Chapter 8 TERMS

Question 1

define accelerator

Answer 1

A compound that speeds up the reaction; also refers to the catalyst in the reaction of impression materials.

Question 2

define addition reaction

Answer 2

A polymerization reaction in which each polymer chain grows to a maximal length in sequence.

Question 3

define base paste

Answer 3

The component that forms the main three-dimensional structure of a final impression.

Question 4

define cast

Answer 4

A dimensionally accurate reproduction of a part of the oral cavity or extraoral facial structures that is produced in a durable hard material.

Question 5

define catalyst paste

Answer 5

A component of a polymerization reaction that decreases the energy required for the reaction and usually does not become part of the final product; however, the term catalyst has been used for the structural component of dental materials that initiates the polymerization reaction.

Question 6

define colloid

Answer 6

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.

Question 7

define condenstation reaction

Answer 7

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.

Question 8

define crosslinking

Answer 8

The process of joining polymer chains to form a three-dimensional network structure.

Question 9

define elastomer

Answer 9

Any of various polymers having the elastic properties of natural rubber.

Question 10

define fusion temperature

Answer 10

The temperature below which a definite reduction in plasticity occurs during cooling of an impression compound.

Question 11

define gel

Answer 11

A network of fibrils forming a weak, slightly elastic brush-heap structure of hydrocolloid; also the solid network structure of a cross-linked polymer.

Question 12

define gelation

Answer 12

The process of transforming a hydrocolloid from a sol to a gel.

Question 13

define imbibition

Answer 13

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.

Question 14

define inelastic

Answer 14

Incapable of sustaining significant elastic deformation without fracturing under stress.

Question 15

define initiator

Answer 15

The component that starts a polymerization reaction; types include photoinitiators, chemi- cal initiators, and heat initiators.

Question 16

define micelle

Answer 16

An aggregate of surfactant molecules or ions in solution.

Question 17

define model

Answer 17

A positive full-scale replica of teeth, soft tissues, and restored structures used as a diagnostic aid for the construction of orthodontic and prosthetic appliances; a facsimile used for display purposes.

Question 18

define polymerization

Answer 18

A chemical reaction that transforms small molecules into large polymer chains.

Question 19

define pseudoplasticity

Answer 19

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.

Question 20

define rheology

Answer 20

Pertaining to the science that describes the fluid or flow characteristic of materials.

Question 21

define set

Answer 21

The state of being sufficiently rigid or elastic to permit removal from the mouth without plastic deformation.

Question 22

define setting time

Answer 22

The time from the start of mixing to the point where the material loses its flow poten- tial or plasticity.

Question 23

define shear thinning

Answer 23

The tendency for viscosity to decrease as the shearing rate increases (see pseudo- plasticity and thixotropy).

Question 24

define static mixing

Answer 24

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.

Question 25

define syneresis

Answer 25

The expression of fluid onto the surface of gel structures.

Question 26

define thixotrophy

Answer 26

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.

Question 27

define undercuts

Answer 27

The recessed areas on dental structures, including teeth, edentulous ridges, prostheses, and restorations.

Question 28

define viscoelasticity

Answer 28

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.