Luperox® for Polymerization of Ethylene
Luperox® initiators are available worldwide to offer solutions for polymerization of ethylene and copolymers.
Selecting the proper Luperox® organic peroxide is a critical parameter to consider for polymerization. The "initiator package" affects directly the polymer properties and also the productivity (% conversion, cost per unit of monomer converted).
Polymerization is propagating macroradicals building polymer chains to generate short or long branched structure. Short segments affect mechanical performance of the finished polymer while longer branched chains affect the rheological behavior. The reaction takes place at temperatures between 100 to 300°C and at high pressure (1200-3000 bars). At these pressures ethylene liquefies and acts as a solvent under supercritical conditions.
Molecular weight and density is regulated by controlling the temperature and pressure, and by the addition of a chain transfer agent (e.g. propane).
In order to maximize productivity, multiple peroxides should be introduced throughout the various zones of both tubular and autoclave processes. This will help start, or "light-off" the polymerization, but also maximize productivity as the temperature and pressure reach the maximum in each reactor zone.
The criteria for selecting peroxides for various zones is described by the peroxide half-life:
- Short half-life peroxides alone provide fast light-off, but lower peak temperature.
- Long half-life peroxides alone provide high peak temperature, but are slower to light-off.
The benefits of peroxide initiation over oxygen initiation:
- Lower initiation temperatures
- Extended cooling zones preventing max peak lagging
- Increased conversion rate of ethylene + 20 %
- More efficient use of reactor length
Effect of Temperature
Molecular structure is primarily determined by pressure and temperature. Below summarizes how these parameters impact molecular weight and branching.
* Short Chain branching
** Long Chain Branching
Productivity is proportional to gas inlet and outlet temperatures but temperature profiles impact both properties and productivity.
In the tubular process, polymerization takes place in a long tube (>1000 m) at high pressure. Peroxide is introduced at different zones to optimize productivity. The reaction temperature profile is controlled by the dose and reactivity of the initiators. Addition of peroxide in various zones can increase the conversion rate of ethylene (+ 20 %) over oxygen initiation alone.
Chart of Staged Peroxide Injection / Tubular Process
In the autoclave process, polymerization takes place in a series of stirred reactors (2 to 6). Peroxide is introduced in each of the reactors to optimize productivity and performance. The reaction temperature is controlled by the peroxide dose. Conversion of ethylene to polymer is proportional to the T outlet / T inlet ratio. Quality is dependent on the mean polymerization temperature, but independent of T outlet.
Chart of Staged Peroxide Injection / Autoclave Process
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