UNDERSTANDING THE CHEMICAL TREATMENT OF SOLVENT PAINTS AND EXTRAPOLATIONS TO WATERBORNE COATINGS ; . m 9

1 2&3'73

I P F

Written by: D.B. Mitchell S.M. DeBoo G.A. Tonn T.P. Curran M.J. Deschryver

Presented by: David 6. Mitchell Senior Group Leader GRACE Dearborn Lake Zurich, Illinois

Presented at: "Finishing '93 Conference and Exposition" October 25-28, 1993 Dr. Albert B. Sabin Convention Center Cincinnati , Ohio

Copyrighted by SME; Technical Paper Number to be Assigned

1.0

Understanding the Chemical Treatment of Solvent Paints and Extrapolations to Waterborne Coatings.

Introduction.

The introduction of waterborne coatings has raised a number of concerns as to their treatment in Paint Spray Booths (PSB), and how this may differ from traditional solvent paints. To evaluate this requires a thorough understanding of current chemical treatment protocols for solvent coatings. In this respect two automotive assembly plants using solvent paints were researched. One of these plants (hereafter designated Plant A) did not possess any solids removal equipment, whilst the other (Plant B) had an autoweir and filtering device, Laboratory and field studies were performed upon the PSB sump water from both plants. The waterborne research was completed using a 250 gallon downdraft PSB located at GRACE Dearborn, designed by Daniel L. Bower Co.,Inc. and equipped with a Palin dissolved air floatation (DAF) unit for solids removal.

There are numerous references in the literature disclosing chemical and mechanical treatment methods and their relative merits. Performance is usually evaluated based upon detackification, paint solids f loa ta tion/removal , and dewaterability of the resultant paint sludge. The testing duration is normally 1-2 hours. Whilst the above test parameters are obviously important, the omission of soluble contaminant quantification ensures the test protocol provides only a basic understanding of suspended solids distribution, If soluble contamination is quantified, it is usually via a single parameter, dissolved solids. No attempt is made to verify the importance o f soluble chemistries, and both Chemical Oxygen Demand (COD) and Fats Oils and Grease (FOG) are ignored.

Therefore it can be a cause for concern when the empirical results from a laboratory PSB treatment protocol is extrapolated to the field. When this is done, the programs frequently provide poor performance with respect to solids flotation and removal, microbiological control and foaming. Furthermore knowledge of corrective action is often missing because of the artificial test ~

~

me thodo logy. ~~

The objectives of this study were to:-

a. Determine the effect of soluble contaminants upon suspended solids settling.

b. Determine the settleability of a solvent/waterborne (15:85) paint mixture versus 100% waterborne.

c. Comment upon the expected differences in P.S.B. treatment efficacies and protocols, for solvent and waterborne paints.

- 2.0 Materials and Methods.

2.1 Plant A - Test Methods 2.1.1 Settlinq Column and Particle Sizinq Studies (Plant A),

The perspex settling column used was 3.05 m. high with an internal diameter of 15.54 cm, The distance between the sampling ports was 30.48 cm. and samples were taken by inserting stainless steel tubes (4.5mm diameter) into the center of the column. The column was filled from the base by a centrifugal electric pump. The waste was continuously stirred in a 60 litre aspirator, ensuring uniform distribution of solids, prior to and during transfer to the column. The shaft of the stirrer was 48.26 cm. long with a mixing blade 5.08 cm, in diameter. The sample volume used in the column was 30 liters.

Three static column tests were performed. The first was completed after the P . S . B . sump had been in operation for 3 months following a clean out. The dissolved and suspended solids concentrations were 6715 and 2,450 ppm respectively. The second test was performed upon P.S.B. sump water which had been in the booth for 2 weeks of operation. Consequently the suspended and dissolved contaminant concentrations were significantly lower, 624 ppm and 836 ppm respectively. The final test was based upon a sump water sample where the dissolved solids were diluted from 836 ppm to 575 ppm. This sample was prepared by centrifuging the sump sample to remove the suspended solids and diluting the centrate (dissolved solids) by 30%. The suspended solids were then dosed back into the centrate to reconstitute the approximate original suspended solids concentration (actually 680 ppm) This sample was then stirred for 24 hours prior to performing the

settling column study.

