Document 3eJM5VOLR2GLpXVy25p58Lqen
Table 4 Job Categories of Worker* Production of PVC Resins
Bagging and shipping operator Catalyst makeup man Dryer operator General laborer, custodial Laboratory technician Maintenance personnel Monomer maintenance personnel Monomer operator
Monomer recovery operator Monomer supervisor Polymer supervisor Reactor cleaner Reactor operator Slurry blender Utilities operator Warehouse personnel
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for each job category, but, from the available data. It
appears that reactor cleaners are likely to suffer the greatest
exposure. Operators of the polymerization process
equipment, those Involved In unloading or transferring the
monomer, and maintenance personnel, respectively, have
successively lover exposures to VC. Other personnel are likely
to be exposed to considerably lover levels.
Reactor cleaning poses a special hazard. During the reaction
process, the polymer, with entrapped monomer, builds up on the
surface of the reactor, so that periodic cleaning is usually
required. In the past, most of this cleaning was performed by
hand using a scraper and chisel. Air analyses have shown that VC
concentrations In the reactor prior to ventilation reach 3,000
ppm. After ventilation has been completed and the worker begins
the scraping, concentrations between 50 and 100 ppm may easily be
released. Air analyses have shown concentrations of 600 to
1,000 ppm of VC close to the hand. Since these studies
were completed. Industry has moved to greater use of water-jet
or organic-solvent cleaning.
Industry has also instituted monitoring procedures of
reactor interiors prior to worker entry.
Another exposure problem concerning reactor cleaning has
been that the air left in the reactor disperses into workroom
air, thus producing a major source of vinyl chloride exposure. Substantial efforts are currently underway to recover all
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unreacted monomer prior to opening the vessel, but the low levels
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Involved pose a significant technological difficulty. 3. Compounding of PVC Resins with Additives*-
In most applications, PVC is used with one or more additives to order to process and convert the compound into its final products. The additives used depend on the initial form of the resin, the type of processing to be used, and the desired application. Resins are classified either as general purpose (made by any of the techniques described in the previous section) or as dispersion (made primarily by emulsion polymerization). After the addition of a plasticizer, the general purpose resins can be further categorized into plasticized resins and unplastidzed (l.e. rigid) resins. The dispersion resins, or latexes, may be plasticized for ultimate use as plastisols or organosols, or they may be chlorinated for processing similar to Chat performed on the general purpose resins.
To these basic resins, additives are compounded according to particular requirements. These additives are usually categorized as follows:
o Plasticizers, to increase resin flexibility, softness, and elongation,and in some cases, to extend the basic resin;
o Heat stabilizers, to prevent discoloration during the processing of resin compounds;
o Fillers, to reduce costs, but sometimes to produce opacity, specific electrical properties, resistance
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to ultraviolet light, resistance to blocking, improved dryblending characteristics, and other properties; o Pigments and dves. to color PVC resin compounds; o Processing aids, .to facilitate the achievement of certain processing objectives, such as Increased processing rates, lower processing temperatures, improved fusion, and reduced surface gloss; o Impact modifiers to protect rigid resins against brittle fracture; o Lubricants, to reduce friction of melts with the surfaces of processing machinery and molds; o Light stabilisers, to prevent PVC degradation from continuous exposure to sunlight; o Fungicides, to inhibit the vulnerability of compounded FVC to attack by microorganisms; o Flams retardants, to preserve the nonflammability of PVC compromised by the use of flammable plasticizers; o Antistatic agents, to improve electrical conductivity; o Antioxidants, to provide protection in high-temperature applications; and o Foaming agents, to influence cell structure for resins expended into foams. The compounding of these additives with PVC resins may occur in a plant producing the basic resin, in a plant designed for the purpose of compounding alone, or in a plant which also processes the compounded resins. Trade publications identify
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approximately 200 companies supplying chlorinated, plasticized, and rigid PVC or copolymer resins.
The mechanism of the compounding stage ensures that the additives are fully dispersed in the PVC resins to yield a homogeneous material suitable for further processing. The resultant compounds may be in powder or granular form. The latter form is obtained by heating the compound into granules or pellets. These are then used as feedstock for subsequent extrusion and molding processes. The powder form can be fed directly into fabrication equipment where the PVC resin need be heated only once, thus requiring fewer additives.
