2232
Anal, Chem. 1981, 53, 2232-2234
Infrared Spectrometric Determination of Catalysts Used in the Production of High-Density Polyethylene David R. Battiste," James P. Butler, Joe B. Cross, and Max P. McDaniel 342A PL, Phillips Petroleum Company, Phhillips Research Center, Bartlesville, Oklahoma 74004
An infrared spectrometric method for determination of residual catalyst support in commerlcal polyethylene is described. As low as 100 parts per million (ppm) of residue (corresponding to catalyst productivity of I O 4 g polyethylene/g of catalyst) can be detected by use of the absorbance of elther the 1118or 470-cm-' band of silica support and callbration standards. The method Is more accurate, more rapld, and more convenient than ashing the polymer and weighing the resldue. The time saved In this analysls can be Important in control of the polymerization process in a cornmerclal reactor.
Currently, most of the world's high-density polyethylene is made from catalysts containing a trace of an active ingredient impregnated onto a highly porous silica support. Usually the active component is chromium (2-51, but titanium (6) and vanadium (7) have also been reported. During polymerization the silica fragments become finely dispersed in the polymer. If the productivity is high enough, the catalyst and support are usually left in the product because removal would be difficult and costly. An accurate analysis of catalyst residue is important, to assess the economic productivity of a reactor (its output of polyethylene per part of catalyst) and to determine the concentrations of the transition metals in the products. Ideally, productivity can be assessed by monitoring the catalyst feed rate and the polymer output. However, large continuous commercial units are not always a t equilibrium. Thus, another more accurate check is required. This has formerly been dolie by slowly burning away a sample of the polymer and weighing the ash. Because productivities are high, between lo3 and lo4 g of polymer/g of catalyst, a t least 100 g of polymer must be burned to obtain a measurable quantity of ash. This process takes over 2 h. Analysis time by ashing can be several times the residence period of a catalyst particle in a reactor; therefore, the ashing method is not well suited to reactor control. This paper outlines an alternative to the ashing technique which is faster, more convenient, and also more accurate. EXPERIMENTAL SECTION Polymers. Several series of polyethylene samples were prepared from different catalysts, with productivity ranging from near lo2 t o lo4. The samples were prepared in a 2-L autoclave at approximately 105 "C in a slurry process using isobutane as the diluent. An estimate of the productivity referred to hereafter as "by weight" was made by weighing the catalyst added to the reactor, and then weighing the polymer retrieved after venting the isobutane. Productivity by weight would be much less accurate in a continuous process commercial reactor. Two types of catalysts were studied (1)catalyst A (a Davison Grade 962 silica supported catalyst) and (2) catalyst B (a modified, silica supported catalyst). Both catalysts contained 1% Cr and were activated between 600 and 900 "C. The average particle diameter after polymerization was approximately 8 mp. Particle size distribution by volume was determined by use of a Coulter counter type device. The silica residue remaining after ashing of a series of polymers prepared with catalysts whose
productivity ranged from 20 g of polymer/g of catalyst to loo00 g of catalyst was analyzed with a Celloscope Model 112N5-LTS Particle Data, Inc. (Elmhurst, 1L). In all cases, catalyst fragmentation was complete within the f i t 1-2 min of polymerization. A series of standards was made by blending known amounts of silica into a silica-free polymer. This polyethylene was prepared from a catalyst containing no silica. The silica, Cabosil S-17, was micronized in a propanol suspension and impregnated onto the polymer sample. Final mixing occurred on a roll mill at 160 "C. Ashing Technique. Productivity by ash (PA)was determined by weighing approximately 100 g of polymer into a dry quartz bowl, which was then placed on a small burner, and the sample melted and then gently ignited. After most of the polymer had burned away, the bowl was set in a muffle furnace a t 650 "C to oxidize residual carbon. Finally, the bowl was allowed to cool in a desiccator and then weighed. The burning process is dirty, somewhat tedious in repetitive applications, and takes approximately 140 min to complete. Infrared Method. Polymer films were prepared on a Buehler mold assembly by placing 0.52 g of the sample between aluminum foil disks on a 1-mm spacer. The mold was heated to 160 "C, the pressure set to 6000 psig, and the sample then cooled to