Clearly there are concerns with this procedure, since there is the potential to change the suspended solids particle size. Consequently, an experiment was undertaken to verify the impact of dissolved solids upon suspended solids particle size. The sump water from the third test (dissolved solids concentration of 836 ppm) was filtered through a 1.5 p glass fiber filter and the supernate was diluted 30, 40 and 50% prior to particle sizing (Malvern Zeta Sizer 4) . The purpose for filtering the samples was to remove all particles >20-30 p, which are beyond the detectable size limit of this instrument. A 1.5 p filter was used because a 0.45 pmembrane filter blinded within seconds. Furthermore the integrity of the 0.45 p filter was destroyed by the paint solvents.

2.2 Plant B - Test Methods. 2.2.1 Streamins Current potential (S.C.P.1.

The S.C.P. of the unfiltered paint sump water was determined using a Mutek PCD-02 and automatic titrator (Metrohm 702SM Titrino) . A streaming current arises when an electrolyte is forced through a capillary. This is simulated i n the Mutek PCD-02 with a piston moving back and forth in the cylinder. The liquid in the capillary carries a net charge, which is the mobile part of the diffuse layer composing a part of the electrical double layer. The shear imparted to the electrolyte by the movement of the piston develops a streaming current and consequently a streaming potential (Shaw, 1986). Therefore the S.C.P. is basically a measure of the polarization of the electrical double layer beyond the Stern Plane. The S.C.P. was measured by placing a sample of

the PSB sump water (15 ml) in the cylinder. The reciprocating motion o f the piston is started, and an oppositely charged polymer is titrated until the point of zero charge is determined. This procedure was used to determine how the charge in the system varied with time.

2,3 Waterborne Laboratory Test Methods.

A model downflow PSB and organic polymer treatment was used to investigate the distribution of suspended solids, and accumulation of soluble contaminants with varying paint chemistries. The PSB is a downflow design, with a 210 gallon sump equipped with an autoweir (as at Plant B) and a 40 gallon Palin unit (Dissolved Air Floatation) for solids removal (Figures 8 through 13). The hydraulic retention time in the sump and Palin unit approximated 7 minutes and 6 minutes respectively. Paint detackifier (DT 2430) was added to the recirculation line and a flocculent to the Palin influent. There are sample ports located at three depths (9,32 and 47 cm) on the paint sump influent and

Palin together with the sludge which is scraped (every 15 minutes) into a receiving drum. Paint was sprayed via two Binks guns (Model 611, the pots mounted on balances in order to provide a continuous readout of the paint spray rate, which was maintained at 60 ml/minute for 30 minutes. The solvent paint used was PPG dark green and red mica metallic and the waterborne paint BASF white, red and red metallic basecoat. ~~

-

2.4 Total Susnended Solids (T.S.S.) . The T.S.S. concentrations were determined by filtering a

suitable volume (50 ml) through a preweighed 1.5 p n glass fibre filter (Whatman). The filter was then dried at 105'C and placed in a desiccator for 1 hour, allowing it to cool, and then reweighed (Ohaus). This procedure was repeated until a constant weight was obtained.

2.5 Total Dissolved Solids (T.D.S).

The T.D.S. was determined according to Standard Method Part 200, Section 209 (American Water Works Assoc.).

2.6 Fats Oil and Grease (FOG) and Chemical Oxvsen Demand (COD)

FOG and COD were determined according to Standard Methods Part 200 Section 206, and Part 508 Section 508A respectively.

3,o Results and Discussion.

3.1 Current Practices Concernincr Paint Snrav Booth Contaminant Control . In the operation of a PSB the contaminant that receives the

greatest attention, with regards to ensuring strict control limits, is suspended solids. This is probably because it can potentially impact PSB air/water flows (via deposition etc.) , biocide efficacy and booth aesthetics. It is generally accepted that under normal operating conditions concentrations should not exceed 250 ppm and ideally 100 ppm in a booth equipped with solids removal equipment, such as dissolved air floatation or filtration. Whilst T.S.S. concentration is an important operating factor, dissolved contaminants cannot be ignored since they also impact booth performance, for example foam control and suspended solids settling and collection, via stabilization.

