Synthetic waxes have made inroads since last about 50 years. Polyethylene Waxes (PE Waxes) are one of them. PE Waxes are low molecular weight polymer of the Ethylene and sometimes have other alpha olefins as co-monomer. PE Waxes generally have molecular weight of 10000 Mn. or less and exhibit wax like properties. They are generated by either high pressure or low pressure polymerization. All such waxes have same basic long carbon chain structure with branching. The different production process yield product waxes with different properties due to different molecular configuration (chain linearity, cross-linking, modality (mono or bi) polarity and functional group attached to it. Here it is intended to focus on the PE Waxes – to generate knowledge about their genesis refining and conversion to graded product. It would also be focused on to the effect of external contaminants, oligomers and other co-existing refining related issues. The residual presence of such substances if any could be harmful as well as unsafe.
Polyethylene is produced by Polymerization process using Ethylene monomer and added co-monomer, as may be required. The process of polymerization falls in to two broad categories:
• High Pressure Polymerization. ( Produces Low Density Polyethylene – LDPE; Reaction is carried out at 1000 to 3000 barg. At about 80 to 300 deg. C. Peroxides are used as catalysts).
• Low Pressure Polymerization. ( Produces High Density Polyethylene (HDPE) / Linear Low Density Polyethylene (LLDPE); Uses co-monomer such as Butene 1, Hexene 1 or Octene 1; Reaction is carried out at 10 to 80 barg. At temperature of 70 to 300 deg.C. Using catalyst such as:
o Ziegler Natta
o Cr. /Mo Oxides
Both these processes produce Low Polymer Wax as a byproduct. Low Polymer Wax contains residual catalysts, residual solvent, oligomers and other substances such as water etc. and is of a quality that makes it unsuitable and a safety hazard for most applications.
First we will review the Ziegler Natta – Catalyst and the characteristics of wax produced and refining of same:
Ziegler Natta Catalyst:
Understanding the catalyst is of prime importance as it does have impact on type of Low Polymer Wax generated and the subsequent refining process required to make it suitable for use as a Refined Polyethylene Wax. The technology that is used to produce the polyethylene resin also determines the characteristics of the Low Polymer Wax produced. In particular the branching and density of the Low Polymer Wax is fixed in the polymerization process. In particular polyethylene resin produced with Ziegler Natta Catalyst tends to produce higher density (more crystalline) polyethylene (HDPE) polymer due to reduced chain branching of the polymer. Thus Low Polymer Wax produced with Zeigler Natta Catalyst has high density properties. It was Natta who worked along with Ziegler who had used similar catalyst while experimenting with the polymerization process. Natta explained the mechanism that reactive site of the catalyst is “Ti – C” bond and NOT “”Al – C”bond which gets formed during the initiation step:
Ti ( (L) Mt) + Al ( R – M) à R – (L) Mt
Transition Ti metal compound + Metal Alkyl à Organometallic Complex selective site for Polymerization.
Here, Mt = Ti, Cr, V etc. M = Usually Aluminum.
R – (L) Mt + CH2 – CH2 à (L) Mt – CH2 – CH2R
The ethylene reacts with active site to continue insertion of the monomer at the active site “–“ thus creating “Chain Propagation Step”.
This is also accompanied by typical chain transfer reaction:
Ti – CH2 – CH2 – Polymer + C2 H4 à Ti – CH2 – CH3 + CH2=CH-Polymer
Ti – Polymer + (C2H5)3 Al à Ti – C2H5 + (C2H5)2 Al – Polymer
Ti – Polymer + H2 à Ti – H + Polymer – H
All above Ziegler Natta Catalytic reaction steps gives following configuration:
1. Organometallic compound Ti-Tetrachloride is reacted in a reaction vessel along with metal alkyl at the temperature between 100 ~ 130 deg.C. in presence of neutral solvent (Hexane).
2. Pressure of the reaction vessel is up to 20 atm.
3. Ethylene is fed in to the reactor as gas phase Ethylene; reacts with active site of the catalyst to form HDPE and other polymeric product.
4. Melting point of HDPE is 130 deg. C. plus. Thus it would form slurry with HDPE as suspended solid. Low polymeric waxy substances would remain in liquid phase along with solvent.
5. The heat of polymerization is dissipated in to the solvent in liquid phase. The Low Polymer Wax would remain in liquid phase along with the solvent.
6. The catalyst is deactivated by process of decomposition & neutralization.
7. Standard unit processes / unit operations – of separation – Filtration and drying would yield product HDPE.
8. The solvent is recovered, low polymer wax separated and such solvent is recycle back to the plant process. Low polymer wax containing traces of solvent, residual catalyst (most of it deactivated) and other residual contaminants – is removed as waste byproduct.
9. Polyethylene produced could have molecular weight 20000 to 1.5 million.
10. The main variables that control Molecular weight or degree of Polymerization are:
• Higher pressure less branches
• Temperature of the catalyst preparation. Too high temperature de activates the catalyst. THE ACTIVE CATALYSTS ARE PYROPHORIC.
• Chain transfer reagent
• Ratio of Al / Ti in the catalyst added to the reactor. e.g. data put up hereunder for the atmospheric positively pressured experimental reactor run for 4 hours:
Al / Ti Ratio Yield Avg. Mol. Wt. “Mw” Remark
0.67 107 24000
1.00 510 54000
1.30 450 132000 Ratio has no effect on yield.
1.50 450 188000 Ratio has no effect on yield.
1.70 450 262000 Ratio has no effect on yield.
2.00 450 288000 Ratio has no effect on yield.
Note that Al / Ti Ratio has no effect on yield beyond 1.0 except the impact on “Mw”.
HDPE Technologies based on Ziegler Natta catalysts are licensed by:
• Philips Process: Basic Philips Process is based on Ziegler Catalyst. However, at present it utilizes Chromium Oxide on high surface area silica. Tubular reactor.
• Mitsui Chemical (CX Process): This typical Ziegler Natta Catalyst based technology with three reactor system.
All these process technologies produce Low Polymer Wax to varying degree say o.9 to 2.5 % of the HDPE Polymer production.
(Technology wise typical wax production and characteristics)