VINYL CHLORIDE MONOMER EMISSIONS
FROM THE POLYVINYL CHLORIDE PROCESSING INDUSTRIES
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina
IV. DESCRIPTION OF PROCESSES AND EMISSION POINTSIn this section we describe the details of the various compounding and fabricating operations used in the manufacture of polyvinyl chloride products. For each process we have identified the major sources of vinyl chloride monomer loss where such information was available, or have attempted to estimate the loss based on known losses from similar processes.
In all cases, VCM content or loss is reported in "ppm"--that is, parts by weight of vinyl chloride monomer per million parts of polymer. (This is in contrast to units used to report concentration of VCM in air, where is "ppm" is used to connote parts per million by volume).
At the end of each process description, the amount of VCM emitted per weight of polymer is multiplied by the known amount of polymer processed by this method in the United States each year to arrive at the "total nationwide emissions" from each process. These totals are then summarized in Section V.
The physical and chemical properties of polyvinyl chloride are such that it can be used in only a very few applications in its pure (unmodified) form. In most of its applications, PVC requires the addition of a number of additives to increase its flexibility, ease of processing, resistance to degradation, etc. These additives (which may total as much as 100% of the weight of the raw PVC resin) include:
Plasticizers (such as the phthalates) to increase the flexibility of
the finished product.
Heat stabilizers (such as metallic salts, etc.) to prevent degradation and discoloration of the PVC at the elevated temperatures required for processing.
Lubricants to improve the flow of the molten PVC and to prevent its sticking to metal processing surfaces.
Fillers to increase the bulk and lower the cost of the final material. ,
Pigments and dyes to produce the desired color.
1. Simple mixing of all the ingredients.1. Flexible PVC Compounds
2. Adsorption of liquid ingredients onto the surface layer of resin particles.
3. Complete plasticization of the solvated resin particles.
4. Cohesion between plasticized resin particles.
5. Loss of identity of the individual particles by fusion.
6. Chemical interaction of the polymer with some of the ingredients (e.g., stabilizers).
a. Process Description
Hot compounding is frequently used when larger amounts of plasticizer are to be added to the polymer. In this operation the ingredients are first mixed together (in an operation roughly identical to "dry blending"). The resulting blend is then kneaded and fused to produce a homogeneous melt. The melt is then cooled and diced into pellets or granules.
Figures IV-1 and IV-2 show typical process flow diagrams for the hot compounding processes: a continuous process using a Farrel continuous mixer and a batch process using a Banbury mixer. Figure IV-3 is a cross sectional view of a typical Banbury mixer installation showing the ventilation system.
As shown in these flow diagrams, the process consists of several major steps:
1. Blending of raw resin and additives in a blender, where heat is
usually generated by the process. (In some operations, additional heat
may be added to promote mixing.) A dry powder results.
2. Heat mixing (Farrel Mixer or Banbury) to knead and fuse the powder to produce a homogeneous mass.
3. Milling to produce a ribbon of compound.
4. Cooling of the compound ribbon.
5. Dicing of the ribbon to make pellets.
6. Packaging or storing.
b. Vinyl Chloride Monomer Emission Points in Flexible Compounding Operations
Table IV-1 shows VCM levels measured by a major manufacturer at various points along his compounding operation. As shown by these data, the major point of VCM loss is at the addition of the plasticizer in the dry blending part of the operation. The dry blending alone, even with the addition of considerable heat did not result in significant release of VCM. Finally, after the dry blending operation is complete, a small amount of residual VCM remains; much of this is then lost in the subsequent "melt" phases of the operation.
These data are confirmed by the data of Tables IV-2 and IV-3, gathered by major manufacturers. Although less detailed, they confirm that essentially all of the residual VCM in the input raw resin to a flexible resin compounding operation is lost after the dry blending operation.
Finally, it is important to note that the residual VCM level in fully compounded flexible resins is very low; hence, any further processing of these resins into fabricated products can result in only a very small total amount of VCM emission from the fabrication process.
