Polymer Initiators

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 polymers that can be produced by organic peroxides through several mechanisms:

• Polyethylene:

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

• Polystyrene:

  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.

• Polyvinyl Chloride:

  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. There are four processes used: suspension, emulsion, micro suspension and mass process.

• Polypropylene Controlled Rheology:
  
  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 the hopper of the extruder, or injected directly into the extruder.

Benefits

Process:

• Efficiency
• High purity
• Selectiveness
• Safety
• Different packaging, formulations and solvents available

End Product:

• Molecular weight distribution
• Additional benefits vary by product

Markets and Applications

• Polymerization initiators: PVC, LDPE, PS, SBR, ABS, PMMA, EPS, GPPS, HIPS
• Polypropylene modification
• Reactive processing for PE, crosslinked PE, PP, polymer blends