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(11) | EP 1 502 662 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | A method of electrostatically coating plastic objects |
| (57) The method of applying the coating compositions on the surfaces of the plastics object,
by which the individual basecoat layers and the clear topcoat layers are applied in
electrostatic and/or in pneumatic way on the pre-treated surface of the plastics part
until they reach the required thickness and color. Both the basecoat application and
the clear topcoat application are followed by volatilization, drying and cooling.
Prior to the application of the first basecoat layer, which is done with the standard
water-born paint, a two-component base mixture of the respective standard water-born
paint and a water-born adhesion and conductivity promoter is applied onto the pre-treated
plastics surface as undercoat, at least by one spray, in the first spraying booth,
in which also the basecoat application follows. On the dry basecoat a conductive clear
topcoat is applied, at least by one spray, in the second spraying booth. |
Field of the Invention
[0001] The invention relates to the method of applying the coating compositions on the surfaces of the plastics objects, or application of organic coatings on surface of the polymer substrate.
Background of the Invention
[0002] The plastics objects are not electrically conductive per se and because of their molecular nature adhesion of coating compositions to their surfaces is generally poor (mainly in case of polyolefin). For this reason the possibility to perform their quality coating is much more difficult compared to coating of the metallic objects. It is necessary to provide for enough strong linkage between polymeric substrate and applied coating. It can be achieved by suitable physical methods capable to partly break bonds of polymer macromolecules in a thin surface layer of the plastics object, e.g. by short flaming by a gas burner, whereby the polymer substrate surface molecules are impaired letting active molecular bonds arise able to bind themselves to the applied coating. Regularly a primer is initially applied to the pre-treated plastics surface, which is a substance able to furthermore increase adhesion of the coatings successively applied onto the substrate and to ensure electrical conductivity of the surface. The function of primer layer is to create the possibly strongest bond of the basecoat to the plastics surface and to enable the application of further coatings in electrostatic field, e.g. by electrostatic spraying.
[0003] Onto the dried primer the required coating is applied in one or more layers, consisting usually of more (most often two) different coating compositions having different properties and fulfilling different functions. Regularly a basecoat, which is matt after drying, is applied onto the primer, giving the plastics object the required final finish color and simultaneously ensuring the best possible adhesion of the following clear topcoat. Currently the basecoat paint is mostly a water-born one-component coating composition. Nevertheless, a solvent-born or a multi-component composition can also be used.
[0004] On the dry basecoat the glossy topcoat is applied, usually being clear and colorless, to fit any coloring of the basecoat. This clear topcoat is mostly a solvent-born two-component composition, providing the highest gloss. Water-born clear topcoat can also be used, although it is more difficult to apply. The clear topcoat layer creates the final finish of the plastic part and protects the underneath coating and the plastics object itself from weather effects and mechanical damage.
[0005] To apply coating compositions on plastics parts, both pneumatic spraying and - which is more often - spraying in electrostatic field are used. With pneumatic application it is not necessary for the plastics surface to be conductive. Due to this fact the efficiency of the spray painting is relatively low, as substantial quantity of the paint misses the object not utilized, and it is necessary to liquidate it in an environmentally friendly way. The efficiency of the electrostatic application is much higher, yet a conductive surface is required. In technical practice both methods are usually combined with the aim to obtain a reasonable compromise among technological capabilities, ecological requirements and economical interests.
[0006] Most often, namely due to the possibility of the subsequent electrostatic application, the conductive primer is pneumatically applied on the non-conductive plastics object in the first stage. After this application, and even after drying, the surface remains electrically conductive and further coatings can be done by much more efficient electrostatic application.
[0007] As the rule, each of the coatings - primer, basecoat and clear topcoat - is applied in separate spraying booth equipped with air-exhaustion to prevent mutual contamination of the applied coatings and to eliminate pollution of the surrounding ambiance.
[0009] The surface of a plastics part, contaminated by dirt and especially by separator used in injection molds, is first of all thoroughly washed by detergents and pressure water and dried. After being washed the surface has to be pre-treated to activate the possibly best adhesion of the coat layers. This is done e.g. by short controlled flaming by a gas burner.
[0010] To further increase the adhesion of the next layers, primer is applied by pneumatic or electrostatic application onto this freshly pre-treated surface. Primer can be both water-born (which is in most cases) and solvent-born. Primer layer can be applied in one spray or repeatedly, until the required total layer thickness has been reached. Usually the initial thinner layer is sprayed pneumatically and the next thicker one is applied electrostatically. Volatilizing, drying under increased temperature and cooling to ambient temperature follow.
