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11DBMC International Conference on Durability of Building Materials and Components ISTANBUL, Turkey 11-14 May 2008

Acrylic Foam Structural Glazing Tapes

Steve Austin 1 Uwe Manert 2 T 14 ABSTRACT The use of Acrylic Foam Structural Glazing Tapes to replace conventional structural silicone sealant and conventional structural glazing tape for 4-sided structural glazing applications has grown rapidly over the last few years. While the use of Acrylic Foam Structural Glazing Tapes is new in many parts of the world they have been used successfully since 1990 across the globe. Thousands of buildings have been successfully glazed around the world in countries such as USA, Germany, Brazil, Israel, India and Thailand with Acrylic Foam Structural Glazing Tapes. Conventional structural glazing methods of bonding glass lites to a metal frame use either one-part or two-part structural silicone sealants. Acrylic Foam Structural Glazing Tape provides an alternative bonding method with the performance needed for the application yet with some significant benefits over conventionally glazed systems. The successful application history of Acrylic Foam Structural Glazing Tapes and the testing of glass lites bonded with Acrylic Foam Structural Glazing Tape at third party test institutes demonstrate this alternative bonding material has the capability to meet established requirements for the application. Test results according to application specific ASTM standards show that select Acrylic Foam Tapes have acceptable performance when compared to more traditional structural silicone sealants. Additional testing of Acrylic Foam Structural Glazing Tape according to EOTA ETAG 002 was conducted by the CSTB in France. UV accelerated weathering studies have also been conducted demonstrating the strength and durability of Acrylic Foam Structural Glazing Tapes compared to structural silicone sealants. KEYWORDS Acrylic Foam Structural Glazing Tape, Curtain wall, Structural glazing, Durability 1 3M Industrial Adhesives & Tapes Division, Technical Service Specialist, St. Paul, MN USA 55144, 651-736-1259

[email protected] 2 3M Deutschland GmbH, European Technical Service Specialist, Neuss, Germany D-41453, +49 (0) 2131 14

2669 [email protected]

mailto:[email protected]

mailto:[email protected]


T 14, Acrylic Foam Structural Glazing Tapes,Steve Austin & Uwe Manert

1 INTRODUCTION The use of an adhesive to attach glass lites to glazing profiles independent of mechanical fastening began in the mid 1960s as described by Schmidt et al [1988] in a paper titled Performance Properties of Silicone Structural Adhesives published in ASTM STP 1054 Science and Technology of Glazing Systems. This method of attaching glass lites gained popularity over the next few decades as architects and building owners desired the clean look of an adhesively bonded curtain wall system. This method of glazing is now commonly called structural glazing. Structural silicone sealant was the first adhesive to be used in a structurally glazed curtain wall system. The structural glazing industry has done an excellent job of developing test methods, standards and quality control procedures that has lead to a wide acceptance of this type of glazing method. More recently beginning in 1990 according to 3M™ VHB™ Structural Glazing Tapes Technical Guide [2007], acrylic foam structural glazing tapes have been successfully used in thousands of buildings globally as the adhesive bonding glass lites to glazing profiles in curtain wall and commercial glazing window systems. The use, composition and performance of an acrylic foam structural glazing tape is significantly different than that of a conventional structural glazing tape also known as a spacer tape. Conventional structural glazing tapes are employed in a structural glazing system to create a space between the glass and a typical metal glazing profile. The thickness of the structural silicone sealant is determined by the thickness of the conventional structural glazing tape. This tape keeps the space or face clearance constant during the curing of the liquid applied structural silicone sealant. It does not provide a structural bond of the glass to the glazing profile. The structural bond is provided by the structural silicone sealant once it is fully cured. Conventional structural glazing tapes are characterized by a foam core with a thin adhesive skin on two opposing sides. The only adhesive portion is the thin skin which contacts the glass and glazing profile. The internal foam strength of a conventional structural glazing tape is generally low as high strength is not required from this tape in this application. Conversely, an acrylic foam structural glazing tape used in this application must assume the same bonding role of a structural silicone sealant as it replaces both the sealant and the conventional structural glazing tape. That role is to adhesively bond the glass lite and transfer incidental windloads to the building facade structure. Acrylic foam structural glazing tapes are adhesive throughout their entire construction including the foam core. It is this unique construction that give acrylic foam structural glazing tape the strength and performance properties suitable to be used as the primary structural bond in a structural glazing system. As with the use of a structural silicone sealant, the use of an acrylic foam tape for structural glazing applications requires careful consideration. The performance and durability of any adhesive used for this application should be well demonstrated before ever being considered for a structural glazing project. The objective of this paper is to demonstrate the performance and durability of a class of acrylic foam tapes known as 3M™ VHB™ Structural Glazing Tapes. The primary criteria used to demonstrate this performance is based on the test methods and design standards developed specifically for this application by the structural silicone and commercial glazing industry. 1.1 History of Acrylic Based Pressure Sensitive Adhesives Early use of pressure sensitive adhesives began in the 1800s and was focused on attachment solutions for the medical industry according to Satas in the Handbook of Pressure Sensitive Adhesive Technology [1989]. The early examples were based on simple resins and beeswax. Rubber was later introduced in this century as a key component in pressure sensitive adhesives. Satas also states that more recent advancements were made in the automotive area when designers were looking for a method to hold paper sheets or masks during the painting process. The area of masking tapes was invented by 3M Corporation to address this need in the 1920’s and is now widely accepted for this and many other uses. This tape generation was based on natural



