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Impression materials


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impression materials in dentistry specially those used in Prosthodontics.
Impression compound
Zinc oxide Eugenol impression paste
Elastomeric impression compounds like Polysulfides, Condensation silicones, Addition silicones(PVS), Polyether
detailed description with properties, mixing time, working time, setting time ,physical and properties etc

Published in: Health & Medicine
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Impression materials

  1. 1. IMPRESSION MATERIALS Presented by- Dr Sakshi II Yr PG Dptmnt of Prosthodontics 1
  2. 2. WHAT IS AN IMPRESSION??? • negative replica of the hard and soft oral tissues • Register or reproduce the form and relationship of the teeth and oral tissues 2
  3. 3. Purpose of Impression Materials • Impression –negative reproduction of tissues • Positive cast made by filling the impression with dental stone or other model material 3
  4. 4. Desirable qualities of Impression Materials A pleasant odour, taste and acceptable color Absence of toxic or irritant constituents Adequate shelf life for requirements of storage and distribution 4
  5. 5. Desirable qualities contd… Easy to use with minimum of equipment Setting characteristics which meet clinical requirements Low enough viscosity to adapt to the oral tissues,yet be viscous enough to be contained in the impression tray 5
  6. 6. Desirable qualities contd… Should have adequate wettability of the oral tissues Set impression should show adequate elastic recovery with no permanent deformation upon removal from mouth 6
  7. 7. Desirable qualities contd…. Dimensionally stable after setting over temperature and humidity ranges normally found in clinical and lab procedures until the pouring of the cast Compatible with cast and die materials 7
  8. 8. Desirable qualities contd…. Biocompatible ,non-toxic Must be of color and opacity that allows the dentist to evaluate the impression Readily disinfected without significant loss of accuracy or loss of mechanical properties 8
  9. 9. Desirable qualities contd…. Materials ,associated processing time and equipment –cost effective No release of gas or other by-products during setting of the impression materials 9
  10. 10. 10
  11. 11. CLASSIFICATON: Classified on the basis of setting and elasticity. Mode of setting rigid Elastic Set by chemical reaction(irreversible) Impression plaster, zinc oxide eugenol. Alginate, polysulfide, polyether, silicone, Set by temperature change(reversible) Compound, waxes Agar hydrocolloid. 11
  12. 12. 12
  13. 13. Impression Materials Non-elastic Elastic Aqueous Hydrocolloids Non-aqueous Elastomers Polysulfide Silicones Polyether Condensation Addition Agar (reversible) Alginate (irreversible) Plaster Compound ZnO - Eugenol Waxes O’Brien, Dental Materials & their Selection 199713
  14. 14. NON-ELASTIC IMPRESSION MATERIALS NON-ELASTIC IMPRESSION MATERIALS Impression plaster Zinc-oxide eugenol impression paste Impression compound 14
  15. 15. Impression Plaster • used as mucostatic impression material for making final impressions for edentulous patients • Doesn’t compress and displace tissues during seating of tray due to its fluidity 15
  16. 16. Impression plaster contd… • Applicable to patients with displaceable soft tissues that should be recorded in a passive state 16
  17. 17. Composition • ß-calcium sulphate hemihydrate • Reacts with water to form calcium sulphate dihydrate • W/P ratio– 0.5-0.6 • Expansion and setting times controlled by incorporating compounds designed to mediate handling properties 17
  18. 18. • Potassium sulphate added as an anti-setting expansion agent • Borax(retarder)- added to the powder to balance the setting acceleration caused by Pot. Sulphate and to bring the setting time under control 18
  19. 19. • Alzarin red-to make clear distinction between the impression and model 19
  20. 20. • Custom tray constructed using 1-1.5mm spacer with acrylic resin or shellac • Impression plaster can be used as wash material • Techinque- “Puddling” the impression into place 20
  21. 21. • With remaining plaster in tray, the tray is seated in single movement • Then tray is gently moved from side-to side and antero-posteriorly to take advantage of fluidity of material 21
  22. 22. • Hemihydrate particles absorb moisture from the surface of the oral tissues allowing intimate contact between impression material and the tissues 22
  23. 23. • Plaster impression material –very brittle and fractures easily • When undercut is involved, fracture the impression to facilitate removal from mouth • Fragments reconstructed to form completed impression 23
  24. 24. • Beading of the impression done • Coated with separating medium and cast in fresh plaster • Disinfection- achieved with a 10 min soak in sod hypochlorite solution 24
  25. 25. • Not used regularly due to mechanical limitations • Used frequently as occlusal registration material 25
  26. 26. Zinc-oxide Eugenol Impression Paste • Composition-2 separate pastes dispensed in tubes • One tube contains zinc oxide and vegetable or mineral oil • Other tube contains eugenol and rosin 26
  27. 27. Composition Components Percentage Tube no 1 (base) Zinc-oxide 87 Fixed vegetable or mineral oil 13 Tube no 2 (accelerator) Oil of cloves or eugenol 12 Gum or polymerised rosin 50 Filler(silica type) 20 Lanolin 3 Resinous balsam 10 Accelerator solution(CaCl2) and color 5 27
  28. 28. Setting reaction of ZOE • Ionic in nature • Requires ionic medium in which it can proceed at any desired rate • 1st reaction-hydrolysis of zinc oxide to its hydroxide form 28
