Osmotic BlisteringOsmosis is the process by which mois-ture molecules are transferred througha semi-permeable membrane. In thiscase, the coating film is the semi-per-meable membrane.

Osmotic blistering (Figs. 1 and 2) isprobably the most recognized type ofblistering that occurs in coatingsapplied to carbon steel that is subjectto immersion service or prolongedexposure to high-moisture environ-ments. The period of time that industrialcoating systems are exposed to mois-ture in normal atmospheric service envi-ronments is typically not sufficient toproduce osmotic blistering.

By James D. Machen, PCS, KTA-Tator, Inc.Rich Burgess, KTA-Tator, Inc., Series Editor

paintsquare.com / JPCL July 2014 21

Cases fromthe F-Files

listering formation in high-performance industrial coat-ings is a common phenome-non. Understanding why

blisters form is sometimes very simpleand in other instances quite complex.For example, understanding anddescribing the process by which mois-ture migrates through a coating film ata molecular level to achieve equilibriumcan be very complex, especially to thelayman. On the other hand, under-standing why blisters form as a resultof coatings application on surfaceswith elevated temperatures or onporous substrates (such as concrete)is much simpler.

Frequently in the coating industry, theterms “blistering” and “bubbling” areused synonymously. Technically, this isnot correct. When blisters form by anosmotic mechanism, the term “blister-ing” is more common and correct.However, when they are caused by non-osmotic methods, the term “blistering”is somewhat misleading and the term“bubbling” is more accurate. Blister for-mation is typically a result of increasedpressure from moisture accumulation at

certain points in a coating film, whilebubbles are typically formed as a resultof gas and vapor pressures within thecoating film or the substrate. Despitethis, these terms will likely continue tobe used interchangeably.

This article addresses both osmoticand other non-osmotic methods by whichblisters and bubbles form and identifiesthe roles that environmental conditions,substrate type, generic coating type andsurface contamination can play in theirformation. Because carbon steel andformed concrete are the most commonsubstrates to which industrial coatingsare applied, those substrates will also bethe focus of this article.

B

Common Causes of Blister and BubbleFormation in Industrial Coatings

Mechanismsof Failure

Fig. 1: Dense osmotic blistering. All figures courtesy of KTA-Tator, Inc.

Fig. 2: Spotty osmotic blistering

22 JPCL July 2014 / paintsquare.com

F-Files: Mechanisms of Failure

This webinar will explore issuesassociated with waterjettingprior to application of protectivecoatings. The latest SSPC water-jetting standards (WJ-1, WJ-2,WJ-3 and WJ-4) will be describedalong with information abouttheir use in project specifica-tions. The webinar will concludewith a brief review of technicalliterature relating to the per-formance of coatings appliedover waterjetted steel surfaces.

September 3, 201411:00 a.m.-Noon, EST

Register at paintsquare.com/education

FREE WEBINAR ON PERFORMANCE OF COATINGS OVER WATERJETTED SURFACES

Sponsored by:

WEBINAR EDUCATION SERIES

webinarJPCL1_Layout 1 7/3/14 10:37 AM Page 1

Click our Re

ader e-Card at paintsquare.com

/ric

side of the semi-permeable coating cre-ates osmotic pressure and causeswater molecules to slowly penetratethrough the molecular infrastructure ofthe coating. As moisture penetrates, itmigrates toward and accumulates atthe point where the more concentratedsolution of salt or solvent exists. Theosmotic forces accelerate the transportof water through the coating in anattempt to equalize pressures (reachequilibrium) on each side of the coatingfilm. Depending upon the concentrationof soluble contaminants on either sideof the osmotic cell, pressures canreach high levels (reportedly exceeding15,000 psi). When higher concentrationdifferentials of soluble salt contamina-tion are present on either side of acoating film, a greater accumulation offree moisture results and blisters canbe larger and more concentrated. Whenthese pressures exceed the coating’sadhesive bond to the substrate, a blis-ter forms.

Osmotic Blistering Due to Thermal GradientsThis phenomenon is commonly knownwithin the coatings industry as the “coldwall effect.” Thermal gradients occurwhen the metal or steel substrate in theimmersion zone of a tank or vessel is ata lower temperature than the liquid con-tained within the tank. Blisters form asthe warmer water molecules in thestored liquid penetrate the coating filmand then condense at a cooler interfacewithin the lining or at the lining/sub-strate interface. Ultimately, sufficientquantities of liquid accumulate, creatingpressure that causes liquid-filled blistersto form in the coating. One method tominimize thermal gradients is to useexterior insulation on tanks that maydevelop sufficient temperature differ-ences if otherwise left uninsulated.

