Failure Analysis And Degree Of Cure A

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Failure Analysisand Degree of CureFig. 1: Catastrophic failure of undercured zinc-rich primer topcoated with epoxyAll figures courtesy of the authors anyone involved in failure analysis soon realizes, there aremany different reasons why a coating can fail prematurely.This article will focus on one common and preventablecause of premature coating failure—degree of cure.ABy Dwight G. Weldon,Weldon Laboratories48What Is “Cure”?Before discussing cure, it may be helpful to discuss the concept of molecular weight. The molecular weight of a substance refers to how much a molecule of that substance weighs and isrelated to the size of the molecule. Most of the common chemicalswhich we routinely encounter are small molecules of low molecularweight, such as salt (sodium chloride), which has a molecular weightof 58, or methyl ethyl ketone, which has a molecular weight of 72.However, the molecules which make up the binder of a coating arevery large, very high molecular weight polymers and resins. Someacrylic resins have molecular weights of well over 100,000. Two veryimportant factors related to the chemical and physical properties of acoating include the type of resins or polymers composing its binder,and the molecular weight of these polymers and resins.Coatings are broadly grouped into one of two classes: thermoplasticand thermoset. A thermoplastic coating dries by solvent evaporation.JPCL /July 2005/PCEwww.paintsquare.com

Common examples of such a coatingwould be a vinyl acrylic latex housepaint or a solvent-borne acrylic lacquer. The molecular weights of theresins or polymers in such coatingsare as high as they are going to getwhen the paint is manufactured, andthe drying of the wet paint simplyinvolves the evaporation of the solvent (which, in the case of a latex, ismostly water).Thermoset coatings can also dry bysolvent evaporation. Some peoplemight even consider evaporation to bean initial step in the curing process,since the coating will usually becomeless tacky during the drying stage.However, unlike thermoplastic coatings, thermoset coatings continue tocure through a chemical reaction. Thisreaction is usually, but not always,between various ingredients formulated into the liquid paint, such as apolyamide curing agent reacting withan epoxy base component. One of theoldest of thermoset coatings is the airdry alkyd. Here, a complicated processof oxidation and crosslinking at thecarbon-carbon double bonds (unsaturation) of the alkyd resin allows theresin molecules to react very slowlywith one another, such that the finalmolecular weight of the cured alkydresin is very much higher than its initial molecular weight. A moisturecured urethane is a more modernexample of a thermoset coating thatchemically reacts with some speciesthat is not in the can of paint—atmospheric moisture—to crosslink andcure.As one might expect, the physicaland chemical properties of a thermoset coating can depend greatly onwww.paintsquare.comFig. 2: Infrared spectra of epoxy (top) and alkyd (bottom)its degree of cure. When such a coating is initially mixed and applied, itconsists of low molecular weight fragments or prepolymers. These prepolymers lack properties such as solventresistance, chemical resistance, cohesive strength, impermeability, corrosion resistance, and tensile and flexural properties. Putting a coating intoservice before the prepolymers reactto cure and develop the coating’sproperties would be somewhat likebuilding a brick house without usingany mortar. Over time, the housewould fall apart. If a coating is putinto service before it cures, or if something goes wrong and interferes withthe normal curing process, prematurefailure can often result because thecoating’s properties will not havedeveloped. An air-dry industrial maintenance coating might take severalweeks to cure, and a plural-component sprayed polyurea might take sevJPCL /July 2005/PCEeral seconds to cure. Regardless of thetime the chemical reaction or curingprocess takes, the coating must not beput into service before it cures.Failures Relatedto Incomplete CureThere are several possible explanations for why a coating may not havecured properly. Possible reasonsinclude a defective batch of coating,improper mixing on the part of theapplicator (such as not observing thecorrect mix ratio, or adding the wrongthinner), unfavorable environmentalconditions, and, for heat-cured coatings, improper baking conditions.A very common example of a coating failure due to lack of cure is thesplitting, or cohesive failure, of aninorganic zinc-rich primer topcoatedwith an epoxy intermediate/urethanetopcoat (the so-called “Cadillac” ofbridge coatings). When these failures49