  29. 29. Zinc Oxide Eugenol • When the 2 pastes are mixed,the phenol –OH of the eugenol acts as a weak acid and undergoes an acid-base reaction with zinc hydroxide • Forms a salt- zinc eugenolate 29
  30. 30. ZOE contd… • Two further coordinate bonds are formed by donation of pairs of electrons from methoxy oxygen to zinc 30
  31. 31. ZOE contd… • Disadvantage- • Stinging or burning sensation caused by eugenol • Orthoethoxybenzoic acid -substitute 31
  32. 32. Manipulation • Mixed on oil impervious paper or glass mixing slab • Proper proportion of two pastes obtained by squeezing 2 strips of paste of the same length,one from each tube ,onto the mixing slab 32
  33. 33. Manipulation of ZOE contd… • Flexible stainless steel spatula used for mixing • 2 strips of contrasting colors combined with the first stroke of the spatula ,mixing is continued for approx 1 min, until a uniform color achieved 33
  34. 34. Types of ZOE • Classified as Hard paste(type I) soft paste(type II) • Final set for type I paste-10 min type II paste-15min Actual time shorter when setting occurs in mouth 34
  35. 35. • Shorten the setting time – by adding small amount of Zinc acetate or additional accelerator or a drop of water in the paste before mixing or by extending mixing time • Prolonging the setting time- cool spatula and mixing slab 35
  36. 36. • Paste of thick consistency –compresses the tissues • Thin,fluid material results in little or no compression Advantage of heavier consistency – increased strength 36
  37. 37. Dimensional stability • Negligible shrinkage(less than 0.1%) may occur during hardening • No significant dimensional change • Impression can be preserved indefinitely without change in shape provided the tray material is dimensionally stable 37
  38. 38. Disinfection • 2 % alkaline glutaraldehyde solution • Immersed in solution for required time,rinsed and poured immediately 38
  39. 39. Applications of ZOE • Final impression of edentulous ridges • As a wash impression with other impression • As an interocclusal registration material • As a temporary liner material for dentures • As a surgical dressing 39
  40. 40. Impression compound • Also called “modelling plastic” • Thermoplastic material • Supplied in the form of cakes(red) and sticks (green, gray or red) 40
  41. 41. Composition • Mixture of –waxes(principal ingredient) thermoplastic resins filler(increase viscosity and rigidity) coloring agent • Shellac, stearic acid and gutta-percha added to improve plasticity and workability 41
  42. 42. Types of Impression compound Type-I (Lower fusing) Type- II (Higher fusing) 42
  43. 43. Type I (Lower fusing material) • Cakes- as an impression material for completely edentulous patients, the material is softened by heat, inserted into the tray and placed against the tissues before it cools to a rigid mass 43
  44. 44. • Sticks- as a border molding material for the custom tray ,the material is used before making the final impression 44
  45. 45. Type II( Higher fusing material) • Used as an adaptation material which requires more viscous properties • Used for making primary impression of the soft tissues and then used a tray to support a thin layer of a second impression material such as ZnOE paste, hydrocolloids or nonaqueous elastomers 45
  46. 46. Manipulation • Setting mechanism– reversible physical process • Softening by heat – prerequisite • Preheated and used warm (~450C) • Then cooled to the intraoral temperature(370C) at which it is fairly rigid 46
  47. 47. • Once the impression tray is seated , it should be held gently(passively) in position until the impression cools below the fusion temperature 47
  48. 48. • Softened by heat over flame(green stick) or in a temperature controlled water bath • When direct flame is used, the material should be moved over the flame in such a manner that it will not be allowed to boil or ignite so that the constituents are volatilized 48
  49. 49. • Prolonged immersion or overheating in water bath makes the compound brittle or grainy due to leaching of low molecular weight ingredients 49
  50. 50. • Dimensional stability- allow thorough cooling of the impression before removal from the mouth and to construct the cast or die as soon as possible after the impression has been obtained(at least within the hour) • Disinfection – 2% alkaline glutaraldehyde solution 50
  51. 51. Thermal Properties a)Thermal conductivity-low thermal conductivity Significance • During softening of the material, the outside will soften first and inside last, so to ensure uniform softening, the material should be kept immersed for a long time in water bath • The layer adjacent to tissues will remain soft . Thus it is important to cool the compound thoroughly before removing the impression 51
  52. 52. b)Coefficient of Thermal Expansion- high COTE (0.3% acceptable) c)Glass Transition Temperature • The temperature at which the material loses its hardness or brittleness on heating or forms a rigid mass upon cooling • Approx 39°C 52
  53. 53. d)Fusion temperature– corresponds to a definite reduction in plasticity of the material during cooling • Above this temperature, the material remains plastic while the impression is being made • Approx 43.5°C 53
  54. 54. Significance of Fusion temp and Glass transition temp • Above Fusion temp,the fatty acids are liquid and lubricate the softened material to form a smooth plastic mass while the impression is being obtained. • Thus all impressions with compound should be made above this temperature 54
  55. 55. • Once the impression tray is seated,it should be held firmly in position until first fusion temperature and later the glass transition temperature is reached • Thus, impression is made above the fusion temperature and removed after it cools down to its glass transition temperature 55
