Archaeological Wood Properties, Chemistry, and Preservation

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Consolidation of Dry Archaeological Wood by Impregnation with Thermoplastic Resins Arno P. Schniewind Forest Products Laboratory, University of California, Berkeley, CA 94720

Soluble resins have advantages as well as disadvantages over other consolidants, such as thermosetting resins, for deteriorated wood. Methods of application and choice of solvents are discussed. The effectiveness of soluble resins depends on the degree of deterioration, ranging from little or no effect on the properties of sound wood to substantial strengthening of severely degraded wood. Retained solvents can act as plasticizers and thus influence consolidant effectiveness. The adhesive qualities of consolidants were also investigated. Consolidation with soluble resins, like all real systems, was found to be irreversible in the strict sense, but some of the commonly used consolidants could be removed almost completely, depending on the solvents used.

P R O B L E M S ASSOCIATED WITH DRYING of severely deteriorated and weak-

ened material are not evident when dealing with dry archaeological wood, as distinct from waterlogged wood. Nevertheless, dry wood may have undergone deterioration that makes it weak and friable. If the state of preservation is such that the intended use and exposure would threaten the safety of the object, a consolidation treatment may be indicated. The purposes of such treatment would be to arrest further deterioration, to reattach loose particles, and to provide a general strengthening that enables the object to withstand the stresses of its intended use (I). The use referred to here may be identical with the original use of the object, or it may be simply the use entailed by occasional removal from storage to exhibition areas and vice versa. The nature 0065-2393/90/0225-0361$06.00/0 © 1990 American Chemical Society

In Archaeological Wood; Rowell, Roger M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1989.


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of the consolidation treatment must be tailored to the requirements of the intended use. Materials used as consolidants may be classified as either natural or synthetic in origin. Natural consolidants include water-soluble glues such as hide glues, solutions of natural resins such as shellac, molten waxes such as beeswax, and drying oils such as linseed oil (2). These natural materials have their advantages and limitations, and are still used as consolidants, but will not be discussed further here. Synthetic resins may be either thermoplastic or thermosetting. Thermoplastic resins are sensitive to temperature (i.e., they will melt and flow at elevated temperatures and are generally soluble in a variety of neutral organic solvents). Thermosetting resins, on the other hand, will decompose before they will melt and are not truly soluble. Treatments with thermosetting resins are therefore completely irreversible, but sometimes their use cannot be avoided. Epoxy resins are the best examples of thermosetting resins used in conservation, particularly in architectural preservation, where their excellent strength properties are required for the repair of fully functional structural elements. Thermoplastic resins can be introduced into wood either in solution or as liquid monomers, which are then polymerized in situ (2, 3). Cross-linking agents can be included with the monomer to produce a thermosetting resin upon polymerization, initiated by heat, catalyst, or 7 irradiation (4). Even if there is no cross-linking, the prospects for reversibility are not very good for such systems (3). Surface residuals of poly(methyl methacrylate), polystyrene, and polyester mixtures could only be removed with some difficulty with solvents (5). The present discussion will be limited to thermoplastic resins that can be introduced into wood in solution.

Soluble Resins as Consolidants Materials used as consolidants should (1) have long-term stability, including resistance to cross-linking; (2) not change the appearance of the treated object; (3) not create significant internal stresses during solidification within the object; (4) be effective in fulfilling their purpose of strengthening; (5) achieve good penetration and consolidant loading (consolidant retention, i.e., weight gain after consolidation); and (6) have the characteristics required for a reversible treatment. Soluble resins have the potential of meeting or approaching all of these criteria. Most soluble resins that are being used as consolidants in conservation today are also used in picture varnish. Thus these resins have a history of satisfactory use that, in the case of polyvinyl acetate), extends over almost 60 years (6). Consolidants can cause some darkening of wood surfaces much in the way wetting them with water deepens their color, but with proper technique they will not leave a shiny surface film (7). Because excessive



