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The Suitability of Polymer Composites as Protective Matenals ANTHONY T. DIBENEDETTO One method of minimizing the erosion of stone surfaces is to deposit into the surface pores organic monomers or prepolymers, which are then polymerized into a protective layer. This forms a stone-polymer composite near the surface that is meant to protect the stone from further erosion by water and reactive gases. The effectiveness of such a composite as a protective material depends on the mechanical properties of the composite, the stability of the interface between the stone and the polymer, and the permeation characteristics of the polymeric coating. This paper discusses the properties of composite systems relevant to the preservation of stone. There are no materials of construction that are totally immune to environmental degradation. Even stone, undoubtedly the most durable of all traditional materials, is subject to physical and chemical erosion. The rate of degradation depends on both the type of stone and the nature of the environment. The realization that many historic struc- tures are slowly deteriorating under centuries of exposure to atmo- spheric conditions, a deterioration often accelerated by the gaseous and particulate pollutants so common in today's urban environment, has led to the development of many novel techniques of preservation. Among those techniques is the application of synthetic organic mono- Anthony T. DiBenedetto is Professor of Chemical Engineering, University of Connect- icut, Storrs. 312

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Polymer Composites as Protective Materials 313 mere and polymers for the purpose of repairing, consolidating, and protecting the stone structures. Many properties of polymers and polymer composites make them attractive for these purposes. For example, patching materials may be developed using polyester or epoxy resins filled with both stone par- ticles and pigments, forming composite materials- of strength, dura- bility, and compatibility with the stone substrate. Consolidation and surface protection can be attained by impregnating the stone with low- viscosity monomers and subsequently polymerizing the monomers in the surface pores. This process forms coatings that resist water and gaseous-vapor penetration, in addition to strengthening the material. The suitability of such treatment is strongly dependent on the prop- erties of the composite materials formed. A few of the characteristics that determine whether the treatment will be helpful or harmful are the solids content of the composite, the adhesion between the resin matrix and the filler, the adhesion between the polymeric composite and the stone substrate, the matching of physical properties between the polymer composite and the stone, and especially the choice of resin matrix. POLYMER COMPOSITES AS PATCHING MATERIALS A low-viscosity polyester or epoxy prepolymer can be filled with crushed and graded stone particles and a pigment to produce an adhesive patch- ing compound. Since it is essential to bond the resin to both the filler particles and the stone substrate, it is usually necessary to incorporate a bonding agent into the compound formulation. There are a number of such treatments, the most popular for polymer/silicate-type com- posites being the addition of a few percent of an organosilane. An organosilane coupling agent is a compound of the general form R-Si- {XJ3, where R is a resinophilic group and the X's are organic groups capable of interacting with silanols. Some examples of commercial organosilane coupling agents are listed in Table 1. For example, the three methoxy groups {OCH3) of y-glycidoxy-propyltrimethoxysilane (the seventh compound in Table 1) are capable of reacting with the hydroxyl groups of a silicate surface to form a chemically bonded layer of epoxy groups (CH2CH-) on the inorganic surface (see Figure 11. The other methoxy groups are also capable of reaction with the silicate or can interpolymerize in the presence of water to form a protective organosilane polymeric coating. The pendant epoxy group can react with an epoxy resin, so that the matrix is chemically bounded to the silicate substrate.

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Polymer Composites as Protective Materials Glass Glass R' NH Sio \ cl H2 SiO SiRCHO si-o/ >SIRCH - si-o I , o CH2 NH R' SiOH SiOH SiOH +X3 SiRCH - CH2 ~ Glass SiOH \O/ SiOH Coupling Agent, e.g., X =OCH3 R =CH2 O(CH2 )3 Resin Matrix FIGURE 1 Coupling reaction at a glass/silane/epoxy interface. 315 - si0 - SiO ~ SiRCHCH2 - si-o 'Cal - - Si_O > SiRCHCH2 Ol + Resin Matrix + R NH2 Proper choice of a coupling agent is essential to the development of maximum physical properties and, perhaps more importantly, to long- term stability in the presence of moisture or soluble gases, such as sulfur dioxide. In the absence of good adhesion between the phases, thermal stresses will lead to debonding of the imbedded. particles and a microcavitation of the materials. Water and gases can then collect at the interfacial void spaces, causing an accelerated degradation of the inorganic phase. This is illustrated schematically in Figure 2. Even with the proper choice of components, the wettability of a surface must be considered. When a fluid polymer is placed-on a silicate surface, it will form a contact angle, as shown schematically in Figure 3. A large contact angle, 0, represents poor wetting, while a contact angle of zero represents spontaneous wetting. Wetting is favored when the substrate is free of contamination, when the polymer has an affinity for the substrate, and when the surface tension of the polymer is low. Surface roughness affects the wetting characteristics, since the fluid must move up and over asperities. Most important is the possibility of air being trapped under a spreading fluid, thereby creating many voids at the adhesive interface. The inevitable thermal cycling of the material could then more easily lead to adhesive failure at the joint interface. When patching a porous surface, a strong mechanical bond can be