VCM Levels in Resin at Points along Compounding Process (ppm bar weight)
Rigid (co polymer/ Processing Step Flex Flex Flex Flex Flex Flex Semi-Rigid polymer mix) (1) (2) (3) (4) (5) (6) (7) (8) (a) Resin as charged 24* 18* 33* 58* 134** 374** 356 218*** (a-1) dry blend after heating but before [essentially unchanged] plasticizer added (b) dry blend after additives in. 4 6.4 10 7.6 7.3 3.2 65 190 (c) dry blend after completion of ND 4 5.1 ND 0.3 0.7 34 180 blending cycle (d) belt feeder to continuous mixer " 3.2 5.6 " 0.3 0.6 26 184 (after holding hopper) (e) Out of continuous " 2.1 5.4 " ND 0.6 11 211 mixer (melt) (f) Mill slab " 2.7 4.7 " " 0.4 13 218 (g) Pelletized ND 1.8 2.7 ND ND 0.3 13 229 * Bagged suspension resin stored over two months. ** Suspension resin bought in bulk rather than in bags. *** Relatively non-porous particles ND Non-Detectable (<0.1 ppm).
Vinyl Chloride Concentrations in Film Processing
VCM Concentration - ppm Date Resin Dry Blend Pellets Film 3/6/74 1400 2 21 2 8/22 360 4 7 ND 9/23 240 ND ND ND 9/23 300 5 7 ND 9/23 350 7 4 ND 10/14 210 3 8 ND 3/7 2400 48 23 2 3/7 2600 74 80 8 2/21 590 2 6 2 3/8 800 10 2 - 3/12 38 2 ND ND 3/12 1000 90 7 - 3/25 470 20 ND ND 3/25 560 13 1 ND 3/25 340 4 1 ND 7/9 12 1 ND - 7/25 200 2 ND - 7/26 170 ND ND - 7/26 90 ND ND - 7/26 55 ND ND - 7/26 120 3 ND - 8/13 91 1 4 - 8/13 86 1 - - 10/1 210 4 2 - 10/1 135 1 5 - 10/24 730 ND 2 -
Flexible VCM Compound Production
Input Output VCM Level VCM Level Process (ppm) (ppm) Dryblending 72 15 12(?) 12 157 10 411 2 629 <1 72 6 Pelletizing 15 1 12 <1 10 <1
c. Total Amounts of VCM Emitted from Flexible
PVC Compounding Processes
In studies of suspension resins, residual VCM levels ranged from less than 50 ppm to as high as 2600 ppm vinyl chloride monomer by weight. Tables IV-4 and IV5 show a tabulation of WH contents or resins received in consecutive shipments from suppliers by one major fabricator in late 1974. Suspension resin VCM contents are shown to vary from a low of 30 to a high of 3500 ppm.
Complete statistics on VCM levels in input resins are not available. On the basis of our plant visits we estimate that the majority of flexible resin compounding operations had an input resin VCM content of 200 to 1000 ppm. in 1974. In the latter part of 1974, raw resin manufacturers were beginning to devote considerable effort to lowering the VCM levels in their resins, so that the input resin to many of the operations probably fell in the 200500 ppm range. It has been estimated by manufacturers that by the end of 1975 most suspension resins will have residual VCM levels below 50 ppm. The technology to achieve these low levels appears to be a practical and important "control" measure for reducing VCM release to the atmosphere from flexible resin compounding operations.
For the purposes of estimating the total VCM release rate to the atmosphere from a compounding operation, the following formula may be used:
|100,000 x 300||= 30 lbs/day = 13.6 kg/day|
Assuming as a very rough estimate a nationwide average of 300 ppm in the
input resins to flexible resin compounding operations in 1975, the nationwide
release of VCM last year can be estimated as:
Percent* Vinyl Chloride Monomer in PVC Homopolymer
Identification Type Resin % Vinyl Chloride Monomer* 105-1 Suspension .086 244 Emulsion .003 283 Suspension .082 283 Suspension .081 283 Suspension .14 283 Suspension .13 283 Suspension .031 294 Emulsion .005 309 Suspension .016 309 Suspension .O1 311 Suspension .064 311 Suspension .073 311 Suspension .068 311 Suspension .029 311 Suspension .053 311 Suspension .049 311 Suspension .033 311 Suspension .037 312 Suspension .030 312 Suspension .064 312 Suspension .047 312 Suspension .091 312 Suspension .033 312 Suspension .041 313 Suspension .28 313 Suspension .28 313 Suspension .19 313 Suspension .06 313 Suspension .14 321 Suspension .087 321 Suspension .063 321 Suspension .084 321 Suspension .045 321 Suspension .046 321 Suspension .087 321 Suspension .045 321 Suspension .030 321 Suspension .022 321 Suspension .025 323 Suspension .014 323 Suspension .099 903 Suspension .004 * 1x - 10,000 ppm
Percent* Vinyl Chloride Monomer in PVC-PVA Copolymer
Identification Type Resin *% Vinyl Chloride Monomer 163 Suspension .071 163 Suspension .050 163 Suspension .077 163 Suspension .2560 163 Suspension .2600 163 Suspension .2100 297 Suspension .041 430 Solution .009 430 Solution .007 440 Suspension .054 440 Suspension .041 440 Suspension .100 450 Suspension .058 450 Suspension .079 450 Suspension .097 450 Suspension .039 450 Suspension .034 451 Suspension .078 451 Suspension .074 458 Suspension .062 458 Suspension .14 458 Suspension .080 458 Suspension .080 458 Suspension .054 459 Suspension .10 459 Suspension .082 459 Suspension .083 459 Suspension .136 459 Suspension .16 459 Suspension .15 459 Suspension .12 459 Suspension .118 459 Suspension .17 479 Solution .055 479 Solution .O1 480 Solution .003 532 Suspension .35 * 1x - 10,000 ppm.
