Technology Polypropylene catalyst: a peek from Montedison For obvious reasons, companies go to great lengths to blanket their catalyst research with a cloak of secrecy—an action that holds particularly true in the highly competitive world of olefins polymerization technology. However, Italy's Montedison lifted the veil of secrecy surrounding its propylene polymerization catalyst just a bit last month at a company-sponsored seminar in London. The new catalyst is the outcome of a research program conducted jointly by Montedison and Mitsui Petrochemical Industries in Japan. Dr. Paolo Longi of Montedison's polyolefins research department didn't detail the specific chemistry involved. His remarks suggest, however, that, like the so-called conventional propylene polymerization catalyst, the new one contains titanium and an alkyl aluminum compound. The two parts of the catalyst system are stable until mixed together at the time of the polymerization reaction. The performance characteristics of the new catalyst are quite remarkable, according to Montedison. During solvent polymerization, for instance, polypropylene yield is "at least" 250,000 grams per gram of titanium. Conventional catalysts, under the same reaction conditions, result in about 3000 grams of polymer per gram of titanium, the company points out. An important consequence of the higher yield from the reaction is that the amount of catalyst residue entrained in the polymer is significantly less than it is for polypropylene now produced commercially in which titanium can occur at levels of 300 ppm in the material coming from the reactors. The lower concentration of titanium that the new technology provides rules out the need for a catalyst residue "deashing" step, according to Montedison. The new catalyst results in polypropylene with an isotactic index of about 93. Studies under way at Montedison's plastics research center in Ferrara are aimed at a polymer with a higher index rating (about 95), thereby avoiding the need to remove the rubbery atactic coproduct by solvent extraction. (The isotactic index is a measure of the percentage of the stereoregular isotactic form of the polymer that is insoluble in boiling heptane.) The advantages stemming from the new catalyst system are greatest when a plant is designed specifically to use it. For instance, the simpler process leads to a capital investment saving of as much as 20% over a "traditional" polypropylene plant, Montedison says. Polymer cost can be at least 2 cents per lb less than that of competing material made from conventional catalyst systems. The 120,000 metric-ton-per-year 28
C&EN May 2, 1977
polypropylene plant t h a t Novamont, Montedison's U.S. subsidiary, is building near Houston will incorporate the new technology. So will the 150,000 metricton-per-year unit that Montedison is building jointly with Petrofina near Brussels. Dermot A. O'Sullivan, C&EN London
Papermakers discuss pitch, other problems Of the many problems besetting papermakers, accumulation of pitch here and there throughout the process, control of the overall process itself, and waste treatment for recycle to the process and discharge to the environment are among the most perplexing. All three received due attention at the 1977 Papermakers' Conference, held in Chicago last month by the Technical Association of the Pulp and Paper Industry. There is renewed hope that the problems will become more tractable in the future. In papermaking circles, pitch is a miscellaneous collection of calcium carbonate, calcium soaps from wood components, rosin, and residues from papermaking chemicals, in addition to stray fibers and inorganic materials from plant corrosion. Despite years of effort to avoid it, pitch still accumulates throughout pulp and paper mills, causing occasional production delays and adversely affecting the quality of paper. The usual attacks on the pitch problem have involved periodic plant cleaning and/or the addition of suitable chemicals to either dilute or dissolve the pitch to acceptable levels. But there have been efforts to go even further, particularly through more thorough analysis of the tree-to-product process to permit closer control and the chemical identification of pitch itself. According to E. Abrams of E. F. Houghton Co., a key material in pitch control is calcium. One source of calcium is the wood itself. Another is carryover from the white liquor clarifiers, which are supposed to reduce suspended solids levels to 75 ppm or less. Abrams says that this degree of reduction rarely happens because of clarifier overloading. Solids levels as high as 1500 ppm have been reported. One way to reduce calcium in the clarifier effluent is to operate the clarifier more efficiently. Additional techniques are the improved washing of brown stock to increase recovery of salt cake and tall oil, or the addition of suitable dispersants. The latter doesn't necessarily reduce the pitch level but prevents pitch from settling out. Ultimately the control of pitch deposits will require a better identification of the pitch itself, and Charles E. Farley of American Cyanamid says that some
progress along this line has been made. He identifies pitch as originating in the resinous fraction of the wood and consisting primarily of rosin and the mono-, di-, and triglycerides of fatty acids; free fatty acids; fatty alcohols; and waxes. Up to 5% of the raw wood is composed of pitchforming materials, although the actual amount usually increases through reactions in the pulp and paper mills. Farley believes that the best way to counter the problem of pitch formation is by prevention—through better control of water hardness and better control of temperatures throughout the process. One of the key measures of process control in the paper industry is zeta potential—the electrokinetic charge occurring at the solid-liquid interface of colloidal particles. It has been generally recognized that operation at zero zeta potential is a desirable optimum in most parts of pulp and papermaking plants, since charge becomes less effective in keeping particles apart and agglomeration and flocculation are improved. Such operation, says J. G. Penniman of Pen-Kern Inc., is particularly important in reducing biochemical oxygen demand for the waste treatment plant. Zeta potentials haven't always been easy to measure. According to Penniman, the conventional means of zeta potential measurement limited a technician to fewer than 200 determinations per day, hardly enough to exercise control in a complicated process. But Penniman described a new instrument, the Laser Zee System 3000, with which zeta potential measurements can be made automatically at a rate of 60 per minute. The elements of the system are a measuring chamber, a 2-mw helium-neon laser light source, appropriate circuitry to process signals, and a real-time spectrum analyzer. In describing the new system at the conference, Penniman presented what he said were the first paper-industry particle histograms ever published. They show the relative number of particles plotted against zeta potential for several typical cases in papermaking. (The same instrument can also be used in biomedical applications to provide researchers with the histogram in place of mean mobility of particles.) A laser-Doppler technique, which has been used to measure mean mobility, has been under investigation, Penniman says, but he believes the new system offers more advantages. Any light source, not just a monochromatic light, can be used. High resolution can be achieved without resorting to very small scattering angles, thus minimizing the effect of stray light. The direct viewing of particles is compatible with the image analysis device that can be added to the basic system. A wide variety of optional additional equipment modules is available with the System 3000, including a process computer. With suitable software, which is available, the computer provides a means for optimizing process coagulation chemistry through monitoring of electrokinetics at various sampling points. •
