Low Density Polyethylene LDPE/EVA

Arkema manufactures a wide range of initiators for Low Density Polyethylene (LDPE) or Ethylene-Vinyl Acetate (EVA) production and keeps on developing new products and services to meet customer requirements, such as higher conversion rate, lower global specific consumption (required peroxide quantity to produce a chosen mass of polymer, usually expressed in g/kg or kg/ T), and safety standard during operation.

Peroxide Selection Guidelines: with world-class expertise in technology, applications, and supply chain

Arkema produces all peroxides suitable for LDPE/EVA high pressure technology to cover a wide range of working temperatures. A good first selection guideline is to choose peroxides according to the kinetic of their thermal decomposition, reflected by their half-life temperature in a chosen time, see Figure 1.

Frequently cocktails are used selecting preferentially between the Luperox^® 10, Luperox^® 219, Luperox^® 11, Luperox^® 26, Luperox^® 270 and Luperox^® P or Luperox^® DI.

FIGURE 2: Initiators used at different working temperature range in LDPE/EVA

Two same peroxides with similar HLT 1 min may perform differently in polymerization. This is because the two compared peroxides do not have the same molecular structure, see Figure 2, producing then radicals of different initiation efficiency. As this is hardly predictable, running real polymerization in a pilot, such as Arkema is equipped with, is key to attain performance of peroxides or cocktails of peroxides in high pressure polymerization, see below.

Organic Peroxide	Upper Operating temperature initiator (up to T°C )	Chemical nature	Half life temperature °C for 1 min heat exposure in solvent: a trichloroethylene b n-dodecane c n-decane
Luperox^®546M75	160	Tert-amyl perester	99a
Luperox^®10M75		Tert-butyl perester	102a
Luperox^®223M50		Di-Et2 hexyl Peroxdicarbonate	99a
Luperox^®554M75		Tert-amyl perester	112a
Luperox^®11M75		Tert-butyl perester	116a
Luperox^®575	240	Tert-amyl perester	126b
Luperox^®26	240	Tert-butyl perester	130b
Luperox^®570	240-280	Tert-amyl perester	162b
Luperox^®270		Tert-butyl perester	165c
Luperox^®7M50		Tert-butyl perester	166c
Luperox^®DTA	>280	di-tert-amyl peroxide	184b
Luperox^®DI	>280	di-tert-butyl peroxide	189c

Tertio-amyl proxides benefits: better selectivity and resin properties, less fouling

Tert-amyl peroxides like Luperox^® 546, Luperox^® 554, Luperox^® 575 or Luperox^® 570 are known to generate Ethyl free radicals which are less energetic thus more selective than the Methyl radical coming from tert-butyl peroxides as illustrated in Figure 3.

Therefore, radicals issued by tert-amyl peroxides provide a better selectivity toward initiation of olefinic monomers. This causes less fouling and provides better optical properties of LDPE resins by reducing the chains cross-linking and gel formations due to resin H-abstraction leading to chains coupling, while enabling a lower specific consumption of organic peroxide as compared to tert-butyl homologs.

Micropilot Facility: the tool to optimize productivity and performance

To better understand the behavior of peroxides in specific high pressure LDPE/EVA processes, Arkema operates a micropilot equipped with a batchwise autoclave which mimics the kinetic of a one tubular zone polymerization. The Arkema’s micropilot also runs two continuous reactors, one tubular and one autoclave, for a resin production of 0,5-1kG.

Studies are run according to customer’s conditions (pressure up to 2400 bars, temperature 100-310°C).

Impact of polymerization conditions (T, P, Initiators dosage, conversion) on the polymer structure and characteristics such as Mw, SCB, LCB, crystallinity, Tmelt, % of comonomer reacted … can thus be pre-assessed.

The batch reactor is the ideal tool to tune the composition of an organic peroxides cocktail, thanks to direct viewing of the thermal exotherm curve of poor or good thermal connectivity between peroxides.