-

In those systems where soluble contaminants are controlled, it is accomplished by continuous blowdown of the sump water to the wastewater treatment plant. In these instances the contaminant of interest is usually TDS and COD. The latter is selected because

it can be quantified relatively quickly, and it is assumed to correlate with microbiological control. In addition there is frequently a COD wastewater discharge concentration, or loading, the plant must adhere to. Tertiary treatments, such as biotreatment, which would reduce COD are normally absent in automotive and OEM plants. Consequently COD discharge is frequently controlled via dilution with other waste streams, which limits the maximum permissible discharge from paint sumps. With respect to monitoring COD for performance purposes, results shall be presented which question the validity of this argument.

In the succeeding section, the correlation between insoluble and soluble contaminants shall be discussed for solvent paints. However prior to this it is necessary to review basic colloid chemistry theory.

3.2 Introduction to Colloid Theow.

The stabilization or coagulation of solids or liquids (emulsions) is fundamental to numerous processes within industry. Coagulation between two particles is the result of van der Waals attraction. For two particles of equal radii (R), the van der Waals attractive forces (V,) is given by the following expression, providing the distance of separation (h) between the two droplets is much smaller than the droplet size.

AR 12h

- - VA = (Eq. 1)

where A is the Hamaker constant, which is given by the equation:-

where q is the number of atoms or molecules per unit volume, and Bii is the London dispersion constant, which is related to the polarizability.

It follows from equation 1 that V,, or coagulation, increases rapidly as the distance of separation between the particles decreases. Therefore to prevent coagulation a repulsive force must be created to prevent close approach of the particles. There are at least two stabilization mechanisms. The first is based upon the creation of an electrical double layer surrounding the particle, which may be produced via the adsorption of an ionic surfactant. The surface of the particles are covered with a layer of charged head groups (anionic or cationic). The charge at the surface of the particles is compensated for by counterions, which may closely approach the surface (Stern Layer) or extend into the bulk solution which determines the double layer thickness. The thickness of the double layer depends upon the electrolyte concentration. As two particles approach one another, their double layers overlap resulting in repulsion as a result of an increase in the free energy of the system. In a simple case of two particles and low

potential, yo, the repulsion interaction free energy G, is given by the expression,

G, = 2 AR E , E , Y 02 In [l+exp - (K h,) I (Eq* 3)

where E, is the permittivity of the medium, E, that of free space and k is the Debye-Huckel parameter that is related to electrolyte ~

concentration, C. ~

F - 2 z2 e2 c

= [er E, KT J (Eq. 4 )

where Z is the valency of the electrolyte and e is the electronic charge,

A combination o f van der Waals attraction and double layer repulsion was proposed by Deryaguin, Landau, Verwey and Overbeek and is known as the DLVO theory. The energy-distance curve is represented in Figure 1. It is characterized by two minima and one maximum. At long distances of particle separation, attraction is dominant resulting in a shallow minimum (secondary minimum), the depth of which is related to the particle size, Hamaker constant and electrolyte concentration. The force of attraction in this minimum is usually small. In contrast at very short distances of separation, the attractive force is significantly greater than the repulsive force, resulting in a deep primary minimum. If the particles are allowed to approach one another this close then coagulation results. This attraction is prevented by the presence of an energy maximum at intermediate distances of particle separation, The height of the maximum is directly proportional to the surface potential (Yo), and inversely proportional to the electrolyte concentration (Tadros and Vincent, 1980) .

The second mechanism by which coagulation may be prevented is steric stabilization, This is produced by using nonionic surfactants or polymers whereby the hydrophobic head adsorbs at the solid/liquid interface, and the hydrophilic chain extends into the bulk solution. Polyethylene oxide (PEO), polyvinyl alcohol (PVA) or polyvinyl acetate (PVAc), are examples of chemistries used in paint formulations which can function in this manner. The hydrophilic chains produce repulsion as a result of two main effects. The first which is referred to as mixing interaction (osmotic repulsion) results from the unfavorable mixing of the hydrophilic layers upon close approach of the particles. When the hydrophilic layers approach one another to a distance of separation (h), that is smaller than twice the adsorbed layer thickness (261, overlap of these chains may occur (Napper, 1983). However when these chains are in a good solvent condition such as PEO or PVA in water, such overlap becomes unfavorable as a result of the increase in osmotic pressure in the overlap region. This results in diffusion of the solvent molecules into this overlap region, consequently separating the particles. The second effect that has been reported (Tadros and Vincent, 1980) which results from the presence of adsorbed layers, is the loss in configurational entropy

of the chains when there is significant overlap. This effect is always repulsive and is usually referred to as the entropic, elastic or volume restriction effect.