(1) Process Descriptions A. wide variety of compounding equipment is currently available, most of which uses the resin and the various additives in dry form. Sometimes, the resin is premixed with several additives before it is used in further processing with other additives. The order in which specific compounding equipment is used depends on the specifications the PVC must meet. Two-Roll Mill - The material in the form of a powder blend is fed into a mill consisting of two rolls separated by a small gap. The mill kneads the material for about 10 minutes at a temperature of 150-160C and strips it off as a continuous band. This strip of material is then granulated into small cubes suitable for use in an extruder.
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Banbury Mixer - The mixer consists of a Jacketed mi-ring
chamber fitted with blades rotating in opposite directions.
It mixes batches of the material under pressure and heat to
produce a fused and kneaded mixture. The mixture then is
discharged in a doughlike mass which is converted into sheet
form for granulation. A two-roll mill is utilized for this
purpose. The use of Banbury mixers can lead to significant
variations between batches. This inconsistency is tempered
in some cases by the use of automatic weighing and feeding
systems for the injection of the various mixing materials.
Continuous Mixers - This type of mixing is designed to
enhance the consistency between batches by providing a
continuous feed operation; however, this process requires great
care to ensure the homogeneity of the resultant mixture. The
machines used are basically single or twin-screw extruders,
modified to optimize the handling of the material. The compound
may be heated, fed directly to calenders, extruded through a
die plate for granulation, or extruded through a die with a
die-face cutter for granulation.
Multiscrew Compounders - A twin-screw extruder, specially
adapted for the purpose, is also suitable for FVC compounding.
The primary advantage this equipment offers,as opposed to the
extruders described above,is that its screw sections and kneading
discs can be changed to meet the specific requirements of the
material being compounded.
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High-Speed Mixers - Equipment featuring injection molding and extrusion haa also been modified to accept PVC powder blends. To ensure satisfactory processing such blends are combined In a high-speed mixer by friction-generated heat* When this type of equipment is used* the production of the proper blends must be performed with great care.
Wet Granulation - In this process* the PVC Is taken directly from the polymerlzer while still In an aqueous dispersion. The additives are Introduced Into the granulating vessel along with a plasticizer that had previously been mixed with some of the stabilizer and lubricants. The compounded resin particles are then rapidly plasticized, producing a substance In the form of granules which are easily separated from the suspension. This process is limited to polymer manufacture, since It Is not economically feasible to redlsperse the polymer once it has been dried out.
Mixing of Plastiaols - PVC pastes can be produced using paddle-type mixers that can be operated under a vacuum, such as the vertical paddle mixer, the vertical planetary mixer, or the horizontal Z-blade mixer. The PVC resin is Introduced into the mixer along with sufficient plasticizer to form a paste. Other Ingredients are then incorporated slowly and at low temperatures until a smooth homogeneous paste is obtained.
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(2) Worker Exposure In regard to worker exposure during PVC compounding, the PVC resin and the compounds involved in the processing at this stage contain residual amounts of vinyl chloride in concentrations which may be as high as 3,000 ppm. If the basic resin is stored for some time in bags or drums, it will release an amount of the monomer inside the container which could enter the ambient air upon opening the container. Mixing operations during compounding may allow an additional release of the monomer entrapped in the polymer. Levels in the workroom air are generally below 1 ppm, although evidence presented at the OSHA public hearing of June 25, 1974, shows that excursions above this level are not Infrequent. These studies do not conclusively establish one way or the other whether workers engaged in compounding are exposed to concentrations in excess of 1 ppm. Sufficient Information is not yet available to enable a categorization of plants into those where concentrations above 1 ppm are likely and those where they are not likely. The highest exposures in this phase probably occur in storage areas, places where containers are opened, and near processing equipment where the PVC is heated. The numbers and types of workers exposed to vinyl chloride during these compounding operations are not known. Trade publications list about 200 firms, but give no estimate of their sizes.