  56. 56. Important considerations for proper use of impression compound • Low thermal conductivity- adequate time needed to attain thorough heating and cooling • Incorporation of water(wet kneading)- excessive flow of the material at mouth temp. producing distortion as the impression is removed from mouth 56
  57. 57. • Tray used for impression must be strong and rigid enough to support the material and to avoid distortion of the impression • Relatively high viscosity limits its ability to record fine details • Cast should be poured as soon as possible to minimise distortion due to relaxation of the compound 57
  58. 58. • Safely disinfected by immersion in sod hypochlorite, iodophors or phenolic glutaraldehydes 58
  59. 59. Elastomeric Impression Materials ELASTOMERIC IMPRESSION MATERIALS Aqueous hydrocolloids Agar-agar Alginate Non-aqueous elastomers Polysulfides Silicones Polyethers 59
  61. 61. Agar • Reversible hydrocollloid • Physical change of agar from sol to gel induced by lowering temperature • Gel liquefies to sol when heated to a temperature known as liquefaction temperature(700C-1000C) 61
  62. 62. • When sol is cooled, it becomes gel at a point known as the gelation temperature( btw 370C and 500 C) • Thus called reversible hydrocolloid 62
  63. 63. • Gelation temp- critical for impression making • If too high,heat from the sol may injure the oral tissues • If too low,below oral temperature,impossible to make impression because the sol will not convert to a gel 63
  64. 64. • Polysaccharide- extracted from certain types of seaweed • Water major constituent • Supplied as gel • Available in tray and syringe consistencies • Tubes used to fill water cooled trays and cartridges used with syringes 64
  65. 65. Composition Agar Gelling agent Borax Improves strength Potassium sulphate Gypsum hardener Alkyl benzoates Preservatives Water Reaction medium(>80%) Coloring agents Flavouring agents 65
  66. 66. • Fillers such as diatomaceous earth,wax, clay,silica,rubber and similar inert powders– used to control strength , viscosity and rigidity • Thymol and glycerine added as bactericidal agent and plasticiser 66
  67. 67. Making the Agar impression • Process requires a 3 compartment conditioning unit for the agar tray material • Allows liquefaction,storage and tempering • Syringe material used only in liquefaction and storage compartments 67
  68. 68. Liquefy the hydrocolloid gel in the tube in boiling water at 1000C for minimum 10 min tube then placed in a storage bath at 65°C to retain the sol condition until needed impression tray filled with hydrocolloid sol from the tube taken from storage bath , gauze pad placed over the top of the tray material Tray placed in water filled tempering compartment(at abt 45°C) 68
  69. 69. 69
  70. 70. Just before tempering completed,syringe material taken directly from storage compartment and applied to the prepared teeth • Note--- tempering time-3-10 min • if >10 min,partial gelation occurs • syringe material doesn’t require tempering bcoz maintained in fluid state to enhance adaptation to tissues 70
  71. 71. • Syringe material first applied to the base of the preparation,then remainder of the prepared tooth is covered • Tip of the syringe is held close to the tooth and it remains embedded below the surface of the syringe material to prevent entrapment of air bubbles 71
  72. 72. • Water soaked outer layer of hydrocollloid loaded tray and the gauze covering the tray impression material are removed to ensure firm bonding to the syringe hydrocolloid • Tray immediately brought into position,seated with light pressure and held with a very light force 72
  73. 73. • Gelation accelerated by circulating cool (18- 21°C)through tray for 3-5 min • During gelation process, tray must be held in mouth until gelation has proceeded to a point at which gel strength is sufficient to resist deformation or fracture • Tray removed with a snap 73
  74. 74. Distortion during gelation • Some contraction occurs due to physical change (sol gel) • If held rigidly in the tray,shrink towards the center of its mass,thus creating larger dies • Rapid cooling may cause stress concentration near the tray 74
  75. 75. 75
  76. 76. Dimensional stability • Storage medium- 2% potassium sulfate solution or 100% relative humidity 76
  77. 77. Compatibility with Gypsum • Contains borax- retarder for setting of gypsum products • Deficiency of gypsum setting can be overcome by--Immersing agar impression in a solution containing a gypsum accelerator(2% pot sulfate solution) prior to pouring of the impression • By incorporating gypsum surface hardener in the material such as sulfate 77
  78. 78. • Disinfection House hold bleach or iodophors • Accuracy Most accurate of impression materials 78
  79. 79. Alginate • Irreversible hydrocolloid • Most widely used material in dentistry • Developed as a substitute for agar 79
  80. 80. Advantages • Ease of manipulation • No need of expensive equipments • Relatively low cost • Comfort to patients • High viscosity • Ability to displace tissues Disadvantages 80
  81. 81. Potassium or sodium alginate dissolves in water and reacts with calcium ions Calcium sulphate dihydrate A reactor ,reacts with potassium alginate to form a dihydrate insoluble alginate gel Zinc oxide Filler particles, affects properties and setting time Potassium titanium fluoride Accelerator ,counteracts the inhibiting effect of the hydrocolloid on the setting of stone,ensures good quality surface of the cast Diatomaceous earth Filler particles, controls the consistency of the mix and the flexibility of the set alginate Trisodium phosphate Retarder,controls the settting time to produce either regular or fast set alginates Coloring agents Flavoring agents Composition 81
