Description

Organic peroxides are used extensively as a source of free radicals for the polymerization of unsaturated monomer.

Polymerization of monomers can be done with Free radical chemistry for polymerization of monomers containing double bonds in order to produce polymers like low-density polyethylene (LDPE), polyvinyl chloride (PVC), polyacrylate, polystyrene (EPS, GPPS, HIPS, SAN) and polybutadiene.

Organic peroxides are also used for the rheological control of polypropylene by using vis breaking.

Selection of  the appropriate peroxide is crucial for  good processing control and achievement of the expected   properties of polymer.

Typical monomers that can be polymerized by organic peroxides through several mechanisms :

Free radical polymerization yielding LDPE. Polymeric chains are branched allowing the material to have specific performances (rheology and mechanical properties). 

The Bulk or Mass process: This process is used for polymerization of GPPS and HIPS. In this case, styrene monomer used for polymerization is also acting as the solvent.
Suspension polymerization: This process is used for polymerization of EPS where styrene is dispersed  as fine droplets into water. The free radical initiators are exclusively organic peroxides that need to be soluble in styrene and insoluble in water. The size of the droplets can be adjusted by numerous parameters such as addition of colloids, stirring conditions, reactor geometry, etc.

Polymerization of vinyl chloride is exclusively done via a free radical mechanism
The reaction is generally performed between 40 and 80°C over 3 to 10 hours. The molecular weight and density can be regulated by temperature. Temperature is constant during polymerization for a given grade, for example Kw67 (most produced grade of S-PVC) is manufactured at 57°C (true in U.S.?). There are four processes used : Suspension, emulsion, micro suspension and mass process.

Free radicals degrade the polypropylene backbone by way of a reaction called vis breaking (or beta-scission).
This phenomenon is used by most polypropylene producers to adjust the melt flow, grade, and narrow molecular weight distribution during the final step. The benefits for producers are higher rationalization of the plant and much higher production flexibility.  This allows the producers to provide the market with a wider range of products.
The organic peroxide can be added  to thehopper of the extruder, or injected directly into the extruder.

Benefits

Process :

End Product :

Markets and applications

The main families of organic peroxides used for Polymerization Initiators are :

Dialkyl – higher temperature initiators like Luperox DI used in polyethylene or Luperox 101 used in polyproprylene modification

Peresters & Perketals – which are a wide range of peroxides from temperature controlled peresters like Luperox 610 and Luperox 10 used for PVC polymerization to room temperature stable perketals such as Luperox 231 used for HIPS polymerization

Diacyls – which are peroxides like benzoyl peroxide used in the suspension polymerization of styrene