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4. Fabrication of End Products from PVC Compounds^*^*^"*
The usefulness of polyvinyl chloride arises from its adaptability to many processing techniques used In the production of intermediate and final goods. These processes have been designed to accomodate a wide variety of plastic compounds, including PVC because of the its ease in processing and Its flexibility in allowing for the manufacture of a wide variety of products. In these processes, FVC resins compounded with the necessary additives are converted into products which need no further chemical handling. The products at this stage may be final goods (such as pipes), components of other equipment (such as wires), or materials for other Industries (such as films and sheets for packaging). As a result, products made at this stage may or may not be subjected to further processing, but if so, they will no longer be designated as PVC. The number of firms engaged In this processing stage is not precisely known, but has been estimated to be between 4,000 and 20,000. Such firms range In size from those with few employees and simple equipment to large plants Involving many employees and considerable capital.
(1) Process Descriptions The processing of PVC compounds at this stage Involves the shaping of the compound into its desired form. This is accomplished by subjecting it to Intense levels of heat and considerable pressure. Several types of equipment are used according to the final product desired. The major processes
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used to convert PVC into fabricated products are described below.
Extrusion - Both plasticized and rigid PVC formulations (as powder blends or granules) can be extruded (i.e. forced through a die) into a variety of shapes. Determination of the necessary temperatures and other process characteristics, such as rate at which the extrusion is to be carried out, can only be ascertained by careful experimentation. Extrusion is used primarily to produce wire and cable Insulation^ rigid pipe and tubing, blown film, and unplasticized PVC sheet.
Injection Molding - Generally, low-molecular-weight PVC retting fn Hit: In mu of granular <<r powder Manila are used in
Rt l pw t ypc fn|r-rllMn run Id! rig nini IiIiipq. TIi= mlai Inn ,.r l lie
screw produces a homogeneous melt which is then injected into a mold at high pressure.
Blow Molding - This process is generally limited to the production of bottles, and primarily uses powder blends with some interspersion of granular compounds. Most of the equipment used in this process produces a parison (I.e. a partially formed mass of material still in plastic form) by continuous extrusion or by the use of a reciprocating-screw system. This parlson is then blown into the proper mold.
Compression Molding -- This technique is used primarily for processing rigid PVC compounds into phonographic records. Usually, a copolymer of vinyl chloride and vinyl acetate is
000005285 VVC
60 used to produce a low-viscosity melt. This melt is mixed and fused, passed to a two-roll mill from which it is stripped, and then molded by compression in a record press at high pressure. The initial compound can also be in pellet form from which a premeasured molten extrudate is pressed in a mold. This technique is also the only method used to manufacture thick, high-quality, rigid sheet.
Calendering - This technique is used extensively for the production of flexible and rigid PVC film and sheeting. It is usually found in conjunction with the mixing of additives rather than as a separate stage in the processing of PVC. Pellets or powder are fed directly to the hot rolls of the calender. The resulting molten material is carried through finishing rolls to produce rigid sheets or material laminated to a substrate.
Plastisol Processing (Coating) - In this process a paste (or plastisol) made from PVC resin compounded with various additives is coated on various substrates such as fabrics, sheet steel, and paper. Several types of equipment may be used to perform the coating. The paste is generally fed into a device which applies the material to the substrate, either once or several times to achieve the desired thickness. The material is then passed through a heating oven to produce cosplete fusion of the compounded PVC paste material.
