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ABSTRACT—Barrier coatings are widely used in conservation to add a measure of reversibility to an otherwise irreversible adhesiv e bond. Barr ier materi- als are applied as thin films to mating surfaces prior to application of a primary (irreversible) adhesive. Subsequently, if reversal is required,the barrier layer can be softened or dissolved, releasing the bonded  joint.This inv estigation was undertaken to determine the suitability of two synthetic resins for use as barrier layers in the bonding of wood with epoxy. The two materials in question, Paraloid B-72 and Paraloid B-67, were chosen because of their potential to be practically reversible in low-polarity solvents. The tw o polymers w ere compared, as barri er materi- als, to tw o pr ov en barrier co atings , hide gl ue and Butvar B-98, by measuring their strength in shear according to ASTM D905-98. Inv estigations w ere also undertaken to determine the amount of time necessary for barr ier layers to dry prior to application of epoxy. Finally the practical reversibility of the barr ier c oatings was empirically evaluated. Paraloid B-72 was found t o be a suitable barrier mater ial in all respects, while Paraloid B-67 failed both strength and rev ersibility tests. TITRE—Une évaluation de quatre matériaux employés comme couche isolante lors des réparations structurelles d’objets en bois à l’aide de colle époxyde. RÉSUMÉ—Les cou ches isolantes sont très souvent utilisées en restauration pour aider à la réversibilité d’un lien adhésif autrement irréversible. Le matériau sélectionné est appliqué en un mince film sur les surfaces à coller avant l’application de l’adhésif (irréversible). Éventuel lement, s’il y a besoin de défaire ce lien adhésif, la couche isolante pourra être ramollie ou dissoute de façon à détacher les pièces. Cette rech erche a été entreprise dans le but d’évaluer l’efficacité de deux résines synthétiques lorsqu’elles sont employées comme couche isolante lors de la réparation d’objets en bois avec de la colle époxyde. Les résines Paraloid B-72 et Acryloid B-67 ont été sélectionnées par le fait qu’elles demeurent solub les dans des solv ants peu pola ires. Les deux résines ont été comparées à deux autres matériaux ayant déjà fait leur preuve comme couches isolantes, soit la colle de peau et le Butvar B-98, en mesurant leur résistance au cisaillement selon le standard ASTM D905-98. Des recherch es ont également été menées pour déterminer le temps de séchage requis avant l’application de la co lle époxyde . Finalement, la réversibilité a été év aluée de façon empirique. Les résultats démontrent que le Paraloid B-72 peut ser vir adéquatement comme couche isolante, tandis que l’Acryloid B-67 donne des résultats non-satisfaisants au niveau de la résistance du lien et de sa réversibilité. TITULO—Evaluación de cuatro recubrimientos de barrera y combinaciones de epoxi en la reparación estructural de objetos de madera. RESUMEN—Los recubrimientos de barrera son ampliamente usados en conservación para agregar una medida de reversibilidad a lo que de otro modo sería una unión adhesiva irreversible. Los materiales de barrera son aplicados en forma de películas delgadas a las superficies que se van a unir, previo a la aplicac ión de un adhesi vo primario (irreversible).Si posteriormente se requiere que esta unión s ea rev ersible , la capa de barrer a puede ser ablandada o disuelta, liberando la unión así adherida. Esta investigación fue efectuada para determinar si dos tipos de resinas sintéticas son apropiadas como capas de barrera en la unión de madera con adhesivo epóxi. Los dos materiales en cuestión, Paraloyd B-72 y Acryloid B-67, fueron esc ogidos por tener el potencial de ser reversibles en la practica, en solvent es de baja polaridad. Los dos polímeros fuer on comparados, como materiales barrera, con dos recu brimientos de barrera ya probados: cola anim al y Butvar B-9 8, midien do su fuerza de rot ura, de acue rdo a la norma ASTM D 905-9 8. Se efect uaro n, además, inv estigac iones para determinar el tiempo de secado que necesitan las capas de barrera previo a la aplicación del adhesivo epo xi. Finalmente, fue evaluada empíricamente la reversibilidad práctica de las capas de barrera. Se encontró que el Paralo yd B-72 es un material de barrera apropiado en todos los aspect os, mient ras que el Acrylo id B-67 falló en las pruebas de fuerza y de reversibilidad. AN EVALUATION OF FOUR BARRIER-COATING AND EPOXY COMBINATIONS IN THE STRUCTURAL REPAIR OF WOODEN OBJECTS LISA ELLIS AND ARLEN HEGINBOTHAM  JAIC 43 (2004):21–35

Transcript of Ellis, Heginbotham 2004, Barrier Coating and Eopoxy in Structural Repair of Wooden Objects

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ABSTRACT—Barrier coatings are widely used inconservation to add a measure of reversibility to anotherwise irreversible adhesive bond.Barrier materi-als are applied as thin films to mating surfaces prior to application of a primary (irreversible) adhesive.Subsequently, if reversal is required, the barrier layer can be softened or dissolved, releasing the bonded

 joint.This investigation was undertaken to determinethe suitability of two synthetic resins for use asbarrier layers in the bonding of wood with epoxy.

The two materials in question, Paraloid B-72 andParaloid B-67,were chosen because of their potentialto be practically reversible in low-polarity solvents.The two polymers were compared,as barrier materi-als, to two proven barrier coatings, hide glue andButvar B-98, by measuring their strength in shear according to ASTM D905-98. Investigations werealso undertaken to determine the amount of timenecessary for barr ier layers to dry prior to applicationof epoxy. Finally the practical reversibility of thebarrier coatings was empirically evaluated. ParaloidB-72 was found to be a suitable barrier material in allrespects,while Paraloid B-67 failed both strength andreversibility tests.