and later synthetic rubber often characterized by a short shelf life and durability. In the early 1960s 3M also developed a special long lasting cellophane office tape which could remain on paper without turning yellow. The basis for this new tape was acrylic adhesive chemistry. This new acrylic based adhesive technology was also used to make thick double coated tapes for mirror bonding applications. The first thick double coated tapes consisted of a polyurethane or polyethylene foam core. In this construction the only adhesive component is the thin outer skin of the tape. The 1980s saw further advancement of the acrylic adhesive technology. For example, thick double coated acrylic foam tapes which were based solely on acrylic adhesive technology were developed by 3M as described in 3M™ VHB™ Durability Technical Bulletin [2001]. This construction was different than the previous thick double coated tapes as the acrylic foam tapes were adhesive throughout the entire construction. This characteristic is what leads to its unique viscoelastic high strength properties. These products were named 3M™ VHB™ Tapes as they exhibited higher dynamic and static load strength than conventional double coated foam tapes. 1 ETAG 002 TEST STUDY The European Organization for Technical Approval (EOTA) developed the ETAG 002 Guideline for European Technical Approval for Structural Sealant Glazing Kits (SSGK). A part of this European guideline contains the manner and methodology to which structural silicone sealants are tested and characterized for a structural glazing application (kit). This methodology has acted well to characterize structural silicone sealants and is also a good initial method of evaluating an alternative bonding agent such as acrylic foam structural glazing tapes. However, structural silicone is a cold liquid applied, chemically curing sealant while acrylic foam structural glazing tapes are a fully cured, ready to apply tape. Thus, not all listed test procedures are directly applicable for an evaluation of an acrylic foam structural glazing tape. It must also be noted that 3M™ VHB™ Structural Glazing Tapes have a lower dynamic tension design strength of 85 kPa which is lower than that of a structural silicone sealant which is most commonly listed as 140 kPa. The CSTB institute in France was consulted to conduct an independent evaluation of VHB™ Structural Glazing Tape according to ETAG 002. This study focused on an evaluation according to the appropriate test methods that would help to determine the performance and suitability of this acrylic foam tape for a structural glazing application. The mechanical properties evaluated in this test regime were tensile strength, shear strength, cyclic fatigue, UV accelerated aging resistance and water immersion strength retention characteristics. 2.1 CSTB Test Reports According to ETAG 002 on ER4 – Safety in Use The following tables summarize the data from the standard test regime conducted at the CSTB. All test samples consisted of aluminum bonded to glass. While 3M™VHB™ Structural Glazing Tape is more temperature and rate/time sensitive due to its viscoelastic characteristics than structural silicone sealants this data demonstrates its ability to meet the performance demands of this application. The final version of the EOTA approval will take more time to complete but only to make a precise specification for acrylic foam structural glazing tapes which have different properties than those of liquid applied, chemically curing structural silicone sealant.

Table 1. Dynamic tensile and shear strength at -20°C, +23°C & +80°C (ETAG 5.1.4.1.1&2). Additional testing was conducted at 100°C.

Aluminum/Glass -20°C (-4°F) +23°C (73°F) +80°C (176°F) +100°C (212°F)

Shear Strength 706 kPa 562 kPa 409 kPa - Tensile Strength 3130 kPa 412 kPa 185 kPa 221 kPa



Table 2. Dynamic tensile strength after UV exposure - 4000 hr (ETAG 5.1.4.1.1).

Aluminum/Glass 0 hr 1000 hr 2000 hr 3000 hr 4000 hr Tensile Strength 412 kPa 387 kPa 394 kPa 422 453

Table 3. Dynamic tensile strength after high temperature (45°C) water immersion & UV

exposure (ETAG 5.1.4.2.1).

