Chapter 8 Flashcards
define colloid
A solid, liquid, or gaseous substance made up of large molecules or masses of smaller molecules that remain in suspension in a surrounding continuous medium of different matter.
define condensation rxn
A polymerization process in which bifunctional or multifunctional mono- mers react to form first dimers first, then trimers, and eventually long-chain polymers; the reactions may or may not yield by-products; the preferred term is step-growth polymerization. All condensa- tion impression materials yield by-products.
what is an elastomer?
Any of various polymers having the elastic properties of natural rubber.
what is fusion temperature?
The temperature below which a definite reduction in plasticity occurs during cooling of an impression compound.
define gelation?
The process of transforming a hydrocolloid from a sol to a gel.
define gel
A network of fibrils forming a weak, slightly elastic brush-heap structure of hydrocolloid; also the solid network structure of a cross-linked polymer.
define imbibition
The displacement of one fluid by another immiscible fluid in a hydrocolloid. In the context
of impression materials, it is the uptake by agar or alginate when immersed in water.
define micelle
An aggregate of surfactant molecules or ions in solution.
define pseudoplasticity
The tendency of a material to become less viscous as the shear rate increases and
to recover viscosity immediately upon the elimination of shear stress.
define rheology
Pertaining to the science that describes the fluid or flow characteristic of materials.
what is shear thinning?
The tendency for viscosity to decrease as the shearing rate increases (see pseudo- plasticity and thixotropy).
what is static mixing?
A technique of transforming two fluid (or paste-like) materials into a homogeneous mixture without mechanical stirring; it requires a device that forces two streams of material into a mixer cylinder, such that as the streams move through the mixer, while the stationary elements in the mixer continuously blend the materials.
efine syneresis
The expression of fluid onto the surface of gel structures.
define Thixotropy
The property of certain gels or fluids to become less viscous when sufficient energy in the form of impact force or vibration is applied to overcome its yield stress; at rest they require a specific time to return to the previous viscous state. Both pseudoplasticity and thixotropy are shear- thinning processes; the difference is that changes in pseudoplastic viscosity do not exhibit the time dependency characteristic of thixotropy.
define undercuts
The recessed areas on dental structures, including teeth, edentulous ridges, prostheses, and restorations.
describe viscoelastciity?
The ability of a material to strain instantaneously like an elastic solid during rapid stretching or to resist shear flow and to strain linearly over time (like honey) when a stress is applied slowly.
what are important properties of impression materials?
(1) su ciently uid to adapt to the oral tissues, (2) viscous enough to be contained in a tray, (3) able to transform (set) into a rubbery or rigid solid in the mouth in a reasonable time (less than 7 min), (4) resistant to distortion or tearing when removed from the mouth, (5) dimensionally stable long enough to allow one or more casts to be poured, (6) biocompatible, and (7) cost- e ective in terms of time as well as the expense of the associ- ated processing equipment.
waht are the two setting methods, and define them?
Irreversible implies that chemical reactions have occurred and that the material cannot revert to a previous state in the dental o ce. For example, alginate, zinc oxide– eugenol (ZOE) impression paste, impression plaster, and elas- tomeric impression materials, which set by chemical reactions, are irreversible. On the other hand, reversible materials, such as agar and impression compound, so en upon heating and solidify slightly above body temperature with no chemical change taking place.
what determines the accuracy of impressions material?
extent of the rebound when removed from the mouth/impressions tray?
what are the three types of elastomers?
based on their backbone: polysulfide, silicone, and polyether backbones
what is the accelerator paste for polysulfide?
paste which contains lead dioxide, filler, and plasticizer; also oleic or stearic acid as a retarder to control the rate of setting reaction;
what is the base paste of a polysulfide?
contains a multi- functional mercaptan (-SH) called a polysul de polymer, a suitable ller (such as lithopone or titanium dioxide) to provide the required strength, a plasticizer (such as dibutyl phthalate) to confer the appropriate viscosity to the paste, and a small quantity of sulfur, approximately 0.5%, as an accelera- tor.
how does temperature affect rate of setting for polysulfide impression material?
hot and humid accelerates it
what does the basepaste of condensation silicone contain?
α-ω-hydroxyl-terminated polydimethyl siloxane; e material sets by cross-linking between terminal groups of the silicone polymers and the alkyl silicate to form a three-dimensional network (Figure 8-4). Ethyl alcohol
is a by-product of the condensation setting reaction. Its subsequent evaporation accounts for much of the contraction that takes place in the setting impression.
what is the catalyst for condensation silicone?
stannous octoate
what is the base paste of addition silicone?
a polymerization between divinylpolysiloxane and polymethylhydrosiloxane
what is the catalyst of addition silicone?
platinum salt
what is one disadvantage of silicone impression materials?
is their inherent hydrophobic nature.
