ADHESIVES, GLUES AND SEALANTS
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:
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.
Formation of radical-R --- RAD> RAD * + * R
Chain initiation RAD * + A ---> RAD-A *
RAD-A * + A ---> RAD-A-A *
RAD-A-A * + A ---> RAD-A-A-A * ...
Chain termination - There are several ways to end the reaction by either:
Combine 2 radicals RAD-AAA-AAA * + * --- RAD> RAD-AAAAAA-RAD
Using inhibitors / regulators – external agents of the polymerization which react with the radical making terminate the reaction.
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:
Shorter chains of the polymers and decreasing their mechanical properties (strength and elongation)
Reduction of working time, pot life and curing.
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.
Requires less activation energy than the radical polymerization.
It is not as temperature dependent.
The chain termination occurs only for the use of inhibitors, regulators or other agents that stop the reaction.
Depending on the charge of the ion, ionic polymerization is classified into:
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-
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:
Acrylic curing by ultraviolet.
Anionic - Cyanoacrylates
Cationic - Radiation curing Epoxy
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