Moisture Issues with Solvent Free Epoxies

Epoxy tank lining issues with solvent free epoxy paint and moisture during cure.

SNOPSIS:

Technical trade magazine article about moisture issues during cure of solvent free epoxy paints.

Keywords: epoxy, tank lining, epoxy paint

"Value Based Epoxies"

Progressive Epoxy Polymers, Inc.48 Wildowood DrivePittsfield, NH 03263

www.epoxyproducts.comwww.epoxyusa.cominfo@epoxyproducts.com

EPOXY LININGS – SOLVENT-FREE BUT NOT PROBLEM-FREE

Mark B. Dromgool, Managing DirectorKTA-Tator Australia Pty Ltd, Melbourne, Australia

Abstract: Solvent-free epoxy coatings havebeen widely specified and used over recentyears in Australia as linings for a variety ofimmersion service exposures, specifically forpotable water storage and tank linings for somepetroleum products. However, they have notalways delivered the durability or performancehoped for. There have been disappointmentsand some failures which could have beenprevented if a wider understanding existedabout the nuances of specifying and applyingsolvent-free epoxy materials. This paper willoutline some of the less-understood aspects ofsolvent-free epoxies that can result in poor oreven disastrous performance once these liningsare put into service.

Introduction

Solvent-free epoxy tank linings are a brilliantidea. They are usually a one-coat lining system;they can be applied at high film builds; there isno risk of solvent entrapment; some willtolerate early immersion; they can save a greatdeal of time and labour; and they combine thegenerally excellent adhesion of epoxies toprepared steel substrates with a hard tile-likefinish. Another major feature is that they haveminimal OH&S (occupational, health andsafety) issues, i.e., there are no worker exposureor solvent LEL (lower explosive limit) concernsas they do not release any solvent in changingfrom a liquid to a solid; and they are seen asbeing environmentally friendly. Other logisticand quality benefits are that they require theshipping, handling and mixing of much smallervolumes of paint on large projects; and they areeasy to inspect as they can be made in lightcolours which aids quality control duringapplication and inspection whilst in service ascompared to black coal tar epoxies or otherdark-coloured materials.

All of these positive features sound great,and clearly, that was the brilliant idea. What

then, are the down sides? There are drawbacks,but too often these appear to be overlookedbecause they weren’t part of the brilliant idea.One of the largest negatives can be the relativelack of tolerance that many solvent-free epoxymaterials have to mixing, handling, applicationand curing conditions when compared tosolvent-containing epoxy coatings.

Mixing and handing many solvent-freeepoxies is more difficult due to the higherviscosities of one or both components, andthese steps generally involve a lot morecomplicated equipment such as paint heaters,proportioning pumps, static mixers, etc. Themore complicated the equipment, the greaterthe chance that things will not always go rightin the field. If mixing and preparation of thecoating prior to spraying is not performedcorrectly within fairly narrow and sometimesunforgiving boundaries, the applied lining canbe physically and/or chemically compromised.Spraying solvent-free epoxies is more difficultand much less tolerance exists to propertiessuch as paint temperature and viscosity, spraypump capacity, lengths and sizes of spray lines,tip sizes, continuity of work, etc.

Climatic and substrate conditions are alsocritical during certain phases of curing. Itappears that certain solvent-free epoxy coatingsare susceptible to high relative humidityand/or condensing moisture during a certainwindow of time after application. This cancreate intercoat adhesion problems or blisteringif the coating system involves multipleapplications of solvent-free materials.

If these critical issues are not properlyaddressed or understood, the coating may looksatisfactory but it might not work as well or fora satisfactory length of time.

Another negative is the lower chemicalresistance of many solvent-free epoxies. Highvolume solids technology generally involves

the use of low molecular weight resins andreactants. A low molecular weight means moreepoxide per weight or length of resin whichshould mean more reactive cross link sites; butto offset this, solvent-free epoxies containreactive diluents and monoepoxides and thesefunction as internal plasticisers which can lowerthe chemical or water resistance of the curedfilm.

Because of the foregoing less obviousdrawbacks, the reasonably good rate of successof a few Australian facility owners withsolvent-free epoxy linings has not alwaystranslated well to other regions, contractors orowners. This means that giving a solvent-freeepoxy material to a coating applicationcontractor who has not had any previousexperience with it, can result in poorperformance and a lot of frustration.

History

Modern solvent-free epoxies that can beapplied by single-feed airless spray equipmentcould be seen as an ultimate evolution of theearly catalysed epoxy materials that were firstdeveloped in the late 1930s for the dentalindustry and were adopted as adhesives andthen into the protective coatings field nearly 60years ago. Amongst many other changes, thisevolution involved a sequence of incrementalformulation changes to progressively lower thequantity of solvent in the wet coating. As aconsequence, the volume solids rose from alevel of around 45 – 50% in the early years up tothe point where liquid epoxy resins wereavailable which could provide 100% solidscoating products, that were still liquid untilcured.

100% volume solids epoxies did exist as farback as the late 1950s, but these were mainlypatching and casting compounds. Sprayablesolvent-free epoxies (using plural componentequipment) and quite a range of high solidsproducts existed in the 1970s, but someconsiderable development has happened sincethat time. The transition, however, from highsolids epoxies, at say 85 – 90%, up to 100%solids (or theoretically solvent-free) has not

been a small technical challenge. Modernsolvent-free epoxy materials are muchadvanced from their earlier counterparts in thatthey are formulated using different resins,curatives, additives and pigmentation and theirapplication equipment requirements havechanged.

In Australia, the first use of solvent-freeepoxy materials as tank linings for potablewater started in about 1989. A major urbanwater authority who managed a large quantityof tanks had a number of OH&S (occupationalhealth and safety) incidents – principally burnsfrom hot materials – with their workers andcontractors using a hot-applied bitumen lininginside steel water reservoirs. It was alsodeemed untenable to continue using thesematerials in confined spaces due to theinevitable vapour and fumes from the heatedbitumen.

At the time, there were many other potablewater tank lining products that were still inwide use throughout Australia. Besides thehot-applied bitumen, the principal alternativeswere a thin coat of either a solution vinyl or anepoxy primer applied over zinc silicate, somepure epoxy systems or the more ubiquitous coaltar epoxy linings. Some authorities still hadpipelines and tanks lined with coal tar enamel.

A local coating manufacturer introduced thewater authority to a new material that they hadobtained from their overseas parent/affiliate.This material was a two-component, amine-cured solvent-free epoxy. After an initialexperiment where the solvent-free product wasapplied by brush and was seen as working verywell, the authority decided to widen the scopeof the trial and use this coating as the sole liningfor most of their steel potable water reservoirs.

Other coating manufacturers soon enteredthe market with their own solvent-freematerials, most sourced from their respectiveU.S.- or European-based coatings technologypartners. Unfortunately, most coating suppliersand almost all coating contractors who becameinvolved with these solvent-free materials, didnot fully appreciate the differences that existed

between high solids epoxy linings – which mosthad some reasonable experience with – and the100% solids products that had become the newrage.

In essence, the facility owner did not knowthat much about the peculiarities of the newproducts he was specifying; the coatingsuppliers were learning as they went and werenot able to provide an appropriate level ofpractical advice to coating contractors; and theapplicators themselves had to try and adapt tolining products that proved to be quite difficultto handle and relatively intolerant to slightchanges in climatic and substrate conditions,e.g., changes in relative humidity, temperaturedrop after application, etc., as compared to themore common high solids epoxy linings.

In hindsight, the decision by the waterauthority to universally adopt the use ofsolvent-free epoxy linings, was more on thebasis of a perceived environmental benefit; i.e.,to reduce solvent emissions to the atmosphere,to minimise possible taint of the water,(assuming that solvent would be the main itemcausing taint), to avoid extractables from thebitumen or the other lining types affectingwater quality, and/or to reduce OH&S risks;rather than for any quantifiable benefit as atank lining that could not be offered by othermaterials. This means that the water authoritychose solvent-free epoxy for their ownperceived set of reasons, and this had little ifanything to do with how difficult theseproducts might be to apply or what othernegative consequences could result.