- 3.3 Affect of Dissolved Solids upon Solvent Paint (Solids) Settlins Velocities.

The settling velocity of solvent paint particles in P.S.B. sump water (Plant A) was determined at three different dissolved solids concentrations, The first two tests were completed upon undiluted sump water, with a dissolved solids concentration of 6715 (T.S.S. 2,450 ppm) and 836 ppm (T.S.S. 624 ppm) respectively. The explanation for the concentration variance was previously explained in Section 2.1.1. The final settling column test was completed by diluting the dissolved solids of the second run by 30% (T.S.S. 680 ppm), as outlined in Section 2.1.1.

The settling profiles for each of the column tests are shown in Figure 2. It is apparent that there is a significant increase in the particle settling velocities, as the dissolved solids concentration is reduced from 2,450 ppm to 836 ppm. The occurrence of flocculent settling was indicated by the curvature of the isoconcentration lines, which are plotted in a settling column study, enabling the results portrayed in Figure 2 to be calculated. The T.S.S. concentration is also significantly different for both of these tests but this shall be temporarily ignored.

It may appear that the settling velocity should have decreased as the dissolved solids concentration (or more specifically the electrolyte concentration) was reduced, since this would extend the electrical double layer thickness, therefore increasing repulsion. Presumably an additional stabilization mechanism is operative, independent and of greater significance than ionic double layer compression. Paint formulations, whether solvent or waterborne, obviously contain relatively high concentrations of both charged and uncharged stabilizers, such as PVA. etc. Dilution of the stabilizer below a critical concentration will negate or reduce their function, consequently increasing coagulation. Comparison of the second and third settling column studies showed very similar settling rates. The results did indicate increased settling at the beginning of the tests, although this could also be explained as experimental variation. As time progressed there was no difference in the settling rate. Therefore either the dissolved solids concentration was below the critical stabilizer concentration, or the experimental protocol, outlined in Section 2.1.1, has an inherent flaw and the particle size of the T.S.S. was altered (ie. reduced).

To verify the impact of dissolved solids upon stabilizing paint particles, filtrate of paint sump water was diluted and particle size analysis completed as outlined in Section 2.1.1. The results shown in Table 1 confirm that as the dissolved solids concentration was reduced, paint particles coagulated. A 50% dilution resulted in a 60% increase in particle size (approx.).

Table 1 Relationshin between Particle Size and Dissolved Solids Concentration.

Dilution (%I of 1 . 5 ~ Dissolved Solids Particle Size (nm). Filtrate Concentration

~

l.E€&?- ~

0 30 40 50

836 585 501 418

1601 1662 1793 2636

As previously mentioned the T.S.S. of the first settling column test (2,450 ppm) was significantly greater than the two succeeding tests. An experiment was not performed whereby T.S.S. concentrations were varied whilst T.D.S. remained constant. However it is extremely unlikely that suspended paint particles would stabilize themselves. This would only occur if the T.S.S. concentrations approximated high percent values, where the distance between particles was reduced to overcome the secondary minimum.

- 3.4 Streamins Current Potential (S.C.P.) of Paint Booth sum^ Water (Plant B).

