By Kenneth M. Hart

For bonding in applications that do not require structural strength but in which resistance to dynamic shear is very important, the viscoelastic nature of acrylic foam tape allows it to be used in place of traditional mechanical fasteners. One such application, the construction of traffic signs, suggests further uses of this tape as a fastener.

Comparison of the Denham Springs, La., highway sign using 3M VHB acrylic foam tape and rivets. Note (below) the improved legibility associated with the tape sign.

Acrylic foam tape belongs to the category of pressure-sensitive adhesives (PSA's). Within this category, a wide range of performance characteristics are available—from the adhesive used in repositionable Post-it Notes to those used in bonding wear strips to the exterior shell of airplane wings exposed to temperature cycles from -40°C to 65°C.

In order of increasing performance, the materials used for PSA's in the joining and fastening area include natural rubber, synthetic rubber, acrylic, and silicone polymers. PSA's are commercially available in a variety of thicknesses from 0.025 to 4 millimeters and widths up to 240 centimeters. Single-coated PSA's (tape constructions with a carrier and a PSA on one side), such as Scotch Masking Tape, are not widely used in the joining and fastening industry and will not be addressed here.

PSA's that are less than about 0.25 millimeter thick are typically coated as a pure adhesive (without a carrier) on a liner or as a double-coated tape, with the PSA coated on both sides of a carrier layer such as polyester. These tapes are used where a thin bond line is required.

PSA's thicker than about 0.25 millimeter typically have a foam core or contain a foamed adhesive. These foam tapes are generally used where there may be inconsistencies in the materials that are being bonded, as the foam fills in the gaps. This is especially important where relatively large pieces are being bonded and long bond lines will be present. Another advantage of the thicker PSA's is that they allow the joining of materials that have different coefficients of thermal expansion, which create dynamic shear under large temperature variations.

One important advance in PSA's is a hybrid system that combines an acrylic PSA with a curable epoxy. The PSA allows precise placement of the hybrid adhesive, while the epoxy provides structural strength once it's cured. The major uses of these products are in the bonding of glass, metals, ceramics, and engineered plastics in general industrial applications.

The use of acrylic foam tape for traffic sign construction resulted from the need to reduce the overall manufacturing cost of signs and improve their durability and legibility in bad weather. Although a number of alternatives were evaluated, acrylic foam tape has properties that enable it to meet and exceed the design criteria.

Acrylic foam tape can replace bolts and rivets because of the unique viscoelastic properties of the tape. Acrylic PSA's are very viscous liquid polymers, so they are able to flow and "wet out," or completely coat, the surface of the substrate that is to be bonded. A very strong bond develops with the substrate as all of the substrate's microstructures are filled. The elasticity of foam tape allows it to adsorb and distribute the load by acting as a shock absorber between the substrates. Because the core of acrylic foam tape is itself a cross-linked acrylic PSA, it can rapidly contract and expand under very high dynamic shear conditions without the foam core splitting.

A typical traffic sign has one or more posts permanently mounted to the roadway with a concrete footing. To reduce the potential for human injury from striking a post, the support posts are designed to break away at the base and, depending on the size of the sign, directly under the sign. One or more panel stiffeners are often used to prevent the sign from buckling under high winds. Acrylic foam tapes are being used to join smaller panels together through the use of a batten strip on the backside to make larger signs, and to attach the panel stiffeners.

The National Cooperative Highway Research Program Report (NCHRP) 230 and the 1985 American Association of State Highway and Transportation Officials (AASHTO) Guide, Standard Specifications for Structural Supports for Highway Signs, Luminaries and Traffic Signals, provide specifications for crash testing and static load testing for highway signs to ensure the safety of motorists. To evaluate the suitability of acrylic foam tapes in the construction of highway signs, the Safety and Structural Systems Division of the Texas Transportation Institute at Texas A&M performed two crash tests that followed the NCHRP and AASHTO guidelines (at 32 and 98 kilometers per hour, or 20 and 60 miles per hour, respectively).

