D08.
Define initiator and free radicals and describe three ways that a molecule can
be activated.
Free
radicals are compounds with a free electron. Such compounds can be formed when a
bigger compound is split into two smaller compounds. During such a reaction, a
bond is split, and when that split occurs, the smaller segments retain one free
electron each from the bond that was broken.
Free
radicals are formed when certain molecules interact with electromagnetic fields.
Such fields are formed by electromagnetic waves such as infra red (heat) light,
visible light, ultra violet light or x-ray. Certain compounds, such as tertiary
amines, can also interact with larger compounds and cause them to produce two
free radicals
For
example, benzoyl peroxide, C6H6-C(O)-O-O-C(O)-C6H6, when exposed to heat or tertiary
amines (different activators), decomposes into two C6H6-C-O* units, where *
represents an electron. These two units are two free radicals, I*, that will
initiate the first step of the polymerization process.
Activation
Activation can occur after exposure to heat, x-rays, light and certain chemicals. In all cases, an exchange of electromagnetic wavelengths occurs during the activation process. A chemical activator that has been used in self-curing dental resins is N,N-dimethyl-p-toluidine. However, because this amine tends to cause discolorations, other more color stable tertiary amines have been introduced.
N,N-Dimethyl-p-Toluidine
Initiator
Initiator
is the chemical compound that is capable of forming free radicals. Benzoyl
peroxide is an example. Another example includes campheroquinone, which is used
in light-activated composites.
Benzoyl peroxide
Free
radical
A
free radical is a compound that has a free electron that is available for bond
formation.
Free radical
When the tertiary amine interact with the benzoyl peroxide, two free radicals are from from each benzoyl peroxide molecule.
Question:
Where are the activator and the initiator kept before the self-curing material is mixed?
