In today's global economy significant effort is expended to continually lower the cost of products and services. Attempts to reduce costs while maintaining or improving quality and performance are nothing new to business. Most manufacturing firms have sought to squeeze cost out of their products since the inception of commerce. The addition of terms to the business lexicon like "lean manufacturing " and "cost-downs " attest to the fact that efforts to reduce costs are not going to diminish any time soon. Thus, in the coatings industry it can be expected that both original equipment manufacturers and custom coaters will continually seek meaningful cost metrics as well as ways to minimize measurable costs. In the paint and coatings market, sophisticated product finishers have for a long time looked beyond a coating's price per unit of volume or mass (i.e., dollars per gallon or pound) and have attempted to assess its applied cost. Applied cost has typically been expressed as a unit of currency per unit of coverage, such as $0.03 /ft2 .In assessing the comparative cost of two similar coatings or coating methods (i.e., ones with similar performance and application properties), a starting point has been to compare their theoretical cost per unit of coverage. Yet, as experienced users of paint are regrettably aware, every drop of paint introduced into a facility does not actually go out the door on a finished work piece. A significant challenge for most coaters is to obtain an accurate number for the cost of a painted unit. To accomplish this objective, a coater must first answer the following question, "How much raw paint is needed to obtain a given volume of cured paint film?" This is not an easy question to answer as the concepts and calculations can be relatively complicated. Most coaters rely on information provided by their paint suppliers and use very basic methods to estimate their applied cost per unit. In order to determine applied paint cost per unit, a coater must first calculate the volume of cured paint film needed (i.e., paint thickness x area covered). The area covered is a function of the raw coating's percent volume solids, the application transfer efficiency, and the coating's dry film thickness. Transfer efficiency is primarily a function of the paint application method or equipment. (e.g. There is an inherent efficiency advantage of electrocoat over spray-applied liquid coatings.) In the electrodeposition market, it has long been appreciated that post-rinsing and a well-run system allow coaters to attain transfer efficiencies in the range of 95--98%. However, this efficiency rating only takes into account the relative amount of coating on work pieces that enter the curing oven. The efficiency rating does not incorporate some attributes specific to all coating formulations (including electrocoat) that may be apparent only during and after the curing process. In applied coating cost calculations, it was frequently assumed that all electrocoats are basically the same in terms of coverage, once one adjusts for volume solids. However, recent studies indicate that all electrocoats do not offer equal coverage. In fact, applied coverage can vary significantly based on the coating formulation examined. Thus, the coater's challenge is to understand what causes this unequal coverage between different electrocoats. Film Density The more significant problem with using a dry film density measurement to evaluate paint coverage is simply that it is an obfuscating concept when applied to electrocoat. An uncured coating density might be relevant to a discussion of applied cost in a situation where paint is purchased by weight rather than by volume (i.e., purchasing powder coatings by the pound). In such an instance it is accepted that a given weight of an uncured coating with a lower density covers a greater area than does a higher density product. Intuitively this makes sense, because we understand that a pound of feathers will cover more surface area than a pound of lead. But in a situation where an uncured liquid coating is purchased by volume (i.e., by the gallon) rather than by weight, a product 's density is irrelevant in understanding its applied coverage. A lead-containing paint film is intuitively more dense than is a lead-free paint film. But a gallon of paint at 100%volume solids still theoretically covers 1,604 ft2 at a thickness of one mil whether it contains lead or not. The same fallacy (that density affects coverage) can confuse electrocoat users in situations where electrocoat is purchased by weight, or even on a cost per unit basis. One might assume that a lower density coating covers a greater area than does a higher density one. But paint coverage is a function of volume solids, transfer efficiency, and dry film thickness since it is a measure of a volume of paint (DFT over a specified area).The density of the paint making up that volume does not affect coverage. This leads us into a discussion of an additional factor that affects an electrocoat's film thickness and has significant impact on coverage and applied cost. Weight Loss The importance of weight loss in calculating a coating's coverage, and subsequently its applied cost, can be seen through a review of a typical paint cost calculation. The following formula has typically been used to calculate paint coverage and applied cost: (1,604 ft2 /gal) x (%Volume Solids)x (%Transfer Efficiency) = Coverage However, this formula either assumes that weight loss is approximately zero, or else that it is irrelevant to the coverage calculation. But weight loss does affect the dry film thickness. Some paint goes into the oven but does not come out on the finished work piece. Thus, weight loss is relevant to coverage and applied cost calculations. The following formula is suggested for a more accurate estimate of applied cost: (1,604) x (%VS) x (1 -%weight loss) x (%TE) = Coverage The difference is $0.00176/ft2 ($0.02496 -$0.02320). While this number may seem relatively small, it is approximately 7.6% of the initially estimated applied cost ($0.02320/ft2). Thus, a finisher estimating his paint cost by the conventional formula will likely encounter a negative variance of 7.6% in his applied paint cost. Using the second formula, an electrocoat applicator can more accurately estimate his actual paint cost and build that into his costing model. Study Methodology These coatings span five generations of formulations that were developed during a period of approximately 15 years. Each material was applied at its recommend pigment to binder ratio and percent bath solids. All six paints were coated over zinc phosphated cold rolled steel lab panels at dry film thicknesses of 0.8 -0.9 mil. Panels were cured at the bake parameters (time at metal temperature) recommended for each individual coating. Additionally, panels were prepared at a 25F undercure, 25F overbake, and 425F (supercure) bake. So a total of four bake schedules were considered for each of six different electrocoats. The dry film density was measured per a Dry Film Density test method (Appendix 1). The weight loss was measured per a Weight Loss test method (Appendix 2). Details From the data and charts one observation is readily apparent. Dry film density is similar for all six paints, but the weight loss varies drastically. At the standard cure condition for each formulation, the dry film density ranges from 1.37 to 1.42 g/cm ³. The mean density was 1.40 and the standard deviation was only 0.023. Thus, the standard deviation of the six film densities was only 1.7% of the mean value. The weight loss, on the other hand, ranges from 7.1--16.6% at the standard cure temperatures. The mean was 12.7 and the standard deviation was 3.5. Thus, the standard deviation of the weight loss measures was a full 27.9% of the mean value. One can conclude that the variation between products in weight loss is significantly greater than the variation in density. The general trend is that weight loss decreases as the coating generations progress (Newer generation products tend to have lower weight loss than older generations.) However, the dry film density remains consistently near 1.4 g/cm ³ for all six products. Our study confirmed that there is very little correlation between dry film density and coating weight loss. Examining coating weight loss is a much more meaningful method of distinguishing different electrocoat paints than is considering their film density. This distinction is important in calculating an electrocoat's applied cost. Weight loss has more impact on applied coverage and affects the applied cost of an electrodeposition coating more than does coating density. The weight loss of an electrocoat product is a meaningful and significant measurement that should be taken into account to accurately estimate applied cost. Obviously, all else being equal, a coating with lower weight loss is better than a coating with higher weight loss as it yields a lower applied cost. A paint applicator that ignores the weight loss of a coating is prone to underestimate both his paint usage and the actual cost of applying that coating. Acknowledgement Appendix 1. Dry Film Density Test Method Scope Measures the dry film density of a cured coating. Safety Equipment /Resources Instructions Figure 1 (see text) Appendix 2.Weight Loss Test Method Scope This method is used to determine the weight loss of electrocoats curing at 300F (149C) or higher. Safety Equipment /Resources Instructions Table 1.Summary of Results (see text) Serving the Finishing Industries. Since 1936. PF Online and all contents are properties of Gardner Publications, Inc. All Rights Reserved. |