ADHESIVES, GLUES AND SEALANTS


Polymerization - polymerization reactions

What is polymerization?

Polymerization reactions are the set of chemical reactions in which an initiator or hardener monomer active other monomer starting a chain reaction which forms the polymer.

Consider the wick of an explosive, when approaching a heat source such as a match or a lighter, this reacts quickly burning all along the wick, in this example lighter or match would be the initiator monomer and wick burned would be the final polymer that has obtained.

The polymerization reactions are classified as:

glue point Radical Polymerization

Ionic Polymerization:

Radical Polymerization.

In order to produce radical polymerization it´s necessary that initiator monomer or hardener, which activates and causes the reaction, contains free radicals, namely unpaired electrons which react with the resin monomer in order to form the polymer.

Radical is defined as chemical material extremely unstable therefore with a high reactive power due it has unpaired electrons.

The steps that occur in the radical polymerization are:

Initiation of the reaction - free radical is formed by the action of chemical, thermal, electrochemical or photochemical energy.

Chain growth

Chain termination - There are several ways to end the reaction by either:

One of the main disadvantages of radical polymerization is that it can not control the molecular weight and size of the final polymer, because each reaction terminates in an undefined way.

The polymers that are produced by radical polymerization are strongly temperature dependent, if temperature increase will speed up the polymerization reactions causing:

Ionic polymerization.

Ionic polymerization operation is similar to the radical polymerization; the radical in this case is an ion (atom or molecule) in which one of its parts is charged positively or negatively by the absence or presence of electrons.

Features:


Depending on the charge of the ion, ionic polymerization is classified into:

glue point Anionic polymerization - The ion is an atom or molecule with excess of electrons - negatively charged.

ION-+ A -> ION-A-

ION-A-+ A -> ION-A-A-

ION-A-A-+ A -> ION-A-A-A-

glue point Cationic Polymerization - The ion is an atom or molecule with lack of electrons - positively charged.

ION + + A -> ION-A +

ION-A + + A -> ION-A-A +

ION-A-A + + A -> ION-A-A-A +

Adhesives cure by polymerization:

glue point Radical polymerization

glue point Ionic Polymerization:


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