Overview

Introduction

The Second ASTM Symposium on Durability of Building and Construction Sealants and Adhesives (2005-DBCSA) was held on June 15-16,2005 in Reno, Nevada. The symposium brought together ar­chitects, engineers, scientists – researchers and practitioners. Their aim was to transfer new ideas, gained from laboratory research and field work, to the study of sealant and adhesive durability and the development of new products and test methods.

Nineteen papers were presented at the symposium. This book contains a selection of twelve sympo­sium papers published by the Journal of ASTM International (JAI) prior to February 2007. JAI is an online, peer-reviewed journal for the international scientific and engineering community. Publication in JAI allows rapid dissemination of the papers as soon as they become available, while publication in this Special Technical Publication (STP) is intended to provide easy access to the condensed in­formation in a single volume for future reference.

Since the commercial introduction of the first elastomeric sealants and adhesives about fifty years ago, major advancements have been made in our understanding of their durability and the factors gov­erning it. The progress of sealant and adhesive technology in building and construction structures has brought with it many new materials, products, systems, designs and concepts. It has also brought an awareness of new or formerly unrealized problems relating to the durability of building and con­struction sealants, which ASTM C 24 Committee on Buildings Seals and Sealants is addressing.

Against a background of national and international efforts to harmonize testing and approval of build­ing materials and structures, ASTM C 24 Committee has been looking for ways of bringing together the experience of international experts gathered in the application and testing of building and con­struction sealants.

The current series of ASTM symposia on Durability of Building and Construction Sealants and Adhesives is a continuation of tri-annual symposia which were inaugurated by the RILEM Technical Committee 139-DBS Durability of Building Sealants in 1994. Today, this continuing series of sym­posia provides the best scientific forum globally in the building and construction industry for peer-re­viewed papers on all aspects of sealant and adhesive durability.

As with most scientific disciplines, substantial advances often occur through a series of small steps, rather than in giant leaps. This is also the case for the papers presented at the ASTM Symposium on Durability of Building and Construction Sealants and Adhesives (2005-DBCSA). Many of the papers reflect progress reports on on-going research.

This volume contains twelve contributions reflecting the wide spectrum of current state-of-the-art re­search into sealant and adhesive durability. The symposium papers cover the following topics:

• Factors Influencing the Durability of Sealed Joints and Adhesive Fixations

• Durability Studies of Sealants and Adhesives

• Development of New Test Methods and Performance Based Specifications

Copyright® 2006 by ASTM International www. astm. org

Below is a short overview of the papers which were published in JAI in the above three categories.

Factors Influencing the Durability of Sealed Joints and Adhesive Fixations

While our understanding of the factors influencing the durability of sealed joints and adhesive fixa­tions has progressed substantially over the past decades, there is still much to learn. A number of pa­pers therefore focus on this topic.

The modulus of a sealant is a key property influencing the durability of weatherproofing joints that undergo movement. For sealants subjected to cyclic movements, formulation changes resulting in a reduction in modulus will provide higher durability, if sealant fatigue and other performance proper­ties, such as adhesion, are not simultaneously degraded. J. M. Klosowski and J. E. Plott show the util­ity of a melamine resin additive in reducing the modulus of polyurethane and silicon-curable polyether sealants. The authors demonstrate that addition of relative small amounts of the melamine resin results in modulus reduction and improved adhesion in the specific sealants studied. However, the short-term nature of the study does not allow a sound assessment of the long-term durability of these sealants, since potential negative effects of the additive on the long-term behavior of physical properties were not studied.

Differential thermal movement between the spacer frame and the glass panes is a key contributor to the aging of the insulating glass edge seal and of the insulating glass unit (IGU) itself. Using finite el­ement analysis, J. E. Stewart, W. R. O’Brien, and A. T. Wolf model the thermal movements occur­ring in the edge seal of a large IGU as a result of temperature variations (-30°C to +60°C) for three commercially available spacer bars of different material (aluminum, galvanized steel, and stainless steel) and comer design (comer keys or bent comers). As expected, at the low temperature, the cor­ners are pulled inward, resulting in a bending angle >90°; while at the high temperature, the comers are pushed outwards, resulting in a bending angle <90°. Monitoring the changes occurring in the thickness of the polyisobutylene primary seal along the circumference of the IGU, the authors find that the stainless steel spacer has, by far, the least effect on the change in the cross-sectional area, while the aluminum spacer has the most substantial effect. This finding is in keeping with the ex­pected performance based on the difference in thermal expansion coefficients between spacer mate­rial and float glass. Thus, changes in the effective cross-sectional area of the primary seal available for diffusion that arise from differential thermal movements are likely to account for the observed per­formance differences of IGUs manufactured with different spacer materials.