The water authority’s specification requiredthat the contractor apply the solvent-free epoxyin at least two coats, to achieve a final minimumDFT of 500 microns (20 mils). All work was tobe closely supervised by an independentinspector representing the owner’s interests,and high voltage spark (continuity) testing wasmandatory. The use of an epoxy holdingprimer was optional, but was frequentlyadopted by contractors as it usually assistedwith logistics, i.e., each day’s blasting could beprimed with a quick-dry coating, thereafterallowing a much larger surface to be coated

with the solvent-free epoxy with fewer joins,once all the surface preparation and primingwas completed. Most reservoir painting wasperformed in the winter months (July andAugust) as water demands were typically lowerthrough this period.

All too often during these early tank paintingcontracts, many of the people involved hadvery little prior experience with solvent-freeepoxy materials. There was no shortage ofhistory with more conventional epoxy linings,and few people realised or were told that thesenew materials were subtly but significantlydifferent. Typically, the coating supplier’stechnical support person was the local salesrepresentative who was still quite low on thelearning curve with these products; theinspector had inspected epoxy linings beforebut not the new solvent-free types; and thecoating contractor’s staff had painted tankswith all types of epoxies in the past, and hadgenerally coped quite well. Whilst confidencelevels were probably fairly high; there wasmore than enough inexperience and ignoranceto go around. In truth, no one knew what theydidn’t know.

Not aiding this situation, was the scarcity ofgood practical information from the coatingmanufacturers. Probably one of the reasons forthis is that many of the manufacturers simplydid not know what it took to apply theseproducts under field conditions. Quite anumber of product data sheets from the periodhad no guidance on how to mix and preparethe coating for spraying; and most quotedoverseas spray equipment setups that were astraight copy of the information provided bytheir overseas technology partners and wascommonly not available locally or had no directequivalent. Similarly, information on suitableclimatic and substrate conditions before, duringand after the coating was to be applied wasgenerally absent or worthless; anddehumidification (DH) equipment or climatecontrol was typically not mentioned.Consequently, the manufacturer’s sales staff,the inspector and the coating contractors wereoften left to their own devices.

Problems

Even though the water authority that firstused these materials (and has continued to usethem almost exclusively) is essentially happywith the performance and durability of thesolvent-free epoxy-lined reservoirs, theselinings have actually been far from problem-free. The list of problems that have appearedover the years includes areas of soft coating;delamination between coating layers; blisteringfrom the substrate or between coating layers;the formation of an exudate or sweat;crocodiling of the film’s outer surface; poorperformance along stripe-coated areas; poorfilm flow or formation resulting in pinholes orother defects; and the appearance of a “chalky”film.

Other disadvantages of solvent-free epoxiesas compared to their solvent-basedcounterparts are shorter pot lives duringapplication; inferior wetting of the substrate;lower bond or adhesion strengths; lowerchemical resistance; a less uniform filmstructure; higher internal stress levels; higherrisk of delamination if multi-coat applicationsare required; and less effective maintenancecharacteristics. A potentially lower chemicalresistance is very important, because highquality (e.g., potable) water, is one of the mostdifficult cargoes to hold in a mild steel tank – itis more damaging to an organic film thanseawater or even many chemical products.

A Case History Example

After only three years of service in a potablewater reservoir, large blisters were noted inparts of the epoxy lining on the tank shell. Theblisters were water-filled and originatedbetween the two coats of solvent-free epoxy.Additionally, large areas of the second coat ofsolvent-free epoxy had disbonded and theadhesion between these coating layers wasvariable, generally between fair to very poor.The first coat of solvent-free epoxy was securelybonded to the thin-film epoxy primer and nocorrosion of the steel substrate had occurred.The visible problems were principally confinedto the immersed surfaces below the normal

upper water level. The tank was about 44metres diameter by 12 metres high (144 feet by40 feet) and consisted of a steel shell and roofstructure fitted to a concrete floor.

When the pattern of disbonding andblistering was studied, the extent and type ofproblems seemed to be strongly related to thevertical bands of the coating on the inner tankshell that corresponded with the repetitivesequence of applying the first coat of solvent-free epoxy from a scissor lift platform. Where avertical band with very bad blistering andadhesion was found, ones of lesser intensitywere usually alongside, improving somewhatwith successive bands, and then the patternrepeated itself.

For example, working clockwise around thetank, the first vertical band between 12 and 1o’clock was quite good, between 1 and 2 hadminor problems, between 2 and 3 was worseand from 3 to 4 o’clock was seriously affectedwith blisters and delamination. The bandbetween 4 and 5 o’clock was good, between 5and 6 had minor problems, and so on. The fieldrecords indicated that both coating layers ofsolvent-free epoxy had taken about three daysto complete right around the tank, and thevertical bands which were in the worsecondition, were usually the last areas coated ina work shift, typically ending in the lateafternoon. Adjacent vertical bands in bettercondition were sprayed earlier on the same day.The reservoir was located at the top of a hilland as was typical, the relining work wascarried out over the winter months, July andAugust.

Underneath the disbonding second coat ofsolvent-free epoxy, could be seen vertical linesof dots or tear-shaped streaks. The fingerpointing between the owner, the coatingsupplier and the contractor includedaccusations of painting over water, given thequite unique pattern of dots. The contractorinsisted that the tank was always dry whenthey applied the coating materials, althoughafter some searching of the records it was foundthat the tank was sometimes wet withcondensation first thing in the morning due to

an overnight drop in temperature andconsequential rise in relative humidity.Predictably, the owner’s inspector and thecontractor’s QA system did not allow paintapplication to start on the following day untilconditions improved and the surface was dryand warm, which was typically by latemorning. The contractor was very insistent thathe had applied the coating fully in accordancewith the owner’s specification and themanufacturer’s written product data sheet.

The facility owner’s specification containedtypical details with respect to applicationconditions, humidity and restrictions of climaticand substrate temperatures, however, it was farfrom comprehensive. The specificationcontained the following instructions beyond therequirements to comply with themanufacturer’s product data sheet and writtenrecommendations:

“ … no coating application shall be carriedout if the temperature is likely to drop belowthe minimum application temperature orbefore the previous coating has cured. Epoxypaints shall not be applied with ambienttemperatures below 10°C, … or when thetemperature of the steel surface … is lessthan 3°C above the dewpoint, or at a relativehumidity of greater than 80%.”

“The (blast holding) primer should not beallowed to become wet with ponded water,etc.”

Ironically, the specification included theabove comment that the epoxy primer was notallowed to be wetted, yet it was silent withrespect to the same restriction for thesubsequent solvent-free epoxy layers.Specifically noteworthy was the absence in thespecification of instructions relating to climaticor substrate conditions after coating applicationand during the curing phase, and there was norequirement or suggestion to use DHequipment.

The manufacturer’s product data sheet thatwas in force during the contract periodcontained no details at all of climatic or

substrate conditions that should exist during orafter application.

The coating supplier insisted emphaticallythat there was no potential for their solvent-freeepoxy to suffer from any exudate problems,their reasons being that it was cycloaliphaticamine-cured, and …“these materials don’t dothis”. The MSDS for the solvent-free epoxycuring agent noted the presence of an organicamine, listed as Isophorone Diamine.

However, the pattern of the failure wasrevealing, and the subsequent laboratoryanalysis using IR (infrared) spectroscopy andATR (attenuated total reflectance) IR wasconvincing. The laboratory results showed thepresence of an amine or an amine salt materialand a hydrocarbon-based exudate between thetwo solvent-free epoxy coating layers. Thesematerials were found in the blister liquid andbetween the two high build coating layers bothabove and below the waterline.