By the end of 1975, assuming the-goal of 50 ppm is reached, the annual rate of VCM release from flexible resin compounding operations may be estimated (assuming the same rate of production):
2. Compounding of Rigid Formulations
a. Process Description
A minor amount of rigid compounds is also produced by a process involving fusing of the powder and dicing to produce rigid pellets.
By far the largest application for rigid PVC resins is in the production of PVC pipe. The great majority of PVC pipe producers do their own compounding on the same site as the pipe production. Mixing the additives with the raw resin takes place in a high intensity "hot" mixer through the use of an air sweep or a vacuum, considerably increasing the amount of VCM given off. Older installations do not have this feature, but the pressures of the OSHA regulations and the hazards of exceeding the lower explosive limit for VCM will probably result in more and more facilities providing for VCM removal in the "hot" mixer.
Figure IV-4 Dry Blend Compounding
From the "hot" mixer, the blended powder is sent to a cooler, which may be a high intensity mixer with a cooling jacket, or a ribbon blender from which it is transferred to a storage silo.
A variation of this process is the "double batching" method of compounding in which only a portion (usually 50%) of the batch of, raw resin is mixed with additives in the hot miser. The remaining portion of raw resin is sent directly to the cold mixer where it is blended with the material from the hot mixer. A typical double batching compounding set-up is shown in Figure IV-5. Double hatching is efficient in that energy bangs are possible and the heat exposure of the resin is minimized.. Single hatching which is much more widely practiced in the pipe industry, removes a considerably large quantity of VCM since the major point of VCM emission appears to be the hot mixing stage.
Data on VCM loss in rigid formulation compounding is scarce. Almost all of the data which do exist came from the compounding of pipe compound, which is by far the largest application of rigid PVC.
b. Points of VCM Loss and Amounts of Loss from Compounding of Rigid PVC Formulations
It appears that under some circumstances, very high removal rates for VCM are possible in the dry blending operation due to the relatively high temperatures in the hot mixer (as high as 138°C (280°F)) and the large surface area of the resin powder. The amount of VCM given off in the compounding operation appears to be highly variable, and dependent upon mining temperature and time, mixing intensity, resin particle size and porosity, and aspiration within the hot mixer. Some manufacturers show negligible amounts of VCM lost in the compounding operation, while one manufacturer reported 94 to 98% removal of VCM during rigid compounding. Data from a manufacturer of an internal aspiration system for hot blenders indicates that the loss is highly dependent upon the amount of aspiration, ranging from 86% removal of VCM without aspiration to 99% removal with aspiration and air stripping. These data are all summarized in Table IV-6.
Raw resins to pipe compounding operations in late 1974 (which are presumed to be typical of PVC resins for other rigid formulations) typically contained 300 to 500 ppm VCM, with VCM levels in compounded resins about . 50 ppm. As a very rough estimate therefore, the total loss of VCM from PVC compounding manufacturing operations is estimated to be about 250-450 lbs per million lbs of pipe produced. At an estimated 1.9 billion lbs of rigid PVC produced in 1974, this corresponds to a total VCM loss of 216,000-388,000 kg (475,000855,000 lbs) of VCM lost/year from rigid PVC compounding.