The settling column work indicated that the T.S.S. were stabilized by dissolved solids. This is a broad parameter and does not indicate if the suspended solids were charge or sterically stabilized. The S.C.P. of the P.S.B. sump water was determined during two 20 hour periods spaced 1 week apart, upon two systems at Plant B. One paint booth (Topcoat) was operating well with respect to T.S.S. removal (e50 ppm), whilst the other (plastics) was maintaining a higher than desirable T.S.S. concentration (>250 ppm) in the sump. The S.C.P. of both systems is illustrated in Figure 3. The charge for topcoat was neutral, whilst that for plastics was 8 times more anionic. However it was constant throughout the first study period, as was the T.S.S. concentration in the sump. During the second monitoring period, Dec. 30 1992, the S.C.P. decreased. There was no painting at this time although P.S.B. maintenance (cleaning) was ongoing. The increase in the anionic S.C.P. was a result of paint solids being flushed from the spray booths to the sump. The S.C.P. data inferred that the stabilizer of the paint solids was uncharged, i.e. steric. If this were not the case a decrease in the S.C.P. during the first monitoring period (when paint was sprayed) would have been expected.

__

3.5 Importance of Monitorins Dissolved Contaminants for Effective P.S.B. Operation (SOLVENT PAINTS).

As previously mentioned effective P.S.B. operations include T.S.S. removal, foam control, detackification (for solvent paints), reduced adhesion (for waterborne paints), microbiological control and sludge which dewaters well (>40-50% solids). In the plastics sump at Plant B the T.S.S. temporarily and gradually exceeded the control limits. Addition of the correct polymer can eliminate the problem, whereas an incorrect polymer selection will make matters worse as indicated by Table 2 .

Table 2 . Relationship between Soluble/Particulate Contaminants and Foamins Potential.

Polymer Concn. T.S.S. T.D.S FOG - COD Foam T h e 0 hz!LL hEml hz!LL -&.l (secs)

Blank ..... 225 5940 57 1.1 43

AF' 4 3 1 1 2 5 80 53 80 1 2 1 . 2 37

DT 2430 5 0 0 80 6050 1 8 1.1 37 DT 2430 1000 9 0 5890 1 4 1 .0 56 DT 2430 2 0 0 0 1 2 0 5970 1 4 1.1 86 DT 2430 4 0 0 0 100 6050 8.7 1 . 0 94

In numerous cases T.S.S. is the only chemical parameter which is considered to quantify the performance of a P.S.B. program. The soluble contaminant is often determined for alternative reasons, as previously mentioned. The results shown in Table 2 indicate the T.S.S. concentrations can be reduced by addition of AF 431, although the P.S.B. operation would remain out of control due to foaming. Furthermore monitoring T.D.S and COD for performance is completely superfluous and redundant, since there is no correlation in this instance to foam control. If the wrong polymer was added (eg. AF 431) the result would be 2 - 4 feet of foam in the P.S.B. sump, and the potential for foaming in the paint booths and lost production via increased reject rates, etc. The parameters which should be monitored are T.S.S. and m. The polymer to use is DT 2430. This treatment was added at slightly higher concentrations than usual, in order to reduce T.S.S. together with FOG. Since the addition of DT 2430, the T.S.S. in Plant B Plastics sump has frequently been as low as 6 ppm and never exceeded 50 ppm. As the T.S.S. in the paint sump was reduced there was no foam, since the DT 2430 also removed FOG.

Fats Oils and Grease (FOG) which are directly correlated with foam control, are probably comprised of ffsolublen nonionic polymers or surfactants which are also responsible for sterically stabilizing T.S.S. (Section 3 . 4 ) . It is likely that if FOG had been monitored in the T.S.S. settling column tests, there would have been a good correlation between poor settling and high FOG.

In Plant B there was not a foaming problem when the T.S.S. was outside the control limits, because the paint particles were acting as defoamers. This may have been expected since a layer of solvent probably surrounds the paint particles (To S. S. 1 . Consequently as the paint particles adhered to any bubbles, the surface tension of the bubble film would be reduced. This would create a thin spot and the pressure within the bubble would burst the wall, thus ~

dissipating the foam. With respect to waterborne paints this ~

mechanism will not be operative. Consequently it shall be essential to utilize a chemical program that prevents the increase of soluble contaminants (eg FOG) , providing of course the correlation is also true for waterborne paints. If this is not ensured, there could be an inordinate number of problems which may affect production. The chemistry and theory behind the effectiveness of DT 2430 has been reported previously (Mitchell and TOM, 1992).

