Elastic Impression Materials PDF

Elastic Impression materials By Dr. Sherif Refat Abd El Monem Lecturer of Biomaterials  Classification of impression materials Impression materials Rigid impression materials...

Elastic Impression materials By Dr. Sherif Refat Abd El Monem Lecturer of Biomaterials  Classification of impression materials Impression materials Rigid impression materials Elastic impression Impression materials Zn O/eugenol compound Elastomers Hydrocolloids Condensation polysulfide Polyether Addition Silicon Agar Alginate silicon  Classification of Dental Impression Materials Setting Mechanism Rigid (inelastic) elastic Chemical reaction -Alginate (irreversible) Zinc oxide– -Polysulfide eugenol -Polyether -Condensation silicone -Addition silicone Thermally induced Impression Agar physical reaction compound (reversible)  The impression materials after setting can be rigid (inelastic) or elastic. A set rigid material is highly resistant to flexure, and it fractures suddenly under stress (like upon removal from mouth), like chalk as : ZOE impression paste, impression plaster, and impression compound. elastic impression materials means that the material is flexible and can be deformed and still return to its original form to a great extent when unstressed. Examples include : agar, alginate, and elastomers.  Q: Can we use rigid impression materials to take impressions for intraoral areas with undercuts ? A: No, because on setting they would be locked in undercut areas (e.g. interproximal areas) and upon removal they would be permanently deformed and fractured Diagram illustrating how an impression material is placed under stress during removal from an undercut area. (a) Impression in place before removal. (b) During removal https://pocketdentistry.com/16-impression-materials-classification-and- requirements/  I-Elastic impression materials A-Hydrocolloids What are colloids? - Substances are called colloids when they consist of two or more phases with the units of at least one of the phases having a dimension slightly greater than simple molecular size - The range of size is usually recognized as being approximately 1 to 500 nm measured in maximum dimension However, there are many variations between different references about determining the range of particle size of the colloidal phases - Colloid is a Heterogenous mixture with dispersed phase having particle size intermediate between true solution and suspension https://www.priyamstudycentre.com/2021/03/colloid.html Important notes about colloids: 1-True solution = Single phase 2-Colloid = two separate phases = Dispersed phase + Dispersion phase 3-When dispersion phase of a colloid is water it is called Hydrocolloid 4-Colloids can exist in two states sol or gel 5-Colloids can experience sol to gel or vise versa transformation under certain conditions and this transformation could be reversible or irreversible. 6- In sol state the colloid exists as a viscous liquid 7- Gel exists as an elastic solid or semisolid or jellylike. A sol is a colloidal dispersion in a continuous liquid medium and a gel is a suspension that behaves like an elastic solid. If a hydrocolloid contains an adequate concentration of the dispersed phase, the sol, under certain conditions change to a gel. In the gel state, the dispersed phase forms agglomerates in the form of chains or fibrils, also called micelles. https://www.mdpi.com/gels/gels-03-00004/article_deploy/html/images/gels-03- 00004-g006.png Quiz : If a hydrocolloid is an impression material so it must be inserted in the mouth in ------ state to be sufficiently fluid to accurately catch intraoral details, and left in the mouth sufficient time to be converted into ----- state to develop enough elastic properties to be removed from mouth without distortion Alginate (irreversible hydrocolloids) When alginate powder is fluffed the diatomaceous earth, with fine porous silica particles will become airborne as the cover is removed. Long-term exposure by inhalation can cause Silicosis and pulmonary hypersensitivity. So, you have to wear mask before removing cover Manufacturers have introduced a “dustless” alginate polyethylene glycol or polypropylene glycol in the alginate powder agglomerate the particles powder becomes denser and less able to become airborne. The setting of alginate hydrocolloid is a process of crosslinking alginic acids with calcium ions. The alginic acid, which is extracted from brown seaweed, is a linear copolymer of β-D-mannuronic acid and α-L-guluronic acid  2-Dispensing the Alginate: The recommended amount of water is measured into rubber bowl using the manufacturer’s liquid measuring set. The water temperature should be at room temperature (22° to 23° C). a full measure free from large voids by The manufacturer’s powder scoop is taken. Finally, the excess powder is scraped off with a spatula. The powder is put into the rubber bowl that contains the water.  