Figure IV-5 Typical Double Batch Compounding of Pipe Resin
VCM Loss During Dry Blend
Compounding of Rigid PVC Formulations
VCM Content (After Stripping) Blend Blend Input from from % VCM Removal Resin Mixer Cooler After Cooler (ppm) (ppm) (ppm)
Manufacturer A 218 190 180 17 Manufacturer B Batch 1 1014 33 30 98 Batch 2 413 - 26 94 Manufacturer C 550 - 74 87 Manufacturer D Batch 1 300 - 80 73 Batch 2 530 - 92 83 Batch 3 390 - 200 49 Werner-Pfluderer "Exorsta" Data Mixing @ 120°C (237°F) No Aspiration 1000 565 160 84 Aspiration 1000 205 36 96 Aspiration b Air Stripping 1000 68 11 99
3. Plastisol and Organosol Compounding
Plastisols and organosols are liquid systems consisting of dispersions of. PVC resins in additives. Plastisols typically contain 30 to 50% plasticizer plus other additives such as stabilizers and fillers. Organosols differ from plastisols in that the former are thinned with solvents to control the viscosity. The VCM emissions from plastisol and organosol compounding appear to be negligible because the VCM content of the input resin is extremely low. Most organosols and plastisols are made from emulsion resins. Data from manufacturers indicate that raw emulsion resins typically have VCM content of less than 10 ppm. Thus, the total amount of VCM which could be emitted could not exceed 10 pounds of VCM per million pounds of emulsion resins used to produce plastisols or organosols or a total of 2,000 kg (4,400 lb) per year.
Extruded products account for a large fraction of the consumption of PVC resins, and include both flexible and rigid formulations. Approximately 80% of rigid PVC is processed by extrusion; major products include pipe and conduit, panels and siding, windows and other profiles and rigid sheet. Extrusion also accounts for almost 40% of flexible PVC fabrication with major products being wire and cable sheathing, weather stripping, medical tubing, garden hose and film. (A breakdown of the type and quantity of products made by extrusion of PVC is given in Table II-3.) Extrusion takes place at temperatures ranging from 120 to 190°C (Table IV-7). In general, extruders processing powder blends will operate at the upper temperatures, and those processing granulated compounds at the lower end. Unplasticized PVC is processed at a somewhat higher temperature than plasticized PVC.
1. Extrusion of Flexible PVC
Extruders of flexible PVC operate in either of two modes: they may purchase compounded flexible PVC to be fed directly into their extruders, or they may purchase raw resin and do their own compounding in-house. The VCM emissions from the plants of extruders of flexible PVC is totally dependent upon which of these choices is made. As discussed under "Flexible PVC Compounding" above, almost all the residual vinyl chloride monomer in raw resin compounded into flexible formulations is lost during the hot blending portion of the compounding operation, when plasticizer is added. The amount of VCM remaining after completion of compounding is usually less than 10 ppm. Thus, the extruder of flexible PVC resin will have VCM emissions of a maximum of only ten parts of VCM per million parts of resin processed if he starts with compound. His counterpart who purchases and compounds from raw resin will have emissions as high as 200500 parts per million.
Typical Extruder Temperatures for PVC
Plasticized Compounds Temp, oC feed end of screw 120-140 front end of screw 140-160 head 150-170 die 160-180 Unplasticized (rigid) Compounds feed end of screw 140-150 front end of screw 155-165 head 165-175 die 170-190
We estimate that approximately 70 to 75% of manufacturers of extruded flexible PVC products do their am compounding. (A breakdown of this is shown in more detail in Table III-4 of Section III.) The only major purchasers of ready-made flexible compound for extrusion are makers of film and medical tubing, who buy their compound from the resin producers; wire and cable coating extruders also buy a minor fraction of their feed as compound, from independent compounders.
Wire and cable coating and film extrusion are good examples of products made by extrusion of flexible PVC. Wire and cable coating is the single largest extrusion process for flexible PVC and accounts for approximately 354 million lbs per year, or about 22% of the 1.6 billion lbs of flexible PVC consumed in the United States each year. The major resin used is a homopolymer of medium to high molecular weight, with an additive content of 40 to 60 percent based on the final compound. About 125-135 million pounds of PVC were used in 1973 for the fabrication of flexible film for packaging applications.Wire and Cable Coating
The producers of PVC-insulated wire and cable generally purchase raw resin and produce granulated compound themselves. (About 70 million lbs--or 20x--is bought from independent compounders who prepare special formulations. for the wire and cable industry.) The total process is shown schematically in Figure IV-7.
