Method for the recovery of secondary polyols from paint sludges

The invention relates to a method for the recovery of secondary polyols from paint sludge coagulate, formed from paint overspray which has first been dewatered by mass action, by chemical treatment in a manner such that the recovered product can be reused in the manufacture of paints. According to the recovery method, the organic components contained in the paint coagulate are glycolytically split at an elevated temperature into reusable polyols. Preferably, a mixture of different glycols is used, preferably in an amount of about 20 to 50% by weight based on the dewatered paint coagulate. The addition of glycol dilutes the paint coagulate and makes it flowable. The water contained in the paint coagulate is evaporated and removed during the glycolytic splitting because of the increase in temperature. The impurities contained in the paint coagulate, such as pigments, coagulants or flocculants, can be comminuted and finely dispersed by intensive shearing of the paint coagulate. At the end of the process, these impurities can also be filtered out from the polyols formed.

SUMMARY OF THE INVENTION 
The present invention relates to a method for the recovery of secondary 
polyols from paint sludges formed from paint overspray collected during 
spray painting to which a coagulant has been added, as disclosed, for 
example, in U.S. Pat. No. 5,281,629 (counterpart of EP 556 670). 
When paints are applied by spraying, a significant portion of paint, about 
30 to 50% of the amount used, is unavoidably sprayed past the substrate 
that is to be painted. This so-called overspray is collected on the walls 
of the painting booth, down which it drains to be collected. Usually, the 
overspray is collected by a film of water which runs down the walls of the 
booth to be discharged. The collected paint particles are then coagulated 
by the addition of a coagulant, usually clay or a similar substance, and 
in some cases, a flocculant is also added, so that a paint sludge is 
formed from the collected paint particles. This method of collecting 
overspray can be used for all types of paint, namely for water-based 
paints such as acrylate paints, solvent paints, one-component paints and 
two-component paints such as polyurethane paints. Epoxide paints may also 
be mentioned as an example of such two-component paints. 
After dewatering and, in some cases, drying, the paint sludge is usually 
disposed of as hazardous waste. This method incurs very high disposal 
costs. As a result, attempts have been made to find ways of using the 
paint sludge, in order to not only avoid disposal of the paint sludge and 
the high costs associated therewith, but also, in addition, to be able to 
recycle any materials of value contained in the paint sludge, as a raw 
material. 
Published German Patent Application DE 43 28 157 discloses a method of 
dewatering and drying paint sludge, which is then introduced in granulate 
form as a source of carbon and slag-forming agent with blast air into a 
blast furnace. Published German Patent Application DE 29 35 103 discloses 
an attempt to produce a composition which can be processed into articles 
by injection molding, by mixing reinforcing fibers with well dewatered 
paint sludge. According to Published German Patent Application DE 43 20 
566, paints can also be concentrated in the discharged water by 
electrophoresis, wherein the paint adheres to an anode roller or belt and 
is scraped off and separated in a concentrated form from the water 
discharged from the spray booth. It is also possible to concentrate the 
paint particles in the water discharged from the spray booth by membrane 
ultrafiltration. 
The resin or paint vehicle is also commonly extracted from paint sludge by 
mixing the paint sludge, which has largely been dewatered, with a solvent 
and subsequently diluting the mixture so that the resulting liquid can be 
filtered. As a result, the soluble vehicle can be separated from the 
coagulant in the coagulated paint. The recovered vehicle can then be 
formulated into new paints. 
The recovery of paint from paint sludge is, however, only possible for 
one-components paints, in which the resin or the paint vehicle remains 
reactive for a relatively long time at room temperature and in a moist 
environment. When two-component systems are involved, these methods are 
not possible, because the two components react with one another during the 
time when the paint is collected and dewatered, causing the resin to set, 
so that it can no longer be used as a reactive raw material. Therefore, 
aside from the expense of disposal as hazardous waste, paint sludges of 
two-component paints could generally in the best case scenario be 
hydrogenated, incinerated or disposed of in a blast furnace, in which the 
actual value of the material, which is obtained in relatively large 
amounts, is largely wasted. 
U.S. Pat. No. 5,281,629 (counterpart of EP 556 670) discloses a method for 
recovering the paint vehicle from paint coagulates of a two-component 
paint based on polyurethanes. According to this method, the cross-linking 
reaction between the vehicle component, namely the polyol, and the curing 
agent component, namely the polyisocyanate, is prevented by the addition 
of a blocking agent such as an amine. That is, the reactivity of the 
reactive end groups of the curing agent is blocked, so that the blocked 
polyisocyanate can no longer react with the polyol component at room 
temperature. This method proceeds according to the following steps: 
discharging the coagulate from the booth water, 
preliminarily dewatering the paint sludge by centrifuging, decanting, etc. 
blocking the ability of the end groups of the curing agent to react and 
diluting the mixture with a solvent, 
separating the insoluble coagulant components, for example, the coagulant, 
the pigments, etc. by filtration, 
removing the residual water from the filtrate, which contains the resin 
components and the blocked curing agent component and adjusting the 
rheological properties of the recyclate to the desired values when 
reformulating a paint. 
