Michael Golek looks at a new development in automotive painting which eliminates the necessity for a separate primer layer in automotive OEM coatings.
Automotive coatings fall within an extremely wide range of specifications set by automobile manufacturers. In order to meet the high surface finishing standards demanded by the automotive industrycoatings carefully matched for compatibility are applied in several layers.
Searching for a more ecologically sustainable and cost-efficient solutionBASF’s experts asked themselves whether the traditional coating processproven in millions of applicationscould be improved. Nextthey asked themselves if it was possible to create a cutting-edge system that offers significant cost-cutting and ecological benefitsand can meet the constantly tightening VOC standards around the world? The answer to these questions is ‘yes’.
Although every automotive manufacturer has different finish specificationsthe coating lines in which cathodic electrocoatscolour layers and clearcoats are applied to vehicle bodies have become longer and increasingly complex.
From the starting pointwhere bodies-in-white enter the painting processto the end where the painted bodies enter the final assembly stagethe painting line can be more than a mile long.
Neverthelesshaving delivered successful results for yearseach of the steps in the traditional process is now considered firmly established. The disadvantage of this processhoweveris the high cost required to achieve maximum processing reliability together with all the required quality characteristics.
In addition to improving the coating productsefforts to optimise the painting process are focused on shortening the linesimplifying application processesreducing costs and reducing the amount of solvents used. For examplewith Integrated ProcessIIthe length of the conveyor line can be reduced by 17percent.
Early attempts at paint line optimisation sought to streamline the standard process in general. The sequence of the individual process steps was analysed. The basic premises of the process chainand their variations among automobile manufacturerswere closely studied. System-specific feasibility analyses and process studies led specialists to take a hard look at the primer stage as an area in which significant savings could be achieved.
The problem is that the electrocoats (or e-coats) that underlie the primer layer are light-sensitive. In conventional paint systemsthe unique structure of the primer protects the e-coat by absorbing any remaining UV light that penetrates through the clearcoat and the basecoat.
Eliminating or changing the primer allows this residual UV light to attack the e-coat and impair adhesioncausing the paint finish or the e-coat film to separate partially from the substrate.
In field tests and laboratory simulations of three- to four-year weatheringthis loss of adhesion occurred over large areas.
Neverthelesseliminating the primer remained a promising approach to reducing costs in the painting process. The knowledge gained in previous tests and studiesand the simple question – what does the primer actually do? – led BASF researchers to the idea of cutting out the primer and the process steps needed to apply itwhile maintaining its critical functionality. To retain this functionalitythe properties of the primer had to be integrated into the other components of the paint system.
The result was Integrated ProcessIIa new process technology that can also be incorporated into existing lines.
Integrated ProcessII streamlines automotive production line coating – in particularthe overall length of painting linesthe number of process steps involvedand cycle times.
This technology eliminates an entire coating layerand yet can be seamlessly integrated into existing plants. With the elimination of a coating layervolatile organic compound (VOC) emissions are reduced – without limiting the available colour range.
BASF’s Integrated Process II enables the elimination of the entire primer application section of a coating line-along with the associated cleaningsandingdust extractionwater treatmentwaste disposal and pre- and post-treatment operations-while retaining all of the primer's functions. The result is a much shorter coating line.
The functions of the primer are performed by a new water-based paint system consisting of two components: ColorPro I (which absorbs ultraviolet radiation) and ColorProII. In combinationthe ColorProI and ColorProII layers perform all the functions normally associated with primersfrom ensuring protection against stone chipping to providing a base for an excellent overall finish.
In a conventional processthe primer is applied in one stepand then baked. This is followed by the application of the basecoat in two steps. In the Integrated ProcessII system the primer is eliminated. Thenin place of the traditional basecoatColorProI is applied in the first stepand ColorProII in the second step.
Existing equipment can continue to be used. All that is needed is the installation of a second paint supply system (consisting of three or four circulation lines) in the first booth.
The design of ColorProI and ColorProII guarantees that all common basecoat application methods can be used.
Eliminating the primer not only eliminates expensive processes and system technologybut it also eliminates related solvent emissions. BASF’s Integrated ProcessII can make a significant contribution
to meeting the more stringent requirements of the future.
Primers are traditionally applied by spraying. They even out irregularities and offer additional protection from corrosion and stone chips.
Primers are also highly efficient in protecting the electrocoat from the effects of UV radiation. Their ability to fill in irregularitiesas well as their sandabilityis an important property.
The colour of a primer is also important for the basecoat and clearcoat layers that go over it. Properly matched primers allow the subsequent basecoats to be thinner. Because of their multiple functionsthe formulation of primers is an expensive and demanding task.
In generalthe application of primers begins with a manual and/or automated cleaning process. Then the primer is applied by means of electrostatically assisted high-rotation ‘bells’ and robots equipped with nozzles that atomise the paint pneumatically.