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61 Plastisol Processing (Molding Methods) - Por molding purposes, several methods may be employed to shape a PVC paste Into Its desired shape. A measured amount of paste may be charged Into a mold. The mold Is then rotated to cause the paste to gravitate to the walls where it is fused to form a uniform thickness. The paste may also be poured into a tvld where It is fused. Low-pressure Injection molding is also used to produce PVC shoe soles. Foams - Flexible or rigid PVC foams can be produced by subjecting PVC plastisol to mechanical or chemical blowing. In this process, the plastisol is mixed with an Inert gas or air which is then fed through spray nozzles under carefully controlled conditions to produce a slowly expanding foam. The plastisol is whisked and the resulting froth Is stabllzed. One of these mixtures Is then subjected to heat, while being calendered, extruded or molded, so that it is fused into the desired form. (2) Worker Exposure Exposure to vinyl chloride in the fabricating stage arises from the escape of residual monomer entrapped In the polyvinyl chloride. The monomer escapes at its fastest rate when the compound is heated. In general, the rate and the amount released depend on the residual amount, the extent to which the PVC has been diluted by additives, the type of equipment used for processing, and the temperature at which the PVC is processed. Worker exposure depends on the rate and the amount
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62 released, as well as the extent to which the processing area is ventilated. These factors have yet to be definitively delineated for each type of processing, so that it Is not possible at this time to state which processes might result In excess exposures*
Information presented at the OSHA public hearing of June 25, 1974,indicated that the concentrations of vinyl chloride in fabricating plants fall generally below 1 ppm with a small number skirting this level. An even smaller number of readings showed concentrations In excess of 1 ppm, ranging as high as 40 or 50 ppm on occasion. However, the information available as yet shows no clear-cut pattern which could permit a full characterization of exposure levels according to process type* In general the numbers and types of workers exposed to vinyl chloride in Its fabricating stages cannot be estimated until a scientific sample of the Industry Is performed.
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NOTES 1. C. A. Brighton, J. L. Benton, C. C. Marks, and J. P. Dux
In N. M. Bikales, ed., Encyclopedia of Polymer Science and Technology, Volume 14, Interscience Publishers, a division of John Wiley & Sons, Inc., New York, 1971, pp. 305-483.
2. D. P. Keane, R. B. Stobaugh, and P. L. Townsend, "Vinyl chloride: how, where, who - future," Hydrocarbon Processing, February 1973, pp. 99-110.
3. Statement of R. J. Reynolds, Shell Chemical Company, at OSHA hearing on February 15, 1974.
4. "Tight monomer supply plagues PVC producers," Chemical and Engineering News, May 28, 1973, pp. 6-7.
5. D. W. F. Hardie in A. Standen, ed., Kirk-Othmer Encyclopedia of Chemical Technology. 2nd ed.. Volume 5, Interscience Publishers, a division of John Wiley & Sons, Inc., New York, 1964, pp. 171-178.
6. "Production processes for vinyl chloride," Hydrocarbon Processing, November 1971, pp. 220-223.
7. L. F. Albright, "Manufacture of vinyl chloride," Chemical Engineering, April 10, 1967, pp. 219-226.
8. Division of Health Standards, Office of Standards Development, OSHA, "Industrial Hygiene Survey Report on Vinyl Chloride and Polyvinyl Chloride Manufacturing Facilities," March 1974.
9. Statement of V. K. Rowe, Dow Chemical Company, at OSHA hearing on February 16, 1974, Appendix II.
10. Statement of the Manufacturing Chemists Association at OSHA hearing on February 15, 1974.
11. Statement of PPG Industries, Inc., at OSHA hearing on February 15, 1974.
12. H. E. Frey, "Polyvinyl chloride resins," Chemical Economic Handbook, Stanford Research Institute, Menlo Park, California, September 1973.
0** .. c vl.C. 00 00
64 13* M. J. R. Cantov in A. Standen, ed., Kirk-Othmer
Encyclopedia of Chemical Technology* 2nd ed., volume 21, Interscience Publishers, a division of John Wiley & Sons, Inc., New York, 1970, pp. 369-412. 14. United States Tariff Commission, "Preliminary Report on U.S. Production of Selected Synthetic Organic Chemicals, November, December, and Cumulative Totals, 1973," Washington, D.C., February 6, 1974. 15. "pyc*" Plastics Engineering, December 1973, pp. 25-40. 16. W. A. Cook, P. M. Giever, B. D. Dinman, and H. J. Magnuson, "Occupational acroosteolysis: II. An industrial hygiene survey." Archives of Environmental Health. Volume 22, January 1971, pp. 74-82. 17. R. J. Galluch, "Polyvinyl chloride," Plastics World, August 20, 1973, pp. 66-67, 149-152. 18. Chemical Marketing Reporter, May 20, 1974. 19. Comment of Union Carbide Company on Draft Environmental Impact Statement Included in Appendix C.
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