Document 4vz2vd51LNgZMd0Lj2BK04p41

Environmental Health Perspectives Vol. 41, pp. 123-198, 1981 Poly(vinyl Chloride) Processes and Products by R. N. Wheeler, Jr.* Polytvinyl chloride) resins are produced by four basic processes: suspension, emulsion, bulk and solution polymerization. PVC suspension resins are usually relatively dust-free and granular with varying degrees of particle porosity. PVC emulsion resins are small particle powders containing very little free monomer. Bulk PVC resins' arc similar to suspension PVC resins, (hough the particles tend to be more porous. Solution PVC resins are smaller in particle Size than' suspension PVC with high porosity particles containing essentially no free monomer.-The variety - of PVC resin products does, not lend itself to broad generalizations concerning health hazards. In studying occupational hazards the particular PVC.process and th.e product must be considered and identified in the study. Poly(vinyl chloride) is a ubiquitous part of our environment today, in that it appears in clothing, upholstery, flooring, wire insulation, food contain.rs, phonograph records and an almost infinite variety of other items. Despite this wide applica tion and the size of the industry there is little public understanding of the term. A part of this confusion stems from the human inclination to abbreviate terms--thus, we say PVC when discussing poly(vinyl chloride) resins, poly(vinyl chloride) latexes, poly(vinyl chloride) compounds, poly(vinyl chloride) film and so forth. This confusion has been accentu ated by governmental regulators, who define PVC as a polymer containing any amount of vinyl chlo ride. On taking these two factors into account it has become almost impossible to distinguish what is meant when the term PVC is used in the various media. In order to study possible PVC industryrelated health problems one must know something of the various processes and products to properly evaluate study results. Many study results can have significance only if the source and composition of the PVC is stated. Manufacture of synthetic resins from vinyl chlo ride and other monomers involves reacting these monomers in agitated pressure vessels in the pres ence of catalysts and converting these liquids and/or gases to solid resins. A considerable amount of heat Union Carbide Corporation, P. 0. Box 8361, South Charleston, West Virginia 25303. October 1981 is generated by the reaction. This is removed by cooling the vessel. As the monomer is converted to polymer during the reaction, the rate of reaction slows down; thus, after some optimum reaction time, the unconverted remaining monomer is re moved from the reacting mass by heat and vacuum and the resin (PVC) is recovered as a dried white powder or as a liquid latex or solution. This polymerization reaction may take place in pure monomer, in a solution, in a water-monomer emul sion or in a water suspension of monomer. The nature of the polymerization process determines the nature of the subsequent recovery process, and the nature of the resin particles produced. Current ly there are four basic vinyl chloride polymerization techniques which give use to the following four processes: (1) suspension polymerization, (2) emul sion polymerization, (3) bulk polymerization, and (4) solution polymerization. Suspension polymerization is the major process used for the manufacture of PVC resins and is used for about 82-85% of U.S. production (Fig. 1). It involves the charging of one or two parts water and one part vinyl chloride monomer or comonomer mixture to an agitated reactor along with initiator and suspending agents such as poly(vinyl alcohol). The mass is reacted at 50-65C until about 85-90% of the contained monomer is converted to resin. The resin-water mixture is heated, sometimes under vacuum, until the unconverted monomer is substantially removed. The resin is then removed 123 - moldings produced is remote. agitated reactor and reacted at 30-0C to form a All PVC processes have a problem with reactor synthetic latex. The reactor agitation must be system fouling to some degree. The conversion of a sufficiently vigorous to emulsify the monomer- liquid monomer to a high molecular weight solid . water mixture but not so vigorous that the latex is polymer while contacting reactor walls, baffles and coagulated. When 80-95% of the monomer is con- so forth results in the deposition of some of that verted to polymer, the latex may be gently stripped solid polymer on surfaces. This deposition or foul- of the unconverted monomer with heat and vacu- ing interferes with the