Лако-красочные материалы — производство

Polymerisation may be initiated by one of three mechanisms: a) Free Radical Initiation

b) Anionic Initiation

B+A“ + CH2 = CHX -> A-CH2-CHX“B+

c) Cationic Initiation

A~B++CH2 = CHX -> B-CH2-CHX+A“

Although most vinyl and acrylic monomers will undergo any of the above types of initiation, some of the polyether monomers can only be polymerised when ionic initiation techniques are used.

In practice ionic initiation is restricted to the production of the so called "high" polymers used in structural applications. Polymers for surface coating applications are normally prepared exclusively using free radical initiation techniques.

The initiator is consumed during the course of the polymerisation and the reaction mixture requires “topping up” or “spiking” with initiator at regular intervals to maintain the free radical concentration at the desired level, to ensure a high degree of conversion of monomer to polymer.

Free radicals are extremely reactive, and the monomer units in the reaction mixture have to compete with other species for the free radical. This includes competition with other free radicals.

The rate of initiation is defined in terms of the rate of formation of polymer chains and is expressed as:

Ri = 2fkd[l]

where: Ri is the rate of initiation

f is the frequency factor for free radicals reacting with monomer units kd is the decomposition rate for the initiator

[I] is the molar concentration of the initiator

The number of chain ends present at any given time can be determined by:

Ni = 2fkd[l]t

where: Ni is the number of initiated chains at time t

The number of chain ends per monomer unit is given by

.. Ni Mm

Мі = ~мГ

where: Mi is the number of chain ends per monomer unit

Mm is the molecular weight of the monomer Mp is the mass of polymer formed during time t

Ni (number of chain ends present) = 3.8 x 10-3