6 UNSATURATED POLYESTER
7 EMULSION POLYMERISED STYRENE BUTADIENE RUBBER
7.2 Applied processes and techniques in the production of emulsion styrene butadiene rubber
An overview of the production process is given in the flow chart in Figure 7.3.
Mechanical
Figure 7.3: Flow diagram of the ESBR production process
Above a critical concentration, surfactant molecules form aggregates called micelles. One exam-ple of this phenomenon is a solution of the potassium or sodium salts of fatty or rosin acids.
These are usually known as soaps. The addition of monomers which are insoluble in water, such as styrene or butadiene, to the stirred soap solution results in droplets of monomer stabilised by soap molecules being formed. These droplets are approximately 1000 nm in diameter.
MP/EIPPCB/POL_BREF_FINAL October 2006 123 Despite being highly insoluble, the monomer is able to diffuse through the water to the soap mi-celles where it enters the hydrocarbon rich interior of the micelle. The addition of a free radical catalyst results in a polymerisation reaction occurring within the micelle. A high molecular weight polymer is rapidly formed and terminated. The polymerisation is fuelled by more mono-mer diffusing from the droplets to, what now is called, the growing latex particle.
The latex particle is stabilised by soap molecules adsorbed on the surface. As the particle grows, more soap is required and this is taken from inactivated micelles. At between 10 and 20 % of conversion from monomer to polymer, no micelles are left, as the soap concentration has dropped below the critical micelle concentration. At about 60 % conversion, the monomer droplets disap-pear.
Polymerisation is terminated before total conversion in order to avoid unwanted effects such as long chain branching and the formation of gel. Termination is effected by the addition of chemi-cal substances known as a short stop, which immediately kills all free radichemi-cals.
About ten years ago, all ESBR plants modified their processes to eliminate the presence of vola-tile nitrosamines. These potentially carcinogenic chemicals were present at concentrations in parts per billion. The changes made eliminated the use of sodium nitrite and one component of the short stop system, sodium dimethyl dithiocarbamate.
The molecular weight of the polymer molecules is regulated by the addition of a chain transfer agent or modifier. This has the effect of terminating one growing molecule and initiating another.
The more modifier that is added, the lower the molecular weight of the final product.
The reaction is carried out continuously in a series of continuously stirred tank reactors (CSTR) under moderate pressure. The latex is then stripped of unreacted monomers. Butadiene is re-moved in flash tanks, the first at atmospheric pressure and an optional second under vacuum. The latex then passes to steam stripping columns, where styrene is removed.
The resultant latex contains about 1015 particles/cm3 and each particle is about 60 nm in diameter.
The content of solids is typically between 20 and 25 %. The basic technology of emulsion polym-erisation has remained more or less unchanged since the 1940s, when the introduction of the re-dox catalyst system saw the production of the so-called ‘cold SBR’. The rere-dox system allows the production of free radicals at a low temperature of 5 oC instead of 50 ºC (hot SBR), resulting in a better controlled reaction and a rubber with improved mixing characteristics and better final properties.
7.2.1 Preparation of rubber bales
The stripped latex is blended with an antioxidant emulsion prior to coagulation. By changing the pH of the latex from alkaline to acidic, the soap is converted to organic acid whereupon the latex immediately coagulates. The organic acid remains in the rubber. The typical concentration of organic acid in the final product is about 5.5 %. The coagulation is caused by the addition of sul-phuric acid and coagulation aids, and the rubber appears as small crumbs suspended in water.
After leaching to remove the acid, the rubber crumb suspension passes over screens where most of the water is removed and recycled back into the coagulation process. The wet crumbs pass to a dewaterer which reduces the water content to about 10 %. Then it is conveyed to a dryer where the water content is further reduced to <1.0 %. The dry crumbs are pressed into bales which are wrapped in polyethylene or ethylene vinyl acetate (EVA) film and packed automatically into crates.
The antioxidant is added to protect the rubber during the drying and baling process and to give it an adequate storage life. The typical antioxidant concentration lies in the range of 0.5 to 2.0 %.