What everybody ignored and failed toappreciate, was that the climatic and substrateconditions after paint application are as vitallyimportant as those before and during. Whatapparently happened after each day of sprayingthe first coat of epoxy high build, was thatfreshly applied coating system was subject tocondensation with water from the coolingnight-time air, within hours of being applied.The last vertical band of the day’s work wasclearly the most seriously affected with bandsthat were sprayed earlier in the day being lessso. With the coating still being “green” in theseareas, i.e., whilst it was still in the process ofcrosslinking and drying, its outer surface wasliterally subject to immersion conditions withvery pure condensed water. The water dropletson the fresh coating reacted with some of theamine-containing ingredients in the film, and,due to the water solubility of the amine, thesematerials were pulled from the film by thecondensation.

Some amine compounds have good watersolubility and can be drawn into condensedmoisture on the surface of a partly cured epoxycoating film. The next process involves a

chemical reaction between dissolved CO2

(carbon dioxide) in the water and the amine inthe epoxy. CO2 is much more soluble in waterthan is O2 (oxygen), so even though there isnearly 21% O2 in the atmosphere and only avery small percent of CO2, there is a goodchance that there can be enough CO2 dissolvedin the water. This forms carbonic acid whichchemically reacts with the amine, neutralising itand forming a class of compounds calledcarbamates. This is the archetypal amine blushor amine sweat. The presence of moistureprovides the opportunity to extract the amine;the dissolved CO2 allows it to be converted to acarbamate – without water or carbonic acid thisdoes not happen. Carbamates are oily or greasyand do not react with epoxy. The presence of acarbamate does not necessarily hurt theperformance of the first coat of epoxy, fromwhich it was formed, i.e., the loss of a smallquantity of amine is insignificant. The primaryconcern is for disbonding of a subsequent coatlayer due to the presence of the greasy film.

In some instances the condensation becameso heavy that drops started to run down thevertical walls, enlarging in volume as theywent. These became the vertical lines of dots orstreaks that were visible after the second coat ofsolvent-free epoxy had delaminated. Whilstwater is good at solubilising the amine,carbamates are much less soluble in water. Themost effective way to remove carbamates fromthe surface of a curing epoxy is to dissolve themin either an alcohol or a ketone solvent,however, no one knew that they were present.Certainly, neither the specification or theproduct data sheet made any mention or gaveany warning of the conditions that could causethese exudate materials to develop. Even thecoating supplier, who was the one party whocould or should have known that an exudate ofamine leading to a carbamate was possible,absolutely believed that the coating productwas immune to this effect. If the manufacturerdoesn’t know or doesn’t tell, who else is likelyto?

If in the unlikely event there was not enoughCO2 present in the condensing moisture – andgiven a very large collective surface area of the

small water droplets, this is doubtful – then theamine that did not form into a carbamate film,would essentially remain as amine. It is quiteprobable that the amine would be dissolvedinto the film of condensed water, but it wouldnot be removed from the surface unless thewater dripped off under the influence ofgravity. It is expected that any dissolved aminewould be redeposited onto the outer surface ofthe first coat of solvent-free epoxy when thewater droplets evaporated.

The carbamate material would be also bepresent, but it would be beneath the film ofcondensation until the water evaporated in thewarmth of the next morning’s weather. It wouldessentially remain there unless it was exposed toa material in which it was soluble, i.e., either analcohol or a ketone solvent, which neverappeared because the next coating layer wassolvent-free. Even though the surface probablyappeared to be satisfactory when it wasovercoated, it still had a thin film of exudatepresent.

Even if the inspector or the coating supplierhad taken the opportunity to look at the coating,there is a very good chance that the presence ofan exudate would have been missed, primarilybecause it was only every third or fourth verticalband inside the tank wall that was seriouslyaffected. Due to the pot life of the coating andthe time to reach a surface dry condition wherewetting the surface with condensation would nothave a negative influence, many of the verticalbands of coating would probably have beenquite alright.

The exudate that remained after the waterevaporated would be mostly carbamate. If a newcoating layer was applied, this material has noability to react with or be absorbed by the freshlayer of wet epoxy – it will remain sandwichedbetween the coating layers as a greasy film. It isworth stressing that the solvent-free epoxy doesnot contain any solvent, and particularly, it doesnot contain any alcohol or ketones, which somesolvent-based epoxies do have. Therefore, thecarbamate will persist. If there is any unreactedamines present on the surface, these will likelyreact with epoxide groups in the new coating

layer, this becoming part of the regular coat-to-coat bonding mechanism of epoxies.

The laboratory analysis showed that there wasa “an amine or amine salt” present in the blisterliquid and also on the underside of the outercoating layer of solvent-free epoxy. There wasalso an organic oily material that the analystsimply indicated was “aliphatic carbon-hydrogen,” in nature without being specific as towhat this was. If a basic (alkaline) material isreacted with an acid and neutralised, the productis often described as a “salt” or sometimes(depending on the reactants) as a “soap.” The IRspectras detected the reaction product from thebasic amine and the carbonic acid and describedthis as an amine salt. In fact, this is thecarbamate. It is also noteworthy that thehydrocarbon is also a by-product of the samereaction. As the amine (NH2) contains nitrogenand hydrogen, and the carbonic acid logicallycontains carbon; one reaction product is ahydrocarbon, which would explain the oily film.

Why had the coating film responded the waythat it did after it was immersed? In essence, theoutcome was totally predictable. Even thoughthe cured system of an epoxy primer and twocoats of solvent-free epoxy had been tested by avariety of means after it was cured – destructiveadhesion testing excepted – the presence of avery thin film of a greasy carbamate between thetwo high build layers would be very hard todetect. Once the potable water was introducedinto the tank, water vapour permeation throughthe lining would have started. This is a totallyexpected phenomena and is typical of a semi-permeable membrane when exposed to anaqueous cargo.

As the water vapour reached the carbamatelayer it will have found an interfacial zone whereit could start to accumulate, simply because theadhesion was lower along the plane where thecarbamate was present, and therefore, aconvenient cleavage zone existed. Theaccumulation of molecular water and thesubsequent blistering was not due to solublematerial per se between the coating layers –because carbamates have very low solubility – so

the water transmission was not osmotic butsimply regular permeation.

It is important to stress that the particularsolvent-free epoxy coating formulation that wasused on this tank is not defective. It is just thatthe coating contains certain amines that areabsolutely vulnerable to the well recognisedproblem of amine blush. The curing coating isonly susceptible to specific conditions for acertain length of time after application, and inthis instance, the problem was mostly limited tothe last vertical band or so that was appliedduring a daytime work shift. Coatings that areapplied late in a day are often a concern from asolvent retention point of view – this is a verydifferent problem.

Who should bear most of the responsibility ofthe foregoing case history? Of all the partiesinvolved; the facility owner, the coatingmanufacturer, the inspector and the contractor;who should have had the most knowledge abouthow these new coating products should behandled and applied, and who should havemade it their business to be the disseminators ofthat knowledge? It is worthwhile going throughsome of the history of the project and relatingproduct data sheets, specifications and otherwritten information to a timeline to see if apattern and an answer emerges.

The tank was prepared and painted in July1993. The manufacturer’s product data sheet thatwas current at that time was dated May 1992.This data sheet contained absolutely no guidanceinformation at all regarding climatic andsubstrate conditions.

When the lining problem was discovered, itwas three years later, i.e., in August 1996. Therehad been two updated versions of the productdata sheet over the intervening period, the firstdated October 1993 and the second one datedApril 1995. These two latter data sheets werealmost identical to each other, the maindifference being an extension to an overcoatingtable if the lining had been exposed to directsunlight. With respect to recommendations orrestrictions for climatic and substrate conditions,both of these versions were equally silent. There

was more information regarding the heating ofthe paint through inline heaters or heated hoses,but essentially, there was no more applicationadvice than was contained in the May 1992version. There was an additional systeminformation sheet that was referenced on the twolatter data sheets, but this similarly had noguidance information on climatic or substrateconditions. Even a separate section in the coatingdata sheet manual on relative humidity andsubstrate temperature, offered no extra advice.This was extremely general and referred mainlyto climatic conditions whilst the coating or liningwas being applied, rather than afterwards. Theonly oblique reference was the following:

“In general reduction in temperature leads to arisk of condensation. For instance steel cooleddown during the night will often showcondensation and this will not evaporate untilthe steel is heated up again by sunlight orother means.”