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shrinkage leading to internal stresses would be objectionable in a picture varnish as well, we may presume that this shrinkage does not become a problem in wood consolidation. Effectiveness, a relative matter involving both adhesion to wood and cohesion within the resin, will be addressed in more detail later. Substantial penetration is possible, but polymer loading is limited by resin concentration and the void volume in the wood. Finally, some evidence suggests that these treatments are reversible, at least to a first approximation (8). The particular advantages of soluble resins as consolidants are that they are easy to apply by a variety of methods and that they offer the potential of reversibility, at least in the short term. The main disadvantage is that their effectiveness in strengthening deteriorated wood is somewhat limited. This is not to say that they are ineffective. However, they are introduced in the form of solutions, typically with a solids content of 20% or less. Therefore, the maximum polymer loading that can be achieved is limited to 20% or less of the total void volume. In addition, the resin itself may not be nearly as strong as sound (undeteriorated) wood parallel to grain, so that the achievable amount of strengthening is also limited. The greatest potential for effective strengthening is therefore found with the most severely deteriorated material. According to Grattan (9), the most common polymers used in solvent-resin consolidant systems are acrylics, polyvinyl acetates), polyvinyl butyrals), and soluble nylon. Of these, soluble nylon has since been shown to be unsuitable because it inevitably cross-links and embrittles within as little time as a few weeks (JO). The others are known to have excellent longterm stability (9). Most likely the stability of polymers placed inside the wood may be even better, because they would be protected from lightcatalyzed degradation reactions. Possible wood-polymer interactions, however, are unknown at this time.

Methods of Application Although commonly used resins are sometimes sold as solutions, they are usually obtained in powder or pellet form. No single solvent is suitable for all types of resin, so that the choice of solvent must be tailored to the particular resin used. For wood, the first choice might be between polar and nonpolar solvents. The latter would allow resin solutions to penetrate wood better because polar solvents tend to get adsorbed on cell walls, and adsorption reduces their mobility (II). Additionally, solvents with low boiling points are preferable because they minimize problems with residual vapors. Another consideration is the viscosity of a given solution. L o w viscosity is necessary if good penetration is desired. Some commonly used solvents are ethanol, methanol, acetone, and toluene. Solution concentration needs to be decided by balancing the require-



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merits of good penetration and adequate polymer loading. Given complete saturation and a constant fraction of void volume, loading will be in direct relation to polymer concentration in solution, but at high concentrations penetration may be incomplete because of high viscosity (12). If consolidation is required only for surface layers of an object, application by brushing may be sufficient. Consolidant may be applied in several coats, but eventually a saturation of surface layers will lead to formation of undesirable glossy films. More resin can be introduced by application of many coats of low-concentration solutions than by use of solutions in high concentration, because the latter lead to early gloss formation (9). Barclay (7), for example, used a 5% solution of polyvinyl butyral) resin (Butvar B90) in ethanol for brush treatment of parts of an English fire engine. Better penetration and loading can be obtained either by soaking in consolidant solution or by continuous (or intermittent) spraying of solution within a closed system (13). Spraying is advantageous for large objects where it would be neither practical nor desirable from a safety viewpoint to prepare the quantity of solution required for soaking. Consolidation of a dugout canoe 5 m long was accomplished by spraying with a 13% solution (weight basis) of a polyvinyl acetate) resin (AYAF) in methanol within a temporary enclosure (14) . Another variation is to apply consolidant dropwise over a period of time (15) . Soaking or spraying may achieve complete penetration, but this will take time. The best chance of getting maximum penetration and maximum loading as well is to use vacuum impregnation. For small objects this method would not require elaborate equipment, but it may not be practical for large objects. Objects need to be evacuated while submerged in consolidant solution. After the air is removed from the wood, the vacuum is released and consolidant solution is pushed into the wood by atmospheric pressure. Vacuum impregnation was used for detachable parts of the fire engine mentioned previously. In that case the concentration of consolidant was raised to 20%, as it leads to greater loading and the restrictions that apply to brushing are of no concern (7).