[0011] Provided the conductive primer has been applied, application of the basecoat onto thus prepared surface follows, being performed either in electrostatic or in pneumatic way. The electrostatic application is used for the initial layers, while the pneumatic one usually for the final ones, particularly in case of metallic paints. The basecoat paint can be applied in one spray or repeatedly to achieve required total thickness of the basecoat layer, that affects the final nuances of the respective color. Volatilizing, drying under higher temperature and cooling follow.
[0012] Provided conductive primer was applied before, a non-conductive colorless clear topcoat is finally applied on the dry basecoat layer, the application being performed in the electrostatic way. Should the pneumatic application be required due to the geometry of the object, then in the first instance the critical areas are treated pneumatically, and only after that step electrostatic application follows for the whole surface. The clear and colorless topcoat can be applied in one spray or repeatedly, until the required layer thickness has been achieved. Volatilizing, drying under higher temperature and cooling to ambient temperature follow. After having passed the whole process the painted part is checked for quality and dispatched.
[0013] It is apparent from this that the existing method of coating the plastics parts is a rather complicated process from many points of view. The target of the presented invention therefore is to reduce the demands regarding the paint shop technological equipment, to increase the productivity of the labor and to make the whole process more effective.
Summary of the Invention
[0014] The target can be solved to a great extend by the method of applying the coating compositions on the surfaces of the plastics objects, by which the individual basecoat layers and the clear topcoat layers are applied either in electrostatic and/or in pneumatic way on the pre-treated surface of the plastics part until they reach the required thickness and color, both the basecoat application and the topcoat application being followed by volatilization, drying and cooling, according to the invention. The principle of the invention consists in the fact that, prior to the application of the first basecoat layer, for which the standard water-born paint is used, a two-component base mixture, consisting of the respective standard water-born paint and a water-born adhesion and conductivity promoter, is applied onto the pre-treated plastics surface as undercoat, at least by one spray, in the first spraying booth, in which also the basecoat application follows. After the basecoat application is finished, a conductive clear topcoat is applied on the dry basecoat layer in the second spraying booth.
[0015] In contrast to the routinely used processes of spray painting, there is no need to apply primer as the ground coat, which results in absence of further operation of volatilizing, drying and cooling. The advantage of the coating method described in this invention is also the fact that due to the conductivity of the two-component base mixture it is possible to apply it in highly effective electrostatic way even as undercoat provided the geometry of the part is suitable.
[0016] The principle of the method according to the invention consists further in the fact that the basecoat layers can be easily applied immediately after the application of the adhesive and conductive two-component base mixture in the same spraying booth, by spraying so to say "wet on wet". Thus, in comparison to the technological equipment needed with the existing method of the spray painting, the spraying booth routinely used for the primer application is eliminated now. Parameter setting of the application equipment remains the standard one.
[0017] The quantity of adhesion and conductivity promoter in the mixture with the water-born basecoat paint is highly variable. The promoter quantity in the base mixture can directly control the efficiency of the application and thus the economic parameters of the painting process can be significantly optimized.
[0018] Practically the two-component base mixture may - depending on the sort of the standard water-born paint, metallic or universal - contain from 1 up to 38 % of weight of the adhesion and conductivity promoter, which is based on binding and pigment agents and solvents, the rest is a water-born standard basecoat paint. This reflects the ratio of the mixture components reaching as a maximum 60 weight parts of the promoter to 100 weight parts of the water-born basecoat paint. Higher volume of the promoter increases the efficiency of the application even by 28 %, nevertheless, the basecoat layers of some paints require in such case application of higher thickness to reach the needed color.
[0019] In contrast to the routinely used processes, where the dry basecoat layer is nearly non-conductive, with the method according the invention a conductive clear topcoat (either water-born or solvent-born) with conductivity enhancing admixtures in the form of solvents, solvents for conductivity increase, hardeners and similar is applied, enabling thus solely electrostatic application instead of the former pneumatic one, provided the geometry of the part is suitable. Compared to the standard process it is possible to increase the efficiency of the topcoat application at least by one half, which is the next important feature of the invention.
Brief Description of the Drawing
[0020] To illustrate the invention more thoroughly a block diagram giving examples of the application of the paint layers onto the plastics surfaces according to the invention shall be presented.
Description of the prefered embodiment
Example 1
[0021] The cleaned and washed surface of the plastics part, in this case an automobile bumper, is pre-treated in the conventional way by short controlled flaming using natural gas burner, whereby physical condition for adhesion of the subsequently applied layers is improved.
