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.



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

CEFIC
Avenue E Van Nieuwenhuyse 4
B- 1160 Brussels

Patrick Detournay
T +32 2 676 72 05
E pdt@cefic.be