The test signs were constructed by joining two identical 1.2 x 1.8-meter panels with 1.1-millimeter (0.045 inch)-thick 3M VHB (very high bond) acrylic foam tape and a batten strip on the back side of the sign. Aluminum stiffeners were fastened with the tape to the back of the completed sign to add rigidity and facilitate attachment to the mounting posts with 13 x 65 mm A325 bolts. The mounting posts were two S4 x 7.7 A36 steel supports positioned 1.8 meters apart.

Ordinarily, two-legged sign installations are crash-tested by striking both legs. But in the tests described here, a passenger car weighing 800 kilograms was used to strike only one leg, in order to evaluate how well the acrylic foam tape bonded the stiffeners and batten strips to the sign. One-legged impact tests are typically more severe than two-legged tests. The test vehicle was equipped with transducers to measure roll, pitch, and yaw rates, and with a triaxial accelerometer near the vehicle's center of gravity, to measure acceleration levels.

The sign installation suffered negligible damage when struck in the left support at 32 km/hr. The left support yielded while remaining attached to the stiffeners and sign panel. The right sign support did not disengage from its base anchor. While the whole sign pivoted clockwise around the right support, the installation remained upright and serviceable. Occupant impact velocities, ride-down accelerations, and change in momentum were well within the recommended limits of NCHRP Report 230 and the 1985 AASHTO Standards.

The sign installation received moderate damage from the 98 km/hr. crash test. Upon impact, the right side support disengaged from the ground support, traveled upward, and ultimately struck the sign panel 180 degrees from the point of contact at 0.37 second after impact. The sign rotated counterclockwise and became disengaged from the left support at 0.42 second after impact, leaving the left support standing upright. The right support remained attached to the sign until both the right support and the sign hit the roadway.

The mechanism of failure for both posts was either by the shearing of the attachment clips connecting the panel to the stiffeners or by the pulling of the attachment clips through the aluminum stiffener. Even though the sign was struck at 98 km/hr. and landed about 2 meters behind the initial installation, there was no evidence that the batten strip or stiffeners, which were attached to the panels with acrylic foam tape, had separated. When failure occurred, it was the bolts and attachment clips or the substrate that failed, not the tape.

As with the test at 32 km/hr., occupant impact velocities, ride-down accelerations, and change in momentum were well within the recommended limits of NCHRP Report 230 and the 1985 AASHTO Standards.

By contrast, when a riveted sign was struck by a vehicle under similar conditions to the 98 km/hr. described above, the rivet heads pulled through the sign upon impact, causing extensive damage and ruining the sign. Unlike acrylic foam tape, the rivets were not able to dissipate the energy associated with the high dynamic shear force.

To simulate wind shear, static load testing was completed in accordance with the 1985 AASHTO Standard on a 0.6 x 1.2 meter x 2 mm aluminum sign. Three horizontal stiffeners were attached to the sign using 1.1-mm (0.045 inch)-thick 3M VHB acrylic foam tape. The stiffeners were attached to a support post using standard slotted aluminum clips. Four 12 x 5-cm steel plates were attached to the sign to distribute the static load so that the bolts would not pull through the 2-mm-thick aluminum sign. A standard Instron device with a load cell system was used to apply and record the load.

Based on data from the 1985 AASHTO Standard, a 1.2 x 2.4-meter sign exposed to a wind speed of 145 km/hr. undergoes a load of 121 kg. Two consecutive tests simulating a steady 145 km/hr. (90 mph) wind were conducted in order to simulate the dynamic conditions of a storm. A third test was run at steadily increasing loads until the system failed. From the same AASHTO data, the ultimate wind load was back-calculated for the destructive test.

These static load tests presented three potential sources of failure: 1) the adhesive bond between the aluminum sign panel and the stiffeners, 2) the aluminum clips clamping the stiffeners to the steel U-post, or 3) the U-post itself.

