The present invention relates to a process for spray-coating of mass-produced goods with frequent changes between at least one aqueous and at least one non-aqueous coating agent inside a spray-coating chamber.
The coating of vehicles in a production line takes place in fully automated vehicle coating lines whereby the application of the colour-determining coating layer (as finishing top coat layer or as base coat layer with a transparent top coal layer) is carried out with a number of different colour shades with frequent changes of colour. The change of colour or colour shade results from the manufacturing schedule and can take place after each coating operation of an automotive body, or the bodies are assembled in small groups of the same colour shade. Each coating agent with a different colour shade is supplied to the spray-coating devices from a coating supply reservoir via a dedicated circulation line connected thereto.
To obtain an automatic change of colour shades, it is sufficient to switch from one circulation line to another circulation line, so long as the feeding lines, the metering devices, as well as the spray-coating devices themselves, are automatically rinsed between applications of different colour shades. When changing colour in spray-coating chambers, in which non-aqueous coating agents of different colour shades are handled, this automatic rinsing is carried out with mixtures of water-immiscible organic solvents conventionally used in coating agents. However, the colour change in the spray-coating chambers, in which aqueous coating agents with different colour shades are handled, is followed by automatic rinsing with rinsing media which contain as main constituents water and water-miscible organic solvents conventionally used in aqueous coating agents, and which contain optional additional auxiliary agents in small quantities. The respective rinsing medium is also supplied from the supply reservoir to the colour changer via a dedicated circulation line and can be supplied from there to the spray-coating devices via the supply lines and the metering devices.
The time available for the actual change of colour, and therefore for the rinsing operation, is dependent upon the speed at which the vehicles are being conveyed through the spray chamber and on the distance between the automotive bodies. In practice, the conveyor speed and the distance are adjusted to the period of time required for the change of colour and the rinsing process, which totals for example 5 to 20 seconds. Quite apparently, productivity suffers with any delay, so there is a need to decrease the amount of time needed between application of different coating colours or colour shades.
There has been a focus in recent years, particularly in the vehicle manufacturing/assembly plants, to replace non-aqueous (i.e., solvent-based) coating systems with aqueous coating systems due to environmental considerations. The actual replacement of non-aqueous coatings with aqueous coatings, however, has been problematic, especially with basecoats and topcoats which provide, respectively, a particular colour or a desired property or effect to the article (e.g., an automobile) being coated. The need to apply both solvent based and aqueous based coatings adds considerable complexity to the coating process, particularly if the same spray chamber needs to be used to apply both types of coatings. In such situations, rinsing of the supply lines, metering devices and spray-coating devices themselves, can't be carried out with either the conventional rinsing agents used for solvent based coatings nor those conventionally used for aqueous based coatings alone. Instead, the two types of rinsing media need to be used in sequential rinsing steps and in the proper order. A change of colour from an aqueous to a non-aqueous coating agent, for example, would result in the following sequence:
termination of the application of an aqueous coating agent with the colour shade A; PA1 rinsing of the supply lines, the metering devices and of the spray devices with PA1 a rinsing medium which contains, as essential constituents, water and water-miscible organic solvents conventionally used in aqueous coating agents and which contains optional additional auxiliary agents in small quantities; PA1 rinsing with mixtures of water-immiscible organic solvents conventionally used in non-aqueous coating agents; and PA1 application of a non-aqueous coating agent with the colour shade B.
The same applies for a change of colour from a non-aqueous to an aqueous coating agent. This multiple step process, with an additional rinsing step, obviously results in lower productivity as it is impossible to realize such a sequence for each change of colour in the short time that is available for the rinsing operation. In addition to the decrease in efficiency and productivity, the consumption of rinsing media as well as the quantity and costs for waste disposal would increase. It is, of course, possible to utilize two separate spray apparatus in a single spray chamber, one dedicated to aqueous coatings and the other dedicated to non-aqueous coatings. However, this alternative clearly adds capital costs to the vehicle manufacturing process and is thus not a satisfactory solution.
Solvent mixtures are known in the art which are said to be suitable for cleaning surfaces contaminated with solvent coatings or water-borne coatings. For example, U.S. Pat. No. 5,700,330 discloses a solvent mixture consisting of 15 to 35% by weight of one or more glycol ethers and of 65 to 85% by weight of one or more aliphatic ketones, which can be used for cleaning instruments that have been contaminated either by coating agents containing solvents or by water-dilutable coating agents. An improved solvent mixture is taught by DE A 198 23 852, which discloses a solvent mixture consisting of 20 to 65% by weight of one or more glycol ethers, 9 to 40% by weight of one or more aliphatic ketones, 10 to 35% by weight of one or more aliphatic monohydric alcohols and 5 to 20% by weight of one or more organic solvents, selected from the group of the N-substituted pyrrolidones and/or butyrolactone.
It is an object of the invention to provide a process for spray-coating mass-produced substrates, in particular automotive bodies and their parts, using aqueous as well as non-aqueous coating agents inside a spray-coating chamber. The process is intended to overcome the aforementioned problems and to be suited particularly for coating operations that require frequent changes in the coatings being applied. A solution to the problem is obtained by coating mass-produced substrates with aqueous and non-aqueous coating agents inside one and the same spraying chamber and by rinsing the installations to be rinsed of that particular coat-applying unit with a universal non-aqueous rinsing medium of a given composition each time the coating agent is changed. By "installations to be rinsed" are meant in particular the supply lines from the coat changer to the spray-coating devices, including the metering devices as well as the spray-coating devices themselves.