~

3.6 Settlins of Waterborne Paint T.S.S.

The introduction of waterborne paints has increased concern in the industry with respect to their treatment, in regards to their propensity to foam, particulate floatability, sludge dewaterability, impact upon COD, etc. The discussion of these concerns shall focus upon the floatation of the T.S.S., although the other criteria shall be discussed briefly, It will be apparent that waterborne paint systems have narrower control limits, and exceeding these will cause significant problems. Briefly, solvent paint systems are typically treated with either organic or inorganic paint detackifiers at an elevated pH (8-5-9.2). The chemical stabilizers/surfactants in waterborne paints are different, with respect to both chemistry and concentration. Consequently at an elevated pH (above 8.5) they foam to a great extent. Furthermore in a paint booth which sprays 100% waterborne, the settling velocity of the paint particles is greater than for solvent paints. This is perhaps not unexpected when one considers and accepts a solvent layer surrounds the particles of solvent paint, which enhances the buoyancy of the solids. This feature is also responsible for the poor performance of hydrophilic paint detackifiers, such as inorganic chemistries (Mitchell and Tonn, 1992).

The settling rate of waterborne paints was investigated in the 250 gal P.S.B. at GRACE Dearborn. The results indicated in Figures 4 and 5 illustrate particulate settling in a P.S.B. that sprays a ratio of waterborne:solvent paint (85: 15) and 100% waterborne respectively. A total of 1 gallon of paint was sprayed in 30 minutes, During this test there was no solids removal. Following spraying the paint booth was turned off and the solids allowed to settle in the sump for 2 hours. Samples were taken every 15 minutes from the top and bottom sump sample ports. Comparison of Figures 4 and 5 indicate that spraying a combination of solvent and waterborne paint reduces the settleability of the waterborne paints. The T . S . S . concentrations for 100% waterborne paint

(Figure 5) sampled in the bottom port (10 cm. from the sump floor) of the paint sump, were an order of magnitude greater than a combination of solvent and waterborne paint. It was evident during the test, that simultaneous spraying of the solvent paint with the waterborne coatings, resulted in a floating paint raft in the sump. The reason for a reduction of the waterborne paint T.S.S. concentration (sampled at the lowest port, Figure 5 ) after 50 minutes, was because the particles settled past the port and reached the sump floor. Soluble contaminants (TDS, COD and FOG) were also monitored during this study. It is not possible to conclude at this point about the particular relationships of these contaminants with T.S.S. removal. However T.D.S. does not appear to correlate as closely as FOG with foam control,

A further paint booth test was performed upon 100% waterborne paints with the purpose of reducing T.S.S. sedimentation and foam in the sump, Therefore an additional chemical was added to the sump to aid flotation, and solids were removed continuously whilst the paint was sprayed. The purpose of removing the T.S.S. during the experiment was to establish if T.S.S. contributed to foam. The chemical additive referred to above does not reduce FOG. Sample locations to determine T.S.S. concentrations, were at both the top and bottom (ports) of the influent and effluent to the sump (Figure 6). The T.S.S. concentrations of the weir box supplying the DAF, and the DAF effluent were also quantified (Figure 7). Finally the percent solids of the Palin sludge raft was determined,

Comparison of Figures 5 and 6 indicates that T.S.S. concentrations from the bottom sample ports, for both influent and effluent, are an order of magnitude less for the latter experiment. This is indicative that there is considerably less settlement of the solids, due to the chemical addition referenced above. Additional evidence that there is limited settling, is provided by the negligible variation between top and bottom sump T.S.S. concentrations. The results illustrated by Figure 7, indicate that a consolidated solids raft accumulated (and floated) in the sump. The T.S.S. concentration in the weir box exceeded 2750 ppm which represents a concentration factor of >11 TIMES (maximum sump T.S.S. concentration equaled 230 ppm). Comparison of Figures 6 and 7, shows that improved floatation of the paint, allows the Palin unit to reduce the overall T.S.S. concentration in the sump before termination of paint spraying (reduction of T.S.S. in Figure 6 at 20 minutes).

The Palin unit removed T.S.S. extremely effectively, reducing concentrations in the sump to e100 ppm in only 10 minutes after cessation of spraying paint. The weir box T,S,S, concentration was reduced 97%, and particulate solids concentration in the Palin effluent reached a minimum of 10 ppm. Optimization of the chemical floatation aid to the sump will improve the Palin performance further, and reduce T.S.S. carryover to an expected concentration of e50 ppm.