This does not warn against the possible effectsof water condensing on the coating after it hasbeen applied, and this information sheet wasdated February 1994, at least 20 months after theoriginal application.

During the finger pointing after the tank wasfirst emptied and inspected, the coatingmanufacturer went into print to both thecontractor and the tank owner, and:

• Accused the contractor of painting overwater, stating that the existence of water inthe blisters and the vertical lines of tear-shaped drops indicated that water waspresent when the second coat of solvent-freeepoxy was applied;

• Presented and compared weather bureauinformation from an outdoor monitoringstation several kilometres away from the site,to the contractor’s field records from insidethe roofed tank and claimed inconsistencies;

• Advised that their tests of the blister liquidshowed the presence of a solvent, and theyestimated it to be present at a level of 20%(implying that the contractor had addedsolvent against specification and data sheetinstructions);

• Later amended their accusation that thecontractor had added solvent and admittedthat the organic components in the blisterliquid were actually from the curing agent ofthe epoxy, which leached out when in contactwith water condensation (which was knownto have been present in the early mornings onsome days);

• Concluded that, even though substratetemperatures would have risen to well abovethe dewpoint later in the day, that “moisturewould still have been present and it is thepresence of this moisture that is the likelycause of the blistering and poor intercoatadhesion”;

• Introduced a new set of climatic guidelines(not previously published) that required thatthe relative humidity is to be below 50% atapplication temperatures between 5 and10°C. Note that the temperature at the timeof application was always above 10°C, as perthe specification;

• Implied that the contractor had notconformed with the new applicationguidelines for climatics, even though thesewere first issued three years after the coatingsystem was applied;

• Advised that their solvent-free epoxy “is notdesigned for underwater curing or curingwith a wet surface.” This advice was firstprovided nearly six weeks after the blisteringproblem was first discovered; and

• Confirmed again, in writing, quoting theiroverseas technology source, that theirsolvent-free epoxy is not susceptible to aminebloom, and neither was this phenomenaobserved whilst their National TechnicalManager was on site and performed apersonal inspection.

The facility owner’s specification did havesome omissions and shortcomings, specificallyrelated to the climatic and substrate conditionsthat should have existed after the coating systemhad been applied, not just prior to and during. Italso failed to call for dehumidification or anyother forms of climate control. However, it isquite likely that some of the key details in theowner’s specification were actually led orprovided by one or more of the suppliers of ashort list of approved solvent-free epoxy

products that were contained in an appendix tothe specification. If the manufacturer of theproduct used on this tank, who had beensecuring the largest share of the tank liningsupply over the previous few seasons, had nottaken the opportunity to advise the owner ofmeasures that the specification should containthat would ensure a higher quality product andlower both of their risk of problems developing,then this could be seen to be an act of omission.

It is also worth questioning where the coatingsinspector and the contractor would turn to obtainrelevant and reliable information on new coatingproducts so they can do their workprofessionally and accurately. It is easy toassume that having experience with other highsolids epoxy tank linings would suffice for bothof these parties, but the new solvent-free epoxymaterials are very much different to many of theold products, so equipment, practices,assumptions and understandings are not oftensatisfactory.

Clearly, the manufacturer of any new coatingproduct should provide comprehensiveinformation that will reliably aid coatingsinspectors and contractors that are reasonablyexperienced, to properly prepare andsatisfactorily apply high performance coating orlining products. In the example of this casehistory, it seems that the coating manufacturerdid not adequately do this important task.

There is an ironic twist in this case whichinvolved the owner dissenting with one of thekey recommendations of the solvent-free epoxymanufacturers. The owner wished to have well-applied and defect-free linings for his watertanks. To ensure or encourage this, the ownerspecified that the contractor was to apply thehigh build system (excluding the primer) in atleast two coats, the theory being that there wouldbe less possibility of concurrent pinholes if thesystem was applied in more than oneapplication. High voltage spark (continuity)testing of all surfaces was also required. Whenthe program of using solvent-free epoxy liningswas initiated, a number of potential coatingsuppliers advised the owner that their productswere normally used as a single-coat system.

Presumably, this was on the advice of theiroffshore technology sources. Not only did thismake sense for a technical point of view,(discussed shortly) but it also had a majorcommercial element.

One of the main selling points for the use ofsolvent-free epoxy materials as linings is thatthey would save application time and labour,even if they were slightly more tricky to apply.This had all the predictable benefits of a quickerturn-around for the tank, less cost for scaffolding,a lower labour input, etc. In spite of thearguments by the manufacturers, the ownerwould not initially relent to a one-coat system.(He did much later when some contractorsproved that they could apply a defect-free liningin a single application.) In fact, the owner wouldnot list as an approved product, any solvent-freeepoxy material that was required to be applied inone coat. Due to the potential volume ofbusiness and so they were not excluded from theapprovals list, most manufacturers issuedproduct data sheets that had a recommendedcoating system that complied with the owner’spreference for a two-coat application.

It appears that the technical reasons againstthe use of two-coat systems was not clearlytelegraphed to the owner, to a degree where itcould influence the specification for this andother water tanks.

The first question that this raises, is: whatexactly was the state of knowledge of the coatingmanufacturer with respect to the parameters forsuccessfully using their new product at the timethat the lining was being applied and when itwas later found to have a major problem? Atthose same times, what knowledge andexperience did the sales or field tech serviceperson have versus the formulators or seniortechnical people? Did the backroom technicalpeople provide good and complete advice andguidance to the sales person to aid him toproperly sell and service this new product in thefield, i.e., giving accurate, practical guidance tothe contractor, the inspector and to the owner?

In short, the tech service function wasprobably performed by someone who was very

low on the learning curve with solvent-freeepoxies. Similarly, the inspector most likelydidn’t have any relevant knowledge of theproducts he was commissioned to look over, andthe coating contractor was learning as he wentalong because no one had provided him withany advice or guidance, written or otherwise,about how these new materials need to behandled.

What was driving this insane and fatedmission? The facility owner wanted to get awayfrom the OH&S issues of using hot-appliedbitumen linings; he felt that there wereenvironmental brownie points by using a liningthat did not release any solvent; he predicted thatwater taint from solvent or extractables from thebitumen will go away if he used the newsolvent-free epoxy technology that has beenpromoted to him; and he wanted a defect-freelining so his specification was written to insiston a two-coat system. The supplier hadprobably rushed a new lining product tomarket and was either really uncertain or evenconfused about how it should work. His staffwere grossly inexperienced on these newmaterials and most of their learning wascoming at the contractor’s and the owner’sexpense. The contractor was thrown in thedeep end with a new product that everyone elsewas waxing lyrically about, but no one had everactually learnt its mixing and applicationparameters and limits. The contractor relied ona grossly incomplete product data sheet and thespecification, and they were both silent on keypoints, because almost all the people involvedsimply didn’t know what they were trying todo and achieve. The problems that appeared inthis (and in many other cases) were allunexpected and undesired, however, they werealmost totally predictable and inevitable.

Settlement of the Problem

The owner wanted the total coating systemfully removed and reapplied, on the basis that hehad not received what he believed he hadspecified or paid for. The coating supplierinsisted that his materials were alright and theapplicator must have mistreated them. In truth,the coating contractor was about the only entity

who undertook his work in accordance with thespecification and the product data sheets as theywere written.

