Patent Application
20050170116
1. Field of the Invention
The invention relates to a process which permits the continuous chromium-free coating of pipe.
2. Description of Related Art
Coated pipes for the automotive industry have hitherto been produced by coating processes that include the use of chromium VI compounds (chromates). Chromates are used in order to achieve very good adhesion values in the extrusion process employed to manufacture such pipes. Chromate-treated pipes as well as chromate-treated aluminum pipes and steel pipes that are first aluminum-treated and then chromate-treated have been manufactured in this manner. However, the automotive industry recently began a switch to chromium-free pipes.
Processes for the continuous coating of pipes are described in, for example, the journal “Kunststoffe”, Volume 57, No. 1, pages 21-24, which discloses a process which uses fluidized-bed coating to coat pipes with PVC. This process is not disclosed to provide good adhesion values or layers of homogeneous thickness. This process does not therefore provide a means of meeting strict automotive industry standards.
Accordingly, it is an object of the invention to provide a coating process which provides good adhesion values and layers of homogeneous thickness, especially thin layers (120-150 μm).
A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawing.
The system preferably operates automatically and continuously to externally coat pipes by fluidized-bed coating. The process may also coat any pipe surface including internal, external or both. The process includes:
Depending on the desired layer thickness, the induction coils (II), (III) and (IV) may be operated in various combinations and with varying power including
In all cases, the pipes are heated by medium-frequency induction. Formulae for the electrical energy required, the coating rate of the process, and the powder usage are provided herein. Using the process it is possible to couple together pipe sections of any desired length to give a continuous line, which is externally coated with polymer powder as it passes horizontally through the process. Suitable coating materials are fusible polymers capable of fluidization, and mixtures of these. Polyamide powders are particularly suitable, especially those based on nylon-11 or nylon-12. Powders which give particularly good processing are described in DE 29 06 647 (Hüls AG), the trade name for which is VESTOSINT (Degussa AG). These powders have a particularly round granular shape due to preparation by a precipitation process.
A commercially available adhesion promoter (e.g. VESTOSINT adhesion promoter WS 5) is first applied to the pipe surface. Suitable primers here include any of the familiar grades for polymers, in particular those for polyamides. They may be applied in solution, suspension, or in powder form. Particularly suitable adhesion promoters for VESTOSINT are those specifically adapted to VESTOSINT. The layer thickness of the primer after air-drying is typically from 5 to 8 μm. If a solvent-containing adhesion promoter is used the solids content is generally about 8%. If use is made of a solvent-containing adhesion promoter whose solids content is about 8%, the layer thickness of the primer after air-drying is from 5 to 8 μm. The process of the invention can achieve uniform layer thicknesses of from 50 to 1000 μm. Preferred coating layer thicknesses are from 50 to 300 μm. The process of the invention can achieve tolerances of +30% in the coating layer thickness.
Pipes produced by the process of the invention are particularly suitable as hydraulic piping and brake piping, e.g. for the automotive industry.
Medium-frequency induction heating provides a heating method which is readily controllable but nevertheless very fast, and gives the further advantage that the induction coil which heats the pipe as it passes through the system can be placed directly within the fluidized powder, with the result that there are no heat losses. At 10,000 Hz and with a pipe wall thickness of 2 mm, heating by 300° C. takes 1 s. At lower frequencies the greater penetration depth makes the heating process even faster, and at 2000 Hz the time would be only 0.73 s under the same conditions.
The induction coil takes the form of coiled tubing and is cooled by passage of water. The induction coil remains cold, as does the powder. The generator system is composed of a machine generator which generates the high frequency, a control cabinet with a control desk, a capacitor battery, and an induction coil. The system may simply be regarded as a transformer with high-frequency electrical energy fed into its primary side and with the workpiece forming its secondary side composed of just one winding. The resultant very high current density in the secondary circuit results in fast heating. Workpieces passing through the system preferably have uniform cross section and uniform wall thickness, including articles with rotational symmetry, such as wires, pipes, rods, and the like.
The throughput rate (advance rate) of the pipes depends on pipe diameter and wall thickness, i.e. on the weight of the pipe per unit length, and also on the generator power. The generator efficiency and the required degree of heating of the pipe also play a role in throughput rate. However, since these latter variables can initially be regarded as constant or at least not subject to great variation, appropriate average numerical values can be assumed. The generator power required is:
where N is the generator power in kW, G is the pipe weight in kg/h passing through the system, cp is the specific heat of steel (˜0.12 kcal/kg degree), Δt is the required pipe temperature increase, and η is the overall efficiency of the generator system (approximately from 0.6 to 0.75). The coefficient 860 derives from the conversion 1 kW=860 kcal/h. If Eq. (1) is solved for G with η=0.7 and Δt˜240° C. the result is a very useful rule-of-thumb formula for the maximum amount of steel in kg/h which can be heated with a particular generator power (valid for the present conditions and similar conditions). This value may be greater or smaller depending on the overall process conditions.
G≈20 N (2)
Using a 36 kW medium-frequency generator, therefore, about 720 kg/h of steel pipe can be heated by 240° C.
For quick approximations, an approximate guideline value for power required (under the present conditions or similar conditions) is:
These rule-of-thumb formulae are naturally not dimensionally correct since the numerical value has been used for quantities which have dimensions (e.g. specific heat cp). However, these dimensionally incorrect equations have proven to be very useful for operating requirements. If the formulae for the weight of pipe passing through the system are combined with the formula for the generator power required, the result is a simple relationship for the maximum throughput rate (advance rate) of a steel pipe of density y=7.85 kg/dm3 for a given generator power. If, for example, there is a frequent need for pipes of various diameters and wall thicknesses to be coated in a pipe-coating system, the following formula rapidly gives a guideline value for the maximum throughput rate. The numerical factor has to be altered somewhat for other conditions. The actual throughput may be greater or smaller depending on the overall process conditions:
The throughput rate vmax has to be inserted here in m/min, the generator power N in kW, the external pipe diameter de in mm, and the pipe wall thickness s likewise in mm.
A pipe-coating process was used to prepare coated pipes. The process includes:
The results of a series of experiments on the system described are given in Table 1. All of Examples 1 to 7 used VESTOSINT 2157 precipitated nylon-12 powder from Degussa AG. In all the examples given there was no chromate-pretreatment.
Experiments on Continuous Pipe-Coating Using PA 12
Coil I Primer drying I
Coil II Pipe preheating
Coil III Fluidized-bed coating pan
Coil IV Smoothing
Coil V Primer drying V
Tests on Primed Pipes
Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
German Application 1023345.9 filed on Jul. 23, 2002 is incorporated herein by reference in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1023345.9 | Jul 2002 | DE | national |
