Polymers for the Conservation of Cultural Heritage - ACS Publications

Chapter 27

Polymers for the Conservation of Cultural Heritage

Downloaded by UNIV OF NORTH CAROLINA on July 4, 2013 | http://pubs.acs.org Publication Date: September 29, 2005 | doi: 10.1021/bk-2005-0916.ch027

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M. Cocca , L. D'Arienzo , G. Gentile , E. Martuscelli , and L. D'Orazio 2

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CAMPEC, Consortiumforthe Application of Polymeric Materials, P.le Ε. Fermic/oCRIAI, 80055 Portici (ΝΑ), Italy Institute of Chemistry and Technology of Polymers, National Research Council of Italy, Via Campi Flegrei 34, 80078 Pozzuoli (NA), Italy

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In this paper the results of a research aimed at developing radically innovative process tailored for the restoration of both stone and textiles are reported. For stone, a new polymerisation procedure of poly(urethane-urea) by in situ polymerisation inside stone is described. Through the method set up a good penetration depth of the polymer into the stone pores is achieved, and the material obtained is characterized by high aggregative and consolidating efficiency. For textile materials, a new consolidating procedure, based on the grafting copolymerisation of acrylic monomers on the cellulose substrate has been set up and a high volume grafting chamber has been designed and realized, in order to carry out the grafting procedure onto large textile items of historical interest.

370 In New Polymeric Materials; Korugic-Karasz, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Objects of historical and artistic value undergo to an inevitable degradation due to physical, chemical, mechanical and biological deterioration of the constituent materials. The most important purpose of restorers and conservators is to slow down these degradation processes through conservative interventions, consisting of restoration, protection and periodical treatments of maintenance (1). After evaluating the status of an artwork and its final placement, and after pointing out the necessary operations to restore it (cleaning, integration of lost fragments, consolidation, protection), the restorer usually faces the problem of the choice of the most appropriate materials useful for the restoration intervention. Since the late sixties polymeric materials have been considered by restorers as the answer to many problems. Polymer based products have been applied, with more or less satisfactory results, on different substrata (stone, wood, textiles, paper, paintings on wall and on canvas) and they are still widely used as consolidating or pre-consolidating agents, as adhesives, as integrating and supporting materials, as co-adjuvant for cleaning (ionic exchange resins, antiredeposition agents on textiles) or as protective agents (2, 3, 4, 5, 6). Nowadays chemists' and restorers' common experience have pointed out the main characteristics of synthetic materials to be used for restoration. First requirement is the mechanical compatibility between synthetic materials and materials constituting the object: for example adhesive and consolidating agents should "reply" to dimensional changes undergone by artworks under environmental variations. Another requirement is reversibility, i.e. the possibility of removing the material used for the restoration, even long time after the intervention and with the chance of coming back to the status before the intervention. Nowadays the main requirement of synthetic materials for conservation purposes is durability; all the materials, both natural and synthetic, undergo to degradation processes that induce chemical modifications. The decreasing in their flexibility (with loss of mechanical compatibility), and the loss of solubility (with loss of reversibility) are phenomena often strictly related each other. An example is the behaviour of polyacrylates, a class of polymers widely used in restoration, that undergo to a photo-oxidative degradation which leads to a strong yellowing, a partial cross-linking and a subsequent decreasing of their solubility (7, 8). The requirement of durability also includes that conservative treatments have to be carried out by using materials that, in principle, will not preclude in the future further restoration interventions (9). In general, macromolecular substances are suitable for application in conservation field for the relevant diversification of their properties.

In New Polymeric Materials; Korugic-Karasz, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.


