Structural Silicone Considerations: Mechanical Properties

There are numerous structural silicone products sold in the global marketplace each with an individual mechanical property profile. To highlight to the reader the wide diversity of properties, we can take the examples of tensile adhesion strength coupled with elongation capacity. Figure 5 below shows recent labora­tory test values of ultimate tensile adhesion strength (wider bars) and ultimate elongation value at maximum load (narrower bars) of 11 commercially avail­able (at the time of this paper) structural silicone sealants. The specimen type was the “tensile adhesion" specimen as outlined in the ASTM C1135 Test Method for Determining Tensile Adhesion Properties of Structural Sealants [10], with one noted change in the specimen configuration: the thickness between bonded plates was changed from the test method default of 1/2 in. (12.7 mm) to 1/4 in. (6.4 mm), which more accurately reflects a real-life SSG application. Each graph represents an average of a minimum of ten specimens, all of which exhibited cohesive failure at rupture.

The 11 products represented in Fig. 5 are identified as follows:

1. Single-component, medium modulus silicone, manufactured in the U. S.

2. Single-component, high modulus silicone, manufactured in Europe.

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FIG. 4—Detail of sliding anchoring system.

3. Single-component, high modulus silicone, manufactured in the U. S.

4. Single-component, high modulus silicone, manufactured in Europe.

5. Dual-component, high modulus silicone, manufactured in the U. S.

6. Single-component, high modulus silicone, manufactured in Europe.

7. Single-component, medium modulus silicone, manufactured in U. S.

8. Single-component, high modulus silicone, manufactured in the U. S.

9. Dual-component, high modulus silicone, manufactured in the U. S.

10. Single-component, high modulus silicone, manufactured in the U. S.

11. Dual-component, high modulus silicone, manufactured in the U. S. Looking at these two basic properties of any sealant, and comparing the

extremes, one will recognize that product No. 11 is more than double the tensile

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Tensile & Elongation Comparison C1135: t=6mm

□ Elongation Capacity

Э60

I 40 0 20

Э00

FIG. 5—Tension and % elongation comparison showing wide variability between prod­ucts of these two measured properties.

strength of product No. 1. The values of the weakest versus the strongest are 0.60 MPa (87.1 psi) and 1.32 MPa (191.1 psi), respectively, the latter being 120 % stronger than the weaker. Regarding flexibility, one will find even wider vari­ability within the group with the least flexible at 41 % and the most flexible 317 %, a 673 % difference. Such wide variation in mechanical properties will undoubtedly lead to performance differences when tested in some protective glazing applications. Protective glazing applications as defined by ASTM C1564 [11] include those subject to: earthquakes, hurricanes, windstorms, blasts, and other similar events.

When assessing the strength versus elongation profile of any given formula­tion, it is generally the case that filled rubber sealants attain physical strength at the cost of reduced flexibility. That is to say, the stronger the material, the stiffer and less flexible it becomes, and vice versa. Further, in these sealants, a given strain will result in higher stress in a stiff (less flexible) sealant and lower stress in a softer (more flexible) sealant. Arguably, it would be advantageous to lower the stresses imposed from strains imparted into a structural seal during a seis­mic event such that the sealant is not grossly overstressed leading to tearing or rupture of the sealant. Looking again at Fig. 5, some of the tested structural sili­cones possess a closer balance of these two properties with a combination of both high strength and high elongation capacity. In a material of this type, a

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larger strain could be absorbed with a lower stress within the sealant when com­pared to others. This is interesting for consideration in protective glazing appli­cations, which can impart larger strains (and subsequent stresses) into the system than do wind load and thermal expansion/contraction forces more typi­cally seen in common SSG applications.

The shear strain behavior of a structural sealant is a reflection of its relative modulus, whereby a lower modulus structural sealant (medium modulus) will have more strain capacity than a stiffer less flexible (high modulus) structural sealant (capacity being point of rupture). The behavior of a structural glazing bead in shear has been previously reviewed by Klowsowski and Wong [12]; how­ever, the information in the Klowsowski paper is relevant to testing on speci­mens at standardized (relatively slow) strain rates. An earthquake would impart shear strains into the structural seals at higher velocities than lab-tested rates. The test data presented below represents load-deformation values of a struc­tural sealant tested at both standardized and higher strain rates.