Лако-красочные материалы — производство

Polymerisation can only proceed efficiently and economically if sufficient free radicals are present throughout the polymerisation. However, the presence of too many free radicals can have a deleterious effect upon the polymer, resulting in too low a molecular weight, or excessive chain grafting etc. Thus, it is desirable to know how the number of free radicals relates to the initiator concentration, initiator type, and reaction conditions, including reaction temperature. This relationship is expressed as the initiator half-life, and is defined as the time taken for half of a given mass of initiator to decompose.

The classical derivation of half-life is:

tv2 = к Г dt = — ln-/— since Ct =

со ^ г

where: C0 = concentration at time = 0

and Ct = concentration at time = t

but for initiators (the decomposition constant) is temperature dependent, and is related to the activation energy of decomposition (E), the gas constant (R) and the temperature in degrees absolute (T).

This is the standard Arhenius equation. If a maximum velocity constant (kmM) is incorporated, then:

kd = kmax Є

thus:

(2) Q’fym kmax

+ e/rt = constant + e/rt

The term ln(ln(2)/kmax) is considered to be constant, and a plot of the Naperian logarithm of Ц against 1/T will be linear, with a gradient of E/2.3R.

Organic peroxides have activation energies in the region 100-150 KJ/molecule.

Peroxy compounds with low activation energies have decomposition rates which are moderately linear with respect to temperature, and hence, are particularly suited to reaction conditions where the reaction temperature is not controlled between narrow limits.

Initiators with high activation energies show large increases in decomposition rate for a small temperature rise, and hence the half-life is shorter. In terms of reaction kinetics t^ is a first order reaction. As will become apparent, free radical formation does not follow the ideal for first order kinetics, due to the influence of factors other than temperature on the decomposition rate. In practice, the rate of decomposition has been found to lie between that predicted for first order and second order reactions.

Manufacturers and suppliers provide comprehensive data on the half-life of initiators over a wide range of temperatures. This information is essential when formulating acrylic copolymers for surface coating applications.

Examples of for some of the more commonly used free radical initiators, taken from the literature, have been reproduced below to illustrate the effect of the temperature on the decomposition rate.

However, for any given initiator does not only depend upon the reaction temperature, but also upon the reaction environment, and the concentration of initiator present. This can be illustrated by the following example for di-benzoyl peroxide.

Ц for di benzoyl peroxide used in the polymerisation of styrene monomer at 90°C, is 85 minutes, but when used in the same concentration, and at the same temperature as a cross linking agent, ti^ is 65 minutes.

The half-life is dependent on the concentration of initiator used. In general, the lower the molar concentration, the longer ил.

It is normal to quote for an initiator in solutions of 0.1 molar concentration in a suitable non-polar solvent. Primary radicals formed during decomposition can degrade

further to form secondary radicals under the influence of certain solvents. These so called secondary radicals can then initiate decomposition of other peroxide molecules. This phenomenon is termed “induced decomposition”, and this can have a significant effect upon for the initiator.