In the extrusion process for wire coating, high rates of output are of primary importance. Figure IV -7 is an illustration of the crosshead type die used for wire coating. The wire to be coated passes straight through a crosshead die at right angles to the length of the extruder. The polymer melt (melted granules) enters the crosshead from the extruder and is directed around the wire and merges through the die. After emerging, the wire may be preheated electrically or flamed to remove lubricants and to improve adhesion.VCM Emissions from Wire and Cable Coatings
As discussed above, the primary emissions of VCM from wire and cable coating operations will occur in the compounding steps. The primary point of this emission would be in the addition of the plasticizers and additives to the raw resin during the hot mixing portion of the compounding operation (discussed above). Emission of VCM at later points in the process is negligible. Taking as a very rough average a net emission from the entire coating operation of 300 parts of VCM per part of resin processed in 1974, the emissions from a 1,000 lb per hour extrusion line would be 0.3 lbs per hour of VCM, or 3.3 kg/day (7.2 lb/day). The total nationwide emissions from the process (assuming 354 million lbs of PVC used per year) would be approximately 48,000 kg VCM per year (106,000 lb/year).
Method 1 Silos ---> Henschel type --> Twin Screw ---> Granulators ---> Wire Mixers 1000 lbs. Comp. Extruders Extruders Method 2 Resin ---> Blenders ---> Banbury ---> Mill ---> Strip ---> Granulator ---> Extruders Storage Mill
Figure IV-7 Crosshead Die For Wire Coating
Most flexible PVC packaging film is made by blown film extrusion. These products are used for a variety of consumer and industrial applications. Consumer applications include meat and produce wrap; industrial applications include wrapping for small parts or loose paper type products. A particular advantage of the PVC films for these applications is their ability to be oriented and then shrunk during subsequent exposure to heatto produce a so-called shrink wrap film.
It is possible to extrude film from either compounded powder or pellets. Most flexible PVC extruded film is formed from pellets (which are made using conventional powder blending techniques described earlier), followed by extrusion of a strand which is cooled and pelletized. Film is made from either purchased pellets or via in-house compounding at the fabrication plant. Flexible film is also made from powder which is compounded on-site using standard techniques.
Both of these methods are indicated in the flow chart of a typical flexible
film extrusion plant, shown in Figure IV-8. (Figure IV-8 also indicates the
sources of VCM emission.) The major source of emission will be from the hot
stage of the compounding operation. Other emission points of less
importance as shown in Figure IV-8 are from:
- venting to the atmosphere from raw powder and pellet storage silos,
- the vacuum port of the pelletizing extruder, and
- the fume collector which surrounds the film bubble.
Thus, emissions from this portion of the industry can be estimated by assuming all of the VCM entering as raw resin leaves the film operation. One major manufacturer's sampling of incoming (uncompounded) resin between June and November of 1974 measured VCM concentrations ranging from 2 to 325 ppm, with typical readings of 65 ppm.
Assuming this figure is typical and that 130 MM lbs of flexible PVC packaging film are processed annually, the emissions from this sector of the industry are 65 x 130 - 8450 lbs/yr (3840 kg/year). It should be noted that approximately 90% of this arises from the compounding portion of the operation, and less than lox arises from the actual extrusion process.
Figure IV-8 Flexible PVC Film Extrusion With In-Plant Compounding
2. Extrusion of Rigid PVC Resinsa. General
As in the case of processors of flexible PVC resins by extruders, manufacturers of rigid PVC resins by extrusion may purchase ready-made compound from the resin producers, or may compound their resins themselves. (The independent compounder of rigid PVC resins for extrusion is virtually nonexistent.) Manufacturers of pipe and conduit-which account for 78% of the total consumption of rigid PVC for extrusion--compound about 99x of their resin themselves. Most other manufacturers of extruded rigid PVC products purchase all of their compound from the resin producers. (An exception to this rule are the producers of foam molding, many of whom formulate their own compounds.)
b. Major Examples of Extruded Rigid PVC Products
Production of PVC pipe represents about 55% of the total extrusion of PVC in
this country (or about 78% of extrusion of rigid PVC). Most PVC pipe
manufacturers compound raw material at the same site, and the majority of the
VCM loss is from the hot mixing step of the compounding operation.
A typical PVC pipe manufacturing facility produces 20-25 million pounds of extruded pipe per year using 4805 extruders. A schematic of a typical plant's operations is shown in Figure IV-9. (Note that in Figure IV-9 compounding via both simple and double batching is indicated.)