  82. 82. Modified alginates • Dustless alginates • Include polyethylene glycol or polypropylene glycol on the alginate powder to agglomerate the particles • Color indicators added to reveal the stage of setting reaction 82
  83. 83. Modified alginates • Two paste alginate materials • One paste contains sol of alginate,fillers,retarders and other ingredients like glycols and dextrose • Other paste contains gypsum dihydrate, fillers, retarder , glycerol or glycol, gypsum surface modifier and some silicone oil 83
  84. 84. Gelation process • Typical sol-gel reaction • Soluble alginate reacts with calcium ions from calcium sulphate and forms insoluble calcium alginate • Production of calcium alginate- rapid,doesn’t allow sufficient working time • Retarder trisodium phosphate added to extend working time 84
  85. 85. Setting reaction of alginate (a) K2n-Alginate+ n CaSO4 nK2SO4 + Can- alginate (b)2Na3PO4 + 3CaSO4 Ca3(PO4)2 + 3Na2SO4 85
  86. 86. 86
  87. 87. Controlling Setting Time • Ideal W/P ratio- 20 ml water/8gms of powder 40 ml water/16gms of powder • Powder should be weighed not measured • Approx 2.5:1 • Slight modification in W/P ratio affects 2 important properties--- tear strength elasticity 87
  88. 88. • Thus setting time best regulated by amount of retarder added during manufacturing • Fast-set alginate- 1.5-3min • Regular set alginate- 3-4.5 min • Can also be influenced by altering the temperature of water 88
  89. 89. • Cool water in hot weather • Precool mixing bowl and spatula • Tap water-contains certain levels of metallic ions(Ca,Mg) • Tap water with a high hardness may accelerate setting time 89
  90. 90. Preparation of Alginate Impression Materials • Measured powder added slowly to premeasured water already poured into clean rubber bowl • Powder incorporated into water by carefully mixing with a metallic spatula flexible enough to adapt well to the wall of the mixing bowl 90
  91. 91. • Avoid incorporating excessive air into the mix • Vigorous figure of 8 stropping motion • Mixing time- 45sec to 1 min • Result should be a smooth creamy mixture that doesn’t drip off the spatula when raised from bowl 91
  92. 92. • Mechanical mixing devices • Include rotating mixing bowl , mechanical mixer with time-control unit, a vaccum mixer for water/powder mixing • Advantages- convenience, speed and reduction of human error 92
  93. 93. Making the alginate impression • Perforated metal tray preferred • Thickness of alginate impression between the tray and the tissues should be at least 3mm • Compressive strength doubles during first 4 min after gelation,but doesn’t increase appreciably thereafter • Improve elasticity over time which minimizes distortion of the material during impression removal(undercut areas) 93
  94. 94. Compressive strength of an Alginate gel as a function of Gelation time Time from Gelation(min) Compressive strength(KPa) 0 330 4 770 8 810 12 710 16 740 94
  95. 95. • Alginate impression should not be removed from the mouth for at least 3 min after gelation has occurred • Tear strength increased when the impression is removed along a vertical path with a snap • Speed of removal- between rapid movement and a slower rate 95
  96. 96. STRENGTH • Manufacturer’s directions should be followed • Any deviation from instructions can have adverse effects on the gel strength SHELF LIFE • 2 factors affecting shelf life- storage temperature and moisture contamination 96
  97. 97. Dimensional stability • Syneresis- loss of water when exposed to air at room temperature associated with shrinkage • Imbibition- swelling of the impression if immersed in water 97
  98. 98. How can distortion be minimised?? • Poured immediately after making impression • If pouring delayed, then rinsed in tap water,disinfected wrapped in a surgical paper towel saturated with water and placed in a sealed plastic bag or humidor 98
  99. 99. Compatibility with Gypsum • Poatssium titanium fluoride-surface hardener or gypsum hardener • Solubility-1.3gm per 100 ml of water at 20°C • Fluoride on surface of alginate will form fine calcium fluoride precipitates with the calcium from the gypsum • Fine calcium fluoride particles become nuclei that accelerate the setting of gypsum 99
  100. 100. • Rough stone surface will result if excess rinsing water collected on the surface of the impression at the time of pouring the stone mixture • A dried gel results in its adherence to the surface of the cast which results in tearing upon removal 100
  101. 101. • Surface of impression should be shiny but with no visible water film or droplets at the time of pouring • Stone cast or die should be kept in contact with the impression for a minimum of 30 min, preferably for 60 min before the impression is separated from the cast 101
  102. 102. Disinfection • Household bleach(1-10 dilution) iodophors or synthetic phenols • After rinsing,disinfectants sprayed on exposed surface • Impression immediately wrapped in a disinfectant-soaked paper towel and placed in a sealed plastic bag for 10 min 102
  103. 103. • Wrapped impression removed from the bag, unwrapped, rinsed and shaken to remove excess water • Then poured with stone of choice Accuracy Not capable of reproducing the finer details compared with other impression materials 103
  104. 104. 104
  105. 105. Laminate technique(Alginate -Agar method) • Modification of agar procedure • Agar in tray replaced with a mix of chilled alginate that bonds to the agar expressed from a syringe • Alginate gels by chemical reaction while agar gels by means of contact with the cool alginate rather than water circulating through the tray 105
