The problem of meeting the thickness of dry film specifications

In recent years, there have been some interesting developments in the way marine coatings and linings are specified, which have inadvertently created a situation that makes it challenging to meet the paint specifications currently written.

First of all, there have been challenges in meeting paint specifications due to the subjective nature of certain inspection assessments, such as visual assessment of surface cleanliness, rust and rust removal, dust removal, etc.

Secondly, as of 2008, there was considerable demand for ships from the shipbuilding boom and strong shipping market, and attractive charter prices encouraged shipowners to embrace newbuilding as soon as possible. Now that market conditions have deteriorated, owners are more cautious and more cautious about what is unacceptable and what is not. As a result, the standards have tightened.

In addition, the advent of the International Maritime Organization (IMO) Performance Standard for Protective Coatings (IMO PSPC) has increased the focus on protective coatings in all areas of ships, especially ballast tanks. In particular, PSPC introduced the concept of a minimum dry film thickness based on the 90:10 rule.

This article focuses on the application of a problem that meets the requirements of a specified dry film thickness (DFTExamples of problems encountered, it is considered to be well understood and the most objective elements of application. The paper will show that even this most basic aspect of this paint specification is neither well understood nor clearly defined.

introduce

Due to the subjective nature of some assessments, there are concerns about meeting paint specifications. For example, when using a surface comparator, visually assess surface cleanliness, rust and rust removal, dust and profile height.

During times of economic prosperity, there is huge demand for shipping, and attractive charter fees encourage shipowners to accept ships as soon as they are ready. Now that market conditions have deteriorated significantly, owners are more cautious and acceptance standards are becoming more stringent, especially for paints. Owners are frustrated with what is now seen as a poor quality performance, which is acceptable in times of economic prosperity, but unacceptable in the recession of the shipbuilding industry.

In addition to market changes, the advent of the IMO Performance Standard for Protective Coatings (IMO PSPC) is increasingly focusing on coatings in all areas of ships, especially ballast tanks. In particular, PSPC introduces a minimum dry film thickness based on the 90:10 rule (DFTThe concept of ).

This article focuses on satisfying the requirements for a specified dry film thickness (DFT), which is considered to be well understood and the most objective element of paint application. The paper will show that even this most basic aspect of this paint specification is neither well understood nor clearly defined. If the coating process does not meet the requirementsDFT，Then the service life of the coating system will be affected to a certain extent. This degradation in performance can manifest itself in a number of ways, for example, lowDFTThe chance of corrosion is increased or highDFTThe probability of cracking increases.

This study reveals that the current way of specifying paints is insufficient,TDSonDFTProbably quite misleading.

For more information on how to do this in the Technical Data Sheet (TDS) and other guidance literatureDFTSuggestion.

The conclusion was reachedDFTThe range is more specific than the specificDFTThe value is more important. The range will reflect any minimum/maximum values recommended by the paint supplier. The challenge is specifying what the application can achieve.

specification

The key elements of any coating specification are required for the individual coatings that make up the protective coating systemDFT。 The values used in the specification are usually taken from the paint manufacturer's technical data sheet (TDS）。 atTDSA value or range of values is usually provided, for example, 125 microns or 125 - 150 microns, and usually refers to a single coating. If the protocol is 2 x 125 μm, it is specifiedDFT（S DFT) is 250 μm. Consider a typical specification, as required by the IMO PSPC for seawater ballast tanks in most areas, but do not include repairs or installation of joints.

This may seem fairly straightforward, as do the underwater hull specifications detailed below. However, the actual situation may differ significantly from the specification, as shown in Figure 1 below.

If the application of the scheme is based on the cleanliness of the surface and the ultimateDFTCheck for evaluation, then the scheme is likely to be accepted, despite the low values of epoxy corrosion protection and self-polishing antifouling layer thickness.

Keep in mind that many major commercial shipyard programs now only give the owner's representative an assessment of cleanliness and finalityDFTchance, then the application has little or no relevance to the given scenario. Combined with the complete lack of application documentation (even if the coating technical documentation needs to be developed in accordance with the requirements of the PSPC), considerable problems arise when trying to determine the cause of service failures.