reactor operation so that um, or it may be subjected to a second initiator periodically it has to be removed. In earlier times, treatment and reacted to essentially 100% mono- this was removed manually by a man working in a mer conversion. The product of this polymerization shutdown reactor. These reactor cleaners were may be simply filtered and shipped to consumers as exposed to very high concentrations of vinyl chlo- a latex for coatings, mastics, and the like, or the ride monomer (several thousand ppm). It was polymer may be recovered as a dry resin. Recovery within the group of men in this cleaning operation techniques vary. The most commonly used is sim- that the excess of angiosarcoma was observed by ply spray-drying of the latex, though some resins Creech (1). More recently, techniques to minimize are recovered by coagulating the latex and dewa- fouling and techniques for cleaning such as solvent tering with subsequent drying of the coagulum washing and hydroblasting have reduced or elimi- (resin). nated the need for reactor entry and cleaning. In the oil-soluble initiator system, one part - Eraulsipn polymerization is the second most -wide- monomer containing an organic peroxide is emulsified ly used process for the .manufacture of PVC resins in one to twb parts water containing 0.01 to 0.03 and comprises 10-12% of total U.S. production. One parts surfactant. The resulting emulsion is reacted .. of the important things to understand about emul- . at 30-60C to form a synthetic latex. Approximate- sion polymerization is that it is not a single process' ly 80-90% of the mdnomer is converted,to polymer, but a large family of processes, each producing After reaction, the latex is gently heated and- specialized products that are defined or specified in vacuum-treated to remove the unconverted mono terms of performance in a particular application. In mer. After stripping, the latex may be shipped as a the interest of brevity,- the two major process product though most of it is converted to a dry families will be discussed: the water-soluble initiator powder by spray-drying. The basic difference be- system and the oil-soluble initiator system (Fig. 2). tween the oil-soluble initiator process and the In the water-soluble initiator system, one to two water-soluble initiator is that the size of the emulsified parts water, one part monomer, 0.01 to 0.03 parts monomer particle determines the resin particle size surfactant and water-soluble initiator (a redox in the oil-soluble initiator process, while polymer- system or a persulfate salt) are charged or fed to an ization technique determines the resin particle size VC MONOMER WATER V MONOMER recovery AIR '' BAG HOUSE REACTOR REACTOR DRYER October 1981 PVC LATEX Figure 2. Emulsion polymerization process. '' PVC RESIN 125 ss and poorer removal of residual VCM. These oulk process resins are generally, directly competi tive with suspension process PVC homopolymers. The relatively large porous resin particles (50 to 150 p.m) tend'not to retain monomer, thus removal of residual VCM is theoretically easy. The transfer of heat from the reactor wall to the resin particle for stripping is poor thus offsetting the advantages gained by the high particle porosity in removing residual VCM. In the early 1970s, residual VCM in this type of resin was of the order of 1000 ppm when produced, but more recently these resins have less than 50 ppm in residual VCM when produced. Solution polymerization is a process unique to Union Carbide Corporation and accounts for about 2% of the total resin produced (Fig. 4). Vinyl chloride monomer, comonomer, solvent and initia tor are fed to a continuous reactor system. The polymer formed is soluble in the reacting mass so that the reactor product is a viscous resin solution. This solution is distilled to remove the unconverted vinyl chloride monomer, and the resin product is recovered by treating the resin solution with water and drying the product. The resin particle is veryporous, is always a copolymer, is free of soaps and suspending agents, has a median particle size of 75 pm and contains less than 0.2 ppm residual VCM. Manufacturing investment is high, and the product rids its greatest use as a coating material, i.e., paints and lacquers, that utilize its good dissolving qualities. It is used in relatively small quantities, .and is nearly always shipped in bags rather than bulk. In addition to the basic PVC resins described earlier, there is a wide variety of resin powders, pellets, liquids and latexes in commerce that fall under the general designation of PVC. These are