Fig. 3: Infrared spectrum of an inorganic zinc-rich primer. Note that there are practically no carbon-hydrogenabsorption bands near 2900 cm -1, a feature typical of a fully cured coating.happen, they can often be catastrophic (Fig. 1). The undercured zinc-richprimer not only has poor solventresistance, but also has low cohesivestrength. It can be softened and weakened by the strong solvents in theepoxy intermediate coat.Furthermore, when coatings cure,they shrink, resulting in a stressknown as shrinkage stress. Since theundercured zinc-rich primer hasalready been weakened by the solvents in the epoxy intermediate coat,when the epoxy itself cures andshrinks, the resulting stress is sufficient to tear apart the underlying50primer, sometimes resulting in largesheets of disbonded coating.1,2Even the simple oil-based alkyd coatings mentioned above are not immuneto failure because of lack of cure. Suchcoatings cure by reaction as the carbon-carbon double bonds with oxygenin the atmosphere. The process hasbeen studied extensively but is still notcompletely understood. However, it isclear that it can be sped up by the useof certain catalysts, or “driers,” whichare often organometallic compoundssuch as cobalt naphthenate. Althoughfailures involving alkyds that do notdry properly are rarely as catastrophicJPCL /July 2005/PCEas the inorganic zinc-rich primer failures discussed above, it can be veryfrustrating to paint a bridge, or a housefor that matter, and not have the paintdry. Not only can it be messy, but thesoft, sticky alkyd will retain excessiveamounts of dirt, turning an otherwisebeautiful paint job into an unsightlyproblem. The lack of cure could be dueto the wrong type or amount of catalyst formulated into the coating, to theuse of the wrong type of alkyd resin, orperhaps to excessive coating thickness.Urethanes are another type of thermoset coating whose properties, andhence serviceability, can be dramatiwww.paintsquare.com

cally decreased by lack of cure. Typesof failures related to lack of cureinclude poor chemical resistance,poor adhesion, blistering, inferiorweathering properties such as chalking or color fading, and, for the flexible variety, decreased tensile and flexural properties.Lack of cure is not restricted to airdry coatings. Some coatings will notreact or cure properly unless heatedto a certain temperature for a certainperiod of time. The elevated temperature speeds up the chemical crosslinking reaction, which would occur slowly or not at all, at room temperature.Therefore, the underbaking of coatings such as fusion-bonded epoxypowder coatings or polyestermelamine coil coatings can result inthe coatings being undercured.Undercured epoxy powder coatingson buried underground pipe canresult in poor adhesion, increasedpermeability, and a tendency to blister. An occasional, isolated blister isprobably not a problem, particularlyif the pipeline is also under cathodicprotection. However, excessive blistering of hundreds of feet ofpipeline—or even miles in the worstcase scenario—can result in the drawof very large amounts of current. Thismay exceed the capacity of thecathodic protection system, orbecome very expensive as greaterand greater amounts of electricity arerequired to protect the exposed steel.Undercured coil coatings can sufferfrom excessive dirt retention, and significantly undercured or even overcured ones can show inferior tensileand flexural problems, resulting incracking upon forming. Furthermore,lack of cure of coil coatings applied togalvanized sheet results in a coatingwww.paintsquare.comFig. 4: Infrared spectra of a two-component urethane coating cured for 24 hours (bottom) and 2 weeks(top). Note that as the coating cures, the isocyanate band near 2270 cm-1 becomes dramatically weaker.that is more permeable to water. Theauthor is familiar with at least oneinstance where this led to blisteringof the coating as a result of excessivewhite rust which formed under thepermeable coating during storage (the“storage stain” familiar to many usersof coil-coated galvanized stock).Detecting Curing ProblemsThere is more than one way to investigate the possibility that a failingcoating has not cured properly.Field InvestigationPerhaps the most common procedure,and certainly the least expensive, isto perform some type of solvent resistance test. Since thermoset coatingscure by a chemical reaction, whichtransforms very low molecularweight starting material into veryhigh molecular weight coatings, theability of the coating to resist solventimproves as the coating cures. OneJPCL /July 2005/PCEsimple way to evaluate this isdescribed in ASTM D5402 (StandardPractice for Assessing the SolventResistance of Organic Coatings UsingSolvent Rubs). The Significance andUse section of this practice states that“Coatings which chemically changeduring the curing process, such asepoxies, vinyl esters, polyesters,alkyds, and urethanes, become moreresistant to solvents as they cure.These coatings should reach specificlevels of solvent resistance beforebeing topcoated and before placing inservice.”Briefly, ASTM D5402 is performedby saturating cheesecloth with methylethyl ketone (MEK), or another specified solvent, and rubbing the clothback and forth (one back and forthstroke is referred to as a “double rub”)on the coating, for at least 25 doublerubs. The test area is then evaluatedfor such things as changes in appearance, hardness, or thickness, and the51