This deviation from the scenario specification often results in degraded performance of the entire scenario in the service. The degree of performance degradation may vary depending on the type of product and its performance requirements.

Figure 1: DisplayDFTand the scheme of the coating cross-section

Figure 2: Total number of read pairsDFTImpact of the assessment- Source: Francis RA

standard

Determine the coatingDFTOne of the first issues is to assess the total number of structures required in order to have a multi-faceted view of the structure in question. Francis I provide a good analysis of the requirements of each key criterion, as shown in Table 1:

Table 2 shows how the readings are taken and the minimum of the various standardsDFTRequest.

As shown in Figure 2, Francis also shows how the number of readings in a region affects the overall results. The chart shows the average of the coating despite specifying a thickness of 85DFTReadings in μm can be approximated by a relatively small number of readings, and the minimum and maximum values recorded are significantly influenced by the readings taken.

Technical Data Sheet

There are guidelines and standards, and what matters is the recommendations made by the paint manufacturer. The key document is the Technical Data Sheet (TDS）。 When considering the main paint suppliers of ballast tank coatingsTDS, which can make some interesting observations:

One. Technical Data Sheet (TDS）

b. System guidelines/recommendations

The commonly used information is:TDS, which is rarely given to on-site personnel, or only mentioned when something goes wrong. For example, the typical problem with cargo hold coating is the time period after the coating is applied before the first cargo is loaded, such as coal. This is rarely includedTDS,But it often appears in the guidelines for the use of coated products.

This is what comes upTDSand what information should be included. For example, many anticorrosive coatings will be designated for non-cargo hold use, so the timing of the first batch of coal loading is irrelevant to these specifications.

The end result will be an increase in the number of data tables required or the length of data tables under different service conditions. The requirements are inTDSA number of references to the coating system guidance document are clearly identified in a prominent location on (i.e. "This datasheet should be read in conjunction with the Marine Systems Guidance").

TDSonDFTA number of factors are commonly considered, including:

• Optimized product performanceDFTvalue (although the paint supplier will test a range of thicknesses to reflect real-world conditions).

• Assigned to a specific product for a specific purposeDFTUses offered by competitors will also be considered. If there is a significant difference, then this increases the cost (higher) relative to the competitionDFT) or lower cost (lowerDFT）。

• If applied by airless spraying, the coating will coalesce to a minimum.

• Other commercial or practical considerations.

atTDSGet it oftenDFTValues or sometimes ranges are used for a given typical termDFT The data sheets on the main paint suppliers are:

• Typical thickness

• Recommended dry film thickness

• Film thickness

• The indicated film thickness

• Recommended system dry film thickness (minimum and maximum)

None of these terms are consistent with the "nominal" used in the IMO PSPCDFT Terminology.

For more general uses, work specifications may deviateTDSgivenDFTNot unusual. For example, a typical system for the cargo hold of a bulk carrier can be 2 x 150μmDFTwhereasTDS125-150 μm can be givenDFTRange. However, the maintained specification may be 3 x 100 μm. Therefore, it is specified for bulk carriersDFTIt seems to be lowerTDSThe minimum value given.

In general,TDSThe content of the documents makes it clear that they are advisory and often carry a statutory disclaimer indicating that the values they give are based on laboratory tests and can be updated in the light of practical experience.

What is not clear isDDSatTDSThe role of value. Is it a minimum, a nominal value, an average or something else? What does typical mean? If it is recommendedDFTHow does the coating performance change if the application deviates from the recommended value? What does a given range mean, is it a maximum/minimum or simply some guideline value?

Paint company guidelines often don't address these ambiguities. This can make it difficult for end users in the event of subsequent failures. (One could argue that providing relatively vague data may be appropriate for paint suppliers, as it is difficult to assess in the event of a failure of subsequent claims).

When reviewing the recommendation form, most paint suppliers believe that it is goodDFTIt should not exceed 2 times as specified (per coating and the entire scheme) and allow up to x2.5 access within the area (complex structural area).