Effectiveness of Soluble Resin Consolidants The effectiveness of consolidation treatments depends on the depth of penetration and the loading of consolidants, on the type of consolidant and the type of solvent used, and on the extent of wood deterioration. For small objects where the only concern might be stabilizing the surface against abrasion during handling, a surface treatment with shallow penetration may be quite sufficient. If, on the other hand, some overall strengthening is required, it will be advantageous to obtain full or nearly full penetration. Given full penetration, effectiveness as measured by improvements in strength of the wood after consolidation would be expected to increase in



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some proportion to polymer loading. This increase was found to be true to some extent for deteriorated wood treated with polyvinyl butyral) and acrylics, but in some cases little or no difference was found between loading of about 20 versus 30% (12). When sound ponderosa pine wood was treated with polyvinyl acetate) (AYAF) in methanol at either 5.67 or 13.2% concentration (weight basis), no measurable increase in strength could be observed (14). However, significant increases in strength have been found for deteriorated Douglas-fir taken from bacterially degraded foundation piles. Figure 1 shows some data taken from several studies that indicate a functional relationship between the degree of deterioration and the improvement that could be achieved by treatment with polyvinyl butyral) resin (Butvar B98) as 12.5 or 15% solutions (weight basis) to give approximately 20% polymer loading (12, 14, 16). Samples with the lowest residual bending strength (highest degree of deterioration) show the greatest improvements in strength following consolidation.

DC

1.0

I 40

1

50

60

70

MOR of untreated control, MPa Figure 1. Improvement factor; ratio of modulus of rupture (MOR) (bending strength) values of wood treated with Butvar B98 and untreated wood for deteriorated Douglas-fir at various levels of residual bending strength (degrees of deterioration).

Table I shows some physical data for several commonly used consolidants. The improvement factors of the first four resins—i.e., the polyvinyl butyrals) (Butvar B90 and B98), the acrylic (Acryloid B72), and the first polyvinyl acetate) (AYAT), which has the highest molecular weight—were shown statistically to be significantly different from 1.0 (1.0 implies no i m -


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Table I. Properties of Resins and Their Tensile Molecular Strength, Resin Weight MPa Butvar B90 45,000 46 Butvar B98 34,000 46 Butvar B98 34,000 46 Acryloid B72 AYAT 167,000 29 AYAF 113,000 18 AYAA 83,000 10 b AYAC 12,800 Downloaded by UNIV OF CINCINNATI on January 12, 2016 | http://pubs.acs.org Publication Date: May 5, 1989 | doi: 10.1021/ba-1990-0225.ch013

-

-

Strengthening Capability

°c 68 68 68 40 28 24 21 16

Improvement Factor* 1.14 1.19 1.20 1.16 1.13 1.10 1.03 1.11

Ref. 12 12 14 14 14 14 14 14

"Improvement factor is the ratio of bending strength (MOR) values of treated wood vs. untreated controls. ''Listed as "not practical to measure."

provement); the others were not significant. The explanation for the lack of improvement with the other polyvinyl acetates) lies with their relatively low tensile strengths—sound Douglas-fir wood has a tensile strength of about 125 M P a — a n d their low glass-transition temperatures, T , which are near or even below room temperature. A rough estimate of the properties of a composite can be obtained from a volume-weighted average of the properties of the components, which makes the properties of the resin so important (12). Polymers near or above their glass-transition temperature lose stiffness and become rubbery, i n which state they can contribute little to improving the strength of a much stiffer component. The best improvement factors were obtained for Butvar B98, followed by Acryloid B72 (trade name for an acrylic resin), and Butvar B90. Similar conclusions were reached by other investigators (9, 15). The major disadvantage of polyvinyl butyrals) is the relatively high viscosity of their solutions (9), which appears to make them somewhat more difficult to remove again (15). g

