Isotactic polypropylene is a semi-crystalline polymer.  Free radicals generated using Luperox^® organic peroxides degrade the polypropylene backbone via beta-scission. This chemical process is used to adjust the melt flow, grade, and to narrow molecular weight distribution.  This operation is commonly called "vis breaking" or "PP controlled rheology."

The organic peroxide can be dry-blended with PP in the hopper of an extruder or injected in the extruder throughout several zones as:

• a liquid (pure or diluted in a solvent)
• a solid (peroxide + PP carrier)

Peroxide is also useful for grafting maleic anhydride onto PP.  These grafted polymers are useful for:

• Tie layers
• Glass and filler
• Printability and paintability
• Increased compatibility with other homopolymers

The modification of polypropylene is achieved in a 3 step process:

1. Initiation: Free radical formation by thermal decomposition of organic peroxide

2. Macro radical formation: transfer of alkoxy radical to PP backbone

3. Beta scission: Macro radical rearrangement leading to chain scission

The macro radical can then be transferred to another PP chain.

For the selection of the right organic peroxide and its concentration, the following parameters need to be considered:

• Technology of injection (liquid or solid)
• Extruder temperature
• Extruder residence time
• Residues in the final resin and food-contact regulations
• PP characteristics before degradation (homo, copolymer or heterophasic copolymer, MFI, additives type and level)
• Impact on mechanical properties

Generally the extruder residence time is 20-50 seconds and processing temperature is between 200 and 240°C.

The chart shows how Luperox^® 101 pure peroxide liquid can quickly alter the melt rheology of polypropylene. Accurate metering can be difficult, therefore a dilute form of the peroxide in a solvent or dispersed on a solid is recommended.

Efficient grafting of maleic anhydride by reactive extrusion can be achieved while also changing the melt rheology of the polypropylene.  This same performance is available with pure liquid Luperox^® 101 or the various solvent-added and powder formulations.