It is important to note that since there is no recommendation from the paint supplier, the ISO 12944 standard refers to the specifiedDFTThe maximum value is ×3, while it is known that for Korean manufacturers the maximumDFTValues are typically specified as ballast tank coatings up to 2,000 μm, approximately the PSPC nominalDFT 6x, and higher than the recommended x2 designationDFTGuide. While this can be convenient for yard production capacity, when nominalDFTAt 320μm, how does this affect the performance of the coating system?

It seems necessary for paint suppliers to carefully review the content and details of the datasheets and recommendation sheets to capture the actual issues at hand, especially with regard:DFT。

What is specifiedDFT？

When paint specifications are given onlyDFTvalue, e.g. 2 x 160μm, what is the interpretation of that particular value? Is it a minimum, average, pattern, or maximum?

Most people think of this number as a "nominal" or "average (average)" number, i.e. it is not an exact number. It understood that the "good practice" limits set in the guidelines and recommendations would vary (the maximum value was usually specified at x2DFT), and the minimum value can be determined by the physical ability of the paint film to be bound or by the minimum value rule, such as the IMO 90:10 rule. However, the author has met inspectors, shipyards, owners and paint companies, who often think that specifiedDFTis the minimum.

The Chambers Dictionary definition of a noun includes: a property or property of a name that is called, not actual, belonging to or relating to the name. In the engineering sense, the term "nominal" is often used in conjunction with dimensions and in the DFTThe case is usually accepted as meaning nominalDFTMay not match any of the appliedDFTReading.

This means that the nominal size is accompanied by a tolerance. In shipbuilding, we can see that the maximum value recommended by Paint Company is:DFT 2 fold, so we have an upper limit on the nominal value. Rules are often applied to set minimum values, such as 80:20 or 90:10 rules.

Let's consider this for 2 x 160 μmDFTWhat the specs mean. First of all, it is assumed that 160 μm is nominalDFT。 (Maybe it should be a meaning or a pattern.) ）

The mean will require that the average readings in a given sample (sample size see: SSPC-PA2, ISO19840, IMO PSPC) will be given by the arithmetic mean, while the pattern is a common set of readings.

For example, take the following set of numbers: 1, 2, 3, 3, 3, 3, 5, 5, 6, 7, 10, 10

Figure 3: Mean of the normal distribution

The difference between the average and the pattern can also be illustrated in figures 3 and 4.

For a normal distribution, the average means that you can expect to find 50%DFTThe value of the reading, below which 50% of the reading can be found.DFTThe meter software is normally distributed and provides statistical summaries, typically including:

• averageDFT

• utmost

• Minimally

• standard deviation

• range

• reading

The average is the average of the sets of readings taken, with the maximum being the highest reading, the minimum being the lowest reading, and the range being the difference between the maximum and minimum readings.

Figure 4: Skew distribution shows the difference between the pattern and the mean

The standard deviation is a measure of the distribution distribution shown by the curve. A low standard deviation will indicate a delicate process that can accurately perform low variations, and a minimum of the range (the difference between the maximum and minimum values). A large standard deviation indicates poor control over the application, resulting in larger variations and larger ranges.

For a normal distribution, use the following approximations:

• 66.6% of all values are within ±1σ (σ-standard deviation),

• 95.4% of all values are in the range of ±2σ

• 99.75% of all values are in the range of ±3σ

Depending on the number and distribution of readings, the pattern can be below or above the average, and will produce a curve of similar shape to the normal curve, but "skewed" towards the pattern value.

Process stability and control

In shipbuilding, the importance of accuracy in all processes cannot be overstated. It has been said that "the successful application of precision control technology, shipbuilding is quite fundamental to achieving a high level of productivity" II. We all know that it is impossible to be good, especially when it comes to coating on board. This is due to the fact that many factors in the processing create an inherent variability, which makes it difficult to control III. There are two elements to the variability of any process, the assignable cause and the random variable.