After both replications of the 121-kg load, the sign installation returned to its pretest state: The adhesive bonds between the aluminum sign panel and stiffeners remained intact even though the U-post bent upward about 15 cm and rotated about 39 degrees around the longitudinal axis of the U-post. When a 240-kg load was applied (simulating a 260-km/hr. or 160 mph wind shear), plastic deformation of the steel U-post occurred. Again, the adhesive bonds between the aluminum sign panel and the stiffeners remained intact. On the basis of these test results, the installation using 3M VHB acrylic foam tape is acceptable according to the 1985 AASHTO Standards at wind speeds of at least 145 km/hr.

The static and dynamic load performance of acrylic foam tape in highway signs has been vividly seen in an experimental sign that the Louisiana Department of Transportation (DOT) built before 1992 with 3M VHB acrylic foam tape. In 1992, Hurricane Andrew generated winds of up to 250 km/hr. (155 mph) in the Baton Rouge area, destroying several signs that had been constructed using rivets and bolts, but not the test sign constructed with acrylic foam tape.

The Louisiana DOT is now using the 3M tape in the construction of all panel-style new signs. For the fiscal year ended in June 1999, this amounted to about 100 square meters (10,000 square feet) of traffic signs built using the tape. Attaching the sign and supports with the tape has eliminated drilling through the sign face to attach fasteners, thus reducing areas where moisture can penetrate and cause premature delamination. The Maintenance Section of the Louisiana DOT has realized a 60 percent decrease in the amount of time it takes to attach the supports to the signs.

Further "live" test data on highway signs was obtained during Hurricanes Erin (1995) and Nicole (1998) in Florida. The Orlando-Orange County Expressway Authority has been using 3M VHB acrylic foam tape for its signs since 1992. Hurricane Nicole not only produced winds up to 153 km/hr. (95 mph), but also brought tornadoes that destroyed virtually everything along a 2-km stretch of the Beeline Expressway, including large trees, vegetation, and highway signs.

Examination of the destroyed traffic signs indicated that the supporting U-posts were sheared off near the ground from the torsional stress of the tornadic winds. There was no evidence of adhesive or cohesive failure on the signs. It was estimated that the signs withstood wind gusts up to 320 km/hr. (200 mph). The expressway authority also reported that the signs were quicker and easier to manufacture than conventional riveted signs, making them more cost effective.

Acrylic foam tape provides the following advantages that may apply to various industrial uses. It eliminates the need for drilling or welding, leaving a surface smooth and free of blemishes. It eliminates the potential of galvanic corrosion with the use of dissimilar materials in bonding. It uniformly distributes the stress over the bonded area, eliminating the concentrated stress areas that mechanical fasteners can cause. It allows the use of thinner materials whose thickness is not reliant on the fastening. It allows the joining of materials that have different coefficients of thermal expansion. Its resistance to extreme temperatures and ultraviolet light allows its use in very demanding applications. And, it helps reduce assembly time, which translates into reduced total cost.

The use of acrylic foam tape to attach stiffeners is widely applicable to the construction of elevators, emergency vehicles, trucks, trailers, and electronic component housings. This type of tape is also useful in bonding dissimilar materials (for example, plastic to metal) in the aerospace, appliance, window and door, and electronics markets. Other attributes that make acrylic foam tape suitable for the electronics industry are its waterproofing and dustproofing.

While there are many advantages to using acrylic foam tape, it is certainly not intended as a general-purpose fastener. This is especially true when joining materials that are below 0°C at the time of application or that have to withstand temperatures greater than 150°C for a long time. Acrylic foam tape is not good for joining materials that are dirty or oily.

The raw material cost of using acrylic foam tape is higher than that of traditional fasteners, but the total manufactured cost may actually be lower when labor, equipment, and overhead are factored in. A total cost-benefit analysis should be made on a case-by-case basis.


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