The owner’s specification was somewhatcontributory in that it specified that at least twocoats of solvent-free epoxy were to be applied.As will be shown shortly, this conflicts with theparameters of these materials. The specificationwas reasonably descriptive about relativehumidity, dewpoint and surface temperatureconditions before and during application, butwas silent as to conditions beyond the time thatthe coating was applied, i.e., during its curingphase. For example, there was no requirementsto have humidity control or to ensure that thecoating be kept free from surface moisture beforebeing recoated or offered for service. Similarly,various versions of the coating manufacturer’sproduct data sheet (as outlined earlier, it hadsince changed several times in the years since thecontract was performed) had conflictinginformation regarding its preference for a one-coat or a two-coat system and was silent on mostkey application recommendations. The datasheet version of the original date had noinformation at all regarding climatic andsubstrate conditions, dehumidification or anypost-application condition control.

In spite of the poor appearance of the lining,with large blisters and sheets of disbondingpaint, it was actually deemed to be workingvery well. The lining was technically fit-for-service, with no compromises, in the two keyattributes that a lining inside a potable watertank must possess:

• It must protect the steel tank from corrosionand ensure that it retains its structuralintegrity; and

• The quality of the water inside the tankmust not be contaminated or negativelyimpacted upon by the lining.

Only when the major contributors to theproblem were actually identified, (and this wasnot the coating contractor as was stronglypromoted in the early weeks) was it wasultimately agreed that the lining had nottechnically failed (if the right definition of

failure was used) and it still had many years ofuseful life ahead of it, purely on the obviousintegrity of the primer and the first coat ofsolvent-free epoxy. If the second coat of epoxywas ignored, as it was paying no effective rolein the lining system, it was found that the dryfilm thickness of the balance of the system,actually closely conformed to the coatingmanufacturer’s normally recommended single-coat system of solvent-free epoxy over a thin-film primer. Additionally, the owner’srequirements for a durable and serviceablelining were still being met and it still hadresidual value, therefore there was no realbenefit in fully removing and reapplying it.The coating manufacturer agreed to underwritethe balance of the life expectancy that the owneroriginally required for the lining as a formalwarranty. The coating application contractorhad his retention monies returned and the tankwas not required to be reblasted. The tank isstill in service and is said to be still in excellentcondition, some eight or so years later.

Chemistry

Some of the specific problems that solvent-free materials can face, relate to the chemicaladjustments that need to be made in order tomake the transition from high solids (i.e.,containing up to about 90% solids) to a full100% solids or solvent-free material. In simpleterms, because epoxies work by a chemicalcrosslinking of a resin containing an epoxidering, (an oxygen atom bonded to two differentcarbon atoms which are themselves joined by asingle bond) and a curing agent or converterwhich contains amine (NH2) groups; a sufficientnumber of amine compounds are needed toensure that the less-molecularly mobile epoxidegroup is reacted.

With regular high solids (i.e., solvent-based)epoxies, the coating formulator has a choice of awide variety of curing agents. In past years,these were commonly aromatic amines and/orpolyamides, depending on the propertiesdesired in the cured film. In later times,especially as the demand for higher volumesolids materials increased, formulators movedto multi-functional polyamines such as

cycloaliphatic amines, and on to polycyclicaliphatic polyamines. This is because to make atrue solvent-free coating, all of the liquidingredients (the resin, the curing agent, anydiluents, accelerators, plasticisers, etc.) must notcontain any solvent. Because liquid epoxyresins are now quite commonplace, one of thebiggest challenges, then, has been to find ordevelop solvent-free curing agents that stillhave the desired reactivity.

The choices available for cycloaliphaticamine curatives were essentially limited to twocommercial products: cyclohexyl diamine andisophorone diamine. These are single ringstructures. To get the polycyclic aliphatics,several companies employed hydrogenationtechnology to aromatic amines. Explainedsimply, this involves putting hydrogen in andtaking double bonds or aromatics out.

Whilst the cycloaliphatic amines are said tohave better molecular mobility and a fasterspeed of reaction as compared to the moretraditional aromatic amines (1), this can becounter productive as there still exists the issueof ensuring that each epoxide group is properlyreacted in the drying film, with a functionalamine that is in the right place at the right time.This problem arises because of the inevitableincrease in viscosity and the thixotropy of afreshly mixed (and applied) solvent-free film.

Having some solvent in the wet coatingmight be seen by some as being undesirable,(especially in the drive to truthfully call thecoating solvent-free), but there are manydistinct advantages with having a measure ofsolvent present, at least during the mixing,induction, application, reaction, drying andcuring stages of a coating’s transformation fromwet to dry. Solvents have much lower surfacetensions than resins, and the beneficial wettingaction of solvents is not available in a solvent-free film, often to the coating’s detriment (1).

This lower surface tension means that asolvent-based coating is more inclined to fullywet out a profiled substrate. An intimately andcompletely wet out surface profile is essential ifa lining is to develop and retain good wet and

dry adhesion. It also helps that most solvent-containing products take longer before they gel.Rapid-setting polymer films, especially thosethat contain larger sized pigment particles, cansometimes not fully wet out deep and angularsurface profiles. This can lead to accumulationand cleavage points for molecular water vapourthat has permeated the film, which is often aprecursor to a drop in wet or dry adhesionstrengths. These accumulation points are thelocations where corrosion can commence ifoxygen is present.

Even a small amount of solvent in a wetcoating ensures that at both a macro and at amolecular level, an acceptable level of mobilityis retained. The solvent provides lubricitywithin the wet film – a key feature whenencouraging reactive species to get closeenough to chemically crosslink before the filmfreezes. A good analogy is a bowl of freshlycooked pasta, e.g., spaghetti. This is sticky andstarchy and does not slide or move freely. Infact, the molecular chains of an epoxy resin arenot unlike cooked spaghetti: long, intertwined,sticky and somewhat lacking in ability to move.Add some pasta sauce or some olive oil to thepasta, and the lubricity is provided (1). Thesauce or oil is very like solvent. The solventallows the molecular chains to move and slide.If molecular movement can be retained duringthe time that the coating is still wet, there is amuch better chance that the epoxide groups onthe resin and the amine molecules on the curingagent, will get physically close enough to react.

With any reactive organic coating to be usedas a chemically-resistant lining, the aim is toencourage more complete crosslinking of thereactive species that are present. This meansthat with an epoxy material, every epoxidemolecule needs access to a reactive aminegroup and the reaction must run to completion.If both viscosity and thixotropy are high,physical and molecular movement of thereactive species in the film, when the coating isapplied to the substrate, may be restricted. Ifthis happens, there will be a greater presence ofunreacted, monomer-like zones in the film thatare less chemically resistant (1). This is causedby having some unreacted epoxide groups on

the resin backbone and amine curing agentmolecules not finding partners and becomingfrozen in the setting film. This inevitably leadsto a higher permeability to oxygen andmoisture vapour – both of which are highlyundesirable in an immersion-grade lining.With a solvent-based coating, and particularlyone that has a nice slow gel time, the chances ofa very high order of reaction occurring, areusually very good.

However, in the drive to formulate a solvent-free coating, many of the highly desirablefeatures that the solvent brings to the party aredenied. Higher volume solids demands lowermolecular weight resins and reactants. Thisadds to the problem of poor molecular mobility,and the almost inevitable faster gel times thatsolvent-free materials have, will accentuate thedifficulty in ensuring that the two reactivecomponents can get together, or that thesubstrate can be totally wet out by the coating,to the exclusion of any uncoated zones, beforethe coating dries and then cures.

At the molecular level, curing is not smoothand uniform, instead it is nucleated andautocatalytic. This means that once it starts, itproceeds more rapidly in those specific areas. Italso means that where it starts, it then goes alittle faster. This is very much like starting tosolving a crossword puzzle, where the presenceof a few horizontal words makes it easier to addthe vertical cross-links. Whole patches of thepuzzle can be done while nearby areas remainuntouched. From a practical point of view,there is no such thing as a complete cure. Thechemical cure of epoxy resins is a clusterphenomenon and the idea is to make theuncured clusters as small and as uniformlydistributed as possible.

