Molecular weight adjustment of polypropylene after polymerization is today intensively practiced by producers in order to offer a wide range of grades to customers and rationalise polymerization operations. This operation, commonly called "vis-breaking" or "controlled rheology", is generally performed at the finishing step into an extruder by addition of peroxide. Viscosity reduction is controlled by the amount of peroxide introduced.
Key parameters in PP decomposition are the active oxygen content and the half-life time (t1/2) of the peroxide. t1/2 represents the time needed to decompose half of the peroxide. Temperature accelerate decompositon of peroxide (cf table1), however in industrial compounding conditions, temperature doesn't appear so critical but as many external parameters may disturb the process, resin producers generally use on-line rheometer for an accurate control of MFI.DHBP is one of the most used. Due to his high molecular weight the volatility is limited and flash point is reasonably high. Organoleptic properties are better than DCUP. Ditertiobutyl peroxyde is also efficient, much cheaper than DHBP and also used industrialy by some producers.
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