16 Impression Materials: Classification and Requirements
Polyether impression material is an elastic-type material, as are the polysulfide and silicone materials. These materials have demonstrated good accuracy in clinical evaluations and are thixotropic, which provides good surface detail and makes them useful as a border molding material. Mar 14, · Polymer clays or plasticine clays are made of synthetic plastics and require “curing” or low heat to harden. They stay moldable for a very long time if stored properly in a plastic or metal box (no paper or fabric that would absorb the moisture). Polymer .
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No notes for slide. Impression materials 1. 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. 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.
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 oc 6 7. Desirable qualities contd…. Dimensionally stable after what is an antique rose 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.
Biocompatible how to get rid of stye in eye fast 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. Materials ,associated processing time and equipment —cost effective No release of gas or other by-products during setting of the impression materials 9 Mode of setting rigid Elastic Set by what type of impression material is modelling plastic reaction irreversible Impression plaster, zinc oxide eugenol.
Alginate, polysulfide, polyether, silicone, Set by temperature change reversible Compound, waxes Agar hydrocolloid. Sulphate and to bring the setting time under impeession 18 Composition Components How to download sbsettings without cydia 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 shat 20 Lanolin 3 Resinous plxstic 10 Accelerator solution CaCl2 and color 5 matsrial Thus it is important to cool the compound thoroughly before removing the impression 51 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 off produce either regular or fast set alginates Coloring agents Flavoring agents Composition 81 Compressive strength of an Alginate gel as a function of Gelation time Time from Gelation min Compressive strength KPa 0 4 8 12 16 94 How can distortion be minimised??
Based on the backbone of polymer chains Elastomers Polysulfide Silicones Polyethers Composition The Base Polysulfide polymer -SH,mercaptan gp Principal ingredient Titanium oxide and Zinc Fillers Sulphate ,copper how to draw zangetsu sword 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 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 R2 Aziridine ring Chain propagation mipression 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 Preparing a tray 2.
Managing tissue 3. Preparing the material 4. Making the impression 5. Removing the impression 6. Preparing stone casts and dies 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 plaetic 6. Dimensional stability 7. Disinfection 8. Wettability and hydrophillization 9. Biocompatibility 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 how to remove base stand from samsung tv retarded by the sluggishness of dashpot D2 E- as time passes, S2 spring recovers and extends dashpot D2 slowly near to its original plaatic. Dashpot How to get apps for free on jailbroken iphone remains unchanged 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 Comparative properties of elastomeric impression materials Property Polysulfide Condensation silicone Addition silicone Polyether WT min 2.
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The accuracy of the model depends on the accuracy of the impression in which it was cast. The impression stage is the first of many stages involved in the production of dentures, crowns, bridges, orthodontic appliances etc.
It is of great importance, therefore, that inaccuracies are minimized at this stage, otherwise they will be carried through and possibly compounded later on. The tray is required because the materials are initially quite fluid and require support. Many criteria may be used to classify impression materials. The most widely used and understood method is to classify them according to chemical type.
Hence, we have silicone materials, alginates, etc. Most dentists are able to associate a material from a particular chemical group with a particular set of characteristics or properties which render it suitable for some applications but not for others. Other methods of classification are sometimes used and these may be based upon consideration of the properties of the materials either before or after setting.
Before setting, the property most normally used to characterise materials is viscosity. This may effect the fine detail which can be recorded in impressions of hard tissues and may influence the degree of tissue compression or displacement achieved with soft-tissue impressions. Thus, materials which are initially very fluid are often classified as mucostatic impression materials because they are less likely to compress soft tissues, whilst materials which are initially more viscous are classified as mucocompressive.
It should be remembered however, that viscosity often varies with the applied stress p. Thus, certain materials which appear fairly viscous whilst under low stress conditions may become more fluid during the recording of the impression, when the material is placed under higher stress. When a substance behaves in this way, it is said to be pseudoplastic.
Another complicating factor is the spacing of the impression tray. This controls the thickness of the impression material and hence the pressure transmitted to the underlying tissues. A relatively fluid impression material confined in a close-fitting impression tray will compress the soft tissues to a greater extent than the same material used in a loosely-fitting tray.