Aluminum/Glass 0 hr 504 hr 1008 hr Tensile Strength 412 kPa 400 kPa 325 kPa

Table 4. Dynamic tensile strength after NaCL immersion (ETAG 5.1.4.2.2).

Aluminum/Glass 0 hr 480 hr Tensile Strength 412 kPa 416 kPa

Table 5. Dynamic tensile strength - cyclic load/fatigue (ETAG 5.1.4.6.5).

Aluminum/Glass 0 cycles 5350 cycles Tensile Strength 412 kPa 406 kPa

3 VISCOELASTIC NATURE OF 3M™ VHB™ SRUCTURAL GLAZING TAPE The main purpose of the test study according to ETAG 002 was to gain a further understanding of the mechanical properties of 3M™ VHB™ Structural Glazing Tape. The challenge now with the data is to take the viscoelastic properties of this unique adhesive into consideration. Adhesives typically are elastic in nature. This means in general it is easy to determine maximum stress and modulus. This changes significantly if testing is done with a viscoelastic adhesive like 3M™ VHB™ Structural Glazing Tape. In Fig. 1 is an example of a stress/strain curve of a typical elastic adhesive compared to 3M™ VHB™ Tape when they are extended to a certain point of strain. This graph also demonstrates the capability of 3M™ VHB™ Structural Glazing Tape to stress relax when a constant strain is applied to a glazing joint for example from a differential thermal expansion and contraction type stress. This unique property of a viscoelastic adhesive will act to protect the bond line interface. This property is not typical of elastic adhesives and the associated stress relaxation effect would not be expected as observed with a viscoelastic adhesive.

Figure 1. Typical stress vs. strain curves of elastic adhesive compared to viscoelastic adhesive for concept representation.

As stated earlier the main challenge is to find a significant characterization model for these types of adhesives and to accurately describe viscoelastic behavior. The following characteristic of viscoelastic 3M™ VHB™ Tape is used to help explain this. 3M™ VHB™ Tape will exhibit

1

2

1

2

Elastic model

Viscoelastic model

2Stress



different modulus and ultimate strength based on the rate (speed) of the applied load. This means that 3M™ VHB™ Structural Glazing Tape tested at 500 mm/min shows much higher values than when tested at a slower rate of 5 mm/min (an elastic adhesive would be relatively constant). It also must be noted that a faster strain rate is associated with a high wind event as described in a section titled “Pull Rate” by the Schmidt et al [1988] paper referenced earlier in Section 1 of this paper. Tests conducted with 3M™ VHB™ Structural Glazing Tape at various strain rates yield a logarithmic curve as described below in Fig. 2.

0

200

400

600

800

1000

1200

0 50 100 150 200 250 300 350

δ (kPa)

strain rate (mm/min)

Figure 2. Ultimate stress vs. strain rate of 3M™ VHB™ Structural Glazing Tape. Figure 2 is an accurate depiction of the viscoelastic adhesive. By using the maximum elongation (strain) for each strain rate one can then calculate the load duration. This graph is then transferred into an exponential formula. This formula is used to calculate the dynamic and the static design strength of 3M™ VHB™ Structural Glazing Tape. This characterization data confirms the currently used 3M design guideline strength values of 85 kPa for dynamic loads such as windload and 1.7 kPa for static loads such as panel bonding for curtain walls. 4 ASTM STRUCTURAL PERFORMANCE MOCK-UP TEST STUDY The test study conducted by the CSTB demonstrates several of the material property characteristics of 3M™ VHB™ Structural Glazing Tape and the methodologies used for determining appropriate and safe design guidelines. A test study was also conducted at an accredited independent 3rd party test facility (Winwall Technology Pte Ltd - Singapore) to demonstrate the validity of these design guidelines in a real life based on ASTM mock-up tests under stresses and environmental conditions that glass panels would typically experience in a glazed curtain wall system. This test study consisted of bonding double glazed unit glass panels (insulated glass) as well as monolithic glass panels to a typical structural glazing metal profile. A side by side comparison was conducted comparing glass panels bonded with 3M™ VHB™ Structural Glazing Tape to samples bonded with a one-part structural silicone sealant - Dow Corning® 795. The glazed panels were constructed following the respective design guidelines for both 3M™ VHB™ Structural Glazing Tape and the structural silicone sealant for a windload design pressure of 2.9 kPa. The first test sequence consisted of a polyvinyl butyral (PVB) laminated glass panel bonded with 3M™ VHB™ Structural Glazing Tape, a double glazed unit (DGU) bonded with 3M™ VHB™ Structural Glazing Tape and a DGU bonded with the structural silicone sealant. The test protocol began with air infiltration testing according to ASTM E 283 Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen and water penetration according to ASTM E 331 Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference. Thermal cycling was also conducted for 20 cycles from -25°C to 70°C followed again by the air infiltration and water penetration test methods. After these initial tests there were no water or air leakages identified in any of the three window units. Testing then progressed with the same glazed panels