The economics of PVC pipe extrusion dictate that individual processors purchase raw PVC resin powder and add stabilizers, lubricants, and processing aids and pigments in a central compounding operation at the plant. The powder blend to feed the extruders is typically prepared in 400-1,000 lb (180-455 kg) batches.
The capacity of the extruders is typically 600-700 lbs/hr (270-320 kg/hr)
although some plants, particularly those processing manufacturing larger
diameter pipe, use extruders with capacities as high as 1,300 lbs/hr (590
kg/hr). Simple twin-screw machines typically consist of two twinscrew machines
operating in series. In this mode of operation, the first extruder's function is
to melt and mix the powder and extrude it into the hopper of the second extruder
via an intermediate evacuated pelletizing stage (in which the molten strands of
resin are cut into pellets using a hot face cutter.) Evacuation occurs from this
intermediate "pelletizing" stage.
After emerging from the second extruder, the melt passes through the annular orifice of the die, and then is cooled in a water bath, cut into lengths and stored. Some types of pipe require a secondary finishing or shaping operation prior to storage.
Figure IV-9 Typical PVC Pipe Extrusion Operation
c. Sources of VCM Loss in Pipe Extrusion
In practice, resin is transferred from the storage
silo to the hot mixer via an intermediate weighing station in which additives
are mixed. The batch is processed using either the single or double-batch method
which have been described previously. From the processor's-viewpoint,
doublebatching is efficient in that energy savings are possible and the heat
exposure of the resin is minimized. Single batching, however, removes a
considerably larger quantity of VCM. In estimating the quantity of VCM
discharged by a particularly plant, it is therefore essential to determine which
practice is used.
Sources of VCM losses from pipe extrusion facilities are from vents in the following four areas:
1. Resin Handling
Hopper car, transfer devices, and raw resin storage silos
In-plant conveying systems
Extruder vent pump
4. Pipe Handling/Storage
Pipe cutting station
Pipe storage facility
Hot miser. As discussed previously, considerable quantities of VCM can be removed during the hot blending stage. Modern installations remove VCM directly from the bowl of the hot mixer through the use of an air sweep or a vacuum. Although there are still a considerable number of installations which do not follow this practice, it appears that the pressures of OSHA regulations and the hazards of exceeding the lowef explosive limit for VCM will result in provision for VCM removal from the bowl of the intensive mixer.
Cold mixer. After hot mixing the powder compound is transferred to the cooling stage. Removal of VCM at this stage is comparable to that achieved in the hot mixer.
Extruder vent. Elimination of volatiles from the molten pipe extrudate is crucial to the production of quality pipe. This removal occurs from an evacuated port at a stage in the extruder at which the resin is molten. Typically, the molten resin is exposed to a vacuum of 12-14" Hg., although vacuums as high as 23" Hg. have been observed during our field visits. In the case of pipe production using a single extruder (either single screw or twin screw), the volatiles are removed from a vacuum port located along the barrel.
Ventilation. In modern pipe production plants the ventilation systems from the storage/transfer and compounding stages are collected at a central location - often this is a rooftop collector containing a bag for filtering powder particles. This is known as the bag house and is important for economical operation since considerable quantities of PVC powder can be recovered. The bag house is also the major concentrated location of VCM in a typical pipe processing plant.
Robintech, Inc., has the capability of blending additives in the polymerization kettle. These resins are referred to as in-house compounded (IHC) resins and they do not require compounding at the pipe extrusion facility. The VCM discharge from such plants should be considerably less since the compounding steps are eliminated.
VCM Loss in Pipe Extrusion. Although data on VCM levels in pipe from extruders are not available, an estimate of the amount of VCM lost may be made from the measured VCM levels in the air exiting from the extrusion process. Typical concentrations of VCM between 23.5 and 430 ppm in air were reported at a flow rate of 3.5 SUM of air, corresponding to a total loss of 4.2 x 10-4 - 77.4 x 10-4 lbs/hr (1.9 x 10-4 - 35 x 10-4 kg/hr) of VCM at an extrusion rate of 1,000 lbs/hr (454 kg/hr) of pipe. This corresponds to a VCM loss of 0.4 - 7 lbs VCM per million lbs of PVC pipe extruded--a negligible quantity.
