TechArchive

Original article date: April 1998

The first of 3M’s very high bond (VHB) tapes was introduced 15 years ago. How has this alternative to mechanical fastening techniques developed in the interim? Alan Quinn sticks to the trail like glue!

The technology developed the existing pressure-sensitive adhesive tapes that had then been around for some time. What made the difference was the exceptional strength of the bond. These tapes had a peel-adhesion range of 12-16lb/in width (or up to 600psi in normal tensile strength). This made them at least six times stronger than the conventional double-sided tapes had been.

VHB tapes have a number of benefits over alternative fastening methods. They require no drilling or punching and no mixing or formulating. This makes production quicker and simpler and also eliminates dust and swarf.

Where some fastening methods require secondary finishing steps, such as filling, smoothing and painting, tape joints are concealed and do not damage surfaces. They are lighter and less bulky than mechanical fastening systems. They also reduce the potential for rust formation, because no holes are needed in metal surfaces for screws or bolts to pass through.

In many cases, tape speeds up assembly, being quick to apply with relatively little effort. The tapes are packaged with a removable liner that protects the adhesive surface prior to use. During assembly, pressure-sensitive tapes can be used in several ways, according to production volume. In the simplest form, foam tape lengths can be cut and applied and applied by hand. Tape may also be supplied in die-cut shapes.

The acrylic foam tapes also have insulative properties which might be utilised in the design of a thermal break. A thermal break avoids differences in temperature within a structure, so reducing condensation and also the stresses associated with differential rates of expansion, such as those between aluminium and polycarbonate.

The rigid characteristics of conventional mechanical fastening may result in twisting or buckling with temperature variations. In contrast, VHB tape will tolerate movements up to six times its thickness without incurring any deterioration in bond performance. In applications where assembled parts are subject to vibrational and shearing forces, the ability to damp down these forces, much like a shock absorber, is a great benefit. Rattling of metal parts is prevented and VHB tape can also prevent electrolytic corrosion between dissimilar metals (see feature article on page 00).

No single tape can handle every holding and fastening task. Over the 15 years, a selection of tape formulations has emerged. Two types of adhesives are commonly used for pressure-sensitive tape – acrylic and rubber. Acrylic formulations meet high strength holding requirements. They are made from a combination of raw materials that undergo a chemical reaction to become a stable new material with predictable and controllable characteristics. Various combinations of synthetic materials give combinations of performance characteristics.

By contrast, rubber adhesives adhere more easily to a wider variety of surfaces, but cannot be made with bond strength or temperature and solvent resistance characteristics that match those of acrylic adhesives. Because of their good wetting ability, soft adhesives require less rub down pressure to form a bond. The homogeneous nature of a rubber adhesive is less sophisticated chemically than acrylic.

Degree of firmness is an important characteristic of any adhesive. Those formulated to be soft (and which tend to feel more sticky) have higher initial adhesion, while firm adhesives may have reduced holding power initially, but increase in bond strength over a period of time to give a much higher final bond strength. Many variations of relative firmness and softness exist for different applications.

The proper selection of an adhesive is closely related to the type of materials to be joined and the nature of physical loading to be encountered (peel, shear and tensile forces). Some important properties are unfortunately mutually exclusive: for example, if high temperature stability is important, it may be difficult to incorporate high initial holding power in the same tape.

Tape thickness must also be considered. Smooth and regular surfaces could be jointed with a thin tape layer, while irregular surfaces will require a thicker material, a double-faced foam tape for example. As an adhesive carrier, foam adds mechanical stability to a pressure-sensitive adhesive and contributes to peel, shear and tensile strength. The holding strength required for a particular application will be determined in large part by the surface area available for the tape. A large bonding area permits the use of a lower bond strength (perhaps with other properties critical to the application), while a small adhesive area could require the adhesive with the highest available bond strength.

One vehicle builder uses VHB tape to join 10m long fibreglass body panels on the sides of a motor home. According to the manufacturer, the VHB tape forms a bond that is stronger than the fibreglass itself.

Similarly, a manufacturer of emergency vehicles uses VHB tape to fasten body panels. Preferred to welding for finished appearance and for the shock-absorbing properties of the tape, it also eliminates rattle from the metal-to-metal construction.

Evidence for the durability of tape assemblies comes from a manufacturer of truck trailers who evaluated options for mounting panels to frame members. The evaluation was prompted by damage to cargo being caused in transit by assembly welds. The test was designed to represent 15 years of over-the-road usage. By the end of test, all of the riveted panels had reportedly failed along with half the welded panels, but all the panels bonded with VHB tape were intact with no separation. Assembly was also faster.

An ambulance manufacturer credits the tape with increasing production and cutting costs by approximately 50%. VHB tape was used to bond exterior panel skins and trim. The tape reduced the time needed to manufacture each ambulance by about 25 hours, while cutting rejects, downtime and cleanup.

• 3M Specialty Tapes & Adhesives
• 0161 237 6363

April 1998