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;