372 Unfortunately, most of the polymers do not fulfil the above-described prerequisites. The use of these substances in conservation field is due to the need of the restorers to find materials with high consolidating or protective properties compared with old natural resins and waxes. Furthermore, the application of synthetic substances is fast and easy. It can be stated that until few decades ago the application of polymeric materials on artworks was made without taking into account their characteristics and thus without foreseeing their eventual negative consequences. Recently, much has been done to explain the mechanisms of action of the products used and the efficacy of the treatments. Nevertheless a methodical study on their intrinsic chemical properties is still needed. In fact, there are many ambiguous situations and it is not always possible to associate the trade name of a product to its chemical composition. The technical sheets often seem to be drawn up for exclusive use of manual operators, because, apart from the mandatory safety data, they only report applicative information. Information about chemical characteristics and composition of products are often missed. A very interesting example is the case of the commercial polyacrylate dispersions Primal AC33, a copolymer ethylacrylate-methylmetacrylate made and commercialised by Rohm and Haas, widely used as consolidating agent for stones and mortar (10, 11), for parchments (12), for fossilized wood (13), as adhesive of paint layers on different type of substrates (wood, wall, ceramic) (14) and as adhesive agent for lining processes on textiles (15). Primal AC33 was introduced on the U.S.A. market by Rhom and Haas in 1953 with the trade name Rhoplex AC33. Nowadays this commercial dispersion has been discontinued by Rohm and Haas, but several suppliers of products for conservators commercialise different materials in order to replace the original Primal AC33. In one of the most recent research (16) the results of the comparison among the chemico-physical characteristics of Primal AC33 and its substitutes are reported. The investigated materials showed remarkable differences in durability: aging treatments under xenon-arc lamp induce structural modifications evidenced by progressive broadening of the carbonyl peak at 1731 cm" , with simultaneous increasing of new components at about 1780 cm" and 1665 cm' . In Figure 1 FT-IR spectra (in the range 1600-1900 cm' ) are showed for thin films of different commercial products, both un-aged and aged for exposure under xenon-arc lamp for up to 250 hours. It can be observed that the relative intensities of the convoluted absorption bands showed by aged materials indicate that these products, all used by the restorers, show very different extents of photooxidative degradation phenomena. Nevertheless, in conservation, the trend is that both the choice and the final evaluation of the materials applied is carried out by restorers; in general a strict scientific support is missed. These considerations bought us to start a research programme to find and test new polymers and application technologies with 1

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Wavenumber (cm-1) Figure 1. FT-IR spectra ofdifferent acrylate-based commercial products, unaged and aged by exposure to Xenon-arc lamp for 250 hours; A) Primal AC3 3 un-aged; B) Acrilem IC79 aged; C) Primal AC33 aged; D) Primal B60A aged.


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advanced properties tailored for specific restoration treatments. Our goal was to fill the gap between the high level of specialization reached in the researches in the macromolecular field and the comparatively low level of specificity and performance of the materials usually applied in conservation. The ideal aim is to promote new routes for qualified restoration interventions that, starting from the real needs of an artwork, will be carried out by using the most efficient and modern technology.

A New Route for the Consolidation of Stone Among the materials not usually applied for conservation of stone, polyurethanes show very satisfactory results (17). In fact most of the polymers used in conservation are characterised by Tg values close to or slightly higher than room temperature. This can be easily explained by considering that a polymer coating with a Tg value considerably higher than room temperature is unable to respond to dimensional changes of the treated items and could thus damage them. On the contrary, polymers with Tg value lower than room temperature is too soft to act as a consolidating agent, tending moreover to pick up dirt. Polyurethane, characterised by hard and soft domains, often show two glass transition, the first one at temperature lower than the room temperature, and the second one at higher values. At room temperature soft segments are above their Tg, while hard micro-domains are below their Tg. Specific Tg values and other chemico-physical properties can be easily obtained by a rational choice of the monomers and their ratio during the synthesis. Furthermore polyurethanes are in general materials with excellent tensile properties, abrasion resistance, good transparency and high photo-oxidative resistance. Unfortunately, polyurethanes show the same problems of poor penetration inside the stone pores, already shown by other polymeric materials. Usually, polymers are applied on stone in solution of organic solvents or in water dispersion; then solvent or water evaporate through the external surface of the treated item. For polymeric solutions, the high viscosity strongly reduces the penetration of the material inside stone; moreover part of the material applied often rises up to the surface during the evaporation process of the solvent. For water dispersions the most relevant problem is the size of the dispersed particles. In Figure 2 an example of bad application of polyurethane from water dispersion is reported. The average size of the water dispersed particles applied is too big to let the material penetrate inside the stone pores. Trie result is an external coating that has not consolidating effect on the inner layer of the degraded stone.