Retained Solvents as PL·sticizers Retained solvents may act as plasticizers (i.e., unless the solvent evaporates completely, the deposited resin may not have the same properties as the original solid in its pure state). Retained solvent lowers the glass-transition temperature. If the T of the mixture is at or below room temperature, rubbery behavior will result, and consolidant effectiveness will be reduced. Acryloid B72 seemed to retain solvent in films cast from acetone and toluene solutions and then air-dried (12). Further study showed that Acryloid B72 may retain measurable amounts of solvent even after 30 days at room temperature (16, 17). Results are shown in Table II. After 1 day of air-drying, substantial amounts of solvent were retained and T values were too low to measure. After 30 days, only traces of acetone g

g


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Table Π. Glass-Transition Temperatures and Retained Solvent Concentrations of Cast Films Drying Condition 1 Day, 30 Days, 30 Days, 20 °C 20 °C 50 °C Solvent Residual Residual Residual Solvent, B.R, T , Solvent, T , Solvent, % % Polymer Type % °C °C °C °C 0.0 40 39 0.3 B72 Acetone 5.8 56 0.2 40 2.2 B72 77 6.3 Ethyl acetate 0.3 75 54 3.2 B98 19.7 65 Methanol 1.5 66 49 4.8 B98 12.3 Toluene-ethanol 74* g

g


-

SOURCE: Adapted from ref. 16. "Azeotrope boiling point of toluene-ethanol-water.

remained in Acryloid B72, and T values approached those of the original resins. The fact that some of the T values in Table II exceed those for the same rësins given in Table I probably reflects the method (differential scanning calorimetry, DSC) used for the resins in Table II. D S C tends to give values 5-8°C greater than the method known to have been used for the Butvar data in Table I. The remaining resin-solvent combinations show more substantial residual solvents, and T for the Acryloid B72-ethyl acetate combination was still too low to measure. Films dried for 30 days at 50°C showed normal values of T for Acryloid B72 and only a trace of retained solvent. For Butvar B98 the solvent removal was not as complete. The data also show that the solvents with higher boiling points (i.e., ethyl acetate and the ethanol-toluene mixture) are more difficult to remove than acetone or methanol. When the same process of drying at 50°C was applied to wood specimens treated with consolidants, the Butvar B98 specimens dried at the elevated temperature had greater improvement factors than those dried at room temperature, whereas for Acryloid B72 the opposite was true (16). The data of Table II would have indicated improvements connected to drying at elevated temperature for both resins. The difference in behavior probably results from the fact that the heat treatment was carried out above T for Acryloid B72 and below T for Butvar B98. Such a heat treatment or annealing can lead to improved properties if it is carried out below T (18). Similar effects take place at room temperature (if the T is above it), but at a lower rate. Of course, prolonged heating at temperatures as high as 50°C could rarely, if ever, be justified for dry archaeological artifacts. g

g

g

g

g

g

g

g

Adhesive Qualities of Consolidants A n effective consolidant must adhere well to the material to be treated (2, 9, 19) and must act as an adhesive to reattach loose fragments (7, 15). A n


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acrylic (Paraloid B72, the European designation for Acryloid B72) was tested for use as a conservation adhesive in the form of a 50% (weight basis) solution and performed satisfactorily (20). In another study, the adhesive qualities of Acryloid B72, AYAT, and Butvar B98 in the form of 15% solutions (weight basis) (i.e., a concentration suitable for consolidation) were investigated. A t this concentration the viscosity of the consolidant solution is much lower than that of most adhesive formulations. Tests were made with polar solvents (acetone for Acryloid B72 and ethanol for AYAT and Butvar B98) and also with nonpolar solvents (toluene for Acryloid B72 and AYAT and a 60:40 mixture of toluene and ethanol for Butvar B98, for which a suitable nonpolar solvent i n its pure form could not be found). Some of the results are summarized in Figure 2.