There are some well-known mecha-nisms or driving forces that fuel osmoticblister formation. In simple terms, theactions of these forces (discussed ingreater detail later in this column) resultin the accumulation or concentration ofmoisture at specific points within thecoating film. These driving forces are:• contamination of the steel substrateby water-soluble salts;• water-soluble solvents trapped withinthe applied coating film; and• thermal gradients (temperature differ-ences) across the coated surface.

Osmotic Blistering Due to Water-Soluble Salts and Entrapped SolventsOsmotic blisters can form as a result ofwater-soluble salt contamination on thesurface of the steel being coated orwater-soluble solvents that are retainedor “trapped” within an applied coatinglayer. Because water-soluble salts (suchas chlorides, sulfates and nitrates) aretypically non-visible, detection of thesecontaminants in blister liquids requiresspecific analytical test methods suchas ion chromatography (IC) to confirmtheir presence. While retained ortrapped solvents can often be detectedby the telltale solvent smell of liquidtaken from blisters (blisters are oftenliquid-filled), their presence is verifiedby using laboratory analytical methodsfor the solvent detection such as gaschromatography/mass spectroscopy(GCMS).

Osmotic blisters resulting from thepresence of water-loving soluble saltsor retained solvents occur when thereis ample moisture in contact with acoating film (i.e., a coating in immer-sion service) and when there is a differ-ence on each side of the film in theconcentration of dissolved salts or sol-vents. This relative difference on each

paintsquare.com / JPCL July 2014 23

Non-Osmotic Blistering: BubblesEven though many blistering problemsare commonly associated with coatingsin immersion service and high continu-ous moisture exposures, blisters canand do form by other mechanisms.These non-osmotic blisters, which wecall bubbles, are often associated withcharacteristics of the substrate or envi-ronmental conditions during coatingapplication.

Coating Application During Highand Low TemperaturesIn many locations, the coating applica-tion season is limited to a time whenenvironmental conditions are favorable(i.e., typically in late spring, summerand early fall). Those “blue sky” daysmust be productive. The environmentalconditions that give us these kinds ofdays are usually advantageous; howev-er, they also have their pitfalls. Forexample, applying a coating to a sur-face in direct sunlight exposure orapplying the coating thicker than recom-mended can result in bubble formation.Bubbles typically form because the heatfrom the sun causes the surface of theapplied coating to dry more rapidly thanthe body of the coating film. This rapidsurface drying process creates a rigid,“skinned over” surface layer that pre-vents solvent within lower levels of thefilm from escaping. As the solvent in thelower layers heats, it volatilizes andexpands and creates vapor pressurewithin the coating film. It is the vaporpressure that causes bubbles to form.

In other instances, bubbles can form incoatings due to cooler temperatures orhigh relative humidity during application.When application occurs during coolertemperatures or elevated humidity, thedrying and curing of the coating film isslowed considerably. As a result, theproper release of solvents from the film

does not occur. Consequently, if the nextcoating layer is applied too soon, the sol-vent in the underlying film becomestrapped and bubble formation can occur.However, the bubbles may not occurimmediately, and their appearance canbe delayed until the environmental condi-

tions become warm enough to cause theentrapped solvents to volatilize.

Coating Application Over Porous SubstratesCoating application on porous substrates such as formed concrete

Click our Reader e-Card at paintsquare.com

/ric

24 JPCL July 2014 / paintsquare.com

F-Files: Mechanisms of Failure

Click our Re

ader e-Card at paintsquare.com

/ric

densing moisture) from the interior ofthe structure.

When a non-breathable coating isapplied over these porous substrates,the air and moisture often becomes“sealed” in the substrate. As a result,any condition (i.e., sunlight) that causesthe air to warm and the moisture tovaporize causes expansion andincreased pressure within the concrete.The increased pressure on the back-side of the coating often causes bub-bles to form.

Coating Application Over MoistureBubbles do not always appear on thesurface of the coating. Bubbles some-times form within the coating or on thebackside of the coating film. For exam-ple, moisture-cured urethane (MCU)coatings that dry and cure by reactionwith atmospheric moisture and othercoating types (i.e., aliphaticpolyurethanes) that are formulated withmoisture sensitive components canalso bubble.