TITRE—Une évaluation de quatre matériauxemployés comme couche isolante lors des réparationsstructurelles d’objets en bois à l’aide de colleépoxyde.RÉSUMÉ—Les couches isolantes sont trèssouvent utilisées en restauration pour aider à laréversibilité d’un lien adhésif autrement irréversible.Le matériau sélectionné est appliqué en un mincefilm sur les surfaces à coller avant l’application del’adhésif (irréversible). Éventuellement, s’il y a besoinde défaire ce lien adhésif, la couche isolante pourra

être ramollie ou dissoute de façon à détacher lespièces. Cette recherche a été entreprise dans le butd’évaluer l’efficacité de deux résines synthétiqueslorsqu’elles sont employées comme couche isolantelors de la réparation d’objets en bois avec de la colleépoxyde. Les résines Paraloid B-72 et Acryloid B-67ont été sélectionnées par le fait qu’elles demeurentsolubles dans des solvants peu polaires. Les deux

résines ont été comparées à deux autres matériauxayant déjà fait leur preuve comme couches isolantes,soit la colle de peau et le Butvar B-98, en mesurantleur résistance au cisaillement selon le standardASTM D905-98. Des recherches ont également étémenées pour déterminer le temps de séchage requisavant l’application de la colle époxyde. Finalement, laréversibilité a été évaluée de façon empirique. Lesrésultats démontrent que le Paraloid B-72 peut servir adéquatement comme couche isolante, tandis que

l’Acryloid B-67 donne des résultats non-satisfaisantsau niveau de la résistance du lien et de sa réversibilité.

TITULO—Evaluación de cuatro recubrimientos debarrera y combinaciones de epoxi en la reparaciónestructural de objetos de madera. RESUMEN—Losrecubrimientos de barrera son ampliamente usados enconservación para agregar una medida de reversibilidada lo que de otro modo sería una unión adhesivairreversible. Los materiales de barrera son aplicados enforma de películas delgadas a las superficies que se vana unir, previo a la aplicación de un adhesivo primario(irreversible). Si posteriormente se requiere que estaunión sea reversible, la capa de barrera puede ser ablandada o disuelta, liberando la unión así adherida.Esta investigación fue efectuada para determinar si dostipos de resinas sintéticas son apropiadas como capas debarrera en la unión de madera con adhesivo epóxi.Losdos materiales en cuestión, Paraloyd B-72 y AcryloidB-67, fueron escogidos por tener el potencial de ser reversibles en la practica, en solventes de baja polaridad.Los dos polímeros fueron comparados,como materialesbarrera, con dos recubrimientos de barrera ya probados:cola animal y Butvar B-98, midiendo su fuerza derotura, de acuerdo a la norma ASTM D905-98. Se

efectuaron, además, investigaciones para determinar eltiempo de secado que necesitan las capas de barreraprevio a la aplicación del adhesivo epoxi. Finalmente,fue evaluada empíricamente la reversibilidad práctica delas capas de barrera. Se encontró que el Paraloyd B-72es un material de barrera apropiado en todos losaspectos, mientras que el Acryloid B-67 falló en laspruebas de fuerza y de reversibilidad.

AN EVALUATION OF FOUR BARRIER-COATING ANDEPOXY COMBINATIONS IN THE STRUCTURAL REPAIR

OF WOODEN OBJECTS

LISA ELLIS AND ARLEN HEGINBOTHAM

 JAIC 43 (2004):21–35

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TÍTULO—Uma avaliação de quatro revestimentos deproteção e combinações de epóxi no reparo estruturalde objetos de madeira. RESUMO—Revestimentos deproteção são amplamente usados em conservação paraaumentar o grau de reversibilidade de uma aderência,de outro modo, irreversível. Materiais de proteção sãoaplicados como finas películas para unir superfíciesantes da aplicação de um adesivo principal(irreversível). Posteriormente, se a reversão for necessária, a camada de proteção pode ser amolecida oudissolvida,liberando a junção.Esta pesquisa foi realizadaa fim de determinar a conveniência do uso de duasresinas sintéticas como camadas de proteção na

aderência de madeira com epóxi.Os dois materiais emquestão, Paraloid B-72 e Acryloid B-67, foramescolhidos por seu potencial reversibilidade emsolventes de baixa polaridade. Os dois polímeros,enquanto materiais de proteção, foram comparados adois revestimentos de proteção testados, cola animal eButvar B-98,através da medição de sua força de adesão,de acordo com ASTM D905-98.Também foram feitaspesquisas para determinar a quantidade de temponecessária para as camadas de proteção secarem antes daaplicação de epóxi. Finalmente,a reversibilidade práticados revestimentos de proteção foi empiricamenteavaliada. Paraloid B-72 revelou-se um material deproteção apropriado em todos os aspectos, enquantoAcryloid B-67 falhou em ambos os testes de resistênciae reversibilidade.

1. INTRODUCTION

In the conservation of wooden artifacts, it is oftennecessary to repair broken wooden elements thatserve a structural or load-bearing function. Suchrepairs must have high strength, yet be reversible inthe future.Where the break in question is recent andthe mating surfaces are clean and undisrupted, animal

hide glue is widely believed to be a suitable adhesive,though, in practice, reversal of intact hide glue bondscan be problematic. In cases where the matingsurfaces are dirty or damaged, or a gap-filling adhe-sive is needed, animal hide glue may have greatlyreduced strength, and an alternative adhesive may berequired.Bulked epoxy resins have found wide use insuch instances,and some have the advantage that after 

setting they can be carved,sawn,sanded,and finished,allowing them to be used simultaneously as bothadhesive and fill material.These qualities, along withtheir high strength,can recommend the use of bulkedepoxies over laboratory-prepared acrylic adhesivessuch as Paraloid B-72 bulked with glass microspheresor fumed silica. One commercially available bulkedepoxy, widely used by furniture conservators, isAraldite 1253, a carvable paste epoxy, bulked withtitanium dioxide, amorphous silica, iron oxide, andphenolic resin (Ciba 2001). The primary disadvan-tage of using epoxies in conservation is that, oncecured, the bonds formed can be extremely difficult to

reverse due to the insolubility of the cross-linkedpolymer in solvents.