Examples of these coating application jobs could be:

While assignable causes can be addressed and managed, the inherent variability of the process can only be improved by changing the process technology. Therefore, in order to improve process capabilities, as many assignable reasons as possible need to be identified and managed.

Process stability/performance is evaluated using control charts. There are various control charts, and the simple control charts are shown in Figure 5. The key elements of a control chart are to set the upper and lower tolerances and the average. This is usually a specified value or a target value.

Figure 5 shows the concentration situation, where specification constraints or tolerances are set outside of process capability limits. If so, the process can be said to be capable of taking on the work of tolerance. However, if the specifications are limited to the upper and lower capacities, the process will not be able to perform all the work within the required tolerances. Obviously, the further constraints within the range of capabilities can set the specification limits, the more likely it is to be able to meet the specifications. In most processes, it is normal for tolerance limits to be set around the average value, such as a typical coating value of 150 μ±50 μm.

Figure 5: Simple control chart

The effect of the minimum and maximum values

You need to understand the actual impact of the minimum and maximum values. A number of coating inspections have been evaluated to see what is actually feasible.

DFTControl depends on many factors, such as worker skills, equipment, access considerations, and complexity of the structure. Very good performance is likely to be on a flat surface, while more complex surfaces will tend to increase the rangeDFTValue IV. The size of the region will also have an impact.

Data presented by Francis in 2013 showed that for small areas, the coating reading range for a single layer of inorganic zinc silicate was 85 μm nominalDFTThe range is 240 μm, with a minimum thickness of about 20 μm and a maximum of about 260 μm.

On a larger scale, the work carried out by Whitaker, Wimmer and Bohlander on the underwater hull of the US Navy carrier gives the results shown in Table 3 below.

Data from Safinah's case study on the hull of a new-build commercial vessel gives a mean of 0.18 for the ratio of standard deviation to mean (also known as coefficient of variation):

• prescribedDFT：610μm

• averageDFT：990μm

• Standard deviation: 170 μm

• The process capacity reaches 3σ480 - 1500μm

Thus, even in the relatively simple area of the underwater hull, there is a considerable range for the quality of application achieved, in which the ratio of standard deviation to mean ranges from a relatively good 0.11 for the US Navy to a relative difference of 0.44. In short, the closer the standard deviation value is to the mean (the higher the ratio), the larger the distribution of the curve. So, the process isn't well controlled for a number of reasons, so the more likely you are to get or not get an application, such as:

Weather/wind conditions, worker skills, equipment capacity/maintenance, surface roughness.

A high value of this ratio indicates that the process is not well controlled, resulting in an over-application of coatings, which can penalize a site in a number of ways:

• Increased cost of paints and thinners/cleaners

• Increase the application time

• Increase curing/drying time

• Emissions increase

• Waste increases

• Delay in establishing schedules

• Increase the utilization rate of the facility

In more complex areas, the results of the Safina study showing cargo space give a ratio of 0.19 standard deviation to the mean value.

• prescribedDFT：250μm

• averageDFT：649μm

• Standard deviation: 133 μm

• The process capacity reaches 3σ250 - 1048μm

Thus, while the cargo hold process does exhibit greater variability (higher standard deviation) than the outer shell, the ratio of mean to standard deviation is approximately the same (0.18 to 0.19).

The reason for this is relatively simple. Exterior hull schemes typically include 4 or more coats of paint compared to 2 coats in the cargo hold. of each coatingDFTThe variability is additive, so the more coats applied, the greater the variability included.

Therefore, the more steps in a process (i.e., the more paint coats in the scheme), the greater variability that should be expected, regardless of the complexity of the surface to be coated. The result of ballast tanks is also usually two coat schemes and is a more complex area, so a better comparison with the cargo tank should be provided. The ratio of the standard deviation to the mean is 0.26

• prescribedDFT：320μm

• averageDFT：602μm

• Standard deviation: 162 μm

• The process capacity reaches 3σ116 - 1088μm

Thus, for ballast tanks in the cargo holds, the standard deviation and ratio are quite high, albeit with only two coats of paint. This means that design complexity has a greater impact on the variability of the coating process rather than the number of coats. This also means that in order to maximize the possibilities of coating application, the design complexity and number of coatings should be as small as possible, but simplifying the design will bring greater benefits.