Classification of materials according to viscosity is not, therefore as simple as it may seem. Figure There are often significant variations between different brands of the same type of material and these variations can spread across the divisions between different levels of viscosity.
A more widely used classification of materials involves consideration of the properties of the set material. This factor is primarily responsible for governing the principal applications of the materials. The properties which are most important are rigidity and elasticity , since they determine whether an impression material can be used to record undercuts.
When standing teeth are to be recorded, or when the patient has deep soft-tissue under-cuts, the set impression material must be flexible enough to be withdrawn past the undercuts and elastic enough to give recovery and an accurate impression. Hence impression materials are classified as being elastic or non-elastic. The term elastic as applied to impression materials is fairly unequivocal since the materials which form this group all possess the ability to be stretched or compressed and give a reasonable degree of elastic recovery following strain.
They could be described as possessing rubbery characteristics. The term non-elastic however, is not a particularly good term with which to describe a group of products which in some cases are clearly plastic e. It may be less confusing if the terms rubbery and non-rubbery were used instead of elastic and non-elastic. However, the latter terms have been used for many years and are therefore likely to be familiar to dentists.
Classification according to elastic properties and chemical type. The requirements of impression materials can be conveniently discussed under four main headings:. This requires a low viscosity or a degree of pseudoplasticity. The way in which the material interacts with saliva is another factor affecting fine-detail reproduction. Some products are hydrophobic and may be repelled by moisture in a critical area of the impression. For such products, a dry field of operation is essential.
Other materials are more compatible with moisture and saliva and no special precautions are necessary. In cases where the impression is made from a hydrophobic material the hydrophilic slurry of calcium sulphate hemihydrate in water may not be able to approach closely enough to the surface of the impression on a microscopic scale.
This can result in blow holes and loss of fine detail. The ability of impression materials and gypsum products to reproduce detail in the cast is normally determined by measuring the contact angle which a drop of aqueous calcium sulphate solution makes with the surface of the impression material. A low contact angle is favourable as it indicates good wetting.
Materials which expand during setting result in undersized dies or casts. The effect on the accuracy of fit of the resultant restoration depends on the type of restoration and the complexity of shape involved. For the simple crown preparation, illustrated in Fig. For greatest accuracy, the dimensional change should be minimal.
This results in thermal contraction, the magnitude of which depends on the value of coefficient of thermal expansion of the impression material and impression tray to which it is attached. It is difficult to calculate the precise value of the thermal contraction or to predict accurately the direction in which it operates since the contraction of the tray and that of the material act in opposite directions, providing the impression material remains attached to the tray.
This is illustrated in Fig. The effects of thermal changes are minimized if the values of coefficient of thermal expansion of the impression material and tray material are small.
It is important that the impression material remains attached to the impression tray during the recording of the impression. Partial detachment may cause gross distortions of the impression which may remain undetected and will almost certainly lead to ill-fitting appliances or restorations. Manufacturers of impression materials often supply tray adhesives which are used to enhance bonding.
Additional retention is achieved by using perforated trays. In addition to the requirements given above, there are two further requirements which apply specifically to materials used for recording undercuts.
These materials must have adequate elastic properties and adequate tear resistance, coupled with a rigidity which is low enough to enable the impression to be removed. The thickest parts of the impression are compressed against the tray when they pass the widest part of the tooth crown. As the impression is withdrawn it is likely that the material is also subjected to tensile stresses as the trapped material is stretched. If a material is rigid after setting it may not be possible to remove it from undercut areas.
This obviously has a negative effect on the ability to achieve an adequate impression, but more seriously may undermine the viability of the remaining teeth as they may be subjected to a considerable stress if an attempt is made to remove the impression. The impression recorded with the elastic material accurately records the true shape of the tooth with the correct degree of undercut. The impression recorded with the plastic material has been grossly distorted during removal and has not recorded any undercut.
The impression recorded with the viscoelastic material gives a distorted shape. The degree of distortion depends on the severity of the undercut, the thickness of the impression material and the time for which the impression is maintained in a compressed state Fig. The behaviour of viscoelastic materials is described on p.
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