2. Product Information and Characteristics
2.4.1. Safety and Quality
Styrene is a reactive monomer, undergoing many reactions of an
unsaturated compound (such as addition) and of an aromatic compound
(such as substitution). Polymerisation generally takes place by
free-radical reactions initiated thermally or catalytically.
Polymerisation occurs slowly even at ambient temperatures. To maintain
the monomer content for quality as well as safety reasons the
polymerisation reaction is retarded by phenolic inhibitors. The most
commonly used inhibitor is tertiary-butyl catechol (4-tert-butylcatechol
(TBC) or p-tert-butyl catechol; CAS RN 98-29-3; EC No.: 202-653-9), but
hydroquinone (1,4-dihydroxybenzene; CAS RN: 204-617-8; EC No.:
123-31-9; Annex I Index No.: 604-005-00-4) works also. The
polymerisation reaction is exothermic and if contained may become
violent. If the heat is not removed, the bulk styrene temperature may
rise to a level at which polymerisation is self-sustaining and very
rapid, evolving the release of large quantities of heat together with
volumetric expansion.
As pictured in Figure 4, oxygen and inhibitor must be maintained at
certain minimum levels to prevent oxidative degradation and subsequent
(poly)peroxide formation. The presence of oxygen is required for the
inhibitor to function properly, therefore styrene needs to be stored
under air or inert gas with sufficient oxygen levels (see Table 2 for
details on the solubility of gases in styrene).
A recent study on the inhibition chemistry confirmed that free peroxide
radicals, such as benzaldehyde and formaldehyde, are formed at the same
time. An oxygen level of 5%-vol. in the vapour phase is regarded as
being sufficient for inhibition of the liquid phase. This means that
limited exposure to air is the best way to keep the content of
oxygenates as low as possible.
Figure 4: Flammability Diagram for "Styrene/Oxygen/Nitrogen"
It should be noted that in climate zones and in seasons with significant
temperature differences between night and day, the styrene vapours
evolved in the headspace at higher temperatures will condense on roofs,
walls and internal fittings of storage tanks when it cools off. The
phenolic inhibitors have high boiling points and stay in the liquid
phase, resulting in the condensed styrene vapours containing no
inhibitor. Precautions must be taken to prevent polymer build-up on
surfaces above the liquid level of tanks and containers, which could
result in plugging of pressure-relief valves.
Table 2: Solubility of Gases in Styrene at 25 oC
GAS CC |
GAS/CC STYRENE |
ppm by wt. |
Oxygen |
0.4 |
581 |
Nitrogen |
0.08 |
102 |
Air |
0.15 |
198 |
O2 from air* |
0.032 |
50 |
* amount of oxygen when saturated with air
In warm climates, consideration can be given to applying a coating on
the internal surfaces of tanks, thereby preventing discolouration of the
styrene from oxidised iron and minimizing polymer build-up in the
headspace. US patent 4,161,554 describes a coating system employing TBC,
which upon one single application is expected to last for several
years.
It is recommended that the maintenance and inspection schedule for
storage of styrene be followed as indicated in Table 3, which includes
auxiliaries such as vapour return lines and safety valves.
Table 3: Schedule for Storage Maintenance of Styrene
CHECK POINT |
INTERVAL |
REMARKS |
Inhibitor Analysis |
2-3 times weekly |
|
|
Daily |
if above 25-30°C |
Polymer Analysis |
2-3 times weekly |
|
|
Daily |
if above 25-30°C |
Colour |
2-3 times weekly |
|
Oxygen |
Weekly |
easier in vapour phase |
Monomer Temperature |
Daily |
|
Monomer Vapour Temperature |
Daily |
|
Inspection Points for Polymer |
Air Vent |
at least quarterly |
|
Foam Reservoir |
within 5 years |
|
(Internal) Roof Seal, Sides and Floor |
when technically possible |
|
Vacuum Pressure Relief Valve |
at least quarterly |
|
Flame Arrestor |
at least quarterly |
|
Vapour Return Line |
at least quarterly |
|
Long Product Lines |
pending residence time |
See Table 3 |
Product Turnover |
Date of Last Filling |
running log |
|
Volume Before Filling |
running log |
|
Volume After Filling |
running log |
|