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DRAFT

VINYL CHLORIDE MONOMER EMISSIONS 
FROM THE POLYVINYL CHLORIDE PROCESSING INDUSTRIES

Report to
U. S. Environmental Protection Agency
Research Triangle Park, North Carolina


IV.      DESCRIPTION OF PROCESSES AND EMISSION POINTS

In 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.

A.     COMPOUNDING

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:

Successful compounding of these ingredients to achieve satisfactory properties in the final product depends on the ability to blend all of the additives sufficiently well that a homogeneous material results. The compounding process has been divided into the following separate processes, all or some of which may take place in any given operation:

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1. Simple mixing of all the ingredients.
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).
1.    Flexible PVC Compounds There are two major methods for the preparation of plasticized PVC: dry blending and hot compounding. In the dry blending operation, the liquid additives are simply mixed with the resins and stirred rapidly below the fusion temperature at 93-107C (200-225F). The resin-particles "soak up" (or adsorb) the liquid and the result is a dry powder barely distinguishable in appearance from the original resin. Although the resulting mixture is technically not yet "fully compounded", it may be stored and then fed directly to fabricating equipment where the resulting high temperatures melt the dry powder and produce a fused compound in the process of fabrication.

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:

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Figure IV-1.       Continuous Hot Compounding Line.

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Figure IV-2.       Batch Hot Compounding operation.

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Figure IV-3.        Banbury Mixer

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The major point of VCM emission in the compounding of flexible resin is in the addition of plasticizer during the dry blending operation. (This conclusion holds whether or not the dry blended powder is later subjected to fusion.) Experiments by resin and compound manufacturers appear to show that little or no VCM is released during the simple process of stirring and heating in the dry blender. (if no aspiration is used), but that upon addition of the plasticizer, most of the residual monomer is released. Our limited data appear to indicate that with the initial VCM contents of the input resin in the range of 20 to over 1,000 ppm, the VCM content of the dry powder after plasticizer addition is usually reduced to less than 20 ppm. Minor amounts of VCM are emitted at later points of the compounding operation, notably in the additional blending of the dry powder, and in the processing of the melt after the melt mixers. (Although it had originally been hypothesized that the major points of VCM emission would be at the points at which the resin was melted, this is apparently not so for two reasons: (1) very little VCM is left by the time the compound reaches the melt mixer and (2) the surface area of polymer exposed to the atmosphere during the melt is very small compared to the large surface area exposed by the fine particles in the dry blender.)

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.

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Table IV-1
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).

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TABLE IV-2
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         -

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TABLE IV-3
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

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The amount of vinyl chloride monomer emitted from flexible resin compounding processes is almost directly proportional to the amount of residual VCM in the input resin to the operation, since practically all, of the residual VCM in the resin is released upon compounding. The amount of residual VCM in the incoming resin is highly variable, and dependent upon a number of factors, notably the details of the polymerization and post-stripping process, and the storage and shipping history of the resin prior to compounding. Storage time of the resin before use strongly affects the residual VCM levels: it has been estimated that approximately 23 6o 50Z of the original VCM content of raw resin is lost during the first 30 days of shipping and storage under ordinary conditions. This loss may be accentuated under conditions of high ventilation. Because of this effect of storage, VCM losses will, in general, be higher from compounding operations which take place at the same location at which the raw resin is manufactured, and lower from these operations which purchase or ship input resins from other locations. 

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
106

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:

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TABLE    IV-4
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

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TABLE IV-5
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.

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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):

As discussed in Section III (Table III-4) approximately 83% of flexible PVC compound is produced by the fabricators of semi-finished products, about 11% is produced by the raw resin producers, and about 6% by independent compounders. Table III-4 of Section III further subdivides the compounding operations by type of final product.

2.       Compounding of Rigid Formulations

In rigid dry blend formulations, the resin, filler, lubricant and stabilizers are mixed in an intensive (high speed) mixer where considerable heat is generated. It is then cooled in a ribbon blender or in a lower speed cooling mixer similar in design to the hot, high intensity mixer, and transferred for packaging or storage. In this operation, the compound does not go through the melt phase. Figure IV-4 shows a schematic of a dry blend process.

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.

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Figure IV-4 Dry Blend Compounding

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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.

b.      Points of VCM Loss and Amounts of Loss from Compounding of Rigid PVC Formulations

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.

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 138C (280F)) 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.

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Figure IV-5 Typical Double Batch Compounding of Pipe Resin

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TABLE IV-6

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 @ 120C (237F)

No Aspiration                   1000     565     160          84
Aspiration                      1000     205      36          96
Aspiration b Air
Stripping                       1000      68      11          99

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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.

B.       EXTRUSION

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 190C (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.

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TABLE IV-7

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

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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).

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Figure IV-6. Schematic of Wire and Cable Coating Process.

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

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Figure IV-7 Crosshead Die For Wire Coating

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         Film Extrusion

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: 

We have obtained data on residual VCM content of extrusion-blown film from two major manufacturers. In one case the manufacturer reported that during a six month period VCM concentrations in film leaving the plant never exceeded 0.02 ppm. The second manufacturer indicated that with rare exception the concentrations were below the levels detectable by gas chromatographic methods.

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.

IV-23


Figure IV-8 Flexible PVC Film Extrusion With In-Plant Compounding

IV-24


2.     Extrusion of Rigid PVC Resins

          a.     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

        (1)     Pipe

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.

IV-25


Figure IV-9 Typical PVC Pipe Extrusion Operation

IV-26


          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

    2.      Compounding

    3.      Extrusion

    4.     Pipe Handling/Storage

The major locations of VCM removal and discharge to the atmosphere are (in decreasing order of importance): hot mixer, cold mixer, extruder vent, resin unloading and transfer.

IV-27


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:

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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 380C (100F)], 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).

      C.    CALENDERING

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.

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Figure IV-10 Rigid Profile Extrusion

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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 200C 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 380F. 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.

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Figure IV-11 Typical Calendering operation


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TABLE IV-8

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)

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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.

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     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

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Figure IV-12 Solvent Cast PVC Film Production

IV-36


production process, the loss of VCM from this segment of the industry is estimated to be:

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[Section   V | Table of Contents]

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