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316 CONSERVATION OF HISTORIC STONE BUILDINGS H2O or Gas Penetration Collection of H2O at Interfaces m_ 1. In`. ~ Composite Patch ~o: ' ~ '-' ~ d ~ ~ _~ - o O o ~ . Q Substrate FIGURE 2 With poor adhesion between phases in a silicate-resin patching material, thermal stresses will lead to debonding of imbedded particles and microcavitation of matenals. Water and gases can then collect at interracial voids and cause accelerated degradation of inorganic phase. cleveloped by forcing the fluid into the capillary passages leading to the interior. First, the fluid must wet the capillary passages in order to displace the air in the pores (see Figure 41. Second, enough time must be allowed for penetration to occur. In a cylindrical, open pore of diameter, d, the depth of penetration is equal to: l l /COS ~ ~LV d t V 4~ ' 11) where ~ is the contact angle, Kiev is the surface tension of the fluid, ~ is the viscosity of the fluid, and t is the time of penetration. Most patching compounds will be highly filled, so that the viscosity is ex- tremely high, causing penetration to be very slow and in many cases negligible. Even when the mechanical joint is perfectly made, one still has to accept the fact that there is a mismatch of physical properties between the patch and the substrate. Perhaps the most important is the differ- ence in thermal expansion coefficients. The polymer composite will always have a higher thermal expansion coefficient, and thus normal thermal cycling will lead to internal stresses at all particle/polymer interfaces and at the composite/substrate boundaries. Under adverse conditions this could lead to microcracking of the patch material and even accelerated damage to the substrate. Thus, a great deal of care

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Polymer Composites as Protective Materials 317 Moderately Good Wetting Poor Wetting FIGURE 3 Contact angle is a measure of wettability. Fluid polymer forms contact angle (~) at a surface. The smaller the contact angle, the better the wetting; a contact angle of zero represents spontaneous wetting. In this schematic, LV is the liquid-vapor interface, SL is the solid-liquid interface, and SV is the solid-vapor interface. must be taken in choosing the proper patching component for a given . . app 1catlon. A critical variable is the choice of polymer matrix. Polyester and epoxy are generic names for a multitude of different resin formulations with vast differences in both physical properties and environmental stability. Some polyesters, for example, have relatively Tow resistance to atmospheric humidity, while others are highly resistant. Filled Pore 1 Polymer Coating Stone Substrate Coated Pore FIGURE 4 Bonding strengthened by forcing polymer into pores. Patching material can form strong mechanical bond with porous substrate when forced into capillary passages. Depth of penetration depends on contact angle, surface tension of fluid, viscosity of fluid, and time of penetration {see equation 1 and Figure 31.

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318 CONSERVATION OF HISTORIC STONE BUILDINGS TABLE 2 Illustrative Sorption and Transmission Rates of Water in Polymers at 40 C Water transmission % H2O rate 24-hour immersion 90-95% RH Polymer 1/8-inch-thick sample {g/m2/24 hours/mil) Polyethylene {0.92 glrnl) 0.01 28 Polyethylene 10.96 g/ml) 0.01 4 Polylvinyl chloride) 0.03 32 Polylvinyl choride) Plasticized) 0.4 88 Poly~methyl methacrylate) 0.2-0.4 550 Polytethylene terephthalate) Mylar) 0.03-2.5 30 Silicone rubber 0.1-0.15 Epoxy resin 0.01-0.5 The five varieties of polyester resins commercially available are (11 general purpose, {2) isophthalic polyester, (31 bisphenol-A-based poly- ester, (41 chIorine-bearing polyesters, and (51 viny} ester resins. The general purpose types, composed of phthalic anhydride, maleic anhydride, and propylene glycol, are the lowest-cost resins, but they generally offer the least corrosion resistance. The isophthalic types use a phthalic acid monomer and exhibit better resistance in both acidic environments and saltwater. The other types have higher temperature resistance and can be formulated for chemical resistance under very severe conditions. The bisphenol-A-based polyesters are commonly used and are highly resistant to a wide variety of harsh environmental conditions. TABLE 3 Some Gas Permeation Values Through Polymer Films at 30 C P x 10~ (cc (STp)/mm/cm2/sec/cm Hg) 02 H2S CO2 H2O Film Poly(vinylidene chloride) (Saran) 0.05 0.29 0.31 15-100 Polyester (Mylar A) 0.22 0.71 1.50 1300 Polychlorotrifluoroethylene (Kel-F) 5.60 12.50 2.9 Polyethylene (0.92 g/ml) 55.0 448.0 350.0 800 Natural rubber 230.0 1200.0 1330.0