Sulfur contamination from natural latex gloves inhibits the setting of addition silicone.
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why are alginate substitute impressions better than irreversible hydrocolloid ones?
The advantage of these so-called alginate substitutes is the ability to make multiple, accurate diagnostic casts from one impres- sion. Laboratory studies have reported that they exhibited better detail reproduction and less variability in linear dimen- sional change than irreversible hydrocolloid.
what does the base paste of a polyether ring-opening system impression matieral contain?
The base paste contains the polyether polymer, colloidal silica as filler, and a plasticizer such as glycol ether or phthalate.
what does the accelerator paste of a polyether ring-opening system impression material contain
The accelerator paste contains an alkyl-aromatic sulfonate in addition to the filler and plasti- cizer.
e ether-dominated polymer backbones of polyether ring-opening impression materials the most hydrophilic of all elastomeric impression materials.
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f severe undercuts are present, the use of an impressions tray sohould be avoided
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polysulfide impression material is the most elastic; good for when undercuts are severe–>can remove with less change of breakage
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An increase in temperature accelerates the rate of polymerization of all elastomeric impression mate- rials; therefore, the e ect of temperature on working and setting time should be taken into consideration.
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Normally, having more base materials in the mixture tends to increase the working and setting times. One should be aware that this is not economical, as a portion of the paste is not used. Moreover, since the accelerator paste contains a retarder as well as a reactor, increasing the base/accelerator ratio may not produce a predictable change in the polymer- ization rate.
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compare and contrast pseudoplasticity and thixotrophy
A pseudoplastic material displays decreasing viscosity with increasing shear stress, and recovers its viscosity imme- diately upon a decrease in shear stress. A thixotropic material does not ow until su cient energy in the form of an impact force or a vibration force is applied to overcome the yield stress of the material. At rest it takes a speci c time to regain its previous viscous state.
what materials exhibit shear thinning properties?
add. silicone and polyether
An impression with a su ciently high elastic limit should not sustain permanent deformation.
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list the materials in order of increasing relative amount of permanent deformation in compression (following strain from removal):
addition silicone, condensation silicone, polyether, and polysulfide; polysulfide recovers slower when exposed to elastic stress/deformation
The characteristics of a viscoelastic solid are intermediate between those of an elastic solid and a viscous liquid
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how to remove viscoelastic materials properly?
very quickly–limit the time for deformation when removing; a rocking/teasing method should be avoided as that can deform the impression; however, some slight teasing will need to be done to break the seal between the impression material and object
what is the order of impression materials with increasing elastic modulus?
polysulfide, condensation silicone, addition silicone, and polyether.
Although the new polyether materials are signi cantly more exible than that of PVS at the time of removal, their adhesion, which results from the inherent hydrophilicity, can still make the impres- sion removal di cult.
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define tear strength
The amount of force needed to tear a specified test specimen divided by the thick- ness of the specimen
what is the order of increasing tear strength
silicones (addition and condensation), poly- ether, and polysulfide. However, the literature has shown that some polyethers or silicones have a higher tear strength than certain polysulfides.
what is tear energy?
dividing the force (F) needed to keep the crack propagating divided by t/(1+λ), where t is the thickness of the specimen and λ is the extension ratio
why can tear strength not be used to define resistance to tear?
idk lol confused
contract dimensional accuracy vs dimensional stability
e former refers to the lack of dimensional change during curing and shortly after removal from the mouth. The latter refers to the lack of dimensional changes over time.
what are the six major causes of distortional change/inaccuracy/instability?
(1) polymerization shrinkage, (2) loss of a condensation reac- tion by-product (water or alcohol), (3) thermal contraction from oral temperature to room temperature, (4) absorption of water or disinfectant over a period of time, (5) incomplete recovery of deformation because of viscoelastic behavior, and (6) incomplete recovery because of plastic deformation
One variable that has a negative e ect on the polyether impression is the absorption of water or uids and the simul- taneous leaching of the water-soluble plasticizer. us, the polyether impression should be stored in a dry (relative humidity below 50%), cool environment to maintain its accu- racy and should never be le for protracted periods in disin- fecting solutions.`
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It is important to note that a long immersion time of condensation silicones, addition silicones, and polysulfides in disinfectants may cause the surfactant in the hydrophilic polyvinyl siloxane to leach out and render the impression less hydrophilic.
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