0.0 AYAT

B72

B98

White glue

Figure 2. Static shear strength values for adhesive joints in deteriorated Douglas-fir made with AYAT Acryloid B72, Butvar B98, and PVA emulsion "white glue". Key: F, polar solvent; N, nonpolar solvent. (Adapted from refi 21). y

Figure 2 shows average static shear strength properties for deteriorated Douglas-fir (21). When polar solvents are used, the shear strength of joints made with the three consolidants is about the same. By comparison, the shear strength obtained with consolidant solutions was only about two-thirds of the value of joints made with a commercial PVA "white glue". The difference is hardly surprising because the white glue is specifically formulated as an adhesive, which implies a high enough viscosity to prevent excessive penetration. The consolidant solutions, on the other hand, are formulated to penetrate well. The solution tends to do just that, leaving what is commonly termed a "starved joint". O n the basis of these considerations, the adhesive qualities of all three consolidants can be termed excellent.


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The conclusion of excellence, however, is reserved for consolidants dissolved in polar solvents. When toluene was used as the solvent, very little adhesive quality remained for AYAT and Acryloid B72. The use of toluene-ethanol as solvent reduced the shear strength for Butvar B98 by a relatively small amount, compared to ethanol alone as the solvent. These results agree with previous consolidation tests, in which the improvement factor tended to be larger for polar solvents than for nonpolar solvents (12). The poor adhesion of systems with nonpolar solvents may in part result from the superior penetration of nonpolar solvents, leaving little or no "adhesive" on the surfaces to be bonded.

Reversibility of Consolidation Treatments Consolidation treatments with soluble resins can be considered reversible in principle, but it has been questioned (J, 9) whether this reversibility would ever be practically feasible. Experiments with an acrylic and a polyvinyl butyral) consolidant showed that most of the resin could be removed from treated wood, and that the acrylic could be removed more readily than the polyvinyl butyral) (15). In another study, a consolidation treatment of archaeological wood with Acryloid B72 in toluene could be substantially but not completely extracted again, as evidenced by scanning electron microscopic examination (22). Several processes take place when wood is treated with solvents. Solvents may swell wood, depending on their nature; they may extract some of the adsorbed moisture; some of them may be retained in wood by tenacious adsorption within; and most importantly, the solvents may remove wood extractives (8). Thus, some of the extractives may be removed or redistributed within the wood during consolidation treatment, and more may be removed along with the consolidant when attempts are made at treatment reversal. It is therefore not possible to return the wood to exactly the same state as before the initial consolidation treatment. Loss of extractives implies loss of potential diagnostic features, but otherwise does not necessarily have any practical significance for archaeological wood because the amount is usually small. It is therefore desirable to i n vestigate how closely reversibility can be approached, even if it cannot be reached in the strict sense. Results of experiments to explore this question (23) are summarized in Table III. The experiments were made with deteriorated Douglas-fir. Specimens measuring 0.25 X 1 X 2 in. were first conditioned to a constant moisture content of nominally 12%, treated by vacuum impregnation with a 15% solution (weight basis) to a polymer loading of 21-28%, reconditioned, extracted, and reconditioned again for final weight determination. Extractive content was determined on parallel control specimens. Extraction was accomplished by Soxhlet extraction or by soaking, with or without agitation and several changes of fresh solvent (23).



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Table III. Reversibility of Consolidation Treatments as Indicated by Residual Resin Content Residual Resin, % Extraction Polymer Solvent Uncorrected Corrected Method Butvar B98 0.8 Methanol -0.3 Soxhlet Toluene-ethanol 2.9 Soak and agitate 2.7 Toluene-ethanol Soak only 6.0 5.7 AYAT Acetone -1.0 -0.3 Soxhlet Toluene Soxhlet 1.2 1.9 Acryloid B72 Toluene 0.2 0.9 Soxhlet Acetone Soxhlet -0.1 0.6 Acetone Soak and agitate -0.3 0.4 Acetone Soak only 0.7 1.3 SOURCE: Adapted from ref. 23.

