Using Rheology Test Methods to Assess Durability of Cured Elastomers Undergoing Cyclic Deformation

ABSTRACT: The current measurement test method to assess elastomeric sealant durability is ASTM C719. This method requires a minimum of five weeks of curing and conditioning before being subjected to ten movement cycles at room temperature and then ten movement cycles at variable tem­peratures. This method is a fine predictor of sealant movement capability for products used in moving joints in commercial construction applications. ASTM E1886 suggests that building assemblies be subjected to 9000 cycles of wind pressure. Sealant materials are typically used to anchor glazing as­semblies into frames, and the choice of the correct sealant is critical to pass­ing the test criteria specified in ASTM E1866. Rheological instruments have the capability to characterize the dynamic mechanical behavior of elasto­meric materials undergoing oscillatory (cyclic) deformation under controlled test conditions and, therefore, provide a laboratory tool for assessing dura­bility. Cyclic testing can be conducted under controlled strain (deformation) conditions at frequencies that simulate joint movement due either to thermal expansion differentials or seismic events, or under controlled stress (load) that model hurricane-force wind loads or design pressures. An immediate stress-softening response was observed from controlled-strain experiments at 15 % movement that was ascribed to the Mullins effect; however, three of the four cured silicone sealants exhibited a modest recovery over the re­maining four days of cyclic testing. Under controlled-stress cycling at 0.138 MPa for 150 minutes at 0.5 Hz, the silicones exhibited ultimate deformations well below their rated movement capabilities. The results from both types of

rheology test methods did not reveal outward signs of fatigue and suggest which elastomeric materials will perform better under the drastic cycling that occurs in ASTM E1866 and ASTM C719 testing.

KEYWORDS: cycling, deformation, durability, elastomers, movement, rheology, rheometry, sealant, silicone

Introduction

Durability evaluation of sealant-materials is a science unto itself. The existing assessments of sealant durability include the ASTM C719-93 “Standard Test Method for Adhesion and Cohesion of Elastomeric Joint Sealants under Cyclic Movement (Hockman Cycle)” [1], “JIS-A 5758 Sealing Compounds for Sealing and Glazing in Buildings" [2], and “ISO 9047:2003 Building construction— Jointing products—Determination of adhesion/cohesion properties of sealants at variable temperatures" [3]. These methods use a block of sealant approxi­mately 12 mm by 12 mm by 50 mm extruded between two parallel plates made from a selection of available substrate material(s). The sealant is allowed to cure and the adhesion and cohesion properties are evaluated after extension and compression cycles. For example, ASTM C719 requires a minimum of five weeks of curing and conditioning before being subjected to ten movement cycles at room temperature and then ten movement cycles at variable tempera­tures. These methods are used by the global industry to assess movement ca­pability of high performance sealants. Other custom assessments of durability use the same joint design and will subject the specimens to conditions that are specified in structural glazing specifications such as the European Organization for Technical Approval (EOTA) ETAG 002 Guideline for European Technical Approval for Structural Sealant Glazing Systems [4] and ASTM C1184-00a Standard Specification for Structural Silicone Sealants [5]. These specifications assess the properties of structural sealants after aging in environments such as salt fog, water immersion, heat, cold, hot water, and SO2 environments by comparing the stress-strain properties and adhesion to control samples.

After Hurricane Andrew blew by South Florida in 1992, the destruction and devastation that occurred prompted the development of glazing standards for the hurricane-prone areas in the United States. AsTM E1886-02 “Standard Test Method for Performance of Exterior Windows, Curtain Walls, Doors, and Im­pact Protective Systems Impacted by Missile(s) and Exposed to Cyclic Pressure Differentials” [6] proposes that building assemblies be subjected to a total of 9000 cycles of wind pressures after the glazing is impacted with specified flying debris, simulating hurricane conditions. This full-scale test requires the proper glazing, frame design, and structural attachment of the glazing. The spirit of the test is to ensure that the glazing remains in the frame after being impacted during a hurricane. When the glazing remains in the frame after the impact event, the interior of the building is protected from the windblown debris and rain damage that can occur.

ASTM E1886 requires the sealants, framing system, and glazing system to harmoniously perform for the duration of the test. This harmonious perfor­mance has several times been a study in trial and error. There is no sealant durability test that really predicts the performance of a sealant in this full size assembly. This paper is an attempt to take a different approach to determining a method that can predict sealant performance undergoing either strain – or stress-dependent movement cycles.

Rheology, the study of the deformation and flow of matter, provides test methods to determine material functions that describe the viscous or elastic response of a material. For example, a rheology test method was applied to examine the temperature dependence of the dynamic tensile properties of vari­ous sealants as a diagnostic tool in durability assessment [7]. Building upon this data, an attempt is made to further understand the use of this tool to predict durability of elastomeric materials.