4-Loading the Tray: The spatula is used to transfer the alginate from the bowl to the tray. The tray is filled to the top , make sure that there are no voids. A useful method is to start at the posterior to anterior by adding more and more to the posterior. Alginate is The most flexible elastic impression So, Remove by sharp rapid snap from patient mouth to: decrease permanent deformation Effect of consistency of the mix of alginate and time of removal on the tear strength of alginate impressions. https://pocketdentistry.com/8-impression-materials/  Definition : The resistance of impression Tear strength material to fracture (tear) when subjected to a tensile force Hydrocolloids have relatively low tear strength, so:- - Thickness must be from 4-6 mm “look at tray selection” - During removal from patient mouth, remove by sharp rapid snap The tear strength of alginates varies from 3.7 to 6.9 N/cm. Take care: many sections of an impression are thin. You must follow recommendations for minimum thickness dimensions.  Teared impression Inaccurate cast https://support.clearcorrect.com/hc/en- us/articles/216946298-Impressions-Problems#9 3- Dimensional stability: Syneresis and imbibition problems Storage in either air or liquid results in high dimensional changes due to syneresis (in air) and imbibition (in liquid). You have to pour alginate imp. as soon as possible after disinfection (about 10 min), if that is not possible store in 100% relative humidity for shortest time or wrap it by wet towel. As, Storage in humid air about 100% relative humidity results in the least dimensional changes. So, the most appropriate procedure is to pour alginate after disinfection immediately (10-15 min.)  4-Dimensional accuracy: The impression material must record the detail of the oral tissues, but this detail also must be transferred to the model or die. the minimum ability must be to transfer a line only 0.075 mm wide to a gypsum model or die material. - Alginate undergoes slight shrinkage during setting due to agglomeration of dispersed phase (like Agar)  4- COMPATIBILITY WITH GYPSUM After imp. rinse well in cold water to remove saliva and any blood, and then disinfect. Next after disinf. , all free surface water should be removed before pouring a gypsum model. Take care : 1- Saliva , blood interfere with gypsum setting, 2- free accumulated water dilute gypsum and produces soft chalky gypsum surface. 3- Don t leave the cast too long in contact with impression after setting (set gypsum will absorb water from impression that results in cast with chalky surface).  B- Elastomers Introduction: Elastomeric impression materials when set become flexible cross-linked polymers. Elastomeric impression materials dominate the market mainly Due to their : 1-greater accuracy, 2- dimensional stability with time, and 3- ability to record details as compared with the hydrocolloid materials. - In this LECTURE we concentrate on the most commonly used elastomeric impression materials : addition silicones and polyethers extra low, low, medium, heavy, and putty, in increasing order of filler content Extra-low and putty forms are only for condensation and addition silicones According to ISO : type 0: putty, Type 1: heavy-bodied, type 2: medium-bodied, and type 3: light-bodied  1- Hand mixing: -the same lengths of materials onto a mixing pad - The catalyst paste is first collected on a stainless steel spatula , then spread over the base paste - Till uniform in color - The two-putty systems for condensation and addition silicone are dispensed by an equal number of scoops of each material. https://mydentaltechnologynotes.wordpress.com/2018/07/05/ impression-materials-non-aqueous-elastomers/  2-static automixing: The base and catalyst are in separate cylinders of the plastic cartridge. (all except putty) without mechanical mixing By flow division of streaming mix over alternating left and right turn helixes Uniform mix with no air-bubbles No motor driven static automixing system  3- dynamic mechanical mixer : ease of use, speed, and good mix, but more expensive than hand and automixing -The impression materials are supplied in collapsible plastic bags placed in a Cartridge -uses a motor to drive parallel plungers, forcing the materials into a mixing tip to an impression tray or syringe. - Motor driven mixing tip.  Three common methods for making impressions for a fixed restoration are:1) a simultaneous, dual-viscosity technique,2) a single-viscosity or monophase technique, and 3) a putty-wash technique. In general, Impression material is injected directly into the prepared teeth (low viscosity)and a tray containing the bulk of the impression material(higher viscosity) is placed thereafter.  