So, what do we mean by cure? We speak interms of adequately cured, satisfactorily cured,and fully cured, and we determine this withcomparatively crude tests for hardness andsolvent resistance, for example, using pencilhardness and double rub solvent wipe testing.Unfortunately, a completely cured coating withmaximum cross-link density for the utmost inbarrier protection sounds good but its

achievement is a fallacy. Coatings are notperfect barriers to the environment. Water,hydrocarbon solvents, and gases willeventually penetrate a lining and when thathappens, the coating needs to have a balance ofproperties, both physical and chemical, totolerate the condition.

With epoxy coatings, one possible way tominimise the negative consequences of highthixotropy and poor molecular ability, is for theformulator to add slightly more curing agentthan is numerically needed, (i.e., above thestoichiometric ratio) so that there is a betterchance that all of the epoxides are reacted, evenif this means that some unreacted amine is leftover. With the amine groups being molecularlysmaller than the polymer containing theepoxide, they are generally perceived to be themore mobile of the pair. However, there is aslight danger in this as unreacted aminecompounds that stay locked in a cured filmhave an affinity for water which cancompromise the film when it is used with anaqueous cargo in immersion. By contrast, someunreacted epoxide linkages in the film areusually quite stable in that they are not polarand do not hydrogen bond with water.

Having a slight excess of curing agent in thefilm, i.e., above what is stoichiometricallyrequired for complete crosslinking with theepoxide, can be tolerated in some situations, butnot in many others. If the solvent-free liningapplication is to be a one-coat system, this cansometimes be of minimal consequence. Inpractice, some of the excess material (which isprincipally amine) can be exuded or leachedfrom the surface of the curing film. Onceoutside the film, if moisture and CO2 arepresent, the amine may react to form an aminesalt, or more correctly, an amine soap. If so, itwill lie on the outer surface as a slightly greasybut principally unreactive layer of carbamate.

The presence of a carbamate film is usually aproblem only if it is to be overcoated. This isone of the main reasons why most solvent-freeepoxy linings should be specified to be appliedas single-coat systems. (It is worth stressingthat the generation of an amine sweat or

exudate is not limited to solvent-free epoxies –even many high solids materials, especiallythose designed for early water immersion, cansweat particularly badly – but products basedon some cycloaliphatic curing agents are knownto be problematic.)

If, however, it is desired or specified that thesolvent-free epoxy system is to be applied intwo or more coats, then the formation of acarbamate layer on the surface of the first coat,will often be a very undesirable unless it can besatisfactorily removed. The carbamate materialis relatively insoluble in water but is quitereadily dissolved in either an alcohol or aketone solvent. Unfortunately, the specifiedthinning or cleaning solvent for many epoxyproducts, (and which are likely be available ona jobsite) usually contain quite large amounts ofan aromatic such as zylene or toluene, and onlysmall quantities (if at all) of any ketone and/oralcohol. These epoxy solvents will notadequately dissolve the carbamate so it willremain on the outside of the epoxy film. Ifovercoated without being removed it can act asa very effective bond-breaker between the twocoating layers.

When the new lining is being inspected andtested after application, this condition may wellnot be visible or detectable. However, after thecoating is immersed, particularly with anaqueous cargo, serious consequences can occur.

Organic lining films are semi-permeablemembranes. Water vapour will start topermeate into and move through the molecularmatrix between the atoms and molecules of theresin and around pigment particles of a newepoxy lining within hours of being immersed(2), especially if the density or completeness ofchemical crosslinking is low or if themicroscopic structure results in an excess ofinterstitial voids. It is believed that after severaldays of being wet (even with pondedrainwater) in the case of many epoxy films, thequantity of water intake is already quite high.The time taken to reach an equilibrium withwater entering and leaving the film at the samerate, may take longer, i.e., days or weeks,however many variables will affect this.

If there is a plane in the lining systembetween two separately applied epoxy layerswhere adhesion is poor due to the presence ofan exudate, water will start to accumulate atthis interfacial zone. This is not an osmoticinfluence in this case because the carbamate isnot particularly water-soluble so it is not astrong osmotic agent, rather, it is simply theresult of regular moisture vapour permeation.In many cases, this will result in the formationof either water-filled blisters or evenwidespread detachment of the second coatinglayer if the hydraulic pressure of theaccumulating water is greater than the adhesionstrength between the coating layers.

The potential for an exudate to develop onsolvent-free epoxies is aggravated by a numberof conditions. The most pronounced influenceis the substrate and climatic conditions duringand after the coating system is applied. Thisincludes the substrate temperature, the airtemperature, the relative humidity, dewpointand airflow. Providing DH prior to, during andafter application so as to control the dew point,prevent condensation for the narrow windowof time when the solvent-free epoxy is at risk,and the problem is essentially solved.

Mixing and Application

Another major influence is the preparation,mixing, induction time, heating and spraying ofthe coating, and with respect, this is the areawhere many coating manufacturers seemed tolet down their applicator clients for quite sometime in the early years by not being able todirect and advise on these crucial aspects. As aconsequence, many contractors had to learn attheir own expense, often by redoing work orhaving to suffer other contractual penaltieswhen information or instructions (fromspecification and/or product data sheets) wereoften incomplete or wrong.

Mixing and preparing the coating beforeapplication is very important and is often notgiven the consideration that it deserves. Theindividual components of most solvent-freeepoxy materials are extremely viscous. There is

also potential for settling out of some pigmentcomponents after manufacture, either in storageor during transport. Even in the smaller kitsizes that these products are often supplied in(due to limits of weight nominated in OH&Sregulations for manual handling), mixing thesecomponents to a state of acceptablehomogeneity (both individually and oncecombined) is a real challenge. These materialsare in effect, liquid solids, and there is minimallubricity because there is no solvent.

Preferably, the individual componentsshould be premixed thoroughly before they areblended and then further mixed. It helps tohave the components at a warm enoughtemperature before mixing, and if this is notachieved, then many negative consequences canarise. The ideal paint liquid temperature is 20 –25°C (68 – 77°F), and although this often meansthat the pot life is shorter, the lowering of theviscosity usually means that the mixing is moreefficient and quicker, and the energy that thereactive species have, is higher.

Getting the paint components to the idealpre-mixing temperature can be difficult,especially in the winter. If paint is storeddirectly on a concrete floor, it will soon adoptthe temperature of the floor. This can often bewell below 10°C (50°F). A hot water immersionbath is one way of elevating the paint’stemperature prior to mixing. Another way is tohave a dedicated storage shed that is heated, oreven a compressor room or boiler house that isthoroughly warm. In this case, the paint needsto be in the warm storage at least overnight,(say 12 hours minimum) prior to use. Puttingthe paint on a pallet with a canvas tent frombehind the aftercooler of a portable compressor,or leaving the drums in the sun is not effectiveas the heat distribution and transfer isextremely variable and inefficient, even withindifferent parts of the same drum. Having paintat too cold a temperature prior to mixing,induces the biggest variability in the curedcoating and causes the largest problems withsolvent-free epoxies.

Many application and performance issuescan be minimised if a continuous process of

mixing, heating and spraying the coating isfollowed, such as would occur during theairless application of multiple drums of coatingto large flat steel surfaces. In this case, the timebetween physically mixing the product and itsapplication to the substrate, would usually bevery consistent, especially if the work scopeexisted over a number of hours.

For example, with good access a spraypainter can probably apply 20 litres (5 gallons)of a well mixed solvent-free epoxy in 10 or sominutes. If a continuous process of mixing andspraying is established – which is the only waythat these products can be handled – each drumof paint would probably be in a mixed liquidstate before being deposited on the surface, forbetween, say, 15 and 25 minutes. If spraying isnot interrupted, then this time would berelatively consistent right through the fullwindow of application. (We will later showthat this consistency in time, evaporates whenspot repair painting work or stripe coating isperformed using hand application methods.)

