Adhesive tapes offer greater design freedom and reduced costs

Paul Boughton

Double-sided adhesive tapes can be found in products all around us today, from automobiles and mobile telephones, to domestic appliances and furniture. One of the advantages of adhesive tapes is that they can be used invisibly, with no outward sign of their presence. For designers not familiar with the use of tapes, it can therefore be more difficult to be inspired to use this method of joining components rather than alternatives, yet modern adhesive tapes are remarkably versatile and the technology is continuing to advance at a rapid pace.

Furthermore, adhesive tapes can be used for a variety of applications beyond simply joining two components: they can be used for sealing, spacing components at a defined distance from each other, laminating different materials, improving EMC (electromagnetic compatibility) and EMI (electromagnetic interference) performance of electrical/electronic devices, improving noise and vibration performance, providing thermal or electrical insulation, and temporarily holding components together to simplify complex assembly processes.

Designers may have specified adhesive tapes in the past, only to suffer product failures or difficulties during manufacture. Suppliers of tapes will say that such problems may have been due to incorrect specification or application equipment. They will also say today's adhesive tapes deliver higher performance and the application experience now available among suppliers means that projects are unlikely to fail.

Double-sided tape types

Many different types of double-sided tape are available, though they generally fall into five categories: filmic tapes with a thin carrier film sandwiched between two layers of adhesive; non-woven tapes that have a non-woven carrier sandwiched between adhesive layers or impregnated with adhesive; foam tapes that have a thicker foam carrier between the adhesive layers; cloth tapes that use cloth/fabric in place of film or foam; and transfer tapes that are purely adhesive supported by a release liner. In addition, converters can create custom tapes by laminating any of these with other materials. Adhesives are normally pure acrylic, modified acrylic, natural rubber or synthetic rubber, with a range of characteristics available to suit different substrates, operating environments and production requirements. Although the release liner may seem a minor component in the make-up of the tape, it has an important role to play in terms of the ease with which the tape can be processed and handled. For example, a polyethylene-coated paper liner will be better suited to kiss-cutting than a low-cost glassine (silicone-treated) paper liner, whereas a polypropylene film liner is transparent, allowing for visual inspection of the applied adhesive before final assembly.

Suppliers of tapes - with which we will include here converters that purchase tapes from manufacturers and process them by slitting, laminating it and die cutting as necessary - have largely evolved to serve the demands for customised products. Lohmann, which describes itself as one of Europe's largest manufacturer of technical pressure-sensitive adhesive tape systems, says that around 85 per cent of its sales are of tailored products, converted locally to suit customers' specific requirements. Lohmann processes tapes from 3M, Avery, Adhesive Research, Scapa and others, meaning that customers have an extensive choice of adhesive technologies and performance characteristics. These tapes will often be attached to non-adhesive materials including metal foils, felts and foams, Poron microcellular urethane foams, Bisco silicone foams, and hook-and-loop fasteners.

One of Lohmann's recent developments is ESD protection films for electronic applications. These films are covered with a special antistatic coating that is insensitive to moisture and has a controlled surface resistance. Also for the electronic market, the company has created an electrical conductive tape range called DuploCOLL EC (electrical conductive). Typical tapes in this range consist of a conductive non-woven material, copper or aluminium foil as the carrier, with a conductive acrylic adhesive. These adhesive tapes have high electrical conductivity as well as providing EMI shielding. Applications are found in automotive infotainment devices, navigation systems, displays, CD/MP3 players, mobile telephones, disk drives, televisions and hi-fi systems.

Optically clear tapes can be used for bonding screens, lenses, prisms and other assemblies featuring clear or coloured glass or plastic components. Such tapes are available from, among others, Lohmann, 3M and Adhesives Research. Other specialist characteristics that can be specified for adhesive tapes today include flame retardance and moisture barrier properties.

Niche markets: from solar to crime prevention

The introduction of so many new products may come as a surprise to many design engineers, but Tesa UK, another leading supplier in this field, claims that half of its turnover derives from products developed within the past five years. As an example of the types of niche markets in which Tesa operates, the company offers tapes that are TÜV-approved for bonding solar modules. For high-temperature operating environments, Tesa has a range of heat-activated films (known as the HAF series) that can withstand temperatures of up to 500°C. In a bid to reduce certain types of vehicle crime, the company has also developed a tape for fixing VIN (vehicle identification number) plates; a die mixed into the adhesive leaves a 'footprint' in the substrate surface that fluoresces under ultraviolet light if the plate is removed.

Ultraviolet (UV) light has traditionally caused problems for adhesives tapes, especially in outdoor applications or those where the product is exposed to sunlight, as the UV light changes the composition of the adhesive - which can result in premature product failure unless the joint is designed adequately to exclude UV light. However, companies such as Tesa are addressing this problem by launching standard products for such applications, often as a spin-off from work in the automotive industry. For example, Tesa 62852, 62854 and 62855 are based on automotive tapes, with a closed-cell polyethylene foam backing and pure acrylic adhesive. These offer better UV performance, water-, solvent- and temperature-resistance and anti-ageing properties than alternative tapes.

Manufacturers of adhesive tapes have also made significant advances in creating tapes for bonding to substrates with low surface energy - such as polyethylene, polypropylene, polyurethane, EPDM (ethylene propylene diene M-class) elastomers, PTFE (polytetrafluoroethylene) and silicone. In the past it was difficult to achieve high bond strengths on these, unlike on polyester, glass and metals that have a high surface energy. Plasticisers used when compounding plastics to improve flexibility can also cause problems for adhesive tapes, as they migrate from the substrate to the adhesive, degrading it and leading to joint failures. Suitable adhesive tapes are available for bonding plasticised plastics, hence it is important to understand the substrate composition before specifying an adhesive tape. Indeed, it used to be the case that bonding to 'difficult' plastics was problematic but today there are adhesive tapes suitable for use with virtually any plastic.

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