Barrier coatings add a measure of reversibility toadhesive bonds that might otherwise be impossible torelease. They are applied as thin films to matingsurfaces prior to the application of the difficult-to-reverse, primary adhesive. In the conservation of wooden artifacts, animal hide glue has been used as abarrier material for epoxy joins due to its highstrength, its ease of use, and its familiarity amongfurniture conservators. Hide glue, however, hascertain disadvantages when used as a barrier material.First and foremost, it is not always reversible in a safeand practical manner. Reversal depends on moistureand/or heat, both of which can cause damage towood and associated finish materials. Some promis-ing work has been reported using microwave radia-tion to reverse hide glue bonds (Neher 1996).However, the equipment necessary is quite expen-sive, and the technique has not gained wide accept-ance. In addition, hide glue is known to weakenwhen exposed to extremes of humidity (Buck 1990)and may degrade over long periods of time.

Recognizing the difficulty in reversing repairsmade using hide glue as a barrier, and seeking an

appropriate alternative, Anderson and Podmaniczky(1990) tested the suitability of Butvar B-98 (polyvinyl butyral) as a barrier layer for epoxy joins inwood. Butvar B-98 is often referred to simply as apolyvinyl butyral, but it is actually a copolymer of polyvinyl butyral, polyvinyl alcohol, and polyvinylacetate in a ratio of approximately 40:10:1 (Horie1987, 101–102; Monsanto 1994).This polymer was

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recently shown experimentally by Spirydowicz et al.(2001) to be a suitably stable material for the consol-idation of dry archaeological wood. Reporting onthe results of their testing, Anderson and Podman-iczky (1990) noted that while barr ier coatings shouldhelp make epoxy repairs more easily reversible, theymust also maintain the overall strength of the bond.The results of their work demonstrated that Butvar B-98 dissolved in ethanol is a suitably strong barrier material when used in conjunction with the bulkedepoxy Araldite 1253.

Butvar B-98, while a good alternative to hideglue as a barrier material, nevertheless has significant

limitations with regard to its reversibility; it is solublein polar solvents such as alcohols and in certainmixtures of polar and nonpolar solvents (Monsanto1994; Spirydowicz et al. 2001). Unfortunately, manyvarnishes and paints used traditionally to coatwooden artifacts are also sensitive to this range of solvents, making it difficult or impossible to dissolvea Butvar B-98 barrier layer without damaging anadjacent surface coating. This is particularly truebecause extended exposure may be necessary toallow the solvent (present as either a liquid or vapor)to penetrate deep into the repair and dissolve thebarrier. Even after Anderson and Podmaniczky’simportant study, therefore, a need remained for awell-tested barrier coating of high strength thatcould be reversed in low-polarity solvents.

In this study, the authors chose Paraloid B-72 andParaloid B-67 for comparison with the other provenbarrier adhesives because they have advantageousdissolution properties, they are readily available, andthey are well known and widely used by conserva-tors. Paraloid B-72, formerly known as Acryloid B-72, a copolymer of ethyl methacrylate andmethylacrylate, is a Feller Class A material and is notknown to become insoluble or degrade over time

(Horie 1987, 106). Its inclusion in this study seemedobvious: it is a mainstay in the conservator’s studio,and its strength, when used in combination withepoxies in the bonding of stone, has recently beenclearly established and published by Podany et al.(2001). In addition, it is soluble in a wide range of solvents, ranging from low-polarity solvents such asxylenes to high-polarity solvents such as acetone. In

the context of wooden artifacts conservation, thisproperty confers the significant advantage that abarrier layer of Paraloid B-72 could be applied inacetone—a solvent that is fast-drying and low intoxicity, while remaining reversible in xylenes, asolvent that will not dissolve or damage most associ-ated historic furniture finishes or paints.

Paraloid B-67, poly isobutyl methacrylate,formerly known as Arcyloid B-67, is also considereda Feller Class A material even though it is known tocross-link over time (Horie 1987,108). It was consid-ered in this study because it is reversible in low-aromatic hydrocarbons that present less of a health

hazard than the fully aromatic solvents necessary toreverse Paraloid B-72.

The authors conducted comparative shear-strength testing using Araldite 1253 bulked epoxywith all four of the mentioned barrier coatings (hideglue,Butvar B-98, Paraloid B-72, and Paraloid B-67).Results were compared with the measured strengthof Araldite 1253 used without a barrier coating. Itwas hoped that if Paraloid B-72 or Paraloid B-67proved to be of comparable strength to the other twoproven barrier materials, the results of this workwould provide conservators with more options inchoosing a barrier coating when factors such as thesolubility of an original finish need to be considered.To confirm that the adhesive bonds using barrier layers were in fact reversible, the authors alsoconducted empirical reversibility testing.

2. METHODOLOGY

This study was organized into four components.First, a barrier application protocol was established,and the amount of time required for barrier layers todry was determined experimentally. Second, theshear strength of adhesive bond made with Araldite

1253 epoxy and each of the barrier materials, as wellas that of the Araldite 1253 alone, was determinedquantitatively according to ASTM D905-98 (ASTM2001).Third, the patterns of failure in the test sampleswere analyzed. Both strength testing and failureanalysis were conducted according to ASTM standardD 905-98. Fourth, the practical reversibility of thebarrier materials was tested empirically.

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2.1 BARRIER APPLICATIONPROTOCOL

The following solutions of barrier mater ials werechosen for testing:• 10% (w/v) solution of Butvar 98 in ethanol• 17% (w/v) solution of Paraloid B-72 in acetone• 17% (w/v) solution of Paraloid B-67 in Shell

Mineral Spirits 135• Titebond Liquid Hide Glue direct from manu-

facturer’s container The solutions were formulated as such for two

reasons: they needed to be concentrated enough to

leave a significant amount of material on the surfaceof the wood, but they also had to be applied in acontinuous, even coat with a brush. Anderson andPodmaniczky used a 20% (w/v) solution of Butvar B-98 in ethanol in their study. In our experience,however, this mixture proved too viscous to brush onconveniently, and a concentration of 10% in ethanolwas used instead.The 17% solution of Paraloid B-72(w/v) in acetone was chosen because it had beenused successfully in the 2001 study by Podany et al.and was found to be easy to apply.The 17% solutionof Paraloid B-67 in Shell Mineral Spirits 135 waschosen to be comparable to the Paraloid B-72 solu-tion.The Titebond hide glue was chosen because it iswidely available in a reasonably standardized formu-lation and was found to be of a strength comparableto typical hot animal hide glues in moderate humid-ity environments (Buck 1990). While some contestthe comparability of hot and liquid hide glues(Podmaniczky 2003), it was assumed that for thepurposes of this study the liquid hide glue could beconsidered generally representative of the highlyvariable class of collagen-based glues.The viscosity of the liquid hide glue was found to be suitable directlyfrom the manufacturer’s bottle.

The barrier coatings were applied as consistentlyas possible to samples of hard maple similar to thosecalled for in the ASTM shear-strength testingmethod. For each kind of coating, a new brush wasdipped into the jar containing the solution; the bris-tles were then brushed against the rim of the jar; andthe brush was flipped over and excess solutionbrushed away. The sample was then brushed once

along its length and again across the grain.The coat-ings applied this way appeared to completely saturatethe surface of the wood, leaving no bare or dry areas.

After the first application had dried, the barrier coatings were examined. While the hide glue layer appeared coherent and glossy over the entire surface,the three synthetic resin layers did not; therefore, asecond coat of each synthetic resin was applied over the first. Upon drying, all three of these samplesappeared to have a reasonably thin, yet coherent andglossy film over the test surface.We therefore decidedthat in preparing the sample blocks for shear-strengthtesting, the hide glue barrier layer would be applied

in a single layer, while the three synthetic resinswould be applied in two layers.

2.2 SOLVENT EVAPORATION FROMTHE BARRIER COATINGS

We next tried to determine the length of timerequired for the barrier layers to dry, prior to theapplication of the epoxy adhesive. It is generallyaccepted that there should be little to no solventremaining in the coating, as retained solvent can actas a plasticizer within the resin and thus weaken thebarrier film (Podany et al. 2001, 27).

2.2.1 TESTING METHOD

A simple test was designed to establish when thesolvent had evaporated from barrier layers. Smallwafers of hard maple (the wood called for in theASTM shear-strength testing method) were paintedwith the barrier coatings and then weighed periodi-cally until there was no more significant weightchange.The maple was cut into 20 samples measur-ing approximately 5 x 7.5 x 0.4 cm. Five pieces of wood were set aside to be used as controls to track

the changes in weight of the substrate caused by fluc-tuations in the ambient relative humidity. The 15remaining samples were divided into three groups.Each group was coated with two coats of thesynthetic resin barrier solutions, the second coatfollowing the first by three days. Hide glue was nottested for solvent evaporation time because of antic-ipated complications due to its continual weight

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change with fluctuations in ambient relative humid-ity. Changes in weight for all 20 samples wererecorded in the same order, using an Ohaus PrecisionStandard scale, which is accurate up to 1 mg (0.001g). Since the weight of solvent added in each coat wastypically about 0.3 g, the scale was effectively accu-rate to approximately 0.3% for measuring solventloss. The five wafers of each group were weighedindividually, and their weights were then averaged.Initially, weight measurements were taken every half hour.The interval between measurements increasedwith time until, five days after the second applicationof barr iers, measurements were taken twice a day.We

felt that once the weight stopped changing measura-bly, it could be concluded that the barrier adhesiveswere essentially free of solvent.

To better understand the dynamics of solventevaporation from wood substrates, the same test wascarried out using two different substrates,Douglas fir wood and 4 mil Mylar polyester film (essentiallynonabsorbent).These results were compared to thoserecorded for hard maple.

2.2.2 Results of Solvent Evaporation Tests

The solvent evaporation testing on mapleshowed that all three nonaqueous barrier-coatingmaterials would be essentially solvent free within fivedays of the application of the second coat. Asexpected, acetone and ethanol, the faster-evaporatingsolvents, yielded dry films more quickly than theslower-evaporating Shell Mineral Spir its 135.Table 1shows the times required for the 98% and 100%

evaporation of solvent from the second coat of barrier solution.Based on this result,sample blocks of maple that had been coated with the barrier layersand allowed to dry for three days, then recoated andallowed to dry for five days, were considered suitablefor strength testing according to ASTM standards.

Two interesting phenomena were observedduring evaporation testing. First, it became clear thatthe nature of the substrate played a large role in theevaporation rate of solvent from the resin layer.Figures 1 and 2 illustrate the extremely differentevaporation rates for three different substrates wheninitially coated with resin solutions. Figure 1 shows

the progress of drying for the first coat of 17% (w/v)Paraloid B-72 in acetone when applied to Mylar,maple, and Douglas fir test panels. On the Mylar substrate,which is essentially nonabsorbent, over 99%of the solvent applied had evaporated within oneminute. In contrast, on the test wafers of maple, adense and even-grained wood, it took approximately21 hours for 98% of the solvent to evaporate.Withthe fir substrate, which is lighter than maple and hasdistinct hard and soft zones in each annual ring, ittook approximately 29 hours for 98% of the solventin the first coat to evaporate. Figure 2 shows theprogress of drying for 17% (w/v) Paraloid B-67 inShell Mineral Spir its 135 (a much slower-evaporatingsolvent than acetone) on the same three substrates.On the Mylar substrate, 98% of the solvent appliedhad evaporated after only 1 hour and 40 minutes.Onthe test wafers of maple, it took approximately 60hours for 98% of the solvent to evaporate, and withthe fir substrate, it took over 91 hours for 98% of the

Table 1. Time Required for Solvent Evaporation from Second Coat of 

Barrier Material on Hard Maple Substrate

98% Evaporated

Terminal Weight

(100% evaporated)

Paraloid B-72 (17% [w/v] in acetone) 2.5 hours 28 hours

Butvar B-98 (10% [w/v] in ethanol) 26 hours 51 hours

Acryloid B-67 (17% [w/v] in Shell 135) 75 hours 124 hours

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solvent to evaporate. It is interesting to note that inthe drying curves for Paraloid B-67 on maple and fir substrates, the fir samples initially dried more quicklythan the maple samples, but were overtaken by themaple after about a day.The cause of this phenome-non is unknown, but it suggests that the mechanismsof solvent evaporation from wooden substrates arecomplex and may depend on a wide range of vari-ables, such as the anatomical characteristics of thewood, interactions with bound water in the wood,the condition of the wood surface, the affinity of theparticular solvent for both the resin and the wood,the volatility of the solvent, and the film thickness.

The second phenomenon noted during evapora-tion testing was that, on wooden substrates, the firstand second coats of resin solutions dried at differentrates.With the fast-evaporating solvents, acetone andethanol, the second coat of barrier material clearlydried more quickly than the first. Presumably this isbecause the first layer of resin seals the wood, so thatwhen the second coat is applied, the solvent is notabsorbed into the wood to the same degree. Figure 3shows the percent of solvent evaporated vs. length of 

time for the first and second coats of 17% (w/v)Paraloid B-72 in acetone applied to maple substrate.While the first coat did not reach 98% evaporationuntil 21 hours after application, the second coat was98% dry after only 2.5 hours. In contrast to theresults with fast-evaporating solvents, the second coatof barrier in slow-evaporating mineral spirits driedmore slowly than the first.Figure 4 shows the percentof solvent evaporated vs. length of time for the firstand second coats of 17% (w/v) Paraloid B-67 in ShellMineral Spirits 135 applied to maple substrate.Whilethe drying rates are much more similar than withParaloid B-72 in acetone, it is clear from the graph

that the first coat evaporated more quickly than thefirst.This result presumably indicates that some of thesolvent in the second coat penetrated the first coatand was absorbed into the wood.The increased over-all thickness of resin after the second coat may thenhave contributed to an overall slower drying of thecoating layer.

The degree to which barrier layers on woodshould be allowed to dry before final assembly with

Fig. 1. Evaporation of solvent from 17% (w/v) Paraloid B-72 inacetone applied to three different substrates

Fig. 2. Evaporation of solvent from 17% (w/v) Paraloid B-67in Shell Mineral Spirits 135 applied to three different substrates

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epoxy is difficult to judge with certainty. Both for convenience and because contamination of thesurface by airborne pollutants could result in a weak-ened bond, it is better to close joints soon after thesurfaces are prepared. However, premature bondingwhen using barrier coatings could result in a jointthat is initially weak due to placticizing effects of retained solvent. It might also result in bonds that areweakened even after the eventual drying of thesolvent (the retained solvent could impair the bond-ing of the epoxy to the resin,or the shrinkage of theresin film during drying could cause internal stresseswithin the joint). Based on our results, it should also

be considered that when applied to wood, a signifi-cant portion of the solvent may be retained, not inthe resin layer but in the wood itself. In this case, anyremaining solvent would not be likely to contributeto plasticizing or weakening the barrier layer. In anyevent,more testing is clearly warranted to determinethe relationship between solvent retention and thestrength of barrier coatings on wood substrates.

2.3 STRENGTH TESTING

Unfortunately, there is no ASTM standard for determining the strength of combinations of barrier coatings and adhesives. Where possible, this studyused the testing methodology as specified by ASTMD905-98,“Standard Test Method for Strength Prop-erties of Adhesive Bonds in Shear by CompressionLoading,” and modifications were made where neces-sary.Modifications were in some cases based on thosemade by Podany et al. in their 2001 study and byAnderson and Podmaniczky in 1990.

2.3.1 Determination of Specific GravityASTM D905-98 specifies the use of hard maple

for shear-strength testing of adhesives. It further stip-ulates that the maple used fall within a certain rangeof specific gravity.To measure the specific gravity of the maple obtained for this test, two small pieces of the wood were oven-dried, according to the specifi-

Fig. 3. Comparison of solvent evaporation rates from the firstand second coats of 17% (w/v) Paraloid B-72 in acetone whenapplied to hard maple

Fig. 4. Comparison of solvent evaporation from the first andsecond coats of 17% (w/v) Paraloid B-67 in Shell MineralSpirits 135 when applied to hard maple

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cations of ASTM D143-94 (2001), “Standard TestMethods for Small Clear Specimens of Lumber.”Thetwo samples were weighed and then placed in anoven at 103°C until their weight loss ceased chang-ing. The moisture content of the wood was thendetermined by dividing the samples’ loss in mass bythe oven-dry mass.These results were used to calcu-late the specific gravity of the blocks as shown inAppendix X1 of ASTM D905-98 (ASTM 2001,25).The specific gravity of the hard maple stock fell in anacceptable range.

2.3.2 Sample Preparation

The size of the samples specified by ASTMD905-98 was too large for the Getty ConservationInstitute’s Instron tensile testing-machine, which islimited by a 10 kilonewton (kN) load cell. Based onthe ultimate strength of trial samples of different sizesprepared for this purpose, it was calculated that themachine would be able to run samples of approxi-mately one-quarter of the specified size. The finalconfiguration of the test blocks (fig. 5) results in abond area of 1 sq. in.

The 3/4 in.x 9 1/4 in. hard maple stock was cutinto 1 1/4 in. strips across the grain on a table saw.Surfaces to be glued were prepared by sanding lightlywith 320 grit abrasive paper.The wooden strips weredivided into five groups, four of which were coatedon one face with each of the respective barrier mate-rials in the manner described in section 2.1.The fifthgroup was not coated with any barrier material andserved as a control.After drying, pairs of strips fromeach group were bonded together using Araldite1253 in the manner described below. The twocomponents of the epoxy were measured beforemixing by weight according to the product datasheet, provided by manufacturer Vantico (Vantico Inc.

2002), which specifies the optimal resin:hardener weight ratio at 100:82.One of the strips, already coated with the barrier,

was covered with the epoxy paste and laid, face up, ina jig built for this experiment.The second strip wasplaced into the jig above the first, overhanging byapproximately a quarter of an inch.The upper stripwas pushed down in the jig so that the adhesive layer 

measured 0.030 in. or 30 mils thick.This adhesive-layer thickness was chosen to approximate a typicalgap-filling bond as might be required in woodenartifacts conservation. The samples were removedfrom the jig and left to cure for eight days. Excessepoxy was removed from the edges of the strips, firstusing a shoulder plane.

Some suggest that the bond between glue andepoxy may be strengthened if the epoxy is appliedbefore the glue is completely dry or if the surface of the dried glue is scuffed prior to application of theepoxy (Podmaniczky 2003). The sample blocksprepared with a hide glue bar rier layer for this study,however, were assembled after the glue had thor-oughly dried, without any surface preparation.

Once the samples had been cleaned of excessadhesive, the bonded wooden strips were then cut

into smaller pieces, approximately 1 in. wide, on atable saw. Ten small test blocks were prepared andlabeled for each barrier material. The bond area of each sample block was then calculated by measuringthe width and length of the bonded area.These datawere recorded for use in determining the final loadat failure for each test block.

Fig. 5. Dimensions of hard maple samples prepared for shear-strength testing

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2.3.3 Shear-Strength Measurement

The samples were tested in batches according tosample type by the same operator dur ing a four-hour run. The samples were held in sealed polyethylenebags until just before testing.Ten samples per sampletype were run. Both parts of each sample werelabeled in pencil according to adhesive type andnumbered in sequence for post-testing analysis of thebreak edge.

A Model 4201 Instron with a 10 kN load cell,belonging to the Getty Conservation Institute, wasused to test the samples (fig. 6). Instron Series IX

Automated Materials Tester software,version 8.06.00,was used to run the tests and to partially analyze thedata. Before each sample was run, the operator entered the width and thickness of each sample.TheInstron’s moving crosshead was configured to pushdown on one of the two bonded sample blocks whilethe other was held in a fixed position,creating a shear stress on the adhesive bond (fig.7).The crosshead wasset to move down at a constant rate of 5 mm per minute, the ASTM standard specified speed, until thesample failed.

For each sample, the Instron Series IX Auto-mated Materials Tester software then calculated themaximum load, displacement of the crosshead atmaximum load, and stress at maximum load, as wellas the mean and standard deviation of the samplesgrouped together.The software also produced graphsshowing these results.

The quantitative results of the Instron testingwere then subjected, by group, to the “Q” test for outliers at the 90% confidence level.This test is partof ASTM E178-94 (2001). The test considers thenumber of samples and the distribution of the results.Any individual result deemed incompatible with the

spread of the others is excluded within a determinedconfidence level.

2.3.4 Results of Shear-Strength Testing

Table 2 and figure 8 show the results of theInstron shear-strength testing. Araldite 1253 epoxybonds prepared with Butvar B-98 and Paraloid B-72barrier layers proved to be as strong as or stronger than bonds prepared with no barrier. Bonds preparedwith liquid hide glue barrier layers were weaker on

Fig. 6.The Instron Model 4201 with 10 kN load cell at the Getty Conservation Institute

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average, but nearly as strong as bonds with no barrier,while bonds prepared with Paraloid B-67 barrier layers were much weaker than any other type.Theseresults indicate that Butvar B-98, Paraloid B-72, andliquid hide glue barrier layers yield high-strengthepoxy bonds and can be considered suitable barrier materials for use with wood and epoxy adhesive.Conversely, the use of Paraloid B-67 was shown toresult in consistently weak bonds and to be clearlyunsuitable for use as a barrier material.The failure of Paraloid B-67 to produce a sufficiently strong bondwas disappointing, since it could have provided abarrier method reversible in solvents of low polar-ity—safe for objects, and with low toxicity, safe for conservators. This failure may be directly related tothe low polarity of the Paraloid B-67 polymer. It maybe that epoxy resin, which is a highly polar material(Down 2001), is unable to bond satisfactorily to sucha low-polarity material.This hypothesis is supportedby the fact that 100% of failure occurred between theParaloid B-67 and epoxy layers (see below, sec. 2.4).If this outcome is in fact the case, then the search for a strong barrier material soluble in low or nonaro-

matic solvents may be inherently unlikely to succeed.

2.4 FAILURE ANALYSIS: ESTIMATEDPERCENTAGE WOOD FAILURE

ASTM D905-98 specifies that an estimatedpercentage wood failure be calculated. In a simpleadhesive-testing scenario, this calculation serves to

distinguish between areas in which the adhesivehas failed and areas where the substrate hasfailed. This study, however, presented a morecomplex situation than anticipated by ASTMstandards because of the use of the barrier coat-ings.The control samples prepared with simpleepoxy bonds were analyzed according to theASTM standard. For samples prepared withbarrier adhesives, failure was divided into thefollowing four groups:1. wood failure, wherein wood was removedfrom one of the faces of the wooden sample2. barrier coating–wood failure,meaning that

the barrier coating was pulled from thewooden face, sometimes, but not always, taking

tiny wood fibers along with it3. barrier coating–epoxy failure, where the join

failed in the interface between the two4. Epoxy failure, where the epoxy adhesive was

pulled apart, and remains were found on bothfaces of the sample.For all samples, the percentage failure of each type

per sample was defined using a gridded, transparentplastic ruler (fig. 9). The grid, which divides squareinches into 254 units,was placed on top of each bondsurface after failure and examined under a stereomi-croscope.The number of square units in which eachtype of failure occurred were counted, and thepercentage of failure for each type was calculated.

In the event that an epoxy bond on a woodenartifact is stressed to the point of failure, it is prefer-able that the adhesive break away cleanly from thesubstrate without causing additional damage to thewood. The results of the failure analysis, shown intable 3, indicate that all four barrier materials testedoffer some protection to the underlying wood byreducing wood failure when the bond is broken.Wood failure, which was 6.8% for wood bonded

directly with epoxy, was reduced to nil or virtually nilwhen any of the four barriers was used. Butvar B-98and liquid hide glue samples tended to fail at theinterface between the epoxy and the barrier layer,while Paraloid B-72 tended to fail at the interfacebetween the wood and the barrier. Paraloid B-67,which failed the overall strength testing, always failedat the interface between the epoxy and the barrier,

Fig. 7. Detail of the Instron’s crosshead with a sample in place

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indicating that the cause of failure was poor adhesion

between the Paraloid B-67 and the epoxy.

2.5 REVERSIBILITY OF BARRIERCOATINGS IN SOLVENT VAPORS

To test the reversibility of the barrier coatings,spare sample blocks of maple, coated with the barri-ers, were bonded to one another using Araldite 1253.

The three barrier materials reversible in organicsolvents (Paraloid B-72,Paraloid B-67,and Butvar B-98) were tested for reversibility in solvent vapor chambers with no direct application of liquid solvent.After the epoxy had cured,brass screws were insertedinto the upper and lower pieces of wood. Four largesteel washers were suspended from the bottomscrews. The total weight of the four washers wasapproximately 88 grams.The samples were suspendedby the upper screws on bamboo skewers lying on therim of glass beakers, which were enclosed in sealed,clear polyethylene bags (fig. 10).Approximately 2 mlof the appropriate solvent was placed in the bottomof the beakers. Xylenes were used for the samplebonded with Paraloid B-72, ethanol for the Butvar B-98 sample, and Shell Mineral Spirits 135 for theParaloid B-67 sample.

The Butvar B-98 sample fell apart on its own inthree to four days. The Paraloid B-72 sample cameapart with gentle pressure after five days in thesolvent-rich environment.The Paraloid B-67 coatingdid not fall apart, even with gentle pressure, after fivedays, at which point the test was suspended. Theseresults suggest that Butvar B-98 and Paraloid B-72barrier layers are practically reversible in ethanol and

xylenes respectively, even without direct applicationof liquid solvent.The disadvantages of using ethanolfor reversal on objects with painted or varnishedsurfaces have been discussed above (see sec. 1).Whilexylenes should be safe to use with many painted or varnished surfaces, they will not be safe with all.Furthermore, the health hazards associated withxylenes makes them a less than ideal choice for 

Table 2. Results of Shear-Strength Testing Using Epoxy Adhesive Alone

 and With Four Different Barrier Layers

Mean pressure

at failure

Standard

deviation

Samples tested Results excluded

 No barrier 1301 psi   ±233 10 1

Butvar B-98 1403.1 psi   ±134.6 9 1

Paraloid B-72 1350.4 psi   ±327 10 ---

Liquid hide glue 1153.8 psi   ±366.9 9 ---

 Acryloid B-67 251.8 psi   ±78 9 ---

0

200

400

600

800

1000

1200

1400

1600

1800

No barrier Butvar B-

98

Paraloid

B-72

Liquid

hide glue

Paraloid

B-67

Barr ier ateria l

        Stength

 oaaFureinPSI 

Fig. 8. Results of Instron shear-strength testing of epoxybonds with different barrier materials. The graph showsthe mean strength of the 10 samples tested and indicatingthe range of one standard deviation.

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barrier reversal.The failure of the Paraloid B-67barrier layer to be reversed by the vapor of mineral spirits (in which it had previously beendissolved) is somewhat mysterious, though itmay be related to the low vapor pressure of thesolvent. Had it not been for the failure of Paraloid B-67 to perform adequately in strengthtesting (see sec. 2.3.4), further testing of morevolatile and/or more polar solvents might havebeen warranted.

2.6 MICROWAVEREVERSIBILITY OF HIDE GLUE

The possible utility of microwave technol-ogy for reversing hide glue joints has not goneunnoticed, especially by furniture conservators(Neher 1996; Anderson and Podmaniczky1990).Theoretically, microwave radiation can beused to excite the hide glue’s water molecules,heating and weakening the glue line to thepoint that it either falls apart or comes apartwith gentle pressure. In practice, consumer microwave ovens can be used to deliver themicrowaves if an object is small enough; other-wise there are hand-held devices (available atconsiderable expense), such as the WorkRiteWood Welder, that generate radio frequenciesfor use in industrial applications.

A simple experiment was undertaken to

Fig. 9.The gridded, transparent ruler used for failure analysis

Fig. 10. Empirical reversibility testing using improvised vapor cham-bers

Table 3. Failure Analysis

Wood

Failure

Barrier Coating– 

Wood Failure

Barrier Coating– 

Epoxy Failure

Epoxy Failure

No barrier 6.8% n/a n/a 93.2%*Butvar B-98 0.3% 16.6% 71.8% 11.3%

Paraloid B-72 nil 62.1% 37.9 % nil

Liquid hide glue nil 27.3% 64.7% 8

Acryloid B-67 nil nil 100% Nil

* Indicates nonwood failure according to ASTM D905-98.

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separate sample blocks that had been bonded withepoxy using Titebond Liquid Hide Glue as a barrier coating. The samples were heated in a microwaveoven; some had water injected into the bond linewith a very fine syringe.All samples came apart easilyafter 20–30 seconds. Intentional overexposure in themicrowave oven resulted in scorching of the woodblocks, demonstrating that this method of reversal hassome potential dangers. Microwave reversal is notsuitable for joints in close proximity to metal fasten-ers or ornaments. While the reversal of hide gluebarrier layers with microwave radiation cannot beuniversally recommended, further study is called for 

in this area.

4. CONCLUSION

This study demonstrates that Paraloid B-72 is a suit-able material for use as a reversible barrier layer for epoxy joins in wood. It offers strength comparable toepoxy used alone as well as to the other proven andwidely used barrier materials, Butvar B-98 and hideglue.Like these other barrier materials,Paraloid B-72appears to offer some protection to underlying woodin the event that the epoxy bond is broken. Addi-tionally, Paraloid B-72 was shown in practice to be areversible barrier material in xylenes vapor. Thisquality factor can offer significant advantages over Butvar B-98 and hide glue when making repairs near finished, painted, or otherwise solvent-sensitivesurfaces.This study also demonstrates that Paraloid B-67 is not a suitable material for use as a reversiblebarrier layer for epoxy joins in wood. Paraloid B-67failed both strength and reversibility tests.

ACKNOWLEDGMENTS

The authors would like to especially thank the J. Paul

Getty Museum’s Brian Considine, Jane Bassett, JulieWolfe, George Johnson, and Mark Mitton of Deco-rative Arts and Sculpture Conservation for their unstinting support and great help throughout theproject; Jerry Podany of Antiquities Conservation for sharing his knowledge and experience with barrier coatings; Stefan Simon of the Getty ConservationInstitute for permission to use the institute’s equip-

ment, and the institute’s Urs Mueller for providingcheerful advice, Instron training, and setup; and JaneDown of the Canadian Conservation Institute for her insight and guidance.