chemically or mechanically converted PVC resins such as post-chlorinated resins and compounded resins containing plasticizers, stabilizers and the like. As a general rule, the additional processing has removed essentially all of the residual VCM and agglomerated the dusts. -Converted PVC resins have no involvement with problems related to vinyl chloride monomer exposure. Conclusions ,Poly(vinyl chloride) resins are produced by four basic techniques: suspension, emulsion, bulk andsolution polymerization. The variety of PVC resin products does not lend itself to broad generalizations concerning hazards to worker health.' In evaluating occupational hazards, the.PVC process, the PVC product and other materials present must be considered before valid conclusions can be reached. WATER VENT October 1981 WASTE WATER Figure 4. Solvent vinyl resin process. 127 k,' KtCElVED APR 4 {977 J- f.e. SPECIALTY VINYL CHLORIDE RESIN PROCESSES EFFECTS OF GOVERNMENTAL REGULATIONS R. N. WHEELER, JR. September I, 1976 3. Reactor product changeovers were made without equipment cleanup; thus, any VCM contained in the reactor was reacted into a non-PVC latex. This technique requires careful study anH product selection to maintain overall product quality. 4. Emissions during monomer unloading were reduced by inert gas purging of the unloading hoses to the supplier's truck and using dry disconnect couplings. These couplings contain check valves that close automatically when the hose is disconnected; thus, the VCM is held in the hose rather than emitted to the air. This is a feasible solution when small hoses can be used. Items which are troublesome under the OSHA standard and will be affected by the proposed EPA standard are: 1. All rotary pumps handling VCM or mixtures of VCM will have to be equipped With double mechanical seals instead of their present single seals. No measureable change in VCM emissions is expected though, theoretically, emissions caused by seal failure are lower with double seals. 2. Revision of reciprocating compressor and pump packing glands in a manner acceptable to EPA. As mentioned earlier, this portion of the proposed EPA standard is unclear. No measure able change in emissions is expected. 3. VCM emissions caused by control of equipment maintenance for items in monomer service is awaiting promulgation of the EPA standard. Since only the monomer storage tank and piping are involved, the investment will be minimal. The effect of the VCM health and air emission regulations on latex plants has been to tax the engineers ingenuity to the utmost. Many of the controls used make full utilization of available facilities and knowledge. This in one advan tage of the program discussed earlier in that investment for control equipment is optimized. CONTINUOUS BULK POLYMERIZATION - UC c Continuous bulk polymerization of vinyl chloride produces a fine, very porous resin particle containing no surfactants, suspending aids or other extractible material. The porous fine particle gives up residual vinyl chloride monomer very readily; thus, these resins as sold contain 0 to 10 ppm vinyl chloride monomer. The resins find use in plastisols, slush and powder coatings and in medical devices such as blood bags. h '!' ` 12. The flow diagram (Figure III) shows four steps: polymerization, monomer stripping, dewatering and drying. The process operates on the principle that polyvinyl chloride is insoluble in vinyl chloride. Vinyl chloride, catalyst and comonomer are fed to an agitated reactor; as the monomer polymerizes the polymer precipitates so that the reaction mass is a slurry of resin sus pended in monomer. Some heat of reaction is removed by jacket cooling, but most is removed by evaporation of monomer, Resin slurry is removed to the stripper where it contacts hot water driving off the unconverted monomer and yielding a resin water slurry. This is vacuum stripped to remove more mono mer. The resin is dewatered and dried in an air drier. The major problem contributing to emissions is equipment fouling with resin deposits. These deposits must be removed manually from the reactor and accessory pipelines. Reactor cleaning is required about every five days. The continuous bulk polymerization is relatively old in terms of technology and plant. The fact that it is still operating is indicative of the unique proper ties of its resins. Age has, however, not prevented progress toward meeting the current and proposed regulations. Table DC shows the change in personal exposure from 1973 to 1976. TABLE DC CONTINUOUS BULK FVC PERSONAL VCM EXPOSURE Operator Year 1973 Year 1<?76 Number 1 7. 