1-Simultaneous, dual-viscosity technique: low and high viscosity imp. Mat are mixed at the same time Low viscosity is injected by syringe on prepared teeth and critical areas High viscosity is placed in tray Then tray is placed in mouth https://mydentaltechnologynotes.wordpress.com/2018/07/05/ impression-materials-non-aqueous-elastomers/  An elastomeric addition-silicone impression. Turquoise material is of a low or injection consistency, and maroon material of a high or tray consistency. (Courtesy Dr. Charles Mark Malloy, Portland, OR.) (craig’s textbook )  2- Single-viscosity or monophase technique, impressions are taken with a medium-viscosity imp. Mat. Pseudoplastic materials :a decreased viscosity when subjected to high shear rates (shear thinning) e.g. addition silicon and polyethers https://mydentaltechnologynotes.wordpress.com/2018/07/05/ impression-materials-non-aqueous-elastomers/  3- The putty-wash technique : a two-step impression procedure a preliminary impression is taken in high- or putty-consistency material before preparation. Space is provided for a low consistency material, and after preparation, a low-consistency material is syringed into the area and the preliminary impression reinserted. https://www.youtube.com/watch?v=3iUUG18nxho  ADDITION SILICONE IMPRESSION MATERIALS Addition silicone impression materials (also known as vinyl polysiloxanes [VPS]) Composition and Setting Reaction: The material is supplied as a two-paste or a two-putty system. One contains a low-molecular-weight silicone with terminal vinyl groups, reinforcing filler, and a chloroplatinic acid catalyst. and the other contains a low-molecular-weight silicone with silane hydrogens and reinforcing filler. https://pocketdentistry.com/8-impression-materials-2/  No volatile by-product (such as water or ethanol) is formed in this addition polymerization reaction, So, minimal dimensional change occurs If hydroxyl groups are present in the addition silicone , that can results in the formation of hydrogen gas. https://pocketdentistry.com/12-replicating-materials-impression-and-casting/ The hydrogen can produce bubbles in gypsum dies or casts if poured in less than 30 minutes after the impression is taken. So, it is recommended to wait at least 30 minutes for the setting reaction to be completed before the gypsum models and dies are poured.. Epoxy dies should not be poured until the impression has stood overnight Some products allow immediate pouring by the inclusion of a hydrogen absorber, such as palladium. Therefore, the manufacturer’s directions should be followed.  Properties: The dimensional change in 24 hours is very low. The elastic recovery is about 99.8% (permanent deformation of 0.2%) which is the highest between all the impression materials. High tear strength when compared to hydrocolloids Elastomerics are viscoelastic So, Snap rapid sharp removal of imp. is required  Compatibility with gypsum products: Nearly all addition silicones contain surfactants, so they are hydrophilic with contact angles of about 20° to 40°. C Addition silicone impression materials are often supplied in cartridges of base and catalyst (A) that are auto-mixed through a dispensing tip and using a mixing gun (A, B). Alternatively, the material may be supplied as “sausages” (C) that fit into a power mixer. (Courtesy Y-W Chen, University of Washington Department of Restorative Dentistry, Seattle, WA.) (dental material foundations and applications) Dental materials foundations and applications 11ed. ; Ch. Impression materials  DIMENSIONAL CHANGES ON SETTING : 1-the major cause for contraction during setting is cross-linking and rearrangement of bonds between polymer chains (for both addition silicon and polyethers PE) 2-Addition-silicone AND PE materials undergo polymerization shrinkage. Dimensional stability : 1- high in air when compared to hydrocolloids 2-The addition silicones have small linear dimensional change after 24 hrs 3- The dimensional change in 24 hours of about - 0.1% is very low 4- some types of addition silicon can be poured after 1 week  POLYETHER IMPRESSION MATERIALS (PE) Polyether materials have properties similar to that of the addition silicones. The polyether polymer is hydrophilic and exhibits good wetting properties. The setting reaction mechanism is : Cationic addition polymerization by opening of the reactive terminal rings.  