Unfortunately, practical experience has shown that this method functions 
only for very small paint booths with short residence times of one or at 
most two hours of paint overspray or paint coagulate for the first two 
steps of the method before the blocking agent is added. The powerful foul 
smell of the blocking agent and the resulting discomfort during painting 
makes it is impossible to add the blocking agent much earlier. As a 
result, the blocking agent may only be added when the paint coagulate 
reaches a part of the plant which is completely and permanently closed off 
and/or is appropriately accommodated in an open space or in a separate 
room from which odors cannot escape into the work areas. In the case of 
larger industrial-size paint shops, the residence time of the paint 
coagulate until the conclusion of the preliminary dewatering step is more 
likely to be days rather than hours. In such paint shops, the two 
components of the paint sludge of a two-component paint have already 
completely reacted with one another by the time the preliminary dewatering 
process has been concluded. The blocking agent, which can only be added 
after the preliminary dewatering is concluded, cannot reverse the 
completed reaction. For this reason, this recovery method is not 
applicable to larger paint shops. A further disadvantage of this method is 
that as the blocked curing agent is contained in the recycled material, at 
higher temperatures, for example, when the recycled paint is heated, the 
blocking action disappears and the curing action comes into play once 
again. This must be taken into consideration when reformulating the 
recycled paint, whether it is a one-component or a two-component paint. 
This makes it practically impossible to introduce the recycled material 
into a manufacturing process for primary paints during the production of 
paints. 
It is an object of the present invention to provide a method, which can be 
used universally for the recovery of paint from paint sludges of 
one-component and two-component paints in small as well as in large paint 
shops, in a manner such that the recycled material can also be used to 
produce primary paints of any kind. 
Pursuant to the present invention, these and other objects are accomplished 
by a method for the recovery of the paint vehicle from a paint sludge 
coagulate of paints, which paint coagulate is formed from overspray 
collected during the application of the paint by spraying by the addition 
of a coagulant, wherein the paint coagulate, which has first been 
dewatered, is chemically treated and a vehicle product, which can be used 
for the production of paint, is produced, comprising heating the paint 
coagulate to a temperature above 100.degree. C. and glycolytically 
hydrolyzing or splitting its organic components into polyols, which can be 
reused as a vehicle for the production of paints, wherein the water, 
contained in the paint coagulate, is evaporated and removed during the 
glycolytic splitting. 
According to these features, the organic components of the paint coagulate 
are glycolytically split at an elevated temperature into reusable polyols, 
from which the vehicle resins of a new paint can be produced. This 
recovery of polyols is possible with sludges from all types of paints and 
the recovered polyols can be further processed into any possible type of 
new paint. 
For the glycolysis, a mixture of glycols is preferably used and, moreover, 
in an amount which dilutes the paint coagulate, preferably about 20 to 50% 
by weight of the paint coagulate. The water contained in the paint 
coagulate is evaporated during the glycolytic splitting because of the 
increase in temperature and discharged. The other materials contained in 
the paint coagulate, such as pigments, coagulants or flocculants, can be 
comminuted and finely dispersed by intensive shearing of the flowable, 
diluted paint coagulate during the glycolytic splitting. Alternatively, 
these components can be filtered out of the polyols formed at the end of 
the process. 
In order to supply the heat required for the glycolysis of the paint 
sludge, the temperature of the paint coagulant is raised by an externally 
supplied source as well as by internal friction resulting from the 
intensive shearing in a shearing reactor. If necessary, impurities in the 
paint sludge, such as pigments, coagulants or flocculants can be 
comminuted and dispersed by the shearing, so that they no longer interfere 
in the subsequent raw paint material. The temperature of the mass during 
glycolytic splitting is 150.degree. to 250.degree. C., and preferably to 
200.degree. C. Preferably, a mixture of glycols is used for the glycolysis 
of the paint sludges which is composed: 
diethylene glycol (DEG) 
dipropylene glycol (DPG) and 
butylene glycol (BG), preferably 1,3-butylene glycol. 
The amount of glycol mixture added corresponds to 10 to 100% by weight of 
the paint to be converted. Glycolytic splitting can be carried out as a 
batch operation or as a continuous operation. Preferably, the glycolysis 
is carried out in a batch reactor. The reactor is usually equipped with a 
stirrer, a distillation unit, an inlet for the paint sludge and an inlet 
for the glycol. Furthermore, it is possible to integrate a shearing 
reactor into the course of the process. 
A secondary polyol obtained from polyurethane paint sludges has the 
following analytical data after the glycolysis: 
OH number: 324 
acid number: 4.3 
viscosity (20.degree. C.): 2,500 mPas. 
Generally, the OH number, that is, the number of polyol reactive end 
groups, determines the hardness of the cured paint and, for conventional 
paint polyols, ranges from 150 to 400. The acid number indicates the 
number of free carboxylic acid groups and should be as low as possible. 