Any material, that is an effective catalyst for an equilibrium reaction, catalyzes both the straight re­action as well as the reversion of the reaction. This much can be gleaned from any basic chemistry textbook. What makes predicting the durability of sealants and adhesives complicated is the fact that under weathering exposure in general several reactions occur simultaneously. Y. Cai in his paper studies the effect of tin and aminosilane – both well-known silicone condensation catalysts – on the durability of a silicone elastomeric coating. The author shows that the tin catalyst has the strongest in­fluence on detrimental changes in elastomeric properties and on chalking, but that the aminosilane also contributes to these changes.

Durability Studies of Sealants and Adhesives

The intent of this section is to present recent studies of the durability of sealant and adhesive materi­als and systems.

Customers increasingly challenge sealant formulators to develop high weatherability construction sealants with ever-higher performance. The trend towards higher service-life sealants is most visible in Japan and has led there to a gradual substitution of conventional sealants, such as polyurethanes, by higher performance sealants, such as silane curable organic sealants. Building on experience gained in Japan, C. Urban, T. Matsumoto, S. Tomari, and F. Adeleu in their paper discuss some of the factors that influence weatherability and durability of one-component sealants, such as binder, pig­ments, stabilizers, and catalysts. The authors compare conventional polyurethane sealants to silane

curable sealants based on polypropylene oxide or polyacrylate backbones and mixtures of these two backbones. As expected, binder and stabilizer have the most notable influence on weathering resis­tance; however, catalyst, titanium dioxide, and plasticizer also affect the weathering behavior. For op­timum weathering performance, all raw materials influences need to be carefully tested. Combinations of raw materials, especially in the case of stabilizers, may have synergistic effects, but may also reduce the weathering resistance. The authors also highlight the fact that a minimum of 1500-2000 hours of accelerated weathering should be used to assess the durability of construction sealants, and that for more demanding applications, 5000 hours, 10 000 hours, or an even longer du­ration of accelerated weathering may be required.

The second paper by Y. Masaoka, Y. Nakagawa, T. Hasegawa, and H. Ando also deals with the dura­bility of silane curable organic sealants. In this paper, the authors discuss the durability and perfor­mance of sealants based on a novel telechelic silane curable acrylate polymer in contact with photo­catalytic self-cleaning glass. Conventional sealants, when used in this application, often lack sufficient weatherability or involve the risk of hydrophobic staining of the self-cleaning glass. Sealants based on the novel telechelic silane curable acrylate polymer retain good adhesion to the self-cleaning glass even after more than 10 000 hours of exposure to UV irradiation in a super-accel­erated xenon-arc weathering machine. Based on outdoor exposure of the test samples for two months and measurement of contact angles before and after exposure the authors conclude that these novel sealants have very low staining potential on photocatalytic coatings.

Since its introduction nearly four decades ago, structural silicone glazing (SSG) has become a popu­lar glazing method for curtain wall construction. The major difference between SSG systems and the more widely used ‘dry-glazed’ systems is that glass lights or panels in SSG systems are adhered to the supporting glazing frame with structural silicone sealant along either two edges of the glass panel or all four edges. It is generally believed that SSG systems perform well in seismic regions due to the ‘resilient attachment’ of glass to the glazing frame. This notion has merits in four-sided SSG systems, but has not been previously substantiated for two-sided systems, wherein the top and bottom edges are typically captured in metal glazing pockets. The research presented by A. M. Memari, X. Chen,

P. A. Kremer, and R. A. Behr in their paper therefore is aimed at characterizing the serviceability and ultimate behavior of two-sided SSG curtain walls under cyclic racking displacements. Serviceability drift capacities corresponding to damage states such as gasket distortion, weather-seal and structural seal failures leading to air leakage and glass cracking are identified. Based on the results obtained in this study and comparisons with data collected during comparable studies on dry-glazed curtain walls, the authors conclude that serviceability and ultimate drift capacities of two-sided SSG systems under seismic conditions are significantly higher than their dry-glazed counterparts.

Leave a reply

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>