Underbody Coating
Automotive coatings and the processes used to coat automobile surfaces exemplify the Avant-grade of technologies that are capable of producing durable surfaces, exceeding customers expectations of appearance, maximizing efficiency, environmental regulations. These accomplishments are rooted in 100 years of experience, trial-and-error approaches, technique and technology advancements, and theoretical assessments. Because of advancements directed at understanding the how, why, when, and where of automobile coatings, the progress in controlling droplets and their deposition attributes, and the development of new technologies and paint chemistries, a comprehensive and up-to-date review of automobile coating and coating technologies was considered to be a value to industrial practitioners and researches. Overall, the critical performance factors driving the development and use of advanced automotive coatings and coating technologies are (a) aesthetic characteristics; (b)corrosion protection; (c) mass production; (d) cost and environmental requirements; (e) appearance and durability. Although the relative importance of each of these factors is debatable, the perfection of any one at the expense of another would be unacceptable. Hence, new developments in automotive coatings are described and discussed in the following review, and then related to improvements in production technologies paints.
At the beginning of the automotive industry about100 years ago, cars were painted with a varnish-like product that was brushed onto the vehicle surfaces; this coating was sanded and smoothed, and then varnish was reapplied and refinished to establish several layers of the coating. After multiple layers of varnish were applied, vehicles were polished to produce shiny surfaces. Some manufactures, including Ford in the model T line, employed a combination of brushing, dipping, & even pouring to fully cover and protect various parts of car. All of these coating steps were implemented manually, and it was not uncommon for the period between it beginnings of the coating process to the end, when the coating was dry and the car was ready to sell, to take up to 40 days. Between the 1920s & 1940s, automotive transitioned to using spray equipment & ‘stoving enamels’ that were based on alkyd resins; these two advancements decreased application and drying times to a week or less. Because of the newly developed spray coating technologies, the surface finishes were more even & less sanding was needed. Automotive coatings continue to evolve as they either satisfy or are anticipated to meet customer expectations & environmental regulations while also lowering manufacturing & ownership costs. One of these evolutions is in the use of smart coatings because the offer the potential to significantly improve surface durability while adding additional functionalities or properties like self-healing, super-hydrophobicity, self-stratifying, self-sensing, sound proofing, & vibration damping. For example, a smart coating could respond to its environment to enhance the coating life; a smart coating with self-healing properties would be useful in response to an abrasive, mechanical trigger or to a corrosive event in which the coating is self-healing as result of UV, heat, or mechanical activation. Self-healing can also be achieved by employing shape memory polymers that are triggered with temperature and humidity manipulations, or with UV radiation; self-healing associated with the swelling of special clays such as montmorillonite is also possible. Other smart coatings include those with internal sensing capabilities that entail the passive or active triggering of fluorescent molecules or quantum dots. In the former, the sensing system signals & activates changes in or repair of the coating by sending data to an external detect or, in the latter, the sensing system itself would be responsible for outputting the response signal.
TRENDS IN AUTOMOTIVE COATING PROCESSES
The current trends in automotive coating process are motivated by lowering manufacturing costs, delivering customer satisfaction via aesthetic features and corrosion protection, and mitigating environmental concerns. At tremendous amount of consideration has been put into current automotive coating systems, and they have a level of sophistication that satisfies most customers around the world. Compared to 30 years ago, the problem of corrosion is almost alleviated, and the durability & appearance of the topcoats have become acceptable for the lifetime of a car. In fact, with the worldwide emergence of two-layer topcoats, the colour, gloss, & chip resistance of automobile coating remain in relatively excellent condition during the first 7-10years of an automobile’s use. Aesthetic features
are also trending with automotive fashions. Hence, a closer look at recent trends in automotive coatings is presented in thefollowing:1. POWDER COATINGFig.1. Powder Coating One solution to the emission of VOCs has been the replacements of liquid coatings with coatings in the form of dry, particulate solids, commonly called ‘powder coatings. Their compositions contain very low concentrations of volatile solvents, on the order of 2% substantially less than any other paint system. Currently, the automotive industry uses powder coating on wheels, bumpers, hubcaps, doorhandles, decorative trim & accent parts, truck beds, radiators, & numerous engine parts. A clear powder topcoat has been developed; BMW & Volvo are using it on their new model cars, & GM, Ford, & Chrysler have formed a consortium to test it on their production lines.
MODERN AUTOMOTIVE COATINGPROCESSES
Modern automotive coating methods consists of five main steps. They include the following:
- PRETREATMENT.
Pre-treatment sequences for the body-in-white the first sequence in preparing the BIW for subsequent coating is pre-treatment sees Fig.2. Pre-treatment removes & clean excess metal & forms an appropriate surface structure enabling bonding of a corrosion protection layer. It consists of cleaning the body surface to remove remaining oils from the stamping process & welding residues through three primary liquid dip processes of degreasing, conditioning, & phosphating. Pre-treatment helps the primer to bond onto the metal. A phosphate treatment applies an inert layer of metal phosphate, providing resistance to corrosion spread. The degreasing solution is composed of alkaline salts &surfactants & can include caustic soda, sodium carbonate. The surfactants are types of detergents for emulsifying oils &lubricants on the BIW. In surface conditioner which creates nucleation sites for phosphate crystal growth. It increases the number of crystallization nuclei on the metal surface that enhance bonding mechanism for the subsequent phosphatase quince. Finally, the liquid dip phosphate ions, nitrate ions, zinc & hydrogen ions. The free acid etches the steel surface, causing hydrogen to be released while metal phosphate ion sore precipitated onto the surface in a crystalline form.
- ELETRODEPOSITION.
Electrodeposition coating process E-coating involves dipping automobile bodies into the coating solution & passing an electric current through the body & the liquid ED paint solution. Because of the charged
nature of this coating process, the ED paint penetrates into
places a spray would not reach. During operation, a mixture of resin, binder, & paste containing the pigments is fed into the ED tank. The automobile body is lowered into the tank, & an electric current applied; the solution in the tank consists of 80-90%deionised water & 10-20% paint solids. The resin is the backbone of the final paint film & provides corrosion protection, durability, & toughness. Pigments are used to provide colour & gloss. After E-coat, the automobile body enters a bake oven, in which heating & keeping at a temperature of 160_Cfor 10 min causes film curing to promote maximum performance properties. The oven temperature & heating time primarily enhance chip resistance & film adhesion to the body; corrosion protected is affected less by these conditions. Because surface roughness influences the smoothness & brilliance of the topcoat, some sanding of the surface is usually performed to remove or eliminate defects before the body enters the next application step. A common challenge in the current E-coating process is that of water spots that contaminate coated surfaces. If water spots are present, they have removed NY sanding. Hence, it is necessary to use well-deionised water& routinely monitor its conductivity. Alternately, operators also add surfactants or ultrafiltrates to the water rinse zone to eliminate or manage water spots
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