Under normal storage conditions (dry, mild temperature and indirect sunlight), ESBR will have a storage life of at least one year, providing that the packaging remains undisturbed.
7.2.2 Oil extension
Another important development was made in 1951 with the discovery of oil extension, where a very high molecular weight rubber has its viscosity significantly reduced by the addition of ap-proximately 28 % of compatible oil. An oil emulsion is prepared which is coagulated with the rubber latex. At the moment the emulsion is broken, the oil transfers quantitatively to the rubber – no free oil being observed at any time. Oil extended rubber allows highly filled compounds to be mixed easily whilst maintaining final properties at a high level.
7.2.3 ESBR latex
Some ESBR plants also produce latex as a finished product. The polymerisation plants are used to produce basic latex which has a low solids content and a small particle size. For practical and economic reasons, it is necessary to increase the solids content of the latex. Straightforward evaporation of the base latex only allows a solids content of around 50 % to be reached before the viscosity becomes too high. This problem can be overcome by increasing the particle size in an agglomeration process. Subsequent evaporation of the agglomerated latex enables solids contents of more than 60 % to be achieved whilst maintaining a practical viscosity. These high solids lati-ces are principally used to produce foam mattresses and pillows, foam-backed carpets, adhesives and sealants.
In the case of plants that produce ESBR latex as a finished product, it could be possible use dif-ferent technical parameters and difdif-ferent processes in addition to those described in Table 7.2 and Figure 7.3 [27, TWGComments, 2004].
MP/EIPPCB/POL_BREF_FINAL October 2006 125 7.2.4 Technical parameters
Product type ESBR
Reactor type Continuously stirred tank reactors in series
Reactor size 10 - 40 m3
Number of reactors in use up to 15 Polymerisation pressure up to 0.5 MPa
Polymerisation temperature 5 – 10 °C (50 ºC is used to produce the so-called
‘hot SBR’)
Emulsifying agent Various anionic surfactants, usually fatty or rosin acid soaps.Nonylphenols are used at some sites
(see footnote)
Modifier Tertiary dodecyl mercaptan
Shortstops Sodium polysulphide
Isopropyl hydroxylamine, diethyl hydroxylamine
Catalyst/initiators Hydroperoxides/iron peroxide salts for hot SBR
% solids at end of reaction 15 - 30 % Conversion of monomer to polymer 50 – 70 %
Antioxidant p-phenylenediamine derivatives, phenolic types, phosphite types
Extender oil
Highly aromatic, naphthenic, treated distillate aromatic extract (TDAE), mild extract solvate
(MES)
Capacity per reactor line Typically 30000 - 60000 t/yr.
Note: Nonylphenol is harmful for aquatic ecosystems, and it has been declared as a ‘hazardous priority substance’ under the Water Framework Directive, meaning that discharge to all water bodies should be stopped by 2015.
Table 7.2: Technical parameters of the ESBR process
7.3 Aktuelle Emissions- und Verbrauchswerte
Die inTable 7.3 angegebenen Werte stammen von sechs Anlagen in Europa. Jeder Emissions- und Verbrauchsbereich ist als Spanne wiedergegeben, die durch Streichung des jeweils angege-benen niedrigsten und höchsten Wertes erhalten wurde. Alle Werte beziehen sich auf die emittier-te oder verbrauchemittier-te Menge pro Tonne Produkt.
Parameter Einheit Niedrigster Wert Höchstwert
GJ 3 8
GJ 1 2
Energie- und Wasserverbrauch:
Dampf Elektrizität
Wasser m3 5 50
Emissionen luftseitig:
VOC, gesamt g 170 540
Ablauf der Abwasserbehandlung:
Abwassermenge m3 3 5
CSB-Genehmigungswert für den Standort g/t 150 200 Industrielle Abfälle:
Gefährlich kg 3,0 5,0
Nicht gefährlich kg 0,24 3,6
Kautschukabfälle kg 1,5 5,2
Table 7.3: Emissions- und Verbrauchsangaben bei ESBR-Anlagen (pro Tonne Produkt)
MP/EIPPCB/POL_BREF_FINAL October 2006 127