9 ppm 0. 5 ppm Number 2 3. 6 ppm 0. 05 ppm Number 3 7. 95 ppm < 0. 43 ppm Number 4 2.4 ppm 0. 14 ppm Number 5 16. 6 ppm 0. 14 ppm The low TWA exposures for 1976 raise a question. The OSHA standard states that TWA exposures of less than 0. 5 ppm require no control action yet on the day these measurements were made four out of the five men wore a respirator for a part of the day. The plant, as a safety policy, chooses to avoid employee exposure if there is a possibility of an emission. The continuous bulk polymerization plant is a part of a larger suspension resin operation; therefore, no specific data on ambient air emissions is available. Operational changes caused by the regulations are as follows: a: a.Vw* w CONTINUOUS BULK \ YL RESIN PROCESS p- 1. Solvent scrubbing of the monomer recovery system vent reduced the VCM concentration of this stream to less than 0. 5%. The vent from this system was then incinerated in the local steam power plant. The solvent scrubbing system oper ates at 100 psi cleaning 400 pounds of vent gas per hour with methyl ethyl ketone. 2. The process control rooms where the operators spend most of their time were pressurized and ventilated with air from a remote location. When an operator has to go to a particular plant location he checks the VCM monitor and uses a respirator if the need is indicated. 3. All rotary pumps in vinyl chloride service were equipped with double mechanical seals as required by the proposed EPA stan dard. In this plant there was a decrease in the work space air concentration as a result of the change from packing and single seals to double seals. Process development and new investment remain to be done in this process to achieve full compliance with the OSHA standard and the proposed EPA standard. Major items for further work are: 1. Improved resin slurry stripping. The present two-stage continuous system is barely meeting the minimum EPA limit for stripping. The monomer left in the very porous resin particle readily leaves the resin in the drier; thus, the resin leaves the process in the 0 to 10 ppm RVCM range. Improved stripping would reduce the dried resin to 0 ppm RVCM and would reduce potential employee VCM exposure in the drying area. The number of stripping stages and operating conditions are not yet fully determined. 2. Improved monomer recovery. Replacement of the existing reciprocating compressors with rotary units is in the engineer ing stage. This change will be a part of an overall revision of the recovered monomer system to provide more surge capacity, to reduce safety hazards caused by vacuum operation, to reduce vinyl chloride emissions and to reduce the energy requirements of the process. This low conversion process has a major portion of its investment in monomer recovery and handling equipment; thus, this could be said to be a large project. 3. Reactor agitator stuffing box revision. The proposed EPA stan dard requires that agitator packing consist of double mechanical seals with a pressurized sealing fluid between the seals. Since the reacting material has the potential for complete conversion jOC- 15. 045^20 to polymer, mechanical seals quickly become inoperative. The present system uses dual sets of mechanical packing with a sealing fluid which hopefully EPA will accept as equivalent. 4. Equipment maintenance. As in the preceding discussion of other processes the application of EPA's proposed regu lation to maintenance of equipment is not wholly clear; thus, a problem of interpretation and application of the regulation remains. As in the solvent and latex processes the continuous bulk process has many other smaller emission problems that have been or are being corrected. This discussion of the effect of governmental regulations on the specialty PVC processes is more of a brief progress report than it is a technical presentation. The industry has many problems that are similar to the larger segment of the PVC and it also has some problems that are peculiar to its operations. A great deal of progress has been made, but the end is not yet in sight. In summary, the specialty vinyl chloride resin area has attempted to respond to the need for public and worker safety as well as to the resulting governmental regulations. The key factors in this response have been and will continue to be: 1. Availability of technical and financial resources. 2. Personal commitment on the part of the people involved, 3. Aid and cooperation of the regulatory bodies. With these tools almost any problem including vinyl chloride control can be overcome with a lot of hard work. 16.