It was also apparent during this experiment that removal of

waterborne paint solids on a continuous basis, substantially reduced foaming in the sump. The foam/solids depth was approximately 5 cm compared to 30 cm during the first experiment (solids removal after spraying paint) . Therefore effective removal of T.S.S. may eliminate catastrophic P.S.B. operational problems associated with foam, but as will be discussed shortly will be insufficient to improve sludge dewatering. It should be noted that

are not available at present. Furthermore the test duration must be increased to enable the soluble contaminants to concentrate, whilst T.S.S. are removed continuously. Therefore it would be inappropriate to diminish their importance. However it can be concluded, that the mechanism of solvent paint solids acting as defoamers is absent with waterborne paints, as previously mentioned.

~-

~

results quantifying soluble contaminants during this experiment, ~~

The percent solids of the Palin raft was determined at 5, 20, 30, 50, and 60 minutes. The average percent cake solids approximated 6%. This is well below the value expected for a solvent and waterborne/solvent paint mixture (approximately 25 and 15% respectively). The reason for this is the foamy nature of the sludge. This issue shall be addressed in the near future.

4.0 Conclusions ,

At the beginning of the 1980's when high solids enamel solvent paints were introduced, new problems were encountered, These were a consequence of poor detackification with the inorganic treatment programs used at that time, This resulted in high maintenance costs as paint booth floodsheets and eliminator sections, etc. required frequent maintenance/cleaning. Periodically production was affected, as the scrubbing efficiency of booths or odor control was not maintained due to high paint sump T.S.S. concentrations. In addition, inadequate P , S , B . designs which had insufficient flow through velocities in troughs supplying water to the floodsheets, resulted in solids settling even when T.S.S. were less than 50 ppm. However, it was frequently possible to ignore the problems of poorly operated paint booths, so long as production was not affected. This situation may no longer be tenable, since it is more likely that problems will not be confined to the paint sump, but will also occur in the production area, the Paint SDrav Booth. This may occur due to the potential for excessive foaming if control is not maintained.

The introduction of organic polymer programswas a significant improvement over the old inorganic chemistries such as clay, silicate, colloidal silica, aluminates etc,, and eliminated many of the problems However there were still instances of misapplication, because of a myopic focus upon controlling T.S .S . at the expense of soluble contaminants. With respect to solvent paints, the soluble parameter which should be monitored to optimize the operational performance of a P,S.B is FOG, On those occasions where corrosion of the paint booth is a concern, then obviously

dissolved solids (or specifically chlorides etc.) should be quantified.

The advent of waterborne paints presents a combination of new and old treatment concerns. The major difference between these paints and coatings of the past, is that operational windows or control limits are considerably narrower. As previously mentioned, the target T.S.S. sump concentration for a solvent paint P.S.B., equipped with solids removal equipment, is generally 100-250 ppm. Booths without solids removal capabilities can exceed 2000 ppm T.S.S. and perform well. This does not appear possible with a waterborne paint P.S.B. Although additional work is required a guesstimated target T.S.S. (sump) concentration would be 50-100 ppm.

Waterborne paints do not require detackification in the accepted sense, although their adherence to P.S.B. components must be controlled. This requires a similar chemistry as for solvent paints. The single most important difference between these coating technologies, is the propensity for waterborne coatings to foam. Further differences include increased settling of solids and reduced paint sludge dewaterability. The tendency for the coatings to foam can be reduced by ensuring that the pH of the system does not exceed 8.3. If a solvent paint is sprayed into the same booth, this pH is on the low side to ensure perfect detackification. Alternative polymers may therefore be required. These have already been developed by GRACE Dearborn, and commencing October 1993 are commercially available. However operating the paint booth at this pH will not eliminate the foam. Unlike solvent paints where the foam is caused by soluble contaminants, it appears at this stage that both T.S.S. and soluble paint chemistries may be responsible for foaming of waterborne paint coatings. It is not conclusive at this stage which soluble component (FOG or COD) should be monitored to ensure foam control, although preliminary results indicate it is FOG. If this is the case, then focussing upon COD would be as misplaced an effort as it was for solvent paint systems, unless

Upon there are Wastewater discharge criteria to consider. verification of which soluble contaminant should be monitored, the specific chemistries responsible shall be identified by Gas Chromatography/Mass Spectroscopy.

