Durability of Bonded Stainless Steel Button Point Fixing on Glass

In an earlier paper, some data on the durability of the cured structural silicone film adhesive itself as well as of single-lap-joint shear specimens and stainless steel button point fixing on glass made with the film were reported [15]. As can be seen from the data presented in this paper, the structural silicone film adhe­sive combines high transparency, strong adhesion performance, thermal stabil­ity, and excellent weatherability with high tensile and shear strengths, unmatched by RTV (condensation curing) structural silicones. For illustration, Fig. 6 summarizes the data from the previous paper obtained on stainless steel

TABLE 2—Typical properties of cured silicone film adhesive.

Property

Test Method

Typical Value

Unit

Indentation hardness

JIS K 6253 [25] Durometer

70

JIS A

100% modulus

JIS K 6251 [22] (dumbbell 3)

4.0

MPa

Young’s modulus

ISO 527 Parts 1 and 2 [23,24]

9.3

MPa

Max. tensile strength

JIS K 6251 (dumbbell 3)

9.0

MPa

Elongation at break

JIS K 6251 (dumbbell 3)

250

%

Tear strength

JIS K6252 [26] (crescent specimen)

35

N/mm

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FIG. 5—Tensile stress-strain curve and corresponding graphs for secant modulus (stress-strain ratio) and tangent modulus (slope of the stress-strain curve) of the cured silicone film adhesive at room temperature (dotted lines show extrapolation of data to Young’s modulus).

button point-fixing specimens on glass combined with some new data on the short-term resistance to constant load exposure.

Specimens for testing the durability of point fixing on glass were prepared by bonding a 20 mm diameter stainless steel “button" with a threaded socket head (see Fig. 3) to a glass plate (cross section of adhesive interface: 314 mm2). The structural silicone film adhesive (1 mm initial and 0.8 mm final thickness) was cured between the button and the glass by placing the complete assembly into an industrial autoclave operated at a pressure of 1.275 MPa and a tempera­ture of 130°C for a total of 25 min. Separate test specimens were exposed to hot water immersion, to accelerated weathering, and to outdoor weathering, allowed to recondition for 1 day, then fixed to a tensile test machine, and pulled vertically to the glass surface at a rate of 50 mm/min. The hot water immersion was carried out at 50°C for a period of up to 8 weeks. Accelerated weathering occurred for up to 12,000 h in a machine with a fluorescent light source [ATLAS UVCON UC-1 Ultraviolet Condensation Weathering Device with UV-A 340 (340 nm) fluorescent bulbs]. The specimen was positioned in the tester such that the glass surface was exposed to the irradiation (accelerated weathering of

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the silicone through the glass) and the machine was operated according to ASTM G154-06 [29] with a cycle of 4 h UV at 60°C and 4 h of dark (no UV) and condensation occurring at 40° C. Outdoor weathering was carried out for up to 36 months in Chiba, Japan, in a weathering rack oriented towards the southeast with an inclination angle of 33° and the solar irradiation of the silicone occur­ring through the glass surface. Exposure to a constant load of 1.25 MPa occurred for a period of up to 16 weeks, after which the test specimens were tested to destruction in the tensile tester using a pull rate of 50 mm/min.

As can be seen from Fig. 6, after 16 weeks of loading the test specimen with a constant stress at 1.25 MPa, a reduction in the ultimate failure strength to 4.55 MPa from an original value of 4.77 MPa was noted. However, this decline may very well be within the error of the test when comparing all of the data shown in Fig. 6.