Cleaning and liquid contact with solids – Processes – Using solid work treating agents
Reexamination Certificate
2001-10-23
2003-12-09
El-Arini, Zeinab (Department: 1746)
Cleaning and liquid contact with solids
Processes
Using solid work treating agents
C134S007000, C134S010000, C134S016000, C134S019000, C134S027000, C134S028000, C134S029000, C134S038000
Reexamination Certificate
active
06660100
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a method for removing paint from plastic parts coated with layers of paint.
2. The Prior Art
A method for removing paint from painted plastic parts is known from German Patent No. DE 43 44 582 A1. In this known process, the plastic parts are first mechanically crushed and subsequently circulated at an elevated temperature jointly with alkalinized glycol and in the presence of chemically stable abrasive particles. The alkalinized glycol causes the paint to swell, so that its adhesive power to adhere to the plastic parts is reduced accordingly. Due to the circulation of the plastic parts together with the abrasive particles, the swollen paint is scraped off from the plastic parts, which are subsequently rinsed with water, dried, and then separated from the scraped off paint particles. This known method achieves adequate removal of paint for most of the commonly used coats of paint.
However, the required circulation time can be kept within justifiable limits only by heating the paint removing agent to temperatures in the range of 80° C. and 100° C. with high expenditure of energy, which requires substantial amounts of paint remover in relation to the bulk material of crushed plastic pieces. This is because customarily employed paint removers, in particular alkalinized glycol, only lead to very poor wetting of the surface of the paint, which substantially reduces the duration for which the paint remover effectively acts on the coats of paint vis-à-vis the actual duration of the whole process. Furthermore, the abrasive particles required in this known process have a considerable size, so that they have to be completely separated from the now-naked plastic particles with substantial expenditure if the quality of the new plastic parts produced from such plastic particles is not be impaired.
In order to separate the naked plastic pieces from the paint removing agent, the plastic pieces are rinsed with water and subsequently dried. This requires substantial amounts of water. Furthermore, this treatment procedure renders the required drying process very costly in terms of energy.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a process of the type specified above in which the expenditure of energy required for removing the paint from painted plastic parts is reduced.
In connection with the method defined by the invention, the plastic parts consisting of thermoplastic material and having coats of paint are first crushed to bulk size, whereby at least 90 percent by weight of all pieces have an overall length of less than 20 cm following the crushing step. The plastic part from which the paint is to be removed is destroyed in this way. However, such crushing offers the advantage that the transport and treatment measures known in the field of bulk material technology can be applied without any problems to the plastic bulk material so produced. Furthermore, the predominating number of plastic parts from which the paint has to be removed have defects that make it impossible to directly reuse the entire plastic part.
However, the size of the pieces is selected to be larger than about 3 mm, preferably larger than 10 mm in order to keep the enlargement of the surface area of the plastic parts small by the crushing process. It is possible in this way to keep the amount of chemicals required for wetting the surface of the plastic pieces small as well. The goal of the method of the invention is to recover the plastic mass of the plastic pieces free of paint in order to be able to produce new plastic parts by known methods and to then paint such parts without the coats of paint adhering thereto being in the way. Since plastic components are often coated with thermosetting plastic paints, it is practically impossible to dissolve the coat of paint directly because the cross-linkage of the paint polymer would have to be broken up in this case. This, however, would require chemicals that may possibly attack the thermoplastic part itself, which should be avoided. A paint removing agent is therefore added to the bulk material of plastic pieces and circulated together with such bulk material. The paint removing agent causes swelling of the coat of paint and thus reduces its adherence to the plastic pieces. After the paint removing agent has acted for an adequate length of time, the adhesion of the coat of paint has been reduced to such an extent that it can be easily knocked off from the plastic pieces.
After the paint removing agent has acted on the coat of paint of the plastic piece, the paint removing agent is no longer needed for carrying out the subsequent steps of the method. In fact, the paint remover would interfere with the removal of the coats of paint from knocking, because it would dampen the knocks accordingly. Furthermore, there is always the risk that the paint removing agent might damage other processing equipment installed downstream. It is therefore preferable if the paint removing agent is separated from the plastic pieces after it has acted on them. It is basically conceivable to wash the plastic pieces with a simple solvent, in particular with water. However, this requires very large amounts of water and energy in the subsequent step for drying the plastic pieces. It is simpler and more efficient, on the other hand, if the separation between the paint removing agent and the painted plastic pieces is carried out by adding a solid substance absorbing liquid. Such a solid substance can be easily separated from the plastic pieces, for example by screening without requiring any substantial expenditure of energy for that purpose. As the paint removing agent is stored in the interior of the solid substance, it is possible also to employ chemically aggressive paint removing agents without causing damage to the treatment equipment downstream by residues of the paint removing agent.
In order to keep the energy expenditure as low as possible, the paint removing agent employed according to the invention is an emulsion comprised of at least one benzyl-substituted alkanol, an alkyl-glycol acetate and/or an N-alkyl-pyrrolidone with an aqueous lye. These components can not be completely mixed. The mixing ratio of the substances is selected so that it is in the range of a mixing ratio of between 1:2 and 2:1. In this way, the alkanol, the acetate or the pyrrolidone are prevented from completely dissolving in the aqueous lye, so that the result is an emulsion. Such an emulsion possesses high viscosity vis-a-vis the pure substances or the solution, so that a pulpy mass is obtained.
This provides the paint removing agent with high adhesive power for adhering to the painted surfaces of the plastic pieces, so that the pieces are wetted even if they are disposed in the circulation process far above any possible level of the liquid. This results in the advantage that the plastic pieces are wetted by the paint removing agent throughout the duration of the circulation process, which means that a short duration of action of the paint removing agent of about 7 to 8 hours is obtained even near room temperature. This eliminates the necessity of having to heat the paint removing agent or the plastic pieces in the course of the circulation process, so that the required feed of energy is reduced accordingly.
It is also possible to carry out the process for swelling the coats of paint at an elevated temperature, which reduces the required duration of action of the paint removing agent if the temperature is raised only slightly. A temperature rise to about 60° C. is adequate for reducing the duration of action of the paint removing agent to a few minutes, which additionally results in substantial energy savings vis-à-vis the prior art. In addition, this results in the benefit that only a very small amount of paint removing agent of about 3 to 4% by weight of the fill of bulk material is required because the paint removing agent always precipitates in the form of a thin film. This means that it suffices to feed
Collard & Roe P.C.
El-Arini Zeinab
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