Cationic Coagulants Plus Collidal Silica for Use in Paint Detackification

This invention relates to a wet spray booth treating agent and to a method for the treatment of a wet spray booth by the use of the agent, and more particularly relates to a wet spray booth treating agent for diminishing the tackiness of coating material entrained in the circulating water to be sprayed for collecting surplus paint and precluding the coating material from adhering fast to the interior of the circulating water systems and to a method for the treatment of the circulating water by the use of the treating agent.

Generally in the coating process of the automotive industry, the yield of the paint applied to an automobile body ranges from 50 to 80% and the remainder 50 to 20% of the used paint constitutes an excess paint to be removed in the subsequent process. For the collection of the surplus paint sprayed excessively, the automobile body is treated in a wet spray booth adapted to give a wash with water and the water so used for the washing is circulated for re-use.

In the wet spray booth constructed as described above, since the surplus paint which is collected in the washing water has high tackiness, it adheres to the water-film panel, the piping system, the spray nozzle, etc. of the spray booth, clogs the piping system and the nozzle, and seriously degrades the efficiency of water washing. Further when the surplus paint so deposited clogs the waste gas discharge system and interferes with the flow air in the spray booth, the vapor of the solvent of the paint fills up the interior of the booth to produce a state dangerous to safety and hygiene and seriously degrade the shop environment. Moreover, the greater part of the surplus paint settles to the bottom of the booth and the bottom of the circulation pit in the spray booth system and the sediment of paint so formed hardens into a rubbery mass with elapse of time and the removal of this hardened mass and the cleaning of the bottoms take up much time and labor.

The paint collected in the washing water not merely undergoes adhesion or sedimentation within the system but also dissolves into components or disperses into fine particles. An increase in TDS accelerates the progress of corrosion inside the system. Further, the solvent in the paint not merely increases the hydrocarbon (HC) content of the waste gas from the booth but also dissolves into the cleaning water and increases the COD of the water being circulated. The solvent of the paint, depending on the kind thereof, acidifies the circulating water and consequently accelerates the corrosion of the system.

U.S. Pat. No. 4,564,464--The compositions are pumpable, hectorite clay containing slurries. In addition to hectorite, which has been proven as a most effective detackifying agent, the slurries include thinning agents such as water-soluble aliphatic and/or alicyclic amines. Water conditioning agents, such as water-soluble polyphosphates are also made part of the slurry formulation.

U.S. Pat. No. 4,629,572--Paint spray booth wastes from clear oil-based coating compositions, such as lacquers, which have been collected in a water wash are easily detackified and removed from the wash water by contacting the wastes in the wash water with

(1) a water-dispersible polymeric condensation reaction product of a urea or an amino triazine and an aldehyde and

(2) a water-swellable clay.

U.S. Pat. No. 4,504,395--The compositions comprise a hectorite clay, an anti-foaming agent and optionally, an aluminum oxide material and a montmorillonite clay. These compositions are introduced into a paint spray booth water wash system so as to detackify oversprayed paint.

U.S. Pat. No. 4,153,548--An acidic silica sol and coagulating polymer are used to reduce the tackiness of paint solids of electrodeposition paints and to condition such solids so that they can be separated and removed from the drippings and wash water obtained as a result of the application of electrodeposition paints to various types of products. The polymers used in the practice of this invention are usually acrylamide polymers or anionic acrylamide polymers.

U.S. Pat. No. 3,515,575--An improvement in the process of deactivating and collecting paints with a water curtain in a water wash booth comprises maintaining in the water at least 0.5 ppm of a water-soluble polymer having an average molecular weight of from 1,000 to 15,000,000 and having repeated groups with the formula ##STR1## wherein R is a hydrogen or methyl group and R' is an amide or carboxyl group.

A number of polymers are suggested in col. 4 of the specification. Mentioned in passing in that the polymers may be used in conjunction with absorbents. A wide variety of absorbents are mentioned, one of which is colloidal silica. No other reference to colloidal silica is made.

U.S. Pat. No. 3,738,945 discloses preferred polymers used in the invention, e.g. epichlorohydrin dimethylamine ammonia polymers. The patent does not teach or suggest treating paint spray booth waters.

The drawing is of a laboratory scale paint spray recirculation tester.

