2.6 The Role of Acrylic Polymers in Acrylic/Alkoxysilane Stone Consolidants and a Pilot Study on the Feasibility of Long-Chain Alkyl-Trialkoxysilanes and Mixed Alkoxytrialkoxysilanes as Stone Consolidants

The Metropolitan Museum of Art
The Getty Conservation Institute

George Wheeler
Charles Selwitz
Period of Activity: 1/1/88 to present

Project Abstract
Subsequent to the work of Larson, acrylic polymer/alkoxysilane mixtures attained near regal status for the consolidation of limestones and marbles in museums. The addition of an acrylic polymer such as B72 B72 ;to an alkoxysilane such as methyltrimethoxysilane (MTMOS) imparts adhesive properties to the consolidant which aid in the bonding of loose grains of stone, and provides a surface compatible with other adhesives. It is not known to what degree the B72 affects the complex set of reactions which the alkoxysilane must undergo to form a solid network polymer. Part of this research project is designed to reveal the interactions which may exist and to discover what beneficial mechanical properties might accrue from these interactions. (C.S.)

Major Findings and Recommendations
The nature of the interaction of the acrylic polymer, B72, and silane esters was probed by vapor pressure, viscosity, weight loss and strength (modulus of rupture) measurements, gel permeation chromatography, Si29 NMR, and vapor analysis. Vapor pressure and weight loss measurements show that B72 has little deterring effect on silane esters evaporation. The lower alkyl (up to C4) trimethoxysilanes have extremely low viscosities which are substantially increased by adding B72. GPC discloses no reaction between the two components. Si29 NMR however shows that B72 strongly inhibits the (water) curing of the silane ester probably due to the hydrophobicity imparted to the solution. Headspace analysis confirms this. Mechanical testing shows that the acrylic polymer weakens the cured product.

From these findings it is concluded that there is no beneficial phase interaction of the B72 with the silane esters in neat mixtures and any advantage accruing from B72 must derive from the adhesive properties of the acrylic polymer. To investigate this further, acrylic/silane mixtures were added to limestone and sandstone of identical porosity. MTMOS had virtually no effect in limestone and any improvement in strength from mixtures can be attributed to B72 alone. On the other hand, neat MTMOS does improve the strength of sandstone and acrylic/silane mixtures do prove to be better than neat MTMOS alone.

Primary Publications
Wheeler, G., "Sol-Gel Technology and its Application to the Improvement and Development of Stone Consolidation Systems," Presentation at the Materials Research Society, Spring Meeting, Reno, Nevada, 1988.

ABSTRACT-A review of research in the development of new stone consolidation systems shows that little progress has taken place in recent years. In sharp contrast, the field of sol-gel technology has experienced rapid progress in the development of glass-like solids from organometallic precursors based on silicon, aluminum, zirconium, sodium, boron, germanium, titanium, and mixtures thereof. The more successful consolidation systems to date are based on the same or similar materials, namely, tetraalkoxy- and methyl-trialkoxysilanes. Sol-gel chemistry, in fact, has much to contribute to the stone consolidation arena and this paper charts common ground, points of divergence, and future directions for a concerted effort.

Wheeler, G., "The Chemistry and Physics of B72 Acrylic Resin/Alkoxysilane Stone Consolidants and Examination of Alternative Alkoxysilanes as Stone Consolidants," Report to the Getty Conservation Institute, June, 1989.

ABSTRACT-Advancing the front of stone consolidation has proven difficult in recent years. Since the development of WACKER OH and H in the 1960s, BRETHANE, the "Bologna Cocktail" and acrylic resins dissolved in methyltrimethoxysilane (or just neat MTMOS) in the 1970s, there has been no evolution in stone consolidation. This fact was lamented in the closing remarks by V. Furlan at the Lausanne "Stone Conference" in 1985. What was not pointed out at that time was how little is known about the chemistry and physics of any of the above-mentioned consolidation systems. The research presented in this report is designed to understand such properties by B72/alkoxysilane mixtures and to examine the potential of some other alkoxysilanes for stone consolidation.

Wheeler, G., and G. L. Shearer, "B72/Methyltrimethoxysilane Stone Consolidants: The Role of B72 Acrylic Resin, Materials Issues in Art and Archaeology II, Vol. 185, 1990, Proceedings of the Materials Research Society, Spring Meeting, San Francisco, California, pp. 209-226.

ABSTRACT-This paper discusses the interactions which exist between the acrylic resin B72 and methyltrimethoxysilane (MTMOS) and the properties which accrue from these interactions.

Wheeler, G. S., "Further Work on the Consolidation of Stone for the Getty Conservation Institute," Final Report to the (Conservation at the Getty) Institute, January 1993.

ABSTRACT-The purpose of this research project is to evaluate and compare and, in some cases, attempt to understand the chemistry of several stone consolidants: (1) methytrimethoxysilane (MTMOS)-based systems including neat MTMOS and solutions (sols) of water, MTMOS, and alcohols; (2) acrylic-, amine-, and expoxytrimethoxysilanes; (3) solutions or sols and two-step applications of types 1 and 2; (4) WACKER (Conservare) (H and OH); (5) an epoxy resin; (6) an isocyanate dimer. The means of evaluation and comparison is primarily mechanical testing of consolidated stone samples. A key feature of this testing program is a comparison of not only one consolidant type to another, but also how well each consolidant performs on both sandstone and a limestone. The means of understanding the chemistry of the alkoxysilane-based consolidants is liquid and solid state 29Si nuclear magnetic resonance (NMR) spectroscopy.