  106. 106. • Agar contacts the prepared teeth ,maximum detail reproduced • Equipment cost low, less preparation time needed • Main disadvantages- bond btw agar and alginate not always sound higher viscosity of alginate displaces agar during seating dimensional inaccuracy of alginate limits its use to single units 106
  108. 108. Elastomeric impression materials • Comprise a group of synthetic polymer- based impression materials that are chemically cross-linked when set and can be stretched • Rapidly recover to their original dimensions 108
  109. 109. Based on the backbone of polymer chains Elastomers Polysulfide Silicones Polyethers 109
  110. 110. Elastomers • Supplied in 2 components: base paste catalyst paste • Formulated in several consistencies in increasing content of filler Extra light body Light body Medium or regular body Heavy body Putty(extra heavy) 110
  111. 111. • Extra low and putty available only for condensation and addition silicones • Polysulfide provided only in light body and heavy body • No heavy body for condensation silicone 111
  112. 112. • Different colored pastes dispensed either through a spiral mixing tip or in equal lengths on a mixing pad • Setting occurs through a combination of chain-lengthening polymerization and chemical cross-linking by either a condensation or addition reaction 112
  113. 113. • 3 types of systems available to mix the catalyst and base : hand mixing, static automixing, dynamic mechanical mixing • To record soft tissues for edentulous patients under minimum compression: polysulfide or addition silicones (free flowing, minimum viscosity) 113
  114. 114. • If moderate compression required: medium- viscosity polysulfides, addition silicones, polyethers • Undergo shrinkage upon polymerisation • Condensation type silicones undergo additional contraction 114
  115. 115. • Polysulfides and condensation silicones : highest dimensional changes during setting • Addition silicones and polyethers: lesser dimensional changes 115
  116. 116. Polysulfides • First synthetic elastomeric impression material introduced in 1950 • 2 paste system • Available in low, medium and high consistencies • Made up of a base and accelerator/reactor • Brands- COE-FLEX,PERMALASTIC,NEOPLEX etc 116
  117. 117. Composition The Base Polysulfide polymer(-SH,mercaptan gp) Principal ingredient Titanium oxide and Zinc Fillers Sulphate ,copper carbonate or silica Strengthener Dibutyl phthalate Plasticizer (confers viscosity to base) The Accelerator Lead dioxide, hydrated copper oxide or organic peroxide Reactor Sulfur Promoter, accelerates the reaction Oleic acid or Stearic acid Retarder, controls setting reaction 117
  118. 118. • Working time: 5-7 min( longest among elastomers) • Setting time: 8-12 min • Pouring the cast: impression must be poured within 30 min to 1 hr 118
  119. 119. • Each paste supplied in a dispensing tube with approx sized bore diameters at the tip • Equal lengths of paste extruded from each tube to provide the correct ratio of polymer to cross-linking agent 119
  120. 120. • Reaction starts at the beginning of mixing and reaches its maximum rate soon after spatulation is complete • Resilient network started to form • During final set, a material of adequate elasticity and strength is formed that can be removed past undercuts 120
  121. 121. • Polymerisation results in chain lengthening and cross –linking with an increase in molecular weight • Setting indicated by change of the color of the paste to dark- brown or gray- brown • Color- presence of lead oxide 121
  122. 122. 122 Polymerisation of ploysulfide impression material SH groups interact with oxygen released from lead dioxide Completion of the condensation reaction results in water as a by-product Pendant –SH for cross-linking and terminal for chain lengthening
  123. 123. • Hot and humid conditions accelerate the setting of polysulfide impression material • Reaction slightly exothermic and yields water as a by-product 123
  124. 124. • Good flexibility • High tear strength • Hydrophobic • Messy, stains clothes and has an offensive odor • Uses—impression for crown and bridge edentulous impressions 124
  125. 125. Condensation silicone • Followed in 1955 • Supplied as two-paste system or base-paste and a low viscosity liquid catalyst or a two-putty system • Putty used as tray material in conjunction with a low-viscosity silicone • Referred to as the Putty-wash technique • Brands- Speedex 125
  126. 126. Composition The Base paste - -hydroxyl-terminated polydimethyl siloxane High molecular weight polymer Silica or calcium carbonate Fillers The Liquid Accelerator Tin octoate Metal organic ester Orthoalkyl silicate Oil-based diluents Thickening agents Increase viscosity 126
  127. 127. Condensation silicone • Working time: 3 min • Setting time: 6-8 min • Impression must be poured as soon as possible within first 30 min 127
  128. 128. • Curing involves a reaction of tri- and tetra- functional alkyl silicates in the presence of stannous octoate as a catalyst • Sets by cross-linking between terminal groups of the silicone polymers and the alkyl silicate to form a 3-D network 128
  129. 129. 129 Condensation polymerisation of alpha-omega hydroxy – terminated poly(dimethyl siloxane) with tetraethyl orthosilicate in the presence of stannous octoate(catalyst) This reaction results in the release of ethanol molecules
  130. 130. • By-product: Ethyl alcohol • Subsequent evaporation accounts for much of the contraction that takes place in the setting impression • Extra-heavy or putty consistency developed to counteract the large polymerisation shrinkage 130