One point that must be stressed in relation tothe time that the catalysed coating remainsmixed, is that an airless spray pump will oftenonly pull paint from the centre of the feedcontainer, irrespective of whether it is a siphonfeed or a bottom-outlet hopper. If sequentialdrums of fresh paint are added as the liquidlevel drops in this container, some of the oldermixes of paint will remain against the outerwalls. These will have a totally differentinduction time than the fresh material and it isimportant that a regular sequence of scrapingthis older material down and remixing theproduct is performed. If not, parts of thecoating in this container will get beyond its potlife and will gel and/or cure. This leads to poorfilm chemistry and risks tip blockages.

The second most important aspect ofpreparing the solvent-free epoxy coating forspraying involves getting the mixed paint up toan appropriate temperature to apply. Thistemperature is higher than what is ideal forstorage prior to mixing. This involves theheating of the mixed product as it passesthrough an inline heater after the spray pump

and before the fluid tip. Elevating thetemperature of the mixed paint toapproximately 35 – 40°C (95 – 104°F) lowers itsviscosity and significantly increases molecularmobility. Both of these are very positivefeatures, but consistency and balance is veryimportant.

A lower viscosity coating will atomise easierand better through the spray tip. It will alsoflow out better to form a continuous film andwill wet out the profiled substrate in a superiormanner. Elevating the temperature lowers theviscosity, increases the speed of theepoxy:amine reaction as well as increasing theenergy that individual chemical groups have toallow them to find a reactive partner. It alsoshortens the pot life, which is why it is vital tohave consistency in this temperature. At 40°C(104°F), the pot life of many commercialsolvent-free epoxies can be less than 10minutes. It is important to understand that aninline heater will achieve the requiredtemperature of the pumped coating within theaccepted tolerance, only if the rate of flow of thecoating through the spray lines and itsincoming temperature are constant. However,depending on the physical arrangement of theinline heater, it may be possible to recycle thepaint until the right temperature is reached.

The rate of flow of the coating through theheater needs to be consistent, and the only wayto achieve this is to be spraying continuously.It is not acceptable to be working on multiplelevels of a scaffold, for instance, and have thespray painter put down the gun whilst heclimbs from one scaffold level to the next beforehe starts again. This stops the flow of paint andrisks overheating the coating in the heater. Thebest approach is to have another spray painteralready positioned on the next level, the spraygun is passed to him and he starts sprayingwithout a break. Spray tip blockages are a largeproblem unless quick-reverse tips are usedwhich allow the blockage to be cleared andspraying to continue within seconds.

Even with the above continuous mixing,heating and spraying sequence, it is commonthat the spray lines would need to be flushed

out with solvent after a number of litres havebeen sprayed. During a day’s spraying, thelines may need to be flushed twice or maybeeven more often. Sometimes, after 100 – 120litres (say 25 – 30 gallons), the lines need to beflushed with solvent. If this is not donereligiously, some of the older mixes of paintmaterial that have remained against the outsideof the container from where the airless pump isfeeding, or have been in contact with the innerwalls of the spray lines, will gel and start tocure. This will slow down the rate of flow,change the transfer rate of heat to the flowingmaterial and will raise the incidence of spraytip blockages. It could even cost the applicatora set of spray lines, the fluid section of theairless spray pump or the inline heater; if thehot epoxy cures in the line.

The other variable in the heating andspraying process, (besides the flow rate) is theincoming temperature of the paint as describedearlier. The more constant that this is, the moreconsistent is the heat pick-up from the inlineheater. This helps explain why unevenlyheated paint materials that have not beenstored in a controlled and heated storage, give avariably cured product.

The last item to discuss on the topic ofapplication, is the spray tip. In spite of theadvice of many manufacturers with respect totip sizes, practice has shown that a smaller tip isusually much better. Many product data sheetsrecommend tip sizes between 0.53 – 0.79mm(0.021 – 0.031 inches). Experienced solvent-freeepoxy applicators have learnt that smaller tipsizes, typically 0.53mm (0.021 inches) or less aremuch better at atomising the coating, but thesewill only work if the paint is close to 40°C(104°F) at the tip. Some contractors report thatthey start spraying with an 0.63mm (0.025 inch)tip and then switch to a smaller size as theinline heater and paint temperature rises andstabilises. It seems that forcing the hot coatingthrough a small tip induces a better break-up,more movement, agitation and shear atmolecular level, and better wetting of thesubstrate. It also appears that the combinationof the right spray temperature and a small tipsize, gives a film that has less voids and a more

closed film, resulting in less defects that wouldbe detected with a continuity tester (highvoltage spark tester) after curing.

Conversely, if the paint temperature is toolow and the tip size too large, the coating willappear to spray satisfactorily, but will not flowout as well. This often results in an applicationthat does not form a closed film when wet, evenat quite a high wet film thickness. Thisinvariably results in many spark-through zoneswhen continuity testing is performed.

In our opinion, the key issues of paintstorage temperature, spraying temperaturesand pressures, tip sizes and the number andthicknesses of coats in the solvent-free epoxysystem; were aspects that were not very wellunderstood by some coating manufacturers inthe early days of marketing these products, andstill too commonly, many field tech service staffdo not know which combinations of equipmentand material preparation are required forsatisfactory application and performance.

Stripe Coating

The foregoing describes what can andshould happen when continuously sprayapplying solvent-free epoxy materials. A verydifferent set of conditions occur when spotrepair or stripe coating is attempted.

Unlike most solvent-based epoxies, virtuallyall solvent-free materials have a very short potlife. This is typically between 30 to 60 minutesafter mixing at about 25°C (77°F). If areasonable pot life is to be achieved to allow foreffective spot painting or stripe coating, thecoating cannot afford to be too elevated intemperature. The consequence of this is thatthe high in-can viscosity prevails which is aserious impediment to adequate mixing.Adding solvent to lower the viscosity and to aidmixing is forbidden as these products have avery low tolerance to field-added thinners asthis changes the chemical nature of thematerial, risks solvent entrapment even morethan with a conventional solvent-based epoxylining, and alters the rate and degree of cure.

The applicator therefore has a quandary:does he leave the materials as cold as possibleto get enough pot life, risk not having completemixing and struggle with applying a viscousmaterial; or does he heat the components,thereby lowering the viscosity which aidsmixing and makes application easier, butseriously shortens the pot life? There is no bestanswer but the former procedure normallyprevails in spite of its negatives.

The best procedure if stripe coating isrequired, is to use a compatible solvent-basedepoxy as the stripe coat. In spite of argumentsto the contrary, this does not jeopardise theintegrity of a lining for potable water service asthere are many solvent-based epoxy coatingproducts that are fully potable water-approved.

A second issue is effective proportioning ofthe components when a mix is to be made thatis not a full kit. Solvent-free epoxies aretypically packaged in 10 litre or 20 litre kits. Tomix up one to two litres for touch-up, some skillis needed to ensure that the correct ratio isproportioned from each can. With extremelyhigh viscosities present, this is almostimpossible and experience shows that this iswrong much more often than it is right. Wrongmix ratios result in solutions which arestoichiometrically incorrect by a significantmargin which causes an increase in unreactedcomponents and compromised chemical andphysical properties.

The consistency in time between mixing andapplication now gets stretched in bothdirections. As soon as touch-up material ismixed, the applicator must start doing spotrepairs, and all too commonly, the balance ofthe product goes off in the paint can byreaching its pot life before it is all consumed.This means that the time of application aftermixing will range from about 5 minutes to 45 –60 minutes, or even less on a hot day.

A point often forgotten is that pot lifedepends on ambient temperature but it alsodepends on material temperature and thatmeans exotherm needs to be taken intoconsideration. Mix the same epoxy material in

a four litre (one gallon) kit and also in a 20 litre(five gallon) bucket and compare the pot life.The key is the paint volume to surface arearatio. Small volumes have a longer pot lifebecause they have a much better volume tosurface area ratio.