Moisture-Cured Urethane BubblingWhen applying an MCU coating undermoist conditions, such as conditionswhere residual moisture remains on thesurface, relative humidity is too high orcondensing moisture or rain contactthe uncured MCU surface, the coatingperforms as designed, readily reactingwith moisture to achieve curing.Unfortunately, when moisture is abun-dant, the reaction occurs rapidly andcarbon dioxide gas (CO2) is generated(commonly termed “out-gassing”) as abyproduct of this reaction. Whenexcessive moisture accelerates curing,the CO2 is often trapped in the coatingfilm, and the resulting increase invapor pressure can produce bubbleswithin the coating. In fact, when across section of the coating is viewed

enters from the exterior or interior ofthe structure. Exterior moisture typical-ly enters through the natural porosityof the concrete substrate and alongcracks, crevices or control joints, andinterior moisture can result from“vapor drive” (i.e., humidity and con-

and concrete block (CMU) can alsoresult in bubbling. In such instances,the inherent porosity of the concretesubstrate often contains trapped air ormoisture. In that regard, air is presentbecause it will occupy any open spacethat is not under vacuum, and moisture

microscopically, it often has a Swisscheese-like appearance.

Aliphatic Polyurethane Bubbling

Two-part, aliphatic urethanes cure bypolymerization of the two components(polyol and isocyanate). However, whenapplied over moisture, bubbles canform on the backside of the urethanefilm (Fig. 3). This occurs because theisocyanate component of the urethaneformulation reacts with moisture. Aswith the MCU reaction discussed earli-er, CO2 gas is formed. The gas is typi-cally trapped in the lower layers of theurethane coating film and also at theinterface of any previously applied coat-ing where the moisture was present.Again, the pressure created by the gasformation causes bubbles to form. Onedifference with this formation processis that the bubbles can be very fine andare not always visible to the unaidedeye. When viewed microscopically in across section, the fine bubbles oftenhave a foam-like appearance. For thisreason, this phenomenon is typicallyreferred to as “foaming.”

In the case of bubbles with MCU andaliphatic urethane formulations, the out-gassing that occurs between layers candisrupt adhesion. Typically, other formsof bubbling have little to no adverseeffect on coating adhesion, unless veryintense and concentrated.

paintsquare.com / JPCL July 2014 25

Click our Reader e-Card at paintsquare.com

/ric

Fig. 3: Bubbling on the backside of a polyurethanepaint film

Click our Reader e-Card at paintsquare.com/ric

26 JPCL July 2014 / paintsquare.com

Click our Re

ader e-Card at paintsquare.com

/ric

F-Files: Mechanisms of Failure

There are differing opinions as to thedamage that can result from blistersand bubbles. One opinion is that if theyremain unbroken, they may not be con-sidered a coating defect that requiresrepair. For example, some more recent-ly developed 100%-solids, thick-film

coating formulations such as elastomer-ic urethane, urethane hybrids andpolyurea are often applied to form athick, monolithic and flexible protectivefilm. While blistering and bubbling maystill occur in these films, the monolithicand flexible nature of the film preventsthem from fracturing and the underlyingsubstrate often remains uncompro-mised. In addition, when used in immer-sion service, the actual weight andpressure of the liquid contained withinthe vessel can serve to hold the mono-lithic coating film in place on the sub-strate being protected. Under such cir-cumstances, whether there is any bene-fit to cutting out and repairing the blis-ters requires careful consideration.Even though blisters or bubbles are pre-sent, the flexible and monolithic filmoften has sufficient structural integrityof its own that allows the coating to ful-fill the originally expected service life.

About the AuthorJames D. Machen isa senior coatingsconsultant with KTA-Tator, Inc., a coatingsconsulting engineer-ing firm and distribu-tor of inspectioninstruments, where

he has been employed for over 20years. Machen is an SSPC-certifiedProtective Coatings Specialist, a NACE-certified Coatings Inspector Level 3(Peer Review) and a Level II Inspector inaccordance with ASTM D4537. He per-forms coating failure analyses, coatingsystem recommendations, specificationpreparation and major project manage-ment for a variety of clients in the trans-portation, water and waste, power gen-eration, chemical processing andmarine industries. He is a graduate ofThe Pennsylvania State University.

ConclusionIn summary, blistering and bubbling arecommon coating problems that occurfor several different reasons. While thisarticle presents and discusses theosmotic mechanisms that produce blis-ters, bubbles have several causes.

Click our Reader e-Card at paintsquare.com/ric

SealForLife in Quark_Layout 1 6/12/14 9:06 AM Page 1