavy-bodied material, it is placed in a tray. Custom made acrylic trays may be constructed with a space of 2mm for the impression material and a rubber cement adhesives should be painted. The adhesive should be allowed to evaporate as otherwise the Polysulfide material may separate from the tray during removal from the mouth. Sometimes holes can be drilled through the tray to provide mechanical retention.
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78. A variety of techniques are used to make impressions.
In case of light bodied materials the light bodied material is injected into the preparation and then the heavy bodied material is placed in the tray and both the material sets together.
The same impression may also be accomplished with a single mix of regular-bodied material.
When the material is set it should be removed with a steady force. If tearing of the impression occurs the second time the impression is left in the mouth for a longer time to obtain higher tear strength.
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79. After the removal of the impression it is checked for completeness and detail, thoroughly cleansed with water, disinfected and cleansed with water again.
The excess water is shaken off and moisture removed by a gentle stream of air.
The next step is the preparation of the die or model and should not be delayed.
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80. Silicone Rubber Impression Materials
Disadvantages of Polysulfides as Compared to Silicones:
Objectionable odor.
Staining of uniforms by Lead Dioxide.
Amount of effort required to mix the base with the
accelerator.
Long setting times.
Moderately high shrinkage on setting.
Fairly high permanent deformation.
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81. Composition Classification: There are two types:
Condensation Silicones
Addition Silicones
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82. Condensation Silicones:
Supplied as a base and Accelerator/Catalyst.
The base is a paste containing a moderately low molecular weight Silicone liquid called a Dimethylsiloxane, which has reactive -OH groups.
Reinforcing agents such as Silica are added to give proper consistency to the paste and stiffuess to the set rubber.
The accelerator is usually supplied as a liquid but may be provided as a paste by the use of thickening agents.
The accelerator consists of a Tin Organic Ester suspension.
The silicone pastes are supplied in light, regular, and heavy-bodied consistency as well as a very heavy consistency called a Putty.
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83. Ethyl Alcohol….by product of setting reaction.
The consistency is controlled by the selection of the molecular weight of the Dimethylsiloxane and the concentration of the reinforcing agent. Higher molecular weights are used with the heavier bodied materials.
The concentration of reinforcing agents increases from 35% for light-bodied consistency to 75% for the putty consistency.
Ethyl Alcohol….by product of setting reaction.
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84. Setting Reaction:
The ethyl alcohol produced as a by-product in the reaction gradually evaporates and contributes to the high shrinkage during he first 24hrs after setting.
The incorporation of large amounts of filler in the putty consistency results in a reduction of the dimensional change.
The setting reaction is sensitive to moisture and heat, with increases in either resulting in shorter setting and working times.
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85. Addition Type Silicones:
The material is supplied as a two-paste system or a two­putty system, with one containing a low molecular weight Silicone having terminal Vinyl Groups, Reinforcing filler and the other containing a low molecular weight Silicone having Silane Hydrogens and reinforcing filler.
The two are mixed in equal lengths of paste or equal quantities of putty, and the addition reaction occurs between the vinyl and hydrogen groups with no by product being formed.
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86. No volatile by-product is formed in this reaction, and minimal dimensional changes occur during polymerization.
Increases in temperature increase the rate of reaction and shorten the setting time.
If the hydroxyl groups are present in the addition silicone, a side reaction occurs that results in the formation of hydrogen.
The hydrogen is gradually released from the set impression material and produces bubbles in gypsum dies prepared less than 1 hour or epoxy dies less than 24hrs, after the impression is made.
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87. Note
Some products permit the immediate pouring of dies by controlling the presence of Hydroxyl Groups or by the inclusion in the impression material a hydrogen absorber such as Palladium. Thus, the manufacturer's directions for the time of preparation of die from a particular Silicone should be followed.
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88. ~ It has moderate working time (6-8min).
Properties:
Condensation types:
~ It has moderate working time (6-8min).
~ They have pleasant odor, a moderate high tear strength and excellent recovery from deformation; the permanent deformation is lower than that of polysulphide.
