Polypropylene PP | Arkema Luperox®

The addition of organic peroxides to the molten polypropylene during extrusion is called vis-breaking or controlled rheology of polypropylene (CRPP).
Main organic peroxides to be used : di-tert-butyl peroxide, di-amyl-butyl peroxide, and 2,5-dimethyl-2,5di(tert-butylperoxyl)-hexane.

Peroxide Selection Guidelines

The goal is to improve the melt flow properties of poly-propylene (PP) after the polymerization process. The peroxide can be added in the hopper or in the extruder as a liquid (pure or diluted with solvent) or as a solid masterbatch (peroxide is absorbed or coated on PP carrier).

Luperox^® 101, Luperox^® DTA or Luperox^® DI can be used, see Figure 1 and 2.

Figure 1 half life-time_temperature_PP.jpg

FIGURE 2: Characteristics of organic peroxides CR-PP

Organic Peroxide	Advantage
Luperox^® 101	Available in liquid and solid formulations
Luperox^® DTA	No tert-butanol by product
Luperox^® DI	Cost efficiency

The main advantages of organic peroxides in polypropylene modification are:

•The excellent control of the melt flow index (MFI) increase, see Figure 3.

•The narrowing of the molecular weight distribution (MWD) and the flexibility in its control.

•A high efficiency of the selected organic peroxide at operational temperature.

•A low cost process to achieve required MFI.

•A variety of grades can be produced from one single PP resin.

•A PP resin with increased clarity and better heat distortion.

Main criteria to select the most efficient peroxide

Technology of injection (physical form of peroxide grade)

•Extruder temperature (200-250°C): above 240°C, thermal degradation becomes significant

•Extruder residence time: 8-10 x half-life time of the peroxide at a given process temperature is required to get a complete decomposition without residual

•Final MFI and MWD polypropylene properties

•Acceptable residual peroxide level.

MFI versus peroxide loading_PP.jpg

A broad range of organic peroxide formulations

The main advantage of a diluted peroxide grade is to provide a better accuracy in dosage and a better homogeneity, especially when the required quantity of peroxide is low. To make the handling and the dosage for operators easier, these different formulations of organic peroxides can be supplied in larger containers like big bags for solids (up to 500 kgs) or like IBC (1 ton) for liquids.

Formulation	Assay	Physical form
Luperox^® 101MO32	32%	Liquid
Luperox^® 101MO50	50%	Liquid
Luperox^® 101MO70	70%	Liquid
Luperox^® 101PP7,5G	7,5%	Granules (2-3mm)
Luperox^® 101PP10G	10%	Granules (2-3mm)
Luperox^® 101PP20G	20%	Granules (2-3mm)

When the addition of solid peroxide is required, Arkema is proposing masterbatches in granules on a PP carrier. Various solid or liquid (in mineral oil) peroxide predispersions, based on Luperox^® 101, with different concentration levels are described in Figure 4.

Solid dispersions of peroxides offer some advantages related to:

•Productivity: excellent dosing accuracy and dispersion in the extruder;

•Quality: no lumping and acceptable odor levels;

•Handling: free flowing powders;

•Safety: lower risk associated to flash point.

High melt strength polypropylene (HMS – PP)

Melt strength of polypropylene in molten state may be too low especially for applications requiring stretching. For that reason, it may become very interesting to use an organic peroxide able to limit the visbreaking and favor the grafting of specific chains and/or agents to enhance the melt strength and improve the stretching behavior of polypropylene during extrusion step.

Luperox^® TBIC (t-butyl isopropyl monoperoxycarbonate) is particularly well suited for such an application.