The total nationwide emissions of VCM from PVC pipe extrusion (accounting for 1.26 billion lbs/year of PVC resin) are estimated to be:
VCM loss from compounding: 142,000-257,000 kg/yr (250-450 ppm lost) (315,000-567,000 lbs/year) VCM loss from extrusion: 218-3,800 kg/year (negligible) (480-8,400 lbs/year)(2) Profiles and Siding
Profiles and siding account for almost 100 million lbs of rigid PVC extrusion per year. Manufacturers typically buy pelletized compounds from the resin manufacturers who supply custom formulations to the large fabricators.
Compound arrives to the fabricator in trucks and is stored in vented silos.
The residence time of the compound in the silo can vary from three days to two
months. From the silos, the resin is conveyed into vented surge hoppers where it
is warmed slightly [to 380°C (100°F)], and then into the extruder. From the
extruder, the profile is conveyed through a cooling system--either water or
air-cooled--and thence to a cutter. Scrap from the cutting operation (averaging
about 1.5% of the product) is sent to the grinding room for recycling. (Figure
IV-10 shows a schematic of the operation.)
Essentially no data are available on the VCM emission from these operations. One manufacturer quoted an input compound level of 100 ppm VCM as received from the resin manufacturer. The amount of further loss during the extrusion step is not known. One source of loss to the atmosphere is from the storage silo. This loss may be relatively small since the resin is in pellet form rather than in powder form. Some loss probably occurs over the heated extrusion section. However, this appears to be quite small, since the measured levels of VCM in the air exhaust over the extrusion is very small --less than 0.1 ppm (volume of VCM vapor per vole of sir).
Calendering is used for the production of both plasticized and rigid PVC
sheet as well as coated fabrics and unsupported flexible films. It is capable of
producing high quality material at very high rates of output. The resin
formulations typically contain 20-30X plasticizer. Its major application is in
the production of flexible PVC sheet and film, and accounts for the consumption
of approximately 867 million pounds per year of flexible resin--or about 54% of
the total U.S. consumption of flexible compound.
Essentially all calendering compound is produced by the fabricator rather than the resin producer. Typically, the raw (suspension or bulk process) resin and additives are fed to a hot blender, thence to a Banbury mixer where it is melted; it is then milled and discharged directly into the calender. Often a screening extruder is used before the calender. After the calender, the sheet is cooled and finished.
Calendering can also be used for coating fabric and paper with plasticized
PVC sheet; the substrate is fed into the calender nip of the last roll to carry
out the lamination.
In calendering, the PVC is subjected to fairly high temperature because of the high shear; molecular weight polymers can therefore-be used. In rigid sheet production, extreme pressures and high roll temperatures--approaching 200°C for homopolymers--must be used.
Figure IV-11 shows a schematic of a typical calendering operation which could be used for manufacturing flexible unsupported films or coated fabrics These products are used for shower curtains, baby pants, wall coverings, swimming pool lining, tape, surgical drapes and book covers.
As in all processing of flexible PVC resins, the majority of the residual vinyl chloride monomer loss in PVC calendering plants occurs during the compounding portion of the operation. In the past, raw resin arrived with a residual VCM level typically between 100 and 500 ppm. Even at these input levels it is possible to reduce VCM in outgoing film to below 1 ppm. Table IV-8 shows data obtained from a manufacturer of unsupported film which shows the reduction in VCM at different stages in the process. This data was obtained on a process which has two mills following the Banbury. Unfortunately data were not obtained directly after the Banbury. The data does indicate however that the major portion of VCM is removed either by the Banbury alone or in combination with the first run. The fact that very little reduction in VCM content is measured between the first and second mill supports the conclusion that the major portion of the VCM is eliminated by the Banbury. This loss is not surprising, since the polymer during this operation is hot, molten, plasticized, and has a high surface-tovolume ratio--all optimum conditions for the release of monomer.
Based on these data, the nationwide emissions from calendering of flexible PVC can be estimated to be:
Compounding portion: 39,000-195,000 kg/year (100-500 ppm) (86,700-430,000 lbs/year) Calendering portion: 3,900-7,900 kg/year (10-20 ppm) (8,670-17,340 lbs/year)D. BLOW MOLDING
Rigid PVC bottles are produced by the blow molding process. All blow molded PVC bottles are made from compounded pellets purchased by the blow molder from the resin supplier. These pellets are stored after delivery and then vacuum-conveyed to a hopper which feeds a single-screw extruder. The compound is melted in the extruder, reaching a temperature of 380°F. From the extruder, the molten compound passes through a die to a mold where it is blown and cooled. After cooling the flashings are cut from the bottle and recycled into-a grinder and thence to the extruder hopper. About 30% of the feed is recycled as ground flashings.