tested according to ASTM E 330 Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by Uniform Static Air Pressure Difference. No failure was observed with either the 3M™ VHB™ Structural Glazing Tape glazed panels or the structural silicone sealant glazed panel in this wind load structural test at ambient as well as cold and hot temperatures (32ºC, -25ºC, and 70ºC) up to the design pressure of 2.9 kPa, corresponding to a wind speed of 250 kph (155 mph). After this, positive and negative pressures were gradually increased in 1 kPa increments up to 8.4 kPa at ambient temperature conditions. At this negative pressure point the laminated glass failed and blew out of the chamber. However, glass was still attached and bonded to the 3M™ VHB™ Structural Glazing Tape around the perimeter of the glazing frame. Testing was then stopped at this point. A second test sequence patterned after the first test sequence was run consisting of two monolithic tempered glass lites bonded with either 3M™ VHB™ Structural Glazing Tape or the structural silicone sealant. The DGU panel bonded with 3M™ VHB™ Structural Glazing Tape was also subjected to this second test sequence after surviving the first test sequence. No failure was observed with either the 3M™ VHB™ Structural Glazing Tape glazed panels or the structural silicone sealant glazed panel in any of the tests including wind load structural tests up to a positive and negative pressure of 10 kPa which corresponds to a sustained wind speed of 467 kph (290 mph). The ability of the panels to survive pressure loads well beyond the original design pressure demonstrates the use of safety factors for both structural silicone sealant and 3M™ VHB™ Structural Glazing Tape. This testing protocol also demonstrated that no air or water leakage can be obtained with proper assembly methods. The glazed panels constructed with the acrylic foam structural glazing tape provided excellent performance overall compared to control panels glazed with a well accepted one-part structural silicone sealant. Table 6 below is a summary of this 3rd party test study.

Table 6. Summary of application specific performance tests.

Test Sequence Test Method Laminated Glass -

3M VHB SGT

Insulated Glass (DGU) - One Part

SS

Insulated Glass (DGU) - 3M VHB

SGT

8mm Tempered Glass - One Part SS

8mm Tempered Glass - 3M VHB

SGT

Air Infiltration ASTM E283 (at +0.3kPa)

No air leakage from panel





Water Penetration

ASTM E331 (at +0.72kPa) No water leakage No water leakage No water leakage No water leakage No water leakage

Temperature Cycling

20 cycles (-250C to 700C)

Subjected to 40 cycles







Water Penetration

ASTM E331 (at +0.72kPa) No water leakage No water leakage No water leakage No water leakage No water leakage

Windload Structural

ASTM E330 (-250C, 320C,

700C, hold for 1 minute)

+/-2.9kPa (60psf, 250kph)







Windload Structural Max.

ASTM E330 (320C, hold 10

sec)

+/- 6kPa (125psf, 360kph )

+/- 6kPa (125psf, 360kph )

+/- 8kPa (167psf, 416kph)

+/- 8kPa (167psf, 416kph)

+/- 8kPa (167psf, 416ph)

Windload Structural Destructive

Maximum Pressure (+/-)

Glass burst at -8.4kPa (175psf,

426kph)

> -8.4kPa (175psf, 426kph)

>10kPa (209psf, 465kph)

>10kPa (209psf, 465kph)

>10kPa (209psf, 465kph)

Subjected to 2x test cycles

+/-2.9kPa (60psf, 250kph) +/-2.9kPa (60psf, 250kph)

For each cycle, temperature is maintained at -250C for 15 minutes and

700C for 15 minutes

For each cycle, temperature is maintained at -250C for 15 minutes and

700C for 15 minutes



5 DURABILITY OF 3M™ VHB™ STRUCTURAL GLAZING TAPES The best measure of the durability of a structural silicone sealant or acrylic foam structural glazing tape is how it performs in real life applications. Structural silicone sealants have demonstrated durable performance over the last four decades when used properly in structural glazing applications. Acrylic foam structural glazing tapes such as 3M™ VHB™ Structural Glazing Tape has demonstrated durable performance since 1990 for this same application. Thus, it was appropriate to conduct an accelerated weathering study to evaluate the durability of 3M™ VHB™ Structural Glazing Tape compared to that of a two-part structural silicone sealant -Dow Corning® 983. This test study was conducted at the 3M Weathering Resource Facility in St. Paul, Minnesota. Test samples consisted of 6 mm clear float glass bonded to black anodized aluminum with either the two-part structural silicone sealant or 3M™ VHB™ Structural Glazing Tape. These samples were then placed in a high intensity UV light chamber (Xenon Arc lamp) with cycling heat and humidity (water spray). Samples were then taken out of the chamber at various time intervals and tested for tensile strength. The maximum exposure time in this cycling chamber was 10,000 hours. Figure 3 below depicts the tensile strength retention of both the two-part structural silicone sealant and 3M™ VHB™ Structural Glazing Tape in this test study.