When waterborne or solventborne primers are usedthe vehicle body moves to the flash-off zone and the baking oven after leaving the paint booth.
After leaving the oventhe vehicle passes through the inspectionsanding and correction zonesand any necessary reworking is performed. The surface is then cleaned again and the body enters the topcoat application area.
With the Integrated Process II systemthe current practice of finesse sanding and the quality inspection stage are eliminated. In additionthe so-called ‘conveyor accumulator’where vehicles await the next step after primingis no longer needed.
A complete profitability analysis of the primer stagethenmust cover not only the application processbut also all the booth cleaning work that is required in a primer application booth.
The frequency of booth cleaning as well as the cost of the recovery of the overspray from the booth water is a function of the transfer efficiency of the application process – iethe ratio of sprayed product that ends up on the body to the amount that ends up as overspray. Seen as a wholethe primer application process generates a substantial share of costs.
As already notedthe protection of the e-coat from the damaging effects of UV radiation is essential. Because of its high energy contentUV radiation is able to attack macromolecular materials and coatings through the formation of ‘free radicals’leading to adhesion failure and embrittlement.
Normallysuch damage does not occur in the e-coat unless the high-energy radiation is able to penetrate the overlying layers. Thereforeif the primer is to be eliminatedthe other layers applied over the e-coat must achieve the absorption of UV light.
BASF decided to pursue a system strategy with its Integrated ProcessII. This decision was based on previous experience with coordinated combinations of light stabilisers in metallic paints (involving durabilitycolor stabilitygloss retention and resistance to cracking for more than 10years).
Transferring the UV absorption function to the clearcoat proved to be possible only to a limited extent becausebeginning at a specific level in the nano rangethe clearcoat loses its transparency and colourless properties. It takes on an undesirable green or yellow cast.
For this reasonthe approach utilising the ColorProI and ColorProII basecoats as a system seemed to make the most sense.
Integrated ProcessII offers automotive OEMs the following advantages:
- Eliminates the primer and its application processes.
- Can be integrated in existing facilities.
- Performs all the functions of the primer layer.
- Absorbs UV radiation.
- Covers sanding marks and substrate textures.
- Prevents delamination after aging and stone chips.
- Prevents other damage from reaching the substrate.
- Offers cost savings over the conventional processwith high first-through capability and high efficiency.
- Represents a state-of-the-art environmental solution.
- Permits the same paint formulation for OEM manufacturers and add-on parts manufacturers.
- Delivers a finish nearly identical to that of conventionally applied coatings.
- Allows unrestricted repairability.
The ColorProI and ColorProII layers are applied in the existing topcoat booth layout without increasing the flash time between application steps.
In additionthere is no change required for the flash tunnel conditions prior to clearcoat application. Moreoverthe ColorProI layer is colour-matched to the ColorProII layerwith no color limitationsthus permitting a substantially thinner overall paint finish than is possible with a conventional approach.
The system can be applied with existing electrostatic equipment andin generalis fully compatible with two-component clearcoats and BASF’s SlurryGloss clearcoat.
Extensive studies involving UV exposure of automotive coatings in different climates and over varying periods of time revealed major differences among colours in their ability to absorb UV radiation.
Dark colours are inherently UV-absorbingbut many currently popular colours – in particularsilver – allow varying amounts of UV radiation to pass through the coating. The ColorProI and II system was matched in such a way thatfor some coloursit is now possible to achieve UV protection with a single coat of ColorPro-in other wordswith only one layer that combines the properties of ColorProI and ColorProII. With more difficult colourshowevermaximum functionality and reliability still requires both system components in combination.
These variations can only be determined through extensive laboratory testsclimatic and aging simulationsand application tests-all of which have to be carried out for every colour.
The BASF system consisting of ColorProI and ColorProII can be varied for optimum performance for any given set of requirements. BASF continues to research system-based approaches to deliver further improvements in automotive finishes.
The latest results from system modifications show that the brief infrared flash time between ColorProI and ColorProII applications can be eliminatedand that the combination of ColorProI and ColorProII is only necessary for certain colours.
Among future objectives is the adjustment of all BASF coating componentsfrom electrocoats to clearcoatsto create a fully integrated system encompassing the entire paint process. In the meantimehoweverany coatings currently on the market can be used with the components in the Integrated ProcessII.
The process is leading the way for the entire automobile industry. It was first implemented as the Integrated ProcessI at the DaimlerChrysler plant in Rastatt (Germany) in the production of A-Class cars. The shortening of the application linecost savings and a general simplification of the painting process are the results of BASF innovationsadvantages that can now be realised in existing facilities in the form of Integrated ProcessII.
In additionwith the reduction in VOCsIntegrated ProcessII demonstrates that eco-efficiency can be achieved in automotive paint operations without any negative impact on manufacturing technologyquality or design freedom.
Michael Golek is with BASFMünsterGermany
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