Residual resin percentages in Table III are given both before and after correction for the amount of extractives, which are presumed to have been extracted along with the consolidant. In several cases the uncorrected values are negative, which would be expected if all consolidant and some extractives had been removed. After the correction, the only negative value is for the AYAT-acetone system, which must therefore be considered the most re versible. The least reversible treatment was the Butvar B98-toluene-ethanol system, especially when removal was attempted by soaking only, which left corrected residual resin of 5.7%. For AYAT and Acryloid B72, the polar acetone was much more effective than the nonpolar toluene in removing consolidant. When the extraction method was by soaking, agitation using a magnetic stirrer was very beneficial. For Acryloid B72 and acetone, soaking with agitation was just as effective as Soxhlet extraction. This finding is important because Soxhlet extraction would not be a practical method in all but the most exceptional cases and with very small objects. The data of Table III show, however, that wiih proper choice of solvent and extraction method, residual resin content can be held to less than 1%.

References 1. Williams, M. A. Wooden Artifacts Group Preprints; New Orleans Meeting, American Institute for Conservation: Washington, DC, 1988. 2. Werner, A. E. A. Proceedings of the First International Symposium on the Conservation and Restoration of Cultural Property; Tokyo National Research Institute of Cultural Property, 1977; p 17. 3. Unger, Α.; Unger, W. Holztechnologie 1987, 28, 234-238. 4. Meyer, J. A. In Wood Technology: Chemical Aspects; Goldstein, I. S., Ed.; ACS Symposium Series 43; American Chemical Society: Washington, DC, 1977; p 301. 5. Unger, Α.; Reichelt, L.; Nissel, D. Neue Museumskunde 1981, 24, 58-64.



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6. Feller, R. L.; Stolow, N.; Jones, Ε. H. In On Picture Varnishes and Their Solvents; National Gallery of Art: Washington, DC, 1985; p 125. 7. Barclay, R. Stud. Conserv. 1981, 26, 133-139. 8. Schniewind, A. P. Preprints; Vancouver Meeting, American Institute for Con servation: Washington, DC, 1987; p 107. 9. Grattan, D. W. Proceedings of the Furniture and Wooden Objects Symposium; Canadian Conservation Institute: Ottawa, 1980; p 27. 10. Bockhoff, F. J.; Guo, K.-M.; Richards, G. E.; Bockhoff, E. In Adhesives and Consolidants; Brommelle, N. S.; Pye, Ε. M.; Smith, P.; Thomson, G., Eds.; The International Institute for Conservation of Historic and Artistic Works: Lon don, 1984;p81. 11. Nicholas, D. D. For. Prod. J. 1972, 22(5), 31-36. 12. Wang, Y.; Schniewind, A. P. J. Am. Inst. Conserv. 1985, 24, 77-91. 13. Chemical Section and Section for Repairing Technique Sci. Conserv. 1968, 4, 39-46. 14. Schniewind, A. P.; Kronkright D. P. In Adhesives and Consolidants; Brommelle, S.; Pye, Ε. M.; Smith, P.; Thomson, G., Eds.; The International Institute for Conservation of Historic and Artistic Works: London, 1984; p 146. 15. Nakhla, S. M. Stud. Conserv. 1986, 31, 38-44. 16. Carlson, S. M.; Schniewind A. P. Stud. Conserv., in press. 17. Carlson, S. M. M.S. Thesis, University of California, Berkeley, 1987. 18. Mininni, R. M.; Moore, R. S.; Flick, J. R.; Petrie, S. Ε. B. J. Macromol. Sci., Phys. 1973, B8, 343-359. 19. Rosenqvist, A. M. Recent Advances in Conservation; International Institute for Conservation: London, 1961; p 140. 20. Koob, S. P. Stud. Conserv. 1986, 31, 7-14. 21. Sakuno, T.; Schniewind A. P. unpublished manuscript, 1988. 22. Hatchfield, P. B.; Koestler, R. J. Scanning Microsc. 1987, 1, 1059-1069. 23. Schniewind, A. P. Wooden Artifacts Group Preprints; New Orleans Meeting, American Institute for Conservation: Washington, DC, 1988. RECEIVED for review January 24, 1989. ACCEPTED revised manuscript June 12, 1989.


Archaeological Wood Properties, Chemistry, and Preservation

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