2.12 Basic Research on the Chemistry and Interaction of Aliphatic Isocyanates with Adobe, Stone, and Terracotta

The Getty Conservation Institute
Loyola Marymount University

Neville Agnew
Richard Coffman
Charles Selwitz
Michael Schilling
Michele Derrick
Eric Doehne
Eric Hansen
Michael Geis
Period of Activity: 6/87 to 12/91

Project Abstract
Polyisocyanates interact with the water inherently present in clays to form intractable cross-linked polymeric networks, and this makes these compounds promising candidates for the consolidation of adobe. Basic research in this interaction was undertaken as an aid to developing optimum formulations and procedures for use in adobe preservation programs such as the "Fort Selden Test Wall Program." The research consisted of three phases: (a) the reaction of polyisocyanates with pure clays, (b) the interaction of polyisocyanates with clay-sand mixtures, interaction of polyisocyanates with clay-sand mixtures,; and (c) the treatment of natural adobe adobe ;compositions with polyisocyanates.

Major Findings and Recommendations
The first phase has been completed and is the basis for two papers which were written and submitted for publication (see section 9.1: Agnew 1987b and Agnew 1987c). The work with hexamethylene diisocyanate and the more practical isocyanurate trimer of hexamethylene diisocyanate has shown that the interaction with pure clay results in a polyureau, due to the interaction of the polymer with adsorbed water, but this does not result in the consolidation of the clay to an aggregate with any mechanical strength even though the bonding between the clay is occurring. The ability of a clay to expand due to liquid water absorption is greatly reduced with increasing polymer loading to the extent that a normally hydrophilic clay such as sodium montmorillonite can be made hydrophobic. However, the ability of a diisocyanate-treated clay to absorb water vapor does not appear to differ greatly from an untreated clay. Absorbing clays can take in more polymer than kaolinites or calcium montmorillonites. In the thermal analysis portion of the study, the use of advanced procedures demonstrated that water can be thermally desorbed from clays at temperatures 100 C to 150 C lower than previously described.

Work on the second phase, the interaction of isocyanates with clay-sand mixtures, has shown that the addition of sand will lead to structures with mechanical integrity and water repellency. Natural adobe and polymer are even stronger products as a result of the silt content. Procedures for minimizing color formation were developed. Deep penetration, a critical requirement for successful consolidation, is achieved by using more dilute solutions of isocyanates in ketone solvents and using more applications.

Primary Publications
Coffman, R. L., N. Agnew, M. Geis, and C. Selwitz, "The Effects of Hexamethylene Diisocyanate-Derived Polymers on the Physical Properties of Selected Natural Clays," 1989, manuscript under consideration.

ABSTRACT-Several natural clays were treated with hexamethylene diisocyanate- derived polymers to determine the effect treatment would have on the physical properties of the clays. Polymer loadings ranged from 0.05% up to 9% by weight. The clays were examined by X-ray diffraction, scanning electron microscopy, electron microprobe, and infrared spectroscopy, and tested for moisture absorbency, water repellency, and degree of consolidation. X-ray diffraction analysis indicates that with treatment there is no modification of the clays' structure. Examination of polymer-treated clays by scanning electron microscopy and electron microprobe was unsuccessful in imaging and/or providing a compositional analysis of the polymer coating. Despite this inability to image or obtain a compositional analysis, infrared spectroscopy revealed that there was chemical interaction between the clays and the polymer. Treatment of expandable clays (e.g., Na-montmorillonite and bentonite) resulted in a decrease in their expandability, as demonstrated by the treatment of highly absorbent clays (typically absorbing up to twelve times their weight in water) which were rendered nearly water repellent with moderate to low polymer levels (2-4% by weight). While those clays were rendered hydrophobic to liquid water they were still capable of transmitting and absorbing water vapor. Little or no bulk consolidation resulted from treatment, although particle agglomeration did occur in polymer-treated samples.

Coffman, R. L., "Modification of the Physical Properties of Adobe and Other Natural Building Material by Chemical Consolidation," Materials Issues in Art and Archaeology II, Vol. 185, 1990, Materials Research Society, Spring Meeting, San Francisco, California, pp. 201-208.