Fig. 5: A DSC heating curve of an uncured powder coating, displaying a strong exothermic peak from 145 to 205 Ccloth can be examined for color transfer of the coating. The author oftenuses a slightly modified version of thisprocedure, substituting cotton swabsfor cheesecloth. ASTM D4752(Standard Test Method for MeasuringMEK Resistance of Ethyl Silicate(Inorganic) Zinc-Rich Primers bySolvent Rub) is similar to D 5402, butas the title suggests, was developedspecifically for inorganic zinc-richprimers.Solvent rub testing is a fairly roughway of measuring degree of cure. It is52quite capable of detecting moderate tolarge differences in cure, but not smalldifferences. It should also be kept inmind that not every properly curedcoating has the same degree of solventresistance. Thus, it would not be fairto assume that an alkyd coatingwould have the same degree of solvent resistance as an epoxy-phenolic.Therefore, it is useful for the persondoing the testing to have a controlsample of the coating in question, orat least a non-failing sample, in orderto establish what the “normal” degreeJPCL /July 2005/PCEof solvent resistance is for the particular coating.Laboratory InvestigationIn the laboratory, infrared spectroscopy is often a very useful technique for investigating degree of cure.In some instances it can be muchmore sensitive than a solvent rub test.Very briefly, infrared spectroscopyrelies upon the fact that molecules arein a constant state of motion, or vibration. Indeed, it is convenient to thinkof a molecule as a collection of ballswww.paintsquare.com

(atoms) connected by springs (thechemical bonds holding the moleculetogether). Depending on the type ofatoms and the type of chemical bondsholding them together (springs), portions of the molecule will vibrate atdifferent frequencies, often measuredin units of cm-1 (reciprocal centimeters). The frequency of vibration isvery dependent on the structure ofthe molecule. Therefore, a carbon atomsingle bonded to a hydrogen atom willalways show a stretching type ofvibration near 2,800–2,950 cm-1.Likewise, a carbon atom double bonded to an oxygen atom will typicallyvibrate at about 1,650 to 1,750 cm-1.The significance of this is that wheninfrared light is focused on or through asample, if the frequency of the infraredlight matches one of the vibrations inthe molecules making up the sample,the sample will absorb some of thatparticular light. Since there are manydifferent structural features in a typicalorganic molecule (or, in our cases, polymer), which are often referred to as“functional groups,” the sample willabsorb infrared light at numerous frequencies and to differing degrees. Theresult is an infrared spectrum, which isvery characteristic of the sample beinganalyzed. Fig. 2 shows the infraredspectrum of both an alkyd and anepoxy, and clearly illustrates that thetwo spectra differ substantially inappearance.Not only are the positions of thebands in an infrared spectrum indicative of the chemical structure, or functional groups, of the sample, but alsothe intensity of the bands is proportional to the concentration of that particular functional group. It is thisquantitative aspect of infrared spectroscopy that makes it a useful tool inwww.paintsquare.commeasuring degree of cure.The catastrophic failure of anundercured inorganic zinc-rich primertopcoated with an epoxy intermediatecoat has been previously discussed.While solvent rub testing might beable to show that the primer wasundercured, infrared spectroscopycan detect smaller variations indegree of cure. A detailed discussionof this procedure has been previouslypresented. 3 Briefly, when ethyl sili-“One of the main reasonswhy epoxy coatings donot cure properly isbecause they have notbeen mixed properly”cate zinc-rich primers cure, they do soby reaction with atmospheric moisture. This results in a cleavage of theethoxy groups (CH3CH2O-) in the silicate prepolymer and their evaporation from the coating in the form ofethyl alcohol. Since the ethoxy groupsare rich in carbon-hydrogen bonds,the absorption bands in the infraredspectrum due to carbon-hydrogenvibrations in the 2,800–2,950 cm-1region decrease upon curing (Fig. 3).This decrease can be measured quantitatively by infrared spectroscopy,allowing one to determine the primer’sdegree of cure.JPCL /July 2005/PCEInfrared spectroscopy is also ideallysuited to monitoring the degree ofcure of urethane coatings. Urethanecoatings consist of a component A,which is typically a hydroxyl (-OH)functional polyester or acrylic resin,and a component B, which contains anisocyanate (an isocyanate contains the-NCO functional group). When mixedtogether, these two ingredients reactto form the actual urethane.Fortunately for the analytical chemist,the isocyanate group has a verystrong and very characteristic band inthe infrared spectrum near 2,270 cm-1.There is very little else that absorbs inthis region of the spectrum. Since theisocyanate group is consumed duringthe curing reaction, resulting in theproduction of the urethane polymer,the intensity of the 2,270 cm-1 isocyanate band decreases, as shown inFig. 4. Therefore, the degree of cure ofa urethane can be monitored by measuring the decrease in the intensity ofthis band, usually relative to someother band in the spectrum.It should be pointed out that whendoing failure analysis, alternative possibilities should be considered. Thus, itis entirely possible that the spectrumof a failing urethane could have notrace of any unreacted isocyanate, andyet the coating could be completelyuncured. This could happen if the contractor neglected to add any of theisocyanate curing agent to the component A. Fortunately, an experiencedinvestigator would normally recognizethis by other clues in both the physical nature of the sample and the features of the infrared spectrum.Although infrared spectroscopy cantheoretically be used to monitor thedegree of cure of epoxy coatings,4,5the author has found it to be of very53