  131. 131. Addition silicone • Commonly referred to as Polyvinyl siloxanes(PVS) • Supplied as low, medium, high and very high consistencies • Based on silicone prepolymers that carry vinyl and hydrogen side groups which can polymerize by addition polymerization • Brand name- Aquasil 131
  132. 132. Composition The Base Polymethylhydrosiloxane Low molecular weight polymer Fillers The Accelerator Divinyl polymethyl siloxane Other siloxane pre-polymers Platinum salt Catalyst Reatrder Controls working and setting times 132
  133. 133. • Working time: 2-4.5 min • Setting time: 3-7 min • Cast can be poured upto 1 week after making the impression 133
  134. 134. • Reaction activated by a platinum salt catalyst (chloroplatinic acid) without the release of by- products • In presence of impurites or moisture, secondary reaction takes place between the residual hydrides and moisture leading to evolution of hydrogen gas 134
  135. 135. 135 Hydrogen atoms along the backbone str of PVS chain move to the vinyl group during addition polymerisation(top) Final str after platinum salt has initiated the addition polymerisation reaction (bottom) The zigzag line at the other end of divinylpolysiloxane represents repeating units of dimethylsiloxane with a vinyl terminal
  136. 136. • This can cause minute gaseous voids in the gypsum casts and reduce the effectiveness of cross-linking polymer structure • Automatic mixing systems simplified their manipulation, reduced voids in impressions, reduced the amount of material wasted and reduced the sensitivity of their mixing technique 136
  137. 137. 137
  138. 138. Advantages • Most elastic of currently available materials • Virtually negligible distortion upon removal from undercuts • Exceptional accuracy in reproducing anatomic details • Dimensional stability allows pouring long after impression making • Excellent occlusal record registration material 138
  139. 139. Disadvantages • Inherent hydrophobic nature Non-ionic surfactant wetting agent added to silicone paste rendering the surface of the impression more hydrophillic and called hydrophilized addition silicone 139
  140. 140. • Sulfur contamination from natural latex gloves inhibits the setting of addition silicone • Touching the tooth with latex gloves before seating the impression can inhibit the setting of critical surface next to tooth 140
  141. 141. Polyether • Supplied as two-paste system in low, medium and high consistencies • 2 types: 1. based on ring-opening polymerization of aziridine rings which are at the end of branched polyether molecules 2. based on an acid-catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups • Brands- IMPREGUM 141
  142. 142. 1st type • Main chain probably a copolymer of ethylene oxide and tetrahydrofuran • Cross-linking and setting promoted by an initiator and an aromatic sulfonate ester • R alkyl group • Produces cross-linking by cationic polymerisation via the imine end groups • Supplied as 2 pastes: base and accelerator 142
  143. 143. Composition The Base Polyether polymer Colloidal silica Filler Glycol ether or pthalate Plasticizer The Accelerator Alkyl aromatic sulfonate Initiator Filler Plasticizer 143
  144. 144. • Working time: 2.5 min • Setting time: 4.5 min • Poured upto 1 week of storage 144
  145. 145. 145 Initiator ,aromatic sulfonate ester dissociates and forms alkyl cations that bind the nitrogen atoms of the azridine ring terminals of the prepolymer(top,left) The arrows indicate binding between the cations (R) with nitrogen atoms This action opens up the ring,and the reacted pre-polymer (center) now has 2 ethylene imine terminals(-NR-CH 2-C+H2) which can react with nitrogen atoms of adjacent unreacted prepolymers.(R2 Aziridine ring Chain propagation polymerization reaction yields a larger molecule(right) which continues growing by binding with aziridine rings of additional unreacted prepolymers Polymerisation reaction terminates when the growing chain combines with a counter ion
  146. 146. 2nd type • Based on an acid-catalyzed condensation polymerization of polyether prepolymer with alkoxysilane terminal groups • Mechanism similar to condensation silicones • Material often called hybrid • Behave very much like the 1st type due to ether linkages 146
  147. 147. • High degree of wettability • Inherent hydrophillic nature • Relative stiffness Excellent material for good duplication of fine details and rigid support for pick-up copings 147
  148. 148. Making impressions with elastomeric materials • Fabrication of gypsum models ,casts and dies involves 6 major steps: 1. Preparing a tray 2. Managing tissue 3. Preparing the material 4. Making the impression 5. Removing the impression 6. Preparing stone casts and dies 148
  149. 149. 1.Impression trays • Custom tray recommended to reduce the quantity of material required • In case of severe undercuts, custom tray avoided • Prior to impression making, uniform thickness of tray adhesive applied 149
  150. 150. 2.Tissue management • Displace the gingival tissues, control gingival haemorrhage and control sulcular fluids to ensure access for the tooth preparation and making impression • Gingival retraction cord- most commonly used 150
  151. 151. 3.Manipulation of impression materials • Supplied for 3 modes: hand mixing, static mixing and dynamic mechanical mixing 151
  152. 152. Hand mixing • Dispense the same length of materials onto a mixing pad or glass slab • Catalyst paste first collected on stainless steel spatula and then spread over base paste • Mixture is then spread over the mixing pad • Mass is then scraped up with the spatula blade and spread uniformly back and forth on the mixing pad 152