VCM Losses from Flexible PVC Calendering
VCM Concentration (ppm) PVC resin to Banbury 400 PVC compound from Banbury - (No value vas measured) PVC compound from first mill 32 PVC compound from second mill 26 Film from calender ND (<1 ppm)
Resin manufacturers typically control the VCM levels in compound for bottles to extremely low values. Ethyl Corporation, who is a major supplier of blow molding resins for example, currently produces to a specification of less than 1 ppm VCM for their food grade bottles and to a specification of less than 10 ppm in its general purpose bottle compound. They estimate and we agree, that less than 15% of the VCM in the compounded pellets is removed during blow molding.
These figures indicate that the total loss of VCM from blow molding is very small (probably less than 1 lb VCM loss per million lbs of PVC processed by this route). The major source of this loss may be at the point at which the bottles are blown. Prior to this, the process is essentially totally enclosed. Little, if any, VCM can escape from the extruder, and the residence time at the die (where the molten compound is first exposed to the atmosphere) is too low-typically about 4 seconds--to allow much escape of VCM.
E. INJECTION MOLDING
Injection molding is an intermittent, cyclic process in which particles of compound are heated until they become molten. The melt is then forced into a closed mold where it cools, solidifies and is ejected as a finished or semi-finished part.
Both flexible and rigid PVC compounds are injection molded. Both homopolymers and copolymer resins are used, with homopolymers predominating. Although it is possible to mold powder blends, most injection molders use compounded pellets. Shoe components account for the majority of the injection molding of flexible compound and pipe fittings account for the majority of rigid compound which is injection molded.
VCM Loss. VCM loss during the injection molding of flexible PVC pellets is slight or negligible. Little monomer remains in the compound granules which are put into the injection molding process since most has been removed during compounding. In addition, the injection molding process is essentially close to the atmosphere allowing little or no monomer to escape.
We have no data on the VCM losses during injection molding of rigid PVC compound. In contrast to flexible compound, the VCM content of rigid compound is sometimes substantial (possibly ranging as high as 100 ppm in late 1974 compound). However, the opportunity for VCM loss is relatively slight. We would estimate that the major source of loss in the injection molding process would be at the point at which the pellets are melted.
F. COMPRESSION MOLDING
Phonograph records are the major PVC product fabricated by compression
molding. In the record molding process either compounded resin or dry blend is
fed to a small extruder where it is melted. A measured amount of material is
then extruded between the labels that go onto the record. The operator picks
this up from the extruder and places the sandwich in a press. The press closes
and the finished record is removed some 15 to 30 seconds later.
VCM Loss. Records are made from a polyvinyl chloride/polyvinyl acetate copolymer which in early 1974 had a relatively high monomer content (greater than 500 ppm). However, manufacturers of records tell us that in late 1974 the monomer content in raw resin was reduced to 50 to 100 ppm. Compounding of semi-rigid compound for records may take place either at the resin producers or at the record manufacturing site. It appears that approximately half of the input resin monomer content is lost in the compounding process. (This is a very rough estimate, based on a minimal amount of data.) We cannot estimate the additional VCM loss during the compression molding process since no data are available.
G. SOLVENT CAST FILM
The solvent casting process is used to produce packaging films of higher
quality than those made by blown film extrusion. The solvent cast product has
better gauge control and improved clarity.
The solvent cast processing consists of dissolving powdered resin and casting the solution onto a belt. The casting belt is totally enclosed thereby permitting complete recovery of solvent. The wet film is then passed to a drying oven. A high percentage of solvent recovery is essential to the economics of the process. The details of the solvent recovery system are considered proprietary by the film manufacturers.
A generalized flow chart for solvent cast film production showing potential emission points for VCM is shown in Figure IV-12. The sources of VCM loss are primarily from the solvent recovery operation with minor quantities during resin transfer and storage. We have not been able to obtain quantitative data on VCM concentrations in streams leaving the processes.
One manufacturer reported the results of several months monitoring of incoming resin at solvent casting operations, indicating an average VCM concentration of 20 ppm. Typically they found no detectable concentration of VCM in film leaving the process, but they have not been able to isolate the specific source of VCM lose from the process.
We estimate that 50 million lbs of solvent cast PVC film are manufactured in the U.S. Assuming that an average concentration of 20 ppm enters the process in the raw PVC powder, and that it is completely lost in the
production process, the loss of VCM from this segment of the industry is estimated to be:
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