ABSTRACT-The use of commercially available chemical consolidants, hexamethylene diisocyanate (HDI)-derived polymers, and silanes, has been shown to increase the compressive strength of adobe and other natural building materials. Laboratory and field testing of chemically treated adobe and sand-clay mixtures has also revealed an enhanced resistance to disintegration by water. In HDI-treated materials this resistance to disintegration is attained without a significant loss of porosity. Because porosity is not lost and the adobe can still breathe, the uptake of water by capillary rise is less destructive than for an adobe whose surface is sealed against moisture penetration. The treatment of other natural building materials with HDI-derived polymers is currently being evaluated and more research is still necessary before widespread usage could be recommended. hexamethylene diisocyanate (HDI)-derived polymers, and silanes, has been shown to increase the compressive strength of adobe and other natural building materials. Laboratory and field testing of chemically treated adobe and sand-clay mixtures has als;

Coffman, R. L., C. Selwitz, and N. Agnew, "The Getty Adobe Research Project at Fort Selden II: A Study of the Interaction of Chemical Consolidants with Adobe and Adobe Constituents," Adobe 90, Vol. 1, 1990, Proceedings of the 6th International Conference on the Conservation of Earthen Architecture, Las Cruces, New Mexico, October 14-19, 1990, pp. 250-254.

ABSTRACT-Treatment of adobe with hexamethylene diisocyanate-derived polymers and silanes increases compressive strength and consolidation and enhances resistance to disaggregation by water. To better understand the modifications of the properties of natural adobe, the effects of treating individual components of adobe(clay, silt, and sand) with chemical consolidants were examined. Several different clays (Na- and Ca-montmorillonites montmorillonites; and kaolin kaolin;) were treated with isocyanate and silanes as were mixtures of clay and quartz sand, sand alone, and adobe. X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), grain size analysis, water sorption analysis, and compressive strength testing were used to evaluate the effects of the chemical on consolidation, modification of compressive strength, and resistance to disaggregation by water in clays, sand-clay mixtures, and adobe. The results of these evaluations indicate that the clay type plays a significant role in the properties of isocyanate- and silane-treated adobe.

Coffman, R. L., N. Agnew, G. Austin, and E. Doehne, "Adobe Mineralogy: Characterization of Adobes from Around the World," Adobe 90, Vol. 1, 1990, Proceedings of the 6th International Conference on the Conservation of Earthen Architecture, Las Cruces, New Mexico, October 14-19, 1990, pp. 424-429.

ABSTRACT-A mineralogical survey of adobes from several historic and archaeological earthen structures in different parts of the world was undertaken to evaluate the variability in durability and resistance to weathering. The mineral composition (including clay type and quantity) and overall particle size distribution was determined for each sample. A study of the effectiveness of two chemical consolidants (a silane and an isocyanate) on the adobe samples were also performed. Preliminary results indicate that variation in clay mineralogy clay mineralogy; and grain size distribution play significant roles in the success or failure of chemical consolidation.

M. R. Schilling, F. Preusser, and G. Gutnikov, "Thermogravimetric Analysis of Calcium Montmorillonite Treated with Hexamethylene Diisocyanate," Journal of Thermal Analysis, Vol. 38, 1992, pp. 1635-1643.

ABSTRACT-The product of the reaction between calcium montmorillonite and hexamethylene diisocyanate in acetone, catalyzed with dibutyltin dilaurate, was studied by thermogravimetry to determine the amount of polymer produced. The method developed to determine polymer content in the clay possessed excellent accuracy and precision, and produced more consistent results than traditional gravimetric methods.

Schilling, M. R., "Analysis of Polymeric and Composite Materials Using Thermogravimetry," M.S. Thesis, California State Polytechnic University, Pomona, California, 1990.

ABSTRACT-See Section 5.1.

Hansen, E. F., and N. Agnew, "Consolidation with Moisture-Curable Isocyanates: Polyureas and Polyurethanes," Proceedings of the ICOM 9th Triennial Meeting, Resins Working Group, Dresden, Vol. 2, August 1990, pp. 557-562.

ABSTRACT-Commercially available aliphatic polyfunctional isocyanates (such as Mobay Corporation's Desmodur N3390, a prepolymer of hexane-1,6-diisocyanate) and other chemically related systems show promise in the consolidation of a wide range of cultural materials. Based upon preliminary experimental results and the chemical literature, particularly suitable materials may include adobe, stone, paint on ethnographic objects, leather, and wool. The polymer, a polyurea, may be formed in situ by reaction with atmospheric and adsorbed water. It is colorless with resistance to yellowing and degradation. There is ample opportunity for modifying the physical properties of the polymer by incorporating urethane linkages through the use of polyhydroxy compounds as co-reactants. Unlike many other reactive monomers (typically volatile silanes) which are difficult to use on a large scale, the prepolymer of hexane-1,6-diisocyanate, which is chemically an isocyanurate with three pendant isocyanate groups, is involatile and with normal care is safe to use. The low cost and consistency of quality of the product offer particular advantages for further developmental work in this area of conservation. The Getty Conservation Institute is actively pursuing some of these research directions.