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Figure 2. SEM micrograph ofsandstone treated with a commercial polyurethane-based water dispersion.

To solve these problems, in the work here reported (18) we focused our attention on the in situ polymerisation, consisting of the application of monomers that react during their penetration inside the stone pores. The consolidating effect is obtained through the formation of a layer more or less continuous of polymer, coating the inner walls of the pores within die stone and linking up the disaggregated grains with each other. A typical reaction scheme of the synthesis of polyurethanes is reported as follows:

Scheme 1

R - N = C = 0 + HO—R,

R-NH-C—O—Ri



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T(°C) Figure 3. Thermogravimetric curve ofNeapolitan yellow tuff; WR = weight residual; WLR = weight loss rate.


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The most important parameter in this in situ polymerisation procedure was the room temperature, to reproduce the operative conditions of restorers, and, overall, the presence of bonded water into the stone. In Figure 3 a typical thermogravimetric trace of Neapolitan yellow tuff is reported: it can be observed the large amount of water, evidenced by the weight loss during heating. This water cannot be removed from stone and it has to be considered as a reactive component of our system, leading to the proceeding of the side reaction giving urea bonds, as reported as follows: Scheme 2 -co

HO 2

R-N=e=o

R-N-C—OH

2

RNCO R-NH

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R-N-C—N-R H

H

H

Despite to the side reaction, the use of a catalyst selective for the reaction isocyanate-alcohol in respect to the side reaction isocyanate-water, and the right choice of the monomers and their ratio, let us to obtain a poly(urethane-urea) characterised by high depth of penetration and homogeneous distribution into the stone pores. Moreover treated samples showed optimal values of water vapour permeability and a strong decrease in the rate of water capillary absorption (from 32 to 14 mg cm" s" ), thus indicating a good protective effect (19). Furthermore poly(urethane-urea) in situ polymerised inside stone forms a regular homogeneous thin film covering the grains of the stone without modifying their morphological features. Also mechanical properties of treated stone were very promising. The aggregative efficiency (AE) of this treatment was calculated following the equation: 2

l/2

A E (%) = 100*(AW - ΔΨρο,)/ AW

(1)

where: AW = average weight loss of 10 untreated tuff samples after 300 abrasion cycles; AW j = average weight loss of 10 tuff samples treated by in situ polymerisation, after 300 abrasion cycles. Tuff samples treated by in situ polymerisation showed aggregative efficiency values up to 83%. po


378 Moreover compression teste carried out on tuff samples, previously undergone to accelerated weathering by freeze-thaw cycles and then treated by in situ polymerisation procedure, showed high recover in the compression strength.


The recovery in the compression strength (RCS) was calculated following the equation: RCS (%) = 100*(CSpo, - CS )/(CS - CSft) ft

