Development of catalytic systems for PE preparation
The supported Ti-, Cr-, or Zr-based catalyst systems used in the commercial conditions are in the centre of our attention. Study of simpler so called "model" systems based on the same transition metal salts provides highly valuated information utilized then in the optimization process of developmental or commercial catalyst samples. The support in these systems contributes to the final catalyst polymerisation performance and from that point of view several techniques of its characterization had been developed within the time.
Evaluation of the catalyst systems is based mainly on the catalyst polymerisation performance assessment and characterization of the polymer sample obtained. The polymerisation tests are carried out in reactors of volume 1.8 and 3.0 L. The polymer is characterized on the basis of mechanical tests and inner structure.
Development of two proprietary catalyst Cr- and Ti-based systems resulted form a systematic research in the field. The Cr-based catalyst has been utilizing in production unit for more than 10 years for production of film, pipe grades and nowadays its modified version is available as well as for blow-moulding grades.
Study of the poisons effect on the particular catalyst systems represents another field of our research activities. The studies are carried out under the conditions closed to commercial one and, thus the results may serve as a basis for arrangement of the set of purifying system. A special arrangement of dosing system allows us to work even at the level of 0.1 ppm - that is especially important when working with poisoning agents like e.g. O2, COS. The poison admission could be done in continuous mode or batch-wise.
Methods that are used within the catalyst development or characterization:
- catalyst synthesis or its handling is carried out under strictly inert conditions; the high vacuum lines or glove box together with highly purified N2 are used
- silica activation is carried out in lab-scale quartz activator (the amount is around 15 grams) and in pilot plant scale facility (up to 5 kgs)
- characterization of the silica gels is based on chemical analysis, further analyses of specific area (S.A.), particle size distribution (PSD) - Malvern, pore volume (P.V.), DTA, SEM and TEM microscopy, analyses of OH group content or arrangement by means of IR, NIR and DRIFT techniques accomplish the image of support quality
- study of the effect of activation procedure of the support (silica) on the catalyst polymerisation performance (activation temperatures up to 850 °C)
- determination of the number and arrangement of OH groups on the support surface (reaction with R3Al, IR analysis)
- development and optimization of polymerisation processes in lab scale in order to simulate commercially used processes
- continuous innovation of properties of PE products in UNIPETROL RPA´s assortment
- several proprietary catalyst systems were developed in PIB, some of them have been implemented in commercial use for various applications
- development of one-phase supported catalyst for ethylene polymerisation and its copolymerisation with 1-alkenes in gas phase. Development of a proprietary highly active two-phase catalyst for slurry polymerisation
- improvement or development of a proprietary supported catalyst based on Cr, especially suited for HDPE pipe and film grades
- moreover, two proprietary Ti-based catalysts were disclosed for syndiospecific polymerisation of styrene
We are capable of catalyst synthesis based on the request of the customer and its evaluation for the developmental or commercial purposes.
Development and optimization of polymerisation processes in lab-scale
The polymerisation tests are carried out in stainless steel reactors of proprietary design, the process is PC controlled and monitored. Design of the reactor together with arrangement of the whole polymerisation facility give us an extended flexibility resulting in smooth transitions when testing in the gas-phase or slurry phase polymerisation processes is required. The reactor originally intended for testing in the gas-phase process with a mechanically stirred fluidized bed could easily be adapted for working in the slurry process.
Connection of the reactor with PC makes possible:
- a new controlling system enables to keep pressure in the reactor constant during the polymerisation run
- connection of the reactor to GC apparatus secures that replenishment of the consumed gases is carried out in a very similar way as it is done in commercial conditions maintaining the required molar ratios of H2/C2, comonomer/C2
- to save all the required data
Connection of the reactor to GC gives possibility of continuous replenishment of the consumed gas components so that the polymerisation is carried out at a constant mixture composition. The above mentioned conditions enable to make comparison of commercial catalysts with the developmental ones under real conditions, to study the kinetic profiles, to watch even a formation of agglomerates, deposits on the reactor walls or even formation of side products (oligomeration or hydrogenation). This could easily be compared with the structure of polymer obtained and a complex view on the tested catalyst system could be figured out.
Analysis of composition distribution (CD) of polyethylene copolymers is essential for understanding of the relation between the structure and physico-mechanical properties of the polyethylene resins.
The following techniques are used for evaluating CD:
- TREF - temperature rising elution fractionation (solution method)
- SIS - stepwise isothermal segregation (fractionation in melt)
The result of the CD analysis is short-chain branching/molecular weight relation.
Innovation of polymer properties
The process of material improvement is in principal based on knowledge of relation "catalyst polymerisation performance - the inner structure of the produced polymer - mechanical properties of the matrix. The sequence of the proposed steps is then related to the extent of the required changes. The easiest way - when only a mild change of polymer properties is needed - is to optimize polymerisation conditions, while for a substantial improvement of polymer grade the catalyst optimization or even development of a new one is undertaken.
The following sequence or combination of method is available for polymer properties evaluation:
- analysis of structural parameters by means of the following techniques (GPC, FTIR, DSC-SSA/SIS)
- critical mechanical properties are evaluated by using standard techniques (tensile flexural tests, ESCR, methods for evaluation of RCP (S4 test) and SCG (FNCT test) are used for pipe grade, development of a set of alternative methods is inevitable (PCR by means of PSI, or SCG by means of modified tensile test)