~ Some of the materials are hydrophobic (makes cast formation a problem).
~ Ideally it should be poured within one hour, most of the dimensional changes occur during this period.
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89. Reproduce fine details. Non-toxic, allergic reactions have been noted.
Setting reaction is sensitive to moisture and heat.
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90. Addition type: The most accurate of the elastic impression materials.
They have less polymerization shrinkage, low distortion, fast recovery from deformation and a moderately high tear strength.
It has a working time 3-5 min.
They are available in both hydrophilic (adding surfactant) & hydrophobic forms; have no smell or taste .
It can be poured up to one week after impression making with acceptable clinical results.
Increases in temperature increase the rate of reaction and shorten the setting time .
The putty consistency should not be mixed with latex rubber gloves.
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91. Polyether Rubber Impression Materials
The polyether impression material is the stiffest of the impression of the elastomers when set.
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92. Composition:
Polyether rubber impression materials are supplied as a base & catalyst system. The base is a moderately low molecular weight polyether containing ethylene amine terminal groups.
The liquid polymers are mixed with fillers to provide rigidity. Oils are added as Plasticizers to enhance the mixing and handling characteristics of the material. The set materials also are distributing as two- paste system.
The catalyst paste contains an aromatic Sulfonic acid that enhances the further polymerization & cross- linking of the polymer chains during the chemical reaction. This reaction is responsible for transforming the paste into a stiff polyether rubber.
Some clinicians suggest using this material for impression of a few teeth or a quadrant, because of its tremendous stiffness.
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93. Properties
Working time is short enough that the viscosity noticeably increases during the mixing of the material.
The dimensional changes of polyether is lower than that of any other rubber impression material except the addition silicones, but if it stored in contact with water it absorbs water & changes dimension .
The permanent deformation is less than that of Polysulphide.
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94. Hydrophilic (good wettability for easy cast forming) .
The aromatic Sulfonic acid catalyst can cause skin irritation & direct contact with the catalyst should be avoided.
Hydrophilic (good wettability for easy cast forming) .
Good surface detail, it has low tear strength.
Some have unpleasant taste.
It should be poured within 2 hours .
High stiffness is indicated by low flexibility.
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95. Disinfection of rubber impressions
A variety of disinfectants were used:
Neutral Glutaraldehyde.
Acidified Glutaraldehyde.
Phenol
Iodophor
Chlorine Dioxide.
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96. Rubber Materials For Bite Registration
In the past it was done using bite registration Waxes.
The following Shortcoming were associated with the use of Bite registration Waxes:
Distortion on Removal.
Release of Internal Stress on Storage.
High Flow Properties.
Large dimensional changes from mouth to room temperature.
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97. Recently, polyether and addition Silicone rubbers have been used to register the bite. Based on their properties (Short working time, low strain in compression, low dimensional changes) the rubber bite registration materials are superior to waxes
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98. Failure with impression
Poor reproduction of surface details
- rough or uneven surface, due to uncompleted set, premature removal, unproper mixing, contamination.
- air bubbles, due to rapid set, unproper mixing, surface contamination.
- irregular shaped voids, due to surface contamination and premature movement.
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99. 2. Poor fit
- distortion… due to adhesive failure between tray and imp material, tray not sufficiently rigid, excessive seating pressure, movement of tray during setting.
- casting too big or small….inappropriate impression technique, imp poured too late or to early (insufficient time for elastic recovery), impression stored under wrong conditions.
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100. Choice and use of tray Avoid flexible trays Use perforated trays
Make sure the tray is clean
Apply only a thin layer of adhesive
Wait appropriate time before taking impression
Do not mix adhesives and impression materials
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101. Bad impression Extra chair-side time Repeat visit for patient
Patient distress
Waste of expensive materials
Remake bill from laboratory
Spoil relationship with laboratory
It was considered impossible to produce satisfactory restorations in up to 13% of the impressions.
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104. Shrinkage of imp material towards the tray resulting in the production of a model which is slightly larger than original
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105. We finished Biomaterial course!
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