a Ideally , a ther mal free radical initiator should be relatively stable at room temperature but should decompose rapidly enough at the polymer-processing temperature to ensure a practical reaction rate. In addition to temperature, the decomposition rate (k d ) of the initiator will depend on the solvent/monomer system used. The confining effect of solvent molecules (the cage effect) causes secondary “wastage” reactions including recombination of radicals to regenerate the initiator . The cage effect becomes mor e significant as viscosity increases. The most important indicator of activity of an initiator is its half-life (t 1/2 ). It is the time required to reduce the or iginal initator content of a solution by 50%, at a given temperature. Assuming first order decomposition kinetics, which is true for most free radical organic initiators, the half-life (t 1/2 ) is related to the initiator decomposition rate (k d ) as follows: t 1/2 = ln2/k d Table Ilists the decomposition rate (k d ) data 1 for several commonly used free radical initiators, at specific tempera- tures and solvents. T o further ass ist you in your selection of a thermal initiator, the temperature corresponding to a 10 hour half-life in a specific solvent is also shown. Fig. 1 relates the initiator decomposition rate (k d ) in s -1 to the half-life for a broad range of k d values, including those found in T able I. Thermal Initiators: Decomposition Rate and Half-Life Fig 1: Relationship of Half Life (t F F F F F ) to Decomposition Rate (k d ). k x 10 8 x 10 7 t =85d 1/2 200d 20d 40d 40h 10d 24h 8.5d 20h 120m 12m 70s 7s 80s 0.75s 0.07s 10s 1s 0.12s 0.25s 0.35s 0.5s 5s 3s 20d 60h 200m 20m 120s 5h 30m 40m 200s 20s 30s 40s 10m 80m 10h 120h 60d 1y 2y x 10 6 x 10 5 x 10 4 x 10 3 x 10 2 x 10 1 x 10 0 1 2 3 4 5 6 7 8 9 10= This chart is reprinted with permission from Wiley Interscience. How to read the chart:See circled example: Half lives are to the left of each vertical line. A half life of 200s has a rate constant of 3.4 x 10 -3 Key:y =years d = days h = hours m = minutes s = seconds
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Ideally, a thermal free radical initiator should be relativelystable at room temperature but should decompose rapidly
enough at the polymer-processing temperature to ensure apractical reaction rate. In addition to temperature, the
decomposition rate (kd) of the initiator will depend on thesolvent/monomer system used. The confining effect of
solvent molecules (the cage effect) causes secondary“wastage” reactions including recombination of radicals toregenerate the initiator. The cage effect becomes more
significant as viscosity increases.
The most important indicator of activity of an initiator is itshalf-life (t
1/2). It is the time required to reduce the original
initator content of a solution by 50%, at a giventemperature.
Assuming first order decomposition kinetics, which is true
for most free radical organic initiators, the half-life (t1/2
) isrelated to the initiator decomposition rate (k
d) as follows:
t1/2
= ln2/kd
Table I lists the decomposition rate (kd) data1 for severa
commonly used free radical initiators, at specific tempera-
tures and solvents. To further assist you in your selection oa thermal initiator, the temperature corresponding to a 10hour half-life in a specific solvent is also shown. Fig. 1
relates the initiator decomposition rate (kd) in s-1 to the
half-life for a broad range of kd values, including those found
in Table I.
Thermal Initiators: Decomposition Rate and Half-Life
Fig 1: Relationship of Half Life (tFFFFF
) to Decomposition Rate (kd).
k x 108 x 107
t =85d1/2
200d 20d
40d
40h
10d 24h
8.5d20h 120m 12m
70s 7s
80s
0.75s
0.07s
10s 1s
0.12s
0.25s
0.35s
0.5s5s
3s
20d
60h
200m 20m 120s
5h 30m
40m
200s 20s
30s
40s
10m80m
10h120h
60d
1y
2y
x 106x 105 x 104 x 103 x 102
x 101 x 100
1
2
3
4
5
6
7
8
9
10 =
This chart is reprinted with permission from Wiley Interscience.
How to read the chart:
See circled example: Half lives are to
the left of each vertical line.
A half life of 200s has a rate constantof 3.4 x 10-3
Thermal Initiators: Decomposition Rate and Half-Life (continued)
1”Polymer Handbook”, Eds. Brandrup, J; Immergut, E.H.; Grulke, E.A., 4th Edition, John Wiley, New York, 1999, II/2-69; Aldrich Catalog No. Z412473.2Amines can significantly increase the decomposition rates of peroxides, e.g., addition of N,N -dimethyl aniline to benzoyl peroxide causes the latter to
decompose rapidly at room temperature.3Persulfate decomposition is pH dependent.
Table I: Decompositon Rates and 10-Hour Half-lif e Temperatures of Common Thermal Initiators
(arranged alphabetically by initiator)
Aldrich
Cat. No. Initiator Solvent T(°C) kd(s-1) 10h Half-life °C (Solvent)
441465 tert -Amyl peroxybenzoate 99 (benzene)
118168 4,4-Azobis(4-cyanovaleric acid) Acetone 70 4.6 x 10-5 69 (water)Water 69 1.9 x 10-5
Water 80 9.0 x 10-5
380210 1,1'-Azobis(cyclohexanecarbonitrile) Toluene 80 6.5 x 10-6 88 (toluene)95 5.4 x 10-5
102 1.3 x 10-4
441090 2,2'-Azobisisobutyronitrile (AIBN) Benzene 50 2.2 x 10-6 65 (toluene)70 3.2 x 10-5
100 1.5 X 10-3
179981 Benzoyl peroxide2 Benzene 60 2.0 x 10-6 70 (benzene)78 2.3 x 10-5