For paint polyols, the dynamic viscosity is usually between 1,000 and 
10,000 mPas. If the viscosity of the paint during processing deviates from 
the desired value, it is adjusted by the addition of solvents. Preferably, 
the amount of solvent used should be as low as possible. The secondary 
polyols recovered pursuant to the present invention, can be converted into 
a raw material for paints by admixing with processing additives as well as 
by blending with a primary polyol. 
Contamination of the waste product with coagulants and/or flocculants does 
not interfere with the glycolytic splitting of the paint sludge according 
to the present invention. The impurities either are finely dispersed by 
intensive shearing during the glycolysis, so that they no longer interfere 
during later processing of the paint, or they are removed by a 
solid/liquid separation, for example by filtration, after glycolysis, if 
the secondary polyols are to be processed further to a light colored paint 
or to a clear lacquer. If they are to be processed further into a black 
paint for painting axles or engine parts for example, or to a paint of 
dark color, such as a filler paint, the impurities can remain in the 
polyols. In this case, it is advantageous to finely disperse the 
impurities in order to ensure trouble-free processing of the recycled 
paint in plants with fine valves, metering pumps and spraying equipment. 
A shearing reactor, suitable for comminuting and finely dispersing 
inorganic components, such as pigments, coagulants or flocculants, could 
be formed, for example, by a disk rotor with a plurality of aligned, 
axially perpendicular perforated, circular disks spaced a predetermined 
distance from each other and by a stator, which is provided with 
perforated stator disks, which are inserted between the rotor disks with 
little axial clearance (for example, about 0.1 to 0.5 mm) between the 
rotor and stator disks. The paint coagulate is passed at a low flow rate 
essentially axially through the shear reactor, with the rotor rotating at 
rates of 5,000 to 20,000 rpm. This leads to average peripheral speeds of 
30 to 100 m/sec, depending on the rpm and the diameter. Because of the 
internal friction generated in the paint sludge, not only are the 
particles comminuted and finely dispersed, but the paint is also heated by 
at least 30.degree. C. 
The paint sludge usually has a relatively high residual moisture content 
(for example, about 50% by weight). However, this water should not be 
contained in the newly formulated paint and thus, also not in the 
recovered polyols. Water-containing mixtures can also be processed in 
accordance with the recovery method of the present invention in such a 
manner that the recovered polyol is largely anhydrous, even without adding 
a special dewatering step. According to the present invention, the water 
is automatically distilled off during glycolysis, which takes place at 
elevated temperatures. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the preferred 
embodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Example 1 
The paint sludge concentrate used for Example 1, originates from the 
painting of aircraft parts. The waste is obtained as floating sludge after 
the coating process. The paint concentrate is composed of the following: 
polyurethane: 71.4% by weight 
water: 20.5% by weight 
solvent: 2.5% by weight 
epoxide: 5.6% by weight. 
This concentrate of reacted polyurethane paint is transferred to a stirred 
reactor to which a glycol mixture, which consists of diethylene glycol and 
1,3-butylene glycol, has been added, and is heated with constant stirring 
to a mass temperature of 195.degree. C. During the process, care is taken 
to ensure that the temperature in the reactor does not drop below 
190.degree. C. During glycolytic splitting, the volatile residual moisture 
of the paint sludge is automatically distilled off and a pre-glycolysate 
is obtained. The ratio to which the paint sludge is mixed with glycol 
mixture depends on the desired end product of the recovery method, which 
can be a raw paint material either for a filler paint or, for example, for 
a structural paint. For the secondary polyol produced in this example, the 
paint sludge and the glycol were mixed in a ratio of 80% to 20% by weight. 
Subsequently, in a separation step, the pre-glycolysate is separated from 
the nonvolatile components (epoxides) by a liquid-solid separation 
(filtration). After the glycolysis reaction is complete and the impurities 
have been removed, the secondary polyols obtained from the paint sludge, 
have the following analytical data: 
OH number: 237 
acid number: 3.4 
viscosity (20.degree. C.): 908 mPas 
After admixture with processing additives, the secondary polyols can be 
reacted alone or after being blended with a primary polyol. 
Example 2 
The paint sludge concentrate used in Example 2, originates from painting 
automobiles. The waste is obtained as a sludge coagulate from the 
processing of a two-component clear lacquer based on polyurethane. The 
paint concentrate has the following composition: 
paint coagulate: 41.9% by weight 
water: 55.0% by weight 
coagulant: 3.1% by weight 
The procedure and corresponding process parameters for glycolytic splitting 
of the polyurethane and the processing of the paint sludge is identical to 
the procedure and parameters mentioned in Example 1. After the glycolysis 
reaction is completed and the impurities are removed, the secondary polyol 
obtained from the paint sludge has the following analytical data: 
OH number: 324 
acid number: 4.3 
viscosity (20.degree. C.): 2,500 mPas 
After being mixed with processing additives, these secondary polyols can 
also be reacted alone or blended with a primary polyol. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.