[0022] The two-component base mixture containing 77 % of weight of metallic water-born standard basecoat paint and 23 % of weight of water-born conductivity and adhesion promoter is applied by electrostatic method on the pre-treated surface by one spray in the first spraying booth. The promoter is a combination of binders with appropriate pigment and solvents, besides the usual organic solvents mainly water. In the described case the promoter is known under the commercial name Wörwag Haftzusatz A 1819. For the next paint layer electrostatic application of identical pure standard metallic basecoat paint follows in the same spraying booth immediately, no adhesion and no conductivity promoter being used in this case. If necessary, critical areas of the plastics part can be additionally treated by pneumatic application to reach the required thickness of the basecoat layer and proper color.
[0023] As the next steps 10 minutes volatilizing under the temperature of 45°C and relative humidity of 65 %, 18 minutes drying under 85°C, and 14 minutes cooling under 24°C follow.
[0024] To reach a glossy topcoat, colorless conductive clear glossy topcoat mixture is applied by electrostatic way, by one spraying, in the second spraying booth. This mixture contains 5 % of weight of special solvent to enhance the conductivity and 40 % of weight of standard solvent, to 100 weight parts of this mixture another 37 weight parts of hardener are added. Thus a very high efficiency of the application is achieved, being increased by 85 % compared to the application of a standard non-conductive clear topcoat. If necessary, critical areas of the plastics part can also be treated by additional pneumatic application to reach the required coat thickness. There is no difference whether this pneumatic application is done as the first step or as the last step when applying the topcoat. Then 10 minutes volatilizing under temperature of 25°C and relative humidity of 65 %, 50 minutes drying under 88°C, and finally 14 minutes cooling under 24°C follow.
Example 2
[0025] According to the second example the two-component base mixture containing 63 % of weight of universal water-born standard basecoat paint (no metallic) and 37 % of weight of water-born conductivity and adhesion promoter is applied by electrostatic way on the pre-treated surface, by one spray, in the first spraying booth. Then the application of identical pure standard universal basecoat paint follows in the same booth immediately. In this case two changes are obvious - the universal paint has replaced the metallic one and the content of the adhesion and conductivity promoter in the two-component mixture has been increased. With the exception of these changes the further procedure is identical with previous example. The efficiency of the electrostatic application of the conductive clear topcoat in the second spraying booth is 75 % higher compared to the application of a standard non-conductive clear topcoat.
[0026] The above said procedures are illustrated by the presented block diagram, where the block 1 stands for surface treatment, block 2 represents the first spraying booth with the activity divided into two phases, 2a standing for the application of the two-component base mixture and 2b standing for the application of the pure standard basecoat, block 3 stands for volatilizing, block 4 for drying and block 5 for cooling. Block 6 follows representing the second spraying booth with electrostatic clear topcoat application, block 7 stands for volatilizing, block 8 for drying and block 9 for cooling.
[0027] The adhesion and resistance of the coating applied in the described two examples are fully conforming and it is evident that this method enables to maximally use the electrostatic application, save one spraying booth and reach higher productivity and efficiency of the whole process compared to the existing routinely used method.
Field of the Application
1. The method of applying the coating compositions on the surfaces of plastics objects
by applying the paint layers onto the surface of the plastics parts, by which the
individual basecoat layers and the clear topcoat layers are applied in electrostatic
and/or in pneumatic way on the pre-treated surface of the plastics part until they
reach the required thickness and color, both the basecoat application and the clear
topcoat application being followed by volatilization, drying and cooling, characterized by the fact, that prior to the application of the first basecoat layer, for which the
standard water-born paint is used, a two-component base mixture of the respective
standard water-born paint and a water-born adhesion and conductivity promoter is applied
onto the pre-treated plastics surface as undercoat, at least by one spray, in the
first spraying booth, in which also the basecoat application follows, whereupon after
the basecoat application is finished, a conductive clear topcoat is applied on a dry
basecoat layer in the second spraying booth.
2. The method according to the claim 1, characterized by the fact, that the basecoat layers are applied immediately after the application
of the two-component base mixture onto the plastics part surface by so called "wet
on wet" system.
3. The method according to the claim 1, characterized by the fact, that the two-component base mixture applied as undercoat onto the pre-treated
surface of the plastics part contains from 1 % of weight up to 38 % of weight of water-born
adhesion and conductivity promoter, which is based on binders, pigments and solvents,
with the rest of the mixture being standard water-born basecoat paint.
4. The method according to at least one of above stated claims, characterized by the fact, that the conductive clear topcoat, which creates the top layer consists
of clear substance mixed with both the standard solvent and the special solvent for
conductivity enhancement, and with the hardener to make the conductivity effective.