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where: CSft = average compression strength of 15 tuff samples aged by 50 freezethaw cycles; CSpoi = average compression strength of 15 aged tuff samples, consolidated by die in situ polymerisation technique; CS = average compression strength of 15 unaged, untreated tuff samples. Freeze-thaw aged tuff samples treated by in situ polymerisation showed recovery in the compression strength up to 85%. The research is still in progress to evaluate durability properties of this consolidating procedure and efficiency on different stone. Moreover, preliminary tests carried out on different stone substrata gave us other very promising results. As an example SEM micrographs of sandstone, both untreated and after in situ polymerisations procedure are reported in Figure 4. In Figure 4A thefracturedsurface of untreated sandstone is characterised by a high roughness; furthermore small disaggregated grains (diameter < 10 μπι) are deposited on the stone surface. In Figure 4B it is well evident the film of polymeric material homogeneously covering the stone: die stone surfaces appears smoothed by the presence of the polymeric film in which disaggregated small grains are embedded. Finally, in Figure 5, a comparison between the treatment developed in the present work and a traditional consolidating method based on the application of ethyl silicate derivatives is reported. In Figure 5A the external surface of untreated sandstone is showed. In Figure 5B the micrograph of sandstone treated by silicate-based material is reported, showing highly extended cracking phenomena on the treated surface. In Figure 5C the morphology of sandstone treated by in situ polymerisation of poly(urethane-urea) is reported: the polymeric film is regular, homogeneous, and does not drastically modify the morphological features of the substratum on which it is applied.



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Figure 4. SEM micrographs offracture surface of: (A) untreated sandstone; (Β) sandstone treated by in situ polymerisation ofpoly(urethane-urea).



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Figure 5. SEM micrographs of external surfaces of untreated sandstone (A), sandstone treated by application ofsilicate-based material (B) and sandstone treated by in situ polymerisation ofpoly(urethane-urea) (C).


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New Polymer-based Processes for the Restoration of Textiles Polymeric materials have been subjected to many studies to evaluate their usefulness in replying to specific requirements for conservation of artworks constituted by stone. Their efficacy in restoration treatments oftextilesmaterials is still an open field for investigations. Acrylic and vinylacetate dispersions have been used as consolidating and adhesive agents on textiles for more than 20 years. There is much anecdotal information about unsuccessful treatments but there are few quantitative data on die effects of the applications of these materials for consolidating and adhesion purposes (2, 4, 5, 8, 20). The aim of consolidation is to hold together degraded fibres improving physical strength of yarns and fabrics. Nowadays a large number of polymers are applied on ancient textiles by impregnation to increase their tearing and breaking strength (5). In adhesive techniques a support fabric is coated with an adhesive material and put in contact with the verso of the textile object to be conserved. Two adhesive techniques are currently used by restorers: heat sealing, for textiles, and cold vacuum table, for the lining of paintings on canvas (21, 22). The work started from the needs of better understanding the phenomena related to the application of polymeric materials on textiles, with particular attention to the extent of absorption phenomena, to the morphological analysis of the coatings and to the correlation among the physico-chemical properties of the material applied and the effects obtained on the treated items (23). All the materials tested, both acrylic and acetovinylic, applied on cotton textiles, are absorbed on the textile materials with amounts mainly related to the dilution of the commercial water dispersions used for impregnation. The relative amount A of polymeric materials impregnating the yarns is well fitted by a sigmoid curve, whose equation is here reported: A - A A — 0 ^max ι A (3) A

""l

+ e

0c-Xc«ter)/dx

+

A

™ *

W

where: A = percent relative amount of dried polymers impregnating the yarns; χ = dry content (content of polymeric material) of the water dispersion, at different dilutions, used for impregnation; A = A value for χ = 0; in our case: A = 0; A = asymptote of the impregnation curve at high values of the dry content in water dispersions; in our case: A = value of A corresponding to impregnation with the as-supplied water dispersions; ,χ*.^ " x value for A == 0.5*A ; dx = parameter related to shape of sigmoid curve. 0

0

m a x

m a x

max


382 In figure 6 the sigmoid curve of the percent relative amount of dried polymer impregnating the yarns as a function of the polymer content in the water dispersion used for the impregnation (RH = 50%, Τ = 25°C) is reported for the commercial vinylacetate-co-butylacrilate water dispersion Mowilith SDM5.


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Polymers for the Conservation of Cultural Heritage - ACS Publications

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