Composition : base paste:- long-chain polyether copolymer with alternating oxygen atoms and methylene groups (O–[CH ] ) and reactive 2 n terminal groups - Silica fillers - plasticizers - triglycerides Catalyst paste : aliphatic cationic starter as a cross-linking agent. + silica filler and plasticizers The reaction mechanism is shown (PE2): Cationic polymerization by opening of the reactive terminal rings. The backbone of the polymer is supposed to be a copolymer of ethylene oxide and tetramethylene oxide units. The reactive terminal rings open by the cationic initiator of the catalyst reactive terminal ring becomes cation itself to open more rings then chain lengthening goes on (PE3)  Properties: The elastic recovery of the polyethers is slightly less than that of the addition silicones. PE is viscoelastic so Sharp rapid removal of imp. From mouth is necessary Higher tear strength than hydrocolloids The polyether absorbs water and changes dimensions if stored in contact with water. So, polyether impressions should not be stored in water and should be washed and dried after removal from the mouth and disinfectants the polyether impression should be stored in a dry (relative humidity below 50%) away from sunlight,and cool environment to maintain its accuracy  Biocompatibility: The aromatic sulfonic acid ester catalyst type can cause skin irritation, and direct contact with the catalyst should be avoided. Thorough and good mixing of the catalyst with the base should be accomplished to prevent any irritation of the oral tissues. Polyethers (PE) show defined working time with a sharp transition into the setting phase. This behavior is often called snap-set. Note : working and setting times of elastomeric impression materials are shortened by increases in temperature and humidity  Polyethers are hydrophilic , they are compatible with gypsum products, producing excellent cast surface. Viscosity : - viscoelastic pseudoplastic properties as addition silicones , which allows use of monophase impression materials - shear-thinning due to a weak network of triglyceride crystals in PE. - The crystals align when the impression material is sheared, but form a random network when not sheared. https://www.researchgate.net/figure/Pour-the-cast-with-compatible- dental-plaster_fig1_282184121  https://pocketdentistry.com/12-replicating-materials- impression-and-casting/ Demonstration of the mechanism for the property of shear thinning or pseudoplasticity in polyethers.The trigliceride network, A, within the impression material aligns when sheared as with syringing, and , to achieve a lower viscosity B Once the shear force is removed, the viscosity increases with randomization of the triglyceride network, C. The polyether materials are supplied in two mixing systems: (1) as a cartridge with a mixing gun or (2) as a sausage with a dynamic mechanical mixer  POLYSULFIDE IMPRESSION MATERIALS The polysulfide impression materials (also called rubber base or mercaptan) are supplied as two pastes, with one tube labeled catalyst or accelerator and the other marked base. The base material consists of about 80% low-molecular weight organic polymer and contains reactive mercaptan groups (-SH) and 20% reinforcing agents The accelerator, or catalyst, tube contains a compound that causes the mercaptan groups to react to form a polysulfide elastomer. It contains lead dioxide, filler, plasticizer, and oleic or stearic acid as a retarder to control the rate of the setting reaction  Three viscosities; light, regular and heavy The most common catalyst is lead dioxide Condensation polymerization , water by- product The permanent deformation values of 2% to 3% (elastic recovery of 97% to 98%) Polysulfide impression materials shrink 0.3% to 0.4% during the first 24 hours Studies indicate that accuracy is improved if the models or dies are prepared 30 minutes after removal, but no more long delays should occur. Can be electroplated The use of lead-free polysulphide is present the highest tear strength  CONDENSATION SILICONE IMPRESSION MATERIALS used as an accurate duplicating material in the dental laboratory. The base paste contains a moderately low-molecular- weight silicone liquid, which has reactive –OH groups, Reinforcing agents such as silica The accelerator consists of a tin organic ester suspension and an alkyl silicate.  The multifunctional ethyl silicate produces a network or cross-linked structure that causes the low values of permanent deformation and flow. The ethyl alcohol produced as a by-product and gradually evaporates and cause high shrinkage during the first 24 hours after setting. The setting reaction is sensitive to moisture and heat, which when increased resulting in shorter setting and working times Putty-wash technique is preferred The dimensional change during 24 hours after setting is relatively high. About 50% of the dimensional change occurs during the first hour after setting, and the remaining 50% occurs between 1 and 24 hours THANK YOU