  153. 153. • Process continued until the mixed paste is uniform in color with no streaks of the base or catalyst appearing in the mixture • 2 putty systems(condensation and addition silicone) dispensed by volume using equal number of scoops of each material • Knead the material with fingers until a uniform color is obtained 153
  154. 154. Static mixing • Transforms 2 fluid(or paste-like) materials into a homogenous mixture without mechanical mixing • Device used- gun for compressing materials into a 2- cylinder cartridge, which contains the base and catalyst separately, as well as mixing tip 154
  155. 155. • Mixing tip is made of helical mixer elements in a cylindrical housing • Mixer elements are series of alternating right and left –turn 180°helixes positioned so that leading edge of one element is perpendicular to the trailing edge of the next • Length of each material is the same as the inner diameter of the cylinderical housing 155
  156. 156. Dynamic mechanical mixing • Device uses motor to drive parallel plungers,forcing the materials into a mixing tip and out into an impression tray or syringe • Motor driven impeller mixes the materials as they are extruded through the tip • Materials supplied in collapsible plastic bags housed in cartridge • Polyether and addition silicone 156
  157. 157. 4.Making an impression • 3 techniques: • Multiple-mix technique • Monophase technique • Putty wash technique 157
  158. 158. Multiple-mix technique • Syringe material(light body)and tray material(heavy body) • Lighter material injected within or around the tooth preparation • Filled tray then inserted in the mouth and seated over the syringe material • Tray material force the syringe material to adapt to the prepared tissues 158
  159. 159. Monophase technique • Medium body polyether and addition silicone • Only one mixture is made and a part of the material is placed in the tray and another portion in syringe for injection in the prepared tissues • Success depends on pseudoplastic (shear thinning) property of material 159
  160. 160. Putty –wash technique • Originally developed for condensation silicone to minimize the effect of associated dimensional changes • Thick putty material placed in stock tray and a primary impression made • Space for light-body “wash” material provided • Mixture of thin consistency wash material placed into putty impression and preparation 160
  161. 161. 5.Removal of the impression • Shouldn’t be removed until curing progressed sufficiently to provide adequate elasticity ,so distortion doesn’t occur • Typically impression should be ready for removal within at least 10 min from time of mixing,allowing 6-8 min for impression to remain in mouth 161
  162. 162. • Mechanics of removing impression– separation at the impression/tissue interface and stretching of the impression • 1st step to break the physical adhesion between the tissue and the impression • Polyether requires extra effort • 2nd step stretches the impression enough to pass under the height of contour of hard tissue to remove impression 162
  163. 163. 6.Preparation of stone casts and dies • Silicones– hydrophobic • Surfactant sprays- debubblizers improve surface wettability of silicone impression material for stone slurry • Dilute solution of soap also acts as surfactant • Polyether n polysulfide- don’t require surfactant 163
  164. 164. Properties of elastomeric impression materials 1. Working and setting time 2. Reproduction of oral structure detail 3. Rheological properties 4. Elasticity and visco-elasticity 5. Tear strength 6. Dimensional stability 7. Disinfection 8. Wettability and hydrophillization 9. Biocompatibility 164
  165. 165. 1. Working and setting time Mean working time(min) Mean setting time(min) Impression material 23°C 37°C 23°C 37°C POLYSULFIDE 6.0 4.3 16.0 12.5 CONDENSATION SILICONE 3.3 2.5 11.9 8.9 ADDITION SILICONE 3.1 1.8 8.9 5.9 POLYETHER 3.3 2.3 9.0 8.3 165
  166. 166. 2.Reproduction of detail • Record detail to the finest degree • When stone poured on the surface, finest details not always reproduced 166
  167. 167. 3.Rheological properties • Viscosity and flow behaviour depends upon - ease of mixing -air entrapment during mixing -tendency of trapped air to escape before the impression is made • All elastomers exhibit shear-thinning characteristics before setting 167
  168. 168. • 2 categories of shear thinning phenomena— • Pseudoplasticity • Thixotropy • Pseudoplastic material- displays decreasing viscosity with increasing shear stress and recovers its viscosity immediately upon a decrease in shear stress 168
  169. 169. • Thixotropic material- doesn’t flow until sufficient surface energy in the form of an impact force or vibration force is applied to overcome the yield stress of material • Extreme shear thinning- material retains an immobile state at rest but flows freely under stress 169
  170. 170. Significance of shear thinning • Exhibited by addition silicone and polyether impression materials • Enable the clinician to use a monophase impression making technique to capture details needed for fixed prostheses 170
  171. 171. 4. Elasticity and viscoselasticity • Explained well by Maxwell- Voigt model • Relative amount of permanent deformation • Addition silicone<polyether<polysulfide • Recovery of elastic deformation following strain is less rapid for the polysulfide material than for other three types of impression materials 171