The significant effect of this long variation intime after mixing is the effective length of theinduction time. Whilst still in the can, theexothermic heat of reaction will slowly lowerthe viscosity (up to a point) which aidsmolecular mobility and the rate of chemicalcrosslinking. As soon as the coating is applied,it will adopt the temperature of the substrateand will start to gel, but the molecular mobilitywill be retarded by the lack of fluidity and amuch smaller mass of uncured coating.

If a single mix of touch-up material isapplied over a time range of (say) 5 minutes to45 minutes after mixing, there will be adiscernable difference in the chemical nature ofthe cured film with it being pre-optimumbefore some mid point, and post-optimumafter. This chemical difference will affectproperties such as the rate and completeness ofcure, the adhesion, permeability, the potentialto generate an exudate, and most definitely, thechemical resistance of the resulting film. Thiscondition is visible to an experienced eye as aprogressive change in colour of spot repairareas or stripe coats from a single mix ofcoating.

Also common is the delamination of a highbuild spray-applied solvent-free epoxy coatingoff the stripe-coated areas inside a tank, purelybecause the earlier stripe coating was brush-applied by hand using batch mixes of material.Oftentimes this has a different constitutiondepending on how long the material has beenmixed and can sometimes be seen to changealong the length of one single longitudinal weldinside a large tank.

Stripe coating the edges, welds and cornersis an important aspect of successfully lining astorage tank, and if a solvent-free epoxy has alow tolerance to this task because it is difficultto mix, is hard to brush, has poor wetability and

varies chemically due to the length of itsinduction time; then this adds to the overallintolerance and lack of user-friendliness ofsolvent-free epoxy materials.

Riveted tanks or deeply pitted substrates,(whether concrete or steel) are not the best itemsto be coated using solvent-free epoxy linings.In both cases, the amount of stripe coating orspot repair that is needed, can negate the otherpossible benefits of considering solvent-freeepoxies. Solvent-containing, high solidsepoxies are much better for these types of tanksor surfaces, because they mix better and morethoroughly, have a longer gel time and a lowerviscosity, which means that they flow into pits,wet the substrate better and can be appliedmore successfully along welds and aroundrivets.

In spite of arguments to the contrary (mainlyby the solvent-free epoxy manufacturers) thereis minimal evidence of solvent entrapmentwhere localised pits or minor overbuild hasoccurred where a high solids, solvent-containing epoxy tank lining has been properlyapplied and correctly ventilated. The solventthat is contained in a high solids epoxy, has amuch better chance of being released from thecuring film, than if an even smaller amount ofsolvent happens to be added (againstinstructions) into a solvent-free material. Highsolids epoxies are designed to release solventbut 100% volume solids materials are not.

Summary

Tank lining work performed in colderclimates throughout the winter months, has ahigher risk of problems developing problems ifsolvent-free epoxy lining materials are used,unless specific and regimented protectionmethods to control climatic and substrateconditions are enforced, and very detailedmixing, preparation and application proceduresare followed.

Solvent-free epoxy coatings are specialisedmaterials, but they are more difficult to handleand are far less tolerant to mixing, applicationand weather/substrate conditions as tank

linings as compared to solvent-containing highsolids epoxies. The transition in properties andrequirements, especially through the mixingand application stages, is a quantum leap fromwhat is required for (say) a 90% volume solidsmaterial, to one that is solvent-free. Fewcontractors have fully learnt the key parametersof these products, and if the correct sequencesand methods are not followed, poorperformance will usually result. There are fewother coating materials that are so difficult tolearn and pick up as an applicator becausemany of the well-proven and long-knownprinciples of mixing and applying high solidsepoxies, do not relate in any way to what isneeded for solvent-free products. Also, manymanufacturer’s representatives, unless theyhave spent many days on site with their armsdeep into the paint trying to solve applicationproblems, have not advanced too far from the“it works fine in the laboratory, why can’t youget it to go properly out in the field” retort.

Often the comment that is made by thesuppliers is that there are OH&S andenvironmental benefits with using a materialthat is 100% solids, but is the lining systemactually solvent-free if an epoxy primer is used(as it usually is) that is solvent-based? Also, inAustralia (at present) there is no VOCregulations covering solvent emissions frompaint materials. Even if there was, does a smallamount of solvent being emitted compensatefor a solvent-free lining that does not work aswell or for as long, and is required to be redoneearlier?

In summary, solvent-free epoxy linings:

• Are viscous, thixotropic and have short potlives;

• Are difficult to mix fully;• Are highly intolerant of field-added solvent

as this compromises the rate and degree ofcure and lessens the service life (1);

• Are critical with respect to their temperatureprior to mixing;

• Require heating to about 40°C (104°F)through an inline heater or similar prior tobeing sprayed;

• Require more expensive applicationequipment with a higher risk that thecoating may set in the lines or pumpsrendering them unserviceable;

• Should usually be applied as one-coatsystems;

• Are much more likely to allow an exudate tobe released from the coating;

• Are intolerant to disruptions duringspraying;

• Are very sensitive to high relative humidityand/or temperature depression during andafter application;

• Generally require dehumidification toensure that climatic conditions remainacceptable within a very tight range

• Are very intolerant materials to use as stripecoats or for spot repair;

• Are less chemically resistant because of alower density of crosslinking and the higherchance of having unreacted epoxide oramine species in the cured film;

• Can have a higher permeability due to alower completeness of crosslinking;

• Require a higher film build, and hence morecoating product, to provide equalpermeability resistance;

• Have higher internal stress levels;• Exhibit inferior wetting properties to a

profiled substrate;• Have lower wet and dry adhesion; and• Have less flow as a wet film due to their

viscosity.

By contrast, well formulated solvent-containing high solids epoxy tank linings:

• Are easier to mix and have a longer andmore usable pot life;

• Can accept some site-added solvent to helpmake the coating system adjust for differentclimatic or substrate conditions and to suitthe available application equipment;

• Usually result in more uniform film buildand less chances of overbuild;

• Have a slower gel time after applicationwhich aids flow into the surface profile andassists release of air from the film;

• Are quite tolerant of a variety of weather,substrate and other application conditions;

• Allow a more controlled cure to develop;• Multiple coats minimise the chances of

concurrent pinholes or defects;• Are designed with tolerance to being

applied as multiple coats withoutcompromise;

• Are less likely to form an exudate from thecuring coating;

• Have superior wetting due to the lowersurface tension that the solvent brings to theresin binder;

• Apply well as stripe coats without thecompromises to their chemistry andintegrity due to variations in the effectiveinduction time;

• Can achieve higher crosslink densities andmore complete cure due to the lowerviscosity, more lubricity and hence bettermolecular mobility of the reactive species;

• Have lower internal stress levels; and• Are much more easily handled by coating

application contractors with a variety of skilllevels.

On the basis of the above, we question thewidespread and even blind acceptance ofsolvent-free epoxy systems for tank linings andsimilar immersion service, as compared to thetolerance and forgiveness of a range of manyexcellent solvent-based epoxy linings. Can weafford as an industry, to be experimenting withstate-of-the-art materials at the facility owner’sexpense, when the owner is likely to be theparty disadvantaged if the latest technologydoes not deliver service lives or performancethat is any better than what we know worksextremely well?

In closing, an appropriate quote from MikeO’Donoghue et al. (1):

“Small mistakes with one-coat systems have thepotential for far bigger consequences than smallmistakes with multicoat systems.”

And another from Mark S. Schilling:

“New and poorly understood plus inexperienceequals Ouch.”

References:

(1) M. O’Donoghue, R. Garrett, V.J. Datta, E.Swanson and B. Dillingham; “Fast-CureAHC Epoxy Coatings for Tank and PipeApplications,” Materials Performance,June 2000, NACE International, HoustonTexas.

(2) E. Hemmings, Ameron (Australia) PtyLtd, private communication, July 2004.

Considerable appreciation and credit is given toMark S. Schilling for his extensive input, advice,editing and reviewing of this technical paper.

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