The previous discussion provides a brief outline of the potential problems which manufacturing facilities using waterborne coatings may encounter. These potential upsets can be avoided with a correctly designed paint booth, and complimentary chemical treatment program. Operational problems of the past could be compensated for by an increased cleaning and maintenance effort. This is unlikely to provide acceptable resolutions for poor P.S.B. performance in the future, where waterborne paints are utilized, because it may prove difficult to isolate the effects of poor control to areas which will not affect production. These coatings shall require a quality program based upon an effective chemistry, knowledge upon its application and a meaningful monitoring program. As stated in the title of this paper, the intention was not to

answer all questions regarding waterborne coatings, but to illustrate that many assumptions for solvent paints were erroneous, and therefore these should not simply be extrapolated to the future.

- 4.0 Biblioqraphv

Mitchell, D.B, and TOM, G.A. (1992) . , Organic Paint Detackifiers and Associated Benefits. Proceedings of the Nineteenth WaterBorne, Higher-Solids,& Powder Coatings Symposium.

Napper, D.H, (1983) . , Polymeric Solubilization of Dispersion.

Shaw, D.J. (1983) ., Colloid and Surface Chemistry. 3rd edition.

Academic Press, London.

Butterworth, London.

Tadros, Th.F. and Vincent, B. (1980) ., Encyclopedia of Emulsion Technology, Editor P, Becher, Vol I, Marcel Dekker, New York.

Energy Distance Curve According to the DLVO Theory

Figure 1

I :

80

80

70

60

SO

40

30

20

10

0

a

3

Correlation Between Suspended Solids Settling And Dissolved Solids

F i g *..,.......... . ........................................*.,*,

o 676

0 /.:A 6716 TDS

TDS TDS

0 A

0 25 SO 75 100 125 150 175 200 225 250 Time in Mintues

v) r 0 0

c

c,

m .cI,

e *= I

CD c

cb a,

m-

E

5 L

. ..

Settling of Waterborne and Solvent Pa (Ratio of Waterborne:Solvent Paint-

int Suspended Solids. 35: 1 5)

\ . . . . . . . . . . . . . . . . . . . . . . . . . . . - . .. . . . . . . . . . . . . . .

Figure 4

TSS sample location

-.- TSS (Influent-Top)

+ TSS (Influent-Bottom)

* TSS (Eff luent-Top)

* TSS (Eff luent-Bottom)

\

0 15 30 45 60 75 90 105 120 Time (mins)

.

I ,

Total Suspended Solids Settling Studies for Waterborne Paints (1 00% waterborne)

2,000 I

Time (mins)

Figure 5

TSS sample location

--e TSS (influent-top)

+ TSS (influent-bottom)

* TSS (eff luent-top)

* TSS (eff luent-bottom)

.. .

Total Suspended Solids Settling Studies for Waterborne Paints (100% watefbome-suspended solids removed continuously)

_ _ ~

-TSS (infiuent-top)

+ TSS (infiuent-bottom)

* TSS (effluent-top)

* TSS (effluent-bottom)

Figure 6

TSS sample Io cation

Time (mins.)

Total Suspended Solids. Removal via DAF (1 00% waterborne-suspended soiids removed continuously)

Figure 7

TSS sample location

-Weir TSS

* Palin effluent TSS

Time (mins.)

Figure 8

PSB tes t u n i t .

Figure 9

A i r scrubbing s e c t i o n s t imu la t e s cu r ren t s tandards f o r a i r f l o w .

Figure 10

Self-adjust ing w e i r .

Figure 11

Ph c o n t r o l l e r keeps p rec i se con t ro l of Ph under var ious condi t ions.

Figure 1 2

Spray guns a d j u s t t o s imulate d i f f e r e n t spray rates i n t h e f i e l d .

Figure 13

Pa in t po t s on d i g i t a l scales provide exact d a t a on pa in t usage.