in a small aluminumpan, which is typically crimped shut.Inside the samplecompartment of thefurnace there are twosmall platforms, onefor the sample andone for the reference.(The reference isoften just an emptyaluminum pan.)Attached to theundersides of thesample and referenceplatforms are temperature sensors, orthermocouples. Thefurnace is programmed to heat upover a certain range,at a certain rate (forinstance, from 20 to200 C at 10C/minute).If nothing hapFig. 6: A DSC heating curve of an epoxy coating. Note the sudden change in the position of the baseline in the 60-80 C regiondue to its glass transition (measured at 68.9 C).pens to the sample,one essentially gets astraight line,although possibly sloping in either thelimited practical use. However, one ofdetermine the approximate mix ratio.positive or negative direction, in a plotthe main reasons why epoxy coatingsMention was made previously ofof heat flow versus temperature.do not cure properly is because theyblistering failures of fusion-bondedHowever, if a thermal “event” occurs,have not been mixed properly. Forepoxy pipeline coatings, and failures ofthere will be a deviation from this baseinstance, many polyamide epoxyoven-cured coil coatings that have notline. If the thermal event is curing ofcoatings are to be mixed in the ratiobeen properly cured. While solvent rubthe sample due to reaction betweenof 1.0 parts of the A component totesting might reveal deficiencies in theresidual unreacted components, the1.0 parts of the B component, by volcure of such coatings, and while theresample will liberate heat. (The curingume. One of these components conis a possibility that infrared specreactions of paints and coatings aretains the epoxy resin and the othertroscopy might also prove useful, difexothermic, meaning that they evolvecontains the polyamide curing agent,ferential scanning calorimetry (DSC) isheat.) The evolution of heat by the samor hardener. For instance, if only halfthe technique of choice for these typesple will be detected by the thermocouof the curing agent is used, the coatof coatings.ple attached to the underside of theing will not cure properly and willA differential scanning calorimetersample platform, and the heating curvelikely fail prematurely. Solvent rubis, in essence, a very expensive andwill show a peak in the exothermictesting may indicate lack of cure, andprecisely controlled furnace. The samdirection. An example of such aninfrared spectroscopy can oftenple (usually a few milligrams) is placed54JPCL /July 2005/PCEwww.paintsquare.com

exotherm is shown in Fig. 5.The presence of an exotherm in aDSC curve of a suspect coating sampleclearly means that the sample has notbeen fully cured and that residual functional groups remain for continuedreaction. However, it is also quite possible that the coating might show noexotherm, yet still be undercured. Thiscould happen, for instance, in the caseof a two component epoxy if it had notbeen mixed correctly. There might verywell be unreacted epoxy resin present,but there will be no chemical reaction,and hence no exotherm, if there is nocuring agent left to complete the cure.Or perhaps the wrong curing agentwas used, either during the factory production of the coating or by the applicator at the jobsite.Fortunately, a DSC curve usuallygives an additional piece of informationbesides a curing exotherm: the glasstransition temperature. There are various definitions for the glass transitiontemperature (often abbreviated Tg).The most common, and perhaps mostpractical definition, is that the glasstransition temperature is that temperature below which a polymer is in ahard, glassy state, and above which it isin a flexible or rubbery state. The glasstransition temperature can vary fromone type of polymer to another.However, for any specific thermosetting polymer type, the glas

weeks to cure, and a plural-compo-nent sprayed polyurea might take sev-eral seconds to cure. Regardless of the time the chemical reaction or curing process takes, the coating must not be put into service before it cures. F a i l u r es Related to Incomplete Cure There are several possible explana-tions for why a coating may not have cured properly.

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