Development of a Practical Method to Evaluate the Fatigue Properties of Structural Silicone Glazing Adhesives

ABSTRACT: Structural silicone sealants have been used since the mid 1960s in the construction industry to attach glass to curtainwall framing systems. Design standards for these sealants have been in practice since the early 1970s and continue to be developed today. Most of these standards require tests that are based on a one-time destructive test of the Joint material in tension. The industry established structural silicone design stress is intended to be reached only during the highest windload for the specified return wind period. The existing practice has had great success because the industry guidelines minimize stress on the silicone, and dictate quality assurance procedures that are instituted by curtainwall producers. In real life situations the sealant is additionally fatigued through a cyclic shear mechanism caused by daily thermal movement differences between the glass pane and the aluminum frame. This paper discusses a method for testing the effect of fatigue in sealants used in structural designs and comparing the data to control samples.

KEYWORDS: fatigue, structural silicone, durability, shear strain

Introduction

Design standards for structural silicone glazing (SSG) have been in practice since the early 1970s. [1] The existing practice has had great success because of the low design stress on the structural silicone (SS) along with the industry standard quality assurance procedures.

The success of SSG can be attributed to the fact that panels of glass and or metal are attached to a frame with a continuous silicone rubber adhesive. The weatherability, and durability of the silicone makes it an excellent product for the application, and the continuous adhesion keeps air and water out of the system. The resultant product is an architecturally pleasing facade that has a performance unmatched by panels captured by gaskets.

Figure 1 shows a typical SS design incorporated into a commercially available curtainwall system.

Although the SS design stress is designed for a certain return wind period, its use has been based on a tension test to destruction. The most popular methods of testing are based on the ASTM С1135 Standard Test Method for Determining Tensile Adhesion Properties of Structural Sealants and ISO 8339 Building Construction-Jointing Products-Sealants-Determination of Tensile Properties.

Sandberg performed fatigue testing of SS published data on the Creep Rupture and Fatigue of Struc­tural Sealants. [2] The testing was performed to determine if the constant load designs of SS were appropriate for the applications based on Civil Engineering standards and logic. The conclusion was that the existing designs were conservative and appropriate.

Later on in 1990, the International Conference of Building Officials in Whittier California published the Acceptance Criteria for Type 1 Structural Silicone Glazing Sealants (Adhesive) [3]. This criteria required cyclic load tests between 0.5 and 1.0 times the design stress for 50 cycles. The condition of acceptance was the fatigued samples must show 95 % of the control sample strength and modulus. The spirit of this test was to show that the structural sealant would allow more than a once-in-50-years return windload at design stress without being affected.

The European Organization for Technical Approval developed the ETAG 002 Guideline for European

Manuscript received June 3, 2005; accepted for publication November 22, 2006; published online January 2007. Presented at ASTM Symposium on Durability of Building and Construction Sealants and Adhesives, Second Symposium on 15-16 June 2005 in Reno, NV; A. T. Wolf, Guest Editor.

1 Associate Industry Scientist, Dow Coming Corporation, P. O. Box 994, Midland, MI 48686-0994, U. S.A.

2 Technical Marketing Engineer, GE Advanced Materials-Silicones, 260 Hudson River Rd„ Waterford, New York 12188 U. S.A.

3 Vice President Technology, Tremco Inc, 3777 Green Rd., Beachwood, OH 44122 U. S.A.

Copyright © 2007 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.

Technical Approval for Structural Sealant Glazing Systems [4], and it also contains a mechanical fatiguing requirement. It states that the test pieces are to be subjected to repetitive tensile loads with a cycle time of 6 s for 100 times from 0.1 to 1.0 times the design stress, then 250 times from 0.1 to 0.8 times the design stress, and then 5000 times from 0.1 to 0.6 times the design stress. The condition of acceptance is that after testing the fatigued sealant must have greater than 75 % of the strength compared to the control.

These two initial standards are performed by fatiguing the samples in tension based on an anticipated windload for the structure. However, daily thermal movement differences between glass and aluminum framing will cause the structural silicone sealants to experience shear. The logic of tension fatigue testing is easy to understand because the equipment used in these tests was developed for tension testing.

Members of ASTM Committee C24 on Building Seals established a task group to develop a logical method of shearing SS joints and comparing the strength and modulus after testing to control samples. The amount of shear should be based on the thermal cycles placed upon the system for the life of the system.

Large lites of monolithic glass that are adhered to aluminum curtainwall frames can be as large as 1800 mm wide by 3650 mm tall. These lites are typically fabricated in a shop at temperatures of 15-30°C. A thermal cycle in Minneapolis, Minnesota can cause the exterior wall to experience -40°C in the winter and can reach up to 80°C in the summer. Thus, thermal movement in Minnesota would be driven by a temperature difference of 65 C° (80-15). Phoenix, Arizona may not have the winter cold and the exterior skin may only experience 0°C in the winter, but may reach 100°C in the summer. The Phoenix thermal cycle could be as large as 85 C° (100-15).

When considering a worse case scenario of a silicone structurally glazed 3650 mm tall lite experienc­ing an 85 C° temperature change, the dimensional change of the glass will be absorbed by the thickness of the SS. It is assumed that the thermal movement of the glass is in one direction only, because the glass is supported with setting blocks to absorb gravity loads. This thermal movement of the glass would be equivalent to 2.79 mm at the top of the lite. If this 2.79 mm of movement is induced through a 6.0-mm SS

FIG. 2—Fatigue testing device.

FIG. 3—Samples in fatigue device, sheared longitudinally.

adhesive thickness, the resultant strain on the sealant is 10.3 %. This is determined by comparing the thermally induced dimension to the original dimension using the Pythagorean relationship for a right triangle.

It is assumed that the thermal cycling on the curtainwall frame could happen twice per day. If the life span of a curtainwall is 50 years and the SSG system cycles twice per day, 36 512 cycles could occur. At one time per day, 36 525 cycles could occur in 100 years.

The task group chose to evaluate specimens for strength and modulus after 36 500 cycles of 15 % shear strain. A rate of five cycles per minute was selected to minimize any potential internal heat build-up in the specimens. The device that was used was a machine that operates on a cam to perform the cycling.