Accelerated Weathering - Tensile StrengthHigh Intensity UV Light, Cycling Heat and Humidity

Pull Rate of 0.5"/min @ 70oF/50% RH Conditions

0

20

40

60

80

100

120

140

160

180

200

VHB™ Structrual Glazing Tape 2-Part Structural Silicone (9.5 mm)

Bonding Material

% O

rigin

al B

ond

Stre

ngth

Initial1000 hrs2500 hrs5000 hrs7500 hrs10000 hrs

Figure 3. Tensile bond strength after accelerated weathering. Both the two-part structural silicone sealant and the 3M™ VHB™ Structural Glazing Tape exhibit good tensile strength retention even after 10,000 hours in this harsh accelerated weathering chamber. This 10,000 hour test is a more severe UV cycling test with a higher total dose of radiation than that of the accelerated weathering tests described in both ETAG 002 and ASTM C 1184 Standard Specification for Structural Silicone Sealants. 6 SUMMARY Select grades of Acrylic Foam Tapes have proven their performance and durability for structural glazing applications since 1990 as described earlier in Section 1 Introduction of this paper. While these tapes may be new to the world in some locations they have earned acceptance in the marketplace in other parts of the world and their growth continues.



The data presented in this paper demonstrates the properties, performance and durability of an Acrylic Foam Structural Glazing Tape. The contract glazer, fabricator or curtain wall designer must be sure to use only an appropriate Acrylic Foam Structural Glazing Tape if they intend to use a tape for glass bonding in structural glazing applications. It is also critical to follow the design guidelines and quality control procedures as outlined by the Acrylic Foam Structural Glazing Tape manufacturer. The material properties of a viscoelastic Acrylic Foam Structural Glazing Tape are different than those of a structural silicone sealant. Both have demonstrated their performance in structural glazing applications for many years. The standards, codes and specifications currently written for structural glazing are focused on the properties of cold liquid applied, chemically curing elastomeric structural silicone sealants. While these documents are useful as a starting point to help characterize an Acrylic Foam Structural Glazing Tape, there exists a great need to develop appropriate standards, codes and specifications for Acrylic Foam Structural Glazing Tapes to ensure only suitable products are used for this critical application. ACKNOWLEDGEMENTS The authors wish to acknowledge Christophe Magniez for his work with the CSTB (France), Ken Goh for his work with the Winwall Technology Pte Ltd (Singapore) and Dick Fischer of the 3M Weathering Resource Center. REFERENCES EOTA 2005, ETAG 002 Guideline for European Technical Approval For Structural Sealant Glazing Kits (SSGK), Kunstlaan 40 Avenue des Arts, B, 1040 Brussels. Schmidt, C.M., Schoenherr, W.J, Carbary, L.D., & Takish, M.S., 1989, ‘Performance Properties of Silicone Structural Adhesives’, ASTM STP 1054 Science and Technology of Glazing Systems, pp. 22-45, ASTM International, West Conshohocken, PA, USA. 3M Corporation, 2007, 3M™ VHB™ Structural Glazing Tapes Technical Guide, pp. 5. Satas, D. 1989, Handbook of Pressure Sensitive Adhesive Technology, Van Nostrand Reinhold, New York, pp. 2-3. 3M Corporation, 2001, 3M VHB Durability Technical Bulletin, pp. 1. ASTM E283-04 2004, ‘Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen’, ASTM International, West Conshohocken, PA, USA. ASTM E331-00 2000, ‘Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference’, ASTM International, West Conshohocken, PA, USA. ASTM E330-02 2002, ‘Standard Test Method for Structural Performance of Exterior Windows, Doors, Skylights and Curtain Walls by Uniform Static Air Pressure Difference’, ASTM International, West Conshohocken, PA, USA. ASTM C1184-05 2005, ‘Standard Specification for Structural Silicone Sealants’, ASTM International, West Conshohocken, PA, USA.

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Acrylic Foam Structural Glazing Tapes - Project Vision ...projectvisiondynamics.com/wp-content/uploads/2017/01/DBMC-Paper … · Acrylic foam structural glazing tapes are ... The