  172. 172. MAXWELL-VOIGT MODEL 172 A-Maxwell-Voigt model in a stress-free state B-during loading,only S1 spring contracts in response to load C-when loading continues,the pistons in dashpot D1 and D2 move proportionally to the duration of loading . S2 spring contracts alongwith dashpot D2. No change in S1 D-the moment the load is released,S1 spring recovers instantly, whereas rest of the elements remain unchanged. S2 should also recover instantly but retarded by the sluggishness of dashpot D2 E- as time passes, S2 spring recovers and extends dashpot D2 slowly near to its original position. Dashpot D1 remains unchanged
  173. 173. • Polyvinyl siloxane- exhibit most elastic recovery • Distortion on removal from undercuts is virtually non-existent • If material is in advanced stage of elasticity, and compressed excessively during seating of impression,distortion can occur when the material elastically rebounds 173
  174. 174. • MOE in increasing order • Polysulfide<condensation silicone<addition silicone< polyether 174
  175. 175. 5.Tear strength • Low viscosity materials used in interproximal and subgingival areas • Measures the resistance of an elastomeric material to fracture when subjected to a tensile force acting perpendicular to a surface flaw • Tear strength in increasing order • Silicones<polyethers<polysulfides 175
  176. 176. • Influenced by consistency and manner of removing the materials 176
  177. 177. 6. Dimensional stability • Dimensional accuracy v/s dimensional stability • 6 major sources of dimensional change 1. Polymerisation shrinkage 2. Loss of condensation reaction by-product 3. Thermal contraction from oral temp to room temp 4. Absorption of water or disinfectant over a period of time 5. Incomplete recovery of deformation because of plastic deformation 6. Incomplete recovery of deformation because of viscoelastic behaviour 177
  178. 178. • Absorption of water or fluids- negatively affects polyether impression • Simultaneous leaching of water soluble plasticizer • Stored in dry(relative humidity<50%),cool environment to maintain its accuracy • Should never be left for protracted periods in disinfecting solutions 178
  179. 179. 7.Disinfection • Polysulfides and silicones--Glutaraldehydes, chlorine compounds ,iodophors,phenolics • Polyether- chlorine compounds or iodophors 179
  180. 180. 8.Wettability and hydrophilization • Silicones-most hydrophobic • Polyethers-hydrophillic • Spray surfactant on hydrophobic impressions prior to pouring with gypsum • Non-ionic surfactant added during manufacturing 180
  181. 181. • Hydrophyllized PVS- ether group –hydrophillic and oriented towards the surface when the surfactant migrates by diffusion to the surface region • Depends on the contact angle made by water droplets with the impression surface and surface energy 181
  182. 182. • As soon as hydrophillized PVS encounters oral fluids during impression making, begins to release surfactant to its surroundings • Adequate amount of surfactant trapped on the surface of set PVS impressions ,thus providing hydrophilicity for pouring of gypsum dies 182
  183. 183. 9.Biocompatibility • Tests covered in ISO 10993-5,Biological Evaluation of Medical Devices:Tests for in-vitro cytotoxicity • Polysulfide – lowest cell death count • Polyether –highest cell toxicity scores 183
  184. 184. • Elastomer induced biocomaptibility problem– fragment of impression material trapped in patient’s gingival sulcus • Causes severe gingival imflammation • Can also occur in 2nd stage implant surgery • Contact dermatitis from Polyether in dentists or dental technicians 184
  185. 185. 10. Shelf life • Don’t deteriorate appreciably in tube or container before the expiration date when stored in dry , cool environment • Clear liquid expressed along material- plasticizer segregation • Indicates manufacturing error or excessive temperature extremes during storage 185
  186. 186. Effects of mishandling • Failure to produce an accurate epoxy or gypsum die or cast 186
  187. 187. Common failures occurring with use of elastomeric impression materials 187 Type of failure Causes Rough or uneven impression surface Incomplete polymerization,improper ratio or mixing of components,presence of oil or plaque on teeth Surface agents (latex for PVS) inhibit polymerization Too rapid polymerization from high humidity or temp Excessively high accelerator /base ratio(cond silicone) Bubbles Air incorporated during mixing Irregularly shaped voids Moisture debris on teeth surfaces Rough or chalky stone casts Inadequate cleaning of impression Excess water not blown off impression Excess wetting agent left on impression Premature removal of cast,improper w/p ratio of stone Failure to delay pour(PVS) that doesn’t contain Palladium salt for 20 min
  188. 188. Distortion Resin tray not aged sufficiently , still undergoing polymerisation shrinkage Lack of adhesion of elastomer to tray Excessive bulk of material Lack of mechanical retention to the impression tray Excessive bulk of material Insufficient relief for the reline material Development of elastic properties in material before tray is fully seated Continued pressure against impression material that developed elastic properties Movement of tray during polymerization Premature/improper removal from mouth Delayed pouring of polysulfide or condensation silicone impression 188
  189. 189. Comparative properties of elastomeric impression materials Property Polysulfide Condensation silicone Addition silicone Polyether WT(min) 4-7 2.5-4 2-4 3 ST(min) 7-10 6-8 4-6.5 6 Tear strength(N/m) 2500-7000 2300- 2600 1500-4300 1800-4800 Percent contraction(at 24h) 0.40-0.45 0.38-0.60 0.14-0.17 0.19-0.24 Contact angle(°) 82 98 98/53 49 Hydrogen gas evolution N N Y N Automatic mixing N N Y Y Custom tray Y N N N Unpleasant odour Y N N N Multiple casts N N Y Y Stiffness 3 2 2 1 Distortion on removal 1 2 4 3189
  190. 190. s 190