In practice, the problem is further exacerbated by the fact that not all painting work will be done by the same team. In fact, there may be more than one team per region, and skills/competencies and equipment, as well as local conditions, can vary widely.

Of course, these figures are also subject to change for ships of different sizes, while smaller vessels offer more complex/tighter structures. The authors suggest that consideration could be given to the use of a compensated gross tonnage factor (OECD Council Working Group on Shipbuilding, Compensated Gross Tonnage (CGT) System) to determine the complexity of different ship types and sizes.

Impact on the coating program

According to the requirements of the IMO PSPC and as shown in the data sheets of most paint suppliers, consider 2 x 160 μmDFTSpecification. In this case,TDSThe values on are not "nominal" and have been interpreted by the authors as either target values or averages/averages.

utmostDFT: Good practices from the Paint Company guidelines mean that the application is maximumDFTIt should be specified for each coating and the entire programDFT x2 。 This will give the maximum scheme thickness of 2×320 μm, 640 μm.

leastDFT: When applying the 90:10 rule or the 80:20 rule, the following minimum values are given:

• The 90:10 rule - 2 x 144 microns or 288 microns in total

• 80:20 rule - 2 x 128 μm or 256 μm in total.

The standard deviation of the water ballast tank application has been deduced from the previous example and is 162 μm. Therefore, if the minimum acceptable value is 288 μm according to the IMO PSPC 90:10 rule, the addition of 3 standard deviations will indicate a mean value of 774 μm (given by: minimum + three standard deviations = 288 + 3 (162) μm) and a maximum value that can be expected is 1260 μm (by mean plus three standard deviations = 774 + 3(162) μm).

The resulting average thickness of 744 μm exceeds the recommended maximum system thickness recommended by most coating suppliers, i.e. x2 is specifiedDFT(640 μm in this example), which also exceeds the x3 value specified in ISO 12904.

To achieve the required specifications:

• Minimum 288 microns

• Up to 640 microns

According to the reported Safinah data, the standard deviation needs to be 58.7 μm or about 36% of the standard deviation achieved in the field.

Reach highDFTThe problem is further complicated by the fact that if it is found low during the detection processDFTarea, for example, identifies a 250 μm area, and if it is touched by airless spraying, it will not be acquired to 288 microns or 320 microns, but may increase the thickness by 160 microns to 410 microns, compounding the overapplication problem. If a patch coating is applied through a brush, an additional 80 μm can be added. As a result, the application of any "build" outer coating can be minimalDFT，This may increase the averageDFT，and to further extend the programme beyond the recommended guiding principles.

Actual distribution: Actually, applicationDFTThe data does not result in a normal distribution, but rather a skewed distribution as shown in Figure 4. The actual data set of ballast tanks is given in Figure 6 below.

This ballast Water Tank coating is regulated according to the IMO PSPC and therefore its nominalDFTIt should be 320 μm. Analysis of these data shows that:

• Total reading: 566

• leastDFT：272μm

• utmostDFT 1326μm

• Range 1100 microns

• The average is 611 microns

• The standard deviation is not correlated

• Mode 564.5 μm

Figure 6: Thickness data from ballast tanks

The breakdown of the thickness readings in the 200 micron width band is as follows:

Given that the recommended practice would recommend a maximum of 640 microns, then 193 readings (34%) exceeded the maximum and a very small number of readings were below the minimum.

Although the average and pattern are below the maximum of 640 microns. thereforeDFTThe actual distribution of the readings results in a value greater than the specified value. Specifically, the actual distribution will tend to be higherDFTThe value is skewed, and this is minimally skewedDFTrules and intensified.