The invention comprises a method for the treatment of circulating water which collects surplus paint in a paint spray booth which comprises treating this water with a paint detackifying amount of a paint detackification agent composition comprising

(A) a water soluble cationic polymeric coagulant having a molecular weight less than 500,000, and;

(B) An aqueous colloidal silica sol having an average particle size between about 1-150 nm, with the ratio of A to B being from 1/50 to 50/1 (actives basis).

The invention also provides as novel compositions described above as (A) and (B), set forth.

The invention detackifies a wide degree of paint products including water-based paints, oil-based paints, lacquers, enamels, and the newer high-solid paints.

The invention is particularly adapted to detackifying high-solid paints and clear coats which are applied in the form of organic solvent suspensions. A particularly noteworthy advantage of the invention is that it is capable of coagulating pigmented base coat and clear overcoats when such contaminations are present together in the circulating waters of paint spray booths.

These cationic polymeric coagulants have molecular weights less than 500,000, preferably less than 100,000, and usually within the range of 10,000-200,000.

A preferred water-soluble cationic polymeric coagulant of the type described above is the subject of U.S. Pat. No. 3,738,945, the disclosure of which is incorporated herein by reference and is made a part hereof.

A preferred composition of this type is the reaction product of epichlorohydrin dimethylamine and ammonia in accordance with the teachings of U.S. Pat. No. 3,738,945, which product has a molecular weight of about 50,000. This particular polymer is in the form of a 48% by weight aqueous solution and is referred to hereafter as Polymer 1.

Another useful polymer is a 20% by weight solution of polydiallyldimethyl ammonium chloride (DADMAC) having a molecular weight of about 100,000. This particular polymer is referred to hereafter as Polymer 2.

Other cationic polymeric coagulants include such low molecular weight polymers as melamine formaldehyde resins of the type described in U.S. Pat. No. 4,656,059. Similarly, water-soluble urea formaldehyde resins may be used. Also useful are the polyethylene imines or any of the water-soluble polymers described in Canadian Pat. No. 731,212, the disclosure of which is incorporated herein by reference.

The Polymers are preferably employed as dilute aqueous solutions, e.g. 5-15% weight. Commercially, they are usually supplied as concentrates and can be diluted on-site. The amount of polymers used in the practice of the invention vary between 1 and 1,000 ppm by weight of the circulating water in the paint spray booth. Routine experimentation will determine the optimum dosage.

These products are well known and are produced by the neutralization with demetalization or deionization and subsequent concentration. Typical commercial silica sols are those set forth below in Table I. The products listed below are all commercially available from Nalco Chemical Company, One Nalco Center, Naperville, Ill. 60566-1024.

The preferred particle size range used in this invention is between 1-20 nm, although larger particle sizes up to about 150 nm may be used. The amount of colloidal silica used in the compositions of the invention vary between 1 to 5,000 ppm.

Polymeric Coagulants to Colloidal Silica

The ratio of the water-soluble polymeric coagulants to colloidal silica may generally be within the range of 1/50 to 50/1, preferably it is 1/5 to 5/1 (actives basis).

In this test (see the drawing), desired amounts of detackifiers are added to the water manually or using a chemical pump to monitor the chemical demand. These detackifiers are allowed to mix in the tank through water recirculation for 5 minutes. The paint being tested is then sprayed into the chamber at a rate of 2 ml/min. through an air atomized spray gun located 12 inches above the center of the scrubbing section. The test paint is sprayed for 5 minutes, then the paint kill is tested for tackiness or stickiness. The testor wets his hand with the water solution in the test tank, then takes a sample of the floating paint sludge from the water tank, squeezes, and rubs tightly between his fingers. Any tackiness or stickiness present is a poor paint kill. A chart for degree of paint kill is provided here to assure consistent description. A minimum degree of kill of 6 is required to assure booth cleanliness.

At the conclusion of a good paint kill, paint is continuously sprayed, and the paint sludge conditions tested every few minutes. At any given time, if any trace of tackiness of sludge is present, paint spray should be discontinued and the program re-examined. The amount of paint sprayed should be recorded, the chemical dosage, and condition of sludge.

Additional chemicals should be added and paint spray resumed. Repeat this process at least four times.

The detackification effects were tested against high solids enamels. The results are set forth below in Tables II and III.

The new system has been checked several times and gave reproducible results. Other paint samples were also checked.

All systems were charged initially with 500 ppm of chemical and maintained with 100 ppm increments to 900 ppm total of chemical added. This equals the type of charge necessary for 8000/8001 to detackify DCT 3000 PPG clear coat.

The following results were yielded when using synthetic Chicago tap water: