epoxy resin-consolidated stone

Epoxy Resin-Consolidated Stone: Appearance Change on AgingAuthor(s): William S. Ginell and Richard CoffmanReviewed work(s):Source: Studies in Conservation, Vol. 43, No. 4 (1998), pp. 242-248Published by: International Institute for Conservation of Historic and Artistic WorksStable URL: http://www.jstor.org/stable/1506733 .Accessed: 11/05/2012 11:12

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EPOXY RESIN-CONSOLIDATED STONE: APPEARANCE CHANGE ON AGING

William S. Ginell and Richard Coffman

Summary-Dilute solutions of epoxy resins have been used successfully in the past to consolidate deteriorated, porous stone. However, the appearance change (darkening, yellowing) on curing has proved to be a deterrent to the wider use of these materials by conservators. In the study reported here, it is shown that solvent washing to remove surface-deposited resin and exposure to sunlight are effective methods for restoring the original visual appearance of consolidated outdoor stone.

Introduction

The principal, common uses of epoxy resins in the preservation of outdoor architectural and monu- mental stone are as an adhesive for rejoining detached parts and as an injection grout for filling cracks. In both applications, the high adhesive and cohesive strengths of the cured resin are responsible for helping to restore some of the degraded structural properties of the original stone. The usually viscous resin remains at the interface between the stone surfaces and does not penetrate the stone to any great extent. Because the major portion of the cured resin is shielded and is not exposed to light, photodegradation and possible changes in appearance are not considered to be significant problems.

Another application for epoxy resins in architectural conservation is as a consolidant for porous, fragile, deteriorated stone [1-7]. Here, a solution of the resin and the hardener, in an appropriate solvent system, is applied to the stone and deep pene- tration of the low viscosity, low surface tension solution is achieved. Following solvent evaporation, the cured epoxy resin acts to strengthen and reduce fragility of the stone and its susceptibility to water damage.

Although the adhesive and crack-filling applications of epoxy resins have been generally accepted for stone preservation, many practitioners have objected to the use of this material as a consolidant. The main reasons for this seem to be the darkened appearance of the stone surface following treatment and curing, and the reported yellowing that occurs with time. The generally irreversible nature of cured epoxy resins is becoming less objec- tionable because of the realization that even deeply penetrating consolidants, that are normally considered to be soluble, cannot be removed completely from porous stone.

Received May 1997

Yellowing of cured epoxy resins is a common observation [8-12]. The effect has been attributed to the photodegradation of the amine hardener [6], to the degradation of the resin itself [13], to the degradation of additives or accelerators [8], and to the presence of impurities*. Multiple amine groups in typical hardeners, carbonyl groups, and the presence of phenoxymethylene groups on cured epoxy resins could be the principal sources of the chro- mophores that result from photo-oxidation reac- tions.

The darkening of the surface of a porous stone following treatment with an epoxy resin solution is an optical effect. The original, diffusely scattering, high surface roughness or high porosity surface is modified by the epoxy solution and the degree of specular, or mirror-like, reflection increases. As the solvent evaporates, uncured resin is deposited on the surface and, after curing, forms a film that con- tinues to exhibit a relatively high degree of specular reflectance. For thermoplastic polymer consolidant solutions, curing occurs only after solvent evaporation, and it is difficult to avoid back-diffusion of the solution to the stone surface where polymer deposits build up as the solvent evaporates. However, as the epoxy resin in solution cures, its solubility in the solvent decreases as crosslinking takes place and, eventually, precipitation occurs. If solvent evaporation can be delayed until curing has occurred, the polymer will be deposited within the stone and back-diffusion and excessive darkening due to surface deposition will be reduced [1].

*A reference in the catalog of Conservation Materials Ltd, Sparks, Nevada, states that HXTAL NYL-1 (a cycloaliphatic epoxy resin system that is similar to Shell Eponex 1510-Jeffamine D230) owes its stability to the absence of traces of metal ions that have been removed during manufacture and that are responsible for color development.

Studies in Conservation 43 (1998) 242-248 242

Epoxy resin-consolidated stone: appearance change on aging

To minimize the problems of appearance change and thus overcome the principal objection to the use of epoxy resin solutions as stone consolidants, it is necessary:

(a) to decrease the amount of epoxy resin that remains on the stone surface (either by physical removal or by delaying solvent evaporation); or

(b) to select resins that are relatively resistant to yellowing [5, 14] if complete removal is not fea- sible; or

(c) to use an epoxy resin that will degrade and chalk when exposed to oxygen and ultraviolet radiation.

To evaluate the relative effectiveness of these concepts, a series of tests was performed using two types of epoxy resin-hardener systems and a variety of stone surface treatments after impregnation of the stone with the epoxy solution. The treated stone samples were then subjected to natural aging by exposure to sunlight and to artificial aging in the laboratory. Appearance changes were measured instrumentally before, during, and following aging.

Materials and treatment procedure

Resins

The two epoxy resins selected for these tests com- prised a standard aromatic bisphenol-A type that is widely available from a number of manufacturers, and a hydrogenated version of a bisphenol-A that can be described as a cycloaliphatic resin (Table 1).

The cycloaliphatic resin-hardener system used in this study is relatively resistant to yellowing, but the phenolic GY6010-HY965 epoxy is known to be sus- ceptible to photo-oxidation.

Epoxy resin solutions were prepared using a resin-to-hardener weight ratio of 4:1 in a toluene

(methylbenzene)/propan-2-ol solvent (1:5 by vol- ume). The solution concentration was 16-5wt% resin.

Stone sample treatments

The stones used in the tests were 54 x 54 x 9mm samples of a light-colored Salem limestone, which is a homogeneous, calcite-cemented, oolitic limestone that is quarried in Indiana [15]. It is almost pure calcium carbonate, with small amounts of quartz, haematite and magnesium carbonate. The stone has a water-accessible porosity of 17.4% and a com- pressive strength of 35MPa.

The samples were wet-sawed and the surface saw- marks removed by wet sanding. After oven drying to constant weight, stone samples were immersed, face down, to a depth of about 3mm in each of the two impregnation solutions, in closed containers for about one hour. On removal from the epoxy solutions, the superficial solution on the samples was blotted off with absorbent paper. Complete pene- tration of the stone was achieved in all cases (Figure 1).

The samples were then subjected to a variety of treatments that were designed to modify the nature of the exposed surfaces. The treatments included wrapping in polyethene film and solvent washing to remove surface deposits. The various treatments that were carried out are described in Table 2.

The wrapping procedure was used to prevent solvent evaporation and back-diffusion of soluble, uncured resin. This procedure allowed time for curing and precipitation of the resin to occur within the stone pore spaces [11]. Acetone (propanone) washing before or after curing was intended to remove most of the soluble resin from the stone surface.

After the wrappings were removed, all samples were exposed to air and allowed to dry until constant weight was achieved. The consolidant loading was determined gravimetrically after drying for two months and amounted to about lwt%.

Table 1 Epoxy resin systems used

Aromatic (AR)

Diglycidylether of bisphenol-A (resin) Alkoxylated triethylenetetramine (hardener)

Ciba-Geigy Araldite GY 6010 Ciba-Geigy HY956

Aliphatic (AL)

Hydrogenated diglycidylether of bisphenol-A (resin) Polyoxypropylene diamine (hardener)

Shell Eponex 1510 Texaco Jeffamine D-230


W.S. Ginell and R. Coffman

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b Figure 1 Appearance of Salem limestone samples before and after consolidation with epoxy resin: (a) untreated stone, (b) treated, cured in air without wrapping.

Solvent-treated and untreated controls were also prepared.

Appearance measurements were performed on all samples using a Minolta (Model CM1000) spec- trophotometric chromameter that viewed a repro- ducible 1.lcm-diameter area of the stone surface through an orienting mask. The relatively large viewing area of this instrument served to average out small local appearance differences inherent in the limestone surface.

CIELAB color coordinates (L*a*b*) (ASTM: E308) were determined, from which AE values were derived using the relationship:

AE = V/(AL*)2 + (Aa*)2 + (Ab*)2

AE rather than the Yellowness Index (ASTM: E313) was used in these tests because of the over- riding contribution of L* to the change in appearance. The Yellowness Index, furthermore, is generally applicable to near-white materials, which was not the case for Salem limestone.

All AEs were referenced to the L*a*b* coordinates of the original, untreated stone. Appearance measurements were made before and after outdoor aging for two, seven and 18 months. The overall uncertainty of AE can be as high as ?1 0, which includes errors relating to differences in the appearance of the original stone from one surface location to another and to variations in surface deposits. AAE -1 is about the limit of visually detectable differences at the wave- lengths to which the human eye is most sensitive.

Artificial aging of one set of samples was performed in an Atlas Ci-35 Weather-Ometer at an irradiance of 0.35 watts.meter-2 (measured at

Table 2 Salem limestone sample treatments after epoxy solution impregnation

Sample Treatment group

2 Control samples were treated with solvent only and air dried for two weeks

3 Samples were wrapped tightly with polyethene film, sealed with tape, and allowed to cure for two weeks

4 Sample surfaces were wiped with an acetone-impregnated cloth, wrapped with polyethene film, and cured for two weeks

5 Sample surfaces were scrubbed using acetone and a soft brush, wiped to remove surface solvent, wrapped, and cured for two weeks

6 Samples were wrapped and cured for two weeks; after curing, surfaces were scrubbed with acetone and brush and wiped dry

7 Samples were cured in air for two weeks; no wrapping or wiping

8 Samples were cured in air for two weeks, then the surfaces were scrubbed with acetone and brush and wiped dry


Epoxy resin-consolidated stone: appearance change on aging

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b Figure 2 Appearance of Salem limestone sample exposed in Weather-Ometer: (a) sample 8AR, treated by immersion and shielded from light for 800 hours; (b) reverse side of sample 8AR, masked and exposedfor 800 hours.

340nm), a black panel temperature of 45 ? 5?C, and a relative humidity of 55 + 5%. The samples were mounted in holders that served to mask part of the stone surface so that border areas were not exposed to light, but otherwise these areas received the same treatment as the center portion of the sample (Figure 2).

Natural weathering was carried out on a second set of samples that were mounted at a 45? angle, facing south, on the laboratory roof at Marina del Rey, California. Included in this group were addi-

b Figure 3 Appearance of Salem limestone sample exposed to sunlight: (a) sample 7AL, treated by cap- illary rise and shielded from light for 18 months; (b) reverse side of sample 7AL, exposed to sunlight for 18 months.

Studies in Conservation 43 (1998) 242-248

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Table 3 Appearance change (AE) of Salem limestone following epoxy consolidation and both natural and artificial aging

Treatment AE after curing AE after outdoor aging

0-2 3.5 42 4.7 3.6 2-1 2-6 1.9 3.3

16.1 13-2 14-6 20-0

t = 2 months t = 7 months t = 18 months

03 0-4 20 2-3 1-3 4-1 1.4 03 06

12-2 30

10.4 7.7

1.1 0-3 1-2 1-1 29 32 1 2 0-5 1-5 8-8 1-2 7.2 5.3

00 00 05 08 1 2 1-0 1.4 1-1 1 8 4.6 10 3-5 1-8

AE after artificial aging

800 hours (-2 months)

09 24 09 3.5 22 1-2 07 1-0

11.7 7.5

11-5 60

AE = V/(AL*)2 + (Aa*)2 + (Ab*)2 AE referenced to untreated limestone *Control sample treated with toluene/isopropanol solvent only **AL = cycloaliphatic resin, AR = aromatic resin

The results are summarized in Table 3, in which AE values are given for samples before aging; after natural aging outdoors for two, seven and 18 months; and after artificial aging for 800 hours in the Weather-Ometer. After seven months, the treated control samples that were stored in the dark showed AE values of <1.0, referenced to the color values measured before the start of the exposure period.

It can be seen that the initial appearance changes for the wrapped samples (3, 4, 5, 6) immediately after curing are much smaller than those for the unwrapped, air-cured samples (7 and 8). Surface scrubbing with acetone before and after curing was most effective in minimizing initial, pre-aging appearance changes. After exposure for two months, many of the wrapped samples had nearly recovered their original appearance (AE -+ 1), whereas the unwrapped samples still retained much of their darkened appearance. After an outdoor exposure of 18 months, the appearance of all wrapped samples was indistinguishable, both instrumentally and visually, from the untreated limestone. The unwrapped aromatic-resin-treated stone samples had recovered to a great extent, but the aliphatic samples had not recovered completely.

There do not seem to be important differences among the initial appearances of the limestone samples that were treated with the two types of resin- hardener combination. All surfaces appeared to be

noticeably darker than the untreated stone. However, for the unwrapped samples (7 and 8) the rate of lightening of the aromatic epoxy resin appeared to be greater than that of the aliphatic resin [4]. This is in agreement with the superior light stability of the aliphatic compound and the assumption that lightening is due to oxidation and loss of surface-deposited epoxy. Insofar as the effects of surface washing before or after wrapping are concerned, washing does not appear to be as important a parameter as wrapping, which mini- mizes back-diffusion and the deposition of resin on the surface.

On comparison of the changes in L*, a* and b*, it was found that the principal contribution to the large change in AE following resin curing, and its recovery after aging, was the change in L*, the lightness or darkness of the surface. Changes in yellowing, as evidenced by Ab*, were noted but they seem to be of secondary importance, and little change in a*, a measure of the red-green contribution to the color, was observed.

Conclusions

The data show that exposure to sunlight and air of limestone that has been consolidated using dilute solutions of either a light-stable cycloaliphatic or a more commonly used aromatic epoxy resin that

Studies in Conservation 43 (1998) 242-248

t = 0

2* 3AL** 3AR 4AL 4AR 5AL 5AR 6AL 6AR 7AL 7AR 8AL 8AR

246

Epoxy resin-consolidated stone. appearance change on aging

tends to yellow, results in the gradual recovery of the original, pre-treatment appearance of the stone. The initial appearance change resulting from the treatment, primarily darkening of the surface, is due to the accumulation of cured resin on the stone surface. Over time, photo-oxidation and chalking remove this surface layer, resulting in the restoration of the original appearance of the stone. The initial darkening of the stone can be reduced to a very great extent by wrapping to prevent back-diffusion of the uncured resin solution to the surface as the solvent evaporates. Removal of surface- deposited resin, either cured or uncured, is also an effective method for minimizing initial appearance changes, but not as effective as preventing deposition at the start.

The use of consolidants that are stable to degradation by light will result in prolonging the exposure time required to restore the original stone appearance if precautions are not taken to remove surface-deposited resins. The loss of surface epoxy resin during aging should not impair the consolidation effectiveness of the treatment and it can be concluded that the use of epoxy resin solutions for consolidation of fragile or delaminating porous stone that will be exposed to air and light should not be discarded because of temporary appearance changes or possible yellowing. Both effects have been shown to diminish with exposure time.

Acknowledgements

We would like to acknowledge with thanks the con- tributions to this study of Brian Amiri, Eric Hansen, Marie Labinis, Blanche Kim and Charles Selwitz.

Materials

Araldite GY 6010 and HY 956: Ciba Specialty Chemicals, 4917 Down Avenue, East Lansing, MI 48823-5691, USA.

Shell Eponex 1510: Miller Stephenson Co. Inc., 12261 Foothill Blvd, Sylmar, CA 91342, USA.

Jeffamine D-230: Huntsman Corp., PO Box 27707, Houston, TX 77227, USA.

References

1 DOMASLOWSKI, W., 'Consolidation of stone objects with epoxy resins', Monumentum 4 (1969) 51-64.

2 GAURI, K.L., 'Efficiency of epoxy resins as

stone preservatives', Studies in Conservation 19 (1974) 100-101.

3 KOTLIK, P., JUSTA, P., and ZELINGER, J., 'The application of epoxy resins for the consolidation of porous stone', Studies in Conservation 28 (1983) 75-79.

4 CAVALLETTI, R., LAZZARINI, L., MARCHESINI, L., and MARINELLI, G., 'A new type of epoxy resin for structural consolidation of badly decayed stone' in Proceedings of the Fifth International Congress on Deterioration and Conservation of Stone, Lausanne (1985) 769-778.

5 LAZZARINI, L., 'The use of epoxy resins in the restoration of stone', Quaderni di conser- vazione e restauro 2 (1989) 95-109.

6 SELWITZ, C., Epoxy Resins in Stone Conservation, Research in Conservation 7, Getty Conservation Institute, Marina del Rey, California (1992).

7 KUMAR, R., and GINELL, W.S., 'Evaluation of consolidants for stabilization of weak Maya limestone' in Proceedings of International Colloquium on Methods of Evaluating Products for the Conservation of Porous Building Materials in Monuments, Rome (1995) 163-178.

8 TENNENT, N.H., 'Clear and pigmented epoxy resins for stained glass conservation: light aging studies', Studies in Conservation 24 (1979) 153-164.

9 DOWN, J.L., 'The yellowing of epoxy resin adhesives: report on natural dark aging', Studies in Conservation 29 (1984) 63-76.

10 DOWN, J.L., 'The yellowing of epoxy resin adhesives: report on high intensity light aging', Studies in Conservation 31 (1986) 159-170.

11 DOMASLOWSKI, W., 'Light resistance of stone consolidated with epoxy resins', Zabytkoz- nawstwo i Konservatorstwo 14 (1989) 3-21 [English summary in Art & Archaeology Technical Abstracts 29 (1992) #2922]

12 RODRIGUEZ BLANCO, J., and FORT GONZALEZ, R.Y., 'Treatment-induced changes of the stones in Santa Maria La Real de Nieva cloister (Segovia, Spain)' in IIIrd Inter- national Conference on Restoration of Build- ing and Architectural Heritage, Granada (1996) 488-493.

13 DELGADO RODRIGUES, J., COSTA, D., SA DA COSTA, M., and EUSEBIO, I., 'Behavior of granites under aging tests' in Degradation and Conservation of Granitic Rocks in Monu- ments, Research Report No. 5 (1994) 355- 360.

14 BRADLEY, S.M., 'Evaluation of HXTAL NYL-



1 and Loctite 350 adhesives for glass conservation' in ICOM Committee for Conservation 9th Triennial Meeting, Dresden (1990) 669-674.

15 McGEE, E.S., 'Mineralogical characterization of the Shelbourne marble and the Salem limestone test samples used to study the effects of acid rain', U.S. Geological Survey, Federal Center, Box 25425, Denver, CO 80225, USA.

Authors

WILLIAM S. GINELL received his PhD in physical chemistry from the University of Wisconsin in 1949. Since then, he has held research positions at Brookhaven National Laboratory, Atomics Inter- national, Aerospace Corporation, and McDonnell Douglas Astronautics Company. He joined the Getty Conservation Institute in 1984 as Head of Materials Science and, currently, is Senior

Conservation Research Scientist. His principal research interests are conservation in humid, tropi- cal environments; stone and earthen materials conservation; seismic stabilization of historic structures; and architectural conservation. Address. Getty Conservation Institute, 1200 Getty Center Drive, Suite 700, Los Angeles, CA 90049, USA.

RICHARD COFFMAN received an MSc in geological sciences from the University of California at Riverside and a PhD in geological sciences from the University of California at Santa Barbara. While completing his doctorate, he was a Senior Fellow in the Scientific Research Program at the Getty Conservation Institute. His studies at GCI included research on the consolidation of stone and adobe, and it was at this time that the research on epoxy resins was initiated. He is currently a California Registered Geologist and a senior project manager with Tait Environmental Management Inc., Orange, California.

Resum&-Des solutions diluees de resines epoxy ont ete utilisees avec succes dans le passe pour la consolidation de la pierre poreuse deterioree. Cependant, les changements d'aspect (noircissement, jaunissement) au cours du traitement se sont reveles un obstacle d une extension de l'usage de ce type de materiaux par les restaurateurs. Dans la presente etude, on montre que le nettoyage par un solvant pour deliminer la resine residuelle en surface ainsi que l'exposition aux rayons du soleil sont des methodes efficaces pour restaurer I'aspect original de la pierre en aeuvre consolidee.

Zusammenfassung-In der Vergangenheit haben Restauratoren verduinnte Epoxidharz-Ldsungen mit Erfolg bei der Festigung von porosem Gestein eingesetzt. Die durch derartige Behandlungen ausgeldste Veranderung der Gesteinsoberfldche (Nachdunkelung, Gilbung) hat den weiteren Gebrauch dieser Methode jedoch verhin- dert. Die Autoren der hier beschriebenen Untersuchung zeigen, daJf Versuche zur Entfernung der auf der Oberfldche verbliebenen Harze durch Abwaschen mit Losungsmittel und Einwirkung von Sonnenlicht wirksame Schritte zur Wiederherstellung des originalen Erscheinungsbildes bereits konsolidierter Gesteine, die sich im Freien befinden, sein konnen.

Resumen-En el pasado han sido usadas, con exito, soluciones de resinas epoxi con elfin de consolidar piedra deteriorada o porosa. Sin embargo, el cambio en la apariencia (oscurecimiento, amarilleamiento) durante el curado se ha mostrado como un gran inconveniente para el uso extensivo de estos materiales por los restau- radores. En el estudio que aqui se presenta se muestran dos metodos efectivos para devolver la apariencia visual original a la piedra consolidada en exteriores: el lavado con disolventes con el objeto de eliminar los dep6sitos de resina en la superficie y la exposicion a la luz del sol.


epoxy resin-consolidated stone

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