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Polymer Composites as Protective Materials 319 The epoxy resins are formed by the reaction of epichIorohydrin with a hydroxyI-containing compound, such as bisphenoT A, and then cured to a thermoses material with anhydride or amine curing agents. They have somewhat better thermal properties than the polyesters end: are generally more resistant to corrosion, except in the presence of strong oxidizing agents. Once again, however, the specific properties are highly dependent on the monomer fo~ulation. The epoxy chain can be ar- omatic-based (starting with bispheno} Al or cycloaliphatic or aliphatic (starting. with glycerol!. These prepolymers can be cured with long- chain or short-chain amines or a variety of acid or anhydride catalysts. Other commercially available formulations are simply too numerous to mention. It is possible to formulate epoxies with softening points ranging from 50 C to 200 C, with water;solubilities ranging from less than 0.1 percent to more than 5 percent, with gas-transmission rates varying by orders of magnitude, and with mechanical properties rang- ing from high ductility and impact resistance to extreme brittleness. Water and gas permeability depend on both the solubility and the _ diffusivity of the penetrant in. the polymer matenat. Corn ot. anise properties can be varied over wide ranges by choosing the prepolymer components appropriately. Tables 2 and 3 give some idea of the range of gas and water vapor transmission rates possible with different types of polymer films. In any particular application..these properties would have to be measured for the compounds being considered as treating agents. Epoxy formulations, for example, could be highly aromatic or highly aliphatic and have properties at either end of the spectrum illustrated by Tables 2 and 3. CONSOLIDATION OF STONE USING MONOMERS AND PREPOLYMERS An increasingly popular method of protecting and strengthening the surface of a stone structure is to impregnate the surface with an organic monomer or prepolymer and then to polymerize the material in the. surface pores of the stone. If this is done properly, the monomer wiB coat deeply the internal surface pores, adhere to them, and then po- lymerize into a tough polymer film~with low permeability to~water and corrosive gases. Many polymers have been tried, including epoxies, silicones, fluorocarbons and poly~methy} methacrylatel. The. consoli- dated surface is a composite structure the properties of which depend on the constituent phases as well as Ol1 the adhesion between the two phases.

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320 CONSERVATION OF HISTORIC STONE BUILDINGS The first step of the impregnation process requires that the monomer penetrate the stone pores. As previously expressed by equation 1, the depth of penetration is a function of contact angle, surface tension, viscosity, pore size, and time. Very often, some of the properties de- sirable in the consolidated structure cause difficulties in the processing. For example, a silicone might be desirable because of its water resist- ance, while that very characteristic may mean it is relatively non- wetting (high contact angle d. Thus, it may be difficult to make a silicone penetrate the surface pores, which would~ result in shadow impreg- nation, clogging of pores, and a surface fiLn that could be subject to peeling. Some of the most satisfactory epoxy prepolymers also have very high viscosities, which decrease the depth of penetration. Under certain conditions it might be necessary to dilute the epoxy with either a reactive or nonreactive solvent to permit penetration. Upon curing, however, the presence of a solvent wiD result in greater shrinkage, leading to higher internal stresses and, perhaps, cracking of both the polymer coating and the stone. Thermoplastic materials, such as poly~methy] methacrylateJ, have a combination of good wetting char- acteristics, mechanical strength, and impermeability and can be ap- plied in a low-viscosity monomeric form. Upon long-term exposure to wet, polluted air, and under the internal stresses created at the pore surfaces, however, there can be a tendency for the polymeric coating to craze, thus resulting in loss of protection. The above-mentioned rlifficulties should not discourage the use of polymers in stone consolidation. It is possible to choose a resin with proper characteristics for both processing and long-term stability. The environmental stability, so strongly dependent on the permeability characteristics, is determined by choosing the proper resin for the known environmental conditions. When choosing a material for consolidation, one should seek expert advice. Also, regardless of the degree of expertise, one should elect to obtain and analyze accelerated test data on the composite system. The improper choice of consolidation matenals can lead to a bigger problem than was there onginally. Our historic structures are too important for us to make hasty decisions on the means of saving them. BIBLIO GRAPHY Anonymous. Chemistry arid Physics of Interfaces, Americal Chemical Society: Wash- ington, D.C., 1965. DePuy, G.W., L.E. Kukacka, A. Aushem, W.C. Cowan, P. Colombo, W.T. Lockman, A.J. Romano, W. G. Smoak, M. Steinberg, F.E. Causey. Concrete-Polymer Materials, Fifth Topical Report, BNL 50390 and USER REC-ERC-73-12, 1973.

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Polymer Composites as Protective Materials Gauri, K. Lal. The preservation of stone. Scientific American, June 1978. Manson, J.A., and L.H. Sperling. Polymer Blends and Composites, pp. 335 371 in Filled Porous Systems. Plenum Press: New York, 1976. Sternman, S., and J.G. Marsden. Bonding Organic Polymers to Glass by Silane Coupling Agents. 1h Fundamental Aspects of Fiber Reinforced Plastic Composites, R.T. Schwartz and H.S. Schwartz, eds. Interscience: New York, 1968. 321