Once the minimum rule is introduced, then the average is achievedDFTwill end up much higher than specifiedDFT。 This, combined with the difficulty of coating complex spaces, leads to averageDFTClose to or greater than the x2 provided in the paint company's guide DFTMaximum.

conclusion

The variability of the coating process, the number of paint coats, the complexity of the surface and the minimum useDFTRules all result in an actual averageDFTMuch greater than the specified value. The change in the average value may be close to or even exceed the x2 commonly recommended by paint companies DFTMaximum.

It is clear that the shipyard applies more paint to make up for the lowDFTthan inDFTIt is easier to remove paint in cases of too much. Although for reaching the specific minimum of the coating to be performedDFTIt is important, but the use is minimalDFTRules lead to higher than expectedDFTThere is a real danger in readings, and this can also lead to degradation or even failure of the coating.

The problem for yards is that this extra paint not only increases the man-hours and cost of coating, but also extends the coating and drying time and increases VOC emissions. The problem faced by the owners is,DFTThe implementation may exceed the recommendations of the paint supplierDFT, and in excessDFTThe impact, if any, on the performance of the coating may not be fully understood.

Therefore, the reality is that unless current coating application techniques are improved, the range of readings that will be obtained in practice for any given specification will depend on the number of coatings, the complexity of the structural design, the technology of the applicator, the equipment used, and so on.

Paint supplier'sTDSIt needs to be very explicit for what is being citedDFTValue. It is advisable to quote a range of acceptable minimum to maximum values for each coating, rather than some vague single value, average, or some other measure such as a nominal value that can be interpreted as a minimum. Paint suppliers should be cautious in testing their products as intendedDFTThe case may be realized on the spot and provides information about the estimated thicknessTDSData.

Therefore, the IMO PSPC specification may be better written in the range of 288 μm - 640 μm. This translates to an average of about 464 μm. The problem is that, given the complexity of some aspects of the ship's structure, for the practically attainable range, it needs to have a larger maximum, more like x3 with a nominal value of 320 μm, so the range is given 288 – 960 microns, which means an average of 624 microns (assuming a normal distribution).

Of course, this is just one example. As far as the author understands, for ballast tank coatings, in particular, the maximum limit set by South Korean shipyards is 2000 μm. While this may seem high, it is clearly an attempt to cap the spec beyond the capacity limits of the application to ensure that reprocessing is no longer necessary in such a way as to remove overly thick paint.

Introducing and using minimum rules in the specification, such as the 90:10 or 80:20 rule, results in an averageDFTExcessive increase, which is partly due to:DFTNon-normal distribution of values.

DFTThe meter is set to assume that the readings collected are represented by a normal distribution and provide statistical results related to this distribution. But, as it has been proven, the ship'sDFTReadings tend to be off-distribution, which can cause pairsDFTConcerns about how the meter's data is presented.

However, more statistics can be used for our assistance, but there is a need to review how we collect themDFTReading. If a single reading is not considered, then the requirements of SSPC-PA 2 are taken into account, and 3 readings are required for point measurements. This "grouping" of readings often results in the data being forced into a normal distribution. (This is a result of the Central Restriction Theory-CLT), which is why the SSPC-PA 2 approach results in the need for lessDFTReading. If you don't group the readings, you'll need more readings to call the CLT to generate a normal distribution. This was the basis for Shewhart's work in 1960 on statistical process control – the theory of control charts proposed by Grant and Leavenworth, McGraw Hill. The more readings are collected, the better the overall condition of the coating in a particular area. However, in the case of time constraints, a small subset of data collected correctly can give a reasonable overview.

This study shows that the current way of specifying coatings is not sufficient,TDSonDFTmay be misleading.

suggestionTDSShould be included onlyDFTmaximum and minimum values, rather than some individual fuzzy values. This will allow each paint supplier to be sure that their product will provide the required performanceDFTRange. Of course, this adds some complexity because of the drying time, curing time, and may be affectedDFTOther data impacted, such as hours of service, will need to reflect the scope provided.

Those who are developing coating specifications should also consider:DFTThe range is more specific than the specificDFTThe value (nominal, average, or otherwise) is more important. The range will reflect any minimum/maximum values recommended by the paint supplier. The challenge will be to specify what the application can achieve.