SOURCES OF MATERIALS

Paraloid B-67, polyisobutyl methacrylateConservation Support SystemsSanta Barbara, Calif. 93101

Butvar B-98,polyvinyl butyral resinConservation Support Systems

Santa Barbara, Calif. 93101

Ciba Araldite AV 1253,Vantico Inc.Conservation Support SystemsSanta Barbara, Calif. 93101

Paraloid B-72, copolymer of ethyl methacrylate andmethylacrylate

Conservation Support SystemsSanta Barbara, Calif. 93101

Shell Mineral Spirits 135, slow evaporating, 15%aromatic content

Conservation Support SystemsSanta Barbara, Calif. 93101

Titebond Liquid Hide GlueFranklin InternationalColumbus, Ohio 43207

REFERENCES

ASTM. 2001. Standard practice for dealing withoutlying observations, E178-94. In  Annual book of 

 ASTM standards. Philadelphia: American Society for 

Testing and Materials.

ASTM. 2001. Standard test methods for small clear specimens of lumber, D143-94 (reapproved 2000). In

 Annual book of ASTM standards. Philadelphia: Ameri-can Society for Testing and Materials.

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ASTM. 2001. Standard test method for strengthproperties of adhesive bonds in shear by compressiveloading,D905-98.Philadelphia:American Society for Testing and Materials.

Anderson, Mark J., and Michael S. Podmaniczky.1990. Preserving the artifact: minimally intrusiveconservation treatment at the Winterthur Museum.Papers from the Wooden Artifacts Group,ed.M.J.Ander-son.Wooden Artifacts Specialty Group of the Amer-ican Institute for Conservation of Historic & ArtisticWorks and the Foundation of the AIC.

Buck, Susan L. 1990. A study of the properties of commercial hide glue and traditional hot hide glue inresponse to changes in relative humidity and temper-ature. Papers from the Wooden Artifacts Group,ed.M.J .Anderson. Wooden Artifacts Specialty Group of theAmerican Institute for Conservation of Historic andArtistic Works and the Foundation of the AIC.

Ciba Specialty Chemicals Corp., North America.2001. Araldite 1253 packaging materials. East Lans-ing, Mich.

Down,2001. Review of CCI research on epoxy resinadhesives for glass conservation. Reviews in Conserva-

tion 2:39–46.

Horie, C.V. 1987. Materials for conservation. London:Butterworths.

Monsanto Co. 1994. Butvar polyvinyl butyral resin:Properties and uses, pub. 2008084B. St. Louis, Mo.

Neher, Albert. 1996. Radio frequency heating andthe reversibility of animal glue in furniture joinery.UKIC Conservation News 59: 35–37.

Podany, J., K. M. Garland, W. R. Freeman, and J.Rogers. 2001. Paraloid B-72 as a structural adhesiveand as a barrier within structural adhesive bonds:Evaluations of strength and reversibility. Journal of the 

 American Institute for Conservation 40:15–33.

Podmaniczky, M. S. 2003. Personal communication.Furniture conservation department, Winterthur Museum,Winterthur, Delaware.

Spirydowicz, K., E. Simpson, R. A. Blanchette, A.Schniewind, M. K. Toutloff, and A. Murray. 2001.Alvar and Butvar:The use of polyvinyl acetal resinsfor the treatment of the wooden artifacts fromGordion, Turkey. Journal of the American Institute for 

Conservation 40: 43–57.

Vantico Inc. 2002. Product data, Araldite AV-1253/HV-1253.

FURTHER READING

Blackshaw, S. M., and S. E.Ward. 1983. Simple testsfor assessing materials for use in conservation. In The 

Proceedings of the Symposium Resins in Conservation,ed. J. O.Tate, N. H.Tennent, and J. H.Townsend. Edin-burgh: Scottish Society for Conservation andRestoration. 2.1–2.15.

Bradley, S. 1984. Strength testing of adhesives andconsolidants for conservation purposes. In  Adhesives

and Consolidants, ed. N. S. Brommelle et al. London:International Institute for Conservation of Historicand Artistic Works. 22–24.

Down, J. L., J. MacDonald, J. Tetreault, and R. S.Williams. 1996. Adhesive testing at the CanadianConservation Institute, past and future. Studies in

Conservation 41:19–44.

Koob, Stephen P. 1986.The use of Paraloid B-72 as anadhesive:Its application for archaeological ceramics andother materials. Studies in Conservation 31:7–14.

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LISA ELLIS is currently the Mellon Fellow at theSociety for the Preservation of New England Antiq-uities. The work described in this paper wasconducted while she was a graduate intern in theDecorative Arts and Sculpture Conservation depart-ment at the J. Paul Getty Museum. She has graduatedegrees in art history from the University of Torontoand art conservation from Queen’s University,Kingston, Canada. She has held internships at the ArtGallery of Ontario, Parks Canada, the Institute of Nautical Archaeology’s Bodrum Laboratory inTurkey, and the Agora Excavations in Athens.Address:Museum of Fine Arts, Boston, Objects Conservation,

465 Huntington Ave., Boston, Mass. 02115;[email protected]

ARLEN HEGINBOTHAM is assistant conservator of decorative arts and sculpture at the J. Paul GettyMuseum in Los Angeles, California. Since taking aninterest in art conservation in 1990,he has worked inseveral private furniture conservation studios, includ-ing most recently Robert Mussey Associates inBoston, Massachusetts. He has also held internshipsand technician positions at the Yale Peabody Museumof Natural History, the Isabella Stuart Gardner Museum, and the Philadelphia Museum of Art.Heginbotham received his B.A. in East Asian studiesfrom Stanford University in 1989 and his M.A. in artconservation from Buffalo State College in 1999.Address: Decorative Arts and Sculpture ConservationDepartment, J. Paul Getty Museum, 1200 GettyCenter Dr., Suite 1000, Los Angeles, Calif. 90049-1687; [email protected]

Received for review on January 2, 2003. Revisedmanuscript received June 16, 2003. Accepted for publication June 25, 2003.